WO2004049056A1

WO2004049056A1 - Method and apparatus for generating narrowband coherent light output from an optical oscillator

Info

Publication number: WO2004049056A1
Application number: PCT/AU2003/001570
Authority: WO
Inventors: Brian John Orr; Yabai He
Original assignee: Macquarie University
Priority date: 2002-11-22
Filing date: 2003-11-24
Publication date: 2004-06-10

Abstract

An apparatus and a method are described for providing a narrowband coherent beam of optical radiation, along an optical path that is different from that of any broadband radiation which is also generated. The apparatus (10) includes an OPO (12) pumped by a laser (14) and wavelength selection means (30) disposed outside of the optical cavity of the OPO (12). The wavelength selection means (30) selects a narrowband component from a coherent output beam (28) generated by the OPO (12), and introduces the narrowband component (32) into the OPO cavity (12) as injection-seeding radiation in such a manner that, upon the introduction into said optical cavity of a second pump beam (14.2), the narrowband component (32) overlaps spatially and temporally in the nonlinear-optical gain medium of the OPO (12) with the second pump beam (14.2), thereby interacting with the nonlinear-optical gain medium of the OPO (12) to generate a narrowband second coherent output (34) in a single longitudinal mode.

Description

OPTICAL PARAMETRIC OSCILLATOR AND METHOD OF USE

Technical Field

The present invention relates to optics and its applications. More particularly, the invention relates to pulsed, tunable coherent light sources such as optical parametric oscillator (OPO) devices, of which the output wavelengths can be controlled. Such light sources are effectively the nonlinear-optical counterpart of lasers. Pulsed, tunable OPOs enable spectroscopic diagnostic sensing of chemical substances in industrial, clinical, or environmental situations. Other examples of the use of such light sources are in photonics and optical telecommunications applications (e.g., wavelength division multiplexing), or in remote sensing the atmosphere (e.g., by lidar), or in basic scientific measurements.

Background of the Invention During the past four decades, tunable coherent light sources (lasers and their nonlinear-optical counterparts) have played a vital role in spectroscopic sensing of chemical processes, in industrial and environmental diagnostics and in basic science. One such form of tunable coherent light source, the pulsed optical parametric oscillator (OPO), had an impressive impact in the first 15 years of that period [1 - 4], followed by sporadic progress in the next decade [4 - 8]. The reason for this temporary decline of interest was probably because the early tunable nanosecond-pulsed OPOs were almost invariably bulk lithium niobate (LiNbO₃) devices, which proved difficult to operate and were damage- prone, largely because intracavity losses from gratings and etalons caused the operating •threshold to approach the- damage -threshold of optical materials.such. as. lithium niobate.. Within the last 15 years, there has been a dramatic resurgence of interest in pulsed tunable OPOs, owing to various advances in optical and laser technology that make them more efficient and trouble-free.

A variety of nanosecond-pulsed, continuously tunable OPOs, with various wavelength-control strategies that are useful for numerous spectroscopic sensing applications [4, 9 - 25], are known to the applicant. Among the most recent advances [19, 20, 22 - 25] is the generation of single-longitudinal-mode tunable coherent radiation from a pulsed OPO system comprising an actively controlled ring cavity with a quasi- phase-matched (QPM) nonlinear-optical material (such as periodically poled lithium niobate, PPLN) as the OPO gain medium, and a continuous-wave, single-longitudinal- mode tunable diode laser to injection-seed the OPO. Such a tunable-diode-laser-seeded, quasi-phase-matched OPO is pumped by a pulsed Nd:YAG laser which is either a high- performance single-longitudinal-mode system [19, 20, 24] or a compact, low-cost multi- mode system [22 - 24].

Within the last 15 years, OPOs have regained acceptance as high-power pulsed tunable sources suitable for many spectroscopic applications such as atmospheric sensing (e.g., by infrared lidar [21, 25]). This revival in interest in OPO technology [4 - 8] is attributable to new nonlinear-optical materials [26] such as beta barium borate (BBO) and potassium titanyl phosphate (KTP), and further enhanced by the more recent availability of quasi-phase-matched nonlinear-optical media such as periodically poled lithium niobate (PPLN) [27, 28]. The implementation of pulsed tunable OPOs has been further facilitated by improved pump lasers (many of them commercially available), and also by advanced OPO system designs offering tunable narrowband operation by means of intra- cavity gratings and/or etalons [3, 29] or by injection seeding (reviewed recently in refs [22] and [25]).

OPOs typically involve coherent three-wave nonlinear-optical processes in non- centrosymmetric solid-state media [1 - 7, 26], with a single laser input wave (also referred to as a 'pump' wave, having a frequency ω_P) and two coherent output waves (also referred to as a 'signal' wave, having a frequency ω_s, and an 'idler' wave, having a frequency co_τ, where ω_s > ω_τ). These three waves obey both energy conservation and phase-matching conditions: ωp = ω_s + ω_j ; and k_P - k_s - k_j - Δk = 0 (1) wherein k_j is a wave vector with the subscript j denoting pump radiation (P), signal radiation (S) or idler radiation (I), respectively. The magnitude of k_j equals n_j (BJ / c (= 2π ΓLJ / λ_j), where n_j is the refractive index at vacuum wavelength λ;, and c is the speed of light. The phase-mismatch increment Δk must be minimised to optimise OPO conversion efficiency and thereby to control the output signal and idler wavelengths, λ_s and λ_τ.

Equation (1) applies to conventional birefringently phase-matched nonlinear-optical media, in which phase matching is achieved by adjusting the angle and/or temperature of a birefringent nonlinear-optical crystal via its ordinary- and extraordinary-ray refractive indices [26], resulting in a birefringently phase-matched OPO. Various high-quality bulk OPO materials (such as lithium niobate, beta barium borate and potassium titanyl phosphate) may be used. However, different cuts of OPO crystal are required for different spectral regions [26]. A more recently implemented alternative, for which a slightly modified version of equation (1) is required, is to use quasi-phase-matched media tailored for specific wavelengths by periodic optical structuring; periodically poled lithium niobate (PPLN) is a prominent example [19, 20, 22, 24, 25, 27, 28]. Other promising quasi-phase-matched OPO materials include periodically poled potassium titanyl phosphate (PPKTP), and periodically poled rubidium titanyl arsenate (PPRTA). All of these quasi-phase-matched OPO media offer higher nonlinear-optical coefficients, lower operating thresholds, and smaller size than birefringently phase-matched OPO materials. Another recently developed quasi-phase-matched nonlinear-optical material is orientation-patterned gallium arsenide (GaAs), which offers (as yet unrealised) OPO tunability out to about 16 μm in the far-infrared wavelength range with a conveniently short pump wavelength at about 1 μm [25].

Optical cavities, in which light is reflected resonantly between carefully aligned mirrors, play a central role in controlling the optical bandwidth and output wavelength of tunable coherent light sources, both lasers and nonlinear-optical systems (such as OPOs). Optical cavities are also vital in applying such light sources to high-resolution spectrometry, by means of devices such as Fabry-Perot etalons or Fourier-transform (e.g., Michelson) interferometers. Cavity-enhanced spectroscopy is prominent in various forms optical sensing science, for example, cavity ringdown spectroscopy [30, 31] with either pulsed OPOs [22, 23] or continuous-wave tunable lasers [32 - 35].

Various types of optical cavity design may be used for pulsed OPOs. At one extreme of operational simplicity are 'free-running' pulsed OPOs, with no intra-cavity wavelength-selective components. Such pulsed OPOs usually employ a simple two- mirror optical cavity (resonant at either λ_s or λ_τ) and are usually pumped at λ_P by a pulsed monochromatic coherent source (e.g., harmonics of a Nd:YAG laser). Free-running pulsed OPOs yield broadband tunable output radiation and may be based on nonlinear- optical materials [26] that are either birefringently phase-matched (using angle- or temperature-tuned bulk media such as lithium niobate, beta barium borate, or potassium titanyl phosphate) or they may be quasi-phase-matched (such as periodically poled forms of lithium niobate, potassium titanyl phosphate or rubidium titanyl phosphate). Optical bandwidths of the pulsed OPO output signal and idler radiation are typically 5 - 50 cm^"1 [1 - 7, 13, 15, 36 - 40]. The output of such a free-running pulsed OPO (with no wavelength-selective mechanism) is therefore broadband, comprising many frequency components determined by the resonance frequencies of the OPO cavity. The output from such a light source is suitable for low-resolution or multiplex spectroscopy, but unsuitable for use in many spectroscopic applications or in most optical communications situations. For many higher-resolution spectroscopic applications, or in many optical communications situations, it is necessary to use additional ways to narrow the optical bandwidth and control the output wavelengths. The conventional approach, at the other extreme of operational complexity, is to employ intracavity gratings and/or etalons [3 - 7, 29, 46 - 51]. Various continuously tunable nanosecond-pulsed OPO designs of this type have been used to generate single-longitudinal-mode signal or idler output radiation of narrow optical bandwidth for a variety of high-resolution spectroscopic applications. A disadvantage of this approach is that intracavity wavelength-selection elements such as gratings and etalons tend to introduce optical losses that cause the threshold for OPO operation to approach the optical damage threshold of the nonlinear-optical medium. Such complications were particularly prominent during the first twenty years of OPO development. They are still problematic now, although the problems have been diminished by improvements in pump-laser design, in nonlinear-optical materials, in mirror coatings, and in tunable OPO cavity design.

Within the last 15 years, a popular alternative approach to narrowband nanosecond- pulsed OPO tuning has been injection seeding with a dye laser [4, 5, 9 - 12, 52], a single- mode tunable diode laser [14 - 20, 53 - 61], or other forms of tunable coherent light source [61 - 64]. This mjection-seeding approach has the advantage of eliminating optical losses due to intracavity wavelength-selection elements such as gratings and etalons. However, it is limited by the need for suitable tunable lasers or other coherent light sources for injection-seeding. Such mjection-seeding sources are often more expensive than intracavity gratings and etalons and, moreover, they are generally more limited in their optical tuning range.

The versatility and flexibility in design of pulsed tunable OPOs are attributable to the fact that an OPO is a nonlinear-optical device, not a laser. This facilitates methods of temporal and wavelength control (and, consequently, appropriate spectroscopic detection schemes) that are not possible with lasers. Lasers generally depend on population inversion of an optical gain medium, with associated optical lifetime and saturation limitations. However, the contrasting processes of optical parametric gain, oscillation and amplification are more amenable to modular system design because they depend on nonlinear-optical coefficients and phase-matching conditions.

High-resolution spectroscopic applications require the narrowest possible optical bandwidth and high spatial beam quality. Active control of the length of an injection- seeded OPO cavity is generally necessary for stable, continuously tunable single- longitudinal-mode operation. This has been achieved by actively varying the length of the OPO cavity synchronously with the wavelength scan of the seed source, using some form of opto-electronic feedback to stabilise the process. However, progress on this front has been limited, with only a few published reports of high-resolution spectra actually recorded by actively tuning output radiation from an injection-seeded nanosecond-pulsed OPO (either continuously [19, 20, 47, 49] or in fine wavelength steps [55]).

Higher output power (which may be required for applications such as atmospheric lidar sensing [21] or nonlinear-optical wavelength conversion [11, 22] or molecular state preparation [22]) is attainable by means of one or more additional optical parametric amplifier (OP A) stages [19, 20, 22, 60]. Oscillator/amplifier configurations are already adopted in commercial pulsed, tunable OPO systems (such as the Continuum Mirage system [29]) to meet a recognised consumer demand for a combination high output power, continuous tunability, and narrow optical bandwidth (e.g., to make OPO/OPA's at least competitive with comparable pulsed tunable dye or titanium: sapphire laser systems). A modular high-performance pulsed spectroscopic system for some applications has been developed and reported by the applicants. It comprises a tunable-diode-laser-seeded periodically poled lithium niobate OPO with an actively controlled ring cavity, and a bulk-lithium niobate OP A stage [19, 20, 24]. That modular periodically poled lithium niobate OPO/OPA system is pumped at 1.064 μm by a single-longitudinal-mode nanosecond-pulsed Nd:YAG laser, injection-seeded by a continuous wave tunable diode laser at about 1.5 μm, and generates coherent narrowband signal and idler outputs that are continuously tunable in the vicinities of about 1.5 μm and about 3.5 μm, respectively. An intensity-dip control scheme is used to lock the length of the OPO ring cavity to the single-longitudinal-mode tunable diode laser seed radiation. Spectroscopic experiments with output from this nanosecond-pulsed periodically poled lithium niobate OPO confirm a remarkably narrow effective scanning bandwidth as small as about 100 MHz (about 0.003 cm^-1) and good beam quality. This enables high-resolution, time-resolved laser spectroscopy, particularly in molecular-beam environments or in gases at low pressure. The OPO-based spectroscopic systems described above are typically pumped by high-performance nanosecond-pulsed solid-state lasers (many of them commercially available from suppliers such as Continuum, Positive Light, Quantel, Spectra-Physics, and Spectron). A frequently used OPO pump laser is a 1.064-μm flashlamp-pumped, Q- switched Nd:YAG oscillator/amplifier system that is equipped with an injection seeder for single-longitudinal-mode operation, special cavity optics to yield a quasi-Gaussian beam profile, and nonlinear-optical stages to generate harmonics at 532 nm and 355 nm. Typical operating parameters are: pulse duration, about 8 ns; repetition rate, 10 Hz; pulse energies: >1 J at 1.064 μm, >0.7 J at 532 nm, >400 mJ at 355 nm. The optical bandwidth of one such single-longitudinal-mode pulsed laser has been measured with a confocal Fabry-Perot etalon to be 45 ± 5 MHz (0.0015 ± 0.0002 cm^"1) fwhm.

As an alternative to an OPO, an optical parametric generator (OPG) or amplifier (OP A) [65] may be used, thereby eliminating the need to enclose the nonlinear-optical gain medium in an optical cavity. Examples of this approach include pulsed OPG/OPA systems based on either birefringently phase-matched [66] or quasi-phase-matched [67, 68] nonlinear-optical media.

Injection seeding of a pulsed OPO may also be arranged for simultaneous generation of two or more adjustable output wavelengths. The corresponding injection- seeding wavelengths can be controlled by spectroscopic tailoring, for instance, to match on- and off-resonance wavelengths of a spectrum of interest. Such spectroscopically tailored pulsed OPOs have been demonstrated and potential applications proposed by the inventors [16, 21, 25]. Dual- wavelength injection seeding of OPOs is particularly relevant to atmospheric remote sensing techniques such as DIAL (differential absorption lidar) [21, 25], where simultaneous monitoring of characteristic on-resonance and off- resonance wavelengths is accessible by means of a spectroscopically tailored OPO system. Another application of a spectroscopically tailored OPO, already realised [16], entails thermometric sensing of nitrogen gas in furnace air by OPO-based coherent anti- Stokes spectroscopy (CARS).

US Patent No 5,659,419 describes a tunable OPO that is excited in pulsed fashion to generate tunable narrowband radiation. The OPO employs wavelength-selective means, disposed outside a resonant cavity, to reduce the bandwidth of the OPO radiation generated. A pumping pulse is divided by a beam splitter into at least two partial pulses, and a delay between the partial pulses is adjusted by means of a delay distance in such a manner that the first partial pulse will pump the crystal of the OPO generating broadband output, part of which is passed through the wavelength-selective means and returned to the OPO as narrowband radiation, to pass through the crystal when it is pumped by the second partial pulse. The narrowband output from the OPO caused by the second partial pulse is passed through a diaphragm, separating broadband radiation from narrowband radiation. This patent depends on the use of a two-mirror resonant OPO cavity, which results in narrowband output and broadband output being in the same physical space. The apparatus disclosed in this patent suffers from the disadvantage that, because the narrowband and broadband outputs are in the same physical space, the diaphragm is generally incapable of achieving a reliable separation thereof, despite the difference in timing of the broadband and narrowband pulses arriving at the diaphragm. No means for separating the narrowband and broadband outputs into different optical paths are disclosed. This patent also does not disclose the use of means for controlling the optical path length of the resonant cavity, nor does it disclose any means for controlling the operating conditions of the crystal or for controlling the optical delay. It does not specify any means to control the timing of the returned narrowband radiation by the wavelength- selective means.

Object of the Invention It is an object of the present invention, in achieving narrowband operation of a tunable OPO, either pulsed or continuous-wave, to overcome or substantially ameliorate at least one of the above disadvantages.

Summary of the Invention According to a first aspect of the present invention, there is provided an apparatus for providing a narrowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous- wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising:

- an optical parametric oscillator including an optical cavity comprising at least three optically interconnected reflectors or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed from one to the other, and a nonlinear-optical gain medium, said nonlinear-optical gain medium being capable of generating, from a first pump introduced into said optical cavity in a first direction, a first coherent output beam pulse that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a narrowband component comprising signal and/or idler radiation, from said first coherent output beam pulse, and for introducing said narrowband component into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction and at a time such that, upon the introduction into said optical cavity of a second pump pulse, said narrowband component overlaps spatially and temporally in said nonlinear-optical gain medium with said second pump pulse thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent output beam pulse;

- means for decoupling said first coherent output pulse from said cavity, along a first optical path, into said selection means;

- means for coupling said narrowband component from said selection means into said cavity; and - means for decoupling said narrowband second coherent output pulse from said cavity along a second optical path that differs from the first optical path.

The intensity and the wavelength of the second pump pulse is preferably such as to generate sufficient gain in the nonlinear optical gain medium at an OPO output wavelength that corresponds with the wavelength of at least a portion of said narrowband component whereby it interacts with said nonlinear optical gain medium to generate a narrowband second coherent output beam pulse.

In all aspects of the invention, the optical cavity may be arranged as a ring cavity comprising three or more reflectors and/or deflectors, or as bow-tie cavity, for example, where reflectors are arranged in such a way as to resemble a bow-tie. The means for coupling and the means for decoupling may each be a suitable reflector. Any one or more of the means for decoupling may be a suitable deflector such as a prism.

The reflector may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s). In all aspects of the invention, the reflectors may comprise reflective surfaces. The reflectors may be mirrors, for example.

The deflector(s) may be a prism(s) having at least two surfaces disposed at a first angle relative to each other, so as to cause the prism to deflect a beam of optical radiation through a second angle. Where deflectors are used, an optical cavity may be configured that has a simple mechanical construction and that may improve the mecahnical stability of the apparatus. Also, a deflector may provide a means for decoupling desired radiation out of the optical cavity and for coupling input radiation into the optical cavity.

The optical cavity may comprise three or more optically interconnected reflective surfaces or mirrors. In a preferred embodiment of the invention, the optical cavity comprises four mirrors, arranged in "bow-tie" fashion or at the four corners of a rectangle. In order to control the optical path length inside the optical cavity, the position of at least one of the mirrors may be adjustable. This may be achieved by means of a piezo- electrically operated positioning device. Alternatively or additionally, to control the orientation of at least one of the mirrors may be adjustable, in order to facilitate tuning of the OPO output wavelengths. One or more of the optical cavity mirrors may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s), in order to decouple the narrowband second coherent output beam pulse from the cavity. One or more of the optical cavity mirrors may be highly reflective.

The apparatus may further comprise means to control the wavelength band centres of the OPO output by adjusting the operational parameters (eg temperature, orientation, position, etc of the non-linear optical medium) which affect the phase-matching conditions of the optical parametric process.

The narrowband component selected from said first coherent output beam pulse may be either single-feature or multiple-feature narrowband coherent light. When such radiation is used to injection-seed the optical parametric oscillator, said second coherent output beam pulse will have a corresponding spectral composition. The narrowband component may be selected from either the signal or the idler radiation forming part of the first coherent output beam pulse. In the event that it is desirable to use injection- seeding having a multiple-feature narrowband, said narrowband component may be selected from the signal and idler radiation of the first coherent output beam pulse.

The means for selecting said multiple-feature narrowband component from either the signal or the idler radiation of said first coherent output beam pulse may conveniently comprise means for separating one or more desirable single-feature components from any undesirable wavelength components, and for discarding or not using such undesirable wavelength components. The apparatus according to the first aspect of the invention may further comprise continuous tuning means for controlling and/or varying the wavelength of each feature of said narrowband component.

The apparatus may further comprise means for introducing said selected narrowband component into said optical cavity as injection-seeding radiation, for the generation of said second coherent output beam pulse.

The first and second pump pulses may originate from the same input pulse or from different input pulses. In the event that they they originate from the same input pulse, the apparatus may comprise a beam splitter to split the input pulse into the two components. In the event that the first and second pump pulses originate from different input pulses, the different pulses may originate from the same or different sources of radiation. In either such events, the apparatus may comprise suitable switching and control means to ensure that the desired time delay between the first and second pump pulses is achieved.

The first and second pump pulses may have the same or different pulse durations. In one embodiment, the first pump pulse is a continuous wave pump beam, and the second beam is a pulsed or continuous-wave beam. In the event that both pump beams are pulsed, the two pulsed beams may be synchronised.

The first and second pump pulses may have the same or different wavelengths.

The selection means may be operatively disposed relative to the optical cavity of the optical parametric oscillator. Conveniently, it is operatively disposed between the first output means and the second input means.

As an alternative, first directive means may be provided to cause the first output coherent pulse to be directed to said selection means and second directive means to cause said narrowband component to be directed to said optical cavity. The first directive means and the second directive means may be the same or different.

The wavelength(s) of the narrowband component may be wholly the wavelength(s) having sufficent gain in the OPO process of the second pump pulse or may be partly within the range of wavelength(s) having sufficent gain in the OPO process of the second pump pulse. The first and second directions may be the same or different.

The timing of the introduction, into the nonlinear-optical gain medium, of the narrowband component and the second pump pulse is important for the efficient operation of the apparatus in accordance with the invention. The timing for the introduction of the two pulses may be such that there is a sufficient overlap for the narrowband component to be capable of injection-seeding the optical parametric oscillator. Put in a different way, there is preferably at least sufficient energy originating from the narrowband component left in the optical cavity for the nonlinear-optical gain medium to be in an injection- seeded condition slightly before and during the period that the second pump pulse interacts with the nonlinear-optical gain medium. In a preferred embodiment of the mvention the narrowband component is introduced into the nonlinear-optical gain medium just prior to the introduction into said optical cavity of said second pump pulse.

In order to achieve a desired timing for the introduction of said narrowband component into the optical cavity, the apparatus in accordance with the invention may comprise optical delay means between the optical cavity and the selection means for selecting a narrowband component from said first coherent output beam pulse. This optical delay means may be adapted to delay the arrival of the narrowband component to achieve a desired overlap with the second pump pulse. Alternatively or additionally, means may be provided for achieving a desired time delay between said first pump pulse and said second pump pulse. Such means may comprise a second pump laser source and a suitable timing control device adapted to ensure that the second pump pulse arrives at the optical cavity at the desired time. By using a second pump laser source, it becomes possible to use first and second pump pulses of different characteristics, including different wavelengths and pulse durations.

Similarly, the intensities of the narrowband component and the second pump pulse are important for the efficient operation of the apparatus in accordance with the invention. As stated above, there is preferably at least sufficient energy originating from the narrowband component left in the optical cavity to injection-seed the nonlinear gain medium when the introduction of the second pump pulse commences.

The second pump pulse and the first pump pulse may use the same region of the nonlinear-optical gain medium, or a different region of the nonlinear-optical gain medium, or a separate nonlinear-optical gain medium which may be the same or different material or the same material cut at a different angle or with a different periodic or regular internal structure.

By using a ring or bow-tie optical cavity, in which the direction of propagation of a given component of light is uni-directional, it is also possible to use more than one intracavity nonlinear-optical gain medium, each independently pumped. The different media may be made of the same or different materials. Alternatively, they may comprise crystals cut at different angles or with different periodic or regular internal structure. Optionally, different regions of the same crystal may be used as different media. As still another option, the different media may be operated at different operating conditions such as temperature. These variations in the construction of the optical cavity may be employed to extend the range of wavelengths that can be handled by the apparatus.

According to a second aspect of the present invention, there is provided a method for provoding a narrowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous-wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first pump pulse, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said first pump pulse, a first coherent output beam pulse that has a broad band of signal and idler wavelengths;

- decoupling said first coherent output beam pulse from said cavity, along a first optical path;

- selecting, in a location outside of said optical cavity, a narrowband component comprising signal and/or idler radiation, from said first coherent output beam pulse;

- coupling said narrowband component into said cavity and into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction and after a suitable time delay; and

- introducing a second pump pulse into said nonlinear-optical gain medium in such a manner and at such a time that said narrowband component overlaps spatially and temporally in said nonlinear-optical gain medium, with said narrowband component, thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent output beam pulse; and

- decoupling said narrowband second coherent output beam pulse from said cavity along a second optical path which differs from the first optical path. According to a third aspect of the present invention, there is provided an apparatus for providing a narrowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising: - an optical parametric oscillator including an optical cavity comprising at least three optically interconnected reflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected from one to the other, and a nonlinear-optical gain medium, said nonlinear-optical gain medium being capable of generating, from a first continuous-wave pump beam introduced into said optical cavity in a first direction, a first coherent continuous-wave output beam that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a narrowband component comprising signal and/or idler radiation, from said first coherent continuous- wave output beam, and for introducing said narrowband component into said nonlinear- optical gain medium as injection-seeding radiation, in a second direction such that, upon the introduction into said optical cavity of a second continuous-wave pump beam, said narrowband component overlapping spatially in said nonlinear-optical gain medium with said second continuous-wave pump beam thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent continuous-wave output beam; - means for decoupling said first coherent output beam from said cavity, along a first optical path, into said selection means;

- means for coupling said narrowband component from said selection means into said cavity; and

- means for decoupling said narrowband second coherent continuous-wave output beam from said cavity along a second optical path that differs from the first optical path.

The second continuous-wave pump beam conveniently generates sufficient gain at an OPO output wavelength that corresponds with the wavelength of at least a portion of said narrowband component, so as to generate the narrowband second coherent continuous-wave output beam. The means for decoupling may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s).

The optical cavity may comprise three or more optically interconnected reflective surfaces or mirrors. In a preferred embodiment of the invention, the optical cavity comprises four mirrors, arranged in "bow-tie" fashion or at the four corners of a rectangle. In order to control the optical path length inside the optical cavity, the position of at least one of the mirrors may be adjustable. This may be achieved by means of a piezo-electrically operated positioning device. Alternatively or additionally, to control the orientation of at least one of the mirrors may be adjustable, in order to facilitate tuning of the OPO output wavelengths. One or more of the optical cavity mirrors may be partially reflective at the appropriate wavelength(s) and partially transmissive at the appropriate wavelength(s), in order to decouple the narrowband second coherent continuous-wave output beam from the cavity. One or more of the optical cavity mirrors may be highly reflective.

The narrowband component selected from said first coherent continuous-wave output beam may be either single-feature or multiple-feature narrowband coherent light. When such radiation is used to injection-seed the optical parametric oscillator, said second coherent continuous-wave output beam will have a corresponding spectral composition. The narrowband component may be selected from either the signal or the idler radiation forming part of the first coherent continuous-wave output beam. In the event that it is desirable to use mjection-seeding having a multiple-feature narrowband, said narrowband component may be selected from the signal and idler radiation of the first coherent continuous-wave output beam. The means for selecting said multiple-feature narrowband component from either the signal or the idler radiation of said first coherent continuous-wave output beam may conveniently comprise means for separating one or more desirable single-feature components from any undesirable wavelength components, and for discarding or not using such undesirable wavelength components. The apparatus according to the third aspect of the invention may further comprise continuous tuning means for controlling and/or varying the wavelength of each feature of said narrowband component.

The apparatus may further comprise means for introducing said selected narrowband component into said optical cavity as injection-seeding radiation, for the generation of said second continuous-wave coherent output beam.

The first and second continuous-wave pump beam may originate from the same input beam or from different input beam. In the event that they they originate from the same input beam, the apparatus may comprise a beam splitter to split the input beam into the two components.

In the event that the first and second pump beams originate from different input beams, the different beams may originate from the same or different sources of radiation. The first and second pump beams may have the same or different wavelengths.

As an alternative, first directive means may be provided to cause the first coherent continuous-wave output beam to be directed to said selection means and second directive means to cause said narrowband component to be directed to said optical cavity. The first directive means and the second directive means may be the same or different.

The wavelength(s) of the narrowband component may be wholly the wavelength(s) having sufficent gain in the OPO process of the second continuous-wave pump beam or may be partly within the range of wavelength(s) having sufficent gain in the OPO process of the second continuous- wave pump beam.

The first and second directions may be the same or different.

Similarly, the intensities of the narrowband component and the second continuous- wave pump beam are important for the efficient operation of the apparatus in accordance with the invention. As stated above, there is preferably at least sufficient energy originating from the narrowband component left in the optical cavity for an efficient injection-seeded operation of the OPO process of the second continuous-wave pump beam.

The second continuous-wave pump beam and the first continuous-wave pump beam may use the same region of the nonlinear-optical gain medium, or a different region of the nonlinear-optical gain medium, or a separate nonlinear-optical gain medium which may be the same or different material or the same material cut at a different angle or with a different periodic or regular internal structure.

By using a optical cavity, in which the direction of propagation of a given component of light is uni-directional, it is also possible to use more than one intracavity nonlinear-optical gain medium, each independently pumped. The different media may be made of the same or different materials. Alternatively, they may comprise crystals cut at different angles or with different periodic or regular internal structure. Optionally, different regions of the same crystal may be used as different media. As still another option, the different media may be operated at different operating conditions such as temperature. These variations in the construction of the optical cavity may be employed to extend the range of wavelengths that can be handled by the apparatus. According to a fourth aspect of the present invention, there is provided a method for providing a narrowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first continuous-wave pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected by the reflectors, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said first continuous-wave pump beam, a first coherent continuous- wave output beam that has a broad band of signal and idler wavelengths;

- decoupling said first coherent continuous-wave output beam from said cavity, along a first optical path;

- selecting, in a location outside of said optical cavity, a narrowband component comprising signal and/or idler radiation, from said first coherent continuous-wave output beam;

- coupling said narrowband component into said cavity and into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction; and

- introducing a second continuous-wave pump beam into said nonlinear-optical gain medium in such a manner that said narrowband component overlaps spatially in said nonlinear-optical gain medium, with said narrowband component, thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent continuous-wave output beam; and

- decoupling said narrowband second coherent continuous-wave output beam from said cavity along a second optical path which differs from the first optical path. The expression "single-feature narrowband", as used in this specification in relation to electromagnetic radiation, shall be construed as referring to radiation having a narrow band of wavelengths centred at one discrete wavelength only. The expression "multiple-feature narrowband", as used in this specification in relation to electromagnetic radiation, shall be construed as referring to radiation having two or more discrete narrowband components, each component being narrowband and centred at a desired wavelength. One advantage of the third and fourth aspects of the invention is that they do not require synchronisation of various related optical beams or the use of optical delay lines.

In one embodiment of the first and second aspects of the invention, the first pump pulse beam and the second pump pulse beam are generated by two separate pulsed lasers, with suitably synchronised firing circuits. The time interval between the two laser pulses typically needs to be in the range of 5 - 50 nanoseconds (i.e., between 5 x 10^"9 and 5 x 10^"

⁸ s).

The apparatus according to the first aspect of the invention may alternatively comprise an integrated double-pulse pump laser system for generating both the first pump pulse beam and the second pump pulse beam. Suitable double-pulse Nd:YAG laser systems are commercially available (e.g., Continuum Surelite), having been designed for particle imaging velocimetry (PIV) applications.

As another option for generating the first pump pulse beam and the second pump pulse beam, the apparatus according to the first aspect of the invention may comprise a single laser system that produces a pulse train (e.g., by mode locking). The single laser system may be provided with an electro-optical switch to select from the train pulses that are suitably separated in time.

Alternatively and preferably, the apparatus according to the first aspect of the mvention may comprise a single-pulse laser system, with each laser pulse split into a first portion that is fed into the OPO as the first pump pulse beam and a second portion that is beamed along an optical delay line and is introduced into the OPO cavity as the second pump pulse beam. The second pump pulse beam may be delayed sufficiently to arrive at the OPO slightly after and during the injection-seeding of the OPO by the said narrowband wavelength component.

In one embodiment of the invention, the apparatus according to the first aspect of the invention comprises a beamsplitter for splitting pulses generated by the pump laser. As one alternative, every pulse generated repetitively by the pump laser may be split into the first pump pulse beam and the second pump pulse beam. As another option, alternate pulses of the pulse laser may be used as the first pulse beam, whilst the remaining pulses may be used as the second pump pulse beam.

In another embodiment of the invention, the apparatus according to the first aspect of the invention comprises a fast optical switch, which may be driven electro-optically or acousto-optically or by other suitable means, for splitting a single pulse from the pump laser, which may be every pulse generated repetitively by the pump laser, into an earlier portion (with a short decay time) and a later portion to be used as the first and second pump pulse beams, respectively.

In order to achieve a difference in timing of the first and second pump pulse beams, the apparatus according to the first aspect of the invention may also comprise two optical delay lines for delaying the second pump pulse beam and the selected narrowband seed radiation, so that they arrive synchronously at said OPO cavity after a suitable time delay.

Because light travels at a known speed in any medium of known refractive index, the length of the optical delay line determines the time delay of the second pump pulse beam. Depending on the duration of the pulses generated by the pulsed pump laser, the optical delay line may be longer or shorter, so as to cause the second pump pulse beam to be delayed by a desired time interval relative to the first pump pulse beam. As a practical guide, it has been calculated by the inventors that an air-filled optical delay line should be about 0.3 m long for every nanosecond that the second pump pulse beam is to be delayed. The apparatus according to the first aspect of the invention will have a natural time delay between the second pump pulse beam and the first pump pulse beam, depending on their different path lengths after they leave the beamsplitter or fast optical switch that separates them. The delay line through which the second pump pulse beam is passed will impose an additional, adjustable time delay to augment the natural time delay, yielding the overall time delay between the first and second pump pulse beams.

An overall time delay in the range of from about 1 nanosecond to about 100 nanoseconds has been found to be sufficient for purposes of the invention. A preferred time delay is in the range of from about 5 nanoseconds to about 50 nanoseconds. The ideal overall time delay will depend on the type of wavelength-selective feedback unit that is employed.

The overall time delay may be selected such that it exceeds the time it takes for the first pump laser pulse beam to decay to a point where it does not interfere with the second pump laser pulse beam. The time interval by which the second pump pulse beam needs to be delayed may conveniently be expressed in terms of the decay characteristics of the first output coherent light pulse beam. The overall time delay between the second and first laser pulses is preferably shorter than the time it takes for a pulse of the first output coherent light beam to decay to zero.

The time delay line may be an optical fibre of a suitable length. Alternatively, it may be provided by a series of mirrors spaced from one another and arranged in such a fashion that the second pump pulse beam may be reflected from one mirror to another along an optical path whose length determines the time delay. In the apparatus according to the first aspect of the invention, there is an intrinsic delay in the wavelength-selective feedback means that depends on the length of the optical path traversed by first output coherent light pulse beam in and around the wavelength-selective feedback unit. It is necessary to provide an additional adjustable delay of the selected narrowband seed radiation to ensure that the second pump pulse and the selected narrowband seed radiation are synchronised.

The methods according to the second and fourth aspects of the invention do not require the step of filtering the narrowband second output beam pulse emerging from the cavity, for the purpose of removing a broadband component therefrom.

The apparatus according to the first and third aspects of the invention do not require means to filter the narrowband second coherent output beam pulse emerging from the cavity or to have a filter in the same optical path for the purpose of removing a broadband beam. In particular, no external diaphragm is required to separate the narrowband second coherent output radiation emerging from the cavity from broadband radiation.

Preferably, in the apparatus according to the first aspect of the invention, the pulsed pump source is a high-performance nanosecond-pulsed solid-state laser equipped with an injection seeder for single-longitudinal-mode operation. A commercially available laser of this type that is frequently used for OPO pumping [20, 22, 24] is a 1.064-μm flashlamp- pu ped, Q-switched, injection-seeded Nd:YAG oscillator/amplifier system, with special quasi-Gaussian cavity optics and nonlinear-optical stages to generate harmonics at 532 nm and 355 nm.

As a less ideal alternative, the pulsed pump source may conveniently be a multimode laser, whereby single-longitudinal-mode tunability of either signal or idler output radiation from the wavelength-selective characteristics of the OPO cavity may be attained [22 - 24]. It is preferably a simple, compact, multimode Nd:YAG laser. A Continuum Minilite II laser, delivering pulses at about 50 mJ per pulse at a wavelength of about 1.064 μm and a repetition rate of about 10 Hz, has been found by the inventors to work well. Such a laser may .be used to pump a nanosecond-pulsed periodically poled lithium niobate OPO system. The multimode Nd:YAG laser may oscillate on several longitudinal modes, yielding an optical bandwidth of about 1 cm^" and a rapidly modulated temporal profile (about 6 ns fwhm). It may be driven off regular mains power and it may be air-cooled, thereby facilitating field-based OPO applications. It conveniently uses the resonance properties of the OPO ring cavity to constrain the resonated wave (the signal is resonant in the instance investigated by the inventors) to a single longitudinal mode of the OPO cavity and to ensure that it is continuously tunable without mode hops as the cavity length and tunable-diode-laser injection-seeding wavelength are scanned. The idler output may remain multimode, consistent with the characteristics of the multimode pump laser. The second output coherent light pulse beam from the OPO cavity preferably has a narrow optical bandwidth. It has been found that a second output coherent light pulse beam having an optical bandwidth of about 100 MHz (about 0.003 cm^-1) [24], can be produced. Such output light has a quality that is comparable to that of the output from a more elaborate, tunable-diode-laser-seeded periodically poled lithium niobate OPO pumped by a single-longitudinal-mode Nd:YAG laser. A pulsed OPO output wavelength in the nfrared region has-been -found to be suitable for spectroscopic applications [22 - 25].

The pump laser may be an all-solid-state (e.g., diode-pumped) nanosecond-pulsed laser such as systems described by various authors [41 - 45, 50, 51]. Such lasers are able to offer high repetition rates (>1 kHz) and may be readily transportable.

As a nonlinear-optical gain medium, a multi-grating quasi-phase-matched element may be used. For instance, a suitable periodically poled lithium niobate element, commercially available from Crystal Technologies Inc, has a set of eight parallel quasi- phase-matched gratings of varying periodicity disposed on a single substrate [28]. At a fixed OPO crystal temperature, a single periodically poled lithium niobate grating generates broadband signal and idler output spread over about 5 cm^-1 (about 150 GHz) fwhm. With 1.064-μm pump radiation, this combination of eight periodically poled lithium niobate grating periods and temperature variation over about 50 C° provides uninterrupted quasi-phase-matched OPO tuning ranges from about 1.45 μm to about 1.55 μm for the signal output and from about 4.0 to about 3.4 μm for the idler output [20, 24].

The external wavelength-selective feedback means is preferably a tunable optical filter, of which several embodiments are possible. One such embodiment comprises a high-resolution diffraction-grating device. Another embodiment comprises a high-finesse tunable optical filter operating on the cavity ringdown principle [30, 31]. In all such embodiments, the tunable optical filter may conveniently be adapted to resonate with, store and feed back a narrowband wavelength component of the broadband first coherent light pulse from the OPO. During the time delay, the narrowband coherent light pulse emerging from the tunable optical filter employed for external wavelength-selective feedback may conveniently be used to injection-seed the OPO on a single longitudinal mode of whichever wave (signal or idler) is resonated in the cavity.

In one embodiment of the external wavelength-selective feedback means, the tunable optical filter may be or may comprise a suitable diffraction grating mounted at nearly grazing incidence in the Littman-Metcalf configuration [69], with a tuning mirror arranged to reflect the first-order-diffracted light back onto the grating surface and (for the selected wavelength) back along the path of the incident light. Wavelength selection is achieved by fine tuning of the angular setting of the tuning mirror and the optical bandwidth of the feedback light is resolved by a suitable spatial filter (e.g., a pinhole, slit or optical fibre). It has been found that the small dimensions of the optical interaction region in the nonlinear-optical medium of the OPO provides an additional mode of spatial selection, so that a separate spatial filter may not necessarily be required. It is customary to use an intracavity Littman-Metcalf grating-based wavelength selector in established narrowband tunable pulsed OPO designs (e.g., the Bosenberg-Guyer OPO design [29], which is commercially available in the form of the Continuum Mirage system). In contrast, the Littman-Metcalf grating-based wavelength selector in this embodiment is external to the cavity of the narrowband tunable pulsed OPO.

In another embodiment of the external wavelength-selective feedback means, the tunable optical filter may be a high-finesse Fabry-Perot optical cavity that is able to resonate with pulsed OPO output light, store selectively a resonate narrowband component of that light for a period of time, and emit the stored light gradually through both front and back cavity mirrors with a characteristic exponential ringdown decay time. This property of a high-finesse optical cavity is regularly used in cavity ringdown spectroscopic techniques [30, 31], which enable ultra-sensitive absorption spectroscopy by enhancing the effective path length of light traversing a sample contained in the cavity.

High-finesse tunable optical filters are used as resonant optical cavities and are well-established tools in many branches of science and industry, including metrology, spectroscopy and astronomy. Simple Fabry-Perot cavities (either fixed-length or scanning) provide a useful means of calibrating either the optical bandwidth or the relative wavelength of laser or OPO radiation as it is scanned. More significantly, the dynamic response of a very-high-finesse optical cavity to a light pulse plays a fundamental role in the present invention [32 - 35], because it determines the characteristic build-up and decay of light in the cavity and the rate at which narrowband wavelength-selected light can be fed back to the gain medium in the cavity-ringdown embodiment of the invention.

The output coherent light may have a narrow band of frequencies. More preferably, it corresponds to single-longitudinal-mode operation (effectively single-frequency).

The apparatus according to the first and third aspects of the invention may conveniently also comprise means for varying the wavelength of the narrowband portion that is selected out of the first coherent output of the OPO cavity.

Alternatively, the injection-seeding of the pulsed OPO may be configured for spectroscopic applications requiring a coherent source that simultaneously generates two or more adjustable output wavelengths, for instance, to match on- and off-resonance wavelengths of a spectrum of interest [16, 21, 25].

Spectroscopic tailoring of OPO output by multi-wavelength injection seeding is most readily implemented with a birefringently phase-matched medium in an OPO cavity that is slightly misaligned to reduce its finesse. It is also possible in quasi-phase-matched media with grating channels wide enough to allow different non-collinear phase-matching angles for each of the OPO output wavelengths. Conventional multi-wavelength OPOs require a source of coherent seed radiation that can simultaneously generate a structured set of discrete mjection-seeding wavelengths; an array of tunable diode lasers is a suitable injection-seeding source for such a purpose. In this invention, the role of the multi- wavelength injection-seeding source is accommodated by suitable design of the external wavelength-selective feedback unit, such that the optical filter determines which individual narrowband output wavelengths are generated. As an example of a suitable quasi-phase-matched nonlinear-optical medium, a single periodically poled lithium niobate grating (on a multi-grating substrate [28]) allows continuous tunability over the following ranges in the 1.5-μm region: about 400 GHz (about 13 cm^-1) at constant temperature; about 7.5 THz (about 250 cm^-1) with additional temperature tuning.

While the preferred form of nonlinear-optical medium is quasi-phase-matched, (i.e., a crystal with alternating structural domains), it is also possible to use a birefringently phase-matched nonlinear-optical medium (i.e., a homogeneous, non-centrosymmeric crystal). Another option is to use more than one nonlinear-optical crystal, such as a matched pair birefringently phase-matched crystals that are aligned to minimise nonlinear-optical walk-off effects.

The geometric arrangements for nonlmear-optical phase-matching are typically collinear, with the pump beam, injection-seeding beam and coherent output beam co-- propagating in the gain medium. As an alternative, noncollinear phase matching may be employed, in which the pump beam, injection-seeding beam and coherent output beam are each in different directions determined by the phase-matching conditions as in Equation (1); this can be advantageous in increasing the optical bandwidth of the free- running OPO to yield a wider range of wavelengths and hence facilitate multi-wavelength injection-seeded operation of the OPO. The apparatus according to the first and third aspects of the invention has the advantages that it is an OPO-based system, that it is tunable and that it eliminates the need for ez^'tber intracavity optical bandwidth-narrowing elements (such as gratings, prisms or etalons) or an independent, external injection-seeding tunable source (such as a tunable diode laser). Further advantages of the apparatus according to the first and third aspects are its simphcity (largely due to its modular construction) and its versatility, in that it is capable of providing an operating output wavelength that is tunable over a wide range and not limited by the tuning range of an available injection-seeding laser.

A further important advantage of all aspects of the invention is that no diaphragm, slit or other spatial filter is required to separate narrowband output from broadband output, as is the case with the output of the apparatus disclosed in US Patent No 5,659,419.

In the apparatus according to the first aspect and the method according to the second aspect of the invention, a single OPO stage is first pumped by a laser pulse beam to operate in a free-running mode, thereby generating a multimode OPO output. The same OPO stage is then subsequently pumped with a delayed laser pulse beam and injection-seeded by external wavelength-selective feedback of narrowband radiation from a tunable optical filter (of which there are several possible embodiments). The tunable optical filter processes the broadband OPO output radiation produced previously during the free-running mode of the OPO operational cycle. Because the seed radiation is generated originally by the same OPO cavity, it is automatically in resonance with the OPO cavity. The resonance-frequency match between the OPO cavity and the tunable optical filter could be maintained by tracking the tunable optical filter configuration to the OPO output, such that the wavelength-selective feedback maintains a high OPO output intensity.

For a pulsed pump laser system with a pulse duration of up to several nanoseconds, the time delay between the pre and main pump pulses can be readily produced by beamsplitters and an optical delay line. For pump pulses with longer pulse duration, the requirement on the physical space and mechanical stability for the delay line could be difficult to meet. One solution for such a system would be to use an optical switch such as a combination of a Pockels cell and a polarising beamsplitter to separate a pump laser pulse into two parts. The earlier part of the beam will be used as a pre-pump pulse, whereas the later part of the beam will be used as the main pump pulse. Using an external narrowband tunable optical filter for wavelength-selective feedback, in accordance with the teachings of this invention, effectively generates suitable injection-seeding radiation and extends the range of available laser sources as injection seeders. The outcome is a narrowband, pulsed, tunable OPO system that eliminates the need for either intracavity optical-bandwidth-narrowing elements (such as gratings, prisms or etalons) or an independent injection-seeding tunable laser (such as one or more tunable diode lasers). Recent experiments have demonstrated the feasibility of the invention and established the performance characterisics of various embodiments of this OPO system.

The approach of the first and second aspects of the invention is thus to select a narrowband component from the multimode output of a pulsed free-running OPO by using a suitable external wavelength-selective feedback unit in the form of a narrowband tunable optical filter (of which there are several possible embodiments) and then to use this selected narrowband radiation as an injection-seeding source for the generation of a subsequent OPO pulse. This approach eliminates the need for an independent injection- seeding tunable laser (such as a tunable diode laser) and offers the prospective advantages of instrumental simplicity, modularity, and versatility of operating wavelength range.

An implementation of this idea is to use one tunable optical filter and two separate OPO stages pumped by two synchronised pump pulses. Apart from the requirement of two OPO stages, the match of the resonance frequencies among the two OPO cavities and the narrowband tunable filter is also difficult to maintain in such an implementation.

Another implementation of the apparatus according to the first aspect of the invention may be based on the principles of optical cavity ringdown spectroscopy [30,31] in designing the external wavelength-selective feedback unit. The initial pump pulse may generate a multimode OPO output which may be coupled to a high-finesse tunable ringdown cavity. The ringdown cavity may store selectively one frequency component near the centre of the multimode distribution of the OPO signal output. The transmitted OPO signal intensity of the ringdown cavity may be monitored by a photodetector. A penta prism may be provided behind the ringdown cavity to separate the unwanted residual pump and idler beams from the OPO signal output. A match of the resonance wavelength between the ringdown cavity and the OPO cavity may be maintained by dithering and tracking the ringdown or the OPO cavity length to maximise the averaged transmission intensity. Leakage of light through the front mirror of the ringdown cavity may be returned coUinearly with the delayed pump pulse to the OPO cavity and to served as a narrowband seeding radiation to the OPO cavity for the delayed pump pulse. The wavelength of this seeding radiation may automatically be in resonance with the cavity because it was generated from the same OPO cavity by a previous pump pulse.

The ringdown cavity may comprise two high reflective flat mirrors (Newport 10CM00SR.70T). To enable the ringdown cavity to select only one longitudinal mode out of a multimode free-running OPO output, the FSR of the ringdown cavity may be larger than the optical bandwidth of a free-running PPLN OPO. The free-running bandwidth of such an OPO has been determined to be approximately 150 GHz (5 cm^"1) FWHM. Therefore, a FSR of about 450 GHz (15 cm^"1 ) should be adequate. This corresponds to a mirror separation of 0.333 mm ( c/(2*FSR) ). To have the ringdown time τ of the cavity in the order of the pulse duration of - 10 ns of a Nd:YAG pump laser, the reflectivity of the mirrors need to be higher than 99.9889%. Commercial mirrors of this quality are available (eg. Newport 10CM00SR.70T). The optical bandwidth (FWHM) of the ringdown cavity as a tunable optical filter is 16 MHz ( =l/(2πτ) ).

Light entering a high-finesse cavity may be reflected back-and-forth by the highly reflective cavity mirrors for a large number of times before leaking out of the cavity completely. As a result, a short laser pulse could be trapped inside the cavity for a significantly longer time than its original pulse duration. The decay of light trapped inside the cavity may be described by an exponential function. The time τ taken for the intensity to decrease to the 1/e of its initial value, is expressed by the following equation: τ = d / [c*(l-R-α)] (2) wherein d is the distance between the cavity mirrors; c is the speed of light, R is the reflectivity of the cavity mirrors; and α is the single-path absorption coefficient, of light, of the medium contained in the cavity.

This decay property of a high-finesse cavity may be utilised in the cavity ringdown spectroscopy method [30, 31] to measure molecular absorption spectra of samples contained in a high-finesse optical cavity. The cavity ringdown technique may involve the use of either a pulsed [22, 23] or a tunable continuous wave coherent source [32 - 34].

In the present application, the cavity ringdown technique is used not for spectroscopic detection but as a novel way to injection-seed a pulsed OPO. In addition to its ringdown property, the high-finesse optical cavity is also operating as a tunable optical filter. Coherent interference between the radiation stored inside the cavity and the incoming -light makes the optical -cavity wavelength-selective. Only radiation having a wavelength, an integer multiple of which wavelength equals the round-trip optical path length of the cavity, is built up and coupled effectively in the cavity. All other wavelength components of the radiation are strongly rejected. If the pulse duration is not much shorter than the cavity ringdown time, the radiation stored in the cavity during the pulse duration is of narrow optical bandwidth. Its full-width-at-half-maximum (FWHM) intensity could be estimated by the following equation:

FWHM = l / (2πτ) (3)

In this application, broadband light generated by a free-running pulsed OPO is sent to such a high-finesse cavity which resonates with and stores a narrowband wavelength component of the broadband multimode OPO output. The leakage of this narrowband coherent radiation stored inside the cavity is returned to injection-seed narrowband operation of the OPO by a delayed subsequent pump laser pulse. According to a fifth aspect of the invention, there is provided an apparatus for providing a narrowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous-wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising: - an optical parametric oscillator including an optical cavity and a nonlinear-optical gain medium, the optical cavity comprising at least three optically interconnected reflectors or deflectors disposed in such a manner relative to one another that optical radiation entering said optical cavity is sequentially directed from one reflector to the other, said nonlinear- optical gain medium being capable of generating, from a first pump beam introduced into said optical cavity in a first direction, a first coherent output beam that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a narrowband component comprising signal and/or idler radiation, from said first coherent output beam, and for introducing said narrowband component into said nonlinear-optical gain medium as mjection-seeding radiation, in a second direction such that, upon the introduction into said optical cavity of a second pump beam, said narrowband component overlaps spatially in said nonlinear-optical gain medium with said second beam thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent output beam; - means for decoupling said first coherent output beam from said cavity, along a first optical path, into said selection means;

- means for decoupling said narrowband second coherent output beam from said cavity along a second optical path that differs from the first optical path.

According to a sixth aspect of the invention, there is provided an apparatus for providing a pulsed, narrowband, coherent beam of optical radiation, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising: - an optical parametric oscillator including an optical cavity and a nonlinear-optical gain medium, the optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such a manner relative to one another that optical radiation entering said optical cavity is sequentially reflected or directed from one reflector or deflector to the other, said nonlinear-optical gain medium being capable of generating, from a first pump pulse introduced into said optical cavity in a first direction, a first coherent output beam pulse that has a broad band of signal and idler wavelengths; and

- means for decoupling said narrowband second coherent output pulse from said cavity along a second optical path that differs from the first optical path. According to a seventh -aspect- -of -the invention, there is provided an -apparatus for providing narrowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising:

- an optical parametric oscillator including an optical cavity comprising a nonlinear- optical gain medium and at least three optically interconnected reflectors or deflectors disposed in such manner relative to one another that optical radiation entering said optical cavity is sequentially reflected or deflected from one to the other, said nonlinear-optical gain medium being capable of generating, from a first continuous-wave pump beam introduced into said optical cavity in a first direction, a first coherent continuous-wave output beam that has a broad band of signal and idler wavelengths; and - selection means, disposed outside of said optical cavity, for selecting a narrowband component comprising signal and/or idler radiation, from said first coherent continuous- wave output beam, and for introducing said narrowband component into said nonlinear- optical gain medium as injection-seeding radiation, in a second direction such that, upon the introduction into said optical cavity of a second continuous-wave pump beam, said narrowband component overlaps spatially in said nonlinear-optical gain medium with said second continuous-wave pump beam thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent continuous-wave output beam;

- means for decoupling said first coherent output beam from said cavity, along a first optical path, into said selection means;

- means for decoupling said narrowband second coherent continuous-wave output beam from said cavity along a second optical path that differs from the first optical path. According to an eighth aspect of the invention, there is provided a method for providing a narrowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous-wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of: - introducing a first .pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectros disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected and/or deflected from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity; - generating, from said first pump beam, a first coherent output beam that has a broad band of signal and idler wavelengths;

- decoupling said first coherent output beam from said cavity, along a first optical path;

- selecting, in a location outside of said optical cavity, a narrowband component comprising signal and/or idler radiation, from said first coherent output beam; - coupling said narrowband component into said cavity and into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction; and

- introducing a second pump into said nonlinear-optical gain medium in such a manner and at such a time that said narrowband component overlaps spatially and temporally in said nonlinear-optical gain medium, with said narrowband component, thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent output beam; and

- decoupling said narrowband second coherent output beam from said cavity along a second optical path which differs from the first optical path. According to a ninth aspect of the invention, there is provided method for providing a pulsed, narrowband coherent beam of optical radiation, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first pump pulse, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected or directed from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said first pump pulse, a first coherent output beam pulse that has a broad'band of signal and idler-wavelengths;

- selecting, in a location outside of said optical cavity, a narrowband component comprising signal and/or idler radiation, from said first coherent output beam pulse; - coupling said narrowband component into said cavity and into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction and after a suitable time delay; and

- decoupling said narrowband second coherent output beam pulse from said cavity along a second optical path which differs from the first optical path. According to a tenth aspect of the invention, there is provided a method for providing a narrowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first continuous-wave pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity;

- introducing a second continuous-wave pump beam into said nonlinear-optical gain medium in such a manner that said second continuous-wave pump beam overlaps spatially in said nonlinear-optical gain medium, with said narrowband component, thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent continuous-wave output beam; and

- decoupling said narrowband second coherent continuous-wave output beam from said cavity along a second optical path which differs from the first optical path. According to an eleventh aspect of the invention, there is provided a method for providing a narrowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of: - introducing a continuous-wave, first pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially reflected by the reflectors, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said continuous-wave first pump beam, a first coherent continuous-wave output beam that has a broad band of signal and idler wavelengths;

- coupling said narrowband component into said cavity and into said nonlinear- optical-gain medium-as injection-seeding radiation, in a second direction; and

- introducing a pulsed, second pump beam into said nonlinear-optical gain medium in such a manner that said second pump beam overlaps spatially in said nonlinear- optical gain medium, with said narrowband component, thereby interacting with said nonlinear-optical gain medium to generate a pulsed, narrowband second coherent output beam; and

- decoupling said pulsed, narrowband second coherent output beam from said cavity along a second optical path which differs from the first optical path.

Brief Description of the Drawings A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic block diagram of one embodiment of an apparatus in accordance with the first aspect of the invention;

Figures 2 to 4 are diagrammatic representations of other embodiments of an apparatus in accordance with the first aspect of the invention; Figures 5 (a) to 5 (e) are representations of the amplitude, over time, of optical beams at various points in the apparatus of Figure 1;

Figures 5(f) to 5(j) are representations of the spectral distributions of the corresponding optical beams shown in Figures 5 (a) to 5 (e);

Figures 6 to 15 are diagrammatic representations of different embodiments of tunable filter means for providing a wavelength-selective feedback, forming part of the apparatus in accordance with the first aspect of the invention;

Figures 16 to 18 are diagrammatic representations of other embodiments of an apparatus in accordance with the first aspect of the invention;

Figures 19 to 22 are diagrammatic representations of embodiments of apparatus in accordance with the third aspect of the invention;

Figure 23 is a schematic diagram of a narrowband, ns-pulsed QPM OPO tuned by a wavelength-selective feedback module external to the OPO ring cavity; and

Figure 24 shows another embodiment of an apparatus in accordance with the invention, of which the optical cavity is formed by two mirrors and a prism. Detailed Description of the Drawings

Referring to Figure 1, there is shown an apparatus 10 in accordance with the invention, comprising an optical parametric oscillator (OPO) cavity 12 pumped by a pulsed pump laser 14. The output pulse beam 14.1 from the pump laser 14 is fed through an optical isolator 16 to prevent the feedback of potentially damaging laser light towards the laser 14, before it passes through a beam splitter 18 which reflects a portion 14.2, referred to hereinbelow as the delayed pump pulse beam, into an optical delay line 20.

Another portion 14.3, refened to hereinbelow as the initial pump pulse beam, is fed through an optical attenuator 22 (comprising a polarisation and intensity controller) into the OPO cavity 12. The delayed pump pulse beam 14.2 is steered by the mirrors 19.1 and 19.2, attenuated by another optical polarisation and intensity controller 24 and reflected by a beamcombiner 26 into the OPO cavity 12 from the opposite direction as that from which the initial pump pulse beam 14.3 enters the OPO cavity 12. The time delay t_<j_ei_ay

(measured at the position of the OPO gain medium) between the initial pump pulse beam 14.3 and the delayed pump pulse beam 14.2 is typically set to be half of the time duration of the pump laser pulse beam 14.1. The time delay t_de y between the initial pump pulse 14.3 and the delayed pump pulse 14.2, each measured at the gain medium of the OPO 12, should be ideally longer than double the full-width-at-half-maximum (FWHM) time delay (ΔtF_WHM) of the pump laser pulse 14.1, so that the delayed pump pulse beam 14.2 and the initial pump pulse beam 14.3 interact with the optical medium separately. A reduced time delay down to a half of the ΔtF_WHM is also acceptable.

The OPO cavity 12 comprises at least one nonlinear-optical medium (not shown in Figure 1, but described below with reference to Figures 2 - 4 and 16 -18) and two reflective mirror-surfaces (not shown in Figure 1, but also described below with reference to Figures 2 - 4 and 16 -18). The operational parameters (eg. Temperature, orientation and/or position) of the nonlinear optical medium ar adjustable by means of control units (not shown in Figure 1, but described below with reference to Figures 2 to 4 and 16 to 18) for controlling the phase matching condition of the optical parametric nonlinear interaction process.

The nonlinear-optical medium may either be a non-centrosymmeric crystal of homogeneous composition for birefringent phase-matching of the OPO process or it may be a crystal with alternating structural domains, such as periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP), for quasi-phase- matching of the OPO process.

The OPO cavity 12 is resonant at certain specific signal and idler output wavelengths which depend on and are determined by the geometry of the OPO cavity 12.

An initial broadband OPO output 28, consisting of both a signal beam and an idler beam, is generated when the nonlinear-optical medium is pumped by the initial pump pulse beam 14.3.

An external wavelength-selective feedback unit 30 selects one or several narrowband wavelength components from the initial broadband OPO output 28, and returns it, as a narrowband seed beam 32, to the OPO cavity. The narrowband seed beam 32 is combined with the delayed pump laser beam 14.2 by the beamcombiner 26, such that it co-propagates with the delayed pump laser pulse beam 14.2. An optical delay line 31, similar to the optical delay line 20, is placed between the wavelength-selected feedback unit 30 and the OPO cavity 12 to produce a time delay of amount (t _eι y - t_prior) with reference to Figure 5. Alternatively, only one optical delay line 20 may be provided, that is, in an alternative embodiment the delay line 31 is not provided. The narrowband seed beam 32 is used to injection-seed the OPO cavity 12 slightly before and during the delayed pump pulse beam 14.2 there. Because of the injection seeding effect of the narrowband seed beam 32, the delayed pump pulse beam 14.2 generates a wavelength- tailored OPO output 34 which matches the spectral components of the narrowband seed beam 32. The wavelength of the nanowband seed beam 32 is preferably variable over a range of wavelengths by means of the external wavelength-selective feedback unit 30, as will be described in greater detail below in relation to Figures 6 to 15.

Referring to Figure 2, there is shown an apparatus 10' in which a single pulsed pump source 14' and a single optical nonlinear-optical medium 12.1' are provided in an optical ring cavity 12' formed by mirrors Ml', M2', M3' and beam splitter M4'. The beam splitter M4' is also used as output decoupler for the generated OPO radiation and as an input coupler for the wavelength-selected injection-seeding radiation.

The pulsed pump source 14' includes an optical isolator 16' to prevent the feedback of potentially damaging light towards the pump source 14' .

A first or initial portion 14.3' of the output of the pump source 14' passes through a beamsplitter 18' and a polarisation and intensity control unit 22' to form the first pulse pump beam 14.3'. Another portion is reflected by the beamsplitter 18' and is fed through an optical delay line 20' and a polarisation and intensity control unit 24' to form the second pump beam 14.2'.

The first or initial pump pulse portion 14.3' enters the optical cavity through the mirror Ml ' and is reflected by the minor M3 ' to the minor M4' and by the minor M4' to the minor M2' in the direction shown by the broken arrow.

The second pump pulse 14.2' passes through the minor M2' and is reflected consecutively by the minors M4' and M3' so as propagate in the cavity in the direction of the solid anow. It thus pumps the nonlinear-optical crystal 12.1' from the opposite direction as that from which the first pump pulse beam 14.3' does. The time delay t _ei_ay (measured at the position of the OPO gain medium 12.1') between the first pump pulse beam 14.3' and the second pump pulse beam 14.2' is typically set to be half of the time duration of the pump laser pulse beam 14.1'. The time delay t_dei_ay between the initial pump pulse 14.3' and the delayed pump pulse 14.2', measured at the gain medium 12.1' of the OPO, is preferably longer than double the full-width-at-half-maximum (FWHM) time delay (ΔtFW_Hivi) of the pump laser pulse 14.1', so that the delayed pump pulse beam 14.2' and the initial pump pulse beam 14.3' interact with the gain medium 12.1' separately. A reduced time delay down to a half of the ΔtFWH_M is, however, still acceptable.

In order to ensure that the desired time delay is achieved, the length of the delay path 20' can be adjusted. In addition to the optical time delay path 20', a delay path 31 ' is provided to ensure that the selected nanowband component from the external wavelength selective feedback unit 30' can be delayed to arrive at the gain medium 12.1' at the desired moment, ie just prior to the arrival of the delayed pump pulse 14.2', so that it can seed the gain medium 12.1'. Alternatively, only one optical time delay path 20' may be provided, that is, in an alternative embodiment the delay path 31 ' is not provided.

The length of the optical path in the optical cavity 12' is accurately controlled by a piezoelectric translator 12.3' (also shown as PZT in other Figures) attached to minor M3', for controlling the position of the minor M3' relative to the minors Ml', M2' and M4'. The use of an optical delay line to delay the arrival of an optical pulse could sometimes become inconvenient and mechanically unsuitable. For example, a 30-ns long delay requires an optical path length of 9 meters, which is cumbersome. Alternatively, two pump sources may be provided as is described below in relation to Figure 3.

Referring to Figure 3, there is shown an embodiment of the apparatus in which the optical cavity 12" comprises two nonlinear-optical media 12.11" and 12.12". Similar parts to those in Figures 1 and 2 are indicated by similar reference numerals.

The operational parameters (eg. temperature, orientation, position) of the two nonlinear-optical media 12.11" and 12.12" are adjustable by control units 12.21" and 12.22", for controlling the phase-matching condition of the optical parametric nonlinear interaction process.

The first pump beam 14.3" is generated from the output of a first pump source 14" after having been passed through a polarisation and intensity control unit 22".

The second pump beam 14.2" is generated from the output of a second pump source 15" after having been passed through a polarisation and intensity control unit 24". The pump sources 14" and 15" may be either continuous-wave or pulsed light sources. One prefened combination of pump source types is a configuration in which both pump sources are pulsed. Another alternative combination of pump source types is a configuration in which pump source 14" is continuous-wave whilst pump source 15" is pulsed. Another combination is where both pump sources are continuous-wave. In the case that both pump sources are pulsed, a optical delay and synchronising unit 17" may be provided to control the time delay of pump beam 14.2" relative to pump beam 14.3".

Coherent radiation that has a broad band of signal and idler wavelengths is generated when the nonlinear-optical crystal 12.11" is pumped by the pump source 14". The coherent radiation circulates insides the ring cavity 12" in a first direction indicated by the broken line anow. A portion of the coherent radiation is coupled out of the ring cavity 12" through the beam splitter M4" as the first coherent output beam pulse radiation. An external wavelength-selective feedback unit 30" selects one or several nanowband wavelength components from the first coherent output beam pulse radiation, and returns it, as a nanowband component, to the optical cavity 12" such that it co- propagates with the delayed pump beam 14.2". The one or several nanowband wavelength components may be returned to the ring optical cavity 12" via delay path 31" and minor 19.2" or in an alternative embodiment, in which the delay path 31" is not provided, via minor 19.2". The nanowband seed beam is used to injection-seed the OPO cavity. An optical delay and synchronising unit 17" ensures that narrowband component and the delayed pump beam 14.2" overlap in both time and space in the second gain medium 12.12", such that coherent nanowband radiation is generated in the second gain medium 12.12", which is circulated in the optical cavity until it exits at the minor M4" in a different direction to that of the initial broadband output.

Because of the injection seeding effect of the nanowband component, wavelength- tailored nanowband OPO radiation is generated when the nonlinear crystal 12.12" is pumped by the second pump beam 14.2". The nanowband OPO radiation circulates insides the ring cavity in the opposite direction to that of the initial radiation generated in the crystal 12.11", as is indicated by the solid line anow. A portion of the nanowband OPO radiation is coupled out of the cavity through the beam splitter M4" as wavelength- tailored nanowband coherent output radiation.

The nonlinear crystals 12.11" and 12.12" may be in the form of two separate and individual crystals or, alternatively, they may be in the form of two different locations on or regions of a single crystal substrate.

In the event that two separate and individual crystals 12.11" and 12.12" are used, the two optical parametric processes associated with respectively the first pump beam 14.3" and the second pump beam 14.2" can be separated from each other. This provides flexibility in the design and optimisation of the performance of the OPO. For example, the choice of crystal 12.11" and the associated pump source 14", as well as the optical parametric processes occurring in them, may be optimised for lowering the pump threshold of that part of the apparatus, whilst the choice of the crystal 12.12" and the associated pump source 15" as well as the optical parametric processes occurring in them may be optimised for generating high OPO output energy and/or for one or more desired output wavelengths.

One factor that binds the two optical parametric processes is that they need to share one common OPO output wavelength selected by the wavelength selective means 30".

A further advantage of using two crystals is that the possibility of interference between the first pump beam 14.3" and the second pump beam 14.2", when they overlap in time and space in a nonlinear optical crystal, can be eliminated. Furthermore, the use of two crystals provides flexibility in the design of the OPO cavity and the beampath anangement of the first and second pump beams 14.3", 14.2".

Referring to Figure 4, there is shown an embodiment of the apparatus 10'" in which the first pump pulse and second pump pulse are generated by the use of an optical switch to direct an earlier part of the output pulse of the pump source into a first pump beam 14.3'", and to direct a later part of the output pulse of the pump source into the second pump beaml4.2'". The switching point between the earlier part and later part is set by a timing control unit 17'". A widely-used form of optical switch, which may be provided as part of the timing control unit 17'", is a combination of polarising beam splitter and Pockels cell.

Figures 5(a) to 5(e) illustrate the amplitudes, over time, and Figures 5(f) to 5(j) illustrate the spectral distributions (with amplitude on the y-axis and wavelength on the x- axis) of conesponding optical pulses at various points in the apparatus 10.

Figures 5(a) and 5(f) respectively represent the initial pump pulse 14.3. The time it takes for the initial pump pulse beam 14.3 at the full-width-at-half-maximum (FWHM) of its amplitude curve is indicated as Δt_FWHM in Figure 5(a). As is shown in Figure 5(f), the spectral distribution of the laser output pulse beam 14.1 is narrow. The wavelength at peak amplitude is indicated as V_PUMP-

Figures 5(b) and 5(g) respectively represent amplitude over time and the spectral distribution of the initial broadband output 28 of the OPO cavity 12. It reaches maximum amplitude at almost the same time as the initial pump pulse 14.3 reaches maximum amplitude, and is characterised by an initial sharp rise in amplitude followed by a gradual decay, as is illustrated by Figure 5(b). The temporal profiles of the OPO output pulse beams 28 and 34 are dependent on temporal profiles of the pump pulse beams 14.3 and 14.2, respectively. Figure 5(g) illustrates the broadband nature of the initial broadband output 28. The initial broadband output 28 has several peaks representing a number of different wavelengths, with the peaks further away from the centre having progressively lower amplitudes. This applies to both the signal and idler radiation emitted from the OPO cavity 12. Figure 5(c) shows the rapid build up of the nanowband seed beam 32 and its gradual decay over time. Figures 5(c) and 5(h) respectively represent amplitude, over time, and the spectral distribution of the nanowband seed beam 32. Figure 5(c) shows that the maximum amplitude of the nanowband seed beam 32 has been timed to arrive at the OPO cavity 12 prior to the arrival of the delayed pump pulse 14.2, as shown in Figures 5(d), so as to be able to act as seed radiation in the OPO cavity 12. The time t_pri_0r between the maximum amplitude of the nanowband seed beam 32 and the arrival, at the OPO cavity 12, of the maximum amplitude of the delayed pump pulse 14.2, is typically about half of the Δt_FWHM of the delayed pump pulse 14.2.

Figures 5(e) and 5(j) respectively represent amplitude, over time, and the spectral distribution of the nanowband output beam 34. As can be seen, it reaches maximum amplitude at the same time as the delayed pump pulse 14.2, but it has a nanow spectral distribution. Although not shown in Figure (j), the signal or idler wavelength of the nanowband OPO output beam 34 will be the same as wavelength of the nanowband seed beam 32. This output wavelength conesponds to the particular longitudinal mode of the OPO cavity that has been selected by the wavelength-selective feedback unit 30, so that the wavelength of nanowband seed beam 32 will automatically coincide with the signal or idler wavelength of the nanowband OPO output beam 34. The resulting single- longitudinal-mode OPO output wavelength may be continuously tuned by concerted variation of OPO control parameters (such as the cavity length, the crystal temperature, the pump laser wavelength) and by the setting of the wavelength-selective feedback unit 30. In this way, a nanowband (typically single-longitudinal-mode) output beam 34 is generated by the OPO cavity 12. This nanowband output beam 34 may be employed in the spectral analysis of atmospheric air or in other high-resolution spectroscopic applications where it is desirable to have radiation having a nanow band of wavelengths. In addition, the wavelength-selective feedback unit 30 may be used to generate a nanowband output beam 34 of which the wavelength is variable, as will be described hereinbelow. The wavelength-selective feedback unit 30 may be an optical grating, a prism, an etalon, a fibre grating or any other optical component having wavelength-dependent selectivity.

Figure 6 shows how an optical grating 40 as used in a Littman-Metcalf configuration, may be employed to generate the nanowband seed beam 32. The broadband OPO output beam 28 from the OPO cavity 12 is passed through a beamsplitter 36 and a spatial filter 38 comprising a pin-hole 38.1 and co-operating with lenses L3, L4. The spatial filter 38 improves the spatial quality of the initial OPO output beam 28. The initial OPO output beam 28 is focussed through the pinhole 38.1 by lens L3 and then collimated by lens L4 onto a grating 40. The grating 40 is illuminated near its grazing incidence angle, so as to achieve a high dispersion (angular spread) of the spectrum. A diffracted beam 41, which is radiated from the grating 40, is reflected back to it by a tuning minor 42, for a second pass. Different wavelength components of the initial OPO output beam 28 are spread angularly during the second pass. The spatial filter 38 blocks any unwanted spectral components diffracted off the grating 40. A portion of the radiation diffracted from the grating 40, which is nanowband, is allowed to return in the direction of the OPO cavity 12, passing through the lenses L3, L4 and the spatial filter 38 in the opposite direction to the initial OPO output beam 28. The pin-hole 38.1 blocks the unwanted spectral components and allows only a selected radiation of nanow wavelength distribution to pass through the spatial filter as the nanowband seed beam 32. Wavelength selection is achieved by tuning the angle at which the tuning minor 42 is disposed relative to the grating 40. The intensity of the selected nanowband seed beam 32 may be monitored by a beamsplitter 36 and photodetector 44 combination, which detects the intensity of the beam 32, enabling optimisation of the intensity of the wavelength-selected nanowband seed beam 32 by varying the angle of the tuning minor 42 and/or control parameters for the OPO 12 (such as the cavity length, the crystal temperature, the pump laser wavelength).

Figure 7 shows an embodiment of the invention which is the same as that shown in Figure 6, except that the tuning minor 42 has been dispensed with, such as in a Littrow configuration. In this embodiment, the grating 40 itself is tunable. Backward-diffracted radiation passes through the pin-hole 38.1 and is treated in the same way as described with reference to Figure 6. Wavelength selection is thus achieved by varying the orientation angle of the grating 40 itself. Figure 8 shows an embodiment of the invention which is the same as that shown in

Figure 6, except that the grating 40 and the tuning minor 42 have been replaced by a prism 46 and a tuning minor 42'.

Figure 9 shows an embodiment of the invention which is the same as that shown in Figure 6, except that the grating 40 and the tuning minor 42 have been replaced by a transmission type of tunable optical filter in the form of an etalon 48 and a return minor 42". The reflectivity of the minor 42" may be chosen to be less than 100%, conveniently to be about 50% to about 70%, so as to reduce potential interference interaction by multiple reflections between the etalon 48 and the minor 42". Other transmission types of optical filter include thin-film filters, fibre-grating filters and filters based on birefringent media including liquid crystals.

Figure 10 shows an embodiment of the invention which is the same as that shown in Figure 6, except that the grating 40, the tuning minor 42, the spatial filter 38 and the lens L4 have all been replaced by a reflection type of tunable optical filter in the form of an optical fibre grating 50. By manipulating and structuring the refractive indices of the fibre forming part of the fibre grating 50, its response to the initial OPO output light beam becomes wavelength-dependent. An enlarged view 51 of the optical fibre grating 50 is also depicted in Figure 10, to illustrate the periodic or regular variations of refractive index in the optical fibre core. The fibre grating 50 may be manipulated such that only components having a desired wavelength are reflected back to the OPO cavity 12, in the form of the nanowband seed beam 32, whereas components of the radiation having other wavelengths are transmitted through the fibre. Optical fibre gratings of this type are widely used in fibre-optical telecommunication devices. Other reflection types of optical filter include thin-film filters and filters based on birefringent media including liquid crystals. Figures 11 and 12 show alternative optical configurations that may be employed to determine the intensity of the nanowband radiation that is reflected from the grating 40 and tuning minor of the embodiment of the apparatus shown in Figure 6. In Figure 11, a beamsplitter 52 is provided between the spatial filter 38 and the lens L4. The beamsplitter 52 reflects radiation on to a multi-element photodetector anay 54 for detecting the intensity distribution of the radiation that is reflected. This distribution is effectively an image of the return beam on the plane of the pinhole 38.1. photodetector anay 54 is pre-calibrated (e.g., by means of the photodetector 44 shown in Figures 6 - 9) to indicate when the peak of the spatial distribution of the radiation diffracted by the grating 40 is optimally adjusted to pass back through the pin-hole 38.1 to form the nanowband seed beam 32. As the orientation of the tuning minor 42 is varied, the central wavelength of the selected nanowband seed beam 32 will vary accordingly. The wavelength of beam 32 conesponds to the particular longitudinal mode of the OPO cavity that has been selected by the wavelength-selective feedback unit 30; it may be continuously tuned by concerted variation of OPO control parameters (such as the cavity length, the crystal temperature, the pump laser wavelength) and the setting of the wavelength-selective feedback unit 30. The photodetector array 54 could less ideally be a four-element detector or a two-element split photodiode.

This output wavelength conesponds to the particular longitudinal mode of the OPO cavity that has been selected by the wavelength-selective feedback unit 30, so that the wavelength of nanowband seed beam 32 will automatically coincide with the signal or idler wavelength of the nanowband OPO output beam 34. The resulting single- longitudinal-mode OPO output wavelength may be continuously tuned by concerted variation of OPO control parameters (such as the cavity length, the crystal temperature, the pump laser wavelength) and the setting of the wavelength-selective feedback unit 30.

In Figure 12, the spatial filter section 38 is varied relative to that in Figures 6 - 9 and 11 , by replacing the pin-hole 38.1 by a suitable length of single-mode optical fibre 38 , into which the beams 28 and 32 are imaged by suitable (e.g., microscope objective) lenses.

If desired, the seed beam 32 may comprise several portions or components, each being of nanowband or single-longitudinal-mode radiation. Such a beam may be prepared for certain spectroscopic applications, such as atmospheric sensing or combustion diagnostics, that require multi-wavelength spectroscopic tailoring. Figure 13 shows an anangement for combining nanowband radiation components in a serial fashion, whilst Figure 14 shows an anangement for combining nanowband radiation components in a parallel fashion. In each case, the two wavelength-selective feedback units generate two separate nanowband outputs for dual-wavelength injection-seeding of the OPO. This approach can be extended to multi-wavelength seeding by using multiple wavelength- selective feedback units, ananged either in series (as in Figure 13) or in parallel (as in Figure 14). Figure 15 shows a serial configuration based on two grating and tuning minor combinations 40, 42 and 40'", 42'". The grating 40 and tuning minor 42 forms a first wavelength selector whereas grating 40' " and tuning minor 42'" forms a second wavelength selector. The zero-order diffraction from grating 40 is used further by grating 40' " and tuning minor 42' ". The wavelength-selected radiation component from grating 40'" and tuning minor 42" ' is combined together by the zero-order diffraction at grating 40 with the wavelength-selected radiation component from grating 40 and tuning minor 42.

Figure 16 is a schematic diagram of an embodiment of an apparatus in accordance with the invention, using wavelength-selective feedback incorporating a Littman-Metcalf- style filter unit with pin-hole spatial filter, as shown in Figure 6.

The OPO cavity 12 is pumped by a pulsed pump laser 14, which is typically a single-longitudinal-mode pulsed Nd: YAG laser producing pulses having a repetition rate of 10 Hz and a wavelength of 1.06 microns. The output pulse beam 14.1 from the pump laser 14 passes through a beamsplitter 18 which allows a portion 14.2, refened to herein as the second or delayed pump pulse beam, to be fed into an optical delay line 20. Another portion 14.3, refened to herein as the initial or first pump pulse beam, is fed through an optical attenuator 22 (comprising a thin-film polariser TFP1 and a half- wave plate HWP1) into the OPO cavity 12.

The delayed pump pulse beam 14.2 is attenuated by an optical attenuator 24 (comprising a thin-film polariser TFP2 and a half-wave plate HWP2) and is reflected by a series of minors 26.1, 26.2, 26.3, 26.4, 26.5 and a beamcombiner 26.6, placed at preselected positions along the optical delay line 20 so as to provide an optical path length which will give a desired delay time, into the OPO cavity 12 from the opposite direction as that from which the initial pump pulse beam 14.3 enters the OPO cavity 12. The OPO cavity 12 comprises an nonlinear-optical medium 12.1 and four reflective minor-surfaces Ml, M2, M3 and M4.

The nonlinear-optical OPO medium 12.1 is a quasi-phase-matched crystal such as periodically poled lithium niobate (PPLN). A typical PPLN element is 19 mm long by 11 mm broad by 0.5 mm thick with a set of eight parallel quasi-phase-matched gratings of varying periodicity (28.5 - 29.9 μm, in 0.2-μm steps) on a single substrate [28]. In other embodiments of the nonlinear-optical OPO medium, it is possible to use a birefringently phase-matched nonlmear-optical crystal or to use a pair of nonlinear-optical crystals co- aligned to minimise walk-off effects.

The OPO cavity 12 is provided with a temperature and position control means 12.2 for controlling the temperature and position of the nonlinear-optical medium 12.1.

The minors Ml, M2, M3 and M4 are ananged in a bow-tie ring-cavity configuration. Minors Ml and M2 are flat, whereas minors M3 and M4 are concave with a radius of curvature of 100 mm and are AR-coated at the pump wavelength of 1.06 μm. Ml, M2 and M4 are highly reflective at OPO signal wavelength of about 1.5 μm, whereas M3 has a reflectivity of 50% at the OPO signal wavelength and is used as both input and output coupler.

The optical length of the optical cavity 12 is accurately controlled by a piezoelectric translator 12.3 attached to minor Ml, driven by an electronic control unit 12.4, for controlling the position of the minor Ml relative to the minors M2, M4 and M3.

The output of the pulsed single-longitudinal-mode Nd:YAG pump laser 14 is divided by beamsplitters (BSl, BS2) into two beams 14.3 and 14.2 which pump the PPLN crystal 12.1 from opposite directions. Both pump beams 14.3 and 14.2 are focused to the centre of the PPLN crystal 12.1 by two identical lenses (LI, L2) with a focal length of about 40 cm. The waist diameter of each of the pump beams 14.2 and 14.3 is around 100 μm. Intensities of these two pump pulses 14.2 and 14.3 could be individually adjusted by two sets of attenuators 22, 24, each comprising a half-wave plate (HWPl, HWP2) and a thin-film polarizer (TFP1, TFP2).

The two portions 14.2 and 14.3 of the output from the pump laser 14 are separated , and the portion 14.2 is time-delayed by a delay line 20, so that they interact with the PPLN crystal 12.1 successively. The portion 14.3 generates a multimode OPO output beam 28 which is coupled to a wavelength-selective feedback system 30' as described with reference to Figure 6. In the wavelength-selective feedback system 30' a desired wavelength is selected by operating a control system 43 adapted to control the angular position of the tuning minor 42 relative to the grating 40. The wavelength-selected radiation in the form of the nanowband seed beam 32 is returned coUinearly with the delayed pump pulse beam portion 14.2 to the OPO cavity 12 and serves to seed the OPO cavity 12 before the arrival of the delayed portion 14.2 of the pump pulse 14.1. The wavelength of this seeding radiation 32 is automatically in resonance with the OPO cavity 12 because it was generated from the same OPO cavity 12 by a previous pump pulse 14.3.

The initial broadband OPO output beam 28 usually consists of both a signal beam and an idler beam, although it is possible that either the signal beam or the idler beam

(whichever is not to be filtered by the wavelength-selective feedback unit 30') will not exit from the OPO cavity 12, owing to factors such as minor reflectivity and substrate transmission. Both signal and idler beams are necessarily generated simultaneously inside the OPO cavity 12 when the nonlinear-optical medium 12.1 is pumped by the initial or first pump pulse beam 14.3.

The wavelength-selective feedback unit 30' selects one or several nanowband wavelength components from the initial broadband OPO output beam 28, and returns it, as a nanowband seed beam 32, to the OPO cavity through the minor 26.6, just prior to the delayed pump pulse 14.2, as indicated in Figures 5 (c) and 5 (d). The desired time delay for the nanowband seed beam 32 is achieved either by locating the wavelength- selective feedback unit 30' at a suitable distance from the OPO cavity 12 and/or by insering an additional optical delay line (not shown explicitly in Figure 16) of suitable path length between the wavelength-selective feedback unit 30' and the OPO cavity 12. The narrowband seed beam 32 is used to injection-seed the OPO cavity 12 before the delayed pump pulse 14.2 arrives there. Because of the injection seeding effect of the nanowband seed beam 32, the delayed pump pulse beam 14.2 generates a wavelength- tailored OPO output beam 34 which matches the spectral distribution of the nanowband seed beam 32.

The wavelength selected for the nanowband seed beam may be in the near-infrared wavelength region, around about 1.5 μm, where suitable high-reflectivity minors, photodetectors, nonlinear-optical materials, and other optical components are readily available from commercial suppliers. Most of the optical mounts and piezoelectric control devices required for the OPO cavity 12 are also standard commercial components. The form of wavelength-selective feedback unit 30' that is depicted in Figure 16 is as shown in Figure 6. It can equally well be replaced by any of the other embodiments of a wavelength-selective feedback unit shown in any of Figures 7 - 15. Two other embodiments are fully depicted in Figures 17 and 18 and discussed below.

Figure 17 is a schematic diagram of another embodiment of an OPO system in accordance with the invention. Like reference numerals refer to like components. The wavelength-selective feedback 30' in this embodiment is different to that of Figure 16, whereas the other parts of this system are the same as those shown in Figure 16. The wavelength-selective feedback 30' comprises a tunable etalon and storage device 60 and a prism 62. A photodetector 64 is provided to measure the intensity of the components penetrating the prism 62 and directed by it towards the photodetector 64. An etalon controller 66 is provided to control the tunable etalon and storage device 60. A single- mode optical fibre 68 is provided to conduct the initial output beam 28 and the nanowband seed beam 32, as well as creating a suitable time delay for the nanowband seed beam 32.

The tunable etalon and storage device 60 is effectively an optical ringdown cavity comprising two carefully aligned, highly reflective flat minors (e.g., Newport 10CM00SR.70T), one of which is attached to a suitable piezoelectric translator (e.g., Piezomechanik GmbH, model HPStl50/20-15/25VS35). To enable the ringdown cavity to select only one longitudinal mode out of a multimode free-running OPO output beam, the free spectral range (FSR) of the ringdown cavity should be larger than the optical bandwidth of a free-running periodically poled lithium niobate OPO. The free-running bandwidth of the OPO cavity 12 is about 150 GHz (5 cm^"1) FWHM. Therefore, a FSR of about 450 GHz (15 cm^"1) should be adequate. This conesponds to a minor separation of 0.333 mm (c/(2*FSR) ). To have the ringdown time τ of the cavity in the order of the pulse duration of about 10 ns of a Nd:YAG pump laser, the reflectivity of the minors need to be >99.9889% (see Equ. 2). The optical bandwidth (FWHM) of the ringdown cavity as a tunable optical filter may be about 16 MHz (= l/(2πτ).

Depending on the type of wavelength-selective feedback unit 30 that is used, the nanowband seed beam 32 may be prolonged significantly in time scale. This is particularly true when the apparatus uses a high-finesse etalon as shown in Figure 9 or a ringdown cavity as shown in Figure 17.

Referring to Figure 18, there is shown an apparatus as shown before in other embodiments of the invention such as Figure 16, except that it is provided with two sets of wavelength-selective units which are coupled together in series, each comprising a grating 40, 40' and a tuning minor 42, 42'. The initial broadband OPO output is coupled to the second wavelength-selective combination of grating 40' and tuning minor 42' through the zero-order diffraction of the broadband OPO output by the grating 40. The nanowband component selected by the second wavelength-selective unit is combined together with the narrowband component selected by the first wavelength-selective combination of grating 40 and tuning minor 42 through a zero-order diffraction in opposite direction by the grating 40. The wavelength-selective function of each of the two wavelength-selective units could be disabled individually by beam flags 45, 45' which block the diffracted beams 41, 41'. The tuning minors 42, 42' and the beam flags 45, 45' are controlled by control unit 43.

In the passages that follow, embodiments of the third and fourth aspects of the invention are described with reference to Figures 19 to 22. Figures 19 to 22 are adaptations of Figures 1 to 3 and 16 respectively. In adapting the aforementioned embodiments, which can be used for both pulsed and continuous wave operation, to embodiments of the third and fourth aspects of the invention, which operate on continuous wave radiation only, the optical delay lines 20 and 31 are either eliminated or otherwise simply shown as optical links A20 and A31 as they are not required, for continuous wave operation, to cause an optical delay. Although not described below in the same manner as the aforementioned drawings that had been adapted, it should be understood that the same adaptation can be applied to any of the other embodiments of the first and second embodiments shown in any of the other drawings. In addition to these changes, continuous wave pump lasers are used for the third and fourth aspects of the invention, instead of pulsating pump lasers.

Referring to Figure 19, which represents an embodiment according to the third and fourth aspects of the present invention, there is shown an apparatus AlO in accordance with the invention, comprising an optical parametric oscillator (OPO) cavity A12 pumped by a continuous- wave pump laser A14, preferably with stable output intensity and nanow optics bandwidth, most preferably with stable single-longitudinal-mode output. The output beam A14.1 from the continuous-wave pump laser A14 is fed through an optical isolator A16 to prevent the feedback of potentially damaging laser light towards the continuous-wave pump laser A14, before it passes through a beam splitter A18 which reflects a portion A14.2, refened to hereinbelow as the second pump beam. Another portion A14.3, refened to hereinbelow as the initial pump beam, is fed through an optical attenuator A22 (comprising a polarisation and intensity controller) into the OPO cavity 12. The pump beam A14.2 is steered by the minor A19.1, an optical line A20 and minor A19.2, attenuated by another optical polarisation and intensity controller A24 and reflected by a beam combiner A26 into the OPO cavity A12 from the opposite direction as that from which the initial pump beam A14.3 enters the OPO cavity A12.

The OPO cavity A12 comprises at least one nonlinear-optical medium (not shown in Figure 19, but described with reference to Figures 20 - 21) and two reflective minor- surfaces (not shown in Figure 19, but described with reference to Figures 20-21). The nonlinear-optical medium could either be a non-centrosymmeric crystal of homogeneous composition for birefringent phase-matching of the OPO process or it may be a crystal with alternating structural domains, such as periodically poled lithium niobate (PPLN) or periodically poled potassium titanyl phosphate (PPKTP), for quasi-phase-matching of the OPO process.

The OPO cavity A12 is resonant at certain specific signal and idler output wavelengths which depend on and are determined by the geometry of the OPO cavity A12.

An initial broadband OPO output A28, consisting of both a signal beam and an idler beam, is generated when the nonlinear-optical medium is pumped by the initial pump beam A14.3. An external wavelength-selective feedback unit A30 selects one or several nanowband wavelength components from the initial broadband OPO output A28, and returns it, as a nanowband seed beam A32, to the OPO cavity. The nanowband seed beam A32 is combined with the second pump beam A14.2 by the beam combiner A26, such that it co-propagates with the second pump beam A14.2. An optical line A31 connects optically the wavelength-selected feedback unit A30 (not shown in Figure 19) and the OPO cavity A12 together. The narrowband seed beam A32 is used to injection- seed the OPO cavity A12. Because of the injection seeding effect of the nanowband seed beam A32, the second pump beam A14.2 generates a wavelength-tailored OPO output A34 which matches the spectral components of the nanowband seed beam A32. The wavelength of the nanowband seed beam A32 is preferably variable over a range of wavelengths by means of the external wavelength-selective feedback unit A30, as has been described in greater detail above in relation to Figures 6 to 15. Referring to Figure 20, representing an embodiment according to the third and fourth aspects of the present invention, there is shown an apparatus A10' in which a single continuous-wave pump source A14' and a single optical nonlinear-optical medium

A12.1' are provided in an optical ring cavity A12' formed by minors AMI', AM2', AM3' and AM4'.

The continuous-wave pump source A14' includes an optical isolator A16' to prevent the feedback of potentially damaging light towards the pump source A14'.

A portion A14.3' of the output of the pump source A14' passes through a beamsplitter A18' and a polarisation and intensity control unit A22' to form the initial pump beam A14.3'. Another portion is reflected by the beamsplitter A18' and is fed through an optical line A20' and a polarisation and intensity control unit A24' to form the second pump beam A14.2'.

The initial pump beam A14.3' enters the optical cavity through the minor AMI' and is reflected by the minor AM3' to the minor AM4' and by the minor AM4' to the minor AM2' in the direction shown by the broken anow.

The second pump beam A14.2' passes through the minor AM2' and is reflected consecutively by the minors AM4' and AM3' so as propagate in the cavity in the direction of the solid anow. It thus pumps the nonlinear-optical crystal A12.1' from the opposite direction as that from which the initial pump beam A14.3' does. The length of the optical path in the ring optical cavity A12' is accurately controlled by a piezoelectric translator A12.3' (also shown as PZT in other Figures) attached to minor AM3', for controlling the position of the minor M3' relative to the minors AMI',

AM2' and AM4'. An optical line A31' connects optically the wavelength-selected feedback unit A30' and the OPO cavity A12' together. Figure 21 is the same as Figure 3, but without the optical delay and synchronising unit 17". In Figure 21, which represents another embodiment according to the third and fourth aspects of the present invention, there is shown an embodiment of the apparatus in which the ring optical cavity A12" comprises two nonlinear-optical media A12.l l" and

A12.12". Similar parts to those in Figures 19 and 20 are indicated by similar reference numerals.

The first pump beam A14.3" is generated from the output of a first continuous-wave pump source A14" after having been passed through a polarisation and intensity control unit A22". The second pump beam A14.2" is generated from the output of a second continuous-wave pump source A15" after having been passed through a polarisation and intensity control unit A24".

Coherent radiation that has a broad band of signal and idler wavelengths is generated when the nonlinear-optical crystal A12.ll" is pumped by the pump source A14". The coherent radiation circulates insides the ring cavity A12" in a first direction indicated by the broken line anow. A portion of the coherent radiation is coupled out of the ring cavity A12" through the beam splitter AM4" as the first coherent output beam radiation. An external wavelength-selective feedback unit A30" selects one or several nanowband wavelength components from the first coherent output beam radiation, and returns it, as a nanowband component, to the ring optical cavity A12" such that it co- propagates with the second pump beam A14.2". The nanowband seed beam is used to injection-seed the OPO cavity. An optical line A31" connects optically the wavelength- selected feedback unit A30' and the OPO cavity A12" together.

The nanowband seed beam and the second pump beam A14.2" overlap in both time and space in the second gain medium A12.12", such that coherent nanowband radiation is generated in the second gain medium A12.12", which is circulated in the ring optical cavity until it exits at the minor AM4" in a different direction to that of the initial broadband output.

Because of the injection seeding effect of the nanowband component, wavelength- tailored narrowband OPO radiation is generated when the nonlinear crystal A12.12" is pumped by the second pump beam A14.2". The nanowband OPO radiation circulates insides the ring cavity in the opposite direction to that of the initial radiation generated in the crystal A12.ll", as is indicated by the solid line anow. A portion of the narrowband OPO radiation is coupled out of the cavity through the beam splitter AM4" as wavelength-tailored nanowband coherent output radiation.

The nonlinear crystals A12.ll" and A12.12" may be in the form of two separate and individual crystals or, alternatively, they may be in the form of two different locations on or regions of a single crystal substrate.

In the event that two separate and individual crystals A12.l l" and A12.12" are used, the two optical parametric processes associated with respectively the first pump beam A14.3" and the second pump beam A14.2" can be separated from each other. This provides flexibility in the design and optimisation of the performance of the OPO. For example, the choice of crystal A12. l l" and the associated pump source A14", as well as the optical parametric processes occurring in them, may be optimised for lowering the pump threshold of that part of the apparatus, whilst the choice of the crystal A12.12" and the associated pump source A15" as well as the optical parametric processes occurring in them may be optimised for generating high OPO output energy and/or for one or more desired output wavelengths.

One factor that binds the two optical parametric processes is that they need to share one common OPO output wavelength selected by the wavelength selective means A30". A further advantage of using two crystals is that the possibility of interference between the first pump beam A14.3" and the second pump beam A14.2", when they overlap in time and space in a nonlinear optical crystal, can be eliminated. Furthermore, the use of two crystals provides flexibility in the design of the OPO cavity and the beampath anangement of the first and second pump beams A14.3", A14.2". Referring to Figure 22, which represents an embodiment according to the third and fourth aspects of the present invention, there is shown a schematic diagram of an embodiment of an apparatus in accordance with the invention, using wavelength-selective feedback incorporating a Littman-Metcalf-style filter unit with pin-hole spatial filter, as shown in Figure 6. The OPO cavity A12 is pumped by a continuous- wave pump laser A14, which has typically a single-longitudinal-mode output wavelength. The output beam A14.1 from the pump laser A14 passes through a beamsplitter A18 which allows a portion A14.2, refened to herein as the second pump beam, to be fed into an optical link line A20. Another portion A14.3, refened to herein as the initial or first pump beam, is fed through an optical attenuator A22 (comprising a thin-film polariser TFPl and a half- wave plate HWPl) into the OPO cavity A12.

The second pump pulse beam A14.2 is attenuated by an optical attenuator A24 (comprising a thin-film polariser TFP2 and a half-wave plate HWP2) and is reflected by a minor A26.5 and a beamcombiner A26.6, placed at preselected positions along the optical link A20, into the OPO cavity A12 from the opposite direction as that from which the initial pump pulse beam A14.3 enters the OPO cavity A12.

The OPO cavity A12 comprises an nonlinear-optical medium A12.1 and four reflective minor-surfaces AMI, AM2, AM3 and AM4. The minors AMI, AM2, AM3 and AM4 are ananged in a bow-tie ring-cavity configuration. AM3 is used as both input and output coupler.

The OPO cavity A12 is provided with a temperature and position control means A12.2 for controlling the temperature and position of the nonlinear-optical medium A12.1.

The optical length of the optical cavity A12 is accurately controlled by a piezoelectric translator A12.3 attached to minor AMI, driven by an electronic control unit A12.4, for controlling the position of the minor AMI relative to the minors AM2, AM4 and AM3.

The output of the continuous-wave pump laser A14 is divided by beamsplitters (ABSl, ABS2) into two beams A14.3 and A14.2 which pump the crystal A12.1 from opposite directions. Both pump beams A14.3 and A14.2 are focused to the centre of the crystal A 12.1 by two identical lenses (LI, L2). Intensities of these two pump pulses A14.2 and A14.3 could be individually adjusted by two sets of attenuators A22, A24, each comprising a half- wave plate (HWPl, HWP2) and a thin-film polarizer (TFPl, TFP2). The two portions A14.2 and A14.3 of the output from the pump laser A14 are separated. The portion A14.3 generates a multimode OPO output beam A28 which is coupled to a wavelength-selective feedback system A30' as described with reference to Figure 6. In the wavelength-selective feedback system A30' a desired wavelength is selected by operating a control system A43 adapted to control the angular position of the tuning minor A42 relative to the grating A40. The wavelength-selected radiation in the form of the nanowband seed beam A32 is returned coUinearly with the second pump beam portion A14.2 to the OPO cavity A12 and serves to seed the OPO cavity A12. The wavelength of this seeding radiation A32 is automatically in resonance with the OPO cavity A12 because it was generated from the same OPO cavity A12 by a previous pump pulse A14.3.

The initial broadband OPO output beam A28 usually consists of both a signal beam and an idler beam, although it is possible that either the signal beam or the idler beam (whichever is not to be filtered by the wavelength-selective feedback unit 30') will not exit from the OPO cavity A12, owing to factors such as minor reflectivity and substrate transmission. Both signal and idler beams are necessarily generated simultaneously inside the OPO cavity A12 when the nonlinear-optical medium A12.1 is pumped by the initial or first pump pulse beam A14.3. The wavelength-selective feedback unit A30' selects one or several nanowband wavelength components from the initial broadband OPO output beam A28, and returns it, as a nanowband seed beam A32, to the OPO cavity through the minor A26.6. The nanowband seed beam A32 is used to injection-seed the OPO cavity A12. Because of the injection seeding effect of the narrowband seed beam A32, the second pump beam A14.2 generates a wavelength-tailored OPO output beam A34 which matches the spectral distribution of the nanowband seed beam A32.

The form of wavelength-selective feedback unit A30' that is depicted in Figure 22 is as shown in Figure 6. It can equally well be replaced by any of the other embodiments of a wavelength-selective feedback unit shown in any of Figures 7 - 15. In Figure 23 a wavelength-control of a pulsed tunable OPO has the versatility and wavelength-agility of a grating or etalon with the modularity of injection seeding. A wavelength-selective tunable filter (WSTF) external to the OPO cavity is employed, enabling access to a wider range of wavelengths than with convenient TDL seed sources. The new nanowband tunable OPO system is based on quasi-phase-matched (QPM) NLO media ananged in a simple, piezoelectrically scanned "bow-tie" ring cavity with no intracavity tuning elements and pumped by a ns-pulsed Nd:YAG laser. The OPO system in Fig. 23 has an optical feedback from an external WSTF, which is configured as a Litmann-Metcalf diffraction grating. The spatial filter feeds back a low-power, nanowband portion of the broadband light from the earlier free-running (unseeded) phase of the pulsed OPO, to injection-seed the same OPO after a suitable time delay, synchronising it with a counter-propagating later part of the delayed pump pulse.

The experimental implementation of Fig. 23 comprises an OPO B10 based on periodically poled KTiOPO₄ (PPKTP), pumped by a frequency-doubled SLM Nd:YAG laser with a pulse duration of ~8 ns at 10 Hz. Pump laser output is split into two beams which pump the PPKTP crystal successively from opposite directions, with the later, higher-power pump pulse traversing an optical delay line (typically 5-10-ns delay). Intensities and polarisations of these two pump beams are individually adjustable by combinations of half-wave plate (HWP) and thin-film polariser (TFP). The earlier pump pulse generates a broadband, free-running OPO output that is coupled to the WSTF, the narrowband output from which is returned coUinearly, via time delay B31, with the delayed pump pulse, which is delayed by delay line B20, to the OPO.

Referring to Figure 24, there is shown an apparatus as described before in other embodiments of the invention such as that shown in Figure 16, except that the OPO cavity C12 of Figure 24 comprises two minors CMl, CM2 and a prism P.

A nonlinear-optical gain medium C12.1 is provided in the optical cavity C12. The operational parameters (eg. temperature, orientation, position) of the nonlinear-optical medium are adjustable by a control unit C12.2. The operational parameters (eg. temperature, orientation, position) of the prism are adjustable by a control unit C12.3.

An initial pump beam C14.3 passes through a beamsplitter DM1 and then enters the optical cavity C12 through the minor CMl, before pumping the nonlinear-optical crystal C12.1 from the right. The generated initial broadband OPO radiation propagates in the direction of the broken anow C15. The initial broadband OPO signal component C28.1 resonants in the optical cavity C12. The initial broadband OPO idler radiation component C28.2 is deflected by the prism P at a smaller angle than that of the signal radiation, and exits the optical cavity C12. The initial broadband OPO output radiation C28 generated from the initial broadband OPO signal radiation C28.1 passes through the partially transmissive minor CMl, and is reflected to a wavelength-selective feedback unit C30 by a minor C19.2. The wavelength-selective feedback unit C30 selects a nanowband component from the initial broadband OPO output beam C28, and returns it via the minor C19.2, as a nanowband seed beam C32, to the optical cavity C12. The desired time delay for the nanowband seed beam C32 is achieved by an optical delay line C31.

The narrowband seed beam C32 is coupled through the partially transmissive minor CMl, deflected by the prism P and reflected by the minor CM2 to injection seed the nonlinear-optical gain medium C12.1 in such a manner that, upon the introduction into the optical cavity C12 of a delayed or second pump beam C14.2 through the minor CM2, the nanowband seed radiation C32 overlaps spatially and temporally in the nonlinear-optical gain medium C12.1 with the delayed pump beam C14.2, thereby interacting with the nonlinear-optical gain medium C12.2 to generate nanowband second OPO radiation, comprising a narrowband second OPO signal component C34.1 and a second OPO idler component C34.2. The nanowband second OPO signal component C34.1 resonants in the cavity in the direction of the solid anow C17. The second OPO idler component C34.2 is deflected by prism P at a different angle to that at which the signal radiation is deflected and exits the optical cavity C12. A portion of the nanowband second OPO signal component C34.1 passes through the partially transmissive minor CMl and may be directed off the beam path of the initial pump beam C14.3 by a dichroic minor DM1.

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Claims

1. An apparatus for providing a nanowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous-wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising:

- an optical parametric oscillator including an optical cavity and a nonlinear- optical gain medium, the optical cavity comprising at least three optically interconnected reflectors or deflectors disposed in such a manner relative to one another that optical radiation entering said optical cavity is sequentially directed from one reflector or deflector to the other, said nonlinear-optical gain medium being capable of generating, from a first pump beam introduced into said optical cavity in a first direction, a first coherent output beam that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a nanowband component comprising signal and/or idler radiation, from said first coherent output beam, and for introducing said nanowband component into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction such that, upon the introduction into said optical cavity of a second pump beam, said nanowband component overlaps spatially in said nonlinear- optical gain medium with said second beam thereby interacting with said nonlinear-optical gain medium to generate a narrowband second coherent output beam;

- means for decoupling said first coherent output beam from said cavity, along a first optical path, into said selection means; - means for coupling said narrowband component from said selection means into said cavity; and

- means for decoupling said nanowband second coherent output beam from said cavity along a second optical path that differs from the first optical path.

2. An apparatus as claimed in claim 1, wherein the optical cavity is ananged as a ring cavity comprising three or more reflectors and/or deflectors.

3. An apparatus as claimed in claim 1, wherein the optical cavity is ananged as a bow-tie cavity.

4. An apparatus as claimed in claim 1, wherein a surface of the nonlinear-optical gain medium acts as a reflector.

5. An apparatus as claimed in claim 1, wherein the nonlinear-optical gain medium acts as a deflector.

6. An apparatus as claimed in claim 1, wherein at least one of the reflectors or deflectors of the optical cavity is a deflector and wherein the deflector is a prism.

7. An apparatus as claimed in claim 6, wherein the prism is the nonlinear-optical gain medium.

8. An apparatus as claimed in claim 1, wherein the means for decoupling is partially reflective at the wavelength(s) of the optical radiation in the optical cavity and partially transmissive at the wavelength(s) optical radiation in the optical cavity.

9. An apparatus as claimed in claim 1, wherein, in order to control the optical path length inside the optical cavity, the position of at least one of the reflectors and/or deflectors is adjustable.

10. An apparatus as claimed in claim 1, wherein the orientation of at least one of the reflectors and/or deflectors is adjustable.

11. An apparatus as claimed in claim 1, wherein the reflectors are minors.

12. An apparatus as claimed in claim 1, wherein the nanowband component comprises more than one wavelength feature, and the apparatus further comprises continuous tuning means for controlling the wavelength of each feature of the nanowband component.

13. An apparatus as claimed in claim 1, further comprising means for introducing said selected nanowband component into said optical cavity as injection-seeding radiation, for the generation of said second coherent output beam pulse.

14. An apparatus as claimed in claim 1, wherein a multi-grating quasi-phase- matched element is used as the nonlinear-optical gain medium.

15. An apparatus as claimed in claim 1, wherein the nonlinear-optical gain medium is a periodically poled lithium niobate element.

16. An apparatus as claimed in claim 1, wherein the nonlinear-optical gain medium is a periodically poled potassium titanyl phosphate.

17. An apparatus as claimed in claim 1, wherein the selection means is a tunable optical filter.

18. An apparatus as claimed in claim 1, wherein the selection means is a high- resolution diffraction-grating device.

19. An apparatus as claimed in claim 1, wherein the selection means is a high- finesse tunable optical filter operating on the cavity ringdown principle.

20. An apparatus as claimed in claim 1, wherein the tunable optical filter comprises a diffraction grating mounted, at nearly grazing incidence, in the Littman- Metcalf configuration, with a tuning minor ananged to reflect the first-order- diffracted light back onto the grating surface along the path of the incident light.

21. An apparatus as claimed in claim 1, wherein the tunable optical filter comprises a high-finesse Fabry-Perot optical cavity that is able to resonate with pulsed OPO output light, store selectively a component of that light for a period of time, and emit that light gradually through both front and back cavity minors with a characteristic exponential ringdown decay time.

22. An apparatus as claimed in claim 1, wherein the output coherent light has a nanow band of frequencies, conesponding to single-longitudinal-mode operation.

23. An apparatus as claimed in claim 1, comprising means for varying the wavelength of the nanowband portion that is selected out of the first coherent output of the OPO cavity.

24. An apparatus as claimed in claim 1, comprising a birefringently phase-matched medium in the OPO cavity that is slightly misaligned to reduce its finesse.

25. An apparatus for providing a pulsed, nanowband, coherent beam of optical radiation, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising: - an optical parametric oscillator including an optical cavity and a nonlinear- optical gain medium, the optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such a manner relative to one another that optical radiation entering said optical cavity is sequentially directed from one to the other, said nonlinear-optical gain medium being capable of generating, from a first pump pulse introduced into said optical cavity in a first direction, a first coherent output beam pulse that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a nanowband component comprising signal and/or idler radiation, from said first coherent output beam pulse, and for introducing said nanowband component into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction and at a time such that, upon the introduction into said optical cavity of a second pump pulse, said nanowband component overlaps spatially and temporally in said nonlinear-optical gain medium with said second pump pulse thereby interacting with said nonlinear-optical gain medium to generate a nanowband second coherent output beam pulse;

- means for decoupling said first coherent output pulse from said cavity, along a first optical path, into said selection means; - means for coupling said nanowband component from said selection means into said cavity; and

- means for decoupling said narrowband second coherent output pulse from said cavity along a second optical path that differs from the first optical path.

26. An apparatus as claimed in claim 25, wherein the wavelengths of radiation generated by the second pump pulse conesponds with the wavelength of at least a portion of said nanowband component.

27. An apparatus as claimed in claim 25, wherein the intensity of the second pump pulse is such as to generate sufficient gain in the nonlinear optical gain medium at an OPO output wavelength that conesponds with the wavelength of at least a portion of said nanowband component, whereby it interacts with said nonlinear optical gain medium to generate a nanowband second coherent output beam pulse.

28. An apparatus as claimed in claim 25, wherein the first and second pump pulses originate from the same input pulse and the apparatus comprises a beam splitter

5 to split the input pulse into the first and second components.

29. An apparatus as claimed in claim 25, wherein the first and second pump pulses originate from different input pulses.

30. An apparatus as claimed in claim 25, comprising optical delay means between the optical cavity and the selection means or between the selection means and o the optical cavity, or between both.

31. An apparatus as claimed in claim 25, wherein the optical delay means is adapted to delay the arrival of the nanowband component, so as to achieve a desired overlap with the second pump pulse.

32. An apparatus as claimed in claim 25, comprising a delay path for achieving a s desired time delay between the first pump pulse and the second pump pulse.

33. An apparatus as claimed in claim 25, wherein the second pump pulse and the first pump pulse use the same region of the nonlinear-optical gain medium.

34. An apparatus as claimed in claim 25, wherein the second pump pulse and the first pump pulse use different nonlinear-optical gain media. 0

35. An apparatus as claimed in claim 25, comprising an integrated double-pulse pump laser system for generating both the first pump pulse beam and the second pump pulse beam.

36. An apparatus as claimed in claim 25, wherein the pulse pump laser system is a

Nd:YAG laser system. 5 37. An apparatus as claimed in claim 25, comprising a single laser system that produces a pulse train by mode locking, the single laser system being provided with an electro-optical switch to select pulses from the pulse train that are suitably separated in time.

38. An apparatus as claimed in claim 25, comprising a single-pulse laser system, o with each laser pulse split into a first portion that is fed into the OPO as the first pump pulse beam and a second portion that is beamed along an optical delay line and is introduced into the OPO cavity as the second pump pulse beam.

39. An apparatus as claimed in claim 25, wherein the second pump pulse beam is delayed sufficiently to arrive at the OPO slightly after the arrival and during the injection-seeding of the OPO by the said nanowband wavelength component.

40. An apparatus as claimed in claim 25, comprising a beamsplitter for splitting pulses generated by the pump laser.

41. An apparatus as claimed in claim 25, comprising a fast optical switch, for splitting pulses from the pump laser, into an earlier portion and a later portion to be used as the first and second pump pulse beams, respectively.

42. An apparatus as claimed in claim 25, comprising two optical delay lines, one for delaying the second pump pulse beam and the other for delaying the selected nanowband seed radiation, so that they each arrive at the OPO cavity after a suitable time delay.

43. An apparatus as claimed in claim 25, wherein the overall time delay of the second pump pulse beam is from about 1 nanosecond to about 100 nanoseconds.

44. An apparatus as claimed in claim 25, wherein the overall time delay of the second pump pulse beam is shorter than the time it takes for a pulse of the first output coherent light beam to decay to zero.

45. An apparatus as claimed in claim 25, wherein time delay line is an optical fibre of a suitable length.

46. An apparatus as claimed in claim 25, wherein time delay line is a series of minors spaced from one another and ananged in such a fashion that the second pump pulse beam is reflected from one minor to another along an optical path whose length determines the time delay.

47. An apparatus as claimed in claim 25, wherein the time delay of the selected nanowband seed radiation is adjustable to ensure that the arrival times at the optical cavity of the second pump pulse and the selected nanowband seed radiation are synchronised.

8. An apparatus for providing nanowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said apparatus comprising:

- an optical parametric oscillator including an optical cavity comprising a nonlinear-optical gain medium and at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that optical radiation entering said optical cavity is sequentially directed from one to the other, said nonlinear-optical gain medium being capable of generating, from a first continuous-wave pump beam introduced into said optical cavity in a first direction, a first coherent continuous-wave output beam that has a broad band of signal and idler wavelengths; and

- selection means, disposed outside of said optical cavity, for selecting a nanowband component comprising signal and/or idler radiation, from said first coherent continuous-wave output beam, and for introducing said nanowband component into said nonlinear-optical gain medium as injection-seeding radiation, in a second direction such that, upon the introduction into said optical cavity of a second continuous-wave pump beam, said nanowband component overlaps spatially in said nonlinear-optical gain medium with said second continuous-wave pump beam thereby interacting with said nonlinear-optical gain medium to generate a nanowband second coherent continuous-wave output beam;^'

- means for coupling said nanowband component from said selection means into said cavity; and

- means for decoupling said na owband second coherent continuous-wave output beam from said cavity along a second optical path that differs from the first optical path.

49. An apparatus as claimed in claim 48, wherein the second continuous-wave pump beam generates sufficient gain at an OPO output wavelength that conesponds with the wavelength of at least a portion of said nanowband component, so as to generate the nanowband second coherent continuous-wave output beam.

50. A method for providing a nanowband coherent beam of optical radiation, selected from the group consisting of pulsed and continuous-wave, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said first pump beam, a first coherent output beam that has a broad band of signal and idler wavelengths; - decoupling said first coherent output beam from said cavity, along a first optical path;

- selecting, in a location outside of said optical cavity, a nanowband component comprising signal and/or idler radiation, from said first coherent output beam;

- coupling said nanowband component into said cavity and into said nonlinear- optical gain medium as injection-seeding radiation, in a second direction; and

- introducing a second pump into said nonlinear-optical gain medium in such a manner and at such a time that said nanowband component overlaps spatially and temporally in said nonlinear-optical gain medium, with said nanowband component, thereby interacting with said nonlinear-optical gain medium to generate a nanowband second coherent output beam; and

- decoupling said nanowband second coherent output beam from said cavity along a second optical path which differs from the first optical path.

51. A method for providing a pulsed, nanowband coherent beam of optical radiation, along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a first pump pulse, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed from one to the other, and a nonlinear-optical gain medium disposed in said optical cavity;

- selecting, in a location outside of said optical cavity, a nanowband component comprising signal and/or idler radiation, from said first coherent output beam pulse;

- coupling said narrowband component into said cavity and into said nonlinear- optical gain medium as injection-seeding radiation, in a second direction and after a suitable time delay; and

- introducing a second pump pulse into said nonlinear-optical gain medium in such a manner and at such a time that said nanowband component overlaps spatially and temporally in said nonlinear-optical gain medium, with said nanowband component, thereby interacting with said nonlinear-optical gain medium to generate a nanowband second coherent output beam pulse; and

- decoupling said narrowband second coherent output beam pulse from said cavity along a second optical path which differs from the first optical path.

52. A method for providing a nanowband coherent continuous-wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of: - introducing a first continuous-wave pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed by the reflectors, and a nonlinear-optical gain medium disposed in said optical cavity;

- generating, from said first continuous-wave pump beam, a first coherent continuous-wave output beam that has a broad band of signal and idler wavelengths; - decoupling said first coherent continuous-wave output beam from said cavity, along a first optical path;

- selecting, in a location outside of said optical cavity, a nanowband component comprising signal and/or idler radiation, from said first coherent continuous-wave output beam; - coupling said nanowband component into said cavity and into said nonlinear- optical gain medium as injection-seeding radiation, in a second direction; and

- introducing a second continuous-wave pump beam into said nonlinear-optical gain medium in such a manner that said second continuous-wave pump beam overlaps spatially in said nonlinear-optical gain medium, with said nanowband component, " thereby interacting- with said nonlinear-optical -gain - medium -to generate a nanowband second coherent continuous-wave output beam; and

- decoupling said nanowband second coherent continuous-wave output beam from said cavity along a second optical path which differs from the first optical path.

53. A method for providing a nanowband coherent pulsed- wave optical radiation along an optical path that is different from that of any broadband radiation which is also generated by the apparatus, said method including the steps of:

- introducing a continuous-wave, first pump beam, in a first direction, into an optical parametric oscillator comprising an optical cavity comprising at least three optically interconnected reflectors and/or deflectors disposed in such manner relative to one another that light radiation entering said optical cavity is sequentially directed, from one to the other and a nonlinear-optical gain medium disposed in said optical cavity;

- selecting, in a location outside of said optical cavity, a nanowband component comprising signal and/or idler radiation, from said first coherent continuous-wave output beam;

- introducing a pulsed, second pump beam into said nonlinear-optical gain medium in such a manner that said second pump beam overlaps spatially in said nonlinear- optical gain medium, with said nanowband component, thereby interacting with said nonlinear-optical gain medium to generate a pulsed, nanowband second coherent output beam; and

- decoupling said pulsed, nanowband second coherent output beam from said cavity along a second optical path which differs from the first optical path.