WO1994025896A1

WO1994025896A1 - A tunable optical oscillator

Info

Publication number: WO1994025896A1
Application number: PCT/GB1994/000915
Authority: WO
Inventors: Miles John Padgett; Malcolm Harry Dunn; Wilson Sibbett; Finlay George Colville
Original assignee: The Secretary Of State For Trade And Industry Acting Through Director, National Physical Laboratory; University Court Of The University Of St. Andrews Of College Gate
Priority date: 1993-04-30
Filing date: 1994-04-29
Publication date: 1994-11-10
Also published as: GB9309031D0; AU6576994A

Abstract

An optical oscillator has first (13) and second (14) resonant cavities with at least a portion of each cavity sharing a common volume. A characteristic of at least one of the cavities is variable. A non-linear material (20) is positioned within the 'common volume'. Variation of the or each resonant cavity enables a continuously tunable output signal to be obtained. The invention overcomes the problem associated with prior art systems in which phase matching, resonance of a common cavity at two wavelengths and energy conservation were required simultaneously.

Description

A TUNABLE OPTICAL OSCILLATOR This invention relates to tunable optical oscillators. More particularly, but not exclusively, it relates to a tunable optical parametric oscillator which is capable of smooth and continuous frequency tuning of output light.

In recent years, the development of new non-linear optical materials, coupled with advances in laser technology, has led to the possibility of compact, coherent light sources that are tunable over the whole of the visible spectrum and beyond. These light sources are based on Optical Parametric Oscillators (0P0) which use a conventional laser as an 0P0 pump source. OPOs use a non-linear optical material to split an incoming photon (pump photon) into two photons of lower energy (signal and idler photons). The exact ratio between the signal and idler frequencies is determined by phase matching conditions in the non-linear optical material which is usually a crystal and the modes of the resonating cavity.

OPOs, like other laser systems, fall into two main categories. The first is pulsed systems which emit their light as a stream of short, or ultrashort pulses of light. The second is Continuous-Wave (CW) systems which emit light at a constant intensity. The present invention has applications within both types of systems. Investigations into pulsed OPOs began as far back as 1965 when Giordmaine and Miller demonstrated tunable optical parametric oscillation in lithium niobate (A. Giorda ine ans R.C. Miller, Phys. Rev. Lett., vol 4, p973, 1969). Despite considerable effort directed in this field, progress was slow due to lack of suitable non-linear materials. In addition to a large non-linear coefficient and a high damage threshold, the ideal material is a non-linear crystal which needs to exhibit a wide transparency range and an adequate degree of birefringence. The latter two properties govern the possible tuning range, since in order to obtain^* parametric oscillation, the signal, idler and pump fields need to be phase-matched within the non-linear material crystal.

Since the early 'proof-of-principie' demonstration, there has been the availability of better non-linear materials which offer increased efficiencies. A significant breakthrough occurred in the mid 1980' s when Tang and co-workers developed an OPO based on crystalline urea, pumped in the UV range giving a visible output (W.R. Donaldson and C.L. Tang. Appl. Phys. Lett., vol 44, p25, 1984). This was superseded by Byer and co-workers who demonstrated the use of beta barium borate (BBO) as a non-linear crystal (Y.X. Fan, R.C. Eckardt. R.L. Byer, J. Nolting and R. Wallenstein. Appl. Phys. Lett, vol 53, p2014, 1988). Having better long term damage properties than urea, the discovery of BBO acted as a catalyst for future work. Most recently Chen and co-workers succeeded in using the related crystal lithium triborate (LBO) as a non-linear material (C. Chen, Y.Wu, A. Jiang, B. Wu, G. You, R. Li and S. Lin. J. Opt. Soc. Amer., vol B6, p616, 1989). Energy entering Into or emanating from a signal, idler or pump cavity, will have a particular characteristic frequency and therefore an associated wavelength. For the purposes of this specification the term signal, idler or pump wavelength will be used to describe these energies.

Experiments on CW OPOs began in the late I960's with Smith using crystals of Ba∑Na Nbs Oιs(BANANA) (R.G. Smith, J.E. Geusic, H. Levinstein, J.J. Rubin, S. Singh, and L.G. Van Uitert, Appl. Phys. Lett vol 12, p308, 1968) and Byer using lithium niobate (R.L. Byer, A. Kovrigin, and J.F. Young, Appl. Phys. Lett, vol 15, pi36, 1969). Both Smith and Byer succeeded in obtaining CW output. During the 1980' s, attention switched to the newly developed crystals such as magnesium oxide doped, lithium niobate or potasium titanylphosphate (KTP) and their implementation within various novel geometries, such as Byer's diode-pumped monolithic ring laser. Recently the use of LBO as the non-linear crystal in a CW OPO (F. Colville, A. Henderson, M. Padgett, J. Zhang and M. Dunn. Optics Letters, vol. 18 pp 205, 1993) has been demonstrated.

Within the context of a CW system, a critical design parameter is the pump power required to reach threshold. This is proportional to the product of the losses at signal, idler and pump wavelengths. For pulsed operation it is often sufficient for the OPO cavity to be resonant at either the signal or the idler wavelength (i.e. low loss for signal or idler). Such devices are described as singly-resonant oscillators (SRO). However, to reduce the threshold to a level compatible with available CW pump powers, it is usually necessary to make the OPO cavity resonant at two of the three possible fields i.e. signal and idler; or signal and pump; or idler and pump, fields. These devices are called doubly-resonant oscillators (DRO). A low threshold design is particularly relevant when using a diode-pumped, all solid-state laser as the pump source.

In DROs the output wavelengths of signal and idler fields are over-constrained. This means that in addition to satisfying the requirement for energy conservation ω^ + ω_s = ω_p), (where ω^ is the idler frequency, where ω_s is the signal frequency, and ω_p is the pump frequency) the lowest overall threshold is attained when the signal and idler frequencies match those of a cavity mode and also satisfy the phase-matching condition (i.e. k^ + k_s = k_p) (where

, k_s is 2ιr/λ_s and k_p 1s 2τr/λ_p). When using the same cavity to resonate the signal and idler fields, it is difficult to satisfy all these conditions simultaneously. Lowest loss is achieved as a compromise between detuning of the signal and idler frequencies away from the cavity modes and the phase-matched condition. As the cavity length or the pump frequency is scanned, the output of the OPO hops from one signal and idler mode pair to the next. Many such mode hops occur within the free spectral range (FSR) of the OPO cavity and the exact behaviour depends critically on the mismatch between the FSRs associated with the signal and idler fields. This results in the output frequency of a conventional DRO being very difficult to tune. To date all smoothly tunable OPOs have been based on an SRO geometries.

International Patent Application WO-A1-9303523 (LASERSIGHT INC.) describes -a multi-wavelength solid state laser using frequency conversion techniques.

As discussed above, when using a fixed frequency pump source and a single cavity to resonate both the signal and idler fields, continuous tuning over an appreciable range of the OPO is not possible. This is because the signal and idler frequencies are required to tune in opposite directions to maintain energy conservation (ω^ + ω_s = ω_p). A single cavity cannot be used to tune continuously the OPO in this way so as to maintain both the signal and idler fields on resonance. To date these constraints have been a limiting factor on the development of CW OPOs. The present invention arose from a consideration of the problem of obtaining smooth tuning and a low threshold within a DRO geometry.

According to the present invention there is provided: a cavity oscillator having at least two resonant cavities which have at least some volume in common, a non-linear material positioned such that at least a portion of the non-linear material is within the common volume; the oscillator being such that simultaneous generation of a signal field and an idler field is achieved by virtue of an input from a pump field. For applications within doubly resonant OPOs a multiple cavity oscillator (MCO) arrangement allows smooth and continuous tuning while maintaining both the signal and idler fields at resonance. This reduces the pump power required to reach threshold compared to a singly resonant cavity geometry. Control of the wavelength of one of the fields can be achieved independently of a second, or further, field. Simultaneous control of the wavelength of the, or each, field may be achieved independently of its, or their, polarisation. It is understood that by careful selection of the cavity parameters, as described below, that independent modulation of wavelengths of more than two fields is also achievable. The above fields termed "signal" and "idler^"" may be any set of fields at different wavelengths and/or polarisations to one another.

The resonator may form part of an optical parametric oscillator (OPO). When employed in an OPO the longitudinal mode frequencies of the signal and idler fields can be arranged to be varied independently of one another.

Preferably the signal and idler fields are coupled into different sections of the resonator cavity. A dichroic optical element which is dimensioned and arranged to separate the signal and idler sections of the resonator. A polarisation sensitive optical element can be used to separate the signal and idler sections of the or each resonator. The pump field may be resonantly enhanced within one of the existing sections of the resonator or within an additional section. The resonator may be arranged such that the Optical Parametric Oscillator is operated continuously. Mismatch of the free spectral ranges for the signal and idler sections of the resonator may be used to govern the tuning behaviour. Such a resonator may have a tuning element incorporated into either Its signal or Idler section.

The resonator can be arranged such that the Optical

Parametric Oscillator is operated in a pulse or Q-switched manner. Alternatively the resonator can be arranged such that the Optical Parametric Oscillator is operated in a mode locked fashion.

The resonator may be used in projection TV systems or instrumentation in which more than one colour light signal is required. Some of these are described below.

An OPO based instrument is a superior technology in many application areas currently using dye lasers. Compared to dye laser based systems, the OPO technology offers significantly higher 'wall plug' efficiences, ease of use in being an all-solid-state system, and, perhaps most importantly, a much greater tuning range. Compared to systems based on frequency-doubled Tirsapphire lasers, which use an additional frequency mixing stage to obtain full spectral coverage, the OPO technology covers an uninterrupted tuning range. Further examples of potential applications include:

Isotope Separation: Isotope Separation has applications in the nuclear and medical industries. The wide tunability offered by the OPOs will have particular application in the medical field where the variety of isotopes to be separated and/or detected requires a range of wavelengths.

Photo-Dynamic Therapy: Photo-Dynamic Therapy requires a tunable light source to activate the or each photosensitive drug(s) which is/are often used to combat cancer. After migrating to the target cells the drug is light triggered into the release of a free-radical that destroys the cell.

Spectroscopy: Many areas of Physics and Chemistry use tunable dye lasers to investigate interactions between atoms and molecules. A rugged OPO based instrument would be easier to use and offer a much wider tuning range.

Environmental Monitoring/Trace Element Detection: A narrow (sub Doppler) linewldth, widely tunable source would be invaluable for the detection of a wide range of trace elements and molecules.

Colour Separation: Colour Separation is currently the largest application for ion laser/dye laser systems. The printing industry uses tunable laser sources for matching the available pigments to an artist's original. OPOs offer ease of use, wider tuning range within a single device and a more compact package.

As briefly mentioned above, the dual cavity resonator may be incorporated into video display or projection equipment, used in telecommunication applications or indeed any equipment which requires a light source of a particular colour. For example the resonator may be used with a relatively cheap and rugged, widely available, laser sources to provide a light signal of any colour. The wavelength of the input laser light must be greater than that/those of the output colour(s). Accordingly economies of scale can be used to manufacture such light sources more efficiently and cheaply and they have the added advantage that they may be tuned to a different wavelength (colour).

Other features, advantages and benefits are described below. The resonator may have means for controlling the group velocity dispersion of the or each output signal. This control of the signal may be achieved in the signal and/or idler sections of the resonator.

Continuous tunability can be achieved by coupling the signal and idler fields into different cavities, thereby allowing their mode frequencies to be controlled independently. These devices are referred to as multiple-cavity oscillators (MCO).

Two different phase matching types exist. For type II phase matching (where the signal and idler fields have orthogonal polarisations), a polarising beam splitter can be used to couple the signal and idler fields into different cavities. However, in the geometry required for type I phase matching, (where the signal and idler fields share the same polarisation), wavelength selective optics are used to split the signal and idler cavities. This can be achieved by using a dichroic beam splitter or alternatively a dichroic mirror. The latter enables a linear cavity to be employed.

When operating near degeneracy in type I geometry, splitting the signal and idler cavities is not possible because the signal and idler have the same polarisation and wavelength characteristics. Therefore, an alternative method is employed.

One such alternative method involves applying a technique developed for ultra-short pulse lasers in which prism combinations compensate for dispersion. This technique may be used within an OPO to control the mismatch between the FSR of the signal and idler cavities. However, other techniques may also be employed.

Embodiments of the invention will now be described, by way of examples only, and with reference to the Figures, in which:- Figure 1 shows a diagramatical view of a dual cavity resonator which uses a wavelength dependent optic to separate signal and idler fields;

Figure 2 shows a diagramatical view of a dual cavity resonator which uses a polarisation separation optic to separate signal and idler fields;

- Figure 3 shows a diagramatical view of one embodiment of the present invention which employs a double cavity oscillator (DCO);

Figure 4 shows a diagramatical view of an alternative embodiment of the present invention;

Figure 5 shows diagrammatical ly an experimental arrangement; and

Figure 6 shows a graph of a stable frequency output from an OPO. An example of dual-cavity oscillators (DCO) (10) incorporating a pair of dichroic mirrors (11a and b), is shown in

Figure 1. A similar arrangement, in Figure 2, has a dichroic or polarisation sensitive beam splitter (12).

A first cavity (14) defined between mirrors (9) and (lib) is controlled to give fine adjustment of an output frequency. The first cavity (14) is referred to as the master cavity. A second cavity -(13), defined between mirrors (9) and (11a), referred to as the slave cavity, is locked to a local maximum in the OPO output power, thus defining a particular signal and idler mode pair and hence a single frequency output.

Two main modes of operation for the dual-cavity oscillator are possible: Firstly, by incorporating a deliberate mismatch between FSRs of the master (14) and slave (13) cavities, only one signal and idler mode pair is resonant within the phase matching bandwidth. Through a combination of master (14) cavity tuning and phase matching, smooth tuning is obtained. Secondly, by exactly matching the FSRs, all the signal and idler mode pairs within the phase matching bandwidth can be brought to resonance simultaneously. A single etalon (not shown) may be placed within one of the cavities, selects any one of the mode pairs. In this case, tuning is not continuous. By using the etalon (15) to select different mode pairs, the output frequency is tunable in increments equal to the Free Spectral Range (FSR). These techniques can also be adapted to include resonant pump enhancement. This increases the effective pump field by resonating the pump frequency within an OPO crystal (16) and it can be accomplished by making either the signal or the idler cavity resonant at the pump wavelength. This reduces the pump power required to reach threshold and/or the resonant requirements on the signal and idler fields. It follows that if the pump resonance is sufficiently high then the resonance requirement on the signal or idler field can be removed completely. The resultant device is still described as a doubly resonant OPO but one in which the resonant fields are the pump field and one of the signal or idler fields. Alternatively, an additional section of the cavity can be added specifically for resonating the pump field in a triple-cavity oscillator (TCO) (not shown). One practical implementation of a DCO is described below with reference to Figure 3. A pump output wavelength of 364 nm was obtained from a single, longitudinal mode argon-ion laser (not shown). The near room temperature, noncritlcal phase-matching in a Lithium Triborate (LBO) crystal (20) gives signal and idler wavelengths of 500nm and 1340nm respectively. Coarse tuning is achieved by changing the temperature of the LBO crystal (20) which alters the phase-matching condition. Fine tuning is accomplished by adjusting the length (LI) of the master cavity (14). The length (LI) is defined between two mirrors (Ml and M2). A dichroic beam splitter (21) is used to split the two beams used is used to reflect light (at around 500nm) to a third mirror M3. The path length L2 between mirrors Ml and M3 defines the characteristic length of the second resonant cavity or slave cavity (13). A feedback control system comprises a detector (23), PID contrβl (24) and a voltage source (not shown) connected to a piezoelectric drive (26). As the change in frequency of the light reflected off the beam splitter (21) is proportional, amongst other things, to the change in length L2 of the cavity (13), tuning may be achieved by displacing the mirror M3 in the direction of double headed arrow A-A. Similarly tuning of the other light source which is in the range of 1.3 - 1.4μm can be achieved by displacing mirror M2 in the direction of double headed arrows B-B with respect to Ml, thereby varying the length LI of the first resonant cavity (14). Frequencies fl and f2 of the light sources are constrained so as to conserve energy. That is the sum of the photon energies of each output signal (E=hf) is equal to the photon energy of the input source. Alternatively said the signal frequency plus the idler frequency is equal to the pump frequency.

When mirror M2 is displaced so as to effect tuning, the feedback control (24) displaces mirror M3 so as to maintain both signal and idler cavities at resonance. Smooth and continuous tuning of the frequency of both signal and idler waves is thereby achieved.

A pulsed system is of a similar geometry to a CW system, where the use of a pulsed pump source results in a pulsed output from the OPO. Such pump sources will frequently be Q-switched lasers and therefore have sufficient peak power to enable an OPO to be configured in an SRO geometry. However, the extra mode selection offered by a DRO geometry gives a more controllable output. The possible modes of operation are the same as those described above for the CW implementations.

A method of obtaining a sequence of ultrashort laser pulses is mode-locking. In mode-locking a laser is forced to oscillate simultaneously on many longitudinal modes. Even for CW pumping, by ensuring that the modes maintain a fixed phase relationship, the output of the laser takes the form of a sequence of discrete ultrashort pulses. The time between pulses is the round-trip time of a pulse within the laser cavity and duration of the pulse depends on the number of modes which are locked together (increasing the number of locked modes shortens the pulse).

As first demonstrated by Tang and co-workers in the late 1980's it is also possible to obtain an ultrashort pulse, mode-locked output from an OPO (D.C. Edelstein, E.S. Wachman, CL. Tang, Appl. Phys, Lett, vol 54, pl728, 1989). To date, these devices have all been based on SRO geometries, where only one of the signal or idler frequencies is resonant. As discussed above, the lengths (LI and L2) of the two cavities (13 and 14) of a DCO geometry are adjusted so that the frequencies (fl and f2) of the two oscillating cavity modes satisfy the condition for energy conservation. A DRO mode-locked device has an additional constraint called walk-away. Walk-away occurs within the OPO crystal (20) where the mismatch in group velocities between the pump, signal and idler results in a reduction in the overlap between the pulses as they propagate through the length of the crystal. To minimise the effects of walk-away, the crystal orientation can be selected so that the differences in the group velocities between the pulses can be reduced. Within mode-locked laser systems, group velocity dispersion (GVD) causes pulse broadening. GVD is a wavelength dependent phenomenon. Group velocity dispersion results in the different spectral components of a single laser pulse exhibiting different round-trip times within the cavity. GVD compensation is frequently included within a mode-locked laser cavity and is used to optimise the laser operation which leads to a reduction in pulse duration.

Within a mode-locked, dual-cavity OPO, GVD and/or walk-away for both the signal and idler pulses, can be controlled by including one or more compensating prisms 30a, b, c, d, e, f, g, h or other arrangement in both sections of the cavity shown in Figure 4. Care is required over the adjustment of relative cavity lengths LI' and L2¹ and GVD compensation so that in addition to the longitudinal mode frequencies satisfying the requirement for energy conservation, the round-trip times of the signal and idler pulses are equal. The latter ensures that the pulses can be appropriately synchronised within the non-linear crystal (20).

As with the CW systems, the pump laser (not shown) may be resonantly enhanced either within the signal, idler or an external cavity. Increased pump resonance may reduce or remove the need for a resonance signal or idler wave. For enhanced operation, the pump source may itself be synchronously mode-locked to the OPO. All previous pump sources for continuous-wave optical parametric oscillators (CW OPOs) have been confined to the spectral region 488 - 532nm. A detailed description of UV-pumped CW OPO with reference to Figures 5 and 6 appears below. Pumped at 364nm by a single-frequency argon-ion laser (not shown) and using LBO as the non-linear crystal (33), a threshold of 115mW is demonstrated together with temperature tuning from 502 - 494nm (signal) and 1.32 - 1.38μm (idler).

A single lens (29) mode-matches pump signal (31) into an OPO cavity (32). An LBO crystal (33), measuring 3 x 3 x 20mm along x, y, z axes respectively, is cut θ = 0°, φ = 90° for type II non-critical phase-matching (NCPM), and has a triple-AR coating applied to both faces. The coating has >99.7% transmitting at the OPO wavelengths and >97% transmitting at the pump. The OPO cavity is formed by a 15mm radius of curvature (r.c.) mirror (34) and two 300m r.c. (35a and 35b). The crystal (33) is in an oven (36) to allow temperature tuning. The OPO employs a DCO linear, standing-wave cavity with waist sizes for a signal and idler fields of 32 and 51μm respectively. The OPO mirrors (34 and 35) are 55% reflecting at 364nm and >99.7% reflecting at both 502nm and 1.32μm. A polarising beam splitter (37) is used to decouple the signal and idler cavities. Taking d_eff =-l J 5pm/V, predicts a threshold pump power of around 50mW. The experimental threshold is approximately 350mW.

Such CW OPOs, which operate single-frequency with widely spaced signal and idler wavelengths, are attractive sources for optical frequency standards for which narrow linewidth radiation is required over a comb of frequencies covering the entire optical region. The type II geometry also allows polarization decoupling of the signal and idler cavities. Single frequency operation has been demonstrated in the above described device (as illustrated by the graph in Figure 6). Further, by applying the feedback control system, as shown in Figure 3, to mirror 35b of Figure 5, smooth and continuous tuning of the single frequency outputs at both signal and idler wavelengths has been obtained. It will be appreciated that the above embodiments are described by way of example only and modifications to them may be made without departing from the scope of the invention.

Claims

1. A cavity oscillator having at least two resonant cavities which have at least some volume in common, a non-linear material positioned such that at least a portion of the non-linear material is within the common volume; the oscillator being such that simultaneous generation of a signal field and an idler field is achieved by virtue of an input from a pump field.

2. A cavity oscillator according to Claim 1 having at least two resonant cavities arranged such that there is simultaneous resonance of the pump field and the signal field.

3. A cavity oscillator according to Claim 1 having at least two resonant cavities arranged such that there is simultaneous resonance of the pump field and the idler field.

4. A cavity oscillator according to Claim 1 having at least two resonant cavities arranged such that there is simultaneous resonance of the signal field and the idler field.

5. A cavity oscillator according to any of Claims 1 to 4 wherein tuning is achieved by varying a characteristic of one or both of the cavities independently of the other cavity so as to tune their respective frequencies.

6. A cavity oscillator according to Claim 5 wherein one of the resonant fields is taken from the group of the signal field, idler field and pump field.

7. A cavity oscillator according to any preceding claim wherein the resonant fields have different polarisations.

8. A cavity oscillator according to any preceding claims wherein the resonant fields have different wavelengths.

9. An optical parametric oscillator including the cavity oscillator according to any preceding claim. 19. An optical parametric oscillator according Claim 9 wherein the longitudinal mode frequencies of the at least two resonant cavities are arranged to be varied independently of one another. 11. An optical parametric oscillator according to Claim 9 or 10 wherein two or more of the signal, idler and pump fields, are coupled into different regions of the resonator cavity. 12. An optical parametric oscillator according to Claim 11 wherein coupling is achieved by use of at least one dichroic optical element which is adapted to separate two of the signal, idler and pump sections of the resonator cavity.

13. An optical parametric oscillator according to Claim 11 wherein coupling is achieved by use of at least one polarisation sensitive element which is adapted to separate two of the signal, idler and pump sections of the resonator cavity.