WO2005081430A2

WO2005081430A2 - Apparatus and method for the delivery of high-energy ultra-short optical pulses from a fibre amplifier

Info

Publication number: WO2005081430A2
Application number: PCT/IB2005/000150
Authority: WO
Inventors: John Clowes; Anatoly Grudinin; Ian Godfrey
Original assignee: Fianium Ltd
Priority date: 2004-01-24
Filing date: 2005-01-21
Publication date: 2005-09-01
Also published as: WO2005081430A3; GB0401571D0

Abstract

The present invention is directed at providing short, high-energy optical pulses of high spectral and temporal quality at both a signal and frequency-doubled wavelength. One embodiment of the present invention comprises a short-pulse master-oscillator (MO), a pulse conditioner (PC), a parabolic amplifier, a compressor and a frequency converter. The implementation of parabolic amplification and pulse conditioning of the seed pulses form a master oscillator, which provides high-quality compressed pulses at very high energies, comparable to those generated by femtosecond solid-state systems.

Description

APPARATUS AND METHOD FOR THE DELIVERY OF HIGH-ENERGY ULTRA-SHORT OPTICAL PULSES FROM A FIBRE AMPLIFIER

CROSS REFERENCE TO RELATED APPLICATIONS The present application claims benefit, under 35 U.S.C. § 119(a) through (d) and (f), of U.K. Application Serial No. 0401571.5, filed January 24, 2004, which is expressly incorporated fully herein by reference.

FIELD OF THE INVENTION The present invention relates generally to the field of laser technology, and more specifically, to apparatus and methods for generating high-energy femtosecond ⁽ pulses from an optical system comprising an oscillator, pulse-conditioner, a parabolic amplifier, a compressor and a frequency converter.

DESCRIPTION OF THE RELATED ART Ultra-fast high-power laser sources, generating pulses of sub-picosecond duration are becoming increasingly important in application areas ranging from micro-machining to medical and bio-detection. Until now, the intensity required for driving highly nonlinear processes could only be provided by Ti:sapphire or similar bulk lasers. This has been particularly true for high-energy pulsed laser systems in which Ti:sapphire lasers are traditionally used as injection seed sources for regenerative amplifiers (U.S. Patent No. 5,530,582). While Ti:sapphire lasers may be useful, they may have significant undesirable features relating to high repetition rate due to relatively short cavity lengths, high- cost, large footprint, high power consumption, and poor reliability. Ti:sapphire modelocked lasers are referred to as "laser pumped laser" devices in that they require a large-frame Argon Ion laser or frequency doubled Nd.YAG laser or similar laser to pump the T sapphire material. Precision alignment must be maintained between the pump beam and the gain volume in the Ti:sapphire material to achieve stable mode- locking. This places additional restrictions on both the mechanical and thermal stability of the environment in which they can be operated. Fibre lasers make an attractive alternative for many applications in ultra-fast optics that currently rely on Ti.sapphire lasers. The undesirable features of bulk lasers such as the Ti:sapphire system can be eliminated using a fibre laser system which is inherently compact, can be designed to have relatively low repetition rates, does not require water-cooling and has a small footprint. Techniques for the generation of ultra-short optical pulses from fibre-lasers have been known for a number of years. Unfortunately, there are no diode-pumped fiber laser sources with emission wavelengths in the near IR where most of the broad gain bandwidth materials like Ti:Sapphire operate. Such wavelengths can be produced by frequency-doubling of the pulses from a MOPA (Master Oscillator Power Amplifier), short-pulse Erbium-doped fibre laser (EDFL) (Nelson et al., "Efficient frequency-doubling of a femtosecond fiber laser", OPTICS LETTERS, Vol. 21, pp 1759 (1996)). Minimum pulse energies on the order of 30 picojoules are required for injection sources in regenerative amplifiers in high-energy pulse systems. Pulse energies achievable from frequency-doubled, MOPA/compressor fibre systems of the order of a few hundred picojoules make them suitable for injection seeding. However, with higher energy seed pulses, the complexity, size and cost of the regenerative amplifier maybe significantly reduced. A fibre laser source, with pulse- energy greater than a few nanojoules may be desirable and may compete with Ti:sapphire and solid state lasers on available pulse energy, size, low cost (both material and maintenance) and reliability. The achievable pulse energy from MOPA fibre lasers is limited by the output energy that can be extracted from the Erbium Doped Fibre Amplifier (EDFA) before nonlinear effects in the fibre cause distortion of the pulse. The pulse-energy threshold where nonlinear effects become problematic, can be extended through the use of multi-mode fibre (Hofer et al„ "High-power 100 fs pulse generation by frequency doubling of an erbium-ytterbium-fiber master oscillator power amplifier", OPTICS LETTERS, V 23, PP1840, (1998)) or large mode-area amplifier fibres. Using multi- mode fibre amplifiers, Hofer et al., reported pulse energies of greater than 2 nanojoules at 780 nm. However, excitation of the fundamental mode within the multi-mode fibre amplifier, and maintenance of polarization, requires accurate and reliable launching optics, which complicate the system design and adversely affect reliability. Using large-mode area doped fibre in the amplifier the available pulse energy can be slightly increased. Pulse energies on the order of 1 nanojoule can be expected from such a configuration. As reported by Kurokawa et al. in Wavelength-dependent Amplification Characteristics of Femtosecond Erbium-Doped Optical Fiber Amplifiers, 58 APPLIED PHYSICS LETTERS, 2871 (1999) and Richardson et al. in Amplification of Femtosecond Pulses in a Passive All-Fiber Soliton Source, 17 OPTICS LETTERS 1596 (1992), pulse amplification and compression can be achieved in a negative dispersion, Erbium- doped fibre amplifier. In such an amplifier, referred to as a Soliton Raman Compressor (SRC), sub-lOO-femtosecond, Raman shifted solitons can be produced. Furthermore, the resulting pulses have a degree of wavelength tunability through control of the amount of Raman self-frequency shift by varying the amplifier pump power. The soliton Raman compressor has been adopted by Fermann et al. (U.S. Patent No. 6,014,249) to provide 100-femtosecond optical pulses at 780 nm through frequency-doubling of a modelocked erbium-doped fibre laser. Using this approach, the available pulse power at 780 nm is limited to a few hundred picojoules, dependent on the efficiency of the frequency-doubling process and the mode-area of the doped fibre in the amplifier. Furthermore, the non-linear crystal (used for frequency- doubling), using this approach, has a large role in improving the quality of the optical pulse at the frequency-doubled wavelength. Due to the high-nonlinearity of the SRC amplifier-compressor, the pulse quality (both spectral and temporal) at the signal wavelength (prior to frequency doubling) is compromised. Various approaches have been used to avoid nonlinear effects in the amplifier and further extend the available pulse-energy from fibre lasers. One approach, for example, has used chirped pulse amplification (CPA), where pulses are stretched in time (reducing the peak-power of the pulse) prior to amplification. The amplified pulses are subsequently compressed in a bulk-optic compressor where nonlinear effects are not present. A variation of CPA uses amplifiers with normal (positive) dispersion to simultaneously amplify and stretch the pulses. Due to the combination of gain, nonlinearity and normal dispersion in this type of amplifier, the pulses acquire a parabolic shape in time and linear chiφ (Kruglov et al., Self-Similar Propagation of High-Power Parabolic Pulses in Optical-Fiber Amplifiers, 25 OPTICS LETTERS 1753, (2000)). This system is referred to as a parabolic amplifier (PA). The linearity of the chiφ may be reflected in the compressibility of the pulse (Tamura and Nakazawa, Pulse Compression by Nonlinear Pulse Evolution with Reduced Optical Wave

- ^■b - Breaking in erbium-Doped Fiber Amplifiers, 21 OPTICS LETTERS 68 (1996)), and as such, the quality of a compressed pulse from a parabolic amplifier may be significantly better than that of a pulse compressed after chiφed pulse amplification. The linearity of the chiφed pulse, and hence the quality of the compressed pulse, may be dependent upon the pulse shape and amplitude prior to parabolic amplification as well as the length, dispersion and gain of the amplifier. The shape and amplitude of the output pulses from the master oscillator is crucial for the generation of a high-quality parabolic amplifier-compressor system. As such, the master oscillator requires a special design to include pulse conditioning that provides pulses that are short enough and of sufficient amplitude to seed the parabolic amplifier and produce output pulses of highly linear chiφ. This may be very difficult to achieve while maintaining a stable self-starting oscillator, particularly if relatively high powers and narrow pulse widths are required to seed the parabolic amplifier in an optimized way. The pulse width can be passively controlled using dispersion management including the use of dispersion shifted fibre (DSF), dispersion decreasing fibre (DDF) (See Chernikov et al., Soliton Pulse Compression in Dispersion Decreasing Fiber, 18 OPTICS LETTERS 476 (1993)) or comb-like dispersion profiled fibre (CDPF) (See Chernikov et al., Comb-like Dispersion Profiled Fiber for Soliton Pulse Train Compression, 19 OPTICS LETTERS 539 (1994)). Alternatively, pulse compression may be achieved using specially designed photonic crystal fibres (PCF), also referred to as photonic bandgap fibres (PBF). A PCF, with a low-nonlinearity, and high anomalous (negative) dispersion can be used to build an in-fibre compressor that is capable of handling optical pulses with extremely high peak-powers (Matos et al., All Fiber Chirped Pulse Amplification using Highly-Dispersive Air-Core Photonic Bandgap Optical Fiber", 11 OPTICS EXPRESS 2832 (2003)). The in-fibre compressor presents many benefits over the bulk-optic design for example, no alignment optics are required and the PCF compressor is spliced to the end of the parabolic amplifier fibre. The achievable power levels of femtosecond pulses, compressed after chiφed- pulse and parabolic amplification is limited by the available pump power from single- mode pump laser-diodes, typically 500 to 600 mW. Using approximately 30% fibre efficiency in typical erbium-doped fibres, using two pump diodes, a maximum average power of 350 to 400 mW is expected at 1560 nm. Assuming a compression and frequency doubling efficiency of the order of 50%, a maximum of 180 to 200 mW average power at 780 nm is expected. Spectral combining of pumps, for example 980 and 1480 nm can result in increased output powers. The amount of available power from a core-pumped, rare-earth doped parabolic fibre amplifier can be greatly increased through the use of fibre lasers as pump sources where pump powers of several watts can be provided to the amplifier. Erbium-doped fibre amplifiers for example, can be pumped by radiation at 980 nm or by 1480 nm to provide both high gain efficiency and low ESA (excited state absoφtion). High-power fibre lasers operating around 980 nm may be fabricated from

Ytterbium doped fibre, core pumped by a Neodymium fibre laser operating at a wavelength around 920 nm. Alternatively, a Raman fibre laser may be made to operate at a wavelength close to 980 nm, through Raman conversion of radiation around a wavelength of 920 nm, possibly using a 920 nm fibre laser as the pump source for the Raman laser. Similarly, a Raman fibre laser, operating around 1480 nm may be fabricated by Raman conversion of pump radiation at or around 1060 nm. A fibre laser operating around 1060 nm may be used as a pump source for such a Raman fibre laser system. Typically, a 980 nm or 1480 nm fibre laser, fabricated using one of the above-mentioned methods, is capable of providing in excess of 3W, a vast increase in the pump power available from conventional single-mode pump sources. Alternatively, clad pumping has been utilized to deliver high-power pump light from multimode diode modules into the core of fibre amplifiers (U.S.Patent No. 4,829,529). Using multimode laser diodes and laser diode arrays, several watts of optical pump power can be delivered to the amplifier fibre. A greater number of double-clad fibres, with varieties of rare-earth dopants are becoming commercially available and the pump power achievable from laser diode arrays and diode bars is rapidly increasing, bringing the price per watt down considerable. Core-pumping, however, may provide additional flexibility. The doped-fibres are more readily available and easier to fabricate with greater control and flexibility in parameters such as core size and numerical aperture. Furthermore, for operation at 1550 nm, core pumped, Er3+ doped fibre is more readily available and easier to fabricate than double-clad ErYb+ fibre. Polarisation may be a major issue when using bulk Grating compressors and quasi-phase-matched nonlinear materials such as periodically-poled Lithium Niobate (PPLN) for frequency doubling. Both compressor and frequency-doubling stages require incident light of linear polarisation and specific orientation. Polarisation issues can be minimised by using polarisation maintaining (PM) fibre throughout the amplifier or, in non-PM amplifiers, by using Faraday rotating mirrors (FRM's), as disclosed by Duling et al. in U.S. Patent No. 5,303,314, issued April 12, 1994.

SUMMARY OF THE INVENTION The present invention relates generally to the field of laser technology, and more specifically, to apparatus and methods for generating high-energy femtosecond pulses from an optical system comprising an oscillator, pulse-conditioner, a parabolic amplifier, a compressor and a frequency converter. An embodiment of the present invention relates to apparatus and methods for the delivery of high-energy ultra-short optical pulses comprising an optical source that generates optical signal pulses; a pulse conditioner that receives optical signal pulses from the optical source and conditions the amplitude, phase and temporal width of the optical signal pulse; a nonlinear parabolic fibre amplifier having a positive dispersion at the wavelength of the signal pulses, wherein the amplifier receives the optical signal pulses from the pulse conditioner, and wherein the amplifier amplifies and chiφs the optical signal pulses producing high-power, linearly chiφed, optical signal pulses; a pulse-compressor that receives optical signal pulses from the nonlinear parabolic fibre amplifier and disposed to compress the optical signal pulses; and a frequency converter that receives optical signal pulses from the pulse converter and outputs high-power optical pulses. The apparatus may further comprise a polarisation controller positioned between the optical source and the parabolic fibre amplifier. This polarisation controller may adjust the state of polarisation of the optical signal pulses received by the parabolic fibre amplifier. The optical source may further comprise a fibre oscillator or a rare earth doped fibre amplifier. The parabolic fibre amplifier may be doped with Er3+, Yb+, ErYb+, Nd, Pr, Tm, or Ho. The parabolic fibre amplifier may comprise a double-clad fibre pumped by a broad area laser comprising one or more multimode laser diodes. The double-clad fibre may also be doped with Yb+, ErYb+, Nd, Pr, Tm, or Ho. The parabolic fibre amplifier may be a Raman fibre amplifier that is be pumped by more than one pump laser operating at different wavelengths such that the Raman gain spectra at each pump laser wavelength, when summed together, forms a gain bandwidth greater than the bandwidth of the signal pulses being amplified. The pulse conditioner may comprise one or more optical amplifiers, a pulse compressor, and a dispersive optical element. The optical amplifiers of this embodiment may be a rare earth doped fibre amplifier, which may be doped with Er3+, Yb+, ErYb+, Nd, Pr, Tm, or Ho. The pulse-compressor may comprise a dispersion decreasing fibre (DDF), a comb-like dispersion profiled fibre (CDPF), or a dispersion shifted fibre (DSF). The dispersive optical element may comprise a chiφed fibre Bragg grating that conditions the phase of the optical signal pulses launched into the parabolic fibre amplifier. This embodiment may further comprise a first pump coupled to the parabolic fibre amplifier through a first wavelength division multiplexer coupler, which may provide pump light to the parabolic fibre amplifier. The apparatus may further comprise a second pump coupled to the parabolic fibre amplifier through a second Wavelength division multiplexer coupler, which may provide pump light to the parabolic fibre amplifier. These first and second pumps may be, for example, laser diodes or fibre lasers. Such fibre lasers may be, for example, Raman fibre lasers operating around 980 nm or 1480 nm. These fibre lasers may also be rare-earth doped fibre laser operating around 980 nm and pumped through a wavelength division multiplexer by a second rare-earth-doped fibre laser operating around 920 nm. The pulse-compressor of the present invention may comprise a soliton Raman compressor (SRC) that may be a rare earth doped fibre amplifier. The fibre amplifier may, fore example, have a core area greater than 50μm² and may be a doped fibre amplifier with a polarisation maintaining fibre. The pulse-compressor may be, for example, a bulk grating pair. The pulse-compressor may also be a photonic crystal fibre that has a low nonlinearity and high anomalous dispersion in comparison to standard optical fibres. The frequency converters of the present invention may be, for example, periodically poled nonlinear crystals that may be lithium niobate (PPLN) or potassium titanyl phosphate (KTP). The apparatus of the present invention may further comprise a polarisation maintaining isolator disposed between the pulse conditioner and the parabolic amplifier. This polarisation maintaining isolator may provide linearly polarised light to the parabolic amplifier. The apparatus may further include a polarisation maintaining isolator disposed between the parabolic amplifier and the pulse compressor. This polarisation maintaining isolator may provide linearly polarised light to the pulse compressor. The apparatus may also comprise a polarisation maintaining isolator disposed between the polarisation controller and the parabolic amplifier. The adjustment of the polarisation controller may provide linearly polarised light of variable amplitude at the output of the polarisation maintaining isolator at the input to the parabolic amplifier. The frequency converter of the present invention may comprise a plurality of frequency conversion stages cascaded in series. These frequency conversion stages may use the same nonlinear crystal. The plurality of frequency conversion stages may also produce frequency doubled light at the same wavelength. Each of the frequency stages may use signal light that was not frequency converted in the previous conversion stage. The nonlinear crystal may, for example, comprise periodically poled lithium niobate (PPLN). The apparatus according to the present invention may include a wavelength selective output coupler disposed between the frequency conversion stages. The wavelength selective output coupler may provide an optical output at the frequency-converted wavelength after each frequency conversion stage. This wavelength selective output coupler may be a bulk optical grating or a dichroic mirror. The nonlinear crystal may be a periodically poled KTP. The plurality of frequency conversion stages may be comprised of different nonlinear crystals. These nonlinear crystals may be selected to frequency double the frequency doubled light from the previous frequency conversion stage in the series. The different nonlinear crystals may be, for example, (PPLN), periodically pooled KTP or periodically pooled beta barium borate (BBO). Another embodiment of the present invention relates to a method for generating high-power optical pulses, comprising the steps of generating optical signal pulses; conditioning the pulse amplitude, phase, and temporal width of the optical signal pulses; amplifying and chiφing the optical signal pulses producing high-power, linearly chiφed, optical signal pulses; compressing the linearly chiφed optical signal pulses to provide high spectral quality, high temporal quality, and high- power compressed optical pulses; and frequency-converting the amplified, compressed optical signal pulses to produce high-power optical signal pulses at a different wavelength to the optical signal pulse. The method may also comprise the step of controlling the state of polarisation of the signal light and/or frequency- converting signal light that may not be converted efficiently in the previous frequency converter with cascading frequency converters. This last step may provide more than one train of output optical pulses at the frequency converted wavelength. These cascaded frequency conversion stages may be designed such that the output pulses at the frequency-converted wavelength of each frequency conversion stage are of approximately the same amplitude. The method may, for example, also include the step of frequency-converting the frequency converted optical pulses to provide optical pulses at more than 2 optical wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incoφorated in and constitute a part of the specification, schematically illustrate embodiments of the present invention. These drawings together with the general description given above and the detailed description of some of the embodiments of the present invention, which follow, serve a exemplars of embodiments of the present invention. Figure la illustrates a schematic of one embodiment of the present invention, utilising a parabolic amplifier and a soliton Raman compressor (SRC) to produce high-power optical pulses from a fibre amplifier. Figure lb illustrates a schematic of one possible configuration of a pulse conditioner, comprising an amplifier and pulse compressor. Figure lc illustrates an exemplary schematic of another possible configuration of a pulse conditioner, comprising an amplifier and a chiφed fibre Bragg grating. Figure 2a shows an example of the spectrum at the output of a fibre oscillator after pulse conditioning. Figure 2b shows an example of the autocorrelation trace at the output of a fibre oscillator after pulse conditioning. Figure 3 shows an example of the spectrum of the optical pulses at the signal wavelength at the output of the SRC. Figure 4 shows an example of the autocorrelation trace of the optical pulses at the signal wavelength at the output of the SRC. Figure 5 shows an example of the optical spectrum of the frequency-doubled optical pulses from an experimental demonstration of one embodiment of this invention. Figure 6 illustrates an exemplary schematic of the system described in an embodiment of the present invention, utilising a high-power parabolic amplifier and bulk-grating-pair compressor to produce high-power optical pulses from a fibre amplifier. Figure 7 illustrates an exemplary schematic of the system described in an exemplary embodiment of the present invention, utilising a high-power parabolic amplifier, co-pumped and counter-pumped by two pump lasers and bulk-grating-pair compressor. Figure 8 illustrates an exemplary schematic of the system described in an exemplary embodiment of the present invention, including a PCF compressor stage. Figure 9 illustrates an exemplary schematic of the system described in an exemplary embodiment of the present invention, including multiple frequency- conversion stages, cascaded in series to provide multiple outputs at the frequency- doubled wavelength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS By way of example only, certain embodiments of the present invention are described herein, without limitation. A first embodiment optimizes the system size and reliability while maintaining high temporal and spectral quality optical pulses. A system in the second embodiment provides a high pulse-energy and average power, at the frequency-doubled wavelength while maintaining a high temporal and spectral quality. A third embodiment provides maximum pulse-energy and control over the quality of the parabolic amplification process. A fourth embodiment of the present invention provides high-pulse energy while maintaining an all fibre, compact design of the femtosecond fibre laser. A fifth embodiment of the present invention provides more than one optical output at a frequency-converted wavelength, enabling a single optical source to seed multiple regenerative amplifiers. Figure 1 describes the configuration of a femtosecond pulse generating system according to one embodiment of the present invention. A fibre oscillator (master oscillator MO) 101 provides seed pulses at a signal wavelength. The MO can be, for example, a passively modelocked erbium-doped fibre laser operating at a signal wavelength around 1550 nm. The MO produces transform-limited pulses of duration of 300 to 500 femtosecond and at a repetition rate from 30 MHz to 100 MHz. The average output power for a 50 MHz repetition rate MO may be between 0.1 and 5 mW depending on the design of the MO and the output coupler used in the cavity design. Pulses from the MO pass through an optical isolator 102 for the signal light at 1550 nm and through a pulse conditioner 103. Figure lb illustrates one possible configuration of a pulse conditioner. The pulse conditioner 103 in this example comprises an amplifier fibre 118, pumped by a laser diode 116 via a wavelength division multiplexer (WDM) 117. Amplified pulses from the amplifier enter a pulse-compressor 119, which may comprise a length of dispersion decreasing fibre (DDF). Alternatively, instead of a DDF, a dispersion shifted fibre (DSF) or comb-like dispersion profiled fibre (CDPF) may be used as a pulse compressor. In a first embodiment of the present invention, a second isolator 104 at the signal wavelength may follow the pulse conditioner. The gain of the pulse- conditioner pre-amplifier may be varied to control and set the pulse amplitude at the input to the parabolic amplifier. The pulse-compressor may be designed to provide pulse widths of less than 350-femtoseconds at the input to the parabolic amplifier. The pulse conditioner may also comprise a fibre amplifier and a dispersive element such as, for example, a chiφed fibre Bragg grating (CFBG) to condition the phase of the pulses prior to parabolic amplification. Figure lc illustrates an example of such a pulse conditioner. The pulse conditioner 103 comprises an amplifier fibre 118 pumped by a laser diode 116 via a WDM 117. The amplified pulses pass to a chiφed fibre Bragg grating (CFBG) 121 via an optical circulator 120. The orientation and level of dispersion of the CFBG determines the magnitude and sign of the chiφ imparted onto the optical pulse. The pulse conditioner may be subsequently followed by a fibre polarisation controller 105, with which the state of polarisation of light launched into the parabolic amplifier fibre 106 may be adjusted. The parabolic amplifier fibre may be pumped by a laser pump 108 through a wavelength division multiplexer 107. In this embodiment of the invention, the parabolic amplifier may be, for example, a rare-earth doped fibre amplifier and the pump source to the parabolic amplifier may be a 980 nm laser diode emitting up to 100 mW of power. Alternatively, the parabolic amplifier may be, for example, a Raman fibre amplifier comprising positive dispersion fibre pumped by one or more pump lasers of wavelength such that the peak of the resulting Raman gain spectrum corresponds to the wavelength of the signal pulses. The parabolic amplifier in this exemplary embodiment of the present invention, may comprise a length of approximately 1.5m of erbium doped fibre amplifier with NA ~ 0.2 and normal (positive) dispersion. In this embodiment, the parabolic amplifier fibre may be non-polarisation maintaining and may be, for example, ER3+ doped. The parabolic amplifier fibre, however, may also be doped with ErYb+ or any rare earth doped fibre combined with an appropriate master oscillator. Furthermore, the parabolic amplifier fibre may be, for example, a polarisation maintaining fibre. As shown in Figure 1, the output of the parabolic amplifier may pass through a polarisation maintaining isolator 109, which may launch linearly polarised light into a second amplifier fibre 110 via an optical tap coupler 111, which may extract, for example, about 1% of the optical signal for monitoring puφoses. Using the polarisation controller 105 the state of polarisation through the parabolic amplifier may be adjusted such that most of the optical power may be launched into the transmitted eigenmode of the PM isolator. The top coupler 111 may monitor the pulse amplitude. The second amplifier fibre, in this embodiment, may also serve as the soliton Raman compressor (SRC). The SRC may be pumped via a wavelength division multiplexer (WDM) 113 by a laser diode 112, which, for example, may emit up to about 500 mW of optical power at 980 nm. The SRC amplifier fibre may also be Er3+ doped, with an NA of approximately 0.12 and with anomalous (negative) dispersion. The fibre may be designed to have a large mode-area for propagation of higher-energy Raman soliton-like pulses. The SRC fibre, for example, could equally be a double-clad amplifier fibre, pumped via the cladding by one or more multi-mode laser diodes or a laser diode bar. The output of the SRC may be terminated by a fibre cleave and light from the SRC may be launched to the frequency-doubler 114 through a set of confocal focusing optics 115. The frequency-doubler in this embodiment of the invention may be a length of periodically poled lithium niobate (PPLN), of poling period and length tailored to the wavelength and pulse duration of the input pulses to the frequency doubler. In experiments conducted with the system described in this first embodiment, after the SRC amplifier, up to 60 mW of average power at the signal wavelength (pulse energy ~ 1 nJ) was measured. The seed source in this system may produce stable, 350-femtoseconds, transform limited pulses at 1560 nm with an average power of approximately 0.5 mW. The pulse-conditioner may include a pre-amplifier stage, following the master source, to boost the average power to approximately 4mW (optimised for the parabolic amplification process). In another embodiment, a dispersion-managed fibre may be removed from the pulse conditioner in this system if the seed pulses from the master oscillator are of sufficient duration to seed the parabolic amplifier. Figure 2a shows the spectrum and Figure 2b, the autocorrelation trace for pulses after the pulse-conditioning stage. Figure 3 shows an example spectrum at the output of the SRC 301. Also shown in Figure 3 is the spectrum of the input pulse to the parabolic amplifier 302. The optical spectrum at the output of the soliton Raman compressor (SRC) shows the formation of a self-frequency shifted Raman soliton. In addition to the soliton, there may remain some non-soliton spectral components at the fundamental wavelength that were not compressed in the SRC. The level of self-frequency shift and the relative amount of non-soliton components is, for example, highly dependent on the drive currents in the parabolic amplifier and SRC pump diodes. A Raman soliton of approximately 90-femtosecond duration, for example, may be self-frequency shifted by approximately 50 nm to canter at 1612 nm. The pump powers to the parabolic amplifier and SRC may be adjusted and set to provide a very low non-soliton component, constituting less than 10% of the total power at the output. A feature of this system, for example, is little pump power is wasted in the SRC power amplifier on amplifying non-Soliton spectral components. Figure 4 shows the autocorrelation trace for a laser, with sub-100-femtosecond pulses and virtually no pedestal. Figure 4 further shows a high-quality optical pulse as produced by this embodiment of the present invention. By reducing the pump power to the parabolic amplifier and increasing the pump to the SRC, the quality of compression in the SRC may be reduced. Also, the level of non-soliton spectral components may be increased and significantly more pump power may be consumed. With this embodiment, for example, a PPLN of length 500 μm and of poling period 20.5 μm may be used to frequency double the optical pulses at approximately 1612 nm. The PPLN crystal may be heated to fine-tune the frequency-doubling wavelength and avoid photo-refractive damage in the PPLN. The length of the PPLN crystal may also be chosen such that its spectral acceptance bandwidth, for example, is comparable to that of the input pulse at the signal wavelength. With this embodiment, for example, transform limited, frequency-doubled pulses at a wavelength of 806 nm, of duration of less than 90-femtoseconds and of average power of 13 mW were produced, corresponding to a pulse-energy of approximately 250 pJ. Figure 5 shows the spectrum of such exemplary frequency- doubled pulse. A conversion efficiency of approximately 20% was achieved with this embodiment. Such results may be obtained using, for example, a relatively small core-area doped fibre in the SRC. For example, a fibre with core diameter of approximately 9μm may feasibly increasing the core diameter by a 50 to 60%. For a fixed pulse width, the energy of the Raman soliton-like pulse scales proportionally with the core area. An SRC could therefore be implemented to provide an increase in average power at the signal wavelength by a factor of up to around 2.5. By using a large mode-area SRC fibre, pulse energies of InJ may be possible at the frequency- doubled wavelength. A second exemplary embodiment of the present invention maximizes pulse- energy at the frequency-doubled wavelength. This embodiment allows high-energy fibre systems to be utilized as direct replacements of Tirsapphire seed sources in many regenerative amplifier systems. Figure 6 describes the configuration of a femtosecond pulse generating system according to this embodiment of the present invention. The design of the master-oscillator, pulse-conditioner and frequency-doubling systems of the second embodiment of the invention may be similar to those described in the first embodiment. However, the amplifier-compressor systems may differ. In this second embodiment, the output from the pulse-conditioned master oscillator passes through the second isolator 104 in the system and into the parabolic amplifier fibre 106 via a polarisation controller 105. In some embodiments, the second stage isolator may be a polarisation maintaining (PM) isolator and the relative positions of this isolator 104 and the polarisation controller may be 105 interchanged so that the PM isolator follows the polarisation controller. In this configuration, by altering the state of polarisation with the polarisation controller, the amplitude of the single polarisation pulses after the PM isolator may be varied and pre-set, providing further control over the amplitude of the input pulses to the parabolic amplifier. The parabolic amplifier fibre may be, for example, core-pumped in the co- propagation direction, by a high-power laser 608 via a wavelength division multiplexer (WDM) 607. The parabolic amplifier fibre may be, for example, a double-clad, rare-earth doped fibre, pumped by one or more multimode laser diodes or a diode bar configuration. In a second exemplary embodiment, the pump laser may be, for example, a high-power Raman fibre laser operating around 1480 nm, a high-power Raman fibre laser operating around 980 nm, or an Ytterbium-doped fibre laser operating around 980nm, each core-pumped by a Neodymium fibre laser operating around 920 nm. After the WDM, a pump power in excess of two Watts may be delivered to the normal dispersion, parabolic amplifier fibre. The highly-chhped output pulses from the parabolic amplifier fibre, collimated by optical lenses 609 may be compressed in a bulk grating pair (BGP) compressor 610 conFigured to provide the pulse-width required to seed the regenerative amplifier. The compressed pulses may be frequency-doubled in a PPLN crystal 114. As with the first embodiment, the poling period and thickness of the PPLN may be chosen to coincide with the peak signal wavelength and pulse width respectively. In this second embodiment of the invention, the compression does not result in self-frequency shift, therefore the peak signal wavelength of the compressed pulses should coincide with the peak wavelength of the seed pulses from the master oscillator. For example, based upon a 50% frequency doubling conversion efficiency, a compression efficiency of up to 80% and a parabolic amplifier with an absoφtion efficiency of approximately 30%, with 2 Watts of pump power into the parabolic amplifier, pulse-energies in excess of 4 nJ may be, for example, achievable at the frequency doubled wavelength. With further increased pump power, higher pulse- energies may be possible. In a third embodiment of the present invention the pump configuration of the parabolic fibre amplifier may be modified to allow for better control of the gain profile and for higher output powers from the amplifier. Figure 7 illustrates the design of a third embodiment of the invention. The output from the pulse-conditioned master oscillator may pass through the second isolator 104 in the system and into the parabolic amplifier fibre 106 via a polarisation controller 105. The parabolic amplifier may be core-pumped in both the co-propagating direction by a first pump laser 608 via a first WDM 607 and in the counter-propagating direction by a second pump laser 702 via a second WDM 701. In this third embodiment, the pump lasers may be, for example, s high-power 1480 nm fibre lasers, but these may also be high- power fibre lasers operating at 980 nm. Pump power in excess of 4 Watts may be delivered to the parabolic amplifier with this configuration. Furthermore, by adjusting the relative pump powers from the two pump lasers, the gain profile along the parabolic fibre amplifier may be controlled to provide the optimum chiφ characteristics of the output pulse from the amplifier. A fourth embodiment of the present invention comprises a passive in-fibre compressor rather than a bulk-optic compressor. Figure 8 illustrates an exemplary design of the fourth embodiment of this invention. The master oscillator-parabolic amplifier-compressor design may be identical to that shown in Figure 7. Optical pulses at the signal wavelength leaving the parabolic fibre amplifier 106, enter a length of photonic crystal fibre (PCF) 801 wherein compression of the linearly chiφed pulses occurs. The PCF may be designed to have a low non-linearity (such as an air-core PCF) but high anomalous (negative) dispersion. Due to the low non- linearity, this fibre may support higher energy optical pulses than conventional single- mode fibres. Compressed optical pulses from the PCF may be launched into the frequency converter 114 via the confocal focusing optics 115. With careful fabrication, the non-linearity, dispersion and effective mode-area of the PCF may be designed such that the soliton compression results in Raman self-frequency shift. The level of self-frequency shift can then be controlled via the pump power to the parabolic amplifier and by the length of PCF fibre used in the compressor. Such a design provides the additional advantage of being tuneable, over a very broad bandwidth if the PCF has the right design parameters. In the fifth embodiment of this present invention multiple frequency conversion stages may be included after the master oscillator-parabolic amplifier- compressor system. Figure 9 illustrates a design of the fifth embodiment of the present invention. The master oscillator-parabolic amplifier-compressor design is identical to that shown in Figure 7, described in the third embodiment of this invention. Optical pulses at the signal wavelength leaving the parabolic fibre amplifier 106 are compressed in the bulk-grating compressor 610 and then may be focused onto a first frequency conversion stage 114 using a set of confocal lenses 115. The output of the first frequency converter includes optical pulses at both the signal and frequency doubled wavelengths, the ratio of which may be dependent on the conversion efficiency of the nonlinear crystal. A bulk-optic wavelength selective output coupler 901, comprising, for example, an optical grating or dichroic mirror, may be placed after the first stage frequency converter and before the confocal focusing lenses 902 and nonlinear crystal 903 of a second stage frequency converter.

The output coupler directs light at the frequency-doubled wavelength to an output, subsequently directed to seed a regenerative amplifier. The non-frequency-converted signal pulses may be transmitted by the output coupler 901 to the second stage frequency converter. A second output coupler 904 may provide an additional output at the frequency-doubled wavelength from the second stage frequency converter 903.

A third output coupler 907 may provide an additional output at the frequency-doubled wavelength from the third stage frequency converter 906. Additional frequency conversion stages can be included, to the point where the frequency-doubled pulses have insufficient energy to be deemed useful. If each frequency conversion stage is optimised, then the frequency-doubled output pulses will be progressively reduced in energy after each stage since the signal input pulses become weaker after each conversion stage. Alternatively, the frequency conversion efficiency of each conversion stage may be tailored by selecting PPLN crystals of a certain thickness or poling period, such that all outputs have approximately equal pulse energies. It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. The disclosed embodiments are therefore considered in all respects to be illustrative examples and not restricted. The scope of the invention is indicated by the appended claims.

Claims

What is claimed is:

1. An apparatus for the delivery of high-energy ultra-short optical pulses comprising: an optical source that generates optical signal pulses; a pulse conditioner that receives optical signal pulses from said optical source and conditions at least one selected from the group consisting of amplitude, phase and temporal width of said optical signal pulse; a nonlinear parabolic fibre amplifier having a positive dispersion at the wavelength of said optical signal pulses, wherein said amplifier receives said optical signal pulses from said pulse conditioner, and wherein said amplifier amplifies and chiφs said optical signal pulses producing high-power, linearly chiφed, optical signal pulses; a pulse-compressor that receives optical signal pulses from said nonlinear parabolic fibre amplifier and compresses said optical signal pulses; and a frequency converter that receives optical signal pulses from said pulse converter and outputs high-power optical signal pulses.

2. The apparatus according to claim 1, further comprising a polarisation controller positioned between said optical source and said parabolic fibre amplifier, wherein said polarisation controller adjusts the state of polarisation of the optical signal pulses received by said parabolic fibre amplifier.

3. The apparatus according to claim 1, wherein said optical source comprises a fibre oscillator.

4. The apparatus according to claim 1, wherein said parabolic fibre amplifier comprises a rare earth doped fibre amplifier.

5. The apparatus according to claim 4, wherein said parabolic fibre amplifier is doped with one selected from the group consisting of: Er3+, Yb+, ErYb+, Nd, Pr, Tm, and Ho.

6. The apparatus according to claim 1, wherein said parabolic fibre amplifier comprises a double-clad fibre pumped by a broad area laser comprising one or more multimode laser diodes.

7. The apparatus according to claim 6, wherein said double-clad fibre is doped with one selected from the group consisting of: Yb+, ErYb+, Nd, Pr, Tm, and Ho.

8. The apparatus according to claim 1, wherein said parabolic fibre amplifier comprises a Raman fibre amplifier.

9. The apparatus according to claim 8, wherein said Raman fibre amplifier is pumped by more than one pump laser operating at different wavelengths such that the Raman gain spectra at each pump laser wavelength, when summed together, forms a gain bandwidth greater than the bandwidth of the optical signal pulses being amplified.

10. The apparatus according to claim 1, wherein said pulse conditioner comprises one or more selected from the group consisting of an optical amplifier, a pulse compressor, and a dispersive optical element.

11. The apparatus according to claim 10, wherein said optical amplifier comprises a rare earth doped fibre amplifier.

12. The apparatus according to claim 11 wherein said fibre amplifier is doped with one or more selected from the group consisting of Er3+, Yb+, ErYb+, Nd, Pr, Tm, and Ho.

13. The apparatus according to claim 10, wherein said pulse-compressor comprises one or more selected from the group consisting of a dispersion decreasing fibre (DDF), a comb-like dispersion profiled fibre (CDPF), and a dispersion shifted fibre (DSF).

14. The apparatus according to claim 10, wherein said dispersive optical element comprises a chiφed fibre Bragg grating; wherein said chiφed fibre Bragg grating conditions the phase of the optical signal pulses launched into said parabolic fibre amplifier.

15. The apparatus according to claim 1, further comprising a first pump coupled to said parabolic fibre amplifier through a first wavelength division multiplexer coupler, wherein said first pump provides pump light to said parabolic fibre amplifier.

16. The apparatus according to claim 1, further comprising a second pump coupled to said parabolic fibre amplifier through a second wavelength division multiplexer coupler, wherein said second pump providing pump light to said parabolic fibre amplifier.

17. The apparatus according to claim 15, wherein said first pump comprises a laser diode.

18. The apparatus according to claim 16, wherein said second pump comprises laser diode.

19. The apparatus according to claim 15, wherein said first pump comprises a fibre laser.

20. The apparatus according to claim 16, wherein said second pump comprises a fibre laser.

21. The apparatus according to claim 19, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 980nm.

22. The apparatus according to claim 20, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 980nm.

23. The apparatus according to claim 19, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 1480nm.

24. The apparatus according to claim 20, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 1480nm.

25. The apparatus according to claim 19, wherein said fibre laser comprises a rare- earth doped fibre laser operating around 980nm and pumped through a wavelength division multiplexer by a second rare-earth-doped fibre laser operating around 920nm.

26. The apparatus according to claim 20, wherein said fibre laser comprises a rare- earth doped fibre laser operating around 980nm and pumped through a wavelength division multiplexer by a second rare-earth-doped fibre laser operating around 920nm.

27. The apparatus according to claim 1, wherein said pulse-compressor comprises a soliton Raman compressor (SRC).

28. The apparatus according to claim 27, wherein said SRC comprises a rare earth doped fibre amplifier.

29. The apparatus according to claim 28, wherein said fibre amplifier comprises a core area greater than 50μm .

30. The apparatus according to claim 28, wherein said rare earth doped fibre amplifier comprises a polarisation maintaining fibre.

31. The apparatus according to claim 1, wherein said pulse-compressor comprises a bulk grating pair.

32. The apparatus according to claim 1, wherein said pulse-compressor comprises a photonic crystal fibre, said photonic crystal fibre having a low nonlinearity and high anomalous dispersion in comparison to standard optical fibre.

33. The apparatus according to claim 1, wherein said frequency converter comprises a periodically poled nonlinear crystal.

34. The apparatus according to claim 33, wherein said nonlinear crystal comprises one of the group consisting of lithium niobate (PPLN) and potassium titanyl phosphate (KTP).

35. The apparatus according to claim 1, further comprising a polarisation maintaining isolator disposed between said pulse conditioner and said parabolic amplifier, wherein said polarisation maintaining isolator provides linearly polarised light to said parabolic amplifier.

36. The apparatus according to claim 1, further comprising a polarisation maintaining isolator disposed between said parabolic amplifier and said pulse compressor, wherein said polarisation maintaining isolator provides linearly polarised light to said pulse compressor.

37. The apparatus according to claim 2, further comprising a polarisation maintaining isolator disposed between said polarisation controller and said parabolic amplifier, wherein adjustment of said polarisation controller provides linearly polarised light of variable amplitude at the output of said polarisation maintaining isolator.

38. The apparatus according to claim 1, wherein said frequency converter comprises a plurality of frequency conversion stages cascaded in series.

39. The apparatus according to claim 38, wherein said plurality of frequency conversion stages comprise the same nonlinear crystal, wherein each of said plurality of frequency conversion stages produce frequency doubled light at the same wavelength, and wherein each of said frequency stages use signal light not frequency converted in the previous conversion stage.

40. The apparatus according to claim 39, wherein said nonlinear crystal comprises periodically poled lithium niobate (PPLN).

41. The apparatus according to claim 39, wherein a wavelength selective output coupler is disposed between frequency conversion stages, wherein said wavelength selective output coupler provides an optical output at the frequency- converted wavelength after each frequency conversion stage.

42. The apparatus according to claim 41, wherein said wavelength selective output coupler comprises one of the group consisting of a bulk optical grating and a dichroic mirror.

43. The apparatus according to claim 39, wherein said nonlinear crystal comprises a periodically poled KTP.

44. The apparatus according to claim 38, wherein said plurality of frequency conversion stages comprise different nonlinear crystals, wherein said nonlinear crystals are selected to frequency double the frequency doubled light from the previous frequency conversion stage in the series.

45. The apparatus according to claim 44, wherein said different nonlinear crystals are fabricated from one of the group consisting of periodically poled lithium niobate (PPLN), periodically poled KTP, and periodically pooled beta barium borate (BBO).

46. A method for generating high-power optical pulses, comprising the steps of: generating optical signal pulses; conditioning at least one property of said optical signal pulses selected from the group consisting of amplitude, phase, and temporal width; amplifying and chiφing said optical signal pulses; compressing said optical signal pulses; and frequency-converting said optical signal pulses.

47. The method according to claim 46, further comprising the step of: controlling the state of polarisation of said optical signal pulses.

48. The method according to claim 46, further comprising the step of: frequency-converting said optical signal pulses that was not converted efficiently in the previous frequency converter with cascading frequency converters..

49. The method according to claim 48, wherein said cascading frequency converters are designed such that the output pulses at the frequency-converted wavelength of each frequency conversion stage are of approximately the same amplitude.

50. The method according to claim 48, further comprising the step of: frequency-converting the frequency converted optical pulses to provide optical pulses at more than 2 optical wavelengths.

51. The method according to claim 46, wherein said optical signal source is generated by a fibre oscillator.

52. The method according to claim 46, wherein said amplifying and chiφing step is performed with a parabolic fibre amplifier comprises a rare earth doped fibre amplifier.

53. The method according to claim 52, wherein said parabolic fibre amplifier is doped with one selected from the group consisting of: Er3+, Yb+, ErYb+, Nd, Pr, Tm, and Ho.

54. The method according to claim 52, wherein said parabolic fibre amplifier comprises a double-clad fibre pumped by a broad area laser comprising one or more multimode laser diodes.

55. The method according to claim 54, wherein said double-clad fibre is doped with one or more selected from the group consisting of: Yb+, ErYb+, Nd, Pr, Tm, and Ho.

56. The method according to claim 52, wherein said parabolic fibre amplifier is a Raman fibre amplifier.

57. The method according to claim 56, wherein said Raman fibre amplifier is pumped by more than one pump laser operating at different wavelengths such that the Raman gain spectra at each pump laser wavelength, when summed together, forms a gain bandwidth greater than the bandwidth of the signal pulses being amplified.

58. The method according to claim 46, wherein said conditioning step is performed with a pulse conditioner.

59. The method according to claim 58, wherein said pulse conditioner comprises one or more selected from the group consisting of an optical amplifier, a pulse compressor, and a dispersive optical element.

60. The method according to claim 59, wherein said optical amplifier comprises a rare earth doped fibre amplifier.

61. The method according to claim 60, wherein said fibre amplifier is doped with one or more selected from the group consisting of Er3+, Yb+, ErYb+, Nd, Pr, Tm, and Ho.

62. The method according to claim 59, wherein said pulse-compressor comprises one or more from the group consisting of a dispersion decreasing fibre (DDF), a comb-like dispersion profiled fibre (CDPF), and a dispersion shifted fibre (DSF).

63. The method according to claim 59, wherein said dispersive optical element comprises a chiφed fibre Bragg grating; wherein said chiφed fibre Bragg grating conditions the phase of said optical signal pulses launched into said parabolic fibre amplifier.

64. The method according to claim 46 further comprising the step of providing pump light to a parabolic fibre amplifier, with a first pump coupled to said parabolic fibre amplifier through a first wavelength division multiplexer coupler.

65. The method according to claim 64, further comprising the step of providing pump light to a parabolic fibre amplifier, with a second pump coupled to said parabolic fibre amplifier through a second wavelength division multiplexer coupler.

66. The method according to claim 64, wherein said first pump comprises a laser diode.

67. The method according to claim 65, wherein said second pump comprises laser diode.

68. The method according to claim 64, wherein said first pump comprises a fibre laser.

69. The method according to claim 65, wherein said second pump comprises a fibre laser.

70. The method according to claim 66, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 980nm.

71. The method according to claim 67, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 980nm.

72. The method according to claim 66, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 1480nm.

73. The method according to claim 67, wherein said fibre laser comprises a Raman fibre laser operating at a wavelength around 1480nm.

74. The method according to claim 66, wherein said fibre laser comprises a rare-earth doped fibre laser operating around 980nm and pumped through a wavelength division multiplexer by a second rare-earth-doped fibre laser operating around 920nm.

75. The method according to claim 67, wherein said fibre laser comprises a rare-earth doped fibre laser operating around 980nm and pumped through a wavelength division multiplexer by a second rare-earth-doped fibre laser operating around 920nm.

76. The method according to claim 46, wherein said compressing step is accomplished with a pulse compressor.

77. The method according to claim 76, wherein said pulse-compressor comprises a soliton Raman compressor (SRC).

78. The method according to claim 77, wherein said SRC comprises a rare earth doped fibre amplifier.

79. The method according to claim 78, wherein said fibre amplifier comprises a core area greater than 50μm².

80. The method according to claim 78, wherein said rare earth doped fibre amplifier comprises a polarisation maintaining fibre.

81. The method according to claim 76, wherein said pulse-compressor comprises a bulk grating pair.

82. The method according to claim 76, wherein said pulse-compressor comprises a photonic crystal fibre, said photonic crystal fibre having a low nonlinearity and high anomalous dispersion in comparison to standard optical fibre.

83. The method according to claim 46, wherein said frequency-converting step is accomplished with a frequency converter.

84. The method according to claim 83, wherein said frequency converter comprises a periodically poled nonlinear crystal.

85. The method according to claim 84, wherein said nonlinear crystal comprises one of the group consisting of lithium niobate (PPLN) and KTP.

86. The method according to claim 46, further comprising the step of polarisation-maintaining said optical signal pulses with a polarisation maintaining isolator.

87. The method according to claim 86, wherein said polarisation-maintaining step occurs between said conditioning and amplifying steps.

88. The method according to claim 86, wherein said polarisation-maintaining step occurs between said amplifying and compressing steps.

89. The method according to claim 86, wherein said polarisation maintaining isolator provides linearly polarised light to said parabolic amplifier.

90. The method according to claim 46, wherein said frequency-converting step comprises a plurality of frequency-converting steps cascaded in series.

91. The method according to claim 90, wherein said plurality of frequency-converting steps comprise the same nonlinear crystal, wherein each of said plurality of frequency-conversion steps produce frequency doubled light at the same wavelength, and wherein each of said frequency stages use signal light not frequency converted in the previous conversion stage.

92. The method according to claim 91, wherein said nonlinear crystal comprises a periodically poled lithium niobate (PPLN).

93. The method according to claim 91, wherein said nonlinear crystal comprises a periodically poled KTP.

94. The method according to claim 91, wherein plurality of frequency-conversion steps comprise different nonlinear crystals, wherein said nonlinear crystals are selected to frequency double the frequency doubled light from the previous frequency conversion step in the series.

95. The method according to claim 94, wherein said different nonlinear crystals are fabricated from one of the group consisting of periodically poled lithium niobate (PPLN) and periodically poled KTP or BBO.