Optical amplifier and method for amplifying a signal beam - Patents.com

Abstract

The present invention generally relates to an optical parametric chirped pulse amplifier. The optical parametric chirped pulse amplifier includes a first optical parametric amplifier stage and a second optical parametric amplifier stage that sequentially amplify an expanded signal beam. A pulsed laser provides a fundamental beam. The second amplifier stage is pumped by the full power of a second-harmonic beam generated from the fundamental beam. The residual fundamental beam is used to generate another second-harmonic beam that pumps the first amplifier stage.

Description

[Technical field]

[0001] (Priority) This application claims priority to U.S. Patent Application No. 16 / 546,178, filed August 20, 2019, the disclosure of which is incorporated herein by reference in its entirety.

[0002] (Technical field of the invention) The present invention relates generally to optical parametric chirped pulse amplifiers, and more particularly to optical pumping methods and apparatus for multi-stage optical parametric chirped pulse amplifiers. [Background technology]

[0003] Discussion of Background Art In optical parametric amplification, an optically nonlinear crystal arranged for difference frequency generation is used to provide amplification of a beam of pulsed laser radiation. A pulsed "pump beam" and a shorter wavelength pulsed "signal beam" propagate together through the nonlinear crystal. Difference frequency generation converts photons of the pump beam into photons of the signal beam and into photons of the "idler beam." The energy of the idler photons is the difference in energy between the pump and signal photons. Power is transferred from the pump beam to the signal and idler beams, adding photons into the signal beam, thereby amplifying the signal beam. Because each pump photon is completely converted into one signal photon and one idler photon, the nonlinear crystal is not significantly heated by the optical parametric amplification process.

[0004] There is no energy storage in nonlinear crystals as there is in conventional optical gain media. Therefore, for optimal efficiency of the amplification process, the pulse durations of the pump and signal beams should be closely matched and the pulses should overlap in time. Optical parametric amplification can provide optical gain of several orders of magnitude in nonlinear crystals that are only a few millimeters (mm) long. This relatively high gain means that in most cases no more than two amplification stages are required.

[0005] Optical parametric chirped pulse amplification is used to amplify beams of ultrashort pulsed laser radiation. Ultrashort pulsed laser beams have broad spectral bandwidths. For example, the pulses have durations of less than 300 femtoseconds (fs) and corresponding spectral bandwidths of more than 3 nanometers (nm). In chirped pulse optical parametric amplification, the pulses in the signal beam are stretched in time prior to amplification in order to maintain the peak intensity of the signal beam below the optical damage threshold during amplification. In particular, to maintain the peak intensity below the optical damage threshold of the nonlinear crystal. Stretching the signal pulse to a single-digit nanosecond duration allows for the application of much higher pump pulse energies and thus obtain much higher amplified signal pulse energies. After amplification, the signal pulses can be compressed in time, thereby producing ultrashort amplified signal pulses with high pulse energies and high peak powers.

[0006] Stretching the signal pulse also eliminates the need to use an ultrashort pump pulse to match the ultrashort signal pulse. Powerful and relatively simple lasers can provide the pump beam of nanosecond pump pulses. For example, diode-pumped Q-switched solid-state lasers use neodymium (Nd) lasers such as yttrium aluminum garnet (YAG) or yttrium orthovanadate (YVO4). 3+ ) or Ytterbium (Yb 3+ These lasers have a gain medium doped with . Typically, these lasers deliver a "fundamental beam" with a wavelength in the near-infrared region of the electromagnetic spectrum, from about 950 nm to 1,100 nm. To provide pulses with a sufficiently short wavelength to be used as pump pulses for optical parametric amplification, the near-infrared fundamental beam must be converted into a "second harmonic beam" with a wavelength that is one-half the wavelength of the fundamental beam. For example, the second harmonic beam has a wavelength of about 475 nm to 550 nm, in the visible region of the electromagnetic spectrum. Another optically nonlinear crystal is used to generate the second harmonic beam from the fundamental beam.

[0007] In a typical prior art two-stage optical parametric chirped pulse amplification arrangement, the second harmonic beam is split into a higher power beam and a lower power beam. The small amount of fundamental beam that is not converted to a second harmonic beam is discarded. The lower power beam is used to pump the first stage amplifier, and the higher power beam is used to pump the second stage amplifier. Approximately 80% of the second harmonic beam is directed into the higher power beam, and approximately 20% of the second harmonic beam is directed into the lower power beam. A power ratio of 80% to 20% is typical, but other power ratios can be provided by suitable specifications of the beam splitting elements. The chirped signal beam is amplified successively in the first and second stage amplifiers. An extensive detailed description of the design criteria for optical parametric chirped-pulse amplifiers is provided in the review article “Design criteria for ultrafast optical parametric amplifiers” (C. Manzoni and G. Cerullo, J. Opt. 18(2016)103501).

[0008] A significant drawback of prior art arrangements is that the efficiency conversion of the fundamental beam to the second harmonic beam is at best about 50%. This conversion efficiency is limited by the spectral bandwidth of the signal beam being amplified and by the temporal "walk-off" as the beam propagates through the nonlinear crystal. Both limits constrain the nonlinear crystal length to a few millimeters or less. There is a significant waste of power in the fundamental beam. There is a need for a multi-stage optical parametric chirped pulse amplifier that transfers power from the fundamental beam to the signal beam with higher overall efficiency. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] "Design criteria for ultrafast optical parametric amplifiers" (C. Manzoni and G. Cerullo, J. Opt. 18(2016)103501) Summary of the Invention [Means for solving the problem]

[0010] In one aspect, an optical amplifier apparatus according to the invention comprises a pulsed laser generating a beam of fundamental laser radiation. A first optically nonlinear crystal partially converts the fundamental beam into a first beam of second harmonic laser radiation leaving a residual beam of unconverted fundamental laser radiation. A second optically nonlinear crystal partially converts the residual fundamental beam into a second beam of second harmonic laser radiation. A first optical parametric amplifier stage is optically pumped by the second second harmonic beam and arranged to amplify a signal beam. A second optical parametric amplifier stage is optically pumped by the first second harmonic beam and arranged to further amplify the signal beam. The present invention provides, for example, the following: (Item 1) 1. An optical amplifier comprising: a pulsed laser generating a beam of fundamental laser radiation; a first optically nonlinear crystal that partially converts the fundamental beam into a first beam of second harmonic laser radiation and leaves a residual beam of unconverted fundamental laser radiation; a second optically nonlinear crystal that partially converts the residual fundamental beam into a second beam of second harmonic laser radiation; and a first optical parametric amplifier stage optically pumped by the second second harmonic beam and arranged to amplify a signal beam; a second optical parametric amplifier stage optically pumped by the first second harmonic beam and arranged to further amplify the signal beam; An optical amplifier comprising: (Item 2) 2. The optical amplifier of claim 1, wherein the signal beam amplified in the first amplifier stage is generated by a short pulse laser and expanded by a pulse stretcher. (Item 3) 3. The optical amplifier of claim 2, wherein pulse generation in the pulsed laser and the short pulse laser is synchronized to temporally overlap pulses of the second second harmonic beam and pulses of the signal beam in the first amplifier stage. (Item 4) 2. The optical amplifier of claim 1, further comprising an optical delay in the second amplifier stage arranged to overlap in time the pulses of the first second harmonic beam and the pulses of the signal beam. (Item 5) 2. The optical amplifier of claim 1, wherein the amplified signal beam from the second optical amplifier is directed into a pulse compressor arranged to compress in time the amplified signal beam. (Item 6) 2. The optical amplifier of claim 1, wherein the first and second nonlinear crystals are made of lithium triborate or beta barium borate. (Item 7) 2. The optical amplifier of claim 1, wherein the first and second amplifier stages are in the form of nonlinear crystals made from beta barium borate. (Item 8) 2. The optical amplifier of claim 1, wherein a pulse of the second second harmonic beam has a longer duration than a pulse of the first second harmonic beam. (Item 9) 2. The optical amplifier according to item 1, wherein the fundamental beam has a wavelength of about 950 nanometers to about 1,100 nanometers. (Item 10) 10. The optical amplifier of claim 9, wherein the fundamental beam has a wavelength of about 1,030 nanometers. (Item 11) 1. An optical amplifier comprising: a first optical parametric amplifier stage arranged to receive the signal beam and to amplify said signal beam; a second optical parametric amplifier stage arranged to receive the amplified signal beam and further amplify the signal beam; a pulsed laser generating a beam of fundamental laser radiation; a first optically nonlinear crystal that partially converts the fundamental beam into a first beam of second harmonic laser radiation and leaves a residual beam of unconverted fundamental laser radiation, the first second harmonic beam being delivered to the second amplifier stage; a second optically nonlinear crystal that partially converts the residual fundamental beam into a second beam of second harmonic laser radiation, the second second harmonic beam being delivered to the first amplifier stage; an optical delay located between the first nonlinear crystal and the second amplifier stage, the optical delay being arranged such that pulses of the amplified signal beam and pulses of the first second harmonic beam overlap in time at the second amplifier stage; Equipped with An optical amplifier, wherein generation of the fundamental beam pulse and the signal beam pulse is synchronized in the first amplifier stage to overlap in time with the second second harmonic beam pulse and the signal beam pulse. (Item 12) 2. The optical amplifier of claim 1, wherein the signal beam amplified in the first amplifier is generated by a short pulse laser and expanded by a pulse stretcher. (Item 13) 2. The optical amplifier of claim 1, wherein the amplified signal beam from the second optical amplifier is directed into a pulse compressor arranged to compress in time the amplified signal beam. (Item 14) 2. The optical amplifier of claim 1, wherein the first and second nonlinear crystals are made of lithium triborate or beta barium borate. (Item 15) 2. The optical amplifier of claim 1, wherein the first and second amplifier stages are in the form of nonlinear crystals made from beta barium borate. (Item 16) 2. The optical amplifier of claim 1, wherein a pulse of the second second harmonic beam has a longer duration than a pulse of the first second harmonic beam. (Item 17) 2. The optical amplifier according to item 1, wherein the fundamental beam has a wavelength of about 950 nanometers to about 1,100 nanometers. (Item 18) Item 18. The optical amplifier of item 17, wherein the fundamental beam has a wavelength of about 1,030 nanometers. (Item 19) 1. A method for amplifying a signal beam, comprising: generating a beam of fundamental laser radiation; converting a portion of the fundamental beam into a first beam of second harmonic laser radiation and leaving a residual beam of unconverted fundamental laser radiation; converting a portion of the residual fundamental beam into a second beam of second harmonic laser radiation; directing the second second harmonic beam to a first optical parametric amplifier stage, the first optical parametric amplifier stage being optically pumped thereby and arranged to amplify the signal beam; directing the first second harmonic beam to a second optical parametric amplifier stage, the second optical parametric amplifier stage being optically pumped thereby and arranged to further amplify the signal beam; A method comprising: (Item 20) 20. The method of claim 19, wherein the signal beam is pulsed, the pulses of the signal beam being expanded prior to amplification in the first optical parametric amplifier stage, and the pulses of the signal beam being compressed after amplification in the second optical parametric amplifier stage. (Item 21) 20. The method of claim 19, wherein the signal beam and the fundamental beam are pulsed and generation of the signal beam pulses and the fundamental beam pulses are synchronized in the first parametric amplifier stage to overlap in time the pulses of the second second harmonic beam and the signal beam pulses. [Brief description of the drawings]

[0011] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0012] [Figure 1] FIG. 1 diagrammatically illustrates a preferred embodiment of an optical parametric chirped pulse amplifier according to the present invention, the optical parametric chirped pulse amplifier including a pulsed laser delivering a fundamental beam, a first nonlinear crystal generating a first second harmonic beam from the fundamental beam, and a second nonlinear crystal generating a second second harmonic beam from the residual fundamental beam, the first second harmonic beam and the second second harmonic beam optically pumping a second amplifier stage and a first amplifier stage, respectively. [Figure 2A] 2A and 2B are graphs that diagrammatically illustrate the normalized calculated power as a function of time for one pulse of each of the fundamental beam, the residual fundamental beam, the first second harmonic beam, and the second second harmonic beam in one embodiment of the optical amplifier of FIG. [Figure 2B] 2A and 2B are graphs illustrating diagrammatically the normalized calculated power as a function of time for one pulse of each of the fundamental beam, the residual fundamental beam, the first second harmonic beam, and the second second harmonic beam in one embodiment of the optical amplifier of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Detailed Description of the Invention Turning now to the drawings, in which like features are identified by like reference numerals, Figure 1 diagrammatically illustrates a preferred embodiment of an optical parametric chirped pulse amplifier 10 in accordance with the present invention. The optical amplifier 10 comprises first and second optical parametric amplifier stages 12 and 14, respectively, in the form of optically nonlinear crystals.

[0014] The optical amplifier 10 also includes a pulsed laser 16, which generates a beam of pulsed fundamental laser radiation, identified by a single arrowhead F. A first optically nonlinear crystal 18 arranged for second harmonic generation partially converts the fundamental beam F into a beam of pulsed second harmonic laser radiation, identified by a double arrowhead SH1. The wavelength of the second harmonic beam SH1 is half the wavelength of the fundamental beam F. The second harmonic generation leaves a residual beam of unconverted fundamental laser radiation RF1. As an example, the second harmonic beam SH1 has about 50% of the power of the original fundamental beam F. Equivalently, each original pulse of the fundamental beam F is divided into a pulse of the second harmonic beam SH1 and a pulse of the residual fundamental beam RF1, each of which has about 50% of the energy of the original pulse.

[0015] The second harmonic beam SH1 and the residual fundamental beam RF1 are spatially separated by a dichroic mirror 20, which reflects the second harmonic beam SH1 and transmits the residual fundamental beam RF1. A second optically nonlinear crystal 22, also arranged for second harmonic generation, partially converts the residual fundamental beam RF1 into a beam of pulsed second harmonic laser radiation SH2. Again, a residual beam of unconverted fundamental laser radiation RF2 is also present. The second harmonic beam SH2 and the residual fundamental beam RF2 are spatially separated by another dichroic mirror 24, which reflects the second harmonic beam SH2. The dichroic mirror 24 transmits the residual fundamental beam RF2, which can be discarded or used to generate a pump beam for an additional preamplifier stage.

[0016] The residual fundamental beam RF1, which is depleted by second harmonic generation in the first nonlinear crystal 18, generally has a longer pulse duration and a lower beam quality than the fundamental beam F. For these reasons, second harmonic generation is generally less efficient in the second nonlinear crystal 22 than in the first nonlinear crystal 18. As an example, the residual fundamental beam RF1 is converted into the second harmonic beam SH2 with an efficiency in the range of about 10% to about 20%, the residual fundamental beam RF2 having about 80% to about 90% of the power of the residual fundamental beam RF1. The second harmonic beam SH2 therefore has about 10% to 20% of the power of the second harmonic beam SH1. It is preferable to optimize the efficiency of second harmonic generation in the first nonlinear crystal 18 with respect to the optimal overall efficiency of the optical amplifier 10. Nevertheless, a conversion efficiency of more than 20% in the second nonlinear crystal 22 can still be achieved by careful optimization.

[0017] The short pulse laser 26 generates a beam of pulsed seed laser radiation, identified by a large single arrowhead S, which is amplified in the optical amplifier 10. The seed beam S has an ultrashort pulse and a wide spectral bandwidth. The seed beam S has a longer wavelength than the second harmonic beams SH1 and SH2. As an example, the short pulse laser 26 may include an ultrafast laser resonator and use supercontinuum generation to produce the seed beam S.

[0018] The seed beam S is directed to a pulse stretcher 28 arranged to stretch the ultrashort pulses in time to produce the stretched seed beam S1. The pulses in the stretched seed beam S1 have durations comparable to the pulses of the second harmonic beams SH1 and SH2. Fixed and variable pulse stretcher arrangements are well known in the art. A detailed description thereof is therefore not necessary for an understanding of the principles of the invention and is therefore not presented herein.

[0019] The dichroic mirror 30 receives the second harmonic beam SH2, reflects it from the dichroic mirror 24, and directs it along a common path with the expanded seed beam S1 into the first amplifier stage 12. The pulse generation in the pulsed laser 16 and the short pulse laser 26 is synchronized so that the pulses of the second harmonic beam SH2 and the pulses of the expanded seed beam S1 overlap in time. During the passage of these co-propagating beams through the first amplifier stage 12, the time overlap is required for the optical parametric amplification. The second harmonic beam SH2 is the pump beam and the expanded seed beam S1 is the signal beam in the first amplifier stage 12. The optical parametric amplification of the first stage produces the amplified signal beam S2.

[0020] The dichroic mirror 20 directs the second harmonic beam SH1 through an optical delay 32. The second harmonic beam SH1 continues to propagate onto another dichroic mirror 34, which directs the second harmonic beam SH1 along a common path with the amplified signal beam S2. The optical delay 32 is arranged such that the pulses of the second harmonic beam SH1 and the pulses of the amplified signal beam S2 overlap in time during the passage of these co-propagating beams through the second amplifier stage 14. The second harmonic beam SH1 is the pump beam and the amplified signal beam S2 is the signal beam in the second amplifier stage 14. The optical parametric amplification of the second stage produces a further amplified signal beam S3, which is the output beam from the optical amplifier 10.

[0021] The amplified signal beam S3 may be delivered to an optional pulse compressor 36, which is arranged to compress in time the amplified stretched pulses to produce an amplified compressed signal beam S4. The pulses in the amplified compressed signal beam S4 may have durations comparable to the durations of the pulses in the original seed beam S. Pulse compressor arrangements are well known in the art and a detailed description thereof is not necessary for an understanding of the principles of the present invention.

[0022] As an example, the fundamental beam F has a near-infrared wavelength, and the second harmonic beams SH1 and SH2 have corresponding visible wavelengths. For example, the near-infrared wavelength is in the range of 950 nm to 1,100 nm. These beams may have pulse durations in the range of hundreds of femtoseconds to a few nanoseconds. The first and second optically nonlinear crystals may be made of lithium triborate (LBO) or beta barium borate (BBO). The nonlinear crystals in the first and second amplifier stages may also be made of LBO or BBO.

[0023] The amplified signal beams S2 and S3 may be tuned over visible and infrared wavelengths by varying the wavelength of the seed beam and rotating the nonlinear crystals in the first and second amplifier stages. The central wavelengths of the seed and signal beams may be any wavelength that can be phase matched for parametric amplification in the nonlinear crystals of the first and second amplifier stages. With a fundamental wavelength of about 1,030 nm and a second harmonic wavelength of about 515 nm, a properly oriented crystal made from LBO or BBO can amplify beams with wavelengths ranging from 600 nm up to the wavelength at which the material becomes opaque, which is above 2,000 nm.

[0024] 2A and 2B are graphs that diagrammatically illustrate normalized calculated power as a function of time for a fundamental wavelength of 1,030 nm and a second harmonic wavelength of 515 nm in one embodiment of the optical amplifier 10 of FIG. 1. In the embodiment, the second harmonic laser radiation is generated by type-1 second harmonic generation in a BBO crystal. The figures depict one pulse in each of the fundamental beam F, the residual fundamental beam RF1, the second harmonic beam SH1, and the second harmonic beam SH2. These pulses are overlapped in the figures for comparison purposes.

[0025] FIG. 2A shows that the second harmonic generation of the fundamental beam F in the first nonlinear crystal 18 preferentially depletes the peak of the pulse of the residual fundamental beam RF1, which effectively increases the duration of this pulse. FIG. 2B shows that the pulse of the second harmonic beam SH2 generated in the second nonlinear crystal 22 therefore has a longer duration than the pulse of the second harmonic beam SH1 generated in the first nonlinear crystal 18. Lengthening the pulse of the second harmonic beam SH2 in turn lengthens the pulse of the amplified expanded seed beam S2 generated in the first amplifier stage 12, compensating for the narrowing temporal gain in the first stage optical parametric amplification. The longer pulse of the amplified expanded seed beam S2 has an improved temporal overlap with the pulse of the second harmonic beam SH1 in the second amplifier stage 14, thereby increasing the efficiency of the second stage optical parametric amplification.

[0026] The two-stage optical parametric chirped pulse amplifier of the present invention provides a significant increase in overall efficiency over the two-stage arrangement of the prior art. All of the power in the second harmonic beam SH1 is directed into the second amplifier stage 14. In contrast, in the prior art arrangement, only about 80% of the power in the second harmonic beam is directed into the final amplifier stage. Directing all of the power of the second harmonic beam into the second amplifier stage can provide a gain of more than about 25% to the signal beam in the second amplifier stage. Meanwhile, the power in the residual fundamental beam RF1 is utilized to generate a pump beam for the first amplifier stage 12, rather than being discarded as in the prior art arrangement. As discussed above, the pulses of the amplified expanded beam S2 have good temporal overlap with the pulses of the second harmonic beam SH1 in the second amplifier stage 14 of the present invention. In the prior art arrangement, the narrowing temporal gain in the first amplifier stage creates a signal pulse that is shorter than the pump pulse in the final amplifier stage, which limits the power transfer from the pump pulse to the signal pulse. Overall, the amplifier of the present invention is approximately 50% more efficient than the prior art arrangement.

[0027] In summary, the expanded signal beam is amplified in a first optical parametric amplifier stage and then further amplified in a second optical parametric amplifier stage. The first nonlinear crystal converts the fundamental beam into a second harmonic beam to pump the second amplifier stage, leaving a residual fundamental beam. The second nonlinear crystal converts the residual fundamental beam into another second harmonic beam to pump the first amplifier stage.

[0028] The present invention is described above in terms of preferred embodiments. The present invention is not, however, limited by the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. An optical amplifier, comprising: a pulsed laser generating a beam of fundamental laser radiation; a first optically nonlinear crystal that partially converts the beam of fundamental laser radiation into a first beam of second harmonic laser radiation and leaves a residual beam of unconverted fundamental laser radiation; a second optically nonlinear crystal for partially converting the residual beam into a second beam of second harmonic laser radiation; a first optical parametric amplifier stage optically pumped by the second beam of second harmonic laser radiation, the first optical parametric amplifier stage being arranged to amplify a signal beam; a second optical parametric amplifier stage optically pumped by the first beam of second harmonic laser radiation, the second optical parametric amplifier stage being arranged to further amplify the signal beam; An optical amplifier comprising:

2. 2. The optical amplifier of claim 1, wherein the signal beam is a seed beam, the seed beam being generated by a short pulse laser and then temporally stretched by a pulse stretcher prior to amplification in the first optical parametric amplifier stage.

3. 3. The optical amplifier of claim 2, wherein pulse generation in the pulsed laser and the short pulse laser is synchronized to temporally overlap pulses of the second beam of second harmonic laser radiation and pulses of the signal beam in the first optical parametric amplifier stage.

4. 2. The optical amplifier of claim 1, further comprising an optical delay arranged to temporally overlap pulses of the first beam of second harmonic laser radiation and pulses of the signal beam in the second optical parametric amplifier stage.

5. 2. The optical amplifier of claim 1, wherein the amplified signal beam from the second optical parametric amplifier stage is directed into a pulse compressor arranged to compress in time the amplified signal beam.

6. 2. The optical amplifier of claim 1, wherein the first optically nonlinear crystal and the second optically nonlinear crystal are made from lithium triborate or beta barium borate.

7. 10. The optical amplifier of claim 1, wherein the first optical parametric amplifier stage and the second optical parametric amplifier stage are in the form of nonlinear crystals made from beta barium borate.

8. 2. An optical amplifier as claimed in claim 1, wherein a pulse of the second beam of second harmonic laser radiation has a longer duration than a pulse of the first beam of second harmonic laser radiation.

9. The optical amplifier of claim 1 , wherein the beam of fundamental laser radiation has a wavelength between about 950 nanometers and about 1,100 nanometers.

10. 10. The optical amplifier of claim 9, wherein the beam of fundamental laser radiation has a wavelength of about 1,030 nanometers.

11. An optical amplifier, comprising: a first optical parametric amplifier stage arranged to receive the signal beam and to amplify said signal beam; a second optical parametric amplifier stage arranged to receive the amplified signal beam and further amplify the signal beam; a pulsed laser generating a beam of fundamental laser radiation; a first optically nonlinear crystal that partially converts the beam of fundamental laser radiation into a first beam of second harmonic laser radiation and leaves a residual beam of unconverted fundamental laser radiation, the first beam of second harmonic laser radiation being delivered to the second optical parametric amplifier stage; a second optically nonlinear crystal that partially converts the residual beam into a second beam of second harmonic laser radiation, the second beam of second harmonic laser radiation being delivered to the first optical parametric amplifier stage; and an optical delay disposed between the first optically nonlinear crystal and the second optical parametric amplifier stage, the optical delay being arranged such that pulses of the amplified signal beam and pulses of the first beam of second harmonic laser radiation overlap in time at the second optical parametric amplifier stage; Equipped with an optical amplifier, wherein generation of the pulses of the beam of fundamental laser radiation and the pulses of the signal beam are synchronized to temporally overlap the pulses of the second beam of second harmonic laser radiation and the pulses of the signal beam in the first optical parametric amplifier stage.

12. 12. The optical amplifier of claim 11, wherein the signal beam is a seed beam, the seed beam being generated by a short pulse laser and then temporally stretched by a pulse stretcher prior to amplification in the first optical parametric amplifier stage.

13. 12. The optical amplifier of claim 11, wherein the amplified signal beam from the second optical parametric amplifier stage is directed into a pulse compressor arranged to compress in time the amplified signal beam.

14. 12. The optical amplifier of claim 11, wherein the first optically nonlinear crystal and the second optically nonlinear crystal are made from lithium triborate or beta barium borate.

15. 12. The optical amplifier of claim 11, wherein the first optical parametric amplifier stage and the second optical parametric amplifier stage are in the form of nonlinear crystals made from beta barium borate.

16. 12. An optical amplifier as claimed in claim 11, wherein a pulse of the second beam of second harmonic laser radiation has a longer duration than a pulse of the first beam of second harmonic laser radiation.

17. The optical amplifier of claim 11 , wherein the beam of fundamental laser radiation has a wavelength between about 950 nanometers and about 1,100 nanometers.

18. 20. The optical amplifier of claim 17, wherein the beam of fundamental laser radiation has a wavelength of about 1,030 nanometers.

19. 1. A method of amplifying a signal beam, the method comprising: generating a beam of fundamental laser radiation; converting a portion of the beam of fundamental laser radiation into a first beam of second harmonic laser radiation, leaving a residual beam of unconverted fundamental laser radiation; converting a portion of the residual beam into a second beam of second harmonic laser radiation; directing the second beam of second harmonic laser radiation to a first optical parametric amplifier stage, the first optical parametric amplifier stage being optically pumped by the second beam of second harmonic laser radiation and arranged to amplify the signal beam; directing the first beam of second harmonic laser radiation to a second optical parametric amplifier stage, the second optical parametric amplifier stage being optically pumped by the first beam of second harmonic laser radiation and arranged to further amplify the signal beam; A method comprising:

20. 20. The method of claim 19, wherein the signal beam is pulsed, and the pulses of the signal beam are expanded prior to amplification in the first optical parametric amplifier stage, and the pulses of the signal beam are compressed after amplification in the second optical parametric amplifier stage.

21. 20. The method of claim 19, wherein the signal beam and the beam of fundamental laser radiation are pulsed and generation of the pulses of the signal beam and the pulses of the beam of fundamental laser radiation are synchronized to temporally overlap pulses of the second beam of second harmonic laser radiation and pulses of the signal beam in the first optical parametric amplifier stage.

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