Apparatus And Method For Tunable Frequency Parametric Down Conversion Of High Peak Power Lasers Through Dual Chirp Pulse Mixing

Abstract

A laser architecture for selectively producing short high-energy laser pulses having octave-spanning, continuous tunability. Two oppositely chirped pulses are used in combination with a pair of tunable pulse stretcher/compressors to produce a short, high-energy, tunable, broadband pulse.

Claims

1. A tunable pulse stretcher/compressor, comprising: a first grating and a second grating, each of the first and second gratings having tunably separable grating features therein, the first grating further being tunably rotatable relative to an incoming laser pulse; and a retroreflector; wherein the grating features in first grating tunably reflect an incoming laser pulse incident on the first grating at a plurality of angles relative to the first grating so as to form a first plurality of reflected pulses having a corresponding plurality of frequencies that travel to the second grating; wherein the grating features in the second grating tunably reflect the first plurality of reflected pulses incident on the second grating at a plurality of angles relative to the second grating so as to form a second plurality of reflected pulses that travel to the retroreflector, at least one of the second plurality of reflected pulses having a frequency different from a frequency of at least one of the first plurality of reflected pulses; wherein the retroreflector directs the second plurality of reflected pulses back into the second grating as a third plurality of reflected pulses; wherein the grating features of the second grating tunably reflect the third plurality of reflected pulses incident on the second grating at a plurality of angles relative to the second grating so as to form a fourth plurality of reflected pulses that travel back to the first grating, at least one of the second plurality of reflected pulses having a frequency different from a frequency of at least one of the first plurality of reflected pulses; and wherein the grating features of the first grating tunably combine the fourth plurality of reflected pulses into a final pulse that is output from the stretcher compressor; wherein the final pulse has a predetermined positive or negative chirp produced by the tunings of the gratings in the first and second grating.

2. The tunable stretcher/compressor according to claim 1, wherein the retroreflector comprises a curved mirror; wherein an angle of the grating features in the first and second gratings relative to the laser pulses incident thereon is tuned to tune a wavelength of pulses reflected from the grating features; and wherein a separation between the second grating and the retroreflector is tuned to produce a predetermined positive or negative chirp of the final pulse.

3. The tunable stretcher/compressor according to claim 1, wherein an angle of the grating features in the first and second gratings relative to the laser pulses incident thereon is tuned to tune a wavelength of pulses reflected from the grating features; and wherein a separation between the first and second grating is tuned to produce a predetermined negative chirp of the final pulse.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a block schematic illustrating aspects of optical pump amplification (OPA) of a source laser pulse in accordance with the prior art.

[0019] FIG. 2 is a block schematic illustrating aspects of optical parametric chirped pulse amplification (OP-CPA) of a source laser pulse in accordance with the prior art.

[0020] FIGS. 3A and 3B are block schematics illustrating aspects of a Dual-Chirp Optical Parametric Chirped Pulse Amplification (DC-OPCPA) system for amplification of a source laser pulse in accordance with the present invention.

[0021] FIG. 4 is a block schematic illustrating aspects of a tunable pulse stretcher used in a DC-OPCPA system in accordance with the present invention.

[0022] FIG. 5 is a block schematic further illustrating aspects of a tunable pulse stretcher used in a DC-OPCPA system in accordance with the present invention.

[0023] FIG. 6 is a block schematic further illustrating aspects of a tunable pulse compressor used in a DC-OPCPA system in accordance with the present invention.

[0024] FIGS. 7A and 7B are plots showing the results of a simulation of pulse amplification and compression using dual-chirp optical pulse amplification (DC-OPCPA) in accordance with the present invention.

DETAILED DESCRIPTION

[0025] The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.

[0026] The present invention provides a high efficiency, tunable, high-contrast, broad-bandwidth laser amplifier with carrier envelope phase locking that can enable the generation of short high-power laser pulses at wavelengths where appropriate gain materials do not exist.

[0027] The laser amplifier scheme of the present invention, Dual Chirp Optical Parametric Chirped Pulse Amplification (DC-OPCPA), utilizes a high-energy, chirped pulse as a pump to amplify lower frequency, broadband pulses within a nonlinear crystal. The amplification is done within a second-order nonlinear material under difference frequency generation or optical parametric amplification arrangements known in the art. However, as described in more detail below, the use of a single initial pulse to produce oppositely signed chirped pump and signal pulses is a new feature of the DC-OPCPA scheme in accordance with the present invention, and leads to the generation of an idler pulse having increased bandwidth from the initial signal pulse, where the idler pulse which enables passive carrier envelope phase (CEP) locking. For a positively chirped pump and negatively chirped signal, a positively chirped idler pulse is produced that can be compressed using standard dispersive optical elements. In addition, as described in more detail below, the laser amplifier scheme in accordance with the present invention further includes a novel tunable pulse stretcher/compressor that enables the length of the pulse to be tuned over the transmission region of the nonlinear second-order material. This is achieved by allowing both the stretcher/compressor systems to be designed to allow for both rotation and changes in the grating separation.

[0028] The present invention provides a new laser architecture for selectively producing short high-energy laser pulses having octave-spanning, continuous tunability. Unlike the prior art techniques for pulse amplification discussed above, in accordance with the present invention, two oppositely chirped pulses are used in combination with a pair of the novel tunable pulse stretcher/compressors of the present invention to produce a short, high-energy, tunable, broadband pulse.

[0029] The envisioned mode of operation is that for signal amplification that a positive chirp is applied, the signal is amplified and then recompressed. For idler amplification, a negative chirp is applied to the signal, that generates an amplified positively chirped idler, and then the idler is compressed.

[0030] A tunable DC-OPCPA system in accordance with the present invention requires (1) a broad-bandwidth, ultrashort seed pulse and (2) a tunable pulse stretcher and compressor to access the various operational wavelengths. The seed pulse can be provided by means of supercontinuum generation, wherein a small portion of an initial high energy broadband pump pulse can be compressed to generate a low energy ultrashort pulse. This ultrashort pulse can be focused into a material (e.g., fused silica) and through strong self-phase modulation generates an ultrabroadband (white light) source extending over the range of wavelengths over which the DC-OPCPA can be tuned. The desired operational wavelength can then be selected from this white light source.

[0031] In order to amplify the ultrashort pulses to high energy, they need to first be stretched temporally, amplified, and then recompressed. This general process is known as chirped pulse amplification (CPA). To accommodate a changing operational wavelength, both the stretcher and compressor needs to be tunable.

[0032] The block schematics in FIGS. 3A and 3B illustrate aspects of the novel laser architecture and method for selectively producing short high-energy laser pulses in accordance with the present invention.

[0033] As illustrated in FIGS. 3A and 3B, the laser architecture used in the method of the present invention includes a high-energy broadband laser source 301, a white light continuum optical parametric amplifier (WLC OPA) 304, a tunable stretcher 306, a dual-chip optical parametric amplifier (DC OPA) 308, and a tunable compressor 310.

[0034] As illustrated in FIGS. 3A and 3B, broadband laser source 301 emits a high-energy broadband pump pulse 302 having a frequency ω.sub.p and a pulse length on the order of about 100 to about 200 ps. In the exemplary case described herein pulse 302 is positively chirped, but the method and apparatus in accordance with the present invention can also process a negatively chirped pulse in a manner comparable to that described herein. In accordance with the present invention, a small portion of the energy from this pulse is split off and is then compressed and used to generate a white light continuum (WLC). WLC is a nonlinear process where a short pulse is propagated through a material (e.g., fused silica) where it undergoes strong self-phase modulation. This leads to significant enhancement of the pulse's spectral bandwidth, producing the wavelengths necessary to seed the amplifier system. The WLC process produces a seed pulse referred to herein as signal pulse 303, having a frequency ω.sub.s and a pulse length on the order of about 10 fs. See Jiun-Cheng Wang and Juen-Kai Wang, “Experimental and theoretical analysis of white-light seeded, collinear phase-matching, femtosecond optical parametric amplifiers,” J. Opt. Soc. Am. B 21, 45-56 (2004). Signal pulse 303 can be used as is, or it can be further amplified through OPA 304 to produce an amplified signal (idler) (s+i) pulse 305 having both the original signal pulse energy plus a small amount of idler energy. Signal (idler) pulse 305 is then input into tunable pulse stretcher 306 where is it selectively stretched and chirped to produce negatively chirped signal (idler) pulse 307 having a pulse length on the order of about 100 ps.

[0035] Positively chirped initial pump pulse 302 and negatively chirped signal (idler) pulse 307 are then directed into dual-chirp optical parametric amplifier (DC-OPA) 308 which contains the novel tunable stretcher/compressor described below, that can actively adjust allowing for operation at varying wavelengths. Both the signal and idler are amplified in DC-OPA 308 until the pump energy begins to deplete, with the signal and idler mixing to produce a high-energy, positively chirped idler (signal) pulse 309 having a frequency ω.sub.i=ω.sub.p−ω.sub.s and a pulse length on the order of about 100 ps. This idler pulse has a spectral bandwidth that is greater than that of both the pump and the signal, and further has a pulse-to-pulse stable carrier envelope phase (CEP) offset because both the pump and signal arise from a single pulse and therefore have a fixed phase difference.

[0036] Pulse 309 is then directed into tunable compressor 310, where it is compressed to produce the final short, compressed high-energy pulse 311 having a pulse length on the order of about 10 fs, while the residual energy from the pump pulse 302 and signal (idler) pulse 307 are output into energy dump 312.

[0037] As noted above, this short high-energy pulse is generated from the initial longer, lower-energy pulse through the use of a pair of novel tunable pulse stretcher/compressors in accordance with the present invention that can selectively operate at varying wavelengths.

[0038] The block schematic in FIG. 4 illustrates aspects of the novel tunable pulse stretcher/compressor in accordance with the present invention.

[0039] In the exemplary embodiment illustrated in FIG. 4, a tunable pulse/stretcher in accordance with the present invention incorporates a two-grating pulse stretcher, but other dispersion based stretchers such as prism-based or multi-grating stretcher could also be used.

[0040] Thus, as illustrated in FIG. 4, in an exemplary embodiment, two gratings 410 and 420 are used to spatially disperse and collimate the frequency content of an incoming broadband pulse 401. Thus, as illustrated in FIG. 4, incoming pulse 401 is reflected from grating 410 and is fanned out into a continuous spectrum having multiple frequency components such as frequency components 402a and 403a, which are reflected from grating 410 at different angles relative to the grating. Frequency components 402a and 403a reflect off grating 420 such that they travel in a parallel path, shown in FIG. 4 by lines 402b/403b. Grating 410 directs frequencies 402b/403b to retroreflector 430, which reflects the pulses back into grating 420 as frequency components 402c/403c. Grating 420 then reflects frequency components 402c/403c back to grating 410 as frequency components 402d/403c. Finally, grating 410 reflects frequency components 402d/403d onto the same path producing output pulse 404. The different path lengths traveled by the frequency components 402/403 results in a relative time delay between the frequency components. For a continuous spectrum, this results in an elongated pulse with a temporal frequency dependence, or “chirp.” By tuning the angle θ of grating 410 with respect to the initial pulse 401 and the separation (d) between gratings in each of gratings 410 and 420, the pulse stretcher/compressor can be selectively tuned to process input pulses having various wavelengths and to produce output pulses having predetermined chirp rates.

[0041] The use of such an adjustable pulse stretcher deviates from prior art CPA architectures, and its tunability is key for optimizing the DC-OPCPA process in accordance with the present invention. While prior art architectures often use a final pulse compressor, such a compressor architecture is typically reserved for compression of the final pulse because although it can compress high energy pulses, it can only introduce a negative chirp. A positive chirp architecture is more complicated, limiting tunability and pulse energy and is thus reserved as the stretcher to compliment the negative chirp architecture for the compressor. For DC-OPCPA, the required positive chirp for compression is produced by the nonlinear interaction.

[0042] A basic design of the stretcher is further illustrated by the block schematic shown in FIG. 5. In the stretcher, the grating angle θ, i.e., the angle of the grating relative to the incident pulse, is used to tune the wavelength of the stretched pulse, while the separation L between the grating and the curved mirror is used to control the chirp. The operational wavelength is changed by rotating the grating, while the chirp is changed by changing the separation between the curved mirror and the grating. In this arrangement both positive and negative chirps can be applied, however it is not suitable for high energy pulses and thus reserved for as a stretcher. The change in the distance L changes the relative path lengths traveled by different frequency components, allowing access to varying chirp rates. In this arrangement, an L less than the focal length of the mirror produces a positive chirp, while an L greater than the focal length produces a negative chirp.

[0043] The basic compressor design is shown in FIG. 6. The operational wavelength is changed by rotating the setup about the first grating. The chirp is adjusted by changing the distance between the first grating and the second grating/retroreflector pair. In this arrangement only negative chirps can be applied, however it can handle high energy pulses and so can be used for pulse compression. As with the pulse stretcher described above, the angle θ between the input pulse and the first grating can be tuned to produce a predetermined wavelength of the compressed pulse, while the separation L between the first grating and the second grating can be tuned to control the chirp, wherein an increase in L leads to an increase in the negative chirp. The whole setup is rotated about the center axis of the first grating to maintain alignment through the whole system.

EXAMPLES

[0044] 2-D axisymmetric simulations of pulse generation in accordance with the present invention were run with the MATLAB Sandia Nonlinear Optics (m1SNLO) code. The simulation used a positively chirped, 800 nm pump and a negatively chirped, 1500 nm signal in a 7 mm, type I beta barium borate (BBO) crystal to produce a positively chirped 1714 nm idler and amplified signal pulse. The pump parameters are 2.0 J, 200 ps with 18 THz of bandwidth (chirp parameter 0.09 THz/ps), while the signal parameters are 2.4mJ, 100 ps also with 18 THz of bandwidth (chirp parameter −0.18 THz/ps).

[0045] The results of this simulation are shown in FIGS. 7A and 7B.

[0046] FIG. 7A is a plot of the normalized spectral power density of the pump (701), signal (702), and idler (703). Depletion of the pump energy can be observed as a dip in the pump spectrum, and a slight increase in spectrum. While the signal maintains its spectrum, the resulting idler pulse's spectrum is larger than both the pump and the signal. From the simulation, the amplified signal is 247mJ, 100 ps, with 18 THz of bandwidth (chirp parameter −0.18 THz/ps), while the idler is 214 mJ, 100 ps, with 28 THz of bandwidth (chirp parameter 0.28 THz/ps). FIG. 7B shows the result when the chirp is removed from the idler pulse. The idler's pulse duration is reduced from 100 ps to 23fs, shorter than both the pump and signal when they are individually compressed.

[0047] Simulated runs were also made at other signal wavelengths. The resulting pulse energy for the signal and idler pulses is summarized in Table I below:

TABLE-US-00001 TABLE I Signal (nm) Idler (nm) Es (mJ) Ei (mJ) 1500 1714.3 247 214 1400 1866.6 207 153 1300 2080 139 85.6 1200 2400  50 23.8

[0048] The amplified, positive chirped idler is then directed into a simulated tunable pulse compressor in accordance with the present invention that can selectively compress the pulse to provide a predetermined pulse power and/or pulse duration. Through tuning of the compressor, the CEP of the passively locked idler can be actively tuned. See E. Treacy, “Optical pulse compression with diffraction gratings,” in IEEE Journal of Quantum Electronics, vol. 5, no. 9, pp. 454-458 (1969). Assuming an efficiency of about 70%, CEP pulses having a power of about 10 TW can be tunably produced from initial pulses having a wavelength of 1.6-2.6 μm. The same scheme can be applied using a negatively chirped idler pulse to produce an ˜10 TW tunable signal from initial pulses having a wavelength of about 1.1-1.6 μm. Either of these pulses can be frequency converted through either harmonic generation or OPA/OPCPA/DC-OPCPA to generate tunable pulses in the visible or mid-wave through long-wave infrared, respectively.

[0049] Advantages and New Features

[0050] This technique combines the benefits of both OPA and OPCPA technology with the addition of active CEP control and increased bandwidth that leads to potentially shorter, transform-limited pulses. In summary, DC-OPCPA produces high-energy, high-contrast pulses with increased bandwidth at high quantum efficiency allowing operation at high average powers. Combined with tunable stretcher/compressors, the system supports tunable, ultrashort pulses, with active CEP management of the idler pulse without the complication of a CEP controlled pump system. Such an approach is general and can be adapted to any chirped laser system operating at arbitrary wavelengths and repetition rate.

[0051] Alternatives

[0052] As discussed above, OPA and OPCPA are the only alternatives that do not rely on a lasing material. There are no known lasing materials that can possibly provide the tunability that this system provides. OPA is limited to lower intensity pulses while OPCPA is limited by seed pulses, reduced bandwidth, and does not provide CEP locking.

[0053] In cases where beam quality is a concern, the idler can be first produced in a pre-amp, spatially filtered and then used to seed a final amplifier.

[0054] There is no known technique that provides the flexibility of this approach for producing high power laser pulses.

[0055] The present disclosure describes various particular aspects, embodiments and features of an architecture and method for producing compressed, high-power laser pulses. Although particular embodiments, aspects, and features have been described and illustrated, one skilled in the art would readily appreciate that the invention described herein is not limited to only those embodiments, aspects, and features but also contemplates any and all modifications and alternative embodiments that are within the spirit and scope of the underlying invention described and claimed herein. The present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein, and all such modifications and alternative embodiments are deemed to be within the scope and spirit of the present disclosure.