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High-conversion-efficiency optical parametric chirped-pulse amplification system using spatiotemporally shaped pump pulses L. J. Waxer, V. Bagnoud, I. A. Begishev, M. J. Guardalben, J. Puth, and J. D. Zuegel University of Rochester, Laboratory for Laser Energetics, 250 East River Road, Rochester, New York 14623-1299 Received November 6, 2002 High-conversion-efficiency, high-stability optical parametric chirped-pulse amplification is demonstrated with a spatiotemporally shaped pump laser system. Broadband 5-mJ pulses are produced at a 5-Hz repetition rate with a pump-to-signal conversion efficiency of 29% and energy stability better than 2% rms. To our knowledge this is the highest conversion efficiency and stability achieved in an optical parametric chirped-pulse amplification system. © 2003 Optical Society of America OCIS codes: 140.7090, 190.4970, 140.3280.
Since its invention in the early 1990s,1 optical parametric chirped-pulse amplification (OPCPA) has become an attractive alternative to Ti:sapphire-based regenerative amplif iers for producing millijoule-level broadband laser pulses for injection of high-energy, short-pulse lasers that operate at a 1-mm wavelength.2 – 4 It has been noted, however, that the pump-to-signal conversion eff iciency of the OPCPA process can be poor,5 especially for systems pumped by commercial Q-switched lasers. One approach to overcoming this concern is using a pump laser that is better suited to the OPCPA process. Careful tailoring of the pump and seed spatiotemporal prof iles to uniformly saturate the pump can produce high optical parametric amplif ication (OPA) conversion eff iciency.6,7 In addition, it has been shown theoretically that saturation in the OPA process can produce an OPCPA output with better stability than that of the pump laser.8 Much of the previously reported OPCPA work used pump lasers that offered limited control over the pulse characteristics.3,4,9,10 In this Letter we experimentally demonstrate that improved control over the temporal and spatial prof iles of the pump laser significantly increases the efficiency of the OPCPA process as well as the OPCPA output stability. To optimize the OPCPA conversion eff iciency and stability, a three-dimensional modeling code was developed that predicts the output of an OPCPA system given arbitrary pump and seed input energies and spatiotemporal prof iles. Optimal conversion efficiency is obtained by homogeneously depleting the pump in space and time. In our setup this requires pump spatial and temporal prof iles that are highorder super-Gaussian in shape with FWHMs that are approximately equal to that of the Gaussian-shaped seed pulse. A diagram of the experimental OPCPA setup is shown in Fig. 1. The OPA pump laser, shown in the largest shaded box, is seeded by a diode-pumped, single-longitudinal-mode Nd:YLF master oscillator producing ⬃100-nJ pulses at 1053 nm with a 300-Hz repetition rate.11 The temporal pulse-shaping system, described in detail elsewhere,12 uses an aperture0146-9592/03/141245-03$15.00/0
coupled electronic signal to drive high-speed electro-optic modulators. The shape of the aperture determines the temporal shape of the pulse that is sliced out of the master-oscillator output pulse. This temporally shaped pulse is then amplif ied from ⬃50 pJ to 3 mJ in a diode-pumped Nd:YLF regenerative amplifier.13 The output of the regenerative amplif ier is spatially apodized with a serrated-tooth apodizer to produce a 5-mm-FWHM beam that has a f lat spatial prof ile at the output of the frequency-doubling crystal. The apodized pulse is then amplif ied from ⬃400 mJ to 40 mJ in a two-pass, f lash-lamp-pumped, 7-mm Nd:YLF amplifier operating at 5 Hz. The output of the amplif ier is relay imaged to a 3-mm b-barium borate (BBO) frequency-doubling crystal that produces 526.5-nm pulses with energies as high as 25 mJ. A Pellin–Broca prism separates the second-harmonic light from the unconverted 1.053-mm light so that the latter does not serve as a seed for the OPA. As shown in Fig. 2(a), at the output of the BBO frequency-doubling crystal the pump laser produces temporally f lat 1-ns pulses with a 10– 90 rise time of ⬃160 ps. Measured at the input to the OPA, the
Fig. 1. Diagram of the OPCPA experimental setup. The largest shaded box includes all the components of the OPA pump laser. FR, Faraday rotator; IP, image plane. © 2003 Optical Society of America
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Fig. 2. (a) A 1-ns FWHM square temporal prof ile of the OPA pump laser. (b) Spatial prof ile of the OPA pump laser. (c) Streak camera measurement of the spatiotemporal prof ile of the pump, showing little spatiotemporal coupling.
time-integrated spatial profile of the pump laser, shown in Fig. 2(b), has a tenth-order super-Gaussian spatial prof ile. Figure 2(c) shows the spatiotemporal prof ile of the pump pulse as measured by a streak camera, demonstrating that there is no spatiotemporal coupling in the OPA pump pulse, which can lead to a reduction in conversion eff iciency.4 The broadband seed laser for the OPCPA system is a mode-locked Nd:glass laser (Time-Bandwidth Products, Inc.) that produces pulses with a center wavelength of 1054 nm and a FWHM bandwidth of ⬃6 nm. The laser is synchronized to an external master clock, resulting in a jitter between the pump and the broadband seed and pump lasers that was measured to be less than 23 ps peak to valley. The 160-mW output of the seed laser is temporally stretched to ⬃700 ps in a double-passed Öffner-triplet-based pulse stretcher,14 providing ⬃800 pJ of seed energy at the OPA input. The seed energy stability was measured (over 2000 shots) to be better than 0.5% rms. Our simulations show that when the OPA process is highly saturated these f luctuations have a negligible effect on the OPA output stability. The OPA consists of two wedged (2±) 5 mm 3 5 mm clear-apeture lithium triborate (LBO) crystals (25 and 23 mm long) cut at 11.8± for type I angular phase matching. LBO was chosen as the OPA nonlinear medium because of its relatively high nonlinearity, high angular acceptance, and low walk-off between the pump beam (extraordinary wave) and the seed beam (ordinary wave). Dielectric antiref lective coatings were provided by the vendor (Conex Systems, Inc.). The temporally stretched seed and the relay-imaged pump beams have a FWHM of ⬃1.5 mm, which provides a maximum pump intensity of ⬃1.1 GW兾cm2 . The two LBO crystals are used in a walk-off compensated configuration15 with an ⬃4-mm air gap between them. This gap introduces a negligible amount of dephasing that does not affect the conversion process. The angle (measured in air) between the pump and
seed pulses is 0.5±, which allows the signal and idler beams to be separated after the OPA process. Figure 3 shows the conversion eff iciency of the two-crystal OPCPA system as a function of pump energy. At maximum conversion efficiency 17.7 mJ of pump energy produces 5.1 mJ of amplified signal energy, providing an overall gain of greater than 6 3 106 and a conversion eff iciency of 29%. Excellent agreement between modeling and experimental results is shown in Fig. 3. At the point of maximum conversion efficiency, the OPCPA output energy stability is 1.6% rms (measured over 100 shots) and is better than that of the pump, as shown in the inset table in Fig. 3. The spectrum of the input stretched seed pulse and the amplif ied signal pulse is shown in Fig. 4(a). The FWHM of the amplified signal spectrum is ⬃8 nm, centered at ⬃1055 nm. Whereas the spectrum of the unamplified seed pulse is roughly Gaussian in shape, the shape of the amplif ied spectrum is square because of saturation in the OPCPA process. The amplified output of the OPCPA system is compressed in a double-passed, single-grating compressor. The autocorrelation of the compressor output, as measured with a scanning autocorrelator, is shown in Fig. 4(b). Also shown is the intensity autocorrelation of the transform-limited pulse, which is estimated by calculating the Fourier transform of the spectrum measured at the output of the compressor. Using this calculated pulse, we estimate the temporal duration (FWHM) of the compressed, amplif ied signal to be 470 fs, which is approximately 1.3 times the Fourier transform limit. The wings in the experimental autocorrelation trace are also present in the autocorrelation of the stretched and recompressed unamplified seed. These wings dominate any differences in the temporal shapes of the compressed amplif ied and unamplif ied seed, and we believe that they are caused by uncompensated spectral phase in the stretcher –compressor combination. We measured the far field of our amplified signal beam at the focus of a 500-mm-focal-length lens with a 12-bit beam analyzer (Spiricon LBA 500PC and Cohu 4800 CCD camera). By comparing the FWHM of the Fourier transform of the measured near-f ield prof ile
Fig. 3. Conversion eff iciency as a function of pump energy. The solid curve shows model predictions, with squares representing experimentally measured values. Error bars show f luctuations in measured energies. The inset table shows pump and signal energies and stabilities at the 29% conversion eff iciency point.
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tem produces 5 mJ of amplified signal at a 5-Hz repetition rate with a pump-to-signal conversion eff iciency of 29%. To our knowledge this is the highest conversion efficiency ever achieved in an OPCPA system. In addition to high conversion efficiency, the system output is highly stable, with a rms stability of 1.6%, which is better than that of the pump laser and is a significant improvement on systems reported in the literature. This work was supported by the U.S. Department of Energy (DOE) Off ice of Inertial Conf inement Fusion under cooperative agreement DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority. The support of the DOE does not constitute an endorsement by the DOE of the views expressed in this Letter. L. Waxer’s e-mail address is
[email protected]. References
Fig. 4. (a) Spectrum of stretched seed pulse (dashed) and amplified signal pulse (solid). Clipping in the seed-pulse spectrum is due to the f inite extent of the stretcher optics. (b) Measured autocorrelation (diamonds) of the amplified signal shown along with an autocorrelation trace (solid curve) calculated from a fast Fourier transform of the spectrum of the amplified signal shown in (a).
with that of the measured far-f ield prof ile, we determined that the FWHM of the amplified signal beam is ⬃1.3 times the diffraction limit in the horizontal direction and ⬃1.1 times the diffraction-limited width in the vertical direction. These numbers match closely with those measured for the stretched seed pulse, indicating that the OPA did not introduce additional phase errors into the spatial prof ile of the amplif ied pulse. The maximum pump-to-signal conversion efficiency possible in a degenerate OPA such as the one presented here is 50%. In our experiment the conversion eff iciency was limited by a number of factors, including the finite slopes of the spatiotemporal prof ile of the pump pulse, walk-off between the seed and the pump, and noise in the pump spatiotemporal prof ile. Our simulations show that decreasing the rise time of the temporal prof ile to ⬃50 ps and increasing the order of the super-Gaussian spatial prof ile to 30 would increase the conversion efficiency of our system to approximately 40%. We have demonstrated an OPCPA system that uses a spatiotemporally shaped pump pulse to maximize the conversion efficiency of the OPCPA process. This sys-
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