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Abstract
We describe a parametric-amplification–based front end for seeding high-energy Nd:glass laser systems. The front end delivers up to 200 mJ by parametric amplification in 2.5-ns flat-in-time pulses tunable over more than 15 nm. Spectral tunability over a range larger than what is typically achieved by laser media at similar energy levels is implemented to investigate cross-beam energy transfer in multibeam target experiments. The front-end operation is simulated to explain the amplified signal’s sensitivity to the input pump and signal. A large variety of amplified waveforms are generated by closed-loop pulse shaping. Various properties and limitations of this front end are discussed.
© 2017 Optical Society of America
1. Introduction
High-energy laser systems are used in a large range of physics experiments [1–4]. Nd:glass is currently the only solid-state technology with which kilojoules of energy can be produced in a single beam. Disk amplifiers with good spatial gain uniformity and wavefront can be produced at the required large aperture, e.g., ~38 cm square, for the National Ignition Facility (NIF), the Laser Mégajoule, and OMEGA EP [5–7]. The use of Nd:glass amplifiers for high-energy amplification restricts the operation wavelength to ~1.05 μm, a wavelength range at which there are not many other amplification media. A large number of beams are required to produce more than 1 MJ of energy for laser inertial confinement fusion. After frequency conversion to the UV, they either directly compress the target or are used to generate x rays that then compress the target [8,9]. When the high-energy UV beams cross, resonant processes lead to detrimental coupling that decreases the amount of energy actually coupled into the target [10,11]. A practical path to reduce cross-beam energy transfer (CBET) is to introduce a relative spectral detuning between laser beams that have significant spatial overlap close to the target [12–14]. This has been shown numerically and experimentally to reduce the energy loss caused by CBET and lead to more-efficient and more-uniform laser–matter interaction.
High-energy amplification has been demonstrated on the NIF at wavelengths offset from the gain peak by several nanometers [15,16]. Implementing a large detuning is difficult in the front end, where the high gain is generally synonymous with spectral gain narrowing. For example, the Nd:glass regenerative amplifier in the NIF preamplifier module has a fluorescence bandwidth of the order of 1.5 nm, which can be expanded to ~4 nm via intracavity gain equalization [16], a strategy originally demonstrated for chirped-pulse amplification in a Ti:sapphire regenerative amplifier [17]. Front-end laser systems with different Nd-doped glasses could be used to cover a broader spectral range, but this is not a very versatile solution. Ti:sapphire is a very broadband material that has been used for amplification around 1053 nm [18,19]. Regenerative amplifiers are required at that wavelength because of the low laser gain, typically leading to poor contrast and energy scalability concerns.
We demonstrate the generation of spectrally tunable nanosecond pulses with energies exceeding 100 mJ using fiber technology and a sequence of two parametric amplifiers pumped by a 5-Hz frequency-doubled Nd:YLF laser system. Spectral tunability over more than 15 nm around 1053 nm and closed-loop pulse shaping are demonstrated. Section 2 presents the architecture of the system, with a description of the signal generation, pump generation, and the parametric amplifiers. Section 3 presents simulations of the operation of the parametric amplifiers. Section 4 describes the general performance of the system and the closed-loop, pulse-shaping results. Section 5 discusses various technical aspects related to this source and its use.
2. System description
2.1 Overall system
The primary goal of these investigations is the generation of flat-in-time high-energy pulses at a wavelength tunable over 10 nm (1050.6 nm to 1060.2 nm). A similar seed source will be used for the OMEGA EP Nd:glass amplifiers [7]. After amplification, the pulses will be frequency tripled and used in combination with nontunable frequency-tripled pulses originating from the OMEGA Laser System [20]. The front end is composed of three distinct parts: signal generation, pump generation, and optical parametric amplifier (OPA) stages (Fig. 1). The pump laser and parametric stages are part of the Multi-Terawatt laser (MTW) [21] and are similar to those deployed on the OMEGA EP laser [22], where they are used for optical parametric chirped-pulse amplification (OPCPA).
Fig. 1 Front-end schematic showing signal and pump generation for the optical parametric amplifier (OPA) stages. AWG: arbitrary waveform generator; MZM: Mach–Zehnder modulator; LBO: lithium triborate; cw: continuous wave.
2.2 Signal-pulse generation
The signal pulse is generated at 300 Hz by direct temporal pulse shaping of a tunable monochromatic cw (continuous-wave) laser source. The source is an external cavity grating-tuned diode, which can provide more than 20 mW of average power between 1040 and 1070 nm (Newport Velocity laser [23]). Its linewidth is smaller than 200 kHz and its spectral-tuning resolution is 0.01 nm, which should be suitable for a wide range of applications. A small fraction of its output power is sent to a wavemeter that precisely measures wavelengths, while the remainder is gated by an acousto-optic modulator (AOM) to an ~100-ns pulse. The 200-MHz frequency of the acoustic wave induces a wavelength shift of 0.7 pm on the diffracted wave, but the resulting shift of the amplified pulse wavelength relative to the wavelength determined by the wavemeter is negligible for the planned investigations. The gated pulse is then amplified in a double-pass Yb-doped fiber amplifier. A 25-nm bandpass filter between the first and second passes reduces the amplified spontaneous emission around 1030 nm, where the laser gain is much higher. The amplified pulse is sent to a sequence of two LiNbO3 Mach–Zehnder modulators (MZM’s) integrated in a single structure. Each MZM biased for zero transmission provides an extinction approximately equal to 25 dB. The typical approach to driving these modulators is to drive one with a flat-in-time voltage pulse to gate the incoming signal and the other one with a time-varying voltage from an arbitrary waveform generator (AWG) to shape the incoming signal [24]. In these experiments, the two MZM’s were driven by two identical signals originating from a Tektronix 70000 AWG (50 GS/s) [25] after broadband amplification, splitting, and proper relative-timing adjustment. This is done to increase the achievable extinction ratio for pulse shaping, noting that the zero-transmission voltage of the AWG approximately corresponds to 25 dB and 50 dB when driving one or two MZM’s, respectively. The shaped pulse is then amplified in an Yb-doped amplifier similar to the previously described amplifier.
The seed pulse is transported via fiber to the launch site, where the beam exiting the fiber is first collimated by a commercial collimator to a fixed ~2.5-mm-diam beam. A zoom then precisely collimates the beam and sets its size to match the size of the broadband seed used for OPCPA operation. The beam propagates in a Faraday isolator that prevents retroreflections of the amplified beam from disrupting the front-end operation and damaging the launch fiber or fiber front end.
2.3 Pump-pulse generation
The pump laser is based on a fiber front end followed by amplification in a Nd:YLF regenerative amplifier and Nd:YLF ring laser [26]. The fiber front end temporally shapes a monochromatic seed source at 1053 nm with a sequence of two MZM’s: one driven by a rectangular 2.5-ns pulse, the other driven by a Kentech Instruments AWG (10 GS/s) [27]. Pulse shaping precompensates for square pulse distortion in subsequent amplifiers. The shaped pulse is amplified by an Yb-doped fiber amplifier and then amplified at 5 Hz in a diode-pumped Nd:YLF regenerative amplifier to a few millijoules. The Gaussian output beam is then apodized to a square high-order super-Gaussian profile by a binary pixelated apodizer. This defines a reference plane that is re-imaged at each round-trip in the ring laser’s cavity. The cavity contains two flash-lamp–pumped Nd:YLF heads operating at 5 Hz and a Pockels cell that keeps the optical pulse in the cavity for four round-trips. After the fourth round-trip, the beam is ejected and re-imaged onto a lithium triborate (LBO) crystal for second-harmonic generation (SHG) to 526.5 nm. The SHG energy variation is 0.5% rms. The SHG beam is split into two beams that are independently imaged and down-collimated to the preamplifer (beam size: 2.5 mm) and power-amplifier (beam size: 5 mm) parametric stages. These beams are high-order square super-Gaussian beams defined by the apodizer at the input reference plane of the ring laser.
2.4 Parametric amplifiers
The two parametric stages are based on Type-I phase matching in LBO, which was originally chosen for the OPCPA front end because of its relatively large angular acceptance. The preamplifier is composed of two crystals in a walk-off compensating configuration, with a total length of 66 mm. The power amplifier has one 16.5-mm LBO crystal. The two stages are operated in a slightly noncollinear configuration (external pump-signal angle set to 8 mrad) to facilitate idler removal. A two-lens imaging system between the preamplifer and power amplifier provides the necessary magnification to match the size of the amplified signal beam from the preamplifier to the power-amplifier pump beam size. Optical paths are precisely matched for good temporal overlap between the amplified signal pulse from the preamplifer and the power-amplifier pump pulse. A half-wave plate and polarizer on the pump beam of each amplifier allows one to set the mean energy levels independently, but relative variations of the pump energy or pulse shape, including those induced by the front-end pulse-shaping system, lead to identical relative variations in each amplifier at a given operating point.
3. Simulations
The OPA system was simulated to understand its overall performance and behavior. The nonlinear equations describing the propagation of pump, signal, and idler in each crystal have been numerically solved using the fourth-order Runge–Kutta method. The simulations take into account only the longitudinal propagation direction in the crystals, not the two transverse coordinates and one temporal coordinate, because the spatial and temporal walk-offs are not significant relative to the spatial extent and temporal duration of the waves, respectively. In these conditions, the waves are represented by the scalar values of their electric field and corresponding intensity. The maximal input intensities of the pump (IP,max = 0.6 GW/cm2) and signal (IS,max = 5 W/cm2) used in the simulations are representative of what is typically used on this system.
The spectral acceptance of the preamplifier at its nominal pump intensity is shown in Fig. 2(a) in unsaturated conditions, where the pump is not depleted, resulting in a gain of 3 × 108 at 1053 nm, and in saturated conditions, where the input signal intensity is sufficient to saturate the pump, resulting in a gain of 6 × 107 at 1053 nm. The bandwidth at half maximum is 80 nm and 102 nm for unsaturated and saturated operation, respectively. With signal and pump intensities set to IS,max and IP,max, respectively, the preamplifier yields amplification to intensities higher than 90% of what is observed at 1053 nm over 80 nm. The power amplifier, which relies on a much shorter length of LBO, has much broader spectral acceptance and therefore does not restrict the operation bandwidth. The parametric stages, as currently configured, therefore allow for efficient operation over a large bandwidth that easily surpasses Nd:doped materials.
Fig. 2 (a) Simulated gain of the preamplifier, normalized to the gain at 1053 nm, as a function of the seed wavelength. (b) Output preamplifier signal intensity versus pump intensity at three wavelengths, with both intensities normalized to their values at saturation for the central wavelength. (c) Output power-amplifier signal intensity versus pump intensity at three wavelengths, with pump intensity normalized to IP,max and signal intensity normalized to the value reached at 1053 nm at IP,max.
The typical behavior of the preamplifier as a function of pump intensity, for a fixed signal intensity and three wavelengths, is shown in Fig. 2(b). The amplifier reaches saturation for practically the same pump intensity (0.6 GW/cm2), allowing for stable operation without the need for a wavelength-specific pump set point. Assuming preamplifier operation at the pump set point leading to saturation at 1053 nm, the output preamplifier intensity is used at the input of the power amplifier after scaling by 0.125 to take into account the magnification between the two amplification stages, losses, and nonideal operation of the preamplifier that typically leads to lower-than-simulated output energy. Operation of the power amplifier is essentially identical at these three wavelengths [Fig. 2(c)], again leading to similar amplified intensities at the output of the system. This is of practical importance since it allows one to operate the amplifier over a wide range of input wavelengths without modifying the pumping conditions of the two amplification stages, provided that the seed pulse intensity is maintained to its nominal value over that range.
Figure 3 shows the simulated behavior of the signal at 1053 nm amplified by the combined preamplifier and power amplifier. For the plot in Fig. 3(a), the horizontal axis corresponds to scaling the signal intensity at the input of the preamplifier between a very low value, for which all amplifiers are unsaturated, and IS,max. The vertical axis corresponds to scaling the pump intensity in the two amplifiers between zero and IP,max. The intensity in the two amplifiers is simultaneously modified to show the effect of pump intensity variations on the amplified signal intensity, in particular in terms of noise and pulse-shaping sensitivity. The signal is not amplified to a significant intensity until the pump reaches approximately 60% of IP,max in the two amplifiers. At the nominal pump intensity, the signal is amplified to its maximal intensity for a large range of input signal intensities between 20% and 100% of IS,max, showing that the front end is not very sensitive to the input signal intensity. Figures 3(b) and 3(c) represent lineouts of Fig. 3(a) going through (IS,max, IP,max). The derivatives and , which represent the relative variations of the output signal with respect to relative variations of the input pump and input signal, respectively, are plotted on these figures. These plots highlight the expected properties of the amplifiers for pulse shaping of the amplified signal by controlling either the signal or the pump:
Fig. 3 (a) Normalized signal intensity at the power amplifier output as a function of the normalized input signal and pump intensities. (b) Signal intensity at the power amplifier output as a function of the input signal intensity (blue curve) and corresponding sensitivity ΔS (red curve). (c) Signal intensity at the power amplifier output as a function of the input pump intensity (blue curve) and corresponding sensitivity ΔP (red curve).
- • The amplified signal does not strongly depend on the input signal intensity. The sensitivity ΔS is always smaller than 1, meaning that relative variations of the amplified signal are smaller than the relative variations of the input signal. The sensitivity is approximately zero for values of the input signal intensity higher than 20% of IS,max, indicating that there is no practical control of the amplified signal intensity. At a lower input signal intensity, i.e., below 20% of its nominal value, the amplified signal intensity can be controlled by modulating the input signal intensity, indicating that the pulse shape of the amplified signal can be controlled by the signal-generation system, in particular the MZM’s driven by the AWG. Since the parametric stages can be operated at full pump intensity without a seed and have an output that is limited by photon-energy conservation laws, signal shaping is unlikely to lead to optical damage. One drawback of signal pulse shaping is that operating the pump at nominal intensity with a lower seed level leads to higher absolute levels of parametric fluorescence in time slots where the amplifier is not saturated.
- • The amplified signal strongly depends on the input pump’s intensity. The sensitivity ΔP takes large values, typically of the order of 5, indicating that the pump variations are magnified in the amplified signal. This high sensitivity shows that pulse shaping of the output signal by controlling the pump is subject to tighter control requirement. Shaping only the pump allows in principle for a very large dynamic range. The pump intensity can be set from a low value approaching zero, where the signal is not amplified, to its nominal value, and the dynamic range of the output signal is in principle set by the amplification gain of the amplifiers. Because the input signal and parametric fluorescence experience approximately the same gain in the amplifiers, this approach to pulse shaping leads to lower relative parametric-fluorescence levels. Temporally modulating the pump pulse is practically difficult because the high-energy pump amplifiers might require different set points for different input pulse shapes, for example, to prevent optical damage.
Pulse shaping of the input signal was chosen for this work, but shaping the pump pulse or shaping both the signal and pump pulses is not fundamentally precluded.
4. Experimental results
4.1 Energy, wavelength tunability, and beam profile
The preamplifier and power amplifier have been run at a constant pump energy—140 mJ and 500 mJ, respectively—with the tunable monochromatic cw laser at constant power (20 mW) and fiber amplifiers operated at constant pump current. With flat-in-time input signal and pump pulses, the amplified, approximately flat-in-time 2.5-ns pulse reaches an energy of 200 mJ at 1051.5 nm, and has more than 100 mJ over 17 nm [Fig. 4(a)]. The typical energy fluctuation is 1% rms (root mean square). Examples of amplified beam profiles are shown in Fig. 4(b). Although the input signal beam is Gaussian, the amplified beam is defined by the pump’s super-Gaussian beam profile.
Fig. 4 (a) Measured energy at the output of the power amplifier versus seed wavelength, at constant pump and seed energies. (b) Beam near field at four different wavelengths.
The parametric fluorescence generated by the amplifiers, measured by a spectrometer when the amplifiers are unseeded and pumped at full energy, extends over ~100 nm, indicating that spectral acceptance is not the cause of the observed bandwidth limitation. The output energy of the fiber front end decreases by a factor of 5 between 1051.5 nm, where it reaches its maximal value, and 1060 nm, and by a factor of 50 between 1051.5 nm and 1070 nm, because of the gain slope of the Yb-doped amplifiers. This reduction explains the lower amplified energy observed at longer wavelengths. The lower-wavelength limitation is caused by the spectral filters in the fiber front end generating the signal, which induce a sharp cutoff below 1050 nm. The demonstrated performance meets the tunability requirement (1050.6 nm to 1060.2 nm) for the intended application, and is approximately four times the demonstrated bandwidth of a Nd:glass regenerative amplifier with intracavity gain equalization [16]. Operation at high output energy over more than the 17-nm bandwidth demonstrated here could be achieved by hardware modifications and changes in operational procedures. Broader filters or tunable filters could be used in the fiber amplifiers to allow for signal propagation over a larger range of wavelengths. The seed laser output power and the currents for the fiber-amplifier pump diodes could be increased to compensate for the lower gain at higher wavelengths. Finally, the design of Yb:doped fiber amplifiers can be optimized for broadband operation in particular wavelength ranges [28] in order to improve the performance over what has been obtained here with amplifiers designed for narrowband operation at 1053 nm.
4.2 Closed-loop pulse shaping
Pulse shaping of the amplified signal was performed with closed-loop optimization. The shape of the pump pulse was precompensated to give a 2.5-ns flat-in-time pulse after amplification for all these results. The amplified signal pulse shape was measured after the power amplifier with a photodiode and real-time oscilloscope and compared to the target pulse shape, after taking into account an arbitrary calibrated time delay between the AWG and oscilloscope time bases. Because the MZM’s are initially biased at extinction, any voltage increase on the modulators leads to a transmission increase. We therefore start with a constant null voltage out of the AWG; then we increase the voltage at each time sample by a quantity proportional to the normalized difference between the target and measured waveform, after normalization. The proportionality constant was empirically chosen to provide convergence in ~20 iterations. Considering an oscilloscope acquisition at 5 Hz and averaging, the process typically took a few minutes for each target shape and was manually stopped when a good match was observed.
Pulse-shaping results are shown in Figs. 5 and 6, where all displayed waveforms were obtained at full pump energy. Figures 5(a) and 5(b) demonstrate the generation of a 2.4-ns power ramp, for which the optical power is a linear function of time. For these measurements, a 12-GHz oscilloscope was used, and the plotted waveforms are averages over 20 acquisitions. AWG voltages were first optimized with the seed source operating at 1055 nm, resulting in good agreement with the target pulse shape. The seed source was then tuned to 1060.2 nm, which led to poorer agreement of the measured and target pulses. The accumulated optical phase in an electro-optic material is proportional to the propagation length and optical index, which is controlled by the applied voltage, and inversely proportional to the wavelength. No significant wavelength dependence of the induced time-varying transmission is expected for an ideal MZM with perfectly symmetric arms; therefore, the dependence observed in this experiment, which arises from a sequence of two integrated MZM’s, is attributed to fabrication-related nonideal characteristics of these devices. It could be addressed in future implementations by MZM’s with tighter fabrication tolerance. Closed-loop pulse shaping was run again at 1060.2 nm, to determine an optimal set of AWG voltages that led to better agreement with the target pulse shape. This demonstrates that closed-loop pulse shaping is an efficient and powerful tool for precise pulse shaping over a large wavelength range.
Fig. 5 [(a),(b)] Measured linear power ramp at 1055 nm (red curve) and 1060.2 nm (yellow curve) generated at the output of the power amplifier compared to the target pulse shape (blue). (a) AWG voltages optimized at 1055 nm were used at both wavelengths. (b) AWG voltages optimized specifically for each wavelength were used. [(c),(d)] Measured waveforms for flat-in-time and Gaussian target waveforms with various durations.
Fig. 6 Measured pulse shapes after closed-loop pulse shaping. On each plot, 100 acquired waveforms are represented by a color-coded histogram at each temporal point; their average is plotted with a thick black line, and the target pulse shape is indicated by solid black circles.
The source wavelength was then kept at 1053 nm, and various pulse shapes were chosen as target pulse shapes for the closed-loop pulse shaping. Figures 5(c) and 5(d) demonstrate the generation of flat-in-time and Gaussian pulse shapes with a full width at half maximum ranging from 0.1 ns to 2.4 ns. The plotted waveforms are single-shot acquisitions obtained with a 70-GHz Tektronix DPO77002SX oscilloscope [25] and a Discovery Semiconductors DSC10 photodiode [29], which results in a photodetection impulse response of the order of 13 ps. The measured rise time (10% to 90%) is 40 ps, and the shortest generated optical pulses have a duration of 100 ps. This duration is consistent with the finite bandwidth of the MZM modulator and the driving electronics (AWG and voltage amplifiers), which leads to a sub-10-GHz analog bandwidth. Other variables that limit the pulse-shaping bandwidth in this particular demonstration include the delay matching between the AWG voltages driving the two sequential MZM’s and the relative delay between the amplified signal from the preamplifier and pump pulse in the power amplifier. The Fourier transform of the measured waveforms reveals that the observed noise is broadband over the detection bandwidth and is consistent with the expected level of noise from the oscilloscope.
Finally, Fig. 6 demonstrates more-advanced pulse-shaping capabilities representative of what might be required for operation on high-energy laser systems, such as the generation of linear ramps with various slopes, sequences of 200-ps Gaussian pulses, and a tailored pulse shape composed of two short optical pulses followed by a nonlinear ramp.
5. Discussion
This subsection describes and discusses potentially limiting physical processes and possible paths for mitigation.
- • Generation of parametric fluorescence. The parametric amplifiers generate broad parametric fluorescence [Fig. 2(a)]. When the input signal is present at nominal intensity, the fluorescence photons experience the saturated gain in the amplifiers instead of the higher unsaturated gain; consequently, optimizing the temporal and spatial overlap of the signal and pump reduces the amount of parametric fluorescence. Fluorescence is generated in a wide range of angles [22] and can therefore be spatially filtered in the far field, but the fluorescence generated at angles close to the signal cannot be angularly filtered. A relatively narrowband tunable spectral filter operating on the amplified beam after either the preamplifier or power amplifier could be used. Amplification in relatively narrowband Nd:glass amplifiers will strongly restrict the spectral support of the parametric fluorescence. Parametric fluorescence might therefore not correspond to a significant fraction of the overall amplified signal after subsequent amplification, but it might have detrimental effects on front-end diagnostics that are not spectrally selective, for example, spatial and temporal measurements. This can be alleviated by spectral filtering in the diagnostics optical path.
- • Amplification of pump light at the fundamental frequency. Interference between pump light at the fundamental wavelength and the tunable seed has been observed when the latter is injected at lower-than-nominal energy. This interference was unambiguously traced back to these two optical sources by measuring the corresponding beating frequency as a function of the tunable-source wavelength. On this system, the unconverted pulse at 1053 nm is separated from the second-harmonic pulse after the SHG crystal by a Pellin Broca prism and two dichroic mirrors. The Pellin Broca prism provides a relative angular deviation between fundamental and the SHG beam of 22 mrad in the SHG crystal space, i.e., 62 mrad in the preamplifier stage space, which is significantly larger than the 8-mrad noncollinear angle chosen for noncollinear interaction in the preamplifier. The pump light at the fundamental wavelength reaching the preamplifier stage and being phase matched in it is therefore a result of scattering in optical components. Although the interference was not observed when the input signal was injected in the preamplifier at nominal energy level because the signal gets amplified to a much higher energy and also depletes the gain seen by the scattered light, it might be beneficial to decrease the amount of scattered light. This could be achieved by additional separation between fundamental and upconverted photons after the SHG stage, e.g., by dichroic mirrors, or angular filtering in the imaging system between the SHG crystal and preamplifier stage, noting that only the scattered light at the fundamental wavelength that is at ~8 mrad from the pump beam is phase matched in the preamplifier.
- • Parametric amplification of fiber front-end–amplified spontaneous emission (ASE). The fiber front end generates ASE over ~20 nm. The ASE and seed have identical spatial properties, and the fluorescence photons present in the parametric amplifiers during the pump pulse experience the large parametric gain. This was not found to be a limitation in this front end, but it could be remediated, if needed, by a tunable spectral filter either in the fiber front end or after the parametric stages.
- • Parasitic second-harmonic generation. Second-harmonic generation of the signal or idler in the parametric amplifiers is a wavelength-dependent loss mechanism that can affect the amplification efficiency, but this can be mitigated by choosing a suitable orientation of the optical waves and crystals [30,31].
- • Amplification in subsequent amplifiers. Amplifying the front-end output pulses to significantly higher energy with broadband parametric optical amplifiers is technically feasible, but providing the required pump pulse becomes increasingly complex and expensive. Amplification in Nd: glass amplifiers faces the challenge of limited bandwidth of these materials. Spectral gain variations are much less drastic than in the high-gain front end, and beamlines based on Nd:glasses are likely capable of amplifying pulses over tens of nanometers at kilojoule energies [15,32,33].
- • Conversion of frequency modulation to amplitude modulation (FM-to-AM conversion). High-energy laser systems require phase modulation to broaden the spectrum of the amplified optical pulses and prevent stimulated Brillouin scattering in optical components [34]. Phase modulation is also used for smoothing by spectral dispersion, a strategy for providing time-averaged smooth fluence distributions on targets [35]. FM-to-AM conversion occurs when the spectral modes of the phase-modulated pulse are modulated, e.g., by chromatic dispersion or spectrally varying gain [36,37]. This results in deleterious amplitude modulation, which can lead to optical damage or nonideal laser–target interaction. Cancelling the overall chromatic dispersion caused by propagation and providing a flat spectral gain over the phase-modulated pulse bandwidth (up to 200 GHz, i.e., 0.7 nm at 1053 nm, but most commonly below 100 GHz) alleviate FM-to-AM conversion, following, for example, what was done in Ref [24]. Noting that it is not necessary to perform these tasks over the full spectral tuning range of the source simultaneously, but only around the central wavelength of the pulse for a particular laser shot, this is not a difficult task, particularly with reconfigurable spectral-modulation systems [38].
6. Conclusions
The architecture and operation of a system producing temporally shaped spectrally tunable optical pulses around 1053 nm have been described. The fiber front end and parametric amplifiers make it possible to operate over more than 15 nm in a range suitable for seeding high-energy Nd:glass laser systems. Closed-loop pulse shaping has been used on the signal to generate various temporal pulse shapes representative of pulse shapes of interest for high-energy laser systems. Various trade-offs and limitations of the system have been described. A similar front end is being deployed on the OMEGA EP laser [7] to study CBET mitigation. Similar system architectures can be used in other experimental situations to generate multiple beams at different wavelengths for high-energy target physics or wavelength-tunable temporally shaped high-energy pulses, e.g., for remote sensing.
Disclaimer
This report was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
Funding
Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944; University of Rochester; New York State Energy Research and Development Authority.
Acknowledgments
This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.
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