We propose and demonstrate an OPCPA architecture emitting few-cycle pulses at 3070 nm and 1550 nm based on a high-energy femtosecond ytterbium-doped fiber amplifier pump. The short pump pulse duration allows direct seeding by a supercontinuum in the 1.4 – 1.7 µm signal range, generated in bulk YAG. It also allows a simplified dispersion management along the system and broad optical gain bandwidth. The dual output system delivers 20 µJ, 49 fs signal pulses at 1550 nm and 10 µJ, 72 fs idler pulses at 3070 nm. Power scaling limitations due to beam distortion in the last MgO:PPLN-based OPCPA stage are discussed and investigated.
© 2016 Optical Society of America
The development of high repetition rate few-cycle pulse sources in the mid-infrared (mid-IR) is currently of great interest for an ever growing number of applications: multidimensional molecular spectroscopy , high intensity physics experiments such as high harmonic generation in gases  or solids  and laser-induced electron diffraction . In strong field physics, the use of mid-IR driving wavelengths allows higher ponderomotive energies and high-harmonic generation energy cutoff. In all these applications, operation of the driver source at a high repetition rate / average power allows larger photon flux or better signal to noise ratio in subsequent experiments.
These applications have triggered the development of a number of such sources, based on optical parametric chirped pulse amplification (OPCPA) pumped by 1-10 picosecond bulk Yb:YAG- or Nd:YVO4-based laser sources that are capable of generating 100 µJ – 1 mJ pulses at repetition rates greater than 100 kHz [1,5–7]. These OPCPA systems are seeded with either erbium-doped fiber-based femtosecond sources [6,7], a difference-frequency generation process allowing passive carrier-envelope phase (CEP) stability  or an optical parametric oscillator . Although these solutions all lead to the generation of large optical spectra around either 1.5 µm or 3 µm, they involve complex and costly setups, and often exhibit non negligible spectral structure within the useful bandwidth.
The necessary tight synchronization between OPCPA pump and seed is obtained either through active electronic locking of independent laser systems , or through optical synchronization [1,5,7], where the pump seed is derived from the signal oscillator via a nonlinear optical interaction or vice versa. However, in this case, the use of a long optical amplifying chain to reach the final pump energy often requires active optical stabilization of the delay between pump and signal and / or precise control of the initial oscillator frequency .
A very elegant way to both decrease the cost and complexity of the OPCPA seed system and provide robust passive optical synchronization between pump and seed is to use a small fraction of the amplified pump laser energy to generate a supercontinuum (SC). It thereby provides broadband, smooth and compressible spectral content at the desired signal wavelength. This is routinely achieved in Ti:Sapphire pumped OPCPA systems , and has been extended to high-power OPCPA systems operating at 800 nm that are pumped by frequency-doubled Yb-based laser sources . The ability to use this attractive architecture depends on the available pump source, in particular the pump duration, and the SC generation mechanism. Indeed, the complex filamentation process at work in SC generation can prevent the generation of significant spectral content at the target seed wavelength: the damage threshold of the nonlinear material is reached before the spectrum extends to the desired wavelength. In the particular case of interest in this work, it has been observed that, starting from pulses at 1030 nm, the extension of the SC toward 1600 nm requires pulses shorter than 1 ps and loose focusing conditions .
Another research area that has seen tremendous progress in the past decade is high-energy Yb-doped femtosecond fiber amplifiers. Large-mode-area fibers, chirped pulse amplification architecture, and the recent implementation of coherent combining architectures  have allowed the design of compact systems delivering pulse energies in the 100 µJ - 10 mJ range at high repetition rates. And, compared to Yb:YAG high-power sources, the pulse duration is shorter, usually around 300 fs.
In this article, we describe an OPCPA architecture built around a state of the art Yb-doped fiber femtosecond pump source delivering 400 fs, 400 µJ pulses at 125 kHz repetition rate. The short pulse duration compared to previous Yb- or Nd-based systems results in a number of important advantages. First, it allows efficient seeding at 1550 nm using SC generation directly from the pump pulses, resulting in extremely robust, passive pump – signal synchronization. The short pump pulse duration also allows the use of few-mm lengths of bulk materials to provide stretching and compression for the signal and idler, which minimizes the accumulation of higher-order spectral phase. Finally, the shorter pump pulse duration increases the damage peak intensity, permitting the use of shorter nonlinear crystals to perform the amplification, which increases the spectral bandwidth of the parametric process. The OPCPA stages are all operated in collinear geometry, allowing the use of both signal and idler without the introduction of angular chirp on the latter. These points result in the dual generation of 49 fs, 20 µJ signal pulses at 1550 nm and 72 fs, 10 µJ idler pulses at 3070 nm from a simple and robust setup, with the added benefit of inherent CEP stability of the idler pulses. We first describe in details the source architecture and associated experimental results, and then discuss power scaling issues that have been encountered in magnesium oxide-doped periodically poled lithium niobate (MgO:PPLN) crystals used for the parametric process.
2. Experimental setup and results
The experimental setup is depicted in Fig. 1. It starts with an industrial-grade femtosecond ytterbium-doped fiber amplifier system (Tangerine, Amplitude Systemes) which generates 400 µJ, 400 fs pulses at a repetition rate of 125 kHz, corresponding to an average power of 50 W. The pump pulse spectrum and autocorrelation are shown in Fig. 2, along with the beam profile. Operation of the ytterbium-doped fiber amplifier in nonlinear regime results in a low pedestal in the autocorrelation trace.
A small fraction (approximately 5 µJ) of the overall pump pulse energy is used to generate the signal seed through SC generation. Beam diameters throughout this article are given at 1/e2. The 3.1 mm diameter collimated beam is focused in a 10 mm-long YAG crystal with a 150 mm lens. The input intensity is set to be just below the multifilamentation regime for which the spectrum became modulated due to interferences. The power contained in the spectral range from 1.4 µm to 1.7 µm in the SC beam is measured to be 400 µW, corresponding to an energy of 3.2 nJ. This spectral content is smooth, stable, and spatially homogeneous, as studied in details in . It is important to note that if the pump pulse is stretched to 700 fs by detuning the pump laser compressor with residual positive or negative dispersion, SC generation still occurs in the visible, but the spectral content generated at 1550 nm dramatically drops and becomes negligible until double filamentation occurs. The short pump pulse duration is therefore a key characteristic to allow this seed generation process.
The SC seed is then amplified in three stages, all based on MgO:PPLN with anti-reflection (AR) coatings for pump, signal and idler wavelength on both sides. The first stage is located at the output of SC generation, and consists in a 1 mm-long crystal with a poling period of 30.16 µm, operated at 150°C. The pulse pump energy is 10 µJ, corresponding to 1.25 W average power. Signal and pump beams are collinearly combined and separated using dichroic mirrors. The signal and pump beam dimensions are 180 µm × 180 µm and 200 µm × 215 µm respectively. The pump intensity is roughly estimated to be 160 GW/cm2 assuming Gaussian temporal and spatial profiles. The spectrum of the signal at the output of the first OPA stage is shown in Fig. 3 (green trace). The gain bandwidth of this first stage generates a very smooth output spectrum with a full width at half maximum (FWHM) of 115 nm. The signal energy at the output is 130 nJ (corresponding to 16 mW of average power). The autocorrelation FWHM of the signal at this point is 70 fs, corresponding to a pulse duration of 50 fs assuming a Gaussian profile. This signal pulse is stretched with a 2 mm-thick AR-coated silicon window to a measured duration of 280 fs, to match the pump pulse duration and allow better energy extraction in further stages. A dichroic mirror is used to remove the idler and the signal is fed to the second stage.
This second stage is built around a 750 µm long PPLN crystal with a 29.62 µm poling period and operating at 150 °C. It is pumped with an energy of 58 µJ (7.3 W average power). The PPLN poling period is here chosen to maximize the spectrum bandwidth. The signal and pump beam parameters inside the nonlinear crystal are 860 µm × 950 µm and 660 µm × 930 µm respectively. This corresponds to a pump intensity of 58 GW/cm2. The signal spectrum at the output of the second stage is plotted on Fig. 3 (blue trace). At the output of this stage, the signal energy is 1.8 µJ. An extra 1 mm-long silicon window is used to provide additional stretching to the signal to reach a tradeoff between energy extraction and bandwidth in the third stage. The pulse duration at the output of this window is measured to be 320 fs, assuming a Gaussian pulse shape.
The final stage uses a 1 mm-long crystal with a poling period of 29.62 µm operated at 185°C. Signal and pump beam dimensions are 1350 µm × 1410 µm and 1300 µm × 1440 µm respectively. This leads to a maximum pump intensity of 115 GW/cm2 for an energy of 327 µJ, corresponding to 40 W of average power. Optical gain in the last stage has to be limited to avoid beam distortions as will be discussed in the next part. The signal spectrum is plotted in Fig. 3 (red trace) and exhibits a FWHM of 110 nm, and a Fourier-transform limited (FTL) duration of 40 fs. At the output of the third stage, dichroic mirrors are used to separate the signal and idler beams and to remove the pump.
The signal is compressed using an AR-coated 110 mm-long block of fused silica. A telescope is implemented before the compressor in order to increase the beam diameter to 4 mm to avoid self-focusing in fused silica. Figure 4 shows the second-harmonic frequency resolved optical gating (SHG-FROG) temporal characterization of the compressed signal. The pulse duration is compressed down to 49 fs, slightly longer than the FTL duration (40 fs), due to a small amount of residual third-order spectral phase accumulated in the dispersive sequence throughout the system. The beam quality is good, with an M2 value of 1.2 and 1.1 in X and Y directions respectively. The output compressed signal energy is 20 µJ with a compression efficiency of 92%.
On the second output port, the idler located at a central wavelength of 3070 nm exhibits an inverted chirp sign compared to the signal, and is therefore compressed with a 12 mm-long window of AR-coated silicon plate. A telescope is implemented before the compressor in order to increase the beam dimension to 5 mm to avoid nonlinear effects in bulk silicon. The compression efficiency is 90%. The measured spectra and retrieved SHG-FROG temporal profiles together with spectral phases of compressed idler are plotted in Fig. 5 for two different energies. For 5.5 µJ [Figs. 5(a) and 5(b)], the spectral FWHM is 260 nm, corresponding to a FTL duration of 43 fs, while the measured compressed pulse duration is 55 fs. This corresponds to 5.5 optical cycles at 3070 nm. As previously, a small amount of non-compensated residual third-order spectral phase remains in these conditions. For 10 µJ idler pulses [Figs. 5(c) and 5(d)], the spectral FWHM increases slightly to 290 nm, corresponding in this case to a FTL duration of 35 fs, and the measured compressed pulse duration is 72 fs. The same silicon window length is used in both cases to obtain the shortest pulses. An additional nonlinear high-order spectral phase is clearly responsible for pulse duration increase. It might result from self-phase modulation in the Si compressor or nonlinear effects in the PPLN crystal. Idler output long-term power stability is monitored at full power (a frame every 2 s) and shows a stability better than 1.5% RMS over 180 min as shown in Fig. 6.
3. Power limitations: discussion
We now discuss power scaling issues that have been encountered in the last OPCPA stage. Although beam distortions issues arising at high power have been reported in MgO:PPLN , no clear physical explanation has been provided yet. Here, we describe a few experimental observations that contribute to further understand this phenomenon.
A first separate experiment is conducted by simply focusing a pump beam down to a 400 µm diameter in a 1 mm-long MgO:PPLN crystal with two poling periods: 29.5 µm and 31.6 µm, the latter being close to a multiple-period quasi phase matching for second-harmonic generation (SHG) of the pump wavelength. The pump beam characteristics are as follows: 400 fs pulse duration, 500 kHz repetition rate, 1030 nm wavelength, average power up to 10 W (up to 20 µJ energy per pulse). The beam diameter at 1030 nm in the far field is recorded as a function of input average power at 1030 nm and SHG power at the output at 515 nm. The corresponding data is shown in Fig. 7. For low SHG generation, the beam diameter only starts to increase at 8 W of input average power, and this increase is very limited. However, in the case of high parasitic SHG generation, the beam size starts increasing at 3 W input power, and is almost doubled at 10 W. The same data is also plotted as a function of parasitic SHG power, clearly showing that the beam size increases at lower SHG power when the fundamental power is higher.
To interpret this data set, we make the following assumptions. First, the beam size increase is due to a thermal lens proportional to the absorbed power, resulting in a beam size linearly dependent on the thermal dioptric power. Second, the absorbed power is determined by linear residual absorption at 515 nm, 1030 nm, and green-induced infrared absorption (GRIIRA). We can then fit the experimental data to the following expression:Eq. (1) for the two poling periods considered. The third equation corresponds to the ratio between absorptions at 1030 nm and 515 nm. Indeed, although measured residual absorption values can widely differ in the literature, there is usually an order of magnitude difference between these two parameters, with α2ω / αω ≈10. Using this allows to determine the value for the GRIIRA coefficient αG = 0.025 cm/MW. This is consistent with literature data: the coefficient is a factor of two below that reported in  for undoped lithium niobate, as expected for 5% doped MgO:PPLN . The GRIIRA effect therefore seems to be preponderant for the beam size increase in this experiment. However, this analysis does not explain the ring-shaped beam distortions that appears on the signal beam when operating the OPCPA at high power when no additional stretching between the second and the third stages is used. In particular, their appearance do not follow the simple scaling rules derived here. In the next paragraph, we report investigations on the beam distortion performed directly on the third OPCPA stage before adding the 1 mm-long silicon window to provide additional stretching.
The first observation is made by operating the third OPCPA stage at a power where the signal beam clearly exhibits multiple ring pattern as shown in the Fig. 8 (b). This ring pattern is visible on the signal, but also on the parasitic green SHG beam and red sum-frequency beam. A simple but crucial information is brought by temporally detuning the signal and pump beams: the ring structure disappears on the signal [Fig. 8(a)], green, and red beams. This points to a physical origin related to the presence of the amplified signal and/or idler in the crystal, clearly different from the GRIIRA beam distortions observed above. As a second experiment, we vary the repetition rate of the pump laser using an external acousto-optic modulator, and, for each value of the repetition rate, record the pump average power (equivalently the pump energy), idler average power [Fig. 9(a)], and idler pulse energy [Fig. 9(b)] at which the ring structure appears. The green line represents the available pump energy limit, meaning that any point on this line corresponds to a case where the full pump energy could be used without observing any spatial degradation. No limitations are indeed visible under 60 kHz in this configuration. From the power threshold observed at full repetition rate (125 kHz), we draw two lines: the blue dashed line corresponds to a limiting phenomenon only related to average power (such as linear absorption thermal lensing), while the orange dashed line represents a limiting phenomenon only related to peak power (such as Kerr lens). It is clear that the observed power limitation cannot be explained by these limiting cases, it clearly shows a dependence on both quantities. A previous study of pump beam distortions with ring structures in a similar experiment is available . The authors’ conclusion is that the pump beam degradation is average power dependent and they listed possible physical effects responsible for the beam degradation, such as GRIIRA, linear absorption, photorefractive effect and pyroelectric effects. In this work, we show that there is a peak power dependence and that the beam degradation is related to the presence of signal and/or idler at large intensities inside the PPLN, not to the pump. A nonlinear absorption phenomenon would explain this behavior, although no clear mechanism is identified yet. Although GRIIRA would also explain such a dependence, the disappearance of the effect when pump and signal are separated in time rules out this mechanism.
We have proposed and implemented a simplified OPCPA architecture to generate few-cycle high-power mid-IR pulses. It relies on a femtosecond pump source allowing efficient and coherent SC generation in the 1.4 – 1.7 µm signal range, and bulk stretching – compression solution minimizing residual high-order spectral phase. This alleviates the need for a temporal pulse shaping element. This architecture allows the generation of a dual output consisting of 20 µJ, 49 fs pulses at 1550 nm and 10 µJ, 72 fs pulses at 3070 nm at a repetition rate of 125 kHz. Although CEP stability has not been characterized yet, it is related to the stability of the interferometer formed by the signal and pump paths between SC generation and last OPA stage. Other systems relying on the same difference frequency generation mechanism have shown excellent CEP stability properties, as reviewed in . Additional elements are provided to sort out the phenomenon that limits power scaling in MgO:PPLN crystals. The use of other nonlinear crystals such as KTA as a low gain last stage seems a promising way to further scale the output energy, involving a tradeoff with the output pulse bandwidth.
Fonds Unique Interministériel FUI project STAR (501100003391); LABEX PALM project MIR OPCPA.
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