Overcoming Intra-Cavity Nonlinear Phase Limitations in Cavity-Enhanced Optical Parametric Chirped Pulse Amplification through Cavity-Locking

Aleem M. Siddiqui, Jeffrey Moses, Kyung-Han Hong, Franz X. Kärtner
2013 CLEO: 2013   unpublished
In cavity-enhanced OPCPA, nonlinear phase shifts imparted on the intracavity pump pulse limit pump power loading and degrade system performance. We show that cavity-locking offsets these effects, maintaining dramatic bandwidth extension and high conversion efficiency. OCIS codes: (190.4970) Parametric oscillators and amplifiers; (320.7110) Ultrafast nonlinear optics Optical parametric chirped-pulse amplification (OPCPA) has emerged as a means of providing intense few-cycle optical pulses for
more » ... tical pulses for applications including ultrafast time-resolved spectroscopy, and high-harmonic generation. The bell-shaped pump intensity profile and the time-varying wave-vector mismatch of the interacting pulses, however, result in a non-uniform small-signal gain limiting conversion efficiency and bandwidth. We have proposed cavityenhanced OPCPA (C-OPCPA) [1] as a method to overcome these limitations. In C-OPCPA, pump pulses are coherently combined in a low finesse enhancement cavity transparent to signal and idler containing an OPA crystal in which a signal is amplified (see Fig 1a) . If the pump has a narrow bandwidth (BW) and the seed is sufficiently chirped, the cavity passively shapes the intra-cavity pump profile to attain optimized gain profiles. Additionally, through impedance matching, conversion efficiencies and gain bandwidths beyond that of the single-pass OPCPA are achievable. Our previous numerical study, however, did not include cavity-locking dynamics or the strong nonlinear phase shifts due to n 2 generated in a PPLN crystal at high intracavity intensity. Here, employing a numerical model treating three-wave mixing in the presence of both realistic intensity-dependent nonlinear-phase shifts and cavity-loading, we find that cavity-locking can be used to offset nonlinear phase-shifts detrimental to operation of C-OPCPA, allowing octave-spanning gain while maintaining high conversion efficiency. C-OPCPA dynamics can be understood by considering the intracavity parametric conversion as a nonlinear, time-varying, intracavity loss, where each temporal coordinate is independent (when, for instance, the interacting pulses are long enough for dispersion to be neglected). Temporal coordinates with initially low conversion (i.e., nonlinear loss) experience greater enhancement, which increases conversion and compensates the initially low loss. In steady state, C-OPCPA therefore reshapes the interacting pump pulse according to the time varying enhancement: , (1) where T/R is the transmission/reflection coefficient of the output/input coupler, loss(t) captures linear loss as well as nonlinear loss via conversion, and δ(t) captures the cumulative roundtrip phase (linear and nonlinear). C-OPCPA relies on loading sufficient pump power to maintain high conversion. At low pump powers when nonlinear phase effects in the enhancement cavity are negligible, the pump power is fully loading at each temporal coordinate and is limited only by linear losses. However, at high powers when intracavity nonlinear conversion occurs, a phase profile develops on the intracavity pump arising from the parametric process and from self phase modulation (SPM). These phase effects can prevent cavity loading, limiting enhancement. Phase shifts from parametric amplification are relatively small since the they are proportional to the fractional pump depletion, which is low compared to the intracavity pump pulse, since low intracavity conversion per pass is its stationary point of operation and corresponds to high overall conversion efficiency. SPM, however, is particularly detrimental since the resulting phase shifts are strongest at high intracavity powers. Temporal coordinates with initially low conversion may never attain the full enhancement necessary to drive up the conversion due to intensity dependent, self-phase-modulation-induced phase shifts manifesting during the build up process that limit pump power loading. Thus, under the conditions of an interferometrically stable enhancement cavity, the relative strength of quadratic and cubic nonlinear effects, (i.e., d eff , and n 2 ), determines the ability of the cavity to load pump power and overcome limitations set by the gain profile in parametric amplifiers. This is illustrated by the blue and black curves in Fig 1b which show the intracavity pump profile, reflection, fractional conversion and intracavity pump phase for various simulation conditions. A 100-nm wide, 1.55-µm, 6ps signal with 2µW of power is amplified in a C-OPCPA system pumped with narrowband, 8-W, 6ps, 1.03 µm pulses. The cavity has a 10% output/input coupler and a 5-mm PPLN crystal. The laser repetition rates and cavity FSR are
doi:10.1364/cleo_at.2013.jw2a.32 fatcat:vg7igthf7jaxriwxsq3pepdqze