Optimization of gain in traveling-wave optical parametric amplifiers by tuning the offset between pump- and signal-waist locations

Abstract

We present experimental demonstration and modeling of the optimization of a phase-sensitive optical parametric amplifier by tuning the relative position between the pump-and signal-beam waists along the propagation direction. At the optimum position, the pump beam focuses after the signal beam, and this departure from co-located waists increases with increasing pump power. Such optimization leads to more than 3 dB improvement in the measured de-amplification response of the amplifier. c 2017 Optical Society of America OCIS codes: 190.4410, 230.4320, 230.7020. Optical parametric amplifiers (OPAs) are ubiquitous these days both in pure research and in industrial applications. Hence, it is important to adjust these OPA systems for optimum matching with the input signal and detector modes. In squeezing experiments, a matched local-oscillator (LO) that properly extracts the distorted spatiotemporal squeezed mode can optimize the detected amount of squeezing [1]. In the same spirit, we report here on the performance optimization of an OPA-based system by adjusting the longitudinal offset between the waist locations of the pump and signal beams. We find that the optimal offset is significantly different from zero, contrary to conventional wisdom.In Fig. 1 we show the key elements of the experimental setup. Light from a telecom-band (1560 nm) continuouswave (CW) distributed-feedback (DFB) laser is fed into a pulse carver that outputs an 8 MHz train of 160 ps flattop pulses. These pulses are subsequently amplified by a series of erbium-doped fiber amplifiers (EDFAs) to reach peak powers of >2 kW. This pulsed vertically polarized light is then collimated into a free-space beam and focused into a 1-cm-long periodically-poled potassiumtitanyl-phosphate (PPKTP) crystal for second-harmonic generation (SHG). Conversion efficiencies of >65% produce a pulse train at 780 nm with >1 kW peak power that is then used to pump a 3-cm-long PPKTP-based OPA stage. A small signal to be amplified by the OPA is tapped from the main beam prior to the SHG stage, because after the SHG stage the spatiotemporal mode at 1560 nm is severely distorted due to the high conversion efficiency. Both crystals (Raicol Crystals Ltd.) are poled for type-0 interaction (i.e., all fields are copolarized). The OPA stage is operated as a degenerate OPA (DOPA). Each of the crystals is held at the phase-matching temperature with better than 10 mK stability. The pump and signal beams are combined on a dichroic mirror (DM) and are directed into the OPA crystal. The focusing lens of the pump beam is mounted on a micrometer-controlled translation stage, allowing for fine adjustment of the pump-waist location inside the crystal along the propagation direction. A tap is placed before the crystal, which directs both beams to a spatial profiler (DataRay Inc., model BMS2-CM4-IGA) capable of measuring beam waists down to 1 µm with 0.1 µm accuracy and the focal-point positions with ±1 µm repeatability. The profiler is placed near the focal plane and is used to determine t...

Devices for quantum communication over mode-division-multiplexing systems

“…First, based on our previous studies of the nonlinear mode interactions in spatially-multimode optical parametric amplifiers in free space [6][7][8][9] and multimode waveguides [10], we have developed a scheme for a spatial-mode demultiplexer based on sum-frequency generation (SFG) in a two-mode PPLN waveguide [11,12]. We have experimentally demonstrated such a demultiplexer, in which, by adjusting the spatial profile of a 1560-nm pump wave, we could selectively upconvert either mode TM00, or mode TM01, or any superposition of these two modes of a 1540-nm signal to TM01 mode at 775 nm, for both classical [13,14] and single-photon-level [14,15] signals.…”