Abstract
Supercontinuum generation (SCG) has emerged as a useful tool in a multitude of applications, such as gas sensing [1] and frequency metrology [2]. Methods for SCG have been developed in the past two decades by the use of photonic crystal fibers [3], microstructured fibers [4] and multiple-plate continuum (MPC) [5]. Among these methods, MPC is receiving increasing attention in the realm of energetic few-cycle pulse generation because of the easier optical alignment, lower cost, and free from beam-pointing induced spectral distortion. MPC-based single-cycle pulses have been generated at 800 nm and 1030 nm [6,7]. In this work, we investigate MPC and dispersion compensation (DC) at 1.55 μm (where material dispersion becomes predominantly anomalous) by using a home-built optical parametric chirp pulse amplifier (OPCPA) source. To the best of our knowledge, it is the first time that 1.55 μm MPC pulse is compressed close to its transform limit. Our setup has three main parts [Fig. 1(a)]. The first part contains a difference frequency generator (DFG) and an optical parametric amplifier (OPA-1) pumped by a common Yb:KGW amplifier (Light Conversion, Pharos), producing 1 kHz, 44 fs, 10 μJ, carrier-envelope phase (CEP) stable pulse at 1.55 μm. The second part is a two-stage OPCPA pumped by a 1 kHz, 85 ps, 30 mJ, Nd:YAG amplifier (EKSPLA, APL 2003) at 1064 nm and sandwiched between a Martinez (grating-lens pair) pulse stretcher and a grating-pair pulse compressor, delivering 1 kHz, 80 fs, 3 mJ pulses at 1.55 μm. Finally, a nonlinear compressor stage is in charge of MPC and DC. Limited by the space and damage threshold of the spatial light modulator (SLM), we take 0.7 mJ from OPCPA for proof-of-concept test. We use 0.5 m focusing length (focus spot size is 0.19 mm in diameter) and 9 pieces of 200μm-thick quartz plates in MPC, generating spectrum from 1.25 μm to 1.9 μm at -20 dB level [Fig. 1(b)]. Total transmission after removing the conical emission rings is 53.6%. A pulse shaper consisting of a 4-F system and a liquid crystal-based SLM is used for all-order DC. In our polarization-gating cross-correlation FROG (PG-XFROG) measurement [Figs 1(c) and 1(d)], the pulse width is compressed from 80 fs to 20 fs (3.8 cycles). Energy up-scaling can be realized by using looser focusing and chirp mirrors. Such intense light source paves the way to generation of soft x-ray laser at water window for label-free bio-imaging. This work is supported by Ministry of Science and Technology (MOST) at Taiwan under grants 104-2112-M-007-012-MY3, 105-2112-M-007-030-MY3 and 107-2112-M-007-006. Authors acknowledge Dr. C. H. Lu and Prof. A. H. Kung for discussion of MPC.