Crystal-Configuration Considerations for Higher-Sensitivity Picosecond-Pulse SHG FROG

Peter Susnjar,Rick Trebino,Rok Petkovsek,Travis Jones

doi:10.1109/jqe.2020.2971761

Abstract

We experimentally compare a thick BIBO crystal, a walk-off compensating configuration of thin BBO crystals, and a thin PPLN crystal for the use as a nonlinear medium in SHG FROG, aiming for increased sensitivity required for complete pulse characterization of low-energy (below pJ) pulses a few picoseconds long at wavelengths near 1030 nm. We investigate the dependence of the spectral bandwidth of the frequency-doubled input ultrashort pulse and the conversion efficiency on focal-spot size in the nonlinear crystal. Both numerical and experimental results confirm an increase of the bandwidth with tighter focusing in a thick BIBO and a more-than-threefold increase of conversion efficiency compared to a thin BIBO crystal. A PPLN crystal is found to be the most efficient among the three, but very thin (<; 0.3 mm) crystals would be required to extend the measurement capabilities of SHG FROG to sub-ps pulses. A SHG FROG using a PPLN crystal for characterization of picosecond pulse is demonstrated with a sensitivity of 2 ×10 -3 (mW)2.

Highlights

M OST modern applications of ultrashort laser pulses require, or at least benefit from, detailed information about both its intensity and phase
We have investigated three possible SHG geometries and crystals – a thick birefringent crystal BIBO, a walk-off compensating configuration of BBO crystals, and quasi-phase matched (QPM) in PPLN, to be used in a SHG Frequency-resolved optical gating (FROG) with improved sensitivity for characterization of low energy pulses in the picosecond range
We have studied the dependence of the conversion bandwidth on focusing in a 10-mm-thick BIBO and confirmed that it can be significantly increased by the tight focusing

Summary

INTRODUCTION

M OST modern applications of ultrashort laser pulses require, or at least benefit from, detailed information about both its intensity and phase. Some of the most sensitive FROG setups reported so far include SHG in a chirped, aperiodically poled lithium niobate (APPLN) waveguide [9], [10], four wave mixing (FWM) in a 22 km long dispersion-shifted fiber [11] or in a semiconductor amplifier [12], and electro-optical amplitude modulation [13] They have all been demonstrated at telecom wavelengths (around 1550 nm) and all depend on technology not readily available at other wavelengths. Several other experimental studies have shown efficient frequency conversion over the entire bandwidth of femtosecond pulses using SHG in a thick nonlinear crystal, when very tight focusing is employed [14]–[17]. The first results of a SHG FROG with the thin PPLN crystal, the most efficient among the three compared, are shown in section V demonstrating its sensitivity by measuring a 20 fJ, 7.8 ps-long pulse

EXPERIMENTAL SETUP

CONVERSION EFFICIENCY OF SHG WITH ULTRASHORT PULSES

SPECTRAL BANDWIDTH OF UP-CONVERTED ULTRASHORT PULSE

TEST OF A HIGHLY SENSITIVE SHG FROG USING A PPLN CRYSTAL

Findings

CONCLUSION