High-resolution spectral-shearing interferometry

Abstract

We present spectral-shearing interferometry with a high group-delay resolution that is defined as a ratio of the maximum temporal window and the shortest pulse duration. This technique allows the resolution of 10 4 .The spectral-shearing interferometry (SSI) directly measures a frequency difference of Fourier-phase i. e. groupdelay (GD). A GD resolution of SSI is defined as a ratio of a temporal window for the identified shear-frequency and the shortest pulse-duration. SIPDER [1] uses a chirped pulse for quasi-cw, so that a temporal window is limited by chirp rate.In this presentation, an implementation of SSI for a high group-delay resolution is proposed. Our method gives a shear frequency by frequency mixing with a signal pulse and a pair of monochromatic pulses that have different colors [2]. This technique allows measuring a huge group-delay with any decay of short-time information, so that we can apply the technique to reconstruction of waveform for a pre-pulse or encoded pulse-train. Our method requires a low resolution of a spectrometer to measure a large group-delay, so that we apply an angular-dispersive frequencymixing [3] to our technique for simple setup. Fig. 1 shows experimental setup for spectral-shearing interferometry with a high GD-resolution. A 10-fs Ti: sapphire oscillator is used for a light source. A part of a signal pulse passes through an etalon to generate a pair of monochromatic pulses. We named this technique for Spectrally-Interferometric Measurement with a Preset Length Etalon (SIMPLE). A pair of monochromatic pulses and a signal pulse are focused onto a 1-mm-thick BBO crystal with the type-I phase matching.A purity of spectral shear GQ shear corresponds to a bandwidth of the etalon (120 GHz), so that a duration of temporal window is 8.3 ps. Spectral shear Q shear is given by a free spectral range (5.8 THz), because two adjacent longitudinal modes are selected with a band pass filter. Effective bandwidth is given by Q 2 shear /GQ shear (280 THz) that corresponds to a shortest pulse of 3.5 fs.We calculate a phase-match angle of BBO crystal as a function of sum frequency generation (SFG) frequency. Phase match angle is proportional to SFG frequency, so that a linear-image sensor can be used with no frequencycalibration. An angular distribution of SFG is given by n 'T PM , where n is refractive index and 'T PM is a slope of phase-match angle, respectively. A beam divergence of 6.5 deg. corresponds to a bandwidth of 100-THz at the type-I phase matching ('T PM = 25.4 THz/deg.). A frequency resolution of a thick-nonlinear crystal is given by ('T PM 'T accept )/L c , where 'T accept is an acceptance angle and L c is crystal thickness, respectively. A frequency resolution is 5THz at the type-I phase matching ('T accept = 3.5 mm mrad) and L c = 1 mm. The BBO crystal and a camera operate as a spectrometer. Fig. 1. Experimental setup for spectral-shearing interferometry with a wide-temporal window.Signal t Etalon BPF t t Linear Image sensor FT lens Focusing lens c-axis T PM Q BBO Side view Top v...