Frequency-resolved optical gating with the use of second-harmonic generation

Abstract

We discuss the use of second-harmonic generation (SHG) as the nonlinearity in the technique of frequencyresolved optical gating (FROG) for measuring the full intensity and phase evolution of an arbitrary ultrashort pulse. FROG that uses a third-order nonlinearity in the polarization-gate geometry has proved extremely successful, and the algorithm required for extraction of the intensity and the phase from the experimental data is quite robust. However, for pulse intensities less than-1 MW, third-order nonlinearities generate insufficient signal strength, and therefore SHG FROG appears necessary. We discuss the theoretical, algorithmic, and experimental considerations of SHG FROG in detail. SHG FROG has an ambiguity in the direction of time, and its traces are somewhat unintuitive. Also, previously published algorithms are generally ineffective at extracting the intensity and the phase of an arbitrary laser pulse from the SHG FROG trace. We present an improved pulse-retrieval algorithm, based on the method of generalized projections, that is far superior to the previously published algorithms, although it is still not so robust as the polarization-gate algorithm. We discuss experimental sources of error such as pump depletion and group-velocity mismatch. We also present several experimental examples of pulses measured with SHG FROG and show that the derived intensities and phases are in agreement with more conventional diagnostic techniques, and we demonstrate the highdynamic-range capability of SHG FROG. We conclude that, despite the above drawbacks, SHG FROG should be useful in measuring low-energy pulses.

Ultrashort-pulse fiber ring lasers

“…Experimental data was taken by spectrally resolving the autocorrelation of the pulse (SHG FROG [121]). Figure 17 shows an experimental SHG FROG trace of a 72-fs fundamental pulse (compressed with a 150 lines/mm grating pair) along with the best calculated fit, obtained with the generalized projections algorithm [122].…”

Optimised design of fibre-based pulse compressorfor gain-switchedDFB laser pulses at 1.5 µm

“…The laser was biased below threshold at 5 mA, and gain-switched at a repetition rate of 500 MHz using electrical pulses from a step recovery diode. The electrical pulses had a duration of 80 ps and an amplitude of 13 V. The DFB laser pulses were characterised using spectral and autocorrelation measurements, as well as a second-harmonic generation FROG set-up to determine the pulse intensity and chirp [6].…”

“…In this paper we report a systematic approach to fibre compressor design based on the complete intensity and chirp characterisation of the initial gainswitched laser pulses using the technique of frequency-resolved optical gating (FROG) [6]. The complete characterisation of the electric field of the laser pulses allows the design of the fibre-pulse compressor to be optimised by examining the propagation of the pulses in different types of optical fibre using numerical 3 simulations based on the nonlinear Schrödinger equation (NLSE).…”

Characterization of nonlinear switching in a figure-of-eight fiber laser using frequency-resolved optical gating

“…In this letter, we apply the measurement technique of frequency-resolved optical gating (FROG) [5] to determine the switching characteristics of a NALM during the steady-state operation of a modelocked F8L. We use FROG measurements to determine the intensity and phase of pulses coupled out from the NALM in both the clockwise and anticlockwise directions, and using the nonlinear Schrödinger equation, we numerically propagate these pulses back into the NALM to determine the intracavity propagating fields.…”

“…The F8L output pulses were characterized using an experimental FROG technique based on the spectral resolution of a second harmonic generation autocorrelation signal in a BBO crystal, and our experimental setup was similar to that in [5]. The pulse intensity and phase characteristics were retrieved from the measured FROG signal using standard phase-retrieval techniques [5]. Fig.…”