Transient-grating Frequency-resolved Optical Gating Research Articles

Single-shot complete characterization of synthesized laser pulses and the nonlinear frequency-conversion process

High-energy-synthesized laser pulses through a nonlinear frequency-conversion process with different characteristics, such as polarization, central wavelength, and pulse duration, play important roles in materials science, high-energy physics, and ultrafast optics. In this study, we present an improved transient-grating frequency-resolved optical gating based on a self-referenced and reflective structure, which enables the single-shot complete measurement of complex high-power synthesized laser pulses in the broadband range and analysis of the nonlinear frequency-conversion process of ultrashort pulses. The waveform/spectrum evolution of both the fundamental and second harmonic pulses in a nonlinear frequency-conversion process with different injected energies was studied for the first time using this method. Moreover, the method was numerically and experimentally verified to be able to completely characterize double pulses with spectral and temporal separation, including the relative phase between the two components. This method has considerable potential for studying the complex physical processes of high-power synthesized laser fields.

Characterization of two-color ultrashort laser pulses using polarization-gating and transient-grating frequency-resolved optical gating

Two-color ultrashort laser pulses have emerging applications in numerous areas of science and technology. In many cases, the slightest change in the combined electric field of a two-color pulse greatly affects its interaction mechanism with the system. Therefore, a precise characterization of the temporal/spectral profile of the combined electric field is of great importance. In this work, we demonstrate that a full characterization is possible using the well-known transient-grating (TG) or polarization-gating (PG) frequency-resolved optical gating (FROG) techniques, and by employing the recently developed Retrieved-Amplitude N-grid Algorithmic (RANA) approach for the retrieval process. We demonstrate the validity of using these techniques and this approach for multi-cycle and few-cycle pulses in the absence and presence of noise.

Convolutional neural network for transient grating frequency-resolved optical gating trace retrieval and its algorithm optimization* *Project supported by the National Key R&D Program of China (Grant No. 2017YFB0405202) and the National Natural Science Foundation of China (Grant Nos. 61690221, 91850209, and 11774277).

A convolutional neural network is employed to retrieve the time-domain envelop and phase of few-cycle femtosecond pulses from transient-grating frequency-resolved optical gating (TG-FROG) traces. We use theoretically generated TG-FROG traces to complete supervised trainings of the convolutional neural networks, then use similarly generated traces not included in the training dataset to test how well the networks are trained. Accurate retrieval of such traces by the neural network is realized. In our case, we find that networks with exponential linear unit (ELU) activation function perform better than those with leaky rectified linear unit (LRELU) and scaled exponential linear unit (SELU). Finally, the issues that need to be addressed for the retrieval of experimental data by this method are discussed.

Simple method for simultaneous long-term stabilization of relative timing and carrier-envelope phase in waveform synthesis

We present an f-to-2f interferometry method to simultaneously measure and control the relative timing (RT) and carrier-envelope phase (CEP) of an arbitrarily tailored optical-field waveform. Long-term stabilization of the phase-locking system results in a CEP stability of 280 mrad and a RT stability of 110 attosecond over 8 h at a repetition rate of 1 kHz. The synthesized optical field characterized by a transient-grating frequency-resolved optical gating delivers 3.8-fs near single-cycle waveforms. This technique constitutes a versatile tool for coherent pulse synthesis, which can be applied in experiments using a dual-color light field for high-harmonic generation and attosecond timing precision pump-probe spectroscopy.

Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy.

Broadband supercontinuum (SC) pulses in the few cycle regime are a promising source for spectroscopic and imaging applications. However, SC sources are plagued by poor stability, greatly limiting their utility in phase-resolved nonlinear experiments such as 2D photon echo spectroscopy (2D PES). Here, we generated SC by two-stage filamentation in argon and air starting from 100fs input pulses, which are sufficiently high-power and stable to record time-resolved 2D PE spectra in a single laser shot. We obtain a total power of 400μJ/pulse in the visible spectral range of 500-850nm and, after compression, yield pulses with duration of 6fs according to transient-grating frequency-resolved optical gating (TG-FROG) measurements. We demonstrate the method on the laser dye, Cresyl Violet, and observe coherent oscillations indicative of nuclear wavepacket dynamics.

Supercontinuum pulse shaping in the few-cycle regime

The synthesis of nearly arbitrary supercontinuum pulse forms is demonstrated with sub-pulse structures that maintain a temporal resolution in the few-cycle regime. Spectral broadening of the 35 fs input pulses to supercontinuum bandwidths is attained in a controlled two-stage sequential filamentation in air at atmospheric pressure, facilitating a homogeneous power density over the full spectral envelope in the visible to near infrared spectral range. Only standard optics and a liquid crystal spatial light modulator (LC-SLM) are employed for achieving pulse compression to the sub 5 fs regime with pulse energies of up to 60 μJ and a peak power of 12 GW. This constitutes the starting point for further pulse form synthesis via phase modulation within the sampling limit of the pulse shaper. Transient grating frequency-resolved optical gating (TG-FROG) allows for the characterization of pulse forms that extend over several hundred femtoseconds with few-cycle substructures.

Dispersion-free transient-grating frequency-resolved optical gating

We have developed a compact dispersion-free TG (transient-grating) FROG (frequency-resolved optical gating) by utilizing a mask that separates the input beam into three distinct beams focused into fused silica to create the FROG signal. Two of the beams are reflected off the same set of mirrors to ensure identical optical paths, eliminating the difficulty in establishing zero time delay between the beams. In addition, the use of only reflective optics avoids material dispersion in the FROG except for the mixing crystal. This TG FROG is capable of operating with an intensity of 1 x 10(11) W/cm(2) and has resolutions less than 0.5 and 1.3 fs for 25- and 10-fs input pulses, respectively.

Design and implementation of a TW-class high-average power laser system

We describe the design, modeling and characterization of a titanium-doped sapphire multipass, kilohertz amplifier system with output pulses of energy 4.4 mJ and duration 17 fs, giving a peak power of 0.26 TW. The thermal lensing in the second amplifier stage is virtually eliminated by cryogenic cooling of the laser crystal. Gain-narrowing and shifting of the amplified spectrum are reduced by tailoring the output spectrum of the oscillator and by using a low-loss multipass amplifier chain. Fourth-order spectral dispersion was completely eliminated by using a prism pair in addition to adjusting the stretcher and compressor grating separation and angle. We also numerically modeled the evolution of the pulse energy and spectral phase and amplitude through the amplifier system. The results of the model are in excellent agreement with measurements made using the technique of transient-grating frequency-resolved optical gating.

02-TW laser system at 1 kHz

We have developed a 1-kHz repetition-rate Ti:sapphire laser system that can simultaneously generate high peak and average powers of 0.2TW and 4W, respectively. The laser system generates 4-mJ energy pulses with a 20-fs pulse width. We eliminated thermal lensing in the system by cooling the Ti:sapphire crystal to 125K. The output 20-fs pulses were fully characterized by use of the new technique of transient-grating frequency-resolved optical gating. We demonstrate experimentally that the pulse duration at the output is limited only by fifth-order dispersion.

Transient-grating frequency-resolved optical gating

We introduce a transient-grating beam geometry for frequency-resolved optical-gating measurements of ultrashort laser pulses and show that it offers significant advantages over currently used geometries. Background free and phase matched over a long interaction length, it is the most sensitive third-order pulse-measurement geometry. In addition, for pulses greater than ~300 fs in length and ~1 microJ in energy, the nonlinear medium can be removed and the nonlinearity of air can be used to measure the pulse.