Enhanced dynamic range spatio-spectral metrology of few-cycle laser pulses

Abstract

Accurate spatio-temporal and spatio-spectral metrology is critical to the characterization and use of ultra-short, high-power lasers. The emergence of few cycle pulses, with bandwidths of tens or hundreds of nanometers, poses a significant challenge to existing metrology techniques. This is due both to large discrepancies in the sensitivities of the measurements at different wavelengths and to variation in the spectral intensity at those wavelengths. In this paper, the authors propose spectral filtering and stitching of the measurements as a robust, simple solution that enhances the dynamic range of the measurements, allowing accurate few-cycle pulse reconstruction. This enhancement is demonstrated using INSIGHT -- the most commonly used spatio-spectral measurement device -- as well as using IMPALA and spatially resolved Fourier transform spectrometry.

Figures (6)

• Figure 1: Pulse characterization of the LUCID laser. (a) Dispersion scan measurement. (b) Retrieval of the temporal pulse shape. (c) Retrieved spectrum and spectral phase of the laser.
• Figure 2: (a) Schematic of the experimental setup. Laser path is shown, as well as the spectral filter (F), IMPALA mask (IP), and focusing off-axis parabola (OAP1). A removable mirror (RM1) channels the laser into the INSIGHT measurement. A different removable mirror (RM2) channels the beam to the focal spot/IMPALA camera (FC). Finally, without both mirrors the beam goes to another off-axis parabola (OAP2) and is steered into the SRFTS measurement. (b) Detailed schematic of the INSIGHT measurement: Mirrors M1 and M2 together with BS1 form a Michelson Interferometer. Mirror M3 and BS2 are used to look after the focal spot on the Z+ camera. BS3 and M4 redirect the beam towards Z- and Z0 cameras. The Direct Camera DC is used for pointing stabilization. (c) Detailed schematic of the SRFTS measurement.BS1 is a 10/90 beamsplitter cube, M denotes flat mirrors, the arrow denotes movement of the piezo stage, P is a 50 mm focusing parabola, BS2 is a 2% reflection beam splitter and CCD denotes the camera. .
• Figure 3: Overview of IMPALA. (a) Pinhole mask designed by genetic algorithm for LLC laser system measurement. (b) Interference speckle pattern that results from passing LLC beam through pinhole mask and then focusing. Red colorscale shows relative intensity. (c) Streak pattern obtained by performing a spatial Fourier transform on the interference speckles. These streaks contain spatially separated chromatic information about the beam. (d) Retrieved wavefront for one particular spectral band of the LLC laser. Colorscale shows phase from $-\pi$ to $\pi$. Blue dots indicate the spatial points at which the wavefront is sampled.
• Figure 4: Spectral (a, b, c) and temporal (d, e, f) properties inferred from the measurements: (a) Spatially integrated INSIGHT spectra from the measurements without filter, and the one with 850 nm cutoff filter (solid lines with shading), and the spectrum measured with a spectrometer (dotted line); Spatio-spectral reconstruction of the direct INSIGHT measurement (b), and stitched data from the two measurements (c). (d) Fourier-transform-limited (FTL) pulse duration obtained from the spatially-integrated direct INSIGHT measurement (solid line with shading) and from the spectrometer data (dotted line). Spatio-temporal reconstruction using the direct INSIGHT measurement (e), and using the stitched data from the measurements (f).
• Figure 5: Impact of dynamic range enhancement on IMPALA data. (a) IMPALA streaks (blue) for LLC laser without any spectral filtering as well as the simulated quasi-monochromatic streaks centered at 720 nm, 760 nm, 800 nm, 840 nm, and 880 nm (shown in pink). The insert shows an enlarged example of a streak. (b) IMPALA streaks for LLC laser with 850 nm spectral filter in place. (c) IMPALA streaks obtained by summing the results of the no filter and 850 nm filter cases. All areas with signal are normalized to the same value. (d) IMPALA streaks obtained by summing the results of the no filter and 850 nm filter cases. The blue colorbar shows the places where just one signal is present or where overlapping signals are present. (e) Retrieved 760 nm wavefront for the LLC laser. Colorscale shows phase from $-\pi$ to $\pi$. Blue dots indicate the spatial points at which the wavefront is sampled. (f) Retrieved 840 nm wavefront.