Spatio-spectral metrology at focus of ultrashort lasers: a phase-retrieval approach.

Antonin Borot,Fabien Quéré

doi:10.1364/oe.26.026444

Abstract

The complete characterization of an ultrashort laser beam ultimately requires the determination of its spatio-temporal electric field E(x, y, t), or its spatio-spectral counterpart Ẽ(x, y, ω). We describe a new measurement technique called INSIGHT, which determines Ẽ(x, y, ω), up to an unknown spatially-homogeneous spectral phase. Combining this information with a temporal measurement at a single point of the beam then enables the determination of the spatio-temporal field E(x, y, t). This technique is based on the combination of spatially-resolved Fourier-transform spectroscopy with an alternate-projection phase-retrieval algorithm. It can be applied to any reproducible laser source with a repetition rate higher than about 0.1 Hz, relies on a very simple device, does not require any reference beam, and circumvents the difficulty associated with the manipulation of large beam diameters by working in the vicinity of the beam focus. We demonstrate INSIGHT on a 100 TW-25 fs laser, and use the measurement results to introduce new representations for the analysis of spatio-temporal/spectral couplings of ultrashort lasers.

Highlights

Ultrashort laser sources have become major scientific tools, used in very different research fields ranging from femtochemistry to material science, high-precision frequency metrology, or plasma physics [1]
We present the results of INSIGHT measurements performed on a high-power femtosecond laser, and use these results to address one of the difficulties that has hindered the development of spatio-temporal/spectral beam analysis so far: while the complex 3D matrix retrieved with techniques such as INSIGHT in principle contains all possible information on the laser E-field, in its raw form it does not readily shed light on the nature of the couplings affecting the beam
We applied the INSIGHT technique to the UHI100 laser system at CEA, delivering 100TW pulses of 25 fs duration with a spectrum centered at 800 nm, at a repetition rate of 10 Hz, Intensity ω1=2.27PHz (c) ω2=2.34PHz (d) ω3=2.40PHz (e) ω4=2.46PHz (f) in a beam of about 7 cm diameter

Summary

Introduction

Ultrashort laser sources have become major scientific tools, used in very different research fields ranging from femtochemistry to material science, high-precision frequency metrology, or plasma physics [1] The development of these advanced light sources has been intimately coupled with progress in optical metrology. The ability, since the late 90’s, to accurately measure the temporal evolution of the local electric field E(t) of ultrashort pulses [2] has played a crucial role in the optimization and applications of these lasers Such measurements, once at the forefront of optical metrology, can be routinely achieved thanks to a variety of elegant techniques [3,4,5,6]. The complete characterization of ultrashort laser beams requires measurement of the complex-valued electric field E(x, y, t) in space-time, or equivalently of its counterpart E(x, y, ω) in space-frequency [8]

Results

Discussion

Conclusion