Towards an in situ, full-power intensity profiler for petawatt-class lasers

A technique to measure the intensity profile of a focused laser pulse at full power is a long-standing desire. Nearly 40 years ago, Sarachik and Schappert suggested an approach, and worked out many of the details, to measure the intensity that is based on relativistic Thomson scattering (RTS), which consists of a rich spectrum of Doppler shifted radiation of the laser light, and its harmonics. It is straightforward to show that intensities can be extracted from the n^th-order RTS harmonic spectrum from: I(λ,θ;n)=2πm_ec³/[r₀λ₀²(1-cos²θ)](nλ/λ₀-1), where λ₀, λ, m_e, r₀, θ, n and c are the laser wavelength, n^th harmonic wavelength, the electron mass, classical electron radius, angle of observation, harmonic order and speed of light respectively. This expression was recently used to measure the intensity of VEGA 2, 200 TW pulses generated at the Centro de Láseres Pulsados (CLPU) facility in Salamanca, Spain. Specifically, 30 fs pulses with energies between 1 and 2.5 J were focused to w₀ ~ 20 μm and captured by microscope objectives and dispersed by an Andor iStar spectrometer. This presentation will include a description of the apparatus, our initial results and a discussion of prospects for exploiting RTS to develop an in situ intensity profiler for intensities above 10¹⁸ W/cm².