We propose a new idea to measure the temporal contrast of laser pulses in this paper. Unlike the traditional time-to-space transformation method, the design in this paper can be called a somewhat time-to-time transformation. A single-shot pulse beam is simultaneously divided into two separate beams by a beam splitter. The two beams then enter a pair of optical replicators respectively to generate two pulse sequences that have equal time intervals respectively. The time interval of the two sequences alters by about 100fs that we can use one sequence to scan the other at a time resolution of 100fs through a nonlinear crystal. The time resolution depends on the altering of time interval and the time window depends on the pulse number of the sequence. This method can avoid the spatial modulation of the laser pulse. On the other side, the time resolution and time window are adjustable. It is also possible to measure the high contrast with a low-dynamic-range detector by attenuating the power of each pulse in the sequence respectively.
The transient radial shearing interferometry technique based on fast Fourier transform (FFT) provides a means for the measurement of the wavefront phase of transient light field. However, which factors affect the spatial bandwidth of the wavefront phase measurement of this technology and how to achieve high-precision measurement of the broad-band transient wavefront phase are problems that need to be studied further. To this end, a theoretical model of phase-retrieved bandwidth of radial shearing interferometry is established in this paper. The influence of the spatial carrier frequency and the calculation window on phase-retrieved bandwidth is analyzed, and the optimal carrier frequency and calculation window are obtained. On this basis, a broad-band transient radial shearing interference phase-retrieval method based on chirp Z transform (CZT) is proposed, and the corresponding algorithm is given. Through theoretical simulation, a known phase is used to generate the interferogram and it is retrieved by the traditional method and the proposed method respectively. The residual wavefront RMS of the traditional method is 0.146λ, and it is 0.037λ for the proposed method, which manifests an improvement of accuracy by an order of magnitude. At the same time, different levels of signal-to-noise ratios (SNRs) from 50 dB to 10 dB of the interferogram are simulated, and the RMS of the residual wavefront is from 0.040λ to 0.066λ. In terms of experiments, an experimental verification device based on a phase-only spatial light modulator is built, and the known phase on the modulator is retrieved from the actual interferogram. The RMS of the residual wavefront retrieved through FFT is 0.112λ, and it decreases to 0.035λ through CZT. The experimental results verify the effectiveness of the method proposed in this paper. Furthermore, the method can be used in other types of spatial carrier frequency interference, such as lateral shearing interference, rotational shearing interference, flipping shearing interference, and four-wave shearing interference.
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