Instantaneous optical-phase-recording in a two-photon gated Bragg grating

Abstract

Temporal phase structure of an optical pulse has been recoded with a 10 fs temporal resolution in a two-photon formed Bragg grating. The refractive index modulation of 0.01 has been achieved with 6 nano-joule IR laser pulses.The frequency domain phase conjugator (FDPC) generates time-reversed replica of a phase modulated (frequency chirped) pulse [1]. The time-reversed replica regenerates initial pulse shape after passing though again the same GDD element. We have demonstrated FD phase conjugation by the Time Ordered Pulse Recording/Readout in a Periodic Diffractive Optics (TOPEDO) where the temporal phase structure of optical pulse is recorded as a second-order cross correlation function in space as shown in Fig. 1. The TOPEDO requires two-photon sensitive recording medium that effectively changes its index of refraction by low intensity irradiation. One of the advantages of the two-photon interference is that the two pulses can be interacted each other with a long time delay exceeding their coherent length. Moreover, the interference efficiently produces deep amplitude modulation even with small gate pulse energy. We have numerically examined the interaction length and the contrast ratio in the two-photon Reference pulse mom, cross correlation between the TL-reference and chirped pulses. A visibility of 0.5 has been evaluated when the ratio of pulse duration is 50.The first FDPC experiment has been demonstrated in semiconductor doped colored-glasses having An=105with sub-mJ laser energy [2]. In this paper, we have demonstrated deep modulation of refractive index with a few nano-joule laser pulses from a mode-locked oscillator. In general, change of refractive index is produced as a result of absorption breaching due to laser irradiation. Thus an ideal material should have large absorption change with small laser energy. The medium should be initially optically thick at the two-photon wavelength and should has a large absorption cross section producing low saturation fluence. We have developed dye-doped thermo plastic film for this purpose. Pulses coming from a 10 fs mode-locked laser was splitted into two-beams and interacts at the focal plane of a microscope objective as shown in Fig.2. The laser pulse bound 22 times between a pair of chirped mirror to compensate a GDD of 1260 fS2 produced by the microscope objective. The pulse duration and Fourier phase were monitored by frequency shearing interferometory (SPIDER). An interference fringes at the focal plane is 13 micron wide and 207 micron thick. The average 2 laser intensity at the plane was 3.7 GW/cM Fig.1 Schematics illustration of two-photon-gated pulse recording and time-reversed readout in a periodic diffractive optics. 1604