We report what is to our knowledge the first experimental demonstration of a continuous coherent transient optical processor. A 13-bit pattern was stored as a spectral population grating in the Eu3+-ion ground state in a Eu3+:Y2SiO5 crystal. A 3120-bit data stream was processed continuously, yielding a cross-correlation signal that agreed well with theory. The data stream’s duration exceeded both the absorbing transition’s homogenous transverse dephasing time and upper-state lifetime. This experiment showed that by permanently storing the pattern as a population grating in the absorber’s ground state, coherent transient optical devices provide continuous, real-time data processing capability.
Recent works [Chong et al., Phys. Rev. Lett. 105, 053901 (2010); Wan et al., Science 331, 889 (2011)] have shown that the time-reversed process of lasing at threshold realizes a coherent perfect absorber (CPA). In a CPA, a lossy medium in an optical cavity with a specific degree of dissipation, equal in modulus to the gain of the lasing medium, can perfectly absorb coherent optical waves at discrete frequencies that are the time-reversed counterpart of the lasing modes. Here the concepts of time reversal of lasing and CPA are extended for optical radiation emitted by a laser operated in an arbitrary regime, i.e., for transient, chaotic, or periodic coherent optical fields. We prove that any electromagnetic signal
E(t) generated by a laser system
S operated in an arbitrary regime can be perfectly absorbed by a CPA device ${S}^{\ensuremath{'}}$ which is simply realized by placing inside
S a broadband linear absorber (attenuator) of appropriate transmittance. As examples, we discuss CPA devices that perfectly absorb a chaotic laser signal and a frequency-modulated optical wave.