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Applied Optics

Coherent optical image delay device using a BSO phase-conjugate mirror and its applications

Takuso Sato, T. Suzuki, Peter John Bryanston-Cross, Osamu Ikeda, Takeshi Hatsuzawa

Published Mar 15, 1983• 6 citations
Publisher

Abstract

The Bi12SiO20 (BSO) crystal has been used as an optical storage medium for a 2-D optical field to obtain a delayed coherent optical image. The method used the BSO crystal in a four-wave mixing configuration to produce a phase-conjugated wave front. It has been demonstrated that the image of a vinyl film could be compared with its stored image over a time delay varying from 40 msec to a few seconds. Measurements have also been made of the changes of optical path length within this inhomogeneous medium.

DOI
10.1364/ao.22.000815

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The Bi12SiO20 (BSO) crystal has been used as an optical storage medium for a 2-D optical field to obtain a delayed coherent optical image. The method used the BSO crystal in a four-wave mixing configuration to produce a phase-conjugated wave front. It has been demonstrated that the image of a vinyl film could be compared with its stored image over a time delay varying from 40 msec to a few seconds. Measurements have also been made of the changes of optical path length within this inhomogeneous medium.
This thesis describes theoretical and experimental aspects as well as potential applications in the field of coherent optics known as Phase Conjugate Optics (PCO). By utilizing nonlinear optical techniques, real-time or wavefront reversal of an arbitrary incident electromagnetic field can be realized. The nonlinear optical interaction gives rise to what is referred to as the phase conjugate (of the incident monochromatic wave) by performing the operation of complex conjugation upon the incident wave's complex spatial amplitude in real time. This conjugate wave, which is also designated as being a wavefront, has the property of exactly retracing the path of the incident field. The ability of the conjugate wave to correct for inhomogeneous linear and nonlinear (intensity-dependent) aberrations as well as polarization distortions is proved. In particular, the theory of a degenerate four-wave nonlinear optical interaction as providing for the conjugator is presented. The effects of linear and nonlinear losses upon this interaction are discussed. The quantum mechanical origin of the third order nonlinear optical susceptibility responsible for the four-wave mixing process is analyzed for both single and two-photon allowed transitions in an atomic (or molecular) system. The analogies of four-wave mixing with that of real-time holography are discussed. The theory of conjugation via four-wave mixing in optical waveguides is presented. Several of the above characteristics of conjugate fields are verified experimentally where conjugate fields via degenerate four-wave mixing were observed both in the bulk and in waveguide geometries, using carbon disulfide as the nonlinear medium. Amplified time-reversed wavefront generation as well as a mirrorless optical parametric mode of oscillation have been observed, both in agreement with theoretical predictions. Potential applications of PCO are discussed in three different regimes: spatial-frequency, temporal-frequency, and spatial/temporal frequency domains. In the first category, the ability of PCO to correct for image modal dispersion in optical waveguides as well as the use of PCO to perform real-time coherent image processing and nonlinear microscopy is discussed. Temporal-frequency domain applications of PCO to be analyzed include the use of a nearly degenerate four-wave mixing process as a narrowband, wide field-of-view optical filter, capable of an amplified bandpass. The ability of a PCO interaction to renarrow (transform limited) optical pulses which have been temporally spread due to propagation through (group velocity) dispersive channels is analyzed. A potential application of PCO in the field of nonlinear laser spectroscopy is presented. Specifically, the scattering of a probe photon off a two-photon coherent state (created in a three-level atomic system) is shown to yield a conjugate replica. This conjugate replica is capable of providing sub-Doppler width resolution of the two-photon resonance. Further, in the transient regime, the optical free-induction decay of the conjugate wave is capable of yielding detailed spectral features of the three-level system such as the anharmonic contribution to a (nearly) harmonic potential. This technique, performed in the time domain, is known as [alpha]-beat spectroscopy. Finally, a detailed theoretical and experimental study is presented of a laser resonator in which one (or both) of the mirror(s) comprising the optical cavity is replaced by a conjugate mirror (PCM). This novel resonator, which is termed a conjugate resonator (PCR), combines many of the spatial- and temporal-frequency aspects of PCO interactions discussed above. The stability criterion, transverse and longitudinal mode spectra, and the PCR output energy, as well as the frequency locking features of the laser modes to the PCM are discussed.
The slowing down of light pulses is achieved using backward-wave four-wave mixing in a medium with local response. A Bi12TiO20 crystal with an external dc field is used in the experiment as a proof-of-concept material. The delay and shape transformation of output pulses are studied and compared for the transmitted and phase conjugate channels. It is shown that the phase conjugate pulse achieves a longer delay under typical experimental conditions with equal intensities of the pump beams. This advantage of the phase conjugate beam is especially pronounced for short pulses with half-widths smaller than the response time of the medium. The agreement of the experimental results with numerical calculations confirms the validity of the previously developed theoretical model. Analytical solutions are derived for input pulses with triangular shapes.
We present the results of an experimental study of the effects of laser coherence on the operation of the double phase-conjugate mirror using barium titanate as the nonlinear medium. In our experiment the two input beams that drive the interaction originate from a single helium-neon laser, and the degree of coherence between the input beams at the crystal is varied through the use of a time-dependent phase delay in one of the beam paths. As the two input beams make the transition from being mutually incoherent to mutually coherent, the phase-conjugate reflectivity decreases dramatically and becomes unstable. These results are significant to laser phase-locking applications where changes in the coupling intensity can affect the quality of phase locking.
A nondegenerate phase-conjugate wave was generated via stored atomic coherence in a Pr3+:Y2SiO5{\mathrm{Pr}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Pr3+:Y2​SiO5​ crystal based on the electromagnetically induced transparency effect, and its capability for wave-front reconstruction of phase distortion was demonstrated experimentally. The phase-matching condition during the storage-retrieval process of the phase-conjugate wave was characterized both experimentally and theoretically in detail. Theoretical simulations fit the experimental data very well. Such a scheme of storage and retrieval of the phase-conjugate wave may have potential applications in optical signal processing and information security.
We demonstrate a new method to implement holographic associative memory based on a coherently induced double phase conjugate mirror that has phase-conjugate reflectivity larger than unity for weak reference signal over a large range of pumping-reference intensity ratio. The phase conjugator, operated with a copper-doped KNSBN crystal, serves as a novel nonlinear feedback and gain element for the associative memory system. A vanadium-doped KNSBN crystal as storage medium improves the response speed of the holographic recording process. A stored image has been completely retrieved when a partial input addresses the associative memory system.