Efficient Phase Conjugation Research Articles

Efficient low-intensity optical phase conjugation based on coherent population trapping in sodium

We have observed optical phase-conjugate gain (>50) in sodium vapor, using low-intensity pump lasers (1 W/cm2), with a response time of the order of 1 μs. Coherent population trapping is experimentally identified as the phase-conjugate mechanism. A theoretical model is presented that supports these observations by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.

Self-pumped phase-conjugation properties of cerium-doped BaTiO_3 crystals in the near infrared

Studies of a new Ce-doped BaTiO(3) crystal as an efficient self-pumped phase conjugator in the near infrared are presented. The internal beam production during the establishment of self-pumped phase conjugation (SPPC) is closely observed, and the corresponding mechanisms of SPPC are identified accordingly. When the incident beam is in an a-face-incidence geometric arrangement, it is observed for the first time that SPPC mechanism has a transition from stimulated photorefractive backscattering and four-wave mixing to total internal reflection when λ is varied from 706 to 733 nm. Variations of the SPPC reflectivity with the λ, incident angle, and position of the input beam have been investigated, and a high reflectivity of approximately 80% has been obtained. In addition, SPPC reflectivity of approximately 40% is obtained when the incident beam enters the crystal by the +c face, and a variation with λ of optical beam patterns in the crystal has also been observed. Qualitative explanations of the phenomenon of SPP mechanism transition with λ that will be very useful in practical applications are given.

Highly efficient self-pumped phase conjugation in barium titanate crystals

We report a highly efficient self-pumped phase conjugation (SPPC) at near-infrared wavelengths with an undoped BaTiO3. A 80% SPPC reflectivity at 741 nm was achieved. We made a detailed investigation on the buildup process of the phase-conjugate signal. A possible mechanism for this highly efficient SPPC mirror is also discussed.

Two-Wave Mixing and Self-Pumped Phase Conjugation with Photorefractive BaTiO3 at Near-Infrared Wavelengths

We studied the wavelength dependence of the exponential gain coefficient and response time of the two-wave mixing at near-infrared regime with an undoped BaTiO3. We also report a highly efficient self-pumped phase conjugation (SPPC). A 80% SPPC reflectivity at 741 nm was achieved.

Phase conjugate reflectivity and diffraction efficiency of gain gratings in Nd:YAG

It is shown that gain gratings can be used as efficient phase conjugate and diffractive optical elements. Experimental results are presented of saturable gain four-wave mixing in a Nd:YAG amplifier using orthogonally polarised pump beams of pulse duration 16 ns. Phase conjugate reflectivity and diffraction efficiency are measured for both the reflection and transmission grating cases. Results show a higher efficiency for the transmission geometry. Transient (experimental results are comjpaed to a steady state theory.

Efficient Optical Phase Conjugation in Methyl-Orange-Doped Polyvinyl Alcohol Film

An efficient optical phase conjugate (PC) wave in methyl-orange (MO)-doped polyvinyl alcohol (PVA) films by degenerate four-wave mixing (DFWM) is reported. As the wavelength concerned was far from the absorption center of the MO dye, a PC wave was generated more efficiently. For the configuration of mutually orthogonal-polarized pump and probe waves the PC reflectivity reached 30% at a wavelength of 531 nm and an intensity of 1 W/cm2 of a cw krypton-ion laser, by using a film with absorbance (=the product of the small-intensity absorption coefficient and the film thickness) of 2.1 at 531 nm. It was found that the photoinduced birefringence resulting from trans-cis isomerization, i.e., the real part of the nonlinear susceptibility χ2112, mainly contributed to generation of the PC wave at 531 nm.

Highly efficient self-pumped phase conjugation at near-infrared wavelengths by using nominally undoped BaTiO_3

Using a nominally undoped crystal of photorefractive BaTiO(3), we have examined self-pumped phase conjugation at near-infrared wavelengths. We report reflectivities as high as 74% between 720 and 800 nm. As expected, the crystal response time increases significantly at longer wavelengths. We believe that this value of reflectivity in the self-pumped geometry is the highest reported nominally undoped crystal.

Coherence restriction on phase conjugation in droplets

Successful observation of wave mixing and stimulated scattering phenomena in droplets implies that the time reversed wave should be generated. The authors have shown that efficient phase conjugation requires a narrow signal linewidth whereas morphology dependent resonance generation requires a relatively wide linewidth of the stimulated signal. It has been shown that these two requirements work against one another so that phase conjugation by stimulated backscattering in droplets is not observed.

Review of four-wave mixing and phase conjugation in plasmas

Four-wave mixing (FWM) in plasmas and its applications to phase conjugation (PC) and plasma diagnostics have been extensively studied. Plasmas have received considerable attention, since they become more efficient four-wave mixing and phase conjugation mediums at longer wavelengths (far-infrared approximately 10 mu m to radiowave approximately 10 m) for which there are few other nonlinear mediums. In addition, FWM shows great potential as a plasma diagnostic. Recent advances in the experimental and theoretical study of FWM and PC in plasmas are reviewed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Efficient self-pumped phase conjugation at near-infrared wavelengths using cobalt-doped BaTiO_3

Using BaTiO(3) crystals doped with cobalt, we have measured the reflectivity of self-pumped phase-conjugate mirrors operating in the total-internal-reflection geometry as a function of wavelength in the range between 633 and 933 nm. We find that for cobalt concentrations above 25 parts in 10(6), the reflectivity is higher than 50% for all the investigated wavelengths. At a wavelength of 800 nm the reflectivity is independent of incident power over a range of 12 to 600 mW.

Real-time generation of the Wigner distribution of complex functions using phase conjugation in photorefractive materials

An optical processor for real-time generation of the Wigner distribution of complex amplitude functions is introduced. The phase conjugation of the input signal is accomplished by a highly efficient self-pumped phase conjugator based on a 45 degrees -cut barium titanate photorefractive crystal. Experimental results on the real-time generation of Wigner distribution slices for complex amplitude two-dimensional optical functions are presented and discussed.

Phase conjugation under conditions of degenerate four-wave interaction involving simulated Raman scattering in a resonator

A system for phase conjugation of Stokes radiation by a four-wave interaction involving intracavity stimulated Raman scattering (STRS) in a Raman-active medium with a nonresonant nonlinearity. This method can be used in construction of high-power sources of highly directional Stokes radiation on the basis of round-trip STRS amplifiers coupled to a phase-conjugating element. It is shown that an STRS converter utilizing organic liquids and subjected to laser pumping of less than 100 MW/cm2 intensity inside a resonator can ensure a highly efficient phase conjugation with a peak reflection coefficient in excess of unity. Depending on the ratio of the Raman and nonresonant susceptibilities of a medium, there is an optimal value of the coefficient representing the degree of conversion of laser pump radiation into STRS radiation, which ensures the maximum phase conjugation efficiency.

Enhanced photorefractive performance from 45°-cut BaTiO_3

We have characterized the photorefractive performance of a special 45 degrees -cut crystal of BaTiO(3) and compared it with the conventional 0 degrees -cut material. An increased peak gain coefficient of 26 cm(-1) was measured using near normal beam incidence. Response times measured were ~1 order of magnitude faster than similar 0 degrees -cut crystals. The effects of an externally applied electric field parallel to the grating wave vector on the gain and response time were investigated. As an application example, the crystal was used to produce efficient and stable self-pumped image phase conjugation.

Ultrasonic phase conjugation

A real-time ultrasonic phase conjugator is reported. The scheme is based on sampling the wave front in space by means of a transducer array and mixing the signal from each array element with a second harmonic signal using nonlinear circuit components. The same array then retransmits the phase conjugated wave front into the medium. The technique is demonstrated at 300 kHz using a 20-element transducer array, and it is shown that efficient phase conjugation is possible up to spatial frequencies of ±80°.

Efficient phase conjugation of a cw low-power laser diode in a short Cs vapor cell at 852 nm

Up to 50% cw phase-conjugate reflectivity has been obtained with a 30-mW diode laser at 0.85 microm by means of resonant degenerate four-wave mixing on an optically thick, 1-mm-long Cs vapor cell. Self-oscillation of a Fabry-Perot cavity containing this Cs cell is demonstrated.

CW Four-Wave Mixing in Synthetic Diamond

ABSTRACTWe report efficient CW optical phase conjugation and resonantly enhanced four-wave mixing spectroscopy of the N-V color center in synthetic diamond. An ultra-narrow resonance is observed in the room temperature nearly degenerate four-wave mixing spectrum upon excitation within the vibronic band at 579 nm, furnishing the triplet manifold decay rate.

Optical nonlinearities in semiconductor-doped glasses

We present a phenomenological theory of the intensity-dependent dielectric function of semiconductor-doped glasses near the absorption edge. It is shown that efficient phase conjugation by degenerate four-wave mixing should be possible in these materials.

Efficient phase conjugation by Brillouin enhanced four wave mixing

It is shown that frequency detuning and phase mismatch effects can strongly effect the reflection coefficient of Brillouin enhanced four-wave mixing. Extremely large reflectivities ( R > 10 6) can be obtained if the intensities and frequency of the pump beams and the length of the interaction region all take on certain critical values. This theory accounts for the high reflectivity which has been reported by another author.

Efficient phase conjugation of an ultraviolet XeF laser beam by stimulated Brillouin scattering

We report efficient, diffraction-limited, phase conjugation of an XeF (3511-A) laser beam using stimulated Brillouin scattering. Approximately 70% of the 1-GHz bandwidth-locked portion of an injection-locked XeF laser output is phase conjugated by focusing the laser beam at 5 GW/cm (2) into hexane or isopropanol.

Mode-locked laser oscillation using self-pumped phase-conjugate reflection

Efficient phase conjugation of 40-psec Nd:YAG laser pulses in Eastman Kodak mode-locking dyes A 9740 and A 9860 has been obtained. Variation of the phase-conjugate reflectivity with pump intensity is quadratic, without saturation effects, and variation with dye concentration agrees with a simple analytical model. We have used this reflectivity to obtain self-sustained oscillation and mode locking of a novel self-pumped phase-conjugate laser resonator.