Phase-conjugate Reflectivity Research Articles

Beam drift reduction by straightness measurement based on a digital optical phase conjugation.

One of the greatest challenges of long distance measurement is the beam drift caused by the air refractive index gradient. It has been established in many researches that optical phase conjugation (OPC) can be used to compensate for the beam bending. However, this method is limited to responding speed, phase conjugate reflectivity, flexibility, and specific source and medium. To reduce beam drift, instead of OPC, this study applies a digital OPC (DOPC) method, which is also creatively applied to collimation and flatness measurements. The main devices in the wavefront correction unit are the spatial light modulator and the Shack-Hartmann wavefront sensor. For the straightness measurement unit, the collimation and flatness of the optical rail are measured through the prism system and a position-sensing detector. After wavefront compensation, the root mean square is decreased from 0.0029λ to 0.0005λ. The beam drift is decreased from 1.22mm to 0.70mm in the x direction and from 2.49mm to 1.55mm in the y direction. The experimental data indicate that the straightness measurement system based on DOPC can effectively decrease the beam drift.

Triphenylmethane dye-doped gelatin films for low-power optical phase-conjugation

We have studied degenerate four-wave mixing (DFWM) based optical phase-conjugation (OPC) in few triphenylmethane (Acid blue 7, Acid blue 9, Acid blue 1 and Methyl green) dye-doped gelatin films using a 633nm He–Ne laser radiation of total power 35mW. Phase-conjugate (PC) reflectivity from the dye films was measured by varying the experimental parameters such as incident angle between the pump-probe beams in the DFWM geometry, dopant concentrations of the gelatin film, probe beam intensity and read-out beam intensity. The maximum PC reflectivity was observed in the Acid blue 7, Acid blue 9, Acid blue 1 and Methyl green sensitized gelatin films were 0.22%, 0.24%, 0.07% and 0.13%, respectively. The origin of the mechanism of the PC wave generation from these dye films is also reported.

Optimizing experimental parameters on OPC in PMMA polymer doped azo dye

In this paper, the optical phase conjugation (OPC) using degenerate four wave mixing (DFWM) technique in a nonlinear DR1 chromophore doped in polymer PMMA thin films (by 8% W/W) is experimentally investigated. A CW Nd:YAG laser (532nm) with 50mW incident power has been used for (DFWM) technique to produce optical phase conjugation beam. The optimized reflectivity (R) relates to signal, forward and backward pump intensity titled with parameters M=IsignalIFP and q=IFPIBP. Furthermore, we have discussed temporal behavior and the maximum of phase conjugation reflectivity. The photo-induced reorientation of the DR1 by trans–cis–trans photoisomerization is the dominant mechanism for the OPC wave generation.

Study of Degenerate Four-Wave Mixing in Disperse Orange Dye-Doped Polymer Film

Nonlinear optical phase conjugation by degenerate four-wave mixing (DFWM) is an important technique with applications in many fields of science and technology. Phase conjugation by Degenerate four-wave mixing is observed in Disperse Orange-25 dye doped in Polymethyl methacrylate – metacrylic acid (PMMA-MA) polymer film under low-power, continuous-wave laser irradiation. A maximum phase conjugate efficiency of 0.22% has been obtained for probe beam intensity at 0.11 W/cm2. Phase conjugation is observed for both parallel- and orthogonally-polarized probe and pump beams. The maximum PC reflectivity is achieved when the angle between probe and forward pump beam is 7 degrees. The effects of dye concentration, intensity of backward, forward pump and inter beam angle between probe and forward pump beam on phase conjugation reflectivity are also studied. PC signal strength first increases and then decreases. PC reflectivity is increased by increasing the intensity of the backward and forward pump beam. The polarization and intensity profile are verified to be preserved in the conjugate signal. The predominant phase conjugation signal is attributed to the facts that reverse saturable absorption and large third order susceptibility of the dye molecules.

Optical phase conjugation reflectivity via stimulated Brillouin scattering in acousto-optic diffusive semiconductor plasma crystal

Analytical investigation has been made for optical phase conjugation (OPC) reflectivity via stimulated Brillouin scattering (SBS) in acousto-optic diffusive semiconductor plasma crystal. Our analysis is based on hydrodynamic model and coupled mode approach of interacting waves. The numerical estimations are made for n-type InSb semiconductor plasma crystal duly irradiated by CO2 laser. The magnitude of the third-order nonlinear optical susceptibility for III-V semiconductor obtained from our analysis is found to agree well with the experimental values. Maximum OPC reflectivity is obtained when cyclotron frequency is in resonance with applied pump frequency.

Two-dimensional modeling of transient gain gratings in saturable gain media

A transient two-dimensional model describing degenerate four-wave mixing inside saturable gain media is presented. The new model is compared to existing one-dimensional models with their qualitative results confirmed. Large quantitative differences with respect to peak reflectivity and optimum pump fluence are observed. Furthermore, the influence of the beam focus size, the transverse position and the crossing angle on the reflectivity of the grating is investigated using the improved model. It is demonstrated that the phase conjugate reflectivity depends sensitively on the transverse features of the interacting beams with a transverse shift in the position of the pump beams yielding a threefold improvement in reflectivity.

Frequency detuning dependence of oscillation condition for a semilinear photorefractive optical resonator in a photorefractive material with reflection gratings

Frequency detuning dependence of four-beam coupling in a photorefractive crystal pumped with two counter-propagating waves for a semilinear coherent optical resonator on the oscillation conditions has been analyzed in the case of non-degenerate-wave mixing under the slowly varying amplitude approximation method. Self oscillation can be achieved when the gain arising from the four-beam coupling is large enough to overcome the cavity loss. The effects of frequency detuning (i.e., non-degeneracy), dielectric constant and photoconductivity of the photorefractive materials on the performance of the semilinear photorefractive coherent resonator with the reflection grating configuration have also been studied in detail. The phase-conjugate reflectivity of the pumped crystal and oscillation intensity has been calculated for different input pump beam intensity ratio, intensity reflectivity of the conventional mirrors, degenerate energy coupling strength of the interacting beams. It has been found that for the higher value of the photoconductivity σp(>2.0pS/cm) of photorefractive crystal, the semilinear resonator can oscillate at almost any frequency detuning (Ω) of the oscillation beam with respect to the fixed frequency of the pump waves whereas for the lower value of photoconductivity σp(<0.1pS/cm) oscillation occurs only when the frequency detuning is limited to small region around Ω=0. But reverse of the case is found for dielectric constant (ɛ), pump intensity ratio (p) and conventional mirror reflectivity (R).

Double Phase Conjugate Mirror with Sb-Doped Sn2P2S6 Crystal for Different Incident Angles, Crystal Thickness, Total Incident Intensity and Polarization

We developed photorefractive Sb-doped Sn2P2S6 samples of different crystal thickness to investigate the properties of the media for double phase conjugation. The Sn2P2S6:Sb crystals have high coupling coefficient of 20 cm−1, and show fast response time of 1.3 ms at total incident intensity of 0.3 W/cm2. The maximum phase conjugate reflectivity of 32% was observed with a 4 mm thickness sample. To investigate the characteristics of Sn2P2S6:Sb for double phase conjugation, on the basis of the coupled wave equations, we showed the dependence of phase conjugate reflectivity on incident angle, crystal thickness, total incident intensity and polarization. From these results, the optimum incident condition was clarified for obtaining the highest phase conjugate reflectivity.

Influence of the volume speckle on fiber specklegram sensors based on four-wave mixing in photorefractive materials

In this work, the dependence on the speckle size in the performance of a micro displacement sensor based on fiber specklegrams stored in a photorefractive BSO (Bi 12SiO 20) crystal is experimentally demonstrated. In our experimental setup, a plastic optical fiber (POF) was used to generate a subjective speckle pattern which was recorded in the crystal by using a four-wave mixing arrangement in transmission geometry. The speckle size was controlled by modifying the diameter of a pupil aperture adjacent to a lens producing the image of the speckle. The signal speckle beam was mixed into the crystal with two counter propagating pump beams to generate a fourth beam which is proportional to the conjugate of the original speckle beam. Real time fringe patterns were obtained at the output of the system by producing micro displacements of the fiber output end. Increases of the phase conjugation reflectivity and the visibility of the fringe patterns were appreciated when the speckle length was increased by decreasing the pupil aperture diameter. This behavior allowed recovering the autocorrelation functions of fringe patterns associated to micro displacements that initially led to decorrelation, and therefore, to improve the dynamic range of the metrological system. Until the best of our knowledge this is the first report about the influence of the speckle size on fiber specklegrams sensors recorded on photorefractive materials by four-wave mixing.

Enhancement of phase-conjugate reflectivity using Zeeman coherence in highly degenerate molecular systems

A comprehensive theoretical analysis is developed for the vectorial phase conjugation using resonant four-wave mixing (FWM) in a highly degenerate rotational vibrational molecular system. The dynamic Stark shifts, saturation, and Doppler broadening are included for a realistic analysis. It is shown that the electromagnetically induced multilevel coherence controls the nonlinear wave mixing yielding interesting results for the phase conjugate (PC) reflectivity. It turns out that the efficiency of the PC reflectivity is decided by the relative phase of the Zeeman coherence and the population grating. When these two contributions are aligned in phase by a small detuning of the pump frequency, a large PC reflectivity ($~$20%) is obtained with moderate pump intensity ($~$500 mW/cm${}^{2}$).

Optical phase conjugation of picosecond pulses at 106 μm in Sn_2P_2S_6:Te for wavefront correction in high-power Nd-doped amplifier systems

We report, for the first time to our knowledge, on picosecondpulse optical phase conjugation using photorefractive Sn(2)P(2)S(6) crystals. For 7.2-ps pulses at 1.06 mum, we have achieved phase-conjugate reflectivities of up to 45% with very fast build-up times, about 15 ms at an intensity of 23 W/cm(2) using Te-doped Sn(2)P(2)S(6). We furthermore demonstrate aberration-free 5 W optical output of 8-ps pulses at 1.06 mum from a side pumped Nd:YVO(4) amplifier using the Sn(2)P(2)S(6)-based phase-conjugate feedback.

Enhancement of nonlinear effects at the degenerate band edge of two-dimensional photonic crystals

The ability of two-dimensional (2D) photonic crystals (PC) for high enhancements of nonlinear processes is analyzed in the case of a degenerate band edge when two symmetrical diffracted beams are generated by Bragg diffraction in the 2D-PC. Calculations are performed using the very simple Bragg-coupled wave theory which only involves three coupled waves (the incident wave and the two diffracted waves) for the linear interaction. The validity of the approximation is proved for wavelengths lying at the neighborhood of the band edges of 2D-PC. Very large local-field intensities are predicted around the band-edge wavelengths, in particular for the upper band edge. Nonlinear propagation is studied through the analysis of degenerate four-wave mixing. For counterpropagating pump beams orthogonally sent onto the 2D-PC huge improvement of the phase conjugate reflectivity are predicted at least for small incidence angles of the signal beam. These results represent an improvement by a factor of 20 when compared to the case of a one-dimensional PC of the same thickness made of the same materials. As three intense phase conjugate beams are generated in the four-wave mixing interaction, the 2D-PC could be very interesting for the purpose of dense parallel optical signal processing. Moreover, the simple theoretical analysis developed in the paper can be used for any kind of 2D-PC.

Double phase conjugate mirror using Sn_2P_2S_6 for injection locking of a laser diode bar

We demonstrate double phase-conjugation in pure and Te-doped Sn(2)P(2)S(6), a semiconducting ferroelectric material, at the wavelength of 685 nm. We observe a phase conjugate reflectivity of more than 800% at an intensity ratio of the pump beams of 44 for Te-doped Sn(2)P(2)S(6). Using a laser diode bar emitting at 685 nm, we demonstrate double phase conjugation of three independent emitters of the laser diode bar with a single mode master laser. By adjusting the center wavelength of the master laser to the center wavelength of an emitter with an accuracy of less than 0.1 nm, locking of any emitter of the laser diode bar is demonstrated. We improve the spectral width of the emitter from 0.5 nm to below 2.5 x 10(-4) nm.

Phase analysis of stimulated Brillouin scattering in long, graded-index optical fiber

A continuous-wave beam was wavefront-split by a prism and propagated through separate paths before being coupled into a long, graded-index fiber. Stimulated Brillouin scattering (SBS) was generated in the fiber and the phase of the reflection was compared to that of the pump using lateral shearing interferometers immediately after reflection and also after propagating back through the separate paths. To analyze the phase conjugating properties of SBS in the fiber, one of the paths included a pathlength oscillation. It was found that SBS from the long, graded-index fiber did not conjugate the phase of the pump. SBS formed a phase-locked beam immediately after reflection from the fiber, but did not lock the phases of the two beams after recombination as would be expected from a phase conjugate reflection.

Study on two-cell stimulated Brillouin scattering system with new media

According to the relationships between the chemical structure of the medium and the absorption coefficient, a series of new media with small absorption coefficients has been found, which are suited for use as the amplifier media. The stimulated Brillouin scattering (SBS) parameters of these media have been calculated or measured when the pump wavelength is 1.064 μm. Choosing C2Cl4 and C4Cl6 as amplifier medium and C2H5OH as generator medium, we investigated the performances of two-cell SBS system pumped by Nd:YAG Q-switched laser. The C2H5OH has a high optical breakdown threshold and the Brillouin frequency shift of C2H5OH is close to those of C2Cl4 and C4Cl6. The results indicate that the system has the characteristics of high power-load ability, high phase-conjugation fidelity and high energy reflectivity stability.

INFLUENCE OF PIEZOELECTRICITY AND MAGNETIC FIELD ON STIMULATED BRILLOUIN SCATTERING IN III–V SEMICONDUCTORS

Using electromagnetic treatment, a detailed analytical investigation of stimulated Brillouin scattering (SBS) has been made for a semiconducting crystal in the presence of an external magnetostatic field. The effect of piezoelectricity (β) and magnetic field [Formula: see text] has been introduced through equation of motion of lattice vibration and Lorentz force, respectively. The analysis is applied to both cases viz. non-piezoelectric (β = 0) and piezoelectric (β ≠ 0) in the absence (B0 =0) and the presence (B0 ≠ 0) of external magnetostatic field. The numerical estimates are made for n-type InSb crystals, taken as representative III–V semiconductor, duly shined upon by pulsed 10.6 μm CO 2 laser. The inclination of applied magnetostatic field with respect to the direction of propagation of pump beam is found to augment the gain coefficient for the onset of stimulated Brillouin scattering. Moreover, the SBS gain coefficient increases with increasing scattering angle and results in a maximum value for the backscattered mode. The backward Brillouin gain is found to be nearly 104 times larger than forward gain when β ≠ 0 and B0 = 10T. The analysis also suggests the possibility of observing optical phase conjugation reflectivity as high as 106 in the weakly piezoelectric doped semiconductors with moderate magnetostatic field. The numerical estimation suggests that piezoelectric doped III–V semiconductors in the presence of magnetic field are candidate materials for fabrication of cubic nonlinear devices.

Optical phase conjugation by degenerate four-wave mixing in basic green 1 dye-doped gelatin film using He–Ne laser

Optical phase conjugation in gelatin film doped with basic green 1 has been measured using CW laser radiation ( λ=632.8 nm) generated by He–Ne laser of total power 35 mW. The degenerate four-wave mixing (DFWM) experiment allowed for measurement of phase conjugate reflectivity as a function of dye concentration, backward beam intensity, forward beam intensity, probe beam intensity, mean pumping beam intensity and angle between the forward pumping beam and probe beam. For 1 mM concentration of basic green 1-doped gelatin film, 0.1% phase conjugate reflectivity has been observed.

Highly efficient phase-conjugation of a 1 µm pico-second Laguerre-Gaussian beam

We have demonstrated highly efficient Laguerre Gaussian beam generation by using a ring self-pumped phase conjugate mirror in the pico-second regime for the first time. The phase conjugate reflectivity was typically ~55%. We have also investigated the conservation of optical angular momentum.

Optical phase conjugation and double-exposure phase-conjugate interferometry in dye-doped thin film using He–Ne laser

We have experimentally demonstrated optical phase conjugation and double-exposure phase-conjugate interferometry in a methyl green dye-doped gelatin film via degenerate four-wave mixing using a low-power He–Ne laser for the first time. The origin of phase conjugation associated with this dye-doped film is discussed. A phase-conjugate reflectivity of 0.13% is obtained with a He–Ne laser of total power 35 mW.

Transfer-matrix modeling of four-wave mixing at the band edge of a one-dimensional photonic crystal

We present a modeling of a degenerate four-wave-mixing nonlinear process in one-dimensional photonic crystals. The model is based on the nonlinear extension of the transfer-matrix description of propagation in the structure. The influence of light localization, near the band edge of the structure, on the enhancement of the phase-conjugate reflectivity is studied. The phase-conjugate reflectivity is shown to increase as the eighth power of the number of layers with an additional large dependence on the index contrast of the structure. In both cases the enhancement is accompanied by a strong reduction of the resonance width, which may lead to some limitation of the enhancement when ultrashort pulses are used. A strong influence of the losses on the nonlinear efficiency of the structure is also predicted with a great importance of scattering losses at the multiple interfaces of the structure.