Photorefractive Two-beam Coupling Research Articles

Self-adaptive amplified spontaneous emission suppression with a photorefractive two-beam coupling filter.

Optical bandpass filters can be utilized to suppress parasitic broadband spectral power prior to laser amplification but are typically designed around specific frequencies or require manual adjustment, thus limiting their compatibility with highly tunable or integrated laser systems. In this Letter, we introduce a self-adaptive volume holographic filter using the dynamic two-beam coupling interaction in photorefractive BaTiO3, demonstrating -10dB suppression of amplified spontaneous emission noise surrounding a tunable 780 nm diode laser peak, with <2nm filter bandwidth and 50% power throughput. The spectral filtering is automatically centered on the lasing mode, with an estimated auto-tuning rate of 100 GHz/s under typical conditions. Furthermore, the filter suppression and bandwidth can be optimized via the two-beam coupling intensity ratio and angle, respectively, for versatile control over the self-adaptive filter characteristics.

EO properties and photorefractive two-beam coupling in AlN at 375 nm

Aluminium nitride (AlN) is a wide bandgap semiconductor with an intrinsic band-to-band absorption edge at ∼200 nm wavelength. Therefore, the material is promising for optical use in the ultraviolet region. With a wurtzite-type, noncentrosymmetric crystal structure, AlN is also interesting for second order nonlinear optics and electro-optics. As grown crystals of AlN show relatively large absorption starting below 650 nm. With the material properties of AlN supporting the photorefractive effect we examined AlN as a photorefractive beam coupler. No effect was observed in the visible spectral range. Using the UV wavelength of 375 nm, where the absorption constant is (17 ± 2) cm-1 for the extraordinary polarization beam coupling was realized. The results show relatively weak and slow photorefractive effect.

Photorefractive two-beam coupling joint transform correlator: modeling and performance evaluation.

The photorefractive two-beam coupling joint transform correlator combines two features. The first is embedded semi-adaptive optimality, which weighs the correlation against clutter and noise in the input, and the second is the intrinsic dynamic range compression nonlinearity, which improves several metrics simultaneously without metric trade-off. Although the two beam coupling correlator was invented many years ago, its outstanding performance was recognized on only relatively simple images. There was no study about the performance of this correlator on complicated images and using different figures of merit. In this paper, the study is extended to more complicated images. For the first time, to our knowledge, we demonstrate simultaneous improvement in metrics performance without metric trade-off. The performance was evaluated compared to the classical joint transform correlator. A typical experimental result to validate the simulation results was also shown in this work. The best performing operation parameters were identified to guide the experimental work and for future comparison with other well-known optimal correlation filters.

Influence of heavy central atom on photoelectric, nonlinear optical, and photorefractive properties of metal phthalocyanines

Measurement of the photoelectric and photorefractive characteristics of 2,3,9,10,16,17,23,24-tetra(15-crown-5)phthalocyaninatoacetatoyttrium(III) [(15C5)4Pc]Y(OAc) has made it possible to reliably establish the features of decrease in the quantum yield of generation of charge carriers φ0 and photorefractive two-beam coupling gain Γ with an increase in the atomic weight of the central metal in a series of related gallium(III), yttrium(III), ruthenium(II), and indium (III) phthalocyaninates. These features can be due to an increase in the spin-orbit interaction constant with increasing atomic weight of the central metal, a change that leads to an increase in both the contribution of triplet excited associates to the formation of mobile charge carriers and the rate of the reverse reaction T1 → S0, which effectively inhibits the formation of charge carriers through the excited triplet state. The measurement of photoelectric and photorefractive characteristics at 1064 nm has shown that the quantum yield of electron-hole pairs in the composite polyvinylcarbazole/supramolecular assemblies of yttrium complex is φ0 = 0.6 and the two-beam coupling gain coefficient Γ is about 35 cm−1 at E0 = 120 V/μm. The dielectric susceptibility measured by the z-scan technique in a solution of the yttrium complex in tetrachloroethane (1 mg in 1 mL) is χ(3) = 4.2 × 10−10 esu.

Slow light with photorefractive four-wave mixing

A slowing down of light pulses using backward-wave four-wave mixing is achieved in photorefractive crystals with different coupling strength. The delay and width of the output pulse are studied as a function of the input pulse width and pump intensity ratio for the amplified transmitted beam and for the phase-conjugated beam. The delay characteristics are compared with those of the two-beam coupling. It is demonstrated that the four-wave mixing process ensures a larger slowing down of short pulses (pulses with width shorter than the photorefractive response time) as compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners. The delay of long pulses in the four-wave mixing configuration is almost the same as the one that can be reached with two-beam coupling. The effect of absorption on the nonlinear shape transformation of the output pulses is discussed qualitatively.

Slow light with degenerate backward-wave four-wave mixing

We report on a slowing down of light pulses using degenerate backward-wave four-wave mixing in a photorefractive crystal. The delay and width of the output pulse for the amplified transmitted beam and for the phase-conjugated beam are studied as a function of the input pulse width. We demonstrate that the four-wave mixing process ensures a larger slowing down of short pulses compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners, which are among the principal drawbacks for slowing down of short pulses with two-beam coupling. The technique may be extended to slowing down of light with degenerate or nearly degenerate backward-wave four-wave mixing based on other types of nonlinearities.

Influence of the Physical Properties of Ferroelectric Liquid-Crystal Mixtures on the Photorefractive Effect

The relationship between the thermal properties and the photorefractive effect of various ferroelectric liquid-crystal compositions was investigated. Mixtures of phenyl pirimidine derivatives were used as host liquid-crystal materials to which chiral dopants and photoconductive compounds were added. The photorefractive two-beam coupling gain coefficient was strongly dependent on the magnitude of spontaneous polarization and the uniformity of the surface-stabilized state.

Photorefractive polymer composites based on ruthenium (II) tetra-15-crown-5-phthalocyanate axially coordinating ethylisonicotinate molecules photosensitive in telecommunication range

Polyvinylcarbazole layers containing ruthenium (II) tetra-15-crown-5-phthalocyanate axially coordinating ethylisonicotinate molecules are found to have photoelectric and photorefractive sensitivity at 1550 nm. The effect is determined by the formation of supramolecular ensembles with the electronic optical absorption and photoelectric sensitivity in the near IR range up to 1600 or 1700 nm and nonlinear optical properties. A photorefractive two-beam coupling gain coefficient Γ measured at 1550 nm increases with an increase in the Ru(II) complex content in the polymer layer and equals 25 cm−1 at 6 wt %. The difference between the coupling gain and absorption coefficients (the actual gain coefficient) is Γ − α = 19 cm−1. Additionally introducing 3 wt % C60 fullerene into the layer does not change the characteristics, but the subsequent preirradiation (irradiation of the whole layer with a 633-nm laser beam in a range of the optical absorption of C60 before recording photorefractive curves) results in an increase in the gain coefficient at 1550 nm up to Γ = 48.3 cm−1 and, hence, in Γ − α = 42.3 cm−1. A time constant of the diffraction grating formation is about τ = 0.8 s.

Dynamic phase-contrast stereoscopy for microflow velocimetry

A nonlinear dynamic phase-contrast stereoscope has been developed for the measurement of all three velocity components of a microfluidic flow field. The stereoscope system captures simultaneously two images of different off-axis views of the same region of interest in a microflow, seeded with tracer particles. Two independent photorefractive two-beam coupling novelty filters, one in each stereoscope channel, are employed to enhance the contrast of tracer particle images. A subsequently applied particle tracking algorithm extracts the velocity information from the images, and in first experiments the axial velocity components could be determined with an error of less than 5%. Finally we report on the determination of the velocity field in a rectangular microchannel with a 170 μm high microstep with the dynamic phase-contrast stereoscope.

Investigation of the dielectric, optical and photorefractive properties of Sb-doped Sn2P2S6 crystals

In this work we report results on electro-physical, optical and photorefractive investigations for Sb-doped Sn2P2S6 crystals. The crystals are obtained by two methods: the vapour-transport technique and the Bridgman technique using stoichiometric Sn2P2S6 composition with different amounts of antimony in the initial compound. The good optical quality of the crystals obtained with the Bridgman technique is underlined. The dependences of the photorefractive two-beam coupling coefficient and the grating build-up time are investigated at the wavelength of 633 nm. It is found that the sample doped with 1.5% of Sb is characterized by an optimal combination of the main photorefractive parameters exhibiting a fairly high two-beam coupling coefficient (up to 20 cm−1) and a short response time (1.3 ms) that is the shortest among all the previously studied Sn2P2S6 crystals in the red spectral region.

Recent advances in photorefractive two-beam coupling

In this article, we review two approaches that we have taken to improve photorefractive two-beam coupling: (1) the enhancement of the effective trap density in a well-known inorganic photorefractive crystals (KNbO3:Fe) and (2) the improvement of a relatively new concept of organic–inorganic hybridized liquid crystal cells.

Spectrally variable two-beam coupling nonlinear deconvolution

In previous work, we introduced a dynamic range compression-based technique for image correction using nonlinear deconvolution; the impulse response of the distortion function and the distorted image are jointly transformed to pump a clean reference beam in a photorefractive two-beam coupling arrangement. The Fourier transform of the pumped reference beam contains the deconvolved image and its conjugate. Here we extend our work to spectrally variable dynamic range compression. This approach allows the retrieval of distorted signals embedded in a very high noise environment and does not require one to work with a very high beam ratio as in our previous work. Resolution recovery of blurred noisy images is demonstrated for several different types of image blur.

Optical amplification in multilayer photorefractive liquid crystal films

Photorefractive two-beam coupling was performed in fullerene C60-doped nematic liquid crystal cell. Photorefractive gain coefficient as high as 1386cm−1 was obtained at an input pump power as weak as 16mW. However, the thin film nature led to a small gain of 16. By the use of several such cells stacked together to increase the overall interaction length, the optical gain was improved greatly, and the pump power to obtain the highest optical gain was reduced. At 1.2V, the highest gain of 141 was obtained for the 40μW signal at an input pump power of 12mW.

Photovoltaic Field-Induced Self-Phase Modulation of Light in Liquid Crystal Cells

We show that photovoltaic fields generated internally in LiNbO3:Fe substrates are capable of efficiently reorienting liquid crystal molecules leading to new concepts of optically addressable light modulators. Using an arrangement consisting of a liquid crystal layer in contact with one or more LiNbO3:Fe photovoltaic substrates, we observe spatial filtering due to self-phase modulation in a planar oriented cell. The LiNbO3:Fe substrates are arranged such that light propagates along the +c axis, allowing a secondary process of power transfer to occur through photorefractive contra-directional two-beam coupling.

Anisotropy of nonlinear coupling of two counter-propagating waves in photorefractiveFe:KNbO3

The dependence of self-pumped photorefractive two-beam coupling in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ with crystal angle has been measured at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the $a$-$c$ and $b$-$c$ crystal planes using a combination of oil immersion and relay imaging. A significant deviation between existing theory and experiment is found for large crystal angles. We propose this is due to a highly anisotropic effective charge trap density in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ of almost two orders of magnitude variation with crystal angle in the $a$-$c$ plane.

Self-activated liquid-crystal cells with photovoltaic substrates

We show that photovoltaic fields are capable of efficiently reorienting liquid crystals, leading to new concepts of optically addressable light modulators. Using an arrangement consisting of a liquid-crystal layer between LiNbO3:Fe photovoltaic substrates, we observed spatial filtering due to self-phase modulation in a planar-oriented cell and nonlinear transmission between crossed polarizers in a twist-oriented cell. These processes do not require an external electric field. The substrates are arranged such that light propagates along the +c axis in each substrate, allowing a secondary process of power transfer to occur through contradirectional photorefractive two-beam coupling.

Understanding the optical and electrical properties of a new photorefractive potassium niobate crystal

Optical and electrical measurements have been made on a new codoped potassium niobate crystal that yields a significant increase in linear absorption, photocurrent, and photorefractive beam coupling efficiency. The Ag impurity enters the K site, changing the local field in the lattice. The Fe in the Nb site, perturbed by the replacement of the next near-neighbor K with Ag, is responsible for the enhanced linear and nonlinear properties. The modified material parameters result in an increase of the photorefractive counter-propagating two-beam coupling efficiency.

Chromophore Orientation Dynamics, Phase Stability, and Photorefractive Effects in Branched Azobenzene Chromophores

The increased solubility and uniform dispersal of branched azobenzene chromophores over their monomeric analogues have been shown to improve the electrooptic performance of high glass transition temperature (Tg) blended polymers. We report here the application of these branched chromophores as guest nonlinear optical molecules in the plasticized low-Tg photoconducting host polymer poly(vinylcarbazole) and demonstrate the presence of orientationally enhanced photorefractive index gratings. When compared with their monomeric analogues, branched chromophores were compatible over a broader range of concentrations and resulted in higher quality optical films; these films have retained their optical clarity for 1 year. A new branched electrooptic chromophore with r33 of 14 pm/V at 1550 nm was synthesized and exhibited photorefractive two-beam coupling over a range of applied fields, including a net two-beam coupling amplification coefficient of 6.4 cm-1 at 780 nm.

Sn2P2S6 Crystals for Fast Near-Infrared Photorefraction

Optical and physical properties of Sn 2 P 2 S 6 ferroelectric crystals suitable for photorefractive applications in the red and near-infrared spectral range are reported. The main advantage of Sn 2 P 2 S 6 is its high beam-coupling gain combined with considerably faster response in the near infrared compared to conventional oxide-type photorefractive materials. We show that modifying the growth conditions and doping with Te can efficiently improve the photorefractive properties of Sn 2 P 2 S 6 . Photorefractive two-beam coupling gain at 860 nm increases from about 2 cm − 1 in conventional “yellow” Sn 2 P 2 S 6 to about 7 cm − 1 in Te-doped samples and 14 cm−1 in modified “brown” samples. We examine the self-pumped optical phase conjugation properties of Sn 2 P 2 S 6 in a ring-cavity scheme. Phase-conjugate signals with amplitudes close to the theoretical limit and rise times of about 10 ms for 4 W/cm 2 pump intensity at 780 nm and 50 ms for 2.5 W/cm 2 pump intensity at 860 nm are obtained. *Present Address: Laboratoire Matériaux Optiques, Photonique et Systèmes (LMOPS), University of Metz and Supélec, F-57070 Metz, France.

The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers

In liquid crystal cells with photoconductive poly(N-vinlylcarbazole) polymer layers, an external dc field can be completely screened by surface charge layers that develop at the liquid crystal–polymer interface. Under spatially modulated illumination, surface charge layers can be discharged in bright areas and lead to reorientation and spatially modulated Freedericksz transition. As a result, an asymmetric energy exchange in the photorefractive two-beam coupling process can take place. We propose a model to explain the origin of reorientation and phase shift in the two-beam coupling process, based on the profile and tilt of the refractive index grating. We also show that cells with just one photoconducting layer are more efficient than a typical design with two layers.