Refractive Index Gratings Research Articles

Coupled resonator optical waveguides

Properties of the recently introduced family of coupled resonator optical waveguides (CROWs) are reviewed, particularly with reference to CROWs designed as planar waveguides in two-dimensional photonic crystal slabs to enhance nonlinear interactions and develop novel all-optical information processing devices. Topics covered include: pulse propagation both in the nondispersive approximation and to all orders of dispersion, and the coupled mode theory of nonlinear optics with pulses in CROWs and its applications to second-harmonic generation and wave coupling via field-induced refractive-index gratings. We also review recent experimental progress in the fabrication and characterization of CROWs, and applications of the CROW concept to fiber gratings and microwave waveguides.

Effect of periodic background loss on grating spectra.

The effect of periodic loss on the performance of refractive-index gratings has been studied in detail. It is shown that the loss periodicity and relative phase strongly affects the symmetry of the reflection, transmission, and loss spectra. This asymmetry is explained successfully through consideration of the overlap between the standing-wave intensity distribution and the periodic loss pattern.

Intracavity refractive index bragg gratings in absorbing and amplifying media

The influence of absorption or gain on the Bragg diffraction efficiency of intracavity refractive index gratings is analyzed. Losses drastically reduce the diffraction efficiency of the device which can nevertheless be optimized in order to limit their inauspicious influence. On the contrary, in amplifying intracavity media very large diffraction efficiencies are obtained even with small integrated refractive index modulations. The optimization of the device for maximum diffraction efficiency or vanishing reflectivity could find applications in tunable lasers and in high contrast amplifying optical signal processing.

Photochromic polymers for the optical homodyne detection of ultrasonic surface displacements

We present a novel scheme with which to detect small ultrasonic surface displacements by use of a photochromic polymer instead of a photorefractive material as an adaptive beam combiner in a two-wave mixing geometry. Poly(methyl methacrylate) is doped with a derivative of zinc tetrabenzoporphyrin that possesses a long-lived triplet state that can be efficiently populated in a reversible manner. The resulting dynamic hologram consists of local absorption and refractive-index gratings, which can process speckled beams reflected from rough surfaces. We believe that this is the first use of a local nonlinear medium for adaptive homodyne detection of ultrasonic surface displacements.

Influence of an ultrafast transient refractive-index grating on nonlinear optical phenomena

With two interfering femtosecond pulses it is possible to produce an ultrafast transient refractive-index grating with relatively high contrast in a transparent material. The grating has a significant impact on nonlinear optical effects, it is responsible for a loss of surface sensitivity in second-harmonic generation from centrosymmetric materials, an enhancement of the generated third harmonic due to self-phase matching, and for the diffraction of the generated harmonics. The latter phenomenon is determined by the nonlinear equivalent of the diffraction law. In a nonresonant material, the refractive-index grating is a thin slide with the thickness of the spatial equivalent of the pulse duration, propagating through the crystal with the group velocity. The duration of the grating depends only on the pulse width, in our case 80\char21{}100 fs. Besides offering a fundamental point of view, this opens the way to a variety of interesting applications, in particular in telecommunications, such as any kind of beam manipulation, optical switching, or demultiplexing, all of them on a femtosecond time scale.

Silicon-containing polymers for non-linear optics

A variety of novel poly[ethynediyl-arylidene-ethynediyl-silylene]s (PEAES), including those containing hypercoordinate silicon atom as well as donor and acceptor groups, has been synthesized. The presence of hypercoordinate silicon favours high values of fast Kerr-type non-linearities, and the incorporation of amide groups gives rise to good optical quality nanocomposites in a silica matrix. The inorganic-organic hybrid sol-gel film incorporating 5–14 mass% of the polymer containing pentacoordinate silicon showed Reχ (3) = −9.5 ×10−11 esu. The predominant role of the electron nonlinear mechanism has been demonstrated by using test beams with linear and circular polarizations. The photorefractive properties of composites based on PEAES as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm without preliminary electric poling. The photorefractive origin of the refractive index changes was confirmed by the high two-beam gain under an externally applied electric field. Refractive index gratings affording two-beam gain can also be written under zero field conditions. To explain this unique property of the materials it is suggested that chromophore orientation asymmetry arises either during film formation or is induced by the longitudinal intensity gradient.

Wave-mixing solitons in ferroelectric crystals

Although the sine-Gordon equation was originally obtained for the description of four-wave mixing in transmission geometry, it describes self-diffraction of the wave from shifted gratings as well. The sine-Gordon equation governs soliton propagation. The photoinduced amplitude of the refractive-index grating exhibits also a soliton shape in the crystal volume. The origin of this effect is the change of the contrast of light due to energy transfer between coupled waves during their propagation, which occurs in bulk crystals with strong photorefractive gain. The theoretical description shows the possibility to control the soliton properties by changing the input intensity ratio and/or input phase difference of the wave. The effect can lead to diffraction efficiency management, auto-oscillations and bistability of the output waves due to wave-mixing in ferroelectrics. Results on the first experimental observation of non-uniform distribution of the grating amplitude profile and its changes versus input intensity ratio are presented.

Photorefractive properties of new polymer composites incorporating poly[ethynediyl-arylene-ethynediyl-silylene

The photorefractive properties of low glass transition temperature, Tg, composites based on oligomeric poly[ethynediyl-arylene-ethynediyl-silylene]s as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm. The photorefractive origin of the refractive index changes was confirmed by the achievement of a two-beam gain as high as 67 cm−1 under an externally applied electric field (E0 = 16 V µm−1). Refractive index gratings affording two-beam gains up to 51 cm−1 can also be written under zero-field conditions. A possible explanation for this unique property of the materials is that chromophore orientation asymmetry can be induced by the longitudinal intensity gradient.

Persistent refractive index gratings in bismuth germanium oxide doped with chromium

The kinetics of the production and dark-decay of index gratings in chromium-doped bismuth germanium oxide Bi12GeO20 (BGO) and undoped BGO were investigated as a function of temperature from room temperature to about 500 K. During the write process in undoped BGO the gratings show a fast leading edge peak, which drops quickly to a steady saturated value. The leading-edge peak does not change significantly at the higher temperatures. Grating production was much slower in the BGO:Cr samples and the gratings grew monotonically throughout the 0.5 s write interval. At room temperature the gratings in BGO:Cr are highly persistent. A grating written at room temperature retains 70% of its initial strength after 24 h when it was read only once an hour. Optical erasure plays a major role in the decay of such gratings; reading the grating every 6 min reduced its strength to 48% of its initial value after 24 h. The grating written at room temperature and monitored as the sample was heated decayed within the same 10 K temperature range as the photochromic absorption bands. As expected, undoped BGO shows a rapid dark decay at room temperature that becomes faster at higher temperatures.

Electro-optically reconfigurable waveguide superimposed gratings

A new concept of electro-optically reconfigurable waveguide superimposed gratings with their widely controllable transmission characteristics are present. It is based on two types of superimposed refractive index gratings, electronically induced in an electro-optically active core. These gratings can be independently switched ON and OFF or both simultaneously activated with the controllable weighting factor. In this case its transmission characteristics represent double-dip rejection band spectrum with independent control of the dip positions and bandwidth. This simple concept opens opportunities for developing a number of tunable devices for integrated optics by use of the proposed design as a building block.

Nonlocal dynamic gratings and energy transfer by optical two-beam coupling in a nematic liquid crystal owing to highly sensitive photoelectric reorientation

We investigate a new ultrahighly sensitive nonlinear optical response in pure nematic liquid crystals composed of rodlike molecules characterized by conjugated acetylene groups. Under the combined application of low-intensity optical waves and dc electrical fields, a photorefractivelike grating formation has been found in degenerate four-wave mixing experiments. Polarization-dependent pump–probe experiments reveal the underlying orientational effect of a space-charge field. The photorefractive-like origin of the gratings was proved by two beam-coupling experiments in which a refractive-index grating was phase shifted by π/2 with respect to the interference grating. The huge optical nonlinearity found in such materials makes them promising for future applications in all-optical image processing and adaptive optics.

Surface-assisted optical storage in a nematic liquid crystal cell via photoinduced charge-density modulation

Abstract This study reports the observation of quasi-permanent recording of holographic gratings in a twisted nematic liquid crystal cell by He–Ne 632.8 nm laser light. The cell is equipped with blocking SiO 2 layers and polyimide insulating/alignment layers and filled with liquid crystalline 6CHBT, doped with an anthraquinone derivative dye. Photogeneration in the bulk of the liquid crystal, photocurrent flow, and loading of the surface with spatially modulated charge density are the major mechanisms which lead to surface electric field induced director reorientation. The novelty of the system described by us is the possibility of ac frequency tuning of the intensity of light diffracted at the so formed refractive index grating.

Boundary conditions for optical rectification and application to degenerate four-wave mixing in a novel geometry

We derive general boundary conditions for optical rectification of Gaussian beams and discuss the effect on cascaded optical rectification and the linear electro-optic effect as well as terahertz generation. These conditions are applied to degenerate four-wave mixing in a novel geometry in which we experimentally demonstrate, for the first time to our knowledge, that the combination of optical rectification of a single pump beam and the linear electro-optic effect produces a refractive-index grating in the transparent spectral region of the inorganic crystal KNbO3. The intensity of a probe beam diffracted by this grating was determined and corresponds to the theoretically expected result obtained from the known material parameters.

Ultrafast active phase conjugation in broad-area semiconductor laser amplifiers

We predict and investigate ultrafast optical phase conjugation generated by the spatiospectral mixing of two picosecond light pulses in the active area of a broad-area laser. The microscopic internal processes linked to these phenomena are described on the basis of multiwave Maxwell–Bloch equations and include, in particular, the dynamic interactions of the counterpropagating light fields self-consistently with the dynamics of the active nonlinear charge-carrier plasma within the laser amplifier. Microscopic simulations demonstrate the spatiotemporal dynamics of the fields and reveal the underlying physical processes of the ultrafast refractive-index grating formation: dynamic spatiospectral hole burning, carrier heating, and coupled diffraction and interference of the light beams by means of the nonlinear induced changes in the spatiospectral carrier and refractive-index distribution.

Analysis of multiple wavelength DFB fiber lasers

The behavior of multiwavelength distributed feedback fiber lasers is simulated in a comprehensive numerical model. Multiple fundamental modes can coexist, even though the coupling coefficients of their respective refractive index gratings are not equal, due to induced dynamic gain gratings. We have investigated the effect on the lasing state of variations in coupling strengths, gain parameters, grating Bragg frequency separation, relative phase between the gratings, length of the structure relative to the beat lengths between the gratings, grating center phase-shift errors, and saturation level of the UV-induced refractive index change.

250-W average-power Nd:YAG laser with self-adaptive cavity completed by dynamic refractive-index gratings

A high average power laser oscillator with a cavity completed by a nonlinear mirror formed by refractive index gratings which accompany population gratings induced in Nd:YAG laser crystals by generating beams themselves is investigated numerically and experimentally. The role of different noise sources at the initial stage of nonlinear cavity formation is studied. The adaptation of the cavity formed by nonlinear dynamic mirrors to intracavity distortions is demonstrated. The generation of beams with average power up to 250 W, near-diffraction-limited divergence, and long coherence length has been achieved.

Photoconducting Polymer - Nematic Liquid Crystal Hybrid Structures the Promising Choice for Optical Information Processing

We present recent progress in development of liquid crystalline optically addressed spatial light modulator, its characterisation and performances. The spatial light modulator is based on photoconducting polymer - nematic liquid crystal hybrid structure and is used in various configurations of holographic recording and reconstruction. The refractive index gratings arising in a system with photoconducting electrode layer are tilted and phase shifted with respect to the light intensity pattern therefore allowing for an extremely efficient energy transfer between the incoming beams. Depending on optical systems used, several useful optical devices have been constructed by us, like: phase conjugate mirrors, Fourier optical correlators for pattern recognition, coherent light amplifiers, incoherent-to-coherent light converters. Their performances are directly linked with physical properties of used materials and the principal photorefractive mechanism which will discussed.

Permanent narrow-band reflection holograms for infrared light recorded in LiNbO3:Ti:Cu channel waveguides

Permanent refractive-index gratings are generated by thermal fixing of holograms in photorefractive lithium niobate channel waveguides. The guides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high-temperature recording with green light, refractive-index modulations exceeding Δn=8×10-5 for light of the telecommunication wavelength 1550 nm appear without the need of any development process of the written holograms. The gratings are stable in the dark and no compensation mechanism via dark conductivity is observed. Thus this method may be well suited for long-time applications in holography and integrated optics.

Efficient self phase matched third harmonic generation of ultrashort pulses in a material with positive dispersion

An ultrafast transient refractive index grating, produced in barium fluoride, a material with positive dispersion, promotes very efficient third harmonic generation. The significant enhancement of the generation up to a conversion efficiency of about 3% is due to self phase matching, involving the instantaneous grating. At the same time several diffraction orders of the third harmonic signal are observed behind the sample.

Increase in phase-conjugate reflectivity of a holographic Nd:YAG oscillator due to resonant refractive-index grating

We have studied both experimentally and numerically the role of resonant refractive-index gratings (RIGs) in a self-pumped phase-conjugate mirror based on an Nd:YAG amplifier with nonreciprocal feedback loop. In experiment the generation of the phase-conjugating laser was delayed with respect to the recording of a holographic nonlinear mirror by an external beam. An increase in the phase-conjugate reflectivity at a delay time of about 3 μs was measured. The numerical calculation confirmed the additional contribution that the resonant RIG formed in a pumped Nd:YAG crystal because of excitation of a higher-lying level of Nd 3+ ions make to the efficiency of phase conjugation.