Surface periodic domain structures for waveguide applications

We report the results of fabrication and investigations of surface periodic domain structures created by a set of quasi-point e-beam irradiations both on the Y- and X-cuts of LiNbO(3), and on Ti:LiNbO(3) and Zn:LiNbO(3) planar waveguides. Domain gratings with spatial periods from 4.75 to 7.25 μm were formed by a 25-keV e-beam. Doses from 500 to 2000 μC/cm(2) were used for different structures to estimate optimal fabrication conditions. The investigations allowed the visualization of the formed surface domain structures, estimation of their uniformity, and determination of waveguide generation of the second optical harmonic. The surface structures can be used in optical devices for the realization of quasi-phase-matched frequency conversion, which includes the creation of compact radiation sources based on waveguides.     

Does photosensitivity pave the way towards the fabrication of miniature coherent light sources in inorganic glass waveguides?

Photosensitivity designates the ability to permanently change the refractive index of a glass by optical processing. The phenomenon allows the fabrication of numerous phase structures, the simplest of which is the Bragg grating obtained by photoimprinting a periodic index modulation within the material. Bragg gratings have changed the way in which optical fibre (or planar waveguide) lasers are now used. Distributed Bragg Reflector (DBR) or Distributed Feed Back (DFB) lasers, when intracore Bragg gratings are used for linear cavity feedback, are commonly fabricated in rare earth doped optical silica-based fibres. On the other hand, photosensitivity can also result in effects which can prove to be detrimental to the fabrication of miniature coherent light sources. The paper will cover some of the advances that have been made in improving the photosensitivity of inorganic glasses, in searching new photosensitive materials and in characterizing Bragg grating properties relevant to laser applications.     

Tools for synthesising and characterising Bragg grating structures in optical fibres and waveguides

Optical waveguide filters based on optical fibre Bragg gratings are used for a wide range of applications in communications systems, fibre lasers and optical fibre sensors. The technical specifications for many of these grating based devices are becoming increasingly challenging. To make technological advances in this area a number of tools, such as: grating synthesis, advanced fabrication techniques, characterisation and reconstruction techniques, are required to improve grating structures. Closing the loop between design and physical realisation has the potential to achieve an unprecedented degree of accuracy in device fabrication. Grating design, fabrication and characterisation has moved from merely an inexact experimental procedure to a highly controlled engineering process. The mathematical and physical tools to achieve this are described and examples of applications are used to illustrate the potential of these capabilities to accelerate further understanding and development of photosensitivity and gratings in novel optical fibres and waveguides in the future.     

Negligible birefringence in dual-mode ion-exchanged glass waveguide gratings

Polarization dependence of UV-written Bragg gratings in buried ion-exchanged glass waveguides is investigated. A polarization-dependent shift in Bragg wavelength of less than 0.02 nm is measured, both for the even and the odd modes of a laterally dual-mode waveguide. The measured wavelength shift corresponds to a waveguide birefringence of the order of 10(-5), which is negligible for most applications in optical communications. It is observed that the UV-induced birefringence is small, within the limits of the measurement accuracy. The thermal stability of the fabricated gratings is also very good. The results are of particular importance for devices considered here since they require a polarization-independent mode-converting waveguide Bragg grating. Polarization-independent performance of these gratings enables the fabrication of a new class of integrated optical devices for telecommunication applications.     

Single-pulse ultraviolet laser recording of periodically poled structures in polymer thin films

A simple fabrication technique for nonlinear polymeric optical waveguide patterns is introduced based on the two-beam interference method. We determined that the second-order nonlinearity of poled polymer films is erased by single-pulse ultraviolet (UV) laser irradiation. The erasure mechanism for second-order nonlinearity is discussed. To form a periodic structure in an optical polymer waveguide, two types of optical configuration of two-beam interference were arranged, and a single-pulse UV laser was exposed directly onto poled films. We prove that this method provides a simple way to fabricate volume-type and ridge-type periodically poled structures, i.e., chi((2)) gratings, from the submicrometer to the millimeter range.     

Embossed Bragg Gratings Based on Organically Modified Silane Waveguides in InP

Considering the large variety of applications for optical glass waveguide gratings, the effective production method of embossing for micropatterning, and the unique advantages of InP-based materials, we expect that hybridization of embossed optical glass waveguide gratings and InP substrates will inevitably lead to new applications in integrated optics. We present our preliminary results of research toward the development of solgel-derived glass waveguide gratings made by embossing on InP. Theoretically, the dependence of the stop-band FWHM and transmission contrast of the grating filter on the grating length, and the relationship between the Bragg grating's reflective wavelength and the dopant concentration in the solgel waveguide, are obtained. Experimentally, using organically modified silane, we solve the problem of mismatching of SiO(2) and InP, and successfully fabricate an embossed glass grating with a second-order Bragg reflection wavelength of 1580 nm and a FWHM of 0.7 nm fabricated upon a solgel waveguide on an InP substrate.     

Fabrication of optical gratings by shrinkage of a rubber material

Ordered wavy surface structures generated by deposition of a metal thin film on a pre-stretched PDMS plate were fabricated and its potential application for optical gratings was proposed. The orientation of the generated structures was always perpendicular to the pre-stretched direction and the pitch of the structure could be adjusted ranging from 4.5 μm to 6.8 μm by controlling the strength of the pre-stretched strain and the thickness of the surface metal film. Based on these periodic structures, various optical gratings were demonstrated. With a slight modification of the fabrication scheme, gratings with different orientations can be fabricated on both sides of the PDMS plate, the double-sided gratings, could be fabricated. It is believed the current method has the potential for the fabrication of a large-scale diffractive grating at lower costs.     

Radiation characteristics of waveguide diffractive doublets

The radiation characteristics of waveguide diffractive doublets consisting of double gratings located on two surfaces of waveguide cladding film are modeled based on a singular perturbation method. We determine the conditions under which the presence of the upper grating does not affect the radiation characteristics of the waveguide diffractive doublet as a whole. This allows independent performance of the upper grating, which may be replaced by a general diffractive optical element, and of the lower grating as a waveguide grating coupler. The results obtained provide an alternative method for determining the thickness of cladding film in the waveguide diffractive doublets for guided-wave manipulation.     

Photonic crystal waveguides by direct writing of e-beam on self-assembled photonic crystals

Direct electron beam lithography technique is used for writing a variety of waveguide structures on thin films of polymethyl methacrylate (PMMA) and self-assembled three-dimensionally ordered photonic crystals made up of PMMA colloidal spheres. The waveguide structures fabricated on both these type of samples are characterized by scanning electron microscope and optical microscope images.     

Embossable grating couplers for planar waveguide optical sensors

Planar optical waveguides are an attractive tool for use in analytical chemistry and spectroscopy. Although similar to fiber optics, planar waveguides have been slow to be commercially accepted due to the difficulty of coupling light into the guide. Generally, prism coupling is the method of choice in the laboratory, as efficiencies approaching 80% can be reached. However, prisms are impractical for routine use for several reasons: expensive positioning equipment is required, coupled power is sensitive to environmental fluctuations, and prism coupling prohibits the fabrication of a truly planar device. The use of thin gratings on the surface of the waveguide allows for a two-dimensional structure to be maintained, while providing enough efficiency to be useful as a sensor. Our research efforts focus on developing a technique to make inexpensive, reproducible gratings that are easy to fabricate. By chemically modifying the surface of a commercial grating with a suitable release agent, it is possible to emboss replica gratings onto a variety of waveguide types. The fabrication of embossed gratings will be described, and their performance on glass, ion-diffused, polymer, and semiconductor waveguides will be presented.     

Molecular Azo Glasses as Grating Couplers and Resonators for Optical Devices

Multifunctional molecular glasses are versatile tools for organic optoelectronics. Their optical waveguide properties are of increasing interest. By using photopatterned molecular azo glass gratings, optical waves within multilayer devices can be controlled, optically excited, and brought to resonance.     

基于光纤光栅的铝合金腐蚀监测研究

铝合金腐蚀是导致航空器性能下降的主要原因之一.铝合金腐蚀初期以点蚀为主,体积几乎不改变,结合铝合金腐蚀的这一特点,设计了基于光纤光栅的薄片型和应力束缚型两种腐蚀监测结构,对铝合金腐蚀进行了实验研究.腐蚀发生前,对光纤光栅施加一定预应力,随着腐蚀的发生应力被逐渐释放,通过测量传感波长的漂移量就可以得到铝合金的腐蚀情况.实验表明,基于光纤光栅的腐蚀监测结构能够真实的反应铝合金的腐蚀情况,并且光纤光栅本身具有体积小、抗电磁干扰、耐腐蚀的特点,非常适合于航空器的铝合金腐蚀监测.     

Waveguide analysis of organic light-emitting diodes fabricated on surfaces with wavelength-scale periodic gratings

Numerical techniques for the analysis of multilayer waveguide structures were used to study the modes that exist in organic light-emitting diode (OLED) devices. The analysis revealed that waveguide modes of the OLED structure could be grouped, according to the behavior of modal-field profiles in the air cover and the glass substrate, into one of four different "families": (i) bound mode, (ii) semibound modes, (iii) leaky modes, and (iv) nonphysical modes. Four different OLED samples were fabricated on glass substrates on which photoresist gratings had been previously formed. The theory was used to compute the angles at which light from these devices should exit into the air. Theory and data agreed well for the semibound modes for all samples; however, they did not agree so well for the leaky modes. Further investigation revealed that better agreement between theory and data could be obtained with these modes being analyzed as Fabry-Perot cavity modes. The theoretical relation between leaky waveguide modes and Fabry-Perot cavity modes is discussed.     

掺Er3+:Al2O3平面光波导的溶胶-凝胶制备

掺铒光波导主要用来制作光波导放大器。采用溶胶-凝胶法制备了透明、稳定的掺铒的铝硅酸盐玻璃溶胶;使用浸渍提拉法或旋转涂敷法,并通过放在乙醇蒸汽中干燥,可在玻璃表面得到均匀的凝胶膜;最后样品在一定温度下烧结;通过多次重复涂敷-烧结这一工艺流程可以形成厚的硅酸盐玻璃膜层并得到波导结构。使用显微镜研究了凝胶膜表面性质,通过棱镜耦合法测量了波导的有效折射率Nm,也测定了所制备波导的吸收光谱。溶胶-凝胶法在制作均匀性好、高掺铒浓度的铝硅酸盐玻璃光波导上具有优势。     

Optical masks prepared by using a liquid-crystal light valve for light-induced photorefractive waveguides

The optical mask prepared by using a LCLV (liquid-crystal light valve) are first proposed for light-induced photoreactive waveguides in photorefractive materials. Employing this technique, various waveguide structures can be fabricated, e.g., Y- or multiple-branches waveguides, fiber-like waveguides, and Mach-Zehnder-like switches, and even whole optical circuits may be formed. A Y-branches waveguide and a fiber-like waveguide were demonstrated in a LiNbO3:Fe crystal. Several technical problems, such as intensities, resolutions, writing speed, and so on, were also discussed in detail. Using a LCLV with a fast response and a writing beam with a high intensity, the waveguide structures may be changed in real time.     

The Finite-conjugate Self-imaging and the Generalized Lau Effect Due to Complex Gratings

Abstract

Laser induced gratings in nonlinear optical media, which are complex or phase gratings, are the key structures in many applications involving laser beam-nonlinear material interactions. The concept of coded imaging of self-imaging structures, developed elsewhere, has been utilized to make a comprehensive study of the self-imaging properties, at finite conjugates, of model complex or phase gratings. The same approach has been used to extend our previous work on the generalized Lau effect. An interesting application of the self-imaging process has been made to visualize, at a certain primary magnification, laser induced thin photorefractive grating in a BSO crystal. Possible applications of this technique include visualization of dynamic or developed laser induced gratings in thin nonlinear optical materials.     

Coupled mode theory: Limitations and applicability

Abstract

Coupled mode theory (CMT) is commonly employed in the analysis of optical waveguide structures involving gratings. As an approximate theory, there are expected limitations to the applicability of this technique. The limitations put forward in the literature are mostly intuitive as the common derivation of the coupled mode equations is of a heuristic nature. It relies upon a slowly varying approximation, leading to the expectation that CMT fails in structures displaying rapidly varying characteristics. Here, CMT is tested in the geometry of a coupler incorporating a grating where rapid and large variations of the perturbed modes occur. Its applicability in this situation is confirmed.     

Electro-optic polymer waveguide grating with fast tuning capability

A novel fast tunable electro-optic (EO) polymer waveguide grating is proposed and designed. Its resonant wavelength can be linearly tuned via the first-order EO effect with a high sensitivity of 6.1 pm/V. We find that the spectrum characteristics of EO polymer waveguide gratings depend strongly on many grating parameters, such as refractive-index modulation, modulation function, grating period, and period number. Material selection, fabrication technology, EO tuning ability, and polarization dependence of EO polymer waveguide gratings are also discussed. Such a waveguide grating not only overcomes the disadvantages of fiber-optic gratings, such as slow wavelength tuning ability and large-scale integration inconvenience, but also has many advantages, such as high resonant-wavelength tuning sensitivity, the same fabrication technology used for semiconductors, and polarization independence.     

Optical add-drop multiplexers based on the antisymmetric waveguide Bragg grating

A novel optical add-drop multiplexer (OADM) based on a null coupler with an antisymmetric grating was designed and experimentally demonstrated. The antisymmetric grating exclusively produces a reflection with mode conversion in a two-mode waveguide. This improves the performance compared with previous demonstrations that used tilted Bragg gratings. Our design minimizes noise and cross talk produced by reflection without mode conversion. In addition, operational bandwidth and, versatility are improved while the compactness and simplicity of the null coupler OADM are maintained.     

Implementation of a graded-index medium by use of subwavelength structures with graded fill factor

We present a novel configuration for the implementation of subwavelength-based graded-index devices. The proposed concept is based on the etching of one-dimensional subwavelength gratings into a high-index slab waveguide to achieve the desired effective index distribution. A graded-index profile can be achieved by gradually modifying the duty ratio of the grating along the horizontal axis, while the beam is confined in the vertical direction by the slab waveguide. On the basis of this concept, novel graded-index lenses and waveguides are both proposed and characterized numerically by use of finite-difference time-domain and finite-element analysis. The proposed devices can be used for guiding, imaging, optical signal processing, mode matching, coupling, and other applications while offering the intrinsic advantages of on-chip integration such as miniaturization, eliminating the need to align each component separately, and compatibility with standard microfabrication techniques for manufacturability.