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.     

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.     

Surface-grating distributed bragg reflector quantum-well lasers fabricated in AlGaAs-GaAs asymmetric epitaxial waveguides

Ridge waveguide lasers, integrated with single deep-surface distributed Bragg reflectors (DBR's) in the passive section, were fabricated with a GaAs-AlGaAs double-quantum-well structure in an asymmetric waveguide. Third-order gratings, with a period of 389 nm and defined by holographic lithography, were formed by low-damage reactive ion etching processes. The grating losses and optical coupling coefficients were estimated, in particular, by use of the relationship between the real and the effective grating lengths that were computed and reexamined by measurements of grating periodicity and mode spacing. By use of two different geometries, we produced guide lines for obtaining high-performance lasing properties for these surface-grating DBR lasers. Additionally, a detailed analysis of lasing wavelength shifts was carried out for this study. It was found that injected-carrier-induced effects shift the lasing wavelength much more than gain-loss competition within an extended DBR laser cavity.     

Dual wavelength fibre laser with tunable channel spacing using an SOA and dual AWGs

In this paper, we propose and demonstrate a dual wavelength fibre laser (DWFL) based on the use of an inhomogeneously-broadened semiconductor optical amplifier (SOA) gain medium as well as two arrayed waveguide gratings (AWGs) together with two optical channel selectors (OCSs) and a broadband fibre Bragg grating (FBG) to generate dual wavelength output at variable channel spacings. The widest spacing obtained from the DWFL is 12.21 nm, while the narrowest spacing is 1.16 nm. The DWFL has good stability with only minor power fluctuations of less than 2 dB and a side mode suppression ratio (SMSR) of approximately 38.5 dB with fluctuations of less than 0.5 dB.     

Wavelength response of waveguide volume grating couplers for optical interconnects

The wavelength response of a waveguide volume grating coupler (WVGC) is analyzed for coupling light from a slab waveguide into the superstrate. A leaky-mode approach is used in conjunction with rigorous coupled-wave analysis. A quantitative theoretical study of the effect of index modulation, waveguide index, and grating thickness on the wavelength bandpass of a WVGC is also presented. The FWHM wavelength bandpasses found for high-efficiency couplers range from 173 to 525 nm. The various Bragg conditions that can be used in designing a WVGC are also presented and compared. The use of the propagation constant of the mode being outcoupled as the incident wave vector in the Bragg condition is shown to produce the highest coupling efficiency.     

Narrowband Bragg reflectors in Ti:LiNbO3 optical waveguides

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.     

Acousto-optic superlattice modulation in fiber Bragg gratings

The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings, treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters.     

Holographic Bragg grating input-output couplers for polymer waveguides at an 850-nm wavelength

Bragg gratings used as input-output couplers in polymeric waveguides have been demonstrated at infrared wavelengths. These Bragg grating couplers were holographically formed volume phase gratings with a near-45 degrees fringe slant angle embedded directly into a waveguide layer. A photopolymer was used for both producing a planar waveguide and constructing the embedded Bragg grating coupler. A coupling efficiency of 23% for input and 5% for output has been achieved at 850 nm. The output-coupling beam profiles are also discussed.     

Hot embossing of grating-based optically variable images in thermoplastic acrylic lacquer

We report the novel use of thermoplastic acrylic lacquer (automotive paint) in the hot embossing of nanoscale structures. Replicas of grating arrays have been produced in coatings of acrylic lacquer using a standard embossing process. The master dies for the experiments comprised grating arrays fabricated by electron beam lithography. Grating patterns with a pitch of 0.7–1.3 μm were configured to produce diffractive images over an area of ∼25 × 25 mm. The embossing experiments used a replicated Ni shim as a die and were performed at 100–150 °C and 80 kN force. A temperature above the glass transition temperature for the lacquer, T = 120–150 °C, was required in order to achieve a uniform impression across the embossed area of ∼80 × 80 mm. The diffractive grating patterns which were embossed into acrylic lacquer have shown optical effects suitable as a security feature including image switching and color movement.     

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.     

Passive temperature-compensating package for optical fiber gratings

We demonstrate a compact, passive temperature-compensating package for fiber gratings. The grating is mounted under tension in a package comprising two materials with different thermal-expansion coefficients. As the temperature rises the strain is progressively released, compensating the temperature dependence of the Bragg wavelength. A fiber grating mounted in a package 50 mm long and 5 mm in diameter exhibited a total variation in Bragg wavelength of 0.07 nm over a 100 °C temperature range, compared with 0.92 nm for an uncompensated grating.     

Polymer-based waveguides and optical switching

The fabrication and linear optical characterization of a Bragg reflector in a planar polymeric waveguide, suitable for an all-optical switching device, is reported. Surface corrugation gratings with a grating constant of 349 nm were produced in different polymeric films of poly(phenylene-vinylene), polydiacetylene and polystyrene by excimer laser photoablation. Furthermore, the gap in the linear transmission spectrum of the modulated waveguide was measured and compared with the result of numerical simulations. A good agreement can only be found if a non-uniform grating with a small chirp is assumed. This small deviation from the uniform grating cannot be resolved with a scanning electron microscope. However, this linear optical investigation helps to further improve the fabrication process and opens the possibility to yield a well-defined grating with a prescribed modulation strength, which is a vital requirement for the proper operation of a nonlinear all-optical switch.     

Narrow band fiber-optic phase-shifted Fabry-Perot Bragg grating filters for atmospheric water vapor lidar measurements

A unique ultranarrowband fiber-optic phase-shifted Fabry-Perot Bragg grating filter for atmospheric water vapor lidar measurements was designed, fabricated, and successfully tested. Customized optical fiber Bragg gratings were fabricated so that two transmission filter peaks occurred: one (89% transmission, 8 pm FWHM) near the 946-nm water vapor absorption line and the other peak (80% transmission, 4 pm FWHM) at a region of no absorption. Both transmission peaks were within a 2.66-nm stop band. Demonstration of tension tuning to the 946.0003-nm water vapor line was achieved, and the performance characterization of custom-made optical fiber Bragg grating filters are presented. These measurements are successfully compared to theoretical calculations using a piecewise-matrix form of the coupled-mode equations.     

单光纤光栅实现窄带全光纤反射器的分析

提出了一种由单个光纤光栅和一个光纤方向耦合器组成的新型全光纤反射器,推导出了当光栅为均匀 Bragg 光栅、器件任意端口输入时,任何一端口的输出解析式。分析表明器件具有法布里-珀罗腔干涉仪的特点,耦合器的耦合比系数类似于法布里-珀罗腔的反射率, 耦合比系数越大,输出光谱半高全宽度(FWHM)越窄, 消光比越好。当耦合比系数大于 0.8 时,FWHM 可以窄到0.02nm,消光比大于 0.9。如果光栅是“强”耦合,器件具有均匀分布的多通道梳状输出特性;光栅为“弱”耦合时,则能实现 FWHM 小于 0.02nm 的单频输出。器件只需单个光栅,克服了制作两个完全相同光栅的困难。     

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.     

Transient wavelength performance of 1.53 microm InP laser diodes for pumping of Er3+-doped solid-state lasers

The transient wavelength performance of high-power InP laser diode stacks with internal Bragg gratings is studied experimentally. Because of the wavelength selective nature of the internal grating, which is decoupled from the actual gain profile dynamics of the laser diode, the wavelength drift with respect to temperature and current is greatly reduced and steady-state conditions are reached after a short settling time of less than 50 ms.     

Fiber Bragg grating flow sensors powered by in-fiber light

This paper presents an active fiber Bragg grating temperature and flow sensor based on self-heated optical hot wire anemometry. The grating sensors are directly powered by optical energy carried by optical fibers. In-fiber diode laser light at 910 nm was leaked out from the fiber and absorbed by the surrounding metallic coating to raise the temperature and change the background refractive index distribution of the gratings. When the diode laser is turned off, the grating is used as a temperature sensor. When the diode laser is turned on, the resonance wavelength and spectral width change of the self-heated grating sensor is used to measure the gas flow velocity. The grating flow sensors have been experimentally evaluated for different grating length and input laser power. The grating flow sensors have demonstrated a 0.35- m/s sensitivity for nitrogen flow at atmosphere pressure.     

Electrically controlled integrated optical filter

The possibility of electric control over the spectral transfer function of an integrated optical filter comprising a reflection Bragg grating in a photorefractive waveguide is demonstrated for the first time. The filter has a spectral selectivity of 0.15 nm, ensures continuous tuning within ～0.1 nm, and can be electrically switched on and off. The proposed method of control is effective and has good prospects for practical applications.     

Theoretical performance of Bragg gratings based on long-range surface plasmon-polariton waveguides

A plasmon-polariton Bragg grating (PPBG) concept, based on the propagation of the long-range ss0b mode in structures comprising a thin metal film of finite width embedded in a homogeneous background dielectric, is discussed theoretically. The PPBGs are operated in an end-fire arrangement with access plasmon-polariton waveguides or optical fibers being directly butt-coupled to their input and output ports. A model for the PPBGs, which was recently proposed and validated experimentally for third order structures, is used to generate theoretical results describing their expected performance for various architectures. First order uniform periodic, interleaved, and apodized grating structures are considered and compared. Third order uniform periodic designs are also considered. The gratings investigated are based on a 20 nm thick Au film embedded in SiO2 and have a Bragg wavelength near 1550 nm. First order uniform periodic gratings provide the strongest reflection, with a maximum reflectance of about 97% being achievable over a length of a few millimeters and over a full width at half-maximum bandwidth of about 0.8 nm. The off-resonance insertion loss of the gratings can be as low as a few decibels. Specific Bragg wavelengths can easily be attained using interleaving without requiring extraordinary resolution from the fabrication process. Apodized designs providing low sidelobe levels are also investigated.     

Surface-relief fiber Bragg gratings for sensing applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.