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We present a stable and switchable dual-wavelength erbium-doped fiber laser. In the ring cavity, an inverse-Gaussian apodized fiber Bragg grating serves as an ultranarrow dual-wavelength passband filter, a semiconductor optical amplifier biased in the low-gain regime reduces the gain competition of the two wavelengths, and a feedback fiber loop acts as a mode filter to guarantee a stable single-longitudinalmode operation. Two lasing lines with a wavelength separation of approximately 0.1 nm are obtained experimentally. A microwave signal at 12.51 GHz is demonstrated by beating the dual wavelengths at a photodetector. [c] 2010 Optical Society of America OCIS codes: 060.2330, 060.3510, 060.3735.

An all-fiber sensor scheme for simultaneous strain and temperature measurement is presented. The sensing head is formed by serially connecting a polarization maintaining photonic-crystal-fiber-based intermodal interferometer (IMI) with a fiber Bragg grating (FBG). The IMI, exhibiting an opposite strain response as compared to that of the FBG, is highly sensitive to strain, while it is insensitive to temperature. This has potential for improving the strain and temperature measurement resolutions. A sensor resolution of [+ or -] 8.3 [micro][epsilon] in strain and [+ or -] 2[degrees]C in temperature are experimentally achieved within a strain range of 0-957.6 [micro][epsilon] and a temperature range of 24[degrees]C-64[degrees]C, respectively. OCIS codes: 060.2310, 060.2370.

We propose an electrically controlled terahertz switch based on metallic grating--liquid crystal--metallic grating (MG-LC-MG) structures. The switching mechanism is realized by modifying the effective refractive of the LC using different bias electric fields. In our design, the MGs not only support supertransmittance at certain frequencies, but they also act as the electrodes. Simulation results show the proposed structure has the potential to realize an electrically controlled terahertz switch with a high extinction ratio and low insertion loss. A prototype design is also proposed for practical implementation. OCIS codes: 040.2235, 050.2230, 050.2770, 050.6624, 160.3710.

A three-parameter optical sensor based on a tilted fiber Bragg grating is proposed. Through the monitoring of the wavelength shift of the core mode resonance and the ghost mode resonance, it is possible to discriminate strain and temperature. In addition, the refractive index can be determined by calculating the normalized transmission spectrum area. With the current approach, resolutions of up to 5.7 x [10.sup.-4], 4 [mu][epsilon], and 3.1[degrees]C were achieved, for refractive index, strain, and temperature, respectively. The developed sensor can be an important tool in several areas of engineering, namely, biomedical, biological, and environmental sensing. OCIS codes: 060.2310, 060.2370, 060.3735.

A compact Si-nanowire-based reflective arrayed-waveguide grating (AWG) for dense wavelength-division multiplexing is proposed. At the end of each waveguide in the array, there is an individual photonic crystal (PhC) reflector, which makes the AWG layout design very flexible. All the PhC reflectors are with the same design. With such a design, the total size of the AWG (de)multiplexer is reduced by more than a half. The reflection efficiency of the used PhC reflectors is enhanced by optimizing the taper between the arrayed waveguides and the PhC reflector. A 200 GHz AWG (de)multiplexer is designed and fabricated as an example. The total size is only about 193 [micro]m x 168 [micro]m. OCIS codes: 230.7408, 230.5298.

Double femtosecond laser pulses are usually generated by using a Michelson structure. We propose a novel and simple device by using two high-density transmissive gratings for generation of double pulses and conversion between single and double laser pulses by shifting one of the two gratings by a quarter period. The apparatus has the advantages of compact volume, simple structure, and convenience in manipulation. Experimental outputs of the double laser pulses are well verified in experiment, which can be properly explained by numerical simulation with the rigorous coupled-wave theory. This structure provides an interesting approach for generation of double pulses and conversion between single and double laser pulses for practical applications of the femtosecond laser. [c] 2010 Optical Society of America OCIS codes: 320.7080, 050.1950.

We propose and demonstrate a displacement and angular drift simultaneous measurement technique based on a defocus grating. The displacement and angular drift of the incident beam can be detected by monitoring the movements of [+ or -] 1 diffraction order spots of the defocus grating. The relationship between drift of the incident beam and movements of [+ or -] 1 diffraction order spots is studied in detail. Compared with other methods, this technique eliminates the requirement of two or more detecting systems for measuring displacement and angular drift simultaneously. The proof-of-principle experiment shows that the root-mean-square errors of displacement and angular drift measurements are less than 0.5 [micro]m and 0.84 [mu]rad, respectively. OCIS codes: 220.1140, 110.2970, 050.1950.

Phase sensitive X-ray imaging methods can provide substantially increased contrast over conventional absorption-based imaging and therefore new and otherwise inaccessible information. The use of gratings as optical elements in hard X-ray phase imaging overcomes some of the problems that have impaired the wider use of phase contrast in X-ray radiography and tomography. So far, to separate the phase information from other contributions detected with a grating interferometer, a phase-stepping approach has been considered, which implies the acquisition of multiple radiographic projections. Here we present an innovative, highly sensitive X-ray tomographic phase-contrast imaging approach based on grating interferometry, which extracts the phase-contrast signal without the need of phase stepping. Compared to the existing phase-stepping approach, the main advantages of this new method dubbed 'reverse projection' are not only the significantly reduced delivered dose, without the degradation of the image quality, but also the much higher efficiency. The new technique sets the prerequisites for future fast and low-dose phase-contrast imaging methods, fundamental for imaging biological specimens and in vivo studies. X-ray imaging | differential phase contrast | grating interferometer | tomography doi/ 10.1073/pnas.1003198107

This study proposes a novel process for fabricating a sandwiched long-period fiber grating/SLPFG) using a SU-8 thick photoresist technique. The SLPFG consists of a thin cladding optical fiber sandwiched with a double-sided periodical grating coating. By varying the external loads on the SLPFG, the transmission dip of the resonance wavelength is tuned according to a squared-harmonic curve. The SLPFG can thus be utilized as a loss tunable filter or sensor. The resonance dip wavelength is related to the cladding thick-nesses of the optical fiber and the periods of the grating. A maximum transmission resonant dip of 34.61 dB was achieved. OCIS codes: 060.2370, 120.2440, 220.4000.

We demonstrate a polarization-independent tunable optical filter based on switchable polarization gratings (PGs) formed using reactive and nonreactive liquid crystals (LCs). PGs are anisotropic diffraction gratings that exhibit unique properties, including a zero-order transmittance that is independent of incident polarization and that can vary from ~0% to ~100%, depending on wavelength and applied voltage. A stack of several PGs of varying thicknesses combined with an elemental angle filter yields polarization-independent bandpass tuning with minimal loss. We introduce a novel hybrid PG consisting of both reactive and nonreactive LC layers, which allows very thick gratings to be created with thin active LC layers. We demonstrate a tunable optical filter with a peak transmittance of 84% of unpolarized light, a minimum full width at half-maximum of 64 nm, and a maximum tuning range of 140 nm. OCIS codes: 120.2440, 230.3720, 230.0250.

X-ray phase contrast imaging is a very promising technique that may lead to significant advancements in a variety of fields, perhaps most notably, medical imaging. The radiation physics group at University College London is currently developing an x-ray phase contrast imaging technique that works with laboratory x-ray sources. This system essentially measures the degree to which photons are refracted by regions of an imaged object. The amount of refraction that may be expected to be encountered in practice impacts strongly upon the design of the imaging system. In this paper, we derive an approximate expression between the properties of archetypal imaged objects encountered in practice and the resulting distribution of refracted photons. This is used to derive constraints governing the design of the system. OCIS codes: 050.1960, 110.7440, 340.7430.

We report the construction and testing of a chirped diffraction grating, which serves as a substrate for surface plasmon-enhanced optical transmission. This grating possesses a spatial variation in both pitch and amplitude along its surface. It was created by plasma oxidation of a curved poly(dimethoxysilane) sheet, which resulted in nonuniform buckling along the polymer surface. A gold-coated replica of this surface elicited an optical response that consisted of a series of narrow, enhanced transmission peaks spread over the visible spectrum. The location and magnitude of these transmission peaks varied along the surface of the grating and coincided with conditions where surface plasmons were excited in the gold film via coupling to one or more of the grating's diffracted orders. A series of measurements were carried out using optical diffraction, atomic force microscopy, and normal incidence optical transmission to compare the grating topology to the corresponding optical response. In addition, the impact of a thin dielectric coating on the transmission response was determined by depositing a thin silicon oxide film over the grating surface. After coating, wavelength shifts were observed in the transmission peaks, with the magnitude of the shifts being a function of the film thickness, the local grating structure, and the diffracted order associated with each peak. These results illustrate the ability of this surface to serve as an information-rich optical sensor whose properties can be tuned by control of the local grating topology. 10.1021/ac100497w

A cryptosystem for plaintext messages was developed with gratings and their spectral and directional optical properties. Although there were many applicable grating types and optical responses, this manuscript took an example of a binary metallic surface relief and its specular reflectance at three wavelengths to demonstrate the working principles and the capabilities of the cryptosystem. For one, a series of numbers and the grating characteristics served as the ciphertext containing plaintext messages and the information of the sender's signature. Confidential, high-density, and authentic messages could be, therefore, delivered with tiny or even virtual gratings. Second, four unique encryption/decryption keys here significantly reduced the risk of the ciphertext being easily recovered by eavesdroppers. Further manipulation of keys not only offered several strategies of enhancing the system's safety, but also allowed the coexistence of many two-party, or even multiple-entity, communications. The reflectance spectra shown here were mainly attributed to the Wood's anomaly and were numerically obtained from programs based on rigorous coupled-wave analysis. [c] 2010 Optical Society of America OCIS codes: 050.1950, 060.4785, 160.4760.

Since accurate alignment is essential for projection lithography, an extended dual-grating based alignment scheme is proposed. This method is an extension of the basic dual-grating alignment model, the mechanism of which is explained to make it clear how the extended scheme performs in projection lithography. The framework of the extended alignment scheme for projection lithography is constructed, and the process of key parameter determination is then detailed. In both cases, a tiny shift of the wafer during the alignment process can be resolved by a conspicuous displacement or phase variation of corresponding fringes. Analytical results indicate that alignment is independent of the gap between wafer and mask, disturbance from the fluctuation in illumination can be neglected, and alignment resolution in sub-nanometers can be realized with this scheme. [c] 2010 Optical Society of America OCIS codes: 050.0050, 110.5220, 220.1140.

We describe a novel method of noncontact mode area measurement at long distance of 11.25 m by borrowing the concept of a circular Dammann grating (CDG). The area of objects can be determined accurately by measuring the circular spectrum diameter of the CDG. This noncontact mode measurement requires neither a large amount of image data nor any pattern recognition approach. The spectrum diameter is derived from simple lens formulas. From the fractional Fourier transform, we find that there exists a linear relationship between the spectrum diameter and the distance traveled by the CDG. Compared with the conventional methods, this technique has the advantages of a simple design with good accuracy of better than 3%, low cost, noncontact mode, and a more compact design. Finally, we present several experimental results demonstrating the effectiveness of this system. OCIS codes: 050.1950, 140.3560, 220.4000, 040.0040, 120.4640.

A novel design of monolithically integrated diplexers and triplexers for fiber-to-the-home applications is presented. A bilevel etched asymmetrical 2 x 2 optical coupler is analyzed for efficient couplings of both upstream and downstream signals. The design of the diplexer is extended to a triplexer by adding an etched diffraction grating as an additional downstream demultiplexing element. The total size of the integrated diplexer and triplexer is smaller than 500 [micro]m x 500 [micro]m. OCIS codes: 130.3120, 060.4510.

A transmission-type surface plasmon resonance configuration with dielectric gratings regularly patterned on a silver film was investigated with the aim of enhancing the diffraction efficiency of radiative surface plasmons. The theoretical work was conducted using rigorous coupled-wave analysis in terms of first order diffraction efficiency and conversion efficiency (CE). The results show that pyramid gratings can produce a higher transmittance compared with other grating profiles. Design optimization of the pyramid grating at a wide range of grating thicknesses and periods resulted in a maximum transmittance that was larger than 77% and a peak CE of about 85%. This study demonstrates the potential of using transmitted surface plasmon waves in various optical devices, such as optical biosensors, optical imaging systems, and polarization filters. OCIS codes: 050.2770, 240.6680, 310.0310.

Fiber Bragg gratings (FBGs) have previously found many applications as strain and vibration sensors. Here we demonstrate that they may also be employed as pickups for musical instruments and, specifically, for acoustic guitars and solid-body electric guitars. By fixing the FBG to a vibrating part of the instrument's body, e.g., near the bridge of an acoustic guitar or on the headstock of a solid-body guitar, a number of sound recordings were made and compared to those obtained with either piezoelectric pickups or with magnetic induction pickups. The change in attenuation at the FBG's midreflection point is found to be correlated to the amplitude of vibration of the vibrating structure of the instrument. Acoustic frequency spectrum analysis supports the observation that the FBG acoustic transducer has a frequency response range that is comparable to those of commercial piezoelectric pickups. The recordings made with FBG pickups were of comparable quality to those obtained with other recording methods. OCIS codes: 060.3735, 120.7280, 230.1040.

We present the design and fabrication of a novel dual-function subwavelength fused-silica grating that can be used as a polarization-selective beam splitter. For TM polarization, the grating can be used as a two-port beam splitter at a wavelength of 1550 nm with a total diffraction efficiency of 98%. For TE polarization, the grating can function as a high-efficiency grating, and the diffraction efficiency of the-1st order is 95% under Littrow mounting. This dual-function grating design is based on a simplified modal method. By using the rigorous coupled-wave analysis, the optimum grating parameters can be determined. Holographic recording technology and inductively coupled plasma etching are used to manufacture the fused-silica grating. Experimental results are in agreement with the theoretical values. OCIS codes: 050.1950, 230.1360, 230.5440, 050.6624, 060.4510..

Laser beam shaping is an important subject for industrial and medical applications since different applications may require different laser intensity distributions. We present an all-fiber beam-shaping device that could transform a Gaussian-shaped laser beam into an intensity uniform beam in the 1.0 [micro]m wavelength region using a long-period grating (LPG). The device can be used to shape the beam of a Yb-doped high-power fiber laser. The mode structure of the LPGs made of HI1060 fiber is analyzed. The design parameters that affect the beam shaping are discussed, and the results of the beam shaping using two cladding modes, [LP.sub.03] and [LP.sub.04], are presented. OCIS codes: 060.2340, 060.2310, 140.3300, 230.2285.

A method for testing the physical and optical properties of hydrogel thin films is reported based on using long-period fiber gratings. For humidity levels from 50% to 70% RH, a wavelength decrease of 11.0 nm is observed, with a sensitivity of 0.6 nm/%RH. For humidity levels from 70% to 98% RH, a wavelength increase of 6.3 nm is seen, with a sensitivity of 0.2nm/%RH. For humidity levels greater than 60% RH, the transmission loss at resonance increases rapidly, showing a high degree of sensitivity of 0.5 dB/%RH. The blueshift of the wavelength was seen to be due to a thickening of the overlay, while the redshift of wavelength is due to the contribution of the drop in refractive index of the overlay. The results match well with what is expected theoretically. OCIS codes: 310.6860, 310.7060, 060.2300, 060.2310, 060.2370.

We propose a novel approach to designing an optical filter with a flat-top and ultranarrow transmission band by using a sampled fiber Bragg grating (SFBG) having multiple equivalent phase shifts (EPSs). In the proposed approach, multiple EPSs are fabricated by adjusting several sampling periods of the SFBG. The obtained multiple EPSs are then arranged properly to get a flat-top transmission band in the narrow--1st reflection band of the SFBG, leading to the generation of a flat and ultranarrow transmission band. Filters with two or five EPSs are designed by simulation and experiment, with low ripples and ultranarrow l dB bandwidths observed in transmission bands. OCIS codes: 050.2770, 120.2440, 060.2340, 050.5080.

We present theory, design, and preliminary experimental studies for a compact wavefront sensor based on lateral shearing interferometry using a binary phase grating, image sensor, and Fourier-based processing. The integrated system places a diffractive element directly onto an image sensor to generate interference fringes within overlapping diffraction orders. The shearing ratio and the interferogram signal-to-noise ratio directly affect the reconstruction accuracy of wavefronts with differing spatial variations. Optimal shearing parameters associated with the autocorrelation of the input encourage placing a spatial light modulator as the diffractive element allowing adaptive wavefront sensing. Experimental results from a fixed-grating system are presented as well as requirements for next-generation adaptive systems. OCIS codes: 070.6120, 110.3175, 130.3120.

We present a technique for improving the pulse-delay performance of a stimulated Brillouin scattering (SBS) based broadband slow-light system by combining it with fiber Bragg gratings (FBG). We optimize the physical device parameters of three systems: (1) broadband SBS, (2) broadband SBS + a single FBG, and (3) broadband SBS + a double FBG for maximizing the delay performance. The optimization is performed under distortion and system resource constraints for a range of bit rates from 0.5 to 8.5 Gbps. We find that an optimized broadband SBS + a double FBG system improves the fractional delay 1.8 times that of the broadband SBS system at an optimum bit rate of 3 Gbps. Also, pump power consumption is reduced by 15% as compared to the broadband SBS system at the same bit rate. OCIS codes: 290.5900, 260.2030, 060.3735, 060.4370.

We propose a miniature pulse compressor that can be used to compensate the group velocity dispersion that is produced by a commercial femtosecond laser cavity. The compressor is composed of two identical highly efficient deep-etched transmissive gratings. Compared with prism pairs, highly efficient deep-etched transmissive grating pairs are lightweight and small. With an optimized groove depth and a duty cycle, 98% diffraction efficiency of the -1 transmissive order can be achieved at a wavelength of 800 nm under Littrow conditions. The deep-etched gratings are fabricated in fused silica by inductively coupled plasma etching. With a pair of the fabricated gratings, the input positively chirped 73.9fs pulses are neatly compressed into the nearly Fourier transform-limited 43.2 fs pulses. The miniature deep-etched grating-based pulse compressor should be of interest for practical applications. OCIS codes: 320.5520, 050.0050.

We have theoretically proposed and experimentally demonstrated a new kind of ultranarrow identical-dual-bandpass sampled fiber Bragg gratings (SFBGs) with a [pi] phase shift technique. The spacing of two bandpasses of the proposed grating can be flexibly adjusted by changing the sampled period, and any desired spacing can be achieved in principle. An experimental example shows that the transmission peaks of two narrow transmission-band are near 1549.1 and 1550.1 nm. Based on the proposed SFBG, an ultranarrow identical-dual-channel filter is designed. Two channels of the proposed filter have an equal bandwidth, an even strength, and the same group delay. The bandwidth of each channel of our filter is as small as 1 pm and up to [10.sup.-3] pm (corresponding to ~0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of [10.sup.2]-[10.sup.4]. The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers. OCIS codes: 060.3735, 050.2770, 060.2340.

We propose a thermally actuated tunable grating for measuring the beam profile of a C[O.sub.2] laser. The grooves of a transmissive grating are filled with a liquid whose refractive index depends on temperature. A visible laser as a probe and a C[O.sub.2] laser as a heat source are illuminated on the grating. The C[O.sub.2] laser is absorbed, and depending on its beam profile, a temperature profile is induced on the grating. The refractive index of the heated liquid is changed, resulting in a change of efficiency of the grating for the probe laser. By using the 1st orders of diffraction in a 4f imaging system, the beam profile of the C[O.sub.2] laser is imaged onto a CCD camera by the probe laser. OCIS codes: 050.1950, 050.5080.

A fiber Bragg grating (FBG) vibroacoustic sensor exploiting an intensity-based interrogation principle is presented. The optical system is complemented by signal processing techniques that allow disturbances to be mitigated and improve the spectral estimation. The sensor is capable of performing frequency analysis of sounds up to 3 kHz, with top sensitivity in the 100-500 Hz frequency range, and of dynamically tracking pulsed phenomena that induce a strain to the FBG. The sensor has been applied to the detection of voice, showing a great intelligibility of the speech despite the low-quality environment, and to the monitoring of the heartbeat rate from the wrist. OCIS codes: 120.4640, 060.3735, 060.2370.

Two-dimensional (crossed) diffraction gratings with sinusoidal or truncated pyramidal (trapezoidal) profiles are proposed to have diffraction efficiency almost independent of the incident polarization inside the optical communication spectral window 1.5-1.6 [micro]m The gratings are characterized by different periods in the two orthogonal directions, chosen to support only one dispersive diffraction order in addition to the zeroth (specular) one. OCIS codes: 060.4230, 050.1950, 050.2770.

An optical temperature sensor was created using a femtosecond micromachined diffraction grating inside transparent bulk 6H-SiC, and to the best of our knowledge, this is a novel technique of measuring temperature. Other methods of measuring temperature using fiber Bragg gratings have been devised by other groups such as Zhang and Kahrizi [in MEMS, NANO, and Smart Systems (IEEE, 2005)]. This temperature sensor was, to the best of our knowledge, also used for a novel method of measuring the linear and nonlinear coefficients of the thermal expansion of transparent and nontransparent materials by means of the grating first-order diffracted beam. Furthermore the coefficient of thermal expansion of 6H-SiC was measured using this new technique. A He-Ne laser beam was used with the SiC grating to produce a first-order diffracted beam where the change in deflection height was measured as a function of temperature. The grating was micromachined with a 20 [micro]m spacing and has dimensions of approximately 500 [micro]m x 500 [micro]m (l x w) and is roughly 0.5 [micro]m deep into the 6H-SiC bulk. A minimum temperature of 26.7[degrees]C and a maximum temperature of 399 [degrees]C were measured, which gives a AT of 372.3 [degrees]C. The sensitivity of the technique is [DELTA]T = 5 [degrees]C. A maximum deflection angle of 1.81[degrees] was measured in the first-order diffracted beam. The trend of the deflection with increasing temperature is a nonlinear polynomial of the second-order. This optical SiC thermal sensor has many high-temperature electronic applications such as aircraft turbine and gas tank monitoring for commercial and military applications. Society of America OCIS codes: 280.6780, 050.7330.

The usage of two static gratings for obtaining super-resolved imaging dates back to the work by Bachl and Lukosz in 1967. However, in their approach a severe reduction in the field of view was the necessary condition for improving the resolution. We present an approach based on two static gratings without sacrificing the field of view. The key idea for not paying with the field of view is to use white light illumination to average the ghost images obtained outside the region of interest since the positions of those images are wavelength dependent. Moreover, large magnification is achieved by using a commercial microscope objective instead of a test system with a unity magnification as presented in previous works. Because of the large magnification, the second grating has a low spatial period. This allows us to create an intermediate image and use a second imaginglens with low resolution capability while still obtaining an imaging quality as good as that provided by the first imaging lens. This is an important improvement in comparison with the original super-resolving method with two fixed gratings. OCIS codes: 100.6640, 110.4850.

A novel polarization converter using reflective metallic gratings and a polarization beam splitter is introduced for LCD backlight illumination. These two optical elements form a polarization rotation resonator. Broadband and high optical efficiency of polarization conversion in the visible region is achieved through the resonance of the refracted light and the surface plasmon wave in metallic surface-relief gratings. For wide-angle illumination, the conversion efficiency with arbitrary incident angle is studied. This device can convert unpolarized light to linear polarization with over 85% efficiency. OCIS codes: 230.5440, 240.6680, 050.6624.

We describe a Luus--Jaakola (LJ) optimization algorithm approach for the design of wavelength division multiplexing (WDM) optical communication systems by using phase-only sampled-fiber Bragg gratings. The LJ method is used to optimize the phase grating as well as the number of segments which form the grating. The numerical example of WDM is studied. The method is easily applicable and shows promising results with low refractive index modulation. OCIS codes: 060.2340, 230.1480, 050.2770.

An all-fiber sensor capable of simultaneous measurement of temperature and strain is newly presented. The sensing head is formed by a fiber Bragg grating combined with a section of multimode fiber that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. The strain and temperature coefficients of multimode fibers vary with the core sizes and materials. This feature can be used to improve the strain and temperature resolution by suitably choosing the multimode fiber. For a 10 pm wavelength resolution, a resolution of 9.21[micro][epsilon] in strain and 0.26 [degrees]C in temperature can be achieved. OCIS codes: 060.2310, 060.2370.

A theoretical study is conducted regarding the propagation properties of Lamb waves in a film loaded with different liquids on its two sides using two laser ultrasonic methods. Results showed that the proposed experiment techniques allow for an accurate characterization of Lamb mode propagation in a noncontact way.

Subwavelength metallic and dielectric diffraction gratings which rotate the linear polarization of incident light by 90[degrees] are examined. Using rigorous diffraction theory in total-internal-reflection configuration, it is shown that full conversion from incident transverse electric field to transverse magnetic zero-order field can be achieved with both dielectric and metallic elements, but dielectric gratings provide higher efficiency and are thus preferable. The fabrication aspects and constraints are discussed in detail and the behavior of the gratings over broad wavelength bands is presented. OCIS codes: 230.1950, 260.2110, 260.5430.

W/C and Co/Si[O.sub.2] multilayer gratings have been fabricated by depositing a multilayer coating on the surface of laminar-type holographic master gratings. The diffraction efficiency was measured by reflectometers in the energy region of 0.6-8.0 keV at synchrotron radiation facilities as well as with an x-ray diffractometer at 8.05 keV. The Co/Si[O.sub.2] and W/C multilayer gratings showed peak diffraction efficiencies of 0.47 and 0.38 at 6.0 and 8.0 keV, respectively. To our knowledge, the peak efficiency of the W/C multilayer grating is the highest measured with hard x rays. The diffraction efficiency of the Co/Si[O.sub.2] multilayer gratings was higher than that of the W/C multilayer grating in the energy range of 2.5-6.0 keV. However, it decreased significantly in the energy above the K absorption edge of Co (7.71 keV). For the Co/Si[O.sub.2] multilayer grating, the measured diffraction efficiencies agreed with the calculated curves assuming a rms roughness of ~1 nm. OCIS codes: 230.1950, 230.4170, 340.7480.

Fabrication of a thick analog profile with photoresist is a difficult task in photolithography. We demonstrate that a binary phase-grating photomask with an appropriate period and duty cycles is capable of manipulating the exposure illumination in an analog fashion and can be used for fabrication of the desired analog micro-optics profiles on the surface of a thick photoresist. By choosing the proper period and variation of duty cycle of the phase-grating mask, one can create the desired analog intensity of exposure illumination for an optical stepper. This allows the formation of a wide range of analog micro-optics profiles with an SPR 220-7 photoresist. The numerical convolution of the diffraction efficiency curve and resist exposure characteristics is used to predict the final resist profile and also to design the appropriate duty-cycle distribution for the binary phase grating. As a demonstration of this technology, we fabricated a variety of micro-optical elements, such as a positive lens, ring lens, prism, and vortex of ~100-200 [micro]m diameter, by using a phase-grating mask fabricated in a poly(methyl methacrylate) electron-beam OCIS codes: 050.1970, 110.5220, 220.4610.

Optically writable, thermally erasable surface relief gratings in thin Disperse Red 1 polymethyl methacrylate azopolymer films were used to demonstrate an arbitrarily reconfigurable fiber Bragg filter. Gratings were optically written on azopolymer-coated side-polished fiber blocks, and a write-erase-write cycle was demonstrated. Finite difference time domain simulations reveal that this optically reconfigurable device concept can be optimized in a silicon-on-insulator waveguide platform. OCIS codes: 160.5470, 230.1480.

Long-period fiber Bragg gratings fabricated by exposure of hydrogen-loaded fiber to UV laser light exhibit large-scale dynamic evolution for approximately two weeks at room temperature. During this time two distinct features show up in their spectrum: a large upswing in wavelength and a substantial deepening of the transmission minimum. The dynamic evolution of the transmission spectrum is explained quantitatively by use of Malo's theory of UV-induced quenching [Electron. Lett. 30, 442 (1994)] followed by refilling of hydrogen in the fiber core and the theory of hydrogen diffusion in the fiber material. The amount of hydrogen quenched by the UV irradiation is 6% of the loaded hydrogen. OCIS codes: 050.2770, 060.2310, 060.2340, 160.5340.

A method is presented to sense the wave front at the exit of an optical surface. This method uses a set of diffracted rays generated when a He-Ne laser impinges on a rectangular diffraction grating. The grating was placed near the curvature center of the surface to be tested. After they are reflected in the test surface, the diffracted rays have the information of the slopes of the wave front, like in the Hartmann test. The Hartmann pattern was registered near the curvature center and captured with a CCD camera. The slopes for each ray are measured from the experimental pattern, and they are compared with the ideal ones simulated in a computer. The evaluation was carried out by use of Seidel polynomials to obtain the information of the aberrations of a mirror 53 cm in diameter. OCIS codes: 220.4840, 050.1950.

Single-shot transient-grating measurements for thermometry in pressurized reacting flows are examined in the context of rapid digital signal processing. Simple approaches are discussed for temperature determination and rejection of unwanted signals in real-time measurement applications. Examples of temperature data in pressurized postflame gases are presented in the form of probability-density functions (PDFs). Three contributions to the PDF half-widths are discussed. Analysis of phase-matching requirements indicates that beam steering as a result of density fluctuations affects the signal amplitude but not the grating period. Therefore, such stochastic beam deviations have little effect on the derived temperatures. Mode noise on the cw probe beam as well as linear light scattering are found to be insignificant in the frequency range of the observed transient-grating acoustic signature. Use of a single-mode laser for the pump beams is shown to enhance the signal intensity. OCIS codes: 120.1740, 290.5890, 300.6430, 050.7330.

A sensitive interferometric sensor scheme that is based on coherent imaging of a first phase grating onto a second phase grating, their periods accurately matched, is suggested. Experimental data, obtained with a setup based on the suggested scheme, are presented. The sensor was found capable of measuring an angular tilt of a mirror less than 0.5 [micro]rad. Compared with a previously suggested measuring scheme, the novelty of the one presented here is the inclusion of a second set of gratings, which eliminates measurement ambiguity. Some characteristics of the sensor scheme are discussed. OCIS codes: 050.1950, 120.3180.

Bragg gratings are recorded in doped and partially polymerized poly(methyl methacrylate) with green light (wavelength, 532 nm) in transmission geometry, and the gratings are read in reflection geometry with infrared light (wavelength, ~1550 nm). Diffraction efficiencies of more than 99% with a wavelength bandwidth of ~1 nm are obtained for single gratings with a typical length of 15 mm. Superposition of four gratings in a volume sample has been demonstrated as well. The material is promising for use in the fabrication of add--drop filters, attenuators, switches, and multiplexers--demultiplexers for optical networks that use wavelength division multiplexing. OCIS codes: 050.7330, 090.2900, 160.5470, 230.1480.