Identical-dual-bandpass sampled fiber Bragg grating and its application to ultranarrow filters

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(-3) pm (corresponding to approximately 0.1 MHz), which is less than the bandwidth of the conventional SFBG filters by a factor of 10(2)-10(4). The proposed grating and filter can find potential applications with slow light and dual-wavelength single-longitudinal-mode fiber lasers.     

Characterization of chromatic dispersion and polarizationsensitivity in fiber gratings

Fiber gratings have already become key passive components in fiber optic communication systems. We have characterized gratings used in reflection for dispersion compensation and long period gratings used in transmission for gain flattening using a low-loss, low-noise experimental setup having a picometer optical wavelength resolution. Our measurements include reflection or transmission response, group delay and polarization dependent loss. We have scanned the spectrum of our devices using a very narrow linewidth tunable laser. A network analyzer is used for the chromatic dispersion measurements. Time delays corresponding to the design values have been measured within the useful bandwidth of the gratings for dispersion compensation and the devices have been found to have reasonably small ripples that increase in magnitude toward the shorter wavelength range. The long period gratings for gain flattening have very small group delays. Polarization dependent loss has been measured for the first time in these devices. A polarization analyzer was used and Jones matrix analysis was applied to obtain the measurements. The gratings for dispersion compensation have small a polarization dependent loss within their useful bandwidth, while the long period gratings exhibit higher values and a stronger wavelength dependency in the polarization dependent loss     

Independence of orientation and size in spatial discriminations

This study of form vision explores the relationships between orientation and spatial frequency in suprathreshold discrimination tasks. Orientation discrimination thresholds for sine-wave gratings were 0.3-0.5 deg, much less than the roughly 10-24-deg orientational bandwidth of channels; spatial-frequency discrimination thresholds were 3-7%, much less than the roughly 1.2-octave spatial-frequency bandwidth of channels. We find that spatial-frequency discrimination between two gratings was as acute when the two gratings were orthogonal as when they were parallel. Orientation discrimination between two gratings was as acute when the two gratings had the same spatial frequencies as when they had different spatial frequencies. Thus orientation and spatial frequency are independent dimensions at the discrimination stage of spatial information processing.     

Path-reversed substrate- guided-wave optical interconnects for wavelength-division demultiplexing

A path-reversed substrate-guided-wave holographic interconnection scheme is investigated for a wavelength-division demultiplexing application. Using a beveled edge of a waveguiding plate allows optical signals to be coupled into the waveguiding plate and then to be coupled out of the plate by a waveguide hologram. Theoretical analyses are given for dispersion, bandwidth, and recording parameters of various guided-wave holographic gratings. A device is fabricated with a 45 degrees incident angle and a 45 degrees diffraction angle by use of a 20-microm photopolymer film. The 3-dB bandwidth of the device is measured to be 20 nm. Four-channel wavelength demultiplexing is demonstrated at 796, 798, 800, and 802 nm with no cross talk observed. A one-to-five cascaded four-channel wavelength-division demultiplexer with +/-5% energy uniformity under s polarization is also demonstrated to increase the user-sharing capacity. Twenty fan-out channels (5 x 4) are achieved experimentally.     

Second-harmonic generation in second-harmonic fiber Bragg gratings

We consider the production of second-harmonic light in gratings resonant with the generated field, through a Green's function approach. We recover some standard results and obtain new limits for the uniform grating case. With the extension to nonuniform gratings, we find the Green's function for the second harmonic in a grating with an arbitrary phase shift at some point. We then obtain closed form approximate expressions for the generated light for phase shifts close to π/2 and at the center of the grating. Finally, comparing the uniform and phase-shifted gratings with homogeneous materials, we discuss the enhancement in generated light and the bandwidth over which it occurs, and the consequences for second-harmonic generation in optical fiber Bragg gratings.     

Dispersion control with use of long-period fiber gratings

Chirped long-period fiber gratings are analyzed for management of dispersion in optical fiber communications systems. A ray model is used to derive simple analytic expressions that describe the transmission, chromatic delay, and dispersion properties of chirped long-period fiber gratings. A numerical model based on coupled-mode theory is used to verify the accuracy of the analytic expressions and explore design issues of the chirped long-period grating. With certain reasonable restrictions, chirped long-period gratings are found to be a viable and desirable alternative to existing dispersion compensation techniques.     

基于光束扫描方法制作取样光栅的实验研究

报道了一种基于光束扫描方法采用普通均匀相位模板和连续的 2 4 4nm倍频氩离子激光器 ,制作波长间隔为 0 .8nm和 1.6nm取样光栅的新技术 ,并对得到的实验结果进行了分析。研制的取样光栅波长间隔均匀 ,具有良好的谱特性。采用的方法操作简单、灵活 ,可方便地制作不同类型的取样光栅。     

Diffraction properties of volume holographic gratings for an ultrashort pulsed beam with different polarization states readout

The diffraction properties of volume holographic gratings are studied when the gratings are illuminated by an ultrashort pulsed beam with different polarization states. The developed coupled wave theory of Kogelnik is used. Considering the dispersion effect of the grating media, solutions for the diffracted and transmitted intensities, diffraction efficiencies and the bandwidths of the gratings are given in transmission volume holographic gratings and reflection volume holographic gratings. The bandwidths of the gratings are reduced by the dispersion effect of the grating media. They also have different influences on the diffraction of an ultrashort pulsed beam with different polarization states. For different values of the ratio of the spectral bandwidth of the input pulse to that of the grating, the changes of the spectral and temporal distributions of the diffracted intensities, as well as the diffraction efficiencies of the gratings are shown.     

Self-adaptive noise suppression for characterizing the dispersion of chirped fiber Bragg grating

We propose a method to suppress the beat noise generated in fiber low coherence interferometry (LCI) systems for characterizing the chromatic dispersion of chirped fiber Bragg gratings. The beat noise is considered as the dominant noise in the system because of the spectrum mismatch between interference arms due to the broad bandwidth of the light source and the introduction of dispersive components into the measurement arm, and is unfavorable for the signal quality. An experimental system is set up and interferograms of various situations are provided. Experiment results indicate that our method is feasible and effective, as it improves the signal to noise ratio effectively by a factor of more than 3, thereby expanding the measurement range of the LCI system.     

基于狭缝整形的低损耗飞秒光纤光栅制备

为了解决飞秒激光逐点法制备的光纤布拉格光栅（Fiber Bragg Gratings, FBG）损耗较高的问题,利用逐面法完成飞秒光纤光栅制备的实验研究。运用高斯光束传播的基本理论,通过狭缝整形技术对聚焦的飞秒激光能量分布进行等高宽整形,突破在光纤横截面内诱导产生圆形折射率调制的难点,最终制备得到低损耗的飞秒光纤光栅。开展不同狭缝宽度制备FBG的光谱特性对比实验,结果表明：利用光斑直径为5.0 mm的飞秒激光光束刻写FBG时,采用宽度为1.7 mm的狭缝制备得到的FBG插入损耗降低至0.15 dB,短波损耗降低至0.5 dB,验证了基于狭缝整形的低损耗飞秒光纤光栅制备方法的有效性。针对狭缝法制备的FBG反射率分散问题,提出控制折射率匹配液填充量以及调整飞秒激光能量的方法,并优化光束聚焦流程,成功降低FBG反射率的分散度。本研究对推动飞秒光纤光栅在大容量、高链路损耗等环境中的实际应用具有重要意义。     

Dependence of fringe spacing on the grating separation in a long-period fiber grating pair

The spectral spacing of the interference fringes formed by a pair of long-period fiber gratings was investigated. The variation of the fringe spacing was measured while the separation between the gratings was changed from 22 to 500 mm. When the grating separation was much longer than the length of the individual grating, the inverse of the fringe spacing became linearly proportional to the grating separation and to the differential effective group index of the fiber. In the third stop band of the grating pair, made along a dispersion-shifted fiber centered at 1.55 mum, the differential effective group index was calculated to be ~6.4 x 10(-3), which is approximately twice the differential effective index of the fiber. The discrepancy between the two indices was observed to decrease with the band order, a phenomenon that is explained by the first-order dispersion of the fiber. The measured interference fringes were not regularly spaced in the frequency domain, but regular spacing is required in wavelength-division multiplexing communication systems. Analysis of the second-order dispersion of the fiber and the grating-induced nonlinear phase shift within grating regions as the factors that induce chirping on the fringe spacing is presented.     

Wideband true-time-delay beam former that employs a tunable chirped fiber grating prism

A fiber grating prism that consists of four tunable chirped-grating delay lines for wideband true-time-delay beam forming is proposed and demonstrated. The chirped gratings are produced by use of the grating bending technique in which a uniform grating is surface mounted on a simply supported beam. We obtained chirped gratings with different chirp rates by bending the uniform gratings with different beam deflections. Four linear chirped fiber gratings with identical spectral width but linearly increased grating length are fabricated. The spectra and time-delay responses of the tunable chirped gratings are measured. A chirped-grating prism for wideband true-time-delay beam forming by use of four chirped gratings is constructed and tested experimentally. We obtained different time delays by tuning the wavelength of the optical carrier. The proposed true-time-delay beam former with a four-element phased-array steerer is suitable for continuous beam forming at microwave frequencies up to 20 GHz.     

Chromatic dispersion measurement using a multiwavelength frequency-shifted feedback fiber laser

In this paper, we present the realization of a fiber laser source emitting simultaneously over 17 wavelengths spread over the whole C-band. An acoustooptic frequency shifter is placed in the laser ring cavity to suppress the cross-gain saturation effects of the erbium-doped fiber. The emitted wavelengths are fixed by a set of fiber Bragg gratings (FBGs). A power uniformity reaching 6 dB is achieved by inscribing the FBGs while monitoring the laser output. We demonstrate the reliability of this laser as a source for characterization of optical components and networks by the measurement of optical fiber chromatic dispersion. The measurement is performed over the whole telecommunication C-band (1530-1560 nm) using the time-of-flight method. We perform the measurement on three different fibers with different levels of dispersion, namely a standard fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The results are compared with measurements obtained using a standard network analyzer. The agreement between the two methods is better than /spl plusmn/1%, thus proving the suitability of the developed laser source for this application.     

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.     

Polarization-insensitive high-dispersion total internal reflection diffraction gratings

We report the first experimental realization of total internal reflection (TIR) diffraction gratings. Performance of less than 0.7-dB insertion loss (IL) for both TE and TM polarizations and 0.5-dB polarization-dependent loss (PDL) are predicted over a 50-nm spectral bandwidth with simultaneous fabrication tolerances on the depth and the duty cycle of binary gratings of +/-5% and +/-14%, respectively. Nineteen gratings were fabricated that met these specifications, yielding IL and PDL values less than 0.6 and 0.2 dB, respectively, across the entire 50-nm bandwidth. Measurements made under the Littrow configuration resulted in high efficiency and low PDL across a 100-nm bandwidth, with up to 100% diffraction efficiency within the experimental measurement error.     

啁啾FBG和DCF对16×10Gb/s系统进行色散补偿的性能研究

对负色散光纤法和啁啾光纤光栅法实现色散补偿的两种方案进行了模拟研究 ,结论是 :(1 )对各信道完全色散补偿的前提下 ,入纤光功率较小时 ,啁啾光纤光栅色散补偿系统误码性能较好 ;入纤光功率较高时 ,用负色散补偿光纤进行色散补偿效果更好 ;(2 )适当的欠补偿比完全补偿效果好 ,入纤光功率一定时 ,用啁啾光纤光栅比用负色散补偿光纤进行欠补偿的效果好     

Universal method for holographic grating recording: multimode deformable mirrors generating Clebsch-Zernike polynomials

Recording methods for making aberration-corrected holographic gratings are greatly simplified by use of a plane multimode deformable mirror (MDM) upon one of the two recording beams. It is shown that MDM compensators easily provide the superposition of many interesting active optics modes, which we have named Clebsch-Zernike modes. When we apply only a uniform loading or no loading at all onto the rear side of the MDM clear aperture, the available Clebsch-Zernike modes are made to belong to a subclass of the Zernike modes that includes the three modes of the third-order aberration theory as well as a well-defined part of the Zernike higher-order modes. Such a recording method is considered to be universal, since it does not require the use of a sophisticated optical system such as a compensator. Active optics 12-arm MDM's in the vase form have been designed from the elasticity theory. The design of six-arm MDM's is currently carried out with theoretical results. As an example of the method, the recording of three holographic gratings of the Hubble Space Telescope Cosmic Origins Spectrograph has been investigated. Substantial improvements in image quality have been found by use of a six-arm MDM as recording compensator. The result is that aberrations of much higher order can simultaneously be corrected so that the residual blur images of the spectra occupy areas approximately 10 (direction of dispersion) x 3 (cross dispersion) = 30 times smaller-also in terms of pixel number-than those obtained by our American colleagues. Therefore the active optics recording method appears to provide substantial gains in resolving power and in sensitivity: (i) For all three gratings the spectral resolution would be increased by a factor of 10, and (ii), in addition, for the two higher dispersion gratings, the limiting magnitude on the sky appears to be increased by a magnitude of approximately 1-1.2.     

Simultaneous multiwavelength real-time optical spectrum analysis

We propose a technique for performing a real-time Fourier transformation simultaneously over all the channels of a multiwavelength signal traveling in a single optical fiber. The technique requires only the reflection of the original signal in an appropriately designed structure of superimposed chirped fiber gratings. The potential and the limitations of superimposed fiber-grating structures for implementing known applications of fiber gratings over various multiwavelength channels (including the application proposed herein) are analytically and numerically studied. To demonstrate our proposal we design a real-time optical spectrum analyzer operating on three different wavelength channels. Numerical calculations show that the design works properly, and we use joint time-frequency signal representations to get a better understanding of the physical processes that determine the behavior of the system.     

Characterization of apodized fiber Bragg gratings for rejection filter applications

Apodized fiber Bragg gratings with >/=30-dB sidelobe suppression +/-1 nm from the Bragg center wavelength are spatially characterized with side-scatter and side-diffraction techniques. Resonant side scatter is suppressed, indicating that little dispersion arises from reflections off the grating boundaries.     

Broadband, lossless, dispersion-compensating asymmetrical twin-core fiber design with flat-gain Raman amplification

We report here a broadband, lossless, dispersion-compensating asymmetrical twin-core fiber design with flat-gain Raman amplification that uses a single pump. Simulations show that broadband Raman amplification, with +/- 0.1-dB gain ripple, is achievable over a 31-nm bandwidth (1504-1535 nm) by use of a single pump. Amplifier characteristics have been modeled, with the effects of wavelength-dependent splice and background attenuation loss taken into account. The fiber also has a high negative-dispersion coefficient [-230 to -330 ps/(km nm)] over the operating wavelength range and, hence, only 12.5 km of this fiber can compensate for an accumulated dispersion of 240 km of standard transmission fiber. The device is thus proposed as a lossless dispersion-compensating module wherein lossless operation is achieved by use of inherently gain-flattened Raman amplification.