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.     

Robust chirped mirrors

Optimized chirped mirrors may perform suboptimally, or completely fail to satisfy specifications, when manufacturing errors are encountered. We present a robust optimization method for designing these dispersion-compensating mirror systems that are used in ultrashort pulse lasers. Possible implementation errors in layer thickness are taken into account within an uncertainty set. The algorithm identifies worst-case scenarios with respect to reflectivity as well as group delay. An iterative update improves the robustness and warrants a high manufacturing yield, even when the encountered errors are larger than anticipated.     

Design and comparative performance analysis of different chirping profiles of tanh apodized fiber Bragg grating and comparison with the dispersion compensation fiber for long-haul transmission system

Various dispersion compensation units are presented and evaluated in this paper. These dispersion compensation units include dispersion compensation fiber (DCF), DCF merged with fiber Bragg grating (FBG) (joint technique), and linear, square root, and cube root chirped tanh apodized FBG. For the performance evaluation 10 Gb/s NRZ transmission system over 100-km-long single-mode fiber is used. The three chirped FBGs are optimized individually to yield pulse width reduction percentage (PWRP) of 86.66, 79.96, 62.42% for linear, square root, and cube root, respectively. The DCF and Joint technique both provide a remarkable PWRP of 94.45 and 96.96%, respectively. The performance of optimized linear chirped tanh apodized FBG and DCF is compared for long-haul transmission system on the basis of quality factor of received signal. For both the systems maximum transmission distance is calculated such that quality factor is ≥ 6 at the receiver and result shows that performance of FBG is comparable to that of DCF with advantages of very low cost, small size and reduced nonlinear effects.     

A comparison of wavelength dependent polarization dependent lossmeasurements in fiber gratings

We have measured wavelength dependent Polarization Dependent Loss (PDL) in chirped fiber Bragg gratings for dispersion compensation, grating filters for wavelength add/drop multiplexing and long period gratings for EDFA gain flattening. The PDL is measured in devices used in reflection and in transmission by applying the Jones matrix method, the Mueller matrix method and the polarization scanning method. A comparison of the experimental results and an analysis of the sources of errors are presented     

Ray-tracing model for stretcher dispersion calculation

We propose a ray-tracing model that provides a clear physical picture and simple formulas for grating pair stretcher dispersion calculations. With this model we can easily demonstrate why and to what extent the stretcher and compressor are opposite quantitatively without using a Fourier transform. The dispersion calculation shows that the spherical aberration in the stretcher decreases fourth-order dispersion compared with an aberration-free stretcher. In a chirped pulse amplification system, this fourth order can help to reduce residual fourth-order dispersion. The effect of the finite beam size and the misalignment are also considered.     

Improving the performance of fiber gratings with sinusoidal chirps

Sinusoidal chirps are introduced in fiber gratings to improve their performance as dispersion compensators and multichannel filters. The sinusoidally chirped fiber gratings exhibit a flattop spectrum with steep edges and high reflectivity. The bandwidth utilization defined as the ratio of -1:-30-dB bandwidth could be very high (>0.85). This structure can be applied to the sampled fiber gratings to enhance channel uniformity. We demonstrate 264 uniform channels with a 25-GHz-spacing for high-density-wavelength multiplexing applications. Multichannel dispersion compensations with seven uniform channels of 50-GHz-spacing in short fiber gratings are also demonstrated. The impact of possible fabrication errors on the spectra of the gratings is discussed.     

Chirped optical X-shaped pulses in material media

We analyze the properties of chirped optical X-shaped pulses propagating in material media without boundaries. We show that such ("superluminal") pulses may recover their transverse and longitudinal shapes after some propagation distance, whereas the ordinary chirped Gaussian pulses can recover their longitudinal width only (since Gaussian pulses suffer a progressive transverse spreading during their propagation). We therefore propose the use of chirped optical X-type pulses to overcome the problems of both dispersion and diffraction during pulse propagation.     

相位掩模版接缝相差对光栅反射峰的影响

电子束刻蚀啁啾相位掩模版采用的是分步啁啾的方法,而掩模版制造过程中分步之间的接缝误差是啁啾光纤光栅反射峰不平坦的一个重要因素.本文以不同分步步长,总长度140mm的相位掩模版为例,理论分析和比较了不同接缝误差对啁啾光纤光栅反射峰的影响.分析表明,最大相位误差为3°和7°时,光栅反射峰波动的统计平均分别为0.98%和2.31%.进行相关试验研究以上相位掩模版写入光栅的反射特性,得出结论与理论分析一致.     

Optimization of chirped mirrors

We demonstrate that a highly efficient global optimization of chirped mirrors can be performed with the memetic algorithm. The inherently high sensitivity of chirped-mirror characteristics to manufacturing errors can be reduced significantly by means of the stochastic quasi-gradient algorithm. The applicability of these algorithms is not limited to chirped mirrors.     

Temporal Talbot effect in fiber gratings and its applications

We show that a temporal effect equivalent to the spatial Talbot effect (self-imaging) applies to the reflection of periodic pulse trains from linearly chirped fiber gratings (LCFG's). For specific input repetition periods the reflected signal is an exact replica of the input signal. Input repetition period values that give rise to this effect depend on the dispersion coefficient of the grating. We propose to use this effect as an alternative for dispersion measurement in LCFG's. Furthermore, by using the properties of the temporal Talbot effect, we can design linear passive devices (LCFG's) for use as frequency multipliers, able to multiply the repetition rate of a given pulse train.     

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.     

Generation of a train of ultrashort pulses using periodic waves in tapered photonic crystal fibres

We consider a light wave propagation in tapered photonic crystal fibres (PCFs) wherein the wave propagation is described by the variable coefficient nonlinear Schrödinger equation. We solve it directly by means of the theta function identities and Hirota bilinear method in order to obtain the exact periodic waves of sn, cn and dn types. These chirped period waves demand exponential variations in both dispersion and nonlinearity. Besides, we analytically demonstrate the generation of a train of ultrashort pulses using the periodic waves by exploiting the exponentially varying optical properties of the tapered PCFs. As a special case, we discuss the chirped solitary pulses under long wave limit of these periodic waves. In addition, we derive these types of periodic waves using the self-similar analysis and compare the results.     

The use of fast initial response features on the homogeneously weighted moving average chart with estimated parameters under the effect of measurement errors

Fast initial response (FIR) features are generally used to improve the sensitivity of memory-type control charts by shrinking time-varying control limits in the earlier stage of the monitoring regime. This paper incorporates FIR features to increase the sensitivity of the homogeneously weighted moving average (HWMA) monitoring schemes with and without measurement errors under constant as well as linearly increasing variance scenarios. The robustness and the performance of the HWMA monitoring schemes are investigated in terms of numerous run-length properties assuming that the underlying process parameters are known and unknown. It is found that the FIR features improves the performance of the HWMA monitoring scheme as compared to the standard no FIR feature HWMA scheme, and at the same time, it is observed that the simultaneous use of a recently proposed FIR feature and multiple measurements significantly reduces the negative effect of measurement errors. An illustrative example on the volume of milk in bottles is used to demonstrate a real-life application.     

Evaluation of optimal dispersion conditions for CNT reinforced epoxy composites using cyclic voltammetry measurements

The optimum dispersion time of nanoparticles is important for obtaining uniform dispersion of fillers or other additives in a matrix. In this study, the optimal dispersion time of carbon nanotube (CNT) in a matrix was investigated using cyclic voltammetry (CV), measurement for different dispersion methods and times. In addition, the mechanical properties of CNT composites manufactured using different dispersion methods were evaluated by tensile and flexural tests. The CV and mechanical test results were correlated to the dispersion condition of CNT in the composites. It was found that tip-type sonication resulted in better dispersion than bath-type sonication. Improved CNT dispersion resulted in composites with both enhanced CV measurements and improved mechanical properties. In the study reported here, improvements in dispersion were generally accompanied by higher electrical currents. This suggests that the CV measurement method is an effective tool for determining optimal dispersion times, for different CNT dispersion processes.     

Dispersion compensation technologies for femtosecond fiber system

Developed highly chirped broadband fiber Bragg gratings and suspended-core fibers are shown to offer flexible dispersion management and allow for femtosecond oscillators with transform-limited pulse quality. Such dispersion compensators could have potential, particularly for all-fiber chirped-pulse amplification systems.     

Optimization of distributed resistive metal film heaters in thermally tunable dispersion compensators for high-bit-rate communication systems

Tunable dispersion compensators are an essential component for optical networks operating at 40 Gbits/s and beyond. One fiber-based tunable dispersion compensator that has proved to be effective consists of a chirped fiber Bragg grating tuned by a thin-film distributed resistive heating element. We describe several modifications to the heater design that minimize temperature-induced higher-order dispersion, eliminate the need for a second stabilization heater when the device is operated at constant ambient temperature, and significantly lower its maximum operating temperature. We demonstrate a tunable dispersion compensator with a single thin-film heater that provides over 500 ps/nm of tunable dispersion over a fixed 100-GHz bandwidth with a maximum operating temperature of less than 125 degrees C above ambient.     

Chirped-cavity dispersion-compensation filter design

A new basic structure of a dispersive-compensation filter, called a chirped-cavity dispersion-compensator (CCDC) filter, was designed to offer the advantages of small ripples in both reflectance and group-delay dispersion (GDD). This filter provides a high dispersion compensation, like the Gires-Tournois interferometer (GTI) filter, and a wide working bandwidth, like the chirped mirror (CM). The structure of the CCDC is a cavity-type Fabry-Perot filter with a spacer layer (2 mH or 2 mL) and a chirped high reflector. The CCDC filter can provide a negative GDD of -50 fs2 over a bandwidth of 56 THz with half the optical thickness of the CM or the GTI.     

Segmented chirped grating for coarse wavelength division demultiplexing with multimode optical fibers

A chirped grating segmented into partitions each having a constant blaze angle to use in a demultiplexer for coarse wavelength division multiplexing with multimode optical fibers is developed. Its designed configuration utilizes a resonance region to achieve high diffraction efficiency and large dispersion. The width, blaze angle, and diffraction order of each partition were optimized by vector diffraction analysis. The diffraction loss of the manufactured grating was less than 1.5 dB, and polarization-dependent loss was less than 0.6 dB within a wavelength width of at least 70 nm. It is confirmed that a demultiplexer with the developed chirped grating had a wide passband and low cross talk.     

Solitary Pulses in an Amplified Nonlinear Dispersive Medium in the Presence of Gain Saturation

Abstract

The steady state solitary solution in a two-level homogeneously broadened amplifier with host nonlinearity and dispersion is perturbatively studied by consideration of gain saturation up to second-order energy terms normalized to the saturation energy of the transition. Carrier frequency shift occurs as a result of the saturation. The chirped solitary pulses derived exist in the anomalous group velocity dispersion region only.