Mode-locked performance of hybrid soliton pulse source utilizing fiber grating external cavity lasers

Mode-locking characteristic of hybrid soliton pulse source (HSPS) utilizing linearly chirped raised-cosine flat top apodized fiber Bragg grating (FBG) is investigated by using coupled-mode equations. It is found that the fundamental repetition frequency range of HSPS is significantly extended by using linearly chirped raised-cosine flat top apodized FBG instead of linearly chirped Gaussian apodized FBG. The range of repetition frequencies over which proper mode-locking is obtained is 2-3.3 GHz with linearly chirped raised-cosine flat top apodized grating whereas this range is 2.1-2.95 GHz with linearly chirped Gaussian apodized grating.     

Intensity noise of mode-locked fiber grating external cavity semiconductor lasers

The noise of hybrid soliton pulse source (HSPS) with linearly chirped Gaussian apodized fiber Bragg grating is analyzed by couple-mode equations including spontaneous emission noise when HSPS is mode-locked. Relative intensity noise (RIN) is calculated using numerical solutions of these equations. It is shown that transform-limited pulses are generated over a wide tuning range around the fundamental mode-locking frequency with low spontaneous noise. However, high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable over a wide tuning range. Linewidth enhancement factor and spontaneous coupling factor are the most effective noise parameters and noise increases with increasing value of these parameters.     

Relative intensity noise of mode-locked fiber grating external cavity semiconductor lasers

The noise of hybrid soliton pulse source (HSPS) with linearly chirped Gaussian apodized fiber Bragg grating is analyzed by couple-mode equations including spontaneous emission noise when the HSPS is mode-locked. Relative intensity noise is calculated using numerical solutions of these equations. It is shown that transform limited pulses are generated over a wide tuning range around the fundamental mode-locking frequency with low spontaneous noise. However, a high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable over a wide tuning range. It is also shown that noise is extremely sensitive to the RF and DC currents, linewidth enhancement factor, gain saturation parameter and spontaneous coupling factor.     

Performance analysis of all optical swapping networks with a label eraser made of a Gaussian filter

In this paper, a simple label eraser employing Gaussian apodized fiber Bragg grating (FBG) for all optical label swapping (AOLS) networks is proposed. Relying on the analysis of the payload through multi-stage erasers, this kind of eraser significantly improves the cascadability in comparison with the traditional fiber-loop mirror (FLM) eraser. The influences of the residual labels on the intermediate swapping node labels through the eraser are also investigated. It is shown that the power penalty is only less 1 dB when the optical power ratio of residual label to the new label signal arrives at -8 dB. The influence is negligible due to the sharp notch filtering effect of the Gaussian apodized FBG.     

Effect of Spontaneous and Carrier Noise on Mode-Locked Hybrid Soliton Pulse Source

Noise characteristics of mode-locked hybrid soliton pulse source (HSPS) where fiber Bragg grating is used as external cavity is presented. Model is based on time-domain solution of coupled-mode equations including spontaneous noise. Relative intensity noise (RIN) is calculated by using numerical solution of these equations. It is shown that inclusion of carrier density noise in the rate equations is necessary for the accurate analysis of noise in mode-locked lasers. It is also found that transform-limited pulses are not generated over a wide tuning range around the fundamental mode-locking frequency with spontaneous and carrier density noise sources if noise level is high. Therefore mode-locking range where transform limited pulses are obtained reduces.     

Characterization of small-signal intensity modulation of single-mode fiber grating Fabry-Perot laser source

A comprehensive study on the small-signal intensity modulation (IM) characteristics of a fiber grating Fabry-Perot (FGFP) laser is numerically investigated. The effect of external optical feedback (OFB), temperature, injection current, cavity volume, nonlinear gain compression factor, and fiber grating (FG) parameters on IM characteristics are presented. The temperature dependence (TD) of IM is calculated according to the TD of laser cavity parameters instead of using the well-known Parkove relationship. It has been shown that the optimum external fiber length (L _ext) is 3.1 cm. The optimum range of working temperature for FGFP laser is between 23 to 27 °C. We also show that by increasing the laser injection current from 10 to 60 mA, the IM peak amplitude decreased from 6.3 to 0.2 dB and the relaxation-oscillation frequency (ROF) is shifted from 1.2 GHz towards higher frequency of 5.48 GHz. In addition, the AR coating reflectivity and gain compression factor have no significant effect on the IM. The study indicates that a stable operation and excellent modulation characteristic can be obtained after optimization process.     

双光纤光栅外腔半导体激光器相干失效研究

根据双光纤Bragg光栅(FBG)外腔半导体激光器相干失效的物理过程, 运用速率方程和双FBG耦合模理论, 分析了双FBG外腔半导体激光器相干失效产生和控制的条件, 提出了实现和控制双FBG外腔半导体激光器相干失效多模稳定工作的方法. 双FBG外腔半导体激光器在相干失效下具有多模的稳定工作状态, 相干失效长度缩短, 相干失效长度内光谱稳定. 实验测量结果表明, 外腔反射率为3%时, 从非相干失效状态到相干失效状态, 半峰值全宽度从0.5 nm突然展宽到0.9 nm. 在相干失效状态下, 功率稳定, 边模抑制比大于45 dB, 在0℃–70℃工作温度范围内峰值波长漂移小于0.5 nm, 最小相干失效长度小于0.5 m. 双FBG外腔半导体激光器相干失效的应用对提高光纤放大器和光纤激光器的性能具有重要意义. 关键词： 非线性 半导体激光器 双光纤Bragg光栅 相干失效     

Numerical analysis of apodized fiber Bragg gratings formation using phase mask with variable diffraction efficiency

Scalar diffraction theory is applied to analyze the intensity distribution in the fiber core during fiber Bragg grating (FBG) fabrication using an apodized phase mask. The averaged diffraction field distribution in fiber core was calculated as a function of optical fiber position. The results show that in a case of apodized FBGs fabrication, averaged field intensity profile, and thus refractive index changes in fiber core have complex form. Moreover, it was shown that the influence of optical fiber position behind the phase mask on average intensity distribution in fiber core decreases with increasing of its diameter.     

高斯切趾型光纤布拉格光栅外腔半导体激光器的混沌输出特性

基于光反馈半导体激光器(SL)速率方程模型,理论仿真研究了高斯切趾型光纤布拉格光栅(GAFBG)反馈SL(GAFBGF-SL)混沌输出的延时特征(TDS)以及混沌带宽特性.结果表明:随着反馈强度的增加,GAFBGF-SL表现出由准周期进入混沌的动力学演化路径;通过合理选择GAFBG布拉格频率与SL中心频率之间的频率失谐及反馈强度,GAFBGF-SL混沌输出的TDS能得到有效抑制(低于0.02);通过进一步绘制混沌信号TDS及带宽在GAFBG布拉格频率与SL中心频率之间的频率失谐和反馈强度构成的参量空间中的分布图,确定了获取弱TDS、宽带宽光混沌信号的参数范围.     

Generation of millimeter-wave in optical pulse carrier by using an apodized Moiré fiber grating

A novel scheme is proposed to transform a Gaussian pulse to a millimeter-wave frequency modulation pulse by using an apodized Moiré fiber Bragg grating in radio-over-fiber system. The relation between the input and output pulses is analyzed theoretically by Fourier transformation method and the requirements for the proposed fiber grating are presented. An apodized Moiré fiber Bragg grating is designed and its characteristics are studied. It is shown that the proposed device is feasible, and the new scheme is believed to be an effective solution for the generation of millimeter-wave sub-carrier in future radio-over-fiber systems.     

Asymmetrically Apodized Fiber Bragg Gratings for Applications in Dispersion-Less Fabry-Pérot Fiber Cavities

Abstract

This article presents a theoretical study of the dispersion properties of the asymmetrically apodized fiber Bragg gratings. It is shown that the dispersive behavior of these gratings may be changed from normal to anomalous, depending on the apodization function and the grating strength. The asymmetrical Bessel functions were chosen in order to demonstrate these properties of the gratings. The application of this effect in a dispersion-less Fabry-Pérot fiber cavity formed by two asymmetrically apodized fiber Bragg gratings is discussed.     

Spectrum apodization of a time-dependent Gaussian pulse incident on the apodized circular aperture in far-field

The diffraction spectral intensity characteristics of a time-dependent Gaussian pulse from an apodized circular aperture in the far field are studied both theoretically and numerically. It is shown that the apodization of the diffracted Gaussian pulse spectrum can be found. Also the red or blue shift of the spectral intensity maximum of the incident pulse is presented.     

Theoretical investigation into spectral characteristics of a semiconductor laser with dual-FBG external cavity

A new theoretical model of the dual-FBG external cavity based on the couple-mode theory has been proposed for controlling the output spectral characteristics of the semiconductor laser with dual-FBG external cavity. The output spectral characteristics have been studied through the analyses of the external cavity length, the FBG length and the fiber length between the two FBGs. The results reveal that the side mode suppression ratio (SMSR), the full wave at half maximum (FWHM), the longitudinal mode interval (LMI) and the stability of the central wavelength are mainly determined by the effective external transmissivity, the fiber length between the two FBGs and the external cavity length, respectively. Furthermore, the rising/falling tendencies of the SMSR and the FWHM are exactly opposite. There exist the maximum of the SMSR and the minimum of the FWHM in a semiconductor laser with dual-FBG external cavity. The increase of the fiber length between the two FBGs results in the exponential decrease of the LMI. The increase of the external cavity length leads to the better stability of the central wavelength.     

Improving the characteristics of the modulation response for fiber Bragg grating Fabry–Perot lasers by optimizing model parameters

A unified and comprehensive study on the small-signal intensity and frequency modulation characteristics of a fiber Bragg grating Fabry–Perot (FBG–FP) laser are numerically investigated. The effect of injection current, temperature, external optical feedback (OFB), nonlinear gain compression factor, fiber grating (FG) parameters and spontaneous emission factor on modulation response characteristics are presented. The rate equations of the laser model are presented in the form that the effect of temperature (T) and external optical feedback (OFB) are included. The temperature dependence (TD) of laser response is calculated according to the TD of laser cavity parameters instead of directly using the well-known Parkove equation. It is shown that the optimum external fiber length (L_ext) is 3.1 cm and the optimum range of working temperature for FGFP laser is within ±2 °C from the FBG reference temperature (T_o). Also, the antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the modulation spectra. It is also show that modulation response is extremely sensitive to the OFB level, high injection current and gain compression factor. The study indicates clearly that good dynamic characteristic can be obtained by system parameters optimization.     

光纤光栅激光器激射波长的研究

一般认为，用光纤光栅作选频元件的光纤激光器，激射波长与光纤光栅中心反射波长一致，本文报道了不同的实验研究结果。通过细致的实验研究，发现光纤光栅激光器激射波长相对于光纤光栅中心反射波长有一定的偏移。激射波长可以出现在光栅中心反射波长的长波端，也可以出现在其短波端。对不同腔结构的掺镱、掺铒光纤光栅激光器的深入研究证明，谐振腔的各向异性对激光器的激射波长偏移起到决定性的作用，波长最大偏移量主要受限于光纤光栅的反射带宽。通过激光腔内的偏振控制器改变谐振腔的各向异性，可以在光纤光栅的反射带宽内控制激射波长的位置。     

Wavelength Tuning Characteristics of a Vertival Cavity Surface Emitting Laser Diode with an External Short Cavity

The characteristics of a wavelength tunable vertical cavity surface emitting laser diode (VCSEL) with an external short cavity are analyzed, in which the oscillation wavelength can be changed over several tens of nanometers with a nearly constant optical power by slightly altering the external cavity length. Analysis is based on rate equations for the optical power and carrier density, taking the effect of carrier-induced refractive index change into consideration, together with the study of behaviors of a complex resonator. The reflection coefficient r₂ of a laser facet facing the external mirror is shown to affect notably the characteristics of wavelength tuning, optical power and carrier density for a change of the external cavity length. It is also noticed that the wavelength change for this length becomes slower with relatively larger r₂ due to an increasing contribution of the effect of carrier-induced refractive index change, within the optical gain spectrum of the laser diode.     

Wavelength tuning characteristics of a fabry-perot semiconductor laser with an external short cavity

Wavelength tuning characteristics of a Fabry-Perot semiconductor laser with an external short cavity are analyzed, in which the oscillation wavelength can be changed by slightly altering the external cavity length. Analysis is based on rate equations for an optical power and carrier density. It is shown that the wavelength tuning range is dominated by a change of carrier density through the effect of carrier-induced refractive index change in the active layer of a laser diode. This depends on effective coupling coefficients of the optical field iteratively reflected back to the laser diode by the external mirror, and on reflection coefficients of an anti-reflection coated laser facet and the external mirror which compose the external cavity. The effective coupling coefficient is also derived using the waveguide theory and Kirchhoff ’s theory. Finally, an unstable condition which may limit a stable wavelength tuning range is shown by results of a linear stability analysis of rate equations.     

Remote fiber sensor based on cascaded Fourier domain mode-locked laser

We propose a high-speed remote fiber Bragg grating (FBG) sensor interrogation system based on a 1.3-μm cascaded Fourier-domain mode-locked (FDML) laser. It consists of multiple FBGs connected to an optical circulator in the laser cavity. The cascaded FDML laser with these multiple FBGs is operational when the scanning frequency of the fiber Fabry-Perot tunable filter matches the fundamental frequency of the laser cavity. Each FBG provides a separate laser cavity for the FDML laser. The scanning frequencies of each laser cavity are 30.5314, 31.5393, 32.7108, and 33.8023 kHz. Using the cascaded FDML laser, we measure the performance of the long-distance static strain FBG sensor interrogation system in both the time and spectral domains. The slope coefficients of the measured relative wavelength difference and the relative time delay from the static strain are found to be 0.95 pm/μstrain and 0.15 ns/μstrain, respectively. We also demonstrate the dynamic response of the interrogation system with 80-Hz modulation strain using the cascaded FDML laser. Thus, an FBG sensor interrogation system for high-speed and high-sensitivity long-distance monitoring systems can be realized using a cascaded FDML laser.     

Wavelength stabilization of a 980-nm semiconductor laser module stabilized with high-power uncooled dual FBG

An optimized dual fiber Bragg grating (FBG) is proposed for 980-nm semiconductor lasers without thermoelectric coolers to restrict temperature-induced wavelength shift.The mathematical model of the temperature-induced wavelength shift of the laser with the dual FBG is built using the external cavity feedback rate equations.The external cavity parameters are optimized for achieving the stability mode-locking laser output.The spectral characteristics of the dual FBG stabilized laser are measured to range from 0 to 70 °C.The side mode suppression ratio (SMSR) is more than 45 dB,while the full-width at half-maximum (FWHM) is less than 1 nm.The peak wavelength shift is less than 0.1 nm.The dual FBG wavelength shift proportional coefficient is between 0.1086 and 0.4342.     

Generation of millimeter-wave sub-carrier optical pulse by using a Fabry-Perot interferometer

A novel scheme is proposed to transform a Gaussian optical pulse to a millimeter-wave (mm-wave) frequency modulation pulse by using a Fabry-Perot interferometer (FPI) for radio-over-fiber (ROF) system.It is shown that modulation frequency of mm-wave is determined by the optical path of the Fabry-Perot (F-P) cavity, and amplitude decay time and energy transfer efficiency are related to the reflectivity of the F-P cavity mirror. The effect of pulse train extension on inter-symbol interference is also discussed.