Polarization-independent all-optical clock division using asemiconductor optical amplifier/grating filter switch

We propose and experimentally demonstrate, for the first time to our knowledge, polarization-independent all-optical clock division (CD) of an optical pulse train at 2.88 GHz. This is achieved using a semiconductor optical amplifier (SOA)/grating filter switch, where the SOA acts as a spectral shifter by a self phase modulation and the grating filter acts as a spectral shutter. The proposed scheme is very insensitive to the polarization of input pulses and requires very low switching energy (~102 fJ). After the all-optical CD operation, we have obtained the output pulse train at 1.44 GHz, which is half the input repetition rate, with high extinction ratio (>20 dB)     

Methods for Ultra-Wideband Pulse Generation Based on Optical Cross-Polarization Modulation

Optical methods for different type ultra-wideband (UWB) pulse generation based on cross-polarization modulation (CPM) are proposed and demonstrated in this paper. Two polarity-reverse pulses can be obtained by CPM and birefringence time delay to form a monocycle pulse. A semiconductor optical amplifier (SOA) is placed after the monocycle pulse process for doublet pulse generation. These two kinds of pulses can be employed in single-band impulse radio UWB (IR-UWB) systems. Two kinds of multi-band UWB pulses can be generated based on monocycle pulse train with proper apodization profiles, realized by hybrid photonic microwave filter and synchronous polarization modulation respectively. Experimental results show that these pulses can be used in multi-band UWB (MB-UWB) over fiber systems.      

基于半导体光放大器交叉增益调制效应的主动锁模光纤激光器

提出一种腔内损耗小的基于半导体光放大器(SOA)交叉增益调制效应(XGM)的主动锁模光纤激光器结构。使用光环行器成功减小了激光器的腔内损耗,提高了激光器的输出功率。从理论上对有理数谐波锁模过程中腔内脉冲复合的物理机制进行了详细分析。利用有理数谐波锁模技术,在调制频率为10 GHz下,得到了重复频率为30 GHz的皮秒级光脉冲序列输出,其峰值功率约0.5 mW。由于半导体光放大器的宽增益谱与滤波器的较大可调谐范围,使得激光器输出可以在较大的波长可调谐范围内保持较大功率输出。成功实现了调制频率为20 GHz的谐波锁模短光脉冲输出,可调谐范围达40 nm,峰值功率大于0.65 mW。半导体光放大器和激光器的短腔长保证了激光器的长期稳定性。     

Dual Orthogonal Polarization States in an Active Mode-Locked Birefringent Fiber Ring Laser

We report the generation of dual amplitude pulses in an active mode-locked fiber laser within a birefringent cavity. Different to normal mode-locked pulses with identical amplitude and polarization state, and pulses polarized on both x- and y-axes simultaneously exist in the output pulse train. The two orthogonal pulse sequences have different amplitudes and lase at different wavelengths. Dual wavelengths are the result of red shift and blue shift of the x- and y-polarization states of the generated pulses, respectively, due to the detuning phenomena. Locking to individual x- or y-polarized pulse is also obtained by adjusting polarization controllers     

基于SOA非线性偏振旋转效应谐波锁模光纤激光器的数值研究

基于半导体光放大器(SOA)的锁模光纤环行激光器因其能得到高质量、高重复率的光脉冲而受到人们的广泛关注.在考虑SOA非线性偏振旋转效应下,建立了基于SOA的谐波锁模光纤激光器理论模型,利用该模型,着重研究了在不同SOA注入电流情况下,SOA非线性偏振旋转效应对谐波锁模光纤激光器输出脉冲波形、峰值功率及脉冲宽度的影响.研究结果表明:当SOA注入电流分别为102 mA和106 mA时,可以获得峰值功率为0.16 mW和0.56 mW、脉冲宽度为6 ps和19.8 ps的稳定锁模脉冲.     

基于半导体光放大器环形腔的全光频率倍增/恢复

提出了一种利用具有干涉作用的半导体光放大器(SOA)环形腔实现全光频率倍增/恢复的新方法,该方法同相关实验比较具有显著优势.数值结果表明:2.5 GHz的光脉冲序列注入SOA环形腔,可输出重复频率为5～25 GHz振幅均衡、与入射光偏振无关的光脉冲序列;SOA的偏置电流对SOA环形腔输出脉冲振幅的均衡度影响显著,对于基频为2.5 GHz和10 GHz输入脉冲序列分别存在一最佳的SOA偏置电流值;从传输速率为2.5 Gbit/s的27-1伪随机信号中可提取出重复频率为分别2.5 GHz和5 GHz的幅度均衡的时钟信号.     

利用非线性偏振旋转锁模技术产生0.7nJ, 1.5ps光脉冲

为了研究锁模光纤激光器以增益平坦型掺铒光纤放大器作为增益介质对输出特性的影响,采用增益平坦型掺铒光纤放大器结合光纤偏振控制器、偏振相关光隔离器组成锁模光纤激光器,基于非线性偏振旋转锁模技术,实现稳定、自起振锁模运转,得到了中心波长1560nm、重复频率6.495MHz、单脉冲能量0.7nJ、脉宽1.5ps的超短光脉冲。同时实验观察到峰值波长为1557nm和1570nm的双峰值波长锁模脉冲的产生。结果表明,采用增益平坦型掺铒光纤放大器替代普通掺铒光纤组成锁模光纤激光器,可获得较高单脉冲能量的超短光脉冲,锁模脉冲的输出光谱可能出现双峰结构,从而可为超短脉冲光纤激光器设计及实用化提供参考。     

Switching-wavelength pulse source constructed from a dispersion-managed SOA fiber ring laser

A new approach to generate switching-wavelength picosecond pulses is developed based on a dispersion-managed semiconductor optical amplifier (SOA) fiber laser cavity. Gain modulation on a 1.55-/spl mu/m SOA is used together with subharmonic gating of pulses in the cavity. An 8-GHz, eight-wavelength output is obtained with a constant spacing of 0.91 nm between neighboring wavelengths. The wavelength channels have nearly identical peak powers. The total amplitude and time jitter of the output is measured to be smaller than 0.4 ps. The spacing and the number of output wavelengths can be easily tuned by changing the operating frequency and the subharmonic ratio without modifying the optical cavity.     

Theory and simulation of the dynamics and stability of actively modelocked lasers

A new model is proposed for the active modulation component of a mode-locked laser cavity. By using Jacobi elliptic functions to capture the periodic forcing to the cavity, we are able to construct exact solutions representing a mode-locked pulse train. Two families of pulse-train solutions are generated: one in which neighboring pulses are in-phase and a second in which neighboring pulses are out-of-phase. We show that only out-of-phase solutions allow for stable mode-locked pulse trains. Further, pulse-to-pulse interactions can generate instabilities that destroy the pulse train altogether or lead to Q-switching.     

Analytical Characterization of Optical Pulse Propagation in Polarization-Sensitive Semiconductor Optical Amplifiers

In this paper, we perform analytical characterization of optical pulses propagating through a polarization-sensitive semiconductor optical amplifier (SOA). We derive analytical expressions for the carrier density, gain and phase evolution along the SOA and show how these expressions prove useful in optical signal processing applications. The propagation of counter-propagating pulses as well as pulse streams across SOAs have been analysed and expressions for energy gain have been derived in all these cases. We also show that our analytical results reduce to corresponding results of polarization insensitive SOAs already published. The analytical results are in excellent agreement with detailed numerical simulations done in MATLAB using the NIMROD portal. The analytical calculations lead to significant savings with regard to simulation time and processing capacity requirements. We further prove that the energy gain difference for counter-propagating pulse streams is directly proportional to the difference delay between in them and hence can be used as a measure of the delay difference. This theoretical result agrees well with experimental results.     

10 GHz主动锁模光纤激光器

报道了由电吸收调制器 (EAM)和半导体光放大器 (SOA)构成的 10GHz光纤锁模激光器 ,可输出脉宽为 10ps,谱宽为 0 4nm近变换极限的锁模光脉冲序列。该激光器在未加反馈控制回路情况下 ,可稳定工作 6h以上。并对EAM和SOA工作参数对锁模脉冲性能的影响进行了分析。     

调Q及连续掺Yb光纤激光器中的自锁模研究

在用半导体激光器抽运的单包层掺Yb调Q光纤激光器中观察到了清晰稳定的自锁模脉冲序列。脉冲包络形状为调Q脉冲。每个锁模脉冲的幅值由其在调Q脉冲中的相应位置决定。经过分析，认为自相位调制是调Q光纤激光器中产生锁模的主要原因。自相位调制的存在使得光脉冲的频谱被展宽，当这种展宽和腔的模式间隔相差不多时，腔内的模式便能相互作用，直到它们之间产生一个固定的相位关系。也即形成锁模。在此基础上。去掉声光晶体，并用两个光栅作为腔镜，实现了全光纤法布里-珀罗(F-P)腔锁模光纤激光器。改变腔结构，分别采用光栅和光纤反射圈作为前后腔镜，同样观察到了锁模脉冲。经过观察发现，锁模脉冲的产生和掺Yb光纤的浓度、长度、抽运功率的大小有着密切的关系。这为锁模脉冲的产生提供了一种新的方法。     

泵浦抽空状态下的同步泵浦光学参量振荡器

用计算机数字模拟了Nd:YAG锁模激光器倍频光同步泵浦下LiNbO_3光学参量振荡器的脉冲形成过程。泵浦抽空状态下,输出信号脉冲序列中各脉冲的波形与宽度各不相同,起始脉冲窄(～7微微秒),后面脉冲宽(～20微微秒)而且有亚结构。腔长失谐主要降低输出能量,对脉冲加宽作用不大。器件增益对脉冲波形、宽度和腔长失谐量均有影响。     

Stability and compression of pulses in the soliton laser

We present a theory of the soliton laser which examines how an external optical fiber enables a laser to produce shorter pulses than it could produce alone. We begin by discussing a phenomenological laser model which shows how the lower limit to the mode-locked pulse width can arise. This model is coupled to an external optical fiber cavity, into which a part of the output beam is launched. The returning pulse from the fiber cavity is then mixed with a circulating pulse in the laser at the output mirror. We have found stable solutions which are nearly periodic in the external cavity. We also find more than one solution for a given set of model parameters, depending on the initial conditions. The radiation (non-soliton) part of the propagation in the optical fiber cannot be ignored. It acts as a buffer between the tendency of the fiber to produce solitons, and the fact that a pure soliton is not an exact fixed point of the laser model.     

Characterization of ultrashort pulse formation in passivelymode-locked fiber lasers

Pulse formation in an all-solid-state passively mode-locked neodymium glass fiber laser is investigated by employing characterization techniques in the time, optical, and radio-frequency domains. The pulses are generated by a combination of passive amplitude modulation arising from nonlinear polarization evolution and passive frequency modulation due to quasi-soliton-like pulse shaping. Several distinct operation modes for ultrashort pulse generation are described and pulses as short as 38 fs are directly generated in the cavity     

Analysis of optical DEMUX characteristics based on four-wave mixingin semiconductor optical amplifiers

We have analyzed the basic characteristics of all-optical demultiplexing (DEMUX) based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) by solving a modified nonlinear Schrodinger equation by the finite-difference beam propagation method. Amplified spontaneous emission noise was not included in our model. The optimum pump pulsewidth for obtaining the high ON-OFF ratio is 1~3 ps for 1 ps, 250 Gb/s probe pulses. The shorter limit of the pulsewidth is due to detuning between the pump and probe frequencies, which is determined by the gain bandwidth of the SOA. In order to achieve faster DEMUX operation, an SOA with broader gain bandwidth is required. We also simulated pattern effects in the FWM signal. Power fluctuation in the FWM signal can be reduced by using a strong energy pump pulse and/or weak energy probe pulse. The energy fluctuation of the FWM signal decreases to less than 1% for a 30-bit, 250-Gb/s input probe pulse train with a pulse energy of 0.01 pJ. This small fluctuation should not disturb DEMUX operation. We have also examined DEMUX from time multiplexed signals by repetitive pump pulses. Strong energy pump pulses decrease the FWM signal intensity. However, there is no pattern effect due to gain saturation because the pump pulses are injected continuously     

All-optical ultra-wideband doublet signal source based on the cross-gain modulation in a semiconductor optical amplifier

We propose a novel scheme to generate the ultra-wideband (UWB) doublet signal pulse based on the cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA). In the scheme, only an optical source and an SOA are needed. As there is only one wavelength included in the output doublet signal pulse, no time difference between the upper and down pulses is introduced during the transmission process. By using the software of Optisystem 7.0, the impacts of the optical power, the SOA current, the wavelength and the input signal pulse width on the generated doublet pulse are simulated and tudied numerically. The results show that when the pulse width of the input signal pulse is larger, the output signal pulse is better, and is insensitive to the change of wavelength. In addition, the ultra-wideband positive and negative monocycles can be generated by choosing suitable optical source power and SOA current.     

An optically programmable mode-locked laser

We demonstrate an optically mode-locked laser that generates a repetitive pattern of ultrashort pulses when driven by a repetitive pattern of optical pulses from an external source. Two modes of operation are described: in one mode of operation the generated pattern is determined directly by the driving pulse train; in the other mode it represents the result of logic operations between elements of the driving pulse train     

SOA反射率对谐波锁模光纤激光器的影响

基于半导体光放大器(SOA)的锁模光纤环行激光器因其能得到高质量、高重复率的光脉冲而受到人们的广泛关注。计及SOA端面剩余反射率,建立了基于SOA的谐波锁模光纤环行激光器的理论模型。利用该模型研究了SOA的端面剩余反射率对输出脉冲的波形、峰值功率及其脉冲宽度的影响。结果表明,SOA的端面剩余反射率对锁模脉冲影响较大;随着端面剩余反射率的增大,峰值功率逐渐增加,脉冲宽度逐渐变宽。     

全保偏非线性偏振环形镜锁模掺铒光纤激光器

研究了基于非线性偏振环形镜锁模的全保偏光纤激光器锁模机制。在非线性偏振环形镜中,用偏振分束器取代传统的非线性放大环形镜锁模激光器中的光纤耦合器,并辅以非互易性元件和增益光纤,作为全保偏光纤激光器中实现稳定锁模的核心器件。构建了一台基于非线性偏振环形镜的掺铒光纤锁模激光振荡器,实现了重复频率75 MHz,时域脉冲宽度141 fs,总输出功率约30 mW的稳定锁模脉冲序列输出。该激光器具有双向输出,且通过调节腔内波片可调节输出功率。此外,对激光器输出功率和重复频率的稳定性进行了评价,在自由运转情况下,1 h内输出脉冲序列的平均功率波动小于0.05%,重复频率的1 s相对稳定度为2.010-8。该结构的全保偏光纤激光器可开机自启动锁模,且环境稳定性高、重复频率较高、脉冲宽度窄,能满足激光测距、激光加工、激光光谱成像、航天等应用对超短脉冲光源的需求。