电吸收调制激光器产生短光脉冲的实验研究

介绍了电吸收调制激光器(EML)产生超短光脉冲的原理,对输出脉冲进行了数值模拟和实验研究。分析了反向偏置电压和调制深度对于脉冲宽度和消光比的影响,对驱动参数进行了优化;根据模拟结果在实验中得到了10GHz重复频率的类高斯型光脉冲输出,脉宽为21．4ps,最大消光比为14．3dB,并与LiNbO3调制器(LN MOD)在相同调制频率下产生的光脉冲进行了比较。     

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)     

An Interferometric Method for High Extinction Ratio Measurements With 60-dB Dynamic Range

In this letter, we introduce a novel interferometric method for dynamic extinction ratio measurement of temporally demultiplexed pulses. A high extinction intensity modulator is used in order to temporally demultiplex the pulse train. The resulting extinction ratio is measured to be 44 dB. Our method has a dynamic range of ${sim}$ 60 dB. To our knowledge, this is the highest dynamic extinction ratio measurement method reported to date for high-speed optical modulators.      

输出大啁啾脉冲的展宽脉冲锁模光纤激光器

报道了一种大啁啾脉冲输出的全光纤展宽脉冲锁模激光器,以非线性偏振旋转(NPR)实现自启动锁模。激光器其余部分为全单模光纤(SMF)结构,提供很大的正色散,光栅对提供色散补偿,输出展宽脉冲。实验中得到了重复频率36.96MHz,单脉冲能量1.81nJ的稳定锁模脉冲序列,使用频谱分析仪观测得到脉冲序列一次谐波信噪比(SNR)达到80dB。直接输出脉冲有很大的正啁啾,脉宽为2.17ps,经过腔外压缩可获得70fs的脉冲。这种能压缩到百飞秒量级的大啁啾脉冲非常适用于光纤啁啾脉冲放大(CPA)系统。     

利用相位和强度调制器产生高消光比超短光脉冲

提出了一种产生高消光比超短光脉冲的新方法.利用相位调制器调制连续光生成啾啁光,而后利用M-Z强度调制器的倍频调制抑制对压缩不利的啁啾部分的影响,再通过等效啁啾光纤光栅进行压缩产生光脉冲.理论和仿真结果表明,该方法可以很好地消除光脉冲的基底及减小旁辦,产生消光比大于30 dB、波形理想的光脉冲,具有很强的可实现性.最后利用实际制作的色散系数为-380 ps/nm的等效啁啾光纤光栅对该方法进行了实验验证,结果表明,在重复频率为2.5 GHz、相位调制系数为9时,可产生脉宽小于18 ps的高质量光脉冲.     

基于45°倾斜光栅的重复频率可切换被动谐波锁模光纤激光器

基于非线性偏振旋转（NPR）技术,搭建了一台重复频率可切换的被动谐波锁模掺铒光纤激光器。腔内45倾斜光纤光栅（45 TFG）作为一种理想的光纤型起偏器,与两侧偏振控制器（PC）一起实现非线性偏振旋转效应,实现激光器的稳定锁模输出。在673 mW恒定泵浦功率下,通过调节腔内的PC,观测到了从基频到37阶谐波等多种锁模状态。该激光器可产生稳定的最高重复频率脉冲为783 MHz,对应的谐波阶数是37阶,且具有41 dB的边模抑制比（SSR）。高重复频率且稳定性好的脉冲可用于特定的应用中,如现代光通信系统、光学传感等。     

基于偏置马赫-曾德调制器的微波光子信号倍频

高速光信号源在现代光通信中不可或缺,目前倍受研究者关注。提出了利用一种具有偏置控制的马赫-曾德尔调制器(MZM),采用倍频方案产生高速微波光子信号,并进行了实验研究。通过在MZM 上施加一定的直流偏置引起两臂光脉冲的相位差,使光脉冲发生分裂实现倍频。实验中,利用5 GHz 的射频信号源,成功获得了频率增加一倍的10 GHz 高质量高速光信号。同时,也可以观察到在不同偏置电压下会产生不同的脉冲序列,发现优化偏压是实现高质量倍频的必要条件。该方案可用于产生40 GHz 以上的高频率光脉冲,可广泛应用于高速光通信。     

基于相位调制和光参量放大的光短脉冲源实验研究

提出了一种基于光参量放大和相位调制加色散压窄产生高消光比短脉冲源的新方法.连续抽运光首先经过LiNbO<,3>强度调制器,利用强度调制的倍频调制特性产生占空比为33%的光脉冲,再经相位调制器在脉冲的主体部分引入最大线性正啁啾,经参量放大后所生成的闲频光的啁啾为抽运光的2倍,通过合适的色散介质得到了11.4 ps的短脉冲...     

Optical pulse shaping by cross-phase modulation in a harmonic mode-locked semiconductor fiber ring laser under large cavity detuning

The authors experimentally investigate the phenomena of the pulse-shape distortion due to the drift of cavity length or modulation frequency in a harmonically mode-locked semiconductor fiber ring laser (SFRL). Also, it is experimentally demonstrated that the optical pulse shaping by cross-phase modulation (XPM) can be applied to enhance the robustness of a harmonic mode-locked semiconductor ring laser to the environmental factors, e.g., the modulation-frequency drift and/or the cavity-length drift. The mechanism of pulse-shape distortion due to the modulation-frequency drift or cavity-length drift is explained and experimentally verified. As the locking condition of the SFRL goes to the rational harmonic mode-locking (RHML) regime, the pulse-shape distortion becomes dominant. To validate the explanation, a simple equation for estimating and comparing the amount of cavity-length detuning in terms of frequency is proposed for comparison, which is called the equivalent frequency detuning (EFD). The distortion mostly appeared in the trailing edge of the pulse. By selectively filtering the XPM-induced red-chirped wavelength components in the rising edge of the pulse, the mode-locked pulse train with large pulse-shape distortion can be reshaped to regenerate a mode-locked pulse train with less pulse-shape distortion than the pulse train without the proposed scheme. This method can be used to reduce the pulse-shape distortion effectively, maintaining the pulse repetition rate under the large modulation-frequency detuning, or equivalently, large cavity-length detuning. The proposed scheme is able to generate the pulse-trains robust to the modulation-frequency drift and/or cavity-length drift of the mode-locked SFRL.     

功率和谐波同步检测的马赫曾德调制器偏压控制技术

为了克服马赫曾德调制器工作点随温度、压力等外界因素的漂移,需要对它的直流偏压进行补偿。以往采用微扰信号谐波检测的方案偏置点定位精度高,但由于需要在回路中附加小幅抖动信号,会引入一定的噪声,影响信号的纯度;而功率检测的方案易受光源功率自身扰动的影响,稳定性不高,且偏置点定位性能差。提出了一种平均功率和谐波检测相结合的MZ调制器偏置点控制技术,通过谐波探测确定最佳工作点,并以电光调制器前后光功率比值作为基准值进行反馈控制,提高了偏压控制的定位精度和噪声性能,同时克服了激光功率自身扰动的影响。在此过程中还提出了一种通过操控误差信号实现偏置点任意调节的方案,实现了大范围灵活的偏置点控制。实验发现,所提出的方案具有定位精度高、稳定性好和偏置点调节范围广的优点,能够很好地满足马赫曾德调制器偏置点控制的要求。     

Condition for short pulse generation in ultrahigh frequency mode-locking of semiconductor lasers

It is shown that although it is possible to obtain mode-locking without self-pulsation when certain criteria are satisfied, the shortest pulses are almost always generated at or close to the onset of self-pulsation. Thus, the amplitude of the optical pulse train is modulated by the (relatively) low-frequency envelop of a few gigahertz under this condition. This observation was obtained by simultaneously measuring the pulsewidth using an autocorrelator and monitoring the optical intensity using a high-speed photodiode and a microwave spectrum analyzer. It is concluded that while it is possible to generate picosecond optical pulses in ultrahigh-frequency mode-locking of quantum-well lasers, very short pulses ( to 1 ps) are almost always accompanied by self-pulsation which is manifested as low-frequency (gigahertz) envelope modulation of the optical pulse train.<>     

Electrooptic polymeric digital optical switches (DOSs) with adiabatic couplers

Electrooptic polymeric digital optical switches with an adiabatic coupler is proposed and demonstrated. The tip portion of the Y-branch is replaced by a full 1 /spl times/ 2 adiabatic coupler. By having this design, the switching voltage can be lowered and the devices are easier to fabricate. The fabricated switches show a characteristic step-like response to the driving voltage. The switching voltage is 5.9 /spl sim/ 6.4 V for dual-arm driving and the extinction ratio is higher than 20 dB. With the improvement of the bias voltage, this nonresonant optical switch design can be driven in a push-pull configuration without bias.     

Characteristics of 10-Gb/s Electroabsorption Modulator Integrated Distributed Feedback Lasers for Long-Haul Optical Transmission Systems

In this study, we review the characteristics of 10-Gb/s electroabsorption modulator integrated (EAMI) distributed feedback (DFB) lasers for long-haul optical transmission systems. The recent progress and issue of extinction ratio, chirp, and frequency response are described, which are important parameters to improve transmission performance. Except for the extinction ratio, frequency response and chirp characteristics are discussed further. The frequency response of packaged EAMI-DFB lasers has been improved using the novel impedance matching technique. The large signal chirp characteristics are calculated with varying bias voltage, facet reflectance, grating phase, and a coupling coefficient (κL) to predict the measured chirp characteristics. Finally, transmission performance (BER, eye margin, and eye opening penalty) is measured and estimated using the calculated chirp characteristics.     

Reliability estimation from zero-failure LiNbO/sub 3/ modulator bias drift data

Since x-cut LiNbO/sub 3/ (LN) optical modulators exhibit bias drift that rolls over around a voltage limit of most systems, their laboratory aging tests sometimes yield zero-failures and do not generate sufficient estimates of device reliability. In this regard, a modified calculation method is proposed and applied to data of OC192 x-cut LN modulators. The calculation presumes a time-independent probability that modulators are expected to have a voltage peak higher than the failure criterion. The obtained small failure rate is considered to be consistent with a rolling-over type drift behavior of these modulators.     

Reproducible optical second-harmonic generation using a mode-locked laser

Experiments have shown a one-to-one correspondence between the intensity of a fundamental pulse of light and its second harmonic. This correspondence is observed when the fundamental is each pulse of light in one train of pulses generated by a mode-locked laser. Such fundamental light shows no variation in the spatial distribution of intensity from pulse to pulse in one train of pulses and no randomness in the phases and amplitudes of the locked modes. As a result of these conditions, the ratio of harmonic intensity to the square of the fundamental shows little or no deviation from the mean. To contrast this, the wide fluctuations in this ratio observed when the laser is not mode-locked are also presented. In addition, the deviation from a square law in the relatianship between harmonic and fundamental intensities in one burst laser pulse is demonstrated. These deviations are found less often when the laser cavity is short (40 cm) than when it is long (3 meters). They may be attributed to the appearance or disappearance of different modes of oscillation during the pulse and possible variations in the mode amplitudes.     

Optimum operating points for electroabsorption modulators in 10Gb/s transmission systems using nondispersion shifted fiber

The chirp, optical extinction ratio and insertion loss of an electroabsorption modulator (EAM) depend on the properties of the bulk or multiple-quantum-well absorption layer of the device and on the bias voltage and modulating voltage waveform. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for five different EAM's. To comprehensively explore this issue, the measured dependence of the absorption and α-parameter on applied voltage is used to accurately model an EAM in a system simulator. The effects of group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity are included to permit assessment of the dependence of the optimum bias and modulation voltages on the average transmitted optical power for a given fiber length. The improvement in receiver sensitivity, relative to that obtained with maximum optical extinction ratio, depends quite significantly on the transmitted optical power and the specific properties of the modulator. This makes it difficult to determine optimum operating conditions which apply generally     

A lithium niobate light modulator for fiber optical communications

An electrooptic intensity modulator using lithium niobate has been developed for applications in binary fiber optical digital communications at the wavelength of 1.06 µm. We have shown that many shortcomings generally associated with electro-optic modulators can be surmounted. The modulator was driven by a compact transistor amplifier, temperature dependence of the static birefringence was minimized, and the optical bias was made adjustable by a dc voltage superposed on the signal. The modulator has been operated at 70-Mb/s pulse rate and 100-percent modulation, its extinction ratio is better than 40 to 1 and the optical insertion loss is about 1 dB.     

Electrooptic spectral shearing interferometry using a Mach-Zehnder modulator with a bias voltage sweeper

We propose a novel implementation of electrooptic spectral shearing interferometry (EOSSI) for complete characterization of optical pulses. We measure the spectrum of the output pulse of a Mach-Zehnder intensity modulator by sweeping the bias voltage. By doing this, we can synthesize a spectral fringe, with which we can reconstruct the waveform of the pulse under test. Compared to the previous EOSSI, our proposal simplifies the setup and enables us to measure high-repetition rate trains of optical pulses. In the experiment, we characterize picosecond optical pulse trains at a repetition rate of 10 GHz with high sensitivity and reliability.     

A novel scheme for generating optical dark return-to-zero pulses and its application in a label switching optical network

We experimentally demonstrate a novel and simple method to generate a dark return-to-zero (RZ) pulse with tunable pulsewidth and extinction ratio by using a dual-arm LiNbO /sub 3/ intensity modulator. Our experimental results show that this dark RZ pulse signal can be used in a 10-Gb/s optical packet switching system as an optical label. In addition, we demonstrate that this dark RZ label can be easily erased using the gain saturation effect in a semiconductor optical amplifier.     

Polymeric digital optical modulator based on asymmetric branch

A digital optical modulator based on an asymmetric Y-branch waveguide is proposed and fabricated by using an electro-optic polymer. The operating point is initially shifted to the off-state utilising the asymmetry in the branch to provide an initial zero-state with no electrical bias. It has been confirmed that the high extinction ratio can be obtained with a low drive voltage. An extinction ratio of 25 dB is demonstrated for a drive voltage of 20V using a polymer PMMA-DR1 with r_ss of 5 pm/V at 1.3 μm