Influence of fiber nonlinearity on the fiber transfer function:theoretical and experimental analysis

The influence of fiber nonlinearity on the fiber transfer function is investigated theoretically and experimentally. A rigorous expression for the fiber transfer function using a directly modulated semiconductor laser as an optical transmitter is presented. Very good agreement of fiber transfer function between experimental data and theoretical predictions is achieved. Furthermore, the fiber parameter values extracted from fitting are the same as those obtained from relative intensity noise measurements, and the fiber nonlinearity coefficient value is compared with other published results. A thorough physical explanation for the effect of fiber nonlinearity on the fiber transfer function at low-medium frequencies is provided. Results reveal that in the anomalous dispersion regime the fiber nonlinearity enhances the high-pass behavior typical of the fiber transfer function using a directly modulated laser operating at high bias current, shifts the transfer function dips toward higher frequency, and consequently leads to a larger bandwidth. Furthermore, it is shown that the fiber nonlinearity can provide partial or total compensation of the nonuniform frequency response at low-medium frequencies resulting from a directly modulated laser operated at low bias current     

Two Different Operation Regimes of Fiber Laser Based on Nonlinear Polarization Rotation: Passive Mode-Locking and Multiwavelength Emission

We have proposed and demonstrated a nonlinear polarization-rotation-based fiber laser with two different operation states: passive mode-locking and multiwavelength emission. The intensity-dependent transmission or loss induced by nonlinear polarization rotation accounts for the distinct operation regimes. Our experiment results indicate that both passively mode-locked pulses and continuous-wave multiwavelength can be generated from the same fiber laser just through adjusting polarizations. Another characteristic of the current multiwavelength laser is that the used periodic filter is a birefringence fiber filter, which facilitates all-fiber integration of the fiber laser, so it is a potential multifunction laser source with all-fiber configuration and convenient manipulation.     

L-band fiber laser with wide tuning range based on dual-wavelength optical bistability in linear overlapping grating cavities

A new mechanism to tune an erbium-doped fiber laser in the L-band is presented. The technique is based on the dual-wavelength bistability in overlapping linear cavities, which allows two lasing lines to be switched from one to another by a triggering pulse superimposed to the dc bias current applied to the pump source. The two wavelengths can be independently tuned to give the laser a total tuning range of 33 nm that covers the whole L-band, while the tuning range of either of the tunable fiber Bragg gratings (FBGs) used is only half of that range. An output dynamic range of 7 dB, and a maximum output power of up to 11 dBm with high optical signal-to-noise ratios are obtained. A study of the switching transient responses shows that the typical switching time is about 4 to 12 ms, depending on the dc bias current applied to the pump source. Moreover, our wide tuning technique is implemented by only applying ordinary commercial tunable FBGs which normally would have given rise to only about half of the tuning range; no additional filtering mechanisms are required to achieve almost uniform outputs.     

光纤激光器中锁模孤子序列的长周期脉动

实验观察到非线性偏振旋转锁模光纤激光器中锁模孤子及孤子簇的长周期脉动现象.单孤子与孤子簇的脉动周期达103和102次脉冲周期量级.脉动周期及脉动包络形状与抽运功率及腔内偏振设置有关.孤子脉冲的长周期脉动现象归因于不严格的1阶偏振态设置.不严格的1阶偏振态设置使得偏振相关隔离器对每次腔内循环的锁模孤子脉冲进行了微弱的周期偏振滤波.积累的偏振滤波损耗导致了锁模脉冲序列出现长周期脉动.     

A switchable dual-wavelength erbium-doped fiber laser based on Sagnac loop inserted with two FBGs

A novel switchable dual-wavelength erbium-doped fiber （EDF） laser is demonstrated. The wavelength selection ele- ment consists of two fiber Bragg gratings （FBGs）, a polarization controller （PC） and a 3 dB coupler forming a Sagnac loop inserted with two FBGs. We study the effect of coupling ratio on filtering performance in this paper. By adjusting PC, we can change the wavelength-dependent loss, and then using nonlinear polarization rotation effect to suppress the mode competition caused by the homogeneous broadening of EDF, we obtain single- and double-wavelength laser outputs. At room temperature, under 200 mW pump power, dual-wavelength laser is achieved, and the center wave- lengths are 1545.34 nm and 1548.20 nm, respectively. The peak power values are -13.36 dBm and -14.58 dBm, and side mode suppression ratios （SMSRs） are 41.10 dB and 39.88 dB, respectively. Within two hours, the maximum fluctuation of peak power is less than 0.7 dB, which shows that the demonstrated fiber laser is stable. Moreover, by adjusting PC, singel-wavelength laser output is obtained, the peak power is -6.27 dBm or -5.45 dBm, and SMSR is 40.03 dB or 39.96 dB at 1545.34 nm or 1548.20 nm, respectively.     

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.     

基于激光瞬态特性的内腔气体浓度光纤传感器

报道了一种基于掺铒光纤激光器瞬态特性的内腔吸收式气体浓度传感器.介绍了掺铒光纤激光器内腔吸收谱的基本原理,以及激光激射延迟时间与气体浓度的关系.用光纤环全反镜和光纤Bragg光栅构成F-P线形腔,通过调节光栅的反射波长,使激光激射波长落在乙炔的一个较强的吸收峰1532.83 nm上.通过测量激光激射延迟时间可以获得气体的浓度.实验证实了方案的可行性,该装置的灵敏度可以达到10×10~(-6).     

外差锁定式相干光通信

提出并研制了一种新的外差锁定激光频率调制系统,可用于相干光通信,并进行了闭环调频、调幅实验。对实用的外差锁定式半导体激光器光纤通信系统的技术条件作了讨论。     

Erbium-doped fiber compound-ring laser with a ring filter

A new all-fiber, diode-pumped laser with a compound-ring structure has been demonstrated. A double-coupler fiber ring filter formed by two in-line low-loss couplers is used as an intracavity mode selection element. It functions as a filter which provides a larger free spectral range for the compound laser cavity and serves as a ready port for the laser output. The fiber laser is pumped at 980 nm with a threshold of 7.9 mW and a slope efficiency of 0.11. The condition for a maximum output power can be obtained by tuning an in-line polarization controller     

An 800 Mbit/s optical transmission experiment using a single-mode fiber

It has been confirmed by a preliminary transmission experiment that a repeater spacing at 800 Mbits/s can be lengthened up to 7.3 km, by using a combination of a low-loss single-mode fiber and a well designed AlGaAs laser. The laser is modulated by a signal current superimposed on a dc bias current above the threshold. This modulation scheme avoids the spectrum broadening of the laser at the cost of extinction ratio degradation. A discrepancy of about 13 dB exists between the measured and the calculated error-rate performance curves. The major part of the discrepancy, 10 dB, is due to the extinction ratio degradation.     

Analysis of laser diodes as transmitter and preamplifiers in timecompression multiplexing systems

It is proposed that a laser diode used as a transmitter in the transmission mode be used as a low bias current preamplifier in the receiving mode for time compression multiplexing transmission. By adopting the low bias current approach, the preamplifier is wavelength and polarization independent. The maximum bias current is determined to be about 0.5 I_th (I_th=threshold current) by experiments. A transmission experiment at 3.5 Mb/s reveals that about 4 dB of improvement in receiver sensitivity can be obtained by using a laser preamplifier at I=0.54 I_th when compared to an unbiased laser. The receiver sensitivity of the proposed configuration (laser amplifier at low bias current and photodiode) is estimated by considering the laser preamplifier noise and circuit noise. The laser preamplifier noise is calculated by treating the laser as a traveling wave amplifier with highly reflecting facets     

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。     

Measurements of mode partition noise of laser diodes

Mode partition noise may become the dominant performance limitation for single-mode fiber systems. We define the mode partition coefficientkfor laser diodes which can be used as a universal index, like threshold current and spectral width, to characterize laser diodes. Thekvalue varies between 1 and 0, and for largerk, the mode partition noise will be higher. We also demonstrate two simple techniques for measuringk: the low-pass filter method and the sampling method. We discuss thekvalues measured under different operating conditions (pulse width, dc bias) for several types of laser diodes,kvalues are around 0.4-0.6 with short pulse width (1.5-5ns), and decreases to 0.14 pulse width of 50 ns. The largestkvalues were obtained for a laser exhibiting strong relaxation oscillation. For that laser,kdid not depend on the pulse width. Thekvalue is an important factor for system and fiber design as well as laser diode design.     

Full-duplex optical transmission using self-heterodyne lasertransceivers

The use of semiconductor diode lasers simultaneously as heterodyne receivers and lightwave transmitters in a full-duplex single optical fiber transmission system is reported. A transmission distance of 16 km, limited by Rayleigh backscattering, is obtained at a bit rate of 40 Mb/s for this simple system, which requires no optical couplers or photodiodes. The proposed operation is analogous to that of the so-called autodyne radio receiver in which the local oscillator circuit also performs the heterodyne mixing. The optical counterpart is called a self-heterodyne mixer and offers the same frequency selectivity and tunability as a conventional heterodyne detector but with reduced receiver complexity. In the self-heterodyne receiver, an optical signal to be detected is introduced directly into the lasing cavity by coupling the signal into the laser through an uncoated cleaved end facet. Changes in the optical cavity field, which result from interference between the lasing mode and the injected signal, modulate the rate of stimulated carrier recombination and thus produce a change in the injection current. This alternating current component is detected and amplified in the laser active region bias     

Suppression of relaxation oscillations in tunable fiber lasers witha nonlinear amplified loop mirror

The concept of relaxation oscillation damping in a tunable fiber laser with a nonlinear absorber or loss element is demonstrated, using a dual-cavity erbium-doped fiber laser. A nonlinear amplified loop mirror was employed as the nonlinear loss element to facilitate damping. Experimental results exhibiting complete elimination of relaxation oscillations are presented     

A stabilized and tunable erbium-doped fiber ring laser with double optical filter

We have proposed and experimentally demonstrated a stabilized and tunable erbium-doped fiber ring laser by using a Fabry-Pe/spl acute/rot laser and a fiber Fabry-Pe/spl acute/rot filter. The sidemode suppression ratio of >30 dB/0.1 nm and the output power of >2.2 dBm can be achieved while this ring laser is tuned from 1528.28 to 1559.64 nm with the step of 1.12 nm. The wavelength variation of less than 0.01 nm and the power fluctuation of /spl les/0.04 dB have also been obtained. When the constant output power is controlled by adjusting the bias current of 980-nm pump laser, the power variation of less than /spl plusmn/0.05 dB can be obtained over the entire tunable range.     

External cavity controlled operation of a semiconductor diode gain element in series with an optical fiber

The series combination of a semiconductor diode gain element (a diode laser whose end facets have been antireflection coated) and an optical fiber has been placed inside an external cavity and the combined system has lased in a single spectral line whose width was less than the1.7 times 10^{-5}-nm (7.5-MHz) resolution of the scanning Fabry-Perot interferometer used in the measurement. This result has been achieved with either a single-mode or a multimode optical fiber in series with the diode gain element and for experiments in which a polarizer, oriented so its polarization is parallel to the diode gain element polarization, was placed at the other end of either fiber. The output of the external cavity is temporally stable as measured both by a 225-MHz-bandwidth detector system and a spectrum analyzer. Over a one minute interval the maximum fluctuation in the output frequency of the external cavity was found to be 2 MHz. Analysis of the threshold behavior of the external cavity for the cases where the multimode or single-mode fiber are in the cavity indicates that the coupling coefficients for either fiber are nearly the same. Without an external cavity, the coupling coefficient from the diode gain element (or from a similar, not anti-reflection-coated, laser) to the multimode fiber is over five times larger than that to the single-mode fiber. These coupling results are explained by postulating that for the multimode fiber, only one or a selected number of the many modes of the fiber can participate in the laser action of the external cavity. It is believed that only these mode(s) are reflected back into the fiber by the spherical mirror in appropriate phase and angle to participate in the laser emission.     

基于可变激光源和啁啾M-Z调制器的双调谐光电振荡器

设计了一种利用微波光子滤波器(MPF)实现频率可调谐的光电振荡器(OEO)。该模型通过双驱动Mach-Zehnder调制器(DD-MZM)和啁啾光栅形成高Q值的MPF,得出OEO的振荡频率是关于光源波长和DD-MZM直流偏置电压的函数,通过对光源波长或DD-MZM偏置电压的调整,可实现振荡频率调谐功能。同时,在电域上,利用RF耦合器和RF延时线形成两路具有延时差的反馈信号,通过合并后反馈至DD-MZM,从而有效地抑制边模。理论和仿真实验表明,针对不同的调谐范围可实现粗调和微调的功能。     

波长可调谐光纤激光器

已成功研制了一台大功率宽调谐范围光纤激光器,利用主动锁模技术实现了光纤激光器的稳定脉冲输出。实验中调节偏振控制器到合适位置和调制器偏压为3．8V时,获得了平均输出功率2．160mW,重复率9．970754GHz,中心波长为1561．200nm的锁模脉冲输出,输出脉冲中心波长在1528．015-1571．330nm之间连续可调。     

Spontaneous oscillations in the InP-InGaAsP lasing optoelectronicswitch (LOES)

We present the first comprehensive experimental and theoretical analyses of spontaneous electrical and optical oscillations that occur when the InP-InGaAsP lasing optoelectronic switch (LOES) is biased in or near the negative differential resistance region of the device characteristic. For a device with a switching voltage and current of ~7 V and 0.5 mA, respectively, electrical oscillations are observed which result in current spikes of up to 613 mA, with a FWHM of 0.6 μs. The repetition rate varies from 900 Hz to 0.22 MHz, increasing with bias current. Pulses of laser light correlated to the current pulses are emitted from the LOES for some circuit configurations. A lumped circuit element model is presented which agrees well with the experimental results, and illustrates the effects of the bias circuit parameters on the device oscillations