位于第三近红外窗口的平坦光纤超连续谱产生

报道了一种基于非线性偏振旋转效应的被动锁模光纤激光器。采用980 nm分布式反馈激光器作为泵浦源,0.5 m长的高掺杂掺铒光纤作为增益介质。实现了脉冲宽度为822 fs的传统孤子锁模脉冲,输出脉冲的平均功率为2.8 mW,信噪比为55.8 dB。通过微调腔内的偏振控制器,实现了传统孤子脉冲和孤子分子脉冲间的切换,孤子分子的脉冲宽度为312 fs,信噪比为53.86 dB。孤子分子脉冲经掺铒光纤放大器放大后泵浦一段57 m长的高非线性光纤,产生了位于第三近红外窗口（1600 nm～1870 nm）的超连续谱,其20 dB谱宽为355.8 nm。     

Large amplitude noise reduction in ultrashort pulse trains using a power-symmetric nonlinear optical loop mirror

We analyse numerically the capabilities of a power-symmetric nonlinear optical loop mirror (NOLM) in the ultrashort pulse regime for high-quality amplitude regeneration of an optical signal. The device, which operates through nonlinear polarisation rotation, includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. For particular adjustments of the retarder orientation, and a circularly polarised input beam, the output energy characteristic flattens near the switching energy, a property that can be used to eliminate large amplitude fluctuations in an optical signal. The group velocity mismatch between polarisation components introduced by twist is mitigated by the interplay between anomalous dispersion and the nonlinear Kerr effect, although strong twist should be avoided as it still introduces substantial pulse distortion. Contrary to other designs, where a plateau characteristic requires a large power imbalance between the counter-propagating beams, both pulses in the present scheme can be simultaneously close to fundamental solitons, which allows a substantial widening of the plateau for particular pulse parameters. Good quality, nearly transform-limited pulses are obtained in this case at the NOLM output. The device is applicable for the regeneration of ultrafast data streams in which the signal-to-noise ratio is severely deteriorated.     

Experimental investigation of a passively mode-locked fiber laser based on a symmetrical NOLM with a highly twisted low-birefringence fiber

We experimentally investigate the passive mode-locking operation of a figure eight-fiber laser based on a symmetrical nonlinear optical loop mirror (NOLM) with a highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by the polarization asymmetry between the counterpropagating beams in the loop. The most efficient switching is obtained when we have linear polarization for one of the beams and the circular polarization for the other. We used a quarter-wave retarder (QWR) in the NOLM loop to break the polarization symmetry. Through the QWR position, it is possible to adjust the transmission behavior from a maximum to a minimum at a low input power. With our configuration, it is possible to get self-starting mode-locking operation at a specific position of the QWR. This QWR position corresponds to a value close to the minimal transmission. The pulse repetition frequency was 0.8 MHz. The mode-locked laser ran in a stable operation for hours. We achieved a stable generation of picosecond pulses with milliwatts of an average output power.     

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.     

Fiber torsion sensor demodulated by a high-birefringence fiber Bragg grating

A new high-birefringence (Hi-Bi) fiber torsion sensor demodulated by a Hi-Bi fiber Bragg grating (FBG) is proposed in this study. The twisted Hi-Bi fiber sensor characteristics are analyzed using the Jones matrix. The intensity ratio from two reflected Bragg wavelengths is associated with the twist angle of the measured Hi-Bi fiber. It is found that the twist angle resolution is estimated at around 0.3° under ±0.1 dB readout from an optical spectrum analyzer if the polarization state of the light source is stable. The advantages of this new torsion sensor are: (1) insensitivity to intensity variations from the light source, (2) insensitivity to the torsion gauge length, and (3) absolute measurement in the twist angle. However, the polarization state of light in the proposed method needs to be controlled, and any birefringence change in the twisted Hi-Bi fiber needs to be prevented.     

All-fiber passive mode-locked laser to generate ps pulses based in a symmetrical NOLM

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ～25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.     

全光纤高功率被动锁模掺铥光纤激光器

利用非线性光环形镜(NOLM)的可饱和吸收特性实现了可自启动的2μm全光纤高能量被动锁模掺铥光纤激光器。当泵浦功率大于3W时,激光器工作在连续或不稳定脉冲运转状态;泵浦功率达到4.69W后,输出为自启动锁模脉冲,重复频率4.26MHz,中心波长2 061.5nm,光谱半极大宽度18.1nm,平均输出功率8.8mW;继续增加泵浦功率到最大值7.56W,可以得到中心波长2 062.2nm、光谱半极大宽度17.1nm、斜率效率为6.2%、脉冲宽度和能量分别为424fs和65.6nJ的稳定锁模脉冲。这是目前已报道的在未经放大情况下脉冲能量最高的2μm锁模脉冲光纤激光器。     

滤波对8字腔掺铒光纤激光器锁模运转的影响

构建了基于损耗非对称非线性光学环镜的8字腔掺铒光纤锁模激光器,并讨论了腔内滤波带宽对腔内脉冲演化和激光器输出特性的影响.在非线性光学环镜中引入双向输出耦合器,耦合器和传输光纤位置的不对称产生非互易性,实现锁模运转.利用自制的可调谐滤波器实验研究了滤波带宽对激光器的影响.当滤波带宽为2.1 nm时,腔内脉冲的演化过程受滤波和孤子效应的共同作用,激光器顺时针和逆时针输出脉冲半高全宽分别为583.7fs和2.94 ps.随着滤波带宽增大,滤波的作用逐渐减弱,激光器两路输出脉冲参数逐渐接近,并接近傅里叶变换极限脉冲.当滤波带宽较大时,腔内脉冲的演化过程受增益谱和孤子效应的共同作用,激光器顺时针和逆时针输出脉冲均为变换极限脉冲,半高全宽约为440 fs.通过调节滤波器中心波长实现了对激光器输出脉冲光谱的连续调谐,调节范围大于30 nm.     

Figure-eight actively-passively mode-locked erbium-doped fiber laser

The advantages of using nonlinear optical loop mirror (NOLM) to compress pulse with slight amplitude fluctuation and reflected energy loss are analyzed in theory. Experimentally the NOLM is placed in an actively mode-locked erbium-doped fiber ring laser to form a figure-eight actively and passively mode-locked fiber laser. 12 ps mode-locked pulses centered at 1.543 μm were obtained with the modulation frequency of 2.498748700 GHz. 3.715 mW output power is achieved with 50 mW pump power.     

高速OCDM系统中的全光阈值技术分析

分析了高速OCDM系统中基于非线性光学环境(NOLM)和非线性放大(NALM)环境的全光阈值技术。考虑了NOLM环境输入脉冲峰值功率对其阈值判决功能的影响,数值模拟了由高非线性光纤构成的NOLM和NALM环境的脉冲整形和旁瓣抑制功能,并和实验结果进行了比较。结果表明基于NOLM和NALM环境的全光阈值技术都能有效地抑制40 Gbit/s的OCDM系统中的干扰噪声,最优的NOLM环境的脉冲峰值输入功率为2 W,而NALM环境只需要mW量级的脉冲峰值输入功率即能够更好地抑制干扰噪声。     

High-energy femtosecond stretched-pulse fiber laser with a nonlinear optical loop mirror

A stretched-pulse fiber laser with a nonlinear optical loop mirror (NOLM) that produces 100-fs pulses with 1-nJ energy is demonstrated. These results constitute a 30-fold increase in pulse energy over previously reported femtosecond fiber lasers with a NOLM. Compared with previous stretched-pulse lasers, this laser offers a cleaner spectrum and improved stability, with comparable pulse duration and energy. Implications for the construction of truly environmentally stable lasers are discussed.     

Demonstration of sharp switching from a power-symmetric NOLM and a polarizer

We demonstrate theoretically, experimentally and numerically that a steep all-optical transfer characteristic can be obtained from a power-symmetric NOLM, including a quarter wave retarder and highly twisted fibre, followed by a polarizer. We first develop a theoretical analysis to show that, if the input polarisation is chosen linear, then for a correct choice of the wave plate and input polarisation angles the transmission grows from zero with a 4 dB/dB slope as power is increased, two times faster than can be obtained with a NOLM alone. An experimental study is then realised, which supports the theoretical results. Steep switching is demonstrated for reasonably low peak power levels that do not exceed 8 W, less than one third of the NOLM critical power. In addition, using the same setup we demonstrate the possibility to obtain a sinusoidal transmission characteristic that cancels out for a particular nonzero value of input power, which can be easily adjusted simply through rotation of the wave plate and output polarizer. This setup would be very useful in the frame of optical pulse shaping and signal processing applications, among others.     

Optical pulse shaping at moderate power using a twisted-fibre NOLM with single output polarisation selection

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8-10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.     

Short optical pulse profile characterization using a nonlinear optical loop mirror

We propose in this work a technique for short pulse profile retrieval based on the Kerr effect in a fiber nonlinear optical loop mirror (NOLM). Under some assumptions, the profile can be determined from the energy transfer characteristic of the pulses through the NOLM, which can be measured with a low-frequency detection setup. Two numerical approaches are considered, both relying on the resolution of a system of nonlinear algebraic equations, and which differ in the way that the profiles are discretized. We show numerically that both approaches allow proper profile retrieval for a wide variety of pulse shapes. The two techniques are compared, and their advantages and drawbacks are discussed. The effect of the amplitude noise of the pulses is assessed, as well as the impact of an inaccurate knowledge of the NOLM transfer function, or of the energy transfer characteristic. The technique is demonstrated in the frame of the characterization of both ns and ps pulses.     

Generation of cavity-birefringence-dependent multi-wavelength bright–dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright–dark pulse pair in a mode-locked thulium-doped fiber laser(TDFL). The nonlinear polarization rotation(NPR) and nonlinear optical loop mirror(NOLM) were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation. By incorporating different lengths of high birefringence polarization-maintaining fiber(PMF), the fiber laser could operate stably in a multi-wavelength emission state. Compared with the absence of the PMF, the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright–dark pulse pair.     

A tunable nonlinear optical loop mirror with feedback using linear highly birefringent fiber in the loop

The dynamics of a nonlinear optical loop mirror (NOLM) with feedback using a high birefringence fiber in the loop are investigated. The effect of rotating input polarization angle on the output power and polarization angle is numerically examined, illustrating the sensitivity of the NOLM with feedback to the input’s polarization state, as well as the polarization chaos present with sufficient input powers. The inclusion of a polarization-dependant loss in one or both arms of the coupler is also shown to change the output dynamical behaviour of the system.     

Ultralow-birefringence measurement in optical fibers by the twist method

We describe a simple method for measuring ultralow birefringence in optical fibers. It allows one to measure birefringence in the range of 4x10(-6) 1.25x10(-8) , which corresponds to the 0.2580-m range of the polarization beat length at the wavelength of 1 microm . A fiber section of a length shorter than the polarization beat length can be used for measurement. The measuring procedure involves measurement of the light intensity only and does not require an analysis of the light-polarization state. Experimental results for an optical fiber with a beat length of 1.9m are presented.     

Highly nonlinear optical fibre loop mirror for reshaping femtosecond pulses from a soliton-compressed electro-absorption modulator source over a wide repetition rate and wavelength range

Highly nonlinear fibre (HNF) applied to a nonlinear optical fibre-loop mirror (NOLM) proves very effective for high quality femtosecond pulses shaping. The HNF exhibiting about four times larger nonlinearity than either dispersion flattened fibre or dispersion-shifted fibre excites higher-order soliton propagation enabling significant pulse compression, while shortening the loop length required for efficient switching, which advantageously reduces both the detrimental higher-order chromatic dispersion effects and sensitivity to environmental perturbations. A short NOLM consisting of a 60 m HNF demonstrates effective reshaping and compression of 254–394 fs pulses from a fibre-soliton compressed electro-absorption modulator source to produce pedestal-free and transform-limited 186–243 fs pulses over a wide wavelength (1550–1570 nm) and repetition-rate (6.8–18 GHz) range.     

非线性光环路镜去复用理论研究

本文从非线性光环路镜(NOLM)的一般原理出发,研究了NOLM的一般特性,发现利用光纤的双折射李效应并通过合理地选择波长能减小群色散的影响,降低开关光功率.在忽略群色散条件下,研究了光开关功率、消光比与控制脉冲抖动大小和文中定义的参数d的变化关系.     

Generation of high-energy pulses from an all-normal-dispersion figure-8 fiber laser

We propose and study numerically an all-normal-dispersion Ytterbium-doped figure-eight fiber laser scheme for generation of high-energy pulses. The monotonous pulse stretching that takes place in the fiber under the combined actions of normal dispersion and nonlinear Kerr effect is compensated by the amplitude modulation effect of a bandpass filter inserted in the ring section of the laser. The Nonlinear Optical Loop Mirror (NOLM) also contributes to shorten the pulses. An output coupler with a large output coupling ratio is inserted at the amplifier output in order to extract the maximal energy from the laser. A short segment of Ytterbium-doped fiber compensates for the losses. Stable single-pulse operation is predicted over a wide range of values of the laser parameters. If the laser parameters (ring and NOLM length, dispersion, filter bandwidth, output coupling ratio) are optimized, pulses with several tens of nanojoules energy are readily obtained, with picosecond duration and a large positive chirp which is linear near the peak. If small-signal gain is large enough, the use of very large output coupling ratios opens the way to pulse energies close to 100 nJ and, after dechirping outside the laser, to durations of ˜50 fs and peak powers of 1 MW.