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.     

High-energy pulses from a figure 8 fiber laser with normal net dispersion

In this paper we show numerically that high-energy pulses can be obtained with a figure-eight Erbium-doped fiber laser with large normal net dispersion, and in which an anomalous-dispersion Nonlinear Optical Loop Mirror (NOLM) is used as the effective saturable absorber. One advantage of this configuration over the ring cavity is the possibility to adjust the length of the NOLM loop to avoid overdriving the saturable absorber. The ring section of the laser includes a bandpass filter to balance the combined effects of Kerr nonlinearity and normal dispersion. Strict polarization control is performed in the NOLM as well as in the ring section of the laser. The NOLM is a power-symmetric scheme whose switching relies on nonlinear polarization rotation. This architecture allows a precise control of the low-power NOLM transmission through the orientation of a quarter-wave retarder, whose adjustment is shown to be critical for stable pulsed operation. Pulse formation appears to depend critically on the filter width. If it is wide enough, ps pulses with a large positive linear chirp are produced. After dechirping outside the laser, nearly transform-limited pulses with durations down to 240 fs, energies up to 10 nJ and peak powers beyond 40 kW are predicted.     

10 and 20 Gb/s all-optical RZ to NRZ modulation format and wavelength converter based on nonlinear optical loop mirror

We present experimental and theoretical results on all-optical 10 and 20 Gb/s RZ to NRZ modulation format and wavelength converter based on a nonlinear optical loop mirror (NOLM). A vector model of converter was developed and the shape of converted pulses was found analytically for particular choice of polarization states. In the experiment, non-zero dispersion shifted fiber with a length 1200 m was used as a nonlinear medium. Pulses from a 10 GHz mode-locked semiconductor laser diode were modulated to form pseudorandom RZ signal and eventually time division multiplexed to 20 Gb/s. RZ pulses were subsequently converted to NRZ signal. The performance of the converter was evaluated experimentally using the data communication analyzer and bit error ratio tester.     

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.     

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.     

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.     

自启动被动锁模掺铒光纤激光器的研究

在非线性光纤环形镜非线性开关效应和块状半导体波导饱和吸收效应的共同作用下，实现了掺铒光纤激光器的自启动被动锁模，获得了十分稳定的锁模脉冲序列，观察到高次谐频锁模脉冲输出。分析了非线性光纤环形镜的非线性开关反射特性。     

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.     

A triple function linear fiber laser with passive mode-locking and continuous-wave single-wavelength and multiwavelength lasing

We propose a fiber laser that can operate with passive mode-locking pulse output, continuous single-wavelength emission, and continuous multiwavelength emission. It consists of a nonlinear optical loop mirror (NOLM) in a linear cavity. The NOLM is used to induce different kinds of intensity-dependent feedback, which determine the operation mode of the laser. We have experimentally demonstrated that the fiber laser can easily be switched between the operation modes. The results may find useful applications in optical communication and fiber sensing.     

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.     

Fiber laser mode locked by a Sagnac interferometer with nonlinear polarization rotation

We describe a new fiber laser configuration based on a nonlinear optical loop mirror with a symmetrical coupler, a quarter-wave retarder, and highly twisted, birefringent fiber in the loop. The nonlinear optical loop mirror configuration operates by nonlinear polarization rotation. We have achieved stable generation of subpicosecond pulses with milliwatts of average output power.     

有理数谐波主被动锁模掺铒光纤激光器

在理论上详细分析了利用非线性光学环形镜（NOLM）来减小输出脉冲幅度波动,消除噪声并对脉冲进行压缩整形的物理机制。在主动锁模掺铒光纤环形激光器中（AHML－EDFL）接入一个非线性光学环形镜,形成结构新颖的主被动锁模掺铒光纤激光器（APHML－EDFL）,利用非线性光学环形镜所具有的饱和吸收体功能,成功地制抑了4阶有理数谐波锁模（RHML）中较大的幅度噪声,在1GHz量级的调制频率下,由主被动锁模掺铒光纤激光器获产生重复频率为5．1GHz,幅度相当稳定的4阶有理数谐波锁模脉冲序列。     

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.     

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.     

A linear cavity multiwavelength fiber laser with adjustable lasing line number for fixed spectral regions

We report an experimental demonstration of muliwavelength erbium-doped fiber laser with adjustable wavelength number based on a power-symmetric nonlinear optical loop mirror (NOLM) in a linear cavity. The intensity-dependent loss (IDL) induced by the NOLM is used to suppress the mode competition and realize the stable multiwavelength oscillation. The controlling of the wavelength number is achieved by adjusting the strength of IDL, which is dependent on the pump power. As the pump power increases from 40 to 408 mW, 1–7 lasing line(s) at fixed wavelength around 1601 nm are obtained. The output power stability is also investigated. The most power fluctuation of single wavelength is less than 0.9 dB, when the wavelength number is increased from 1–7.     

Widely tunable sum-frequency generation in PPLN waveguide pumped by a multi-wavelength Yb-doped fiber laser

We present some experimental results on tunable sum-frequency generation in a periodically poled lithium-niobate waveguide using a multi-wavelength fiber laser pump stabilized by a nonlinear optical loop mirror. We are able to up-convert to about 629 nm a continuous-wave infrared signal varying from 1497 nm to 1525 nm. Such a wideband conversion efficiency is ensured by the multiple spectral components of the laser pump, which is controlled by an adjustable Fabry-Perot filter. Potential applications, in particular for stellar imaging, are discussed.     

Measuring chromatic dispersion of optical fiber using time-of-flight and a tunable multi-wavelength semiconductor fiber laser

We have developed a tunable multi-wavelength semiconductor fiber laser (SFL) for chromatic dispersion measurements of optical fiber based on the time-of flight (TOF) or pulse-delay technique. The SFL incorporates a programmable high-birefringence fiber Sagnac loop to select the separation of the lasing wavelengths between 3.2 and 1.6 nm. The SFL emits 5 and 11 wavelengths for separations of 3.2 and 1.6 nm, respectively, all within the C-band and with an output power uniformity within 3.2 dB. Results from TOF measurements are compared with standard phase-shift techniques; the percent differences between the two methods are within ±1.34% for measurements on various lengths of standard single mode fiber.     

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。     

Rational harmonic figure-eight actively-passively mode-locked erbium-doped fiber laser

The nonlinear optical loop mirror (NOLM) is used as a saturable absorbed to reshape pulses. Experimentally, an actively mode-locked erbium-doped fiber ring laser with figure-eight structure is set up. 2 - 4 order harmonic pulse train with stable amplitude has been obtained when the RF modulation frequency is about 2.5 GHz.     

利用非线性光学环路镜实现多个波长的同时变换

利用非线性光不路镜（ＮＯＬＭ）成功地实现了多个波长的同时变换,最大波长变换间距大于２５ｎｍ。实验系统中采用增益开关分布反馈半导体激光器（ＧＳ ＤＦＢ－ＬＤ）产生的超短光脉冲作为控制光,频谱分割法得到多波长激光作为信号光。改变控制光的输入功率或非线性光学环路镜中的偏振控制器的偏振方向能够改变不同变换波长信号的性能。