High-repetition-rate optical pulse generation using a rationalharmonic mode-locked fiber laser

Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency f_m=(n+1/p)f_c, where n and p are both integers and f_c is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when f_m ≈1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency f_m=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier     

Theoretical and experimental study of pulse-amplitude-equalizationin a rational harmonic mode-locked fiber ring laser

A simple technique, based on nonlinear polarization rotation, for the amplitude equalization of high repetition rate pulses generated from a rational harmonic mode-locked fiber ring laser was demonstrated. Amplitude equalized pulses at 10 GHz (the fourth rational harmonic) were obtained with a harmonic component suppression ratio up to 30 dB. A theoretical explanation was given in detail, which indicates that, after pulse amplitude equalization, the situation of unequal pulse spacing can be improved to a certain extend     

高次谐波锁模飞秒掺镱光纤激光器的噪声情况

高重复频率的飞秒激光在高速激光测距和三维成像等领域有着非常重要的作用。其中基于飞秒光纤激光器的高次谐波锁模是获得GHz量级以上高重复频率脉冲的重要手段之一。基于含腔内光栅对色散补偿的非线性偏振旋转（NPR）锁模的掺镱（Yb）光纤激光器,在180 mW泵浦光时获得了稳定的143 MHz基频锁模脉冲序列,当泵浦光功率升至1 W时获得了最高20次谐波（2.86 GHz）锁模脉冲序列输出。系统地对比研究了基频锁模与高次谐锁模状态下,脉冲重复频率精密锁定后的艾伦偏差和相位噪声,7次谐波锁模状态下重复频率锁定精度能够保持在10?13 Hz@1 s的稳定度,为高次谐波锁模飞秒激光脉冲序列用于精密测量提供了实验依据。     

80-160-GHz mode-locked fiber ring laser synchronized to external optical pulse stream

The authors present a novel scheme to generate ultrahigh repetition rate picosecond pulses synchronized to an optical pulse stream in an actively mode-locked fiber ring laser. A monolithic Mach-Zehnder interferometer with integrated semiconductor optical amplifiers serves as an 80-GHz optically controlled modulator. 80-GHz transform-limited 2.1-ps Gaussian pulse with low timing jitter, extinction ratios up to 25 dB, and 5 mW of average output power are demonstrated. The laser is wavelength tunable over more than 15 nm around 1555 nm. At 160 GHz, 2.5-ps pulse trains with 3-mW output power are generated by rational harmonic mode locking.     

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。     

Higher order repetition rate multiplication for multiwavelength pulsed source

We demonstrate a polarization-insensitive all-fiber approach for higher order repetition rate multiplication of laser pulses using a birefringence loop mirror filter. Owing to the periodicity and wide bandwidth characteristic of the filter, simultaneous repetition rate multiplication of pulses at different wavelengths can be achieved. The repetition rates of a mode-locked fiber laser and an externally modulated multiwavelength laser source have been multiplied, respectively, by a factor of four. A 160-GHz total capacity, with four individual channels each operating at 40 GHz, has been experimentally demonstrated.     

Dual-wavelength 10-GHz actively mode-locked erbium fiber laser incorporating highly nonlinear fibers

We demonstrated a dual-wavelength actively mode-locked erbium-doped fiber ring laser with 0.8-nm wavelength spacing at a repetition rate of 10 GHz. A 1-km highly nonlinear fiber is introduced in the nonpolarization-maintaining cavity to suppress the gain competition of the homogeneously broadened gain medium in a self-driven manner. Output pulses at 1557.36 and 1558.17 nm are generated simultaneously. Amplitude fluctuations less than 0.45% and 1.7% of two wavelengths are measured. The corresponding timing jitters are less than 80 and 90 fs, respectively.     

Synchronous mode-locking in passively mode-locked semiconductor laser diodes using optical short pulses repeated at subharmonics of the cavity round-trip frequency

Synchronous mode-locking was achieved in passively mode-locked semiconductor lasers using optical pulses with repetition rates at subharmonics of the cavity round-trip frequency. Stable and continuous mode-locked pulses at 8.5 GHz were generated when the repetition rate of the control optical pulses was 4.25 GHz (2nd subharmonic). The timing jitter of the mode-locked pulses was reduced to 1 ps. The relationship between repetition rates of the control pulses and the realization of stable and continuous mode-locking was examined.     

Stable 80-GHz short pulse generation using the cascaded polarization-maintaining fiber loop mirrors

In this work, an optical short pulse generator is designed consisting of a pulse compressor and cascaded notch filter type repetition rate doublers. The performance characteristics such as pulsewidth and peak power as a function of design parameters are studied. The pulse compressor is optimized based on the simulation results. The 6-ps-wide pulses at 20-GHz repetition rate directly generated from mode-locked fiber laser are compressed to 1.25-ps-wide pulses. Using a set of polarization-maintaining fiber loop mirrors, the repetition rate is quadrupled and stable 1.45-ps-wide pulses at 80 GHz are achieved.     

Pulse-amplitude equalization in a rational harmonic mode-locked fiber ring laser by using modulator as both mode locker and equalizer

A simple method, by letting the pulses pass through an electrooptic intensity modulator twice, for the amplitude equalization of high repetition rate pulses generated from a p-order rational harmonic mode-locked fiber ring laser when p<5 was proposed. The modulator was used as both a mode locker and a pulse-amplitude equalizer in the cavity. A theoretical explanation based on time-domain analysis was used to find appropriate parameters to obtain an equalized pulse train and the experimental results agree with the theory.     

Mode-locking at very high repetition rates more than terahertz inpassively mode-locked distributed-Bragg-reflector laser diodes

We propose a technique for achieving mode-locking at very high repetition rates more than terahertz with high output power using a distributed-Bragg-reflector (DBR) laser, including an intracavity saturable absorber. This method is based on harmonic passive mode-locking at high gain levels and the selectivity of harmonic numbers related to the spectrum-filtering property of the intracavity Bragg reflector. Transform-limited pulses at repetition rates from 500 GHz to 1.54 THz were generated with an output power exceeding 15 mW. We studied stability of harmonic pulses by using conventional self-consistent mode-locking equations. We also discuss the effect of the spectrum-filtering properties of the intracavity Bragg reflector to the characteristics of mode-locked pulses     

Numerical study on a frequency-tunable actively mode-locked fiber laser

We have numerically presented an actively mode-locked fiber laser with tunable repetition rate based on phase modulator. By finely optimizing intra-cavity parameters, the ultrashort pulses with tunable repetitive frequency at giga hertz level can be easily generated due to the balance between dispersion and nonlinearity in the fiber laser cavity. When the pulse frequency is changed from 1.0 GHz to 4.2 GHz, the spectral width increases from ~15.65 nm to ~27.25 nm. In addition, the corresponding pulse duration decreases from ~81.59 ps to ~31.57 ps. Moreover, these output pulses with giga hertz repetitive rates and the picosecond widths can be further compressed by using the reasonable dispersion medium. For the pulse regime with repetition frequency at giga hertz level, the obtained smallest pulse duration is about ~62 fs based on chirp pulse compression. We hope that these simulation results can promote further research and application in the ultrashort pulse lasers with high repetition rate.     

有理数谐波锁模掺铒光纤环形激光器

在主动谐波锁模光纤环形激光器实验中获得数倍于调制频率ｆｍ的高重复率脉冲序列,其中ｆｐ＝２ｆｍ的输出脉冲序列极为稳定,所得的最高锁模脉冲重复频率ｆｐ＝４ｆｍ≈６ＧＨｚ。实验证实了一种被称之为有理数谐波锁模（ＲＨＭＬ—ＲａｔｉｏｎａｌＨａｒｍｏｎｉｃＭｏｄｅ－ｌｏｃｋｉｎｇ）新技术在较低的调制频率和极简单的腔构条件下也同样成立。实验观察到ＲＨＭＬ激光器允许失谐范围不小于±２ｋＨｚ,失谐比大于１０－６。     

主动有理谐波锁模光纤激光器

利用有理谐波锁模,实现了锁模光纤激光器输出光脉冲重复频率的倍增,得到了重复频率10GHz~50GHz输出脉冲.     

Er-Yb femtosecond ring fiber oscillator with 1.1-W average power and GHz repetition rates

We report an all-fiber passively mode-locked femtosecond laser oscillator based on the heavily doped Er-Yb phosphate-glass active fiber. Only 20 cm of the gain fiber is sufficient to produce as much as 1.1 W of average output power at 1.5 /spl mu/m directly from the oscillator. The laser can be harmonically mode-locked at repetition rates ranging from 1.7 to 7.2 GHz by adjusting the polarization bias in the cavity. The pulsewidth varies from 300 to 570 fs at the lowest and the highest repetition rate, respectively, and the maximum peak pulse power exceeds 1 kW.     

Pulse-amplitude equalization in a rational harmonic mode-locked fiber laser using nonlinear modulation

In this letter, a simple technique for the pulse-amplitude equalization of high repetition rate pulses generated from a rational harmonic mode-locked fiber ring laser was demonstrated. The principle is based on nonlinear modulation of the modulator and a theoretical explanation based on time-domain analysis was given. Amplitude equalized short pulses up to the seventh rational harmonic mode-locking were obtained with an optimum bias level and modulation depth of the modulator, which agrees well with the simulation result and demonstrates the efficiency of this method.     

6 MHz重频调谐范围的稳定锁模掺镱光纤激光器

针对精密测量的实际应用需求,开发了具有6 MHz重复频率调谐范围的掺镱锁模光纤激光器。该光纤激光器利用啁啾光纤光栅实现色散补偿,可以实现不同色散域的锁模。实验中,系统研究了不同腔内净色散对锁模激光器输出特性和稳定性的影响,发现当具有一定负腔内净色散时,在不同的重复频率下都可以输出相同的光谱,同时输出光谱具有较好的高斯型。根据以上研究,特殊设计了稳定的简化腔结构和空间延迟线,同时优化了腔参数,保证了大范围的重复频率调谐和稳定的锁模运转。当中心重复频率为26 MHz 时,调谐比率达到23%。激光器稳定输出平均功率为3.23 mW的飞秒激光脉冲,去啁啾后脉冲宽度为347 fs。基于此,将光纤锁模激光器重复频率锁定于铷原子钟,在1 s的平均时间内获得了2×10?10的艾伦方差。     

Wavelength and Repetition-Rate Tunable Mode-Locked Semiconductor-Seed-Source for Yb-Fiber-Amplifier Systems

We present a wavelength and repetition-rate tunable passively mode-locked semiconductor-seed-source for ytterbium-doped fiber amplifiers (YDFAs). Ultrashort pulses 2-6 ps in duration are generated over a broad tuning range of 1030-1090 nm, while passive harmonic mode-locking permits pulse repetition rates to vary from 500 MHz to over 5 GHz. We compare the amplification characteristics under fundamental and eighth harmonic mode-locking using a single-mode YDFA.     

脉幅稳定的有理数谐波锁模光纤激光器

通过调节振幅调制器的静态偏压和驱动功率来控制调制器的驱动频率边频分量 ,在 2 5GHz的射频调制频率下 ,有效地抑制了 3～ 5阶有理数谐波锁模脉冲序列的幅度起伏 ,直接由激光器输出幅值稳定的有理数谐波锁模脉冲序列