Impact of external optical feedback on the beat notes of dual-polarization fiber lasers

We present a theoretical analysis and an experimental study of the impacts of external optical feedback on dualfrequency fiber lasers. The external optical feedback can effectively suppress the phase noise of the beat notes of dual-frequency fiber lasers, provided some requirements are satisfied. The polarization of the optical feedback is important for the fiber laser's stability, and it can also tune the beat note frequency. A side effect of external optical feedback, as demonstrated in the experiments, is lowering the sensitivity of the dual-frequency fiber laserbased sensors, although such degradation is not obvious.     

Pulse-to-pulse coherent beat note generated by a passively Q-switched two-frequency laser

We demonstrate experimentally the pulse-to-pulse coherence of the beat note produced by a dual-polarization passively Q-switched Nd:YAG laser subjected to a frequency-shifted, polarization-rotated, optical feedback. The reinjection of one laser eigenstate into the other eigenstate ensures the phase-locking of the beat note against an external acoustic reference wave at the onset of each pulse, circumventing the intrinsic memory loss of the optical phase between successive pulses. It opens the possibility to generate optically a coherent pulsed beat note in the radio-frequency range with a subhertz linewidth, i.e., over thousands of pulses. An application to lidar radar is discussed.     

Intracavity electromagnetically induced transparency

The effect of intracavity electromagnetically induced transparency (EIT) on the properties of optical resonators and active laser devices is discussed theoretically. Pronounced frequency pulling and cavity-linewidth narrowing are predicted. The EIT effect can be used to reduce classical and quantum-phase noise of the beat note of an optical oscillator substantially. Fundamental limits of this stabilization mechanism as well as its potential application to high-resolution spectroscopy are discussed.     

Multiple four-wave mixing in optical fibers: 1.5-3.4-THz femtosecond pulse sources and real-time monitoring of a 20-GHz picosecond source

In this work, we report recent progress on the design of all-fibered ultra-high repetition-rate pulse sources for telecommunication applications around 1550 nm. The sources are based on the non-linear compression of an initial beat-signal through a multiple four-wave mixing process taking place into an optical fiber. We experimentally demonstrate real-time monitoring of a 20 GHz pulse source having an integrated phase noise 0.01 radian by phase locking the initial beat note against a reference RF oscillator. Based on this technique, we also experimentally demonstrate a well-separated high-quality 110 fs pulse source having a repetition rate of 2 THz. Finally, we show that with only 1.4 m of standard single mode fiber, we can achieve a twofold increase of the repetition rate, up to 3.4 THz, through the self-imaging Talbot effect. Experimental results are supported by numerical simulations based on the generalized non-linear Schrödinger equation.     

Bidirectional, synchronously pumped, ring optical parametric oscillator

We report the operation of a bidirectional femtosecond pulsed ring optical parametric oscillator based on periodically poled lithium niobate, pumped alternately with nonsimultaneous pulses from a Ti:sapphire mode-locked laser. A beat note between the two counterpropagating beams attests to a gyro response without dead band. The sensitivity of the device to differential phase changes is demonstrated by measurement of the nonlinear index of lithium niobate.     

Studies of phase characteristics of various photon echoes with a phase sensitive detection method

Various optical coherent transient signals in ruby have been examined by using an optical phase sensitive detection (PSD) technique. The PSD is performed by comparing the phase of the heterodyne beat signals obtained by the Stark switching technique with that of a local oscillator whose frequency is equal to the heterodyne beat frequency. The optical FID, two-pulse photon echoes, radiation locked echoes and notched echoes have been detected as υ-mode signals with small u-mode signals probably due to the asymmetry of the inhomogeneous broadening. The dependence of the phase of the radiation locked and notched echoes upon the driving pulses is also examined. The experimental results are in good agreement with theoretical predictions based on Bloch equations.     

基于平行偏振光注入的1550nm波段垂直腔表面发射激光器获取窄线宽光子微波的理论和实验研究

提出一种基于1550 nm垂直腔表面发射激光器(1550 nm-VCSEL)获取高质量微波信号的全光方案.在该方案中,采用波长位于VCSEL中被抑制模式的中心波长附近、振动方向与VCSEL中主导模式相同的偏振光注入(即平行注入)1550 nm-VCSEL获取高频微波,并借助双光反馈对该高频微波的线宽进行窄化.一方面,基于VCSEL的自旋反转模型,从理论上分析了采用该方案产生微波信号的可行性;另一方面,通过构建相应的实验系统,对该方案产生的微波的特性进行初步实验研究.实验结果表明:在合适的注入条件下,1550 nm-VCSEL能够产生30 GHz左右的微波信号,但该信号的线宽较宽(百兆水平);引入双光反馈后,微波线宽可被压窄两个数量级以上,得到了线宽低于1 MHz、信噪比大于40 d B的微波信号.     

Long-term optical phase locking between femtosecond Ti:sapphire and Cr:forsterite lasers

Long-term optical phase-coherent two-color femtosecond pulses were generated by use of passively timing-synchronized Ti:sapphire and Cr:forsterite lasers. The relative carrier-envelope phase relation was fixed by an active feedback loop. The accumulated phase noise from 10 mHz to 1 MHz of the locked beat note was 0.43 rad, showing tight phase locking. The optical frequency fluctuation between two femtosecond combs was submillihertz, with a 1 s averaged counter measurement over 3400 s, leading to a long-term femtosecond frequency-comb connection.     

Full stabilization of a microresonator-based optical frequency comb

We demonstrate control and stabilization of an optical frequency comb generated by four-wave mixing in a monolithic microresonator with a mode spacing in the microwave regime (86 GHz). The comb parameters (mode spacing and offset frequency) are controlled via the power and the frequency of the pump laser, which constitutes one of the comb modes. Furthermore, generation of a microwave beat note at the comb's mode spacing frequency is demonstrated, enabling direct stabilization to a microwave frequency standard.     

一种新型双环路光电振荡器

利用偏振分束器(PBS)和合束器(PBC)在不增加有源器件的基础上设计了一种双环路光电振荡器。该振荡器可以同时产生光、电两种输出。由于没有增加有源器件,可以避免附加电噪声的引入。此外,光载波在光域中是正交耦合,消除了随机干涉和拍噪声对振荡频率的影响。通过理论分析,双环结构的光电振荡器可以有效抑制每一单环路中起振的边模。经过实验对比,采用双环结构可以把边模抑制比提高30~70 dB。最后,利用该方案和普通商用器件得到了中心频率为12 GHz的振荡,测得边模抑制比为50~70 dB,相位噪声为-93 dBc/Hz at10kHz,Q值达到1010。     

短外腔偏振旋转光反馈下1550nm垂直腔面发射激光器的动力学特性研究

基于扩展的自旋反转模型,对短外腔偏振旋转光反馈下1550 nm垂直腔面发射激光器的动力学特性进行了数值仿真和理论分析.研究结果表明:增加反馈强度会导致多个偏振开关现象出现,中等反馈强度下呈现丰富的动力学状态,譬如单周期、倍周期、准周期及混沌态,增加注入电流使Y方向线偏振模的工作区域被压缩;随着反馈延迟时间的增加,在弱光反馈时,偏振模跳变现象将会以特定的频率发生,施加中等的反馈强度将会导致模式跳变的频率增加,并且出现各种新的动力学状态,包括拍频单周期、拍频脉冲包络、拍频准周期和拍频混沌态.新动力学行为的出现是由于短外腔区激光器的动力学特性对相位变化非常敏感,从而使外腔模式间的拍频效应发挥了关键作用.此外还发现各种动力学状态之间会伴随模式间的跳变而发生相互跳变.     

Monolithic 0–f Scheme-Based Frequency Comb Directly Driven by a High-Power Ti:Sapphire Oscillator

A monolithic O-f scheme-based femtosecond optical frequency comb directly driven by a high-power Ti:sapphire laser oscillator is demonstrated.The spectrum covering from 650 nm to 950 nm is generated from the Ti:sapphire oscillator with a repetition rate of 170 MHz.The average output power up to 630 mW is delivered under the pump power of 4.5 W.A 44-dB signal-to-noise ratio(SNR) of the carrier-envelope phase offset(CEO) beat note is achieved under the resolution of 100 kHz and is long-term stabilized to a reference source at 20 MHz.The integrated phase noise(IPN) in the range from 1 Hz to 1 MHz is calculated to be 138 mrad,corresponding to the timing jitter of 63 as at the central wavelength of 790 nm.     

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry (BOTDR) are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser (FL) with 4-kHz linewidth at 3 dB and a distributed feedback (DFB) laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals (backscattered Brillouin light and local oscillator (LO)) is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.     

Passively mode-locked Raman laser

We report on the observation of a mode-locked laser generated with a crystalline whispering gallery mode resonator pumped with a continuous wave laser. Optical pumping of the resonator generates an optical frequency comb with phase locked components at the Raman offset of the resonator host material. Phase locking of the modes is confirmed via measurement of the radio-frequency beat note produced by the comb on a fast photodiode. Neither the conventional Kerr comb nor hyperparametric oscillation is observed when the comb is present. We present a theoretical explanation of the effect.     

2 μm semiconductor disk laser with a heterodyne linewidth below 10 kHz

We demonstrate a 2 μm semiconductor disk laser emitting in a single longitudinal mode with a linewidth in the <10 kHz range. A heterodyne detection scheme was used for precise linewidth measurements. In these experiments, the output beams of two identical laser cavities were superposed in order to generate a beat note signal on a photodiode. In the absence of active frequency stabilization, a linewidth of 45 kHz was measured at an output power of 100 mW. When using a frequency stabilization consisting of a feedback loop with a Fabry-Perot interferometer as wavelength reference, the linewidth could be further reduced to 9 kHz.     

Quantum state tomography with array detectors

I propose a method for measuring the quantum state of an optical field that occupies a mode having a complicated spatial structure. The technique uses array detectors and a single, plane-wave local oscillator beam. The advantage of using array detectors is that the local oscillator is not mode matched to the field being measured, yet the deleterious effects of this mismatch on the effective detection efficiency are greatly reduced compared to using single detectors. Indeed, when the spatial mode of the signal field is describable by a real function, the effective mode-matching efficiency is unity.     

Direct frequency comb generation from an octave-spanning, prismless Ti:sapphire laser

We describe a self-referenced optical frequency comb generator based on an octave-spanning, prismless Ti:sapphire laser. Dispersion compensation is provided by novel double-chirped mirror pairs and BaF2 wedges. Current versions operate at 80 and 150 MHz. The compact prismless design allows system scaling to a gigahertz repetition rate. Its carrier-envelope beat note is intrinsically stable with a signal-to-noise ratio of 30 dB in a 100-kHz bandwidth. The octave is reached at 25 dB below the average power level. The in-loop accumulated phase error is 1.4 rad (20 mHz to 1 MHz). The technique has the advantages of simplicity and stability compared with previous designs.     

Optical feedback cavity ring-down technique for accurate measurement of ultra-high reflectivity

A novel cavity ring-down (CRD) technique, based on the optical feedback effect of Fabry–Perot diode lasers, is developed for accurate reflectivity measurement of highly reflective cavity mirrors. The strong optical feedback, including the direct reflection from the front cavity mirror of a linear ring-down cavity, is re-injected into the oscillator cavity of the diode laser, and large resonant peaks are observed in the ring-down cavity output signals. The diode laser is switched off by a threshold circuit when the amplitude of a resonant peak exceeds a pre-defined threshold. Exponentially decayed signals recorded immediately after switching off the laser are used to determine the cavity decay time. The ultra-high reflectivity of cavity mirrors is measured to be 99.99606% with a reproducibility of 0.00003%. Compared with the previous phase-shift CRD technique, the signal-to-noise ratio of the cavity output signals is improved with an enhancement factor of over 100.     

双反馈F-P光振荡现象

本文介绍一种带有两个光电反馈迴路的F-P被动式光振荡器。该振荡器将连续的光输入转变成交变的光学多谐振荡输出和光学正弦信号输出。本文分析了产生振荡的原理,还给出了与分析相一致的实验结果。 关键词：