Frequency-locking and threshold current-lowering effects of a quantum cascade laser and an application in gas detection field

In this paper,the frequency-locking and threshold current-lowering effects of a quantum cascade laser are studied and achieved.Combined with cavity-enhanced absorption spectroscopy,the noninvasive detection of H_2 with a prepared concentration of 500 ppm in multiple dissolved gases is performed and evaluated.The high frequency selectivity of 0.0051 cm^-1 at an acquisition time of 1 s allows the sensitive detection of the（1-0） S（l） band of H_2 with a high accuracy of（96.53±0.29）%and shows that the detection limit to an absorption line of 4712.9046 cm^-1 is approximately（17.26±0.63） ppm at an atmospheric pressure and a temperature of 20 ℃.

Numerical study of converting beat-note signals of dual-frequency lasers to optical frequency combs by optical injection locking of semiconductor lasers

The optical injection locking of semiconductor lasers to dual-frequency lasers is studied by numerical simulations.The beat-note signals can be effectively transformed to optical frequency combs due to the effective four wave-mixing in the active semiconductor gain medium. The low-noise Gaussian-like pulse can be obtained by locking the relaxation oscillation and compensating the gain asymmetry. The simulations suggest that pulse trains of width below 30 ps and repetition rate in GHz frequency can be generated simply by the optical injection locking of semiconductor lasers. Since the optical injection locking can broaden the spectrum and amplify the optical power simultaneously, it can be a good initial stage for generating optical frequency combs from dual-frequency lasers by multi-stage of spectral broadening in nonlinear waveguides.

Precise determination of characteristic laser frequencies by an Er-doped fiber optical frequency comb

Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently.Therefore,any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency.As a result,the length unit“meter”is directly related to the time unit“second”.This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region.Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequencydoubling scheme.The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633,698,729,780,1064,and 1542 nm is better than 30 d B.The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1×10^(-13)at 1-s averaging time.The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylenestabilized 1542-nm laser.The results are within the uncertainty range of the international recommended values.Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb,which is not only important for the precise and accurate traceability and calibration of the laser frequencies,but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Stable operation is one of the most important requirements for a laser source for high-precision applications.Many efforts have been made to improve the stability of lasers by employing various techniques,e.g.,electrical and/or optical injection and phase locking.However,these techniques normally involve complex experimental facilities.Therefore,an easy implementation of the stability evaluation of a laser is still challenging,especially for lasers emitting in the terahertz(THz)frequency range because the broadband photodetectors and mature locking techniques are limited.In this work,we propose a simple method,i.e.,relative phase locking,to quickly evaluate the stability of THz lasers without a need of a THz local oscillator.The THz laser system consists of a THz quantum cascade laser(QCL)frequency comb and a single-mode QCL.Using the single-mode laser as a fast detector,heterodyne signals resulting from the beating between the singlemode laser and the comb laser are obtained.One of the heterodyne beating signals is selected and sent to a phase-locked loop(PLL)for implementing the relative phase locking.Two kinds of locks are performed by feeding the output error signal of the PLL,either to the comb laser or to the single-mode laser.By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition,we,in principle,are able to experimentally compare the stability of the emission frequency of the single-mode QCL(f s)and the carrier envelope offset frequency(f CEO)of the QCL comb.The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser.The work provides a simple heterodyne scheme for understanding the stability of THz lasers,which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

Dual-mode stabilization for laser to radio-frequency locking by using a single-sideband modulation and a Fabry-Pérot cavity

The dual-mode stabilization scheme has been demonstrated as an efficient way to stabilize laser frequency.In this study,we propose a novel dual-mode stabilization scheme that employs a sizable Fabry-Pérot cavity instead of the microcavity used in previous studies and has enabled higher bandwidth for locking.The results demonstrate a 30-fold reduction in laser frequency drift,with frequency instability below 169 kHz for integration time exceeding 1 h and a minimum value of 33.8 kHz at 54 min.Further improvement could be achieved by optimizing the phase locking.This scheme has potential for use in precision spectroscopic measurement.

Modulation-free laser frequency ofset locking using bufer gas-induced resonance

We experimentally demonstrate a simple modulation-free scheme for ofset locking the frequency of a laser using bufer gas-induced resonance. Our scheme excludes the limitation of low difraction efciency and laser input intensity when an acousto-optic modulator is applied to shift the laser frequency from the resonance. We show the stabilization of a strong 795- nm laser detuned up to 550 MHz from the 87Rb 5S1/2 F=2→5P1/2F'=2 transition. The locking range can be modifed by controlling the bufer gas pressure. A laser line width of 2 MHz is achieved over 10 min.

Diode end-pumped self-Q-switched and mode-locked Nd,Cr：YAG /KTP green laser

We first experimentally demonstrate a laser-diode end-pumped self-Q-switched and mode-locked Nd,Cr：YAG green laser with a KTP crystal as the intra-cavity frequency doubler. The device produces an average output power of 680 mW at 532 nm. The corresponding pulse width of the Q-switched envelope of the green laser is 170 ＋ 20 ns. The mode-locked pulses have a repetition rate of approximately 183 MHz and the average pulse duration is estimated to be around sub-nanosecond. It is found that the intra-cavity frequency doubling greatly improves the modulation depth and stability of the mode-locked pulses within the Q-switched envelope.

Pump-induced carrier envelope offset frequency dynamics and stabilization of an Yb-doped fiber frequency comb

In this paper, we demonstrate a carrier envelope phase-stabilized Yb-doped fiber frequency comb seeding by a nonlinear-polarization-evolution（NPE） mode-locked laser at a repetition rate of 60 MHz with a pulse duration of 191 fs.The pump-induced carrier envelope offset frequency（ f0） nonlinear tuning is discussed and further explained by the spectrum shift of the laser pulse. Through the environmental noise suppression, the drift of the free-running f0 is reduced down to less than 3 MHz within an hour. By feedback control on the pump power with a self-made phase-lock loop（PLL）electronics the carrier envelope offset frequency is well phase-locked with a frequency jitter of 85 m Hz within an hour.

Chirp Extraction of Self-Similar Pulses in a Passively Mode-Locked Er-doped Fiber Laser

In this paper, we adopt cloud computing in a specific scientific computing field for its virtualization, distribution and dynamic extendibility as follows: We obtain high-energy parabolic self-similar pulses by numerical simulation using our non-distributed passively mode-locked Er-doped fiber laser model. For researching characteristics of these wave-breaking-free self-similar pulses, chirp of them must be extracted. We propose several time-frequency analysis methods adopted in chirp extraction of ultra-short optical pulses for the first time and discuss the advantages and disadvantages of them in this particular application.

Laser frequency locking method for Rydberg atomic sensing

Based on the Rydberg cascade electromagnetically induced transparency,we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature.The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator,realizing real-time feedback for both lasers.We studied the effect of the different probe and coupling laser powers on the error signal.In addition,the Allan variance and a 10 kHz amplitudemodulated signal are introduced to evaluate the performance of the laser frequency stabilization.In principle,the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

Widely tunable laser frequency offset locking to the atomic resonance line with frequency modulation spectroscopy

A simple and robust technique is reported to offset lock a single semiconductor laser to the atom resonance line with a frequency difference easily adjustable from a few tens of megahertz up to tens of gigahertz. The proposed scheme makes use of the frequency modulation spectroscopy by modulating sidebands of a fiber electro-optic modulator output. The short-term performances of a frequency offset locked semiconductor laser are experimentally demonstrated with the Allan variance of around 3.9 × 10-11 at a 2 s integration time. This method may have many applications, such as in Raman optics for an atom interferometer.

Design of a high frequency accuracy heterodyne laser source working in a wide temperature range

To ensure the frequency accuracy of a heterodyne laser source in the ambient temperature range of-20℃ to 40℃, a duallongitudinal-mode thermally stabilized He–Ne laser based on non-equilibrium power locking was designed. The ambient adaptive preheating temperature setting scheme ensured the laser could operate normally in the range of-20℃ to40℃. The non-equilibrium power-locked frequency stabilization scheme compensated for the frequency drift caused by different stabilization temperatures. The experimental results indicated that the frequency accuracy of the laser designed in this study could reach 5.2 × 10^(-9)in the range of-20℃ to 40℃.

Long-term frequency-stabilized optical frequency comb based on a turnkey Ti:sapphire mode-locked laser

We report a long-term frequency-stabilized optical frequency comb at 530–1100 nm based on a turnkey Ti:sapphire modelocked laser.With the help of a digital controller,turnkey operation is realized for the Ti:sapphire mode-locked laser.Under optimized design of the laser cavity,the laser can be mode-locked over a month,limited by the observation time.The combination of a fast piezo and a slow one inside the Ti:sapphire mode-locked laser allows us to adjust the cavity length with moderate bandwidth and tuning range,enabling robust locking of the repetition rate(f_(r)) to a hydrogen maser.By combining a fast analog feedback to pump current and a slow digital feedback to an intracavity wedge and the pump power of the Ti:sapphire mode-locked laser,the carrier envelope offset frequency(f_(ceo)) of the comb is stabilized.We extend the continuous frequency-stabilized time of the Ti:sapphire optical frequency comb to five days.The residual jitters of f;and f;are 0.08 m Hz and 2.5 m Hz at 1 s averaging time,respectively,satisfying many applications demanding accuracy and short operation time for optical frequency combs.

Atmospheric N2O gas detection based on an inter-band cascade laser around 3.939 μm

N2O is a significant atmospheric greenhouse gas that contributes to global warming and climate change.In this work,the high sensitivity detection of atmospheric N2O is achieved using wavelength modulation spectroscopy(WMS)with an inter-band cascade laser operating around 3.939μm.A Lab VIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system.In order to eliminate the interference from water vapor,the detection was performed at a pressure of 0.1 atm(1 atm=1.01325×10^5 Pa)and a drying tube was added to the system.To improve the system performance for long term detection,a novel frequency locking method and 2 f/1 f calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations,respectively.The Allan deviation analysis of the results indicates a detection limit of^20 ppb(1 ppb=1.81205μg/m3)for a 1 s integration time,and the optimal detection limit is^5 ppb for a 40-s integration time.

Frequency difference lock-in phenomenon's weakening by transverse magnetic field in Y-shaped cavity dual-frequency laser

We report experimental progress in weakening the frequency difference lock-in phenomenon in a Y-shaped cavity dual-frequency laser. A cube coil pair is chosen to provide a uniform magnetic field for tunability and uniformity of magnetic field strength. When the transverse magnetic field intensity is 9 m T, the frequency difference lock-in phenomenon is evidently weakened and the frequency difference can be continuously tuned in the range of0.12 MHz to 1.15 GHz. Moreover, the relationship between the minimal frequency difference and magnetic field intensity are investigated and discussed. Then a Y-shaped cavity dual-frequency laser is expected to be utilized as an optimum light source for heterodyne interferometric sensing and precise laser measurement.

基于光学反馈频率锁定的窄线宽稳定中红外激光产生技术研究

中红外精密激光光谱技术在痕量气体检测、基本物理常数测定等领域都有重要应用,然而由于缺乏窄线宽、稳定的中红外光源,很难实现中红外精密光谱测量.本文介绍了一种基于光学反馈频率锁定的窄线宽稳定中红外激光产生技术,分析了光学反馈实现激光到F-P腔锁定的可行性,利用一个高精细度中红外超稳F-P腔作为频率参考,基于光学反馈技术实现了量子级联激光器到该超稳腔的锁定.经过评估得到激光器线宽被压窄到1.1 Hz,压窄激光线宽的同时稳定了激光频率,将激光器的长期漂移控制在20 kHz/12 h.其中,为了获取长时间稳定的光学反馈,基于PDH技术获取了误差信号,用于对反馈相位的实时伺服控制.

基于注入锁定技术的单频连续高功率1342 nm Nd:YVO_(4)激光器

研究了一种基于注入锁定技术的888 nm半导体激光器(LD)泵浦的高功率单频可调谐1 342 nm Nd:YVO_(4)激光器。采用最大输出功率20 mW分布式反馈单频半导体1 342 nm激光器作为注入种子,利用lock-in (LI)技术,对LD端泵的Nd:YVO_(4)环形腔激光器进行种子注入,实现了单频可调谐激光输出。激光器最大平均输出功率为13.9 W,测量的线宽为41 MHz,调谐范围为1 341.677 4~1 341.802 5 nm。x轴和y轴的光束质量M2因子分别为M_(x)^(2)=1.30和M_(y)^(2)=1.23。实验结果表明:与先前文献报道的注入锁定1 342 nm可调谐激光的结果相比,所需种子功率大幅减小,输出功率也有所提升。

基于声光调制器的光学锁相环He-Ne激光稳频方法研究

报道了一种基于声光调制器与光学锁相环相结合的高稳定度激光稳频方法,用于提高热稳频He-Ne激光器的频率稳定度和准确度。为了克服全内腔热稳频He-Ne激光精密调谐困难的缺点,发展了基于声光调制器两次移频的频率调谐光路,有效地消除声光调制器移频光束路径对衍射角θ的依赖。自行研制了具有高灵敏度与捕获带宽的光学锁相环系统,利用声光调制器的高频率响应特性实现热稳频He-Ne激光高速、准确的锁定。成功实现了热稳频He-Ne激光器偏频锁定至碘稳频HeNe激光器。实验结果表明,环路锁定后拍频频率波动在±0.2 Hz范围内,频率抖动的标准差为0.04。偏置频率为30 MHz时,系统在1 s和1000 s积分时间的相对阿伦方差分别为3.3×10^(-9)和1.4×10^(-12)。系统锁定后,压缩后的拍频线宽小于2 kHz。该研究表明,采用基于声光调制器与光学锁相环相结合的激光稳频方法可以实现亚赫兹级的激光频差控制,通过将热稳频He-Ne激光器偏频锁定至高稳定度的参考激光源可以显著提升其频率稳定度和准确度。

自注入锁定激光器的频率热调谐方法改进

为了提高自注入锁定激光器的频率连续可调谐范围,对法布里-珀罗(FP)微腔在频率热调谐过程中注入锁定相位的变化关系进行研究。在传统频率热调谐的基础上,对自注入锁定激光器频率和相位等参数特性进行研究,提出一种在频率热调谐时加入自注入锁定相位补偿和DFB芯片电流补偿的改进算法,并在一台基于FP微腔自注入锁定激光器上对此算法进行验证实验。这台激光器的波长为1550 nm,3 dB线宽为785 Hz,通过一对加热电阻对FP微腔进行频率热调谐。实验结果表明:激光器硬件部分未作任何修改的情况下,改进后的算法在激光器原有驱动控制电路的单片机程序中实现了6 GHz的频率连续调谐范围。该工作为自注入锁定激光器提供一种简单高效且稳定性好的频率调谐方案,具有较高的实用性和市场前景。

光泵磁力仪中垂直腔面发射激光器激光波长锁定

针对光泵磁力仪(OPM)对小型化、低功耗以及激光光源波长稳定性的要求,提出一套垂直腔面发射激光器激光波长锁定控制方案.所提基于多普勒吸收的光反馈波长锁定方案以133 Cs原子D1线F_(g)=4→F_(e)=3超精细能级跃迁波长为参考波长,OPM的原子蒸汽气室同时作为波长锁定的工作气室,无需任何额外装置即可将激光波长锁定在该D1线跃迁波长.使用数字比例积分微分控制与模糊控制算法进行激光的温度控制,使温度波动在±0.005℃内;采用基于电流镜的激光电流驱动方案,使电流波动在±50 nA内,为激光波长锁定提供了良好的硬件基础.最后,在实验室环境下实现OPM长达2 h的稳定信号输出.