基于双波长激光器的集成化中红外双光梳系统(特邀)

Integrated Mid-infrared Dual-comb System Based on a Dual-wavelength Laser(Invited)

中红外双光梳光谱检测系统因其高分辨、高灵敏、快速测量的特性为极低浓度气体的标定带来了革新技术。本文用单腔双波长激光器输出的异步双波长脉冲替代两台锁模激光器,结合非线性差频技术,发展了集成实用化的中红外双光梳系统。共腔产生的1034 nm与1039 nm双波长脉冲序列,其重复频率差约为1.18 kHz,且因共模噪声被抑制,脉冲间的相对稳定性较高。利用级联放大器将种子脉冲光功率提升至1.1 W后,与2 W的1549.315 nm的连续激光非线性差频,将激光器输出波段拓展至中红外。产生的中红外激光功率可达3.5 mW,光谱覆盖范围超过50 nm。对中红外双光梳的相干性进行测量,与1μm的底层双光梳相比,中红外双光梳拍频梳齿频率间隔、信噪比、线宽等无明显劣化,可为复杂环境下的痕量分析提供可行方案。

The mid-infrared dual-comb spectroscopy has innovated the detection of trace gases due to its high resolution,high sensitivity,and short response time.The traditional dual-comb system based on mode-locked lasers,electro-optic modulation or nonlinear optical micro-resonators is always combined with shortcomings of complex structure,high cost,few comb teeth,and poor practicability.In recent years,dual-wavelength mode-locked lasers that allow two asynchronous pulse trains to oscillate simultaneously have attracted increasing attention owing to their great potential in facilitating robust and precise dual-comb spectroscopy.In this paper,with two collimators and a segment of polarizationmaintaining fiber acting as a Lyot filter,two asynchronous pulse trains centered at 1034 nm and 1039 nm respectively can emit from a single-cavity dual-wavelength laser,which is based on a Nonlinear Amplifying Loop Mirror structure.The repetition rate difference between the dual-wavelength pulses is about 1.18 kHz and the 3 dB spectral width per wavelength is~1.6 nm.The mutual coherence is maintained owing to a shared laser cavity,and common-mode noise is canceled.The radio frequencies of the dual-wavelength pulses are traced and~60 Hz frequency drift of each wavelength is measured within 10 hours yet only a 3.5 Hz frequency shift and 0.45 Hz standard deviation of the repetition rate difference is observed.The difference in magnitude indicates the high coherence and great environmental stability of the dual-wavelength pulses.To demonstrate the coherence between the dual-wavelength pulses,the multiheterodyne beat notes are detected.Because of the optical spectral overlap,the mode-resolved beat notes with a frequency interval of 1.18 kHz and~25 dB signal-to-noise ratio are observed.Limited by the resolution bandwidth of the spectrum analyzer,the obtained full width at half maximum of each beat note is about 12 Hz.The average power of the seed pulses is scaled up to 1.1 W with a cascade amplifier to avoid excessive spontaneous emission noise and maintain high coherence.Then,the amplified pulses transform into a mid-infrared band by difference frequency generation with a 2 W continuous laser centered at 1549.315 nm in the periodically polarized lithium niobite crystal.To improve the conversion efficiency,the temperature of the periodically polarized lithium niobite crystal is accurately controlled at 125℃by high precision temperature control furnace.The power of the generated mid-infrared laser reaches 3.5 mW,and the corresponding spectrum covers more than 50 nm.The spectral width could be expanded by tuning the wavelength of the continuous laser.Moreover,the coherence of the mid-infrared dual-comb is measured and its frequency interval,signal-to-noise ratio and linewidth are consistent with the fundamental dualcomb.According to the Lambert-Beer theory,the absorption intensity of light is proportional to the concentration of the gas molecules and the length of the optical path.Hence,a multi-pass gas cell with an optical path of up to 10 m is employed.The generated mid-infrared laser experiences 50 times of reflection in the gas cell,which can greatly extend the interaction length and is conducive to the detection of trace gases.In a word,we have developed an integrated mid-infrared dual-comb system based on a singlecavity dual-wavelength laser.The two asynchronous pulse trains centered at 1034 and 1039 nm with a repetition rate difference of 1.18 kHz can replace the traditional complicated mode-locked lasers and show high coherence due to the suppression of the common-mode noise.The seed pulses are transformed into a mid-infrared band by difference frequency generation in the periodically polarized lithium niobite crystal.The generated mid-infrared dual-comb characteristics are consistent with the fundamental dual-comb.The integrated mid-infrared dual-comb system demonstrated in this paper can provide a potential approach for trace gases detection under complicated conditions.