减反膜外腔半导体激光器特性的研究

研究了镀减反膜GaAs半导体激光器在Littrow 式外腔结构中的输出特性.采用电流补偿技术,得到了约30 GHz的连续无跳模调谐范围.利用外差拍探测技术获得激光器输出线宽约为120 kHz.还研究了激光器的强度噪声分别随着电流与波长变化的特点并对此进行了分析.该类激光器可以广泛应用在冷原子物理、激光光谱以及量子光学等许多领域. 关键词： 减反膜 半导体激光器 线宽 强度噪声     

Control and active stabilization of the linewidth of an ECDL

The linewidth of a polarization-locked external cavity diode laser (ECDL) is explored employing heterodyne as well as self-heterodyne measurements. We use a model capable of providing the individual contributions of white, pink and red noise to the overall linewidth to analyze the measured beat spectra. These spectra are obtained while tuning the external cavity as well as a function of pump current and feedback level. By virtue of our locking technique, we find that the linewidth can be adjusted and minimized by simply altering the setpoint of the closed-loop control. This control of the linewidth is applicable for any ECDL using polarization locking. For our particular ECDL, we are able to tune the overall linewidth from ??8?kHz to 20?kHz. Moreover, we achieve a lower white noise level of the ECDL compared to a free running one.     

A 657-nm narrow bandwidth interference filter-stabilized diode laser

We present a 657-nm external cavity diode laser (ECDL) system,where the output frequency is stabilized by a narrow-band high transmission interference filter.This novel diode laser system emits laser with an instantaneous linewidth of 7 kHz and a broadened linewidth of 432 kHz.     

利用自相位延迟方法测量单块腔反馈半导体外腔激光器的线宽

提出了采用单块折叠法布里-珀罗(F-P)腔作为外腔反馈元件实现窄线宽半导体激光器,采用单块腔的光学反馈来锁定外腔激光器,使用自相位延迟法测量该窄线宽激光器的线宽。实验结果表明,激光器线宽小于35kHz。实验还观测到由于单块腔耦合面上各耦合点的几何量和物理量误差不一样,随着折叠面兼输入输出耦合面上的耦合点的变化,外腔激光器的线宽发生改变。     

基于自制超稳定F-P腔压窄632.8nm外腔半导体激光线宽的实验研究

窄线宽激光由于其具有单色性好、稳定度高、相干长度长等优点,广泛应用于光电检测领域,包括相干通信、精密测量、光学频率标准、吸收光谱计量以及光与物质相互作用研究等。目前频率稳定的氦氖激光器线宽可以达到MHz量级,分布反馈式（DFB）光纤激光器线宽可达kHz量级,DFB半导体激光器线宽可以达到MHz量级,然而光栅反馈半导体激光器可以实现百kHz量级线宽的输出。为了进一步压窄各类激光器线宽,需要通过反馈控制技术来锁定激光到某一频率参考。该研究将自行设计的超稳腔作为频率参考,实现了632.8 nm外腔半导体激光器（ECDL）线宽的有效压窄。本窄线宽激光产生系统的研制包括超稳腔设计、光路设计、ECDL频率控制以及系统集成。超稳腔采用两镜法布里-珀罗腔（F-P腔）结构,腔体是膨胀系数约为10-6 K-1的微晶玻璃,腔镜为一对反射率达99.988 5%(±0.003 5%）的平面镜和凹面镜。为进一步减小外界环境对F-P腔腔长的影响,需要对腔体进行温度控制,本系统采用四片总功率为96 W的半导体制冷片以及水冷散热设计。同时为了降低声音和空气流动对腔模频率的影响,将F-P腔置于真空度为10-5 torr的真空室中;另外为了有效隔振,腔体与真空室用硅橡胶材料隔离。该系统采用的ECDL为德国Toptica公司的DL pro系列激光器,其具有压电陶瓷（PZT）和电流调制两个频率控制端,响应带宽分别为1 kHz和100 MHz。激光器的频率控制采用了Pound-Drever-Hall （PDH）锁频技术,18 MHz的调制频率加载到激光器的电流调制端,通过对F-P腔的反射信号进行解调获得误差信号,通过两路反馈控制,实现了近1 MHz的锁定带宽。通过对系统的不断优化,最后将自由运转状态下约300 kHz的激光线宽压窄到了10 kHz量级,并且系统运行稳定,连续12小时锁定的频率漂移量约为30 MHz。该研究研制的632.8 nm窄线宽激光源不仅可以应用到吸收光谱计量领域,同时也可以在光学面型精密测量领域发挥重要作用。     

激光线宽对单次通过PPMgO:LN晶体倍频效率的影响

通过单次穿过PPMgO:LN晶体产生了2.06 W的780 nm可调谐的连续倍频光. 采用1560 nm的分布反馈式(DFB)半导体激光器、光栅外腔半导体激光器(ECDL)和分布反馈式掺铒光纤激光器(DFB-EDFL)分别作为掺铒光纤放大器(EDFA)的注入光源, 所用的EDFA具有保持窄线宽的功能, 因此可以忽略它对基波线宽的展宽. 研究了激光线宽对单次通过PPMgO:LN 晶体的倍频效率的影响. 控制三台激光器各自注入EDFA的功率一致, 同时也保持EDFA 的输出功率. 在基波功率为12.42 W 时, 使用DFB半导体激光器注入EDFA时得到了1.36 W的780 nm倍频光输出, 转换效率为11.0%; 使用ECDL作为种子源时得到了1.78 W 的780 nm倍频光输出, 转换效率为14.3%; 使用DFB-EDFL作为种子源时得到了2.06 W的780 nm倍频光输出, 转换效率为16.6%. 测得三台种子激光器的线宽分别为1.2 MHz (DFB), 200 kHz (ECDL)和600 Hz (DFB-EDFL). 线宽越窄, 倍频效率越高, 实验结果与理论分析一致.     

An external cavity diode laser using a volume holographic grating

This study presents an external cavity diode laser (ECDL) system, utilizing a volume holographic grating (VHG) and a microfabricated silicon flexure as the VHG holder. The laser design is aimed for easy assembly, controllability, and better stability of the laser cavity. The laser frequency was stabilized to a D₂ transition of rubidium at 780.247 nm, with a mode-hop-free tuning range of 16 GHz and 9.6 GHz with and without feed-forward on the diode injection current. The measured linewidth was 850 kHz in 500 s, qualified for laser cooling experiments.     

Stable Narrow Linewidth 689nm Diode Laser for the Second Stage Cooling and Trapping of Strontium Atoms

We report stable narrow linewidth laser systems based on self-developed Littman configuration external cavity diode lasers （ECDLs）. The frequency of the ECDL is stabilized to a high fineness ultralow-expansion glass reference cavity with the Pound-Drever-Hall technique. By heterodyne beating of two identical systems, we conclude that the linewidth 4.3× 10^-14 at an averaging measurement time. of each ECDL is reduced to lower than 150 Hz and its frequency stability reaches time of 1 s, the averaged long-term frequency drift is less than 0.2 Hz/s over 30 h     

Reducing the linewidth of a diode laser below 30 Hz by stabilization to a reference cavity with a finesse above 10(5)

An extended-cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized through its injection current to a reference cavity with a finesse of more than 10(5) and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few megahertz to a value below 30 Hz. The compact and robust setup appears ideal as a portable optical frequency standard that uses the calcium intercombination line.     

Frequency stabilization of blue extended cavity diode lasers by external cavity optical feedback

Optical feedback from a high-finesse V-resonator, developed for this study, results in efficient coupling with an extended cavity diode laser, stabilizing its emission frequency and strongly decreasing the laser linewidth. This in turn enhances resonator output power, thus increasing the signal-to-noise ratio when used for the detection of gas phase species by absorption spectroscopy. This effect was directly measured by heterodyning two extended cavity diode lasers at a wavelength of 409 nm with and without the influence of optical feedback from a high-finesse V-resonator. The heterodyne signal of freely running lasers is composed of a set of sharp peaks whose envelope shows a width on the order of 4.5 MHz at a sweep rate of 80 MHz/0.8 s, leading to a laser linewidth of 3 MHz. Optical feedback from the high-finesse V-resonator reduces the heterodyne signal to a single peak with a mean width of 10 kHz, leading to a laser linewidth of 7 kHz. This is the lowest value of linewidth, reported thus far, for diode lasers operating in this wavelength region.     

243 nm稳频窄线宽半导体激光器

243 nm是氢原子1S-2S能级跃迁光谱波长. 本文利用Pound-Drever-Hall稳频技术将972 nm光栅反馈外腔半导体激光稳定在一个高精细度低膨胀系数的超稳法布里-珀罗腔上, 通过锥形放大器放大和腔内两次共振增强倍频得到243 nm激光, 最终实现用于探测氢原子1S-2S双光子跃迁的243 nm窄线宽激光.     

Narrow bandwidth interference filter-stabilized diode laser systems for the manipulation of neutral atoms

We present and investigate different external cavity diode laser (ECDL) configurations for the manipulation of neutral atoms, wavelength-stabilized by a narrow-band high transmission interference filter. A novel diode laser, providing high output power of more than 1 W, with a linewidth of less than 85 kHz, based on a self-seeded tapered amplifier chip has been developed. Additionally, we compare the optical and spectral properties of two laser systems based on common laser diodes, differing in their coating, as well as one, based on a distributed-feedback (DFB) diode. The linear cavity setup in all these systems combines a robust and compact design with a high wavelength tunability and an improved stability of the optical feedback compared to diode laser setups using diffraction gratings for wavelength discrimination.     

High repetition-rate wavelength tuning of an extended cavity diode laser for gas phase sensing

A method for rapid wavelength tuning of an extended cavity diode laser (ECDL) is presented providing for high resolution, narrow bandwidth output over limited spectral regions. The method permits tuning over isolated spectroscopic features at repetition rates of tens of kHz, greatly exceeding conventional ECDL tuning speeds. In this paper we present high repetition rate laser induced fluorescence (LIF) spectroscopy of the 5²P_1/2 to 6²S_1/2 transition in indium at 410 nm, to demonstrate the technique. The presented ECDL design is very easy to implement, cheap and robust, as it employs no moving parts and can be used over all wavelength regions where FP diode lasers are available. This extends the usefulness of standard FP diode lasers to high speed sensing applications. Advantages and disadvantages of the technique are discussed. PACS 42.55.Px; 42.60.Fc; 42.62.Fi; 32.50.+d     

Diode laser using narrow bandwidth interference filter at 852 nm and its application in Faraday anomalous dispersion optical filter

We demonstrate an 852-nm external cavity diode laser(ECDL) system whose wavelength is mainly determined by an interference filter instead of other wavelength selective elements. The Lorentzian linewidth measured by the heterodyne beating between two identical lasers is 28.3 k Hz. Moreover, we test the application of the ECDL in the Faraday atomic filter.Besides saturated absorption spectrum, the transmission spectrum of the Faraday atomic filter at 852 nm is measured by using the ECDL. This interference filter ECDL method can also be extended to other wavelengths and widen the application range of diode laser.     

High-resolution laser spectroscopy of ultracold ytterbium atoms using spin-forbidden electric quadrupole transition

We have successfully observed high-resolution spectra of spin-forbidden electric quadrupole transition (¹ S ₀→³ D ₂) in ytterbium (¹⁷⁴Yb) atoms. The differential light shifts between the ¹ S ₀ and the ³ D ₂ states in a far-off resonant trap at 532 nm are also measured. For the spectroscopy, we developed simple, narrow-linewidth, and long-term frequency stabilized violet diode laser systems. Long-term drifts of the excitation laser (404 nm) is suppressed by locking the laser to a length stabilized optical cavity. The optical path length of the cavity is stabilized to another diode laser whose frequency is locked to a strong ¹ S ₀→¹ P ₁ transition (399 nm) of Yb. Both lasers are standard extended-cavity diode lasers (ECDLs) in the Littrow configuration. Since the linewidth of a violet ECDL (～10 MHz) is broader than a typical value of a red or near infra-red ECDL (<1 MHz), we employ optical feedback from a narrow-band Fabry–Perot cavity to reduce the linewidth. The linewidth is expected to be <20 kHz for 1 ms averaging time, and the long-term frequency stability is estimated to be ～200 kHz/h.     

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.     

Spectral filtering of dual lasers with a high-finesse length-tunable cavity for rubidium atom Rydberg excitation

We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity(HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound–Drever–Hall(PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 k Hz, meanwhile the obtained lasers' phase noise around 750 k Hz are suppressed about 30 d B. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of(67 ± 37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ～ 3.3 k Hz and the maximum long-term laser frequency drift of a single laser is ～ 46 k Hz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Composite cavity semiconductor fiber ring laser

A passive subring cavity that served as mode selector was inserted into a semiconductor fiber ring laser to generate a stable lasing spectrum. A narrow linewidth of less than 10 kHz with unidirectional operation in the composite cavity ring laser was achieved. Tuning over a range of 20 nm was obtained by application of 0-30 V dc to a piezoelectric transducer in the subring cavity.     

电光晶体调谐的外腔反馈半导体激光器

报道一种用电光晶体实现快速调谐和凋制激光频率的方法.在Littrow型外腔反馈半导体激光中插入LiNbO3晶体,利用LiNbO3晶体的电光效应,通过改变晶体电压来调节激光器的有效腔长,可以对激光频率进行快速的调谐和调制.采用该方法,自制外腔反馈半导体激光器的调谐频率可达到2 kHz,它的调谐范围为350 MHz,激光频率调谐系数约为1.06 MHz/V,用饱和吸收光谱观测频率调谐的效果.快速激光频率调制可以应用在稳频技术上,将外腔反馈半导体激光器调制在5～100 kHz频率下,均获得了87Rb原子D2线的饱和吸收光谱的色散信号,并实现了激光频率在饱和吸收峰上的长期稳定.