Diode laser spectrometer for high-resolution spectroscopy in the visible range

A high-resolution diode laser spectrometer operating at 657 nm is described. To achieve a narrow linewidth and a high power, a master-slave laser system is employed. The master laser is an extended cavity diode laser whose linewidth is reduced to less than 100 Hz by the FM sideband technique. The slave laser is an AR-coated diode laser and characteristics of injection locking are experimentally studied. The injection current of the slave laser is utilized to stabilize the output power or to produce pulsed output. Using this spectrometer, we probed the intercombination line of Ca and observed high-contrast optical Ramsey fringes with a linewidth of 10 kHz. A velocity-selective Ramsey fringe is also observed in the pulse-mode operation.     

Frequency stabilised grating feedback laser diode for atom cooling applications

The free running linewidth of an external cavity grating feedback diode laser is on the order of a few megahertz and is limited by the mechanical and acoustic vibrations of the external cavity. Such frequency fluctuations can be removed by electronic feedback. We present a hybrid stabilisation technique that uses both a Fabry–Perot confocal cavity and an atomic resonance to achieve excellent short and long term frequency stability. The system has been shown to reduce the laser linewidth of an external cavity diode laser by an order of magnitude to 140 kHz, while limiting frequency excursions to 60 kHz relative to an absolute reference over periods of several hours. The scheme also presents a simple way to frequency offset two lasers many gigahertz apart which should find a use in atom cooling experiments, where hyperfine ground-state frequency separations are often required.     

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.     

A diode laser system for spectroscopy of the ultranarrow transition in ytterbium atoms

A narrow-line diode laser system has been developed for high-resolution spectroscopy of the magnetic quadrupole transition in ytterbium (Yb) atoms at 507 nm. The system consists of an extended-cavity laser diode at 1014 nm, a tapered amplifier and a periodically poled lithium niobate nonlinear crystal which converts the wavelength from 1014 nm to 507 nm. We have stabilized the laser frequency and reduced the linewidth below 1 kHz by tightly locking the laser to a high-finesse optical cavity. By using the developed laser system, we have successfully observed the ultranarrow transition in Yb atoms. Furthermore, our simple and compact laser system could be a high performance and portable frequency reference using iodine spectra whose linewidth becomes less than 50 kHz around 507 nm. We have also demonstrated spectroscopy of iodine molecules. PACS 32.30.-r; 07.60.-j     

External cavity diode laser with kilohertz linewidth by a monolithic folded Fabry-Perot cavity optical feedback

We present a extended-cavity diode laser (ECDL) with kilohertz linewidth by optical feedback from a monolithic folded Fabry-Perot cavity (MFC). In our experiments, an MFC replaces the retroreflecting mirror in the traditional ECDL configuration. Beat-note measurements between this MFC-ECDL and a narrow-linewidth reference laser are performed and demonstrate that the linewidth of this MFC-ECDL is about 6.8 kHz. Phase locking of this MFC-ECDL to the reference laser is achieved with a unity gain as small as 10.2 kHz.     

用于高分辨率光谱研究的窄线宽半导体激光器及其特性研究

报道了一种新型结构的弱反馈外腔半导体激光器，利用此结构将煌激光线宽压窄至５００ｋＨｚ以下。这种激光器可用于高分辨率激光光谱，磁光阱囚禁和原子和原子喷泉量子频标等方面。另外还从理论上研究了其线宽，电调率等物理参量间的关系，得出了有用的关系式，并通过实验对此作了验证。     

Reduce of the Linewidth of a Diode Laser by Locking to a High-Finesse Fabry-Perot Cavity

We report frequency locking of a commercial 657nm diode laser to a high finesse Fabry-Perot cavity by the_ Pound-Drever-Hall method. The laser linewidth relative to the cavity is estimated to be about 6 kHz.     

Locking lasers with large FM noise to high-Q cavities

We demonstrate that a laser can be directly locked to a cavity when the laser linewidth is much greater than the cavity linewidth. We lock an external-cavity diode laser with more than 1 MHz of added frequency noise to a 3.5 kHz wide cavity resonance. Our analog servo acquires lock even though the laser frequency sweeps through the cavity resonance in less than the cavity buildup time. Our theoretical analysis fully describes our measurements and explains why lock can be acquired.     

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.     

External-cavity lasers based on a volume holographic grating at normal incidence for spectroscopy in the visible range

We present two external-cavity diode lasers that utilize a volume holographic grating as the frequency selective feedback element. By using the grating at normal incidence, it is possible to design simple and compact external-cavity diode lasers that have sufficient tunability for molecular spectroscopy. The first design utilizes a long-cavity designed for narrow linewidth and good long-term stability. The laser operates near 635 nm and it has a PZT-controlled tuning range of 28 GHz and a 1-s linewidth of 900 kHz. The second design utilizes a grating attached very close to the laser diode, making the laser compact, robust and easy to operate. The short external-cavity laser operates near 658 nm and it has a linewidth of 30 MHz. Continuous and mode-hop free tuning range of 145 GHz can be obtained by using a simple temperature tuning method.     

Optimization of 780?nm DFB diode lasers for high-power narrow linewidth emission

We demonstrate the realization of narrow linewidth, high-power ridge waveguide DFB diode lasers emitting near 780?nm. The effects of the coupling coefficient, the laser chip length, and the fabrication process onto the spectral linewidth are discussed. By optimizing both the cavity length and the coupling coefficient, we achieve an intrinsic spectral linewidth as small as 35?kHz at an output power of 270?mW.     

Low-noise, tunable diode laser for ultra-high-resolution spectroscopy

We demonstrate the excellent spectral properties of a diode laser setup that combines good tunability with superb short-term frequency stability and controllability. It is based on merging two concepts, the diode laser with resonant optical feedback and the grating stabilized diode laser. To characterize the short-term performance we beat two essentially identical diode lasers and find a short-term linewidth of ~11 kHz. Phase locking between these lasers is achieved with a servo bandwidth as small as 46 kHz, although an analog phase detector is used that requires subradian residual phase error. Despite small phase error detection range and small servo bandwidth, cycle-slip-free phase locking is accomplished for typically many 10 min, and the optical power is essentially contained in a spectral window of less than 20 mHz relative to the optical reference. Due to the excellent performance this laser concept is well suited for atomic or molecular coherence experiments, which require phase locking of different lasers to each other, and as part of a flywheel for optical clocks.     

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.     

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.     

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.     

全频域下窄线宽激光器光谱线宽的分析

窄线宽激光器的线宽表征方式通常采用延时自外差法测量技术。该技术是通过延时光纤差拍产生一个与待测激光线宽相关的洛伦兹频谱,因此该频谱只具有单一的线宽表现形式。为了能够观察到激光器的线宽和频率噪声在其傅里叶频率分布下的完整特性,报道了一种基于β算法计算窄线宽激光器线宽的方法。该方法是结合频率噪声中的白噪声和1/f噪声分别诱导不同激光线型的理论,从而确定激光线宽。首先,对β算法的基本原理进行了详细的分析说明。通过基于维纳-辛钦定理,分析了窄线宽激光器不同频率范围内的频率噪声和激光线宽的依赖关系。阐明了在截止频率趋于0和无穷大的两个范围条件时,激光频谱特性从高斯线型向洛伦兹线型演变。同时推导出使两种线型转换的截止频率表达式,并将其转换为频率噪声函数,该函数定义为β分子线。此时频率噪声分量中高斯线型的总和即为激光线宽计算公式;其次,对窄线宽激光器的频率噪声和激光线型进行数值仿真。将通过OEwaves公司的OE4000互相关零差相位/频率噪声自动测试系统测得的频率噪声谱密度,带入β算法理论公式中。结果显示：1/f噪声导致激光呈现高斯线型,线宽随截止频率的增加而增大。而白噪声将导致洛伦兹线型,线宽不再随截止频率而改变。此外,在低频区域,频率噪声电平远大于其傅里叶频率,噪声调制系数较高,该部分噪声可以决定线宽大小。因此,高斯线型区域对应的频率噪声的积分,即为待测激光器的线宽;在高频区域,频率噪声电平与其傅里叶频率相差较小,频率波动较快,噪声对线宽影响可以忽略。并且频率带宽在截止频率范围内,计算的线宽误差较小。最后,实验上运用β算法对RIO公司的1 550 nm低噪声窄线宽激光器的频率噪声功率谱密度进行积分计算,成功获得了其不同傅里叶频率分布下对应的激光线宽值。其中β分子线将频率噪声中的白噪声和1/f噪声分隔两部分：当频率噪声谱密度大于β分子线时,激光即为高斯线型,线宽随频率积分带宽的增加而减少;而频率噪声谱密度小于β分子线时,激光呈现洛伦兹线型,线宽为定值不再改变。同时为了对β算法进行实验验证,搭建了延迟光纤为50 km、移频频率为60 MHz的延时自外差法测量系统。对注入电流为110 mA的RIO 1 550 nm低噪声窄线宽激光器的线宽进行实验测量,测量结果表明激光线宽为1.8 kHz,与上述β算法中2.8 kHz的频率带宽积分结果一致。充分证明了此算法的准确性。β算法可以对任意类型的窄线宽激光器进行线宽表征,对窄线宽激光器的研究具有重要意义。     

半导体泵浦铷蒸气激光器出光实验

针对中心波长780 nm的商用连续波半导体激光器,使用斩波器将连续激光变为脉冲输出形式,用2400 line/mm的平面全息光栅搭建Littrow外腔将线宽压窄至0.2 nm以下。采用透镜组对线宽压窄后的半导体激光进行光束整形,半导体激光经线宽压窄和光束整形后,经透镜聚焦进长约8 mm的铷蒸气泡进行半导体泵浦碱金属激光实验。首次出光得到17.5 mW的基模线偏振铷激光;在最新的改进实验中,半导体泵浦铷激光输出功率已达到2.8 W。     

A novel discrete mode narrow linewidth laser diode for spectroscopic based gas sensing in the 1.5 μm region

The characteristics of a novel Discrete Mode laser diode are described with particular reference to its capability in spectroscopic based gas sensing. In this regard, its emission linewidth, Side Mode Suppression Ratio and tuneability are of particular note. The measured narrow linewidth of 550?kHz has applications with respect to spectrally narrow absorption features found, for example, in saturated absorption lines, in hyperfine structure of some rotational manifolds and in whispering gallery mode features associated with optical microspheres. Other notable measured characteristics show mode-hop-free temperature tuning of 11 nm at a linear tuning rate of 0.1 nm/°C and a Side Mode Suppression Ratio in the range 56 to 46 dB and a long term (72?hours) wavelength stability of 1?pm per day. In essence, this novel Discrete Mode laser displays excellent potential for diode laser based sensing applications in the near infrared.     

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。