⇦Chinese Physics Letters, 2017, Vol. 34, No. 7, Article code 074211⇨ A 420 nm Blue Diode Laser for the Potential Rubidium Optical Frequency Standard * Sheng-Nan Zhang(张盛楠)1, Xiao-Gang Zhang(张晓刚)1, Jian-Hui Tu(涂建辉)2**, Zhao-Jie Jiang(蒋招杰)1, Hao-Sen Shang(商浩森)1, Chuan-Wen Zhu(祝传文)1, Wei Yang(杨炜)2, Jing-Zhong Cui(崔敬忠)2, Jing-Biao Chen(陈景标)1** Affiliations 1State Key Laboratory of Advanced Optical Communication Systems and Networks, and Institute of Quantum Electronics, School of Electronics Engineering and Computer Science, Peking University, Beijing 100871 2National Key Laboratory of Science and Technology on Vacuum Technology & Physics, Lanzhou Institute of Physics, China Academy of Space Technology, Lanzhou 730000 Received 24 April 2017 ^*Supported by the China Academy of Space Technology Foundation under Grant No CAST-2015-5-10, the National Hi-Tech Research and Development Program, and the National Natural Science Foundation of China under Grant No 91436210.
^**Corresponding author. Email: tujianhui@spacechina.com; jbchen@pku.edu.cn Citation Text: Zhang S N, Zhang X G, Tu J H, Jiang Z J and Shang H S et al 2017 Chin. Phys. Lett. 34 074211 Abstract We report a 420 nm external cavity diode laser with an interference filter (IF) of 0.5 nm narrow-bandwidth and 79% high transmission, which is first used for Rb optical frequency standard. The IF and the cat-eye reflector are used for selecting wavelength and light feedback, respectively. The measured laser linewidth is 24 kHz when the diode laser is free running. Using this narrow-linewidth IF blue diode laser, we realize a compact Rb optical frequency standard without a complicated PDH system. The preliminary stability of the Rb optical frequency standard is $2\times10^{-13}$ at 1 s and decreases to $1.9\times10^{-14}$ at 1000 s. The narrow-linewidth characteristic makes the IF blue diode laser a well suited candidate for the compact Rb optical frequency standard. DOI:10.1088/0256-307X/34/7/074211 PACS:42.55.Px, 32.70.Jz, 06.20.fb © 2017 Chinese Physics Society Article Text External cavity diode lasers (ECDLs) are becoming ideal light sources for many fields such as precise laser spectroscopy[1-7] and atomic frequency standard[8-12] because of their compactness and robustness. Linewidths of ECDLs can be narrowed to hundreds of kilohertz or smaller,[1,13] which meet numerous experimental demands of atomic molecular optical (AMO) physics without a complex PDH[14] frequency stabilization system. The ECDL designs mainly include three configurations: diffraction grating, Faraday anomalous dispersion optical filter (FADOF) and interference filter (IF). Littrow[15,16] and Littman[17,18] are two kinds of diffraction grating configurations. For the common Littrow configuration, the direction or position of the output beam depends on the wavelength. They require the precise alignment of grating angle, and are therefore sensitive to acoustic and mechanical disturbances. The extended cavity Faraday laser system developed in recent years[19-22] is a novel ECDL which uses a laser diode as the gain medium and FADOF as the frequency selective device. For the IF configuration,[23-28] a narrow bandwidth filter is used to select the wavelength, and a cat-eye reflection geometry serves as the feedback mirror. The two tasks of wavelength selection and feedback reflection are carried out by two different optical elements. Therefore, the IF configuration is less sensitive to the misalignment induced by mechanical and thermal disturbances and also reduces the angle change when the laser wavelength is tuned. As we know, now the most reported IF diode lasers are working in the 600–1000 nm range.[23-28] In the wavelength region 420 nm, there are some important spectroscopic lines such as Rb 5$S_{1/2}$–6$P_{1/2}$ and 5$S_{1/2}$–6$P_{3/2}$ transition lines.[29] The traditional method for achieving blue light is using a frequency-doubled diode laser. However, the whole size of the frequency-doubled laser system is very large and its frequency noise and power noise are comparatively higher than that of a single diode laser. With the 420 nm frequency-doubled diode laser, the relative stability of $2.8\times10^{-10}$ is achieved based on the modulation transfer spectroscopy (MTS) method.[30] Limited by the characteristics of the frequency-doubled diode laser, it is difficult to further improve the stability. In this Letter, we demonstrate a compact IF diode laser at 420 nm with a narrow bandwidth filter of 79% transmission and 0.5 nm full width at half maximum (FWHM). The output frequency could be tuned smoothly over 20 GHz and the output power could be 20 mW enough for our experiment. The most probable linewidth of this diode laser is 24 kHz with 52 groups of the measured beat signal data between two identical lasers while both are free running. The measured linewidth is narrow enough for the Rb optical frequency standard without a complicated PDH frequency stabilization system. The main mechanical and optical construction of the IF blue diode laser is depicted in Fig. 1(a). This construction is similar to what is mentioned in the above references. Here we briefly described the IF blue diode laser as follows: the light emitted from the laser diode is collimated by an objective lens (LC, C230TME-A) with short focal length (4.51 mm) and high numerical aperture (0.55). Then the light transmits the IF and a lens (L1, C220TME-A) focuses the collimated light on the partial reflector to form a cat's eye reflector. This reflector reflects 50% of light back to the laser diode. The cavity length can be changed by a piezo-electric transducer (PZT) attached to the reflector. The focused light is collimated by the second lens (L2, C280TME-A). A thermoelectric cooler (TEC) and a thermistance put in the laser diode package stabilize the laser diode's temperature. The photograph of the diode laser is shown in Fig. 1(b). Compared with the Littrow and Littman ECDLs, the IF diode laser configuration can more easily adjust laser frequency and optimize optical feedback.

Fig. 1. (a) The schematic diagram of the blue diode laser with an IF for wavelength selection. LC: collimating lens, PZT: piezo-electric transducer, L1: lens forming a cat's eye, and L2: lens providing a collimated output beam. (b) The photograph of the IF blue diode laser setup. The length of the blue diode laser is about 140 mm.

Fig. 2. Interference filter transmission at an incident angle of 12$^{\circ}$ as a function of the incident wavelength. The FWHM of filter is 0.5 nm, and the peak transmission is 79%.

The IF transmission is measured as shown in Fig. 2. The peak transmission of 79% can be achieved at a nominal wavelength of 420.3 nm with a 0.5 nm FWHM. The angle of the highest transmission of the 420.3 nm light is designed to be 12$^{\circ}$ to avoid the light reflected from the filter surface feeding back into the laser diode. The output laser wavelength can be coarsely controlled by rotating the filter. The transmitted wavelength depends on the angle of incidence,[24,27] $$\begin{align} \lambda =\lambda _{\max } \sqrt {1-\frac{\sin ^2\theta }{n_{\rm eff}^2 }},~~ \tag {1} \end{align} $$ where $\theta$ is the angle of incidence, $\lambda _{\max}$ is the wavelength at normal incidence, and $n_{\rm eff}$ is the effective index of refraction.

Fig. 3. An example of the spectra of the beat signal between two identical IF diode lasers with dots and the Lorentzian fitting (solid curve). The resolution bandwidth of the spectrum analyzer is set to 30 kHz, and sweep time to 50 ms.

Fig. 4. Linewidth statistical distribution of the beat signal between two identical blue diode lasers and the Gaussian fitting (solid curve).

To determine the linewidth of this laser, we measured the beat signal between two identical free-running lasers by an rf spectral analyzer (Agilent 9320B). Two laser beams are superimposed on a high-speed photo detector (HAMAMATSU C5658), and the detected beat signal is fed into the spectral analyzer. We then randomly recorded many groups of beat signal data at different parameters of the resolution bandwidth. An example of the spectra of the beat signal with dots is shown in Fig. 3. The Lorentzian fitting (solid curve) shows that the linewidth is 31 kHz when the resolution bandwidth of the spectrum analyzer is set to 30 kHz, and sweep time to 50 ms, corresponding to the linewidth of 22 kHz of each laser. We measured 52 groups of beat signal and each group was fitted with a Lorentzian function to obtain the linewidth. The statistical distribution of the linewidth is shown in Fig. 4. When we fitted the 52 groups statistical data distribution with a Gaussian function, we obtained that the most probable linewidth[31] of the beat signal between two lasers is 34 kHz, corresponding to 24 kHz linewidth of each laser. The blue diode laser with a narrow-bandwidth IF is an ideal candidate for the compact Rb optical frequency standard. To fully exploit the narrow-linewidth IF blue diode laser, we used it as the light source of the 420 nm Rb optical frequency standard where the high-performance modulation transfer spectroscopy (MTS)[30,32-36] was used to stabilize the laser frequency. The experimental setup of the compact Rb optical frequency standard is shown in Fig. 5.

Fig. 5. Experimental setup of 420 nm Rb optical frequency standard. P1, P2, P3, halfwave plates; PBS1, PBS2, polarized beam splitters; M1, M2, M3, mirrors; L, lens; ISO, isolator; PD, photoelectric detector.

Evaluated by the residual error signal after locking, the Allan deviation of the Rb optical frequency standard against the averaging time is shown in Fig. 6. The preliminary stability is $2\times10^{-13}$ at 1 s and decreases to 1.9$\times$10$^{-14}$ at 1000 s. The detailed information about the compact optical frequency standard with an Rb cell will appear elsewhere soon.

Fig. 6. Allan deviation of Rb optical frequency standard. The preliminary 1 s stability is 2$\times$10$^{-13}$ at 1 s and decreases to 1.9$\times$10$^{-14}$ at 1000 s.

In conclusion, we have demonstrated an IF blue diode laser at 420 nm with a narrow bandwidth filter of 79% transmission and 0.5 nm FWHM. The output laser can be tuned smoothly over 20 GHz and the output power could be 20 mW. We also fitted 52 groups of the beat signal between two identical free-running lasers with a Lorentzian function. The most probable linewidth of each diode laser is 24 kHz. Based on this 420 nm IF blue diode laser, we realized a compact Rb optical frequency standard. The preliminary stability is $2\times10^{-13}$ at 1 s and decreases to $1.9\times10^{-14}$ at 1000 s. The signal-to-noise ratio up to 100000 in the Rb microwave clock has been achieved.[37] With some detailed improvements, the stability of this Rb optical frequency standard is $1.2\times 10^{-14}/\sqrt \tau$ from 0.1 s to 80 s. It is better than most of the commercial microwave clocks. Another advantage is that it can be integrated to a small volume without a complicated PDH frequency stabilization system. The Rb optical frequency standard provides a premium option because it extends the Rb frequency standard from microwave to optical band. References Optically stabilized narrow linewidth semiconductor laser for high resolution spectroscopyUsing diode lasers for atomic physicsA compact grating-stabilized diode laser system for atomic physicsCs 455 nm Nonlinear Spectroscopy with Ultra-narrow LinewidthApplication of Origen2.1 in the decay photon spectrum calculation of spallation productsSensitive absorption measurements of hydrogen sulfide at 1.578 ?m using wavelength modulation spectroscopyPassively Q-Switched Nd,Cr:YAG Laser Simultaneous Dual-Wavelength Operation at 946 nm and 1.3 μmStable Narrow Linewidth 689 nm Diode Laser for the Second Stage Cooling and Trapping of Strontium AtomsExperimental realization of an optical second with strontium lattice clocksActive Faraday optical frequency standardNarrow bandwidth interference filter-stabilized diode laser systems for the manipulation of neutral atomsLaser phase and frequency stabilization using an optical resonatorA simple extended-cavity diode laserApplication of diode lasers as a spectroscopic tool at 670 nmExternal-cavity diode laser using a grazing-incidence diffraction gratingSelf-aligned extended-cavity diode laser stabilized by the Zeeman effect on the cesium D_2 lineNote: Demonstration of an external-cavity diode laser system immune to current and temperature fluctuationsAn all-optical locking of a semiconductor laser to the atomic resonance line with 1 MHz accuracyDiode laser operating on an atomic transition limited by an isotope ^87Rb Faraday filter at 780 nmFaraday laser using 1.2 km fiber as an extended cavityOptical characteristics of a deformable-mirror spatial light modulatorInterference-filter-stabilized external-cavity diode lasersExternal cavity diode laser setup with two interference filtersA 657-nm narrow bandwidth interference filter-stabilized diode laserNarrow linewidth tunable external cavity diode laser using wide bandwidth filterDiode laser using narrow bandwidth interference filter at 852 nm and its application in Faraday anomalous dispersion optical filterIsotope ^87Rb Faraday anomalous dispersion optical filter at 420 nmDoppler-free modulation transfer spectroscopy of rubidium 52S1/2–62P1/2 transitions using a frequency-doubled diode laser blue-light sourceFrequency stabilization of Nd:YAG lasers with a most probable linewidth of 06 HzHigh-Frequency Optically Heterodyned Saturation Spectroscopy Via Resonant Degenerate Four-Wave MixingSPIE ProceedingsModulation transfer spectroscopy in atomic rubidiumFrequency stabilization of a 1083 nm fiber laser to ⁴ He transition lines with optical heterodyne saturation spectroscopiesPractical choices in the FRF measurement in presence of nonlinear distortionsMetrological characterization of the pulsed Rb clock with optical detection

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