Sub-MHz linewidth of 633 nm diode lasers with internal surface DBR gratings

Abstract

We report on two ultrastable lasers each stabilized to independent silicon Fabry-Pérot cavities operated at 124 K. The fractional frequency instability of each laser is completely determined by the fundamental thermal Brownian noise of the mirror coatings with a flicker noise floor of 4 × 10 −17 for integration times between 0.8 s and a few tens of seconds. We rigorously treat the notorious divergences encountered with the associated flicker frequency noise and derive methods to relate this noise to observable and practically relevant linewidths and coherence times. The individual laser linewidth obtained from the phase noise spectrum or the direct beat note between the two lasers can be as small as 5 mHz at 194 THz. From the measured phase evolution between the two laser fields we derive usable phase coherence times for different applications of 11 s to 55 s. It is well known that frequency is the physical quantity that can be measured with by far the highest accuracy. "Never measure anything but frequency!" was the advice of Arthur Schawlow [1]. The high accuracy results from the fact that the phase of a purely periodic signal can be measured in the simplest case by counting the zero crossings of the signal within a given time or with even increased accuracy by a phase measurement that interpolates the signal between the zero crossings. Hence, the generation of truly phase coherent signals over long times is the key to precision measurements and enabling technologies. In the most advanced optical atomic clocks [2-5] pre-stabilized lasers serve as oscillators to interrogate ultranarrow optical transitions with linewidths of a few mHz. Oscillators with coherence times of tens to hundreds of seconds will allow for investigations of extremely small energy shifts in the clock transition, caused by sources such as interactions amongst atoms [6, 7]. Ul-trastable oscillators beyond the state of the art will find useful applications in sub-mm very long baseline interfer-ometry (VLBI) [8], atom interferometry and future atom-based gravitational wave detection [9-11], novel radar applications [12], the search for dark matter [13], and deep space navigation [14]. Consequently, large effort has been put into the development of extremely coherent sources based on highly stable optical Fabry-Pérot resonators [15-18]. Alternative schemes are currently being investigated using cavity-QED systems [17, 19] and spectral-hole burning in cryogenically cooled crystals [20]. Here we report on the coherence properties of two cavity-stabilized laser systems operating at a wavelength of 1542 nm. Our systems are based on well-isolated single-crystal silicon Fabry-Pérot resonators, temperature stabilized at 124 K. For a system that has well designed locking electronics, the fractional frequency stability of the laser is given by the fractional stability of the optical length of the cavity. Fundamentally, the cav-ities' length stability is limited by statistical Brownian noise of the mirror coatings, substrates, and spacer [21]. Due to the...

Advances in narrow linewidth diode lasers

“…Narrow linewidth red laser (∼620-650 nm band) is an important light source for laser spectroscopy, coherent measurement, and holographic technology. To meet this demand, FBH [34] proposed a narrow linewidth DBR laser in the 633 nm band. The total length of the laser resonator is 2 mm; the ridge gain region is 1.5 mm; the grating region is 0.5 mm; the laser power is 10 mW at 150 mA; and the spectral linewidth is less than 1 MHz.…”

Advances in narrow linewidth diode lasers

“…2010 年德国 FBH 研究所 [31] 采用 6 阶表面布拉格光栅, 研制了 4 µm 条宽、4 mm 腔长的 1064 nm 波段 DBR 结 构激光器, 实现了线宽为 180 kHz@180 mW, 测出固有线宽为 2 kHz, 阈值电流为 65 mA@25 • C, 斜率 效率为 0.41 W/A@25 • C.针对高性能光纤激光器对 975 nm 波段窄线宽半导体激光泵浦源的需求, 美国伊利诺伊大学 (University of Illinois) 的 Coleman 等[32] 采用在宽条波导 (40 µm) 表面刻蚀光栅的方法, 研制出腔长 1.5 mm 的 974.8 nm 波段 DBR 激光器, 实现激光功率 500 mW, 激光线宽 350 kHz, SMSR > 40 dB 的 高功率输出. 随后, 德国 FBH 研究所[33] 采用普通紫外光刻和反应等离子体刻蚀技术制备出 80 阶表面 光栅 DBR 激光器, 在 970 nm 波段实现激光功率 6 W, 电光转化率 > 50%, 光参量积 < 1.8 mm×mrad, 激光线宽仅为 0.41 nm (130.7 GHz), 十分适用于泵浦光纤激光器.窄线宽红光激光器 (620∼650 nm 波段) 是激光光谱学、相干测量和全息技术的重要光源, 针对这 一需求, 德国 FBH 研究所[34] 提出一种 633 nm 波段的窄线宽 DBR 激光器, 激光器谐振腔总长 2 mm, 其中脊形增益区为 1.5 mm、光栅区为 0.5 mm, 实现了激光功率 10 mW@150 mA, 光谱线宽 < 1 MHz 的输出, 可连续工作 1700 h@14 mW. 随后, 该研究所的 Paschke 等[35] 报道了一种脊型波导 DBR 激 光器, 通过步进光刻和反应离子刻蚀在激光结构中引入光栅, 研制出波长为 626.5 nm 的 DBR 激光器, 实现输出功率 > 50 mW@ 0 • C, 激光线宽 < 1 MHz@150 mA, SMSR > 20 dB, 该技术可替代传统全固 态激光器作为量子信息实验光源, 通过降低光源模块体积, 可有效提升量子信息系统的微型化程度.…”