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.     

A simple scheme for precise relative frequency stabilization of lasers

We present a simple scheme for tuneable relative frequency stabilization of lasers. A highly sensitive and accurate frequency-to-voltage converter is used to derive an error signal from the beat note between two lasers. We analyze in detail detector noise and drift, modulation detection bandwidth, and cross-talk from power modulation. The results indicate that sub-kHz relative linewidth and a locking point drift on the order of 100 Hz for times scales of 1 h are achievable. The scheme can, therefore, be applied to situations where up to now only optical PLLs could provide sufficient accuracy and precision. To demonstrate its potential for high-resolution, high-precision spectroscopy we lock a diode laser to a fs-frequency comb and find a relative linear drift of 314 Hz during a 2.8 h period. PACS 42.62.Fi; 42.62.Eh     

Stable diode lasers for hydrogen precision spectroscopy

We report on an external cavity diode laser at 972 nmstabilized to a mid-plane mounted Fabry-Perot (FP) resonator with afinesse of 400000. The 0.5 Hz optical beat note line width betweentwo similar lasers (Allan deviation 2 × 10^-15) is limitedby thermal noise properties of two independent FP resonators. Thelong term drift of the FP resonator and mirror substrates made fromUltra-Low-Expansion glass (ULE) is small and can be well predictedon time intervals up to many hours if the resonator is stabilized atthe zero thermal expansion temperature T_c. Using a Peltierelement in a vacuum chamber for temperature stabilization allowsstabilization of the FP cavity to T_c which is usually below theroom temperature. Beat note measurements with a femtosecond opticalfrequency comb referenced to a H-maser during 15 hours have shown awell defined linear drift of the FP resonance frequency of about 60 mHz/s with residual frequency excursions of less than ±20 Hz.     

Optical phase locking of two extended-cavity diode lasers with ultra-low phase noise for atom interferometry

We present a phase coherent laser system with ultra-low phase noise with a frequency difference of 6.9 GHz. The laser system consists of two extended-cavity diode lasers that are optically phase-locked with electrical feedback to the injection current of a slave laser. The bandwidth of the optical phase-locking loop is extended up to 8 MHz. We achieve the residual phase noise of two phase-locked lasers of below ?120 dBrad²/Hz in the offset frequency range of 100 Hz–350 kHz and a flat phase noise of ?127 dBrad²/Hz from 700 Hz to 20 kHz. These results are, to the best of our knowledge, the lowest phase noise level ever reported with two extended-cavity diode lasers.     

An Yb-fiber frequency comb phase-locked to microwave standard and optical reference

We present a fully stabilized Yb-fiber frequency comb locked to a microwave standard and an optical reference separately. The carrier-envelope offset frequency is generated by a standard f–2f interferometer with 40 dB signal-tonoise ratio. The offset frequency and the repetition rate are stabilized simultaneously to the radio frequency reference for more than 30 hours, and the fractional Allan deviation of the comb is the same as the microwave standard of 10^-12 at 1 s.Alternatively, the comb is locked to an ultra-stable optical reference at 972 nm using an intracavity electro-optic modulator,exhibiting a residual integrated phase noise of 458 mrad(1 Hz–10 MHz) and an in-loop tracking stability of 1.77× 10^-18 at 1 s, which is significantly raised by six orders comparing to the case locked to the microwave frequency standard.     

Ultra-stable microwave generation with a diode-pumped solid-state laser in the 1.5-μm range

We demonstrate the first ultra-stable microwave generation based on a 1.5-μm diode-pumped solid-state laser (DPSSL) frequency comb. Our system relies on optical-to-microwave frequency division from a planar-waveguide external cavity laser referenced to an ultra-stable Fabry–Perot cavity. The evaluation of the microwave signal at ~10 GHz uses the transportable ultra-low-instability signal source ULISS^®, which employs a cryo-cooled sapphire oscillator. With the DPSSL comb, we measured ?125 dBc/Hz phase noise at 1 kHz offset frequency, likely limited by the photo-detection shot-noise or by the noise floor of the reference cryo-cooled sapphire oscillator. For comparison, we also generated low-noise microwave using a commercial Er:fiber comb stabilized in similar conditions and observed >20 dB lower phase noise in the microwave generated from the DPSSL comb. Our results confirm the high potential of the DPSSL technology for low-noise comb applications.     

Elimination of pump-induced frequency jitter on fiber-laser frequency combs

Optical frequency combs generated by femtosecond fiber lasers typically exhibit significant frequency noise that causes broad optical linewidths, particularly in the comb wings and in the carrier-envelope offset frequency (f(ceo)) signal. We show these broad linewidths are mainly a result of white amplitude noise on the pump diode laser that leads to a breathing-like motion of the comb about a central fixed frequency. By a combination of passive noise reduction and active feedback using phase-lead compensation, this noise source is eliminated, thereby reducing the f(ceo) linewidth from 250 kHz to <1 Hz. The in-loop carrier-envelope offset phase jitter, integrated to 100 kHz, is 1.3 rad.     

Generation of phase-locked and tunable continuous-wave radiation in the terahertz regime

Broadly tunable phase-stable single-frequency terahertz radiation is generated with an optical heterodyne photomixer. The photomixer is excited by two near-infrared CW diode lasers that are phase locked to the stabilized optical frequency comb of a femtosecond titanium:sapphire laser. The terahertz radiation emitted by the photomixer is downconverted into RF frequencies with a waveguide harmonic mixer and measurement-limited linewidths at the Hertz level are demonstrated.     

基于飞秒光频梳的双频He-Ne激光器频率测量

利用光纤飞秒光频梳和外腔可调谐半导体激光器, 建立了一套双频He-Ne激光器频率测量系统. 选用铷钟作为系统的频率基准, 通过将外腔半导体激光锁定至光频梳使得其频率溯源至铷钟, 再利用外腔可调谐半导体激光与双频He-Ne激光器输出的正交偏振激光拍频, 同时测量两路正交偏振激光频率. 将可调谐半导体激光器锁定至光频梳第1894449个梳齿, 其绝对频率为473612190000.0±2.7 kHz, 相对不确定度为5.7×10^-12. 对商品双频He-Ne激光器进行频率测量实验, 双频He-Ne激光器水平方向偏振激光频率均值为473612229934 kHz, 竖直方向偏振激光频率均值为473612232111 kHz, 平均时间为1024 s的相对Allan标准差为5.2×10^-11, 频差均值为2.177 MHz, 标准偏差为2 kHz.     

Optical amplifier for femtosecond frequency comb measurements near 633 nm

An optical amplifier operating at the 633-nm wavelength region is designed for absolute frequency measurements of iodine-stabilized He-Ne lasers with an optical femtosecond frequency comb generator. The same setup can also be used in other applications that require relatively high (15 mW) laser powers with good spectral characteristics. Radiation of a master laser is amplified by a microlens-coupled diode laser, which is injection locked to the frequency of the master laser. The amplifier can be reliably operated over several hours with very low phase noise. Optical amplification of more than 39 dB is demonstrated. PACS 06.20.Fn; 06.30.Ft; 42.55.Px; 42.62.Eh     

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.     

Improved stabilization of a 1.3 microm femtosecond optical frequency comb by use of a spectrally tailored continuum from a nonlinear fiber grating

We report significant enhancement (+24 dB) of the optical beat note between a 657 nm cw laser and the second-harmonic generation of the tailored continuum at 1314 nm generated with a femtosecond Cr:forsterite laser and a nonlinear fiber Bragg grating. The same continuum is used to stabilize the carrier-envelope offset frequency of the Cr:forsterite femtosecond laser and permits improved optical stabilization of the frequency comb from 1.0 to 2.2 microm. Using a common optical reference at 657 nm, a relative fractional frequency instability of 2.0 x 10(-15) is achieved between the repetition rates of Cr:forsterite and Ti:sapphire laser systems in 10 s averaging time. The fractional frequency offset between the optically stabilized frequency combs of the Cr:forsterite and Ti:sapphire lasers is +/-(0.024 +/- 6.1) x 10(-17).     

Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10^–14.     

Noise properties of an optical frequency comb from a SESAM-mode-locked 1.5-μm solid-state laser stabilized to the 10?13 level

We present a detailed investigation of the noise properties of an optical frequency comb generated from a femtosecond diode-pumped solid-state laser operating in the 1.5-??m spectral region. The stabilization of the passively mode-locked Er:Yb:glass laser oscillator, referred to as ERGO, is achieved using pump power modulation for the control of the carrier envelope offset (CEO) frequency and by adjusting the laser cavity length for the control of the repetition rate. The stability and the noise of the ERGO comb are characterized in free-running and in phase-locked operation by measuring the noise properties of the CEO, of the repetition rate, and of a comb line at 1558?nm. The comb line is analyzed from the heterodyne beat signal with a cavity-stabilized ultra-narrow-linewidth laser using a frequency discriminator. Two different schemes to stabilize the comb to a radio-frequency (RF) reference are compared. The comb properties (phase noise, frequency stability) are limited in both cases by the RF oscillator used to stabilize the repetition rate, while the contribution of the CEO is negligible at all Fourier frequencies, as a consequence of the low-noise characteristics of the CEO-beat. A?linewidth of ??150?kHz and a fractional frequency instability of 4.2×10^?13 at 1?s are obtained for an optical comb line at 1558?nm. Improved performance is obtained by stabilizing the comb to an optical reference, which is a cavity-stabilized ultra-narrow linewidth laser at 1558?nm. The fractional frequency stability of 8×10^?14 at 1?s, measured in preliminary experiments, is limited by the reference oscillator used in the frequency comparison.     

利用光频梳提高台阶高度测量准确度的方法

提出一种利用光频梳和可调谐半导体激光器提高台阶高度测量准确度的方法. 通过将可调谐激光器锁定至光频梳,可对激光器的输出波长进行精确锁定与测量.基于可调合成波长链原理,利用锁定后的半导体激光器构建了一套台阶高度测量方案,该方案可消除合成波长误差对台阶高度测量不确定度的影响. 采用一台可调谐半导体激光器和光频梳进行了5000 s的连续锁定实验, 结果表明,锁定后的可调谐半导体激光器的频率稳定度达 1.8×10^-12.该方法的理论测量不确定度约为7.9 nm, 且测量结果可溯源至时间频率基准.     

Widely tunable THz synthesizer

The generation of cw-THz radiation by photomixing is particularly suited to the high resolution spectroscopy of gases; nevertheless, until recently, it has suffered from a lack of frequency metrology. Frequency combs are a powerful tool that can transfer microwave frequency standards to optical frequencies and a single comb has permitted accurate (10⁻⁸) THz frequency synthesis with a limited tuning range. A THz synthesizer composed of three extended cavity laser diodes phase locked to a frequency comb has been constructed and its utility for high resolution gas phase spectroscopy demonstrated. The third laser diode allows a larger tuning range of up to 300 MHz to be achieved without the need for large frequency excursions, while the frequency comb provides a versatile link to be established from any traceable microwave frequency standard. The use of a single frequency comb as a reference for all of the cw-lasers eliminates the dependency of synthesized frequency on the carrier envelope offset frequency. This greatly simplifies the frequency comb stabilization requirements and leads to a reduced instrument complexity.     

Low phase noise diode laser oscillator for 1S-2S spectroscopy in atomic hydrogen

We report on a low-noise diode laser oscillator at 972?nm actively stabilized to an ultrastable vibrationally and thermally compensated reference cavity. To increase the fraction of laser power in the carrier we designed a 20?cm long external cavity diode laser with an intracavity electro-optical modulator. The fractional power in the carrier reaches 99.9%, which corresponds to an rms phase noise of φ(rms)2=1?mrad2 in 10?MHz bandwidth. Using this oscillator, we recorded 1S-2S spectra in atomic hydrogen and have not observed any significant loss of the excitation efficiency due to phase noise multiplication in the three consecutive two-photon processes.     

On the role of shot noise in carrier‐envelope phase stabilization

This paper represents a systematic investigation of detection shot noise in carrier‐envelope phase (CEP) stabilization. Numerical simulations are conducted to calculate the influence of shot noise in laser oscillators. These results are compared with experimental results for Ti:sapphire lasers. It is found that shot noise imposes a limitation for obtaining sub‐100 mrad CEP jitters. Careful interferometer design is necessary to push this limit toward 10 mrad. In contrast to oscillator stabilization, shot noise appears to play a much more restrictive role in amplifier stabilization. Using spectral interferometry together with spectral broadening in sapphire, it already appears practically challenging to reach sub‐100 mrad jitters. Adaption of the optical nonlinearity in the broadening step appears key to further improvements of the CEP jitter of amplified systems. We believe that these improvements open a perspective for currently unfeasible applications of CEP stabilized pulses. Moreover, our considerations can be easily adapted to CEP stabilization of other laser types beyond Ti:sapphire.     

Demonstration of a transportable 1 Hz-linewidth laser

We present the setup and test of a transportable clock laser at 698 nm for a strontium lattice clock. A master-slave diode laser system is stabilized to a rigidly mounted optical reference cavity. The setup was transported by truck over 400 km from Braunschweig to Düsseldorf, where the cavity-stabilized laser was compared to a stationary clock laser for the interrogation of ytterbium (578 nm). Only minor realignments were necessary after the transport. The lasers were compared using a Ti:Sapphire frequency comb as a transfer oscillator. The generated virtual beat showed a combined linewidth below 1 Hz (at 1156 nm). The transport back to Braunschweig did not degrade the laser performance, as was shown by interrogating the strontium clock transition.     

Accuracy of optical frequency comb generators and optical frequency interval divider chains

We compared two methods for measuring large optical frequency differences: an optical frequency comb generator, which creates a large number of sidebands from a single-mode laser through electro-optic modulation, and an optical frequency interval divider chain, which divides a frequency gap successively by two until it becomes accessible to a radio-frequency counter. By locking two diode lasers to two modulation sidebands of a comb generator, ~1 THz apart, and measuring this interval with a chain of four phase-locked interval dividers, we demonstrate for the first time to our knowledge the accuracy of both techniques within an experimental limit of 6.8 x 10(-15).