Optical coherence transfer over 50-km spooled fiber with frequency instability of 2×10~(-17) at 1s

We demonstrate coherent transfer of an ultra-stable optical frequency at 192.8 THz over 50-km spooled fiber. Random phase noise induced by environmental disturbance through fiber is detected and suppressed by feeding a correctional signal into an acousto-optic modulator. After being compensated, the fiber-induced frequency instability is 2×10-17 at 1-s averaging time and reaches 8×10-20 after 16 h. The noise floor of the compensation system could be as low as 2×10-18 at 1-s averaging time.     

Laser frequency instability of 6×10^-16 using 10-cm-long cavities on a cubic spacer

We demonstrate two ultra-stable laser systems at 1064 nm by independently stabilizing two 10-cm-long Fabry–Pérot cavities.The reference cavities are on a cubic spacer,which is rigidly mounted for both low sensitivity to environmental vibration and ability for transportation.By comparing against an independent ultra-stable laser at 578 nm via an optical frequency comb,the 1064 nm lasers are measured to have frequency instabilities of 6×10^-16 at 1 s averaging time.     

A 532 nm molecular iodine optical frequency standard based on modulation transfer spectroscopy

We report construction of an iodine-stabilized laser frequency standard at 532 nm based on modulation transfer spectroscopy(MTS) technology with good reproducibility. A frequency stability of 2.5 × 10~(-14) at 1 s averaging time is achieved,and the frequency reproducibility has a relative uncertainty of 3.5×10~(-13), demonstrating the great stability of our setup.The systematic uncertainty of the iodine-stabilized laser frequency standard is evaluated, especially the contribution of the residual amplitude modulation(RAM). The contribution of the RAM in MTS cannot be evaluated directly. To solve this problem, we theoretically deduce the MTS signal with RAM under large modulation depth, and prove that the nonsymmetric shape of the MTS signal is directly related to the MTS effect. The non-symmetric shape factor r can be calibrated with a frequency comb, and in real experiments, this r value can be obtained by least-squares fitting of the MTS signal,from which we can infer the RAMinduced frequency shift. The full frequency uncertainty is evaluated to be 5.3 k Hz(corresponding to a relative frequency uncertainty of 9.4×10~(-12)). The corrected transition frequency has a difference from the BIPM-recommended value of 2 k Hz, which is within 1σ uncertainty, proving the validity of our evaluation.     

Bidirectional erbium-doped fiber amplifiers used in joint frequency and time transfer based on wavelength-division multiplexing technology

In an experiment on the round-trip fiber transfer of joint frequency and time signals based on wavelengthdivision multiplexing technology, a specific bidirectional erbium-doped fiber amplifier(Bi-EDFA) with low noise and high symmetry simultaneously is designed and applied to compensate for the loss of the link. The Allan deviation(ADEV) deterioration of the 1 GHz frequency signal induced by the Bi-EDFA is only 8 × 10-15 at 1 s and 9 × 10-18 at 104s in the forward direction, and is 1.7 × 10-14 at 1 s and 1.2 × 10-17 at 104s in the backward direction. The degraded time deviation(TDEV) caused by the asymmetry of the Bi-EDFA is only 0.8 ps at an average time of 103 s. With the proposed Bi-EDFA, in the field experiment on the 110 km fiber transfer of joint frequency and time signals, the ADEV of the 1 GHz frequency signal is 7.3 × 10-14at1 s and 2.5 × 10-17 at 104s. The TDEV of the 1 pulse per second time signal is 6.8 ps with an average time of 103 s.     

Coherence transfer from 1064 nm to 578 nm using an optically referenced frequency comb

A laser at 578 nm is phase-locked to an optical frequency comb(OFC) which is optically referenced to a subhertzlinewidth laser at 1064 nm. Coherence is transferred from 1064 nm to 578 nm via the OFC. By comparing with a cavitystabilized laser at 578 nm, the absolute linewidth of 1.1 Hz and the fractional frequency instability of 1.3 × 10-15 at an averaging time of 1 s for each laser at 578 nm have been determined, which is limited by the performance of the reference laser for the OFC.     

Transferring the stability of iodine-stabilized diode laser at 634 nm to radio frequency by an optical frequency comb

An optical flequency comb phase-locked on an iodine frequency stabilized diode laser at 634 nm is constructed to transfer the accuracy and stability from the optical domain to the radio frequency domain. An external-cavity diode laser is frequency-stabilized on the Doppler-free absorption signals of the hyperfine transition R（80）8-4 using the third-harmonic detection technique. The instability of the ultra-stable optical oscillator is determined to be 7 ×10^-12 by a cesium atomic clock via the optical frequency comb＇s mass frequencv dividing technique.     

Widely tunable laser frequency offset locking to the atomic resonance line with frequency modulation spectroscopy

A simple and robust technique is reported to offset lock a single semiconductor laser to the atom resonance line with a frequency difference easily adjustable from a few tens of megahertz up to tens of gigahertz. The proposed scheme makes use of the frequency modulation spectroscopy by modulating sidebands of a fiber electro-optic modulator output. The short-term performances of a frequency offset locked semiconductor laser are experimentally demonstrated with the Allan variance of around 3.9 × 10~(-11) at a 2 s integration time. This method may have many applications, such as in Raman optics for an atom interferometer.     

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     

A Hertz-Linewidth Ultrastable Diode Laser System for Clock Transition Detection of Strontium Atoms

The frequencies of two 698 nm external cavity diode lasers （ECDLs） are locked separately to two independently located ultrahigh finesse optical resonant cavities with the Pound Drever-Hall technique. The linewidth of each ECDL is measured to be -4.6 Hz by their beating and the fractional frequency stability below 5 × 10^-15 between 1 s to lOs averaging time. Another 698nm laser diode is injection locked to one of the cavity-stabilized ECDLs with a fixed frequency offset for power amplification while maintaining its linewidth and frequency characteristics. The frequency drift is H1 Hz/s measured by a femtosecond optical frequency comb based on erbium fiber. The output of the injection slave laser is delivered to the magneto-optical trap of a Sr optical clock through a iO- ta-long single mode polarization maintaining fiber with an active fiber noise cancelation technique to detect the clock transition of Sr atoms.     

A long-term frequency-stabilized erbium-fiber-laser-based optical frequency comb with an intra-cavity electro-optic modulator

We demonstrate an optical frequency comb based on an erbium-doped-fiber femtosecond laser with the nonlinear polarization evolution scheme.The repetition rate of the laser is about 209 MHz.By controlling an intra-cavity electrooptic modulator and a piezo-transducer,the repetition rate can be stabilized with a high-bandwidth servo in a frequency range of 3 k Hz,enabling long-term repetition rate phase-locking.The in-loop frequency stability of repetition rate is about1.6×10-13 in an integration time of 1 s,limited by the measurement system;and it is inversely proportional to integration time in the short term.Furthermore,using a common path f –2 f interferometer,the carrier envelope offset frequency of the comb is obtained with a signal-to-noise ratio of 40 d B in a 3-MHz resolution bandwidth.Stabilized carrier envelope offset frequency exhibits a deviation of 0.6 m Hz in an integration time of 1 s.     

Frequency Stability of Atomic Clocks Based on Coherent Population Trapping Resonance in ^85Rb

An atomic clock system based on coherent population trapping （CPT） resonance in 85Rb is reported, while most past works about the CPT clock are in ST Rb. A new modulation method （full-hyperfine-frequency-splitting modulation） is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in SSRb. The experimental results show that the short-term frequency stability of the CPT clock in SSRb is in the order of 10^-10/s and the long-term frequency stability can achieve 1.5 × 10^-11/80000s, which performs as well as 87 Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of SSRb indicates that an atomic clock based on CPT in SSRb should be a promising candidate for making the chip scale atomic clock.     

利用传输腔技术实现镱原子光钟光晶格场的频率稳定

为了获得高稳定度和高精确度的原子光晶格钟,光晶格场的频率必须得到锁定,线宽必须控制到特定水平用来消除交流斯塔克频移.本文提出利用传输腔技术来实现对镱原子光钟的光晶格场的频率锁定和抑制频率长期漂移的锁定方案.首先,将一个殷钢材料的传输腔锁定在基于调制转移谱技术锁定的780 nm激光场上,再将759 nm的光晶格光场锁定在传输腔上.实验结果表明,光晶格光场的线宽可以锁定和控制在1 MHz以下.光晶格光场与锁定于氢钟的光梳拍频结果显示,光晶格光场的长期频率稳定度优于3.6×10~(-10),可以确保实现镱原子光钟的不确定度进入10~(-17).     

Single-Photon Detection at Telecom Wavelengths

A single-photon detector based on an InGaAs avalanche photodiode has been developed for use at telecom wavelengths. A suitable delay and sampling gate modulation circuit are used to prevent positive and negative transient pulses from influencing the detection of true photon induced avalanches. A monostable trigger circuit eliminates the influence of avalanche peak jitter, and a dead time modulation feedback control circuit decreases the afterpulsing. From performance tests we lind that at the optimum operation point, the quantum efficiency is 12% and the dark count rate 1.5 × 10^-6 ns^-1, with a detection rate of 500 kHz.     

Improvements and New Evaluation of NIM4 Caesium Fountain Clock at NIM in 2005-2006

The NIM4 caesium fountain clock has been operating stably and sub-continually since August 2003. We present our improvements on NIM4 in 2005-06 and the most recent evaluation for its frequency shifts with an uncertainty of 5 × 10^-15. A 203-day comparison between NIM4 and GPS time shows an agreement of 2 × 10^-14. Finally the construction of the NIM5 transportable caesium fountain clock is briefly reported.     

Wavelength-modulation spectroscopy using a frequency-doubled current-modulated diode laser

We have studied atomic absorption in an argon discharge by wavelength-modulation spectroscopy with a frequency-doubled diode laser. The tunable wavelength-modulated radiation at 430 nm was generated by frequency doubling a current-modulated 860-nm diode laser in a KNbO₃ crystal. 2f-, 4f- and 6f-harmonic spectra as a function of diode laser modulation depth were measured on a Doppler-broadened sample of excited argon atoms produced in a capacitively coupled plasma chamber. Characterisation of the harmonic signals was accomplished. Minimum detectable absorbances of 7.7×10^-5 and 1.9×10^-4 based on a 3σ criterion (σ being the standard deviation of the noise) were estimated for 2f- and 4f-harmonic detection of the frequency-doubled radiation with a time constant of 0.1 s. The concentrations of argon in the 1s₄ state were found to be in the range of 3×10⁸ to 1.2×10¹¹ cm^-3 for the experimental conditions studied. Received: 25 February 2002 / Revised version: 4 April 2002 / Published online: 14 May 2002     

Multilayer Au/TiO2Composite Films with Ultrafast Third-Order Nonlinear Optical Properties

We report on the ultrafast third-order optical nonlinearity in multilayer Au/TiO2 composite films fabricated on quartz substrates by pulsed laser deposition technique. The linear optical properties of the films are determined and optical absorption peaks due to surface plasmon resonance of Au particles are observed at about 590hm. The third-order optical nonlinearities of the films are investigated by z-scan method using a femtosecond laser （50 fs） at the wavelength of 800 nm. The sample showed fast nonlinear optical responses with nonlinear absorption coefficient and nonlinear refractive index being -3.66 × 10^-10 m/W and -2.95 × 10^-17 m^2/W, respectively. The results also show that the nonlinear optical effects increase with the increasing Au concentration in the composite films.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

A narrow linewidth and frequency-stable probe laser source for the <Superscript>88</Superscript>Sr<Superscript>+</Superscript> single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s ² S _1/2–4d ² D _5/2 clock transition of the ⁸⁸Sr⁺ optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the ⁸⁸Sr⁺ clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5×10⁻¹⁶ at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Δν/ν=1×10⁻¹⁴) on the ⁸⁸Sr⁺ clock transition. The effective averaging time during the linewidth measurements was about 100 s.     

Effects of Proton Irradiation on a CMOS Image Sensor

We perform 9 MeV proton irradiation of a complementary metal oxide semiconductor （CMOS） image sensor at doses from 1 × 10^9 to 4 × 10^10 cm^-2. In general, the average brightness of dark output images increases with an increasing dose, and reaches the maximum at 1 × 10^10 cm^-2. The captured colour images become very blurry at 4 × 10^10 cm^-2. These can be explained by change of concentrations of irradiation-induced electron-hole pairs and vacancies in the various layers of CMOS image sensor calculated by the TRIM simulation programme with dose.