Iodine stabilized dye laser system for frequency measurements in the visible and near IR region of the spectrum

An iodine stabilized dye laser system is described that provides traceable measurement of reference frequencies in the visible spectrum from 540 to 670 nm and in the near infrared at 1.15 /spl mu/m. The system allows calibration of the widely used 633 nm, 612 nm, and 543 nm HeNe laser systems. Also, frequency measurements of a polarization stabilized 1153 nm HeNe laser have been performed via frequency doubling and comparison with the dye system operating on the corresponding 576 nm lines. Studies of the shift sensitivities of the system at various wavelengths of interest are described for variation of iodine cell pressure, laser modulation amplitude, and optical saturation power. The dye system was also stabilized to hyperfine components associated with the 6-3 P(33) iodine transition and compared with a 633 nm iodine stabilized HeNe standard.     

633nm He—Ne碘稳频激光波长的国际比对

１９９７年７月,在北京中国计量科学研究院进行了由国际计量局组织的碘稳频６３３ｎｍＨｅ－Ｎｅ激光波长基准的多边国际比对,进行了频率稳定度、频差及相应的压力位移、功率位移和调制位移测量实验。测量工作以ＢＩＰＭＰ３激光器为参考,最后给出以ＢＩＰＭ４激光器为基准的最终测量结果。     

Iodine-stabilized extended cavity diode laser at λ=633 nm

An extended cavity diode laser at 633 nm has been frequency stabilized to I₂-Doppler-free absorption signals of the P(33)6-3 transition using a third-harmonic detection technique. The frequency was measured by the beat-frequency technique with an iodine-stabilized He-Ne laser as reference. A minimum value for the two-probe relative standard uncertainty of 1× 10^-11 (5 kHz) is reached after 100 s. We also report measurements of the hyperfine splittings of the P(33)6-3 transition and laser frequency dependence on modulation amplitude and iodine pressure     

Three-longitudinal-mode He-Ne laser frequency stabilized at 633 nm by thermal phase locking of the secondary beat frequency

We present a simple scheme for thermal frequency stabilization of a three-longitudinal-mode He-Ne laser at 633 nm with improved short-term (5 x 10(-11) at 1-s average time) and long-term (124-kHz standard deviation for 10 h) frequency stability. A stabilized output power of 3 mW was readily obtained from the central mode by polarization-mode selection. The optical frequency of the central mode could be precisely tuned by an external frequency reference over 160 MHz with high precision or could be optically phase locked to a reference laser by use of a thermal frequency-stabilization circuit and an external acousto-optic modulator. The laser will be useful in various applications for which a laser source with high power and frequency stability is necessary.     

Precision frequency measurement of the 2S1/2 2D5/2 transition of Sr+ with a 674-nmdiode laser locked to an ultrastable cavity

We have measured the frequency of the 5s ²S_1/2 -4d ²D_5/2 clock transition of a single Sr ion confined in a Paul trap. A diode laser locked to an ultrastable Fabry-Perot (F-P) cavity was used to probe the transition with a resolution of 3.5 kHz. The absolute frequency was determined from heterodyne measurements referenced to an iodine stabilized HeNe laser and a CO₂ laser yielding a value for the S-D transition of (444 779 043 963±30) kHz. This work could lead to the development of a new optical frequency standard at 674 nm     

Frequency stabilization of a diode-pumped Nd:Yag laser at 532 nm to iodine by using third-harmonic technique

A compact frequency standard was constructed by stabilizing the frequency of a diode-pumped Nd:YAG laser to the Doppler-free spectrum of iodine at 532 nm. The performance of the laser and the stability and repeatability of the stabilization scheme were investigated. The dependence of the laser frequency on such parameters as pressure of the iodine cell, modulation amplitude, and pressure were also studied. The results show that by using standard third-harmonic locking technique stability and repeatability comparable to more elaborate iodine-stabilized Nd:YAG laser systems can be achieved in a portable, and relatively simple and inexpensive, setup.     

Optical Frequency Metrology of an Iodine-Stabilized He-Ne Laser Using the Frequency Comb of a Quantum-Interference-Stabilized Mode-Locked Laser

We perform optical frequency metrology of an iodine-stabilized He-Ne laser using a mode-locked Ti:sapphire laser frequency comb that is stabilized using quantum interference of photocurrents in a semiconductor. Using this technique, we demonstrate carrier-envelope offset frequency fluctuations of less than 5 mHz using a 1 s gate time. With the resulting stable frequency comb, we measure the optical frequency of the iodine transition [¹²⁷I₂ R(127) 11-5 i component] to be 473 612 214 712.96 ± 0.66 kHz, well within the uncertainty of the CIPM recommended value. The stability of the quantum interference technique is high enough such that it does not limit the measurements.     

Intracavity iodine cell spectroscopy with an extended-cavity laserdiode around 633 nm

The purpose of the work described was to develop a tunable laser diode which is as easy to use as a He-Ne laser stabilized on iodine. The particularity of this experiment is the use of I₂ cell placed inside the extended cavity laser (ECL). The experimental set-up mounted takes the form of a lambdameter which gives the wavelength to one part in 10⁶, a Fabry-Perot (FP) interferometer for mode analysis and a beat-frequency measurement system. The FP interferometer has the particular feature of a second I₂ cell which makes it possible to detect the iodine transitions by simple visual inspection of an oscilloscope. Many transitions, some much more intense than the usual R(127)11-5 transition, are under study. We worked mainly on P(33) 6-3 since this is inside the range of our beat-frequency system. Results are promising: the free running relative frequency stability is about three parts in 10¹⁰ and the locked-laser relative frequency stability about three parts in 10¹¹ after 80 s. This is already good enough for use as a wavelength standard for length and for interferometric measurement     

Frequency shifts of iodine stabilized He-Ne lasers at higherharmonic order stabilization

The frequency shifts of an ¹²⁷I₂ stabilized He-Ne laser at 633 nm have been measured at the third, fifth, and seventh harmonic stabilization using a digital signal processing lock-in amplifier. The observed frequency shifts confirmed the previous theoretical prediction that the laser frequency depends on the harmonic order of stabilization. The lower effects of modulation amplitude shift, power shift, and iodine pressure shift at the fifth harmonic stabilization indicate a potential improvement of the frequency reproducibility of the laser standard. A model is suggested to describe the power shifts of the laser. The mechanism explains the lower power shift of some lasers when they are operated at relatively high intracavity power     

Optical frequency standard at 532 nm

We have constructed a frequency-doubled Nd:YAG laser source stabilized via modulation transfer spectroscopy to hyperfine absorption peaks in molecular iodine. The system is suitable for use as an optical frequency standard because of the excellent stability (σ<1×10^-13 for τ⩾1 s) and reproducibility (75 Hz per ir laser) observed between two independent dissimilar systems. With heterodyne techniques, we have measured the hyperfine splittings of seven rovibrational transitions in ¹²⁷I₂. A computer fits the data for the hyperfine coupling constants based on a four term Hamiltonian. The residuals from these fits have a standard deviation as low as 520 Hz, displaying the best reported agreement to date with the quadrupolar hyperfine Hamiltonian. Modulation transfer lineshapes have been accurately recorded using rf offset phase-locking techniques with our two systems. Pressure shift (-1.3 kHz/Pa) and broadening (74 kHz/Pa) of the a₁ component of R(56)32-0 have been derived from these lineshape measurements     

Frequency stabilization of internal-mirror He-Ne lasers by air cooling

Instead of the traditional heating method, the cavity length of an internal-mirror He-Ne laser is controlled by air cooling which is implemented by a mini cooling fan. The responsive property of the cooling fan and the thermal expansion of the internal-mirror laser tube are investigated. According to these investigations, a controlling system is designed to drive the cooling fan controlling the cavity length of the laser. Then the frequency is stabilized by comparing the light intensities of two operating longitudinal modes. The results of beating with an iodine stabilized He-Ne laser show that a relative uncertainty (Δf/f-) of 4.3×10^-9 in 5 months, a frequency fluctuation of <1.4 MHz, and an Allan deviation of 6×10^-11 (τ=10,000 s) in 20?h are obtained.     

New Developments in Iodine-Stabilized He-Ne Lasers

The improvement of the performance of a 633-nm He-Ne laser stabilized by saturated absorption in iodine, which results from heating the iodine cell is described. A simple modification of the laser which permits frequency-stabilized operation at two other visible wavelengths which may be useful for interferometry is also reported.     

Absolute frequency control of a 1560 nm (192 THz) DFB laser lockedto a rubidium absorption line using a second-harmonic-generated signal

We demonstrate second-harmonic generation from a DFB laser at 1560 nm in a type I critically phase-matched KNbO₃ crystal. We obtain 2.2 nW at 780 mm with 11.3 mW at 1560 nm incident on the crystal. The conversion efficiency is 17.2 pW/(mW)². The 780 nm beam is used to interrogate a resonance of the ⁸⁷Rb-D₂ line at 780 nm and lock the laser frequency. To characterize the absolute frequency stability, two 1560 nm DFB lasers are respectively stabilized on a Doppler resonance of the ⁸⁷Rb-D₂ line (780.246 nm) and of the ⁸⁵Rb-D₂ line (780.244 nm). The square root of the Allan variance measured from the beat note is around 1.5×10^-9 for averaging times between 3 and 100 s. To improve the precision of the frequency locking, we realize a setup to observe a saturated absorption profile. We use a 780 nm stabilized laser as a pump and the SHG signal as a probe. A saturated absorption profile is observed over the Doppler envelope. Work is under progress to use this saturated resonance for an improved frequency control     

1.5μm光纤通信波段乙炔稳频激光技术研究进展

1.5μm乙炔饱和吸收谱线是国际计量委员会(CIPM)正式推荐的光纤通信波段复现‘米’定义的频率参考标准。乙炔稳频激光依据稳频方法可分为线性吸收和饱和吸收两大类,饱和吸收相比线性吸收,能够消除乙炔分子的多普勒效应,获得线宽更窄、频率稳定度和复现性更高的稳频激光,1s频率稳定度能够达到10^-13量级,波长漂移为10^-12量级。利用13C2H2(ν1+ν3)P(16)谱线,研发的微型气室有望实现稳频激光的全光纤链路传播,为高度集成化、抗干扰能力强的稳频激光源提供了新的发展方向。高性能的1.5μm近红外稳频激光直接为密集波分复用系统、精密光纤传感等多个领域提供波长参考源,结合飞秒激光频率梳技术可进一步完善光纤通信中激光波长量值传递溯源体系,提升激近红外波段光波长的测量能力,为光纤波段的精密测量提供量值保障。     

Improved frequency stability of an external cavity diode laser by eliminating temperature and pressure effects

The construction of a passively stabilized external cavity diode laser operating at 780 nm is reported. The sensitivity of laser frequency to changes in air pressure was studied and subsequently eliminated. The relative frequency stability obtained was 4 × 10(-9) for an integration time of 4000 s.     

拉线传感器变角度工作条件下的校准

为了解决静强度变形测量试验中,拉线传感器两维角度偏转引入的测量问题,采用转台提供标准偏转角度、拉线传感器正交安装测试的方法,突破了变角度工作条件下的拉线传感器校准技术,提出了基于正交安装的灵敏度系数校准方法,构建了校准装置,并通过两个正交方向的灵敏度系数合成,提出了拉线传感器正交误差模型的建立方法,最终为拉线传感器变角度工作条件下的现场误差测算提供了方案,实现了静强度试验中变形量测量的快速评定。拉线传感器校准方法的探索,对其在测试领域的应用起到了促进作用。     

Frequency stabilization of an external cavity diode laser to molecular iodine at 657.483 nm

The saturation spectrum of the P(84) 5-5 transition of 127I2 at 657.483 nm is obtained with the third-harmonic demodulation method using an external cavity diode laser. The laser frequency is modulated by modulating the diode current instead of modulating the cavity length with a piezoelectric transducer (PZT). Current modulation allows a modulation frequency that is higher than PZT modulation. The signal-to-noise ratio of 1000 is better than previous results presented in the literature. The laser is frequency stabilized to the hyperfine component o of the P(84) 5-5 transition with a frequency stability of better than 10 kHz (2.2 x 10(-11) relative stability).     

Frequency stabilization of a diode laser with a thin Cs-vapor cell

We propose a novel method of stabilizing laser oscillation frequency that uses a sub-Doppler spectrum of atoms in a thin vapor cell. An extended-cavity diode laser is frequency-locked to a hyperfine component of the Cs-D(2) line. In the Allan-variance measurements on the beat note between two lasers thus stabilized, a frequency stability of 6.6x10(-11) is achieved at an averaging time of 5.8 s. The frequency can be controlled even when the laser beam intensity is as small as 70 nW/cm(2).     

A Stabilized HCN Laser for Infrared Frequency Synthesis

Infrared frequencies have recently been synthesized in suitable diodes up to 88 THz with accuracies of parts in 109. Stabilized lasers are necessary in order to make frequency measurements of higher accuracy. The hydrogen-cyanide laser is the lowest frequency basis laser used in these synthesis schemes, and its stabilization has been the subject of recent interest. The laser is stabilized by locking it to a phase-locked microwave reference chain. Two servo loops are utilized. The first loop is a relatively slow frequency-lock loop with the correction applied to a piezoelectric-translator driver. This loop not only accommodates thermal expansion of the laser, but also serves as an acquisition aiding loop for the second servo. The latter is a phase-locked system with the correction applied to the laser discharge current controller. Data regarding the system stability are presented.     

Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering

A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency (<500 MHz) region with excellent long-term stability has been demonstrated for distributed measurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouin fiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shifted local oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scattering with high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distributed measurements of both temperature and strain over long fiber has been developed utilizing advanced digital signal processing techniques.