High Long-term Frequency Stability Research Articles

Frequency stabilization of multiple wavelength lasers based on a broadband spectrum

We report on frequency stabilization of multiple wavelength lasers operating at 1389 and 1695 nm simultaneously on a broadband spectrum. These lasers are implemented in ytterbium optical lattice clock experiments, which need to have a narrow enough linewidth and maintain high long-term frequency stability. A 1560 nm femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser, which is referenced to a local ultrastable optical cavity with the instability better than 1 × 10−15 at 1 s averaging time. Through the combination of erbium-doped fiber amplifier and high nonlinear fiber, the spectral width of the maser laser is broadened from 10 nm to more than 300 nm. The range of the broadened spectrum can cover 1389 and 1695 nm. Meanwhile, the spectral intensity at the corresponding wavelength can ensure that the signal-to-noise ratio of the beat signals between the two lasers and the broadened spectrum is about 30 dB at a resolution bandwidth (RBW) of 100 kHz. After phase locking the 1389 and 1695 nm lasers on the broadband spectrum, the residual linewidths are obtained to be about 0.8 Hz at 1 Hz RBW, and the stabilities are 3.5 × 10−16 and 4.7 × 10−16 at 1 s averaging time respectively, improving about six orders of magnitude. Our result can be conducive to obtaining the stabilized laser sources for the atomic optical clock, and will be of great significance for simplifying and miniaturizing the optical clock system.

Frequency shift of an optical frequency standard as a function of probe modulation of the radiation frequency, pressure, and gas temperature in an absorbing cell

Frequency shifts of an optical Nd : YAG/I2-standard are measured in a pressure range of molecular iodine 5 – 22 mTorr and deviation of probe modulation frequency 480 kHz – 4 MHz for the hyperfine structure component a1 of the molecular iodine absorption line R(56) 32 – 0. The frequency shift of the optical standard is estimated under varied temperatures of a cell and cell finger, which determines the pressure of the molecular iodine vapour. The requirements are defined to temperature stability of the cell and its finger for obtaining high long-term frequency stability of the optical frequency standard.

Continuous-Wave Sub-THz Photonic Generation With Ultra-Narrow Linewidth, Ultra-High Resolution, Full Frequency Range Coverage and High Long-Term Frequency Stability

We report on a photonic system for generation of high quality continuous-wave (CW) sub-THz signals. The system consists on a gain-switching-based optical frequency comb generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using Fabry-Perot tunable filters (FPTFs) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth <;10 Hz at 120 GHz, complete frequency range coverage (60-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz (10-12 of generated frequency), high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, offers a very high performance, especially suited for sub-THz photonic local oscillators and high resolution spectroscopy.

Investigation of Short-term Amplitude and Frequency Fluctuations of Lasers for Interferometry

One of the limiting factors of accuracy and resolution in laser interferometry is represented by noise properties of the laser powering the interferometer. Amplitude and especially frequency fluctuations of the laser source are crucial in precision distance measurement. Sufficiently high long-term frequency stability of the laser source must be achieved especially in applications in fundamental metrology. Furthermore, the short-term frequency variations are also important primarily for measurements done at high acquisition speeds. This contribution presents practical results of measurements of short-term amplitude and frequency noises of a set of laser sources commonly used in laser interferometry. The influence of the interferometer design and electrical parameters of the detection system are also discussed.

Optical Frequency Comb as a general-purpose and wide-band calibration source for astronomical high resolution infrared spectrographs

A spectrally filtered Optical Frequency Comb (OFC) laser is proposed as a versatile calibration source for astronomical spectrometers in the 1–2 μm spectral range. Such a source overcomes the limitations of current calibration lamps providing a uniform spectrum of equally spaced lines with similar intensity and extremely high long-term frequency stability. We present preliminary studies and results of a system which filters the OFC from a 100 MHz comb spacing to 16 GHz one, an adequate spacing for spectrometers with resolving power Δλ/λ >30000. The first approach employs two Fabry-Perot cavities in series, made of dielectric coated mirrors, followed by a non-linear optical broadening system. The limitations of such a filtering process are discussed. These can be overcome by the second approach, based on filtering cavities with metallic coated mirrors.

Cryogenic sapphire oscillator with exceptionally high long-term frequency stability

The authors report on the development of a sapphire cryogenic microwave resonator oscillator with long-term fractional frequency stability of 2×10−17√τ for integration times τ&amp;gt;103s and a negative drift of about 2.2×10−15∕day. The short-term frequency instability of the oscillator is highly reproducible and also state of the art: 5.6×10−16 for an integration time of τ≈20s.

Laser frequency standards

This paper gives a brief review of the present status of the problem of developing optical frequency standards. The basic principles of laser-frequency stabilization based on narrow nonlinear optical resonances are presented. The basic physical factors determining the shape and the shift of narrow resonances are discussed. The design of the modern laser frequency standard, based on the use of supernarrow resonances with a relative width of 10−11 and smaller as the reference, is studied. The currently used techniques for stabilizing the frequency of lasers for different ranges and the methods used for measuring the characteristics of the frequency stability are described. Specific achievements in the development of lasers with a narrow radiation line and high long-term frequency stability and reproducibility are presented. Special attention is devoted to the He–Ne laser. For this laser a radiation line width of less than 0.1 Hz and a long-term stability of 10−14–10−15 have been obtained, and the effect of different factors on the shifts of the stabilized frequency are studied in detail. Results achieved in the development of the first optical clocks in the world are presented. The progress achieved in the absolute measurement of submillimeter, IR, and visible range laser frequencies is examined. Some applications of laser frequency standards and the prospects for their further development are described briefly.