Absolute Laser-Frequency Measurements

Several other recently developed techniques for absolute determination of the wavelengths of single-mode lasers have already been discussed, in Sect.4.4. A very precise method competing with these wavelength measurements 

This method [6.34] relies on the fact that of all physical quantities, it is the frequency which can be measured most accurately. With modern fast counters, frequencies up to 500 MHz can be counted directly and calibrated against frequency standards. At higher frequencies, a heterodyne technique may be used whereby the difference between the unknown frequency and a known, nearly equal frequency is generated by a nonlinear detector and can be counted directly. The known frequency is synthesized from two or more known lower frequencies by a nonlinear device which can generate harmonics or which can sum different frequencies [6.35]. 

Several of these nonlinear devices have been developed to generate sum 

Assume that two lower frequency radiations, with known frequencies and 2 (e.g., the radiation from two far infrared lasers), impinge on the diode, together with the laser of unknown frequency v. The nonlinear response of the diode generates harmonics mv and nv2 of the lower frequency radiations, and by difference-frequency generation, the beat frequency 

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Laser Physics (ILP), Novosibirsk, including different methods of frequency stabilization [3,4], measurements of hyperfine line separations or frequency intervals between absorption lines [5,6, ] and absolute optical frequency measurements [8,9,10,11]. As a result of these efforts, the Comite Consultatif des Longueurs (CCL) meeting in 1997 recommended the frequency of one particular component, the aio hyperfine structure (HFS) component of the R(56)32-0 transition, for the realization of the metre with a relative standard uncertainty of 7 x 10-11 [ ] ... 

Udem, T. Reichert, J. Holzwarth, R. Hansch, T.W. Absolute optical frequency measurement of the caesium D1 line with a mode-locked laser. Phys. Rev. Lett. 1999, 82, 3568-3571. 

The data recorded as the laser frequency is scanned consists of the fluorscence signal from the PMT, a Doppler-free I2 spectrum and frequency markers from the etalon. The etalon provides a calibration of the frequency scan. The Doppler-free I2 spectra provides an absolute frequency reference used to correct for small laser frequency drifts, separator voltage drifts and to determine the absolute acceleration voltage of the separator for the Doppler shift corrections. We are thus able to record data over long periods of time, e.g. 3 hours, and maintain a reasonable resolution of 100 MHz. Some of the first online data recorded with this system is shown in Figure 2. The overall detection efficiency has been measured to be 1/1000, i.e. one detected photon per 1000 atoms, for the largest transition in the nuclear spin 1/2 isotopes. 

The absolute frequency of the fundamental IS — 2S transition in atomic hydrogen has now been measured to 1.8 parts in 1014, an improvement by a factor of 104 in the past twelve years. This improvement was made possible by a revolutionary new approach to optical frequency metrology with the regularly spaced frequency comb of a mode locked femto-second multiple pulsed laser broadened in a non-linear optical fiber. Optical frequency measurement and coherent mixing experiments have now superseded microwave determination of the 2S Lamb shift and have led to improved values of the fundamental constants, tests of the time variation of the fine structure constant, tests of cosmological variability of the electron-to-proton mass ratio and tests of QED by measurement of g — 2 for the electron and muon. 

Abstract. We present a review of the helium spectroscopy, related to transitions between 23S and 23P states around 1083 nm. A detailed description of our measurements, that have produced the most accurate value of the 23Po — 23Pi fine structure interval, is given. It could produce an accurate determination (34 ppb) of the fine structure constant a. Improvements in the experimental set up are presented. In particular, a new frequency reference of the laser system has been developed by frequency lock of a 1083 nm diode laser to iodine hyperfine transitions around its double of frequency. The laser frequency stability, at 1 s timescale, has been improved of, at least, two orders of magnitude, and even better for longer time scales. Simultaneous 3He —4 He spectroscopy, as well as absolute frequency measurements of 1083 nm helium transitions can be allowed by using the Li-locked laser as frequency standard. We discuss the implication of these measurements for a new determination of the isotope and 23 5 Lamb shifts. 

Moreover, the tunability of the developed green laser and the rich density of I2 absorptions give a grid of frequency references at 1083 nm as we show in Fig. 9. It can be used for other frequency difference measurements, such as 3He hyperfine splittings and 3He-4He isotope shifts. This shift is large enough to allow a precise determination of the relative nuclear radii of these atoms, and it is also used to test QED corrections [23]. But, perhaps, the most important advantage of the new reference, is the possibility of absolute frequency measurements of... 

Frequency Comparison and Absolute Frequency Measurement of 12-stabilized Lasers at 532 nm... 

Abstract. We present a frequency comparison and an absolute frequency measurement of two independent -stabilized frequency-doubled Nd YAG lasers at 532 nm, one set up at the Institute of Laser Physics, Novosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the l2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R.(56)32-0 aio component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured. 

We present a frequency comparison of two independent -stabilized Nd YAG lasers at 532 nm and an absolute frequency measurement of the laser frequencies which were locked to different HFS components of the R(56)32-0 and P (54)32-0 iodine absorption line. The absolute frequencies have been determinded using a phase-coherent frequency chain which links the 12-stabilized laser frequency to a CH4-stabilized He-Ne laser at 3.39 pm. This laser had been calibrated before the measurements against an atomic cesium fountain clock. 

In order to measure the absolute frequencies of the iodine spectrometers, we employed a frequency chain which links the Nd YAG laser frequencies to a CH4 stabilized He-Ne laser at 3.39 /rm (Fig. 3). This laser was set up at the Institute of Laser Physics in Novosibirsk, Russia [17], and has been calibrated previously (1996) for a measurement of the hydrogen IS - 2S absolute frequency [18]. In... 

In order to verify this result, we measured the frequency gap using a different technique while one ILP laser was locked to the R(56)32-0 aio transition another ILP Nd YAG laser with slightly worse characteristics was first locked to the same transition to subtract frequency shifts due to the use of different iodine cells and then alternately locked to the ai, aio and ai5 component of the P(54)32-0 line. The beat frequency between the two lasers of about 47 GHz was detected by a fast photodetector (New Focus model 1006) and measured by mixing the signal down with a Rb-clock synchronized high-frequency synthesizer. Within the uncertainty of the two measurements, the results of the absolute frequency measurement using the frequency chain were confirmed (see Fig. 4). According to this measurement, the frequency differences between the aio HFS component of the (R56)32-0 line and the ai, aio and ai5 HFS component of the (P54)32-0 line are ... 

Fig. 4. Results of the absolute frequency measurements shown are the absolute frequencies and hyperfine line separation of the hyperflne components ai, aio and ais of the P(54)32-0 and the hyperflne component aio of the R,(56)32—0 iodine absorption line. Numbers in brackets correspond to an independent heterodyne frequency measurement where the beat between two ILP lasers, locked to corresponding HFS components of the P(54)32-0 and the R,(56)32-0 line, were observed (for details see text)...

The absolute frequency position of the two-photon transition is measured by comparing the infrared dye laser wavelength with an I - stabilized He-Ne reference laser at 633 nm (see Fig.2). The hey of the wavelength comparison is a nonconfocal etalon Fabry-Perot cavity (indicated as FPE in Fig.2) kept under a vacuum better than 10-6 mbar. This optical cavity is built with two silver-coated mirrors, one flat and the other spherical (R = 60 cm), in optical adhesion to a zerodur rod. Its finesse is 60 at 633 nm and 100 at 778 nm. An auxiliary He-Ne laser as well as the dye laser are mode-matched and locked to this Fabry-Perot cavity. Simultaneously the beat frequency between the auxiliary and etalon He-Ne lasers is measured by a frequency counter. 

In 1986, R. BEAUSOLEIL and D. McINTYRE completed their thesis research at Stanford with an absolute frequency measurement of the F = 1 component of hydrogen 1S-2S [11,20], As frequency reference they employed a 486 nm cw dye laser, locked to a narrow absorption line of 1J Te2 vapor. This line was chosen near a reference line, calibrated to within 4 parts in 1010 by A. FERGUSON et al. [21]. Its second harmonic coincides very nearly with the resonance frequency of the hydrogen two-photon transition, so that the frequencies can be precisely compared by observing a radio frequency beat signal. 

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