Optical Frequency Reference Research Articles

Development of an optical frequency comb around 1556 nm referenced to an Rb frequency standard at 778 nm

To transfer and expand in the optical telecommunication range the accurate 5S1/2(F = 3) – 5D5/2 (F = 5) rubidium frequency reference at 778 nm (385.285 THz), we employ a second harmonic generation (SHG) process coupled to an optical frequency comb (OFC) both based on the use of a low power and narrow linewidth laser diode operating around 1556 nm. The Rb optical frequency reference is known with an accuracy of 2.6 × 10−12 and exhibits a frequency stability of 4 × 10−13τ−1/2, with a Flicker plateau of 2 × 10−14 for τ > 200 s. With 20 mW of laser power at 1556 nm, we perform a SHG process in a periodically poled lithium niobate (PPLN) crystal and we obtain 6.5 μW of harmonic radiation at 778 nm, used to phase lock the IR laser against a Rb optical frequency standard (OFS). This Rb-referenced fundamental radiation at 1556 nm is duplicated as 34 lines spaced by 10 GHz using an electro-optic phase modulator (EOM). This preliminary result is mainly limited by unexpected high optical losses in the Fabry Perot cavity, which comprises the EOM.

Hyperfine structure and absolute frequency determination of the R(1 2 1)35-0 and P(1 4 2)37-0 transitions of [formula omitted] near 532 nm

Hyperfine structures of the R(1 2 1)35-0 and P(1 4 2)37-0 transitions of molecular iodine at the blue end of the tuning range of a frequency-doubled Nd:YAG laser are measured by using two I 2-stabilized lasers. The absolute frequencies of the observed transitions are determined by a frequency link to an optical frequency reference (the R(5 6)32-0:a 10 hyperfine transition), using an optical frequency comb generator. The observed hyperfine transitions are good frequency references for both frequency-doubled Nd:YAG and Nd:YVO 4 lasers in the 532 nm region. High-accurate hyperfine constants are obtained for the P(1 4 2)37-0 transition by fitting the measured hyperfine splittings to a four-term Hamiltonian, which includes the electric quadrupole, spin–rotation, tensor spin–spin, and scalar spin–spin interactions.

Vibration dependence of the tensor spin–spin and scalar spin–spin hyperfine interactions by precision measurement of hyperfine structures of ^127I_2 near 532 nm

Hyperfine structures of the R(87)33-0,R(145)37-0, and P(132)36-0 transitions of molecular iodine near 532 nm are measured by observing the heterodyne beat-note signal of two I2-stabilized lasers, whose frequencies are bridged by an optical frequency comb generator. The measured hyperfine splittings are fit to a four-term Hamiltonian, which includes the electric quadrupole, spin–rotation, tensor spin–spin, and scalar spin–spin interactions, with an accuracy of ∼720 Hz. High-accurate hyperfine constants are obtained from this fit. Vibration dependences of the tensor spin–spin and scalar spin–spin hyperfine constants are determined for molecular iodine, for the first time to our knowledge. The observed hyperfine transitions are good optical frequency references in the 532-nm region.

Rotation dependence of the excited-state electric quadrupole hyperfine interaction by high-resolution laser spectroscopy of ^127I_2

Hyperfine structures of the R(58)32–0,P(55)32–0, and P(104)34–0 transitions of molecular iodine near 532 nm are measured by heterodyne beating of two I2-stabilized lasers. The measured hyperfine splittings are fitted to a four-term Hamiltonian, which includes the electric quadrupole, spin–rotation, tensor spin–spin, and scalar spin–spin interactions, with an accuracy of ∼600 Hz for the R(58)32–0 and P(55)32–0 transitions. Highly accurate hyperfine constants are obtained from this fit. The rotation dependence of the excited state (the B state, v′=32) electric quadrupole constant eQq′ is found to be eQq′(J)=-544.049(14)-0.0002110(43)J(J+1) MHz. The observed hyperfine transitions are good candidates for an optical frequency reference in the 532-nm region.

Optical frequency references for wavelengthdivision-multiplexed optical communication systems

Optical frequency references for wavelengthdivision-multiplexed optical communication systems

Hyperfine structures of the R(122)35-0 and P(84)33-0 transitions of [formula omitted] near 532 nm

Hyperfine structures of the R(122)35-0 and P(84)33-0 transitions of molecular iodine near 532 nm are observed at the red end of the tuning range of the frequency-doubled Nd:YAG laser. Frequency separations between the hyperfine components are measured by observing the heterodyne beat between two I 2-stabilized lasers. The measured hyperfine splittings are fit to a four-term Hamiltonian, which includes the electric quadrupole, spin-rotation, tensor spin–spin, and scalar spin–spin interactions, with an accuracy of about 660 Hz. A new empirical formula for the electric quadrupole constant as a function of the vibrational and rotational energies in the B state is proposed. The observed hyperfine transitions are good candidates for an optical frequency reference in the 532 nm region.

Observation of the2S1/2−2F7/2electric octupole transition in a single171Yb+ion

The ${}^{2}{S}_{1/2}{(F=0)\ensuremath{-}}^{2}{F}_{7/2}(F=3)$ electric octupole transition in ${}^{171}{\mathrm{Yb}}^{+}$ has been detected by observing quantum jumps in the fluorescence of a single ion held in an electrodynamic trap. The transition frequency has been measured as 642 121 498.1 (0.8) MHz. The extreme weakness of this transition makes it ideally suited as an optical frequency reference of subhertz linewidth. The absolute frequencies of a series of Doppler-free absorptions in tellurium vapor are also reported. One of these absorptions was used as a local frequency reference while locating the electric octupole transition.

Sub-systems for optical frequency measurements: application to the 282-nm /sup 199/Hg/sup +/ transition and the 657-nm Ca line

We are developing laser frequency measurement technologies that should allow us to construct an optical frequency synthesis system capable of measuring optical frequencies with a precision limited by the atomic frequency standards. The system will be used to interconnect and compare new advanced optical-frequency references (such as Ca, Hg(+ ), and others) and eventually to connect these references to the Cs primary frequency standard. The approach we are taking is to subdivide optical frequency intervals into smaller and smaller pieces until we are able to use standard electronic-frequency-measurement technology to measure the smallest interval.

Accurate measurement of the frequency of the 6S–8S two-photon transitions in cesium

Accurate optical frequency measurement is realized on the hyperfine components of the 6S–8S two-photon transition in cesium with an uncertainty of 3×10 −10. Light shift and pressure shift are studied. These measurements show that these transitions can be used for the realization of an optical standard at 822 nm, in order to provide new optical frequency references in the visible range.

Measurement of the thermal expansion coefficient of an all-sapphire optical cavity

We present measurements of the frequency-temperature dependence of an all-sapphire Fabry-Perot optical cavity to be used as an optical frequency reference. Measurements were made by tracking the frequency of the cavity relative to a second high-stability cryogenic sapphire-spaced cavity-a technique with impressive resolution for measurements of the coefficient of thermal expansion (CTE). Measurements presented here cover the temperature range of 11 K to 26 K. We find that the CTE of the all-sapphire cavity for these temperatures is given by /spl alpha//sub cavity/=(7.7/spl plusmn/0.9)/spl times/10/sup $/ -/sup 13/T/sup (3.23/spl plusmn/0.05/).

Ultrastable optical frequency reference at 1.064 μm using a C/sub 2/HD molecular overtone transition

Recently we have developed a new frequency modulation technique for detection of weak resonances of molecules located inside a high finesse cavity. We match the FM sideband frequency to the exact splitting of the cavity free spectral range. With this method we are now detecting saturated molecular overtone transitions with very high sensitivities. The resulting excellent signal-noise ratio for these weak but narrow overtone lines has opened the door for realizing high quality optical frequency references in many spectral regions in the visible and near infrared. Here we demonstrate its potential by presenting our recent frequency stabilization result of locking a diode-laser-pumped Nd:YAG laser to the P(5) line of the HCCD (/spl nu//sub 2/+3/spl nu//sub 3/) band at 1064 nm.

Observation of an Electric Octupole Transition in a Single Ion

The $^{2}S_{1/2}\ensuremath{-}^{2}F_{7/2}$ electric octupole $(E3)$ transition in ${}^{172}{\mathrm{Yb}}^{+}$ has been detected by observing quantum jumps in a single laser cooled ion, stored in an electrodynamic trap. The transition frequency is 642 116 785.3(0.7) MHz $(1\ensuremath{\sigma})$. Consideration of the transition rate and laser parameters implies a $^{2}F_{7/2}$ level lifetime of 3700 days. This is the first time an atomic $E3$ transition has been driven. This transition has applications as an optical frequency reference.

Measurement of the coefficient of thermal expansion of a cryogenic, all-sapphire, Fabry-Perot optical cavity

We present measurements of the coefficient of thermal expansion (α) of an all-sapphire, cryogenic, Fabry-Perot optical cavity between 5 K and 77 K. The cavity is to be used as a secondary optical frequency reference. Measurements were made by tracking the frequency of the cavity relative to a free-running Nd:YAG non-planar ring laser. We find that α = (1.56 ±0.84) × 10 −12 T (3.0 ± 0.2) [K −1].

Sub-Doppler optical frequency reference at 1064 μm by means of ultrasensitive cavity-enhanced frequency modulation spectroscopy of a C_2HD overtone transition

Using our cavity-enhanced frequency modulation technique, we obtained the saturated signal of the weakly absorbing gas C(2)HD for the P(5) line of the (nu(2) + 3nu(3)) overtone band at 1.064 microm. An absorption sensitivity of 1.2 x 10(-10) (1.8 x 10(-12)/cm) has been obtained. The absolute frequency of the line center is established (within +/-2.6 kHz) with respect to a doubled Nd:YAG laser/I(2) frequency reference. The short-term stability, limited by the available signal-to-noise ratio, currently is 1 + 10(-11) at 1 s, improving to 5 x 10(-13) at 1000 s. The measured pressure-broadening rate is 34.7(0.8) kHz/mTorr (260 +/- 6.1 kHz/Pa). Selection of slow molecules gave a linewidth four times narrower than the room-temperature transit-time limit.

A microwave temperature control system dedicated to a cryogenic sapphire-spaced Fabry - Pérot optical frequency reference

The extremely high frequency stability attainable with a cryogenic all-sapphire Fabry - Pérot cavity is limited by temperature-induced length fluctuations of the sapphire spacer. This paper presents a method of measuring the temperature in the sapphire spacer at cryogenic temperatures. The two systems described are analysed to have precisions of 750 and 75 nK over measurement times of 1 s. The temperature is detected by monitoring the AC susceptibility at microwave frequencies. The temperature coefficient of the susceptibility originates from the paramagnetic contribution of residual chromium in the sapphire. Two microwave detection circuits are proposed and the noise limitations of each discussed. The optimal doping levels of the paramagnetic ion are found to be commonly available as residuals in commercially available sapphire crystals. We show that an optical frequency reference with an Allan variance of a few times for integration times up to 100 s can be achieved using this system.

Comparison of Three Optical Frequency References for HDWDM Systems under Field Conditions

The relative optical frequency stabilities of three optical reference units at 1547.825 nm were examined. The reference units were located at different places in Berlin and connected via several kilometeres of optical fiber. The three optical signals were heterodyned to measure the frequency stabilities, which were better than 2×10−11at 1000 s sample time.

Cryogenic system for a sapphire Fabry-Perot optical frequency standard

This paper describes the design and performance of a cryogenic system for a sapphire Fabry-Perot optical frequency reference cavity. The cryogenic system utilises a single vacuum system, a liquid helium coupled heat exchanger, low mechanical vibration mounting and an axial, bottom-entering optical path. The cryogenic and thermal performances are also described.

Cryogenic, all-sapphire, Fabry–Perot optical frequency reference

This paper presents a design for a Fabry–Perot optical frequency reference resonator which utilizes the high dimensional stability of cryogenically cooled sapphire. We show that cryogenic sapphire cavities can achieve substantial improvements in frequency stability compared with room-temperature cavities. The design of a laser stabilization system based on such a resonator is discussed. Estimates of fundamental and practical limitations on the frequency stability of such a system suggest that a fractional frequency stability of 10−16 over integration times from 10 to 104 s is possible. The fundamental limits to stability from quantum shot noise, radiation pressure fluctuations, and thermal noise are overwhelmed by practical limits which arise due to mechanical, thermal, and optical effects.

Optical frequency determination of the hyperfine components of the [formula omitted] two-photon transitions in rubidium

An absolute optical frequency measurement is made of the hyperfine components of the 5S 1 2 -5D 3 2 two-photon transitions in rubidium with an uncertainty of 1.3×10 -11. This is carried out in a simple way by taking advantage of the near coincidence between the two-photon frequency at λ=778 nm in Rb and the frequency difference of two HeNe lasers, stabilized respectively to an iodine line at λ=633 nm and a methane transition at λ=3.39 μm used as reference. This measurement provides a new optical frequency reference in the visible range.

Multiwavelength sources using laser diodes frequency-locked to atomic resonances

It is shown that low-cost compact disk AlGaAs laser diodes make it possible to realize manifold optical sources with precise and stable wavelengths when locked to rubidium vapor resonances. Many optical frequency references can be obtained with the use of absorption cells containing a foreign gas or by placing cells in static magnetic fields. The same approach can be used with other wavelengths required by optical fiber communications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>