L . Ma

Delivery of high stability optical and microwave frequency standards over an optical fiber network

Optical and radio frequency standards located in JILA and National Institute of Standards and Technology (NIST) laboratories have been connected through a 3.45-km optical fiber link. An optical frequency standard based on an iodine-stabilized Nd:YAG laser at 1064 nm (with an instability of ∼4×10-14 at 1 s) has been transferred from JILA to NIST and simultaneously measured in both laboratories. In parallel, a hydrogen maser-based radio frequency standard (with an instability of ∼2.4×10-13 at 1 s) is transferred from NIST to JILA. Comparison between these frequency standards is made possible by the use of femtosecond frequency combs in both laboratories. The degradation of the optical and rf standards that are due to the instability in the transmission channel has been measured. Active noise cancellation is demonstrated to improve the transfer stability of the fiber link.

Stabilization and frequency measurement of the I/sub 2/-stabilized Nd:YAG laser

We report improved stabilization results for and progress toward a more accurate frequency measurement of the 532 nm iodine-stabilized system based on a frequency-doubled Nd:YAG ring laser. We confirm the CCL-adopted frequency well within its stated uncertainty (/spl plusmn/40 kHz).

Absolute frequency atlas of molecular I/sub 2/ lines at 532 nm

With the aid of two iodine spectrometers, we report for the first time the measurement of the hyperfine splittings of the P(54)32-0 and R(57)32-0 transitions near 532 nm. Within the tuning range of the frequency-doubled Nd:YAG laser, modulation transfer spectroscopy recovers nine relatively strong ro-vibrational transitions of /sup 127/I/sub 2/ molecules with excellent SNR. These transitions are now linked together with their absolute frequencies determined by measuring directly the frequency gaps between each line and the R(56)32-0:/spl alpha//sub 10/ component. This provides an attractive frequency reference network in this wavelength region.

Optical heterodyne spectroscopy enhanced by an external optical cavity: toward improved working standards

A study is made of the use of an external resonator to enhance sub-Doppler signals observable with the high-sensitivity techniques of optical heterodyne spectroscopy. The case of modulation-transfer spectroscopy in a ring resonator is considered in detail. Exciting the /sup 127/I/sub 2/ resonance at 612 nm with a low-power He-Ne laser made it possible to observe a S/N of 250:1 in a 10-kHz bandwidth. Used in an optimal control loop, this performance would provide a laser stability of 10 Hz at 1 s. Such a greater-than-hundredfold improvement in stability should lead to increases in accuracy as well. >

Dye-Laser Frequency Stabilization Using Optical Resonators

We describe a study, performed using heterodyne techniques, of the frequency fluctuations of two completely independent ring dye lasers locked to independent reference cavities. Single laser linewidths of less than 750 Hz were achieved, the principal limitation being residual vibrations from the noisy laboratory environment. With future design and environmental improvements, ultranarrow linewidths are expected thus providing a useful tool for a great variety of high precision experiments.

Thermally induced self-locking of an optical cavity by overtone absorption in acetylene gas

Strong self-locking phenomena are observed when laser power is converted into heat by a weakly absorbing medium within a high-finesse cavity. Deposited heat leads to increased temperature and, for the case of weakly absorbing intracavity gases studied here, to an associated reduction of density and refractive index. This thermal change in refractive index provides self-acting cavity tuning near resonant conditions. In the experiments reported here a Fabry‐Perot cavity of finesse 274 was filled with acetylene gas and illuminated with a titanium:sapphire laser tuned to the P(11) line of the n1 1 3n3 overtone band near 790 nm. The dependencies of maximum frequency-locking range on gas pressure, laser power, and laser frequency sweep rate and direction were measured and could be well unified by analysis based on the thermal model. In the domain of strong self-tuning an interesting self-sustained oscillation was observed, with its several sharp frequencies directly and quantitatively linked to the acoustic boundary conditions in our cylindrical cell geometry. The differences between the behavior of acetylene near 790 nm and molecular oxygen with electronic transition near 763 nm are instructive; whereas the absorbed powers were similar, they differed strongly in their rates for internal to translational energy conversion by collisional relaxation.

Frequency comparison of /sup 127/I/sub 2/-stabilized Nd:YAG lasers

A first international comparison of I/sub 2/-stabilized Nd:YAG lasers has been made between the National Research Laboratory of Metrology (NRLM), Tsukuba, Japan, the Joint Institute for Laboratory Astrophysics JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, the National Institute of Standards and Technology (NIST), Boulder, and the University of Colorado, Boulder. The results of the comparison show that the square root Allan variance of the portable NRLM laser has reached 2/spl times/10/sup -14/ when the integration time is larger than 300 s. Matrix measurements were made for five hyperfine components from a/sub 6/ to a/sub 10/ of the R(56)32-0 line. The averaged frequency difference between the NRLM and JILA lasers for four measurements made on three separate days was -4996 Hz (NRLM-JILA, at 532 nm) with a standard deviation of 88 Hz.

Experimental implementation of optical clockwork without carrier-envelope phase control

We demonstrate an optical clockwork without camer-envelope phase control using sum-frequency generation between a CW optical parametric oscillator at 3.39 μm and a modelocked Tisapphire laser with dominant spectral peaks at 834 and 670 nm.

Frequency Comparison of Two Ca-40(+) Optical Clocks with an Uncertainty at the 10(-17) Level

Based upon an over-one-month frequency comparison of two (40)Ca(+) optical clocks, the frequency difference between the two clocks is measured to be 3.2×10(-17) with a measurement uncertainty of 5.5×10(-17), considering both the statistic (1.9×10(-17)) and the systematic (5.1×10(-17)) uncertainties. This is the first performance of a (40)Ca(+) clock better than that of Cs fountains. A fractional stability of 7×10(-17) in 20,000 s of averaging time is achieved. The evaluation of the two clocks shows that the shift caused by the micromotion in one of the two clocks limits the uncertainty of the comparison. By carefully compensating the micromotion, the absolute frequency of the clock transition is measured to be 411 042 129 776 401.7(1.1) Hz.

Optically pumped lasers between high-lying states in the sodium dimer

More than 25 laser lines around 2.5 μm from theC1∏u state to (3)1∑g+ and 7 laser lines around 0.9 μm from the high-lying1∑u state to (2)1∑g+ in the sodium dimer were detected using a nitrogen laser as the pumping source. Most of the optically pumped laser lines have been identified.