Shin-ichi Ohshima

Development of an optically pumped Cs frequency standard at the NRLM

Describes development of an optically pumped Cs frequency standard aiming at a primary frequency standard. Using this laboratory-type standard, a Ramsey resonance spectrum was observed. It has been shown that a laser power of about 1 mW is almost sufficient for two-frequency optical pumping. It has also been shown that the C-field strength can be measured using Zeeman coils by establishing a population difference among the Zeeman sublevel with sigma /sup +/ polarized laser light. >

Short Term Frequency Stability Tests of Two Cryogenic Sapphire Oscillators

Ultra-high short-term frequency stability has been realized in microwave oscillators based on liquid helium cooled sapphire resonators which operate on the same Whispering Gallery mode. Two cryogenic sapphire oscillators were built to evaluate their stability at short averaging times. These oscillators exhibited a fractional frequency stability of 1.1×10-15 at an averaging time of 1 s, which is more than 100 times better than that of a hydrogen maser. For averaging times between 2 and 640 s the measured oscillator fractional frequency instability was below 10-15 with a minimum of 5.5×10-16 at an averaging time of 20 s. The noise floors of the control servos which contribute to the short-term frequency stability are also discussed.

Time and frequency transfer and dissemination methods using optical fiber network

In the time and frequency transfer and dissemination field, it is important to provide cost effective remote frequency calibration services with an uncertainty of around 10-12 for end users. It is also required to develop ultra precise transfer methods with an order of 10-15 or better uncertainty for the comparison between ultra stable frequency standards which are under developing. This study shows two methods using optical fiber networks to satisfy these demands. First, it is an economical remote calibration method using existing synchronous optical fiber communication networks. The measured frequency stability (the Allan deviation) of the transmission clock is 2times10-13 for an averaging time of one day. The result indicates the method is promising for the simple remote frequency calibration service. Second, it is an ultra precise two-way optical fiber time and frequency transfer method using a newly proposed bi-directional optical amplifier. In this method, wavelength division multiplexing (WDM) signals are transmitted along a single optical fiber. The preliminary measured frequency stability is less than 1015 (tau =104 s) for a 100-km-long fiber with the bi-directional amplifier. It suggests that the method has capability for improving TAI (International Atomic Time) and UTC (Coordinated Universal Time)

Spectral width of saturated absorption spectra of Cs with a laser diode

The saturated absorption spectra and line widths of the Cs-D 2 line were observed as a function of the incident power. A minimum spectral width of 20 MHz was obtained when the incident power was under 0.1 mW and all of the hyperfine spectral lines and the cross-over resonance lines were resolved.

Phase-coherent optical frequency division by 3 of 532-nm laser light with a continuous-wave optical parametric oscillator

Phase-locked 3:1 division of an optical frequency was achieved with a continuous-wave monolithic optical parametric oscillator (OPO) pumped by a 532-nm Nd:YAG laser, by use of 5% MgO-doped LiNbO(3) as a nonlinear optical crystal. The OPO generated signal light (798 nm) with 4-mW power and idler light (1596 nm) with 3-mW power for a pump power of 68 mW. Approximately 2microW of second harmonics (SHs) of the idler light was produced by external-cavity-enhanced SH generation by use of a periodically poled LiNbO(3) crystal. The beat signal between the signal light and the SH of the idler light was observed with a signal-to-noise ratio of 40 dB at a 10-kHz bandwidth and was successfully phase locked to a signal from a synthesizer through the electro-optic effect of the crystal.

Accuracy of an Optical Parametric Oscillator as an Optical Frequency Divider

Abstract The coherence and the accuracy of a cw optical parametric oscillator (OPO) as an optical frequency divider was measured. The phase coherence between the pump and the signal or the idler was confirmed. The accuracy of the OPO as an optical frequency divider was found to be better than 5 × 10−18.

Simple Time and Frequency Dissemination Method Using Optical Fiber Network

This paper describes a simple and cost-effective method of frequency dissemination. In current digital communication networks, node clocks are hierarchically synchronized to the atomic master clock through fiber links. This synchronized network is used as an intermediate link for remote calibration services like the global positioning system common-view method. A prototype reference signal generator has been developed for recovering the communication clock signal and synthesizing a 10-MHz signal from it. The generator output frequency at the client site can be traced to coordinated universal time (UTC) National Metrology Institute of Japan (NMIJ) with some uncertainty, depending on the stability of the node clocks and the distance from the master clock. The stability performance of the generated reference signal has been tested at Okinawa-the farthest prefecture from Tokyo, where the master clock is located (baseline distance of 1500 km). The primary rate (1.544 MHz) for telecommunication services was chosen for the 10-MHz signal generation in the experiment. A sinusoidal phase fluctuation within a one-day period is dominantly observed. This fluctuation is mainly caused by fiber expansion and contraction due to normal daily temperature changes. It degrades the stability (Allan deviation) to the level of 5 X 10-13 (t = 40 000 s, which is almost half a day). However, the major part of the phase fluctuation can be canceled by averaging a full days data. In this case, the Allan deviation becomes 1 X 10-13, which is obtained at Okinawa over ten consecutive days of measurement. The worst average frequency offset relative to UTC (NMIJ) (one-day averaging) is -6.3 X 10-13. The results indicate that this method promises to be suitable for most applications, providing an uncertainty of less than 1 X 10-12 at an averaging time of one day.

Light shifts in an optically pumped Cs beam frequency standard

Frequency shifts caused by light, which are called light shifts in an optically pumped Cs beam frequency standard, were estimated. Frequency shifts due to monolithic light were measured by introducing laser light along the Cs beam. The relative dependence of the shift on the laser frequency agreed very well with the theory, but the absolute shift was between one and two times that of the theory. The light shifts due to the optical pumping and optical detection in the standard are estimated to be less than 2*10/sup -15/ and 1*10/sup -16/, respectively, and both are negligible at the present state of development. >

Characteristics of an optically pumped Cs frequency standard at the NRLM

Characteristics of an optically pumped Cs frequency standard developed at the National Research Laboratory of Metrology (NRLM) are reported. The short-term frequency stability was estimated to be 1.1*10/sup -12// square root pi when the optical feedback technique was used for laser diode stabilization. Frequency shifts due to microwave power and pumping conditions were measured and their characteristics are described. >

Long-term, mode-hop-free operation of a continuous-wave, doubly resonant, monolithic optical parametric oscillator

Abstract More than 24-h mode-hop-free operation of a continuous-wave doubly resonant monolithic optical parametric oscillator (OPO) was demonstrated using 5% MgO-doped LiNbO 3 as a nonlinear crystal. The OPO was pumped by a 532-nm Nd:YAG laser with 68 mW power, and generated 4 mW of signal power at 807 nm and 3 mW of the idler power at 1561 nm. The OPO maintained a single longitudinal signal and idler mode pair even under the free running condition, owing to the high mechanical stability of the monolithic structure and the strong thermal self-locking mechanism. By stabilizing the crystal temperature so as to maintain the output power constant, the drift of the output power could be suppressed completely and a power fluctuation of 1.3% rms was achieved.