Kenichiro Okumura

Towards an accurate frequency standard at 1.5 /spl mu/m based on the acetylene overtone band transition

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.

Frequency measurements and hyperfine structure of the R(85)33–0 transition of molecular iodine with a femtosecond optical comb

Absolute frequency measurements of the R(85)33‐0 transition of molecular iodine at the blue end of the tuning range of a frequency-doubled Nd:YAG laser are implemented with a femtosecond optical comb based on a mode-locked Ti:sapphire laser. The hyperfine structure of the R(85)33‐0 transition is observed by use of high-resolution laser spectroscopy and is measured by the femtosecond optical comb. The observed hyperfine transitions are good frequency references for both frequency-doubled Nd:YAG and Nd:YVO4 lasers in the 532-nm region. High-accuracy hyperfine constants are obtained by our 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.

Optical Frequency Synthesis From a Cryogenic Sapphire Oscillator Using a Fiber-Based Frequency Comb

The authors have demonstrated optical frequency synthesis using a fiber-based frequency comb system, referenced to an ultrastable microwave oscillator. The oscillator is based on a high Q-factor cryogenic sapphire resonator cooled with liquid helium. The 100-MHz signal synthesized from the 10.8-GHz oscillation frequency of the cryogenic sapphire oscillator (CSO) was used to stabilize the repetition frequency of the mode-locked fiber laser. When the synthesized optical frequency was compared with a rubidium two-photon stabilized laser at 778 nm, the measured fractional frequency stability was ~6 10-14 tau -12 for the averaging times tau between 1 and 100 s, and the best frequency stability was 3.0 10-15 for an averaging time of 1280 s

Optical frequency measurement of the acetylene transitions at 1.5 /spl mu/m

We report on an optical frequency measurement of the acetylene transitions at 1.5 /spl mu/m. Using an optical frequency comb generator and second harmonic generation, we can directly connect an acetylene-stabilized diode laser to a 778 nm diode laser stabilized on a reference Rb two-photon transition.

Frequency stabilization under very small modulation and its stability estimation of a PbSnTe diode laser

The frequency of a PbSnTe diode laser around 10.6 μm was stabilized under very small frequency modulation to one of the absorption lines of C 2 H 4 in the ν 7 band. The frequency control and modulation were performed by the injection current. The minimum modulation width in this experiment was analytically determined to be 1.1 MHz. From analysis of a linear approximate model for the experimental system, the frequency fluctuation, which represents peak-to-peak width during 5 min, including the modulation width, was estimated to be 3.8 \times 10^{-8} . This value was about two orders larger than the value obtained by the ordinary method, which is a simple conversion of the control error signal.

Accurate Frequency Atlas of 1.5 μm Band of Acetylene Measured by a Mode-Locked Fiber Laser

We have been developing optical frequency standards based on absorption lines of acetylene molecules. Using optical frequency comb generators such as a mode-locked fiber laser, we are now measuring difference frequencies between each rotational-vibrational transition and a P(16) line of acetylene molecule. These transitions can be used as accurate frequency references for the calibration at telecom region

Unmodulated-Type Frequency Stabilization of a PbSnTe Diode Laser

The frequency of a PbSnTe diode laser around 10.6 µm was stabilized to one of the absorption lines of C2H4 without frequency modulation by controlling the injection current. Two different methods were tried: one used light after absorption and the other used light after absorption and reference light for comparison. From the analysis of the linear approximate models of the experimental systems, the frequency fluctuations, which here are taken to represent the peak-to-peak widths over a period of 5 minutes, were estimated to be 9.3×10-8 in the former case and 2.2×10-8 in the latter. The analysis also clarified that frequency fluctuations are underestimated when obtained by the usual method, which is a simple conversion of the control error signals. These values were over one order smaller than the values obtained from the linear approximate models.

Saturation spectroscopy of acetylene molecule towards frequency standard at 1550 nm region

We have observed saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/ using an intracavity cell. These narrow lines can be expected to use as a frequency standard at 1550 nm region. We also demonstrate it is possible to saturate a weak molecular transition using a fiber amplifier.

A frequency measurement system for an optical frequency standard at 1.5 /spl mu/m

We: are making a frequency measurement system for an optical frequency standard operating at 1.5 /spl mu/m region using a saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/ molecule. After the first preliminary measurement reported at CPEM98, we have been improving the system to make it more accurate and reliable. The status report will be presented including preliminary results of the frequency measurement using a mode-locked fiber laser.

An accurate optical frequency measurement system at telecommunication region

We are now developing an accurate optical frequency measurement system at the telecommunication region using an optical frequency standard with Doppler-free acetylene absorption and a mode-locked fiber laser.