Youichi Bitou

Phase-locked widely tunable optical single-frequency generator based on a femtosecond comb

We present an arbitrary optical single-frequency generator based on a femtosecond optical frequency comb. The functions of this device are comparable to those of a radio-frequency synthesizer. However, this device operates at hundreds of terahertz. The absolute frequency accuracy of this synthesizer is approximately 1 kHz at a 282 THz carrier frequency. The stability is approximately 2 x 10(-14) at 100 s, and the tuning speed exceeds 30 GHz/s. This source demonstrates the integration of a phase-locked optical comb into a versatile and easy-to-use system for the generation of tunable, absolute optical frequencies. By using downconversion, one could generate tunable terahertz frequencies that are phase locked to a microwave reference, such as a Cs atomic clock, and high-precision interferometry could benefit greatly from the stability and accuracy of this widely tunable source.

Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer.

We report on a displacement metrology setup that provides sub-pm resolution in air. The setup is based on a Fabry-Perot cavity. However, unlike current Fabry-Perot cavity based displacement setups we incorporate a novel fs-laser based arbitrary wavelength synthesizer that provides efficient suppression of atmospheric disturbances while providing very wide and precise tuning of the output wavelength. The wavelength synthesizer provides sub-10 attometer wavelength resolution. The setup provides subpm length stability for integration times of up to one minute and sub-10 pm for up to half an hour without airtight enclosure of the Fabry-Perot cavities.

Digital phase-shifting interferometer with an electrically addressed liquid-crystal spatial light modulator

A digital phase-shifting interferometer with a liquid-crystal-display coupled, parallel aligned, nematic liquid-crystal spatial light modulator is developed. The optical phase shift in the Michelson-type polarization interferometer is achieved by a digital phase shift of a grating displayed on the spatial light modulator. Accurate experimental results of using the heterodyne system for pi/2 phase steps were demonstrated. A phase-shifting interferometer with no moving parts and no requirement for calibration of the value of the phase shift was achieved.

Accurate wide-range displacement measurement using tunable diode laser and optical frequency comb generator

A laser-frequency-based displacement measurement system with sub-nanometer uncertainty using an optical frequency comb generator is developed. In this method, the optical frequency of a tunable laser is locked to the resonance of a Fabry-Perot cavity. One of the two mirrors of this Fabry-Perot cavity is connected to the element whose displacement is to be measured. Wide range optical frequency and displacement measurements were realized by using an optical frequency comb generator, which consists of an electro-optic modulator placed inside of an optical resonator. We demonstrate a displacement measurement of up to 10 mum with 220 pm uncertainty under the stable condition.

Doppler-free spectroscopy using a continuous-wave optical frequency synthesizer

A continuous-wave (cw) optical frequency synthesizer is demonstrated by using a monolithic-type cw optical parametric oscillator (cw-OPO) and an optical frequency comb. The cw-OPO is phase locked to an optical frequency comb that is phase locked to an atomic clock. The output frequency of the cw-OPO is frequency shifted with an electro-optic modulator, which makes it possible to tune the frequency continuously over 10 GHz. Furthermore, Doppler-free spectroscopy is performed using the optical frequency synthesizer for a cesium D1 line at 895 nm. The observed linewidth of 5 MHz is the natural linewidth of cesium. The center frequency of the line is consistent with a previous report.

Phase-shifting interferometry with equal phase steps by use of a frequency-tunable diode laser and a Fabry–Perot cavity

A phase-shifting interferometry (PSI) with equal phase steps by use of a frequency-tunable diode laser and a Fabry-Perot cavity is proposed for the Carré algorithm. The measurement accuracy of the Carré algorithm depends on the equality of the phase steps. Using the Fabry-Perot cavity as a highly stable optical frequency reference, a high degree of phase step equality can be realized in PSI with an optical frequency shift. Our experimental scheme realizes an optical frequency step equality higher than 5.1 x 10(-5) and a measurement repeatability of lambda/800.

Gauge block interferometer using three frequency-stabilized lasers

We have developed a gauge block measurement system that uses three frequency-stabilized lasers. The stabilized lasers are as follows: an I2 stabilized offset locked He-Ne laser, an I2-stabilized Nd:YAG laser, and a Rb-stabilized diode laser. The I2-stabilized offset locked He-Ne laser is commercially available and its relative wavelength uncertainty is 2.5 X 10-11. An I2-stabilized Nd:YAG laser and a Rb-stabilized diode laser was developed in our institute and their relative wavelength uncertainties are 5 X 10-12 and 1 X 10-9, respectively. In the measurement system, laser beams were introduced to the interferometer using an optical multimode fiber. An interferometer fringe pattern was taken using a CCD camera and the excess fraction parts were calculated from the fringe pattern using the Fourier transform method. The excess fraction part obtained from the Rb-stabilized semiconductor laser was used only to determine the integer part of the fringe order, because the accuracy and stability of the wavelength were not sufficient for the long gauge block measurements. This interferometer can measure gauge blocks of up to 1000 nm long and the standard uncertainty of the interferometer is about 75 nm for a 1000 mm long gauge block.

Gauge Block Measurement Using a Wavelength Scanning Interferometer.

A gauge block measurement system that uses a wavelength scanning interferometer was developed. By using a wavelength scanning technique, we determined the integer part of the interference fringe order. From that integer part, we determined the absolute length of a gauge block using only a single light source (wavelength scanning laser diode), even when no information on the nominal length of the gauge block was available. Using the Fourier transform technique to analyze the interference fringe, we measured the excess fraction parts of the interference fringe order with a resolution of over 0.003, and then we determined the integer part of the fringe order in the test measurement of a 10 mm gauge block. It was estimated that we could determine the integer part when the nominal length was shorter than approximately 100 mm, which can be expanded by improving the frequency stability of the light source. The final uncertainty was similar to that obtained with a conventional system based on a multiple wavelength interferometer.

Discontinuous surface measurement by wavelength-tuning interferometry with the excess fraction method correcting scanning nonlinearity

Wavelength-tuning interferometry can measure surface shapes with discontinuous steps using a unit of synthetic wavelength that is usually larger than the step height. However, measurement resolution decreases for large step heights since the synthetic wavelength becomes much larger than the source wavelength. The excess fraction method with a piezoelectric transducer phase shifting is applied to two-dimensional surface shape measurements. Systematic errors caused by nonlinearity in source frequency scanning are fully corrected by a correlation analysis between the observed and calculated interference fringes. Experiment results demonstrate that the determination of absolute interference order gives the profile of a surface with a step height of 1 mm with an accuracy of 12 nm.

Rubidium-stabilized diode laser for high-precision interferometer

An Rb-stabilized diode laser has been developed for use in a high-precision interferometer. The light source is a commercially available external-cavity tunable diode laser. The laser frequency is stabilized to a Doppler-free absorption line of Rb by the third-harmonic technique. The laser emits an output beam with a high power (more than 7 mW) and fast frequency modulation (10 kHz). The relative optical frequency uncertainty of 4.3×10 –10 is achieved for a 0.01-s averaging time.