National Metrology Institute of Japan (NMIJ), the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has improved the reliability and the performance of an optical frequency measuring system based on atomic clock and optical frequency comb under the project "Development of Broadband Optical Synthesizer" (fiscal years 2002-04) supported by the Science and Technology Promotion and Coordination Fund of the Ministry of Education, Culture, Sports and Technology (MEXT), and started the calibration service for optical frequency from February 15, 2005.

"Optical comb" is generated from an ultra-short pulse laser (Fig. 1) and contains various color components which are regularly spaced. The optical comb can be used as "optical frequency ruler" by use of cesium atomic clock to stabilize the spacing. The optical frequency measurement is one of applications for the "ruler".

Up to now, the equivalency of laser standard has to be confirmed only by comparing lasers, and a determination of an absolute frequency has been difficult. The newly developed technology ensures the absolute frequency measurement and confirmation of their equivalency.

In addition, all the color components of the optical comb are equally accurate. Therefore, various wavelengths and types lasers will be available for length standards.

The service will be applicable to lasers used for measuring pitch length of semiconductor, and contribute to the semiconductor industry through improving of fabrication accuracy.

Before 1983, optical frequency measurement had been necessary to determine the velocity of light. Now it is indispensable technique for realizing the definition of length. Before 1999, a roomful of equipments and multiple researchers had been needed for optical frequency measurement. In 1990s, the optical frequency was determined regularly only in a few laboratories in the world because of its technical difficulty. The NMIJ-AIST has been maintaining and providing the wavelength of iodine-stabilized helium-neon lasers as national length standards since 1980s, and keeping their accuracy through the international comparison of laser frequency.

In 1999 and 2000, German and US laboratories propose absolute frequency measurements for lasers using the optical frequency comb generated from ultra-short pulse lasers, which caused a very important technological innovation in this field. Subsequently, the NMIJ-AIST has succeeded the absolute frequency measurement of lasers.

The research work has been carried out under the project "Development of Broadband Optical Synthesizer" (fiscal years 2002-04) supported by the Science and Technology Promotion and Coordination Fund from the Ministry of Education, Culture, Sports and Technology (MEXT).

In this project, NMIJ-AIST have improved the reliability and stability of a system through the noise reduction in signal transmission from the atomic clock to the optical comb, and the optimization of optical and electrical systems, as well as the protection against dust. As a result, NMIJ-AIST began the calibration service for "broadband optical frequency" on February 2005.

The optical frequency is extremely high, as high as hundreds of tera-hertz (1 tera-hertz (=THz) =10¹² Hz), and cannot be converted directly to electrical signals. For the conversion, optical beat measurements are used. When two optical beams of frequency (f₁) and (f₂) are mixed, optical beat occurs at a frequency (f₁ - f₂) and is observed if (f₁ - f₂) is adequately low. The optical frequency is measured in combination with the optical beat measurements.

An ultra-short pulse train generated from a mode-locked laser contains fine spectral lines spaced by a frequency interval determined by repetition frequency (f_rep) as illustrated in Fig. 1. As the spectral lines seem like a comb, the spectrum is named "optical frequency comb". The optical frequency comb has lines spaced at precisely uniform interval, and if the intervals are stabilized to the frequency based on the cesium atomic clock, the optical frequency comb can be used as a optical frequency ruler. Particularly, in case of optical comb of greater span than an octave (double frequency), it may be possible to set frequencies of comb lines to multiples of (f_rep), by selecting conditions appropriately. In this condition, all the frequencies of the optical frequency comb are the integral multiples of the repetition frequency (f_rep). Where n is a large integer. Finally, a laser frequency to be measured (f_laser) can be determined as f_laser = n·f_rep + f_beat, by measuring an optical beat (f_beat).

Fig. 1 A schematic diagram of optical frequency comb and optical frequency measurement

In order to convert stable signals of atomic clock into optical signals (n·f_rep) in the optical comb, the following steps are traced: (1) microwave frequency synthesis of f_rep from a microwave frequency generated from an atomic clock (Process A), (2) control of optical comb (Process B), and (3) n-time multiplication by optical comb (Process C). The MIJ-AIST has studied the bottleneck steps, Process A and Process B, and improved these. Consequently, improved system could measure the most stable laser in NMIJ/AIST by using the most stable atomic clock as the reference. That is, this "optical frequency ruler" is confirmed not to degrade the signal.

In addition, signal-to-noise (S/N) ratio of the optical beat is of great significance. The optical system has been optimized so that the S/N ratio is stabilized for a long time by increasing adjusting points, and by selecting optical components of reduced loss. Besides, the accuracy for measuring signals of lower S/N ratio has been improved by checking the frequency behavior.

The absolute frequency of a laser can be determined from 500 nm to 1064 nm as far as an optical frequency comb exits. Therefore, laser of any wavelength, and of any type, semiconductor, solid-state and others can be used technically as a standard laser for length.

The provision of the calibration service will upgrade the reliability of lasers for length standard, and make it possible to use various kinds of laser as reference for precise length measurement, such as green gas laser, which has not been calibrated so far, and semiconductor lasers of various colors. Consequently, it is expected that compact and lower-price laser-interferometers will appear.

So far, the national standards for length are iodine-stabilized helium-neon lasers of 633 nm. This optical frequency calibration service can determine the absolute wavelength (optical frequency), for any wavelength and for any type. It is expected that various wavelength standards will be appear. In addition, if the calibration service is available for ultraviolet laser of shorter wavelength (193 nm) in the future, the accuracy of the measurement for semiconductor pitch length will be improved and can contribute to the semiconductor industry.

On the other hand, the appearance of the optical comb has stimulated studies to realize frequency standards in the optical frequency range. In near future, the advent of "optical clock" with an error as small as 1 second in a hundred million years, is expected as a post-cesium atomic clock.