Many manufacturers evaluate whether their products are manufactured in accordance with the designed values and guarantee their quality. Mitutoyo provides users with measuring instruments that are indispensable for quality assurance of products after calibrating them with traceability connected to the national standards.
The national length standards in Japan today are in accordance with the atomic clock and optical frequency comb device synchronized to Coordinated Universal Time ^{[1] [2]} (optical comb device) owned by the National Institute of Advanced Industrial Science and Technology (AIST). In comparison with the former national standards of the iodine-stabilized He-Ne laser, the level of uncertainty is notably lower, and rulers for wavelengths in a wide range from visible light to near infrared light can be provided by taking advantage of spectral characteristics. Thus the optical comb is innovative technology useful for developing length standard technology and acquiring traceability.
Mitutoyo has developed a laser frequency calibration system using the optical comb device equivalent to the national standards jointly with AIST, further improved the reliability of in-house standards and worked on maintaining an efficient traceability system. Then, Mitutoyo was approved as a company registered with the Japan Calibration Service System (JCSS) first in the calibration of laser wavelength (frequency) with the use of an optical comb device in 2017. This following section will introduce a laser frequency calibration system using an optical comb device that was developed by Mitutoyo. One meter is now defined by multiplying the physical constant of the light velocity c by time t (=1/299792458s) ^[3]. For realizing length more precisely, a laser wavelength with a stabilized frequency in a vacuum is actually used by taking advantage of the relationship among light velocity c, wavelength λ and frequency γ. Making a ruler with a precise length is measuring a frequency precisely.
A laser that was representative of a length standard was an iodine-stabilized He-Ne laser with a wavelength of 633 nm, which was used as a national standard of Japanese length until July 2009. The uncertainty was said to be 4.2 Χ 10-11 (k=2) ^[4]; even now it is used as a standard in private companies. The optical comb is a laser with an extremely equal interval between modes, frep. The absolute frequency can be expressed by using the residual frequency FCEO when frep and frep are virtually extended to the domain of zero frequency and mode order N. Therefore, the absolute frequency γ laser of a laser to be calibrated can be measured by measuring the beat frequency fbeat generated by interference with the laser to be calibrated, specifying the mode order N, and substituting frep and fCEO ^[5]. AIST that owns the optical comb device provides laser calibration service with the uncertainty of 1.4 Χ 10-14 (k=2) as the length specified standard instrument (national standard) ^[6]。 Japan has the JCSS; when a company conforms to and is registered with the JCSS, the company can provide calibration service traceable to the national standards. The specified secondary standard owned by a company registered with the JCSS as an in-house standard must be calibrated with the use of the specified standard (national standard) on a regular basis. The calibration cycle can be extended up to five years only when the retention of its performance can be confirmed by evaluating it in comparison with a laser equivalent to the specified secondary standard in the case of a laser, for example. Like this, a company registered with the JCSS is required to strictly manage calibration instruments. The applied research of the optical comb is conducted in fields such as frequency standards, spectroscopic measurement and geometric measurement ^{[7] [8] [9]}. One industrial application is utilization for JCSS wavelength calibration. We developed an optical comb device that can calibrate a laser wavelength traceable to the national standards, even by others than AIST, in cooperation with AIST ^{[10] [11]}.
In response to the results of this research and Mitutoyo's request, this method was added to the JCSS length traceability system in December 2015 ^[12].
Figure 1 shows the current length traceability system. In the former system, the specified secondary standard for length was the only molecular absorption line wavelength stabilized laser calibrated by the optical comb device synchronized to the national standard for frequency UTC (NMIJ). In the added traceability system, a frequency standard is also the specified secondary standard for length only when the optical comb device is used by a company registered with the JCSS. This is a mechanism in which a laser standard in a lower position of the traceability is calibrated by the optical comb device by setting a frequency standard as a standard instrument ^{[12] [13]}.
In both systems, the sources of each standard are likewise the national frequency standards. The added system is compliant because the standard instrument measuring a laser frequency is the frequency standard that is traceable to the national frequency standards.

Figure 1 Length traceability system

Figure 2 shows the length traceability system that Mitutoyo started to utilize on the basis of this added system. We regard a frequency standard calibrated with UTC (NMIJ) that is the national frequency standard as the specified secondary standard and use the optical comb device to calibrate a laser of the reference standard for common use. Then, we connect a standard instrument in a lower position and a measuring instrument with calibration linkage and secure the traceability in many commodities to be provided to users. When we conduct laser JCSS calibration with the optical device, we apply to the National Institute of Technology and Evaluation (NITE) and undergo the proficiency test in the presence of AIST. As a result, we are certified to calibrate a laser frequency with the uncertainty of 1.1 X 10-13 (k=2, on average in a day). The three advantages of using the optical comb device are shown as follows:

Figure 2 Length traceability system in Mitutoyo

Figure 3 shows the external view of the laser frequency measuring system with the optical comb device; Fig. 4 shows the system configuration outline. The optical comb frequency is stabilized by synchronizing and controlling the phase of the optical comb with respect to the reference frequency of frep and fCEO supplied from the frequency synthesizer synchronized with the frequency standard calibrated with UTC (NMIJ). Then, the optical comb device becomes traceable to the national standards.
Integration of the widened spectral range and harmonic generation technology into the optical comb oscillator that oscillates in the 1.5- μmm wavelength band enables an optical comb with the wavelength band of 633 nm or 532 nm to be acquired ^{[10] [11]}. The beat frequency generated by the interference between the optical comb and laser to be calibrated is measured with the frequency counter; the absolute frequency of the laser to be calibrated is measured.

Figure 3 Laser frequency measuring system with optical comb	Figure 4 System configuration outline

The frequency standard in this device is based on UTC (NMIJ) according to the GPS Common View method ^[14] and is remotely calibrated in the operating environment every day.
According to the GPS Common View method, the GPS time signals are simultaneously measured with the standards 1 and 2 to be compared to each other and compared; accordingly, comparative measurement at a high precision level can be conducted regardless of if GPS error exists or not. By taking advantage of this principle, AIST provides remote calibration service for time frequency. The uncertainty of remote calibration depends on distance; the distance between AIST and Mitutoyo is less than 50 km. Therefore, it is 1.1 Χ 10-13 (K=2) ^[14] at the average measurement in a day. Our frequency standard has the uncertainty of 1.1 Χ 10^-13 (k=2) because the frequency fluctuation is smaller than the remote calibration uncertainty.

Figure 5 Principle of GPS Common View method