Smart Industry and Society System Using Compact and Unique Optical Frequency Combs

Introduction

At present, there is a need for factories that consist the clusters of smaller facilities rather than a single massive factory. While there is a strong emphasis on people’s daily lives, the declining birthrate has a major issue. In this context, in addition to enabling people to work in the areas with a good living environment, proximity to the workplace is also important for creating an environment where both women and men can work. Furthermore, it is believed that the involvement of the senior generation would be beneficial.

To ensure traceability under the measurement work and eliminate the tariff barriers in trade through mutual recognition agreements, we have established a method for measuring length using optical fibers for business optical communication, thereby enabling remote measurement [1].

Around 1996, as shown in Figure 1, we conducted a demonstration experiment using a 20-km section of Tokyo Electric Power Company (TEPCO) of commercial optical fiber. As a result, we were able to measure a block gauge with a nominal length of 251 mm within an accuracy of 30 nm. The light source is broad spectrum and so tandem interferometer is utilized. However, although Japan currently has a vast network of optical fibers and very much dark fibers that are not actually in use, operations had reached a standstill because these resources were not being made available to the public at reasonable prices.

However, recently photonic fiber capable of generating femtosecond pulses was developed at the University of Bath in the UK, and researchers succeeded in broadening the spectrum. We also participated in the collaborative research. This broad-spectrum light, which has a constant repetition frequency, was termed an optical frequency comb (optical comb) possesses numerous functions applicable across many fields, leading to its adoption as a specified length standard under Measurement standard. These functions make it effective for the standard in a wide range of fields [2]. Here, we have developed a low-cost, user-friendly optical comb designed for ease of use by general users, thereby contributing to the current industrial and social landscape through a comprehensive measurement system technology framework.

Key Smart Technologies

Optical Frequency Comb Light Source

As shown in Figure 3, we have minimized the number of components required for optical comb laser oscillation as much as possible. Among these, the polarization modulator accounts for a significant portion. This device applies tension and torsion to the optical fiber to induce phase shifts. It consists of three sections, each providing λ, λ/2, and λ/4 plate functions, respectively.

Furthermore, the air gap within the resonator was eliminated. As a result, not only were component costs (approximately $2,000) reduced, and so the effects of external vibrations and atmospheric fluctuations were also minimized in small box. This provides enabled cost reduction to us.

The polarization function is realized by deformation of optical fiber. Therefore, the air gap in the laser resonator has been eliminated. As a result, the cost of the optical comb is reduced, and so the effects of external vibrations and atmospheric fluctuations were also minimized. This cheap optical comb is simple and stable. The laser box measures 33 cm × 23 cm × 9 cmt, which is compact laser system. Furthermore, if necessary, the overall volume can be reduced to one-third or less. The laser has an output power of 1 to 2 mW and a central wavelength of about 1.55 µm.

The reputation frequency fr of the optical frequency comb is very steady since the heating components is only the laser diode which is automatically stabilized at the proper current and the optical comb repetition frequency fr is 34 to 37 MHz. Generally, an optical fiber etalon is used in the case of increasing the repetition frequency [3]. However, if a tandem laser interferometer is used as the interferometer to increase the optical power available for measurement, it is not a problem even if the original repetition frequency is low. Furthermore, the fluctuation in fr was on the order of a few Hz and so the relative frequency stability is less than 10-7. The frequency drift due to the room temperature is less than 10 Hz/â??. The spectrum of the optical frequency comb, as shown in Figure 3, has a bandwidth of 30–80 nm. Furthermore, to investigate the stability of the optical comb’s repetition frequency fr, beat signals with fr1 = 34.876 MHz and fr2 = 34.709 MHz produced a stable beat, as shown in the Figure 4.

Since these are lasers of the same type, even if there is a slight drift, it is in the same direction of drift and is therefore expected to cancel out. Furthermore, it is presumed that the drift is caused by temperature effects. In practical applications, phase locking system is applied to ensure that the beat signal between the GPS frequency and the optical comb frequency remains constant, and so the optical comb is highly stable.

Fundamental Knowledge for Application

Two Optical Comb Interferometry

In the case to apply optical comb to distance metrology, it is easy to utilize the interferometric technique and interference fringe is processed to that in Figure 1. However, since the signal in this case consists of interference fringes, a low-pass filter with a large time constant is required to eliminate the fine structure of these fringe patterns. Furthermore, the width of the photodetection signal is approximately 10 times larger. However, since the signal-to-noise ratio (SNR) of the detection signal is significantly better, the resolution is expected to be more than 10 times higher. Additionally, since the travel range of the scanning stage is reduced by a factor of several hundred, a precision stage with a constant speed can be used. Additionally, since the travel range of the scanning stage is reduced by a factor of several hundred, a precision stage with a constant speed can be used. Taking all these factors into account, a resolution of several tens of nanometers is expected.Since numerous papers have been published on measurement technologies using optical combs as light sources, this paper will not cover them in detail. However, when the focus is on cost reduction rather than ultra-high precision, tandem interferometers offer advantages over methods using Fabry-Pérot etalons. This is because they allow for the use of lasers with higher power in actual measurements. Furthermore, the effective use of optical fibers contributes to the stability of the interferometer. Furthermore, single-mode optical fibers are immune to external disturbances and take up little space. This makes it easy to house the interferometer in a compact box. The peak of the interference fringes of an optical comb presents a significant challenge [4]. The interference fringes of comb 1 and comb 2 are several tens of micrometers wide, making measurement impossible when combs 1 and 2 are close together. However, the trigger pulse signal is very narrow (several tens of nanometers), allowing for accurate determination of the pulse interval length across various length ranges. Furthermore, the trigger signal can be automatically generated, enabling fast measurements. Figure 5 illustrates these aspects and the repetition frequencies of comb 1 and comb 2, showing the relationship between the pulse signals.

Finding the peak of the interference fringes of an optical comb presents a significant challenge [4]. The interference fringes of comb 1 and comb 2 are several tens of micrometers wide, making measurement impossible when combs 1 and 2 are close together. However, the trigger pulse signal is very narrow (several tens of nanometers), allowing for accurate determination of the pulse interval length across various length ranges. Furthermore, the trigger signal can be automatically generated, enabling fast measurements. Figure 5 illustrates these aspects and the repetition frequencies of comb 1 and comb 2, showing the relationship between the pulse signals.

In wide interference fringe signals, precise measurement of two closely spaced interference fringes requires ingenuity. Here, as shown in Figure 6, we developed a method in which the peaks of the interference fringes of comb 1 and comb 2 are measured separately and then mixed. The pulse measurement technique shown in Figure 5 can perform precise measurements without problems even for very closely spaced interference fringes.

As part of the interference fringe analysis procedure, as shown in Figure 7, we consider a virtual triangle using combs 1 and 2 and the difference between them. First, since the distance L due to the repeating combs themselves can be calculated, the angle α of the triangle is constant and can be determined with high resolution. Once the difference between comb 1 and comb 2 (L1-L2) is determined, L1 can be determined using angle α. Since (L1-L2) can be determined with an accuracy of 30 nm, L1 can be determined with an accuracy of 10^-6.

Tandem Interferometer for Distance Measurement

Here, a tandem interferometer was considered with cost reduction in mind. As the first interferometer, as shown in Figure 8, the reflectivity of the incident mirror (set to 25% here) was lowered, and the reflectivity of the other mirror was set to 100j%. In other words, it can be considered a Fizeau interferometer. Therefore, since the laser beam path is linear, the effects of atmospheric turbulence can be considerably reduced by using sealed tubes or vacuum tubes.

This is a schematic diagram of the interference fringe pattern shown in the figure. It is close to the Fizeau type, and the laser power of the optical pulse gradually decreases as the order increases. The repetition frequency fr of the developed optical comb is around 35 MHz, so this is for interference orders of 3 to 10. At this level, the signal-to-noise ratio of the interference fringe pattern is sufficiently good. Here, if the interference fringe order is set to 10, all the necessary interference fringes can be formed by moving the stage in the second interferometer by about 45 cm. This Fizeau type is easier to understand if you think of it as the delay of the optical pulse. Then, by evaluating the intensity of the interference fringe signal, the order of the delay can be correctly determined. As shown in Figure 8, a tandem interferometer is used for distance measurement. The lasers from comb 1 and comb 2 pass through an optical fiber switch; when the laser from comb 1 is passing, the laser from comb 2 is stopped.

After passing through this switch, the output passes through a circuit and is then split by a fiber beam. One beam is reflected by a ball lens (refractive index; 2.0) on a linearly moving stage. The other beam is reflected by a ball lens for distance measurement. These reflected beams are mixed at the BS and, via a circulator, are incident on the photodetector. At this time, the fiber switch is operated remotely.

High Accuracy by Optical Comb with GPS Frequency

Since the electrical power consumption of this optical comb system is less than 1 W, heat generation is minimal. The primary factor affecting performance is the environmental conditions at the laser installation site. Therefore, covering the device with thermal insulation keeps the fr value fairly constant. The frequency of GPS signals is used as the frequency standard. Recently, it has become possible to receive GPS signals (frequency: 1575.42 MHz) using a simple, small antenna. We will discuss phase locking between the GPS frequency in the air and the optical comb frequency fa. First, the signal acquired from the optical comb using a broadband frequency detector contains a signal at a frequency close to 1575.42 MHz. In other words, mixing the fa signal with the GPS signal generates a beat signal with a frequency of 17.5 MHz or less.

Let's go and trust each other and let's meet and learn. This makes handling easier. Next, to stabilize the optical comb, it is sufficient to stretch a portion of the optical fiber in the resonator with a telephone distortion element PZT. However, the location where the Light comb is installed should be within ±1 â?? of the current temperature and the temperature is lower. Let's do it, let's do it, let's do it, let's make it bright, let's do it together. Temperature control of the fiber is simple. As a result, the fr value can be kept with an accuracy of 10⁻⁸ or less.

Future Paradise of Factory

With the rapid advancements in 3D printing technology, quality control of products has become essential. This technology now enables the fabrication of large objects, such as houses. In such cases, since quality control relies primarily on image measurement, an accurate measuring tool is required, making optical comb interferometers particularly effective. In this context, inspections for age-related changes are also necessary. Furthermore, for small objects, the scale is in the nanometer range. Optical comb applications are also effective for measurements on the picometer scale [5]. These manufactured objects may require a relative accuracy of 1 ppm.

As for delivery, drones are expected to play a significant role. They are particularly effective for small items, which often require urgent delivery. In large-scale facilities such as the semiconductor factories currently under construction, it is also effective to establish small businesses near the factories. While remote work is becoming increasingly common, it primarily involves desk-based tasks such as design. In the field of experimentation, the design of patents and the production of new products through remote experimentation are also considered important. In particular, the child-rearing generation can choose from a variety of occupations, thereby contributing to societal development. Figure 10 is a schematic diagram illustrating these points.

Summary

We demonstrated that the development of low-cost, smart optical combs, linked with GPS, makes it possible to build new industrial and social systems. To build a prosperous society, collaboration between 3D printers and drones is expected. Furthermore, by implementing traceability under the Measurement Law, mutual recognition agreements can be met, thus avoiding trade tariff barriers.

References