JP7812146B2 - Multiplexed optical frequency comb generator - Google Patents

Description

The present invention relates to a multiplexed optical frequency comb generator that generates multiple optical frequency combs.

Traditionally, in the field of mobile communications, technological innovation has been a constant pursuit of improving communication speeds. In recent years, fourth-generation mobile communication systems (4G communication) and fifth-generation mobile communication systems (5G communication) have become mainstream, and compatible devices are gradually becoming available. However, with sixth-generation mobile communication systems (6G communication), which will offer communication speeds that surpass those of 5G, the expected frequency band (terahertz band) may reach the technical limits (upper frequency limit) of electrical methods. This raises concerns that issues such as lower wireless carrier output, increased phase noise, and increased signal transmission loss may become apparent.

To overcome these challenges in commercializing 6G communications and realize ultra-high-speed, high-capacity communications, there is a strong need for a paradigm shift that goes beyond the upper frequency limits imposed by electrical methods. Meanwhile, while 6G communications has the potential to significantly reduce the transmission speed gap between optical and radio communications, there is a technological gap between the two due to the differences between optical and electrical technologies, and time delays occur when converting optical and electrical signals (optical-to-electrical conversion). There are concerns that such time delays could be a fatal obstacle to 6G, which requires ultra-low latency.

As such, in order to realize 6G communications, there is a strong demand for a "seamless connection between optical and radio communications" that achieves excellent connection with optical communications while achieving ultra-low latency. These challenges arise from the fact that radio communications are based on electronics. Therefore, if wireless communications could be realized with as little electronic intervention as possible, these challenges could be resolved.

Optical communications using optical fiber networks enable extremely high information transmission speeds, and recent advances have led to the development of silicon photonics technology, which achieves ultra-high speeds, large capacity, low latency, and low power consumption by replacing the electronic wiring inside devices with optical wiring. Given this background, optical devices are sometimes used as carrier generators in wireless communications, and optical communications technology is sometimes incorporated into part of communication systems. For example, an example has been disclosed in which light beams with different wavelengths are modulated and then combined to generate terahertz waves, which are then used in wireless communications (see, for example, Non-Patent Document 1).

Terahertz wave generation techniques using light/terahertz wave conversion technology are expected to be a method for generating ultra-high frequency, low phase noise terahertz waves. This method makes it possible to modulate terahertz waves in the optical domain using existing optical devices, so modulation methods used in optical communications can be adapted to modulate terahertz waves. Furthermore, frequency multiplexing is a technology that is directly linked to increasing the capacity of wireless communications, and is therefore considered a necessary technological element for 6G communications.

One method of frequency multiplexing has been disclosed in which a filter is used to extract any optical frequency mode with a desired frequency interval from an optical frequency comb (see, for example, Patent Document 1). It is also effective to use a multi-frep micro optical comb resonator equipped with multiple microresonators that generate optical frequency combs with different frequency intervals (see, for example, Patent Document 2).

JP 2009-004858 A Japanese Patent Application Laid-Open No. 2023-170363

Tadao Nagatsuma, "Ultra-high-speed wireless communication pioneered by terahertz waves," Journal of the Japan Society for Precision Engineering, Vol. 82, No. 3, 2016

However, conventional multi-frep micro optical comb resonators have a structure in which each microresonator is excited in parallel by an individual excitation laser light source corresponding to that resonator, which requires multiple excitation laser light sources and multiple optical systems, resulting in a complex device configuration and high costs.

In addition, the energy conversion efficiency from pump laser light to a micro-optical comb is typically less than 1%, so most of the output from a multi-frep micro-optical comb resonator is residual pump light that is not converted into a micro-optical comb, posing a challenge for improving the utilization efficiency of the pump laser light.

This invention was proposed in light of the above-mentioned problems, and aims to provide a multiple optical frequency comb generator that can generate a multiple optical frequency comb with a simple configuration and at low cost, and that can increase the utilization efficiency of the excitation laser light.

In order to solve the above problems, a multiple optical frequency comb generator according to one embodiment of the present invention proposes the following means.
(1) A multiple optical frequency comb generator according to a first aspect of the present invention includes at least a laser light source that emits continuous wave laser light, a first micro-optical resonator that generates a first optical frequency comb by being excited by the continuous wave laser light, an optical filter that is disposed downstream of the first micro-optical resonator and that separates the first optical frequency comb generated by the micro-optical resonator from residual excitation light, and a second micro-optical resonator that generates a second optical frequency comb by being excited by the residual excitation light separated by the optical filter, wherein the first micro-optical resonator and the second micro-optical resonator are formed on a semiconductor substrate .

(2) Aspect 2 of the present invention is characterized in that, in the multiplexed optical frequency comb generator of aspect 1, the first optical microresonator and the second optical microresonator generate optical frequency combs with different frequency intervals.

(3) Aspect 3 of the present invention is characterized in that, in the multiplexed optical frequency comb generator of aspect 1 or 2, the first optical frequency comb and the second optical frequency comb have a frequency interval in the range of 100 GHz to 1 THz.

(4) A fourth aspect of the present invention is the multiple optical frequency comb generator according to any one of the first to third aspects, characterized in that the first and second micro-optical resonators are made of a medium having a nonlinear optical effect, and are made of one or more media selected from the group consisting of silicon _nitride ( _Si3N4 ), aluminum gallium arsenide (AlGaAs), lithium niobate ( _LiNbO3 ), tantalum pentoxide ( _Ta2O5 ), _and gallium nitride (GaN).

(5) Aspect 5 of the present invention is characterized in that, in the multiple optical frequency comb generator of any one of aspects 1 to 4, the difference in frequency spacing between the first optical frequency comb and the second optical frequency comb is in the range of 10 GHz to 50 GHz.

The present invention makes it possible to provide a multiple optical frequency comb generator that can generate multiple optical frequency combs at low cost with a simple configuration and that can increase the utilization efficiency of excitation laser light.

1 is a schematic diagram illustrating a configuration of a multiple optical frequency comb generator according to an embodiment of the present invention. FIG. 1 is an explanatory diagram illustrating an optical frequency comb. FIG. 1 is an explanatory diagram illustrating an embodiment of the present invention.

The following describes a multiplexed optical frequency comb generator according to one embodiment of the present invention, with reference to the drawings. Note that the embodiment described below is specifically described to provide a better understanding of the spirit of the invention, and does not limit the present invention unless otherwise specified. Furthermore, the drawings used in the following description may, for convenience, show enlarged essential parts to make the features of the present invention easier to understand, and the dimensional proportions of each component may not necessarily be the same as in reality.

FIG. 1 is a schematic diagram showing the configuration of a multiple optical frequency comb generator according to an embodiment of the present invention.
The multiplexed optical frequency comb generator 10 of this embodiment includes one laser light source 11 and a plurality of pumping units 12A, 12B, 12C, and 12D (four in this embodiment).
Furthermore, if necessary, it is possible to easily generate a micro optical comb by arranging a high-speed wavelength scanning mechanism or an optical amplifier in front of the pumping unit 12A or each of the pumping units 12B, 12C, and 12D.

The laser light source 11 may be any laser light device that emits continuous wave laser light (CW laser light) at a single frequency. In this embodiment, a DFB laser that emits laser light with an emission wavelength tuned to 1550 nm or a wavelength therearound is used.

Each of the excitation units 12A, 12B, 12C, and 12D includes a micro-optical resonator 21 and an optical filter 22 arranged downstream of the micro-optical resonator 21.

For example, excitation unit 12A is composed of a first micro-optical resonator 21A and a first optical filter 22A. Similarly, excitation unit 12B is composed of a second micro-optical resonator 21B and a second optical filter 22B, excitation unit 12C is composed of a third micro-optical resonator 21C and a third optical filter 22C, and excitation unit 12D is composed of a fourth micro-optical resonator 21D and a fourth optical filter 22D.

Of these, the first optical microresonator 21A of the excitation unit 12A is connected to the laser light source 11. The second optical microresonator 21B of the excitation unit 12B is connected to the side of the first optical filter 22A of the excitation unit 12A from which residual excitation light is emitted. The third optical microresonator 21C of the excitation unit 12C is connected to the side of the second optical filter 22B of the excitation unit 12B from which residual excitation light is emitted. The fourth optical microresonator 21D of the excitation unit 12D is connected to the side of the third optical filter 22C of the excitation unit 12C from which residual excitation light is emitted.

The first micro-optical resonator 21A is excited by the continuous wave laser light (first micro-optical resonator 21A) emitted from the laser light source 11 to generate a first optical frequency comb. The second micro-optical resonator 21B is excited by residual excitation light, separated by the first optical filter 22A of the excitation unit 12A, that contains the same optical components as the continuous wave laser light emitted from the laser light source 11, to generate a second optical frequency comb. Similarly, the third micro-optical resonator 21C is excited by residual excitation light separated by the second optical filter 22B of the excitation unit 12B to generate a third optical frequency comb. Similarly, the fourth micro-optical resonator 21D is excited by residual excitation light separated by the third optical filter 22C of the excitation unit 12C to generate a fourth optical frequency comb.

In addition, the first to fourth micro-optical resonators 21A to 21D each convert approximately 1% of the output value of the incident continuous wave laser light and residual excitation light into their respective optical frequency combs, and the remaining approximately 99% is output together with the optical frequency comb as residual excitation light with the same optical components as the continuous wave laser light.

Here, the optical frequency comb formed by each of the first to fourth optical microresonators 21A to 21D is light having an ultra-discrete multispectral structure in which a series of optical frequency modes are arranged in a comb-like pattern at equal frequency (f _rep ) intervals and with aligned optical phases, as shown in Figure 2.

The first to fourth optical microresonators 21A to 21D may be optical microresonators including microring optical waveguides formed on a semiconductor substrate by a semiconductor process. The diameter of the microring may be, for example, approximately 20 μm to 400 μm.

The microring optical waveguide may be made of any medium having a nonlinear optical effect, and may be made of, for example, one or more media selected from the group consisting of silicon nitride (Si ₃ N ₄ ), aluminum gallium arsenide (AlGaAs), lithium niobate (LiNbO ₃ ), tantalum pentoxide (Ta ₂ O ₅ ), and gallium nitride (GaN).

The optical frequency combs generated by the first to fourth micro-optical resonators 21A to 21D each have a short optical resonator length, allowing for a large frequency spacing (frep) between adjacent optical frequency modes. The frequency spacing (frep) between adjacent optical frequency modes may be, for example, 100 GHz or more and 3 THz or less. From the perspective of application to terahertz communications, it may more preferably be 100 GHz or more and 1 THz or less.

The first to fourth micro-optical resonators 21A to 21D may be configured to generate optical frequency combs with the same frequency interval, or may be configured to generate optical frequency combs with different frequency intervals.

When the first to fourth optical microresonators 21A to 21D are configured to generate optical frequency combs with the same frequency interval, the first to fourth optical filters 22A to 22D of each of the excitation units 12A, 12B, 12C, and 12D can output optical frequency combs with the same frequency interval.

In addition, by placing an optical phase control device after each excitation unit 12A, 12B, 12C, and 12D and generating terahertz waves using individual independent optical/terahertz wave conversion elements, it is possible to enhance terahertz waves through coherent synthesis, control the beam profile, and control the beam deflection using a phased array antenna.

Furthermore, if the first to fourth optical microresonators 21A to 21D are configured to generate optical frequency combs with different frequency intervals, the first to fourth optical filters 22A to 22D of each of the excitation units 12A, 12B, 12C, and 12D can output optical frequency combs with four different frequency intervals.

When terahertz waves are generated using an optical frequency comb with different frequency intervals and an optical/terahertz wave conversion element, it becomes possible to generate frequency-multiplexed terahertz waves. It can also be used as a dual optical comb spectroscopic light source in the near-infrared wavelength region or terahertz region.

The first optical filter 22A to the fourth optical filter 22D separate and output approximately 1% of the optical frequency comb generated by the first micro-optical resonator 21A to the fourth micro-optical resonator 21D, respectively, from the remaining approximately 99% of the residual excitation light.

Of these, the residual excitation light separated by the first optical filter 22A of excitation unit 12A is incident on excitation unit 12B, the residual excitation light separated by the second optical filter 22B of excitation unit 12B is incident on excitation unit 12C, and the residual excitation light separated by the third optical filter 22C of excitation unit 12C is incident on excitation unit 12D. The residual excitation light separated by the fourth optical filter 22D of excitation unit 12D can be input to a further excitation unit located below, or returned to excitation unit 12A and incident on excitation unit 12A together with the laser light emitted from the laser light source 11.

Each of the first optical filter 22A to fourth optical filter 22D may be composed of a dichroic mirror. Such a dichroic mirror can be formed in the same optical waveguide as the excitation unit, for example, using a fiber Bragg grating.

The operation and effects of the multiple optical frequency comb generator configured as above will now be described.
According to the multiplexed optical frequency comb generator 10 of this embodiment, continuous wave laser light emitted from the laser light source 11 is excited by the first micro-optical resonator 21A to generate a first optical frequency comb, residual excitation light that was not converted into the first optical frequency comb is used to excite the second micro-optical resonator 21B to generate a second optical frequency comb, the residual excitation light that was not converted into the second optical frequency comb is used to excite the third micro-optical resonator 21C to generate a third optical frequency comb, and the residual excitation light that was not converted into the third optical frequency comb is used to excite the fourth micro-optical resonator 21D to generate a fourth optical frequency comb.

In this way, the residual excitation light that was not converted into an optical frequency comb by the previous-stage micro-optical resonator 21 is used in the subsequent-stage micro-optical resonator 21 to generate further optical frequency combs. This makes it possible to generate multiple optical frequency combs that are output by each of the cascaded excitation units 12A to 12D, whereas optical frequency combs that normally generate only about 1% of the input continuous wave laser light are normally generated.

This significantly increases the efficiency of generating an optical frequency comb from a single laser light source 11. For example, if up to 10 pumping units are connected to form a multiplexed optical frequency comb generator, the total output of the optical frequency combs output from each pumping unit will be approximately 9.55% of the output of a single laser light source, which is 100%, making it possible to significantly increase the conversion efficiency of the optical frequency comb for a single laser light source.

Furthermore, according to the multiple optical frequency comb generator 10 of this embodiment, by changing the frequency spacing of the optical frequency combs generated by the first to fourth micro-optical resonators 21A to 21D, it is possible to output a variety of optical frequency combs from multiple channels according to the number of micro-optical resonators 21 formed (generation of a multi-frep optical frequency comb).

By extracting two adjacent modes from the multiplexed optical frequency comb generated in this way using an optical filter, then independently modulating them and combining them to perform optical-to-terahertz conversion, it becomes possible to generate ultra-high frequency, low phase noise, frequency-multiplexed terahertz waves with high energy utilization efficiency.

If we could generate such ultra-high frequency, low-phase-noise, frequency-multiplexed terahertz waves, it would enable a paradigm shift that transcends the limits of communication electronics and enable seamless connectivity between optical and mobile communications. Furthermore, it would be possible to significantly simplify frequency multiplexing compared to electrical approaches and increase the efficiency of optical energy utilization.

As shown in the upper part of Figure 3, an experimental device was created that used a pump laser device (emission wavelength 1560 nm) as the light source, installed a micro-optical resonator between the first and second optical amplifiers, and equipped with a high-speed wavelength scanning mechanism downstream of the light source.

Using this experimental setup, the light intensity and optical spectrum were measured in the wavelength range of 1470 nm to 1650 nm at the output side of the high-speed wavelength scanning mechanism (measurement position 1), the output side of the first optical amplifier (measurement position 2), the output side of the micro-optical resonator (measurement position 3), and the output side of the micro-optical resonator (measurement position 4).

As a result, as shown in the lower part of Figure 3, the laser light emitted from the pump laser device had a wavelength of 1560 nm and an output of 1 mW (measurement position 1). This laser light passed through the first optical amplifier to become laser light with a wavelength of 1560 nm and an output of 500 mW (measurement position 2). This amplified laser light then passed through a micro-optical resonator to generate a multiplexed optical frequency comb with an output of 1.5 mW (measurement position 3). This multiplexed optical frequency comb then passed through the second optical amplifier to generate a multiplexed optical frequency comb with an output of 30 mW (measurement position 4).

The present invention contributes to a paradigm shift that transcends the limits of mobile (wireless) communication electronics, and to wireless communication technology that can support, for example, sixth-generation mobile communication systems (6G) by seamlessly connecting optical and mobile communications. It can also be used as a dual optical comb spectral light source in the near-infrared wavelength region or terahertz region. Therefore, it has industrial applicability.

REFERENCE SIGNS LIST 10... Multiplexed optical frequency comb generator 12A, 12B, 12C, 12D... Excitation unit 21A... First micro-optical resonator 21B... Second micro-optical resonator 21C... Third micro-optical resonator 21D... Fourth micro-optical resonator 22A... First optical filter 22B... Second optical filter 22C... Third optical filter 22D... Fourth optical filter

Claims (5)

a laser light source that emits continuous wave laser light;
a first micro-optical resonator that is excited by the continuous wave laser light to generate a first optical frequency comb;
an optical filter disposed downstream of the first micro-optical resonator, for separating the first optical frequency comb generated in the micro-optical resonator from residual pump light;
a second micro-optical resonator that is excited by the residual excitation light separated by the optical filter to generate a second optical frequency comb ;
The multiple optical frequency comb generating device is characterized in that the first micro-optical resonator and the second micro-optical resonator are formed on a semiconductor substrate . The multiple optical frequency comb generator described in claim 1, characterized in that the first optical microresonator and the second optical microresonator generate optical frequency combs with different frequency intervals. The multiple optical frequency comb generator according to claim 1 or 2, characterized in that the first optical frequency comb and the second optical frequency comb have a frequency spacing in the range of 100 GHz to 1 THz. 3. The multiple optical frequency comb generating device according to claim 1, wherein the first micro-optical resonator and the _second micro-optical resonator are made of a medium having a nonlinear optical effect, and are composed of one or more media selected from the group consisting of silicon nitride ( _Si3N4 ), aluminum gallium arsenide (AlGaAs), lithium niobate ( _LiNbO3 ), _tantalum pentoxide ( _Ta2O5 ), and gallium nitride (GaN). The multiple optical frequency comb generator according to claim 1 or 2, characterized in that the difference in frequency spacing between the first optical frequency comb and the second optical frequency comb is in the range of 10 GHz to 50 GHz.