Multiple optical frequency comb generation apparatus

Abstract

To provide a multiple optical frequency comb generation apparatus capable of generating multiple optical frequency combs at low cost with a simple configuration and of improving the utilization efficiency of excitation laser light.SOLUTION: The multiple optical frequency comb generation apparatus includes: a laser light source for emitting continuous-wave laser light; a first micro optical resonator configured to be excited by the continuous-wave laser light and to generate a first optical frequency comb; an optical filter disposed downstream of the first micro optical resonator and configured to separate the first optical frequency comb generated by the micro optical resonator from residual excitation light; and a second micro optical resonator configured to be excited by the residual excitation light separated by the optical filter and to generate a second optical frequency comb.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multiple optical frequency comb generator that generates multiple optical frequency combs. [Background technology]

[0002] Traditionally, technological innovations aimed at improving communication speeds have been a constant focus in the field of mobile communications. 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 emerging. However, with sixth-generation mobile communication systems (6G communication), which will achieve 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.

[0003] To overcome these challenges in the practical application of 6G communications and achieve ultra-high-speed, high-capacity communications, a paradigm shift that goes beyond the upper frequency limits imposed by electrical methods is strongly required. While 6G communications has the potential to significantly reduce the transmission speed gap between optical and radio communications, there exists a technological gap between the two due to the differences between optical and electrical technologies, which results in time delays associated with the conversion of 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.

[0004] Thus, 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 we could achieve wireless communications that involve as little electronics as possible, these challenges could be resolved.

[0005] Optical communications using optical fiber networks offer extremely high information transmission speeds, and recent advances have been made in silicon photonics technology, which achieves ultra-high speeds, large capacity, low latency, and low power consumption by replacing electronic wiring within 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 terahertz waves are generated by modulating and then combining light beams with different wavelengths, and these terahertz waves are used in wireless communications (see, for example, Non-Patent Document 1).

[0006] A terahertz wave generation method using light / terahertz wave conversion technology is 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 used 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.

[0007] As a frequency multiplexing method, a method has been disclosed in which an arbitrary optical frequency mode with a desired frequency interval is extracted from an optical frequency comb using a filter (see, for example, Patent Document 1). Also, it is effective to use a multi-frep micro optical comb resonator equipped with multiple micro resonators that generate optical frequency combs with different frequency intervals (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-004858 [Patent Document 2] Japanese Patent Publication No. 2023-170363 [Non-patent literature]

[0009] [Non-Patent Document 1] Tadao Nagatsuma, "Ultra-high-speed wireless communication pioneered by terahertz waves," Journal of the Japan Society for Precision Engineering, Vol. 82, No. 3, 2016 Summary of the Invention [Problem to be solved by the invention]

[0010] 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 the individual microresonator, which requires multiple excitation laser light sources and multiple optical systems, resulting in a complex device configuration and high costs.

[0011] In addition, the energy conversion efficiency from excitation laser light to a micro-optical comb is usually less than 1%, so most of the output from the multi-frep micro-optical comb resonator is residual excitation light that is not converted into a micro-optical comb, posing a challenge to improve the utilization efficiency of the excitation laser light.

[0012] The present invention has been proposed in view 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. [Means for solving the problem]

[0013] 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 is characterized by comprising at least a laser light source that emits continuous wave laser light, a first optical microresonator that generates a first optical frequency comb by being excited by the continuous wave laser light, an optical filter disposed downstream of the first optical microresonator that separates the first optical frequency comb generated by the optical microresonator from residual excitation light, and a second optical microresonator that generates a second optical frequency comb by being excited by the residual excitation light separated by the optical filter.

[0014] (2) A second aspect of the present invention is characterized in that, in the multiple optical frequency comb generator of the first aspect, the first micro-optical resonator and the second micro-optical resonator generate optical frequency combs with different frequency intervals.

[0015] (3) A third aspect of the present invention is characterized in that, in the multiple optical frequency comb generator of the first or second aspect, the first optical frequency comb and the second optical frequency comb have a frequency interval in the range of 100 GHz or more and 1 THz or less.

[0016] (4) A fourth aspect of the present invention is characterized in that, in the multiple optical frequency comb generator of any one of the first to third aspects, the first optical microresonator and the second optical microresonator are media 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).

[0017] (5) A fifth aspect of the present invention is characterized in that, in the multiple optical frequency comb generation device of any one of the first to fourth aspects, the difference in frequency spacing between the first optical frequency comb and the second optical frequency comb is in the range of 10 GHz or more and 50 GHz or less. [Effects of the Invention]

[0018] According to the present invention, it is possible 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. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a schematic diagram illustrating a configuration of a multiple optical frequency comb generator according to an embodiment of the present invention. [Figure 2] FIG. 1 is an explanatory diagram illustrating an optical frequency comb. [Figure 3] FIG. 1 is an explanatory diagram illustrating an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] A multiple optical frequency comb generator according to one embodiment of the present invention will be described below with reference to the drawings. The following embodiment 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 show key components enlarged for ease of understanding the features of the present invention, and the dimensional proportions of the components may not necessarily be the same as those in the actual device.

[0021] FIG. 1 is a schematic diagram showing the configuration of a multiple optical frequency comb generator according to one 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.

[0022] The laser light source 11 may be a laser light device that emits continuous wave laser light (CW laser light) of a single frequency, and in this embodiment, a DFB laser that emits laser light with an emission wavelength adjusted to 1550 nm or a wavelength therearound is used.

[0023] Each of the excitation units 12A, 12B, 12C, and 12D includes a micro-optical resonator 21 and an optical filter 22 disposed in the subsequent stage of the micro-optical resonator 21.

[0024] For example, the pumping unit 12A is composed of a first micro-optical resonator 21A and a first optical filter 22A. Similarly, the pumping unit 12B is composed of a second micro-optical resonator 21B and a second optical filter 22B, the pumping unit 12C is composed of a third micro-optical resonator 21C and a third optical filter 22C, and the pumping unit 12D is composed of a fourth micro-optical resonator 21D and a fourth optical filter 22D.

[0025] Of these, the first micro-optical resonator 21A of the pumping unit 12A is connected to the laser light source 11. The second micro-optical resonator 21B of the pumping unit 12B is connected to the side of the first optical filter 22A of the pumping unit 12A from which the residual pumping light is emitted. The third micro-optical resonator 21C of the pumping unit 12C is connected to the side of the second optical filter 22B of the pumping unit 12B from which the residual pumping light is emitted. The fourth micro-optical resonator 21D of the pumping unit 12D is connected to the side of the third optical filter 22C of the pumping unit 12C from which the residual pumping light is emitted.

[0026] First micro-optical resonator 21A is excited by continuous wave laser light (first micro-optical resonator 21A) emitted from laser light source 11 to generate a first optical frequency comb. Second micro-optical resonator 21B is excited by residual pumping light separated by first optical filter 22A of pumping unit 12A, which has the same optical components as the continuous wave laser light emitted from laser light source 11, to generate a second optical frequency comb. Similarly, third micro-optical resonator 21C is excited by residual pumping light separated by second optical filter 22B of pumping unit 12B to generate a third optical frequency comb. Similarly, fourth micro-optical resonator 21D is excited by residual pumping light separated by third optical filter 22C of pumping unit 12C to generate a fourth optical frequency comb.

[0027] In addition, each of the first to fourth micro-optical resonators 21A to 21D converts approximately 1% of the output value of the incident continuous wave laser light or residual excitation light into the respective optical frequency comb, 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.

[0028] Here, the optical frequency comb formed by each of the first to fourth micro-optical resonators 21A to 21D is a comb in which a series of optical frequency modes are arranged at the same frequency (f rep ) spacing and optical phase are aligned like the teeth of a comb, and the light has an ultra-discrete multispectral structure.

[0029] The first to fourth micro-optical resonators 21A to 21D may be micro-optical resonators including micro-ring optical waveguides formed on a semiconductor substrate by a semiconductor process, for example. The diameter of the micro-ring may be, for example, about 20 μm to 400 μm.

[0030] The microring optical waveguide may be made of any medium having a nonlinear optical effect, such as 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).

[0031] The optical frequency combs generated by the first to fourth micro-optical resonators 21A to 21D have short optical resonator lengths, so the frequency spacing (frep) between adjacent optical frequency modes can be increased. 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.

[0032] The first to fourth micro optical resonators 21A to 21D may be configured to generate optical frequency combs with the same frequency intervals, or may be configured to generate optical frequency combs with different frequency intervals.

[0033] When the first to fourth micro-optical resonators 21A to 21D are configured to generate optical frequency combs with the same frequency intervals, the first to fourth optical filters 22A to 22D of the excitation units 12A, 12B, 12C, and 12D can output optical frequency combs with the same frequency intervals.

[0034] In addition, by placing an optical phase control device after each excitation unit 12A, 12B, 12C, and 12D and generating terahertz waves using individually independent optical / terahertz wave conversion elements, it is possible to enhance the terahertz waves through coherent synthesis, control the beam profile, and control the beam deflection using a phased array antenna.

[0035] Furthermore, when the first to fourth micro-optical resonators 21A to 21D are configured to generate optical frequency combs with different frequency intervals, the first to fourth optical filters 22A to 22D of the excitation units 12A, 12B, 12C, and 12D can output optical frequency combs with four different frequency intervals.

[0036] By generating terahertz waves 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. This can also be used as a dual optical comb spectroscopic light source in the near-infrared wavelength region or the terahertz region.

[0037] The first to fourth optical filters 22A to 22D separate and output approximately 1% of the optical frequency comb generated by the first to fourth micro-optical resonators 21A to 21D, respectively, and the remaining approximately 99% of the residual pump light.

[0038] Of these, the residual excitation light separated by the first optical filter 22A of the excitation unit 12A is incident on the excitation unit 12B, the residual excitation light separated by the second optical filter 22B of the excitation unit 12B is incident on the excitation unit 12C, and the residual excitation light separated by the third optical filter 22C of the excitation unit 12C is incident on the excitation unit 12D. The residual excitation light separated by the fourth optical filter 22D of the excitation unit 12D can be input to a further excitation unit disposed below, or can be returned to the excitation unit 12A and incident on the excitation unit 12A together with the laser light emitted from the laser light source 11.

[0039] Each of the first to fourth optical filters 22A to 22D may be made up of a dichroic mirror, which may be formed in the same optical waveguide as the excitation unit by, for example, a fiber Bragg grating.

[0040] 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 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, residual excitation light 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 residual excitation light 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.

[0041] In this way, by using the residual excitation light that was not converted into an optical frequency comb in the previous-stage micro-optical resonator 21 in the subsequent-stage micro-optical resonator 21 to generate further optical frequency combs, it becomes possible to generate multiple optical frequency combs that are output from each of the multi-stage connected 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.

[0042] This significantly increases the efficiency of generating an optical frequency comb from a single laser light source 11. For example, if a multiplexed optical frequency comb generator is configured by connecting up to 10 pumping units, the total output of the optical frequency combs output from each pumping unit will be approximately 9.55% of the output of one laser light source, which is 100%, significantly increasing the conversion efficiency of the optical frequency comb for a single laser light source.

[0043] Furthermore, according to the multiple optical frequency comb generator 10 of this embodiment, by changing the frequency intervals of the optical frequency combs generated by the first to fourth micro-optical resonators 21A to 21D, it becomes 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).

[0044] The multiplexed optical frequency comb generated in this way is then used to extract two adjacent modes using an optical filter, which are then optically modulated independently and combined to perform optical-to-terahertz conversion. This makes it possible to generate ultra-high frequency, low phase noise, frequency-multiplexed terahertz waves with high energy utilization efficiency.

[0045] 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 connection between optical and mobile communications. Furthermore, it would be possible to significantly simplify frequency multiplexing compared to electrical approaches and improve the efficiency of optical energy utilization. [Example]

[0046] As shown in the upper part of Figure 3, an experimental device was fabricated using a pump laser device (emission wavelength 1560 nm) as the light source, with a micro-optical resonator between the first and second optical amplifiers, and a high-speed wavelength scanning mechanism downstream of the light source.

[0047] Using this experimental equipment, the optical 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).

[0048] 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 and became laser light with a wavelength of 1560 nm and an output of 500 mW (measurement position 2). This amplified laser light then passed through an optical microresonator 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 a second optical amplifier to obtain a multiplexed optical frequency comb with an output of 30 mW (measurement position 4). [Industrial Applicability]

[0049] The present invention contributes to a paradigm shift that transcends the limitations 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 the terahertz region. Therefore, it has industrial applicability. [Explanation of symbols]

[0050] 10...Multiple optical frequency comb generator 12A, 12B, 12C, 12D...Excitation units 21A…1st micro optical cavity 21B…Second micro optical resonator 21C...Third micro optical cavity 21D…4th microscopic optical cavity 22A...First optical filter 22B...Second optical filter 22C...Third optical filter 22D...Fourth optical filter

Claims

1. 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 optical microresonator that is excited by the residual excitation light separated by the optical filter to generate a second optical frequency comb.

2. 2. The multiple optical frequency comb generator according to claim 1, wherein the first and second micro-optical resonators generate optical frequency combs with different frequency intervals.

3. 3. The multiple optical frequency comb generator according to claim 1, wherein 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. The first micro-optical resonator and the second micro-optical resonator are media having a nonlinear optical effect, and are made of silicon nitride (Si ３ N ４ ), aluminum gallium arsenide (AlGaAs), lithium niobate (LiNbO ３ ), tantalum pentoxide (Ta ２ O ５ 3. The multiple optical frequency comb generator according to claim 1, wherein the multiple optical frequency comb generator is made of one or more media selected from the group consisting of gallium nitride (GaN), gallium nitride (GaN), and gallium nitride (GaN).

5. 3. The multiple optical frequency comb generator according to claim 1, wherein a difference in frequency interval between the first optical frequency comb and the second optical frequency comb is in the range of 10 GHz to 50 GHz.

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