Multiplex optical frequency comb generation device

Abstract

The present invention is characterized by having at least: a laser light source (11) that emits continuous oscillation laser light; a first micro-optical resonator (21A) that generates a first optical frequency comb through excitation based on the continuous oscillation laser light; an optical filter (22A) that is disposed downstream of the first micro-optical resonator (21A) and separates residual excitation light and the first optical frequency comb generated in the micro-optical resonator; and a second micro-optical resonator (21B) that generates a second optical frequency comb through excitation based on the residual excitation light having been separated by the optical filter (22A).

Description

Multiplexed optical frequency comb generator

[0001] The present invention relates to a multiple optical frequency comb generator that generates multiple optical frequency combs. This application claims priority to Japanese Patent Application No. 2024-046015, filed on March 22, 2024, the contents of which are incorporated herein by reference.

[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 realize 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 differences between optical and electrical technologies, resulting 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 wireless communications could be realized with as little electronic intervention as possible, these challenges could be resolved.

[0005] Optical communications using optical fiber networks offer extremely high information transmission speeds, and recent advances in silicon photonics technology have enabled the replacement of electronic wiring within devices with optical wiring, achieving ultra-high speeds, large capacity, low latency, and low power consumption. Against this backdrop, optical devices are sometimes used as carrier generators in wireless communications, and optical communications technology is sometimes incorporated into communication systems. For example, a case 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 generate terahertz waves at ultra-high frequencies and with low phase noise. This method allows terahertz waves to be modulated 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 of extracting an arbitrary optical frequency mode with a desired frequency interval from an optical frequency comb using a filter has been disclosed (see, for example, Patent Document 1). Also, it is 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).

[0008] JP 2009-004858 A JP 2023-170363 A

[0009] Tadao Nagatsuma, "Ultra-high-speed wireless communication pioneered by terahertz waves," Journal of the Japan Society for Precision Engineering, Vol. 82, No. 3, 2016

[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, since the energy conversion efficiency from excitation laser light to a micro-optical comb is usually less than 1%, 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, which posed a challenge in improving 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.

[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 aspect 1 of the present invention is characterized by including 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 in 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.

[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 to 1 THz.

[0016] (4) Aspect 4 of the present invention is the multiple optical frequency comb generator according to any one of Aspects 1 to 3, wherein 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 ), and gallium nitride (GaN).

[0017] (5) A fifth aspect of the present invention is characterized in that, in the multiple optical frequency comb generator 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 to 50 GHz.

[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.

[0019] The present invention relates to a multiplexed optical frequency comb generator, a method for generating multiple optical frequency combs, and a method for generating multiple optical frequency combs.

[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] 1 is a schematic diagram showing a multiple optical frequency comb generator according to one embodiment of the present invention. The multiple optical frequency comb generator 10 of this embodiment includes one laser light source 11 and multiple pumping units (four in this embodiment: 12A, 12B, 12C, and 12D). If necessary, a high-speed wavelength scanning mechanism or an optical amplifier can be installed in front of the pumping unit 12A or each of the pumping units 12B, 12C, and 12D to facilitate the generation of a micro optical comb.

[0022] The laser light source 11 may be any 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 optical microresonator 21A and a first optical filter 22A. Similarly, the pumping unit 12B is composed of a second optical microresonator 21B and a second optical filter 22B, the pumping unit 12C is composed of a third optical microresonator 21C and a third optical filter 22C, and the pumping unit 12D is composed of a fourth optical microresonator 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 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 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 residual pumping light is emitted.

[0026] 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 pumping light, which has been separated by the first optical filter 22A of the pumping unit 12A and has 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 pumping light, which has been separated by the second optical filter 22B of the pumping unit 12B, to generate a third optical frequency comb. Similarly, the fourth micro-optical resonator 21D is excited by residual pumping light, which has been separated by the third optical filter 22C of the pumping unit 12C, to generate a fourth optical frequency comb.

[0027] In addition, each of the first to fourth optical microresonators 21A to 21D converts approximately 1% of the output value of the incident continuous wave laser light or residual excitation light into its 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 optical microresonators 21A to 21D may be optical microresonators including microring optical waveguides formed on a semiconductor substrate by a semiconductor process, with the diameter of the microring being, for example, approximately 20 μm to 400 μm.

[0030] The micro-ring optical waveguide may be made of a medium having a nonlinear optical effect, such as silicon nitride (Si ３ N ４ ), aluminum gallium arsenide (AlGaAs), lithium niobate (LiNbO ３ ), tantalum pentoxide (Ta ２ O ５), 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 optical microresonators 21A to 21D are configured to generate optical frequency combs with the same frequency intervals, optical frequency combs with the same frequency intervals can be output from the first to fourth optical filters 22A to 22D of each of the excitation units 12A, 12B, 12C, and 12D.

[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, optical frequency combs with four different frequency intervals can be output from the first to fourth optical filters 22A to 22D of each of the excitation units 12A, 12B, 12C, and 12D. In this case, there may be an arbitrary difference in the frequency interval of the optical frequency combs generated between any two micro-optical resonators selected from the first to fourth micro-optical resonators 21A to 21D, and this difference may be, for example, in the range of 10 GHz to 50 GHz.

[0036] By using an optical frequency comb with different frequency intervals and an optical / terahertz wave conversion element to generate terahertz waves, 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 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.

[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. Note that 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 formed 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 described above will now be described. According to the multiple 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, and residual excitation light that was not converted to 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 to the second optical frequency comb is then 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 to the third optical frequency comb is then 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 excitation units 12A to 12D connected in multiple stages, 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 generation efficiency of an optical frequency comb generated 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 taken as 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 interval of the optical frequency combs generated by the first to fourth optical microresonators 21A to 21D, it becomes possible to output a variety of optical frequency combs from multiple channels according to the number of optical microresonators 21 formed (generation of multi-frep optical frequency combs).

[0044] The multiplexed optical frequency comb generated in this way is then optically modulated independently to extract two adjacent modes using an optical filter, and then 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.

[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 provided between the first optical amplifier and the second optical amplifier, and equipped with a high-speed wavelength scanning mechanism downstream of the light source.

[0047] Using this experimental apparatus, 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 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 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).

[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.

[0050] 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

1. A multiple optical frequency comb generator comprising at least: a laser light source that emits continuous wave laser light; a first optical microresonator that is excited by the continuous wave laser light to generate a first optical frequency comb; an optical filter that is arranged downstream of the first optical microresonator and separates the first optical frequency comb generated in the optical microresonator from residual excitation light; and 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. 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. A multiple optical frequency comb generator according to claim 1 or 2, 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 and second micro-optical resonators 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. A multiple optical frequency comb generator according to claim 1 or 2, characterized in that the 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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