Microcomb control device

Abstract

Provided is a microcomb controller that, by using a coupled ring microresonator and without the use of feedback control, generates a soliton comb that is insensitive to heat, as a passive and simple means.　The present invention comprises: a laser light source device (100) that radiates excitation light that is a continuous wave; a coupled resonator that comprises a primary microresonator (102) that generates a microcomb through coupling of the excitation light and a secondary microresonator (103) that is disposed in parallel to the primary microresonator and has a resonant frequency different from that of the primary microresonator; a heater (104) that heats the secondary microresonator; and a temperature controller that controls the temperature of the heater. The temperature controller adjusts the temperature of the heater so that the AMX, which is produced when the resonant frequency of the primary microresonator and the resonant frequency of the secondary microresonator mutually approach within a prescribed range, is produced in a resonant frequency mode that fulfills the condition dfrep / dT = 0 and with a nearest-approach resonant frequency difference, to reduce low-phase noise.

Description

Microcom Control Unit

[0001] The present invention relates to a microcomb controller, and more particularly to a microcomb controller that reduces phase noise resulting from thermal fluctuations.

[0002] An optical frequency comb is a type of ultrashort-pulse mode-locked laser that has a discrete, evenly spaced comb-like spectrum. Among optical frequency combs, those that generate a comb-like spectrum using a microscale microresonator are called microcombs (see, for example, Non-Patent Document 1). Microcombs, particularly soliton combs in a mode-locked state, have attracted attention as compact, mass-producible frequency combs and are expected to be used in optical / THz communications, LiDAR chip-scale optical atomic clocks, and other applications (see, for example, Non-Patent Document 2). Reducing the phase noise of microcombs reduces the noise of terahertz transmitters in optical / THz communications, enabling advanced terahertz communications and radar.

[0003] P. Del'Haye, et al. , Nature, 450, pp. 1214-1217 (2007) T. Herr, et al. , Nature Photonics, 8, pp. 145-152 (2014) T. Tetsumoto et al. , Nature Photonics, 15, 516 (2021) N. Kuse, et al. , Communications Physics, 5:312 (2022) J. R. Stone, et al. , Phys. Rev. Lett. , 125, 153901 (2020)

[0004] However, the microcomb repetition frequency f ｒｅｐ and carrier envelope offset frequency f ＣＥＯ The phase noise of a synchronous optical fiber is limited by the thermo-optic effect resulting from thermal noise, resulting in poor frequency accuracy and phase noise precision. From the perspective of phase noise, a method of stabilizing the phase noise using two Brillouin lasers (see Non-Patent Document 3) and a method of stabilizing the phase noise using a long fiber (see Non-Patent Document 4) have been demonstrated to reduce the phase noise at the repetition rate by 40 dB or more compared to when these methods are not used, but this has the drawback of making the system complex.

[0005] Furthermore, when constructing a feedback loop, the frequency of the pump light is modulated, so if attention is paid to each individual comb mode, the phase noise may worsen.

[0006] In view of the above problems, an object of the present invention is to provide a microcomb control device that generates a thermally insensitive soliton comb using a coupled-ring microresonator in a passive and simple manner without using feedback control.

[0007] A first aspect of the present invention is a microcomb control device, comprising: a laser light source device that irradiates excitation light that is a continuous wave; a coupled resonator that includes a waveguide into which the excitation light is incident and a main microresonator that generates a microcomb by optically coupling with the waveguide; a sub-microresonator that is optically coupled with the main microresonator and has a resonance frequency different from that of the main microresonator; a heater that heats the sub-microresonator; and a temperature controller that controls the temperature of the heater, wherein the temperature controller controls the AMX that is generated when the resonance frequency of the main microresonator and the resonance frequency of the sub-microresonator are close to each other within a predetermined range and are df rep The main point is to adjust the heater temperature so that the resonance frequency mode that satisfies / dT=0 and the closest resonance frequency difference are generated, thereby reducing the low phase noise.

[0008] In the first aspect of the present invention, the temperature controller may control the temperature of the heater to change the resonant frequency mode generated by the AMX, thereby searching for a suitable temperature.

[0009] In the first aspect of the present invention, the temperature controller adjusts the temperature of the heater to |df ｒｅｐ After identifying the resonant frequency mode generated by the AMX in which |df| is minimum, the temperature of the heater is finely adjusted to adjust the closest frequency difference in the AMX, and |df ｒｅｐ The temperature of the heater may be adjusted so that | / dT|=0.

[0010] In the first aspect of the present invention, a photodetector is further provided for detecting transmitted light from the waveguide, and the comb mode frequency f detected by the photodetector is rep Change in temperature df repThe heater temperature may be adjusted based on / dT.

[0011] According to the present invention, by using a coupled-ring microresonator, it is possible to provide a microcomb control device that generates a soliton comb that is insensitive to heat as a passive and simple method without using feedback control.

[0012] 1 is a block diagram showing an example of the configuration of a microcomb control device according to an embodiment of the present invention; 2 is a cross-sectional schematic diagram of an example of a microresonator and a microheater constituting the microcomb control device according to the present embodiment; 3 is a diagram showing the spectrum of an optical frequency comb in the frequency domain; 4 is a diagram explaining AMX; 5 is an enlarged view of a part of FIG. 4; 6 is a graph showing the df when the nearest resonant frequency difference is fixed and the mode of the microcomb in which AMX occurs is changed; ｒｅｐ 10 is a graph showing the behavior of df / dT when the position of the comb mode where AMX occurs is fixed and the nearest frequency difference is changed. ｒｅｐ 1 is a graph showing the behavior of / dT when random thermal fluctuations are applied to the system. ｒｅｐ df ｒｅｐ 1 is a graph showing the power spectrum of the signal with respect to frequency.

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings relating to the embodiment, the same or similar parts are designated by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each component, etc., may differ from the actual ones. Therefore, specific thicknesses and dimensions should be determined with reference to the following explanation. Furthermore, it goes without saying that the drawings may include parts with different dimensional relationships and ratios.

[0014] Furthermore, the embodiments are merely examples of devices and methods for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention does not limit the configuration, arrangement, layout, etc. of each component to those described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0015] (Embodiment) A microcomb control device according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 shows an example of the configuration of a microcomb control device according to an embodiment of the present invention. As shown in Fig. 1, the microcomb control device 10 according to this embodiment is composed of a CW laser light source device 100, a waveguide 101, a main microresonator 102, a sub-microresonator 103, and a microheater 104. The microheater 104 is connected to an electrode (not shown) and is further connected to a signal control device (not shown).

[0016] FIG. 2 shows an example of the structure of a microresonator used in the microcomb control device according to this embodiment. FIG. 2 is a partial cross-sectional view of an example of a main microresonator 102, a sub-microresonator 103, and a microheater 104. As shown in FIG. 2, the main microresonator 102 and the sub-microresonator 103 are formed on a substrate 201. A cladding layer 202 is formed on the microresonator 103 and the substrate 201, and the microheater 104 is placed on the sub-microresonator 103 with the cladding layer 202 sandwiched between them. The above structure can be fabricated, for example, using semiconductor manufacturing equipment.

[0017] The microheater 104 is connected to a signal control device (not shown) via electrodes (not shown). The signal control device controls the temperature of the sub-microresonator 103 by controlling the current flowing through the microheater 104. Changing the temperature of the microresonator 103 changes the refractive index of the sub-microresonator 103 due to the temperature dependency of the refractive index of the medium constituting the sub-microresonator 103, and changes the resonant frequency of the sub-microresonator 103 due to changes in the resonator length of the sub-microresonator 103. The signal control device functions as a temperature control device that controls the temperature of the microheater 104.

[0018] The main microresonator 102 and the sub-microresonator 103 are made of, for example, silicon nitride (Si ３ N ４ ), tantalum pentoxide (Ta ２ O ５The main microcavity 102 and the sub-microcavity 103 are made of a medium having a third-order nonlinear optical effect, such as Si, gallium nitride (GaN), or aluminum gallium arsenide (AlGaAs). ３ N ４ , Ta ２ O ５ In the case of AlGaAs, a SiO (not shown) is formed on a Si substrate. ２ The substrate 201 is formed with the film, and the main microcavity 102 and the sub-microcavity 103 are formed thereon. If the medium of the main microcavity 102 and the sub-microcavity 103 is GaN, Al ２ O ３ is used as the substrate 201. The cladding layer 204 is made of, for example, SiO ２ The micro-heater 104 is made of a metal such as titanium (Ti) or platinum (Pt).

[0019] In this embodiment, the main microresonator 102 and the sub microresonator 103 are ring-shaped waveguides formed on a substrate, but any microresonators that can generate a soliton comb and sweep a comb mode can be used, and are not limited to ring-shaped waveguides formed on a substrate.

[0020] As shown in FIG. 1, the waveguide 101 and the main microresonator 102 are spaced apart by a distance d １ The main microresonator 102 and the sub-microresonator 103 are spaced apart by a distance d ２ The waveguide 101 and the main microresonator 102, and the main microresonator 102 and the sub-microresonator 103 are optically coupled to each other, forming a coupled-ring microresonator. The waveguide 101 and the sub-microresonator 103 are formed at positions where they are not optically coupled to each other, and the sub-microresonator 103 is optically coupled only to the main microresonator 102.

[0021] The ring-shaped main microresonator 102 and the sub microresonator 103 each have a circumference length of L. １ , and L ２ , the refractive index of the medium is n １ , and n ２, m, p are positive integers, and the speed of light is c,

[0022]

[0023]

[0024] Multiple resonant frequencies ν satisfying ｍ , ν ｐ The resonator is made of a medium having a third-order nonlinear optical effect, and when the wavelength of the pump light coupled to the resonator matches one of the resonant frequencies of the resonator, four-wave mixing occurs due to the optical Kerr effect, and thus a microcomb is generated. The main microresonator 102 and the sub-microresonator 103 are designed so that all of the resonant frequencies do not match each other. That is, n １ L １ ≠n ２ L ２ It is designed to be.

[0025] The pumping light 105 emitted from the CW laser light source device 100 propagates through the waveguide 101 and is coupled to the main microresonator 102. The frequency difference between the pumping light 112 emitted from the CW laser light source device 100 and one of the multiple resonant frequencies of the main microresonator 102 is set to a predetermined range f ｏｕｔ If kept within this range, a soliton comb will be generated.

[0026] The spectrum of an optical frequency comb in the frequency domain is shown in Figure 3. As shown in Figure 3, it is represented by a comb-like spectrum with discrete and evenly spaced modes. The frequency interval between the modes of the comb-like spectrum shown in Figure 3 is a repetition frequency f ｒｅｐ Assume that there are modes of the optical frequency comb up to zero frequency, and the mode closest to zero frequency is the 0th mode. The frequency of the 0th mode is the carrier envelope offset frequency f ＣＥＯ and the frequency of the nth mode is f ｎ = nf ｒｅｐ +f ＣＥＯ It is expressed as:

[0027] Here, the main microresonator 102 and the sub microresonator 103 each have a resonant frequency, and when these resonators are optically coupled to each other, if the resonant frequencies of each resonator become close to each other, the modes with close resonant frequencies repel each other, and the resonant frequencies change so that the close resonant frequencies move away from each other.

[0028] This state is shown in Figure 4. Figure 4 shows an example of the resonant frequencies of the main microresonator 102 and the sub-microresonator 103, with the horizontal axis representing frequency. The resonant frequency mode shown in the lower row is that of the main microresonator 102, and the resonant frequency mode shown in the upper row is that of the sub-microresonator 103. The main microresonator 102 and the sub-microresonator 103 are designed so that all of their resonant frequencies do not coincide with each other, and the interval between the resonant frequencies of the main microresonator 102 and the resonant frequency of the sub-microresonator 103 are different. Therefore, when comparing the modes in the upper and lower rows shown in Figure 4, there are cases where the resonant frequencies are close to each other, as seen in the resonant frequency mode 405 of the main microresonator 102 and the resonant frequency mode 404 of the sub-microresonator 103 surrounded by the dashed line 401.

[0029] 5 is an enlarged view of the mode surrounded by the dashed line 401 shown in FIG. 4. The mode 405 of the resonant frequency of the main microresonator 102 is shifted to the mode 403, and the mode 404 of the resonant frequency of the sub-microresonator 103 is shifted to the mode 402.

[0030] This phenomenon is called avoided mode crossing (AMX). AMX can occur not only between two optically coupled resonators, but also when, for example, multiple different spatial modes exist in a space such as a waveguide and the frequencies of the multiple modes are close to each other.

[0031] The resonant frequency when AMX occurs is given by the following equation:

[0032]

[0033] Here, ω± are the resonance frequencies on the high and low frequency sides when AMX occurs, and ω １ , ω ２are the resonant frequencies on the high frequency side and the low frequency side before AMX occurs, respectively, and ε is a constant representing the strength of interaction between the resonant frequency groups. １ , ω ２ The frequency difference is called the nearest resonant frequency difference.

[0034] Next, we will explain how to reduce the phase noise. Thermal noise reduces the frequency spacing f between the comb modes of the microcomb. ｒｅｐ Phase noise occurs in f ｒｅｐ The phase noise of f is due to the change in the physical cavity length caused by thermal noise and the change in detuning caused by thermal noise. ｒｅｐ The phase noise of is given by the following equation:

[0035]

[0036] where f ｒｅｐ is the comb intermode frequency, T is the temperature of the heat bath, L is the microcavity length, and K ｄｉｓｐｅｒｓｉｏｎ is the change in detuning f ｒｅｐ Among the changes in K, the contribution of higher-order dispersion is Ｒａｍａｎ is the change in detuning f ｒｅｐ Among the changes in K, the contribution from Raman scattering ＡＭＸ is the change in detuning f ｒｅｐ The change in is due to AMX, α is the magnitude of detuning, ν ｃ is the center frequency of the microcomb. ｒｅｐ / dT is the thermal f ｒｅｐ The first term on the right side is the change in the cavity length due to heat. ｒｅｐ The second term on the right side is the change in f due to the change in detuning. ｒｅｐ This shows the change in

[0037] Adjust the right side of the above equation and add df to the left side. ｒｅｐ If / dT=0, the thermal noise ｒｅｐ The microcomb control device of the present invention is expected to have no change in df ｒｅｐ In order to achieve / dT=0, the frequency at which AMX shown by Equation 3 occurs is adjusted by adjusting the resonant frequency of the sub-microresonator 103 shown in FIG. ＡＭＸ By adjusting dfｒｅｐ The resonant frequency of the sub-microresonator 103 shown in FIG. 1 is adjusted by changing the temperature of the sub-microresonator 103 using the microheater 104 installed on the sub-microresonator 103 to a suitable temperature, i.e., df ｒｅｐ This is achieved by searching for the temperature at which / dT=0.

[0038] In this embodiment, the principle was demonstrated by numerical calculation. The temperature-dependent Lugiato-Lefever Equation (LLE) (see Non-Patent Document 5) is shown below as an equation describing the electric field in the resonator of the microcomb.

[0039]

[0040] The effect of temperature-dependent detuning is introduced into the above equation, and the actual temperature-dependent effect f ｒｅｐ (T) was calculated. f when the temperature of the sub-microcavity was changed ｒｅｐ Find the change in the slope of df ｒｅｐ / dT f ｒｅｐ The important thing in the proof of principle is that df ｒｅｐ The aim is to find out whether there exists a situation in which the sum of / dT is zero.

[0041] In this embodiment, first, the nearest resonant frequency difference is fixed, and df is calculated when the mode of the microcomb in which AMX occurs is changed. ｒｅｐ / dT behavior and df ｒｅｐ We searched for the mode of the microcomb where AMX occurs, such that / dT is closest to zero. ｒｅｐ The temperature of the sub-microcavity is adjusted within a certain range so that AMX occurs in the mode of the microcomb where / dT is closest to zero, and df when the closest resonant frequency difference is changed ｒｅｐ The behavior of / dT was calculated.

[0042] Figure 6 shows the relationship between df and AMX when the closest resonant frequency difference is fixed and the microcomb mode in which AMX occurs is changed. ｒｅｐ The vertical axis of the graph shown in FIG. 6 represents thermal sensitivity, and the change in temperature by 1 K isｒｅｐ The change in df ｒｅｐ / dT. The horizontal axis indicates the frequency at which AMX occurs, and the value on the horizontal axis indicates the order of the comb mode in which AMX occurs, when the comb mode of the frequency of the pump light 112 is set to zero. When AMX occurs in a negative order comb mode (A), when AMX occurs at a frequency far from the pump frequency, the order of AMX does not change even if the order of AMX is changed. ｒｅｐ / dT did not change significantly, and df by AMX ｒｅｐ As the frequency at which AMX occurs approaches the pump frequency, df ｒｅｐ When AMX occurs in a positive-order comb mode (B), if the frequency at which AMX occurs is large and the frequency at which AMX occurs is gradually brought closer to the frequency of the pump light 112, the effect of AMX gradually becomes more pronounced, and at point C, df ｒｅｐ After / dT = 0, df ｒｅｐ It can be seen that / dT becomes smaller rapidly.

[0043] df ｒｅｐ To make / dT=0, AMX must be df at point C. ｒｅｐ It is considered that the position of the comb mode where AMX occurs is preferably at point C, that is, df ｒｅｐ / dT=0 and df when the nearest frequency difference is changed ｒｅｐ The behavior of / dT was obtained. The results are shown in Figure 7. The vertical axis of the graph shown in Figure 7 is thermal sensitivity, and the horizontal axis is the change in the nearest frequency difference. df ｒｅｐ It can be seen that there exists the closest frequency difference for which / dT=0.

[0044] Finally, when a random thermal fluctuation of 5 mK is applied to the system, the f ｒｅｐ The fluctuation of f was calculated. The calculation results are shown in Figure 8. The f ｒｅｐ(data 801) fluctuates with a standard deviation of 0.35 kHz (data 802), and ｒｅｐ It can be seen that the fluctuation has been reduced to one hundredth of the original value.

[0045] The f shown in FIG. ｒｅｐ The fluctuation of f is Fourier transformed. ｒｅｐ The power spectrum of f is plotted against frequency in FIG. 9. The power spectral density (data 801) is reduced by 40 dB (data 802), and ｒｅｐ It can be seen that the fluctuations in

[0046] As described above, by adjusting the temperature of the sub-microresonator, the resonant frequency of the sub-microresonator can be adjusted, and the frequency position at which AMX occurs can be adjusted, thereby reducing phase noise caused by thermal fluctuations regardless of the design, such as the configuration and shape, of the main microresonator.

[0047] In this embodiment, the reduction in phase noise due to thermal fluctuations caused by adjusting the frequency position at which AMX occurs by adjusting the resonant frequency of the sub-microcavity was confirmed by numerical calculation. However, when confirming this by actual experiment, for example, the transmitted light output from the waveguide 101 is detected by a photodetector, and the comb mode frequency f detected by the photodetector is ｒｅｐ df for temperature ｒｅｐ The temperature of the sub-microcavity may be adjusted based on / dT.

[0048] As mentioned above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description.

[0049] 10 Microcomb control device 100 CW laser light source device 101 Waveguide 102 Main microresonator 103 Sub-microresonator 104 Microheater 105 Pumping light 201 Substrate 202 Cladding layer 402 to 405 Mode 801, 802 Data

Claims

1. A laser light source device that irradiates excitation light that is a continuous wave; a coupled resonator that includes a waveguide into which the excitation light is incident, a main microresonator that generates a microcomb by optically coupling with the waveguide, and a sub-microresonator that is optically coupled with the main microresonator and has a resonance frequency different from that of the main microresonator; a heater that heats the sub-microresonator; and a temperature controller that controls the temperature of the heater, wherein the temperature controller controls the AMX that is generated when the resonance frequency of the main microresonator and the resonance frequency of the sub-microresonator are close to each other within a predetermined range and are equal to df ｒｅｐ A microcomb control device that adjusts the temperature of the heater so that a resonance frequency mode that satisfies / dT=0 and the closest resonance frequency difference occurs, thereby reducing low phase noise.

2. The microcomb control device according to claim 1, wherein the temperature controller controls the temperature of the heater to change the resonant frequency mode generated by the AMX, thereby searching for a suitable temperature.

3. The temperature controller adjusts the temperature of the heater to |df ｒｅｐ After identifying the resonant frequency mode generated by the AMX in which |df| is minimum, the temperature of the heater is finely adjusted to adjust the closest frequency difference in the AMX, and |df ｒｅｐ 2. The microcomb control device of claim 1, wherein the temperature of the heater is adjusted so that |dT|=0.

4. A photodetector is further provided to detect transmitted light from the waveguide, and the comb mode frequency f detected by the photodetector ｒｅｐ Change in temperature df ｒｅｐ 2. The microcomb controller of claim 1, wherein the temperature of the heater is adjusted based on / dT.

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