Wavelength filter and laser device

Abstract

To provide a wavelength filter, etc., with which it is possible to pass a high peak selected wavelength through.SOLUTION: A wavelength filter 3 comprises a first filter circuit 17 and a second filter circuit 13. The first filter circuit 17 is composed of a plurality of optical ring resonators 17A, 17B mutually differing in transmission wavelength intervals, that are connected in series, in which a transmission band obtained therefrom by a vernier effect is within the gain band of an optical amplifier, with lights of a selected wavelength from the gain band and a recursion mode wavelength occurring on the short wavelength or long wavelength side of the selected wavelength passing through. The second filter circuit 13 connects in series to the first filter circuit 17, and suppresses light of a recursion mode wavelength from light having passed through the first filter circuit 17.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wavelength filter and a laser device. [Background technology]

[0002] In recent years, digital coherent optical communication using phase modulation has become mainstream in high-capacity optical communication systems. This communication system requires laser devices that meet requirements such as single wavelength oscillation, narrow linewidth, wide wavelength tuning range, and compact size. To simultaneously satisfy all these requirements, the integration of optical elements is essential. Therefore, wavelength-tunable laser devices that hybridly combine a wavelength filter with wavelength selection functionality arranged on a silicon substrate with an optical amplifier such as an SOA (Semiconductor Optical Amplifier) ​​have attracted attention.

[0003] Wavelength filters have a wavelength selection function that transmits specific selected wavelengths from a predetermined communication wavelength band using the Vernier effect of, for example, two optical ring resonators with different resonant wavelength intervals. Each optical ring resonator resonates at its respective resonant wavelength, with maximum transmittance at that resonant wavelength and minimum transmittance at wavelengths intermediate between the resonant wavelengths. The interval between the resonant wavelengths is constant, and this interval, the interval between the transmitted wavelengths, is called the FSR (Free Spectrum Range). The Vernier effect is expressed as the FSR, which is the least common multiple of the FSRs transmitted by each optical ring resonator, i.e., the FSR of all optical ring resonators, expressed as FSRtotal.

[0004] 7 is an explanatory diagram showing an example of the filter characteristics of the conventional eleventh and twelfth optical ring resonators. For convenience of explanation, the FSR of the eleventh optical ring resonator is designated as FSR11, and the FSR of the twelfth optical ring resonator is designated as FSR12.

[0005] 7, the transmission band T11 of the 11th optical ring resonator is shown by a solid line, the transmission band T2 of the 12th optical ring resonator is shown by a dotted line, and the transmission bands T1*T2 of the 11th and 12th optical ring resonators are shown by a thick line. The FSR11 of the 11th optical ring resonator is different from the FSR12 of the 12th optical ring resonator. Due to the Vernier effect of FSR11 and FSR12, the total transmission band T11*T12 has the highest peak of transmittance (first mode M1) at the wavelength where the resonant wavelengths of the 11th and 12th optical ring resonators overlap the most.

[0006] A peak of the second mode M2 ​​is formed adjacent to both sides of the peak of the first mode M1, and a peak of the third mode M3 is formed adjacent to both sides of the peak of the first mode M1. Since the second mode M2 ​​has less overlap of resonance wavelengths than the first mode M1, the peak of the second mode M2 ​​is lower than the peak of the first mode M1. Furthermore, since the third mode M3 also has less overlap of resonance wavelengths than the second mode M2, the peak of the third mode M3 is also lower than the peak of the second mode M2.

[0007] 8 is an explanatory diagram showing an example of a recursive mode for the first mode M1. The first mode M1 has a transmittance of a wavelength at which the resonant wavelengths of the eleventh optical ring resonator and the twelfth optical ring resonator overlap most closely. Within the range of ±FSRtotal from the first mode M1, there is only one wavelength at which the transmission band T11 of the eleventh optical ring resonator and the transmission band T12 of the twelfth optical ring resonator completely match. Then, the first mode M1 (M4) where the transmission band T11 and the transmission band T12 overlap is repeated for each FSRtotal. This repeated first mode M1 (M4) is a recursive mode for the first mode M1.

[0008] Therefore, the transmission bands of the 11th optical ring resonator and the 12th optical ring resonator are designed so that FSRtotal is greater than the operating wavelength range, so that overlap with wavelengths other than the selected wavelength does not occur within the operating wavelength range used for optical communications.

[0009] Furthermore, the transmission band T11 of the eleventh optical ring resonator and the transmission band T12 of the twelfth optical ring resonator have a periodic bandwidth. In addition to the first mode M1, the FSRtotal also includes overlapping transmission bands, such as the second mode M2 ​​and the third mode M3. Examples of FSRtotal include the transmission ratio between the peak of the first mode M1 and the peak of the second mode M2, and the modal gain difference, which is the transmission ratio between the peak of the first mode M1 and the peak of the third mode M3. The modal gain difference is an index for evaluating wavelength monotony. It is generally considered desirable for the modal gain difference of a laser device to be, for example, 3 dB or greater.

[0010] However, by adjusting the FSR of the eleventh and twelfth optical ring resonators, the range of the FSRtotal is widened within the range of wavelengths used, and as a result, although the recursive mode can be ignored, the widened range of the FSRtotal reduces the modal gain difference between the peak of the selected wavelength and the peak of a wavelength adjacent to the selected wavelength.

[0011] Therefore, since the wavelength range used in optical communications generally extends beyond 30 nm, the modal gain difference can be reduced by widening the range of FSRtotal to a level where the retroreflecting mode can be ignored. Fig. 9 is an explanatory diagram showing an example of the transmittance of the first mode M1, the second mode M2, and the third mode M3 of the conventional eleventh optical ring resonator, the twelfth optical ring resonator, and the AMZI.

[0012] The wavelength filter is configured by connecting an eleventh optical ring resonator, a twelfth optical ring resonator, and an AMZI in series. The dotted lines in Figure 9 represent the transmission bands T11*T12 of the eleventh and twelfth optical ring resonators, the solid lines represent the transmission band T13 of the AMZI, and the bold lines represent the transmission bands T11*T12*T13 of the eleventh optical ring resonator, the twelfth optical ring resonator, and the AMZI. In this case, the FSRtotal range is wide, so the recursive mode can be ignored.

[0013] However, even if the recursive mode can be ignored, a modal gain difference of 3 dB or more must be ensured. Therefore, the AMZI, for example, sets a transmission band T13 that blocks wavelengths near the second mode M2. Because the AMZI blocks wavelengths near the second mode M2, the modal gain difference between the first mode M1 and the second mode M2 ​​improves from -2.5 dB to -10 dB or less. As a result, even if the FSRtotal range is widened, a modal gain difference of 3 dB or more is ensured.

[0014] Therefore, in order to block wavelengths near the second mode M2, FSR13, which is the FSR of AMZI, is set to 1.5 to 3 times the average value of FSR11 of the 11th optical ring resonator and FSR12 of the 12th optical ring resonator. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-102097 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-75767 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-154052 Summary of the Invention [Problem to be solved by the invention]

[0016] However, in the conventional laser device, FSR13 of the AMZI is close to FSR11 of the eleventh optical ring resonator and FSR12 of the twelfth optical ring resonator, resulting in a steep filter characteristic as shown in Fig. 9. As a result, for example, if the transmission band of the AMZI is shifted, the peak of the first mode M1, where the transmittances of the eleventh optical ring resonator and the twelfth optical ring resonator overlap most, will be outside the transmission band.

[0017] FIG. 10 is an explanatory diagram showing an example of the transmittance of the first, second, and third modes when the transmission band of the AMZI is shifted. If the transmission band of the AMZI is shifted by, for example, approximately 2 nm, light in the second mode M2 ​​falls within the transmission band of the AMZI, resulting in partial transmission of the light in the second mode M2. Furthermore, the peak of the first mode M1 falls outside the transmission band of the AMZI, resulting in suppression of the light of the selected wavelength in the first mode M1. As a result, even a slight shift in the transmission band of the AMZI results in a 1 dB drop in the light of the selected wavelength in the first mode M1.

[0018] The disclosed technology has been made in view of the above points, and aims to provide a wavelength filter or the like that can transmit light of a selected wavelength with a high peak. [Means for solving the problem]

[0019] In one aspect, the wavelength filter disclosed herein includes a first filter circuit and a second filter circuit. The first filter circuit has a transmission band obtained by serially connecting multiple optical ring resonators with different transmission wavelength intervals to produce a Vernier effect within the gain band of the optical amplifier, and transmits light of a selected wavelength and a recursive mode wavelength generated on the shorter or longer wavelength side of the selected wavelength from the gain band. The second filter circuit is connected in series with the first filter circuit and suppresses light of the recursive mode wavelength from the light transmitted by the first filter circuit. [Effects of the Invention]

[0020] According to one aspect of the wavelength filter disclosed in the present application, a selected wavelength with a high peak can be transmitted. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a block diagram showing an example of the configuration of a laser device according to this embodiment. [Figure 2]FIG. 2 is an explanatory diagram showing a comparative example of the FSRtotal of the conventional eleventh and twelfth optical ring resonators and the FSRtotal of the first and second optical ring resonators of this embodiment. [Figure 3] FIG. 3 is an explanatory diagram showing an example of the filter characteristics of the first filter circuit and the second filter circuit of this embodiment. [Figure 4] FIG. 4 is an explanatory diagram showing an example of the transmittance of the first mode, the second mode, and the retroreflection mode in the first optical ring resonator, the second optical ring resonator, and the AMZI of this embodiment. [Figure 5] FIG. 5 is an explanatory diagram showing an example of the transmittance of the first mode, the second mode, and the retroreflection mode when the transmission bands of the AMZI are shifted. [Figure 6] FIG. 6 is an explanatory diagram showing the relationship between FSR3 and FSRtotal of AMZI in this embodiment. [Figure 7] FIG. 7 is an explanatory diagram showing an example of the filter characteristics of a conventional optical ring resonator. [Figure 8] FIG. 8 is an explanatory diagram showing an example of a recursive mode relative to the first mode. [Figure 9] FIG. 9 is an explanatory diagram showing an example of the transmittance of the first mode, the second mode, and the third mode in the conventional eleventh optical ring resonator, the twelfth optical ring resonator, and the AMZI. [Figure 10] FIG. 10 is an explanatory diagram showing an example of the transmittance of the first mode, the second mode, and the third mode when the transmission band of the conventional AMZI is shifted. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the wavelength filter and laser device disclosed in the present application will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments. [Example]

[0023] Fig. 1 is a block diagram showing an example of the configuration of a laser device according to this embodiment. The laser device 1 shown in Fig. 1 is a wavelength laser device having an SOA (Semiconductor Optical Amplifier) ​​2 and a wavelength filter 3, in which the SOA 2 and the wavelength filter 3 are hybrid-mounted. The laser device 1 realizes a stimulated emission state that exceeds the loss due to hybrid mounting, the propagation loss in the wavelength filter 3, and the absorption loss inside the SOA 2.

[0024] The SOA2 is an optical amplifier with a gain band. The gain band is a wavelength band within the wavelength range used in optical communications that includes the selected wavelength and the wavelength of the return mode generated on the longer or shorter wavelength side of the selected wavelength. The connection section within the SOA2 is precisely hybrid-mounted to minimize optical loss when connected to the wavelength filter 3. The SOA2 generates amplified spontaneous emission (ASE) in response to the drive current. When the drive current of the SOA2 increases, the ASE increases. The ASE is wavelength-selected by the wavelength filter 3 controlled by a phase shifter, reflected by the reflector 18 within the wavelength filter 3, and re-input to the SOA2. The return light re-input to the SOA2 has a single wavelength.

[0025] The wavelength filter 3 is a wavelength filter that transmits light of a selected wavelength from the light within the gain band input from the SOA 2 and outputs the transmitted light of the selected wavelength back to the SOA 2. The SOA 2 amplifies the light of the selected wavelength input from the wavelength filter 3 and outputs the amplified light of the selected wavelength as laser light to, for example, a transmitter or a receiver.

[0026] The wavelength filter 3 has an input / output unit 11, a first optical waveguide 12, a second filter circuit 13, a second optical waveguide 14, a third optical waveguide 15, a fourth optical waveguide 16, a first filter circuit 17, and a reflecting mirror 18. The wavelength filter 3 is a closed loop circuit in which light from the input / output unit 11 is reflected by the reflecting mirror 18, and the returned light reflected by the reflecting mirror 18 is output from the input / output unit 11.

[0027] The input / output unit 11 is an input / output unit of the wavelength filter 3 that optically couples the SOA 2 and the first optical waveguide 12. The second filter circuit 13 has an AMZI (Asymmetric Mach-Zehnder Interferometer) 13A. The first filter circuit 17 has a first optical ring resonator 17A and a second optical ring resonator 17B.

[0028] The first optical waveguide 12 is an optical waveguide that optically couples the input / output unit 11 and the AMZI 13A. The second optical waveguide 14 is an optical waveguide that optically couples the AMZI 13A and the first optical ring resonator 17A. The third optical waveguide 15 is an optical waveguide that optically couples the first optical ring resonator 17A and the second optical ring resonator 17B. The fourth optical waveguide 16 is an optical waveguide that optically couples the second optical ring resonator 17B and the reflecting mirror 18. The reflecting mirror 18 is a folding section of the wavelength filter 3 that is optically coupled to the fourth optical waveguide 16.

[0029] The AMZI 13A includes a first coupler 131, a first arm 132A, a second arm 132B, a second coupler 133, and a first phase shifter 134 disposed in the second arm 132B. The first coupler 131 has a branching function that branches and outputs light from the first optical waveguide 12 to the first arm 132A and the second arm 132B. The first arm 132A and the second arm 132B change their optical refractive indexes in response to a drive voltage to adjust the transmission bands of light passing through each arm 132A and 132B. The second coupler 133 has a multiplexing function that multiplexes light passing through the first arm 132A and light passing through the second arm 132B and outputs the multiplexed light to the second optical waveguide 14. The first phase shifter 134 is disposed on the second arm 132B and is, for example, a heater that adjusts the temperature of the second arm 132B to thereby adjust the phase of light passing through the second arm 132B.

[0030] The second coupler 133 has a branching function of branching and outputting the return light from the second optical waveguide 14 to the first arm 132A and the second arm 132B. The first coupler 131 also has a multiplexing function of multiplexing the return lights after transmission from the first arm 132A and the second arm 132B and outputting the multiplexed light to the first optical waveguide 12. The AMZI 13A is an asymmetric Mach-Zehnder interferometer in which the first arm 132A and the second arm 132B are different from each other by adjusting the first phase shifter 134.

[0031] The second optical waveguide 14 is an optical waveguide through which light from the second coupler 133 in the AMZI 13A passes. A second phase shifter 14A is disposed in the second optical waveguide 14. The second phase shifter 14A is, for example, a heater that adjusts the temperature of the second optical waveguide 14, thereby adjusting the optical length of the second optical waveguide 14.

[0032] First optical ring resonator 17A in first filter circuit 17 transmits light with FSR1 from the light passing through second optical waveguide 14 and outputs the transmitted light to third optical waveguide 15. FSR is the transmission wavelength interval of the transmittance peak. FSR1 is the FSR of first optical ring resonator 17A. First optical ring resonator 17A includes third phase shifter 17A1 that shifts the phase of first optical ring resonator 17A. Third phase shifter 17A1 is, for example, a heater that adjusts the temperature of first optical ring resonator 17A to adjust the FSR1 of first optical ring resonator 17A. For example, increasing the heater power increases the temperature of the silicon wire in first optical ring resonator 17A, increasing the refractive index of the silicon wire. As a result, the resonant wavelength of first optical ring resonator 17A and the wavelength of first mode M1 become longer.

[0033] Second optical ring resonator 17B in first filter circuit 17 transmits light with FSR2 from the light passing through third optical waveguide 15 and outputs the transmitted light to fourth optical waveguide 16. FSR2 is the FSR of second optical ring resonator 17B. Second optical ring resonator 17B has fourth phase shifter 17B1 that shifts the phase of second optical ring resonator 17B. Fourth phase shifter 17B1 is, for example, a heater that adjusts the temperature of second optical ring resonator 17B to adjust the FSR2 of second optical ring resonator 17B. For example, increasing the heater power increases the temperature of the silicon wire in second optical ring resonator 17B and the refractive index of the silicon wire. As a result, the resonant wavelength of second optical ring resonator 17B becomes longer, and the wavelength of first mode M1 becomes longer.

[0034] First optical ring resonator 17A and second optical ring resonator 17B have different FSRs, and therefore obtain a transmission band by the Vernier effect when connected in series. The transmission bands of first optical ring resonator 17A and second optical ring resonator 17B are within the gain band of SOA2, and transmit light of a selected wavelength and a wavelength of a retroreflection mode generated on the shorter or longer wavelength side of the selected wavelength from the gain band.

[0035] The reflecting mirror 18 corresponds to the return part of the closed circuit of the wavelength filter 3, and is a loop mirror that reflects the light input from the fourth optical waveguide 16 and returns the reflected returning light to the fourth optical waveguide 16. The reflecting mirror 18 may also be provided with a phase shifter that adjusts the optical length, and can be modified as appropriate.

[0036] Second optical ring resonator 17B transmits light of FSR2 from the return light input from fourth optical waveguide 16, and outputs the transmitted return light to third optical waveguide 15. Furthermore, first optical ring resonator 17A transmits light of FSR1 from the return light input from third optical waveguide 15, and outputs the transmitted return light to second optical waveguide 14.

[0037] The AMZI 13A has a transmission band that suppresses the retroreflection mode light occurring on the shorter and longer wavelength sides of the selected wavelength. The AMZI 13A suppresses the retroreflection mode light of the selected wavelength from the return light input from the second optical waveguide 14, and outputs the return light of the selected wavelength to the first optical waveguide 12. The first optical waveguide 12 outputs the return light of the selected wavelength to the input / output unit 11. The input / output unit 11 outputs the return light of the selected wavelength from the first optical waveguide 12 to the SOA 2. The SOA 2 optically amplifies the return light of the selected wavelength input from the first optical waveguide 12, and outputs the optically amplified return light of the selected wavelength as laser light.

[0038] That is, the light from the SOA2 passes through the input / output unit 11, the first optical waveguide 12, the AMZI 13A, the second optical waveguide 14, the first optical ring resonator 17A, the third optical waveguide 15, the second optical ring resonator 17B, and the fourth optical waveguide 16 before being output to the reflecting mirror 18. Furthermore, the returning light from the reflecting mirror 18 passes through the fourth optical waveguide 16, the second optical ring resonator 17B, the third optical waveguide 15, the first optical ring resonator 17A, the second optical waveguide 14, the AMZI 13A, the first optical waveguide 12, and the input / output unit 11 before being output to the SOA2.

[0039] The core material of the optical waveguides of the first optical waveguide 12, the second filter circuit 13, the second optical waveguide 14, the first filter circuit 17, the third optical waveguide 15, and the fourth optical waveguide 16 of the wavelength filter 3 is formed of silicon (Si). Furthermore, the cladding covering the cores of the first optical waveguide 12, the second filter circuit 13, the second optical waveguide 14, the first filter circuit 17, the third optical waveguide 15, and the fourth optical waveguide 16 is formed of, for example, silicon oxide (SiO2). Furthermore, the SOA 2 is formed of InP or the like.

[0040] 2 is an explanatory diagram showing a comparison between the FSRtotal of the conventional eleventh and twelfth optical ring resonators and the FSRtotal of the first and second optical ring resonators 17A and 17B of this embodiment. In FIG. 2, the FSRtotal of the conventional eleventh and twelfth optical ring resonators is shown by a dotted line, and the FSRtotal of the first and second optical ring resonators 17A and 17B of this embodiment is shown by a solid line. Note that, as a simulation condition, when comparing the conventional FSRtotal with the FSRtotal of this embodiment, the FSRtotal of this embodiment is set to be twice the conventional FSRtotal.

[0041] In the FSRtotal of this embodiment, peaks of the recursive mode M4 occur at long and short wavelengths sufficiently away from the peak of the selected wavelength M1, which did not occur in the conventional FSRtotal. The peak of the recursive mode M4 has a transmittance similar to that of the peak of the first mode M1. Furthermore, in the FSRtotal of this embodiment, the second mode adjacent to the selected wavelength M1 changes from M2 to M2', improving the modal gain difference. As a result, the modal gain difference of this embodiment is improved from 2.5 dB in the conventional embodiment to 7.3 dB, as shown in FIG. 2. Furthermore, if the FSR difference between FSR1 of the first optical ring resonator 17A and FSR2 of the second optical ring resonator 17B is widened, the FSRtotal becomes narrower, thereby improving the second mode M2.

[0042] The transmittance of the AMZI 13A can be calculated as T(λ) = 1 / 2(1 + cos(2π / λ·nΔL)), where T(λ) is the transmittance of the AMZI 13A, n is the effective refractive index of the waveguides in the first arm 132A and the second arm 132B, and ΔL is the optical path difference between the first arm 132A and the second arm 132B of the AMZI 13A. If the optical path difference ΔL is designed to be small, phase rotation becomes gradual even when λ fluctuates, and the AMZI 13A becomes a wideband filter. As a result, as pointed out in the problem of the prior art, the transmission band of the AMZI 13A is no longer steep, and the sensitivity of the transmission band decreases, so even if the transmission band shifts slightly, the slight shift can be absorbed.

[0043] The retroreflecting mode M4 shown in Figure 2 occurs at wavelengths approximately 30 nm away from the first mode M1 of the selected wavelength on the longer and shorter wavelength sides. Therefore, as shown in Figure 3, the FSR3 of the AMZI 13A is set to a range of 60 nm, which is ±30 nm from the first mode M1 of the selected wavelength. As a result, the AMZI 13A can suppress the retroreflecting mode from the gain band.

[0044] FIG. 3 is an explanatory diagram showing an example of the filter characteristics of the second filter circuit 13 and the first filter circuit 17 of this embodiment. T1*T2 shown in FIG. 3 is the transmission band of the first optical ring resonator 17A and the second optical ring resonator 17B. T3 shown in FIG. 3 is the transmission band of the AMZI 13A. As a result, a selective wavelength ratio attenuation effect of 20 dB can be expected from the vicinity of the retroreflection mode M4. The transmission band of the AMZI 13A shown in FIG. 3 is gentler than the transmission band of the conventional AMZI shown in FIG. 9.

[0045] 4 is an explanatory diagram showing an example of the transmittance of the first mode M1, the second mode M2, and the retroreflection mode M4 in the first optical ring resonator 17A, the second optical ring resonator 17B, and the AMZI 13A of this embodiment. Fig. 4 shows the transmission band T3 of the AMZI 13A, the transmission bands T1*T2 of the first optical ring resonator 17A and the second optical ring resonator 17B, and the transmission bands T1*T2*T3 of the AMZI 13A, the first optical ring resonator 17A, and the second optical ring resonator 17B. In the transmission band T3, the change in the transmission characteristics near the selected wavelength M1 is small, so almost no change in the second mode M2 ​​is observed, but it can be confirmed that the retroreflection mode M4 is sufficiently suppressed.

[0046] Figure 5 is an explanatory diagram showing an example of the transmittance of the first mode M1, the second mode M2, and the retroreflection mode M4 when the transmission band of the AMZI 13A is shifted. For example, even when the transmission band T3 of the AMZI 13A is shifted by 5 nm, the suppression effect of the retroreflection mode M4 due to the transmission band T3 of the AMZI 13A remains almost unchanged, and the peak reduction of the selected wavelength M1 is within approximately 0.3 dB. As a result, it can be seen that the tolerance for widening the transmission band T3 of the AMZI 13A is much looser than that of the transmission band T13 of the conventional AMZI.

[0047] The FSRtotal of the first optical ring resonator 17A and the second optical ring resonator 17B is set to be within the wavelength range used, ensuring a modal gain difference close to the selected wavelength. Furthermore, an AMZI 13A is used as a filter to suppress the light of the recursive mode M4 generated in the wavelength range used.

[0048] The FSR of the AMZI13A is FSR3. The relationship between FSR3 and FSRtotal is FSR3 = 1.5 to 3 x FSRtotal. By widening FSR3 compared to conventional methods and narrowing FSRtotal, the modal gain difference between the selected wavelength M1 and the second mode M2 ​​close to the selected wavelength is increased, while the light of the retroreflecting mode M4 is suppressed by the AMZI13A. Because the transmission band of the AMZI13A is wide, the change in the wavelength direction of the transmitted wavelength is small.

[0049] 6 is an explanatory diagram showing the relationship between FSR3 and FSRtotal of the AMZI 13A of this embodiment. The FSR3 of the AMZI 13A is, for example, the transmission band (solid line) when it is set to about twice the FSRtotal. When the FSR3 of the AMZI 13A is set to about twice the FSRtotal, the AMZI 13A can completely suppress light in the retroreflection mode M4 on the short wavelength side of the selected wavelength M1 and the retroreflection mode M4 on the long wavelength side.

[0050] The FSR3 of the AMZI 13A is, for example, the transmission band (dashed line) when it is set to about 1.5 times the FSRtotal. When the FSR3 of the AMZI 13A is set to about 1.5 times the FSRtotal, the AMZI 13A can suppress the light of the retroreflection mode M4 on the short wavelength side of the selected wavelength M1 and the retroreflection mode M4 on the long wavelength side to 3 dB or less.

[0051] The FSR3 of the AMZI 13A is, for example, the transmission band (two-dot chain line) when it is approximately three times the FSRtotal. When the FSR3 of the AMZI is approximately three times the FSRtotal, the light of the retroreflection mode M4 on the short wavelength side of the selected wavelength M1 and the retroreflection mode M4 on the long wavelength side can be suppressed to 3 dB or less.

[0052] The transmission band T3 of the AMZI 13A is set so that the transmittance is maximum at the wavelength corresponding to the first mode M1 and minimum at the wavelengths of the recursive modes M4 generated on the shorter and longer wavelength sides of the first mode M1. The first phase shifter 134 is controlled to adjust the optical path difference between the first arm 132A and the second arm 132B so that the FSR3 of the AMZI 13A is 1.5 to 3 times the transmittance FSRtotal of the first optical ring resonator 17A and the second optical ring resonator 17B. As a result, by controlling the optical path difference using the first phase shifter 134, the transmittance peak of the AMZI 13A, which suppresses the recursive mode light, can be shifted.

[0053] The wavelength filter 3 of this embodiment has a first filter circuit 17 and a second filter circuit 13 connected in series. The first filter circuit 17 connects a first optical ring resonator 17A and a second optical ring resonator 17B in series, each having a different FSR. The transmission band obtained by the Vernier effect of the first and second optical ring resonators 17A and 17B connected in series is within the gain band of the SOA 2, and transmits light at a selected wavelength and at wavelengths of a recursive mode generated on the shorter or longer wavelength side of the selected wavelength from the gain band. As a result, the modal gain difference can be ensured by widening the range of the total FSR. Furthermore, the second filter circuit 13 is connected in series with the first filter circuit 17 and suppresses light at wavelengths of the recursive mode from the light transmitted by the first filter circuit 17. As a result, the transmission band of the second filter circuit 13, which suppresses light in the recursive mode, is broad and gentle, allowing light at a high-peak selected wavelength to pass even if the transmission band is slightly shifted.

[0054] The second filter circuit 13 is configured with an AMZI 13A having a transmission characteristic of FSR3 that is 1.5 to 3 times the FSRtotal between the peak of the selected wavelength M1 transmitted by the first filter circuit 17 and the peak of the retroreflection mode M4 for that selected wavelength M1. As a result, the transmission band of the AMZI 13A, which suppresses the retroreflection mode light, is broad and gentle, so that even if the transmission band of the AMZI 13A is slightly shifted, light of the selected wavelength with a high peak can be transmitted.

[0055] First filter circuit 17 adjusts the transmission band for transmitting light of a selected wavelength in accordance with the adjustment of the resonant wavelength of at least one of first optical ring resonator 17A and second optical ring resonator 17B, thereby adjusting the transmission band for transmitting light of a selected wavelength in accordance with the adjustment of the resonant wavelength.

[0056] First filter circuit 17 adjusts the transmission band for transmitting light of a selected wavelength in response to temperature adjustment by the phase shifter of at least one of first optical ring resonator 17A and second optical ring resonator 17B, thereby adjusting the transmission band for transmitting light of a selected wavelength in response to temperature adjustment by third phase shifter 17A1 or fourth phase shifter 17B1.

[0057] The wavelength filter 3 has a closed loop circuit that connects the first filter circuit 17 and the second filter circuit 13 in series, and an input / output unit 11 that is arranged at the input stage of the closed circuit and is optically coupled to the SOA 2 to input and output light to and from the SOA 2. As a result, the wavelength filter 3 can be realized as a closed circuit with the same input and output.

[0058] The wavelength filter 3 includes a closed loop circuit that connects the first filter circuit 17 and the second filter circuit 13 in series, an input / output unit 11 that is arranged at the input stage of the closed circuit and optically couples with the SOA 2 to input and output light to and from the SOA 2, and a reflector 18 that is arranged at the return part of the closed circuit and reflects light. As a result, the wavelength filter 3 can be realized as a return closed circuit.

[0059] The first filter circuit 17 and the second filter circuit 13 are formed on the same substrate, and the first filter circuit 17 and the second filter circuit 13 are connected in series by an optical waveguide, which results in a reduction in the size of the wavelength filter 3.

[0060] Furthermore, wavelength filter 3 has a silicon core and a silicon oxide cladding that covers the core. The difference in refractive index between the core and the cladding is increased. As a result, for example, light is more easily bent within first optical ring resonator 17A and second optical ring resonator 17B, which allows wavelength filter 3 to be made smaller.

[0061] First optical ring resonator 17A, second optical ring resonator 17B, and AMZI 13A are connected in series within a closed loop circuit of wavelength filter 3. As a result, the resonator length of the optical ring resonator can be shortened, allowing wavelength filter 3 to be miniaturized.

[0062] In this embodiment, the transmission band of the AMZI 13A is wide, so once the setting is made so that the modal gain difference is sufficient, there is no need to control the transmission band of the AMZI 13A, and a stable wavelength filter 3 and a laser device 1 equipped with the same can be realized.

[0063] Once FSR3 is set, AMZI13A has a very wide wavelength shift tolerance as long as the oscillation wavelength is not changed, so adjustment of the first phase shifter 134 of AMZI13A is not required during operation, simplifying the wavelength stability control algorithm.

[0064] In this embodiment, the AMZI 13A is directly connected between the first optical waveguide 12 and the second optical waveguide 14. However, the AMZI 13A may be placed at any position as long as it is connected in series to the first optical ring resonator 17A and the second optical ring resonator 17B, and this can be changed as appropriate.

[0065] Furthermore, although the example has been given in which two first optical ring resonators 17A and two second optical ring resonators 17B are arranged, the number is not limited to two and may be, for example, three or more, and can be changed as appropriate.

[0066] In this embodiment, the AMZI 13A is disposed between the first optical waveguide 12 and the second optical waveguide 14. However, the present invention is not limited to the AMZI 13A, and a filter with a transmission band that suppresses the recursive mode M4 with respect to the first mode M1 may be disposed, and other suitable modifications can be made.

[0067] The second filter circuit 13 and the first filter circuit 17 are connected in series to the wavelength filter 3. Since the wavelength filter 3 is a linear circuit, the order of connection does not matter.

[0068] It is not necessary to provide both the first phase shifter 134 of the AMZI 13A and the second phase shifter 14A of the second optical waveguide 14; either one of them is sufficient.

[0069] The wavelength filter 3 has a risk of wavelength shifting from the set wavelength due to aging during operation, environmental fluctuations, etc. To avoid this, the wavelength filter 3 may be configured to detect the amount of wavelength shift and suppress the wavelength shift by feeding back the detected amount to a phase shifter in order to stabilize the laser oscillation wavelength and operate the filter, which is called a wavelength locker.

[0070] In wavelength filter 3, a configuration is also possible in which the path branched off in a Y-shape from input / output unit 11 is made into a closed loop circuit, and two optical ring resonators, a first optical ring resonator 17A and a second optical ring resonator 17B, and AMZI 13A are connected in series within the closed circuit, but the basic concept is the same.

[0071] Furthermore, in order to obtain a high output, a boost SOA may be installed in place of the SOA2, and modifications can be made as appropriate. [Explanation of symbols]

[0072] 1. Laser device 2. SOA 3 wavelength filters 11 Input / output section 13 Second filter circuit 13A AMZI 17 First filter circuit 17A First optical ring resonator 17B Second optical ring resonator 18 Reflector

Claims

1. a first filter circuit that transmits, within a gain band of the optical amplifier, a transmission band obtained by the Vernier effect when a plurality of optical ring resonators having different transmission wavelength intervals are connected in series, and transmits, from the gain band, light of a selected wavelength among a plurality of peak wavelengths that are periodically generated and have the maximum transmittance of each optical ring resonator, and light of a recursive mode peak wavelength that occurs at a peak wavelength on the short wavelength side or a peak wavelength on the long wavelength side nearest to the selected wavelength among the plurality of peak wavelengths; a second filter circuit connected in series with the first filter circuit and configured to suppress light of the peak wavelength of the retroreflection mode from light transmitted through the first filter circuit; The second filter circuit comprises: A wavelength filter characterized in that it is an asymmetric Mach-Zehnder interferometer having transmission characteristics of a transmission wavelength interval of 1.5 to 3 times the transmission wavelength interval between the selected wavelength and the peak wavelength of the retroreflection mode for the selected wavelength.

2. The first filter circuit comprises:

2. The wavelength filter according to claim 1, wherein a transmission band for transmitting light of the selected wavelength is adjusted in accordance with adjustment of a resonant wavelength of at least one of the plurality of optical ring resonators.

3. The first filter circuit comprises:

2. The wavelength filter according to claim 1, wherein a transmission band for transmitting light of the selected wavelength is adjusted in response to adjustment of the temperature of at least one of the plurality of optical ring resonators.

4. a closed loop circuit connecting the first filter circuit and the second filter circuit in series; an input / output unit that is arranged at an input stage of the closed circuit, optically coupled to the optical amplifier, and inputs and outputs the light between the optical amplifier and the input / output unit; 4. The wavelength filter according to claim 1, wherein the wavelength filter comprises:

5. a closed loop circuit connecting the first filter circuit and the second filter circuit in series; an input / output unit that is arranged at an input stage of the closed circuit, optically coupled to the optical amplifier, and inputs and outputs the light between the optical amplifier and the input / output unit; a reflecting portion disposed at a folded portion of the closed circuit and reflecting the light; 4. The wavelength filter according to claim 1, wherein the wavelength filter comprises:

6. The first filter circuit and the second filter circuit 6. The wavelength filter according to claim 1, wherein the first filter circuit and the second filter circuit are formed on the same substrate, and the first filter circuit and the second filter circuit are connected in series by an optical waveguide.

7. The same substrate is a core formed of silicon; 7. The wavelength filter according to claim 6, further comprising: a cladding covering the core and made of silicon oxide.

8. an optical amplifier having a gain band; a wavelength filter optically coupled to the optical amplifier; The wavelength filter is a first filter circuit that transmits, within a gain band of the optical amplifier, a transmission band obtained by the Vernier effect when a plurality of optical ring resonators having different transmission wavelength intervals are connected in series, and transmits, from the gain band, light of a selected wavelength among a plurality of peak wavelengths that are periodically generated and have the maximum transmittance of each optical ring resonator, and light of a recursive mode peak wavelength that occurs at a peak wavelength on the short wavelength side or a peak wavelength on the long wavelength side nearest to the selected wavelength among the plurality of peak wavelengths; a second filter circuit connected in series with the first filter circuit and configured to suppress light of the peak wavelength of the retroreflection mode from light transmitted through the first filter circuit; The second filter circuit comprises: A laser device characterized in that it is an asymmetric Mach-Zehnder interferometer having transmission characteristics with a transmission wavelength interval of 1.5 to 3 times the transmission wavelength interval between the selected wavelength and the peak wavelength of the retroreflection mode for the selected wavelength.

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