Optical delay circuit and wavelength tunable light source

Abstract

When realizing a small size and high controllability of a delay amount in an optical delay circuit, if a conventional waveguide-type optical circuit that uses a ring resonator is used, the realization is hampered by the limit in the the size of the ring resonator. Therefore, this optical delay circuit according to one embodiment of the present invention is a waveguide-type circuit that includes delay lines having different lengths similarly to the conventional optical fiber-type optical circuit. Said optical delay circuit is characterized in that controllability of a delay amount is increased by controlling phase change amounts from delay lines and light transmitted from the delay lines is required to be multiplexed near the same phase for controlling the phase change amounts, and is characterized by including a mechanism for reflecting light at a position half the length of each of the delay lines in order to realize the multiplexing.

Description

[Technical field]

[0001] The present disclosure relates to an optical circuit that applies optical delay, and more particularly to an optical delay circuit that controls the amount of optical delay with high precision and continuous control. [Background technology]

[0002] Optical delay is a technology that adjusts the optical path length along which light propagates to slow down the speed of light in order to control the timing of optical signals, etc. Optical circuits using this technology, i.e., optical delay circuits, are being put to practical use in optical recording media, optical measuring instruments, etc. in optical communications, etc.

[0003] The most primitive method of delaying light is the optical fiber delay circuit, which connects appropriate optical path switches to delay lines of different lengths. This optical fiber delay circuit has the advantages of being simple in configuration and being able to obtain a large delay amount depending on the length of the fiber delay line, but on the other hand, it has the disadvantages of being difficult to control because the delay amount is discrete, and the circuit is large.

[0004] To overcome the drawbacks of such optical fiber type optical delay circuits, that is, to realize a small size and high controllability of the delay amount, it is effective to apply a waveguide type optical circuit that controls light on a substrate made of an appropriate material. As a conventional technique related to a waveguide type optical circuit, an optical delay circuit that uses the group delay characteristics of a ring resonator has been proposed (for example, Non-Patent Document 1). In this waveguide type optical circuit that uses a ring resonator, the refractive index of the waveguide is controlled by a heater electrode provided on the ring type waveguide, thereby controlling the characteristics as an optical delay circuit.

[0005] However, a problem with waveguide-type optical circuits using ring resonators is that they are limited in size by the relationship between the free spectral range (FSR) and the bending radius of the ring resonator when it comes to controlling the optical delay.

[0006] The FSR of a ring resonator is the periodic band of the optical frequency of the resonance characteristics, and is expressed by (Equation 1).

[0007]

number

[0008] where λ0 is the reference wavelength in the wavelength band of interest, R is the radius of the ring resonator, and n g is the group refractive index of the ring waveguide. For example, assuming the C-band (wavelength: 1.53 μm to 1.65 μm) which is a typical communication wavelength, and the reference wavelength is 1.55 μm, the FSR required to have only one resonance characteristic in the band of interest is 35 nm or more. In this case, the group refractive index of the ring, n g If we take is 3.8, then from (Equation 1), the bending radius of the ring is calculated to need to be approximately 3 μm or less, which shows that a waveguide-type optical circuit with an extremely small bending curvature of the ring is required.

[0009] On the other hand, most of the reports on ring resonators with wide FSR to date are related to silicon photonics, which allows strong light confinement, i.e., a small bending radius. If a similar structure could be realized as a ring resonator using compound semiconductors instead of silicon, the optical delay circuit could be realized on the same substrate as the light-emitting element and the light-receiving element, which would have the advantages of miniaturization, low cost, and low connection loss. However, it is difficult to reduce the bending radius of the ring with such compound materials, and the minimum bending radius is currently limited to about 100 μm. Thus, it is currently difficult to realize a small, continuous, and high delay controllability in a waveguide-type optical circuit using a ring resonator. [Prior art documents] [Non-patent literature]

[0010] [Non-Patent Document 1] J. Yang, AO Karalar, SS Djordjevic, NK Fontaine, C. Yang , W. Chen, S. Chu, BE Little, and SJB Yoo, “Variable slowlight buffers in all-optical packet switching routers”, in Proc. Optical Fiber Communication / National Fiber Optic Engineers Conference 2008, OTuF2, 2008. Summary of the Invention

[0011] The present disclosure has been made in consideration of such problems, and an object of the present disclosure is to provide a waveguide-type optical delay circuit in which the delay amount can be controlled highly accurately and continuously without depending on the bending radius design as in the case of a ring resonator.

[0012] In order to solve the above problems, the present disclosure provides an optical delay circuit comprising: an M×N optical switch having M ports and N ports, where M is an integer equal to or greater than 1 and N is an integer equal to or greater than 2; a plurality of optical delay lines having different lengths connected to the N ports of the M×N optical switch; and reflecting mirrors installed at ends of the plurality of optical delay lines, wherein the M×N optical switch comprises a plurality of arm waveguides and a plurality of switch electrodes formed on the plurality of arm waveguides that control the phase of light propagating through the arm waveguides, and wherein the spectroscopic characteristics of light propagating from the M ports to the N ports in the M×N optical switch and the multiplexing characteristics of light propagating from the N ports to the M ports change in response to signals applied to the plurality of switch electrodes. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing a schematic diagram of an optical delay circuit exemplified in consideration of one embodiment of the present invention. [Diagram 2]FIG. 2 is a schematic diagram showing a mechanism for reflecting light at half the length of each delay line in an optical delay circuit according to one embodiment of the present invention. [Diagram 3] FIG. 1 is a schematic diagram illustrating an optical delay circuit including an optical switch according to an embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram showing an optical switch using refractive index control of a directional coupler. [Diagram 5] FIG. 13 shows the results of calculating the optical transmittance to the first output port and the second output port when the phase difference of the light traveling from the 2×2 optical switch to the first delay line and the second delay line is changed in an optical delay circuit according to one embodiment of the present invention. [Figure 6] This is the result of calculating the ratio of the light intensity returning to the first input port and the second input port with respect to the incident light according to the change in the switching characteristics of the 2×2 optical switch. [Figure 7] 13 shows the calculation results of the phase change amount of the first arm waveguide and the effective length at an optical wavelength of 1.55 μm. [Figure 8] 1 is a diagram illustrating an optical delay circuit according to an embodiment of the present invention; [Figure 9] FIG. 11 is a diagram showing the intensity of light returning to the first input port and the second input port when the phase change amount of the first arm waveguide is fixed at π / 2 and the phase of the light traveling back and forth through the first delay line is changed by the first delay line electrode. [Figure 10] 13 is a diagram showing the intensities of light returning to a first input port and a second input port when the wavelength of the input light is 1.56 μm. [Figure 11] FIG. 1 is a diagram illustrating a schematic configuration of a general wavelength tunable light source. [Figure 12] 1 is a diagram showing the feedback ratio by a filter, the gain of a longitudinal mode, and the gain of a longitudinal mode as a wavelength tunable laser with respect to the wavelength. [Figure 13] 1 is a diagram showing a schematic diagram of a tunable light source to which an optical delay circuit according to an embodiment of the present invention is applied; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. One embodiment of the present invention is a waveguide-type circuit including delay lines of different lengths, like a conventional optical fiber-type optical circuit. However, it differs from the conventional technology in that the controllability of the delay amount is improved by controlling the amount of phase change from each delay line. In addition, in order to control the amount of phase change, it is necessary for the light sent from each delay line to be multiplexed in the vicinity of the same phase. To achieve this, the present invention is characterized by including a mechanism for reflecting light at a position half the length of the delay line.

[0015] Fig. 1 is a diagram showing a schematic diagram of an optical delay circuit exemplified in considering one embodiment of the present invention. The optical delay circuit shown in Fig. 1 includes an M-input N-output (hereinafter, referred to as MxN) optical spectrometer 12 that splits incident light 11, an NxM optical multiplexer 14 that combines the light split by the MxN optical spectrometer 12 and outputs output light 13, and a delay line group 15 that connects the MxN optical spectrometer 12 and the NxM optical multiplexer 14 and guides the light split by the MxN optical spectrometer 12 to the NxM optical multiplexer 14. Here, M is an integer of 1 or more, and N is an integer of 2 or more. In addition, 1st to Mth input ports 16 are provided at the end of the MxN optical spectrometer 12 on the side where the incident light 11 is input, and 1st to Mth output ports 17 are provided at the end of the NxM optical multiplexer 14 on the side where the output light 13 is output. Further, the delay line group 15 is made up of N delay lines of different lengths.

[0016] In the optical delay circuit configured in this manner, if the transfer matrices in M×N optical splitter 12, N×M optical multiplexer 14 and delay line group 15 are A, B and D, respectively, each transfer matrix is ​​given by (Equation 2).

[0017]

number

[0018] Here, L a is the length of the M×N spectrometer 12, L bis the length of the N×M optical combiner 14, L n is the length of the nth delay line. Also, a n,m Or b n,m (m=1,2,.,M, n=1,2,.,N) is a complex number obtained by removing the phase factor due to the length from the M×N optical splitter 12 and the N×M optical combiner 14, and β is the propagation constant in the circuit.

[0019] 1, assume that light enters the i-th port of the M×N optical spectrometer 12 and exits from the j-th port of the N×M optical multiplexer 14. The transfer constant H at this time is expressed by Equation 3.

[0020]

number

[0021] In general, the effective length L of an optical circuit with a transfer function H is eff is expressed as the phase characteristic of the transfer function H and the differential of the optical propagation constant β, so the effective length L eff is expressed by (Equation 4). Note that, since the spatial phase factor is defined here to decrease with distance, (Equation 4) is differentiated and the sign is inverted.

[0022]

number

[0023] Here, considering the condition that the light from each delay line is strongly output from the jth output port of the N×M optical multiplexer 14 shown in FIG. 1, that is, the condition that the optical phase from each delay line is close to the same phase, the third term of (Equation 4) can be expressed as (Equation 5).

[0024]

number

[0025] Therefore, from (Equation 4) and (Equation 5), the effective length L effIt can be seen that |a is expressed as a weighted average of the intensity of light from each delay line when the electric field of the light passing through each delay line split by the M×N optical splitter 12 is combined by the N×M optical combiner 14. That is, the factor of the electric field intensity from each delay line, |a k,i b j,k It can be seen that by controlling |, that is, by controlling the amount of phase change of the light, the amount of delay of the light can be controlled.

[0026] As mentioned above, in the transformation from (Equation 4) to (Equation 5), the condition that the light from each delay line is close to the same phase and is multiplexed by the downstream multiplexer was taken into consideration. This has a certain degree of freedom, but at a certain propagation constant, βL k This can be achieved by choosing a wavelength and delay line length such that is approximately 0, and at the same time designing the transfer matrices A and B in (Equation 4) so ​​that φ = 0. A simple way to achieve this is to reflect light at half the length of each delay line in Figure 1, and send it back and forth through the delay line.

[0027] 2 is a schematic diagram showing a mechanism for reflecting light at half the length of each delay line in an optical delay circuit according to one embodiment of the present invention. This optical delay circuit has a structure in which a reflecting mirror 21 is provided at a position where the length of each delay line is half in the delay line group 15 of the optical circuit shown in FIG. 1. With this configuration, light incident on the input port 16 of the M×N optical spectrometer 12 is split at an appropriate splitting ratio through a split wave, then travels back and forth through each delay line, and returns to the input port 16 again. In other words, the light that has traveled back and forth has an effective length L that differs depending on the characteristics of the M×N optical spectrometer 12. eff Since the light sensed by the optical delay circuit is the light that can control the amount of delay with high precision.

[0028] (First embodiment) A first embodiment of the present disclosure will be described below with reference to the drawings. The optical delay circuit in this embodiment is an optical circuit including an optical switch using a multi-mode interference (hereinafter, referred to as MMI) waveguide having two 2×2 ports.

[0029] 3 is a diagram showing a schematic diagram of an optical delay circuit including an optical switch according to an embodiment of the present invention. In the optical delay circuit in this embodiment, a first MMI 301 and a second MMI 302 are installed in a 2×2 optical switch 30 that switches between optical demultiplexing and optical multiplexing characteristics, and two ports of the first MMI 301 and two ports of the second MMI 302 are connected by a first arm waveguide 303 and a second arm waveguide 304. A first switch electrode 305 and a second switch electrode 306 that control the phase of light are installed midway through the first arm waveguide 303 and the second arm waveguide 304, respectively. In addition, a first input port 307 and a second input port 308 are installed at the tip of the first MMI 301 (opposite the port to which the first arm waveguide 303 and the second arm waveguide 304 are connected) to guide the incident light 11 from the outside into the 2×2 optical switch 30 and to emit the light that has returned after going back and forth to the outside, as described later. On the other hand, a first output port 309 and a second output port 310 are provided at the end of the second MMI 302 (opposite the port to which the first arm waveguide 303 and the second arm waveguide 304 are connected), respectively. Furthermore, a first delay line 311 and a second delay line 312 having different lengths are connected to the first output port 309 and the second output port 310, respectively, and a reflection mirror 21 for reciprocating light is provided at each end of the first delay line 311 and the second delay line 312. A first delay line electrode 321 and a second delay line electrode 322 for controlling the phase of the branched light optically branched by the 2×2 optical switch 30 are provided midway through each of the first delay line 311 and the second delay line 312. In this embodiment, the length of the 2×2 optical switch 30 is 400 μm, and the structure of the 2×2 optical switch 30 is a Mach-Zehnder Interferometer (hereinafter, abbreviated as MZI) optical switch.

[0030] In the optical delay circuit having such a configuration, by transmitting a control signal to the first switch electrode 305 and the second switch electrode 306 and controlling the phase of the light, the optical demultiplexing characteristic and the optical multiplexing characteristic of the 2×2 optical switch 30 change, and the branching ratio of the emitted light can be adjusted as desired. A method for the first switch electrode 305 and the second switch electrode 306 to control the phase of the light includes a change in refractive index as is normally used in a waveguide-type circuit, and if the relevant waveguide is a semiconductor, for example, a method includes carrier injection, heating by a heater, or application of an electric field.

[0031] Fig. 4 is a schematic diagram of an optical switch using refractive index control of a directional coupler. As described above, the structure of the 2 × 2 optical switch 30 in this embodiment is an MZI switch, but the structure of the switch in this disclosure is not limited to this, and may be a structure in which the branching ratio is adjusted by controlling the refractive index of the directional coupler itself, as shown in Fig. 4.

[0032] In the optical delay circuit of this embodiment, it is assumed that an InP-based semiconductor multilayer substrate is used, and the refractive index of the first delay line 311 and the second delay line 312 for light with a wavelength of 1.55 μm is 3.4, and the group refractive index is 3.8.

[0033] In the optical delay circuit having such a configuration, the lengths of the first delay line 311 and the second delay line 312 are adjusted so that the phases of the light traveling back and forth through them are equal at a specific wavelength. In general, in an optical circuit, it is desirable to control the effective length of the incident light 11 while keeping the intensity of the light constant. The wavelength of light at which the effective length changes while the light is sufficiently small during the control of the effective length is called the optimum operating wavelength here, and the optical delay circuit in this embodiment is configured to design the optimum operating wavelength to be 1.55 μm. That is, when the diffraction order is set to 40, the refractive indexes of the first delay line 311 and the second delay line 312 are 3.4 as described above, so that when the difference in the optical path length traveling back and forth through each line is about 1.55×40 / 3.4=18.24 μm, the light returns to the first input port 307 and the second input port 308 again. The reason why the diffraction order is set to 40 here is to realize an optical delay circuit with the above-mentioned FSR of about 35 nm.

[0034] Considering that the reflecting mirror 21 is installed at the end of the first delay line 311 and the second delay line 312, if the length of the first delay line 311 is 100 μm, the length of the second delay line 312 should be 100+18.24 / 2=109.12 μm. Since what is important is the difference in optical path length, the lengths of the first delay line 311 and the second delay line 312 are not necessarily limited to this value. In addition, since the length relationship does not matter, the second delay line 312 does not necessarily have to be longer.

[0035] In addition, a first delay line electrode 321 and a second delay line electrode 322 may be provided in each of the first delay line 311 and the second delay line 312 to control the phase of the branched light optically separated by the 2×2 optical switch 30, and by using these first delay line electrode 321 and the second delay line electrode 322, the phase of the light traveling back and forth through the first delay line 311 and the second delay line 312 may be controlled.

[0036] FIG. 5 shows the results of calculating the optical transmittance to the first output port 309 and the second output port 310 when the phase difference of the light traveling from the 2×2 optical switch 30 to the first delay line 311 and the second delay line 312 is changed in the optical delay circuit according to this embodiment. Here, since the first switch electrode 305 performs the phase change, the phase of the light in the first arm waveguide 303 is changed. As shown in FIG. 5, as the amount of phase change increases, the optical transmittance transitions from the second output port 310 to the first output port 309, and after the optical transmittance to the first output port 309 reaches 100%, the optical transmittance to the second output port 310 increases again. That is, it can be seen that the branching rate of the branched light can be controlled according to the amount of phase change of the light. Note that, although a phase change in the first arm waveguide 303 is described here, in an MZI, the relative phase relationship between the first arm waveguide 303 and the second arm waveguide 304 affects the optical switch characteristics, so that a similar effect can be obtained by phase control in the second arm waveguide 304.

[0037] 6(A) to 6(E) show calculation results of the ratio of the light returning to the first input port 307 and the second input port 308 in terms of intensity with respect to the incident light 11 incident in accordance with the change in the switch characteristics of the 2×2 optical switch 30. Fig. 6(A) shows the calculation results when the amount of phase change of the light in the first arm waveguide 303 is 0, Fig. 6(B) shows the calculation results when the amount of phase change of the light in the first arm waveguide 303 is π / 4, Fig. 6(C) shows the calculation results when the amount of phase change of the light in the first arm waveguide 303 is π / 2, Fig. 6(D) shows the calculation results when the amount of phase change of the light in the first arm waveguide 303 is 3π / 4, and Fig. 6(E) shows the calculation results when the amount of phase change of the light in the first arm waveguide 303 is π. 6, in the vicinity of a typical wavelength of 1.55 μm used for communication, it can be seen that almost all of the incident light returns to the first input port 307 regardless of the amount of phase change in the first arm waveguide 303. This is because the difference in the round trip length of light in the first delay line 311 and the second delay line 312 shown in FIG. 3 is designed using the diffraction order 40 at a wavelength of 1.55 μm.

[0038] In addition, since the half period (half of the FSR) of both the first output port 309 and the second output port 310 is slightly more than 17 nm, it can be seen that the FSR is 35 nm when the optical delay circuit shown in Fig. 3 is used as a filter, as intended. In terms of the bending radius of a ring resonator, this corresponds to a bending radius of several μm, which is an unrealistic bending waveguide radius for a compound semiconductor waveguide. In other words, this means that the optical delay circuit according to this embodiment achieves control of the delay amount for light with a wavelength of 1.55 μm, which could not be achieved with a ring resonator.

[0039] 7 shows the calculation results of the phase change amount in the first arm waveguide 303 and the effective length at an optical wavelength of 1.55 μm. As shown in FIG. 7, when the phase change amount in the first arm waveguide 303 is 0, the effective length is 1018.24 μm. The value indicated by this effective length corresponds to twice the sum of the length of the 2×2 optical switch 30, 400 μm, and the optical path length of the second delay line 312, 109.12 μm. That is, in the optical delay circuit according to this embodiment, when the phase change in the first arm waveguide 303 is 0, 100% of the light output from the 2×2 optical switch 30 is output to the second output port 310, and 100% of the incident light 11 is guided to the second delay line 312. On the other hand, when the phase change amount of the first arm waveguide 303 is 0.5 or 1, it can be seen that the effective length is reduced to 1009.12 μm and 1000 μm, respectively. When the phase change is 0.5, light is equally incident on the first delay line 311 and the second delay line 312, so from (Equation 5), the effective length of the delay line part at this time is twice the sum of the average value of the first delay line 311 and the second delay line 312 and the length of the 2×2 optical switch 30. Also, when the phase change amount is 1, all light is guided to the first delay line through the first output port 309, so the effective length is twice the sum of the length of the first delay line and the length of the 2×2 optical switch 30. In this way, the optical delay circuit according to this embodiment can continuously control the delay amount by continuously controlling the phase change of the light in the 2×2 optical switch 30.

[0040] From the above, the optical delay circuit according to this embodiment is capable of continuously controlling the delay amount over a relatively wide wavelength range, which could only be achieved with a ring resonator with an extremely small bending radius.

[0041] Second Embodiment A second embodiment of the present disclosure will be described below. As in the first embodiment, this embodiment is an example in which the intensity of light returning to the second input port 308 is used to adjust the optimum operating wavelength of an optical delay circuit having a 2×2 optical switch 30.

[0042] 8 is a schematic diagram of the optical delay circuit of this embodiment. The optical delay circuit of this embodiment has a structure in which a power monitor 41 is installed at the second input port 308 in the optical delay circuit having the 2×2 optical switch shown in FIG.

[0043] As shown in Fig. 6(A) to (E), when the incident light 11 is incident on the first input port 307, the intensity of the light returning to the first input port 307 and the intensity of the light returning to the second input port 308 change when the relative phase relationship between the first arm waveguide 303 and the second arm waveguide 304 is controlled. In particular, when the phase change amount is other than 0 and π, it can be seen that the intensity of the light reflects the FSR of the optical circuit and changes depending on the wavelength. Here, when the intensity of the light returning to the first input port 307 and the intensity of the light returning to the second input port 308 are compared, it can be seen that the intensities of the two lights are in a complementary relationship with respect to the optical wavelength. In other words, if the intensity of the light returning to the second input port 308 is known, the intensity of the light returning to the first input port 307 can also be found based on that.

[0044] By utilizing this property and controlling the first delay line electrode 321 and the second delay line electrode 322 connected to the first delay line 311 and the second delay line 312, it is possible to adjust the optimum operating wavelength according to the wavelength of the incident light 11. To achieve this, in this embodiment, a power monitor 41 is connected to the second incident port 308.

[0045] 9(A) to 9(E) are diagrams showing the intensities of light returning to the first input port 307 and the second input port 308 when the phase change amount of the first arm waveguide 303 is fixed at π / 2 and the phase of the light traveling back and forth through the first delay line 311 is changed by the first delay line electrode 321. Fig. 9(A) shows the intensities of light when the phase change amount of the light traveling back and forth through the first delay line 311 is 0, Fig. 9(B) shows the intensities of light when the phase change amount of the light traveling back and forth through the first delay line 311 is π / 4, Fig. 9(C) shows the intensities of light when the phase change amount of the light traveling back and forth through the first delay line 311 is π / 2, Fig. 9(D) shows the intensities of light when the phase change amount of the light traveling back and forth through the first delay line 311 is 3π / 4, and Fig. 9(E) shows the intensities of light when the phase change amount of the light traveling back and forth through the first delay line 311 is π. It can be seen that the peaks of the light intensity at the first input port 307 and the second input port 308 shift to longer wavelengths by changing the phase of the light in the first delay line 311. This means that the optimum operating wavelength shifts to longer wavelengths according to the amount of change in the phase of the light in the first delay line 311. In other words, the optimum operating wavelength can be adjusted according to the wavelength of the incident light 11 by controlling the amount of phase change of the light with the first delay line electrode 321.

[0046] FIG. 10 is a diagram showing the intensity of light returning to the first input port 307 and the second input port 308 when the wavelength of the incident light 11 is 1.56 μm. Here, when focusing on the intensity of the light returning to the second input port, it can be seen that the intensity of the light is minimized when the phase change amount of the light in the first delay line 311 is around 0.6π. As described above, the intensity of the light returning to the first input port 307 at this time is close to the maximum value. In other words, if the first delay line electrode 321 is controlled so that the phase change amount of the light in the first delay line 311 is 0.6π, it is possible to adjust the optimum operating wavelength to match 1.56 μm, which is the wavelength of the incident light 11. In this way, by adopting the configuration of this embodiment, it is possible to appropriately control the optimum operating wavelength for the wavelength of the incident light 11.

[0047] (Third embodiment) A third embodiment of the present disclosure will be described below with reference to the drawings. The optical delay circuit according to this embodiment is an example of a wavelength-tunable light source used in laser oscillation, etc. Prior to the description of this embodiment, the configuration and function of a general wavelength-tunable light source will be described below.

[0048] Fig. 11 is a diagram showing a typical configuration of a typical tunable light source. As shown in Fig. 11, the tunable light source has a resonator structure consisting of an optical gain waveguide 61 that generates and amplifies light, a phase adjuster 62 that finely adjusts the resonator length in the tunable light source, a tunable filter 63 that selectively feeds back light of a specific wavelength to the light emitted from the optical gain waveguide 61, and mirrors 64 arranged to sandwich them. Note that the tunable filter 63 is often configured to adjust the reflection spectrum of a distributed Bragg reflector or the like, and in this case, the tunable filter 63 also serves as the mirror 64.

[0049] Fig. 12(A) shows the feedback ratio and longitudinal mode gain by the filter versus wavelength, and Fig. 12(B) shows the longitudinal mode gain as a wavelength tunable laser (wavelength tunable light source) versus wavelength. The feedback ratio shown in Fig. 12(A) is the feedback ratio of light to the optical gain waveguide 61 by the wavelength tunable filter 63, and can also be said to be the reflectance spectrum of light as seen from the optical gain waveguide 61. The longitudinal mode gain is derived from the fact that light feels a large gain in the optical gain waveguide 61 when the light creates a standing wave (longitudinal mode) in the resonator length of the wavelength tunable light source.

[0050] If the wavelength of light is λ and m is the longitudinal mode order (a positive integer), the wavelength of the longitudinal mode is expressed by (Equation 6).

[0051]

number

[0052] where n k and L kare the refractive index and physical length of each component that constitutes the tunable light source. In other words, in terms of the tunable light source shown in Fig. 11, these correspond to the refractive index and physical length of the phase shifter 62 (k=1), the optical gain waveguide 61 (k=2), and the tunable filter 63 (k=3). For simplicity, the lengths of these three components are all set to 400 µm, and the refractive indices are all set to n. In addition, the tunable light source is made of semiconductors, and the wavelength of the light from the semiconductor laser is set to 1.55 µm, which is a typical wavelength in optical communications.

[0053] As can be seen from (Equation 6), since there are an infinite number of longitudinal mode orders, in order to oscillate a tunable light source at a single wavelength, it is necessary to select a specific longitudinal mode and feed it back to the optical gain waveguide. Therefore, as shown in FIG. 12(B), a tunable filter 63 that selectively feeds back light of a specific wavelength can be provided in the tunable light source to select a specific longitudinal mode. The adjustment of the tunable filter 63 varies depending on the type, but as can be seen from (Equation 6), the longitudinal mode can be adjusted by adjusting the combined length of all three components, the phase adjuster 62 (k=1), the optical gain waveguide 61 (k=2), and the tunable filter 63 (k=3). The phase adjuster 62 in FIG. 11 adjusts this length. In other words, the tunable light source obtains a laser of the required wavelength by adjusting the tunable filter 63 and the phase adjuster 62.

[0054] Here, for a laser oscillating at a certain longitudinal mode order m, if the tunable filter 63 is adjusted to select, for example, a longitudinal mode order of m+1, a large wavelength change can be obtained. However, the change in longitudinal mode order m is called mode hopping, and the light intensity becomes unstable before and after the mode hopping, and the spectral purity deteriorates. For example, in gas sensing, in which light from a wavelength-tunable light source with a swept wavelength is transmitted through a gas and the gas type is identified from the transmission spectrum, if a mode hop occurs during the wavelength sweep, the continuity of the transmission spectrum is lost, making analysis difficult.

[0055] There is also a method of using a tunable light source as a non-temperature controlled laser to compensate for changes in the laser oscillation wavelength due to temperature changes by sending an electrical signal to the tunable light source, but even in this method, it is important to have wavelength control that does not cause mode hopping in the tunable light source. Non-temperature controlled lasers are very useful for reducing the size and power consumption of laser modules, but if mode hopping occurs during operation, it will significantly impair communication quality. Therefore, for the above reasons, wavelength control that does not cause mode hopping, that is, wavelength control at the same longitudinal mode order (called continuous tuning), is important in terms of applications. However, as will be described later, the small tuning amount is a problem with conventional continuous tuning.

[0056] For example, consider a case where laser oscillation occurs at wavelength λ and longitudinal mode order m in the wavelength tunable light source in Fig. 11. In this case, if the refractive index of phase adjuster 62 is changed from n to n+Δn to control the laser wavelength from λ to λ+Δλ, the ratio of the change amount Δλ of the laser wavelength to the wavelength λ can be expressed as shown in (Equation 6) to (Equation 7).

[0057]

number

[0058] As mentioned above, L1, L2, and L3 are all 400 μm. In addition, since the assumed wavelength-tunable light source is a semiconductor, the rate of change in the refractive index of the semiconductor due to heat or current (Δn / n) is about 0.5%. Therefore, from (Equation 7), Δλ / λ is less than 0.2%, and if the wavelength under consideration is 1.55 μm, the wavelength change amount Δλ in continuous tuning is only about 3 nm, which means that there is almost no change. In other words, with conventional continuous tuning, it is difficult to efficiently tune light in the typical wavelength band used in optical communications, for example.

[0059] Therefore, in this embodiment, the phase adjuster 62 in the wavelength tunable light source shown in FIG. 11 is replaced with the optical delay circuit described in the first and second embodiments.

[0060] FIG. 13 is a schematic diagram showing a tunable light source to which the optical delay circuit according to this embodiment is applied. In this embodiment, the tunable light source has a structure in which the phase adjuster 62 shown in FIG. 11 is replaced with the optical delay circuit described in the first and second embodiments. The first input port 307 of the 2×2 optical switch 30 is connected to the optical gain waveguide 61, and as shown in FIG. 8, a first delay line 311 and a second delay line 312 having different lengths are connected to the output side of the 2×2 optical switch 30. Furthermore, a first delay line electrode 321 and a second delay line electrode 322 that control the amount of phase change of light are connected to the first delay line 311 and the second delay line 312, and a reflection mirror 21 that reflects the light and makes it go back and forth is installed at the end. In addition, a power monitor 41 that monitors the intensity of the returned light is connected to the second input port 308 of the 2×2 optical switch 30. In a tunable light source with such a configuration, the amount of wavelength change at the same longitudinal mode order m is determined by the amount of change in effective length. In this case, the amount of wavelength change Δλ corresponding to (Equation 7) is expressed by (Equation 8).

[0061]

number

[0062] Here, L1' is the effective round trip length of the replaced optical delay circuit.

[0063] For example, consider that L1' can be changed from 500 μm to 510 μm. For comparison, the lengths of the optical gain waveguide 61 and the tunable filter, L2 and L3, are the same at 400 μm as described above. Then, from (Equation 8), Δλ / λ is about 0.75%. If the assumed wavelength of light is 1.55 μm, the wavelength change amount is about 12 nm, and it can be seen that about four times the continuous tuning amount can be obtained compared to the above-mentioned conventional technology. In this way, the optical delay circuit according to one embodiment of the present invention can continuously and highly accurately control the effective length for a relatively wide range of wavelength bands, and therefore contributes to improving the efficiency of continuous tuning of a tunable light source. [Industrial Applicability]

[0064] Compared to conventional optical delay circuits, such as waveguide-type optical delay circuits using ring resonators, this technology can control the amount of delay with high precision and continuously, and is expected to be applied to optical circuits in optical communications, laser oscillators, and other devices.

Claims

1. An M×N optical switch having M ports and N ports, where M is an integer equal to or greater than 1 and N is an integer equal to or greater than 2; a plurality of optical delay lines of different lengths connected to the N ports of the M×N optical switch; a reflecting mirror disposed at an end of each of the optical delay lines; An optical delay circuit comprising: The M×N optical switch is A plurality of arm waveguides; a plurality of switch electrodes formed in the plurality of arm waveguides for controlling a phase of light propagating through the arm waveguides; Equipped with a spectroscopic characteristic of light propagating from the M ports to the N ports in the M×N optical switch and a multiplexing characteristic of light propagating from the N ports to the M ports are changed in response to signals applied to the plurality of switch electrodes; The light emitted from the plurality of optical delay lines is multiplexed in the M×N optical switch in the vicinity of the same phase. An optical delay circuit configured as above.

2. 2. The optical delay circuit according to claim 1, wherein the length of each of said plurality of optical delay lines is adjusted so that the phases of the light traveling round trip through said optical delay line are equal at a specific wavelength of incident light.

3. 3. The optical delay circuit according to claim 1, further comprising a delay line electrode in at least one of said plurality of optical delay lines for controlling a phase of said light propagating through said optical delay line.

4. 4. The optical delay circuit according to claim 1, further comprising a power monitor for monitoring the intensity of said light in at least one of said M ports.

5. An optical delay circuit according to any one of claims 1 to 4, an optical gain waveguide for generating and amplifying light, said light being input to said optical delay circuit via at least one of said M ports; a tunable filter disposed on the opposite side of the M ports of the optical gain waveguide, for selectively feeding back light of a specific wavelength with respect to light from the optical gain waveguide; a second reflecting mirror disposed on an opposite side of the tunable filter from the optical gain waveguide; A tunable light source comprising:

Images