Wavelength conversion circuit and multi-wavelength light source

Abstract

Provided are a wavelength conversion circuit and a multi-wavelength light source that are compact and generate two wavelengths having a greater frequency difference.　In this wavelength conversion circuit, input light is demultiplexed into a first arm waveguide and a second arm waveguide by a first coupler, a phase difference is applied between light beams propagated through each of the first and second arm waveguides by a phase modulation means provided to each of the first and second arm waveguides, and the light beams are multiplexed by a second coupler. A loop mirror is provided so as to switch, into an input port and an output port, a pair of ports of the second coupler on the opposite side from the pair of ports of the second coupler to which the first and second arm waveguides are connected. Output light is output from a port paired with an input port of the first coupler on the opposite side from the pair of ports of the first coupler to which the first and second arm waveguides are connected. The multi-wavelength light source includes the wavelength conversion circuit, and another end of a semiconductor optical amplifier, of which one end is optically coupled to a mirror, is optically coupled to an input port of the first coupler via a wavelength filter.

Description

Wavelength conversion circuit and multi-wavelength light source

[0001] The present invention relates to a wavelength conversion circuit that generates light of multiple wavelengths and a multi-wavelength light source using the same.

[0002] In optical communications used for transmitting and receiving large amounts of data in data centers and other locations, the light emitted from a light source is modulated, and the signal is transmitted through an optical waveguide, including an optical fiber. For example, multiple lasers emitting different wavelengths are used, each light is modulated, and then combined in an optical multiplexer to output to a single optical waveguide. However, using multiple lasers results in a large and complex device, so wavelength conversion circuits have been proposed that can obtain multiple wavelengths from a single light source (for example, Patent Document 1).

[0003] Furthermore, in recent years, optical communication using multiple wavelengths with frequency differences ranging from 100 GHz to 800 GHz has been considered through MSAs (Multi-Source Agreements), and wavelength conversion circuits that emit multiple wavelengths corresponding to such large frequency differences of several hundred GHz are required.

[0004] For example, Non-Patent Document 1 describes an LN(LiNbO) using the electro-optic effect. ３ This paper describes a method for emitting two wavelengths of light from a single light source by using SSB (Single Sideband) modulation for an optical modulator and further creating a dual-carrier configuration using a 2x2 coupler on the output side of the optical modulator. The light generated by this method yields two wavelengths corresponding to a frequency difference of twice the frequency modulated by the optical modulator.

[0005] Japanese Patent Publication No. 2006-30732

[0006] H. Yamazaki et al., “Dual-carrier IQ modulator with a complementary frequency shifter,” Opt. Express 19, B69 (2011).

[0007] In the method described above, which uses an LN optical modulator to emit two wavelengths, there is a limit to the modulation speed, making it difficult to increase the frequency difference of the generated light. While it is possible to increase the frequency difference of the modulated frequencies by connecting multiple modulators in series, this increases the circuit size and power consumption.

[0008] The present invention has been made in view of the above circumstances, and its objective is to provide a wavelength conversion circuit and a multi-wavelength light source that are compact and capable of generating two wavelengths with a larger frequency difference.

[0009] The wavelength conversion circuit according to the present invention is a wavelength conversion circuit that separates input light into a first arm waveguide and a second arm waveguide using a first coupler, applies a phase difference between the light propagating through the first arm waveguide and the second arm waveguide using phase modulation means provided to each of the first arm waveguide and the second arm waveguide, and combines them using a second coupler, wherein the first coupler and the second coupler are 2x2 couplers, and the second coupler is provided with loop mirrors to swap the input port and output port between a pair of ports on the opposite side of the pair of ports to which the first arm waveguide and the second arm waveguide are connected, and outputs output light from an output port of the first coupler that is paired with the input port to which the input light was input, on the opposite side of the pair of ports to which the first arm waveguide and the second arm waveguide are connected.

[0010] With these characteristics, by passing the signal through a single MZ-type optical modulator twice, two wavelengths corresponding to a frequency difference four times the modulation frequency can be obtained as output light. Therefore, even at the same modulation speed, two wavelengths with a larger frequency difference than before can be obtained, and it is possible to generate two wavelengths with a larger frequency difference while maintaining a compact design. In particular, an optical multiplexer for guiding the output light into a single waveguide is not required, making it even more compact.

[0011] In the invention described above, the loop mirror may be characterized by including a directional coupler. Alternatively, the loop mirror may be characterized by including an MMI type optical waveguide. With such features, it is possible to generate two wavelengths with a larger frequency difference while being more compact.

[0012] The multi-wavelength light source according to the present invention includes the wavelength conversion circuit described above, and is characterized in that the other end of a semiconductor optical amplifier, to which a mirror is optically coupled at one end, is optically coupled to the input port of the first coupler via a wavelength filter.

[0013] These characteristics allow for the generation of two wavelengths with a larger frequency difference, while maintaining a compact size.

[0014] In the invention described above, the semiconductor optical amplifier may be characterized by being of the quantum well type. With this characteristic, since only a single wavelength can be fed back to the SOA, even if a quantum well type is used for the SOA, the instability of laser oscillation caused by longitudinal mode competition can be suppressed.

[0015] This is a circuit diagram showing a wavelength conversion circuit according to one embodiment of the present invention. This is a circuit diagram for explaining the principle of the wavelength conversion circuit according to one embodiment of the present invention. This is a diagram of the wavelength spectrum of light generated by the wavelength conversion circuit. This is a circuit diagram showing a multi-wavelength light source according to another embodiment of the present invention. This is a diagram of the wavelength spectrum of light generated by the multi-wavelength light source.

[0016] Hereinafter, a wavelength conversion circuit, which is one embodiment of the present invention, and a multi-wavelength light source, which is another embodiment, will be described in detail with reference to Figures 1 to 5.

[0017] <Example 1> As shown in Figure 1, the wavelength conversion circuit 10 includes a first coupler 11 that splits the input light from the input waveguide 1 into the first arm waveguide 3 and the second arm waveguide 4, respectively; a second coupler 12 that combines and splits the light from the first arm waveguide 3 and the second arm waveguide 4 again; and a loop mirror 8 that loops the light from the second coupler 12.

[0018] The first coupler 11 includes a port 11a connected to the input waveguide 1 and a port 11b paired with port 11a. Port 11b is connected to the output waveguide 2 and propagates the output light to the output waveguide 2 as described later. The first coupler 11 also includes a pair of ports 11c and 11d on the opposite side of the pair of ports 11a and 11b. These ports 11c and 11d are connected to the first arm waveguide 3 and the second arm waveguide 4, respectively. In other words, the first coupler 11 can split the input light from the input waveguide 1 and propagate it to the first arm waveguide 3 and the second arm waveguide 4, respectively. The first coupler 11 is a 2x2 coupler.

[0019] Furthermore, optical modulators 5 and 6 are provided in the first arm waveguide 3 and the second arm waveguide 4, respectively. In addition, a heater 7 is provided in at least one of the first arm waveguide 3 and the second arm waveguide 4 (in this case, the first arm waveguide 3). Optical modulators 5 and 6 are, for example, optical modulators that modulate the amplitude of light. The heater 7 can also modulate the phase, for example, by the thermo-optic effect. These phase modulation means can introduce a phase difference between the light propagating in the first arm waveguide 3 and the second arm waveguide 4, respectively.

[0020] For example, the optical modulator 5 can be an MZ (Mach-Zehnder) type optical modulator. In this case, it has a branch that separates the light propagating through the first arm waveguide 3, and each branch has a phase modulation element 5a, 5b that modulates the phase, and at least one of the branches is further equipped with a heater 5c.

[0021] The phase modulation elements 5a and 5b can be formed by doping impurities into the waveguide of a Si core to create a pn structure or a pin structure semiconductor, thereby obtaining a change in refractive index through the carrier plasma effect. Alternatively, the phase modulation elements 5a and 5b may be compound semiconductors of group III-V elements such as InP, achieved through hybrid bonding. Furthermore, the waveguide of an LN (lithium niobate) core or SiO ２When the core is used as a waveguide, LN can be used for the phase modulation elements 5a and 5b. In the case of a waveguide whose core is formed of a semiconductor made of a group III-V element such as InP, a compound semiconductor made of a group III-V element such as InP can be used for the phase modulation elements 5a and 5b.

[0022] Furthermore, the heater 5c is disposed to adjust the phase via the thermo-optic effect. In the case of a Si core waveguide, the heater 5c can be formed, for example, such that a voltage can be applied to TiN or an impurity-doped Si core. Instead of the heater 5c, a pn structure or a pin structure of the Si waveguide may be provided to cause a refractive index change. In the case of a waveguide with a core made of a material other than Si, a refractive index change may be caused by a structure similar to that of the phase modulation element 5a instead of the heater 5c.

[0023] The optical modulator 6 is also the same as the optical modulator 5. That is, it has a branch for separating light from the second arm waveguide 4, and each branch is provided with phase modulation elements 6a and 6b that modulate the phase, and at least one branch is further provided with a heater 6c. Other detailed materials and the like can also be selected from a plurality of structures and materials in the same manner as the optical modulator 5. Furthermore, the heater 7 provided in the first arm waveguide 3 described above can also be selected from a plurality of structures and materials in the same manner.

[0024] The second coupler 12 has a pair of ports 12a and 12b connected to the first arm waveguide 3 and the second arm waveguide 4 respectively, combines the light propagated from the first arm waveguide 3 and the second arm waveguide 4, demultiplexes the combined light, and outputs the light from a pair of ports 12c and 12d. That is, the second coupler 12 is a 2×2 coupler.

[0025] The loop mirror 8 is connected such that the input side and output side of the pair of ports 12c and 12d on the opposite side of the pair of ports 12a and 12b connected to the first arm waveguide 3 and the second arm waveguide 4 of the second coupler 12 are swapped. That is, the loop mirror 8 is disposed such that the pair of ports 12c and 12d of the second coupler 12 are connected to each other via a loop. A device including a directional coupler or a multi-mode interference (MMI) type optical waveguide can be used for such a loop mirror 8.

[0026] With these arrangements, the wavelength conversion circuit 10 can obtain output light from input light through the following operation.

[0027] First, input light is guided from the input waveguide 1 to the first coupler 11 to be demultiplexed, and then propagates to the first arm waveguide 3 and the second arm waveguide 4. In the first arm waveguide 3 and the second arm waveguide 4, the phase of the respectively propagated light is modulated by the optical modulators 5, 6 and the heater 7 to provide a phase difference.

[0028] The light propagating through the first arm waveguide 3 and the second arm waveguide 4 respectively enters the second coupler 2 from ports 12a and 12b respectively, is combined, then is demultiplexed, and is looped by the loop mirror 8 from ports 12c and 12d of the second coupler 12. That is, the light propagated from the port 12c to the loop mirror 8 travels clockwise (CW) on the paper surface and is guided to the port 12d. Conversely, the light propagated from the port 12d to the loop mirror 8 travels counterclockwise (CCW) on the paper surface and is guided to the port 12c.

[0029] The light looped by the loop mirror 8 is combined and then demultiplexed by the second coupler 12, and is guided to the first arm waveguide 3 and the second arm waveguide 4. The light propagated to the first arm waveguide 3 and the second arm waveguide 4 is further provided with a phase difference by the optical modulators 5, 6 and the heater 7, and is combined by the first coupler 11. Then, the light is guided from the port 11b of the first coupler to the output waveguide 2, and output light is obtained.

[0030] Here, also referring to FIG. 2, electrical waveforms of φsinΩt and -φsinΩt are respectively applied to phase modulation elements 5a and 5b of the optical modulator 5 to perform optical modulation. Further, electrical waveforms of φcosΩt and -φcosΩt are respectively applied to phase modulation elements 6a and 6b of the optical modulator 6 to perform optical modulation, where φ is the modulation degree and Ω is the modulation frequency. Then, it is assumed that a phase difference of π is provided by the heater 5c for the upper and lower branches of the optical modulator 5. It is also assumed that a phase difference of π is provided by the heater 6c for the upper and lower branches of the optical modulator 6. Further, it is assumed that a phase difference of π / 2 is provided between the first arm waveguide 3 and the second arm waveguide 4 by the heater 7.

[0031] Here, as input light, a light with frequency ω ｃ is input into the input waveguide 1. Then, the input light is split into equal powers by the first coupler 11. The light propagated from the port 11c to the first arm waveguide 3 is amplitude-modulated by the optical modulator 5, then phase-delayed by π / 2 by the heater 7, and then enters the second coupler 12. On the other hand, the light propagated from the port 11d to the second arm waveguide 4 is amplitude-modulated by the optical modulator 6 and then enters the second coupler 12. At this time, in the second coupler 12, the lights respectively incident from the first arm waveguide 3 and the second arm waveguide 4 are converted into dual carriers in the process of being combined and split, resulting in two lights with different frequencies ω ｃ +Ω and ω ｃ -Ω, which correspond to two lights with different wavelengths. At this time, from the port 12c of the second coupler 12 to the loop mirror 8, the light with frequency ω ｃ +Ω propagates clockwise (CW), and the light with frequency ω ｃ -Ω propagates counterclockwise (CCW). It should be noted that higher-order wavelength-converted lights corresponding to frequencies ω ｃ -3Ω and ω ｃ +3Ω are also generated despite low conversion efficiency.

[0032] The clockwise light and the counterclockwise light are incident on the second coupler 12 by the loop mirror 8 such that the input port and the output port of the second coupler 12 are exchanged. That is, the clockwise light is incident on the second coupler 12 from the port 12d, and the counterclockwise light is incident on the second coupler 12 from the port 12c, they are combined, and then split into equal powers.

[0033] One of the lights split from the clockwise light is phase-delayed by π / 2 by the heater 7 in the first arm waveguide 3, amplitude-modulated by the optical modulator 5, and then incident on the first coupler 11. On the other hand, the other of the split lights is amplitude-modulated by the optical modulator 6 in the second arm waveguide 4 and then incident on the first coupler 11. In the first coupler 11, the respectively incident lights are combined and split again. In this process, dual carrier conversion occurs again, resulting in two different frequencies ω ｃ +2Ω and ω ｃIt generates light of two different wavelengths corresponding to ω. Then, the output waveguide 2 has a frequency ω ｃ Light with a wavelength corresponding to +2Ω is propagated and emitted as output light.

[0034] On the other hand, the light separated from the counterclockwise light also undergoes a similar process in the first arm waveguide 3 and the second arm waveguide 4, and is then combined and separated again in the first coupler 11. In this process, dual carrier formation occurs again, resulting in two different frequencies ω ｃ And it generates light of two different wavelengths corresponding to ωc - 2Ω. And the output waveguide 2 has a frequency ω ｃ Light with a wavelength corresponding to -2Ω is propagated and emitted as output light.

[0035] In other words, the output waveguide 2 outputs light of frequency ω ｃ Light of a wavelength and frequency ω corresponding to +2Ω ｃ Light with a wavelength corresponding to -2Ω can be obtained. Furthermore, as mentioned above, higher-order wavelength-converted light (clockwise light with frequency ω) can be obtained in the loop mirror 8. ｃ - 3Ω, counterclockwise light with frequency ω ｃ For light with a wavelength corresponding to +3Ω, wavelength conversion occurs again during the return process through the first arm waveguide 3 and the second arm waveguide 4, but this squares the already low generation efficiency. As a result, the generation efficiency is negligibly small. In other words, the generation of unwanted wavelength-converted light can be suppressed.

[0036] The above operation will be further explained using mathematical formulas.

[0037] If the amplitude of the input light is A, the electric field Ein of the input light is given by the following equation (1).

[0038] At this time, the electric field E of clockwise light ｃｗ , counterclockwise light field E ｃｃｗ This can be calculated as shown in equations (2) and (3) below.

[0039] Here, if m is an integer, then J ｍ This represents the m-th order Bessel function. Therefore, the incident frequency ω is the input light. ｃLight of the corresponding wavelength is clockwise light (corresponding to frequency ωc + Ω) and counterclockwise light (frequency ωc + Ω). ｃ It is shown that it is converted to a wavelength of -Ω. Furthermore, higher-order wavelength-converted light (clockwise light with frequency ω) is shown. ｃ - 3Ω, counterclockwise light with frequency ω ｃ Light with a wavelength equivalent to +3Ω is also generated, although with low conversion efficiency.

[0040] The electric field Eout for light that returns from the loop mirror 8 to the first arm waveguide 3 and the second arm waveguide 4, and then to the output waveguide 2, can be calculated as shown in equation (4) below.

[0041] Therefore, the frequency ω of the incident light is ｃ Light of a wavelength corresponding to this is two different wavelengths (frequency ω ｃ +2Ω, and frequency ω ｃ It is shown that the output light (corresponding to -2Ω) is emitted into the output waveguide 2.

[0042] Figure 3 shows the results of calculating the wavelength spectrum generated by the wavelength conversion circuit 10 based on the above-mentioned formulas. The results for the "clockwise light (CW)" and "counterclockwise light (CCW)" of the "loop mirror" are given by formulas (2) and (3), respectively, and the result for the "output light" is given by formula (4). For example, assuming that the modulation frequency by optical modulators 5 and 6 is 50 GHz, two wavelengths of light corresponding to a frequency difference of 200 GHz will be obtained as output light.

[0043] As described above, the wavelength conversion circuit 10 allows for the production of output light with a wavelength corresponding to a frequency difference of four times the modulation frequency by passing the light through optical modulators 5 and 6 twice. Therefore, even at the same modulation speed, it is possible to obtain output light of two wavelengths with a larger frequency difference than conventional circuits. Furthermore, because the wavelength conversion circuit 10 passes the light through optical modulators 5 and 6 twice, it eliminates the need for series multi-stage connections, allowing for a compact design and contributing to low power consumption. In particular, since the output light is guided to a single output waveguide 2, an optical multiplexer for guiding the output light to a single waveguide is unnecessary, making it even more compact. In short, the wavelength conversion circuit 10 is compact while being able to generate output light of two wavelengths with a larger frequency difference.

[0044] <Example 2> As shown in Figure 4, the multi-wavelength light source 20 includes the wavelength conversion circuit 10 of Example 1, and the other end of a semiconductor optical amplifier (SOA) 22, which has a mirror 23 optically coupled to one end, is optically coupled to port 11a, which is the input port of the first coupler 11, via a wavelength filter 21.

[0045] A ring resonator can be used as the wavelength filter 21. A heater, pn structure, or pin structure may be formed in the ring resonator to change the refractive index. The wavelength filter 21 may also be formed by a grating. In addition, multiple wavelength filters 21 may be arranged in a multi-stage configuration.

[0046] The semiconductor optical amplifier 22 can be, for example, a quantum well type, but it may also be a quantum dot type. Instead of the combination of the semiconductor optical amplifier 22 and the mirror 23, a reflective semiconductor optical amplifier (RSOA), which is an SOA with a reflective multilayer film formed on it, can also be used.

[0047] Here, the wavelength filter 21 has a frequency ω ｃ Assume that the filter is transparent to light of a wavelength corresponding to ω. Furthermore, assume that the semiconductor optical amplifier 22 has sufficient gain for at least that wavelength of light. Then, the input light has a frequency ω ｃWhen light is input to the input waveguide 1, similar to the case of the wavelength conversion circuit 10, the output waveguide 2 receives two different wavelengths of light (frequency ω) as output light. ｃ +2Ω, ω ｃ A (equivalent to -2Ω) is emitted. Meanwhile, in the input waveguide 1, the light of the other wavelength (frequency ω) is generated by simultaneously dual-carriing each of the two wavelengths mentioned above. ｃ (equivalent to) returns as return light.

[0048] Then, in the multi-wavelength light source 20, the frequency ω ｃ In the light, resonance occurs between mirror 23 and loop mirror 8. The feedback light is returned to the input waveguide 1 after wavelength conversion relative to the input light, resulting in optical loss in the wavelength conversion circuit and attenuation due to the feedback efficiency. Here, if the semiconductor optical amplifier 22 has a gain that exceeds the optical loss and attenuation due to the feedback efficiency of the wavelength conversion circuit, the output light will have two different wavelengths (frequency ω ｃ +2Ω, ω ｃ It becomes possible to simultaneously generate laser oscillation of light (corresponding to -2Ω). At this time, even though two wavelengths of light are being generated simultaneously, the semiconductor optical amplifier 22 has wavelength ω ｃ Only single-wavelength light can be fed back.

[0049] Next, we will explain the operation of the multi-wavelength light source 20 using mathematical formulas.

[0050] In the multi-wavelength light source 20, the electric field E of the feedback light ｒｅｆ This is shown by the following equation (5).

[0051] According to the same formula, the input light is input to the input waveguide 1 with frequency ω ｃ It is shown that when light is incident on the input waveguide, light of the same wavelength as the input light returns to the input waveguide 1 as feedback light. The feedback efficiency of the feedback light relative to the input light is approximately 4 J. １ (φ) ４ It is represented as follows.

[0052] Figure 5 shows the results of calculating the wavelength spectrum generated by the multi-wavelength light source 20 based on the above-mentioned formulas. The "input light" is calculated using formula (1), the "clockwise light (CW)" and "counterclockwise light (CCW)" of the "loop mirror" are calculated using formulas (2) and (3), respectively, the "output light" is calculated using formula (4), and the "feedback light" is calculated using formula (5).

[0053] Here, for example, let's assume that the optical loss due to the wavelength conversion circuit 10 is 8 dB, the modulation frequency by the optical modulators 5 and 6 is 50 GHz, and φ = 1. Then, the feedback efficiency of the light returning to the input waveguide 1 as feedback light will be approximately 15%, which translates to an optical loss of 8.2 dB. Therefore, if the gain of the semiconductor optical amplifier 22 exceeds the sum of this optical loss of 8.2 dB and the optical loss of the wavelength conversion circuit 10, which is 16.2 dB, then laser oscillation will be possible. As a result, two wavelengths of light corresponding to a frequency difference of 200 GHz will be obtained as output light. Furthermore, since the semiconductor optical amplifier 22 can only receive feedback of a single wavelength as described above, even when using a quantum well type SOA, the instability of laser oscillation caused by longitudinal mode competition can be suppressed.

[0054] Thus, the multi-wavelength light source 20, like the wavelength conversion circuit 10, is compact yet capable of generating two wavelengths with a larger frequency difference.

[0055] Although embodiments and modifications based thereon have been described above, the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the invention or the scope of the attached claims.

[0056] 1. Input waveguide 2. Output waveguide 3. First arm waveguide 4. Second arm waveguide 5. Optical modulator 6. Optical modulator 7. Heater 8. Loop mirror 10. Wavelength conversion circuit 20. Multi-wavelength light source 22. Semiconductor optical amplifier

Claims

1. A wavelength conversion circuit that separates input light into a first arm waveguide and a second arm waveguide using a first coupler, applies a phase difference between the light propagating through the first arm waveguide and the second arm waveguide using phase modulation means provided to each of the first arm waveguide and the second arm waveguide, and combines them using a second coupler, wherein the first coupler and the second coupler are 2x2 couplers, and the second coupler is provided with loop mirrors to swap the input port and output port between a pair of ports on the opposite side of the pair of ports to which the first arm waveguide and the second arm waveguide are connected, and outputs output light from an output port on the opposite side of the pair of ports to which the input light was input, the input port to which the input light was input.

2. The wavelength conversion circuit according to claim 1, characterized in that the loop mirror includes a directional coupler.

3. The wavelength conversion circuit according to claim 1, characterized in that the loop mirror includes a multimode interference (MMI) type optical waveguide.

4. A multi-wavelength light source comprising a wavelength conversion circuit according to one of claims 1 to 3, wherein the input port of the first coupler is optically coupled to the other end of a semiconductor optical amplifier, which has a mirror optically coupled to one end, via a wavelength filter.

5. The multi-wavelength light source according to claim 4, characterized in that the semiconductor optical amplifier is of the quantum well type.

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