JP2014119468A - Wavelength variable light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength variable light source such that even with a constitution in which a number of LDs are used, a cost increase can be prevented.SOLUTION: A wavelength variable light source 100 includes: 12 LDs 1; two MMI elements 10 with six input ports and one output port; and an MMI element 20 with two input ports and two output ports. The MMI element 20 is connected to two connection waveguides 51 connected to the two MMI elements 10 and two connection waveguides 52. The two connection waveguides 52 are each connected to arm portions 31a and 31b included in a Mach-Zehnder modulator 30.

Description

  The present invention relates to a wavelength tunable light source using a plurality of LDs and a Mach-Zehnder modulator.

  Along with the dramatic increase in communication demand in recent years, the development of a wavelength division multiplex communication system that enables large-capacity transmission with a single optical fiber by multiplexing a plurality of signal lights having different wavelengths has been promoted. Yes. In order to realize a wavelength division multiplexing communication system, a low-cost tunable light source that covers the entire wavelength band is required. Therefore, an optical semiconductor device that can be monolithically integrated has attracted attention.

  In a wavelength tunable light source using an optical semiconductor, a configuration in which an LD array is coupled to one optical waveguide by an MMI (Multi Mode Interference) element is applied. The MMI element is a multimode interference (MMI) type optical coupler (optical multiplexing / demultiplexing element). The LD array is composed of a plurality of LDs (Laser Diodes) having different oscillation wavelengths. In addition to the LD array, a configuration in which a modulator for generating a data signal is integrated in one optical semiconductor element is realized.

  In the following, an MMI element of K (natural number) input L (natural number) output is also expressed as K × LMMI.

  Patent Document 1 discloses a technique (hereinafter also referred to as related technique B) that solves the problems of a technique (hereinafter also referred to as related technique A) in which a Mach-Zehnder modulator is arranged at the subsequent stage of the MMI element. In the related art A, in the configuration in which the LD is connected to each of the 12 input ports of the 12 × 1 MMI, when one LD emits light, in principle, only light having an intensity of 1/12 is output. Couple to the waveguide. In this case, the remaining light of 11/12 intensity is emitted from 12 × 1 MMI. Therefore, 12 × 1 MMI has a problem that a loss of 10.8 dB occurs.

  Related technology B is a technology for realizing miniaturization of an element while suppressing optical loss. Specifically, Related Technology B is configured to connect two output waveguides connected to K × 2 MMI to two arms of a Mach-Zehnder modulator. With such a configuration, it is possible to improve the loss by, for example, 3 dB.

JP 2007-065357 A

  However, Related Technology B has the following problems. Specifically, in Related Technology B, when a large number of LDs are used, it is necessary to use an expensive MMI element that can process light from the LDs at a time. For example, in the related art B, when 12 LDs are used, it is necessary to use an expensive MMI element such as 12 × 2 MMI. In this case, the related technique B has a problem that the cost of the MMI element or the like is significantly increased.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a wavelength tunable light source capable of suppressing an increase in cost even in a configuration using many LDs.

  In order to achieve the above object, a wavelength tunable light source according to an aspect of the present invention includes M (natural number of 2 or more) × N (natural number of 2 or more) LDs (Laser Diodes) having different oscillation wavelengths, Are connected to the M first input waveguides and one first output waveguide, and N first MMI (Multi Mode Interference) elements with M inputs and one output, An N-input S-output second output connected to the N first output waveguides connected to the first MMI element and S (even N or less) second output waveguides. An MMI element and a Mach-Zehnder modulator having S arm portions respectively connected to the S second output waveguides, and connected to the N first MMI elements (M × N) A different LD is connected to each of the first input waveguides.

  According to the present invention, M (natural number greater than or equal to 2) × N (natural number greater than or equal to 2) LDs, N first MMI elements with M outputs and 1 output, and second MMI with N inputs and S outputs An element. The second MMI element is connected to the N first output waveguides connected to the N first MMI elements and the S (even N or less) second output waveguides. Is done. The S second output waveguides are respectively connected to S arm portions of the Mach-Zehnder modulator.

  In other words, the configuration of the MMI element provided between the M × N LDs and the arm portion of the Mach-Zehnder modulator is such that the N first MMI elements are the first stage MMI elements, and the second MMI element is the second MMI element. This is a two-stage configuration as a second-stage MMI element. Thereby, even when the number of LDs to be used is large (for example, 12), there is no need to use an expensive MMI element capable of processing many LDs. Therefore, even in a configuration using many LDs, an increase in the cost of the wavelength tunable light source can be suppressed.

It is a figure which shows the structure of the wavelength variable light source which concerns on Embodiment 1 of this invention. It is sectional drawing of an embedding structure. It is sectional drawing of a high mesa structure. It is a figure which shows the structure of the wavelength variable light source which concerns on the modification of Embodiment 1 of this invention. It is a figure which shows the structure of the wavelength variable light source which concerns on Embodiment 2 of this invention. It is a figure which shows the structure of the wavelength variable light source which concerns on Embodiment 3 of this invention. It is a figure which shows the principle of phase modulation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof may be omitted.

  It should be noted that the dimensions, materials, shapes, relative arrangements, and the like of the constituent elements exemplified in the embodiments are appropriately changed depending on the configuration of the apparatus to which the present invention is applied and various conditions. It is not limited to those examples. Moreover, the dimension of each component in each figure may differ from an actual dimension.

  In each of the following embodiments, the MMI element is a multimode interference (MMI) type optical coupler (optical multiplexing / demultiplexing element). In the following, an MMI element with K (natural number) input and L (natural number) output is also expressed as K × LMMI. The waveguides and connection waveguides described in the following embodiments are optical waveguides that propagate light.

<Embodiment 1>
FIG. 1 is a diagram showing a configuration of a wavelength tunable light source 100 according to Embodiment 1 of the present invention. The wavelength variable light source 100 is formed on an InP substrate 21 described later.

  The wavelength tunable light source 100 includes a semiconductor laser group LX, MMI elements 10a, 10b, and 20 and a Mach-Zehnder modulator 30.

  The semiconductor laser group LX is composed of semiconductor laser groups LX1 and LX2. Each of the semiconductor laser groups LX1 and LX2 is composed of six LD1s. That is, the semiconductor laser group LX including the semiconductor laser groups LX1 and LX2 includes 12 (6 × 2 = 12) LD1s.

  Each of the 12 LDs 1 is a semiconductor laser having a different oscillation wavelength. Each LD1 emits light.

  Each of the MMI elements 10a and 10b is a 6-input 1-output MMI element (6 × 1 MMI). The MMI element 10a has six input ports IP1 and one output port OP1. The six connection waveguides 50 are respectively connected to the six input ports IP1. The connection waveguide 50 connected to each input port IP1 is an input waveguide of the MMI element 10a.

  The six LD1s constituting the semiconductor laser group LX1 are connected to the six connection waveguides 50, respectively. That is, the six LDs are connected to the six input ports IP1 by the six connection waveguides 50, respectively.

  Since the MMI element 10b has the same configuration as the MMI element 10a, a detailed description of the MMI element 10b will not be given. That is, the MMI element 10b has six input ports IP1 and one output port OP1. The six LD1s constituting the semiconductor laser group LX2 are each connected to six input ports IP1 (MMI element 10b).

  A connection waveguide 51 is connected to the output port OP1 of each of the MMI elements 10a and 10b. The connection waveguide 51 is an output waveguide of each of the MMI elements 10a and 10b. That is, each of the MMI elements 10a and 10b is connected to six connection waveguides 50 (input waveguides) and one connection waveguide 51 (output waveguide).

  Hereinafter, each of the MMI elements 10 a and 10 b is also simply referred to as an MMI element 10. That is, twelve LD1s are connected to twelve (2 × 6 = 12) connecting waveguides 50 (input waveguides) connected to the two MMI elements 10, respectively. That is, different LD1s are connected to each of the twelve connection waveguides 50 (input waveguides of the MMI element 10) connected to the two MMI elements 10. Accordingly, light emitted from different LDs 1 is input to each of the twelve input ports IP1 included in the two MMI elements 10.

  The MMI element 20 has two input ports IP2 and two output ports OP2. That is, the MMI element 20 is a 2-input 2-output MMI element (2 × 2 MMI).

  The output port OP1 of the MMI element 10a is connected to one input port IP2 of the MMI element 20 by a connection waveguide 51. That is, the connection waveguide 51 connected to the MMI element 10a is connected to one input port IP2 of the MMI element 20. Further, the output port OP1 of the MMI element 10b is connected to the other input port IP2 of the MMI element 20 by the connection waveguide 51. That is, the connection waveguide 51 connected to the MMI element 10 b is connected to one input port IP 2 of the MMI element 20.

  The connection waveguide 51 is an output waveguide of the MMI element 10 and an input waveguide of the MMI element 20.

  Two connection waveguides 52 are connected to the two output ports OP2 of the MMI element 20, respectively. Each connection waveguide 52 is an output waveguide of the MMI element 20.

  That is, the two output ports OP1 included in the two MMI elements 10 are connected to the two input ports IP2 of the MMI element 20 by the connection waveguide 51, respectively. In other words, the MMI element 20 includes two connection waveguides 51 (output waveguides of the MMI element 10) connected to the two MMI elements 10 and two connection waveguides 52 (output guides of the MMI element 20). To the waveguide).

  The Mach-Zehnder modulator 30 includes arm portions 31 a and 31 b and an MMI element 33. Hereinafter, each of the arm portions 31 a and 31 b is also simply referred to as an arm portion 31.

  That is, the Mach-Zehnder modulator 30 has two arm portions 31. Each arm part 31 propagates light. A modulation electrode 32 is formed on a part of each arm portion 31. A voltage is applied to the modulation electrode 32 from the outside. The arm unit 31 controls the phase of the propagating light according to the magnitude of the voltage applied to the modulation electrode 32.

  The two connection waveguides 52 are connected to the arm portions 31a and 31b, respectively. That is, the two connection waveguides 52 (output waveguides) connected to the MMI element 20 are connected to the arm portions 31a and 31b (Mach-Zehnder modulator 30), respectively. In other words, the two output ports OP2 of the MMI element 20 are connected by the two arm portions 31 and the two connection waveguides 52, respectively.

  The MMI element 33 has two input ports IP3 and one output port OP3. That is, the MMI element 33 is a 2-input 1-output MMI element (2 × 1 MMI).

  The arm portion 31 a is connected to one input port IP <b> 3 of the MMI element 33 by the connection waveguide 53. The arm portion 31 b is connected to the other input port IP 3 of the MMI element 33 by the connection waveguide 53. A connection waveguide 54 is connected to the output port OP3 of the MMI element 33.

  With the above configuration, in the wavelength tunable light source 100, the light emitted from each LD 1 is input to the Mach-Zehnder modulator 30 via the MMI element 10 and the MMI element 20. The Mach-Zehnder modulator 30 outputs the input light from the connection waveguide 54.

  The wavelength tunable light source 100 is not limited to the above configuration, and may be realized by a configuration based on the following expression (hereinafter referred to as configuration A).

  The configuration A includes M (natural number greater than or equal to 2) × N (natural number greater than or equal to 2) LDs, N first MMI elements, a second MMI element with N inputs and S outputs, a Mach-Zehnder modulator, . S is an even number equal to or less than N.

  In the configuration A, each of the M × N LDs has a different oscillation wavelength. In the configuration A, each of the N first MMI elements is an M-input 1-output element. In the configuration A, each of the N first MMI elements is connected to the M first input waveguides and the one first output waveguide.

  In the configuration A, the second MMI element is connected to the N first output waveguides connected to the N first MMI elements and the S second output waveguides. Is done. In the configuration A, the Mach-Zehnder modulator has S arm portions respectively connected to the S second output waveguides. In the configuration A, each different LD is connected to each of the (M × N) first input waveguides connected to the N first MMI elements.

  The configuration of the wavelength tunable light source 100 according to the present embodiment is a configuration in which M = 6, N = 2, and S = 2 in the above configuration A.

  In the related art A described above, the intensity of light that can be coupled to the output waveguide of 12 × 1 MMI is 1/12 of the intensity of light input to the 12 × 1 MMI. That is, a theoretical loss of 10.8 dB occurs. The principle loss is a fundamental light loss.

  On the other hand, in the wavelength tunable light source 100 having the configuration of FIG. 1, the intensity of light that can be coupled to the output waveguide of the 6 × 1 MMI (MMI element 10) is 1/6 of the intensity of the light input to the 6 × 1 MMI. Become. That is, a principle loss of 7.8 dB occurs.

  In the 2 × 2 MMI (MMI element 20), the light from the 6 × 1 MMI output waveguide is only branched into two by the 2 × 2 MMI, and therefore no loss occurs. The loss is a loss in light propagation (transmission). Even when one of the 12 LDs emits light (emits light) and light is input to one of two 2 × 2 MMI input waveguides, each output waveguide of 2 × 2 MMI Light is output evenly without loss of principle. Therefore, compared with the above-mentioned related art A, in Embodiment 1 of FIG. 1, a loss can be reduced by 3.0 dB.

  In Related Technology B, a special MMI such as 12 × 2 MMI is applied. On the other hand, in the present embodiment, since it is configured by using general-purpose MMIs such as 6 × 1 MMI and 2 × 2 MMI, implementation is easy.

  In the wavelength tunable light source 100, the output waveguide of 2 × 2 MMI (MMI element 20) is connected to each arm portion 31 of the Mach-Zehnder modulator 30. The Mach-Zehnder modulator 30 can generate and output signal light by intensity modulation such as RZ modulation and NRZ modulation, or phase modulation such as DPSK modulation. Specifically, the Mach-Zehnder modulator 30 generates and outputs signal light by applying a voltage to the modulation electrode 32 and performing the intensity modulation or phase modulation.

  Further, the wavelength tunable light source 100 applies a voltage to the modulation electrode 32 formed on each arm portion 31 of the Mach-Zehnder modulator 30 to inject current into each arm portion 31. Thereby, the phase of the light propagating through each arm part 31 is controlled.

  Here, for example, it is assumed that the phase difference (hereinafter also referred to as phase difference A) between the light propagating through the arm portion 31a and the light propagating through the arm portion 31a is zero. In this case, the light is strengthened in the 2 × 1 MMI (MMI element 33) on the output side of the Mach-Zehnder modulator 30, and the light is output from the output waveguide (connection waveguide 54). When the phase difference A is π, the light is weakened within the 2 × 1 MMI (MMI element 33). In this case, no light is output from the MMI element 33.

  Therefore, intensity modulation can be performed by injecting current from the modulation electrode 32 to each arm portion 31 to control the phase of light. At this time, the light phase of only the arm portion 31 on one side may be changed by π. Further, push-pull driving may be performed in which the phase of the light of the arm portion 31a is changed by π / 2 and the phase of the light of the arm portion 31b is changed by −π / 2.

  For example, when the phase of the light of the arm unit 31 a is changed by π and the phase of the light of the arm unit 31 b is changed by −π, the intensity of the light output from the Mach-Zehnder modulator 30 does not change. However, since the phase changes by π, a phase modulation signal can be generated.

  In the above description, the plasma effect by current injection is used to change the light phase of each arm 31. However, the present invention is not limited to this. For example, a quantum confined Stark effect, a Pockels effect, or the like generated by applying a reverse voltage to the modulation electrode 32 may be used. Further, the light phase may be changed by forming a heater electrode on the arm portion 31 and applying heat to the heater electrode.

  The above-described device can be manufactured using, for example, an InP-based material. FIG. 2 is a cross-sectional view of the connection waveguide 51 taken along the line A1-A2 of FIG. As described above, the wavelength variable light source 100 is formed on the InP substrate 21. That is, the connection waveguide 51 is formed on the InP substrate 21.

  Here, the connection waveguide 51 has a buried structure in which InGaAsP22 is buried with InP23 and 24 on the InP substrate 21. Light is confined in InGaAsP22 having a higher refractive index than InP23 and 24, and the light propagates in the longitudinal direction of the element (connection waveguide 51). Note that the connection waveguides 50, 52, 53, 54 also have the same structure as the connection waveguide 51.

  Note that a high mesa structure as shown in FIG. 3 may be applied to the structure of the connection waveguide (for example, the connection waveguide 51) according to the present embodiment. When the LD has a buried structure, it is more advantageous in terms of connection loss if the connection waveguide structure is also a buried structure. However, when the structure of the connection waveguide is a high mesa structure, the light confinement effect is increased, and radiation loss in the curved connection waveguide can be suppressed.

  1 may be a buried structure, and the Mach-Zehnder modulator 30 may be a high mesa structure. By making the structure of the Mach-Zehnder modulator 30 a high mesa structure, the electric capacity of the Mach-Zehnder modulator 30 is reduced and high-speed modulation is possible.

  In the present embodiment, M = 6, N = 2, and S = 2 in the above configuration A, but the present invention is not limited to this. For example, in configuration A, M = 8, N = 2, and S = 2 may be set. In this case, the principle loss of 8 × 1 MMI is 9.0 dB, the principle loss of 2 × 2 MMI is 0.0 dB, and the total principle loss is 9.0 dB. Therefore, compared with FIG. 1, the loss increases by 1.2 dB, but the number of LD1 is 16, and the wavelength range of light that can be covered can be expanded 1.3 times. Thereby, a modulation signal in a wide wavelength range can be generated.

  As described above, according to the present embodiment, in the wavelength tunable light source 100, the configuration of the MMI element provided between the 12 (M × N) LDs 1 and the arm portion 31 of the Mach-Zehnder modulator 30 is as follows. Two MMI elements 10 are the first stage MMI elements, and the MMI element 20 is the second stage MMI element. Thereby, even when the number of LDs to be used is large (for example, 12), it is not necessary to use an expensive MMI element (for example, 12 × 2 MMI) that can process many LDs. Therefore, even in a configuration using many LDs, an increase in the cost of the wavelength tunable light source can be suppressed.

  In the related art B, which is the above-described prior art, in the case of a configuration using 12 LDs, it is necessary to use a special MMI such as 12 × 2 MMI. Note that 12 × 2 MMI is complicated in design and lacks feasibility.

  On the other hand, in the present embodiment, as described above, a general-purpose MMI with proven results such as 6 × 1 MMI and 2 × 2 MMI is used. Therefore, it is easy to realize the configuration of the wavelength variable light source. Moreover, in the wavelength tunable light source 100 of the present embodiment, a loss of 3.0 dB can be reduced as compared with the related art A described above. That is, according to the present embodiment, general-purpose MMI can be applied, and loss of light transmission can be reduced.

(Modification of Embodiment 1)
Note that the variable wavelength light source 100 according to Embodiment 1 is not limited to the configuration shown in FIG. For example, the wavelength tunable light source according to Embodiment 1 may be a wavelength tunable light source 100A shown in FIG.

  FIG. 4 is a diagram showing a configuration of a wavelength tunable light source 100A according to a modification of the first embodiment of the present invention. Referring to FIG. 4, the wavelength tunable light source 100 </ b> A further includes a Mach-Zehnder modulator 30 </ b> A instead of the Mach-Zehnder modulator 30 and an SOA (Semiconductor Optical Amplifier) 62 compared to the wavelength-tunable light source 100 of FIG. 1. It differs from the point to prepare. The rest of the configuration of wavelength tunable light source 100A is the same as that of wavelength tunable light source 100, and therefore detailed description will not be repeated.

  The Mach-Zehnder modulator 30A is different from the Mach-Zehnder modulator 30 in that it includes SOAs 61a and 61b. Since the other configuration of the Mach-Zehnder modulator 30A is the same as that of the Mach-Zehnder modulator 30, detailed description thereof will not be repeated.

  The SOA 61a is formed on a part of the arm portion 31a. The SOA 61b is formed on a part of the arm portion 31b. The SOA 62 is formed in the connection waveguide 54. That is, the SOA 62 is formed after the Mach-Zehnder modulator 30A.

  In the wavelength tunable light source 100A having the above configuration, in the SOA (for example, the SOA 61a), the intensity of light input to the SOA can be amplified by injecting a current into the SOA. Therefore, it is possible to compensate for a loss generated by 6 × 1 MMI and increase the light output.

  The wavelength tunable light source 100A is not limited to the configuration shown in FIG. For example, the variable wavelength light source 100A may be configured not to include the SOA 62. For example, the wavelength variable light source 100A may be configured not to include the SOAs 61a and 61b. That is, the wavelength tunable light source 100A may have a configuration in which an SOA is formed in at least one of the arm part of the Mach-Zehnder modulator 30A and the subsequent stage of the Mach-Zehnder modulator 30A.

<Embodiment 2>
FIG. 5 is a diagram showing a configuration of a wavelength tunable light source 100B according to Embodiment 2 of the present invention.

  The wavelength tunable light source 100B is expressed by the configuration A described above. The wavelength tunable light source 100B has a configuration in which M = 3, N = 4, and S = 2 in the above configuration A.

  Specifically, referring to FIG. 5, wavelength tunable light source 100B includes semiconductor laser group LXA instead of semiconductor laser group LX, as compared with wavelength tunable light source 100 of FIG. The difference is that MMI elements 11a, 11b, 11c, and 11d are provided instead of 10b, and an MMI element 20A is provided instead of the MMI element 20. Since the other configuration of wavelength tunable light source 100B is the same as that of wavelength tunable light source 100, detailed description will not be repeated.

  The semiconductor laser group LXA is composed of semiconductor laser groups LX11, LX12, LX21, and LX22. Each of the semiconductor laser groups LX11, LX12, LX21, and LX22 is composed of three LD1s. That is, the semiconductor laser group LXA has 12 (6 × 2 = 12) LD1s.

  Each of the MMI elements 11a, 11b, 11c, and 11d is a three-input one-output MMI element (3 × 1 MMI). Hereinafter, each of the MMI elements 11a, 11b, 11c, and 11d is also simply referred to as an MMI element 11. Note that the input port and output port of the MMI element 11 are different only in number from the MMI element 10 of FIG. 1, and therefore will not be described in detail.

  Three connection waveguides 50 and one connection waveguide 51 are connected to each of the four MMI elements 11. Each connection waveguide 50 is an input waveguide of the MMI element 11. The connection waveguide 51 is an output waveguide of the MMI element 11.

  The twelve LDs 1 are respectively connected to twelve input waveguides connected to the four MMI elements 11. Specifically, the semiconductor laser groups LX11, LX12, LX21, and LX22 are connected to the MMI elements 11a, 11b, 11c, and 11d, respectively.

  Hereinafter, the semiconductor laser group LX11 will be described. The three LDs 1 constituting the semiconductor laser group LX11 are each connected to three connection waveguides 50 (MMI elements 11a). That is, the three LDs 1 constituting the semiconductor laser group LX11 are connected to the MMI element 11a by the three connection waveguides 50.

  The semiconductor laser groups LX12, LX21, and LX22 are also connected in the same manner as the semiconductor laser group LX11 described above, and will not be described in detail.

  The MMI element 20A is a 4-input 2-output MMI element (4 × 2 MMI). The input port and output port of MMI element 20A differ only in number compared to MMI element 20 in FIG. 1, and thus detailed description will not be repeated.

  Two connection waveguides 51 and two connection waveguides 52 are connected to the MMI element 20A. Each connection waveguide 51 is an input waveguide of the MMI element 20A. Each connection waveguide 52 is an output waveguide of the MMI element 20A.

  The two connection waveguides 52 (output waveguides) of the MMI element 20A are connected to the arm portions 31a and 31b (Mach-Zehnder modulator 30), respectively.

  In the related art A described above, the intensity of light that can be coupled to the output waveguide of 12 × 1 MMI is 1/12 of the intensity of light input to the 12 × 1 MMI. That is, a theoretical loss of 10.8 dB occurs.

  On the other hand, in the wavelength tunable light source 100B having the configuration of FIG. 5, the intensity of the light that can be coupled to the output waveguide of the 3 × 1 MMI (MMI element 11) is 1/3 of the intensity of the light input to the 3 × 1 MMI. Become. That is, a principle loss of 4.8 dB occurs.

  In the 4 × 2 MMI (MMI element 20A), the light intensity that can be coupled to the output waveguide is ½, and a principle loss of 3 dB occurs. That is, a total of 7.8 dB of principle loss occurs in the MMI element 11 and the MMI element 20A.

  Therefore, compared with the above-described related art A, the loss can be reduced by 3.0 dB in the second embodiment of FIG.

  In Related Technology B, a special MMI such as 12 × 2 MMI is applied. On the other hand, in the present embodiment, since it is configured by using general-purpose MMIs such as 3 × 1 MMI and 4 × 2 MMI, it is easy to realize.

  In the present embodiment, M = 3, N = 4, and S = 2 in the configuration A, but the present invention is not limited to this. For example, in the configuration A, M = 4, N = 4, and S = 2 may be set. In this case, the principle loss of 4 × 1 MMI is 6.0 dB, the principle loss of 4 × 2 MMI is 3.0 dB, and the total principle loss is 9.0 dB. Therefore, compared with FIG. 5, the loss increases by 1.2 dB, but the number of LD1 becomes 16, and the wavelength range of light that can be covered can be expanded 1.3 times.

  As described above, the wavelength tunable light source 100B according to the present embodiment includes two MMI elements provided between the LD 1 and the arm portion 31 of the Mach-Zehnder modulator 30, as in the first embodiment. The MMI element 11 is a first stage MMI element, and the MMI element 20A is a second stage MMI element. Therefore, even in a configuration using many LDs, an increase in the cost of the wavelength tunable light source can be suppressed.

  The configuration of the wavelength tunable light source 100B of FIG. 5 is similar to the configuration of the modification of the first embodiment (FIG. 4), in which one or both of the arm portion of the Mach-Zehnder modulator 30 and the subsequent stage of the Mach-Zehnder modulator 30 are SOAs. It is good also as a structure which formed. In the case of this configuration, the intensity of light input to the SOA can be amplified by injecting a current into the SOA. Therefore, it is possible to compensate for the loss generated by 3 × 1 MMI and increase the light output.

<Embodiment 3>
FIG. 6 is a diagram showing a configuration of a wavelength tunable light source 100C according to Embodiment 3 of the present invention. Referring to FIG. 6, the wavelength tunable light source 100 </ b> C includes an MMI element 20 </ b> B instead of the MMI element 20 </ b> A as compared with the wavelength tunable light source 100 </ b> B of FIG. 5 and a phase modulator 40 instead of the Mach-Zehnder modulator 30. Is different from the point provided. Since the other configuration of wavelength tunable light source 100C is the same as that of wavelength tunable light source 100B, detailed description will not be repeated.

  The MMI element 20B is a 4-input 4-output MMI element (4 × 4 MMI). Note that the input port and output port of the MMI element 20B differ only in number from the MMI element 20 of FIG. 1, and thus will not be described in detail.

  The phase modulator 40 includes Mach-Zehnder modulators 30 a and 30 b, an MMI element 41, and a phase shifter 42.

  The Mach-Zehnder modulators 30a and 30b are provided in parallel. Since each of Mach-Zehnder modulators 30a and 30b has the same configuration as that of Mach-Zehnder modulator 30 in FIG. 5, detailed description will not be repeated.

  Hereinafter, each of the Mach-Zehnder modulators 30 a and 30 b is also simply referred to as a Mach-Zehnder modulator 30. That is, in the phase modulator 40 (wavelength variable light source 100C), two Mach-Zehnder modulators 30 are provided in parallel. That is, the two Mach-Zehnder modulators 30 are provided so as to be arranged in parallel.

  Each of the Mach-Zehnder modulators 30a and 30b has arm portions 31a and 31b. Hereinafter, each of the arm portions 31 a and 31 b is also simply referred to as an arm portion 31. That is, the two Mach-Zehnder modulators 30 provided in parallel have four arm portions 31.

  In the wavelength tunable light source 100C, as in the second embodiment, the twelve LDs 1 are respectively connected to twelve input waveguides connected to the four MMI elements 11.

  Four connection waveguides 51 and four connection waveguides 52 are connected to the MMI element 20B. Each connection waveguide 51 is an input waveguide of the MMI element 20B. Each connection waveguide 52 is an output waveguide of the MMI element 20B. That is, the four output waveguides (connection waveguides 51) connected to the four MMI elements 11 are connected to the MMI element 20B.

  The four connection waveguides 52 (output waveguides) connected to the MMI element 20B are connected to the four arm portions 31 (arm portions of the two Mach-Zehnder modulators 30), respectively. That is, the four connection waveguides 52 (output waveguides) connected to the MMI element 20B are respectively connected to the four arm portions 31 included in the two Mach-Zehnder modulators 30.

  The MMI element 41 is a 2-input 1-output MMI element (2 × 1 MMI). Connection waveguides 54, 55, and 56 are connected to the MMI element 41. The connection waveguide 54 connected to the MMI element 33 is an output waveguide of the Mach-Zehnder modulator 30 a and an input waveguide of the MMI element 41. That is, the output waveguide (connection waveguide 54) of the Mach-Zehnder modulator 30a is connected to the MMI element 41.

  The connection waveguide 55 is an input waveguide of the MMI element 41. The connection waveguide 56 is an output waveguide of the MMI element 41. A phase shifter 42 is provided between the Mach-Zehnder modulator 30 b and the MMI element 41.

  The phase shifter 42 changes the phase of light by π / 2. The phase shifter 42 is connected to the output waveguide (connection waveguide 54) of the Mach-Zehnder modulator 30b and the input waveguide (connection waveguide 55) of the MMI element 41.

  The wavelength tunable light source 100C is not limited to the above configuration, and may be realized by a configuration based on the following expression (hereinafter referred to as configuration B).

  The configuration B includes M (natural number greater than or equal to 2) × N (natural number greater than or equal to 2) LDs, N first MMI elements, N input S output second MMI elements, and two Mach-Zehnders. And a modulator. S is an even number equal to or less than N.

  In the configuration B, each of the M × N LDs has a different oscillation wavelength. In the configuration B, each of the N first MMI elements is an M-input 1-output element. In the configuration B, each of the N first MMI elements is connected to the M first input waveguides and one first output waveguide.

  In the configuration B, the second MMI element is connected to the N first output waveguides connected to the N first MMI elements and the S second output waveguides. Is done. In the configuration B, each of the two Mach-Zehnder modulators has S arm portions respectively connected to the S second output waveguides. In the configuration B, different LDs are connected to each of the (M × N) first input waveguides connected to the N first MMI elements. In the configuration B, two Mach-Zehnder modulators are provided in parallel.

  The configuration of the wavelength tunable light source 100C according to the present embodiment is a configuration in which M = 3, N = 4, and S = 4 in the configuration B.

  The tunable light source 100C having a configuration in which two Mach-Zehnder modulators 30 are connected in parallel can further reduce the principle loss compared to the tunable light source 100 or the tunable light source 100B to which one Mach-Zehnder modulator 30 is applied.

  In the configuration of FIG. 6, the intensity of light that can be coupled to the output waveguide of the 3 × 1 MMI (MMI element 11) is 1/3 of the intensity of the light input to the 3 × 1 MMI. That is, a principle loss of 4.8 dB occurs.

  In the 4 × 4 MMI (MMI element 20B), no loss occurs because light from the output waveguide of 3 × 1 MMI is only branched into 4 by the 4 × 4 MMI. Even when any of the 12 LDs 1 emits light (emits light) and light is input to any of the 4 × 4 MMI input waveguides, each 4 × 4 MMI output waveguide has Light is output evenly without loss of principle.

  Therefore, compared with the configuration of FIGS. 1 and 5 in which the principle loss of 7.8 dB occurs, the configuration of FIG. 6 can reduce the loss by 3.0 dB. Further, 3 × 1 MMI or 4 × 4 MMI is generally used and can be easily realized.

  Next, the principle of phase modulation will be described. In DQPSK modulation, two Mach-Zehnder modulators 30 arranged in parallel are used for phase modulation of the I channel and the Q channel, respectively. In the wavelength tunable light source 100C, the above-described phase shifter 42 is provided in order to shift the phase between the lights output from the Mach-Zehnder modulators 30a and 30b by π / 2.

  FIG. 7 is a diagram illustrating the principle of phase modulation. Hereinafter, the arm portions 31a and 31b of the Mach-Zehnder modulator 30a are also referred to as arms A and B, respectively. In the following, the arm portions 31a and 31b of the Mach-Zehnder modulator 30b are also referred to as arms C and D, respectively.

  Referring to FIG. 7, on the I channel side, the Mach-Zehnder modulator 30a applies a voltage of 0 to Vπ to the modulation electrode 32 of the arm A. On the I channel side, the Mach-Zehnder modulator 30 a applies a voltage of 0 to −Vπ to the modulation electrode 32 of the arm B. On the Q channel side with a phase difference of π / 2, the Mach-Zehnder modulator 30 b applies a voltage of 0 to Vπ to the modulation electrode 32 of the arm C. On the Q channel side, the Mach-Zehnder modulator 30 b applies a voltage of 0 to −Vπ to the modulation electrode 32 of the arm D.

  Here, the synthesis of arm A and arm B (hereinafter also referred to as synthesis AB) changes on the real axis in FIG. The composite AB is indicated by the vector VC1. Also, the combination of arm C and arm D (hereinafter also referred to as composite CD) varies on the imaginary axis and represents 0 and 1. The composite CD is indicated by the vector VC2. By combining the composites AB and CD, a 2-bit signal of (0, 1) (1, 0) (1, 1) (0, 1) corresponding to four types of phases can be generated.

  In the present embodiment, M = 3, N = 4, and S = 4 in Configuration B above, but the present invention is not limited to this. For example, in the configuration B, M = 4, N = 4, and S = 4 may be set. In this case, the principle loss of 4 × 1 MMI is 6.0 dB, the principle loss of 4 × 4 MMI is 0.0 dB, and the total principle loss is 6.0 dB. Therefore, compared with FIG. 4, the loss increases by 1.2 dB, but the number of LD1 is 16, and the wavelength range of light that can be covered can be expanded 1.3 times.

  Further, for example, in the configuration B, M = 8, N = 4, and S = 4 may be set. In this case, the principle loss of 8 × 1 MMI is 9.0 dB, the principle loss of 4 × 4 MMI is 0.0 dB, and the total principle loss is 9.0 dB. Therefore, the loss is increased by 4.2 dB compared to FIG. 4, but the number of LDs is 32, and the wavelength range of light that can be covered can be expanded by 2.7 times.

  Further, in the wavelength tunable light source 100C of the present embodiment, the configuration of the MMI element provided between the LD 1 and the arm portion 31 of the Mach-Zehnder modulator 30 is similar to that of the first embodiment. A first stage MMI element is used, and the MMI element 20B is a second stage MMI element. Therefore, even in a configuration using many LDs, an increase in the cost of the wavelength tunable light source can be suppressed.

  It should be noted that within the scope of the invention, the present invention can be freely combined with the embodiments and modifications of the embodiments, or can be appropriately modified and omitted with reference to the embodiments and modifications of the embodiments. is there.

  For example, in the first to third embodiments, the configuration of the MMI element provided between the LD 1 and the arm portion 31 of the Mach-Zehnder modulator 30 is a two-stage configuration, but is not limited to this. The configuration of the MMI element may be three or more stages.

  1 LD, 10, 10a, 10b, 11, 11a, 11b, 11c, 11d, 20, 20A, 20B, 33, 41 MMI element, 30, 30A, 30a, 30b Mach-Zehnder modulator, 31, 31a, 31b arm part, 32 modulation electrode, 40 phase modulator, 42 phase shifter, 50, 51, 52, 53, 54, 55, 56 connecting waveguide, 100, 100A, 100B, 100C wavelength variable light source.

Claims (2)

M (natural number of 2 or more) × N (natural number of 2 or more) LDs (Laser Diodes) having different oscillation wavelengths,
N first MMI (Multi Mode Interference) elements each having M inputs and one output, each connected to M first input waveguides and one first output waveguide;
N input S outputs connected to the N first output waveguides connected to the N first MMI elements and S (even N or less) second output waveguides. A second MMI element of
A Mach-Zehnder modulator having S arm portions respectively connected to the S second output waveguides,
Each of the different LDs is connected to each of the (M × N) first input waveguides connected to the N first MMI elements. S is 4,
The wavelength variable light source is provided with two Mach-Zehnder modulators in parallel,
The wavelength tunable according to claim 1, wherein the four second output waveguides connected to the second MMI element are respectively connected to the four arm portions of the two Mach-Zehnder modulators. light source.

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