JP6381507B2 - Optical coupler, wavelength tunable light source and wavelength tunable light source module - Google Patents

Description

  The present invention relates to an optical coupler, and a wavelength tunable light source and a wavelength tunable light source module including the same.

  2. Description of the Related Art In recent years, with a dramatic increase in communication demand, a wavelength division multiplexing communication system has been realized in which capacity transmission of an optical fiber per one is enhanced by multiplexing a plurality of signal lights having different wavelengths. A wavelength division multiplexing communication system includes a laser diode (hereinafter referred to as “LD”) and a multi-mode interference type optical coupler (hereinafter referred to as “MMI”). A light source is used.

  As an LD light source used in the above-described wavelength division multiplexing communication system, a single mode LD (hereinafter referred to as “single mode LD”) capable of obtaining a high side mode suppression ratio (SMSR) of at least 30 to 40 dB or more. Is preferred). The single mode LD includes, for example, a distributed feedback laser diode (hereinafter referred to as “DFB-LD”) and a distributed Bragg reflector laser diode (hereinafter referred to as “DBR-LD”). ) Etc.

  In order to realize the wavelength division multiplexing communication system, a low cost covering a wide wavelength range (30 nm or more), for example, the entire wavelength band of about 30 to 40 nm called C band (Conventional band) or L band (Long band). The wavelength tunable light source is required. As an LD light source of such a wavelength tunable light source, an LD light source monolithically integrated on the same substrate has attracted attention.

  In general, the variable wavelength light source is used in combination with an external modulator module that generates a data signal. In addition to the above-described wavelength tunable light source, research is also being conducted on monolithically integrating an electro absorption (EA) optical modulator and a Mach Zehnder (MZ) optical modulator on the same substrate. ing.

  Various techniques have been proposed for the above-described wavelength tunable light source. For example, a wavelength tunable light source configured such that input light coupled (multiplexed) by MMI is output from an output waveguide in a configuration in which the output sides of a plurality of DFB-LDs are connected to an MMI input waveguide is proposed. (For example, refer to Patent Document 1). Hereinafter, in this specification, an MMI having K inputs (K is a natural number) and L outputs (L is a natural number) is referred to as “K × L-MMI”. That is, K input means K input waveguides connected to the MMI, and L output means L output waveguides connected to the MMI.

  Further, for example, the output side of a plurality of LDs is connected to N input waveguides of N × 2-MMI (N is a natural number of 3 or more), and the two output waveguides of the MMI are two Mach-Zehnder modulators. A variable wavelength light source connected to an arm has been proposed (see, for example, Patent Document 2). According to the technique of Patent Document 2, it is possible to improve the optical loss as compared with the case of using N × 1-MMI.

  Further, for example, in an MMI in which the output sides of a plurality of DFB-LDs are connected to a plurality of input waveguides of the MMI, after the phase adjustment of the output light is performed in each of the two output waveguides of the MMI, the light A wavelength tunable light source configured to be input to another MMI and output from one output waveguide of the other MMI is proposed (see, for example, Patent Document 3). According to the technique of Patent Document 3, it is possible to increase the output and improve the signal-to-noise ratio (SNR).

Japanese Patent No. 3888744 Japanese Patent No. 4728746 JP 2011-44581 A

  In a wavelength division multiplexing communication system having a transmission rate of 100 Gbps or higher in a trunk line system, digital coherent communication using optical phase modulation has recently been put into practical use. In a wavelength division multiplex communication system employing digital coherent communication, a narrow linewidth tunable light source having a laser oscillation linewidth of 500 kHz or less is used as a light source for transmission and reception.

  In wavelength division multiplexing communication employing the above-described digital coherent communication, if the wavelengths of optical signals used for transmission and reception are different, separate wavelength variable light sources are required for transmission and reception. However, in such a configuration, the power consumption of the entire transmission / reception apparatus including the wavelength variable light source increases or the mounting area increases.

  On the other hand, in wavelength division multiplex communication employing digital coherent communication, the wavelengths of optical signals used for transmission and reception may be the same. In such a case, it is desirable to use one tunable light source as a light source for transmission and reception from the viewpoint of reducing power consumption of the entire transmission and reception device and reducing the mounting area.

  Here, for example, a configuration is assumed in which the variable wavelength light source disclosed in Patent Documents 1 to 3 is also used as a light source for transmission and reception. Since the wavelength tunable light sources of Patent Documents 1 to 3 have one output, it is necessary to branch the one output into two outputs by the polarization maintaining coupler, but branching loss occurs in the polarization maintaining coupler. In order to compensate for this branching loss, the output light of the wavelength tunable light source is increased by increasing the injection current to a semiconductor optical amplifier (Semiconductor Optical Amplifier, hereinafter referred to as “SOA”) provided on the output side of the wavelength tunable light source. Need to be increased. However, supplementing the branching loss of the polarization maintaining coupler with the SOA causes an increase in power consumption and laser oscillation line width.

  Further, Patent Document 2 shows an N × 2-MMI configuration, and states that the output branches equally to two output ports at a specific wavelength. However, there has been a problem in that input light from the entire wavelength range (30 nm or more) and from any input port cannot be output sufficiently equally to the two output ports.

  Therefore, the present invention has been made in view of the above-described problems, and an object thereof is to provide a technique capable of sufficiently outputting light to a plurality of output waveguides.

  An optical coupler according to the present invention includes a multimode waveguide, a plurality of input waveguides connected to an input end face that is one end face of the multimode waveguide, and an output end face that is the other end face of the multimode waveguide. And a plurality of output waveguides connected to each other. The output end surface is a flat surface, and a length between an end near the end in the width direction of the input end surface and the output end surface is a center near the center in the width direction of the input end surface and the output A length between the central portion of the input end surface and the output end surface is constant over the central portion, and the length between the end portion of the input end surface and the central portion is longer than the length between the end surface and the central portion. The length between the intermediate portion and the output end surface is shorter than the length between the center portion of the input end surface and the output end surface.

  According to the present invention, the output end surface is a flat surface, and the length between the end portion of the input end surface and the output end surface is longer than the length between the center portion of the input end surface and the output end surface. The length between the central portion and the output end surface is constant over the central portion, and the length between the intermediate portion between the end portion and the central portion of the input end surface and the output end surface is the center of the input end surface. Shorter than the length between the part and the output end face. Thereby, light can be output to the plurality of output waveguides sufficiently evenly.

3 is a diagram illustrating an example of a configuration of a wavelength tunable light source according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. 6 is a diagram for explaining the effect of a comparative example of the optical coupler according to Embodiment 1. FIG. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. FIG. 6 is a diagram for explaining the effect of the optical coupler according to the first embodiment. 2 is a diagram illustrating an example of a configuration of a wavelength tunable light source module according to Embodiment 1. FIG. 2 is a diagram illustrating an example of a configuration of a transmission / reception device according to Embodiment 1. FIG. 6 is a diagram illustrating an example of a configuration of an optical coupler according to Embodiment 2. FIG. It is a figure for demonstrating the effect of the optical coupler which concerns on Embodiment 2. FIG. 6 is a diagram illustrating an example of a configuration of an optical coupler according to Embodiment 3. FIG. 6 is a diagram illustrating an example of a configuration of a wavelength tunable light source according to Embodiment 4. FIG. It is a figure which shows an example of a structure of a related wavelength variable light source. It is sectional drawing which shows an example of a structure of a related wavelength variable light source. It is sectional drawing which shows an example of a structure of a related wavelength variable light source. It is a figure which shows an example of a structure of a transmission / reception apparatus. It is a figure which shows an example of a structure of a wavelength variable light source module.

<Related technologies>
Before describing the wavelength tunable light source according to the embodiment of the present invention, a wavelength tunable light source related thereto (hereinafter referred to as “related wavelength tunable light source”) will be described.

  FIG. 20 is a diagram illustrating an example of a configuration of a related wavelength variable light source. The related variable wavelength light source 15 of FIG. 20 is a variable wavelength light source using a plurality of single mode LDs having different oscillation wavelengths, and includes the DFB-LD array 1, the optical coupler 2, and the SOA 6.

  The optical coupler 2 is connected to a multimode waveguide 3 that is a multimode region, N MMI input waveguides 4 connected to the input end face of the multimode waveguide 3, and an output end face of the multimode waveguide 3. The single MMI output waveguide 5 is provided. The optical coupler 2 configured in this way is N × 1-MMI.

  The DFB-LD array 1 is composed of N (N is a natural number of 3 or more) DFB-LDs 1a having different oscillation wavelengths, and is connected to the MMI input waveguide 4.

  The SOA 6 is connected to the MMI output waveguide 5.

  Here, when any one DFB-LD 1a in the DFB-LD array 1 is caused to oscillate, 1 / N of the light output from one DFB-LD 1a (hereinafter referred to as "LD output light"). Are coupled at the MMI output waveguide 5 and the remaining (N−1) / N is radiated out of the MMI output waveguide 5. Compensation for branch loss, coupling loss, and the like is performed by injecting current into the SOA 6. Specifically, the current output to the SOA 6 increases (amplifies) the light output from the MMI output waveguide 5. The light increased (amplified) by the SOA 6 is output to the outside as output light 7.

  FIG. 21 is a cross-sectional view showing an example of the A-A ′ cross section of FIG. 20 and shows an example of the configuration of the MMI input waveguide 4.

  The MMI input waveguide 4 includes a first InP lower cladding layer 41, a second InP current blocking layer 44 and an InGaAsP waveguide layer 42, and a third InP upper cladding layer 43 on an InP substrate 40. Are stacked in order. The InGaAsP waveguide layer 42 is made of an InGaAsP-based material that does not substantially absorb LD output light. The InGaAsP waveguide layer 42 may be a bulk epitaxial layer or a multiple quantum well (MQW) layer.

  21 shows the configuration of the MMI input waveguide 4, the configuration of the MMI output waveguide 5 is the same as the configuration of FIG. Further, in the configuration of the multimode waveguide 3 which is a wide multimode region, the width of the InGaAsP waveguide layer 42 (width in the horizontal direction in FIG. 21) is wider than that of the MMI input waveguide 4 and the MMI output waveguide 5. Except for this, the configuration is the same as that of the MMI input waveguide 4 and the MMI output waveguide 5.

  FIG. 22 is a cross-sectional view showing an example of the B-B ′ cross section of FIG. 20, and shows an example of the configuration of the SOA 6.

  The SOA 6 is formed on the InP substrate 40, the first InP lower cladding layer 41, the second InP current blocking layer 44 and the InGaAsP active layer 45, the third InP upper cladding layer 43, and the fourth layer. The InGaAsP contact layer 46 is sequentially laminated. For the InGaAsP active layer 45, an InGaAsP-based material having a gain with respect to light passing through the MMI output waveguide 5 (hereinafter referred to as “guided light”) is used. The InGaAsP active layer 45 may be a bulk epitaxial layer or a multiple quantum well (MQW) layer.

  FIG. 22 shows the configuration of the SOA 6, but the configuration of the DFB-LD 1a is the same as the configuration of FIG. In the DFB-LD 1a and the SOA 6, when current is injected into the InP substrate 40 and the InGaAsP contact layer 46 through electrodes (not shown) provided thereon, a gain is generated in the InGaAsP active layer 45, and spontaneous emission light is generated. Occur. In the DFB-LD 1a, spontaneous emission light having a specific wavelength reflected by the diffraction grating becomes stimulated emission seed light, and laser oscillation occurs when a threshold current determined by the balance between gain and loss is exceeded. On the other hand, the SOA 6 has a function of an amplifier that amplifies the guided light, and is designed not to oscillate alone.

  Referring back to FIG. 20, the oscillation wavelength of the DFB-LD 1a changes at a rate of about 0.1 nm / ° C. according to the temperature of the DFB-LD 1a (hereinafter referred to as “element temperature”). Using this property, when the element temperature is changed within a certain range (for example, 10 to 50 ° C.), the DFB-LD array 1 (for example, the number of MMI input waveguides 4 N = 10 to 16) The interval of the oscillation wavelength of each DFB-LD 1a is designed so that the oscillation wavelength of any DFB-LD 1a matches the oscillation wavelength of another adjacent DFB-LD 1a. According to such a design, by combining the selection of the DFB-LD 1a and the device temperature adjustment, the entire wavelength band (about 30 to 40 nm) of the C band or the L band can be obtained with the DFB-LD array 1 formed by one chip. Can be covered.

  FIG. 23 is a diagram illustrating an example of a configuration of the transmission / reception device 8, and illustrates a configuration of the transmission / reception device 8 for a digital coherent communication system. Although not shown in FIG. 23, each of the two wavelength variable light source modules 9 is provided with the related wavelength variable light source 15 of FIG.

  The output 7a for transmission output from the wavelength tunable light source module 9 (the output light 7 output from the related wavelength tunable light source 15 in FIG. 20) is modulated by the modulator module 10 and then output to the outside as the transmission signal 11. Is done.

  The reception signal 12 received from the outside is input to the reception module 14 together with the reception output 7b output from the wavelength tunable light source module 9 (output light 7 output from the related wavelength tunable light source 15 in FIG. 20). Restored after processing. In general, an insertion loss occurs in the modulator module 10 in the transmission / reception apparatus 8, so that the transmission output 7 a requires a high output, but the reception output 7 b may be a relatively low output.

  Here, when the wavelengths of the optical signals used for transmission and reception may be the same, one wavelength variable light source module from the viewpoint of reducing the power consumption and the mounting area of the entire transmission / reception device 8 It is desirable to use 9 as a light source for transmission and reception.

  FIG. 24 is a diagram illustrating an example of the configuration of the variable wavelength light source module 9 that can also be used as a light source for transmission and reception.

  The wavelength tunable light source module 9 includes the related wavelength tunable light source 15 of FIG. 20, a coupling optical system 16, a beam splitter 17, and a monitor 18. Further, an optical fiber 19 and a polarization maintaining coupler 20 are sequentially connected to a portion where the wavelength variable light source module 9 outputs light.

  The related wavelength tunable light source 15 emits single mode output light 7 (FIG. 20), and the emitted output light 7 is optical fiber 19 through a coupling optical system 16 including a lens, an optical isolator (not shown), and the like. To join. The output light 7 coupled to the optical fiber 19 is branched at a predetermined ratio by the polarization maintaining coupler 20, and one of the branched output lights 7 is output as a transmission output 7a, and the other is output as a reception output 7b. The

  Further, a part of the output light 7 passing through the coupling optical system 16 is extracted by the beam splitter 17, and the wavelength and output level of the LD output light from the monitor 18 including a wavelength filter, a photodiode and the like (not shown). Used for detection.

  Since the number of outputs (MMI output waveguides 5) of the related variable wavelength light source 15 is one, in order to use the variable wavelength light source module 9 as a light source for transmission and reception, one output is output by the polarization maintaining coupler 20. Although it is necessary to branch to two outputs, a branching loss occurs in the polarization maintaining coupler 20. In order to compensate for this branching loss, it is necessary to increase the LD output light (output) of the related wavelength variable light source 15 by increasing the injection current to the SOA 6 of FIG. 20 provided on the output side of the related wavelength variable light source 15. There is. However, there is a problem that compensating for the branching loss of the polarization maintaining coupler 20 with the SOA 6 causes an increase in power consumption and laser oscillation line width. On the other hand, according to the wavelength tunable light source according to the embodiment of the present invention described below, such a problem can be solved.

<Embodiment 1>
FIG. 1 is a diagram showing an example of the configuration of a wavelength tunable light source according to Embodiment 1 of the present invention. Hereinafter, in the wavelength tunable light source 28 and the like according to the first embodiment, components that are the same as or similar to the related wavelength tunable light source 15 are denoted by the same reference numerals, and different components are mainly described.

  In the following description, a line passing through the center of the input end face 22a and the center of the output end face 22b of the multimode waveguide 22 (light travel direction axis) is defined as the centerline CL, and the centerline CL of the multimode waveguide 22 A parallel direction is defined as an extending direction, and a direction perpendicular to the center line CL of the multimode waveguide 22 is defined as a width direction.

  In FIG. 1, the length in the width direction of the multimode waveguide 22 is longer than the length in the extending direction of the multimode waveguide 22. However, this is only for convenience of explanation, and in general, the length of the multimode waveguide 22 in the extending direction is longer than the length of the multimode waveguide 22 in the width direction. It is assumed that

  1 includes a DFB-LD array 1 shown in FIG. 20, an optical coupler 21, a first SOA (first optical amplifier) 23 similar to the SOA 6 shown in FIG. A second SOA (second optical amplifier) 24 similar to the SOA 6 shown in FIG. 20 is provided.

  1 includes a multimode waveguide 22 that is a multimode region, and N (N is a natural number of 3 or more) MMIs connected to an input end face 22a (one end face) of the multimode waveguide 22. The input waveguide 4 and M (M is a natural number of 2 or more) MMI output waveguides 5 connected to the output end face 22 b (the other end face) of the multimode waveguide 22 are provided. The optical coupler 21 configured in this manner is N × M-MMI. In the following, the optical coupler 21 will be described as N × M-MMI where N = 16 and M = 2, that is, 16 × 2-MMI. However, the values of N and M are not limited to this. Absent.

  Here, before describing the components of the optical coupler 21 in detail, the DFB-LD array 1, the first SOA 23, and the second SOA 24 will be described in detail.

  The DFB-LD array 1 includes the same number of DFB-LDs 1 a as the MMI input waveguides 4 and is connected to the MMI input waveguides 4.

  The first SOA 23 is connected to one MMI output waveguide 5, and the second SOA 24 is connected to the other MMI output waveguide 5. A current injection mechanism (not shown) is independently connected to each of the first SOA 23 and the second SOA 24. For example, the first SOA 23 and the second SOA 24 are formed by removing a predetermined portion of the MMI output waveguide 5 by etching, and then performing a regrowth technique called butt joint growth on the cross section of the MMI output waveguide 5 and the second SOA 24. The first SOA 23 and the second SOA 24 are formed so as to be directly bonded to each other.

  Next, components of the optical coupler 21 will be described in detail.

  Each MMI input waveguide 4 is a single mode waveguide. The 16 MMI input waveguides 4 are disposed between the end 22a1 and the end 22a2 of the input end face 22a at an input waveguide interval (Din). The 16 MMI input waveguides 4 are arranged along the width direction of the multimode waveguide 22 with an input waveguide interval (Din). The 16 MMI input waveguides 4 are arranged symmetrically with respect to the center line CL.

  The two MMI output waveguides 5 are disposed along the width direction of the multimode waveguide 22 at the output waveguide interval (Dout). The two MMI output waveguides 5 are arranged symmetrically with respect to the center line CL.

  The output end face 22b of the multimode waveguide 22 is a flat surface, and the output waveguide side boundary position is the same regardless of the position in the width direction. In the first embodiment, a non-reflective coating (not shown) is applied to the output end face 22b in order to suppress the generation of reflected return light at the output end. By suppressing the generation of reflected return light, an increase in the laser oscillation line width can be suppressed.

  The input end face 22a of the multimode waveguide 22 is provided with irregularities, and the input waveguide side boundary position is substantially different depending on the position in the width direction. Specifically, the length between the end portions 22a1, 22a2 near the ends in the width direction of the input end face 22a and the output end face 22b is equal to the center portion near the center in the width direction of the input end face 22a and the output end face 22b. It is longer than the length between. The length between the center portion of the input end face 22a and the output end face 22b is constant over the center portion. The length between the intermediate portion between the end portions 22a1, 22a2 and the central portion of the input end surface 22a and the output end surface 22b is shorter than the length between the central portion of the input end surface 22a and the output end surface 22b. Yes. The multimode waveguide 22 has a shape that is line-symmetric with respect to the center line CL.

  With this configuration, the multimode region length (the length between the portion of the input end face 22 a connected to the MMI input waveguide 4 and the output end face 22 b) L varies depending on the position of the MMI input waveguide 4. Yes. Here, as shown in FIG. 1, the multimode region lengths of the 16 MMI input waveguides 4 are represented by symbols L1, L2,..., L16 from top to bottom.

  In the configuration of FIG. 1, the multimode region length (L1, L16) of the MMI input waveguide 4 at the array end is the longest. The multimode region length (L6 to L11) of the MMI input waveguide 4 at the center of the array is constant and is shorter than the multimode region lengths (L1, L16) of the MMI input waveguide 4 at the end of the array. The other multimode region lengths (L2 to L5, L12 to L15) of the MMI input waveguide 4 are shorter than the multimode region lengths (L6 to L11) of the MMI input waveguide 4 at the center of the array. In particular, the multimode region length (L3, L4, L13, L14) of the MMI input waveguide 4 is shorter than the other multimode region lengths (L1, L2, L5 to L12, L15, L16).

  Next, the operation of the wavelength tunable light source 28 in FIG. 1 will be described.

  When an arbitrary DFB-LD 1a is selected and current injection equal to or greater than the threshold current is performed, laser oscillation occurs in the selected DFB-LD 1a. The LD output light output from the DFB-LD 1 a is input to the multimode waveguide 22 of the optical coupler 21 via the MMI input waveguide 4 and then coupled in each of the two MMI output waveguides 5. . The guided lights combined in the two MMI output waveguides 5 are increased (amplified) by the first SOA 23 and the second SOA 24, respectively, and then output as two output lights 7 from the wavelength variable light source.

  Here, when the LD output light is input to the multimode waveguide 22 through the MMI input waveguide 4, an interference pattern resulting from the superposition of a plurality of eigenmodes is shown, but the multimode region lengths L1 to L16 are appropriately set. If designed, light can be coupled at an approximately equal ratio to the two MMI output waveguides 5 which are single mode waveguides.

  Since the light is coupled to the two MMI output waveguides 5 at an approximately equal ratio, the optimum multimode region length varies depending on the position of the MMI input waveguide 4. FIG. 1 shows an appropriate multimode region length arranged by linear approximation when the input waveguide interval (Din) is 3 μm and the width W of the multimode waveguide 22 is 48 μm in the C band or the L band. L1 to L16 are shown. According to such a configuration, as shown in FIG. 1, the difference between the concave and convex portions of the input end face 22a of the multimode waveguide 22 can be made within 10 μm.

  Next, the multi-mode region lengths L1 to L16 of the optical coupler 21 configured as shown in FIG. 1 are appropriate, that is, light is coupled to the two MMI output waveguides 5 at an approximately equal ratio. explain.

  2 to 7 are diagrams showing simulation results of the branching characteristics of the optical coupler (hereinafter referred to as “comparative optical coupler”) compared with the optical coupler 21 according to the first embodiment. As the comparative optical coupler, an optical coupler having an input end face 22a changed to a flat surface was used.

  2 to 7 correspond to the two MMI output waveguides 5, and the outputs of the multimode region lengths L1 to L16 in each MMI output waveguide 5 are shown. When the length of the multimode waveguide 22 (the length along the center line CL) is Le, FIG. 2 shows Le = 300 μm, FIG. 3 shows Le = 310 μm, and FIG. 4 shows Le = 320 μm. 5 is Le = 330 μm, FIG. 6 is Le = 340 μm, and FIG. 7 is Le = 350 μm.

  In the comparative optical coupler, at Le = 310 μm (FIG. 3), the outputs of the multimode region lengths L1 to L16 are substantially equal, and light is coupled to the two MMI output waveguides 5 at an approximately equal ratio. Other than Le (FIGS. 2 and 4 to 7), the outputs of the multimode region lengths L1 to L16 vary.

  8 to 11 are diagrams illustrating simulation results of the branching characteristics of the optical coupler 21 according to the first embodiment, as in FIG. 2 and the like. 8 is Le = 310 μm, FIG. 9 is Le = 315 μm, FIG. 10 is Le = 320 μm, and FIG. 11 is Le = 325 μm.

  In the first embodiment, in any Le, the outputs of the multimode region lengths L1 to L16 are approximately equal, and light is coupled to the two MMI output waveguides 5 at an approximately equal ratio.

  FIG. 12 is a diagram showing an actual measurement result of the branching characteristic in the optical coupler in which the input end face 22a is changed to a flat surface. FIG. 13 shows an actual measurement result of the branching characteristic in the optical coupler 21 according to the first embodiment. FIG. Here, the input waveguide interval (W / N) is 3 μm, and the SOA drive current is constant.

  The actual measurement results in FIG. 12 and FIG. 13 almost agree semi-quantitatively with the simulation results shown in FIG. As can be seen from the results shown in FIGS. 8 to 11 and 13, according to the optical coupler 21 according to the first embodiment, the multimode region is independent of the position (input port number) of the MMI input waveguide 4. The outputs of the lengths L1 to L16 can be made almost equal. That is, light can be coupled at an approximately equal ratio in the two MMI output waveguides 5 that are single mode waveguides. Further, according to the first embodiment, suppression of Le dependency can also be expected.

  Although the results are not shown, similar results can be obtained even if the shape of the optical coupler 21 in FIG. 1 is slightly changed. For example, in FIG. 1, the multimode region length of the MMI input waveguide 4 at the center is L6 to L11. However, the length is not limited to this, and may be L5 to L12, or L7 to L10. Also good.

  FIG. 14 is a diagram illustrating an example of the configuration of the variable wavelength light source module 30 according to the first embodiment. The wavelength variable light source module 30 includes the wavelength variable light source 28 shown in FIG. The configuration and operation of the wavelength tunable light source module 30 are the same as those of the wavelength tunable light source module 9 shown in FIG.

  The output light of the wavelength tunable light source 28 (the output light 7 from the first SOA 23 and the second SOA 24 in FIG. 1) is separated and imaged on the two optical fibers 19 by the coupling optical system 16, and the wavelength tunable light source Light is output from the two output ports (two optical fibers 19) of the module 30. That is, in the wavelength tunable light source module 30 according to the first embodiment, each of the plurality of output lights 7 output from the wavelength tunable light source 28 can be extracted separately. Light output from the two output ports (two optical fibers 19) is used as a transmission output 7a and a reception output 7b.

  Accordingly, the polarization maintaining coupler 20 provided on the output side of the wavelength tunable light source module 9 in FIG. 24 is not necessary in the wavelength tunable light source module 30 according to the first embodiment. Moreover, it is possible to couple the LD output light that is substantially equal to each other in the two MMI output waveguides 5 over the entire wavelength band and is substantially the same as the related wavelength variable light source 15 at a ratio of about 1 / N. I found out by the survey. This is because the driving current of the DFB-LD 1a according to the first embodiment may be approximately the same as that of the related wavelength tunable light source 15, and it is not necessary to increase the driving current as compared with the DFB-LD 1a of the related wavelength tunable light source 15. means.

  From the above, according to the configuration of the first embodiment, the power consumption and the laser oscillation line width are higher than those of the related wavelength variable light source 15 to the extent that the related wavelength variable light source 15 compensates the branching loss of the polarization maintaining coupler by the SOA 6. Can be suppressed.

  FIG. 15 is a diagram illustrating an example of a configuration of the transmission / reception device 32 according to the first embodiment. The transmission / reception apparatus 32 of FIG. 15 includes one wavelength variable light source module 30 of FIG. The transmission output 7 a output from the wavelength tunable light source module 30 is modulated by the modulator module 10 and then output to the outside as the transmission signal 11. The reception signal 12 received from the outside is input to the reception module 14 together with the reception output 7b output from the wavelength variable light source module 30, and restored after the signal processing.

  According to the first embodiment configured as described above, the wavelength tunable light source module 30 can also be used as a light source module for transmission and reception. For this reason, power consumption can be reduced compared with the transmission / reception apparatus 8 of FIG.

<Embodiment 2>
FIG. 16 is a diagram illustrating an example of the configuration of the optical coupler according to Embodiment 2 of the present invention. In the second embodiment, in the 16 × 2-MMI optical coupler 21 described in the first embodiment, the interval between the MMI output waveguides 5 is optimized. Since the operation of the optical coupler 21 according to the second embodiment is the same as that of the first embodiment (FIG. 1), description thereof is omitted here.

  Also in the second embodiment, as in the first embodiment, the width (length in the width direction) of the multimode waveguide 22 is W, and the length of the multimode waveguide 22 (the length along the center line CL). ) Will be described as Le. The length Le of the optical coupler 21 may be substantially the same as the length Le of the optical coupler 21 in FIGS. 8 to 11 (the optimum value of Le is 325 μm).

  The arrangement positions of the N MMI input waveguides 4 are centered on the center (center line CL) in the width direction of the multimode waveguide 22 of the optical coupler 21 (coordinate 0) (centered on W / 2 of the input end face 22a). ) And are arranged at intervals of W / N. Further, the two outer MMI input waveguides 4 are positioned inward by W / (2N) from the end of the multimode region of the optical coupler 21, that is, coordinates (W / 2−W / (2N)). , −W / 2 + W / (2N)). Since N = 16 in the second embodiment, the two outer MMI input waveguides 4 are arranged at coordinates (W / 2−W / 32, −W / 2 + W / 32).

  On the other hand, the two MMI output waveguides 5 are arranged symmetrically with respect to W / 2 of the output end face 22b and at an interval of 1.1 to 1.3 times W / N.

  FIG. 17 is a diagram illustrating the dependence of the minimum output value on the output waveguide interval in the wavelength tunable light source 28 according to the second embodiment. Here, the input waveguide interval (W / N) is 3 μm, and the SOA drive current is constant. As can be seen from FIG. 17, when the output waveguide interval (Dout) is 3.3 to 3.9 μm, the minimum value of the output can be increased, and in particular, it can be increased most in the vicinity of 3.4 μm. As described above, in the second embodiment, the two MMI output waveguides 5 are symmetrical with respect to W / 2 of the output end face 22b and are spaced 1.1 to 1.3 times W / N. Therefore, the output of the wavelength tunable light source 28 can be increased.

<Embodiment 3>
FIG. 18 is a diagram showing an example of the configuration of the optical coupler according to Embodiment 3 of the present invention. The optical coupler 21 according to Embodiment 3 includes the tapered waveguide 25 disposed between the multimode waveguide 22 and each of the MMI output waveguides 5. 2 is different from the optical coupler 21 according to the second example.

  Of the tapered waveguide 25, the width of the connection portion with the MMI output waveguide 5 is almost the same as the width of the MMI output waveguide 5, and the width of the connection portion with the multimode waveguide 22 is the same as that of the MMI output waveguide 5. It may be larger than the width and not more than the interval (Dout) of the MMI output waveguide 5. If the length of the taper waveguide 25 is about 20 to 50 μm, excess loss can be almost ignored. Since the configuration and operation of the optical coupler 21 according to the third embodiment other than the tapered waveguide 25 are the same as those in the first or second embodiment, the description thereof is omitted here.

  According to the third embodiment configured as described above, since the light near the MMI output waveguide 5 can be coupled by the tapered waveguide 25, the coupling efficiency to the MMI output waveguide 5 is improved. be able to.

<Embodiment 4>
FIG. 19 is a diagram showing an example of the configuration of the wavelength tunable light source 28 according to Embodiment 4 of the present invention.

  In the fourth embodiment, the first SOA 23 and the second SOA 24 are different from the configurations described so far. The optical coupler 21 and the DFB-array 1 according to the fourth embodiment are the same as those described so far.

  That is, the optical coupler 21 is connected to the multimode waveguide 22, the 16 MMI input waveguides 4 connected to the input end face 22 a of the multimode waveguide 22, and the output end face 22 b of the multimode waveguide 22. And two MMI output waveguides 5. Then, the optical coupler 21 combines the LD output light input from each MMI input waveguide 4 and outputs the combined LD output light to each MMI output waveguide 5.

  According to the optical coupler 21 according to the first to third embodiments, as described above, since the multimode region length is appropriate, the two MMI output waveguides 5 are substantially equal over the entire wavelength band. LD output light can be coupled.

  The 16 DFB-LDs 1a included in the DFB-array 1 are connected to the opposite ends of the multimode waveguides 22 of the respective MMI input waveguides 4 and can each perform single mode oscillation at different wavelengths.

  The waveguide-type first SOA 23 and the second SOA 24 are respectively connected to the output end sides of the two MMI output waveguides 5. The first SOA 23 and the second SOA 24 have curved waveguides 23a and 24a on their output end sides. Except for the length of the first SOA 23 and the second SOA 24 (the length in the direction in which the LD output light passes), the first SOA 23 and the second SOA 24 are configured identically.

  Here, when current injection is performed on the first SOA 23 and the second SOA 24, the LD output light traveling through each MMI output waveguide 5 is amplified, and when the current value is increased, the first SOA 23 and the second SOA 24 are increased. The amplification factor in the SOA 24 also increases. The amplification factor is expressed by logarithmically expressing the output light intensity based on the input light, but generally exhibits a non-linear behavior. Specifically, the gain is substantially constant when the input light is weak (low output), but the gain decreases when the input light is strong (high output). For this reason, the maximum output tends to be saturated with the input light intensity.

  The saturation output per unit length of the first SOA 23 and the second SOA 24 depends on the optical confinement coefficient and the current density of the active layers (corresponding to the InGaAsP active layer 45 in FIG. 22) of the first SOA 23 and the second SOA 24. Determined. In other words, if the optical confinement coefficients and current densities of the active layers of the first SOA 23 and the second SOA 24 are the same, the maximum outputs of the first SOA 23 and the second SOA 24 are the first SOA 23 and the second SOA 24, respectively. This is determined by the length of the SOA 24.

  In the configuration of FIG. 19, the length of the first SOA 23 is longer than the length of the second SOA 24, and the widths and optical confinement factors of these active layers are the same. Therefore, the maximum output at the same current density is higher in the first SOA 23 than in the second SOA 24. That is, the amplification factor of the first SOA 23 is higher than the amplification factor of the second SOA 24.

  As described above, in the transmission / reception apparatus for the digital coherent communication system, the reception output 7b may be lower than the transmission output 7a. In view of this, in the fourth embodiment, light amplified by the first SOA 23 having a high amplification factor is used as the transmission output 7a, and light amplified by the second SOA 24 having a low amplification factor is used for reception. It is configured to be used as the output 7b.

  According to such a configuration, if the length of the first SOA 23 is the same as the length of the SOA 6 in FIG. 20, the driving current of the DFB-LD 1 a necessary for obtaining the transmission output 7 a equivalent to the SOA 6 can be obtained. The injection current for the first SOA 23 is the same as that of the related variable wavelength light source 15. On the other hand, the driving current of the DFB-LD 1a and the injection current for the second SOA 24 necessary for obtaining the receiving output 7b equivalent to the SOA 6 are smaller than those of the related wavelength variable light source 15. Therefore, when one wavelength tunable light source 28 according to the fourth embodiment shown in FIG. 19 is used in the transmission / reception device 32 (FIG. 15), the transmission / reception device 8 in FIG. As compared with the above, power consumption can be reduced to, for example, 1/2 or less. As described above, according to the fourth embodiment, it is possible to further suppress an increase in the laser oscillation line width and the power consumption.

  It should be noted that the present invention can be freely combined with each other within the scope of the invention, and each embodiment can be appropriately modified or omitted.

  4 MMI input waveguide, 5 MMI output waveguide, 21 optical coupler, 22 multimode waveguide, 22a input end face, 22a1, 22a2 end, 22b output end face, 23 first SOA, 24 second SOA, 25 Tapered waveguide, 28 wavelength variable light source, 30 wavelength variable light source module, CL center line.

Claims (5)

A multimode waveguide;
A plurality of input waveguides connected to an input end face which is one end face of the multimode waveguide;
A plurality of output waveguides connected to an output end face which is the other end face of the multimode waveguide;
The output end surface is a flat surface;
The length between the end in the width direction of the input end face and the output end face is longer than the length between the center in the width direction of the input end face and the output end face. ,
The length between the central portion of the input end surface and the output end surface is constant over the central portion,
The length between the output end surface and the intermediate portion between the end portion and the central portion of the input end surface is shorter than the length between the central portion of the input end surface and the output end surface. vessel. The optical coupler according to claim 1,
The plurality of input waveguides are:
Arranged symmetrically with respect to a center line passing through the center of the input end face and the center of the output end face, and arranged at an interval of W / N;
The plurality of output waveguides are:
Arranged symmetrically with respect to the center line, and arranged at an interval of 1.1 times to 1.3 times W / N,
here,
W is the width of the multimode waveguide with respect to the direction of the center line,
N is the number of the plurality of input waveguides, and is an optical coupler that is a natural number of 3 or more. The optical coupler according to claim 1 or 2, wherein
An optical coupler further comprising a tapered waveguide disposed between the multimode waveguide and each of the output waveguides. The optical coupler according to any one of claims 1 to 3,
A wavelength tunable light source comprising: first and second optical amplifiers connected to two of the plurality of output waveguides and having different amplification factors. A wavelength tunable light source according to claim 4,
A wavelength tunable light source module capable of separately taking out each of a plurality of output lights output from the wavelength tunable light source.

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