Optical Multiplexing Circuit and Light Source

Abstract

To provide an optical multiplexing circuit that can accurately monitor light of a plurality of wavelengths, and that can tolerate degradation of LDs. An optical multiplexing circuit includes m sets of multiplexers configured to multiplex light output from n connection waveguides being a plurality of connection waveguides wherein a multiplexing unit configured to input and multiplex light output from the m sets of the multiplexers from m input waveguides, an output waveguide configured to output light multiplexed by the multiplexing unit, and n×m or m branching units being inserted into n×m connection waveguides of the plurality of connection waveguides or the m input waveguides are provided on a same substrate.

Claims

1. An optical multiplexing circuit comprising: m sets of multiplexers configured to multiplex light output from n connection waveguides being a plurality of connection waveguides wherein a multiplexing unit configured to input and multiplex light output from the m sets of the multiplexers from m input waveguides, an output waveguide configured to output light multiplexed by the multiplexing unit, and n×m or m branching units being inserted into n×m connection waveguides of the plurality of connection waveguides or the m input waveguides are provided on a same substrate.

2. A light source with a monitoring function, comprising: the optical multiplexing circuit according to claim 1; n×m laser diodes each optically coupled to a corresponding one of the connection waveguides; and a plurality of photodiodes each optically coupled to a corresponding one of a plurality of monitoring waveguides each being configured to output light divided by the branching units, wherein one laser diode of n laser diodes is selected and operated for each m set.

3. The light source with a monitoring function according to claim 2, wherein a light emission direction from the n×m laser diodes is configured to be generally perpendicular to an incident direction of light at the plurality of photodiodes.

4. The light source with a monitoring function according to claim 2, wherein the n×m laser diodes are three sets (m=3) of laser diodes that output light of three primary colors of red light (R), green light (G), and blue light (B).

5. The light source with a monitoring function according to claim 3, wherein the n×m laser diodes are three sets (m=3) of laser diodes that output light of three primary colors of red light (R), green light (G), and blue light (B).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0021] FIG. 1 is a diagram illustrating a typical light source of a projector using LDs.

[0022] FIG. 2 is a diagram illustrating a basic structure of an RGB coupler using a PLC.

[0023] FIG. 3 is a diagram illustrating a configuration of an RGB coupler using two directional couplers.

[0024] FIG. 4 is a diagram illustrating a light source with a monitoring function according to a first embodiment of the present invention.

[0025] FIG. 5 is a diagram illustrating a light source with a monitoring function according to a first example of a second embodiment of the present invention.

[0026] FIG. 6 is a diagram illustrating a light source with a monitoring function according to a second example of the second embodiment of the present invention.

[0027] FIG. 7 is a diagram illustrating a light source with a monitoring function according to a third example of the second embodiment of the present invention.

[0028] FIG. 8 is a diagram illustrating a light source with a monitoring function according to a fourth example of the second embodiment of the present invention.

[0029] FIG. 9 is a diagram illustrating an example of a multiplexer according to the fourth example of the second embodiment.

DESCRIPTION OF EMBODIMENTS

[0030] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the present embodiment, description is given for the case of a method using a directional coupler as a multiplexer, but the present invention is not limited to a multiplexing method. An RGB coupler that multiplexes wavelengths of three primary colors of light is described as an example, but it goes without saying that the present invention can be applied to optical multiplexing circuits that multiplex a plurality of other wavelengths.

First Embodiment

[0031] FIG. 4 illustrates a light source with a monitoring function according to a first example of a first embodiment of the present invention. A light source with a monitoring function includes first to third LDs 201.sub.1 to 201.sub.3 that respectively output light of respective colors of R, G, and B, a PLC-type RGB coupler 210, and first to third PDs 202.sub.1 to 202.sub.3 optically connected to the RGB coupler 210.

[0032] The PLC-type RGB coupler 210 includes first to third input waveguides 211.sub.1 to 211.sub.3 optically connected to the first to third LDs 201.sub.1 to 201.sub.3, first to third branching units 212.sub.1 to 212.sub.3 that divide light propagating through the waveguide into two, a multiplexing unit 214 that multiplexes one beam of the light divided by each of the first to third branching units 212.sub.1 to 212.sub.3, first to third monitoring waveguides 213.sub.1 to 213.sub.3 that output the other beam of the light divided by each of the first to third branching units 212.sub.1 to 212.sub.3 to the first to third PDs 202.sub.1 to 202.sub.3, and an output waveguide 215 that outputs the light multiplexed by the multiplexing unit 214.

[0033] In the PLC-type RGB coupler 210, light incident on each of the first to third input waveguides 211.sub.1 to 211.sub.3 is divided into two by each of the first to third branching units 212.sub.1 to 212.sub.3. One beam of the divided light is output to the first to third PDs 202.sub.1 to 202.sub.3 via the first to third monitoring waveguides 213.sub.1 to 213.sub.3, and the other beam of the divided light is multiplexed by the multiplexing unit 214 and output to the output waveguide 215.

[0034] An optical multiplexing circuit using the directional coupler illustrated in FIG. 3 can be used as the multiplexing unit 214. In this case, the first to third input waveguides 211.sub.1 to 211.sub.3 are coupled to the first to third input waveguides 101 to 103 illustrated in FIG. 3, respectively, and the output waveguide 215 is coupled to the output waveguide 106 illustrated in FIG. 3. However, the multiplexing unit 214 is not limited thereto, and another multiplexing unit of a waveguide type (for example, a Mach-Zehnder interferometer, a mode coupler, or the like) may be used.

[0035] As illustrated in FIG. 4, when light propagating through the first to third input waveguides 211.sub.1 to 211.sub.3 is divided by the first to third branching units 212.sub.1 to 212.sub.3, respectively, a coupling characteristic between the first to third LDs 201.sub.1 to 201.sub.3 and the first to third input waveguides 211.sub.1 to 211.sub.3 may be monitored. In addition, it is possible to adjust white balance as a light source by using a monitoring value of the first to third PDs 202.sub.1 to 202.sub.3 by recognizing a multiplexing characteristic of the multiplexing unit 214 in advance.

Second Embodiment

[0036] As described above, the degradation of the emission end surfaces of LDs depends on the wavelengths, and the degree of degradation of LDs on the short wavelength side (B) is greater than that of LDs on the long wavelength side (R). According to the first example of the first embodiment, the first to third PDs 202.sub.1 to 202.sub.3 can respectively monitor light of the respective colors of R, G, and B. Thus, even if the degrees of degradation of LDs of respective colors are different from each other, a white balance can be adjusted with high accuracy since feedback control can be performed individually.

[0037] However, in a case where the degree of degradation of LDs is out of a range for feedback control, accurate white balance adjustment cannot be made. Thus, in the second embodiment, a plurality of LDs of the same color is prepared, and a degraded LD is switched to a backup LD. In this way, a configuration is employed in which individual accurate monitoring is possible even in actual operation of a light source. The present invention is not limited to an optical multiplexing circuit using an RGB coupler, and may be an optical multiplexing circuit that includes n LDs for each m set of wavelengths, i.e., a plurality of wavelengths, and can select one LD of the n LDs for each m set for multiplexing, and a light source with monitoring function.

First Example

[0038] FIG. 5 illustrates a light source with a monitoring function according to a first example of the second embodiment of the present invention. In the light source according to the first example, an RGB coupler 310 having the same configuration as the RGB coupler 210 of the first embodiment and a plurality of LD fixed to an LD mount 303 are optically coupled. On the LD mount 303, two (n=2) LDs for each color (m=3), i.e., LDs 301.sub.11 to 301.sub.31, being LDs for each color in an active system and LDs 301.sub.12 to 301.sub.32 being LDs for each color in a standby system. By changing the fixed position of the LD mount 303 relative to the RGB coupler 310, the LDs of the active system can be switched to the LDs of the standby system.

[0039] The first example illustrates an example in which LDs of respective colors are mounted on one substrate, and the three primary color LDs are simultaneously switched from the active system to the standby system. LDs may be mounted on different LD mounts for respective colors to switch the LDs of the active system to the LDs of the standby system for each color. As described above, the shorter the wavelength the severe the degradation of LDs, so only the LDs of the blue light B on the short wavelength side or only the LDs of the blue light B and the green light G may include LDs of the active system and the standby system.

[0040] In the first example, the LDs 301 of each color are fixed to the LD mount 303 and mechanically switched. Mechanical switching may increase a coupling loss between the LDs 301 and input waveguides 311. Thus, switching can be performed while active alignment is performed for each LD. Meanwhile, the work step increases in the active alignment, and thus the LDs 301 may be arranged so that a beam diameter is larger than a core diameter at the end surfaces of the input waveguides 311. Although the coupling loss before switching also increases, sensitivity due to misalignment associated with the switching of the LDs 301 is reduced, so there is less variation in the coupling loss due to mechanical switching.

[0041] Furthermore, although one standby system is implemented with respect to the active system (n=2) in the above description, a plurality of standby systems may be implemented (n>2). Of course, it is possible to configure not only an optical multiplexing circuit for three colors, three wavelengths, but also an optical multiplexing circuit for a plurality of wavelengths (m>2).

[0042] With such a configuration, a degraded LD can be easily switched to an unused LD. Thus, even when used in actual operation without sealing the LDs and the RGB coupler, it is possible to extend the life expectancy by the number of LDs mounted. Compared to the first example, the end surface of the RGB coupler facing the LD mount needs to be larger for alignment with the LD mount, but there is no change to the process of producing the RGB coupler, and there is no increase in manufacturing cost.

Second Example

[0043] FIG. 6 illustrates a light source with a monitoring function according to a second example of the second embodiment of the present invention. The light source with the monitoring function includes a plurality of LDs that output light of respective colors R, G, and B, a PLC-type RGB coupler 310, and first to third PDs 302.sub.1 to 302.sub.3 optically connected to the RGB coupler 310. The plurality of LDs include LDs 301.sub.11 to 301.sub.31, which are LDs for respective colors of the active system, and LDs 301.sub.12 to 301.sub.32, which are LDs for respective colors of the standby system (n=2, m=3). The RGB coupler 310 differs from the RGB coupler 310 of the first example in that multiplexers 317.sub.1 to 317.sub.3 and connection waveguides 316.sub.1 to 316.sub.3 for connecting the multiplexers and LDs are added to the first to third input waveguides 311.sub.1 to 311.sub.3, respectively.

[0044] The outputs of the LDs 301.sub.11 to 301.sub.31, which are LDs of respective colors of the active system, and the outputs of the LDs 301.sub.12 to 301.sub.32, which are LDs of respective colors of the standby system, are connected to a multiplexing unit 314 via the multiplexers 317.sub.1 to 317.sub.3 and the first to third branching units 312.sub.1 to 312.sub.3, respectively. One beam of the divided light is output to the first to third PDs 302.sub.1 to 302.sub.3 via the first to third monitoring waveguides 313.sub.1 to 313.sub.3, and the other beam of the divided light is multiplexed by the multiplexing unit 314 and output to the output waveguide 315.

[0045] In the second example, switching between the active system and the standby system is performed by electrically switching drive circuits of the LDs. Thus, there is no need to align between the LDs and the RGB coupler when switching between the active system and the standby system.

[0046] With such a configuration, a degraded LD can be easily switched to an unused LD. Thus, even when used in actual operation without sealing the LDs and the RGB coupler, it is possible to extend the life expectancy of LDs. Compared to the first example, the circuit size of the RGB coupler is larger, but because optical circuits are fabricated on the same wafer or chip, there is no increase in manufacturing cost and no additional components are needed because it can be made simultaneously in a single process.

Third Example

[0047] FIG. 7 illustrates a light source with a monitoring function according to a third example of the second embodiment of the present invention. The configuration of the light source with the monitoring function is the same as the configuration in the second example, except for the circuit configuration of the RGB coupler 320 being different from the second example. The light source with the monitoring function includes a plurality of LDs that output light of respective colors R, G, and B, a PLC-type RGB coupler 320, and first to third PDs 302.sub.1 to 302.sub.3 optically connected to the RGB coupler 320. The plurality of LDs include LDs 301.sub.11 to 301.sub.31, which are LDs for respective colors of the active system, and LDs 301.sub.12 to 301.sub.32, which are LDs for respective colors of the standby system (n=2, m=3).

[0048] The PLC-type RGB coupler 320 includes connection waveguides 316.sub.11 to 316.sub.32 optically connected to a plurality of LDs, branching units 322.sub.11 to 322.sub.32 that divide light propagating in the connection waveguides into two, and multiplexers 327.sub.1 to 327.sub.3 that multiplex the outputs of the branching units 322.sub.11 to 322.sub.31 and the outputs of the branching units 322.sub.12 to 322.sub.32. The outputs of the multiplexers 327.sub.1 to 327.sub.3 are input to a multiplexer 324 via first to third input waveguides 211.sub.1 to 211.sub.3, respectively, and light multiplexed by the multiplexing unit 314 is output from an output waveguide 325.

[0049] In the third example, two outputs of the first to third monitoring waveguides 3231 to 3233 are output to first to third PDs 302.sub.1 to 302.sub.3. In a case where the effective light-receiving area in the light-receiving surface of each PD is wide, the light emitted from all of the two monitoring waveguides can also be received by the PDs by disposing the two monitoring waveguides at 5 to 20 μm intervals at the end surface of the RGB coupler 320. In other words, multiplexers that multiplex the outputs of the two monitoring waveguides may be omitted.

[0050] In the third example as well, switching between the active system and the standby system is performed by electrically switching drive circuits of the LDs. Thus, there is no need to align between the LDs and the RGB coupler when switching between the active system and the standby system.

[0051] With such a configuration, a degraded LD can be easily switched to an unused LD. Thus, even when used in actual operation without sealing the LDs and the RGB coupler, it is possible to extend the life expectancy of LDs. Compared to the first example, the circuit size of the RGB coupler is larger, but because optical circuits are fabricated on the same wafer or chip, there is no increase in manufacturing cost and no additional components are needed because it can be made simultaneously in a single process.

[0052] In the third example, the emission direction of the light from the LD 301 is configured to be generally perpendicular to the incident direction of the light at the PD 302, and thus it is possible to avoid stray light entering PD 302. Stray light is light that has leaked into the RGB coupler 310 without the output of the LD 301 being able to couple to the input waveguide 311, or the like.

Fourth Example

[0053] FIG. 8 illustrates a light source with a monitoring function according to a fourth example of the second embodiment of the present invention. The light source of the fourth example differs in the connecting order of the branching units and the multiplexing units of the RGB coupler 310. This is a case where three LDs (n=3) are implemented for each color (m=3). The PLC-type RGB coupler 310 includes three input waveguides of first to third input waveguides 311.sub.1 to 311.sub.3 optically connected to the three LDs of first to third LDs 301.sub.1 to 301.sub.3, respectively, multiplexing units 314.sub.1 to 314.sub.3 that respectively multiplex light of the respective colors input to the first to third input waveguides 311.sub.1 to 311.sub.3, first to third branching units 312.sub.1 to 312.sub.3 that divide the outputs of the multiplexing units 314.sub.1 to 314.sub.3 into two, a multiplexer 317 that multiplexes beams each being one beam of the light divided by each of the first to third branching units 312.sub.1 to 312.sub.3, and first to third monitoring waveguides 313.sub.1 to 313.sub.3 that output the other beam of the light divided by each of the first to third branching units 312.sub.1 to 312.sub.3 to the first to third PDs 302.sub.1 to 302.sub.3.

[0054] In the fourth example, light in which light of the respective colors R, G, and B is multiplexed is output to the first to third monitoring waveguides 313.sub.1 to 313.sub.3. Thus, in a case where light of the respective colors of R, G, and B is monitored, it is necessary to use a wavelength filter or the like in a preceding stage of the first to third PDs 302.sub.1 to 302.sub.3 to separate.

[0055] Switching the LDs for each color is performed by electrically switching the drive circuits of the LDs. Thus, there is no need to align between the LDs and the RGB coupler at the time of switching.

[0056] Furthermore, a single mode needs to be maintained in order to output light of each of colors of R, G, and B multiplexed by the multiplexing units 314.sub.1 to 314.sub.3 to the output port 316. Thus, an optical circuit in which Y branch circuits illustrated in FIG. 9(a) are connected in two stages, a three-branch circuit illustrated in FIG. 9(b), or an optical circuit combining a Multi-mode Interference (MMI) with a mode converter illustrated in FIG. 9(c) is applied to the multiplexer 317.

[0057] Note that, in the RGB coupler, the branching units for monitoring, the multiplexing units, and the multiplexer that multiplexes the outputs of the plurality of multiplexing units have various connection configurations as illustrated in the first to fourth examples, and the present invention is not limited to these examples.

Other Examples

[0058] In the third example, the emission direction of the light from the LD 301 is configured to be generally perpendicular to the incident direction of the light at the PD 302. In the first, second, and fourth examples as well, in a case where the output ends of the first to third monitoring waveguides 313.sub.1 to 313.sub.3 are disposed on the end surface of the side orthogonal to the side coupled to the LD 301, it is possible to avoid stray light from entering the PD 202 or 302. At this time, it is also possible to remove light that is not multiplexed by the multiplexing unit 314 or stray light that has leaked out therefrom or stray light that has leaked out to the interior of the RGB coupler 310 via a disposal port of the multiplexing unit 314.

REFERENCE SIGNS LIST

[0059] 1 to 3, 21 to 23, 201, 301 LD [0060] 4 to 6 Lens [0061] 7 to 9 Half mirror [0062] 10 to 12 Dichroic mirror [0063] 13 to 15, 202, 302 Photodiode (PD) [0064] 16 MEMS [0065] 17 Screen [0066] 30, 100, 210, 310, 320A, 320B RGB coupler [0067] 31 to 33 Waveguide [0068] 34, 35 Multiplexer [0069] 101 to 103, 211, 311, 321 Input waveguide [0070] 104, 105 Directional coupler [0071] 106, 215, 315, 325 Output waveguide [0072] 212, 312, 322 Branching unit [0073] 313, 323 Monitoring waveguide [0074] 316, 326 Connection waveguide [0075] 317, 327 Multiplexing unit