Patent Publication Number: US-2022221649-A1

Title: Optical Multiplexing Circuit and Light Source

Description:
TECHNICAL FIELD 
     The present invention relates to an optical multiplexing circuit and a light source, and more particularly to an optical multiplexing circuit capable of multiplexing light of a plurality of wavelengths such as three primary colors of light and monitoring the intensity of light of each wavelength, and a light source including the optical multiplexing circuit. 
     BACKGROUND ART 
     In recent years, a small light source including laser diodes (LDs) that output light of three primary colors of red light (R), green light (G), and blue light (B) as a light source to be applied to a glasses-type terminal and a small pico projector has been developed. Since LDs have a higher directionality than LEDs, a focus-free projector can be realized. Further, since LDs have a high light emission efficiency and a low power consumption, and also a high color reproducibility, LDs have recently been attracting attention. 
       FIG. 1  illustrates a typical light source of a projector using LDs. The light source for the projector includes LDs  1  to  3  that output light of a single wavelength of respective colors of R, G, and B, lenses  4  to  6  that collimate the light output from the LDs  1  to  3 , and dichroic mirrors  10  to  12  that multiplex the respective light and output the light to a MEMS mirror  16 . RGB light combined into a single beam is swept by using the MEMS mirror  16  or the like and is synchronized with modulation of the LDs, and thus an image is projected onto a screen  17 . Half mirrors  7  to  9  are respectively inserted between the lenses  4  to  6  and the dichroic mirrors  10  to  12 , and white balance is adjusted by monitoring the divided light of each color by using photodiodes (PDs)  13  to  15 . 
     In general, an LD emits light in a longitudinal direction of a resonator; however, because the accuracy when monitoring the rear side is poor, it is common to monitor the front side from which light is emitted (front monitoring). As illustrated in  FIG. 1 , for use as an RGB light source, bulk optical components such as the LDs  1  to  3 , the lenses  4  to  6 , the half mirrors  7  to  9 , and the dichroic mirrors  10  to  12  need to be combined with a spatial optical system. Furthermore, for monitoring for an adjustment of white balance, since bulk components such as the half mirrors  7  to  9  and the PDs  13  to  15  are needed and the optical system increases in size, there is a problem in that a reduction in the size of the light source is hindered. 
     On the other hand, an RGB coupler using a planar lightwave circuit (PLC) instead of a spatial optical system with bulk components has been attracting attention (for example, see Non Patent Literature 1). In a PLC, an optical waveguide is produced on a planar substrate such as Si by patterning by photolithography or the like, and reactive ion etching, and a plurality of basic optical circuits (for example, a directional coupler, a Mach-Zehnder interferometer, and the like) are combined, and thus various functions can be realized (for example, see Non Patent Literatures 2 and 3). 
       FIG. 2  illustrates a basic structure of an RGB coupler using a PLC. An RGB coupler module including LDs  21  to  23  of respective colors of G, B, and R and a PLC-type RGB coupler  20  is illustrated. The RGB coupler  20  includes first to third waveguides  31  to  33  and first and second multiplexers  34  and  35  that multiplex light from two waveguides into a single waveguide. As methods using a multiplexer in an RGB coupler module, there are a method of using symmetrical directional couplers having the same waveguide width, a method of using a Mach-Zehnder interferometer (for example, see Non Patent Literature 1), and a method of using a mode coupler (for example, see Non Patent Literature 4), and the like. 
     By using a PLC, a spatial optical system using a lens, a dichroic mirror, or the like can be integrated on one chip. Further, since the LD of R and the LD of G have a weaker output than that of the LD of B, an RRGGB light source in which two LDs of R and two LDs of G are prepared is used. As described in Non Patent Literature 2, by using mode multiplexing, light of the same wavelength can be multiplexed in different modes, and an RRGGB coupler can also be easily realized by using a PLC. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [Non Patent Literature 1] A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated Waveguide-type Red-green-blue Beam Combiners for Compact Projection-type Displays”, Optics Communications 320 (2014) 45-48 
         [Non Patent Literature 2] Y. Hibino, “Arrayed-Waveguide-Grating Multi/Demultiplexers for Photonic Networks,” IEEE CIRCUITS &amp; DEVICES, November, 2000, pp. 21-27 
         [Non Patent Literature 3] A. Himeno, et al., “Silica-based Planar Lightwave Circuits,” J. Sel. Top. Q. E., vol. 4, 1998, pp. 913-924 
         [Non Patent Literature 4] J. Sakamoto et al. “High-efficiency Multiple-light-source Red-green-blue Power Combiner with Optical Waveguide Mode Coupling Technique,” Proc. of SPIE Vol. 10126 101260 M-2 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
       FIG. 3  illustrates a configuration of an RGB coupler using two directional couplers. An RGB coupler  100  using the PLC includes first to third input waveguides  101  to  103 , first and second directional couplers  104  and  105 , and an output waveguide  106  connected to the second input waveguide  102 . 
     A waveguide length, a waveguide width, and a gap between the waveguides are designed such that the first directional coupler  104  couples light of λ 2  incident from the first input waveguide  101  to the second input waveguide  102 , and couples light of λ 1  incident from the second input waveguide  102  to the first input waveguide  101  and back to the second input waveguide  102 . A waveguide length, a waveguide width, and a gap between the waveguides are designed such that the second directional coupler  105  couples light of λ 3  incident from the third input waveguide  103  to the second input waveguide  102 , and passes light of λ 1  and λ 2  coupled to the second input waveguide  102  in the first directional coupler  104 . 
     For example, green light G (wavelength λ 2 ) is incident on the first input waveguide  101 , blue light B (wavelength λ 1 ) is incident on the second input waveguide  102 , red light R (wavelength λ 3 ) is incident on the third input waveguide  103 , and the three colors of light R, G, and B are multiplexed by the first and second directional couplers  104  and  105  and output from the output waveguide  106 . Light of 450 nm, light of 520 nm, and light of 638 nm are used as the wavelengths of λ 1 , λ 2 , and λ 3 , respectively. 
     Thus, the application of such an RGB coupler to configure a light source including a monitoring function for an adjustment of white balance is demanded. 
     On the other hand, in an optical connection between each of the LDs  21  to  23  and the RGB coupler  20  illustrated in  FIG. 2 , optical axes are generally aligned via a gap. However, the LDs  21  to  23  for visible light used in the light source has shorter wavelengths than LDs in communication wavelength bands, and a smaller mode field diameter. Thus, even when light output power is the same as that in the communication wavelength bands, power density is higher by one digit. Furthermore, energy from visible light to ultraviolet light is higher than energy of light in the communication wavelength bands, and thus, an emission end surface is severely degraded due to the dust collection effect of light, and life expectancy of the LD is shortened. 
     Thus, even with a light source with increased accuracy of monitoring, it may be out of a range for feedback control when the light source is in actual operation with degradation of LDs. Sealing the LDs and the RGB coupler can suppress such degradation, but there has been a problem in that manufacturing cost of such a light source is increased. 
     Means for Solving the Problem 
     An object of the present invention is to provide an optical multiplexing circuit including a multiplexing unit constituted by a PLC, which can accurately monitor light of a plurality of wavelengths and can tolerate degradation of LDs, and a light source including the optical multiplexing circuit. 
     In order to achieve such an object, an embodiment of 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. 
     Effects of the Invention 
     According to the present invention, one laser diode of n laser diodes can be selected and operated for each m set to tolerate degradation of LDs. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a typical light source of a projector using LDs. 
         FIG. 2  is a diagram illustrating a basic structure of an RGB coupler using a PLC. 
         FIG. 3  is a diagram illustrating a configuration of an RGB coupler using two directional couplers. 
         FIG. 4  is a diagram illustrating a light source with a monitoring function according to a first embodiment of the present invention. 
         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. 
         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. 
         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. 
         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. 
         FIG. 9  is a diagram illustrating an example of a multiplexer according to the fourth example of the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     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 
       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   1  to  201   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   1  to  202   3  optically connected to the RGB coupler  210 . 
     The PLC-type RGB coupler  210  includes first to third input waveguides  211   1  to  211   3  optically connected to the first to third LDs  201   1  to  201   3 , first to third branching units  212   1  to  212   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   1  to  212   3 , first to third monitoring waveguides  213   1  to  213   3  that output the other beam of the light divided by each of the first to third branching units  212   1  to  212   3  to the first to third PDs  202   1  to  202   3 , and an output waveguide  215  that outputs the light multiplexed by the multiplexing unit  214 . 
     In the PLC-type RGB coupler  210 , light incident on each of the first to third input waveguides  211   1  to  211   3  is divided into two by each of the first to third branching units  212   1  to  212   3 . One beam of the divided light is output to the first to third PDs  202   1  to  202   3  via the first to third monitoring waveguides  213   1  to  213   3 , and the other beam of the divided light is multiplexed by the multiplexing unit  214  and output to the output waveguide  215 . 
     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   1  to  211   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. 
     As illustrated in  FIG. 4 , when light propagating through the first to third input waveguides  211   1  to  211   3  is divided by the first to third branching units  212   1  to  212   3 , respectively, a coupling characteristic between the first to third LDs  201   1  to  201   3  and the first to third input waveguides  211   1  to  211   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   1  to  202   3  by recognizing a multiplexing characteristic of the multiplexing unit  214  in advance. 
     Second Embodiment 
     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   1  to  202   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. 
     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 
       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   11  to  301   31 , being LDs for each color in an active system and LDs  301   12  to  301   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. 
     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. 
     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. 
     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&gt;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&gt;2). 
     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 
       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   1  to  302   3  optically connected to the RGB coupler  310 . The plurality of LDs include LDs  301   11  to  301   31 , which are LDs for respective colors of the active system, and LDs  301   12  to  301   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   1  to  317   3  and connection waveguides  316   1  to  316   3  for connecting the multiplexers and LDs are added to the first to third input waveguides  311   1  to  311   3 , respectively. 
     The outputs of the LDs  301   11  to  301   31 , which are LDs of respective colors of the active system, and the outputs of the LDs  301   12  to  301   32 , which are LDs of respective colors of the standby system, are connected to a multiplexing unit  314  via the multiplexers  317   1  to  317   3  and the first to third branching units  312   1  to  312   3 , respectively. One beam of the divided light is output to the first to third PDs  302   1  to  302   3  via the first to third monitoring waveguides  313   1  to  313   3 , and the other beam of the divided light is multiplexed by the multiplexing unit  314  and output to the output waveguide  315 . 
     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. 
     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 
       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   1  to  302   3  optically connected to the RGB coupler  320 . The plurality of LDs include LDs  301   11  to  301   31 , which are LDs for respective colors of the active system, and LDs  301   12  to  301   32 , which are LDs for respective colors of the standby system (n=2, m=3). 
     The PLC-type RGB coupler  320  includes connection waveguides  316   11  to  316   32  optically connected to a plurality of LDs, branching units  322   11  to  322   32  that divide light propagating in the connection waveguides into two, and multiplexers  327   1  to  327   3  that multiplex the outputs of the branching units  322   11  to  322   31  and the outputs of the branching units  322   12  to  322   32 . The outputs of the multiplexers  327   1  to  327   3  are input to a multiplexer  324  via first to third input waveguides  211   1  to  211   3 , respectively, and light multiplexed by the multiplexing unit  314  is output from an output waveguide  325 . 
     In the third example, two outputs of the first to third monitoring waveguides  3231  to  3233  are output to first to third PDs  302   1  to  302   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. 
     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. 
     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. 
     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 
       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   1  to  311   3  optically connected to the three LDs of first to third LDs  301   1  to  301   3 , respectively, multiplexing units  314   1  to  314   3  that respectively multiplex light of the respective colors input to the first to third input waveguides  311   1  to  311   3 , first to third branching units  312   1  to  312   3  that divide the outputs of the multiplexing units  314   1  to  314   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   1  to  312   3 , and first to third monitoring waveguides  313   1  to  313   3  that output the other beam of the light divided by each of the first to third branching units  312   1  to  312   3  to the first to third PDs  302   1  to  302   3 . 
     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   1  to  313   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   1  to  302   3  to separate. 
     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. 
     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   1  to  314   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 . 
     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 
     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   1  to  313   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 
     
         
           1  to  3 ,  21  to  23 ,  201 ,  301  LD 
           4  to  6  Lens 
           7  to  9  Half mirror 
           10  to  12  Dichroic mirror 
           13  to  15 ,  202 ,  302  Photodiode (PD) 
           16  MEMS 
           17  Screen 
           30 ,  100 ,  210 ,  310 ,  320 A,  320 B RGB coupler 
           31  to  33  Waveguide 
           34 ,  35  Multiplexer 
           101  to  103 ,  211 ,  311 ,  321  Input waveguide 
           104 ,  105  Directional coupler 
           106 ,  215 ,  315 ,  325  Output waveguide 
           212 ,  312 ,  322  Branching unit 
           313 ,  323  Monitoring waveguide 
           316 ,  326  Connection waveguide 
           317 ,  327  Multiplexing unit