Patent Publication Number: US-2022229230-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 λ2 incident from the first input waveguide  101  to the second input waveguide  102 , and couples light of λ1 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 λ3 incident from the third input waveguide  103  to the second input waveguide  102 , and passes light of λ1 and λ2 coupled to the second input waveguide  102  in the first directional coupler  104 . 
     For example, green light G (wavelength λ2) is incident on the first input waveguide  101 , blue light B (wavelength λ1) is incident on the second input waveguide  102 , red light R (wavelength λ3) 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 λ1, λ2, and λ3, 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. Meanwhile, because the wavelength of the RGB coupler  100  using the PLC is shorter than the wavelength of the optical coupler in the communication wavelength bands, the allowable error in manufacturing is small. Thus, even with a light source with increased accuracy of monitoring, in a case where the error in manufacturing is large, it may be out of a range for feedback control when the light source is in actual operation. When the allowable error in manufacturing is set to be smaller, the yield becomes low, and there is a problem in that the manufacturing cost of the 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 mitigate allowable errors in manufacturing, and a light source including the optical multiplexing circuit. 
     According to the present invention, in order to achieve such an object, an embodiment of an optical multiplexing circuit includes: a plurality of branching units each configured to divide light output from a corresponding one of a plurality of input waveguides; a multiplexing unit configured to multiplex beams each being one beam of the light divided by each of the plurality of branching units; an output waveguide configured to output the light multiplexed by the multiplexing unit; and a plurality of monitoring waveguides each configured to output another beam of the light divided by each of the plurality of branching units, wherein a plurality of optical multiplexing circuits including multiplexing units having different multiplexing characteristics are provided on a same substrate. 
     An embodiment of a light source with a monitoring function includes: the optical multiplexing circuit; a plurality of laser diodes each optically coupled to a corresponding one of the plurality of input waveguides; and a plurality of photodiodes each optically coupled to a corresponding one of the plurality of monitoring waveguides, wherein the multiplexing unit is switched by changing a fixed position of the optical multiplexing circuit relative to the plurality of laser diodes and the plurality of photodiodes. 
     Effects of the Invention 
     According to the present invention, it is possible to easily switch between multiplexing units with different characteristics, and thus even an optical multiplexing circuit having a small allowable error in manufacturing is capable of individual accurate monitoring even in a case of being subjected to actual operation without reducing the yield. 
    
    
     
       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 an example of a multiplexer according to the second example of the second embodiment. 
         FIG. 8  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. 9  is a diagram illustrating a light source with a monitoring function according to a fourth example of the second embodiment of the present invention. 
     
    
    
     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  is a diagram illustrating 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  can 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 
     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, for example, deviation from a design value of an RGB coupler is different between the short wavelength side (B) and the long wavelength side (R) due to an error in manufacturing, a white balance can be adjusted with high accuracy since feedback control can be performed individually. However, in a case where it is out of a range for feedback control due to the error in manufacturing, accurate white balance adjustment cannot be made. Thus, in a second embodiment, a configuration is employed in which individual accurate monitoring is possible even at a time of actual operation of a light source without setting a small allowable error in manufacturing. 
     First Example 
       FIG. 5  is a diagram illustrating a light source with a monitoring function according to a first example of the second embodiment of the present invention. The light source of the first example can be said to have a configuration in which three of the RGB coupler  210  of the first embodiment are integrated into an RGB coupler  310  on the same PLC substrate. A light source with a monitoring function includes first to third LDs  301   1  to  301   3  that respectively 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 PLC-type RGB coupler  310  includes first to third input waveguides  311   1  to  311   3 , first to third branching units  312   1  to  312   3 , multiplexing units  314   1  to  314   3 , first to third monitoring waveguides  313   1  to  313   3 , and output waveguides  315 . The first to third input waveguides  311   1  to  311   3  are optically connected to the first to third LDs  301   1  to  301   3 . The first to third branching units  312   1  to  312   3  divide light propagating through the first to third input waveguides  311   1  to  311   3  into two. The multiplexing units  314   1  to  314   3  multiplex one beam of the light divided by the first to third branching units  312   1  to  312   3 . The other beam of the light divided by the first to third branching units  312   1  to  312   3  propagates through the first to third monitoring waveguides  313   1  to  313   3  and is output to the first to third PDs  302   1  to  302   3 . The light multiplexed by the multiplexing unit  214  propagates through the output waveguide  315  and is output to an output port  316 . 
     The multiplexing units  314   1  to  314   3  may use, for example, an RGB coupler illustrated in  FIG. 3 , and are multiplexing units with the same circuit format. However, the multiplexing units are designed such that the wavelength at which the transmittance of the multiplexing units is greatest is shifted toward the long wavelength side at the multiplexing unit  314   3  and shifted toward the short wavelength side at the multiplexing unit  314   1  with respect to the multiplexing unit  314   2 . For example, for the waveguide length, the waveguide width, and the gap between the waveguides of the RGB coupler  310 , the multiplexing unit  314   2  in accordance with the design values and the multiplexing units  314   1  and  314   3  with the design values ±0.05 μm are fabricated on the same substrate. 
     In the state illustrated in  FIG. 5 , the first to third LDs  301   1  to  301   3  and the output port  316  are connected to the multiplexing unit  314   1 , and the first to third PDs  302   1  to  302   3  respectively monitor the outputs of the first to third LDs  301   1  to  301   3 . In a case where it is out of a range for feedback control when the light source is in actual operation, as illustrated in  FIG. 5 , the fixed position of the RGB coupler  310  can be changed relative to the LDs and PDs, and can be switched from the multiplexing unit  314   1  to the multiplexing unit  314   2  or the multiplexing unit  314   3 . 
     With such a configuration, it is possible to easily switch between multiplexing units with different characteristics, and thus even an RGB coupler having a small allowable error in manufacturing is capable of individual accurate monitoring even in a case of being in actual operation without reducing the yield. 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. 
     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 configuration of the light source with a monitoring function is the same as that of the first example, except that the light source is different from that of the first example in that three outputs from the multiplexing units  314   1  to  314   3  are multiplexed by a multiplexer  317  and output to the output port  316 , and three outputs of the respective first to third monitoring waveguides  313   1  to  313   3  are multiplexed by multiplexing units  318   1  to  318   3  and output to the first to third PDs  302   1  to  302   3 . The first to third LDs  301   1  to  301   3  are fixed to an LD mount  319 , and by changing the fixed position of the LD mount  319  relative to the RGB coupler  310 , it is possible to switch from the multiplexing unit  314   1  to the multiplexing unit  314   2  or the multiplexing unit  314   3 . 
     With such a configuration, it is possible to easily switch between multiplexing units with different characteristics, and thus even an RGB coupler having a small allowable error in manufacturing is capable of individual accurate monitoring even in a case of being in actual operation without reducing the yield. Compared to the first example, the circuit size of the optical circuit of the RGB coupler is slightly larger, but required locations of alignment between the RGB coupler and external optical elements can be reduced. 
       FIG. 7  illustrates an example of a multiplexer according to the second example of the second embodiment. 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. 7( a )  are connected in two stages, a three-branch circuit illustrated in  FIG. 7( b ) , or an optical circuit combining a Multi-mode Interference (MMI) with a mode converter illustrated in  FIG. 7( c )  is applied to the multiplexer  317 . 
     Third Example 
       FIG. 8  is a diagram illustrating a light source with a monitoring function according to a third example of the second embodiment of the present invention. The light source of the third example differs in the connecting order of the branching units and the multiplexing units of the RGB coupler  310 . The PLC-type RGB coupler  310  includes first to third input waveguides  311   1  to  311   3  optically connected to the first to third LDs  301   1  to  301   3 , 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 third 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. The multiplexer  317  uses the multiplexer of  FIG. 7  illustrated in the second example. 
     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 third examples, and the present invention is not limited to these examples. 
     Fourth Example 
       FIG. 9  illustrates a light source with a monitoring function according to a fourth example of the second embodiment of the present invention. The configuration of the light source with the monitoring function is the same as that of the first and second examples, except that the RGB coupler  320  is divided into two PLC substrates of an RGB coupler  320 A and an RGB coupler  320 B. 
     The RGB coupler  320 A includes first to third input waveguides  321   1  to  321   3  optically connected to first to third LDs  301   1  to  301   3 , and first to third branching units  322   1  to  322   3  that divide light propagating in the waveguide into two. Then, one beam of the light that is divided by each of the first to third branching units  322   1  to  322   3  is output to the RGB coupler  320 B. The other beam of the light is output to the first to third PDs  302   1  to  302   3  via a plurality of monitoring waveguides  313   1  to  313   3 . 
     The RGB coupler  320 B includes three sets of multiplexing units  324   1  to  324   3  that multiplex beams each being one beam of light that is divided by the first to third branching units  322   1  to  322   3 . By changing the fixed position of the RGB coupler  320 B relative to the RGB coupler  320 A, it is possible to switch from the multiplexing unit  324   1  to the multiplexing unit  324   2  or the multiplexing unit  324   3 . 
     With such a configuration, it is possible to easily switch between multiplexing units with different characteristics, and thus even an RGB coupler having a small allowable error in manufacturing is capable of individual accurate monitoring even in a case of being in actual operation without reducing the yield. Compared to the first example, the number and intersection of the waveguides in the RGB coupler  320 A and  320 B can be reduced, and the circuit size of the optical circuit can be reduced. 
     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. 
     Other Examples 
     In the second example, three outputs of the first to third monitoring waveguides  313   1  to  313   3  are multiplexed by the multiplexers  318   1  to  318   3  and output to the 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 three monitoring waveguides can also be received by the PDs by disposing the three monitoring waveguides at 5 to 20 μm intervals at the end surface of the RGB coupler  310 . In other words, the multiplexers  318   1  to  318   3  can be omitted. Similarly, for optical coupling from the output waveguide  315  to the output port  316 , in a case where the three output waveguides  315  are disposed at 5 to 20 μm intervals, a spatial optical system of the output port  316  may be fine tuned, and the multiplexer  317  can be omitted. 
     In the first example as well, in a case where the first to third monitoring waveguides  313   1  to  313   3  and the three output waveguides  315  can be arranged as described above, the multiplexing units  314   1  to  314   3  can be switched by changing only the relative positional relationship between the RGB coupler and the LDs. 
     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 and second example 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 
           213 ,  313  Monitoring waveguide 
           214 ,  314 ,  324  Multiplexing unit 
           316  Output port