Patent Publication Number: US-2022229235-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, an optical circuit using a PLC is an embedded waveguide, which has weak confinement of light, and the minimum bend radius of the waveguide is limited. Accordingly, when a monitoring function is added to the RGB coupler  100 , there has been a problem in that the accuracy of monitoring is limited due to design constraints of the optical circuit. 
     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 with the size being reduced, 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 at least one monitoring waveguide of the plurality of monitoring waveguides includes a bent waveguide constituted by a rib-shaped waveguide. 
     Effects of the Invention 
     According to the present invention, the monitoring waveguide includes a bent waveguide constituted by a rib-shaped waveguide, so that the accuracy of the monitoring can be maintained while the size of the optical wave circuit can be reduced without limiting the minimum bend radius of the waveguide. 
    
    
     
       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 second embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a monitoring waveguide according to a third embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a method of preparing the monitoring waveguide according to the third embodiment of the present invention. 
         FIG. 8  is a diagram illustrating a modified example of the monitoring waveguide according to the third 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  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 
       FIG. 5  illustrates a light source with a monitoring function according to a second embodiment of the present invention. According to 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 the PD  202  is disposed to face the emission surface of the LD  201 , the PD  202  may be incident with stray light and accurate monitoring values may not be achieved. Stray light is light which has leaked out to the interior of the RGB coupler  210  without the output of the LD  201  coupling to the input waveguide  211 , light that is not multiplexed by the multiplexing unit  214  or light that has leaked out therefrom, light that has leaked out to the interior of the RGB coupler  210  via a disposal port of the multiplexing unit  214 , or the like. 
     Thus, in the second embodiment, the first to third monitoring waveguides  313   1  to  313   3  are bent waveguides for optical path conversion of 90 degrees so that the LD  301  and the PD  302  does not face each other. The emission direction of the light from the LD  301  and the emission direction of the light from the multiplexing unit  314  are 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 the PD  302 . 
     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 , a multiplexing unit  314 , 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 waveguides into two. The multiplexing unit  314  multiplexes 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  to output. 
     Third Embodiment 
     As described above, an optical circuit using a PLC is an embedded waveguide, which has weak confinement of light, and the minimum bend radius of the waveguide is limited. In the second embodiment, the first to third monitoring waveguides  313   1  to  313   3  are bent waveguides for optical path conversion of 90 degrees. However, in a case of a small bend radius, the light divided by the first to third branching units  312   1  to  312   3  leak out and the accuracy of the monitoring falls. On the other hand, in a case of large bend radius, the chip size of the RGB coupler  310  is large. 
     Therefore, in a third embodiment, a rib-shaped waveguide is used as a bent waveguide for optical path conversion, and the lateral clad of the waveguide core is formed by air. Thus, a highly confined waveguide can be achieved. As a result, the minimum bend radius is reduced, and the expansion of the chip size of the RGB coupler  310  is suppressed. 
       FIG. 6  illustrates a monitoring waveguide according to the third embodiment of the present invention.  FIG. 6  is an enlarged view of the vicinity of the connection between the branching unit  312  and the monitoring waveguide  313 . As illustrated in  FIG. 6( a ) , the branching unit  312  is a directional coupler having a single input and two outputs, which divides light propagating through the input waveguide  311  into two, and outputs the light to the monitoring waveguide  313  and the multiplexing unit  314 . 
     As illustrated in  FIG. 6( b ) , the input waveguide  311  is a single mode embedded waveguide in which a core  403   a  is embedded in a clad  402  on a substrate  401 . As illustrated in  FIG. 6( c ) , the branching unit  312 , which is a directional coupler, is also constituted by an embedded waveguide in which the two cores  403   b  and  403   c  are disposed in close proximity. 
     The output of the branching unit  312  connected to the monitoring waveguide  313  is connected to a bent waveguide portion  313   a  with the waveguide width being widened in a tapered manner. A cross section of the bent waveguide portion  313   a  of the monitoring waveguide  313  is illustrated in  FIG. 6( d ) . The bent waveguide portion  313   a  is a multi-mode rib-shaped waveguide in which the width of the core  403   d  is enlarged than the core  403   a.    
     According to such a configuration, the light divided by the branching unit  312  is output to the PD  302  in a multi-mode via the monitoring waveguide  313  including the bent waveguide portion  313   a . However, since the PD  302  directly receives the light emitted from the waveguide end surface of the RGB coupler  310 , the measurement of optical power is not impaired. Therefore, by using a bent waveguide with a high confinement, it is possible to reduce the size of the RGB coupler  310  without reducing the accuracy of the monitoring. 
       FIG. 7  illustrates a method of preparing the monitoring waveguide according to the third embodiment of the present invention. A bottom clad layer  402   a  is deposited on the substrate  401  ( FIG. 7( a ) ), and the core layer is further deposited and etched to form a core pattern  403  ( FIG. 7( b ) ) so as to form a desired waveguide pattern ( FIG. 7( b ) ). At this time, in the portion for creating the bent waveguide portion  313   a , a large core layer is left in consideration of the accuracy of the subsequent photolithography. Here, the periphery of the bent waveguide portion  313   a  is left in a rectangular shape. 
     An upper clad layer  402   b  is deposited so as to cover the bottom clad layer  402   a  and the core pattern  403  to complete the embedded waveguide ( FIG. 7( c ) ). Next, the clad and the core of the portion for creating the bent waveguide portion  313   a  is etched to form a rib-shaped waveguide ( FIG. 7( d ) ). 
     In this way, only an etching step for the portion of the bent waveguide of the monitoring waveguide may be added in the PLC-type RGB coupler created by the conventional method, and in addition, a multi-mode waveguide with large allowable errors in manufacturing may be created, so it is possible to reduce the size of the RGB coupler  310  by the addition of a simple process. 
       FIG. 8  illustrates a modified example of the monitoring waveguide according to the third embodiment. The waveguide width is enlarged at the bent waveguide portion  313   a  without connecting a tapered waveguide to the output of the branching unit  312 . The size of the RGB coupler  310  can be further reduced by removing the tapered waveguide. 
     Other Examples 
     For the monitoring waveguide  313 , the bent waveguide portion  313   a  may be applied to all of the first to third monitoring waveguides  313   1  to  313   3  of the respective colors of R, G, and B in the RGB coupler  310  in  FIG. 5 . Meanwhile, the bent waveguide portion  313   a  may be applied to only the first monitoring waveguide  313   1  for which a small bend radius is required or only the first and second monitoring waveguides  313   1  and  313   2 . 
     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  RGB coupler 
           31  to  33  Waveguide 
           34 ,  35  Multiplexer 
           101  to  103 ,  211 ,  311  Input waveguide 
           104 ,  105  Directional coupler 
           106 ,  215 ,  315  Output waveguide 
           212 ,  312  Branching unit 
           213 ,  313  Monitoring waveguide 
           214 ,  314  Multiplexing unit 
           401  On substrate 
           402  Clad 
           403  Core