Patent Publication Number: US-2023152541-A1

Title: Optical Multiplexing Circuit and Optical Source

Description:
BACKGROUND ART 
     The present invention relates to a light combining circuit and a light source. More particularly, the present invention relates to a light combining circuit for combining light of multiple wavelengths such as light of three primary colors and monitoring the light intensity of each wavelength, and a light source including the light combining circuit. 
     As a light source for an eyeglass-type terminal or a small projector, a light source has become prevalent that includes laser diodes (LDs) outputting three primary colors light of R (red light), G (green light), and B (blue light). 
       FIG.  1    illustrates an exemplary light source for a projector using an LD. The projector light source includes LDs  1  to  3  that output single wavelength light of each color of R, G, and B, lenses  4  to  6  that collimate the respective light output from the LDs  1  to  3 , and dichroic mirrors  10  to  12  that combine the respective light and output it to a MEMS mirror  16 . The RGB light is bundled in one beam and swept by the MEMS mirror  16  or the like to project a video on a screen  17  while synchronizing LD modulation and sweep. Half mirrors  7  to  9  are inserted between the lenses  4  to  6  and dichroic mirrors  10  to  12 . Split light of each color is monitored by photo diodes (PDs)  13  to  15  to adjust the white balance of the projected video. 
     It is general that in the LD, the light is monitored (front monitoring) before a resonator that emits light. As shown in  FIG.  1   , a light source including the RGB light sources includes bulk optical components combined with free space optics, 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 . In addition, the monitoring for adjusting the white balance needs bulk components such as the half mirrors  7  to  9  and PDs  13  to  15 . Inclusion of a lot of bulk components leads to a larger optical system, which makes it difficult to provide a smaller light source. 
     Then, instead of the free space optics with the bulk components, an RGB coupler using a planar lightwave circuit (PLC) has drawn attention. The PLC includes a flat substrate such as Si that is subjected to patterning such as photolithography and reactive ion etching processing to fabricate an optical waveguide. A plurality of basic light circuits (such as a directional coupler and a Mach-Zehnder interferometer) can be combined to achieve various functions. 
       FIG.  2    illustrates a basic structure of an RGB coupler using the PLC.  FIG.  2    illustrates an RGB coupler module including LDs  21  to  23  of G, B, and R colors and a PLC type RGB coupler  30 . The RGB coupler  30  includes first to third waveguides  31  to  33  and first and second combiners  34  and  35  that combine light from two waveguides into one waveguide. Combined output light  36  is output from the chip end face. As a combiner, there are ways of using a symmetrical directional coupler having the same waveguide width, the Mach-Zehnder interferometer, and a mode coupler or the like. 
     Using the PLC shown in  FIG.  2   , a free space optics using the lenses and dichroic mirrors or the like as in  FIG.  1    can be integrated on one chip  30 . Because the LDs of R and G output less than the LD of B, an RRGGB light source is used that includes two LDs for each of R and G. Because a multiplex mode can be used to combine light of the same wavelength in different modes, the PLC can be used to easily achieve an RRGGB coupler as well. 
       FIG.  3    illustrates the 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 . 
     The first directional coupler  104  couples light λ 2  incident from the first input waveguide  101  to the second input waveguide  102 , couples light λ 1  incident from the second input waveguide  102  to the first input waveguide  101 , and couples it again to the second input waveguide  102 . The second directional coupler  105  couples light λ 3  incident from the third input waveguide  103  to the second input waveguide  102 , and transmits light of λ 1  and λ 2  coupled to the second input waveguide  102  at the first directional coupler  104 . The waveguide length, waveguide width, and inter-waveguide gap in each unit are designed to achieve the above operation. 
     For example, green light G (wavelength λ 2 ) is incident to the first input waveguide  101 , blue light B (wavelength λ 1 ) is incident to the second input waveguide  102 , and red light R (wavelength λ 3 ) is incident to the third input waveguide  103 . Light of three colors R, G, and B are combined by the first and second directional couplers  104  and  105  into RGB combined light that is output from the output waveguide  106 . For the wavelengths of λ 1 , λ 2 , and λ 3 , light of 450 nm, 520 nm, and 638 nm is used, respectively. The inventors have proposed a configuration of the RGB coupler including the PLC that includes a monitoring function for the white balance adjustment (PTL 1). 
       FIG.  4    illustrates the configuration of a light source with the monitoring function in the conventional technologies. A light source with a monitoring function  210  includes first to third LDs  201   1  to  201   3  that output light of colors R, G, and B, respectively, a PLC type RGB coupler  210 , and a PD  202  optically connected to the RGB coupler  210 . The RGB coupler  210  includes input waveguides  211   1  to  211   3  corresponding to the respective colors, a wave-combining unit  214 , and an output waveguide  215 , and further includes a splitting unit  212  near the output waveguide  215 . After propagating the output waveguide  215 , combined light  203  is output. A part of the combined light  203  is split by the splitting unit  212  and input in the PD  202 . 
     A configuration is also contemplated in  FIG.  4    that light is split from the input waveguides  211   1  to  211   3  and monitored by three independent PDs for the respective colors.  FIG.  4    shows a configuration that by directly monitoring the output from the output waveguide  215 , there is no need to provide a monitoring circuit for each color. A smaller light source can be achieved, and by understanding the combining characteristics of the wave-combining unit  214 , the white balance can be adjusted using the monitoring value of the PD  202 . 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent Application Publication No. 2018-180513, description 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Unfortunately, in the splitting unit  212 , the split ratio largely depends on the wavelength, and the split ratio of R and the split ratios of G and B are non-uniform because R has a wavelength far from those of G and B. For example, for a splitting unit including a directional coupler that has a Normalized Index Difference of 0.45%, a waveguide width of 2 μm, an inter-waveguide gap of 2 μm, and a length that is set such that the split ratio for blue is 1%, the split ratio for three colors is R:G:B=23:5:1. Such a non-uniform split ratio makes it essential to correct the monitoring detection value in the PD  202  to use the light source in  FIG.  4    in a video display device or the like and adjust the white balance. There is a problem that a detection value with a large width in the PD  202  is difficult to use as a monitoring function. Also, the PD requires a large dynamic range. 
     In view of the foregoing, it is an object of the present invention to provide a coupler with a monitoring function and a light source that are easily to use in a video display device and have a simplified configuration. 
     Means for Solving the Problem 
     To achieve the above purpose, one embodiment of the present invention is a light combining circuit comprising: a first splitting unit for splitting first wavelength light; a preliminary wave-combining unit for combining second wavelength light and third wavelength light; a second splitting unit for splitting the combined second wavelength light and third wavelength light; a main wave-combining unit for combining first split light from the first splitting unit and first split light from the second splitting unit; an output waveguide for outputting the combined light from the main wave-combining unit; a first monitoring waveguide for outputting second split light from the first splitting unit; and a second monitoring waveguide for outputting second split light from the second splitting unit. 
     In addition, another embodiment may be a light source with a monitoring function comprising: the above light combining circuit; three laser diodes for outputting the first wavelength light, the second wavelength light, and the third wavelength light, respectively; and a photo diode optically coupled to the first and second monitoring waveguides. 
     Effects of the Invention 
     As described above, the light combining circuit and light source of this disclosure may solve or reduce the problem of the wavelength dependence of the split ratio and provide a monitoring function that is easy to use in a video device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a configuration of an exemplary light source for a projector using an LD. 
         FIG.  2    illustrates the basic structure of an RGB coupler using a PLC. 
         FIG.  3    illustrates a configuration of an RGB coupler using two directional couplers. 
         FIG.  4    illustrates a configuration of an RGB coupler with a monitoring function in the conventional technologies. 
         FIG.  5    illustrates another configuration of an RGB coupler with a monitoring function in the conventional technologies. 
         FIG.  6    illustrates a configuration of a light source with a monitoring function of a first embodiment. 
         FIG.  7    illustrates a configuration of a light source with a monitoring function of a second embodiment. 
         FIG.  8    illustrates a configuration of a light source with a monitoring function of a third embodiment. 
         FIG.  9    illustrates a configuration of a light source with a monitoring function of a fourth embodiment. 
         FIG.  10    illustrates a configuration of a light source with a monitoring function of a fifth embodiment. 
         FIG.  11    illustrates a cross-section near an emitting end of a monitoring waveguide and a PD. 
         FIG.  12    illustrates a configuration of a light source with a monitoring function of a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A light combining circuit and a light source of this disclosure may achieve a very simplified configuration while avoiding the problem of the wavelength dependence of the splitting unit in the conventional technologies. A video device using the light source of this disclosure may largely reduce the wavelength dependence of the monitoring value to simplify the adjustment process of the white balance. 
     Referring again to the configuration of the light source in the conventional technologies in  FIG.  4   , the wave-combining unit  214  combines the three colors of R, G, and B and then the splitting unit  212  splits the monitor light, thus making it possible to monitor the output light  203  after passing through all components of the RGB coupler circuit  210 . This configuration reflects all of the characteristics of the RGB coupler circuit  210  including the wave-combining unit  214 , and thus is more preferable as a method of detecting the monitor light. Unfortunately, due to the wavelength dependence of the split ratio of the splitting unit  212 , it is not easy to handle the monitoring value in the video device using this light source. To avoid the problem of the wavelength dependence of the split ratio of the splitting unit  212 , it is also contemplated to split the three colors of R, G, and B by independent splitting units. 
       FIG.  5    illustrates another configuration of an RGB coupler with a monitoring function in the conventional technologies. The light source in  FIG.  5    includes first to third LDs  301   1  to  301   3 , a PLC type RGB coupler  310 , and first to third PDs  302   1  to  302   3  optically connected to the RGB coupler  310 . The RGB coupler  310  includes input waveguides  311   1  to  311   3  corresponding to the first to third LDs, first to third splitting units  312   1  to  312   3  that split light propagating the waveguides into two, a wave-combining unit  314  that combines first one of the respective light split by the first to third splitting units  312   1  to  312   3 , and an output waveguide  315  that outputs the combined light. 
     Additionally, the RGB coupler  310  includes first to third monitoring waveguides  313   1  to  313   3  that output second one of the respective light split by the first to third splitting units  312   1  to  312   3  to the respective first to third monitoring wave-combining units  316   1  to  316   3 , and first to third monitoring waveguides  317   1  to  317   3  that output the outputs of the respective first to third monitoring wave-combining units  316   1  to  316   3  to the corresponding PDs  302   1  to  302   3 . 
     In the configuration of the light source shown in  FIG.  5   , the individual splitting units  312   1  to  312   3  split the respective light of R, G, and B before combining them. Therefore, the configuration of the two splitting units may be adapted for each color to provide a uniform split ratio and input the monitoring light split by the splitting units to the corresponding PDs directly. Unfortunately, inputting the monitoring light to the corresponding PDs directly may not reflect to the monitoring value the characteristics of the wave-combining unit  314  that outputs the RGB combined light  303 . Therefore, in the configuration of the conventional technologies in  FIG.  5   , the split light is passed through dummy monitoring wave-combining units  316   1  to  316   3  of the same configuration as the wave-combining unit  314 . The configuration of the RGB coupler in  FIG.  5    needs three dedicated PDs for the respective colors and four three-input wave-combining units of the same configuration, which complicates and enlarges the entire configuration of the light source. 
     In discussing a way to avoid the problem of the wavelength dependence of the splitting unit  212  without the complicated configuration of the light circuit as shown in  FIG.  5   , the inventors focus on the fact that the split ratio of three colors departs to a significantly large value at the wavelength of R. The actual video device using the LDs has a combination of RGB wavelengths for the best color reproduction, but indeed the combination is determined by the wavelengths of R, G, and B of an LD that is easy to mass produce. For example, LDs of B (blue) near 445 nm, G (green) near 515 to 520 nm, and R (red) near 638 nm are mass produced. As B may damage eyes more for shorter wavelengths, it may be used at longer wavelengths (close to G), which will not increase the difference between the wavelengths of B and G. Then, the inventors have reached an idea of splitting R, G, and B separately. We have found a configuration of the light combining circuit that may solve or significantly reduce the problem of the wavelength dependence of the splitting unit, while keeping the simple entire configuration of the light source by using one PD detector as in the configuration of the RGB coupler in  FIG.  4   . 
     A light combining circuit and a light source of this disclosure include a first splitting unit for splitting light of wavelength R and a second splitting unit for splitting combined light of G and B, and split monochromatic light of R and combined light of G and B, independently. Therefore, each light of G and B from the LDs is first combined by a preliminary wave-combining unit before being split. The split light of each wavelength is combined by a main wave-combining unit to output combined light of RGB. Two split lights from the two splitting units are detected by a single PD. Splitting the monochromatic light of R and the combined light of G and B independently may provide a more uniform split ratio between three wavelengths of R, G, and B, while the single PD receives each light and monitors their levels. The light source of this disclosure may be combined with a video device that controls the three light sources by time division to largely simplify the video adjustment process such as the white balance. 
     First Embodiment 
       FIG.  6    illustrates a configuration of a light source with a monitoring function of a first embodiment of this disclosure. A light source  400  in the first embodiment includes first to third LDs  401   1  to  401   3 , an RGB coupler  410  that is a PLC type light circuit, and a PD  402  optically connected to the RGB coupler  410 . The RGB coupler  410  includes input waveguides  411   1  to  411   3  corresponding to the first to third LDs, a main wave-combining unit  416  (a wave-combining unit  2 ) combining three colors of R, G, and B, and an output waveguide  417  outputting combined light  403 . The RGB coupler  410  of this disclosure splits the three colors of R, G, and B separately using a first splitting unit  413  dedicated for R and a second splitting unit  414  for GB before combining them using the main wave-combining unit  416 . Before splitting G and B, a preliminary wave-combining unit  412  (a wave-combining unit  1 ) combines each light of G and B from the second LD  401   2  and third LD  401   3 . The second splitting unit  414  splits the combined GB light. 
     For the R light split by the first splitting unit  413  dedicated for R, first light is provided to the main wave-combining unit  416  through a split waveguide  415   1  and second light is provided to the PD  402  through a monitoring waveguide  418   1 . For the GB light split by the second splitting unit  414  for G and B, first light is provided to the main wave-combining unit  416  through a split waveguide  415   2  and second light is provided to the PD  402  through a monitoring waveguide  418   2 . The RGB coupler  410  functions as a light combining circuit. 
     As the waveguide distance between the two monitoring waveguides  418   1  and  418   2  may be for example as close as 30 μm, the single PD  402  may receive the split light from the two monitoring waveguides. In other words, the first and second monitoring waveguides  418   1  and  418   2  have respective terminations juxtaposed close to each other on one side end face of the substrate of the PLC circuit. As a normal PD has a size of about 0.75 mm square, one PD may receive lights from two waveguides. Therefore, a single PD may monitor lights as in the configuration in the conventional technologies shown in  FIG.  4   . 
     Therefore, the light combining circuit  410  of this disclosure may be implemented as including the first splitting unit  413  for splitting first wavelength light, the preliminary wave-combining unit  412  for combining second wavelength light and third wavelength light, the second splitting unit  414  for splitting the combined second wavelength light and the third wavelength light, the main wave-combining unit  416  for combining first split light from the first splitting unit and first split light from the second splitting unit, the output waveguide  417  for outputting the combined light from the main wave-combining unit, the first monitoring waveguide  418   1  for outputting second split light from the first splitting unit, and the second monitoring waveguide  418   2  for outputting second split light from the second splitting unit. 
     The light circuit of this disclosure, i.e., the RGB coupler  410  is different from the configuration in the conventional technologies in  FIG.  4    in that the three colors of R, G, and B are split separately by the first splitting unit  413  dedicated for R and the second splitting unit  414  for GB. In the conventional technologies shown in  FIG.  4   , after the three colors of R, G, and B are combined, the RGB light is split by the single splitting unit in which the split ratio largely depends on the wavelength. In contrast, in the light circuit of this disclosure, the first splitting unit  413  dedicated for R splits only the monochromatic light of the wavelength R and the second splitting unit  414  for GB splits the combined light of G and B. By dividing the splitting units into two parts, the split ratios in the R waveband and the G and B wavebands may be set separately. Splitting the three colors of RGB by the single splitting unit as described above will provide a very non-uniform split ratio like R:G:B=23:5:1. Particularly, the split ratio at the R wavelength is larger than the split ratio at the wavelengths G and B. However, as shown in  FIG.  6   , dividing the splitting unit into two parts may optimize the split structure for each wavelength to largely reduce the wavelength dependence of the split ratio. 
     To divide the splitting unit into two parts as shown in  FIG.  6   , each light of G and B from the second and third LDs  401   2  and  401   3  needs to be combined previously by the preliminary wave-combining unit  412 . In comparison with the configuration in  FIG.  4   , it is necessary to divide the splitting unit into two parts in the RGB coupler  410  and add the preliminary wave-combining unit  412 . However, it is possible to keep the light source and the interface with PD and thus the PLC chip area only needs to be increased slightly. With the two splitting units  413  and  414 , it is easy to set the split-side coupling rate appropriately. It is possible to control the wavelength dependence between the wavelengths G and B to about 2:1 at the most at the second splitting unit  414  for GB. Thus, it is possible to control the deviation of the split ratio between the three wavelengths of R, G, and B within twice. Additionally, by setting the split ratio of the first splitting unit  413  dedicated for R smaller than that of the second splitting unit  414  for GB, it is possible to control the wavelength dependence of the split ratio to about R:G:B=1:1:0.5. 
     In the configuration of the conventional technologies in  FIG.  4   , the deviation between the three wavelengths reaches 23 times (26 dB). In contrast, in the configuration of the light combining circuit of this disclosure in  FIG.  6   , it is possible to control the monitoring value deviation in the PD at the wavelengths of R, G, and B within twice (6 dB). It is also possible to directly use the monitoring electrical signal from the PD  402  on the video device without physical conversion such as amplification/attenuation. In addition, it is also possible to use the monitoring signal on the video device side only by adding a slight correction to the monitoring signal. Thus, the detection signal of the PD  402  may become much easier to use as the monitoring function of the device side. The specific usage of the monitoring signal at the PD  402  will be described below. 
     As described above, in the light source with a monitoring function in the first embodiment, the first splitting unit  413  dedicated for R light splits only the monochromatic light of R and the second splitting unit  414  for GB light splits the combined light of G and B, before combining the RGB light. Additionally, by the single PD receiving the two split lights, it is possible to largely reduce the split ratio deviation between RGB while maintaining the small and simple configuration and provide an easy-to-use monitoring signal. 
     Second Embodiment 
     In the above first embodiment, the deviation of the split ratio between RGB may be largely reduced. But with respect to each split monitoring light received by the PD  402 , the split light of R does not pass through the main wave-combining unit  416 . Therefore, the monitoring output of R light does not reflect the combining characteristics (such as the wavelength dependence) that the RGB combined light  403  receives from the main wave-combining unit  416 . The monitoring outputs of G and B also do not pass through the main wave-combining unit  416 , but they pass through the preliminary wave-combining unit  412 . Therefore, if the two wave-combining units  412  and  416  have the same characteristics, the monitoring outputs of G and B reflect the characteristics of the RGB combined light  403 . 
     As described above, in the split light received by the PD  402 , the split light of R does not correctly reflect the RGB combined light  403  obtained through the actual combining characteristics at the main wave-combining unit  416 . As it does not correctly reflect the actual RGB combined light to be monitored, the configuration in the first embodiment is a little bit disadvantageous over the configuration in  FIG.  4    in which light is split at the final part of the RGB coupler. Then, the light combining circuit of this embodiment has a configuration that reflects the characteristics of the main wave-combining unit by improving the first embodiment. 
       FIG.  7    illustrates a configuration of a light source with a monitoring function of a second embodiment of this disclosure. A light source  500  of the second embodiment includes first to third LDs  501   1  to  501   3 , an RGB coupler  510  that is the PLC type light circuit, and a PD  502  optically connected to the RGB coupler  510 . The RGB coupler  510  includes input waveguides  511   1  to  511   3  corresponding to the first to third LDs  501   1  to  501   3 , a main wave-combining unit  516  (a wave-combining unit  2 ) combining the three colors of R, G, and B, and an output waveguide  517  outputting combined light  503 . The RGB coupler  510  functions as a light combining circuit. 
     Again in the second embodiment, the RGB coupler  510  splits the three colors of R, G, and B separately by a first splitting unit  513  dedicated for R and a second splitting unit  514  for GB before combining them at the main wave-combining unit  516 . Before splitting G and B, each light of G and B from the second and third LDs  501   2  and  501   3  is combined at a preliminary wave-combining unit  512  (a wave-combining unit  1 ) as in the first embodiment. The difference from the first embodiment is that first split light of R light that is split by the splitting unit  513  is provided to the main wave-combining unit  516  through a waveguide  515   1  and second split light is provided to the PD  502  through a monitoring waveguide  518   1  and a dummy wave-combining unit  519  (a wave-combining unit  3 ). In other words, the second embodiment includes, in addition to the configuration in the first embodiment, the dummy wave-combining unit  519  (the wave-combining unit  3 ). The dummy wave-combining unit  519  is a monitoring wave-combining unit that is disposed in the middle of the first monitoring waveguides  518   1  and  520  and has the same configuration as the main wave-combining unit  516 . 
     With respect to the GB light split by the second splitting unit  514  for G and B, first split light is provided to the main wave-combining unit  516  through a split waveguide  515   2  and second split light is provided to the PD  502  through a monitoring waveguide  518   2 . The split light of R light from the dummy wave-combining unit  519  through the monitoring waveguide  520  and the split light of GB light from the monitoring waveguide  518   2  are received by the PD  502 . 
     In the configuration of the light combining circuit in  FIG.  7   , the split light of R light toward the PD passes through the same transmission characteristics (the combining characteristics) as that of the RGB combined light  503  by passing through the dummy wave-combining unit  519  having the same configuration as the main wave-combining unit  516 . For example, even if the combining characteristics depends on the wavelength, the misbalance between the wavelengths in the RGB combined light to be monitored is reflected to the monitoring detection value in each light at the PD. With the three wave-combining units  512 ,  516 , and  519  that are the wave-combining units of the same configuration, each split monitoring light received by the PD reflects the same characteristics as the combining characteristics received by the actual RGB combined light  503 , making it possible to monitor the RGB combined light  503  more accurately. 
     In comparison with the light combining circuit in the first embodiment, one more wave-combining unit is added, while it is possible to reduce the deviation of the split ratio between RGB more than in the conventional technologies and reflect the characteristics of the actual RGB combined light to the monitoring output more correctly. 
     Third Embodiment 
     In both of the above first and second embodiments, the light combining circuit includes a plurality of wave-combining units. Light not combined by the wave-combining unit provides stray light. Stray light entering the RGB light beam at the emitting part of the light circuit will disturb the beam profile. The stray light includes light that is output from the LD, not coupled to the input waveguide, and leaked inside the RGB coupler, light that is leaked or not completely combined at the wave-combining unit, and light that is leaked inside the RGB coupler through the discarding port of the wave-combining unit light, or the like. In this embodiment, the wave-combining unit is tilted from the emitting direction of the RGB combined light to minimize the effects of the stray light. 
       FIG.  8    illustrates a configuration of a light source with a monitoring function of a third embodiment of this disclosure. A light source  600  in the third embodiment includes first to third LDs  601   1  to  601   3 , an RGB coupler  610  that is the PLC type light circuit, and a PD  602  optically connected to the RGB coupler  610 . The RGB coupler  610  includes input waveguides  611   1  to  611   3  corresponding to the first to third LDs, a main wave-combining unit  616  (a wave-combining unit  2 ) combining the three colors of R, G, and B, and an output waveguide  617  outputting combined light  503 . The RGB coupler  610  functions as a light combining circuit. 
     The RGB coupler  610  in the third embodiment includes similar components to those in the RGB coupler  510  in the second embodiment: a preliminary wave-combining unit  612 , a main wave-combining unit  616 , a dummy wave-combining unit (a monitoring wave-combining unit)  619 , a first splitting unit  613  dedicated for R, and a second splitting unit  614  for GB. The mutual connection relationship between the components is the same as that in the second embodiment, so its detailed description is omitted here. The difference from the RGB coupler in the second embodiment is the orientation of the preliminary and dummy wave-combining units  612  and  619  and the crossing positions of the waveguides. At the combiner, the stray light is often generated in the oblique direction to the emitting direction from its output port. Therefore, the output guided-wave direction (the emitting direction of the output light) of the preliminary and dummy wave-combining units  612  and  619  are tilted from the output guided-wave direction of the main wave-combining unit  616 . Although  FIG.  8    shows that the orientations of the two wave-combining units  612  and  619  are tilted, only one of them may be tilted. 
     By tilting two wave-combining units  612  and  619 , the emitting directions of the two wave-combining units may be shifted from the direction of the RGB combined light  603  (the lateral direction in  FIG.  8   ) to prevent the stray light from coupling to the RGB output light. There is also a merit that the chip size may be reduced in the direction (the lateral direction) from the light source to the emitting part in  FIG.  8   . Typically, in the RGB coupler including the PLC, the LD is optically coupled to one side of the chip and the RGB combined light  603  is emitted from the opposite side. Therefore, the chip is elongated in the direction along the RGB combined light  603 . As in  FIG.  8   , the emitting directions of the two wave-combining units may be shifted from that direction to provide a small chip size in the emitting direction of the RGB combined light  603 . 
     Fourth Embodiment 
     In the above third embodiment, the orientations of some of the wave-combining units may be tilted from the emitting direction of the RGB combined light to prevent the stray light from the wave-combining unit from coupling to the RGB combined output. However, the waveguides connecting the wave-combining units need to be bent to tilt the orientations of the wave-combining units. By bending the waveguides, light may be leaked at the bend sections of the waveguides. In such a case, the emitting position of the RGB combined light at the chip end may also be shifted while keeping the orientation of the wave-combining unit along the emitting direction of the RGB combined output. 
       FIG.  9    illustrates a configuration of a light source with a monitoring function of a fourth embodiment of this disclosure. A light source  700  in the fourth embodiment includes first to third LDs  701   1  to  701   3 , an RGB coupler  710  that is the PLC type light circuit, and a PD  702  optically connected to the RGB coupler  710 . The RGB coupler  710  includes input waveguides  711   1  to  711   3  corresponding to the first to third LDs, a main wave-combining unit  716  (a wave-combining unit  2 ) combining the three colors of R, G, and B, and an output waveguide  717  outputting combined light  703 . The RGB coupler  710  functions as a light combining circuit. 
     The difference from the RGB coupler  610  in the third embodiment shown in  FIG.  8    is only the configuration of the output waveguide  717  that emits the RGB combined light  703 . In the RGB coupler  710  in this embodiment, the termination of the output waveguide from the main wave-combining unit  716  is shifted from the extension of the output point along the chip end. In other words, the output waveguide  717  has a termination at the position  722  shifted from the extension  721  that extends from the output point of the main wave-combining unit  716  in the guided wave direction. 
     In the example in  FIG.  9   , the direction of light input/output from the main wave-combining unit  716  matches the directions of the four sides of the chip. However, even if the orientation of the main wave-combining unit that emits the RGB combined light is tilted from the four sides of the chip, the termination position of the output waveguide may be shifted from the crossing point between the extension  721  of the output point of the main wave-combining unit and the chip end. In this case, as the extension and the sides of the chip are not orthogonal, the output waveguide is bent near the termination so that the waveguide is terminated perpendicular to the chip end face. 
     The configuration of the output waveguide  717  may be directly applied to the output waveguide  417  in the first embodiment, the output waveguide  517  in the second embodiment, and the output waveguide  617  in the third embodiment. 
     Fifth Embodiment 
       FIG.  10    illustrates a configuration of a light source with a monitoring function of a fifth embodiment of this disclosure. A light source  800  in the fifth embodiment is the same as the light source  400  in the first embodiment only except that a PD  803  is mounted in a different way. Therefore, a description is given only about how to mount the PD. The light source  800  includes a flip-up mirror  804  at the emitting end of the two monitoring waveguides  418   1  and  418   2 . The flip-up mirror  804  converts the optical path of the incident light by 90 degrees. 
       FIG.  11    illustrates the cross-section structure near the emitting end of the monitoring waveguide and the PD.  FIG.  11    shows the cross-section perpendicular to the substrate surface of the PLC taken along line XI-XI in  FIG.  10   . The RGB coupler  410  includes an underclad layer  802  and a core layer that are sequentially stacked on a substrate  801 . Light circuits are formed in the core layer, including the waveguide, the splitting unit, and the wave-combining unit. Finally, an overclad layer  805  is formed to cover these formed light circuits. 
     This embodiment uses the PD  803  of a surface mount type. The flip-up mirror  805  reflects the light emitted from the monitoring waveguides  418   1  and  418   2  above the substrate into the PD  803 . The flip-up mirror  805  includes a substrate separately made of Si or the like. The substrate has an inclined surface of 45 degrees and is attached to the exit end plane of the RGB coupler  410 . Moreover, the flip-up mirror  805  may also be made by fabricating an inclined surface of 45 degrees in the middle of the monitoring waveguide by dry etching a substrate tilted at 45 degrees. 
     In the configuration of the light source in the fifth embodiment, the PD  803  may be disposed not to face the exit planes of the LDs  401   1  to  401   3 , making it difficult for the stray light to enter the PD  803  and reducing the mounting area of the light source  800 . Of course, it will be apparent that the method of mounting the PD on the light source in the fifth embodiment shown in  FIGS.  10  and  11    may apply to any of the first to fourth embodiments described above. 
     Sixth Embodiment 
       FIG.  12    illustrates a configuration of a light source with a monitoring function of a sixth embodiment of this disclosure. A light source  900  in the sixth embodiment is the same as the light source  400  in the first embodiment except for the position of a PD  902 . The light source  900  in the first embodiment includes first to third LDs  901   1  to  901   3 , an RGB coupler  910  that is the PLC type light circuit, and a PD  902  optically connected to the RGB coupler  910 . The RGB coupler  910  has a configuration in which two monitoring waveguides  918   1  and  918   2  terminate on a side perpendicular to the chip side for an output waveguide  917  of an RGB combined light  903 , as bent waveguides for optical path conversion of  90  degrees. 
     As shown in the first to fourth embodiments, disposing the PD to face the exit planes of the three LDs will allow the stray light to enter the PD, so that inaccurate monitoring values may be provided. Therefore, in the light source and RGB coupler in this embodiment, the two monitoring waveguides  918   1  and  918   2  are used as bent waveguides for optical path conversion of 90 degrees so that the LDs and PD should not face each other. With the configuration in which the emitting direction of each light from the three LDs and the emitting direction of the light from the main wave-combining unit  916  are generally perpendicular to the incident direction of the light to the PD  902 , it is possible to avoid the stray light from entering the PD  902 . 
     It will be appreciated that the configuration in which the monitoring waveguide in this embodiment is used as a bent waveguide for optical path conversion of 90 degrees may apply to any of the above first to fifth embodiments. 
     In the light source with the monitoring function in each of the above embodiments, the single PD may monitor each light of R, G, and B. As it is possible to control the deviation of the split ratio between light of each color detected by the PD to twice at the most (within 6 dB), knowing the value of the inter-wavelength variation of the split ratio in advance makes it possible to easily correct the detection value of the PD on the device side using the light source of this disclosure. As the signal value detected by the PD has a difference of at most 6 dB between the three colors, it is not necessary to even physically change (amplify/attenuate) the detected electrical signal from the PD for each color. In the RGB coupler in each of the above embodiments, the split ratio is generally set to main: monitoring=9:1 as an example. As the variation of the light level observed in the monitoring waveguide is within the deviation of 6 dB, no special specifications are required for the PD detection dynamic range. It may help reduce the cost as the RGB light source. 
     Additionally, in a video device using the light source of this disclosure, if the R light source, the G light source, and the B light source are operated independently at different times during the period not used as the actual video signal, the monitoring value may be obtained for the monochromatic light. This may associate (calibrate) a known calibrated output level from the monochromatic LD with the monitoring value by the PD correctly. For example, at the sweep time of a video signal in a peripheral area not displayed to the user as a video, only a single light source may emit light. By repeating multiple measurements during a predetermined measurement period for each scan of the video signal and at a constant number of scans, a monitoring value for each color alone may be easily obtained. Typically, the response speeds of the PD and LD are about several μsec to some ten μsec, which are much higher than the frame speed of the above video signal. Therefore, without affecting the display of the actual video, it is possible to monitor the monochromatic light of R, G, and B. The light combining circuit and light source of this disclosure make it possible to monitor LD light with very high accuracy and adjust the white balance with higher accuracy, by cooperating with an operation of controlling each LD by time division using the device with which the light combining circuit and light source are used in combination. 
     As described in detail above, the light combining circuit and light source of this disclosure may solve or reduce the problem of the wavelength dependence of the split ratio and provide a monitoring function that is easy to use in a video device. 
     INDUSTRIAL 
     The present invention may be generally used in an optical communication system. 
     REFERENCE SIGNS LIST 
       1  to  3 ,  21  to  23 ,  201   1  to  201   3 ,  301   1  to  301   3 ,  401   1  to  401   3 ,  501   1  to  501   3 ,  601   1  to  601   3 ,  701   1  to  701   3 ,  901   1  to  901   3  LD 
       13  to  15 ,  202 ,  302   1  to  302   3 ,  402 ,  502 ,  602 ,  702 ,  803 ,  902  PD 
       31  to  33 ,  101  to  103 ,  211   1  to  211   3 ,  311   1  to  311   3 ,  411   1  to  411   3 ,  511   1  to  511   3 ,  611   1  to  611   3 ,  711   1  to  711   3 ,  911   1  to  911   3  Input waveguide 
       36 ,  203 ,  303 ,  403 ,  503 ,  603 ,  703 ,  903  RGB combined light 
       106 ,  215 ,  315 ,  417 ,  517 ,  617 ,  717 ,  917  Output waveguide 
       200 ,  300 ,  400 ,  500 ,  600 ,  700 ,  800 ,  900  Light source with a monitoring function 
       210 ,  310 ,  410 ,  510 ,  610 ,  710 ,  810 ,  910  RGB coupler 
       212 ,  312   1  to  3123 ,  413 ,  414 ,  513 ,  514 ,  613 ,  614 ,  713 ,  714 ,  913 ,  914  Splitting unit 
       213 ,  317   1  to  317   3 ,  418   1 ,  418   2 ,  518   1 ,  518   2 ,  520 ,  618   1 ,  618   2 ,  620 ,  720 ,  918   1 ,  918   2  Monitoring waveguide 
       214 ,  314 ,  416 ,  516 ,  616 ,  716 ,  916  Main wave-combining unit 
       316   1  to  316   3 ,  519 ,  619 ,  719 ,  919  Dummy wave-combining unit (Monitoring wave-combining unit) 
       412 ,  512 ,  612 ,  712 ,  912  Preliminary wave-combining unit 
       804  Flip-up mirror