Optical multiplexing circuit and light source

Abstract

To reduce the size while being able to accurately monitor light of a plurality of wavelengths. 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.

Claims

1. An optical multiplexing circuit comprising: 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, wherein the plurality of input waveguides are single mode embedded waveguides in which a core is embedded in a clad on a substrate, and wherein the plurality of branching units are embedded waveguides having two cores that are disposed in close proximity to each other while embedded in the clad on the substrate.

2. The optical multiplexing circuit according to claim 1, wherein the rib-shaped waveguide is a multi-mode waveguide.

3. The optical multiplexing circuit according to claim 2, wherein the at least one monitoring waveguide is configured such that an optical axis of the plurality of input waveguides and an emission direction of light from the multiplexing unit is generally perpendicular to an optical axis of the at least one monitoring waveguide.

4. A light source with a monitoring function, comprising: the optical multiplexing circuit according to claim 2; 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.

5. The optical multiplexing circuit according to claim 1, wherein the at least one monitoring waveguide is configured such that an optical axis of the plurality of input waveguides and an emission direction of light from the multiplexing unit is generally perpendicular to an optical axis of the at least one monitoring waveguide.

6. A light source with a monitoring function, comprising: the optical multiplexing circuit according to claim 3; 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.

7. A light source with a monitoring function, comprising: the optical multiplexing circuit according to claim 1; 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.

8. The light source with a monitoring function according to claim 7, wherein the plurality of laser diodes are three laser diodes that output light of three primary colors of red light (R), green light (G), and blue light (B).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram illustrating a typical light source of a projector using LDs.

(2) FIG. 2 is a diagram illustrating a basic structure of an RGB coupler using a PLC.

(3) FIG. 3 is a diagram illustrating a configuration of an RGB coupler using two directional couplers.

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

(5) FIG. 5 is a diagram illustrating a light source with a monitoring function according to a second embodiment of the present invention.

(6) FIGS. 6(a)-6(d) diagrams illustrating a monitoring waveguide according to a third embodiment of the present invention.

(7) FIGS. 7(a)-7(d) are diagrams illustrating a method of preparing the monitoring waveguide according to the third embodiment of the present invention.

(8) FIGS. 8(a)-8(d) are diagrams illustrating a modified example of the monitoring waveguide according to the third embodiment.

DESCRIPTION OF EMBODIMENTS

(9) 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

(10) 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.sub.1 to 201.sub.3 that respectively output light of respective colors of R, G, and B, a PLC-type RGB coupler 210, and first to third PDs 202.sub.1 to 202.sub.3 optically connected to the RGB coupler 210.

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

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

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

(14) As illustrated in FIG. 4, when light propagating through the first to third input waveguides 211.sub.1 to 211.sub.3 is divided by the first to third branching units 212.sub.1 to 212.sub.3, respectively, a coupling characteristic between the first to third LDs 201.sub.1 to 201.sub.3 and the first to third input waveguides 211.sub.1 to 211.sub.3 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.sub.1 to 202.sub.3 by recognizing a multiplexing characteristic of the multiplexing unit 214 in advance.

Second Embodiment

(15) 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.sub.1 to 202.sub.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.

(16) Thus, in the second embodiment, the first to third monitoring waveguides 313.sub.1 to 313.sub.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.

(17) The PLC-type RGB coupler 310 includes first to third input waveguides 311.sub.1 to 311.sub.3, first to third branching units 312.sub.1 to 312.sub.3, a multiplexing unit 314, first to third monitoring waveguides 313.sub.1 to 313.sub.3, and output waveguides 315. The first to third input waveguides 311.sub.1 to 311.sub.3 are optically connected to the first to third LDs 301.sub.1 to 301.sub.3. The first to third branching units 312.sub.1 to 312.sub.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.sub.1 to 312.sub.3. The other beam of the light divided by the first to third branching units 312.sub.1 to 312.sub.3 propagates through the first to third monitoring waveguides 313.sub.1 to 313.sub.3 and is output to the first to third PDs 302.sub.1 to 302.sub.3. The light multiplexed by the multiplexing unit 214 propagates through the output waveguide 315 to output.

Third Embodiment

(18) 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.sub.1 to 313.sub.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.sub.1 to 312.sub.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.

(19) 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.

(20) 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.

(21) As illustrated in FIG. 6(b), the input waveguide 311 is a single mode embedded waveguide in which a core 403a 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 403b and 403c are disposed in close proximity.

(22) The output of the branching unit 312 connected to the monitoring waveguide 313 is connected to a bent waveguide portion 313a with the waveguide width being widened in a tapered manner. A cross section of the bent waveguide portion 313a of the monitoring waveguide 313 is illustrated in FIG. 6(d). The bent waveguide portion 313a is a multi-mode rib-shaped waveguide in which the width of the core 403d is enlarged than the core 403a.

(23) 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 313a. 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.

(24) FIG. 7 illustrates a method of preparing the monitoring waveguide according to the third embodiment of the present invention. A bottom clad layer 402a 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 313a, a large core layer is left in consideration of the accuracy of the subsequent photolithography. Here, the periphery of the bent waveguide portion 313a is left in a rectangular shape.

(25) An upper clad layer 402b is deposited so as to cover the bottom clad layer 402a 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 313a is etched to form a rib-shaped waveguide (FIG. 7(d)).

(26) 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.

(27) 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 313a 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

(28) For the monitoring waveguide 313, the bent waveguide portion 313a may be applied to all of the first to third monitoring waveguides 313.sub.1 to 313.sub.3 of the respective colors of R, G, and B in the RGB coupler 310 in FIG. 5. Meanwhile, the bent waveguide portion 313a may be applied to only the first monitoring waveguide 313.sub.1 for which a small bend radius is required or only the first and second monitoring waveguides 313.sub.1 and 313.sub.2.

REFERENCE SIGNS LIST

(29) 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