Optical Multiplexing Circuit and Light Source

Abstract

An optical multiplexing circuit includes a plurality of branching units configured to each divide light output from a corresponding one of a plurality of input waveguides, a multiplexing unit configured to multiplex a plurality of first beams of the light, each obtained by dividing the light by a corresponding one of the plurality of branching units, an output waveguide configured to output the light multiplexed by the multiplexing unit, a plurality of monitoring filters configured to individually input, via a first monitoring waveguide, a corresponding one of a plurality of second beams of the light, a wavelength through each of the plurality of monitoring filters having a transmittance of 50% being set to be a center wavelength of the plurality of second beams of the light, and a change in wavelength due to an assumed change in temperature being set to be less than half of an FSR, and a plurality of second monitoring waveguides.

Claims

1. An optical multiplexing circuit, comprising: a plurality of branching units configured to each divide light output from a corresponding one of a plurality of input waveguides; a multiplexing unit configured to multiplex a plurality of first beams of the light, each obtained by dividing the light by a corresponding one of the plurality of branching units; an output waveguide configured to output the light multiplexed by the multiplexing unit; a plurality of monitoring filters configured to individually input, via a first monitoring waveguide, a corresponding one of a plurality of second beams of the light, each obtained by dividing the light by a corresponding one of the plurality of branching units, a wavelength through each of the plurality of monitoring filters having a transmittance of 50% being set to be a center wavelength of the plurality of second beams of the light, and a change in wavelength due to an assumed change in temperature being set to be less than half of an FSR; and a plurality of second monitoring waveguides that each output an output of a corresponding one of the plurality of monitoring multiplexing units.

2. The optical multiplexing circuit according to claim 1, wherein the plurality of monitoring filters are each a directional coupler.

3. The optical multiplexing circuit according to claim 1, wherein the plurality of monitoring filters are each a Mach-Zehnder interferometer.

4. A light source with a monitoring operation, 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 second monitoring waveguides.

5. The light source with a monitoring operation according to claim 4, 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).

6. A light source with a monitoring operation, 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 second monitoring waveguides.

7. A light source with a monitoring operation, 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 second monitoring waveguides.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a diagram illustrating a typical light source of a projector using an LD.

[0020] FIG. 2 is a diagram illustrating a basic structure of an RGB coupler using a PLC.

[0021] FIG. 3 is a diagram illustrating a configuration of an RGB coupler using two directional couplers.

[0022] FIG. 4 is a diagram illustrating a light source with a monitoring function according to a first embodiment of the present invention.

[0023] FIG. 5 is a diagram illustrating a light source with a monitoring function according to a second embodiment of the present invention.

[0024] FIG. 6 is a diagram illustrating a dependence of the transmittance of a directional coupler on wavelength.

DESCRIPTION OF EMBODIMENTS

[0025] 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.

First Embodiment

[0026] 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 200 with a monitoring function includes first to third LDs 201.sub.1 to 201.sub.3 that respectively output light of respective colors of G, B, and R, 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.

[0027] An output of the RGB coupler 210 is taken out of a window 203 provided in a housing, and, for example, when the output is applied to a projector, a MEMS mirror is irradiated with the output.

[0028] Furthermore, the light source 200 with a monitoring function includes a thermistor 204. Since an oscillation wavelength of each of the LDs 201 fluctuates due to a change in temperature, feedback control is performed on the LDs 201 in accordance with the change in temperature.

[0029] 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 a plurality of first beams of the light, each obtained by dividing the light by a corresponding one 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 a plurality of second beams of the light, each obtained by dividing the light by a corresponding one of the first to third branching units 212.sub.1 to 212.sub.3, to corresponding ones of 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.

[0030] 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. A plurality of first beams of the divided light are 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, respectively, and a plurality of second beams of the divided light are multiplexed by the multiplexing unit 214 and output to the output waveguide 215.

[0031] 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.

[0032] 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

Configuration of Light Source

[0033] In the first embodiment, the thermistor 204 is disposed near the LDs 201 inside a package of the light source 200 with a monitoring function. However, in terms of heat radiation, the LDs 201 are mounted on the package via a mounting having excellent heat conductivity. Therefore, even when the thermistor 204 is disposed near the LDs 201, a temperature of each of the LDs 201 itself is not accurately measured. Further, it is common to perform a measurement with one thermistor without disposing a thermistor on each individual LD 201 due to mounting restrictions of the package. Therefore, a distance between each individual LD 201 and the thermistor 204 is also different, and a temperature of each individual LD 201 cannot be accurately measured.

[0034] Thus, in the second embodiment, a configuration is adopted where feedback control can be performed on the LD 201 by accurately monitoring a change in wavelength due to a change in temperature.

[0035] FIG. 5 illustrates a light source with a monitoring function according to the second embodiment of the present invention. A light source 300 with a monitoring function includes first to third LDs 301.sub.1 to 301.sub.3 that respectively output light of respective colors G, B, and R, a PLC-type RGB coupler 310, and first to third PDs 302.sub.1 to 302.sub.3 optically connected to the RGB coupler 310. An output of the RGB coupler 310 is taken out of a window 303 provided in a housing, and, for example, when the output is applied to a projector, a MEMS mirror is irradiated with the output. The PLC-type RGB coupler 310 includes first to third input waveguides 311.sub.1 to 311.sub.3 optically connected to the first to third LDs 301.sub.1 to 301.sub.3, first to third branching units 312.sub.1 to 312.sub.3 that divide light propagating through the waveguide into two, a multiplexing unit 314 that multiplexes a plurality of first beams of the light, each obtained by dividing the light by a corresponding one of the first to third branching units 312.sub.1 to 312.sub.3, and an output waveguide 315 that outputs the light multiplexed by the multiplexing unit 214.

[0036] Furthermore, the RGB coupler 310 includes first to third monitoring waveguides 313.sub.1 to 313.sub.3 that output a plurality of second beams of the light, each obtained by dividing the light by a corresponding one of the first to third branching units 312.sub.1 to 312.sub.3, to corresponding ones of first to third monitoring filters 316.sub.1 to 316.sub.3, and first to third monitoring waveguides 317.sub.1 to 317.sub.3 that output an output of the first to third monitoring multiplexing units 316.sub.1 to 3163 to the first to third PDs 302.sub.1 to 302.sub.3.

[0037] The first to third monitoring filters 316.sub.1 to 316.sub.3 can measure, as a change in light intensity of the light received by the PDs 302, a change in the oscillation wavelength of the LDs 201 due to a change in temperature. Therefore, the monitoring filters 31 may be circuits having dependence on wavelength to the extent to which a change in the oscillation wavelength of the LDs 201 can be measured as a change in light intensity, and measurement is easy when the circuit has a strong dependence on wavelength. A change in temperature of the LDs 201 is estimated from the change in light intensity, and feedback control is performed on the LDs 201.

[0038] With such a configuration, a change in the temperature of the LDs 301 of the respective colors of R, G, and B can be accurately monitored without using a thermistor. As a result, color control can be performed with high accuracy, and white balance as a light source can also be adjusted with high accuracy.

[0039] Monitoring Filter

[0040] A specific example of a case where a directional coupler is applied as the monitoring filters 316 having dependence on wavelength will be described.

[0041] FIG. 6 illustrates a dependence of the transmittance of the directional coupler on wavelength. The first monitoring filter 316.sub.1 for monitoring green light G (wavelength λ2=520 nm) is illustrated. In the directional coupler illustrated in FIG. 6(b), a core thickness H of a waveguide=1.75 μm, a specific refractive index difference Δ between waveguides=1.0%, a length L of the directional coupler=222 μm, a width W of the waveguide=1.5 μm, and a gap G between the waveguides=1.0 μm, and FIG. 6(a) illustrates transmittance when a crossport is an output with respect to an input port. When the first LD 3011 has an oscillation wavelength of 520 nm, transmittance is designed to be 49.6%.

[0042] In general, the wavelength of the semiconductor LD of visible light changes by approximately 3 nm with respect to a change in temperature of approximately 50 degrees. For example, when it is assumed that the oscillation wavelength changes by ±3 nm with respect to a change in temperature of 50 degrees, the output of the directional coupler illustrated in FIG. 6(b) has light intensity changed by approximately ±1.5% with respect to the change in the oscillation wavelength of ±3 nm. Therefore, sufficient measurement can be performed by a monitoring function using a conventional PD.

[0043] When a directional coupler is used, it is preferable to set the center wavelength of each of the LDs 301 at a point of transmittance of 50% at which a fluctuation of power is the greatest with respect to a change in wavelength. Further, it is necessary to set an FSR so as to monotonically decrease or monotonically increase in a range of an assumed change in wavelength. In other words, it is preferable to set a change in wavelength due to an assumed change in temperature so as to be less than half of the FSR.

[0044] Further, a monitoring filter to which a Mach-Zehnder (MZ) interferometer is applied can also be used instead of the directional coupler. In an asymmetric MZ, an output on a crossport side with respect to an input port is set to cos 2 (πn(λ) ΔL/λ) with respect to a path length difference ΔL of two arm waveguides.

[0045] Here, λ is a center wavelength of each of the LDs 301, and n(λ) is a refractive index. A change in oscillation wavelength and, furthermore, a change in temperature can be indirectly determined by adjusting AL such that a necessary fluctuation of light intensity can be obtained for a desired wavelength range of the asymmetric MZ.