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.
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.
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
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).
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 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.
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, 2, and 3, respectively.
However, the application of such an RGB coupler to configure a light source including a monitoring function for an adjustment of white balance has not been studied from the viewpoint of size reduction of the light source and accuracy of monitoring.
An object of the present invention is to provide an optical multiplexing circuit capable of accurately monitoring light of a plurality of wavelengths by measuring a change in oscillation wavelength of a laser diode due to a change in temperature, 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 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 configured to each output an output of a corresponding one of the plurality of monitoring multiplexing units.
According to the present invention, since a change in oscillation wavelength of a laser diode due to a change in temperature can be measured as a change in light intensity of light received by a photodiode via a monitoring filter, it is possible to accurately monitor light of a plurality of wavelengths.
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 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.
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.
The PLC-type RGB coupler 210 includes first to third input waveguides 2111 to 2113 optically connected to the first to third LDs 2011 to 2013, first to third branching units 2121 to 2123 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 2121 to 2123, first to third monitoring waveguides 2131 to 2133 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 2121 to 2123, to corresponding ones of the first to third PDs 2021 to 2023, 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 2111 to 2113 is divided into two by each of the first to third branching units 2121 to 2123. A plurality of first beams of the divided light are output to the first to third PDs 2021 to 2023 via the first to third monitoring waveguides 2131 to 2133, 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.
An optical multiplexing circuit using the directional coupler illustrated in
As illustrated in
Configuration of Light Source
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.
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.
Furthermore, the RGB coupler 310 includes first to third monitoring waveguides 3131 to 3133 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 3121 to 3123, to corresponding ones of first to third monitoring filters 3161 to 3163, and first to third monitoring waveguides 3171 to 3173 that output an output of the first to third monitoring multiplexing units 3161 to 3163 to the first to third PDs 3021 to 3023.
The first to third monitoring filters 3161 to 3163 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.
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.
Monitoring Filter
A specific example of a case where a directional coupler is applied as the monitoring filters 316 having dependence on wavelength will be described.
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
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.
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.
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 ΔL such that a necessary fluctuation of light intensity can be obtained for a desired wavelength range of the asymmetric MZ.
Filing Document | Filing Date | Country | Kind |
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PCT/JP2019/016361 | 4/16/2019 | WO |
Publishing Document | Publishing Date | Country | Kind |
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WO2020/213064 | 10/22/2020 | WO | A |
Number | Name | Date | Kind |
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20170146744 | Katsuyama | May 2017 | A1 |
20170357054 | Sugiyama | Dec 2017 | A1 |
20180128979 | Heanue | May 2018 | A1 |
20210152794 | Yamada | May 2021 | A1 |
20220229235 | Sakamoto | Jul 2022 | A1 |
Number | Date | Country |
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2005-70365 | Mar 2005 | JP |
WO-2020240798 | Dec 2020 | WO |
Number | Date | Country | |
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20220196912 A1 | Jun 2022 | US |