The present disclosure relates to an optical device and, more particularly, to a wavelength multiplexing circuit in an optical circuit.
In the field of information processing using light (for example, Non Patent Literature) and in the field of optical communications, filters and switches using waveguides have been developed. For example, in a quartz-based planar lightwave circuit (PLC), a glass film that is an undercladding is deposited on a Si substrate, and a glass film with an adjusted refractive index so as to have a desired refractive index difference (Δ) is deposited on the glass film. The glass film is patterned by photolithography and reactive ion etching to produce a core. Finally, the periphery is embedded with a glass film (overcladding) having a lower refractive index than the core to form a waveguide. PLC is characterized by having a high transmittance in a range from visible to infrared, and various functions are achieved with a low loss by combining a plurality of basic optical circuits (for example, directional couplers, Mach-Zehnder interferometers, and the like). In recent years, research and development that utilize PLC not only in optical communication but also in the visible light field by taking advantage of the feature that the PLC is transparent (low propagation losses) even in visible light is attracting attention. For example, a plurality of RGB couplers that multiplex red (R), green (G), and blue (B), which are three primary colors of light, are reported, and the development in the field of video has been studied.
By using a polymer waveguide rather than a quartz-based waveguide, the cost reduction of the waveguide-type RGB coupler can be expected. The polymer waveguide is produced by spin coating and patterning by using the cladding polymer and core polymer having a refractive index difference adjusted. Examples of a patterning technique that is promising for lower costs include a direct exposure method and a light nanoimprint method. Because the spin-coated core polymer is directly patterned, these methods can simplify the producing process, without dry etching and the like. On the other hand, because patterning is performed using a reaction caused by absorption of UV light, there is a problem that the loss of light on the short wavelength side such as blue is great, and when broadband wavelength is handled as an RGB coupler, the transmittance is biased by the wavelength (color). Actually, for an embedded polymer waveguide, which is made by the present inventors on trial, with SU-8 material as a core and adjusted to have a refractive index difference (Δ) of 0.8%, propagation losses are 0.8 to 4.4 dB/cm for light with wavelength 465 to 638 nm.
[Non Patent Literature 1] A. Nakao, et al., “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 (2014) 45-48
When an RGB coupler is produced using a polymer waveguide, the propagation losses differ depending on the wavelength (color), and therefore, even when the transmittance of the multiplexing portion is approximately equivalent, there is a problem that the output is biased.
A circuit for transmittance adjustment is formed in each of a green waveguide and a red waveguide, for example, based on a blue waveguide having maximum propagation losses.
An optical circuit of the present disclosure for solving the above problems includes a semiconductor substrate, a multiplexing circuit on the semiconductor substrate, a first waveguide including a polymer, which is connected, on the semiconductor substrate, to the multiplexing circuit and propagates red light, a second waveguide including the polymer, which is connected, on the semiconductor substrate, to the multiplexing circuit and propagates green light, a third waveguide including the polymer, which is connected, on the semiconductor substrate, to the multiplexing circuit and propagates blue light, and an output waveguide connected, on the semiconductor substrate, to the multiplexing circuit and located opposite to the first waveguide, the second waveguide, and the third waveguide, in which each of the first waveguide and the second waveguide is provided with a loss portion (an adjustment circuit of transmitted light) that causes an excessive loss so that the power of each of the first waveguide and the second waveguide becomes the same as the output power of the third waveguide.
According to the present disclosure, a polymer waveguide type RGB coupler having different propagation losses depending on the wavelength (color) has an effect that the output can be balanced.
Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that, in the drawings, components with the same function are denoted with the same reference signs for the sake of clear description. However, it is obvious to those skilled in the art that the present disclosure is not limited to the description of the embodiments described below, and the mode and the detail thereof can be modified in various ways without departing from the spirit of the disclosure in this specification and the like. Further, configurations according to different embodiments can be implemented appropriately in combination.
A method of producing a waveguide of the present embodiment will be described briefly. A cross-sectional structure of the waveguide is illustrated in
Assuming that propagation losses for wavelengths of red light (R), green light (G), and blue light (B) respectively emitted from the first light source 107, the second light source 108, and the third light source 109 are Rloss (dB/cm), Gloss (dB/cm), and Bloss (dB/cm), respectively, the transmittances of the multiplexing circuit 110 for wavelengths of red light (R), green light (G), and blue light (B) respectively emitted from the first light source 107, the second light source 108, and the third light source 109 are Rcouple (dB), Gcouple (dB), Bcouple (dB), respectively, and the path lengths for wavelengths of red light (R), green light (G), and blue light (B) respectively emitted from the first light source 107, the second light source 108, and the third light source 109 are LR (cm), LG (cm), and LB (cm), respectively, the total transmittances Rtrans, Gtrans, and Btrans of wavelengths of the RGB coupler are calculated as follows.
R
trans
: R
couple
−R
loss
×L
R
G
trans
: G
couple
−G
loss
×L
G
B
trans
: B
couple
−B
loss
×L
B
When the transmittances of wavelengths RGB in the multiplexing circuit is made equal (Rcouple=Gcouple=Bcouple), because Rloss<Gloss<Bloss, the output varies depending on the color. In the present embodiment, as illustrated in
This results in RGB light with no output variation from the output waveguide 111. In the present embodiment, by increasing the path for R and G, the light of color input from each of the first waveguide 103, the second waveguide 104, and the third waveguide 105 can be adjusted to have the same output power from the output waveguide 111.
In the present embodiment, by adjusting the wave multiplexing efficiency of the multiplexing circuit, RGB output variation is eliminated. As an example, an adjustment method by using a mode coupler in a multiplexing circuit will be described. The mode coupler is configured as illustrated in
This configuration not only achieves RGB light with no output variation, but also eliminates the need for extra circuits and allows the elements to be miniaturized.
The present disclosure relates to an optical device, and more particularly, can be applied to a wavelength multiplexing circuit in an optical circuit.
101 Semiconductor substrate
102 SiO2 film
103 First waveguide
103a Adjustment circuit
104 Second waveguide
104a Adjustment circuit
105 Third waveguide
106 Cladding polymer
107 First light source
108 Second light source
109 Third light source
110 Multiplexing circuit
111 Output waveguide
Number | Date | Country | Kind |
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2019-013035 | Jan 2019 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2020/002450 | 1/24/2020 | WO | 00 |