Optical multiplexer / demultiplexer and RGB coupler

Abstract

In the present invention, refractive index fluctuation due to short wavelength and high power in a quartz-based planar lightwave circuit is suppressed. Provided is an optical multiplexing circuit for multiplexing light having different wavelengths, the optical multiplexing circuit comprising an asymmetric Mach-Zehnder interferometer (MZ) which is constituted from two waveguides and in which two arms having different lengths are formed between two coupler portions, the optical multiplexing circuit being characterized in that a second waveguide core width of an arm in which light on a short wavelength side propagates is larger than a first waveguide core width of the waveguides constituting the asymmetric MZ.

Description

Technical Field

[0001] The present invention relates to an optical multiplexing / demultiplexing circuit and an RGB coupler, and more particularly, to an optical multiplexing / demultiplexing circuit using an asymmetric Mach-Zehnder interferometer and an RGB coupler including the same.

Background Art

[0002] On a substrate such as Si, a silica-based planar optical waveguide (PLC) in which a core having a high refractive index and a cladding having a low refractive index are formed using a glass film formation technique and a semiconductor microfabrication technique is known. Many optical devices such as optical splitters, wavelength multiplexers / demultiplexers, and optical switches using PLCs have been put into practical use. In recent years, PLCs have been studied for application in the visible wavelength range by taking advantage of their transparency not only to light with a wavelength of 1.55 μm used in optical communication but also to visible light. For example, an RGB coupler that multiplexes red (R), green (G), and blue (B), which are the primary colors of light (see, for example, Patent Document 1). The wavelengths of RGB are generally around R = 638 nm, G = 520 nm, and B = 450 nm.

[0003] An RGB coupler is an optical circuit that multiplexes a plurality of lights input from each input port and outputs them by multiplexing them into a single waveguide via a directional coupler and a mode coupler. Laser diode (LD) bare chips corresponding to each color are integrated at each input port, and their application to smart glass and the like as an ultra-small RGB light source is being studied. A specific optical multiplexing / demultiplexing circuit for multiplexing and demultiplexing light is configured by combining a directional coupler, a mode coupler, an asymmetric Mach-Zehnder interferometer (MZ), and a multimode interferometer (MMI).

[0004] FIG. 1 shows the configuration of a conventional RGB coupler. The RGB coupler 10 includes a B+G multiplexing circuit composed of an asymmetric MZ formed from waveguides 11 and 12, and a BG+R multiplexing circuit composed of a mode coupler formed from waveguides 11, 13, and MMI 14. Although the PLC is generally transparent to the visible wavelength range, since it confines light in a very small region of several microns, the energy density in the waveguide is very high. In particular, for light with high-energy wavelengths such as violet and blue, characteristic variations in the core have been confirmed (see, for example, Non-Patent Document 1).

[0005] The characteristic variations in the core are considered to be caused by the formation of color centers due to two-photon absorption in dopants (such as GeO2 and HfO2) for refractive index adjustment, and become more prominent as the wavelength is shorter and the power is higher. Also, the characteristic variations start from a change in the refractive index (the refractive index increases), and when this change becomes large, it is observed as loss (Kramers-Kronig relation). Therefore, in a circuit that uses light interference, the transmittance varies due to a change in the interference state caused by the refractive index change.

[0006] FIG. 2 shows the configuration of a conventional B+G multiplexing circuit composed of an asymmetric MZ. The asymmetric MZ is formed from waveguides 11 and 12, and two arms with different lengths are formed between coupler portions 15 and 16. The asymmetric MZ realizes optical multiplexing and demultiplexing by using the interference of light controlled by the coupling ratio of the coupler portion and the optical path length difference between the two arms. Here, for the coupler portions 15 and 16 of the asymmetric MZ, the waveguide width is 1.75 μm, the gap is 1.5 μm, the core thickness is 2.0 μm, and the relative refractive index difference Δ is 1% so that almost no blue light is coupled. In the case of the asymmetric MZ, when strong blue light propagates from Port2 to waveguide 11, the refractive index of one side arm 17 composed of waveguide 11 increases, changing the optical path length difference. As a result, as the FSR of the asymmetric MZ changes, the positions of the peaks and valleys in the spectrum shift. In particular, a B+G multiplexing circuit with a small wavelength interval is sensitive to refractive index variations and often becomes a bottleneck for characteristic variations.

[0007] FIG. 3 shows the transmittance of a conventional B+G multiplexer when blue light is transmitted through it. Blue light was incident from Port2 and adjusted so that the output of Port4 became 30 mW. When blue light is input to Port2, the optical path length difference of one arm 17 changes. For example, when green light is input, the peak position of the output spectrum shifts to the short wavelength side in a linear relationship with the light transmission time. Therefore, if a PLC continues to be used under short wavelength and high power conditions, due to the characteristic fluctuations of the core, it will pose a major problem for application to optical functional circuits. Although the asymmetric MZ has been described, it goes without saying that if the refractive index changes in a directional coupler or a mode coupler, the optimal coupling condition cannot be obtained and losses will occur.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Non-Patent Documents

[0009]

Non-Patent Document 1

Summary of the Invention

[0010] An object of the present invention is to provide an optical multiplexer / demultiplexer circuit and an RGB coupler that can suppress refractive index fluctuations due to short wavelengths and high powers in a PLC.

[0011] In order to achieve such an object, one embodiment of the present invention is an optical multiplexer / demultiplexer that is composed of two waveguides and includes an asymmetric Mach-Zehnder interferometer (MZ) in which two arms with different lengths are formed between two coupler portions, and that multiplexes and demultiplexes lights with different wavelengths. The second waveguide core width of the arm through which light having a short wavelength propagates is thicker than the first waveguide core width of the waveguide constituting the asymmetric MZ. Most of the light of the arm through which Of the waveguide light propagates is Other than the waveguides of the two arms characterized by being thicker than the first waveguide core width of the waveguide constituting the asymmetric MZ. BRIEF DESCRIPTION OF THE DRAWINGS

[0012]

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Figure 10

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0014] Figure 4 shows the transmittance when blue light and green light are transmitted through a conventional B+G multiplexing circuit. For comparison with Example 1 below, it is the calculation result by the three-dimensional beam propagation method when blue light is input to Port2 and green light is input to Port1. In the B+G multiplexing circuit composed of the conventional asymmetric MZ shown in Figure 2, the waveguide widths of waveguides 11 and 12 are 1.5 μm, the gaps of coupler parts 15 and 16 are 1.2 μm, the coupler length L is 300 μm, the core thickness is 2.0 μm, and the relative refractive index difference Δ is 1.0%. It represents the transmittance of the blue light output to Port4 and the green light output to Port4, indicating that it operates as a B+G multiplexing circuit.

[0015] As described above, since the refractive index of arm 17 on the side where blue light propagates increases and the optical path length difference changes, the transmittance fluctuates. As a result, as shown in Figure 3, the peak position of the output spectrum of green light shifts to the short-wavelength side in a linear relationship with the light transmission time.

Example

[0016] Figure 5 shows the configuration of the B+G multiplexing circuit according to Example 1 of the present invention. The B+G multiplexing circuit is an asymmetric MZ composed of two waveguides 21 and 22, and has two arms with different lengths between two coupler parts 25 and 26. The B+G multiplexing circuit is a PLC composed of a lower cladding layer provided on an Si substrate, a core layer having a higher refractive index than the lower cladding layer, and an upper cladding layer provided on the core layer. The core layer contains a dopant for refractive index adjustment. Waveguides 21 and 22 include waveguide cores formed in a desired pattern, and the upper cladding layer is provided so as to surround the waveguide cores. The waveguide widths of waveguides 21 and 22 are 1.25 μm, and the dimensions of coupler parts 25 and 26 are the same as those in the above conventional example.

[0017] In Example 1, green light is input from Port1 into waveguide 22, and blue light is input from Port2 into waveguide 21, and the combined blue and green light is output from Port4. The difference from the conventional example is that the waveguide core width of the arm 27 on the side where the blue light with a shorter wavelength propagates is made wider than the waveguide core width of waveguide 21. The shape of the one-sided arm 27 has a waveguide width conversion section 28a which is a tapered waveguide that gradually widens the waveguide width of the coupler section 25, a waveguide width expansion section 28c with a predetermined thickness, and a waveguide width conversion section 28b which is a tapered waveguide that gradually narrows to the waveguide width of the coupler section 26.

[0018] Fig. 6 shows the relationship between the waveguide width of the arm and the refractive index fluctuation in the B+G multiplexing circuit of Example 1. The horizontal axis is the waveguide width nm of the waveguide width expansion section 28c of the one-sided arm 27, and the vertical axis is the shift amount of the peak position of the output spectrum with respect to the light passing time, in nm / h, as shown in Fig. 3. It can be seen that when the waveguide width is expanded to 5.0μm with respect to the shift amount of the waveguide width of 1.25μm, the shift amount is suppressed to about 1 / 10.

[0019] According to Example 1, by making the waveguide core width of the arm through which the light on the shorter wavelength side of the asymmetric MZ propagates wider than the waveguide core width of the waveguide constituting the asymmetric MZ, the energy density of the light can be reduced, and the change in the optical path length difference due to the refractive index fluctuation can be suppressed.

[0020] As described above, the characteristic fluctuation of the core is considered to be caused by the formation of color centers due to two-photon absorption in the dopant for refractive index adjustment. Therefore, in Example 1, ZrO2 is added to the core layer as the dopant with the least characteristic fluctuation among the oxides whose refractive index increases by addition. According to the configuration of the waveguide of Example 1, even when using conventional dopants (such as GeO2, HfO2, etc.), the change in the optical path length difference due to the refractive index fluctuation can be suppressed, but it is more preferable to apply ZrO2.

[0021] In Example 1, the waveguide width conversion sections 28a and 28b and the waveguide width expansion section 28c are included in one of the arms 27. Among these, the waveguide width conversion section 28a can be provided in the waveguide 21 on the input side of the coupler section 25, and the waveguide width conversion section 28b can be provided in the waveguide 21 on the output side of the coupler section 26. Also in the two coupler sections, the waveguide core width of the waveguide 21 that becomes the arm through which the light on the short wavelength side propagates is made wider to be equal to the waveguide core width of the waveguide width expansion section 28c. Thereby, characteristic fluctuations are suppressed also in the coupler section, and the same effect is obtained also with respect to fluctuations in the optical path length. However, the coupling length of the coupler section becomes long, and the multiplexing circuit becomes large in the direction of the optical axis.

Example

[0022] Fig. 7 shows the configuration of the B+G multiplexing circuit according to Example 2 of the present invention. The B+G multiplexing circuit is an asymmetric MZ composed of two waveguides 31 and 32, and has two arms with different lengths between two coupler sections 35 and 36. The difference from Example 1 is that not only the waveguide core width (second waveguide core width) of the arm 37 through which blue light propagates on one side, but also the waveguide core width (third waveguide core width) of the other arm 39 is made wider than the waveguide core width (first waveguide core width) of the waveguide constituting the asymmetric MZ. That is, the shape of the other arm 39 has a waveguide width conversion section 40a that gradually widens the waveguide width of the coupler section 35, a waveguide width expansion section 40c having a predetermined thickness, and a waveguide width conversion section 40b that gradually narrows to the waveguide width of the coupler section 36.

[0023] In the configuration of Example 1, since blue light passes through the path from Port 2 to Port 4 substantially, only the waveguide core width of the arm 37 on one side was made wider. However, as described above, since the asymmetric MZ utilizes the interference of light, blue light also passes through the other arm. Therefore, the waveguide core width of the other arm 39 is also made wider to reduce the energy density of the light and suppress the change in the optical path length difference due to the refractive index fluctuation.

[0024] As shown in Fig. 6, since there is a correlation between the waveguide width and the shift amount, the waveguide core width of each arm is adjusted so that the change amount of the optical path length of each arm balances with respect to the amount of light passing through the two arms 37 and 39. In the configuration of the B+G multiplexing circuit of the second embodiment, the relationship is that the first waveguide core width < the third waveguide core width < the second waveguide core width. Note that the magnitude relationship of the waveguide core widths varies according to the wavelengths of the two lights to be multiplexed and the interference state of the lights. According to the second embodiment, a structure more resistant to blue light can be achieved.

Embodiment

[0025] Fig. 8 shows the configuration of the B+G multiplexing circuit according to the third embodiment of the present invention. The B+G multiplexing circuit is an asymmetric MZ composed of two waveguides 51 and 52, and has two arms with different lengths between the two coupler portions 55 and 56. The fact that not only the waveguide core width of the arm 57 on the side where blue light propagates but also the waveguide core width of the other arm 59 is thick is the same as in the second embodiment, but the point that the waveguide width expansion portion 60c of the other arm 59 is provided in the straight portion is different. That is, the waveguide width expansion portions of both arms are provided in the straight portions of their respective arms.

[0026] As shown in Fig. 7 of the second embodiment, thickening the bent portion of the arm leads to the excitation of higher-order modes. Therefore, the waveguide width expansion portion 60c having a predetermined thickness is provided in the straight portion of the arm. Note that it is also desirable to form the waveguide width conversion portions 60a and 60b that connect between the waveguide width expansion portion 60c and the waveguides constituting the asymmetric MZ in the straight portions of the arms.

Embodiment

[0027] Fig. 9 shows the configuration of the RGB coupler according to the fourth embodiment of the present invention. It is an RGB coupler in which an R coupler is added to the B+G multiplexing circuit shown in the first to third embodiments. The RGB coupler 70 includes a B+G multiplexing circuit composed of an asymmetric MZ composed of waveguides 71 and 72, and a BG+R multiplexing circuit composed of a mode coupler composed of waveguides 71, 73, and MMI 74.

[0028] The waveguides 71 to 73 are single-mode waveguides. The multiplexing in the B+G multiplexing circuit is the same as in the first to third embodiments, and the multiplexing of the BG+R multiplexing circuit will be described. The red light incident from the waveguide 73 is converted from the guided mode to a higher-order mode (for example, the first-order mode) at the first coupling section 81 and then transferred to the MMI 74. The red light transferred to the MMI 74 is further converted from the guided mode to the fundamental mode (the zero-order mode) at the second coupling section 82 and then transferred to the waveguide 71. As a result, light in which the three wavelengths of RGB are multiplexed is output from the output end of the waveguide 71.

Embodiment

[0029] FIG. 10 shows the configuration of an RGB coupler according to Embodiment 5 of the present invention. It is an RGB coupler in which an R coupler is added to the B+G multiplexing circuit shown in the first to third embodiments. The RGB coupler 90 includes a B+G multiplexing circuit composed of an asymmetric MZ composed of the waveguides 91 and 92, and a BG+R multiplexing circuit composed of a directional coupler composed of the waveguides 91 and 93.

[0030] The multiplexing in the B+G multiplexing circuit is the same as in the first to third embodiments, and the multiplexing of the BG+R multiplexing circuit will be described. The waveguide 91 of the BG+R multiplexing circuit includes first to third portions 101a to 101c having different waveguide widths. Each of the first to third portions 101a to 101c is coupled via waveguide width conversion portions 101d and 101e which are tapered waveguides. The effective refractive index of the red light in the zero-order mode with respect to the waveguide 93 and the effective refractive index of the red light in the higher-order mode with respect to the second portion 101b are made equal, and the effective refractive index of each color light in the higher-order mode with respect to the second portion 101b and the effective refractive index of each color light in the zero-order mode with respect to the waveguide 93 are not made equal, so that the waveguide widths of the waveguide 93 and the second portion 101b are set. As a result, light in which the three wavelengths of RGB are multiplexed is output from the output end of the third portion 101c of the waveguide 71.

[0031] In Examples 4 and 5, red light is multiplexed after the B+G multiplexing circuit. It is known that multiplexing by a directional coupler is more likely to transition even if there is a mismatch in the effective refractive index for light on the longer wavelength side. Therefore, in the RGB coupler, accurate multiplexing can be achieved by multiplexing from the shorter wavelength side.

[0032] Also, in the above examples, the RGB coupler was taken as an example to explain the function as an optical multiplexer. However, the wavelengths to be multiplexed are not limited to the above, and any circuit that multiplexes so-called short-wavelength light can exhibit the effects. Furthermore, this embodiment is not limited to the case of multiplexing due to the symmetry of light, and can also be applied to the case of demultiplexing.

Claims

1. An optical multiplexer / demultiplexer that is composed of two waveguides and an asymmetric Mach-Zehnder interferometer (MZI) in which two arms with different lengths are formed between two coupler portions, and that multiplexes and demultiplexes lights with different wavelengths, wherein a second waveguide core width of the waveguide of the arm through which most of the light on the short-wavelength side propagates is wider than a first waveguide core width of the waveguide that constitutes the asymmetric MZI other than the waveguides of the two arms. The optical multiplexer / demultiplexer is characterized by this.

2. The optical multiplexer / demultiplexer according to claim 1, wherein the arm through which most of the light on the short-wavelength side propagates includes a waveguide width conversion portion that is a tapered waveguide connecting between a waveguide width enlargement portion having the second waveguide core width and a waveguide having the first waveguide core width.

3. The optical multiplexer / demultiplexer according to claim 1, wherein in the two coupler portions, the waveguide core width of the waveguide that becomes the arm through which most of the light on the short-wavelength side propagates is made wider to be equal to the second waveguide core width.

4. A third waveguide core width of the waveguide of the arm different from the arm through which most of the light on the short-wavelength side propagates is wider than the first waveguide core width and is different from the second waveguide core width. The optical multiplexer / demultiplexer according to claim 1, 2, or 3 is characterized by this.

5. The optical multiplexer / demultiplexer according to claim 4, wherein the portion of the different arm having the third waveguide core width is provided in a straight portion.

6. The waveguide constituting the asymmetric MZ is composed of a silica-based planar optical waveguide circuit and contains ZrO as a dopant. 2 The optical multiplexer / demultiplexer circuit according to any one of claims 1 to 5, characterized by multiplexing and demultiplexing at least blue light.

7. An optical multiplexer / demultiplexer according to any one of claims 1 to 6 that multiplexes blue light and green light, and a mode coupler that multiplexes the output of the optical multiplexer / demultiplexer and red light An RGB coupler characterized by including these.

8. An optical multiplexer / demultiplexer according to any one of claims 1 to 6 that multiplexes blue light and green light, and a directional coupler that multiplexes the output of the optical multiplexer / demultiplexer and red light An RGB coupler characterized by including these.

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