JP2018180375A - Broadband branch optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical branching circuit which has a simple structure with a planar lightwave circuit and features small dependency of the branching ratio on wavelength.SOLUTION: An optical branching circuit 10 is an optical circuit formed on a PLC. Optical waveguides 11, 12 both have a waveguide width of 2 μm and a relative refractive index difference of 0.45%, and are crossed to have core central axes thereof intersect at a crossing angle θ of approximately 10°, or greater than 5° and less than 15°. The crossing angle θ of the core central axes is set to approximately 10°.SELECTED DRAWING: Figure 4

Description

  The present invention relates to broadband branch optical circuits, and more particularly, to a broadband branch optical circuit that handles wide wavelength bands of red light (R), green light (G), and blue light (B).

  In recent years, development of compact RGB light sources using laser diodes (LDs) of R, G and B primary colors has been underway for use in eyeglasses-type terminals and pico projectors. Since the LD has high rectilinearity compared to the LED, it is possible to realize a focus-free projector in which focusing is achieved regardless of the distance and angle with the projection surface. In addition, it has high luminous efficiency, low power consumption, high color reproducibility, and has been attracting attention in recent years.

  FIG. 1 shows a typical system outline of a projector 100 using a conventional LD. The LD 101 collimates LD light of each color of R, G and B individually using the lens 102 for single wavelength oscillation, and combines multiple lights into one beam as a RGB light source using the dichroic mirror 105 use. A desired image is projected on the screen 110 by scanning the bundled RGB light in synchronization with the modulation of the LD using a MEMS (Micro Electro Mechanical Systems) mirror 107 or the like.

  At this time, in order to adjust the white balance, part of each LD light is branched by the half mirror 103 or the like, and the intensity of each LD light is monitored by the photodiode (PD) 104. The LD 101 emits light in the front-rear direction, but the accuracy on the rear side monitoring is poor, so the front side (front monitoring) as shown in FIG. 1 is generally used. Therefore, as shown in FIG. 1, in order to use as an RGB light source, it is necessary to spatially and optically combine bulk components such as the LD 101, the lens 102, the half mirror 103, the PD 104, and the dichroic mirror 105.

  Furthermore, since the output of the LD 101 is weaker than that of the R and G in B, an RRGGB light source or the like in which the LD 101 of R and G is doubled and doubled twice may be used. Such a complex optical system is a factor that makes miniaturization difficult.

On the other hand, in recent years, an RGB coupler module 200 using a silica-based planar lightwave circuit (PLC) as shown in FIG. 2 instead of RGB multiplexing by such bulk parts has attracted attention (for example, non-patent document 1) reference). PLC manufactures an optical waveguide on a planar substrate such as Si by patterning with photolithography etc., reactive ion etching, and multiple basic optical circuits (eg directional coupler, Mach-Zehnder interference) Realizes various functions by combining the (For example, see Non-Patent Documents 2 and 3)
The basic structure of the RGB coupler module 200 using PLC is formed by three input optical waveguides and one output optical waveguide as shown in FIG. The combining method is a method using two symmetrical directional couplers, a method using a Mach-Zehnder interferometer (see non-patent document 1), a method using a mode coupler (see non-patent document 2), etc. It is possible to integrate the RGB combined optical system, which is realized by a lens and a dichroic mirror, on one chip.

  In PLC, it is possible to branch off part of light using a directional coupler, a Mach-Zehnder interferometer (see Non-Patent Document 4), or the like. For this reason, it becomes possible to integrate a monitoring function by optically connecting PD to the output part of the branched optical path. Usually, PLC is used in the communication wavelength band (1.55 μm band or 1.31 μm band), so the wavelength dependency of the branching ratio by the directional coupler does not pose a major problem.

A. Nakao, R. Morimoto, Y. Kato, Y. Kakinoki, K. Ogawa and T. Katsuyama, “Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays”, Optics Communications 330 (2014) 45-48 Y. Hibino, "An Array of Photonic Filtering Advantage", IEEE CIRCUITS & DEVICES, Nov., 2000, pp. 21-27 A. Himeno, et al., “Silica-Based Planar Lightwave Circuits”, J. Sel. Top. Q. E., vol. 4, 1998, pp. 913-924 Rei Matsubara, 9 others, "20ch Monolithically Integrated Optical Switch / VOA / Tap Coupler Array Module Using Quartz PLC", IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers, Vol. 106, no. 545, p. 9-12 K. Jinguji, et al., “MACH-ZEHNDER INTERFEROMETER TYPE OPTICAL WAVEGUIDE COUPLER WITH WAVELENGTH-FLATTENED COUPLING RATIO” Electron. Lett., Vol. 26, no. 17, no. 17, pp. 1326-1327, 1990

  However, in the case where the PLC handles the visible light range, in particular, a wide wavelength range of about 450 nm to 650 nm (RGB), the light is split by the conventional simple directional coupler 300 as shown in FIG. Then, as shown in FIG. 3A, there is a problem of wavelength dependency of the branching ratio that the branching ratio k differs by about 25 times between blue (450 nm) and red (650 nm). The directional coupler 300 used here has a waveguide width of 2 μm, a gap of 2 μm, a relative refractive index difference of 0.45%, and a directional coupler such that the branching ratio k in blue (450 nm) is 1%. The length is set. The branching ratio k is the intensity of light emitted from the output end 302 b of the branched optical waveguide 302 when the intensity of light incident from the input end 301 a of the optical waveguide 301 is 1.

  On the other hand, couplers with less wavelength dependency using a Mach-Zehnder interferometer have been proposed (see Non-Patent Document 5), but the difference in branching ratio depending on the wavelength is still large and the structure is complicated. There is a problem that the optical circuit is upsized.

  In the branching of light using a directional coupler, the coupling length is determined by the seepage of light from the waveguide, so that the wavelength dependency of the branching ratio is prominent in a wide wavelength range. Therefore, there has been a problem that light can not be branched at a constant branching ratio over the entire visible wavelength range by a simple circuit using a planar lightwave circuit.

  The present invention has been made in view of such problems, and an object thereof is to provide a branched optical circuit having a simple structure using a planar lightwave circuit and having a small wavelength dependency of the branching ratio. is there.

  In order to solve the above-mentioned problems, the present invention is a broadband branched optical circuit, comprising a planar lightwave circuit in which an optical waveguide composed of two intersecting optical waveguide parts is formed, and the two optical waveguide parts Is characterized in that the crossing angle of the central axis of the core is larger than 5 ° and smaller than 15 °.

  The invention according to claim 2 is a broadband branch optical circuit, wherein an optical waveguide including a first optical waveguide portion and a second optical waveguide portion branched from the first optical waveguide portion is formed. And an angle between the central axis of the core of the first optical waveguide portion and the central axis of the core of the second optical waveguide portion at the branching portion with the first optical waveguide portion, It is characterized by being larger than 5 ° and smaller than 15 °.

  The invention according to claim 3 is a broadband branch optical circuit, which comprises: a first input optical waveguide having a tapered portion; a second output optical waveguide connected to the tapered portion; and a third output light guide A planar lightwave circuit in which an optical waveguide composed of a waveguide portion is formed, and a connection end face of the second output optical waveguide portion and the tapered portion of the third output optical waveguide portion is parallel; The waveguide widths at the connection end faces of the output optical waveguide part and the third output optical waveguide part are different.

  The present invention can reduce the wavelength dependency of the branching ratio of the branching optical circuit, and can be realized with a simple structure using a planar lightwave circuit.

It is a figure which shows the typical system outline of the projector 100 using the conventional LD. It is a figure which shows the basic structure of the RGB coupler module 200 using the conventional PLC. (A) is a figure which shows the calculation result of the branch ratio in wavelength 450nm-650 nm in the branch optical circuit 300, (b) is a figure which shows the structure of the conventional directional coupler 300. FIG. It is a figure which shows the structure of the branch optical circuit 10 which concerns on Embodiment 1 of this invention. It is a figure which shows the calculation result of the branch ratio in wavelength 450nm-650 nm in the branch optical circuit 10 of Embodiment 1 when the crossing angle (theta) is 5 degrees, 10 degrees, and 15 degrees. It is a figure which shows the calculation result of the loss in wavelength 450nm-650nm in the branch optical circuit 10 of Embodiment 1 when the crossing angle (theta) is 5 degrees, 10 degrees, and 15 degrees. It is a figure which shows the structure of another branch light circuit 20 which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the branch optical circuit 30 which concerns on Embodiment 2 of this invention. R (650 nm), G (520 nm) when the waveguide width ratio (t ₁ / (t ₁ + t ₂ )) of the branch portion in the branch optical circuit 30 according to the second embodiment of the present invention is changed ) And B (450 nm) are diagrams showing simulation results of the branching rate k.

  The present invention crosses or branches light waveguides at a shallow angle, thereby physically branching light rather than utilizing light interference.

(Embodiment 1)
Conventionally, in the planar lightwave circuit, the intersections of the waveguides are designed so as to prevent light leakage by increasing the intersection angle θ. On the other hand, in the first embodiment of the present invention, the intersection angle θ of the waveguide intersection portion is reduced to actively use the light leakage to realize the branching of the light with low wavelength dependency.

  Hereinafter, embodiments of the present invention will be described in detail.

  FIG. 4 shows the configuration of the branching optical circuit 10 according to the first embodiment of the present invention. The branching optical circuit 10 is an optical circuit formed on the PLC. The optical waveguides 11 and 12 both have a waveguide width of 2 μm and a relative refractive index difference of 0.45%, and the cross angle θ of the central axes of the cores is about 10 °, that is, larger than 5 ° and smaller than 15 ° So are crossed. The branching ratio k is the intensity of the light emitted from the output end 12 b of the optical waveguide unit 12 when the intensity of the light incident on the input end 11 a of the optical waveguide unit 11 is 1.

  FIG. 5 shows calculation results of the branching ratio at wavelengths 450 nm to 650 nm when the crossing angle θ in the branching optical circuit 10 according to the first embodiment is 5 °, 10 °, and 15 °. Further, FIG. 6 similarly shows the calculation result of the loss in the branch light circuit 10 of the first embodiment.

  When the crossing angle θ is about 5 °, as shown in FIG. 5, the maximum branching ratio is about 2% at a wavelength of 450 nm to 650 nm and about 0.2% at a wavelength of 450 to 500 nm. The wavelength dependency of the branching ratio k is large. In addition, as shown in FIG. 6, a loss of 10% or more occurs at all wavelengths. Further, when the crossing angle θ is about 15 °, as shown in FIG. 5, the light beam is hardly branched regardless of the wavelength over the wavelength of 450 nm to 650 nm.

  On the other hand, if the crossing angle θ of the central axis of the core is set to about 10 ° as in this embodiment, the maximum branching ratio k is about 1.5% at the wavelength 550 nm and the minimum branching ratio at wavelengths 450 nm to 650 nm. k is about 1.1% at a wavelength of 450 nm, and it becomes possible to obtain a branching rate of about 1% regardless of the wavelength. Furthermore, the loss at this time is suppressed to about 5% at maximum at a wavelength of 450 nm as shown in FIG.

  As described above, in order to split a small amount of light for monitoring, etc., it is extremely simple to use a cross waveguide in which the crossing angle θ is set to about 10 °, that is, more than 5 ° and less than 15 °. A branched optical circuit with small wavelength dependency can be realized.

  Although the waveguides are crossed in FIG. 4, in the present embodiment, as shown in FIG. 7, the input end 21a and the output end 21b are used instead of crossing the two optical waveguides having input / output ends. The angle θ is approximately 10 °, that is, larger than 5 ° and more than 15 ° even in the branched optical circuit 20 including the optical waveguide portion 21 and the branched optical waveguide portion 22 branched (branched) from the optical waveguide portion 21. It goes without saying that the same effect can be obtained by reducing the

Second Embodiment
FIG. 8 shows the configuration of the branching optical circuit 30 according to Embodiment 2 of the present invention. In the first embodiment, waveguides having the same width are simply crossed or branched. However, in the branching optical circuit 30 according to the present embodiment, the waveguide width of one input optical waveguide 31 is broadened in a tapered shape, and the enlarged diameter end portion 34 Is a branched optical circuit in which the main output optical waveguide 32 and the branched output optical waveguide 33 are branched. Connection end face of the input optical waveguide 31 of the main output optical waveguide 32 and the branched output optical waveguides 33 are parallel, the main output optical waveguide width t ₁ of the waveguide 32 and the waveguide width t ₂ of the branched output optical waveguides 33 The connection end faces are different from each other. Since the main output optical waveguide 32 has a wider waveguide width (t ₂ > t ₁ ) than the branched output optical waveguide 33 and has a high branching ratio k, higher intensity light propagates. In the branched optical circuit shown in FIG. 8, the main output optical waveguide 32 is a linear waveguide, and the branched output optical waveguide 33 is a curved waveguide and a linear waveguide, but if the bending loss is within the allowable range, the main The output optical waveguide 32 can also be configured to include a curved waveguide. The branching ratio k is the intensity of light emitted from the output end 33 b of the branched output optical waveguide 33 when the intensity of light incident on the input end 31 a of the input optical waveguide 31 is 1.

FIG. 9 shows R (650 nm) when the waveguide width ratio (t ₁ / (t ₁ + t ₂ )) of the branch portion is changed in the branch optical circuit 30 according to the second embodiment of the present invention The simulation result of branching rate k of G (520 nm) and B (450 nm) is shown. The simulation results waveguide width t ₁ of the input optical waveguide 31 and the branched output optical waveguide 32 are those performed by fixing the 2 [mu] m, changing the waveguide width t ₂ of the main output optical waveguide 33. In addition, all of the input optical waveguides 31, the main output optical waveguides 32, and the branch output optical waveguides 33 have a non-refractive index difference of 0.45%. As shown in FIG. 9, R (650nm), G (520nm), B (450nm) none of the branch rate k of change in the same way to changes in waveguide width t ₂ of the main output optical waveguide 32 The wavelength dependency of the change of the branching ratio k is low.

As described above, in the branching optical circuit 30 according to the present embodiment, by changing the waveguide width ratio (t ₁ / t ₂ ), it is possible to change the branching ratio while keeping the wavelength dependency low.

10, 20, 30 Branching Optical Circuits 11, 12, 21, 22 Optical Waveguide 31 Input Optical Waveguide 32 Main Output Optical Waveguide 33 Branching Output Optical Waveguide 100 Projector 101 LD
102, 106 lens 103 half mirror 104 PD
105 dichroic mirror 107 MEMS mirror 110 screen 200 RGB coupler module

Claims (3)

  The planar lightwave circuit is provided with an optical waveguide formed of two intersecting optical waveguide parts, and the intersection angle of the central axes of the cores is larger than 5 ° and smaller than 15 °. Broadband branch optical circuit characterized by   A planar lightwave circuit having an optical waveguide formed of a first optical waveguide portion and a second optical waveguide portion branched from the first optical waveguide portion is provided, and a branch with the first optical waveguide portion An angle between the central axis of the core of the first optical waveguide portion and the central axis of the core of the second optical waveguide portion in the portion is larger than 5 ° and smaller than 15 °. Light circuit.   A planar lightwave circuit including an optical waveguide formed of a first input optical waveguide portion having a tapered portion, and a second output optical waveguide portion connected to the tapered portion and a third output optical waveguide portion, The connection end faces of the second output optical waveguide portion and the tapered portion of the third output optical waveguide portion are parallel, and the connection of the second output optical waveguide portion and the third output optical waveguide portion is performed. A wide-band branch optical circuit characterized in that the waveguide widths at the end faces are different.

Images