JP2020136297A - Ring resonator filter and wavelength variable laser element - Google Patents

Abstract

To provide a ring resonator filter and a variable semiconductor laser element in which a heater electrode can be placed on a flexure waveguide, without using a resin material for flattening.SOLUTION: A ring resonator filter FR1 includes a flexure waveguide 124 forming a circular structure, two arm parts 122, 123 for inputting/outputting light, and two couplers 131, 132 for coupling each arm part 122, 123 and the flexure waveguide 124. The width D1 of a flexure waveguide 134 is wider than the width D2 of the arm parts 122, 123 for connection with the ends of the couplers 131, 132.SELECTED DRAWING: Figure 2

Description

The present disclosure relates to a ring resonator filter and a tunable laser device.

Conventionally, in a tunable laser module, there is known a technique of arranging a heater electrode on a flattened surface using a dielectric film such as benzocyclobutene (BCB) or silicon dioxide (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2011-247926

By the way, as the waveguide of the ring resonator filter having a small bending radius, a waveguide having a large confinement is suitable in order to reduce the bending radius, and for example, a high mesa waveguide is suitable. In such a waveguide, the width of the waveguide needs to be about 2 um or less in order to ensure single mode. Further, even when a waveguide other than the above is used, it is preferable to process the periphery of the waveguide on a mesa to reduce the heating volume from the viewpoint of increasing the thermal efficiency of the heater electrode. However, it is difficult to form a heater electrode on such a thin mesa, and after flattening the periphery of the mesa with a resin material such as polyimide, the heater electrode is formed on the mesa.

However, in a conventional ring resonator, even if flattening is performed with a resin material or the like, stress due to a step break due to a step between the mesa and the resin material or a difference in the coefficient of linear expansion between the resin material and the semiconductor is applied to the high mesa waveguide. As a result, there is a problem that the filter characteristics change.

Further, in the conventional ring resonator filter, when the heater electrode is formed on the mesa without flattening by the resin material, the heater electrode width is limited to the mesa width or less, so that the width is 2 um or less on the bent waveguide. It was difficult to form a heater electrode by patterning a pattern smaller than the waveguide width by photolithography. Further, in the conventional ring resonator filter, in order to form a heater electrode having a required length with a desired resistance value, the electrode becomes a narrow and thick film electrode, and it is more difficult to form a heater.

The present disclosure has been made in view of the above, and provides a ring resonator filter and a variable semiconductor laser device capable of arranging a heater electrode on a bent waveguide without using a resin material for flattening. The purpose is to do.

In order to solve the above-mentioned problems and achieve the object, the ring resonator filter according to the present disclosure includes a bent waveguide forming a circumferential structure, two input / output waveguides for inputting / outputting light, and the above two input / output waveguides. A ring resonator filter comprising each of the input / output waveguides and two couplers coupled to the bent waveguide, wherein the width of the bent waveguide is the input / output connected to the end of the coupler. Thicker than the width of the waveguide.

Further, in the ring resonator filter according to the present disclosure, in the above disclosure, each of the bent waveguide and the input / output waveguide has the same core layer.

Further, in the ring resonator filter according to the present disclosure, in the above disclosure, the bent waveguide has a mesa shape.

Further, the ring resonator filter according to the present disclosure further includes an electrode provided on the bending waveguide in the above disclosure.

Further, in the ring resonator filter according to the present disclosure, in the above disclosure, the width of the electrode is narrower than the width of the bent waveguide.

Further, in the ring resonator filter according to the present disclosure, in the above disclosure, the electrode is a control electrode for controlling the wavelength characteristic of the ring resonator.

Further, in the ring resonator filter according to the present disclosure, in the above disclosure, the electrode is a microheater.

Further, in the ring resonator filter according to the present disclosure, the bending waveguide is described in the above disclosure.
It has a light loss portion on the inner diameter side.

Further, the tunable laser element according to the present disclosure includes the above-mentioned ring resonator filter.

According to the present disclosure, there is an effect that the heater electrode can be arranged on the bent waveguide without using a resin material for flattening.

FIG. 1 is a schematic perspective view of a tunable laser device according to an embodiment. FIG. 2 is a plan view schematically showing the configuration of the ring resonator filter according to the embodiment. FIG. 3 is a diagram showing the relationship between the light intensity distribution of the input / output waveguide and the bending waveguide. FIG. 4 is a diagram showing the relationship between bent waveguides having different widths and input / output waveguide widths having similar light intensity distributions. FIG. 5 is a plan view schematically showing the wiring in the ring resonator filter. FIG. 6 is a diagram schematically showing a plane of a ring resonator filter according to a modified example of one embodiment.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. It should be noted that the present embodiment is not limited to this embodiment. Further, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals. Furthermore, it should be noted that the drawings are schematic, and the relationship between the dimensions of each element, the ratio of each element, and the like may differ from the actual ones. Furthermore, there are cases where parts having different dimensional relationships and ratios are included between the drawings. In addition, the xyz coordinate axes are appropriately shown in the drawings, and the directions will be described thereby.

FIG. 1 is a schematic perspective view of a tunable laser device according to an embodiment. The tunable laser element 100 shown in FIG. 1 outputs a laser beam by oscillating a laser in the 1.55 μm band, for example. The tunable laser element 100 includes a first waveguide 110 and a second waveguide 120 formed on a common base B.

[Structure of the first waveguide]
First, the first waveguide 110 will be described.
The first waveguide section 110 includes a waveguide section 111, a semiconductor laminated section 112, a p-side electrode 113, and a microheater 114. The first waveguide 110 has an embedded waveguide structure as the first waveguide structure.

The waveguide portion 111 is formed so as to extend in the z direction in the semiconductor laminated portion 112. Further, the waveguide portion 111 has a gain portion 111a arranged in the semiconductor laminated portion 112 and a DBR type diffraction grating layer 111b.

The gain unit 111a has a multiple quantum well structure including a plurality of well layers and a plurality of barrier layers that are alternately laminated, and a multiple quantum well structure including upper and lower lower and upper light confinement layers. It emits light when a current is injected. The well layer and the barrier layer constituting the multiple quantum well structure of the gain portion 111a are made of InGaAsP having different compositions. In the first embodiment, the emission wavelength band from the gain unit 111a is the 1.55 μm band. The lower light confinement layer is made of n-type InGaAsP. The upper light confinement layer is made of p-type InGaAsP. The bandgap wavelengths of the lower light confinement layer and the upper light confinement layer are set to be shorter than the bandgap wavelength of the gain unit 111a.

The diffraction grating layer 111b has a structure in which a sampled diffraction grating is formed in the p-type InGaAsP layer along the z direction, and the groove of the diffraction grating is embedded with InP. The diffraction grating layer 111b is sampled although the lattice spacing of the diffraction grating is constant, so that it exhibits a substantially periodic reflection response with respect to the wavelength. The bandgap wavelength of the p-type InGaAsP layer of the diffraction grating layer 111b is preferably shorter than the bandgap wavelength of the gain portion 111a, for example, 1.2 μm.

The semiconductor laminated portion 112 is configured by laminating semiconductor layers. The semiconductor laminated portion 112 has a function of a clad portion and the like with respect to the waveguide portion 111. The semiconductor laminated portion 112 has the same material and structure as the portion including the gain portion 111a in the portion including the gain portion 111a, but the p-type InGaAsP layer is replaced with the p-type InP layer and the y direction. The difference is that the p-type semiconductor layer and the n-type semiconductor layer have a laminated structure in which the positions of the p-type semiconductor layer and the n-type semiconductor layer are reversed with the gain portion 111a interposed therebetween. Further, the semiconductor laminated portion 112 is replaced with an optical waveguide layer made of InGaAsP in the portion including the diffraction grating layer 111b, and the p-type semiconductor layer and the n-type semiconductor layer sandwich the diffraction grating layer 111b in the y direction. It differs in that it has a laminated structure in which the positions of and are reversed. Further, a SiN protective film is formed on the semiconductor laminated portion 112.

The p-side electrode 113 is arranged on the semiconductor laminated portion 112 along the gain portion 111a. The p-side electrode 113 is in contact with the semiconductor laminated portion 112 via an opening formed in the SiN protective film of the semiconductor laminated portion 112. Further, an n-side electrode (not shown) is formed on the surface of the semiconductor laminated portion 112 opposite to the surface on which the p-side electrode 113 is formed.

The microheater 114 is arranged along the diffraction grating layer 111b on the SiN protective film of the semiconductor laminated portion 112. The microheater 114 functions as a first refractive index changer, generates heat when a current is supplied, and heats the diffraction grating layer 111b. That is, the microheater 114 changes the temperature of the diffraction grating layer 111b by changing the amount of the supplied current, and changes the refractive index of the diffraction grating layer 111b.

[Structure of the second waveguide]
Next, the second waveguide section 120 will be described.
The second waveguide portion 120 includes a bifurcated portion 121 having a mesa shape, an arm portion 122 and an arm portion 123 having a mesa shape and functioning as two linear waveguides, and a ring-shaped bend having a mesa shape. A waveguide 124 and a microheater 125 made of Ti are provided.

The two-branch portion 121 is composed of a 1 × 2 type bifurcated waveguide including a 1 × 2 type multi-mode interference type (MMI) waveguide 121a, and the two port side is connected to each of the arm portion 122 and the arm portion 123. At the same time, the 1-port side is connected to the 1st waveguide 110 side. The two-branch portion 121 integrates one end of each of the arm portion 122 and the arm portion 123, and is optically coupled to the gain portion 111a.

Each of the arm portion 122 and the arm portion 123 extends in the z direction and is arranged so as to sandwich the bent waveguide 124. The arm portion 122 and the arm portion 123 are close to the bending waveguide 124, and both are optically coupled to the bending waveguide 124 with the same coupling coefficient κ. The value of κ is, for example, 0.2. The arm portion 122, the arm portion 123, and the bent waveguide 124 constitute the ring resonator filter RF1. Further, the ring resonator filter RF1 and the bifurcated portion 121 form a reflection mirror M1. The microheater 125 as the second refractive index changer has a ring shape and is arranged on a SiN protective film formed so as to cover the bent waveguide 124.

As described above, the first waveguide section 110 and the second waveguide section 120 constitute a laser resonator C1 composed of a diffraction grating layer 111b optically connected to each other and a reflection mirror M1. .. The gain portion 111a is arranged in the laser resonator C1.

Further, in the tunable laser element 100, due to its configuration, as shown by the optical path OP in FIG. 1, the optical feedback in the laser resonator C1 is from the diffraction grating layer 111b to the bifurcated portion 121 and the ring resonator filter RF1. This is performed by a path that returns to the diffraction grating layer 111b via one of the arm portion 122 and the arm portion 123, the bending waveguide 124, the other of the arm portion 122, and the other two branch portions 121 in order. In addition, it orbits in the bending waveguide 124 during one optical feedback. There are two optical paths for optical feedback, a clockwise optical path and a counterclockwise optical path. As a result, the optical feedback length becomes long, so that the effective resonator length can be lengthened, and the line width of the laser beam L1 can be narrowed.

[Structure of ring resonator filter RF1]
FIG. 2 is a plan view schematically showing the configuration of the ring resonator filter RF1.

As shown in FIG. 2, the ring resonator filter RF1 has a bent waveguide 124 forming a circumferential structure, and two arm portions 122 and arms that function as input / output waveguides for inputting and outputting light to and from the ring resonator filter RF1. The unit 123 includes a coupler 131 and a coupler 132 that connect the arm portion 122, the arm portion 123, and the bending waveguide 124. Further, the width D1 of the bent waveguide 124 is thicker than the width D2 of the input / output waveguide connected to the end of the coupler 131 (D1> D2). Specifically, the width D1 of the bent waveguide is formed with a width of 2 to 3 um, and the width D2 of the arm portion 122 and the arm portion 123 is formed with a width of 1.1 um.

FIG. 3 is a diagram showing light intensity distributions in the basic modes of the input / output waveguide and the bending waveguide. FIG. 4 is a diagram showing the relationship of the waveguide widths in which the light coupling between the basic modes is the largest in the bent waveguide and the linear waveguide having different widths. In FIG. 3, the horizontal axis shows the position (um) in the width direction in the waveguide, and the vertical axis shows the light intensity distribution. In FIG. 3, the horizontal axis 0 indicates the center of the waveguide, and the positive direction of the horizontal axis is defined as the outside of the bend with respect to the curve L10. Further, in FIG. 3, curve _{L 1} represents a light intensity distribution of the fundamental mode of the straight waveguide of width 1.1Um, the light intensity distribution of the fundamental mode curve _{L 10} is the bending waveguide width 1.1Um bending radius 10um Is shown. Further, in FIG. 4, the horizontal axis indicates the width (um) of the bent waveguide 124, and the vertical width is the closest to the light intensity distribution in the basic mode (the coupling ratio is the largest when the center position of the waveguide is appropriately shifted). The linear waveguide width (um) is shown. Further, in FIG. 4, curve L ₁₀ shows a bending radius of 10 um, curve L ₁₅ shows a bending radius of 15 um, curve L ₂₀ shows a bending radius of 20 um, and curve L ₂₀ shows a bending radius of 20 um. Reference numeral ₂₅ denotes a bent waveguide having a bend radius of ₂₅ um.

As shown in Figure 3 and the curve L _10, light distribution, the peak on the outside of the bend waveguide 124 to move. Further, as shown by the curve L ₁₀ in FIG. 4, the bend radius is 10um, be wider width and 2um, the spread of the light intensity distribution of the fundamental mode is a straight waveguide and as wide 1.1um .. For example, as shown in curve _{L 10} shown in FIG. 2, the bending even when the width D1 of the waveguide 124, the arm portions 122, 123 and the width D1 is straight waveguide of width D2 = 1.1um The bent waveguide 124 can be optically coupled with low loss via the coupler 131 and the coupler 132.

[Wiring of ring resonator filter]
Next, the wiring in the ring resonator filter RF1 described above will be described. FIG. 5 is a plan view schematically showing the wiring in the ring resonator filter RF1.

As shown in FIG. 5, the ring resonator filter RF1 is provided on the island-shaped mesa 140 formed inside the bending waveguide 124, the electrode 141 provided on the bending waveguide 124, and the coupler 131. It has an electrode 142 and the like. The bent waveguide 124 and the island-shaped mesa 140 are formed as a series of semiconductor mesas connected by a bridging portion. The width D10 of the electrode 141 is formed to be thinner than the width D1 (mesa width) of the bending waveguide 124. The electrode 141 and the electrode 142 are electrically connected. The electrode 141 and the electrode 142 are control electrodes that control the wavelength characteristics of the ring resonator filter RF1. Specifically, the electrode 141 is a microheater 125 (see FIG. 1) that generates heat when electric power is supplied, and the electrode 142 is a wiring electrode that electrically connects the electrode pad and the microheater 125. is there. Further, the electrode 142 is formed so as to overcome the mesa of the coupler 131. The periphery of the electrode 142 overcoming the coupler 131 may be flattened by using a resin material such as polyimide to prevent disconnection of the electrode 142. Even with such a configuration, since the resin material is located away from the heating portion of the microheater 125, the heating of the resin material is small and the influence of stress due to the difference in linear expansion coefficient can be reduced.

According to one embodiment described above, the width D1 of the bent waveguide 124 is larger than the width D2 of each of the arm portion 122 and the arm portion 123 that function as input / output waveguides connected to the ends of the coupler 131. Since it is thick, the microheater 125 can be arranged on the bent waveguide 124 without using a resin material for flattening (for the heater heating portion).

Further, according to one embodiment, since each of the bending waveguide 124 and the arm portion 122 and the arm portion 123 functioning as the input / output waveguide is composed of a high messa waveguide having the same core layer, one crystal. It can be formed by growth and one-time semiconductor etching, and the manufacturing process can be simplified.

(Modification example)
Next, a modified example of one embodiment will be described. FIG. 6 is a diagram schematically showing a plane of a ring resonator filter according to a modified example of one embodiment.

The ring resonator filter RF1A shown in FIG. 6 has a light loss portion 400 on the inner diameter side of the bending waveguide 124A. The light loss portion 400 is formed unevenly on the inner diameter side of the bending waveguide 124A. Since the higher-order mode of the bent waveguide has a distribution having light intensity on the inner diameter side of the basic mode, the light loss section 400 can selectively increase the light propagation loss in the higher-order mode. In the ring resonator filter RF1A, if the light orbiting the ring includes a higher-order mode instead of a single mode, the wavelength characteristics of the filter are disturbed.

According to the modified example of the above-described embodiment, by providing the light loss portion 400 on the inner diameter side of the bending waveguide 124A, the scattering loss in the higher-order mode can be increased by roughening the shape of the waveguide. Therefore, even when the bending waveguide 124A is thickened, the propagation in the higher-order mode can be suppressed and the wavelength characteristics of the filter can be stabilized.

In the modified example of the above-described embodiment, a configuration for increasing the scattering loss is disclosed as a method for suppressing propagation in the higher-order mode, but the absorption loss on the inner diameter side is increased by ion implantation or an absorption layer. The configuration can also be changed.

(Other embodiments)
The present disclosure is not limited by the above-described embodiment. The present disclosure also includes a configuration in which the above-mentioned components are appropriately combined. Further, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present disclosure is not limited to the above-described embodiment, and various modifications can be made.

Although some of the embodiments of the present application have been described in detail with reference to the drawings, these are examples, and various modifications are made based on the knowledge of those skilled in the art, including the embodiments described in the columns of the present disclosure. , It is possible to implement the present disclosure in other forms with improvements.

100 Tunable laser element 110 First waveguide 111 waveguide 111a Gain 111b Diffraction grating layer 112 Semiconductor lamination 113 n-side electrodes 114, 125, 126 Microheater 120 Second waveguide 121a MMI waveguide 122 , 123 Arm part 124, 124A Bending waveguide 128 Overclad layer 131, 132 Coupler 140 High Mesa waveguide 141, 142 Electrode 400 Light loss part C1 Laser resonator L1: Laser light M1 Reflection mirror OP Optical path RF1, RF1A Ring resonator filter

Claims (9)

A ring resonance including a bent waveguide forming a circumferential structure, two input / output waveguides for inputting / outputting light, and two couplers for coupling each of the two input / output waveguides with the bent waveguide. It is a vessel filter
The width of the bent waveguide is larger than the width of the input / output waveguide connected to the end of the coupler.
A ring resonator filter characterized by that. The ring resonator filter according to claim 1.
Each of the bent waveguide and the input / output waveguide has the same core layer as each other.
A ring resonator filter characterized by that. The ring resonator filter according to claim 1 or 2.
The bent waveguide has a mesa shape.
A ring resonator filter characterized by that. The ring resonator filter according to any one of claims 1 to 3.
Further provided with electrodes provided on the bent waveguide.
A ring resonator filter characterized by that. The ring resonator filter according to claim 4, wherein the ring resonator filter is used.
The electrode is
The width is narrower than the width of the bent waveguide,
A ring resonator filter characterized by that. The ring resonator filter according to claim 5.
The electrode is
A control electrode that controls the wavelength characteristics of the ring resonator.
A ring resonator filter characterized by that. The ring resonator filter according to claim 6.
The electrode is
It is a micro heater
A ring resonator filter characterized by that. The ring resonator filter according to any one of claims 1 to 7.
The bent waveguide
It has a light loss part on the inner diameter side,
A ring resonator filter characterized by that. The ring resonator filter according to any one of claims 1 to 8 is provided.
Tunable laser element.

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