JP7353766B2 - Ring resonator filter and wavelength tunable laser element - Google Patents

Description

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

Conventionally, in a wavelength tunable laser module, a technique is known in which a heater electrode is arranged on a flattened dielectric film made of benzocyclobutene (BCB), silicon dioxide, or the like (see, for example, Patent Document 1).

JP2011-247926A

By the way, for the waveguide of a ring resonator filter having a small bending radius, in order to reduce the bending radius, a waveguide with large confinement is suitable, such as a high mesa waveguide. In such a waveguide, the width of the waveguide needs to be approximately 2 um or less in order to ensure single mode property. Further, even when using a waveguide other than the above, from the viewpoint of increasing the thermal efficiency of the heater electrode, it is preferable to process the periphery of the waveguide into a mesa shape to reduce the heating volume. 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 conventional ring resonators, even if flattening is performed using a resin material, etc., stress is applied to the high mesa waveguide due to step breakage due to the step difference between the mesa and the resin material, and due to the difference in linear expansion coefficient between the resin material and the semiconductor. There is a problem in that the filter characteristics change as a result.

In addition, in conventional ring resonator filters, when forming a heater electrode on a mesa without flattening it with a resin material, the width of the heater electrode is limited to less than the mesa width, so It was difficult to form a heater electrode by patterning a pattern smaller than the width of the waveguide using photolithography. Furthermore, in the conventional ring resonator filter, in order to form a heater electrode of the necessary length and a desired resistance value, the electrode must be narrow and thick, making it even more difficult to form the heater.

The present disclosure has been made in view of the above, and provides a ring resonator filter and a tunable semiconductor laser element in which a heater electrode can be placed on a bent waveguide without using a resin material for flattening. The purpose is to

In order to solve the above-mentioned problems and achieve the purpose, a ring resonator filter according to the present disclosure includes a bent waveguide forming a circular structure, two input/output waveguides for inputting and outputting light, and a ring resonator filter according to the present disclosure. A ring resonator filter comprising input/output waveguides and two couplers coupled to the bent waveguides, the width of the bent waveguides being equal to the width of the input/output waveguides connected to the ends of the couplers. 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.

Moreover, the ring resonator filter according to the present disclosure further includes an electrode provided on the bent 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 that controls wavelength characteristics 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, in the above disclosure, the bent waveguide is
It has an optical loss part on the inner diameter side.

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

According to the present disclosure, it is possible to arrange a heater electrode on a curved waveguide without using a resin material for planarization.

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

Embodiments of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to this embodiment. Further, in the description of the drawings, the same or corresponding elements are given the same reference numerals as appropriate. Furthermore, it should be noted that the drawings are schematic and the dimensional relationship of each element, the ratio of each element, etc. may differ from the actual drawing. Furthermore, the drawings may include portions with different dimensional relationships and ratios. In addition, xyz coordinate axes are shown as appropriate in the drawings, and directions will be explained using these.

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

[Configuration of first waveguide section]
First, the first waveguide section 110 will be explained.
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 section 110 has a buried waveguide structure as a first waveguide structure.

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

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

The diffraction grating layer 111b has a configuration in which a sampling diffraction grating is formed in a p-type InGaAsP layer along the z direction, and the grooves of the diffraction grating are filled with InP. The diffraction grating layer 111b has a constant grating interval but is sampled, and thus exhibits a substantially periodic reflection response with respect to 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 section 111a, and is, for example, 1.2 μm.

The semiconductor laminated portion 112 is configured by laminating semiconductor layers. The semiconductor laminated portion 112 has a function such as a cladding portion for the waveguide portion 111 . The semiconductor laminated part 112 has the same material and structure as the part including the gain part 111a, except that the p-type InGaAsP layer is replaced with a p-type InP layer, and the y-direction It differs in that it has a stacked structure in which the positions of the p-type semiconductor layer and the n-type semiconductor layer are reversed with the gain section 111a in between. In addition, the semiconductor laminated portion 112 has the following points: a portion including the diffraction grating layer 111b is replaced with an optical waveguide layer made of InGaAsP, and a p-type semiconductor layer and an n-type semiconductor layer are sandwiched between the diffraction grating layer 111b in the y direction. It differs in that it has a laminated structure with the positions reversed. In addition, a SiN protective film is formed on the semiconductor laminated portion 112.

The p-side electrode 113 is arranged on the semiconductor stack 112 along the gain section 111a. The p-side electrode 113 is in contact with the semiconductor stack 112 through an opening formed in the SiN protective film of the semiconductor stack 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 on the SiN protective film of the semiconductor laminated portion 112 along the diffraction grating layer 111b. The microheater 114 functions as a first refractive index changer, generates heat when supplied with electric current, 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 current supplied, and changes the refractive index of the diffraction grating layer 111b.

[Configuration of second waveguide section]
Next, the second waveguide section 120 will be explained.
The second waveguide section 120 includes a two-branch section 121 having a mesa shape, an arm section 122 and an arm section 123 having a mesa shape and functioning as two straight waveguides, and a ring-shaped bent part having a mesa shape. It includes a waveguide 124 and a microheater 125 made of Ti.

The 2-branch section 121 is composed of a 1×2 type branch waveguide including a 1×2 type multimode interference type (MMI) waveguide 121a, and the 2-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 first waveguide section 110 side. By the two-branch section 121, one end of each of the arm section 122 and the arm section 123 is integrated and optically coupled to the gain section 111a.

Each of the arm portion 122 and the arm portion 123 extends in the z direction and is arranged to sandwich the bent waveguide 124 therebetween. The arm portion 122 and the arm portion 123 are close to the bent waveguide 124, and both are optically coupled to the bent 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 a ring resonator filter RF1. Furthermore, the ring resonator filter RF1 and the two-branch section 121 constitute a reflection mirror M1. The microheater 125 as a second refractive index changer has a ring shape and is arranged on a SiN protective film formed to cover the bent waveguide 124.

In this way, the first waveguide section 110 and the second waveguide section 120 constitute a laser resonator C1 constituted by the diffraction grating layer 111b and the reflection mirror M1 that are optically connected to each other. . Gain section 111a is arranged within laser resonator C1.

In addition, in the wavelength tunable laser element 100, due to its configuration, as shown by the optical path OP in FIG. This is performed in a route that returns to the diffraction grating layer 111b via one of the arm portions 122 and 123, the bent waveguide 124, the other of the arm portions 122 and 123, and the two-branch portion 121 in this order, And it circulates within the bent waveguide 124 during one optical feedback. Note that there are two optical paths for optical feedback: a clockwise optical path and a counterclockwise optical path. This increases the optical feedback length, making it possible to increase the effective resonator length and narrowing the linewidth of the laser beam L1.

[Configuration 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 includes a bent waveguide 124 forming a circular structure, and two arm portions 122 and an arm that function as input/output waveguides for inputting and outputting light to and from the ring resonator filter RF1. 123, and a coupler 131 and a coupler 132 that couple the arm part 122, the arm part 123, and the bent waveguide 124. Furthermore, 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 bent waveguide has a width D1 of 2 to 3 um, and the arm portions 122 and 123 have a width D2 of 1.1 um.

FIG. 3 is a diagram showing the light intensity distribution of the fundamental mode of the input/output waveguide and the bent waveguide. FIG. 4 is a diagram showing the relationship between the waveguide widths in which the coupling of light between fundamental modes is maximized in a curved waveguide and a straight waveguide that have different widths. In FIG. 3, the horizontal axis indicates the position (um) in the width direction within the waveguide, and the vertical axis indicates the light intensity distribution. In FIG. 3, the horizontal axis 0 indicates the center of the waveguide, and with respect to the curve L10, the positive direction of the horizontal axis is defined on the outside of the bend. Furthermore, in FIG. 3, curve _L1 shows the light intensity distribution of the fundamental mode in a straight waveguide with a width of 1.1 um, and curve _L10 shows the light intensity distribution of the fundamental mode in a bent waveguide with a width of 1.1 um and a bending radius of 10 um. shows. In addition, in FIG. 4, the horizontal axis indicates the width (um) of the bent waveguide 124, and the vertical width indicates the light intensity distribution of the fundamental mode is closest (the coupling rate is highest when the waveguide center position is appropriately shifted). The straight waveguide width (um) is shown. Further, in FIG. 4, a curve _L10 represents a bent waveguide with a bending radius of 10 um, a curve _L15 represents a bent waveguide with a bending radius of 15 um, a curve L20 represents a bent waveguide with a bending radius of 20 um, and a curve _L20 represents a bent waveguide with a bending radius of 20 um. ₂₅ indicates a bent waveguide with a bending radius of 25 um.

As shown in curve _L10 in FIGS. 3 and 3, the peak of the light distribution shifts to the outside of the curved waveguide 124. Furthermore, as shown in curve _L10 in Fig. 4, when the bending radius is 10 um, even if the width is as wide as 2 um, the spread of the light intensity distribution of the fundamental mode is comparable to that of a straight waveguide with a width of 1.1 um. . For example, as shown in curve _L10 shown in FIG. 2, even if the width D1 of the bent waveguide 124 is increased, the arm portions 122, 123, which are straight waveguides with a width D2=1.1 um, and the width D1 The bent waveguide 124 can be optically coupled through the coupler 131 and the coupler 132 with low loss.

[Wiring of ring resonator filter]
Next, the wiring in the ring resonator filter RF1 mentioned above will be explained. 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 includes an island-shaped mesa 140 formed inside the bent waveguide 124, an electrode 141 provided on the bent waveguide 124, and an electrode 141 provided on the coupler 131. and an electrode 142. The bent waveguide 124 and the island-shaped mesa 140 are formed as a series of semiconductor mesas connected via a bridging portion. The width D10 of the electrode 141 is formed smaller than the width D1 (mesa width) of the bent waveguide 124. Electrode 141 and electrode 142 are electrically connected. Electrode 141 and electrode 142 are control electrodes that control the wavelength characteristics of ring resonator filter RF1. Specifically, the electrode 141 is a microheater 125 (see FIG. 1) that generates heat when power is supplied, and the electrode 142 is a wiring electrode that electrically connects the electrode pad and the microheater 125. be. Further, the electrode 142 is formed to cross over the mesa of the coupler 131. A configuration may also be adopted in which disconnection of the electrode 142 is prevented by flattening the area around where the electrode 142 crosses the coupler 131 using a resin material such as polyimide. Even in this configuration, since the resin material is located away from the heating section 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 the embodiment described above, the width D1 of the bent waveguide 124 is larger than the width D2 of each of the arm portions 122 and 123 that function as input/output waveguides connected to the end of the coupler 131. Since it is thick, the micro-heater 125 can be placed on the bent waveguide 124 without using a flattening resin material (for the heater heating section).

Further, according to one embodiment, each of the bent waveguide 124 and the arm portions 122 and 123 functioning as input/output waveguides is formed of a high mesa waveguide having the same core layer, so that one crystallization is possible. It can be formed by growth and one-time semiconductor etching, and the manufacturing process can be simplified.

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

The ring resonator filter RF1A shown in FIG. 6 has an optical loss section 400 on the inner diameter side of the bent waveguide 124A. The optical loss section 400 is formed in an uneven manner on the inner diameter side of the bent waveguide 124A. Since the higher-order modes of the bent waveguide have a distribution in which the optical intensity is closer to the inner diameter than the fundamental mode, the optical loss section 400 can selectively increase the optical propagation loss of the higher-order modes. In the ring resonator filter RF1A, if the light circulating around the ring is not in a single mode but includes a higher-order mode, the wavelength characteristics of the filter will be disturbed.

According to the modification of the embodiment described above, by providing the optical loss portion 400 on the inner diameter side of the bent waveguide 124A, it is possible to increase the scattering loss of higher-order modes by roughening the waveguide shape. Therefore, even if the curved waveguide 124A is made thicker, propagation of higher-order modes can be suppressed and the wavelength characteristics of the filter can be stabilized.

Note that in the modification of the embodiment described above, a configuration is disclosed in which the scattering loss is increased as a method of suppressing the propagation of higher-order modes, but it is also possible to increase the absorption loss on the inner diameter side by ion implantation or an absorption layer. The configuration can also be changed.

(Other embodiments)
Note that the present disclosure is not limited to the above-described embodiment. The present disclosure also includes configurations in which the above-mentioned components are appropriately combined. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present disclosure are not limited to the one embodiment described above, and various modifications are possible.

As mentioned above, some embodiments of the present application have been described in detail based on the drawings, but these are merely examples, and various modifications may be made based on the knowledge of those skilled in the art, including the embodiments described in the section of the present disclosure. However, it is possible to implement the present disclosure in other forms with improvements.

100 Tunable wavelength laser element 110 First waveguide section 111 Waveguide section 111a Gain section 111b Diffraction grating layer 112 Semiconductor laminated section 113 N-side electrode 114, 125, 126 Micro heater 120 Second waveguide section 121a MMI waveguide 122 , 123 Arm part 124, 124A Bent waveguide 128 Over cladding layer 131, 132 Coupler 140 High mesa waveguide 141, 142 Electrode 400 Optical loss part C1 Laser resonator L1: Laser light M1 Reflection mirror OP Optical path RF1, RF1A Ring resonator filter

Claims (9)

A bent waveguide forming a circular structure, two input/output waveguides for inputting and outputting light, and two couplers for mechanically coupling each of the two input/output waveguides to the bent waveguide. A ring resonator filter comprising:
The width of the bent waveguide is wider than the width of the input/output waveguide connected to the end of the coupler,
In the bent waveguide, a part of the width of the bent waveguide becomes narrow at the location of the coupler, and the narrow width part is connected to an end of the coupler.
A ring resonator filter characterized by: The ring resonator filter according to claim 1,
each of the bent waveguide and the input/output waveguide has the same core layer;
A ring resonator filter characterized by: The ring resonator filter according to claim 1 or 2,
the bent waveguide has a mesa shape;
A ring resonator filter characterized by: The ring resonator filter according to any one of claims 1 to 3,
further comprising an electrode provided on the bent waveguide;
A ring resonator filter characterized by: The ring resonator filter according to claim 4,
The electrode is
the width is narrower than the width of the bent waveguide;
A ring resonator filter characterized by: 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: The ring resonator filter according to claim 6,
The electrode is
It is a micro heater.
A ring resonator filter characterized by: A bent waveguide forming a circular structure, two input/output waveguides for inputting and outputting light, and each of the two input/output waveguides and the bent waveguide are coupled mechanically or by being brought close to each other. A ring resonator filter comprising two couplers,
The width of the bent waveguide is
thicker than the width of the input/output waveguide connected to the end of the coupler,
The bent waveguide is
Has an optical loss part on the inner diameter side,
A ring resonator filter characterized by: comprising a ring resonator filter according to any one of claims 1 to 8,
Tunable wavelength laser element.

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