JP2015220358A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical element that can improve a flat mode distribution of light to increase the efficiency of coupling with an optical fiber.SOLUTION: An optical element 1 has an optical resonator, an inclined mirror 12 for reflecting light emitted in an optical resonator axis direction from the optical resonator in a direction vertical to the optical resonator axis direction, and a lens 18 for converging reflected light. The inclined mirror 12 has a reflection face having a curvature, and the mode radius is corrected by reflecting light from the reflection face having the curvature so that the mode distribution of light has a circular shape.

Description

  The present invention relates to an optical element, and more particularly, to an optical element that converts an emission direction of output light from an optical resonator into a vertical direction of a semiconductor substrate.

(Conventional technology)
In general, the mode radius of a semiconductor laser is greatly different from the mode radius of an optical fiber. It was necessary to focus on the fiber. Conventionally, by using a surface emitting type semiconductor array integrated with InP lenses and an external coupling lens for the output light, the mode radius is converted and the optical fuzzer can be mounted simply without using an external lens. High optical coupling to an optical fiber can be realized (Non-Patent Document 1).

FIG. 1 is a diagram showing a configuration of a conventional semiconductor laser.
The semiconductor laser shown in FIG. 1 includes a laser resonator that emits light in the horizontal direction of a substrate, a 45 ° tilting mirror, and an InP lens (Inp-bottom Lens).
The mode radius of the waveguide structure of the laser resonator is different in the vertical and horizontal directions of the substrate and has a flat mode distribution. For this reason, the mode distribution of the light output to the outside of the semiconductor laser through the 45 ° tilt mirror and the InP lens is also flattened, and this semiconductor laser has an optical fiber having a cylindrical waveguide structure and complete light. As a result, the optical coupling efficiency could be reduced.

  As a semiconductor laser that enables high optical coupling with an optical fiber, there is a surface emitting laser in which a resonator, an inclined mirror, and an InP lens are integrated on a substrate. FIG. 2 shows a configuration of a surface emitting semiconductor laser including this InP lens.

2A and 2B are diagrams showing a configuration of a conventional surface emitting lens integrated laser 100, where FIG. 2A is a top view of the integrated laser 100, and FIG. 2B is a side view of the integrated laser 100. FIG.
In FIG. 2, the integrated laser 100 includes an upper electrode 101, an inclined mirror 102, a diffraction grating 103, an upper SCH layer 104, a multilayer quantum well layer 105, a lower SCH layer 106, a highly reflective film 107, and a lens. 108 and a lower electrode 109. The semiconductor laser 1 further includes an InP substrate 110 and two cladding layers 111 and 112.

  In this integrated laser 100, the laser resonator is configured to operate in the DFB mode by the diffraction grating 103 with the direction parallel to the InP substrate 110 as the waveguide direction.

  The output light emitted from the end of the laser resonator propagates through the InP medium as a Gaussian beam and reaches the tilt mirror 102. The inclined mirror 102 is a plane formed so as to form 45 ° with respect to the emission direction of the resonator, and converts the traveling direction of the incident light by total reflection into the substrate vertical direction. Further, the output light reaches the InP lens 108 formed on the back surface of the substrate on the optical axis.

  The InP lens 108 collects light in accordance with the mode radius (about 4.5 μm) of the single mode fiber, and the light is output to the outside of the semiconductor laser 100. As described above, in the semiconductor laser 100, by integrating the InP lens 108, the mode radius of the waveguide structure of the resonator is converted according to the mode radius of the optical fiber, and the optical fiber can be simply mounted without using an external lens. Can be combined.

  In general, the mode distribution in a laser resonator is not symmetric in the vertical and horizontal directions of the substrate, but is an asymmetric distribution depending on the waveguide structure. That is, it has different mode radii in the vertical direction and the horizontal direction.

FIG. 3 is a cross-sectional view of a laser resonator 200 having a buried hetero structure which is one of general waveguide structures.
In FIG. 3, in the vertical direction of the substrate 210, the multilayer quantum well layer includes an upper and lower SCH layer 201, an upper p-type InP layer 202, a lower n-type InP layer 203, and a quantum well layer 204. Have a laminated structure. Each SCH layer 201 is made of AlGaInAsSCH, and the quantum well layer 204 is made of AlGaInAs.

The thickness of the core layer including the quantum well 204 and the upper / lower SCH layer 201 is generally about 200 to 300 nm. In the horizontal direction of the substrate 210, the insulating InP layers 201 and 203 function as cladding layers.
In FIG. 3, the waveguide width (stripe width) Ws is generally set to about 1.5 to 2.0 μm according to the oscillation wavelength size. In FIG. 3, hc represents the thickness of the core layer, and hm represents the mesa height.

  The difference in the mode radius is caused by the difference in the optical confinement structure in the horizontal direction and the vertical direction in the waveguide described above. This point shows the simulation result of the mode distribution of the laser resonator 200 in FIG. 4 described later.

  FIG. 4 is a diagram illustrating a simulation result of the mode distribution of the laser resonator 200. In the example of FIG. 4, the stripe width Ws is 1.5 μm, the core layer thickness hc is 200 nm, and the mesa height hm is 2.6 μm as the waveguide structure of the resonator.

  An integrated laser 100 as shown in FIG. 2 was used as the integrated laser at the time of simulation. It was confirmed that the mode radius at this time was 1.1 μm in the horizontal direction of the substrate 210 and 0.55 μm in the vertical direction. Furthermore, a flat mode distribution having a mode radius of about twice in the horizontal direction was confirmed. The light emitted from the resonator at this time propagates with different divergence angles in the vertical direction and the horizontal direction, and reaches the inclined mirror 102 (FIG. 2).

  The tilt mirror 102 is a plane, and reflects light in the vertical direction of the substrate 210 (FIG. 3) while maintaining the vertical and horizontal mode distribution of the beam. Light having a flat mode distribution that has propagated at different beam divergence angles in the vertical and horizontal components is incident on the InP lens 108. The condensing distance and the converging angle after passing through the lens 108 differ depending on the vertical component and the horizontal component.

K. Adachi et al., “A 1.3-μm Lens-Integrated Horizontal-Cavity Surface-Emitting Laser with Direct and Highly Efficient Coupling to Optical Fibers,” Optical Fiber Communication Conf./National Fiber Optic Engineers Conf., San Diego, CA , Paper jThA31, Mar. 22-26, 2009

  In the conventional condensing laser, the light emission direction is changed by an inclined mirror. However, since the mode distribution of light is different between the vertical direction and the horizontal component, there is a problem that coupling loss occurs in the optical fiber.

  The present invention has been made under such circumstances, and an object of the present invention is to provide an optical element that can improve the mode distribution of flat light and improve the coupling efficiency with an optical fiber.

  An invention for solving the above-described problems includes: an optical resonator formed on a substrate; and light emitted from the optical resonator in an optical resonator axial direction in a direction perpendicular to the optical resonator axial direction. A tilting mirror that reflects the light and a lens that collects the reflected light, and the tilting mirror has a reflecting surface, and the reflecting surface having the curvature reflects the light, thereby The mode radius is corrected so that the mode distribution is circular.

  Here, the tilt mirror may be configured to reflect the light on the surface of the substrate, and the lens may be formed on the surface side of the substrate.

  The tilt mirror may be configured to reflect the light on a back surface of the substrate, and the lens may be formed on the back surface side of the substrate.

  The reflective surface of the inclined mirror may have a curvature in the horizontal direction with respect to the substrate so that the mode radius in the horizontal direction is corrected.

  When the distance from the optical resonator end to the tilt mirror is 4.0 μm or less, the curvature of the tilt mirror may be set to 30 μm or less.

  The substrate and the lens may be made of InP.

  According to the present invention, the mode distribution of flat light can be improved and the coupling efficiency with an optical fiber can be improved.

It is a figure which shows the structure of the conventional semiconductor laser. It is a figure which shows the structure of the conventional surface emitting type lens integrated laser. It is sectional drawing of the laser resonator which has a general buried heterostructure. It is a figure which shows the simulation result of the mode distribution of the laser resonator of FIG. It is a figure which shows the structural example of the semiconductor laser of 1st Embodiment. It is a figure which shows an example of the relationship between the propagation distance of the Gaussian beam which propagates in the InP layer, and a beam radius. It is a figure which shows an example of the dependence of the coupling efficiency with respect to the inclination mirror curvature in case propagation distances differ. It is a figure which shows an example of the dependence of the coupling efficiency with respect to the inclination mirror curvature in case a mode distribution is flat. It is a figure which shows the structural example of the semiconductor laser of 2nd Embodiment.

<First Embodiment>
The optical element according to the first embodiment of the present invention will be described below. In the description of this embodiment, for example, an optical element is applied to a semiconductor laser.

[Configuration of semiconductor laser]
5A and 5B are diagrams illustrating a configuration example of the semiconductor laser 1 according to the present embodiment, in which FIG. 5A is a top view of the semiconductor laser 1, FIG. 5B is a side view of the semiconductor laser 1, and FIG. FIG.

  As shown in FIGS. 5A to 5C, the semiconductor laser 1 includes an n-InP substrate 10, an upper electrode 11, an inclined mirror 12, a diffraction grating 13, an upper SCH layer 14, and a multilayer quantum well. A layer 15, a lower SCH layer 16, a highly reflective film 17, an InP lens 18, and a lower electrode 19 are provided. The semiconductor laser 1 further includes InP cladding layers 20 and 21.

  The tilting mirror 12 has an angle of 45 ° with respect to the light emission direction from the resonator (optical resonator axial direction). This angle is formed by etching.

  In this semiconductor laser 1, the curvature of the curved surface of the inclined mirror 12 is set to Rm in order to correct the mode distribution (having a horizontal component and a vertical component) emitted from the resonator. In FIG. 5A, for example, the shape of the boundary of the etching mask that forms the reflecting surface of the inclined mirror 12 is set to be a curve. A circle that draws this curve (indicated by a two-dot chain line in FIG. 5A) has a center o at a position that extends on the extension line of the resonator stripe on the substrate surface. The radius of the circle is Rm.

  Note that the distance z in FIG. 5B represents the propagation distance of light propagating in the InP medium in the horizontal direction of the substrate, and d1 in FIG. 5B represents the propagation distance of light propagated in the vertical direction of the substrate. ing.

  In this semiconductor laser 1, the output light emitted from the end of the resonator propagates through the InP medium as a Gaussian beam and reaches the inclined mirror 12. The inclined mirror 12 is formed with a reflecting surface so as to form 45 ° with respect to the emitting direction of the resonator, and the traveling direction of incident light is converted into the vertical direction of the substrate 10 by total reflection on this surface. Thereby, the output light emitted from the end of the resonator reaches the InP lens 18 formed on the back surface of the substrate on the optical axis via the reflection mirror 12.

[Method of manufacturing semiconductor laser]
Next, a method for manufacturing the semiconductor laser 1 will be described with reference to FIG. 5 again.
When the semiconductor laser 1 is manufactured, a substrate in which the n-InP cladding layer 22, the lower SCH layer 16, the multiple quantum well layer 15, and the upper SCH layer 14 are grown on the SI-InP substrate is used as an initial substrate.

  First, after forming the diffraction grating 13 on the surface of the substrate described above, the p-InP clad layer 22 is formed by burying the diffraction grating 13 by regrowth. Next, a mesa structure is formed by etching, and a semi-insulating InP layer doped with Fe is formed on both sides of the mesa by burying regrowth again.

  The resonator includes, in the vertical direction, a core layer (total thickness of 200 nm) composed of a multiple quantum well layer 15, a lower SCH layer 16, and an upper SCH layer 14, and an InP cladding layer 20 sandwiching the core layer from above and below, 21 (each having a layer thickness of 2 μm). And in the horizontal direction of the resonator, it has a buried hetero structure in which InP layers are formed on both sides of the mesa.

  In the semiconductor laser 1 of this embodiment, the resonator length is 200 μm and the stripe width is 1.5 μm. The semiconductor laser 1 operates in the DFB mode associated with the diffraction grating 13 formed in such a resonator.

  Next, after etching one end of the resonator to form the p-side and n-side electrodes 11 and 19 that perform buried regrowth of i-InP, in the i-InP region where buried regrowth was performed, Etching is performed at an angle of 45 ° with the laser stripe. At this time, the inclined mirror 12 having a curved surface in the horizontal direction of the substrate 10 is formed by making the boundary of the etching mask a curved surface having a curvature Rm.

  The tilting mirror 12 is disposed at a distance z = 1.0 μm from the end of the resonator. By the inclined mirror 12, output light from the resonator is emitted from the back surface of the substrate. In the semiconductor laser 1 of this embodiment, the curvature Rm of the inclined mirror 12 is a curved surface shape with Rm = 3.8 μm.

  Then, the InP substrate 10 is polished to set d1 shown in FIG. 5 to 100 μm. Thereafter, an InP lens 18 having a curvature R1 of 200 μm is formed on the back surface of the substrate by etching. In this way, the semiconductor laser 1 is manufactured. When this semiconductor laser 1 was used to conduct a coupling experiment with a single mode fiber, it showed good characteristics with a coupling loss of 2 dB or less. This indicates that the efficiency is improved by 20% or more compared to the conventional InP lens integrated laser (FIG. 2) having a plane mirror.

Next, various characteristics of the semiconductor laser 1 described above will be described.
FIG. 6 is a diagram showing the propagation distance and beam radius of a Gaussian beam propagating in the InP medium. 6, when the DFB mode operation of the semiconductor laser 1, the mode radius beam emission becomes 0.55μm vertically _{w 0y,} have become 1.1μm horizontally _{w 0x.}

  In general, since the vertical mode radius of the resonator is small, the spread angle in the vertical direction is larger. For example, when the propagation distance z (FIG. 5B) is long, a part of the mode distribution leaks outside the cladding layer due to the beam spread in the vertical direction. Therefore, when the upper clad layer 21 is 2 μm, the distance z at which 90% or more of the light distribution exists in the InP medium needs to be 4 μm or less.

  Next, the calculation result of the coupling efficiency between the inclined mirror 12 and the optical fiber will be described with reference to FIG. FIG. 7 is a diagram illustrating an example of the dependence of the coupling efficiency on the curvature of the inclined mirror 12 when the propagation distance z is different. In FIG. 7, the horizontal axis indicates the propagation distance, and the vertical axis indicates the beam radius.

In FIG. 7, the calculation results of the curvature R ₁ and the coupling efficiency of the inclined mirror 12 when the propagation distance z from the resonator end to the inclined mirror 12 is 0.5 μm, 1.0 μm, 2.0 μm, and 4.0 μm are shown. It is shown.
The curvature R ₁ of the InP lens 18 is designed in accordance with the mode radius w _{1y in} the vertical direction (0.55 μm in the example of FIG. 7). In the example of FIG. 7, R _l is a value at which light is most coupled to the optical fiber, that is, R _l = 200 μm.
As described above, considering that the output light from the resonator has a beam spread during propagation, the propagation distance z needs to be 4.0 μm or less.

  It can be seen from FIG. 7 that high coupling of 90% or more can be obtained by designing the curvature Rm of the inclined mirror 12 according to the propagation distance z. For example, when the propagation distance z to the tilting mirror 12 is 4.0 μm or less, high coupling of 90% or more can be obtained when the curvature Rm of the tilting mirror 12 is 30 μm or less.

  Next, the result of calculating the difference in coupling efficiency with respect to the mode radius of the resonator will be described with reference to FIG. FIG. 8 is a diagram illustrating an example of the dependence of the coupling efficiency on the tilt mirror curvature when the mode distribution is flat.

In FIG. 8, the InP lens when the vertical mode radius w _1y in the resonator is 0.55 μm and the horizontal mode radius w _1x is 0.5 μm, 1.0 μm, 1.5 μm, and 2.0 μm. A curvature R _{l of} 18 and coupling efficiency is shown.
The curvature R _l of the InP lens 18 is designed in accordance with the mode radius w _{1y in the} vertical direction (0.55 μm in the example of FIG. 8), and R _l = 200 μm. The distance z from the end of the resonator to the tilting mirror 12 was 1 μm.

 FIG. 8 shows that even when the mode radius is greatly different between the horizontal direction and the vertical direction, a high coupling efficiency of 90% or more can be realized by setting the curvature Rm of the inclined mirror 12 to 20 μm or less.

  As described above, according to the optical element 1 of the present embodiment, the inclined mirror 12 has a reflection surface having a curvature Rm, and light is reflected by the reflection surface having the curvature Rm, so that the mode distribution of light is circular. The mode radius is corrected so that This improves the mode distribution of flat light and improves the coupling efficiency with the optical fiber.

Second Embodiment
In the semiconductor laser 1 of the first embodiment, the case where the light emission direction is converted to the back side of the substrate by the inclined mirror 12 has been described. However, the semiconductor laser may be configured to convert to the substrate front side. .

  FIG. 9 is a diagram illustrating a configuration example of the semiconductor laser 1A according to the second embodiment, in which (a) is a top view of the semiconductor laser 1A, (b) is a side view of the semiconductor laser 1A, and (c) is a semiconductor laser. The front view of 1A is shown. In the following description, the symbols used in the description of the first embodiment are used as they are unless otherwise specified.

As shown in FIGS. 9A to 9C, the semiconductor laser 1A includes a substrate 10, an upper electrode 11, a diffraction grating 13, an upper SCH layer 14, and a multilayer as in the first embodiment. A quantum well layer 15, a lower SCH layer 16, and a highly reflective film 17 are provided. Further, the semiconductor laser 1A includes InP cladding layers 20 and 21. Although the lower electrode is not shown in FIG. 9, the semiconductor laser 1A is also provided with the lower electrode in the same manner as that shown in FIG.
On the other hand, unlike the first embodiment, the semiconductor laser 1A includes an inclined mirror 12A having an angle of + 45 ° with respect to the light emitting direction from the resonator. The semiconductor laser 1A includes an InP lens 18A that condenses the light reflected by the tilt mirror 12A on the upper surface side (surface side of the substrate 10) of the tilt lens 12A.

  The production of the semiconductor laser 1A is almost the same as that in the first embodiment. That is, a substrate obtained by growing the lower SCH layer 16, the multiple quantum well layer 15, and the upper SCH layer 14 on the SI-InP substrate is used as the initial substrate.

  First, after forming the diffraction grating 13 on the substrate surface described above, the p-InP cladding layer 22 is formed by burying the diffraction grating 13 by regrowth. Next, a mesa structure is formed by etching, and a semi-insulating InP layer doped with Fe is formed on both sides of the mesa by burying regrowth again.

  The resonator includes, in the vertical direction, a core layer (total thickness of 200 nm) composed of a multiple quantum well layer 15, a lower SCH layer 16, and an upper SCH layer 14, and an InP cladding layer 20 sandwiching the core layer from above and below, 21 (each having a layer thickness of 2 μm). And in the horizontal direction of the resonator, it has a buried hetero structure in which InP layers are formed on both sides of the mesa.

The resonator length can be 150 μm and the stripe width can be 1.6 μm. The semiconductor laser 1A operates in the DFB mode associated with the diffraction grating 13 formed in such a resonator. The optical mode radii of the output light emitted from the end of the resonator were w _1y = 0.55 μm and w _1x = 1.16 μm in the vertical and horizontal directions, respectively. Subsequently, one end of the resonator was etched, and i-InP buried regrowth was performed to form an InP lens 18A having a curvature R ₁ of 9 μm on the substrate surface in the i-InP region.

  After forming the n-side and p-side electrodes, etching is performed at an angle of 135 ° with the emission direction of the resonator. At this time, an inclined mirror 12A having a curved surface in the horizontal direction of the substrate 10 is formed by making the boundary of the etching mask a curved surface having a curvature Rm.

  Further, the tilting mirror 12A is disposed at a position where the distance z = 0.5 μm from the end of the resonator. Output light from the resonator is emitted from the substrate surface by the inclined mirror 12A. In the semiconductor laser 1A of this embodiment, the inclined mirror 12A has a curved surface shape with a curvature Rm = 2.3 μm, and the length from the inclined mirror 12A to the InP lens 18A is 4 μm.

  Also in the semiconductor laser 1A manufactured in this way, in the coupling experiment to the single mode fiber, the coupling efficiency showed a good characteristic of 3 db or less as in the first embodiment. It was also found that the efficiency was improved by 50% or more compared to the conventional InP lens integrated laser (FIG. 2) having a plane mirror.

  In addition, each said embodiment is not restricted to the example mentioned above, It can change. For example, in the semiconductor lasers 1 and 1A, even when the stripe width and the core layer thickness are changed regardless of the structure of the waveguide 200 shown in FIG. 3, the mirror curvature corresponding to the flat mode distribution in that case is used. Thus, high coupling efficiency can be realized.

  In addition, the semiconductor lasers 1 and 1A have a high coupling efficiency by using a mirror curvature corresponding to a flat mode distribution even in a resonator having a different waveguide structure such as a ridge structure or a high mesa structure regardless of a buried hetero structure. Can be realized. Further, the material and size of the substrate 10, the lens 18, and the resonator may be appropriately changed as long as high coupling efficiency can be realized by the inclined mirrors 12 and 12A.

  In each of the above embodiments, the case where the optical element is applied to a semiconductor laser has been described. However, the present invention can also be applied to, for example, a waveguide or a modulator.

1,1A Semiconductor laser 10 Substrate 12 Tilt mirror 18 Lens

Claims (6)

An optical resonator formed on a substrate;
An inclined mirror that reflects light emitted from the optical resonator in the optical resonator axial direction in a direction perpendicular to the optical resonator axial direction;
A lens for collecting the reflected light, and
The inclined mirror has a reflection surface having a curvature, and the light is reflected by the reflection surface having the curvature so that the mode radius is corrected so that the mode distribution of the light is circular. A characteristic optical element.   The optical element according to claim 1, wherein the inclined mirror is configured to reflect the light on a surface of the substrate, and the lens is formed on the surface of the substrate.   The optical element according to claim 1, wherein the inclined mirror is configured to reflect the light on a back surface of the substrate, and the lens is formed on the back surface of the substrate.   The reflective surface of the inclined mirror has a curvature in the horizontal direction with respect to the substrate so that the mode radius in the horizontal direction is corrected. Optical element.   5. The light according to claim 1, wherein when the distance from the end of the optical resonator to the inclined mirror is 4.0 μm or less, the curvature of the inclined mirror is set to 30 μm or less. 6. element.   6. The optical element according to claim 1, wherein the substrate and the lens are made of InP.