JP2009231423A - Wavelength variable light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a wavelength variable light source capable of securing high reliability even during a high output operation. <P>SOLUTION: In the wavelength variable light source provided with an optical semiconductor element 1 to be a gain medium, a wavelength selection means 3 and a ring resonator (constituted of an isolator 2, lenses 4 and 5 and an optical fiber 6, for instance) and configured so that light L inside the resonator may pass through the optical semiconductor element 1 only in one direction, a current non-injection area is provided at least on the end part of the element 1 on the side of an end face 1b where the light L is incident from the outside of the element out of both end faces 1a and 1b of the optical semiconductor element 1 where the light passes through. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a tunable light source, and more particularly to a ring laser type tunable light source.

  In recent years, a variable wavelength light source using an optical semiconductor element has attracted attention in fields such as measurement and medical diagnosis. The wavelength variable light source is a laser capable of changing the oscillation wavelength, and a laser called the Littman method and a laser called the Littrow method are well known. In addition to them, a ring laser type described in Non-Patent Document 1, for example, is also known. This ring laser type wavelength tunable light source basically includes an optical semiconductor element serving as a gain medium, a ring resonator including an isolator for determining the traveling direction of light in one direction, a wavelength filter for wavelength selection, and a ring resonator. And an optical coupler that extracts light from the outside.

  In the ring laser type wavelength tunable light source, since each component is usually connected by an optical fiber, this wavelength tunable light source is characterized in that it is easy to manufacture and has excellent stability. The Littman type and Littrow type have a light emitting element inside the external resonator, and the oscillation light reciprocates inside the light emitting element, whereas in a ring laser type wavelength tunable light source, the light traveling direction is an isolator. Therefore, the oscillation light has a feature that it passes through the light emitting element only in one direction.

  Here, the operating principle of the ring laser will be described with reference to FIG. First, light is emitted from both end faces of the light-emitting element 1, which is an optical semiconductor element serving as a gain medium. Then, by the isolator 2 arranged outside the light emitting element 1, only the light emitted from one end face of the element 1 circulates in the ring resonator. The light emitted from the light emitting element 1 enters the wavelength filter 3, and only light having a specific wavelength passes therethrough. This wavelength-selected light is guided through the optical fiber 6 with the optical coupler 7 interposed in the middle, and returned to the light emitting element 1 again. Light incident on the light emitting element 1 is amplified by a stimulated emission phenomenon. This is repeated, and laser oscillation starts when the gain balances the loss inside the ring resonator and the loss of light exiting from the ring resonator. The oscillated laser beam is extracted from the optical coupler 7 to the outside of the resonator. For example, when an etalon is used for the wavelength filter 3, the selected wavelength changes by swinging it in the direction of arrow A, so that the wavelength of the extracted laser light can be controlled depending on the setting of the swing position. It becomes.

The light emitting device used here basically has a structure in which laser oscillation is suppressed by removing a resonator from a normal semiconductor laser. Specifically, in order to suppress light reflection at the end face, an anti-reflection film is applied, or the optical waveguide itself is formed obliquely with respect to the end face to suppress laser oscillation inside the element. . The characteristics of this type of wavelength tunable light source largely depend on the characteristics of the light emitting element, and the gain spectrum width determines the wavelength variable width, and the gain of the light emitting element determines its output.
SH Yun, et al. “High-speed wavelength-swept semiconductor laser with a polygon-scanner-based wavelength filter” Optics Letters, Vol. 28, Issue 20, pp. 1981-1983 (2003)

  In the above-described ring laser type wavelength tunable light source, it is expected that the demand for higher output will increase as the wavelength tunable width increases. In order to meet this requirement, it is necessary to improve the output of the light emitting element mounted on the wavelength tunable light source.

  Increasing the light confinement rate in the active layer is an effective technique for increasing the output of the light emitting element, but this may cause a characteristic end face deterioration called optical damage (COD). This phenomenon is particularly noticeable in light-emitting devices in which a semiconductor layer is formed on a GaAs or GaN substrate, and this is a major factor that hinders the high output of wavelength tunable light sources and the reliability during high output operation. . In particular, in a wavelength tunable light source, the light confinement rate inside the light emitting element changes due to a change in the oscillation wavelength, and a failure is likely to occur in a wavelength region where the light confinement rate increases.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a wavelength tunable light source that can ensure high reliability even during high output operation.

As described above, the wavelength tunable light source according to the present invention includes an optical semiconductor element serving as a gain medium, a wavelength selection unit, and a ring resonator, and light inside the resonator passes through the optical semiconductor element only in one direction. In the wavelength tunable light source configured as follows,
A current non-injection region is provided at an end of the optical semiconductor element, at least on an end face side where light enters from the outside of the element, of both end faces through which the light passes. is there.

  Here, the knowledge that has led to the above configuration will be described. The optical semiconductor element end face that emits light in the forward direction with respect to the traveling direction of light directed by the isolator in the ring resonator has only a problem of heat generation due to self-emission, whereas In addition to the heat generated by self-emission, the element end face on the light exit side (that is, the light incident from the outside), the light that has circulated around the ring laser is incident on the inside of the element from the outside. The light density increases and the amount of heat generation increases. From the above, the present inventor has found out that the end face of the optical semiconductor element on which light is incident from the outside is likely to deteriorate after intensive efforts. More specifically, the light incident on the end face of the optical semiconductor element from the outside causes localization of light due to nonlinear interaction with the light exiting from the end face, so that the end face of the element is likely to be deteriorated. It is thought that.

  Based on the knowledge described above, in the present invention, a current non-injection region is provided at the end of the optical semiconductor element on the end surface on the side where the light circulated in the ring resonator is incident again.

  Note that, unlike a semiconductor laser, a gain medium that does not itself have a resonator has no output amplification due to a resonance phenomenon. Therefore, if a current non-injection region is provided, light loss increases due to light absorption in that region. Therefore, it is desirable to provide the current non-injection structure only on the end face side where light enters from the outside of both end faces of the optical semiconductor element.

  The above-mentioned current non-injection region refers to a structure that suppresses current injection in the vicinity of the end face in order to reduce heat generation at the end face of the optical semiconductor element due to higher output, and is generally called a window structure. One of things. This current non-injection structure is proposed for application to a semiconductor laser in Japanese Patent Application Laid-Open No. 3-187284, Japanese Patent Application Laid-Open No. 5-29706, and the like. However, it has not been proposed at all to apply such a current non-injection region to a gain medium in an external resonator.

  More specific current non-injection structures include those in which the electrode in the vicinity of the end surface is removed, the electrode in which the second conductivity type contact layer in the vicinity of the end surface is removed, and the insulating film so as to cover the second conductivity type contact layer Are formed, and any of them can be suitably employed in the present invention.

  Here, the current non-injection region is desirably in a range from 5 μm to 50 μm from the element end face toward the inside of the element. If the area is smaller than 5 μm, the spread current may reach the end face, which is not preferable because it does not substantially become a current non-injection area but causes deterioration of the end face. Further, when the current non-injection region is larger than 50 μm, the light loss due to light absorption in this region becomes large and the light output is reduced, which is not desirable.

  Further, the present invention is particularly preferably applied to a wavelength tunable light source using an optical semiconductor element in which a semiconductor layer is laminated on a GaAs or GaN substrate.

  In the wavelength tunable light source of the present invention, since the current non-injection region is provided as described above, the end face of the optical semiconductor element on which the light inside the resonator is incident is increased in light density even when operated at a high output. By this, destruction is prevented and high reliability can be obtained. Therefore, as described above, it is possible to sufficiently cope with the problem that a failure is likely to occur particularly in a wavelength region where the optical confinement rate is high. Thus, according to the present invention, it is possible to realize a wavelength tunable light source with high output and high stability.

  As described above, optical damage (COD) occurs particularly prominently in an optical semiconductor device in which a semiconductor layer is formed on a GaAs or GaN substrate. Therefore, the present invention uses such an optical semiconductor device. It is particularly effective when applied to a wavelength tunable light source.

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

  FIG. 1 shows a wavelength tunable light source according to an embodiment of the present invention. As shown in the figure, this wavelength tunable light source includes a light emitting element 1 that is an optical semiconductor element serving as a gain medium, an isolator 2 that defines a traveling direction of light L in a ring resonator configured as described below, The wavelength filter 3 that selects the wavelength of the light L that passes through the ring resonator, the lens 4 that collimates the light L emitted from the one end face 1a of the light emitting element 1, and the light L that is converted into parallel light is condensed. An optical fiber 6 in which an optical path is set so as to return the light L emitted from the one end surface 1a to the other end surface 1b of the light emitting element 1, and an optical coupler 7 interposed in the middle of the optical fiber 6. And have.

  In addition, lenses 4 and 5 similar to the above are disposed on the other end surface 1 b side of the light emitting element 1, and the isolator 2 is disposed between the lenses 4 and 5. On the other hand, the wavelength filter 3 is disposed between the lenses 4 and 5 on the one end face 1 a side of the light emitting element 1. In the present embodiment, a ring resonator is constituted by the lenses 4 and 5 and the optical fiber 6 described above. The wavelength filter 3 can be reciprocally swung manually or automatically in the direction of the arrow A, and the passing wavelength range changes when the swiveling position is changed.

  Hereinafter, the operation of the wavelength tunable light source will be described. Light L is emitted from one end face 1a and the other end face 1b of the light emitting element 1, respectively. The light L emitted from the one end face 1a passes through the lens 4, the wavelength filter 3 and the lens 5, enters the optical fiber 6, is guided there, and enters the light emitting element 1 again from the other end face 1b. On the other hand, since the light L emitted from the other end face 1b of the light emitting element 1 is blocked by the isolator 2, the traveling direction of the light in the ring resonator is limited to only one direction.

  In the wavelength filter 3, only light of a specific wavelength among the incident light L passes. The light L incident on the light emitting element 1 after the wavelength is selected in this way is amplified by the stimulated emission phenomenon. This is repeated, and laser oscillation starts when the gain balances the loss inside the ring resonator and the loss of light exiting from the ring resonator. The oscillated laser beam L is extracted from the optical coupler 7 to the outside of the resonator. At this time, since the wavelength of the light L passing therethrough is changed by changing the oscillation position of the wavelength filter 3, the wavelength of the extracted laser light L is changed to a desired value by changing the oscillation position of the wavelength filter 3. Can be controlled. Although a case where an etalon is applied to the wavelength filter 3 is described here, a wavelength filter made of a diffraction grating may be used.

  Here, in the present embodiment, on the end face 1b side of the light emitting element 1, the above-described current non-injection region is provided at the end of the element. Hereinafter, the layer structure of the light-emitting element 1 will be described together with the manufacturing method.

First, as shown in FIG. 2A, as an example, an n-GaAs buffer layer 12, an n-In _0.48 Ga _0.52 P lower clad layer 13 on the (1 0 0) plane of an n-GaAs substrate 11 by metal organic chemical vapor deposition, n or i-In _x1 Ga _1-x1 As _1-y1 P _y1 optical waveguide layer 14, compressive strain In _x3 Ga _1-x3 As _1-y3 P _y3 quantum well active layer 15, p or i-In _x1 Ga _{1- x1} As _{1-y1 Py1} upper optical waveguide layer 16, p-In _0.48 Ga _0.52 P first upper cladding layer 17, GaAs etching stop layer 18, n-In _0.48 Ga _0.52 P constriction layer 19 and GaAs cap layer 20 grown Let

Thereafter, a resist pattern in which an opening region having a 3 μm stripe is formed in the forward mesa direction with respect to the substrate 11 by using a photolithography method, and the GaAs cap layer 20 is etched with a tartaric acid mixed solution. After the resist is peeled off, the n-In _0.48 Ga _0.52 P constriction layer 19 is etched with a hydrochloric acid solution using the GaAs cap layer 20 with the opening formed as a mask. Further, the remaining GaAs cap layer 20 and the portion of the GaAs etching stop layer 18 exposed by etching the constriction layer 19 are removed with a tartaric acid mixed solution. Thus, the state shown in FIG. 2B is obtained.

Next, as shown in FIG. 2C, a p-In _0.48 Ga _0.52 P second upper cladding layer 21 and a p-GaAs contact layer 22 are formed again by metal organic vapor phase epitaxy. A photolithography method is used to form a resist pattern having an opening on the heel side from the broken line (that is, covering from the end surface to the position of the broken line). The width of the resist pattern is set to 60 μm, for example, so that a resist is formed between a position corresponding to a distance of 30 μm from the cleavage plane position of the light emitting element and the element end face.

Next, a Ti / Pt / Au electrode material is deposited on the resist pattern by vacuum deposition, and the electrode material deposited on the resist is peeled off together with the resist, so that a p-electrode is formed in the region that was the opening of the resist pattern. Form 23. Further, an NH ₃ : H ₂ O ₂ aqueous solution is used with the p electrode 23 as a mask, and the p-GaAs contact layer 22 on the outer periphery of the device is removed by etching. As a result, the state shown in FIG. 2D is obtained.

  The region from which the contact layer 22 has been removed becomes a current non-injection region. On the other hand, the contact layer 22 remains, and the portion that is almost directly above the region where the constriction layer 19 is etched becomes the current injection region. In this embodiment, the current non-injection region is formed only at one end of the light-emitting element 1. However, in order to reduce the process complexity, the current non-injection region is formed at both ends of the light-emitting element 1. It doesn't matter.

Then, after alloying at 500 ° C., the n-GaAs substrate 11 is polished to a thickness of 100 μm, an n electrode 24 made of AuGe / Ni / Au is vacuum-deposited on the polished surface, and alloyed again at 400 ° C. After that, the sample is cleaved at the both end face setting positions, and the non-reflective film is formed on both end faces of the bar (the end faces that become the cavity faces in the case of a normal semiconductor laser, and the end faces 1a and 1b after the element is completed). Apply a coat. In this example, a plurality of portions (current injection regions) in which the above-described n-In _0.48 Ga _0.52 P constriction layer 19 is etched are formed in one sample, and thus the bar includes a plurality of light emitting portions. . Therefore, the bar is then cleaved so as to include one light-emitting portion, so that the light-emitting element 1 is completed.

  As described above, the light emitting element 1 is provided with a current non-injection region in which the p-GaAs contact layer 22 is removed in a region near the end face 1b. In the wavelength tunable light source of the present embodiment, by using the light emitting element 1 having such a current non-injection region, the current density in the vicinity of the end face 1b can be reduced, and the heat generation at the end face 1b can be reduced. Can be reduced. Therefore, even if this wavelength variable light source is used at a high output, it is possible to prevent problems such as end face destruction due to an increase in light density, and to ensure high reliability. Such a wavelength tunable light source can be widely applied in fields such as communication, measurement, and medicine.

  In the embodiment described above, the light-emitting element 1 made of the GaAs substrate 11 that easily causes optical damage is applied. However, the present invention is also an optical semiconductor made of a GaN substrate that is also likely to cause optical damage. Even when applied to the case of using an element, it is particularly effective. Furthermore, the present invention can also be applied to the case where an optical semiconductor element made of a substrate different from the above two substrates is used, and even in such a case, the effects described above can be obtained similarly.

  Further, the current non-injection region formed in the optical semiconductor element is not limited to the structure in which the electrode 23 is removed as in the above embodiment, but in addition, the structure in which the second conductivity type contact layer in the vicinity of the end surface is removed, A structure in which an insulating film is formed so as to cover the conductive contact layer can also be applied.

1 is a schematic configuration diagram showing a wavelength tunable light source according to an embodiment of the present invention. 3A and 3B illustrate a process for manufacturing a light-emitting element applied to the wavelength tunable light source in FIG. 3A and 3B illustrate a process for manufacturing a light-emitting element applied to the wavelength tunable light source in FIG. 3A and 3B illustrate a process for manufacturing a light-emitting element applied to the wavelength tunable light source in FIG. 3A and 3B illustrate a process for manufacturing a light-emitting element applied to the wavelength tunable light source in FIG.

Explanation of symbols

1 Light emitting element
2 Isolator
3 wavelength filter
4, 5 lens
6 Optical fiber
11 n-GaAs substrate
12 n-GaAs buffer layer
13 n-In _0.48 Ga _0.52 P lower cladding layer
14 n or i-In _x1 Ga _1-x1 As _1-y1 P _y1 optical waveguide layer
15 Compressive strain In _x3 Ga _1-x3 As _1-y3 P _y3 quantum well active layer
16 p or i-In _x1 Ga _1-x1 As _1-y1 P _y1 Upper optical waveguide layer
17 p-In _0.48 Ga _0.52 P First upper cladding layer
18 GaAs etching stop layer
19 n-In _0.48 Ga _0.52 P constriction layer
20 GaAs cap layer
21 p-In _0.48 Ga _0.52 P Second upper cladding layer
22 p-GaAs contact layer
23 p electrode
24 n electrode

Claims (3)

In a wavelength tunable light source configured to include an optical semiconductor element serving as a gain medium, a wavelength selection unit, and a ring resonator, and configured so that light inside the resonator passes through the optical semiconductor element only in one direction,
Wavelength variable, characterized in that a current non-injection region is provided at an end of the optical semiconductor element, at least on an end face side where light enters from the outside of the element, of both end faces through which light passes. light source.   2. The wavelength tunable light source according to claim 1, wherein the current non-injection region is provided only on an end face side where light enters from the outside of the element.   3. The wavelength tunable light source according to claim 1, wherein the optical semiconductor element is formed by laminating a semiconductor layer on a GaAs or GaN substrate.

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