KR100642628B1 - Single mode vertical resonant surface emitting laser - Google Patents

Abstract

The single - mode characteristics of vertical cavity surface emitting lasers were improved by the simultaneous use of the selective light absorbing layer and the ion implanted layer in the transverse direction and their relative size differences. The light absorbing layer absorbs the light at the edge of the mode, thereby increasing the light loss in the higher order transverse mode and preventing laser oscillation in the higher order transverse mode. In particular, since the center of the light absorbing layer is removed by the patterning and etching processes, a small light loss occurs in the basic mode and a larger light loss occurs in the high-order mode. When the diameter of the light absorbing layer is slightly smaller than the diameter of the ion implanted layer, the basic mode operates in a single mode in a much wider range.

VCSEL, surface emission, laser, single mode, semiconductor, transverse mode, ion implant,

Description

Single mode vertical cavity surface emitting lasers

1 is a cross-sectional view illustrating a single mode vertical cavity surface emitting laser structure according to an embodiment of the present invention.

2 is a comparative cross-sectional view comparing a conventional vertical resonant surface emitting laser structure (A) with a vertical resonating surface emitting laser structure (B) according to an embodiment of the present invention.

Description of the Related Art [0002]

10 ... semiconductor substrate 20 ... lower Bragg reflector

30 ... laser active layer 40 ... spacer

50 ... ion implanted layer

60 ... electric conductive layer 70 ... light absorbing layer

80 ... electrode

90 ... upper Bragg reflector

The present invention relates to a single mode vertical resonance surface emitting laser, and more particularly, to a single mode vertical resonance surface emitting laser, in which an absorption layer is placed in a vertical resonance surface emitting laser so that absorption does not occur in a specific diameter, Mode vertical resonant surface emitting lasers that are capable of sustaining the mode.

Vertical_Cavity Surface-Emitting Lasers (VCSELs) are used as light sources for short / near-field optical communication systems, optical connections, optical storage devices, and laser printing. In particular, due to the high optical fiber coupling efficiency of surface emitting lasers, low unit cost due to wafer fabrication process, low mounting cost, and two-dimensional array characteristics, It is predicted that it will be quickly positioned as a light source.

However, since the conventional surface emitting lasers have a characteristic of multiple transverse mode, there is a transmission distance limitation due to chromatic dispersion in ultra-high speed optical communication and data communication. Therefore, Development efforts are being made on single transverse mode light sources.

Conventional surface emitting lasers employ either ion implantation or oxide apeture for current limiting. Ion implantation has been used in the early days of VCSEL development and is a stable technology used in commercial device manufacturing. However, the disadvantage of this structure is that as the current increases, heat is generated in the center of the laser resonator and the size of the optical mode is changed. This is called a thermal lens effect, and it has a large influence on optical characteristics in an ion implantation type laser having no special refractive index fluctuation structure.

On the other hand, the oxide laser oxidizes the AlAs layer to form an insulating film composed of AlxOy, thereby achieving stable optical mode characteristics by controlling current and refractive index. However, since the refractive index difference between AlAs and AlxOy is large, it is difficult to control the oxide film diameter to a very small diameter of 2-3 μm in order to obtain a single mode. Therefore, a multi-mode output having a large oxide film diameter is mostly used. In addition, the lifetime of a device is basically short in a small-diameter oxide film, which has a fundamental weakness in addition to a difficulty in the process.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a vertical cavity surface emitting laser in which a single mode output is maintained in a wide operating current range through a simple structure.

It is another object of the present invention to provide a structure capable of eliminating the mode instability during operation of the ion implantation structure and avoiding shortening of the lifetime of the single mode oxide film laser.

As a constituent means for achieving the above object, the present invention is a vertical resonant surface emitting laser including an active layer between a lower Bragg reflector vertically stacked on a semiconductor substrate and an upper Bragg reflector, wherein the upper Bragg reflector upper side A current confinement structure for horizontally limiting the current introduced from the current confinement structure to the center of the active layer; And an upper Bragg reflector disposed adjacent to the current confinement structure and having an outer light loss greater than the inner diameter at a boundary with a predetermined diameter, for supplying a current to the central portion of the active layer and being oscillated from the active layer And a light absorbing layer absorbing a part of light to cause selective loss.

Preferably, the light absorbing layer may be formed between the current confinement structure and the upper Bragg reflector, or may be formed between the active layer and the current confinement structure.

Preferably, the boundary portion diameter of the light absorbing layer is smaller than the current confined portion diameter of the current confinement structure.

Preferably, the current confinement structure is formed by injecting ions into the current confinement structure to form a resistive region having a predetermined diameter in a central portion thereof and a resistive region Is an ion-implanted layer for forming a region.

Also preferably, the light absorbing layer may be formed by coating a light absorbing material on the upper surface of the current confinement structure, or by growing GaAs on the current confinement structure upper surface.

Preferably, the light absorbing layer is formed by removing the central portion of the light absorbing layer to a predetermined diameter by etching after the pattern formation so that light absorption does not occur at the central portion.

Preferably, the current confinement structure is formed with an oxide layer for forming a resistivity region of a predetermined diameter and a resistance region outside the current confinement structure at a central portion of the current confinement structure It is possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The surface emitting laser emits light rays perpendicular to the surface of the semiconductor substrate to the lower surface or the upper surface of the substrate. A VCSEL is a surface emitting laser having a reflector disposed parallel to the wafer surface and an optical cavity formed therebetween. In general, a VCSEL includes a substrate having a lower Bragg reflector and an upper Bragg reflector arranged therebetween and having a quantum well active region therebetween. Distributed Bragg Reflectors (DBRs) typically have reflectance above 99%.

1, a vertical cavity surface emitting laser according to the present invention includes a laser active layer 30 having a lower Bragg reflector on a semiconductor substrate 10 and a spacer 40 interposed therebetween. The semiconductor substrate 10 is a substrate formed of a suitable semiconductor material (e.g., gallium arsenide (GaAs), indium phosphide (InP), or a combination of such materials). Although not shown, a lower contact portion is formed on the lower surface of the semiconductor substrate 10, which is formed of a suitable metallic or other conductive material.

The lower Bragg reflector 20 arranged on the semiconductor substrate 10 alternately exhibits a low refractive index and a high refractive index material and can be n-doped. A laser active layer 30 is formed on the lower Bragg reflector 20. The active layer 30 may be composed of a plurality of quantum well layers and a space layer and is a portion into which current is injected and converted into light.

An electrically conductive layer 60 for supplying an electric current is formed on the active layer 30. The electrically conductive layer has a current confinement structure for limiting the current in the lateral direction and supplying the current only to the center portion. On the electrically conductive layer 60, a light absorbing layer 70 and an electrode 80 are formed on the upper side thereof. An upper reflector 90 is formed on the electrode 80 of the light absorbing layer to form a laser resonator together with the lower reflector 20. The upper reflector 90 can be manufactured by a variety of different methods, preferably by a dielectric deposition method.

In addition, the light absorption layer may be formed between the current confinement structure and the upper Bragg reflector as described above, or may be formed between the active layer and the current confinement structure.

In the surface emitting laser structure of the present invention, the current confinement structure is preferably an ion implanted layer (50). The ion implantation layer 50 is formed by implanting hydrogen, deutron, or various ions into the electrically conductive layer 60 to form a resistive layer in which only crystals in the implanted region are destroyed, To form a layer.

The light absorption layer 70 functions to supply current to the central part of the laser active layer 30 by gathering the current starting from the upper electrode 80 on the funnel through the ion implantation layer. And at the same time a selective optical loss of the light oscillated therefrom. Therefore, it is most preferable that the light absorption layer is arranged adjacent to the ion implantation layer, and the arrangement position thereof may be the upper side or the lower side of the ion implantation layer. The light absorbing layer according to the present invention absorbs light emitted from the active layer. For example, it is possible to coat a material such as Si, which is an absorbing material for absorbing light at 850 nm, It is also possible to grow the absorbent layer using a material. Such a light absorption layer is patterned at a central portion thereof with a diameter smaller than the center diameter of the current confinement structure, and is etched and removed. The light absorbing layer according to the present invention is not limited to this, and any other structure capable of absorbing light can be understood by those skilled in the art.

In the present invention, the light absorbing layer 70 is used for absorbing the oscillation light while using the ion-implanted layer 50 as the current confinement structure. The ion implantation layer 50 functions not only to limit the current but also to increase the refractive index by generating heat in the current path.

The diameter at which the refractive index increases and the diameter at the ion implantation structure are almost the same. As the temperature increases, the difference between the central refractive index and the peripheral refractive index increases. As the refractive index difference increases, the optical index guiding effect becomes larger, so that the optical loss is reduced, and the transverse mode is generated, so that the single mode operation becomes difficult.

To prevent this, the light absorbing layer 70 absorbs the light at the edge of the mode, thereby increasing the light loss in the higher order transverse mode, thereby preventing laser oscillation in the higher order transverse mode. In particular, the light absorbing layer is formed such that the outer light loss is larger than the inner light loss with a certain diameter as a boundary. That is, the light absorbing layer is formed by removing the middle portion by the patterning and etching process, thereby increasing the light loss in the high-order transverse mode and reducing the light loss in the basic mode.

In contrast to the present invention, when the current confinement structure is formed in an oxide film structure, there is a problem that the difference in refractive index increases and the number of existing modes increases. That is, when AlAs is used as the oxide film structure, the refractive index of the center portion is about 2.8, and the refractive index of the oxidized AlOx is about 1.6. Thus, the refractive index difference is significantly increased.

On the other hand, if the ion confinement structure is used as the current confinement structure as in the present invention, the refractive index difference inherent to the material can be substantially eliminated. However, the ion implantation layer has a difference in effective refractive index generated by heat, and this value is 0.02 or less. Therefore, according to the principle that the difference in the refractive index between the inner side and the outer side is smaller, it is easier to maintain the single mode. Thus, the oxide film structure has a more unfavorable condition than the ion implantation structure in the single mode oscillation.

In the present invention, a light absorption layer is used for absorbing the multi-mode oscillation that passes through the center of the ion implantation layer while using the ion implantation layer as the current confinement structure.

When the diameter is about 10 microns in the ion implanted structure, multimode occurs and the optical output transitions between the various transverse modes, causing noise. However, in a single mode, such a mode transition does not occur and a stable output can be obtained. In order to obtain a single-mode oxide laser, a diameter of about 2 microns is required, and the life time of such a small oxide vertical cavity surface-emitting laser is greatly shortened.

Therefore, in the present invention, a current confinement structure is formed with a relatively large diameter using an ion implantation structure, and a light absorption layer is formed on the current confinement structure to absorb the multi-mode light output. The use of the structure of the present invention to obtain the single mode characteristic makes it possible to solve the problem of shortening the life time.

The light absorption layer 70 of the present invention is formed such that the outer light loss is larger than the inner light loss with a certain diameter as a boundary and the diameter of the light absorption layer 70 is slightly smaller than the diameter of the ion implantation layer 50 The base mode can be made to operate in a much wider range of single modes.

In Fig. 2, Structure A is a structure with only a 10 占 퐉 ion implantation diameter (a) without a selective light loss layer. Structure B is a structure including a selective light absorbing layer according to the present invention, an ion implantation diameter (b) of 10 μm, and a selective light loss layer diameter (c) of 6 μm.

Table 1 below is a table comparing basic mode selection ratios of VCSELs of structure A and structure B. Here, the basic mode selection ratio represents the ratio of the light output between the basic mode and the next higher-order transverse mode. The larger this value is, the better the characteristic of operating in single mode.

Basic mode selection ratio = optical output (LP01 mode) / optical output (LP11 mode)

Α (difference between optical gain and optical loss between two modes) / (critical optical gain value)

This value is tabulated according to the structure and refractive index change Δn as follows.

Δn Structure A Structure B 0.00 (when device temperature does not rise) 0.126 1.80 0.01 (device temperature rise corresponds to 30 to 40 degrees) 0.02 1.55

As a result of calculation, when the ion implantation diameter of 10 μm for current confinement was included in the structure with the selective light absorption layer having a diameter of 6 μm, the fundamental mode selection ratio increased by about 70 times as compared with the case without the light absorption layer. This is to prove that the single mode characteristic is drastically improved.

In this case, it is assumed that the selective absorption region absorbs only at a radius of 3 μm or more, does not absorb the inside of the selective absorption region, and only slightly increases the refractive index (0.002). This is because the light distribution of the fundamental mode is gathered in the inside and the higher order modes are slightly distributed outside, so that if the diameter of the light absorption layer is made slightly smaller than the diameter of the ion implantation, different modes are caused depending on the mode.

Therefore, the present invention suffers from laser oscillation because it experiences a larger optical loss in the higher-order mode. In fact, if the diameter of the selective light loss layer is equal to or greater than the diameter of the ion implantation, the higher order transverse mode will experience a larger light loss than the single mode, but the effect will be greater if the light loss layer diameter is slightly smaller than the ion implantation diameter.

The change in refractive index 0.01 is an effect due to an increase in the refractive index at the center of the device when a current is passed through the device and is a typical value in an ion-implanted VCSEL. At this time, the ratio of the basic mode selection ratio between the structure A and the structure B is about 70 times different. This means that the mode suppression ratio between the two modes is 70 times different in the device operation.

As described above, according to the present invention, a single mode output is maintained in a wide operating current range. In order to achieve this, the present invention provides a current confinement structure in which an ion implantation layer is formed and used simultaneously with the photoabsorption layer, and the diameter of the photoabsorption layer is formed to be smaller than the diameter of the ion implantation layer to prevent laser oscillation in the high- So that it can be used as a single mode. The present invention also provides a surface emitting laser with a simpler structure than the prior art, so that the single mode output is maintained in a wider operating current range.

In addition, the present invention provides an effect of eliminating the mode instability during operation of the ion implantation structure and avoiding shortening of the lifetime of the single mode oxide film laser.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims It will be understood by those skilled in the art that it is readily known.

Claims (9)

A vertical resonant surface emitting laser comprising an active layer between a vertically stacked lower Bragg reflector and an upper Bragg reflector on a semiconductor substrate, A current confinement structure for limiting the current introduced from the upper side of the upper Bragg reflector in the lateral direction to supply the current to the center of the active layer; And A current confinement structure arranged in the lower portion of the upper Bragg reflector so as to have a larger outside light loss at a boundary with a predetermined diameter and to supply a current to a central portion of the active layer, And a light absorbing layer absorbing a part of the light absorbing layer to cause selective loss, The current confinement structure is formed by injecting ions into the current conduction layer to form a resistivity region having a predetermined diameter and a resistance region outside the center region in the center of the current confinement structure, Wherein the ion-implanted layer is an ion-implanted layer. The vertical cavity surface emitting laser according to claim 1, wherein the light absorbing layer is formed between the current confinement structure and the upper Bragg reflector. The vertical cavity surface emitting laser according to claim 1, wherein the light absorption layer is formed between the active layer and the current confinement structure. 2. The vertical cavity surface emitting laser according to claim 1, wherein a boundary portion diameter of the light absorbing layer is smaller than a current limiting portion diameter of the current confinement structure. delete The vertical cavity surface emitting laser according to claim 1, wherein the light absorbing layer is formed by coating a light absorbing material on the current confinement structure upper surface. The vertical cavity surface emitting laser according to claim 1, wherein the light absorbing layer is formed by growing GaAs on the upper surface of the current confinement structure. 2. The vertical cavity surface emitting laser according to claim 1, wherein the light absorbing layer is formed by etching after the pattern formation to remove the central portion to a predetermined diameter so that light absorption is not caused at the center portion. delete

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