JP2004119831A - Vertical cavity type surface emitting laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a variation in a time and spacial lateral mode caused by a large variation of current distribution from a change in temperature, modulation and the like, and to restrict mode noise caused by the difference of transmission characteristics for each mode at a high-speed signal transmission time. <P>SOLUTION: This vertical cavity type surface emitting laser element for generating a laser beam along a laminated direction includes a first DBR reflective mirror 2, a light emitting region layer 4 containing an active layer, a transparent buffer layer 5, and a second DBR reflective mirror 7 on a semiconductor substrate 1. The active layer 4-1 has an optical gain capable of laser oscillation and a concentric circular structure which is separated by an inactive layer 4-2 with non-light emission or a low optical gain. The buffer layer 5 is set to a thickness for separating 0-th diffracted light and primary diffracted light spatially for diffracted light in a concentric circular structure. The second reflective mirror 7 is selectively set to a range for reflecting the 0-th diffracted light and not reflecting higher order diffracted light. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a vertical cavity surface emitting laser device used as a light source for optical communication and optical transmission technology.
[0002]
[Prior art]
At present, in fields such as optical communication and optical transmission, semiconductor lasers are widely used as light sources because of high coherency of emitted light and high-speed operation, or extremely small size. Above all, the beam is circular, the output light can be extracted in the direction perpendicular to the element substrate, the threshold current can be reduced, the inspection in the wafer state can be performed during manufacturing, and the cost is low. 2. Description of the Related Art A cavity type surface emitting laser element (hereinafter, referred to as VCSEL) has been widely used.
[0003]
Unlike an edge-emitting semiconductor laser, a VCSEL has a structure in which a higher-order transverse mode is likely to exist because the active layer extends in the plane direction sufficiently larger than the wavelength. In fact, most commercial VCSELs oscillate in a number of transverse modes. Each transverse mode has different transmission characteristics with respect to a transmission line such as a multimode fiber. Therefore, when a large number of transverse modes oscillate, temperature fluctuations and high-speed modulation by digital signals cause energy transfer between modes to occur, and the transmission characteristics fluctuate over time (so-called modal mode). Noise) and a phenomenon (mode dispersion) in which the time axis jitter increases due to the transmission delay time difference between the modes occurs, and the transmission characteristics deteriorate.
[0004]
In order to suppress the deterioration of the transmission characteristics, the size of the active layer (in-plane direction) is reduced so that the transverse mode oscillates in a single mode as much as possible. One horizontal mode. Ideally, the active region is narrowed down to about 波長 wavelength in the in-plane direction, and light is confined, so that only the fundamental transverse mode can exist as a standing wave. Since the current needs to be injected into the region, the element resistance becomes extremely high, and high-speed response becomes impossible. Further, a gain is lowered due to a temperature rise caused by injecting a current into the high resistance layer.
[0005]
In view of this, in a wide active layer to some extent, a single transverse mode oscillation is obtained by utilizing a gain difference for each mode. For example, (1) utilizing the fact that only the fundamental transverse mode has a peak near the center of the active layer region, the upper (output side) DBR is partially etched to control the reflectivity and provide a spatial distribution of gain. And (2) a method of attaching an external resonator or a wavelength filter by utilizing a delicate wavelength difference between the transverse modes.
[0006]
As an example of (1), there is a structure shown in FIG. 13 (Non-Patent Document 1). On a semiconductor substrate 101, a lower DBR reflector 102, a light emitting region layer 103 including an active layer and upper and lower cladding layers, and an upper DBR reflector 104 are stacked. A selective oxidation layer 105 is formed below the upper DBR reflector 104, thereby forming a current confinement structure. A ring-shaped contact electrode 106 is formed on the uppermost portion, and a solid contact electrode 107 is formed on the lower surface of the substrate 101.
[0007]
Further, a ring-shaped peripheral portion 108 is formed on the surface of the upper DBR reflector 104 by etching except for the vicinity of the central axis of the light emitting region. Here, the vicinity of the center of the upper DBR reflector 104 is set to have a high reflectance, and the reflectance of the ring-etched portion 108 is set to be low.
[0008]
In this structure, the zero-order fundamental transverse mode has a light emission peak near the center axis of the light-emitting region, and the first-order higher-order transverse mode has a light emission peak near the periphery. As a result, the gain for the higher-order transverse mode is reduced, the gain difference between the modes can be increased, and only the fundamental transverse mode can be selectively oscillated.
[0009]
As an example of (2), there is a structure as shown in FIG. 14 (Patent Document 1). 14A shows the element structure, FIG. 14B shows a near-field pattern during light emission, and FIG. 14C shows a spectrum during light emission. A lower DBR reflecting mirror 202, a lower cladding layer 203, an active layer 204, an upper cladding layer 205, an upper DBR reflecting mirror 206, and an upper electrode 207 are sequentially formed on a semiconductor substrate 201, and an opening 208a is formed on the back surface of the semiconductor substrate 201. The lower electrode 208 is formed, and a wavelength filter 209 is provided in the opening 208a.
[0010]
In this example, utilizing the fact that there is a slight difference in the resonance wavelength between the fundamental transverse mode and the higher-order transverse mode, the higher-order transverse mode is optically filtered by the wavelength selection filter 209, and the size of the aperture 208a is increased. Is slightly larger than the beam diameter of the fundamental transverse mode (indicated by the broken line in FIG. 14B), thereby masking the higher-order transverse mode with electrodes. This limits light extracted to the outside to the basic transverse mode.
[0011]
However, in any case, since the transverse mode is defined by the resonator characteristics, the gain for each mode fluctuates due to fluctuations in temperature, current distribution, etc. May even show strong luminescence.
[0012]
In the example of FIG. 13, when the injection current value increases or modulation is performed with a large amplitude, the gain for each transverse mode fluctuates due to the current distribution in the active layer and the accompanying temperature distribution. As a result, the gain difference between the higher-order transverse mode and the fundamental transverse mode becomes smaller or reversed, and unintended higher-order transverse mode oscillation may occur.
[0013]
In the example of FIG. 14, as described above, the spatial gain distribution in the active layer surface due to the current distribution and temperature distribution changes, and the position of the basic lateral mode fluctuates, or only unintended higher-order transverse mode oscillates. There is even. All of these problems stem from the fact that the distribution of gain in the active layer region fluctuates due to current and temperature distributions.
[0014]
On the other hand, as an example of defining light emission in the active layer plane, an attempt has been made to introduce a periodic structure into the active layer to restrict light emission in the in-plane direction and increase the light emission component in the plane direction relative to the entire light emission. (Patent Document 2). In this example, as shown in FIG. 15, a lower DBR reflecting mirror 312, a buffer layer 313, an active layer 321, a buffer layer 314, and an upper DBR reflecting mirror 315 are stacked on a substrate 311 to form a VCSEL structure. ing. FIG. 15A is a perspective view, and the buffer layers 313 and 314 are not shown. FIG. 15B is a sectional view.
[0015]
By forming the active layer 321 into a concentric photonic structure, a photonic band in the horizontal direction is generated to limit light emission in the horizontal direction. With this structure, the light emission pattern from the concentric active layer 321 has a structure in which a change in spatial position in the active layer surface due to a current distribution or the like is suppressed.
[0016]
However, in this case as well, the limitation of light emission in the stacking direction (vertical direction) is due to the resonator, and there are many possible transverse modes, and no measures against the transverse modes are taken. For this reason, the unification of the transverse mode irrespective of the fluctuation of the current distribution or the temperature distribution cannot be realized.
[0017]
Similarly, there has been proposed a method of controlling the transverse mode by forming an active layer gain distribution in a plane using a quantum wire structure or the like (Japanese Patent Laid-Open No. 6-177480). However, in order to form a quantum structure (quantum wire or quantum dot), a fine structure smaller than the wavelength of light is generally required, and its size is very small as compared with a normal active layer (quantum well structure). Therefore, there is a difficulty in reproducibility and uniformity during manufacturing. Further, since the volume is very small, it is difficult to increase the light emission from the active region.
[0018]
[Patent Document 1]
JP-A-8-236852
[0019]
[Patent Document 2]
JP-A-8-213711
[0020]
[Non-patent document 1]
IEEE Photonics Technology Letters, Vol. 11, No. 12, pp. 1536-1538, 1999
[0021]
[Problems to be solved by the invention]
As described above, conventionally, in the vertical cavity surface emitting laser element, the transverse mode changes due to a large current distribution variation due to a temperature change or modulation, and the spatial distribution and the intensity distribution are not constant over time. During high-speed signal transmission and communication, there is a problem that signal fluctuation noise occurs due to lateral mode fluctuation and transmission characteristics deteriorate.
[0022]
The present invention has been made in consideration of the above circumstances, and has as its object the ability to suppress temporal and spatial lateral mode fluctuations due to large current distribution fluctuations due to temperature changes and modulation. It is an object of the present invention to provide a vertical cavity surface emitting laser device capable of suppressing mode noise caused by a difference in transmission characteristics between modes during signal transmission.
[0023]
[Means for Solving the Problems]
(Constitution)
In order to solve the above problems, the present invention employs the following configuration.
[0024]
That is, the present invention provides a first reflecting mirror, a light emitting region layer including an active layer formed on the first reflecting mirror, a transparent buffer layer formed on the light emitting region layer, and a transparent buffer layer formed on the light emitting region layer. A vertical cavity surface emitting laser element that outputs a laser beam along a stacking direction of each reflector and each layer, wherein the active layer is capable of laser oscillation. And has a concentric structure divided by an inactive layer having no light emission or low optical gain, and each of the divided active layers has a single lateral direction in a radial direction from a center point of the concentric structure. The width of the active layer is limited to the width of mode oscillation, and the interval between the divided active layers is a dimension capable of performing phase-locked oscillation with each other. It is set to a thickness that can spatially separate the next diffraction light, 2 of the reflection mirror, the 0 reflects the diffracted light, characterized in that it is selectively formed in a range that does not reflect the higher-order diffracted light.
[0025]
The present invention also provides a first reflecting mirror, a light emitting region layer including an active layer formed on the first reflecting mirror, a transparent buffer layer formed on the light emitting region layer, and a transparent buffer layer formed on the light emitting region layer. A vertical cavity surface emitting laser element that outputs a laser beam along a stacking direction of each reflector and each layer, wherein the active layer is capable of laser oscillation. And has a concentric structure divided by an inactive layer having no light emission or low optical gain, and each of the divided active layers has a single lateral direction in a radial direction from a center point of the concentric structure. The width of the active layer is limited to a mode oscillation width, and the interval between the divided active layers is a dimension capable of performing phase-locked oscillation. The buffer layer includes a zero-order diffracted light, a first-order diffracted light, and a diffracted light of the concentric structure. Thickness capable of spatially separating high-order diffracted light of second or higher order Is set, the second reflecting mirror, the one reflecting the diffracted light, characterized in that it is selectively formed in a range that does not reflect the 0-order diffracted light and higher-order diffracted light.
[0026]
Here, preferred embodiments of the present invention include the following.
[0027]
(1) The reflector should be a distributed Bragg reflector (DBR).
[0028]
(2) The concentric structure is a concentric structure composed of a circle and a ring.
[0029]
(3) Assuming that the thickness of the buffer layer is L, the diameter of the concentric structure is Da, and the diffraction angle of the first-order diffracted light by the concentric structure is θ, the center axis of the second reflecting mirror is equal to the center axis of the concentric structure. And the diameter Dm of the second reflecting mirror satisfies Da <Dm <Ltanθ. Further, when the beam spread angle of the first-order diffracted light is θ1, the central axis of the second reflecting mirror coincides with the central axis of the concentric structure, and the diameter Dm of the second reflecting mirror is Da <Dm <Ltan ( θ−θ1).
[0030]
(4) A conical prism for converting the first-order diffracted light into parallel light is provided on the second reflecting mirror.
[0031]
(5) The concentric structure is a concentric elliptical structure composed of an ellipse and an elliptical ring, and the long side direction and the short side direction of the ellipse and the elliptical ring coincide with each other. Match with large crystal orientation.
[0032]
(6) A crystal in which the concentric structure is a concentric rectangular structure composed of a rectangle and a rectangular frame, and each side of the rectangle and the rectangular frame is parallel to each other, and the longer side direction of the rectangle and the rectangular frame has a large gain of the active layer. Must match direction.
[0033]
(7) Between the buffer layer and the light emitting region layer, there is provided a partial reflector having a low reflectance such that the active layer does not cause laser oscillation in combination with the first reflector. Part of the second reflecting mirror is formed as a partial reflecting mirror between the buffer layer and the light emitting region layer.
[0034]
(8) An insulating film or a high resistance film is selectively formed between the buffer layer and the light emitting region layer for current confinement.
[0035]
(Action)
According to the present invention, the periodic structure is introduced into the active layer to control the distribution of light emission in the active surface, thereby restricting the transverse mode to a spatially stable and separable pattern, and providing a desired mode by the resonator. By adopting a structure capable of spatially selecting the transverse mode, the variation of the transverse mode due to the variation of the temperature and the current distribution can be reduced.
[0036]
More specifically, the active layer has a concentric structure including, for example, a center circle and a plurality of rings, and the center circle diameter and the width of each ring are small enough to emit light in a single fundamental mode. As a result, an effect similar to that of the diffraction grating is obtained, and the light emission pattern in the far field is separated into a central circle (zero-order light) and a ring-like high-order light. By selectively disposing DBR resonators at positions where these modes of each order are spatially separated, a structure that selectively emits light only in a desired transverse mode is realized. It is possible to solve.
[0037]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.
[0038]
(1st Embodiment)
FIGS. 1A and 1B are diagrams for explaining an element structure of a semiconductor laser according to a first embodiment of the present invention, in which FIG. 1A is a cross-sectional view illustrating an entire configuration, FIG. 1B is a plan view illustrating an active layer configuration, (C) is a diagram showing a radiation light intensity distribution.
[0039]
As shown in FIG. 1A, an n-type AlAs / Al doped with Si is formed on a semiconductor substrate 1 made of n-type GaAs. _0.15 Ga _0.85 Lower DBR reflector 2, made of a quarter-wave stacked semiconductor layer of As, Al _0.2 Ga _0.8 A light emitting region layer 4 including an active layer composed of a multiple quantum well of As / GaAs is stacked. On the light-emitting region layer 4, transparent Al near the oscillation wavelength _x Ga _1-x A thick buffer layer 5 made of As (0.3 ≦ x ≦ 0.6) is formed, and a C-doped p-type AlAs / Al _0.15 Ga _0.85 A circular upper DBR reflector 7 composed of an As quarter-wave laminated semiconductor layer is partially formed.
[0040]
Then, a ring electrode 8 of AuZn for electrical contact is formed on the buffer layer 5 so as to surround the upper DBR reflecting mirror 7. On the back surface of the semiconductor substrate 1, a back electrode 9 made of AuGe or the like is formed. As a result, a vertical cavity surface emitting laser device that outputs laser light in the stacking direction is obtained.
[0041]
The light emitting region layer 4 including the active layer is made of Al _x Ga _1-x Between the lower cladding layer 4-3 and the upper cladding layer 4-4 made of As (0.3 ≦ x ≦ 0.6), laser oscillation consisting of a central circle and a ring as shown in FIG. It has a concentric structure composed of an active layer 4-1 having a high optical gain and an inactive layer 4-2 having no light emission or low optical gain. The diameter of the active layer 4-1 or the width of the ring has a width capable of oscillating in a single transverse mode in which the light intensity distribution in the ring is unimodal in the radial direction of the concentric circle. Further, the respective active layers 4-1 are optically coupled to each other, and are arranged at intervals capable of phase synchronization.
[0042]
Due to this concentric structure, when current is injected into the active layer 4-1, coherent light emission from each active layer 4-1 due to stimulated emission interferes with light emission from each active region, which is shown in the figure. The zero-order light 10 and the higher-order light 11 having a certain diffraction angle are spatially separated. Actually, since the presence of a resonator is required to cause stimulated emission, the arrow shown in the figure is virtual light. For example, when the width of the active layer 4-1 is 0.8 μm and the width of the inactive layer 4-2 is 0.4 μm, the diffraction angle of ± 1st-order light is about 12.5 °. FIG. 1C shows the light intensity distribution with respect to the diffraction angle by the concentric structure in this case. The numbers of circles and rings are calculated for the center circle and the quadruple ring.
[0043]
Therefore, by placing the upper DBR reflecting mirror 7 only at a spatial position corresponding to a desired mode, it is possible to set so that only the desired mode causes stimulated emission and laser oscillation is possible. At this time, it is necessary to take the following positional relationship as a condition for placing the higher-order transverse mode at a separable position.
[0044]
In FIG. 2, reference numerals 11-1 and 11-2 denote + 1st-order and -1st-order lights diffracted by concentric structures, respectively. These are not spatially separated including the zero-order light beam 10 near the active layer, and are in an overlapping state. Then, the distance L from the active layer 4-1 in the normal direction is
L> Da / tan θ (= Lo) (1)
Within the range that satisfies That is, at the position of L ≦ Lo, the respective diffracted lights are not separated spatially, and therefore oscillate at the same time. Here, Da is the outer diameter of the outermost ring of the active layer 4-1. Further, since the thickness of the upper cladding layer 4-4 is extremely thin as described later, the distance L from the active layer 4-1 in the normal direction is substantially the thickness of the buffer layer 5.
[0045]
In addition, since the higher-order diffracted light forms a beam having a certain angular spread (± θ1 = 2θ1) in the diffraction angle direction, the upper DBR reflection is actually separated from Lo by a distance in consideration of the effect of this spread. A mirror 7 needs to be placed. Thereby, the thickness of the buffer layer 5 is determined. Therefore, each dimensional relationship is
Da <Ltan (θ−θ1) (2)
Can be written.
[0046]
For example, in the above-described example, if the number of rings around the circle is 4, Da = 10.4 μm. At this time, Lo is about 47 μm. If the divergence angle of the primary light is defined as an angle that is 1/10 or less of the primary light peak, the angle is about 1 °. Therefore, the distance L between the active layer 4-1 and the upper DBR reflector 7 is about 51 μm or more is required. That is, the thickness of the buffer layer 5 needs to be about 51 μm or more. This thickness is extremely thick as compared with a normal VCSEL, and it is difficult to manufacture the crystal by crystal growth. However, as described later, it can be realized by applying a substrate bonding technique.
[0047]
In addition, as shown in FIG. 2, when the zero-order light is selected, the upper DBR reflecting mirror 7 is placed near a center including the central axis of the concentric active layer 4-1 at a position where only the zero-order light is reflected. At this time, it is preferable that the central axis of the upper DBR reflecting mirror 7 coincides with the central axis of the concentric active layer 4-1, and the diameter Dm is larger than Da. Because, in the annular active layer 4-1 corresponding to the outside of the upper DBR reflector 7, no resonator is formed in the vertical direction, and oscillation is not possible even in the fundamental mode, so that the active layer does not function effectively. Is simply a loss medium.
[0048]
Therefore, in combination with the above dimensional relationship, the desirable range of Dm is
Da <Dm <Ltan (θ−θ1) (3)
Is defined as Further, in order to prevent stray light from a portion other than the upper DBR reflector 7 from being output from the surface and becoming noise, a portion of the light output surface other than the upper DBR reflector is masked by the ring-shaped electrode 8 as much as possible. It is desirable.
[0049]
1 and 2 described above are realized by the following method. For example, after the lower DBR reflector 2 and the lower cladding layer 4-3 are formed on the semiconductor substrate 1 by using a normal crystal growth technique or the like, an active layer is formed on the entire surface. After that, the upper cladding layer 4-4 is formed.
[0050]
Here, ZnAs is used with a resist or a dielectric film such as SiNx as a mask so that the active layer formed on the entire surface remains in a concentric structure. ₂ Using Zn as a diffusion source, Zn is diffused at about 600 to 700 ° C. to be disordered, and the inactive layer 4-2 is formed. If the upper cladding layer 4-4 is thick at this time, if the concentric structure becomes a fine structure, and if the concentric structure is diffused to the depth of the active layer, lateral diffusion also occurs at the same time, so that a fine structure cannot be formed. For this reason, the thickness of the upper cladding layer 4-4 is set to be as thin as the width of the ring of the concentric structure. The formation of the inactive layer 4-2 can also be realized by performing annealing after ion implantation of Zn or Si to perform disordering, or by increasing the resistance by proton implantation.
[0051]
Next, the thick buffer layer 5 bonded to another support substrate is bonded to the upper cladding layer 4-4 of the light emitting region layer 4 by using a technique such as substrate bonding. After that, the support substrate is removed. The buffer layer 5 may be made of the same material as the upper clad layer 4-4.
[0052]
Next, the upper DBR reflector 7 is formed on the entire surface of the buffer layer 5, and the upper DBR reflector 7 is patterned only at a desired position by a method such as dry etching. At this time, when forming the upper DBR reflector on the entire surface, if the temperature becomes high and there is a possibility that the fine structure of the concentric circle may be broken by the indentation diffusion, the buffer layer 5 on which the DBR reflector 7 is formed may be bonded to the substrate. Finally, the laser element is completed by forming the electrode 8.
[0053]
As described above, according to the present embodiment, the active layer 4-1 has a concentric circular structure including a central circle and a plurality of rings, and the central circle diameter and the width of each ring are small enough to emit light in a single fundamental mode. As a result, an effect similar to that of the diffraction grating is obtained, and the light emission pattern in the far field is separated into a central circle (zero-order light) and a ring-like high-order light. By selectively arranging the upper DBR reflecting mirror 7 at a position where the modes of each order are spatially separated, it is possible to realize a structure for selectively emitting light only in a desired transverse mode. Therefore, the variation of the transverse mode due to the variation of the temperature or the current distribution can be reduced.
[0054]
In the configuration of the present embodiment, under the condition that the active layer 4-1 oscillates only in the fundamental transverse mode in the radial direction, the output light is always limited to only the desired mode. By making the width of each active layer 4-1 such that oscillation in a single transverse mode is possible in the radial direction as much as possible with respect to current injection and the accompanying temperature distribution, current injection and An element capable of operating in a single transverse mode regardless of temperature can be realized.
[0055]
On the other hand, in the conventional active layer structure, if it is confined in a small area such as 0.8 μm in diameter, the resistance value increases extremely, and the temperature rise when current flows is also increased. On the other hand, according to the present embodiment, since the element resistance depends on the area of the entire active layer 4-1, the increase in resistance can be suppressed. For example, if the number of circles and rings is four in the above example, the area is almost the same as a circle having a diameter of 8.8 μm, a low resistance value can be obtained, and high-speed driving is possible. Become. To further reduce the resistance, the number of rings may be increased and the buffer layer 5 may be made thicker.
[0056]
(Second embodiment)
FIG. 3 is a sectional view showing an element structure of a semiconductor laser according to the second embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0057]
This embodiment differs from the first embodiment described above in that a high resistance region (current constriction structure) 6 made of a selective oxide film or the like is provided between the light emitting region layer 4 and the buffer layer 5. is there.
[0058]
In the light-emitting region layer 4, the current flowing through the inactive layer 4-2 does not contribute to laser oscillation. Therefore, if the current occupies a large proportion, the threshold value increases. Therefore, as in the case of the normal VCSEL, the high-resistance region 6 is formed in a portion other than the circular light-emitting region on the upper cladding layer 4-4 as in the present embodiment, so that the current flowing through the inactive layer 4-2. And the rise of the threshold can be suppressed.
[0059]
(Third embodiment)
FIG. 4 is a sectional view showing an element structure of a semiconductor laser according to the third embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0060]
This embodiment is different from the first embodiment in the method of forming an active layer having a concentric structure. In the first embodiment, the active layer 4-1 having a concentric structure is formed by forming the inactive layer 4-2 by diffusion of impurities. However, in the present embodiment, the active layer itself is left as it is. The high-resistance region 6 is formed on the active layer corresponding to the portion where the inactive layer 4-2 having the concentric structure is to be formed.
[0061]
In this structure, by reducing the current density injected into a portion other than the active layer 4-1 having the concentric structure, the optical gain of the portion can be substantially reduced. The same effect as when forming No. 2 is obtained. Note that the high resistance region 6 is not limited to being provided above the active layer, and may be provided below or above and below. Although the upper cladding layer is omitted in the figure, it goes without saying that the upper cladding layer may be provided as in the first and second embodiments.
[0062]
In the present embodiment, the portion of the inactive layer 4-2 is not disordered, but it is also effective to form a current confinement structure similar to that of the present embodiment on the disordered inactive layer 4-2. is there. That is, since there is no current confinement effect in the disordered region, a leak current flows, and the oscillation threshold current effectively increases. Therefore, by forming the high resistance region 6 on the disordered inactive layer 4-2, there is an effect that the inactivation of the active layer and the current confinement can be realized at the same time.
[0063]
(Fourth embodiment)
FIG. 5 is a sectional view showing an element structure of a semiconductor laser according to a fourth embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0064]
This embodiment is different from the first embodiment in that the upper DBR reflecting mirror 7 is provided separately in the stacking direction.
[0065]
In the structure of the first embodiment, as the optical resonator in the vertical direction, a resonator structure longer than that of the conventional VCSEL is required. For this reason, not only the instability of the longitudinal mode is increased, but also the active layer 4-1 may come to a node of the standing wave in the longitudinal direction, and the phase matching may not be achieved. In this case, as shown in FIG. 5, the upper DBR reflecting mirror 7 is divided into a partial reflecting mirror 7-1 for achieving phase matching and a partial reflecting mirror 7-2 for obtaining a high reflectance and forming a resonator. Then, the structure is sandwiched between the buffer upper cladding layers 5.
[0066]
With such a configuration, it is possible not only to obtain the same effect as in the first embodiment, but also to stabilize the standing wave and select only the fundamental transverse mode while maintaining phase matching. , More stable operation becomes possible.
[0067]
(Fifth embodiment)
FIG. 6 is a plan view showing an active layer structure of a semiconductor laser according to a fifth embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0068]
This embodiment differs from the first embodiment in that the active layer 4-1 having a concentric structure is formed separately.
[0069]
In the structure of the first embodiment, when the radius of the ring of the active layer is small, only the fundamental mode oscillates in the center circle, so that the phase-locked oscillation is maintained by the interaction in each ring. When the radius of the ring becomes large, the length of the circumference becomes long, and the lateral mode in the circumferential direction becomes unstable, so that there is a high possibility that a phase shift occurs in each ring. In this case, as shown in FIG. 6, it is possible to stabilize the mode by cutting a part of the ring to form a fan shape.
[0070]
In the structure of this embodiment, when viewed in one round in the circumferential direction, the mode becomes a higher-order mode instead of the basic lateral mode. However, since the basic lateral mode is maintained in the radial direction, the effect of interference is small. The same effects as in the above-described embodiments are exhibited, and a stable single-peak output is obtained.
[0071]
(Sixth embodiment)
FIG. 7 is a sectional view showing an element structure of a semiconductor laser according to a sixth embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0072]
In the first to fifth embodiments, an example has been described in which the basic lateral mode is selected as the radial lateral mode. However, the present invention is not limited to this, and it is also possible to select a higher-order lateral mode.
[0073]
In the present embodiment, as shown in FIG. 7, the upper DBR reflecting mirror 7 is arranged in a ring shape so that the central axis coincides with the active layer 4-1 having the concentric structure, so that the position corresponding to the higher-order transverse mode is obtained. Was placed. In FIG. 7, reference numeral 10 denotes 0-order light, 11 denotes desired ± 1st-order light, and 12 denotes second-order or higher-order light. The primary output light 11 has an inverted conical emission having a certain width with respect to the substrate normal axis. As a result, only the first-order light can be reflected by the upper DBR reflecting mirror 7, and the 0-order light 10 and higher-order light of the second or higher order can be prevented from being reflected by the upper DBR reflecting mirror 7.
[0074]
Therefore, according to the present embodiment, laser oscillation can be performed by selecting only the primary light. In this case, since the electrode 8 can be arranged at a position facing the active layer 4-1 having the concentric structure, the uniformity of the injection current distribution can be increased as compared with the case of using an annular electrode, and a stable state can be obtained. There is an effect that an optical output can be obtained.
[0075]
(Seventh embodiment)
FIG. 8 is a sectional view showing an element structure of a semiconductor laser according to a seventh embodiment of the present invention. The same parts as those in FIG. 7 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0076]
This embodiment is different from the sixth embodiment in that a conical prism is provided on the light extraction side.
[0077]
In the configuration of the sixth embodiment, considering that the primary output light is coupled to an optical fiber or the like, a large divergence angle of the output light beam lowers the coupling efficiency. Therefore, in the present embodiment, as shown in FIG. 8, a conical prism 13 is provided on the upper surface of the element, and an output light beam having a certain spread with respect to the normal axis of the substrate is refracted by the prism 13 so that the light parallel to the axial direction is emitted. Beam. Thereby, an annular output light along the axial direction can be obtained, and coupling with high efficiency can be achieved.
[0078]
(Eighth embodiment)
FIG. 9 is a sectional view showing an element structure of a semiconductor laser according to the eighth embodiment of the present invention. The same parts as those in FIG. 7 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0079]
In this embodiment, a prism structure is monolithically integrated in the laser element itself as shown in FIG. 9, instead of attaching a separate body such as a conical prism. That is, the prism layer 15 is integrally formed on the upper DBR reflecting mirror 7. The prism layer 15 is made of a transparent material having an oscillation wavelength of AlGaAs or the like, and is set at an angle such that the first-order diffracted light becomes a parallel beam due to refraction of the surface.
[0080]
The structure of the present embodiment is realized by the method shown in FIG. In FIG. 10, for simplicity, only the upper part of the buffer layer 6 is illustrated. After the buffer layer 6 and the upper DBR reflector 7 are formed on the entire surface in FIG. 10A, a prism layer 15 is further formed on the entire surface. In this case, the upper DBR reflecting mirror 7 needs to have a laminated structure in consideration of the fact that the outermost layer is not in contact with air and is in contact with the prism layer 15.
[0081]
Next, as shown in FIG. 10B, after the resist 16 is patterned, baking is performed, and the resist 16 is deformed into a hemispherical shape as shown in FIG. 10C. Thereafter, the pattern of the resist 16 is transferred to the prism layer 15 by dry etching using a chlorine-based reaction gas 17 or the like, and the shape shown in 15-1 in FIG. 10D can be realized. Further, the unnecessary portion of the prism layer 15 is removed together with the upper DBR reflecting mirror 7 by dry etching using a resist, a dielectric film or the like as a mask 18, whereby the structure shown in FIG. 9 can be manufactured.
[0082]
(Ninth embodiment)
FIG. 11 is a sectional view showing an element structure of a semiconductor laser according to the ninth embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0083]
In the above embodiments, only the active layer having a concentric structure has been described, but the present invention is not necessarily limited to the concentric structure. In the present embodiment, as shown in FIG. 11, the active layer 4-1 is formed in a concentric elliptical structure. As in the first embodiment, the portion 4-2 may be disordered by diffusion of impurities so that the active layer formed on the entire surface remains in a concentric elliptical structure as in the first embodiment.
[0084]
In the case of the present embodiment, since the beam shape is an ellipse or an elliptical ring, it is necessary to change the shape of the upper DBR reflecting mirror 7 accordingly. That is, an elliptical shape is preferable when the zero-order diffracted light is selected, and an elliptical ring shape is preferable when the first-order diffracted light is selected. By making the active layer have a concentric elliptical structure, the diffracted light becomes a concentric elliptical beam in which the major axis direction of the active layer is the minor axis direction. However, since only the portion where the upper DBR reflecting mirror 7 exists can exist as a mode, an elliptical beam is obtained by selecting the 0th-order diffracted light, and an elliptical ring beam is obtained by selecting the 1st-order diffracted light.
[0085]
Here, when the major axis direction of the ellipse is made to coincide with the direction in which the gain of the multiple quantum well active layer is high, there is an effect that the polarization is stabilized. For example, if the substrate is a (100) GaAs substrate and the substrate is Al _0.2 Ga _0.8 When the active layer of the As / GaAs multiple quantum well is used, the [011] or [01-1] direction is made to coincide with the major axis direction to increase the gain of the polarization mode in each direction. The effect is that waves can be obtained.
[0086]
(Tenth embodiment)
FIG. 12 is a sectional view showing an element structure of a semiconductor laser according to the tenth embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0087]
In the present embodiment, as shown in FIG. 12, the active layer 4-1 has a concentric rectangular structure. Also in the case of such a concentric rectangular structure, the effect of polarization stabilization can be obtained as in the ninth embodiment. In this case, the polarization can be effectively stabilized by matching the long side direction of the concentric rectangular structure with the direction in which the gain of the active layer is high. In the diffracted light obtained at this time, the zero-order light is near the center of the optical axis, and the first-order light is divided into four beams having inclinations in the direction orthogonal to each side of the concentric rectangular structure. Therefore, when the primary light is selected, it is possible to obtain a multi-beam having the same phase.
[0088]
(Modification)
Note that the present invention is not limited to the above embodiments. In the embodiment, an example in which the active layer is formed in a concentric circle, a concentric ellipse, or a concentric rectangular structure has been described. However, the present invention is not necessarily limited thereto. You may.
[0089]
In the embodiment, a device using a GaAs-based material and having an emission peak at a wavelength of about 850 nm is described. However, the present invention is not limited to these materials and can be applied to other material systems. is there. For example, in the case of an InP-based material, a material having a wide band gap can be buried as a near-flat state as the inactive layer 4-2 after directly etching the active layer. Therefore, the upper cladding layer can be formed thinner thereafter, and a thicker upper cladding layer can be formed by a technique such as substrate bonding.
[0090]
In the case of a GaN-based material, a method in which a thin clad layer is formed and then a thick clad layer is adhered to a substrate by etching a part of the active layer with AlN after removing the part of the active layer, or a method of thermally annealing a part of the active layer in hydrogen. This can be realized by forming an inactive layer by increasing the resistance by such a method as described above, and then forming a thin clad layer and bonding another substrate having a thick clad by a substrate bonding technique. Further, the pattern of the active layer may be a pattern consisting of only a ring as long as the diffraction conditions and the dimensional relation conditions described in the embodiment are satisfied.
[0091]
In addition, various modifications can be made without departing from the scope of the present invention.
[0092]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to suppress the fluctuation of the temporal and spatial transverse modes due to the fluctuation of the current distribution caused by the temperature fluctuation and the modulation while suppressing the rise of the element resistance and to unify it. . As a result, it is possible to suppress mode noise caused by a difference in transmission characteristics for each mode during high-speed signal transmission, which has the effect of enabling higher-speed signal transmission.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view and a plan view showing a device structure of a semiconductor laser according to a first embodiment, and a characteristic diagram showing a radiation light intensity distribution.
FIG. 2 is a schematic diagram for explaining a dimensional relationship of each part in the first embodiment.
FIG. 3 is a sectional view showing an element structure of a semiconductor laser according to a second embodiment.
FIG. 4 is a sectional view showing an element structure of a semiconductor laser according to a third embodiment.
FIG. 5 is a sectional view showing an element structure of a semiconductor laser according to a fourth embodiment.
FIG. 6 is a sectional view showing an element structure of a semiconductor laser according to a fifth embodiment.
FIG. 7 is a plan view showing an active layer structure of a semiconductor laser according to a sixth embodiment.
FIG. 8 is a sectional view showing an element structure of a semiconductor laser according to a seventh embodiment.
FIG. 9 is a sectional view showing an element structure of a semiconductor laser according to an eighth embodiment.
FIG. 10 is a sectional view showing a manufacturing process of a prism structure according to the eighth embodiment.
FIG. 11 is a sectional view showing an element structure of a semiconductor laser according to a ninth embodiment.
FIG. 12 is a sectional view showing an element structure of a semiconductor laser according to a tenth embodiment.
FIG. 13 is a perspective view and a cross-sectional view showing the element structure of a conventional semiconductor laser.
FIG. 14 is a cross-sectional view showing a device structure of a conventional semiconductor laser and a characteristic diagram showing an emission spectrum.
FIG. 15 is a sectional view showing an element structure of a conventional semiconductor laser.
[Explanation of symbols]
1 .... Semiconductor substrate
2. Lower DBR reflector
4-1 Active layer
4-2: Inactive layer
4-3 ... Lower cladding layer
4-4: Upper cladding layer
5 ... Buffer layer
6 High resistance layer
7 Upper DBR reflector
8. Ring electrode
9 Back electrode
10 ... 0th order light
11: 1st order light (higher order light)
12 ... Higher order light of 2nd or higher
13 ... Conical prism
15 ... Prism layer
16, 18 ... resist
17 ... Reaction gas

Claims (9)

A first reflecting mirror, a light emitting region layer including an active layer formed on the first reflecting mirror, a transparent buffer layer formed on the light emitting region layer, and a transparent buffer layer formed on the transparent buffer layer A vertical cavity surface emitting laser element that includes a second reflecting mirror and outputs laser light along a laminating direction of each reflecting mirror and each layer,
The active layer has an optical gain capable of lasing, has a concentric structure divided by an inactive layer having no light emission or low optical gain, and each of the divided active layers has a center point of the concentric structure. Is limited to a width in which a single transverse mode oscillation is generated in a radiation direction from the device, and the interval between the divided active layers is a dimension capable of performing phase-locked oscillation with each other,
The buffer layer is set to a thickness capable of spatially separating the 0th-order diffracted light and the 1st or higher-order diffracted light of the diffracted light by the concentric structure,
The vertical cavity surface emitting laser device, wherein the second reflecting mirror is selectively formed in a range that reflects the zero-order diffracted light and does not reflect the high-order diffracted light. A first reflecting mirror, a light emitting region layer including an active layer formed on the first reflecting mirror, a transparent buffer layer formed on the light emitting region layer, and a transparent buffer layer formed on the transparent buffer layer A vertical cavity surface emitting laser element that includes a second reflecting mirror and outputs laser light along a laminating direction of each reflecting mirror and each layer,
The active layer has an optical gain capable of lasing, has a concentric structure divided by an inactive layer having no light emission or low optical gain, and each of the divided active layers has a center point of the concentric structure. Is limited to a width in which a single transverse mode oscillation is generated in a radiation direction from the device, and the interval between the divided active layers is a dimension capable of performing phase-locked oscillation with each other,
The buffer layer is set to a thickness capable of spatially separating zero-order diffracted light, first-order diffracted light, and second-order or higher-order diffracted light of the concentric structure,
A vertical cavity surface emitting laser device, wherein the second reflecting mirror is formed selectively in a range that reflects the first-order diffracted light and does not reflect the zero-order diffracted light and the high-order diffracted light. 3. The vertical cavity surface emitting laser device according to claim 1, wherein the concentric structure of the active layer is a concentric structure including a circle and a ring. When the thickness of the buffer layer is L, the diameter of the concentric structure is Da, and the diffraction angle of the first-order diffracted light by the concentric structure is θ, the center axis of the second reflecting mirror is the same as the center axis of the concentric structure. 4. The vertical cavity surface emitting laser element according to claim 3, wherein the diameters Dm of the second reflecting mirror are equal to each other and satisfy Da <Dm <Ltanθ. When the thickness of the buffer layer is L, the diameter of the concentric structure is Da, the diffraction angle of the first-order diffracted light by the concentric structure is θ, and the beam spread angle of the first-order diffracted light is θ1, the second reflecting mirror 4. The vertical resonator type according to claim 3, wherein the central axis of the vertical mirror coincides with the central axis of the concentric structure, and the diameter Dm of the second reflecting mirror satisfies Da <Dm <Ltan (θ−θ1). Surface emitting laser device. 3. The vertical resonance according to claim 2, wherein the concentric structure of the active layer is a concentric structure including a circle and a ring, and a conical prism that converts the first-order diffracted light into parallel light is provided on the second reflecting mirror. Type surface emitting laser device. The concentric structure of the active layer is a concentric elliptical structure composed of an ellipse and an elliptical ring, and the long sides and short sides of the ellipse and the elliptical ring coincide with each other. 3. The vertical cavity surface emitting laser device according to claim 1, wherein the gain of the layer matches the crystal orientation. The concentric structure of the active layer is a concentric rectangular structure composed of a rectangle and a rectangular frame, each side of the rectangle and the rectangular frame is parallel to each other, and the longer side direction of the rectangular and the rectangular frame is the active layer. The vertical cavity surface emitting laser device according to claim 1, wherein the crystal orientation coincides with a crystal orientation having a large gain. The method according to claim 1, further comprising, between the buffer layer and the light emitting region layer, a partial reflector having a low reflectance such that the active layer does not cause laser oscillation in combination with the first reflector. A vertical cavity surface emitting laser device according to any one of claims 1 to 8.

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