JP6650961B2 - Surface emitting laser and optical coherence tomography using the same - Google Patents

Description

  The present invention relates to a surface emitting laser and an optical coherence tomography using the surface emitting laser.

  One of the surface emitting lasers is a vertical cavity surface emitting laser (hereinafter, may be referred to as VCSEL). The VCSEL sandwiches the upper and lower portions of an active layer between two reflectors, forms a resonator in a direction perpendicular to the surface of the substrate, and emits laser light in a direction perpendicular to the surface of the substrate. Further, there is a wavelength-variable VCSEL that can change the wavelength of emitted light. As an example, a cavity is provided between an upper reflecting mirror and an active layer in a VCSEL, and the upper reflecting mirror is moved in a direction of an optical path of a laser beam, thereby changing a resonator length and changing a wavelength of emitted light. There are lasers that can do this. The surface emitting laser capable of changing the wavelength of the emitted light may be hereinafter referred to as a wavelength tunable VCSEL.

  On the other hand, a tunable VCSEL is known to be suitable as a light source for an optical coherence tomography (hereinafter sometimes abbreviated as OCT). When a wavelength tunable VCSEL is used as a light source for OCT, it is desired to further widen the wavelength tunable width in order to improve the OCT depth resolution. In order to widen the wavelength tunable width of the wavelength tunable VCSEL, it is necessary to provide high reflectivity over a wide reflection band in the reflectors provided above and below the active layer.

  Non-Patent Document 1 discloses a wavelength tunable VCSEL having a configuration in which a pair of Distributed Bragg Reflectors (hereinafter sometimes abbreviated as DBRs) are provided above and below an active layer.

IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 6, No. 6, Nov. 2000

  Here, the wavelength dependence of the intensity of the light output from the VCSEL is mainly determined by factors such as the wavelength dependence of the gain spectrum of the active layer and the wavelength dependence of the reflectance of the upper reflector and the lower reflector.

  On the other hand, in the case of OCT using a variable wavelength light source, it is preferable that the light output intensity of the light source does not greatly differ depending on the wavelength. Therefore, when a tunable VCSEL that is one of the tunable light sources is used for OCT, it is preferable that the gain spectrum of the active layer and the reflectivity of the upper and lower reflecting mirrors have small wavelength dependence.

  However, it is known that in a tunable VCSEL, the gain spectrum of the active layer has wavelength dependence. For example, the gain spectrum of the active layer may have a smooth convex shape with a large gain on the long wavelength side of the peak wavelength. Moreover, since the shape of the gain spectrum of the active layer is determined by quantum mechanics, it cannot be easily changed.

  When such a wavelength tunable VCSEL is used, the wavelength dependence of the optical output intensity may increase, and an accurate OCT image may not be obtained.

  In view of the above problems, an object of the present invention is to provide a surface emitting laser whose light output intensity or threshold current does not greatly differ depending on the wavelength.

The surface emitting laser according to the present invention includes a lower reflecting mirror, an active layer, a gap, and an upper reflecting mirror in this order, and changes a distance between the lower reflecting mirror and the upper reflecting mirror. A surface emitting laser that varies the wavelength of light emitted by the light emitting device, comprising a driving unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in an optical axis direction of the emitted light, The relationship between the wavelength λ _g at which the gain at the time of laser oscillation of the active layer is the maximum, the central wavelength λ _{0 of} the emitted light, and the wavelength λ _r at which the reflectance of the reflector on the side from which the light is emitted is maximized is λ _r <λ ₀ <λ _g or λ _g <λ ₀ <λ _r .

Another surface emitting laser according to the present invention has a lower reflecting mirror, an active layer, a gap, and an upper reflecting mirror in this order, and the distance between the lower reflecting mirror and the upper reflecting mirror is reduced. A surface emitting laser that changes a wavelength of light emitted by changing the wavelength, and has a driving unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in an optical axis direction of the emitted light. The relationship between the emission wavelength λ _s of the ground level of the active layer, the center wavelength λ _{0 of} the emitted light, and the wavelength λ _r at which the reflectance of the reflector on the side from which the light is emitted is maximized is λ _r <characterized in that it is configured to satisfy the lambda ₀ <lambda _s or _{λ s <λ 0 <λ r} .

Another surface emitting laser according to the present invention has a lower reflecting mirror, an active layer, a gap, and an upper reflecting mirror in this order, and the distance between the lower reflecting mirror and the upper reflecting mirror is reduced. A surface emitting laser that changes a wavelength of light emitted by changing the wavelength, and has a driving unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in an optical axis direction of the emitted light. When the wavelength dependence of the gain of the active layer is G (λ) and the wavelength dependence of the reflectance of the reflector on the side from which the light is emitted is R (λ), d ² G (λ) / A region where dλ ² <0 and a region where d ² R (λ) / dλ ² > 0 are at least partially overlapped with each other.

  According to the surface emitting laser according to the present invention, it is possible to provide a surface emitting laser having a small wavelength dependence of light output intensity or threshold current.

FIG. 1 is a cross-sectional view illustrating a configuration of a wavelength tunable VCSEL according to an embodiment of the present invention. FIG. 9 is a diagram for describing a problem of a conventional wavelength tunable VCSEL. FIG. 4 is a diagram for explaining the wavelength dependence of the gain spectrum, the reflectance, and the optical output intensity of the tunable VCSEL according to the embodiment of the present invention. FIG. 4 is a diagram for explaining the wavelength dependence of the gain spectrum, the reflectance, and the optical output intensity of the tunable VCSEL according to the embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a configuration of an upper reflecting mirror according to the embodiment of the present invention. FIG. 4 is a diagram for explaining a mechanism in which the wavelength tunable VCSEL according to the embodiment of the present invention produces an effect. FIG. 4 is a diagram for explaining a mechanism in which the wavelength tunable VCSEL according to the embodiment of the present invention produces an effect. FIG. 1 is a diagram for explaining a configuration of an OCT having a tunable VCSEL according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a gain spectrum of an active layer according to the first embodiment of the present invention. (A) A diagram showing the reflection spectrum of the upper reflector, (b) a diagram showing the wavelength dependence of the threshold current of the tunable VCSEL, and (c) a diagram showing the wavelength dependence of the optical output intensity of the tunable VCSEL in the first embodiment of the present invention. FIG. FIG. 6A is a cross-sectional view illustrating a configuration of an upper reflecting mirror according to a second embodiment of the present invention. FIG. 6B is a drawing illustrating a reflection spectrum of the upper reflecting mirror. FIG. 7A is a cross-sectional view illustrating a configuration of an upper reflecting mirror according to a third embodiment of the present invention. FIG. 8B is a diagram illustrating a reflection spectrum of the upper reflecting mirror.

  Hereinafter, a tunable VCSEL according to an embodiment of the present invention will be described. FIG. 1 is a sectional view of a surface emitting laser according to the present embodiment.

  As shown in FIG. 1, the surface emitting laser according to this embodiment includes a lower electrode 101, a substrate 102 formed on the lower electrode 101, a lower reflector 110 formed on the substrate 102, and a lower reflector. A lower cladding layer 103 formed on the lower cladding layer 110, an active layer 104 formed on the lower cladding layer 103, an upper cladding layer 120 formed on the active layer 104, and a lower cladding layer 120 formed on the upper cladding layer 120. And the upper electrode 105 formed. The insulating layer 106 is formed on the upper cladding layer 120, and the upper reflecting mirror 130 is formed on the insulating layer 106. The upper cladding layer 120 and the upper reflecting mirror 130 are separated from each other, and have a gap 107. When a current is injected into the active layer 104 using the lower electrode 101 and the upper electrode 105, light emission occurs there, and the light reciprocates in a resonator formed by the lower reflector 110 and the upper reflector 130. Causes stimulated release. The light stimulated emitted in the resonator passes through the upper reflector 130 in a direction perpendicular to the surface of the substrate 102, that is, in the y direction, and is emitted as laser light of a specific wavelength. Further, since the upper cladding layer 120 has the current confinement layer 121, the current supplied from the electrode 105 is injected into the active layer 104 through the opening 122 of the current confinement layer 121.

  Here, if the distance (distance α in FIG. 1) from the interface between the upper cladding layer 120 and the gap 107 to the interface between the upper reflecting mirror 130 and the gap 107 changes, the resonator length changes and oscillation occurs. The wavelength of the laser beam can be changed. Therefore, if the driving unit 140 that changes the distance α is used, the wavelength of the emitted laser light can be changed. Stated another way, the wavelength of the laser beam can be changed by displacing the upper reflecting mirror 130 in the optical axis direction (the y direction in FIG. 1) using the driving unit 140.

In the surface emitting laser according to the present embodiment, the wavelength λ _g at which the gain of the active layer at the time of laser oscillation is maximum, the central wavelength λ _{0 of} the emitted light, and the reflectance of the reflecting mirror at the side from which the light is emitted are maximized. The relationship between the following wavelengths λ _r satisfies λ _r <λ ₀ <λ _g or λ _g <λ ₀ <λ _r . The reason why the configuration having the above-described features produces an effect will be described below with reference to FIGS.

Here, when the reflectivity of the reflecting mirror on the side from which light is emitted is constant irrespective of the wavelength (FIG. 2B), the wavelength λ _{g at} which the gain is maximized is a wavelength as shown in FIG. If there is a dependence, the intensity of the emitted light has a wavelength dependence as shown in FIG. Usually, the wavelength lambda _g is greater gain on the long wavelength side with respect to the center of the gain spectrum as shown in FIG. 2 (a), reduced by the shorter wavelength side. Therefore, the emitted light intensity is large on the long wavelength side and small on the short wavelength side.

  Therefore, by taking into account the wavelength dependence of the gain spectrum and giving the wavelength dependence of the reflectance of the reflector on the side from which light is emitted so that the wavelength dependence of the light output intensity is reduced as a result, the light The wavelength dependence of the output intensity can be reduced.

For example, when λ _g is larger than λ ₀ (FIG. 3A), if λ _r is smaller than λ ₀ (FIG. 3B), the wavelength dependence of the optical output intensity becomes smaller (FIG. 3B). FIG. 3 (c)).

Conversely, when λ _g is smaller than λ ₀ (FIG. 4A), λ _r is increased relative to λ ₀ (FIG. 4B). Thereby, the wavelength dependence of the light output intensity is reduced (FIG. 4C).

As an example of a wavelength-dependent reflecting mirror configured to satisfy the above λ ₀ <λ _r or λ _r <λ ₀ , a distributed Bragg reflector (hereinafter abbreviated as DBR) may be mentioned. Can be

In the surface emitting laser according to the present embodiment, both the lower reflecting mirror 110 and the upper reflecting mirror 130 are DBRs. The lower reflecting mirror 110 is composed of a multilayer film in which layers having a high refractive index and layers having a low refractive index are alternately stacked. In the present embodiment, the upper reflecting mirror 130 is a reflecting mirror having a wavelength dependency configured to satisfy λ ₀ <λ _r or λ _r <λ ₀ .

The upper reflecting mirror 130 in the present embodiment will be described with reference to FIG. 5, which is an enlarged view of the upper reflecting mirror 130. The upper reflector 130 has a stacked body 134 in which first layers 131 and second layers 132 are alternately stacked, and the refractive index (n ₂ ) of the _second layer 132 is equal to that of the first layer 131. It is smaller than the refractive index (n ₁ ) (n ₁ > n ₂ ). The layers at both ends of the stacked body 134 in the stacking direction are the first layers 131. Note that the number of layers of the upper reflecting mirror 130 shown in FIGS. 1 and 2 is merely an example, and actually has more layers. Note that the first layer 131 and the second layer 132 have an optical thickness of １ ／ of the center wavelength λ ₀ of the surface emitting laser according to the present embodiment.

  In the present specification, the term “center wavelength” is used to mean a wavelength at the center of a wavelength range of light emitted from a surface emitting laser. That is, it means the wavelength at the center between the shortest wavelength and the longest wavelength of the light emitted from the surface emitting laser. The wavelength of light emitted from the laser is determined by factors such as the fluctuation width of the resonator length, the reflection band of the reflector, and the gain band of the active layer. At the time of design, basically, a center wavelength is set, and the configuration of each element is determined according to the center wavelength.

  By increasing the number of laminations of the first layer 131 and the second layer 132 constituting the upper reflecting mirror 130, the upper reflecting mirror 130 can have high-reflectance reflection characteristics over a wide band.

The surface emitting laser according to this embodiment, the both ends of the stack 134, the optical thickness nd is, lambda _0/4 <nd or ₀ <nd <λ _0/4 Third configured to meet the Layer. The third layer 133 is smaller than the first layer 131, and is a layer on the opposite side of the layered body 134 of the layer (131 or 108) adjacent to the third layer 133, that is, the air layer 108. It has a refractive index (n ₃ ) greater than the refractive index. The refractive indices n ₁ , n ₂ , and n ₃ are all larger than the refractive index (≒ 1) of the air layer 108.

Next, the effect of providing the third layer 133 will be described with reference to FIGS. First, as shown in FIG. 6 (a), when the third layer 133 is not provided, the light L ₁ reflected at the interface between the first layer 131 and the air layer 108 and second layer 132 second a light L ₂ reflected by the interface between the one layer 131, intensify each other to have the same phase, reflection occurs. Because the light L ₁ is light L ₂ optical path length lambda _0/2 larger than the, and the light L ₂ is reflected at the interface layer of high refractive index comes from a layer of low refractive index (n ₂₎ (n ₁₎ This is because the phase is λ _0/2 shifts at the time. Since the intensification of such light occurs most at the center wavelength λ ₀ , the reflectance of the reflecting mirror is the largest at λ ₀ . On the other hand, as the wavelength becomes larger or smaller than λ ₀ , the reflectance becomes smaller (FIG. 6B).

On the other hand, when the third layer 133 is provided as shown in FIG. 6C, the light L ₁ ′ reflected at the interface between the third layer 133 and the air layer 108 and the first layer 131 light L _{2 'and} is reflected at the interface between the third layer 133, phase weaken lambda _0/2 differ, reflection is suppressed. Because the light L _{1 'is} the light L _2' optical path length compared to the lambda _0/2 increases, and the light L ₂ at the interface layer of low refractive index comes from the layer of high refractive index (n ₁₎ (n ₃₎ This is because the phase does not shift when reflected. Further, suppressive effect of such a reflection is larger closer to the center wavelength lambda _0, the lower the central wavelength lambda ₀ around reflectance, as the wavelength is shorter than the center wavelength lambda _0, or, the reflection as a long The suppression effect is small (FIG. 6D). When the wavelength becomes shorter or longer, a wavelength having a high reflection suppressing effect appears again, and the reflectance decreases.

Here, as in FIG. 7 (a), when the optical thickness of the third layer 133 is λ _0/4 <nd, at a longer wavelength than the center wavelength lambda _0, since most antireflection effect becomes higher, As shown in FIG. 7B, a reflecting mirror having a low reflectance in a long wavelength band can be realized. This is because the optical thickness with the highest antireflection effect is at a quarter of the wavelength, and conversely, the thicker the optical thickness, the longer the wavelength with the higher antireflection effect.

As shown in FIG. 7 (c), the case where the optical thickness of the third layer 133 is _{0 <nd <λ 0/4} , the short wavelength than the center wavelength lambda _0, since most antireflection effect becomes higher, Figure As shown in FIG. 7D, a reflector having a low reflectance in a short wavelength band can be realized. This is because, as before, the optical thickness with the highest antireflection effect is at 1/4 of the wavelength, and conversely, the thinner the optical thickness, the shorter the wavelength at which the antireflection effect is high. is there.

  In other words, the convex portion of the graph of the wavelength dependence of the gain spectrum of the active layer and the concave portion of the graph of the wavelength dependence of the reflectance of the reflector on the side from which light is emitted partially overlap. In addition, the wavelength dependence of the light output intensity or the threshold current can be reduced.

In this embodiment, when the wavelength dependence of the gain of the active layer is G (λ) and the wavelength dependence of the reflectance of the reflecting mirror on the side from which light is emitted is R (λ), d ² G A region where (λ) / dλ ² <0 and a region where d ² R (λ) / dλ ² > 0 are at least partially overlapped. As a result, the wavelength dependence of the light output intensity can be reduced.

  The above is shown in a graph. When an active layer having a gain spectrum as shown in FIG. 3A is used, the wavelength dependence of the light output intensity is obtained by using a reflecting mirror as shown in FIG. Can be reduced.

Conversely, when an active layer having a gain spectrum as shown in FIG. 4A is used, the wavelength dependence of the optical output intensity or the threshold current can be reduced by using a reflecting mirror as shown in FIG. 7C. it can. Further, as another configuration of such a reflecting mirror having a wavelength dependency designed in consideration of the gain spectrum (the upper reflecting mirror in the present embodiment), the following is mentioned. That is, the reflecting mirror on the side from which light is emitted, the first layer and the second layer having a smaller refractive index than the first layer are alternately laminated, and the layers at both ends are the first layer. Wherein the optical thickness of one of the first layers is greater than の of the center wavelength λ ₀ .

Here, when the optical thickness of one of the first layers is の of the center wavelength λ ₀ , the reflection spectrum of the upper reflector has a low reflectance at the center wavelength λ _0. .

On the other hand, the optical thickness of the first layer, 1/2 and larger than the central wavelength lambda _0, the reflectance is low in the wavelength region longer than the central wavelength lambda _0. Similarly, the optical thickness of one layer of the first layer, 1/2 is smaller than the central wavelength lambda _0, the reflectance is low in the wavelength region longer than the central wavelength lambda _0. Therefore, by changing the optical thickness of the first layer without providing the third layer, it is possible to change the wavelength at which the reflectivity decreases, and it is possible to design a reflecting mirror according to the gain spectrum.

From another point of view, in the surface emitting laser according to the present embodiment, the ranges of λ _r <λ ₀ <λ _g and λ _g <λ ₀ <λ _{r correspond to} the emission wavelength of the ground level of the active layer. When λ _s is set, λ _r <λ ₀ <λ _s or λ _s <λ ₀ <λ _r can be rephrased.

(Upper reflector and lower reflector)
In the present embodiment, at least one of the upper reflecting mirror and the lower reflecting mirror may be a laminate having the above-described multilayer film, and both may be a laminate having the above-described multilayer film. Good. The structures and materials of the upper reflector and the lower reflector according to the present embodiment can be independently selected.

  In addition, one of the upper reflecting mirror and the lower reflecting mirror may be a diffraction grating, for example, a high refractive index difference sub-wavelength grating (hereinafter, abbreviated as HCG) mirror. The HCG mirror has a configuration in which a material having a high refractive index and a material having a low refractive index are periodically arranged alternately in an in-plane direction. As an example of the HCG mirror, there is a periodic structure of a high refractive index region (AlGaAs portion) and a low refractive index region (void portion) in which a semiconductor layer such as an AlGaAs layer is processed to provide a periodic gap.

  In the case of a wavelength-selectable VCSEL, it is preferable to make the moving reflecting mirror (the upper reflecting mirror in FIG. 1) a lightweight mirror from the viewpoint of increasing the wavelength variable speed. Therefore, in the present embodiment, it is preferable to use a thin (light) HCG mirror instead of a multilayer (mirror) mirror having a thick (heavy) configuration as the upper reflecting mirror.

An example of the dielectric multilayer mirror is a dielectric multilayer having a plurality of pairs of a SiO ₂ layer as a silicon oxide layer and a TiO ₂ layer as a titanium oxide layer.

On the other hand, when the semiconductor multilayer mirror, that is, the first layer, the second layer, and the third layer are all semiconductor layers, the material forming the semiconductor layer is AlxGa (1-x) As (0 ≦ x It is preferable to have a material represented by ≦ 1). For example, a semiconductor multilayer film having a plurality of sets of pairs of _Al x _Ga as GaAs layer and the low refractive index layer as a high refractive index layer _(1-x) As layer (0.9 ≦ x ≦ 1). Alternatively, AlAs where x = 1 can be used as the high refractive index layer.

  It should be noted that by appropriately changing the number of pairs of multilayer mirrors, it is possible to control the reflection bandwidth and the reflectance of high reflectance.

  In the embodiment of the present invention, the movable mirror can use a MEMS (Micro Electro Mechanical System) structure such as a silicon cantilever driven by electrostatic attraction.

  In the surface emitting laser according to the present embodiment, the upper reflecting mirror 130 is used as a light-extracting-side reflecting mirror, but the lower reflecting mirror 110 may be used as a light-extracting-side reflecting mirror. The peak reflectance of the reflector on the light extraction side is lower than the reflectance of the other reflector.

  Here, as the reflecting mirror on the side from which light is extracted, it is preferable that the value of the reflectance be between 99.0% and 99.5%. In a normal design, the reflectance is increased by increasing the number of pairs of the DBR, that is, the first layer 131 which is a high refractive index layer and the second layer 132 which is a low refractive index layer. When the material of the low refractive index layer is determined, the approximate number of pairs is determined. In the present embodiment, the number of pairs of the upper reflecting mirror 130 is set to be larger than the number of pairs of the DBR designed for the normal reflecting mirror on the light extraction side. That is, a stack of several tens of pairs of the same material is further added to the DBR composed of the number of pairs so that the peak of the reflectance becomes 99.5% or more. The number of pairs of reflectors on the light extraction side should be between 99.0% and 99.5%, which is the optimal range of reflectance when the third layer is arranged. Is preferred.

In the embodiment of the present invention, the third layer 133 is shown to be in contact with the air layer 108. However, a dielectric material such as SiO ₂ having a lower refractive index than the third layer 133 may be used. Good. Even when SiO ₂ is used, the difference in the refractive index between the third layer and the third layer can be increased, and the use of a dielectric material allows the semiconductor layer (third layer) to be more resistant than when the semiconductor layer (third layer) is in direct contact with air. Environmental characteristics can be improved.

  Further, in general, a DBR made of a dielectric material and a DBR made of a semiconductor make it easier to increase the difference in refractive index between the dielectric material and the DBR. On the other hand, the number of pairs is larger in a DBR made of a semiconductor than in a DBR made of a dielectric. However, due to processes such as being able to form a film at the same time during crystal growth and allowing a current to flow by doping, etc. There are advantages. In the case where a DBR is formed of a semiconductor whose refractive index difference cannot be made larger than that of a dielectric, a high reflectance and a wide reflection band can be obtained by increasing the number of layers. For example, it is preferable to have a structure in which 35 pairs or more of the first layer which is a high refractive index layer and the second layer which is a low refractive index are alternately laminated.

(Active layer)
The active layer in the present embodiment is not particularly limited as long as it is a material that generates light by injecting a current. Case of emitting light in the wavelength band around 850 _nm, it is possible to use a material having a quantum well structure composed of _{Al n Ga (1-n)} As (0 ≦ n ≦ 1). Also, if the light is emitted in the wavelength band around 1060 _nm, or the like can be used materials composed of _{In n Ga (1-n)} As (0 ≦ n ≦ 1).

  Further, the active layer in the present embodiment preferably has a sufficiently wide gain, and more specifically, preferably has a gain in a wavelength region wider than the reflection band of the upper reflecting mirror and the lower reflecting mirror. An example of such an active layer is an active layer having a quantum well structure capable of emitting light at at least two or more different energy levels. Further, the quantum well structure may be formed of a plurality of layers so as to have a single quantum well or a multiple quantum well.

  The material and structure of the active layer in the present embodiment can be appropriately selected according to the wavelength to be oscillated.

  Further, the active layer in the present embodiment may emit light when irradiated with light and excited, or may emit light when injected with current. Therefore, the surface emitting laser in this embodiment or the optical coherence tomography described below may have an excitation light source for exciting the active layer, or may have a power supply for injecting current into the active layer. May be.

(First cladding layer and second cladding layer)
In the embodiment of the present invention, a cladding layer is provided to confine light and carriers. In the embodiment of the present invention, the cladding layer also plays a role as a spacer for adjusting the length of the resonator.

As the first clad layer and the second clad layer in the present embodiment, an AlGaAs layer in which the composition of Al is appropriately selected depending on the wavelength band to be emitted can be used. For example, to emit light in a wavelength band around 850 nm, an Al _0.8 GaAs layer can be used. To emit light in a wavelength band around 1060 nm, an Al _0.4 GaAs layer and a GaAs layer are used. Can be used. Note that the first clad layer and the second clad layer have different conductivity types. The resonator length can be a λ resonator or a long resonator of about 5λ in a fixed wavelength VCSEL. Therefore, the thickness of the cladding layer is adjusted to secure the resonator length. On the other hand, in a wavelength tunable VCSEL, a 3 to 4λ resonator is preferably used in consideration of a movable area (a gap portion described later) of the movable mirror, driving, and a current confinement structure, and the thickness of the cladding layer is adjusted. When the thickness of the cladding layer is adjusted, the thickness of the first cladding layer and the thickness of the second cladding layer do not necessarily have to be the same, and can be appropriately selected as long as the resonator length can be adjusted.

(Current constriction layer)
In the present embodiment, a current confinement layer for limiting the region where the current injected into the laser flows can be provided as necessary. The current confinement layer is formed by implanting hydrogen ions or selectively oxidizing an AlGaAs layer having an Al composition of 90% or more provided in the cladding layer.

(Void)
Usually, no solid exists in the voids in the present embodiment. Therefore, depending on the atmosphere, the void may be in a vacuum or a fluid such as air, an inert gas, or a liquid such as water may be present. Note that the length of the gap (α in FIG. 1) can be determined in consideration of the wavelength variable bandwidth and the pull-in of the movable mirror. For example, in a 3 or 4λ resonator in which the wavelength is tunable with a wavelength tunable bandwidth of 100 nm around 1060 nm with air as the center, the length of the air gap is about 1 μm.

(Drive part)
The drive unit is not particularly limited as long as it can change the resonator length of the wavelength tunable VCSEL according to the present embodiment. For example, a driving unit that drives by applying a voltage using a MEMS mechanism and a driving unit that drives using a piezoelectric material such as piezo are given. Further, although the present embodiment has a cantilever structure, it may have a doubly supported structure.
The drive unit in the present embodiment may have a configuration that displaces the upper reflector, a configuration that displaces the lower reflector, or a configuration that displaces both.

(Optical coherence tomography)
Optical coherence tomography (hereinafter sometimes abbreviated as OCT) using a wavelength tunable light source does not use a spectrometer, so that it is expected to acquire a tomographic image with a small SN loss and a high SN ratio. ing. An example in which the surface emitting laser according to the embodiment is used in the light source unit of the OCT will be described with reference to FIG.

  The OCT apparatus 8 according to the present embodiment has at least a light source unit 801, an interference optical system 802, a light detection unit 803, and an information acquisition unit 804, and the above-described surface emitting laser can be used as the light source unit 801. . Although not shown, the information acquisition unit 804 has a Fourier transformer. Here, the form in which the information acquisition unit 804 includes a Fourier transformer is not particularly limited as long as the information acquisition unit has a function of performing Fourier transform on input data. One example is a case where the information acquisition unit 804 has a calculation unit, and the calculation unit has a function of performing Fourier transform. More specifically, the arithmetic unit is a computer having a CPU, and the computer executes an application having a Fourier transform function. Another example is a case where the information acquisition unit 804 includes a Fourier transform circuit having a Fourier transform function. Light emitted from the light source unit 801 is output as interference light having information on the object 812 to be measured via the interference optical system 802. The interference light is received by the light detection unit 803. The light detection unit 803 may be a differential detection type or a simple intensity monitor type. Information on the time waveform of the intensity of the received interference light is sent from the light detection unit 803 to the information acquisition unit 804. The information acquisition unit 804 acquires the peak value of the time waveform of the intensity of the received interference light, performs Fourier transform, and acquires information on the object 812 (for example, information on a tomographic image). Note that the light source unit 801, the interference optical system 802, the light detection unit 803, and the information acquisition unit 804 can be arbitrarily provided.

  Hereinafter, the process from the emission of light from the light source unit 801 to the acquisition of tomographic image information of the object to be measured will be described in detail.

  Light emitted from the light source unit 801 for changing the wavelength of light passes through the fiber 805 and enters the coupler 806, and is converted into irradiation light passing through the irradiation light fiber 807 and reference light passing through the reference light fiber 808. Branched. The coupler 806 is configured to operate in a single mode in the wavelength band of the light source, and various fiber couplers can be configured to be 3 dB couplers. The irradiation light passes through the collimator 809, becomes parallel light, and is reflected by the mirror 810. The light reflected by the mirror 810 irradiates the object 812 through the lens 811 and is reflected from each layer of the object 812 in the depth direction. On the other hand, the reference light passes through the collimator 813 and is reflected by the mirror 814. In the coupler 806, interference light is generated by the reflected light from the object 812 and the reflected light from the mirror 814. The interfering light passes through the fiber 815, is collected through the collimator 816, and is received by the light detection unit 803. Information on the intensity of the interference light received by the light detection unit 803 is converted into electrical information such as a voltage and transmitted to the information acquisition unit 804. The information acquisition unit 804 processes the data of the intensity of the interference light, specifically, performs Fourier transform to obtain information of the tomographic image. The data of the intensity of the interference light to be Fourier-transformed is usually data sampled at equal wave number intervals, but it is also possible to use data sampled at equal wavelength intervals.

  Information on the obtained tomographic image may be sent from the information acquisition unit 804 to the image display unit 817 and displayed as an image. Note that a three-dimensional tomographic image of the object 812 to be measured can be obtained by scanning the mirror 811 in a plane perpendicular to the direction in which the irradiation light is incident. The light source unit 801 may be controlled by the information acquisition unit 804 via the electric circuit 818. Although not shown, the intensity of light emitted from the light source unit 801 may be sequentially monitored, and the data may be used for correcting the amplitude of the signal of the intensity of the interference light. The surface emitting laser according to the embodiment of the present invention can oscillate laser light over a wide band while suppressing an increase in threshold current for emitting laser light. And a tomographic image with high depth resolution can be obtained.

The OCT apparatus according to the embodiment is useful when acquiring a tomographic image of a living body such as an animal or a human in fields such as ophthalmology, dentistry, and dermatology. The information on the tomographic image of the living body includes not only the tomographic image of the living body but also numerical data necessary for obtaining the tomographic image.
In particular, it is preferable to use the measurement target as the fundus of the human body and acquire information on a tomographic image of the fundus.

(Other uses)
The surface emitting laser according to the embodiment of the present invention can be used as a light source for optical communication or a light source for optical measurement in addition to the above-described OCT.

  Hereinafter, examples of the present invention will be described. The active layer structure and the layer structure shown in the following examples are merely examples, and the present invention is not limited thereto. Although the manufacturing method is specifically shown in the embodiments, the dimensions of the components of the light source, the manufacturing steps, the apparatuses, and various parameters are not limited to the embodiments. Further, semiconductor materials, electrode materials, dielectric materials, and the like are not limited to those disclosed in the embodiments.

(Example 1)
A configuration of a wavelength tunable VCSEL having a center wavelength of 1060 nm according to the first embodiment of the present invention will be described with reference to FIG. The tunable VCSEL according to the present embodiment has a cantilever structure as shown in FIG.

  The surface emitting laser in this embodiment includes an n-type GaAs substrate as the substrate 102, an n-type semiconductor DBR as the lower reflector 110, an InGaAs layer having a quantum well structure as the active layer 104, a GaAs layer as the insulating layer 106, and an upper reflector 130 Is used as an n-type semiconductor DBR.

  The lower reflector 110 has a structure in which 40 pairs of GaAs and AlAs are alternately stacked.

In addition, an Al _0.98 Ga _0.02 As layer provided by selective oxidation is used as the current confinement layer 121. As the laser driving electrode, an n-type electrode made of AuGe / Ni / Au is used as the lower electrode 101, and a p-electrode made of Ti / Au is used as the upper electrode 105. With these electrodes, a current is injected into the active layer 104 to emit light and cause laser oscillation. Further, by driving the MEMS using the driving unit 140 to displace the upper reflecting mirror 130 in the y direction, the wavelength of the laser beam can be changed.

In the range of standard carrier density is ^3x10 18 ^cm -1 when the laser is operated in FIG. ^9, 4x10 ¹⁸ cm ^-1, indicating the gain spectrum of the active layer 121 when the ^5x10 18 ^{cm -1,} respectively. The active layer 121 has a quantum well of InGaAs / GaAs, and there are three quantum wells from the lower cladding layer 103 to the upper cladding layer 120. The ground level of the quantum well has a wavelength of 1080 nm. FIG. 9 shows that there is a gain peak at a wavelength near the ground level at any carrier density. Then, the gain sharply decreases as the wavelength becomes longer than the wavelength of 1080 nm. On the other hand, the gain gradually decreases as the wavelength becomes shorter. As described above, the gain spectrum shown in FIG. 9 is determined by quantum mechanics and cannot be easily changed. Therefore, in the present embodiment, the wavelength dependence of the optical output intensity or the threshold current is reduced by designing the reflectance spectrum of the upper reflecting mirror according to the shape of the gain spectrum as shown in FIG.

The configuration and characteristics of the upper reflector according to the present embodiment will be described. The upper reflecting mirror 130 according to the present embodiment includes a first layer (Al _0.2 Ga _0.8 As layer) which is a high refractive index layer and a second layer (Al _0.8 Ga layer) which is a low refractive index layer. _0.2 As layer) are alternately laminated, and the first layer is on both surfaces of the laminate. Since the number of pairs of the first layer and the second layer is 47 pairs and the number of the first layer is one layer, they are referred to as 47.5 pairs here. Then, a third layer (Al _0.55 Ga _0.45 As) is provided on both ends of such a laminate. The optical thickness of each of the first and second layers was set to １ ／ of the center wavelength (1060 nm), and the optical thickness of the third layer was set to 1 / 3.8 of the center wavelength (1060 nm). The refractive indices of the first layer, the second layer, and the third layer are n ₁ = 3.35, n ₂ = 3.05, and n ₃ = 3.14, respectively.

  FIG. 10A shows a graph of the wavelength dependence of the reflectance of the upper reflecting mirror in the present embodiment (Example 1 (47.5 pairs + third layer)). Further, as a conventional example in FIG. 10A, a first layer is further laminated on a structure in which the first layer and the second layer are alternately laminated in 22 pairs so that both end layers become the first layer. 3 shows the wavelength dependence of the reflectance of the reflecting mirror (22.5 pairs). Further, a reflecting mirror (47.5 pairs) in which a first layer is further laminated on a structure in which 47 pairs of the first layer and the second layer are alternately laminated so that the layers at both ends become the first layer. 3) shows the wavelength dependence of the reflectance. The upper reflecting mirror (47.5 pairs + third layer) in this embodiment has a higher reflectance in a low wavelength region than the conventional example (22.5 pairs) and the conventional example (47.5 pairs), It can be seen that the reflectance is low in the long wavelength region.

  The threshold current and the wavelength dependence of the optical output intensity at the drive current (four times the threshold) when the upper reflecting mirror having the characteristic as shown in FIG. 10A is applied to the wavelength tunable VCSEL according to the present embodiment. Graphs are shown in FIGS. 10 (b) and 10 (c).

  According to FIG. 10B, comparing the conventional example (22.5 pairs) with the present embodiment, it can be seen that the present embodiment has a smaller wavelength dependence of the threshold current in a wider wavelength band. Further, according to FIG. 10C, comparing the conventional example (22.5 pairs) and the conventional example (47.5 pairs) with the present embodiment, the present embodiment has a wider wavelength band, specifically, It can be seen that the wavelength dependence of the light output intensity is small from around 1030 nm to around 1080 nm.

  As described above, by introducing a wavelength-dependent reflecting mirror in consideration of the gain spectrum, the laser characteristics are stabilized, and specifically, the wavelength dependence of the threshold current and the optical output intensity is reduced as compared with the conventional example. It turns out that it becomes.

  The tunable VCSEL according to the present embodiment can be manufactured using epitaxial growth and selective wet etching. The void 107 can be formed by using GaAs as a sacrificial layer and performing selective wet etching. By using a mixed solution of water, citric acid and hydrogen peroxide as an etchant of GaAs, selective etching according to the Al composition of AlGaAs can be performed. In this embodiment, a mixture of water and citric acid (weight ratio 1: 1), and a mixture of 30% hydrogen peroxide solution at a ratio of 4: 1 were used as an etchant. By controlling the etching time, it is possible to form the tunable VCSEL according to the present embodiment in which the GaAs layer 106 supporting the beams of the upper reflector 130 and the upper reflector 130 are left.

(Example 2)
A tunable VCSEL according to a second embodiment of the present invention will be described with reference to FIG. In the second embodiment, differences from the first embodiment will be described, and description of common points will be omitted.
In this embodiment, a desired reflection spectrum is constituted only by two layers of a high refractive index layer and a low refractive index constituting the upper reflecting mirror. At this time, the optical thickness of one of the constituent high refractive index layers is set to 0.513 times the center wavelength. That is, the optical thickness of one of the first layers is larger than 中心 of the center wavelength λ ₀ . Otherwise, the configuration is the same as that of the first embodiment.

  FIG. 11A shows a specific configuration of the upper reflecting mirror. The upper reflecting mirror 1130 has a stacked body 1133 in which 46 pairs of first high refractive index layers 1131 having a refractive index of 3.35 and low refractive index layers 1132 having a refractive index of 3.05 are alternately stacked. The optical thicknesses of the first high refractive index layer and the low refractive index layer are both １ ／ of the center wavelength (1060 nm).

  Then, a second high refractive index layer 1134 having a thickness of 0.513 times the center wavelength (1060 nm) and a refractive index of 3.35 is formed on the laminate, and a low refractive index layer 1132 having the same thickness as the above is formed thereon. A refractive index layer 1131 is provided. That is, there are 47.5 pairs.

  FIG. 11B shows the wavelength dependence of the reflectance realized by such an upper reflecting mirror. FIG. 11B shows a reflection spectrum of a conventional example (22.5 pairs) shown in FIG. 10A of the first embodiment as a conventional example.

  From FIG. 11B, it can be seen that the reflection spectrum of the upper reflecting mirror in the present embodiment has the peak of the reflection spectrum located on the shorter wavelength side than the center wavelength, similarly to the first embodiment.

  FIG. 11 (c) shows a comparison of the threshold current of the wavelength tunable VCSEL when the two types of reflectors shown in FIG. 11 (b) are used. FIG. 11B shows that the threshold current is more stable in a wider wavelength tunable band by using the reflecting mirror of the present embodiment than in the conventional example. Therefore, the wavelength dependence of the light output intensity is also reduced.

(Example 3)
Third Embodiment A tunable VCSEL according to a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, differences from the first embodiment will be described, and description of common points will be omitted. In the present embodiment, the gain of the active layer and the structure of the upper reflecting mirror are different from those of the first embodiment.

  The tunable VCSEL according to the present embodiment differs from the first embodiment in that the active layer 104 has an asymmetric quantum well structure. Like the first embodiment, the active layer 104 has three InGaAs / GaAs QW layers having a gain peak near 1080 nm, and additionally has one InGaAs / GaAs QW layer having a gain peak near 1040 nm.

Next, a specific configuration of the upper reflecting mirror 1230 is shown in FIG. The upper reflecting mirror 1230 includes a first layer (Al _0.2 Ga _0.8 As layer) which is a high refractive index layer and a second layer (Al _0.8 Ga _0.2 As layer) which is a low refractive index layer. ) Are alternately laminated, and the layers at both ends are the laminate 1234 as the first layer. That is, since the pair of the first layer and the second layer is 47 pairs and the first layer is one layer, they are referred to as 47.5 pairs here. Then, one of the layers at both ends of such a laminate has a refractive index of 3.1 and an optical thickness of 1 / 4.3, which is smaller than 1/4 of the center wavelength (1060 nm). An intermediate refractive index layer 1233 is provided. On the other surface of the laminate 1234, a second intermediate refractive index layer 1235 having an optical thickness of more than 厚 い of the center wavelength (1060 nm) and a refractive index of 3.12, which is 3.92 times, is provided. Have been.

FIG. 12B shows the wavelength dependence of the gain of the active layer in this embodiment, and FIG. 12C shows the wavelength dependence of the reflectance of the upper reflector having the above-described configuration. The gain spectrum of the active layer is obtained when the carrier density is 3 × 10 ¹⁸ cm ⁻¹ . FIG. 12C shows that the active layer in this example has gain peaks near 1040 nm and 1080 nm. In addition, it can be seen that the reflection spectrum of the upper reflecting mirror in this example has local minimum values at 1050 nm and 1075 nm.

  In the present embodiment, since the active layer is provided with quantum wells having different base orders, the gain spectrum of the active layer has a plurality of peaks. Therefore, two kinds of different intermediate refractive index layers having different optical thicknesses are introduced in order to match the gain peaks. Since the optical thickness of the first intermediate refractive index layer 1233 is thinner than 中心 of the center wavelength, a region having a lower reflectance on the shorter wavelength side than the center wavelength is formed. On the other hand, since the second intermediate refractive index layer 1235 has an optical thickness larger than １ ／ of the center wavelength, the second intermediate refractive index layer 1235 forms a region having a lower reflectance on the longer wavelength side than the center wavelength.

  With such a configuration, the wavelength dependence of the optical output intensity or the threshold current can be reduced according to the above-described principle.

DESCRIPTION OF SYMBOLS 101 Lower electrode 102 Substrate 103 Lower clad layer 104 Active layer 105 Upper electrode 106 Insulating layer 107 Void 120 Upper clad layer 121 Current confinement layer 130 Upper reflector 131 First layer 132 Second layer 133 Third layer 134 Stack Body 140 drive

Claims (17)

A lower reflector,
An active layer,
Voids,
And an upper reflector, in this order, the lower reflector, a surface-emitting laser that varies the wavelength of light emitted by changing the distance between the upper reflector,
A drive unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in the optical axis direction of the emitted light,
The relationship between the wavelength λ _g at which the gain at the time of laser oscillation of the active layer becomes maximum, the central wavelength λ _{0 of} the emitted light, and the wavelength λ _r at which the reflectance of the reflector on the side from which the light is emitted becomes maximum. , Λ _g <λ ₀ <λ _r .
The convex portion of the graph showing the wavelength dependence of the gain of the active layer and the concave portion of the graph showing the wavelength dependence of the reflectance of the reflector on the side from which light is emitted are configured to partially overlap. ,
The reflecting mirror on the side from which the light is emitted, a first layer and a second layer having a smaller refractive index than the first layer are alternately laminated, and both end layers are the first layer. has a laminate is a layer, on at least one of both ends of the laminate, provided the optical thickness nd _{is, 0 <nd <λ 0/} 4 further has a third layer that is configured to meet the The refractive index of the third layer is smaller than the first layer, and among the layers adjacent to the third layer, the laminate has a higher refractive index than the layer on the opposite side,
Wherein the third layer is provided with the third layer so that the wavelength dependence of the light output intensity or the threshold current of the surface emitting laser is reduced. . A lower reflector,
An active layer,
Voids,
And an upper reflector, in this order, the lower reflector, a surface-emitting laser that varies the wavelength of light emitted by changing the distance between the upper reflector,
A drive unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in the optical axis direction of the emitted light,
The convex portion of the graph showing the wavelength dependence of the gain of the active layer and the concave portion of the graph showing the wavelength dependence of the reflectance of the reflector on the side from which light is emitted are configured to partially overlap. ,
G (λ) is the wavelength dependence of the gain of the active layer, R (λ) is the wavelength dependence of the reflectance of the reflector on the side from which the light is emitted, and the maximum gain of the active layer during laser oscillation is maximum. Where d ² G (λ) / dλ ² <0, and d ² R (λ) / dλ ² > 0, where λ _{g is} the central wavelength λ ₀ of the emitted light. Is configured to at least partially overlap, the λ _g is smaller than the λ ₀ , and the reflecting mirror on the side from which the light is emitted has a first layer and a refractive index smaller than the first layer. A second layer and a second layer are alternately laminated, and a layer at both ends has a laminate as the first layer, and at least one of both ends of the laminate has an optical thickness nd of 0 <0. nd <λ _0/4 further provided a third layer that is configured to meet the refractive index of the third layer is the first layer Small, and has a refractive index greater than the opposite side of the layer and the laminate of the adjacent layers of the third layer,
Wherein the third layer is provided with the third layer so that the wavelength dependence of the light output intensity or the threshold current of the surface emitting laser is reduced. . The surface emitting laser according to claim 2 , wherein the adjacent layer is air. The surface emitting laser according to claim 2 , wherein the first layer, the second layer, and the third layer are all semiconductor layers. The surface emitting laser according to claim 4 , wherein a material constituting the semiconductor layer includes a material represented by AlxGa (1-x) As (0≤x≤1). The surface emitting laser according to any one of claims 2 to 5 , wherein the third layer is provided on one of both ends of the stacked body. The surface emitting laser according to claim 2 to 5, characterized in that said third layer is provided on both ends of the laminate. The reflecting mirror on the side from which the light is emitted has a first layer and a second layer having a smaller refractive index than the first layer are alternately stacked, and the first layer is the stacked layer. Having a laminate laminated on both surfaces in the direction,
The surface emitting laser according to claim 1, wherein an optical thickness of one of the first layers is larger than の of the center wavelength λ ₀ . The reflecting mirror on the side from which the light is emitted has a first layer and a second layer having a smaller refractive index than the first layer are alternately stacked, and the first layer is the stacked layer. Having a laminate laminated on both surfaces in the direction,
The surface emitting laser according to claim 1, wherein an optical thickness of one of the first layers is smaller than の of the center wavelength λ ₀ . The drive unit includes a surface emitting laser according to any one of claims 1 to 9, characterized in that for displacing the upper reflector. The surface emitting laser according to any one of claims 1 to 10 , wherein the upper reflecting mirror is a diffraction grating. The drive unit includes a surface emitting laser according to any one of claims 1 to 11, characterized in that for displacing the lower reflecting mirror. The active layer is a surface emitting laser according to any one of claims 1 to 12 emits light when light is excited by irradiation. The active layer is a surface emitting laser according to any one of claims 1 to 12 emits light when current is injected. A lower reflector,
An active layer,
Voids,
And an upper reflector, in this order, the lower reflector, a surface-emitting laser that varies the wavelength of light emitted by changing the distance between the upper reflector,
A drive unit that displaces one of the lower reflecting mirror and the upper reflecting mirror in the optical axis direction of the emitted light,
The convex portion of the graph showing the wavelength dependence of the gain of the active layer and the concave portion of the graph showing the wavelength dependence of the reflectance of the reflector on the side from which light is emitted are configured to partially overlap. ,
The relationship among the emission wavelength λ _s of the ground level of the active layer, the center wavelength λ _{0 of} the emitted light, and the wavelength λ _r at which the reflectance of the reflecting mirror from which the light is emitted is maximized is λ _s < is configured to satisfy λ ₀ <λ _r ,
The reflecting mirror on the side from which the light is emitted, a first layer and a second layer having a smaller refractive index than the first layer are alternately laminated, and both end layers are the first layer. has a laminate is a layer, on at least one of both ends of the laminate, provided the optical thickness nd _{is, 0 <nd <λ 0/} 4 further has a third layer that is configured to meet the The refractive index of the third layer is smaller than the first layer, and among the layers adjacent to the third layer, the laminate has a higher refractive index than the layer on the opposite side,
Wherein the third layer is provided with the third layer so that the wavelength dependence of the light output intensity or the threshold current of the surface emitting laser is reduced. . The surface emitting laser according to any one of claims 1 to 15 , wherein the reflectivity of the reflecting mirror on the side from which the light is emitted is between 99.0% and 99.5%. A light source section for changing the wavelength of light,
An interference optical system that splits irradiation light and reference light that irradiate light from the light source unit onto the object, and generates reflected light of the light irradiated to the object and interference light by the reference light,
A light detection unit that receives the interference light,
An information acquisition unit that processes a signal from the light detection unit to acquire information on the object,
Optical coherence tomography having
An optical coherence tomography, wherein the light source unit is the surface emitting laser according to any one of claims 1 to 16 .

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