JP2002076513A - Bragg's reflecting mirror distributed independently of temperature and planar optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the temperature dependency of reflection characteristics of a DBR mirror concerning a temperature independent distributed Bragg's reflecting mirror and a planar optical element. SOLUTION: At least a pair of film 2 having the positive temperature coefficient of refraction factor and film 3 having the negative temperature coefficient of refraction factor are laminated, and the wavelength of one pair is made into the half integer multiple of a central wavelength so that a distributed Bragg's reflecting mirror can be configured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature-independent distributed Bragg reflection type mirror and a surface optical element, and more particularly to a distributed Bragg reflection type mirror having a multilayer structure used for a surface optical semiconductor element such as a surface emitting laser. The present invention relates to a temperature-independent distributed Bragg reflection type mirror and a surface type optical element which are characterized by selecting a multilayer film material for reducing temperature dependence in a (DBR) mirror.

[0002]

2. Description of the Related Art In recent years, with the rapid increase in the number of Internet users, maintenance of optical communication systems has been progressing at a rapid pace, and optical communication systems used for optical interconnections in trunk-line or access-type optical communication systems have been developed. Devices are mainly divided into waveguide devices and surface devices.

[0003] Among them, a surface type device is required to be able to reduce power consumption, to be able to form a two-dimensional high-density array element, and to be compatible with an LSI in mounting surface.
Alternatively, there is an advantage that coupling to an optical fiber is easy.

In such a surface-type device, for example, a surface-emitting semiconductor laser or a surface-type semiconductor optical amplifier, a multilayer DBR mirror is used to form a resonator or the like. A conventional surface emitting semiconductor laser will be described with reference to FIG. First, an n-type AlAs λ / 4 is formed on an n-type GaAs substrate 51 as shown in FIG.
The first semiconductor DBR mirror 54 is formed by alternately stacking the films 52 and the n-type GaAs λ / 4 films 53. The first
One pair of n-type AlA constituting the semiconductor DBR mirror 54
The total optical length of the sλ / 4 film 52 and the n-type GaAs λ / 4 film 53 is configured to be an integral multiple of half a wavelength, and in this case, the optical length of one pair is λ / 2.

[0005] Referring to FIG. 10 (b), next, apart from this first semiconductor DBR mirror wafer, a laser wafer is first prepared with an In composition ratio of 0.2.
6 on an InGaAs substrate 61 having an In composition ratio of 0.24
After alternately stacking a p-type InAlAs λ / 4 film 62 of No. 5 and a p-type InGaAs λ / 4 film 63 of 0.26 in In composition ratio to form a second semiconductor DBR mirror 64, p
A type InAlGaAs cladding layer 65, a strained quantum well active layer 66, and an n-type InAlGaAs cladding layer 67 are sequentially stacked. Note that a pair of p-type InAlAs λ / 4 films 62 and a p-type I
The total optical length with the nGaAs λ / 4 film 63 is configured to be an integral multiple of a half wavelength, and in this case, the optical length of one pair is λ / 2.

Next, an n-type InA formed on an InGaAs substrate 61 is formed as shown in FIG.
The lGaAs cladding layer 67 and the first semiconductor DBR mirror 54 formed on the n-type GaAs substrate 51 are brought into contact with each other, and both wafers are bonded by applying heat and applying heat. Then, the InGaAs substrate 61 is selected. By this, the basic structure of the surface emitting semiconductor laser is completed.

In the above-mentioned conventional surface emitting semiconductor laser, the first semiconductor DBR mirror 54 and the second semiconductor DBR mirror 64 are used as reflectors in order to perform the entire film forming process by epitaxial growth, but they are not necessarily used. The present invention is not limited to the semiconductor DBR mirror, and a similar surface emitting laser can be formed by using dielectric multilayer films having different refractive indexes.

[0008]

However, when the DBR mirror is formed of a semiconductor or a dielectric as described above, the oscillation threshold I _th of the surface emitting laser, the light output characteristic, However, there is a problem that the optical amplification characteristics of the surface type semiconductor optical amplifier change.

That is, the refractive index of a semiconductor or a dielectric depends on temperature, and the reflection characteristic fluctuates when the optical length of the layer constituting the DBR mirror changes with temperature. It will be described with reference to FIG.

FIG. 11 is an explanatory diagram of the temperature dependence of the reflectivity of the DBR mirror. The solid line shows the reflection spectrum at a normal use environment temperature, and depends on the film thickness of the layers constituting the DBR mirror. High reflection characteristics with respect to the laser oscillation wavelength can be obtained.

However, since the temperature coefficients of the refractive indexes of the semiconductor and the dielectric are positive, as the temperature rises, the forbidden band width decreases and the refractive index increases.
This is because the optical length of the layer constituting the R mirror increases, and as a result, the reflection spectrum shifts to the longer wavelength side as shown by the broken line in the figure. Conversely, when the temperature decreases, the forbidden band width increases and the refractive index decreases. However, when the refractive index decreases, the optical length of the layer constituting the DBR mirror shrinks, and as a result, the reflection spectrum becomes short. This shifts to the wavelength side.

Accordingly, it is an object of the present invention to reduce the temperature dependence of the reflection characteristics of a DBR mirror.

[0013]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. FIG.
1A is a schematic sectional view of a distributed Bragg reflection type mirror, and FIG. 1B is a view showing one pair of multilayer films constituting the distributed Bragg reflection type mirror.
(C) is a diagram schematically showing a change in the refractive index and a change in the optical length of each film when the temperature rises.

1 (a) to 1 (c) (1) The present invention relates to a temperature-independent distributed Bragg reflection type mirror, a film 2 having a positive temperature coefficient of refractive index and a film having a negative temperature coefficient of refractive index. 3 are alternately laminated at least one pair, and the optical length of one pair is set to an integral multiple of half the center wavelength.

As described above, by alternately laminating at least one pair of films 2 having a positive temperature coefficient of refractive index and films 3 having a negative temperature coefficient of refractive index on the substrate 1, temperature fluctuation of the optical length is reduced. They can cancel each other, thereby reducing the temperature change of the reflection characteristic.

That is, as shown in FIG. 1B, the film 2 having a positive refractive index temperature coefficient is d ₁ , the refractive index is n ₁ , and the film temperature of the negative refractive index film 3 is a film 3. The thickness is d ₂ , the refractive index is n ₂ ,
Temperature coefficient (dn ₁ / dT), where T is the absolute temperature
And the temperature coefficient (dn ₂ / dT) are, by definition, dn ₁ / dT> 0 and dn ₂ / dT <0. When the center wavelength is λ and m is an integer, n ₁ d ₁ + n ₂ d ₂ = m (λ / 2) is satisfied.

As shown in FIG. 1C, when the temperature rises, the refractive index n ₁ of the film 2 having a positive refractive index temperature coefficient increases, and the optical length increases, while the temperature coefficient of the refractive index increases. Is negative membrane 3
Since the refractive index n ₂ of the optical fiber increases, and the optical length shrinks, the fluctuation of the optical length is offset as a whole.

In particular, by setting the relationship between the temperature coefficient of the refractive index and the film thickness so as to satisfy (dn ₁ / dT) d ₁ + (dn ₂ / dT) d ₂ = 0, the temperature change of the reflection characteristic can be reduced. Can be eliminated.

(2) Further, according to the present invention, in the above (1), the film 2 having a positive temperature coefficient of refractive index is either a semiconductor or a dielectric, and the temperature coefficient of refractive index is negative. The film 3 is characterized by being either a metalloid or a polymer.

As described above, the film 2 having a positive refractive index temperature coefficient is preferably a semiconductor such as a III-V compound semiconductor or polycrystalline Si, or a dielectric film such as SiO ₂ or TiO ₂ . The film 3 having a negative temperature coefficient of refractive index is B
A semimetal such as i or TlGaAs, or a polymer such as PMMA (polymethyl methacrylate) is preferred.

(3) The present invention is characterized in that, in the above (2), the semimetal is a group III-V compound containing Tl.

As described above, when a III-V group compound containing Tl such as TlGaAs is used as a semimetal, the distribution of temperature can be increased by using a III-V group compound semiconductor as the film 2 having a positive temperature coefficient of refractive index. The whole Bragg reflection (DBR) mirror can be formed by epitaxial growth,
In particular, when the present invention is applied to a surface-emitting optical element such as a surface-emitting semiconductor laser or a surface-type semiconductor optical amplifier, the entire film forming process can be performed by epitaxial growth.

(4) Further, the present invention is characterized in that the surface-type optical element includes the temperature-independent distributed Bragg reflection type mirror according to any one of the above (1) to (3). .

As described above, by applying the temperature-independent distributed Bragg reflection mirror according to any one of the above (1) to (3) to a surface optical element, a surface optical element with less temperature fluctuation can be obtained. Can be configured. In this case, as the surface optical element, a surface emitting semiconductor laser, a surface semiconductor optical amplifier, a surface optical filter, or the like is typical.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A manufacturing process according to a first embodiment of the present invention will now be described with reference to FIGS. Referring to FIG. 2A, first, the MBE method (Molecular
Beam epitaxy) to form the AlAsSb film 12
The first DBR mirror 14 is formed by alternately stacking 1 to 40 pairs, for example, 25 pairs of TlGaAs films 13. In this case, the thickness of one pair of the AlAsSb film 12 and the TlGaAs film 13 is formed such that the optical length is an integral multiple of 1/2 of the laser oscillation wavelength λ, for example, λ / 2. In the figure, only four pairs are shown for simplicity.

Referring to FIG. 2B, on the other hand, apart from the DBR mirror wafer, a laser wafer is first formed of InGaAs having an In composition ratio of 0.26.
MOVPE method (metal organic chemical vapor deposition) on the s substrate 15
Using Zn, a Zn concentration of 5 × 10 ¹⁷ cm ⁻³ and a thickness of 17
A p-type InAlGaAs cladding layer 16 having an In composition ratio of 0.256 and an Al composition ratio of 0.21 and a strained quantum well active layer 17 having an In composition ratio of 0.21 and an Si concentration of 5 × 10 ¹⁷ c
m ⁻³ , thickness 178.2 nm, In composition ratio 0.25
6. An n-type InAlGaAs cladding layer 18 having an Al composition ratio of 0.21 is sequentially laminated.

FIG. 2C is a schematic enlarged view of the inside of the rectangle shown by the broken line in FIG. 2B, and shows the strained quantum well active layer 1 in this case.
7, the thickness is 10 nm, and the In composition ratio is 0.25
6. Non-doped i-type In with Al composition ratio of 0.21
Three AlGaAs barrier layers 19, a thickness of 7 nm,
A non-doped i-type InGaAs strain well layer 20 having a composition ratio of 0.47 is formed by alternately stacking two layers. The emission wavelength of the strained quantum well active layer 17 is 1.3 μm
Becomes

Next, an n-type InA film formed on the InGaAs substrate 15 is formed as shown in FIG.
The lGaAs cladding layer 18 and the first DBR mirror 14 formed on the InAs substrate 11 are brought into contact with each other,
A weight is introduced into a heat treatment furnace (not shown). H ₂ gas is introduced into the heat treatment furnace, and the two wafers are bonded by performing a heat treatment at a temperature of, for example, 650 ° C. for, for example, 10 minutes.

Next, after selectively removing only the InAs substrate 11, a second DBR similar to that shown in FIG.
Using the wafer mirror, a p-type InAlGaAs cladding layer 16, are laminated alternately and TlGaAs film 21 and the AlAsSb film 22 is contacted to face the first 2DBR mirror 23 formed by the weighted with H ₂ gas atmosphere Then, the two wafers are bonded by performing a heat treatment at, for example, a temperature of 650 ° C. for, for example, 10 minutes. Finally, the substrate of the second DBR mirror wafer (not shown)
Is completed, the basic structure of the surface emitting semiconductor laser according to the first embodiment of the present invention is completed.

FIG. 4 is a diagram for explaining the correlation between the forbidden band width and the lattice constant of AlAsSb and TlGaAs (if necessary, see H. Koh, et.
al. , Record of the 16th El
electronic Materials Sympos
ium, Minoo, pp. 19-20, July 9-
11, 1997), and AlAsSb lattice-matched to InAs has a positive bandgap of about 1.6 eV, while TlGaAs lattice-matched to InAs is-
It has a negative bandgap of about 0.8 eV, and such TlGaAs becomes a semimetal.

A semimetal having such a negative bandgap is
Since the temperature coefficient of the refractive index is also negative, by combining with AlAsSb having a positive temperature coefficient of the refractive index, the DBR
The temperature dependency of the mirror can be reduced.

[0032] In particular, refractive index n _AlAsSb of AlAsSb film, the film thickness d _AlAsSb, refractive index n _TlGaAs of TlGaAs film, the film thickness and d _TlGaAs, when the T absolute temperature, temperature coefficient of the refractive index, _Respectively , dn _AlAsSb / dT, d
represented by _{n TlGaAs / dT, (dn AlAsSb} / dT) d
By setting the film thickness so as to satisfy the _{AlAsSb + (dn TlGaAs / dT)} d TlGaAs = 0, it is possible to completely eliminate the temperature dependence of the DBR mirror.

Next, a DBR mirror according to a second embodiment of the present invention will be described with reference to FIG. First, PMMA (polymethyl methacrylate) is applied on a glass substrate 31 by a spin coating method to form PMMA.
After forming the film 32, polycrystalline Si
The DBR mirror 34 is completed by forming the film 33 and forming such a process for 1 to 10 pairs, for example, 5 pairs. The PMMA film 32 and the polycrystalline Si film 33
Is formed to have an optical length of an integral multiple of 1/2 of the laser oscillation wavelength λ, for example, 1 × (λ / 2).

In this case, since the refractive index difference between PMMA and polycrystalline Si is about 2.0, it is possible to obtain a reflectance of 99% or more in five pairs, and PMMA has a temperature coefficient of refractive index. Is negative, so that a DBR mirror with small temperature dependence can be realized as in the first embodiment.

Next, a DBR mirror according to a third embodiment of the present invention will be described with reference to FIG. First, a PMMA film 32 is formed on a glass substrate 31 by applying PMMA by spin coating, and then a TiO ₂ film 35 is formed by a vacuum deposition method. For example, the DBR mirror 36 is completed by forming seven pairs. The PMMA film 3
2 and a film thickness of one pair of the TiO ₂ film 35 are an integral length of 1/2 of the laser oscillation wavelength λ as an optical length, for example, 1 ×
(Λ / 2).

Also in this case, since the temperature coefficient of the refractive index of PMMA is negative, it is possible to realize a DBR mirror having a small temperature dependence as in the second embodiment.

Next, a DBR mirror according to a fourth embodiment of the present invention will be described with reference to FIG. First, a Bi film 37, which is a semimetal, is formed on a glass substrate 31 by a vacuum evaporation method, and then an SiO ₂ film 38 is formed again by a vacuum evaporation method.
By forming 10 pairs, for example, 5 pairs, D
The BR mirror 39 is completed. Note that the Bi film 37 and Si
The film thickness of one pair of the O ₂ film 38 is formed as an optical length of an integral multiple of の of the laser oscillation wavelength λ, for example, 1 × (λ / 2).

Also in this case, since the temperature coefficient of the refractive index of Bi, which is a semimetal, is negative, a DBR mirror with small temperature dependence can be realized as in the first embodiment.

Next, a surface type semiconductor optical amplifier according to a fifth embodiment of the present invention will be described with reference to FIG. FIG. 8 is a schematic cross-sectional view of a surface-type semiconductor optical amplifier according to a fifth embodiment of the present invention, and selectively removes the InAs substrate 11 of the surface-emitting semiconductor laser shown in FIG. Therefore, the description of the manufacturing process will be referred to. Illustration of the electrodes is omitted. In this case, the strained quantum well active layer 1
The InAs substrate 11 is removed to prevent absorption of light of about 1.3 μm, which is the oscillation wavelength of No. 7.

In this surface type semiconductor optical amplifier, when a laser beam enters from one DBR mirror, for example, the first DBR mirror 14 side, it is amplified and emitted from the second DBR mirror 23 side.

Also in this case, since the temperature dependence of the reflection characteristics of the first DBR mirror 14 and the second DBR mirror 23 is small, the optical amplification characteristics are not affected by the temperature fluctuation.
It is possible to realize stable optical amplification characteristics.

Next, a surface optical filter according to a sixth embodiment of the present invention will be described with reference to FIG. FIG. 9A is a schematic cross-sectional view of a surface optical filter according to a sixth embodiment of the present invention. First, as in the above-described fourth embodiment, glass is used. A Bi film 41 and a SiO ₂ film 42 using a vacuum deposition method are alternately formed on a substrate 41 by, for example, 5
The first DBR mirror 44 is formed by forming a pair. Also in this case, the film thickness of one pair of the Bi film 41 and the SiO ₂ film 42 is formed as an optical length to be an integral multiple of 1/2 of the wavelength λ of the transmitted light, for example, 1 × (λ / 2).

[0043] Then, an integral multiple of the thickness d _f through the InGaAsP layer on an InP substrate (both not shown) wavelength of the transmitted light lambda, for example, after forming the InP layer 45 of 1 × lambda, the As in the case of the first embodiment, both wafers are bonded to each other by performing heat treatment in a state where the InP film 45 and the first DBR mirror 44 face each other and pressurized.

Next, after removing the InP substrate and the InGaAsP layer, the SiO ₂ layer was formed again using the vacuum evaporation method.
The basic structure of the optical filter is completed by forming the second DBR mirror 48 by forming five pairs of films 46 and Bi films 47 alternately.

FIG. 9B is a diagram schematically showing the reflectance characteristics of the surface optical filter shown in FIG. 9A, and the thickness of the InP film 45 is shown. Only light having a wavelength corresponding to the wavelength can be selectively transmitted.

The embodiments of the present invention have been described above. However, the present invention is not limited to the configuration described in each embodiment, and various modifications are possible. For example, in the above-described first embodiment, the MBE method is used as the method for forming the DBR mirror. However, the method is not limited to the MBE method, and the MOVPE method may be used.

In the first embodiment, AlAsSb and Tl lattice-matched to the InAs substrate are used.
Although GaAs is used, as is apparent from the figure, Ga
AlAs lattice matched to GaSb substrate using Sb substrate
Sb and TlGaAs may be used.

In the first embodiment, TlGaAs is used as a material having a negative temperature coefficient of refractive index. However, the material is not limited to TlGaAs, and an InP substrate is used and lattice matching is performed with the InP substrate. TlGaP or TlGaAsP may be used. By using such a group III-V compound containing Tl, all film forming steps can be performed by an epitaxial method.

In the second to fourth embodiments, since the epitaxial growth method is not used,
Although an inexpensive glass substrate is used as the substrate, the substrate is not limited to the glass substrate, and any substrate may be used. However, depending on the application, it is desirable to use a substrate that is transparent to the wavelength of the target light.

In the second to fourth embodiments, the polycrystalline S
Although i, TiO ₂ , and SiO ₂ are used, and the temperature coefficient of the refractive index is a film having a negative temperature coefficient, three combinations are shown using PMMA and Bi. However, the present invention is not limited to such three combinations. Since the temperature coefficients of the refractive indexes of ordinary semiconductors and dielectrics are positive, and the temperature coefficients of the refractive index of ordinary semimetals are negative, various semiconductors or dielectrics and polymers or semi-conductors such as PMMA are used. Any combination with metal may be used.

In the second to fourth embodiments, a simple DBR mirror has been described.
Actually, it is used as the surface emitting semiconductor laser of the first embodiment, the surface semiconductor optical amplifier of the fifth embodiment, or the DBR mirror of the surface optical filter of the sixth embodiment. When a transparent glass substrate is used as in the second to fourth embodiments, it is not always necessary to remove the substrate.

In the sixth embodiment, a glass substrate is used, but an InP substrate is used.
AlAsSb and T lattice matching DBR mirror to InP
It may be made of lGaP and formed entirely by an epitaxial growth method.

[0053]

According to the present invention, the DBR mirror is constructed by alternately stacking thin films having a positive temperature coefficient of refractive index and thin films having a negative temperature coefficient of refractive index.
It is possible to reduce the temperature dependence of the reflection characteristics of the mirror,
As a result, a surface-emitting semiconductor laser and a semiconductor optical amplifier that oscillate stably in a wide temperature range can be realized, which greatly contributes to a higher density or lower power consumption of an optical communication system.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is an explanatory diagram of a manufacturing process partway through the first embodiment of the present invention.

FIG. 3 is an explanatory view of a manufacturing process of the first embodiment of the present invention after FIG. 2;

FIG. 4 is an explanatory diagram of a correlation between a forbidden band width and a lattice constant of AlAsSb and TlGaAs.

FIG. 5 is an explanatory diagram of a DBR mirror according to a second embodiment of the present invention.

FIG. 6 is an explanatory diagram of a DBR mirror according to a third embodiment of the present invention.

FIG. 7 is an explanatory diagram of a DBR mirror according to a fourth embodiment of the present invention.

FIG. 8 is a schematic sectional view of a surface type semiconductor optical amplifier according to a fifth embodiment of the present invention.

FIG. 9 is an explanatory diagram of a surface optical filter according to a sixth embodiment of the present invention.

FIG. 10 is an explanatory diagram of a manufacturing process of a conventional surface emitting semiconductor laser.

FIG. 11 is an explanatory diagram of the temperature dependence of the reflectance of a conventional DBR mirror.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Substrate 2 Film with positive refractive index temperature coefficient 3 Film with negative refractive index temperature coefficient 11 InAs substrate 12 AlAsSb film 13 TlGaAs film 14 First DBR mirror 15 InGaAs substrate 16 n-type InAlGaAs cladding layer 17 Strain quantum well active layer Reference Signs List 18 p-type InAlGaAs cladding layer 19 i-type InAlGaAs barrier layer 20 i-type InGaAs strain well layer 21 TlGaAs film 22 AlAsSb film 23 second DBR mirror 31 glass substrate 32 PMMA film 33 polycrystalline Si film 34 DBR mirror 35 TiO ₂ film 36 DBR mirror 37 Bi film 38 SiO ₂ film 39 DBR mirror 41 glass substrate 42 Bi film 43 SiO ₂ film 44 first 1DBR mirror 45 InP layer 46 SiO ₂ layer 47 Bi film 48 first 2DBR mirror 51 n-type GaAs substrate 52 n AlAs λ / 4 film 53 n-type GaAs λ / 4 film 54 first semiconductor DBR mirror 61 InGaAs substrate 62 p-type InAlAs λ / 4 film 63 p-type InGaAs λ / 4 film 64 second semiconductor DBR mirror 65 p-type InAlGaAs cladding layer 66 strain quantum well Active layer 67 n-type InAlGaAs cladding layer

Claims (4)

[Claims] 1. A film having a positive temperature coefficient of refractive index and a film having a negative temperature coefficient of refractive index are alternately laminated at least one pair or more, and the optical length of said one pair is set to an integral multiple of half the center wavelength. Temperature-independent distributed Bragg reflection mirror, characterized by: 2. The film having a positive temperature coefficient of refractive index is either a semiconductor or a dielectric, and the film having a negative temperature coefficient of refractive index is one of a metalloid and a polymer. The temperature-independent distributed Bragg reflection mirror according to claim 1, wherein: 3. The temperature-independent distributed Bragg reflection mirror according to claim 2, wherein said semimetal is a III-V compound containing Tl. 4. A surface optical element comprising the temperature-independent distributed Bragg reflection type mirror according to claim 1. Description: