JP2004134486A - Semiconductor laser equipped with diffraction grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser which is capable of realizing a κ modulation profile in the direction of a laser resonator necessary for obtaining a high optical signal output as it keeps a single mode yield as high as the other semiconductor laser where a λ/4 phase shifter or a phase control region is formed at the center of a laser resonator. <P>SOLUTION: A diffraction grating 102 equipped with a λ/4 phase shifter 103 is formed on a semiconductor substrate 101, and an optical guide layer 104 is formed thereon. The composition and/or the thickness of the optical guide layer 104 is modulated in the direction of an optical waveguide. The optical guide layer 104 is maintained at a level lower than the crest of the diffraction grating 102. A modulation profile of a diffraction grating coupling coefficient κ is precisely controlled not by the height of the diffraction grating 102 but by the optical guide layer 104. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor laser having a diffraction grating, and more particularly to a semiconductor laser having a diffraction grating having a λ / 4 phase shift portion and a phase adjustment region.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent Publication No. 1-38772
[Patent Document 2]
JP 2001-36192 A
[Patent Document 3]
JP-A-2-105593
A distributed feedback semiconductor laser (DFB-LD: Distributed Feedback Back Laser Diode) in which a λ / 4 phase shift unit is arranged at the center of the laser resonator and both end faces have a non-reflection structure so as not to be affected by the phase of the diffraction grating. Can theoretically achieve a single-mode yield of 100%. FIG. 19 shows an example of such a distributed feedback semiconductor laser.
[0003]
As shown in FIG. 19, a diffraction grating 1102 for selecting an oscillation wavelength is formed on the surface of a semiconductor substrate 1101 at a predetermined period. The diffraction grating 1102 is provided with a λ / 4 phase shift unit 1103 corresponding to a quarter wavelength (a half cycle of the diffraction grating 1102) of the standing wave in the resonator of the laser light. The position of the λ / 4 phase shift unit 103 is at the center of the laser resonator. An optical guide layer 1104 is formed on the diffraction grating 1102, and an active layer 1106 having a multiple quantum well (MQW) structure and a cladding layer 1107 are sequentially formed thereon. On the cladding layer 1107, a p-electrode 1109 is formed. On the back surface of the substrate 1101, an n-electrode 1110 is formed. Both the front end face and the rear end face are covered with a non-reflection (anti-reflection, AR) film 1111.
[0004]
In the conventional distributed feedback semiconductor laser shown in FIG. 19, the forward optical output at which the signal light is output and the backward optical output at which the monitor light is output are theoretically the same. However, there is a problem that it is inferior to a distributed feedback semiconductor laser having a high reflection structure in which one end face is covered with a high reflection film (High-Reflection, HR).
[0005]
As one method for solving this problem, a λ / 4 phase shift portion is formed so as to be shifted backward from the center of the laser resonator, and the value of the diffraction grating coupling coefficient κ is determined before the λ / 4 phase shift portion. A distributed feedback semiconductor laser in which a diffraction grating having two different heights is formed in a laser resonator so as to be small and large at the rear has been proposed (see Patent Document 1: Japanese Patent Publication No. 1-37872). This configuration is shown in FIG.
[0006]
The semiconductor laser of FIG. 20 has the same configuration as that of the semiconductor laser of FIG. 19 except that the position of the λ / 4 phase shift section is different. Is omitted.
[0007]
As shown in FIG. 20, in the diffraction grating 1102A, the λ / 4 phase shift unit 1103 is slightly shifted backward from the center of the laser resonator. The height of the diffraction grating 1102A is relatively low before the λ / 4 phase shift unit 1103 and relatively high after the λ / 4 phase shift unit 1103. Therefore, the value of the diffraction grating coupling coefficient κ is small before the λ / 4 phase shift unit 1103, and large after the λ / 4 phase shift unit 1103. In other words, the part before the λ / 4 phase shift unit 1103 is the low κ region 1112, and the part after the λ / 4 phase shift unit 1103 is the high κ region 1113. As a result, as compared with the conventional semiconductor laser shown in FIG. 19, it is possible to increase the amount of signal light output in front of the laser resonator, that is, to increase the output of the signal light.
[0008]
However, in the structure of FIG. 20, since the value of κ rapidly changes before and after the phase shift unit 1103, the tolerance of the κ value for obtaining a single mode yield and high output characteristics as designed is very narrow. There's a problem. Further, since the control of the κ value is realized by changing the height of the diffraction grating 1102A, there is a problem that it is difficult to manufacture the semiconductor laser with a high yield.
[0009]
Furthermore, since the equivalent refractive index is slightly different between regions where the height of the diffraction grating 1102A is different, as a result, the single mode characteristics are extremely deteriorated, and at the time of high output, the effect of hole burning is high. As a result, the Bragg reflection condition changes, so that there is also a problem that mode jump is likely to occur.
[0010]
As another method for solving the above-described problem of the semiconductor laser of FIG. 19, the height of the diffraction grating is continuously reduced from the rear to the front of the laser resonator, and the phase shift unit is provided in the laser resonator. A distributed feedback semiconductor laser formed so as to be shifted rearward from the center has been proposed (see Patent Document 2: JP-A-2001-36192). This configuration is shown in FIG.
[0011]
The semiconductor laser of FIG. 21 has the same configuration as the semiconductor laser of FIG. 19 except that the position of the λ / 4 phase shift unit and the configuration of the diffraction grating are different. The description is omitted here.
[0012]
As shown in FIG. 21, in the diffraction grating 1102B, the λ / 4 phase shift unit 1103 is shifted backward from the center of the laser resonator. The height of the diffraction grating 1102B continuously decreases from the rear of the laser resonator to the front. As a result, as compared with the conventional semiconductor laser of FIG. 19, the single mode yield as designed and the κ value tolerance (allowable width) for obtaining high output characteristics can be increased, and the semiconductor laser can be manufactured at a high yield. It is expected that it can be produced.
[0013]
[Problems to be solved by the invention]
However, the conventional semiconductor laser shown in FIG. 21 is similar to the conventional semiconductor laser shown in FIG. 20 in that a modulation profile of a desired κ value along the resonator is realized by controlling the height of the diffraction grating. Therefore, it is difficult to precisely control the κ value despite the fact that the κ value tolerance is expanded as compared with the case of the semiconductor laser of FIG. 19, and it is still impossible to manufacture the semiconductor laser at a high yield. Can not.
[0014]
The specific method of forming the diffraction grating in the conventional semiconductor laser shown in FIG. 21 is as follows. That is, first, the diffraction grating 1102B is formed on the surface of the semiconductor substrate 1101 by etching, and then the optical guide layer 1104 is grown on the diffraction grating 1102B by MOVPE (Metal-Organic Vapor Phase Epitaxy). Let it. At this time, the semiconductor crystal forming the light guide layer 1104 grows in both the concave and convex portions of the diffraction grating 1102B. The light guide layer 1104 has irregularities reflecting the irregularities of the diffraction grating 1102B at the beginning of the growth process (while the thickness of the light guide layer 1104 is small). Go. In the growth step of the light guide layer 1104, the height of the diffraction grating 1102B is adjusted to a desired value by optimizing the standby condition. That is, the thickness of the light guide layer 1104 is set to be equal to or higher than the top of the diffraction grating 1102B. Eventually, as shown in FIG. 21, the unevenness of the diffraction grating 1102B is buried by the light guide layer 1104.
[0015]
In this method of controlling the mass transport by changing the standby condition in the growth step of the light guide layer 1104, it is difficult to precisely control the height of the diffraction grating 1102B. Therefore, the fluctuation of the height of the diffraction grating 1102B greatly changes the κ modulation profile in the cavity direction, and as a result, the oscillation characteristics and single mode yield of the semiconductor laser are greatly affected.
[0016]
The present invention has been made in view of the above-mentioned problems of the conventional semiconductor laser shown in FIG. 21, and an object of the present invention is to provide a λ / 4 phase shift unit and a phase adjustment region in a laser resonator. Provides a semiconductor laser that can realize a κ modulation profile in the laser cavity direction required for obtaining high signal light output with high accuracy while maintaining a single mode yield equivalent to that of a semiconductor laser formed in the center. Is to do.
[0017]
Another object of the present invention is to provide a semiconductor laser capable of improving reproducibility and uniformity of oscillation characteristics.
[0018]
Still other objects of the present invention not specified herein will become apparent from the following description and accompanying drawings.
[0019]
[Means for Solving the Problems]
(1) The semiconductor laser of the present invention
A semiconductor substrate;
A diffraction grating having a λ / 4 phase shift portion or a phase adjustment region formed on the semiconductor substrate;
A light guide layer formed on the semiconductor substrate adjacent to the diffraction grating,
An active layer formed adjacent to the light guide layer,
In a semiconductor laser in which an optical waveguide that propagates laser light is formed including the active layer,
At least one of the composition and the thickness of the light guide layer is modulated along the optical waveguide direction, and the light guide layer is at a level lower than the peak of the diffraction grating over the entire surface. Things.
[0020]
(2) In the semiconductor laser of the present invention, at least one of the composition and the thickness of the light guide layer is modulated along the direction of the optical waveguide, and the light guide layer extends over the entire surface from the top of the diffraction grating. Is also at a low level. For this reason, the modulation profile of the value (κ value) of the diffraction grating coupling coefficient κ is not at the height of the diffraction grating, but at a level lower than the peak of the diffraction grating, and at least one of the composition and the thickness is different. It is precisely controlled by the modulated light guide layer. Therefore, the tolerance for the modulation profile of the κ value is widened.
[0021]
As a result, the laser resonator required to obtain a high signal light output while maintaining a single mode yield equivalent to that of the semiconductor laser in which the λ / 4 phase shift portion and the phase adjustment region are formed in the center of the laser resonator. The κ modulation profile in the direction can be realized with high accuracy. Further, since the κ modulation profile can be realized with high accuracy, the reproducibility and uniformity of the oscillation characteristics can be improved.
[0022]
(3) In Patent Document 3 (Japanese Patent Laid-Open No. 2-105593), in a semiconductor laser having a diffraction grating, an InGaAsP layer having a thickness smaller than the height of the peak of the diffraction grating is provided on the diffraction grating. In addition, there is disclosed a technique in which a film is provided on the InGaAsP layer with a thickness equal to or higher than the height of the peak of the diffraction grating. However, when this technique is used to form an active layer on a diffraction grating, the technique is intended to suppress the influence of the diffraction grating and improve the crystallinity of the active layer. .
[0023]
(4) In a preferred example of the semiconductor laser of the present invention, the thickness of the light guide layer at the peak of the diffraction grating is equal to or less than half the thickness at the trough of the diffraction grating.
[0024]
In another preferable example of the semiconductor laser of the present invention, the composition wavelength of the light guide layer is shorter than the oscillation wavelength of the semiconductor laser, and the distributed feedback operation by the diffraction grating is mainly realized by the periodic change of the refractive index. .
[0025]
In still another preferred example of the semiconductor laser of the present invention, the thickness of the semiconductor modified layer at the regrowth interface of the diffraction grating is set to 20% or less of the volume of the light guide layer.
[0026]
In still another preferred embodiment of the semiconductor laser of the present invention, the phase shift portion is located at a position behind the center of the semiconductor laser along the optical waveguide, and the diffraction is performed from a rear end to a front end of the semiconductor laser. The coupling coefficient (κ value) of the lattice is continuously reduced.
[0027]
In still another preferred example of the semiconductor laser of the present invention, a region where the κ value changes in the optical waveguide direction (κ value modulation region) is in a range of 1/10 or more and 以下 or less of the entire length of the optical waveguide. It is formed.
[0028]
In still another preferred embodiment of the semiconductor laser of the present invention, a current confinement structure having a pnpn thyristor structure is formed on both sides of the active layer.
[0029]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[0030]
(1st Embodiment)
FIG. 1 is a schematic longitudinal sectional view along a resonator (optical waveguide) showing a configuration of a distributed feedback semiconductor laser according to a first embodiment of the present invention. The operating wavelength (oscillation wavelength) of this distributed feedback semiconductor laser is preferably set in the range of 0.3 to 1.7 μm.
[0031]
As shown in FIG. 1, a diffraction grating 102 for selecting an oscillation wavelength is formed at a period of about 0.243 μm on a (001) plane of a semiconductor substrate 101 made of n-type InP. The height of the diffraction grating 102 is constant from the rear end face to the front end face of the laser resonator. In the diffraction grating 102, a λ / 4 phase shift unit 103 is formed in the middle thereof, and the λ / 4 phase shift unit 103 causes a quarter wavelength (λ / 4) of a standing wave in the resonator of the laser light. (Ie, a half cycle of the diffraction grating 102) is given a phase shift. Therefore, the phase of the diffraction grating 102 is shifted by (λ / 4) on both sides of the λ / 4 phase shift unit 103.
[0032]
The total length of the resonator (optical waveguide) of the semiconductor laser is 600 μm. The position of the λ / 4 phase shift unit 103 is set such that the side of the optical output from which the signal light of the semiconductor laser is emitted (the left side in FIG. 1) is forward, and the side of the optical output from which the monitor light is output is the small side (in FIG. The right side is defined as the rear side, and with reference to the front end face (the left side end face in FIG. 1), it is formed at a position of about 360 μm toward the rear side. Therefore, the λ / 4 phase shift unit 103 is located behind the center of the resonator.
[0033]
On the n-type InP substrate 101 on which the diffraction grating 102 is formed, a light guide layer 104 made of n-type InGaAsP is formed so as to overlap the diffraction grating 102. The composition of the light guide layer 104 is set such that the bandgap wavelength (wavelength composition) gradually decreases from 1.13 μm to 1.03 μm from the rear end to the front end of the laser. The thickness of the light guide layer 104 is set so as to gradually decrease from about 30 nm to about 10 nm from the rear end to the front end of the laser. In this way, the refractive index fluctuation along the optical waveguide is mainly realized by the light guide layer 404 existing only in the valley of the diffraction grating 102.
[0034]
An n-type InP buffer layer (thickness: about 0.1 μm, impurity concentration: 1 × 10 4) is formed on the light guide layer 104 made of n-type InGaAsP and the diffraction grating 102 exposed therefrom. ¹⁸ cm ^-3 ) 105 is formed. On the buffer layer 105, a separate confinement heterostructure (SCH) layer (wavelength composition: 1.1 μm, thickness: about 50 nm) made of non-doped InGaAsP, and an MQW structure (well layer) made of non-doped InGaAsP : Wavelength composition 1.5 μm, thickness about 5 nm, barrier layer: wavelength composition 1.2 μm, thickness about 10 nm). On the active layer 106, a p-type InP cladding layer (thickness: about 0.1 μm, impurity concentration: 1 × 10 ¹⁸ cm ^-3 ) 107 are formed. The n-type InP buffer layer 105, the active layer 106 made of non-doped InGaAsP, and the p-type InP cladding layer 107 form a striped optical waveguide extending in the front-rear direction of the semiconductor laser.
[0035]
With the above configuration, the distribution of the value of the coupling coefficient κ of the diffraction grating 102 is as shown in FIG. That is, κ is 10 cm at the front end face (distance = 0) of the semiconductor laser. ^-1 Degree, 100 cm at the rear end face (distance = 600 μm) ^-1 And continuously increasing from the front end face to the rear end face.
[0036]
As clearly shown in FIG. 10, the semiconductor laser of the first embodiment has a stripe-shaped ridge structure, and the buffer layer 105, the active layer 106, and a part of the cladding layer 107 are inside the ridge structure. positioned. On both sides of the ridge structure, a current block layer 121 made of n-type InP (thickness: about 0.7 μm) and a current block layer 122 made of p-type InP (thickness: about 0.7 μm) are laminated in this order. ing. The remaining portion (thickness: about 3 μm) of the cladding layer 107 extends not only to the upper part of the ridge structure but also to the upper part of the current blocking layer 122.
[0037]
On the cladding layer 107, a cap layer 108 (thickness: about 0.3 μm) made of p-type InGaAs is formed. On the cap layer 108, a p-electrode 108 made of TiAu is formed. On the back surface of the substrate 101, an n-electrode 109 also made of TiAu is formed.
[0038]
Both the front end face and the rear end face of the semiconductor laminated structure having the above configuration are made of TiO. ₂ And SiO ₂ And is covered with an anti-reflection (AR) film 111 having a reflectance of 0.1% or less.
[0039]
Next, a method of manufacturing the distributed feedback semiconductor laser according to the first embodiment having the above configuration will be described with reference to FIGS.
[0040]
First, as shown in FIG. 2, electron beam (EB) lithography and HBr: H are formed on a (001) plane of a semiconductor substrate 101 made of n-type InP. ₂ O ₂ : H ₂ A diffraction grating 102 for selecting an oscillation wavelength is formed with a period of about 0.243 μm by wet etching using an O-based etchant. At this time, the λ / 4 phase shift unit 103 is formed on the diffraction grating 102 at the position described above.
[0041]
Thereafter, as shown in FIG. 3, SiO 2 is placed on the n-type InP substrate 101 having the diffraction grating 102 formed on the surface. ₂ A growth inhibition mask 302 having a thickness of 100 nm is formed. This growth blocking mask 302 is formed by thermal CVD, photolithography, and etching using hydrofluoric acid. ₂ Made from. The mask 302 is composed of two trapezoidal (tapered) mask portions in which a stripe-shaped opening 304 is arranged at the center. The two mask portions are formed symmetrically with respect to the opening 304. The width of each mask portion of the mask 302, that is, the mask width 303 continuously decreases from about 50 μm to about 3 μm from the rear end face to the front end face. The width of the opening 304 between the opposing mask portions is about 2.0 μm, and is constant from the rear end face to the front end face. The opening 304 is used for selectively growing a predetermined semiconductor layer for an optical waveguide as described later.
[0042]
Subsequently, as shown in FIG. 4, an optical guide layer 104 made of n-type InGaAsP is grown on the diffraction grating 102 exposed from the opening 304 of the growth prevention mask 302 by selective MOVPE growth. At this time, due to the two trapezoidal mask portions of the mask 302, the composition of the light guide layer 104 continuously changes from the 1.13 μm wavelength composition to the 1.03 μm wavelength composition from the rear end face to the front end face. Can be changed. Further, the thickness of the light guide layer 104 is continuously changed from about 30 nm to about 10 nm from the rear end face to the front end face. The light guide layer 104 having a wavelength composition and a thickness that continuously changes in this manner can be obtained by appropriately selecting the growth conditions using the mask 302 shown in FIG.
[0043]
As is clear from FIG. 4, the thickness of the light guide layer 104 is smaller than the height of the peak of the diffraction grating 102 over its entire length. This is because, in the selective MOVPE growth process for the light guide layer 104, the thickness of the n-type InGaAsP crystal growing at the peak of the diffraction grating 102 is less than half the thickness of the n-type InGaAsP crystal growing at the valley. By optimizing the growth conditions, it can be realized. As a result, the refractive index fluctuation along the optical waveguide (laser resonator) is mainly realized by the n-type InGaAsP crystal existing in the valley of the diffraction grating 102, that is, the optical guide layer 104. Further, a distribution of the coupling coefficient κ of the diffraction grating 102 as shown in FIG. 5 is obtained.
[0044]
In the selective MOVPE growth of the n-type InGaAsP crystal on the diffraction grating 102, almost no growth occurs on the surface of the diffraction grating 102 having a high step density in the initial stage of growth, and most of the supplied raw material has a low step density. Grow on the surface. After that, as the valleys of the diffraction grating 102 are filled with the n-type InGaAsP crystal, the n-type InGaAsP crystal gradually grows on the peaks of the diffraction grating 102. Therefore, the modulation profile of the κ value in the direction along the resonator can be precisely controlled, and the reproducibility, uniformity, and single-mode yield of the oscillation characteristics can be greatly improved.
[0045]
Note that a metamorphic layer is formed at the regrowth interface of the n-type InGaAsP crystal in the diffraction grating 102, and its thickness is preferably set to 20% or less of the volume of the light guide layer 104. This is because a desired light guide layer 104 as shown in FIG. 4 can be reliably obtained. Further, both the composition and the thickness of the light guide layer 104 are changed from the rear end face to the front end face, but any one of the composition and the thickness may be changed.
[0046]
After growing the light guide layer 104 as described above, photolithography and wet etching using hydrofluoric acid are performed to remove the SiO 2 on the diffraction grating 102. ₂ The growth inhibition mask 302 manufactured by the method is shaped to obtain a growth inhibition mask 603 as shown in FIG. That is, the trapezoidal mask portion 302 of the growth blocking mask 302 is selectively etched so that the mask width 303 is uniformly 3 μm in the direction along the laser resonator. As a result, a growth inhibition mask 603 as shown in FIG. 6 is obtained, in which two stripe-shaped mask portions are arranged on both sides of the stripe-shaped opening 605 (width 2 μm). The width of each mask portion, that is, the mask width 604 is 3 μm and is constant from the front end face to the rear end face.
[0047]
After that, the SiO ₂ As shown in FIG. 7, a buffer layer of n-type InP (thickness: about 0.1 μm, impurity concentration: 1 × 10 5) is formed by selective MOVPE growth using a growth inhibition mask ¹⁸ cm ^-3 ) 105, on which an SCH layer made of non-doped InGaAsP (wavelength composition: 1.1 μm, thickness: about 50 nm) and an MQW structure made of non-doped InGaAsP (well layer: wavelength composition: 1.5 μm, thickness about 5 nm) , Barrier layer: an active layer 106 having a wavelength composition of about 1.2 μm and a thickness of about 10 nm), and a p-type InP cladding layer (thickness: about 0.1 μm, impurity concentration: 1 × 10 3) ¹⁸ cm ^-3 ).
[0048]
The n-type InP buffer layer, the non-doped InGaAsP active layer 106 and the p-type InP thus formed constitute an optical waveguide. Since the semiconductor crystal does not grow in the portion of the growth blocking mask 603 covered by the mask portion, the optical waveguide thus formed has a stripe-shaped ridge structure extending from the front end to the rear end of the substrate 101. After that, the growth inhibition mask 603 is removed using hydrofluoric acid. The state at this time is as shown in FIG.
[0049]
Subsequently, SiO 2 is deposited over the entire surface of the substrate 101 by thermal CVD. ₂ After depositing a film (not shown), the SiO ₂ The film is patterned using hydrofluoric acid to form a stripe-shaped growth inhibition mask 801 that covers only the top of the ridge structure (optical waveguide), as shown in FIG. Then, selective MOVPE growth is performed using this mask 801, and as shown in FIG. 9, a current blocking layer made of p-type InP (thickness: about 0.7 μm) is formed on a buffer layer 105 made of n-type InP. A current block layer 121 (thickness: about 0.7 μm) 122 made of n-type InP is formed in order. As a result, the current blocking layers 121 and 122 are formed on both sides of the ridge structure (optical waveguide). After that, the growth blocking mask 801 is removed. The state at this time is as shown in FIG.
[0050]
Next, as shown in FIG. 10, the remaining portion of the cladding layer 107 made of p-type InP is grown to a thickness of about 3 μm. The surface of the cladding layer 107 is flat. Further, a cap layer 108 made of p-type InGaAs is grown thereon to a thickness of about 0.3 μm.
[0051]
Subsequently, a p-electrode 108 made of TiAu is formed on the surface of the cap layer 108, and an n-electrode 109 made of TiAu is formed on the back surface of the substrate 101. Thereafter, a heat treatment for alloying the p-electrode 109 and the n-electrode 110 is performed at 430 ° C. Thus, a laser structure is completed on the semiconductor substrate 101. Actually, since a large number of laser structures are simultaneously formed on the semiconductor substrate 101 through the above-described steps, each laser structure is divided so that the cavity length becomes 600 μm. Then, TiO was added to the front and rear end faces of the obtained laser structure. ₂ / SiO ₂ By coating and forming an anti-reflection (AR) film 111 having a reflectance of 0.1% or less, the distributed feedback semiconductor laser of the first embodiment having the configuration shown in FIG. 1 is completed.
[0052]
In the distributed feedback semiconductor laser according to the first embodiment having the above configuration, both the composition and the thickness of the light guide layer 104 are modulated along the direction of the optical waveguide, and the light guide layer 104 is formed on the entire surface thereof. Over a level lower than the peak of the diffraction grating 102. For this reason, the modulation profile of the value (κ value) of the diffraction grating coupling coefficient κ is not at the height of the diffraction grating 102 but at a level lower than the peak of the diffraction grating 102 and its composition and thickness are modulated. It is precisely controlled by the light guide layer 104. Therefore, the tolerance for the modulation profile of the κ value is widened.
[0053]
As a result, the [lambda] / 4 phase shifter 1-3 maintains the single mode yield equivalent to that of the semiconductor laser formed at the center of the laser resonator, while maintaining the laser resonator direction necessary for obtaining a high signal light output. Can be realized with high precision. Further, since the κ modulation profile can be realized with high accuracy, the reproducibility and uniformity of the oscillation characteristics can be improved.
[0054]
Further, in the distributed feedback semiconductor laser according to the first embodiment, a region (low κ region) 112 having a small coupling coefficient κ of the diffraction grating 102 is located forward, and a region 113 having a large coupling coefficient κ (high κ region) 113. Is behind, the electric field strength inside the semiconductor laser is large in the front and small in the rear as shown in FIG. Further, in the λ / 4 phase shift unit 103 (located at a distance of 360 μm from the front end face) introduced to improve the single mode yield, a peak of the electric field intensity occurs. That is, electric field concentration occurs in the λ / 4 phase shift unit 103. As a result, as shown in FIG. 12, the current-light output characteristics in front of the element from which the signal light is emitted are improved as compared with the conventional structure in which the value of the coupling coefficient κ is uniform in the laser resonator.
[0055]
In the first embodiment, both the wavelength composition and the thickness of the light guide layer 104 are continuously modulated between the rear end face and the front end face, but the wavelength composition or the thickness of the light guide layer 104 is modulated. Almost the same effect can be obtained by simply performing the operation.
[0056]
In the first embodiment, the region where the κ value changes in the direction along the optical waveguide (κ value modulation region) extends over the entire length of the optical waveguide, but is not limited to this. The optical waveguide may be formed so as to be limited to a range of 1/10 or more and 1/3 or less of the entire length of the optical waveguide.
[0057]
(2nd Embodiment)
In the above-described first embodiment, the optical waveguide including the n-type InP buffer layer 105, the non-doped InGaAsP active layer 106, and the p-type InP clad layer 107 is directly formed by selective MOVPE growth. The optical waveguide can be formed by wet etching or dry etching using a mask. The distributed feedback semiconductor laser according to the second embodiment is manufactured by such a method.
[0058]
A method for manufacturing the distributed feedback semiconductor laser according to the second embodiment will be described with reference to FIGS.
[0059]
The steps from the step of forming the diffraction grating 102 on the semiconductor substrate 101 to the step of forming the light guide layer 104 thereon are the same as in the first embodiment. That is, first, as shown in FIG. 2, in the same manner as in the first embodiment, the diffraction grating 102 having the λ / 4 phase shift portion 103 on the (001) plane of the semiconductor substrate 101 made of n-type InP. And then SiO 2 ₂ An n-type InGaAsP light guide layer 104 having a modulated wavelength composition and thickness is grown on the diffraction grating 102 by using a growth inhibition mask 302 made of GaAs. The thickness of the light guide layer 104 is smaller than the height of the diffraction grating 102 over its entire length.
[0060]
After that, after completely removing the growth blocking mask 302, the optical waveguide layer 130 is formed over the entire surface of the semiconductor substrate 101 by MOVPE growth. The optical waveguide layer 130 includes an n-type InP buffer layer 105 formed on the n-type InGaAsP light guide layer 104, a non-doped InGaAsP active layer 106 formed on the n-type InP buffer layer 105, and a non-doped InGaAsP active layer. And a part of the p-type InP cladding layer 107 formed on the upper surface 106. Here, the active layer 106 includes a first SCH layer (wavelength composition: 1.1 μm, thickness: about 50 nm) made of non-doped InGaAsP and an MQW structure (well layer: wavelength composition 1.5 μm, thickness 5 nm) made of non-doped InGaAsP. And a barrier layer: a wavelength composition of about 1.2 μm and a thickness of about 10 nm) and a second SCH layer made of non-doped InGaAsP (a wavelength composition of about 1.1 μm and a thickness of about 50 nm).
[0061]
Subsequently, as shown in FIG. 13, a stripe-shaped mask having a width of 2 μm is formed on the p-type InP cladding layer 107 so that the region where the diffraction grating 102 is formed together with the light guide layer 104 is completely protected. 201 is formed. Then, using the mask 201 as an etching mask, the optical waveguide layer 130 is selectively etched to form a groove 131. The groove 131 forms a striped optical waveguide (ridge structure) from the optical waveguide layer 130.
[0062]
Then, using the same mask 201 as a growth preventing mask, a current blocking layer 141 made of p-type InP and a current blocking layer 142 made of n-type InP are formed in the trench 131 by MOVPE selective growth, as shown in FIG. The layers are sequentially formed. At this time, the entire groove 131 is prevented from being buried by the current blocking layers 141 and 142.
[0063]
Further, after removing the mask 201, the remaining portion of the p-type InP cladding layer 107 is formed on the entire surface of the semiconductor substrate 101 to fill the entire groove 131, and the cladding layer 107 having a predetermined thickness is formed thereon. To be placed. On the cladding layer 107, a cap layer 108 made of p-type InGaAs is formed.
[0064]
Subsequent steps are the same as in the first embodiment. That is, a p-electrode 108 made of TiAu is formed on the surface of the cap layer 108, an n-electrode 109 made of TiAu is formed on the back surface of the substrate 101, and then heat treatment for alloying the two electrodes 109 and 110 is performed. Then, each laser structure on the substrate 101 is divided so that the cavity length becomes 600 μm, and an anti-reflection (AR) film 111 is applied and formed on both front and rear end surfaces of the obtained laser structure. A distributed feedback semiconductor laser is completed. The cross section along the resonator of the semiconductor laser of the second embodiment is the same as that shown in FIG.
[0065]
In the distributed feedback semiconductor laser of the second embodiment having the above-described configuration, the same diffraction grating 102 and light guide layer 104 as those of the first embodiment are formed on the semiconductor substrate 101. The same effect as in the case of is obtained.
[0066]
In the second embodiment, the remaining portions of the current blocking layers 141 and 142 and the cladding layer 107 are formed by MOVPE growth, but may be formed by LPE (Liquid Phase Epitaxy, liquid phase growth method). Good.
[0067]
(Third embodiment)
In the first and second embodiments described above, the diffraction grating 102 is formed below the MQW active layer 106 (on the side of the substrate 101). In the present invention, the diffraction grating is formed after growing the active layer. It is also effective in the structure. The distributed feedback semiconductor laser according to the third embodiment has such a configuration.
[0068]
A method of manufacturing the distributed feedback semiconductor laser according to the third embodiment will be described with reference to FIGS.
[0069]
First, as shown in FIG. 16, a buffer made of n-type InP is formed on the (001) plane of the n-type InP substrate 101 on which the diffraction grating and the light guide layer are not formed in the same manner as in the first embodiment. Layer (thickness: about 0.1 μm, impurity concentration: 1 × 10 ¹⁸ cm ^-3 ) 105 is formed. Thereafter, a first SCH layer (wavelength composition: 1.1 μm, thickness: about 50 nm) made of non-doped InGaAsP and an MQW structure made of non-doped InGaAsP (well layer: wavelength composition 1.5 μm, thickness) are formed on the buffer layer 105. 5 nm, barrier layer: wavelength composition: 1.2 μm, thickness: about 10 nm) and a second SCH layer made of non-doped InGaAsP (wavelength composition: 1.1 μm, thickness: about 50 nm) are laminated in this order, and MQW activity is increased. A layer 106 is formed. Further, on the MQW active layer 106, a cladding layer made of p-type InP (thickness: about 0.1 μm, impurity concentration: 1 × 10 ¹⁸ cm ^-3 ) 107 is formed. The n-type InP buffer layer 105, the non-doped InGaAsP active layer 106, and the p-type InP clad layer 107 thus formed constitute an optical waveguide layer 130. The buffer layer 105, the MQW active layer 106, and the cladding layer 107, that is, the optical waveguide layer 130 are formed over the entire surface of the substrate 101.
[0070]
Thereafter, a diffraction grating (not shown) having a depth of 30 nm is formed on the cladding layer 107 by EB exposure and etching. This diffraction grating has a λ / 4 phase shift portion, and extends in the direction perpendicular to the plane of FIG. On this diffraction grating, a growth blocking mask 202 is formed. The growth blocking mask 202 has the same configuration as the mask 302 shown in FIG.
[0071]
Subsequently, an optical guide layer is formed by selective MOVPE growth so as to overlap the diffraction grating such that the maximum thickness is 50% of the depth of the diffraction grating. The state at this time is the same as in the case of FIG. After that, the growth blocking mask 202 is removed.
[0072]
Next, the remaining portion of the p-type InP cladding layer 107 is grown to a thickness of about 3 μm on the previously formed p-type InP cladding layer 107, and a p-type InP cap layer 108 is further formed thereon to a thickness of 0 μm. It grows to about 3 μm. Both the p-type InP cladding layer 107 and the p-type InP cap layer 108 are formed on the entire surface of the substrate 101. The state at this time is as shown in FIG.
[0073]
Then, on the p-type InP cap layer 108, SiO 2 ₂ After a mask 203 having a stripe-shaped mask portion is formed, the waveguide layer 130 is selectively removed by wet etching or dry etching using the mask 203. Thus, as shown in FIG. 18, grooves 131 are formed on both sides of the stripe-shaped mask portion, thereby forming an optical waveguide having a ridge structure.
[0074]
After that, a current block layer 141 made of p-type InP, a current block layer 142 made of n-type InP, and a current block layer 143 made of p-type InP are formed in this order so as to fill the trench 131. As a result, the current blocking layers 141, 142, and 143 are formed on both sides of the ridge structure (optical waveguide). The state at this time is as shown in FIG. After that, the growth blocking mask 203 is removed.
[0075]
The p-type InP current blocking layer 141, the n-type InP current blocking layer 142, and the p-type InP current blocking layer 143 together with the n-type InP substrate 101 provide a pnpn thyristor configuration.
[0076]
The steps from the subsequent steps of forming the p-electrode 108 and the n-electrode 109 to the steps of applying and forming the anti-reflection (AR) films 111 on the front and rear end faces of each laser structure are the same as those in the second embodiment. Thus, the distributed feedback semiconductor laser according to the third embodiment is completed.
[0077]
In the distributed feedback semiconductor laser of the third embodiment having the above configuration, the same diffraction grating 102 and light guide layer 104 as in the first embodiment are formed on the n-type InP cladding layer 107. The same effect as in the first embodiment can be obtained.
[0078]
(Fourth embodiment)
In the above-described first to third embodiments, a λ / 4 phase shift unit is introduced into a part of the diffraction grating in order to improve the single mode yield. A phase adjustment region using a grating may be introduced. The distributed feedback semiconductor laser according to the fourth embodiment is obtained by replacing the λ / 4 phase shift section with this phase adjustment region, and the other configuration is the same as the distributed feedback semiconductor laser according to the first or second embodiment. It has the same configuration as a laser. Therefore, illustration and description of the configuration are omitted.
[0079]
In the phase adjustment region, the period of the diffraction grating is made shorter than the other portions, and the accumulated phase change amount, which is the sum of the phase change amounts in the diffraction grating period modulation region, is a quarter of the wavelength of the standing wave in the resonator of the laser light. 1 (λ / 4). In the semiconductor laser manufactured in this manner, the period in the phase adjustment unit is short, and the wavelength of oscillation is slightly shorter than the period in the front and rear parts of the semiconductor laser in which the period of the diffraction grating is uniform. Since the change in the refractive index is felt as the period, the amount of phase shift is slightly larger than λ / 4 with respect to the periods of the diffraction gratings at the front and rear portions of the semiconductor laser. In this semiconductor laser, since the electric field concentration in the phase adjustment region is suppressed, the semiconductor laser is stable up to a higher light output range as compared with the structure having the λ / 4 phase shift portion shown in the first embodiment or the second embodiment. There is an advantage that an excellent single mode oscillation characteristic can be obtained.
[0080]
Note that the cumulative phase shift amount in the phase adjustment region is not limited to λ / 4, but may be 3λ / 4, 5λ / 4, or the like. In addition, in order to obtain stable single-mode characteristics at a high yield, it is desirable that the phase adjustment region of the diffraction grating is 1/10 or more and 1/3 or less of the entire length of the semiconductor laser.
[0081]
(Modification)
In the first to fourth embodiments described above, the present invention is applied to a distributed feedback (DFB) type semiconductor laser, but the present invention is not limited to this. The present invention provides a distributed Bragg reflector (DBR) type semiconductor laser obtained by integrating a semiconductor laser and a Bragg waveguide on which a diffraction grating is formed, and an electric field in which a semiconductor laser and a modulator are integrated. The present invention can be applied to an optical integrated device such as an absorption modulator integrated semiconductor laser.
[0082]
In the first to fourth embodiments described above, the MQW active layer is made of an InGaAsP / InP-based material, but the present invention is not limited to this. The MQW active layer may use an AlGaAs / GaAs-based material, an AlGaInP / GaInP-based material, a GaN-based material, a ZnSe-based material, or another compound semiconductor material.
[0083]
The present invention can be applied not only to a semiconductor laser having a diffraction grating but also to an optical integrated device such as a modulator integrated light source in which the semiconductor laser is integrated.
[0084]
It is needless to say that the present invention is not limited to the above embodiments, and can be appropriately changed within the scope of the technical idea of the present invention. Further, the number, position, shape, and the like of the constituent members are not limited to the above-described embodiment, but can be set to a number, position, shape, and the like suitable for carrying out the present invention.
[0085]
【The invention's effect】
In the semiconductor laser of the present invention, the λ / 4 phase shift portion and the phase adjustment region are necessary for obtaining a high signal light output while maintaining a single mode yield equivalent to that of the semiconductor laser formed in the center of the laser resonator. A highly accurate κ modulation profile in the laser cavity direction can be realized with high accuracy. In addition, the reproducibility and uniformity of the oscillation characteristics can be improved.
[Brief description of the drawings]
FIG. 1 is a vertical longitudinal sectional view along a resonator showing a configuration of a distributed feedback semiconductor laser according to a first embodiment of the present invention.
FIG. 2 is a schematic perspective view illustrating a method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention.
FIG. 3 is a schematic perspective view showing a method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, and is a continuation of FIG. 2;
FIG. 4 is a schematic cross-sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, which is a continuation of FIG.
FIG. 5 is a diagram showing a distribution of a coupling coefficient κ in an axial direction of the distributed feedback semiconductor laser according to the first embodiment of the present invention.
FIG. 6 is a schematic sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, which is a continuation of FIG.
FIG. 7 is a schematic cross-sectional view showing the method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, which is a continuation of FIG. 6;
FIG. 8 is a schematic cross-sectional view showing the method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, and is a continuation of FIG. 7;
FIG. 9 is a schematic cross-sectional view showing a method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, and is a continuation of FIG.
FIG. 10 is a schematic sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the first embodiment of the present invention, which is a continuation of FIG. 9;
FIG. 11 is a graph showing a normalized electric field intensity distribution in a direction along a resonator of the distributed feedback semiconductor laser according to the first embodiment of the present invention.
FIG. 12 is a graph showing a relationship (current-light output characteristic) between forward light output P and injection current I of the distributed feedback semiconductor laser according to the first embodiment of the present invention.
FIG. 13 is a schematic sectional view illustrating a method for manufacturing a distributed feedback semiconductor laser according to a second embodiment of the present invention.
FIG. 14 is a schematic cross-sectional view showing a method for manufacturing the distributed feedback semiconductor laser according to the second embodiment of the present invention, and is a continuation of FIG.
FIG. 15 is a schematic sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the second embodiment of the present invention, which is a continuation of FIG.
FIG. 16 is a schematic sectional view illustrating a method for manufacturing a distributed feedback semiconductor laser according to a third embodiment of the present invention.
FIG. 17 is a schematic sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the third embodiment of the present invention, which is a continuation of FIG. 16;
FIG. 18 is a schematic sectional view illustrating the method for manufacturing the distributed feedback semiconductor laser according to the third embodiment of the present invention, which is a continuation of FIG. 17;
FIG. 19 is a vertical longitudinal sectional view taken along a resonator, showing a structure of an example of a conventional distributed feedback semiconductor laser.
FIG. 20 is a vertical longitudinal sectional view taken along a resonator, showing a structure of another example of a conventional distributed feedback semiconductor laser.
FIG. 21 is a vertical longitudinal sectional view along a resonator, showing a structure of still another example of the conventional distributed feedback semiconductor laser.
[Explanation of symbols]
101 semiconductor substrate
102 diffraction grating
103 λ / 4 phase shift unit
104 Light guide layer
105 buffer layer
106 MQW active layer
107 cladding layer
108 cap layer
109 p electrode
110 n electrode
111 Anti-reflection (AR) film
112 Low κ region
113 High κ region
121, 122 Current block layer
130 Optical waveguide layer
131 groove
141, 142, 143 Current block layer
201, 202, 203 mask
302 mask
303 Mask width
304 mask opening
603 mask
604 mask width
605 mask opening
801 mask

Claims (7)

A semiconductor substrate;
A diffraction grating having a λ / 4 phase shift portion or a phase adjustment region formed on the semiconductor substrate;
A light guide layer formed on the semiconductor substrate adjacent to the diffraction grating,
An active layer formed adjacent to the light guide layer,
In a semiconductor laser in which an optical waveguide that propagates laser light is formed including the active layer,
At least one of the composition and the thickness of the light guide layer is modulated along the optical waveguide direction, and the light guide layer is at a level lower than the peak of the diffraction grating over the entire surface. Semiconductor laser. 2. The semiconductor laser according to claim 1, wherein a thickness of the light guide layer at a peak of the diffraction grating is equal to or less than half a thickness at a valley of the diffraction grating. 3. The semiconductor laser according to claim 1, wherein a composition wavelength of the light guide layer is shorter than an oscillation wavelength of the semiconductor laser, and the distributed feedback operation by the diffraction grating is mainly realized by a periodic change in a refractive index. 4. The semiconductor laser according to claim 1, wherein a thickness of the semiconductor metamorphic layer at a regrowth interface of the diffraction grating is set to 20% or less of a volume of the light guide layer. 5. The phase shift unit is located at a position behind the center of the semiconductor laser along the optical waveguide, and a coupling coefficient (κ value) of the diffraction grating continuously decreases from a rear end to a front end of the semiconductor laser. The semiconductor laser according to claim 1, wherein the semiconductor laser is provided. The semiconductor laser according to any one of claims 1 to 5, wherein a region where a κ value changes in the optical waveguide direction is formed in a range of 1/10 or more and 1/3 or less of the entire length of the optical waveguide. . 7. The semiconductor laser according to claim 1, wherein a current confinement structure having a pnpn thyristor structure is formed on both sides of the active layer. 8.

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