JP3700245B2 - Phase-shifted distributed feedback semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a distributed feedback semiconductor laser which is a kind of semiconductor laser element, and more particularly to a distributed feedback semiconductor laser suitable for use in an optical communication module, an optical communication system, and an optical network.
[0002]
[Prior art]
At present, there is an urgent need to opticalize not only trunk lines but also subscriber networks (introduction of so-called optical communication systems using light as a communication medium). Distributed feedback semiconductor lasers capable of realizing stable single longitudinal mode operation even during high-speed modulation have been put into practical use as basic transmission devices for such optical communication systems. A semiconductor laser device in which a diffraction grating (grating) is formed along an active layer called a distributed feedback type (or DFB type) is described in, for example, Japanese Patent Laid-Open No. 63-122188. Among the distributed feedback semiconductor lasers, the λ / 4 phase shift distributed feedback semiconductor laser having a phase shift type diffraction grating is particularly preferable for realizing a stable single longitudinal mode operation with good reproducibility. This mainstream structure of the transmitting device is expected.
[0003]
A normal distributed feedback laser has a diffraction pattern consisting of periodic irregularities formed along the cavity length direction for oscillating laser light (the direction extending from the laser light emitting end face to the other end face facing it). A grid is provided. In contrast, the diffraction grating of a λ / 4 phase shift distributed feedback semiconductor laser forms a phase shift region that generates a phase shift of a quarter wavelength (λ / 4) in the center of the laser resonator. It has a structure called a λ / 4 phase shift type diffraction grating. Its features are repeated from both ends of the resonator with a period of L (distance between the tops of adjacent convex parts or the bottom of the concave part) and an amplitude of H (height difference between the top of the convex part and the bottom of the concave part). The regular pattern of irregularities formed is a convex part having a height of about H / 2, a width L / 2 formed between the concave parts at the approximate center of the resonator length, or a depth formed between the convex parts. It is the structure cut | disconnected by the recessed part of about width H / 2 and width L / 2. Laser oscillation is performed by shifting the phase of light of wavelength λ generated from the active layer by λ / 4 by the intermittent portions of the uneven pattern.
[0004]
For example, Applied Physics Letter, Vol. 55, No. 5 (July 1989), pp. 415-417 (Applied Physics Letters, Vol. 55, No. 5, July). 1989, pages 415-417).
[0005]
[Problems to be solved by the invention]
In fabricating the above-mentioned λ / 4 phase shift type distributed feedback semiconductor laser, it is required to accurately form the width of the phase shift region (intermittent portion of the concavo-convex pattern) of the diffraction grating. However, the basic technology for mass production of λ / 4 phase shift type diffraction gratings with high yield has not been established yet. Therefore, at present, the λ / 4 phase shift distributed feedback semiconductor laser has not been widely put into practical use.
[0006]
The present invention provides an element structure of a phase-shifted distributed feedback semiconductor laser that can be fabricated with a simpler fabrication method and better reproducibility (with higher yield) than conventional phase-shifted distributed feedback semiconductor lasers, and a fabrication method therefor The purpose is to do. Another object of the present invention is to provide an optical device such as an optical module capable of operating at high cost at a low cost.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have provided at least one region in which no diffraction grating is formed in the laser resonator, and the propagation constant of the light wave in this region is a region in which the diffraction grating is formed. A structure (hereinafter referred to as a partial diffraction grating structure) set to a value different from the above is adopted, and the sum Ls of the difference Δβ between the propagation constants of these two regions and the length of the region not formed by the diffraction grating (length in the resonator direction) The semiconductor laser device was configured so as to satisfy the relationship expressed by Equation 1.
[0008]
0.4π <Δβ · Ls <0.6π (Formula 1)
By configuring the semiconductor laser element in this way, a distributed feedback semiconductor laser having an equivalent λ / 4 phase shift type diffraction grating was realized by a simpler method than before. In short, the semiconductor laser device of the present invention includes a first semiconductor layer called an active layer, and a second semiconductor layer (for example, a light guide layer, which has a refractive index smaller than that of the active layer bonded to the active layer). A resonator for laser oscillation is formed with a cladding layer, a buffer layer, or a combination of these layers), and a diffraction grating is formed in the resonator length direction at the position where the light generated in the first semiconductor layer reaches. Since it suffices to form it intermittently, it is possible to avoid the strict dimensioning operation of the intermittent portion of the concavo-convex pattern as in the prior art. Of course, in intermittent formation of the diffraction grating (formation of the partial diffraction grating structure), (1) a diffraction grating is formed on both end faces of the resonator, and (2) a gap region that separates the diffraction gratings (diffraction grating non-diffraction). The formation region) may be one place or a plurality of places, but it is necessary to consider two points that the length in the resonator length direction (the sum of the lengths if there are a plurality of places) satisfies the relationship of the above-mentioned formula 1. There is. When the partial diffraction grating structure is formed in either the buffer layer or the cladding layer, the controllability of the phase shift amount is further improved.
[0009]
An example of the design of the semiconductor laser device of the present invention will be described with reference to a sectional view of the semiconductor laser device in the cavity length direction of FIG. As shown in the figure, a layered structure (hereinafter referred to as a diffraction grating supply layer) having a partially formed buried diffraction grating 13 is realized in a distributed feedback semiconductor laser having a wavelength of 1.3 μm. The origin of the “diffraction grating supply layer” is that a “semiconductor layer” having a different composition from that of the cladding layer is formed on the principal surface of the cladding layer as will be described later in Example 1, and an uneven pattern is formed on this semiconductor layer. It is in process characteristics. Of the resonator length of length Lc, the propagation constant of the light wave is slightly different between the region 18 where the embedded diffraction grating exists and the region 19 where the embedded diffraction grating does not exist. Therefore, in the element structure of FIG. 1, the product Δβ · Ls of the propagation constant difference Δβ and the length Ls of the region 19 where the diffraction grating does not exist is a phase shift amount with respect to the light wave. By adjusting Ls so that this value becomes π / 2, an equivalent λ / 4 shift type diffraction grating can be obtained. The normalized optical coupling coefficient is given by κ (Lc−Ls) using the optical coupling coefficient κ. Incidentally, kappa is the reflectivity of the unit length of the specific diffraction grating (here the resonator length direction) (unit: m ~ ¹⁾ shows a.
[0010]
2 shows the Ls value and the normalized optical coupling coefficient κ (Lc−Ls) value for obtaining the phase shift amount π / 2 with respect to the composition wavelength of the InGaAsP quaternary of the diffraction grating supply layer 13 in the case of FIG. It is the result of calculation. The composition wavelength is a value determined by the composition ratio of the compound semiconductor composed of four elements of In, Ga, As, and P, and the wavelength value is a physical quantity corresponding to the forbidden band width. It can be seen that the phase shift amount π / 2 can be obtained when Ls is about 60 to 100 μm. Since this length is as small as 1/7 to 1/4 of the cavity length of a normal laser of 400 μm, the normalized optical coupling coefficient κ (Lc−Ls) is not significantly reduced and is 2.5 or more and 6 It can be seen that the following can be realized. On the other hand, the phase shift amount changes substantially in proportion to the duty ratio of the diffraction grating. Normally, the duty ratio can be set in the range of 0.4 to 0.6, so the phase shift amount is distributed in the range of about 0.4π to 0.6π. However, within this range, there is no adverse effect on the single mode characteristics of the distributed feedback laser. I know it.
[0011]
By the way, the propagation constant β shown in the above Equation 1 is obtained as Equation 2 by the effective refractive index n _eff of the medium and the wavelength λ of the light guided through the medium.
[0012]
β = 2πn _eff / λ (Formula 2)
From Equation 2, it can be seen that the propagation constant β in the semiconductor laser device of the present invention depends on the semiconductor material constituting the propagation constant β. For example, the wavelength λ is determined by the material of the active layer (well layer in the case of a multiple quantum well structure) in the first semiconductor layer. Incidentally, the speed v of light propagating through a medium having an effective refractive index n _eff is determined by n _eff (c is the speed of light: 2.99792458 × 10 ⁸ m / s) as shown in Equation 3.
[0013]
v = c / n _eff (Equation 3)
In the semiconductor laser device of the present invention, it is clear from Equation 3 that the phase of laser oscillation is modulated by modulating the speed of light between the diffraction grating forming region and the diffraction grating non-forming region having different propagation constants.
[0014]
In the element configuration using the embedded diffraction grating 13 shown in FIG. 1, the propagation constant difference Δβ between the diffraction grating formation region and the diffraction grating non-formation region depends on the thickness and refractive index (depending on the composition) of the diffraction grating supply layer 13. ). Currently, methods with extremely high controllability of film thickness and composition in crystal growth, such as metal organic vapor phase epitaxy and molecular beam crystal growth, have been established, and these can be applied to the fabrication of the semiconductor laser device of the present invention. A phase shift type distributed feedback semiconductor laser having a very high controllability of the phase shift amount can be realized.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described more specifically with reference to Examples 1 to 3 showing the embodiment of the present invention and FIGS. 3 to 5 related thereto.
[0016]
<Example 1>
FIG. 3 shows an example in which a distributed feedback semiconductor laser having a wavelength of 1.3 μm is manufactured using the present invention. As shown in FIG. 3 (A), an n-type InP buffer layer 1.0 μm 102 and an n-type InGaAsP lower guide layer (composition wavelength 1.10 μm) 0.05 on an n-type (100) InP semiconductor substrate 101 by metal organic vapor phase epitaxy. Multi-quantum well layer with μm and 5 periods (6.0 nm thick 1% compressive strain InGaAsP (composition wavelength 1.37 μm well layer, 10 nm thick InGaAsP (composition wavelength 1.10 μm) barrier layer) and InGaAsP (composition wavelength 1.10 μm) upper side An active layer 103 composed of an optical guide layer 0.05 μm, a first p-type InP cladding layer 0.1 μm 104, an undoped InGaAsP (composition wavelength 1.15 μm) diffraction grating supply layer 0.05 μm 105, and a p-type InP cap layer 0.01 μm 106 are sequentially formed. The emission wavelength of the multiple quantum well active layer 103 is about 1.31 μm.
[0017]
Next, parts of the cap layer 106 and the diffraction grating supply layer 105 (substantially the center of the resonator length) are removed by photolithography and selective wet etching as shown in FIG. Here, the stripe direction formed by etching is [01-1], and the width of the removed region is 80 μm. Subsequently, as shown in FIG. 3C, a diffraction grating having a uniform period of 241 nm is formed over the entire upper surface of the semiconductor layer stacked on the substrate by using an interference exposure method and wet etching. The depth of the diffraction grating is about 80 nm so that the diffraction grating penetrates the diffraction grating supply layer 105 and reaches the first p-type InP cladding layer 104.
[0018]
Subsequently, a second p-type InP cladding layer 1.7 μm 108 and a high-concentration p-type InGaAs cap layer 0.2 μm 109 are sequentially formed by metal organic vapor phase epitaxy. In this step, the recess formed in the InP clad layer 104 is filled by newly regrowing the InP clad layer 108 on the upper surface, and the trace of the regrowth interface almost disappears. Therefore, the InP cladding layers 104 and 108 are formed substantially integrally, and there is almost no influence on the optical waveguide in the cladding layer by the regrowth interface.
[0019]
As a result, as shown in FIG. 3D, a distributed feedback laser structure is formed in which the diffraction grating is embedded in the cladding layer and the diffraction grating is not formed over the resonator central portion of 80 μm. After processing into an embedded laser structure having a lateral width of about 1.5 μm or a ridge waveguide laser structure having a lateral width of about 2.2 μm, an upper electrode 110 and a lower electrode 111 are formed. As shown in FIG. 3E, after cutting into an element having an element length of 400 μm by a cleavage process, low reflection films 112 having a reflectance of about 1% are formed on both end faces of the element by, for example, a sputtering method.
[0020]
The manufactured 1.3 μm band distributed feedback semiconductor laser element had a threshold current of 10 mA and an oscillation efficiency of 0.45 W / A at room temperature and in continuous conditions. Reflecting simple fabrication, good oscillation characteristics were obtained with a threshold current of 25 mA and an oscillation efficiency of 0.30 W / A even at a high temperature of 85 ° C. The spectrum shape when forward bias is applied below the oscillation threshold is shown in FIG. Reflecting the λ / 4 shift type diffraction grating, a typical spectrum in which an oscillation main mode appears in the center of the stop band was obtained. As a result, a stable single mode operation with a sub-mode suppression ratio of 40 dB or higher was achieved at a high manufacturing yield of 95% or higher even at a high temperature of 85 ° C. This structure is applicable not only to the 1.3 μm band but also to the distributed feedback semiconductor laser in the 1.55 μm band and other wavelength bands.
[0021]
<Example 2>
FIG. 4 is an example (cross-sectional view in the cavity length direction) of a 1.3 μm band distributed feedback semiconductor laser having a plurality of phase shift regions according to the present invention. The structure is the same as that of Example 1 except that the region where the diffraction grating is formed is different. As shown in FIG. 4, the diffraction grating supply layer is removed at two locations inside the laser resonator, and the phase shift amount in each region is about π / 2. In the manufactured element, in addition to the characteristics of Example 1, the light intensity distribution in the resonator is flattened compared to the case of Embodiment 1, so that it is excellent in single mode operation even at high output operation of 40 mW or more. A distributed feedback semiconductor laser was realized. This structure is applicable not only to the 1.3 μm band but also to the distributed feedback semiconductor laser in the 1.55 μm band and other wavelength bands.
[0022]
<Example 3>
In this embodiment, an example in which the distributed feedback semiconductor laser described in Embodiment 1 or 2 is applied to an optical device will be described. FIG. 5 is a structural diagram of a module in which a distributed feedback semiconductor laser 201 is mounted on a heat sink 202, and then an optical lens 203, a rear end face light output monitoring photodiode 204, and an optical fiber 205 are integrated. The threshold current was 10 mA and the oscillation efficiency was 0.20 W / A under continuous conditions at room temperature. Reflecting simple fabrication, good oscillation characteristics were obtained with a threshold current of 25 mA and an oscillation efficiency of 0.13 W / A even at a high temperature of 85 ° C. In addition, a stable single mode operation with a submode suppression ratio of 40 dB or higher was achieved at a high manufacturing yield of 95% or higher even at a high temperature of 85 ° C. In this laser, since the normalized optical coupling coefficient can be set as high as 2.5 or more, the oscillation characteristics are not degraded at all by the return light from the fiber end, which is the biggest problem in module mounting.
[0023]
【The invention's effect】
According to the semiconductor light emitting device of the present invention, a stable single-mode high output distributed feedback semiconductor laser and an optical module equipped with the same can be realized by an extremely easy method. By using the present invention, not only the device performance and the yield are dramatically improved, but also the price reduction, the capacity increase, and the distance increase of the optical communication system to which this device is applied can be easily realized.
[Brief description of the drawings]
FIG. 1 is a sectional view in a cavity length direction for explaining a semiconductor laser device according to the present invention.
FIG. 2 is a diagram showing a relationship between a composition wavelength, a phase shift region length, and a normalized optical coupling coefficient of a diffraction grating supply layer (semiconductor material forming a diffraction grating).
FIGS. 3A and 3B are diagrams for explaining a first embodiment of the present invention, in which FIGS. 3A to 3E are flow charts showing a manufacturing process of a semiconductor laser device, and FIG. 3F is a diagram of oscillation light of the manufactured laser device; FIGS. It is a figure which shows a spectrum.
FIG. 4 is a cross-sectional view of the semiconductor laser device according to the second embodiment of the present invention in the cavity length direction.
FIG. 5 is a cross-sectional view of an optical module according to Embodiment 3 of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... n-type (100) InP semiconductor substrate, 12 ... Active layer, 13 ... Diffraction grating supply layer, 14 ... Cladding layer, 15 ... Low reflection film, 16 ... Upper electrode, 17 ... Lower electrode, 101 ... N-type (100 ) InP semiconductor substrate, 102... Buffer layer, 103... Compression-strained multiple quantum well active layer, 104... First cladding layer, 105 ... diffraction grating supply layer, 106 ... cap layer, 107 ... diffraction grating, 108 ... second cladding layer , 109 ... cap layer, 110 ... upper electrode, 111 ... lower electrode, 112 ... low reflection film, 201 ... distributed feedback semiconductor laser, 202 ... heat sink, 203 ... optical lens, 204 ... monitor photodiode, 205 ... optical fiber.

Claims (5)

A first conductive type buffer layer on a first conductive type semiconductor substrate; a diffraction grating along a traveling direction of light for distributing and reflecting light emitted to the light emitting layer and a layer near the light emitting layer; And having at least one region where the diffraction grating is not formed in the laser resonator, the propagation constant of the light wave in this region is determined as the region where the diffraction grating is formed. In a distributed feedback semiconductor laser having a partial diffraction grating structure set to a different value, between the difference Δβ between the propagation constants of the two regions and the sum Ls of the lengths of the regions where the diffraction grating is not formed,
0.4π <Δβ · Ls <0.6π
Relationship there is,
The diffraction grating is provided in either one of the buffer layer or the cladding layer, and each of the gratings constituting the diffraction grating has a constant thickness and is provided intermittently. A distributed feedback semiconductor laser characterized in that a space between the one grating and another adjacent grating is filled with a material constituting the buffer layer or the cladding layer .   2. The distributed feedback semiconductor laser according to claim 1, wherein a conductivity type of the light guide layer for forming the diffraction grating is different from that of the surrounding layers. Normalized optical coupling coefficient κ (Lc-Ls) of the diffraction grating (where κ is the optical coupling coefficient, and the reflectance (unit: m ⁻ ) of the unit length of the diffraction grating (here, the cavity length direction)) ¹ ), Lc; the length of the supply, Ls; the length of the portion where the phase shift amount is greater than 0.4π and less than 0.6π and no diffraction grating is provided) is 2 3. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser is 6 or less. A first conductive type buffer layer on a first conductive type semiconductor substrate; a diffraction grating along a traveling direction of light for distributing and reflecting light emitted to the light emitting layer and a layer near the light emitting layer; And having at least one region where the diffraction grating is not formed in the laser resonator, the propagation constant of the light wave in this region is determined as the region where the diffraction grating is formed. In a distributed feedback semiconductor laser having a partial diffraction grating structure set to a different value, between the difference Δβ between the propagation constants of the two regions and the sum Ls of the lengths of the regions where the diffraction grating is not formed,
0.4π <Δβ · Ls <0.6π
There is a relationship
The diffraction grating is provided in either one of the buffer layer or the cladding layer, and each of the gratings constituting the diffraction grating has a constant thickness and is provided intermittently. The space between the one lattice and another adjacent lattice is filled with the material constituting the buffer layer or the cladding layer,
Normalized optical coupling coefficient κ (Lc-Ls) of the above periodic diffraction grating (where κ is the optical coupling coefficient, and the reflectance of the diffraction grating unit length (here, the cavity length direction) (unit: m ⁻¹ ), Lc: length of the supply, Ls: length of a portion where the phase shift amount is larger than 0.4π and less than 0.6π and no diffraction grating is provided) Is a distributed feedback type semiconductor laser characterized by being 2 or more and 6 or less.   5. The distributed feedback semiconductor laser according to claim 4, wherein a conductivity type of the light guide layer for forming the diffraction grating is different from that of the surrounding layers.

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