JPH0552072B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a distributed feedback semiconductor laser, and more particularly to a distributed feedback semiconductor laser in which the width of a diffraction grating in a plane parallel to an active layer is varied according to a desired functional form. .

Semiconductor lasers are widely used as light sources for optical fiber communications, but the problem is how to eliminate signal deterioration due to speckle noise in the combination of semiconductor lasers and multimode optical fibers. Recent advances in mode control technology for semiconductor lasers have significantly improved the coherence of semiconductor laser light compared to conventional methods, and the oscillation spectrum has improved as shown in Figure 1 for modulation frequencies of several tens to 100 Mb/s or less. As shown, an oscillation spectrum 10 in a state close to single longitudinal mode oscillation is realized approximately at the center of the gain distribution curve 100. However, when such semiconductor laser light with good coherence is propagated through a multimode optical fiber, as a result of irregular mode conversion that occurs in the optical fiber, the light that has passed through different optical paths at the output end of the optical fiber interfere with each other, exhibiting irregular fluctuations, and also causing disturbances in the pulse waveform for pulse-modulated light. There are currently three known methods for removing such speckle noise: (1) employing high frequency superimposed modulation, (2) using a light emitting diode as a light source, and (3) employing a mode scrambler. Among these, method (1) modulates a semiconductor laser with a high frequency signal of several hundred MHz, and the oscillation spectrum 20, 21, 22, 23 with a spread corresponding to a gain distribution of 200, as shown in Figure 2, is generated. , 24, . . . and the coherence of the laser beam decreases. However, this method is troublesome in that the semiconductor laser must be constantly modulated with a high frequency signal. Method (2) uses incoherent light from a light emitting diode instead of coherent semiconductor laser light.
Although it is effective in removing modal noise, the modulation band is only a few tens of Mb/s at most, and the amount of light coupled to the optical fiber cannot be increased, so the repeating interval cannot be made very long. Since the method (3) is not directly related to the present invention, its explanation will be omitted. For details, please refer to "Optical Fiber Communication" by Soejima and Kaibuchi, published by Telecommunication Technology News Company, pp. 503-504.

It is an object of the present invention to eliminate the above-mentioned drawbacks of conventional methods for speckle noise removal due to coherence of light sources in multimode optical fiber communication systems, and to eliminate optical The object of the present invention is to provide a light source having an emitted light distribution capable of coupling light into a fiber at a level comparable to that of a conventional semiconductor laser.

Another object of the present invention is to provide a semiconductor laser having a continuous oscillation spectrum as shown in FIG. 31 within a defined spectral band.

According to the present invention, at the boundary between any two layers of the active layer, the waveguide layer, the cladding layer, and the first growth layer on the substrate side, the width is set according to a desired function shape in a plane parallel to the active layer. A diffraction grating with a corrugate-like periodic structure along the light emission direction is
A distributed feedback type or distributed Bragg reflector type double heterojunction semiconductor laser characterized in that more than one pair is formed is obtained.

Next, the principle of the present invention will be explained using drawings and mathematical formulas.

Generally, what is called a distributed feedback type (hereinafter abbreviated as DFB) or a distributed Bragg reflector type (hereinafter abbreviated as DBR) semiconductor laser consists of an active layer, a waveguide layer,
The period at the boundary between any two layers, the cladding layer and the first growth layer on the substrate side, is Λ=N・λ _p /2n _e (1) where N = integer λ _p = the period of the laser beam in vacuum. A corrugated periodic structure given by wavelength _ne = effective refractive index is formed as a diffraction grating on the entire surface or a portion thereof.
A feature of the DFB or DBR semiconductor laser is that it uses Bragg diffraction using a diffraction grating instead of a Fabry-Perot optical resonator for optical feedback inside the laser, so the oscillation wavelength is λ _p given by equation (1). It can be fixed to , and single frequency oscillation can always be obtained. For a more detailed explanation of DFB or DBR semiconductor lasers, see H.C.C. II., M.B. Punish, "Heterostructure Lasers," Academic Press, 1978, Part A, pp. 90-106. Please refer to Since DFB or DBR semiconductor lasers emit laser light with good coherence, speckle noise is more likely to occur, and they are not suitable for multimode optical fiber communications as they are.

By the way, the above-mentioned corrugated synchronous structure effectively introduces a refractive index distribution of the form n(x) = n ₀ + n ₁ cos (2π/Λx) (2) along the light emitting part of the laser. It is equivalent to
The spatial frequency spectrum distribution obtained by Fourier transforming n(x)−n ₀ ≡△n(x) given by equation (2) shows a sharp delta function-like peak exactly at the position of ω = 2π/Λ. , this reflects the fact that single frequency oscillation can be obtained. Therefore, if we make the distribution of △n(x) different from Equation (2) and select it so that its spatial frequency spectrum is continuous within a certain band, it will be continuous only in the frequency band corresponding to that band. This makes it possible to realize laser oscillation with a specific oscillation spectrum distribution. That is, if we take the origin of the x-axis at the center of the semiconductor laser, then △n(x) and its spatial frequency spectrum F(ω) are △n(x)=1/√2π∞ ∫ −∞e ^-ix 〓F( ω)dω (3) F(ω)=1/√2π∞ ∫ −∞e ^ix 〓△n(x)dx (4) The relationship is as follows. Furthermore, it can be safely assumed that there is a relationship between F(ω) and the oscillation spectrum distribution S(ν) approximately as follows: S(ν)∝F(ω=2n _e /N·ν/C) (5). So S(ν)
If a desired distribution is given as , then the distribution of n〓 to be realized can be found using equations (5) and (3). Various distributions can be selected for S(ν), but the simplest distribution suitable for our purpose is a rectangular shape that extends almost completely to the gain curve 30 of the semiconductor laser, as shown in FIG. 31. The distribution S(ν)=1 ν ₀ −Δν<ν<ν ₀ +Δν =0 ν>ν ₀ +Δν or ν<ν ₀ −Δν (6) is adopted. However, ν ₀ =C/λ ₀ . This and equation (5), and (3)
From the formula, △n〓∝〓 _0- △〓 ∫ 〓 ₀ ^- △〓e ^-ix 〓・dω ∝e ^-ix 〓 ⁰・sinx・△ω/x (7) However, ω ₀ = 2n _e /Nν ₀ / C, △ω=2n _e /N△ν/C(7
)′ becomes. In reality, we only need to consider the real part of equation (7), so as the refractive index distribution, n==n ₀ +n ₁ cos(xω ₀ )・(sin(x・△ω
)/x) (8). Specifically, as shown in Fig. 4, the amplitude of the irregularities of the corrugated periodic structure given by the function cos(xω ₀ ) is indicated by dotted lines 41 and 42.
As a result of modulation by the function sin(x·Δω)/x as shown in FIG. FIG. 5 shows an example of an InGaAsP-DFB semiconductor laser having a diffraction grating with a corrugated periodic structure in which the amplitude in the depth direction perpendicular to the active layer 50 changes in proportion to a function corresponding to the curve 40 in FIG. FIG. In Figure 5, 5
0 is an In _0.6 Ga _0.4 AS _0.86 P _0.14 active layer, 51 and 53 are p-type and n-type In _0.7 Ga _0.3 As _0.6 P _0.4 cladding layers, respectively, 54 is an n-type InP substrate, 56 is a p-type InP layer, 5
5 is a p-type In _0.6 Ga _0.4 As _0.86 P _0.14 cap layer;
A corrugated periodic structure 52 is formed at the boundary between the clad layer 51 and the p-InP layer 56, and the amplitude in the depth direction perpendicular to the active layer 50 changes in proportion to a function corresponding to the curve 40 in FIG. ing.

The semiconductor laser shown in Figure 5 would be completely suitable for our purposes if it could actually be manufactured, but in reality, it is necessary to process a corrugated structure with complex irregularities in the depth direction, such as 52. The situation is extremely difficult.

In the present invention, the above-mentioned difficulty can be solved by changing the width of the diffraction grating in a plane parallel to the active layer in proportion to the function corresponding to curves 41 and 42 in FIG. 4, as shown in FIG. This problem is solved by realizing a refractive index change as expressed by equation (8) in an average sense. In FIG. 6, 60 is a view from above of the interface between any two layers of the active layer, the waveguide layer, the cladding layer, and the first growth layer on the substrate side, and 61 and 62
indicates the range in which the laser oscillation light is distributed, 63 is a function form proportional to the function corresponding to the curves 41 and 42 in FIG. 4, and 64 is a corrugated diffraction grating formed on the boundary surface 60. Each element of the diffraction grating 64 may protrude upward from the boundary surface 60 or may be recessed in the shape of a groove.
There are no absolute criteria for determining the height or depth, and it is usually within the range of 0.01 to 1 μm. For example, if we look at the part indicated by A-A' in Fig. 6, the amplitude of the diffraction grating 64 at that part is
W' is smaller than the width W of the distribution of laser oscillation light.
If the effective refractive index amplitude corresponding to the peak (1) of the corrugation of the diffraction grating 64 is n ₁ , then the average effective refractive index amplitude in the A-A' portion is n ₁ ×W'/W. It is possible to think that. Therefore, it can be considered that the diffraction grating 64 shown in FIG. 6 achieves an effective refractive index distribution as shown in equation (8) on average. The length L of the main part of the diffraction grating 64 is given by equation (7).
From equation (7)', it is given by L=N/2n _e・C/△ν・2π = N/n _e・λ ² / ₀ /△λ・π (9). Now, n _e =3.456, N=2, λ ₀ =
If 1.3μm△λ=20Å, L=2×1.3 ² ×10 ^-8 cm×π／3.456×20×10 ^-8 cm
= 154 μm, which is a reasonable value considering the normal dimensions of semiconductor laser chips.

Next, embodiments of the present invention will be described using the drawings.

FIG. 7 is a partially cutaway view of one preferred embodiment of a semiconductor laser according to the present invention. In FIG. 7, 76 is an n-type InP substrate, 73 is an n-type InP layer, 71 and 72 are n-type and p-type, respectively.
Type In _0.7 Ga _0.3 As _0.6 P _0.4 cladding layer, 70 is In _0.6
Ga _0.4 As _0.86 P _0.14 active layer, 74 is p-type InP layer, 74
1 is p-type In _0.6 Ga _0.4 As _0.86 P _0.14 cap layer, 79 is
SiO ₂ insulating film, 78 is Cd diffusion part for ohmic contact,
77 and 791 are Au-Zu and Au-Ge respectively
-Ni electrode. 75 is the cladding layer 71 and n-type
It is formed on the boundary surface of the InP layer 73, and the width in the plane parallel to the active layer 70 is the curve 41, 42 in FIG.
It is a diffraction grating with a corrugated periodic structure that changes in proportion to a function corresponding to . A semiconductor laser having the structure shown in FIG. 7 can be manufactured by the steps shown in FIG. First, as shown in Figure 8a, the n-type
An n-type InP layer 81 is grown on an InP substrate 80 by liquid crystal epitaxial growth. Next, as shown in FIG. 8B, a SiO ₂ film 812 is applied thereon, and a photoresist film 813 is spinner coated on top of the SiO 2 film 812, followed by baking. Next, as shown in FIG. 8c, this wafer is patterned with a shape corresponding to the change in width in a plane parallel to the active layer 70 of the diffraction grating 75 in FIG. Exposure is performed using a photomask 814 that has a pattern that fills in the area surrounded by the opposing dotted lines (in the case of a positive resist) or a pattern that fills in the area other than the area (in the case of a negative resist). When the development process is performed, a wafer is obtained in which a portion of the SiO ₂ film corresponding to a pattern corresponding to a width change in a plane parallel to the active layer 70 of the diffraction grating 75 is exposed as shown in FIG. 8d. Further, this wafer is etched with a buffer etching solution to remove the exposed portion of the SiO ₂ film, and the photoresist film 813 is also removed with a stripping agent, as shown in Fig. 8e.
A wafer as shown in is obtained. Next, as shown in FIG. 8f, a photoresist film 815 is spinner-coated on this wafer, and after a baking process is performed, Ar laser light or He
-Two beams 816 and 817 using Cd laser beam
A diffraction grating pattern is holographically recorded in the photoresist film 815 by the interference. When this is developed and processed, a wafer as shown in Figure 8h is obtained,
By etching this with dilute hydrochloric acid and removing the photoresist film 815 with a stripper, a wafer with a diffraction grating 82 as shown in _FIG .
By removing the wafer, a wafer as shown in FIG. 8j is obtained. On top of this, n-type In _0.7 Ga _0.3 As _0.6 P _0.4 cladding layer 83 and In _0.6 are sequentially formed by liquid phase epitaxial growth.
Ga _0.4 As _0.86 P _0.14 Active layer 84, p-type In _0.7 Ga _0.3 As _0.6
P _0.4 cladding layer 85, p-type InP layer 86, p-type In _0.6
Ga _0.4 As _0.86 P _0.14 cap layer 861 is grown (8th
Figure k) Furthermore, after applying a SiO ₂ insulating film 862 and diffusing Cd (not shown in Figure 8), the Au-Zn electrode 87,
By attaching the Au--Ge--Ni electrode 88, a wafer as shown in FIG. 8l is obtained. By cleaving this into a size one to several times larger than L in FIG. 6, a semiconductor laser as shown in FIG. 7 can be obtained.

FIG. 9 is a partially cutaway diagram showing another embodiment of the present invention. In Figure 9, 96 is n type
InP substrate, 93 is n-type InP layer, 91 and 92 are n-type and p-type In _0.7 Ga _0.3 As _0.6 P _0.4 clad layers, 90 is In _0.6 Ga _0.4 As _0.86 P _0.14 active layer, 94 is p
941 is a p-type In _0.6 Ga _0.4 As _0.86 P _0.14 cap layer, 99 is an SiO ₂ insulating film, 991 is a Cd diffusion part for ohmic contact, and 98 and 97 are Au-
Zn and Au-Ge-Ni electrodes. The difference from the embodiment shown in FIG. 7 is that the diffraction grating 95 does not extend over the entire light emitting part of the semiconductor laser, but instead
It is located outside the light emitting part surrounded by. That is, the embodiment shown in FIG. 7 is a DFB semiconductor laser, and the embodiment shown in FIG.
The semiconductor laser shown in the figure is a DBR semiconductor laser.
The semiconductor laser shown in FIG. 9 can also be manufactured by the process shown in FIG.

Although the present invention has been described above using two embodiments, the present invention is not limited to the above two embodiments, and the width of the diffraction grating can be changed as shown by curves 41 and 42 in FIG. There is no problem in adopting any function form without being limited to curves.
Furthermore, it should be added that the semiconductor laser material and the mode control structure and current limiting structure of the semiconductor laser are not limited in any way.

[Brief explanation of the drawing]

Fig. 1 shows the oscillation spectrum 10 and gain distribution 100 of a typical mode-controlled semiconductor laser, and Fig. 2 shows the oscillation spectrum 20, 21, 22, 23, 24 of a semiconductor laser modulated by high frequency superposition.
FIG. 3 is a diagram showing the oscillation spectrum 31 and gain distribution 30 of the semiconductor laser according to the present invention, and FIG. 4 is the amplitude distribution corresponding to the real part of equation (7) on page 8. 4 is a diagram showing a periodic refractive index distribution whose amplitude is modulated by 41 and 42. FIG. FIG. 5 is a cross-sectional view of a DFB semiconductor laser whose amplitude in the depth direction changes in proportion to a function corresponding to curve 40 in FIG.
50 is a substrate, 50 is an active layer, 51 is a waveguide layer, 53 and 56 are cladding layers, and 55 is a cap layer. FIG. 6 shows a corrugated periodic structure diffraction grating 6 whose width in a plane parallel to the active layer surface 60 changes in proportion to a function corresponding to curves 41 and 42 in FIG.
4 and the corresponding light emitting parts 61 and 62,
7 and 9 show a semiconductor laser according to the present invention.
In this figure, 75 and 95 have widths in a plane parallel to the active layer that correspond to the curve 41 in FIG.
42 is a corrugated periodic structure diffraction grating that changes in proportion to a function, 76 and 96 are InP substrates, 73 and 93 are n-type InP layers, 70 and 90 are
InGaAsP active layers 71, 72, 91 and 92 are
InGaAsP cladding layer, 74 and 94 are p-type InP layers,
741 and 941 are p-type InGaAsP cap layers, 7
9 and 99 are SiO ₂ insulating films, 78 and 991 are Cd diffusion parts, 77 and 98 are An-Zn electrodes, and 791 and 97 are
It is an Au-Ge-Ni electrode. FIG. 8 is a diagram showing the manufacturing process of a semiconductor laser according to the present invention.

Claims (1)

[Claims] 1. The width in a plane parallel to the active layer changes according to the shape of an arbitrary function other than a function shape having a value of 1 in a part with a diffraction grating and 0 in a part without a diffraction grating, and The time-frequency spectrum obtained by converting the spatial frequency ω into the time frequency ν in the spatial frequency spectrum of the arbitrary function is given by a predetermined frequency width Δν with a predetermined center frequency ν ₀ in between. A distributed feedback semiconductor laser characterized by having a corrugated periodic structure diffraction grating that includes a frequency band [ν ₀ −Δν, ν ₀ +Δν]. 2. The width in a plane parallel to the active layer changes according to the shape of an arbitrary function other than the function form which has a value of 1 in the part with the diffraction grating and 0 in the part without it, and the space of the arbitrary function. The time-frequency spectrum obtained by converting the spatial frequency ω into the time frequency _ν in the frequency spectrum is a frequency band [ν ₀ − Δν, ν ₀ +Δν], the active layer,
The semiconductor laser according to claim 1, wherein the semiconductor laser is formed over the entire laser emitting region at the interface between any two layers of the cladding layer, the waveguide layer, and the first growth layer on the substrate side. . 3. The width in a plane parallel to the active layer changes according to the shape of any function other than the function form which has a value of 1 in the part with the diffraction grating and 0 in the part without it, and the space of the said arbitrary function. The time-frequency spectrum obtained by converting the spatial frequency ω into the time frequency _ν in the frequency spectrum is a frequency band [ν ₀ − A distributed Bragg reflection semiconductor laser characterized by having a corrugated periodic structure diffraction grating including [Δν, ν ₀ +Δν]. 4. The width in a plane parallel to the active layer changes according to the shape of an arbitrary function other than a function shape having a value of 1 in a part with a diffraction grating and 0 in a part without a diffraction grating, and the space of the arbitrary function. The time-frequency spectrum obtained by converting the spatial frequency ω into the time frequency _ν in the frequency spectrum is a frequency band [ν ₀ − Δν, ν ₀ +Δν], the active layer,
4. The semiconductor laser according to claim 3, wherein the semiconductor laser is formed outside the laser emitting region at the interface between any two layers of the cladding layer, the waveguide layer, and the first growth layer on the substrate side.