JPS5914689A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a light source which has radiation light distribution without spectral noise by forming at least one or more corrugated diffraction gratings which are specified in length in parallel direction to the propagating direction of a laser light in the boundary between the two arbitrary layers of an active layer, a clad layer, a waveguide and the first grown layer of substrate side. CONSTITUTION:A corrugated structure diffraction grating of L=(Nlambda<2>0)/4neXDELTAlambda, where lambda0 is central oscillation wavelength of a laser, DELTAlambda is spectral width, ne is equivalent refractive index and N is order number of diffractions in a semiconductor laser of multilayer structure which contains an active layer, is provided. In other words, an N type InP layer 73, N type an P type In0.7Ga0.3As0.6P0.4 clad layer 71, 72, an In0.6Ga0.4As0.86P0.1 active layer 70, a P type InP layer 74, a P type In0.6Ga0.4As0.86P0.14 cap layer 741, an SiO12 film 79, an ohmic contact Cd diffused part 79, and electrodes 77, 791 are formed on an N type InP substrate 76, and the diffraction grating is provided on the boundary between the layers 71 and 73.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser having one or more diffraction gratings whose length in a direction parallel to the direction in which laser light travels is limited to a finite value as distributed feedback means.

Semiconductor lasers are widely used as light sources for optical fiber communications, but the problem is how to eliminate signal degradation caused by speckle noise when combining 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. At the center of the gain distribution curve 100, an oscillation spectrum 10 in a state almost like single-longitudinal mode oscillation is realized.

However, when such a semiconductor laser beam with good coherence is propagated through a multimode optical fiber 1, different optical paths are formed at the output end of the optical fiber as a result of irregular mode conversion that occurs in the y0 fiber. The transmitted light interferes with each other, exhibiting irregular fluctuations, and pulse waveform disturbances occur 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 methods, method (1) modulates a semiconductor laser with a high frequency signal of several hundred Ml (z), and as shown in FIG.
This method takes advantage of the fact that the coherence of the laser beam decreases in the following states. However, this method is troublesome in that the semiconductor laser must be constantly modulated with a high frequency signal. (2)
This method uses incoherent light from a light emitting diode instead of coherent semiconductor laser light, and is effective for removing modal noise, but the modulation band is only a few tens of Mlt/s at most, and it is difficult to connect optical fibers. Since the amount of coupled light cannot be increased, the relay interval cannot be made too long. Since method (3) has no direct effect on the present invention, its explanation will be omitted. For details, see "Optical Fiber Communication" by Soejima and Kaibuchi, published by Telecommunications Technology 21-S Co., Ltd., 503-
Please refer to 504 Sada.

It is an object of the present invention to eliminate the above-mentioned drawbacks of conventional methods for speckle noise removal due to light source coherence in multimode optical fiber communication systems;
To provide a light source having a degree of coherence so low that it does not cause speckle noise and having a radial light distribution capable of coupling into an optical fiber light at a level comparable to that of a conventional semiconductor laser.

It is in.

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, a corrugated diffraction grating whose length in the direction parallel to the traveling direction of the laser beam is limited to a finite value can be used in any two of the active layer, cladding layer, waveguide layer, and first growth layer on the substrate side. A distributed feedback or distributed Bragg reflector semiconductor laser in which at least one semiconductor laser is formed at the boundary between two layers can be 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, a cladding layer, and a first growth layer on the substrate side. A corrugate-like periodic structure is formed as a diffraction grating on the entire surface or a part thereof, with a period at the boundary between the two layers, where N = integer λo, wavelength of laser light in vacuum, jle - effective refractive index. be.

The particular disadvantage of DFB or DBR semiconductor lasers is that they use Bragg diffraction using a diffraction grating instead of a Fabry-Perot type optical resonator for optical feedback inside the laser.
The oscillation wavelength is λ given by equation (1). It can be fixed to , and single frequency oscillation can always be obtained. DFB or DB
For a more detailed explanation of R semiconductor lasers, please refer to H.C. Cage II and M.B. Banish, "Heterostructure Lasers", Academic Press, 1978, Part A, pp. 190-106. Since DFB or DBI-L 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 periodic structure effectively has 2π n(x) = no-)-n, cos (-x) along the laser emission part.
This is equivalent to introducing a refractive index distribution of the form (2).

n(x) −no3Δn(x) given by equation (2)
The spatial frequency spectrum distribution obtained by Fourier transform shows a sharp delta function-like peak exactly at the position of ω=2r/A, which reflects that single frequency oscillation can be obtained. Therefore, if we make the distribution of Δn(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.

The easiest way to realize a diffraction grating with a continuous spatial frequency spectrum is to make Δn (x) a function that varies sinusoidally only in a finite interval, rather than a function that extends infinitely as in equation (2) above. If it is assumed that this is not the case, then the spatial frequency spectrum F(ω) is as follows.

Actually, we only need to consider the case where ω〉0, so the above (
Focusing only on the second term in the parentheses of equation (4), the spatial frequency spectrum F(ω) expressed by equation (3) is ω=ω, as shown in FIG. Mi2(2Δω=2Δω in the band with a peak at q)
It has a continuous spectrum of 2π/a. Figure 3 31
The correspondence relationship between the oscillation spectrum, R, and the spatial frequency antivector of the previous Re equation (4) can be determined as follows. In other words, the center wavelength of the oscillation spectrum is λ. , where Δλ is the spectral width, is given by using equation (1) above. Now, for example, λo: 1.3μm1Δλ=
io people, N = 2, no = 3.456, then A =
0.376 XIOCIIL= 0.376 tim
= 128.5cm-' a = 0.0245cm = 24°5 μm. In other words, it is sufficient to cut off the length of the diffraction grating at L=2a=49 μm. Of course, in an actual semiconductor laser, the length L of the diffraction grating does not have to strictly satisfy the relationship derived from the above equation (6), but may just approximately satisfy the relationship. In general, converting a function defined in an infinite interval [-■, ω] and having a finite value in that interval into a form truncated in the finite interval (-a, a) is called truncating the function. In this specification, the diffraction grating whose length is determined by the function equation shown in equation (7) above will be referred to as a truncated diffraction grating. Note that there may be cases where the amount of reflection necessary for distributed feedback cannot be obtained with the value of L determined from the above equation (7), but in that case, a plurality of diffraction gratings with the same value of L may be formed.

Figure 5 shows an In with two truncated diffraction gratings.
1 is a cross-sectional view of a GaAsP-DB semiconductor laser. In Figure 5, 50 is Ino, r Gao4Aso, 5s
Po, t4 active layer, 51 and 53 are p-type and n-type, respectively
Type 1no, yGao, s ASo, 11 Po, 4
Cladding layer, 54 is n-type InP substrate, 56 is p-type InP
lii, 55 is p-type Ino, s Gao, 4 ASo,
86P0.14 cap layer, 2 rad layers 51 and p
- Truncated diffraction gratings 52 and 521 are formed at the boundary of InP ff156.

Next, an embodiment ζ of the present invention will be explained with reference to the drawings.

In Figure 7, 76 is an n-type 1nP substrate, 73 is an n-type I
nP layers 71 and 72 are n-type and p-type 1no, respectively;
y ()ao, aA80. S P, 0.4 cladding layer,
7o is In, o, e, Gao, 4Aso, 5sPo,
la active layer, 74 is p-type 1nP layer, 741 is p-type 1no
, 5 Gao, a Aso, se PO, 14 cap layer, 79 is Sin, insulating film, 78 is Cd for ohmic contact
Diffusion parts 77 and 791 are Au-Zu and Au, respectively.
-(g)-Ni electrode. 75 is the cladding layer 71 and n
This is a truncated diffraction grating formed at the interface of the type InP layer 73. Although not shown in this figure, 75
A similar shape is also formed at the output end on the reflection side.

The semiconductor laser having the structure shown in FIG. 7 is produced by epitaxially growing a liquid crystal as shown in FIG. Next, as shown in FIG. 8(b), a sio film 812 is applied thereon, and a photoresist film 813 is spinner coated thereon, followed by a baking process. Next, this wafer is coated with a photomask 814 as shown in FIG. 8(C).
When the exposure and development process is carried out using a wafer, a wafer is obtained as shown in FIG. 8 ((i), in which the SiU film and film in the portion corresponding to the pattern in FIG. 6 are exposed. Furthermore, this wafer is buffered. 810 of the part that was etched with etching liquid and won a prize.
By removing the two films and also removing the photoresist film 813 using a stripping agent, a wafer as shown in FIG. 8(e) is obtained. Next, as shown in FIG. 8(f), a photoresist film 815 is spinner-coated on this wafer, and after a baking process is performed, Ar laser beam or l
A diffraction grating pattern is formed on the photoresist film 81 by interference of two beams 816 and 817 using an ie-Cd laser beam.
Record a hologram within 5. By developing and processing this, a wafer as shown in Figure 8 (h) is obtained, which is etched with dilute hydrochloric acid and the photoresist film 815 is removed with a stripper, resulting in a diffraction grating as shown in Figure 8 (h). A wafer having 82 and 821 formed thereon is obtained, and the Sin film 812 is further removed using a buffer etching solution to obtain a wafer as shown in FIG. 8U). On top of this, by liquid phase epitaxial growth, n-type 1no, y Gao, x Aso,
aPo, 4 cladding layer 83.1no,s (Jao4A
S0.86 pH, +4 active layer 84.1) type Ino, y
Gaax, Aso, s Po, 4 Krat layer 85,
p-type InPli86, p-type 1no, s Gao, <A
After growing SO, 116P (1,14 cap layer 861 in FIG. 8(k)), and adding a Si and insulating film 862 and diffusing Cd (not shown in FIG. 8), an Au-Zn electrode 87 is formed.
, by attaching Au-0e-Nl electrode wings, Figure 8 (
Ueno 1 as shown in J-) is obtained. By dividing 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.

Although the present invention has been described above using examples, the present invention is not limited to the above-mentioned examples, and of course, DFB
It should be added that the present invention is also applied to semiconductor lasers, and there are no limitations regarding the semiconductor laser material and the modulated control structure and current limiting structure of the semiconductor laser.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the oscillation spectrum 10 and gain distribution 100 of a typical mode-controlled semiconductor laser, and FIG. 2 is a diagram showing the oscillation spectrum 20 of a high-frequency superposition modulated semiconductor laser.
．． 21.22, n124 and a diagram showing the gain distribution 200, FIG. 3 is the oscillation spectrum 31 of the semiconductor laser according to the present invention.
FIG. 4 is a diagram showing the spatial frequency spectrum of a truncated diffraction grating. FIG. 5 is a cross-sectional view of a DFB semiconductor laser having diffraction gratings 52 and 521, where 54 is a substrate, 50 is an active layer, 51 is a waveguide layer,
53 and 56 are cladding layers J-155 are cap layers. FIG. 6 shows a mask pattern, and 60, 61, 62, . . . are patterns corresponding to the outer periphery of the truncated diffraction grating, respectively. FIG. 7 is a cutaway perspective view of a portion of the semiconductor laser 11 according to the present invention, and 75 is a truncated diffraction grating 76 formed at the boundary between the n-type 1rtP layer 73 and the InGaAsP cladding layer. The substrate 73 is an n-type InP layer, 70 is a 1nGaAsP active layer 71 and 72 is a 1nOaAsP crat layer 74 is a p-type InP layer
The P layer 741 is p-type 1nGaAs; the P cave layer 79 is made of Sin;
The insulating film 78 is a Cd diffusion part 77 and the Au-Zn electrode 791 is Au-Ge-Ni [pole]. FIG. 8 is a diagram showing the manufacturing process of a semiconductor laser according to the present invention. Representative Patent Attorney Uchihara Oto 5+l Diagram O Fang 2 Mouth. Enter 3rd O figure 8 (α) (J) (d) (t) (i) r 7cn I
EI 116 (C) (f) (i> 2 I6

Claims (1)

[Claims] 1. In a semiconductor laser having a multilayer structure including an active layer, the center oscillation wavelength of the laser is λo1, the spectral width is Δ
λ, the isolateral refractive index is n, and the diffraction order is N, the active layer is a corrugated diffraction grating whose length L in the direction parallel to the traveling direction of the laser beam at least approximately satisfies the relational expression. , a cladding layer, a waveguide, and a first grown layer on the substrate side, at least one of which is formed on the interface between any two of them.