JPS5914690A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a light source which has no spectral noise by disposing one or more sets of diffraction gratings of corrugated periodic structure which is disposed in a plane parallel to the active layer and has apodized amplitude distribution on the boundary between two arbitrary layers of an active layer, a waveguide layer, a clad layer, and the first grown layer of substrate side. CONSTITUTION:A diffraction grating which has a corrugated periodic structure represented by N(lambda0/2ne) where N is integer number, lambda0 is wavelength in vacuum of laser light, ne is effective refractive index is provided on the boundary between two arbitrary layers of an active layer, a waveguide layer, a clad layer, and the first grown layer of a substrate side. In other words, an N type InP layer 73, N type, P type and I type In0.7Ga0.3As0.6P0.4 clad layers 71, 72, an In0.6Ga0.4As0.86P0.14 active layer 70, a P type InP layer 74, a P type In0.6Ga0.4 As0.86P0.14 cap layer 741, an SiO2 layer 79, an ohmic contacting Cd diffused layer 78, and electrodes 77, 791 are formed on an N type InP substrate 76, and a diffraction grating 75 is formed on the boundary between the layers 71 and 73.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a distributed feedback semiconductor laser, and more particularly to a cloth feedback conductor laser in which a diffraction grating is twisted.

Semiconductor laser L is widely used as a light source for optical fiber communication, but it is difficult to carefully remove signal deterioration due to spatial noise in a combination of a semiconductor laser and a multimode optical fiber.511 # has become a problem trap.

Recent advances in mode control technology for semiconductor lasers have significantly improved the coherence of semiconductor laser light compared to conventional methods, and the development spectrum has improved from several tens to 100 Mb.
For modulation frequencies below /s, as shown in Figure 1,
An oscillation spectrum 10 in a state close to single-longitudinal mode oscillation is realized approximately at the center of the gain distribution surface ls1υO. However, when such a well-coherent semiconductor laser beam is propagated through a multimode optical fiber, it passes through different optical paths at the output end of the optical fiber as a result of irregular mode conversion that occurs in the original fiber. The incoming lights interfere with each other, exhibiting irregular fluctuations, and pulse waveform disturbances occur for pulse-modulated light. Currently, three methods are known to remove such speckle noise: (1) use of high frequency superimposed modulation, (2) use of a light emitting diode as a light source, and (3) use of a mode scrambler. Among these methods, method (1) modulates a semiconductor laser with a surface frequency signal of several hundred MHz, resulting in a gain distribution of 20 as shown in the tR2 diagram for one oscillation spectrum.
Multimode oscillation 20, 21 with a spread corresponding to 0
．． 22, 23, 24. . . , which takes advantage of the fact that the coherence of the laser beam decreases. However, this method is troublesome in that the semiconductor laser must be constantly modulated with a local wave 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 Mb/s each, 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, see "One-Year Fiber Communication" by Soe, Ku, and Yuanbuchi, published by Telecommunications Technology News, 503-
See page 504.

It is an object of the present invention to eliminate the above-described drawbacks of the conventional method for removing smooth and dull noise caused by the coherence of a light source in a multimode optical fiber communication system, and to achieve a low degree of coherence that does not cause speckle noise. The object of the present invention is to provide a light source having an emitted light distribution capable of coupling into an optical fiber the same level of light as that of a conventional semiconductor laser.

Another object of the present invention is to provide a semiconductor diode which exhibits a continuous oscillation spectrum within a defined spectral band.

According to the present invention, the active layer, waveguide layer, cladding layer, substrate@
At the boundary between any two layers of the first growth layer, a Purgout-like periodic structure having an aborted amplitude distribution in a plane parallel to the active node and along the light emission direction is formed. A distributed M type or distributed flag reflector type double heterojunction semiconductor laser is obtained, which is characterized by forming one or more sets of diffraction gratings.

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

Generally, what is called a distributed feedback reflector (hereinafter abbreviated as DPB) or distributed flag reflector type (hereinafter abbreviated as DBR) semiconductor laser consists of an active layer, a waveguide layer, a layer, a layer, and a first layer on the substrate side. There is a period at the boundary between any two layers of the layers, where N = integer λ = wavelength of laser light in vacuum ne - effective refractive index. Or it is formed in one part.

The characteristic of DI"B or DB semiconductor laser is that it uses optical feedback inside the laser. Since it uses a Bragg dome with a diffraction grating instead of a bellows optical resonator, the oscillation wavelength is determined by formula 11). λ given by.

It can be fixed to , and a single-major wave number fingering can always be obtained. DFB or DBI (For a detailed historical explanation of semiconductor lasers, see
Co-authored by Bee Vanish, 1 Heterostructure, 7 Racers”, Academic Press, 1978, Bart A.
, pp. 90-106. Dfi'B Mata
Since DBI (DBI) emits laser light with good coherence, speckle noise is more likely to occur, and it is not suitable for multimode optical fiber communication as it is.

Incidentally, the above-mentioned corrugated synchronous structure is effectively equivalent to introducing a refraction amount distribution along the light emitting portion of the laser.

n(X) n,=△, (,) given by formula (2)
The spatial frequency spectrum distribution obtained by performing the 7-lier transform shows a sharp peak in the form of a delta function exactly at the position r (ω = 2π/A), which reflects the fact that single-frequency oscillation can be obtained. Therefore, the distribution of △, (,) is expressed as (2)
If the form is different from the formula and the spatial frequency spectrum is selected to be a continuous spectrum within a certain band, it is possible to realize laser oscillation with a continuous oscillation spectrum distribution only in the frequency band corresponding to that band. It's going to happen.

That is, if the origin of the X-axis is taken at the center of the semiconductor laser, △,
(X) and its spatial frequency spectrum I'''(ω) are in the following relationship.F←) and oscillation spectrum distribution S(ν
), it is safe to assume that there is a general relationship between the two. Therefore, if a desired distribution is given as 8(ν), the distribution of (,) to be realized using equations (5) and (3) can be found. As S(ν), the distribution of each persimmon can be selected, but the distribution of each persimmon is suitable for our purpose.
- As shown in FIG. 31, the most common distribution is a rectangular distribution S(ν)=1 ν that extends almost completely to the gain curve 30 of the semiconductor laser. −Δν×ν×ν. +Δν(6) =0 ν〉ν. +△shi, cross νkuν. −Δν is adopted. However, ν. =C/λ. From ξ, equation (5), and equation (3), =. -ixωo, simouth wandering (7). Actually, since only the factor part of equation (7) can be considered t, the refractive index distribution is n (x) = n6 + nl cos (xω.,
, (gxn (x ''') ) t8).Specifically, as shown in Figure 4, the function 0
Is the amplitude of the unevenness of the full-gate periodic structure given by 0G (xω.) a point? As shown in #41.42, the function '5in (
As a result of modulation by x・△ω)/X, it is sufficient to realize a bend 1f1 minute a6 as shown in song @40. Giving a certain 11d number a distribution of curve 40 or a shape similar to it in this way is called 7podization. Fifth
The figure is a cross-sectional view of an example of an InGaAsP-DPB semiconductor laser having a diffraction grating having a full-dart periodic structure shown by curve 40. In Figure 5, 50 is Lie-11GaO
-4Aso-1@p11.14 active layer, 51 and 53 are pm and °'%nlL"0.?G80.8"0-
11pO-49, 54 is n11. InP substrate,
56 tj: p fli InP layer, 55 a p
ff1I n6. @oa(1,4Aso-s@po・
Goose 4′? r layer, cladding layer 51 and p I
A full-gate periodic structure 52 is formed at the boundary of the nP layer 56 and is 7-voted.

If it could be fabricated, it would be perfect for our purpose, but in reality, it is extremely difficult to fabricate a full gate structure like 52, which has multiple convex and convex changes in the depth direction.

In the present invention, the difficulty of iL-like description is solved by employing a 7-box grid in a plane as shown in FIG. In FIG. 6, 60 indicates the active layer, the waveguide layer, the cladding layer, and the first layer on the substrate side! any 2 of khA
6 is a view from above of the lump interface between two layers, 61 and 6.
2 indicates the range iyt in which the laser oscillation light is distributed, 63 the functional form of the 7-bottom 1'zu circle@st++ (xbld/x), and 64 a corrugated diffraction grating formed on the boundary 1fi60. Each it of the diffraction grating 64 is the current surface 60
It doesn't matter whether it protrudes upward from the surface or convexes to 4-1K, and there is no absolute criterion for determining its quotient or depth, and it is usually 0.01.
-1 μm range V@ may be sufficient. In FIG. 6, for example, if we look at the part indicated by A-A', the amplitude W't of the diffraction grating 64 in that part is 1JW of the distribution of the laser oscillation light.
It's smaller than that. If n is the effective refractive index amplitude corresponding to the peak of the corrugation of the diffraction grating 64, then A-A
' On average, the effective refractive index amplitude is nI XVw
'/When you become a WK, you can become an island. Therefore, in the end M6
The diffraction grating 64 shown in the figure can be considered to have an average lapsed refractive index distribution as shown in equation (8). The length L of the main part of the diffraction grating 64 is calculated from equation (7) and (7Y), assuming that C At first glance, this is a reasonable value.

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 a thin InP substrate, 73 B n
m-InP layers 71 and 72 are nm and p, respectively.
BlI j'l o*y G a ops A liO
, I Po, 4 Kura, Do layer, 70 is Ino, eGao-
*Aso-aaPo, ta tA layer, 74 is p m
InP layer, 741 is P deaf I no, s Ga(1,4
AsO-all Po, t+kya, p layer, 79 is Sin
, an insulating film, 78 is an ohmic contact CD diffusion part, 77 and 791 are Au-Z and Au-GeN, respectively.
It is an i-electronic test. 751d, cladding layer 71 and n-type InP
This is a diffraction grating with a corrugated periodic structure having an abodized amplitude distribution formed at the boundary surface of the layer 73. 7th
A semiconductor laser having the structure shown in the figure can be manufactured by the process shown in FIG.

First, as shown in FIG. 8(a), an n kl InP layer 81 is epitaxially grown as a liquid crystal on an n-inch InP substrate 80 .

Next, as shown in FIG. 8(b), a SiO film 81 is placed on top of it.
2 is applied, and a resist film 813 (7t) is spinner coated on top of the resist film 813, and a baking process is performed. Next, as shown in Fig. 8 (cl) for this query, a photomask 81 is formed with a pattern in the shape of the 7-bodize function of the diffraction grating 75 in Fig. 7.
When the light exposure and development process is performed using 8i0.
Sea urchin I. with exposed membranes is obtained.

Furthermore, this wafer is treated with a buffer treatment, tinted liquid, etched to remove the Sin and film in the Shimada parts, and the photoresist layer 813 is also removed with a stripping agent. is obtained. Next, as shown in Figure 8(f),
Further, a photoresist layer 815 is spinner-coated on this wafer and subjected to a baking process, as shown in FIG. 8(g).
As shown in FIG. 3, a diffraction grating pattern is recorded in a hologram in a photon strike film 815 by interference of two beams 816 and 817 using an Ar laser beam or a He--Cd laser beam. When we process this, we obtain the formula I in Figure 8 (h) <p, which is rare.

Process and etch with hydrochloric acid, and remove with a peeling agent.
5 is removed, a sea urchin I. having a diffraction grating 82 as shown in FIG. 8(i) is obtained, which is further buffered,
After removing the StO film 812 with a quenching solution, the film 812 is removed (Fig. 8(j)).
Eno is obtained as shown in . On top of this, n-type InO, 7G a 64 was sequentially grown by liquid phase epitaxial growth.
A S 04 PG・4 cladding layer 83, In
o-sGao-4Aso-sgPO, t4 active layer 84,
pya Z n 6. y G a O4A s 6. @I
P6.4 cladding layer 85, pmInPmInP substrate
p mIno,. Ga 0.4 people 3°, 11+IP
0.14 Grow layer 861 and C Fig. 8(k)
) Circle 8i0. After ca diffusion (not shown in Figure 8) on the insulating film 8621, Au-z, electric j1M87, Au
By adding -Ge Nit&88, an evaporator as shown in Fig. 8 (1) can be obtained. This is L in Figure 6.
A semiconductor laser as shown in FIG. 7 can be obtained by reducing the size to one to several times the size of the semiconductor laser.

FIG. 9 is a partially cutaway diagram showing another embodiment of the present invention. 'In the ninth meat, 96 is n-type 1, P substrate, 9
3 td, 'n fJ InP layer, 91 and 92Fi respectively n-type 2 and p W Ino, yGao, 5Aso
-sPo-+ cladding layer, 90 is In(1-11Ga(
1-4A1i0-alIPo, 14 active layer, 94 p-type 1nP layer, 941 p m I no, lI Ga
@, 4As646 P6. ．． 4 cap, p layer, 99 is S
iO, insulating film, 991 is ohmic, Cd diffusion part for contact, '98 and 97 are A, u Zn and A, -G, respectively.
, -Ni electrode. The difference from the embodiment shown in FIG. 7 is that the interpolated grating/cedar 5 does not extend over the entire emission line of the semiconductor laser, but is located outside the light emitting section surrounded by the dotted line 992. That is, the embodiment of the seventh cause is a DFB semiconductor laser, and the semiconductor laser in FIG. 9 is a DBR semiconductor laser.

The semiconductor laser shown in FIG. 9 can also be manufactured by the steps shown in FIG. 8.

Although the present invention has been described above using two embodiments, the present invention is not limited to the above two embodiments;
In particular, it should be added that there are no limitations on the semiconductor laser material and the mode control structure and current limiting structure of the semiconductor laser.

[Brief explanation of drawings]

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 of a single-frequency superposition modulated semiconductor laser.
, 21.22, 23.24 and a diagram showing the gain distribution 200,
Figure 3 shows the oscillation spectrum 3 of the semiconductor laser according to the present invention.
1 and a gain distribution 30, and FIG. 4 is a diagram showing a periodic refractive index distribution modulated in amplitude with amplitude distributions 41 and 42 corresponding to a 7-bodies function. FIG. 5 is a cross-sectional view of a Dk'B semiconductor laser having a diffraction grating 52 having an amplitude distribution divided into 7 bodies in the depth direction, where 54 is a substrate, 50 is an active layer, and 51
5 is a waveguide layer, 53 and 56 are CL and D layers, and 55 is a cap layer. FIG. 6 is a diagram showing a cortate-shaped periodic structure diffraction grating 64 whose amplitude is modulated by a 7-podize function 63 in a direction parallel to the active layer portion 60, and the corresponding light emitting portions 61 and 62. A semiconductor laser according to the present invention
In the cutaway diagram, 75 and 95 are full-gate periodic structure diffraction gratings having an amplitude distribution aboded in the yc direction parallel to the active layer, 76 and 96 are I'nJ', j4叡, 73 and 93. is an n-type InP layer, 70 and 90 are In(j,
lA31' active layer, 71, 72, 91 and 92 are InG
BA! IP layer, layer 74 and 94 are p-type In
PtV! , 741 and 941 are p-type 1 n Ga
A s P cap layer, 79 and 99 are SiO. Insulating film, 78 and 991 are Cd diffusion parts, 77 and 98 are AH
-Z(1t@, 791 and 97 are Au-Ge-Ni% poles. Figure 8 is a diagram showing the manufacturing process of a semiconductor laser according to the present invention. Figure 8 (d) <e) Product l θ6 (C) (f)

Claims (1)

[Claims] 1. A semiconductor laser having a distribution reflector and a reflector having an amplitude distribution that is divided into seven bodies in a direction parallel to the active layer. 2. A full-gate periodic structure with an amplitude distribution 7podized in the direction parallel to the active layer is placed on the interface between the active layer, the CL layer, the D layer, the waveguide layer, and any 2 # of the growth layer on the substrate side. ,
A semiconductor laser according to claim 1, characterized in that the semiconductor laser is formed over the entire laser emission region. 3. A full-gate periodic structure with an abbreviated amplitude distribution in the direction parallel to the active layer is used as the active layer.
, the waveguide layer, and the first Q-long layer on the substrate side, the semiconductor laser is formed outside the laser emission region at the interface between any 2M of the substrate side first Q-long layer.