JPH02106084A - Distributed reflector type semiconductor laser - Google Patents

Abstract

PURPOSE:To enable the title laser to be stably oscillated in single vertical mode thereby facilitating the automatic output control by a method wherein a distributed reflector is provided with a quadratic or higher degree diffracting lattice as well as a photodetector detecting any diffracted beams from the distributed reflector on the upper part thereof. CONSTITUTION:A distributed reflector 43 is composed of a quadratic diffracting lattice 43a and a photoconductive channel layer 45. When normal bias is given to an n type electrode 63 and a p type electrode 65 for a semiconductor laser, an active layer 53a is fed with current to cause luminescent recoupling. Then, the emitted beams are respectively reflected on the end 53b of an active region side formed by cleavage and the distributed reflector 43 to cause laser oscillation. At this time, the distributed reflector 43 increases the reflectance only on the beams in wave length of lambda so that the semiconductor laser may be single mode-operated stably in the wave length of lambda. Furthermore, photodetector 55 detects the linear diffracted beams from the distributed reflector 43 so that potential difference may be caused between a p type electrode 55d and an n type electrode 55c for the photodetector resultantly enabling the laser beams to be monitored. Through these procedures, the semiconductor laser can be oscillated in vertical mode stably thereby facilitating the automatic output control.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a distributed reflection type semiconductor laser which can be used as a light source for optical communication, for example, and particularly relates to a distributed reflection type semiconductor laser that integrates a light receiver for monitoring the output of the semiconductor laser. This invention relates to a reflective semiconductor laser.

(Prior Art) A light source with a small spectral width is essential to realize long-distance optical communication and large-capacity optical communication. Therefore, a zero-distribution reflection type semiconductor is desired, and a semiconductor laser that oscillates in a single longitudinal mode is desired. A laser is known as a type of such a semiconductor laser, and a conventional example thereof is described in, for example, the literature (IEICE technical research report 0QE85-7 (+985)) P
, 47).

FIGS. 4(A) to 4(C) are diagrams schematically showing the structure of a distributed reflection semiconductor laser (hereinafter sometimes abbreviated as semiconductor laser) disclosed in this document. Figure 4(A) is a cross-sectional view of this semiconductor laser cut along the laser stripe, and Figure 4(B) is a cross-sectional view of the semiconductor laser cut along the direction perpendicular to the laser stripe. FIG. 4C is a cross-sectional view of a portion of this semiconductor laser where the distributed reflector 15 is located, taken along a direction perpendicular to the laser stripe. Hereinafter, the structure of a conventional distributed reflection semiconductor laser will be briefly explained with reference to these figures.

This conventional semiconductor laser includes an active region 13 and a first-order diffraction grating 15a, which is a distributed reflector to which light from the active region is coupled, on an n-type InP substrate 11 as a base.
r and a distributed reflector 15 were integrated. This structure will be explained in detail as follows.

An InGaAsP active layer 13a and a p-type InP low refractive index protective layer 17 are laminated in this order on a predetermined region of the n-type InP substrate 11, and a region adjacent to the active layer 13a of this substrate 11 in the waveguide direction A first-order diffraction grating 15a was formed. Roughly, on this p-type InP low refractive index layer 17 and on the first-order diffraction grating 15a, a p-type InGaAsP optical waveguide layer 19 and a zinc-doped p-type InP cladding layer 21 are formed.
and a zinc-doped p-type InGaAsP cap layer 23 were laminated in this order. Further, an n-type electrode 25 is formed on the lower side of the substrate 11, an n-type electrode 27 is formed on the active layer 13a of the cap layer 23 (a region corresponding to this region), and a p-type electrode 27 is formed on the lower side of this p-side electrode 27. A zinc diffusion region 29 (region 1fi indicated by a dotted pattern in the figure) was formed that reached the InP cladding layer 21.

Furthermore, in this semiconductor laser, InGaAs
P active layer 13a, p-type 1nP low refractive index layer 17, p-type In
GaAsP optical waveguide layer 19, p-type InP cladding layer 21
The laminate consisting of the laminate and the can layer 23 had a stripe shape, and the cross section taken in the direction orthogonal to the stripe direction had an inverted mesa shape. Then, the P -type 1NP current stenosis layer 33 and N type 1NP so that this laminated body 31 is embedded in both sides of this striped laminated body 31 of the substrate 11.
The current confinement layer 35 was formed in this order.

Here, the above-mentioned distributed reflector 15 is composed of a diffraction grating 15a and a p-type 1nGaAsP optical waveguide layer 19, but as is well known, it has a large It has a reflectance (however, the input in formula 0 is the wavelength of light,
n *tt is the effective refractive index of the distributed reflector, △ is the period of the diffraction grating, and m is the order of the diffracted light (m = 1.2, 3.-...
...), respectively.

m·λ=2 n off ·△...■ Therefore, in this semiconductor laser, only the longitudinal mode with λ that satisfies Equation 0 is stably launched. In addition, in this semiconductor laser, in order to obtain good output characteristics, a p-type InGaAsP optical waveguide layer 19 is provided on the active layer 13a and the diffraction grating 15a.
BIG:Bundle-In
It is called teqrated-Guide. ) The coupling of the waveguide between the active region 13 and the distributed reflector 15 was increased.

Note that the semiconductor laser shown in Figure 4 has a structure in which a distributed reflector is provided only on one side of the active region, but in the above document, distributed reflectors are provided on both sides of the active region. things are also shown.

(Problems to be Solved by the Invention) However, in the structure of the conventional distributed reflection semiconductor laser described above, automatic output control (Δauto
ntrol (hereinafter abbreviated as APC),
Laser output at a level effective as monitor light can only be obtained from the end face of the active region or the end face on the side of the distributed reflector.
Therefore, it is necessary to separately install a photodetector externally, and therefore, in order to monitor the laser output with high efficiency, the light receiving surface of the photodetector and the optical axis of the semiconductor laser must be aligned with high precision. However, there was a problem in that the work was extremely difficult. Further, there is a problem in that the light (return light) returning from the light receiver facing the end face to the semiconductor laser has an adverse effect VI on laser emission.

In addition, since laser light of a level that can be used for chip inspection can only be obtained from the end facets, the characteristics of a large number of semiconductor lasers formed on a substrate cannot be measured until after the substrate has been thoroughly tested. Another problem was that it took a long time to sort the chips.

The present invention has been made in view of the above points, and an object of the present invention is to provide a distributed reflection semiconductor laser that eliminates the above-mentioned problems, oscillates in a single longitudinal mode, and can easily perform PC. It's about doing.

(Means for Solving the Problem) In order to achieve this object, according to the present invention, a distributed reflector comprising an active region on a substrate and a distributed reflector to which light from the active region is coupled. In a type semiconductor laser, the distributed reflector includes a second-order diffraction grating or a higher-order diffraction grating, and a light receiver is provided above the distributed reflector to receive the diffracted light from the distributed reflector. Features.

(Function) The function of the distributed reflection type semiconductor laser of the present invention will be explained by giving a specific example. FIG. 2 shows the situation when the distributed reflector of the semiconductor laser of the present invention is equipped with a second-order diffraction grating.
In particular, we focused on the area around the distributed reflector, and it also shows the behavior of light at this time. In addition, 41 in FIG. 2 is an InP substrate as a base, 43a is a second-order diffraction grating, 4
5 is an InGaAsP layer as an optical waveguide layer, 47
The distributed reflector 43 is composed of a zero diffraction grating 43a, which is, for example, an InP layer as a cladding layer, and a leading waveguide layer 45. Further, the second-order diffraction grating is a diffraction grating having a pitch of 80, which is determined by setting n to 2 in the following formula (1). However, n5ff is the effective refractive index of the portion where the distributed reflector 43 is located, and λ is the oscillation wavelength.

△n=(λ・n)/2 nafy... ■In the structure shown in FIG. 2, the light 51 emitted from the active region (not shown) undergoes distributed reflection with the second-order diffraction grating 43a. When entering the vessel 43, the second-order diffracted light 51a of the incident light 51 returns to the active region side, the zero-order diffracted light 51b is transmitted as is, and the first-order diffracted light 51c is directed perpendicularly to the incident light 51. In this case, a phenomenon occurs in which the light is diffracted in the direction of the InP substrate 41 side and the direction of the side on which the cladding layer 47 and the like are laminated. Therefore, a certain amount of light is emitted on the side other than the end face, and in particular, the light diffracted toward the crystalline layer side is incident on a light receiver (not shown) provided above the distributed reflector 43. become. Therefore, in the receiver, this first-order diffracted light 51
Since this first-order diffracted light is proportional to the intensity of the laser beam emitted from the active region, the intensity of the laser beam can be monitored. Further, the second-order diffracted light 51a returns to the active region, so that the distributed reflector 43
Selectivity in the reflectance occurs, and this semiconductor laser oscillates in a single longitudinal mode as in the conventional case.

Even when the diffraction grating is a diffraction grating of an order higher than the second order, diffracted light corresponding to the order of the diffraction grating is emitted on the cladding layer side, so it can be monitored with a light receiver in the same way as when a second order diffraction grating is provided. I can do it.

(Embodiments) Hereinafter, embodiments of the distributed reflection semiconductor laser of the present invention will be described with reference to the drawings. The drawings used in the following explanation are merely schematic illustrations to facilitate understanding of the present invention, and therefore, the present invention is not limited to the dimensions of each component in the drawings,
It should be understood that the invention is not limited to shape, size ratio, etc. Further, the following embodiments will be explained using examples in which the present invention is applied to the distributed reflection type semiconductor laser described using FIG. However, this is only an example, and the conductivity type may be the opposite conductivity type. Roughly speaking, it is also possible to use InGaAsP/
It is clear that the present invention can also be applied to semiconductor lasers using materials other than InP, such as GaAs1fr.

゛Layer 1 First, the structure of the distributed reflection type semiconductor laser of the embodiment will be explained. 1(A) to 1(C) are diagrams schematically showing the structure, and FIG. 1(A) is a cross-sectional view of this semiconductor laser taken along the laser stripe, and FIG. B
) is a cross-sectional view of the part where the active region 53 of this semiconductor laser is (near the area indicated by P in the figure) taken along the direction perpendicular to the laser stripe, and FIG. 1(C) is the distribution of this semiconductor laser. The part where the reflector 43 is located (near the area indicated by Q in the figure)
FIG. 3 is a cross-sectional view taken along a direction perpendicular to the laser stripe.

The schematic structure of the semiconductor laser of the embodiment shown in FIG. 1 is as follows. On an n-type InP substrate 41 as a base, an active region 53 whose region is mainly determined by an active layer 53a;
Distributed reflector 43 to which light from this active region 53 is coupled
In this case, a second-order diffraction grating 43a and a distributed reflector 43 mainly composed of an optical waveguide layer 45 are integrated,
However, the structure is such that a light receiver 55 is provided above the distributed reflector 43. Its structure will be explained in detail as follows.

An InGaAsP active layer 53a and a p-type InP low refractive index protective layer 57 are laminated in this order on a predetermined region of the n-type InP substrate 41, and a region of the substrate 41 that is in contact with the active layer 53a by 1IsI in the waveguide direction. is a second-order diffraction grating 4
3a is formed. Note that the pitch of this second-order diffraction grating 43a is approximately 4,800 people. This pitch is calculated by setting n to 2 in the following 0 formula to find the threshold △,l of the n-th diffraction grating.
The oscillation wavelength λ of the semiconductor laser is 1.55 μm, and the effective refractive index of the portion where the distributed reflector 43 is located is neff =
It was determined by setting it to 3.2 to 3.3.

Δn = (λ-n) /2 neff-...■Also, the above-mentioned p-type 1nP low refractive index #5? Yo and 2nd order diffraction grating 4
A p-type rnGaAsP optical waveguide layer 45, a zinc-doped p-type InP cladding layer 59, and a zinc-doped p-type 1nGaAsP cap layer 61 are laminated on the layer 3a in this order. Further, in a predetermined area on the area corresponding to the distributed reflector 43 of the cavity layer 61, a p-type InGaAs light absorption layer 55a for forming a light receiver 55 having a composition having a smaller energy gap than the active layer 53a is provided. , n-type I
nGaAs light absorption layer 55b and n-type electrode 55 for light receiver
c are laminated in this order. Note that the light absorption layers 55a, 55
The predetermined area where b is provided is an area that can efficiently absorb the diffracted light from the distributed reflector 43, and can be changed depending on the design, but in this case it includes the area directly above the distributed reflector 43. It is considered as an area. In addition, the light absorption layers 55a, 5
If the area of 5b is too wide, it will be easily influenced by noise, and if it is too narrow, the desired output will not be obtained, so the area of 5b should be an appropriate area taking this point into consideration.

Further, an n-type electrode 63 is provided on the lower side of the n-type InP substrate 41.
On the region of the cap layer 61 corresponding to the active layer 53a, p
A type electrode 65 is formed, and a p-type electrode 55d for the light receiver 55 is formed near the light receiver 55 on the cap layer 61, and a p-type electrode 55d is formed below each p-type electrode 55d, 65. Zinc diffusion region 67a reaching p-type InP cladding layer 59,
67b (area shown in a dotted pattern in the figure).

Moreover, the stacked body consisting of the InGaAsP active layer 53a, the I) type 1nP low refractive index layer 57, the p-type InGaAsP optical waveguide layer 45, the p-type InP cladding layer 59, the cap layer 61, and the photodetector light absorption layers 55a and 55b is Although it is a live shape, the cross section taken in the direction perpendicular to the stripe direction has an inverted mesa shape, and p-type 1nP current confinement layers are embedded in both side regions of this stripe-shaped stack. 69 and n-type 1nP low refractive index layer 71 are formed in this order.

In the distributed reflection type semiconductor laser having the structure shown in FIG. 1, when a forward bias is applied to the n-type electrode 63 and the p-type electrode 65 for the semiconductor laser, a current is injected into the active layer 53a and radiative recombination occurs. . And the light emitted here! !
Laser oscillation occurs when the light is reflected by the end face 53b on the active region side formed by g-opening and the distributed reflector 43, respectively. At this time, the distributed reflector 43 has a high reflectance only for light having a wavelength λ that satisfies the equation 0, which has already been explained.
．． 55 μm) and operates in a stable single mode. Also, the second
As already explained using the figure, since the light receiving section 55 receives the first-order diffracted light from the distributed reflector 43, a potential difference is generated between the p-type electrode 55d and the n-type electrode 55c for the light receiver. As a result, laser light can be monitored.

Next, in order to deepen the understanding of the present invention, an example of a method for manufacturing the distributed reflection semiconductor laser of the embodiment shown in FIG. 1 will be described. 3(A) to 3(J) are manufacturing process diagrams for explaining the process, and show the state of the element at the main steps in the manufacturing process. J) is a perspective view, and FIGS. 3(G) and 3(H) are side views. For convenience of explanation, these figures are shown as if the opening has already been completed, but in actual manufacturing, after a large number of semiconductor lasers are fabricated on a substrate, opening is performed to obtain individual semiconductor lasers. Please understand that this is a procedure.

First, an InGaAsP layer 5 for an active layer is formed on an n-type InP substrate 41 by a suitable crystal growth method such as liquid vapor phase epitaxy.
3y, p-type 1nP layer 57y and p-type I for low refractive index protective layer
The nGaAsP surface protective layer 72 is grown in this order (FIG. 3(A)').

Next, the SiO□ film 7 covers a predetermined area on the surface protection layer 72.
3 is formed by a conventionally known method, and then the SiO□ film 7 is formed.
The InP layer is etched with a hydrochloric acid-based etchant until the n-type 1nP substrate 41 is exposed.
The InGaAsP layer is etched using a sulfuric acid-based etchant (FIG. 3(B)).

Next, a second-order diffraction grating 43a (with a pitch of about 4800) is formed on the exposed surface of the n-type InP substrate 41 by a suitable method such as two-beam interference exposure method (FIG. 3(C)).
.

Next, after removing the 5in2 film 73, the p-type InGaAs
The P surface protection layer 71 is removed using a hydrochloric acid-based selective etchant, and then a second crystal growth process is performed to form a p-type 1nGaAsP layer 45y for the optical waveguide layer, a zinc-doped p-type InP layer 59V for the cladding layer, and a zinc-doped cap layer. p type 1
nGaAsP layer any, InGaAs for p-type light absorption layer
The layer 55x and the InGaAs layer 55y for n-type light absorption layer are grown in this order (FIG. 3(D)).

Next, a 5i02 film 751Fr covering the region corresponding to the diffraction grating 43a of the InGaAs layer 55y for the n-type light absorption layer is formed by a conventionally known method, and then a sulfuric acid-based etchant is used to form the n-type and p-type light absorption layers. Each semiconductor layer 55y.

The exposed portion of the 55x SiO2 film 75 is etched (FIG. 3(E)).

Next, the SiO□ film 75 is removed and restored by a conventionally known method so that the stripe direction is <011> across the p-type InGaAsP layer 61y for the cap layer and the InGaAs layer 55y for the n-type light absorption layer. Width W is 3~4um
A striped SiO□ film 77 is newly formed. Then, this striped SiO□ film 77 is used as an etching mask, and the portion exposed from this SiOz film 77 is etched with n-type In
Etching is performed so that the cross-sectional shape becomes an inverted mesa up to the P substrate 41. As a result, InGaAsP active layer 53a
, O-type InP low refractive index protective layer 57, p-type InGaA
An sP optical waveguide layer 45, a zinc-doped InP cladding layer 59, a zinc-doped p-type InGaAsP cap layer 61, and a distributed reflector 43 are obtained (FIG. 3(F)).

Next, with the 5i02 film 778 still attached, the n-type In
In the third crystal growth step, a p-type InP current confinement layer 69 and an n-type InP current confinement layer 71 are grown in this order on both sides of the reverse mesa stack of the P substrate 41. Figure 3 (G) is
A side view of the semiconductor laser intermediate after the growth of each current confinement layer is shown in the direction shown in FIG. 3(F), and FIG. It is a side view seen from the direction shown by R in F).

Next, the SiO□ film 77 is removed by a conventionally known method, and then the semiconductor laser p-type electrode 65 and the light receiver p-type electrode 55d of the p-type InGaAsP cap layer 61, which were explained using FIG. 1, are removed. The planned area to be formed (in this case, the third
From this surface, p-type In
Zinc is diffused so as to reach the P cladding layer to form zinc diffusion regions 67a and 67b (FIG. 3(I)).

Next, an n-type electrode 63 and a p-type electrode are placed on the lower side of the n-type InP substrate 41.
A p-type electrode 65 for a semiconductor laser and a p-type electrode 55d for a light receiver are provided on a predetermined region of the type InGaAsP cap layer 61.
Each is formed by a conventionally known method (FIG. 3(J)).

By the above procedure, the distributed reflection semiconductor laser of the embodiment shown in FIG. 1 can be obtained. By the way, regarding the size of the semiconductor laser in this example, in this example, Fig. 3 (J
) to W. The total width represented by is 300 to 4 oum, the length of the active region is 200 to 400 L1 m, and the length of the distributed reflector is 200 to 400 um, which is represented by β2. However, it is clear that these values are not limited to these and can be changed depending on the design.

Although this invention was made after the distributed reflection type semiconductor laser, the structure of the active region does not matter. A similar effect can be expected even with a semiconductor laser integrated with a distributed reflector equipped with a diffraction grating or a higher-order diffraction grating.

(Effects of the Invention) As is clear from the above description, the distributed reflection semiconductor laser of the present invention has a distributed reflector equipped with a second-order diffraction grating or a higher-order diffraction grating. Therefore, the diffracted light of the laser beam also comes out in the lamination direction in which the active layer, cladding layer, etc. are laminated. Since the structure has a light receiver built in the direction in which such diffracted light comes out, it is easy to perform APC in which the semiconductor laser and light receiver are integrated. There is no need for such troublesome optical axis alignment, and there is no need for any trouble caused by returning light.In addition, the laser oscillation itself is performed on the same principle as conventional distributed reflection semiconductor lasers, so only a single longitudinal mode is generated. oscillates stably.

Furthermore, when manufacturing a distributed reflection type semiconductor laser, a large number of semiconductor lasers are generally fabricated on a substrate.

Therefore, even before the folds, active regions and distributed reflectors are arranged alternately, so when electricity is applied to the electrodes of a particular chip, the active region uses the distributed reflectors on both sides as reflecting mirrors to emit a laser beam. Oscillation occurs. Therefore, since the diffracted light is incident on the light receiver of the chip, the chip can be inspected using this diffracted light.

Therefore, it is also possible to sort chips before opening.

[Brief explanation of drawings]

FIGS. 1(A) to (C) are diagrams for explaining the structure of a distributed reflection semiconductor laser according to an embodiment. FIG. 2 is a diagram showing details of the present invention in the case where the distributed reflector is provided with a second-order diffraction grating. FIGS. 3(A) to 3(J) are process diagrams showing an example of a method for manufacturing a distributed reflection semiconductor laser according to an embodiment, and FIGS. 4(A) to (J) are diagrams for explaining the process.
C) is a diagram providing a first explanation of the structure of a conventional distributed reflection type semiconductor laser. 41... Base (n-type InP substrate) 43-... Distributed reflector 43a... Second-order diffraction grating or higher-order diffraction grating 45-... Guide waveguide layer (p-type InGaAsP layer) 45y
... p-type InGaAsP layer for optical waveguide layer 51 ... incident light, 51a ... second-order diffracted light 51b ...
・0th order diffracted light, 51c...1st order diffracted light 53...
・Active region 53a''-active layer CInGaAsP layer) 53y...
- InGaAsP layer 55 for active layer...light receiver 55a''-light absorption layer (p-type 1nGaAs layer) 55b-
...Light absorption layer (n-type InGaAs layer) 55c...p-type electrode for light receiver 55d...n-type electrode for light receiver 55X =p-type InGaAs layer for light absorption layer 55y...
- N-type InGaAs layer for light absorption layer 57...Low refractive index protective layer (p-type 1nP layer) 57y =p-type InP layer for low refractive index protective layer 59...Clad layer (p-type 1nP layer) 59y
...P-type InP layer for cladding layer 61--Kit layer (p-type 1nGaAsP layer) 61y...Cap layer side p
Type InGaAsP layer 63...n-type electrode for semiconductor laser 65...p-type electrode for semiconductor laser 6? a, 67b = sub-18 diffusion region 69...p-type current confinement layer, 71...n-type current confinement layer 72...surface protection layer (p-type 1nGaAsP layer)
73, 75...5IO2 film 77-...5i02 film in stripe shape. Patent applicant: Oki Electric Industry Co., Ltd. 41... Base (n-type InP substrate) 43... Distributed reflector 43a... Second-order diffraction grating or higher-order diffraction grating 45-- Guide waveguide layer (p-type 1nGaAsP layer) 53.
...Active region 53a - active layer (InGaAsP layer) 55... Light receiver 55a -=light absorption layer (p-type InGaAsP layer) 55b
---Light absorption layer (n-type rnGaAsP layer) 55c...
P-type electrode for light receiver 55d...N-type electrode for light receiver 57...Low refractive index protective layer (p-type InP layer) 59...
Cladding layer (p-type InP layer) 61...Key p-tube layer (
p-type InGaAsP layer) 63...n-type electrode for semiconductor laser 65...p-type electrode for semiconductor laser 6h/7b...zinc diffusion region 69...p-type current confinement layer 71...n-type current Narrowing layer 51: Incident light 51a: 2nd order diffracted light 51b: 0th order diffracted light 1c 1st order diffracted light Figure 2 for detailed explanation of this invention 1 <O1+> Steps showing an example of the manufacturing method Figure 3: A process diagram showing an example of the manufacturing method Figure 3: A process diagram showing an example of the manufacturing method

Claims (1)

[Claims] (1) In a distributed reflection semiconductor laser that includes an active region on the side of a substrate and a distributed reflector to which light from the active region is coupled, the distributed reflector is a second-order diffraction grating or a higher-order diffraction grating. 1. A distributed reflection semiconductor laser comprising a diffraction grating, and a light receiver above the distributed reflector for receiving diffracted light from the distributed reflector.