JPH03136388A - Semiconductor optical amplifier - Google Patents

Abstract

PURPOSE:To provide low reflectivity at input and output end faces and to efficiently couple an input and an output to an exterior by employing as a semiconductor substrate in which various semiconductor layers are laminated, an oblique substrate having a substrate surface orientation inclined with respect to the crystal axis direction of the semiconductor substrate. CONSTITUTION:A GaAs oblique substrate 1 having a surface orientation 20 inclined at an angle theta from (x-axis) 21 of a plane orientation [100] to (y-axis) direction of a plane orientation [010] is provided. A first clad layer 17 of n-type AlGaAs, an optical waveguide 2 used also as an active layer of GaAs and a second clad layer 15 of a p-type AlGaAs are sequentially laminated on the substrate 1 by a MBE method. Thereafter, the layer 15 is formed in a ridge state by an etching process using a photolithography method, then insulating layers of SiNx are provided at both sides of the ridge, a cap layer 14 of a p<+> type AlGaAs is provided on the layer 15, and electrodes 12, 18 of Au are attached to the upper and lower surfaces.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a traveling wave semiconductor optical amplifier having a semiconductor laser structure and having input and output end faces with a non-reflection structure.

[Prior Art] Conventionally, traveling wave type semiconductor optical amplifiers have been manufactured by using a semiconductor laser structure and applying an AR coating to input and output end faces. By lowering the reflectance of the AR coat, gain ripples can be reduced and high gain can be obtained.

Let Gs be the single-pass gain of the optical amplifier.

Considering that a gain ripple of 2 dB is allowed, the conditions for the reflectance R, R, are a, "Kτr, < o, 1 given by 0. For example, single-pass gain G, = 30 d
If B is required, the required AR coat reflectance is “1”.
τI"';-R<0.01%.

To realize such an extremely low reflectance AR coating, the film thickness should be set to λ.
/4 condition, and it is essential to set the refractive index to a desired value. Conventionally, such extremely low reflectance AR
To prepare the coat film, a film of Sin, etc. was formed by electron beam evaporation. The refractive index of the AR coating film can be controlled by controlling the gas atmosphere during electron beam evaporation (for example,
Regarding film thickness control, methods include monitoring the optical film thickness in real time, and monitoring efficiency changes and threshold changes in real time by depositing it on the end face of an actual semiconductor laser. It is done using.

In order to construct such an AR coat film with extremely low reflectance, the setting conditions for both the refractive index film thickness and the thickness are extremely strict and the tolerance thereof is small. Therefore, there are disadvantages in that the yield of device fabrication is significantly reduced and the variation is large.

Furthermore, if the AR coating film with extremely low reflectivity is composed of a simple single-layer film, its wavelength band will be relatively narrow, so it will satisfy the desired performance (gain and gain ripple) of the traveling wave optical amplifier. It has the disadvantage of being limited in the wavelength range that can be detected.

Several methods have been proposed to alleviate such severe AR coating conditions. For example, there is a method of providing a window structure near the end face to reduce the effective reflectance.

FIG. 6 shows a device cross-sectional view of a semiconductor optical amplifier having an AR/window end face structure, and is shown in the following literature.

Automobiles, etc.: “Traveling wave semiconductor optical amplifier with window edge structure”
IEICE Optical and Quantum Electronics Study Group Material 0QE89-17 (1989) In the figure, 1α0 is lnP
In the substrate, 101 is an optical waveguide section including an active layer, 102 is a window structure section, and 103 and 104 are AR coats.

In the window structure section 102, light waves are not confined and, on the contrary, diffraction spread occurs, so that the coupling efficiency of reflected light due to the light emitted from the optical waveguide section 101 returning to the optical waveguide section 101 again is reduced. It has been reported that in this configuration, a traveling wave semiconductor optical amplifier was fabricated in which the effective reflectance was reduced to 0.003% under relatively easy coating conditions of about 2% AR coating.

However, in such a window structure,
Since the beam waist position is located inside the laser, away from the input and output end faces, there is a problem in that the coupling efficiency is significantly reduced when butt coupling is performed from the end face with an external object, such as input/output coupling using a spherical fiber. be.

Another conventional technique is to form the cleavage surface obliquely with respect to the waveguide.

FIG. 7 is a top view showing the structure of the second conventional example. In FIG. 7, 110 is a laser stripe structure, and 111 and 112 are AR coats.

The light waves emitted from the stripe structure 110 are as follows.

Since the light is reflected at the oblique end face and the wavefront is inclined twice as much as the inclination angle of the oblique end face, the reflection coupling efficiency is greatly reduced. The rate of reduction is a function of the waveguide distribution width and tilt angle. In particular, the following literature reports a semiconductor optical amplifier configured so that the tilt angle satisfies the Brewster condition with the aim of achieving no reflection in the TE moat.

(J.T., Chang et at, “Trav
eling-waveBrewster-angle
d 5tripe InGaAgP LaserAmp
litier at 1. :lpm”J, Modern
0pTics: 15°3, pp355~:164
(1988),) In this method, the surface orientation of the substrate is orthogonal to the surface orientation of the deep end face, and the stripe structure, which is a waveguide, is formed so that its propagation direction is not perpendicular to the deep end face. ing.

[Problems to be Solved by the Invention] Among the conventional semiconductor optical amplifiers described above, the one shown in FIG. 6 has a drawback that the coupling efficiency when performing input/output coupling with an external device is significantly reduced. In the structure shown in FIG. 7, the plane orientation of the substrate is perpendicular to the plane orientation of the narrow end face, and the stripe structure is formed such that its propagation direction is not perpendicular to the narrow end face. Conventionally, in many of the processes for manufacturing semiconductor lasers, etc., they are formed perpendicularly or parallel to the cleavage plane, but when forming a stripe structure so that they are not perpendicular to each other, a manufacturing process different from the conventional one is required. The disadvantage is that the manufacturing process becomes complicated.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has low reflectance at the input/output end faces, can efficiently perform input/output coupling with external objects, and requires no special manufacturing process. The purpose of the present invention is to provide a semiconductor optical amplifier that does not

[Means for Solving the Problems] A semiconductor optical amplifier of the present invention is a semiconductor optical amplifier having a semiconductor laser structure, in which a semiconductor substrate on which various semiconductor layers are laminated is tilted with respect to the crystal axis plane direction of the semiconductor substrate. A tilted substrate with a substrate surface orientation is used.

In this case, AR:
ff-t may be applied.

Furthermore, a reflective end face formed by etching may be provided between the input and output end faces.

[Operation] Since the substrate is an inclined substrate whose surface orientation is inclined with respect to the crystal axis plane direction, the stacking direction of various semiconductor layers epitaxially grown on the inclined substrate is also not perpendicular to the crystal axis plane direction. Therefore, when the substrate is cleaved, the cleaved end face has an angle with respect to the stacking direction of various semiconductor layers including the optical waveguide (active layer), similar to the conventional one shown in Figure 7. The AR effect becomes large.

In the case shown in FIG. 7, the epitaxially grown film has an inclination angle (horizontal inclination angle) in the plane, whereas in the case of the present invention, the inclination angle (horizontal inclination angle) is set in the plane perpendicular to the epitaxial film.
This is achieved by a normal process except for the use of a tilted substrate.

[Example] Fig. 1 is a side view showing the configuration of the first embodiment of the present invention, Fig. 2 is a top view of the first embodiment, and Fig. 3 is a diagram showing the ridge portion of the first embodiment. FIG. 3 is a diagram showing a cross-sectional structure.

In this embodiment, instead of the commonly used GaAs substrate having a [100] plane orientation, a substrate having a plane orientation shifted from the [100] plane direction is used as the substrate on which growth is performed.

In FIG. 1, reference numeral 1 denotes a GaAs tilted substrate whose substrate surface orientation 20 is inclined from the [100] surface orientation (X-axis) 21 toward the [01G] surface orientation (y-axis) by an angle θ. Such a substrate can be easily obtained by cutting out a crystal and polishing it.

In this embodiment, as shown in FIG. 3, a first cladding layer 17 of n-AlGaAs, a Ga+A
s, an optical waveguide 2 which also serves as an active layer, and a p-AlGaA
After sequentially depositing the second cladding layer 15 using the MBE method, the second cladding layer 15 is processed into a ridge shape by an etching process using the photolithography method, and then 5iN is deposited on both sides of the ridge. , l are provided, and po-AlGa is provided on the second cladding layer 15.
After providing the cap layer 14 made of As, the upper and lower surfaces are
The electrode 12.18, which is U, is attached.

In this way, a semiconductor optical amplifier equipped with the ridge waveguide section 5 as shown in FIG. 2 was formed.

In this embodiment, a laser resonator is fabricated by cleavage in the same way as in a normal semiconductor laser. At this time, since the plane orientation of the inclined substrate 1 is shifted by θ from the [100] plane orientation, the cleavage end face where the (010) plane appears is inclined by θ with respect to the substrate plane orientation. Furthermore, by electrobeam (EB) evaporation, Sin, λ is applied to the input and output end faces.
AR coatings 3.4 are formed by depositing a thickness corresponding to /4, respectively, to complete the AR structure.

In this embodiment, the propagation direction of the input and output light waves is tilted in the vertical direction due to refraction.

The wavefront normal of the reflected light is tilted by 20, the phase shift of the overlap integral becomes large, and the coupling efficiency decreases, so the effective reflectance decreases significantly, making it impossible to realize a high-performance traveling wave semiconductor optical amplifier with little gain ripple. Ta.

The reduction coefficient ξ of the reflectance R is assumed to be ξ=R/RO, assuming that the light intensity distribution of the light wave in the layer direction (X direction) has a Gaussian distribution and the I/e2 distribution width is 2W.

Here, β is the propagation constant of the waveguide, where λ is the wavelength in vacuum and neff is the equipolarized refractive index, β = (2π/λ)・n
It is expressed as eff. Further, R is the reflectance in the case of normal incidence (θ=0). - As an example, 2w=2μm, λ=
0.83μm, neff=3.2, ξ=1/
Determining the tilt angle θ such that e”=0.135 is as follows. Therefore, in the parameters of a normal 0.83 μm laser structure, by using a tilted substrate tilted several degrees, the reflectance can be reduced by about one order of magnitude. This is possible and extremely effective for forming an AR structure.

Figure 4 shows the angle of inclination of the substrate and the reflectance reduction coefficient L=R/RO
FIG.

As shown in the figure, the reflectance reduction coefficient ξ is determined depending on the degree of inclination formed and the wavelength used.

FIG. 5 is a diagram showing the configuration of a second embodiment of the present invention.

This embodiment is a superluminescent diode by forming an etched mirror surface 33 on the structure shown in the first embodiment. Since the other configurations are the same as those of the first embodiment, the same reference numerals as in FIG. 1 are given, and the explanation will be omitted.

A rectangular shape hollowed out until reaching the inclined substrate 1
is formed using dry etching, and its wall surface is an etched mirror surface 33.

The device of this embodiment operates in the LED mode by injecting current between each electrode. In this case, by determining the upper limit of the reflectance of the front AR coat 4 in consideration of the optical output intensity and gain, it becomes possible to stably operate the device to emit light without causing laser oscillation. In addition, if necessary, the reflectance on the etched mirror surface 33 can be adjusted based on the inclination of the mirror surface (horizontal and vertical directions).
It is also possible to perform control using this method.

In each of the embodiments described above, A
Although the R coat is provided on all of the surfaces, if high AR efficiency is not required, it is of course possible to use the narrow end surfaces as they are.

[Effects of the Invention] Since the present invention is configured as described above, it has the following effects.

In the first aspect, the AR effect is enhanced by forming a laser structure using a tilted substrate having a substrate surface orientation that is tilted from a normal surface orientation, and using the cleavage surface as an input/output end surface. It is possible to realize a semiconductor optical amplifier with extremely low reflection, and it is possible to obtain a high-performance traveling wave optical amplifier with little gain ripple and high gain.

In addition, the only change in this manufacturing process is that the substrate itself is a tilted substrate, and the other processes can be manufactured in the same way as normal devices, so there are advantages such as no complications during device manufacturing. There is.

In the thing according to claim 2, the AR in the above effect
There are effects that can further enhance the effect.

According to the third aspect of the present invention, there is an effect that a superluminescent diode having the above-mentioned effect can be constructed.

[Brief explanation of the drawing]

FIG. 1 is a side view showing the configuration of the first embodiment of the present invention, FIG. 2 is a top view of the first embodiment, and FIG. 3 is a diagram showing the cross-sectional structure of the ridge portion of the first embodiment. , FIG. 4 is a diagram showing the relationship between the tilt angle of the substrate and the reflectance reduction coefficient in the first embodiment, FIG. 5 is a diagram showing the configuration of the second embodiment of the present invention, and FIG.
FIG. 7 and FIG. 7 are diagrams each showing the configuration of a conventional example. DESCRIPTION OF SYMBOLS 1... Inclined substrate, 2... Optical waveguide, 3.4-AR coat, 5... Ridge waveguide part, 12.18-- Insulation layer 4i1 13-... Insulating layer, 14-... Cap layer, 15--Second cladding layer, 17-Axis first. cladding layer.

Claims (1)

[Claims] 1. In a semiconductor optical amplifier having a semiconductor laser structure, the semiconductor substrate on which various semiconductor layers are laminated is an inclined substrate having a substrate plane orientation that is inclined with respect to the crystal axis plane orientation of the semiconductor substrate. A semiconductor optical amplifier characterized by the following. 2. The semiconductor optical amplifier according to claim 1, wherein an AR coating is applied to an end face of the substrate that serves as an input/output end face. 3. The semiconductor optical amplifier according to claim 1 or 2, further comprising a reflective end face formed by etching between the input and output end faces.