Semiconductor optical amplifying element

A semiconductor optical amplifying element is disclosed which has a semiconductor multilayer structure including at least a first semiconductor layer for providing an optical gain in response to the injection of carriers thereinto, and a p-side electrode and an n-side electrode for the carrier injection. A first reflecting surface and a second reflecting surface are disposed thickwisely of the semiconductor multilayer structure and opposite to each other thereacross. The element is designed so that light incident thereon from the thickwise direction of the semiconductor multilayer structure is amplified by propagating through the element perpendicularly to the thickwise direction of the semiconductor multilayer structure while being multiple-reflected between the first reflecting surface and the second reflecting surface.

BACKGROUND OF THE INVENTION 
The present invention relates to a semiconductor optical amplifying element 
for use in fiber optic communication. 
Since a semiconductor optical amplifier has a high internal gain and a 
small size and is highly reliable, much study is being given it for use 
not only as a direct optical amplifying element of a nonregenerative 
repeating system in an intensity modulation-direct detection (IM-DD) 
system for fiber optic transmission and in a coherent transmission system 
but also as a pre-amplifier of a photodetector and a booster amplifier in 
an optical circuit. From the viewpoint of enhancement of system 
characteristics, the realization of a semiconductor optical amplifying 
element is highly desired which is excellent in terms of high saturation 
output, high gain, low noise, wide bandwidth and low polarization 
dependence. In this case, incident light, even if initially linearly 
polarized, generally becomes elliptically polarized after its long 
distance propagation through a fiber, and besides, the elliptically 
polarized wave fluctuates with the lapse of time. On this account, the 
intensity of output light amplified by the semiconductor optical 
amplifying element undergoes irregular variations, and hence no fixed gain 
can be obtained. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
semiconductor optical amplifying element which is extremely low in its 
dependence on the plane of polarization of incident light. 
The present invention has its feature in that light is incident on the 
element from the direction of the plane of the active layer, not on the 
end face of the element and along the active layer as in the prior art 
example, and the incident light is subjected to multiple reflection 
between a pair of reflecting surfaces provided opposite to each other 
across the active layer. 
In accordance with the present invention, there is provided a semiconductor 
optical amplifying element which has a semiconductor multilayer structure 
including at least a first semiconductor layer for providing an optical 
gain in response to the injection of carriers thereinto, and a p-side 
electrode and an n-side electrode for the carrier injection, characterised 
in: 
that a first reflecting surface and a second reflecting surface are 
disposed thickwisely of the semiconductor multilayer structure and 
opposite to each other thereacross; and 
that the element is designed so that light incident thereon from the 
thickwise direction of the semiconductor multilayer structure is amplified 
by propagating through the element perpendicularly to the thickwise 
direction of the semiconductor multilayer structure while being 
multiple-reflected between the first reflecting surface and the second 
reflecting surface.

DETAILED DESCRIPTION 
To make differences between prior art and the present invention clear, 
prior art will first be described. 
In FIG. 6 there is shown, by way of example, a known typical semiconductor 
optical amplifying element and an arrangement for actuating it. This will 
be described on the assumption that the element has a traveling wave type 
structure and is formed using InGaAsP for a 1 .mu.m wavelength band. 
Reference numeral 101 indicates an n-InP substrate, 102 an InGaAsP active 
layer, 103 a p-InP clad layer, 104 a p-InGaAsP cap layer, 105 and 106 a 
p-side electrode and an n-side electrode, and 107 and 107' anti-reflection 
(AR) films. Incident light enters the element from the one end face 
thereof and is amplified by controlling a voltage across the p-side 
electrode 105 and the n-side electrode 106 within the range in which it 
does not reach an oscillation threshold value, and the thus amplified 
light is emitted from the other end face. 
With such a conventional optical amplifying element as mentioned above, its 
gain varies several dBs depending on whether the incident light is 
polarized in the direction of the longer or shorter side of the cross 
section of the active layer 102, because the active layer 102 has a 
rectangular cross section usually about 0.1 .mu.m thick and 1 .mu.m wide 
and because the AR films 107 and 107', even if having minimum reflectivity 
obtainable with presently available manufacturing techniques, provide a 
residual reflection of 1% or so. Furthermore, incident light, even if 
initially linearly polarized, generally becomes elliptically polarized 
after its long distance propagation through a fiber, and besides, the 
elliptically polarized wave fluctuates with the lapse of time. On this 
account, the intensity of output light amplified by the semiconductor 
optical amplifying element undergoes irregular variations, and hence no 
fixed gain can be obtained. 
On the other hand, a multiple quantum well (MQW) structure, in which are 
laminated active layers 102 each having a thickness of 200 .ANG. or 
thinner, that is, smaller than the de Broglie wavelength of electrons, has 
a large state density, and consequently, as compared with an ordinary 
structure having a single thick active layer, the MQW structure provides a 
higher gain for the same injected carrier density; conversely speaking, 
this structure requires a lower injected carrier density for obtaining the 
same gain. It is therefore expected that the application of the MQW 
structure to the semiconductor optical amplifying element will provide 
high gain, low noise characteristics. However, the gain coefficient of the 
MQW structure significantly depends on the direction of polarization of 
light, and when the incident light is polarized perpendicularly to the 
quantum well film, the above-mentioned high gain characteristic is 
impaired, and besides, the output light intensity also varies irregularly 
owing to the afore-mentioned fluctuation in the polarization of the 
incident light. 
Thus there has been a strong demand for the materialization of a 
semiconductor optical amplifying element whose amplification factor is 
always constant independently of the polarization characteristic of 
incident light, but such a semiconductor element has not been proposed so 
far. 
The present invention effectively eliminable of the above defects of prior 
art will hereinafter be described in detail with respect to its 
embodiments. 
(EMBODIMENT 1) 
FIG. 1 illustrates a first embodiment of the present invention, FIG. 1A 
being a schematic diagram of the semiconductor optical amplifying element 
according to the present invention and FIG. 1B a sectional view taken on 
the line 1B--1B in FIG. 1A. This embodiment is formed using InGaAsP. 
Reference numeral 1 indicates an n-InP substrate, 2 an n-InGaAsP layer, 3 
an n-InP clad layer, 4 an InGaAsP active layer (a first semiconductor 
layer), 5 a p-InP clad layer, 6 a p-InGaAsP cap layer, and 7 and 8 a 
p-side electrode and an n-side electrode, respectively. The portion of the 
n-side electrode 8, indicated by 9, is made flat by selective etching, and 
the metallic films of the p-side electrode 7 and the flat portion 9 form a 
pair of highly reflective surfaces. Reference numerals 10 and 10' 
designate anti-reflection films. The respective layers are formed by an 
epitaxial growth method such as a liquid phase epitaxial (LPE) growth 
method, vapor phase epitaxial (VPE or MO-CVD) growth method, or molecular 
beam epitaxial (MBE) growth method. In FIG. 1B, incident light P.sub.in 
enters the element at an angle .theta..sub.0 thereto through the region 
covered by the left-hand anti-reflection film 10 and the incident light is 
repeatedly reflected in zigzag between the highly reflective surfaces 7 
and 9 which also act as the electrodes, as shown. At the same time, for 
each reflection the incident light P.sub.in passes through the active 
layer 4 which has a gain based on the injection of carriers thereinto, and 
as a result of this, the incident light is amplified and is then emitted 
as output light P.sub.out from the region covered by the right-hand 
anti-reflection film 10'. In this instance, since the propagating light 
passes through the active layer 4 substantially perpendicularly to its 
surface (in its thickwise direction) when the angle of incidence 
.theta..sub. 0 is selected to be small to some extent, the amplification 
factor of the element can be made almost free from its dependence upon the 
plane of polarization of the incident light. 
Now, an example of the amplification characteristic will be described in 
conjunction with the schematic diagram shown in FIG. 2. Reference numerals 
11 and 13 indicate clad layers, 12 an active layer, and 14 and 15 highly 
reflective films. For the sake of brevity, the anti-reflection films 10 
and 10' shown in FIG. 1 are omitted. Letting the incident angle of the 
incident light P.sub.in be represented by .theta..sub.0, the angle of 
travel .theta. of the light in the element, with respect to the normal to 
the element surface, is given by Snell's Law as follows: 
EQU sin .theta.=n.sub.0 /n.sub.1 sin .theta..sub.0 (1) 
where n.sub.0 and n.sub.1 are the reflective indices of air and the clad 
layer 13. In this case, the distance x.sub.0 over which the light crosses 
the active layer 12 of a thickness d is as follows: 
EQU x.sub.0 =d/cos .theta. (2) 
Assuming that the incident light P.sub.in has been reflected m times prior 
to its emission from the element of a length L, the total distance x.sub.t 
of the passage of light across the active layer 12 is as follows: 
##EQU1## 
where t is the distance between the both reflecting surfaces 14 and 15. 
For example, in a case where .theta..sub.0 =34.degree. 
(.theta.=10.degree.), L=1 mm, d=0.5 .mu.m, and t=10 .mu.m, then x.sub.t 
=288 .mu.m. In this case, a maximum gain coefficient g.sub.max obtainable 
in the active layer 12 is a gain coefficient g.sub.th until an oscillation 
perpendicular to the both reflecting surfaces 14 and 15 occurs, and this 
is given by the following equation: 
##EQU2## 
In the above, .alpha..sub.c is the absorption loss in the clad layer, and 
R.sub.1 and R.sub.2 are given in terms of the reflectivity of the 
reflecting surfaces 14 and 15, respectively. For instance, with R.sub.1 
=R.sub.2 =1 and .alpha..sub.c =20 cm.sup.-1, then g.sub.max =380 
cm.sup.-1. Consequently, such as large value as follows can be obtained as 
the amplification degree G of the element: 
##EQU3## 
Incidentally, this element has no resonators on its incidence and emission 
end faces and, in this sense, has a substantially ideal traveling type 
structure, and consequently, the anti-reflection films 10 and 10' are not 
essentially requisite. In this embodiment the anti-reflection films are 
used to prevent an increase of an unnecessary insertion loss. As will be 
seen from Eq. (5), the amplification factor G of the element also depends 
upon the total distance x.sub.t of the passage of light across the active 
layer 12. Hence, the total distance x.sub.t can be equivalently increased 
simply by reducing the incident angle .theta..sub.0 (the angle of travel 
.theta.) as referred to previously, or increasing the length L of the 
element. 
(EMBODIMENT 2) 
While the embodiment of FIG. 1 employs, as the first and second reflecting 
surfaces, the metallic films (7, 9) which also serve as electrodes, this 
embodiment, shown in FIG. 3, contemplates further reduction of loss 
through utilization of distributed Bragg reflectors which have periodic 
multilayer structures of different refractive indices. In FIG. 3 reference 
numeral 21 indicates an n-InP substrate, 22 a distributed Bragg reflector 
formed by a periodic multilayer structure comprising n-InGaAsP layers 23 
and n-InP layers 24, 25 an n-InP clad layer, 26 an InGaAsP active layer, 
27 a p-InP clad layer, 28 a distributed Bragg reflector made up of 
p-InGaAsP layers 29 and p-InP layers 30, 31 a p-InP layer, 32 a p-InGaAsP 
cap layer, 33 and 34 a p-side electrode and an n-side electrode, and 35 
and 35' anti-reflection films. The InGaAsP layers 23 and 29 in the 
distributed Bragg reflectors 22 and 28 become transparent to incident 
light by increasing the energy gaps of the layers 23 and 29 as compared 
with the energy gaps of the InGaAsP active layer 26. Further, a high 
reflecting can be obtained by optimizing the period of lamination of the 
layers into each distributed Bragg reflector structure. The other 
principles of operation are the same as those of the embodiment of FIG. 1. 
(EMBODIMENT 3) 
FIG. 4 illustrates a third embodiment of the present invention, FIG. 4A 
being a schematic diagram of the semiconductor optical amplifying element 
according to the present invention and FIG. 4B a sectional view taken on 
the line 4B--4B in FIG. 4A. 
The embodiment of FIG. 4 utilizes, for injecting carriers into the active 
layer, what is called a Transverse Junction Stripe (TJS) structure, 
instead of using the structure shown in FIGS. 1, 2 and 3 in which a pn 
junction is formed in the interface of the active layer and the clad layer 
for injecting carriers into the active layer. This embodiment employs the 
multiple quantum well (MQW) structure for the active layer (a first 
semiconductor layer), a dielectric multilayer film of high reflectivity 
for the one reflecting surface, and the afore-mentioned distributed Bragg 
reflector for the other reflecting surface. 
Reference numeral 41 indicates a semi-insulating (SI) InP substrate, 42 a 
distributed Bragg reflector made up of non-doped or SI-InGaAsP layers 43 
and InP layers 44, 45 an active layer of a multiple quantum well (MQW) 
structure made up of InGaAsP well layers 46 and InP barrier layers 47 and 
having a thickness (&lt;200 .ANG.) less than the de Broglie wavelength of 
electrons (which active layer will hereinafter be referred to as the "MQW" 
layer), 48 an n-InP layer, 49 an n-InGaAsP cap layer, 50 and 51 a p-side 
electrode and an n-side electrode, respectively, 52 a dielectric 
multilayer film of high reflectivity, 53 and 53' anti-reflection films, 54 
a Zn diffused region, and 55 a region in which the MQW layer 45 has been 
alloyed with Zn diffused. Since carriers are injected into the MQW layer 
45 from the Zn diffused region, a gain region of a width substantially 
equal to the diffusion length of the carriers (several microns) is 
provided along the diffusion front. The gain region has a smaller energy 
gap than those of the adjoining regions, and consequently, by a suitable 
selection of the wavelength of incident light, its loss in the MQW layer 
45 is reduced, and at the same time, a waveguiding action is created, by 
which the incident light is effectively confined and is amplified. 
Further, the use of the MQW layer as the active layer provides a high 
gain. This wafer can be manufactured by the MBE, MO-CVD or similar growth 
method which exhibit low growth rate and consequently has an excellent 
uniform wafer growth capability. 
(EMBODIMENT 4) 
The present invention is essentially almost free from mixing of 
spontaneously emitted light, because the incidence and emission of light 
take place in the direction of its top surface. Nonetheless, an optical 
filter may also be provided in the optical path for further reduction of 
noise and for lessening the influence of degradation of the gain of an 
optical amplifying element of another stage. In FIG. 5, reference numeral 
61 indicates an n-InP substrate, 62 a distributed Bragg reflector made up 
of n-InGaAsP layers 63 and n-InP layers 64, 65 an n-InP clad layer, 66 and 
70 InGaAsP carrier confining layers, 67 an MQW layer comprised of InGaAs 
well layers and InGaAsP barrier layers 69, 71 a p-InP clad layer, 72 a 
p-InGaAsP cap layer, 73 a metallic film which is used both as a p-side 
electrode and as a reflecting surface, 74 an n-side electrode, 75 and 75' 
anti-reflection films, 76 and 76' band-pass type optical filters, each 
composed of a dielectric multilayer film, 77 and 77' rod lenses, 78 and 
78' optical fibers. According to this embodiment, since the optical 
filters 75 and 75' can be formed by sputtering or a similar process on the 
wafer surface, the manufacturing process is very simple and optical filter 
films of excellent reproducibility can be fabricated. Besides, since an 
optical axis alignment for incident light and emitted light can be 
achieved by mounting the rod lenses 77 and 77' on the wafer surface, high 
coupling of light can easily be provided. 
Although in any of the above-described embodiments light is entered into 
and emitted from the element through the wafer surface, the same results 
as mentioned above could be obtained even in the cases where light is 
entered into and emitted from the side of the substrate, where light is 
incident on the wafer surface but is emitted from the substrate side, and 
where light is incident on the substrate surface but is emitted from the 
wafer surface. While no particular reference has been made to a method for 
suppressing a transverse oscillation along the active layer, the 
oscillation can be prevented by depositing an anti-reflection films on 
either end face, forming the end faces aslant, or adding a non-exciting 
region. Incidentally, the embodiments have been described to be formed of 
the InGaAsP/InP material, but the invention can also be implemented using 
other materials such as InGaAlAs, AlGaAs and AlGaAsSb series. 
As described above, according to the present invention, a semiconductor 
multilayer structure including at least an active layer and a pair of 
reflecting surfaces (reflectors) opposite to each other across the 
multilayer structure are provided and light is entered from a window made 
in one of the reflecting surfaces (reflectors) so that the light is 
multiple-reflected (substantially perpendicularly) in the direction 
thickwise of the active layer. By this, it is possible to obtain a 
semiconductor amplifying element which is extremely low in the dependence 
upon the plane of polarization of the incident light and is substantially 
constant in its amplification degree. 
Further, the mixture of spontaneously emitted light is decreased by 
providing band-pass type optical filters in the interface for entering 
incident light into the element and in the interface for emitting output 
light from the element. 
Moreover, it is possible to freely select, as the first and second 
reflecting surfaces, economically advantageous metallic films (7, 9, 73), 
which can also be used as electrodes, or semiconductor surfaces, 
semiconductor multilayer films (22, 28, 42, 62) which affords reduction of 
loss, or dielectric multilayer films (52) of high reflectivity. 
A high gain can be obtained by using the MQW layer as the active layer 
which is the first semiconductor layer. The use of the TJS structure for 
injecting carriers into the active layer permits effective carrier 
injection even if the active layer is formed thick. Thus, the element of 
the present invention is very promising as a direct optical amplifying 
element in the IM-DD and the coherent transmission system, and hence it is 
of great utility when put to practical use.