Injection laser with an inverted waveguiding rib

A transverse mode stabilized injection laser with a planar active layer is provided with a transverse waveguiding effect by the presence of a rib of intermediate refractive index material protruding through a blocking layer overlying the active layer. Optionally the blocking layer may include high refractive index material to provide additional waveguiding effect and controlled attenuation of higher order transverse modes.

BACKGROUND OF THE INVENTION 
This invention relates to semiconductor lasers, and in particular to 
transverse mode stabilized lasers with planar active layers. 
One example of such a laser is described by T. Furuse et al in a paper 
entitled "Transverse Mode Stabilised AlGaAs BH laser having a Built-in 
Plano-convex Waveguide" presented at The Optical Communication Conference, 
Amsterdam, Sept. 17-19, 1979, 2.2-1 to 2.2-4. That paper discloses a 
construction in which a transverse waveguiding effect is provided for a 
planar active layer by the presence of an inverted rib of intermediate 
band gap material grown before the growth of the active layer. However, 
this structure has several disadvantages, especially as far as the smooth 
and proper epitaxial growth of the layers is concerned, because of the 
presence of the inverted rib on the substrate before the formation of the 
active layer. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to avoid the 
disadvantages of the prior art. 
More particularly, it is an object of the invention to provide an injection 
laser element of the type here under consideration which does not possess 
the disadvantages of the conventional laser elements of this type. 
It is yet another object of the present invention to develop a laser 
element of the above-type which has an improved waveguiding effect in 
comparison with conventional laser elements. 
In pursuance of these objects and others which will become apparent 
hereafter, one feature of the present invention resides in an injection 
laser element having a plurality of layers epitaxially grown upon a 
substrate, this plurality of layers including a planar active layer 
sandwiched between lower refractive index higher band gap upper and lower 
passive layers, a blocking layer, an intermediate index layer whose 
refractive index is intermediate that of the active layer and that of the 
upper passive layer, wherein the upper passive and the intermediate index 
layers both have the same conductivity type which is the opposite of that 
of the blocking layer, wherein the blocking layer covers the upper passive 
layer except in the region of a stripe thereof extending normally from 
laser end face to laser end face, above which stripe the blocking layer is 
absent, wherein the intermediate index layer extends in the region of the 
stripe into material of lower refractive index and is contiguous with the 
material of the upper passive layer, and wherein the refractive indices of 
the intermediate index, blocking, upper passive and active layers, in 
relation to their thickness are such that the intermediate index layer is 
optically coupled at the laser emission wavelength with the active layer 
in such a way as to provide a guided wavelength in the direction of light 
propagation that is shorter in the region of the active layer overlaid by 
the stripe than in the adjacent regions of the active layer. 
The present invention is thus concerned with a structure in which a 
transverse waveguiding effect is provided by the presence of an inverted 
rib of intermediate band gap material grown after the growth of the active 
layer. One particular advantage of this structure is that it employs a 
planar substrate surface on which to grow the epitaxial layers. This means 
that the substrate surface can be etched back with an unsaturated melt 
before growth of the first epitaxial layer is commenced, and furthermore 
this first layer can be a relatively thick layer to ensure that growth is 
proceeding properly. Moreover the provision of self alignment of a current 
confining structure with the waveguide is relatively straightforward. 
With this structure, it is possible to provide additional discrimination 
against the higher order transverse modes by making a part of the 
thickness of the blocking layer out of higher refractive index material 
than that of the intermediate index layer so that the higher order modes 
are preferentially attenuated by leakage into this part of the blocking 
layer. 
A particularly advantageous construction of the injection laser element is 
obtained when the plurality of layers further includes a low index layer 
covering the intermediate index layer and being of a material with a lower 
refractive index than the material of the intermediate index layer. 
The material of the blocking layer may advantageously have substantially 
the same refractive index as the material of the upper passive layer. 
However, it is especially advantageous when the material of the blocking 
layer has a refractive index exceeding that of the material of the 
intermediate index layer. It is also advantageous in this context when the 
material of the blocking layer has a refractive index substantially equal 
to that of the material of the active layer. 
According to an advantageous aspect of the present invention, the blocking 
layer has a first portion contiguous with the upper passive layer and 
being of a material of substantially the same refractive index as the 
material of the upper passive layer, and a second portion covering the 
first portion and being of a material with a refractive index exceeding 
that of the material of the intermediate index layer, and especially 
substantially equal to that of the material of the active layer. Generally 
speaking, the blocking layer has at least a portion of a refractive index 
exceeding that of the material of the intermediate index layer which is 
contiguous with the upper passive layer and which is so optically coupled 
with the material of said active layer as to provide discrimination 
against higher order transverse modes propagating under the stripe. 
Especially good results are obtained when the active layer is of quaternary 
(In,Ga) (As,P), where the upper and lower passive layer is of 
arsenic-substituted quaternary (In,Ga)(As,P) in which the proportional 
amount of arsenic substituted for phosphorus is between one-half and 
one-third of the equivalent substitution amount in the material of the 
active layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing in detail, it may be seen that the reference 
numeral 1 has been used to identify a substrate on which the laser element 
of the present invention is epitaxially grown. The substrate 1 of FIG. 1 
is an n-type indium phosphide substrate. In the following discussion, the 
present invention will be explained as being used in a laser element 
having an active layer 3 of the quaternary (In,Ga) (As,P) material. 
However, it will be apparent that the structures employed here can be used 
in other semiconductor material systems or combinations as well. 
The substrate surface upon which the layers of the device are to be 
epitaxially grown is given a final clean-up preparation by etching it with 
an unsaturated melt of indium phosphide, and then the first epitaxial 
layer, an n-type indium phosphide lower passive layer 2, is grown, 
typically several microns thick. Next the quaternary (In.sub.x, 
Ga.sub.1-x) (As.sub.y, P.sub.1-y) active layer 3 is grown on the lower 
passive layer 2. This active layer 3 is typically about 0.2 microns thick 
and p-type, y may typically be 0.6. The active layer 3 is covered by the 
epitaxial growth of a thin, typically about 0.3 microns thick, upper 
passive layer 4 of p-type indium phosphide, and this in turn is covered by 
an n-type blocking layer 5 of indium phosphide. The blocking layer 5 is 
typically about 0.6 microns thick. Standard photolithographic techniques 
are used to etch through the blocking layer to expose a stripe 6 of the 
material of the underlying upper passive layer 4, this stripe extending 
normally from one end face of the laser element to the other. This stripe 
6 may be typically about 2 to 4 microns in width. Next an intermediate 
index layer 7 is grown to cover the blocking layer and the exposed stripe 
6 of the passive layer 4. This layer 7 is of p-type material and is made 
of quaternary (In,GA) (As,P) having a refractive index intermediate that 
of indium phosphide and that of the quaternary material of the passive 
layer 3. Preferably the proportional amount of arsenic substituted for 
phosphorus in the material of the layer 7 is between one half and one 
third of the equivalent substitution in the material of the active layer 
3. The intermediate index layer is covered with a p-type low index layer 8 
whose refractive index is less than that of the intermediate index layer 
7. Conveniently the layer 8 is made of indium phosphide, and this is 
covered with a p.sup.+ -type contact layer 9 to which contact is made. The 
layer 9 is typically of ternary (In,Ga) As or may be of quaternary 
(In,Ga)(As,P). 
The composition and construction of this laser element is designed so that 
the optical field penetrates to a not insignificant extent from the active 
layer 3 through the upper passive layer 4 and into the region of the 
intermediate index layer 7 overlying the stripe 6. This penetration of the 
optical field into the intermediate index material is designed to provide 
a transverse waveguiding effect by shortening the guided wavelength 
.lambda..sub.Z of the laser emission in the region of the active layer 3 
underlying the stripe in comparison with that in the adjacent regions of 
the active layer 3. The guided wavelength is related to the effective 
refractive index n.sub.eff by the formula .lambda..sub.Z n.sub.eff 
=.lambda..sub.o, where .lambda..sub.o is the free space wavelength of the 
light. Neglecting fringe effects, the effective refractive index is 
determined in relation to the variation of optical intensity I and 
refractive index n.sub.y in the direction normal to the layers (the 
y-direction) , with the refractive index at any point making its 
contribution to the effective index approximately in proportion to the 
optical intensity at that point. 
Optical coupling between a first waveguide constituted by the active layer 
3 bounded by its associated upper and lower passive layers 4 and 2, and a 
second waveguide constituted by the region of the intermediate index layer 
5 overlying the stripe 6 bounded by the low index and upper passive layers 
8 and 4 results in an increase in n.sub.eff and hence a shortening of the 
guided wavelength .lambda..sub.Z provided that n.sub.eff for the first 
guide computed for an infinite separation of the two guides is greater 
than n.sub.eff for the second guide similarly computed for an infinite 
separation of the two guides. This condition is readily met provided that 
the condition if avoided in which the intermediate index layer 5 is thick 
in comparison with the active layer 3 and/or too close in value of 
refractive index to that of the active layer 3. In certain circumstances 
the presence of the low index layer 8 can be dispensed with in the 
formation of the second waveguide referred to earlier in this paragraph. 
The blocking layer 6 is made of n-type material so that it presents a 
reverse biased p-n junction to confine current flow through the device and 
concentrate its flow across the active layer 3 to the region registering 
with the stripe 6. In this way the current confinement is self aligned 
with the transverse waveguiding effect. 
For many applications it is desirable to discriminate against the higher 
order transverse modes, usually restricting operation to the zero order 
transverse mode. For these applications the above described laser element 
can be designed so that, in the absence of electrical drive, only the zero 
order mode is below cut-off. This will provide a measure of discrimination 
against the higher order transverse modes, but it may be found that under 
these circumstances the cut-off modes are not sufficiently heavily 
attenuated to prevent their propagation by gain guiding at drive levels 
for which single mode operation is required. This problem increases with 
increasing stripe width. For stripe widths in the region of 1 to 2 microns 
1st order and higher transverse modes are cut off with a structure 
providing relatively strong transverse waveguiding. For wider stripes 6, 
the transverse waveguiding is weaker and correspondingly the higher order 
modes are not so heavily attenuated. 
This problem may be ameliorated by making modifications to the structure of 
FIG. 1 to include high index material in the blocking layer into which the 
optical field of the higher order transverse modes will preferentially 
penetrate and contribute additional attenuation discriminating 
preferentially against higher order transverse modes. These modifications 
are depicted in FIG. 2. 
Referring to FIG. 2, the preparation of this laser element follows the same 
pattern of growth at least as far as the growth of the active layer 3 is 
concerned, The next layer to be grown is an upper passive layer 24 of 
p-type indium phosphide. The layer 24 is covered with an n-type blocking 
layer 25 of quaternary (In,Ga) (As,P) and then standard photolithographic 
techniques are used to etch through this blocking layer 25 and part way 
through the underlying upper passive layer 24 to expose a stripe 26 of the 
material of the upper passive layer 24. This stripe 26 is just like the 
stripe 6 of the device of FIG. 1, that is to say it is typically 2 to 4 
microns wide, and extends normally from one end face of the laser element 
to the other. The preparation of the laser element then continues in the 
same way as that of the laser element of FIG. 1, with the growth of the 
intermediate index, low index and contact layers 7, 8 and 9. 
Just as in the laser element of FIG. 1, the penetration of the optical 
field from the active layer 3 through the upper passive layer 24 and into 
the region of the intermediate index layer 7 defined by the stripe 26 is 
designed to increase the effective refractive index in comparison with the 
value it would have if this optical field did not penetrate as far as the 
intermediate index material of the layer 7. The laser element is also 
designed so that, outside the region of the stripe 26, the optical field 
will penetrate into the region of the quaternary material of the blocking 
layer 25. This is so that the blocking layer 25 shall guide light 
transversely away from the stripe region 26 and thus provide attenuation 
which is stronger for higher order modes. The material of the blocking 
layer 25 is made of higher index material than that of the intermediate 
index layer 7. Its refractive index may be smaller or greater than that of 
the material of the active layer 3, but generally it is preferred to make 
it equal to it. 
It will be appreciated that the high index blocking layer 25 bounded by the 
upper passive layer 24 and by the intermediate index layer 7 constitutes 
another waveguiding structure optically coupled to the waveguiding 
structure constituted by the active layer 3 bounded by the two passive 
layers 2, 24. This optical coupling will have an effect both upon the 
guided wavelength of light propagating in the regions of the active layer 
3 outside that overlaid by the stripe 26, and also upon the guided 
wavelength of any wave that may be excited in the blocking layer 25 
itself. For the blocking layer 25 to carry light obliquely away for the 
central stripe region 26, it must be made to have an effective refractive 
index that is greater than that of the active layer 3 under the stripe 6. 
Its effect on the guided wavelength in the regions of the active layer 3 
beneath it is therefore the opposite of that produced by the layer 26, so 
that the effective index is lowered. As a result the waveguiding action 
for light propagating in the region of the active layer under the stripe 
26 is somewhat augmented. 
Because there is no effective lateral waveguiding effect for light coupled 
into the "blocking layer waveguide", this waveguide provides radiative 
loss. This loss is wanted because it is greater for the higher order 
transverse modes, but since it also contributes some loss to the zero 
order mode, the blocking layer 25 does not approach as close to the active 
layer 3 as does the intermediate index layer 7 in the region of the stripe 
26. By this means the loss to the zero order mode is kept to an acceptable 
value. 
Any required increase in transverse waveguiding consequent upon narrowing 
the stripe width can be provided by reducing the spacing between the 
intermediate index layer 7 and the active layer 3, leaving the spacing 
between the high index material of the blocking layer 25 and that of the 
active layer 3 unchanged. 
The use of high index material in a blocking layer for preferentially 
attenuating higher order transverse modes has been previously described by 
H. Nishi et al in the paper entitled "Self-aligned Structure ih 
InGaAsP/InP DH lasers"0 appearing in Applied Physics Letters, Volume 35, 
No. 3, pages 232-4 (August 1979), but in that instance low index material 
fills the space left by the etching of the stripe in the blocking layer. 
This means that transverse waveguiding is provided solely by the effect of 
the blocking layer being optically coupled with the active layer. The 
limitation of this is that the technique becomes increasingly inefficient 
with decreasing stripe width because a decreased stripe width requires a 
correspondingly stronger transverse waveguiding effect and, if this is 
provided solely by the blocking layer, it produces an increased 
attenuation of the zero order transverse mode. 
It should be appreciated that complementary conductivity type versions of 
both structures can be grown using p-type substrates. The use of a p-type 
substrate can be advantageous because this necessitates the use of n-type 
material for the intermediate index layer. Under these circumstances the 
problem of carrier leakage by diffusion from the active layer into the 
nearby intermediate index material is less on account of the fact that the 
mobility of holes is less than that of electrons. 
One difficulty in manufacturing a laser element as depicted in FIG. 2 
concerns terminating the etching so as to have the requisite thickness of 
the upper passive layer 24 beneath the stripe 26. For materials for which 
selective etches exist which will etch material of one conductivity type 
significantly faster than material of the other, this problem can be 
ameliorated by adopting the construction illustrated in FIG. 3, in which 
the upper passive layer 4 is essentially the same as that of the device 
described with reference to FIG. 1. This is covered with a blocking layer 
deposited in two parts comprising a first part or portion 35A of low index 
indium phosphide. covered by a second part or portion 35B of high index 
quaternary (In,Ga)(As,P) of the same conductivity type and having the same 
composition as blocking layer 25 of the device of FIG. 2. This allows the 
etching to be terminated upon exposure of the opposite conductivity type 
material of layer 4. In all other respects, the construction of this 
device is the same as that of the device previously described with 
reference to FIG. 2, with the proviso, of course, that the choice of which 
of the two complementary conductivity type versions of the structure to 
construct is determined by the need to have the layer 4 composed of 
material of the conductivity type less readily etched by the particular 
etch used for the selective etching. 
The structure of FIG. 3 also supplies a more reliable current blocking 
action since its reverse biassed junction is located in InP material. 
Because InP is of higher bandgap than the active layer, no leakage current 
is generated at the p-n junction by absorption of spontaneous electro 
luminescence emitted by the active layer 3. 
While I have described above the principles of my invention in connection 
with specific apparatus, it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of my invention as set forth in the accompanying claims.