Coating for DFB/DBR laser diodes

In DFB/DBR semiconductor diode lasers, competition may arise between the DFB (Distributed Feed-Back) mode corresponding to the period of the grid present and the FP (=Fabry-Perot) mode determined by the relative distance of the mirror surfaces, as a result of which the laser does not operate in one single mode. By providing an antireflex layer, this problem is suppressed, but other disadvantages are obtained, such as a large line width. However, by providing a phase layer on the antireflection coating, the operation of the laser in a single mode is combined with a comparatively narrow line width. Furthermore, a reflective coating can be applied to the phase layer. In this case, both the module and the phase of the effective reflection can be adjusted substantially independently of each other, as a result of which a narrow line width and SLM can be more readily combined. The antireflex coating, the phase layer and the reflective coating can be manufactured by means of not more than two materials having a suitable refractive index. Two particularly suitable materials are hafnium oxide and silicon dioxide, which can be applied to the mirror surfaces by means of the comparatively simple and attractive vapor deposition technique.

BACKGROUND OF THE INVENTION 
The invention relates to a semiconductor diode laser surrounded by a medium 
and comprising a semiconductor body having a pn junction which at a 
sufficiently high current strength in the forward direction can produce 
coherent electromagnetic radiation in a strip-shaped active region located 
within a resonant cavity, the resonant cavity being constituted at least 
over part of its length by a periodical variation in the effective 
refractive index in the longitudinal direction, while the resonant cavity 
is limited by surfaces substantially at right angles to the active region, 
at least one of these surfaces being provided with an antireflection 
coating. 
Such a semiconductor diode laser is described in Philips European Patent 
Application No. 87201626.6 which was laid open to public inspection under 
No. 0259919 on March 16, 1988. 
Semiconductor diode lasers of various constructions are used in many 
fields. The resonant cavity can be formed in different ways. In many 
cases, it is constituted by two parallel extending mirror surfaces, for 
which mostly cleavage surfaces of the semiconductor crystal are used. By 
repeated reflection on these mirror surfaces, radiation modes known under 
the designation Fabry-Perot (FP) modes are produced. 
According to another embodiment, the resonant cavity is obtained by a 
periodical variation of the effective refractive index for the radiation 
produced along at least part of the length of the resonant cavity. Instead 
of reflection on mirror surfaces, reflection at a grid (constituted by the 
said periodical variation of the refractive index) is used. Lasers in 
which this is the case are designated as DFB (Distributed FeedBack) 
lasers. They exist in various constructions and are known under the 
designation "Distributed FeedBack"(DFB) lasers, of which construction the 
semiconductor diode laser described in the aforementioned European Patent 
Application is an example, and as "Distributed Bragg Reflection" (DBR) 
lasers. In this application, for the sake of simplicity they will all be 
indicated by the designation "DFB" laser. 
DFB lasers have, as compared with the aforementioned Fabry-Perot lasers, 
inter alia the advantage that they can oscillate more readily in a single 
stable longitudinal oscillation mode (Single Longitudinal Mode or SLM 
mode) within a large temperature range and with a high output power. This 
is especially important with the use in optical telecommunication because 
in the SLM mode the chromatic dispersion is minimal so that the signal can 
be transported over a large distance without disturbance through the 
optical glass fiber. Further, DFB lasers can be integrated comparatively 
readily within an electrooptical monolithic circuit. 
However, since in general a DFB laser has at the ends of the active region 
end faces at right angles to the active layer, Fabry-Perot oscillations 
can occur between them so that in principle the DFB laser has at least one 
FP mode besides at least one DFB mode with substantially equal 
amplification. 
It is very difficult to manufacture lasers by means of the usual 
technologies in which the position of the mirror surfaces is exactly in 
phase with the period of the grid. Moreover, the usual processes result in 
a spread of the properties of lasers manufactured within a wafer. An 
example of such a property in which spread occurs is the position of the 
mirror surfaces with respect to the grid. As a result, there will be among 
the lasers manufactured from one wafer a number of multimode lasers or 
lasers passing from one mode to another mode, while the yield of SLM 
lasers will be low. The yield moreover also depends upon the so-called KL 
product, in which L is the length of the resonant cavity and K is equal to 
.pi.*.DELTA.n/.lambda..sub.b, where .DELTA.n is the amplitude of the 
refractive index variation and .lambda..sub.b is the Bragg wavelength. 
With a KL product of 2 to 3, the yield of SLM lasers is, for example, 5 to 
10% of the yield of FP lasers. With smaller values of this product, the 
yield approaches zero. 
In order to suppress the FP mode not desired in DFB lasers, various 
measures have been suggested, among which, as described in the 
aforementioned European Patent Application, the use of an antireflection 
coating. Substantially the whole quantity of radiation produced by the 
laser now emanates at the mirror surface (or the mirror surfaces) and the 
FP mode is suppressed. A disadvantage of this method is that the line 
width of the SLM strongly increases. The sensitivity to reflection 
variations also strongly increases. Due to a larger line width and an 
increased sensitivity to reflection, use, especially at high modulation 
frequencies, is limited, which, like the fact that the lasers do not 
operate in the SLM mode, limits the use within the field of optical 
telecommunication. 
SUMMARY OF THE INVENTION 
The invention has inter alia for its object to obviate this disadvantage 
and to combine a high yield with a narrow line width, at least a line 
width as narrow as possible. 
The invention is based inter alia on the recognition of the fact that this 
object can be achieved in that the feedback of radiation into the resonant 
cavity is increased. The feedback can be increased in that a part of the 
radiation emanating through the mirror surfaces is thrown back via a 
reflection. The invention is further based on the recognition of the fact 
that such a reflection must and also can take place in such a manner that 
the radiation fed back has a phase which is correct for the SLM mode of 
operation, that is to say that the phase corresponds to the effective 
refractive index variation in the resonant cavity. This is connected with 
the recognition of the fact that it is possible and advantageous to adjust 
the phase (.phi.) and the amplitude (.vertline.r.sub.eff .vertline.) of 
the effective reflection [r.sub.eff 
(=.vertline.r.sub.eff.vertline.*e.sup.1.phi.)], which is to be understood 
to include the addition of all the reflection at the various interfaces, 
independently or at least as independently as possible of each other. 
According to the invention, a DFB semiconductor diode laser of the kind 
mentioned above is for this purpose characterized in that a layer which is 
designated as a phase layer, is applied to the antireflection coating, as 
a result of which at least a part of the radiation transmitted by the 
antireflection coating is fed back into the resonant cavity, the 
refractive index and the thickness of said layer being chosen so that for 
effective reflection there is provided a phase optimal for single mode 
operation. 
When the material from which the phase layer is manufactured has been 
chosen, the phase of the effective reflection can be caused to correspond 
to the period of the grid by varying the thickness of the phase layer, as 
a result of which SLM operation is obtained. Before the antireflection 
coating or the phase layer is applied, the lasers already operating in the 
SLM mode can be selected from the lasers manufactured from one wafer. The 
remaining lasers are now provided with the antireflection coating and the 
phase layer. The antireflection ensures that substantially no radiation is 
fed back into the active layer before the radiation has the desired phase. 
The phase layer ensure that the desired phase of the effective reflection 
is obtained. The desired phase can be derived from measurements or 
calculations. The phase layer may also be given the desired thickness in a 
stepwise manner, i.e. in a cycle of coating and measuring. As soon as a 
laser operates in the SLM mode, this laser is then put aside. 
In a first embodiment, the antireflection coating comprises, like the phase 
layer, a single layer. The thickness of the antireflection layer must then 
correspond approximately to the optical path length of a quarter 
wavelength for the radiation produced by the laser. With respect to the 
reflections at the various interfaces between the various layers, it holds 
that: 
EQU .vertline.r.sub.1 .vertline.-.vertline.r.sub.2 .vertline.=0. (1) 
Herein .vertline.r.sub.i .vertline. is the module of the reflection at the 
interface between the i.sup.th and the i-1.sup.th layer. The 0.sup.th 
layer is the semiconductor body. Elaboration of equation (1) gives the 
following condition, which must be fulfilled by the refractive indices: 
EQU n.sub.1 =(n.sub.0 * n.sub.2).sup.1/2. (2) 
Herein n.sub.0 is the refractive index of the semiconductor body, which for 
A.sub.III -B.sub.v materials and the usual wavelengths of radiation is 
about 3.2; n.sub.1 is the refractive index of the antireflex layer and 
n.sub.2 is the refractive index of the phase layer. If the phase layer is 
made of hafnium oxide (HfO.sub.2), of which material the technological 
advantages are described in the aforementioned European Patent 
Application, the value of n.sub.2 is equal to 1.76. It follows then from 
equation (1) that n.sub.1 must be approximately equal to 2.4. Titanium 
dioxide (TiO.sub.2), whose refractive index is 2.2, fulfils this 
requirement fairly accurately. 
The choice of materials for manufacturing the antireflection coating can be 
enlarged in that the latter is composed of several layers, for example two 
layers. Besides hafnium oxide, silicon dioxide (SiO.sub.2) is an 
attractive material from a viewpoint of manufacturing technique. The 
refractive index of the latter material is about 1.45. By means of these 
materials, a two-layer antireflex coating can be composed. 
In a second embodiment of the semiconductor diode laser according to the 
invention, the antireflection coating comprises two layers successively 
having a higher and a lower refractive index, which refractive indices 
both lie between the refractive index of the semiconductor body and that 
of the medium, the thickness of said antireflection coating corresponding 
approximately to an optical path length of a quarter wavelength for the 
radiation produced by the laser, while the phase layer comprises a layer 
having a refractive index lying between that of the semiconductor body and 
that of the medium and higher than that of the second layer of the 
antireflection coating. The chosen order of succession of the refractive 
indices of the various layers is of importance besides the thicknesses 
thereof for the operation both of the antireflection coating and of the 
phase layer according to the invention. The refractive indices of the 
three layers are chosen so that the reflection at the first three 
interfaces is a minimum, that is to say: 
EQU .vertline.r.sub.1 .vertline.-.vertline.r.sub.2 .vertline.-.vertline.r.sub.3 
.vertline.=0. (3) 
Herein .vertline.r.sub.i .vertline. again is the module of the reflection 
at the interface between the i.sup.th layer and the i-1.sup.th layer The 
0.sup.th layer is again the semiconductor body. 
In order to limit as far as possible the number of materials used, a third 
embodiment is characterized in that the first layer of the antireflection 
coating and the phase layer consist of a first material, while the second 
layer of the antireflex coating consists of a second material. The 
refractive index of the phase layer therefore now has the subscript 1. 
Elaboration of the equation (3) for this case gives the following relation 
for the refractive indices: 
EQU (n.sub.0 *n.sub.2 -n.sub.1)-1/3*n.sub.1 *(n.sub.0 -n.sub.2)=0. (4) 
It appears that inter alia the combination of hafnium oxide (HfO.sub.2, 
n.sub.1 =1.76) and silicon dioxide (SiO.sub.2, n.sub.2 =1.45) satisfies 
this relation accurately. Also certain other material combinations satisfy 
accurately equation (4), but, as already set out hereinbefore, the said 
combination offers practical advantages. The phase (.phi.) of the 
effective reflection can be calculated by the following equation: 
EQU .epsilon.=(4.pi./.lambda.)*(n.sub.1 *d.sub.3). (5) 
Herein .lambda. is the wavelength of the radiation, d.sub.3 is the 
thickness and n.sub.1 is the refractive index of the phase layer. 
Therefore, a fourth embodiment of a semiconductor diode laser according to 
the invention is characterized in that the first material is hafnium oxide 
(HfO.sub.2) and the second material is silicon dioxide (SiO.sub.2). 
The phase of the effective reflection can thus be chosen substantially 
independently of the module of the effective reflection, it is true, but 
the module of the effective reflection is determined by the transition 
from the phase layer to the medium. In view of the fact that the medium 
generally will be air and that a given material is chosen for the phase 
layer, the value of the module of the effective reflection is fixed. In 
order to obtain a maximum yield and optimum properties, that is to say a 
largest possible number of SLM devices having a comparatively narrow line 
width, it is favorable if the effective reflection upon emanation of the 
radiation from the laser can be chosen freely, that is to say 
independently of the phase chosen. In fact, if, for example, after a cycle 
of measuring, coating and re-measuring, it has been ascertained that the 
phase of the effective reflection by means of the phase layer has reached 
the desired value and if, in order to obtain a smaller line width, the 
effective reflection should be further increased, a phase variation 
occurring during this increase of the effective reflection is undesirable. 
A fifth embodiment of a semiconductor diode laser according to the 
invention is for this purpose characterized in that a reflective coating 
is applied to the phase layer, by means of which the module of the 
effective reflection is adjusted independently of the phase. More 
particularly, this can be achieved if a laser according to the invention 
is characterized in that the reflective coating comprises lasers having a 
refractive index lying between that of the semiconductor body and that of 
the medium, whose number is even and whose thickness approximately 
corresponds to an optical path length of a quarter wavelength for the 
radiation produced by the semiconductor diode laser, which layers have 
alternately a lower and a higher refractive index, while the first layer 
has a lower refractive index than the phase layer. 
Similar considerations again apply also to the reflective coating as given 
above with respect to the choice of the materials of which the 
antireflection coating can be composed. In order to limit the number of 
materials, for all layers having a lower refractive index a first material 
can be chosen and for all layers having a higher refractive index a second 
material can be chosen. For practical reasons, the combination of hafnium 
oxide (HfO.sub.2) and silicon dioxide (SiO.sub.2) is to be preferred. The 
desired value of the effective reflection of the emanating radiation will 
mostly lie between 10 and 60%. 
It should be noted that the thickness of the "1/4.lambda." layers, i.e. 
layers whose thickness corresponds to an optical path length of the 
radiation of a quarter wavelength, also includes thicknesses which 
slightly deviate therefrom, that is to say that the thicknesses may lie, 
for example, between 0.15.lambda. and 0.35.lambda.. It should be noted 
that with the considerations essential to the invention and with the 
formulae used the occurrence of multiple reflections in the layers is 
neglected. In practice, this provides an amply sufficient accuracy, also 
with respect to the aforementioned tolerances of the thickness of the 
"1/4.lambda." layers. 
Further embodiments are therefore characterized in that the reflective 
coating comprises two or four layers.

The Figures are schematic and not drawn to scale, while especially the 
dimensions in the direction of thickness are exaggerated for the sake of 
clarity. Corresponding parts are generally designated by the same 
reference numerals in the various embodiments. Semiconductor regions of 
the same conductivity type are generally cross-hatched in the 
cross-sections in the same direction. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows partly in cross-section and partly in perspective view a first 
embodiment of a semiconductor diode laser according to the invention. A 
diagrammatic cross-section of the semiconductor diode laser of FIG. 1 
taken on the line II--II is shown in FIG. 2. FIG. 3 shows diagrammatically 
the refractive index profile of FIG. 2 at the area of point A. The 
semiconductor diode laser (cf. FIG. 1) comprises a semiconductor body 
having a substrate 1 of a first conductivity type and a layer structure 
disposed on it. This layer structure comprises at least a first passive 
layer 2 of the said first conductivity type, a second passive layer 3 of 
the second opposite conductivity type and an active layer 4 lying between 
the first and the second passive layer. A pn junction (whose position 
depends upon the conductivity type of the semiconductor region located 
between the layers 2 and 3) is situated between the layers 2 and 3 in the 
layer structure. This pn junction can produce, at a sufficient current 
strength in the forward direction, coherent electromagnetic radiation in a 
strip-shaped region 4A of the active layer located within a resonant 
cavity. The first and second passive layers 2 and 3 both have for the 
laser radiation produced a lower refractive index than the active layer 4 
and have a larger band gap than the layer 4. In this embodiment, a third 
passive layer 20 is present between the layer 4 and the layer 3. This 
layer is a so-called anti-melt-back layer, which serves to avoid 
back-dissolving--a problem occurring in liquid phase epitaxy. Since this 
problem does not always occur, the presence of this layer is not 
absolutely necessary. 
The resonant cavity is constituted by a periodical refractive index 
variation in the longitudinal direction and over at least part of the 
length of the active region 4A. This refractive index variation is 
obtained by a grid 7, which is etched into the substrate surface and whose 
grooves are filled with material of the layer 2, which has a refractive 
index for the emitted radiation different from that of the substrate 1. 
The active region 4A is further bounded by end faces 5 and 6, which are 
substantially at right angles to the active region and one of which (the 
face 5) is provided with an antireflection coating 27 and a phase layer 
30. The second passive layer 3 and the substrate 1 are electrically 
connected (through the intermediate semiconductor regions) to metal layers 
(16, 17), which are provided on the upper and the lower surface and serve 
as connection conductors. 
In this embodiment, the substrate 1 consists of indium phosphide (InP) of 
the n-conductivity type. The layer 2 consists of n-type indium gallium 
arsenic phosphide (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). The active 
layer 4 also consists of indium gallium arsenic phosphide. The layer 3 
consists of p-type indium phosphide. 
The laser according to this embodiment is of the so-called DCPBH type and 
comprises a current-limiting layer structure. The laser has two grooves 9 
and 10, which limit the active region 4A and in which a layer 11 of p-type 
InP and a blocking layer 12 of n-type InP are provided. The layers 11 and 
12 do not extend on the strip-shaped part 3A of the layer 3 located 
between the grooves 9 and -0. Further, a layer 13 of p-type Inp is formed 
over the assembly and a top layer 14 of In.sub.x Ga.sub.1-x As.sub.y 
P.sub.1-y is formed thereon, on which a silicon oxide layer 15 is 
disposed. This oxide layer 15 is provided with a slot-shaped opening, 
within which an electrode layer 16 provided on the surface contacts the 
layer 14. The lower surface is contacted with the electrode layer 17. 
The antireflection coating 27 comprises a first layer 28 and a second layer 
29. The layer 28 and the phase layer 30 both consist of hafnium oxide 
(HfO.sub.2), while the layer 29 consists of silicon dioxide (SiO.sub.2). 
The refractive index of hafnium oxide for the radiation produced by this 
laser (.lambda.=1.55 .mu.m) is n.sub.1 =1.76, while the corresponding 
value for silicon dioxide is n.sub.2 =1.45. The thickness of the layers 28 
and 29 approximately corresponds to an optical path length of a quarter 
wavelength for the radiation produced by the laser and is about 220 nm for 
the layer 28 and about 267 nm for the layer 29, respectively. The 
thickness of the phase layer 30 depends upon the desired phase of the 
effective reflection, which in turn depends upon the phase which is 
necessary to cause the laser to operate in the SLM mode. This can be 
determined by measurements and/or calculations. For the semiconductor 
diode laser used in this embodiment, this phase was 90.degree.. The 
thickness of the phase layer 30 can now be calculated by means of equation 
(5) and is in this embodiment about 110 nm. The module of the effective 
reflection (.vertline.r.sub.eff .vertline.) in this embodiment depends 
only upon the material of which the phase layer 30 consists and upon the 
medium. Since the former is hafnium oxide and the latter is air, the 
reflection coefficient R.sub.f in this embodiment is about 9% (R.sub.f 
=.vertline.r.sub.eff .vertline..sup.2). 
The operation of the coating described in this embodiment was ascertained 
as follows. From the DFB semiconductor diode lasers manufactured from 
eight slices with a KL product of 1 to 2.5, first the SM lasers were 
selected. Then all the remaining operating lasers were covered with a 
one-layer antireflection coating consisting of hafnium oxide and having a 
thickness approximately corresponding to an optical path length of a 
quarter wavelength of the radiation produced by the laser. This coating 
resulted in a R.sub.f of 0.5% and a .vertline..phi..vertline. of 
180.degree. for the effective reflection. Subsequently, the SM lasers were 
selected again. However, these lasers had a line width of 120 MHz on an 
average at an output power of 4 mW, while the line width of the uncoated 
SM lasers was 20 MHz on an average in the same measuring conditions. After 
this cycle of measuring, providing the coating and measuring, the coating 
on the lasers not operating in the SM mode was extended to the coating 
described in this embodiment. The line width of the DFB lasers thus 
obtained was on an average 35 MHz in the same measuring conditions. This 
means that the coating described in this embodiment results in a 
substantial improvement of the yield and the line width of the SM lasers 
with respect to SM lasers provided with a one-layer antireflection coating 
(a factor of 3.4 improvement), while the resulting line width is only a 
factor of 1.7 larger than that of uncoated SM lasers. 
FIG. 4 shows diagrammatically in a polar diagram the module 
(.vertline.r.sub.eff .vertline.) and the phase (.phi.) of the effective 
reflection in the semiconductor diode laser of FIG. 1 for different values 
of the thickness of the phase layer (d.sub.3). The circles 40, 41 and 42 
correspond to a .vertline.r.sub.eff .vertline. value of 1.0, 0.5 and 0.1, 
respectively. The corresponding values for the reflection coefficient 
R.sub.f (=.vertline.r.sub.eff .vertline..sup.2) are then 1.0, 0.25 and 
0.01, or 100%, 25% and 1%, respectively. In the Figure, there are points 
43 approximately on a circle whose radius corresponds to a R.sub.f value 
of 9%. These points indicate the phase of the effective reflection as 
determined by the equation (5), as variable only the tbickness of the 
phase layer 30 (d.sub.3) being considered. The Figure shows that for the 
same R.sub.f value the effective reflection can be given any desired phase 
between 0.degree. and 360.degree.. 
For the compositions of the semiconductor layers and for the manufacture of 
the semiconductor diode laser described here, reference is made to the 
aforementioned European Patent Application. The layer 28 of hafnium oxide, 
the layer 29 of silicon dioxide and the layer 30 of hafnium oxide are 
successively applied by vapor deposition to one of the end faces of the 
laser, in this case the end face 5. The thicknesses of the layers 28, 29 
and 30 are 220, 267 and 110 nm, respectively. The application by vapor 
deposition can be effected in a usual manner. 
FIG. 5 shows diagrammatically in cross-section taken on the line II--II in 
FIG. 1 a second embodiment of a semiconductor diode laser according to the 
invention, which differs from the first embodiment only by the coating. 
FIG. 6 shows diagrammatically the refractive index profile of the 
semiconductor diode laser of FIG. 5 at the area of point B. Construction 
and manufacture of the semiconductor diode laser described in this 
embodiment are--except the construction of the coating--equal to those of 
an earlier embodiment, to which reference is made here. A reflective 
coating 35 is applied to the phase layer 30 and comprises a first layer 31 
and a second layer 32. For the materials of which these layers are made, 
use is made of the same materials of which the antireflex coating 27 is 
composed, but in the reverse order of succession. The layer 31 consists of 
silicon dioxide, while the layer 32 consists of hafnium oxide. The 
thicknesses of these layers again approximately correspond to an optical 
path length of a quarter wavelength of the radiation produced by the laser 
and are therefore approximately equal to the thicknesses of the 
corresponding layers in the antireflection coating: d.sub.4 =267 nm and 
d.sub.3 =220 nm. The thickness of the phase layer follows from the desired 
phase of the effective reflection and can be calculated by means of the 
equation (5). FIG. 7 shows diagrammatically in a polar diagram the module 
(.vertline.r.sub.eff .vertline.) and the phase (.phi.) of the effective 
reflection in the semiconductor diode laser of FIG. 5 for different values 
of the thickness of the phase layer (d.sub.3). The circles 40, 41 and 42 
have already been described with reference to FIG. 4. In FIG. 7, the 
points 44 corresponding to different phases all lie on a circle having a 
value R.sub.f of 25%. By the use of a reflective coating between the phase 
layer and the medium, as in this embodiment, the value of the effective 
reflection can be varied substantially independently of the phase, while 
by means of the phase layer the phase can be varied independently of the 
effective reflection. The advantage thereof has already been set out 
above. 
FIG. 8 shows diagrammatically in cross-section taken on the line II--II in 
FIG. 1 a third embodiment of a semiconductor diode laser according to the 
invention, which differs from the preceding embodiments by the coating. 
FIG. 9 shows diagrammatically the refractive index profile of the 
semiconductor diode laser of FIG. 8 at the area of point C. FIG. 10 shows 
diagrammatically in a polar diagram the module (.vertline.r.sub.eff 
.vertline.) and the phase (.phi.) of the effective reflection in the 
semiconductor diode laser of FIG. 8 for different values of the thickness 
of the phase layer (d.sub.3). The considerations made with respect to the 
construction and the manufacture of the second embodiment also apply to 
this embodiment. The reflective coating 35 is extended with respect to the 
preceding embodiment with a further silicon dioxide layer 33 and a further 
hafnium oxide layer 34. The thickness of the layers 29, 31 and 33 is about 
267 nm and the thickness of the layers 28, 32 and 34 is about 220 nm. 
These thicknesses again approximately correspond to an optical path length 
of a quarter wavelength of the radiation produced by the laser in the said 
materials. As appears from FIG. 10, any desired phase can now be adjusted 
with a value R.sub.f of 40%. 
Although in this Application the invention is described with reference to 
the DCPBH laser structure very important for optical telecommunication, 
the use of a .phi.-coating according to the invention is also of great 
importance for other laser structures of the DFB or DBR type for the same 
reasons as in the structure described (here), in order to obtain a 
suppression as efficient as possible of Fabry-Perot modes and radiation in 
one single mode of oscillation. The invention is therefore not limited at 
all to the given embodiments, but relates to all forms of lasers in which 
the resonant cavity is constituted by a periodical variation of the 
effective refractive index over at least part of the length of the active 
region or over at least part of that part of the resonant cavity which 
lies outside the active region.