Narrowband laser transmitter having an external resonator from which the output power can be taken

A narrowband laser transmitter which has a semiconductor laser and an external optical resonator coupled to the semiconductor laser so that the output power of the transmitter can be taken from the resonator, characterized by the laser transmitter being both a micro-optical realization or implementation as well as executed with a free beam propagation. To this end, the resonator is composed of an optical grating arrangement arranged in the beam path of the laser emission from the semiconductor laser and this optical grating arrangement will conduct one part of the supplied laser emission back to the semiconductor laser while conducting the other or second part of the emission to a coupling location at which the other part can be taken as the output power of the transmitter.

BACKGROUND OF THE INVENTION 
The present invention is directed to a narrowband laser transmitter 
comprising a semiconductor laser and an external optical resonator coupled 
to the semiconductor laser with the outpower power of the transmitter 
being taken from the external optical resonator. This arrangement for the 
transmitter enables a wavelength selectivity for the transmitter. 
A transmitter of a semiconductor laser and an external optical resonator is 
proposed in an earlier filed German patent application No. P 36 00 726.9, 
whose disclosure was incorporated in U.S. patent application Ser. No. 
906,503, filed Sept. 12, 1986. In this proposed transmitter, the external 
optical resonator is fashioned in the form of an optical directional 
coupler, which is composed of two waveguides extending a slight distance 
from one another and with a defined coupling length so that crossover of 
power between the two waveguides will occur in the coupling length. One of 
the two waveguides is coupled to the semiconductor laser. The laser 
emissions coupled into these waveguides is supplied to a partially 
reflective feedback device which conducts a part of the laser emission 
back into the coupling length. That part of the laser emission that the 
feedback device allows to pass is conducted to another waveguide of a 
directional coupler by a loop-shaped optical waveguide. The output power 
of the transmitter is taken at this other waveguide. 
An extremely narrowband, single-mode operating condition is required for 
future fiber-optic communication systems, particularly with a heterodyne 
or homodyne reception, can be achieved with such a laser transmitter. 
Such a laser transmitter exhibits the advantage that the coupling of the 
resonator and of the system fiber to the semiconductor laser occurs at 
only one side of the semiconductor laser, in contrast to other embodiments 
of the laser transmitter wherein the system fiber is coupled at one side 
and the resonator is coupled to the semiconductor laser at the other side. 
As a result of the single-sided coupling, adjustment problems are 
considerably reduced and the requirement of a high mechanical stability of 
the fiber of the system and other external resonator can be easily met. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a structure for a 
narrowband laser transmitter wherein both a micro-optical implementation 
or execution, as well as an execution with a free beam propagation of the 
laser transmitter, are possible. 
This object is achieved by an improvement in a narrowband laser transmitter 
comprising a semiconductor laser and an external optical resonator coupled 
to the semiconductor laser with the output power of the transmitter being 
taken from the resonator. The improvements are that the resonator is 
composed of an optical grating means, which is arranged in the beam path 
of the laser emission of the semiconductor laser, for conducting a first 
part of the laser emission back to the semiconductor laser and conducting 
a second part of the emission to a coupling location at which the second 
part can be taken as the output power of the transmitter. 
A single-mode laser operation with a narrow line width can be forced by the 
back reflection of the first part of the emitted laser emission. 
Preferable and advantageous development of the grating used in the laser 
transmitter of the invention are that the grating means comprises a 
partially reflective optical grating from which a narrowband first part of 
the laser emission is diffracted and reflected back in the direction 
towards the semiconductor laser and the second part is transmitted or 
conducted in a direction toward said coupling location. The reflective 
optical grating is preferably fashioned on one side face of a prismatic 
member of a material which is transparent for the laser emission and the 
grating is preferably a phase grating. 
The grating means comprises a reflective grating from which the narrowband 
first part is diffracted back in the direction towards the semiconductor 
laser and works in conjunction with a beam splitter which is arranged in 
the beam path of the emitted laser emission with the beam splitter 
splitting a part of the emission out of the beam and deflecting this part 
in the direction towards the coupling location as the second part. The 
reflective grating is fashioned on one side of a prismatic member, which 
is composed of material that is transparent for the laser emission and is 
arranged in the beam path of the laser emission and the beam splitter is 
mounted to this prismatic member. 
In another embodiment of the invention it is particularly important that a 
collimated optics for collimating the laser emission be arranged in the 
beam path of the emitted laser radiation between the semiconductor laser 
and the grating means. This collimating optic simultaneously focuses a 
narrowband part of the first part of the laser emission conducted back to 
the semiconductor laser onto the semiconductor laser. The collimated laser 
emission generated by the collimating optics optimizes the 
wavelength-selective effect of the grating means. As a result of the 
combination of the grating means with the collimating optics, only a 
narrow wavelength region is conducted back to and focused on the 
semiconductor laser. This is particularly the case when the collimating 
optics is composed of a graded index or gradient lens (see in this regard 
IEEE J. Quant. Electronics QE-16 (1980) pp. 11-22). The embodiment with 
the gradient lens, particularly in its development in accordance with it 
being fixed to a prismatic member is suited for micro-optical execution of 
the laser transmitter of the present invention. 
Another embodiment of the laser transmitter of the invention has a free 
beam propogation, preferably comprising collimating optics being composed 
of a microscopic objective arranged in the free beam path of the laser 
emission between a semiconductor laser and the grating means. 
Expediently, a focusing optics is arranged in the beam path of said second 
part of the laser emission that is conducted to the coupling location. The 
focusing optics are arranged for focusing this second part onto the 
coupling location. This focusing optics is preferably composed of a 
gradient lens and is especially used for the micro-optical execution, but 
also can be used for the execution having a free beam propogation. 
It is especially advantageous to design the gradient lens, which is 
arranged in the beam path of the second part of the laser emission and is 
connected to the coupling location, in such a fashion so that the focus of 
the second part coincides with an end face of the gradient lens. As a 
result thereof, a system fiber for forwarding the output power of the 
laser transmitter can be directly butt-coupled to the gradient lens. 
It is expedient in view of the micro-optical execution of the laser 
transmitter to construct the transmitter in accordance with the use of a 
gradient lens which may be of a length so that the focal point of the lens 
is on an end surface of the lens as a focussing optic which is fixed to 
the prismatic member of material which is transparent to the laser 
emission. The prismatic member is arranged in the beam path of the second 
part of the laser emission that is to be conducted to the coupling 
location. 
When an additional monitoring output is required, for example for control 
purposes, then an additional beam splitter can be provided in one of the 
beam paths of the laser of the invention. This additional beam splitter 
cuts or splits a third part of the radiation out for monitoring purposes 
so that this additional beam splitter is preferably arranged in the beam 
path of the emitted laser emission in accordance with an arrangement 
wherein it is between the semiconductor laser and the grating means. 
When utilizing the additional beam splitter, other advantages are that it 
is arranged in the beam path to cut out a third part for monitoring 
purposes and that it is arranged between the collimating optics and the 
grating means and is fixed to the prismatic member which is positioned in 
the beam path of the emitted laser radiation. Focussing optics for this 
third part of the laser emission, which is to be cut out for monitoring 
purposes, is a gradient lens, which has its surface fixed to the prismatic 
member and whose length is dimensioned so that the focus of this third 
part coincides with the end face of the gradient lens. 
A wavelength balancing or tuning in a laser transmitter of the invention is 
possible with a lateral displacement of the overall resonator relative to 
the semiconductor laser. A fine balancing or tuning is possible by 
adaptation of the resonator length, for example, the optical distance 
between the semiconductor laser and the grating arrangement. For example, 
given a wavelength of 1.3 pm and an optical path of 15 mm in the external 
resonator, the frequency modification is 15 GHz/pm. 
Since the line width of the resulting single mode spectrum is inversely 
proportional to the length of the external resonator (see IEEE J. of 
Quant. Electronics, QE-20 (1984) pp. 486ff.), a gradient lens arranged in 
front of the grating arrangement can be lengthened by a multiple of what 
is referred to as its "pitch" length in order to achieve smaller line 
widths. This will not influence the beam quality, nor the stability of the 
arrangement. 
The micro-optical embodiment of the laser transmitter emission can be 
constructed in an extremely compact fashion. 
Other advantages and features will be readily apparent from the following 
description of the drawings, claims and disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principles of the present invention are particularly useful in a laser 
transmitter generally indicated at LS in the FIGS. and includes a 
semiconductor laser HL and an external optical resonator ER. 
The laser transmitter LS of FIG. 1 includes the semiconductor laser HL, 
which has a beam path St of divergent emission extending on an axis A. A 
collimated or first gradient lens KO is arranged in the beam path St and 
collimates the divergent radiation into an essentially parallel radiation. 
The gradient lens KO, at an end surface, is fixed to a side face or 
surface SFO of a transparent prism PK. The side face SFO faces the 
semiconductor laser HL and is arranged to extend perpendicular to the axis 
A of the beam path. A partially reflecting optical grating TG in the form, 
for example, of a phase grating having, for example, a saw tooth profile 
is fashioned on a side surface or face SF of the prism PK which faces away 
from the semiconductor laser HL and is arranged to extend obliquely at an 
angle relative to the axis A. A second transparent prism PK2, which 
augments the prism PK to form a transparent cuboid is arranged on the 
surface SF of the prism PK. The grating TG could also be formed in the 
second prism PK2. These two prisms PK and PK2 and the grating TG form the 
grating means GE and are part of the resonator ER. 
A focusing or second gradient lens OF has one of its end faces fixed or 
mounted at a side SF2 of the second prism PK2 that faces away from the 
semiconductor laser HL. This second or focusing gradient lens OF focuses a 
second part St2 of the laser emission that has passed through the grating 
TG and the second prism PK2 onto a coupling location KS. The length L of 
this second gradient lens OF is selected so that a focus point F of the 
focused second part St2 of the laser emission coincides with the end face 
EF of the second gradient lens OF that faces away from the semiconductor 
laser HL. 
This has the advantage that a system fiber SyF, which is to be coupled to 
the coupling location KS and which forwards the output power of the 
transmitter LS, can be directly butt-coupled to the gradient lens OF, as 
shown in FIG. 1 at the point F on the surface EF. 
The grating arrangement GE is formed of the double prism arrangement and 
guarantees a simple beam path and, thus, a good coupling efficiency into 
the fiber SyF of the system. 
The grating TG also conducts a narrowband part ST1 of a laser emission 
output by the semiconductor laser HL back to the semiconductor laser HL. 
The wavelength-selective effect of the grating TG is optimized by the 
firmly connected collimating or first gradient lens KO, since this lens KO 
generates a collimated beam St given a suitable distance from the 
semiconductor laser HL. Due to the combination of the first gradient lens 
KO and of the grating arrangement GE, only a narrow wavelength region is 
reflected back into itself and is re-focused onto the semiconductor laser 
HL. 
As a result of the structural elements employed, the arrangement of FIG. 1 
is constructed in an extremely compact fashion. 
In FIG. 2, the embodiment of the laser transmitter LS has a free beam 
propagation to the grating means GE and also therefrom. As illustrated, 
the grating TG of the grating means GE is a partially reflective grating. 
The collimating optics KO in the embodiment of FIG. 2 is composed of a 
microscope objective arranged in the beam path St of the laser emission, 
which is divergently emitted from a semiconductor laser HL. The microscope 
objective converts the divergent laser emission into parallel beams that 
propagate freely following the objective. The grating means GE is arranged 
in this beam path St of the laser emission. This grating means GE is 
composed of a cuboid, which is a transparent member PK1 that is arranged 
at an angle obliquely relative to the axis A of the beam path and has a 
partially reflecting grating TG which, for example, is fashioned here on 
the side face SF1 of the member PK1, which face SF1 faces the 
semiconductor laser HL. The grating is fashioned, for example, in the form 
of a phase grating. 
That part of the laser emission conducted back to the semiconductor laser 
HL by the grating TG is focused onto the semiconductor laser HL by the 
microscope objective KO. That part St2, which is the second part of the 
laser emission that has passed through the grating TG and the cuboid 
member PK1, is focused onto a coupling location KS by a focusing optics 
OF. This focusing optics OF can also blank out a part of the supplied 
radiation. It can be integrated in the grating means GE and, as in the 
embodiment of FIG. 1, can be a second gradient lens, whose length is 
appropriately selected so that the focal point F of the second part St2 
focused by the second lens coincides with an end face EF facing away from 
the semiconductor laser HL. 
In the embodiment of FIG. 2, the maximum amount of power that can be 
coupled out is dependent on the quality of the coupling via the microscope 
objective, as well as on the reflecting property of the grating TG. 
Changes in the resonator length and in the degree of feedback are easily 
possible in this embodiment. 
Let it be pointed out that the free beam propagation with feedback is 
practiced by a grating in Littrow configuration in many known arrangements 
in order to achieve narrow line widths and single-mode behavior. Examples 
of this are shown in Electron. Lett. 19 (1983) pp. 110-112; IEEEJ. Quant. 
Electr. QE-18 (1982) pp. 259 ff., and Electron. Lett. 21 (1985) pp. 
658-659. All of these arrangements use the two-sided coupling to the 
semiconductor laser wherein the output power of the transmitter is taken 
at one side of the semiconductor laser and the external resonator is 
coupled to the other side of the semiconductor laser. 
In the micro-optical embodiment of the laser transmitter of FIG. 3, the 
essential differences from the embodiments of FIG. 1 is that the grating 
of the grating means GE is a reflecting grating RG that should have the 
highest possible reflectivity. As in the embodiment of FIG. 1, the 
divergent laser emission is collimated by a first gradient lens KO and the 
grating means GE is composed of transparent prism PK on whose surface SF, 
which faces away from the semiconductor laser HL has the reflecting 
grating RG applied, for example, in the form of a relief-like grating 
having a sawtooth profile. 
A beam splitter ST is required for coupling out the output power of the 
laser transmitter LS. This beam splitter ST is expediently arranged 
between the first gradient lens KO and the prism PK in the beam path St of 
the laser emission and deflects the second part St2 of the laser emission 
out of the beam path at an angle of approximately 90.degree.relative to 
the axis A. The beam splitter ST is expediently fixed or mounted to the 
prism PK, for example on a surface SFo which faces toward the 
semiconductor laser HL and can be composed of a beam splitter cube. 
The second part St2 of the laser emission deflected or split out is focused 
onto a coupling location KS by a focusing optics OF. The focusing optics 
is likewise expediently secured to the prism PK. As in the embodiment of 
FIGS. 1 and 2, it can be composed of a second gradient lens of a 
corresponding length, wherein the focal point F of the focused second part 
St2 of the laser emission which has been deflected out coincides with the 
end face EF facing away from the beam splitter ST. The fixing of the 
gradient lens expediently occurs so that one end face of the lens is fixed 
to the side face SFo of the prism PK facing toward the semiconductor laser 
HL. 
The beam splitter ST also deflects a part of the first part St1 of the 
laser emission that is conducted back to the semiconductor laser HL and 
this reflected first part is not shown or exploited in FIG. 3. The 
embodiment of the laser transmitter LS of FIG. 3 is also constructed in a 
very compact form 
In yet another embodiment of the laser transmitter LS of FIG. 4 differs 
from the embodiment of FIG. 1 only in that an additional beam splitter St1 
is arranged between the collimation optics KO and the grating means GE in 
the beam path St of the laser emission. This additional beam splitter St1 
reflects out a third part St3 of the laser emission for monitoring 
purposes, and this third part is focused by a focusing optics F01, which 
is arranged in the beam path of the third part St3. Like the beam splitter 
ST in the embodiment of FIG. 3, the additional beam splitter ST1 can be 
fixed to the prism PK and can be a beam splitter cube. Like the focusing 
optics OF of the embodiment of FIG. 3, the focusing optics FO1 can also be 
fixed to the prism PK and can be a third gradient lens wherein the focal 
point F1 of the focused split-out third part St3 of the laser emission 
coincides with an end face EF1 facing away from the additional beam 
splitter ST1. As a result thereof, a monitor fiber MoF can be directly 
butt-coupled to the end face EF1 of the focusing optics FO1. 
The embodiment of FIG. 4 is also distinguished by a very compact structure. 
The arrangement for monitoring purposes, as shown by way of example in 
FIG. 4, can be provided in every embodiment of the laser transmitter. 
Let it again be pointed out in conclusion that the special advantage of the 
laser transmitter described herein lies in the turnability or matchability 
of the emission spectrum on the basis of the grating and of the resonator 
length. It is expedient to provide mirrors on the sides of the 
semiconductor laser HL facing away from the external resonator ER. As a 
result thereof, the optical power given the same pump system is increased 
due to the lowering of the threshold current and the sensitivity with 
respect to parasitic reflections is reduced. This is true of all laser 
transmitters comprising external resonators and a single-sided out 
coupling, for example, in which the output power of the transmitter is 
taken at the side of the resonator. 
Although various minor modifications may be suggested by those versed in 
the art be understood that we wish to embody within the scope of the 
patent granted hereon all such modifications as reasonably and properly 
come within the scope of our contribution to the art.