Diffraction optical coupler

A device for optically coupling an optical fiber (1) forming part of an optical communication system, to an optical semiconductor laser amplifier (4) having an input facet (5) and an output facet (7). The optical fiber has an end surface (2) arranged opposite to at least one of the facets. A characteristic feature of the invention is an diffraction optics element (11) arranged between the end surface of the fiber and the surface of the facet in order to adapt the nearfield of the fiber end to the nearfield of the facet surface and for providing optical filtering reducing spontaneous emission noise. Preferably the diffraction optics element is a phase hologram.

FIELD OF THE INVENTION 
The present invention relates to an optical coupling device for coupling an 
optical fiber of an optical communication system to an optical 
semiconductor amplifier having an input and an output facet. The optical 
fiber has an end surface provided opposite to at least one of the facets. 
DESCRIPTION OF RELATED ART 
Optical coupling devices of the kind referred to above have a fair 
efficiency and a high noise figure. Attempts have been made in order to 
improve the fair efficiency of the coupling between the end of the fiber 
and the facet of the semiconductor laser amplifier by melting the end of 
the optical fiber into the form of a lens focusing the light onto the 
facet. In spite of this the losses of this known optical coupling device 
is typically in the order of 5 dB. To compensate for said losses the 
amplification of the semiconductor laser amplifier must be increased. This 
means that for a predetermined, desired fiber-semiconductor laser 
amplifier-fiber-amplification the amplification of the semiconductor laser 
amplifier must be increased. This, however, will increase the so called 
amplification ripple. Amplification ripple manifests itself as non-desired 
amplification fluctuations. The amplification fluctuations should be less 
than 3 dB. Should the fiber-semiconductor laser 
amplifier-fiber-amplification be e.g. 26 dB it is therefore in practice 
required that the so called modal reflectance of the semiconductor laser 
amplifier is less than 4.times.10.sup.-5 which only can be achieved if 
extensive technological measures are taken. It is also difficult to 
reproduce the results with a reasonable economic investment. Altogether 
this implies that the desired high amplification must be reduced. 
This high, non-desired, noise figure of the known optical coupling device 
depends on spontaneous emission noise from light of all wavelengths within 
the wavelengths range transmitted by the coupling device, i.e. light of 
all frequencies within the complete gain characteristic of the 
semiconductor laser amplifier. Typically this gain characteristic extends 
over 40 nm. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical coupling device 
having enhanced coupling efficiency while simulataneously its noise figure 
is reduced. The losses of the optical coupling device should be in the 
order of 1 dB and less. 
Another object of the present invention is to provide a simple enclosure 
for an optical amplifier, particularly a laser amplifier, said enclosure 
also including a small number of components. The less the number of 
components that are present within the enclosure the simplier will be the 
alignment, i.e. the line up, of the components within the enclosure. This 
follows from the fact that the accuracy during line up should be better 
than 1.mu.m. 
Still another object with the invention is to image the wavefront that 
propagates from one end of the fiber optics element onto one end of the 
active region of the semiconductor laser amplifier. In accordance with the 
invention this is achieved by using a wavelength dependence diffraction 
optics element, preferably a phase hologram which at the same time works 
as a narrow-band optical filter. 
Another object of the present invention is to image the wavefront which 
propagates from a second end surface of the active region of the 
semiconductor laser amplifier onto the end surface of an optical fiber by 
using a diffraction optics element, preferably a phase hologram.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In FIG. 1 there is shown a conventional optical coupling device for an 
optical fiber 1 having a melt end surface 2 commonly referred to as a 
taper. Light propagating through the optical fiber and schematically shown 
at arrow 3 is directed by the end surface 2, working as a lens, to a 
schematically shown semiconductor laser amplifier 4, more particularly to 
an active region 6 of one end surface 5 of the semiconductor laser 
amplifier. The semiconductor laser amplifier amplifies the light and 
amplified light leaves the active region through its opposite end surface 
7. Amplified light, schematically shown at arrow 8, strikes a second melt 
end surface 9, also referred to as taper 2, of an output optical fiber 10. 
End surfaces 5, 7 are commonly referred to as facets. 
The coupling efficiency of this previously known coupling device is 
improved in accordance with the present invention by adapting the 
nearfield of each taper 2, 9 to the nearfield of each end surface 5 and 7 
respectively of the active region. 
The term nearfield as used herein refers to the power distribution of light 
in planes which are perpendicular to the propagation direction of light 
and which are situated in the nearfield of the end surface in question. 
The nearfield of a taper and the nearfield of an end surface of the active 
region are not equal neither as regards to extension or to symmetry. This 
has been illustrated in FIGS. 3A-C and 4A-C. FIG. 3 is illustrating the 
nearfield of a taper. FIG. 3A is illustrating the power distribution of 
the electromagnetic wave along the x-direction and FIG. 3B the power 
distribution along the y-direction of an electromagnetic wave propagating 
in the z-direction. The directions referred to above correspond to the 
directions of the x-y-z-coordinate system shown schematically in FIG. 1. 
As appears from FIGS. 3A and 3B the two curves are rising and descending 
with the same inclination and an imaginary image of the power distribution 
in a plane perpendicular to the propagation direction of the wave has the 
appearance shown in FIG. 3C. 
A corresponding imaginary image of the nearfield of any of the end surfaces 
5, 7 is shown in FIGS. 4A-C. The curve shown in FIG. 4A is rising and 
falling with less inclination than the curve of FIG. 4B, this resulting in 
a power distribution, in a plane perpendicular to the propagation 
direction of the wave, having the appearance shown in FIG. 4C. 
Accordingly, if the nearfield shown in FIG. 3C is reproduced or imaged on 
the nearfield shown in FIG. 4 the two nearfields will not completely 
overlap each other, this reducing the coupling efficiency. 
In FIG. 2 an optical coupling device in accordance with the present 
invention is shown in a side view. Elements corresponding to each other in 
FIGS. 1 and 2 are indicated with the same reference designations. In 
accordance with the present invention a diffraction optics element 11 and 
12 is each provided between end surfaces 2, 5 and 7, 9 respectively. Each 
diffraction optics element is preferably a phase hologram, a kinoform or 
any wavefront-reconstructing device. The diffraction optics element has a 
transfer function that adapts the nearfield of the end of the optical 
fiber to the nearfield of the end surface of the semiconductor laser 
amplifier. The transfer function is a mathematical expression known to the 
man skilled in the art and will therefore not be described in detail here. 
It is a simple matter to manufacture computer generated holograms having 
the described transfer function. Once the original has been produced 
copies of the original can be produced at a low price. The desired 
transfer function is impressed in the diffraction optics element in the 
form of grooves or impressions of varying form, depth and density. Usally 
the diffraction optics element is a film or plate of optically transparent 
material such as glass or plastics. 
In FIG. 2 it is shown that the optical coupling device in accordance with 
the present invention comprises a metal member 13 on which the 
semiconductor laser amplifier is mounted in such manner that heat 
generated in the amplifier is conducted to the metal member. In this 
manner the metal member works as a heat sink for the semiconductor laser 
amplifier. From FIG. 2 it is also apparent that the diffraction optics 
elements 11, 12 each are oblique relative to the optical axis of the 
coupling device in order to prevent non-desired reflection. Such 
non-desired reflection can take place if light emitted from the 
semiconductor laser amplifier 4 is reflected back into the amplifier. 
FIG. 5 is schematically illustrating how the nearfield of an optic fiber 
taper has been adapted to the nearfield of an end surface of the 
semiconductor laser amplifier with the aid of a diffraction optics element 
11 in accordance with the invention. Solid lines 14 represent the 
propagation of the electromagntic wave in the z-direction, i.e. in the 
propagation direction of the wave, while reference designation 15 refers 
to the propagation direction of the wave after its passage through the 
defraction optics element 11. 
In FIG. 6 the influence of the diffraction optics element 11 on the angle 
of refraction of a light beam relative to the optical axis of the system 
is shown. From Bragg's relation: 
EQU 2 d sin .phi.=.lambda. 
where d.sup.-1 =the frequence of the diffraction optics element, i.e. the 
distance between two adjacent grooves or the distance between two maxima 
of refraction index, 
.phi.=the direction of the beam relative to the optical axis and 
.lambda.=wavelength 
it follows that incident light of different wavelengths 
.lambda..sub..degree., .lambda..sub..degree. ' are refracted differently. 
In accordance with the present invention this is used to an advantage. 
Light of predetermined wavelengths will thus have a reduced coupling 
efficiency upon entrance into the end surface 5 of the semiconductor laser 
amplifier. This is illustrated at broken lines 16 in FIG. 6. Therefore, by 
properly selecting the frequency of the grooves the diffraction optics 
element will work as a band pass filter for the light. The advantage 
achieved in doing so is that spontaneous emission noise will come from 
light of wavelengths falling within the pass band of the filter and not, 
as is the case with the known construction, from light of wavelengths 
falling within the complete wavelength range of the amplification spectrum 
of the laser amplifier. Thus by proper selection of said frequency of the 
diffraction optics element the bandwidth can be restricted, for example to 
.+-.5 nm, around a centre wave length .lambda..sub..degree.. This has been 
illustrated in FIG. 7. 
In FIG. 7 the coupling efficiency as a function of the wavelength around a 
center wavelength .lambda..sub..degree. is shown. Should the wavelength 
filtering effect referred to above be absent, then the bandwidth would 
normally be in the order of 40 nm. 
An enclosure in accordance with the invention comprises, further to the 
laser amplifier 4, the two diffraction optics elements 11, 12 and the 
optic fibers 1, 10. If desired the enclosure may also comprise the metal 
member 11. 
The invention may be modified and varied in many ways within the scope of 
the appending claims.