Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a tap for monitoring an optical
signal in a fiber. The tap finds particular application for monitoriny
the output power of optical transmitters.
Optical fiber waveguide transmitters are often required
to be equipped with a transmitted power monitor. The most common
application of such a monitor is that of alarm diagnosis in the event
of optical source failure. Other applications include the
stabilization of output power and feedback linearization.
~ asically, in a fiberoptic monitoring tap, a
predetermined fraction of light from an optical fiber is diverted from
the fiber and directed to a photodetector where its power level is
measured.
A known fiberoptic tap is made by twisting together two
lengths of optical fiber, heating the twisted pair in a twist region,
and pulling the twisted pair from either end to encourage fusion at the
heated region. The resulting component has four ports. Typically,
when monitoring transmitter power, one port is coupled to the optical
source, a second port is spliced to the output fiber, a third port is
redundant and so is placed in a reservoir of index-matched oil, and
~O from the last port is taken the monitored light signal from which the
transmitted output power can be calculated.
Like other known taps, this power tap incurs a penalty in
the form of reduction of transmitted power~ Some power is diverted to
the monitor photodetector and some is lost by device coupling
imperfections.
A fiberoptic tap is now proposed in which the principle
of conservation of radiance is used and which, when practically embodied
'~
as a transmitter power monitor, draws only optical power which is
superfluous to that which can be transmitted into the output fiber.
According to the invention there is provided an optical
fiber tap comprising first and second optical fibers, the first fiber
having a higher mode volume than the second fiber whereby otle end of
the second fiber located to receive light emitted from one end of the
first fiber receivès only a fraction of the emitted light, and means
for directing to a photodetector light radiated from the first fiber
end other than said fraction directed into the second fiber.
The first and second fibers can be spliced together.
Preferably a large area photodiode is located close to a junction
region between the spliced fiber ends so as to receive light radiated
from the junction region. The photodiode can be maintained in position
by a mass of transparent adhesive surroundin~ the junction region, such
mass acting to transmit radiated light towards the photodiode.
The photodetector can alternatively be maintained at a
remote location. In such an embodiment, a transparent prism can be
sealed into a mass of transparent adhesive adjacent the junction region
so that light transmitted to the transparent block is directed
predominantly from one face of the prism. The photodetector can be
located adjacent to the prism face or can be located at the end of
monitoring optics coupled to the emitting face.
In most practical fiberoptic installations, output or
line fibers are of the graded index type, such fiber being
characterized by a relatively lower mode volume in comparison to a
step-index fiber of equivalent core size. Thus for coupling light from
a light emitting source to a graded index output fiber, the first fiber
of the tap can be a pigtail of step-index fiber with the second fiber
being a piqtail of graded index fiber ~Jhich cannot support all of the
modes qenerated in the step-index fiber. However, the only requiremerlt
of the tap is that some modes transmitted through the first fiber
should not he supportable in the second fiber. Consetluently, so long
as the mode volumes differ appropriately, the transition can be
step-index to step-index or graded index to graded index.
Embodiments of the invention will now be described by ~ay
of example, with reference to the accompanying drawings in which:-
Fi~ure 1 show a fiberoptic tap according to the
invention, the tap incorporating a large area photodiode; and
Figure 2 shows an alternative embodiment of fiberoptictap for use with a remote photodetector.
Referring in detail to Figure 1, there is sho~n a light
emittinq diode 10, a fiberoptic tap 12, and a line fiber 14. The power
tap has two pigtail fibers 16 and 18 of substantially identical core
area, the fibers being spliced together, for example by the well-known
fusion splicintl technique. Immediately adjacent to a splice zone 20 is
a lartle area photodiode 22 oF a type available From SILONEX under the
2~ mo~el number NSL 703, the spliced fiber being fixed in position
adjacent the photodiode 22 by a mass 24 of adhesive. A suitable
adhesive is made by Norland under the model No. NOA 61. The adhesive
is not brittle and flows easily so producing a very smooth surface when
it cures. The Pi9tail fiber 16 is a relatively high mode volume
step-index fiber, the mode volume basically being a measure oF the
maximum light which the light can carry, this being a function both of
the Fiber numerical aperture and the core diameter. The fiber 18 has a
lower mode volume owing to its graded index core. The light emitting
t.~
diode 10 has an ennitting diameter equal to or greater to the tore
diameter of the piqtail fiber 16 to which it is coupled. This ensures
that the step-index pigtail fiber 16 is fully excited. In particular
it supports rnodes which cannot be supported in the other piytail fiber
18. Power transmitted across the splice zone 20 is only 50% of the
power transmitted by fiber 16 because of the difference in refractive
index profiles of the two fibers.
Most important from a practical viewpoint however, is the
1n fact that the pigtail fiber 18 carries no less power than if it had
been directly coupled to the light emitting diode 10. The rejected
power is radiated away from the splice zone 2n. Upwardly directed
light is reflected at the surface of the adhesive mass 24 and together
with directly radiated light, passes through a thin layer of the
adhesive and empinges on the adjacent photodiode 22. The photodiode 22
can be connected into a diaqnostic alarm circuit, a linearization
circuit or a stabilizing circuit all being well known in the optical
transmitter control art. The pigtail fiber 18 should itself have a
mode volume marginally larger than the line fiber 14 so that even if
liqht is lost at splice or connector imperfections, the line fiber
~n transmits the maximurn optical power of which it is capable.
The power tap described possesses several advantageous
features compared to known power taps. Firstly, it taps ofF a large
proportion of power, 50% in the example of Figure 1, without lessening
the power that would, in any practical embodiment, be launched into the
~raded index line fiber. Secondly, since the power tap is essentially
a standard splice, the insertion loss is minimal, and both size and
cost are modest. Thirdly, power detected is very closely correla-ted
with the transmitter power. Finally, so long as modes are excited
~ ~5~ ~ ~ 3 ~
throuqhout the area of the first fiber core, the tap ratio is
determined lar~ely hy the respective mode volumes of the fibers 16 and
18, the mode volume being a standard pararneter of comrnerciall~
available fiher. The tap ratio is not closely dependent upon assembly
tolerances. It should be mentioned that although LEU's ~ill normally
excite modes throughout the first fiber core, semiconductor lasers
often will not. In the latter case, though the tap is quite adequate
as a message tap, it will have a less predictable power tap ratio.
Referrinq to Figure 2, there is shown an example of po~Jer
1n tap in which the photodetector is located remote from the power tap
splice. Features equivalent to those appearing in the Figure 1
embodiment are designated by like numerals. The Figure 2 optical power
tap has a transparent block 28 of glass adhering to the fibers in the
position occupied by the photodiode in the Figure 1 embodiment. The
block has an inclined plane face 30. The angle of the face is chosen
to be roughly perpendicular to the direction in which optical radiation
(shaded) is emitted from the splice zone 20. The light is concentrated
at a lens system 32 to the input end of a supplementary fiber 34 whence
it is taken to a monitoring photodetector.
2n The examples of tap shown are for a specific use as power
taps. The invention can also be used in, for example, a series of
message taps in which a certain amount of light is drawn off at each
tap. In such an embodiment, at each tap a short length of relatively
lower mode volume Fiber will be inserted between and spliced to
conti~uous len~th of a line fiber. The arrangement obtained can be
used as a tap for optical siganls propagating in both directions along
the fiber since two unidirectional tap regions are providedD
In the embodiments sho~n, the function of the adhseive 24
is manifold. Thus, the reqion of adhesive belo~J the splice zone 20 in
the Figure 1 and 2 embodiments acts to couple light out of the cladding
of pigtail fiber 18 to transmit it to a photodiode or transparent block
as appropriate. rlext, the top surface of the adhesive acts to
supplement the downward directed light by reflecting upwardly radiating
light from its top surface. Additionally the adhesive has a protection
function. Lastly, the adhesive acts to support the photodiode or the
transparent block in the desired position adjacent the splice zone 20.
nbviously, althou~h not particularly convenient from a practical
viewpoint, these three functions can be performed by two or three
components instead of jointly by the adhesive 24. Additionally, there
is no reason, in principle, why the photodiode cannot be fabricated
directly on the fiber in the splice zone.
2n
