Note: Descriptions are shown in the official language in which they were submitted.
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Fiber-Optic Ener~y Transmission Monitor
The present invention relates to a method and system of monitoring the intensityof light within a fiber optic cable, and more particularly to a passive photo detector
S l]tili7ing cladding modes to optically couple the fiber to a photodetector.
It is we~l known in the art to tap light from an optical fiber for various purposes.
One use of such optical fiber taps is to monitor light intensity in the optical fiber. The
purpose of such monitoring will often be to control the quantity of light which a laser
10 introduces to the optical fiber when the transfer function between the laser and the optical
fiber is variable or non-linear. In this capacity, the tap will normally form part of a
feedback circuit to the laser.
Optical taps of this nature are also used to tap light from an optical fiber, either to
15 be fed into another fiber or simply to make use of the signal while not terrnin:~ting the
fiber.
Various arrangements have been used in the past to perform this function. Most
previous arrangements have relied on microbending the fiber and attaching an optical
20 coupler to the fiber of similar refractive index to the fiber itself. This optical coupler acts
as a mode stripper so that a portion of the light leaks through the optical coupler and into
a photoreceptive device attached to the optical coupler. Devices of this nature are shown
in US S,080,506 Campbell et al, US 4,768,854 Campbell at al, US 4,741,585 Uken et al,
US 4,728,169 Campbell et al and US 4,586,783 Campbell et al.
Using this technique to tap the light from the fiber presents problems with
saturation of the photo detector and improper calibration. Using filters to prevent
saturation creates problems in determining the appropriate attenuation for signal
calibration and is labor intensive. A technique is required which prevents saturation of the
3() detector and also allows lor more precise calibra~ion of the laser diode power/energy
. .
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emitted for use as a feedback control. Mcans are also required for replacing the photo
diode without compromising the laser diode package.
Furthermore, attaching an optical coupler to a fiberoptic fibre is not straightforward
S and it would therefore be advantageous to avoid using an optical coupler if possible.
According to the invention there is provided in combination: (a) an optical fiber for
carrying light from a light source comprising: a core arranged to propagate light along its
length; a cladding layer surrounding the core; a buffer layer surrounding the cladding
10 layer; said buffer layer being removed from a discreet area along the length of the surface
of the fiber to expose the surface of the cladding layer, the exposed cladding layer being
polished; said fiber being bent over part of its length, upstream of the exposed cladding
layer, to cause light to leak from the core into the cladding layer and, in turn, from the
polished cladding layer; (b) a photodetector for detecting the intensity of light escaping
15 from the cladding layer; and (c) means for mounting said photodetector in operative
relation to said exposed cladding layer.
Further according to the invention there is provided a method of monitoring the
intensity of light tr~n.cmi~ted from a light source through an optical fiber which has a core,
20 a cladding layer surrounding the core and a buffer layer surrounding the cladding layer,
said fiber being bent over part of its length to an extent that light escapes from the core
into the cladding layer, said method comprising the steps of: (a) removing the buffer layer
from a discreet area along a part of the length of the fiber in which light is being
propagated in the cladding layer, whereby to expose the surface of the cladding layer; (b)
25 polishing part of the exposed cladding layer whereby light leaks from the cladding layer;
and (c) detecting the intensity of the light escaping from the optical fiber.
The present invention provides a passive tap for an optical fiber which allows aphotodetector to monitor light in an optical f1ber without the need for an optical coupler,
3() thu.s simplil~!ing the construction of the optical tap. Fur~hermore. the ]ig}lt le~el in the i'iher
can be monitored withoul inducing signiiicant loss in the iiber.
.
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A portion of the buffer coating of the optical fiber is removed. The cladding layer
beneath is polished with coarse grit, and minim~l distortion is induced in the optical fiber
upstream of the coarsely polished area allowing light leakage into the cladding layer and
then out of the cladding layer through the coarsely polished area of the surface. A
S photodetector is directly mounted in proximity to the coarsely polished area of the
cladding to detect light leakage.
The invention will now be described by way of example with reference to the
accompanying drawings in which:
Figure 1 shows the operation of an optical f1ber;
Figure 2 shows an optical fiber in a condition of light frustration;
Figure 3 shows an arrangement according to a first embodiment of the present
invention;
Figure 4 shows a close up of part of the fiber of the first embodiment shown in
Figure 2;
Figure 5 shows a feedback circuit according to a modification of the first
embodiment of the present invention;
Figure 6 shows an arrangement according to a second embodiment of the present
25 invention;
Figure 7 shows a cross section view of the arrangement according to the second
embodiment of the invention;
3() Figures 8 and ~ show side and overhcad vicws of a third embodimcnl of the present
invention: and
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Figures 10 and 11 show side and bottom views of a photodetector array inline
package according to a modification of the second and third embodiments of the present
invention.
S According to a first embodiment of the present invention, as shown in Figures 1
and 2, there is provided an optical fiber 1 comprising a glass core 2 of diameter 200,um,
a cladding layer 3 surrounding the core having an outside diameter of 245~m and
thickness 45,um comprising glass of a different refractive index to that of the core 2 and
a nylon buffer layer 4 of outside diameter 265~m and thickness 20,um.
A laser module, not shown in the Figures, i,s provided for tr~nsmitting light into the
glass core 2 via an optical interface 10. As the laser light entering the core is coherent, all
of the light entering the fiber 1 will be travelling in substantially the same direction. The
interface between the core 2 and the cladding layer 3 is substantially flat at the scale of the
wavelength of the light so that scattering of light does not normally occur at this interface.
The light in the glass core 2 travels along the core until it reaches the surface of the core
where there is an interface with the cladding layer. Due to the difference in refractive
index between the core and the cladding layer, light will be refracted. If the light is
incident on the surface of the core at an angle greater than the critical angle, defined as
follows:
~c=sin-l(nlln2)
nl = refractive index of the core
n2 = refractive index of the cladding layer
it will be totally internally reflected and continue its passage along the core. If the core is
a straight cylinder, the ray will always hit the surface of the core at the same angle, will
always be reflected, and will continue to propagate along the core, as shown in Figure 2.
30 The laser is arranae~ in such a manner that the light travclling along the core hits thc
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interface at an angle greater than the critical angle, and propagates along the fiber as
discussed above.
As illustrated in Figure 3, if the fiber 1 is bent, the angle of incidence of the light
5 on the interface between the core 2 and the cladding on the surface of the fiber 3 facing
away from the center of curvature will be decreased, and the angle of incidence on the
surface of the fiber facing toward the center of curvature will be increased. Such bending
is known as light frustration. At a certain radius of curvature, the angle of incidence on the
outer facing surface will pass below the critical angle and light will escape from the core
10 2 as shown in Figure 3. Light passing into the cladding layer 3 will then hit the interface
between the cladding layer 3 and the buffer layer 4. The buffer layer 4 has a refractive
index much lower than the cladding layer and therefore total internal reflection will occur
at this interface. l'he light reflected from the buffer layer will then reach the cladding
layer/core interface, but due to the high angle of incidence, little refraction occurs, and
15 most of the light is reflected again and remains in the cladding layer 3. Most of the light
which leaks from the core is trapped in the cladding layer and continues to internally
reflect along the fiber, even if the fiber is straightened downstream of the light frustration,
as further shown in Figure 3.
As shown in Figure 4, a section of the buffer layer may be removed at a location11 downstream from a point of light frustration. A region 12 of the cladding layer 3 is
polished. The polishing roughens the surface of the cladding layer allowing light which
reaches the roughened surface to escape from the cladding layer despite the large change
in refractive index between the glass cladding layer 3 and the atmosphere.
A section of the fiber upstream of the polished section of fiber is light frustrated
to a radius of curvature at which light leakage into the cladding layer starts to occur. Due
to the polishing of the cladding layer, a significant proportion of the light entering the
cladding layer 3 will escape therefrom at the location 11 of the polishing 12.
3()
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As illustrated in Figure 1, an InGaAs photodetector 13 is placed in proximity to the
exposed section of cladding 11 and is isolated from other light sources. The level of light
emitted from the fiber is monitored by the photodetector 13. The fiber is terminated by an
optical interface 14 and the level of energy in the f1ber is monitored by a pyroelectric
5 detector 15. By polishing the fiber and bending the f1ber to an appropriate degree, it is
straightforward to ensure that the light intensity escaping from the polished portion of the
fiber does not saturate the photodetector, so that the signal output by the photodetector is
representative of the intensity of light in the f1ber.
According to an advantageous development of the first embodiment of the
invention, shown in Figure 5, the output signal from the photodetector 13 is compared with
two high and low voltages VH and VL between which the output should be m:~int~in~d. If
the voltage falls outside this range, an LED warning light 22 is lit to alert the user.
In a further advantageous development of this embodiment, the output of the sensor
is fed back to the laser module control mechanism so that it can correct for non-linear
characteristics of the laser or fiber or other variations in the interface, to keep the light
level in the fiber 1 at the intended level.
A second embodiment of the present invention is shown in Figures 6 and 7. A
plastic connector housing 30 is provided. It comprises two opposing end surfaces 31,32.
The surface 31 at one end receives a plurality of optical fibers from a laser module via an
umbilical assembly. Connecting the fibers in the umbilical to the fibers in the connector
can be achieved through an optical interface at the side 31 of the plastic connector. This
would include a positioning pin with a stainless steel plate molded into the housing that
would mate with a female type connector linked to the umbilical. However, the plastic
molding could alternatively be molded onto f1bers emerging from the umbilical. This
would eliminate the need for any optical interfaces. As shown best in Figure 7, optical
f1bers pass throu h a set of bores 33 which extend from the end surface 31 to the other
3() surl'dce 3~ thr()ugll the plastic housing 3() and arc terminaled ill Fiber O~tic interlaces 37.
The fibers are molded illtO ~he connector during creation of the conneclor. Modifications
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of the embodiment are envisaged in which the connector is made in two parts which are
bonded together enclosing the fiber. Signals that enter the plastic connector housing are
accordingly propagated through the plastic connector housing, to the second fiber-optic
interface 37 and into a second fiber-optic cable.
s
The path of the fibers is bent in at least one location upstream of the photodetector
wells, in such a way that each of the optical fibers 1 in the plastic connector housing are
held in a light frustrated arrangement as previously described in connection with Figure
3. In this embodiment, bending will inherently occur between the umbilical and the
10 connector, as the fibers have to be brought into line with the bores through the plastic
connector housing, and will generally also be bent to an extent within the umbilical if the
umbilical is not held prefectly straight upstream of the connector. No specific provisions
therefore need to be made for bending the fibers. However, in an embodiment where this
was not the case, the bore through the plastic connector housing could be curved upstream
15 of the photodetector wells to light frustrate the fiber. The optical fibers 1 passing through
the bores are stripped and polished as in the first embodiment so that a small amount of
light escapes from the fibers. This may be done prior to molding or bonding of the plastic
connector around the fibers. A set of photoreceptor wells 34 extend perpendicularly from
a top surface of the plastic connector housing 30 into the housing. Each well intercepts
20 one of the bores 33 holding the optical fibers. The wells intercept the fiber holding bores
in the section of the fiber where light is travelling in the cladding layer close to where the
fiber is light frustrated.
InGaAs photodetectors 13 are mounted on the upper surface of the plastic
25 connector housing 30 above each of the photodetector wells 34. They are each arranged
to receive light from the well over which they are mounted and not to receive any light
from anywhere outside the well. Calibration of the signals generated by these
photoreceptors is likely to be required, as each of the fibers upstream of the detectors are
likely to be bent to different degrees, allowing ditferent amounts of light to enter the
~() cladding layer. However, the calihration should onlv need to be carried out once as the
libers should remain in the same state once the assembly has been completed. The
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calibration would normally be carried out empirically, so that the same value is output by
each calibra~ed photodetector when the light intensity through each f1ber is the same.
The outputs of the photodetectors 13 are monitored to determine the intensity of5 the light being carried by the fiber. A feedback circuit to the laser producing the light in
the fiber is arranged to m~3int~in the intensity of the light at a level proportional to the
signal that the fiber is intended to carry.
A third embodiment of the invention is shown in Figures 8 and 9. Figure 8 shows
10 a side view of an aluminum connector housing 50 of the third embodirnent while Figure
9 is a top view. The housing of this embodiment differs from that of the second
embodiment in that no photodetector wells are provided in the housing. The optical fibers
1, passing through the alllmimlm housing 50 are exposed over a section of the surface of
the housing, making it easier to remove the buffer layer and polish the exposed cladding
15 layer. Photodetectors 52 are mounted directly adjacent to the fibers to detect the light
therein, along with a signal conditioning electronics board to process the signals from the
photodetectors. Two rows of fibers are provided in the housing, one row being exposed
on the top surface and one row exposed on the bottom surface. A row of photodetectors
52 is mounted on the top surface, and a row on the bottom surface. By arranging the fibers
20 in this way, twice as many fibers can be monitored in a similar sized package.
By an advantageous modification of this third embodirnent, the photodetectors 13may be in an in-line package 54 as shown in Figures 10 and 11. This makes m~mlf~cture
of the units much cheaper and allows closer mounting of the photodetectors whereby more
25 fibers can be monitored in a given sized unit. Furthermore, any circuitry associated with
each of the photodetectors can be mounted inside the inline package.
While the embodirnents described above use InGaAs photodetectors, silicon,
germanium or any other type of photodetector operating at the frequencies of the light in
3(:) the l'iber wou]d he acceptahle alterna~ es Optical interl'aces are accomplished by methods
known ~ell in the art such as Ihe ~T and FC standards.
