GB2332050A

GB2332050A - Fibre-optic energy transmission monitor

Info

Publication number: GB2332050A
Application number: GB9825877A
Authority: GB
Inventors: Brian L Peterson; John E Jackson
Original assignee: Videojet Systems International Inc
Current assignee: Videojet Technologies Inc
Priority date: 1997-12-05
Filing date: 1998-11-27
Publication date: 1999-06-09
Also published as: GB9825877D0; DE19856219A1; CA2255282A1

Abstract

Fiber-optic tap for monitoring fight intensity in a fiber-optic cable (1) constructed by removing the buffer layer (4) and rough polishing the cladding layer (3) beneath so that light in the cladding layer (3) is emitted therefrom. A photodetector (13) is located in proximity with the polished surface (12) and optically isolated from other light sources. The signal produced by the photodetector (13) indicates the light intensity in the fiber. The signal produced by the photodetector (13) can trigger an alarm and can also be fed back to the light source to maintain the light intensity in the fiber (1). An array of photodetectors (13) can be provided in a unit to monitor multiple fibers (1).

Description

1 Fiber-Optic Energy Transmission Monitor 2332050 The present invention

relates to a method and system of monitoring the intensity of light within a fiber optic cable, and more particularly to a passive photo detector utilizing cladding modes to optically couple the fiber to a photodetector.

It is well 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. Th purpose of such monitoring will often be to control the quantity of light which a laser 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 be fed into another fiber or simply to make use of the signal while not terminating 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 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 5,080,506 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 detector and also allows for more precise calibration of the laser diode power/energy 2 emitted for use as a feedback control. Means 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 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 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 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 transmitted from a light source through an optical fiber which has a core, 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) 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 a photodetector to monitor light in an optical fiber without the need for an optical coupler, thus simplifying the construction of the optical tap. Furthermore, the light level in the fiber can be monitored without inducing significant loss in the fiber.

C Z i 3 A portion of the buffer coating of the optical fiber is removed. The cladding layer beneath is polished with coarse grit, and minimal 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 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 fiber; 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 invention; Figure 7 shows a cross section view of the arrangement according to the second embodiment of the invention; Figures 8 and 9 show side and overhead views of a third embodiment of the present invention.. and 4 Figures 10 and I I 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.

According to a first embodiment of the present invention, as shown in Figures I and 2, there is provided an optical fiber I comprising a glass core 2 of diameter 2001m, a cladding layer 3 surrounding the core having an outside diameter of 245pm and thickness 45pm comprising glass of a different refractive index to that of the core 2 and a nylon buffer layer 4 of outside diameter 265pm and thickness 20pm.

A laser module, not shown in the Figures, is provided for transmitting 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:

6c=sin-l(nl/n2) ril = 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.

The laser is arranged in such a manner that the light travelling along the core hits the 1 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 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 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. The 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 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 location 11 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.

6 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 fiber is monitored by a pyroelectric detector 15. By polishing the fiber and bending the fiber 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 fiber.

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 maintained. 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 Wwr 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 fibers emerging from the umbilical. This would eliminate the need for any optical interfaces. As shown best in Figure 7, optical fibers pass through a set of bores 33 which extend from the end surface 31 to the other surface 32 through the plastic housing 30 and are terminated in Fiber Optic interfaces 37.

The fibers are molded into the connector during creation of the connector. Modifications 1 7 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.

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 I 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 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 of the photodetector wells to light frustrate the fiber. The optical fibers I 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 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 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 different amounts of light to enter the cladding layer. However, the calibration should only need to be carried out once as the fibers should remain in the same state once the assembly has been completed. The 8 calibration would normally be carried out empirically, so that the same value is output by each calibrated photodetector when the light intensity through each fiber is the same.

The outputs of the photodetectors 13 are monitored to determine the intensity of the light being carried by the fiber. A feedback circuit to the laser producing the light in the fiber is arranged to maintain 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 a side view of an aluminum connector housing 50 of the third embodiment 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 aluminum 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 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 embodiment, the photodetectors -13 may be in an in-line package 54 as shown in Figures 10 and 11. This makes manufacture of the units much cheaper and allows closer mounting of the photodetectors whereby more 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 embodiments described above use InGaAs photodetectors, silicon, germanium or any other type of photodetector operating at the frequencies of the light in C the fiber would be acceptable alternatives. Optical interfaces are accomplished by methods known vvell in the art such as the ST and FC standards.

1 9

Claims

CLAIMS:
2. A combination according to claim I further including means responsive to the photodetector to produce a signal proportional to the intensity of the light detected, whereby the signal can be used to control light intensity in said fiber.
In combination:

a) an optical fiber (1) for carrying light from a light source comprising:

a core (2) arranged to propagate light along its length; a cladding layer (3) surrounding the core (2); a buffer layer (4) surTounding the cladding layer (3); said buffer layer (4) being removed from a discreet area (11) along the length of the surface of the fiber (1) to expose the surface of the cladding layer (12), the exposed cladding layer (11) being polished; said fiber (1) being bent over part of its length, upstream of the exposed cladding layer (11), to cause light to leak from the core (2) into the cladding layer and, in turn, from the polished cladding layer (12); b) a photodetector (13) for detecting the intensity of light escaping from the cladding layer; and c) means for mounting said photodetector (13) in operative relation to said exposed cladding layer (11).

A unit including a plurality of the combinations of claim 1.
4. A unit according to claim 3 including a housing (30) including a plurality of bores (33) arranged to hold said fibers (1), and wherein each of said bores (33) is provided with an aperture (34) at a location where the optical fiber (1) held by the bore (33) is polished, and wherein each of said photodetectors. (13) is mounted to receive light through each of said apertures (34).
5. A unit according to claim 4, wherein said apertures (34) are of sufficient dimension to allow all of said fibers (1) to be simultaneously polished once said fibers are held in said housing (30).
6. A unit according to claim 4 or 5, wherein said detectors (13) are all mounted in a single package.
7. A method of monitoring the intensity of light transmitted from a light source through an optical fiber (1) which has a core (2), a cladding layer (3) surrounding the core (2) and a buffer layer (4) surrounding the cladding layer (23), said fiber being bent over part of its length to an extent that light escapes from the core (2) into the cladding layer (3), said method comprising the steps of a) removing the buffer layer (4) from a discreet area (11) along a part of the length of the fiber (1) in which light is being propagated in the cladding layer (3), whereby to expose the surface of the cladding layer (3); b) polishing part of the exposed cladding layer (11) whereby light leaks from the cladding layer (3); and C) detecting the intensity of the light escaping from the optical fiber (1).
8. A method according to claim 7 further comprising the step of generating a signal representative of said light intensity and using said signal to monitor the intensity of the light source.
9. A method according to claim 8 further comprising the step of varying the light intensity of said source in accordance with the magnitude of said signal.

1
10. A method of monitoring the intensity of light transmitted from a light source through an optical fiber substantially as hereinbefore described with reference to the accompanying drawings.
11. In combination an optical fiber, a photodetector and means for monitoring the photodetector substantially as hereinbefore described with reference to the accompanying drawings.