Optical fiber moisture sensor

An optical fiber moisture sensor comprising a mechanical system having a stationary jaw and a moving jaw between which said fiber is mounted, the sensor also including an element made of moisture-sensitive material coupled to said moving jaw to allow it to be urged towards said stationary jaw in the presence of moisture, thereby applying compression to said optical fiber, wherein said element is made of a material that looses mechanical strength with increasing moisture and that is mounted to oppose the action of resilient means urging said moving jaw towards said stationary jaw.

The present invention relates to moisture sensors using an optical fiber 
and designed to enable the state of optical links to be monitored. Such 
sensors are used in particular at special points on an optical connection, 
such as splice boxes, where possible ingress of water or moisture is 
prejudicial to the quality of transmission. Such sensors may also be used 
in any kind of industrial system that requires monitoring for moisture or 
water. 
BACKGROUND OF THE INVENTION 
Optical fiber sensors, per se, are already known. They use a material 
which, in the presence of moisture, absorbs the moisture and swells, 
thereby compressing an optical fiber to a greater or lesser extent as a 
function of the amount of moisture absorbed, which fiber is used for 
monitoring the connection. The stress applied to the optical fiber gives 
rise to a local attenuation in transmission and this is detected. 
The article entitled "Optical fiber line support system" published in NTT 
Review, Vol. 3, No. 1, January 1991 describes such a sensor. The optical 
fiber passes through a mechanical system having a stationary jaw and a 
moving jaw between which the fiber passes, and having absorbent material 
that swells in the presence of moisture, thereby optionally actuating the 
moving jaw. The system applies no stress on the fiber in the absence of 
any absorbed moisture, and it compresses it with increasing moisture 
absorption by the material. 
That system has the advantage of being particularly simple. However, its 
reaction time is relatively long and it is not very accurate when it comes 
to monitoring quickly-varying stresses due to variations in ambient 
humidity which may be prejudicial to the quality of transmission on the 
optical line monitored by means of the sensor. 
An object of the present invention is to remedy these drawbacks. 
SUMMARY OF THE INVENTION 
The present invention provides an optical fiber moisture sensor comprising 
a mechanical system having a stationary jaw and a moving jaw between which 
said fiber is mounted, the sensor also including an element made of 
moisture-sensitive material coupled to said moving jaw to allow it to be 
urged towards said stationary jaw in the presence of moisture, thereby 
applying compression to said optical fiber, wherein said element is made 
of a material that losses mechanical strength with increasing moisture and 
that is mounted to oppose the action of resilient means urging said moving 
jaw towards said stationary jaw. 
The sensor of the present invention preferably also includes at least one 
of the following features: 
said element made of moisture-sensitive material is mounted as a spacer 
between said jaws, thereby moving them apart against the action of said 
resilient element without compressing said fiber in the absence of 
moisture, but at least partially releasing the action of said resilient 
element in the presence of moisture; 
the said material is selected from materials based on cellulose, 
polyacrylamide, saccharose, and carbon hydrate, in particular; and 
the said optical fiber is an optical fiber independent from an optical link 
being monitored or it is one of the optical fibers of said optical link 
being monitored.

DETAILED DESCRIPTION 
In the sole FIGURE, the moisture sensor has an optical fiber 1. It is 
mounted in a box 2. The box is not watertight. In particular, the box 2 
also contains splices between two lengths in a multifiber optical 
connection to be monitored, with the sensor being associated therewith in 
use. The optical fiber 1 may be a fiber independent from the connection 
being monitored and it may be a monomode or a multimode monitoring fiber. 
Alternatively, it may be a fiber belonging to the connection being 
monitored, being a monomode fiber or a multimode fiber, that is used at a 
first wavelength for transmission purposes, and at a second wavelength for 
monitoring purposes. 
The sensor is constituted by a mechanical system for compressing the 
optical fiber 1 as a function of the moisture in the box 2. It comprises a 
stationary jaw 3 and a moving jaw 4 on opposite sides of the optical fiber 
1 which passes between them. Each of these two jaws has a middle side 
shoulder 3A or 4A on its side facing the other jaw, which shoulder is 
itself serrated as shown, or corrugated, or merely convex, being smooth or 
otherwise. 
The stationary jaw is carried by one of the walls of the box and it is held 
stationary relative to said wall. The moving jaw is urged by resilient 
means towards the stationary jaw. It is held relative to said stationary 
jaw against said resilient means by a counteracting element whose 
counteraction varies with moisture conditions, having high mechanical 
strength in the absence of moisture in the box but loosing said mechanical 
characteristic with increasing moisture. 
In the embodiment shown, this counteracting element is mounted to hold the 
jaws apart and is constituted by two pairs of spacers or abutments 6 or by 
two spacing rings between the jaws 3 and 4 that act against a compression 
spring 7. The spring 7 is mounted between the moving jaw and the facing 
wall of the box. 
This counteracting element is made of a material based on cellulose, 
polyacrylamide, saccharose, or a carbon hydrate, in particular, having 
high mechanical strength when dry but loosing strength in the presence of 
and as a function of ambient moisture, such that its strength may become 
substantially zero, thereby releasing the resilient element to act 
correspondingly on the moving jaw. 
Any such mechanical stress exerted on the optical fiber 1 is detected and 
located by backscattering, in particular. 
Naturally, any other equivalent disposition of the resilient element and of 
the counteracting element could be adopted without going beyond the ambit 
of the present invention, providing said counteracting element opposes the 
action of the resilient element when dry and offers reduced mechanical 
opposition thereto in the presence of moisture.