Thermal measuring and testing – Temperature measurement – Nonelectrical – nonmagnetic – or nonmechanical temperature...
Reexamination Certificate
2001-04-30
2003-01-28
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Nonelectrical, nonmagnetic, or nonmechanical temperature...
C374S137000, C374S120000, C385S012000, C250S227140
Reexamination Certificate
active
06511222
ABSTRACT:
The present invention relates to the field of temperature sensors and concerns more particularly a compact optical fibre temperature sensor.
Currently, optical fibre temperature sensors are divided into two categories: the first category concerns so-called extrinsic optical sensors and the second those called intrinsic optical sensors. In extrinsic optical sensors, the optical fibre is a passive element assuring a simple transmission line function and it must therefore be perfectly insensitive to the physical variable to be measured. Conversely, for intrinsic optical sensors, it is the optical fibre itself which is the element sensitive to the variable to be measured, which acts directly on the actual physical characteristics of the fibre.
There is known, in particular from French Patent No. 2 664 695, an optical fibre temperature sensor of the intrinsic type wherein the multimode type optical fibre which includes an outward connection length (from the optical source) and a return connection length (to the detection and exploitation circuit) is formed in its sensitive portion in a winding of determined curvature with several turns in the medium to be monitored. This sensors relies on the birefringence properties of a curved optical fibre. It is known that the curvature of a fibre, by inducting stress in the fibre, causes losses and thus a drop in the luminous intensity transmitted.
However, such a temperature sensor also has numerous drawbacks. First of all, the configuration of the sensitive portion of the optical fibre, wound over several turns, influences the space requirement and lifetime of the sensor. Each optical fibre in fact has a critical radius of curvature Rc for which the fibre will fracture (for example for an entirely silica fibre with an external radius r, this critical radius of curvature is equal to 100 r/3.3). Hence, the space requirement of the sensor is imposed by this minimum dimension and the lifetime of the sensor will be shorter the closer the winding diameter is to this critical fracture diameter. Further, the use of a multimode fibre generates particularly severe exploitation conditions. Indeed, a fibre of this type includes a large number of propagation modes, which greatly depend both on the optogeometrical properties of the fibre (refraction index and core radius, refraction index profile, numerical aperture), on the initial injection conditions of the optical source (solid angle and emitted wavelength source-fibre distance, axial and angular alignment), on the conditioning of the transmission line (rectilinear or curved path), and on the surrounding temperature. Hence, it is very difficult to obtain identical modal distributions from one sensor to another. Finally, in such curved optical guides, the temperature response is directly linked to the modal distribution at the input end of the fibre and it is assumed that only the first curvature lengths efficiently contribute towards generating losses, the losses virtually stabilising beyond a certain length.
The present invention thus concerns a compact optical fibre temperature sensor which nonetheless has great linearity and a large measuring range, and is arranged so as to permit industrial exploitation.
These objects are achieved by an optical fibre temperature sensor including an optical source for supplying an optical signal, a first fibre optic transmission line connected to the optical source, a sensitive portion of the optical fibre connected to the first fibre optic transmission line, a second fibre optic transmission line connected to the sensitive portion of the optical fibre, and an optical detection and processing circuit connected to the second fibre optic transmission line to receive and analyse the optical signal transmitted by the optical source and passing through the optical fibre, characterised in that said sensitive portion of the optical fibre is mounted on a plane support and bent over a determined length (N
1
) and with a determined curvature amplitude (A
1
). Preferably, the sensitive portion is periodically bent (T
1
).
Via this particularly simple structure wherein the sensitive portion of the optical fibre is shaped in an optical guide having perfectly determined characteristics, it is possible to obtain particularly precise temperature measurements. Moreover, the use of an optical fibre offers such a sensor complete immunity as regards perturbation of electromagnetic origin and complete security in a sensitive, and particularly an explosive, medium.
In an alternative embodiment, a second transmission line can be omitted from this optical fibre temperature sensor and replaced by a plane reflective element directly connected to the sensitive optical fibre portion, which is mounted on a plane, periodically bent (T
1
) support over a determined length (N
1
) and with a determined curvature amplitude (A
1
).
Depending on the optical fibre employed to form the sensitive portion, the period of curvature T
1
is selected so as to satisfy either the following inequality:
2&pgr;T
1
≧1/&rgr;
2
kn
1
+2&Dgr;/&rgr; for a step index fibre, or the following equality:
T
1
=&rgr;&pgr;(2/&Dgr;) for an optical fibre with parabolic index profile, where K=2&pgr;/&Dgr; is the module of the wave vector and &lgr; the optical wavelength;
n
1
is the refraction index of the core material of the optical fibre;
&Dgr;=n
1
−n
2
2
is the relative difference of the refraction indices of the core and cladding materials of the optical fibre; and
&rgr; is the optical fibre core radius.
According to a preferred embodiment, the sensitive portion of the optical fibre of the temperature sensor according to the invention is preceded and followed by a section of fibre, respectively upstream and downstream, mounted on a plane support and periodically bent (T
2
, T
3
) over a determined length (N
2
, N
3
) and with a determined amplitude of curvature (A
2
, A
3
). Preferably, these determined lengths of the upstream and downstream sections of optical fibre are identical.
The determined periods of curvature of the upstream and downstream sections of optical fibre are determined so as to obtain optimum coupling between all the trapped modes without causing coupling with the radiated modes.
Advantageously, the optical fibre is a multimode fibre and is formed of core and cladding materials (including an outer cladding) of decreasing refraction indices. It is surrounded, at its sensitive portion, by at least one additional layer of a material having a lower refraction index than that of the outer cladding and with different optical properties as regards temperature to those of the optical fibre core. According to a first embodiment, the layer of additional material may have an optical index which decreases with temperature when the core material itself has an optical index which is constant or increases with temperature. According to a second embodiment, this layer of additional material may have an optical index which increases with temperature when the core material itself has an optical index which is constant or decreases with temperature.
Preferably, the first and second fibre optic transmission lines are interlaced to form a network of bends of low amplitude and determined period. This determined period is selected so as to obtain optimum coupling between the trapped modes, without causing coupling between the radiated modes.
Advantageously, the optical source is selected from among the following three sources: a coherent light source of the laser type, a partially coherent light source of the superluminescent diode type, or a slightly coherent light source of the luminescent diode type.
The first and second fibre optic transmission lines preferably form a single optical fibre. However, these first and second fibre optic transmission lines may also be formed of two distinct optical fibres connected by an optical guide forming the sensitive optical fibre portion.
The present invention also concerns a temperature measuring device provided with two temperature sensors such as
Gutierrez Diego
Sughrue & Mion, PLLC
Talltec Technologies Holdings S.A.
Verbitsky Gail
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