Optical waveguides – With optical coupler – Input/output coupler
Patent
1988-09-23
1991-11-05
Sikes, William L.
Optical waveguides
With optical coupler
Input/output coupler
385 15, 385123, G02B 602, G02B 616
Patent
active
050626868
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates generally to optical sensors, that is to sensors in which the output is an optical signal, and is also separately concerned with optical fibre networks for monitoring such sensors.
BACKGROUND ART
A number of sensors have been proposed in which the output signal is transmitted along an optical fibre. In some cases, the fibre itself is used as the transducer element, while others use an optical effect.in a material which is addressed by an optical fibre to enable that effect to be sensed. The latter group includes mechanical shutter/mirror configurations, for example U.S. Pat. No. 4,547,728, and devices using optically active liquid crystals. Grattan et al (Rev Sci. Instrum 57 (6), 1175) have described a sensor using a transducer constructed from a piece of doped glass whose absorption profile is measurably temperature-dependent over a range of interest. The device operates in a transmission configuration and also utilizes a piece of neodymium glass to generate a reference wavelength. U.S. Pat. No. 4,671,651 to Toyoda et al proposes the use of the semiconductor compound CdInGaS.sub.4 which absorbs various amounts of light of specific frequencies as a function of temperature.
A serious disadvantage of transmission mode sensors of this type is the great accuracy required in their construction to ensure that the parts are closely enough aligned that at least an adequate proportion of the light received from the incident fibre reaches the endface of the detection fibre. The problem remains if the device is converted to reflection mode in which a single fibre transmits both the incident and detected light: see, e.g., the aforementioned U.S. Pat. No. 4,671,651 and U.S. Pat. No. 4,689,483 to Weinberger. In this case, even slight misalignment of the reflector, which can easily arise during assembly or in use, can seriously diminish or eliminate the received signal.
U.S. Pat. No. 4,575,259 to Bacci et al discloses a slightly different arrangement, viz an optical fibre thermometer that utilizes the temperature dependent absorption characteristics of a thermochromic solution. Light enters the sensing probe by way of a first optical fibre after which it passes through the thermochromic solution and is reflected by a reflective substance. Some of the reflected light is captured by a second fibre which is connected to either a beam splitter or a star coupler to form two separate beams of light. The beams of light are then routed to separate filters and photodetectors for comparison and ultimate calculation of the temperature.
U.S. Pat. No. 4,673,299 to Dakin discloses a temperature sensing assembly that includes a doped optical fibre that absorbs transmitted light as a function of temperature. Unlike the previously described references, the Dakin apparatus does not use a reflective surface to return the incident light. Instead, naturally occurring back-scattered light is routed to a wavelength separator and detection elements. Respective lasers produce light of different wavelengths to compensate for any variation in dopant concentration. By comparing the detector reading with the time it is detected, it is possible to obtain the temperature at any location along the optical fibre. A second embodiment presents a similar design with the exception that one laser is used in conjunction with a light-emitting fibre and a wavelength filter. The sensing fibre is doped with a material that partially absorbs the incident light and in turn produces fluorescent light covering two wavelength bands. The wavelength filter separates these bands and the relative intensities are compared with a ratiometer.
Back-scattering, in this case from a matrix of randomly distributed chips of semiconductor crystals, is also relied upon in U.S. Pat. No. 4,288,159. U.S. Pat. No. 4,652,143 to Wickersheim et al discloses a temperature measurement apparatus in which a surface of interest is coated with a temperature sensitive luminescent material. When excited by incident light from a pulse generator and
REFERENCES:
patent: 4176551 (1979-12-01), Hammer
patent: 4437761 (1984-03-01), Kroger
patent: 4523092 (1985-06-01), Nelson
patent: 4575259 (1986-03-01), Bacci et al.
patent: 4594504 (1986-06-01), Coursolle et al.
patent: 4649271 (1987-03-01), Hok
patent: 4652143 (1987-03-01), Wickersheim
patent: 4673299 (1987-06-01), Dakin
Barrow Ross
Elias Martin C.
Jacob Peter G.
Lyons Peter R. A.
Rashleigh Scott C.
Commonwealth of Australia
Sikes William L.
Wise Robert E.
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