CA2408043A1 - Optical sensor and readout apparatus - Google Patents

Description

Optical sensor and readout apparatus s FIELD OF THE INVENTION
The present invention generally relates to a method and an apparatus for measuring and monitoring various physical parameters, such as temperature, strain or pressure, with an optical sensor.
io BACKGROUND OF THE INVENTION
Optical sensors in general, and fiber optic sensors in particular, have is proven to be useful in a number of situations where access to the measurand is problematic with electric wiring, or where electromagnetic interference prevents the use of electric or electronic sensors. Optical sensors use the correlation between the change of the optical properties of a material and the parameter to be measured. The material in question is interrogated by a beam of light in free 2o space, or by light guided by a waveguide such as an optical fiber, and the transmitted, reflected, re-emitted or scattered light is measured by a remote photodetector or other type of photosensor.
Many optical sensors use a material with bandpass properties, that is a 2s material where light is either transmitted, reflected or absorbed over a narrow band of wavelength. The shift of the wavelength corresponding to maximum or minimum transmission or reflection is then correlated with the parameter to be measured. To measure that wavelength, one can disperse the transmitted or reflected spectrum of a broadband source with a spectrometer, and use, for example, a photodetector 3o array to detect the peak wavelength. Spectrometers, however, are quite bulky and expensive, especially if you need a good wavelength resolution. Alternatively, one

2 can use a tunable monochromatic source, and scan the spectrum band of interest.
Tunable sources, however, are typically quite expensive, and often include fine mechanical parts that make them not very suitable for use in the field.
s One example of a bandpass-type sensor is a so-called Fiber Bragg Grating (FBG). This is a narrowband filter imprinted into the core of a single mode optical fiber, using the interference pattern created by two intersecting beams of UV
light.
Due to the photosensitive nature of some optical fibers, this results in the creation of a permanent periodic refractive index pattern along the fiber, which reflects light io over a very narrow range of wavelengths centered on ~,m~= 2 n P, where n is the effective index of the mode guided by the fiber, and P is the period of the grating.
Since the wavelength of maximum reflection is dependent on the period of the grating as well as the refractive index of the fiber, FBGs can be used to sense temperature, strain, pressure, and many other parameters. Their reflective nature is imply that the source and detector can easily be collocated. The fact that they are imprinted inside the fiber itself lends to very small sensor gauges, since the fiber diameter is typically only 125 microns. Furthermore, multiple FBGs can be imprinted at different locations along the same optical fiber, and interrogated with the same instrument. However, the cost of the readout instrumentation has 2o remained so far quite high due to the complexity and cost of the components used (tunable lasers or spectrum analyzers).
Methods that involve measuring the peak wavelength in fact discard a large part of the information available from the sensor, that is the shape of the bandpass 2s spectrum itself. Given that that shape is very often a constant, its knowledge provides a means to track the position of the peak wavelength, just by measuring the amount of light that is transmitted or reflected by the sensor. The reason this is not done is that the transmissivity through the rest of the optical system that carries the light from the light source through the sensor and to the photosensor, is 3o rarely known with good accuracy, and furthermore can vary with environmental conditions or over time.

3 Accordingly, it would be desirable to provide a method and an apparatus for measuring and monitoring various physical parameters (such as temperature, strain, pressure, etc...) that alleviated some of the above-mentioned drawbacks of s the prior art. Furthermore, it would be desirable to provide such a method and apparatus requiring very few optical components, all of which being robust and compact enough in such a way that the apparatus can be made to be portable and is able to operate in various harsh environments.
to SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, there is provided a method and an apparatus for measuring and monitoring various is physical parameters, like temperature, strain, pressure, etc... The apparatus comprises an optical sensor, whose spectral reflection or transmission properties vary with the parameter to be measured (the measurand) in a well known and defined manner, and a readout apparatus that translates this variation into an electronic signal.
More specifically, the present invention provides such a method and apparatus that is able to track the difference between the wavelength of a fixed optical source and the wavelength of maximum reflection or transmission of a bandpass-type optical sensor, using a knowledge of the spectral shape of the 2s sensor. It is particularly suitable to the readout of FBG sensors, since the spectral shape of an FBG can be tailored almost at will at fabrication time.
The present invention and its advantages will be better understood upon reading of preferred embodiments thereof with reference to the appended 3o drawings.

4 BRIEF DESCRIPTION OF THE DRAWINGS
A detailed non-restrictive description of preferred embodiments will be given herein below with reference to the following drawings, in which like numbers refer s to like elements:
Figure 1 is a schematised representation of an optical sensor and readout apparatus used in transmission according to a preferred embodiment of the invention.
Figure 2 is a schematised representation of an optical sensor and readout ~o apparatus used in reflection according to a preferred embodiment of the invention.
Figure 3 is a schematised representation of an arrangement of a plurality of optical sensors for using the same optical source to probe the plurality of sensors.
Figure 4 is a schematised representation of an arrangement of a plurality of optical sensors for using a plurality of wavelength multiplexed optical sources to probe is several sensors along the same optical fiber according to a preferred embodiment of the invention.
Figure 5 is a graph of a wavelength (nanometers) versus the function S'/S for a 0.4 mm long uniform Fiber Bragg Grating.
DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The present invention provide a method and an apparatus for measuring and monitoring various physical parameters, like temperature, strain, pressure, 2s etc... As illustrated on Figures 1 and 2, the apparatus comprises an optical sensor S, whose spectral reflection or transmission properties vary with the parameter to be measured (the measurand) in a well known and defined manner, and a readout apparatus that translates this variation into an electronic signal.
3o The optical sensor S is a device having spectral reflection or transmission properties such that reflection or transmission is maximum at a given wavelength of light, denoted by ~.maX, and decreases monotonically for wavelengths greater and smaller than a,",ax, at least over a range of wavelengths 0~,. The shape of the transmission or reflection curve over that wavelength range is called the spectral signature of the sensor, and can be described by a function of wavelength ~,, s which we call S(~,), normalized so that S(~.maX) = 1. The transmission or reflection of the sensor itself is given by Tm~ *S(~.), or RmaX *S(~.), where TmaX and RmaX are the value of transmission or reflection respectively at ~,max. Furthermore, the optical sensor is made such that when the parameter to be measured varies, ~,~,~"~
varies in a well defined manner, but the spectral signature S(~,) remains unchanged.
to Furthermore, the function S(~,) must be such that its slope, or derivative S'(~,), when divided by S(~,), varies monotonically over the range 0~, around ~,m~.
For example, if S(~,) is a gaussian function exp(-(~,/8)Z), then S'(~,)/S(~,) is a linear function of ~,. The value of the function S'(~,)/S(~,) is then unambiguously attributed to the difference between ~, and ~,meX, when ~.m~"~ varies over a range ~~..
The range is of wavelengths 0~, corresponds to the range of variation of ~,m~ that can be unambiguously measured by the readout apparatus described below, and therefore corresponds to the measurement range of the sensor. A sensor having a Gaussian signature is advantageous since S'(~,)/S(~,) is a linear function of ~, and, then, there is only a single easily determined value of ~, corresponding to 2o S'(~,)/S(~,). However, the spectral signature S(~,) of the optical sensor may have another shape than a Gaussian function as long as it is well known and well defined and that S(~,) decreases monotonically for wavelengths greater and smaller than ~,m~.
Referring again to Figures 1 and 2, there is shown a schematised 2s representation of an optical sensor S and a readout apparatus according to the present invention. As a preferred embodiment, the optical sensor S is a fiber Bragg grating (FBG). Advantageously, the FBG can be designed to have a spectral signature approaching closely a gaussian function. This is achieved by giving the FBG a so-called "apodization profile" which is a gaussian function, and 3o keeping the FBG reflectivity low enough (typically less than 10%). If the FBG

b reflectivity is too strong, its spectral shape is almost flat around ~,m~, making its slope very small, so that the signal S'(~,)IS(7~) is also always very small.
However, even a uniform FBG of low enough reflectivity will have a usable spectral signature. The FBG can be designed to have a smaller or larger value of 0~..
If the s FBG has a uniform period, then o~, is inversely proportional to its length.
On the other hand, if the period of the FBG is linearly chirped (i.e. Increases or decreases in a linear fashion along the length of the FBG), then e~, is proportional to its length. The use of an FBG gives a lot of flexibility in the choice of 0~,. For example, a l0mm long FBG with a gaussian apodization profile having a full-width at half-io maximum (FWHM) of 3mm, and a uniform period, will have a usable e~, of about 0.5 nm, while the same FBG with a chirped period of 15 nm/cm will have a ~~, of 7 0 nm. Sensors of the same length can therefore have measurement ranges varying by a factor of more than 20. Of course, any other optical sensor presenting a well known spectral signature having a reflection peak or a is transmission sag such as a long period fiber grating for example could be conveniently used.
The readout apparatus, which is schematized in Figures 1 and 2 for sensors used in transmission or reflection respectively, comprises a wavelength-locked 20 optical source (WLOS), itself comprised of an optical source OS emitting over a very narrow range of wavelengths, much smaller than 0~,. Preferably OS is a laser.
Furthermore, the central wavelength of the optical source ~,o is modulated at a frequency f, over a range 8~,, which is also much smaller than 0~,. Therefore the wavelength is a function of time described by: ~,(t) _ ~.o + (8a,/2) sin(2~ft).
For example, and as a preferred embodiment, the source can be a DFB
(Distributed FeedBack) laser diode with a current driver (D), whose central emission wavelength ~,o can be modulated by modulating the driving current, or the operating temperature. The emission wavelength of a DFB laser is normally quite 3o stable in time if the driving current and temperature are kept constant.
However, to counter possible long-term drifts of the laser wavelength, the readout apparatus can also comprise a means for keeping ~,o constant (locking the wavelength).
This can be achieved by separating the light emitted by the diode with a partially transmitting optical element (a beam splitter BS1 for example), and sending one s part onto a filter (F) with bandpass-like reflection or transmission properties, and a wavelength of maximum reflection or transmission ~,f, but with a bandwidth smaller than o~., the bandwidth being nevertheless larger than 8~,. The light reflected or transmitted by the filter F is measured by a photodetector PD1. The current generated by the photodetector PD1 is treated by the electronic circuit EC1:
it is 1o amplified and converted to a voltage, and filtered to keep only the part that is modulated at the frequency f. This filtered signal is used as an error signal in an electronic feedback loop PID to act on the diode driving current and maintain the diode at the central wavelength of the filter F. The filter F can be, for example, an optical etalon, a thin film bandpass filter, or a gas absorption line. This is arrangement for "locking" the laser diode wavelength is well known in the art, and commercial devices are available to perform that task.
The other part of the light from the optical source OS is directed onto the optical sensor S either through free space with appropriate redirecting optics, or 2o through an optical waveguide. If the sensor is used in transmission, the light transmitted by the sensor is collected by a second photodiode PD2 as can be seen on Figure 1. If used in reflection, another beam splitting element BS2 is used to redirect the reflected light onto the photodiode PD2 as can be seen on Figure 2.
If the sensor S is an FBG, then the light from the laser diode can be launched info 2s an optical fiber into which the FBG is provided, and optical fiber couplers can be used as beam splitting elements.
The photocurrent from the photodiode is then treated by the electronic circuit EC2 in the following way. The current is first amplified and converted to a 3o voltage V proportional to the photodiode current. One part of this signal is filtered with a low pass filter so as to remove all modulation frequencies above a value fm;n g < f, which gives a voltage Vd~, proportional to rd~*Po*T*S(~,), where Po is the power emitted by the light source, T is the transmission of the optical system from the source to the photodiode, and rd~ is the responsivity of the diode, amplification and filtering circuit in Volts per Watt of incident power. The other part is filtered to keep s only the signal modulated at the frequency f, which gives a voltage Vas. If 8~, is much smaller than ~~,, then that part is equal to ray*Po*T*S'(~.)*8~,, where rep is the responsivity of the photodiode, amplification and ac filtering circuit. Vas is then divided by Vd~, either numerically after digitizing both signals with an analog-to-digital converter, or by controlling the gain of the amplifier to maintain Vd~
at a to preset value, with an automatic gain control circuit, in which case ray is also proportionally affected, so that Vas can be directly related to S'(~,)/S(~,).
The value of Vas thus extracted can be correlated to the difference between ~,m~ and ~.o, and to the value of the measurand through calibration curves.
is For increased accuracy, the signal obtained from the light collected through the locking filter F can be treated in the same way by EC1, so that the error signal is made to be proportional to the difference between ~,o and ~,f. The error signal is always small, as it is kept close to zero by the feedback loop, however it is a more precise estimation of ~,o than just using ~,f. Furthermore, if ~,f is the central 2o wavelength of a gas absorption line, then its value can be known with extreme accuracy, and is not prone to variations in time, as opposed to an optical etalon or thin film filter, whose central wavelength depends on temperature, angle of incidence, and refractive index, all quantities which can vary with environmental conditions.
The accuracy and resolution of the readout system is then limited by the noise from the photodiode and electronic amplification and filtering circuit, by the precision and accuracy of the calibration curves, and by the calibration factor proportional to s~,. One must also ensure that the wavelength modulation 3o amplitude S~, does not vary over time, or with environmental conditions.

The main advantage of this readout system is that the signal is independent of both Po and T, and only depends on the value of the measurand. Therefore, the readout system is unaffected by variations in the emitted power of the photodiode, s or in the transmission of the optical system that carries the light from the source to the sensor, and from the sensor to the photodiode. Another advantage is that the wavelength of the source does not need to be tunable over the range ~~, of the sensor. It only needs to be tunable over a small range within 0~,, which makes a standard, commonly available, and relatively low cost DFB laser diode suitable as io the optical source. Other readout systems for FBG sensors typically use expensive tunable lasers, or tunable filters, to scan the spectrum, and locate the wavelength of maximum reflection.
Referring now to Figure 3, there is shown an arrangement where the same is wavelength-locked optical source is used to probe multiple sensors, by splitting it into multiple beams with optical splitter SP. For each sensor, one photodiode and an electronic treatment circuit is required.
Figure 4 shows how the optical source and photodiode arrangement can be 2o combined with other similarly locked sources at wavelengths ~,~ outside of the range ~,o+/-(d~,/2), by using wavelength division multiplexing devices (WDM), such that multiple sensors with corresponding central wavelengths ~~ can be placed along the path of the light. As an example, multiple FBGs can be used at different locations along the same piece of optical fiber, or can even be overwritten at the 2s same location on the fiber, and measured simultaneously with the multiple sources. Since each sensor can be designed to operate over a different range 0~,, this scheme gives a lot of flexibility in the design of a sensing system. For example, one could use one FBG to measure temperature over a given range corresponding to one value of ~~., and another one to measure strain over a range ~o corresponding to another value of 0~,, without being constrained by a readout system that is better suited to the range of one parameter only. One could also use two or more FBG's with slightly different central reflection wavelengths, and measure the same parameter over different ranges. In this arrangement, a tunable laser could be used for probing each FBG in a sequential manner, by being locked s to a different, sequentially increasing or decreasing wavelength, using a locking filter with periodic transmission properties, although this would not give any additional advantage over the method described above and it would be much more expensive to achieve. A typical DFB laser diode, however, can be tuned over a limited range of wavelengths (2-3 nm). If the filter used for locking the to wavelength of the laser has a periodic response function, the DFB laser could be locked to a different peak of the locking filter, and as a result the reading apparatus would operate over a different measurement range. This can be used to extend the measurement range. For example, if the measurement range is 2 nm around the central wavelength of the laser, and if the locking filter has peaks is separated by 1.5 nm, the DFB laser could be set at either one of three peaks of the locking filter, given that it can be tuned over three nanorneters. The total measurement range would then be 5 nm. The instrument could be programmed to automatically tune the laser to a different peak of the locking filter when the measurand approaches certain values near the end of the measurement range.
Another useful feature is that multiple FBG's can be overwritten at the same location in the fiber, or written very near to one another so that they are affected in the same way by the measurand. The central wavelengths of the different FBG's can be slightly different so that their measurement ranges nearly overlaps. In this 2s configuration, the wavelength of the DFB laser only needs to be tuned to the center of one of the multiple measurement ranges. This has the practical advantage that the central wavelength of the DFB laser does not need to be specified with great accuracy, which means that a cheaper, non-wavelength-selected laser can be used.

In the preferred embodiment, the optical sensors are fiber Bragg gratings (FBGs), but any other component having a welt defined spectral signature including a known transmission or reflection peak could equally be used. Long period fiber grating or an appropriately selected gas cell could for example be s considered. The optical source is preferably a DFB laser diode, and the locking filter is an etalon filter, or a gas cell such as acetylene which has many absorption bands in the range 1500-1550nm. The beam splitters are fused fiber couplers in the case of an all-fiber embodiment, but could alternatively be embodied by any appropriate optical element or arrangement.
io As an example, consider an FBG used for temperature sensing. The dependence of the central wavelength of the FBG on temperature is approximately 0.01nm1°C. To make a sensor with a measurement range of 200°C, one needs the range 0~, to be 2nm. This can achieved with a uniform FBG of length 0.4 mm, and is reflectivity of 10%. Figure 5 shows the function S'!S over the 2 nm range, which is nearly linear, and can be described by a calibration function derived from a 3~d or 4t" order polynomial fit, so that only 4 or 5 parameters are required for calibrating the sensor. A 16 bit digitization of the output voltage would give more than 10,000 points, for a resolution better than .02°C.
While embodiments of this invention have been illustrated in the accompanying drawings and described above, it will be evident to those skilled in the art that changes and modifications may be made therein without departing from the essence of this invention. All such modifications or variations are believed 2s to be within the scope of the invention.

Claims