WO2008034225A1 - Optical sensor and method for making same - Google Patents

Abstract

There is provided a sensor system for measuring a change in an environmental condition, the sensor system comprising: an optical source unit for sending an optical interrogation signal in an optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range; an optical sensor device in the waveguide, the optical sensor device comprising an optical grating having an optical response spectrum comprising a substantially linear slope within the wavelength range, the optical sensor device for outputting an optical output signal substantially at the peak wavelength in response to the optical interrogation signal, the optical output signal having an output intensity according to the slope; and an optical detecting device for measuring a variation in the output intensity over time, the variation being indicative of the change.

Description

OPTICAL SENSOR AND METHOD FOR MAKING

SAME CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority of US provisional Patent Application No. 60/845,166, entitled "Optical Sensor and Method for Making Same" and filed on September 18, 2006.

TECHNICAL FIELD

[0002] The description relates generally to optical sensor technology, more specifically in the context of structural monitoring and vibration measurements.

BACKGROUND OF THE ART

[0002] In order to capture or to detect events such as an onset of volcanic activities or the onset of a potential Tsunami wave well below the surface of the ocean, or to measure various types of vibrations such as vibrations in an airplane during flight or of space vehicles during launch, vibrations of a cooling water pipe of a nuclear reactor, vibrations of rail cars, vibrations caused by flow obstruction in oil, gas, water and chemical pipelines or any other vibrations indicative of events typically occurring in hostile environments under extreme temperatures and pressures, monitoring solutions require high speed measurement capabilities. Such capabilities permit the capturing of some wide spectrum vibration signatures for example.

[0003] High speed sensing units have been designed using broad band light sources or various sweep lasers and various tunable filters such as movable Fabry Perot cavities which are tuned mechanically, or a stack of thin film filters tuned using thermo-optic effects or any other temperature dependent tunable filters. Some designs also use an analog to digital converter which is time shared amongst many sensors. All of these solutions are however limited in speed, and even more so when the number of sensors is increased.

[0004] Recent developments have introduced the use of serially multiplexed Fiber Bragg Grating (FBG) sensor arrays in a Wavelength division multiplexed (WDM) sensor network. These may require the use of peak searching algorithms or other measurement techniques which are either complex or require the collection of a considerable amount of data which increases with the number of sensors. The time required to scan through all the sensors as well as the time needed to perform the measurements therefore limits the measurement speed. Other approaches have been taken but face several issues.

[0005] The present system and method attempts to meet the issues faced by the prior art, such as speed of measurement, distance spans between sensors and base units, levels of noise, weight and ease of physical deployment or installation considerations, sensor energizing issues, lifespan of the devices, accuracy of measurements, stability and other practical issues.

SUMMARY

[0006] According to an embodiment, there is provided a method for fabricating an optical grating device comprising: providing a light source emitting ultraviolet (UV) light; providing photosensitive material; providing a UV light- blocking material having two apertures; providing a phase mask between the photosensitive material and the UV light-blocking material; and sending the UV light from the light source through the two apertures and the phase mask, onto the photosensitive material, to produce the optical grating device in the photosensitive material in a single UV light exposure using Fraunhofer diffraction effects, the optical grating device comprising a pair of optical gratings and an optical interference cavity therebetween.

[0007] According to another embodiment, there is provided a sensor system for measuring a change in an environmental condition, the sensor system comprising: an optical source unit for sending an optical interrogation signal in an optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range; an optical sensor device in the waveguide, the optical sensor device comprising an optical grating having an optical response spectrum comprising a substantially linear slope within the wavelength range, the optical sensor device for outputting an optical output signal substantially at the peak wavelength in response to the optical interrogation signal, the optical output signal having an output intensity according to the slope; and an optical detecting device for measuring a variation in the output intensity over time, the variation being indicative of the change.

[0008] According yet to another embodiment, there is provided a method for measuring a change in an environmental condition of a sensing device comprising an optical grating device within an optical waveguide, the method comprising: sending an optical interrogation signal in the optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range for which the optical grating device has a substantially linear slope in its optical response spectrum; the optical grating device outputting an optical output signal substantially at the peak wavelength in response to the interrogation signal, the optical output signal having an output intensity according to the slope; and measuring a variation in the output intensity over time, the variation corresponding to the change in the environmental condition.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Reference is now made to the accompanying Figures depicting aspects of the present description, in which:

[0010] Fig. 1 a is an edge sensing device with a gratings structure as shown in accordance with an embodiment.;

[0011] Fig. 1 b illustrates a method of fabricating a fiber Bragg grating according to the prior art;

[0012] Fig. 1c illustrates a method of fabricating one of the gratings illustrated in Fig. 1a and in accordance to an embodiment;

[0013] Fig. 1 d illustrates a method of fabricating the pair of the gratings illustrated in Fig. 1a in a single exposure and in accordance to an embodiment; [0014] Fig. 1e is a flow chart showing the steps of the fabrication method as in Fig. 1 c;

[0015] Fig 2a shows a desirable reflection spectrum of a grating and illustrates the operation principle of the sensor system according to an embodiment;

[0016] Fig. 2b shows a spectrum of symmetrical edge sensor grating design according to an embodiment;

[0017] Fig 3 compares the spectrum of gratings fabricated with the method of Fig. 1 b and with the method of Fig. 1c under various conditions;

[0018] Fig. 4 shows various cavity lengths in optical grating sensor devices formed by different pairs of gratings according to various embodiments;

[0019] Fig. 5 is a sensor system according to an embodiment;

[0020] Fig. 6a shows how a reflection spectrum of an optical grating sensor reacts under a change in an environmental condition in accordance with an embodiment;

[0021] Fig. 6b shows how a reflection spectrum of a different optical grating sensor reacts under a change in an environmental condition in accordance with an embodiment, the optical grating sensor being apodized;

[0022] Fig. 7 shows how sensor devices are organized in a network having a star-like architecture;

[0023] Fig. 8a is the sensor system of Fig. 5 wherein the optical interrogation source unit has an array of controlled lasers and a switch;

[0024] Fig. 8b illustrates how a fast tunable laser scans the various sensor devices in Fig. 8a and in accordance with an embodiment;

[0025] Fig. 8c shows how an ultra-fast optical switch switches between the lasers of Fig. 8a in accordance with an embodiment; [0026] Fig. 9 is the sensor system of Fig. 5 adapted to time division multiplexing in accordance with an embodiment;

[0027] Fig. ioa and Fig. 10b illustrate the principle behind three-dimensional sensing EDM in accordance with an embodiment;

[0028] Fig. 11a shows an optical sensor device mounted on a mechanical amplifier in accordance with an embodiment;

[0029] Fig 11 b shows an optical sensor device mounted on a substrate in accordance with an embodiment;

[0030] Fig. 1 1 c shows an optical sensor device mounted on aluminum foil in accordance with an embodiment; and

[0031] Fig. 12 is a flow chart illustrating the measurement method in accordance with an embodiment.

[0032] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0033] In the following description, technical terms involving fiber optics have been used and are understood by those professionals skilled in the art.

[0034] A high-speed network of vibration sensors is used to capture impact events, acoustic emissions and provide early warnings for structural failures. Vibration sensors made up of FBGs (Fiber Bragg Grating) have the advantage of small size and light weight; immunity to electromagnetic radiation and lightning, remote and distributed sensing, low noise, long life and the capability to operate at high temperatures and in hostile environments. These FBG based vibration sensors can be used to capture vibration signatures at preset locations of a large structure, constantly comparing the data to a known reference, thereby giving an overview of the global structural health and thus provide an early warning in case the structure has been compromised. [0035] The FBG design forms the proposed sensing device. Besides having a sensing element which responds to vibrations, it also contains an edge filter function, which translates intensity changes into wavelength changes. A twin grating structure forms an optical amplifier causing interference fringes. These fringes add up to a stronger reflection. Vibrations cause changes in the cavity length. Hence, the cavity length changes with respect to time. Changes in cavity length appear in the form of a frequency modulation of these fringes in opposite polarities. From the data obtained, velocity and acceleration measurements can be derived, and the signal can be analyzed in the form of an amplitude and/or frequency distribution. In such a network, the interrogation unit consists of a continuous wave DFB (Distributed Feedback) laser held stable at a temperature within ±0.005 ⁰C, thereby forming an ultra stable light source. The source is split into 8 or 16 channels using standard coupler technology. Changes in the sensors move the intersecting point between the DFB laser center wavelength and the envelope of the grating structure, giving different amplitude responses. A look-up table of each sensor is recorded as part of the calibration procedure, and is stored inside a computer. The complete system contains the DFB light source, optical splitters, FBG sensors and also the electrical amplifier and detector arrays. The detector arrays output to a datalogger structure, or a data memory unit, which can support frequencies in the range of 50KHz to 2 MHz. Twin Bragg gratings, placed on top of thin layers of Kapton, form a small coupon. This coupon can be mounted onto or inside the structure to be measured using silicon grease as a coupling agent. A vibration in the structure is transmitted from the structure to the sensing element. The design of the sensor can be done for different ranges: the distance between the twin gratings, the size and weight, and method of attachment of the coupon may all be varied depending on the specific environment to be monitored.

[0036] The present sensing system may be configured as a sensor array using the proposed FBG technology. The sensor array includes a plurality of sensor elements, each of which is made of a specially designed grating with a spectrum having a gentle rising slope for dynamic strain, pressure and temperature measurements. For higher speed signals, a pair of gratings form a cavity of varying lengths. In the first case, as the FBG experiences vibration, the slope is shifted either to the left or to the right with respect to the original spectral position. Since the interrogating DBF laser, or source, is controlled at a very stable temperature , its fixed center wavelength will intercept the slope at different positions during the vibration. Hence, the reflection coming from a vibrating FBG changes in intensity with time. The change in intensity can be related to a stored calibration curve or look up table accurately relating the change in intensity to those in wavelength. Depending on sensor design, it will further relate to displacement or strain, changes in displacement over time gives a velocity measurement and changes in velocity over time gives changes in acceleration in the form of vibration : microns per seconds square. In the case of the twin grating cavity and optical amplifier configuration, vibrations will cause the optical fringes to increases, the increase in optical fringes will increase the intensity of the total reflected light. The broad band sensor FBG will keep the cavity structure intact over a wide temperature range.

[0037] The interrogation process is performed by the laser light source, which is a continuous-wave source whose center emitting wavelength is maintained by the use of a Thermoelectric Controller (TEC) inside the DFB laser package. The laser driver is placed under constant power control mode, keeping the power changes of the source less than 0.002 of 1OmW, or 0.02mW, which translates in a driving current change less than 0.01 mA at a supply voltage of 2 volts. From the detailed characterization of the DFB lasers, the center emitting wavelength is known to shift at the rate of 10 Picometers (pm) per mA driving current; the change in DFB wavelength is therefore less than 0.1 pm.

[0038] Since the scheme is directed at a high measurement speed of at least 12KHz, it is expected that at such a small time interval, the temperature of the laser module, or the environment surrounding the interrogation unit, will not be able to change. The DFB is expected to hold its center wavelength constant. In the case of a twin-FBG sensor head, both FBGs will move together in the same direction and under the same temperature change, making the measurement independent of temperature but more sensitive. [0039] Any change in intensity of the reflected light is therefore a good indicator that the slope of the sensor has moved due to vibration.

[0040] In the case of the FBG cavity, any changes in the number of fringes would also be a good indicator that the sensors have been subjected to vibration or impacts.

[0041] Various FBG designs having varying grating slopes, can be done to optimize the sensitivity of the sensor. As an example, for a 149 micron slit gratings, the slope is found to be 5nm to 6nm, giving a possible detection range of ±2nm or more. By using a longer grating length, the slope becomes steeper and the measurement range becomes smaller. Through this optimization, systems and sensors can become an integrated solution matching to the exact customer requirement.

[0042] Twin gratings of different cavity length can be designed to adapt to various applications. For example, the cavity length of 5mm would be useful for high frequency Acoustic Emission measurements, where the signal is expected to be from 150 to 300 KHz. Since this configuration has a wide-band frequency response, it can also be used to perform dynamic strain and pressure measurement. In another embodiment, a cavity length of 0.2m to several meters is useful for impact events and for measurements requiring sensitivity enhancements. Finally, ultra long cavities of IOOmeters to 1000 meters are useful for detecting the slightest of disturbances along the entire cavity length, which is useful for perimeter security , Seismic activities, hydrophone .pipeline monitoring and impact detection in a wind turbine.

[0043] The basic structure of this EDM technology is a star network. The interrogation system features one sensor per channel, the DFB laser can be split into 8, 16 or even 32 channels each having their corresponding detector.

[0044] Yet another application of the proposed sensor system has a combination of several DFB lasers to form a source, thereby enabling the combination of EDM with wavelength-division multiplexing (WDM). Depending on the FBG sensor design (slope versus bandwidth), it is possible to incorporate several FBG into each of the fiber channels. A two DFB system with 2 different center wavelengths will allow measurements at normal and elevated temperatures using the same interrogation unit.

[0045] For high-speed concurrent measurements using this hybrid star network, the detection circuit contains a wavelength demultiplexer for each detector. Another approach is to connect the DFB lasers to a 1 by N optical switch. In such an embodiment, any one DFB laser is allowed to interact with the network at any time. This system switches banks of sensors across all the fiber channels; one DFB addressing a set of prescribed sensors. This configuration might be suitable for applications where the sensors need to be monitored in a pseudo real-time fashion, where some cycle stealing is allowed.

[0046] The FBG can be adapted to become a three-dimensional 3D sensing element, a three-dimensional EDM (3DEDM) sensor. This embodiment uses the property of polarization in polarization-maintaining fiber. For example, the sensor EDM gratings are written around 1550nm, and another overlapping EDM gratings is also written at 1310nm. The interrogation unit, or source, hence contains two light sources; one is a FBG laser with polarized output. The output of the laser goes through a polarization switch. Light polarized in the slow axis will first be allowed to interact with the slow axis EDM grating. Then, the laser light is switched to the fast axis polarization, which will only interact with the corresponding fast axis gratings structure. The two axis are independent and, through this scheme, one is able to differentiate vibration changes in both the X and Y directions. The third axis, Z, is simply measured by a 1310nm DFB laser in combination with the 1550nm source. At the detector end, a simple 1310nm / 1550nm demultiplexer separates the 1310nm output which corresponds to the Z axis measurement. Since all the FBG structures are EDM, they will give simple and direct measurements without any complex calculation. Again, since the DFB lasers emit at a minimum power of 1OmW, their output can be split into many fiber channels, each made of polarization- maintaining fiber.

[0047] High-speed measurement using the above-described 3D sensor system can be done using a twin DFB structure. The detection circuit will start with a 1310nm/1550nm wavelength demultiplexer for separating the 1310nm and the 1550nm outputs, and a polarization demultiplexer for separating the slow axis form the fast axis. Therefore, three detectors will be required for each 3D sensor.

[0048] Another extension of the 3D vibration sensor network is to incorporate a tunable laser for the 1550nm and another tunable laser in the 1310 nm. The 3D sensor can be connected in series forming a hybrid 3D/EDM/WDM sensor network.

[0049] The EDM and 3DEDM concepts can also be combined with TDM (3D/EDM/TDM), in which case each sensor is separated by a minimum distance of 1 to 2 meters. Up to a hundred of the same sensors can be connected in series giving a large-scale sensor network. The detection circuit design consists of fully integrated detector arrays where the light source is pulsed at various pulse widths (light requires around 9ns to make a round-trip down a one meter fiber). These pulses of light propagate down the fiber and interact with all the sensors. The reflected pulses of light hence return at different time intervals. Each reflected pulse is then accumulated into a capacitor. After the reception of each reflection pulse, the capacitor is read and discharged in order to be ready to sample the next pulse coming from the next sequential FBG. The EDM structure allows fast measurements and direct interpretations. There is no need for complex calculations. It is also ideal for 3D impact studies of large structures like airplanes and space vehicles during take-off. EDM/TDM will address vibrations of pipelines, thereby giving a much clearer idea for determining the cause of specific problems.

[0050] Finally, by combing EDM with TDM and WDM, it will be possible to create a very large-scale sensor network involving thousands of sensors. Such a network requires tunable laser transmitters and complex receiver designs. Adapting the EDM technology to 3D EDM by using two wavelengths and polarization maintaining fibers, it is possible to develop a very large-scale 3D vibration sensor network and install that network system on the surface of a space shuttle, for example. [0051] High speed measurement sensor networks can also be configured as follows: 1 ) EDM for small sensor networks, where the number of vibration sensors in the networks can be anywhere from 8 to 16; 2) WEDM for medium scale networks, where the number of sensors can go up to 32 to 64; 3) TDM for large scale sensor networks where the number of sensors is in the hundreds; 4) WTDM for very large scale networks where the number of sensors can be up to a thousand or more.

[0052] Another embodiment concerns the adaptation of the EDM technology to 3D by writing EDM gratings on Polarization-Maintaining (PM) fibers at around 1550nm, and overlapping gratings at 1310nm. All sensors involve EDM technology in cases where there is no need for intensive data-logging, or data- recording, and of any complex algorithms to identify the center wavelength of the sensor gratings. This system has a speed improvement of two to three orders of magnitude.

[0053] Referring now to Fig. 1a, there is illustrated a sensing element. The sensing element is a very short length EDM FBG grating written into a standard single mode fiber length. The sensing element can have one optical grating A or B or two optical gratings A and B placed a length L apart. The fiber used may be another type of photosensitive material 8 (refer to Fig. 1c) and may also have been Hydrogen loaded to increase its photosensitivity.

[0054] The grating length of A and B is in the range of a few hundred of microns, such as between 145 to 150 microns, which contrasts with conventional WDM technology where it is custom to utilize 8 mm to 10 mm long gratings to provide a narrow spectrum having a full width half maximum (FWHM) of around 0.2 to 0.3nm.

[0055] Fig. 1 b shows how FBGs are fabricated in accordance with the prior art. A slit (also referred to as an aperture) is introduced between the incident UV beam and the phase masks. As a skilled person in the art understands that the narrow slit acts as a pin hole and thus has a dispersive effect on the incoming light. The parallel beam coming in from the UV laser is transformed into a cone of light with different angles of incidence. This causes deviations from the well known FBG equation where the center wavelength of the FBG is given by the product between the effective index of the fiber and the period of phase-masks. Because of the spread of the incidence angle, the period size of the image that has been written into the core of the fiber now produces a continuous chirp and a variable dose. This chirping results in a broadening of the (Full-Width-Half-Maximum) FWHM of the FBG. In addition, the variable dose causes the slow amplitude rise in the upward slope. This is called the pinhole effect. The smaller the pin-hole, the larger the spreading angle of incidence. However, the narrow slit with limited energy limits the strength of the FBG. As a result, only small reflectivity FBG will form.

[0056] In the proposed method, the distance 22 between the aperture 14 formed in the UV-light-blocking material 15 and the phase mask 16 is made larger and chosen such that the diffraction has a very different effect, as illustrated in Fig. 1 c and Fig. 1d.

[0057] For example and referring to Fig. 1c, the distance 22 is of the order of tens of millimeters. In one embodiment, it is chosen to be 20 mm. If the distance 22 and the size of the aperture 14 is chosen appropriately, the intensity rather than the period becomes much more affected. Because of the longer distance between the UV-light blocking material 15 having the aperture 14 and the phase mask 16, the UV light emanating from the UV light source 10 and passing through the aperture forms a beam 12 which spreads according to the Fraunhofer diffraction effect. The intensity of the exit beam 12 forms a Gaussian distribution, and thus the FBGs written as such become self-apodized. If the energy density becomes very low, the EDM grating would not be formed. The light focusing device such as the cylindrical lens 20 focused the light to increase the intensity of the incident light beam and thus decrease the fabrication time needed to inscribe the gratings.

[0058] The method for creating the gratings sensor may use a 248nm excimer laser to create the pinhole effect for dispersing the UV (Ultraviolet) light; and writing the fiber gratings while the UV light is being dispersed, thereby providing for a very broad-band, low reflectivity gratings sensor. [0059] The above fabrication method can be extended to fabricate the pair of gratings of Fig. 1 a in a single exposure as illustrated in Fig. 1d. In such a case, two apertures 14 are used instead of one, as illustrated. Multiple gratings can be written in the same way.

[0060] Fig. 1e is a flow chart showing the steps of the proposed fabrication method.

[0061] In step 30, a light source emitting ultraviolet (UV) light is provided.

[0062] In step 32, a length of photosensitive material is provided. This can be any type of photosensitive waveguide to be mounted on a structure to be monitored for example, comprising optical single mode and multi-mode as well as polarization-maintaining fiber.

[0063] In step 34, a UV light-blocking material having two apertures is provided and in step 36, a phase mask is placed between the photosensitive material and the UV light-blocking material.

[0064] In step 38, the UV light is sent from the light source through the two apertures and the phase mask, onto the photosensitive material, to produce the optical grating device in the photosensitive material in a single UV light exposure using Fraunhofer diffraction effects. The optical grating device thus comprises a pair of optical gratings and an optical interference cavity therebetween.

[0065] In the above-described method, an optional step provides an optical focusing device between the UV light-blocking material and the phase mask. The sending step 36 can thus also comprise increasing the intensity of the UV light through the phase mask and onto the photosensitive material using the optical focusing device.

[0066] Once fabricated the optical grating device can be interrogated according to the edge sensing operation, with an optical signal having a peak wavelength and as described below. [0067] Fig. 2a illustrates a desirable shape of an optical reflection intensity spectrum of a grating and how the edge sensing operation functions. An interrogation optical signal is sent into the FBG. An interrogation optical signal produced by a laser at a center (or peak) emitting wavelength 40 set within a wavelength range 42 (also referred to as a measurement range) for which the grating's optical reflection spectrum has a substantially linear slope, as illustrated. For example, the peak wavelength of the interrogation signal can be set approximately 3nm lower than the center wavelength 44 of the edge sensor grating. The center wavelength 44 of the grating is the wavelength for which the reflected intensity is at a maximum. The peak reflectivity of the grating is at least 3.5 to 4% higher than the reflectivity at the non-reflected wavelengths in the reflection spectrum, as illustrated by the difference 46. To maintain the peak wavelength 40 stable, the interrogation source or laser is held under strict temperature control (in the order of +/- 0.005 degree Celsius). There is hence an intersection between the spectrum and the interrogation signal around the middle of the rising slope.

[0068] Fig. 2b shows a symmetrical edge sensor design spectrum. Here, the slit size goes up to 400 microns resulting in a smaller but symmetrical spectrum. This design is desirable for a WDM adaptation where more sensors can be connected in series in each of the fiber channels.

[0069] Fig. 2b also shows the placement of the peak wavelength 40 of the interrogation signal on a narrower Gaussian type reflection spectrum of edge- sensing FBGs. The peak wavelength 40 is generally set by reducing the center wavelength 44 of the FBG reflection spectrum by half of the rising slope of the spectrum.

[0070] If the grating's each have a reflection spectrum with a narrower slope, more sensors can be connected in series, and the sensor system thus becomes more sensitive to vibrations (or any change in environmental conditions for example).

[0071] The gratings can each be customized and incorporated into calibration curves which are stored inside a memory of a processing device such as a computer (Laptop PC) connected to a data-logger or a memory unit recording data measured from an optical detecting device. The self-apodization of the grating provides for a larger range of measurements. The size of the aperture is thus selected according to the design and purpose of the grating. For example, the type of self-apodized gratings, a larger aperture is used.

[0072] Fig. 3 shows various Edge Detection Measurement FBGs formed by different apertures sizes and fabricated by the prior art and proposed method. Spectrum 50 illustrates a reflection spectrum of a 148 micron length optical grating. The slope is very gentle on the shorter wavelength side and there is no self apodization effect. The measurement range 42 is 6 nm wide. Spectrum 52 shows a spectrum of a grating fabricated using the proposed method and with an aperture size of 4mm. The grating having spectrum 52 is self-apodized. The slope shows very smooth curve with a much reduced measurement range 42 of 2nm span. This reduced range allows for much greater sensitivity and for the fabrication of gratings side by side to cover a large band of frequencies.

[0073] Fig. 4 describes the basic structure of a passive optical amplifier formed by a pair of gratings forming a cavity (also referred to as a twin EDM FBG cavity): a short optical interference cavity 60 of a length L around 5mm is suitable for acoustic emission (AE) measurements at high frequencies such as high speed vibrations, or dynamic strain and pressure measurements. As the cavity gets longer, under the influence of vibrations, there is an amplitude-to- frequency conversion that occurs. The longer the cavity, the more sensitive the sensor head becomes. Sensor head 62 has a medium length cavity of around 0.1 to 10 meters and is used for impact studies for example.

[0074] For sensor heads 64 of up to 1 km long, disturbances due to the earth's movements can be measured. This type of sensor head can be used for perimeter security applications, and in pipelines or wind turbine blades for example. Applications involving long sensor heads require complex signal processing circuits at the detection end. Essentially, it involves frequency-to- amplitude conversion circuits, and a high-pass filter is used to remove any slow moving signals. After processing, the signals are passed to the standard analogue to digital conversion circuits. With this approach, there is no need to use very high speed analogue to digital at the front end.

[0075] Fig. 5 is a sensor system 70 according to an embodiment. The following description illustrates the operation of the sensor system 70 with the EDM sensor 72 (also referred to as an optical sensor device) according to an embodiment. The sensor device 72 has an optical grating 74 which is interrogated with an interrogation source unit 76 having an optical source emitting at a wavelength such as a DBF laser source 78. The source 78 is controlled by the TEC controller 80 and the laser driver 82. A power splitter 84 can be used to simultaneously direct the interrogation signal in various channels (or optical waveguides) 86 each having a sensor device 72 comprising one or more gratings 74.

[0076] The laser 78 is a standard 14 pin standard TEC packaged DFB. The TEC 80 is controlled to within 0.005⁰C, which is equivalent to a 0,5 picometer center or peak wavelength stability. The power is also maintained to be constant: variations within 60 seconds can be reduced to 0.02%. The DFB line width is typically 20MHz, so it is less than 0.16pm and it intersects with the slope of the EDM gratings at only one point. The reflected light intensity is thus modulated by the edge and is registered by an optical detector device 88 optionally having an amplifier circuit (not shown). The Data logger 90 is a memory device for storing the measured data for each channel 86. A laptop PC or any other type of processing device 92 is then connected to the data logger 90 to further process the data.

[0077] Still referring to Fig. 5, the system 70 can use a USB connection between the processing device 92 and the memory device or data logger 90 for transferring all the signals to the processing device 92.

[0078] Since each measured change is only represented by a single point of data, there is no need for a DSP to do signal processing and number crunching. In an embodiment, the data-logging circuit is chosen to have a 16-bit resolution, and a sampling rate of 12KHz per channel if all the channels are used. If only two channels are used, the sampling rate can go up to 50KHz for those two channels. The total capacity of the analogue to digital conversion device (A/D) is a maximum of 100KHz.

[0079] As mentioned before, the laser is under strict temperature and constant power control from devices 80 and 82. The feedback signal of the DFB power is obtained by a monitoring of the back facets of the laser source 78. The output of the laser 78 is split into 8 (or any given number) of fiber channels 86, each of the channels is equipped with a 3 port coupler 94 and the input comes from the 1 by N splitter 84. Then one of the output of the coupler is connected to EDM sensor 72 through the front panel and then further connected to the physical sensor head. The other output of the coupler goes into a single PIN detector 88, or into one of the channels of an array detector 88. The output of the detector 88 is connected to an electronic amplifier circuit (not shown). The sensitivity of the sensors is partly controlled by the design of the EDM grating, and by the programmable gain in the amplifier circuit, thereby providing customization in measurement ranges and sensitivities.

[0080] The above paragraph serves to illustrate that it is possible to select different data-loggers 90 having sampling speeds that meets the requirements of the signal to be captured. For speeds higher than 20Khz, the system architecture is changed such that the data-logger is now in the form of a PCI card, and a PCI based industrial PC is necessary. Such systems provide for the measurement of AE signals in metal structures, like rail bridges, partial discharges in power transformers, pressure vessels and storage tanks, the signals will be in the range of 200 KHz to 300 KHz. In such applications, the sampling speed needs to be at least 2M samples per second. At this speed, the USB connection described above is no longer sufficient to transfer the data to an external processing device 92.

[0081] The slope of the FBG is the sensing mechanism. Referring now to Fig. 6a and Fig. 6b, as the FBG responds to changes in environmental conditions, the slope moves whereas the peak emitting wavelength of the interrogation optical signal is kept at the same location. In a specific embodiment, for example, power fluctuations are limited to about 0.02% or less. Hence, at a typical power of 10 mW, a change of no more than 0.02 mW will occur. Assuming that the laser voltage is around 1.8 volt, this translates into a driving current change of 0.01 mA. The rule of the thumb for standard DFB lasers is that each mA of driving current can cause a wavelength shift in the order of 10 picometers. So the wavelength change is less than 0.1 picometer.

[0082] Still referring to Fig. 6a and Fig. 6b, an EDM sensor responds in the following manner: the vibration causes a dithering of the rising slope of the gratings in the sensor. As the slope moves, the point where it intersects with the fixed DFB wavelength changes with time. The intensity of the reflected light therefore also changes accordingly. When the vibration is such that the slope moves to the right, the intercept becomes smaller in intensity. When the slope moves to the left, the reflected light increases in intensity. The change in intensity is therefore representative of the displacements due to vibrations or environmental changes over time. The velocity of this displacement can be calculated as a derivative of the displacement with respect to time (dx/dt), and acceleration can be calculated by performing a double derivative (d2x/dt2).

[0083] A reflection spectrum of EDM gratings having a 148 microns grating length is shown in Fig. 6a to have a bandwidth of more than 7nm. A self- apodized gratings fabricated with the slit far from the phase mask is shown in Fig. 6b. Note the symmetry of the gratings and the narrowing of the bandwidth. In Fig 6b, the measurement range of such a sensor head will be correspondingly smaller. Since the slope is steeper, the sensor will have higher sensitivity. The slope of the sensor together with an adjustable amplification at the detector provides the ultimate higher sensitivity of the sensor network.

[0084] In general, the higher the sensitivity, the smaller the measurement range. A design combination can be achieved by customizing the EDM FBG.

[0085] Referring to Fig. 7, multiple EDM sensing devices are shown to be connected to the interrogation source unit 76 using 8 channels 86, each channel 86 contains an EDM sensing device. Each sensor cable and connector is assigned a given color or code (1 to 8) such that the system can identify where the signal comes from. The color or code assignment is done during system configuration. It can be easily imagined that two or three DFBs with center wavelengths 10 nm apart within the C-band can be combined by an optical switch matrix, incorporating one DFB laser output at a time. For example, various existing optical switch technologies have switching speeds from 20 nanoseconds to several micro seconds. Systems equipped with fast switches forms a seamless scanning light source for interrogating an array of EDM sensors at each fiber channel. With such architecture, the number of sensors in the network can be increased by two to four times with very little change in system cost.

[0086] To differentiate each of the sensors 72, the fiber channels are color- coded and the cables that connect the sensors to the system unit shall match the same color. This will make sure that the insertion/connections remains the same as the calibration is performed EX factory.

[0087] Note that in the above-described sensor system 70 (referring to Fig. 5 and Fig. 7), it is possible to use individual detectors to form the receiver (or measuring) unit. The differences between the detectors are minimized as we always match the cable color or code with the channel color or code. It is also possible to use a detector chip array, where the chips will be characteristically very close from one another. In doing so, channel to channel variations are further reduced.

[0088] Fig. 8a to Fig. 8c shows an embodiment wherein EDM and WDM concepts are combined to form EDM/WDM networks.

[0089] Referring to Fig. 8a, each EDM gratings 74 is made by using different phase mask periods. The data-logger 90 is equipped with digital (input/output) I/O to control an optical switch 96. At each switch position, the system software stored and executed by the processing device 92 relates the signal obtained to the different DFB center wavelengths. The capacity of the sensor network increases by N where N is the number of DFB lasers. In such a sensor network, each of the fiber channels can handle several wavelength multiplexed EDM sensors; the number of multiplexed sensors depends on the measurement range specified. For example, the 148micron EDM sensor 72, having a rising slope span of 7 nm, and a falling edge of 3 nnri, will require a spectral separation of 10 nanometers between the sensors. Whereas, in the case of self-apodized FBGs with narrower bandwidths, the sensor 72 can be spaced at 4 nm if the amplitude of the measured vibrations remain smaller than the equivalent of ±500 micro strains.

[0090] The network of sensor devices 72 can be organized in a sequential network or a concurrent sensing network. In Fig. 8a, an array of DFB lasers is being switched in and out of the sensor array. In Fig. 8b, a fast tunable laser scans the various EDM sensors 72. In Fig. 8c, an ultra-fast optical switch N X 1 (here represented as 4 by 1 ) switches between the group of DFB lasers of the sensor network. With a switching speed of 20ns, all the sensors of the different wavelengths can be interrogated concurrently at a 12KHz sampling speed. The pulsed laser in this case supports only one center wavelength, the output can however be split into at least 8 to 16 channels or more. A local Digital Signal Processing (DSP) controller is used to do a part of the signal processing, and a Universal Serial Bus (USB) transfers the processed data to the processing device 92 ( Laptop or PC).

[0091] As best seen in Fig. 9, a time-division multiplex (TDM) scheme can be adapted to the EDM technology to support an even larger sensor network. In this case, EDM sensors 72 are physically spaced apart, in each channel, at a minimum of 1 meter. The DFB laser which used to be CW in the simple EDM configuration is now configured as a pulsed laser source. The pulsed laser is set at 1 MHz pulse rate, the pulse width is 5ns. The reflection by the EDM FBGs will return at different times. For example, the first reflection will return at 9ns, followed by the reflection from the second FBG at 18ns, and so forth. Supposing the network has 100 of these sensors, for example, the total elapsed time would be 900 ns. All the reflected lights are returned into a time gated detection circuit synchronized by a starting pulse when the pulsed laser starts to fire the pulse. The output of the detector circuit is a programmable electronic amplifier circuit making adjustments for the effect of attenuation due to sensor positions in the network. [0092] The outputs of the amplifier are fed to an A/D converter and measured data is continuously transferred, in an embodiment, to a local DSP unit (DSP Board and channel high speed A/D converter) 98 which performs part of the signal processing using the timing and control of device 100. The processed data is then sent to a laptop or a computing device such as device 92, where it is stored into files carrying fiber channel identifications, time stamps, user identifications and session or experiment identification information. Just like the High Performance Switch (HPS) communication system, the TDM equivalent also has 4 to 8 channels. In such an embodiment, a total of 400 or 800 sensors can be supported by a single interrogation unit, each channel having up to 100 sensors 72.

[0093] A further extension of the TEDM network is to incorporate WDM technology on top of the architecture.

[0094] In this case, more sensors can be added to each fiber channel having different center wavelengths. The spectral separation of the second set of hundred sensors is about 10nm higher or lower than the first set of EDM gratings. The pulsed laser source supports a tunable laser structure, with switching times of less than 1 microsecond. During the time the source is switching, no pulse is fired. The tunable pulsed laser can support up to 16 wavelengths; in this case, the EDM gratings are the self-apodized designs with smaller sensing ranges, and are placed at 4 nanometer spectral intervals. In this case, with the telecom C band, 1525 nm to 1565, there is up to 10 sensors that can be installed, giving each fiber an optical capacity of a 1000.

[0095] In practice, as light propagates over the TEDM FBGs, there will be some signal loss. To support an extended sensor network, a pulsed laser with higher power becomes necessary. The alternative is to place an optical amplifier stage before the power splitting circuit.

[0096] Finally, by writing the EDM gratings in polarization maintaining fiber 150, as in Fig. 10a, the source laser is still the CW DFB laser but this time equipped with a polarization switch. [0097] As illustrated in Fig. 10b, the system would first set the incident light to the fast axis (refer to graph 200), at a peal wavelength of 1550nm for example. This fast axis light will interact with the same grating having its center wavelength represented by the product of its lower effective index and twice the gratings period.

[0098] Then, (refer to graph 202), the system can switch to the slower axis and the interrogation signal interact with the FBG in the slow axis using the same peak wavelength or not, which is typically 0.4nm higher that the center wavelength of the FBG.

[0099] Essentially, the EDM gratings in PM fiber will give two different slopes. These two slopes react with the transverse load differently. So it is possible to differentiate stress and vibration between the X and Y axis.

[00100] Referring to graph 204, a third EDM gratings having center wavelength at 1310nm can be written directly over the original 1550nm EDM gratings, which will have yet another slope. A simple 1310nm and 1550nm demultiplexer can separate the two different wavelengths for interrogation and detection purposes. The 1310nm reflected light can be fed into another detector. The measurements from the 1310nm FBG thus represent changes of the environmental disturbance in the Z axis.

[00101] Of course, the light source also has to include a 1310nm DFB laser which is combined concurrently with the 1550nm DFB laser. In this case, the 1310nm light path does not include a polarization switch. Hence, a modified light source subsystem is included in the sensor system in order to obtain three- dimensional measurements.

[00102] Fig. 1 1a, 1 1 b and 11c shows various forms of adaptation of the EDM sensors from the EDM FBGs. Fig. 11a shows an EDM FBG mounted on a mechanical amplifier. This adaptation is suitable for laboratory experiments where enhanced sensitivity is required. [00103] Fig 1 1 b shows an EDM pressure sensor where the FBG is mounted onto a pre-shaped Kapton onto a carbon fiber composite, thereby forming a vibrometer.

[00104] Fig. 1 1c shows the light-weight vibrometer (sensor) design where the FBGs are mounted on a small piece of substrate (Kapton, aluminum foil or stainless steel pad) using epoxy thereby allowing the vibration to be freely transmitted to the sensor head. The choice of a substrate will depend on the design of the frequency response, the maximum range of vibration and the sensitivity.

[00105] Hence, according to an embodiment of the above description, where a pair of FBGs are used as a sensor device (also referred to as twin FBGs), each characterized by the above-described construction, various cavity lengths form amplitude to frequency conversion devices. An array of sensors with structures can be used for non-destructive monitoring of large infrastructures, using nonlinear elastic wave spectroscopy or the capturing of vibration signatures that can be related to delamination and crack formation in monitored infrastructures for example.

[00106] A plurality of these discrete sensors and twin FBGs sensors form a star network architecture, the sensor heads consists of these very short length FBG and also twin FBG cavity, each mounted on a variety of substrates having different size, area, weight and material composition such as Kapton, aluminum Silicon Carbide and stainless steel.

[00107] In an embodiment, a multiple channel data-logger connected to these fiber channels receives the signal as changes in the reflected light, and in the form of oscillating fringes. These signals can be further processed to give the desired parameters that are being measured. The sampling rate of the datalogger may vary from 6KHz, 12KHz, 50KHz, 150KHz, up to 2MHz. In the case of a long cavity device of up to 100 meters and 1000 meters, a frequency to amplitude conversion circuit supporting high speed signals from 125M to 500MHz handles the changes of reflected light fringes caused by disturbances in the cavity. [00108] In the proposed sensor system, an optical switch can be used to combine the output of several fiber or semiconductor DFB lasers (optical interrogation sources). The speed of this switch will depend on the required system design. For example, if some requirements need real-time capturing of the signatures from all of the sensors, then the system needs a fast optical switch. Whereas, in another situation, the switch creates a scanning function or cycle, taking turns in monitoring different parts of the structure. This will enable several of the above-mentioned sensors to be multiplexed on the same optical fiber.

[00109] A tunable laser can be used instead of the multiple DFB lasers, such tunable DFBs are typically tuned by changing the phase section of the four segment DFB lasers. A single tunable laser will replace many DFB laser, thus enabling lower power consumption and achieving a lower cost WDM version of the optical sensor system.

[00110] A dual DFB laser system uses a 1 by 2 coupler in reverse operation, combining their outputs and simultaneously feeding the optical circuit. The wavelength of the first DFB laser is used to measure vibrations and disturbances at room temperature, and the center wavelength of the second DFB laser is chosen such that, at elevated temperature, the slope of the gratings sensor intercepts this second DFB laser's center-wavelength. Using this method, it is possible to construct an interrogation system which can measure and confirm the unit function at normal ambient temperature, and yet be able to operate at 400 degrees Celsius when polyimide-coated fiber is used in the fabrication of the FBG having the characteristic slope.

[00111] With a tunable laser, the sensor array mentioned above can be used in a wavelength demultiplexing mode so that each fiber can contain several of the mentioned sensors placed in series, and limited only by the normal optical amplifier spectrum width of 1525 to 1565nm.

[00112] The sensor array mentioned above, in the form of single FBGs, can also be utilized in a time-division multiplex configuration where hundreds of these sensors can be connected in the same fiber with a distance of 2 or more meters apart. The time delay introduced with each sensor being further apart will enable to detectors to identify the relative location of the sensors. In this case, the detector will contain a frequency to amplitude converter and, in addition, a charge integration circuit to accumulate the total charge resulting from each reflected pulse of light. The sampling clock of the A/D converter is synchronized to this sample and hold circuit. The A/D circuit is allowed to sample the maximum value of the sample and hold circuit. The sample and hold circuit is then be discharged before it is allowed to interact with the next reflected pulse of light. A digital signal processor subsystem is required to handle these high speed reflected light pulses from the array of FBGs resulting from a single pulse generated by the transmitter. The pulse width of the laser source should be shorter than 5ns (nanosecond), which allows a sensor separation of approximately one meter. In the cases where the sensors are spaced further apart, wider pulses can be used. The time interval between the transmitted pulses depends on the number of sensors connected to the network. Another pulse will not be released from the transmitter until all the reflected pulses have returned. This is done by using a type of pulse counter circuit having a comparator, and the reference value given in a system configuration. A separation of 1 meter between sensors means approximately 9ns for a round trip time. If there are 100 sensors installed, then the total elapsed time should be around 1 microsecond. With a built-in DSP, 5 cycles can be done and the resulting measurements can be averaged to reduce the impact of noise and errors. The effective sampling speed of such a system is usually 200KHz, which is more than enough for most acoustic measurements. For ultrasound measurements, the sampling rate is allowed to go up to a full speed of 1 MHz. For ultra sound measurements, the sampling speed has to go up to 250 to 500 MHz.

[00113] The transmitter of the TDM (Time Division Multiplexing) shall use a Master Oscillator Power Amplifier (MOPA) having a narrow (about 5ns) pulse width. The peak wavelength of the pulse is made to stay constant during the pulse event, and to remain as unaffected as possible by changes in the outside environment, or by changes in drive currents and temperature changes of the thermoelectric cooler control unit.

[00114] The transmitter is triggered by the data-logging unit at the start of a measurement cycle. A fixed delay of 10ns is inserted so that the A/D operation will start synchronized to the arrival of the first reflected pulse, and continuous sampling will occur thereafter at every 10ns.

[00115] The MOPA can be seeded by a tunable laser so that the sensor network can form a hybrid of TDM and WDM (Wavelength Division Multiplexing) system. In such a case, another dimension can be added to the scale of the network. Furthermore, the TDM part can support up to 100 sensors per fiber by joining several other layers of edge sensing FBGs written at different wavelengths. Hence, several hundreds of sensors can be placed on one fiber channel.

[00116] At each tuned wavelength, the TDM part of the network measures the first set of 100 sensors, creating a first set of data. Then, when the wavelength is tuned to another value, the TDM circuit measures a different set of 100 sensors, and so on and so forth. The effective coverage will be reduced by the tuning time of the transmitter and an electronics shutter should be incorporated into the tunable laser. Alternatively, the MOPA transmitter can drive a fast switch matrix of 1 x N, where N is number of fiber channels. Furthermore, the MOPA output can drive into a 1 x N splitter, N being 8 or 16 for example, so that concurrent measurements can be done simultaneously on all fiber channels. In this case the sensor network is expanded in breadth. Here, multiple detection circuits can work in parallel, thereby allowing real-time measurements. Each fiber channel can be supported by its own DSP unit, thereby allowing concurrent signal and data processing.

[00117] Fig. 12 illustrates the steps of the method for measuring a change in an environmental condition of a sensing device comprising an optical grating device within an optical waveguide as described herein above. [00118] In step 320, an optical interrogation signal is sent in the optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range for which the optical grating device has a substantially linear slope in its optical response spectrum;

[00119] In step 322, the optical grating device outputs an optical output signal substantially at the peak wavelength in response to the interrogation signal, the optical output signal having an output intensity according to the slope.

[00120] In step 324, a variation in the output intensity over time is measured, the variation corresponding to the change in the environmental condition.

[00121] The method is then extendable for a sensing system comprising a network of multiple sensing devices and for a sensing system where the optical grating device has a pair of optical gratings and an optical cavity therebetween. In such a case, optional steps 326, 328 and 400 are performed.

[00122] In step 326, the step of amplifying the output intensity according to the change is performed in an optical interference cavity formed between a pair of optical gratings in the optical grating device.

[00123] Step 328 is an extension of step 326, wherein a number of optical interference fringes produced in the optical interference cavity are counted and the number being indicative of the change.

[00124] In step 400, for a network of multiple optical sensing devices each comprising at least a pair of optical grating devices, one of the optical sensing devices for which the variation is measured is identified. This provides for a spatial distribution measurement of the change within an environment of the optical sensing device

[00125] Hence, in accordance with the description provided herein, the sensor array of the sensor system 70 can therefore be almost limitless in size, which is particularly useful for large infrastructure monitoring. The sensor network acts like an array of listening devices and capture the vibration signatures of a large infrastructure which in itself is a source of vibrations. The vibration signature of a known good unit, or a group of known good standard units can be obtained from the measurement of each subsystem, and stored in the software library to form references for future measurements. Hence, continuous, automatic monitoring of these large infrastructures can allow early warning of any degradation, and the onset of structural fatigue. Elaborate studies can be made using structures with known and well characterized defects so that they form a library. A pattern recognition software can reverse the process and identify not only the onset of the defect, but also be able to identify what kind of defects and where they occur in the infrastructure.

[00126] Yet another application of the above optical sensor system network is in the form of nonlinear elastic wave Spectroscopy. Adding to the above sensor network is an input frequency pair of actuators, the frequency pairs consisting of one low frequency and another frequency at around 10 to 20 times higher than the first. They are setup in two radial intercepting field covering the whole area to be inspected within the infrastructure or environment selected. The sensors are placed in a regular grid. If there should be any defects, nonlinear interaction between the two frequencies will occur and be represented by harmonics of the lower and higher frequencies and by the mixing of the high and low frequencies. This measurement method is about 100 to 1000 times more sensitive than conventional amplitude based acoustic measurements.

[00127] The sensor network can also be useful for structural health monitoring of pipelines carrying, for example, oil, gas or water or other chemicals. Defect examples can be the wall thinning of the pipeline due to corrosion, deformations, leakage (in the form of a flow rate changing and thereby causing a change in the vibration signatures), and tampering or intended terrorists acts. An array of vibration sensors can be planted in the perimeter of the pipeline to alert management and owners of unauthorized access. The sensor network can also be used for homeland security, for top safeguarding the perimeter of structures, reservoirs, fuel storages, nuclear reactors, chemical storage sites, such as tank farms holding oil and gas. Long stretch of borders can also be monitored. The sensor network is also ideal for the monitoring aircrafts in service, space vehicles modules constructed as carbon fiber composite modules, and their joints between the modules. The sensor network can also be used to detect weaknesses in submarines hull , marine vessels, liquefied natural gas or oil tankers. The sensor network can also be used for monitoring the vibration signature of power generators, capture partial discharge events in a power transformer, and further diagnosis the location, exact timing and residual life of these events. The sensor network can be used to monitor an electrical power distribution network in the form of overhanging power lines. The sensor network can also be used for seismic studies, on set of earth quakes, to identify cracks and deterioration of the supporting cables in cable stayed bridges. The sensor network can also detect cracks in building foundations, can be used for seismic studies targeting oil and gas exploration, for monitoring the structural health of railway bridges and railway tracks as a result of train traffic and loading, rail distortion etc. The sensors can also be used to perform weight in motion studies of the loading of our highway structures, to monitor vibrations of cooling pumps and distribution pipes of cooling water in nuclear reactors, to monitor the steam pipes in a coal fire generator plant. They can also form a hydrophone array to detect submarine activities, like detecting ocean floor movements, or the onset of a tsunami. Finally, the sensor network can capture vibration signatures of aircraft during flights, vibration patterns of space vehicles during launches, impact events like earthy quakes, or even small movements and sounds in a snow covered slope as a warning for the onset of a avalanche, cracks in the containment wall of a nuclear reactor or in a dam.

[00128] The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the subject matter disclosed. Modifications which fall within the scope of the present description will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for fabricating an optical grating device comprising:

providing a light source emitting ultraviolet (UV) light;

providing photosensitive material;

providing a UV light-blocking material having two apertures;

providing a phase mask between the photosensitive material and the UV light-blocking material;

sending the UV light from the light source through the two apertures and the phase mask, onto the photosensitive material, to produce the optical grating device in the photosensitive material in a single UV light exposure using Fraunhofer diffraction effects, the optical grating device comprising a pair of optical gratings and an optical interference cavity therebetween.

2. The method as in claim 1 , further comprising providing an optical focusing device between the UV light-blocking material and the phase mask and increasing the intensity of the UV light through the phase mask and onto the photosensitive material using the optical focusing device.

3. The method as in claim 1 , wherein each one of the pair of optical gratings is characterized by having a self-apodized optical reflection intensity spectrum comprising a substantially linear slope within a wavelength range, and further comprising selecting a size of the two apertures and a distance between the UV light-blocking material and the phase mask to adjust a steepness of the slope.

4. The method as in claim 3, wherein the sending comprises sending the ultraviolet (UV) light through multiple pairs of apertures of the UV light-blocking material, through the phase mask, and onto the photosensitive material, to produce the multiple optical grating devices in the photosensitive material in a single UV light exposure, each of the multiple optical grating devices comprising the pair of optical gratings and the optical interference cavity therebetween.

5. The method as in claim 3, further comprising:

interrogating the optical grating device with an optical signal having a peak wavelength within the wavelength range;

the optical grating device outputting an optical output signal substantially at the peak wavelength in response to the interrogating, the optical output signal having an output intensity according to the slope; and measuring a variation in the output intensity over time, the variation corresponding to a change in an environmental condition of the optical grating device.

6. The method as in claim 5, further comprising mounting the optical grating device on a structure prior to interrogating to monitor an environmental condition of the structure.

7. The method as in claim 5, wherein the measuring comprises measuring a number of interference fringes produced by the optical interference cavity, the number being indicative of the change.

8. An optical sensor device fabricated according to the method defined by one of claims 1 to 4.

9. A sensor system for measuring a change in an environmental condition, the sensor system comprising:

an optical source unit for sending an optical interrogation signal in an optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range;

an optical sensor device in the waveguide, the optical sensor device comprising an optical grating having an optical response spectrum comprising a substantially linear slope within the wavelength range, the optical sensor device for outputting an optical output signal substantially at the peak wavelength in response to the optical interrogation signal, the optical output signal having an output intensity according to the slope; and an optical detecting device for measuring a variation in the output intensity over time, the variation being indicative of the change.

10. The sensor system as in claim 9, wherein the optical sensor device comprises a pair of optical gratings and an optical interference cavity therebetween, the optical cavity for amplifying the output intensity according to the change.

11. The sensor system as in claim 10, wherein the optical interference cavity comprises an optical interference cavity for producing a number of optical interference fringes, the number being indicative of the change.

12. The sensor system as in claim 10, further wherein the optical interference cavity has a length for which the variation in the output intensity over time generates one of: high-frequency AE measurements, dynamic stain measurements, pressure measurements, an identification of an impact event, and a determination of a disturbance occurring along the length of the optical interference cavity.

13. The sensor system as in claim 1 , wherein the optical sensor device comprises multiple optical sensor devices each being in one of multiple optical waveguides and each comprising one of multiple optical gratings, each one of the multiple optical gratings having an optical response spectrum comprising a substantially linear slope within a wavelength range,

wherein the optical source unit comprises an optical switch for sending multiple optical interrogation signals in respective optical waveguides, each one of the multiple optical interrogation signals having a peak wavelength within the wavelength range of the one optical grating in the respective optical waveguide; and

the multiple optical sensor device each outputting one of multiple optical output signals each substantially at the peak wavelength of its interrogation signal and each with one of multiple output intensities in accordance with the slope of its respective optical grating.

14. The sensor system as in claim 13, further wherein the optical detecting device comprises a demultiplexing device for measuring variations in the output intensities, the variations being indicative of the change, and the optical sensing device for which each of the variations are measured providing for an indication of a spatial distribution of the change.

15. The sensor system as in claim 1 , wherein the optical sensor device comprises an optical sensor device for measuring the change at a frequency at least higher than 12 KHz.

16. A method for measuring a change in an environmental condition of a sensing device comprising an optical grating device within an optical waveguide, the method comprising: sending an optical interrogation signal in the optical waveguide, the optical interrogation signal having a peak wavelength within a wavelength range for which the optical grating device has a substantially linear slope in its optical response spectrum; the optical grating device outputting an optical output signal substantially at the peak wavelength in response to the interrogation signal, the optical output signal having an output intensity according to the slope; and measuring a variation in the output intensity over time, the variation corresponding to the change in the environmental condition.

17. The method as in claim 16, wherein the optical grating device comprises two optical gratings and an optical interference cavity between the two optical gratings, and further comprising the optical interference cavity amplifying the output intensity according to the change.

18. The method as in claim 17, further comprising counting a number of optical interference fringes produced in the optical interference cavity, the number being indicative of the change.

19. The method as in claim 16, wherein the sensing device comprises multiple optical grating devices each within an optical waveguide, wherein the sending comprises switching between multiple optical interrogation signals and sending each one of the multiple optical interrogation signals the optical waveguide of each one of the optical grating devices according to the sending of claim 16.

20. The method as in claim 17, wherein the measuring comprises demultiplexing between multiple optical output signals from the multiple optical grating devices, the measuring comprising measuring according to the measuring of claim 16 for each of the optical output signals, the variation in the output intensity of each of the optical output signals being indicative of the change.

21. The method as in claim 20, further comprising identifying one of the optical sensing devices for which the variation is measured to provide a spatial distribution measurement of the change within an environment of the sensing device.