WO2002016909A1

WO2002016909A1 - A method and apparatus for transverse load sensing by use of pi-phase shifted optical fiber

Info

Publication number: WO2002016909A1
Application number: PCT/US2001/025935
Authority: WO
Inventors: Michael Leblanc; Sandeep Vohra; Tsung Tsai
Original assignee: The Government Of The United States Of America, As Represented By The Secretary Of The Navy
Priority date: 2000-08-18
Filing date: 2001-08-20
Publication date: 2002-02-28
Also published as: AU2001288310A1

Abstract

A method and apparatus for determining birefringence in an optical fiber (14) having a π-phase-shifted Bragg grating (19), as well as changes in fiber birefringence, e.g. caused by mechanical loading (16) on the fiber transverse to the fiber's optical axis (15). Such an optical grating (19) has narrow transmission band within its broader reflection band, whose peak is split in two if the grating (19) is in birefringent fiber, the magnitude of the plit being proportional to the magnitude of birefringence. In addition to permitting direct measurement of the fiber birefringence, monitoring of changes in spcing between the two peaks will indicate corresponding changes in fiber birefringence. This can be a result of mechanical deformation of the fiber, e.g. by loading or application of strain, permitting one to determine that load, strain, etc. by determining the change in spacing between the two peaks.

Description

A METHOD AND APPARATUS FOR TRANSVERSE LOAD SENSING BY USE OF PI-PHASE SHIFTED OPTICAL FIBER

Technical Field

The use of fiber Bragg gratings for sensing strain in the direction parallel to an optical fiber is well known. It is also possible to use Bragg gratings to measure strain, e.g. in a direction transverse to the optical fiber in a host material in which the fiber is embedded, or from a load applied to the optical fiber. This approach is based on the principle that a nonaxisymmetric load applied to the optical fiber will create birefringence in the optical fiber that one can observe by splitting the refection spectrum of the Bragg grating into two peaks (having maxima at λ, and λ₂ respectively), corresponding to the fast and the slow axes of the grating. The relation between grating birefringence B and spectral spectral separation Δλ=(λ_rλ₂) between the peaks is:

B = (Δn/n) = (Δλ) l

where Δn=n_rn₂ is the difference of the index of refraction between the slow and fast axes of the grating, n is the average index of refraction of the grating core, and λ^"=(λ, + λ₂)/2 is the average wavelength of the grating. Because birefringence varies with both grating strain and grating temperature, measurement of both Δλ and λ permits measurement of both axial strain and temperature. However, if one is interested primarily in strain or loading measurement, the temperature dependence of conventional Bragg gratings reduces the reliability with which one can do this.

For transverse strain-sensing applications, one generally desires microstrain resolution, which normally implies changes in fiber birefringence smaller than 10^"6. This level of birefringence corresponds to wavelength separations much smaller than the typical bandwidth of a Bragg grating. For this reason, the use of Bragg gratings written in polarization maintaining fiber has been proposed. Such fiber has an initial birefringence that is sufficient to split completely the grating's spectrum into two separate spectral peaks, and what is monitored are small changes in the spectral separation already present. However, polarization maintaining fiber is unduly expensive.

In contrast to ordinary Bragg gratings, the spectral transmission window in a π-phase-shifted fiber Bragg grating can be made very narrowband (less than 1 pm) and is split in two when the grating is birefringent. This sharpness permits very high-accuracy measurement of grating birefringence, to an accuracy of B=5(10)^"10. Furthermore, the birefringence required for separating the peaks is much smaller than for regular gratings and can be provided by the intrinsic birefringence of a grating written in non-polarization maintaining fiber.

Disclosure of the Invention

Accordingly, an object of the invention is to permit measurement of strain, loading, or the like in an optical fiber directed transverse to the fiber's optical axis.

Another object is to do the foregoing in a manner which is relatively temperature insensitive.

Another object is to do the foregoing with micro strain resolution.

Another object is to do the foregoing without the need for polarization maintaining optical fiber.

In accordance with these and other object made apparent hereinafter, the invention concerns a method and apparatus which employs an optical fiber with at least some birefringence, and in which is a π-phase-shifted fiber Bragg grating, along with a detector disposed to detect, responsive to light incident on the grating, the spectral position of each transmission peak of the grating. These peaks are unique to π-phase-shifted Bragg gratings, and by monitoring the position of, or change in separation of, the spectral positions of the peaks, one can infer fiber birefringence, or change of birefringence over time. Because these spectral peaks are much narrower band than the bandwidth of an ordinary Bragg grating, the resolution which this scheme affords is much higher than obtainable with an ordinary Bragg grating, and without need for polarization maintaining fiber. Because strain or mechanical loading on a fiber alters the fiber's birefringence, one can infer from shifts in the transmission peaks changes in fiber strain or loading, permitting fine- resolution measurement of the strain or loading. Theory suggests that this scheme would prove relatively temperature insensitive, and tests confirm this.

These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein:

Brief Description of the Drawings

Figure 1 is a schematic view illustrating an embodiment according to the invention.

Figure 2 is a graph having three curves, each illustrating aspects of the spectral response of a π-phase-shifted Bragg grating.

Figure 3 is a schematic of an embodiment according to the invention used to test features of the invention.

Figure 4 is a schematic of the apparatus used in the test to load a π-phase-shifted Bragg grating transverse to its optical axis.

Figure 5 is a graph of data taken in the.

Figure 6 is a graph of data taken with an apparatus similar to that of figure 3, illustrating temperature stability of the apparatus.

Modes for Carrying Out the Invention

With reference to the drawing figures, wherein like numbers indicate like parts throughout the several views, figure 1 is a schematic showing an embodiment according to the invention. Laser 10 outputs a relatively broadband signal to optical coupler 12 and ultimately to a length of optical fiber 14 in which is inscribed a π-phase-shifted Bragg grating 19. Fiber 14, and hence grating 19, has a mechanical load 16 directed on it, e.g. from stressing object in which fiber 14 is embedded or by placing a load on fiber 19, which physically distorts the glass of fiber 14 in a direction transverse to direction 15 . This distortion causes corresponding distortion in the fast and slow birefringent axes of fiber 14, changing the fiber's net birefringence. Members 13 and 17 are the individual grating elements of the Bragg grating, illustrated schematically in figure 1 as vertical lines, and are periodically spaced portions of fiber 14 at which the fiber's index of refraction has been changed to vary sufficiently from surrounding glass to set up an optical grating in fiber 14. Forming elements 13, 17, can be done in any conventional manner, e.g. by doping fiber 14 with material having a differing index of refraction, or, more commonly, by inscription using a laser. Disposed between grating elements 13, 17, is a portion 15 of fiber 14 containing no gratings. Spacing 15 is characteristic of a π-phase-shifted Bragg grating, and distinguishes it from an ordinary Bragg grating by setting up, in effect, a narrow bandwidth Fabry-Perot filter within the Bragg grating. Detector 18 receives and detects light transmitted through fiber 14 as a function of optical wavelength, and detector 26 does the same for light reflected from fiber 14 though optical coupler 24 via optical tap 22. Detectors 18, 26 can be any device which can detect optical power or intensity as a function of wavelength, and preferably are simple and conventional spectrum analyzers or oscillographs. Direction 15 represents for sake of illustration the optical axis of fiber 14, and is the direction through which light flows through grating 19.

Figure illustrates the response of a π-phase-shifted Bragg fiber. Curves a, b, and c represent the transmission response of the grating, normalized to a magnitude of one, for light polarized at three different orientations to the gratings fast and slow axes. Each of the curves shows a transmission notch 27 corresponding to the effective bandwidth of the Bragg grating, disposed about a center wavelength 32, which is characteristic of all Bragg gratings. Within bandwidth 27 is a narrowband portion 28, 30 of high transmissivity characteristic of a π-phase-shifted Bragg. Curves 28 and 30 illustrate grating response for light which is polarized directly along the respective fast and slow axes of the optical fiber in which the grating resides. This results in a single transmission peak in each curve (28 or 30) on opposite sides of center wavelength 32. Curve b illustrates grating response for input light which is polarized transverse to the fast and slow axes, resulting in two peaks 28' and 30' narrowly spaced from one another in wavelength, each on the order of half the magnitude of peaks 28, 30, indicating that peaks 28' and 30' resulted from optical energy which split between the two birefringent axes of the fiber carrying the Bragg grating. The wavelength spacing between axes 28 'and 30' thus reflects the amount of birefringence in fiber 14 and permits a means by which to measure it. Because the spacing peaks 28', 30' is much narrower than bandwidth 27 of the Bragg grating, this permits a more sensitive measure of birefringence than is obtainable by ordinary, non-π-phase-shifted, Bragg gratings.

In operation, light from laser 10 is incident on fiber 14 via optical coupler 12. Bragg grating 19 reflects a portion of the light, which couples through member 24 to detector 26, and transmits a portion of the light, which couples via member 20 to detector 18. The light transmitted to detector 18, and that transmitted to detector 26 are complementary to one another: the spectrum detected at 18 is like the curves of figure 2, with bandwidth 27 appearing as a notch removed from the spectrum of laser 12, and peaks 28, 28', 30, 30' appearing as passband peaks; whereas the spectrum detected at 26 is like the curves of figure 2 flipped about the wavelength axis, with a peak within bandwidth 27, and a notched trough corresponding to 28, 28', 30, 30'. For purposes of simplicity, the spectral responses at either of detectors 18 or 26 is hereinafter generically called the spectral response of π-phase-shifted Bragg grating 19, and the bands 28, 28', 30, 30' referred to generically as transmission peaks, with the understanding that the term comprehends as well the corresponding reflection notch as viewed at detector 26.

Example I

A test was performed using the experimental setup illustrated in figure 3. A Phonetics Tunics 1550 tunable laser 10' was used to interrogate grating 19'. Laser 10' had a step accuracy of 1pm, but could be controlled with high resolution over a approximately 30 pm range by use of an external voltage applied to the laser's fine scanning input (FSC in figure 3) (2.0pm/V at 300 Hz). The spectrum of the selected 30 pm region of laser 10's output was displayed as an x-y trace on oscilloscope 31 (TEKl) (the x and y axes defining a plane orthogonal to the optical axis of Bragg grating 19'). Detector 26' (Dl) provided the signal for the y-axis. Polarization controller 32 (PC) was used to adjust the polarization of input light, and phase shifter 34 (Δ(φ) was used to compensate for the laser's voltage-to-wavelength delay. This method of interrogation was selected because of the availability of the components in the test's laboratory, and does not represent an optimal system in terms of either achievable resolution or overall system cost.

The π-phase-shifted grating 19' was written in Naval Research Laboratory photosensitive fiber by use of the ultraviolet re-exposure approach, and figure 2 is the resultant spectral response of grating 19'. The center wavelength of the free grating was measured to be 1566.872 nm, with polarization peaks 28', 30' separated by 6.0 pm and each having a full-wave-half-maximum bandwidth of 3.6 pm. The separation of the peaks was measured by use of oscilloscope 30's built in cursor-voltmeter features, with a 0.1 pm (0.05 V) resolution, corresponding to a 6(10)^"8 birefringence resolution. When the birefringence-induced split was larger than 4 pm, both peaks 28', 30' were present simultaneously. For smaller birefringence, polarization controller 32 was manually adjusted to alternate between the peaks.

To apply a known transverse strain to grating 19', the loading configuration shown in figure 4 was used. Fiber grating 19' in fiber 14', and a fiber 36 of the same diameter (used for force balancing) were diametrically loaded by two glass plates 38, 40. Top glass plate 38 and steel block 42 to which plate 38 was attached had a combined mass of 108.5 g. Additional loading was applied by a fixture instrumented with a load cell (not shown). Each fiber 14, 36 saw half the total force, distributed over the 25.4 mm width of the glass plate. Simple tabs (1 and T2) made with adhesive tapes attached to optical fiber 14' were used to indicate the fiber rotation angle, which was measured with a simple protractor to an accuracy of ±2°.

Figure 5 shows the measured birefringence as a function of the applied load (per unit fiber length) for three different orientations of the π-phase-shifted grating 19'. We found the o° direction by rotating optical fiber 14' to obtain maximum peak separation for a given load. The scale at the top of figure 5 gives the strain difference (6_x-£_y) at the core of fiber 14', with a scale factor of 339 μ£f (N/mm), as provided by theory. (For the theory in detail, see the paper by M. LeBlanc et al., Transverse Load Sensing by use of Pi-Phase-Shifted Fiber Gratings, OPTICAL LETTERS, vol. 24, no 16, at p.1091.) From theory, the wavelength separation Δλ between peaks 28', 30' is:

Δλ = { [Δλ₀Cos ( 2ψ) +λ₀ (n₀ ²/2 ) (P₁₂-P_u) ( e -e ) ] ²+ [Δλ₀Sin ( 2ψ) ] ²}^Vl

where e and β_y are strains which force 16' induces in the x and y directions of fiber 14' (i.e. directions orthogonal to fiber 19"s optical axis, and to each other), ψ is the angle which the principle strain e° of fiber 14' makes with arbitrarily selected x axis, and P₁₂ and P_u are elements of the fiber optic strain tensor for fiber 14'. Note that the effect of the original birefringence is contained in the initial separation term λ₀, which is known irrespective of the mechanism which created it. The dotted lines of figure 5 show a simulation of results super imposed on laboratory results. For the simulation, the following values were used: P_u=0.113, P₁₂=0.252, n=1.46, and λ₀=6pm, and the following plotted:

( e£-e£) = [ 8 (l+v_c) /E_f] [F/nd_e]

Where F is the force on grating 19' per unit length, E_f =70.3 and is Young's modulus of optical fiber 14', d_f =1.25μm and is the diameter of fiber 14', and V O.17, Poisson's ration for fiber 14'.

Depending on the orientation of optical fiber 14', the applied load either increases of decreases birefringence. When the load is applied at 45° to the principal strain directions, the initial effect of the load is to rotate the direction of the principal axes, with no change in the magnitude of the birefringence. In the experimental results shown in figure 5, the slope of the linear portions of the data is 73 ± 2pm/(N/mm), whereas theory predicts 78.5 pm/(N/mm). This discrepancy, it is believed, can be attributed to the lack of precise knowledge during the test of the optical properties of grating 19', and P_π and P₁₂ in particular, at the wavelength used. With this experimental slope, the 0.1 pm accuracy of the setup corresponds to 1.4 (10)^"3 N/mm, or 0.5μ£- of strain difference. The maximum separation of the transmission peaks (while remaining within the stopband of both polarization spectra) is approximately 100 pm for grating 10', corresponding to a -500 μC principal strain difference in the optical fiber, or a 1.4 N/mm transverse load. This is sufficient for many practical applications, particularly so because the transverse strain differences in optical fiber is typically 20 to 100 times smaller than those in a host.

Example II

In a separate test, a second grating with an intrinsic separation (corresponding to peaks 28', 30' for grating 19') of 3.7 was loaded as in the previous example, and the loading setup placed in an environmental chamber so that the temperature of the test could be changed. The experiment was done in two steps: First, the zero-load birefringence was determined for a range of temperatures of 15°C to 55°C. The results indicated no measurable change in birefringence (bottom curve in figure 6). Following this, the slope of the 0° curve at each temperature was measured, by applying a force in the range of 1 to 10 N on the loading block, using a piezo-driven actuator. Within the accuracy of the experiment, the results show that the force-response slope is also constant (top curve, figure 6). Given that the temperature is uniform across the grating and that the materials involved are thermomechanically isotropic (at least compared with polarization maintaining fiber), no changes in birefringence should be induced by changes in temperature. The test results confirm this.

The invention has been described in what is considered to be the most practical and preferred embodiments. It is recognized, however, that obvious modifications to these embodiments may occur to those with skill in this art. Accordingly, the scope of the invention is to be discerned from reference to the appended claims, wherein:

Claims

1. A system comprising; an optical fiber, said fiber having birefringence, said fiber comprising a π-phase-shifted Bragg grating in said optical fiber; and a detector, said detector being operatively disposed, responsive to light incident upon said Bragg grating, to detect the spectral position of each transmission peak of said Bragg grating.

2. The system of claim 1, wherein said detector is further disposed, responsive to said light incident on said Bragg grating, to detect the change in said spectral position of said each peak.

3. The system of claim 1, wherein said system further comprises a physical object, and wherein said Bragg grating is operatively coupled to said object effective to cause mechanical loading of said Bragg grating transverse to the optical path through said grating.

4. The system of claim 3, wherein said detector is further disposed, responsive to said light incident on said Bragg grating, to detect the change in said spectral position of said each peak.

5. The system of claim 4, wherein said change in spectral position is in response to said loading of said Bragg grating.

6. A method for determining birefringence in an optical fiber, comprising: detecting, responsive to light incident upon said Bragg grating, the spectral position of each transmission peak of the spectral response of said Bragg grating; determining the spacing between said each transmission peak; and associating said spacing with a corresponding change in said magnitude of said birefringence.

7. The method of claim 6, further comprising: detecting the change in said spectral position of said each transmission peak; and associating said change with a corresponding change in said magnitude of said birefringence.

8. A method of determining loading by a physical object transverse to the optical axis of an optical fiber, comprising: providing said fiber with a π-phase-shifted Bragg grating; operatively coupling said object to said Bragg grating effective to couple said loading to said Bragg grating transverse to the optical path through said grating; and detecting the change in spectral position of each transmission peak of the spectral response of said Bragg grating responsive to said loading.

9. The method of claim 8, wherein said loading is selected from the group consisting of: pressure, strain.