Compact and fast interrogation unit for fiber Bragg grating sensors

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PROCEEDINGS OF SPIE

SPIEDigitalLibrary.org/conference-proceedings-of-spie

Compact and fast interrogation unit

for fiber Bragg grating sensors

Peter Kiesel, Markus Beck, Oliver Schmidt, Noble

Johnson, Michael Bassler, et al.

Peter Kiesel, Markus Beck, Oliver Schmidt, Noble Johnson, Michael Bassler,

Wolfgang Ecke, Kerstin Schroeder, Hartmut Bartelt, "Compact and fast

interrogation unit for fiber Bragg grating sensors," Proc. SPIE 6758, Photonics

in the Transportation Industry: Auto to Aerospace, 67580A (8 October 2007);

doi: 10.1117/12.734869

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Compact and Fast Interrogation Unit for Fiber Bragg Grating Sensors

Peter Kiesel

a

, Markus Beck

a

, Oliver Schmidt

a

, Noble Johnson

a

, Michael Bassler

a

, Wolfgang Ecke

b

,

Kerstin Schroeder

b

, and Hartmut Bartelt

b

a

_{Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto CA 94304;}

b

_{Institute for Physical High-Technology, Albert-Einstein-Str. 9, D-07745 Jena, Germany.}

ABSTRACT

We present a compact and fast wavelength monitor capable of resolving pm wavelength changes. A photosensor array or position detector element is coated with a linear variable filter, which converts the wavelength information of the incident light into a spatial intensity distribution on the detector. Differential read-out of two adjacent elements of the photosensor array or the position detector is used to determine the centroid of this distribution. A wavelength change of the incident light is detected as a shift of the centroid of the distribution. The performance of this wavelength detector was tested with a wavelength tunable light source. We have demonstrated that our device is capable of detecting wavelength changes as small as ~0.1 pm. The wavelength monitor can be used as read-out unit for any optical sensor that produces a wavelength shift in response to a stimulus. In particular, changes in the reflection properties of one and two-dimensional photonic crystals can been detected. The performance of this interrogation method has been tested for the case of temperature and strain sensors based on Fiber Bragg Gratings (FBG).

Keywords: optical sensor, interrogation, read out, vibration sensor, FBG sensor

1. INTRODUCTION

The overall market for sensors is huge (~$50 billion [1]); however, the fraction of the market for optical fiber sensors is still very small (but continuously growing) (<$200 million [1]). Current inhibitors of large scale commercial acceptance of the technology are the bulkiness and high cost of the interrogation systems. Within the last few years a general trend towards smart sensors and sensors with bus capabilities has begun. Optical sensors offer significant, even compelling, advantages for smart sensor schemes, particularly when connected via an optical bus. A major driving force behind this new demand for modern sensor techniques is the automotive sector, where optical data communication has been established in recent years. Biological, chemical and environmental sensors represent additional rapidly evolving fields

that can benefit from optical sensors

.

We have developed a low-cost and compact wavelength detector suitable for the

read-out of all kinds of optical sensors that are based on an optical wavelength shift caused by an external stimulus, e.g., temperature, strain, stress, acceleration, bio-coating, or chemical environment. Fig. 1 gives an overview of such sensors. Examples are: Fiber Bragg Grating (FBG) sensors, laser cavity sensors, micro-sphere or micro-ring based sensors as well as photonic crystal sensors. In comparison to their electronic counterparts, optical sensors have many distinct advantages. They are very sensitive, allow for remote and distributed sensing, can be used in harsh environments and are immune to electromagnetic interferences. The main disadvantage of optical sensors is their comparatively high cost. Within the last few years there have been many reports on inexpensive fabrication methods for the sensor component itself [2] (e.g., nano-imprint lithography, UV lithography, FBG fabrication with fs-lasers, and fabrication of FBG during the fiber drawing process). However, interrogation systems that are capable of resolving the small wavelength shifts (sub pm to few nm) produced by these sensors are quite bulky and expensive.

Our approach allows leveraging various technologies developed for the consumer market (e.g., inexpensive detector arrays and read-out techniques for camcorders, digicams or webcams) as well as low cost position sensors in order to realize a low-cost interrogation system. We have developed various systems to determine wavelength and wavelength shifts for monochromatic light input (e.g., single fiber with only one Fiber-Bragg-Grating (FBG) in combination with a one-dimensional coated position sensor), as well as systems suitable for multiple wavelength input (e.g., single fiber with several FBG in combination with a one-dimensional photo detector array with smart coating). A coated two-dimensional detector array even enables the simultaneous read-out of multiple wavelength shifts from more than one light source (e.g., fiber bundle with one or several FBG per fiber). In this paper we will mainly concentrate on the single fiber, single wavelength interrogation unit.

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wavelength re fle cti on white light wavelength re fle cti on wavelength re fle cti on white light gain medium fluidic channel mirror n2 n1 wavelength out put int ens ity wavelength out put int ens ity white light λ1 wavelength re flec tion λ1 λ1+∆λ stimulus white light λ1 wavelength re flec tion λ1 λ1+∆λ wavelength re flec tion λ1 λ1+∆λ stimulus λ1 white_light wavelength re flec tion λ1 λ1+∆λ stimulus λ1 white_light wavelength re flec tion λ1 λ1+∆λ wavelength re flec tion λ1 λ1+∆λ stimulus

Photonic crystal sensor

Silicon micro-sphere sensor

Surface plasmon resonance sensor Laser cavity sensor

Fiber Bragg grating sensor

wavelength tr ans mis sio n white light wavelength tr ans mis sio n wavelength tr ans mis sio n white light

Stimulus causes

wavelength shift

of sensor output

Fig. 1: Examples of optical sensors which are based on a wavelength shift. An external stimulus causes a wavelength shift in the optical output signal of the sensor.

2. WAVELENGTH SHIFT DETECTOR

The wavelength detectors are based on light sensitive elements, which are usually used to provide spatial information (e.g., photodiode array, CCD chip or CMOS detector array) in combination with a linear variable coating. Examples for such coatings are mirrors or optical filters that incorporate controlled inhomogeneous (spatially dependent) transmission/reflection properties. The laterally varying transmission/reflection properties of the coating on top of the light-sensing element define a correlation between position and wavelength information. Therefore, the spatially dependent signal of the detector contains information about the incident spectrum. By choosing suitable materials, a wide spectral range from the deep UV to the far IR and even into the THz range is addressable. A compact system for multi-signal analysis can be designed based on these wavelength sensitive detectors. Dependent upon the specific characterization method, detection systems may be designed for either high wavelength resolution or for broadband applications. The size of the spectrometer can be only slightly larger than that of a detector array, and fabrication requires only conventional, readily available processing technology, which should allow for a cost effective manufacturing process.

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Graded Cavity

Homogeneous DBR

Substrate

Photodiode Array or

Position sensor

(b)

(a)

0

5

10

15

930

940

950

960

970 transmi

ssion peak (nm)

position (mm)

shift @ 950 nm: 2.9 nm/mm

Graded Cavity

Homogeneous DBR

Substrate

Photodiode Array or

Position sensor

(b)

(a)

0

5

10

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930

940

950

960

970 transmi

ssion peak (nm)

position (mm)

shift @ 950 nm: 2.9 nm/mm

Fig. 2: (a) A filter with laterally varying transmission properties transforms spectral into spatial information. A photodetector records the intensity distribution transmitted through the filter. (b) Dependence of spectral position of peak transmission on lateral filter position.

We have fabricated various coatings that exhibit the properties described above, by MOVPE, MBE, and electron beam deposition. As an example, Fig. 2a shows a linear variable GaAs/AlAs Fabry-Perot coating. A graded Fabry Perot cavity has been sandwiched between two homogeneous DBR (distributed Bragg reflector) mirrors. The spectral transmission peak of a Fabry Perot structure is determined by the thickness of the cavity. Since the cavity thickness of this structure is laterally varying also the transmission peak is laterally changing. The gradient of this coating is characterized by the lateral shift of the Fabry Perot peaks which is about 3 nm/mm as shown in Fig. 2b. In order to demonstrate a wavelength shift detector with high spectral resolution (~sub pm) we combined this filters with a low-cost position sensor or detector array as depicted in Fig. 2a. The position sensor or detector array is able to very precisely determine the position of the incoming light beam. In combination with the gradient of the coating, this translates into a very high wavelength resolution in the sub picometer range. We have characterized our wavelength detector with a VCSEL (vertical cavity surface emitting laser), where the emission wavelength can be tuned very precisely by adjusting the laser current. Fig. 2b shows the output signal of the wavelength monitor as the incoming wavelength changes. Our wavelength detector follows very precisely the wavelength of the incoming light.

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Tunable VCSEL

λ₀=945-950 nm

Lin. Variable Filter (3 nm/mm)

946 n m 954 n m

Position Sensing Device

(b)

(a)

00 1 2 3 4 2 4 6 8 10 12 14 1.7 1.8 1.9 0.8 1.0 1.2 1.4 1.6 1 µA steps 2 µA steps 5 µA steps wa vele ng th ch an ge (p m) time (s) ~ 0.25pm ~ 0.5pm ~ 1.3pm 0 1 2 3 4 0 2 4 6 8 10 12 14 1.7 1.8 1.9 0.8 1.0 1.2 1.4 1.6 1 µA steps 2 µA steps 5 µA steps wa vele ng th ch an ge (p m) time (s) ~ 0.25pm ~ 0.5pm ~ 1.3pm Tunable VCSEL λ₀=945-950 nm

946 n m 954 n m

(b)

(a)

00 1 2 3 4 2 4 6 8 10 12 14 1.7 1.8 1.9 0.8 1.0 1.2 1.4 1.6 1 µA steps 2 µA steps 5 µA steps wa vele ng th ch an ge (p m) time (s) ~ 0.25pm ~ 0.5pm ~ 1.3pm 0 1 2 3 4 0 2 4 6 8 10 12 14 1.7 1.8 1.9 0.8 1.0 1.2 1.4 1.6 1 µA steps 2 µA steps 5 µA steps wa vele ng th ch an ge (p m) time (s) ~ 0.25pm ~ 0.5pm ~ 1.3pm

Fig. 3: (a) Setup for wavelength shift detection with high spectral resolution. (b) Response of wavelength shift detector to small wavelength shifts of the incident light.

The wavelength detector shown in Fig. 3 was capable to cover a total spectral range of about 5 nm. In order to demonstrate a much broader spectral range we have used a 10 mm position sensing device covered with a coating that has a steep gradient of about 30 nm/mm, as illustrated in Fig.4a. In order to characterize this combination we have used spectrally filtered light from a halogen lamp as a tunable light source. As shown in Fig. 4b this combination is able to detect wavelength changes of the incoming light over a total range of 80 nm. The wavelength resolution in this case was about 1 pm, which was confirmed by using a tunable VCSEL.

Key features of our wavelength detector include compactness (chip-size), no mechanical parts, vibration insensitivity, customized spectral resolution, fast read-out, suitability for broad wavelength ranges (UV to FIR), simultaneous spectral and spatial imaging (not shown in this paper), and monolithic integration.

Spectrally filtered Halogen lamp

91

0 nm

98

0 nm

Length: 10 mm 0 50 100 150 200 -10 -8 -6 -4 -2 0 2 4 6 8 10 980 nm diffe rential sig nal (V) time (s) 910 nm

(b)

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Fig. 4: (a) Setup for wavelength shift detection with broad spectral range. (b) Response of wavelength shift detector to large wavelength shifts of the incident light.

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5 10 15 20

time (sec)

3. READ-OUT OF FIBER BRAGG GRATING (FBG) SENSOR

The strength of this interrogation method has been demonstrated for the case of Fiber Bragg Grating (FBG) sensors, which address a wide range of applications such as structure health monitoring of aircrafts or temperature, pressure or vibration sensing in harsh environments. An FBG is formed by a periodic modulation of the refractive index along a finite length of the core of an optical fiber. This pattern reflects a narrow wavelength spectrum determined by the periodicity of the refractive index profile. An external stimulus (e.g., temperature or strain) changes the periodicity of the grating and, therefore, alters the reflected wavelength spectrum. The resulting wavelength shift is a direct measure of the stimulus. The high sampling rate of our detector enables monitoring the dynamics of strain in a structure. Consequently, this read-out unit can be used for interrogating optical vibration sensors. For demonstration we have monitored the oscillations of various metal springs that were equipped with a FBG strain sensor. As an example, Fig. 5 shows the response of the optical read-out unit to a deflection of a metal spring (at t ~ 1 s) and the subsequent oscillations when the spring was released (at t ~ 2.5 s). The strain can be measured dynamically with a resolution of a few micro strain. In a similar approach an FBG sensor has been used as a temperature sensor with a resolution of less than 0.1 K. For the shown measurements, the speed of the read-out was limited to 15 kHz due to the differential amplifier we used. With a faster amplifier, we were able to achieve 500 kHz.

Fig. 5: Response of an optical FBG vibration sensor to an applied static strain (t ~ 1 s) and the dynamic response when the strain was released (t ~ 2.5 s).

4. SUMMARY

We have demonstrated that our technology is capable of precisely measuring wavelengths and wavelength shifts with sub pm resolution, can monitor wavelength ranges of the order of 100 nm, and is not limited to a specific wavelength range between the deep UV and the far IR. We have employed this scheme to read out sensor elements based on the detection of small wavelength shifts. All the used components can be integrated into very compact systems. Similar to FBG sensors, the optical properties of two and three-dimensional photonic crystals are very sensitive to small perturbations. Our read-out technology is fully applicable to detect these effects with a high resolution.

REFERENCES

1. “Sensor Markets 2008: Worldwide Analyses and Forecasts for the Sensor Markets until 2008” published by Intechno Consulting, Basel (Switzerland) (1999). A more recent analysis by Frost&Sullivan from 8/2004 indicated even higher growth rates and market sizes.

2. 17th International Conference on Optical Fibre Sensors, Marc Voet, Reinhardt Willsch, Wolfgang Ecke, Julian Jones, Brian Culshaw, eds., Proceedings of SPIE Vol. 5855