Closed-loop Interrogation Techniques for Temperature Measurement

using Fibre Bragg Gratings

Felipe Walter D. Pfrimer, Marcela H. Koyama, Elnatan C. Ferreira and Jos´e A. Siqueira Dias

Department of Electronics and Microelectronics, School of Electrical and Computer Engineering, University of Campinas,

Av. Albert Einstein, 400, Campinas, SP, Brazil

Keywords:

FBG Interrogation Circuit, Closed-loop Measurement Technique, Multi-point Interrogation.

Abstract:

Closed-loop schemes which maintain the amplitude of the reﬂected light from the FBG sensors at a constant

level are proposed to implement interrogators for FBG temperature sensors. Two systems are presented: a

very low-cost system where a thermoelectric cooler is used to tune a DFB laser, and a more complex multi-

point system with a broadband laser where n tracking FBGs located inside the interrogator are tuned by

n thermoelectric modules. The variation of the laser wavelength with temperature (or the variation of the

tracking FBGs with the temperature) is used to perform the measurements. Since the values of temperature in

these devices are uniquely associated with the sensing FBG reﬂected wavelength, if the temperature behaviour

of the FBG sensor is known, by measuring the temperature in the laser (or in the tracking FBGs) it is possible

to accurately calculate the temperature in the FBG sensors. A single-point sensor prototype was constructed to

validate the technique and a very high resolution of ±0.08 m

◦

C was measured in a 100

◦

C temperature range.

1 INTRODUCTION

Fiber Bragg Gratings (FBG) optical sensors became

very popular since they are relatively easy to inter-

rogate. Since applying a strain that stretches a FBG

causes a change in its gratings period, the strain re-

sults in a change in wavelength of the light reﬂected

by the FBG. Thus, to interrogate a FBG, it is neces-

sary to measure the variation of the centre wavelength

of the FBG, usually called the Bragg shifts (Hill and

Meltz, 1997).

However, due to the small dependency of the

Bragg shifts with temperature for a naked FBG, inter-

rogators with very high resolutions wavelength mea-

surement are required. For example, since a typ-

ical naked FBG presents a ∆λ/∆T ≈ 10 pm.

◦

−1

(Othonos and Kalli, 1999), the interrogator should

present a 1 pm resolution if a resolution of 0.1

◦

is required in the measurement system. Furthermore,

techniques which employ sophisticated optical com-

ponents in the interrogation system are usually very

expensive (Othonos and Kalli, 1999) and cannot be

used in low-cost applications. In this paper we present

two interrogation techniques: a single-point inter-

rogation technique which uses a narrow band DFB

laser and presents very high resolution and low-cost,

and a multi-point closed-loop electronics interroga-

tion technique which employs a broad band laser and

has a very fast interrogation speed.

2 THE PROPOSED TECHNIQUE

2.1 Principle of Operation

The basics of the proposed closed-loop interrogation

technique is shown in the block diagram of Figure 1,

where a closed-loop interrogation system with a DFB

narrow band laser is shown. The FBG sensor is illu-

minated by a DFB laser with wavelength λ

through

optical coupler OC

. The reﬂected light by the FBG is

sent to the photodiode D

and the photodiode current

is converted to voltage in the transimpedance ampli-

ﬁer A

. To show the principle of operation of the sys-

tem it will be assumed that when the laser wavelength

is at the FWHM (Full Width at Half Maximum) of the

FBG, the output voltage of the transimpedance ampli-

ﬁer will be equal to V

In (Dias et al., 2008), a feedback control of the

operating point of a FBG was proposed, in order to

allow for the interrogation of ac signals around the

operation point. In this paper a similar approach is

used, and a feedback loop is implemented in order to

force the laser’s output wavelength track the changes

Walter D. Pfrimer F., H. Koyama M., C. Ferreira E. and A. Siqueira Dias J..

Closed-loop Interrogation Techniques for Temperature Measurement using Fibre Bragg Gratings.

DOI: 10.5220/0004308300350039

In Proceedings of the International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2013), pages 35-39

ISBN: 978-989-8565-44-0

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

TEC

REF

= V

DFB LASER

FBG

Vo u t

TEC

PID

Controller

TEC

Driver

Vref

Vth

Figure 1: Block diagram of the basic closed-loop interroga-

tion technique.

of the FBG (caused by an external strain, in this case a

temperature variation) so that the output voltage of the

transimpedance ampliﬁer will always be at V

. This

is done by entering the output of the transimpedance

ampliﬁer and a reference voltage (numerically equal

to V

) in a PID controller which output will drive the

thermoelectric cooler (TEC) normally found in DFB

lasers (Cremonezi et al., 2012).

If a characterization of the laser’s wavelength as

a function of the laser’s temperature is performed us-

ing the internal thermistor available in the DFB laser’s

package, by measuring the voltage V

in a volt-

age divider made with the thermistor, it is possible to

know the wavelength of the laser and, consequently,

the temperature of the FBG.

2.2 Multi-Point FBG Sensing with

Narrow Band DFB Lasers

The proposed technique can be used to interrogate

multiple FBGs. However, if the system is illuminated

with a narrow DFB laser and the sensors are chained

in a series conﬁguration, there is a trade-off between

temperature range and number of points possible to

interrogate. Low-cost lasers usually can be tuned over

a 2 nm range when heated/cooled by its internal ther-

moelectric cooler, resulting in a temperature scanning

range of approximately 200

◦

C. This results in a se-

rious limitation because, if, for example, four FBGs

are to be inserted in the same optical ﬁbre, each FBG

will be allowed to have an excursion of only approxi-

mately 50

◦

C or overlapping of the spectra will occur.

If a low excursion range is acceptable, a very sim-

ple and low-cost interrogation can be implemented,

as shown in Figure 2. A single microcontroller con-

trols the whole system, and also implements a digital

PID controller, which is the core of the interrogator.

The system is initialized by forcing the laser to its

minimum temperature, which leads to the minimum

wavelength in the laser’s output. Then an analogue

class AB power ampliﬁer A

, controlled by a micro-

controller’s internal DAC, drives the TEC (heating it)

while the output of the transimpedance ampliﬁer V

out

is monitored by an A/D converter also internal to the

microcontroller.

When the value of V

out

reaches 10% of V

, (that

is, when the laser wavelength is beginning to enter

the FBG spectrum), the output of the DAC is discon-

nected from the TEC driver and the the output of the

PID controller is fed into the input of the TEC power

driverA

. The PID circuit acts and the system reaches

its steady state value, with V

out

= V

. The A/D con-

verter checks if the system is stable while another

channel of the A/D converter measures the voltage at

the thermistor V

. Since V

is uniquely related to

the lasers wavelength output, the exact position (and

consequently the temperature) of the FBG can be eas-

ily calculated.

TEC

REF

= V

DFB LASER

Vo u t

TEC

Driver

Vref

Vth

FBG

microcontroller

Digital PID

Algorithm

DAC

A/D

Scanning

Algorithm

Figure 2: Block diagram of the DFB laser single-point

closed-loop interrogator.

PHOTOPTICS2013-InternationalConferenceonPhotonics,OpticsandLaserTechnology

After the temperature of the ﬁrst FBG is read, the

PID controller is disconnected from the TEC power

driver and the microcontroller DAC starts to heat the

TEC, in order to the laser reach the next FBG. Again,

when the output of the transimpedance ampliﬁer indi-

cates that the laser wavelength is at the beginning of

the spectrum of next FBG, the same procedure done

with the ﬁrst FBG is repeated (the DAC is discon-

nected, the PID takes control of the system, etc.). This

is repeated until the last FBG in the chain is measured,

when the system begins to travel backwards (by cool-

ing the laser), in the direction of the ﬁrst FBG.

The only difference when measuring the FBGs

during the returning path (cooling the laser) is that

now it is necessary to drive the TEC until the laser

crosses the whole FBG spectrum and the condition

out

= 0.9V

is reached, that is, the laser wavelength

is at the left side of the FBG so that the PID takes

control of the system always at the same point. It is

important to notice that this system is relatively slow,

since the measurements cannot be performed simul-

taneously and the time required to scan four FBGS

(heating/cooling time, PID time necessary to stabilize

the system) can take up to 2 minutes.

2.3 Multi-Point FBG Sensing with

Broad Band Lasers

Implementing a closed-loop interrogator using a

broad band laser leads to a system which can accept a

large number of sensing FBGs and a extremely very

fast measurement time, typically in the order of a few

microseconds for each FBG. A block diagram of the

interrogation system using a broad band laser is pre-

sented in Figure 3. In this scheme, the number of

optical components is proportional to the number of

FBGs sensing elements, and the ﬁnal cost of an inter-

rogation system with input for more than 3 FBG tem-

perature sensors is dominated by the cost of the opti-

cal components (tracking FBGs, couplers and photo-

diodes).

The broad band laser illuminates the chain with

n FBGs sensing elements through a optical circula-

tor Circ

. The reﬂected light from the chain of FBG

sensing elements is fed into a series of optical cou-

plers. Each optical coupler let 95% of the incoming

light goes to the next coupler, and 5% of the light

is directed into a small circuit called Signal Tracking

Block (STB

. In each STB there is one FBG (called

tracking FBG, which is glued to a Zn substrate), one

photodiode, one thermoelectric cooler, one semicon-

ductor temperature sensor and one microcontroller

with its the electronic circuitry. Although it is not nec-

essary that the tracking FBGs be matched to the sens-

Figure 3: Block diagram of the broad band laser multi-point

closed-loop interrogator.

ing FBGs, it is easier to design the system if the FBGs

in each pair of tracking/sensing FBGS have their cen-

tre λ close. The basic schematic of each STB block is

presented in Figure 4,

The STB electronics circuits drive the TEC while

the microcontroller acquires the output of the tran-

simpedance ampliﬁer, which value is equal to the con-

volution between the sensing and the tracking FBGs.

A PID algorithm executed in the microcontroller con-

trols a DAC and a TEC driver circuit, forcing the

value of this convolution (output voltage of the tran-

simpedance ampliﬁer V

out

) to be equal to a reference

value V

ref

. If the temperature behaviour of the track-

ing FBG was previously characterized, measuring its

temperature it is possible to determine the centre λ of

the tracking FBG. Knowing the centre λ of the track-

ing FBG and remembering that, in the same FBG pro-

ﬁle (one has to choose to operate on the left or on the

right side of the FBG proﬁle) the convolution value

is uniquely related to the centre wavelength of sens-

Closed-loopInterrogationTechniquesforTemperatureMeasurementusingFibreBraggGratings

Transimpedance

amplifier

Tracking FBG

Microcontroller

STB

TEC

Driver

From STB

n-1

To STB

n+1

IC temperature

sensor

DAC

ADC

Optical Isolator

2x2

Optical Coupler

95% : 5%

Convolution

Figure 4: Schematic of the Signal Tracking Block - STB.

ing FBG, the temperature of the sensing FBG is easily

calculated.

The system may require up to one minute to

have all STBs in steady state, due to the slow heat-

ing/cooling which is used to control the tracking

FBGs. However, once the steady state is reached,

all the signal processing for the stabilization of the n

sensing FBGs is done in parallel, and it requires only

a few microseconds to interrogate each one of the n

channels, since all that has to be done is to convert the

analogue voltage V

out

to digital.

3 EXPERIMENTAL RESULTS

A prototype of a single-point FBG interrogator us-

ing a 1550 nm DFB laser (Eudyna Devices USA,

2004) was implemented to validate the principle of

the closed-loop technique. In the developed prototype

(block diagram shown in Figure 4), an analogue PID

controller was used, and the microcontroller was re-

sponsible only for scanning the system during start-up

and for reading the voltage V

at the thermistor.

The system was tested in the temperature range of

−20

◦

C to 80

◦

C, and the resolution obtained was very

high since, after reaching the steady-state, the output

voltage of the transimpedance ampliﬁer presented a

maximum ﬂuctuation which was less than 10µV, in

an output signal of 2.5 V. The edge of the sensing

FBG proﬁle has a width of approximately 400 pm and

a variation from 0% to 100% of the FBG reﬂected

light reaching the photodiode causes a voltage varia-

tion equal to 5 V in the output of the transimpedance

ampliﬁer. Therefore, the slope of the FBG proﬁle is

about 40 pm/V. Since a voltage change of 20µV can

be measured, it means that a variation of 800.10

−6

pm can be detected. This result indicates that the de-

veloped closed-loop single point interrogator using a

DFB laser presents a resolution of 0.08m

◦

C for a typ-

ical naked FBG with ∆λ/∆T = 10 pm.

◦

−1

A plot of the measured data in the prototype for

the temperature range of 30

◦

C to 47

◦

C is shown in

Figure 5, where it is presented the thermistor voltage

measured as a function of the temperature in the FBG

sensor, acquired with a precision AD590M (Timko,

1976) temperature transducer.

Figure 5: Plot of the thermistor voltage V

as a function

of the temperature measured with an AD590M temperature

transducer.

PHOTOPTICS2013-InternationalConferenceonPhotonics,OpticsandLaserTechnology

4 DISCUSSIONS

AND CONCLUSIONS

Three techniques for the implementation of closed-

loop interrogators for FBG temperature sensing appli-

cations were presented. A very low-cost single-point

interrogator can be constructed with only three opti-

cal components, a DFB laser, an optical coupler and a

photodiode. This interrogator has an extremely high

resolution (80 µ

◦

C for a naked FBG sensor) and with

a 2 nm tunable DFB laser it can measure temperatures

over a 200

◦

C range.

The DFB laser technique can also be used in

multi-point temperature measurements, but the sys-

tem presents limitations: the interrogator is slow (up

to two minutes can be necessary to measure FBGs

which are at the extreme points of the temperature

range), and there is a trade-off between the number of

sensing FBGs and the maximum temperature range

that each FBG can measure. Nevertheless, the sys-

tem presents a very high resolution and is extremely

cheap, since the cost of all components necessary to

implement the interrogator is around 300 Euro at the

time of writing.

A multi-point closed-loop interrogator scheme

that uses a broad band laser was also presented. This

technique is extremely fast and each FBG sensing el-

ement can be interrogated in a few microseconds. The

system is modular and can be easily built to oper-

ate with a custom number of channels. Since part

of the optical power is lost in the series of the opti-

cal couplers inside the equipment, it is expected that

the maximum number of channels would be limited

to 30-40. The multi-point closed-loop interrogation

technique is much more expensive than the technique

which uses the DFB laser because each interrogation

channel requires one FBG, one optical coupler and

one photodiode. The electronics circuits, TECs and

IC temperature sensors which are necessary to imple-

ment the interrogator are only a fraction of the cost of

the optical components and, therefore, the cost of the

interrogator is basically given by the optical compo-

nents.

ACKNOWLEDGEMENTS

This project was partially supported by Conselho Na-

cional de Desenvolvimento Cient´ıﬁco e Tecnol´ogico

- CNPq/Brazil and Coordenac¸˜ao de Aperfeioamento

do Pessoal de N´ıvel Superior - CAPES/Brazil.

REFERENCES

Cremonezi, A., Ferreira, E., Biazon Filho, A., and

Siqueira Dias, J. (2012). A ﬁber bragg grating rms

current transducer based on the magnetostriction ef-

fect using a terfenol-d toroidal shaped modulator. Sen-

sors Journal, IEEEl, PP(99):1.

Dias, J. S., Leite, R., and Ferreira, E. (2008). Electronic

technique for temperature compensation of ﬁbre bragg

gratings sensors. AEU - International Journal of Elec-

tronics and Communications, 62(1):72–76.

Eudyna Devices USA, I. (2004). 1,550nm DFB DWDM

Direct Modulation Laser FLD5F6CX-E Data Sheet.

Hill, K. O. and Meltz, G. (1997). Fiber bragg grating tech-

nology fundamentals and overview. IEEE J.Lightwave

Tech., 15:1263–1276.

Othonos, A. and Kalli, K. (1999). Fiber Bragg Grat-

ings Fundamentals and applications in Telecommu-

nications and Sensing. Artech House, Norwood, MA

- USA.

Timko, M. P. (1976). A two-terminal ic temperature trans-

ducer. IEEE Journal of Solid-State Circuits, SC-

11(6):784–788.

Closed-loopInterrogationTechniquesforTemperatureMeasurementusingFibreBraggGratings