Recent Advances in Fiber Cavity Ring-down Technology

Susana Silva and Orlando Frazão

INESC TEC and Department of Physics and Astronomy, Faculty of Sciences of University of Porto,

Rua do Campo Alegre 687, 4169-007 Porto, Portugal

Keywords: Cavity Ring-down, Optical Fibre Sensors, Strain, Curvature, Refractive Index.

Abstract: A brief review in the fibre cavity ring-down (CRD) technique is presented. It addresses the latest

developments in CRD technique for sensing applications, undergone at INESC TEC. The CRD is based on

the conventional configuration with the possibility of adding amplification in order to compensate the

output signal losses induced by the sensing head. The results obtained for strain, curvature and refractive

index sensing are presented, corresponding to distinct sensing structures, namely, a chirped fibre Bragg

grating (FBG), a long period grating (LPG) and a multimode interference (MMI) based sensor.

1 INTRODUCTION

The cavity ring-down (CRD) technique consists in a

spectroscopy method broadly used in technology.

Being mainly employed in the sensor area, this

methodology enables an accurate analysis of

amplitude behaviour over the time. Throughout the

past decades, the CRD spectroscopy has been

subject of a lot of research, namely in chemical and

molecular analysis in real time (Berden et al., 2000).

The principle involved settles the basis for various

configurations, being also applied to resonant optical

cavities with high reflective mirrors, as a result of

the high evolution of this technique (Herbelin, 1980)

(Herbelin et al., 1980; Anderson et al., 1984).

Through association with the previous developed

fibre loops, there were implemented new fibre optic-

based CRD settings, which, in turn, used a fibre loop

operating as the resonant cavity. This configuration

quickly obtained a lot of popularity in the scientific

community, mainly for presenting an effective

alternative to the usual CRD configuration (Stewart,

2004). By virtue of these conceptual studies, the

CRD technique has been target of a lot of research in

the spectroscopy field, being implemented in the last

decade to the measurement of physical parameters,

such as strain (Tarsa et al., 2004; Ni et al., 2007;

Silva et al., 2015a), temperature (Wang, 2005),

curvature (Lerber et al., 2002; Silva et al., 2015b)

and pressure (Wang and Scherrer, 2004a; Wang and

Scherrer, 2004b; Qiu et al., 2007). More recently, a

fast development of CRD technique allied to the

biochemical sensing field can also be easily

recognized. The implementations in this area are

quite extensive, which can go from detection of

organic dyes (Brown et al., 2002) to unicellular

organisms (Tarsa et al., 2004), or even 1-octyne in

decane solution (Pu and Gu, 2009). The CRD

approach has also been widely used for refractive

index of liquids (Ni et al., 2008; Zhou et al., 2009a;

Zhou et al., 2009b; Wang et al., 2010; Wong et al.,

2012) and gas sensing (Stewart et al., 2001; Vogler

et al., 2003; Zhao et al., 2013; Shimizu and

Noriyasu, 2014).

In addition to these applications, the CRD

technique use is widespread, being implemented in

chemical sensors (Wang, 2009), using frequency-

shifted interferometry (Ye et al., 2011), using

wavelength-tuneable ultrashort pulsed light (Hiraoka

et al., 2013) or even an Optical Time Domain

Reflectometer (OTDR) instead of the usual

implemented laser and modulator setup (Passos et

al., 2014). More recently, a new CRD topology for

remote sensing was demonstrated (Silva et al.,

2016).

This work adresses the latest developments in

fibre optic based CRD technique for sensing

applications, undergone at INESC TEC. The results

obtained for strain, curvature and refractive index

sensing are presented, with focus to the sensing

structures

used and the CRD configuration for such

purpose.

Silva S. and Frazao O.

Recent Advances in Fiber Cavity Ring-down Technology.

DOI: 10.5220/0006258303510355

2 FIBER-OPTIC CRD SETUP

The experimental setup of the main configuration

CRD system used for sensing is presented in Figure

1. The fibre loop is composed of two standard (2 ×

1) 1:99 optical fibre couplers and a sensing head. A

commercial OTDR is used as modulated laser source

in order to send impulses (1 s at 1550 nm) down

into the fibre cavity. The train of pulses is coupled

via 1% arm of the input optical coupler, rings around

inside the fibre loop and is coupled out via 1 % arm

of the output coupler; the amplitude of the output

pulses decay temporally due to the total existing

losses in the fibre loop (fibre loss, fibre couplers

insertion losses, sensing head transmission

attenuation), passes through a photodetector (gain of

40 dB) and is monitored in an oscilloscope.

Figure 1: Experimental setup of the CRD configuration

that uses an OTDR as laser source and a transmission-

based fiber structure as sensing head.

The advantage of using an OTDR is to allow

system optimization either in terms of impulse and

optical power – allowing its use in long distance

applications. Furthermore, the use of a multimode

laser source increases the optical power inside the

fibre loop. Also, the output signal of the OTDR may

be used as laser source or to interrogate the variation

of the output signal.

3 SENSING APPLICATIONS

3.1 Strain Sensing

A fibre-based CRD configuration using an OTDR to

interrogate the output signal has been developed for

the measurement of strain (Silva et al., 2015a). The

sensing head, which is placed inside the fibre loop

cavity (1560 m), is based on a chirped fibre Bragg

grating (chirped-FBG) and acts as a strain sensing

element. The chirped-FBG is centred at 1570 nm,

has 4 nm width and is interrogated in transmission.

The output signal response is strongly dependent

of the wavelength position of the FBG with respect

to the OTDR laser source (operating at 1550 nm). If

the chirped-FBG is placed at lower wavelengths, the

output signal is expected to decrease with increasing

strain. On the other hand, if the FBG is placed at

wavelengths higher than 1550 nm (in this case, the

FBG is centred at 1570 nm) the output signal is

expected to increase with applied strain.

Figure 2 presents the OTDR pulse intensity

decay that is back-reflected by the chirped-FBG

inside the fibre loop. At the beginning, there is

signal saturation that may be avoided by placing a

few meters of singlemode fibre between the OTDR

and the cavity ring. Afterwards, the amplitude of the

signal decays due to the several round trips of the

pulse inside the cavity, as expected.

Figure 2: OTDR waveform of the impulse intensity decay

caused by the chirped-FBG inside the fiber loop.

The operation mode of the CRD interrogated by

the OTDR is based on the second reading of the

back-reflected light from the FBG. In this case, the

light travelled ca.1600 m which corresponds to a

single round trip of 8 s, similar to the one obtained

with the CRD configuration. This also corresponds

to the distance between two consecutive peaks (800

m from observing Fig. 2) – due to signal processing

of the OTDR, the back-reflected and measured light

is divided by 2; therefore, the effective distance

between two consecutive peaks is ca. 1600 m.

The OTDR may also be a viable interrogation

method as depicted in Figure 3.

Photodetector

Optical fiber coupler (2)

1:99

Optical fiber coupler (1)

1:99

Modulated

Laser Source

Oscilloscope

Sensing

head

Fibre

Loop

02468101214161820

Amplitude (dB)

Distance (Km)

Figure 3: Cavity ring-down time versus applied strain to

the chirped-FBG when interrogated by the OTDR.

When strain is applied, the chirped-FBG moves

to longer wavelengths, scanning the several

wavelength peaks of the laser source and thus

resulting in a periodic behaviour as shown in

Figure 3. In this case, the ring-down time is almost

10-fold the one obtained with the conventional CRD

configuration (22.8 µs) and ranges from 180 to

206 µs due to the FBG position regarding the laser

source. Using the CRD configuration it was possible

to obtain a linear response to strain applied to the

chirped-FBG in the range 0-2000 and a sensitivity of

1.34 ns/ was obtained.

3.2 Curvature Sensing

The fibre-based CRD configuration presented in

Figure 1 has also been used for the measurement of

curvature (Silva et al., 2015b). An OTDR is used to

send impulses down into the fibre loop cavity

(800 m), inside of which a long period grating

(LPG) is placed to act as sensing device. The LPG

was written by UV in SMF28e with a period of 335

nm and a length of 25 mm. The LP07 resonance

mode of the LPG is positioned at 1566.2 nm with a

full width half maximum (FWHM) of 33.7 nm. This

resonance is situated in the long-wavelength side of

the multimode laser spectrum which is centred at

1550 nm and has a 50 nm-bandwidth.

In this case, an added-signal was used for

curvature sensing and it was obtained by the sum of

several conventional CRD impulses. Figure 4 shows

the CRD trace when the LPG was interrogated using

(1) the CRD conventional signal processing and (2)

when an added-signal was applied in the fibre loop.

Figure 4: CRD trace for (1) conventional configuration

and (2) added-signal obtained by the sum of several

conventional CRD impulses.

In the first case (1), the OTDR sent impulses of

1 s at 1550 nm down into the fibre cavity; the time

of a single round trip was ca. 3.85 s which is

determined by the cavity length. An exponential fit

was also performed and a ring-down time of 23.7 µs

was attained, with an associated loop loss of 0.7 dB.

In the second case (2), the OTDR sent impulses of

20 s at 1550 nm in order to obtain the added-signal.

After performing an exponential fit, a ring-down

time of 43.3 µs was obtained. Such results show that

the added-signal increases the optical power but

increases as well the ring-down time due to the sum

of the several loops that light travels inside the ring.

The behaviour of this structure as a curvature

sensor was duly characterized. As expected, the

amplitude of the added-signal decreases with

decreasing curvature, as result of bending the LPG.

The ring-down time as a function of curvature was

also determined for both conventional CRD- and

added-signals, as depicted in Figure 5.

Figure 5: Ring-down time versus curvature applied to the

LPG when interrogated by the conventional CRD signal

processing and added-signal.

0 200 400 600 800 1000 1200 1400 1600

175

180

185

190

195

200

205

210

Ring-down time (s)

 ()

0,03

0,04

0,05

0,06

0,07

0,08

-50 0 50 100 150

0,026

0,028

0,030

0,032

0,034

0,036

0,038

0,040

litude

(

)

Amplitude (mV)

Time (s)

(2)

(1)

2,22,42,62,83,03,23,43,6

Added-signal

Conventional CRD signal

Ring-down time (s)

1/R (m

-1

)

Results show that the sensitivities obtained for

the added-signal and conventional CRD signal are

15.3 s/m

-1

and 0.74 s/m

-1

, respectively. The

added-signal configuration presents a sensitivity 20-

fold the one obtained for the conventional CRD

signal processing.

3.3 Refractive Index Sensing

In a recent approach, a CRD system using a

multimode interference (MMI) based fibre sensor

was developed for refractive index (RI) sensing. Due

to the high losses associated to the MMI-based

sensing head, an erbium doped fibre amplifier

(EDFA) was inserted in the fibre loop cavity for

amplification of the output signal. The EDFA is

composed by 2 m of an erbium-doped fibre (losses

of 14 dB/m @ 980 nm) and in order to compensate

the losses from the MMI-based sensing head, a

current of 155.7 mA was applied to the fibre device.

The sensing head is based on multimode

interference and relies on a singlemode – coreless-

multimode – singlemode fibre configuration. The

coreless-MMF is a pure silica fibre with 125 m-

diameter and 20 mm-length, which was spliced

between two SMFs and interrogated in transmission.

The CRD trace obtained with the proposed

configuration is shown in Figure 6. The time of a

single round trip is ca. 7.8 s and is strongly

dependent on the pulse width, fibre length, fibre

losses, MMI losses and others.

Figure 6: CRD trace for pulses sent by the modulated

multimode laser source into the fiber loop with 500 ns

width.

The sensing head was then submitted to RI

changes of water caused by temperature variation.

The MMI-based fibre sensor was placed under water

and submitted to increasing temperature in the range

25–80C. In this experiment, the OTDR sent

impulses of 500 ns at the operation wavelength of

1550 nm, which in turn is located in the slope of the

band-rejection filter centred at 1546 nm. Therefore,

the wavelength variation caused by RI changes of

the external medium will shift the band-rejection

peaks, associated with an amplitude variation of the

acquired signal. In this case, the temperature-

induced RI variation of water causes the amplitude

of the band-rejection peaks to change and

consequently leading to measurable losses in the

acquired CRD signal.

Figure 7 presents the ring-down time variation

according to temperature-induced RI changes of

water. Since the MMI-based fiber sensor is

intrinsically sensitive to temperature but also to RI

of the external medium, one had to remove the

temperature component of the acquired output

signal, in order to obtain the presented results.

Figure 7. Ring-down time versus temperature-induced RI

changes of water.

The behavior of the sensing head to temperature

variation was studied. The MMI fiber sensor was

placed in a furnace and submitted to increasing

temperature and a negligible sensitivity of -1.610

s/C was attained. This allowed eliminating the

temperature component from RI measurement of

water and a linear sensitivity of 580 s/RIU in the

RI range of 1.324-1.331 was obtained.

4 CONCLUSIONS

This brief overview on fiber optic based CRD

technique for sensing applications, undergone at

INESC TEC, shows that one of the new lines of

research is the use of existing equipment on the

market for long range measurement. An example

was recently demonstrated, namely, the use of the

OTDR for the measurement of strain. Another line

of investigation is signal processing that makes the

0 20406080100

100

150

200

250

300

350

400

Amplitude (mV)

Time (s)

1.322 1.324 1.326 1.328 1.330 1.332

Ring-down time (s)

Refractive index of water (RIU)

Temperature increase

CRD a quick acquisition system and allows analysis

in real time. The use of the add-signal as a new

solution presents greater sensitivity when the sensor

is subjected to curvature. Finally it was presented the

use of the CRD for RI measurement. It is clearly that

its application in the future will be in spectroscopy

for medicine, biomedical and chemical areas of

research.

ACKNOWLEDGEMENTS

This work was supported by Project "CORAL –

Sustainable Ocean Exploitation: Tools and Sensors,

NORTE-01-0145-FEDER-000036, financed by the

North Portugal Regional Operational Programme

(NORTE 2020), under the PORTUGAL 2020

Partnership Agreement, and through the European

Regional Development Fund (ERDF). S.S. received

a Pos-Doc fellowship (ref. SFRH/BPD/92418/2013)

also funded by FCT – Portuguese national funding

agency for science, research and technology.

REFERENCES

Berden, G., Peeters, R., Meijer, G., 2000. Int. Rev. Phys.

Chem. 19, 565-607.

Herbelin, J. M., McKay, J. A., Kwok, M. A., Uenten, R.

H., Urevig, D. S., Spencer, D. J., 1980. Applied Optics

19 (1), 144–147.

Anderson, D. Z., Frisch, J. C., Masser, C. S., 1984.

Applied Optics 23 (8), 1238–1245.

Stewart, G., Atherton, K., Culshaw, B., 2004. Opt. Lett. 29

(5), 442-444.

Tarsa, W. P., Brzozowski, D. M., Rabinowitz, P.,

Lehmann, K. K., 2004. Opt. Lett. 29 (12), 1339-1341.

Ni, N., Chan, C. C., Dong, X. Y., Sun, J., Shum, P., 2007.

Meas. Sci. Technol., Special Issue, 18, 3135-3138.

Silva, S., Passos, D. J., Marques, M. B., Frazão, O., 2015a.

Microw. Opt. Technol. Lett. 57 (6), 1442-1444.

Wang, C., 2005. Opt. Eng. Lett. 44 (3), 030503.

Lerber, T., Sigrist, M. W., 2002. Appl. Opt. 41 (18), 3567-

3575.

Silva, S., Biswas, P., Bandyopadhyay, S., Jorge, P. A.,

Marques, M. B., Frazão, O., 2015b. IEEE Photon.

Technol. Lett. 27 (19), 2079-2082.

Wang, C., Scherrer, S. T., 2004a. Opt. Lett. 29 (4), 352-

354.

Wang, C., Scherrer, S. T., 2004b. Appl Opt. 43 (35), 6458-

6464.

Qiu, H., Qiu, Y., Chen, Z., Fu, B., Chen, X., Li, G., 2007.

Microw. Opt. Technol. Lett. 49 (7), 1698-1700.

Brown, R. S., Kozin, I., Tong, Z., Oleschuk, R. D., Loock,

H.-P., 2002. J. Chem. Phys. 117, 10444-10447.

Tarsa, P. B., Wist, A. D., Rabinowitz, P., Lehmanna, K.

K., 2004. Appl. Phys. Lett. 85 (19), 4523–4525.

Pu, S., Gu, X., 2009. Opt. Lett. 34 (12), 1774-1776.

Ni, N., Chan, C., Xia, L., Shum, P., 2008. IEEE Photon.

Technol. Lett. 20 (16), 1351-1353.

Zhou, K. M., Webb, D., Farries, M., Hayes, N., Zhang, L.,

Bennion, I., 2009a. Opt. Lasers Eng. 47, 1023-1027.

Zhou, K. M., Webb, D. J., Mou, C. B., Farries, M., Hayes,

N., Bennion, I., 2009b. IEEE Photonics Technol. Lett.

21 (22), 1653-1655.

Wang, C., Herath, C., 2010. Opt. Lett. 35 (10), 1629-1631.

Wong, W. C., Zhou, W., Chan, C. C., Dong, X., Leong, K.

C., 2012. Sens. Actuat. B 161, 108-113.

Stewart, G., Atherton, K., Yu, H., Culshaw, B., 2001.

Meas. Sci. Technol. 12, 843–849.

Vogler, D. E., Muller, M. G., Sigrist, M. W., 2003. Appl.

Opt. 42 (27), 5413-5417.

Zhao, Y., Bai, L., Wang, Q., 2013. Opt. Communications

309, 328-332.

Shimizu, H., Noriyasu, H., 2014. Japanese J. Appl. Phys.

53, 116601.

Wang, C., 2009. Sensors 9 (10), 7595-7621.

Ye, F., Qi, B., Qian, L., 2011. Optics letters 36 (11), 2080-

2082.

Hiraoka, T., Ohta, T., Ito, M., Nishizawa, N., Hori, M.,

2013. Japanese J. Appl. Physics 52 (4R), 040201.

Passos, D. J., Silva, S. O., Fernandes, J. R. A., Marques,

M. B., Frazão, O., 2014. Photonic Sensors 4 (4) 295-

299.

Silva, S., Marques, M. B., Frazão, O., 2016. Microw. Opt.

Technol. Lett. 58 (11), 2711-2713.