Investigations on Sensitivity of Modal Fibre-Interferometer for

Acoustic Detection

Alexandre C. T. Santos

1,2 a

, Ricardo M. Ribeiro

, Andrés P. L. Barbero

Taiane A. M. G. de Freitas

and Cláudia B. Marcondes

Instituto de Pesquisas da Marinha (IPqM), Rio de Janeiro/RJ, Brazil

Departamento de Engenharia de Telecomunicações, Universidade Federal Fluminense (UFF), Niterói/RJ, Brazil

Centro Federal de Educação Tecnológica Celso Suckow da Fonseca (CEFET-RJ), Rio de Janeiro/RJ, Brazil

claudia.marcondes@cefet-rj.br

Keywords: Acoustic Communications, Michelson Interferometer, Opto-Acoustic Receiver, Modal Interferometer,

Ultrasound, Modalmetric.

Abstract: This paper describes investigations on sensitivity of modal fibre-interferometer named as “modalmetric”,

which is inserted in two optical coupling circuits based on circulator and 2x2 fibre-coupler. The “modalmetric”

in reflective structure is simply a single-mode fusion spliced with a short or long piece of a sensitive multi-

mode fibre (s-MMF) with a cleaved or mirrored end. All the devices presented here were probed in the C-

band and the tests were carried out at ~ 43 kHz frequency. By using a circulator, an increased opto-acoustic

sensitivity could be reached by misalignment of a FC/PC connection in the single-mode fibre, thus suggesting

the excitation of higher order modes in addition to the fundamental LP

. By using an 2x2 fibre-coupler, an

increased sensitivity was observed when one of the arms was made more reflective from the FC/PC ferrule

termination, thus suggesting a combination of modalmetric with Michelson interferometer or alternatively a

type of light recycling that oscillates between the reflective terminations. This paper also shows successful

tests of acoustic transmission through a 9.5cm length metallic billet. The central motivation is future applica-

tions of the modalmetric-based devices on ultrasonic communication and monitoring through solid media.

1 INTRODUCTION

Telecommunication links and networks are almost

always based on the use of electromagnetic carriers.

In the optical domain, the use of fibres to convey the

light carrier predominates despite the distances

involved. In the radiofrequency domain, electro-

magnetic waves typically presenting frequencies from

kHz to many GHz can carrier information through the

free air in many types of wireless services.

However, many physical media hinder or even

prohibit electromagnetic propagation suitable for

communications. Thus, ultrasonic communication is

practically the unique possibility. Some examples

regarding almost only acoustic communications can

a https://orcid.org/0000-0001-9478-179X

b https://orcid.org/0000-0002-3169-6675

c https://orcid.org/0000-0002-9077-8283

d https://orcid.org/0000-0003-0721-0089

e https://orcid.org/0000-0001-8002-1968

be cited as: petroleum (Rudraraju, 2010), solid media

(Heifetz et al, 2018) (Wang et al, 2018), energy cables

(Trane et al, 2015), downhole (Ahmad et al, 2014),

metallic pipes (Chakraborty et al, 2015) and undersea

environment (Farr et al, 2010).

Piezoelectric transducers (PZTs) have been used

as acoustic emitter (and also as detector) due to their

compactness, sensitivity, and availability (Sun et al,

2013). Fibre-optic detectors, especially those based

on fibre Bragg gratings (FBGs) and interferometers

provide the advantageous characteristics of optical

sensors as: immunity to electromagnetic interference

(EMI), large bandwidth, electric isolation, and others.

Interferometric sensors based on single-mode fibre

(SMF) are more common than on multimode (MMF)

Santos, A., Ribeiro, R., Barbero, A., G. de Freitas, T. and Marcondes, C.

Investigations on Sensitivity of Modal Fibre-Interferometer for Acoustic Detection.

DOI: 10.5220/0011778000003408

In Proceedings of the 11th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2023), pages 88-95

ISBN: 978-989-758-632-3; ISSN: 2184-4364

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

owing to sensitivity ~1000 times larger than the latter

(Layton at al, 1979). Hydrophones based on SMF

(Meng at al, 2021), are sofisticated devices and in

general require a great fibre length spam.

The modalmetric device that will be presented

here, is of a very simple construction, and is proposed

intending to be useful for digital communication and

works by selecting/restr and constraitings the

interfering modes traversing the SM/MM splice.It is

recognized that FBG (Wild and Hinckley, 2011) and

interferometric sensors may present comparable or

even higher sensitivity than PZT-based sensors

(Moccia et al, 2012). It was reported the acoustic

communications through a metallic wall using PZT

transducer as a transmitter and an FBG as a receiver

by means of differential detection using PSK

modulation (Wild and Hinckley, 2011). Therefore,

opto-acoustic detectors may present advantages when

compared to a piezoelectric one.

Modalmetric interferometer sensors in the

reflective structure (R-MMI) are fabricated by simply

splicing single-mode fibre (SMF) with the sensitive

multimode fibre (s-MMF) (Freitas et al, 2020). It does

not require the use of a FBG itself neither the careful

control of interrogation, i.e., the “spectral alignment”

between the laser and the grating spectrum. The R-

MMI structures behave like a one-arm interferometer.

Such R-MMIs structures have been used as a

distributed disturbance detector operating in the

electrical frequency’s domain (Oanca et al, 1997)

(Ribeiro and Balod, 2018).

This paper investigates the sensitivity of two

versions of the optical circuit of an opto-acoustic

detector based on a lumped R-MMI structure. The

sensitivity under 42.9 kHz acoustic frequency is

characterized when the ultrasonic wave amplitude

excitation is varied. In a first version, using an optical

circulator to light-coupling, the sensitivity increase is

investigated after the properly adjustment of a FC/PC

connection that is inline inserted with the SMF link.

In a second version, investigations of sensitivity were

performed by using an 2x2 fibre-coupler, where one

of the arms should be made more reflective from the

ferrule termination. Furthermore, the paper not only

shows opto-acoustic detection in a back-to-back

configuration, but as well a transmission along 9.5-

cm length of a metallic billet.

2 EXPERIMENTAL

Figures 1a-1d sketch the four built experimental

setups comprising the two optical circuit versions of

the modalmetric detector: using circulator (Figs.1a

Figures 1: Experimentals setups of the opto-acoustic

modal-interferometric detector.

and 1b) and using an 2x2 fibre-coupler (Figs.1c and

1d). The probe light is launched into port 1 and exits

at port 2 of an optical circulator or into port 1 and exits

port 2 and 3 of an 2x2 fibre-optic coupler. Figure 1a

shows a back-to-back transmission configuration

Investigations on Sensitivity of Modal Fibre-Interferometer for Acoustic Detection

using a continuous probe from an external-cavity

tunable laser (C-band) as light source. Figures 1b-1d

show an acoustic transmission in back-to-back or

through a 9.5-cm length of a metallic billet using a

continuous 1550nm DFB laser as light source.

The used SI-SMF (9μm/125μm) and GI-MMF

(62.5μm/125μm) were both of standard Telecom-

grade. The probe optical signal propagates along the

SMF until reach the sensitive MMF (s-MMF). The s-

MMF is fusion spliced to the output port of the

circulator or fibre-coupler. The SMF length from

splice to the SMF/SMF-FC/PC connector was ~ 65

cm as shown in Fig. 1a. The higher-order > LP

modes excitation even in the SMF (Schulze et al,

2013), is achieved by properly adjusting the

SMF/SMF-FC/PC connector as will be better

explained in the next section. The free end of MMF

was simply cleaved and none mirroring was applied.

The light signal at < 1 mW optical power level, as

reflected by the cleaved end of the s-MMF,

modulated (or not) by the acoustic waves, is

recovered by the optical circulator/2x2 fibre-coupler.

A SMD50T25F45R model PZT ultrasonic

transducer disc from STEMiNC is used to generate

acoustic waves. The disc was made with SM111

ceramic, presents 50 mm and 2.5 mm diameter and

thickness, respectively. The (44 ± 3) kHz was

specified by the manufacturer as the radial mode

resonance frequency. In communications and

monitoring is generally expected the arrival of very

weak acoustic signals. Therefore, there is no effort

here to generate high power ultrasound. An arbitrary

function generator (AFG) directly provides and

excites the PZT disc with a 1-tone 42.9 kHz

frequency electrical carrier that matches the effective

resonance frequency of the disc. The AFG provided 0

– 12.5 Vpp voltage range. The PZT disc generates and

transfers the acoustic waves directly to the s-MMF

(Fig.1a and 1c) or through a metallic billet (Fig. 1b

and 1d).

Figure 1a and 1c show 50 mm length (L

) of s-

MMF strand that was glued over the face and along

the diameter of the PZT disc by using a

cyanoacrylate-based adhesive.

The reflected light by cleaved end of the s-MMF

is collected by the port 3 of the circulator or port 4 of

the 2x2 fibre-coupler and reaches the preamplified

PIN photodetector (Thorlabs - PDA 10CS). In the

setup of Fig. 1a, the PDA was pre-amplified at 30 dB

(4.75 x 10

Ω) and the remained setups (Figs. 1b-1d)

the PDA was set at 40 dB (1.51 x 10

Ω)

transimpedance gain (TIA) level for high impedance

input load corresponding to 775 kHz or 320 kHz

bandwidth, respectively. The output of the

photodetector was connected to the f

= 50 kHz

electrical band-pass filter (EBPF). The filtered output

signals were displayed and recorded by means of

analog or digital oscilloscopes.

The SMF was fusion spliced to a 62.5/125 μm s-

MMF strand leading to reflective modalmetric

device. As can be seen in Figs. 1b and 1d, ~27 cm

length s-MMF was coiled in 5 turns with ~1.7 cm

diameter which was laid and glued using a

cyanoacrylate-based adhesive over the internal

surface of a metallic box with ~ 2 mm thickness just

in contact with the billet with 9.5 cm length and 6.4

cm diameter by means of bottom surface. Therefore,

the s-MMF presents ~27 cm of physical interaction

length with the vibrating surface. It should be pointed

out that the device works in reflective mode and the

effective interaction length is really the double of the

~27 cm s-MMF physical length. By using an acoustic

couplant gel, the PZT disc was put in physical contact

in the top surface of the billet.

3 RESULT AND DISCUSSIONS

3.1 Optical Circulator + Modalmetric

Setup in Back-to-Back Acoustic

Transmission

The laser was tuned to 1551.5 nm wavelength and Pin

= 5.7 mW was the optical power fed to the s-MMF.

Because the s-MMF end was simply cleaved, a

maximum of 0.035 x 5.7 mW ≈ 200 μW (-7 dBm)

optical power could be back reflected to the

photodetector. The TIA gain was set to 30 dB.

Figure 2 shows two dependences of output

voltage amplitude (mVpp) from the detector device

of Fig. 1a when the excitation voltage amplitude

(Vpp) applied on the PZT disc is varied in the ~ 0 -

10 V range. The vertical axis means the output

voltage signal as captured by the oscilloscope. In the

first dependence, the FC/PC connectors were “fully

screwed” thus concentrically aligning both the SMF

cores. The dependence is clearly sublinear. In the

second dependence, it was kindly unscrewed one of

the sides of the adapter that contains the SMF FC/PC

connectors, keeping the other side totally close. The

FC/PC connector were very slightly misaligned and

carefully fitted to obtain a maximum output

amplitude (“fitted max. sensitivity”) when 10Vpp

voltage excitation is applied. Almost none transmitted

optical power variation was observed in such

procedure by using an optical power meter. However,

it was observed that the amplitude of the signal

PHOTOPTICS 2023 - 11th International Conference on Photonics, Optics and Laser Technology

showed at the scope became greater, when compared

with the “fully screwed” connectors. It is likely that

by means of a slightly lateral with/without additional

angular misalignment, higher-order modes than the

fundamental LP

are also excited in the fibre core

(Ivanov et al, 2006) (Freitas et al, 2020) (Schulze et

al, 2013). Since the distance along the SMF from the

SM/MM splice to the FC/PC connector is over <1m,

the high-order modes can survive and excites high-

order modes in the s-MMF. By using this strategy, the

sensitivity could be increased. In the reverse sense, it

was reported the near-field contour of the light

propagating along the SMF after the back coupling

from the MMF (Ivanov et al, 2006). Most of the

modulated content is propagating along the cladding

and higher-order modes through the core. A similar

behavior was reported where an amplitude

enhancement of the signal was achieved after lateral

misalignment from 0.9 to 6.8 μm of the fibre’s core

but in the SMF/MMF fusion splice out of the

concentric fitting (Ribeiro and Balod, 2018).

Similarly, the angular misalignment by using the SM-

FC/APC with MM-PC connection can also enhances

the modalmetric sensitivity (Visagathilagar et al,

2014) (Freitas et al, 2020). However, in this paper is

shown that the mode excitation can be carried out

even in the SMF link, i.e., by merely adjusting the

FC/PC-FC/PC connection and monitoring

(optimizing) the output signal amplitude.

Figure 2: Sensitivity dependences of the modalmetric de-

vice using an optical circulator in B2B.

Figure 2 shows a linear dependence when the

FC/PC connector is adjusted as “fitted max.

sensitivity”. A sensitivity of 1014 mVpp/Vpp was

calculated by linear regression. For excitation voltage

amplitudes > 4 Vpp the sensitivity measured for

“fitted max. sensitivity” is higher than for the “fully

screwed” connector. The 1014 mVpp/Vpp sensitivity

was 1014/87 ≈ 11.7 times greater than the 87

mVpp/Vpp measured for the FBG-based opto-

acoustic detector (Leal et al, 2018). By using the

modalmetric detector, very weak acoustic signals as

those generated by applying < 0.05 Vpp amplitude on

the PZT disc was transduced to the optical domain

with good fidelity. The same experiment was carried

out with the FBG-based detector and only excitations

of > 0.5 Vpp amplitude could be properly detected

(Leal et al, 2018). Roughly speaking, the

modalmetric-based optical detector using circulator

presented ~ 10 dB higher sensitivity than the FBG-

based device.

Figure 3 shows the dependence of output voltage

amplitude (mVpp) from the device when the

wavelength of the probing laser is tuned in the 1525-

1625 nm wavelength range and the FC/PC connection

is in the “fully screwed” status. The excitation voltage

amplitude applied on the PZT disc was 10 Vpp and

5.7 mW was again launched to the s-MMF.

Figure 3: Sensitivity dependence of the modalmetric device

using an optical circulator when the wavelength is made to

vary in the 1525-1625 nm range.

The graded-index MMF is ultimately an optical

guide of multi-mode interference (MMI) type. The

spectral response of such structure is periodic fringes

where the periodicity decreases as the fibre length

increases (Kumar et al, 2003). Because of the short

length of the MMF used here, an incomplete

oscillation occurs as shown in Fig. 3 where the

maximum of transmission is close to1525 nm.

Although the scope trace is not shown here, an

improved sensitivity of 1340 mVpp/Vpp was reached

by properly fitting the SMF/SMF connection and

probing the device with 1525 nm wavelength. This

leads a sensitivity of 1340/87 = 15.4 times (~12 dB)

the 87 mVpp/Vpp reached by the FBG-based opto-

acoustic detector (Leal et al, 2018).

Investigations on Sensitivity of Modal Fibre-Interferometer for Acoustic Detection

3.2 Optical Circulator + Modalmetric

Setup in Acoustic Transmission

Through a Metallic Billet

Now, a DFB laser emitting around 1550 nm

wavelength and Pin < 5 mW was the optical power

launched to the s-MMF. The TIA gain was set to 40

dB. As is shown in the Fig. 1b, the acoustic waves

propagate along the metallic billet thus resulting in a

weak dynamic strain/stress that occurs due the arrival

of ultrasound in the position where the glued

modalmetric s-MMF is placed. The link was

optimized by simple trial, i.e. by slightly changing the

coupling position of the PZT-transmitters over the top

surface of the billet.

Figure 4 shows the acoustic response, i.e. it shows

the output voltage amplitude (Vpp) from the device

detector when the excitation voltage amplitude (Vpp)

applied on the PZT disc is varied in the ~ 0 – 12.5 V

range. The FC/PC connector was “fully screwed”.

The response is nonlinear thus presenting an

approximate agreement with the B2B transmission

shown by Fig. 2.

Figure 4: Sensitivity dependence of the modalmetric device

using an optical circulator after propagation along 9.5 cm

length of a metallic billet.

The average acoustic sensitivity from 0 to ~ 3 Vpp

voltage amplitude excitation was extracted as to be ~

2000 mV/V. This latter is greater than the 1014 mV/V

obtained in B2B as shown in Fig. 2 because now the

TIA gain is 40 dB instead of 30 dB. Therefore, the

normalized sensitivity regarding the propagation

along the billet is given by 2000/3.17 = 631 mV/V.

Furthermore, as was already reported (Freitas and

Ribeiro, 2021), in B2B measurements using the same

PZT-disc and 25 cm length of s-MMF, an average

acoustic sensitivity of 1074 mV/V was achieved but

constrained to the 0-1 V range of excitation. In the

present paper, a normalized acoustic sensitivity of

631 mV/V with a good linearity at least in the 0 - 3V

amplitude range excitation is measured as seen from

Fig. 4. An increase from 0 - 1V to 0 - 3V of the

dynamic range is here observed due the acoustic

attenuation after propagation along the billet thus

reducing the ultrasound amplitude that strikes the s-

MMF.

Although a similar result was previously

published by our group (Ribeiro and Freitas, 2021),

an important difference should nevertheless be

noticed. In (Ribeiro and Freitas, 2021)and section 3.1,

a tunable laser with 100 kHz linewidth, more

powerful and of higher cost was used as a light

source. In this sub-section, a DFB laser with ~10

MHz linewidth was used as the light source. So far,

we have not carried out studies on the effect of the

laser linewidth on the sensitivity of the modalmetric

devices. However, from the results obtained, it seems

that the performance difference is not significant at

least for external-cavity and DFB lasers.

3.3 2x2 Fibre-Coupler + Modalmetric

Setup in Back-to-Back

Transmission

As is shown in the Fig. 1c, the optical circuit to couple

the light is an 70/30 2x2 fibre-coupler where the

acoustic waves are in back-to-back transmission. The

FC/PC connector in the SMF was “fully screwed”.

The optical probe of the s-MMF launched in the arm

3 was at 1.8 mW power level.

Figure 5 shows the acoustic sensitivity (mVpp)

from the modalmetric detector when the excitation

voltage amplitude (Vpp) applied on the PZT disc is

varied in the ~ 0 – 12.5 V range. The acoustic

sensitivity measurements were performed in three

situations considering the arm 2 terminated by a

FC/PC connector. In the first situation, the ferrule was

immersed in a refractive index matching liquid so that

the reflected signal was virtually null. The curve

marked with blue squares shows a reduced sensitivity

of ~0.66/10 = 66 mVpp/Vpp under 10 Vpp amplitude

excitation. If we take into account that 1.8 mW is

probing the s-MMF, 1.8 x 0.035 = 63 μW power level

is reflected by the cleaved end of s-MMF and < 63

μW will reach the photo-detector. From the

calibration plot of modalmetric response under the

probing power level as can be seen from Fig. 4 of

(Freitas et al, 2020), a sensitivity of << 150 mV/V is

extrapolated and is in good agreement with the 66

mV/V measured even taking into account the 40 dB

TIA amplifying gain. The circuit with 2x2 fibre-

coupler, which presents free port (arm 2) terminated

for null reflection, works only on the basis of the

PHOTOPTICS 2023 - 11th International Conference on Photonics, Optics and Laser Technology

modalmetric device as in the case of using the

circulator, but with reduced sensitivity.

Hereafter, the ferrule of arm 2 was cleaned and

dried so that it began to reflect ~3.5% of the light

power. The curve marked with red diamond yields a

sensitivity of 12/12.5 = 960 mVpp/Vpp for 12.5 V

voltage excitation amplitude. There was then a

sensitivity increase of 960/66 = 14.5 times when

compared to the circuit working only as modalmetric.

It should be noted that the behavior of the curve

marked with red diamonds is not linear, which is

consistent with the fact that we have here a “fully-

screwed” FC/PC connector, similar to what was

described in section 3.1. By extending the straight

line between 0 and 4.5 Vpp of the curve marked with

red diamonds, an extrapolated sensitivity of ~20/12.5

= 1600 mVpp/Vpp is obtained for 12.5 Vpp of

voltage excitation amplitude.

Two possible interpretations are outlined here.

The circuit using a 2x2 fibre-coupler with a free port

(arm 2) terminated with ~3.5 % reflection, works as

one of the arms or reference arm of a fibre-optic

Michelson interferometer. The other arm (arm 3) is

the s-MMF, i.e. the reflective modalmetric device.

When arm 2 is terminated with refractive index

matching gel, the light arriving at the PD carries only

the amplitude modulation generated from the splice

when acoustic signals disturb the s-MMF. When the

arm 2 (ferrule) is terminated with a free surface

reflecting 3.5% of the light power, the visibility of the

interferometer increases as the optical powers of arms

2 and 3 reaching the PD are more equalized. Thus, a

possible interpretation for the higher sensitivity

obtained with the Michelson configuration is that

some additional phase modulation generated in the s-

MMF that proceeds through the SMF will be

converted into amplitude modulation when the

optical signals from arm 2 and 3 overlap in the photo-

diode. Therefore, we will have two in-phase

contributions to the output signal: 1

) Amplitude

modulated signal due to the conversion of phase

modulation acquired by modal interference (between

modes) in the s-MMF which is converted into

amplitude modulation from the SM/MM fusion

splice. 2

) Amplitude modulated signal generated

from the Michelson interferometer itself that is

formed by the SM fibers of the 2x2 fibre-coupler.

Another possible interpretation is that there may

be an oscillation of light reflected and modulated on

terminal 3 (cleaved end of the s-MMF) and also

reflected on terminals 1 and 4. In this case, when

terminal 2 starts to reflect some power of light, a

higher level of such power starts to oscillate between

arms 3 – (1 and 4), resulting in an increase in the

oscilloscope signal. This possibility is what is called

“light recycling”, described for a Michelson

interferometers in free-space aiming to detect

gravitational waves (Sato et al, 2000).

In a third experiment, the FC/PC ferrule was

placed in physical contact with a mirrored surface so

that most of the light power level was reflected from

arm 2. What was observed is that the signal

disappeared for all launched acoustic signal values up

to at least 12.5 Vpp. The circuit using a 2x2 fibre-

coupler with a free port (arm 2) terminated with a

mirror is causing virtually total reflection that

saturates the TIA when set at 40 dB. However, when

the TIA gain was reduced to 20 dB, an output signal

could be observed with 2 Vpp amplitude for 10 Vpp

excitation. Since the PDA saturates from 20 Vpp

output, an extrapolation of (3.17)

x 2 Vpp = 20 Vpp

output could be obtained by using 40 dB TIA gain. It

can be inferred here that there must be an optimal

reflection percentage to maximize the sensitivity of

the device as a whole as illustrated in Fig. 1c.

Figure 5: Sensitivity dependence of the modalmetric device

using an 2x2 fibre-coupler in B2B transmission.

3.4 2x2 Fibre-Coupler + Modalmetric

Setup in Acoustic Transmission

Through a Metallic Billet

As is shown in the Fig. 1d, the optical circuit to couple

the light is an 2x2 fibre-coupler 70/30 where the

acoustic waves propagate along the 9.5 cm length

billet thus in principle resulting in a very weak signal

to be detected. The FC/PC connector in the SMF was

“fully screwed”. The optical probe of the s-MMF

launched in the arm 3 was done at 1.8 mW power

level.

Figure 6 shows the acoustic sensitivity (mVpp)

from the device detector when the excitation voltage

amplitude (Vpp) applied on the PZT disc is varied in

the ~ 0 – 12.5 V range. What can be seen from Fig. 6

is that they are essentially attenuated results when

compared to the sensitivity plot of Fig. 5. For

example, the dry ferrule produced sensitivities of

Investigations on Sensitivity of Modal Fibre-Interferometer for Acoustic Detection

12/12.5 = 960 and 5.25/12.5 = 420 mVpp/Vpp for

12.5 Vpp excitation voltage amplitude, respectively.

The physical interpretations are the same as those

given in section 3.3 for the B2B transmission

configuration. We can compare the sensitivity of 420

mVpp/Vpp from Fig.6 referring to the 2x2 coupler,

with 820 mVpp/Vppa from Fig. 4 referring to the

circulator, both showing the sensitivity after

transmission along the billet. The result shown in Fig.

6 refers to a 3.5% reflection from the ferrule of port

2. However, Fig. 6 shows that for 12.5 Vpp excitation

voltage amplitude, an output of ~0.8 Vpp was

achieved with high reflection from the terminal

ferrule in arm 2, but with a TIA gain set to be only 10

dB. Normalizing this last result to that of Fig.5, we

will have an extrapolated sensitivity of (3.17)

x 0.8

~ 25.3 Vpp, which gives, 25.3/12.5 ~ 2023

mVpp/Vpp. Although it could be argued that the 0.8

Vpp signal contains some noise contamination, it

should be noted that the 2x2 coupler offers more

degrees of freedom to achieve increased sensitivity

than when using the circulator.

Figure 6: Sensitivity dependence of the modalmetric device

using an 2x2 fibre-coupler in transmission through a 9.5 cm

length billet.

4 CONCLUSIONS

This paper reported a modalmetric device which is of

very simple construction, high sensitivity and can

works as an opto-acoustic detector. The core of a

modalmetric-based device is a short (or long) length

of s-MMF fusion spliced or connectorized with a

SMF. The optical circuit intended to couple, partially

process and recover the light is here described to be a

circulator or a 2x2 fibre-coupler. Below it is outlined

some conclusions from the experimental results as

described in the present paper:

) By performing a lateral/angular misalignment

in the FC/PC connector between SM fibres, it is

possible to excite LP

as well higher-order modes

(Schulze et al, 2013), thus resulting in a more

sensitive interferometric device. Of course,

performing a misaligned fusion splice instead of

connectorization is more practical and mechanically

stable.

2ª) It is possible to increase the sensitivity of the

modalmetric device by making it operate at a

different wavelength, but closer to 1550 nm. In the

present paper was 1525 nm, which is the lower limit

of the tunable laser. The device sensitivities were

measured as 1340 mVpp/Vpp that is ~12 dB greater

than the sensitivities obtained with the FBG-based

detector (Leal et al, 2018).

3ª) It appears that although the coherence

(linewidth) of an external cavity laser or DFB differs

by ~2 orders of magnitude, the sensitivity

performance of the modalmetric is not significantly

changed. An in-depth analysis of this point is left for

a future research.

) An improved transmission along a 9.5 cm

length metallic billet with circulator and 2x2 coupler

was demonstrated.

5ª) The circuit with 2x2 fibre-coupler presenting a

free port (arm 2) terminated for null reflection, works

only on the basis of the modalmetric device, as in the

previous case of using the circulator.

6ª) The modalmetric circuit using a 2x2 fibre-

coupler but with a free port (arm 2) terminated with

~3.5 % reflectance presented a sensitivity higher than

when is terminated with a null reflectance. In order to

explain such behavior, two suggestions are here

outlined: A superposition of modal and Michelson

fibre interferences; and “light recycling” due

oscillations inside the arms of Michelson

interferometer (Sato et al, 2000). This is an interesting

point to be explored in the future in order to combine

multimodal interference with interference within

single-mode fibre-optic circuits.

7ª) The circuit using a 2x2 fibre-coupler with a

free port (arm 2) terminated with a mirror, causes

virtually total reflection, thus yielding an extrapolated

large sensitivity of 2023 mVpp/Vpp since the TIA

gain is set at 10 dB in transmission through a billet.

It can be inferred here that there may exist an optimal

reflection percentage to maximize the sensitivity.

This is left as a research suggestion for future works.

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