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High-accuracy fiber optical microphone in a
DBR fiber laser based on a nanothick silver
diaphragm by self-mixing technique
Zhengting Du,1 Liang Lu,1,* Wenhua Zhang,1 Bo Yang,1 Shuang Wu,1 Yunhe Zhao,1
Feng Xu,1 Zhiping Wang,1 Huaqiao Gui,2 Jianguo Liu,2 and Benli Yu1
1
2
Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education, Anhui
University, Jiulong Road 111#, Hefei 230601, China
Key Laboratory of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese
Academy of Sciences, Hefei 230031, China
*
[email protected]
Abstract: A high-accuracy fiber optical microphone (FOM) is first applied
by self-mixing technique in a DBR fiber laser based on a nanothick silver
diaphragm. The nanothick silver diaphragm fabricated by the convenient
and low cost electroless plating method is functioned as sensing diaphragm
due to critically susceptible to the air vibration. Simultaneously, microvibration theory model of self-mixing interference fiber optical microphone
is deduced based on quasi-analytical method. The dynamic property to
frequencies and amplitudes are experimentally carried out to characterize
the fabricated FOM and also the reproduced sound of news and music can
clearly meet the ear of the people which shows the technique proposed in
this paper guarantee steady, high signal-noise ratio operation and
outstanding accuracy in the DBR fiber laser which is potential to medical
and security applications such as real-time voice reproduction for throat and
voiceprint verification.
©2013 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (280.3420) Laser sensors.
References and links
1.
L. Mohanty, L. M. Koh, and S. C. Tjin, “Fiber Bragg grating microphone system,” Appl. Phys. Lett. 89, 161109
(2006).
2. J. M. S. Sakamoto and G. M. Pacheco, “Theory and experiment for single lens fiber optical microphone,” Phys.
Proc. 3, 651–658, (2010).
3. H. J. Konle, C. O. Paschereit, and I. R. Rohle, “A fiber-optical microphone based on a Fabry-Perot
interferometer applied for thermo-acoustic measurements,” Meas. Sci. Technol. 21, 015302 (2010).
4. J. H. Churnside, “Laser Doppler velocimetry by modulating a CO2 laser with backscattered light,” Appl. Opt.
23(1), 61–66 (1984).
5. L. Lu, J. Yang, L. Zhai, R. Wang, Z. Cao, and B. Yu, “A self-mixing interference measurement system of a fiber
ring laser with narrow linewidth,” Opt. Express 20(8), 8598–8607 (2012).
6. W. M. Wang, W. J. Boyle, K. T. Grattan, and A. W. Palmer, “Self-mixing interference in a diode laser:
experimental observations and theoretical analysis,” Appl. Opt. 32(9), 1551–1558 (1993).
7. Z. Du, L. Lu, W. Zhang, B. Yang, H. Gui, and B. Yu, “Measurement of the velocity inside an all-fiber DBR laser
by self-mixing technique,” Appl. Phys. B 113, 153–158 (2013).
8. L. Lu, L. Zhai, K. Z. Du, and B. Yu, “Study on self-mixing interference using Er3+–Yb3+ codoped distributed
Bragg reflector fiber laser with different pump power current,” Opt. Commun. 284(24), 5781–5785 (2011).
9. F. Xu, D. Ren, X. Shi, C. Li, W. Lu, L. Lu, L. Lu, and B. Yu, “High-sensitivity Fabry-Perot interferometric
pressure sensor based on a nanothick silver diaphragm,” Opt. Lett. 37(2), 133–135 (2012).
10. E. Yahel and A. A. Hardy, “Modeling and optimization of short Er3+–Yb3+ codoped fiber lasers,” IEEE J.
Quantum Electron. 39(11), 1444–1451 (2003).
11. I. Kelson and A. Hardy, “Optimization of strongly pumped fiber lasers,” J. Lightwave Technol. 17(5), 891–897
(1999).
12. M. Karasek, “Optimum design of Er3+-Yb3+ codoped fibers for large-signal high-pump-power applications,”
IEEE J. Quantum Electron. 33(10), 1699–1705 (1997).
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Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30580
13. L. Lu, Z. Cao, J. Dai, F. Xu, and B. Yu, “Self-mixing signal in Er3+–Yb3+ codoped distributed Bragg reflector
fiber laser for remote sensing applications up to 20Km,” IEEE Photonics Technol. Lett. 24(5), 392–394 (2012).
14. J. A. Bucaro, N. Lagakos, B. H. Houston, J. Jarzynski, and M. Zalalutdinov, “Miniature, high performance, lowcost fiber optic microphone,” J. Acoust. Soc. Am. 118(3), 1406–1413 (2005).
1. Introduction
Interest on fiber optical microphone (FOM) [1, 2] is related to the advantages that an optical
sensor has over conventional sensors, such as intrinsic safety, electromagnetic interference
immunity, capability for remote control and low transmission loss. Furthermore, the
microphone itself does not emit any electromagnetic radiation and thus it is explosion proof.
Additionally long distances between electronic devices and optical head are possible due to
low losses in transmission by using glass fiber optics. These characteristics facilitate new
applications in measurement, surveillance and medicine, e.g. for EMI labs, RFI testing labs,
and MRI application.
In the last years several transduction mechanisms [3, 4] for optical microphone technology
have been developed. Mostly traditional heterodyne or homodyne technique has been applied
to FOM by the way of mixing the scattered light with the light of a constant frequency close
to or equal to the original laser frequency. Compared to the heterodyne or homodyne
interference measuring technology, self-mixing interference (SMI) [5, 6] technique has a lot
of advantages, such as sensitivity and compactness, high accuracy and reliability, self-aligned
and simple to implement. Among the many applications of SMI, ultrahigh sensitivity response
to external optical feedback light in the short cavity configuration of DBR fiber laser has led
to self-aligned optical sensing applications, such as laser Doppler velocimetry [7], vibrometry
[8]. The key idea is the efficient intensity modulation of DBR fiber lasers through the
interference between the pre-existing lasing field and the coherent component of the backreflected light scattered from the measured object. The enhanced sensitivity of self-mixing
approach results from the low ratio of fluorescence to photon lifetime especially in DBR fiber
lasers. As the gain medium in the cavity plays the key important role to amplify the
demodulated SMI signal greatly, the signal-to-noise ratio (SNR) is limited only by the laser
quantum noise of spontaneous emission in the gain medium.
Most recently, we have demonstrated nanothick silver diaphragm fabricated by the
electroless plating method which is more convenient and less costly compared with the microelectromechanical fabrication process. The measured thickness of diaphragm is uniform in a
few hundreds of nanometers which is the thinnest diaphragm that currently exists in the
literature. The sketch of extrinsic Fabry–Perot interferometric sensor constructed by our team
[9] demonstrates a higher pressure sensitivity of 70.5 nm/kPa compared to silica diaphragm
sensors (typically 11 nm/kPa) due to extremely high sensitivity critically susceptible to the air
pressure resulting from the thin size and the small residual stress of the sensing diaphragm.
In this paper, we employ the nanothick diaphragm as sensing diaphragm of fiber optical
microphone by the self-mixing technique in short cavity DBR fiber laser to obtain a steady
and outstanding accuracy real-time voice reproduction. Section2 describes some theoretical
works primarily on the micro-vibration of self-mixing interference. In Section 3, the
experimental setup and results are presented in detail that dynamic property to frequencies
and amplitudes are carried out to characterize the fabricated FOM and also the reproduced
sound of news and music can clearly meet the ear of the people which is potential to medical
and security applications. Finally, the conclusions are drawn in Section 4.
2. Theory model of micro-vibration in self-mixing interference
In this section, we propose and demonstrate a basic theoretical SMI model of micro-vibration
in the DBR fiber laser based on amplifier equations for EYDF [10, 11] and the quasianalytical method results of steady-state equations in the DBR fiber laser [12] firstly. On
account of the boundary conditional equations, we introduce the feedback light from the silver
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30581
diaphragm in the form of seed light, so the output power of micro-vibration SMI can be
deduced.
We build the model of DBR fiber laser with optical feedback presented in Fig. 1 to get the
results of SMI output power based on quasi-analytical solutions to the rate equations. The
pump light with wavelength 980 nm is coupled into the DBR fiber laser cavity by a
wavelength-division multiplexer (WDM). The Fabry–Pérot cavity of the DBR fiber laser
incorporates with a pair of wavelength matched fiber gratings in which Erbium: Ytterbiuum
(Er:Yb) codoped fiber is efficient gain medium. Ytterbium sensitization to Er ions allows
short laser cavity construction critical for output power stability of fiber laser and high
sensitivity to the sensing application due to efficient pump absorption, high peak absorption
out
scattered by the silver
cross-section, and efficient energy transfer. Here the lasing light PLaser
diaphragm reenters into the lasing cavity in the form of Pseed .
PLaser
Loss = 1 − ε1
PLout
PLin
PRin
PRout
Pseed
Loss = 1 − ε 2
R (λs ) = R1
R (λs ) = R2
R (λ p ) = 0
R (λ p ) = 0
out
PLaser
R3
Fig. 1. The principle of SMI output power based on the Boundary conditional equations.
As shown in Fig. 1, P in and P out denote the powers at the input and output of the EYDF.
The subscripts p, s and L, R refer to the pump, lasing light and left, right respectively. Pseed
represents feedback light from external object. 1-ε1 and 1-ε 2 are the total attenuation factor
considering the insertion loss of WDM, connection of fibers and coupling efficiencies. Ri and
ri are corresponding to the reflectivity and reflection coefficient of related apparatus.
Lasing light is scattered from the silver diaphragm influenced by the weak change of
sound pressure and coupled into the laser cavity, then the SMI phenomena occurs. According
to the amplifier equations for EYDF and boundary conditions, we build the following
equations that the expression of output power can be theoretically deduced.
PLout = PLin e
-α s L + Psabs /Pss + Ppabs /Pss
(1)
PRout = PRin e
-α s L + Psabs
(2)
P = ε 2 ε 2 R P
in
L
out
2 R
/Pss + Ppabs
/Pss
+ (1 − R2 ) Pseed 
(3)
PRin = ε12 R1 PLout
(4)
Where P, α are presented the power and small signal absorption coefficient respectively.
The index of abs and ss are powers of the absorbed in one round trip and saturation
respectively. L is the length of the doped fiber.
The transcendental equation for the power PRout in the DBR system is deduced through
iteration from Eq. (1-4) where the scatted light is introduced as the seed light.
PRout = ε12 R1ε 2 [ε 2 R2 PRout + (1 − R2 ) Pseed ]e
-2α s L + 2 Psabs /Pss + 2 Ppabs /Pss
(5)
hν s aeff
, Pseed = (1-R 2 )(r2* ) 2 PRout , r2∗ =r2 +(1-r22 )r3cos(2πLext / λ ) is the
Γ sτ Er (σ seEY + σ saEY )
effective reflectivity at the end of FBG2. νs is the frequency of the signal light, гs is the optical
mode-erbium overlap factor, τEr is the upper level lifetime of erbium ions, σseEY and σsaEY are
the equivalent cross section of signal light stimulated absorption and stimulated emission of
EYDF, aeff is the effective core area of signal light. Equation (5) is a transcendental equation
of PRout, which can be solved by numerical simulation.
With Pss =
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30582
In case of the silver diaphragm is influenced by a sinusoidal change of sound pressure
with the angular frequency of ω0 and amplitude of A, the vibration of the diaphragm is
changed in keeping with the sinusoidal change of sound pressure.
Lext (t ) = L0 +A cos (ω0t )
(6)
The Eq. (5) is solved numerically for PRout and finally the laser output PLaser is given by
-(-∂ L + P abs /P + P abs /P )
PLaser = ε1 (1-R 1 )PRout e s s ss p ss
(7)
The parameters used in our calculations related to the Eqs. (1)-(7) unless stated otherwise,
are given in Table 1. According to the above calculation, we simulate the modulated signal of
micro-vibrations in self-mixing interference system in the follow. The waveform of output
power is shown in Fig. 2.
0.20
Amplitude
0.15
Amplitude
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
0
500
1000
1500
2000
time (us)
output power (mw)
5.10
2500
3000
2500
3000
output power
5.05
5.00
4.95
4.90
0
500
1000
1500
2000
time (us)
Fig. 2. The typical simulated result by quasi-analytical method (Upper trace: vibration signal of
external target, Lower trace: self-mixing interference signal).
Table 1. The parameters used for the DBR fiber laser
Parameter
λ: the oscillation wavelength of signal
σsaEY: the absorption cross-section
σseEY: the emission cross-section
αs: the scattering l osses for the signal
Гs: the power filling factor which represent the fraction of
the signal amplified in the core
aeff: the effective core area of signal light
L: the length of EYDF
τEr: spontaneous lifetime
ε1: attenuation factor
ε2: attenuation factor
r1: the reflectivity at the end of FBG1
r2: the reflectivity at the end of FBG2
r3: is the reflectivity of the external target face
f0: frequency of driving signal launched on the external
target
A: amplitude of driving signal
Value
1550 nm
3 × 10−25 m2
1 × 10−25 m2
0.75 × 10−3 m−1
0.82
1.5 × 10−11 m2
0.1m
10.8 × 10−3 s
0.9
0.9
0.87
0.9
0.001
420 Hz
1.55 × 10−10 m
Figure 2 shows the simulated output power of the nanothick silver diaphragm microvibration in DBR fiber laser based on the results of quasi-analytical solutions to the rate
equations. In the simulation process, we present a procedure to solve numerically
transcendental equation problem with standard root-finding technique, and obtain the emitted
power of the laser in the self-mixing system. For a small value of A = 1.55 × 10−10 m less than
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30583
1/8 wavelength, the simulated output power (lower trace) of micro-vibration approximates to
the driven signal (upper trace) when a sinusoidal waveform is launched on the external
reflector. As the sound signals can be decomposed into the combinations of the sinusoidal
signals, thus the sound signals can be real-time reproduced on the occasion of microvibration.
3. Experimental results and discussions
Based on the theoretical analysis and numerical simulation, we built a set of the self-mixing
sound reproduction system based on a DBR fiber laser with short lasing cavity which is the
key important to the high sensitivity and remote sensing [13]. The scattered light causing selfmixing interference is from the optical head that consists a reflector functioned as an external
cavity. The optical head includes an APC patchcord and a ceramic pipe butt-coupled with
silver diaphragm. And we experimentally get the real-time sound reproduction from microvibration vibrations of the nanothick silver diaphragm based on self-mixing interference in the
DBR fiber laser. The experimental setup of microphone measurement is shown in Fig. 3.
Fig. 3. Experimental setup of fiber optical microphone based on self-mixing in DBR fiber
laser.
As shown in Fig. 3, a Wavelength Division Multiplex (WDM) coupler is employed to
couple the pump power into the gain medium with 3.9cm length. A pair of fiber Bragg
gratings with Bragg wavelength of 1549.77nm and 1549.66nm which are supplied
commercially functioned as short laser resonator and mode selecting apparatus. The short
cavity of the laser with 9.8cm length is preferable to eliminate the multi-longitudinal modes
oscillation and suppress the mode hopping. That is because based on the longitudinal mode
c
spacing expression ( Δvq =
), we can get the value of longitudinal mode spacing
2nl
( Δvq = 1.05 × 109 Hz), where the speed of light in vacuum ( c ) is 3*108 m/s and the refractive
index of optical fiber ( n ) is 1.45. However, the 3dB linewidth is 0.0161nm measured by
envelope analysis patterns with 0.020nm resolution in the spectrum analyzer shown in Fig. 4.
c
Moreover, the lasing modes’ range ( Δν = 2 Δλ ) is 2.01 × 109 Hz based on the
λ
c
equation Δν = 2 Δλ .
λ
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30584
Fig. 4. The linewidth of DBR fiber laser measured by optical analyzer.
To further discuss the effect of the short cavity in the DBR fiber laser, the spectrum shown
in Fig. 5 is recorded every 5 minutes. The signal-to-noise ratio (SNR) is up to 60dB and the
output power difference of oscillate modes is 0.019dBm which illustrates the stable operating
fiber laser of DBR.
0
( 1549.59nm, -8.231dBm)
( 1549.59nm, -8.245dBm)
( 1549.59nm, -8.237dBm)
( 1549.59nm, -8.226dBm)
( 1549.59nm, -8.24dBm)
( 1549.59nm, -8.232dBm)
(1549.59nm, -8.229dBm)
-10
-30
-40
-50
-60
Power (dBm)
-20
-70
-80
Ti
me
(m
inu
te)
5
10
15
20
25
30
35
1546
1548
1550
1552
1554
Wavelength (nm)
Fig. 5. The power spectrum of DBR fiber laser at different times.
On account for the FBGs applied in our setup, the bandwidth and grating diffraction
efficiency reflectance of FBG1 and FBG2 are 0.285nm and 0.213nm, 87% and 90%
respectively. The stimulated gain medium amplifies the signals of λ and the lasing light is
projected to the silver diaphragm which the thickness is 130 nm and the diameter is 1.25 mm,
through an optical APC patchcord which is spliced to the other port of the WDM coupler.
Between the DBR fiber laser and the APC pathcord, a segment of single mode fiber (SMF) is
inserted to segregate the optical sensing part from the electronic devices that is significant to
intrinsic safety, electromagnetic interference immunity, capability for remote control, low
transmission loss and explosion proof. The nano-sickness silver diaphragm in optical head is
critically susceptible to the air vibration caused by the acoustic wave and the reflectivity of
the diaphragm is at the scope from 10% to 90% within the near and intermediate infrared
range of lasing light by changing the reaction time. The thickness and reflectivity of the
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30585
diaphragm at a reaction time of 15 min at room temperature are typically 130 nm and 15%
reflectivity [9]. So, the diaphragm is a good candidate for fiber optical microphone. The
technique of the silver diaphragm is processed by the chemical coating method which is
convenient, low cost and easy to industrialized manufacturing. The other port of lasing light is
fed through the photo diode (PD) to convert the acoustic SMI signal into the photocurrent. For
the sake of the processing circuit, the SMI photocurrent is demodulated to the sound voltage
which can meet the ear by the louder speaker. To quantify the performance of the fiber optical
microphone in this paper, the results of self-mixing sound reproduction system based on a
DBR fiber laser are given in the following part.
SMF
Optical head
Sp
Anechoic Test Box
Type4232
Pump
DBR fiber
laser
f.
Re
er
eak
c.
mi
ou
tp
Signal
Processing
PD
ut
Power Amplifier
Type 2716C
input1
inp
ut
louder
speaker
input2
output
Panel
Type 3160-A-022
PC
Fig. 6. Schematic of the test setup for characterization of the fiber optical microphone by selfmixing technique.
Proof-of-principle experiments are carried out to characterize the fabricated FOM. To
identify the property of the FOM carried out in the paper, the Brüel & Kjær anechoic test
system is exploited as shown in Fig. 6. The application program of Pulse is installed in PC
which connects the “Panel of 2ch Input 2ch output Generator Module” (Type 3160-A-022,
Module Panel) by a cable with the green line. The Module Panel output signal regulated by
the generator in the Pulse program feeds through the “Power amplifier” (Type 2716C) to
amplify the signal and drive the speaker in the “Anechoic Test Box” (Type 4232) by the
Bayonet Nut Connector (BNC) in blue. In the Test Box, the reference microphone (RM) and
the optical head composed by an APC patchcord and a ceramic pipe butt-coupled with silver
diaphragm are laydown, so we can confirm the ultra merits of FOM utilizing the self-mixing
interference (SMI-FOM) in a DBR fiber laser by the comparison to the RM which is a
commercially manufactured with 20uPa / Hz minimum detectable acoustic pressure. The
RM semaphores and the signals of SMI-FOM are jointed to the Panel Input1 and Input2
respectively. Therefore the effects of SMI-FOM can be gained by the Pulse system.
To measure the response of the proposed SMI-FOM, a sinusoidal signal launched to the
speaker in the “Anechoic Test Box” is applied to mobilize the silver diaphragm. In our
experimental setup of fiber optical microphone based on self-mixing, a fast InGaAs
photodiode connects to the output port of the WDM to observe the experimental signal of
SMI-FOM. A typical SMI-FOM signal is obtained in Fig. 7 at the condition of the driven
signal with single frequency of 820 Hz and driving voltage of 1mVrms (mill voltage rootmean-square) with the 0dB magnification of “Power amplifier”. Left curve from the reference
microphone links to the input1 of the Module Panel and right curve from the fiber optical
microphone refers to the input2. The peaks at frequency of 820Hz illustrate the measured
values of reference microphone and SMI-FOM and the results declare the correspondent SNR
with each other. At the same time, the peaks with frequency less than 200 Hz associate with
environmental perturbation and noise of electric supply (fundamental frequency and its
sidebands). But other than that, the noises of RM is generated by the Brüel & Kjær anechoic
test system itself at low frequency, especially lower than 200 Hz. Compared with the
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30586
reference measurement system, the self-mixing signal of fiber optical microphone with silver
diaphragm as the optical head shown in Fig. 7 is accurate, stable with low noise. The results
declare that the SMI-FOM system could provide a fine response to the single frequency at 820
Hz and the silver diaphragm is sensitive to micro-vibration referred to the sound source.
Fig. 7. Measured signal at 820 Hz with 1mVrms. Left curve from the reference microphone
links to the input1 of the Module Panel and right curve from the fiber optical microphone
refers to the input2.
SNR(dB)
To seek for dynamic property of the FOM to frequencies, the range from 120Hz to1020
Hz is mounted on the speaker with 1mVrms amplitude and 0dB gain. The output signals SNR
of RM and SMI-FOM are recorded by the signal analyzer in the Pulse program through the
“2ch Input Module”. Along with the dynamic frequencies of sinusoidal excitation, the SNR is
separately labeled with red solid dot and black hollow circles in polygonal lines. It can be
observed that the SMI-FOM is in the same order of RM on SNR aspect shown in Fig. 8. To
learn more property about SMI-FOM, we obtain the minimum detectable acoustic pressure
with 20uPa / Hz based on the same SNR value with the RM. Thus the minimum detectable
acoustic pressure by self-mixing technique is typical less than the existing fiber optic
microphone [14] with 180uPa / Hz . The difference of SNR is derived from the different
measurement points and the amplitude-frequency response of their circuits. Additionally, the
signal processing circuit is the simple O/E conversion electric circuit with eliminator which
needs to lucubrate.
FOM
RM
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
0
200
400
600
800
1000
1200
frequency (Hz)
Fig. 8. Measured SNR-frequency response of SMI-FOM and RM.
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30587
As a further proof to discuss the effects of the SMI-FOM, the SNR of the fiber optical
microphone and the reference microphone are investigated by different amplitude of the
speaker in the Test Box. The signal generated by the Pulse program is at the frequency of
1020 Hz with an initial voltage of 1mVrms. The different magnifications of the “Power
Amplifier” are load on the speaker with 5 points of the arithmetic sequence at the adjustable
extent, so the SNR to amplitude response of the SMI-FOM is shown in Fig. 9. The hollow
points in black and the solid red dots refer to the measured SMI-FOM and RM respectively.
The red dots are approximately linear dependence on the amplification factor due to the
linearly-reinforced sound pressure, while the SNR of SMI-FOM is not a linear process
resulted from the stimulation of multiple frequencies. The phenomenon is on account of
amplitude-frequency response by the self-mixing interference. In terms of the amplitude of
the vibration larger than 1/8 wavelength, the SMI signal is not a monofrequency signal but
with multiple frequencies. Thus the signals of SMI-FOM can be reproduced with a high
signal-to-noise ratio at different vibration amplitudes of silver diaphragm when a tracking
filter is carried on the signal processing circuit.
FOM
RM
50
SNR(dB)
40
30
20
10
0
0dB
6dB
12dB
18dB
24dB
Gain Amplitude (dB)
Fig. 9. Measured SNR-amplitude response of SMI-FOM and RM at different gain amplitude.
In the sound reproduction experiment, the signal launched on the speaker is replaced by a
sound fragment through editing the generator in Pulse program. The experimental setup is the
same with the measured system as shown in Fig. 6 and the real-time sound reproduction can
be obtained by the loud speaker followed with the signal processing circuit. To fully
comprehend the dynamic sound reproduction of the fiber optical microphone in a DBR fiber
laser at a period of time, we would like to give three examples of the power spectrums shown
in Fig. 10. The upper figures are the power spectrums of SMI-FOM measured by Brüel &
Kjær measurement system at different times, the lower diagrams trace the performance of RM
at the same measured time of the SMI-FOM. By the frequency components comparison of the
lower diagraphs to the upper ones, we achieve the closest propinquity power spectrums
between the SMI-FOM and RM except for the frequencies around 100 Hz originated from
Brüel & Kjær anechoic test system itself noise. The tiny difference of amplitude observed in
the diagrams is resulted from not only the amplitude-frequency response of the circuits
applied to the dual microphones, the frequency distinguishability generated by the sampling
rate of the “2ch Input Module” but also the asynchronized suspension time of the dual Input
channel in the Module. On the other hand, the original music produced directly from the
speaker in the Test Box can be regressed by the loudspeaker and the reversion of the
loudspeaker can clearly meet the ear of the people. In our experiments, the news broadcast
and music are regenerated by the SMI-FOM technique. By the multi-dimensional comparison
between the performance of SMI-FOM and the RM in Brüel & Kjær system, the technique
proposed in this paper guarantees steady, high signal-noise ratio operation and outstanding
accuracy sound reproduction in the DBR fiber laser .
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30588
Fig. 10. the power spectrums of SMI-FOM and RM at different times.
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30589
In this paper we achieve the micro-vibration of the silver diaphragm caused by the sound
source with the technique of the self-mixing interference. The self-mixing interference occurs
due to the scattered light reentering into the laser. In the condition, the SMI-FOM can be
applied to the remote spatial sound transducer which can be used in spy system to guard the
public security. As the stuff of daily essentials could function as the scattered light such as
boxes of paper, mental, slice of paper influenced by the acoustic waves, the self-mixing model
of FOM in DBR fiber laser with outstanding accuracy we have built has a number of potential
applications for high performance in remote spatial sound sensor.
4. Conclusion
We demonstrate a theory analysis and experimental results of self-mixing sound reproduction
system based on a DBR fiber laser with short lasing cavity which is the key important to the
high sensitivity and remote sensing which has never been studied. To investigate the optical
head of the SMI-FOM, the silver diaphragm is applied to scatter the light into the laser cavity
fabricated by the electroless plating method which is convenient and low cost. The nanosickness silver diaphragm is critically susceptible to the air vibration caused by the acoustic
wave. Different movements of the silver diaphragm can be detected from the self-mixing
interference system based on DBR fiber laser and the experimental results of the SMI-FOM
guarantee the outstanding accuracy and steady operation coinciding with the RM results. On
the other hand, the reproduced sound of news and music can clearly meet the ear of the
people. The presented technique may find medical and security applications such as real-time
voice reproduction for throat and voiceprint verification.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No.
61307098, 61275165), the Natural Science Fund of Anhui Province (Grant No.
1208085QF110) and the open foundation of Key Laboratory of Environmental Optics and
Technology of Chinese Academy of Sciences (Grant No. 2005DP173065-2013-2).
#197266 - $15.00 USD
Received 6 Sep 2013; revised 7 Nov 2013; accepted 22 Nov 2013; published 5 Dec 2013
(C) 2013 OSA
16 December 2013 | Vol. 21, No. 25 | DOI:10.1364/OE.21.030580 | OPTICS EXPRESS 30590