Fast and wide tuning range wavelength-swept fiber laser based on dispersion tuning and its application to dynamic FBG sensing

Fast and wide tuning rangewavelength-swept fiber laser

based on dispersion tuning andits application to dynamic FBG sensing

Yuichi Nakazaki and Shinji YamashitaDepartment of Electronic Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,

Tokyo 113-8656, Japan

[email protected], [email protected]

Abstract: We report a wavelength-swept fiber laser with high speed andwide tuning range, and its application to fiber sensors. The laser is based onthe dispersion tuning technique, which does not require any optical tunablefilter in the laser cavity. By directly modulating the semiconductor amplifierand adjusting the dispersion in the cavity, a wide wavelength tuning rangeof 178.7 nm and a fast tuning rate of over 200 kHz are obtained. Thewavelength-swept laser source is applied to a dynamic fiber Bragg gratingsensing system. Dynamic measurement of a 150 Hz sinusoidal strain isdemonstrated with a measuring speed as fast as 40 kHz.

© 2009 Optical Society of America

OCIS codes: (060.2370) Fiber optics sensors; (060.3510) Lasers, fiber; (060.3735) Fiber Bragggratings; (140.4050) Mode-locked lasers.

References and links1. H. Y. Ryu, W. – K. Lee, H. S. Moon, H. S. Suh, “Tunable erbium-doped fiber ring laser for applications of

infrared absorption spectroscopy,” Opt. Commun. 275, 379–384 (2007).2. C. Chong, A. Morosawa and T. Sakai, “High-speed wavelength-swept laser source with high-linearity sweep for

optical coherence tomography,” IEEE J. Sel. Top. Quantum Electron. 14, 235–242 (2008).3. R. Huber, M. Wojtkowski, K. Taira, J. G. Fujimoto, K. Hsu, “Amplified, frequency swept lasers for frequency

domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express 13, 3513–3528 (2005).4. R. Huber, D. C. Adler and J. G. Fujimoto, “Buffered Fourier domain mode locking: unidirectional swept laser

sources for optical coherence tomography imaging at 370,000 lines/s,” Opt. Lett. 31, 2975–2977 (2006).5. S. Yamashita and M. Asano, “Wide and fast wavelength-tunable mode-locked fiber laser based on dispersion

tuning,” Opt. Express 14, 9299–9306 (2006).6. R. Konishi and S. Yamashita, “Widely and fast wavelength-tunable mode-locked linear cavity fiber laser,” The

13th microoptics conference 2007, 2007.7. K. Hotate and Z. He, “Synthesis of optical-coherence function and its applications in distributed and multiplexed

optical sensing,” J. Lightwave Technol. 24, 2541–2557 (2006).8. A. D. Kersey, M. A. Davis, H. J. Patrick, M. LeBlanc, K. P. Koo, C. G. Askins, M. A. Putnam, and E. J. Friebele,

“Fiber Grating Sensors,” J. Lightwave Technol. 15, 1442–1463 (1997).9. K. Hotate and K. Kajiwara, “Proposal and experimental verification of Bragg wavelength distribution measure-

ment within a long-length FBG by synthesis of optical coherence function,” Opt. Express 16, 7881-7887 (2008).10. Turan Erdogan, “Fiber Grating Spectra,” J. Lightwave Technol. 15, 1277–1294 (1997).11. K. O. Hill and G. Meltz, “Fiber Bragg Grating Technology Fundamental and Overview,” J. Lightwave Technol.

15, 1263–1276 (1997).12. A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber

Fabry-Perot wavelength filter,” Opt. Lett. 18, 1370–1372 (1993).13. C. – Y. Ryu and C. – S. Hong, “Development of fiber Bragg grating sensor system using wavelength-swept fiber

laser,” I. O. P. Smart material and structure 11, 468–473 (2002).

#107030 - $15.00 USD Received 3 Feb 2009; revised 15 Mar 2009; accepted 19 Apr 2009; published 1 May 2009

(C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 8310

14. Y. Wang, Y. Cui, and B. Yun, “A Fiber Bragg Grating Sensor System for Simultaneously Static and DynamicMeasurements With a Wavelength-swept Fiber Laser,” IEEE Photon. Technol. Lett. 18, 1539–1541 (2006).

15. E. J. Jung, C. – S. Kim, M. Y. Jeong, M. K. Kim, M. Y. Jeon, W. Jung, and Z. Chen, “Characteristics of FBGsensor interrogation based on FDML wavelength swept laser,” Opt. Express 16, 16552–16560 (2008).

16. S. Li and K. T. Chan, “Electrical wavelength tunable and multiwavelength actively mode-locked fiber ring laser,”Appl. Phys. Lett. 72, 1954–1956 (1998).

17. K. Tamura and M. Nakazawa, “Dispersion-tuned harmonically mode-locked fiber ring laser for self-synchronization to an external clock,” Opt. Lett. 21, 1984–1986 (1996).

18. S. H. Yun, C. Boudoux, M. C. Pierce, J. F. de Boer, G. J. Tearney and B. E. Bouma, “Extended-cavity semicon-ductor wavelength-swept laser for biomedical imaging,” IEEE Photon. Technol. Lett. 16, 293–295 (2004).

1. Introduction

Wavelength-tunable lasers are versatile both in telecom and sensing applications. A wide tuningrange and a fast tuning (sweep) rate of the laser are usefully required in order to enhance the spa-tial resolution, the measurement range, as well as the measurement speed for efficient dynamicsensing. Many different kinds of wide and fast wavelength-swept lasers have been proposed, inwhich most of the schemes consist of a wide gain medium and a fast tunable optical filter in thelaser cavity. The wide gain medium is typically a semiconductor optical amplifier (SOA) or anerbium-doped fiber amplifier (EDFA). EDFA-based lasers can be tunable over 100nm [1], butits wavelength band is limited to 1550nm. The SOA-based lasers can work at many differentwavelength bands with wide gain bandwidth over 100nm. For the sweeping speed there are twolimiting factors which are the sweep speed of the optical filters and the photon lifetime in thelaser cavity, which is inversely proportional to the cavity length. There have been several fasttunable optical filters including piezo-transducer (PZT)-based tunable Fabry-Perot filters (FFP)[1], and polygonal mirror scanners [2]. They are basically mechanically tunable filters and themechanical sweeping speed is normally limited to a few tens of kHz. FFPs have been shownto be able to be swept much faster at a few hundreds of kHz by utilizing the resonance of thespecially-designed PZT [3]. Also, the cavity length should be kept as short as possible for fasttuning. Therefore, the SOA-based lasers are more advantageous than the EDFA-based laserssince the cavity can be shorter. As a different approach to this issue, the Fourier-domain modelocking (FDML) has been proposed recently, in which the sweeping time is set to be equal toone (or integer times of) round trip time of the cavity [4]. With the FDML scheme, a 100 nmtuning range and a 370 kHz sweep rate have been achieved, but such high-speed FFP is notreadily available.

We recently proposed a different type of fast and wide tuning range wavelength-swept fiberlaser using the dispersion tuning scheme [5, 6], which is based on the active mode locking inthe dispersive laser cavity. Since this technique does not require any optical tunable filters, thelaser can be operated at a wide tuning range and a fast tuning speed. With the dispersion tuning,105 nm tuning range and 200 kHz sweeping rate have been previously achieved [5].

Fiber-optic sensors are attractive for health monitoring in many fields such as buildings,ships and airplanes, serving as smart structures and smart materials [7]. In particular, FiberBragg Grating (FBG) sensors have been widely studied in various methods due to their uniquecharacteristics for sensor applications [8-11]. In basic FBG sensor systems, the Bragg wave-length shift of FBG is monitored with broadband sources from a light emitting diodes (LED)or an EDFA. However, these schemes usually suffer from a relatively low signal-to-noise-ratio(S/N) of sensor signals and a slow measurement speed. In order to obtain higher sensor signalpower, a wavelength-tunable laser source with fast tuning speed is always required [12-15].

In this paper, we improve the tuning characteristics of the dispersion tuning-basedwavelength-swept fiber laser at 1.5 μm bands, and apply it to a dynamic FBG sensing. Avery wide tuning range exceeding 170 nm with a fast tuning rate over 200 kHz has been real-



ized. With the swept laser, dynamic measurement of a 150 Hz sinusoidal strain is successfullydemonstrated with a measuring speed as fast as 40 kHz. The results show that the laser is suit-able for dynamic FBG sensing with multiplexed FBG array, and also suitable for other sensingapplications such as the optical coherence tomography (OCT) systems.

2. Dispersion tuning-based wavelength-swept fiber laser

2.1. Principle of dispersion tuning

The free-spectral range (FSR) of the laser cavity F can be expressed as,

F =c

nL, (1)

where L is the cavity length, n is the refractive index in the cavity, and c is the speed of light invacuum. When the cavity contains chromatic dispersion, the FSR is a function of wavelengthλ as shown in Fig. 1. Denoting the FSR at a wavelength λ as F , and ignoring higher orderdispersion, the wavelength λ and the FSR F has the following relation,

λ = − n0

cDF0(F −F0)+λ0 = − n2

0L

c2D(F −F0)+λ0 , (2)

where n is the refractive index at λ , and D is the dispersion parameter. Active mode-locking is atechnique to generate short pulse trains by applying a modulation to the laser cavity. For stableactive mode locking, the modulation frequency fm applied to the cavity has to match with aninteger (N) times of the FSR (= N ·F), where N is the order of harmonic mode locking. Thatis, when we apply a modulation at fm to the dispersive cavity, the laser is forced to operate atthe wavelength λm to meet the harmonic mode-locking condition, which can be expressed as,

λm = − n20L

c2ND( fm − fm0)+λ0 = − n0

cD fm0( fm − fm0)+λ0 , (3)

where fm0 = N ·F . Therefore the lasing wavelength can be tuned by changing the modulationfrequency. This tuning process is depicted in Fig. 1 which is also known as dispersion tuning[16].

Wavelength tuning range Δλmax is determined by the gain bandwidth of the gain medium andlasing at the adjacent harmonic mode, (N-1)-th or (N+1)-th mode. It happens when the changeof modulation frequency exceeds one FSR. From Eq. 3, Δλmax can be expressed as,

Δλmax =n0

c|D| fm0F0 =

1|D|L fm0

, (4)

which indicates smaller fm0, |D|,L is needed for wider tuning range.The pulse of the mode–locked fiber laser in dispersive cavity is expressed with a chirped

Gaussian pulse and its bandwidth δω is given by the following equation [17],

δω =(

πfm0

λ

)1/2 (8πcM|D|L

)1/4

, (5)

where M is the modulation depth. Eq. 5 indicates equation suggests that smaller fm0 and larger|D| and L are needed for narrower instantaneous bandwidth. However, from Eq. 3, smallerfm0 increases the instability of lasing wavelength and causes linewidth broadening which iscommon in the following experiments. Thus fm0 should be as high as the linewidth broadeningaccording to Eq. 5. Since the tuning rate is inversely proportional to the photon lifetime ( ∝



cavity length) of conventional tunable fiber laser, a faster tuning rate can be obtained with ashorter cavity length.

An advantage of applying this method is that wavelength tunable optical filters such as fiberFabry-Perot filters are unnecessary, unlike the conventional methods. Thus, higher tuning rateis expected in this method.

LfncFSR)(

=

fMode-locking freq.

� In large-dispersive resonator

•Lower fm •Higher fm

Cavity resonance

Fig. 1. Concept of the dispersion tuning. The lasing wavelength is tuned as the mode-locking frequency changes.

2.2. Construction and characteristics

Figure 2 shows the schematic setup of the dispersion tuning-based wavelength-swept fiber laser.The laser cavity is constructed with a ring configuration and all the devices adopted are pig-tailed with single-mode fibers. The polarization-independent semiconductor optical amplifier(SOA) module (Covega SOA1013) is used as the gain medium of laser. The SOA has a 3-dBgain bandwidth of 85.7 nm and a 10-dB gain bandwidth of 170.91 nm with an applied currentof 147 mA. Active mode-locking is achieved by directly modulating the injection current tothe SOA with the RF signal from a RF synthesizer (Agilent N5181A), which can effectivelyreduce the intracavity loss and elimination of an external intensity modulator. The triangular orramp waveform from a function generator (FG, Agilent 33250A) is used to modulate the RFsynthesizer for lasing wavelength tuning. In order to provide the chromatic dispersion in thelaser cavity needed for the wavelength tuning, we insert a 100-m-long dispersion compensatingfiber (DCF) with a dispersion parameter of -90 ps/nm/km at λ = 1550 nm. An isolator in theSOA module ensures unidirectional light propagation in the laser cavity. Finally 10 % of thelight in the laser cavity is output via a 9:1 coupler.

In our previous work, we used a polarization-dependent SOA at 1.3 μm wavelength bandregion [5]. Therefore, the laser has to be a sigma-laser configuration using polarization main-

FGIso.

P. C.

DCF:100 m

SOASOA

Coupler

Output10 %

Amp. Mode-locking signal

90%

tt

tt

Around410MHz

Bias-teeDC AC

Synthesizer

Ext-FM signal

147mA

Fig. 2. The schematic construction of the dispersion tuning-based swept laser.



taining fibers (PMF), and its output spectrum was jaggy due to the mode-coupling in the PMF.In this work, we used the polarization-independent SOA so that the laser can be a simple ringconfiguration using standard single-mode fibers. Also, we can optimize the length of the DCFin the cavity and the mode-locking frequency in order to achieve much wider tuning range.

We set the mode-locking frequency at around 410 MHz which is determined by the RFmodulation characteristics of the adopted SOA. Figure 3 shows the change of the lasing spectraand the lasing wavelength as the mode-locking frequency is adjusted manually. The resultsshow that the lasing wavelength shifts almost linearly towards the longer wavelength as themode-locking frequency increases, which is consistent with Eq. 3. The tuning sensitivity is138.1 nm / MHz and the static tuning range is 178.7 nm. The tuning sensitivity calculated byEq. 2 is 133.3 nm / MHz, which is in good agreement with the experimental results. Owingto the small loss in the laser cavity, we can achieve much wider tuning range than the 3-dBbandwidth of the SOA. The output power is about 1.3 dBm and the instantaneous linewidth isabout 1.1 nm when the lasing wavelength is 1540 nm. As discussed in the previous section,an optimum fm0 can be obtained in order to give the narrow linewidth. In our experiment, wehave confirmed that the linewidth can be narrower when fm0 increases. We have realized theinstantaneous linewidth of 0.2 nm by using larger fm0 at 1.0 GHz [5]. Therefore, a narrowerlinewidth is expected by increasing fm0 with a proper SOA.

-60

-50

-40

-30

-20

-10

0

1440 1480 1520 1560 1600 1640

Inte

nsity

[dB

m]

Wavelength [nm]

(a)

178.7 nm

1440

1480

1520

1560

1600

1640

410.4 410.7 411 411.3 411.6 411.9 412.2

Lasi

ng w

avel

engt

h [n

m]

Mode-locking frequency [MHz]

Tuning sensitivity= 138.125 nm/MHz

Raw dataFitting

(b)

Fig. 3. The static characteristics of the swept laser (a): optical spectra and (b): lasing wave-length as a function of the mode-locking frequency

The mode-locking frequency is set to be around 410 MHz in order to modulate linearlywith the triangular waveform and sweep the wavelength linearly as shown in Fig. 4. We use theexternal-FM function of the RF synthesizer to modulate the mode-locking frequency, where thetriangular signal from the function generator is used as the external-FM signal. The triangularwaveform consists of two scan areas, up-scan and down-scan. In the up-scan area, the lasingwavelength shifts toward longer wavelength. Figure 5(a) shows the peak-hold spectra obtainedby the optical spectrum analyzer (Anritsu, AQ6317). Figure 5(b) shows the temporal waveformswhen the mode-locking frequency is modulated at different tuning rates of 200 Hz, 2 kHz, 20kHz, 60 kHz, 100 kHz, and 200 kHz. A dynamic tuning range of over 120 nm is achieved at20 kHz tuning rate. It is observed that the intensity of the peak-hold spectra decreases when thetuning rate increases owing to the integral time of the peak-hold function of the optical spectrumanalyzer. When the scanning speed is below the inverse of the integral time, the peak value isrefreshed at every scan. Therefore the intensity is decreased while the optical power level iskept as shown in Fig. 5(b). On the other hand, when the scanning speed is fast the peak valuebecomes constant regardless of the scanning speed as shown in Fig. 5(a). The limitation of thetuning range at higher scan rate originates from the smaller gain at the edge of gain bandwidth



t

Up-scan Down-scan

Mod

e-lo

ckin

g fre

q.

fm

Time

Fig. 4. Triangular FM waveform to sweep the wavelength linearly

-60

-50

-40

-30

-20

-10

0

1440 1480 1520 1560 1600 1640

Inte

nsity

[dB

m]

Wavelength [nm]

Sweep rate200 Hz2 kHz

20 kHz60 kHz

100 kHz200 kHz

(a)

0.00

0.10

0.20

0.30

0.40

0.50

Volta

ge [V

]

200Hz 2kHz 20kHz 60kHz 100kHz 200kHz(b)

5ms 0.5ms 50µs 16.7µs 10µs 5µs

Fig. 5. The dynamic characteristics of the swept laser. (a): the peak-hold spectra and (b): thetemporal waveform when the mode locking frequency is modulated by triangular waveform

of the SOA. Undulations in the peak-hold spectra in the longer wavelength region are observedas the sweep speed increases due to the difference between the waveforms in the up-scan anddown-scan as shown in Fig. 5(b). The difference between the up-scan and the down-scan isbelieved to be originated from the nonlinear effect of the SOA [18]. An output power of -1.95dBm is measured when this laser is swept at a tuning rate of 200 kHz.

The swept laser is found to be applicable to the dynamic measurement in FBG sensing systemwith a large number of FBGs. The tuning rate of the laser is determined by the photon lifetimeonly since no optical tunable filter is needed in the cavity. Note that higher tuning rate is possibleby reducing the cavity length with a higher dispersive element.

3. Application to FBG sensor

3.1. Principle and experimental setup

In this session, we apply the proposed wavelength-swept laser to a FBG sensing system. Theexperimental setup of our proposed FBG sensor is shown in Fig. 6. The multiplexed FBG array(Fujikura) consists of different Bragg wavelengths, where FBG1 = 1525 nm, FBG2 = 1540 nm,FBG3 = 1550 nm, and FBG4 = 1560 nm. All FBGs are with more than 90 % reflectivity in-cluding fusion splicing loss. The reflected light from the FBG array is launched to a photodiode(New Focus 1611) via a circulator. The sensor signal is then A/D converted with the triggersignal from the function generator. In order to control the A/D-converter (NI PCI6251A) andcalculate the change of the Bragg wavelength, LabVIEW (National Instruments) is used. Asshown in Fig. 7(a), when the optical source is swept, the laser output light scans each FBG.Only the light corresponding to each FBG’s Bragg wavelength is reflected and converted toan electrical signal as shown in Fig. 7(b). In this system, we interrogate the locations of each



Swept laser

PD

Trigger

1 2

3

ReferenceStrain stage

Signal Generator

A/D converter

Labview

A/D converter

LabviewDAQ

Dispersion tuning

FBG1 FBG2 FBG3 FBG4Sync.

Fig. 6. Experimental setup of the FBG sensor

Reference

Swept laser FBG

scan

Spec

trum Δλ

λ1 λ2 λ3 λ4

Volta

ge

Δt

t1 t2 t3 t4One-to-One

Reference

(a) (b)

Fig. 7. Interrogation process of our FBG sensor (a): optical spectrum and (b): detectedtemporal waveform

pulse in the temporal waveform and calculate the relative wavelength using the reference FBG.When the reference FBG is No. 3 as shown in Fig. 7, the relative wavelength Δλ = |λ2 −λ3| isestimated from Δt = |t2 − t3| using the following equation,

Δλ = Δt ·Ss ·Sr , (6)

where Δλ is the relative wavelength from the reference FBG, Δt is the relative time from thereference FBG, Ss is the tuning rate (Hz) of the swept source, and Sr is the tuning range (nm)of the swept source. Note that the Δt(Δλ ) changes in proportion to the strain added in the FBG.

Instead of calculating Δt from the peak values of the waveform in Fig. 7(b), we adopt thedifferentiated waveform in order to reduce the fluctuations by noises and obtain accurate relativewavelengths. The waveform is initially filtered by a low pass filter (LPF, type II Chebyshevfilter) for noise reduction and differentiates against time.

3.2. Results

The temporal waveform of the reflected light from the FBG array and its differentiated wave-form after LPF at 500 kHz are shown in Fig. 8 with the laser sweeps at 40 kHz. We use thedown-scan for interrogation of the FBG array by the negative ramp waveform as shown in Fig.8(a). The cutoff frequency of 500 kHz is chosen experimentally in consideration of sensor signalspectrum. Form the figure, we observe that the multiple traces of the four temporal waveformsare broadened and overlapped at higher scan rates. It is because of the insufficient speed of theA/D-converter as well as the broadened instantaneous linewidth of the laser at high scan rate,which are under investigation and will be reported. Table 1 summarized the static propertiesof the system as well as averages and standard deviations of the relative wavelength from thereference FBG3 measured for 5 seconds.

Figure 9 shows the change of relative wavelength when the FBG2 is stretched manually. Itis observed that the relative wavelength changes linearly as the strain is added, which suggeststhat this sensor system works correctly. The slope of the linear-fitting is 0.99 pm / (μ strain).



Table 1. Static properties of the system

Results \ FBG No. 1 2 3 4

Average 23.91nm 9.20nmReference

9.40nm

Standard deviation 35.37pm 28.10pm 28.93pm

-20-15-10

-5 0 5

10 15 20 25 30 35

0 5 10 15 20 25

Volta

ge [m

V]

Time [μ s]

t

Down-scan

Mod

e-lo

ckin

g fre

q.

fm

Time

-30k

-20k

-10k

0

10k

20k

15 16 17 18 19 20 21 22 23 24 25

Diff

eren

tial i

nten

sity

[a. u

. ]

Time [μ s]

Filtering

Differential

(b)(a)

Fig. 8. (a) temporal waveform of the reflected light (b) differentiated waveform after filter-ing

0 1000 2000 3000

0

1

2

3

4

μ strain

Cha

nge

in

rela

tive

wav

elen

gth

[nm

]

dataFitting

0.99 pm/ strainμ

Fig. 9. Change of relative wavelength when the FBG2 is stretched manually

Figure 10 shows the results of the dynamic sensing when a periodical strain is added to FBG2. The dynamic strain is applied with a PZT stage (PI) driven by a sinusoidal waveform froma RF function generator. When the frequency of the strain changes abruptly from 1 Hz to 10Hz, it is confirmed that this sensor can capture the change of the frequency as shown in Fig.10(a) (Media 1). Figure 10(b) shows that the dynamic strain can be measured correctly when thestrain is added with a 150 Hz sinusoidal vibration. The resolution of this sensor is limited by theFBGs used since our swept laser source has wider instantaneous linewidth than the FBGs. Theresults indicate that this system is capable to work accurately and measure transient distortionat a high measurement rate.



10 Hz10 Hz1 Hz1 Hz(a)

−150−100 −50 0 50 100 1500

1

2

3

4 x 10−7A

mpl

itude

[a.u

.]

Frequency [Hz]

0 2 4 6 8 108

8.2

8.4

8.6

8.8

9

9.2x 10−9

Sen

sor s

igna

l [a.

u.]

Time [s]

(b)150Hz

FFT

Fig. 10. Dynamic response of the system (a): Abrupt change of applied sinusoidal strainfrom 1 Hz to 10 Hz (Media 1) (b): The FFT spectrum when the 150 Hz sinusoidal strain isadded.

4. Conclusion

A fast and wide tuning range wavelength-swept fiber laser has been demonstrated. Withdispersion-tuning in the laser cavity, a wide wavelength tuning range of 178.7 nm and a fasttuning rate of 200 kHz are obtained. The laser is applied to a dynamic FBG sensing system,and a dynamic measurement of a 150 Hz sinusoidal strain has been successfully achieved witha fast measuring speed of 40 kHz. It is expected that the tuning range and speed of the lasercan be improved by shortening the laser cavity, adopting a higher dispersive medium in thecavity, as well as adopting a higher performance SOA. Also, the measurement speed of thesensing system can be further increased by using higher-speed A/D-converter. The results showthat such wavelength-swept fiber laser is promising for efficient dynamic FBG sensing systemswith a high accuracy and fast measuring speed.

Acknowledgements

The authors wish to thank Dr. Koji Omichi of Fujikura for providing part of the FBGs used inthe experiments.



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Fast and wide tuning range wavelength-swept fiber laser based on dispersion tuning and its application to dynamic FBG sensing