Loss-Modulation-Based Wavelength-Range Shifting of Tunable EDF Ring Laser with Cascaded-Chirped Long-Period Fiber Grating for Temperature Measurement †

sensors

Communication Loss-Modulation-Based Wavelength-Range Shifting of Tunable EDF Ring Laser with Cascaded-Chirped Long-Period Fiber Grating for Temperature Measurement †

Koken Fukushima 1,* , Manuel Guterres Soares 1, Atsushi Wada 1 , Satoshi Tanaka 1,* and Fumihiko Ito 2

1 Department of Communications Engineering, National Defense Academy, 1-10-20 Hashirimizu, Kanagawa, Yokosuka 239-8686, Japan; [email protected] (M.G.S.); [email protected] (A.W.) 2 Graduate School of Natural Science and Technology, Shimane University, 1060 Nishikawatsu-cho, Shimane, Matsue 690-8504, Japan; [email protected] * Correspondence: [email protected] (K.F.); [email protected] (S.T.); Tel.: +81-46-841-3810 (ext. 3368) (S.T.) † This paper is an extended version of our paper published in: Fukushima, K.; Guterres Soares, M.; Nakaya, K.; Wada, A.; Tanaka, S.; Ito, F. Wavelength Tunable and Switchable EDF Ring Laser using Cascaded-Chirped Long Period Fiber Grating. In Proceedings of the 25th Opto-Electronics and Communications Conference (OECC2020), Taipei, Taiwan, 4–8 October 2020.

Abstract: A novel tunable Erbium-doped fiber ring laser (EDFRL) with a cascaded-chirped long- period fiber grating (C-CLPG) as a wavelength selection filter is proposed from the viewpoint of the sensor use, in which a variable optical attenuator (VOA) is employed as an intracavity loss modulator to change the oscillation wavelength region so that the resultant tuning wavelength range is widened. In the demonstrative experiment for temperature measurements, oscillation over the wavelength range of 12.85 nm (1557.62~1570.47 nm), which is more than three times range of the previously Citation: Fukushima, K.; Guterres ◦ Soares, M.; Wada, A.; Tanaka, S.; Ito, F. presented laser and is equivalent to 64 C in terms of temperature change, was achieved, while Loss-Modulation-Based a single-wavelength oscillation was maintained. In addition, a practical technique for realizing a Wavelength-Range Shifting of temperature measurement by combining with the VOA control is also discussed. Tunable EDF Ring Laser with Cascaded-Chirped Long-Period Fiber Keywords: tunable lasers; optical fiber sensors; long-period fiber gratings (LPG); cascaded-chirped Grating for Temperature long-period fiber gratings (C-CLPG); Erbium-doped fiber ring lasers (EDFRL); variable optical Measurement . Sensors 2021, 21, 2342. attenuator (VOA); intracavity loss modulation; temperature sensing https://doi.org/10.3390/s21072342

Academic Editor: Kimio Oguchi 1. Introduction Received: 26 February 2021 Among the optical fiber sensors, fiber grating sensors have attracted much attention Accepted: 23 March 2021 Published: 27 March 2021 because they have a lot of advantages for practical sensor use: capability of localized and/or multiplexed sensing, high consistency with transmission fibers, ability to be em-

Publisher’s Note: MDPI stays neutral bedded in structures, etc. In general, fiber gratings are fabricated by inducing a periodic with regard to jurisdictional claims in refractive index modulation along the optical fiber, and are classified into two types based published maps and institutional affil- on their grating period: fiber Bragg grating (FBG) and long-period fiber grating (LPG). iations. The FBG, which is inscribed typically with a submicron grating period, couples a forward- propagating core mode with a backward-propagating one at the Bragg wavelength, and serves as a narrow-band reflection filter [1]. In contrast, the LPG, which is inscribed with a grating period ranging from tens to hundreds of µm, couples a forward-propagating core mode with several codirectional cladding modes at their resonant wavelengths and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. provides a relatively wide and nonreflective band-rejection filter [2]. Especially for sensor This article is an open access article applications, LPGs have advantages such as easier and more flexible in fabrication and distributed under the terms and design than FBGs due to their large grating period. In addition, LPG sensors utilize cou- conditions of the Creative Commons pling characteristics between the core mode and the cladding modes, which makes them Attribution (CC BY) license (https:// sensitive to strain, pressure, temperature, and the environmental refractive index [3]. creativecommons.org/licenses/by/ In recent years, we have proposed several fiber-optic sensors in which a pair of LPGs 4.0/). or chirped LPGs (CLPGs) provides an in-fiber Mach–Zehnder interferometer, called a cas-

Sensors 2021, 21, 2342. https://doi.org/10.3390/s21072342 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 2342 2 of 9

caded LPG (C-LPG) or a cascaded CLPG (C-CLPG), as the sensor element. In such sensors, periodic interference fringes (namely channeled spectra) appear around the attenuation dips of LPGs or CLPGs in a wavelength domain, and to achieve a highly sensitive and accurate sensing, an interference-fringe measurement method based on a Fourier transform technique focusing on the periodic channeled spectrum has been adopted. However, this method requires time-consuming data processing, which limits the measurand to static or quasistatic quantities [4,5]. To solve the above problems, we constructed an Erbium-doped fiber ring laser (EDFRL) with a C-CLPG as a wavelength selection filter and an Erbium- doped fiber amplifier (EDFA) as a gain medium, and have previously presented a method of direct sensing from the oscillation wavelengths [6]. As a result of the demonstrative experiment for the temperature measurement, the oscillation output was obtained at the peak wavelength in the transmittance channeled spectrum of the C-CLPG, and it exhibited a linear response to the temperature variation in accordance with that of the channeled spectrum. The oscillation wavelength range, however, was limited within a fringe spacing of the periodic channeled spectrum so that periodic sawtooth wavelength shifts were observed with the large temperature variations. Although a simple numerical compensation was also proposed to solve the limitation, it was required to know an accurate fringe spacing of the channeled spectrum and the corresponding temperature variation range a priori. Therefore, it was necessary to provide a mechanism to widen or shift the oscillation wavelength range of the laser during the tuning operation [7,8]. The purpose of this study is to extend the range of single-wavelength oscillation by a simple method to solve the problem of the limitation of the oscillation wavelength range mentioned above. In addition, such a laser operation is also very useful in light-source applications as a tunable and switching laser [9,10]. Several simple methods have been reported to extend or shift the oscillation wavelength of fiber ring lasers by adjusting the additional loss in the ring cavity. For the tunable filterless EDFRLs, P. Franco et al. [11] reported tuning the laser by adjusting a variable optical attenuator (VOA) inserted in the cavity to shift the net gain peak, and M. Melo et al. [12] reported that even a bending loss of a tapered single-mode fiber can be effectively used for the tuning operation of the laser. D. Venkatesh et al. [13] proposed a wavelength tuning method by controlling intracavity losses using a VOA as well as a variable optical coupler (VOC), and showed that a more stable output power with the less tuning-induced output variations was obtained by using VOA rather than VOC. For EDFRLs using comblike wavelength selection filters, Y. Liu et al. [14] reported a wavelength-switchable EDFRL using a sampled FBG and a VOA, and F. Li et al. [15] reported that the wavelength range of multiwavelength oscillations can be shifted by tuning the VOA losses. Recently, A. D. Guzman-Chavez et al. [16] demonstrated a wavelength-range tunable and switchable multiline fiber ring laser by using a VOA. In this paper, a programmable VOA was employed as an intracavity loss modulator in the previously proposed EDFRL [6], in which a C-CLPG with a periodic channeled spectrum was used as a wavelength selection filter. In a wavelength-tunable laser using a comblike periodic wavelength filter such as the C-CLPG, the wavelength tunable range is generally limited within the period of the wavelength selective element, which limits the measurement range of the sensor using the oscillating wavelength, but by adjusting the loss and changing the oscillating wavelength range, the wavelength tunable range, or measurement range, can be expanded. We reported at the OECC 2020 international conference [17] that a wavelength-switching operation of the EDF ring laser using C-CLPG can be realized by varying the intracavity loss using a VOA. In this paper, however, we have conducted new experiments, and investigated the intracavity loss dependent characteristics of the laser to clarify the effect on the oscillation wavelength, and demonstrated a detailed examination of the tuning of the tunable wavelength range by loss from the viewpoint of improvement of the sensor applications. By applying a simple method to adjust the additional loss of the VOA used, it was possible to expand the effective tuning range of the oscillation wavelength and extend the measurement range in temperature sensing. In the Sensors 2021, 21, x FOR PEER REVIEW 3 of 8

adjust the additional loss of the VOA used, it was possible to expand the effective tuning range of the oscillation wavelength and extend the measurement range in temperature sensing. In the experiment, we investigated the oscillation wavelength with temperature variation around the C-CLPG by changing the additional loss of VOA and examined the wavelength tuning range. As a result, we succeeded in expanding the maximum oscilla- Sensorstion2021 wavelength, 21, 2342 range to 1557.62~1570.47 nm (12.85 nm), which corresponds to a temper-3 of 9 ature-measurement range of ~64 °C, three times larger than that of the previous laser.

2. Apparatus experiment, we investigated the oscillation wavelength with temperature variation around the C-CLPG by changing the additional loss of VOA and examined the wavelength tuning The experimentalrange. setup As ais result, shown we succeeded in Figure in expanding1. The EDFRL the maximum consist oscillations of a wavelengthC-CLPG rangeas a sensing head, an EDFAto 1557.62~1570.47(Mitsubishi nmCable, (12.85 FA155D nm), which-168FS) corresponds that to include a temperature-measurements built-in input rangeand ◦ output isolators, a programmableof ~64 C, three timesVOA larger (Anritsu, than that MN9611B) of the previous to control laser. the intracavity loss, and a VOC (KS Photonics,2. Apparatus FTDC-N-155-1-FA) with a splitting ratio of 10:90. The oscillation spectrum of the outputThe experimentalfrom the one setup port is shown of the in Figure VOC1. is The measured EDFRL consists by means of a C-CLPG of an as optical spectrum analyzera sensing (OSA; head, Anritsu, an EDFA (Mitsubishi MS9740A) Cable, with FA155D-168FS) a resolution that of includes 0.03 nm, built-in and input the and output isolators, a programmable VOA (Anritsu, MN9611B) to control the intracavity split light from the otherloss, andport a is VOC fed (KS back Photonics, into the FTDC-N-155-1-FA) EDFA through with the a splittingVOA and ratio a of polariza- 10:90. The tion controller. The Coscillation-CLPG spectrumwas fabricated of the output in fromsuch the a oneway port that of the the VOC pair is measuredof CLPGs by meanswas written into a photosensitiveof an optical fiber spectrum (Nufern, analyzer GF1AA) (OSA; Anritsu, by a point MS9740A)-by- withpoint a resolutionmethod ofusing 0.03 nm, a and the split light from the other port is fed back into the EDFA through the VOA and a focused KrF excimer polarizationlaser (GAM controller. laser, TheEX10) C-CLPG beam was (grating fabricated period: in such a way300~305 that the µm pair; separa- of CLPGs tion between two CLPGs:was written 100 mm into; anumber photosensitive of the ﬁber peri (Nufern,od of the GF1AA) grating: by a point-by-point 20; CLPG length: method 5740 µm; chirp ratio:using 0.871 a focused × 10−3 KrF; and excimer the laserfull- (GAMwidth laser, at EX10)half-maximum beam (grating (FWHM) period: 300~305 of theµm; separation between two CLPGs: 100 mm; number of the period of the grating: 20; CLPG attenuation dip of thelength: CLPG 5740 aroundµm; chirp 1560 ratio: nm: 0.871 ~70× 10nm−3);. and The the resultant full-width atchanneled half-maximum spectrum (FWHM) was obtained aroundof 1560 the attenuationnm in the dip transmittance of the CLPG around spectrum 1560 nm: of ~70the nm).C-CLPG. The resultant The EDF channeled was backward-pumped byspectrum a 1.48 was µm obtained laser diode around 1560driven nm in with the transmittance an injection spectrum current of the of C-CLPG. 100 mA, The EDF was backward-pumped by a 1.48 µm laser diode driven with an injection current of so that the oscillation100 was mA, obtained so that the oscillationat the peaks was obtained of the at channeled the peaks of the spectrum channeled spectrumaround around1560 nm that corresponded1560 to nmthe that maximum corresponded gain to the[17]. maximum gain [17].

こ PC Thermal chamber

C-CLPG Digital thermometer Pulley

Weight Fixed VOC stage 10 Heater OUTPUT

Polarization controller 90 SMF EDFA VOA

Figure 1. ExperimentalFigure 1. setupExperimental for the setup tunable for the tunableand switchable and switchable EDFRL EDFRL using using C-CLPG. C-CLPG. In this configuration the VOA enables the control of the amount of optical feedback to In this configurationthe EDFA the by VOA changing enable the additionals the control intracavity of the losses. amount As the intracavityof optical loss feedback is reduced to the EDFA by changingand the the intensity additional of the feedback intracavity light to losses. the EDFA As is the increased, intracavity the gain loss of the is EDFA re- becomes deeply saturated. As a result, the gain profile is flattened over a wide wavelength duced and the intensityrange, of and the the feedback maximum gainlight is to obtained the EDFA at a longer is increased, wavelength sidethe [18gain], so of that th thee EDFA becomes deeplyoscillation saturated. can also As be a obtained result, at the a longer gain wavelength profile is side. flattened On the other over hand, a wide as the wavelength range, and the maximum gain is obtained at a longer wavelength side [18], so that the oscillation can also be obtained at a longer wavelength side. On the other hand, as the intracavity loss is increased, the gain profile becomes similarly to the ASE curve, which is close to small signal gain, resulting in an oscillation obtained at a shorter wavelength side [11,19]. Therefore, the oscillation wavelength range of the EDF laser can be

Sensors 2021, 21, x FOR PEER REVIEW 4 of 8

Sensors 2021, 21, 2342 4 of 9

effectively controlled to the longer or shorter wavelength side by controlling the additional loss of the VOA.intracavity loss is increased, the gain proﬁle becomes similarly to the ASE curve, which is close to small signal gain, resulting in an oscillation obtained at a shorter wavelength 3. Experimental Resultsside [and11,19 Discussions]. Therefore, the oscillation wavelength range of the EDF laser can be effectively controlled to the longer or shorter wavelength side by controlling the additional loss of 3.1. Wavelength-Switchingthe VOA. Operation

Figure 2 shows the3. Experimental typical oscillation Results and spectr Discussionsa for different intracavity losses (colored solid lines) and C-CLPG3.1. Wavelength-Switching transmittance spectrum Operation (black dotted line) at room temperature (~29 °C). It is noted that theFigure oscillation2 shows the spec typicaltrum, oscillation as well spectraas the forchanneled different intracavity spectrum losses of the (colored C-CLPG remains constantsolid lines) without and C-CLPG shifting transmittance if there isspectrum no variation (black dottedin temperature. line) at room Since temperature the oscillation output(~29 is usually◦C). It is notedobtained that the at oscillationone of the spectrum, peaks asof wellthe aschanneled the channeled spectrum, spectrum of the as can be seen from C-CLPGthe figure, remains the constant outputs without of −8.39 shifting dBm, if there −11.91 is no dBm, variation and in temperature.−14.47 dBm Since the oscillation output is usually obtained at one of the peaks of the channeled spectrum, as were respectively obtainedcan be seen at from1569.35 the ﬁgure, nm the(Ch.3), outputs 1563.87 of −8.39 nm dBm, (Ch.2),−11.91 dBm,and and1558.44−14.47 nm dBm were (Ch.1), when the respectiverespectively VOA obtained loss was at 1569.35 set to nm 0,(Ch.3), 7, and 1563.87 15 dB. nm The (Ch.2), reason and 1558.44 for the nm wave- (Ch.1), when length-switching behaviorsthe respective to the VOA shorter loss was wa setvelength to 0, 7, and for 15 dB.the The higher reason VOA for the loss wavelength-switching was that the oscillation wavelengthbehaviors range to the shifted shorter to wavelength the shorter for wavelength the higher VOA side loss as the was intracavity that the oscillation wavelength range shifted to the shorter wavelength side as the intracavity loss of the VOA loss of the VOA increased, as described in the previous section. Furthermore, when the increased, as described in the previous section. Furthermore, when the intracavity loss intracavity loss increasedincreased more more than than 30 30 dB, dB, nono oscillation oscillation occurred, occurred, and only and the only output the corresponding output to corresponding to thethe ASE ASE light light transmitted transmitted through through the the C-CLPG C-CLPG was observed. was observed. 0 100 (c) (b) (a)

80 −20 (d)

60 −40

40 −60 20 Transmittance [%] Transmittance

Output power [dBm]powerOutput −80 0 1450 1500 1550 1600 1650

Wavelength [nm] 1

Figure 2. 2.Oscillation Oscillation spectra spectra of the of EDFRL the EDFRL at room at temperature room temperature (~29 ◦C) with (~29 different °C) with VOA different losses: ( aVOA) 0 dB, (b) 7 dB, (losses:c) 15 dB, (a and) 0 (dB,d) 30 (b dB.) 7 dB, (c) 15 dB, and (d) 30 dB.

Sensors 2021, 21, 2342 5 of 9

3.2. Output Characteristics Dependent on Intracavity Loss The oscillation output behavior was investigated in more detail with the intracavity additional loss of VOA varied by 1 dB. The output power obtained at each channel as a Sensors 2021, 21, x FOR PEER REVIEW 5 of 8 function of the VOA loss is shown in Figure3. As shown in the figure, if the loss of VOA was set to 0 dB, oscillation occurred in Ch. 3. When the intracavity loss was gradually increased, the oscillation output in Ch. 3 decreased and stopped, while oscillation occurred the short wavelengthin Ch.side 2. as As the the VOA intracavity loss lossincreased. was further In addition, increased, oscillationsince the was EDFA obtained was in Ch. 1 as well. These results reflect the fact that the oscillation wavelength range shifted to likely to be a homogeneousthe short broadening wavelength side gain as themedia, VOA single loss increased. wavelength In addition, oscillation since thetended EDFA was to occur essentially. likelyHowever, to be a as homogeneous shown in the broadening figure, gainwhen media, the loss single of wavelength the VOA was oscillation about tended 3 dB and 14 dB, simultaneousto occur essentially. dual-wavelength However, as shown oscillations in the figure, were when obtained the loss in of the the adjacent VOA was about channels Ch. 3 and 3Ch. dB and1, and 14 dB, Ch. simultaneous 2 and Ch. dual-wavelength1, respectively. oscillations These results were obtained indicated in the that adjacent channels Ch. 3 and Ch. 1, and Ch. 2 and Ch. 1, respectively. These results indicated that the laser provided notthe laseronly provided the wavelength-s not only the wavelength-switchingwitching operation operation according according to a topeak a peak of of the the channeled spectrumchanneled of the spectrum C-CLPG of thebyC-CLPG tuning bythe tuning VOA the under VOA underthe constant the constant tempera- temperature, ture, but it also wasbut possible it also wasto switch possible between to switch the between single- the single-and dual-wavelength and dual-wavelength opera- operation, tion, providing a laserproviding with high a laser potential with high as potential a light as source. a light source. 0 Ch. 3 Ch. 2 Ch. 1 −20

−40

Output power[dBm]Output −60

0 5 10 15 20 25 30

VOA loss [dB]

FigureFigure 3. VOA3. VOA loss-dependent loss-dependent output poweroutput at power peak wavelengths at peak wavelengths of channeled spectra of channeled for Ch. 1 (blue spectra circle), for Ch. Ch. 2 (green 1 square),(blue circle), and Ch. Ch. 3 (red 2 diamond).(green square), and Ch. 3 (red diamond).

Sensors 2021, 21, 2342 6 of 9

3.3. Temperature Characteristics Figure4a–d show the temperature dependence of the EDFRL oscillation wavelengths extracted with the threshold at −30 dBm for different intracavity losses (colored symbol plots) and the peak wavelengths of the channeled spectrum of the C-CLPG (black cross plots). It was conﬁrmed that the additional loss in the cavity allowed switching the oscillation wavelength range toward the shorter wavelength side. As can be seen from the ﬁgure, the oscillation wavelength ranges were obtained as follows, 1564.94~1570.68 nm (5.74 nm), 1561.34~1567.76 nm (6.42 nm), 1559.18~1564.89 nm (5.71 nm), and 1557.57~1563.05 nm (5.48 nm), respectively, for the VOA losses of 0 dB (a), 5 dB (b), 10 dB (c), and 15 dB (d). In this study, the oscillation wavelength corresponded to the temperature dependence of the channeled spectrum of the C-CLPG, which showed a linear response to temperature Sensors 2021, 21, x FOR PEER REVIEW ◦ 6 of 8 with a sensitivity of ~−0.2 nm/ C, but a sawtooth response of the oscillation wavelength was obtained due to the limited wavelength range for the periodic channeled spectrum of the C-CLPG, which implies the ambiguity in temperature measurement. As result, this startproperty of the mademeasurement. it necessary These to know properties the initial ar temperaturee in agreement at the with start the of the results measurement. of our previ- ousThese report properties [6]. Furthermore, are in agreement at the with end the of resultsthe tuned of our wavelength previous report range, [6]. Furthermore,dual-wavelength at the end of the tuned wavelength range, dual-wavelength oscillation was obtained instead oscillation was obtained instead of single-wavelength, and the temperature-measurement of single-wavelength, and the temperature-measurement range using single-wavelength rangeoscillation using single-wavelength was even narrower oscillation than the tuned was range even innarrower Figure3, than which the was tuned desired range for in Fig- urepractical 3, which temperature was desired measurement. for practical However, temperature since the measurement. oscillation in different However, wavelength since the os- cillationranges in could different be obtained wavelength by controlling ranges the could VOA be loss, obtained a wide single-wavelength by controlling the oscillation VOA loss, a widerange single-wavelength beyond the limit wasoscillation considered range to be beyond feasible. the limit was considered to be feasible.

1580 VOA : 0 dB 1580 VOA : 5 dB Laser Laser (30~45 ℃) (30~38 ℃) 1575 Laser 1575 Laser (45~69 ℃) (35~62 ℃) Laser Laser 1570 (70~98 ℃) 1570 (58~85 ℃) Laser Laser (97~124 ℃) (84~113 ℃) 1565 1565 Laser Laser (121~130 ℃) (107~130 ℃) Transmittance Transmittance 1560 peaks of C-CLPG 1560 peaks of C-CLPG (30~130 ℃) (30~130 ℃)

Wavelength [nm] Wavelength 1555 [nm] Wavelength 1555

1550 1550 30 50 70 90 110 130 (a) 30 50 70 90 110 130 (b)

Temperature [℃] 1 Temperature [℃]

1580 VOA : 10 dB 1580 VOA : 15 dB Laser Laser (30~46 ℃) (30~32 ℃) 1575 Laser 1575 Laser (46~70 ℃) (32~55 ℃) Laser Laser 1570 (69~98 ℃) 1570 (55~77 ℃) Laser Laser (96~123 ℃) (77~104 ℃) 1565 1565 Laser Laser (119~130 ℃) (102~130 ℃) Transmittance Laser 1560 peaks of C-CLPG 1560 (129~130 ℃) (30~130 ℃) Transmittance peaks of C-CLPG Wavelength [nm] Wavelength 1555 [nm] Wavelength 1555 (30~130 ℃)

1550 1550 30 50 70 90 110 130 (c) 30 50 70 90 110 130 (d) Temperature [℃] Temperature [℃]

FigureFigure 4. Oscillation 4. Oscillation wavelengths wavelengths of ofEDFRL EDFRL (colored (colored symbol symbol plots)plots) and and transmittance transmittance peak peak wavelengths wavelengths of the of C-CLPG the C-CLPG (black(black cross cross plots) plots) as a as function a function of oftemperature temperature for for different different VOAVOA losses: losses: (a ()a 0) dB,0 dB, (b )( 5b) dB, 5 dB, (c) 10(c) dB, 10 anddB, (andd) 15 (d dB.) 15 dB.

Sensors 2021, 21, 2342 7 of 9

As shown in Figure5, the wavelength ranges of single-wavelength oscillation of the measured spectrum for VOA loss of 0 dB (a), 5 dB (b), 10 dB (c), and 15 dB (d) were 1565.45~1570.47 nm (5.02 nm), 1562.26~1566.78 nm (4.52 nm), 1559.71~1564.57 nm (4.86 nm), and 1557.62~1562.78 nm (5.16 nm); and the total single-wavelength oscillation range was found to be 12.85 nm. When the VOA loss was constant, the oscillation wavelength region, which was continuously shifted without jumping, was limited to the temperature range corresponding to the channel spacing of the C-CLPG. However, by shifting the oscillation wavelength range as shown in Figure5, the measurement range could be expanded beyond the limitation of temperature change corresponding to the channel spacing of the C-CLPG. Although the expansion of the measurement temperature range by this method was narrower than that by numerical compensation reported previously in [6], a simple technique without the numerical data processing could be provided. From the results obtained, it was possible to measure the change in oscillation wavelength while Sensors 2021, 21, x FOR PEER REVIEW 7 of 8 maintaining single-wavelength operation simply by controlling the VOA loss in three or four steps according to the shift in oscillation wavelength due to temperature change, resulting in a maximum measurable range of the temperature change of ~64 ◦C. In the range can be furtherpresent extended laser conﬁguration, toward longer depending wavelengths on the performance by reducing of the the VOA entire used, loss the of tunable range can be further extended toward longer wavelengths by reducing the entire loss the cavity. Since the C-CLPG used as a sensor element is subjected to disturbances caused of the cavity. Since the C-CLPG used as a sensor element is subjected to disturbances by temperature changescaused by and temperature other measurands, changes and other the elements measurands, other the elementsthan the other sensor than part the sensor and their connectionspart and need their to connectionsbe protected need with to bethermal protected insulators with thermal and insulatorscushioning and mate- cushioning rials or vibration materialsisolators or to vibration avoid the isolators unnecessary to avoid the disturbances, unnecessary disturbances,which is a problem which is afor problem practical applications.for practical applications.

1570.47 1570

(a) 1566.78

1564.57 1565 1565.45 (b) 1562.78 (c) 1562.26 1560 (d) 1559.71

Wavelength [nm]Wavelength 1557.62 1555 0 5 10 15

VOA loss [dB]

FigureFigure 5. Range 5. Range of single-wavelength of single-wavelength operation operat for differention for VOA different losses: VOA (a) 0 dB, losses: (b) 5 dB,(a) (0c )dB, 10 dB,(b) and5 dB, (d) ( 15c) dB.10 dB, and (d) 15 dB.

4. Concluding Remarks In this paper, we have proposed an oscillation wavelength range shifting technique based on an intracavity loss modulation in a tunable and switchable EDFRL with a C- CLPG and a VOA to expand the available wavelength range of the previously reported tunable laser configuration [6]. The expansion of the oscillation wavelength range of tunable lasers is an important technology for fiber laser sensors that use an oscillation wavelength as a measurement parameter, from the viewpoint of practical sensor applications. In the laser configuration fabricated, the shifting of the tuning range to the arbitrary wavelength within 1557.62~1570.47 nm was successfully demonstrated by controlling the additional intracavity loss of the VOA, i.e., by expanding the oscillation wavelength range. For the application of the temperature measurement, by controlling the VOA according to the shift of the oscillation wavelength, the single-wavelength oscillation could be maintained over an available temperature range without transitioning to the adjacent channel, which was expected to expand the temperature-measurement range compared to the conventional method. In the experiments, it was shown that single-wavelength oscillation was possible in the range of 1564.57~1565.45 nm (12.85 nm), which corresponded to a temperature range of 64 °C and was equivalent to three times the measurement range realized by the previously reported laser. The proposed laser can be used for two-wavelength oscillation by appropriately controlling the VOA loss, making it useful not only as a sensor, but also as a light source. The improvement of the response time and range for the temperature measurement is an important issue for using this type of sensor in rapid and

Sensors 2021, 21, 2342 8 of 9

4. Concluding Remarks In this paper, we have proposed an oscillation wavelength range shifting technique based on an intracavity loss modulation in a tunable and switchable EDFRL with a C-CLPG and a VOA to expand the available wavelength range of the previously reported tunable laser configuration [6]. The expansion of the oscillation wavelength range of tunable lasers is an important technology for fiber laser sensors that use an oscillation wavelength as a measurement parameter, from the viewpoint of practical sensor applications. In the laser configuration fabricated, the shifting of the tuning range to the arbitrary wavelength within 1557.62~1570.47 nm was successfully demonstrated by controlling the additional intracavity loss of the VOA, i.e., by expanding the oscillation wavelength range. For the application of the temperature measurement, by controlling the VOA according to the shift of the oscillation wavelength, the single-wavelength oscillation could be maintained over an available temperature range without transitioning to the adjacent channel, which was expected to expand the temperature-measurement range compared to the conventional method. In the experiments, it was shown that single-wavelength oscillation was possible in the range of 1564.57~1565.45 nm (12.85 nm), which corresponded to a temperature range of 64 ◦C and was equivalent to three times the measurement range realized by the previously reported laser. The proposed laser can be used for two-wavelength oscillation by appropriately controlling the VOA loss, making it useful not only as a sensor, but also as a light source. The improvement of the response time and range for the temperature measurement is an important issue for using this type of sensor in rapid and wide-ranging temperature changes, and is left for further study. In addition, we plan to conduct demonstrations of other parameters that C-CLPG can measure, including refractive index, mechanical deformation, and vibration of the surrounding environment.

Author Contributions: Writing—original draft and editing, K.F.; investigation, K.F.; resources, M.G.S.; validation, A.W.; writing—review, S.T.; conceptualization, S.T.; supervision, S.T.; funding acquisition, F.I. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by JSPS KAKENHI Grant Number JP18H01451. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Othonos, A.; Kalli, K. Fiber Bragg Gratings ~Fundamentals and Applications in Telecommunications and Sensing; Artech House: Boston, MA, USA, 1999. 2. Vengsarkar, A.M.; Lemaire, P.J.; Judkins, J.B.; Bhatia, V.; Erdogan, T.; Sipe, J.E. Long-period fiber gratings as band-rejection filters. J. Lightwave. Technol. 1996, 14, 58–65. [CrossRef] 3. James, S.W.; Tatam, R.P. Optical fibre long-period grating sensors: Characteristics and application. Meas. Sci. Technol. 2003, 14, R49–R61. [CrossRef] 4. Tanaka, S.; Tsukida, O.; Ngo, T.T.; Wada, A.; Takahashi, N. Simultaneous multipoint strain measurement using cascaded long period fiber gratings. In Proceedings of the 24th Optical Fiber Sensors (OFS-24), Curitiba, Brazil, 28 September–2 October 2015. 5. Nagatsuka, M.; Koizumi, M.; Tanaka, S.; Wada, A.; Takahashi, N. Precise measurement technique of long period fiber grating sensors using Fourier transform method. In Proceedings of the 25th Optical Fiber Sensors (OFS-25), Jeju Island, Korea, 24–28 April 2017. 6. Fukushima, K.; Bui, Q.H.; Nakaya, K.; Soares, M.G.; Wada, A.; Tanaka, S.; Ito, F. EDF ring laser using cascaded-chirped long period fiber grating for temperature measurement. Opt. Express 2020, 28, 13081–13090. [CrossRef][PubMed] 7. Xie, W.; Zhang, Y.; Wang, P.; Wang, J. Optical Fiber Sensors Based on Fiber Ring Laser Demodulation Technology. Sensors 2018, 18, 505. [CrossRef][PubMed] 8. Frazão, O.; Correia, C.; Baptista, J.M.; Marques, M.B.; Santos, J.L. Ring fibre laser with interferometer based in long period for sensing applications. Opt. Commun. 2008, 281, 5601–5604. [CrossRef] Sensors 2021, 21, 2342 9 of 9

9. Yan, M.; Luo, S.; Zhan, L.; Zhang, Z.; Xia, Y. Triple-wavelength switchable Erbium-doped fiber laser with cascaded asymmetric exposure long-period fiber gratings. Opt. Express 2007, 15, 3685–3691. [CrossRef][PubMed] 10. Liu, X.; Zhan, L.; Luo, S.; Wang, Y.; Shen, Q. Individually Switchable and Widely Tunable Multiwavelength Erbium-Doped Fiber Laser Based on Cascaded Mismatching Long-Period Fiber Gratings. J. Lightwave Technol. 2011, 29, 3319–3326. [CrossRef] 11. Franco, P.; Midrio, M.; Tozzato, A.; Romagnoli, M.; Fontana, F. Characterization and optimization criteria for filterless erbium- doped fiber lasers. J. Opt. Soc. Am. B 1994, 11, 1090–1097. [CrossRef] 12. Melo, M.; Frazão, O.; Teixeira, A.L.J.; Gomes, L.A.; Ferreira da Rocha, J.R.; Salgado, H.M. Tunable L-band erbium-doped fibre ring laser by means of induced cavity loss using a fibre taper. Appl. Phys. B 2003, 77, 139–142. [CrossRef] 13. Venkitesh, D.; Vijaya, R. A tunable erbium doped fiber ring laser without the use of intra-cavity filters. In Proceedings of the Photonics North 2007, Ottawa, ON, Canada, 4–7 June 2007. 14. Liu, Y.; Dong, X.; Shum, P.; Yuan, S.; Kai, G.; Dong, X. Stable room-temperature multi-wavelength lasing realization in ordinary erbium-doped fiber loop lasers. Opt. Express 2006, 14, 9293–9298. [CrossRef][PubMed] 15. Li, F.; Feng, X.; Lu, C.; Tam, H.Y.; Wai, P.K.A. Characterization of multiwavelength erbium doped fiber lasers with intensity- dependent loss. In Proceedings of the 16th Opto-Electronics and Communications Conference (OECC2011), Kaohsiung, Taiwan, 4–8 July 2011. 16. Guzman-Chavez, A.D.; Vargas-Rodriguez, E.; Martinez-Jimenez, L.; Vargas-Rodriguez, B.L. Switchable multi-wavelength and tunable waveband fiber laser based on a thermal sensitive filter. Opt. Commun. 2021, 482, 126613–126619. [CrossRef] 17. Fukushima, K.; Soares, M.G.; Nakaya, K.; Wada, A.; Tanaka, S.; Ito, F. Wavelength Tunable and Switchable EDF Ring Laser using Cascaded-Chirped Long Period Fiber Grating. In Proceedings of the 25th Opto-Electronics and Communications Conference (OECC2020), Taipei, Taiwan, 4–8 October 2020. 18. Bellemare, A. Continuous-wave silica-based erbium-doped fibre lasers. Prog. Quantum Electron. 2003, 27, 211–266. [CrossRef] 19. Saleh, B.E.A.; Teich, M.C. Theory of laser oscillation. In Fundamentals of Photonics, 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2007; pp. 567–575.