Erbium Ring Fiber Laser Cavity Based on Tip Modal Interferometer and Its Tunable Multi-Wavelength Response for Refractive Index and Temperature

$Erbium Ring Fiber Laser Cavity Based on Tip Modal Interferometer and Its Tunable Multi-Wavelength Response for Refractive Index and Temperature$

applied sciences

Article Erbium Ring Fiber Laser Cavity Based on Tip Modal Interferometer and Its Tunable Multi-Wavelength Response for Refractive Index and Temperature

Yanelis Lopez-Dieguez 1, Julian M. Estudillo-Ayala 1 ID , Daniel Jauregui-Vazquez 1,* ID , Luis A. Herrera-Piad 1 ID , Juan M. Sierra-Hernandez 1, Diego F. Garcia-Mina 2 ID , Eloisa Gallegos-Arellano 3, Juan C. Hernandez-Garcia 1,4 and Roberto Rojas-Laguna 1

1 Departamento de Electrónica, División de Ingenierías Campus Irapuato-Salamanca, Universidad de Guanajuato, Carretera Salamanca-Valle de Santiago km 3.5 + 1.8 km, Salamanca Gto. 36885, Mexico; [email protected] (Y.L.-D.); [email protected] (J.M.E.-A.); [email protected] (L.A.H.-P.); [email protected] (J.M.S.-H.); [email protected] (J.C.H.-G.); [email protected] (R.R.-L.) 2 Departamento de Física, Facultad de Ciencias Básicas, Universidad Autónoma de Occidente, Calle 25 # 115-85, Cali 760030, Colombia; [email protected] 3 Departamento de Tecnologías de la Información y Comunicación, Universidad Tecnológica de Salamanca, Avenida Universidad Tecnológica 200, Ciudad Bajío, Salamanca Gto. 36766, Mexico; [email protected] 4 Catedrático CONACYT, Consejo Nacional de Ciencia y Tecnología (CONACYT), Av. Insurgentes Sur No. 1582, Col. Crédito Constructor, Del. Benito Juárez C.P. 039040, México * Correspondence: [email protected]; Tel.: +52-46-4647-9940 (ext. 2345)

Received: 29 June 2018; Accepted: 7 August 2018; Published: 10 August 2018

Abstract: A tunable multi-wavelength fiber laser is proposed and demonstrated based on two main elements: an erbium-doped fiber ring cavity and compact intermodal fiber structure. The modal fiber interferometer is fabricated using the cost-effective arc splice technique between conventional single-mode fiber and microfiber. This optical fiber structure acts as a wavelength filter, operated in reflection mode. When the refractive index and temperature variations are applied over the fiber filter, the ring laser cavity provides several quad-wavelength laser spectra. The multi-wavelength spectra are tuned into the C-band with a resolution of 0.05 nm. In addition, the spectra are symmetric with minimal power difference between the lasing modes involved, and the average of the side mode suppression ratio is close to 37 dB. This laser offers low-cost implementation, low wavelength drift, and high power stability, as well as an effect of easy controllability regarding tuned multi-wavelength.

Keywords: ﬁber laser; ring laser cavity; tunable multi-wavelength laser

1. Introduction For several decades, the ability to generate tunable or switched multi-wavelength fiber lasers has been one of the focuses pursued by the fiber laser community. These lasers are associated with many reliable applications related to multi-wavelength spectra such as optical fiber sensors [1–9], spectroscopy systems [10], biomedical imaging [11], telecommunications [12], microwave photonics [13], and radio frequency optical domain systems [14]. These applications require achieving compact configuration, stable emission, narrow linewidth, and adequate spacing mode. All these features prevent crosstalk, error measurement, and low resolution. To this end, many methods have been proposed, most of them based on fiber interferometers (Fabry-Perot, Mach-Zehnder, Michelson, and Sagnac) [15–19], comb filters [20,21], Raman effect [20], Brillion scattering [22,23], nonlinear loop mirrors [24], saturable optical absorber elements [25,26], and extrinsic interferometers [27].

Appl. Sci. 2018, 8, 1337; doi:10.3390/app8081337 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1337 2 of 11

These proposed schemes offer compactness, high power stability, minimal wavelength drift, and adequate signal-to-noise ratio, long operation time, and low threshold. Furthermore, some research groups have demonstrated the capability of tunable multi-wavelength [28–33]. The most common techniques to overcome modal competition and achieve multi-wavelength spectra are Four-Wave Mixing (FWM) and Stimulated Brillouin Scattering (SBS). However, these techniques require several meters of highly nonlinear fiber optics, and polarization plays an important role; this entails complex configurations and costly implementation. Moreover, in some arrangements, the multi-wavelength tuning effect undergoes lasing power disparity, small side mode suppression ratio, and power dependence. One of the most common setups used to overcome the undesired effect is the recognized ring fiber laser cavity based on modal fiber interferometers [26–38]. In this work, a cost-effective modal interferometer and simple ring laser cavity are used to demonstrate the multi-wavelength effect. The laser arrangement was governed by a modal fiber structure operated in reflection mode; the structure was set into an erbium fiber ring laser cavity, resulting in a quad-wavelength laser. This initial laser spectrum had a wavelength drift of around 0.008 nm and power variations of 0.46 dB. The refractive index and thermal variations around the modal interferometers were assessed to provide a simple method for tuning multi-wavelength spectra. The spectrum was analyzed when refractive index changes were applied around the fiber filter; here, two dual-wavelength spectra were tuned with a resolution of 0.05 nm. Minimal power and wavelength variations were observed through the tuned range. To assess the thermal effect, a temperature exceeding 90 ◦C was applied over the fiber filter, tuning dual and triple wavelength spectra. Most of the spectra achieved had an adequate side mode suppression ratio, high peak level, lower wavelength drift, and minimal power variation.

2. Fiber Filter Fabrication and Its Principle Operation The modal fiber interferometer (MFI) was fabricated by using the arc splice technique, where the catastrophic splicing of an optical microfiber (<60 µm diameter) at the tip of conventional single-mode fiber (125 µm diameter) is performed. The microfiber used in this work was fabricated using the drawing flame technique [39]. During the fabrication process, only an arc splicer (Fitel S175, Furukawa FITEL Optic Products Co., Ltd., Peachtree, GA, USA) was used in manual mode, setting the single-mode fiber default program to the following parameters: power 94 A.U and Z push distance 11µm. Both fibers were cleaved, set, and aligned into the splicer machine; then, ten discharges were applied until a first catastrophic splice zone (Z1) was achieved (Figure1). Zone Z1 was translated at a specific distance from the electrodes, and the microfiber was exposed at the electrode’s area; then, several discharges are applied to cut the microfiber (≤18 discharges). As a result, the second catastrophic zone, Z2, was obtained; the final structure can be observed in Figure1. The Z1 and Z2 collapse regions had a size of 50µm (this value was estimated using the arc splicer). The distance, D, between Z1 and Z2 oscillated between 400 µm and 500 µm. The fabrication process was monitored in real-time to minimize the insertion loss. Hence, the number of discharges varied, considering the Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 11 minimal insertion loses; consequently, the distance, D, was also affected.

(a) (b)

Figure 1. Physical characteristics of the MFI structure: (a) structure diagram; (b) Scanning Electron Figure 1. Physical characteristics of the MFI structure: (a) structure diagram; (b) Scanning Electron Microscope Image (SEM) of the structure. Microscope Image (SEM) of the structure. The MFI was operated in reflection mode; its interference spectrum is shown in Figure 2a. The spectrum showed visibility oscillating between 8 dB and 10 dB; its free spectral range was 1.6 nm. The Fourier transform was applied to the reflection interference spectrum to identify the modal contribution (see Figure 2b). The fundamental reflected core mode is considered the peak centered at 0 nm−1. Two spatial frequency regions with some intensity peaks can be seen; these modal energies correspond to higher-order reflection modes. All the modes described above are excited by several refractive index interfaces located at zones Z1 and Z2. The first refractive index interface can be found at the Z1 zone, where the bulk of the collapsed material and the core and cladding of the fibers involved produces a refractive index change. At this point, part of the light arriving at the Z1 zone is reflected, and the rest travels through the microfiber. The light then travels to the Z2 zone, where the tip of the bulk collapsed material, and the surrounding medium produces a refractive index interface, resulting in the generation of some reflections.

-3 0.69 FSR=1.6nm -6 0.46 -9 0.23

Reflection(dB) -12

Normalized Amplitude (A.U) Amplitude Normalized 0.00 -15 35 70 105 140 175 1528 1532 1536 1540 1544 Spacial Frequency (nm-1) Wavelength (nm) (a) (b)

Figure 2. MFI characterization: (a) Reflection spectrum of the used MFI obtained using a broadband source at port 1 of an optical fiber circulator (OFC), the questioned MFI at port 2 of the OFC, and the OSA at port 3; (b) Spatial frequency analysis of the reflection interference spectrum.

The amplitude of the total reflected electric vector can be described by [40]: ∑ ∅ 𝐸 𝑅 𝑒 𝑅 1𝑅𝐸 (1) 2 each reflectivity can be determined by the relation 𝑅|𝑛 𝑛⁄𝑛 𝑛| ; here, 𝑛 and 𝑛 represent the effective refractive index involved. 𝑅 is the first reflection and 𝑅 represents the other reflections gene rated by the structure. The phase between the reflected modes generated is

∅ 4𝜋𝑑𝑛⁄ 𝜆 , where 𝜆 is the operation′s wavelength, 𝑑 is the distance of the optical path and 𝑛 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 11

(a) (b)

Figure 1. Physical characteristics of the MFI structure: (a) structure diagram; (b) Scanning Electron Appl.Microscope Sci. 2018, 8, 1337 Image (SEM) of the structure. 3 of 11

The MFI was operated in reflection mode; its interference spectrum is shown in Figure 2a. The spectrumThe MFIshowed was visibility operated oscillat in reflectioning between mode; 8 dB its and interference 10 dB; its spectrum free spectral is shown range inwas Figure 1.6 nm.2a. TheThe spectrumFourier transform showed visibilitywas applied oscillating to the between reflection 8 dB interference and 10 dB; spectrum its free spectral to identify range wasthe modal 1.6 nm. contributionThe Fourier (see transform Figure was2b). The applied fundamental to the reflection reflected interferencecore mode is spectrumconsidered to the identify peak centered the modal at 0contribution nm−1. Two spatial (see Figure frequency2b). The regions fundamental with some reflected intensity core peaks mode can is considered be seen; these the peak modal centered energies at −1 correspond0 nm . Two to spatialhigher-order frequency reflection regions modes. with some All the intensity modes peaksdescribed can beabove seen; are these excited modal by energiesseveral refractivecorrespond index to higher-order interfaces located reflection at zones modes. Z1 and All Z2. the The modes first describedrefractive aboveindex interface are excited can by be severalfound atrefractive the Z1 zone, index interfaceswhere the located bulk of at the zones collapsed Z1 and Z2.material The firstand refractive the core indexand cladding interface of can the be fibers found involvedat the Z1 zone,produces where a refractive the bulk of index the collapsed change. At material this point, and thepart core of the and light cladding arriving of the at fibersthe Z1 involved zone is reflected,produces and a refractive the rest indextravels change. through At the this microfiber. point, part The of thelight light then arriving travels atto the Z1Z2 zonezone, iswhere reflected, the tipand of the therest bulk travels collapsed through material, the microfiber. and the surroundi The lightng then medium travels produces to the Z2 a refractive zone, where index the interface, tip of the resultingbulk collapsed in the material,generation and of thesome surrounding reflections. medium produces a refractive index interface, resulting in the generation of some reflections.

-3 0.69 FSR=1.6nm -6 0.46 -9 0.23

Reflection(dB) -12

Normalized Amplitude (A.U) Amplitude Normalized 0.00 -15 35 70 105 140 175 1528 1532 1536 1540 1544 Spacial Frequency (nm-1) Wavelength (nm) (a) (b)

FigureFigure 2. 2. MFIMFI characterization: characterization: ( (aa)) Reflection Reflection spectrum spectrum of of the the used used MFI MFI obtained obtained using using a a broadband broadband sourcesource at at port port 1 1 of of an an optical optical fiber fiber circulator circulator (OFC (OFC),), the the questioned questioned MFI MFI at at port port 2 2 of of the the OFC, OFC, and and the the OSAOSA at at port port 3; 3; ( (bb)) Spatial Spatial frequency frequency analysis analysis of of the the reflection reflection interference interference spectrum.

The amplitude of the total reflected electric vector can be described by [40]: The amplitude of the total reflected electric vector can be described by [40]:   ∑ ∅  𝐸 𝑅 𝑒n 𝑅 k1𝑅𝐸 (1) k p −2j(∑ = ∅i)p ER =  R1 +∑e i 1 Rk+1∏(1 − Ri)E0 (1) k=1 i=1 2 each reflectivity can be determined by the relation 𝑅|𝑛 𝑛⁄𝑛 𝑛| ; here, 𝑛 and 𝑛 represent the effective refractive index involved. 𝑅 is the first reflection 2 and 𝑅 represents the R = n − n n + n n n othereach reflectivityreflections cangene be rated determined by the by structure. the relation The phase xbetweeny xthe reflectedy ; here, modesx and generatedy represent is the effective refractive index involved. R1 is the first reflection and Rk+1 represents the other reflections ∅ 4𝜋𝑑𝑛⁄ 𝜆 , where 𝜆 is the operation′s wavelength, 𝑑 is the distance of the optical path and 𝑛 gene rated by the structure. The phase between the reflected modes generated is ∅i = (4πdn/λ), where λ is the operation’s wavelength, d is the distance of the optical path and n is the effective refractive indexes of the different zones. More detailed information is presented in reference [41].

3. Results The proposed MFI structure was set into the ring fiber laser cavity. This arrangement consisted of a pump laser diode (QPHOTONICS QDFBLD-980–500); its signal was incorporated into the ring laser cavity by using an optical fiber wavelength division multiplexer (WDM). By using the 980 nm port, the pumped signal was launched to the gain medium (4.5 m of erbium-doped fiber), thus, generating an amplified spontaneous emission (ASE). At the end of the erbium-doped fiber length, a polarization controller (PC) was set using the following configuration: quarter wave plate-half wave plate –quarter wave plate (QWP-HWP- QWP). The purpose of the plates was to increase the system’s Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11 is the effective refractive indexes of the different zones. More detailed information is presented in reference [41].

3. Results The proposed MFI structure was set into the ring fiber laser cavity. This arrangement consisted of a pump laser diode (QPHOTONICS QDFBLD-980–500); its signal was incorporated into the ring laser cavity by using an optical fiber wavelength division multiplexer (WDM). By using the 980 nm port, the pumped signal was launched to the gain medium (4.5 m of erbium-doped fiber), thus, generating an amplified spontaneous emission (ASE). At the end of the erbium-doped fiber length, a polarizationAppl. Sci. 2018, 8controller, 1337 (PC) was set using the following configuration: quarter wave plate-half4 of 11 wave plate –quarter wave plate (QWP-HWP- QWP). The purpose of the plates was to increase the system’s stability. The ASE spectrum was launched to the MFI using an optical fiber circulator (OFC); at this point,stability. the Thereflected ASE spectrum signal from was the launched MFI was to the reincorporated MFI using an opticalinto the fiber ring circulator cavity by (OFC); using at port this 3 of the point, the reflected signal from the MFI was reincorporated into the ring cavity by using port 3 of the OFC. The interference reflection spectrum from the MFI traveled to the optical fiber coupler 90:10. OFC. The interference reflection spectrum from the MFI traveled to the optical fiber coupler 90:10. Here, the 10% coupler port, which was connected to an optical spectrum analyzer (OSA, AQ6370 Here, the 10% coupler port, which was connected to an optical spectrum analyzer (OSA, AQ6370 YokogawaYokogawa ChinaChina Co., Co., Ltd., Ltd., Tokyo, Tokyo, Japan), Japan), monitored monitored the output the output laser. The laser. 90% The signal 90% was signal launched was tolaunched toan an optical optical fiber fiber isolator, isolator, which which provided provided a path a path into theinto cavity. the cavity. Lastly, Lastly, the fiber the isolator fiber isolator output andoutput and thethe 1550 1550 nm nm WDM WDM portport werewere spliced spliced to to close close the the ring ring cavity. cavity. The The ring ring fiber fiber laser laser cavity cavity is shown is shown in Figurein Figure 3. 3.

FigureFigure 3. 3. ExperimentalExperimental setup setup of the ring cavity laser.laser. WDM: wavelengthwavelength division division multiplexer, multiplexer, EDF: erbium-dopedEDF: erbium-doped fiber, ﬁber, PC: PC: polarizatio polarizationn controller, controller, OFC:OFC: optical optical ﬁber fiber circulator, circulator, OC: optical OC: optical coupler, coupler, OSA:OSA: optical spectrumspectrum analyzer, analyzer, OI: OI: optical optical isolator. isolator.

TheThe pumpingpumping diode diode was was set at set 400 at mA, 400 achieving mA, achievin the spectrumg the shown spectrum in Figure shown4a; it isin important Figure 4a; it is importantto stress that to astress lower that current a lower threshold current was threshold observed (70was mA). observed Four lasing (70mA). lines Four constituted lasing thelines initial constituted thelaser initial response. laser This response. spectrum This had spectrum three consecutive had thre lasinge consecutive emissions, one lasing single emissions, line separated one from single line separatedthe triple emissionfrom the by triple 11.7 nm. emission The strong by relationship11.7 nm. Th betweene strong the relationship laser emission between and the interferencethe laser emission andspectrum the interference of the optical spectrum filter is well of recognized;the optical infilter our is work, well this recognized; was governed in our by thework, MFI. this According was governed byto the filterMFI. spectrum,According the to region the filter with lowerspectrum, visibility the didregion not havewith laser lower emissions visibility (see did Figure not4 a);have laser emissionsmoreover, (see the emissionsFigure 4a); were moreover, centered the at the emissions peak interference were centered fringe. at The the triple-laser peak interference emission was fringe. The made up of peak wavelengths centered at 1527.9949 nm (P1), 1529.4903 nm (P2), and 1531.0256 nm (P3). triple-laser emission was made up of peak wavelengths centered at 1527.9949 nm (P1), 1529.4903 nm The spacing mode of the initial multi-wavelength spectrum was 1.6 nm; this value corresponds (P2), and 1531.0256 nm (P3). to the free spectral range of the interference spectrum generated by the MFI. The power peaks fromThe the spacing triple-laser mode emission of the were:initial− multi-wavelength26.1580 dBm (P1), spectrum−16.7160 dBm was (P2),1.6 nm; and this−22.9010 value dBmcorresponds to(P3). the Thefree sidespectral mode range suppression of the interference ratio (SMSR) spectrum values were: generated 33 dB (P1), by 43the dB MFI. (P2), The and power 35 dB (P3).peaks from theThe triple-laser single-lasing emission line (P4) were: had the−26.1580 following dBm characteristics: (P1), −16.7160 wavelength dBm (P2), centered and −22.9010 at 1542.75 dBm nm, (P3). The sidepower mode peak suppression of −19.85 dBm, ratio and (SMSR) SMSR values around were: 41 dB. 33 The dB stability (P1), 43 of dB the (P2), fiber and laser 35 was dB analyzed;(P3). The single- lasingthe laser line spectrum (P4) had was the recordedfollowing every characteristics: 3 min for an wavelength h. The centered centered wavelength at 1542.75 and peaknm, power power peak of −variations19.85 dBm, are and shown SMSR in around Figure4b. 41 As dB. can The be stability observed, of thethe proposedfiber laser fiber was laser analyzed; presents the minimal laser spectrum waswavelength recorded variations every 3 min around for 0.008an h. nm The for centered P3 and P4 wavelength and maximal and for peak P2 (0.02 power nm). variations The minimal are shown inpower Figure fluctuations 4b. As can were be observed, observed atthe P2 proposed (0.46 dB), fi whileber laser the maximal presents intensity minimal fluctuations wavelength were variations presented at P1 (2.5 dB). The initial laser spectrum presented high wavelength stability. It must be around 0.008 nm for P3 and P4 and maximal for P2 (0.02 nm). The minimal power fluctuations were noted that the laser results shown above correspond to the room’s temperature, which was around observed at P2 (0.46 dB), while the maximal intensity fluctuations were presented at P1 (2.5 dB). The 25 ◦C, and the MFI air-suspended (1.00 RIU). Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11

initial laser spectrum presented high wavelength stability. It must be noted that the laser results shown above correspond to the room’s temperature, which was around 25°C, and the MFI air- suspendedAppl. Sci. 2018 ,(1.008, 1337 RIU). 5 of 11

-8dB 1544 -20 40 P4 -10dB Interferometer P2 -16 Spectrum Laser Output 1540 P4 -20 -40 43dB 41dB 20 1536 P3 -24

-60 0 P1 Reflection (dB) Reflection 1532 -28

Wavelength (nm) Wavelength P3 Output Power (dBm)

P2 Power (dBm) Output P P P 1 2 3 P -32 -80 4 -20 1528 P1 1528 1532 1536 1540 1544 0 1530456075 Wavelength (nm) Time (min) (a) (b)

Figure 4. Laser emission: ( a)) Quadruple-wavelength laser laser emission emission output output for for the the initial initial ring ring cavity cavity configurationconﬁguration (black(black line).line). UnequivocalUnequivocal correspondence correspondence between between the the laser laser emission emission (black (black line) line) and and the theMFI MFI reﬂection reflection spectrum spectrum (red (red line); line); (b) One-hour (b) One-hour laser laser emission emission wavelength wavelength stability stability assessment assessment (red) (red)and power and power (blue). (blue).

3.1. Refractive Refractive Index Response The fiber fiber laser response was analyzed when liqu liquidid refractive index variations were applied around the MFI. We set an initial refractive index close to 1.332 RIU (distilled water), obtaining the laserlaser spectra shown in Figure 55.. TheThe P1,P1, P2,P2, andand P4P4 laserlaser peakspeaks underwentunderwent insignificantinsignificant wavelengthwavelength variations (0.4 nm), and a new laser peak was obtained, which had the following characteristics: centered wavelength at at 1543.79 nm (P5), a peak power of −21.2521.25 dBm, dBm, and and SMSR SMSR exceeding exceeding 30 30 dB. dB. Two dual-wavelength emissions comprised this quadruple-laserquadruple-laser spectrum, both of them separated 13.2 nm; thus, each dual emission had a mode offset of 1.6 nm. This spectrum was analyzed during an hour, hour, evaluating evaluating the the wavelength wavelength and and peak peak power power fluctuations fluctuations (Figure (Figure 5b).5 b).According According to the to data, the thedata, P1 the and P1 P3 and lasing P3 lasingmodes modes had power had power fluctuations fluctuations exceeding exceeding 7 dB. However, 7 dB. However, the P2 theand P2 P5 and peaks P5 exhibitedpeaks exhibited lower lowerpower power fluctuations. fluctuations. Additiona Additionally,lly, the the quadruple-laser quadruple-laser spectrum spectrum had had minimal wavelength variations variations (<0.1 (<0.1 nm). nm). The The power power instabilities instabilities observed observed can can be adjusted be adjusted by using by using the PC. the OtherPC. Other techniques, techniques, such such as nonlin as nonlinearear optical optical loop loop mirror mirror and and nonlin nonlinearear fiber fiber optics optics can can also also be implemented to to reduce reduce these these variat variationsions [42,43]. [42,43 However,]. However, these these techni techniquesques will willmodify modify the fiber the laser fiber Appl.response.laser Sci. response.2018 , 8, x FOR PEER REVIEW 6 of 11 Different water-glycerin mixtures were used to increase the liquid refractive index around the n=1.3323 n=1.3323 MFI; their values were estimated using a Refracto 30GS refractometer. The refractive index ranged from 1.3323-20 to 1.3799 RIU; the minimal variation between each sample was 0.003 RIU. When the 1544 P5 -16 liquid refractive-13dB index was increased around the MFI, the laser response was consistently P4 altered -10dB P5 across the wavelength range shown in Figure 6a. It is1540 important to stress that, typically,-24 multi- -40 37dB P2 wavelength fiber laser arrangement exhibit switched31dB lasing mode when any physical effect perturbs 1536 the optical fiber filter. However, our experimental results showed that the entire spectrum displayed-32 -60 P1 a constant wavelength shift to a longer wavelength when1532 linear refractive index increments P4 -40 were

applied. (nm) Wavelength Output Power (dBm) P2 1528 -48 Output Power (dBm) -80 P P P1 P P P 4 5 1 2 3 1528 1532 1536 1540 1544 0 1530456075 Wavelength (nm) Time (min) (a) (b)

FigureFigure 5.5. RefractiveRefractive index response:response: (a ()a Quadruple-wavelength) Quadruple-wavelength laser laser emission emission output output for distilled for distilled water waterimmersion immersion (RIU 1.332);(RIU (1.332);b) Stability (b) analysisStability foranalysis refractive for index refracti surroundingve index alterationsurrounding (wavelength alteration (wavelength(red) and power (red) (blue)).and power (blue)).

By considering the total wavelength shifting, the following tuning parameters can be estimated: 15.71 nm/RIU (P1) 14.33 nm/RIU (P2), 16.13 nm/RIU (P4), and 13.77 nm/RIU (P5) (Figure 6b). Moreover, the following high linear wavelength shifting responses were presented: R2 = 0.97609 (P1), R2 = 0.98706 (P2) R2 = 0.98426 (P3), and R2 = 0.8756 (P4) (Figure 6b). It was also observed that the peaks had different dynamic ranges; for instance, the P5laser peak disappeared for refractive index variations exceeding 1.3476, while, P2 and P4 were present in almost all the refractive index ranges.

P P P P 1 2 4 5 1544 -30 1.3323 1.3446 1.3539 1540 S =15.71 nm/RIU 1.3606 P1 P1 -45 S =14.33 nm/RIU 1.3753 P2 P2 1536 P4 S =16.13 nm/RIU P5 P4 S =13.77 nm/RIU -60 P5 1532 Wavelength(nm) Output Power (dBm) -75 1528

1528 1532 1536 1540 1544 1.34 1.35 1.36 1.37 1.38 Wavelength (nm) Refractive Index (RIU) (a) (b)

Figure 6. Refractive index response: (a) Laser emission response for refractive index ranging from 1.3323 to 1.3757 RIU; (b) Refractive index data analysis for lasing modes observed.

The dynamic range variation is strongly related to the refractive index interference pattern response of the MFI. Here, when the refractive index was applied, the interference pattern underwent a wavelength shift, as well as a visibility decrement [41]; as a result, some peaks start to disappear as the visibility is reduced. These responses were strongly related to the frequencies generated by the MFI (Figure 2b), some of these frequencies disappeared as the refractive index increased [41]; hence, the lasing modes obtained had different dynamic ranges. The P2 peak was selected for the loop hysteresis analysis because of its minimal power and wavelength fluctuations (Figure 7). The lower hysteresis can be attributed to features such as room temperature changes, pumping diode fluctuations, and birefringence changes induced by optical fiber bending. Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11

n=1.3323 n=1.3323

-20 1544 P5 -16 -13dB P4 -10dB P5 1540 -24 -40 37dB P2 31dB 1536 -32 -60 P1 1532 P4 -40 Wavelength (nm) Wavelength Output Power (dBm) P2 1528 -48 Output Power (dBm) -80 P P P1 Appl. Sci. 2018P, 8,P 1337P 4 5 6 of 11 1 2 3 1528 1532 1536 1540 1544 0 1530456075 Different water-glycerinWavelength mixtures (nm) were used to increase the liquid refractiveTime (min) index around the MFI; their values were(a estimated) using a Refracto 30GS refractometer. The(b refractive) index ranged from 1.3323 to 1.3799 RIU; the minimal variation between each sample was 0.003 RIU. When the liquid Figurerefractive 5. Refractive index was index increased response: around (a) theQuadruple-wavelength MFI, the laser response laser was emission consistently output altered for distilled across waterthe wavelengthimmersion range(RIU shown1.332); in (b Figure) Stability6a. It isanalysis important for to refracti stress that,ve index typically, surrounding multi-wavelength alteration (wavelengthfiber laser arrangement (red) and power exhibit (blue)). switched lasing mode when any physical effect perturbs the optical fiber filter. However, our experimental results showed that the entire spectrum displayed a constant Bywavelength considering shift the to atotal longer wavelength wavelength shifting, when linear the refractive following index tuning increments paramete werers applied. can be estimated: 15.71 nm/RIUBy considering (P1) 14.33 the nm/RIU total wavelength (P2), 16.13 shifting, nm/R theIU following (P4), tuningand 13.77 parameters nm/RIU can be(P5) estimated: (Figure 6b). Moreover,15.71 the nm/RIU following (P1) 14.33high nm/RIUlinear wavelength (P2), 16.13 nm/RIUshifting responses (P4), and 13.77 were nm/RIU presented: (P5) R (Figure2 = 0.976096b). (P1), Moreover, the following high linear wavelength shifting responses were presented: R2 = 0.97609 (P1), R2 = 0.98706 (P2) R2 = 0.98426 (P3), and R2 = 0.8756 (P4) (Figure 6b). It was also observed that the peaks R2 = 0.98706 (P2) R2 = 0.98426 (P3), and R2 = 0.8756 (P4) (Figure6b). It was also observed that the had differentpeaks had dynamic different dynamicranges; ranges;for instance, for instance, the theP5laser P5laser peak peak disappeareddisappeared for for refractive refractive index index variationsvariations exceeding exceeding 1.3476, 1.3476, while, while, P2 P2 and and P4 P4 were were presentpresent in in almost almost all theall the refractive refractive index index ranges. ranges.

1528 1532 1536 1540 1544 1.34 1.35 1.36 1.37 1.38 Wavelength (nm) Refractive Index (RIU) (a) (b)

FigureFigure 6. Refractive 6. Refractive index index response: response: (a) ( aLaser) Laser emission emission responseresponse for for refractive refractive index index ranging ranging from from 1.3323 1.3323to 1.3757 to 1.3757 RIU; RIU;(b) Refractive (b) Refractive index index data data analysis analysis for for lasing lasing modes modes observed. observed.

The dynamicThe dynamic range range variation variation is isstrongly strongly related related totothe the refractive refractive index index interference interference pattern pattern responseresponse of the of MFI. the MFI.Here, Here, when when the the refractive refractive index index was applied,applied, the the interference interference pattern pattern underwent underwent a wavelength shift, as well as a visibility decrement [41]; as a result, some peaks start to disappear a wavelength shift, as well as a visibility decrement [41]; as a result, some peaks start to disappear as as the visibility is reduced. These responses were strongly related to the frequencies generated by the visibilitythe MFI is (Figure reduced.2b), someThese of responses these frequencies were st disappearedrongly related as the to refractive the freq indexuencies increased generated [ 41]; by the MFI (Figurehence, the2b), lasing some modes of these obtained frequencies had different disappeared dynamic ranges. as the The refractive P2 peak index was selected increased for the [41]; loop hence, the lasinghysteresis modes analysis obtained because had of different its minimal dynamic power and ranges. wavelength The fluctuationsP2 peak was (Figure selected7). The for lower the loop hysteresishysteresis analysis can bebecause attributed of toits features minimal such power as room and temperature wavelength changes, fluctuations pumping diode (Figure fluctuations, 7). The lower hysteresisand birefringencecan be attributed changes inducedto features by optical such fiber as bending. room temperature changes, pumping diode fluctuations, and birefringence changes induced by optical fiber bending. Appl. Sci. 2018, 8, 1337 7 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11

1529.6 P2 RI Increment P2 RI Decrement Appl. Sci. 2018, 8, x FOR PEER REVIEW Increment 7 of 11 Decrement 1529.4 1529.6 P2 RI Increment P2 RI Decrement Increment Decrement 1529.21529.4 Wavelength (nm) Wavelength

1529.2 1529.0 Wavelength (nm) Wavelength 1.34 1.35 1.36 1.37 1529.0 Refractive Index (RIU) 1.34 1.35 1.36 1.37 Figure 7. Loop hysteresis analysis forRefractive refractive Index index (RIU) variation using the P2 peak. Figure 7. Loop hysteresis analysis for refractive index variation using the P2 peak. Figure 7. Loop hysteresis analysis for refractive index variation using the P2 peak. 3.2. Temperature3.2. Temperature Analysis Analysis To3.2. analyze Temperature the Analysis temperature effects in the ﬁber laser, the MFI was set over a hotplate, and the To analyze the temperature effects◦ in◦ the fiber laser, the MFI was set over a hotplate, and the temperatureTo wasanalyze varied the temperature from 90 C toeffects 210 inC. the This fiber range laser, responded the MFI was to the set cross-sensitivitiesover a hotplate, and presented the temperature was varied from 90 °C to 210 °C. This range◦ responded to the cross-sensitivities by thetemperature MFI [41], whichwas varied requires from temperatures90 °C to 210 °C. over This 100 rangeC to responded recognize to thermal the cross-sensitivities effects. The laser presentedspectrumpresented by the at 90 MFI by◦C the was[41], MFI comprised which [41], which requires of requires triple-laser temperatur temperatur emissions.eses over over For 100 100 P2 °C and °Cto recognize to P3, recognize the spacingthermal thermal effects. mode betweenThe effects. The laserthem spectrumlaser was spectrum around at 90 1.6nm°C at 90was °C and compriwas separated comprisedsed of 13.23 oftriple-laser triple-laser nm from aemissions.emissions. new laser For emission, For P2 andP2 P3,and P6, the whichP3, spacing the was spacing mode centered mode betweenat 1543.33 thembetween nm.was them Thearound was laser around 1.6nm emissions 1.6nm and hadand separated separated the following 13.23 13.23 nm SMSR:nm from from 37 a new dBa new (P2),laser laser emission, 41 dB emission, (P3), P6, andwhich P6, 43 was dBwhich (P6). was centeredTheir atcentered peak 1543.33 wavelengths at 1543.33 nm. The nm. were laser The −laser emissions23.40 emissions dBm had (P2), had th −th19.19e following dBm SMSR: (P3), SMSR: and 37 dB 37− 17.04(P2), dB 41(P2), dBm dB (P3),41 (P6). dB and The(P3), 43 ﬁber and 43 dB (P6). Their peak wavelengths were −23.40 dBm (P2), −19.19 dBm (P3), and −17.04 dBm (P6). The dB (P6).laser’s Their response peak to wavelengths temperature canwere be seen−23.40 in FiguredBm 8(P2),. The − increase19.19 dBm in temperature (P3), and produces−17.04 dBm a shifting (P6). The fiber laser’s response to temperature can be seen in Figure 8. The increase in temperature produces a response in the multi-wavelength. However, at 145 ◦C, the laser spectrum becomes a quadruple-laser fiber laser’sshifting response response to temperaturein the multi-wavelength. can be seen Howe in Figurever, at 8.145 The °C, increasethe laser inspectrum temperature becomes produces a a shiftingemission; responsequadruple-laser this new in spectrumthe emission; multi-wavelength. isthis comprised new spectrum of P2,Howe is P3, comp P5,ver,rised and at of P6. 145P2, As P3,°C, we P5, increasethe and laserP6. theAs spectrumwe temperature, increase thebecomes some a quadruple-laserpeaks,temperature, such as emission; P2, some P5, andpeaks, this P6 suchnew continue as spectrum P2, P5, shifting and isP6 tocomp cont longerinuerised shifting wavelengths. of P2, to longer P3, AtP5, wavelengths. the and same P6. time,As At thewe other same increase laser the emissions start to appear, such as P1 and P0 (wavelength centered at 1528.86 nm (P0) and 1530.33 nm temperature,time, some other laserpeaks, emissions such asstart P2, to P5,appear, and such P6 contas P1inue and P0 shifting (wavelength to longer centered wavelengths. at 1528.86 nm (P0)At the same (P1), peakand 1530.33 wavelength nm (P1),− peak23.08 wavelength dBm (P0) and−23.08− 20.02dBm (P0) dBm and (P1), −20.02 and dBm SMSR (P1), of and 38 dBSMSR (P0) of and38 dB 41 (P0) dB (P1) time, other laser emissions start to appear, such as P1 and P0 (wavelength centered at 1528.86 nm (P0) at 210and◦C). 41 dB (P1) at 210 °C). and 1530.33 nm (P1), peak wavelength −23.08 dBm (P0) and −20.02 dBm (P1), and SMSR of 38 dB (P0) and 41 dB (P1) at 210 °C).

Figure 8. Laser emission response for temperature variation from 90 ◦C to 210 ◦C. Figure 8. Laser emission response for temperature variation from 90 °C to 210 °C.

Figure 8. Laser emission response for temperature variation from 90 °C to 210 °C. Appl. Sci. 2018, 8, 1337 8 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11

InIn analyzing analyzing peaks peaks P2, P2, P3, P3, and and P6, P6, the the following following thermal-wavelength thermal-wavelength relations relations can can be be expected: expected: ◦ ◦ ◦ 21.0121.01 pm/°C pm/ C (P2), (P2), 16.99 16.99 pm/°C pm/ C (P3), (P3), and and 16.97 16.97 pm/°C pm/ C(P6), (P6), as as well well as as adequate adequate linear linear responses responses of of 0.963060.96306 (P2), 0.995070.99507 (P3), (P3), and and 0.9890 0.9890 (P6). (P6). As canAs can be observed be observed in Figure in Figure9 the dynamic 9 the dynamic range is differentrange is differentfor each peak;for each however, peak; however, the temperature the temperature variations variations generated generated different different triple or triple quadruple or quadruple spectra. spectra.The tunable The multi-wavelengthtunable multi-wavelength effect is only effect presented is only inpresented the range in from the range 90 ◦C tofrom 125 90◦C. °C According to 125 °C. to Accordingthe MFI temperature’s to the MFI analysis, temperature’s the visibility analysis, remained the visibility almost constant remained and almost only presented constant wavelength and only presentedshifting. As wavelength a result, the shifting. multi-wavelength As a result, the shifting multi-wavelength range was shorter shifting than range the was one presentedshorter than in the the onerefractive presented index in the analysis. refractive The index appearance analysis. of The other appearance lasing modes of other was lasing evident modes as the was temperature evident as theincreased temperature because increased of the modal because competitions of the modal was competitions similar for was all the similar lasing for modes all the involved. lasing modes It is involved.well recognized It is well that recognized ﬁber-optic interferometersthat fiber-optic exhibit interferometers a suitable sensitivityexhibit a suitable for refractive sensitivity index andfor refractivetemperature index variations and temperature [44]; however, variations the ring laser[44]; cavityhowever, represents the ring an alternativelaser cavity method represents to extract an alternativephase information method [to1– extract9]. phase information [1–9].

FigureFigure 9. 9. TemperatureTemperature data data analysis analysis for for peaks peaks P0 P0 to to P6. P6. Temperature-wavelength Temperature-wavelength response response for for peaks peaks P2,P2, P3, P3, and and P6. P6.

4.4. Discussion Discussion BasedBased on on the the results, results, we we identified identified two two simple simple methods methods to to tune tune multi-wavelength multi-wavelength laser laser spectra spectra generatedgenerated by by a a modal modal fiber-optic fiber-optic interferometer. interferometer. Most Most of of the the features features observed observed in in the the laser laser response response dependdepend onon thethe modal modal interferometer; interferometer; this this one wasone fabricatedwas fabricated using ausing cost-effective a cost-effective technique technique involving involvinginexpensive inexpensive fibers (conventional fibers (conventional single-mode single-mode fiber and fiber microfiber). and microfiber). In this In technique, this technique, the fiber the fiberstructure structure acts as acts a fiber as a filter fiber into filter the into erbium the ringerbium laser ring cavity. laser The cavity. initial The laser initial spectrum laser isspectrum comprised is comprisedof four lasing of four lines lasing (a triple lines consecutive(a triple consecut laserive emission laser emission and one and single-lasing one single-lasing line), whereline), where lower lowerwavelengths wavelengths drift (0.008 drift nm)(0.008 and nm) minimal and minimal power variations power variations of 0.46 dB of were 0.46 observed.dB were observed. When the When modal theinterferometer modal interferometer is immersed is immersed in distilled in water distilled (initial water refractive (initial indexrefractive value), index we value), observed we four observed lasing fourmodes: lasing two modes: dual laser two emissions dual laser separated emissions 13.2 separa nm,ted with 13.2 a spacingnm, with mode a spacing of 1.6 nm,mode and of wavelengths1.6 nm, and wavelengthsvariations lower variations than 0.1nm. lower Whenthan 0.1nm. the refractive When the index refractive is increased, index the is increased, entire quad-laser the entire spectrum quad- laseris tuned, spectrum resulting is tuned, in a tunable resulting resolution in a tunable of 0.05 re nm.solution According of 0.05 to nm. the hysteresisAccording analysis, to the hysteresis this laser analysis,offers minimal this laser wavelength offers minimal variation wavelength when it is operated variation in when a forward it is and operated backward in a refractive forward indexand backwarddirection. refractive Besides, accordingindex direction. to the Besides, results theaccord refractiveing to the index results variations the refractive generate index a 2nm variations tuned generaterange, this a 2nm range tuned can berange, compared this range with can other be works compared [33,45 with,46]. other Here, works it is important [33,45,46]. to Here, stress it that is importantdespite the to higher stress tuningthat despite range th previouslye higher tuning reported range [27 previously,29,32,33,45 reported–49], some [27,29,32,33,45–49], works present irregular some worksspectra, present strong irregular relation spectra, between strong the number relation lasing between modes the andnumber the tunedlasing step,modes lower and resolution,the tuned step,and higherlower resolution, wavelength and drift. higher The wavelength temperature drif responset. The temperature shows tunable response multi-wavelength shows tunable responses multi- wavelengthfor different responses multi-wavelength for different laser multi-wavelength spectra; the resolution laser spectra; is similar the to resolution that observed is similar in refractive to that ° observedindex analysis. in refractive However, index temperature analysis. Howeve incrementsr, temperature must exceed increments 90 ◦C; minimal must exceed temperature 90 C; variations minimal temperature variations will result in lower resolutions. According to the results and considering the high sensitivity of the interferometers [44], the proposed scheme can also be an alternative method Appl. Sci. 2018, 8, 1337 9 of 11 will result in lower resolutions. According to the results and considering the high sensitivity of the interferometers [44], the proposed scheme can also be an alternative method to detect temperature and refractive indexes using a signal acquisition system, which, based on a near-infrared photodetector, can reduce the cost for sensing applications. Moreover, the fiber laser arrangement can be used for remote sensing applications. Despite the power instabilities observed, the laser offers better performance than some works [14,18]. The laser presented in this work is an alternative, cost-effective setup for tuning multi-wavelength spectra using simple and easy-to-control techniques.

Author Contributions: Conceptualization, Y.L.-D. and D.J.-V.; Methodology, D.J.-V. and J.M.S.-H.; Validation, J.M.E.-A., and J.C.H.-G.; Formal Analysis, D.F.G.-M., D.J.-V., Y.L.-D. and L.A.H.-P.; Investigation, Y.L.-D.; Resources, J.M.E.-A., and R.R.-L.; Data Curation, Y.L.-D., E.G.-A.; Writing-Original Draft Preparation, D.F.G.-M., D.J.-V.; Writing-Review & Editing, D.J.-V.; Visualization, D.J.-V., J.M.S.-H., Y.L.-D. and L.A.H.-P.; Supervision, J.M.E.-A., and R.R.-L.; Project Administration, J.M.E.-A., and R.R.-L.; Funding Acquisition, J.M.E.-A., and D.J.V. Funding: This research was funded by CONACYT, grant number [577494/307127], and the Dirección de Apoyo a la Investigación y al Posgrado [UGTO-DAIP CIIC 283/2018]. Acknowledgments: We would like to thank to the National Lab UG for the micrographs. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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