IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 4, APRIL 2012 443

Quantum Electronics Letters

Electrically Tunable Microfiber Knot ResonatorBased Erbium-Doped Fiber Laser

Azlan Sulaiman, Sulaiman Wadi Harun, Fauzan Ahmad, Siti Fatimah Norizan, and Harith Ahmad

Abstract— A compact and tunable fiber laser is demonstratedusing a microfiber knot resonator structure made by a highlydoped Erbium fiber. A stable laser output is achieved at the1533-nm region with a signal to noise ratio of 15 dB using a63-mW 980-nm pump power. With the assistance of a copper wiretouching the circumference of the ring, operating wavelength ofthe proposed laser can be tuned by injecting electric current intothe copper wire. The peak wavelength of the laser can be tunedfrom 1533.3 to 1533.9 nm as the loading current is increasedfrom 0 to 1.0 A. This is due to the thermally induced opticalphase shift attributable to the heat produced by the flow of thecurrent. It is also shown experimentally that the wavelength shiftis linearly proportional to the square of the amount of currentwith a tuning slope of 700 pm/A2.

Index Terms— Erbium-doped fiber laser, microfiber, microfiberlaser, tapered fiber, tunable laser.

I. INTRODUCTION

M ICROFIBERS have attracted growing interest recentlydue to their interesting optical properties, which can

be used to develop low-cost, miniaturized and all-fiber basedoptical devices for various applications [1]–[3]. For instance,many research efforts have focused on the development ofmicrofiber based optical resonators that can serve as opticalfilters, which have many potential applications in opticalcommunication and sensors [4]–[5]. These devices are verysensitive to a change in the surrounding refractive index dueto the large evanescent field that propagates inside the fiberand thus they can be used in various optical sensors aswell as in tunable filters. Of late, many research works havefocused on microfiber knot resonator (MKR), which can be

Manuscript received October 3, 2011; revised December 17, 2011; acceptedJanuary 11, 2012. Date of publication January 16, 2012; date of current versionFebruary 14, 2012. This work was supported in part by the University ofMalaya under High Impact Research Scheme HIR-MOHE D000009-16001,and MOSTI under the BrainGain Program.

A. Sulaiman and S. W. Harun are with the Department of ElectricalEngineering, Faculty of Engineering, University of Malaya, Kuala Lumpur50603, Malaysia (e-mail: azlan27@gmail.com; swharun@um.edu.my).

F. Ahmad is with the Department of Electrical Engineering, Faculty ofEngineering, University of Malaya, Kuala Lumpur 50603, Malaysia. He isalso with the Department of Electrical Engineering, Faculty of Engineering,University Teknologi Malaysia, Kuala Lumpur 54100, Malaysia (e-mail:fauzan.se@gmail.com).

S. F. Norizan and H. Ahmad are with the Physics Department, PhotonicsResearch Center, University of Malaya, Kuala Lumpur 50603, Malaysia(e-mail: sfnorizan@yahoo.com; harith@yahoo.com).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JQE.2012.2184525

assembled by intertwisting and overlapping two microfibersin the resonator via micro-manipulating process [6]. In com-parison with microfiber loop resonator (MLR), MKR doesnot rely on van der Waals attraction force to maintain thecoupling region yet it can achieve stronger coupling due to therigid intertwisted microfibers structure at the coupling region.The knot structure can withstand strong elastic force of themicrofiber and maintain a rigid resonator structure with a morestable resonance condition.

Of late, there is also a growing interest on ultra-compact fiber lasers, which can be achieved using a highlyconcentrated doped fiber. Micro/nano scale lasers are alsorecently demonstrated using semiconductor nano-wire [7] andlaser dye [8] as the gain medium. In an earlier work, Jianget. al. [1] demonstrated a microfiber laser formed by simplytightening a rare-earth-doped microfiber into a knot. Thistechnique requires fabricating the microfiber twice and effortto couple the silica microfiber on the top of the half circle knotof the doped microfiber, which is complicated. In this paper,a simpler MKR-based laser is demonstrated using a highlyconcentrated Erbium-doped fiber (EDF) as a gain mediumin conjunction with flame brushing technique for microfiberfabrication. Tune-ability of the laser is also demonstrated bycontrolling the heat generated from the DC current flowingin the copper wire which physically touches the ring of theErbium-doped MKR. This is the first demonstration of atunable MKR-based laser.

II. EXPERIMENT

Fig. 1 shows the proposed MKR-based tunable EDF laser,which consists of a piece of highly concentrated EDF withan MKR at one end of the fiber and wavelength divisionmultiplexing (WDM) coupler at the other end. A 980-nmlaser diode was used as a pump, which injected the light intothe EDF via the WDM coupler. A 2 m long EDF with anErbium ion concentration of about 2000 ppm was used as thegain medium. A small section of the fiber about 3 cm longnear one of its ends was stripped. The stripped portion of theEDF was horizontally held by two fiber holders where one ofthem was attached to a motorized translation stage that can bemoved to taper the EDF using flame brushing technique. In theprocess, the position of an oxy-butane burner was controlledautomatically so that it can be moved according to a specificalgorithm to soften the uncoated EDF uniformly while the

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444 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 4, APRIL 2012

Fig. 1. Schematic diagram of the MKR-based EDFL setup with a copperwire touching the outer ring of the MKR.

fiber was being stretched until the waist diameter was reducedto ∼2 μm. It was crucial to distribute the heat from the burnerevenly to avoid producing rough surface on the microfiber sothat the transmission loss of the microfiber can be minimized.

The MKR was assembled by micro-manipulating the fabri-cated microfiber. First, the microfiber was cut into 2 unequalparts, where the longer part of the microfiber was twisted toform a large loop and the end of the microfiber was insertedinside the loop to form a knot. The required loop diameter isobtained by gradually pulling one of the fiber ends. The shorterpart of the microfiber was coupled to the knot by van der Wallsforce to collect the light transmitted out from the knot bymeans of evanescent coupling. At least ∼3 mm of couplinglength between the two microfibers was required to achievesufficient van der Waals attraction force to keep them together.The knot configuration was sustained by elastic-bend-inducedtensile force and friction at the intertwisted area and thusensuring the structural stability of the device. The resonatorcavity was formed by this MKR and a perpendicularly cleavedfibre end at the other end of the laser. Owing to the refractiveindex difference between silica glass and air, approximately3–4 % of light was reflected back into the cavity for laseroscillation. A 6 cm long copper wire with 0.16 cm diameterwhich was connected to adjustable power supply was firmlytouching the outer ring of the MKR with 5 mm diameter. Theoutput of the EDFL was tapped from this side of the laserand characterized by an optical spectrum analyser (OSA) witha resolution of 0.015 nm. The performance of the laser wasinvestigated by varying the DC current from 0 to 2.0 A.

III. RESULT AND DISCUSSION

To pump the 2 m long EDF, light from a semiconductordiode laser operating at a wavelength of 980 nm with alinewidth of 1 nm was launched into the fiber. Due to the MKRat the end of the EDF, lasing action was observed at around1533 nm for knots with diameter 3 to 10 mm. Fig. 2 showsthe typical output spectrum of the MKR-based laser, whichoperates at 1533.3 nm when the MKR diameter and 980-nmpump is fixed at 5 mm and 63 mW, respectively. As shown inthe figure, the peak power and signal to noise ratio (SNR) ofthe laser are obtained at −33.8 dBm and 17 dB, respectively.Inset of Fig. 2 shows the measured ASE spectrum, whichis obtained without the MKR. The ASE with peak power ataround 1530 nm oscillates in the loop to allow some fraction

Fig. 2. Stable laser output generated from the proposed MKR-based EDFlaser. Inset is the ASE spectrum of the 2-m EDF before undergoing taperingprocess.

of the light to be reflected back and oscillate in the laser cavity.The operating wavelength is mainly determined by the EDFgain and cavity loss in the cavity whereby the comb-like peaksin the MKR compete to lase. The output laser is consideredsingle mode since the end part of the MKR is a single modeuntapered fiber. Any generated multimode light in the MKRwill be filtered by the untapered fiber.

The operating wavelength of the proposed MKR-based laseris dependent on the resonance wavelength of the MKR. Theoptical resonance is generated due to recirculation of ASE lightinside the circle which forms the resonant cavity for lasingoperation and the MKR serves as both laser filtering elementand mirror. The tuning characteristic of the proposed laser isinvestigated by monitoring the output spectrum from the OSAwhile increasing the amount of DC current flowing throughthe copper wire. When the current flows through the copperwire, heat is produced in the wire causing the temperatureto change. Since the MKR is in contact with the copper wire,any temperature change will influence the refractive index andthe optical path length of the MKR. Fig. 3 shows the lasingspectrum at different current loading in the copper wire, whichis in contact with the MKR. In the experiment, the DC currentis loaded with different values in the range of 0 to 2.0 A instep of approximately 0.5 A. For each change in the currentvalue, 10s settling time is allocated in order to stabilize thewavelength shift before the spectrum is recorded.

As shown in Fig. 3, the peak wavelength of the laser shiftsto a longer wavelength with the increase in conducting currentof the copper wire. For instance, the peak wavelength can betuned from 1533.3 nm to 1533.9 nm as the loading current isincreased from 0 to 1.0 A. The operating wavelength shifts toa longer wavelength by 693 pm and 1200 pm as the currentincreases to 1 A and 2 A, respectively. It is also observedthat the spectrum return to the original state when the currentsupply is cut off. The extinction ratio remains unchanged atabout 15 dB when the value of the flowing current is below1.0 A. However, when the current is increased to 2.0 A, theextinction ratio reduces to about 12 dB. The reduction ofthe extinction ratio is most probably due to the deteriorationof the Van der Waals bonding, which introduces slip at

SULAIMAN et al.: ELECTRICALLY TUNABLE MKR-BASED ERBIUM-DOPED FIBER LASER 445

Fig. 3. Tuning characteristic of the proposed laser at three different loadingcurrents.

700 pm/A2

Fig. 4. Peak wavelength shift against square of the amount of dc current.

the coupling region as the temperature increases above thethreshold level of 1.0 A.

By injecting current, the copper rod acts as a heating ele-ment and the heat generated leads to instantaneous temperaturerise in the MKR. The temperature variation affects both theeffective refractive index and the loop path length of the MKR,which lead to spectral shift. This is attributed to the thermo-optic coefficient of silica fiber which has a positive value in therange of 2 × 10−6/K to 9.8 × 10−6/K [9]. The added heatalso causes thermal expansion to the MKR which increasethe optical path length. In an earlier work, Lim et. al. [3]demonstrated that the shift of the resonant wavelength of MKRis proportional to the square of the conducting current, whichcan be expressed in the form of

�λres

λres∝ ρ I 2

A(1)

where ρ and A represent the conductor resistivity and the crosssectional area of the conductor rod, respectively. The termρ/A in the above equation is equivalent to the resistance perunit length of the conductor material. The resistivity of thecopper rod is 1.68 × 10−8 �·m. This equation is valid forall position of the copper wire. Fig. 4 shows a plot of thepeak laser wavelength shift against the square of the current.The plot shows a linear relation with a slope efficiency of

700 pm/A2. The tuning slope of the MKR-based laser can befurther increased if conductors with higher resistivity are usedsuch as nichrome, constantan, graphite and others which arecommonly used as heating elements. However, the suitabilityof the elements when integrated with the microfiber or otheropto-dielectric device requires further investigation.

IV. CONCLUSION

A compact and tunable fiber laser is demonstrated using a2 m long EDF where 30 mm of its end section is tapered toconstruct an MKR that acts as a wavelength selective filteras well as a mirror. A stable laser output is achieved at1533-nm region with a signal to noise ratio (SNR) of 15 dBusing a 63 mW of 980-nm pump power. With the assistanceof a copper wire touching the circumference of the ring, theoperating wavelength of the proposed laser can be tuned byinjecting electric current into the copper wire. The tuningability is due to the thermally induced optical phase shiftattributable to the heat produced by the flow of the current.The peak wavelength of the laser can be tuned from 1533.3 nmto 1533.9 nm as the loading current increases from 0 to 1.0 A.The wavelength shift is linearly proportional to square of theamount of current with a tuning slope of 700 pm/A2.

REFERENCES

[1] X. Jiang, Q. Yang, G. Vienne, Y. Li, L. Tong, J. Zhang, and L. Hu,“Demonstration of microfiber knot laser,” Appl. Phys. Lett., vol. 89,no. 14, pp. 143513-1–143513-3, Oct. 2006.

[2] M. Sumetsky, Y. Dulashko, J. M. Fini, A. Hale, and D. J. DiGiovanni,“The microfiber loop resonator: Theory, experiment, and application,”J. Lightw. Technol., vol. 24, no. 1, pp. 242–250, Jan. 2006.

[3] K. S. Lim, S. W. Harun, S. S. A. Damanhuri, A. A. Jasim, C. K. Tio,and H. Ahmad, “Current sensor based on microfiber knot resonator,”Sensors Actuat. A, vol. 167, no. 1, pp. 60–62, May 2011.

[4] Y. Jung, G. Brambilla, and D. J. Richardson, “Optical microfiber couplerfor broadband single-mode operation,” Opt. Exp., vol. 17, no. 7, pp.5273–5278, 2009.

[5] Y. Wu, Y.-J. Rao, Y. Chen, and Y. Gong, “Miniature fiber-optic temper-ature sensors-based on silica/polymer microfiber knot resonators,” Opt.Exp., vol. 17, no. 20, pp. 18142–18147, 2009.

[6] Z. Chen, V. K. S. Hsiao, X. Li, Z. Li, J. Yu, and J. Zhang, “Opticallytunable microfiber-knot resonator,” Opt. Exp., vol. 19, no. 15, pp. 14217–14222, 2011.

[7] Q. Yang, X. Jiang, X. Guo, Y. Chen, and L. Tong, “Hybrid structurelaser based on semiconductor nanowires and a silica microfiber knotcavity,” Appl. Phys. Lett., vol. 94, no. 10, pp. 101108-1–101108-3, Mar.2009.

[8] X. Jiang, Q. Song, L. Xu, J. Fu, and L. Tong, “Microfiber knot dye laserbased on the evanescent-wave-coupled gain,” Appl. Phys. Lett., vol. 90,no. 23, pp. 233501-1–233501-3, Jun. 2007.

[9] J. M. Jewell, “Thermooptic coefficients of some standard referencematerial glasses,” J. Amer. Ceramic Soc., vol. 74, no. 7, pp. 1689–1691,Jul. 1991.

Azlan Sulaiman received the B.E. degree in elec-trical engineering from the University of Malaya,Kuala Lumpur, Malaysia, in 1999. He is currentlypursuing the Ph.D. degree with the same university.

He is a Senior Researcher with Telekom MalaysiaResearch and Development, Kuala Lumpur. His cur-rent research interests include electronic and fiberoptic devices.

446 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 4, APRIL 2012

Sulaiman Wadi Harun received the B.E. degree inelectrical and electronics system engineering fromthe Nagaoka University of Technology, Nagaoka,Japan, and the M.Sc. and Ph.D. degrees in photon-ics from the University of Malaya, Kuala Lumpur,Malaysia, in 1996, 2001, and 2004, respectively.

He is currently a Full Professor with the Facultyof Engineering, University of Malaya. His currentresearch interests include fiber optic active andpassive devices.

Fauzan Ahmad received the B.E. degree in mecha-tronics from Universiti Teknologi Malaysia, ShahAlam, Malaysia, and the M.Eng.Sc. degree in signalprocessing from the University of Malaya, KualaLumpur, Malaysia, in 1999 and 2007, respectively.He is currently pursuing the Ph.D. degree in photon-ics with the Department of Electrical Engineering,University of Malaya.

His current research interests include tapered fiber-based optical sensors.

Siti Fatimah Norizan received the B.Sc. degreein computational and electronic physics from theUniversity of Malaya, Kuala Lumpur, Malaysia, in2008. She is currently pursuing the Ph.D. degreewith the Physics Department, Photonics ResearchCenter, University of Malaya.

Harith Ahmad received the Ph.D. degree in laser technology from theUniversity of Swansea, Swansea, U.K., in 1983.

He is a Full Professor with the Department of Physics, University ofMalaya, Kuala Lumpur, Malaysia.

Dr. Ahmad is a fellow of the Malaysian Academic of Science.