Voltage-tuned multiwavelength Raman ring laser withhigh tunability based on a single fiber Bragg grating

Young-Geun Han,1 Sang Bae Lee,2 Chang-Seok Kim,3 and Myung Yung Jeong3,*1Department of Physics, Hanyang University, 17 Haengdang-dong, Seongdong-gu,

Seoul 133-791, Korea2Korea Institute of Science and Technology, Sangwolgok-dong, Seongbuk-gu, Seoul, 131-791, Korea

3Department of Nanosystem and Nanoprocess Engineering,Pusan National University, Busan, 609-735, Korea

*Corresponding author: myjeong@pusan.ac.kr

Received 1 July 2008; revised 30 September 2008; accepted 13 October 2008;posted 14 October 2008 (Doc. ID 97859); published 7 November 2008

A practical scheme for a tunable multiwavelength Raman fiber ring laser based on a single fiber Bragggrating with a voltage-controllable coil heater is investigated. The number of phase-shifted regions with-in a single fiber grating determines the number of reflection peaks and the number of lasing wavelengthsin the multiwavelength Raman fiber ring laser. A stable multiwavelength Raman fiber ring laser withlow output peak-power fluctuation of less than 0:5dB at room temperature is achieved. A multiwave-length Raman fiber ring laser with a high extinction ratio of more than 50dB is realized. High flatnessis obtained for three lasing peaks, and the lasing peak-power difference is measured to be less than0:2dB. A voltage-controllable coil heater with heating elements is used to effectively control three lasingwavelengths in the multiwavelength output, and the tunability of each lasing wavelength is measured tobe 0:11nm=V. © 2008 Optical Society of America

OCIS codes: 060.2310, 060.2351, 060.3735.

Multiwavelength fiber lasers have attracted muchattention because of their great potential for variousapplications in wavelength division multiplexed(WDM) communication systems, optical sensor net-work multiplexing schemes, and instrument testing[1–8]. Multiwavelength fiber lasers have a lot of ad-vantages, such as multiwavelength operation, simplestructure, low cost, and low insertion loss. Variousgain media, such as erbium-doped fiber (EDF) ampli-fiers, semiconductor optical amplifiers, and Ramanamplifiers, were implemented to achieve the multi-wavelength lasing operation. To realize stable opera-tion of the multiwavelength EDF laser at roomtemperature, it is necessary to mitigate homogenousline broadening of erbium ions. The technique of cool-ing the EDF down to cryogenic temperature with li-

quid nitrogen and the frequency-shifted feedbacktechnique are widely utilized [1,2]. To generate themultiwavelength output, multichannel filters, suchas delayed Fabry–Perot or polarization-maintainingfiber-based multichannel filters are the importantkey components [3,4]. Recently, fiber Bragg gratings(FBGs) were proposed to achieve the stable multi-wavelength fiber laser with high functionalities suchas wavelength spacing and lasing wavelength tun-ability [5–8]. Since FBGs have a lot of advantages,such as wavelength selectivity and controllability,simple implementation, and high flexibility, speciallydesigned FBGs such as overlap-written FBGs [5],multimode FBGs [6], sampled FBGs [7], few-modefiber gratings [8], fiber gratings with multipleequivalent phase shifts [9], and cascaded long-periodfiber gratings [10], are proposed for the generation ofmultiwavelength output.

In this paper, we propose and experimentally de-monstrate a simple scheme for a voltage-controllable

0003-6935/08/326099-04$15.00/0© 2008 Optical Society of America

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multiwavelength Raman fiber ring laser based on asingle FBG with multiple phase-shifted regions.Compared with the EDF- or semiconductor-optical-amplifier-based multiwavelength fiber lasers, Ra-man-amplifier-based multiwavelength fiber lasershave a lot of advantages, such as stable operationwithout supplementary techniques at room tempera-ture, lasing wavelength flexibility, depending on thepumping wavelength, and a high extinction ratio. Amultiwavelength Raman fiber ring laser with threelasing channels and a high extinction ratio of morethan 50dB is obtained. The peak-power differenceamong three lasing channels is measured to be lessthan 0:2dB. The output power is stable, and thepeak-power fluctuation is measured to be less than0:5dB. We also demonstrate a voltage-controllablemultiwavelength Raman ring laser based on a coilheater. The three lasing wavelengths are flexibly con-trolled by the voltage-controllable coil heater.Figure 1 shows the experimental setup for the

proposed multiwavelength Raman fiber ring laserbased on a single FBG with multiple phase-shiftedregions. The Raman ring laser consists of the 6kmdispersion-compensated fiber (DCF) for the Ramangain medium, a Raman pump with a high pumppower of ∼1:76W at the operating wavelength of1445nm, a WDM coupler, an isolator for the unidir-ectional operation, a polarization controller (PC), aFBG with two phase-shifted regions for multiple la-ser ring cavities, and a 10=90 coupler. A polarizationcontroller was exploited to control the polarizationstate in the laser cavity. The simultaneous multi-wavelength oscillation was generated stably at roomtemperature because of the inhomogeneous nature ofgain broadening of Raman amplification. The laseroutput through the WDM coupler was monitoredby an optical spectrum analyzer (OSA, Ando,6317B) with the resolution of 0:02nm.

Figure 2(a) shows the measured reflection spec-trum of the FBG with two phase-shifted regions.Based on a beam scanning technique, the FBG withthe length of 2 cm was fabricated after exposing aphotosensitive fiber to a frequency-doubled 244nmArþ laser beam through a phase mask. The originalbeam diameter of the frequency-doubled 244nmArþlaser was∼0:6nm, which was controlled by the sphe-rical lens. The multiple phase shifts within the grat-ing were induced by the direct exposure of the FBG tothe frequency-doubled 244nmArþ laser beam with-out a phase mask. The number of lasing wavelengthswas controlled by the number of phase-shifted re-gions (m) within the fiber grating (the number of las-ing wavelength ¼ mþ 1). The amount of phase shiftwas controlled by the photoinduced refractive indexchange with UV irradiance and the loss of the reflec-tion peak was increased to be ∼8:3dB after inducingtwo phase-shifted regions with the fiber grating.Since the FBG has two phase-shifted regions (m ¼2), it has three resonant peaks at 1552.05, 1553.12,and 1554:11nm. The number of channels in the FBGcan be increased by controlling the length, the chirpratio, and the apodization profile of the FBG.

10

90

PC

Raman pump source6 km DCF

OSA

Isolator

FBG with two phase-shifted regions

phase-shifted regions

WDM coupler

Ni-Cr wire

240 µµµµm120 µµµµm

420 µµµµm

Optical fiber

VV

Fig. 1. Experimental setup for the voltage-controllable multi-wavelength Raman fiber ring laser with a single FBG with twophase-shifted regions. The scheme for the voltage-controllable coilheater is shown in the inset.

Fig. 2. (a) Measured reflection spectrum of the FBG with twophase-shifted regions. (b) Output spectrum of themultiwavelengthRaman fiber ring laser with the pump power change at roomtemperature.

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Figure 2(b) shows the measured output spectrumof the proposed multiwavelength Raman fiber ringlaser. The multiwavelength Raman fiber ring laseris readily realized without additional multichannelfilters because the FBG with two phase-shifted re-gions has multiple resonant peaks correspondingto the number and the position of phase-shifted re-gions within the FBG as shown in Fig. 2(b) [11]. Itis clearly evident that the number of lasing channelsis increased further once the multiply phase-shiftedFBG is utilized. The multiwavelength Raman output

was generated at threshold pump power of approxi-mately 470mW. At the pump power of 600mW, threewavelength channels were obtained with an extinc-tion ratio in excess of 50dB. When the pump powerwas increased, the overall spectral width of theRaman Stokes wave broadened, and the peak powerwas not greatly increased, because the higher-orderRaman Stokes wave in the longer wavelength wasgenerated [12]. High flatness of the multiwavelengthoutput was achieved, and the peak-power differenceof each channel was less than 0:2dB. The bandwidthof each channel was less than ∼0:12nm. Figure 3shows the repeatedly scanned output spectra ofthe multiwavelength Raman fiber ring laser. Wemeasured the stability of the laser by monitoringthe output laser with 5min intervals for period of30min. The output power was stable, and thepeak-power fluctuation was less than 0:5dB.

To fabricate the tunable multiwavelength Ramanfiber ring laser, we utilized the voltage-controllablecoil heater. The diameter of the Ni–Cr wire was120 μmas shown in Fig. 1. The outer diameter of eachcoil, spaced 240 μm apart, was 420 μm. A fan coolerabove the coil heater was employed to prevent heataccumulation and thermal diffusion to the other coil.The appropriate temperature distribution along theFBG is induced by the input electric power, and ther-mo-optic and thermal expansion effects on the FBG

Fig. 3. (Color online) Repeatedly scanned output spectra of themultiwavelength Raman fiber ring laser at room temperature.

Fig. 4. (a) Output spectra of the multiwavelength Raman fiber ring laser with the applied voltage from 1 to 7V. (b) Lasing wavelengthshift as a function of the applied voltage. (c) Induced temperature variation in the proposed coil heater as a function of the applied voltage.

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change the characteristics of the FBG. Consequently,the multiple lasing wavelengths can be controlled bythe applied voltage. Figures 4(a) and 4(b) show themeasured output spectra and the lasing wavelengthshift of the proposed multiwavelength Raman fiberring laser, respectively, as the applied voltagechanges in a range from 1 to 7V. Three lasing wave-lengths shifted into a longer wavelength. A linearshift of the three lasing wavelengths was clearlyobserved with the increase of applied voltage. Thetunability was measured to be 0:11nm=V. If the tem-perature sensitivity of the FBG is increased by con-trolling the doping materials or by etching the FBGs,the feasibility of the proposed tuning technique canbe improved by reducing the operating temperature[13–15]. We measured the temperature variation ofthe proposed coil heater with the applied voltageby using a temperature sensor. Figure 4(c) showsthe induced temperature variation of the proposedcoil heater as a function of the applied voltage.The linear fitting curve of the measured temperaturevariation with the applied voltage was estimated tobe 10:56°C=V as shown in Fig. 4(c).In conclusion, we have investigated experimen-

tally a tunable multiwavelength Raman fiber ring la-ser based on a single FBG with multiple phase-shifted regions. Three Raman laser ring cavitieswere configured simply by a single FBG with twophase-shifted regions. The flattened output of themultiwavelength Raman fiber ring laser with a highextinction ratio of more than 50dB was clearlyachieved. The peak-power difference among thethree lasing wavelengths was measured to be lessthan 0:2dB. The number of lasing wavelengthswas controlled by the number of phase-shifted re-gions. The output power of the proposed multi-wavelength Raman fiber ring laser was stable, andthe peak-power fluctuation was measured to be lessthan 0:5dB. The bandwidth of each channel wasmeasured to be less than 0:12nm. Three lasing wave-lengths were effectively controlled by the voltage-controllable coil heater, and the tunability wasmeasured to be 0:11nm=V.

This work was supported by the IT R&D programof the Ministry of Knowledge Economy, Institute forInformation Technology Advancement (MKE/IITA,2008-F-020-01), Korea.

References

1. N. Park and P. F. Wysocki, “24-line multiwavelength operationof erbium-doped fiber-ring laser,” IEEE Photon. Technol. Lett.8, 1459–1461 (1996).

2. A. Bellemare, M. Karasek, M. Rochette, S. LaRochelle, and M.Tetu, “Room temperature multi-frequency erbium-doped fibrelaser anchored on the ITU frequency grid,” J. Lightwave Tech-nol. 18, 825–831 (2001).

3. H. Chen, “Multiwavelength fiber ring lasing by use of a semi-conductor optical amplifier,” Opt. Lett. 30, 619–621 (2005).

4. G. Sun, Y. Chung, Z. Luo, Z. Cai, and C. Ye, “Optimization ofthe multiwavelength erbium-doped fiber laser in a unidirec-tional cavity without isolator,” Opt. Fiber Technol. 13, 198–201 (2007).

5. D. Wei, T. Li, Y. Zhao, and S. Jian, “Multiwavelength erbium-doped fiber ring lasers with overlap-written fiber Bragggratings,” Opt. Lett. 25, 1150–1152 (2000).

6. S. Fu, L. Si, Z. Guo, S. Yuan, Y. Zhao, and X. Dong, “Switchablemultiwavelength ytterbium-doped double-clad fiber laserbased on a multimode fiber grating,” Appl. Opt. 46, 3579–3582 (2007).

7. J. Yang, S. C. Tjin, and N. Q. Ngo, “Multiwavelength activelymode-locked fiber laser with a double-ring configuration andintegrated cascaded sampled fiber Bragg gratings,”Opt. FiberTechnol. 13, 267–270 (2007).

8. Y. G. Han, D. S. Moon, Y. Chung, and S. B. Lee, “Flexibly tun-able multiwavelength Raman fiber laser based on symmetri-cal bending method,” Opt. Express 13, 6330–6335 (2005).

9. Y. Dai, X. Chen, J. Sun, Y. Yao, and S. Xie, “Dual-wavelengthDFB fiber laser based on a chirped structure and the equiva-lent phase shift method,” IEEE Photon. Technol. Lett. 18,1964–1966 (2006).

10. Y. G. Han, C. S. Kim, J. U. Kang, U. C. Paek, and Y. Chung,“Multi-wavelength Raman fiber ring laser based on tunablecascaded long-period fiber gratings,” IEEE Photon. Technol.Lett. 15, 383–385 (2003).

11. F. Bakhti and P. Sansonetti, “Wide bandwidth, low loss andhighly rejective doubly phase-shifted UV-written fibre band-pass filter,” Electron. Lett. 32, 581–582 (1996).

12. C. S. Kim, R. M. Sova, and Jin. U. Kang, “Tunable multi-wavelength all-fiber Raman source using fiber Sagnac loopfilter,” Opt. Commun. 218, 291–295 (2003).

13. J. Mora, A. Díez, J. L. Cruz, and M. V. Andrés, “A magnetos-trictive sensor interrogated by fiber gratings for DC-currentand temperature discrimination,” IEEE Photon. Technol.Lett. 12, 1680–1682 (2000).

14. Y. G. Han, S. B. Lee, C. S. Kim, Jin. U. Kang, U. C. Paek, and Y.Chung, “Simultaneous measurement of temperature andstrain using dual long-period fiber gratings with controlledtemperature and strain sensitivity,” Opt. Express 11, 476–481 (2003).

15. J. Paul, Z. Liping, B. K. A. Ngoi, and F. Z. Ping, “Improvementof thermal sensitivity of FBG sensors by combined claddingetching and polymer coating,” Proc. SPIE 5272, 49–55 (2004).

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