Wavelength-Switching Erbium-Doped Fiber Laser with aMultimode Fiber Bragg Grating and Faraday Rotator

Mirror

Shinyoung Yoon, Youngsoon Heo, * Se-Jong Baik, *Kigeon Im, ** Yong-San Lee and Hyun-Seo KangETRI, Optical Communications Research Center; 1110-6, Oryong-dong, Buk-gu, Gwangju, 500-480, S. Korea

Phone: +82-62-970-6625, Fax: +82-62-970-6989, Email: yoonnoah(i.etri.re.krDept. ofPhysics, Chonnam National University, Gwangju 500-757, KOREA

Dept. ofPhysics, Daejin University, Poch'on-gun, Kyonggi-do,487-711, Korea

Abstract: Without polarization dependent gain, a Multimode Fiber Bragg Grating (MMFBG)-based wavelength-switching erbium-doped fiber laser (EDFL) as a case of optical source inWDM communication system is demonstrated. By varying spatial-launching position of thesingle-mode fiber against the MMFBG, discrete single-wavelength lasing are obtained.

1. IntroductionWidely tunable single-frequency erbium-doped fiber lasers operating in the 1.55pimwavelength region can bepromising sources for applications such as wavelength-division multiplexing (WDM) communication systems, fibersensors, and spectroscopy [1-3]. Multimode fiber (MMF) has a merit of easy coupling with other light sources suchas light-emitting diodes. If Bragg gratings in MMF's are possible and they have useful characteristics, it wouldcontribute to new applications in optical communication and optoelectronics.

In this paper, we newly propose the use of MMFBG for implementing wavelength-switching EDFL used adouble-path forward scheme. It is well known that a uniform MMFBG reflects different fiber modes at differentwavelengths. As a result, on the transverse plane of the multimode fiber, the distribution of the reflected power atdifferent wavelengths is determined by the spatial distribution of the MMF modes. So the wavelength switching canbe achieved by varying the launching position of the single-mode fiber (SMF) against the MMFBG. On the otherhand, by use of optical faraday rotating mirror (FRM) the polarization dependent gain (PDG) is drastically reducedas compared with conventional fiber type mirror.

2. Characterization of Er-doped fiber laser using a double-path forward configuration and MMFBGDouble-path forward configuration for Er-doped fiber laser operation instead of ring type has some advantages in

that providing an experimental setup without optical isolator, polarization controller etc.Fig. 1 shows the schematic of the experimental fiber laser cavity. In this structure, FRM is utilized to reduce the

polarization dependent gain and 980nm laser diode (7OmA driven) as pump source was used.

|Pump | > WDMILaser Diode lc ouple EDF

)+ )( \ ~cou ler4FRM ..\MMFBG

Fig. 1. Schematic of the experimental fiber laser cavity with MMFBG (FRM: Faraday Rotator Mirror, EDF: Erbium-Doped Fiber)

In this demonstration, the length of the EDF(1O.9m of fiber with 2-dB/m peak gain) with 17.1dB/in of absorptioncoefficient was chosen. Fig. 2. shows the degree of polarization dependence of EDFL. For using FRM, meanwavelength variation through the polarization beam splitter, while varying of polarization controller, is 4.7ppm.

1560.701500

1560.65 vrnvaig1560.45

PC.p,p pC..1560.60 ,5 1560.40- varing varing

(>1560.55 6.7 pp> 1560.35-

4.7 ppm 4.7 ppmU)1560.50 55.156ppm

1560.4 1560__ __25_ ___._ ___.___._

040 80 120 160 200 240 0 40 80 120 160 200 240

Number Number

(a) (b)Fig. 2. Polarization dependence of EDF light source (a) with conventional mirror and (b) with FRM

Wvhen using the axial misalignment technique to launch a broad-band source to the MMFBG from a SMF, thespectral response of the MMFBG varies with the axial misalignment between the SMF and the MMFBG. The fiberend face can be controlled by an automatic precision control system which makes 6-axis alignment control possible.Fig. 3. is the graph for fiber laser output spectra of the wavelength switching EDFL when the axial misalignment(offset) of input fiber varies 0 to A12[tm from MMF's center. Its spectra are a little rougher than ring type fiber laseroutput, but it is enough to adopt for method as CWDM light source and sensor system.

10

E -10

20a)

-300

0~

-60

-70C'1540 1545 1550 1555 1560 1565 1570

Wavelength (nm)

Fig. 3. Fiber laser output spectra of the wavelength switching EDFL at different lasing wavelengths

For reference, we'd like to make sure that above spectra were inevitably limited within about lOnm bandwidthbecause of using the 1:9 coupler with 2Onm-band pass filter.

3. ConclusionIn conclusion, a Multimode Fiber Bragg Grating-based wavelength-switching erbium-doped fiber laser as an

example of optical source in WDM system, which is used double path forward configuration, is newlydemonstrated and characterized. As varying spatial-launching position of the input single-mode fiber, discretesingle-wavelength lasing is obtained. We are also trying to do experiment compared with ring laser.

4. References