Multiwavelength Switchable Fiber Laser Based On A High-Birefringence Fiber Loop Mirror

Recently, with the increasing demand for the information, people expect toreceive the information quickly. The information explosion have promoted theswift and violent growing of global telecommunications business, whereas,the direct result is the phenomenon of the limitless aspiration for thebandwidth which represent telecommunications capacity. With the fastdevelopment of wavelength-division-multiplexed (WDM) systems in opticalcommunications, multiwavelength laser source has become a hot area .The use of semiconductor lasers operating at different wavelengths as theWDM carriers, however, would increase the cost and complexity because theymust be controlled independently. Recently several types of multiwavelengthrare-earth-doped fiber lasers have been reported. People use different kinds ofmethod to get multiwavelength laser, such as Fabry-Perot etalons,[1]wavelength-division multiplexers[2] and fiber loop mirror[3]. In this Letter, wepropose a multiwavelength erbium-doped figure-eight fiber laser based on twoPM fiber loop mirrors for wavelength selection. By using liquid nitrogencooling up to 7 oscillating wavelengths output with 1.9 nm spacing aredemonstrated.TheoryThe theory shown that the reflectivity of a perfect device can be adjustedfrom 0 to 100 percent by controlling the birefringence of the whole fiber loop.A fiber loop reflector is shown in Fig.1(the part in right square). It consistsof a loop of optical fiber between the output port and input port of adirectional coupler. Assume the coupler couples the half of the input powerfrom leg1 to leg3 and couples the other half from leg1 to leg4 (i.e., K=0.5).Light coupled across the waveguides suffers a π/2 phase lag with respect tolight traveling straight through.Fig.1 Experimental setup of multiwavelength fiber laserThe key element in this laser is the PM fiber loop mirror, which serves as awavelength filter. The principle of the PM fiber loop mirror as a filter is asfollows: As shown in Fig.1, the input beam to leg1 is split into twocounter-propagating beams by the 3dB coupler, which recombine at thecoupler after travelling through the fiber loop. As we know , if we use singlemode fiber(SMF) to connect leg 3 and leg4, all light power will reflect fromleg3. When we use Polarization-Maintaining Fibers (PM Fiber) to connect leg3 and leg4, the result will be different. Due to the existence of thebirefringence in PM Fiber, the beams transmitted from the fast axis and slowaxis suffer different optical path length. After coupled into the input port 1,the laser splitted into two lasers, have the equal light intensity. Before theyinterfere on the point of directional coupler the lasers on the fast (slow) axesof ports 3 and 4 suffered same optical path length, after transmitted clockwiseand anticlockwise around the loop. If we place a polarisation controller (PC2)within the PMF loop mirror is set as quarter-wave-plate (QWP) near port 4 (asFig.1) , the result will be different. When the PC2 is set as HWP, means toproduce a pure rotation of 90o in relation to the principal axes of the PM fiberfor beams coming from both directions. The fast-axis-polarised component ofthe anticlockwise beam, after travelling in the PM fiber as the fast mode,becomes slow-axis-polarised by at the exit of PC2,output from port 3. On theother hand, the fast-axis-polarised component of the clockwise beam is firstchanged to slow-axis-polarised by PC2 and then travels in PM fiber as slowmode, output from port 4.The two output slow-axis-polarised components ofthe counter-propagating beams interfere at the point of coupler. Although thetwo components travel through the same length of fiber, they suffer differentoptical path length because they travel as different mode.By using the Jones matrix theory to describe optical characteristic of thefiber loop, we get the reflectivity of the PM fiber loop mirror, namely, whereR (λ) =2K(1?K)[1+COSδφ(λ)] (1)K is the power coupling ratio of the coupler. In this experiment K =0.5 for3dB coupler, L is the length of PM fiber and the ?n is the difference ofrefractive index. Since the reflectivity is the function of λ, and EDF has aquite wide gain bandwidth, generally about 50nm when pumped with 976nmLaser Diode (LD), so we can get several wavelengths oscillatingsimultaneously. We discover that oscillating wavelength spacing is inverseproportioned to the birefringence and the length of PM fiber.Experiment and DiscussThe resonant cavity is composed of two fiber loop mirrors. One of thefiber loop mirrors which make up of PM fiber and a 50:50 coupler serves as awavelength filter. The other fiber loop mirror consists with a polarisationcontroller (PC1), a isolator (ISO), a 10:90 coupler and earth-doped fiber(EDF). In the laser cavity, EDF pumped by a 976-nm pump laser is used asthe gain medium. It is well known that erbium doped fiber is anFig.2. Output optical spectra from fiber laser at 295Khomogeneous gain medium at room temperature, whose homogeneouslinewidth exceeds 10nm. Therefore, we cannot get stable multiwavelengthoscillation at room temperature. The output optical spectrum from the fiberlaser at room temperature (295K) without liquid nitrogen is shown in Fig.2.It is also known that the homogeneous linewidth is reduced as theerbium doped fiber is cooled, and is about 1nm at 77K, so by cooling theerbium doped fiber at 77K with liquid nitrogen, we can expect stablemultiwavelength oscillation. The output from the laser is tapped with a 90:10coupler and send to Optical Spectrum Analyzer(OSA), shown in Fig.3 andFig.4.Fig.3 Output spectra at 77K Fiber loop consist of 3m Δn=4.16x10-4 PM fiberFig.4 Output spectra at 77K Fiber loop consist of 5m Δn=4.31x10-4 PM fiberThe stability is one of the important parameter for the multiwavelengthfiber laser. Fig.5 and Fig.6 shows the scanned output spectrum of laser wherethe birefringence in the fiber loop are 4.16x10-4 and 4.31x10-4. The maximumvariation in the peak powers of the laser modes was measured to be less than1.5 dB through the whole experiment.Fig.5 Repeated scans of output optical spectrum (birefringence is 4.16x10-4)Fig.6 Repeated scans of output optical spectrum (birefringence is 4.31x10-4)To match the channel spacing of 0.8 nm (100 GHz @1550 nm: WDMITU-grid spacing), it is easy to see that the length of PM fiber in our fiber loopis required to be 7m when birefringence is 4.16x10-4.ConclusionIn the proposed fiber laser, stable output laser up to 7 oscillatingwavelengths with 1.9 nm spacing, whose switching can be easily control bythe polarization in fiber loop mirror, is successfully demonstrated. The timestability of the whole system is quite good and the maximum variation ofoutput laser power is less than 1.5dB. The wavelength spacing of the laser canbe varied easily by adjusting the length of the PM fiber. The situation whichmore oscillating wavelengths and smaller wavelength spacing was numericalsimulated.