46 OPTICS LETTERS / Vol. 34, No. 1 / January 1, 2009

Compact Brillouin–erbium fiber laser

S. W. Harun,1,* S. Shahi,2 and H. Ahmad2

1Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia2Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia

*Corresponding author: swharun@um.edu.my

Received October 7, 2008; revised November 8, 2008; accepted November 11, 2008;posted November 24, 2008 (Doc. ID 102444); published December 23, 2008

A single-wavelength Brillouin fiber laser (BFL) is demonstrated at the extended L-band region usingbismuth-based erbium-doped fiber (Bi-EDF) for the first time to the best of our knowledge. A 2.15-m-longBi-EDF is used to provide both nonlinear and linear gains to generate a stimulated Brillouin scattering(SBS) and to amplify the generated SBS, respectively. The BFL operates at 1613.93 nm, which is upshiftedby 0.09 nm from the Brillouin pump with a peak power of 2 dBm and a side-mode suppression ratio of morethan 22 dB. The generated BFL has a narrow linewidth and many potential applications, such as in opticalcommunication and sensors. © 2008 Optical Society of America

OCIS codes: 190.4370, 290.5900.

Stimulated Brillouin scattering (SBS) is a nonlineareffect arising from the interaction between the in-tense pump light and acoustic waves in a medium,giving rise to backward-propagating, frequency-shifted light [1]. The thermally excited acousticwaves generate an index grating that copropagateswith the pump light at the acoustic velocity in themedium. This moving grating reflects the pump lightand causes a frequency downshift in the backscat-tered light owing to the Doppler effect. The frequencyshift with respect to the pump is approximately0.08 nm at the 1550 nm region for silica fibers. Al-though this scattering creates problems for somenonlinear signal processing applications that involveusing a strong cw pump [2,3], SBS can, however, beused for amplification of light propagating in a direc-tion opposite to the pump light. This has led to manyapplications, such as in Brillouin amplifiers, lasers,and microwave signal processors [4,5].

Single-wavelength and multiwavelength lasers canalso be achieved using a hybrid Brillouin–erbium fi-ber laser (BEFL) [6,7], which has recently become atopic of extensive study due to its potential applica-tions in instrument testing and sensing, and as opti-cal sources for dense wavelength division multiplex-ing (DWDM) systems. In these applications involvingthe SBS process, it is desirable to have a mediumthat has a large Brillouin gain coefficient gB to lowerthe power requirements and also to shorten thelength of fiber devices. In an earlier work, a compactBrillouin fiber laser (BFL) has been demonstrated us-ing a chalcogenide fiber, which has the gB coefficientof about 2 orders of magnitude larger than that ofsilica-based fibers [8]. However, the threshold for theBrillouin pump is much larger in this fiber comparedwith a silica fiber.

Recently, a bismuth-based erbium-doped fiber(Bi-EDF) has been extensively studied for use in com-pact amplifiers with short-gain medium lengths. Thisfiber incorporates lanthanum (La) ions to decreasethe concentration quenching of the erbium ions in the

fiber [9], which allows the erbium ion concentration

0146-9592/09/010046-3/$15.00 ©

to be increased to more than 1000 ppm. A fiber withsuch a high erbium dopant concentration is expectedto have enormous potential in realizing a compacterbium-doped fiber amplifiers (EDFAs) and EDFA-based devices. The Bi-EDF has also a very high fibernonlinearity, which can be used for realizing a com-pact BFL. In this Letter, a BEFL is demonstrated un-der a new approach using a Bi-EDF as both linearand nonlinear gains medium for the first time to ourknowledge. The Stokes is generated in the Bi-EDF bythe injection of a narrow linewidth Brillouin pump(BP). With the use of 1480 nm pumping on theBi-EDF, the generated Brillouin Stokes is amplifiedto generate a BEFL.

A configuration of the BEFL is shown in Fig. 1,which consists a circulator, a 2.15-m-long Bi-EDF,and an output coupler. The Bi-EDF has a nonlinearcoefficient of 60 �W km�−1, an erbium concentration of3200 ppm, a cut-off wavelength of 1440 nm, and apump absorption rate of 130 dB/m at 1480 nm. It isforward pumped using a 1480 nm laser diode to pro-vide a linear gain in a long wavelength (L-band) re-gion ranging from 1560 to 1615 nm. A 1480/1550 nmWDM is used to combine the 1480 nm pump andL-band oscillating signal in the ring cavity. TheBi-EDF is also pumped by an external-cavity tunablelaser source with a linewidth of approximately20 MHz and a maximum power of approximately5 dBm to generate a nonlinear gain or Stokes, whichis injected into the ring cavity via optical circulator.

Fig. 1. (Color online) Configuration of the proposed BEFL.

2009 Optical Society of America

January 1, 2009 / Vol. 34, No. 1 / OPTICS LETTERS 47

The injected BP generates backward-propagatingBrillouin Stokes, which is amplified by the linearBi-EDF gain and oscillates in the loop to generate aBEFL in a counterclockwise direction. An optical iso-lator is inserted inside the loop to block the BP fromoscillating in the loop. By using a single fiber for thelinear amplification and Brillouin Stokes generation,we are simultaneously amplifying the BP and Bril-louin signal in this cavity. This allows a shorterlength of active fiber to be used for the BEFL genera-tion, which in turn reduces a total cavity loss and in-creases the output power. A 20% output coupler isused to extract BEFL output, which is characterizedusing an optical spectrum analyzer (OSA).

Figure 2 shows a free-running Bi-EDF laser (withno BP) spectrum of the proposed BEFL, which has afew peaks at the wavelength region of 1614 nm. Thespectrum has multiple peaks because, upon satura-tion, gain competition enables neighboring wave-lengths to acquire a net gain to oscillate, made pos-sible by the inhomogeneously broadened gainmedium. The Bi-EDF laser operates at the extendedL-band region owing to the use of bismuth glass as ahost material of the fiber, which is able to extend theamplification band to a longer wavelength comparedto the conventional silica-based EDF. This is due tothe vibration energy of the bismuth glass lattice be-ing smaller than that of silica, which contributes tolarger emission and lower excited-state absorption inthe extended L-band region [9]. The small Brillouingain necessitates that the wavelength for the opera-tion of the BEFL be close to that at which the Bi-EDFlaser would operate under a free-running condition.Therefore, the BP signal is launched into the Bi-EDFat a wavelength of 1613.84 nm, which is close to theEDF laser peak gain to generate a BEFL in the ex-tended L-band region.

The performance of the BEFL was studied for dif-ferent 1480 nm pumping power. The BP power isfixed at 5 dBm. Figure 3 shows the output spectrumof the BEFL. As shown in the figure, a BEFL wave-length component at a separation of 0.09 nm at alonger wavelength could be easily observed when the1480 nm pump power exceeded �115 mW, and itgrew by more than 45 dB as the pump power wasraised to 152 mW. As shown in the figure, the1480 nm pump power threshold is approximatelywithin 115 to 125 mW. Below this pump power, theerbium gain is very low and cannot sufficiently com-pensate for the loss inside the laser cavity, and thus

Fig. 2. (Color online) Output spectrum of the Bi-EDF laser

(without BP).

no Stokes is observed. The generated BEFL power isobserved to increase as the 1480 nm pump power in-creases, which is attributed to the increment of theerbium gain with pump power. This situation pro-vides sufficient signal power for SBS to generateStokes, which is then amplified by the erbium gain.At the maximum 1480 nm pump power, the BEFLhas a peak power of 2 dBm and a side-mode suppres-sion ratio (SMSR) of more than 22 dB. The incorpo-ration of both optical isolator and circulator in thecavity ensure the unidirectional operation of the BFLand suppresses the residual BP. This prevents thefour-wave mixing from happening and avoids thegeneration of anti-Stokes. The single-wavelengthBEFL has a very narrow linewidth and low technicalnoise, which makes it suitable for sensing applica-tions.

The inset of Fig. 3 shows the output spectrum ofthe BEFL at various BP wavelengths. In this experi-ment, the BP and 1480 nm pump power are fixed at5 dBm and 152 mW, respectively. As shown in thefigure, the BP can be tuned from 1612 to 1615 nm toobtain the maximum output power of approximately2 dBm. The Brillouin gain coefficient gB for theBismuth fiber is also calculated using the followingwell-known equation [8];

gB =21Aeff

PthKLeff. �1�

Here Pth is power corresponding to the Brillouinthreshold, Aeff is the effective cross-sectional area,Leff is the effective length, and K is the constant. Thepeak Brillouin gain coefficient was determined to be3.9�10−10 m/W (using K=0.5, Aeff=3.08 �m2, Leff=1.01 m, and Pth=3.2 mW), which is so much higherthan in the standard silica fibers.

In conclusion, a single-wavelength BEFL is suc-cessfully demonstrated using only a very short lengthBi-EDF as both the linear and nonlinear gains me-dium. The BEFL is obtained at a wavelength of1613.93 nm with a peak power of 2 dBm and a SMSRof more than 22 dB with the BP and 1480 nm pumppowers of 5 dBm and 152 mW, respectively. The spac-ing between the BP and the Stokes is measured to be

Fig. 3. (Color online) Output spectra of the BFL at various1480 nm pump powers. Inset shows the output spectrum atvarious BP wavelengths.

approximately 0.09 nm.

48 OPTICS LETTERS / Vol. 34, No. 1 / January 1, 2009

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