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OPTICS LETTERS / Vol. 34, No. 1 / January 1, 2009
Compact Brillouin–erbium fiber laser S. W. Harun,1,* S. Shahi,2 and H. Ahmad2 1
Department of Electrical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia *Corresponding author:
[email protected]
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 using bismuth-based erbium-doped fiber (Bi-EDF) for the first time to the best of our knowledge. A 2.15-m-long Bi-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 upshifted by 0.09 nm from the Brillouin pump with a peak power of 2 dBm and a side-mode suppression ratio of more than 22 dB. The generated BFL has a narrow linewidth and many potential applications, such as in optical communication and sensors. © 2008 Optical Society of America OCIS codes: 190.4370, 290.5900.
Stimulated Brillouin scattering (SBS) is a nonlinear effect arising from the interaction between the intense pump light and acoustic waves in a medium, giving rise to backward-propagating, frequencyshifted light [1]. The thermally excited acoustic waves generate an index grating that copropagates with the pump light at the acoustic velocity in the medium. This moving grating reflects the pump light and causes a frequency downshift in the backscattered light owing to the Doppler effect. The frequency shift with respect to the pump is approximately 0.08 nm at the 1550 nm region for silica fibers. Although this scattering creates problems for some nonlinear signal processing applications that involve using a strong cw pump [2,3], SBS can, however, be used for amplification of light propagating in a direction opposite to the pump light. This has led to many applications, such as in Brillouin amplifiers, lasers, and microwave signal processors [4,5]. Single-wavelength and multiwavelength lasers can also be achieved using a hybrid Brillouin–erbium fiber laser (BEFL) [6,7], which has recently become a topic of extensive study due to its potential applications in instrument testing and sensing, and as optical sources for dense wavelength division multiplexing (DWDM) systems. In these applications involving the SBS process, it is desirable to have a medium that has a large Brillouin gain coefficient gB to lower the power requirements and also to shorten the length of fiber devices. In an earlier work, a compact Brillouin fiber laser (BFL) has been demonstrated using a chalcogenide fiber, which has the gB coefficient of about 2 orders of magnitude larger than that of silica-based fibers [8]. However, the threshold for the Brillouin pump is much larger in this fiber compared with a silica fiber. Recently, a bismuth-based erbium-doped fiber (Bi-EDF) has been extensively studied for use in compact amplifiers with short-gain medium lengths. This fiber incorporates lanthanum (La) ions to decrease the 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 with such a high erbium dopant concentration is expected to have enormous potential in realizing a compact erbium-doped fiber amplifiers (EDFAs) and EDFAbased devices. The Bi-EDF has also a very high fiber nonlinearity, which can be used for realizing a compact BFL. In this Letter, a BEFL is demonstrated under a new approach using a Bi-EDF as both linear and nonlinear gains medium for the first time to our knowledge. The Stokes is generated in the Bi-EDF by the injection of a narrow linewidth Brillouin pump (BP). With the use of 1480 nm pumping on the Bi-EDF, the generated Brillouin Stokes is amplified to 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 nonlinear coefficient of 60 共W km兲−1, an erbium concentration of 3200 ppm, a cut-off wavelength of 1440 nm, and a pump absorption rate of 130 dB/ m at 1480 nm. It is forward pumped using a 1480 nm laser diode to provide a linear gain in a long wavelength (L-band) region ranging from 1560 to 1615 nm. A 1480/ 1550 nm WDM is used to combine the 1480 nm pump and L-band oscillating signal in the ring cavity. The Bi-EDF is also pumped by an external-cavity tunable laser source with a linewidth of approximately 20 MHz and a maximum power of approximately 5 dBm to generate a nonlinear gain or Stokes, which is 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
The injected BP generates backward-propagating Brillouin Stokes, which is amplified by the linear Bi-EDF gain and oscillates in the loop to generate a BEFL in a counterclockwise direction. An optical isolator is inserted inside the loop to block the BP from oscillating in the loop. By using a single fiber for the linear amplification and Brillouin Stokes generation, we are simultaneously amplifying the BP and Brillouin signal in this cavity. This allows a shorter length of active fiber to be used for the BEFL generation, which in turn reduces a total cavity loss and increases the output power. A 20% output coupler is used to extract BEFL output, which is characterized using an optical spectrum analyzer (OSA). Figure 2 shows a free-running Bi-EDF laser (with no BP) spectrum of the proposed BEFL, which has a few peaks at the wavelength region of 1614 nm. The spectrum has multiple peaks because, upon saturation, gain competition enables neighboring wavelengths to acquire a net gain to oscillate, made possible by the inhomogeneously broadened gain medium. The Bi-EDF laser operates at the extended L-band region owing to the use of bismuth glass as a host material of the fiber, which is able to extend the amplification band to a longer wavelength compared to the conventional silica-based EDF. This is due to the vibration energy of the bismuth glass lattice being smaller than that of silica, which contributes to larger emission and lower excited-state absorption in the extended L-band region [9]. The small Brillouin gain necessitates that the wavelength for the operation of the BEFL be close to that at which the Bi-EDF laser would operate under a free-running condition. Therefore, the BP signal is launched into the Bi-EDF at a wavelength of 1613.84 nm, which is close to the EDF laser peak gain to generate a BEFL in the extended L-band region. The performance of the BEFL was studied for different 1480 nm pumping power. The BP power is fixed at 5 dBm. Figure 3 shows the output spectrum of the BEFL. As shown in the figure, a BEFL wavelength component at a separation of 0.09 nm at a longer wavelength could be easily observed when the 1480 nm pump power exceeded ⬃115 mW, and it grew by more than 45 dB as the pump power was raised to 152 mW. As shown in the figure, the 1480 nm pump power threshold is approximately within 115 to 125 mW. Below this pump power, the erbium gain is very low and cannot sufficiently compensate for the loss inside the laser cavity, and thus
Fig. 2. (Color online) Output spectrum of the Bi-EDF laser (without BP).
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Fig. 3. (Color online) Output spectra of the BFL at various 1480 nm pump powers. Inset shows the output spectrum at various BP wavelengths.
no Stokes is observed. The generated BEFL power is observed to increase as the 1480 nm pump power increases, which is attributed to the increment of the erbium gain with pump power. This situation provides sufficient signal power for SBS to generate Stokes, which is then amplified by the erbium gain. At the maximum 1480 nm pump power, the BEFL has a peak power of 2 dBm and a side-mode suppression ratio (SMSR) of more than 22 dB. The incorporation of both optical isolator and circulator in the cavity ensure the unidirectional operation of the BFL and suppresses the residual BP. This prevents the four-wave mixing from happening and avoids the generation of anti-Stokes. The single-wavelength BEFL has a very narrow linewidth and low technical noise, which makes it suitable for sensing applications. The inset of Fig. 3 shows the output spectrum of the BEFL at various BP wavelengths. In this experiment, the BP and 1480 nm pump power are fixed at 5 dBm and 152 mW, respectively. As shown in the figure, the BP can be tuned from 1612 to 1615 nm to obtain the maximum output power of approximately 2 dBm. The Brillouin gain coefficient gB for the Bismuth fiber is also calculated using the following well-known equation [8]; gB =
21Aeff PthKLeff
.
共1兲
Here Pth is power corresponding to the Brillouin threshold, Aeff is the effective cross-sectional area, Leff is the effective length, and K is the constant. The peak Brillouin gain coefficient was determined to be 3.9⫻ 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 higher than in the standard silica fibers. In conclusion, a single-wavelength BEFL is successfully demonstrated using only a very short length Bi-EDF as both the linear and nonlinear gains medium. The BEFL is obtained at a wavelength of 1613.93 nm with a peak power of 2 dBm and a SMSR of more than 22 dB with the BP and 1480 nm pump powers of 5 dBm and 152 mW, respectively. The spacing between the BP and the Stokes is measured to be approximately 0.09 nm.
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OPTICS LETTERS / Vol. 34, No. 1 / January 1, 2009
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