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Ring cavity multiwavelength Brillouin-erbium fiber laser with a partially reflective fiber Bragg grating Mohammad I. Johari,1 Azin Adamiat,1 Nurul S. Shahabuddin,1 Mohd N. M. Nasir,1 Zulfadzli Yusoff,1,* Hairul A. Abdul Rashid,1 Mohammed H. Al-Mansoori,2 and Pankaj K. Choudhury1 1
Center for Advanced Devices and Systems, Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia 2 Department of Electronics and Communication Engineering, College of Engineering, Universiti Tenaga Nasional, 43009 Selangor, Malaysia *Corresponding author:
[email protected] Received April 17, 2009; revised July 13, 2009; accepted July 13, 2009; posted July 15, 2009 (Doc. ID 110258); published August 4, 2009
Utilization of a partially reflective fiber Bragg grating in the development of a multiwavelength Brillouinerbium fiber laser is proposed. A ring cavity with a preamplified Brillouin pump configuration is implemented to demonstrate much better performance in comparison with the previously reported results obtained with different configurations. The combination of a 7 km long single-mode fiber and a 10 m long erbium-doped fiber ultimately yields the generation of up to twenty-one Brillouin Stokes lines. The laser configuration provides a greater tuning range of 21 nm at a Brillouin pump power of 2 mW and 980 nm pump power of 20 mW, and requires a much lower pump power to initiate lasing. © 2009 Optical Society of America OCIS codes: 140.3510, 290.5900, 060.4370, 140.4480.
1. INTRODUCTION In an attempt of full utilization of wide optical fiber bandwidths, multiwavelength fiber lasers are being recognized as promising optical sources, especially for dense wavelength division multiplexing systems. Interaction between intense pump light and acoustic waves in optical fibers results in the phenomenon of stimulated Brillouin scattering [1], causing the generation of Brillouin Stokes (BS) lines in optical fibers. Such BS lines are essentially of great importance in the development of singlewavelength fiber lasers [2]. By combining the effect that is due to stimulated Brillouin scattering with the usual amplification in erbium-doped fibers (EDFs) in a ring cavity with a reverse-S-shaped fiber section (to feed the BS lines back into the laser cavity), one can achieve a multiwavelength Brillouin-erbium fiber laser (BEFL) [3]. However, the use of two 3 dB couplers on an S-shaped fiber section to tap a portion of the oscillating laser power and reinject into single-mode fiber (SMF) increases the cavity loss. As such, the total output power is observed to be low for this type of BEFL. The insertion of an erbium-doped fiber amplifier (EDFA) in the reverse-S-shaped fiber section serves effectively to enhance the intensity of the laser, but the implementation of two EDFA sections increases the operational complexity in addition to the need for stringent optimization procedures to balance the mode competition between BS lines and self-lasing cavity modes. Moreover, with such configurations, the tuning range of the BEFL also becomes limited [4]. A more robust operation of BEFL over a wider tuning range has recently been reported by the use of a preamplified Brillouin pump (BP) technique [5,6]. In the relevant setups, the injected 0740-3224/09/091675-4/$15.00
BP into the laser cavity is amplified by the EDFA before it enters the SMF; therefore, higher intensity BP and BS lines are generated in the laser cavity. We propose further improvement in the previously reported preamplified BP configuration by utilizing a broadband partially reflecting fiber Bragg grating (FBG) to replace the two 3 dB couplers that construct the reverse-Sshaped fiber section. The partially reflecting FBG functions in the same way as does the reverse-S-shaped fiber section (feeding back a portion of the oscillating laser power into the SMF) but with lower insertion loss. A much better performance, in terms of the number of BS lines, threshold power, maximum output power, slope efficiency, and tuning range is achieved compared with the previously reported BP preamplified BEFL with a reverse-S-shaped fiber section [6].
2. BRILLOUIN-ERBIUM FIBER LASER ARCHITECTURE Figure 1 illustrates the experimental setup of the ring cavity BEFL with the implementation of a partially reflective FBG as a feedback element. In the setup, a 980 nm LD with 200 mW of maximum power was used as the primary pump light for the 10 m long EDF with 440 ppm Er3+ concentration. A wavelength selective coupler (WSC) was used in the experiment to multiplex the pump and the signal lights. The coupling of the BP light into the laser system was achieved by use of a 3 dB coupler (C). An external cavity tunable laser source with a 100 nm tuning range 共1520– 1560 nm兲 and 7.0 dBm maximum pump power was used as the BP light source. For © 2009 Optical Society of America
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Fig. 1. Experimental setup of ring cavity BEFL with a partially reflective FBG.
Fig. 3. Evolution of the BEFL output spectrum corresponding to different pump power values.
high efficiency of BS line generation, the BP linewidth was set narrower than the Brillouin gain spectrum [1]. The Brillouin gain medium was provided by a 7 km long SMF. A broadband partially reflecting FBG with 40 nm bandwidth 共1530– 1570 nm兲 and 50% reflectivity was used in the design to feed a portion of the oscillating laser power back into the SMF. The backreflected portion of the BS line is amplified by the common EDF section to increase the intensity of the redirected BS line and to act as the next (higher-order) BP. A circulator (Cir) was used to extract a portion of the laser as output and feed into an optical spectrum analyzer.
after being reflected by the FBG. Higher-order BS lines would be generated following a similar fashion when lower-order BS lines are injected back into the SMF. A portion of the generated BS lines that pass through the FBG and enter the SMF would also contribute to the generation of higher-order BS lines and the output power. Cascaded BS line generation would continue until the total gain of the laser cavity is less than the cavity loss at the operating wavelength. A stable laser, consisting of the BP and its BS lines, will be produced in a steady-state condition. Figure 3 illustrates the BEFL output spectra at different values of the 980 nm pump power at a fixed BP power of 2 mW. The first-order BS line is clearly observed at the 980 nm pump power of 7 mW with a downshift of 0.088 nm in wavelength. As the pump power increases, the number and the power of the BS lines are also increased because adequate pump power becomes available to amplify the higher-order BS lines to reach their threshold for oscillation in the laser cavity. At the pump power of 150 mW, twenty-one BS lines are generated. Here we observed an approximately three times improvement in comparison with that reported in Ref. [6]. Since a portion of circulating BS lines is amplified before entering the SMF, it creates a higher gain efficiency in the generation of higher-order BS lines. At the same time, this higher intensity of BS lines suppresses the energy extraction by the self-lasing cavity modes. Thus, the output spectrum achieved has no spurious self-lasing cavity modes. Stability of the output BS lines was also investigated at the pump power of 150 mW and the BP power of 2 mW at 1567 nm wavelength. Figure 4 illustrates the fluctuations of the peak power of eight output channels (including the BP) over an observation period of 60 min. During that pe-
3. RESULTS AND ANALYSIS When the BP is not injected into the cavity, the proposed BEFL acts as a normal fiber laser with a self-lasing cavity mode occurring at the point of highest cavity gain, as shown in Fig. 2. The inset shows the output spectrum of the self-lasing cavity mode over a 40 nm spectral range. Unstable oscillation modes are observed due to strong mode competition between self-lasing cavity modes within this region. It is important to carefully optimize the wavelength of the BP to obtain the most stable output. In our experiment, the optimized BP wavelength is found at 1567 nm. With adequate BP power (at the optimized wavelength) launched into the EDFA, through the circulator first (wave propagates in the clockwise direction), then through the 3 dB coupler, and subsequently injected into the SMF, a downshifted BS line will be generated to propagate in the opposite direction and make a round-trip oscillation in the clockwise direction. The firstorder BS line would then be amplified twice by the EDF
Fig. 2. Illustration of the self-lasing cavity modes at the laser peak gain centered at around 1567 nm. Inset: view of the whole spectrum over 40 nm.
Fig. 4. Output peak power fluctuation at 150 mW pump power and 1567 nm BP wavelength with 2 mW power.
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riod, the BP wavelength, BP power, and pump power were kept constant, and the BS lines were scanned at 5 min intervals. The performance of the output channels shows good stability with an average power fluctuation of 0.2 dB. Measurement of the total output power with respect to the pump power (Fig. 5) reveals that the maximum output power at 200 mW and 980 nm pump power is 22 mW (in comparison with 20 mW in Ref. [6] for the same amount of 980 nm pump power) with a slope efficiency of 11% (in comparison with 10% in Ref. [6]). The threshold of the multiwavelength BEFL is found to be around 7 mW, which is less than half of the value reported in Ref. [6] and, to the best of our knowledge, is the lowest value of the ring cavity multiwavelength BEFL threshold reported so far. These improvements are essentially due to much less insertion loss of the FBG in comparison with the reverse-S-shaped fiber section. Figure 6 summarizes the wavelength tuning range characteristics of the proposed BEFL as a function of the 980 nm pump power. In this experiment, the tuning range and also the number of BS lines generated are observed as the BP wavelength is tuned at different 980 nm pump power values. In general, the tuning range of the BEFL system decreases as the 980 nm pump power increases. At 20 mW pump power, the tuning range is approximately 21 nm (in comparison with 14.5 nm, as reported in Ref. [6]), which decreases to only 5 nm at 160 mW pump power. This decrease is essentially the result of strong mode competition (with the increase in 980 nm pump power) between the self-lasing cavity modes and the BS lines. Although higher 980 nm pump power decreases the
Fig. 5. Total output power with respect to the pump power at 1567 nm, and BP wavelength with 2 mW power.
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Fig. 7. Illustration of the tuning range spectrum at 20 mW and 980 nm pump power and BP wavelength with 2 mW power.
Fig. 8. Magnified view of the output channels at BP wavelengths of (a) 1555 nm, (b) 1560 nm, (c) 1567 nm, and (d) 1572 nm.
wavelength tuning range, it increases the number of BS lines generated within the tuning range. At the 980 nm pump power of 160 mW, on average, twenty-one BS lines are generated within the tuning range of 5 nm. Note that only BS lines with an optical signal-to-noise ratio (OSNR) of greater than 20 dB are counted. It can be seen clearly that there is a trade-off between the tuning range and the number of BS lines, which is similar to the previously reported results [7,8]. Figure 7 depicts the tunability of the output spectrum of the proposed BEFL configuration at 20 mW of 980 nm pump power. The tuning range of 21 nm is obtained from 1555 to 1576 nm. At around 1567 nm—where self-lasing cavity modes are most efficient—no self-lasing cavity modes are observed. In addition, Fig. 8 shows clearly the magnified output spectra corresponding to some selected BP wavelengths, namely, 1555, 1560, 1567, and 1572 nm.
4. CONCLUSION
Fig. 6. Plot of tuning range and number of Stokes lines versus pump power.
Finally, from the foregoing discussions, the inference can be drawn that an improved version of a multiwavelength Brillouin-erbium fiber laser utilizing a partially reflecting fiber Bragg grating in a ring cavity has been successfully demonstrated. A clean output spectrum (without any spurious self-lasing cavity modes) has been recorded, and the maximum output power reaches 22 mW, yielding twentyone BS lines with between-line spacings of 0.088 nm. A tuning range of 21 nm was achieved at the BP power of 2 mW and the 980 nm pump power of 20 mW. Such improvements were recorded even with the use of shorter single-mode fiber, low-doped erbium-doped fiber, and
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lower Brillouin pump power in comparison with that used in the reverse-S-shaped fiber section configuration reported before [6].
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