1. Introduction

Nonlinear phenomena in optical fibers have been widely utilized to generate multiple wavelengths fiber lasers such as the use of four-wave mixing in dispersion-shifted and photonic crystal fibers [1–3], Raman scattering in dispersion compensating fiber (DCF) [4–6], Brillouin scattering in single mode fiber (SMF) [7–9], combination of Raman and Brillouin scattering in DCF [10–12], and combination of erbium and Brillouin gains in SMF and DCF [13–16]. Among these technologies, multiwavelength Brillouin-erbium fiber laser (MBEFL) has attracted huge interest due to the inherent advantage of narrow linewidth and fixed Stokes lines spacing of about 0.08 nm [13–18]. The lasing gain of MBEFL is mainly determined by the erbium gain while the Brillouin gain contributes to setting the operating wavelength of the laser. However, the major disadvantage of MBEFL is limited wavelength tunability owing to the mode competition generated from the laser cavity itself.

The presence of free-running modes together with Brillouin Stokes lines in the cavity causes instable operation of the laser and adds to the difficulty in achieving multiwavelength generation in a wide wavelength range [19]. Several schemes employed to overcome this problem include; injection of the Brillouin pump (BP) signal at a wavelength close to the lasing wavelength of the erbium gain [17,18], spectrum filtering [19–21], utilization of large BP power [22], BP pre-amplification technique [23,24], virtual reflectivity [25,26] and deployment of variable optical attenuator to control the cavity modes oscillations [27].

Twelve C-band Brillouin Stokes lines with highest peak power of −25 dBm and wavelength tunability of 14.5 nm using Sagnac loop filter was reported recently [19]. However, incorporating a Sagnac loop filter in a ring cavity to manipulate the spectral loss has led to lower output powers. On the other hand, wide tuning range was achieved by self-seeded MBEFL at the expense of low OSNR of Brillouin Stokes lines [20,21]. Large BP power was utilized to achieve wide tunability at the expense of output Stokes lines number [22]. In another work, single-pass and double-pass BP pre-amplification techniques were employed to enhance the wavelength tunability of Stokes lines [23,24]. Nineteen Stokes lines with a tunability of 10 nm and sixteen Stokes lines with a tunability of 14 nm were obtained for single-pass and double-pass Brillouin pump pre-amplification techniques, respectively. In recent reported works, virtual reflectivity [25,26] and variable optical attenuator [27] were used to reduce the cavity modes oscillations and overcome the tuning range limitation, but both laser structures operate on C-band and provides low Stokes lines power with less number of lasing lines. Although the previous schemes were able to improve the tunability of C-band MBEFL, the improvement of laser tunability was either at the expense of less number of output channels with low peak power or at high threshold power level with low signal-to-noise ratio. In addition, most of the previous improvements of MBEFL tunability focus on C-band, only a few works were reported on the improvement of tunability in L-band MBEFL [23,24].

In this paper, a new design of MBEFL that utilizes nonlinear amplified fiber loop mirror filter (AFLMF) is demonstrated. The nonlinear AFLMF which induces wavelength-dependent cavity loss and serves as spectral amplitude equalizer is employed to shift the erbium-doped fiber gain spectrum. By properly adjusting the polarization controller in the fiber loop in conjunction with the wavelength adjustment of BP, the MBEFL can be tuned over the whole L-band window from 1570 nm to 1610 nm with an average number of 24 stable output channels. To the best of our knowledge, this is the highest number of channels with largest tuning range reported in the development of L-band MBEFL.

2. Experimental setup and operating principle

Figure 1 shows the experimental setup of the proposed MBEFL that exploits a nonlinear AFLMF. In the configuration, the nonlinear AFLMF consists of bidirectional erbium-doped fiber amplifier (EDFA) and a fiber loop mirror filter (FLMF). The EDFA provides the optical gain and consists of 10 m long erbium-doped fiber (EDF) with characteristics of 900-ppm erbium ion concentration, an absorption coefficient of 19 dB/m at 1530 nm and a cut-off wavelength around 1420 nm. The EDF is pumped by a 1480 nm pump laser through a wavelength selective coupler (WSC). The FLMF is formed with the combination of a 3-dB coupler (directional coupler, DC), a length of polarization maintaining fiber (PMF) and two polarization controllers (PCs). At each port of the PCs, the length of PMF connected to the PCs is 0.5 m. Therefore, the total length of PMF in the loop is 2 m. The PMF has a mode file diameter of 10.5 µm at 1550 nm, a cladding diameter of 125 µm, an attenuation coefficient of 0.5 dB/km, a numerical aperture of 0.13, and a birefringence of 4.3 x 10⁻⁴. The reflection and transmission spectra of nonlinear AFLMF are adjusted by controlling the PCs. The Brillouin gain is provided by a 6.7 km of SMF-28 fiber type placed between an optical mirror (M) with reflectivity of 90% and the 3-dB coupler. The BP signal is provided by an external tunable laser source having maximum power and tuning range of 3.5 mW and 100 nm respectively. The output of the laser is measured through the output port of the circulator (port 3) by an optical spectrum analyzer (OSA) having a resolution bandwidth of 0.015 nm.

 

Fig. 1 Experimental setup of multiwavelength BEFL utilizing nonlinear AFLMF.

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The operating principles of the proposed laser can be explained as follows. Without an external BP power injected, the configured laser operates as a bidirectional erbium-doped fiber laser (EDFL) with oscillating modes at the EDF peak gain. By adjusting the PCs in the fiber loop, control can be exerted on the wavelength and power level of EDF lasing (EDFL cavity modes). When an external narrow linewidth BP power is injected through port 1 of the circulator, it travels through port 2 of the circulator to the 3-dB coupler where it splits into two counter-propagating signals. These two counter-propagating signals are both amplified in the loop and reflected or transmitted back according to the reflection and transmission profile of the nonlinear AFLMF. With sufficient transmitted BP power into the SMF-28 fiber, the first-order Brillouin Stokes signal can be formed between the nonlinear AFLMF and the optical mirror once the threshold power of the Brillouin gain medium is exceeded. In addition, the optical mirror recycles the transmitted BP signal back into the SMF-28 fiber, thus provides a bidirectional pumping scheme which initiates more stimulated Brillouin scattering (SBS) emissions. In this case, the threshold power is reduced and the generation of Brillouin Stokes signals is enhanced as reported in [28–30].

The first-order Brillouin Stokes signal propagates in the opposite direction to the BP signal, passes through the nonlinear AFLMF for amplification and then gets reflected back into the SMF-28 fiber. In the proposed laser structure, the oscillation occurs between AFLMF and mirror. When the lasing condition is satisfied, the first-order Stokes signal operates as a laser. This new laser can also be utilized as the BP to generate a higher order Brillouin Stokes signals. Once its power goes beyond the SBS threshold condition, the second-order Stokes signal emerges at a wavelength shifted from the first-order Stokes signal wavelength by 0.089 nm. The higher-order Stokes signals are generated in the same process. This cascading effect is terminated when the lasing condition is not satisfied. Consequently the SBS threshold condition to generate new Stokes signal is also not fulfilled. The laser output is taken from port 1 of the 3-dB coupler that passes through the circulator (from port 2 to port 3). Therefore, with the combination of adjusting the PCs of AFLMF and the wavelength adjustment of the BP, tunable multiwavelength BEFL source can be achieved.

3. Results and discussions

In order to understand the filtering mechanism, the characterization of FLMF is performed beforehand. In the ideal case, the coupler splits the light entering the input port into two waves of equal amplitudes that propagates in the clockwise and counterclockwise direction, both of which propagates around the loop. The PCs acts as a polarization state rotator for both the clockwise and counterclockwise signals. The birefringence in the PMF produces a phase difference between the wave components propagating along the fast and slow axes. The intensity of the output light from the FLMF depends on the phase differences among the interfacing signals which are determined by the settings of the PCs. The difference in phase shift can be expressed as [31],

 

Fig. 2 Transmission spectra of the FLMF for different setting of the polarization controller.

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The typical L-band ASE source spectrum of an EDFA is injected into FLMF through port 1 of the 3-dB coupler as shown in Fig. 1. The transmitted spectrum was measured at port 2 of the 3-dB coupler. Figure 2 shows the typical L-band ASE source and the measured transmission spectra of the FLMF for different setting of the PCs. By adjusting the PCs in the FLMF, the transmission and reflection spectra of the FLMF can be changed so that the amount of the optical feedback to the cavity is controllable and the peak gain of the laser can be tuned. The peak of the bandpass spectrum of the filter which represents the lowest insertion loss of the filter has been tuned within 40 nm, which is in agreement with the wavelength tuning range of the EDFL cavity modes oscillation in Fig. 3 . The measured wavelength tuning of the EDFL cavity modes oscillation at 100 mW of 1480 nm pump power for different settings of the PCs is depicted in Fig. 3. By adjusting the PCs inside the AFLMF, the power level of the optical feedback can be set lower (cavity loss is high), thus the laser oscillates in short wavelength region. On the other hand, the laser oscillates in long wavelength region when the cavity loss inside the laser is low.

 

Fig. 3 Wavelength tuning of the EDFL cavity modes oscillation at 100 mW of 1480 nm pump power.

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The EDFL cavity modes can be shifted over the whole L-band bandwidth from 1570 nm to 1610 nm by adjustments of the PCs in the fiber loop as shown in Fig. 3. By properly setting the PCs in the FLMF, the reflection or transmission spectrum of the nonlinear AFLMF can be tuned to compensate for the variation in the gain profile of the EDFA over a wide bandwidth. Thus, the flexibility on shifting and modifying the spectrum of EDFL operation leads to the ability of tuning the multiwavelength BEFL source over a wide bandwidth.

Figure 4 illustrates the tuning process of the MBEFL output over the whole L-band window from 1570 nm to 1610 nm. In the tuning process, the EDFL oscillation is first shifted by adjusting the PCs, and then the BP signal is injected at the wavelength of this peak gain region as given in Fig. 3. By optimizing the 1480 nm pump power, BP power and careful adjustment of the PCs in the nonlinear AFLMF, wide tunability and higher number of Brillouin Stokes signals are achieved. The optimum point is found when the 1480 nm pump power is driven to 100 mW with 1.1 mW of BP power. An average of 24 output channels can be tuned over 40 nm from 1570 nm to 1610 nm.

 

Fig. 4 Wavelength tuning process at 100 mW pump power, and 1.1 mW of BP power.

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This wide wavelength tunability is achieved by overlapping the cavity peak gain and BP wavelength carefully. Owing to the additional Brillouin gain, the EDFL cavity modes can be efficiently suppressed. In this scenario, the generation of Brillouin Stokes signals always occur at the peak of cavity gain thus eliminating the possibility of co-existing with EDFL oscillating modes at other wavelength. This is unlike to the conventional MBEFL, the peak gain of EDFL is fixed. When the BP wavelength is detuned away from this region, higher Brillouin gains are required to compete with the oscillating modes [23,24]. As a result, the tuning range is limited due to insufficient Brillouin gain to suppress the build-up of EDFL cavity modes.

In our experiment, without introducing the PCs with PMF in the loop and at the same pumping powers of 1480 nm pump and BP at 100 and 1.1 mW respectively, tunability of the output wavelength is found to be only ~2 nm. In this situation (without the PCs with PMF in the loop), the EDFL peak gain is fixed around 1601 nm. Thus with the help of PCs and PMF in the loop to shift the peak gain of EDFL, the tuning bandwidth of the BEFL is extended to 40 nm as depicted in Fig. 4.

This wide tunability with higher number of output channels is the highest tuning range reported in the development of L-band multiwavelength BEFL up to date to the best of our knowledge. The detailed spectra of the sampled multiwavelength BEFL combs in the tuning process are shown in Fig. 5 , in which the output spectra in (a), (b), (c), and (d), correspond to the 1570, 1580, 1600, and 1610 nm respectively. The output spectra presented in Fig. 5 are measured by the optical spectrum analyzer with its resolution bandwidth set at 0.015 nm. The number of the generated output channels and the total laser power against the BP wavelength are also investigated as depicted in Fig. 6 . The BP wavelength is tuned from 1570 to 1610 nm and the 1480 nm pump and BP powers are fixed at 100 and 1.1 mW, respectively.

 

Fig. 5 Output spectra of multiwavelength BEFL in tuning process at BP wavelength of (a) 1570 nm, (b) 1580 nm, (c)1600 nm and (d) 1610 nm.

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Fig. 6 Total output power and the number of generated channels against the BP wavelength.

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Referring to Fig. 6, the number of output channels varies from 23 to 25 channels. An average of up to 24 output channels can be tuned over the whole L-band region. In addition, the total output power is more than 7 mW for the whole tuning range from 1570 nm to 1610 nm. The maximum output power of 9.92 mW occurs around 1580 nm which indicate the highest gain from the Erbium gain block. At BP wavelength of 1610 nm, the output channels dropped to 23 channels with power of 7.22 mW. This observation is a result from the reduction of the erbium gain for BP wavelength higher than 1600 nm, which led to insufficient signal power for the higher order Stokes signals to pump the SMF-28 fiber, thus terminating the multiple generation of output channels. In comparison, the number of output channels produced from the proposed laser structure is double the results presented in [19] and [27]. In addition to this, our tuning range of 40 nm supersedes their reported values of 14.5 nm and 23 nm, respectively. Furthermore, not only the tunable range is extended, but the average output power of the channels is also higher by more than 30 dB.

Lastly, the laser power stability is tested at BP wavelength of 1600 nm with 100 mW and 1.1 mW of 1480 nm pump and BP powers, respectively. Figure 7 shows the variation of peak power for the first ten Stokes lines. In this experiment, the laser spectrum is scanned every 5 minutes over a period of 1 hour. It is found that the Stokes lines show a good stability over the duration. Referring to Fig. 7, from the first-order to the tenth order Stokes lines, the peak powers fluctuate within ± 0.1 dB. This shows that the proposed laser structure has good stability as a result of wavelength selective gain range by AFLMF section.

 

Fig. 7 Spectral stabilities of Stokes lines at 100m W of 1480 nm pump power and 1.1 mW BP power with wavelength at 1600 nm.

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4. Conclusion

We have successfully demonstrated a widely tunable L-band multiwavelength BEFL utilizing a nonlinear AFLMF. The AFLMF provides flexibility in controlling the amount of light reflected and transmitted inside and outside the laser cavity and also provides a mechanism to control and modify the EDF gain profile. Based on this mechanism, the EDF peak gain profile can be shifted over the whole L-band gain spectrum through adjustment of the PCs placed in the nonlinear AFLMF. By proper manipulation of the PCs, in conjunction with manipulating the BP signal wavelength, an average of 24 stable output channels of the configured multiwavelength BEFL are able to be tuned over the entire L-band window from 1570 nm to 1610 nm. We believe that the proposed wide band MBEFL fiber lasers are very useful in various applications to optical component testing and characterization, optical sensor systems, optical switches, and dense wavelength division multiplexing systems.