1. Introduction

Multiwavelength signals generation from a single laser source has attracted lots of interest in dense wavelength division multiplexing transmission system to cater to the large demand of users while constraining the cost. This can be realized with the advent of all-optical fiber lasers which can be constructed mainly from rare-earth doped-fibers or based on non-linear effects in single mode fiber (SMF). Multiwavelength fiber lasers based on stimulated Brillouin scattering (SBS) have been actively investigated due to its narrow linewidth, low threshold power and small wavelength spacing of about 10 GHz (0.08 nm). However, the Brillouin gain is practically low and requires additional amplification medium to overcome the cavity loss in the system. The discovery of Brillouin-erbium fiber laser (BEFL) by Cowle and Stepanov [1] which combined the narrow Brillouin gain with the linear gain from erbium doped fiber (EDF) had opened up a new research dimension for fiber lasers.

The race to produce multiple lasers from this new fiber laser was initiated by introducing a feeding mechanism known as the reverse-S-shaped fiber section [2]. Although multiple lasers were created but the operation was limited to higher path loss for the diverted Stokes lines to be utilized as the Brillouin pump (BP). Therefore an additional erbium-doped fiber amplifier (EDFA) was employed in the reverse-S-shaped fiber section to boost the BP power in order to get higher Brillouin gain in the optical fiber [3]. Although 53 Stokes lines were produced for this improved BEFL structure [4], the BP wavelength must be carefully chosen with regards to the free-running cavity modes. As a result, the BEFL operation is considered as wavelength sensitive. It could not be tuned widely except by employing a tunable bandpass filter in the ring cavity [5]. Therefore, in order to operate a BEFL with tuning capability, the operation is somewhat more complicated as opposed to other laser source technologies.

The mode competition between the generated Stokes lines and self-lasing cavity modes is the main problem in tunability of BEFL. To suppress these unwanted modes, the BP power must be elevated first before entering the Brillouin gain medium [6, 7]. The suppression of cavity modes is achieved owing to the homogeneous saturation of the erbium gain in linear cavity architecture. The same technique was applied for the enhanced reverse-S-shaped BEFL structure by placing the erbium gain block next to the Brillouin gain medium [8]. The requirement of having two EDFAs was eradicated as proposed in [3, 4]. However, the tuning capability observed in this structure was considered small as compared to the previous linear cavity architectures [6, 7]. The most noticeable factor is the amplification process in the erbium gain block. For the ring cavity BEFL, the BP was amplified only once whereas it was amplified twice in the linear cavity BEFL. For both BEFL structures, although a wider tuning range was obtained, the occurrence of self-lasing cavity modes was not completely eliminated. This is due to the existing erbium gain block in the cavity that acts as the primary amplification element. Therefore the laser characteristics are highly influenced by the erbium gain properties that limit the BEFL tuning capability. A possible solution is to remove the erbium gain block from the cavity but it sparks a new challenge; the subsequent BP (diverted Stokes lines) requires amplification in order to generate higher order Stokes lines.

In this paper, we propose a Brillouin fiber laser (BFL) structure with enhanced reverse-S-shaped feedback coupling assisted by out-of-cavity optical amplifier. The laser structure is free from any intrinsic gain properties, in this case the erbium material. The subsequent order of BP is amplified by the erbium gain block relocated in the reverse-S-shaped fiber section. At the optimum output coupling ratio, an average of eleven laser lines can be tuned over 45 nm wavelengths from 1520 nm to 1564 nm.

2. Experimental setup and operation principle

The configuration of the proposed design is shown in Fig. 1 . The main laser cavity is formed in a ring cavity as represented by the blue dashed-box. The Brillouin gain medium is provided by 11 km long dispersion compensating fiber (DCF) with effective core area of 20 μm², nonlinear coefficient of 7.31 (Wkm)^-1, -1328 ps/nm dispersion and 7.28 dB total loss. The reverse-S-shaped is utilized to divert the oscillating signals in the cavity to be used as the higher-order BP signal. This section is formed by a 3 dB coupler (C1), a 90/10 optical coupler (C2) and an EDFA. The 10% port of C2 is used to couple the initial BP signal which is provided by an external cavity tunable laser source (TLS) while the 90% leg is connected to C1. The EDFA is used to amplify the BP and Brillouin Stokes signals as depicted by the dashed box. It consists of 8 m long EDF, a 975-nm laser diode (LD) and a wavelength selective coupler (WSC) as indicated by the dashed blue line in Fig. 1. The EDF has an Er³⁺ ion concentration of 785 ppm, numerical aperture of 0.22, a cut-off wavelength of 900 nm and peak absorption of 7.38 dB/m at 1531 nm. The WSC is employed to couple the 975 nm pump light and BP signals into the EDF for the amplification process. The amplified signal is then routed from port-1 to port-2 of the circulator (CIR) to the Brillouin gain medium (DCF) for the SBS process to take place.

 

Fig. 1 Experimental setup of the enhanced reverse-S-shaped BFL.

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It is important to highlight here that this laser configuration is categorized as BFL instead of BEFL since the EDFA is not placed in the laser cavity resonator. The Brillouin Stokes lines generated in the DCF propagate in the clockwise direction. These signals travel from the DCF and pass through CIR from port-2 to port-3. Then they propagate through a variable optical coupler (VOC) and reach the DCF again via 50% leg of C1. The output of cavity is taken at the output leg of the VOC to the optical spectrum analyzer (OSA). The coupling ratio of the VOC controls the amount of light oscillating in the laser cavity. It is first set at 5% output coupling ratio and a series of experiment is then carried out to optimize the ratio by varying the output coupling ratio from 10% to 80% in 10% increment.

When the BP is injected into the non-resonating fiber section, it is amplified by the EDFA and guided into the DCF through CIR. When the amplified BP reaches the SBS threshold condition, a Brillouin Stoke (BS) line is initiated in the DCF which is downshifted in frequency by about 10 GHz and propagates in the opposite direction of the BP. This BS signal circulates in the primary ring cavity (clockwise direction) until it overcomes the cavity loss and turns into a laser. In the proposed BFL structure, 50% of the laser powers are tapped via C1 in the reverse-S-shaped fiber section. This diverted BS line is then amplified by the EDFA section, propagates to the DCF through port-1 and port-2 of CIR and when its power rise above the SBS threshold, a higher order BS line is generated. Under this situation, the amplified signal becomes the next order BP. Then, the same process is repeated for new laser generation. This BFL operation continues until the Brillouin gain in the cavity becomes insufficient to overcome the cavity loss which naturally terminates the lasing process.

3. Results and discussion

For any fiber lasers, the main characteristic to be observed is the property of free-running cavity modes. In order to investigate this characteristic, the EDFA is activated without injecting the BP. Although a dedicated amplifying medium is not actually in the primary ring cavity, the EDFA section is utilized to amplify the Stokes lines to act as the BP. Due to the homogenous broadening effect in EDF, self-lasing cavity modes are intrinsically generated and dominates the laser cavity [9]. This leads to the instability of the BS signals power when these modes appear together at the laser output. Thus, it becomes the major factor that limits the tuning range of this laser type in most designs. Figure 2 shows the laser output at different output coupling ratios (5%, 10% and 20%) for the proposed design with maximum pump power of 616 mW. It shows the presence of strong modes around 1530 nm wavelength range that coincides with the EDF peak gain. In general, the shape of the spectrum follows the erbium gain shape. Referring to the BFL structure as depicted in Fig. 1, the amplified spontaneous emission (ASE) is generated from the EDFA and propagates in the anti-clockwise direction in the ring cavity. Under this circumstance, there should not be any output due to the orientation of VOC. However, the measured spectrum is taken for any signals that propagate in the clockwise direction. From this result, it is an evident that some portions of ASE circulate in this direction. This is owing to the effect of reflection in the ring cavity. The major contributor to this problem is the existence of Rayleigh scattering effect in optical fibers (DCF). In order to verify this claim, a simple experiment is performed utilizing the setup shown in Fig. 3(a) .

 

Fig. 2 Free running cavity mode at different output coupling ratio of 5%, 10% and 20%.

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Fig. 3 (a) Experimental setup to show the presence of Rayleigh scattering, (b) back-propagated Rayleigh scattering measured at OSA and (c) the power difference with respect to the reference spectrum at 100 mW.

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In this setup, the 975 nm pump light is directly coupled to the EDF via WSC to generate a broadband ASE. Then this ASE light is injected into the 11 km long DCF and the reflected signal through Rayleigh scattering is measured via OSA. An isolator is placed at the other end of DCF to avoid any back-reflected light from the fiber end. This is to ensure that the measured optical spectrum at port-3 of CIR is contributed solely by the Rayleigh back-scattered light. In order to observe the impact of Rayleigh scattering, the EDF is excited at different pump powers as illustrated in Fig. 3(b). The reflected ASE spectrum increases in tandem with the increment of pump power. This shows that the reflected ASE spectrum is dependent on the incident powers. In order to study the increment pattern with respect to wavelength, the Rayleigh backscatter signal at 100 mW pump power is set as the reference spectrum. By comparing the other reflected ASE spectrum to the reference one, the results are depicted in Fig. 3(c). It is seen that the most power increment occurs around 1530 nm which are in agreement with the findings shown in Fig. 2. Since the Rayleigh scattering effect is proportional to the incident power, thus the highest back-scattered power of the ASE peak is recorded in the experiment. Therefore, the Rayleigh scattering effect plays major role in the self-lasing cavity modes measured from the proposed BFL structure.

4. Laser characteristics

To further investigate the impact of Rayleigh-scattering-induced self-lasing cavity modes, an investigation of tuning range is carried out by varying the BP wavelength over the erbium amplification band at different output coupling ratios. Throughout the experiment, the BP power is fixed at 6 dBm. The tuning range is defined as the range of BP wavelength in which the unwanted self-lasing cavity modes are effectively suppressed in the cavity. The plotted graph in Fig. 4 denotes the tuning range with variation in output coupling ratios. In general, the wide tuning range is achieved by efficiently suppressing the weak self-lasing cavity modes that resulted from the Rayleigh back-scattering. This allows the BS signals to become dominant in the cavity and suppresses other potential modes. Tuning range as wide as 44nm (1520-1564 nm) is obtained at 5% and 10% of output coupling ratios. In addition, the tuning range increases from smaller to larger output coupling ratios. This is due to the increment of cavity loss that reduces the dominant effect of self-lasing cavity modes. However, the same tuning range of 47 nm (1520-1567 nm) is obtained from 40% to 80% output coupling ratios which indicates that at these coupling ratios, the tuning range is actually restricted only by the EDFA amplification bandwidth. Figure 4(b) shows the optical spectra taken for the output coupling ratio of 40% for BP wavelength of 1520, 1543 and 1567 nm. The ASE shape around 1530 nm starts to grow when the BP wavelength is fixed at 1520 and 1567 nm. However, its peak power is more than 50 dB lower than the peak of comb lines thus its effect is minimized. This wavelength region is further analyzed by studying its spectral property when the BP wavelength of 1530 nm is injected into the laser structure for various output coupling ratios from 10% to 40% as shown in Fig. 5 .

 

Fig. 4 (a) BFL tuning range at various output coupling ratios, and (b) the optical spectra taken at the output coupling ratio of 40% for BP wavelength of 1520, 1542 and 1564 nm.

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Fig. 5 Output spectra at different output coupling ratios; (a) 5%, (b) 10%, (c) 20% and (d) 40%.

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In our experiment, the laser is counted if its peak power is greater than -20 dBm. Furthermore, only Brillouin Stokes lines are considered as the desired laser. All the anti-Stokes components and the BP itself are not counted. Since the erbium peak gain at 1530 nm has been identified as the main cause of the output instability, its existence within the comb lasers must be carefully analyzed. From Fig. 5, the numbers of channel are 10, 11, 11, and 11 for the output coupling ratio of 5%, 10%, 20% and 40% respectively. Although the number of channel is almost constant, the optical signal-to-noise ratio (OSNR) needs to be taken into account as well. The noise floor does not have any specific profile and it is highly dependent on the interplay between the gain and cavity loss. As more output powers are taken out from the cavity, the oscillating modes get weaker. These modes are then amplified by EDFA and as a result, these modes cannot suppress the build up of ASE during the amplification process. Then the higher ASE level is reflected through the Rayleigh scattering process and propagates together with the oscillating modes. Therefore, the OSNR quality at this erbium peak gain (channel 2-5) deteriorates as depicted in Fig. 5(d).

Figure 6 illustrates the number of generated laser lines with respect to the BP wavelength at different output coupling ratios of 5%, 10%, 20% and 40%. The BP power is set to 6 dBm while the BP wavelength is varied from 1520 to 1570 nm with a step of 1 nm. Referring to Fig. 6, the number of output channels varies from 5 to 12 channels. The highest count of comb lasers is recorded at 20% and 40% output coupling ratios. At 5% and 10% output coupling ratios, an average of 8 and 10 output channels can be tuned over 45 nm from 1520 nm to 1564 nm. In addition, at 20% and 40% output coupling ratios, a corresponding average of 10 and 11 channels can be tuned over the same tuning range of 45 nm. Moreover, at BP wavelength of 1564 nm, the output channels drop to 5, 6 and 8 for 5%, 10% and 20% output coupling ratios, respectively. Therefore, the output coupling ratio of 40% is chosen as the optimum output coupling ratio to produce the highest output channels. In comparison with the results reported in [8–10], the average number of output channels and the tuning range produced from the proposed laser structure is better. As depicted in Fig. 6, the generated output channels of multiwavelength BEFL is influenced by the output coupling ratio and the amplification bandwidth of the EDFA used.

 

Fig. 6 Number of channels generated at different output coupling ratios with variation in BP wavelengths.

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The other critical parameter for fiber laser is the power stability. Figure 7 depicts the stability performance of the proposed BFL for the output coupling ratio of 40%. In this case, the first seven channels are analyzed for the purpose of peak power performance. Its measurements are carried out using OSA. On the other hand, the total power is measured using an optical power meter. The stability experiment is conducted over 1 hour with 5 minutes interval between each measurement. From Fig. 7, the peak power variations for all seven channels are less than 0.2 dB. For the total power, the fluctuation is not greater than 0.1 dB. These findings show that the proposed fiber laser is able to generate stable channels. As opposed to the conventional BEFL structures where the EDFA is in the cavity, our BFL utilizes the out-of-cavity EDFA technique. Thus the self-lasing cavity modes are efficiently suppressed, resulting in good power stability.

 

Fig. 7 Stability performance of the proposed BFL structure in terms of channel peak power and total power at the output coupling ratio is 40%.

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5. Conclusion

We have successfully demonstrated a laser structure for multiwavelength generation utilizing an enhanced reverse-S-shaped feedback coupling assisted by out-of-cavity optical amplifier. The EDFA is utilized to amplify the Brillouin pump and the Brillouin Stokes lines, and it is located in the reverse-S-shaped feedback section. Therefore, the oscillating modes are not affected by other direct amplification in the laser cavity as usually found in multiwavelength BEFL. The proposed laser structure has wide tuning characteristic (45 nm range) that is only limited by the available amplification bandwidth provided by the EDFA. By adjusting the cavity loss, an average of 11 laser lines can be tuned over 45 nm wavelengths at 40% optimum output coupling ratio. Other potential modes are efficiently suppressed thus allowing the lasing lines to become dominant in the cavity at wider wavelength range.