1. Introduction

Stimulated Brillouin scattering (SBS) is a nonlinear process in optical fiber that occurs due to the interaction of light with acoustic waves or vibration phonons. Most efficient scattering manifested in a backward-propagating narrow linewidth Stokes wave with its frequency downshifted from that of the incident light [1]. This unique phenomenon leads to the study of the narrow linewidth (single wavelength and multi-wavelength) laser source. The issue of interest for narrow linewidth technology is to achieve low noise and reduced SBS threshold with its significance in many optical applications [2–4].

Since the Brillouin gain in optical fiber is low [5], a hybrid-gain technique, which integrate Brillouin and Erbium gains in the same laser cavity to produce high power single-wavelength fiber laser has been demonstrated [6]. The combination of these two gain media has successfully produced output with reasonable powers [6–7]. However, the main challenge to achieve high stability in Brillouin-Erbium fiber laser (BEFL) system is to suppress unstable modes (self-lasing cavity modes) oscillating in this BEFL system [8]. These unwanted oscillating modes are sharing the same gain medium with Brillouin Stokes line and therefore, the mode competition always occurred in the laser cavity. In order to overcome this problem, the Brillouin pump (BP) wavelength must be chosen within the peak gain of the laser cavity. However, when the BP is detuned away from the peak gain, the BP must have adequate power to suppress the self-lasing cavity modes which always occur at the peak gain of the self-lasing cavity modes [9]. One of the ways is to integrate a bandpass filter which coincides with the BP wavelength. However, this technique requires precise wavelength tracking to match the wavelength between the BP and the bandpass filter. Alternatively, the injected BP power can be externally increased to compete with the self-lasing cavity modes especially when the BP wavelength is far away from the peak gain of the cavity [10]. Recently, a new approach was reported namely as BP pre-amplification technique to have a wider tuning range [11]. However, the study was limited to a linear cavity laser only.

In this paper, three locations of the EDFA in the laser cavity have been studied. At injected BP power of 1.6 mW while the pump power was set to the maximum value of 135 mW, the maximum Stokes laser power of 25.1 mW (BEFL-1), 15.5 mW (BEFL-2) and 1.4 mW (BEFL-3) was be found. A tuning range of 50 nm (1525 nm to 1575 nm) without appearances of the self-lasing cavity modes was obtained at BEFL-1. For BEFL-2, there are two tuning range regions of 6 nm (1529 nm to 1535 nm) and 22 nm (1541 nm to 1563 nm) can be tuned. On other hand, only 3 nm of the tuning range was obtained in BEFL-3 from 1557 nm to 1560 nm.

2. Brillouin-Erbium fiber laser structure

Figure 1 illustrates the ring cavity BEFL system structures for three different EDFA locations. In general, the BEFL structures consist of dispersion compensating fiber (DCF), optical circulators (Cir), optical coupler and EDFA. The Brillouin gain medium is provided by an 11 km long DCF with effective area of 20 µm², nonlinear coefficient of 7.31 (Wkm)^-1, -1328 ps/nm dispersion and 7.28 dB total loss. The circulator used in the laser system acted as an isolator to direct the propagation of Brillouin Stokes (BS) line in clockwise direction. The measured results were extracted from the laser system by using 80/20 coupler where 80% power is used for monitoring and measurement purpose while the 20% power is circulated back into the laser cavity. An external tunable laser source provided the BP that can be tuned from 1520 nm to 1620 nm (100 nm tuning range). The EDFA is located at three different locations in the ring cavity BEFL as depicted in Fig. 1(a), 1(b) and 1(c). For BEFL-2, it was adopted from Ref. [12]. It was pumped by a 1480 nm pump laser diode (LD) with maximum pump power of 135 mW. It should be noted that, we used the 1480 nm pump LD because the quantum efficiency of 1480 nm pumping is higher compared to the 980 nm pumping [13]. The 1480 nm pump was connected to the Erbium-doped fiber (EDF) through the 1480/1550 nm wavelength selective coupler. The EDF has an absorption coefficient of 4.2 dB/m at 977 nm, a cut-off wavelength around 920 nm and a peak absorption of 5.7 dB/m around 1531 nm. For this study, the length of the EDF was 8.0 meters. An optical spectrum analyzer (OSA) was used for all measurement and observation in the experiment.

 

Fig. 1. Structure of the BEFL system with the three different locations of EDFA: (a) BEFL-1, (b) BEFL-2 and (c) BEFL-3.

Download Full Size | PDF

For BEFL-1, the BP signal was injected directly via the Cir and then amplified by EDFA before entering the DCF. The amplified BP was directed into the DCF to create a narrow linewidth of Brillouin gain and BS line was generated in the opposite direction of the BP propagation with wavelength downshifted by 0.08 nm (10 GHz) from the BP wavelength. Then the BS line was amplified by EDFA and started to circulate in the clockwise direction. Once threshold condition is fulfilled [14], the BS line becomes laser for this system. In this case, only single-wavelength of the BS line was obtained in which the remnant BP that circulated in the anti clockwise direction was blocked by the Cir and the BS is not resonant with the BP, thus the BS cannot act as subsequent BP to create higher order BS signals. The output for this system was taken from the output port of the 80 % optical coupler after BS line passed through the optical Cir as shown in Fig. 1(a). For BEFL-2, the BP was injected through the Cir into the DCF directly to generate the BS line. Once the BS line passed through the Cir, the EDFA amplified the BS line and the output was extracted from the optical coupler at the 80% leg. The same process of generating BS line was repeated for BEFL-3. The BP from optical circulator was guided to the DCF and BS line was propagating in the opposite direction of the BP propagation. The measured results were extracted from the output port of the 80 % optical coupler. For BEFL-3, the output was measured before the EDFA. In general, the cavity loss of the BEFL was constant for all the BEFL structures.

3. Brillouin Stokes laser power

The effect of the EDF pump power on the BS laser power was studied in this experiment. The BS laser power characteristics are illustrated in Fig. 2 for three BEFL structures. The injected BP power and BP wavelength was set at 1.6 mW and 1550 nm respectively. The 1480 nm pump power was varied from 0 mW to the maximum power of 135 mW and the laser output was measured using the OSA with the resolution bandwidth of 0.01 nm. We varied the 1480 nm pump power to obtain the threshold value when the single-wavelength BS line appears in the laser system. The threshold power of BEFL was measured by the requirement of 1480 nm pump power (EDF gain) to get the lasing of the first order Stokes line at a fixed Brillouin gain. This value represents the amount of Stokes line gain required to compensate for the cavity loss to initiate the lasing process. Above the Brillouin threshold, BS laser power increased linearly with the increment of the 1480 nm pump power. In our experiment, the 1480 nm pump threshold was measured around 3 mW for BEFL-1 and BEFL-2. On the other hand, higher 1480 nm pump power of 10 mW was observed for threshold in BEFL-3. This is due to the fact that the BS line experienced the highest propagation loss from the DCF to the input of the EDFA. Therefore, the output power from the EDFA was the lowest compared to BEFL-1 and BEFL-2 structures. As a result, higher 1480 nm pump powers were required to initiate the lasing process.

It can clearly be seen from Fig. 2 that the highest BS laser power of 25.1 mW was recorded at the maximum 1480 nm pump power of 135 mW for BEFL-1. This is due to the fact that the BP was amplified before propagating into the DCF and thus, the BS line power was also increased. Since the BP was not amplified before propagating into the Brillouin gain medium, the BS laser powers of 15.5 mW and 1.4 mW were obtained for BEFL-2 and BEFL-3 respectively. On the other hand, the saturation power occurred when the pump power was higher than 60 mW for the BEFL-3 structure. In this case, the BS line was amplified before entering the DCF. Thus, this strong intensity of BS line generated the second-order Brillouin Stokes line which was propagating in the anti-clockwise direction. As a result, the BS line power was saturated to 1.7 mW as depicted in Fig. 2.

 

Fig. 2. Brillioun Stokes (BS) peak power against 1480 nm pump power for the three locations of EDFA in the BEFL system.

Download Full Size | PDF

The output spectra at 1550 nm of BP wavelength and maximum pump power of 135 nm are depicted on Fig. 3. The highest peak in the spectra is the BS signal and the second peak in the shorter wavelength range is BP line. At higher pump power starting from 120 mW to 135 mW, the Rayleigh backscattered effect originated from the second-order BS line (unshifted Rayleigh scattering) was observed in the output spectra. It is important to highlight the BP was also observed owing to the same effect. In general, DCF has a high germanium (used exclusively as an intermediate for DCF processes) concentration and smaller core than single mode fiber (SMF) [15]. Therefore these factors lead to higher Rayleigh backscattering coefficients.

 

Fig. 3. Output spectra at 1550 nm of the BP wavelength and at maximum pump power of 135 mW.

Download Full Size | PDF

4. Self-lasing cavity modes

The effect of self-lasing cavity modes on the BEFL tuning range for three different EDFA locations was further studied. The tuning range is determined when the BEFL operates without any appearances of the self-lasing cavity modes around the EDF peak gain in the laser system. Therefore the information of the wavelength region of self-lasing cavity modes is important in this investigation. Firstly, the behaviors of all BEFL structures were measured by allowing them to operate freely without the injection of BP. For free-running operation, the strong oscillation modes depend on their round-trip gain. The highest round-trip gain is survived and owing to the Erbium material characteristics, there are only two possibilities of wavelength range for the oscillating modes to appear within the Erbium gain bandwidth; around 1530 nm and 1560 nm regions. This characteristic is determined by the amount of loss experienced by the oscillating modes in the laser cavity. The output spectra for all the three cases are illustrated in Fig. 4. The OSA resolution bandwidth was set at 0.05 nm.

Based on the experimental results, each BEFL structure had its own self-lasing cavity modes signature. From Fig. 4(a) and 4(c), the self-lasing cavity modes appeared around 1530 nm and 1560 nm for BEFL-1 and BEFL-3 respectively. On the other hand, there are two self-lasing cavity modes that appeared around 1530 nm and 1560 nm for BEFL-2 as depicted in Fig. 4(b). The observation can be explained in terms of the average population inversion among three laser structures. The average population inversion is defined as the average of the Erbium ions number at the upper energy level to the Erbium ions number at the low energy level along the EDF in order to maintain the process of lasing. BEFL-1 has the highest average population inversion while BEFL-3 has the lowest average population inversion. On the other hand, the average population inversion of BEFL-2 is in between the other two BEFL structures. It is interesting to note that the number of components was maintained in all BEFL structures. Therefore, the cavity loss of all BEFL structures was fixed during the experiment. However, the experimental findings indicate that cavity loss was not sustained that translated to a variation of average population inversion. This is owing to the characteristic of light propagation in DCF. In this experiment, the intensity of oscillating modes before entering the DCF plays significant role in determining the total propagation loss in the laser cavity. Referring to Fig. 1, our discussions are focused to the point A for all BEFL structures. In this case, the intensity of oscillating modes was the highest at point A for BEFL-3 because the DCF was directly connected to the output of the EDFA. For BEFL-2, the oscillating modes need to propagate through the coupler which has insertion loss of about 7 dB (20% throughput) in the direction of the oscillation. Nevertheless, the output intensity of the oscillating modes is the lowest for BEFL-3 because they propagated through additional device, the circulator. The higher intensity of oscillating light that is propagating in the DCF, experienced higher losses due to the Rayleigh scattering and also to the generation of the second-order Stokes line in the opposite direction to the oscillating Stokes laser in the BEFL-3. Therefore, BEFL-3 has higher cavity loss than BEFL-1 and BEFL-2 which translated to a lower average population inversion, the peak wavelength gain was around 1560 nm. Furthermore, BEFL-1 has the lowest cavity loss which verified the existence of self-lasing cavity modes around 1530 nm peak. It is important to highlight that the two regions of self-lasing cavity modes appeared due to its average population inversion locked to a value in which these two peak gains co-existed. If the loss amount varied to other values, only one peak region will appear. The observation was just specific for the experimental setup used in this study.

 

Fig. 4. Self-lasing cavity modes of the BEFL structures when no BP signal is injected for (a) BEFL-1, (b) BEFL-2 and (c) BEFL-3.

Download Full Size | PDF

5. Tuning range

To investigate the tuning range of the laser, the 1480 pump was set at the maximum power of 135 mW while the BP power was fixed at 1.6 mW and the BP wavelength was tuned over 60 nm span, from 1520 nm to 1580 nm with 1.0 nm steps. The laser output was measured using OSA with its bandwidth resolution set at 0.05 nm. In order to discuss our observation on this matter clearly, the output spectra with 5 nm step are plotted as depicted in Fig. 5.

 

Fig. 5. Output spectrum of (a) BEFL-1, (b) BEFL-2 and (c) BEFL-3 at different BP wavelengths at 135 mW of 1480 nm pump power.

Download Full Size | PDF

In order to discuss the characteristics of Stokes laser power at different BP wavelengths in more details, the wavelength step of 1 nm was used in the preceding analysis as shown in Fig. 6. The tuning range of 50 nm from 1525 nm to 1575 nm was obtained for BEFL-1. On the other hand, there are two wavelength regions from 1529 nm to 1535 nm (region-1) and from 1541 nm to 1563 nm (region-2), which are the tuning range of BEFL-2. Therefore, BEFL-2 has managed to produce the total tuning range of 28 nm. Nevertheless, the tuning range for BEFL-3 was limited from 1557 nm to 1560 nm only. In terms of the output power across the wavelength range, BEFL-1 has the highest value due to its efficiency to amplify the BP before entering the Brillouin gain medium and moreover, its effective cavity loss is the lowest among the BEFL structures understudied in this research work. The experimental findings show that the placement of EDFA in the cavity is critically important to determine the characteristics of the laser output.

 

Fig. 6. Tuning range of Stokes signal at 135 mW of 1480 nm pump power for different locations of EDFA.

Download Full Size | PDF

6. Conclusion

We have successfully investigated the effect of EDFA locations in a ring cavity configured BEFL with dispersion compensating fiber as the Brillouin gain medium. We found out that by placing the EDFA before the Brillouin gain medium (BEFL-1), the BP is amplified first before propagating into the DCF. Experimental results have evidently shown that the amplification of the BP via the EDFA generated higher gain efficiency to produce the BS line in the DCF. For other locations of the EDFA (BEFL-2 and BEFL-3), the weak BP was not amplified before been guided into the DCF, and thus it is not adequate to generate a stronger BS line to be dominant in the BEFL system. At a fixed 1480 nm pump power of 135 mW, a single-wavelength BS line was obtained with a peak power of 25.1 mW. In contrary, only 15.5 mW and 1.4 mW of the BS line power was obtained from BEFL-2 and BEFL-3. For BEFL-1, the tuning range of 50 nm was achieved (1525 nm to 1575 nm) without any self-lasing cavity modes as compared to 28 nm and 3 nm obtained from BEFL-2 and BEFL-3 respectively.