1. Introduction

Hybrid Brillouin-erbium fiber lasers (BEFL’s) have been proposed to achieve multiwavelength oscillation with constant channel spacing of about 11 GHz in a ring cavity laser and a linear cavity [1-6]. This approach takes advantage of the narrow bandwidth of the Brillouin amplification in optical fiber and the high gain from erbium-doped fiber (EDF).

In BEFL with an external Brillouin pump (BP), the generation of Brillouin Stokes lines is achieved when the BP is injected close to the lasing wavelength of the laser structure. This multiwavelength generation is difficult to achieve in a wider wavelength range due to the homogenous broadening effect in EDF [7]. Therefore, the oscillating modes within the peak gain of laser cavity are always dominant. These lasing modes are intrinsically generated and are denoted as the self-lasing cavity modes. As a result, the self-lasing cavity modes cause power instability onto the Brillouin Stokes lines [8].

Owing to the effect of self-lasing cavity modes, the tunable range of BEFL is also limited. The tuning range is defined as the range of BP wavelength which produces the Stokes signals in the absence of self-lasing cavity modes. To overcome this problem, the tuning characteristic of BEFL is achieved using a Sagnac loop filter in the laser cavity to suppress these unwanted modes [9,10]. However, the manipulation of spectral loss to flatten the cavity gain has led to lower output power. We have investigated the BEFL tuning range using high power laser source as the BP [11]. In the experiment, a wider tuning range is achieved at higher BP powers. However, the requirement of higher power from the external laser source is unfavorable due to its higher cost. Thus, the prerequisite of higher BP powers to suppress the self-lasing cavity modes is essential to have wider tuning ranges.

In this paper, an enhanced multi-wavelength BEFL with BP pre-amplification technique within the linear cavity is proposed. The proposed fiber laser eliminates the requirement for high external BP power to create the Brillouin gain because the laser system amplifies the BP within the laser cavity before entering the single-mode fiber. In contrary to the direct-injection of BP into the single-mode fiber, lower BP powers are required to suppress the laser cavity modes in our proposed BEFL structure. In addition to these findings, the tunable range is also enhanced.

2. Brillouin-Erbium fiber laser structure

The configuration of multi-wavelength L-band Brillouin-erbium comb fiber laser utilizing intra-cavity BP pre-amplification technique in a linear cavity is shown in Fig. 1. In the proposed scheme, the linear cavity of the fiber laser is formed by two high reflectivity mirrors (M1 & M2) at both ends of the resonator. The primary amplifying medium consists of a 12 m EDF with an absorption coefficient of 19 dB/m at 1530 nm and it is optimized for the 1480 nm pumping scheme. The EDF is used to provide a large amount of amplification in order to compensate the cavity loss. The pumping scheme of 1480 nm is chosen owing to its efficiency in L-band amplification. Referring to Fig. 1, the gain block (dashed box) consists of the EDF coil, a 1480 nm laser diode and a wavelength selective coupler (WSC). Lastly, the Brillouin gain is provided by 6.7 km long of single mode fiber, SMF-28 fiber. An externalcavity tunable laser source with a maximum power of 3.5 mW and 100 nm tuning range from 1520 nm to 1620 nm is utilized as the BP.

 

Fig. 1. Configuration of BEFL with pre-amplification technique in a linear cavity, the dashed box indicates the EDF gain block.

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The output of the laser structure is taken at the output port of the 3-dB coupler as shown in Fig. 1. The most notable sub-structure of the BEFL is the placement of the EDF gain block whereby it allows the amplification of BP before entering the SMF-28 fiber to generate a Brillouin Stokes line. Therefore, the injected BP accumulates more energy to increase the efficiency of the Brillouin gain in the SMF-28 fiber coil. For the BP direct-injection technique, the EDF gain block is placed between point A and B. Then, the rest of the passive components are configured similarly to the BP pre-amplification technique. In this case, the BP is directly injected into the SMF-28 fiber without experiencing any amplification.

The operating principle of the proposed laser structure is described as follows. The injected BP is pre-amplified by the EDF gain block before propagating into the 6.7 km long SMF-28 fiber coil. Above the threshold condition, the amplified BP light creates a narrow bandwidth of Brillouin gain in the SMF-28 fiber coil. The first-order Brillouin Stokes line is generated in the opposite direction to the BP propagation at a wavelength shifted by 0.089 nm from the BP wavelength. This shifted Stokes line is amplified twice in the EDF gain block when it completes a round-trip propagation. The Brillouin Stokes line starts to oscillate in the laser cavity when its total gain generated from the stimulated Brillouin scattering effect and EDF is equal to the cavity loss. Because of the fact that the Brillouin gain is homogenous, the Brillouin Stokes line can be utilized as a BP for the higher-order Stokes lines [12]. The cascading of Brillouin Stokes lines generation continues until the total gain in the laser cavity is less than the cavity loss at the operating wavelength. At the steady-state condition, a stable laser is produced that consists of the BP and its cascaded Brillouin Stokes lines. For the BP direct-injection technique, the initial BP is directly injected into the SMF-28 fiber coil without any amplification process. The Brillouin Stokes line generated from the SMF-28 fiber coil experiences the same round-trip propagation as previously described in the proposed fiber laser structure.

3. Proof of concept

The effect of self-lasing cavity modes in both laser structures is studied by varying the BP wavelength over a wavelength range around the peak of the EDF gain (pre-determined experimentally). In this experiment, the BP power is set at 1.1 mW with EDF pump power of 90 mW for both techniques; BP direct-injection and intra-cavity BP pre-amplification. The output spectra obtained from this experiment are depicted in Fig. 2 with the optical spectrum analyzer’s resolution bandwidth is set at 0.015 nm. Fig. 2(a) shows the superimposed optical spectra for the BP direct-injection technique for the BP wavelength of 1602 nm and 1608 nm. For both conditions, the presence of self-lasing cavity modes is clearly recorded around 1605-1606 nm (laser cavity peak gain). On the other hand, these eccentrically oscillation modes are completely suppressed for the intra-cavity BP pre-amplification technique as evidently shown in Fig. 2(b).

 

Fig. 2. Output spectrum of the BEFL utilizing (a) BP direct-injection technique and (b) BP pre-amplification technique for the pump power of 90 mW and BP power of 1.1 mW. BP wavelength is set at 1602 nm (blue curve) and 1608 nm (red curve).

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When the initial BP injected into the laser cavity, it accumulates more energies through optical amplification in the EDF coil. Due to this stronger BP intensity along the SMF-28 fiber coil, the intensity of the Brillouin Stokes line generated from the SMF-28 fiber is also significantly increased. With respect to its higher power, the rate of stimulated emission at this injected wavelength is increased and therefore, the EDF gain block is forced to operate into a deep saturation regime. Under this condition, the gain is uniform due to the effect of homogenous saturation across the gain spectrum [13]. Hence the gain of the self-lasing cavity modes is also saturated and their total gain is less than the cavity loss. Therefore, these modes are fully suppressed and are not be able to oscillate above its threshold in the laser cavity.

For the BP direct-injection technique, the generation of Brillouin Stokes lines is only amplified after they propagate through the 3-dB coupler. Then, the amplified Brillouin Stokes lines suffer another 3-dB loss before entering the SMF-28 fiber. Therefore, it cannot generate a higher order Brillouin Stokes line efficiently. By comparing these two laser structures; after one round-trip, the BP intensity before entering the SMF-28 fiber is different thus the magnitude of saturation in the EDF coil is also different. Hence this discrepancy influences the characteristics of the BEFL even though the total cavity loss is kept constant for both laser configurations. Based on the findings, the effect of homogeneous gain saturation must be taken into the account to elaborate the characteristics of the proposed laser structure.

4. Tuning range study

In order to investigate the effect of these self-lasing cavity modes on the tuning range and the number of output channels (including BP), the BP wavelength is varied from 1580 nm to 1620 nm with 1 nm step. In this experiment, the BP power is varied from 0.5 to 3.5 mW with 0.5 mW step and the EDF pump power is also varied from 20 to 140 mW with 10 mW step. Since the tuning range is determined when the BEFL operates without any disturbances from the self-lasing cavity modes, the appearance of these self-lasing cavity modes around the EDF peak gain (1605-1606 nm) is closely monitored. Then from the experimental data, the tuning range is analyzed based on the above definition. In this research work, we analyze the same tuning range achieved from both techniques as shown in Fig. 3.

 

Fig. 3. Number of output channels with variation in BP wavelength for the tuning range of 10 nm at different pump powers, the BP power is 3.5 mW.

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The tuning range of 10 nm is obtained from both techniques at different pump powers, 50 mW and 100 mW for the BP direct-injection technique and the BP pre-amplification technique respectively. In this case, the BP power of 3.5 mW is used during the experiment. The number of channels for the BP pre-amplification technique is higher than that for the BP direct-injection technique. The average values are 17 channels and 11 channels for the BP pre-amplification and the BP direct-injection techniques respectively. These results show that the BP pre-amplification technique is able to suppress the self-lasing cavity modes at higher pump powers. Hence more injected pump powers can be utilized to generate more Brillouin Stokes lines. For the purpose of performance comparison, the tuning range of the BP direct-injection technique is also analyzed at 100 mW of 1480 nm pump power and the BP power of 3.5 mW. Based on the experimental results, the tuning range is limited to only 5.5 nm only. Referring to Fig. 4(a), the appearance of self-lasing cavity modes is observed around 1605 nm and 1606 nm for the BP wavelengths at 1600 nm and 1610 nm respectively. The generation of self-lasing cavity modes occurs at the expense of reduction of the number of Stokes lines. In contrast, the self-lasing cavity modes are efficiently suppressed for the BP pre-amplification technique as depicted in Fig. 4(b). The findings show that the tuning range is enhanced by 4.5 nm for the proposed technique with the same experimental conditions.

 

Fig. 4. Output spectrum of the BEFL utilizing (a) BP direct-injection technique and (b) BP pre-amplification technique for BP wavelengths at 1600 nm and 1610 nm. BP wavelength is set at 1600 nm (blue curve) and 1610 nm (red curve).

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Another analysis used to investigate the benefit of the proposed technique is to analyze the requirement of BP power to produce the same tuning range value. From the previous experiment, the tuning range is calculated and is compared for a fixed pump power value. One of the experimental results is depicted in Fig. 5. In this case, the pump power is tuned to 90 mW and the BP power required to produce this 8 nm tuning range is different for both techniques. In this experiment, the required BP powers are 3.5 mW and 1.0 mW for the BP direct-injection technique and the BP pre-amplification technique respectively. The same tuning range is obtained for the proposed technique at a lower BP power. Furthermore, the proposed technique also produces higher number of channels compared to the BP direct-injection technique.

 

Fig. 5. Number of output channels with variation in BP wavelength for the tuning range of 8 nm at different BP powers, the 1480 nm pump power is 90 mW.

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Finally, the output spectra of Brillouin Stokes lines for the proposed BEFL structure at different BP wavelengths are depicted in Fig. 6. The BP power and pump power are fixed to 3.5 mW and 120 mW respectively. The tunable range is from 1600 nm to 1609 nm (bandwidth of 9 nm) with the number of output channels in the range of 17-19 channels is obtained.

The magnified view of the output at 1603 nm of BP wavelength is depicted on Fig. 6(b). From these experimental results, there are no self-lasing cavity modes observed for all the BP wavelengths within this tuning range. Owing to the effect of homogenous gain saturation, the self-lasing cavity modes experience gain compression. Since the self-lasing cavity modes are the built-up from the weakly resonated modes, they are not be able to oscillate together with the strong injection-locking from the external BP source. It is interesting to note that the tunable range enhancement is obtained without using any manipulation of spectral gain shape as proposed in [9,10]. The inclusion of a spectral filter increases the cavity loss of the laser structure hence the output power is greatly reduced. Due to this reason, the peak power of the Brillouin Stokes signals is mostly higher than -15 dBm. Furthermore, the signal-to-noise ratio of 25 dB is much higher compared to the self-seeded technique as previously reported [5,10].

 

Fig. 6. Output spectra of Brillouin Stokes lines for the BP pre-amplification technique (a) the whole tunable spectrum of output channels at different BP wavelengths and (b) the magnified view of the output spectrum at 1603 nm BP wavelength.

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4. Conclusion

It has been successfully demonstrated that the self-lasing cavity modes of BEFL is effectively reduced by the intra-cavity BP pre-amplification technique. Experimental results have evidently shown that the proposed technique is superior to the conventional BP direct-injection technique. The optical amplification of the initial BP provided by the EDF coil generates a higher intensity of Brillouin Stokes line in the single-mode fiber. Hence the EDF gain is homogeneously saturated by this strong intensity of Brillouin Stokes line. As a result, the self-lasing cavity modes experience gain compression and consequently, these unwanted modes are efficiently suppressed in a wider wavelength range. In addition to this, the effective suppression of self-lasing cavity modes has enabled the BEFL structure to operate at higher pump powers which in turn produces higher number of channels. The tuning range of 9 nm is achieved from 1600 nm to 1609 nm with the number of outputs around 17-19 channels.