1. Introduction

Dense wavelength division multiplexing (DWDM) has emerged as a technique for exploiting the enormous bandwidth of optical fiber. As the transmission capacity of optical communication systems approaches a few Tb/s through DWDM method in recent years, multi-wavelength generation technology becomes more important, considering that the complexity and the cost of the source will increase as the number of DWDM channel increases. DWDM networks require wavelength-tunable transmitters, receivers and/or wavelength routers to enable multiple accesses and switching in the wavelength domain. A fast and widely tunable narrow linewidth laser source is a useful component for such systems.

To date, several multiwavelength Brillouin-erbium fiber lasers (BEFL’s) with different kinds of cavity configuration have been reported in a ring cavity [1–7] and a linear cavity [8–10]. A low tuning range of 2 nm in the 1532 nm region was observed with only one Stokes signal [1]. A single Stokes with 8.85 mW signal peak power around 7 nm tuning range was reported in [3]. An improved tunable Brillouin multiwavelength generation has been reported recently through adjusting the gain profile of Erbium-doped fiber by incorporating a Sagnac loop filter into the fiber ring [7]. As a result, a uniform 12-wavelength comb with 14.5 nm tunable range was obtained. However, all the previous works have limited tunability and the relationship between the number of the Stokes generated and the tuning range was not clearly explained.

In this paper, we report a widely tunable multiwavelength BEFL with up to 60 nm tuning range utilizing a linear cavity. The trade-off between the number of Stokes generated and the tunability of the Stokes signal was investigated. A stable operation of the Stokes was achieved by eliminating the Erbium-doped fiber laser (EDFL) cavity modes through careful optimization of the Brillouin pump power and the 980 nm pump power.

2. Experimental setup

The configuration of the multiwavelength Brillouin-Erbium fiber laser based on a linear cavity is illustrated in Fig. 1. A 980 nm-pump laser of 90 mW maximum output power was used as the primary pump light for the Erbium-doped fiber (EDF). The pump and signal lights were multiplexed via wavelength selective coupler (WSC). The length of EDF used in the experiment was 10 m. The cold cavity loss of 13.45±0.05 dB was measured from a broadband light source from 1530 to 1570 nm wavelength.

The linear cavity of the laser system was formed by two fiber loop mirrors that were constructed using two circulators at both ends of the resonator. The Brillouin pump (BP) was provided by an external-cavity tunable laser with maximum power of 13 dBm and 100 nm tuning range from 1520 to 1620 nm. The BP was coupled into the 8.8 km long of single mode fiber; SMF-28 using a 3-dB coupler (C) as shown in Fig. 1. The output of the system was taken at the output port of the first circulator (Cir1).

 

Fig. 1. Experimental setup of a linear cavity BEFL with an 8.8 km long SMF-28 fiber.

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3. Principle of operation

The operation mechanism of the linear cavity multiwavelength BEFL is described as follows. Without an external BP power launched, the liner cavity operated as a bidirectional EDFL with oscillating modes at the EDF peak gain. Below the threshold power level of the Brillouin-pump and 980 nm pump powers, no stimulated Brillouin scattering (SBS) occurred. Above the threshold condition, the BP light launched through the 3-dB coupler created a Brillouin gain in the SMF-28 fiber, which was down-shifted by 0.088 nm from the BP wavelength. This first-order Brillouin Stokes signal propagated in the opposite direction of the BP at this end of SMF-28 fiber, passed into the EDF and re-injected into the long SMF for double-pass amplification in around. If the total gain generated from SBS and EDF was equal to the cavity loss, a laser oscillation was formed between the second circulator (Cir2) and third circulator (Cir3). Due to fact that the Brillouin gain was homogenous, the Brillouin Stokes signal that operated in a single longitudinal mode could be utilized as the BP for the higher-order Brillouin Stokes signals [11]. The higher-order Brillouin Stokes signals could be generated in both directions by the lower-order Brillouin Stokes signals that passed the long SMF two times in a round. In this case, the excess of lower-order Brillouin Stokes after propagating through third circulator was above the threshold condition to create a higher-order Brillouin Stokes. This cascading of Brillouin Stokes generation continued until the total gain in the laser cavity was less than the cavity loss.

At the steady state condition, a stable laser was produced that consisted of the BP and its Brillouin Stokes signals. The output of the BEFL was taken at the output port of the first circulator (Cir1). It was noted that only a single EDF spool was used in this experiment to amplify the Stokes signal and support multiple wavelengths operation as compared to the utilization of two EDF spools in [4,5]. Furthermore, only a single 980 nm pump laser was deployed instead of two units of pump laser as discussed in the aforementioned reports. This design showed its simplicity with the reduction number of optical fiber components to create a multi-wavelength laser source.

4. Results and discussions

The tuning range of multi-wavelength BEFL was defined as the range of BP wavelength which produced the Stokes signals in the absence of self-oscillation EDFL cavity modes. Without BP power launched, the linear cavity resonator operated as a bi-directional EDFL with only self-oscillation EDFL cavity modes oscillating around the peak gain of EDF. When the BP signal was injected in the cavity; with sufficient power and pumped close to the EDFL cavity modes region, the gain saturation occurred around this region and the self-oscillation EDFL cavity modes were suppressed. By increasing the BP power, the EDFL gain was further saturated thus, the tuning range of BEFL was widened beyond the EDF gain peak.

 

Fig. 2. The presence of the EDFL cavity modes together with the Brillouin Stokes at lower injected BP power of -3.6 dBm.

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Figure 2 shows the presence of the EDFL cavity modes together with Brillouin Stokes at low launched BP power of -3.6 dBm and 980 nm pump power of 90 mW. The resolution bandwidth of the Optical Spectrum Analyzer (OSA) was set at 0.015 nm. However, the presence of EDFL cavity modes showed that the BP power was not enough to fully saturate the EDFL internal gain. Therefore, this caused instability on the Stokes signals and appropriate adjustments of pump power levels were taken to ensure that the BEFL operated without any EDFL cavity modes. As shown in Fig. 3, at a fixed value of BP power and 980 nm pump power of 13 dBm and 50 mW respectively; the BP wavelength was tuned to the region where there were no self-oscillation EDFL cavity modes and it was found to be from 1540.0 to 1570.2 nm. The resolution of OSA was set at 0.5 nm with 60 nm span.

 

Fig. 3. Tuning range of the output spectra of the linear cavity BEFL for BP power fixed to 13 dBm and the 980 nm pump laser was driven to 50 mW.

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Fig. 4. Tuning range of the linear cavity BEFL against 980 nm pump at different BP powers.

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Figure 4 elaborates the impact of the 980 nm pump and BP powers on the tuning range. The tuning range increased with the Brillouin pump power, due to the fact that a higher BP power would result in a higher Brillouin gain that would add to the extra gain required to reach the Brillouin threshold level. This allowed the SBS to take place at a wider spectral range. On the contrary, the tuning range decreased as the 980 nm pump power increased. The result obtained was very much within the expectation since at lower 980 nm pump power, the mode competition between the EDFL cavity modes and Stokes signals could be reduced. Therefore, the system could be tuned wider if the 980 nm pump power was maintained low enough to minimize this mode competition. As shown in Fig. 4, at a lower pump power of 10 mW and at a higher BP power of 13 dBm, the Stokes signal was tuned to a range of about 60 nm. The widely tuning range was achieved as the expense of reduction on the number of the Stokes generated. This reduction on the number of Stokes was due to the fact that the EDF gain was not high enough to produce cascaded Stokes at low 980 nm pump power. Thus the erbium gain was the enabler of producing higher number of Stokes with the expense of the tuning range.

Figure 5 shows the impact of BP power on the number of Stokes and tuning range at a fixed 980 nm pump power of 90 mW. At a higher BP power of 13 dBm up to 6-Brillouin Stokes could be observed to about 25.7 nm wide tuning range. Whereas up to 16 Brillouin Stokes over 4.7 nm could be observed at a lower Brillouin pump power of 1.0 dBm. The reduction on the number of Stokes at a higher BP power was due to the fact that the higher the BP was, the higher was the threshold of the higher-order Brillouin Stokes to become a Brillouin pump [11]. Thus, more EDF pumping powers were needed to achieve the same number of Brillouin Stokes. Increasing the 980 nm pump power would generally increase the number of the Stokes signal generated. This was due to the increment of EDF gain that could easily compete with the Brillouin gain suppression.

 

Fig. 5. Tuning range and number of Stokes generated against BP power at 90 mW pump power.

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In order to operate the BEFL, the BP wavelength should be close to the wavelength at which the EDF laser would operate (in the absence of BP), as the Brillouin gain generated must suppress the EDFL operation. However, BEFL operation was possible over a range of other wavelengths with some efficiency penalties. Figure 6 depicts the number of generated Stokes against BP wavelength at different BP powers. The 980 nm pump power was fixed at 90 mW and the BP wavelength was tuned from 1540 to 1575 nm. The maximum number of the Brillouin Stokes was observed at the peak of the erbium gain around 1558 nm. The number of Stokes reduced when the BP wavelength was detuned away from this wavelength. This was due to the decrement of the Erbium gain in the laser cavity, which led to insufficient signal power for the higher order stokes to pump the SMF-28 fiber, thus terminating the process of multiple generation of the Stokes.

 

Fig. 6. The impact of BP wavelength on the generation of Brillouin Stokes at different BP powers with the 980 nm pump power was set at 90 mW.

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5. Conclusion

A thorough experimental analysis of the tuning range characteristics of a multi-wavelength laser source utilizing a linear cavity of hybrid BEFL system was successfully investigated. The Brillouin and EDFL gain profiles were vital in determining the stability and the tuning range characteristics of the multiwavelength BEFL system. At a fixed BP power, higher EDF pump power caused mode competition and produced smaller tuning range. On the other hand, higher BP power suppressed the EDFL cavity modes, reduced the number of Stokes generated and allowed the Stokes shifted signal to take place at a wider tuning range. Two Brillouin Stokes were simultaneously tuned with a widely tuning range as large as 60 nm at 10 mW and 13 dBm of 980 nm pump and BP powers respectively, while up to six Brillouin Stokes were tuned over a 25.7 nm from 1542.5 to 1568.2 nm at 13 dBm and 90 mW of BP and 980 nm pump powers respectively. The wide tuning range of Brillouin Stokes was achieved by injecting high Brillouin pump powers to saturate the Erbium gain in the laser cavity.