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  • Abstract

SECTION 1

INTRODUCTION

The growing need for very fast information dissemination around the globe has necessitated the growth and exploration of new concepts in fiber optical communications system that is generally accepted as a dominant medium for high speed data/information transmission. This is due to the awesome bandwidth offered by the optical fiber. Dense wavelength division multiplexing (DWDM) systems that are based on the ability of an optical fiber to carry many different wavelengths of light simultaneously without mutual interference evolved to utilize the massive bandwidth of the optical fiber [1]. Fiber laser have been reported to be an attractive candidate to support the DWDM systems because of its ability to generate multiple wavelengths from a coherent single wavelength light source [2]. Among the different technologies of fiber laser design, the Brillouin-erbium fiber laser (BEFL) has been receiving a lot of attention from researchers for its several advantages and potential applications that include low threshold, narrow linewidth, inherent maintained Stokes signal spacing of Formula$\sim$10 GHz [3], a high-power single-frequency laser [4], and optical generation of microwave/millimeter-wave carriers [5].

Multiple-wavelength BEFL (MWBEFL) that covers the conventional band (C-band) and the long-wavelength band (L-band) of the optical communication windows, utilizing ring and linear cavity resonators, have been demonstrated [6], [7], [8]. In an effort to increase the capacity utilization in the DWDM communication systems, researchers considered the L-band MWBEFL as an extension of the C-band MWBEFL. One of the early works on L-band MWBEFL was reported by Harun and Ahmed [8]. In the demonstrated work, 10 channels were obtained using dual ring cavity that utilized two erbium-doped fiber (EDF) and two 980-nm pumps. In another work, Harun et al. reported an L-band MWBEFL that utilized two EDFs in another ring cavity resonator that produced 5 channels [9]. Further increase in the number of output channels produced by a linear cavity L-band MWBEFL to 24 was reported by Haddud et al. in 2005 [10]. In this paper, however, two 980-nm pump lasers were employed in pumping the EDF. In the work reported by Al-Mansoori et al. in 2007, a linear cavity L-band MWBEFL was demonstrated [11]. In this paper, the MWBEFL was pumped by a 1480-nm laser, and the resonator was formed by two fiber loop mirrors at both ends of the laser's cavity. This architecture utilizes the high power conversion efficiency of 1480 nm pump power. In this way, the laser has a reduced threshold value, and thus, 23 Brillouin Stokes lines are generated.

Another approach for enhancing channels generation of L-band MWBEFL had been demonstrated, in which 27 lasing lines were observed. The researchers employed nonlinear amplifying loop mirror in the resonator [12]. Additionally, by utilizing double-pass Brillouin pump (BP) preamplification technique within the laser cavity, 30 lasing lines were obtained by L-band MWBEFL [13]. Recently, we reported widely tunable L-band MBEFL that generates 24 channels by utilizing polarization maintaining fiber (PMF) and two polarization controllers (PCs) placed in a nonlinear amplified fiber loop mirror (AFLM) filter [14], However, the use of PCs in the nonlinear AFLM makes the proposed laser cumbersome for any practical application.

In this paper, we report an efficient L-band MWBEFL that utilizes AFLM in the fiber laser structure. The proposed laser utilizes low pumping powers to generate higher number of lasing lines. We experimentally prove that the laser structure generates 55 lasing lines at BP signal wavelength of 1601.7 nm with corresponding power of −8.5 dBm and 1480-nm laser pump power of 33 mW. This was achieved through the manipulation of the EDF pump power in the AFLM (where the transmission and reflection function of the AFLM is associate with the EDF gain factor) with the assistance of the choice of BP signal wavelength and power such that the effect of gains depletion and saturation of Erbium and Brillouin, respectively, are reduced. To the best of our knowledge, the manipulation of the laser pump powers on the gains depletion and saturation of L-band MBEFL utilizing AFLM has not been investigated. Also, to the best of our knowledge, this is the highest number of lasing lines produced by a linear cavity, L-band MWBEFL.

SECTION 2

LASER STRUCTURE AND PRINCIPLE OPERATION

The experimental setup of a multiwavelength BEFL utilizing AFLM is shown in Fig. 1. The laser cavity comprises AFLM, single-mode fiber (SMF) and a highly reflective mirror (M) spliced to one end of the SMF and an optical circulator. The AFLM is formed by a bidirectional erbium-doped fiber amplifier (EDFA) connected between the output ports of a 50/50 (3-dB) coupler that forms a fiber loop. The EDFA consists of EDF and a wavelength selective coupler (WSC) that was used to multiplex the pump and signal lights. A 1480-nm pump laser with a maximum power of 150 mW was used for the excitation of the Formula$\hbox{Er}^{3+}$-ions. The Brillouin gain is generated in the SMF that is placed between the highly reflective mirror and the 50/50 coupler. An external tunable laser with 1 MHz laser linewidth was used to provide the BP signal, which is injected into the laser's resonator through port 1 of the circulator as shown in Fig. 1. The output of the laser was monitored through an optical spectrum analyzer (OSA) spliced to port 3 of the circulator. The OSA has its spectral resolution set at 0.015 nm.

Figure 1
Fig. 1. Multiwavelength BEFL with AFLM.

In order to design an efficient MWBEFL, the balance between cavity gain and loss must be considered. In the proposed laser structure, the linear gain is provided by a fixed length of EDF. In our case, we chose 10 m long EDF with characteristics of 900-ppm erbium ion concentration, a cutoff wavelength around 1420 nm, and an absorption coefficient of 19 dB/m at 1530 nm. On the other hand, there is a tradeoff between Brillouin gain and propagation loss, and thus, the SMF length must be optimized. In our experiment, seven different spools of SMF with length of 0.1 km, 0.5 km, 1 km, 4.7 km, 6.7 km, 11.5 km, and 12.8 km were tested. We found that there is a significant effect on the characteristics of the laser contributed by the change of SMF length from 0.1 km to 12.8 km. Short length of SMF requires higher laser power for a sufficient acoustic wave to be setup in the fiber to induce scattering. On the other hand, longer SMF length increases the cavity loss. Finally, we found that 6.7 km is the optimum SMF length to achieve the desired results.

The operation characteristics of L-band multiwavelength BEFL utilizing AFLM with intracavity preamplified BP technique is hereby described below. Without the BP signal being injected into the cavity, normal lasing action of the linear cavity EDFL occurred at various oscillating cavity modes at the EDF peak gain. In our laser architecture, shown in Fig. 1, the BP signal is first injected through ports 1 and 2 of the circulator into port 1 of the 3-dB coupler. At the 3-dB coupler, the injected BP signal splits into two beams of equal intensities. 50% of the BP signal propagates in the clock-wise (CW) direction through port-3 of the coupler, around the loop while the other 50% propagates in the counter clock-wise (CCW) direction through port-4 of the coupler. Upon completion of one round trip around the AFLM, the two beams are amplified in the EDF and recombined in the fiber coupler. The transmitted signal available in port-2 of the 3 dB-coupler is therefore the sum of a clockwise field of arbitrary phase Formula$\theta$ and an anticlockwise field of relative phase Formula$\theta$Formula$\pi$ both of equal amplitude [16]. This phase difference between the CW and CCW propagating beams determines the amount of transmitted and reflected light by the AFLM. The transmitted preamplified BP signal is then injected into the 6.7-km SMF through port-2 of the 3-dB coupler to initiate the SBS effect. Above the threshold condition, the amplified BP signal creates a Brillouin gain in the SMF, the effect of which creates the first Brillouin Stokes signal that is down-shifted by 0.089 nm from the BP wavelength.

A laser comb can be formed between AFLM and the highly reflective mirror when the total Brillouin and erbium gains generated are equal to the total cavity loss. This first-order Brillouin Stokes signal propagates in the opposite direction of the transmitted preamplified BP signal. It then divided into two signals of equal intensities by the 3-dB coupler. These two first-order Brillouin Stokes signals are amplified in the EDF, recombined in the fiber coupler, and then reflected to serve as a BP signal for the second-order Brillouin Stokes signal. This process of cascading Brillouin Stokes generation continues until the total gain in the laser cavity is less than the cavity loss at the operating wavelengths. At the steady state condition, a stable laser is produced consisting of the BP and its Brillouin Stokes signals.

SECTION 3

RESULTS AND DISCUSSION

We evaluate the threshold power of the laser, being one of the parameters for the determination of laser efficiency. The lower the threshold power, the better the overall efficiency of the laser system [15]. In multiwavelength BEFL at a fixed BP power, the threshold power was determined by measuring the amount of power of the laser that pumps the EDF at a point when the first Stokes signal appeared in the laser cavity. Fig. 2 illustrates the impact of BP power on the threshold power of the first-order Brillouin Stokes signal at BP wavelengths near the EDF peak gain at 1602 nm and 1605 nm. The results show that lower lasing threshold power of 12.8 mW was achieved at the EDF peak gain at 1605-nm BP wavelength with power of 5.4 dBm. In addition, when the injected BP power is less than −6 dBm, the threshold power is almost equal for 1602 nm and 1605 nm. This is attributed to the efficient switching mechanism of the FLM [16] and to the equal EDF amplification at this particular BP wavelength and power.

Figure 2
Fig. 2. Threshold power with respect to BP power for different BP wavelengths: 1602 and 1605 nm.

Fig. 3 shows the threshold power of the first-order Brillouin Stokes signal at different BP signal wavelengths and power. The lowest threshold power was found at 1605 nm for −8.5 dBm, 0.41 dBm, and 5.4 dBm of BP signal powers, which is attributed to the EDF peak gain. This wavelength range is obtained by investigating the behavior of the linear-cavity laser without injecting any BP signal in which case, the laser cavity acts as a free-running laser. When the BP wavelength is tuned far away from the EDF peak gain, higher pump powers are required to get the first Stokes signal. This is because of the less EDF gain, which results in the reduction of the total EDF and Brillouin gains. In addition, for any value of BP power, it is found that there is a limited range for BP wavelengths to achieve a BEFL lasing condition and to get the Stokes signal. It is shown in Fig. 3 that at −8.5 dBm BP power, if the BP wavelength range is lower than 1595 nm and higher than 1612 nm, no Stokes signals are generated, even if the 1480-nm pump power is at its maximum value. The operational bandwidth of the laser cavity to get the Stokes signal is 18, 30, and 40 nm for −8.5, 0.41, and 5.4 dBm BP powers, respectively.

Figure 3
Fig. 3. Threshold power of the first-order Stokes signal for different BP wavelengths; its power is set at −8.5, 0.41, and 5.4 dBm.

In multiwavelength BEFL, higher BP power depletes the EDF gain and thus prevents efficient use of the EDF gain in the Stokes signals amplification. Therefore, to achieve the generation of the maximum possible number of stable output channels, achieving equilibrium between the 1480 nm pump power and BP signal power is very important. In this regard, characterization has been carried out at low 1480-nm pump and BP powers in order to efficiently utilize the EDF gain, reduce the Brillouin gain saturation and alleviates modes competition.

Fig. 4 shows the impact of BP power on the number of output channels generated at BP wavelength of 1602 nm. The BP was varied from −11.74 dBm to 5.44 dBm at low 1480-nm pump power of 20 mW and 30 mW. It can be seen that the number of output channels decreases with the increment of the BP power. This result is in agreement with the theory and experimental results reported in [17] and [18]. With higher BP power, the number of generated Stokes signals are reduced; thus, the Stokes signals generated closer to the BP power are so dominant that the gain of higher-order Stokes signals are suppressed. Also, due to the fact that the first BP power set a threshold for the consecutive Brillouin Stokes signal to become a BP signal, this threshold value is related to the BP power intensity. The lower the BP power, the lower is the threshold for the higher order Stokes signal to become the BP for the next order Stokes signals. Therefore, efficient utilization of the EDF and Brillouin gains leads to the generation of higher number of output channels. To buttress this argument, it can be seen in Fig. 4 that at low 1480-nm pump power of 20 mW, up to 28 channels are obtained at −10.97 dBm of BP signal power. In addition, a total of 48 stable output channels are observed for 1602-nm BP wavelengths. If we however refer to prior work on L-band MWBEFL reported by [10], [19], no Stokes signal could be generated at this low pump power of 30 mW. The reason was attributed to the higher threshold power required to get the first order Stokes signal, measured at 60 mW in [8] and 33 mW in [19].

Figure 4
Fig. 4. Number of output channels versus BP power at low 1480-nm pump power of 20 and 30 mW for BP wavelength of 1602 nm.

The main objective in this research work is to optimize the MWBEFL operation in order to generate maximum possible number of stable output channels. To achieve this objective, the optimization of Brillouin and erbium gains in the laser cavity structure is critical. In this way, the EDF pump power, BP power and wavelength must be optimized to get the best possible results. In MWBEFL, the intensity of the free-running cavity modes is proportional to the increment of EDF pump power, in this case, the 1480-nm pump power. Higher 1480-nm pump power cause higher mode competition region between the Stokes signals and the free-running cavity modes. The output channels became unstable in this region and operate simultaneously with the free-running cavity modes. In the optimization process, the BP power was fixed to a low value of −8.54 dBm, the 1480 nm pump power was varied to a few values of 25, 30, 33, 40, and 45 mW. Fig. 5 depicts the presence of unstable free-running cavity modes within the Brillouin Stokes signals at 45 mW and −8.5 dBm of 1480-nm and Brillouin power powers, respectively. Referring to Fig. 5, the rise of the noise floor in the BP wavelength range between 1604.8 nm to 1605.6 nm originated from the free-running cavity modes with its peak around 1604.8 nm. The mode competition causes the optical signal-to-noise ratio (OSNR) for the Brillouin Stokes signals in the mentioned wavelength range to be no longer consistent. Under this situation, the free-running cavity modes absorb more pumping energy for their amplification in the laser cavity. The remaining pump energy available for utilization by the Brillouin Stokes signals is therefore inadequate to suppress the cavity modes. Therefore, low 1480-nm pump powers were chosen to reduce the effect of the free-running cavity modes on the laser stability and OSNR of the Stokes signals. In addition, the Brillouin gain saturation and erbium gain depletion can be reduced with low injected 1480-nm and BP pump powers.

Figure 5
Fig. 5. Presence of the free-running cavity modes effecting the Brillouin Stokes signals at 45 mW and −8.5 dBm of 1480 nm and Brillouin power powers, respectively.

Finally, the BP wavelength is varied from 1601 to 1603 nm. For each 1480-nm pump power, the numbers of output channels generated were observed. Based on the optimization process the maximum number of output channels was obtained at 1601.7-nm BP wavelength with −8.54 dBm and 33 mW of BP power and 1480-nm pump power, respectively. Up to 55 stable output channels are obtained with 0.089 nm channels spacing as depicted in Fig. 6. This is the highest number of output channels achieved in the development of L-band MWBEFL system to date to the best of our knowledge. Referring to Fig. 6, the first channel represents the BP power which has higher intensity than the other Brillouin Stokes signals. This is attributed to the reflection and transmission features of the fiber loop mirror as explained early.

Figure 6
Fig. 6. Output spectrum of multiwavelength BEFL with AFLM at 33 mW of 1480-nm pump power and 1601.7-nm BP wavelength with power of −8.5 dBm.

The spectral stabilities of the first twelfth generated output channels is tested at 33 mW of 1480-nm pump power and 1601.7-nm BP wavelength with power of −8.5 dBm. Fig. 7 shows the fluctuations of the first 12 generated output channels (without BP signal). The laser output spectrum was scanned at an interval of 5 minutes for a period of 60 minutes. Referring to Fig. 7, power fluctuations confined to within ±0.11 dB was observed from the first generated output channel (1st Stokes signal) to the twelfth channel. This shows that the proposed laser structure has good stability over the scanned duration.

Figure 7
Fig. 7. Spectral stabilities of the first 12th generated output channels at 33 mW of 1480-nm pump power and 1601.7-nm BP wavelength with power of −8.5 dBm.

For the effect of temperature in Brillouin fiber laser, it has been shown that temperature plays a major role in many different laser characteristics such as lasing frequency, mode hopping, spontaneous pulsing threshold and frequency mode pulling [20], [21]. The temperature dependence of the Brillouin fiber laser output frequency was experimentally measured to be 2.4 Formula$\hbox{MHz}/^{\circ}\hbox{C}$ and the stability of the output beat frequency to the temperature fluctuation was estimated as 1.04 °C/2.5 MHz [21]. When the temperature changed, the fiber refractive index and the acoustic velocity vary, followed by a change in the Brillouin frequency shift. Theoretically the temperature dependence of the Brillouin frequency shift can be estimated based on previous analytical relationship [20], [21] as Formula TeX Source $${dv_{B} \over dT} = {2 \over \lambda}\left[V_{a}{dn \over dT} + n{dV_{a} \over dT}\right]\eqno{\hbox{(1)}}$$ where Formula$v_{B}$ is Brillouin frequency shift, Formula$Va$ is the acoustic velocity, Formula$n$ is the fiber refractive index, Formula$\lambda$ is the wavelength of the pump, and Formula$T$ is the temperature. Equation (1) shows that the Brillouin frequency shift varies with temperature through the change in refractive index and the change in the acoustic velocity.

SECTION 4

CONCLUSION

We have successfully demonstrated an efficient linear cavity L-band MWBEFL that produced 55 stable output channels at low Brillouin and laser pump powers of −8.54 dBm and 33 mW, respectively. The laser structure has the unique advantage of utilizing AFLM and permits preamplification of the BP signal in the laser cavity before being transmitted into the Brillouin gain medium, the SMF. This arrangement has the advantage of utilizing low amount of BP and 1480-nm pump powers that can sustain a large number of lasing signals. The impacts of BP power, BP wavelengths, and 1480-nm pump power on the number of output channels was thoroughly investigated. We discovered that the utilization of AFLM with the manipulation of EDF pump power, BP signal wavelength and power reduces the effect of gains depletion and saturation of Erbium and Brillouin, respectively. Thus, enhances the capacity of L-band MBEFL and leads to the generation of a high number of output channels.

Footnotes

Corresponding author: M. H. Al-Mansoori (e-mail: mmansoori@soharuni.edu.om).

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M. H. Al-Mansoori

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M. Ajiya

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M. A. Mahdi

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