1. Introduction

Over the past decades, multiwavelength fiber lasers have aroused considerable interest due to the comprehensive applications in the fields of dense-wavelength-division-multiplexing systems, fiber optic sensing, microwave photonics and spectroscopy [1–6]. The requirements for multiwavelength sources include a large number of channels over broad bandwidth, precise and stable wavelength spacing, narrow linewidth and small power fluctuation. Both the erbium-doped fiber amplifier (EDFA) and the semiconductor optical amplifier (SOA) have acted as the gain medium to achieve the multiwavelength fiber laser. Compared with the SOA, the EDFA has the characteristics of a higher saturation power, a lower polarization-dependent gain and a flatter gain spectrum. However, the main challenges for the EDFA are a strong homogenous line broadening at room temperature owing to the serious mode competition. A lot of effective solutions have been proposed to overcome the limitation such as utilizing polarization hole burning [7], phase modulation [8], four-wave mixing (FWM) [4] and the cascaded stimulated Brillouin scattering (SBS) [1–3,9–11]. Brillouin-erbium fiber laser (BEFL), which utilizes the hybrid integration of the nonlinear Brillouin gain and the linear gain of the EDFA, has been regarded as a potential and prosperous solution owing to several important advantages, e.g. the ultra-narrow wavelength spacing [12], the stable multiwavelength operation at room temperature [13], low threshold [14], low intensity noise and narrow linewidth [15].

The hybrid BEFL was firstly proposed and demonstrated by Cowle and Stepanov [12]. Recently, different kinds of BEFLs have been developed. On the one hand, a scheme utilizing the extraneous amplified spontaneous emission into a passive EDF section achieved 28 stable output channels [16]. Then, L-band multiwavelength BEFL source employing nonlinear amplifying loop mirror generated 27 lasing signals using a 6.7-km standard single mode fiber (SMF) [17]. In addition, a BEFL, which utilized double-pass amplification technique to preamplify the Brillouin pump (BP) within the laser cavity before entering the SMF, produced 30 lasing lines [18]. On the other hand, the self-seed BEFL can generate a large number of Stokes lines. About 120 Stokes lines incorporating 5-km SMF and a Sagnac loop mirror were achieved by adjusting the polarization controllers (PCs) [3]. Furthermore, the generation of above 160 Stokes lines was observed in a linear cavity with a 12.5-km SMF [1]. However, as previously mentioned experiments, it is not convenient for practical application due to the use of a long SMF or PCs. Recently, multiwavelength fiber laser using only a 20-m highly nonlinear fiber (HNLF) in conjunction with a 215-cm bismuth-based erbium-doped fiber has been demonstrated [19]. The proposed multiwavelength fiber laser generated 13 lasing lines including anti-Stokes.

Although the intensive experiment researches have been explored on the multiwavelength BEFLs, the method exploiting the cascaded SBS and the FWM processes has been seldom investigated since D. S. Lim et al. reported the generation of high-order Stokes and anti-Stokes lines in a Sagnac loop mirror [11]. In this paper, we experimentally demonstrate an approach to generate a stable and tunable multiwavelength comb fiber laser utilizing a simple configuration with a 135-m HNLF. The stable multiwavelength lasing source with an ultra-narrow wavelength spacing of 0.075 nm is achieved in a wide optical spectrum ranging from 1560.2 nm to 1571.8 nm via the cascaded SBS and multiple FWM processes, which significantly increases the number of lasing lines.

2. Experimental setup and principle of operation

The experimental setup of the proposed multiwavelength BEFL is illustrated in Fig. 1 . It is similar to the basic BEFL configuration except that a section of SMF is removed and the change of the incident BP direction [20]. In the scheme, 50% power of the injected BP signal is firstly preamplified and then enters the HNLF. The fiber ring cavity consists of a 3-dB coupler, two optical circulators (Cir1 and Cir2), a 135-m HNLF (Sumitomo electric industries, Ltd) and a bidirectional pump EDFA. The EDFA is composed of a 1480 nm pump laser, 10-m EDF, an isolator and two wavelength-selective couplers (WSCs). The 1480 pump laser is divided into two parts (P1 and P2) by a 3-dB coupler to improve the performance of the EDFA. Two WSCs are acted as multiplex the Brillouin pump and the signal waves. The tunable laser source (TLS, APEX Inc.) with a 3-MHz linewidth and maximum output power of 13 dBm, acts as the BP signal and is injected into the ring cavity via the 3-dB coupler in the clockwise direction. The loss, the zero-dispersion wavelength, the dispersion slope, the effective area and the nonlinear coefficient of the HNLF are 0.51 dB/km, 1552 nm, 0.031 ps/(nm²∙km), 11 μm² and 20 W⁻¹km⁻¹, respectively. An optical spectrum analyzer (OSA, AQ63708B) with a resolution of 0.02 nm monitors the output from one of the ports of the 3-dB coupler. The Cir1 provides a unidirectional path for both the BP signal and the Brillouin Stokes (BS) signal. The Cir2, serving as a high reflectivity optical mirror in order to reflect the BP and signal lights, is used to form the laser cavity that enables laser to oscillation.

 

Fig. 1 Schematic diagram of the proposed multiwavelength BEFL.

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When the intensity of the BP signal is strong enough to excite SBS, the generated first-order BS signal will propagate in the opposite direction of the BP signal, and enter the ring cavity via the Cir1. The forward-transmitted BP signal will be reflected back into the HNLF through the Cir2. SBS will be initiated again if the remaining power of BP is still above the Brillouin threshold of the HNLF. The first-order BS signal amplified by the EDFA will take as the BP signal to create the second-order BS signal. In the same way, next order BS signal will generate the higher-order BS signal. At the same time, the Cir2 permits, BP and BS lines propagating oppositely, in the clockwise direction in the ring cavity. Hence, it excites the degenerate and non-degenerate FWM processes. An intense BP with frequency ${\omega }_p$ generates BS signal with frequency${\omega }_s$, which is a Brillouin frequency shifted by ${\Lambda }_B$ (i.e., ${\omega }_s={\omega }_p-{\Lambda }_B$) with respect to the BP. The two beams interact and hence generate an anti-Stokes signal (i.e., ${\omega }_{i1}=2{\omega }_p-{\omega }_s={\omega }_p+{\Lambda }_B$) and a Stoke signal (i.e., ${\omega }_{i2}=2{\omega }_s-{\omega }_p$ $={\omega }_p-2{\Lambda }_B$) through the degenerate FWM process. Then, these beams produce consecutive higher-order Stokes and anti-Stokes lines by means of the cascaded SBS and multiple FWM processes.

3. Experimental results and discussion

Firstly, we observed the self-lasing cavity modes region to match the BP wavelength. When no BP signal is launched into the ring cavity, unstable self-lasing cavity modes emerge around 1563~1568 nm region due to the strong mode competition. To suppress completely the self-lasing cavity modes, the BP wavelength is firstly set at 1565.5 nm. Figure 2 (a) displays the output optical spectra at both P1 and P2 of 165 mW, BP power of 12.0 dBm, which covers a wide lasing spectrum ranging from 1554.5 nm to 1576.5 nm. The blue curve in Fig. 2 (a) indicates the self-lasing cavity modes, from which one can find that the envelope profile of the output spectrum of the multiwavelength BEFL is similar to the profile of the self-lasing cavity modes. The zoom-in views of the dashed lines from 1563.5nm to 1568 nm and 1572 nm to 1576.5 nm are shown in Figs. 2 (b) and (c), respectively. It can be observed that the wavelength spacing of the multiwavelength BEFL is 0.075 nm, and peak powers of seven Stokes lines, which are generated by the cascaded SBS effect, are greater than −2.5 dBm. Then, the other higher-order Stokes and anti-Stokes lines produced by multiple FWM processes are perfectly symmetric with respect to the seven lower-order BS lines. An interesting phenomenon is that, the peak power of the first-order anti-Stokes line is much lower than that of the second-order anti-Stokes line, and this is because the signal is mostly produced by FWM effect between weak BP signal and the first-order Stokes line. For the peak power and the optical signal-to-noise ratios (OSNRs) of lasing lines that exceed −26 dBm and 10 dB respectively, we can observe that there exist about 80 wavelengths from 1562.5 nm to 1568.5 nm. Then, when both the peak powers and the OSNRs are above −37 dBm and 5 dB respectively, more than 150 lasing wavelengths in 11.6-nm range from 1560.2 nm to 1571.8 nm are achieved. Although more than 200 lasing lines in a broad optical spectrum ranging from 1557.5 nm to 1573.5 nm are observed, the OSNRs are only above 3 dB and there are spurious cavity modes noises at the bottom of these lasing lines, as illustrated in Fig. 2(c).

 

Fig. 2 Spectra of the generated multiwavelength BEFL at BP power of 12.0 dBm with (a) full span and self-lasing cavity modes, and the zoom-in views of (b) 1563.5-1568 nm and (c) 1572-1676.5 nm.

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We have investigated the effect of 1480 nm pump powers and BP power on the multiwavelength generation. With the increasing of 1480 nm pump powers, the intensity and spectral width of the self-lasing cavity modes are increased. As a result, a larger BP power is necessary to suppress the stronger self-lasing cavity modes. When P1 and P2 are both 35 mW and the BP power is −11 dBm, all the 30 lines including 10 anti-Stokes lines are observed, which has shown an obvious multiple FWM effect and low Brillouin threshold power. Figure 3 displays the output spectra under the condition of both P1 and P2 of 80, 90, 100, 110 and 150 mW, and BP power of 4.5, 6.5, 8.5, 10.5 and 11.8 dBm respectively. From Fig. 3, 1480 nm pump powers and BP power have a great influence on the channel number and the output powers. With the increase of 1480 nm pump powers and BP power, the lasing spectrum grows wider. For both P1 and P2 of 80 mW and BP power of 4.5 dBm, 10-nm spectral width is observed. When the powers of both P1 and P2, and BP increase to 150 mW and 11.8 dBm, respectively, the optical spectrum width is broadened to 18 nm. It should be noted that the BP power has a great influence on the OSNRs at the fixed 1480 nm pump power. With the continuous increase of BP power, the number of Brillouin Stokes will reduce gradually [21] and the OSNRs of lasing lines will decrease significantly since the intense power of BP will make the EDFA gain saturation. In addition, the small number of Brillouin Stokes lines weakens the conversion efficiency of multiple FWM processes. Hence, the number of the generated higher-order Stokes and anti-Stokes lines greatly decreases. There exists an optimal BP power to achieve the large OSNRs and the number of lasing lines. In experiments, we find that when the BP power just can suppress completely the self-lasing cavity modes at fixed 1480 nm pump powers, the multiwavelength BEFL obtains a better performance in terms of the OSNRs and the number of lasing lines.

 

Fig. 3 Output spectra of the proposed multiwavelength BEFL for different pump powers and BP powers.

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The limitation of the tuning range for the proposed multiwavelength BEFL, similar to other BEFLs, is largely dependent on the gain competition between Stokes lines and self-lasing cavity modes [22]. On the one hand, the tuning range increases with the BP power. At lower power of 45 mW for both P1 and P2, and BP power range from 2 dBm to 5 dBm, the tuning range increases from 20 nm (1560~1580 nm) to 30 nm (1557~1587 nm) at the expense of the number of output channels, which is in agreement with the reported theory [21]. On the other hand, the tuning range decreases with the increase of the 1480 nm pump power owing to the stronger self-lasing cavity modes. Figure 4 denotes a 6-nm tuning range from 1562 nm to 1568 nm without the occurrence of unwanted self-lasing cavity modes at 80 mW for both P1 and P2, and 5 dBm for the BP power. There exist more than 80 lasing lines with above 5-dB OSNR. While both P1 and P2 increase to 160 mW, the tuning range decreases to only 2 nm.

 

Fig. 4 Tunability of the proposed multiwavelength BEFL at 80-mW both P1 and P2, and 5-dBm BP power.

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Stability is one of the most important properties for the multiwavelength sources. The good stability of the proposed multiwavelength BEFL is experimentally demonstrated. Under the same conditions, as illustrated in Fig. 2, the optical spectrum ranging from 1567 nm to 1569 nm is scanned every 10 minutes during an hour period. The experimental results show that power fluctuations of each lasing line and the total power are within 0.3 dB and 0.01 dB, respectively. Moreover, the operating wavelength drifts, depending on the stability of the experimental environment and BP wavelength, are within 0.002nm. There may be mainly three reasons for the good stability of the multiwavelength source. Firstly, the BP power makes the generated BS lines reach their saturation power level and reduce the power fluctuations for the BS lines. Then, the multiple FWM processes in the fiber decrease the power fluctuations of Stokes and anti-Stokes lines owing to the self-stabilization characteristic in FWM, which has been well demonstrated [23,24]. Furthermore, the FWM effects and high nonlinear gain in HNLF effectively suppress the homogeneous line broadening of the EDFA [4,25,26]. Hence, the power stability of the BEFL is greatly improved in comparison with the conventional BEFLs to some extent.

It is well known that, the conversion efficiency of FWM depends on the phase mismatch. The propagation constant mismatch $\Delta \beta$is expressed as [27]

where is the dispersion slope,${\lambda }_s$, ${\lambda }_p$ and are the signal, pump and the zero-dispersion ${\lambda }_0$wavelength respectively. The power of the first 7 Stokes generated by SBS is much larger than that of other lasing lines. According to Eq. (1) and the previous mentioned parameters of the HNLF, considering pump wavelength of 1565.5 nm and its 1st and 7th Stokes signal, $\Delta \beta$ approximates to 1.8 × 10⁻⁶ m⁻¹ and 9 × 10⁻⁵ m⁻¹ respectively. Therefore, for the 135 m HNLF, the effects of phase mismatch are almost negligible. Similarly, the situation is the same for other pump and the adjacent signals. Thus, the multiple and cascaded FWM processes for the generation of higher-order Stokes and anti-Stokes lines are highly efficient, and more than 20-nm lasing spectral width is obtained.

The multiwavelength BEFL based on Fig. 2 is measured using a high-speed photo-detector followed an electrical spectrum analyzer (Anritsu MS2668c). Figures 5 (a) and (b) illustrate the beat notes at 10 MHz range disconnecting and connecting tunable bandpass filter with 3-dB bandwidth of 0.9 nm in front of the photo-detector respectively. It can be clearly seen in Fig. 5 (a), there are multiple longitudinal modes with a spacing of 0.667 MHz corresponding to the free spectral range of the ring cavity. In contrast, when the bandpass filter is tuned from 1560.2 nm to 1571.8 nm, no beat note emerges, as exhibited in Fig. 5 (b), which illustrates an operation of single-longitudinal-mode [9,28]. Nevertheless, there is an occasional mode hopping owing to the fluctuation of the environmental temperature or the change of polarization states. In the region of 1557.5~1560.2 nm and 1571.8~1573.5 nm, the OSNRs are less than 5 dB and the EDFL cavity modes noises are not completely suppressed, as depicted in Fig. 2 (c). Thus, the lasing lines are not under a rigorous single-longitudinal-mode operation. For the other region of the broad optical spectrum, the EDFL operation is dominant and exhibits the multi-longitudinal-mode operation. Comparing the noise performance of the multiwavelength BEFL without and with the tunable bandpass filter, the noise power of the filtered lasing lines is ~20 dB lower than that of disconnecting the bandpass filter [28].

 

Fig. 5 Electrical spectra of the beating signal observed at the output of the photo-detector (a) disconnecting and (b) connecting a tunable bandpass filter.

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4. Conclusion and outlook

We have successfully demonstrated a simple-structure multiwavelength generation with a 135-m HNLF. The scheme is composed of the conventional optical components, and neither intricate PCs and modulator nor filters are utilized in experiment. The simultaneous presence of the BP signal and Stokes lines within the ring cavity evokes high-order Stokes and anti-Stokes lines via multiple FWM processes. More than 150 Stokes lines in single-longitudinal-mode operation with rigid 0.075-nm wavelength spacing are obtained, and the peak powers and the OSNRs of all lasing lines are greater than −37 dBm and 5 dB respectively. The multiwavelength BEFL can be tuned continuously from 1562 nm to 1568 nm by adjusting the BP wavelength. The experiment demonstrates that the multiwavelength BEFL performs a good performance and stability on both the operating wavelengths and the output powers.

The larger number of lasing lines can be achieved if a suitable longer HNLF is available to decrease the Brillouin threshold. Hence, more BS lines are easier to obtain, which helps to generate multiple FWM processes and increases the number and OSNRs of higher-order Stokes and anti-Stokes. Furthermore, the conversion efficiency of multiple FWM processes will improve and promote the energy transfer from high-peak-power lasing lines to low-peak-power lasing lines. Due to the characteristic of the cascaded SBS in HNLF, the lasing lines of multiwavelength characterize a comb spectrum. Yet if the output multiwavelength BEFL is amplified by adjusting the gain spectrum of an EDFA using long-period fiber Bragg gratings, the weak lasing could be amplified and the uniform power distribution could be improved owing to providing a large gain for the weak signal and a small gain for the large signal. We believe the simple multiwavelength BEFL has potential applications for the optical communications, optical testing and measurement and microwave photonic systems.