We have demonstrated a novel multiwavelength Brillouin-erbium fiber laser (BEFL), in which the Brillouin pump is self-excited within the linear cavity, instead of the injection from the external cavity or direct generation in the intracavity. By using this simple scheme, the generation of more than 160 Brillouin Stokes lines has been experimentally demonstrated, which is the largest one achieved in BEFLs to the best of our knowledge. Also, the single longitudinal mode operation and the low noise performance of output wavelength line have been confirmed. Meanwhile, the experiment demonstrates that the BEFL performs good stability on both the operating wavelengths and the output power of each wavelength.
©2006 Optical Society of America
The multiwavelength fiber laser sources working at the room temperature have been attracted great interests for their applications in many fields, such as precise spectroscopy, photonic component characterization, optical sensing, and dense wavelength-division-multiplexing (WDM) optical communication . This type of lasers have been demonstrated using many different approaches, for instance, using an elliptical core Erbium-doped fiber (EDF) for the anisotropic gain , providing frequency-shifted feedback in cavity to prevent the single mode operation , and using the cascaded stimulated Brillouin scattering (SBS) –. The Brillouin-erbium fiber laser (BEFL) based on the hybrid-gain technique  has been reported to produce stable multiwavelength comb with a spacing of ~11GHz. Since the BEFL integrates the high gain from Erbium ions and the narrowband nonlinear gain of SBS in optical fibers, the total hybrid-gain performs inhomogeneous broadening property for multiwavelength oscillation. Due to its simple configuration, low pump power, the BEFL has become an attractive approach to generate multiwavelength combs. Meanwhile, the BEFL performs the intrinsic properties of rigid frequency spacing around 11GHz and extremely narrow linewidth. It is becoming a competitive candidate for the light source of the next generation ultra-dense WDM systems . Up to date, most works on BEFLs were based on the ring cavity –, only but few BEFL works on the linear cavity  . The linear cavity design exhibits low threshold power for the Erbium pump, and while easily results in the spatial hole-burning (SHB) effect . The SHB effect can avoid mode competition and prompt stable multiwavelength oscillation. Thus, the linear cavity is considered as a relatively more suitable design for multiwavelength fiber laser. Nevertheless, the bandwidth of EDF is limited to a few nanometers in these architectures, which results in a limited number of multiple wavelengths. The largest generated Stokes wavelengths using the linear cavity were limited at 22-line .
Recently, we proposed and demonstrated a self-seeded BEFL using internally self-excited Brillouin pump , which was achieved by incorporating a long single-mode fiber (SMF) as the Brillouin gain medium in the ring cavity. Using this technique, the power distribution was greatly improved, and up to 120-line generation had been observed in the ring cavity BEFL. In this paper, a self-seeded BEFL using the linear cavity was demonstrated. More than 160-line multiwavelength generation has been obtained. To the best of our knowledge, this is the largest wavelength number achieved in BEFLs to date. Generally, the narrow homogeneous Brillouin gain in the BEFLs suppresses the free run process of the EDF, and the combined nonlinear gain and the EDF gain lead to single-mode operation . However, since there is no direct Brillouin pump, the longitudinal mode property in self-seeded BEFLs should be reconfirmed. In this paper, the single longitudinal mode operation of self-seeded BEFL has been experimentally proved. The experiment also demonstrates that the BEFL performs good repetitivity and stability both on the operating wavelengths and the output power of each line.
2. Experiment and Principles
Figure 1 shows the experimental configuration of the new multiwavelength linear cavity self-seeded BEFL. In the configuration, the linear lasing cavity comprises a high-birefringence fiber Sagnac loop mirror, an optical circulator (OC), an EDF and a single mode fiber (SMF). The two-port connected OC and the Sagnac loop mirror serve as two reflectors, and the Sagnac loop mirror also serves as a tunable wide-band reflection filter. The linear gain medium is a 16-m long EDF with 200ppm Erbium ion concentration, which is pumped by a 180-mW 980-nm laser diode through a 980/1550-nm WDM coupler. The Brillouin gain is generated in the 12.5-km long SMF. A 10-dB coupler connected with the port 3 and port 1 of the OC, is used to output the generated multiwavelength laser. The output laser is measured using an optical spectrum analyzer (OSA) with the spectral resolution of 0.06nm. The high-birefringence Sagnac loop mirror is composed of a 3-dB coupler, a length of polarization maintaining fiber (PMF) and two polarization controllers (PCs). Adjusting the PCs can modify the reflection profile of Sagnac loop mirror. By taking the combined advantage of the linear gain in EDF and the nonlinear gain from SBS in long SMF, the Brillouin Stokes line comb is generated in the linear cavity.
The working principle of the new BEFL is described as follows. The oscillating mode is first built in the linear cavity, and then it is narrowed in the long SMF by the dynamic distributed feedback of Rayleigh scattering . The narrowed oscillating laser acts as the Brillouin pump, which propagates the SMF bi-directionally. SBS backscatter cause the generation of Brillouin Stokes with narrow linewidth. The generated Stoked line is amplified in the EDF and then serves as Brillouin pump of the next order Stokes line. Owing to the bidirectional propagation in the cavity, the obtained gain of each Stoke line combines the linear gain in EDF and the nonlinear SBS gain in SMF. The total gain for different Stoke lines exhibits an inhomogeneous broadening mechanism if the Brillouin gain is high enough. This is essential for the multiwavelength comb to generate. The comb can be formed when the total gain exceeds the cavity loss. The first established Brillouin pump wavelength depends on the balance between the cavity loss and the EDF gain. Adjusting the PCs can modify the reflection profile of the Sagnac loop mirror, so the wavelengths of Brillouin Stokes combs, as well as the power distribution can be optimized.
3. Results and Discussion
In the experiment, the power of 980-nm pump laser was fixed at 163 mW, and a 13.5-cm long PMF was incorporated in Sagnac loop mirror. The Brillouin Stokes line comb was observed to generate in the wavelength range of 25.5-nm from 1562.5-nm to 1588-nm. By modifying the reflection profile of the Sagnac loop though adjusting the PCs, in this range the wavelength of combs can be changed. Figure 2 shows the spectra of the generated Brillouin combs of the new BEFL. The generated Brillouin Stokes line combs cover the wavelength range from 1572-nm to 1588-nm. The output powers of Brillouin lines are around -20-dBm. Under any polarization states of two PCs, the stable Brillouin comb with a wavelength spacing of ~0.088nm are observed, though their operating wavelengths and wavelength number may be different. By adjusting the PCs in the Sagnac loop mirror, the output power distribution of Brillouin Stokes lines can be optimized. Figure 2(a) shows the spectrum of the most flattened power distribution achieved in the experiment by adjusting the two PCs. There are 90 Brillouin lines within the 3-dB bandwidth. Meanwhile, as shown in Fig. 2(b), the generation of 160 Brillouin lines beyond -25-dBM powers was observed by carefully adjusting the PCs. Compared with the other previous research works –, the number of the produced Brillouin lines is greatly improved. By using such a simple scheme, we obtained the generation of more than 160 Brillouin Stokes lines, which is the largest wavelength number achieved in BEFLs to the best of our knowledge. This results from the improvement on the power distribution uniformity of the generated comb using the self-seeded Brillouin pumping. Meanwhile, the utility of linear cavity further improves the increase of the wavelength number because of SHB effect.
The effect of the 980-nm pump on the multiwavelength generation was also investigated. During the measurements, the polarization state of the PCs was fixed as the same condition as Fig. 2(a), but the power of 980-nm pump was modified gradually. Figure 3 shows the variation of the output spectra via the 980-nm pump power. It shows that the power of 980nm pump contributes a great effect both on the wavelength number of the generated comb and the output power of the Brillouin lines. The wavelength number at the pump power of 163-mW is almost twice as that at the power of 57-mW. In this laser, the lasing lines can be firstly observed when the pump power exceeds 10-mW, but they are unstable both on the lasing wavelengths and the output power. The laser performs strong hopping between the cavity modes. When the pump power exceeds 57-mW, the output comb becomes stable, and the wavelength spacing begins to fix at the constant of ~0.088-nm. Since the reflection profile of Sagnac loop mirror is determined by the polarization states of PCs, the excited wavelength range is not changed with the pump power. This indicates that the power of 57-mW is the threshold to self-excite the SBS. It also indicates that the threshold of the self-seeded BEFL is 57-mW. Compared with the spectral width above the SBS threshold, the biggest difference is the strongly uneven power distribution on the lasing wavelengths below the SBS threshold. As shown as the blue line in Fig. 3, the total spectral width of the laser without exciting SBS is considerable narrow. Once the SBS is excited, the spectrum is greatly broadened with equal wavelength spacing, and the power distribution becomes even. This is also a distinct proof for self-exciting SBS.
To verify the operation stability of this self-seeded linear cavity BEFL, we present the enlarged spectra of the output combs in Fig. 4, which were repeatedly scanned at the same condition and the same polarization state of the PCs. These spectra were recorded every 10-minutes. From Fig. 4, this self-seeded BEFL exhibits the stability in both the output power and the wavelengths on a long time scale. Furthermore, even if this configuration was reformed again after several days, the same results could still be observed. This means that this self-seeded BEFL performs good long-term stability and repetitivity on operation.
Typically, an erbium-doped fiber laser works on the multiple longitudinal mode status. However, since the narrow homogeneous Brillouin gain in BEFL suppresses the free run process of the EDF, the combined gain of nonlinear gain in the SMF and the linear gain in EDF leads to single-mode operation in BEFLs. Thus, the longitudinal mode beats were strongly suppressed when lasing in a BEFL mode (with SBS exciting) as compared to a free-running mode (without SBS exciting). To verify single-longitudinal-mode operation of the new BEFL, the output signal of one line is measured using a 12-GHz photodetector followed a RF analyzer. Figure 5 shows the RF noise characteristics of the BEFL at the different pump powers for different operation status. For the first status, the power of 980nm pump is set at 30mw, in which the laser works on free-running mode. The result, as shown as the green line, clearly shows the multi-longitudinal-mode operation. The red line shows an immediately remarkable reduction of multi-longitudinal-mode operation when the SBS is excited. However, at this status, the laser is not under a rigorous single-longitudinal-mode operation yet. During the experiment, the stable and rigorous single-longitudinal-mode operation was accomplished when the pump power was beyond 80-mw. As shown in the black line, no longitudinal mode beating is observed at 160-mW pump, and the laser works on the BEFL mode. It is clear that the power in the “BEFL mode” was concentrated in only one longitudinal mode, as indicated by the lack of a longitudinal mode beat spectrum as compared to the free-running case.
The power and wavelength from OSA only mean that the BEFL has good long-term stability. Here, it is obvious that the laser working on the BEFL mode doesn’t include strong high-frequency components on RF spectrum. This means that there is no higher frequency fluctuation on output signal. It indicates that the BEFF has good short-term stability.
We also compare the noise performance of the BEFL under different operation mode. The electrical noise of RF spectrum includes the shot noise, the spontaneous-spontaneous beat noise, and the signal-spontaneous beat noise. Here, the later is the dominant factor. In the BEFL operation mode, owing to the filter effect of the narrow Brillouin gain bandwidth, the high frequency signal-spontaneous and the spontaneous-spontaneous beat noises are greatly reduced. As shown in Fig. 5, the noise power of the BEFL mode is ~8dB lower at higher noise frequencies >10MHz than the free-running mode, but it performs little reduction with the pump power after exciting SBS. At frequencies <10MHz, the noise power of the BEFL mode doesn’t improve with the increase of pump power, because of the strong Brillouin amplification noise  and the spontaneous Brillouin scattering . Since the BEFL takes advantage of the filtering effect of the hybrid amplification, we believe that the relative intensity noise in such a laser can be ~10 dB lower than that of the EDF-based source .
We demonstrated a novel self-seeded linear cavity BEFL. The total gain, combined the linear gain of EDF and the nonlinear gain from SBS, makes it possible for the Brillouin Stokes wavelengths to oscillate. The multiwavelength generation of more than 160 Stokes lines has been obtained. Also, we present the experimental confirmation of the single-longitudinal-mode operation and the low noise performance for the output wavelength line. The experiment demonstrates that the linear cavity self-seeded BEFL performs a good stability both on the operating wavelengths and the output powers.
The authors acknowledge the support from National Natural Science Foundation of China under the grants 60577048, the Science and Technology Committee of Shanghai Municipal under the contracts 04DZ14001, and the Program for New Century Excellent Talents in University of China.
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