A novel multiwavelength erbium-doped fiber laser configuration is proposed and demonstrated. The laser can produce simultaneous four-wavelength lasing oscillations with a minimum wavelength spacing of only 0.36 nm in C-band via using two fiber Bragg gratings written in high birefringence fiber, while ensuring fairly stable room-temperature operation. The laser can also achieve switching modes among four wavelengths by simple adjustment of two polarization controllers in the cavities. The configuration is based on the polarization hole burning and overlapping cavities principle. The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.
©2004 Optical Society of America
Multiwavelength erbium-doped fiber lasers (EDFL) have attracted considerable interest in recent years due to their potential application in wavelength division multiplexed (WDM) fiber-communication systems, fiber sensors and optical instrument testing [1–14]. There are mainly three methods to achieve multiwavelength lasing oscillations. Firstly, a most straightforward approach to produce multiwavelength oscillations is to use different erbium-doped fibers (EDFs) and cavities for different oscillation lines. In this technique, the multi-wavelength lasing lines may be achieved by independent pumping and subsequent multiplexing/ demultiplexing the lasing beams [1,4]. Additionally, multi-wavelength lasers, with up to 16 channel outputs, have also been obtained by using spatially distributed Fabry-Perot resonator in EDF . The technique can produce high performance multiwavelength lasing output because of no interaction between the lasing wavelengths in the cavities. However, construction complexity, high cost and low flexibility are their serious drawbacks. Secondly, multiwavelength oscillations have also been achieved with single EDF by inserting various elements such as comb or interferometer filters into the cavities [6–8]. Multiwavelength lasers with tens of lasing lines can be achieved by this method, but their stability becomes very poor when the spacing between neighboring wavelengths becomes small because of the cross-gain saturation in the homogeneous gain broadening EDF. In order to increase the stability of the multiwavelength oscillations, various techniques for the reduction of wavelength competition have been proposed. One approach is to cool the EDF to 77K with liquid nitrogen [6–7], which is obviously unpractical because the laser cannot operate at room temperature. The second approach is to insert a frequency shifter into the cavity , which is cost expensive and high insert loss. The third approach is to utilize the polarization hole burning (PHB) principle [9–12]. Stable dual-wavelength laser emission at room temperature has been achieved by using a fiber Bragg grating (FBG) written in high-birefringence fiber (HBF) and a few-mode FBG as filter based on the PHB induced by the two orthogonal polarization states of the FBGs [10–11]. C. Zhao et al. proposed a switchable multiwavelength erbium-doped fiber laser based on two cascaded FBGs written in HBF . Though 3-wavelength and 4-wavelength lasing oscillations have been obtained, the laser operated unstably at room temperature. Because the same EDF and the same wavelength selection filter are used in those lasers for all lasing wavelengths, there are problems in balancing the cavity losses for the different wavelengths with the corresponding cavity gains simultaneously. Therefore, it is difficult to obtain stable lasing oscillations with more than two wavelengths at room temperature in those lasers. The third practical approach to produce multiwavelength oscillations is to use active overlapping linear cavities [13–14]. The technique has the advantages of being able to conveniently adjust the cavity losses for the different wavelength and operate at room temperature. However, it increases the system cost and complexity because a FBG and a section of EDF are used for every lasing wavelength. In this paper, a modified multiwavelength EDFL with simple all-fiber configuration based on the polarization hole burning and overlapping cavities principle is proposed and demonstrated. The laser can produce simultaneous four-wavelength lasing oscillations with minimum wavelength spacing of only 0.36 nm in C-band, while ensuring fairly stable room-temperature operation. The four wavelengths are specified by two FBGs written in HBF (HB-FBGs). Furthermore, the laser can also be designed to operate in stable multi-wavelength switching modes by simple adjustment of polarization controllers in the cavities.
2. System configuration and operation principle
The configuration of the proposed EDFL is shown schematically in Fig. 1. The linear cavities of the laser consist of a sagnac fiber loop mirror, a WDM coupler, two sections of EDF, two polarization controllers (PCs) and two HB-FBGs. The sagnac fiber loop mirror acts as a perfect broadband reflection mirror with nearly 100% reflectivity for 1550-nm spectral region. The two HB-FBGs form two overlapping linear cavities with the sagnac fiber loop mirror and the two sections of EDF. The transmission spectra of the two HB-FBGs are shown in Fig.2. The beat length of the PANDA HBF used is 3.1 mm and can produce a corresponding wavelength separation of approximately 0.52 nm in 1550 nm between the orthogonal linear polarization modes of the reflection from the HB-FBG. The PANDA fiber was treated in 120 atm of H2 at 27 □ for 7 days. The HB-FBG1 and HB-FBG2 were then fabricated by using the same phase mask. The HB-FBG2 was made in an unstrained high birefringence fiber. The HB-FBG1 was made in a strained HB fiber (the strain is approximately 6 × 10-4). Seen from the transmission dips of Fig.2, the wavelengths and reflectivities of the HB-FBG1 are 1554.24nm, 37% and 1554.76nm, 31%, respectively. The two wavelengths and reflectivities of the HB-FBG2 are 1555.12nm, 45% and 1555.64nm, 45%, respectively. The minimum wavelength separation among the four wavelengths is 0.36 nm. The two transmission dips located at left-hand in every HB-FBG transmission spectra as shown in Fig. 2 are pure loss peaks (no reflection), which may be in relation to the experimental conditions of inscribing the FBGs. The two sections of EDF with an erbium ion concentration of 400ppm are both pumped by a 1480 nm pigtailed laser diode with 120 mW maximum output power via a 1480/1550 nm WDM coupler. The lengths of the EDF1 and EDF2 are 3.4 m and 4.6 m, respectively. The PCs are used to rotate the polarization state and can achieve continuous adjustment of the polarization losses within the cavities. All fibers in the cavities are standard single mode fiber (SMF) except the two sections of EDF and two sections of 5-cm PANDA HBF, in which FBGs were inscribed. The spectral characteristic of the laser was measured by an ADVANTEST Q8383 optical spectrum analyzer with 0.1 nm resolution.
In the proposed laser configuration, the two FBGs are not normal FBG written in standard SMF as in Refs.  and  but written in PANDA HBF. The HB-FBGs can exploit the difference in the wavelength of Bragg reflection of light populating the orthogonal linear polarization modes of high birefringence fiber. Feedback from a HB-FBG then results in the laser operating on two linearly polarized longitudinal modes that are separated both in wavelength and polarization. The oscillation conditions for the lasing wavelength λ 1, λ 2 , λ 3 and λ 4 specified by the two HB-FBGs, respectively, can be given as 
where R 0 , R 1, R 2 and R 4 are the reflectivities of the sagnac fiber loop mirror, HB-FBG1 for λ 1 and λ 2 , HB-FBG2 for λ 3 and λ 4 , respectively; l 11 , l 21 , l 31 and l 41 are the single-pass loss of λ 1 , λ 2 , λ 3 and λ 4 , respectively, experienced between the fiber loop mirror and FBG1; l 32 and l 42 are the single-pass loss of λ 3 and λ 4 , respectively, experienced between the FBG1 and FBG2; G 11 , G 21 , G 31 and G 41 are the single-pass gain of λ 1 , λ 2 , λ 3 and λ 4 , respectively, obtained from the EDF1; G 32 and G 42 are the single-pass gain of λ 3 and λ 4 , respectively, obtained from the EDF2.
EDF1 and EDF2 in the overlapping cavities are shared by the lasing lines of λ1,2 and λ3,4 as either intracavity gain medium for lasing or as external gain medium for amplification. Because EDF is mainly the homogeneous gain-broadening medium at room temperature, the gain of any wavelength obtained from the EDF1 and EDF2 will be affected by other wavelengths. In addition, the FBG between two neighboring EDFs will also cause the gain of one section of EDF affected by others. Thus, the lasing wavelengths may suppress each other through cross-gain saturations in the EDFs. However, the strengths of the competition effects depend on the lengths of EDFs and the reflectivities of the FBGs, which determine the pump situations and border conditions of the EDFs. Therefore, if the lengths of the EDFs and the reflectivities of the FBGs are optimized, the competition effects between the wavelength λ1,2 and λ3,4 will be greatly weakened. On the other hand, due to the PHB effect introduced by the two wavelengths and the two orthogonal polarization states of the HB-FBGs, the laser can avoid the wavelength competition from the homogeneous gain broadening in the cavity formed by the sagnac fiber loop mirror, EDF1 and HB-FBG1 as well as the cavity formed by the sagnac fiber loop mirror, EDF1, EDF2 and HB-FBG2. Therefore, the oscillation conditions shown in Eqs. (1)–(4) may be satisfied simultaneously under a certain optimizing EDF lengths and HB-FBG reflectivities. So simultaneous four-wavelength lasing oscillations can be achieved. Furthermore, it should also be noticed that one, two, three or all of the oscillation conditions shown in Eqs. (1)–(4) may be satisfied under the given EDF lengths and HB-FBG reflectivities when the single-pass losses in the cavities are altered via adjusting the two PCs. Therefore, the laser can also achieve switching modes among the four wavelengths.
3. Experimental results and discussion
When the output power of the pump laser diode is tuned to 38.4 mW and the PC1 and PC2 were adjusted to an appropriate state, simultaneous four-wavelength lasing oscillations with almost equalized output power spectra as shown in Fig. 3 were obtained. The output peak powers for the four wavelengths are -19.36dBm, -18.94dBm, -18.72dBm and -17.66dBm, respectively. The total output power was measured to be about 3.2 mW. The threshold pump power was measured to be about 3.5 mW. Figure 4 is the results of repeated scan measurements 16 times taken over several minutes. The output power at any wavelength does not vary by more than 0.4dB and the peak wavelengths are stable within the resolution of our OSA. This indicates fairly stable room-temperature operation even for wavelength spacing of only 0.36 nm in the C-band. Higher laser output power could be obtained by increasing the pump power and simultaneous optimizing the lengths of the sections of EDF.
Additionally, in the experiment, four single-wavelength, six dual-wavelength and four three-wavelength lasing oscillations have also been obtained only by carefully adjusting the two PCs. The repeated scanning spectra of one of two-wavelength and three-wavelength lasing oscillations in switching mode are shown in Figs. 5 and 6, respectively. The peak power variations in each wavelength were measured to be also less than 0.4dB. This indicates that they are all very stable at room temperature.
Seen from the experimental results, the stability of this multiwavelength EDFL is much superior to that of the EDFL proposed in Ref.12. The EDFL proposed by C. Zhao et al. is based on two cascaded HB-FBGs and one section of EDF. When the wavelength number of the lasing lines is more than two, the polarization state at other wavelengths must be related to one of the two orthogonal polarization state. The differences in the polarization state at the four wavelengths become less. Therefore, the four-wavelength lasing oscillations were not stable because the lasers at different wavelengths compete severely in homogeneity gain medium-EDF. However, in our proposed laser, the use of overlapped cavities weakens greatly the wavelength competition between the two HB-FBGs from the homogeneous gain broadening, which makes stable four-wavelength lasing oscillations obtained at room temperature.
The neighboring wavelength separations among the four lasing wavelength are 0.36nm and 0.52nm, respectively, in our experiment. They could be modified to uniform wavelength spacing via using two phase-masks with appropriate period or one phase-mask by applying precise axial strain on the HBF with a precise strain controller during the fabrication. The channel separation could also be 0.8nm or 0.4nm by writing the FBGs in the HBF with smaller or bigger beat length.
Furthermore, in principle, an EDFL with simultaneous 2n-wavelength lasing oscillations could be achieved by using n HB-FBGs and n sections of EDF based on the similar configuration and operation principle. The number of FBGs and section number of EDFs used in the proposed EDFL are half of that in Refs.  and  in order to achieve the same multi-wavelength lasing lines because of the use of the HB-FBGs.
In conclusion, the use of two HB-FBGs in an erbium-doped fiber laser with two overlapping cavities allows us to obtain simultaneous four-wavelength laser output with uniform output power by adjusting the polarization losses, pump power and the EDF lengths in the laser cavities. Furthermore, the laser could also operate in stable multi-wavelength switching modes by simple adjustment of the two polarization controllers in the cavities. The laser has fairly stable room-temperature operation for the minimum wavelength spacing of only 0.36 nm in the C-band. In principle, the laser configuration could be extended to realize simultaneous 2n-wavelength lasing oscillations by using n overlapping linear cavities with n HB-FBGs, which is greatly simplified comparing with the laser configuration proposed in the Ref.  and Ref. . The uniform neighboring wavelength spacing such as 0.4nm or 0.8nm could also be obtained by choosing appropriate HBF and phase masks or accurately controlling the strain applied on the HBF during the fabrication of the HB-FBGs.
This work is supported by the National 863 high technology projects under Grant No. 2003AA312100, the National 973 Basic Research Projects under Grant No. 20003CB314906, the National Natural Science Foundation Key Projects under Grant No. 60137010, the Tianjin Natural Science Foundation projects under Grant No. 033183611 and 033800211, and the start-up Foundation of Scientific Research provided by the Personnel Department of Nankai University, P. R. China.
References and Links
1. L. Ding, G. Kai, Y. Xu, B. Guan, S. Yuan, X. Dong, and C. Ge, “A four-wavelength all-fiber laser for wavelength division multiplexing system,” Chin. Phys. Lett. 18, 376–378 (2001). [CrossRef]
2. R. Slavik and S. LaRochelle, Multiwavelength single-mode erbium doped fiber laser for FFH-OCDMA testing, in Proc. OFC 2002, Paper WJ3, 245–246 (2002).
3. L. Talaverano, S. Abad, S. Jarabo, and M. Lopez-Amo, “Multiwavelength fiber laser sources with Bragg-grating sensor multiplexing capability,” J. Lightwave Technol. 19, 553–558 (2001). [CrossRef]
4. T. Miyazaki, N. Edagawa, S. Yamamoto, and S. Akiba, “A multiwavelength fiber ring-laser employing a pair of silica-based arrayed-waveguide-grating,” IEEE Photon. Technol. Lett. 9, 910–912 (1997). [CrossRef]
5. G. Brochu, R. Slavik, and S. LaRochelle, “Ultra-compact 52 mW 50-GHz spaced 16 channels narrow-line and single-polarization fiber laser,” OFC’2004, Paper PDP-22, 1522–1524(2004).
6. S. Yamashita and K. Hotate, “Multiwavelength erbium-doped fiber laser using intracavity etalon and cooled by liquid nitrogen,” Electron. Lett. 32, 1298–1299 (1996). [CrossRef]
7. S. Yamashita and T. Baba, “Spacing-tunable multiwavelength fiber laser,” Electron. Lett. 37, 1015–1017 (2001). [CrossRef]
8. J. N. Maran, S. Larochelle, and P. Besnard, “C-band multi-wavelength frequency-shifted erbium-doped fiber laser,” Opt. Commun. 218, 81–86 (2003). [CrossRef]
9. J. Sun, J. Qiu, and D. Huang, “Multiwavelength erbium-doped fiber lasers exploiting polarization hole burning,” Opt. Commun. 182, 193–197 (2000). [CrossRef]
10. C. Zhao, X. Yang, J. H. Ng, X. Dong, X. Guo, X. Wang, X. Zhou, and C. Lu, “Switchable dual-wavelength erbium-doped fiber-ring lasers using a fiber Bragg grating in high-birefringence fiber,” Microwave and optical Technol. Lett. , 41, 73–75 (2004). [CrossRef]
11. X. Feng, Y. Liu, S. Fu, S. Yuan, and X. Dong, “Switchable dual-wavelength ytterbium-doped fiber laser based on a few-mode fiber grating,” IEEE Photon. Technol. Lett. 16, 762–764 (2004). [CrossRef]
12. C. Zhao, X. Yang, C. Lu, J. H. Ng, X. Guo, J. H. Ng, X. Guo, R. C. Partha, and X. Dong, “Switchable multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber,” Opt. Commun. 230, 313–317 (2004). [CrossRef]
13. Q. Mao and J. W. Y. Lit, “Switchable multiwavelength erbium-doped fiber laser with cascaded fiber grating cavities,” IEEE Photon. Technol. Lett. 14, 612–614 (2002). [CrossRef]
14. Q. Mao and J. W. Y. Lit, “Multiwavelength erbium-doped fiber lasers with active overlapping linear cavities,” J. Lightwave Technol. 21, 160–168 (2003). [CrossRef]