A tunable and switchable dual-wavelength single polarization narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser based on polarization-maintaining chirped moiré fiber Bragg grating (PM-CMFBG) filter is proposed and demonstrated. For the first time as we know, the CMFBG inscribed on the PM fiber is applied for the wavelength-tunable and-switchable dual-wavelength laser. The PM-CMFBG filter with ultra-narrow transmission band (0.1pm) and a uniform polarization-maintaining fiber Bragg grating (PM-FBG) are used to select the laser longitudinal mode. The stable single polarization SLM operation is guaranteed by the PM-CMFBG filter and polarization controller. A tuning range of about 0.25nm with about 0.075nm step is achieved by stretching the uniform PM-FBG. Meanwhile, the linewidth of the fiber laser for each wavelength is approximate 6.5 and 7.1 kHz with a 20dB linewidth, which indicates the laser linewidth is approximate 325 Hz and 355Hz FWHM.
© 2014 Optical Society of America
Dual-wavelength fiber laser with narrow linewidth single-longitudinal-mode (SLM) operation has attracted a lot of research interests owing to its applications in optical communications, microwave photonics systems, optical instrument testing and optical fiber sensors [1–3]. Meanwhile, single polarization laser can be applied in nonlinear frequency conversion, interferometric fiber sensors, pumping of active crystals and optical parametrical devices .
To ensure a stable operation of the SLM dual-wavelength erbium-doped fiber (EDF) laser, two issues need to be carefully addressed. Firstly, the strong homogeneous line broadening and cross-gain saturation in the EDF could lead to unstable dual-wavelength oscillation. The technique such as by cooling the EDF in liquid nitrogen (77K)  has been proposed to solve the issue, but it is not suitable for practical applications. Other solutions to eliminate the homogeneous line broadening and cross-gain saturation contain the use of a hybrid gain medium  and polarization hole burning (PHB) effect . The use of a hybrid gain medium must ensure the careful balance of the gains provided by the two gain media, which may increase the complexity. However, the use of PHB effect is a good method to realize the stable dual-wavelength oscillation. Secondly, since a fiber ring laser usually has a long cavity with closely spaced longitudinal modes, an ultra-narrow band-pass filter (BPF) with two transmission peaks must be incorporated to eliminate multi-longitudinal-mode oscillation and mode hopping. The ultra-narrow BPF can be realized using a phase-shifted fiber Bragg grating (PS-FBG) , a Fabry–Pérot (F-P) filter , FBG pair , or a structured polarization-maintaining chirped fiber Bragg grating (PM-CFBG) . Chirped moiré fiber Bragg grating (CMFBG), which can be seen as the PS-FBG or the distributed F-P resonator possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow transmission band. It can be used to eliminate multi-longitudinal-mode oscillation and mode hopping . Furthermore, various techniques are used to achieve the single polarization operation of the SLM fiber laser such as utilizing polarization dependent losses of the fiber laser , inline broadband polarizer , injection locking  and PM-FBG . The CMFBG written in PM fiber which utilizes the characters of PM-FBG can be used to achieve the stable single polarization SLM operation.
In this letter, we propose a tunable and switchable dual-wavelength single polarization narrow linewidth SLM EDF ring laser based on PM-CMFBG filter. For the first time as we know, the CMFBG inscribed on the PM fiber (PM-CMFBG) is applied for the wavelength-tunable and-switchable dual-wavelength laser. A tuning range of about 0.25nm with about 0.075nm step is achieved by stretching the uniform PM-FBG. The PM-CMFBG filter with ultra-narrow transmission band, a polarization controller (PC) and a uniform PM-FBG are used to ensure stable single polarization SLM operation. Meanwhile, the linewidth of the fiber laser for each wavelength is approximate 6.5 and 7.1 kHz with a 20dB linewidth, which indicates the laser linewidth is approximate 325 Hz and 355Hz FWHM.
2. Experimental principle and structure
The experimental schematic of the wavelength-tunable and switchable dual-wavelength single polarization narrow linewidth SLM EDF ring laser is shown in Fig. 1(a). The ring laser cavity length is about 21 m. A 980 nm/1550 nm WDM is used to launch the 980 nm pumping laser into the ring laser cavity. The 3m long EDF with absorption coefficient 16dB/m serves as the gain medium. The optical circulator (OC) is employed to help the implementation of the ultra-narrow BPF and sustain the unidirectional oscillation in the laser cavity. The PM-CMFBG filter and PM-FBG serve as a longitudinal mode discriminator to realize the stable SLM operation. PC is used to tune the polarization states of the light to adjust the gain and loss of the laser cavity. An isolator (ISO) assures the unidirectional operation. The laser output from 90:10 optical fiber coupler which provides 10% of the optical power for the output and 90% for feedback inside the cavity is measured by an optical spectrum analyzer (OSA; ANDO AQ6317C) with a wavelength resolution of 0.01nm and another OSA (APEX AP2051A) with a wavelength resolution of 0.16pm. The SLM operation and electrical beating signal are monitored by an electrical spectrum analyzer (ESA; Agilent N9010A, 9 kHz~26.5GHz) connected to a photo detector (PD; Tektronix CSA803A SD-48PD subunit, 33GHz). The single polarization operation is measured by using the Agilent 8509 Lightwave Polarization Analyzer (PA).
A CMFBG includes two superimposed linearly chirped Bragg gratings. The refractive index modulation of the grating can be written as15]. Figure 1(b) shows the typical simulation transmission spectrum of the PM-CMFBG filter. The CMFBG can be fabricated by dual-exposure method with linear chirped phase mask . Through adjusting the parameters of phase mask plate and D, the number of the transmission peak of the CMFBG can be selected. The CMFBG can be seen as the PS-FBG or the distributed F-P resonator and possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow transmission band. Thus, it can be used to select the longitudinal modes of the laser cavity effectively. The wavelength interval of the adjacent transmission peak can be expressed as, where λ is the central wavelength of the chirped FBG, n is the effective refractive index of the fiber. In this experiment, we firstly fabricate a 10cm length of PM-CFBG with a 10cm linearly chirped phase mask which has a chirp rate of 0.018nm/cm and a period of 1069.74nm scanned by 248nm KrF excimer laser ultraviolet light. The CFBG is written in a 14 days hydrogen-loaded (10Mpa; at room temperature) germanium-doped PM single-mode fiber. Then we move the mask plate with 11mm and fabricate a 10cm length of PM-CFBG once again. The transmission spectrum of the fabricated PM-CMFBG is shown in Fig. 1(c). The wavelengths of the filter’s transmission peaks which are corresponding to X and Y polarization states are 1544.9203nm and 1545.3002nm, 1545.995nm and 1545.3749nm, 1545.0713nm and 1545.4509nm, 1545.1427nm and 1546.5281nm by using an OSA (APEX AP2051A), respectively. The wavelength interval of the transmission peak is about 0.075nm. Theoretically, the real optical power level of the narrow transmission peaks of the CMFBG should be invariable and same as the optical power level outside the optical transmission band of the CMFBG according to theoretical analysis and simulation model . It should be noted that the observed transmission non-uniformity of the narrow transmission peaks of the CMFBG is, on the one hand, limited by the scanning resolution of the OSA (0.16pm), and on the other hand, may be caused by the unsteadiness of ultraviolet light exposure energy. The index modulation depth of the PM-CMFBG is about, and birefringence coefficient of the PM fiber is. Through analyzing the experimental datum of the PM-CMFBG’s transmission spectrum and simulation model, the 3dB bandwidths of these ultra-narrow transmission peaks are approximate 0.1pm (12.5MHz), as shown in Fig. 1(c). The ring laser cavity length is about 21 m, corresponding to the longitudinal mode spacing of approximately 9.6MHz. Thus, the PM-CMFBG with ultra-narrow transmission peak can be used to realize stable SLM operation.
3. Experimental results and discussion
Through tuning the PC to adjust gain and loss of the laser cavity, we achieved stable dual-wavelength or single wavelength SLM fiber laser operation. The threshold of the fiber ring laser is about 90mW. Through stretching the PM-FBG with two horizontally separated translation stages, we achieved a series of lasing wavelengths with a tuning step of about 0.075nm in a tuning range of about 0.25nm. Figure 2(a) shows the measured optical spectra of the fiber laser under 190mW pump power, and the lasing wavelengths are 1544.918nm and 1545.302nm, 1545.994nm and 1545.378nm, 1545.07nm and 1545.454nm, 1545.142nm and 1546.53nm, respectively, which are almost consistent with the narrow transmission peak wavelengths of the PM-CMFBG filter. The 3dB bandwidths of these laser spectra measured by using an OSA (ANDO AQ6317C) with a wavelength resolution of 0.01 nm are 0.012nm. In the meanwhile, we also measure single-wavelength operation of the fiber laser through adjusting PC, as shown in Figs. 2(b) and 2(c), respectively. The 3dB bandwidths of these laser spectra measured by OSA (APEX AP2051A) with a wavelength resolution of 0.16pm are 0.08pm. The optical signal to noise ratio (OSNR) of these fiber lasers are >55dB. To study the stability of the proposed laser, we measure the optical spectra of the fiber laser with 11 times repeated scans at 3 min intervals in half an hour, respectively. Here we take the fiber laser operating at 1544.994nm and 1545.378nm as the typical illustration in Fig. 2(d). The power amplitude variation of the lasing wavelength is less than 1.5 dB, which means that the dual-wavelength fiber laser operation is stable.
Figures 3(a) and 3(b) show the optical spectra of the proposed laser with PM-CMFBG filter and without PM-CMFBG filter measured by OSA (APEX AP2051A). The SLM operation of the fiber laser in our experiment is proved through the self-homodyne method by the Agilent 9010 ESA. The laser is observed at pump power of 190mW. The scanning frequency range is 100 MHz, which is larger than the longitudinal mode spacing of the fiber laser (9.6MHz). Figures 3(c) and 3(d) show the detected electrical spectra of the beating signals of the fiber laser in the range of 100MHz with PM-CMFBG filter or without PM-CMFBG filter corresponding to Figs. 3(a) and 3(b). Firstly, we measured the beating signal of the fiber laser only using the PM-FBG as the filter but without the PM-CMFBG filter. Here, the fiber laser is no longer stably operating in the SLM operation, as shown in Fig. 3(d), because there are many laser longitudinal modes beating signals. Then we also measured the beating signal of the proposed laser using the PM-FBG and PM-CMFBG filter. We can find that there is no laser longitudinal mode beating signal, as shown in Fig. 3(c). Through comparing Figs. 3(c) and 3(d), we can obtain that the PM-CMFBG filter can be used to select the laser longitudinal modes effectively. Thus, stable SLM operation of the fiber laser is guaranteed by the ultra-narrow PM-CMFBG filter and PM-FBG as we analyzed in section 2.
We use the delayed self-heterodyne method to measure the linewidth of the proposed laser in this experiment, as shown in Fig. 4. The delayed line of the delay self-heterodyne method is about 40km in this experiment, corresponding to a nominal resolution of 4.8 kHz . The RBW of the ESA is 3 kHz. The black line in Fig. 4 shows the measured linewidth of the fiber laser under the 190mW pump power. The Lorentz fitting curve of each lasing wavelength with a 20 dB linewidth of approximate 6.5 kHz and 7.1 kHz is shown in Fig. 4(the red line), which indicates the laser linewidth is approximate 325 Hz and 355Hz FWHM .
Through a segment of 3m commercial optical fiber and a PC, the output port of the proposed laser is connected to the lightwave polarization analyzer (PA, HP 8509B) . We ensure that the single-mode fiber which connects the output of the laser to PA is straight, but not curved and twisty, so that we can reduce the change of the polarization states. The polarization parameters of the laser are measured by the PA, when the fiber laser works at the single wavelength operation. Figure 5 shows the results of the laser for each wavelength testing within 2 min. The apparent degree of polarization (DOP) in percentage term is stable 102.2% and 100.4% (Note that the DOP should be less than 100%; the part of greater than 100% is caused by a calculating error of the instrument). The results indicate that a stable single polarization fiber laser for each wavelength could be successfully achieved with the proposed structure. (Note that they are approximately orthogonal linear polarization states).
In conclusion, we have proposed a tunable and switchable dual-wavelength single polarization narrow linewidth SLM EDF ring laser based on PM-CMFBG filter. The dual-wavelength of the proposed laser can be tuned and switched by the PM-CMFBG filter and PM-FBG. Through stretching the uniform PM-FBG, a tuning range of about 0.25nm with about 0.075nm step is achieved. The PM-CMFBG filter with ultra-narrow transmission band, a PC and a uniform PM-FBG are used to ensure stable single polarization SLM operation. Meanwhile, the 20dB linewidth of the fiber laser for each wavelength is approximate 6.5 and 7.1 kHz, which indicates the laser linewidth is approximate 325 Hz and 355Hz FWHM.
This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 61107094, 61178008, 61275092), and the Fundamental Research Funds for the Central Universities (No.2013JBM017), China.
References and links
1. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
2. X. F. Chen, Z. C. Deng, and J. P. Yao, “Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser,” IEEE Trans. Microw. Theory Tech. 54(2), 804–809 (2006). [CrossRef]
3. L. Jin, Y. N. Tan, Z. Quan, M. P. Li, and B. O. Guan, “Strain-insensitive temperature sensing with a dual polarization fiber grating laser,” Opt. Express 20(6), 6021–6028 (2012). [CrossRef] [PubMed]
4. V. M. Paramonov, A. S. Kurkov, O. I. Medvedkov, and V. B. Tsvetkov, “Single-polarization cladding-pumped Yb-doped fiber laser,” Laser Phys. Lett. 4(10), 740–742 (2007). [CrossRef]
5. D. P. Wei, T. J. Li, Y. C. Zhao, and S. S. Jian, “Multiwavelength erbium-doped fiber ring lasers with overlap-written fiber Bragg gratings,” Opt. Lett. 25(16), 1150–1152 (2000). [CrossRef] [PubMed]
6. S. L. Pan, X. F. Zhao, and C. Y. Lou, “Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier,” Opt. Lett. 33(8), 764–766 (2008). [CrossRef] [PubMed]
7. S. L. Pan and J. P. Yao, “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17(7), 5414–5419 (2009). [CrossRef] [PubMed]
8. L. Bo, J. Meng, T. S. Chuan, and S. Ping, “Tunable microwave generation using a phase-shifted chirped fiber Bragg grating,” IEEE Photon. Technol. Lett. 23(18), 1292–1294 (2011). [CrossRef]
9. X. P. Cheng, P. Shum, C. H. Tse, J. L. Zhou, M. Tang, W. C. Tan, R. F. Wu, and J. Zhang, “Single-longitudinal -mode erbium-doped fiber ring laser based on high finesse fiber Bragg grating Fabry-Pérot etalon,” IEEE Photon. Technol. Lett. 20(12), 976–978 (2008). [CrossRef]
10. D. Chen, H. Fu, W. Liu, Y. Wei, and S. He, “Dual-wavelength single-longitudinal-mode erbium-doped fiber laser based on fiber Bragg grating pair and its application in microwave signal generation,” Electron. Lett. 44(7), 459–461 (2008). [CrossRef]
11. B. Yin, S. C. Feng, Y. L. Bai, Z. B. Liu, L. J. Liang, S. Liu, and S. S. Jian, “Switchable single-polarization dual-wavelength ring laser based on structured PM-CFBG,” IEEE Photon. Technol. Lett. 26(12), 1227–1230 (2014). [CrossRef]
12. S. C. Feng, S. H. Lu, W. J. Peng, Q. Li, T. Feng, and S. S. Jian, “Tunable single-polarization single-longitudinal-mode erbium-doped fiber ring laser employing a CMFBG filter and saturable absorber,” Opt. Laser Technol. 47, 102–106 (2013). [CrossRef]
13. N. Jovanovic, J. Thomas, R. J. Williams, M. J. Steel, G. D. Marshall, A. Fuerbach, S. Nolte, A. Tünnermann, and M. J. Withford, “Polarization-dependent effects in point-by-point fiber Bragg gratings enable simple, linearly polarized fiber lasers,” Opt. Express 17(8), 6082–6095 (2009). [CrossRef] [PubMed]
14. S. Yamashita and G. J. Cowle, “Single-polarization operation of fiber distributed feedback (DFB) lasers by injection locking,” J. Lightwave Technol. 17(3), 509–513 (1999). [CrossRef]
15. L. R. Chen, D. J. F. Cooper, and P. W. E. Smith, “Transmission filters with multiple flattened passbands based on chirped moire gratings,” IEEE Photon. Technol. Lett. 10(9), 1283–1285 (1998). [CrossRef]
16. L. Zhang, K. Sugden, I. Bennion, and A. Molony, “Wide-stopband chirped fibre moiré grating transmission filters,” Electron. Lett. 31(6), 477–479 (1995). [CrossRef]
17. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155(1–3), 180–186 (1998). [CrossRef]
18. S. H. Xu, Z. M. Yang, T. Liu, W. N. Zhang, Z. M. Feng, Q. Y. Zhang, and Z. H. Jiang, “An efficient compact 300 mW narrow-linewidth single frequency fiber laser at 1.5 microm,” Opt. Express 18(2), 1249–1254 (2010). [CrossRef] [PubMed]
19. Z. Meng, G. Stewart, and G. Whitenett, “Stable single-mode operation of a narrow-linewidth, linearly polarized, erbium-fiber ring laser using a saturable absorber,” J. Lightwave Technol. 24(5), 2179–2183 (2006). [CrossRef]