Stable single-polarization single-longitudinal-mode linear cavity erbium-doped fiber laser based on structured chirped fiber Bragg grating Bin Yin,* Zhibo Liu, Suchun Feng, Yunlong Bai, Haisu Li, and Shuisheng Jian Institute of Lightwave Technology, Key Lab of All Optical Network and Advanced Telecommunication of EMC, Beijing Jiaotong University, Beijing, China *Corresponding author: [email protected] Received 4 September 2014; revised 20 October 2014; accepted 15 November 2014; posted 17 November 2014 (Doc. ID 222525); published 22 December 2014
A novel linear cavity erbium-doped fiber (EDF) laser based on a structured chirped fiber Bragg grating (CFBG) filter is proposed for stable single-polarization (SP) single-longitudinal-mode (SLM) operation. For the first time, to the best of our knowledge, a structured CFBG filter with an ultranarrow transmission band which is generated by tapering directly on CFBG is used to select the laser longitudinal mode. The SLM operation is obtained by using the structured CFBG together with an unpumped EDF acting as a saturable absorber. The fluctuations of the laser peak power and center wavelength are less than 0.07 dB and 1 pm in 1 h, respectively. The stable SP operation is achieved by using the inline broadband polarizer. The measured 20 dB laser linewidth is about 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. © 2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (060.3738) Fiber Bragg gratings, photosensitivity; (140.3500) Lasers, erbium. http://dx.doi.org/10.1364/AO.54.000006
1. Introduction
Stable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber lasers (EDFL) have attracted a lot of research interest due to their potential applications in optical communications, optical instrument testing, and optical fiber sensors [1–3]. Meanwhile, many applications such as nonlinear frequency conversion, interferometric fiber sensors, pumping of active crystals, and optical parametrical devices demand that the fiber lasers work on stable single polarization operation [4]. To implement a stable SLM operation of the EDFL, two styles can be carefully adopted. One approach is a short-length cavity to enlarge longitudinal-mode spacing [5]. However, these lasers normally suffer from a low pump absorption and
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hence low slope efficiency. Thus, particular erbiumdoped fibers (EDF) are designed to solve this issue. More importantly, the short-length cavity can result in broad spectral linewidth. Another approach is a long-length ring cavity to offer higher output power without special requirements for the gain media [6]. However, since a fiber ring laser usually has a long cavity with closely spaced longitudinal modes, an ultranarrow bandpass filter (BPF) must be incorporated to eliminate multilongitudinal-mode oscillation and mode hopping [7]. Cusano et al. produce a tapered region in a fiber Bragg grating (FBG) by the arc discharge technique [8]. This method can be used to realize a new ultranarrow BPF which can be used to eliminate multilongitudinal-mode oscillation and mode hopping. Furthermore, to achieve stable SLM operation, several techniques have been proposed to ensure a long-term stable performance in EDFL, for instance, introducing a passive multiplering cavity or a compound ring resonator composed of
a dual-coupler fiber ring to guarantee SLM laser oscillation [9], using an unpumped EDF as a dynamic narrow-bandwidth filter [10], and utilizing selfinjection feedback to control the lasing frequency [11]. At the same time, in order to realize a singlepolarization (SP) fiber laser, several methods have been proposed, such as utilizing polarizationdependent losses of the fiber laser [12], an inline broadband polarizer [6], injection locking [13], and PM-FBG [14]. In this paper, we propose a novel approach for achieving a stable SP SLM linear cavity EDFL. The structured chirped fiber Bragg grating (CFBG) and the saturable absorber of an unpumped EDF are used to ensure stable SLM operation. The SP operation of the fiber laser is guaranteed through using an inline broadband polarizer. The optical signalto-noise ratio (OSNR) of the laser is more than 50 dB, and the fluctuations of the laser peak power and center wavelength are less than 0.07 dB and 1 pm in 1 h, respectively. The measured 20 dB laser linewidth is 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. 2. Experimental Configuration and Principle
Figure 1 shows the experimental setup of the proposed SP-SLM-EDFL with a linear cavity. The fiber laser consists of a 980/1550 nm wavelength division multiplex (WDM), a section of EDF1 of 4 m with absorption coefficient about 16 dB/m at 1530 nm as the gain medium, an optical circulator (OC), a structured CFBG filter, a narrow-band CFBG, a polarization controller (PC), a polarizer with 27 dB extinction ratio, a 90∶10 optical fiber coupler which provides 10% of the optical power for the output and 90% for feedback inside the cavity, and a segment of 2 m unpumped EDF2 with an absorption coefficient about 15 dB/m. The EDF1 is pumped with a 976 nm laser diode (maximum output power 250 mW) through the WDM. The structured CFBG filter and the narrow-band CFBG serve as a longitudinal-mode discriminator to restrict the laser oscillation. The OC is employed to help the implementation of the ultranarrow BPF and sustain the
Fig. 1. Schematic diagram of the proposed SP-SLM-EDFL with a linear cavity.
unidirectional oscillation. The unpumped EDF2 is used as a saturable absorber, and acts as a dynamic narrow-band filter. The PC is used to tune the birefringence status of the laser cavity, and to suppress mode competition through adjusting gain and loss within the laser cavity. The polarizer is used to select one polarized mode of the two orthogonally polarized modes within the laser cavity, in order to get SP laser output, and to suppress the competition of the orthogonally polarized modes. The Exfo/Burleigh WA-1600 wavemeter (WM) is employed to obtain the fluctuations of the operating wavelength and the lasing output power. The laser output is also monitored by the ANDO AQ6317C optical spectrum analyzer (OSA) with a resolution of 0.01 nm and an electrical spectrum analyzer (ESA; Agilent N9010A, 9 kHz– 26.5 GHz) connected to a photo detector (PD; Tektronix CSA803A SD-48PD subunit, 33 GHz). The linewidth of the proposed laser is measured with the delayed self-heterodyne method. The Agilent 8509 Lightwave Polarization Analyzer (PA) is used to measure the SP operation. A 14 cm length of CFBG and a 5 cm length of narrow-band CFBG are fabricated by the standard phase-mask scanning technique. The CFBG is written in a 14 days hydrogen-loaded (10 Mpa; at room temperature) germanium-doped SMF-28 fiber with a 14 cm linearly chirped phase-mask plate which has a chirp rate of 0.018 nm/cm and a period of 1069.74 nm scanned by a 248 nm KrF excimer laser ultraviolet light. The red line and the blue line in Fig. 2(b) show the measured transmission spectrum of the fabricated CFBG and the reflectance spectrum of narrow-band CFBG which has a reflectivity of about 98%, respectively. This fabricated CFBG (the red line) has a 3 dB bandwidth of 0.20 nm and an extinction ratio of 16.6 dB for its transmission response. Then we create the tapered region (approximately 0.35 mm long, and a waist diameter of 110 2 μm) at the middle of the CFBG by a fusing-and-pulling treatment with a fiber fusion splicer (FSU975), and the transmission spectrum of the CFBG (the black line) has one transmission peak generated by tapering directly on the CFBG, as shown in Fig. 2(b). Through analyzing the experimental data in Fig. 2(b), we can find that the insertion losses which are produced by tapering the CFBG are less than 0.02 dB. At the same time, we calculate that the transmission bandwidth of the structured CFBG is approximately 42.5 MHz by analyzing the experimental data of the structured CFBG filter (it is equivalent to PS-CFBG) and the simulation calculation [15]. Figure 2(c) shows the tapered region of the structured CFBG filter under the microscope. The grating of tapered region is removed by the arc discharge technique, as shown in Fig. 2(a), and the tapered region approximates infinitesimal because it just has a length of short region on the CFBG. Thus, the structured CFBG is analyzed by the basis of phase shift theory [16]. The phase delay ΔΦ can be defined as 1 January 2015 / Vol. 54, No. 1 / APPLIED OPTICS
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index modulation depth, and chirp rate) and the taper (position, cavity length, and waist radius). Therefore, the structured CFBG filter with a narrow transmission band can be used to serve as a longitudinal-mode discriminator in the fiber laser. The unpumped EDF is used as a saturable absorber, and acts as a dynamic narrow-band filter. The absorption coefficient of an EDF is inversely proportional to the intensity of the optical light. So when a spatial interference pattern is generated by a standing wave in the EDF, a spatial-holeburning effect is induced, and thus a periodic refractive index change like a weak FBG is formed. A saturable absorber is a self-tracking filter that can stabilize the single-mode output. The bandwidth of the induced FBG (the unpumped EDF) is calculated as [18]
(a)
(b)
c Δf κ λ
(c) Fig. 2. (a) Schematic diagram of the localized grating erasure setup. (b) The experimental transmission spectrum of the structured CFBG filter (red line) without tapering, (black line) with tapering, and (blue line) the reflection spectrum of narrow-band CFBG. (c) The tapered region of the structured CFBG filter under the microscope.
ΔΦ
4·π · Δneff SRI; Dth · Lth ; λB zth
(1)
where λB is the optical wavelength corresponding to the tapered region position zth . Δneff expresses the effective refractive index variation caused by tapering, which depends on the surrounding refractive index (SRI) and the waist diameter Dth. Lth is the length of the tapered region [17]. The chirp rate and the grating length are 0.018 nm/cm and 14 cm, respectively. For any structured CFBG considered, by increasing Lth and thus the phase shift Eq. (1), the narrow passband changes its depth and moves toward lower wavelengths at the same time only to disappear and then to appear again at higher wavelengths. The minimum of the reflectivity of the passband region approaches zero since the phase shift is equal to π 2kπ (k with integer). The relationship between the reflectivity and the length of the passband region is analyzed in Ref. [17]. Meanwhile, the power losses and the reflectivity of the passband region as a function of the taper waist are analyzed in Ref. [8]. Thus, the structured CFBG similar to the phase shift chirped fiber Bragg grating (PS-CFBG) has the characteristics of a narrow-band filter. We can choose the appropriate parameters of BPF through adjusting the CFBG (length, refractive 8
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s 2 Δn 2 1 ; 2neff N
(2)
where N Lg∕Λ is the total number of grating periods, and λ is the center wavelength at which maximum reflectivity occurs. Δn is the periodical spatial variation of the refractive index, which can be given by the well-known Kramers–Kronig relation. The coupling coefficient of the induced grating κ can be estimated from Δn as κ
2Δn : neff λ
(3)
In our experiment, the unpumped EDF-based saturable absorber induces a weak FBG with a bandwidth of less than 21.7 MHz. Thus, with the combination of the structured CFBG filter and 2 m unpumped EDF acting as a saturable absorber, we can realize stable SLM operation of the fiber laser. 3. Experimental Results and Discussion
Through tuning the PC to adjust the gain and loss of the laser cavity, we achieved stable single-wavelength fiber laser operation. At the pump power of 100 mW (980 nm), the measured output spectrum at the wavelength of 1544.178 nm is shown in Fig. 3(a). The output power is about −14 dBm and the OSNR is more than 50 dB. In order to investigate the laser output stability, we measured the optical spectra of the fiber laser with 16 times repeated scans at 2 min intervals in half an hour, as shown in Fig. 3(b). No wavelength variation could be observed within the limited resolution of the OSA in the experimental period. To obtain more detailed properties on the lasing stability, the laser output was measured by using the WM for 1 h with a power resolution of 0.01 dBm and a wavelength resolution of 0.1 pm. The measured results are shown in Fig. 3(c). The fluctuations of wavelength and output power of the proposed fiber laser which are less than
(a) (a)
(b)
(b)
(c)
(c)
Fig. 3. (a) The measured optical spectrum of the proposed laser. (b) The fiber laser operation with 16 times repeated scans in half an hour. (c) The fluctuations of the laser peak power and center wavelength in 1 h measured by WM.
1 pm and 0.07 dB, respectively, represent high-stability lasing operation in the situation of room temperature. The SLM operation of the fiber laser is proved through the self-homodyne method. The detected electrical spectra of the beating signals of the fiber laser in the range of 200 MHz are shown in Fig. 4. The scanning range is much wider than the free spectrum range (FSR), because the cavity length of the fiber ring laser is approximately 22 m, corresponding to FSR of about 9.2 MHz. We can clearly see that the fiber laser is in the stable SLM operation in Fig. 4(d). We measured the beating signal of the fiber laser using the narrow-band CFBG combined with the structured CFBG but without the unpumped EDF. There are only several longitudinal-mode beating signals and the fiber laser is no longer stably operating in the SLM operation, as shown in Fig. 4(a). Then we removed the structured CFBG but added the unpumped EDF. The fiber laser is also no longer
(d)
Fig. 4. Electrical spectra of the beating signals. (a) The narrowband CFBG combined with the structured CFBG but without the unpumped EDF. (b) The narrow-band CFBG combined with the unpumped EDF but without the structured CFBG. (c) The narrow-band CFBG without the structured CFBG and the unpumped EDF. (d) The narrow-band CFBG combined with the structured CFBG and the unpumped EDF. 1 January 2015 / Vol. 54, No. 1 / APPLIED OPTICS
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that the output of the fiber laser is at a stable SP state. The apparent degree of polarization (DOP) in percentage term is as high as 100.3%. (Note that the DOP should be less than 100%; the deviation is induced by the measurement analysis of the equipment.) The results show that a stable SP operation of fiber laser could be successfully achieved with the proposed structure. 4. Conclusion
Fig. 5. Measured linewidth of the proposed laser with delayed self-heterodyne method (black line), and measured experimental data with Lorentz function fitting (red line).
stably operating in the SLM operation, as shown in Fig. 4(b); there is only one longitudinal-mode beating signal. We also measured the beating signal of the fiber laser only using the narrow-band CFBG as the filter but without the structured CFBG and unpumped EDF, as shown in Fig. 4(c); there are many longitudinal-mode beating signals. By comparing Figs. 4(a) and 4(c), we can see that the structured CFBG filter can be used to select the laser longitudinal modes effectively. Thus, with the combination of the structured CFBG filter and 2 m unpumped EDF acting as a saturable absorber, stable SLM operation of the fiber laser is guaranteed. The linewidth of the fiber laser is measured with the delayed self-heterodyne method. The delayed line of the delayed self-heterodyne method is about 40 km, corresponding to a nominal resolution of 4.8 kHz. The RBW of the ESA is 4 kHz. The measured linewidth is shown in Fig. 5 (the black line), and the Lorentz fitting curve with a 20 dB linewidth about 27.7 kHz of the fiber laser is shown in Fig. 5 (the red line), which indicates that the laser linewidth is approximately 1.39 kHz FWHM [19]. Through a segment of 3 m commercial single-mode fiber, the output port of the laser is connected to the Agilent 8509 Lightwave Polarization Analyzer. Figure 6 shows the measured results of the polarization parameters accumulated in 2 min without external disturbance. The Poincare sphere in Fig. 6 shows that there is almost one point, which indicates
Fig. 6. Polarization parameters of the fiber laser. 10
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In conclusion, a stable SP SLM linear cavity EDFL based on structured CFBG has been achieved at room temperature. The structured CFBG together with the unpumped EDF are used to ensure the SLM operation of the fiber laser. Meanwhile, the inline broadband polarizer is used to achieve the stable SP operation. The measured 20 dB laser linewidth is about 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. This work is jointly supported by the Fundamental Research Funds for the Central Universities (No. 2014YJS019). References 1. S. Pan and J. Yao, “A wavelength-switchable singlelongitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17, 5414– 5419 (2009). 2. L. Jin, Y. N. Tan, Z. Quan, M. P. Li, and B. O. Guan, “Straininsensitive temperature sensing with a dual polarization fiber grating laser,” Opt. Express 20, 6021–6028 (2012). 3. G. Jihong, C. Spiegelberg, and J. Shibin, “Narrow linewidth fiber laser for 100-km optical frequency domain reflectometry,” IEEE Photon. Technol. Lett. 17, 1827–1829 (2005). 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, 740–742 (2007). 5. X. He, X. Fang, C. Liao, D. N. Wang, and J. Sun, “A tunable and switchable single-longitudinal-mode dual-wavelength fiber laser with a simple linear cavity,” Opt. Express 17, 21773– 21781 (2009). 6. S. Feng, S. Lu, W. Peng, Q. Li, T. Feng, and 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). 7. B. Yin, S. Feng, Z. Liu, Y. Bai, and S. Jian, “Tunable and switchable dual-wavelength single polarization narrow linewidth SLM erbium-doped fiber laser based on a PM-CMFBG filter,” Opt. Express 22, 22528–22533 (2014). 8. A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, and A. Cutolo, “Photonic band-gap engineering in UV fiber gratings by the arc discharge technique,” Opt. Express 16, 15332– 15342 (2008). 9. C. C. Lee, Y. K. Chen, and S. K. Liaw, “Single-longitudinalmode fiber laser with a passive multiple-ring cavity and its application for video transmission,” Opt. Lett. 23, 358–360 (1998). 10. J. Sun, X. Yuan, X. Zhang, and D. Huang, “Single-longitudinalmode fiber ring laser using fiber grating-based Fabry–Perot filters and variable saturable absorbers,” Opt. Commun. 267, 177–181 (2006). 11. X. Zhang, N. H. Zhu, L. Xie, and B. X. Feng, “A stabilized and tunable single-frequency erbium-doped fiber ring laser employing external injection locking,” J. Lightwave Technol. 25, 1027–1033 (2007). 12. 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, 6082–6095 (2009). 13. S. Yamashita and G. J. Cowle, “Single-polarization operation of fiber distributed feedback (DFB) lasers by injection locking,” J. Lightwave Technol. 17, 509–513 (1999). 14. B. Yin, S. Feng, Y. Bai, Z. Liu, L. Liang, S. Liu, and S. Jian, “Switchable single-polarization dual-wavelength ring laser based on structured PM-CFBG,” IEEE Photon. Technol. Lett. 26, 1227–1230 (2014). 15. X. Chen, J. Yao, F. Zeng, and Z. Deng, “Single-longitudinalmode fiber ring laser employing an equivalent phase-shifted fiber Bragg grating,” IEEE Photon. Technol. Lett. 17, 1390– 1392 (2005).
16. A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, A. Cutolo, and M. Giordano, “Micro-structured fiber Bragg gratings. Part I: spectral characteristics,” Opt. Fiber Technol. 13, 281–290 (2007). 17. M. Pisco, A. Iadicicco, S. Campopiano, A. Cutolo, and A. Cusano, “Structured chirped fiber Bragg gratings,” J. Lightwave Technol. 26, 1613–1625 (2008). 18. K. Zhang and J. U. Kang, “C-band wavelength-swept single-longitudinal-mode erbium-doped fiber ring laser,” Opt. Express 16, 14173–14179 (2008). 19. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155, 180–186 (1998).
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A novel linear cavity erbium-doped fiber (EDF) laser based on a structured chirped fiber Bragg grating (CFBG) filter is proposed for stable single-polarization (SP) single-longitudinal-mode (SLM) operation. For the first time, to the best of our knowledge, a structured CFBG filter with an ultranarrow transmission band which is generated by tapering directly on CFBG is used to select the laser longitudinal mode. The SLM operation is obtained by using the structured CFBG together with an unpumped EDF acting as a saturable absorber. The fluctuations of the laser peak power and center wavelength are less than 0.07 dB and 1 pm in 1 h, respectively. The stable SP operation is achieved by using the inline broadband polarizer. The measured 20 dB laser linewidth is about 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. © 2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (060.3738) Fiber Bragg gratings, photosensitivity; (140.3500) Lasers, erbium. http://dx.doi.org/10.1364/AO.54.000006
1. Introduction
Stable narrow linewidth single-longitudinal-mode (SLM) erbium-doped fiber lasers (EDFL) have attracted a lot of research interest due to their potential applications in optical communications, optical instrument testing, and optical fiber sensors [1–3]. Meanwhile, many applications such as nonlinear frequency conversion, interferometric fiber sensors, pumping of active crystals, and optical parametrical devices demand that the fiber lasers work on stable single polarization operation [4]. To implement a stable SLM operation of the EDFL, two styles can be carefully adopted. One approach is a short-length cavity to enlarge longitudinal-mode spacing [5]. However, these lasers normally suffer from a low pump absorption and
1559-128X/15/010006-06$15.00/0 © 2015 Optical Society of America 6
APPLIED OPTICS / Vol. 54, No. 1 / 1 January 2015
hence low slope efficiency. Thus, particular erbiumdoped fibers (EDF) are designed to solve this issue. More importantly, the short-length cavity can result in broad spectral linewidth. Another approach is a long-length ring cavity to offer higher output power without special requirements for the gain media [6]. However, since a fiber ring laser usually has a long cavity with closely spaced longitudinal modes, an ultranarrow bandpass filter (BPF) must be incorporated to eliminate multilongitudinal-mode oscillation and mode hopping [7]. Cusano et al. produce a tapered region in a fiber Bragg grating (FBG) by the arc discharge technique [8]. This method can be used to realize a new ultranarrow BPF which can be used to eliminate multilongitudinal-mode oscillation and mode hopping. Furthermore, to achieve stable SLM operation, several techniques have been proposed to ensure a long-term stable performance in EDFL, for instance, introducing a passive multiplering cavity or a compound ring resonator composed of
a dual-coupler fiber ring to guarantee SLM laser oscillation [9], using an unpumped EDF as a dynamic narrow-bandwidth filter [10], and utilizing selfinjection feedback to control the lasing frequency [11]. At the same time, in order to realize a singlepolarization (SP) fiber laser, several methods have been proposed, such as utilizing polarizationdependent losses of the fiber laser [12], an inline broadband polarizer [6], injection locking [13], and PM-FBG [14]. In this paper, we propose a novel approach for achieving a stable SP SLM linear cavity EDFL. The structured chirped fiber Bragg grating (CFBG) and the saturable absorber of an unpumped EDF are used to ensure stable SLM operation. The SP operation of the fiber laser is guaranteed through using an inline broadband polarizer. The optical signalto-noise ratio (OSNR) of the laser is more than 50 dB, and the fluctuations of the laser peak power and center wavelength are less than 0.07 dB and 1 pm in 1 h, respectively. The measured 20 dB laser linewidth is 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. 2. Experimental Configuration and Principle
Figure 1 shows the experimental setup of the proposed SP-SLM-EDFL with a linear cavity. The fiber laser consists of a 980/1550 nm wavelength division multiplex (WDM), a section of EDF1 of 4 m with absorption coefficient about 16 dB/m at 1530 nm as the gain medium, an optical circulator (OC), a structured CFBG filter, a narrow-band CFBG, a polarization controller (PC), a polarizer with 27 dB extinction ratio, a 90∶10 optical fiber coupler which provides 10% of the optical power for the output and 90% for feedback inside the cavity, and a segment of 2 m unpumped EDF2 with an absorption coefficient about 15 dB/m. The EDF1 is pumped with a 976 nm laser diode (maximum output power 250 mW) through the WDM. The structured CFBG filter and the narrow-band CFBG serve as a longitudinal-mode discriminator to restrict the laser oscillation. The OC is employed to help the implementation of the ultranarrow BPF and sustain the
Fig. 1. Schematic diagram of the proposed SP-SLM-EDFL with a linear cavity.
unidirectional oscillation. The unpumped EDF2 is used as a saturable absorber, and acts as a dynamic narrow-band filter. The PC is used to tune the birefringence status of the laser cavity, and to suppress mode competition through adjusting gain and loss within the laser cavity. The polarizer is used to select one polarized mode of the two orthogonally polarized modes within the laser cavity, in order to get SP laser output, and to suppress the competition of the orthogonally polarized modes. The Exfo/Burleigh WA-1600 wavemeter (WM) is employed to obtain the fluctuations of the operating wavelength and the lasing output power. The laser output is also monitored by the ANDO AQ6317C optical spectrum analyzer (OSA) with a resolution of 0.01 nm and an electrical spectrum analyzer (ESA; Agilent N9010A, 9 kHz– 26.5 GHz) connected to a photo detector (PD; Tektronix CSA803A SD-48PD subunit, 33 GHz). The linewidth of the proposed laser is measured with the delayed self-heterodyne method. The Agilent 8509 Lightwave Polarization Analyzer (PA) is used to measure the SP operation. A 14 cm length of CFBG and a 5 cm length of narrow-band CFBG are fabricated by the standard phase-mask scanning technique. The CFBG is written in a 14 days hydrogen-loaded (10 Mpa; at room temperature) germanium-doped SMF-28 fiber with a 14 cm linearly chirped phase-mask plate which has a chirp rate of 0.018 nm/cm and a period of 1069.74 nm scanned by a 248 nm KrF excimer laser ultraviolet light. The red line and the blue line in Fig. 2(b) show the measured transmission spectrum of the fabricated CFBG and the reflectance spectrum of narrow-band CFBG which has a reflectivity of about 98%, respectively. This fabricated CFBG (the red line) has a 3 dB bandwidth of 0.20 nm and an extinction ratio of 16.6 dB for its transmission response. Then we create the tapered region (approximately 0.35 mm long, and a waist diameter of 110 2 μm) at the middle of the CFBG by a fusing-and-pulling treatment with a fiber fusion splicer (FSU975), and the transmission spectrum of the CFBG (the black line) has one transmission peak generated by tapering directly on the CFBG, as shown in Fig. 2(b). Through analyzing the experimental data in Fig. 2(b), we can find that the insertion losses which are produced by tapering the CFBG are less than 0.02 dB. At the same time, we calculate that the transmission bandwidth of the structured CFBG is approximately 42.5 MHz by analyzing the experimental data of the structured CFBG filter (it is equivalent to PS-CFBG) and the simulation calculation [15]. Figure 2(c) shows the tapered region of the structured CFBG filter under the microscope. The grating of tapered region is removed by the arc discharge technique, as shown in Fig. 2(a), and the tapered region approximates infinitesimal because it just has a length of short region on the CFBG. Thus, the structured CFBG is analyzed by the basis of phase shift theory [16]. The phase delay ΔΦ can be defined as 1 January 2015 / Vol. 54, No. 1 / APPLIED OPTICS
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index modulation depth, and chirp rate) and the taper (position, cavity length, and waist radius). Therefore, the structured CFBG filter with a narrow transmission band can be used to serve as a longitudinal-mode discriminator in the fiber laser. The unpumped EDF is used as a saturable absorber, and acts as a dynamic narrow-band filter. The absorption coefficient of an EDF is inversely proportional to the intensity of the optical light. So when a spatial interference pattern is generated by a standing wave in the EDF, a spatial-holeburning effect is induced, and thus a periodic refractive index change like a weak FBG is formed. A saturable absorber is a self-tracking filter that can stabilize the single-mode output. The bandwidth of the induced FBG (the unpumped EDF) is calculated as [18]
(a)
(b)
c Δf κ λ
(c) Fig. 2. (a) Schematic diagram of the localized grating erasure setup. (b) The experimental transmission spectrum of the structured CFBG filter (red line) without tapering, (black line) with tapering, and (blue line) the reflection spectrum of narrow-band CFBG. (c) The tapered region of the structured CFBG filter under the microscope.
ΔΦ
4·π · Δneff SRI; Dth · Lth ; λB zth
(1)
where λB is the optical wavelength corresponding to the tapered region position zth . Δneff expresses the effective refractive index variation caused by tapering, which depends on the surrounding refractive index (SRI) and the waist diameter Dth. Lth is the length of the tapered region [17]. The chirp rate and the grating length are 0.018 nm/cm and 14 cm, respectively. For any structured CFBG considered, by increasing Lth and thus the phase shift Eq. (1), the narrow passband changes its depth and moves toward lower wavelengths at the same time only to disappear and then to appear again at higher wavelengths. The minimum of the reflectivity of the passband region approaches zero since the phase shift is equal to π 2kπ (k with integer). The relationship between the reflectivity and the length of the passband region is analyzed in Ref. [17]. Meanwhile, the power losses and the reflectivity of the passband region as a function of the taper waist are analyzed in Ref. [8]. Thus, the structured CFBG similar to the phase shift chirped fiber Bragg grating (PS-CFBG) has the characteristics of a narrow-band filter. We can choose the appropriate parameters of BPF through adjusting the CFBG (length, refractive 8
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s 2 Δn 2 1 ; 2neff N
(2)
where N Lg∕Λ is the total number of grating periods, and λ is the center wavelength at which maximum reflectivity occurs. Δn is the periodical spatial variation of the refractive index, which can be given by the well-known Kramers–Kronig relation. The coupling coefficient of the induced grating κ can be estimated from Δn as κ
2Δn : neff λ
(3)
In our experiment, the unpumped EDF-based saturable absorber induces a weak FBG with a bandwidth of less than 21.7 MHz. Thus, with the combination of the structured CFBG filter and 2 m unpumped EDF acting as a saturable absorber, we can realize stable SLM operation of the fiber laser. 3. Experimental Results and Discussion
Through tuning the PC to adjust the gain and loss of the laser cavity, we achieved stable single-wavelength fiber laser operation. At the pump power of 100 mW (980 nm), the measured output spectrum at the wavelength of 1544.178 nm is shown in Fig. 3(a). The output power is about −14 dBm and the OSNR is more than 50 dB. In order to investigate the laser output stability, we measured the optical spectra of the fiber laser with 16 times repeated scans at 2 min intervals in half an hour, as shown in Fig. 3(b). No wavelength variation could be observed within the limited resolution of the OSA in the experimental period. To obtain more detailed properties on the lasing stability, the laser output was measured by using the WM for 1 h with a power resolution of 0.01 dBm and a wavelength resolution of 0.1 pm. The measured results are shown in Fig. 3(c). The fluctuations of wavelength and output power of the proposed fiber laser which are less than
(a) (a)
(b)
(b)
(c)
(c)
Fig. 3. (a) The measured optical spectrum of the proposed laser. (b) The fiber laser operation with 16 times repeated scans in half an hour. (c) The fluctuations of the laser peak power and center wavelength in 1 h measured by WM.
1 pm and 0.07 dB, respectively, represent high-stability lasing operation in the situation of room temperature. The SLM operation of the fiber laser is proved through the self-homodyne method. The detected electrical spectra of the beating signals of the fiber laser in the range of 200 MHz are shown in Fig. 4. The scanning range is much wider than the free spectrum range (FSR), because the cavity length of the fiber ring laser is approximately 22 m, corresponding to FSR of about 9.2 MHz. We can clearly see that the fiber laser is in the stable SLM operation in Fig. 4(d). We measured the beating signal of the fiber laser using the narrow-band CFBG combined with the structured CFBG but without the unpumped EDF. There are only several longitudinal-mode beating signals and the fiber laser is no longer stably operating in the SLM operation, as shown in Fig. 4(a). Then we removed the structured CFBG but added the unpumped EDF. The fiber laser is also no longer
(d)
Fig. 4. Electrical spectra of the beating signals. (a) The narrowband CFBG combined with the structured CFBG but without the unpumped EDF. (b) The narrow-band CFBG combined with the unpumped EDF but without the structured CFBG. (c) The narrow-band CFBG without the structured CFBG and the unpumped EDF. (d) The narrow-band CFBG combined with the structured CFBG and the unpumped EDF. 1 January 2015 / Vol. 54, No. 1 / APPLIED OPTICS
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that the output of the fiber laser is at a stable SP state. The apparent degree of polarization (DOP) in percentage term is as high as 100.3%. (Note that the DOP should be less than 100%; the deviation is induced by the measurement analysis of the equipment.) The results show that a stable SP operation of fiber laser could be successfully achieved with the proposed structure. 4. Conclusion
Fig. 5. Measured linewidth of the proposed laser with delayed self-heterodyne method (black line), and measured experimental data with Lorentz function fitting (red line).
stably operating in the SLM operation, as shown in Fig. 4(b); there is only one longitudinal-mode beating signal. We also measured the beating signal of the fiber laser only using the narrow-band CFBG as the filter but without the structured CFBG and unpumped EDF, as shown in Fig. 4(c); there are many longitudinal-mode beating signals. By comparing Figs. 4(a) and 4(c), we can see that the structured CFBG filter can be used to select the laser longitudinal modes effectively. Thus, with the combination of the structured CFBG filter and 2 m unpumped EDF acting as a saturable absorber, stable SLM operation of the fiber laser is guaranteed. The linewidth of the fiber laser is measured with the delayed self-heterodyne method. The delayed line of the delayed self-heterodyne method is about 40 km, corresponding to a nominal resolution of 4.8 kHz. The RBW of the ESA is 4 kHz. The measured linewidth is shown in Fig. 5 (the black line), and the Lorentz fitting curve with a 20 dB linewidth about 27.7 kHz of the fiber laser is shown in Fig. 5 (the red line), which indicates that the laser linewidth is approximately 1.39 kHz FWHM [19]. Through a segment of 3 m commercial single-mode fiber, the output port of the laser is connected to the Agilent 8509 Lightwave Polarization Analyzer. Figure 6 shows the measured results of the polarization parameters accumulated in 2 min without external disturbance. The Poincare sphere in Fig. 6 shows that there is almost one point, which indicates
Fig. 6. Polarization parameters of the fiber laser. 10
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In conclusion, a stable SP SLM linear cavity EDFL based on structured CFBG has been achieved at room temperature. The structured CFBG together with the unpumped EDF are used to ensure the SLM operation of the fiber laser. Meanwhile, the inline broadband polarizer is used to achieve the stable SP operation. The measured 20 dB laser linewidth is about 27.7 kHz, which indicates the laser linewidth is approximately 1.39 kHz FWHM. This work is jointly supported by the Fundamental Research Funds for the Central Universities (No. 2014YJS019). References 1. S. Pan and J. Yao, “A wavelength-switchable singlelongitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17, 5414– 5419 (2009). 2. L. Jin, Y. N. Tan, Z. Quan, M. P. Li, and B. O. Guan, “Straininsensitive temperature sensing with a dual polarization fiber grating laser,” Opt. Express 20, 6021–6028 (2012). 3. G. Jihong, C. Spiegelberg, and J. Shibin, “Narrow linewidth fiber laser for 100-km optical frequency domain reflectometry,” IEEE Photon. Technol. Lett. 17, 1827–1829 (2005). 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, 740–742 (2007). 5. X. He, X. Fang, C. Liao, D. N. Wang, and J. Sun, “A tunable and switchable single-longitudinal-mode dual-wavelength fiber laser with a simple linear cavity,” Opt. Express 17, 21773– 21781 (2009). 6. S. Feng, S. Lu, W. Peng, Q. Li, T. Feng, and 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). 7. B. Yin, S. Feng, Z. Liu, Y. Bai, and S. Jian, “Tunable and switchable dual-wavelength single polarization narrow linewidth SLM erbium-doped fiber laser based on a PM-CMFBG filter,” Opt. Express 22, 22528–22533 (2014). 8. A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, and A. Cutolo, “Photonic band-gap engineering in UV fiber gratings by the arc discharge technique,” Opt. Express 16, 15332– 15342 (2008). 9. C. C. Lee, Y. K. Chen, and S. K. Liaw, “Single-longitudinalmode fiber laser with a passive multiple-ring cavity and its application for video transmission,” Opt. Lett. 23, 358–360 (1998). 10. J. Sun, X. Yuan, X. Zhang, and D. Huang, “Single-longitudinalmode fiber ring laser using fiber grating-based Fabry–Perot filters and variable saturable absorbers,” Opt. Commun. 267, 177–181 (2006). 11. X. Zhang, N. H. Zhu, L. Xie, and B. X. Feng, “A stabilized and tunable single-frequency erbium-doped fiber ring laser employing external injection locking,” J. Lightwave Technol. 25, 1027–1033 (2007). 12. 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, 6082–6095 (2009). 13. S. Yamashita and G. J. Cowle, “Single-polarization operation of fiber distributed feedback (DFB) lasers by injection locking,” J. Lightwave Technol. 17, 509–513 (1999). 14. B. Yin, S. Feng, Y. Bai, Z. Liu, L. Liang, S. Liu, and S. Jian, “Switchable single-polarization dual-wavelength ring laser based on structured PM-CFBG,” IEEE Photon. Technol. Lett. 26, 1227–1230 (2014). 15. X. Chen, J. Yao, F. Zeng, and Z. Deng, “Single-longitudinalmode fiber ring laser employing an equivalent phase-shifted fiber Bragg grating,” IEEE Photon. Technol. Lett. 17, 1390– 1392 (2005).
16. A. Cusano, A. Iadicicco, D. Paladino, S. Campopiano, A. Cutolo, and M. Giordano, “Micro-structured fiber Bragg gratings. Part I: spectral characteristics,” Opt. Fiber Technol. 13, 281–290 (2007). 17. M. Pisco, A. Iadicicco, S. Campopiano, A. Cutolo, and A. Cusano, “Structured chirped fiber Bragg gratings,” J. Lightwave Technol. 26, 1613–1625 (2008). 18. K. Zhang and J. U. Kang, “C-band wavelength-swept single-longitudinal-mode erbium-doped fiber ring laser,” Opt. Express 16, 14173–14179 (2008). 19. H. Ludvigsen, M. Tossavainen, and M. Kaivola, “Laser linewidth measurements using self-homodyne detection with short delay,” Opt. Commun. 155, 180–186 (1998).
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