- Email: [email protected]
Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber
Optics Communications 279 (2007) 168–172 www.elsevier.com/locate/optcom
Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber Zhanyuan Liu, Yan-ge Liu *, Jiangbing Du, Shuzhong Yuan, Xiaoyi Dong Key Laboratory of Opto-Electronic Information and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China Received 19 December 2006; received in revised form 21 March 2007; accepted 9 July 2007
Abstract A stable and narrow wavelength spacing multiwavelength erbium-doped fiber laser is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with a narrow wavelength spacing of less than 0.1 nm via using a single fiber Bragg gratings written in polarization-maintaining (PM) fiber. By adjusting polarization controller, the wavelength spacing of dualwavelength lasing oscillations can be tuned to as small as 0.032 nm. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The room-temperature operation principle is based on the polarization hole burning and deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL). The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Multiwavelength fiber laser; Erbium-doped fiber; Polarization hole burning; Fiber Bragg grating; Polarization control
1. Introduction Multiwavelength erbium-doped fiber lasers (EDFLs) attract a lot of interest due to their potential applications in dense wavelength-division-multiplexed (DWDM) fiber communication systems, optical instrument testing and so on. However, Erbium-doped fiber (EDF) is a primary homogeneous gain medium at room temperature, which leads to strong mode competition and unstable lasing, thus it is difficult to obtain simultaneous multiwavelength lasing in erbium-doped fiber lasers (EDFLs). Cooling EDF in liquid nitrogen (77 K) [1] is often used to reduce the homogeneous linewidth of EDF, however, this method is not suitable for practical application. In order to reduce the cross-gain saturation and suppress the mode competition, different techniques have been proposed to realize multiwavelength oscillations at room temperature in EDFLs. These include the introduction of spatial hole burning
*
Corresponding author. E-mail address: [email protected] (Y.-g. Liu).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.07.027
effect [2], polarization hole burning (PHB) effect [3,4], spectral hole burning effect [5] and various nonlinear effects such as four-wave mixing in photonic crystal fiber [6] and stimulated Brillouin scattering [7], in the laser cavity. Other methods by inserting frequency shifter in the laser cavity [8], incorporating a section of multimode fiber [9] or a multimode fiber Bragg grating (FBG) [10] in a laser cavity, employing distributed Fabry-Pe´rot cavities in a strongly chirped fiber Bragg grating [11], and designing specially erbium-doped fibers [12] or cavity structures [13] were also reported. Fiber Bragg gratings (FBGs) are ideal wavelength-selection components for fiber lasers. Several kinds of FBGs have been used in multiwavelength fiber lasers. Among them, FBGs written in polarization-maintaining fiber (PM-FBG) have attracted more attention recently because of their special polarization property. Zhao et al. proposed a switchable multiwavelength erbium-doped fiber laser based on two cascaded PM-FBGs written in HBF [14]. Though three-wavelength and four-wavelength lasing oscillations have been obtained, the laser operated unstably at room temperature. Liu et al. also suggested a novel scheme
Z. Liu et al. / Optics Communications 279 (2007) 168–172
-30 -35
Intensity(dB)
for four-wavelength lasing oscillations using active overlapping linear cavities with two PM-FBGs [15]. However, the wavelength spacing in that report was >0.3 nm and the configuration is also a bit complex. In this paper, only using a single PM-FBG, a multiwavelength EDFL with narrow wavelength spacing (<0.1 nm) based on polarization hole burning and spectral hole burning effect is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with narrow wavelength spacing of only 0.08 nm. Furthermore, the wavelength spacing of dual-wavelength lasing oscillations can be tuned to as small as 0.032 nm, by adjusting the polarization controller. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB.
-40 -45 -50 -55 -60 1544.0
1544.5
1545.0
1545.5
1546.0
Wavelength(nm) Fig. 2. Reflection spectrum of the PM-FBG.
2. Experiment setup
3. Results and discussion In our experiment, by appropriately adjusting the states of PC1, simultaneous triple-wavelength oscillation is obtained. Fig. 3a shows three lasing lines at 1545.056, 1545.370 and 1545.451 nm with the wavelength spacing -5 -10 -15
Intensity(dB)
The proposed laser configuration is shown in Fig. 1. The active medium Er-doped fiber is of length 3.9 m and is pumped by a 980 nm laser diode with a power of 115 mW through a WDM coupler. The filter is a single FBG written in polarization-maintaining (PM) fiber, which offers flexibility for selecting of the lasing wavelength and has the advantage of preserving the long-term stability of fiber laser. The PM-FBG is fabricated using the phasemask method. Due to a long exposure time, the bandwidth of each reflective peak is about 0.1 nm (the reflective spectrum of the FBG has a flattened shape). Fig. 2 shows the reflection spectrum of the PM-FBG. The two center reflective peaks are 1545.058 and 1545.430 nm with bandwidths of 0.1 nm, respectively. The polarization controller (PC1) is used to rotate the polarization state of the incident light into the PM-FBG. The optical circulator (OC) connects the PM-FBG to the laser cavity, and ensures that the oscillation propagates in a single direction. The laser output emerges from the 10% port of a 10:90 optical coupler. The isolator is used to prevent reverse propagating light which is a disadvantage in stabilizing multiwavelength lasing [16]. The spectral characteristics of the laser are measured using an optical spectrum analyzer with 0.01 nm resolution.
169
-20 -25 -30 -35 -40 -45 1544.9 1545.0 1545.1 1545.2
1545.3 1545.4 1545.5 1545.6
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Intensity(dB)
-15 -20 -25 -30 -35 -40 -45 1544.9
1545.0 1545.1
1545.2 1545.3
1545.4
1545.5 1545.6
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Fig. 1. Schematic diagram of the proposed laser.
Fig. 3. Output spectrum of the different triple-wavelength fiber laser by adjusting the PC1.
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of 0.314 and 0.081 nm, respectively. Fig. 3b shows another three lasing lines at 1544.990, 1545.077 and 1545.444 nm, with the wavelength spacing of 0.087 and 0.367 nm. Fig. 4 shows the 16 times repeated scan of output spectra
of EDFL with a 10 s interval. The maximum power fluctuation of the three lasing lines is less than 0.5 dB. This indicates fairly stable room-temperature operation even for wavelength spacing of only 0.08 nm.
-5
-5
-10
-10
-15
-15
Intensity(dB)
Intensity(dB)
Fig. 4. The stability of two triple-wavelength lasers (16 times repeated scans).
-20 -25 -30
-20 -25 -30 -35
-35 -40 -40 -45 -45 1544.7
1544.8 1544.9 1545.0 1545.1
1545.2 1545.3 1545.4
Wavelength(nm)
1545.1 1545.2 1545.3 1545.4 1545.5 1545.6 1545.7 1545.8
Wavelength(nm)
0
Intensity(dB)
-10 -20 -30 -40 -50 -60 1544.7 1544.8 1544.9 1545.0
1545.1 1545.2 1545.3 1545.4
Wavelength(nm) Fig. 5. Output spectrum of dual-wavelength at different wavelength: (a) lasing lines at 1545.019 and 1545.092 nm with a wavelength separation of 0.073 nm; (b) two lasing lines at 1545.368 and 1545.450 nm with a wavelength separation of 0.082 nm; (c) two lasing lines at 1545.063 and 1545.095 with a wavelength separation of 0.032 nm; (d) the stability of dual-wavelength lasing lines with the wavelength separation of 0.032 nm (16 times repeated scans).
Z. Liu et al. / Optics Communications 279 (2007) 168–172
Fig. 6. Schematic diagram of measuring the polarization states.
-10
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-20
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-50 1544.7
1544.8 1544.9 1545.0 1545.11 545.21 545.31 545.4
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Additionally, the stable narrow-line-width dual-wavelength laser arises by carefully adjusting the PC1. Fig. 5a shows two lasing lines at 1545.019 and 1545.092 nm, respectively, with a wavelength separation of 0.073 nm, related to the peak1 of the PM-FBG. Fig. 5b shows two lasing lines at 1545.368 and 1545.450 nm, respectively, with a wavelength separation of 0.082 nm, related to the peak2 of the PM-FBG. Furthermore, through carefully adjusting the state of the PC1, a stable dual-wavelength lasing with a wavelength spacing of as small as 0.032 nm could be obtained as shown in Fig. 5c. Fig. 5d shows the 16 times repeated scans of the laser output spectra with the wavelength spacing of 0.032 nm at a 10 s interval. In our experiment, the dual-wavelength operation can be stable for more than half an hour if the temperature variation and mechanical vibration are reasonably small (i.e., in a laboratory). The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The worst wavelength shift is assumed to be less than 0.01 nm for the limitation of OSA resolution. As mentioned previously, with a single PM-FBG as wavelength selection, three wavelengths with narrow line width could be obtained simultaneously. The performance may be explained in a number of ways. Firstly, the PC1 in the fiber loop is adjusted at various settings of the loop birefringence that is beneficial to the enhancement of polarization hole burning, so a comb filter could be established [17]. In our experiment, the 3 dB bandwidth of the FBG is only 0.1 nm, which is very narrow. By adjusting the PC1, the fine pectinate spectrum is so changeable that can cleave the spectrum of a PM-FBG, by wavelength selecting in the EDFL, into two parts. The narrow-line-width dual- and triple-wavelength could be obtained. Secondly, The PMFBG is fabricated using a long exposure time and the index of refraction is not uniform in each reflection peak. We measure the polarization states of lasers at two wavelengths by using a polarization controller (PC2) and a polarizer (see Fig. 6). In the dual-wavelengths lasing state (see Fig. 5c), we adjust the PC2 to make output2 laser only in wavelength 1545.063 nm, as shown in Fig. 7a. After rotating the PC2 by 90°, the spectrum in output2 became single-wavelength lasing in 1545.095 nm, as shown in Fig. 7b. It is obvious that the laser outputs in two wavelengths are orthogonal in polarization. The reflection characteristic of the PM-FBG depends on the polarization states of the inci-
171
-30
-40
-50 1544.7 1544.8 1544.9 1545.0 1545.1 1545.2
1545.3 1545.4
Wavelength(nm) Fig. 7. Output spectrum of the lasers with different polarization states of the PC2.
dent light, which revealed the strong polarization-dependence of the PM-FBG in one reflection peak. Thus, the dual- and triple-wavelength operation is very stable at room temperature even with wavelength separations of 0.032 nm, due to the enhanced polarization hole burning (PHB) of the laser cavity. Thirdly, spectral hole burning effect is helpful to obtain stable triple-wavelength lasing at room temperature in the EDFRL in Ref. [16]. We investigated the effect of the saturated signal level in the laser cavity on the stability of the triple-wavelength lasing oscillations via changing only the pump power acting on the EDF in Fig. 1. As shown in Fig. 8, the smaller the pump power is, the larger the power fluctuations of the lasing wavelengths are. Furthermore, when the pump power is less than 80 mW, the triple-wavelength lasing oscillations is not obtained. Thus, due to the enhanced polarization hole burning (PHB) of the laser cavity and spectral hole burning effect which becomes obvious at room temperature when the signal input into the EDF are deeply saturated, the stable triple-wavelength can be obtained. For these experimental results, the stable dual-and triple-wavelength lasers with very narrow wavelength spacing
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4. Conclusion
2.0
Power Fluctuation(dB)
1.5 1.0
90mW pump power 1545.056nm 1545.370nm 1545.451nm
In conclusion, we have demonstrated that stable multiwavelength lasing oscillations at room temperature could be achieved by exploiting a PHD effect and a deeply saturated spectral hole burning effect. Multiwavelength lasing oscillations obtained by this method are stable and the maximum power fluctuation over a long-time observation is less than 0.5 dB. Furthermore, the dual-wavelength separation between the lasing lines can also be tuned to as small as 0.032 nm. We believe that this technique provides another simple approach to achieve stable multiwavelength EDF lasing at room temperature.
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -20
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20
40
60
80
100
120
140
160
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The work is supported by the Tianjin Natural Science Foundation under Grant No. 06YFJZJC00300, the National Key Basic Research and Development Programme of China under Grant No. 2003CB314906 and the National Natural Science Foundation of China under Grant No. 10674074.
2.0
Power Fluctuation(dB)
1.5 1.0
Acknowledgements
115mW pump power 1545.056nm 1545.370nm 1545.451nm
0.5 0.0
References
-0.5 -1.0 -1.5 -2.0 -20
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60
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Scan Time(s) Fig. 8. Power fluctuations of the triple-wavelength EDFRL with different pump power. The maximum fluctuations for 90 mW, 115 mW pomp powers are (0.26, 1.98, 2.11) dB and (0.27, 0.36, 0.39) dB at wavelengths of 1545.056, 1545.370 and 1545.451 nm, respectively.
has been generated at room temperature. The device is highly flexible in view of wavelength switching capability, as has been demonstrated by the measured results. Also, the configuration is simple as it requires only adjusting PCs’ states for many kinds of wavelength oscillation operations. Furthermore, the dual-wavelength fiber laser with ultranarrow wavelength spacing and orthogonal polarization has potential applications in microwave/millimeterwave signal generation and modulation of data on the microwave subcarrier [18]. The ultimate limit in this application is the laser source must be single-longitudinal-mode operation. The bandwidth of the generated lasing lines can be further decreased through incorporating ultranarrow dual-transmission-band fiber Bragg grating filter [19] or using a saturable absorber [20] in cavity. Thus, the singlelongitudinal-mode lasing in each wavelength is expected in our configuration.
[1] S. Yamashita, K. Hotate, Electron. Lett. 32 (1996) 1298. [2] M. Ibsen, E. Ronnekleiv, G.J. Cowle, et al., Electron. Lett. 36 (2000) 143. [3] Feng Xinhuan, Sun Lei, Xiong Lingyun, et al., Opt. Fiber Technol. 10 (2004) 275. [4] Xinhuan Feng, Yange Liu, Lei Sun, et al., Chinese Phys. Lett. 21 (2004) 659. [5] E. Desurvire, J.L. Zyskind, J.R. Simpson, IEEE Photon. Technol. Lett. 2 (1990) 246. [6] Xueming Liu, Xiaoqun Zhou, Xiufeng Tang, et al., IEEE Photon. Technol. Lett. 17 (2005) 1626. [7] J. Cowle Gregory, Yu Stepanov Dmitrii, IEEE Photon. Technol. Lett. 8 (1996) 1465. [8] Antoine Bellemare, Miroslav Kara’sek, Martin Rochette, et al., J. Lightwave Technol. 18 (2000) 825. [9] Alistair J. Poustie, Finlayson Neil, Opt. Lett. 19 (10) (1994) 716. [10] Feng Xinhuan, Liu Yange, Yuan Shuzhong, et al., Opt. Express 12 (2004) 3834. [11] R. Slavik, I. Castonguay, S. LaRochelle, et al., IEEE Photon. Technol. Lett. 16 (2004) 1017. [12] O. Graydon, W.H. Loh, R.I. Laming, et al., IEEE Photon. Technol. Lett. 8 (1996) 63. [13] Qinghe Mao, John W.Y. Lit, J. Lightwave Technol. 21 (2003) 160. [14] C. Zhao, X. Yang, C. Lu, et al., Opt. Commun. 230 (2004) 313. [15] Y. Liu, X. Feng, S. Yuan, et al., Opt. Express 12 (2004) 2056. [16] Y. Liu, X. Dong, S. Yuan, et al., Opt. Express 14 (2006) 9293. [17] Xiujie Jia, Yange liu, Libin Si, et al., Chinese Phys. Lett. 23 (2006) 2092. [18] J.S. Leng, Y.C. Lai, W. Zhang, J.A.R. Williamston, IEEE Photon. Technol. Lett. 17 (2006) 1729. [19] X.F. Chen, J.P. Yao, F. Zeng, Z.C. Deng, IEEE Photon. Technol. Lett. 17 (2005) 1390. [20] Y. Cheng, J.T. Kringlebotn, D.N. Payne, Opt. Lett. 20 (1995) 875.
Switchable triple-wavelength erbium-doped fiber laser using a single fiber Bragg grating in polarization-maintaining fiber Zhanyuan Liu, Yan-ge Liu *, Jiangbing Du, Shuzhong Yuan, Xiaoyi Dong Key Laboratory of Opto-Electronic Information and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China Received 19 December 2006; received in revised form 21 March 2007; accepted 9 July 2007
Abstract A stable and narrow wavelength spacing multiwavelength erbium-doped fiber laser is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with a narrow wavelength spacing of less than 0.1 nm via using a single fiber Bragg gratings written in polarization-maintaining (PM) fiber. By adjusting polarization controller, the wavelength spacing of dualwavelength lasing oscillations can be tuned to as small as 0.032 nm. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The room-temperature operation principle is based on the polarization hole burning and deeply saturated effect in an ordinary erbium-doped fiber ring laser (EDFRL). The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Multiwavelength fiber laser; Erbium-doped fiber; Polarization hole burning; Fiber Bragg grating; Polarization control
1. Introduction Multiwavelength erbium-doped fiber lasers (EDFLs) attract a lot of interest due to their potential applications in dense wavelength-division-multiplexed (DWDM) fiber communication systems, optical instrument testing and so on. However, Erbium-doped fiber (EDF) is a primary homogeneous gain medium at room temperature, which leads to strong mode competition and unstable lasing, thus it is difficult to obtain simultaneous multiwavelength lasing in erbium-doped fiber lasers (EDFLs). Cooling EDF in liquid nitrogen (77 K) [1] is often used to reduce the homogeneous linewidth of EDF, however, this method is not suitable for practical application. In order to reduce the cross-gain saturation and suppress the mode competition, different techniques have been proposed to realize multiwavelength oscillations at room temperature in EDFLs. These include the introduction of spatial hole burning
*
Corresponding author. E-mail address: [email protected] (Y.-g. Liu).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.07.027
effect [2], polarization hole burning (PHB) effect [3,4], spectral hole burning effect [5] and various nonlinear effects such as four-wave mixing in photonic crystal fiber [6] and stimulated Brillouin scattering [7], in the laser cavity. Other methods by inserting frequency shifter in the laser cavity [8], incorporating a section of multimode fiber [9] or a multimode fiber Bragg grating (FBG) [10] in a laser cavity, employing distributed Fabry-Pe´rot cavities in a strongly chirped fiber Bragg grating [11], and designing specially erbium-doped fibers [12] or cavity structures [13] were also reported. Fiber Bragg gratings (FBGs) are ideal wavelength-selection components for fiber lasers. Several kinds of FBGs have been used in multiwavelength fiber lasers. Among them, FBGs written in polarization-maintaining fiber (PM-FBG) have attracted more attention recently because of their special polarization property. Zhao et al. proposed a switchable multiwavelength erbium-doped fiber laser based on two cascaded PM-FBGs written in HBF [14]. Though three-wavelength and four-wavelength lasing oscillations have been obtained, the laser operated unstably at room temperature. Liu et al. also suggested a novel scheme
Z. Liu et al. / Optics Communications 279 (2007) 168–172
-30 -35
Intensity(dB)
for four-wavelength lasing oscillations using active overlapping linear cavities with two PM-FBGs [15]. However, the wavelength spacing in that report was >0.3 nm and the configuration is also a bit complex. In this paper, only using a single PM-FBG, a multiwavelength EDFL with narrow wavelength spacing (<0.1 nm) based on polarization hole burning and spectral hole burning effect is proposed and demonstrated. The laser can produce simultaneous dual- and triple-wavelength lasing oscillations with narrow wavelength spacing of only 0.08 nm. Furthermore, the wavelength spacing of dual-wavelength lasing oscillations can be tuned to as small as 0.032 nm, by adjusting the polarization controller. The maximum amplitude variation for every lasing wavelength is less than 0.5 dB.
-40 -45 -50 -55 -60 1544.0
1544.5
1545.0
1545.5
1546.0
Wavelength(nm) Fig. 2. Reflection spectrum of the PM-FBG.
2. Experiment setup
3. Results and discussion In our experiment, by appropriately adjusting the states of PC1, simultaneous triple-wavelength oscillation is obtained. Fig. 3a shows three lasing lines at 1545.056, 1545.370 and 1545.451 nm with the wavelength spacing -5 -10 -15
Intensity(dB)
The proposed laser configuration is shown in Fig. 1. The active medium Er-doped fiber is of length 3.9 m and is pumped by a 980 nm laser diode with a power of 115 mW through a WDM coupler. The filter is a single FBG written in polarization-maintaining (PM) fiber, which offers flexibility for selecting of the lasing wavelength and has the advantage of preserving the long-term stability of fiber laser. The PM-FBG is fabricated using the phasemask method. Due to a long exposure time, the bandwidth of each reflective peak is about 0.1 nm (the reflective spectrum of the FBG has a flattened shape). Fig. 2 shows the reflection spectrum of the PM-FBG. The two center reflective peaks are 1545.058 and 1545.430 nm with bandwidths of 0.1 nm, respectively. The polarization controller (PC1) is used to rotate the polarization state of the incident light into the PM-FBG. The optical circulator (OC) connects the PM-FBG to the laser cavity, and ensures that the oscillation propagates in a single direction. The laser output emerges from the 10% port of a 10:90 optical coupler. The isolator is used to prevent reverse propagating light which is a disadvantage in stabilizing multiwavelength lasing [16]. The spectral characteristics of the laser are measured using an optical spectrum analyzer with 0.01 nm resolution.
169
-20 -25 -30 -35 -40 -45 1544.9 1545.0 1545.1 1545.2
1545.3 1545.4 1545.5 1545.6
Wavelength (nm) -5 -10
Intensity(dB)
-15 -20 -25 -30 -35 -40 -45 1544.9
1545.0 1545.1
1545.2 1545.3
1545.4
1545.5 1545.6
Wavelength(nm)
Fig. 1. Schematic diagram of the proposed laser.
Fig. 3. Output spectrum of the different triple-wavelength fiber laser by adjusting the PC1.
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of 0.314 and 0.081 nm, respectively. Fig. 3b shows another three lasing lines at 1544.990, 1545.077 and 1545.444 nm, with the wavelength spacing of 0.087 and 0.367 nm. Fig. 4 shows the 16 times repeated scan of output spectra
of EDFL with a 10 s interval. The maximum power fluctuation of the three lasing lines is less than 0.5 dB. This indicates fairly stable room-temperature operation even for wavelength spacing of only 0.08 nm.
-5
-5
-10
-10
-15
-15
Intensity(dB)
Intensity(dB)
Fig. 4. The stability of two triple-wavelength lasers (16 times repeated scans).
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1544.8 1544.9 1545.0 1545.1
1545.2 1545.3 1545.4
Wavelength(nm)
1545.1 1545.2 1545.3 1545.4 1545.5 1545.6 1545.7 1545.8
Wavelength(nm)
0
Intensity(dB)
-10 -20 -30 -40 -50 -60 1544.7 1544.8 1544.9 1545.0
1545.1 1545.2 1545.3 1545.4
Wavelength(nm) Fig. 5. Output spectrum of dual-wavelength at different wavelength: (a) lasing lines at 1545.019 and 1545.092 nm with a wavelength separation of 0.073 nm; (b) two lasing lines at 1545.368 and 1545.450 nm with a wavelength separation of 0.082 nm; (c) two lasing lines at 1545.063 and 1545.095 with a wavelength separation of 0.032 nm; (d) the stability of dual-wavelength lasing lines with the wavelength separation of 0.032 nm (16 times repeated scans).
Z. Liu et al. / Optics Communications 279 (2007) 168–172
Fig. 6. Schematic diagram of measuring the polarization states.
-10
Intensity(dB)
-20
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-50 1544.7
1544.8 1544.9 1545.0 1545.11 545.21 545.31 545.4
Wavelength(nm)
-10
-20
Intensity(dB)
Additionally, the stable narrow-line-width dual-wavelength laser arises by carefully adjusting the PC1. Fig. 5a shows two lasing lines at 1545.019 and 1545.092 nm, respectively, with a wavelength separation of 0.073 nm, related to the peak1 of the PM-FBG. Fig. 5b shows two lasing lines at 1545.368 and 1545.450 nm, respectively, with a wavelength separation of 0.082 nm, related to the peak2 of the PM-FBG. Furthermore, through carefully adjusting the state of the PC1, a stable dual-wavelength lasing with a wavelength spacing of as small as 0.032 nm could be obtained as shown in Fig. 5c. Fig. 5d shows the 16 times repeated scans of the laser output spectra with the wavelength spacing of 0.032 nm at a 10 s interval. In our experiment, the dual-wavelength operation can be stable for more than half an hour if the temperature variation and mechanical vibration are reasonably small (i.e., in a laboratory). The maximum amplitude variation for every lasing wavelength is less than 0.5 dB. The worst wavelength shift is assumed to be less than 0.01 nm for the limitation of OSA resolution. As mentioned previously, with a single PM-FBG as wavelength selection, three wavelengths with narrow line width could be obtained simultaneously. The performance may be explained in a number of ways. Firstly, the PC1 in the fiber loop is adjusted at various settings of the loop birefringence that is beneficial to the enhancement of polarization hole burning, so a comb filter could be established [17]. In our experiment, the 3 dB bandwidth of the FBG is only 0.1 nm, which is very narrow. By adjusting the PC1, the fine pectinate spectrum is so changeable that can cleave the spectrum of a PM-FBG, by wavelength selecting in the EDFL, into two parts. The narrow-line-width dual- and triple-wavelength could be obtained. Secondly, The PMFBG is fabricated using a long exposure time and the index of refraction is not uniform in each reflection peak. We measure the polarization states of lasers at two wavelengths by using a polarization controller (PC2) and a polarizer (see Fig. 6). In the dual-wavelengths lasing state (see Fig. 5c), we adjust the PC2 to make output2 laser only in wavelength 1545.063 nm, as shown in Fig. 7a. After rotating the PC2 by 90°, the spectrum in output2 became single-wavelength lasing in 1545.095 nm, as shown in Fig. 7b. It is obvious that the laser outputs in two wavelengths are orthogonal in polarization. The reflection characteristic of the PM-FBG depends on the polarization states of the inci-
171
-30
-40
-50 1544.7 1544.8 1544.9 1545.0 1545.1 1545.2
1545.3 1545.4
Wavelength(nm) Fig. 7. Output spectrum of the lasers with different polarization states of the PC2.
dent light, which revealed the strong polarization-dependence of the PM-FBG in one reflection peak. Thus, the dual- and triple-wavelength operation is very stable at room temperature even with wavelength separations of 0.032 nm, due to the enhanced polarization hole burning (PHB) of the laser cavity. Thirdly, spectral hole burning effect is helpful to obtain stable triple-wavelength lasing at room temperature in the EDFRL in Ref. [16]. We investigated the effect of the saturated signal level in the laser cavity on the stability of the triple-wavelength lasing oscillations via changing only the pump power acting on the EDF in Fig. 1. As shown in Fig. 8, the smaller the pump power is, the larger the power fluctuations of the lasing wavelengths are. Furthermore, when the pump power is less than 80 mW, the triple-wavelength lasing oscillations is not obtained. Thus, due to the enhanced polarization hole burning (PHB) of the laser cavity and spectral hole burning effect which becomes obvious at room temperature when the signal input into the EDF are deeply saturated, the stable triple-wavelength can be obtained. For these experimental results, the stable dual-and triple-wavelength lasers with very narrow wavelength spacing
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4. Conclusion
2.0
Power Fluctuation(dB)
1.5 1.0
90mW pump power 1545.056nm 1545.370nm 1545.451nm
In conclusion, we have demonstrated that stable multiwavelength lasing oscillations at room temperature could be achieved by exploiting a PHD effect and a deeply saturated spectral hole burning effect. Multiwavelength lasing oscillations obtained by this method are stable and the maximum power fluctuation over a long-time observation is less than 0.5 dB. Furthermore, the dual-wavelength separation between the lasing lines can also be tuned to as small as 0.032 nm. We believe that this technique provides another simple approach to achieve stable multiwavelength EDF lasing at room temperature.
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -20
0
20
40
60
80
100
120
140
160
Scan Time(s)
The work is supported by the Tianjin Natural Science Foundation under Grant No. 06YFJZJC00300, the National Key Basic Research and Development Programme of China under Grant No. 2003CB314906 and the National Natural Science Foundation of China under Grant No. 10674074.
2.0
Power Fluctuation(dB)
1.5 1.0
Acknowledgements
115mW pump power 1545.056nm 1545.370nm 1545.451nm
0.5 0.0
References
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Scan Time(s) Fig. 8. Power fluctuations of the triple-wavelength EDFRL with different pump power. The maximum fluctuations for 90 mW, 115 mW pomp powers are (0.26, 1.98, 2.11) dB and (0.27, 0.36, 0.39) dB at wavelengths of 1545.056, 1545.370 and 1545.451 nm, respectively.
has been generated at room temperature. The device is highly flexible in view of wavelength switching capability, as has been demonstrated by the measured results. Also, the configuration is simple as it requires only adjusting PCs’ states for many kinds of wavelength oscillation operations. Furthermore, the dual-wavelength fiber laser with ultranarrow wavelength spacing and orthogonal polarization has potential applications in microwave/millimeterwave signal generation and modulation of data on the microwave subcarrier [18]. The ultimate limit in this application is the laser source must be single-longitudinal-mode operation. The bandwidth of the generated lasing lines can be further decreased through incorporating ultranarrow dual-transmission-band fiber Bragg grating filter [19] or using a saturable absorber [20] in cavity. Thus, the singlelongitudinal-mode lasing in each wavelength is expected in our configuration.
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