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Widely tunable erbium doped fiber ring laser based on loop and double-pass EDFA design
Optics and Laser Technology 124 (2020) 105979
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Widely tunable erbium doped fiber ring laser based on loop and double-pass EDFA design
T
⁎
Serif Ali Sadik , Firat Ertac Durak, Ahmet Altuncu Department of Electrical and Electronics Engineering, Faculty of Engineering, Kutahya Dumlupinar University, 43100 Kutahya, Turkey Photonics Technologies Application and Research Center (FOTAM), Kutahya Dumlupinar University, 43100 Kutahya, Turkey
H I GH L IG H T S
novel EDFRL architectures based on loop EDFA and double-pass EDFA designs are proposed. • Two tuning ranges (C+L band) are achieved using proposed configurations. • Wide flat output power levels with very high OSNRs (~60 dB) are obtained within the tuning range. • Extremely • High stability for laser output power and oscillation wavelength are achieved.
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber ring laser Tunable laser source Erbium doped fiber
This study presents two novel tunable erbium doped fiber ring laser (EDFRL) configurations based on loop EDFA design and double-pass EDFA design, respectively to achieve an ultra-wideband tuning range and stable singlelongitudinal-mode lasing operation. A micro-electro-mechanical system (MEMS) based optical tunable band pass filter was utilized as the wavelength selection element. Output spectral and power characteristics of these EDFRLs were experimentally analyzed and discussed. The EDFRLs had moderate and stable output power level (~ -3 dBm) over 70 nm tuning range that covers C+L band. Moreover, the proposed EDFRLs could also suppress the undesired multi-longitudinal-modes successfully and have a high (~ 60 dB) OSNR level in whole tuning range.
1. Introduction
techniques are based on conventional ring laser configuration with different all-fiber-filters such as fiber Bragg gratings (FBG) [10,11], tapered fibers [12], Fabry-Perot (FP) filters [13] and multimode fibers (MMFs) [14], multiple fiber ring structures [15], saturable absorbers (SAs) with unpumped doped fibers [16]. Since these fiber filters such as FBGs or tapered fibers can be tuned through an applied strain or heating, all-fiber filter based ring lasers come with difficulties for packaging. Also, these tuning mechanisms have low tuning range [6], and multiple fiber ring structures with SAs require additional components making them complex and expensive for simple EDRLs. In our previous work [17], a spectral characterization of the conventional EDFRL with micro-electro-mechanical system (MEMS) based optical tunable bandpass filter (TBPF) was experimentally investigated to optimize the erbium doped fiber (EDF) length and output coupler ratio. In this study, stable and widely tunable two different novel EDFRL architectures based on loop EDFA and double-pass EDFA designs are proposed and demonstrated. To the best of the authors knowledge,
Nowadays, tunable laser sources with a wide tuning range, a narrow linewidth, a selectable and stable laser output signal have attracted great attention for applications such as optical sensor systems, dense wavelength division multiplexing (DWDM) network systems and testing and characterization of optical-fiber components [1–5]. In particular, tunable fiber lasers are preferred due to their advantages of low-intensity noise, high output power, compatibility with optical-fiber components and low cost compared to semiconductor lasers [6]. Especially erbium-doped fiber tunable ring lasers (EDFRLs) ensure a tuning range with a broad emission spectrum over C and L- bands [7–9]. However, with their relatively long cavity length, the problem of multi-longitudinal mode (MLM) oscillation and unstable lasing output usually occurs in the conventional EDFRLs. To achieve single longitudinal mode (SLM) and stable laser output several approaches have been proposed in the literature. These
⁎
Corresponding author at: Department of Electrical and Electronics Engineering, Faculty of Engineering, Kutahya Dumlupinar University, 43100 Kutahya, Turkey. E-mail address: [email protected] (S.A. Sadik).
https://doi.org/10.1016/j.optlastec.2019.105979 Received 24 July 2019; Received in revised form 24 September 2019; Accepted 25 November 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 124 (2020) 105979
S.A. Sadik, et al.
incoming light into its spectrum with a distinct angle for each wavelength. A MEMS mirror reflects the light onto the output collimator, which only couples a small fraction of the spectrum into the output fiber. By modifying the mirror tilt angle, user can choose the tuning wavelength of the filter. The tuning mechanism of the filter uses an integrated micro-mirror with switching time below 50 ms which is also determines the tuning speed of the EDFRL. In this TBPF, a highly reliable tuning mechanism is controlled by a serial bus. Tap couplers with splitting ratio of 90:10 and 50:50 are utilized to form a ring cavity, in which first port is for feedback and second port is for laser output. In the experiments, the optical power and spectrum of the laser output were observed with an optical spectrum analyzer (OSA, Anritsu MS9710B). 3. Experimental results and discussion 3.1. EDFRL based on loop EDFA design Firstly, the output power spectrum versus wavelength of the proposed EDFRL based on loop EDFA design is observed by changing the central wavelength of TBPF which starts from 1525 nm to 1605 nm with 5 nm intervals. It is clear from Fig. 2 that the proposed EDFRL based on loop EDFA design has a total tuning range of 70 nm, covering from 1525 nm to 1595 nm. It is worthwhile to note that, the shortest wavelength which is 1525 nm, is limited only by the tuning range of the TBPF. The inset of Fig. 2 is the enlarged measured lasing spectrum of the proposed EDFRL of 1550 nm. It is obvious that with highly suppressed side-modes and a linewidth of 14.3 pm (OSA minimum resolution is 0.16 pm), the EDFRL based on loop EDFA design is very suitable for narrow linewidth tunable laser applications e.g. DWDM and optical fiber sensing mechanisms. Fig. 3 shows the power and OSNR of the EDFRL based on loop EDFA design, measured through the lasing wavelength tuning range. The wavelength tuning range for this configuration is 1525–1595 nm and the output power is measured −2.48 dBm and −5.35 dBm within this tuning range as the maximum and minimum values, respectively. It can be seen that the laser output power presents low deviation in the tuning range which covers C+L band, with a mean power of −3.13 dBm and a deviation of 0.68 dB at the laser output peak power. Also, OSNR of the laser output signal is varied between the range from 62.18 dB to 65.86 dB which is very sufficient value for low-noise tunable laser applications. In the next experiment, the effect of coupling ratio on the output performance of EDFRL based on loop EDFA design was investigated. It is clearly seen from Fig. 4 that the output power of proposed EDFRL is higher, but the wavelength tuning range is narrower for a 50% output
Fig. 1. The experimental setup of EDFRL based on (a) loop EDFA design (b) double-pass EDFA design. EDF, Erbium-doped fiber; TBPF, Tunable bandpass filter; OSA, Optical spectrum analyser.
none of the previous studies have used these two novel architectures as a tunable laser source. Tuning ranges of wider than 70 nm which covers C+L band are achieved using the proposed EDFRL configurations. Also, extremely flat tuning ranges for laser output signals are obtained with high optical signal to noise ratios (OSNRs) and high stability for laser output power and oscillation wavelength.
2. Experimental setup The proposed EDFRL configurations based on loop EDFA and double-pass EDFA design which have a wide tuning range and a stable laser output signal are shown in Fig. 1. Two bidirectional pump lasers operating at 980 nm region were kept at 120 mW in both configurations. The length of EDF (LIEKKI™ Er30-4/12) used in these experiments was 6 m and the important parameters of EDF are given in Table 1. In the loop EDFA configuration (Fig. 1a), the forward and backward ASE signals of bidirectionally pumped EDFA are combined in 3-dB coupler. In a previous work [18], loop EDFA design was used to realize a band selectable ASE source operating in C-,L-, C+L bands through a C- band seed signal injection. However in this study, the loop EDFA was used in a wavelength tunable fiber ring laser architecture. In the double-pass EDFA configuration (Fig. 1b) the output end of the EDFA is connected to a C+L band optical fiber gold mirror with 100% reflectivity to reinject the forward ASE signal back to the EDFA. In both configurations, the ASE signal is directed to the ring through the second port of a wideband optical circulator (1520–1625 nm). In order to select lasing wavelength from a wide range (from 1525 nm to 1615 nm), an optical MEMS based tunable band pass filter (TBPF) (Sercalo TF1C100) was used in the proposed designs. At the input of the TBPF, light is collimated onto a fused silica grating. The grating diffracts the Table 1 EDF parameters. Parameter
Value
Core diameter Mode field diameter (@1550 nm) Peak core absorption (@1530 nm) Background loss (@1200 nm) Numerical aperture Cut-off wavelength Er3+ concentration
3.67 μ m 6.5 ± 0.5 μ m 30 ± 3 dB/m 23 dB/km 0.2 960 nm 1.9e25 ion/m3
Fig. 2. Output spectra of EDFRL based on loop EDFA design obtained with 5 nm intervals. Tap coupler ratio:10%. Inset:Lasing spectrum of the EDFRL at 1550 nm. 2
Optics and Laser Technology 124 (2020) 105979
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Fig. 5. Short term stability of EDFRL based on loop EDFA design.
Fig. 3. The output power and OSNR of EDFRL based on loop EDFA design as a function of laser tuning wavelength.
lasing wavelengths respectively. The measurements have shown that the output power and lasing wavelength of the loop EDFA based EDFRL design have a high stability in short-term observation. 3.2. EDFRL based on double-pass EDFA design Firstly, the effect of optical fiber mirror on the output power and tuning range of the EDFRL based on double-pass EDFA design is analyzed with and without mirror configurations. As it is shown in Fig. 6 when the optical fiber gold mirror is utilized to make a double-pass EDFA configuration, the tuning range of EDFRL increases from 40 nm to 70 nm. It can be seen that the wavelength tuning range of EDFRL without mirror covers only C- band but when the mirror is added to output end of EDFA, the wavelength tuning range of EDFRL extends to the C+L band. Also, the average output power of EDFRL is raised by 5.51 dB due to the mirror addition, being −3.59 dBm in average power. Fig. 7 shows the measured output power spectra of the proposed EDFRL based on double-pass EDFA design with an optical mirror for the laser tuning range of between 1525 nm and 1610 nm taken at steps of 5 nm while the output coupler tap ratio is 10%. It can be seen from Fig. 7 that the EDFRL has a total tuning range of 70 nm in 3-dB bandwidth, covering from 1530 nm to 1600 nm. The output power spectrum is indicated in the inset of Fig. 7 when the tuning wavelength of TBPF was 1550 nm. The 3-dB linewidth of the laser output at 1550 nm is 16.7 pm which is similar with the EDFRL obtained in based on loop EDFA design. The measured output power and OSNR of the laser output signal in the wavelength tuning range of 1530–1600 nm is shown in Fig. 8. The output power of the proposed EDFRL changes between −2.95 dBm and
Fig. 4. The Effect of output coupling tap ratio on the output power and tuning range for EDFRL based on loop EDFA design.
coupler tap ratio than 10% output coupler tap ratio. Note that a larger output coupler tap ratio leads to a larger intra-cavity loss for a ring laser structure, thus the wavelength tuning range becomes narrower [19]. As it is known, the gain of EDFA is higher at the whole C- band and also at the first part of the L- band. The gain of the EDFA at the second part of the L- band can significantly be increased by a secondary pumping effect provided by ASE reinjection in C- band [20]. When the intra-cavity loss of the ring structure increases, the ASE power feedbacked to the EDFA decreases. This leads to less gain and low lasing output power at the far end of L- band beyond 1600 nm. This is the main reason having a narrower spectral tuning range at the laser output. In this ring laser structure, a tuning range reduction of 10 nm in 3-dB bandwidth was observed when increasing the output coupler tap ratio from 10% to 50%. But also, a higher output coupler tap ratio results in higher laser output power at the expense of narrower wavelength tuning range. With a 50% output coupler tap ratio, the EDFRL based on loop EDFA design exhibits 6.68 dB higher output power in comparison with 10% output coupler tap ratio. The laser stability is an important parameter for practical applications. Therefore, to test the stability of laser, a short-term observation of laser output power and wavelength is performed experimentally. The lasing wavelengths of 1550 nm and 1585 nm which are in C- band and L- band respectively, are employed in the measurements. Fig. 5 shows the output power and lasing wavelength in a 60 min observation time with 5 min intervals. As seen in Fig. 5 the observed output power fluctuations are 0.11 dB and 0.07 dB for the lasing operations at 1550 nm and 1585 nm respectively. Also, one can see that the wavelength deviations are 0.012 nm and 0.011 nm for 1550 and 1585 nm
Fig. 6. The effect of optical fiber gold mirror on the output power and tuning range for EDFRL based on double-pass EDFA design. Output tap coupler ratio is 10%. 3
Optics and Laser Technology 124 (2020) 105979
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Fig. 10. Short term stability of EDFRL based on double-pass EDFA design.
Fig. 7. Output spectra of EDFRL based on double-pass EDFA design obtained with 5 nm intervals. Output tap coupler ratio:10%. Inset:Lasing spectrum of the EDFRL at 1550 nm.
the output tap coupling ratio is increased from 10% to 50%. On the other hand, with a 50% output tap coupling ratio, the EDFRL has a 6.67 dB higher output power in average in comparison with 10% laser output tap coupling ratio. To observe the stability of the EDFRL based on double-pass EDFA design, the laser output was observed for 60 min with 5 min intervals. In the measurements, two lasing wavelengths of 1550 nm and 1585 nm which are in C- band and L- band respectively are selected for demonstration. As illustrated in Fig. 10 the wavelength deviations of the lasing operations at 1550 nm and 1585 nm are 0.013 nm and 0.008 nm, respectively. Furthermore, Fig. 10 also shows the observed output power fluctuations which are 0.09 dB and 0.05 dB for 1550 nm and 1585 nm lasing wavelengths, respectively. As a result, a high output stability of proposed EDFRL based on double-pass EDFA design was achieved based on the measurement results. Finally, to demonstrate the lasing oscillation performance of the EDFRLs, we also have measured the linewidth of the laser outputs using a high resolution OSA (Apex Technologies 2050A) at lasing wavelengths of 1535 nm, 1550 nm, 1565 nm, 1585 nm and 1595 nm. The second OSA used has a minimum resolution of 0.16 pm. As seen in Fig. 11, the linewidths of the proposed EDFRLs are varied from 16.7 pm to 10.2 pm. We have measured that the linewidths of both EDFRL architectures are slightly narrower in L- band (~10.8 pm) in comparison with C- band (~14.2 pm). The laser outputs for the proposed EDFRLs at the lasing wavelength of 1550 nm were also given in Fig. 12. It is clearly seen that both configurations have a narrow (<16.7 pm) lasing oscillation at 1550 nm. Also, undesired side modes were effectively suppressed and mode hopping between neighboring longitudinal modes was not observed.
Fig. 8. The output power and OSNR of EDFRL based on double-pass EDFA design for the tuning range of 1530–1600 nm.
−5.61 dBm with a mean of −3.59 dBm and a deviation of 0.56 dB. Also, the measured OSNR has an average of 59.3 dB that is close to the first EDFRL configuration. The effect of coupling ratio of the output tap coupler on the performance of EDFRL based on double-pass EDFA design is also investigated. As shown in Fig. 9, the output power of EDFRL is higher and wavelength tuning range is narrower for a higher output tap coupling ratio. The tuning range is reduced by 10 nm in 3-dB bandwidth when
Fig. 9. The effect of laser output coupling tap ratio EDFRL power and spectral tuning performance in double-pass EDFA design.
Fig. 11. 3 dB linewidth measurement results of the lasing wavelengths of 1535 nm, 1550 nm, 1565 nm, 1585 nm and 1595 nm for both configurations. 4
Optics and Laser Technology 124 (2020) 105979
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] A. Bellemare, Continuous-wave silica-based erbium-doped fibre lasers, Prog. Quant. Electron. 27 (2003) 211–266. [2] K.C. Reichmann, P.D. Magill, U. Koren, B.I. Miller, M. Young, M. Newkirk, M.D. Chien, 2.5 Gb/s transmission over 674 km at multiple wavelengths using a tunable DBR laser with an integrated electroabsorption modulator, IEEE Photonics Technol. Lett. 5 (1993) 1098–1100. [3] J. Zhang, N. Ansari, Design of WDM PON with tunable lasers: The upstream scenario, J. Lightwave Technol. 28 (2010) 228–236. [4] E.J. Jung, C.-S. Kim, M.Y. Jeong, M.K. Kim, M.Y. Jeon, W. Jung, Z. Chen, Characterization of FBG sensor interrogation based on a FDML wavelength swept laser, Opt. Express 16 (2008) 16552–16560. [5] J. Liu, M. Wang, X. Liang, Y. Dong, H. Xiao, S. Jian, Erbium-doped fiber ring laser based on few-mode-singlemode-few-mode fiber structure for refractive index measurement, Opt. Laser Technol. 93 (2017) 74–78. [6] F. Xiao, K. Alameh, T. Lee, Opto-VLSI-based tunable single-mode fiber laser, Opt. Express 17 (2009) 18676–18680. [7] T. Pfeiffer, H. Schmuck, H. Bulow, Output power characteristics of erbium-doped fiber ring lasers, IEEE Photonics Technol. Lett. 4 (1992) 847–849. [8] S. Yamashita, M. Nishihara, Widely tunable erbium-doped fiber ring laser covering both C-band and L-band, IEEE J. Sel. Top. Quant. Electron. 7 (2001) 41–43. [9] X. Dong, P. Shum, N.Q. Ngo, H. Tam, X. Dong, Output power characteristics of tunable erbium doped fiber ring lasers, J. Lightwave Technol. 23 (2005) 1334–1341. [10] Y.W. Song, S.A. Havstad, D. Starodubov, Y. Xie, A.E. Willner, J. Feinberg, 40-nmwide tunable fiber ring laser with single-mode operation using a highly stretchable FBG, IEEE Photonics Technol. Lett. 13 (2001) 1167–1169. [11] S.-K. Liaw, K.-L. Hung, Y.-T. Lin, C.-C. Chiang, C.-S. Shin, C-band continuously tunable lasers using tunable fiber Bragg gratings, Opt. Laser Technol. 39 (2007) 1214–1217. [12] X. Wang, Y. Li, X. Bao, C- and L-band tunable fiber ring laser using a two-taper Mach-Zehnder interferometer filter, Opt. Lett. 35 (2010) 3354–3356. [13] Q. Wang, Q.X. Yu, Continuously tunable S and C+L bands ultra wideband erbiumdoped fiber ring laser, Laser Phys. Lett. 6 (2009) 607–610. [14] Y. Zhou, S. Lou, X. Liu, Z. Tang, Tunable single-wavelength erbium-doped fiber ring laser using a large-core fiber, Infrared Phys. Technol. 95 (2018) 1–4. [15] C.-H. Yeh, J.Y. Chen, H.Z. Chen, J.H. Chen, C.W. Chow, Stable and tunable singlelongitudinal-mode erbium-doped fiber triple-ring laser with power-equalized output, IEEE Photonics J. 8 (2016) 1–6. [16] C.-H. Yeh, Z.-Q. Yang, T.-J. Huang, C.-W. Chow, Y.-J. Chang, M.-J. Chen, Erbiumdoped fiber dual-ring laser with stable single-longitudinal-mode and 55-nm tuning range, Opt. Laser Technol. 106 (2018) 119–122. [17] S.A. Sadik, F.E. Durak, A. Altuncu, Spectral characterization of an erbium-doped fiber ring laser for wideband operation, in: 2018 Advances in Wireless and Optical Communications (RTUWO), pp. 130–133. [18] A. Altuncu, Band selection in broadband loop ASE source using seed signal injection, IEEE Photonics Technol. Lett. 18 (2006) 1043–1045. [19] Z. Fu, D. Yang, W. Ye, J. Kong, Y. Shen, Widely tunable compact erbium-doped fiber ring laser for fiber-optic sensing applications, Opt. Laser Technol. 41 (2009) 392–396. [20] F.E. Durak, A. Altuncu, The effect of ASE reinjection configuration through FBGs on the gain and noise figure performance of L-Band EDFA, Opt. Commun. 386 (2017) 31–36.
Fig. 12. EDFRL output power spectra at lasing wavelength of 1550 nm for both configurations.
4. Conclusion In this paper, we have presented output power and spectral tuning characteristics of stable and ultra-wideband tunable EDFRLs with two novel configurations based on loop EDFA and double-pass EDFA designs, respectively. To select the lasing wavelength of the proposed EDFRLs, a MEMS based optical TBPF was used that provides a wide electronically controlled tuning range covering C+L bands. In both configurations, a tuning range of wider than 70 nm and an average laser output power level of approximately −3 dBm was achieved for an output tap coupling ratio of 10%. Moreover, the proposed EDFRLs have an OSNR level of higher than 60 dB that can be sufficient for low-noise tunable fiber laser applications. In addition, we have also changed the laser output tap coupling ratio from 10% to 50%, which has increased the output power of the proposed EDFRLs. The output stabilities of oscillation wavelength and power of both proposed EDFRL designs were smaller than 0.013 nm and 0.11 dB respectively, during a short term measurement of 60 min. Consequently, these proposed tunable EDFRL configurations may find vast application fields in ultra-wideband optical WDM transmission systems and in sensor applications. Funding This work was financially supported by the Scientific Research Projects of Kutahya Dumlupinar University, project number 2017/43.
5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Widely tunable erbium doped fiber ring laser based on loop and double-pass EDFA design
T
⁎
Serif Ali Sadik , Firat Ertac Durak, Ahmet Altuncu Department of Electrical and Electronics Engineering, Faculty of Engineering, Kutahya Dumlupinar University, 43100 Kutahya, Turkey Photonics Technologies Application and Research Center (FOTAM), Kutahya Dumlupinar University, 43100 Kutahya, Turkey
H I GH L IG H T S
novel EDFRL architectures based on loop EDFA and double-pass EDFA designs are proposed. • Two tuning ranges (C+L band) are achieved using proposed configurations. • Wide flat output power levels with very high OSNRs (~60 dB) are obtained within the tuning range. • Extremely • High stability for laser output power and oscillation wavelength are achieved.
A R T I C LE I N FO
A B S T R A C T
Keywords: Fiber ring laser Tunable laser source Erbium doped fiber
This study presents two novel tunable erbium doped fiber ring laser (EDFRL) configurations based on loop EDFA design and double-pass EDFA design, respectively to achieve an ultra-wideband tuning range and stable singlelongitudinal-mode lasing operation. A micro-electro-mechanical system (MEMS) based optical tunable band pass filter was utilized as the wavelength selection element. Output spectral and power characteristics of these EDFRLs were experimentally analyzed and discussed. The EDFRLs had moderate and stable output power level (~ -3 dBm) over 70 nm tuning range that covers C+L band. Moreover, the proposed EDFRLs could also suppress the undesired multi-longitudinal-modes successfully and have a high (~ 60 dB) OSNR level in whole tuning range.
1. Introduction
techniques are based on conventional ring laser configuration with different all-fiber-filters such as fiber Bragg gratings (FBG) [10,11], tapered fibers [12], Fabry-Perot (FP) filters [13] and multimode fibers (MMFs) [14], multiple fiber ring structures [15], saturable absorbers (SAs) with unpumped doped fibers [16]. Since these fiber filters such as FBGs or tapered fibers can be tuned through an applied strain or heating, all-fiber filter based ring lasers come with difficulties for packaging. Also, these tuning mechanisms have low tuning range [6], and multiple fiber ring structures with SAs require additional components making them complex and expensive for simple EDRLs. In our previous work [17], a spectral characterization of the conventional EDFRL with micro-electro-mechanical system (MEMS) based optical tunable bandpass filter (TBPF) was experimentally investigated to optimize the erbium doped fiber (EDF) length and output coupler ratio. In this study, stable and widely tunable two different novel EDFRL architectures based on loop EDFA and double-pass EDFA designs are proposed and demonstrated. To the best of the authors knowledge,
Nowadays, tunable laser sources with a wide tuning range, a narrow linewidth, a selectable and stable laser output signal have attracted great attention for applications such as optical sensor systems, dense wavelength division multiplexing (DWDM) network systems and testing and characterization of optical-fiber components [1–5]. In particular, tunable fiber lasers are preferred due to their advantages of low-intensity noise, high output power, compatibility with optical-fiber components and low cost compared to semiconductor lasers [6]. Especially erbium-doped fiber tunable ring lasers (EDFRLs) ensure a tuning range with a broad emission spectrum over C and L- bands [7–9]. However, with their relatively long cavity length, the problem of multi-longitudinal mode (MLM) oscillation and unstable lasing output usually occurs in the conventional EDFRLs. To achieve single longitudinal mode (SLM) and stable laser output several approaches have been proposed in the literature. These
⁎
Corresponding author at: Department of Electrical and Electronics Engineering, Faculty of Engineering, Kutahya Dumlupinar University, 43100 Kutahya, Turkey. E-mail address: [email protected] (S.A. Sadik).
https://doi.org/10.1016/j.optlastec.2019.105979 Received 24 July 2019; Received in revised form 24 September 2019; Accepted 25 November 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 124 (2020) 105979
S.A. Sadik, et al.
incoming light into its spectrum with a distinct angle for each wavelength. A MEMS mirror reflects the light onto the output collimator, which only couples a small fraction of the spectrum into the output fiber. By modifying the mirror tilt angle, user can choose the tuning wavelength of the filter. The tuning mechanism of the filter uses an integrated micro-mirror with switching time below 50 ms which is also determines the tuning speed of the EDFRL. In this TBPF, a highly reliable tuning mechanism is controlled by a serial bus. Tap couplers with splitting ratio of 90:10 and 50:50 are utilized to form a ring cavity, in which first port is for feedback and second port is for laser output. In the experiments, the optical power and spectrum of the laser output were observed with an optical spectrum analyzer (OSA, Anritsu MS9710B). 3. Experimental results and discussion 3.1. EDFRL based on loop EDFA design Firstly, the output power spectrum versus wavelength of the proposed EDFRL based on loop EDFA design is observed by changing the central wavelength of TBPF which starts from 1525 nm to 1605 nm with 5 nm intervals. It is clear from Fig. 2 that the proposed EDFRL based on loop EDFA design has a total tuning range of 70 nm, covering from 1525 nm to 1595 nm. It is worthwhile to note that, the shortest wavelength which is 1525 nm, is limited only by the tuning range of the TBPF. The inset of Fig. 2 is the enlarged measured lasing spectrum of the proposed EDFRL of 1550 nm. It is obvious that with highly suppressed side-modes and a linewidth of 14.3 pm (OSA minimum resolution is 0.16 pm), the EDFRL based on loop EDFA design is very suitable for narrow linewidth tunable laser applications e.g. DWDM and optical fiber sensing mechanisms. Fig. 3 shows the power and OSNR of the EDFRL based on loop EDFA design, measured through the lasing wavelength tuning range. The wavelength tuning range for this configuration is 1525–1595 nm and the output power is measured −2.48 dBm and −5.35 dBm within this tuning range as the maximum and minimum values, respectively. It can be seen that the laser output power presents low deviation in the tuning range which covers C+L band, with a mean power of −3.13 dBm and a deviation of 0.68 dB at the laser output peak power. Also, OSNR of the laser output signal is varied between the range from 62.18 dB to 65.86 dB which is very sufficient value for low-noise tunable laser applications. In the next experiment, the effect of coupling ratio on the output performance of EDFRL based on loop EDFA design was investigated. It is clearly seen from Fig. 4 that the output power of proposed EDFRL is higher, but the wavelength tuning range is narrower for a 50% output
Fig. 1. The experimental setup of EDFRL based on (a) loop EDFA design (b) double-pass EDFA design. EDF, Erbium-doped fiber; TBPF, Tunable bandpass filter; OSA, Optical spectrum analyser.
none of the previous studies have used these two novel architectures as a tunable laser source. Tuning ranges of wider than 70 nm which covers C+L band are achieved using the proposed EDFRL configurations. Also, extremely flat tuning ranges for laser output signals are obtained with high optical signal to noise ratios (OSNRs) and high stability for laser output power and oscillation wavelength.
2. Experimental setup The proposed EDFRL configurations based on loop EDFA and double-pass EDFA design which have a wide tuning range and a stable laser output signal are shown in Fig. 1. Two bidirectional pump lasers operating at 980 nm region were kept at 120 mW in both configurations. The length of EDF (LIEKKI™ Er30-4/12) used in these experiments was 6 m and the important parameters of EDF are given in Table 1. In the loop EDFA configuration (Fig. 1a), the forward and backward ASE signals of bidirectionally pumped EDFA are combined in 3-dB coupler. In a previous work [18], loop EDFA design was used to realize a band selectable ASE source operating in C-,L-, C+L bands through a C- band seed signal injection. However in this study, the loop EDFA was used in a wavelength tunable fiber ring laser architecture. In the double-pass EDFA configuration (Fig. 1b) the output end of the EDFA is connected to a C+L band optical fiber gold mirror with 100% reflectivity to reinject the forward ASE signal back to the EDFA. In both configurations, the ASE signal is directed to the ring through the second port of a wideband optical circulator (1520–1625 nm). In order to select lasing wavelength from a wide range (from 1525 nm to 1615 nm), an optical MEMS based tunable band pass filter (TBPF) (Sercalo TF1C100) was used in the proposed designs. At the input of the TBPF, light is collimated onto a fused silica grating. The grating diffracts the Table 1 EDF parameters. Parameter
Value
Core diameter Mode field diameter (@1550 nm) Peak core absorption (@1530 nm) Background loss (@1200 nm) Numerical aperture Cut-off wavelength Er3+ concentration
3.67 μ m 6.5 ± 0.5 μ m 30 ± 3 dB/m 23 dB/km 0.2 960 nm 1.9e25 ion/m3
Fig. 2. Output spectra of EDFRL based on loop EDFA design obtained with 5 nm intervals. Tap coupler ratio:10%. Inset:Lasing spectrum of the EDFRL at 1550 nm. 2
Optics and Laser Technology 124 (2020) 105979
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Fig. 5. Short term stability of EDFRL based on loop EDFA design.
Fig. 3. The output power and OSNR of EDFRL based on loop EDFA design as a function of laser tuning wavelength.
lasing wavelengths respectively. The measurements have shown that the output power and lasing wavelength of the loop EDFA based EDFRL design have a high stability in short-term observation. 3.2. EDFRL based on double-pass EDFA design Firstly, the effect of optical fiber mirror on the output power and tuning range of the EDFRL based on double-pass EDFA design is analyzed with and without mirror configurations. As it is shown in Fig. 6 when the optical fiber gold mirror is utilized to make a double-pass EDFA configuration, the tuning range of EDFRL increases from 40 nm to 70 nm. It can be seen that the wavelength tuning range of EDFRL without mirror covers only C- band but when the mirror is added to output end of EDFA, the wavelength tuning range of EDFRL extends to the C+L band. Also, the average output power of EDFRL is raised by 5.51 dB due to the mirror addition, being −3.59 dBm in average power. Fig. 7 shows the measured output power spectra of the proposed EDFRL based on double-pass EDFA design with an optical mirror for the laser tuning range of between 1525 nm and 1610 nm taken at steps of 5 nm while the output coupler tap ratio is 10%. It can be seen from Fig. 7 that the EDFRL has a total tuning range of 70 nm in 3-dB bandwidth, covering from 1530 nm to 1600 nm. The output power spectrum is indicated in the inset of Fig. 7 when the tuning wavelength of TBPF was 1550 nm. The 3-dB linewidth of the laser output at 1550 nm is 16.7 pm which is similar with the EDFRL obtained in based on loop EDFA design. The measured output power and OSNR of the laser output signal in the wavelength tuning range of 1530–1600 nm is shown in Fig. 8. The output power of the proposed EDFRL changes between −2.95 dBm and
Fig. 4. The Effect of output coupling tap ratio on the output power and tuning range for EDFRL based on loop EDFA design.
coupler tap ratio than 10% output coupler tap ratio. Note that a larger output coupler tap ratio leads to a larger intra-cavity loss for a ring laser structure, thus the wavelength tuning range becomes narrower [19]. As it is known, the gain of EDFA is higher at the whole C- band and also at the first part of the L- band. The gain of the EDFA at the second part of the L- band can significantly be increased by a secondary pumping effect provided by ASE reinjection in C- band [20]. When the intra-cavity loss of the ring structure increases, the ASE power feedbacked to the EDFA decreases. This leads to less gain and low lasing output power at the far end of L- band beyond 1600 nm. This is the main reason having a narrower spectral tuning range at the laser output. In this ring laser structure, a tuning range reduction of 10 nm in 3-dB bandwidth was observed when increasing the output coupler tap ratio from 10% to 50%. But also, a higher output coupler tap ratio results in higher laser output power at the expense of narrower wavelength tuning range. With a 50% output coupler tap ratio, the EDFRL based on loop EDFA design exhibits 6.68 dB higher output power in comparison with 10% output coupler tap ratio. The laser stability is an important parameter for practical applications. Therefore, to test the stability of laser, a short-term observation of laser output power and wavelength is performed experimentally. The lasing wavelengths of 1550 nm and 1585 nm which are in C- band and L- band respectively, are employed in the measurements. Fig. 5 shows the output power and lasing wavelength in a 60 min observation time with 5 min intervals. As seen in Fig. 5 the observed output power fluctuations are 0.11 dB and 0.07 dB for the lasing operations at 1550 nm and 1585 nm respectively. Also, one can see that the wavelength deviations are 0.012 nm and 0.011 nm for 1550 and 1585 nm
Fig. 6. The effect of optical fiber gold mirror on the output power and tuning range for EDFRL based on double-pass EDFA design. Output tap coupler ratio is 10%. 3
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Fig. 10. Short term stability of EDFRL based on double-pass EDFA design.
Fig. 7. Output spectra of EDFRL based on double-pass EDFA design obtained with 5 nm intervals. Output tap coupler ratio:10%. Inset:Lasing spectrum of the EDFRL at 1550 nm.
the output tap coupling ratio is increased from 10% to 50%. On the other hand, with a 50% output tap coupling ratio, the EDFRL has a 6.67 dB higher output power in average in comparison with 10% laser output tap coupling ratio. To observe the stability of the EDFRL based on double-pass EDFA design, the laser output was observed for 60 min with 5 min intervals. In the measurements, two lasing wavelengths of 1550 nm and 1585 nm which are in C- band and L- band respectively are selected for demonstration. As illustrated in Fig. 10 the wavelength deviations of the lasing operations at 1550 nm and 1585 nm are 0.013 nm and 0.008 nm, respectively. Furthermore, Fig. 10 also shows the observed output power fluctuations which are 0.09 dB and 0.05 dB for 1550 nm and 1585 nm lasing wavelengths, respectively. As a result, a high output stability of proposed EDFRL based on double-pass EDFA design was achieved based on the measurement results. Finally, to demonstrate the lasing oscillation performance of the EDFRLs, we also have measured the linewidth of the laser outputs using a high resolution OSA (Apex Technologies 2050A) at lasing wavelengths of 1535 nm, 1550 nm, 1565 nm, 1585 nm and 1595 nm. The second OSA used has a minimum resolution of 0.16 pm. As seen in Fig. 11, the linewidths of the proposed EDFRLs are varied from 16.7 pm to 10.2 pm. We have measured that the linewidths of both EDFRL architectures are slightly narrower in L- band (~10.8 pm) in comparison with C- band (~14.2 pm). The laser outputs for the proposed EDFRLs at the lasing wavelength of 1550 nm were also given in Fig. 12. It is clearly seen that both configurations have a narrow (<16.7 pm) lasing oscillation at 1550 nm. Also, undesired side modes were effectively suppressed and mode hopping between neighboring longitudinal modes was not observed.
Fig. 8. The output power and OSNR of EDFRL based on double-pass EDFA design for the tuning range of 1530–1600 nm.
−5.61 dBm with a mean of −3.59 dBm and a deviation of 0.56 dB. Also, the measured OSNR has an average of 59.3 dB that is close to the first EDFRL configuration. The effect of coupling ratio of the output tap coupler on the performance of EDFRL based on double-pass EDFA design is also investigated. As shown in Fig. 9, the output power of EDFRL is higher and wavelength tuning range is narrower for a higher output tap coupling ratio. The tuning range is reduced by 10 nm in 3-dB bandwidth when
Fig. 9. The effect of laser output coupling tap ratio EDFRL power and spectral tuning performance in double-pass EDFA design.
Fig. 11. 3 dB linewidth measurement results of the lasing wavelengths of 1535 nm, 1550 nm, 1565 nm, 1585 nm and 1595 nm for both configurations. 4
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] A. Bellemare, Continuous-wave silica-based erbium-doped fibre lasers, Prog. Quant. Electron. 27 (2003) 211–266. [2] K.C. Reichmann, P.D. Magill, U. Koren, B.I. Miller, M. Young, M. Newkirk, M.D. Chien, 2.5 Gb/s transmission over 674 km at multiple wavelengths using a tunable DBR laser with an integrated electroabsorption modulator, IEEE Photonics Technol. Lett. 5 (1993) 1098–1100. [3] J. Zhang, N. Ansari, Design of WDM PON with tunable lasers: The upstream scenario, J. Lightwave Technol. 28 (2010) 228–236. [4] E.J. Jung, C.-S. Kim, M.Y. Jeong, M.K. Kim, M.Y. Jeon, W. Jung, Z. Chen, Characterization of FBG sensor interrogation based on a FDML wavelength swept laser, Opt. Express 16 (2008) 16552–16560. [5] J. Liu, M. Wang, X. Liang, Y. Dong, H. Xiao, S. Jian, Erbium-doped fiber ring laser based on few-mode-singlemode-few-mode fiber structure for refractive index measurement, Opt. Laser Technol. 93 (2017) 74–78. [6] F. Xiao, K. Alameh, T. Lee, Opto-VLSI-based tunable single-mode fiber laser, Opt. Express 17 (2009) 18676–18680. [7] T. Pfeiffer, H. Schmuck, H. Bulow, Output power characteristics of erbium-doped fiber ring lasers, IEEE Photonics Technol. Lett. 4 (1992) 847–849. [8] S. Yamashita, M. Nishihara, Widely tunable erbium-doped fiber ring laser covering both C-band and L-band, IEEE J. Sel. Top. Quant. Electron. 7 (2001) 41–43. [9] X. Dong, P. Shum, N.Q. Ngo, H. Tam, X. Dong, Output power characteristics of tunable erbium doped fiber ring lasers, J. Lightwave Technol. 23 (2005) 1334–1341. [10] Y.W. Song, S.A. Havstad, D. Starodubov, Y. Xie, A.E. Willner, J. Feinberg, 40-nmwide tunable fiber ring laser with single-mode operation using a highly stretchable FBG, IEEE Photonics Technol. Lett. 13 (2001) 1167–1169. [11] S.-K. Liaw, K.-L. Hung, Y.-T. Lin, C.-C. Chiang, C.-S. Shin, C-band continuously tunable lasers using tunable fiber Bragg gratings, Opt. Laser Technol. 39 (2007) 1214–1217. [12] X. Wang, Y. Li, X. Bao, C- and L-band tunable fiber ring laser using a two-taper Mach-Zehnder interferometer filter, Opt. Lett. 35 (2010) 3354–3356. [13] Q. Wang, Q.X. Yu, Continuously tunable S and C+L bands ultra wideband erbiumdoped fiber ring laser, Laser Phys. Lett. 6 (2009) 607–610. [14] Y. Zhou, S. Lou, X. Liu, Z. Tang, Tunable single-wavelength erbium-doped fiber ring laser using a large-core fiber, Infrared Phys. Technol. 95 (2018) 1–4. [15] C.-H. Yeh, J.Y. Chen, H.Z. Chen, J.H. Chen, C.W. Chow, Stable and tunable singlelongitudinal-mode erbium-doped fiber triple-ring laser with power-equalized output, IEEE Photonics J. 8 (2016) 1–6. [16] C.-H. Yeh, Z.-Q. Yang, T.-J. Huang, C.-W. Chow, Y.-J. Chang, M.-J. Chen, Erbiumdoped fiber dual-ring laser with stable single-longitudinal-mode and 55-nm tuning range, Opt. Laser Technol. 106 (2018) 119–122. [17] S.A. Sadik, F.E. Durak, A. Altuncu, Spectral characterization of an erbium-doped fiber ring laser for wideband operation, in: 2018 Advances in Wireless and Optical Communications (RTUWO), pp. 130–133. [18] A. Altuncu, Band selection in broadband loop ASE source using seed signal injection, IEEE Photonics Technol. Lett. 18 (2006) 1043–1045. [19] Z. Fu, D. Yang, W. Ye, J. Kong, Y. Shen, Widely tunable compact erbium-doped fiber ring laser for fiber-optic sensing applications, Opt. Laser Technol. 41 (2009) 392–396. [20] F.E. Durak, A. Altuncu, The effect of ASE reinjection configuration through FBGs on the gain and noise figure performance of L-Band EDFA, Opt. Commun. 386 (2017) 31–36.
Fig. 12. EDFRL output power spectra at lasing wavelength of 1550 nm for both configurations.
4. Conclusion In this paper, we have presented output power and spectral tuning characteristics of stable and ultra-wideband tunable EDFRLs with two novel configurations based on loop EDFA and double-pass EDFA designs, respectively. To select the lasing wavelength of the proposed EDFRLs, a MEMS based optical TBPF was used that provides a wide electronically controlled tuning range covering C+L bands. In both configurations, a tuning range of wider than 70 nm and an average laser output power level of approximately −3 dBm was achieved for an output tap coupling ratio of 10%. Moreover, the proposed EDFRLs have an OSNR level of higher than 60 dB that can be sufficient for low-noise tunable fiber laser applications. In addition, we have also changed the laser output tap coupling ratio from 10% to 50%, which has increased the output power of the proposed EDFRLs. The output stabilities of oscillation wavelength and power of both proposed EDFRL designs were smaller than 0.013 nm and 0.11 dB respectively, during a short term measurement of 60 min. Consequently, these proposed tunable EDFRL configurations may find vast application fields in ultra-wideband optical WDM transmission systems and in sensor applications. Funding This work was financially supported by the Scientific Research Projects of Kutahya Dumlupinar University, project number 2017/43.
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