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All fiber multiwavelength Tm-doped double-clad fiber laser assisted by four-wave mixing in highly nonlinear fiber and Sagnac loop mirror
Optics Communications 456 (2020) 124589
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
All fiber multiwavelength Tm-doped double-clad fiber laser assisted by four-wave mixing in highly nonlinear fiber and Sagnac loop mirror H. Ahmad a,b ,∗, M.H.M. Ahmed a , M.Z. Samion a , S.W. Harun c a
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Physics Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Electrical Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b
ARTICLE
INFO
Keywords: Fiber laser Multiwavelength fiber laser Double-cladding thulium-doped fiber laser
ABSTRACT A four-wave mixing (FWM) based multi-wavelength laser based on a double-cladding thulium-doped fiber laser (DC-TDFL) is proposed and demonstrated. A highly nonlinear fiber (HNLF) in conjunction with a Sagnac loop mirror formed by polarization maintaining fibers (PMFs) is used to induce the FWM effect as well as to mitigate gain-competition in the DC-TDFL. This increases the number of lasing wavelengths that can be generated. Approximately 35, 55, and 68 stable lasing lines are achieved within 10 dB, 20 dB, and 28 dB bandwidths by adjusting the polarization controllers. A uniform channel spacing of 0.38 nm is obtained with minimal wavelength drifts and power fluctuations of 0.056 nm and 2.8 dB. This is, to the best of the author’s knowledge, the first demonstration of large numbers of lasing channels in the 1.97 μm wavelength region from a DC-TDFL.
1. Introduction Multi-wavelength fiber lasers are of great interest due to their high potential for use in a wide variety of applications that include optical telecommunications, in sensors systems and also for optical fiber and component testing [1–5]. Of the many possible fiber laser explored, thulium doped fiber lasers (TDFLs) in particular have gained significant attention due to their ability to operate in the eye-safe 2-μm region, making them highly desirable laser sources for biomedical applications [6]. Due to their popularity, numerous TDFL designs capable of generating single, dual- and multi-wavelength continuous wave (CW) and pulsed outputs have been explored and demonstrated [7–10]. Multi-wavelength fiber lasers (MWFLs) are able to generate a multichannel output with equal spacing, narrow linewidth and with high optical signal to noise ratios (OSNRs) from a single cavity. In theory, MWFLs in the 2 μm region can be realized through different approaches including four-wave mixing (FWM) [11], Lyot filters [12], nonlinear amplifier loop mirrors (NALMs) [13], Sagnac loop mirrors [14] and Mach–Zehnder interferometers (MZIs) [15]. In reality however, this cannot be easily obtained because of the homogeneous broadening effect typical of rare-earth doped fibers lasers that significantly affects multi-wavelength operation stability [13]. In order to overcome this limitation, non-linear effects in the fiber laser cavity can be exploited to suppress the homogeneous broadening effect. In this regard, various approaches have been explored, such as the use of very long lengths of standard single mode fibers (SMFs) to improve the cavity non-linearity
and at the same time suppress mode competition [13,16]. Alternative approaches such as the use of highly non-linear fibers (HNLFs) [17–19] can shorten this length while giving the same result. Another interesting approach towards generating a multi-wavelength output in a TDFL is using the FWM effect. The FWM effect arises due to the interplay between two or three wavelengths which give rise to new wavelengths as was demonstrated by Huang et al. [17] in a MWFL operating at the 1.88 μm region. A similar output was also demonstrated by Al-Alimi et al. [11] who reported 19 output wavelengths near 1.8 μm using an interleaving filter assisted by FWM. In this paper, a multi-wavelength TDFL incorporating a HNLF and a Sagnac loop mirror made from polarization maintaining fibers (PMFs) is demonstrated. The use of the HNLF increases the laser cavity’s nonlinearity at high pump powers while the PMF based Sagnac loop acts as a filter. Up to 35, 55 and 68 lasing wavelengths are obtained within the bandwidth regions 10 dB, 20 dB and 28 dB down from the peak power at room temperature. All wavelengths have a channel spacing of 0.38 nm with an OSNR of 28 dB. The output generated is highly stable, with wavelength drifts and power fluctuations of less than 0.05 nm and 2.8 dB respectively. 2. Experimental setup The experimental setup of the proposed multi-wavelength double cladding — thulium doped fiber laser (DC-TDFL) is shown in Fig. 1.
∗ Corresponding author at: Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad).
https://doi.org/10.1016/j.optcom.2019.124589 Received 14 June 2019; Received in revised form 11 September 2019; Accepted 16 September 2019 Available online 18 September 2019 0030-4018/© 2019 Published by Elsevier B.V.
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
the laser cavity to ensure unidirectional lasing operation, with a 90:10 optical coupler extracting about 10% of the propagating signal in the optical cavity. A spool of 100 m long highly nonlinear fiber (HNLF) with a nonlinear coefficient of ∼10.5 (Wkm)−1 and dispersion slope of 0.016 ps/(nm2 .km) is inserted into the laser cavity in order to provide a nonlinear phase shift and mitigate the mode competition caused by homogeneous gain broadening in the TDF [18]. The light is then coupled to the Sagnac loop, which consists of the 3-dB coupler, a 20-m long PMF with a birefringence of 5×10−4 and polarization controller (PC) designated PC1 as to adjust the transmission properties of the loop mirror. Another polarization controller, designated PC2 is used to control the polarization state in the laser cavity. The 10% output from the laser cavity is extracted for further analysis using a YOKOGAWA-AQ6375 optical spectrum analyzer (OSA) with a resolution of 0.05 nm. The principle transmission of the PMF Sagnac loop mirror can be explained as follows; the PMF has a phase difference between the two propagating fields of the slow axis and fast axis, resulting in a signal spacing which can be given by the equation:
Fig. 1. Experimental setup of the proposed multi-wavelength TDFL.
𝜆2 (1) 𝛥𝑛𝐿 where 𝛥𝑛 and L are the modal birefringence difference between the slow-axis and the fast-axis and the length of the PMF respectively. Based on the equation, the wavelength spacing is inversely proportional to the birefringence and the length of the PMF.
𝛥𝜆 =
3. Results and discussion The comb filter spectra of the 20 m long PMF based Sagnac loop mirror is measured from 1965–1975 nm, with wavelength intervals of 0.38 nm as shown in Fig. 2. The proposed multi-wavelength TDFL has a lasing threshold as low as 5 W, which is achieved by carefully adjusting the two PCs in the cavity. However, at this pump power, the lasing wavelength and peak power is unstable. As the pump power is increased further to 8 W together with the optimization of the PCs, stable multi-wavelength lasing is now obtained. The stability of the lines is attributed to the higher pump power and also by the assistance of the FWM effect. Fig. 3 shows the multi-wavelength operation of the DC-TDFL at a pump power of 8 W, and a total of 35, 55 and 68 lasing lines are obtained within bandwidth regions taken from 10 dB, 20 dB and 28 dB down from the peak power. The channel spacing between two lasing wavelengths is 0.38 nm, which is a function of the PMF length. The OSNR is ∼28 dB. As seen from the figure, more than 80 lasing lines with OSNR values higher than 10 dB can be obtained over a 30 nm bandwidth.
Fig. 2. Transmission spectra of the Sagnac loop mirror.
The proposed system consists of two 793 nm pump laser diodes (LD) whose outputs are combined by a (2+1)×1 multimode pump combiner before being launched into the DC-TDF. The 2.3 m long DC-TDF is a Nufern SM-TDF-10P/130-M fiber with a core and cladding diameter of 10 and 130 μm respectively as well as having a numerical aperture of 0.15. The gain medium has a cladding absorption of 4.5 dB/m at 793 nm and a group velocity dispersion (GVD) of −93 ps2 /km. The other end of the DC-TDF is spliced to a single mode fiber (SMF-28) that serves to attenuate any residual pump. An optical isolator is inserted in
Fig. 3. Multi-wavelength spectrum at a pump power of 8 W.
2
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
Fig. 5. Wavelength drifts for selected lasing lines.
Fig. 4. (a) Multi-wavelength output spectrum and (b) top view of output spectrum.
In order to verify the stability of the proposed multi-wavelength TDFL at room temperature, the optical spectrum output is recorded at intervals of 5 min over a period of 30 min. This is given in Fig. 4(a), which shows a power variation less than 2.8 dB over the entire 30 min test period. Furthermore, it can be observed from the 2-dimensional top view in Fig. 4(b) that multiple lasing wavelengths have a drift of less than 0.056 nm. Both of these observations indicate a very stable multiwavelength output. The slight fluctuations observed are attributed to thermal variations as a result of the high pump power and also from the immediate surroundings. Furthermore, the dielectric materials used in this work are susceptible to thermally induced strain and changes in their refractive indices [20,21]. The temperature dependent phase is accumulated over a single round trip inside the cavity, which causes a temporary change in the effective optical length of the Sagnac loop mirror and hence changes its transmission characteristics. Therefore, these effects influence the optical intensity of the generated lines. In order to further validate the stability of the system, the wavelength drifts and power variations of five selected lasing channels are investigated over a period of 30 min. The five lasing channels are taken within the bandwidth region which is 10 dB down from the peak power, namely 1964.824 and 1968.264 nm (from the shorter region), 1970.153 nm (at the central) and 1974.385 and 1976.667 nm (from the longer wavelength region). Fig. 5 shows the high wavelength stability of the multi-wavelength output, with a wavelength drift of less than 0.056 nm. Fig. 6 shows the peak power variations for the five lasing channels, which are approximately 2.8 dB, 1.5 dB, 0.56 dB, 1.4 dB and 1.0 dB for the lasing wavelengths at 1964.824 nm, 1968.264 nm, 1970.153 nm, 1974.385 nm and 1976.667 nm respectively. The maximum peak power variation is 2.8 dB, with the lowest power fluctation at the central wavelength of 1970.153 nm as compared to the other selected wavelengths. This is due to the FWM threshold, which allows the central peak to gain dominance while suppressing the other wavelengths [19]. The total maximum output power of the multiwavelength laser is 18.6 dBm. Fig. 7 shows the stability of the total maximum output power at intervals of 2 min over a test period of 60 min. From the figure, it can be seen that the output power remains relatively stable between 18.3 and 17.8 dBm. In contrast, the obtained output power is considerably higher than other reported works [19].
Fig. 6. Power fluctuations for selected lasing lines.
Fig. 7. Total laser output power at maximum pump power.
Fig. 8 shows the multi-wavelength spectra obtained at different pump powers. By fixing the polarization state of the PCs and gradually decreasing the 793 nm pump power from 8.0 W to 7.3 W, 6.6 W, 6.2 W, and 5.8 W, the number of lasing lines reduces as expected. This is shown in Fig. 8(a), (b), (c) and (d), respectively. It can also be seen that the bandwidth, OSNR and number of lasing lines of the multiwavelength spectrum are reduced as the pump power is decreased from 7.3 to 5.77 W. This indicates that the laser output performance depends on the gain of the TDF, which decreases as the pump power is lowered. Compared to previous works, the system demonstrated in this work is able to achieve a larger number of lasing channels with comparable laser output lines as shown in Table 1. In order to demonstrate the contribution of the HNLF towards strengthening the generated multiwavelength spectra, the output generated by the same cavity but with the HNLF removed is shown in Fig. 9. For this comparison, the pump 3
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
Table 1 Comparison of multi-wavelength TDFL. Multi-wavelength technique
Wavelength (nm)
Maximum number of channels
Wavelength shifts (nm)
Peak power fluctuations (dB)
OSNR(dB)
Ref.
Interleaving filter Lyot filter Nonlinear amplifier loop mirror FWM in highly Germania doped highly nonlinear fiber PMF Sagnac loop mirror Linear cavity stabilized by HNLF PMF Sagnac loop mirror
1875–1897
19
0.2
1.0
35
[11]
1982–1998 ∼1970
17 42
0.055 0.05
0.237 1.0
35 26
[12] [13]
∼1880
36
0.05
1.6
32
[17]
1990–2007.5
15
0.02
0.5
30
[18]
∼1895
35
0.12
1.3
30
[19]
∼1970
68
0.056
2.8
28
This work
Fig. 8. Multi-wavelength output spectra with pump powers of (a) 7.3 W, (b) 6.6 W, (c) 6.2 W, (d) 5.8 W.
power is maintained at 8 W. As can be seen from the Figure, six lasing lines with OSNR values of more than 41 dB are achieved, although these lines are highly unstable with significant wavelength drifting and in turn resulting in substantial power instability. This is due to the intense gain competition now prevalent in the cavity. As the HNLF has a high nonlinear coefficient, inducing a stronger FWM effect in the cavity which in turn generates a broad gain spectrum and allows for more lasing channels with better stability to be achieved [22]. The proposed all-fiberized multi-wavelength TDFL could find various applications such as in the areas of optical communications and optical sensors. 4. Conclusion In summary, an all-fiber multi-wavelength DC-TDFL using a 100m long HNLF and PMF based Sagnac loop mirror is proposed and demonstrated. The high nonlinearity of the HNLF is used to induce the FWM effect to provide intensity-dependent gain and mitigate the impact of gain competition. By utilizing the PMF Sagnac loop mirror as
Fig. 9. Optical spectra without HNLF at pump power of 8 W.
4
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
a comb filter, up to 35 to 68 lasing lines are obtained over bandwidth regions that are formed at 10 to 28 dB down from the peak power. The lasing wavelengths have a spacing of 0.38 nm and is highly stable, with wavelength drifts of less than 0.056 nm and a maximum power fluctuation of 2.8 dB obtained over a test period of 30 min. The proposed multi-wavelength TDFL would be a potential source for optical communications and optical sensors.
[8] H. Ahmad, A. Muhamad, A. Sharbirin, M. Samion, M. Ismail, Tunable Q-switched thulium-doped Fiber Laser using multiwall carbon nanotube and Fabry–Perot Etalon filter, Opt. Commun. 383 (2017) 359–365. [9] Y. Tang, C. Huang, S. Wang, H. Li, J. Xu, High-power narrow-bandwidth thulium fiber laser with an all-fiber cavity, Opt. Express 20 (2012) 17539–17544. [10] M. Zhang, E. Kelleher, F. Torrisi, Z. Sun, T. Hasan, D. Popa, F. Wang, A.C. Ferrari, S. Popov, J. Taylor, Tm-doped fiber laser mode-locked by graphene-polymer composite, Opt. Express 20 (2012) 25077–25084. [11] A. Al-Alimi, M.A. Bakar, A. Abas, M. Alresheedi, N.Z. Abidin, M. Mahdi, Stable multiwavelength thulium fiber laser assisted by four wave mixing effect, Opt. Laser Technol. 106 (2018) 191–196. [12] S. Liu, F. Yan, F. Ting, L. Zhang, Z. Bai, W. Han, H. Zhou, Multi-wavelength thulium-doped fiber laser using a fiber-based Lyot filter, IEEE Photonics Technol. Lett. 28 (2016) 864–867. [13] W. Peng, F. Yan, Q. Li, S. Liu, T. Feng, S. Tan, A 1.97 μm multiwavelength thulium-doped silica fiber laser based on a nonlinear amplifier loop mirror, Laser Phys. Lett. 10 (2013) 115102. [14] X. Ma, S. Luo, D. Chen, Switchable and tunable thulium-doped fiber laser incorporating a Sagnac loop mirror, Appl. Opt. 53 (2014) 4382–4385. [15] P. Zhang, W. Ma, T. Wang, Q. Jia, C. Wan, Stable multi-wavelength thuliumdoped fiber laser based on all-fiber Mach–Zehnder interferometer, Chin. Opt. Lett. 12 (2014) 111403. [16] X. Wang, Y. Zhu, P. Zhou, X. Wang, H. Xiao, L. Si, Tunable, Multiwavelength Tm-doped fiber laser based on polarization rotation and four-wave-mixing effect, Opt. Express 21 (2013) 25977–25984. [17] T. Huang, X. Li, P.P. Shum, Q.J. Wang, X. Shao, L. Wang, H. Li, Z. Wu, X. Dong, All-fiber multiwavelength thulium-doped laser assisted by four-wave mixing in highly germania-doped fiber, Opt. Express 23 (2015) 340–348. [18] S. Liu, F.-P. Yan, B.-L. Wu, S.-Y. Tan, W.-J. Peng, T. Feng, X. Liang, Q. Li, A multiwavelength thulium-doped silica fiber laser incorporating a highly nonlinear fiber, J. Opt. 16 (2014) 055201. [19] R. Radzali, A. Latif, A. Al-Alimi, M. Mahdi, M.A. Bakar, Highly nonlinear fiberassisted multiwavelength generation in linear cavity thulium-doped fiber laser, IEEE Photon. J. 8 (2016) 1–7. [20] A.M. Flatae, M. Burresi, H. Zeng, S. Nocentini, S. Wiegele, C. Parmeggiani, H. Kalt, D. Wiersma, Optically controlled elastic microcavities, Light Sci. Appl. 4 (2015) e282. [21] J. Gifford, Some temperature refraction coefficients of optical glass, Proc. R. Soc. A 91 (1915) 319–321. [22] A. Ghosh, D. Venkitesh, R. Vijaya, Stability studies on continuous-wave broadband generated in an erbium-doped fiber ring laser using highly nonlinear fiber, IEEE Photon. J. 2 (2010) 703–711.
Acknowledgment Funding for this work was supported by Ministry of Higher Education (MoHE), Malaysia under the grant GA 010–2014 (ULUNG) as well as the University of Malaya under the grants RU 013-2018 and HiCoE Phase II Funding.
References [1] Y.-G. Han, T. Tran, S.-H. Kim, S.B. Lee, Development of a multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings for long-distance remote-sensing applications, Opt. Lett. 30 (2005) 1114–1116. [2] Z. Zhao, Z. Zhang, J. Tan, Y. Liu, J. Liu, 200 Gb/s FSO WDM communication system empowered by multiwavelength directly modulated TOSA for 5G wireless networks, IEEE Photon. J. 10 (2018) 1–8. [3] M. Moghaddam, S.W. Harun, R. Akbari, H. Ahmad, Flatly broadened supercontinuum generation in nonlinear fibers using a mode locked bismuth oxide based erbium doped fiber laser, Laser Phys. Lett. 8 (2011) 369. [4] H. Ahmad, M.Z. Zulkifli, A.A. Latif, S.W. Harun, Tunable dual wavelength fiber laser incorporating AWG and optical channel selector by controlling the cavity loss, Opt. Commun. 282 (2009) 4771–4775. [5] Z. Li, A. Heidt, J. Daniel, Y. Jung, S. Alam, D.J. Richardson, Thulium-doped fiber amplifier for optical communications at 2 μm, Opt. Express 21 (2013) 9289–9297. [6] K. Scholle, S. Lamrini, P. Koopmann, P. Fuhrberg, 2 μm laser sources and their possible applications, in: Frontiers in Guided Wave Optics and Optoelectronics, InTech, 2010. [7] M. Soltanian, H. Ahmad, A. Khodaie, I. Amiri, M. Ismail, S. Harun, A stable dualwavelength Thulium-doped fiber laser at 1.9 μm using photonic crystal fiber, Sci. Rep. 5 (2015) 14537.
5
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
All fiber multiwavelength Tm-doped double-clad fiber laser assisted by four-wave mixing in highly nonlinear fiber and Sagnac loop mirror H. Ahmad a,b ,∗, M.H.M. Ahmed a , M.Z. Samion a , S.W. Harun c a
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Physics Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia c Electrical Department, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b
ARTICLE
INFO
Keywords: Fiber laser Multiwavelength fiber laser Double-cladding thulium-doped fiber laser
ABSTRACT A four-wave mixing (FWM) based multi-wavelength laser based on a double-cladding thulium-doped fiber laser (DC-TDFL) is proposed and demonstrated. A highly nonlinear fiber (HNLF) in conjunction with a Sagnac loop mirror formed by polarization maintaining fibers (PMFs) is used to induce the FWM effect as well as to mitigate gain-competition in the DC-TDFL. This increases the number of lasing wavelengths that can be generated. Approximately 35, 55, and 68 stable lasing lines are achieved within 10 dB, 20 dB, and 28 dB bandwidths by adjusting the polarization controllers. A uniform channel spacing of 0.38 nm is obtained with minimal wavelength drifts and power fluctuations of 0.056 nm and 2.8 dB. This is, to the best of the author’s knowledge, the first demonstration of large numbers of lasing channels in the 1.97 μm wavelength region from a DC-TDFL.
1. Introduction Multi-wavelength fiber lasers are of great interest due to their high potential for use in a wide variety of applications that include optical telecommunications, in sensors systems and also for optical fiber and component testing [1–5]. Of the many possible fiber laser explored, thulium doped fiber lasers (TDFLs) in particular have gained significant attention due to their ability to operate in the eye-safe 2-μm region, making them highly desirable laser sources for biomedical applications [6]. Due to their popularity, numerous TDFL designs capable of generating single, dual- and multi-wavelength continuous wave (CW) and pulsed outputs have been explored and demonstrated [7–10]. Multi-wavelength fiber lasers (MWFLs) are able to generate a multichannel output with equal spacing, narrow linewidth and with high optical signal to noise ratios (OSNRs) from a single cavity. In theory, MWFLs in the 2 μm region can be realized through different approaches including four-wave mixing (FWM) [11], Lyot filters [12], nonlinear amplifier loop mirrors (NALMs) [13], Sagnac loop mirrors [14] and Mach–Zehnder interferometers (MZIs) [15]. In reality however, this cannot be easily obtained because of the homogeneous broadening effect typical of rare-earth doped fibers lasers that significantly affects multi-wavelength operation stability [13]. In order to overcome this limitation, non-linear effects in the fiber laser cavity can be exploited to suppress the homogeneous broadening effect. In this regard, various approaches have been explored, such as the use of very long lengths of standard single mode fibers (SMFs) to improve the cavity non-linearity
and at the same time suppress mode competition [13,16]. Alternative approaches such as the use of highly non-linear fibers (HNLFs) [17–19] can shorten this length while giving the same result. Another interesting approach towards generating a multi-wavelength output in a TDFL is using the FWM effect. The FWM effect arises due to the interplay between two or three wavelengths which give rise to new wavelengths as was demonstrated by Huang et al. [17] in a MWFL operating at the 1.88 μm region. A similar output was also demonstrated by Al-Alimi et al. [11] who reported 19 output wavelengths near 1.8 μm using an interleaving filter assisted by FWM. In this paper, a multi-wavelength TDFL incorporating a HNLF and a Sagnac loop mirror made from polarization maintaining fibers (PMFs) is demonstrated. The use of the HNLF increases the laser cavity’s nonlinearity at high pump powers while the PMF based Sagnac loop acts as a filter. Up to 35, 55 and 68 lasing wavelengths are obtained within the bandwidth regions 10 dB, 20 dB and 28 dB down from the peak power at room temperature. All wavelengths have a channel spacing of 0.38 nm with an OSNR of 28 dB. The output generated is highly stable, with wavelength drifts and power fluctuations of less than 0.05 nm and 2.8 dB respectively. 2. Experimental setup The experimental setup of the proposed multi-wavelength double cladding — thulium doped fiber laser (DC-TDFL) is shown in Fig. 1.
∗ Corresponding author at: Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: [email protected] (H. Ahmad).
https://doi.org/10.1016/j.optcom.2019.124589 Received 14 June 2019; Received in revised form 11 September 2019; Accepted 16 September 2019 Available online 18 September 2019 0030-4018/© 2019 Published by Elsevier B.V.
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
the laser cavity to ensure unidirectional lasing operation, with a 90:10 optical coupler extracting about 10% of the propagating signal in the optical cavity. A spool of 100 m long highly nonlinear fiber (HNLF) with a nonlinear coefficient of ∼10.5 (Wkm)−1 and dispersion slope of 0.016 ps/(nm2 .km) is inserted into the laser cavity in order to provide a nonlinear phase shift and mitigate the mode competition caused by homogeneous gain broadening in the TDF [18]. The light is then coupled to the Sagnac loop, which consists of the 3-dB coupler, a 20-m long PMF with a birefringence of 5×10−4 and polarization controller (PC) designated PC1 as to adjust the transmission properties of the loop mirror. Another polarization controller, designated PC2 is used to control the polarization state in the laser cavity. The 10% output from the laser cavity is extracted for further analysis using a YOKOGAWA-AQ6375 optical spectrum analyzer (OSA) with a resolution of 0.05 nm. The principle transmission of the PMF Sagnac loop mirror can be explained as follows; the PMF has a phase difference between the two propagating fields of the slow axis and fast axis, resulting in a signal spacing which can be given by the equation:
Fig. 1. Experimental setup of the proposed multi-wavelength TDFL.
𝜆2 (1) 𝛥𝑛𝐿 where 𝛥𝑛 and L are the modal birefringence difference between the slow-axis and the fast-axis and the length of the PMF respectively. Based on the equation, the wavelength spacing is inversely proportional to the birefringence and the length of the PMF.
𝛥𝜆 =
3. Results and discussion The comb filter spectra of the 20 m long PMF based Sagnac loop mirror is measured from 1965–1975 nm, with wavelength intervals of 0.38 nm as shown in Fig. 2. The proposed multi-wavelength TDFL has a lasing threshold as low as 5 W, which is achieved by carefully adjusting the two PCs in the cavity. However, at this pump power, the lasing wavelength and peak power is unstable. As the pump power is increased further to 8 W together with the optimization of the PCs, stable multi-wavelength lasing is now obtained. The stability of the lines is attributed to the higher pump power and also by the assistance of the FWM effect. Fig. 3 shows the multi-wavelength operation of the DC-TDFL at a pump power of 8 W, and a total of 35, 55 and 68 lasing lines are obtained within bandwidth regions taken from 10 dB, 20 dB and 28 dB down from the peak power. The channel spacing between two lasing wavelengths is 0.38 nm, which is a function of the PMF length. The OSNR is ∼28 dB. As seen from the figure, more than 80 lasing lines with OSNR values higher than 10 dB can be obtained over a 30 nm bandwidth.
Fig. 2. Transmission spectra of the Sagnac loop mirror.
The proposed system consists of two 793 nm pump laser diodes (LD) whose outputs are combined by a (2+1)×1 multimode pump combiner before being launched into the DC-TDF. The 2.3 m long DC-TDF is a Nufern SM-TDF-10P/130-M fiber with a core and cladding diameter of 10 and 130 μm respectively as well as having a numerical aperture of 0.15. The gain medium has a cladding absorption of 4.5 dB/m at 793 nm and a group velocity dispersion (GVD) of −93 ps2 /km. The other end of the DC-TDF is spliced to a single mode fiber (SMF-28) that serves to attenuate any residual pump. An optical isolator is inserted in
Fig. 3. Multi-wavelength spectrum at a pump power of 8 W.
2
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
Fig. 5. Wavelength drifts for selected lasing lines.
Fig. 4. (a) Multi-wavelength output spectrum and (b) top view of output spectrum.
In order to verify the stability of the proposed multi-wavelength TDFL at room temperature, the optical spectrum output is recorded at intervals of 5 min over a period of 30 min. This is given in Fig. 4(a), which shows a power variation less than 2.8 dB over the entire 30 min test period. Furthermore, it can be observed from the 2-dimensional top view in Fig. 4(b) that multiple lasing wavelengths have a drift of less than 0.056 nm. Both of these observations indicate a very stable multiwavelength output. The slight fluctuations observed are attributed to thermal variations as a result of the high pump power and also from the immediate surroundings. Furthermore, the dielectric materials used in this work are susceptible to thermally induced strain and changes in their refractive indices [20,21]. The temperature dependent phase is accumulated over a single round trip inside the cavity, which causes a temporary change in the effective optical length of the Sagnac loop mirror and hence changes its transmission characteristics. Therefore, these effects influence the optical intensity of the generated lines. In order to further validate the stability of the system, the wavelength drifts and power variations of five selected lasing channels are investigated over a period of 30 min. The five lasing channels are taken within the bandwidth region which is 10 dB down from the peak power, namely 1964.824 and 1968.264 nm (from the shorter region), 1970.153 nm (at the central) and 1974.385 and 1976.667 nm (from the longer wavelength region). Fig. 5 shows the high wavelength stability of the multi-wavelength output, with a wavelength drift of less than 0.056 nm. Fig. 6 shows the peak power variations for the five lasing channels, which are approximately 2.8 dB, 1.5 dB, 0.56 dB, 1.4 dB and 1.0 dB for the lasing wavelengths at 1964.824 nm, 1968.264 nm, 1970.153 nm, 1974.385 nm and 1976.667 nm respectively. The maximum peak power variation is 2.8 dB, with the lowest power fluctation at the central wavelength of 1970.153 nm as compared to the other selected wavelengths. This is due to the FWM threshold, which allows the central peak to gain dominance while suppressing the other wavelengths [19]. The total maximum output power of the multiwavelength laser is 18.6 dBm. Fig. 7 shows the stability of the total maximum output power at intervals of 2 min over a test period of 60 min. From the figure, it can be seen that the output power remains relatively stable between 18.3 and 17.8 dBm. In contrast, the obtained output power is considerably higher than other reported works [19].
Fig. 6. Power fluctuations for selected lasing lines.
Fig. 7. Total laser output power at maximum pump power.
Fig. 8 shows the multi-wavelength spectra obtained at different pump powers. By fixing the polarization state of the PCs and gradually decreasing the 793 nm pump power from 8.0 W to 7.3 W, 6.6 W, 6.2 W, and 5.8 W, the number of lasing lines reduces as expected. This is shown in Fig. 8(a), (b), (c) and (d), respectively. It can also be seen that the bandwidth, OSNR and number of lasing lines of the multiwavelength spectrum are reduced as the pump power is decreased from 7.3 to 5.77 W. This indicates that the laser output performance depends on the gain of the TDF, which decreases as the pump power is lowered. Compared to previous works, the system demonstrated in this work is able to achieve a larger number of lasing channels with comparable laser output lines as shown in Table 1. In order to demonstrate the contribution of the HNLF towards strengthening the generated multiwavelength spectra, the output generated by the same cavity but with the HNLF removed is shown in Fig. 9. For this comparison, the pump 3
H. Ahmad, M.H.M. Ahmed, M.Z. Samion et al.
Optics Communications 456 (2020) 124589
Table 1 Comparison of multi-wavelength TDFL. Multi-wavelength technique
Wavelength (nm)
Maximum number of channels
Wavelength shifts (nm)
Peak power fluctuations (dB)
OSNR(dB)
Ref.
Interleaving filter Lyot filter Nonlinear amplifier loop mirror FWM in highly Germania doped highly nonlinear fiber PMF Sagnac loop mirror Linear cavity stabilized by HNLF PMF Sagnac loop mirror
1875–1897
19
0.2
1.0
35
[11]
1982–1998 ∼1970
17 42
0.055 0.05
0.237 1.0
35 26
[12] [13]
∼1880
36
0.05
1.6
32
[17]
1990–2007.5
15
0.02
0.5
30
[18]
∼1895
35
0.12
1.3
30
[19]
∼1970
68
0.056
2.8
28
This work
Fig. 8. Multi-wavelength output spectra with pump powers of (a) 7.3 W, (b) 6.6 W, (c) 6.2 W, (d) 5.8 W.
power is maintained at 8 W. As can be seen from the Figure, six lasing lines with OSNR values of more than 41 dB are achieved, although these lines are highly unstable with significant wavelength drifting and in turn resulting in substantial power instability. This is due to the intense gain competition now prevalent in the cavity. As the HNLF has a high nonlinear coefficient, inducing a stronger FWM effect in the cavity which in turn generates a broad gain spectrum and allows for more lasing channels with better stability to be achieved [22]. The proposed all-fiberized multi-wavelength TDFL could find various applications such as in the areas of optical communications and optical sensors. 4. Conclusion In summary, an all-fiber multi-wavelength DC-TDFL using a 100m long HNLF and PMF based Sagnac loop mirror is proposed and demonstrated. The high nonlinearity of the HNLF is used to induce the FWM effect to provide intensity-dependent gain and mitigate the impact of gain competition. By utilizing the PMF Sagnac loop mirror as
Fig. 9. Optical spectra without HNLF at pump power of 8 W.
4
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Optics Communications 456 (2020) 124589
a comb filter, up to 35 to 68 lasing lines are obtained over bandwidth regions that are formed at 10 to 28 dB down from the peak power. The lasing wavelengths have a spacing of 0.38 nm and is highly stable, with wavelength drifts of less than 0.056 nm and a maximum power fluctuation of 2.8 dB obtained over a test period of 30 min. The proposed multi-wavelength TDFL would be a potential source for optical communications and optical sensors.
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Acknowledgment Funding for this work was supported by Ministry of Higher Education (MoHE), Malaysia under the grant GA 010–2014 (ULUNG) as well as the University of Malaya under the grants RU 013-2018 and HiCoE Phase II Funding.
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