- Email: [email protected]
S+ band region
Optics Communications 397 (2017) 91–94
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable and switchable Brillouin multi-wavelength thulium fluoride fiber laser in S/S+ band region
MARK
⁎
H. Ahmad , S.I. Ooi, M.Z. Samion, A.A. Jasim, Z.C. Tiu Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia
A R T I C L E I N F O
A BS T RAC T
Keywords: S-band Stimulated Brillouin backscattering Thulium fluoride fiber Micro-air gap
In this work, a multi-wavelength fiber laser with a channel spacing of 0.17 nm operating in the S-band region is proposed and demonstrated. The proposed laser exploits the stimulated Brillouin scattering (SBS) effect to generate the desired output using a 6.9 km long dispersion shifted fiber. A Thulium-Fluoride fiber (TFF) pumped at 1400 nm enables gain and operation in the S-band region. With a Brillouin pump (BP) wavelength of 1503.6 nm and power of 12 dBm, up to 3 well defined even Stokes lines are obtained at the maximum pump power of 250 mW. Furthermore, more even Stokes lines and anti-Stokes lines are also observed at the aforementioned setting, although they are less well defined as the first 3 even Stokes. The laser has an operating range of 1470–1515 nm, with tunability enabled by changing the BP wavelength. Furthermore, the spacing between the lasing wavelengths can be switched from 0.17 nm to 0.08 nm by adjusting the integrated air gap in the laser cavity, making the proposed laser highly suitable for applications such as communications and sensing. This is, the first time to the author's knowledge that such a system has been demonstrated.
1. Introduction The generation of multiple wavelength outputs from a single laser source has long been the focus of significant research efforts due to their enormous potential for a multitude of applications ranging from optical communications, sensing, spectroscopy and testing [1–5]. Multi-wavelength fiber lasers can be realized using multiple approaches, with the most common being the slicing of the amplified spontaneous emission (ASE) spectrum generated by linear gain media such as rare-earth doped gain media [6,7] or semiconductor optical amplifiers [8], as well as by exploiting various non-linear phenomena such as stimulated Brillouin scattering (SBS) [9], nonlinear polarization rotation (NPR) [10–12] and four-wave mixing (FWM) [13,14]. Of the afore-mentioned methods, the generation of multi-wavelength outputs using the SBS technique is of particular interest due to its ability to generate a wavelength comb with a very close spacing of around 0.08 nm [9]. The SBS effect can be induced in a fiber laser system when intense pump light and acoustic waves interact in a nonlinear medium, resulting in a Brillouin comb being generated at a frequency shift of approximately 10 GHz or 0.08 nm [15]. SBS generated outputs are highly suited for dense wavelength division multiplexing (DWDM) applications, with ultra-narrow linewidths [16], low threshold powers [17], overall low noise intensity [18], and a wide tuning range [19].
⁎
Corresponding author. E-mail address: [email protected] (H. Ahmad).
http://dx.doi.org/10.1016/j.optcom.2017.04.009 Received 6 February 2017; Received in revised form 1 April 2017; Accepted 3 April 2017 0030-4018/ © 2017 Published by Elsevier B.V.
A particularly crucial application of SBS-based multi-wavelength fiber lasers is as a signal source for use in the S-band region. The Sband region has, of late, become a focal point for extensive research efforts as a viable alternative to the C- and L-bands, which are quickly reaching their capacity limits. The shorter S-band, which ranges from 1480 nm to 1520 nm [20], creates an effective bandwidth of more than 140 nm when combined with the C- and L-bands, thus providing significant bandwidth capacity to cater to ever-rising demands [21]. However, actual applications in the S-band region still lags behind that of the C- and L-bands due to the significant loss encountered in that region by erbium doped fiber amplifiers (EDFAs). With a cut-off wavelength of about 1530 nm, EDFAs perform very poorly at the Sband region, severely limiting its applications in this wavelength range. While specialized gain media such as depressed cladding Erbiumdoped fibers (DC-EDFs), which allow the EDFA's cut-off wavelength to be tuned to less than 1530 nm can be used to support the development of S-band amplifiers and fiber lasers [20], their limited availability and generally high cost restricts their use. In this regard, Thulium doped fibers (TDFs) have long been demonstrated as an optical amplifier and fiber laser operating in the S-band region [9,10]. The TDF has two significant advantages over current specialized S-band gain media, namely the ability to generate gain in the S-band region as well as a typically broader emission range which allows for the lower S-band region to also be used, from about
Optics Communications 397 (2017) 91–94
H. Ahmad et al.
3. Results and discussion
1460 nm to 1480 nm [21]. However, lasing at this region can be difficult to achieve for TDFs based on conventional silica fibers for two reasons, due to the need for low phonon energy in the host material as well as the difficulty in forming a population inversion by direct pumping at 676 or 790 nm [22,23]. While the latter can be easily resolved by pumping at a higher wavelength, at about 1500 nm, the former limitation is harder to overcome [22]. However, recent advances in fluoride fiber fabrication has made it possible for thuliumfluoride fibers (TFFs) to be fabricated commercially. Fluoride fibers have a low phonon energy, making it a highly suitable host for Tm3+ ions and thus enabling the design and development of various S-band systems centered on TFFs. In this work, an SBS based multi-wavelength Brillouin thuliumfluoride fiber laser (MWBTFFL) is proposed and demonstrated. The proposed system utilizes a 6.9 km long dispersion shifted fiber (DSF) as the Brillouin gain medium (BGM) to generate the desired multiwavelength output, while a 14.5 m long TFF serves as the gain medium to enable S-band operation. Furthermore, the output of the system can be tuned by adjusting the Brillouin pump wavelength, and can switch between single- and double-spaced wavelengths by adjusting a microair gap incorporated into the laser cavity. This is the first time, to the knowledge of the authors, of an SBS based multi-wavelength source operating in the S-band region with wavelength and spacing tunability.
In this setup, the TFF functions as the S-band optical amplifier due to pumping at 1400 nm. The 1400 nm pump is able to induce ground state absorption in the Tm3+ ions, from the 3H6 level to the 3F4 level, which further induces excited state absorption from the 3F4 level to the 3 H4 level. The recombination process from the 3H4 to 3F4 levels provides the desired amplification in the S/S+ band region which covers a region of 1450–1530 nm. The BP wavelength is set at 1503.6 nm, which is around the peak amplification region of the TFFL ring cavity. In order to determine the optimal lasing wavelength, the proposed laser is first operated without the BP active. In this manner, the TFFL achieves self lasing at the afore-mentioned wavelength. Thus, the BP output is set to match this wavelength, and then injected into the cavity to induce the SBS effect. The BP power is fixed at 12 dBm, and the power of pump laser is gradually increased until a Stokes line is obtained at a pump power of 120 mW, as shown in Fig. 2(a). Further increases in the pump power results in more Stokes lines being generated, along with corresponding anti-Stokes, with 2 significant Stokes lines observed at a pump power of 190 mW and 3 significant Stokes lines at a pump power of 250 mW. More Stokes and anti-Stokes are also observed in the figure, although they are less distinct. In the case of a pump power of 250 mW, the BP, 2nd, 4th, 6th and 8th and even 10th Stokes lines are visible, with more even Stokes lines and also anti-Stokes lines being generated, as seen in Fig. 2(b). As in the usual case of generated Stokes lines, the 1st Stokes is created when BP exceed the threshold value of the 1st Stokes. The 1st Stokes will propagate in the opposite direction to the pump, which in this setup is travelling in the clockwise direction. The 1st Stokes will now propagate in an anti-clockwise direction back to the BGM where it will now generate the 2nd Stokes if the power exceeds the threshold value. As before, the 2nd Stokes travels in the opposite direction to the 1st Stokes and in the same direction to the pump. This process creates a set of even and odd Stokes, and will repeat as long as the power of subsequent Stokes exceeds the required threshold value of the next Stokes. In this setup, the tap coupler is placed in the clockwise direction, thus extracting the pump and all subsequent even Stokes lines. The 2nd Stokes is obtained at a wavelength of 1503.8 nm, a redshift of about 0.17 nm with respect to the BP wavelength. In the same manner, this applies to the 4th and 6th Stokes as well as all subsequent Stokes. In this setup, the main focus is to demonstrate the ability to generate Stokes lines in the S/S+ band using a TFF as the gain medium. Fig. 3 shows the stability spectrum of the generated multi-wavelength output from the proposed MWBTFFL. For this observation, the BP is kept fixed at 1503.6 nm at a power of 12 dBm, while the pump power is fixed at 250 nm. The resulting spectrum is observed over a 60min time span, with scans being taken at 10 min intervals. From the figure, it can be seen that there is less than 1 dB of fluctuation throughout the entire observation period. Moreover, the output power is recorded around −0.45 dBm with less than 0.2 dB fluctuation in 60min time span. In addition, the tunability of the MWBTFFL is investigated by changing the BP wavelength. By fixing the BP at 12 dBm and pump power at 250 mW, the BP wavelength is changed from 1470 nm to 1515 nm at 5 nm intervals as shown in Fig. 4. It can be seen from the obtained spectrum that at the shorter wavelength region, only a few distinct Stokes lines can be observed, with the lowest being only 1 Stokes line. However, increasing the BP wavelength now results in the BP and the 2nd Stokes line becoming more clearly defined, as well as the first 2nd anti-Stokes and 4th Stokes lines also beginning to appear. Similarly, as the BP wavelength increases, more distinct Stokes and anti-Stokes lines are observed, along with a significant number of less defined Stokes and anti-Stokes lines. The highest number of Stokes lines achieved is 5 distinct Stokes lines, and more than 8 smaller Stokes
2. Experimental setup The configuration of the proposed MWBTFFL is given in Fig. 1. In this configuration, a 1400 nm laser diode (LD) act as the pump source, and is connected to the 1400 nm port of a 1400/1500 nm wavelength division multiplexer (WDM). An optical isolator (ISO) is placed between the 1400 nm LD and the 1400 nm port of the WDM as to prevent any back-reflected signals which can damage to the LD. The common output of the WDM is then connected to a 14.5 m long TFF, which serves as the primary gain medium for this cavity. The TFF has a Tm3+ concentration of 3200 ppm with an absorption rate of 0.15 dB/m at 1400 nm, as well as a numerical aperture of 0.26 and mode field diameter of 4.5 µm at 1500 nm. The total loss of TFF, which includes the loss of the gain medium and also the silica-fluoride splice joint as well as the FC/PC connecters, is around 7 dB at 1500 nm. The output from the TFF is then connected to one of the input ports of a 3-dB coupler. The other port of the 3-dB coupler connects to tunable laser sources (TLS), which cover a wavelength region of 1360 nm to 1495 nm and 1490 nm to 1630 nm respectively. This gives an effective Brillouin pump (BP) of 270 nm, from 1360 nm to 1630 nm to induce the SBS effect within the cavity. The combined TLS output from the 3-dB coupler is now connected to the 6.9 km long DSF with dispersion of approximately −4 ps/nm km, which serves as the BGM in this setup. The output from the BGM is now connected to an 80:20 optical coupler, with the 20% port connected to a Yokogawa AQ6370B optical spectrum analyzer (OSA) with a minimum resolution of 0.02 nm for analyzing the optical signal. The 80% port of the coupler on the other hand connects to the 1500 nm port of the WDM, completing the optical circuit.
Fig. 1. Schematic of the proposed MWBTFFL.
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Fig. 2. (a) Multi-wavelength output spectrum with BP, even Stokes and anti-Stokes lines against different pump powers, and (b) Multi-wavelength output spectrum with BP, even Stokes and anti-Stokes lines at pump power of 250 mW.
lines being formed at a BP wavelength of 1503.6 nm. At this BP wavelength also, 3 distinct anti-Stokes lines are seen, with more antiStokes lines being generated as well. However, as the BP wavelength continues to increase, the number of distinct Stokes and anti-Stokes lines being generated begins to reduce again. This is attributed to the BP wavelength moving away from its optimal amplification region, and thus reducing in power and no longer being able to overcome the threshold power required to generate the Stokes lines. This however can be overcome by increasing the BP power, the pump power to the TFF or both simultaneously, which will result in the gain peak being made higher and also widening the effective gain. This allows for more Stokes lines to be generated. As shown in Fig. 5, the spacing between the BP and 2nd Stokes constitutes a double Brillouin frequency, due to the emission of 1st Stokes line in the detection system [24]. However, this spacing can be converted to a single Brillouin frequency spacing through the use of the micro air-gap technique generated as demonstrated in Ref. [25]. In this approach, an air gap about 15 µm wide is incorporated between the BGM and the coupler to induce a strong Fresnel reflection effect. The light oscillating through the micro-air gap is forced to propagate in both the clockwise and anti-clockwise directions due to Fresnel reflection. Thus, both the odd and even Stokes are able to oscillate constructively and form single spacing Brillouin spectrum as shown in Fig. 5. From here, it can be seen that with the incorporation of the micro-air gap, the spacing between each peak wavelength reduces from 0.17 nm to about 0.085 nm, resulting in 6 high powered Stokes lines
Fig. 3. Stability of MWBTFFL in an hour time span.
Fig. 4. Tunability of MWBTFFL from 1470 nm to 1515 nm.
Fig. 5. Single-spaced and double-spaced Brillouin output spectrum using micro air-gap.
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now being observed instead of just 3 originally. The same observation can be made for other Stokes lines and also anti-Stokes lines. The inclusion of the micro air-gap does however incur a minor loss of power, which is estimated to be approximately 1.1 dB. The primary advantage of the proposed setup of this work is the ability to generate the Brillouin output both easily and uniformly over a broad tuning range that almost covers the entire S-band region as well as the S+ region. Typically, Brillouin based multi-wavelength outputs in the S-band region could only be realized using depressed cladding EDF (DC-EDF) [26,27] or Brillouin-Raman co-pumping [28,29]. However, both these approaches have certain limitations; the use of the DC-EDF, while able to suppress the gain medium emission to the Sband region [26], suffers from a relatively narrow gain profile, causing non-uniform Stokes peak intensity. Brillouin Raman co-pumping on the other hand has generally inferior performance to that obtained when using a rare-earth gain medium based fiber laser. In addition, the stimulated Raman scattering mechanism results in emissions at frequency downshifted by 13.2 THz [28] from the pump wavelength. Furthermore, both configurations cannot be readily tuned to different wavelengths, particularly when using the DC-EDF in the cavity due to the narrow gain profile emitted. Thus, the proposed system in this work possess a significant advantage in terms of admirable performance in the S/S+ band region and also enabling the tunability of the lasing wavelength outputs. Furthermore, changing the gain medium will allow for similar outputs to be realized at the C- and L-band regions, thus expanding the scope of operation for the proposed laser [30]. The tunable range and number of Stokes lines can be tuned by increasing the laser diode pump power to the TFF. As the pumping power is increased, the gain of the TFF will also increase in tandem, allowing more Stokes and anti-Stokes lines to be generated. Having a longer length TFF at a higher pump power will eventually provide more Stokes lines as well. As such, currently, the present TFF was driven hard enough because of the uncertainty of the fluoride fiber at higher pump powers, which can damage the TFF.
[2]
[3] [4] [5] [6] [7] [8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
[18]
4. Conclusion
[19]
A MWBTFFL using a 14.5 m long TFF and 6.9 km DSF is proposed and demonstrated. The proposed system utilizes the TFF as the S-band linear gain medium, while the DSF serves as the non-linear gain medium to induce the SBS effect in the system, thus generating the desired multi-wavelength output. The system has an SBS threshold of 120 mW, and with a BP of 1503.6 nm at 12 dBm and a maximum pump power of 250 mW, 3 distinct even Stokes lines are obtained. More even Stokes and anti-Stokes lines are also observed, though lacking the distinction of the first three even Stokes lines. The laser's output can be tuned from 1470 nm to 1515 nm by adjusting the BP wavelength, and by incorporating a 15 µm air-gap into the laser cavity, the double-spaced Brillouin output can be made to revert to a singlespaced Brillouin comb by exploiting Fresnel reflection at the air gap. This makes the proposed system highly useful for easy switching between applications requiring dense and less dense wavelength combs.
[20]
[21]
[22]
[23] [24]
[25]
[26] [27] [28]
Acknowledgement [29]
Funding for this work was provided by the University of Malaya under the grants RU 010/2016, and RP 029A – 15 AFR as well as the Ministry of Higher Education, Malaysia under the grants HICOE 2016 and GA 010 - 2014 (ULUNG).
[30]
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erbium-doped fiber lasers anchored on the ITU frequency grid, J. Lightwave Technol. 18 (2000) 825. D. Chen, S. Qin, S. He, Channel-spacing-tunable multi-wavelength fiber ring laser with hybrid Raman and Erbium-doped fiber gains, Opt. Express 15 (2007) 930–935. S.W. Harun, X.S. Cheng, N.K. Saat, H. Ahmad, S-band Brillouin erbium fibre laser, Electron. Lett. 41 (2005) 174–176. F. Mihelic, D. Bacquet, J. Zemmouri, P. Szriftgiser, Ultrahigh resolution spectral analysis based on a Brillouin fiber laser, Opt. Lett. 35 (2010) 432–434. J.J. Veselka, S.K. Korotky, A multiwavelength source having precise channel spacing for WDM systems, IEEE Photonics Technol. Lett. 10 (1998) 958–960. S.W. Harun, F.A. Rahman, K. Dimyati, H. Ahmad, An efficient multiwavelength light source based on ASE slicing, Laser Phys. Lett. 3 (2006) 495. Z. Zhang, L. Zhan, Y. Xia, Tunable self-seeded multiwavelength Brillouin-erbium fiber laser with enhanced power efficiency, Opt. Express 15 (2007) 9731–9736. M.A. Ummy, N. Madamopoulos, A. Joyo, M. Kouar, R. Dorsinville, Tunable multiwavelength SOA based linear cavity dual-output port fiber laser using Lyot-Sagnac loop mirror, Opt. Express 19 (2011) 3202–3211. S.W. Harun, M.R. Shirazi, H. Ahmad, A new configuration of multi-wavelength Brillouin fiber laser, Laser Phys. Lett. 5 (2007) 48. N.S. Shahabuddin, H. Ahmad, Z. Yusoff, S.W. Harun, Spacing-switchable multiwavelength fiber laser based on nonlinear polarization rotation and Brillouin scattering in photonic crystal fiber, IEEE Photonics J. 4 (2012) 34–38. Z. Yan, X. Li, Y. Tang, P.P. Shum, X. Yu, Y. Zhang, Q.J. Wang, Tunable and switchable dual-wavelength Tm-doped mode-locked fiber laser by nonlinear polarization evolution, Opt. Express 23 (2015) 4369–4376. Z. Yan, Y. Tang, B. Sun, T. Liu, X. Li, P.S. Ping, X. Yu, Y. Zhang, Q.J. Wang, Switchable multi-wavelength Tm-doped mode-locked fiber laser, Opt. Lett. 40 (2015) 1916–1919. Y.-G. Han, T.V.A. Tran, S.B. Lee, Wavelength-spacing tunable multiwavelength erbium-doped fiber laser based on four-wave mixing of dispersion-shifted fiber, Opt. Lett. 31 (2006) 697–699. D.S. Lim, H.K. Lee, K.H. Kim, S.B. Kang, J.T. Ahn, M.-Y. Jeon, Generation of multiorder Stokes and anti-Stokes lines in a Brillouin erbium-fiber laser with a Sagnac loop mirror, Opt. Lett. 23 (1998) 1671–1673. J. Tang, J. Sun, L. Zhao, T. Chen, T. Huang, Y. Zhou, Tunable multiwavelength generation based on Brillouin-erbium comb fiber laser assisted by multiple fourwave mixing processes, Opt. Express 19 (2011) 14682–14689. S.P. Smith, F. Zarinetchi, S. Ezekiel, Narrow-linewidth stimulated Brillouin fiber laser and applications, Opt. Lett. 16 (1991) 393–395. S. Norcia, S. Tonda-Goldstein, D. Dolfi, J.P. Huignard, R. Frey, Efficient singlemode Brillouin fiber laser for low-noise optical carrier reduction of microwave signals, Opt. Lett. 28 (2003) 1888–1890. M.R. Shirazi, N.S. Shahabuddin, S.N. Aziz, K. Thambiratnam, S.W. Harun, H. Ahmad, A linear cavity Brillouin fiber laser with multiple wavelengths output, Laser Phys. Lett. 5 (2008) 361–363. Z.A. Rahman, M.H. Al-Mansoori, S. Hitam, A.F. Abas, M.H.A. Bakar, M.A. Mahdi, Optimization of Brillouin pump wavelength location on tunable multiwavelength BEFL, Laser Phys. 19 (2009) 2110. H. Ahmad, M.Z. Zulkifli, A.A. Latif, K. Thambiratnam, S.W. Harun, Bidirectional Sband continuous wave operation in a depressed-cladding erbium doped fiber amplifier, J. Optoelectron. Adv. Mater. 11 (2009) 547–553. R. Caspary, U.B. Unrau, W. Kowalsky, Recent progress on S-band fiber amplifiers, in: Proceedings of the 5th International Conference on Transparent Optical Networks, IEEE, 1, 2003, pp. 236–242. J.Y. Allain, M. Monerie, H. Poignant, Tunable CW lasing around 0.82, 1.48, 1.88 and 2.35 mu m in thulium-doped fluorozirconate fibre, Electron. Lett. 25 (1989) 1660–1662. M.J.F. Digonnet, Rare-Earth-doped Fiber Lasers and Amplifiers, Revised and Expanded, CRC press, Florida, USA, 2001. R. Parvizi, H. Arof, N.M. Ali, H. Ahmad, S.W. Harun, 0.16 nm spaced multiwavelength Brillouin fiber laser in a figure-of-eight configuration, Opt. Laser Technol. 43 (2011) 866–869. Z.C. Tiu, S.N. Aidit, N.A. Hassan, M.F.B. Ismail, H. Ahmad, Single and double Brillouin frequency spacing multi-wavelength Brillouin erbium fiber laser with micro-air gap cavity, IEEE J. Quantum Electron. 52 (2016) 1–5. S.W. Harun, K. Dimyati, K.K. Jayapalan, H. Ahmad, An overview on S-band erbium-doped fiber amplifiers, Laser Phys. Lett. 4 (2006) 10. S.W. Harun, M.Z. Zulkifli, H. Ahmad, A linear cavity S-band Brillouin/Erbium fiber laser, Laser Phys. Lett. 3 (2006) 369. H. Ahmad, M.Z. Zulkifli, M.H. Jemangin, S.W. Harun, Distributed feedback multimode Brillouin-Raman random fiber laser in the S-band, Laser Phys. Lett. 10 (2013) 055102. H. Ahmad, M.Z. Zulkifli, A.A. Latif, K. Thambiratnam, S.W. 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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable and switchable Brillouin multi-wavelength thulium fluoride fiber laser in S/S+ band region
MARK
⁎
H. Ahmad , S.I. Ooi, M.Z. Samion, A.A. Jasim, Z.C. Tiu Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia
A R T I C L E I N F O
A BS T RAC T
Keywords: S-band Stimulated Brillouin backscattering Thulium fluoride fiber Micro-air gap
In this work, a multi-wavelength fiber laser with a channel spacing of 0.17 nm operating in the S-band region is proposed and demonstrated. The proposed laser exploits the stimulated Brillouin scattering (SBS) effect to generate the desired output using a 6.9 km long dispersion shifted fiber. A Thulium-Fluoride fiber (TFF) pumped at 1400 nm enables gain and operation in the S-band region. With a Brillouin pump (BP) wavelength of 1503.6 nm and power of 12 dBm, up to 3 well defined even Stokes lines are obtained at the maximum pump power of 250 mW. Furthermore, more even Stokes lines and anti-Stokes lines are also observed at the aforementioned setting, although they are less well defined as the first 3 even Stokes. The laser has an operating range of 1470–1515 nm, with tunability enabled by changing the BP wavelength. Furthermore, the spacing between the lasing wavelengths can be switched from 0.17 nm to 0.08 nm by adjusting the integrated air gap in the laser cavity, making the proposed laser highly suitable for applications such as communications and sensing. This is, the first time to the author's knowledge that such a system has been demonstrated.
1. Introduction The generation of multiple wavelength outputs from a single laser source has long been the focus of significant research efforts due to their enormous potential for a multitude of applications ranging from optical communications, sensing, spectroscopy and testing [1–5]. Multi-wavelength fiber lasers can be realized using multiple approaches, with the most common being the slicing of the amplified spontaneous emission (ASE) spectrum generated by linear gain media such as rare-earth doped gain media [6,7] or semiconductor optical amplifiers [8], as well as by exploiting various non-linear phenomena such as stimulated Brillouin scattering (SBS) [9], nonlinear polarization rotation (NPR) [10–12] and four-wave mixing (FWM) [13,14]. Of the afore-mentioned methods, the generation of multi-wavelength outputs using the SBS technique is of particular interest due to its ability to generate a wavelength comb with a very close spacing of around 0.08 nm [9]. The SBS effect can be induced in a fiber laser system when intense pump light and acoustic waves interact in a nonlinear medium, resulting in a Brillouin comb being generated at a frequency shift of approximately 10 GHz or 0.08 nm [15]. SBS generated outputs are highly suited for dense wavelength division multiplexing (DWDM) applications, with ultra-narrow linewidths [16], low threshold powers [17], overall low noise intensity [18], and a wide tuning range [19].
⁎
Corresponding author. E-mail address: [email protected] (H. Ahmad).
http://dx.doi.org/10.1016/j.optcom.2017.04.009 Received 6 February 2017; Received in revised form 1 April 2017; Accepted 3 April 2017 0030-4018/ © 2017 Published by Elsevier B.V.
A particularly crucial application of SBS-based multi-wavelength fiber lasers is as a signal source for use in the S-band region. The Sband region has, of late, become a focal point for extensive research efforts as a viable alternative to the C- and L-bands, which are quickly reaching their capacity limits. The shorter S-band, which ranges from 1480 nm to 1520 nm [20], creates an effective bandwidth of more than 140 nm when combined with the C- and L-bands, thus providing significant bandwidth capacity to cater to ever-rising demands [21]. However, actual applications in the S-band region still lags behind that of the C- and L-bands due to the significant loss encountered in that region by erbium doped fiber amplifiers (EDFAs). With a cut-off wavelength of about 1530 nm, EDFAs perform very poorly at the Sband region, severely limiting its applications in this wavelength range. While specialized gain media such as depressed cladding Erbiumdoped fibers (DC-EDFs), which allow the EDFA's cut-off wavelength to be tuned to less than 1530 nm can be used to support the development of S-band amplifiers and fiber lasers [20], their limited availability and generally high cost restricts their use. In this regard, Thulium doped fibers (TDFs) have long been demonstrated as an optical amplifier and fiber laser operating in the S-band region [9,10]. The TDF has two significant advantages over current specialized S-band gain media, namely the ability to generate gain in the S-band region as well as a typically broader emission range which allows for the lower S-band region to also be used, from about
Optics Communications 397 (2017) 91–94
H. Ahmad et al.
3. Results and discussion
1460 nm to 1480 nm [21]. However, lasing at this region can be difficult to achieve for TDFs based on conventional silica fibers for two reasons, due to the need for low phonon energy in the host material as well as the difficulty in forming a population inversion by direct pumping at 676 or 790 nm [22,23]. While the latter can be easily resolved by pumping at a higher wavelength, at about 1500 nm, the former limitation is harder to overcome [22]. However, recent advances in fluoride fiber fabrication has made it possible for thuliumfluoride fibers (TFFs) to be fabricated commercially. Fluoride fibers have a low phonon energy, making it a highly suitable host for Tm3+ ions and thus enabling the design and development of various S-band systems centered on TFFs. In this work, an SBS based multi-wavelength Brillouin thuliumfluoride fiber laser (MWBTFFL) is proposed and demonstrated. The proposed system utilizes a 6.9 km long dispersion shifted fiber (DSF) as the Brillouin gain medium (BGM) to generate the desired multiwavelength output, while a 14.5 m long TFF serves as the gain medium to enable S-band operation. Furthermore, the output of the system can be tuned by adjusting the Brillouin pump wavelength, and can switch between single- and double-spaced wavelengths by adjusting a microair gap incorporated into the laser cavity. This is the first time, to the knowledge of the authors, of an SBS based multi-wavelength source operating in the S-band region with wavelength and spacing tunability.
In this setup, the TFF functions as the S-band optical amplifier due to pumping at 1400 nm. The 1400 nm pump is able to induce ground state absorption in the Tm3+ ions, from the 3H6 level to the 3F4 level, which further induces excited state absorption from the 3F4 level to the 3 H4 level. The recombination process from the 3H4 to 3F4 levels provides the desired amplification in the S/S+ band region which covers a region of 1450–1530 nm. The BP wavelength is set at 1503.6 nm, which is around the peak amplification region of the TFFL ring cavity. In order to determine the optimal lasing wavelength, the proposed laser is first operated without the BP active. In this manner, the TFFL achieves self lasing at the afore-mentioned wavelength. Thus, the BP output is set to match this wavelength, and then injected into the cavity to induce the SBS effect. The BP power is fixed at 12 dBm, and the power of pump laser is gradually increased until a Stokes line is obtained at a pump power of 120 mW, as shown in Fig. 2(a). Further increases in the pump power results in more Stokes lines being generated, along with corresponding anti-Stokes, with 2 significant Stokes lines observed at a pump power of 190 mW and 3 significant Stokes lines at a pump power of 250 mW. More Stokes and anti-Stokes are also observed in the figure, although they are less distinct. In the case of a pump power of 250 mW, the BP, 2nd, 4th, 6th and 8th and even 10th Stokes lines are visible, with more even Stokes lines and also anti-Stokes lines being generated, as seen in Fig. 2(b). As in the usual case of generated Stokes lines, the 1st Stokes is created when BP exceed the threshold value of the 1st Stokes. The 1st Stokes will propagate in the opposite direction to the pump, which in this setup is travelling in the clockwise direction. The 1st Stokes will now propagate in an anti-clockwise direction back to the BGM where it will now generate the 2nd Stokes if the power exceeds the threshold value. As before, the 2nd Stokes travels in the opposite direction to the 1st Stokes and in the same direction to the pump. This process creates a set of even and odd Stokes, and will repeat as long as the power of subsequent Stokes exceeds the required threshold value of the next Stokes. In this setup, the tap coupler is placed in the clockwise direction, thus extracting the pump and all subsequent even Stokes lines. The 2nd Stokes is obtained at a wavelength of 1503.8 nm, a redshift of about 0.17 nm with respect to the BP wavelength. In the same manner, this applies to the 4th and 6th Stokes as well as all subsequent Stokes. In this setup, the main focus is to demonstrate the ability to generate Stokes lines in the S/S+ band using a TFF as the gain medium. Fig. 3 shows the stability spectrum of the generated multi-wavelength output from the proposed MWBTFFL. For this observation, the BP is kept fixed at 1503.6 nm at a power of 12 dBm, while the pump power is fixed at 250 nm. The resulting spectrum is observed over a 60min time span, with scans being taken at 10 min intervals. From the figure, it can be seen that there is less than 1 dB of fluctuation throughout the entire observation period. Moreover, the output power is recorded around −0.45 dBm with less than 0.2 dB fluctuation in 60min time span. In addition, the tunability of the MWBTFFL is investigated by changing the BP wavelength. By fixing the BP at 12 dBm and pump power at 250 mW, the BP wavelength is changed from 1470 nm to 1515 nm at 5 nm intervals as shown in Fig. 4. It can be seen from the obtained spectrum that at the shorter wavelength region, only a few distinct Stokes lines can be observed, with the lowest being only 1 Stokes line. However, increasing the BP wavelength now results in the BP and the 2nd Stokes line becoming more clearly defined, as well as the first 2nd anti-Stokes and 4th Stokes lines also beginning to appear. Similarly, as the BP wavelength increases, more distinct Stokes and anti-Stokes lines are observed, along with a significant number of less defined Stokes and anti-Stokes lines. The highest number of Stokes lines achieved is 5 distinct Stokes lines, and more than 8 smaller Stokes
2. Experimental setup The configuration of the proposed MWBTFFL is given in Fig. 1. In this configuration, a 1400 nm laser diode (LD) act as the pump source, and is connected to the 1400 nm port of a 1400/1500 nm wavelength division multiplexer (WDM). An optical isolator (ISO) is placed between the 1400 nm LD and the 1400 nm port of the WDM as to prevent any back-reflected signals which can damage to the LD. The common output of the WDM is then connected to a 14.5 m long TFF, which serves as the primary gain medium for this cavity. The TFF has a Tm3+ concentration of 3200 ppm with an absorption rate of 0.15 dB/m at 1400 nm, as well as a numerical aperture of 0.26 and mode field diameter of 4.5 µm at 1500 nm. The total loss of TFF, which includes the loss of the gain medium and also the silica-fluoride splice joint as well as the FC/PC connecters, is around 7 dB at 1500 nm. The output from the TFF is then connected to one of the input ports of a 3-dB coupler. The other port of the 3-dB coupler connects to tunable laser sources (TLS), which cover a wavelength region of 1360 nm to 1495 nm and 1490 nm to 1630 nm respectively. This gives an effective Brillouin pump (BP) of 270 nm, from 1360 nm to 1630 nm to induce the SBS effect within the cavity. The combined TLS output from the 3-dB coupler is now connected to the 6.9 km long DSF with dispersion of approximately −4 ps/nm km, which serves as the BGM in this setup. The output from the BGM is now connected to an 80:20 optical coupler, with the 20% port connected to a Yokogawa AQ6370B optical spectrum analyzer (OSA) with a minimum resolution of 0.02 nm for analyzing the optical signal. The 80% port of the coupler on the other hand connects to the 1500 nm port of the WDM, completing the optical circuit.
Fig. 1. Schematic of the proposed MWBTFFL.
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Fig. 2. (a) Multi-wavelength output spectrum with BP, even Stokes and anti-Stokes lines against different pump powers, and (b) Multi-wavelength output spectrum with BP, even Stokes and anti-Stokes lines at pump power of 250 mW.
lines being formed at a BP wavelength of 1503.6 nm. At this BP wavelength also, 3 distinct anti-Stokes lines are seen, with more antiStokes lines being generated as well. However, as the BP wavelength continues to increase, the number of distinct Stokes and anti-Stokes lines being generated begins to reduce again. This is attributed to the BP wavelength moving away from its optimal amplification region, and thus reducing in power and no longer being able to overcome the threshold power required to generate the Stokes lines. This however can be overcome by increasing the BP power, the pump power to the TFF or both simultaneously, which will result in the gain peak being made higher and also widening the effective gain. This allows for more Stokes lines to be generated. As shown in Fig. 5, the spacing between the BP and 2nd Stokes constitutes a double Brillouin frequency, due to the emission of 1st Stokes line in the detection system [24]. However, this spacing can be converted to a single Brillouin frequency spacing through the use of the micro air-gap technique generated as demonstrated in Ref. [25]. In this approach, an air gap about 15 µm wide is incorporated between the BGM and the coupler to induce a strong Fresnel reflection effect. The light oscillating through the micro-air gap is forced to propagate in both the clockwise and anti-clockwise directions due to Fresnel reflection. Thus, both the odd and even Stokes are able to oscillate constructively and form single spacing Brillouin spectrum as shown in Fig. 5. From here, it can be seen that with the incorporation of the micro-air gap, the spacing between each peak wavelength reduces from 0.17 nm to about 0.085 nm, resulting in 6 high powered Stokes lines
Fig. 3. Stability of MWBTFFL in an hour time span.
Fig. 4. Tunability of MWBTFFL from 1470 nm to 1515 nm.
Fig. 5. Single-spaced and double-spaced Brillouin output spectrum using micro air-gap.
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now being observed instead of just 3 originally. The same observation can be made for other Stokes lines and also anti-Stokes lines. The inclusion of the micro air-gap does however incur a minor loss of power, which is estimated to be approximately 1.1 dB. The primary advantage of the proposed setup of this work is the ability to generate the Brillouin output both easily and uniformly over a broad tuning range that almost covers the entire S-band region as well as the S+ region. Typically, Brillouin based multi-wavelength outputs in the S-band region could only be realized using depressed cladding EDF (DC-EDF) [26,27] or Brillouin-Raman co-pumping [28,29]. However, both these approaches have certain limitations; the use of the DC-EDF, while able to suppress the gain medium emission to the Sband region [26], suffers from a relatively narrow gain profile, causing non-uniform Stokes peak intensity. Brillouin Raman co-pumping on the other hand has generally inferior performance to that obtained when using a rare-earth gain medium based fiber laser. In addition, the stimulated Raman scattering mechanism results in emissions at frequency downshifted by 13.2 THz [28] from the pump wavelength. Furthermore, both configurations cannot be readily tuned to different wavelengths, particularly when using the DC-EDF in the cavity due to the narrow gain profile emitted. Thus, the proposed system in this work possess a significant advantage in terms of admirable performance in the S/S+ band region and also enabling the tunability of the lasing wavelength outputs. Furthermore, changing the gain medium will allow for similar outputs to be realized at the C- and L-band regions, thus expanding the scope of operation for the proposed laser [30]. The tunable range and number of Stokes lines can be tuned by increasing the laser diode pump power to the TFF. As the pumping power is increased, the gain of the TFF will also increase in tandem, allowing more Stokes and anti-Stokes lines to be generated. Having a longer length TFF at a higher pump power will eventually provide more Stokes lines as well. As such, currently, the present TFF was driven hard enough because of the uncertainty of the fluoride fiber at higher pump powers, which can damage the TFF.
[2]
[3] [4] [5] [6] [7] [8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
[18]
4. Conclusion
[19]
A MWBTFFL using a 14.5 m long TFF and 6.9 km DSF is proposed and demonstrated. The proposed system utilizes the TFF as the S-band linear gain medium, while the DSF serves as the non-linear gain medium to induce the SBS effect in the system, thus generating the desired multi-wavelength output. The system has an SBS threshold of 120 mW, and with a BP of 1503.6 nm at 12 dBm and a maximum pump power of 250 mW, 3 distinct even Stokes lines are obtained. More even Stokes and anti-Stokes lines are also observed, though lacking the distinction of the first three even Stokes lines. The laser's output can be tuned from 1470 nm to 1515 nm by adjusting the BP wavelength, and by incorporating a 15 µm air-gap into the laser cavity, the double-spaced Brillouin output can be made to revert to a singlespaced Brillouin comb by exploiting Fresnel reflection at the air gap. This makes the proposed system highly useful for easy switching between applications requiring dense and less dense wavelength combs.
[20]
[21]
[22]
[23] [24]
[25]
[26] [27] [28]
Acknowledgement [29]
Funding for this work was provided by the University of Malaya under the grants RU 010/2016, and RP 029A – 15 AFR as well as the Ministry of Higher Education, Malaysia under the grants HICOE 2016 and GA 010 - 2014 (ULUNG).
[30]
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