Photonic crystal fiber based dual-wavelength Q-switched fiber laser using graphene oxide as a saturable absorber H. Ahmad,* M. R. K. Soltanian, C. H. Pua, M. Alimadad, and S. W. Harun Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia *Corresponding author: [email protected] Received 12 February 2014; revised 10 April 2014; accepted 30 April 2014; posted 2 May 2014 (Doc. ID 206372); published 30 May 2014
A Q-switched dual-wavelength fiber laser with narrow channel spacing is proposed and demonstrated. The fiber laser is built around a 3 m long erbium doped fiber as the gain medium and a 10 cm long photonic crystal fiber (PCF) as the element used to generate the dual-wavelength output. The PCF has a solid core approximately 4.37 μm in diameter and is surrounded by microscopic air-holes with a diameter of about 5.06 μm each as well as a zero-dispersion wavelength of about 980 nm. A graphene oxide based saturable absorber is used to generate the desired pulsed output. At the maximum pump power of 72 mW the laser is capable of generating pulses with a repetition rate and pulse-width of 31.0 kHz and 7.0 μs, respectively, as well as an average output power and pulse energy of 0.086 mW and 2.8 nJ, respectively. The proposed fiber laser has substantial potential for use in applications that require longer duration pulsed outputs such as in range finding and terahertz radiation generation. © 2014 Optical Society of America OCIS codes: (060.2410) Fibers, erbium; (060.7140) Ultrafast processes in fibers; (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched; (140.3560) Lasers, ring; (320.7090) Ultrafast lasers. http://dx.doi.org/10.1364/AO.53.003581
1. Introduction
Fiber-based pulse lasers have recently seen widespread use in a multitude of applications including laser ablation, optical telecommunications, sensing and spectroscopy [1–4]. Fiber-based laser sources offer substantial advantages as they are significantly less complex and thus less costly than their bulky optic counterparts. Furthermore, optical-fiber based devices are used for the development of compact lasers that are highly robust and suitable for field work because they eliminate the need for delicate optical alignments. Typically, pulsed fiber lasers are either modelocked or Q-switched. Mode-locked outputs are preferred in scenarios where ultra-fast pulses are desired although this comes at the cost of the pulse 1559-128X/14/163581-06$15.00/0 © 2014 Optical Society of America
power. Q-switched fiber lasers, on the other hand, provide slower pulses but have the advantage of a higher energy per pulse. This type of output is better in applications requiring high-energy outputs such as material processing, or where longer pulses would be more suitable such as in laser range-finding applications. Q-switching can be achieved by either active or passive modulation, although passively Q-switched fiber lasers are inherently more attractive as they can provide the desired Q-switched output without the need for bulky components. Traditionally, passive Q-switching in a fiber laser was accomplished by using exotic crystals such as Cr2 :ZnSe to act as saturable absorbers (SAs) or by employing saturable semiconductor absorber mirrors (SESAMs) in the laser cavity. However, the bulk and complexity of these methods mitigated the benefit of a compact and low-cost system that could be realized by optical fibers. Cr2 :ZnSe crystals required additional components to be integrated into 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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the fiber-laser cavity [5], while SESAMs are expensive and difficult to fabricate [6]. Recent advances however have provided a viable alternative to crystal based SAs and SESAMs in the form of carbon-based SAs. These SAs are formed by using carbon allotropes in the form single-walled carbon nanotubes (SWCNTs) or graphene. SWCNTs enjoy the inherent advantages of high compatibility with optical fibers as well as low saturation intensities, sub-picosecond recovery times, and wide-operating bandwidths [7,8]. Graphene, on the other hand, offers the aforementioned advantages along with a broader operating wavelength due to its gapless band structure, although the graphene based SA can be somewhat difficult to fabricate and also has a low damage threshold. In this regard, graphene oxide (GO) provides a good alternative to graphene, possessing all the characteristics of graphene while at the same time being substantially easier to fabricate and also having a higher damage threshold [9–12]. One of the key advantages of these carbon based SAs is that they allow the development of compact, dual-wavelength fiber laser (DWFL). DWFL has high potential for use in applications encompassing ranging, communications and interferometry [13–15]. The use of a DWFL with a narrow enough channel spacing such as it can be used to generate microwave and terahertz radiation would be of particular interest however. This is because the traditional approaches of optical beat sources such as two tunable distributed feedback fiber lasers or distributed Bragg reflector laser diodes raise issues on the stability of each frequency [16]. As such, a stable dual-wavelength signal such as that generated from a fiber laser will have a significant advantage for microwave and terahertz applications. There have also been earlier reports that use photonics crystal fibers (PCFs) as a medium for generating a dual-wavelength output, such as that demonstrated by Sierra-Hernandez et al. [17] and Chen et al. [18]. In this paper we propose, for the first time to our knowledge, a novel approach of using a very short PCF, approximately 10 cm long, as a means to generate a dual-wavelength output. The output is made to pulse by incorporating a GO based SA into the laser cavity, which generates the desired pulsed output while still maintaining the compact form of the system. The proposed laser is also tunable, allowing a continuous tuning between the two wavelengths. The proposed DWFL will have significant potential as a seeder pulse for pulsed terahertz generation.
the ferrule, which was prepared with a very thin layer of index matching gel (IMG) on its surface. The ferrule with the GO layer is then connected to an FC/PC adaptor, and a fiber ferrule with a clean surface is then connected to this adaptor, thereby sandwiching the GO layer in between the two ferrules and thus creating the required SA. GO has certain advantages in terms of its mechanical properties, as reported in [19,20]. GO is a low-cost and highly attractive alternative to conventional graphene [21]. As it is a precursor for pristine graphene, GO exhibits the same, and in some cases even comparable optical properties of graphene [22,23]. The primary advantage of GO, however, is its ease of fabrication since it can be easily fabricated from graphite, an inexpensive and easily available raw material. Furthermore, GO is also hydrophilic, thus fulfilling the important criterion of stable aqueous colloids for the easy assembly of macroscopic structures. Figure 1 shows the microscopic image of the GO layer on the face of the fiber ferrule. It can be seen from the figure that the GO layer is placed in such a way that it falls on the center of the fiber ferrule, thereby allowing it to completely envelope the propagating beam from the fiber. The GO layer is approximately 1200 μm across its longest length and is kept in place once the other fiber ferrule has been connected to the FC/PC adaptor. This forms the GO-based SA (GOSA) assembly. Figure 2 shows the spectral analysis of the GO paper deposited on the face of the fiber ferrule. The Raman spectroscopy was carried out at a laser output of 50 mW at 532 nm for a period of 10 s. The obtained output spectrum clearly indicates the presence of two peaks at 1353.00 and 1585.35 cm−1 , respectively, and corresponds to D and G peaks of an approximate similar value as reported in [24]. The D intensity is approximately 5000 arbitrary units, while the G intensity is about 5500 arbitrary units. The relatively high D intensity indicates that the edges of the sample form an armchair structure. On the other hand, a low D value would have indicated a predominantly zig-zag edge structure [25]. The relative intensity ratio of both D and G peaks (ID/IG) is computed to be 0.8971, which approximates closely that obtained in [26]. The higher ID/IG indicates that there is a higher level of disorder arising from the formation of covalent
2. Graphene Oxide as Saturable Absorber
The GO based SA used in this work is fabricated by depositing a GO layer on the face of a fiber ferrule. The GO is obtained from Graphene Supermarket Ltd. in the form of a GO paper with each disk having an average diameter of 4.0 cm and a thickness of about 10 μm. For the purpose of this experiment a small piece of the GO paper, about 1 mm × 1 mm in area, is cut from the GO paper and is placed onto 3582
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Fig. 1. Microscopic image of GO layer surrounded by IMG.
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bonds in the GO layer as a result of the functionalizing material used. A small peak is also observed at 2329.00 cm−1 , which is possibly due to the presence of minor amounts of N2 contaminants present during the characterization process. 3. Experimental Setup
The experimental setup of the proposed narrowly spaced dual-wavelength Q-switched fiber laser is shown in Fig. 3. The laser uses a 3 m long erbium doped fiber (EDF) as the active gain medium, which has peak absorption of 18.06 dB∕m in the wavelength of 980 nm. The EDF is pumped by a Lumics 980 nm laser diode through a wavelength-division multiplexer (WDM). One end of the EDF is connected to the common output of the WDM, while the other end is connected to an isolator to ensure no signals propagate in the opposite direction through the EDF. The isolator is in turn connected to a tunable bandpass filter (TBF), which is used as a tuning element to isolate a single wavelength from the dual-
wavelength output. The TBF has a bandwidth of 0.8 nm and a tuning resolution of 0.05 nm. The output of the TBF is connected to a polarization controller (PC), and subsequently to a PCF with a zero-dispersion wavelength at about 1500 nm and an overall diameter of 124 μm as well as a length of 10 cm. The PCF is held in place by a pair of bare fiber holders, which imposed a total loss of 5 dBm, and are used to generate the desired stable and narrow spacing dual-wavelength output by polarization dependence loss (PDL) and fringe spacing effects. The cross-section of the PCF is shown in the inset of the Fig. 3 and it can be seen that the core structure consists of a solid core approximately 4.37 μm diameter surrounded by microscopic air holes, each with an approximate diameter of 5.06 μm and a separation of 5.52 μm between neighboring air holes. The output of the PCF is guided toward a 90∶10 coupler, which extracts a portion of the signal for analysis. The 90% port is connected to the GOSA, which is then connected to the signal port (1550 nm port) of the WDM. This completes the laser cavity. The extracted output is divided into two even powered portions using a 3 dB coupler, with one portion being directed to a high-resolution optical spectrum analyzer (APEX AP2051A) (OSA), while the other spectrum is analyzed separately to obtain the average output power, the spectrum in the time domain and radio frequency spectrum. The dual-wavelength output is obtained by the incorporation of the PCF and PC into the cavity, which results in the dispersion of the oscillating modes in the cavity along one of two possible polarization states, with each state being perpendicular to the other. This will result in the polarization hole burning effect in the EDF, which results in lower mode competition and as a result enhancing dual-wavelength generation. Furthermore, the six-folded PCF will doubly degenerate the fundamental modes leading to strong suppression of those modes when coupling back to the single mode fiber (SMF) [27]. This will further filter out unwanted longitudinal modes until only a few modes propagate in the cavity. Fine-tuning of the PC now allows a dual-wavelength output to be generated. This mechanism has been explained in [28]. 4. Results and Discussion
Fig. 3. Experimental setup of the narrowly spaced dualwavelength Q-switched fiber laser. Inset is the cross-section image of the PCF.
Figure 4(a) shows the dual-wavelength output without the GOSA in the cavity, operating in a continuous wavelength (CW) mode. This operating regime is obtained at a pump power of 66.0 mW, with a launch power into the cavity of about 52.4 mW. The spectrum was monitored using the APEX OSA at the highest resolution of 0.16 pm with a span of 0.164 nm. The desired dual-wavelength output is obtained by carefully adjusting the PC so as to generate a very stable output. A typical result of this dual-wavelength output is shown in the figure, with a spacing of about 0.028 nm between the two wavelengths of 1551.845 and 1551.873 nm. The spacing can be altered or tuned by adjusting the TBF together with the PC. The 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 5. Stability of the system with channel spacing of 170 pm.
Fig. 4. Dual-wavelength laser spectra captured using a highresolution OSA with a resolution of 0.16 pm at (a) CW and (b) Qswitching with GOSA assembly is employed in the laser cavity.
wavelengths are stable and relatively clean, with a high signal-to-noise (SNR) ratio of 53.7 dB. The 3 dB linewidth of the output is about 0.6 pm. Figure 4(b) on the other hand, shows the lasing output of the Q-switched dual-wavelength output, which is obtained by having the GOSA in the cavity. Unlike the spectrum obtained previously, the inclusion of the GOSA generates spikes as observed in the wavelength spectral scan of Fig. 4(b). Typically, a lower resolution OSA, such as one with a resolution of 0.02 or 0.03 nm, would show a cleaner spectrum with less spikes. However, the high resolution of 0.16 pm allows for a substantially more detailed spectrum to be observed, and it is thus to be seen that rather than two output wavelengths with broadened linewidths, the Q-switched laser output in reality consists of a series of spikes around the central wavelength region in the wavelength domain (or frequency domain). Analysis of the output in Fig. 4(b) gives an average wavelength spacing of 0.032 nm, as well as a slightly 3584
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lower SNR of about 49.6 dB, as obtained from the two highest peaks of the spectrum. Overall, the output generated by the system is stable, with little fluctuation in power. For the output generated by the system, a narrow channel output as shown in Fig. 5 is obtained. It can be seen that the output, which has a wavelength spacing of 170 pm between two wavelengths at 1553.85 and 1554.02 nm is highly stable, and shows almost no fluctuation in wavelength or power over a test period of 22 min. Figure 6 shows the pulse train of the generated pulses from the Q-switched laser. As can be seen in Fig. 6, the pulses obtained are relatively stable, with a pulse repetition rate of approximately 27.7 kHz and with average peaks of 3.7 mV. The operating parameters remain unchanged, with the pump power before entering the laser cavity kept constant at 66.0 mW. The pulses are obtained by connecting the open port of the 3 dB coupler to a Thorlabs MOBEL D40FC InGaAs photodetector, which is in turn connected to a LeCroy 500 MHz digital oscilloscope (OSC). Figures 7(a) and 7(b) provide a detailed analysis on a single pulse, which has been extracted from the pulse train. Analysis of a single pulse peak, as shown in Fig. 7(a), reveals a pulse with an average pulse width of 8.4 μs. Figure 7(b) on the other hand, shows an overlap of six consecutive pulses extracted from the same pulse train, approximately 10 s apart. From Fig. 7(b) it can be seen that the pulse widths of the various pulses are stable and consistent at 8.4 μs. Minor fluctuations in the amplitudes of the pulses are observed, with a
Fig. 6. Pulse train of the dual-wavelength Q-switched fiber laser pumped at 66 mW measured using digital oscilloscope.
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power. It can be seen that both the pulse energy and average output power increase as the pump power is raised, with the average output power rising from 0.037 to 0.086 mW as the pump power increases from 54.0 to 72.0 mW. The increase is almost linear at a uniform rate of 0.0027. The pulse energy on the other hand, initially increases at an almost linear rate, rising from 1.7 to 2.6 nJ as the pump power increases from 54 to 66 mW. However, above 66 mW, the rate of increase in the pulse energy becomes shallower,
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deviation of about 0.5 mV from the average amplitude of 3.7 mV. Analysis of the Q-switched pulses is also undertaken by replacing the photodiode and OSC with a (brand) radio frequency spectrum analyzer. Figure 8 shows the first harmonic of the pulse train, which is taken at a resolution of 300 Hz and video bandwidth of 1 MHz. An SNR of about 37 dB is obtained, indicating that the pulse is stable. Figure 9(a) shows the repetition rate and pulse width of the Q-switched DWFL taken at increasing pump powers. The Q-switching threshold of the laser is determined to be approximately 54 mW. The repetition rate behaves as expected for a Q-switched fiber laser, rising linearly at the threshold power to a maximum repetition rate of 31.0 kHz at a pump power of 72 mW. It is important to note that the maximum pump power here is the limitation of the 980 nm LD pump used. An LD with a higher pump would generate a larger repetition rate. The pulse width on the other hand, decreases as the pump power is increased, from 13.2 μs at the threshold power until approximately 7.0 μs at a pump power of 69 mW, after which the pulse width begins to exhibit signs of saturation. Figure 9(b) shows the pulse energy and average output power of the pulses as a function of the pump
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Fig. 7. (a) Single pulse as obtained from pulse train. (b) Overlap of six consecutive pulses from the same pulse train with each pulse taken 10 s apart.
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Fig. 9. (a) Repetition rate and pulse width curves at different pump powers. (b) Pulse energy and average output power curves at different pump powers. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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rising from 2.7 to only 2.8 nJ as the pump power increases from 69 to 72 mW. The stability of the dual-wavelength output and the narrow channel spacing that can be obtained make the proposed DWFL highly suitable for a multitude of applications, in particular, those related to sensing microwave and terahertz generation. 5. Conclusion
In this work a Q-switched fiber laser with a narrowly spaced dual-wavelength output is proposed and demonstrated. The dual-wavelength output is generated using a short, 10 cm long PCF to overcome the modecompetition in the EDF gain medium. The PCF has a zero-dispersion wavelength of about 1500 nm and a solid core approximately 4.37 μm in diameter surrounded by microscopic air-holes with an approximate diameter of 5.06 μm each and separation of 5.52 μm. The pulsed output is obtained using a GO based SA, whereby a GO paper with an average diameter of 4.0 cm and a thickness of about 10 μm is sandwiched between two fiber ferrules. The channel spacing of the output can be tuned, with each pulse having an average 3 dB line width of 0.6 pm. At the maximum, pump-power pulses with a repetition rate, pulse-width, average output power, and pulse energy of 31.0 kHz, 7.0 μs, 0.086 mW, and 2.8 nJ, respectively, are obtained. The proposed DWFL has high potential for applications such as sensing microwave and terahertz radiation generation. The financial support is from University Malaya/ MOHE under grant number UM.C/HIR/MOHE/SC/ 01 and PGRS. References 1. N. M. Fried and K. E. Murray, “High-power thulium fiber laser ablation of urinary tissues at 1.94 μm,” J. Endourol. 19, 25–31 (2005). 2. A. R. Grant, D. P. Holcomb, and T. H. Wood, “Pulsed Yb fiber laser for underwater communications,” in Applications of Lasers for Sensing and Free Space Communications (Optical Society of America, 2011). 3. N. Nishizawa, “Wideband ultra-short pulse fiber lasers and their sensing applications,” in Optical Sensors (Optical Society of America, 2010). 4. R. J. De Young and N. P. Barnes, “Profiling atmospheric water vapor using a fiber laser lidar system,” Appl. Opt. 49, 562–567 (2010). 5. Y. Tang, Y. Yang, J. Xu, and Y. Hang, “Passive Q-switching of short-length Tm3+-doped silica fiber lasers by polycrystalline Cr2+:ZnSe microchips,” Opt. Commun. 281, 5588–5591 (2008). 6. R. Koskinen, S. Suomalainen, J. Paajaste, S. Kivistö, M. Guina, O. Okhotnikov, and M. Pessa, “Highly nonlinear GaSb-based saturable absorber mirrors,” Proc. SPIE 73540, 73540G (2009). 7. D.-P. Zhou, L. Wei, B. Dong, and W.-K. Liu, “Tunable passively Q-switched erbium-doped fiber laser with carbon nanotubes as a saturable absorber,” IEEE Photon. Technol. Lett. 22, 9–11 (2010). 8. B. Dong, J. Hao, J. Hu, and C.-y. Liaw, “Short linear-cavity Qswitched fiber laser with a compact short carbon nanotube based saturable absorber,” Opt. Fiber Technol. 17, 105–107 (2011). 9. C. Liu, C. Ye, Z. Luo, H. Cheng, D. Wu, Y. Zheng, Z. Liu, and B. Qu, “High-energy passively Q-switched 2 μm Tm3+-doped double-clad fiber laser using graphene-oxide-deposited fiber taper,” Opt. Express 21, 204–209 (2013). 3586
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10. Z. Luo, M. Zhou, J. Weng, G. Huang, H. Xu, C. Ye, and Z. Cai, “Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser,” Opt. Lett. 35, 3709–3711 (2010). 11. J. Zhao, Y. Wang, S. Ruan, P. Yan, H. Zhang, Y. H. Tsang, J. Yang, and G. Huang, “Three operation regimes with an Lband ultrafast fiber laser passively mode-locked by graphene oxide saturable absorber,” J. Opt. Soc. Am. B 31, 716–722 (2014). 12. Z. Jun-Qing, W. Yong-Gang, Y. Pei-Guang, R. Shuang-Chen, C. Jian-Qun, D. Ge-Guo, Y. Yong-Qin, Z. Ge-Lin, W. Hui-Feng, L. Jie, and Y. H. Tsang, “Graphene-oxide-based Q-switched fiber laser with stable five-wavelength operation,” Chin. Phys. Lett. 29, 114206 (2012). 13. J. Yu, M.-F. Huang, Z. Jia, T. Wang, and G.-K. Chang, “A novel scheme to generate single-sideband millimeter-wave signals by using low-frequency local oscillator signal,” IEEE Photon. Technol. Lett. 20, 478–480 (2008). 14. Y. Yao, X. Chen, Y. Dai, and S. Xie, “Dual-wavelength erbium-doped fiber laser with a simple linear cavity and its application in microwave generation,” IEEE Photon. Technol. Lett. 18, 187–189 (2006). 15. D. Liu, N. Ngo, G. Ning, P. Shum, and S. Tjin, “Tunable microwave photonic notch filter using a dual-wavelength fiber laser with phase modulation,” Opt. Commun. 266, 240–248 (2006). 16. M. Y. Jeon, N. Kim, J. Shin, C. W. Lee, S.-P. Han, Y. A. Leem, D.-S. Yee, S. K. Noh, and K. H. Park, “Continuous terahertz wave emission using tunable dual-wavelength erbium-doped fiber laser,” in Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on Infrared, Millimeter, and Terahertz Waves (IEEE, 2010), pp. 1–2. 17. J. Sierra-Hernandez, R. Rojas-Laguna, E. Vargas-Rodriguez, J. Estudillo-Ayala, R. Mata-Chavez, D. Jauregui-Vazquez, J. Hernandez-Garcia, J. Andrade-Lucio, and J. GutierrezGutierrez, “A tunable multi-wavelength laser based on a Mach–Zehnder interferometer with photonic crystal fiber,” Laser Phys. 23, 055105 (2013). 18. W. Chen, S. Lou, L. Wang, H. Zou, W. Lu, and S. Jian, “Switchable multi-wavelength fiber ring laser using a side-leakage photonic crystal fiber based filter,” Opt. Laser Technol. 44, 611–616 (2012). 19. D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. Dommett, G. Evmenenko, S. T. Nguyen, and R. S. Ruoff, “Preparation and characterization of graphene oxide paper,” Nature 448, 457–460 (2007). 20. S. Park, K.-S. Lee, G. Bozoklu, W. Cai, S. T. Nguyen, and R. S. Ruoff, “Graphene oxide papers modified by divalent ions— enhancing mechanical properties via chemical cross-linking,” ACS Nano 2, 572–578 (2008). 21. H. Ahmad, A. Z. Zulkifli, Y. Y. Kiat, and S. W. Harun, “Qswitched fibre laser using 21 cm Bismuth-erbium doped fibre and graphene oxide as saturable absorber,” Opt. Commun. 310, 53–57 (2014). 22. X. Zhao, Z.-B. Liu, W.-B. Yan, Y. Wu, X.-L. Zhang, Y. Chen, and J.-G. Tian, “Ultrafast carrier dynamics and saturable absorption of solution-processable few-layered graphene oxide,” Appl. Phys. Lett. 98, 121905 (2011). 23. J. Xu, J. Liu, S. Wu, Q.-H. Yang, and P. Wang, “Graphene oxide mode-locked femtosecond erbium-doped fiber lasers,” Opt. Express 20, 15474–15480 (2012). 24. K. Haubner, J. Murawski, P. Olk, L. M. Eng, C. Ziegler, B. Adolphi, and E. Jaehne, “The route to functional graphene oxide,” Chem. Phys. Chem. 11, 2131–2139 (2010). 25. C. Casiraghi, A. Hartschuh, H. Qian, S. Piscanec, C. Georgi, A. Fasoli, K. Novoselov, D. Basko, and A. Ferrari, “Raman spectroscopy of graphene edges,” Nano Lett. 9, 1433–1441 (2009). 26. L. Chang, S. Wu, S. Chen, and X. Li, “Preparation of graphene oxide–molecularly imprinted polymer composites via atom transfer radical polymerization,” J. Mater. Sci. 46, 2024– 2029 (2011). 27. S. K. Bhadra and A. K. Ghatak, Guided Wave Optics and Photonic Devices (CRC Press, 2013). 28. W. Chen, S. Lou, S. Feng, L. Wang, H. Li, T. Guo, and S. Jian, “Switchable multi-wavelength fiber ring laser based on a compact in-fiber Mach-Zehnder interferometer with photonic crystal fiber,” Laser Phys. 19, 2115–2119 (2009).
A Q-switched dual-wavelength fiber laser with narrow channel spacing is proposed and demonstrated. The fiber laser is built around a 3 m long erbium doped fiber as the gain medium and a 10 cm long photonic crystal fiber (PCF) as the element used to generate the dual-wavelength output. The PCF has a solid core approximately 4.37 μm in diameter and is surrounded by microscopic air-holes with a diameter of about 5.06 μm each as well as a zero-dispersion wavelength of about 980 nm. A graphene oxide based saturable absorber is used to generate the desired pulsed output. At the maximum pump power of 72 mW the laser is capable of generating pulses with a repetition rate and pulse-width of 31.0 kHz and 7.0 μs, respectively, as well as an average output power and pulse energy of 0.086 mW and 2.8 nJ, respectively. The proposed fiber laser has substantial potential for use in applications that require longer duration pulsed outputs such as in range finding and terahertz radiation generation. © 2014 Optical Society of America OCIS codes: (060.2410) Fibers, erbium; (060.7140) Ultrafast processes in fibers; (140.3510) Lasers, fiber; (140.3540) Lasers, Q-switched; (140.3560) Lasers, ring; (320.7090) Ultrafast lasers. http://dx.doi.org/10.1364/AO.53.003581
1. Introduction
Fiber-based pulse lasers have recently seen widespread use in a multitude of applications including laser ablation, optical telecommunications, sensing and spectroscopy [1–4]. Fiber-based laser sources offer substantial advantages as they are significantly less complex and thus less costly than their bulky optic counterparts. Furthermore, optical-fiber based devices are used for the development of compact lasers that are highly robust and suitable for field work because they eliminate the need for delicate optical alignments. Typically, pulsed fiber lasers are either modelocked or Q-switched. Mode-locked outputs are preferred in scenarios where ultra-fast pulses are desired although this comes at the cost of the pulse 1559-128X/14/163581-06$15.00/0 © 2014 Optical Society of America
power. Q-switched fiber lasers, on the other hand, provide slower pulses but have the advantage of a higher energy per pulse. This type of output is better in applications requiring high-energy outputs such as material processing, or where longer pulses would be more suitable such as in laser range-finding applications. Q-switching can be achieved by either active or passive modulation, although passively Q-switched fiber lasers are inherently more attractive as they can provide the desired Q-switched output without the need for bulky components. Traditionally, passive Q-switching in a fiber laser was accomplished by using exotic crystals such as Cr2 :ZnSe to act as saturable absorbers (SAs) or by employing saturable semiconductor absorber mirrors (SESAMs) in the laser cavity. However, the bulk and complexity of these methods mitigated the benefit of a compact and low-cost system that could be realized by optical fibers. Cr2 :ZnSe crystals required additional components to be integrated into 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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the fiber-laser cavity [5], while SESAMs are expensive and difficult to fabricate [6]. Recent advances however have provided a viable alternative to crystal based SAs and SESAMs in the form of carbon-based SAs. These SAs are formed by using carbon allotropes in the form single-walled carbon nanotubes (SWCNTs) or graphene. SWCNTs enjoy the inherent advantages of high compatibility with optical fibers as well as low saturation intensities, sub-picosecond recovery times, and wide-operating bandwidths [7,8]. Graphene, on the other hand, offers the aforementioned advantages along with a broader operating wavelength due to its gapless band structure, although the graphene based SA can be somewhat difficult to fabricate and also has a low damage threshold. In this regard, graphene oxide (GO) provides a good alternative to graphene, possessing all the characteristics of graphene while at the same time being substantially easier to fabricate and also having a higher damage threshold [9–12]. One of the key advantages of these carbon based SAs is that they allow the development of compact, dual-wavelength fiber laser (DWFL). DWFL has high potential for use in applications encompassing ranging, communications and interferometry [13–15]. The use of a DWFL with a narrow enough channel spacing such as it can be used to generate microwave and terahertz radiation would be of particular interest however. This is because the traditional approaches of optical beat sources such as two tunable distributed feedback fiber lasers or distributed Bragg reflector laser diodes raise issues on the stability of each frequency [16]. As such, a stable dual-wavelength signal such as that generated from a fiber laser will have a significant advantage for microwave and terahertz applications. There have also been earlier reports that use photonics crystal fibers (PCFs) as a medium for generating a dual-wavelength output, such as that demonstrated by Sierra-Hernandez et al. [17] and Chen et al. [18]. In this paper we propose, for the first time to our knowledge, a novel approach of using a very short PCF, approximately 10 cm long, as a means to generate a dual-wavelength output. The output is made to pulse by incorporating a GO based SA into the laser cavity, which generates the desired pulsed output while still maintaining the compact form of the system. The proposed laser is also tunable, allowing a continuous tuning between the two wavelengths. The proposed DWFL will have significant potential as a seeder pulse for pulsed terahertz generation.
the ferrule, which was prepared with a very thin layer of index matching gel (IMG) on its surface. The ferrule with the GO layer is then connected to an FC/PC adaptor, and a fiber ferrule with a clean surface is then connected to this adaptor, thereby sandwiching the GO layer in between the two ferrules and thus creating the required SA. GO has certain advantages in terms of its mechanical properties, as reported in [19,20]. GO is a low-cost and highly attractive alternative to conventional graphene [21]. As it is a precursor for pristine graphene, GO exhibits the same, and in some cases even comparable optical properties of graphene [22,23]. The primary advantage of GO, however, is its ease of fabrication since it can be easily fabricated from graphite, an inexpensive and easily available raw material. Furthermore, GO is also hydrophilic, thus fulfilling the important criterion of stable aqueous colloids for the easy assembly of macroscopic structures. Figure 1 shows the microscopic image of the GO layer on the face of the fiber ferrule. It can be seen from the figure that the GO layer is placed in such a way that it falls on the center of the fiber ferrule, thereby allowing it to completely envelope the propagating beam from the fiber. The GO layer is approximately 1200 μm across its longest length and is kept in place once the other fiber ferrule has been connected to the FC/PC adaptor. This forms the GO-based SA (GOSA) assembly. Figure 2 shows the spectral analysis of the GO paper deposited on the face of the fiber ferrule. The Raman spectroscopy was carried out at a laser output of 50 mW at 532 nm for a period of 10 s. The obtained output spectrum clearly indicates the presence of two peaks at 1353.00 and 1585.35 cm−1 , respectively, and corresponds to D and G peaks of an approximate similar value as reported in [24]. The D intensity is approximately 5000 arbitrary units, while the G intensity is about 5500 arbitrary units. The relatively high D intensity indicates that the edges of the sample form an armchair structure. On the other hand, a low D value would have indicated a predominantly zig-zag edge structure [25]. The relative intensity ratio of both D and G peaks (ID/IG) is computed to be 0.8971, which approximates closely that obtained in [26]. The higher ID/IG indicates that there is a higher level of disorder arising from the formation of covalent
2. Graphene Oxide as Saturable Absorber
The GO based SA used in this work is fabricated by depositing a GO layer on the face of a fiber ferrule. The GO is obtained from Graphene Supermarket Ltd. in the form of a GO paper with each disk having an average diameter of 4.0 cm and a thickness of about 10 μm. For the purpose of this experiment a small piece of the GO paper, about 1 mm × 1 mm in area, is cut from the GO paper and is placed onto 3582
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Fig. 1. Microscopic image of GO layer surrounded by IMG.
G
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Fig. 2. Raman spectrum of GO saturable absorber as on the ferrule of the optical connector.
bonds in the GO layer as a result of the functionalizing material used. A small peak is also observed at 2329.00 cm−1 , which is possibly due to the presence of minor amounts of N2 contaminants present during the characterization process. 3. Experimental Setup
The experimental setup of the proposed narrowly spaced dual-wavelength Q-switched fiber laser is shown in Fig. 3. The laser uses a 3 m long erbium doped fiber (EDF) as the active gain medium, which has peak absorption of 18.06 dB∕m in the wavelength of 980 nm. The EDF is pumped by a Lumics 980 nm laser diode through a wavelength-division multiplexer (WDM). One end of the EDF is connected to the common output of the WDM, while the other end is connected to an isolator to ensure no signals propagate in the opposite direction through the EDF. The isolator is in turn connected to a tunable bandpass filter (TBF), which is used as a tuning element to isolate a single wavelength from the dual-
wavelength output. The TBF has a bandwidth of 0.8 nm and a tuning resolution of 0.05 nm. The output of the TBF is connected to a polarization controller (PC), and subsequently to a PCF with a zero-dispersion wavelength at about 1500 nm and an overall diameter of 124 μm as well as a length of 10 cm. The PCF is held in place by a pair of bare fiber holders, which imposed a total loss of 5 dBm, and are used to generate the desired stable and narrow spacing dual-wavelength output by polarization dependence loss (PDL) and fringe spacing effects. The cross-section of the PCF is shown in the inset of the Fig. 3 and it can be seen that the core structure consists of a solid core approximately 4.37 μm diameter surrounded by microscopic air holes, each with an approximate diameter of 5.06 μm and a separation of 5.52 μm between neighboring air holes. The output of the PCF is guided toward a 90∶10 coupler, which extracts a portion of the signal for analysis. The 90% port is connected to the GOSA, which is then connected to the signal port (1550 nm port) of the WDM. This completes the laser cavity. The extracted output is divided into two even powered portions using a 3 dB coupler, with one portion being directed to a high-resolution optical spectrum analyzer (APEX AP2051A) (OSA), while the other spectrum is analyzed separately to obtain the average output power, the spectrum in the time domain and radio frequency spectrum. The dual-wavelength output is obtained by the incorporation of the PCF and PC into the cavity, which results in the dispersion of the oscillating modes in the cavity along one of two possible polarization states, with each state being perpendicular to the other. This will result in the polarization hole burning effect in the EDF, which results in lower mode competition and as a result enhancing dual-wavelength generation. Furthermore, the six-folded PCF will doubly degenerate the fundamental modes leading to strong suppression of those modes when coupling back to the single mode fiber (SMF) [27]. This will further filter out unwanted longitudinal modes until only a few modes propagate in the cavity. Fine-tuning of the PC now allows a dual-wavelength output to be generated. This mechanism has been explained in [28]. 4. Results and Discussion
Fig. 3. Experimental setup of the narrowly spaced dualwavelength Q-switched fiber laser. Inset is the cross-section image of the PCF.
Figure 4(a) shows the dual-wavelength output without the GOSA in the cavity, operating in a continuous wavelength (CW) mode. This operating regime is obtained at a pump power of 66.0 mW, with a launch power into the cavity of about 52.4 mW. The spectrum was monitored using the APEX OSA at the highest resolution of 0.16 pm with a span of 0.164 nm. The desired dual-wavelength output is obtained by carefully adjusting the PC so as to generate a very stable output. A typical result of this dual-wavelength output is shown in the figure, with a spacing of about 0.028 nm between the two wavelengths of 1551.845 and 1551.873 nm. The spacing can be altered or tuned by adjusting the TBF together with the PC. The 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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Fig. 5. Stability of the system with channel spacing of 170 pm.
Fig. 4. Dual-wavelength laser spectra captured using a highresolution OSA with a resolution of 0.16 pm at (a) CW and (b) Qswitching with GOSA assembly is employed in the laser cavity.
wavelengths are stable and relatively clean, with a high signal-to-noise (SNR) ratio of 53.7 dB. The 3 dB linewidth of the output is about 0.6 pm. Figure 4(b) on the other hand, shows the lasing output of the Q-switched dual-wavelength output, which is obtained by having the GOSA in the cavity. Unlike the spectrum obtained previously, the inclusion of the GOSA generates spikes as observed in the wavelength spectral scan of Fig. 4(b). Typically, a lower resolution OSA, such as one with a resolution of 0.02 or 0.03 nm, would show a cleaner spectrum with less spikes. However, the high resolution of 0.16 pm allows for a substantially more detailed spectrum to be observed, and it is thus to be seen that rather than two output wavelengths with broadened linewidths, the Q-switched laser output in reality consists of a series of spikes around the central wavelength region in the wavelength domain (or frequency domain). Analysis of the output in Fig. 4(b) gives an average wavelength spacing of 0.032 nm, as well as a slightly 3584
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lower SNR of about 49.6 dB, as obtained from the two highest peaks of the spectrum. Overall, the output generated by the system is stable, with little fluctuation in power. For the output generated by the system, a narrow channel output as shown in Fig. 5 is obtained. It can be seen that the output, which has a wavelength spacing of 170 pm between two wavelengths at 1553.85 and 1554.02 nm is highly stable, and shows almost no fluctuation in wavelength or power over a test period of 22 min. Figure 6 shows the pulse train of the generated pulses from the Q-switched laser. As can be seen in Fig. 6, the pulses obtained are relatively stable, with a pulse repetition rate of approximately 27.7 kHz and with average peaks of 3.7 mV. The operating parameters remain unchanged, with the pump power before entering the laser cavity kept constant at 66.0 mW. The pulses are obtained by connecting the open port of the 3 dB coupler to a Thorlabs MOBEL D40FC InGaAs photodetector, which is in turn connected to a LeCroy 500 MHz digital oscilloscope (OSC). Figures 7(a) and 7(b) provide a detailed analysis on a single pulse, which has been extracted from the pulse train. Analysis of a single pulse peak, as shown in Fig. 7(a), reveals a pulse with an average pulse width of 8.4 μs. Figure 7(b) on the other hand, shows an overlap of six consecutive pulses extracted from the same pulse train, approximately 10 s apart. From Fig. 7(b) it can be seen that the pulse widths of the various pulses are stable and consistent at 8.4 μs. Minor fluctuations in the amplitudes of the pulses are observed, with a
Fig. 6. Pulse train of the dual-wavelength Q-switched fiber laser pumped at 66 mW measured using digital oscilloscope.
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Fig. 8. First harmonic radio-frequency spectrum of the Qswitched DWFL.
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power. It can be seen that both the pulse energy and average output power increase as the pump power is raised, with the average output power rising from 0.037 to 0.086 mW as the pump power increases from 54.0 to 72.0 mW. The increase is almost linear at a uniform rate of 0.0027. The pulse energy on the other hand, initially increases at an almost linear rate, rising from 1.7 to 2.6 nJ as the pump power increases from 54 to 66 mW. However, above 66 mW, the rate of increase in the pulse energy becomes shallower,
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deviation of about 0.5 mV from the average amplitude of 3.7 mV. Analysis of the Q-switched pulses is also undertaken by replacing the photodiode and OSC with a (brand) radio frequency spectrum analyzer. Figure 8 shows the first harmonic of the pulse train, which is taken at a resolution of 300 Hz and video bandwidth of 1 MHz. An SNR of about 37 dB is obtained, indicating that the pulse is stable. Figure 9(a) shows the repetition rate and pulse width of the Q-switched DWFL taken at increasing pump powers. The Q-switching threshold of the laser is determined to be approximately 54 mW. The repetition rate behaves as expected for a Q-switched fiber laser, rising linearly at the threshold power to a maximum repetition rate of 31.0 kHz at a pump power of 72 mW. It is important to note that the maximum pump power here is the limitation of the 980 nm LD pump used. An LD with a higher pump would generate a larger repetition rate. The pulse width on the other hand, decreases as the pump power is increased, from 13.2 μs at the threshold power until approximately 7.0 μs at a pump power of 69 mW, after which the pulse width begins to exhibit signs of saturation. Figure 9(b) shows the pulse energy and average output power of the pulses as a function of the pump
Repetition rate
Pulse Width (µs)
Fig. 7. (a) Single pulse as obtained from pulse train. (b) Overlap of six consecutive pulses from the same pulse train with each pulse taken 10 s apart.
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Fig. 9. (a) Repetition rate and pulse width curves at different pump powers. (b) Pulse energy and average output power curves at different pump powers. 1 June 2014 / Vol. 53, No. 16 / APPLIED OPTICS
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rising from 2.7 to only 2.8 nJ as the pump power increases from 69 to 72 mW. The stability of the dual-wavelength output and the narrow channel spacing that can be obtained make the proposed DWFL highly suitable for a multitude of applications, in particular, those related to sensing microwave and terahertz generation. 5. Conclusion
In this work a Q-switched fiber laser with a narrowly spaced dual-wavelength output is proposed and demonstrated. The dual-wavelength output is generated using a short, 10 cm long PCF to overcome the modecompetition in the EDF gain medium. The PCF has a zero-dispersion wavelength of about 1500 nm and a solid core approximately 4.37 μm in diameter surrounded by microscopic air-holes with an approximate diameter of 5.06 μm each and separation of 5.52 μm. The pulsed output is obtained using a GO based SA, whereby a GO paper with an average diameter of 4.0 cm and a thickness of about 10 μm is sandwiched between two fiber ferrules. The channel spacing of the output can be tuned, with each pulse having an average 3 dB line width of 0.6 pm. At the maximum, pump-power pulses with a repetition rate, pulse-width, average output power, and pulse energy of 31.0 kHz, 7.0 μs, 0.086 mW, and 2.8 nJ, respectively, are obtained. The proposed DWFL has high potential for applications such as sensing microwave and terahertz radiation generation. The financial support is from University Malaya/ MOHE under grant number UM.C/HIR/MOHE/SC/ 01 and PGRS. References 1. N. M. Fried and K. E. Murray, “High-power thulium fiber laser ablation of urinary tissues at 1.94 μm,” J. Endourol. 19, 25–31 (2005). 2. A. R. Grant, D. P. Holcomb, and T. H. Wood, “Pulsed Yb fiber laser for underwater communications,” in Applications of Lasers for Sensing and Free Space Communications (Optical Society of America, 2011). 3. N. Nishizawa, “Wideband ultra-short pulse fiber lasers and their sensing applications,” in Optical Sensors (Optical Society of America, 2010). 4. R. J. De Young and N. P. Barnes, “Profiling atmospheric water vapor using a fiber laser lidar system,” Appl. Opt. 49, 562–567 (2010). 5. Y. Tang, Y. Yang, J. Xu, and Y. Hang, “Passive Q-switching of short-length Tm3+-doped silica fiber lasers by polycrystalline Cr2+:ZnSe microchips,” Opt. Commun. 281, 5588–5591 (2008). 6. R. Koskinen, S. Suomalainen, J. Paajaste, S. Kivistö, M. Guina, O. Okhotnikov, and M. Pessa, “Highly nonlinear GaSb-based saturable absorber mirrors,” Proc. SPIE 73540, 73540G (2009). 7. D.-P. Zhou, L. Wei, B. Dong, and W.-K. Liu, “Tunable passively Q-switched erbium-doped fiber laser with carbon nanotubes as a saturable absorber,” IEEE Photon. Technol. Lett. 22, 9–11 (2010). 8. B. Dong, J. Hao, J. Hu, and C.-y. Liaw, “Short linear-cavity Qswitched fiber laser with a compact short carbon nanotube based saturable absorber,” Opt. Fiber Technol. 17, 105–107 (2011). 9. C. Liu, C. Ye, Z. Luo, H. Cheng, D. Wu, Y. Zheng, Z. Liu, and B. Qu, “High-energy passively Q-switched 2 μm Tm3+-doped double-clad fiber laser using graphene-oxide-deposited fiber taper,” Opt. Express 21, 204–209 (2013). 3586
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