⇦Chinese Physics Letters, 2018, Vol. 35, No. 2, Article code 024203 Experimental Observation of Bright and Dark Solitons Mode-Locked with Zirconia-Based Erbium-Doped Fiber Laser A. M. Markom1**, S. J. Tan2, H. Haris3, M. C. Paul4, A. Dhar4, S. Das4, S. W. Harun3 Affiliations 1Faculty of Electrical Engineering, Universiti Teknologi MARA, Masai 81750, Malaysia 2School of Engineering, KDU University College, Utropolis Glenmarie, Shah Alam 40150, Malaysia 3Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia 4Fiber Optics and Photonics Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India Received 28 August 2017 **Corresponding author. Email: arnimunira@uitm.edu.my Citation Text: Markom A M, Tan S J, Haris H, Paul M C and Dhar A et al 2018 Chin. Phys. Lett. 35 024203 Abstract We demonstrate the generation of dark and bright solitons with our homemade zirconia-based erbium-doped fiber and graphene oxide (GO) saturable absorber in anomalous dispersion region. The GO is fabricated using an abridged Hummer's method, which is combined with polyethylene oxide to produce a composite film. The film is sandwiched between two optical ferrules and embedded in the laser cavity to enhance its birefringence and nonlinearity. The self-starting bright soliton is easily generated at pump power of 78 mW with the whole length cavity of 14.7 m. The laser produces the bright pulse train with repetition rate, pulse width, pulse energy and central wavelength being 13.9 MHz, 0.6 ps, 2.74 pJ and 1577.46 nm, respectively. Then, by adding the 10 m of single mode fiber into the laser cavity, dark soliton pulse is produced. For the formation of dark pulse train, the measured repetition rate, pulse width, pulse energy and central wavelength are 8.3 MHz, 20 ns and 4.98 pJ and 1596.82 nm, respectively. Both pulses operate in the anomalous region. DOI:10.1088/0256-307X/35/2/024203 PACS:42.81.Qb, 42.55.Wd, 42.60.Fc © 2018 Chinese Physics Society Article Text To date, numerous works have been demonstrated to generate bright pulse ultrafast fiber lasers.[1-4] It was discovered that fiber lasers do emit dark pulses in addition to bright pulses. Dark pulses are defined to have a deep uniform intensity dip in the intensity of a continuous-wave background of the laser emission. Dark pulses are gaining tremendous attention among researchers as it was numerically demonstrated that they are less sensitive to fiber loss compared with bright pulses. Therefore, it is viewed that the dark pulse is more suitable for long distance optical communication. The first dark soliton was experimentally observed in 1988 by Weiner et al. in optical fibers.[5] Later, the first dark soliton mode-locked generation was formed in the large normal dispersion region in conjunction with a fiber Bragg grating (FBG) as a dark pulse shaper.[6] This is then followed by Wang et al., where they successfully generated a dark soliton with the Kelly sidebands using the nonlinear polarization rotation (NPR) technique.[7] After that, the formation of bright-dark and dark-bright mode-locked fiber laser was demonstrated by other researchers using several techniques such as NPR, coupling effect between two different lasing wavelengths, linear and nonlinear intermodulation effect between wavelengths' pulses and nonlinear amplifying loop mirror in a figure of eight resonator.[8-11] The dynamics of both bright and dark pulses are governed by the nonlinear Schrödinger equation (NLSE). Recently, pulse generation with saturable absorber (SA) has been favorable due to simple fabrication of SA, compact cavity design and high performance of pulsed laser.[12] However, only a few types of SAs were experimentally reported to have the capabilities to produce dark pulses. The reported SAs were carbon nanotubes (CNT), graphene oxide (GO) and molybdenum disulfide (MoS$_{2}$).[13-15] Among all the SAs, graphene oxide saturable absorber (GOSA) is an outstanding material in generating ultrafast fiber lasers due to its unique optical properties, large nonlinearity, easy and faster fabrication process, ultrafast recovery time and strong saturable absorber.[16,17] In addition to the well-known erbium-doped fiber (EDF), other types of fiber such as ytterbium-doped fiber (YDF) and tellurite fiber are also capable of generating dark pulse, but the constructed cavity is rather long, ranging from 200 m to 500 m.[15,18] This is not favorable as the repetition rate from this long length cavity is low and the pulse width is broad and is not suitable for applications such as in optical communication and micromachining. The fast-growing need for compact and ultrafast fiber lasers has evolved towards the exploration new materials to act as host or co-dopants and thus to allow very high active ions dopant concentration. Currently, a unique material of zirconia ions attracts great attention due to its outstanding optical, chemical and physical properties. The high erbium concentration in zirconia-doped fiber allows excellent population inversion which leads to the generation of narrow pulse duration without any detrimental effects such as concentration quenching. It also has a large refractive index in the core of the fiber and thus allowing a broad transmission bandwidth to the fourth transmission window of telecommunications.[19,20] Moreover, it was also demonstrated that zirconia-doped erbium fiber (Zr-EDF) exhibits high gain in the C- and L-band region.[20] The maximum recorded gain is 38 dB, with 1-m-long Zr-EDF, at the input power of $-$30 dBm and the pump power of 130 mW in a double pass amplifier configuration.[20] Therefore, the performance of this short length of Zr-EDF with high gain outweighs the conventional EDF. The nonlinear coefficient of Zr-EDF is estimated to be around 14 W$^{-1}$km$^{-1}$.[15] In addition to Zr-EDF as amplifier, bright soliton mode-locked with zirconia-yttria-alumina co-doped erbium-doped fiber (Zr-Y-Al-EDF) with high repetition rate and short pulse duration by utilizing GOSA was successfully demonstrated.[21] To date, there are only several works demonstrating the generation of dark pulse using GOSA. In the experiment conducted by Lin et al., bright and dark square pulses were generated using GOSA and YDF as the gain medium.[12] The bright and dark square pulses were emitted by changing the pump power within the same cavity and the attained fundamental repetition rate was 415.3 kHz. The reported threshold power to initiate a dark square pulse was 450 mW. In another work demonstrated by Zhao et al., bright and dark pulses were also observed.[13] The recorded threshold power to initiate a dark pulse was 131–141 mW. A bright soliton with the Kelly sidebands pulse was observed, but the optical spectrum for dark pulse was no longer soliton pulse with the Kelly sidebands when the pump power was raised beyond the dark pulse threshold power. In our work, we have successfully demonstrated bright and dark soliton pulses which are operating in L-band region with our homemade Zr-EDF using GOSA. By inserting a spool of 10-m-long single mode fiber (SMF) into the existing laser cavity, dark soliton is produced. For the generation of a bright pulse, the measured repetition rate is 13.9 MHz, pulse width 0.6 ps and pulse energy 2.74 pJ, meanwhile for the dark pulse, the measured repetition rate is 8.3 MHz, pulse width 18.3 ns and pulse energy 4.98 pJ. To the best of our knowledge, this is the first demonstration of dark soliton with the Kelly sidebands utilizing GOSA. The soliton pulse is highly desirable for ultrafast optical communication. Figure 1 illustrates an arrangement of the experimental setup for the soliton mode-locked zirconia-erbium-doped fiber laser (Zr-EDFL) with 3-m-long gain medium of Zr-EDF. The improved Zr-EDF is a new sort of zirconia-yttria-alumina (Zr-Y-Al) co-doped EDF with a large amount of erbium ion concentration doping. This high concentration erbium doping does not possess any detrimental effects that are capable of downgrading the laser performance. The preform based on zirconia-yttria–alumina-phospho silica glass was fabricated with doping level of ZrO$_{2}$ and Al$_{2}$O$_{3}$ around 2 wt% and 3.7 wt% to increase refractive index in the core fiber and thus to enhance the concentration doping of erbium ions. The Zr and Al ions in the core improve the solubility of Er$^{3+}$ in the glass structure and also broaden the transmission wavelength regions. The core and cladding diameters of the fiber are 10.04 μm and 126.83 μm, giving a very small refractive index contrast of 0.012 with standard numerical aperture of 0.17. With this, waveguide loss is minimized and this allows more light traveling in the fiber due to smoothly graded refractive index profiles, which is crucial to increase the achievable gain.
cpl-35-2-024203-fig1.png
Fig. 1. Configuration setup for the proposed mode-locked soliton.
A 980 nm laser diode is used to pump the Zr-EDF and to produce excellent population inversion via 980/1550 nm wavelength division multiplexer (WDM). GOSA based on the fabricated graphene oxide-polyethylene oxide (GO-PEO) thin film is sandwiched in the middle of two fiber ferrules inside a physical contact connector. Firstly, sulfuric acid mixture is prepared by blending 18 g phosphoric acid (H$_{2}$SO$_{4}$: H$_{3}$PO$_{4}$), 80 mL graphite flakes and 18 g potassium permanganate (KMnO$_{4}$) with a magnetic stirrer. Again with magnetic stirrer, 1 g PEO that is equivalent to $1\times10^{6}$ g/mol for average molecular weight is liquefied in 120 mL DI water to prepare the polymer on the top of the hot plate stirrer. Next, using the ultra-sonification method, a different quantity of dispersed GO suspension is blended together with a solution of 1 g PEO in deionized water to fabricate GO-PEO composite. The intensity ratio ($I_{\rm D}/I_{\rm G}$) for GO is 0.85, where the capacity of disorder degree is inversely proportional to the $sp^{2}$ cluster average size. Index-matching gel is deposited between the fiber ferrules to reduce insertion loss. An approximation of 3 dB at 1550 nm is recorded for GOSA insertion loss. A polarization controller (PC) is connected to modify the state of polarization oscillating light and to optimize the output power of laser. It connected to a 90:10 coupler, with the 90% port connected to an optical isolator for ensuring the unidirectional operation of the laser. The red dotted 10-m-long single mode fiber (SMF) is an additional fiber that is inserted into the cavity to produce the dark pulse. The output is extracted via a 10% port of the coupler, which is connected to the optical spectrum analyzer (OSA, AQ6317C) for optical characteristics measurement. After the observation from OSA, the output is then unplugged and connected to an oscilloscope (OSC, WaveRunner 104MXI) with a high-speed photodetector for pulse measurement. The details of cavity dispersion are explained as follows: Zr-EDF used is 3 m in length with $-$56 ps$^{2}$/km GVD coefficient, WDM is 3 m in length, with $-$38 ps$^{2}$/km GVD coefficient while the GVD coefficient for the single mode fiber (SMF) is $-$21 ps$^{2}$/km. The PC, isolator and coupler are made from SMF. The total cavity length is approximately 14.7 m whereas the net dispersion and group delay dispersion (GDD) are 0.35 ps/nm and $-$0.46 ps$^{2}$, respectively. This specifies the cavity operates in anomalous dispersion region.[22] When the 10-m-long SMF is inserted into the existing cavity, the total cavity length lengthens to 24.7 m, the estimated dispersion and GDD are 0.59 ps/nm and $-$0.8 ps$^{2}$, respectively. The net GDD is larger (with the 10-m-long SMF) compared with the earlier calculation (without the 10-m-long SMF).
cpl-35-2-024203-fig2.png
Fig. 2. Optical spectrum of bright soliton Zr-EDFL.
Experiment is carried out without the 10-m-long SMF. Figure 2 displays the optical spectrum of Zr-EDFL with GOSA where its spectrum spreads from 1568 nm to 1586 nm in the L-band region at the maximum pump power of 105.5 mW. Further increasing the input pump power above the 105.5 mW leads to a cw operation and no pulsing effect occurs due to the damage threshold of fabricated GOSA. As the input pump power reduced from the above upper limits to 105.5 mW, the mode-locking pulse quickly builds up, indicating that the SA is not thermally damaged due to high intensity inside the cavity. In addition, the maximum input pump power is limited by 980 nm laser diode specifications and for safety reason it is retained at 267.6 mW. From Fig. 2, the center wavelength is located at 1577.46 nm, with peak power of $-$46.02 dBm. The threshold of pump powers for lasing with and without the insertion of GOSA are about 78 mW and 54.4 mW, respectively. This bright soliton has two pairs of Kelly sidebands; the right sidebands are located at 1583.31 nm and 1585.81 nm whereas the left sidebands are located at 1571.89 nm and 1569.24 nm. The first Kelly sideband is visible because of interaction resonant coupling between dispersive wave and soliton, where it is a signature to the solitonic behavior for the appearance of mode-locking pulsed in the spectrum. Meanwhile, for the outer pair of second-order Kelly sideband is produced due to high intensity that is circulating in Zr-EDF pulsed laser cavity.[23,24] As observed, the 3 dB bandwidth of the bright soliton spectrum is measured at 5.4 nm. As observed, the 3 dB bandwidth of the bright soliton spectrum is measured at 5.4 nm. Figure 3(a) shows the stability behavior mode locked pulse operation. The measured optical-signal-to-noise ratio (OSNR) is derived from the top fundamental to the base extinction as shown in Fig. 3(b) for a single rf pulse. It is approximately 51 dB with the repetition rate of 13.9 MHz, which corresponds to the total length cavity. Figure 3(c) shows the bright pulse train with repetition characteristic measured at 13.9 MHz. Meanwhile, Fig. 3(d) reveals the autocorrelator trace and the pulse width is measured to be 0.6 ps, assuming sech$^{2}$ profile. The time-bandwidth product (TBP) of the pulsed laser is estimated to be 0.48, which is insignificantly deviated from sech$^{2}$-shaped laser pulses of 0.315 limit transform. This behavior means that the soliton pulse is chirped due to the harmonizing between dispersion and nonlinearity in laser arrangement setup. At the maximum pump power of 105.5 mW, the output power is 38.1 μW, thus the calculation pulse energy is 2.74 pJ.
cpl-35-2-024203-fig3.png
Fig. 3. The characteristics of bright soliton Zr-EDFL: (a) full rf spectrum with 800 MHz span, (b) single rf pulse, (c) output pulse train with repetition rate of 13.9 MHz, and (d) autocorrelator trace of pulse width.
The 10-m-long SMF spool is inserted into the existing cavity. By adjusting the polarization of light using PC, dark soliton pulse is observed. We do not observe any formation of bright or dark pulses without GOSA. The optical spectrum dark soliton at pump power of 110 mW is shown in Fig. 4. The mode-locked pulse operates at central wavelength of 1596.82 nm with peak power of $-$46.49 dBm and a 3 dB bandwidth spectral of 4.5 nm. Similar to the bright soliton, this dark soliton exhibits two pairs of symmetrical Kelly sidebands; the right sidebands centered at 1601.98 nm and 1604.48 nm while the left sidebands centered at 1591.29 nm and 1588.94 nm. The dark pulse spectrum is shifted slightly to the right due to the changes of cavity length. The operating wavelength of bright pulse emits in a shorter wavelength whereas the dark pulse emits in a longer wavelength of L-band. This is due to the fact that the dark pulse has sufficient gain for amplification that contributes to the wideband transmission meanwhile the bright pulse requires more gain for pulse compression mode-locked fiber laser.[25,26] Then, the mode-locking stability is investigated using an rf spectrum analyzer and the result is shown in Fig. 5(a). The OSNR is approximately around 47 dB as illustrated in Fig. 5(b), which defines a stable pulse operation at the repetition rate of 8.3 MHz.
cpl-35-2-024203-fig4.png
Fig. 4. Optical spectrum of dark soliton Zr-EDFL.
cpl-35-2-024203-fig5.png
Fig. 5. The characteristics of dark soliton Zr-EDFL: (a) full rf spectrum with 180 MHz span, (b) single rf pulse, (c) dark pulse train with 8.3 MHz repetition rate, and (d) full width at half maximum (FWHM) of pulse.
Figure 5(c) illustrates the dark pulse train of the Zr-EDFL at the maximum pump power of 110 mW. The measured repetition rate for this laser is 8.3 MHz, which corresponds to the cavity length of 24.7 m. An attempt is taken to measure the pulse width with an autocorrelator and it is unsuccessful. This could be due to the limited specifications of the commercial autocorrelator where it could only measure a pulse width narrower than 50 ps. However, if transform limited for sech$^{2}$-shaped laser pulses is assumed, the estimation for the narrowest dark pulse should have a pulse width $\sim$0.6 ps while the full width at half maximum (FWHM) from the dark pulse output is estimated to be around $\sim$20 ns as shown in Fig. 5(d). The pulse width of the dark pulse is estimated to be broader than pulse width of bright soliton pulse due to the additional dispersion contributed by the 10-m-long SMF. The TBP for dark soliton pulse is estimated to be larger than the bright soliton pulse. This is because of the broader pulse width observed for dark pulse. The recorded output power is 41.3 μW at the maximum pump power of 110 mW, thus the pulse energy is calculated to be 4.98 pJ. This small energy is also due to the accumulated dispersion in a long cavity, which broadens the output spectrum up to the extended L-band region. The primary cause for the formation of dark pulse in our experiment comes from the 10-m-long SMF. The employment of SMF into the fiber laser cavity increases the nonlinearity, birefringence and net dispersion of cavity and this helps to form dark pulses. In addition to this, the nonlinearity of the new Zr-Y-Al co-doped and saturation absorption effect from GOSA are also contributed to the formation of soliton dark pulse. In conclusion, bright and dark soliton pulses in the L-band region are successfully demonstrated with enhanced Zr-EDF and GOSA. By introducing a 10-m-long SMF into the existing cavity, a dark soliton pulse is observed. The total cavity length required for dark soliton pulse generation is 24.7 m. This corresponds to the repetition rate of 8.3 MHz for the dark pulse. Meanwhile, the cavity length for bright pulse generation (without the 10-m-long SMF) is 14.7 m, which translates to the repetition rate of 13.9 MHz. The pulse width of the dark soliton is much broader compared with the bright soliton due to the additional dispersion contributed by the SMF. References Discrete bisoliton fiber laserDistributed ultrafast fibre laserVersatile multi-wavelength ultrafast fiber laser mode-locked by carbon nanotubesHysteresis phenomena and multipulse formation of a dissipative system in a passively mode-locked fiber laserExperimental Observation of the Fundamental Dark Soliton in Optical Fibers61-GHz dark-soliton generation and propagation by a fiber Bragg grating pulse-shaping techniqueDark pulses with tunable repetition rate emission from fiber ring laserDark-Square-Pulse Generation in a Ring Cavity With a Tellurite Single-Mode FiberBright–Dark Pulse Pair in a Figure-Eight Dispersion-Managed Passively Mode-Locked Fiber LaserObservation of dark pulse in a dispersion-managed fiber ring laserDark pulse emission in nonlinear polarization rotation-based multiwavelength mode-locked erbium-doped fiber laserPerformance Comparison of Mode-Locked Erbium-Doped Fiber Laser with Nonlinear Polarization Rotation and Saturable Absorber ApproachesDomain-wall dark pulse generation in fiber laser incorporating MoS2Dark pulse generation in fiber lasers incorporating carbon nanotubesAn L-band graphene-oxide mode-locked fiber laser delivering bright and dark pulsesGeneration of soliton and bound soliton pulses in mode-locked erbium-doped fiber laser using graphene film as saturable absorberSub 200 fs pulse generation from a graphene mode-locked fiber laserBright and Dark Square Pulses Generated From a Graphene-Oxide Mode-Locked Ytterbium-Doped Fiber LaserFabrication and application of zirconia-erbium doped fibersEnhanced Erbium–Zirconia–Yttria–Aluminum Co-Doped Fiber AmplifierA graphene-based mode-locked nano-engineered zirconia–yttria–aluminosilicate glass-based erbium-doped fiber laserKelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particlesDissipative soliton operation in diode pumped ultrafast Yb:GdYSiO5 oscillatorGraphene mode locked, wavelength-tunable, dissipative soliton fiber laserDark soliton fiber lasers
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