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Dual-Wavelength Dissipative Solitons in an Anomalous-Dispersion-Cavity

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Dual-Wavelength Dissipative Solitons in an Anomalous-Dispersion-Cavity Nanophotonics 2020; 9(8): 2361–2366 Research article Yufeng Song, Guodong Shao, Luming Zhao, Deyuan Shen, Han Zhang and Dingyuan Tang* Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity fiber laser https://doi.org/10.1515/nanoph-2019-0374 laser is through multi-wavelength mode locking. Experi- Received September 19, 2019; revised November 5, 2019; accepted mentally, multi-wavelength mode locking was achieved November 17, 2019 in actively mode-locked fiber lasers [7, 8]. However, due to their low energy, the mode-locked pulses were difficult Abstract: We report on the experimental observation of to be shaped into solitons. Multi-wavelength solitons were dual-wavelength dissipative soliton operation of a fiber first obtained in a passively mode-locked fiber laser with laser with net anomalous cavity dispersion. Different from the nonlinear polarization rotation (NPR) technique [8, 9]. the dual- or multi-wavelength soliton operation of fiber With the development of mode-locking technology, dual- lasers where mode locking is used to initiate soliton for- or multi-wavelength solitons have also been observed mation, no mode locking occurs in our fiber laser. Instead, in fiber lasers passively model-locked by real saturable soliton formation is through the dissipative mechanism absorbers [10–13]. Zhang et al. reported multi-wavelength caused by the effective gain bandwidth limitation. Either dissipative soliton generation in a semiconductor satura- dual-wavelength scalar, or vector, or induced dissipative ble absorber mirror (SESAM) mode-locked fiber laser [11]. solitons are experimentally obtained. Their robustness is A switchable dual-wavelength frequency comb fiber laser experimentally confirmed. passively mode-locked by carbon nanotubes was reported Keywords: pulse propagation and temporal solitons; non- by Zhao et al. [12]. Yun et al. reported multi-wavelength linear fiber optics; fiber lasers. solitons formed in a passively mode-locked figure-eight fiber laser [14]. Multi-wavelength dissipative soliton gen- eration in ytterbium-doped fiber lasers mode-locked by a graphene-deposited fiber taper was reported by Luo et al. 1 Introduction [15]. Very recently, multi-wavelength and wavelength-tun- able dissipative solitons were obtained in an all-normal- Soliton operation of fiber lasers is an interesting topic in dispersion erbium-doped fiber laser by Wu et al. [16]. nonlinear fiber optics [1]. Fiber lasers operating at multiple A characteristic of all reported dual- or multi-wave- wavelengths could have versatile potential applications length solitons is that they are subject to the influence in wavelength division multiplexing optical communi- of the saturable absorber, as it is necessary in the cavity cation [2]. In the past 10 years, dual-wavelength soliton for achieving the laser mode locking. Recently, Tang et al. fiber lasers have been employed as dual-frequency combs demonstrated a novel kind of dissipative soliton forma- for measurement applications [3–6]. A typical method of tion in fiber lasers without mode locking. It was shown achieving temporal multi-wavelength solitons in a fiber that, under effective laser gain bandwidth limitation, a weak periodic modulation could be evolved into a periodic *Corresponding author: Dingyuan Tang, School of Electrical dissipative soliton train in a high-power fiber laser [17]; and Electronic Engineering, Nanyang Technological University, even a high-repetition-rate pulse train could be obtained Singapore 639798, Singapore, e-mail: [email protected] [18]. Vector soliton formation is an intrinsic feature of a Yufeng Song and Han Zhang: International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of single-mode fiber and fiber laser [19–23]. On the basis of Ministry of Education, Institute of Microscale Optoelectronics, theoretical prediction, vector solitons including bright- Shenzhen University, Shenzhen 518060, China. https://orcid. bright vector soliton, dark-bright vector soliton, and dark org/0000-0002-6478-628X (Y. Song) vector solitons have been experimentally studied in fiber Guodong Shao: School of Electrical and Electronic Engineering, lasers. Recently, Thawatchai et al. numerically studied Nanyang Technological University, Singapore 639798, Singapore Luming Zhao and Deyuan Shen: Jiangsu Key Laboratory of Advanced the formation of stable two-component solitons through Laser Materials and Devices, School of Physics and Electronic purely dissipative nonlinearity [24]. However, the study of Engineering, Jiangsu Normal University, Xuzhou, China multi-wavelength vector solitons is still rare, to the best Open Access. © 2019 Dingyuan Tang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. 2362 Y. Song et al.: Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity fiber laser our knowledge. In this letter, we report that, with the tech- spectrum of the laser emission and the soliton pulse nique reported by Tang et al. [17], dual-wavelength dissi- evolution. pative solitons can be generated in a fiber laser. Moreover, The fiber laser has a threshold of ~20 mW. Initially, dual-wavelength scalar or vector dissipative solitons have continuous wave (CW) emission is always obtained. By been obtained and features of the dual-wavelength dissi- changing the orientation of the paddles of the intracav- pative solitons experimentally investigated. ity PC, the emission wavelength of the laser could be shifted. As the pump power is increased, dual-wavelength CW emission of the laser is obtained. Figure 2 shows the optical spectrum of the laser emission in a typical dual- 2 Experimental results wavelength CW operation state. By carefully tuning the paddles of the intracavity PC, the relative spectral The fiber laser setup is schematically shown in Figure 1. strength of the CW emissions could be changed. So far The ring laser cavity is dispersion-managed, and con- we have not fully understood the dual-wavelength opera- sists of a ~3-m erbium-doped fiber (OFS-80) with a group tion mechanism of the fiber laser. It is suspected that, for velocity dispersion parameter D = −48 ps/nm/km and an some reason, a kind of artificial spectral filter could have ~12-m single-mode fiber with D = 18 ps/nm/km. The net formed in the cavity. dispersion of the cavity is anomalous. The fiber laser is When the laser is operating in the dual-wavelength pumped by a high-power Raman fiber laser source (KPS- CW emission regime, by increasing the pump power or BT2-RFL-1480-60-FA) operating at 1480 nm. Through a tuning the paddles of the PC, the CW emission at one fused 1480 nm/1550 nm wavelength division multiplexer wavelength can be changed into soliton emission through (WDM), the pump power is coupled into the fiber ring the same procedure as reported in [17]. A case is presented cavity. To minimize possible effects caused by the residual in Figure 3. Figure 3A shows the polarization-resolved pump light, the reverse pumping configuration is adopted. optical spectra of the laser emission. Figure 3B shows the A 10% output coupler is used to output the signal. A polar- corresponding polarization-resolved oscilloscope traces. ization controller (PC) is employed in the cavity to fine- Initially, the laser emits simultaneously dual-wavelength tune the net cavity birefringence, and an isolator is used CW radiation, one centered at 1573 nm and the other at to force unidirectional operation of the ring. All the com- 1580 nm. As the laser emission intensity increases, the CW ponents in the laser cavity are polarization-independent, emission at ~1580 nm suddenly changes into the vector and no intracavity polarizer is inserted in the cavity. A dissipative soliton emission, characterized by spectral polarization beam splitter is used outside the laser cavity broadening and a synchronized pulse pair on the oscil- to separate the two orthogonal polarizations of the laser loscope traces. The phase-locked vector soliton nature emission. It is to be noted that the components used in of the pulses is identified by the appearance of the Kelly our experiment have very low polarization-dependent sideband in the spectrum, which is a typical characteristic loss (WDM: 0.01 dB, isolator: 0.04 dB, coupler: 0.01 dB). An optical spectrum analyzer (Yokogawa AQ5375) and a 33-GHz oscilloscope (Agilent DSO-X 92804A) together with –15 two 25-GHz photodetectors are used to monitor the optical –20 –25 –30 –35 –40 Spectral intensity (dB) –45 –50 –55 1566 1568 1570 1572 1574 1576 1578 1580 1582 Figure 1: Schematic diagram of the soliton fiber laser. Wavelength (nm) EDF, erbium-doped fiber; WDM, wavelength division multiplexer; SMF, single-mode fiber; PC, polarization controller; ISO, isolator; Figure 2: Optical spectrum of dual-wavelength continuous wave OC, optical coupler. emission of the fiber laser. Y. Song et al.: Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity fiber laser 2363 AB 1 Total output Vertical axis –10 Horizontal axis –20 0 –30 –40 Intensity (a.u.) 1 Intensity (a.u.) Spectral intensity (dB) –50 0 –60 1565 1570 1575 1580 1585 1590 0 50 100 150 200 250 Wavelength (nm) Time (ns) Figure 3: Dual-wavelength operation of the gain-guided dissipative soliton fiber laser: continuous wave and vector gain-guided dissipative soliton. (A) Optical spectrum. (B) Oscilloscope trace. of the soliton operation of lasers [25], and by the peak-dip polarization-resolved spectra shown in Figure 4A suggest spectral sidebands on the polarization-resolved spectra, that the gain-guided dissipative solitons are formed from which shows that there is coherent energy exchange the CW emission centered at 1580 nm and the formed soli- between the two orthogonal polarization components tons are linearly polarized along one of the two orthogonal of the solitons [26]. We emphasize that no mode locking polarization directions of the laser. Figure 4B shows the occurs in the fiber laser. The solitons are formed as a result corresponding polarization-resolved oscilloscope traces. of the periodic modulation under the effective laser gain Indeed, the soliton pulses are also linearly polarized and bandwidth limitation.
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