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 generation 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-tunable 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

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–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

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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. To distinguish the solitons from appear only in one polarization direction. those formed by mode locking, we have named them By increasing the pump power to ~1 W and carefully “gain-guided dissipative solitons” to highlight the role adjusting the net cavity birefringence, the gain-guided played by the effective gain bandwidth limitation on their dissipative solitons could also be formed simultaneously formation. in CW emission at both wavelengths. Figure 5 shows a Besides the dual-wavelength CW and vector gain- typical state of the dual-wavelength gain-guided dis- guided dissipative soliton operation, dual-wavelength sipative soliton operation. Based on the optical spectra CW and scalar gain-guided dissipative soliton opera- shown in Figure 5A, it is easy to see that there are two sets tion have also been obtained, as shown in Figure 4. The of broadband spectra, one centered at ~1577 nm and the

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Figure 4: Dual-wavelength operation of the gain-guided dissipative soliton fiber laser: continuous wave and scalar gain-guided dissipative soliton. (A) Optical spectra. (B) Oscilloscope trace. 2364 Y. Song et al.: Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity ﬁber laser

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Figure 5: Dual-wavelength operation of the gain-guided dissipative soliton fiber laser: scalar and vector gain-guided dissipative solitons. (A) Polarization-resolved optical spectra. (B) The corresponding polarization-resolved oscilloscope trace. other at ~1573 nm. Clear, Kelly spectral sidebands have through the cross-polarization coupling effect. The Kelly appeared on the one centered at 1577 nm, which has a sidebands of the induced solitons have locations different much broader spectral bandwidth, while no obvious Kelly from those of the Kelly sidebands of the strong soliton, spectral sidebands could observed on the band centered indicating that the net cavity birefringence at the wave- at 1573 nm. Figure 5B shows the corresponding polariza- length is large. Because the scalar solitons have a central tion-resolved oscilloscope traces of the laser emissions. It wavelength, which is different from that of the vector is obvious that on the upper trace (green line) there is one pulses, they appear separated in the oscilloscope traces. set of pulses, whereas in the lower trace (blue line) there Note that the induced solitons always move together with are two sets of pulses. We have experimentally identified the inducing solitons. Thus, on the oscilloscope traces that the stronger pulses on lower trace are the solitons at they are always one-to-one related. ~1577 nm, which have a broad spectral bandwidth and are Under even stronger pump power and an appropri- scalar solitons, while the weaker pulses are the solitons ate setting of the PC, a state of dual-wavelength vector at ~1573 nm. They have a narrow spectral bandwidth. The gain-guided dissipative soliton operation could also be weak pulses are vector pulses. We note that, correspond- achieved. Figure 6 shows such a state of the fiber laser ing to each strong pulse on the lower trace, there is also operation. Figure 6A shows the polarization-resolved a very weak pulse appearing on the upper trace. These optical spectra of the laser emission. Figure 6B presents are the solitons induced by the strong scalar solitons the corresponding-polarization resolved oscilloscope

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Figure 6: Dual-wavelength, vector gain-guided dissipative soliton operation of the fiber laser. (A) Polarization-resolved optical spectra. (B) The corresponding polarization-resolved oscilloscope trace. Y. Song et al.: Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity ﬁber laser 2365 traces. The observed spectrum is actually a superposi- relative to each other. The oscilloscope traces shown are a tion of two soliton spectra, one centered at ~1577 nm and snapshot of them in the cavity. the other at ~1581 nm. On both soliton spectra, the Kelly It is to be noted that we have checked that all the intra- sidebands are clearly visible. Moreover, for the soliton cavity components are polarization insensitive, so that centered at 1577 nm, spectral sidebands caused by the soliton formation by NPR mode locking can be excluded. coherent energy exchange are also visible [12]. Experi- Based on our previous work, it is clear under which resid- mentally, the solitons at different center wavelengths ual polarization dependence the NPR mode locking is could be easily identified on the oscilloscope traces. possible [28]. Furthermore, vector solitons are obviously Simply by slightly tuning the intracavity PC, one can sup- observed in our laser cavity, which is not observable in an press solitons at one wavelength. In such a way, we have NPR mode-locking cavity. identified the stronger pulses in the oscilloscope traces to be the solitons centered at ~1581 nm, while the weaker pulses correspond to the solitons centered at 1577 nm. It 3 Summary is worth noting that on the oscilloscope traces the two sets of vector solitons move with different group veloci- In summary, dual-wavelength dissipative soliton opera- ties. They collide with each other frequently. However, tion of a fiber laser without any mode locker in the cavity their collision does not destroy either of them. is experimentally demonstrated. This is different from the Figure 7 shows more results of the dual-wavelength dual-wavelength soliton operation of the conventional vector gain-guided dissipative soliton operation of the mode-locked anomalous-dispersion-cavity fiber lasers, fiber laser experimentally observed. The central wave- where the soliton pulses are generated by the nonlin- lengths of the solitons are at ~1574.5 and ~1577.5 nm. Dif- ear shaping of the mode-locked pulse, either through ferent from the vector gain-guided dissipative solitons the natural balance between the self-phase modulation presented in Figure 6, where the two components of the and anomalous dispersion, or through the dissipative vector solitons have almost comparable strength, the two mechanism. In that case, it is ambiguous whether the components of the vector solitons have a large (>15 dB) formed solitons are the nonlinear Schrödinger equation intensity difference. One may consider the weak compo- type solitons or dissipative solitons. Without a satura- nent part as an induced soliton [27]. Although the induc- ble absorber in cavity, if there is no further action of the ing solitons at different wavelengths have almost the effective gain-bandwidth limitation, no stable solitons same strength, the induced solitons have obviously differ- but soliton breathers could be formed in a fiber laser. ent strengths, as shown by the upper trace on the oscil- Hence, our experimental result is a clear experimental loscope (corresponding to the vertical axis in the optical demonstration of the dual-wavelength dissipative solitons spectrum), where the weak pulses correspond to the soli- in anomalous-dispersion-cavity fiber lasers. Although tons centered at 1577.5 nm while the stronger pulses corre- dual- or multi-wavelength dissipative solitons have been spond to the solitons centered at 1574.5 nm. We note again obtained in normal-dispersion fiber lasers, again they that the solitons with different central wavelengths move are achieved by the mode-locking technique; moreover,

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Figure 7: Dual-wavelength, vector-induced, gain-guided dissipative soliton operation of the fiber laser. (A) Polarization-resolved optical spectra. (B) Polarization-resolved oscilloscope traces. 2366 Y. Song et al.: Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity ﬁber laser dissipative solitons have features different from those [11] Zhang H, Tang DY, Wu X, Zhao LM. Multi-wavelength dissipative obtained in anomalous-dispersion-cavity fiber lasers. Our soliton operation of an erbium-doped fiber laser. Opt Express 2009;17:12692–7. experimental studies have also shown that various states [12] Zhao X, Zheng Z, Liu L, et al. Switchable, dual-wavelength pas- of dual-wavelength dissipative soliton operations could sively mode-locked ultrafast fiber laser based on a single-wall be achieved. And the formation of these states could be carbon nanotube modelocker and intracavity loss tuning. 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