Dual-wavelength dissipative solitons in an anomalous-dispersion-cavity fiber laser

1 Introduction

Soliton operation of fiber lasers is an interesting topic in nonlinear fiber optics [1]. Fiber lasers operating at multiple wavelengths could have versatile potential applications in wavelength division multiplexing optical communication [2]. In the past 10 years, dual-wavelength soliton fiber lasers have been employed as dual-frequency combs for measurement applications [3], [4], [5], [6]. A typical method of achieving temporal multi-wavelength solitons in a fiber laser is through multi-wavelength mode locking. Experimentally, multi-wavelength mode locking was achieved in actively mode-locked fiber lasers [7], [8]. However, due to their low energy, the mode-locked pulses were difficult to be shaped into solitons. Multi-wavelength solitons were first obtained in a passively mode-locked fiber laser with the nonlinear polarization rotation (NPR) technique [8], [9]. With the development of mode-locking technology, dual- or multi-wavelength solitons have also been observed in fiber lasers passively model-locked by real saturable absorbers [10], [11], [12], [13]. Zhang et al. reported multi-wavelength dissipative soliton generation in a semiconductor saturable absorber mirror (SESAM) mode-locked fiber laser [11]. A switchable dual-wavelength frequency comb fiber laser passively mode-locked by carbon nanotubes was reported by Zhao et al. [12]. Yun et al. reported multi-wavelength 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. [15]. Very recently, multi-wavelength and wavelength-tunable dissipative solitons were obtained in an all-normal-dispersion erbium-doped fiber laser by Wu et al. [16].

A characteristic of all reported dual- or multi-wavelength solitons is that they are subject to the influence of the saturable absorber, as it is necessary in the cavity for achieving the laser mode locking. Recently, Tang et al. demonstrated a novel kind of dissipative soliton formation in fiber lasers without mode locking. It was shown that, under effective laser gain bandwidth limitation, a weak periodic modulation could be evolved into a periodic dissipative soliton train in a high-power fiber laser [17]; even a high-repetition-rate pulse train could be obtained [18]. Vector soliton formation is an intrinsic feature of a single-mode fiber and fiber laser [19], [20], [21], [22], [23]. On the basis of theoretical prediction, vector solitons including bright-bright vector soliton, dark-bright vector soliton, and dark vector solitons have been experimentally studied in fiber lasers. Recently, Thawatchai et al. numerically studied the formation of stable two-component solitons through purely dissipative nonlinearity [24]. However, the study of multi-wavelength vector solitons is still rare, to the best our knowledge. In this letter, we report that, with the technique reported by Tang et al. [17], dual-wavelength dissipative solitons can be generated in a fiber laser. Moreover, dual-wavelength scalar or vector dissipative solitons have been obtained and features of the dual-wavelength dissipative solitons experimentally investigated.

2 Experimental results

The fiber laser setup is schematically shown in Figure 1. The ring laser cavity is dispersion-managed, and consists of a ~3-m erbium-doped fiber (OFS-80) with a group velocity dispersion parameter D=−48 ps/nm/km and an ~12-m single-mode fiber with D=18 ps/nm/km. The net dispersion of the cavity is anomalous. The fiber laser is pumped by a high-power Raman fiber laser source (KPS-BT2-RFL-1480-60-FA) operating at 1480 nm. Through a fused 1480 nm/1550 nm wavelength division multiplexer (WDM), the pump power is coupled into the fiber ring cavity. To minimize possible effects caused by the residual pump light, the reverse pumping configuration is adopted. A 10% output coupler is used to output the signal. A polarization controller (PC) is employed in the cavity to fine-tune the net cavity birefringence, and an isolator is used to force unidirectional operation of the ring. All the components in the laser cavity are polarization-independent, and no intracavity polarizer is inserted in the cavity. A polarization beam splitter is used outside the laser cavity to separate the two orthogonal polarizations of the laser emission. It is to be noted that the components used in our experiment have very low polarization-dependent 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 two 25-GHz photodetectors are used to monitor the optical spectrum of the laser emission and the soliton pulse evolution.

Figure 1:

Schematic diagram of the soliton fiber laser.

EDF, erbium-doped fiber; WDM, wavelength division multiplexer; SMF, single-mode fiber; PC, polarization controller; ISO, isolator; OC, optical coupler.

The fiber laser has a threshold of ~20 mW. Initially, continuous wave (CW) emission is always obtained. By changing the orientation of the paddles of the intracavity 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-wavelength CW operation state. By carefully tuning the paddles of the intracavity PC, the relative spectral strength of the CW emissions could be changed. So far we have not fully understood the dual-wavelength operation mechanism of the fiber laser. It is suspected that, for some reason, a kind of artificial spectral filter could have formed in the cavity.

Figure 2:

Optical spectrum of dual-wavelength continuous wave emission of the fiber laser.

When the laser is operating in the dual-wavelength CW emission regime, by increasing the pump power or tuning the paddles of the PC, the CW emission at one wavelength can be changed into soliton emission through the same procedure as reported in [17]. A case is presented in Figure 3. Figure 3A shows the polarization-resolved optical spectra of the laser emission. Figure 3B shows the corresponding polarization-resolved oscilloscope traces. Initially, the laser emits simultaneously dual-wavelength CW radiation, one centered at 1573 nm and the other at 1580 nm. As the laser emission intensity increases, the CW emission at ~1580 nm suddenly changes into the vector dissipative soliton emission, characterized by spectral broadening and a synchronized pulse pair on the oscilloscope traces. The phase-locked vector soliton nature of the pulses is identified by the appearance of the Kelly sideband in the spectrum, which is a typical characteristic of the soliton operation of lasers [25], and by the peak-dip spectral sidebands on the polarization-resolved spectra, which shows that there is coherent energy exchange between the two orthogonal polarization components of the solitons [26]. We emphasize that no mode locking occurs in the fiber laser. The solitons are formed as a result of the periodic modulation under the effective laser gain bandwidth limitation. To distinguish the solitons from those formed by mode locking, we have named them “gain-guided dissipative solitons” to highlight the role played by the effective gain bandwidth limitation on their formation.

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.

Besides the dual-wavelength CW and vector gain-guided dissipative soliton operation, dual-wavelength CW and scalar gain-guided dissipative soliton operation have also been obtained, as shown in Figure 4. The polarization-resolved spectra shown in Figure 4A suggest that the gain-guided dissipative solitons are formed from the CW emission centered at 1580 nm and the formed solitons are linearly polarized along one of the two orthogonal polarization directions of the laser. Figure 4B shows the corresponding polarization-resolved oscilloscope traces. Indeed, the soliton pulses are also linearly polarized and appear only in one polarization direction.

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.

By increasing the pump power to ~1 W and carefully adjusting the net cavity birefringence, the gain-guided dissipative solitons could also be formed simultaneously in CW emission at both wavelengths. Figure 5 shows a typical state of the dual-wavelength gain-guided dissipative soliton operation. Based on the optical spectra shown in Figure 5A, it is easy to see that there are two sets of broadband spectra, one centered at ~1577 nm and the other at ~1573 nm. Clear, Kelly spectral sidebands have appeared on the one centered at 1577 nm, which has a much broader spectral bandwidth, while no obvious Kelly spectral sidebands could observed on the band centered at 1573 nm. Figure 5B shows the corresponding polarization-resolved oscilloscope traces of the laser emissions. It is obvious that on the upper trace (green line) there is one set of pulses, whereas in the lower trace (blue line) there are two sets of pulses. We have experimentally identified that the stronger pulses on lower trace are the solitons at ~1577 nm, which have a broad spectral bandwidth and are scalar solitons, while the weaker pulses are the solitons at ~1573 nm. They have a narrow spectral bandwidth. The weak pulses are vector pulses. We note that, corresponding to each strong pulse on the lower trace, there is also a very weak pulse appearing on the upper trace. These are the solitons induced by the strong scalar solitons through the cross-polarization coupling effect. The Kelly sidebands of the induced solitons have locations different from those of the Kelly sidebands of the strong soliton, indicating that the net cavity birefringence at the wavelength is large. Because the scalar solitons have a central wavelength, which is different from that of the vector pulses, they appear separated in the oscilloscope traces. Note that the induced solitons always move together with the inducing solitons. Thus, on the oscilloscope traces they are always one-to-one related.

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.

Under even stronger pump power and an appropriate setting of the PC, a state of dual-wavelength vector gain-guided dissipative soliton operation could also be achieved. Figure 6 shows such a state of the fiber laser operation. Figure 6A shows the polarization-resolved optical spectra of the laser emission. Figure 6B presents the corresponding-polarization resolved oscilloscope traces. The observed spectrum is actually a superposition of two soliton spectra, one centered at ~1577 nm and the other at ~1581 nm. On both soliton spectra, the Kelly sidebands are clearly visible. Moreover, for the soliton centered at 1577 nm, spectral sidebands caused by the coherent energy exchange are also visible [12]. Experimentally, the solitons at different center wavelengths could be easily identified on the oscilloscope traces. Simply by slightly tuning the intracavity PC, one can suppress solitons at one wavelength. In such a way, we have 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 is worth noting that on the oscilloscope traces the two sets of vector solitons move with different group velocities. They collide with each other frequently. However, their collision does not destroy either of them.

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.

Figure 7 shows more results of the dual-wavelength vector gain-guided dissipative soliton operation of the fiber laser experimentally observed. The central wavelengths of the solitons are at ~1574.5 and ~1577.5 nm. Different from the vector gain-guided dissipative solitons presented in Figure 6, where the two components of the vector solitons have almost comparable strength, the two components of the vector solitons have a large (>15 dB) intensity difference. One may consider the weak component part as an induced soliton [27]. Although the inducing solitons at different wavelengths have almost the same strength, the induced solitons have obviously different strengths, as shown by the upper trace on the oscilloscope (corresponding to the vertical axis in the optical spectrum), where the weak pulses correspond to the solitons centered at 1577.5 nm while the stronger pulses correspond to the solitons centered at 1574.5 nm. We note again that the solitons with different central wavelengths move relative to each other. The oscilloscope traces shown are a snapshot of them in the cavity.

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.

It is to be noted that we have checked that all the intracavity components are polarization insensitive, so that soliton formation by NPR mode locking can be excluded. Based on our previous work, it is clear under which residual polarization dependence the NPR mode locking is possible [28]. Furthermore, vector solitons are obviously observed in our laser cavity, which is not observable in an NPR mode-locking cavity.

3 Summary

In summary, dual-wavelength dissipative soliton operation of a fiber laser without any mode locker in the cavity is experimentally demonstrated. This is different from the dual-wavelength soliton operation of the conventional mode-locked anomalous-dispersion-cavity fiber lasers, where the soliton pulses are generated by the nonlinear shaping of the mode-locked pulse, either through the natural balance between the self-phase modulation and anomalous dispersion, or through the dissipative mechanism. In that case, it is ambiguous whether the formed solitons are the nonlinear Schrödinger equation type solitons or dissipative solitons. Without a saturable absorber in cavity, if there is no further action of the effective gain-bandwidth limitation, no stable solitons but soliton breathers could be formed in a fiber laser. Hence, our experimental result is a clear experimental demonstration of the dual-wavelength dissipative solitons in anomalous-dispersion-cavity fiber lasers. Although dual- or multi-wavelength dissipative solitons have been obtained in normal-dispersion fiber lasers, again they are achieved by the mode-locking technique; moreover, dissipative solitons have features different from those obtained in anomalous-dispersion-cavity fiber lasers. Our experimental studies have also shown that various states of dual-wavelength dissipative soliton operations could be achieved. And the formation of these states could be easily understood on the basis of the nonlinear polarization coupling of light in fibers. Finally, our experimental results have clearly shown the evidence that the formed dissipative solitons are robust and that the collisions between either the scalar or vector dissipative solitons do not destroy them.