1. Introduction

Supercontinuum (SC) generation has attracted continuous research interest due to its various and promising applications, such as optical frequency metrology, spectroscopy, optical coherence tomography, mobile clockwork, device characterization, optical communications [1, 2] and medical sciences. SC generation has an extremely wide spectrum over hundreds or even thousands of nanometers with fine structures and high noise sensitivity [3]. The processes of SC generation involve the combination and interaction of many nonlinear phenomena [4], such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS) [5], modulation instability (MI), Raman self-frequency shift (RSFS), fission of higher-order solitons (HOS), dispersive wave generation [6–8] and and zero dispersion wavelength (ZDW). In addition to the nonlinear medium characteristics, pump source temporal features are also vital to the generation of SC [9]. SRS and FWM effect are mainly responsible in the formation of SC generation for long pulses (pulse width from microsecond to nanosecond), while for the ultrashort pulses (pulse width from picosecond to femtosecond), the primary phenomena are soliton generation and SPM effect [10]. SC spectra can be generated by using continuous wave pumping [11], short pulses [12, 13], ultra-short pulses [14] and noise-like pulses (NLPs) [15–17], and the nonlinear propagation media are also crucial to SC generation, for instance, photonic crystal fiber (PCF) [11, 18], highly nonlinear fiber (HNLF) [19], zero dispersion fiber, tapered fiber [19, 20] and standard fiber (SMF) [15].

In many works, long sections of fiber or special fibers, such as HNLF, fiber gratings, microstructured fibers and PCF are used in fiber laser cavity [8, 11, 19], which have a larger economic cost. Because the production of these schemes requires high-cost technology, as expensive laboratory equipment and material are necessary, and those are uneasily accessible. However, some researchers have reported SC generation using HNLF in order to enhance SC extension and flatness. In the Ref [21], the author used a section of HNLF at the output of laser, a spectrum spanning from 1200 to 2100 nm was generated with average power of around 200 mW. In Ref [22], through a piece of HNLF, spectral broadening more than several hundreds of nm on the short wavelength side was also obtained, achieving a flat spectrum with bandwidth of 500 nm. And the others have reported SC generation by using noise-like pulses in different fiber lasers, describing the advantages of these pump pulses. On the one hand, noise-like pulses are robust against dispersion. In a dispersive medium, noise-like pulses are very long, so that they do not disappear quickly and their peak power remains high over a long distance. Therefore, the nonlinear effects integrate over much longer fiber lengths, allowing SC generation even in fibers whose nonlinear and dispersive properties are not optimized for this purpose, and spectral broadening in such media could also be strongly enhanced by the nonlinear interaction between the noise-like pulse components. And on the other hand, the spectrum of noise-like pulse is broader than the one of solitons. One can expect that further broadening will be reached more easily in comparison with conventional pulses, which is shown in Ref [23]. In fact, some papers have reported noise-like pulses spectra that exceed 100 nm directly at the laser output [24, 25]. If high pump powers are available, flat and broadband spectra covering 200 nm can also be obtained directly at the laser output if a long section of SMF is inserted in the cavity, as shown by some groups [24, 26]. As shown in Ref [24], the authors used noise-like pulses from a figure-eight fiber laser, generating a very wide spectrum covering 200 nm directly at the fiber laser output with average pump power of around 300 mW. Then, these noise-like pulses were launched into a piece of 100 m-long HNLF, the spectrum was extended up to 450 nm and flattened obviously.

In this paper, our work is very interesting as it presents the experimental results of SC generation with an attractive architecture that is figure-eight fiber laser, which is provided by nonlinear transmission characteristic of NOLM. A broad spectrum of SC spanning from 1315 to 1725 nm at the level of 30 dB can be directly achieved by using an Er-doped figure-eight passively mode-locked fiber resonator cavity without any special fibers or amplification system when the pump power is increased to 400 mW. At the pump power of 240 mW, the pulse duration of 4 ns with a fundamental repetition rate of 416.7 kHz corresponding to the laser cavity of 480 m is obtained. The advantage of this configuration is the robustness and flexibility of its nonlinear switching features, which can be adjusted by polarization controller. Hence, that is the reason why we choose the NOLM as the tool for mode-locking. However, the predominant SAs, such as semiconductor saturable mirrors are limited by their narrow tuning range for only few nanometers, and with the high cost in the fabrication process [27]. Due to the utilization of ring cavity and 460-m-long length of SMF-28, the stimulated Raman scattering (SRS) as well as other nonlinear effects especially FWM effect [19] could become effective to expand the range of spectrum to a longer wavelength band. The compact structure may reduce the cost and provide a convenient way to get SC generations.

2. Experimental setup

A schematic of the proposed SC generation setup is shown in Fig. 1. The figure-eight fiber laser consists of a ring laser cavity and a nonlinear optical loop mirror (NOLM) [28]. The total cavity length is about 480 m. In the ring cavity, a 980/1550 nm wavelength division multiplex (WDM) was used to launch the pump power from a 976-nm laser diode with a maximum operation power of 400 mW into the active fiber. The active medium is provided by 7-m erbium-doped fiber (EDF), and this EDF has 8.31 dB/m peak absorption coefficients at 1530 nm. Erbium doped fiber (EDF) [29] is desirable as a gain medium in 1550nm region due to the development of wavelength division multiplexed systems (WDMs). A polarization-independent optical isolator (PI-ISO) is inserted in the cavity to ensure unidirectional propagation of optical signals. A 95/5 optical coupler (OC) was inserted in the scheme, with the 5% output port connected to the combiner. In order to perform two measurements at the same time, a 95.4/4.6 optical coupler is spliced to the laser 5% output port. The 4.6% output port is connected to a 2-GHz photodetector and the detected signal is monitored on a 1 GHz real-time oscilloscope. And the 95.4% port is connected to an optical spectrum analyzer (OSA), whose resolution is set at 0.05 nm for whole measurements. A power-asymmetric, polarization-imbalanced NOLM scheme is served as the saturable absorber (SA), which consists of a 60/40 optical coupler (OC), a long low-birefringence single mode fiber (SMF28, D = 18 ps/nm/km) of about 460 m twisted at a rate of 2 turns per meter and a polarization controller (PC) inserted in the loop in order to adjust the polarization state of light and cavity birefringence. Such a NOLM switching mechanism depends on nonlinear polarization rotation (NPR) [30] in the twisted loop.

 

Fig. 1 Schematic diagram of proposed figure-eight fiber laser.

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3. Experimental results and discussions

The NOLM technique is utilized for obtaining the passive mode-locking state of the fiber laser. In our experiment, the mode-locking threshold of the proposed fiber laser is about 240 mW. It is worth noting that mode-locking can be self-starting in all cases if the pump power is higher than 240 mW with the proper orientation of the PC, and remaining stable for hours. However, the mode-locked pulse can be sustained when the pump power was decreased to 198 mW due to the pump hysteresis phenomenon in fiber laser. A polarizer is usually inserted at the NOLM cavity in those similar schemes, but it is not strictly necessary for mode locking, because that the vast majority of polarization states permit NOLM switching. And without a polarizer in our experiment may be connected with the enhanced noise-like pulses properties, on account of the state of polarization along the cavity is no longer restricted, it means that the laser has greater freedom to self-adjust, adapting its polarization and selecting the NOLM switching power, so as to optimize the mode-locking operation.

The uniform shape of the output pulse train in time domain is shown in Fig. 2(a) depicting less fluctuation from peak to peak with the pulse repetition rate of 416.7 kHz, corresponding to fundamental frequency mode-locking of the 480-m long laser cavity. The output pulse with the durarion of 4 ns and the spectrum with 3 dB bandwidth of 30.6 nm under pump power of 240 mW are shown in Figs. 2(b) and 2(c), respectively. The RF spectra in Fig. 2(d) show that signal/noise ratio (SNR) is about 51dB and the fundamental peak located at the cavity repetition rate is 416.7 kHz as determined by the cavity length with the resolution bandwidth (RBW) set as 1 kHz and the video bandwidth (VBW) set as 300 Hz. The inset of Fig. 2(b) performs the RF spectrum in a 50 MHz range with 1 kHz RBW and 1 kHz VBW.

 

Fig. 2 (a). Optical pulse train from the oscilloscope; (b). The output pulse with width of 4 ns;(c). The output specrtum with 3 dB bandwidth of 30.6 nm ; (d). The RF spectra of the optical pulse.

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The pulse duration broadens as a function of the pump power. Correspondingly, the duration of the mode-locked pulse broadens gradually with the pump power increasing from 240 mW to 390 mW, while the peak of the pulse almost remains constant. In order to investigate the pulse characteristic, we increased the pump power and observed the pulse evolution when the PC was fixed in the experiment. Here, the width of mode-locked pulse broadens from 4 ns to 6 ns. The measured results are shown in Fig. 3(a). Passive mode-locking is able to generate wider spectra due to higher order soliton generation when the pump power is increased. The most significant result in our experiment is the broad and smooth optical spectrum achieved directly from the laser resonator without any amplifier in the fundamental mode-locking regime, extending over more than 400 nm at the level of 30 dB, which can be comparable with those obtained in the previous studies [10, 21, 23]. Based on the structure of NOLM having the wide and flat spectra, the pump powers are increased from 240 mW to 390 mW for additional analyses of the laser phenomena relying on pump powers. The measured results are shown in Fig. 3(b).

 

Fig. 3 (a). Pulse dynamics versus pump powers; (b). Spectrum evolution with the increase of pump power ;(c). The 3 dB spectrum bandwidth; (d). Pulse energy and average output power versus pump power.

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We can find the tendencies of flatness are becoming more flattened with the increasing of pump powers, as shown in Fig. 3(c). The 3 dB spectrum bandwidth is increasing as the augment of pump power. That is, the same amplitude contains more spectrum as pump power increasing. Hence, the spectrum shows “good flatness”under high pump power. We appreciate that the flatness increased obviously with the increasing of pump power and the slight adjustment of PC. The results show that the optical spectrum at the laser cavity output exhibits features such as a smooth spectral width with a wide extension, and a good flatness, these characteristics show the interest of using figure-eight fiber laser resonator directly for SC generation, we can obtain wide spectra in a relatively simple and inexpensive configuration. And it also exhibits a good result in comparison with other works in which some special fibers were used, such as highly nonlinear fiber, fiber grating, micro-structured fibers [3, 8, 11, 19, 20] or the need of higher input pump power. Except that the SC generation can be directly obtained from laser resonator, there is also a number of passively mode-locked sources as the pump for SC generation that have been recently developed, which can generate pulses with a very broad spectral width [10, 15, 21, 23]. We believe that many different laser system configurations with or without amplifier are attractive alternatives for the generation of broad SC exhibiting a high degree of spectral flatness.

The pulse energy changes from 1.6 nJ to 5.6 nJ when the pump power was increased from 114 to 354 mW, which can be seen in Fig. 3(d). At a maximum pump power of 354 mW, the average output power is 2.361 mW, and the measured largest output pulse energy was 5.6 nJ, which was limited by the pump power level.

SC generation can be obtained under the high pump power by adjusting the PC slightly. The center wavelength and the spectral bandwidth are 1559.1 nm and 410 nm, respectively, which can be seen in Fig. 4.

 

Fig. 4 The output spectrum at pump power of 400 mW.

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We measure the output powers and the output spectra every ten minutes under the pump power of 240 mW in order to verify the stability of the figure-eight mode-locked fiber laser, as shown in Figs. 5 and 6. From the following figures we can conclude that such figure-eight mode-locked fiber laser is comparatively stable.

 

Fig. 5 The output power versus the variation of time

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Fig. 6 The output spectrum versus the variation of time

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4. Conclusion

In this work, we have demonstrated the SC generation through a figure eight laser cavity scheme, which includes a polarization imbalanced NOLM. For adjustments of PC and the increasing of pump power, a stable train of nanosecond pulses at 416.7 kHz repetition rate is generated by the laser, indicating fundamental frequency mode locking of the 480-m long laser cavity. The maximum output pulse energy of 5.6 nJ is achieved experimentally, which is limited by pump power level. The experimental results suggest that the flatness and bandwidth of the spectrum was significantly improved through the increasing of pump power and slight adjustment of PC. SC generation with excellent flatness is generated, and with the 30 dB spectral range of about 1315–1725 nm at pump power of 400 mW. The proposed laser system takes advantage of the all-fiber, low-cost, compact and robust features, hence it can find extensive applications in high-energy pulse generation or amplification and SC generation systems. And the SC generation can be applied to optical metrology, optical coherence tomography, optical sensing, fiber gyroscopes, optical communications, and so on.