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Abstract
Supercontinuum (SC) can be generated directly from a random fiber laser (RFL). However, its spectral bandwidth and flatness need to be further optimized for many practical applications. To solve this issue, a RFL based on random distributed Rayleigh scattering in photonic crystal fiber is demonstrated for the first time in this paper. The experimental results revealed that compared with the traditional single or double clad fiber, photonic crystal fiber not only can provide random distributed feedback effectively, but is also a superior nonlinear medium for SC generation which can realize better spectral width and flatness. A flat SC covering 400 nm to 2300 nm is obtained directly from a RFL based on photonic crystal fiber and the corresponding 20 dB bandwidth is more than 1600 nm, which is the widest ever reported to the best of our knowledge. The optical rogue waves caused by solitonic collisions can explain the instability of the output pulses in the time domain. This work proves that photonic crystal fiber can be used in RFL to provide random distributed feedback as well as nonlinear medium for spectrum broadening, and the spectral width and flatness of the generated SC is as good as the conventional method of using a high peak power pulsed laser to pump a piece of photonic crystal fiber, which can greatly reduce the cost of the SC and enrich the research scope of SC as well as RFL.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The novel random fiber laser (RFL) [1–3] toppled the original cognition that an independent resonant cavity is necessary for lasing in conventional fiber lasers once it was reported. By utilizing the inherently or artificially introduced random Rayleigh scattering (RS) as feedback mechanism, the well-defined cavity can be eliminated. Compared with other fiber lasers, RFL shows some unique characteristics such as simple structure, modeless emission, low coherence and favorable stability in time domain [2–4]. Since the first RFL put forward in 2007 [1], RFL shows tremendous vitality and wide applied foreground in speckle-free imaging [5], telecommunications [6,7], distributed sensing [8,9], supercontinuum generation [10–15] and so on.
The supercontinuum (SC) has attracted considerable attentions because of the wide variety of applications such as optical coherence tomography (OCT), hyperspectral lidar, and wavelength division multiplexing (WDM) [16–20]. With the advent of photonic crystal fiber (PCF), the study of SC has entered a new stage of development. Its flexible dispersion properties, especially the zero-dispersion wavelength (ZDW), is facilitate to adjust by changing the air-hole diameter and the hole pitch, thus can be easily designed to satisfy the phase-matching requirements of broad SC. Currently, an independent high peak power pump laser is usually indispensable in conventional SC generation. In 2018, Ma et al., discovered a new SC generation method which is directly from a RFL [10]. Recently, our group reported a RFL directly generates visible to near-infrared SC [15] and the 20 dB bandwidth is about 660 nm. However, compared with the commonly-used SC generation method by using a high peak power pulsed laser to pump a piece of PCF, both the spectral bandwidth and flatness are not as good and which restricts its application to a broader field. Therefore, it is very necessary to carry out further research on the optimization of spectral width and flatness, to not only make the RFL a competitive SC generation method but also enrich the research scope of nonlinear fiber optics and random fiber lasers.
In this paper, a random fiber laser based on random distributed Rayleigh scattering in PCF is reported for the first time. 150 meters of PCF is used in a half-opened RFL, the splicing loss can be effectively reduced by the joint use of transition fiber and controlled hole collapse technique. The process and the reason that a flat and broad SC generated from the RFL are explained. A flat SC spanning 400 nm to 2300 nm with a 20 dB bandwidth of more than 1600 nm is obtained. To the best of our knowledge, it is the broadest SC generated directly from a RFL ever reported.
2. Experimental setup
The experimental setup of the half-opened cavity RFL based on PCF is shown in Fig. 1. A laser diode (LD) with central wavelength of 976 nm is used as the pump source and the maximum average output power is 25 W. An optical fiber mirror (OFM) locates at one end of the cavity has a 1060 nm central wavelength and 40 nm reflective bandwidth. Five meters of double clad ytterbium-doped fiber (YDF) with core/cladding diameter of 10/125 µm and core NA of 0.08 is used to provide active gain and has a 5.34 dB/m absorption coefficient around 976 nm. A customized (2 + 1) × 1 combiner is exploited to connect the three parts mentioned above. A section of 150 meters’ solid core PCF is used to provide random distributed feedback as well as the nonlinear medium for spectral broadening. The PCF has a core/cladding diameter of 7.3/125 µm with 5-layers of hexagonal air holes arranged in the cladding. The micrograph cross-section of PCF is inserted into Fig. 3. Due to the mode field mismatch of the YDF and PCF, a direct splice will bring high loss. Thus, a section of SMF-28 fiber manufactured by Corning Incorporated is used as the transition fiber. It has a length of 0.5 m and core/cladding diameter of 8.2/125 µm. The technique of controlled hole collapse is applied to splice the SMF-28 and PCF and the estimated loss is about 0.25 dB. The free pump end of the combiner and the other side of the PCF are angle-cleaved (more than 8 degrees) to prevent end feedback. The output power and spectra are recorded at the right end of PCF with a power meter (Thorlabs, S314C) and three optical spectra analyzers (Yokogawa, AQ6370, AQ6456 and AQ6375) respectively. To ensure the accuracy of the measure, long-pass filters are used to eliminate the effect of high-order diffraction during the spectra recording.
Fig. 1. Experimental setup of the SC source. LD, laser diode; OFM, optical fiber mirror; YDF, ytterbium-doped fiber; PCF, photonic crystal fiber.
3. Results and discussion
3.1 Output power performance
The output power with variation of the LD pump power is shown in Fig. 2. The output power performs a rising trend with the increase of the pump power at first. When the pump power reaches 2.38 W, the output power drops down rapidly from 0.689 W to 0.257 W. This is due to the transformation of RFL into SC which can be substantiated by the alteration between the yellow line and cyan line in Fig. 4. The threshold pump power of SC is 2.38 W, and then the output power scales linearly with the pump to 2.03 W under the pump power of 17 W. Generally, a longer fiber length in the RFL structure is beneficial for SC generation because it has a lower threshold pumped power. However, the SC threshold power in this paper is lower compared with our previous work [15] even if the length of PCF is shorter than the double clad fiber. This is because PCF has a higher nonlinear coefficient than the double clad fiber, which accelerate the generation of SC.
3.2 Supercontinuum generation
The dispersion curve of the PCF is illustrated in Fig. 3, which is calculated by using the COMSOL MULTIPHYSICS software based on the finite-element method. As can be seen, the zero-dispersion wavelength (ZDW) of the PCF is around 1118 nm. The recorded spectra with different pump powers are presented in Fig. 4. While the pump is below the threshold, the spectra have a broad base from 1000 nm to 1200 nm which can be seen on red and yellow line. This broad base is caused by the amplified spontaneous emission (ASE) after the YDF absorbing 976 nm LD pump. In this process, a new broadband pumping light is formed and its spectrum can be effectively extended to the abnormal dispersion region of the PCF which is essential for the formation of broadband SC. It is also observed that many stochastic spectral spikes with narrow bandwidth superimpose randomly above the broad base at pump power of 1.87 W. Presence of these stochastic spikes corresponds to the interaction between the Rayleigh scattering and stimulated Brillouin scattering (SBS) effect that is an obvious characteristic of RFL near threshold [2]. When the pump power increased to 2.38 W, a wide and flat continuum is generated. Finally, a SC spreading from 400 to 2300 nm is obtained under the pump power of 17 W. Figure 5 is the comparison of spectra between the initial formation of SC and the final output SC. It can be intuitively seen that a 20 dB bandwidth of more than 1600 nm is achieved under 17 W pump power. Furthermore, the overall spectral intensity is enlarged by about 9 dB by the increase of pump power. Even at a lower pump, the spectral characteristics of SC are dramatically improved by comparing with our previous work [15], where the SC spanning from 600 nm to 1700 nm and the cascaded Raman and the cascaded four-wave mixing (FWM) causing the spectral curves fluctuate, like the rise and fall of the waves.
The analysis of this phenomenon can refer to the SC generation using a continuous-wave (CW) pump laser [11]. As we can see in Fig. 3, the spectral bandwidth of the new broadband pump (the broad base mentioned above) ranges from 1000 nm to 1200 nm, which is partially in the normal and partially in the abnormal dispersion region of PCF. Meanwhile, abundant spectral spikes with narrow bandwidth appeared in the abnormal dispersion region of PCF which also plays a role for SC generation. For the case of the broad base pumped in the normal dispersion region, the spectra can be easily extend to the long wavelength by stimulated Raman scattering (SRS). Once the Stokes waves induced by SRS cross above the ZDW of PCF, modulation instability (MI) and soliton related effects will lead to wavelength broadening of the continuum. For the other case pumped in the abnormal dispersion region, MI initially causes the temporal breakup of long pulse and convert them into a large bunch of randomized fs-level solitons before dramatic spectral broadening. Then, soliton dynamics play a significant role to extend the spectrum, where the long wavelength side is driven by the Raman induce soliton self-frequency shift (SSFS) while the corresponding short wavelength side is owing to the dispersive wave generation. As the solitons are being red-shifted under SSFS effect, it will capture a mass of dispersive waves which satisfy the group velocity matching condition, so the dispersive waves are blue-shifted and resulting in the simultaneous extending of the spectra in the long-wave and short-wave directions. Due to the attenuation in silica rises dramatically above ∼2.2 µm, a sharp long wavelength edge can be observed in the SC spectra. Additionally, the short wavelength edge of SC is also limited by the restrictions imposed on the long wavelength edge, because the short wavelength part of the spectra is generated through group velocity matched interactions between the solitons composing of the long wavelength part and the dispersive radiation in the short wavelength components. The peak wave ω (1040 nm) is induced by the combination effects of the emission feature of the Ytterbium ions which have an emission peak near 1030 nm and the OFM have a central wavelength of 1060 nm. And another peak wave ω (1090 nm) is almost located at the Raman gain peak of the wave ω (1040 nm). After SC is generated, the spectral peak near 1090 nm is decreased with the increase of pump power. This is because SSFS effect is more efficient with the increase of pump power which can effectively transfer the power around 1090 nm to the long wavelength band. Because the ASE source has large phase and intensity fluctuations due to its incoherence [21], the solitons are generated with a wide distribution of parameters such as duration, bandwidth and frequency, so that each of them experiences a red-shift with a different rate. This number of solitons exhibiting diverse central frequencies and a superposition of the numerous soliton red-shift spectra forms a smooth spectral extension [22]. Moreover, both the random distributed feedback (RDFB) and OFM have a broad reflective band, closed loop paths of lights with different wavelength can be oscillated successfully in the half-opened cavity when the pump is amplified to provide sufficient gain. Based on the above analysis and discussion, a superior SC source can be realized with excellent spectral flatness and broad bandwidth. To further understand the details of these physical mechanisms, a complete theoretical framework, which comprise both the RFL dynamics and SC generation dynamics, will be established in our next work.
3.3 Temporal characteristics
A digital phosphor oscilloscope (2 GHz bandwidth) and an InGaAs photodetector (1 GHz bandwidth) monitor the temporal characteristics of SC. Near the SC generation threshold, stochastic pulses and irregular giant pulses are emerged in the time domain, which is similar with the characteristics of RFL near threshold [2]. When the threshold is met, the peak power of the stochastic pulses is excited high enough to induce multiple nonlinear effects for SC generation. It is this characteristic of RFL that results in the dramatically expanding of SC while comparing with the ASE source pumping method [21]. The temporal characteristics of output SC at different pump power are illustrated in Fig. 6. Despite utilizing a continuous-wave pump source, pulsing operation of SC is presented by the passive spatial-temporal-modulation of pump light which has been demonstrated in our previous work. However, more obvious instability and larger fluctuation of single-pulse waveform are expressed in this paper compared with Fig. 4 in Ref. [15]. Although the pulse duration and repetition rate also increase with the increase of pump power, it is not stable. An explanation might be as follows. Substantial numbers of solitons with different characteristics are initially created from MI in the above mentioned process. The whole spectro-temporal window above the pump wavelength can be filled with these solitons. Because of their large number and different group velocities, a train of solitons can collide and that give rise in an unusual temporal event, which is identified as optical rogue waves (RWs) [23,24]. Temporally powerful spikes with a peak power many times higher than the mean peak power can be formed through solitonic collisions. The strong solitons have an inclination to capture energy from the weaker ones during such a collision [25]. The immediate effect of the solitonic collision can further enhance the SSFS of the strongest solitons [26], which is also the reason why the SC source can be extends to the almost longest wavelength allowed to be transmitted by the silica fiber. Moreover, the typical features of RWs are the unpredictably and randomness that such rare and strong events can “appear from nowhere and disappear without a trace” [27]. Consequently, as the rogue optical solitons generated in the red edge of the spectrum and with the passive spatial-temporal-modulation of pump light, these extreme and unpredictable pulses can be superimposed randomly in the single pulse and causing the pulse train perform variability and instability in the time domain. With the increase of pump power, the rogue events will occur more frequently and results in greater instability in the pulse train and violent fluctuation in the single-pulse, as shown in Fig. 6. The results are in good agreement with those obtained in Ref. [23] in which the frequency of the occurrence of the rogue events also increases with the average pump power.
Fig. 6. Temporal characteristics of output SC under pump power (a)-(b) 6W; (c)-(d) 11W; (e)-(f) 17W.
4. Conclusion
In this paper, a new type of RFL based on random distributed Rayleigh scattering in PCF is investigated for the first time to optimize the spectrum width and flatness of SC. The Raman-induced soliton self-frequency shift and the trapping of dispersive waves are the main nonlinear mechanisms for spectral broadening. A flat SC spreading from 400 nm∼2300 nm is obtained under 17 W pump and the 20 dB bandwidth is more than 1600 nm. To the best of our knowledge, it is the broadest SC ever reported from a RFL. Temporally unstable pulses derived from optical rogue waves that caused by solitonic collisions. Compared with a conventional SC generation method using a high peak power pulsed laser to pump a piece of PCF, the SC generated by using RFL based PCF in this paper has similar spectral width and flatness. However, a simpler structure and much lower cost, which not only has promising prospect for a wide variety of applications, can enrich the research scope of SC as well as RFL.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2019ZR02).
Disclosures
The authors declare no conflicts of interest.
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