Supercontinuum (SC) generation directly from a random fiber laser (RFL) structure is limited in spectrum span and output power so far. Investigations on wavelength range improvement of SC generated in RFL are analyzed and discussed. The experimental results show that cascaded four wave mixing (FWM) and passive modulation of pump light can explain the appearance of visible components and pulse performance in time domain respectively. To the best of our knowledge, it is the first time a SC covering visible and near-infrared range with 20-dB bandwidth of more than 660 nm is generated directly from a RFL with average output power of 3.4 W, and the spectrum spanning from 600 nm to 1700nm. This work proves that a RFL can be a novel visible to near-infrared SC generation method which has a great potential in various applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Random fiber laser (RFL) has attracted significant attention since the first report by C. J. de Matos et al. in 2007 . Different from conventional fiber lasers, RFL is based on random distributed feedback (RDFB) provided by extremely weak Rayleigh scattering (RS) along hundreds-meters-level even kilometers-level of fiber. Compared with other kinds of fiber lasers, RFL has several special features such as simple and cavity-free structure, robust operation, modeless emission and low coherence [2–4]. These features show excellent potential for various application like speckle-free imaging and free space communication and so on [5,6].
Many works have been carried out to improve the performance of RFL. For the power promoting, the output power has been increased to the record power level of 4 kW with master oscillator power amplification (MOPA) configuration and 919 W in single half-opened cavity [7,8]. For wavelength tuning, up to 8th order Raman Stokes of 1691.6 nm with a spectral purity of more than 90% has been achieved through amplified spontaneous emission (ASE) pumping with good temporal stability . Additionally, novel kinds of fibers have been used to replaced conventional passive fiber. RFL based on tapered fiber has achieved 491 W average output power and 94% spectral purity . A record output power of more than 200 W is recently demonstrated based on 30 m phosphosilicate fiber . In particular, increasing attention has been focused on SC generation by RFL owing to its unique features.
Currently the most used technical setups to obtain visible to near-infrared SC is mainly comprised of two methods: one is using high power laser to pump a piece of photonic crystal fiber (PCF) or high nonlinear fiber and the other is utilizing nonlinear fiber amplifier to get broad spectrum. High splicing loss between double clad fiber and PCF and complicated MOPA configuration are limiting factors for SC in practical applications. In 2018, Ma et al. demonstrated the feasibility of SC generation directly from a typical half-opened RFL and the 20-dB bandwidth of the output SC is 250 nm [12–15]. Recently, our group achieved a near-infrared SC with 20-dB bandwidth of more than 500 nm through RFL . However, more work needs to be done to further broaden the spectrum and increase the output power of SC for practical applications.
In this manuscript, the research on visible to near-infrared SC generation directly form a half-opened RFL cavity is carried out. Longer ytterbium-doped fiber and higher pump power are utilized compared with our previous work . The process of spectrum extending and temporal behaviors of SC are analyzed and discussed. A SC covering visible and near-infrared band is obtained which spans from 600 nm to 1700 nm with 20-dB bandwidth of more than 660 nm. To the best of our knowledge, it is also the broadest SC generation directly from a RFL up to now.
2. Experimental setup
Figure 1 illustrates the schematic diagram of this novel SC source. The experimental setup is constructed in a half-opened RFL configuration. A length of about 1000 meters of germanium-doped fiber (GDF) with a core/cladding diameter of 10/130 µm is used as the medium providing both Raman gain and random distributed feedback. Active gain is supplied by 5 meters of 10/125 double clad ytterbium-doped fiber which exhibits a nominal 5.34 dB/m absorption coefficient around 976 nm. Two different types of gain exist in this cavity, namely the Raman gain and the gain provided by the ytterbium ions, which have been proved to be beneficial for SC generation in our previous work. The pump source is made up of two 976 nm laser diodes (LD) outputting average maximum power of 21.8 W and 22.9 W respectively, corresponding to a total pump power of 44.7 W. The pump power is coupled into the cavity through a (2 + 1) x 1 fiber combiner. An optical fiber mirror (OFM) at one end of the cavity has 40 nm reflective bandwidth with central operating wavelength at 1060 nm. The output fiber end is angle cleaved with more than 8 degrees in case of parasitic feedback.
3. Results and discussion
3.1 Output power performance
The output power of this setup is measured by a power meter (Thorlabs, S314C) with a maximum optical power of 40 W. With the increase of the pump power, the output power performs a trend of ascending firstly, then drops down sharply and rises up again till 3.45 W under the maximum pump power of 44.7 W, which is shown in Fig. 2. The abrupt power decrease can be explained as the power of random lasing is transformed into other wavelengths and the formation of SC , which can be verified by the alteration between olive line and orange line in Fig. 3(a).
3.2 Supercontinuum generation
The chromatic dispersion profiles of the fundamental mode in the GDF is presented in Fig. 3(b) versus wavelength, which is calculated by the COMSOL MULTIPHYSICS software and the MATLAB software. The zero-dispersion wavelength (ZDW) of the GDF is around 1300 nm. The output spectrum is monitored by an optical spectrum analyzer (Yokogawa, AQ6370 and AQ6456) with a resolution of 1 nm. The measured results under different pump power are presented in Fig. 3(a). The broad base of the spectrum is around 1000 nm to 1200 nm which belongs to the ytterbium ions’ emission band can be clearly seen on red and orange line. The broad base can work as new pump source and is beneficial for further spectrum broadening. Substantial random spectral spikes with various bandwidth can be observed over the orange line, which are aroused by the interaction between RS and stimulated Brillouin scattering (SBS) effects. Higher-order SBS Stokes waves are generated in the presence of RS with the increase of pump power, which explains the appearance of stochastic spikes [2,17]. Due to RDFB and OFM with broad reflective band, closed loop paths for lights of different wavelength can be formed through multiple scattering along the ultralong fiber. These numerous closed loop paths vary in loss. Thus, laser oscillation happens in low-loss paths at first, then in higher ones with the increase of the pump power. The emissions from different cavities results in discrete peaks in spectrum. With sufficient pump power, a broad peak is formed since discrete lasing peaks can no longer be distinguished from each other . It may explain that separated Stokes lights in conventional RFLs are connected in this case. After the pump power increased to 4.3 W, SC is generated with a spectrum covering from visible to near-infrared band. Finally, a continuum spanning from 600 to 1700 nm is obtained under a maximum pump power of 44.7 W. Figure 3(c) is the comparison of the spectrum between the initial formation of SC and the final output SC under the maximum pump power. It can be clearly seen that a 20-dB bandwidth of more than 660 nm is achieved under 44.7 W pump power. Meanwhile, the intensity of visible portions is enhanced by about 13-dB by the increase of the pump power.
Compared with our previous work in , visible band spectrum generation is first reported in this paper where cascaded Raman and cascaded four wave mixing (FWM) lead to the generation of visible frequency components [20,21]. Group velocity matching for these processes are feasible by intra-cavity pulses performance (temporal behaviors will be discussed in later paragraphs.) . Partial cascaded FWM phenomena that existed in the half-opened RFL are shown in Table. 1. The first-order to fourth-order FWM processes (No. 1 to No. 4) produce substantial spectral compositions comprising visible portions. From the table we can also find that the peak wave ω (1040 nm) (marked in Fig. 3(c)) participates in most of the nonlinear phenomena which belongs to the emission band of the YDF. Hence, higher gain provided by YDF and higher total power of pump in contrast to our previous setup are all beneficial factors for the generation of ω (1040 nm) and different order of Stokes lights, which makes the spectrum of the SC extending to the visible band feasible. Additionally, the main peak ω (1040 nm) is not in agreement with central wavelength of OFM, which may be caused by the combination effects of the emission feature of ytterbium ions and the OFM. Ytterbium ions have an emission peak near 1030 nm and the OFM has a central wavelength of 1060 nm, so the emission near 1040 nm can be supported by both the ytterbium ions and OFM.
3.3 Temporal behaviors
The temporal output behaviors are measured by an InGaAs photo-detector with a bandwidth of 1 GHz and an oscilloscope with a bandwidth of 2 GHz. Before SC generation, there coexists irregular pulse train and giant pulse in time domain, which is in correspondence with the temporal behaviors of RFL near threshold . The results under different pump power are depicted in Fig. 4 after SC generation. It can be seen from the left row that regular pulse trains are obtained when SC is generated. This kind of time domain character assists the occurrence of cascaded FWM effect mentioned above , which is beneficial for the generation of visible lights. The pulse duration ranges from 76.45 ns to 192.6 ns with the increase of the pump power, the corresponding repetition rate increases from 28.49 kHz to 148.7 kHz. Figure 5 shows the change of pulse duration and repetition rate.
This kind of temporal behaviors in RFLs have not been reported before. In our opinions, this phenomenon may be explained like this: Before SC is generated, ASE with many spikes is emitted around ytterbium ions’ emission band after the 976 nm pump light is absorbed by the YDF. The appearance of spikes is a result of RS-SBS generation near the threshold described in the section 3.2, which also shows irregular pulse train in time domain. The pulse train has a relatively high peak power, which is beneficial for the generation of high order stokes and supercontinuum . When the pump power is sufficient, SC generation is achieved. The SRS effect and the cascaded FWM effect contribute to the spectrum extension to longer and shorter wavelength region respectively. After SC is generated, passive spatial-temporal-modulation of the pump light exists in half-opened cavity . The energy of the pump light is depleted and transferred to the whole spectrum portions of SC through SRS and FWM. These processes need pump light to provide sufficient gain. At a certain moment, the pump power in the cavity cannot offer enough gain for these processes due to consumption [22,23]. So, SC generation cannot be maintained and the system stops lasing. Then, the pump light without depletion can provide enough gain again. In this way, a pulse is formed. Meanwhile, different order Stokes light and other frequency compositions show distinct waveform and duration in the time domain [22–24]. These spectrum components overlap with each other and result in the change of the pulse shape and duration in time domain. Additionally, there exists many power transitions between existing and newly generated spectrum components like high-order and low-order Stokes lights, where existing ones can be seen as a pump source for the newly generated ones. Newly generated waves can acquire energy from various existing waves. The higher the pump power is, the more transfer processes between existing ones and newly generated ones are established. The newly generated compositions appear at different time due to different existing waves. As a result, more pulses are created and result in the increase of the repetition rate.
In this paper, we have investigated the first visible to near-infrared SC generation experiment based on a hybrid-gain and half-opened RFL configuration through cascaded FWM effect. A SC spreading from 600 to 1700 nm is obtained under the maximum pump power of 44.7 W and the 20-dB bandwidth is more than 660 nm. Stable pulse trains are also achieved through intra-cavity passive modulation. To the best of our knowledge, it is the first time that SC generation directly from RFL extents to visible band. At the same time, it is also the broadest SC ever reported by RFL. Compared with conventional SC generation setups, the RFL structure is simple, robust operation with low-coherence and low cost, which has great potential in various applications. Further work will concentrate on the further optimization of the spectrum width and flatness and power scaling of SC.
State Key Laboratory of Pulsed Power Laser Technology (SKL2019ZR02).
The authors would like to acknowledge Xiaoyong Xu, Weide Hong and Sen Guo for their valuable help in experiments.
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