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Abstract
In this study, we presented a high-power widely tunable all-fiber narrowband superfluorescent fiber source (SFS) by employing two tunable bandpass filters and three amplifier stages. More than 935 W output power is achieved, with a slope efficiency of >75% and a beam quality factor of M2=1.40. The tuning of the narrowband SFS ranges from ∼1045 nm to ∼1085 nm with a full width at half maximum linewidth of less than 0.71 nm. The tunable narrowband SFS stably operates without the influence of parasitic oscillation and self-pulsing effects under maximum power. To the best of our knowledge, this study is the first to demonstrate a widely tunable all-fiber narrowband SFS around 1 µm wavelength region with output power reaching kilowatt-level.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power tunable light sources around 1 µm wavelength region have been widely applied in many fields, such as optical fiber sensors, spectroscopy, medical treatment, and spectral beam combining [1,2]. Such high power, wavelength tunable light sources are extensively used as excellent pump sources for tandem pumping, optical parametric amplifiers, and random fiber lasers [3–5]. The emission band of Yb-doped fibers (YDFs) ranges from 976 nm to 1200 nm, which makes them suitable for wavelength tunable operation [6]. In the past few years, many studies have been conducted on conventional tunable Yb-doped fiber lasers (YDFLs) [6–9]. However, the stochastic self-pulse and the interactions of longitudinal modes degrade the temporal stability and performance of tunable YDFLs [10–13].
In recent years, superfluorescent fiber sources (SFSs) have been proven to possess stable temporal property, broad output spectrum, good power scaling potential, and unique performance such as low spectral broadening effects relative to fiber lasers [14]. Narrowband SFSs can be obtained using a spectrum filter. In 2011, Schmidt et al., [15] presented a 697 W narrowband SFS at 1030 nm with full width at half maximum (FWHM) linewidth of 12 pm using two Bragg gratings as the spectrum filter. In 2015, Xu et al., [16] reported a 1.87 kW narrowband SFS at ∼1080 nm with a broad FWHM linewidth of 1.7 nm using fiber circulator and Bragg grating as the spectrum filter. The high output power at 1030 and 1080 nm indicates that the narrowband SFS could operate at high power level and wide wavelength range. Therefore, SFSs provide an ideal option for implementing wide wavelength tunable operation and high-power output. In 2009, Wang et al., [17] presented a tunable narrowband SFS based on spatial structure. The maximum output power is 135 mW. In 2018, Wu et al., [18] reported a 30 W tunable narrowband SFS with a tuning range of 35 nm. The power of the filtered narrowband SFS is excessively low to be amplified because of the low power spectral density (PSD) of the broadband SFS. In 2019, Ye et al., [19] presented a spectrum-manipulable hundred-watt-level high-power SFS with both wavelength and linewidth tenability. The aforementioned studies can only provide tunable narrowband SFSs with output power at hundred-watt-level. Tunable narrowband SFSs with high output power should be developed for various practical applications.
In this study, a wavelength tunable narrowband SFS seed and three amplification stages are used to verify the power scaling and wide wavelength tunable abilities of SFS. Two tunable bandpass filters (TBPFs) with different bandwidths are utilized to gradually reduce the FWHM linewidth of the tunable narrowband SFS during the power scaling process. Thus, a narrowband SFS with adequate power and narrower FWHM linewidth is achieved, that could surmount the low PSD of broadband SFS. The narrowband SFS seed can continuously tune from ∼1026.10 nm to ∼1091.50 nm, and the output power after the main amplifier can be scaled up to kilowatt (kW)-level. A large tuning range of 40 nm (from ∼1045 nm to ∼1085 nm) was obtained at the maximum power by optimizing the parameters of the broadband SFS and amplifiers. Higher output power can be achieved when considerable pump power is launched into the amplifier, and the tuning range can be improved by optimizing the parameters of the broadband SFS.
2. Experiment setup
The experimental setup is depicted in Fig. 1. It consists of a continuously tunable narrowband SFS seed, two preamplifiers and a main amplifier. The continuously tunable narrowband SFS seed mainly contains a broadband SFS, two broadband isolators (ISOs), and a TBPF (provided by Advanced Fiber Resources (Zhuhai) Ltd). The YDF used in the broadband SFS has the length of 10 m with a peak cladding absorption coefficient at 976 nm is 3.9 dB/m. The diameter of the core is 10 µm with 0.075 NA and that of the inner cladding is 130 µm with 0.46 NA. The YDF is pumped by a 25 W fiber coupled laser diode (LD) operating at 976 nm through a (2 + 1) × 1 signal-pump combiner. Two broadband ISOs with 30 dB isolation are fused after the forward output to enhance the ability to prevent the possible backward signal and protect the components of the broadband SFS. The ISOs are followed by a TBPF, to filter the output spectrum for obtaining a series of narrowband SFSs with different wavelengths. The operating wavelength of the TBPF ranges from 1020 nm to 1100 nm with bandwidth of ∼1 nm. Before injecting the narrowband SFS into the preamplifier, a beam splitter with splitting ratio of 99:1 is adopted to monitor the output spectrum after TBPF 1. The backward port is angle-cleaved around 8° to suppress the parasitic laser oscillation. The reflection of residual pump and signal can influence the stability of the continuously tunable narrowband SFS seed. Thus one cladding power stripper (CPS) is adopted to strip the cladding light. The backward port output is measured to monitor the operating state of the system.
Two preamplifiers are used to further power scaling of the narrowband SFS. The YDFs used in the two amplifiers have a length of 5 m. They have the same parameters as that used in the continuously tunable wavelength SFS seed. A 25 W LD and an 85 W LD operating at 976 nm are used in preamplifier 1 and 2, respectively. A CPS, a broadband ISO with >30 dB isolation, and a TBPF with bandwidth of ∼0.5 nm are placed between the preamplifiers. The ISO can protect preamplifier 1 from any unwanted back reflections. The TBPF 2 can make all the narrowband SFSs obtain a narrower linewidth and a higher signal-to-noise ratio (SNR), thereby increasing the performance of the main amplifier. A CPS is also used to strip the cladding light after preamplifier 2. Ultimately, the power of narrowband SFSs are amplified to 1 W-level and 30 W-level, respectively.
The main amplifier is pumped by six 200 W-class non-wavelength-locked LDs at 976 nm through a (6 + 1) × 1 signal-pump combiner. The pump light is injected into a piece of 6 m-long double cladding YDF exhibiting 30/400 µm core/inner clad diameter with 0.06/0.46 NA. The cladding absorption coefficient at 976 nm is 2.8 dB/m. For thermal dissipation and mode instability suppression, the YDF is coiled on the surface of water cooling aluminum plate with a diameter of about 14 cm. A CPS is spliced after the main amplifier to strip out the residual pump and cladding light, and then a quartz block head (QBH) is utilized to deliver the output light of the main amplifier.
3. Experiment results and discussions
The broadband SFS was first investigated. The output power of the broadband SFS reaches 2.23 W with the increase in pump power. The spectral characteristics at different output power are shown in Fig. 2. For the broadband SFS, long gain fiber will enhance the reabsorption of the spontaneous emission. Thus short wavelength light can be used as the pumping light for long wavelength light, and the output spectrum range and FWHM linewidth of the broadband SFS will be expanded. Therefore, a piece of 10 m YDF is used to obtain a broader output spectrum range. At the maximum output power, the output spectrum range of the broadband SFS covers from ∼1020 nm to ∼1100 nm with an FWHM linewidth of 31.1 nm, and the central wavelength of output spectrum is 1047.9 nm.
As shown in Fig. 3(a), continuous wavelength tuning range of ∼65 nm from 1026.1 nm to 1091.5 nm is achieved. The TBPF 1 adopted in the continuously tunable narrowband SFS seed is used for this purpose. The bandwidth of TBPF 1 is set to 1 nm for obtaining a narrowband SFS with a higher power. Figure 3(b) shows the output power of narrowband SFS. The filtered output power varies with the wavelength. The lowest power is 0.706 mW at 1091.5 nm. The maximum power is 50.15 mW at 1035.4 nm. The SNRs of narrowband SFSs are more than 20 dB. The different SNRs of narrowband SFSs are caused by the emission spectrum of the broadband SFS and the filter property.
Although the narrowband SFS seed can be tuned from 1026.1 nm to 1091.5 nm, wavelengths shorter than 1040 nm are not easily amplified because of large net-gain at other wavelength regions. Therefore, significantly higher levels of amplified spontaneous emission is the limitation of the power scaling of the SFS ranging from 1026 nm to 1040 nm. To only use one set of experimental facility, without special parameters designed for the wavelength shorter than 1040 nm, and maximize the power as wide as the wavelength range, five wavelengths longer than 1040 nm namely, 1045, 1055, 1065, 1075, and 1085 nm, were chosen to investigate the amplification characteristics over the tuning range. The narrowband SFSs are amplified by preamplifier 1 and further filtered by TBPF 2. Utilizing TBPF 2 can filter the amplified spontaneous emission (ASE) at other wavelength regions. The output power of the five wavelengths are amplified to 1 W-level, and their FWHM linewidths vary between 0.4 and 0.6 nm. The difference between FWHM linewidths is induced by the property of TBPF 2. Therefore, the FWHM linewidths can be further improved to be more uniform by using a TBPF with better performance. Then, the narrower signals are injected into preamplifier 2 and scaled up to 30 W-level. Figure 4(a) shows the output spectra of preamplifier 2 at the maximum output power, and the narrowband SFSs show a high SNR of more than 25 dB. Figure 4(b) shows the maximum output power and the FWHM linewidths of the power scaling at different wavelengths. The FWHM linewidths of the five wavelengths are 0.60, 0.59, 0.52, 0.48 and 0.40 nm, respectively.
Fig. 4. (a) Measured output spectra at the maximum output power of preamplifier 2 (0.1 nm resolution). (b) Maximum output power and FWHM linewidth of the power scaling at different wavelengths.
Figure 5(a) depicts the output power and slope efficiencies of the main amplifier versus the pump power at different wavelengths. The corresponding maximal output power of the five wavelengths are 962.5 W, 935.7 W, 1019.7 W, 1013.2 W, and 975.5 W, respectively. Due to the dependence of emission/absorption cross sections on wavelength, there are slight differences among the slope efficiencies of all these selected wavelengths, and slope efficiencies of all the five wavelengths are >75%. The results indicate that the widely tunable narrowband SFS can provide 1 kW-level output power at any wavelengths in the range of 1045–1085 nm. During the power scaling process, the output power linearly increases with the increase in pump power, indicating that further power scaling is available in the case of more powerful pump source. However, the 1 kW-level is the highest output power that can be achieved in the experiment because of limited pump LDs. Figure 5(b) shows the FWHM linewidths versus pump power of the main amplifier. All the FWHM linewidths have slightly broadened by approximately 0.1 nm with a coefficient of less than 1.03×10−4 nm/W. The spectral broadening could be attributed to the nonlinear effects, such as self-phase modulation [20–22]. The linear fitting results indicate that the power scaling has well maintained the narrow linewidth characteristics of the narrowband SFS seed.
Fig. 5. (a) Output power and slope efficiency of the main amplifier versus pump power at different wavelengths. (b) FWHM linewidths versus pump power of the main amplifier.
Figures 6(a)–6(e) show the evolution of detailed spectrum versus output power at different wavelengths. Slight ASE composition at other wavelength regions is observed in the amplified light, as shown in Figs. 6(a) and 6(e). The ASE composition at other wavelengths can be suppressed in the main amplifier by increasing the extinction ratio of TPBF. The spectra with an SNR of more than 30 dB at maximum output power are depicted in Fig. 6(f), and no stokes light can be found in the power scaling process (for 1045 nm). Details of the fine spectrum of 1045, 1065, and 1085 nm at full power are plotted in Figs. 6(g), 6(h), and 6(i). The FWHM linewidths of all these narrowband SFSs are less than 0.71 nm. Although the FWHM linewidths in the power scaling process was broadened not obvious, the spectral sideband in the main amplifier was broadened obviously for each wavelength. The spectral characteristics needs be further improved if these types of light sources are applied in some demanding areas, such as spectral beam combining. Several strategies, such as optimizing spectral shapes and spectral linewidths of seed signals [15,22], will help to optimize the spectral properties of the narrowband SFSs to obtain a narrower spectral linewidth without spectral sideband at a specified output power.
Fig. 6. Evolution of detailed spectrum versus output power at different wavelengths: (a) 1045 nm, (b) 1055 nm, (c) 1065 nm, (d) 1075 nm, (e) 1085 nm. (f) Output spectra of the main amplifier at the maximum output power. (g), (h), (i) Details of the fine spectrum of 1045, 1065, and 1085 nm at full power, respectively. (0.1 nm resolution)
A Si photodetector (100 MHz bandwidth) and a digital phosphor oscilloscope (500 MHz bandwidth, 5 GS/s sampling rate) are utilized to measure the temporal characteristic. Figure 7 shows the temporal stability of high power tunable narrowband SFS. The root-mean-square value (RMS) of the temporal signal is calculated to be 0.634% (at 1000 W). The measurement in ms-class domain and Fourier spectrum of the time trace, as depicted in the insertion graph of Fig. 7, can demonstrate that no parasitic oscillation or self-pulsing is generated. A beam quality of M2=1.40 at 1 kW output power and 1065 nm wavelength is measured by Primes laser quality monitor, as shown in Fig. 8. The beam quality can be further improved by employing gain fiber with more appropriate parameters.
4. Conclusions
In this study, we present an all-fiber tunable narrowband Yb-doped SFS with a tunable seed source and three amplification stages. The high-power narrowband SFS can be tuned within the range of 1045–1085 nm with FWHMs of ∼0.5–0.7 nm. The output power at wavelengths between 1045 and 1085 nm can reach kW-level with slope efficiencies higher than 75%. The temporal output of the tunable narrowband SFS is stable and robust. The M2 factor at 1065 nm is measured to be 1.40 at 1 kW output power. Extended power scaling can be available with more powerful pump source, and larger tuning range can be obtained by optimizing the parameters of the narrowband SFS.
Funding
Shaanxi Provincial Science and Technology Department (2018ZDXM-GY-051, 2018ZDXM-GY-060).
Disclosures
The authors declare no conflicts of interest.
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