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Abstract
The amplification of random fiber lasers (RFLs) attracts much attention due to their unique characteristics such as wavelength flexibility and low coherence. We present that, in the kilowatt-level amplification of RFL operating near its lasing threshold, a broad and flat spectral pedestal can co-exist with the narrow spectral peak of RFL. This phenomenon is different from the case in the amplification of fixed-cavity laser seeds. Time-domain measurements show that the broad and flat spectral pedestal, which extends to long wavelengths, is composed of temporal pulses, while few temporal pulses exist in the narrow spectral peak. We attribute the spectral pedestal to intensity fluctuations from the random seed laser and modulation instability in the amplification stage. Control experiments reveal that the working status of the random seed laser and the effective length of the amplifier can influence the spectral bandwidth. By taking advantage of this phenomenon, we propose a novel approach to achieve a high-power broadband light source through the amplification of RFLs operating near the lasing threshold.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power fiber lasers (HPFLs) have attracted much attention from all sectors of military, industrial manufacturing, and scientific research, owing to their excellent output power stability and beam quality [1–4]. Adopting a master-oscillator power amplification (MOPA) configurations, the IPG Photonics Corp. developed the first 10-kW-level and 20-kW-level HPFL in 2009 and 2013, respectively [5,6]. The MOPA configuration is considered the most efficient method for power scaling-up.
Compared to the fixed-cavity (FC) laser with defined resonant cavity, the random fiber lasers have some outstanding properties, including cavity-free structure based on distributed feedback, wavelength tunability, suppression of high-frequency relative intensity noise, and intensity dynamics associated with the pump power [7–13]. Since Turitsyn et al. proposed the novel one-dimensional random fiber laser based on Rayleigh scattering and amplified by stimulated Raman scattering (SRS), the field of random fiber laser (RFL) research has undergone rapid development [7,10,14,15]. During the past decade, the power of RFL oscillator has risen from the milliwatt to the watt level and even beyond that to the kilowatt level [16–20]. For further power scaling of the output power from the RFL, the MOPA configuration with RFL as the seed laser comes into sight.
RFLs have been proved to be a suitable seed laser for power scaling [21–24]. In 2015, Zhou et al. demonstrated a kilowatt-level fiber amplifier seeded by an RFL, and then managed to scale the power from 1 kW to 3 kW based on the MOPA structure [25–27]. In 2017, Li et al. demonstrated a kilowatt-level fiber amplifier seed by a Yb-doped RFL with a narrow 3 dB bandwidth as around 0.4 nm [28]. The output power was scaled up to 2.4 kW in 2018 and the 3 dB bandwidth was managed to decrease to 0.23 nm [23]. In 2019, our group adopted a tandem-pumped MOPA seeded by an RFL and increased the output power to 4 kW [29]. We further achieved a 5.1 kW output in 2021 with a band-pass filter after the random seed to suppress the Raman noise [30].
The spectral evolution, statistical properties and temporal dynamics during the power scaling of the RFL have been widely investigated [31–33]. Spectral-broadening-free property is an attractive advantage for the fiber amplifier seeded by the RFL [25]. The bandwidth of the laser signal can be maintained at a steady level during the amplification of the RFL, while it usually increases as the output power rises in the MOPA seeded by FC oscillator without additional suppression method [29,34]. The absence of the spectral broadening is ascribed to the temporal stability output from the random seed. However, we notice the intensity dynamics and statistical properties of the RFL is associated with its operating status [13]. The RFL laser could suffer serious intensity fluctuations while working near its lasing threshold and near thresholds of cascaded Raman scattering [7,13,33,35,36]. This property indicates that the spectral evolution can be totally different in the fiber amplifiers seeded by RFL operating near lasing threshold and well above lasing threshold, respectively. Therefore, it is vital to study how the operating status of the seed influence the spectral evolution in fiber amplifiers seeded by RFLs, especially the trend of spectral broadening.
Previous studies on kilowatt-level fiber amplifiers seeded by RFLs mainly focus on the evolution of 3 dB bandwidth and 10 dB bandwidth. The evolution of the 20 dB bandwidth is usually disturbed by the spectral peak induced by SRS effect and cannot show the intrinsic spectral evolution of the signal peak power. Fortunately, our previous studies have promise us near-4-kilowatt SRS-free fiber amplifiers seeded by RFLs [30]. Thus, we can study the spectral broadening of signal peak thoroughly without disturbance of the possible spectral Stokes peaks. The study can reveal differences of spectral evolution between high-power fiber amplifiers seeded by random laser and by FC laser in a broad spectral range.
In this paper, we experimentally demonstrate the coexistence of a spectral-broadening-free peak at the seed wavelength and a broad and flat spectral pedestal, around 20-dB-lower than the peak, during near-4-kW amplification of RFL operating near lasing threshold at first. Then, the time-domain measurements illustrate that the spectral-broadening-free peak and spectral pedestal are attributed to continuous power and pulsed power of the output laser, respectively. The underlying mechanism is deduced to be the passive Q-switch induced intensity fluctuations from the random seed combining with the modulation instability in the main amplifier. Moreover, control experiments prove the RFL working near lasing threshold and the long fiber in the main amplifier is favor of spectral broadening during the amplification. Based on experimental results, we propose a novel approach to high-power broadband light source via the amplification of the RFL near the lasing threshold. It is the first time that a quasi-3-kW light source of 191.3-nm 20-dB-bandwidth without spectral peaks induced by SRS is presented to the best of our knowledge. Besides, the unique temporal dynamics of the light source indicate a great potential in various applications.
2. Design of experiments
Experimental setup to investigate the spectral evolution during the amplification of the RFL is illustrated in Fig. 1, which consists of four modules. Three control experiments are designed based on it. A random Raman fiber laser seed with two pre-amplification stages provides seeding random laser. As the comparison, an FBG-based oscillator with equal output power serves as a FC seed. Both of the seeds are compatible with the main amplifier. Only one seed can be connected with the main amplifier at a time. The amplified laser is injected into the measuring system for experimental analysis on power, spectral, and temporal properties.
Fig. 1. Experimental setup for the amplification of either random fiber laser seed or fixed-cavity laser seed and measuring system for time-domain recording after beam splitting. The main amplifier only connects with either random fiber seed or fixed-cavity oscillator at one time. YDFL: ytterbium-doped fiber laser. ISO: isolator. HR-FBG: high-reflectivity fiber Bragg grating. OC-FBG: output-coupling fiber Bragg grating. PC: pump combiner. LD: laser diode. CLS: cladding light stripper. YDF: ytterbium-doped fiber. GDF: germanium doped fiber. BS: beam splitter. PM: power meter. OSA: Optical spectrum analyzer. DM: dichromic mirror. PD: photodiode.
The random fiber seed adopts a half-open cavity composed of a high-reflectivity fiber Bragg grating (HR-FBG) centered at 1070 nm and a piece of 400-meter-long passive fiber (Nufern XP-1060, 6/125 $\mu$m, NA:0.14) offering randomly distributed Rayleigh scattering (RS) feedback. The random fiber seed is pumped by a YDFL centered at 1018 nm so that the working status of the random seed can be controlled by the output power of the YDFL for control experiments. The lasing-threshold pump power of individual RFL is 18.4 W. And the maximum pump power of the random seed without spectrum peaks around 1123 nm induced by SRS is 36.8 W. More detailed properties of the RFL seed is demonstrated in the first section of Supplement 1.
Two isolators are used to protect the pump source and the random fiber laser seed from backward-reflected laser light. Pre-amplifier 1 and 2 are forward- and backward-pumped by 976 nm LDs, respectively. A cladding light stripper (CLS) is inserted between two stages to avoid interference of residual pump. The power from the random seed is amplified over 500 W in order to stabilize the main amplification stage.
The FC oscillator is composed of a pair of HR-FBG and output-coupling fiber Bragg grating (OC-FBG) centered at 1070 nm and a piece of 20-meter-long gain fiber (Nufern YDF-20/400). The oscillator is bidirectionally-pumped by 976 nm LDs and can provide a maximum output power about 500 W. Thus, the FBG-based oscillator serves a substitute of the random seed for control experiments. More detailed properties of the FC seed is demonstrated in the second section of Supplement 1.
Nonlinear effects such as modulation instability (MI) induced by self-phase modulation and cross-phase modulation will cause temporal instability and broaden the spectrum at the same time [37–43]. To study these effects during the power scaling-up process, we not only detect the total output power and spectrum but also measuring the temporal output after beam splitting. Ultra-fast photodiode (Thorlabs DET08C/M) and oscilloscope (Tektronix MDO3104) are used for fast temporal measurements after the light has been attenuated properly. The output laser is collimated by a lens group at first. After the collimation lens, a wedge mirror is used for light-splitting from the amplified laser for proper attenuation. A dichromic mirror (DM) is used to split the 1070 nm laser and the broadened light at longer wavelength so that the temporal behavior can be studied separately. The power, spectral and temporal properties of the complete amplified laser can be recorded by substituting the DM with several wedge mirrors as attenuation scheme.
3. Characteristics of spectral evolution
The amplification process of RFL operating near lasing threshold is studied at first. Individual RFL seed provides 2.7 W signal power at 1070 nm near the lasing threshold. It is connected with the main amplifier after pre-amplification and the residual pump power at 1018 nm of the RFL will be absorbed in the cascaded stages, which makes the output power of the system 5.5 W with only RFL operating near lasing threshold injecting. At the same time, the FC oscillator is disconnected with the main amplifier. The passive fiber in the main amplifier after the gain fiber is set as 40 m.
When the pump power of the main amplifier reaches 5 kW, the output power is 3617 W and the spectral evolution is shown in Fig. 2(a). The spectral peak at 1070 nm remains a narrow bandwidth. However, a broad spectral pedestal can be observed around below 20-dB peak power. The phenomenon indicates that the amplified power has been transferred to longer wavelengths instead of remaining at 1070 nm. To further investigate the evolution of the spectrum and power distribution, the spectra on the short-wavelength and long-wavelength sides of the DM are recorded, respectively.
Fig. 2. (a) Evolution of the spectrum of the whole system during the amplification process over whole wavelength range. (b) Evolution of the spectral peak at 1070 nm after DM. (c) Evolution of the flat region beyond 1080 nm after DM. Red dotted lines at 1083 nm in (b) and (c) represent the cutoff wavelength of the DM.
On one hand, the spectra on the short-wavelength side exhibit a steep spectral peak at 1070 nm in the process of amplification (Fig. 2(b)). The 3dB and 10 dB bandwidths of the peak are nearly constants. On the other hands, the spectra on the long-wavelength side exhibit consistent extension to longer wavelength. The spectrum from 1080 nm to 1250 nm becomes flat when the output power increases (Fig. 2(c)).
The power of short wavelengths and long wavelengths is calculated according to the ratio of the attenuated power after the wedge mirror and the DM. The power evolution is illustrated in Fig. 3(a). The increasing of power at short wavelengths slows down when the total output power reaches 1.5 kW. At the same time, the flat pedestal of long wavelengths becomes obvious. The spectral and power evolution indicates that the power of center wavelength can hardly be scaled-up when reaching a certain level. Instead, the pump power is transferred to longer wavelength and the light source exhibits a broadband characteristic. At the highest total output power of 3617 W, the power of short wavelengths and that of long wavelengths are 1139 W and 2478 W, respectively. Over two thirds of power is transferred to long wavelengths beyond 1070 nm.
Fig. 3. (a) Evolution of the power distribution according to the spectra. (b) Final spectrum with a narrow spectral peak and a broad spectral pedestal. (c) Temporal output at the highest power. (d) Radio-frequency (RF) spectrum of the output at the highest power, the inset declares the repetition frequency of the intensity fluctuations is 24.3 MHz.
The final output spectrum before the DM is shown in Fig. 3(b). The 3 dB bandwidth and 10 dB bandwidth of the 1070 nm spectral peak is 1.8 nm and 4.0 nm, respectively, while the 20 dB bandwidth of the 1070 nm spectral peak reaches 166.0 nm. The amplification process exhibits perfect suppression of spectral broadening on the spectral peak at 1070 nm. Meanwhile, flat and broadband pedestal exists at longer wavelength with a peak power nearly 20-dB-lower than that at 1070 nm.
The underlying mechanism of this phenomenon is spectral broadening induced by MI. The random seed lasing near its threshold exhibits serious intensity fluctuations due to the interaction between stimulated Brillouin scattering (SBS) and Rayleigh scattering (RS) [7,13,33,35,36]. As a result, the seed power is divided into two parts – the pulsed power and the continuous power. In the main amplifier, the continuous power is amplified by the gain of ytterbium ions, which leads to the increase of the power at 1070 nm. The pulsed power, however, is amplified by the gain of MI, which leads to not only the spectral broadening but also the pulsation behavior of the output laser [42]. The MI happens in the normal dispersion region of the fiber and is induced by cross-phase modulation(XPM) [37–39]. The pump lights of the XPM may be the signal light and the Stokes light induced by Raman gain of the pulsed power [38]. The XPM can broaden the Raman-induced Stokes light of the high-peak-power pulses so that no obvious spectral peaks induced by Raman gain of the pulsed power can be spotted during the amplification. This effect will promise the flatness of the spectral pedestal. Besides, XPM can consume the power at 1070 nm so that the power at 1070 nm grows slowly and a narrow spectral peak at 1070 nm free of spectrum broadening coexists with the spectral pedestal. And the continuous power at 1070 nm is too low to induce stable SRS, which will be discussed by control experiments later. The temporal behavior and the RF spectrum of the output laser at the highest power proves the existence of pulses in the amplified laser (Fig. 3(c) and Fig. 3(d)).
To further prove that the spectral pedestal is induced by pulses amplified by MI during the amplification of RFL operating near lasing threshold, we record the temporal behavior of the short-wavelength and long-wavelength laser, respectively. The spectra at the highest output after the DM are recorded in Fig. 4(a) and Fig. 4(b). The short-wavelength side is the spectral peak at 1070 nm. The spectral peaks beyond 1300 nm are because of the defect in the longer wavelength of the reflection band. Considering that the power beyond 1300 nm is rather low for the total output power, the influence of these defects can be neglected. The 3 dB and 10 dB bandwidths remain at the steady level. The long-wavelength side is the flat spectral pedestal beyond 1070 nm. The 3 dB, 10dB, and 20 dB bandwidths reach 97.8 nm, 179.4 nm, and 381.4 nm, respectively. Strong pulses exist only on the long-wavelength side, otherwise the temporal behavior is stable and free of pulses on the short-wavelength side (Fig. 5(a)). The repetition frequency of the pulses is 24.3 MHz and the pulse width is approximately 1 ns (Fig. 5(b) and Fig. 5(c)).
Fig. 4. Spectra after the dichromic mirror of (a) short-wavelength side and (b) long-wavelength side.
Fig. 5. Temporal output of different time scales. Blue and red lines represent the short and long wavelengths, respectively.
The temporal behaviors after the DM prove the MI in the main amplifier. The MI will not only amplify the pulses from the RFL seed operating near lasing threshold but also extend the spectrum to longer wavelength [40,41]. Therefore, the pulses only exist on the long-wavelength side. In contrast, the spectral peak at 1070 nm only consists of the continuous power.
4. Results of control experiments
To understand the triggers and influence factors of MI during the amplification of the RFL operating near lasing threshold, several control experiments are designed.
Firstly, we compare the FC seed with the random seed. The pre-amplified random fiber laser seed and the FBG-based oscillator are connected to the main amplifier, respectively. The fiber in the main amplifier is chosen to be 40-meter-long GDF spliced after 40-meter-long YDF to keep other conditions the same. The amplification of the random seed operating near its lasing threshold induces single peak at 1070 nm and a flat pedestal from 1078 to 1280 nm in the spectrum (red line in Fig. 6). This is because MI causes power transfer from the pump power to the pulsed part at longer wavelength in the amplification process of RFL operating near lasing threshold. As a result, the continuous power at 1070 nm remains at a low level of around 1000 W and cannot reach the threshold of SRS, so no obvious spectral peaks induced by the continuous power at 1070 nm can be spotted in the spectrum. Otherwise, the amplification of the FC seed induces two peaks at 1070 nm and 1123 nm in the spectrum. The spectral peak at 1123 nm is at the peak of Raman gain spectrum corresponding to the 1070 nm laser. Comparison of 3 dB and 10 dB bandwidth (red and yellow line in Fig. 7(a) and Fig. 7(b)) shows that the signal peak keeps broadening in the amplification process of the FC seed, because the continuous power in the main-amplifier stage keeps increasing and the four-wave mixing becomes more serious, which is quite different from the amplification of the random seed as the continuous power can be maintained at a steady level during the amplification process of RFL operating near lasing threshold. However, the 20 dB bandwidth during the amplification of the random seed is always broader than that of the FC seed (red and yellow line in Fig. 7(c)). This phenomenon indicates that the MI in the amplification of the random seed is much stronger than that in the amplification of the FC seed. Because there exist more intensity fluctuations in the random laser operating near lasing threshold, which is a necessary condition of the MI. Without spectral peaks induced by SRS, the random seed is more suitable for a spectrally flat and broadband light source.
Fig. 6. Spectrum of the near-4-kW fiber amplifier seeded by RFL operating near its lasing threshold and spectra of three control experiments.
Fig. 7. Evolution of (a) 3 dB, (b) 10 dB, and (c) 20 dB bandwidth of three control experiments on central wavelength at 1070 nm. The red line overlaps the black line in (b).
Secondly, we study the influence of the working status of the random seed on the output spectrum. The comparison of output spectra at the highest output power (red and blue line in Fig. 6) indicates that the status of operating near lasing threshold is more suitable for a broadband light source. On one hand, the 3dB and 10 dB bandwidth of the random seed working above the threshold with higher pump power of 36.4 W grows more rapidly than that of the random seed operating near the threshold (red and blue line in Fig. 7(a) and Fig. 7(b)). On the other hand, the 20 dB bandwidth of the amplification process of random seed above lasing threshold is much smaller than that of the amplification process of random seed near the threshold (red and blue line in Fig. 7(c)). The intensity fluctuations from the random fiber laser are due to the passive Q-switch induced by SBS and RS in the long passive fiber [7,35]. First, the RS causes a few resonant modes at the maximum gain to reach the SBS threshold when the system operates near lasing threshold. Then, lasing extracts the energy restored in the fiber. Subsequently, the energy needs to accumulate again; therefore, the lasing will pause momentarily. Pulse train generates owing to the constant repetition of this process. When the pump power increases, the spectral broadening caused by the nonlinear effect decreases the spectral power density. As a result, the threshold of SBS will no longer be reached and the pulses will fade and even disappear [7,13]. Therefore, when the random seed works above lasing threshold, more continuous power remains at 1070 nm during the amplification process and 3 dB and 10 dB bandwidth grows more rapidly. Notably, the remaining continuous power even induces a spectral peak at 1123 nm due to SRS effect in amplification of the random seed working above lasing threshold under pump power of 36.4 W. When the random seed operates near lasing threshold, the increase of 20 dB bandwidth due to MI is stronger and a broadband light source beyond the spectral peak at 1070 nm can be obtained.
Finally, the passive fiber spliced after the gain fiber in the main amplifier can enhance the spectrum broadening. For comparison, we utilize 40-meter-long and 70-meter-long GDF once a time. The random seed is set to operate near its lasing threshold. The light source with the longer GDF exhibits a broader spectrum (red and black line in Fig. 6) because the gain spectrum of MI is broader with larger effective length. It is noted that the output power of the system with 40-meter-long GDF is only about 100-watt-higher than that of the system with 70-meter-long GDF. The remaining power of the short wavelengths is nearly the same. Therefore, the 3 dB and 10 dB bandwidths of both systems evolve in the same way (red and black line in Fig. 7(a) and Fig. 7(b)). Moreover, the 70-meter-long GDF can make sure the light source has broader 20 dB bandwidth at the same pump power level (red and black line in Fig. 7(c)).
5. Discussion on a novel broadband light source
The experiments show that the MI will cause spectral broadening during the amplification of the RFL operating near its lasing threshold and the extension shows great flatness without spectral peaks induced by SRS effect. It is further inferred that the amplification of the RFL can provide a high-power broadband light source by enhancing the MI in the main amplifier.
The control experiments show that the working status of the random seed and the length of fiber in the main amplifier can influence the MI effect. When the random seed working above its lasing threshold under pump power of 36.4 W and only 40-meter-long GDF is spliced after the gain fiber in the main amplifier, the 20 dB bandwidth of the light source at the highest output power is 38.6 nm. To set the random seed operating near its lasing threshold and splice 70-meter-long passive fiber after the gain fiber in the main amplifier, the 20 dB bandwidth is increased to 191.3 nm (Fig. 8). The spectrum from 1078 nm to 1200 nm shows great flatness while the spectral peak at 1070.2 nm only has 1.6-nm 3 dB bandwidth and 4.2-nm 10 dB bandwidth. The final spectrum indicates that more than 60 $\%$ of the output power lies in the flat spectral region, which leads to a high-power broadband light source.
The random seed operating near its lasing threshold and the long passive fiber in the main amplifier is in favor of broadening the spectrum methodologically. It is easy to control the spectral bandwidth by controlling the working status of the RFL and the fiber length in the main amplifier. One can simply splice longer passive fiber after the gain fiber in the main amplifier to further broaden the spectrum.
6. Conclusion
In conclusion, we reveal experimentally the difference on spectral evolution between the kilowatt-level amplification of the random seed and FC seed in a broad spectral range. During the amplification of the RFL operating near lasing threshold, a narrow spectral peak is maintained while a flat spectral pedestal extends to long wavelengths around 20-dB-lower level. Time-domain measurements after wavelength division reveal that the spectral pedestal is composed of pulsed power and the spectral peak is composed of continuous power only. The mechanism for spectral broadening is deduced to be the intensity fluctuations from the seed and the MI in the main amplifier. We prove experimentally that spectral broadening could be enhanced by setting the random seed near the lasing threshold and lengthening the fiber in the main amplifier. By utilizing these properties, we demonstrate a kilowatt-level broadband light source with 20 dB bandwidth of 191.3 nm and no obvious Raman-induced spectral peaks can be observed. The light source shows a potential of further spectral broadening via increasing the length of passive fiber in the main amplifier. Besides, unique feature of continuous and pulsed power mixture by wavelengths may make the light source a versatile tool for various applications by means of wavelength division.
Funding
National Natural Science Foundation of China (62122040, 62075113, 61875103).
Acknowledgments
The authors would like to thank Dr. Zhoutian Liu, Lele Wang, and Ying Zhao for help in experimental operations.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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