Open Access
Add to BibSonomy
Abstract
We demonstrate a high power Yb-doped burst-mode all-fiber laser system operating at GHz intra-burst repetition rate. To our knowledge, it is the first report utilizing dissipative soliton resonance (DSR) to generate tunable burst-mode rectangular pulses. Due to the tunable duration and the rapid rise/fall time for DSR pulses, a 1-10 ns adjustable burst pulse duration is achieved. The intra-burst with sinusoidal waveform can be tuned from 0.8 GHz to 1.5 GHz and actively modulated by an electro-optic modulator (EOM). Amplified by a three-stage Yb-doped fiber amplifier (YDFA), the output power achieves 304 W at 10 ns of burst duration, and the maximum peak power reaches over 50 kW at 2 ns of burst duration. This laser system is anticipated to be applied to generate high power arbitrary microwave signal.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Burst-mode is a laser pulse operation generating a bunch of high repetition rate pulse at a much lower repetition rate. Because of the special composite mode of the pulses, burst-mode lasers have attracted strong interests in extensive fields, such as materials processing [1–5], LIDAR [6,7], high-speed imaging [8–10] and precision surgery [11]. In recent years, high power pulsed microwave generation based on photo-conductive semiconductor switches (PCSS) is proved to have the smarter frequency agility and the higher power capability compared to the conventional microwave generation [12,13]. In this mechanism, the time-domain shape of the generated microwave is similar to that of the burst-mode laser pulse which serves as an exciting source of photocurrent [14]. Therefore, in order to achieve high frequency, high power, and tunable microwave signal, the corresponding burst-mode laser with high intra-burst repetition rate, high peak power, and flexible time-domain tunability is required. Besides, the bandwidth of the microwave frequency is determined by the intra-burst laser pulse waveform, hence the sinusoidal waveform is ideal for generation of narrow bandwidth microwave.
The reported burst-mode pulse generation methods mainly include the following. The first is to utilize an Acousto-Optic Modulator (AOM) to periodically cut off the pulse trains generated by a short-pulse laser [15–18]. This method is unable to continuously adjust parameters of burst-mode pulse, and the output power is limited. The second method is to employ two external modulators to modulate the continuous wave (CW) or a pulse laser diode (LD) seed to burst-mode pulses [14,19]. It obtains burst-mode pulses with adjustable parameters, but brings great insertion loss, which further limits the output power. These two methods are not suitable for generating high-power burst-mode pulse due to the lower output power, which leads to severe amplified stimulated emission (ASE) during the amplification process. The third is to multiply additional pulses by a pulse multiplier [20–22]. It is difficult to obtain continuous adjustment parameters of burst-mode pulse as well, and the construction of the laser system is complicated. The fourth method is to directly modulate LD to generate burst-mode pulse [23]. In the case of the envelope less than 5 ns, relatively long circuit response time leads to the disharmonious intensity of sub-pulses on the front and back edges. Therefore, none of the above approaches is appropriate for the burst-mode pulse generation with high repetition rate, high power, flexible tuning, and rapid rising edge.
For high power photoconductive microwave generation, the burst-mode pulse is required to be amplified to high energy. As far as we are aware, the envelope shape of high power amplification using the EOM modulation is very similar to the amplified square-shaped pulse [24,25]. Compared to the square-shaped pulse with the same envelope duration, the modulated pulse has the higher peak power, which made it harder to amplify. Therefore, the burst-mode amplification is more challenging than the square-pulse amplification.
In this paper, we report a hectowatt-level burst-mode all-fiber laser system generating GHz burst-mode pulse with tunable burst duration and intra-burst repetition. A DSR passive mode-locked laser is used to generate rectangular pulse with a repetition of 1.58 MHz, and then an EOM is employed to actively modulate the rectangular DSR pulse to rectangular burst-mode pulse. Tunable burst-mode laser with 0.8-1.5 GHz of intra-burst repetition rate and 1-10 ns of burst duration is achieved. Through a three-stage amplifier, the output power is up to 304 W at 10 ns and peak power reaches over 50 kW at 2 ns. The central wavelength and the FWHM are 1064.3 nm and 0.27 nm, respectively. This laser system can be employed to generate high repetition rate microwave signal at high frequency band. To the best of our knowledge, this is the first time that DSR passive mode-locked laser is employed to generate rectangular burst-mode pulse while combing with the modulation of EOM.
2. Experimental setup
A schematic of the experimental setup is depicted in Fig. 1. The burst-mode laser system is composed of a 1064 nm DSR passive mode-locked fiber laser as a seed source and a master oscillator power amplifier (MOPA) configuration with modulation of EOM. The DSR laser is designed in a dumbbell-shaped configuration, as shown in Fig. 1(a). The cavity contains a piece of Yb-doped fiber (YDF) pumped by a 976 nm LD, a nonlinear optical loop mirror (NOLM) and a Sagnac reflector. A section of 3.2 m long Yb-doped double-clad fiber, of which absorption coefficient ∼4.20 dB/m at 975 nm and core/inner-cladding diameter of 10/130 µm, is used as the gain medium. A 976 nm wavelength-stabilized multi-mode LD with the maximum output power of ∼8 W delivers pump laser into the YDF through a (2 + 1) *1 pump/signal combiner. The NOLM is composed of a 10/90 optical coupler (OC), a ∼100 m long double-clad passive fiber and two in-line polarization controllers (PCs). The PCs are utilized to adjust the intracavity polarization state to realize the mode-locking operation. The NOLM performs not only as an artificial saturable absorber (SA) to start the mode-locked laser, but also as a cavity mirror with reflectivity of 35%. The left Sagnac reflector is constructed by a 50/50 OC and a 1064 nm optical fiber filter with 3-dB bandwidth of 2 nm. The pump dumping point is deployed between the YDF and the 10/90 OC to eliminate the un-absorbed pump laser. In the oscillator, all the fibers are 10/130 µm double-clad fibers except for the LD pump fiber (multi-mode fiber 105/125 µm).
Fig. 1. The schematic diagram of the laser system. (a) The DSR mode-locked fiber laser as a seed. BP: bandpass filter; OC: optical coupler; LD: laser diode; YDF: Yb-doped fiber; PC: polarization controller; (b) The modulation module and the three-stage YDFA. EOM: electro-optic modulator; TIWDM: tap isolator wavelength division multiplexer hybrid; YSF: Yb-doped single-mode fiber; ISO: isolator.
The MOPA system consists of a GHz modulation module and a three-stage YDFA, as shown in Fig. 1(b). The sine-wave oscillator generates a tunable and stable sinusoidal electrical signal, whose frequency can be tune from 0.8 to 1.5 GHz. By employing a modulation driver with a 10 GHz bandwidth, the sinusoidal electrical signal is amplified, and then it is transmitted to a 20 GHz bandwidth EOM as the modulating signal. Thanks to the flexible modulation of the EOM, the rectangular pulse signals can be modulated to burst-mode pulses of tunable intra-burst repetition rate. The YDFA is composed of a core amplifier and two cladding amplifiers in series. A 976 nm LD with maximum pump power of 400 mW forward pumps a piece of 50 cm Yb-doped single-mode fiber (YSF) whose core/ cladding diameter is 4/125 µm via a tap isolator wavelength division multiplexer hybrid (TIWDM), which combines 3 functions: signal detection, return light isolation and wavelength division multiplexing (WDM). One end of the first pre-amplifier is spliced to a 1 W isolator (ISO) to avoid the backward reflections of second pre-amplifier. In the second pre-amplifier, a (2 + 1) ×1 signal-pump combiner is applied to deliver the pump-light of 976 nm from an 8 W LD to a piece of 4.1 m long YDF of which the absorption coefficient is 4.20 dB/m at 975 nm and the core/inner-cladding diameter is 10/130 µm. An isolator bandpass filter (IBP) is spliced before the main amplifier to attenuate the effect of the high-power backward light and filter out the nonlinear spectral broadening arising during the amplification. In the main amplifier, six 70 W 976 nm LDs deliver pump-light via a (6 + 1) ×1 combiner to the gain fiber. The gain fiber used in the stage is 3.9 m long YDF with the absorption coefficient of 4.8 dB/m at 975 nm and the core/inner-cladding diameter of 25/250 µm. The passive fiber and gain fiber are utilized as short as possible in order to reduce the nonlinear accumulation. The output end is angle cleaved and the residual pump laser is dumped out. In order to promote the nonlinearity threshold in the laser system, the parameters (e.g. fiber length and pump power) and devices (e.g. bandpass filters) of the laser system is carefully designed for suppressing the nonlinear effect such as stimulated Raman scattering (SRS).
The waveform characteristics are detected by a 2-GHz digital oscilloscope (Rigol, MSO8204), connected with an 8-GHz InGaAs photodetector. The autocorrelation trace of the DSR pulse is monitored by an autocorrelator (APE, PulseCheck 150) with a measuring range of 150 ps and a resolution of 75 fs. The radio-frequency (RF) spectrum is measured with a high-resolution spectrum analyzer (Rigol, RSA5065). An optical power meter is employed with a 500 mW probe for the seed power and a 400 W probe for the amplified power. An optical spectrum analyzer (Yokogawa, AQ6375) with a resolution of 0.05 nm is utilized to scan the output spectrum.
3. Results and discussions
In the experiment, adjusting statement of the PCs, the stable DSR mode-locked state could be attained when the pump power reaches over 0.68 W. The evolution of the waveform with different pump power is exhibited in Fig. 2(a). As the pump power increasing from 0.68 W to 2.02 W, the rectangular pulse broadens from 1 ns to 10 ns gradually and the peak power remains nearly stable. The output pulse maintains the single pulse state without wave breaking or other unstable nonlinear phenomena. When the pump power is over 2.02 W, the DSR pulse will break up to the multi-pulse state, so the pulse duration is limited to further increase. The flat autocorrelation trace in a 150 ps span showed in the inset of Fig. 2(a) indicates no sub-pulse structure, which distinguishes the DSR pulse from the noise-like pulse [26]. The RF spectrum with a 300 MHz span is presented in Fig. 2(b). The modulation frequency of the envelope is ∼100 MHz, corresponding to the pulse duration of 10 ns. The inset of Fig. 2(b) shows the 1 MHz span of RF spectrum, of which the fundamental repetition rate is 1.58 MHz and the signal-to-noise ratio (SNR) is 90 dB. As given in Fig. 2(c), the central wavelength and 3 dB bandwidth of the corresponding optical spectrum of 5 ns pulse are 1064.28 nm and 0.18 nm, respectively. The spectrum would barely change with variation of the pulse duration. Figure 2(d) shows the linear relationship the pulse duration and the output power changing with the pump power. The output power elevates from 25 to 170 mW with the pump power increasing.
Fig. 2. (a) Output pulse waveforms of mode-locked laser in different pump power; Inset: interferometric autocorrelation trace over a 50 ps span. (b) RF spectrum with a 300 MHz span; Inset: RF spectrum at the fundamental frequency; (c) The spectrum of the DSR pulse; (d) Variation of pulse duration and output power with different pump power.
Employing the EOM working stably in the linear region, the rectangular pulse is modulated into the rectangular burst-mode pulse. By adjusting the output frequency of the oscillator, the intra-burst repetition rate of the burst-mode pulses is continuously tunable, and the temporal shapes of 10 ns burst-mode pulses with different intra-burst repetition rate (0.8, 1, 1.5 GHz) is demonstrated in Fig. 3(a), indicating the sinusoidal waveform. Further, the burst duration can be tuned continuously since the DSR pulse duration adjusted freely by pump power. Figure 3(b) exhibits the waveforms with different burst duration (2, 5, 10 ns) at a constant intra-burst repetition rate (1.5 GHz). Noting that the temporal shape of the burst-mode pulse with 2 ns burst duration is not deformable as the result of the fast rise/fall edge of the burst. Obviously, the temporal parameters of the rectangular burst-mode signal, including intra-burst repetition rate and burst duration, could be tuned easily while maintaining the temporal characteristics of envelope, so that the requirements of generating microwave at high frequency band can be satisfied.
Fig. 3. The temporal shapes of burst-mode pulses output from the EOM in (a) different intra-repetition rate (0.8, 1, and 1.5 GHz, 10 ns) and (b) different burst durations (2, 5, and 10 ns, 1.5 GHz).
Fixing the seed parameter to the burst duration of 10 ns and the intra-burst repetition of 1.5 GHz, the variation of the output power of the burst-mode laser from the main amplifier at different pump power is shown in Fig. 4(a). The slope efficiency is 85.7%, and the output power is up to 304 W at the pump power of 355 W with the corresponding pulse energy of 191.14 µJ and the optical conversion efficiency of 85.1%. As far as we know, this is the highest power reported so far for the GHz burst mode all-fiber laser system. The waveforms in different output power are depicted in Fig. 4(b). In a saturated amplifier, the envelope would be distorted for the large gain of the front end of a pulse [27]. As can be observed, the burst envelopes distort more and more with the increase of the output power. Figure 4(c) shows the corresponding amplified spectra in a 150 nm range. When the output power reaches ∼160 W, the modulation instability (MI) rises on either side of the central wavelength, and the SRS occurs in the 1120 nm band over the output power of 200 W. The calculated peak power reaching the nonlinear threshold is about 20 kW. In the case of the maximum output power, the intensity difference of signal and MI lasing is ∼29.6 dB. The 3-dB spectral bandwidth before and after amplification is respectively 0.23 nm and 0.27 nm, shown in Fig. 4(d). The signal peak of laser basically maintains the narrow bandwidth after the main amplifier. Further loading the pump power, the signal intensity stops to increase, but the nonlinear component elevates rapidly, and the convert efficiency begins to decrease. It is believed that the maximum output power is limited by the enhancement of nonlinear effect.
Fig. 4. Output characteristics of the amplified burst-mode laser (1.5 GHz, 10 ns): (a) The output power versus pump power; (b) The waveforms in different output power (86.4, 230.5, and 304 W); (c) The corresponding amplified spectra in a range of 150 nm; (d) The fine spectra at the signal band of laser.
At the same pump power, it is obviously that the amplified peak power is increasing with the decline of the burst duration, leading to the more and more severe nonlinear effect. The pump power stops increasing when the intensity difference of the signal and the nonlinear component spectrum is less than 30 dB. Therefore, the maximum output power the laser system can reach in different burst durations have a significant difference, as shown in Fig. 5(a). For instance, the maximum output power with the burst duration of 2, 5, and 10 ns at 1.5 GHz intra-burst repetition rate are respectively 85.9, 160.2, and 304 W, and the slope efficiencies are respectively 76.1%, 80.6%, and 85.7%. In addition, the efficiency and the output power are almost consistent in different intra-burst repetition rates. In order to research the distortion of the burst envelope, we define a waveform distortion factor as a standard deviation of the peaks of all intra-burst pulses in a burst pulse. The waveform distortion factors in different output power and seed parameters are depicted in Fig. 5(b). Fixing the intra-burst repetition rate at 1.5 GHz, we compare variation of waveform distortion factor with output power under different burst durations. When the output power falls short of the nonlinear threshold, there exists a rough linear rise for the waveform distortion with the output power, and the waveform distortion is the least in the case of 2 ns and is the largest in the case of 10 ns. However, when the output power exceeds the nonlinear threshold and approaches the maximum power, the rising speed of waveform distortion is slightly accelerated. It indicates that the. If the power goes up further, it can be predicted that the more severe waveform distortion will occur with the further enhancement of non-linearity. In addition, with the same burst duration and the output power, the distortion factors are basically invariable in different intra-burst repetition rates in different intra-burst repetition rates. The measurement of power stability at hectowatt-level output power is depicted in Fig. 6. The relative standard deviation (RSD) of the power fluctuation is 0.23% over a time span of 1 hour at an output power of ∼304 W (1.5 GHz, 10 ns).
Fig. 5. (a) The maximum output power the laser system can reach in different burst durations and the corresponding slope efficiency; (b) The waveform distortion factors in different output power and seed parameters.
4. Conclusion
In conclusion, we have reported a compact all-fiber Yb-doped rectangular burst-mode fiber laser with high intra-burst repetition rate based on DSR. Due to the combination of the passive mode-locked laser and active modulation, the compact laser system structure is achieved. Tunable burst-mode laser with 0.8-1.5 GHz of intra-burst repetition rate and 1-10 ns of burst duration is achieved. The burst-mode pulse can maintain the rectangular envelope less than 2 ns for the fast rise/fall time of DSR. The output power of the burst-mode pulse is amplified to 304 W by a three-stage YDFA. The maximum peak power of the burst-mode pulse is over 50 kW at 2 ns of burst duration. The tunable parameters and hectowatt-level power of the burst mode laser could be intended for the generation of high-power microwave at high frequency band.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR06).
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.
References
1. C. Kerse, H. Kalaycıoğlu, P. Elahi, B. Çetin, D. K. Kesim, Ö Akçaalan, S. Yavaş, M. D. Aşık, B. Öktem, H. Hoogland, R. Holzwarth, and FÖ Ilday, “Ablation-cooled material removal with ultrafast bursts of pulses,” Nature 537(7618), 84–88 (2016). [CrossRef]
2. C. Kerse, H. Kalaycıoğlu, P. Elahi, Ö Akçaalan, and F. Ilday, “3.5-GHz intra-burst repetition rate ultrafast Yb-doped fiber laser,” Opt. Commun. 366, 404–409 (2016). [CrossRef]
3. M. Domke, V. Matylitsky, and S. Stroj, “Surface ablation efficiency and quality of fs lasers in single-pulse mode, fs lasers in burst mode, and ns lasers,” Appl. Surf. Sci. 505, 144594 (2020). [CrossRef]
4. H. Kalaycıoğlu, P. Elahi, Ö Akçaalan, and FÖ Ilday, “High-repetition-rate ultrafast fiber lasers for material processing,” IEEE J. Sel. Top. Quantum Electron. 24(3), 1–12 (2018). [CrossRef]
5. K. Mishchik, G. Bonamis, J. Qiao, J. Lopez, E. Audouard, E. Mottay, C. Honninger, and I. Manek-Honninger, “High-efficiency femtosecond ablation of silicon with GHz repetition rate laser source,” Opt. Lett. 44(9), 2193–2196 (2019). [CrossRef]
6. N. Ma, M. Chen, C. Yang, S. Lu, X. Zhang, and X. Du, “High-efficiency 50 W burst-mode hundred picosecond green laser,” High Power Laser Sci. Eng. 8, e1 (2020). [CrossRef]
7. P. Yi, P. Tong, and Y. Zhao, “Application Comparison of LIDAR Single-pulse and Multi-pulse Mode in High Elevation Difference Area,” in Third International Symposium on Laser Interaction with Matter, Y. M. Andreev, Z. Lin, X. Ni, and X. Ye, eds. (2015).
8. N. Ji, J. C. Magee, and E. Betzig, “Erratum: High-speed, low-photodamage nonlinear imaging using passive pulse splitters,” Nat. Methods 6(3), 197–198 (2009). [CrossRef]
9. L. Tan, W. Jing, G. I. Petrov, V. V. Yakovlev, and H. F. Zhang, “Photoacoustic generation by multiple picosecond pulse excitation,” Med. Phys. 37(4), 1518–1521 (2010). [CrossRef]
10. M. Ekelof, J. Manni, M. Nazari, M. Bokhart, and D. C. Muddiman, “Characterization of a novel miniaturized burst-mode infrared laser system for IR-MALDESI mass spectrometry imaging,” Anal. Bioanal. Chem. 410(9), 2395–2402 (2018). [CrossRef]
11. P. Elahi, H. Kalaycıoğlu, Ö Akçaalan, Ç Şenel, and FÖ Ilday, “Burst-mode thulium all-fiber laser delivering femtosecond pulses at a 1 GHz intra-burst repetition rate,” Opt. Lett. 42(19), 3808–3811 (2017). [CrossRef]
12. Q. Wu, Y. Zhao, T. Xun, H. Yang, and W. Huang, “Initial Test of Optoelectronic High Power Microwave Generation From 6H-SiC Photoconductive Switch,” IEEE Electron Device Lett. 40(7), 1167–1170 (2019). [CrossRef]
13. J. Zhang, D. Zhang, Y. Fan, J. He, X. Ge, X. Zhang, J. Ju, and T. Xun, “Progress in narrowband high-power microwave sources,” Phys. Plasmas 27(1), 010501 (2020). [CrossRef]
14. X. He, B. Zhang, S. Liu, L. Yang, J. Yao, Q. Wu, Y. Zhao, T. Xun, and J. Hou, “High-power linear-polarization burst-mode all-fibre laser and generation of frequency-adjustable microwave signal,” High Power Laser Sci. Eng. 9, e13 (2021). [CrossRef]
15. P. Elahi, S. Yılmaz, Y. B. Eldeniz, and FÖ Ilday, “Generation of picosecond pulses directly from a 100 W, burst-mode, doping-managed Yb-doped fiber amplifier,” Opt. Lett. 39(2), 236–239 (2014). [CrossRef]
16. H. Kalaycioglu, K. Eken, and FÖ Ilday, “Fiber amplification of pulse bursts up to 20 µJ pulse energy at 1 kHz repetition rate,” Opt. Lett. 36(17), 3383–3385 (2011). [CrossRef]
17. Y. Liu, J. Wu, X. Wen, W. Lin, W. Wang, X. Guan, T. Qiao, Y. Guo, W. Wang, X. Wei, and Z. Yang, “> 100 W GHz femtosecond burst mode all-fiber laser system at 1.0 µm,” Opt. Express 28(9), 13414–13422 (2020). [CrossRef]
18. H. Yang, Y. Chen, K. Ding, F. Jia, K. Li, and N. Copner, “3.5 ps burst mode pulses based on all-normal dispersion harmonic mode-locked,” Appl. Phys. B 126(8), 127–135 (2020). [CrossRef]
19. M. Nie, X. Cao, Q. Liu, E. Ji, and X. Fu, “100 µJ pulse energy in burst-mode-operated hybrid fiber-bulk amplifier system with envelope shaping,” Opt. Express 25(12), 13557–13566 (2017). [CrossRef]
20. M. Han, X. Li, S. Zhang, Y. Dan, and Z. Yang, “Generation of Soliton Bursts with Flexibly Controlled Pulse Intervals Based on the Dispersive Fourier-Transform Technique,” IEEE J. Sel. Top. Quantum Electron 25(4), 1–6 (2019). [CrossRef]
21. K. Wei, P. Wu, R. Wen, J. Song, Y. Guo, and X. Lai, “High power burst-mode operated sub-nanosecond fiber laser based on 20/125 µm highly doped Yb fiber,” Laser Phys. 26(2), 025104 (2016). [CrossRef]
22. S. Cai, B. Wu, Y. Shen, and P. Jiang, “Compact narrow bandwidth, linearly polarized burst-mode fiber amplifier with high burst energy of 1 mJ,” Laser Phys. 30(9), 095101 (2020). [CrossRef]
23. T. Chen, H. Liu, W. Kong, and R. Shu, “Burst-mode-operated, sub-nanosecond fiber MOPA system incorporating direct seed-packet shaping,” Opt. Express 24(18), 20963–20972 (2016). [CrossRef]
24. J. Zhao, D. Ouyang, Z. Zheng, M. Liu, X. Ren, C. Li, S. Ruan, and W. Xie, “100 W dissipative soliton resonances from a thulium-doped double-clad all-fiber-format MOPA system,” Opt. Express 24(11), 12072–12081 (2016). [CrossRef]
25. H. Yu, R. Tao, X. Wang, P. Zhou, and J. Chen, “240 W high-average-power square-shaped nanosecond all-fiber-integrated laser with near diffraction-limited beam quality,” Appl. Opt. 53(28), 6409–6413 (2014). [CrossRef]
26. X. Zheng, Z. Luo, H. Liu, N. Zhao, Q. Ning, M. Liu, X. Feng, X. Xing, A. Luo, and W. Xu, “High-energy noiselike rectangular pulse in a passively mode-locked figure-eight fiber laser,” Appl. Phys. Express 7(4), 042701 (2014). [CrossRef]
27. D. N. Schimpf, C. Ruchert, D. Nodop, J. Limpert, A. Tunnermann, and F. Salin, “Compensation of pulse-distortion in saturated laser amplifiers,” Opt. Express 16(22), 17637–17646 (2008). [CrossRef]
