GHz high power Yb-doped picosecond fiber laser and supercontinuum generation Jing Gao,1 Tingwu Ge,1,* Wuyi Li,2 Hongshen Kuang,1 and Zhiyong Wang1 1

Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China 2

State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China *Corresponding author: [email protected]

Received 29 August 2014; revised 15 November 2014; accepted 17 November 2014; posted 18 November 2014 (Doc. ID 221951); published 19 December 2014

We demonstrated a 97 W all-fiber picosecond master oscillator power amplifier seeding by an actively harmonic mode-locked Yb-doped fiber laser. The laser seed pulse duration was 7.7 ps at a 1.223 GHz repetition rate with a central wavelength of 1062 nm. In addition, by launching the amplified pulses into a 5 m long photonic crystal fiber, we obtained a 41.8 W supercontinuum covering the wavelength from 600 to 1700 nm with a 10 dB bandwidth of 1040 nm. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (320.6629) Supercontinuum generation; (060.2320) Fiber optics amplifiers and oscillators. http://dx.doi.org/10.1364/AO.53.008544

1. Introduction

At present, high-power ultrashort pulsed Yb-doped fiber lasers have been extensively used in numerous applications such as material processing, laser marking and welding, military, nonlinear optics, and the pump source for near-infrared supercontinuum (SC) generation [1–6]. In most of these applications, both high average powers and high peak powers are in demand. Although the master oscillator power amplification (MOPA) technique has been successfully adopted in laser average power scaling for many years, the main limitation of pulse fiber laser power scaling is nonlinear effects such as self-phase-modulation (SPM) and stimulated Raman scattering (SRS) [7,8]. Two key approaches to solve the problem are to increase the fiber effective core area and shorten the active fiber lengths by using large mode area (LMA) and highly doped gain fibers [9]. However, in the applications of pumping SC generation, there is a serious conflict between the core diameter and coupling 1559-128X/14/368544-05$15.00/0 © 2014 Optical Society of America 8544

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efficiency. This is because the beam quality might degrade due to the employment of the LMA fiber, and usually the highly nonlinear fibers used for generating a SC exhibit small core areas (the core radii of which are typically smaller than 5 μm [10]). Furthermore, the usage of the highly doped gain fiber brings a big challenge to thermal management at high average power operating conditions. Therefore, increasing the repetition rate or the pulse duration of the laser seed turns out to be an effective way to reach high average powers as well as to suppress the detrimental nonlinear effects. However, for an SC pump application, short pulse duration is beneficial for a wide wavelength range and better flatness. Consequently, researchers tend to reach a high average power SC by utilizing lasers with high repetition rates. For a gain switched laser diode seed, a 21 ps repetition rate of 908 MHz at 1060 nm was demonstrated; by feeding the seed into a MOPA system with a 25 μm core active fiber in the final amplifier, a 100 W average power of multimode output was obtained [11]. After that, a similar MOPA amplifying 28 ps optical pulses up to an average output power

of 200 W at 1040 nm (with a 858 MHz repetition rate) was reported; the gain fiber used in the main amplifier was with a 30 μm core [9]. Although these achievements are significantly impressive, it was proven that the use of a free space pump and signal coupling restrict the output power scaling when it was used in pumping SC generation; the thermal accumulation at the end of the PCF reduced coupling efficiency and maybe lead to serious facet damage [12]. For a semiconductor saturable absorber mirror (SESAM) passively mode-locked Yb-doped fiber laser, by using an extra-cavity repetition rate system, a tri-stage all-fiber amplifier chain with a core diameter of 30 μm (16 ps, 96 W, at a repetition rate of 478 MHz) was obtained [13]. It is should be noted that the capability of the repetition rate improvement depended on the amount of the couplers; the more couplers we used the more seed power we lost, which was detrimental for the subsequent amplification. On the other hand, by utilizing a short linear cavity, an impressive achievement was realized in a picosecond Yb-doped fiber laser with a fundamental repetition rate up to 843 MHz [14], but there is no subsequent amplification result report. In this paper, we present an all-fiberized highpower picosecond pulsed Yb-doped fiber MOPA source for SC generation. The picosecond oscillator was mode-locked by a LiNbO3 intensity modulator in a ring cavity (with a tunable repetition rate from a fundamental repetition rate of 29.83 MHz to a 67th harmonic order repetition rate of 1.998 GHz). In the amplification, we set the repetition rate to 1.223 GHz for the purpose of minimizing the detrimental nonlinearity effects in the later fiber amplifiers. Compact four-stage amplifiers were used to boost the average output power to 97 W with a slope efficiency of 82.6%. Finally, by launching the amplified picosecond pulses into a 5 m long photonic crystal fiber (PCF) with a 1037 nm zero dispersion wavelength (ZDW), we obtained a high-power SC with an average output power of 41.8 W (ranging from at least 600 nm to beyond 1700 nm in the PCF with a 10 dB spectral bandwidth of 1040 nm). The SC optical-to-optical conversion efficiency is 52%. 2. Experimental Setup

The experimental setup of the high average power picosecond, Yb-doped, all-fiber MOPA and SC generation system is shown in Fig. 1. The laser seed, schematically shown in Fig. 2, is an actively harmonic mode-locked fiber oscillator, similar to the one illustrated in [15]. All polarization-maintained (PM) single-mode fibers are used in the ring cavity. The active fiber of the oscillator is a 1 m PM Yb-doped single mode fiber [i.e., a PM Yb-doped double cladding fiber (PM-YDF)], and the core absorption is 250 dB∕m at 975 nm. A 400 mW fiber pigtailed 975 nm laser diode was coupled into the system via a 975/1064 nm wavelength division multiplexer (WDM). The LiNbO3 Mach–Zehnder intensity modulator is used as the mode-locking component; this is controlled by a pulse

Fig. 1. Schematic of the high average power picosecond, Ybdoped, all-fiber MOPA and SC generation system.

pattern generator (PPG). A 1064  5 nm band-pass filter is inserted in the cavity to control the laser wavelength. A PM isolator is employed following the seed source to block the backward propagation light. In the first preamplifier, the gain fiber is a 1 m long Yb-doped fiber (6 μm core and a core absorption of 250 dB∕m at 975 nm) pumped by a maximum power 450 mW fiber pigtailed 975 nm laser diode. In the second preamplifier, a 10 m long LMA–YDF is used as the gain fiber, which exhibits a core diameter of 10 μm, an NA of 0.08, inner cladding diameter of 130 μm, an NA of 0.46, and a cladding absorption of 1.6 dB∕m at 915 nm. The gain fiber is pumped by a 915 nm, 9 W fiber pigtailed laser diode via a 2  1 × 1 fiber combiner. In the third preamplifier, the gain fiber is a 5 m long LMA–YDF with 15/130 μm core/ cladding diameter, and its cladding absorption is 5.4 dB∕m at 975 nm. A 975 nm fiber pigtailed laser diode is coupled into the gain fiber through a 2  1 × 1 fiber combiner which delivers a maximum power of 25 W. In the main amplifier, the gain fiber is the same as the one used in the third amplifier; two 60 W, 975 nm laser diodes are used as the pump source. A 2  1 × 1 high power pump combiner with a coupling efficiency of ∼92% is used to deliver pump light into the active fiber. Strippers with a matched passive fiber are used in all stages to dump the residual pump. A mode field adaptor (MFA) is employed as the coupling components between the MOPA system and the PCF. It exhibits an input pigtail of 15/130 μm double cladding passive fiber and an output pigtail of a 6/125 μm single-mode passive fiber. The output pigtail of MFA is spliced to a piece of 5 m long homemade PCF, which exhibits a core diameter of 4.8 μm and a zero dispersion wavelength at 1037 nm [16]. The output of the PCF is spliced with a 20 cm long 15/130 μm DCF with an 8° angle cleaved facet serving as the end-cap; this reduces back reflection and increases the PCF facet damage threshold. In order to detect the mode-locking state of the laser seed, a 3 GHz spectrum analyzer (Agilent, N9320B) is used to monitor the radio frequency spectrum, and an optical spectrum analyzer (YOKOGAWA, AQ 6370C) is used to measure the spectrum. The pulse duration is measured by an autocorrelator (Femtochrome Research, FR-103WS). The output 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS

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Measured Spectrum Gauss Fit Spectrum

Intensity (dBm)

-20 -30

FWHM=0.3 nm

-40 -50 -60 -70 1056

1058

1060

1062

1064

1066

1068

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Wavelength (nm) Fig. 2. Schematic setup of the tunable actively mode-locked fiber laser seed.

Fig. 4. Gaussian-shaped spectrum of the laser seed.

average power is measured by a power meter produced by Ophir Optronics (500 W).

Measured curve Gauss fit curve

1.0

3. Results and Discussion

By altering the working frequency of the PPG and data length of the radio frequency signals to the intensity modulator, the repetition rate of the laser seed can tune from 29.83 MHz to 1.998 GHz, corresponding to the harmonic order from first to the 67th, while maintaining the same average output power. In this experiment, we set the repetition rate at 1.223 GHz to comprise the average power and peak power. The RF spectrum of the seed at 1.223 GHz is shown in Fig. 3. The signal-to-noise ratio is 53 dB; this indicates that the mode-locking operation state is stable. As the band-pass filter is inserted in the cavity, the seed central wavelength range can be selected from 1059 to 1066 nm; this is realized by adjusting the working frequency of the PPG in a stably mode-locking operation state. The Gauss-shaped output spectrum is shown in Fig. 4, which is centered at 1062 nm with a 0.3 nm, 3 dB bandwidth. The pulse duration is about 7.7 ps if a Gauss pulse profile is assumed (Fig. 5). The output average power of the laser seed is about 20 mW. Then it is amplified to 120 mW, 2.2 W, and 11.2 W in the following three amplifiers,

Intensity (a.u.)

0.8 0.6

7.7*1.414 ps

0.4 0.2 0.0 -40

-20

0

20

40

Time (ps) Fig. 5. Autocorrelation trace of the pulse of the laser seed.

respectively, at a pump power of 450 mW, 5 W, and 21 W. The average output power of the main power amplifier versus incident pump power curve is shown in Fig. 6. The maximum output average power is 97 W when the incident pump power is 117.3 W, which corresponds to a slope efficiency of 76%. As can be seen, the average power increases almost linearly with the increase of incident pump power. Considering the 92% coupling efficiency of the combiner, the main 100

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Signal power Linear fit curve

Signal power (W)

Intensity (dBm)

80

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53 dB

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60

Slope η=76% 40

20

-80

0

0.0

0.5

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Frequency (GHz) Fig. 3. RF spectrum of the seed at a 1.223 GHz repetition rate. 8546

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20

40

60

80

100

120

Pump power (W) Fig. 6. Average output power of the main power amplifier with an increase of the incident pump power.

45

35

pulse duration of the pump laser. The dip around 1380 nm in the spectra is caused by the relatively high OH− absorption peak.

30

4. Conclusion

SC output power (W)

40

25

Slope η=41.1%

20 15

SC output power Linear fit curve

10 5 20

40

60

80

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ps pumped power (W) Fig. 7. Average output power of an SC with an increase of incident pulsed pump power. Inset, SC experiment with 41.82 W average output power.

amplifier efficiency is up to 82.6%. The coupling efficiency of the MFA is in excess of 80% in a high-power operation condition. By launching the picosecond pump laser into the PCF via the MFA, an SC with a maximum power of 41.8 W is generated (Fig. 7). The slope efficiency of the SC to the pump is 41.1% in the experiment. Considering the 80% coupling efficiency of the MFA, the optical-to-optical conversion efficiency of the PCF is estimated as 52%. Figure 7, inset, shows an experimental picture with the corresponding maximum output power. The SC evolution of different pump powers is illustrated in Fig. 8. Figure 8 shows that in the initial process of SC generation, the red- and blue-shifted spectrum broadening components correspond to the solitons and dispersive waves, respectively. As can be seen when the SC output power is 27 W, the continuum width of spectrum wavelength will exceed 1700 nm. As the SC output power is scaled from 27 to 41.8 W, the continuum is getting flatter (with the 10 dB spectral bandwidth increasing from 934 to 1040 nm). The spectrum at the maximum power covers from 600 to 1700 nm (limited by the range of the optical spectral analyzer). Such a good spectrum flatness and wide bandwidth are due to the high repetition rate and the relatively narrower

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Intensity(dBm)

-30 10 dB -40 -50 41.8W 27 W 17 W 11 W 5.5 W 0.6 W

-60 -70 -80 -90 600

800

1000

1200

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Wavelength(nm) Fig. 8. Evolution of the SC spectra with different output powers.

In conclusion, we have experimentally demonstrated a high-power Yb-doped picosecond all-fiber laser source for a SC pump at a 1.223 GHz repetition rate (at 1062 nm). The fiber laser seed is a tunable actively harmonic mode-locked Yb-doped fiber laser. After four-stage amplification, we get a picosecond pulsed laser at an average output power of 97 W, and by launching the pulses into the PCF an SC at an average output power 41.8 W is obtained. The wavelength range is beyond 600–1700 nm with a 10 dB bandwidth of 1040 nm. This is the first demonstration, to our knowledge, of an SC output average power exceeding 40 W, generated from an all-fiber structured actively harmonic mode-locking laser at a GHz repetition rate, with such good flatness covering the visual light components. In the near future, we will amplify the pulses based on this fiber laser at different repetition rates and wavelengths; a promising power tunable with a similar spectral flatness SC source will probably be realized. This paper was supported by the Projects of the Technology Innovation Platform Program under Grant PXM 2011_014204_09_000060, and Development of Industry All-Fiber High Power Lasers Program under Grant 2010 ZX04013-052, China. References 1. B. N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tünnermann, “Femtosecond, picosecond and nanosecond laser ablation of solids,” Appl. Phys. A 63, 109–115 (1996). 2. A. Ancona, F. Röser, K. Rademaker, J. Limpert, S. Nolte, and A. Tünnermann, “Femtosecond and picosecond laser drilling of metals at high repetition rates and average powers,” Opt. Express 34, 3304–3306 (2008). 3. J. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). 4. K. K. Chen, S. U. Alam, J. H. V. Price, J. R. Hayes, D. Lin, A. Malinowski, C. Codemard, D. Ghosh, M. Pal, S. K. Bhadra, and D. J. Richardson, “Picosecond fiber MOPA pumped supercontinuum source with 39 W output power,” Opt. Express 18, 5426–5432 (2010). 5. X. Hu, W. Zhang, Z. Yang, Y. Wang, W. Zhao, X. Li, H. Wang, C. Li, and D. Shen, “High average power, strictly all-fiber supercontinuum source with good beam quality,” Opt. Lett. 36, 2659–2661 (2011). 6. R. Song, J. Hou, S. Chen, W. Yang, and Q. Lu, “High power supercontinuum generation in a nonlinear ytterbium-doped fiber amplifier,” Opt. Lett. 37, 1529–1531 (2012). 7. G. P. Agrawal, Nonlinear Fiber Optics, 4th ed. (Elsevier, 2007). 8. G. P. Agrawal, Applications of Nonlinear Fiber Optics, 2nd ed. (Elsevier, 2008). 9. P. S. Teh, R. J. Lewis, S. Alam, and D. J. Richardson, “200 W diffraction limited, single-polarization, all-fiber picosecond MOPA,” Opt. Express 21, 25883–25889 (2013). 10. http://www.nktphotonics.com/nonlinearfibers. 11. K. K. Chen, S. U. Alam, J. R. Hayes, D. Lin, A. Malinowski, and D. J. Richardson, “Polarisation maintaining 100 W Yb-fiber MOPA producing μJ pulses tunable in duration from 1 to 21 ps,” Opt. Express 18, 14385–14394 (2010). 12. K. K. Chen, S.-U. Alam, D. Lin, A. Malinowski, and D. J. Richardson, “100 W fiberised linearly-polarized picosecond 20 December 2014 / Vol. 53, No. 36 / APPLIED OPTICS

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Ytterbium doped fiber MOPA,” in Conference on Lasers and Electro Optics (CLEO) (2009), paper CWK2. 13. S. Chen, H. Chen, J. Hou, and Z. Liu, “100 W all fiber picosecond MOPA laser,” Opt. Express 17, 24008–24012 (2009). 14. J. Liu, J. Xu, and P. Wang, “High repetition-rate narrow bandwidth SESAM mode-locked Yb-doped fiber lasers,” IEEE Photon. Technol. Lett. 24, 539–541 (2012).

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GHz high power Yb-doped picosecond fiber laser and supercontinuum generation.

We demonstrated a 97 W all-fiber picosecond master oscillator power amplifier seeding by an actively harmonic mode-locked Yb-doped fiber laser. The la...
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