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We report on a passively mode-locked femtosecond fiber oscillator using only fiber-based components without intracavity dispersion compensation. The all-normal dispersion fiber laser operates in the dissipative-soliton regime and utilizes a spectral filter for pulse shaping. The 3.8 ps long pulses with pulse energies of 3.6 nJ can be dechirped with a grating compressor to 76 fs. The output spectrum reveals a full width at half maximum of 39.7 nm and a center wavelength of 1032 nm. The repetition rate is 71 MHz. The influence of pulse energy variation is discussed.
©2011 Optical Society of America
1. Introduction
In the last years, ultrafast ytterbium-doped fiber lasers have proven to be promising candidates to compete with their solid-state counterparts regarding pulse energy and pulse duration. Additionally, they offer the advantages of a waveguide device such as passive air cooling, low cost and in case of an all-fiber-integrated design alignment free operation. The major goal of the current research is to achieve maximum pulse energy with minimum pulse duration for applications such as frequency metrology, material processing, or biophotonics.
This effort led from the stretched-pulse regime [1] with the shortest pulse duration of 28 fs up to date from an ytterbium-fiber laser [2] to the similariton laser [3] and more recently to the development of all-normal dispersion ytterbium-doped fiber lasers. These oscillators have no intracavity dispersion compensation and the pulse dynamic is strongly dependent on group velocity dispersion, fiber nonlinearity, as well as spectral and temporal filtering of chirped pulses [4]. With this approach, pulse energies above 20 nJ with pulse durations below 200 fs were reported using core-pumped normal-dispersive fibers in combination with bulk optical components [5].
Despite the impressive results from these fiber lasers the demonstrated systems contain free-space optics and do not fully profit from the benefits of the waveguide medium such as alignment-free operation and improved decoupling from technical noise. This is mainly due to the lack in performance and availability of fiber-based components at 1 µm wavelength compared to bulk optics with much higher peak and average power capabilities. An all-fiber-integrated femtosecond laser would also be of great interest as a seed source for all-fiber amplifiers for the generation of ultrashort µJ pulses [6].
There are a few reports on fully fiber-integrated Yb based femtosecond lasers without dispersion compensation. Nielsen et al. demonstrated a linear resonator with polarization-maintaining fibers generating 5.6 ps pulses with 1.3 nJ energy [7]. Prochnow et al. used a semiconductor saturable absorber mirror (SESAM) applied to a fiber connector together with nonlinear polarization evolution (NPE) for mode-locking and achieved 0.8 nJ pulses that could be dechirped to 627 fs [8]. Fekete et al. also used a SESAM in combination with NPE to generate 0.2 nJ pulses compressible to 195 fs [9]. Kieu et al. used a saturable absorber based on carbon nanotubes resulting in 3 nJ pulses that could be dechirped to a duration of 235 fs [10]. Similar results were demonstrated by Özgören et al. using a fiber-based Lyot filter resulting in 230 fs 1.5 nJ pulses [11]. Schultz et al. used NPE and a fiber-based filter for stabilizing mode-locking which resulted in 1.8 nJ pulses with a compressed pulse duration of 179 fs [12]. Shorter pulses with 131 fs duration could be demonstrated by Shohda et al. having comparatively low pulse energies of approximately 55 pJ due to the dispersion-managed soliton pulse shaping [13].
Here, we demonstrate to the best of our knowledge the highest pulse energies and shortest pulses from an all-fiber-integrated Yb laser. The oscillator is mode-locked via NPE and utilizes a 90% linear output coupler to reduce the intracavity peak power and therefore prevent damage of the fiber-coupled isolator. The laser generates linearly polarized 3.6 nJ pulses at a repetition rate of 71.3 MHz with an average power of 259 mW that can be dechirped to a duration of 76 fs. The short pulse duration is achieved by minimizing the total normal cavity dispersion [14,15], while at the same time spectral broadening and NPE are strong enough for mode-locking. The laser demonstrates a notable improvement in terms of pulse duration and pulse energy compared to previous results.
2. Experimental setup
The experimental setup is shown in Fig. 1 . The 21 cm long highly ytterbium-doped fiber (4.2 µm mode-field diameter (MFD), 16 dB/cm peak absorption at 976 nm, 3.1 wt. % doping concentration) is core-pumped with a single-mode pump diode via a wavelength-division multiplexer (WDM) combining pump and signal light at 976 nm and 1030 nm. A maximum pump power of 731 mW is available behind the WDM. 58 cm of Corning HI 1060 Flex fiber with a MFD of 4.2 µm follows the active fiber to increase the impact of the fiber nonlinearity; all other fibers have a MFD of about 6.3 µm. A fused fiber coupler with 90% output coupling ratio and an anti-reflection coated angle-polished connector is followed by the spectral filter. It is implemented as a second WDM, whose spectral properties are shown in Fig. 2(a) . On a linear scale, the WDM shows an almost sinusoidal transmission spectrum with a full width at half maximum (FWHM) of 10.6 nm around a transmission maximum of 1033.8 nm.
Fig. 1 Schematic of the fiber ring cavity. WDM, wavelength-division multiplexer; YDF, ytterbium-doped fiber; OC, output coupler; PC, polarization controller.
Fig. 2 (a) Measured transmission spectrum of the small bandwidth WDM serving as a spectral filter. Solid curve, input; dashed curve, output; dotted curve, relative transmission. (b) Photodetector signal of the pulse train.
A fiber-based Faraday isolator including a polarizer causes the suppression of one polarization for the NPE mode-locking. All components are fusion-spliced together. In order to adjust the polarization state in the fiber mechanical polarization controllers in front of and behind the isolator were applied. The total resonator fiber length is approximately 2.9 m, while the fiber of the output coupler is about 1.0 m long. The total cavity dispersion is about 0.067 ps2. Because of the sensitivity of the mode-locking mechanism regarding temperature changes, we implemented the laser in a temperature stabilized housing with passive air-cooled heat-sinks and a 40 × 40 cm2 footprint.
3. Results
Once the polarization controllers are adjusted for clean single-pulse mode-locking the oscillator is self-starting when increasing the pump power. Switching the pump diode off and on again results in the return of the laser to the same state of operation. Figure 2(b) shows the stable 71.3 MHz pulse train at the maximum pump power of 731 mW measured with a photodiode having a 18.5 ps rise time and an oscilloscope with a 6 GHz bandwidth. We verified single-pulse operation by monitoring the pulse train with the same photodiode and a 70 GHz sampling oscilloscope as well as with a long-range autocorrelator having a 150 ps scan-range.
Dechirping of the pulses was accomplished by using reflection gratings with a line density of 300/mm and a total efficiency of 61.3%, which results in compressed pulse energies of 2.2 nJ. The compressor dispersion is calculated to be approximately −0.063 ps2. Because the signal is nonlinearly polarized by the NPE and the grating compressor is polarization sensitive, we added a polarizing beam splitter (PBS) behind the 90% output coupler. A quarter- and a half-wave plate in front of the PBS were adjusted in order to achieve the smallest autocorrelation FWHM. Simultaneously, the corresponding powers of the transmitted and rejected light were measured which resulted in 259 mW for the transmitted polarized signal, while 44 mW are filtered out by the PBS. Thus, 3.6 nJ are accessible in front of the compressor. The transmitted spectrum with the FWHM of 39.7 nm, the rms width of 25.2 nm, and a central wavelength of 1032 nm is shown in Fig. 3(a) and exhibits the typical structure of all-normal dispersion fiber lasers with peaks at the edges. Figure 3(b) shows the spectrum filtered out by the PBS.
Fig. 3 Measured spectra. (a) Transmitted and (b) rejected signal from the PBS behind the output coupler.
The autocorrelation traces of the compressed and uncompressed pulses are shown in Fig. 4(a) and (b) having a FWHM of 102.5 fs and 5.1 ps, respectively. The small side peaks at 9.6 ps in the autocorrelation of the chirped pulses are due to reflections at the approximately 1 mm thick beam splitter of the autocorrelator and are always present independently of the laser operation state. The Fourier-limited autocorrelation has a FWHM of 86.0 fs. Using the deconvolution factor of 1.351, which was calculated from the Fourier-limited pulse and autocorrelation widths, the pulse durations of the compressed and chirped pulses are 76 fs and 3.8 ps, respectively. Thus, the compressed pulse width is 19% above the Fourier-limit of 64 fs.
Fig. 4 (a) Autocorrelation of the compressed pulses (solid curve) and of the Fourier-limited pulses (dotted curve) and (b) detail of the side peak in the autocorrelation. Inset: autocorrelation of the uncompressed pulses.
The prominent modulation in the spectra of Fig. 3(a) and (b) corresponds to a satellite pulse approximately 2.2 ps apart from the main pulse. The Fourier-limited main pulse contains > 97% of the total pulse energy. No further satellite pulses were observed.
To analyze the stability of the generated pulse train we measured the radio-frequency spectra with an RF spectrum analyzer and a fast photodetector. The results are shown in Fig. 5(a) and (b) and indicate stable mode-locked operation without sidebands or harmonic frequencies at least 70 dB below the level at the fundamental repetition frequency. With the applied temperature stabilized housing mode-locking has been sustained and the laser has not shown any changes in spectrum, output power or repetition frequency for several days.
Fig. 5 Radio-frequency spectra at pulse energies of 3.6 nJ. (a) From 0 GHz to 1 GHz. (b) Span of 1 kHz centered at the fundamental repetition rate of 71.3 MHz with a resolution bandwidth of 1 Hz.
4. Influence of pulse energy variation
To study the nonlinear dynamics of the oscillator the pump power was varied, while the laser setup was fixed and the case temperature was held constant. The polarization controllers were adjusted to allow for a wide pump power variation range keeping mode-locked operation. Figure 6(a) reveals a linear dependence of the average power in front of the PBS on the pump power. Continuous-wave operation starts for pump powers above approximately 100 mW, while self-starting mode-locking is achieved above a threshold of 450 mW.
Fig. 6 (a) Measured output power as a function of pump power with a mode-locking threshold of 450 mW. The diagrams (b), (c), and (d) show the output spectra for the indicated pulse energies.
It is notable that with increased pump power the output spectrum broadens and the dechirped pulse duration decreases as is typical for all-normal dispersion fiber lasers. Figure 6(b), (c), and (d) show the spectra of the linearly polarized signal behind the PBS for pulse energies of 2.3 nJ, 3.0 nJ, and 3.6 nJ, corresponding to average powers in front of the PBS of 194 mW, 253 mW, and 308 mW. They reveal that with higher pulse energies the spectra not only become broader but also more structured. For decreasing pulse energies the side peak in the autocorrelation continuously diminishes and below pump powers of approximately 543 mW the satellite pulse is not observable in the autocorrelation anymore.
The dependence of the spectral width on the pulse energy is shown in Fig. 7(a) . The FWHM increases almost linearly with increasing pulse energy from 26.0 nm for 2.3 nJ pulses up to 39.4 nm for pulse energies of 3.6 nJ. Simultaneously, the measured (Fourier-limited) pulse widths shown in Fig. 7(b) decrease from 115 fs (102 fs) for 2.3 nJ pulses to 82 fs (66 fs) for pulse energies of 3.6 nJ. The pulse energies were measured in front of the compressor.
Fig. 7 (a) Dependence of the spectral FWHM on pulse energy. (b) Measured and Fourier-limited pulse durations as a function of pulse energy.
Also, it is obvious from Fig. 8(a) that the high energy pulses cannot be compressed as close to the Fourier-limit as lower energy pulses. The deviation of the measured pulse width from the Fourier-limited width increases from 12.2% for 2.3 nJ pulses to 23.5% for pulse energies of 3.6 nJ. This can be attributed to nonlinear phase terms, which cannot be compensated for by the grating compressor. The frequency chirp of the output pulses and therefore the needed external compressor dispersion strongly depend on the pulse energy. Figure 8(b) shows that with increasing pulse energy less grating dispersion is needed to compress the pulses. The required compressor dispersion varies from −0.078 ps2 for 2.3 nJ pulses to −0.067 ps2 for pulse energies of 3.6 nJ.
Fig. 8 (a) Deviation of the measured pulse FWHM from the Fourier-limited FWHM. (b) External compressor dispersion for minimum pulse width as a function of pulse energy.
5. Conclusion
In summary, we demonstrated an all-fiber-integrated dispersion-compensation free fiber laser, which generates 3.6 nJ pulses at 1032 nm compressible to 76 fs. These are to our knowledge the highest pulse energies and shortest pulse durations from an all-fiber-integrated ytterbium laser.
Acknowledgement
The authors thank the German Research Foundation (DFG) for funding the Centre for Quantum Engineering and Space-Time Research – QUEST.
References and links
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