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We present a mode-locked all-normal dispersion ytterbium fiber oscillator with output pulse energies beyond 0.5 µJ. The oscillator is mode-locked using nonlinear polarization rotation, and stable single-pulse operation is achieved by spectral filtering inside the resonator. The oscillator generates strongly chirped output pulses at a repetition rate of 4.3 MHz which can be compressed down to 760 fs.
©2011 Optical Society of America
1. Introduction
During the last decade the output pulse energies from mode-locked fiber oscillators have been increased by almost three orders of magnitude. This is due to the development of novel fiber geometries, such as double-clad (DC) and photonic crystal fibers (PCF), and the exploitation of different pulse dynamics, e.g. similaritons and dissipative solitons [1,2]. In this regard, the dissipative soliton operation and the use of DC gain fibers enabled output pulse energies of up to 31 nJ with average powers of several watts from standard fiber oscillators [3]. Further power scaling was achieved by use of air-clad PCFs with core diameters of up to 70 µm and resulted in output pulse energies close to 1 µJ and pulse peak intensities above 1 MW [4–7]. However, air-clad PCFs sacrifice the flexibility and simple handling of standard fibers. They are also difficult to splice to other fibers, which prevents the integration of standard fiber-components such as pump-combiners, spectral filters, and output-couplers into the reported setups. From standard fiber oscillators without air-clad PCFs output pulse energies less than 50 nJ have been achieved so far [8].
The pulse energies of dissipative solitons in mode-locked all-normal dispersion fiber oscillators are limited by the total nonlinear phase-shift accumulated over one resonator round-trip. In order to increase the pulse energy, the peak intensities inside the fiber section of the oscillator must be reduced. This can be done either by using fibers with larger mode-field-diameters, as demonstrated in the PCF-setups [4–7], or by increasing the pulse duration. However, mode-field-diameters of at most 25 µm are supported by standard step-index large-mode-area (LMA) fibers. On the other hand, the intracavity pulse durations can be increased significantly by integration of long passive fiber sections into the oscillator. These passive fiber sections increase the total resonator dispersion and result in the generation of giant-chirp pulses with durations of several tens of picoseconds [9,10]. In this paper, we report on our results obtained by the simultaneous increase of mode-field-diameter and pulse duration in a mode-locked all-normal dispersion standard fiber oscillator and demonstrate the generation of output pulse energies exceeding 0.5 µJ.
2. Experimental setup
The experimental setup of the all-normal dispersion ytterbium fiber oscillator is illustrated in Fig. 1 . The fiber section consists of 1.9 m of ytterbium-doped LMA double-clad fiber and 45 m of passive fiber. Estimating the dispersion of both fibers to 20 ps2/km, a total resonator dispersion of 0.94 ps2 is obtained. The ytterbium-doped LMA fiber from Nufern has the maximum commercially available core-diameter of 30 µm with a numerical aperture of 0.06, which corresponds to a mode-field-diameter of approximately 25 µm. To suppress higher-order-modes, this LMA fiber is coiled up to a diameter of 7.5 cm. The passive fiber from Liekki has a core-diameter of 10 µm and a numerical aperture of 0.08. An in-house produced mode-field-adapter (MFA) is used to connect these two fibers. Due to the smaller core-diameter of 10 µm, the passive fiber supports only the fundamental mode and acts as a mode-cleaner after each round-trip.
The free-space section of the oscillator contains four waveplates in combination with a polarizing beam splitter (PBS) to achieve mode-locking by use of nonlinear polarization evolution (NPE) inside the fiber segment. Two spectral filters with bandwidths of 6.5 nm and 12 nm are realized with two birefringent quartz plates (BP) in front of additional PBSs. The second spectral filter is required to suppress two-wavelength operation of the oscillator, which would otherwise perturb mode-locking. A free-space isolator ensures uni-directional operation and the NPE-rejection port at the first PBS is used as the oscillator output port. The pump-light at 975 nm is provided by a multimode diode and is coupled through a dichroic mirror (DM) into the active fiber.
3. Results
Self-starting single-pulse mode-locking is obtained at a pump power of 10.6 W. The oscillator operates at a repetition rate of 4.3 MHz with an average output power of 2.3 W, which corresponds to an output pulse energy of 0.54 µJ. Figure 2(a) shows the power spectrum of the laser with a full-width at half-maximum (FWHM) of 4.2 nm and a Fourier-transform-limited pulse duration of 630 fs. The output pulses have a strong positive chirp and are dechirped with a 1200 groove/mm grating compressor with a transmission efficiency of 55%. From the grating compressor setup an output pulse chirp of 5.2 ps2 can be estimated. The intensity autocorrelations of the chirped and compressed output pulses are shown in Fig. 2(b) and have FWHM of 39 ps and 1.1 ps, respectively. This corresponds to an output pulse duration of 36 ps and an compressed pulse duration of 760 fs, which has been calculated from the optical pulse spectrum by taking the deconvolution factor of 1.48 into account. The pedestal in the autocorrelation function of the compressed pulses contains approximately 13% of the total pulse energy and results from uncompensated higher-order phase terms.
Fig. 2 (a) Optical pulse spectrum on logarithmic and linear (inset) scale. (b) Intensity autocorrelation of the compressed and chirped (inset) output pulses.
To verify stable single-pulse operation of the oscillator, the output pulse train has been recorded with a 10 GHz photodiode in combination with a 6 GHz oscilloscope and an rf-spectrum-analyzer. The oscilloscope trace and the radio-frequency spectrum of the photodiode signal are shown in Fig. 3(a) and (b) . No satellite pulses are visible in the oscilloscope trace. The radio-frequency spectrum at the fundamental repetition rate of 4.3 MHz has been recorded with a resolution of 1 Hz. The constant heights of the radio-frequency peaks, as well as the noise-suppression of 80 dB, confirm that no Q-switching, period-doubling, or higher-harmonic mode-locking are present.
Fig. 3 (a) Oscilloscope traces of the output pulse train. (b) Radio-frequency spectrum at the fundamental repetition rate of 4.3 MHz and over the range of 200 MHz (inset).
Finally, the beam quality of the output beam after the ytterbium-doped LMA fiber section has been characterized by an M2-measurement. Figure 4 shows the measured beam caustic along with the beam profiles in the focal plane and at a distance of 180 mm behind the focus. Both beam profiles have more than 94% overlap with a Gaussian profile on both axes and an ellipticity of less than 7%. Small aberrations in the beam profile at 180 mm result from power attenuators in front of the CCD-camera. The beam diameter is defined as full width at 1/e2 of the peak intensity and from the beam caustic a M2-value of less than 1.1 can be estimated.
Fig. 4 (a) Measured beam caustic (dotted) and corresponding fit with a M2 of 1.07 (line). (b), (c) Beam profiles in the focal plane and at a distance of 180 mm behind the focus.
The output pulse energy has been limited by pulse break-up resulting in multiple-pulse operation at higher pump powers. Nonetheless, to the best of our knowledge, the presented results correspond to an increase by a factor of ten in output pulse energy compared to previously reported non-PCF femtosecond fiber oscillators.
Conclusion
In summary, an ytterbium-doped LMA fiber with a core-diameter of 30 µm and a long passive fiber section of 45 m have been used to simultaneously increase the mode-field-diameter and the pulse durations in a mode-locked all-normal dispersion fiber oscillator. This reduced the pulse peak intensities inside the oscillator, allowing for the generation of output pulse energies of 0.54 µJ. The laser operated at a repetition rate of 4.3 MHz and the strongly chirped output pulses have been compressed down to 760 fs. The demonstrated oscillator setup represents a simple low-cost alternative to commonly used master oscillator power amplifier fiber laser systems.
The authors thank the German Research Foundation (DFG) for funding the Cluster of Excellence Centre for Quantum Engineering and Space-Time Research QUEST.
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