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We demonstrate a monolithic Yb-fiber chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor. For an output energy of 77 nJ, 220 fs pulses with 92% of the energy contained in the main pulse, can be obtained with minimal nonlinearities in the system. 135 nJ pulses are obtained with 226 fs duration and 82 percent of the energy in the main pulse. Due to the good dispersion match of the stretcher to the hollow core photonic bandgap fiber compressor, the duration of the output pulses is within 10% of the Fourier limited duration.
© 2014 Optical Society of America
1. Introduction
Yb-doped femtosecond fiber chirped pulse amplifiers (FCPAs) are extremely attractive to replace their solid-state counterparts when it comes to robustness and ease of operation. However, in most Yb-doped FCPAs delivering high-energy output pulses the advantages of robustness and ease of operation are forfeited since free-space optical components are used to stretch and compress the pulses. The main bottleneck to obtain fiber based stretching and compression is to match the dispersion of the stretcher and compressor. Recent advances have yielded good results with fiber-based stretchers and free space compressors [1–4]. However, the free space compressors require rather large distances between the compressor elements, which reduces the robustness of the system. For several applications, like ultrafast imaging, nonlinear microscopy, tissue photodisruption, endoscopy and several other clinical applications, Yb-fiber based femtosecond systems with a completely monolithic architecture and a flexible – spliced-on – means to deliver near transform-limited femtosecond pulses on target, are highly desirable [5–7]. Although fiber compressors like solid-core photonic crystal fibers (SC-PCF) [8] or higher-order-mode (HOM) fibers have been applied [9,10], such fibers are not suitable as compressor fibers for pulse energies exceeding a few nanojoules. For example, at 1.5 µm up to 14 nJ could be attained in solid core large-mode area (LMA) HOM fibers (effective area 2100 µm2) [10]. In all these cases energy extraction is limited by high nonlinearities experienced in the solid core with increasing peak intensities during the compression process along the fiber. The use of a hollow core (HC) photonic bandgap fiber (PBF) can alleviate this problem, an approach that was first demonstrated in Er-doped fiber lasers [11–13].
At 1 µm wavelength short pieces of HC-PBFs have been used to compress pulses from Yb-fiber oscillators [14]. In a non-monolithic architecture Limpert et al. [15], demonstrated an Yb-FCPA system that uses a short piece of single mode (SM) fiber as a stretcher in combination with a HC-PBF compressor. The Yb-fiber CPA was seeded by an Yb-KGW solid-state oscillator. Pulse energies of up to 80 nJ and pulses as short as 100 fs were generated [15]. Using a similar approach Nielsen et al. [16], achieved 5.3 nJ 158 fs pulses in an Yb-fiber-laser system. Near transform-limited pulses were achieved at this output energy by nonlinear pulse propagation in the amplifier (nonlinear FCPA), which compensated for the residual third order dispersion (TOD) that was not compensated by the HC-PBF compressor. A major drawback of the technique of nonlinear FCPA is that the optimal pulse duration can only be achieved for a certain output energy, and although almost transform-limited pulse durations can be achieved, a considerable portion of the energy resides in pre- and/or post pulses or a pulse pedestal. Properly matching the dispersion of the fiber stretcher and compressor and operating the amplifier with as little as possible nonlinearities will allow generating near transform-limited pulses with much smaller energy content in pre- and post pulses or in a possible pulse pedestal. Additional to properly matching the dispersion of the fiber stretcher and fiber compressor, a major challenge towards developing monolithic Yb-doped FCPA systems is to appropriately splice the fiber compressor to the rest of the system as well. An efficient method to splice polarization maintaining (PM) single mode fibers (SMF) to HC-PBF was reported recently [17]. The splice losses attained with this technique were as low as 0.6 dB. Using this approach the same group demonstrated a monolithic Yb-FCPA with spliced-on compressor delivering 7.3 nJ-297 fs pulses (with 56% of the energy in the main peak). The fiber stretcher was a 35 m long standard PM-SMF and the pulses were compressed in 21 m of HC-PCF [18]. More recently, by using a large mode area amplifier (∅15 µm core) up to 56 nJ output energy with an autocorrelation trace of 710 fs FWHM was demonstrated [6]. The authors reported that at these energy levels the nonlinear Raman effect becomes more pronounced and that the pulses become very difficult to compress in the spliced-on HC-PBF. Pulse degradation is clearly observed in the autocorrelation trace. Side pulses in the autocorrelation trace are attributed to uncompensated TOD.
In this paper we demonstrate a monolithic Yb-fiber CPA system that uses a combination of (PM-)SMF with a dispersion compensating fiber (DCF) [19] as stretcher and a spliced-on HC-PBF as compressor. Compared to a simple (PM-)SMF stretcher, considerable bigger stretching ratios and finer dispersion matching to the HC-PBF compressor can be attained with the DCF. Our system delivers up to 135 nJ pulses with a duration as short as 226 fs with more than 80% of the energy concentrated in the main pulse. Due to the fact that in our system the inclusion of the DCF stretcher warrants that group delay dispersion (GDD) and TOD are compensated, the compressed pulse duration is within 10% of the Fourier transform limited duration. To our knowledge this represents the highest peak power (0.48 MW) so far demonstrated in an Yb-fiber CPA system with a fiber compressor integrated in a completely monolithic architecture.
2. Laser system
The Yb-fiber laser system consists of an all-normal dispersion Yb-fiber oscillator operating at 49 MHz repetition rate, a PM-SMF preamplifier and a ∅ 10 µm core double-clad PM LMA power amplifier, see Fig. 1. The slightly chirped pulses from the oscillator are stretched to ~30 ps duration in a hybrid fiber-pulse stretcher that consists of 20 m of SMF (~8 m of 1060XP in the fiber-pigtails of the fiber-couplers and isolator and ~12 m of Clearlite 980) and followed by 5 m of DCF. After the stretcher a PM fiber polarizer was used in combination with a fiber polarization controller to ensure PM operation of the amplifier with the polarization strictly aligned with the slow axis. After amplification, the pulses are compressed in 25 m of HC-PBF. The measured group delay dispersion of the compressor at 1060 nm is about –1.6 ps2 corresponding to a stretched pulse duration of ~30 ps. Because the effective area of the DCF is rather small (~7 µm2), the seed energy was limited to 0.1 nJ and the pulses were pre-stretched using standard SMF in order to minimize nonlinearities in the DCF stretcher. Thepulses are then fed into the PM-SMF pre-amplifier, which is followed by a fiber pigtailed acousto-optic modulator (AOM) that is used to vary the repetition rate of the laser system. After a high-contrast pigtailed Faraday isolator, the pulses are further amplified in the ∅ 10 µm core double-clad PM LMA power amplifier (using 2 m of Nufern PLMA-YDF-10/125). The output of the power amplifier is directly spliced to the HC-PBF compressor (25 m of HC-1060-02 from NKT Photonics [20]; using a Vytran FFS2000 splicer). The output pulses from the compressor were characterized by second harmonic frequency resolved optical gating (SH-FROG). The shortest compressed pulse duration was achieved by optimizing the fiber length of the pre-stretcher.
Fig. 1 Schematic of the monolithic Yb-fiber chirped pulse amplifier. Before amplification the pulses from the ANDi oscillator are stretched to a duration of ~30 ps in the SMF pre-stretcher and the DCF. After the first amplification stage, an acousto-optic modulator (AOM) can be used to reduce the repetition rate of the system. A high-contrast pigtailed Faraday isolator is used to prevent damage due to possible back-reflections.
We investigate the performance of the laser in the following amplifier-compressor configurations: i) butt-coupling the HC-PBF to the amplifier output, ii) a straight splice where both fiber surfaces were straight cut and iii) angle-splice where both surfaces were angle cleaved. Of the two spliced configurations, the straight splice was the easiest to perform, but with this configuration a loss of efficiency in the amplifier may be expected because the back-reflection at the glass-air-interface at the splice point is guided in the core of the power amplifier and thus amplified traveling backwards, consequently lowering the inversion in the power amplifier. We do not notice a loss in efficiency when operating the laser at the full repetition rate of the oscillator, but at lower repetition rates a loss of efficiency is noticed in the case of a straight splice compared to the case of an angle-splice, which can therefore be mainly attributed to back-reflected amplified spontaneous emission. We have measured the transmission of the spliced-on compressor to be about 50%. The transmission losses of the HC-PBF are roughly 0.1 dB/m, thus accounting for most of the measured transmission losses.
For performing the splice we manually adjusted the fiber position in the transversal direction to the propagation axis for maximum transmission. Although the HC-PBF is nominally non-PM, a preferred polarization orientation was found. We attribute this to the hexagonal structure of the HC-PBF, which is supported by the fact that we found a local optimum in the polarization extinction ratio (PER) every 60 degrees. We observed there was one orientation every 180 degrees in which the PER was optimal, reaching a value of ~16 dB, which as well proved to be the best orientation for the pulse compression. Thus, after optimizing the PER and a second fine optimization of the translational alignment, the two fibers were spliced by slightly heating them while the heating filament was slightly offset towards the active fiber. The splice parameters were chosen in such a way that the applied heat does not collapse the hole-structure of the HC-PBF. In order to observe the splice quality, we continuously monitored the power at the HC-PBF output while operating the laser with only the oscillator switched on. After heating the fibers once, the output power increased by ~10–15% (0.4–0.6 dB). When taking the fibers out from the splicer after heating them once, the splice is rather fragile, and breaks easily, for example when bending the fibers slightly. The splice is more robust by heating the fibers once more, the output power is stabilized at a value about 10% more than before heating the fibers for the first time (i.e. in case after the first time heating the fibers the power increased by more than 10%, the power decreased slightly after the second time heating). Once the fibers are spliced, we fix them on an aluminum v-groove. After applying the heat twice, the robustness of the splice is such that it can be bent slightly without breaking, and handling it in order to fix it to the v-groove does not require extreme caution. Although we have observed an increase in transmission after splicing, in contrast to the reported observed loss in transmission after splicing compared to butt-coupling by Kristensen et al. [17], we do estimate the splice loss to be as well on the order of 0.4–0.6 dB. We base this estimate on the following: Since the light crosses a glass-air interface at the splice point, the losses of the splice should be at least 0.2 dB, and the mode-mismatch of the ∅ 10 µm core amplifier fiber with the ~7 µm mode-field diameter of the HC-PBF should lead to more losses. One may speculate that the increase of power transmitted after splicing in our case, and decrease of power transmitted after splicing the HC-PBF to PM980 fiber reported by Kristensen et al. [17], are caused by a change of mode-field diameter in the HC-PBF close to the splice caused by the heating and local melting, and thus bettermode-matching to the ∅ 10 µm core of our amplifier fiber is obtained, but worse mode- matching to a PM980 fiber. Since the aim of our experiments was however not to investigate the splicing of the HC-PBF to our ∅ 10 µm core amplifier, but the compression of the output pulses from our laser in a spliced-on compressor, we have not attempted to precisely determine the splice loss and the transmission loss of the HC-PBF.
The procedure to perform the angle-splice is the same as to perform the straight splice, with the obvious exception that the angle-splice requires to cut both fibers with the same angle, and to have the same orientation of the surface angle to the optimal polarization axis. This requires a few iterations on performing the angle cleave on one of the two fibers, until the angles and the orientation match. In the case of the angle-splice we have observed the same 10% increase in power after splicing compared to before performing the splice. The beam profile at the output of the HC-PBF after it was angle-spliced to the amplifier is shown in Fig. 2.
Fig. 2 Beam profile at the output of the HC-PBF after it was angle-spliced to the amplifier. On the left and bottom side the integrated (along the horizontal and vertical directions, respectively) profiles (full lines) are shown, together with a Gaussian fit (dashed lines).
3. Results and discussion
In our work, we use about 5 m of specially designed DCF, which allowed us to achieve a much closer match of the higher order dispersion compared to realizations by other groups [6.15,16,18], as highlighted in Fig. 3. Our 25 m of HC-PBF (red curve in Fig. 3) matches the GDD of ~77 m of SMF (blue curve) at the central wavelength of 1060 nm of our Yb-FCPA, but when only compensating for the GDD introduced by the (PM-)SMF, pulse compression will be severely limited by the uncompensated TOD. The black curve in Fig. 3 displays the calculated (using the measured dispersion of the Clearlite 980 SMF for all (PM)-SMF in the system) total dispersion of our stretcher and amplifier, which is close to the dispersion of our compressor fiber.
Fig. 3 Measured dispersion of our HC-PBF compressor (red line) and total dispersion of our stretcher and amplifier (black line). For comparison, the dispersion of 77 m of SMF is shown as well (blue line). The dashed gray line shows the output spectrum of our system.
We characterized the output pulses of the amplifier in terms of pulse energy and temporal profile for the butt-coupled, straight-spliced and angle-spliced configurations. The results ofthe SH-FROG characterization at the full repetition rate of the oscillator (49 MHz) are shown in Fig. 4, for the angle-spliced configuration and Fig. 5, for the butt-coupled and straight-spliced configurations. For a comparison, we also show the envelopes for the Fourier limited (FL, gray lines in Fig. 4(d) and Fig. 5) pulse corresponding to the measured spectrum and for the pulses assuming a spectral phase corresponding to the calculated dispersion mismatch in our system (solid blue lines in Fig. 4(d) and Fig. 5) and corresponding to a system using only SMF as stretcher (dotted blue lines in Fig. 4(d) and Fig. 5).
Fig. 4 SH-FROG characterization of the compressed output pulses in the case of the angle-spliced configuration at the full repetition rate of the oscillator (49 MHz) with a pulse energy of 7 nJ. (a) Measured SH-FROG trace. (b) Reconstructed SH-FROG trace. (c) Measured (black) and reconstructed (blue) spectrum and spectral phase (red). (d) Reconstructed temporal intensity envelope (black) and phase (red). The measured pulse duration is ~230 fs. For a comparison, the Fourier-limited pulse envelope corresponding to the measured spectrum (green, 210 fs) and envelopes assuming a spectral phase corresponding to the dispersion mismatch in our system (solid blue, 220 fs) and a system using only SMF as stretcher (dotted blue, 280 fs) are shown.
Fig. 5 Temporal characterization of the compressed output pulses in the case of the butt-coupled configuration (a) and straight-spliced configuration (b) at the full repetition rate of the oscillator. Both panels show the reconstructed temporal intensity envelope (black) and phase (red). The measured pulse duration is ~230 fs in both cases. For a comparison, the Fourier-limited pulse envelope corresponding to the measured spectrum (green, 210 fs) and envelopes assuming a spectral phase corresponding to the dispersion mismatch in our system (solid blue, 220 fs) and a system using only SMF as stretcher (dotted blue, 280 fs) are shown.
The small deviations observed between the measured pulses and the pulses calculated taking into account the dispersion mismatch between the stretcher and compressor might originate from the oscillator or from nonlinearities just after the output of the oscillator.
As we were limited in pump power (max 10 W from the diode, ~8.5W delivered to the active fiber, ~7.5 W absorbed), an increase of pulse energy from the system was achieved by reducing the repetition rate. For the straight-spliced configuration, a reduction of the repetition rate by a factor of 49 results in an increase of the output pulse energy to 52 nJ. In this case, the output spectrum is broader compared to what is obtained at the full repetition rate of the oscillator, which can be attributed to the combination of positively chirped pulses and a higher gain for shorter wavelengths. The broader output spectrum and well-matched dispersion of the stretcher and compressor lead to an output pulse duration of 178 fs (FL 160 fs), as shown in Fig. 6(a). For the angle-spliced configuration, a reduction of the repetition rate by a factor of 20 results in an increase of the output pulse energy to 77 nJ. Here as well a slight broadening of the output spectrum was observed, leading to slightly shorter pulses (220 fs) after compression compared to the pulses obtained at the full repetition rate of the oscillator, see Fig. 6(b). The fact that 92.5% of the energy is contained in the main pulse in this case, which is about the same as in the case when the amplifier is operated at the full repetition rate of the oscillator (94%), indicates that the nonlinearities in the amplifier are kept very low. Reducing the repetition rate by a factor of 49 in the angle-spliced configuration results in an output pulse energy of 135 nJ. For this case, nonlinearities in the power amplifier and as well in the compressor fiber, lead to a decrease of pulse quality, about 82% of the energy is contained in the main peak, which has a duration of 226 fs. The nonlinearities in the compressor fiber are mainly due to the (small) overlap of the guided mode with the glass cladding structure. This is in accordance with numerical simulations by Lægsgaard and Roberts [21], that show that at energies approaching 100 nJ a considerable nonlinear phase is accumulated in the HC-PBF, limiting the compressed pulse quality.
Fig. 6 Temporal characterization of the compressed output pulses for higher output pulse energies. (a) Straight-spliced configuration at 1 MHz, and output energy of 52 nJ. The retrieved pulse duration is 178 fs. (b) Angle-spliced configuration at 2.45 MHz, and output energy of 77 nJ. The retrieved pulse duration is 220 fs. (c) Angle-spliced configuration at 1 MHz, and output energy of 135 nJ. The retrieved pulse duration is 226 fs.
The stretcher-compressor approach presented in this paper offers great flexibility to further increase the pulse energy of the system to the microjoule level. Because of the losses of about 0.1 dB/m, the length of the HC-PBF compressor in our proof-of-principle system was limited to ~25 m. This limited the stretched pulse duration to ~30 ps, thus at energy levels above ~100 nJ the pulse broadens and pulse compressibility strongly degrades. Bigger stretching ratios can be achieved by for example using a longer DCF stretcher and by adding an HOM fiber to the stretcher even finer dispersion compensation of higher order terms will be possible [19]. Another important parameter limiting the pulse energy of the system presented here is that the main amplifier has a core diameter of only 10 µm, but a larger fiber core diameter can be used efficiently with a correspondingly different HC-PBF design. The advantages of HC-PBFs with a larger core diameter are the lower losses per unit length and lower effective nonlinearities because a smaller fraction of the mode propagates through glass. The dispersion of HC-PBFs with a larger core diameter is generally a bit lower, but because of the smaller losses per unit length a bigger stretching ratio can be used. A drawback of HC-PBFs with larger core diameter is that more modes are supported and thus single-mode operation that is required to obtain good pulse compression is more difficult to achieve.
4. Conclusions
In conclusion we demonstrated for the first time, to our knowledge, a high-peak power ultrafast monolithic Yb-FCPA, where compression is achieved in a spliced-on HC-PBF that compensates for GDD and TOD. In our system, this is achieved by using a hybrid stretcher that consists of standard SMF in combination with a DCF instead of just a simple SMF stretcher. The approach of stretching the pulses in just a SMF or a PM-SMF does not allow for TOD compensation. Compared to this standard approach, inclusion of the DCF stretcher allows us to achieve higher pulse stretching ratios without compromising pulse quality due to excessive uncompensated higher order dispersion terms. A low-loss splice between the HC-PBF and the output of the Yb-FCPA was performed for two different splice modalities: i) straight-splice; ii) angle-splice. In both situations the achieved splice losses are very low (estimated 0.6 dB) and are comparable to the ones reported by Kristensen et al. [17]. Although the angle-splice is more difficult to perform, it has the advantage that a much higher system efficiency can be obtained by minimizing spurious back-reflections to the amplifier. The bigger stretching ratio to keep nonlinearities minimized, together with the better TOD compensation offered by our system design, allowed us to produce cleaner and shorter pulses at higher energy levels as previously reported in a monolithic configuration [6]. We demonstrate 77 nJ, 220 fs pulses with more than 92% of the energy contained in the main pulse (peak power 0.32 MW), and 135 nJ, 226 fs pulses with more than 80% of the energy contained in the main pulse (leading to a peak power of 0.48 MW). Up to an energy of ~80 nJ the energy-content in the main pulse does not change significantly and stays over 92%, illustrating that the nonlinearities in our amplifier are kept as low as possible. At higher energies, nonlinearities are expected to accumulate even in the compressor fiber [21], reducing the compressed pulse quality. The achieved output pulses represent, to our knowledge, the highest peak power generated in a monolithic Yb-FCPA with a spliced on fiber compressor.
Acknowledgments
This work has been supported by the Austrian Science Fund (FWF), grant P23887-N16. A.F. acknowledges support from a Hertha Firnberg Fellowship by FWF (project T420-N16).
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