1. Introduction

Rare-earth-doped fibers have established themselves as a very attractive gain medium for ultrashort pulse amplification. This fact is due to their inherent properties such as high optical-to-optical efficiency, broad gain bandwidth and outstanding thermo-optical behavior, but also due to compactness, robustness and simplicity of operation. The main performance limitation of ultrafast fiber laser systems is nonlinearity in the fiber core, the most important being self-phase modulation and stimulated Raman scattering. These restrictions can be overcome by applying so-called large-mode-area fibers and the well-known chirped-pulse amplification (CPA) technique [1]. Fiber based chirped-pulse amplification has been demonstrated resulting in high energy [2] and high average power femtosecond pulses [3].

In its simplest form such a CPA system consists of a diffraction grating stretcher and compressor unit with the doped fiber as gain medium in between. The use of bulk optics and gratings require free space propagation and therefore need alignment. This diminishes the advantage of a fiber based laser system. In order to build a complete fiber based system, the grating stretcher can be replaced by fiber integrated optical devices such as a standard singlemode fiber [4] or a chirped fiber Bragg grating [5]. Both approaches allow stretching to the nanosecond regime and in particular chirped fiber Bragg gratings provide an engineerability of higher order dispersion. In the case of the compressor the situation becomes more delicate due to the high peak power obtained in the amplifier. Of course, chirped fiber Bragg gratings also offer the possibility of fiber integrated pulse compression. However, the pulse energy is typically restricted to the nanojoule range [6] because of enhanced nonlinear pulse distortion with increasing pulse peak power during the compression. Alternatively, chirped-periodic quasi-phasematched gratings [7] can be applied, offering scalability in peak power but are restricted in achievable time delay (~50 ps).

In this contribution, we report for the first time to our knowledge on an all fiber CPA system based on pulse compression in an air-guiding photonic band gap fiber which possesses anomalous dispersion and a significantly reduced nonlinearity due to the guiding of the laser radiation in air. Using this approach we achieved 0.82 MW peak power pulses out of the fiber compressor without any nonlinear pulse distortion.

2. Photonic band gap fiber

An air-guiding photonic band gap fiber usually consists of a stack of thin-walled capillaries with an extra large hole in the center, as shown in Fig. 1. Light is guided in a well-defined wavelength range and trapped by a 2D photonic band gap (PBG) of the cladding [8,9]. Due to the confinement of the radiation in the hollow core the nonlinearity is significantly reduced. Experiments have shown that the nonlinear coefficient of the hollow core fiber mode is very close to that of air [10]. Therefore, these fibers can be very useful for the delivery of high energy ultrashort laser pulses. Furthermore, the risk of damage is reduced due to the air-guiding.

 

Fig. 1. Microscope image of an air-guiding photonic bandgap fiber. Bright regions are fused silica and dark regions are air.

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The dispersion characteristic of such a bandgap fiber with a well-defined transmission range is in first order determined by the Kramers-Kronig relation and waveguide dispersion [11]. Therefore, the dispersion is anomalous over much of the transmission band. This implies that such fibers can support optical solitons, as recently demonstrated by the soliton-type propagation of 5 MW peak power pulses in the 1.5 µm wavelength region in a 1.7 m long bandgap fiber [11]. In a conventional step-index singlemode fiber the propagation of such pulses over comparable fiber length would be impossible due to enhanced nonlinear pulse distortion.

In our experiment the anomalous dispersion over the bandgap is applied to compensate for the normal dispersion of an ultrashort fiber amplifier.

The air-silica photonic bandgap fiber used (Crystal Fibre AIR-10-1060), shown in Fig. 1, has a core diameter of 10.5 µm and the bandgap ranges from 980 nm to 1080 nm with an attenuation of less than 0.1 dB/m. The spectral attenuation of this fiber is shown in Fig. 2. The group-velocity parameter at the operating wavelength, 1040nm, of the ultrafast fiber laser system is experimentally determined to -0.054 ps²/m by measuring the temporal pulse broadening of an initially unchirped 250-fs pulse in the fiber. The propagation is assumed to be linear (only dispersion) since no changes in the spectrum are observable.

 

Fig. 2. Measured spectral attenuation of the air-guiding photonic bandgap fiber.

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3. Experimental setup and results

The high peak power ultrafast fiber laser system consists of a mode-locked Yb:KGW oscillator, a standard single-mode stretcher fiber, a diode-pumped air-clad mircostructured large-mode-area ytterbium-doped fiber amplifier and the air-core photonic bandgap fiber for dispersion compensation, as shown in Fig. 3.

 

Fig. 3. Experimental setup of the all fiber CPA system; OI: optical isolator, PC LMA: photonic crystal large-mode-area fiber.

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The passively mode-locked Yb:KGW oscillator is running at 73 MHz repetition rate, producing pulses as short as 250 fs at 1040 nm center wavelength and 80 mW average power.

A 1.9 m long step-index single-mode fiber is used for pulse pre-stretching to about 1.9 ps. In addition, a spectral broadening due to self-phase modulation is observed. At a transmitted output power of 40 mW the spectral width is as high as 20 nm. Launching the 250-fs pulses directly into the high gain amplifier fiber leads to an excessive spectral broadening due to self-phase modulation beyond the bandwidth limit of the ytterbium-doped fiber amplifier [12]. This results in a nonlinear chirp and therefore a reduction of re-compressibility of the pulses.

The fiber amplifier is constructed using a 2.1m long air-clad microstructured ytterbium-doped large-mode-area fiber [13]. The single transverse mode core has a mode-field diameter of 21 µm and a 150 µm diameter inner cladding with a numerical aperture of 0.55. The fiber is pumped by a fiber-coupled diode laser emitting at 976 nm. The output characteristic of the amplifier is shown in Fig. 4. The 40 mW average seed power is amplified to a maximum of 8.2 W with a slope efficiency of 62% with respect to the launched pump power. At the highest output power the spectral width is increased to 28.5 nm (shown in Fig. 5) and the temporal width to 4.0 ps.

 

Fig. 4. Output characteristics of the ytterbium-doped fiber amplifier.

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A length of 2 m of the air-guiding photonic bandgap fiber, described in Section 2, is used to recompress the amplified, positively chirped pulses. Figure 5 compares the spectrum of the amplifier output with the output of the photonic bandgap compressor fiber at the highest power level. No changes in spectrum are observed, this leads to the conclusion that neither the transmission characteristics of the bandgap nor nonlinear effects in the air-core fiber affect the pulses at this high peak power level.

 

Fig. 5. Measured spectra after the stretcher, amplifier and compressor fiber.

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The dispersion compensation is fine optimized by changing the length of the single-mode fiber in front of the fiber amplifier. The best compression is found at a stretcher fiber length of 1.9 m. Figure 6 shows the measured autocorrelation trace at the best achievable compression. The autocorrelation width is determined to be 156 fs, which corresponds to a pulse duration (FWHM) of 100 fs - assuming a sech² pulse shape. The wings in the autocorrelation trace can be attributed to nonlinearity (self-phase modulation) in the stretcher and amplifier fiber. They are not due to nonlinearity in the air-guiding compressor fiber which has been proven by the independence of the shape and width of the autocorrelation trace of the transmitted power through the bandgap fiber, what means that the propagation of the pulses in the compressor fiber is determined by dispersion. The maximum average output power of the fiber compressor is 6.0 W, therefore the throughput efficiency is as high as 73% (non-optimized). At a repetition rate of 73 MHz of the system this corresponds to a pulse energy of 82 nJ and a pulse peak power of 0.82 MW.

 

Fig. 6. Measured autocorrelation trace of the fiber re-compressed 0.8 MW pulses.

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4. Conclusions

In conclusion, we have demonstrated for the first time, to our knowledge, a high peak power ultrafast fiber laser system based on compression in an air-guiding photonic bandgap fiber. The obtained 100-fs pulses with an average power as high as 6.0 W corresponds to a peak power of 0.82 MW. No nonlinear pulse distortions are observed in the compressor fiber at this peak power level. Since such air-guiding fibers have approximately three orders of magnitude lower nonlinearity [10] compared to conventional single-mode fibers indicates that this approach can be scaled to the µJ pulse energy level with femtosecond pulse duration. Due to the air-guiding fiber, damage will not limit the scalability. Very high values of anomalous dispersion can be obtained especially on the long wavelength edge of the bandgap. Therefore significantly longer initially stretched pulses can be recompressed with a short length of photonic crystal fiber. The main advantage of this approach is the possibility to build up a very compact and completely fiber integrated high peak power and high energy fiber CPA system.