We report a high-energy femtosecond fiber amplifier based on an air-cladded single-transverse-mode erbium-ytterbium-codoped photonic-crystal fiber with a 26-µm mode-field-diameter. 700-fs, 47-MHz pulses at 1557 nm were amplified and compressed to near-transform-limited 100-fs, 7.4-nJ pulses with 54-kW peak powers without chirped-pulse amplification. A linearly polarized output with an extinction ratio exceeding 42 dB was obtained by double-pass configuration. As an application, supercontinuum spanning from 1000 to 2500 nm was generated by a successive 2-m high-nonlinear fiber with a 140-mW average power.
©2005 Optical Society of America
Femtosecond fiber lasers are widespread to various scientific and engineering fields because of their advantages such as high reliability and fiber delivery. High nonlinearity due to small fiber cores and long-distance propagation has imposed difficulty on power scaling, but use of large-mode-area (LMA) fibers and chirped-pulse amplification (CPA) can overcome this problem and has been extensively investigated [1–4]. Because LMA fibers with step-index profiles are usually multimode, cares must be taken on suppression of higher-order mode excitation to obtain near-diffraction-limit outputs [1–3]. The recent photonic-crystal fiber (PCF) technology can realize single-transverse-mode LMA fibers with ease of handling and high propagation stability [5–7]. Design parameters of the air-hole diameter and spacing can highly control the index difference between the core and clad to be a value small enough for single-mode guidance [5–7], which is difficult by conventional doping control used in step-index fibers. For lasers and amplifiers ytterbium (Yb)-doped PCFs have been investigated [8–10], and a LMA Yb-doped PCF lasers with as large as a 45-µm mode-field diameter (MFD) has been recently reported . Femtosecond and picosecond fiber amplifiers based on LMA Yb-doped PCFs with up to a 60-kW peak power have been reported at a 1-µm region owing to its extensively large nonlinearity limit .
For high-power fiber lasers and amplifiers in a 1.55-µm region, which is important in telecommunications and eye-safe-based applications, erbium-ytterbium (Er:Yb)-codoped fibers are more promising than Er-doped fibers [13,14]. An order of magnitude larger absorption cross section of Yb around 975 nm and higher erbium-doping capability with reduced concentration quenching enable strong core absorption and gain, which is essential for double-clad fibers with higher power launching capability [2,4,13,14]. An air-guided cladding structure that can be easily built in PCFs also enables further pump power scaling [9,10]. Then development of Er:Yb-codoped PCF has been desired for scaling the nonlinearity limits of ultrashort pulses as well as single frequency lasers at the telecommunication band.
In this paper we report an air-cladded, LMA Er:Yb-codoped PCF and its application to the fiber amplifier for 1.55-µm-region high-energy femtosecond pulses. Nearly transform-limited 100-fs pulses with higher than 50-kW peak powers have been generated with perfect single-transverse-mode and single-polarization properties.
2. Fiber specification and experimental setup
Our LMA Er:Yb-codoped PCF has a one-rod core (one hole missing) structure in a triangular air-hole lattice. The effective refractive index of the core rod is closely matched to silica throughout the visible and near-infrared wavelength range. The microscope image of the cross section is shown in Fig. 1(a). The hole spacing (Λ) is 22 µm and the hole-diameter to hole-spacing ratio (d/Λ) is about 0.54 in average, which is slightly above the practical endlessly single-mode cut-off (0.50) . Hence the fiber is multimode on short lengths, whereas the fundamental mode is more bending resistant than in a purely single-mode LMA fiber . Indeed, we found that only the lowest-order mode can propagate on fiber lengths longer than ~1 m with any coiling diameters. The calculated MFD is 26 µm and the numerical aperture (NA) is 0.04. The air-guided clad diameter is 222 µm and the NA is 0.58. The concentrations of Er2O3 and Yb2O3 in the core rod are ~140 ppm and ~2000 ppm, respectively. The pump absorption is 1.6 dB/m at 976 nm.
3. Results and discussion
Figure 3 shows the pulse-energy evolution in both cases of single-pass and double-pass configurations. The pulse energies (average powers) were 3.7 nJ (173 mW) and 7.4 nJ (350 mW), respectively, at a 7.1-W pump power. The relatively low extraction efficiencies of ~2.5% and 5% in single-and double-pass amplification are mainly due to the high core background loss (~0.3 dB/m), which is caused principally by the core rod itself and not by the photonic-crystal structure. Hence, the loss is not fundamentally limited in this fiber, and may be easily improved in future designs. In the single-pass case the polarization extinction ratio (PER) was typically 5~7 dB and was sensitive to the surrounding fluctuation. By use of the double-pass configuration, the PER was improved to >42 dB (detection limit), far above the PER of the PBS itself (26 dB). The polarization state was very stable, which was shown in the stable PBS output power (<0.3% fluctuation). Fluctuation caused by nonlinear polarization rotation has not been observed. The output beam profile was observed with an InGaAs camera and is shown in Fig. 1(b). The beam parameter (M 2) was measured to be less than 1.05, indicating a perfect single-transverse-mode property.
The sech2-fit pulse widths at different energies are also shown in Fig. 3. In both single and double-pass amplification the pulse widths were broader than that of the seed (700 fs) at low pump powers due to chirping. As the pulse energy increased, the pulse width shortened with spectrum broadening by pulse compression in the anomalous dispersion system . Figure 4 shows the autocorrelation traces and spectra of the seed and amplifier outputs at a 7.1-W pump power. The 1535-nm peak observed in double-pass amplification is due to amplified spontaneous emission. The 18-dB suppression ratio against the signal peak is large enough for many applications, but will be further increased by use of angle-polished fiber facets and an appropriate chromatic filter before second amplification.
The pulse width monotonically shortened to as short as 140 fs (~4-nJ pulse energy), where the well-known relationship of τs=3.52|β 2|/γEp=1.76λ Aeff|β 2|/πn2Ep  is almost satisfied and fundamental soliton was formed. Here τs is the fundamental soliton width, β 2 is the group-delay dispersion per unit length, γ=2πn 2/λA eff is the nonlinearity coefficient (0.18 W-1km-1), λ is the wavelength, A eff is the effective mode area, n 2 is the nonlinear index of silica (2.36×10-20 m2/W), and E p is the pulse energy. β 2 was measured to be -29.4 ps2/km at 1550 nm, which is slightly larger than the value of silica itself (-27.9 ps2/km) as is predicted in the LMA PCFs . Above this energy the pulse width became longer and then again shorter as the pump power increased. The corresponding spectrum also showed narrowing and then broadening with dip formation at the center, indicating transition to the second-order soliton. Above E p~6.5 nJ a broad spectral peak appeared in the longer wavelength side due to soliton self-frequency shift and further pulse compression occurred [2,16]. The minimum sech2-fit pulse width of 92 fs was then obtained at the maximum 7.4-nJ pulse energy.
The autocorrelation trace indicates a fraction of energy in the pedestal as is expected from the second-order soliton decay induced by self-frequency shift . In order to determine the peak power and pulse width precisely, second harmonic generation frequency-resolved optical gating (SHG FROG)  was measured as shown in Fig. 5. The grid size is 512×512 and the time step is 23.3 fs (~12-ps temporal range). The retrieval error is 0.005. The retrieved intensity shows a 99.9-fs pulse width, closed to the transform-limited value (91 fs). The peak power is evaluated to be 54 kW by integration over the 12-ps temporal range. This value exactly coincides with the theoretical peak power (53.5 kW) of a 100-fs fundamental soliton by use of β 2=-30.7 ps2/km measured at 1570 nm. The leading low-intensity soliton shows linear phase change in time because of the different carrier frequency from the principal Raman soliton. The autocorrelation trace calculated from the retrieved intensity fairly matches with the experimental [Fig. 4(a)].
Because the soliton energy (or peak power) is proportional to the product of A eff|β 2|, both the large mode area and large anomalous dispersion closed to silica increase the soliton energy by ~40-250 in comparison with standard commercial Er-doped fibers with MFDs of ~5-7 µm and |β 2| of ~3-10 ps2/km. The excitation of second-order soliton and following fission to Raman soliton indicate that the peak power is almost reaching the nonlinearity limit of this fiber, even though further pulse shortening is possible by higher-order soliton compression with serious pulse-shape degradation [17,20]. Much higher peak and average-power operation will be enabled above the presented limit if CPA technique is applied [4,20–23]. The double-pass configuration has advantages such as linear polarization and pulse compression as shown, but on the other hand may reduce the nonlinearity limit by the doubled fiber length . Higher doping with adequate index-increase compensation as well as fabrication of PM LMA PCF  will be the subjects to be pursued for further energy scaling.
As an application of this high-peak power amplifier, supercontinuum spanning over an octave was generated by a successive high-nonlinear dispersion-shift fiber (HN-DSF, Sumitomo Electric.) [20,25]. Both NPM and PM fibers were tested for comparison. Both fiber have γ=21 W-1km-1 and A eff=10 µm2. β 2 at 1550 nm are -0.40 and -1.2 ps2/km, respectively. A fraction of the amplifier output (~220 mW) was injected to the 2-m pieces of HN-DSFs. The intensity-calibrated supercontinuum spectra are shown in Fig. 6. The average power was about 140 mW. The supercontinuum from the NPM-HN-DSF has shorter wavelength cutoffs (994 and 2390 nm) than that of PM-HN-DSF (1005 and 2520 nm). Here the cutoff wavelength is defined as the 20-dB drop from the peak. In NPM-HN-DSF, polarization splitting of the Raman soliton may result in the shorter maximum-wavelength cutoff and larger intensity variation. These will be used for carrier-envelope phase locking of the presented high-power fiber amplifier output [26,27] as well as for fiber-dispersion measurement.
We have presented a femtosecond fiber amplifier at 1.55 µm based on a LMA Er:Yb-codoped PCF. Nearly transform-limited 100-fs, 7.4-nJ pulses were obtained with average and peak powers of 350 mW and 54 kW without CPA, respectively. Robust characteristics such as single-transverse mode (M 2<1.05) and single polarization (PER>42 dB) are preferable for many applications. Over-octave supercontinuum was generated by a successive high-nonlinear fiber for near-future carrier-envelope phase locking. CPA will further scale up the power and pulse energy and is being developed.
The authors thank J. Limpert for helpful discussion and N. Nishizawa for sending us a preprint. This research was supported by Grant-in-Aid for Scientific Research and by the 21st Century COE program of Ministry of Education, Culture, Sports, Science and Technology.
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