Simultaneous pulse amplification and compression in all-fiber-integrated pre-chirped large-mode-area Er-doped fiber amplifier Gong-Ru Lin1*, Ying-Tsung Lin, and Chao-Kuei Lee2 1
Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan R.O.C. 2 Institute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan R.O.C. *Corresponding author e-mail:
[email protected]
Abstract: A large-mode-area Erbium-doped fiber amplifier (LMA-EDFA) based all-fiber-integrated amplified compressor with ultrashort length of 5.37 m and ultralow pumping power (260 mW) is proposed. The LMAEDFA suppresses nonlinear soliton-self-frequency-shift effect happened during femtosecond pulse amplification, in which the fiber laser pulse is reshaped to a low-pedestal hyperbolic-second shape with nearly 100% energy confinement. The pre-chirped amplification from 0.96 to 104 mW and the simultaneous compression of a passively mode-locked fiber laser pulse from 300 to 56 fs is demonstrated. The input pulse energy of 24 pJ is amplified up to 2.6 nJ with shortened pulsewidth of 56 fs and peak power as high as 46 kW. ©2007 Optical Society of America OCIS codes: (060.7140) Ultrafast processes in fibers; (320.5520) Pulse compression; (140.4050) Mode-locked lasers.
References 1.
M. E. Fermann, A. Galvanauskas, and M. Hofer, “Ultrafast pulse sources based on multi-mode optical fibers,” Appl. Phys. B 70, S13-S23 (2000). 2. A. Galvanauskas, M. E. Fermann, and D. Harter, “High-power amplification of femtosecond optical pulses in a diode-pumped fiber system,” Opt. Lett. 19, 1201-1203 (1994). 3. A. Galvanauskas, M. E. Fermann, and D. Harter, “All-fiber femtosecond pulse amplification circuit using chirped Bragg gratings,” Appl. Phys. Lett. 66, 1053-1055 (1995). 4. A. Boskovic, M. J. Guy, S. V. Chernikov, J. R. Taylor, and R. Kashyap, “All-fibre diode pumped, femtosecond chirped pulse amplification system,” Electron. Lett. 31, 877-878 (1995). 5. C. J. S. de Matos, J. R. Taylor, T. P. Hansen, K. P. Hansen, and J. Broeng, “All-fiber chirped pulse amplification using highly-dispersive air-core photonic bandgap fiber,” Opt. Express 11, 2832 (2003). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-22-2832 6. C. J. S. de Matos and J. R. Taylor, “Multi-kilowatt, all-fiber integrated chirped-pulse amplification system yielding 40× pulse compression using air-core fiber and conventional erbium-doped fiber amplifier,” Opt. Express 12, 405-409 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-2-1613 7. J. W. Nicholson, A. D. Yablon, P. S. Westbrook, K. S. Feder, and M. F. Yan, “High power, single mode, all-fiber source of femtosecond pulses at 1550 nm and its use in supercontinuum generation,” Opt. Express 12, 3025-3034 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-23-3025 8. J. Takayanagi, N. Nishizawa, H. Nagai, M. Yoshida, and T. Goto, “Generation of high-power femtosecond pulse and octave-spanning ultrabroad supercontinuum using all-fiber system,” IEEE Photon. Technol. Lett. 17, 37-39 (2005). 9. K. Tamura, H.A. Haus, and E. P. Ippen, Electron. Lett. 28, 2226-2228 (1992) 10. G. P. Agrawal, Nonlinear Fiber Optics (Academic press, San Diego, 2001), Chap. 3. 11. J. Takayanagi, N. Nishizawa, H. Nagai, M. Yoshida, and T. Goto, “High-Peak-Power Ultrashort Pulse Generation Using All-Fiber Chirped Pulse Amplification System with Small Core Multimode Fiber,” Jpn. J. of Appl. Phys. 44, 177-180 (2005).
#77087 - $15.00 USD
(C) 2007 OSA
Received 14 November 2006; revised 1 February 2007; accepted 19 February 2007
19 March 2007 / Vol. 15, No. 6 / OPTICS EXPRESS 2993
1. Introduction Passively mode-locked Erbium-doped fiber lasers (EDFL) are dependable source for generating sub-100fs pulses at 1550nm, however, such EDFLs typically generate pulses with energy lower than those of solid-state lasers (such as Ti:sapphire laser). Owing to the inherent fiber nonlinearity, the EDFL usually meet difficulty in obtaining multi-nJ pulses. To increase the peak power, the multi-mode fibers were employed suppressing the nonlinear effect, and compression of chirped pulses with bulk-optic components after the EDFL is frequently performed [1]. Alternatively, the chirped pulse amplification (CPA) technique were particular designed to stretch ultrashort pulses prior to amplification and to recompress them back after the amplification is completed, with all three types of dispersive elements [2-4]. A vintage configuration which employed a fiber based pulses stretcher in an EDFA, and a pair of bulk diffraction gratings as the compressor after the EDFA was reported to achieve 420-fs pulses with energy of 3 nJ after the grating pair. Nonetheless, such a bulky diffraction grating compressor makes the whole fs laser system incompact. Subsequently, fiber Bragg gratings were inserted after the EDFA to stretch and compress the pulsewidths to 408 fs [3] and 900 fs [4] associated with energies of 3 and 1.6 nJ, respectively. The all-fiber configuration although is alignment-free, which still limits the achievable peak powers due to the fiber nonlinearity. Recently, a photonic bandgap fiber (PBF) based compressor, has been introduced into the CPA system, [5, 6] however, such special structural fibers are difficult to splice with standard optical fibers for high coupling efficiency. In particular, the extremely large GVD of the PBF leads to a strict tolerance on the optimized length, while the large dispersion slope of the PBF also makes the nonlinear chirp hard to be compensated completely. These constrain the peak power of compressed pulses at few kW. More recently, a highly doped EDF [7, 8] was employed, to shorten length of the EDF and to reduce the high-order nonlinear effect during pulse amplification and compression processes in EDFA and single mode fiber (SMF) based soliton compressor, respectively. To date, the ultrashort pulsewidths of 34 fs [7] and 43 fs [8] were obtained with peak power of 140 and 43 kW, respectively, at the cost of low confinement ratio with 1.8 m. per-chirp fiber length was 2.78 m
Pulsewidth (fs)
90 62 85 60
80 75
58
124cm, 59.9fs, Q=96.5
1.25
1.30
1.35
1.40
127cm, 56 fs, Q=100 130cm, 64fs,
1E-3
1E-5 -1000
1.45
-750
-500
Length of SMF (m)
Fig. 4. Compressed pulsewidth and pulse energy confinement ratio as a function of the SMF length.
0
250
500
Delay time(fs)
750
1000
104.0
118cm 121cm 124cm 127cm 130cm
1.0
Power (dBm)
-50
Power (mW)
Intensity (mW)
-250
Fig. 5. Logarithmic plot of the auto-correlated pulses obtained at versatile SMF fiber lengths of nearly optimized soliton condition.
2.0
1.5
Q=92
0.01
1E-4
70 1.20
121cm, 60.1fs, Q=98
0.1
Intensity (a.u.)
95
64
118cm, 60.1fs, Q=95.6
1
100
Energy Confinement Ratio (%)
66
103.8
-60 -70 -80 10
20
30
40
50
60
70
Frequency (MHz)
103.6
0.5
0.0
103.4 1500
1550
Wavelength (um)
1600
1650
Fig. 6. Corresponding spectra of the auto-correlated pulses obtained at versatile SMF fiber lengths of nearly optimized soliton condition
0
10
20
30
40
50
60
Time (min)
Fig. 7. Average power fluctuation and side-mode frequency spectrum of the fiber laser pulse-train.
The logarithmic plot of the auto-correlated pulse shapes obtained at nearly optimized soliton condition are shown in Fig. 5, while the peak powers of the pedestals are sufficiently low although the diagnostic sensitivity of the auto-correlator is limited, such a 100% energy confinement ratio was never observed and reported in previously reported systems to our best knowledge. The fast Fourier transformation of the amplified and compressed EDFL spectrum reveals such a nearly transform-limited shape of the auto-correlated pulse should be somewhat distorted by multi-pedestals. However, these tiny pedestals are unavailable to be resolved due to both the limited sensitivities of the second-harmonic generation based detection and the digitized I/O interfacial data port of the commercial auto-correlator. The average power fluctuation and side-mode frequency spectrum of the fiber laser pulse-train are shown in Fig. 7, which reveal a maximum power drift of 35 dB (determined by a RF spectrum analyzer with resolution of 100 Hz) after the amplified compression in the LMA-EDFA. These results interpret the comparable high stability of such a compact high-power femtosecond fiber laser system with other works reported previously. 4. Comparisons with conventional approaches It is interesting to compare the results obtained in this work with those reported before. Recently, Nicholson and co-workers have demonstrated a chirp-pulse amplifier using four laser diodes to offer forward (backward) pumping power of 1.28 W. Such an EDFA generates #77087 - $15.00 USD
(C) 2007 OSA
Received 14 November 2006; revised 1 February 2007; accepted 19 February 2007
19 March 2007 / Vol. 15, No. 6 / OPTICS EXPRESS 2997
output power up to 800 mW, which supports to obtain 34-fs pulses with pulse energy of 8.7 nJ from a similar EDFL system at a repetition rate of 46 MHz [7]. Although the peak power of pulse can be 140 kW, the pulse compression ratio and the energy confinement ratio is only 7 and 55%, respectively. On the other hand, Takayanagi et al. have demonstrated an alternative approach to obtain a compressed pulsewidth of 41.3 fs at a repetition rate of 48 MHz, providing the average power and the pulse energy of 215 mW and 4.5 nJ, respectively [8]. However, the pumping power required to obtain similar peak power of the compressed EDFL pulse is still as high as 400 mW, while the compressed pulse quality is even worse than 39% and the pulse compression ratio is only 6. In these cases, the high peak power and ultrashort pulse can be obtained under highly pumped EDFA, but half of the pulse energy is scattered to the pedestal. Later on, the same group further demonstrated the pulse compression in a smallcore multimode fiber [11], which increases the energy confinement ratio to 84% at a cost of broadened pulsewidth and small pulse compressing ratio. In contrast, the pre-chirped LMAEDFA needs ultralow pumping power (only 260 mW) for simultaneous amplified compression of the passively mode-locked EDFL pulse from 300 fs to 56 fs. Although the average and peak powers of the generated pulses are 104 mW and 46 kW, the energy of 2.6 nJ can be entirely confined within the central portion of the amplified EDFL pulse. That is, the pulse shape is nearly transform-limited with extremely low pedestal power after simultaneous amplifying and compressing in the LMA-EDFA. Table I Parametric comparison of previous results and our work
Different works
Ref. [7]
Ref. [8]
Ref. [11]
LMA-EDFA
Pforward/backward(mW) frept. (MHz)
610/571 46 250
400/400 48 260
560 48 260
140/120 40 300
34
41.3
80
56
400 8.7
215 4.5
200 4.1
104 2.6
140
42.3
44
46
7 55
6 39
3 84
5.5 ~100
τorigin τcompress Pout,avg Epulse Ppeak
(fs) (fs) (mW) (nJ) (kW)
Rcompress-ratio Qc
(%)
5. Conclusion By using a newly designed large-mode-field-area Er-doped fiber based pre-chirped EDFA with ultrashort length and ultralow pumping power, we primarily demonstrate the simultaneous amplification and compression of a passively mode-locked Erbium-doped fiber laser (EDFL) pulse from 0.96 mW to 104 mW and from 300 fs to 30 fs, respectively. The mixed large mode-field-area and pre-chirping design in an EDFA greatly suppresses the stimulated Raman scattering induced nonlinearly soliton-self-frequency-shift effect happened during the amplification femtosecond laser pulses in conventional EDFA module. The original hyperbolic-second-shape pulse with energy of 24 pJ is generated via the self-started passive mode-locking of EDFL at repetition frequency of 40 MHz. With the specially designed ultrashort-length pre-chirped LMA-EDFA, the energy of EDFL pulse can be greatly amplified to 2.6 nJ with its pulsewidth being compressed to 56 fs, providing a peak power as high as 46 kW after the pre-chirped amplification/compression procedure. Such a simplified pre-chirped LMA-EDFA compressor is able to reshape the EDFL pulse to an ultralow pedestal shape at a relatively high average output power condition.
#77087 - $15.00 USD
(C) 2007 OSA
Received 14 November 2006; revised 1 February 2007; accepted 19 February 2007
19 March 2007 / Vol. 15, No. 6 / OPTICS EXPRESS 2998
Acknowledgments The work was financially supported by National Science Council of Taiwan R.O.C. under grant NSC94-2215-E-002-040 and NSC95-2221-E-002-448.
#77087 - $15.00 USD
(C) 2007 OSA
Received 14 November 2006; revised 1 February 2007; accepted 19 February 2007
19 March 2007 / Vol. 15, No. 6 / OPTICS EXPRESS 2999
