We demonstrate an all-fiber picosecond soliton laser with dispersion management performed by a chirped Bragg grating that generates ~1.6 ps pulses representing the shortest pulsewidth reported to date using this technology. The large anomalous dispersion provided by the grating allows building of a long-length cavity laser with an extremely low fundamental repetition rate of 2.6 MHz. This source allows us to use an original approach for producing energetic pulses that after boosting in a medium power core-pumped amplifier produce an octave-spanning supercontinuum radiation in a nonlinear photonic crystal fiber.
©2008 Optical Society of America
Supercontinuum sources based on the nonlinear broadening of spectrum in microstructured fiber present an attractive alternative to white light sources based on low-power broadband fluorescence, because they can offer radiation with very high spectral brightness and coherence. Many of the supercontinuum sources available today use solid-state femtosecond lasers for pumping the nonlinear media. Supercontinuum sources would benefit greatly if they are based entirely on all-fiber technology [1, 2]. However, fiber sources are either based on 1.55 µm seed lasers and do not extend into the wavelength range below 1 µm, which is required for biological applications, or they rely on environmentally sensitive high-power double clad technology with rather high noise level. Scaling the repetition rate of fiber laser system below 5 MHz would offer a compact and cost-effective core-pumped fiber oscillator with medium average power while with pulse energy sufficient for supercontinuum generation. The repetition rates are still >100 times higher than in Q-switched microchip lasers  and are, therefore, suitable for applications in optical coherence tomography.
The large group velocity dispersion of an optical fiber at wavelengths of 1 µm and below needs to be addressed in order to achieve short pulses from fiber lasers. Dispersion compensators based on fiber technology are highly advantageous because they allow for low-loss and compact cavities [4–6]. Among different solutions demonstrated to date, those based on photonic crystal fiber look most promising [5,6]. Dispersion compensation using chirped fiber Bragg grating (CFBG) is another attractive method that benefits from the flexible and mature technology used for inscription of the gratings in optical fibers .
Here we demonstrate a practical all-fiber supercontinuum source using a picosecond soliton fiber laser as a pumping source with a chirped fiber Bragg grating (CFBG) dispersion compensator. The device is based entirely on low-cost core-pumping technology. The soliton seed laser delivers ~1.6 ps pulses which are, by our knowledge, the shortest pulses reported to date from an all-fiber laser using dispersion compensation by CFBG. The chirped grating offers large anomalous dispersion and allows for a robust soliton pulse regime in a long-length cavity, resulting in a low repetition rate. The refined fiber grating technology allows for novel low repetition-rate oscillator with medium average power but with pulse energy sufficient to exploit efficiently the nonlinear effects in photonic crystal fiber (PCF). The developed pulse source with 1.8 ps width and 2.6 MHz repetition rate used instead of high-power double-clad technology demonstrates an octave-spanning supercontinuum.
2. Mode-locked master oscillator using chirped Bragg grating dispersion compensator
The experimental setup of the supercontinuum source is shown in Fig. 1. The linear cavity of the mode-locked fiber laser (master oscillator) comprises 70 cm of Yb-doped fiber with 500 dB/m absorption at 980 nm pumped with a 980-nm single mode laser diode through a dichroic fiber coupler. A butt-coupled semiconductor saturable absorber mirror (SAM) acting as a cavity end reflector ensured reliable self-starting of the passive mode-locking. The SAM used in this study has been described in detail earlier . The other end of the cavity is terminated by a CFBG.
The CFBG was imprinted into the core of a H2-loaded single-mode fiber using phase-mask technique [9, 10]. The fiber was exposed to 248 nm ultraviolet light from a KrF excimer laser through the phase mask with a length of 10 mm using a beam scanning technique. Optimization of the scanning and writing parameters allowed to a broadband and exceptionally flat reflection response of the CFBG to be achieved. The reflectivity of the CFBG with the center wavelength of 1042 nm was measured to be >97% over the bandwidth from 1030 nm to 1054 nm, as seen from Fig. 2. The grating dispersion was estimated from the measurements to be 4.5 ps/nm using soliton sideband in the mode-locked pulse spectrum . The dispersion of the grating at the wavelength around 1 µm could compensate for the normal dispersion of the standard single-mode fiber with a length of over 50 meters.
The characteristics of the mode-locked pulses generated from the laser with a cavity length corresponding to a repetition rate of 47 MHz are presented in Fig. 3. The laser emits low-noise pulses with duration of 1.95 ps (FWHM) with the spectral resonances, so-called Kelly side-bands that apparently indicates operation in the soliton regime .
To decrease the pulse repetition rate and to maximize the pulse energy after amplification, a piece of single-mode fiber was added to the cavity. The effect of the cavity length on the pulse width and time-bandwidth product was experimentally investigated. The results are summarized in Fig. 4. As can be seen, the pulse width ranges from 2 ps to 1.6 ps and the time-bandwidth product from 0.42 to 0.49 when the length of the fiber cavity varies from 2.5 m to 37 m. 1.6 ps duration pulses has been achieved for a cavity length of 29 m corresponding to a pulse repetition rate of ~3.5 MHz. These are the shortest pulse reported to date for a fiber laser using a CFBG dispersion compensator.
Some increase of the time-bandwidth product with the length of fiber inserted into the cavity, seen from Fig. 4, is due to strong temporal and spectral evolution in a long cavity, as confirmed by the numerical simulation presented below .
The results of the laser simulation presented in Fig. 5 validate the increase of the time-bandwidth product observed in the experiments. The pulse generation and propagation in a fiber laser was described by the nonlinear Schrödinger equation including the effects of dispersion, loss, parabolic gain-bandwidth profile, and SESAM considered as a two-level system. The simulations start from noise and run for a sufficient number of consecutive round trips through the cavity elements until a steady-state is reached . These elements include the saturable absorber, active and passive fiber, and the CFBG. The simulation shows that pulse duration and time-bandwidth product exhibit strong variations in dispersion managed soliton fiber lasers when the value of fiber dispersion becomes close to the value of the dispersion of the CFBG with opposite sign, i.e. the total cavity dispersion approaches zero value. Figure 5(a) shows the pulse duration and Fig. 5(b) the time-bandwidth product for different fiber lengths at different cavity locations. In particular, the pulses are slightly down-chirped at the SAM position, strongly up-chirped before the CFBG, and strongly down-chirped after the CFBG. While the pulse duration continues to decrease gradually with increasing fiber length close to the SAM, it starts to increase at locations near the CFBG. The simulation thus describes the observed increase of time-bandwidth product and pulse duration for fiber lengths longer than ~20 m, since the laser output was set at substantial distance from SAM, as seen from Fig. 1.
With an increase in the length of the fiber in the cavity, the laser tends to oscillate in multiple pulse regime. This behavior is generally expected with soliton pulse laser . For cavity lengths longer than ~37 meters single-pulse operation was not achievable. To achieve pulse energy sufficient for generation octave-spanning spectrum broadening, power scaling of the master oscillator has been performed using a two-stage ytterbium fiber amplifier core-pumped with fiber-coupled 300-mW pump diodes, as shown in Fig. 1. The highly doped ytterbium fibers peak absorption of ~1000 dB/m at 980 nm have lengths of 70 cm and 1.5 m for the 1st and 2nd amplifier stages, respectively. Average output power for a single pulse regime was 1 mW and 78 mW, correspondingly after the first and the second amplifier stages. The changes in the pulse shape and spectrum after the first amplifier stage were found to be negligible owing to long dispersion and nonlinear lengths of 30 m and 400 m, respectively. The efficiency of the second amplifier stage depends on the seed power and was 33% for the strongest seed signal. Parasitic optical back-coupling between the seed laser and the amplifier stages was prevented with the use of fiber-pigtailed optical isolators.
3. Supercontinuum generator
The amplifier output was spliced to the photonic crystal fiber with 0.5-dB excess loss. The 15-m long photonic crystal fiber has a zero dispersion wavelength at 1065 nm and a nonlinear coefficient of 11 (W·km)-1. The master oscillator was optimized for the lowest repetition rate that still allows the multiple pulsing regime to be avoided, using the results presented in the previous section. Seed laser pulses with an energy of 0.004 nJ at a repetition rate of 2.6 MHz were extracted from the cavity with a 20% tap coupler. Figure 6 shows the spectrum (a) and the autocorrelation (b) of these pulses at the laser output (black lines). The autocorrelation reveals a pulse duration of ~1.8 ps assuming a Gaussian fit. The pulses from the master laser were then scaled in the two-stage power amplifier up to energy of 30 nJ. The spectrum and autocorrelation of the pulses at the amplifier output are also shown in Figs. 6(a) and 6(b) (red lines). The spectrum bandwidth increased to 30 nm after the amplifier, while the pulse broadened slightly but acquired a complicated shape, as seen from the Fig. 6.
The pulses from the power amplifier were then coupled to the nonlinear fiber. The spectra at the output of the PCF presented in Fig. 7 correspond to states with different numbers of pulses circulating in the cavity and, consequently, with different pulse energies. The central peak in the broadened spectrum corresponds to the unconverted pump radiation and contains about 34% of the total power. The central peak is believed to be developed largely due to the mismatch between the lasing wavelength and the zero dispersion wavelength of the photonic crystal fiber equal to λZD=1065 nm. Tuning the central wavelength of the CFBG reflectivity bandwidth, and consequently, the master oscillator operation wavelength toward λZD is expected to improve the conversion efficiency .
Since an excessive pump power results in multiple pulsing, the amplifier was found to produce the highest pulse energy when the average output power was limited to 78 mW. With a single pulsed regime, the supercontinuum spectrum extends from 615 nm to 1700 nm (measured at 10-dB level). As seen from Fig. 7, the supercontinuum bandwidth decreases from 1085 nm observed for single-pulse operation to 500 nm for the 8-pulse regime, corresponding to the effective repetition rate of ~20 MHz. Compared to single-pulse operation, the average power for 8-pulse operation increases slightly to 97 mW corresponding, however, to a substantial reduction in the pulse energy to 4.6 nJ.
We have demonstrated a practical all-fiber supercontinuum source generating an octave-spanning spectrum. Both the soliton pulse regime and low fundamental repetition rate of ~3 MHz in a passively mode-locked ytterbium fiber laser have been achieved using a high-performance chirped fiber Bragg grating for dispersion control. The ytterbium-doped fiber laser is capable of producing 1.6 ps pulses, which are the shortest pulses reported to date using CFBG as a dispersion compensator in an Yb-doped fiber laser. Energy scaling of the master oscillator up to 30 nJ at 1040 nm was achieved in a core-pumped two-stage fiber amplifier. These pulses were then spectrally broadened in a photonic crystal fiber to form supercontinuum radiation ranging from 615 nm to 1700 nm. The fiber system demonstrated offers an attractive cost-effective solution for medium power broadband sources.
The authors acknowledge the support of the graduate school of Tampere University of Technology, the Finnish Foundation for Economic and Technology Sciences, the Ulla Tuominen Foundation, the Jenny and Antti Wihuri Foundation, and the Nokia Foundation.
References and links
1. J. W. Nicholson, M. F. Yan, P. Wisk, J. Fleming, F. DiMarcello, E. Monberg, A. Yablon, C. Jørgensen, and T. Veng, “All-fiber, octave-spanning supercontinuum,” Opt. Lett. 28, 643–645 (2003). [CrossRef] [PubMed]
2. G. Genty, S. Coen, and J. M. Dudley, “Fiber supercontinuum sources,” J. Opt. Soc. Am. B 24, 1771–1785 (2007). [CrossRef]
3. J. M. Dudley, L. Provino, N. Grossard, H. Maillotte, R. S. Windeler, B. J. Eggleton, and S. Coen, “Supercontinuum generation in air-silica microstructured fibers with nanosecond and femtosecond pulse pumping,” J. Opt. Soc. Am. B 19, 765–771 (2002). [CrossRef]
6. H. Lim, F. Ö. Ilday, and F. W. Wise, “Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control,” Opt. Express 10, 1497–1502 (2006).
7. O. Katz, Y. Sintov, Y. Nafcha, and Y. Glick, “Passively mode-locked ytterbium fiber laser utilizing chirped-fiber-Bragg-gratings for dispersion control,” Opt. Commun. 269, 156–165 (2007). [CrossRef]
8. O. G. Okhotnikov, L. Gomes, N. Xiang, T. Jouhti, and A. B. Grudinin, “Mode-locked ytterbium fiber laser tunable in the 980-1070-nm spectral range,” Opt Lett. 28, 1522–1524 (2003). [CrossRef] [PubMed]
9. K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication,” Appl. Phys. Lett. 32, 647–649 (1978). [CrossRef]
10. K. O. Hill, B. Malo, F. Bilodeau, D. C. Johnson, and J. Albert, “Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask,” Appl. Phys. Lett. 62, 1035–1037 (1993). [CrossRef]
11. M. L. Dennis and I. N. Duling III, “Experimental study of sideband generation in femtosecond fiber lasers,” IEEE J. Quantum Electron. 30, 1469–1477 (1994). [CrossRef]
12. Y. Chen, F. X. Kärtner, U. Morgner, S. H. Cho, H. A. Haus, E. P. Ippen, and J. G. Fujimoto, “Dispersion-managed mode locking,” J. Opt. Soc. Am. B 16, 1999–2004 (1999). [CrossRef]
13. T. Schreiber, B. Ortaç, J. Limpert, and A. Tünnermann, “On the study of pulse evolution in ultra-short pulse mode-locked fiber lasers by numerical simulation,” Opt. Express 15, 8252–8262 (2007). [CrossRef] [PubMed]
14. A. B. Grudinin, D. J. Richardson, and D. N. Payne, “Energy quantisation in figure eight fibre laser,” Electron. Lett. 28, 67–68, (1992). [CrossRef]