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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 8 — Apr. 14, 2008
  • pp: 5572–5576
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Generation of 30 femtosecond, 900–970 nm pulses from a Ti:sapphire laser far off the gain peak

Claudia Ruppert and Markus Betz  »View Author Affiliations


Optics Express, Vol. 16, Issue 8, pp. 5572-5576 (2008)
http://dx.doi.org/10.1364/OE.16.005572


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Abstract

A 58 MHz femtosecond Ti:sapphire oscillator is optimized for long wavelength operation beyond 900 nm. Sub 30 fs, ~3 nJ pulses with a bandwidth exceeding 20 THz are realized for central wavelengths 900 nm≤λ≤960 nm. This laser opens up new perspectives for the sensitive timeresolved spectroscopy of various semiconductor nanostructures. Moreover, its second harmonic serves as a source of visible multi-milliwatt femtosecond pulses tunable around 475 nm.

© 2008 Optical Society of America

In this article, we report the realization of a Ti:sapphire oscillator optimized for the wavelengths range 900 nm≤λ≤980 nm. The resonator design is depicted in Fig. 1(a) and relies on the standard concept for astigmatically compensated resonators with longitudinal pumping [7

7. D. E. Spence, P. N. Kean, and W. Sibbett, “60 fsec pulse generation from a self-mode-locked Ti:sapphire laser,” Opt. Lett. 16, 42–44 (1991). [CrossRef] [PubMed]

]. The highly doped Ti:sapphire crystal (optical absorption α532nm=6.0 cm-1 corresponding to an estimated doping concentration of 0.8 wt.%) of 3 mm thickness is pumped with a frequency doubled Nd:YVO4 laser (λ=532 nm) operated at a power level between 7.5 W and 10 W. Tight focusing of the pump beam is achieved using a 7.5 cm focal length lens. The arm lengths of the resonator are 90.5 cm and 168.5 cm and give rise to a repetition rate of 58 MHz. Two distinct intervals for the spatial separation of the folding mirrors (focal length: 5 cm) are found to support stable laser operation [1

1. C. Spielmann, et al., “Ultrabroad-band femtosecond lasers,” IEEE J. Quantum. Electron. 30, 1100–1114 (1994). [CrossRef]

]. We note that a folded geometry is chosen solely to provide a compact resonator design. Optical feedback is provided by broadband, low-dispersive mirrors with either HR925 (Laser Components) or HR900–1000 (Layertec) coating. Both types have a very high reflectivity over a spectral region extending from 850 nm to 1000 nm. The efficiency of the output coupler is kept at 6 % to 10 %. Dispersion compensation is achieved by a fused silica prism compressor (tip separation 48.5 cm) in the longer arm of the resonator. Rapid motion of one of the prisms induces stable mode-locking. We note that both the spatial separation of the folding mirrors and the position of the pump focus with respect to the gain medium require a more careful alignment compared to Ti:sapphire lasers at more conventional wavelengths. Despite potential spurious water absorption for long operational wavelengths, we find no purging of the cavity with, e.g., dry air to be necessary.

Fig. 1. (a). Resonator design of the long wavelength femtosecond Ti:sapphire laser. The mirrors used are described in the main text. (b) Optical output spectrum for a central frequency of 904 nm. (c) Intensity autocorrelation of the pulse with sech2-fit. (d) Corresponding interferometric autocorrelation trace with sech2-fit to the envelope.

Fig. 2. (a). Optical spectrum for a central wavelength of 940 nm obtained with a 6 % output coupler. (b) Intensity autocorrelation of the pulse with sech2-fit. (c) Corresponding interferometric autocorrelation trace with sech2-fit to the envelope.

Femtosecond pulses are also obtained at even longer central wavelengths. Wavelength tuning is achieved by adjusting the lateral position of the two prisms in the resonator and slight readjustment of the folding mirror separation. Figure 2 displays the pulse characteristics for a central wavelength of λz=940 nm where the emission cross section of Ti:sapphire is more than a factor of 2 below its peak value at λ=780 nm. In particular, we observe ultrabroadband operation (Δν=23 THz, cf. Figure 2(a)) with an average power of 150 mW obtained with a 6 % output coupler. The intensity auto-correlation trace in Fig. 2(b) reveals a pulse duration as short as tp=29 fs which is corroborated by the corresponding interferometric autorcorrelation trace displayed in Fig. 2(c). These pulse durations are comparable to the shortest ever reported in this spectral range [8

8. E. Riedle, et al., “Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR,” Appl. Phys. B 71, 457–465 (2000). [CrossRef]

,9

9. C. Manzoni, D. Polli, and G. Cerullo, “Two-color pump-probe system broadly tunable over the visible and the near infrared with sub-30 fs temporal resolution,” Rev. Sci. Inst. 77, 023103 (2006). [CrossRef]

] and unprecedented for oscillator systems. We note that the time-bandwidth product of 0.567 and 0.667 for the pulses characterized in Figs. 1 and 2, respectively, is clearly larger than the theoretical limit of 0.315 for sech2 pulses.

Fig. 3. (a). Shortest pulse durations obtained for various central wavelengths. The durations represent FWHM values of sech2-fits to the envelope of interferometric autocorrelation traces. All values are obtained with 6 % output couplers. (b), (c) Corresponding average output powers and FWHM values of the optical bandwidth. (d) Calculated second order roundtrip dispersion of the gain medium (orange line), the fused silica prism compressor (blue) and the entire resonator (green).

We would like to emphasize that the present laser system is well suited for highly sensitive modulation spectroscopy as required, e.g., for time-resolved pump-probe spectros-copy. As an example, the laser noise for modulation frequencies in the order of 10 kHz often used for photomodulation experiments is found to be as low as ΔI/I=1.2×10-6 Hz-1/2. As a result, shot noise limited experiments are possible up to power levels of 460 nW. Clever referencing schemes such as polarization bridges used in electro-optic sampling experiments may even improve this sensitivity in specific experimental geometries.

Fig. 4. (a). Optical spectrum of the second harmonic pulse generated in a 0.5 mm thick BBO crystal. The fundamental pulse of 31 fs duration is centered at 946 nm. (b) Interferometric autocorrelation trace of the second harmonic pulse with a sech2-fit to the envelope.

We now turn to the analysis of second harmonic pulses generated with the present Ti:sapphire laser. Such pulses are tunable around 475 nm, i.e. a wavelengths regime which completely lacks solid state sources of femtosecond pulses with high repetition rates. One example generated from a pulse train centered at λz=946 nm (optical bandwidth Δν=17 THz, pulse duration tp=31 fs, average power 120 mW, output coupling efficiency 6 %) is shown in Fig. 4. These second harmonic pulses are generated in a 0.5 mm BBO crystal with type I-phasematching. As seen in Fig. 4(a), we obtain a remarkably broad spectrum (Δλ=10 nm, Δν=13 THz) centered at a wavelength of λz=472 nm with an average power of 4 mW corresponding to a quantum efficiency of 3.3 % in this conversion process. The pulse duration is again analyzed with an interferometric cross correlation utilizing two-photon absorption in a UV emitting LED. The envelope of the autocorrelation trace in Fig. 4(b) reveals a pulse duration as short as 41 fs. This pulse duration is shorter than the one demonstrated with compact fiber laser sources [11

11. K. Moutzouris, et al., “Multimilliwatt ultrashort pulses continuously tunable in the visible from a compact fiber source,” Opt. Lett. 31, 1148–1150 (2006). [CrossRef] [PubMed]

] while still somewhat longer than the shortest visible pulses generated with the established, but much more complex approach employing kHz Ti:sapphire amplifier and two-stage optical parametric amplification [8

8. E. Riedle, et al., “Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR,” Appl. Phys. B 71, 457–465 (2000). [CrossRef]

,12

12. G. Cerullo, M. Nisoli, S. Stagira, and S. De Silvestri, “Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible,” Opt. Lett. 23, 1283–1285 (1998). [CrossRef]

]. Since the pulse described in Fig. 4 does not take full advantage of the bandwidth of the fundamental spectrum, the choice of a thinner frequency doubling crystal might allow for the generation of even shorter visible pulses on the expense of their power levels.

We would like to acknowledge helpful discussions with M. Wesseli. This work has been supported by the Sonderforschungsbereich 631 of the Deutsche Forschungsgemeinschaft.

References and links

1.

C. Spielmann, et al., “Ultrabroad-band femtosecond lasers,” IEEE J. Quantum. Electron. 30, 1100–1114 (1994). [CrossRef]

2.

P. F. Moulton, “Spectroscopic and laser characteristics of Ti:Al2O3,” J. Opt. Soc. Am. B 3, 125–133 (1986). [CrossRef]

3.

R. Ell, et al., “Generation of 5 fs pulses and octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26, 373–375 (2001). [CrossRef]

4.

S. Raymond, et al., “Excitonic energy shell structure of self-assembled InGaAs/GaAs quantum dots,” Phys. Rev. Lett. 92, 187402 (2004). [CrossRef] [PubMed]

5.

A. Badolato, et al., “Deterministic coupling of single quantum dots to single nanocavity modes,” Science 308, 1158–1161 (2005). [CrossRef] [PubMed]

6.

J. Piel, M. Beutter, and E. Riedle, “20-50 fs pulses tunable across the near-infrared from a blue-pumped noncollinear parametric amplifier,” Opt. Lett. 25, 180–182 (2000). [CrossRef]

7.

D. E. Spence, P. N. Kean, and W. Sibbett, “60 fsec pulse generation from a self-mode-locked Ti:sapphire laser,” Opt. Lett. 16, 42–44 (1991). [CrossRef] [PubMed]

8.

E. Riedle, et al., “Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR,” Appl. Phys. B 71, 457–465 (2000). [CrossRef]

9.

C. Manzoni, D. Polli, and G. Cerullo, “Two-color pump-probe system broadly tunable over the visible and the near infrared with sub-30 fs temporal resolution,” Rev. Sci. Inst. 77, 023103 (2006). [CrossRef]

10.

B. E. Lemoff and C. P. J. Party, “Cubic-phase-free dispersion compensation in solid-state ultrashort-pulse lasers,” Opt. Lett. 18, 57–59 (1993). [CrossRef] [PubMed]

11.

K. Moutzouris, et al., “Multimilliwatt ultrashort pulses continuously tunable in the visible from a compact fiber source,” Opt. Lett. 31, 1148–1150 (2006). [CrossRef] [PubMed]

12.

G. Cerullo, M. Nisoli, S. Stagira, and S. De Silvestri, “Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible,” Opt. Lett. 23, 1283–1285 (1998). [CrossRef]

OCIS Codes
(140.3590) Lasers and laser optics : Lasers, titanium
(140.7090) Lasers and laser optics : Ultrafast lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: February 19, 2008
Revised Manuscript: April 2, 2008
Manuscript Accepted: April 3, 2008
Published: April 4, 2008

Citation
Claudia Ruppert and Markus Betz, "Generation of 30 femtosecond, 900-970 nm pulses from a Ti:sapphire laser far off the gain peak," Opt. Express 16, 5572-5576 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-8-5572


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References

  1. C. Spielmann,  et al., "Ultrabroad-band femtosecond lasers," IEEE J. Quantum. Electron. 30, 1100-1114 (1994). [CrossRef]
  2. P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al2O3," J. Opt. Soc. Am. B 3, 125-133 (1986). [CrossRef]
  3. R. Ell,  et al., "Generation of 5 fs pulses and octave-spanning spectra directly from a Ti:sapphire laser," Opt. Lett. 26, 373-375 (2001). [CrossRef]
  4. S. Raymond,  et al., "Excitonic energy shell structure of self-assembled InGaAs/GaAs quantum dots," Phys. Rev. Lett. 92, 187402 (2004). [CrossRef] [PubMed]
  5. A. Badolato,  et al., "Deterministic coupling of single quantum dots to single nanocavity modes," Science 308, 1158-1161 (2005). [CrossRef] [PubMed]
  6. J. Piel, M. Beutter, and E. Riedle, "20-50 fs pulses tunable across the near-infrared from a blue-pumped noncollinear parametric amplifier," Opt. Lett. 25, 180-182 (2000). [CrossRef]
  7. D. E. Spence, P. N. Kean, and W. Sibbett, "60 fsec pulse generation from a self-mode-locked Ti:sapphire laser," Opt. Lett. 16, 42-44 (1991). [CrossRef] [PubMed]
  8. E. Riedle,  et al., "Generation of 10 to 50 fs pulses tunable through all of the visible and the NIR," Appl. Phys. B 71, 457-465 (2000). [CrossRef]
  9. C. Manzoni, D. Polli, and G. Cerullo, "Two-color pump-probe system broadly tunable over the visible and the near infrared with sub-30 fs temporal resolution," Rev. Sci. Inst. 77, 023103 (2006). [CrossRef]
  10. B. E. Lemoff and C. P. J. Party, "Cubic-phase-free dispersion compensation in solid-state ultrashort-pulse lasers," Opt. Lett. 18, 57-59 (1993). [CrossRef] [PubMed]
  11. K. Moutzouris,  et al., "Multimilliwatt ultrashort pulses continuously tunable in the visible from a compact fiber source," Opt. Lett. 31, 1148-1150 (2006). [CrossRef] [PubMed]
  12. G. Cerullo, M. Nisoli, S. Stagira, and S. De Silvestri, "Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible," Opt. Lett. 23, 1283-1285 (1998). [CrossRef]

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