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
Ti:sapphire lasers are an important cornerstone of today’s ultrafast laser technology . Their importance is predominantly related to the extraordinarily broadband fluorescence of Ti:sapphire between 650 nm and 1000 nm . In particular, this unique bandwidth in combination with clever strategies for dispersion compensation facilitates direct synthesis of ~5fs pulse trains with octave-spanning spectra . These few cycle optical pulses are typically generated with central wavelengths close to the gain maximum at λ=780 nm. However, many spectroscopic applications require ultrashort laser pulses at the long wavelength end of the gain spectrum (900 nm≤λ≤1000 nm). As an example, self-assembled InGaAs/GaAs quantum dots exhibit interband transitions around λ=950 nm . Moreover, high quality GaAs based microcavities  are readily available for photon energies somewhat below the band-gap energy of bulk GaAs (EG,GaAs≈1.5 eV). Ti:sapphire lasers operating at 900 nm≤λ≤1000 nm have, to date, only been realized with pulse durations ≥100 fs. These laser sources typically do not offer sufficient bandwidth for, e.g., non-degenerate pump-probe experiments or electro-optic sampling with multi-THz bandwidth. The generation of significantly shorter pulses in this spectral window relies on parametric frequency conversion with kHz amplified systems  which often do not provide enough sensitivity for time-resolved spectroscopy of semiconductor nanostructures.
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 . 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 . 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.
Figure 1(b) displays the spectrum of the Ti:sapphire laser for an operational central wavelength of λz=904 nm and a 6 % output coupling efficiency. In particular, we find an average power of 200 mW and a pulse bandwidth as large as Δν=21 THz (FWHM). Spectral components with an intensity of at least -10 dB of the peak value can be found over a spectral window of ~52 THz corresponding to as much as ~25 % of an optical octave. Note that the decreased output power compared to oscillators operating, e.g., close to λ=800 nm is largely related to the emission cross section which is reduced by ~40 % compared to the gain maximum of Ti:sapphire . The temporal envelope of the laser pulse is analyzed with a background free intensity autocorrelation based on second harmonic generation in a 100 µm BBO crystal. As depicted in Fig. 1(c) on a logarithmic scale, we find a sech2-fit with a pulse duration of tp=27 fs (FWHM) to agree well with the envelope of the experimental trace. For a more detailed pulse characterization, we also perform interferometric autocorrelation based on two-photon absorption in a photodiode. As seen in Fig. 1(d), this method points towards a very similar pulse duration of tp=29 fs (FWHM). Both methods indicate detectable but very weak pedestals of the ultrashort pulse.
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 [8,9] 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.
We now turn to the analysis of the pulse parameters throughout the interval 900 nm≤λz≤980 nm of central wavelengths λz accessible in mode-locked operation. The lengths of shortest pulses generated with the present resonator are summarized in Fig. 3(a). In particular, ~30 fs pulse duration as determined by sech2-fits to the envelopes of interferometric autocorrelation traces are observed throughout the entire tuning range. The limitations for short and long wavelengths are likely related to the decreasing optical feedback of the present resonator (cf. above discussion of the mirrors in the resonator) and the strongly reduced emission cross section of the gain medium, respectively.
The corresponding average output powers are displayed in Fig. 3(b) and decrease slowly from 180 mW for λz=905 nm to 135 mW for λz=970 nm. It is also possible to generate somewhat longer pulses mainly by adjusting the optical pathlength through the prism tips of the compressor. These pulses can have slightly higher average powers than the values displayed in Fig. 3(b) because such an alignment allows for the use of a larger output coupling efficiency of up to 10 %. Figure 3(c) shows the dependence of the optical bandwidth Δν on the central wavelengths. In particular, the decrease of Δν for longer wavelengths is found to parallel the increasing pulse duration. We also calculate the second and third order dispersion for our laser system using dispersive ray tracing . Figure 3(d) shows the second order dispersion of one roundtrip in the Ti:sapphire gain medium, the fused silica prism compressor and the total roundtrip dispersion of the resonator. The overall negative group velocity dispersion is seen to be almost constant over the spectral region of interest. As a result, fused silica prism compressors appear to be well appropriate for the generation of ultrashort laser pulses. Of course, also the 12 reflections of mirrors per round-trip have to be considered. While we have no precise information on the dispersion properties of these low dispersive mirrors, they are specified to have a second order dispersion of <20 fs2 in the region of interest. As a result, the overall group velocity dispersion of the resonator remains negative. However, the deviation of the time-bandwidth product from the ideal value indicates a clear influence of higher order dispersion likely predominantly related to the mirrors.
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.
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  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 [8,12]. 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.
In conclusion, we have demonstrated a widely tunable ~30 fs Ti:sapphire oscillator in the wavelength range 900 nm≤λ≤980 nm. Taken together with previous results, optical few cycle pulses are now available over almost the entire gain bandwidth of Ti:Al2O3. The present laser system promises new applications among which high-precision ultrafast spectroscopy of semiconductor nanostructures is a prominent example. Moreover, its second harmonic serves as a source of visible multi-milliwatt femtosecond pulses tunable around 475 nm.
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
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