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We present an octave-spanning Ti:sapphire oscillator supporting Fourier-limited pulses as short as 3.7 fs. This laser system can be directly CEO-phase stabilized delivering an average output power of about 90mW with a pulse duration of 4.4 fs. The phase-stabilization is realized without additional spectral broadening using an f-2f interferometer approach allowing for full control of the electric pulse field on a sub-femtosecond time-scale.
©2008 Optical Society of America
1. Introduction
Pulses from octave-spanning Ti:sapphire oscillators are interesting from very different experimental points of view because of their unique spectral and temporal features. These lasers are ideal light-sources for high resolution optical coherence tomography (OCT) [1] and few-cycle pulse shaping experiments as presented in [2]. With this technique it is possible to shape Fourier-limited pulses with less than two optical cycles and flexible pulse sequences for coherent control experiments. Concerning such ultrashort pulse durations close to the single-cycle limit, the influence of the carrier envelope off-set (CEO) phase becomes important. Therefore CEO-phase stabilized few-cycle laser pulses have many fields of application in today’s science. Amplified phase-stabilized femtosecond pulses are e.g. used for efficient high harmonic generation and attosecond science [3, 4, 5], where a controlled electric field evolution of the generating laser pulse is an indispensable requisite. Apart from attosecond science, phase-stabilized pulses are required for frequency metrology and precision spectroscopy [6, 7] or can be directly applied for extreme nonlinear optics [8, 9, 10, 11] and quantum interference control [12].
With octave-spanning laser systems it is possible to realize a direct CEO-phase stabilization [13] without the need for extra-cavity spectral broadening concepts such as using a PCF fiber or monolithic PPLN device [14]. Here we report to the first time on an octave-spanning, CEO-phase stabilized Ti:sapphire oscillator with pulses as short as 4.4 fs and 90mW average output power.
2. Octave-spanning laser system
Our octave-spanning laser system is a prism-less Ti:sapphire oscillator designed for soft aperture Kerr-lens mode locking. The setup, presented in Fig. 1, corresponds to the ones introduced in [16, 17] except for some minor changes. The octave-spanning feature arises here from a unique combination of broadband dispersion compensating mirror pairs (DCMPs) [15] and a specially designed broadband dispersive output-coupling mirror. This output-coupling mirror in combination with the DCMPs is mainly responsible for the laser’s exceptional broadband output spectrum.
Fig. 1. Schematic optical setup of the octave-spanning Ti:sapphire oscillator; BD: beam dump, L: focusing lens, M1–M7: dispersion compensating mirrors (DCMs), OC: output-coupling mirror, P: BaF2 plate, PM: pump aligning mirror, W1/W2: BaF2 wedge pair, X: Ti:sapphire crystal.
The Ti:sapphire crystal is being pumped by approx 5.5W of 532nm light emitted by a frequency-doubled Nd:YVO4 laser (Coherent Verdi series). Around the crystal an astigmatism-compensated z-folded cavity is built by DCMPs, offering both an octave-spanning high reflectivity and negative group delay dispersion to balance the overall net intra-cavity dispersion. A BaF2 wedge pair and an additional plate complete the dispersion management and allow for its fine-tuning. The output spectrum of the oscillator is shown in Fig. 2(A). It covers more than one optical octave, which is reached at approx. -17 dB, supporting Fourier-limited pulses as short as 3.7 fs. At -30 dB the spectral width exceeds 700 nm and even on a linear scale spectral parts beyond 1200 nm are clearly visible. This feature makes the laser an ideal light source for many experiments mentioned before. The average output power is about 100mW at a pulse repetition rate of 80MHz, resulting in pulse energies of 1.25 nJ.
The emitted few-cycle femtosecond pulses were characterized by a home-built SPIDER system [19]. To compensate for the extra-cavity dispersion a pulse re-compression was realized with six reflections on DCMs and some BaF2 bulk material - analog to those components used inside the laser cavity. Pulse durations as short as 4.3 fs (FWHM) have been measured this way; the temporal intensity profile and corresponding phase is shown in Fig. 2(B). A simultaneously performed radio-frequency analysis of the emitted pulse train verified stable and clean mode-locked operation.
Fig. 2. A) Octave-spanning optical output spectrum supporting Fourier-limited pulses as short as 3.7 fs, shown on a linear (left axis) and logarithmic scale (right axis). B) Pulse intensity profile and temporal phase measured with SPIDER, revealing pulse durations as short as 4.3 fs.
3. CEO-Phase Stabilization
CEO-phase stabilization of octave-spanning laser systems is realized directly using the f-2f self-referencing approach without additional spectral broadening [13]. This can be done in a collinear setup [17] with several reflections on DCMs to optimize the temporal delay between the interfering pulses, or in a quasi Michelson type setup, see Fig. 3. The spectral parts centered at 570nm and 1140nm are separated from the oscillator spectrum using a multichroic filter (in our case simply a broadband output-coupling mirror). The center part of the spectrum is reflected for subsequent experiments, but due to the filtering process its width is slightly reduced – mainly in the NIR region, shown in Fig. 5(A) – but still supports Fourier-limited pulses as short as 3.9 fs. The output power decreases to about 90% of the full oscillator power, the good beam quality remains unchanged. Recently also an alternative approach to the separation of the 1f–2f components without impacting the spectral width and output power has been demonstrated from an octave-spanning Ti:Sapphire laser with semitransparent mirrors at the 1f and 2f wavelength [18].
Fig. 3. CEO-frequency f-2f measurment setup; APD: avalanche photodiode detector, DBS: dichroic beam splitter, F: IR-filter, L: focusing lens, M: silver mirror, MCF: multichroic transmission filter, WP: wave plate (λ/4), X: LBO crystal.
Within the f-2f setup the transmitted spectral parts at 570 nm and 1140 nm are separated using a dichroic beamsplitter, allowing for a temporal delay and polarization tuning of the spectral parts with respect to each other. After recombining, the second harmonic of the IR-wing is generated by focusing the radiation into a 1mm LBO crystal, cut for type I second-harmonic generation (SHG) at 1140nm, the “blue-part” of the spectrum is simply transmitted. After re-collimation and filtering with respect to the fundamental IR radiation both signals are focused onto an avalanche photodiode detector for detecting the interference signal.
Fig. 4. RF-frequency measurement; A) Frequency scan showing the laser’s repetition rate frep and the fCEO beat signals, which exhibit a signal-to-noise ratio (SNR) of approx. 30 dB; RBW: 100 kHz. B) Zoomed-in stabilized CEO-frequency needle with a SNR of 60 dB; RBW: 1Hz.
Figure 4(A) shows the measured CEO-frequency beat signals f CEO located at one fourth of the laser repetition rate f rep at approx. 80MHz. Within 100kHz resolution bandwidth (RBW) the f CEO beat is 30dB well above the noise floor, which is sufficient for an active stabilization of the oscillator. This stabilization is realized by regulating the pump power via an acousto-optic modulator (AOM), which is driven by a phase-locked loop (PLL) generated error signal.
Figure 4(B) illustrates the 100Hz span around the stabilized CEO needle with 1Hz resolution. A clean peak with an excellent signal-to-noise ratio of about 60 dB is observed. The stabilized pulses were characterized with SPIDER employing the same compression setup with DCMs and BaF2 as described above, revealing a pulse duration as short as 4.4 fs with 90mWaverage output power, which is to our knowledge the shortest phase-stabilized pulse directly generated from an laser oscillator. The corresponding SPIDER measurement results for this pulse are given in Fig. 5.
As mentioned before, for few-cycle laser pulses the influence of the CEO-phase, the position of the electric field oscillation under the pulse envelope respectively, becomes important. To demonstrate this behavior, the reconstructed electric field of the CEOphase stabilized pulse is presented in Fig. 6. The inlays show the magnified field maxima with assumed zero (A) and π/2 (B) CEO-phase. For this case the peak electric field contrast between CEO-phase 0 and π/2 is at about 3%, with a 11% contrast between positive and negative field maxima at zero CEO-phase. In comparison, the maximum peak contrast for a CEO-phase difference of π/2 for a 7 fs-pulse is less than one percent and the contrast between positive and negative field maxima at zero phase is less than 4%. It is obvious that this difference has direct impact on experiments, which are sensitive to changes in the maximum peak contrast or e.g. for stereo above threshold ionization (Stereo ATI) [9], which is a prominent example for amplified pulses.
Fig. 5. A) Reflected residual spectrum remaining for experiments after extracting the spectral parts for the f-2f measurement, supporting a Fourier-limited pulse duration as short as 3.9 fs, plotted together with the measured spectral phase. B) Pulse intensity profile and temporal phase (measured with SPIDER) for the phase stabilized pulses revealing a pulse duration as short as 4.4 fs.
Fig. 6. Electric field of the CEO-phase stabilized 4.4 fs-pulse from Fig. 5(B). Inlay A) Magnified field maximum with assumed zero CEO-phase; B) Assumed CEO-phase of π/2.
With our home-built prism-based LCD pulse shaper [2] a device allowing for a flexible dispersion compensation to re-compress these pulses for a given experimental setup by shaping a flat spectral phase is available. By phase-shaping only and employing superresolution techniques, pulse durations as short as 3.7 fs can be realized [2]. By expanding this concept to simultaneous phase- and amplitude shaping, a field synthesizer can be established allowing for full control of the electric field [21] on a sub-cycle scale.
4. Conclusion
In conclusion, we demonstrated an octave-spanning Ti:sapphire laser oscillator delivering a ultrabroadband spectral output supporting Fourier-limited pulses as short as 3.7 fs. This spectrum allows for direct CEO-phase stabilization of the emitted pulses without additional spectral broadening, resulting in phase stabilized pulses as short as 4.4 fs with 90mW average output power and 60 dB signal-to-noise ratio in a 1Hz resolution bandwidth for the CEO-needle.
Taking this into account, our laser forms an ideal light source for CEO-phase sensitive and frequency-comb experiments as well as a seed oscillator for few-cycle pulse shaping regards. Using this laser e.g. in combination with a double-LCD pulse shaper, a few-cycle field synthesizer can be realized allowing for control of the entire electric field below the single-cycle scale.
Acknowledgments
The author thanks VENTEON Femtosecond Laser Technologies by Nanolayers for the close cooperation and access to their high quality optics for building this laser. This work was supported by the Bundesministerium für Bildung und Forschung (BMBF) under contract 13N8723, the National Science Foundation (NSF) under contract ECS-0501478, and the Air Force Office of Scientific Research (AFOSR) under contract FA9550-07-1-0014.
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