We investigate the modelocking dynamics of quasi-synchronously pumped, dispersion managed Kerr-lens modelocked Titanium-Sapphire lasers. For the first time, self-starting few-cycle laser pulses with 6 fs pulse duration and ultrabroadband optical spectra are demonstrated without using any intracavity elements like saturable absorbers.
©2005 Optical Society of America
Dispersion managed Kerr-lens modelocked (DM-KLM) Titanium-Sapphire (Ti:sapphire) lasers  are the work horses in the domain of sub-10 fs laser pulses. Since the first observation of Kerr-lens modelocking (KLM) in 1991 , continuous laser development has led to the generation of octave-spanning spectra and 5 fs pulses directly from the oscillator [3,4]. DM-KLM lasers exploit the intensity dependent nonlinear refractive index in conjunction with a careful management of the distribution of discrete dispersive elements inside the cavity. In the time domain, this leads to self-phase modulation and hence additional spectral broadening, whereas in the transverse spatial beam dimensions, the build-up of a Kerr-lens together with a suitable resonator geometry enables efficient gain modulation to favor pulsed operation in comparison to continuous wave (CW) operation. However, KLM lasers with pulse durations below a few tens of femtoseconds are generally not self-starting and usually require external (mechanical) perturbations to initiate modelocking.
In this paper, we report on the detailed dynamics of DM-KLM lasers when quasi-synchronously pumped by a modelocked picosecond Nd:YVO4 laser. We investigate the self-starting behavior and residual power modulation, demonstrating that self-starting is achieved for a wide range of pulse durations and optical bandwidths.
2. General aspects of the starting behavior of Kerr-lens modelocked lasers
Though KLM allows for a large modulation of the effective gain up to several tens of percent, it is generally not self-starting. A parameter that characterizes the self-starting ability is the so-called modelocking driving force and is defined as (d(Δg) / dI) for I → 0, with I the intensity and Δg the gain modulation . In sub-10 fs lasers, the modelocking driving force is designed to be small in order not to overdrive the KLM when the laser transitions from continuous wave operation to pulsed operation.
So far, three different approaches have been published to overcome the self-starting problem in KLM lasers. One approach is to maximize the modelocking driving force by proper cavity alignment [6,7,8]. Cerullo et al. showed that a particular resonator design enables a maximization of the nonlinear mode variation and consequently dynamic loss modulation, achieving self-starting in a KLM Ti:sapphire when operating close enough to the stability edge . This is only possible down to pulse durations of about 40 fs until KLM is overdriven, resulting in a non-continuous modelocking. This means that the nonlinearity in the laser crystal becomes excessive and leads to multiple pulsing and/or modelocked Q-switching.
Alternatively, one can use a semiconductor saturable absorber mirror  (SESAM) or saturable Bragg reflector  (SBR) inside the cavity. The laser then exhibits self-starting modelocking because SESAMs/SBRs provide large modelocking driving forces in the initial pulse build-up phase. Saturation of the SBR after pulse build-up doesn’t harm the laser dynamics, because KLM is taking over the pulse shaping. Another advantage of this method is a relaxed cavity alignment in contrast to purely DM-KLM lasers. A drawback is the bandwidth limitation introduced by these devices that can only be overcome by using non-conventional fabrication procedures which are still under development [11,12].
A third approach to attain self-starting DM-KLM is (quasi-) synchronous pumping [13,14]. The term “quasi” accounts for the fact that precise synchronization of the Ti:sapphire laser is not necessary. Siders et al. have already demonstrated self-starting 30 fs pulses with quasi-synchronous pumping . However, they used hard-aperture KLM not suitable for few-cycle pulses. Such pulses possess optical bandwidths of several hundred nanometers up to one octave and since the mode-size varies strongly with wavelength an efficient use of an hard-aperture is not possible.
We extend this work to the sub-10 fs regime and provide detailed information on the starting transients, as well as residual RF side-band modulation. We also show that only a very small gain modulation induced by the pulsed pump laser is sufficient to initiate soft-aperture modelocking of the shortest pulses directly generated from an oscillator.
3. Motivation for (quasi-) synchronous pumping
Synchronous or quasi-synchronous pumping has many interesting applications. First of all, self-starting is achieved, which is most important when considering the poor starting performance of high-repetition rate DM-KLM lasers. For repetition frequencies of several hundred MHz up to a few GHz , synchronous pumping will significantly improve the starting behavior. We also predict that pumping at a sub-harmonic of the fundamental repetition frequency, which was not done in the context of this publication, will still lead to self-starting of the DM-KLM process. As will be illustrated later on, the residual output power modulation is only on the order of -30 dBc and can even be reduced further by pumping with a higher repetition rate source due to stronger filtering of the gain medium.
Active synchronization of the modelocked pump and Ti:sapphire lasers completely removes the residual output power modulation and is very appealing for many pump-probe experiments because the system simultaneously delivers timing controlled femtosecond near-IR pulses, picosecond green pulses and picosecond IR pulses . In the context of the emerging field of optical frequency metrology, phase-synchronized modelocked pump and Ti:sapphire lasers offer exciting new perspectives by accessing new spectral regions not yet covered by phase-controlled frequency-combs. We have shown earlier  that a fraction of the Ti:sapphire light could be used to modulate a Fabry-Pérot semiconductor structure used as a high-reflector inside the Nd:YVO4 laser cavity, which leads to passive and tight synchronization. We demonstrated back then that one can observe the carrier-envelope difference between both lasers and use such a system for frequency metrology. Phase-synchronization of the picosecond Nd:YVO4 laser with the DM-KLM Ti:sapphire laser is facilitated by the fact that our broadband sub-10 fs pulses have enough optical power at the wavelength of the picosecond laser, around 1064 nm. An interference experiment of the two synchronized lasers provides the difference of the two carrier-envelope offset frequencies . Together with the appropriate scheme for the control of the carrier-envelope offset frequency of the Ti:sapphire laser and a suitable feedback loop, one is readily able to provide fully phase-coherent mode-combs for time domain applications and frequency metrology, either by using the available wavelength ranges directly or by extending the mode-combs to the IR or visible spectral regions by nonlinear frequency mixing. The attractiveness of our approach lies in the fact that by using a modelocked ps-pump laser, we can pump the Ti:sapphire laser and simultaneously use the pump laser for doing experiments in optical frequency metrology. In comparison to the coupling of two independent lasers, this scheme is economically more viable.
4. Experimental results
The experimental setup for the quasi-synchronous pumping is depicted in Fig. 1. The pump source is a frequency doubled (532 nm), picosecond (7 ps), saturable absorber mirror modelocked Nd:YVO4 laser delivering up to 6.5 W of pump power at a repetition rate of 100 MHz and with a diffraction limited beam M2 < 1.2 (picoTrainTM, High Q Laser Production GmbH, Austria, http://www.highqlaser.com/). The Ti:sapphire laser is sketched below, and is a compact, z-folded, prism-less oscillator with broadband optics for octave-spanning spectra (VENTEON OS, NanoLayers Optical Coatings GmbH, Germany, http://www.nanolayers.de/). To control the cavity length of the Ti:sapphire laser, one end mirror was mounted on a piezo-controlled translation stage.
After manually equalizing the resonator lengths to a common repetition frequency of approximately 100MHz, the Ti:sapphire laser immediately starts modelocking, when the cavity is optimized beforehand for KLM operation. In terms of cavity alignment to geometrically optimize the resonator for efficient KLM, there is no difference between CW pumping and (quasi-) synchronous pumping. Changing the total intracavity dispersion by moving or completely removing the glass wedges (and the plate) we had access to different pulse duration regimes and observe reliable self-starting behavior for 100 fs pulses down to 6 fs pulses and ultrabroadband spectra. In the sub-10 fs regime of interest, a typical cavity detuning of about +/- 5 μm, corresponding to +/- 150 Hz, is tolerated while still maintaining self-starting. Both lasers are built on water-cooled breadboards and therefore the passive stability is sufficient to keep the lasers within the self-starting range for many hours without cavity length adjustments.
To study the transition dynamics from CW operation to modelocked operation, a chopper wheel was inserted into the intracavity beam of the Ti:sapphire laser. The average fundamental power was detected with a photodetector (10 MHz bandwidth) as well as the second-harmonic power when the Ti:sapphire laser is modelocked (~1 GHz bandwidth).
The signals were analyzed using a 1.5 GHz LeCroy oscilloscope. Both traces are given in Fig. 2(a) where noise dominantly arises from aliasing problems of the digital oscilloscope. The orange upper trace represents the fundamental power and the blue trace below is the second-harmonic (SH) power of the Ti:sapphire. Since the detector for the fundamental power is slow, the signal shows an average over the individual laser pulses.
The graph illustrates reliable self-starting behavior. Fig. 2(b) shows a closer look on the temporal evolution of the fundamental laser power and reveals some detail on the complex laser dynamics that occurs during the build-up of the 6 fs pulse from the continuous wave running laser over a time span of about 5–10 ms (independently confirmed by the onset of the SH signal, not shown in magnification). This is about one to two orders of magnitude slower than typical build-up times for standard KLM lasers  or SESAM/SBR modelocked lasers . When the system is operated near the edge of the self-starting limits, the self-starting becomes unreliable and stochastic with respect to build-up time.
When using longer pump pulses on the order of several 10 ps to 100 ps, we expect the self-starting behavior to be roughly the same. This is due to the fact that the initial build-up of a pulse is a kind of active modelocking stemming from the periodic and fast gain modulation by the pump pulses. Since the pulse shaping in an actively modelocked laser only weakly depends on the curvature of the loss/gain modulation, self-starting will only cease when approaching the CW pump regime, i.e. for nanosecond and longer pulse durations.
For quasi-synchronous pumping it is important to characterize the output power modulation induced by the mismatch of the pulse repetition rates of the two lasers. When the repetition frequencies are manually adjusted to be equal, no modulation in the output power is observed. If the lasers are not synchronous the output power of the Ti:sapphire will be modulated with the difference frequency of the repetition rates of both lasers. Fig. 3 displays the RF power spectrum of the Ti:sapphire output power with a pump detuning of 5 kHz. It shows side-bands slightly below -60 dBc resulting in a 0.1% power modulation of the optical output. This 60 dB suppression is observable over a wide detuning range from within the self-starting limits up to several MHz. The overall amplitude noise of the Ti:sapphire laser was measured with a vector signal analyzer and the RMS noise was determined to be 0.28 % (from 0.1 Hz to 10 MHz) which is dominated by the noise of pump laser (0.39 % from 0.1Hz to 10 MHz, not representative for the latest commercial version of the picoTrainTM).
In this mode of operation, the laser produces a clean train of modelocked pulses characterized by its spectrum and interferometric autocorrelation (IAC). Fig. 4(a) shows several spectra for which self-starting is observed, from a narrow bandwidth (tunable) 100 fs pulse to ultrabroadband spectra, important for a direct and simple carrier-envelope offset frequency stabilization scheme . The spectrum for which the IAC was measured is represented by the orange curve and has a FWHM bandwidth of 230 nm, with an average power of typically 150 mW at 5W pump power. The measured IAC is represented in Fig. 4(b) by the orange circles. The measured IAC and the corresponding spectrum are used in a phase retrieval algorithm that optimizes the spectral phase to match the calculated IAC with the measured IAC. The black curve on top of the measurement data is the calculated IAC, which fits the measurement very well and corresponds to a pulse duration of 6 fs.
In conclusion, we demonstrated the benefits of quasi-synchronous pumping in terms of a universal self-starting behavior and low residual output power modulation. Self-starting of widely tunable pulses of about 100 fs duration down to 6 fs pulses and ultrabroadband spectra have been shown. Pumping with a 100 MHz picosecond source, we observed a -30 dBc residual optical power modulation when both lasers are not perfectly synchronized. These proof-of-principle experiments open up the possibility of a more general use of modelocked lasers to pump few-cycle femtosecond oscillators. Not only do modelocked pump lasers potentially offer advantages in terms of reduced system complexity and costs (because of the ease of single-pass external doubling) but a completely modelocked and synchronized pump-oscillator system can also prove very valuable for pump-probe experiments, optical frequency metrology and phase controlled few-cycle laser pulses.
References and links
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