Light Sources

Two laser systems were used as light sources. The older one is a syn-chronously pumped ps-system, which has been used by M. Stoerzer in his thesis [St¨o06] and S. Fiebig in her thesis for acquiring the results published amongst others in [ASB⁺07], [ASM06], [FAB⁺08], [SGAM06]. The newer one is a synchronously pumped fs-system.

ps-laser-system

This laser system, being capable of creating pulse durations at the order of 20 ps at a repetion rate of 76 MHz, consists of a customized actively mode-locked Ar-Ion Laser Innova I400 (Coherent) operating at the 514 nm emission line of Argon, which synchronously pumps a dye laser using a Rhodamine 6G methanol-ethyleneglycol-solution in a jet as active material. Active mode locking of the Ar-ion laser is done by using an acousto-optic modulator (AOM) crystal as wavelength selection prism in its cavity. For high qual-ity and short pulse durations, the pulses cycle 30-40 times in the dye cavqual-ity, before they are extracted via Bragg reflection of an intra-cavity AOM, which

4.1 Time-of-Flight Setups Setups

Ar-tube AOM

AOM Dye-jet

cw

Ar-tube AOM

AOM Dye-jet

pulsed

Figure 4.2: Sketch of the ps-system: the upper sketch shows the continuous wave operation mode where both AOM (acousto-optic modulators) are not operating. The lower sketch shows the pulsed mode, where the AOM in the Ar-Ion laser offers pulsed mode for the pump beam. The second AOM in the dye laser deflects the pulses after 30 to 40 cycles in the dye cavity for narrow pulse width and high intensity.

yields a repetition rate of roughly 2,5 MHz. The contrast ratio of main pulse to after-pulse is approximately 200:1. Since Rhodamine 6G has a quite wide band of fluorescence, the resonating wavelength in the Rhodamine cavity can be easily tuned between 575 nm and 620 nm.

By switching off the active mode-locker in the Ar-ion laser and in the dye laser cavity, this laser system can also be operated as continuous-wave (cw) system, thus providing a tunable cw-light source for the Coherent Backscat-tering Cone setup described later in section 4.2. Both operation modes are sketched in fig. 4.2.

fs-laser-system

The fs-second system is a three-step laser system consisting of a high-power pump laser (Verdi V18, Coherent - frequency doubled Nd:YAG) with power output of P = 18 W at 532 nm, which pumps a standard high-power HP-Mira Titanium-Sapphire laser (TiSa) from Coherent. The TiSa offers pulses with a duration of roughly 200 fs at central wavelengths from 760 nm to 820 nm at a cw-power of maximally 4,2 W. These pulses are coupled into a frequency-doubled optical parametric oscillator (OPO) from APE, which uses Rubidium Titanyle Phosphate (RTP) as active crystal and offers 250 fs pulses from 550 nm to 665 nm wavelength at maximally 1,1 W.¹

SHG

RTP-OPO

High power TiSa

Verdi V18

Pulse Picker

Figure 4.3: Sketch of the fs laser system. The Coherent V18 pumps the standard HP Mira TiSa laser. The pulses are converted with an OPO system in ring cavity setup to the desired wavelengths as explained in the text.

As can be seen in fig. 4.3, the OPO is set up in an 8-fold ring cavity.

In the first beam waist, the RTP crystal does down-conversion from the TiSa input, whereas in the second beam waist, second harmonic generation (SHG) takes place in order to frequency double the down-converted light.

The SHG crystal is cool- and heat-able and temperature stabilized in order to provide tuning ability for phase-matching and therefore optimal SHG for a wide frequency band. The pulse length translates to a spectral width of the pulse of roughly 3 nm, as can be measured with the control spectrometer of the OPO. Usually the HP-Mira is operated at 790 nm which corresponds to 590 nm output of the OPO. This is on one hand done for comparison to

1The coherence length of these pulses can be estimated bylcoh.≈c0τpulse/neffto be on the order of 50µm, i.e. roughly 20 scattering events.

4.1 Time-of-Flight Setups Setups older data sets from [St¨o06], on the other hand the efficiency of the RTP crystal used in the OPO is highest at this wavelength. Since the repetition rate is fixed to the HP-Mira’s one, i.e. 76 MHz corresponding to a repetition time of 13,2 ns, a pulse picker from APE is chosen to extract pulses with a contrast of maximal 500:1 between main and after-pulse in order to increase the maximally possible measurement time. The pulse picker, using an AOM, allows for picking at best every other pulse. However, the high contrast can be achieved only, if the pulse picker picks less than each 10th pulse and therefore it is operated normally at each 15th pulse. This still allows for a typical repetition rate of 5 MHz for the experiment.

Both systems have an inherit stability problem, because they rely on syn-chronized pumping. Therefore, the optical cavity lengths of the lasers always have to match exactly for sufficiently large power output, which makes the lasers very sensitive to temperature changes. Moreover, changing humidity is important if the TiSa operates near one of the absorption bands of water vapour. Whereas the old system does not have any stabilization techniques and only manual cavity length tuning, the fs-system can be stabilized in prin-ciple by a piezoelectric driven movable mirror controlling the cavity length of the OPO and several detection units. It turned out, however, that none of the offered stabilization modes (power output, wavelength, pulse width) was sufficiently stable, which is mainly attributed to the fact, that the tempera-ture in the laboratory could not be stabilized to a satisfactory accuracy by the air conditioning unit. The caused power drift is the reason for the size of the error bars in several graphs shown as results, as will be referred to in chapter 5.