Pulse timing jitter

Linearly polarized, near-diffraction-hmited, mode-locked 1319 and 1064 nm pulse trains are generated in separate dual-head, diode-pumped resonators. Each 2-rod resonator incorporates fiber-coupled diode lasers to end-pump the rods, and features intracavity birefringence compensation. The pulses are stabilized to a 1 GHz bandwidth. Timing jitter is actively controlled to < 150 ps. Models indicate that for the mode-locked pulses, relative timing jitter of 200 ps between the lasers causes <5% reduction in SFG conversion efficiency. 

Figure 11 The components of the timing jitter of the laser synchronized subpicosecond pulse radiolysis, crj is the length of the electron pulse (rms), c,- is the length of the probe light (rms), and ctj is the timing fluctuation (rms).

If there is no fluctuation of laser intensity, we have to measure /q only once. Actually, the envelope of laser pulses changes in a relatively long time range (typically from several minutes to a few tens of minutes) because of the change of environmental factors such as room temperature and coolant temperature. There is also an intensity jitter caused by factors such as the mechanical vibration of mirrors and the timing jitter of electronics. Furthermore, in our system, the laser system is located about 15 m from the beam port to prevent radiation damage to the laser system. (Later, it was moved into a clean room, which was installed in the control room to keep the room temperature constant and to keep the laser system clean. The distance is about 10 m.) Therefore it is predicted that a slight tilt of a mirror placed upstream will cause a displacement of the laser pulse at the downstream position where the photodetector is placed. 

The time resolution of the system was measured and found to be about 1 ns. This value was mainly dominated by the pulse width of the ion beam. In fact, because in some cases the pulse width of the ion beam and the transit time jitter of the ion have larger values caused by many factors (ion energy and species, operating conditions of the accelerator and the pulsing system, the type of beam pick up, etc.). 

In a LWA, the electron and laser pulses are inherently synchronized, so the time jitter sources associated with photocathode accelerators are not an issue. The ultimate time resolution should depend only upon the cross correlation between the laser and electron pulses and the physics of the electron beam interaction with the... 

On the Osaka University thermionic cathode L-band linac, a time resolution of two picoseconds was achieved using magnetic pulse compression and time jitter compensation systems (Fig. 13). The time jitter between the Cerenkov light from the electron beam and the laser pulse was measured shot-by-shot with a femtosecond streak camera to accurately determine the relative time of each measurement in the kinetic trace. In this way, the time jitter that would otherwise degrade the time resolution was corrected, and the remaining factor dominating the rise time was the electron-light velocity difference over the 2-mm sample depth. 

Fig. 13. Schematic of high time-resolution pulse radiolysis equipped with a magnetic pulse compression and time jitter compensation system at ISIR, Osaka University. An example of the rise time of the hydrated electron signal at 780 nm is shown.

Access to subpicosecond electron pulses has already been achieved at Osaka University by a new double-decker accelerator concept. In order to reduce the time jitter for the detection of the optical absorption signals in pulse radiolysis studies, the light pulse used for the pump-probe system is Cerenkov emission which is produced in the same cell by a synchronized second electron beam and is concomitant with the electron path. The distance between the axes of the two beams is 1.6 mm. The pulse durations of these electron pulses, which are both produced by delayed beams issued from the same laser, are 430 + 25 fs and 510 20 fs, respectively, and the charge per pulse is 0.65 nC. An electron bunch of 100 fs and 0.17 nC has already been generated. 

Due to the finite width of the lamp pulse (2-5 ns) and the time jitter in the detection system (voltage discriminators, TAC, photomultiplier tube) the experimental decay F(tj) is a convolution of the instrument response function and the tme decay curve... 

Time-resolved studies employed previously-described ( ) equipment but were improved upon as follows. Part of the excitation Nd-YAG laser beam was split off and delivered to the phototube, serving as a marker pulse to trigger our Biomation 6500 for data acquisition the sample emission signal was timed by a laser pulse delay line to arrive vl5 ns after the marker pulse. Laser jitter was thereby minimized, and computer signal averaging more precise. 

For all X-ray diffraction results, we assumed an X-ray beam of 8 keV photon energy with a sin2-pulse shape, and a half-width of 100 fs, which should roughly correspond to the situation at the Linear Coherent Light Source (LCLS) including timing jitter [6]. 

The effective resolution of a TCSPC experiment is characterised by its instrument response function (IRF). The IRF contains the pulse shape of the light source used, the temporal dispersion in the optical system, the transit time spread in the detector, and the timing jitter in the recording electronics. With ultrashort laser pulses, the IRF width at half-maximum for TCSPC is typically 25 to 60 ps for microchannel-plate (MCP) PMTs [4, 211, 547], and 150 to 250 ps for conventional short-time PMTs. The IRF width of inexpensive standard PMTs is normally... 

With the correct delay in the reference channel the time is measured against the laser pulse whieh released the detected photon. The influence of possible pulse period jitter is thus eliminated. 

Of course, a simple leading edge discriminator cannot be used to trigger on such pulses. The amplitude jitter would introduce a timing jitter of the order of the pulse rise time (Fig. 4.1, left). In practice the timing jitter is even larger because any discriminator has an intrinsic delay that depends on the signal slope speed and the amount of overdrive. 

In most TCSPC modules, rate counters complement the CFDs in the photon pulse and reference charmels of TCSPC devices. The CFDs of the photon channel and the reference charmel are often different. The photon ehannel is designed for lowest amplitude-induced timing jitter and the reference ehannel for highest trigger rate. Some TCSPC devices do not use a CFD in the referenee charmel at all and rely instead on the stability of the reference pulses. 

The transit time between the absorption of a photon at the photocathode and the output pulse from the anode of a PMT varies from photon to photon. The effeet is called transit time spread", or TTS. There are three major TTS components in conventional PMTs and MCP PMTs - the emission at the photoeathode, the transfer of the photoelectron to the multiplieation system, and the multiplication process in the dynode system or mieroehannel plate. The total transit time jitter in a TCSPC system also contains jitter indueed by amplifier noise and amplitude jitter of the SER. 

As for the photoelectrons at the cathode, different start veloeities of the secondary electrons at the dynodes result in different transit times. However, because the number of secondary electrons is larger, time-dispersion in the dynode system results in pulse broadening rather than in transit-time jitter. 

Higher pulse amplitudes in general give lower timing jitter because the influence of the background noise is smaller and the influence of the amplitude jitter on the timing is reduced. Therefore TCSPC users often increase the CFD threshold... 

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

Experimental Setup. An obvious extension of the one-color pump-probe experiments is the application of two-color experiments in which two independently tunable dye lasers share the same pump laser. One can use the same high repetition rate and obtain spectral evolutions on excitation at selected wavelengths. The measurements are performed in essentially the same way as one-color experiments.A disadvantage is the broadened instrument function (cross-correlation function) caused by time jitter between the two pulses, since they are not obtained from the same dye laser. This leads to a full-width half-maximum (fwhm) value of the instrument function of approximately 5-10 psec. 

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