We have developed a novel technique which allows for continuous tuning of temporal contrast in petawatt-class lasers . Our method was implemented at PHELIX and has been offered to users as a standard option since 2013. With this technique an arbitrary level of amplified spontaneous emission (ASE) from the standard contrast ratio of 106 which is a typical value for chirped pulse amplification (CPA) lasers to an optimum of 1011 can be applied. Switching between those different ASE levels is possible from day to day if specified in advance of an experiment in the framework of the technical design review (TDR).
If required, this range can be further extended to an ultrahigh temporal contrast of about 1013 by combining our technique with a plasma mirror inside the PHELIX target chamber. However, it should be noted that the plasma mirror setup is not a standard option and has to be planned individually for each experiment (experimenters are asked to contact their dedicated local link scientist for detailed questions and planning).
The adjustable temporal contrast of the PHELIX system offers many possibilities for experimenters since the ASE level can be adapted to their specific application. In particular, several experiments which require the highest contrast, e.g. laser driven ion acceleration with sub-micrometer thick targets  have been realized with our system.
The adjustable temporal contrast is achieved by tuning the gain between an ultrafast optical parametric amplifier (uOPA) and the first of two regenerative amplifiers of the PHELIX femtosecond frontend. Optical parametric amplification is ASE-free and the only source of noise is parametric fluorescence which is confined to the duration of the pump pulse. We developed and use a dedicated laser-diode-pumped laser  to produce 1 ps short pump pulses which allow for parametric amplification of the oscillator pulse without any degradation of its nanosecond and picosecond temporal contrast. With our system a parametric gain of up to 105 is achieved leading to pulse energies of about 100 μJ. We can manipulate the level of produced ASE by tuning the gain between the ASE-free uOPA and the regenerative amplifier, while the overall gain of the system is kept constant. Fig. 1 shows two exemplary pulse profiles. The red curve was recorded with no uOPA gain (the total gain was produced by the regenerative amplifier). In this case the ASE level is about 6 orders of magnitude below the maximum, a typical value for CPA systems. However when a gain of 104 is applied by the uOPA (black curve) the ASE level drops down by four orders of magnitude reaching the detection limit of the used cross-correlator (Sequoia, Amplitude Technologies). Fig 2 shows the ASE levels that are accessible for different gain values of the regenerative amplifier. The minimum ASE level of about 1011 achieved with an uOPA gain of 105 is determined by extrapolating the measured values.
More details about temporal contrast control at PHELIX can be found in Refs. [1,4,5]
 F. Wagner, et. al., Applied Physics B 116, 429 (2014)
 F. Wagner, et. al., Physics of Plasmas 22, 063110 (2015)
 C.P. Joao, et. al., Applied Physics B 118 (3), 401 (2015)
 F. Wagner, et. al., Optics Express 22, 29505 (2014)
 F. Wagner, et. al., proceedings of 42nd EPS conference on Plasma Physics, (2015)