Single cycle midIR pulse: Spatial, temporal and absolute phase characterisation

Summary form only given. 1.7 cycles midIR pulse has been measured via a tunable SEA-F-SPIDER arrangement providing both temporal and spatial characterisation. The intrinsic CEP stability of the pulse is monitored and stabilised from a simple interferometric measurement.A significant part of future development in attosecond science [1] and especially molecular high harmonic generation spectroscopy [2] is likely to require the use of intense and ultrashort infrared laser pulses in the mid-infrared (mid-IR) spectral range. Secondary sources based on Ti:Sapphire laser pumped Optical Parametric Amplifier (OPA) in nonlinear crystals are readily available as commercial systems and their extension to few-cycles mid-IR sources have been demonstrated [3,4]. MidIR pulses at 1.7um are generated by self phase modulation (SPM) of the idler pulse of a HE-TOPAS (Light Conversion) in an Argon filled hollow core fibre and subsequent compression in Fused Silica. The carrier envelope phase (CEP) of such short pulses is of prime interest for strong field physics experiment. Its pulse to pulse passive stability is here assured by the difference frequency generation mechanism in the last OPA stage [4]. Providing a stable pump source (~1% pulse to pulse energy fluctuation) the main source of CEP fluctuation is due to the temporal interferometric jitter between pump and seed in the DFG process.Our temporal characterisation relies on SEA-SPIDER [6] with direct spectral filtering for ancilla preparation (SEA-F-SPIDER [7,8]). A spatio-temporal reconstruction of our single cycle pulse is shown in fig.1 (inset) from which a lineout has been extracted (red curve) and compared to the Fourier Limit (in black). Apulse duration of 9.9fs has been obtained representing 1.7 cycles at 1.7um. Interference fringes at ~700nm are produced from SHG of the seed for one part and SPM of the pump in the last OPA crystal for the other part. The interference of these two almost balanced paths (about 3 meters each)- creates fringes that are stabilised and controlled by a piezo actuator added on the pump arm. The evolution of the interferogram as a function of time with a slow loop stabilisation turned on is visible on figure 1 b) for different relative set phases. The stability in each interval is excellent and on the order of 100 mrad standard deviation. Since the stability of this interferometer is directly coupled to the CEP stability of the generated pulse, this measurement is a monitor of an important source of the CEP fluctuations.