978-1-4244-2120-6/08/$25.00 ©IEEE. Abstract— Recent developments in femtosecond fibre laser combined with the photomixing technique enable THz radiation between 50 GHz up to 3000 GHz to be synthesised. This solid state source THz is dedicated to high resolution spectroscopy in a spectral region no other solution offering the large tunability required. Photomixing is now a powerful technique to explore the THz frequency gap and is well established for high resolution spectroscopy applications. The experiment consists of the conversion of an optical beat into the THz domain by using the non linearity of an ultrafast photodetector, or photomixer. Photomixing is now a realistic solution for the pure rotational or rovibrational spectroscopy, and has been used to characterize the absorption profiles of several asymmetric and symmetric tops molecules such as SO 2 , H 2 S, and OCS [1-3]. The measurement of trace gases in scattering media has been also demeonstrated using this technique [4-5]. When the tedious problem of metrology is solved, the frequency of previously unobserved transitions of H 2 O and NH 3 have been determined [6-7]. In this paper, we propose an alternative to the solution proposed by Q. Quraishi et al. and we use the latest developments in frequency metrology based on femtosecond fiber lasers for frequency measurement in the THz region by avoiding the use of femtosecond Ti:Sa laser and its expensive pump laser [8]. The frequency spectrum of a pulse train emitted by a femtosecond laser consists of a comb of frequencies, the spacing between each line is determined by the repetition rate of the laser, i.e. governed by the length of the cavity. This frequency comb is then used like a ruler, to measure or/and to stabilized the frequency of others lasers. The experiment is presented in figure 1. Two external cavity diode lasers which operate near 780 nm, detuned from each other to the desired THz frequency, are overlapped by use of a beam splitter. A first part of the optical beat passes through a tapered amplifier (not represented on the figure) and then is focused onto the ultrafast photomixer to generate the THz radiation. A silicium lens is placed on the back side of the antenna to extract and pre-collimate the THz beam. An off axis parabolic mirror is used to focus the THz beam onto a cryogenic bolometer. The second part of the optical beat is superposed with the frequency comb by using a second beam splitter to measure radio-frequency beats signals between each diode laser and a part of the frequency comb. The detected beat notes are phase locked to a synthesizer, and so stabilize both lasers. The difference of frequency between the diode lasers can be expressed as : s rr THz f f n f 2 ± ⋅ = where n is an integer, f rr is the repetition rate of the femtosecond laser locked onto a 10 MHz quartz crystal oscillator, and f s the frequency of the synthesizer used to phase lock the beat signals. A classical wavemeter is used to remove the ambiguty of the integer. In the present demonstration the tunability is obtained by scanning the repetition rate of the femtosecond laser. Figure 1: Experimental setup of the synthetised THz source The figure 2 shows an example of a locked beat note between one of the diode lasers and the femtosecond laser. The curve is the result of an average of 100-recorded signal by using a sweep time of 20 ms. The resolution bandwidth is 25 kHz. The full width at -3db is estimated around 500 kHz. The limited bandwidth of our feedback current source prevents the observation of coherent spike. At present, the performance of the phase lock loop is limited by the response of the error input of the current controller. A bias tee directly connected to the diode laser would increase greatly the bandwidth of the error feedback, and so subsequently improve the present spectral purity. Similar experiment when the phase lock loop is open exhibit a full width at -3db around 6 MHz. G. Mouret a , R. Bocquet a , F. Hindle a , A. Cuisset a , Y. Chun a , M. Lours b and D. Rovera b a Université du Littoral Cote d’Opale, Laboratoire de Physico Chimie de l’Atmosphère UMR CNRS 8101, France b Systèmes de Référence Temps-Espace, UMR CNRS 8630, France Frequency measurement in THz domain by using femtosecond laser frequency comb Bolometer Off axis parabolic i Photomixer DL 1 DL 2 fs laser G G PD1 PD2 BS BS PLL PLL Synth