Fast scanning terahertz spectrometer based on synchronized fiber lasers

Mostly common for THz spectroscopy systems is the use of a single ultrafast laser where the original laser beam is split into a part for THz generation and a part for probing. The latter can be mechanically delayed in time to sample the electric field of the THz pulses which is usually realized by a motorized translation stage. An alternative to this approach is the use of fast-scanning devices such as loudspeaker membranes or spring- loaded stages that can achieve scan rates of 10-30 Hz, barely reaching video-rate operation. However, having two synchronized lasers with high timing precision available, it is possible to overcome this mechanical limit. With the technique called asynchronous optical sampling (ASOPS), where one laser has a constant offset in the repletion rate, it was shown that kHz scan rates are possible [1], enabling high speed THz spectroscopy.
Here we demonstrate the operation of a robust and true turnkey fiber-laser based ASOPS system that is used for THz spectroscopy. The laser system consists of two amplified Er-doped fiber lasers (TWIN M-Fiber A, Menlo Systems), emitting 80 fs pulses centered at 1550 nm at a repetition rate fREP of 250 MHz. The repetition rates of both lasers are frequency stabilized and a frequency synthesizer can add an adjustable (1 Hz- 10 kHz) frequency offset Δf to Laser A for ASOPS operation. In ASOPS operation, the entire time between adjacent pulses (1/fREP=4 ns) is scanned during the inverse of the offset frequency, a fact that often has been criticized as for THz spectroscopy usually only a few tens of picoseconds are needed. To circumvent this, the technique called electronically controlled optical sampling (ECOPS) has been proposed: here, both lasers are set to the same repetition rate and one of the laser’s phase relative to the timing stabilization gets modified. By this, a smaller time-window can be selected and electronically sampled.
For efficient THz generation, we excite an interdigitated large-area photoconductive antenna (PCA) with the second harmonic output of one of the lasers and employ electro-optic sampling in a 500 μm thick <110> ZnTe crystal for detection. The result of a single ECOPS scan at a rate of 2.5 kHz is presented in Fig. 1 and shows a signal-to-noise ratio of the electric field of greater than 50. When averaging is applied, the noise floor drops (Fig. 2, solid line) and the dynamic range reaches 50 dB. To compare the ASOPS with the ECOPS technique, the spectrum of an ASOPS scan with identical acquisition time (Δf=2.5 kHz, 1000 averages) is shown as the dashed line. It can be seen that ECOPS reaches a higher signal-to-noise ratio in this case. Also shown in Fig. 2 is the amplitude spectrum of a 680 μm thick DAST crystal excited at 1550 nm as the dotted curve. It demonstrates that using the fundamental wavelength of the lasers can actually enhance the accessible frequency range, a feature only available for fiber-laser based systems.
[1] A. Bartels et al., Rev. Sci. Instrum. 78, 35107 (2007).