  Free space delay lines provide pulses of variable time spacing for optical
experiments such as pump-probe spectroscopy and coherent quantum control,
including spin and photon echo techniques. However, in the terahertz (THz)
region of the spectrum, beam divergence due to diffraction limits the useful
length of traditional free space delay lines. We present a novel double-folded
variable delay line for light in the frequency range 0.24-1.2 THz, which
incorporates a symmetric arrangement of lenses whose spacing can be adjusted to
compensate for diffraction at each delay. Scalable for use in other wavelength
regimes, the design relays an input Gaussian beam waist to the output with up
to 25 ns (~8 m) total delay and is enclosed in a desiccated volume of <0.5 m^3.
The delay line can deliver two or three pulses with relative amplitudes
controlled via variable spacing silicon etalon beam splitters. Beam profiles of
a 0.24 THz beam show good agreement with calculations at long delays, with
insertion loss per delay stage of~3 dB.
