  We demonstrate a terahertz (THz) frequency comb that is flexible in terms of
its frequency range and the number and spacing of comb lines. We use a
combination of near-infrared laser diodes, phase modulation and opto-electronic
frequency conversion. The THz comb lines are characterized to be < 10 MHz by
resolving the pressure dependent collisional broadening of an ammonia molecule
rotational mode.
