Recent advances in the field of ultra-short pulse lasers have led to the development of reliable sources of carrier envelope phase stabilized femtosecond pulses. The pulse train generated by such a source has a frequency spectrum that consists of discrete, regularly spaced lines known as a frequency comb. In this case both the frequency repetition and the carrier-envelope-offset frequency are referenced to a frequency standard, like an atomic clock. As a result the accuracy of the frequency standard is transferred to the optical domain, with the frequency comb as transfer oscillator. These unique properties allow the frequency comb to be applied as a versatile tool, not only for time and frequency metrology, but also in fundamental physics, high-precision spectroscopy, and laser noise characterization. The pulse-to-pulse phase relationship of the light emitted by the frequency comb has opened up new directions for long range highly accurate distance measurement.
We report a new experiment on a high-resolution heterodyne spectrometer using a 3.5 THz quantum cascade laser
(QCL) as local oscillator (LO) and a superconducting hot electron bolometer (HEB) as mixer by stabilizing both
frequency and amplitude of the QCL. The frequency locking of the QCL is demonstrated by using a methanol molecular
absorption line, a proportional-integral-derivative (PID) controller, and a direct power detector. We show that the LO
locked linewidth can be as narrow as 35 KHz. The LO power to the HEB is also stabilized by means of swing-arm
actuator placed in the beam path in combination of a second PID controller.
We experimentally demonstrate that a stabilized femtosecond frequency comb can be applied as a tool for
distance measurement. The scheme is based on optical interference between individual pulses in a Michelson
type interferometer. The cross-correlation functions between individual pulses with a distance of around 15
meter and 30 meter are observed and analysed.