Nonlinear Optics Group - University of St Andrews

Figure 1. Fabry-Perot interferometer

The Fabry-Perot interferometer is illustrated in figure 1. Mirrors are fabricated either from gold-coated mylar film or metallic (nickel) meshes. Mirror translation is based on a stepper motor coupled to a high-ratio gear box providing an inter step translation of 83nm. Figure 2 shows the interferometer incorporated into the THz spectrometer. Radiation from the parametric generator is focussed by the first off-axis parabolic mirror (OAPM) and spatially filtered by passing through a pinhole (not shown) placed in the focal plane of this mirror before being re-collimated by a second off-axis parabolic mirror before enetering the interferometer. The radiation exiting from the interferometer is then focussed into the bolometer detector by the third off-axis parabolic mirror. The signal generated by the bolometer passes to a boxcar integrator to allow both gating of the individual Thz pulses as well as signal averaging. Scanning of the interferometer, data collection and display of the interferometer output as a function of mirror translation are all under computer control (see figure 3). Figure 4 shows acquired inteferometer traces where the source is the line-narrowed version of the parametric generator (see "Line-narrowed non-collinear THz ICOPO" ). The wavelength of the THz radiation is 164 microns (1.8THz), and mesh mirrors with a reflectivity of ~55% are employed. In the top trace the mirror spacing of ~1.3mm results in a free spectral range of 115GHz.

Figure 2 . Scan output of Fabry-Perot interferometer

The fringe width (FWHM) under these circumstances is dominated by the instrumental linewidth, indicating a finesse for the interferomter of around 4 (calculated 3.8). In the case of the bottom trace the mirror spacing has now been increased to 10mm corresponding to a free spectral range of 14.7GHz. On the basis of the measured finesse a fringe width (FWHM) of around 4GHz is expected due to instrumental effects. The measured width of around 8GHz is hence consistent with a linewidth for the line-narrowed parametric generator of <5GHz. The intermediate traces show the progression from an instrumentally dominated linewidth to a source dominated linewidth.

Figure 3. System schematic.

Figure 4. Inteferometer traces for various free spectral ranges (click on image for an enlarged view).