Nonlinear Optics Group - University of St Andrews

Frequency comb generation (FCG) has recently emerged as an important technique in the fields of optical metrology and spectroscopy. It allows an unknown optical frequency to be directly linked to a precise microwave frequency, creating a 'bridge' spanning over 5 orders of magnitude in frequency space. In telecommunications, frequency comb generation has the potential to generate the thousands of optical channels required by the next generation of dense wavelength division multiplexers.

A Comb of Frequencies

As shown left, a frequency comb consists of thousands of equally spaced frequencies over a bandwidth of several THz. It is generated by coupling a laser beam into an optical cavity that contains a modulator. The modulation frequency is set equal to the longitudinal mode spacing of the cavity. As a result, the frequency sidebands generated by the modulator coincide with an axial mode and are resonant. These sidebands therefore pass back through the modulator, generating resonant sidebands of their own and so on. This leads to a cascade process, generating thousands of frequencies equally separated by the cavity free spectral range - a comb of frequencies.

Generating a frequency comb with a modulator (green) inside an optical cavity (grey mirrors).

Frequency comb generation in a cavity only containing a modulator suffers from an exponential power decrease across the modes in the comb. To counteract this, a source of gain is required alongside the modulator. Optical parametric generation is ideal as a source of gain, with a very large bandwidth, low noise characteristics and a coherent phase link between generated waves.

By utilising the many available cavity designs, it is possible to place a modulator directly into an OPO cavity and modulate the signal/idler waves at the longitudinal mode spacing. To obtain the widest possible comb it is necessary to operate the OPO at degeneracy (i.e. signal wavelength = idler wavelength).

Our current research involves the development of a low threshold OPO-FCG by utilising a split-cavity, pump-enhanced scheme. The overall aim is to create a compact device pumped by a single-mode laser diode and therefore suitable as frequency comb source for telecommunications.

Click here for a demonstration of OPO-based frequency comb generation (flash animation).

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