Polarization-stable dual-color DFB fibre laser system for CW terahertz generation

The terahertz, or THz, frequency range represents a gap in technology as well as knowledge in the electromagnetic spectrum. The THz spectral range is here loosely defined as frequencies between 100 GHz and 10 THz. Electromagnetic waves at THz frequencies interact strongly with metals, ceramics, and crystalline materials such as explosives, illicit drugs, and pharmaceutical products.

At the same time, the long wavelength of the radiation (1 THz corresponds to a wavelength of 300 µm) guarantees little scattering in most packing materials and textiles. This combination allows THz sensors to “look through” the walls of non-metallic containers as well as clothing to identify hidden objects.

For this reason, large efforts are currently being put into the exploitation of the “THz gap” to investigate new applications in the safety and security sector as well as industrial environments. We have developed a tuneable dual-wavelength DFB fibre laser system for continuous wave (CW) THz generation in ultrafast semiconductor photomixers, based on low-temperature grown (LT)-GaAsSb or InP-based MQW structures.

The fibre laser system is scalable with respect to output power and is in the latest version fully built of polarization maintaining fibre components. The output power of the laser is at maximum 150 mW and has a polarization extinction ratio of better than 20 dB. Besides has the laser system due to its DFB fibre laser oscillator inherently a narrow linewidth.

This is required in e.g. gas sensing applications relying on detection of narrow spectral absorption lines of toxic gas traces in the THz frequency range.