Terahertz quantum cascade lasers (THz QCL) are a very promising source for efficient frequency comb generation at terahertz frequencies. They do not only provide an output power of the order of milliwatts but are also covering a large spectral bandwidth. Octave spanning devices have recently been reported by our group. They provide a very low intrinsic dispersion due to the flat gain curve and the flat losses of the resonator. This allows frequency comb operation up to more than 600 GHz bandwidth with standard broadband metal-metal waveguide Fabry-Pérot QCLs. Frequency combs at terahertz frequencies are especially interesting for spectroscopic applications employing the powerful dual-comb setup. Such a setup requires a fast detector which is difficult to get with a sufficient sensitivity at terahertz frequencies. We present here an alternative approach, which does not need a fast detector but rather uses one of the two THz QCL frequency combs as an ultrafast multiheterodyne detector integrating local oscillator (LO) and detector in one single device. Two laser ridges are fabricated on the same chip at a distance of 500 um. Part of the light from the sample laser is coupled into the LO laser via the metallic ground plane. The downconverted multiheterodyne beatnote can be measured through the laser power supply line with a bias Tee. The obtained dual-comb covers a bandwidth of 630 GHz with a central frequency of 2.5 THz. The frequency comb spacing was analysed using frequency counting techniques revealing an accuracy down to _frep=fcarrier 10^(-12) at the carrier frequency of 2.5 THz.
Giacomo Scalari, Markus Rösch, Gustavo F. Villares, Lorenzo Bosco, Mattias Beck, and Jérôme Faist, "On-chip THz quantum cascade laser dual frequency combs (Conference Presentation)," Proc. SPIE 10111, Quantum Sensing and Nano Electronics and Photonics XIV, 101110S (Presented at SPIE OPTO: January 30, 2017; Published: 28 April 2017); https://doi.org/10.1117/12.2252212.5393916915001.
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