QOMBS project partner Alpes Lasers has demonstrated high-performance quantum cascade laser frequency combs at λ ∼ 6 μm for the first time. This new wavelength is important because it corresponds to the amide I vibration band of proteins, opening up numerous new biological and medical applications.
Optical frequency combs are laser devices which emit light in a wide spectrum consisting of equidistant peaks in frequency space.
In the mid-infrared range, Quantum Cascade Lasers (QCLs) with specifically engineered optical dispersion have been shown to emit broad and powerful optical frequency combs. The mode spacing of both QCL combs and ultrashort-pulse lasers is given by cavity length. However, in the case of QCLs, the periodic modulation in the time-domain is of the FM rather than AM type and the output power is constant.
The QCL comb is a stand-alone device as it integrates a pump laser and a micro-resonator in its waveguide. This allows QCLs combs to be significantly more compact compared to previous technologies, making them ideal for chemical sensing based on dual-comb spectroscopy.
Being based on QCL technology, comb devices can be manufactured over all the MWIR and LWIR. However, the dispersion compensation which is required for comb generation is more difficult to achieve at wavelengths below 8 µm. This is because the material dispersion increases as the photon energies approach the bandgaps of the constituent materials.
Alpes Lasers addressed this limitation by introducing a novel plasmon-enhanced waveguide design with a thin, highly-doped InP laser inserted in the top cladding. With this technique, QCLs emitting at λ ∼ 6 µm exhibit comb operation from -20ºC up to 50ºC. A maximum output power of 400 mW is achieved at room temperature and 300 mW at 50ºC, showing the robustness of the device. The laser output spectrum is ∼80 cm−1 (∼300 nm) wide at the maximum current, with a mode spacing of 0.334 cm−1, resulting in a total of 240 modes with an average power of 0.8 mW per mode.
The results have been published in Optics Express: https://www.osapublishing.org/oe/abstract.cfm?doi=10.1364/OE.395260.
Figure 1. Top: Measured output power and voltage as a function of current in continuous-wave operation of a 5.5 µm-wide QCL frequency comb at temperatures from −20ºC (blue) to 50ºC (red). Bottom: Frequency comb operation map of the same device. The coloured area indicates the region in which the laser output is a frequency comb. It extends to all measured temperatures (−10 °C to 50 °C), with several hundreds of mA of operational range.
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