Quantum cascade lasers are innovative coherent light sources in the mid-infrared wavelength region. The mid-infrared is of particular importance especially for gas sensing and spectroscopy applications, because the fundamental vibrational modes of gas molecules occur in this spectral range. (e.g.:)
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| Fig. 1:P-I- characteristic (black lines) of a QCL DFB with emission wavelength around 9.5µm at various temperatures up to 280K. |
Conventional semiconductor laser diodes are based on band-to-band transitions within a p-n- junction and are inherently limited in the maximal attainable emission wavelength and operation temperature range by thermal activation of carriers over the bandgap of the semiconductor material used in the active region. For unipolar quantum cascade lasers, the restrictions on long wavelength operation are strongly relaxed. The operating principle of a unipolar laser is based on the amplification of an electromagnetic wave in a semiconductor superlattice structure.
In this structure, mid infrared coherent light is generated by intersubband transitions in quantum wells and tunnelling injection cascading through many identical stages for multiple photon generation. First laser operation based on this principle was demonstrated in 1994 by scientists at Bell Labs (now LUCENT Technologies).
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| Fig. 2: Spectra of a quantum cascade DFB (top) and a quantum cascade FP laser (bottom). |
Based on a license agreement with LUCENT Technologies, Nanoplus fabricates quantum cascade lasers of FP- and DFB-type. For many applications, for example sensing or spectroscopy, single-mode laser emission is very advantageous. As already described for laser diodes, the DFB concept is also used to achieve single mode laser emission in quantum cascade lasers. In a QCL DFB, only laser radiation with one specific laser mode at the desired wavelength is emitted, while other modes are suppressed by a periodic grating structure defined during QCL DFB processing. Both single and multi mode quantum cascade devices are available in the wavelength range from 9 to 12 µm. These devices are operated in pulsed mode (with typical pulse lengths around 100ns) at Peltier temperatures (-15°C) up to room temperature (25°C). A tuning of the emission wavelength can be achieved by a variation in the operating temperature or by using the wavelength shift within the laser pulse (due to device heating).
Nanoplus QCL devices typically are mounted on C-mount, other packaging options are available on request.
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| Fig. 3: Spectra of a 9.35µm DFB QCL at different temperatures up to 280K. inset: emission wavelength versus temperature. |
Figure 1 shows light output versus current characteristics of nanoplus QCL DFBs for temperatures between 250 K and 280 K. At Peltier temperatures around –15°C this Laser shows a maximum output power around 10mW combined with an efficiency of 6.0mW/A.
Figure 2 shows spectra of a multi mode quantum cascade laser (blue line) and a single mode quantum cascade DFB laser (purple line) with emission wavelengths around 9.5µm (suitable e.g. for CO2 sensing applications). In the DFB QCL only laser radiation with one specific laser mode is emitted while other modes are effectively suppressed by the DFB grating structure.
Figure 3 shows single mode emission spectra of a DFB QCL for temperatures between 250 to 280 K together with the temperature dependence of the emission wavelength (inset). The temperature coefficient is 0.54 nm/K for the DFB device (compared to 1.54 nm/K for a quantum cascade FP device).
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