Quantum Cascade Laser
Quantum Cascade Lasers (QCLs) are mide-infrared sources, working in the 3-16 µm spectral range, suitable for trace gas detection and of great interest in terms of system integration.In contrast to conventional diode lasers, QCLs may produce a relatively high output power (up to 1 Watt). The emission wavelength of the QCLs is not determined by the semiconductor materials used, but by the thickness of the different semiconductor layers. This thickness can be manipulated during growth of the devices by molecular beam epitaxial (MBE), so that the desired emission wavelength can be tailored by choosing the layer thickness. In addition, these laser sources are simpler to operate than the present ‘classical' mid-infrared diode lasers and are expected to have a long lifetime (extrapolation shows lifetime up to 20 years).
For spectroscopy, as narrow linewidth lasers are required, Quantum cascade lasers are combined with (Distributed-FeedBack) DFB structure. As a drawback, they have a small scanning range (a few wavenumbers, approximately 100 GHz), which implies that for each gas another QCL is required. However, thanks to their high reliability, relative ease of operation and small size, they are ideally suited for application in monitoring specific molecules as stand-alone equipment.
Quantum cascade laser: physical principles
In traditional semiconductor-based lasers electrons recombine with positively charged "holes" to release single photons with a colour determined by the bandgap and thus the chemical composition of the semiconductor sandwich. The interband transitions between the conduction and the valence bands provide the laser radiation.
The stages of the QC laser consist of an area with closely spaced layers (the injection region) followed by more widely spaced layers (active region). The stack of active regions is clad with two thick semiconductor layers of low refractive index, that serve as a waveguide to direct the produced light along the active regions. Typically 30 to 75 alternating structures of active regions and injector/relaxation regions are stacked. Once an electron is injected from the contact regions, it is forced to pass through all the periods of active regions and injectors sequentially (cascading). Once the device exceeds lasing threshold, it will emit one photon per period and per electron. Adding stages to QC lasers thus increases their output power. In this way QC lasers can provide more than a thousand times the output power of any commercial semiconductor laser operating in the mid-infrared region.
Schematic diagram of the conduction band for a vertical transition quantum cascade laser. The operation of the quantum cascade laser can be understood as follows. The different materials of the semiconductor in the active region have different band gaps, which leads to the creation of quantum wells. These quantum wells have discrete energy levels due to the thinness of the layers. The electrons are quantized in the direction perpendicular to the plane of the layers but can move freely in the plane of the layers. An electron in the upper level of the active region will first scatter to an intermediate subband producing a photon (slow process 3-2) and then fast into the lowest subband (2-1). The energy levels are determined by the thickness of the layers in the active region.
A QC laser can operate in a large number of modes, all at wavelengths around the one determined by the energy difference between the upper and intermediate levels. To produce stable, single-mode emission, as is needed for spectroscopic applications; a grating is integrated into the laser waveguide producing a distributed feedback (DFB) device. The grating selects a single mode that satisfies the Bragg condition. Thus, continuous, single mode emission is produced with tuning ranges of about 100-150 nm (at 3-15 µm).[J1] The tuning here takes place by changing the temperature of the laser, which changes the refractive index of the waveguide material, and thus the wavelength at which the Bragg condition holds.
DFB-QCL can operate either in pulsed mode, operating up to room temperature or in continuous wave mode, operating from cryogenic to above liquid nitrogen temperature. In pulsed mode, heating occurs during the current pulse. This changes the emission wavelength slightly, resulting in a dynamic linewidth of the laser of a few hundred MHz. Therefore, for our application of high resolution spectroscopy in trace gas detection, the laser is preferably used in continuous wave mode, in which case linewidths of a few kHz should be attainable.