DFB - Quantum Cascade Laser Systems
Technology Update - Quantum Cascade Lasers - QCL

Description:
Quantum Cascade Lasers (QCL) are designed for spectroscopy in the MIR spectral range. A schematic description of the gain mechanism is given in the drawing. Opposite to conventional diode lasers, the optical gain is generated via an interband transition of the conduction band. With this band gap design, wavelengths in the MIR spectral region are accessible for spectroscopy applications.

Wavelength Tuning:
Pulsed operated Distributed Feedback Quantum Cascade Lasers (DFB QCLs) show a temperature tuning during the pulse duration. This effect is caused by a change of the temperature within the active area due to the injected current. The benefit of this effect for technical applications is an automatic wavelength tuning. The above figue shows the behavior of a pulsed DFB QCL. The lower trace is a reference curve which shows the transmission ripple of an etalon. The  upper traces show the intensity behavior of the DFB QCL. The dip within the broken lines of the upper trace is caused by the absoption of NO. In summary, a pulsed Distributed Feedback Quantum Cascade Laser System is a ready to use spectroscopic instrument.

System Layout:
Sacher Lasertechnik developed a laser head and driver electronics for QCLs. Typical operation conditions of QCLs are 250K operation temperature, pulsed operation current of 5A .. 10A with pulse length of 100ns and a repetition rate of 10MHz. With this operation equipment, QCLs are excellent tools for MIR spectroscopy.

Available Wavelength:
Sacher Lasertechnik currently offers two difference wavelength for their Distributed Feedback Quantum Cascade Laser (DFB QCL) Systems. Available are 4.8µm for NO absoption and 9.55µm for Methane absorption.

Application: MIR Spectroscopy
The fundamental optical transitions of most molecules are located in the Mid Infra Red (MIR) spectral range. Typical oscillations strength of the fundamental optical transitions are more than a factor of 1000 stronger than their harmonics which are located in the Near Infra Red (NIR) spectral range. Major benefit of the higher value of oscillation strength is a drastical increased detection sensitivity of optical sensors. Publications on QCLs report on sensitivities in the ppb regime.

Future Developments:
Sacher Lasertechnik is currently preparing an External Cavity Quantum Cascade Laser system. The product will be available in early 2005. Please contact us well in advance with your application. 

Acknowledgement:
We gratefully acknowledge financial support by the BMBF with contract FK13N8020 and the technical support of the project partners Fraunhofer IAF and Fraunhofer IPM.