Molecular spectroscopy with a multimode THz quantum-cascade laser

High-resolution molecular spectroscopy is a powerful tool for investigations of the structure and energy levels of molecules and atoms. In addition to scientific utilization, terahertz (THz) spectroscopy is of interest for detection and identification of gases in safety and security applications. While for frequencies below 2 THz many different methods have been developed, spectroscopy above 2 THz is hampered by the lack of frequency-tunable, continuous-wave, powerful, and narrow-linewidth radiation sources. For this frequency range, THz quantum-cascade lasers (QCLs) are promising radiation sources. We report on a THz absorption spectrometer, which combines a grating monochromator, a QCL, and a microbolometer camera. The QCL used for these experiments is based on a single-plasmon waveguide and a Fabry-Pérot cavity with both facets uncoated and is optimized for a low electrical pump power. It operates on several modes centered around 3.4 THz. The laser is mounted in a compact air-cooled cryocooler (model K535 from Ricor). The emitted beam is focused with a TPX lens and guided through a 27 cm long absorption cell onto the monochromator, which spectrally resolves the laser modes. The modes are imaged onto the microbolometer camera. The absorption spectrum of methanol around 3.4 THz is measured by detecting simultaneously the signal of each of the laser modes as a function of the laser driving current

Broadband molecular spectroscopy with a multi-mode THz quantum cascade laser (QCL)

High-resolution molecular spectroscopy is a powerful tool for investigations of the structure and energy levels of molecules and atoms. In addition to the scientific interest, terahertz (THz) spectroscopy is also of interest for the detection and identification of gases in safety and security applications. While for frequencies below 2 THz many different methods have been developed, spectroscopy above 2 THz is hampered by the lack of frequency-tunable, continuous-wave, powerful, and narrow-linewidth radiation sources. For this frequency range, THz quantum-cascade lasers (QCLs) are promising radiation sources. We report on a THz absorption spectrometer, which combines a grating monochromator, a QCL, and a microbolometer camera. The QCL used in these experiments contains a single-plasmon waveguide and a Fabry-Pérot cavity with both facets uncoated. It is optimized for low electrical pumping powers an emits several modes centered around 3.4 THz. The laser is mounted in a compact air-cooled cryocooler (model K535 from Ricor). The emitted beam is focused with a TPX lens and guided through a 27 cm long absorption cell onto the monochromator, which spectrally resolves the laser modes. The modes are imaged onto the microbolometer camera. The absorption spectrum of methanol around 3.4 THz is measured by detecting simultaneously the signal of each of the laser modes as a function of the laser driving current. By this means, frequency multiplexing is achieved

Towards a compact THz local oscillator based on a quantum cascade laser

Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. One example is the OI fine-structure line at 4.7 THz. This is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be operated on board of SOFIA. We report on the development of a compact, easy-to-use source, which combines a quantum-cascade laser (QCL) a compact, lowinput- power Stirling cooler. This work is part of the local oscillator development for GREAT/SOFIA. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. Efficient carrier injection is achieved by resonant longitudinal-optical phonon scattering. The amount of generated heat complies with the cooling capacity of the Stirling cooler. The whole system weighs less than 15 kg including cooler, power supplies etc. The output power is above 1 mW. With an appropriate optical beam shaping, the emission profile of the laser becomes a fundamental Gaussian one. Sub-MHz frequency accuracy can be achieved by locking the emission of the QCL to a molecular resonance. We will present the performance of the QCL-based source along with some application examples in high-resolution molecular spectroscopy

Performance of a compact, continuous-wave terahertz based on a quantum-cascade laser

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum cascade laser (QCL) with a compact, low-input-power Stirling cooler

THz Quantum-Cascade Laser as Local Oscillator for SOFIA

We report on the development of a compact local oscillator (LO) for operation on board of SOFIA, namely for GREAT, the German Receiver for Astronomy at Terahertz Frequencies. The LO combines a quantum-cascade laser (QCL) with a compact, low-input-power Stirling cooler. The output power is sufficient for pumping a hot-electron bolometer mixer. Frequency stabilization is achieved by locking to a molecular absorption line. Detectors operating at room temperature can be used for the stabilization as well. High-resolution molecular spectroscopic experiments demonstrate the usability as LO for SOFIA

A compact, continuous-wave radiation source for local oscillator applications based on a THz quantum-cascade laser

Heterodyne spectroscopy of molecular rotational lines and atomic fine-structure lines is a powerful tool in astronomy and planetary research. It allows for studying the chemical composition, the evolution, and the dynamical behaviour of many astronomical objects. As a consequence, current and future airborne as well as spaceborne observatories such as SOFIA, Herschel or Millimetron are equipped with heterodyne spectrometers. A major challenge for heterodyne receivers operating above approximately 2 THz is the local oscillator, which should be a compact source requiring little electrical input power. THz quantum-cascade lasers (QCLs) have the potential to comply with these requirements. However, until now, THz QCLs operate at rather low temperatures so that cooling by liquid helium or using large cryo-coolers becomes necessary. While these cooling approaches might be acceptable for laboratory experiments, they either result in too many restrictions on airborne or spaceborne heterodyne receivers or are completely unacceptable. We report on the development of a compact, easy-to-use source, which combines a QCL operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output powers and low electrical pump powers [1]. Efficient carrier injection is achieved by resonant longitudinal-optical phonon scattering. At the same time, the operating voltage can be kept below 6 V. The amount of generated heat complies with the cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cryostat. The whole system weighs less than 15 kg including cooler, power supplies etc. The output power is well above 1 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser becomes a fundamental Gaussian one. In addition to the performance of the QCL in the Stirling cooler, we will present results of the application of this source to highresolution molecular spectroscopy

A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum cascade laser (QCL) operating at 3.1 THz with a compact, low input-power Stirling cooler. The QCL, which is based on a two miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. he whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments

A 4.7-THz gas laser local oscillator for GREAT on SOFIA

A particularly important transition for astronomy is the OI fine structure line at 4.7 THz. It is an important cooling line of the interstellar medium and allows studying the chemical composition, the evolution, and the dynamical behavior of astronomical objects. Consequently, this transition is a main target to be observed with GREAT, the German Receiver for Astronomy at Terahertz Frequencies, which will be operated on board of SOFIA. A major challenge for a heterodyne receiver operating at such a high frequency is the local oscillator (LO). Despite significant progress in the development of a quantum-cascade laser based LO [1] the baseline design for GREAT is an optically pumped gas laser operated at 4.7 THz. In this report we will present the design and performance of the 4.7-THz gas laser LO for SOFIA. The LO is based on a radio frequency excited CO2 laser which has a sealed-off gas volume and which is frequency tunable by a grating. The CO2 laser is operated on the 9P12 transition of the CO2 molecule. The output emission is focused into the THz laser resonator. The THz laser is transversely excited. It operates on the 4.75 THz line of 13CH3OH. For frequency stabilization of the CO2 laser a small part of its output radiation is guided into a Fabry-Pérot interferometer (FPI) which serves as a length or frequency reference. In order to compensate for temperature or pressure induced drifts of the FPI length the emission of a frequency stabilized a helium-neon (HeNe) laser is coupled into the FPI as well. The FPI is locked to the emission of the HeNe laser. We will present the design and the performance of the LO with respect to output power, short and long term power stability, and beam profile. The system is ready and awaits implementation in GREAT and operation on board of SOFIA