Abstract-Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

T. Alam, M. Wienold, X. Lü, K. Biermann, L. Schrottke, H. T. Grahn, and H.-W. Hübers

 The schematic diagram of the setup used to stabilize the frequency and output power. The QCL is operated in a mechanical cryocooler. The combination of the absorption cell and the Ge:Ga detector A is used to lock the frequency, and the second Ge:Ga detector B is used as a reference for the output power stabilization. (b) Rear-facet illumination with a low-NA single-mode fiber.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-25-36846

We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers

T. Alam, M. Wienold, X. Lü, K. Biermann, L. Schrottke, H. T. Grahn, and H.-W. Hübers

Fig. 1 (a) Schematics of the experimental setup. The QCL (yellow box) is mounted in a He-flow cryostat. BS - dichroic beamsplitter; OL - objective lens; Ge:Ga - photoconductive Ge:Ga detector. (b) Microscope image of the illuminated QCL facet. The excitation spot with a diameter of approximately 90 μm originates from a multimode diode laser emitting at 809 nm and exhibits essentially a flat-top profile. (c) Calculated profile of the waveguide mode in the vertical (epitaxial-growth) direction for different frequencies (the mode propagates perpendicular to y along the waveguide ridge). The active region (a. r.) has a height of 10 μm and corresponds to the QCL ridge structure in (b).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-4-5420

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a ten-fold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Doppler-free spectroscopy with a terahertz quantum-cascade laser

M. Wienold, T. Alam, L. Schrottke, H. T. Grahn, and H.-W. Hübers

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-6692

We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Characterizing the beam properties of terahertz quantum-cascade lasers

• H. Richter, 
• N. Rothbart, 
• H.-W. Hübers

Terahertz quantum-cascade lasers (QCLs) are very promising radiation sources for many scientific and commercial applications. Shaping and characterizing the beam profile of a QCL is crucial for any of these applications. Usually the beam profile should be as close as possible to a fundamental Gaussian TEM00 mode. In order to completely characterize the laser beam the power and the wavefront have to be measured. We describe methods for characterizing the beam properties of QCLs. Several QCLs with single-plasmon waveguide and emission frequencies between 2 and 5 THz are investigated. The beam profiles of these lasers are shaped into almost fundamental Gaussian modes using dedicated lenses. The beam propagation factor M2 is as low as 1.2. The wavefront is measured along the axis of propagation with a THz Hartmann sensor. Its curvature behaves as expected for a Gaussian beam. The applied methods can be transferred to any other THz beam.

Abstract-Frequency modulation spectroscopy with a THz quantum-cascade laser

R. Eichholz, H. Richter, M. Wienold, L. Schrottke, R. Hey, H. T. Grahn, and H.-W. Hübers  »View Author Affiliations

Optics Express, Vol. 21, Issue 26, pp. 32199-32206 (2013)
http://dx.doi.org/10.1364/OE.21.032199

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

© 2013 Optical Society of America