Axel Roggenbuck

Precisely tunable continuous-wave terahertz source with interferometric frequency control

We realized a tunable continuous-wave terahertz source with megahertz frequency resolution. The system is based on optical heterodyning of two near-infrared distributed feedback diode lasers, each laser being stabilized by electronic feedback from a low-finesse quadrature interferometer. The control loop permits precisely linear laser frequency scans over >1200 GHz, and a beat signal linewidth of 1 MHz at 80 ms time scale. Using GaAs photomixers and log-periodic antennae, we achieve a signal-to-noise ratio of the terahertz power of >70 dB at 100 GHz and 100 ms integration time, and still approximately 30 dB at 1 THz. As an example for high-resolution terahertz spectroscopy, we characterize the transmission properties of a subwavelength metal grating.

Group Delay in THz Spectroscopy with Ultra-Wideband Log-Spiral Antennae

We report on the group delay observed in continuous-wave terahertz spectroscopy based on photomixing with phase-sensitive homodyne detection. We discuss the different contributions of the experimental setup to the phase difference Δφ(ν) between transmitter arm and receiver arm. A simple model based on three contributions yields a quantitative description of the overall behavior of Δφ(ν). Firstly, the optical path-length difference gives rise to a term linear in frequency ν. Secondly, the ultra-wideband log-spiral antennae effectively radiate and receive in a frequency-dependent active region, which in the most simple model is an annular area with a circumference equal to the wavelength. The corresponding term changes by roughly 6π between 100 GHz and 1 THz. The third contribution stems from the photomixer impedance. In contrast, the derivative ∂Δφ/∂ν is dominated by the contribution of periodic modulations of Δφ(ν) caused by standing waves, e.g., in the photomixers’ Si lenses. Furthermore, we discuss the Fourier-transformed spectra, which are equivalent to the waveform in a time-domain experiment. In the time domain, the group delay introduced by the log-spiral antennae gives rise to strongly chirped signals, in which low frequencies are delayed. Correcting for the contributions of antennae and photomixers yields sharp peaks or “pulses” and thus facilitates a time-domain-like analysis of our continuous-wave data.

Enhancing the stability of a continuous-wave terahertz system by photocurrent normalization

In a continuous-wave terahertz system based on photomixing, the measured amplitude of the terahertz signal shows a variability due to drifts of the responsivities of the photomixers and of the optical power illuminating the photomixers. We report a simple method to substantially reduce this variability. By normalizing the amplitude to the DC photocurrents in both the transmitter and receiver photomixers, we achieve a significant increase in stability. If, e.g., the optical power of one laser is reduced by 10%, the normalized signal is expected to change by only 0.3%, i.e., less than the typical uncertainty due to short-term fluctuations. This stabilization can be particularly valuable for terahertz applications in nonideal environmental conditions outside of a temperature-stabilized laboratory.

Self-normalizing phase measurement in multimode terahertz spectroscopy based on photomixing of three lasers

Photomixing of two near-infrared lasers is well established for continuous-wave terahertz spectroscopy. Photomixing of three lasers allows us to measure at three terahertz frequencies simultaneously. Similar to Fourier spectroscopy, the spectral information is contained in an interferogram, which is equivalent to the waveform in time-domain spectroscopy. We use one fixed terahertz frequency νref to monitor temporal drifts of the setup, i.e., of the optical path-length difference. The other two frequencies are scanned for broadband high-resolution spectroscopy. The frequency dependence of the phase is obtained with high accuracy by normalizing it to the data obtained at νref, which eliminates drifts of the optical path-length difference. We achieve an accuracy of about 1–2 μm or 10−8 of the optical path length. This method is particularly suitable for applications in nonideal environmental conditions outside of an air-conditioned laboratory.

Cw terahertz spectrometer with high-precision frequency control

We realized a continuous-wave terahertz spectrometer based on optical heterodyning of two near-infrared distributed-feedback diode lasers. Using active frequency stabilization we achieve 1 MHz resolution and a signal-to-noise ratio up to 80 dB.

Tunable Fabry-Perot THz filter with sub-wavelength grating mirrors

We utilize the reflectivity of one-dimensional metal gratings with sub-wavelength slits to realize mirrors for THz frequencies. Two of them are combined to a Fabry-Perot filter, which features the corresponding transmission bands. By appropriate choice of dimensions, the extraordinary transmission resonance of the sub-wavelength gratings can be superimposed with the Fabry-Perot peak. By varying the resonator length between the grating mirrors, the overlap of both transmission peaks can be controlled. This enables the tuning of the filter bandwidth. The theoretical analysis shows that continuous tuning of the filter bandwidth up to 30% is possible for a two mirror stage. For the performance of comparative measurements, an all-fiber continuous-wave THz system is used. The experimental results are in fairly well agreement with the theoretically predicted tuning properties.

Coherent terahertz imaging with synchronized distributed-feedback diode lasers

We present a heterodyne hybrid terahertz imaging system, which combines electronic narrow-band emitters, operating at 0.2 THz and 0.62 THz respectiveley, with a continuous-wave two-color laser system for electro-optic detection. The laser system employs two distributed-feedback laser diodes, providing a tunable difference frequency which is phase-locked to the emitted terahertz frequency with an offset of 10 MHz.

Cw THz spectrometer with high SNR and MHz frequency resolution

We developed an all-fiber-based continuous-wave terahertz spectrometer based on optoelectronic signal generation and detection. The spectrometer has a very high spectral resolution in the MHz range over the whole tuning range from 50 GHz to 1750 GHz. The signal-to-noise ratio is ≈ 80 dB at 100 GHz and still 50 dB at 1 THz, using a lock-in time constant of 300 ms. We applied the spectrometer to absorption measurements of a-lactose monohydrate and demonstrate the suitability of the instrument to determine the area density of paper samples.

Tunable cw THz source with high-precision frequency control

We realized a tunable continuous-wave terahertz (cw THz) source with MHz frequency resolution. The system is based on optical heterodyning of two near-infrared Distributed Feedback (DFB) diode lasers, each laser being stabilized by electronic feedback from a low-finesse quadrature interferometer. The control loop permits precisely linear laser frequency scans over >1200 GHz, and a beat signal linewidth of 1 MHz @ 80 ms time scale. Using GaAs photomixers and log-periodic antennae, we achieve an SNR of the THz power of > 70 dB at 100 GHz and 100 ms integration time, and still ~ 30 dB @ 1 THz. As an example for high-resolution THz spectroscopy, we characterize the transmission properties of a sub-wavelength metal grating.

Development of a hybrid THz camera using synchronized two-color laser radiation

We demonstrate a hybrid system for THz raster scan imaging using frequency-stabilized DFB lasers for electro-optic detection. The system combines a 0.65-THz micro-electronic narrow-band emitter with two synchronized diode lasers, providing two-color laser radiation. The functional principle promises three-dimensional real-time imaging capabilities.