In this work we demonstrate an open resonator powered for the first time by a frequency-comb locked continuous wave terahertz spectrometer. A sample of a 10 µm thin cling film is placed into the resonator, leading to shifts and attenuation of resonance frequencies, and proving the sensitivity of our instrument to low-absorbing, low-permittivity materials.

Spectrum analyzers and spectrometers are essential for designing sources, analyzing material properties, layer structures and fingerprinting substances. We present an ultra-wideband, continuous-wave photonic receiver with kHz-level spectral resolution in the terahertz domain for both heterodyne and homodyne detection. Employed as a spectrum analyzer front end, it records the emitted spectrum of a source under test, assessing spectral purity, spectral shape and undesired frequency components. It outperforms state-of-the-art electronic systems in terms of frequency coverage and system cost with a competitive spectral resolution and noise floor on the few aW/Hz level at room temperature. It covers the important frequencies above 1.5 THz, yet commercially inaccessible, where sources like quantum cascade lasers operate. When combined with a comb-based photonic source, we demonstrate hetero- and homodyne spectroscopy over an unprecedented frequency range from below 100 GHz to 6.5 THz and a very low noise floor. Locking the photonic system to GPS enables tracing back the measured parameters to SI units, being of key importance for metrological applications. The presented setups offer the broadest continuous-wave frequency coverage to date, combined with a sharp spectral resolution, enabling diverse applications ranging from fast non-destructive testing, astronomic high-resolution spectroscopy, to frequency-modulated RADAR.

We report on the photonic-based broadband characterisation of a 415 GHz band-pass filter (BPF) in the frequency range between 0.3 to 0.8 THz covering several rectangular wave guide bands. BPFs are widely used to reduce out-of-band noise and spurs on the intermediate frequency (IF), e.g. in communication and THz imaging. However, traditional electronic-based characterization methods cannot cover the required bandwidths. Our laser engine is based on a fixed cw laser and a mode-hop-free, widely tuneable external cavity diode lasers optically phased-locked to an erbium-fibre frequency comb at 1550 nm (DFC Core 200+) [1], [2]. The two continuous wave lasers provide a phase-coherent, tuneable difference frequency of up to 10 THz with tuning rates of up to 1 THz/s (Fig. 1a). The difference frequency is converted to terahertz and detected in a free-space spectroscopy setup using photoconductive antennas (PCA) (Fig. 1b). The frequency is linearly swept, and the receiver signal is averaged before applying the Hilbert transform to retrieve amplitude and phase of the transmission. The photonic measurement (Fig1c, blue) is taken at ~0.2 THz/s with ~1.3 MHz frequency resolution in a total measurement time of 200 s. The WM570 band is indicated for comparison. Our sample device under test (DUT) consists of a WM570 filter with centre frequency of 415 GHz and a 3 dB bandwidth of 16.9 GHz (fractional bandwidth of 4.1%) (Fig1c). The filter consists of a symmetrical arrangement of 15 cavities shown in the inset.

Low phase-noise laser sources with robust performance are crucial for numerous applications, including ultraprecision spectroscopy, optical clocks, and trapped-atom quantum computers, where high-fidelity quantum control of atomic qubits is often limited by technical noise in laser systems [1]. Femtosecond frequency combs have proven to be precise optical frequency rulers, capable of transferring the ultra-narrow linewidth of high-finesse-cavity stabilized lasers across the comb spectrum [2]. However, such systems require cost-intensive and complex setups, including a high-finesse cavity within a vacuum chamber, and are prone to long-term drift due to the lack of absolute referencing. As a compelling alternative, we demonstrate a cost-effective locking scheme for the simultaneous short-term and absolute long-term stabilization of a broadband frequency comb, serving as a universal locking reference [3].

We have developed, manufactured, and characterized a WM-570 rectangular waveguide bandpass filter with a central frequency of 415 GHz. Initially, a vector network analyzer (VNA) with millimeter-wave converters was employed to determine the transfer function of the filter within the WM-570 band of 330-500 GHz. Subsequently, we utilized a frequency-comb locked continuous-wave terahertz spectrometer to cover a frequency range from 300 to 800 GHz, spanning several waveguide bands. Using this system, in addition to the designed passband around 415 GHz, we identified several other features at higher frequencies, particularly beyond the measurement range of the VNA converter setup employed. The measurements agree well with the simulation, even at frequencies beyond the operational bandwidth of the WM-570 waveguide. This demonstrates the potential of a tunable optical source to characterize measurement instrumentation up to THz frequencies.

This paper presents the design and implementation of an agile sub-terahertz (sub-THz) broadband time-domain photonic channel sounder, capable of characterizing wireless propagation channels within the 100 GHz to 500 GHz frequency range. The sounder employs a hybrid architecture that combines photonic carrier generation through heterodyne mixing of continuous-wave lasers locked to a frequency comb, along with electronic components for broadband sounding sequence synthesis and digitization. This design enables precise and tunable carrier frequency generation, overcoming the limitations of traditional electronic-only systems. Experimental evaluations highlight the system's high temporal resolution, phase stability, and broadband performance, including over-the-air calibration and monostatic channel measurements across three sub-THz bands (160 GHz, 300 GHz, and 450 GHz). The results establish the feasibility of the time-domain photonic channel sounder as a powerful and versatile tool for measuring various, widely scattered, and spread sub-THz bands.

Optical microwaves have the potential for ultra-low phase noise and high frequency signal generation, which becomes increasingly important for a variety of applications, e.g. in radar [1] and communications [2]. We report on the generation of ultra-low noise and long-term stable 9.6 GHz microwaves through optical frequency division of an offset-free comb, utilizing a novel scheme for GPS referencing. Instead of detecting and locking the carrier envelope offset frequency $(f_{ceo})$, the comb (DFC CORE 200 +) is based on difference frequency generation (DFG) [3], which cancels the $f_{ceo}$ for each optical pulse. Thus, all actuators (temperature, piezo, oscillator) can be used for the stabilization of the comb repetition rate $(f_{rep})$ without the risk of crosstalk between $f_{rep}$ and $f_{ceo}$. The error signal for $f_{rep}$ stabilization is generated by a beat note of one comb mode with a low-phase noise continuous wave (cw) reference laser.

The comb-locked frequency-swept optical synthesizer enables optical metrology based on a phase-coherent widely-tuneable single mode laser source with unprecedented performance in terms of tuning speed and frequency accuracy. The technology and applications are discussed.

Sources for high frequencies in the THz range are of interest in both precision measurements in spectroscopy as well as high carrier frequencies and bandwidth in next-generation communication. Optically generated THz frequencies allow for broad tunability based on telecom technologies. Optical frequency division via an optical frequency comb has the potential to generate ultralow phase-noise THz sources. We present a system that is capable of generating tunable THz frequencies of up to 10 THz at 0.5 THz/s scan rates, phase-locked to a common comb spectrum providing absolute frequency calibration. We show results of combining the photonic source with an electronic receiving system in the waveguide band WR 2.2 (330-500 GHz).

We demonstrate the capabilities of a novel frequency-domain terahertz spectrometer based on a comb-locked frequency synthesizer, which provides absolute frequency calibration. The inherent stability and repeatability of the scans allow for the combination of fast data acquisition with an average time-limited signal-to-noise ratio. We demonstrate kilohertz level frequency resolution in terahertz precision spectroscopy of ultra-high quality whispering-gallery-mode resonators. Spectra covering multiple free spectral ranges (>36 GHz) with sub-20 kHz resolution are acquired in 5 s. We analyze the coupling behavior and temperature tuning of single resonances and, for the first time, observe minute red and blue shifts of different mode families. The experimental results are supported with finite element simulations.

We have designed, fabricated, and characterized a rectangular WR2.2 waveguide bandpass filter with centre frequency of 415 GHz. At first, a vector network analyser was used to determine the filters transfer function between 330 GHz and 500 GHz. Secondly, we use a frequency comb locked continuous wave terahertz spectrometer to cover a broader frequency range between 250 GHz and 1100 GHz. Beside the desired pass band around 415 GHz, we found several other features in the filter’s characteristic at higher frequencies and especially outside the measurement range of the vector network analyser. Our measurements show a high level of agreement with the simulation even at frequencies much larger as the operational bandwidth of the WR2.2 waveguide.

We present precision measurements of terahertz whispering gallery mode resonances using a novel comb-locked frequency domain spectrometer. Absolute long-term frequency stability and resolution allows to accurately resolve temperature shifts across multiple modes.

We report on ultra-high precision terahertz spectroscopy of a spherical microresonator using a novel comb-locked frequency domain spectrometer. With kilohertz frequency resolution and stability, the spectrometer enables the first observation of a minute red and blue shift of whispering gallery modes due to the coupling waveguide.

The high frequency region beyond 100 GHz considered for 6G poses challenges for future communication and measurement equipment. In this work we present a novel tunable THz system based on ultra-stable photonic sources and optical frequency comb technology covering the frequency range from a few GHz up to 500 GHz.

We present a novel frequency domain spectrometer, which is based on two external cavity diode lasers (ECDL) locked to a fibre-based erbium frequency comb, the repetition rate $\mathrm{f}_{rep}}$ of which is in turn referenced to a radio frequency (RF-) oscillator. By using an external endless frequency-shifter [1], one of the lasers is detuned by frequencies up to 10 THz, limited only by the spectral width of the comb. Within the locking bandwidth, the stability and phase noise of the RF reference is transferred to the difference frequency between both ECDLs [2].

Lasers with both a narrow linewidth and high long-term stability play an important role for numerous applications in various fields ranging from precision spectroscopy to preparation and control of quantum gases and atomic clocks in quantum optics. A new locking scheme combines low-phase noise by locking to an optical reference with long-term stability via a GPS disciplined RF-oscillator. Implementing a cost-effective fiber laser as optical reference (OR), the line width of each comb tooth is reduced to few kHz, providing a universal absolute reference to lock cw lasers at any wavelength accessible by the comb spectrum [1], [2].

We exploit a widely tunable comb-locked frequency-swept synthesizer to test a new optical approach to primary gas thermometry based on a global fitting of multiple molecular absorption lines of the same band at different pressures.

Optical Frequency Domain Reflectometry features high resolution and sensitivity. The resolution is limited by the laser scan range, linearity and noise. We introduce a laser source capable of precise > 10 THz wide and > 1 THz/s fast scans remaining phase locked to the optical comb. A resolution of 0.5E-6 at 20 km fiber is demonstrated.

We introduce a frequency-domain terahertz (THz) spectrometer based on a comb locked frequency synthesizer which combines > 3 THz scanning range at rates > 0.3 THz/s. We demonstrate kHz frequency resolution in THz precision spectroscopy of ultrahigh-Q whispering-gallery mode resonators.

In this erratum we correct errors in the scaling and caption of Fig. 4 in the original manuscript [1]. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement