Abstract-Ultra-high-resolution software-defined photonic terahertz spectroscopy

 

Rodolfo I. Hermans, James Seddon, Haymen Shams, Lalitha Ponnampalam, Alwyn J. Seeds, and Gabriel Aeppli

Experiment schematic: a monochromatic telecommunications wavelength laser feeds an optical frequency comb generator (OFCG) with exact tunable spacing. A programmable wavelength selective switch (WSS) selects two bands 12 peaks apart, amplified and mixed in uni-traveling-carrier photo-diode (UTC-PD). A 200 GHz beat frequency is transmitted through horn antennas and lenses through a continuous-flow liquid helium cryostat with thin polypropylene windows and LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_4:{\mathrm{H}\mathrm{o}}^{3+}$ sample. The received signal is down-converted using a sub-harmonic mixer and measured using a microwave spectrum analyzer.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-10-1445

A novel technique for high-resolution 1.5µm$1.5\text{µ}\mathrm{m}$ photonics-enabled terahertz (THz) spectroscopy using software control of the illumination spectral line shape (SLS) is presented. The technique enhances the performance of a continuous-wave THz spectrometer to reveal previously inaccessible details of closely spaced spectral peaks. We demonstrate the technique by performing spectroscopy on LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_4:{\mathrm{H}\mathrm{o}}^{3+}$, a material of interest for quantum science and technology, where we discriminate between inhomogeneous Gaussian and homogeneous Lorentzian contributions to absorption lines near 0.2 THz. Ultra-high-resolution (<100Hz$<100\mathrm{H}\mathrm{z}$ full-width at half maximum) frequency-domain spectroscopy with quality factor 𝑄>2×109$Q>2\times {10}^9$ is achieved using an exact frequency spacing comb source in the optical communications band, with a custom uni-traveling-carrier photodiode mixer and coherent down-conversion detection. Software-defined time-domain modulation of one of the comb lines is demonstrated and used to resolve the sample SLS and to obtain a magnetic field-free readout of the electronuclear spectrum for the Ho3+${\mathrm{H}\mathrm{o}}^{3+}$ ions in LiYF4:Ho3+${\mathrm{L}\mathrm{i}\mathrm{Y}\mathrm{F}}_4:{\mathrm{H}\mathrm{o}}^{3+}$. In particular, homogeneous and inhomogeneous contributions to the spectrum are readily separated. The experiment reveals previously unmeasured information regarding the hyperfine structure of the first excited state in the 5𝐼8${}^5{I}_8$ manifold complementing the results reported in Phys. Rev. B 94, 205132 (2016) [CrossRef]  .

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Exact frequency and phase control of a terahertz laser

Reshma A. Mohandas, Lalitha Ponnampalam, Lianhe Li, Paul Dean, Alwyn J. Seeds, Edmund H. Linfield, A. Giles Davies, and Joshua R. Freeman

Schematic diagram of the experimental arrangement. EDFA, erbium doped fibre amplifier; Tx, photomixer emitter; Rx1 and Rx2, photomixer receivers; PLL, phase lock loop; Δ𝜑$\mathrm{\Delta }\phi$, variable delay line. Electrical connections are shown in black, optical fiber and IR connections in red, and terahertz connections in green.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1143

The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of <1Hz$<1\mathrm{H}\mathrm{z}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range ±0.3𝜋$\pm 0.3\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract- Antenna Integrated THz Uni-Traveling Carrier Photodiodes

Cyril C. Renaud, Michele Natrella,   Chris Graham,  James Seddon,   Frederic Van Dijk,  Alwyn J. Seeds

http://ieeexplore.ieee.org/document/7979505/

High-speed photodiodes are a key element of numerous photonic systems. With the development of potential applications in the THz range such as sensing, spectroscopy, and wireless transmission, devices with integrated antenna covering the frequency range from 0.1 to 3 THz will become essential. In this paper, we discuss the development of uni-traveling carrier photodiodes with integrated antennas to address that need. In particular we develop the key elements to present a simple design tool for the efficient integration of the device with an antenna. We also present fabricated device results that show the highest figure of merit to date for photonic THz emitters. When integrated with well-matched antennas the devices have achieved record level of power up to 1 THz compared to other published photomixers. We also show that these devices can be used as receivers at frequencies up to 560 GHz with conversion losses of the order of 30 dB.