Monolithic 1.5 µm photonic integrated circuits (PICs) — dual-wavelength DFB lasers and mode-locked sampled-grating lasers tunable across 0.254–2.723 THz — can generate the optical beat signals for terahertz (THz) photomixing, but the source paper (Hou et al., IET Optoelectronics 2020) found absolute emitted THz powers of only 0.2–1.4 µW: the photoconductive antennas (PCAs) used as mixers have a room-temperature optical-to-THz conversion efficiency near 10−510^{-5}, most of the divergent THz beam is lost to collection, and an external erbium-doped fibre amplifier is required to pump the mixer. The subsequent literature shows: UTC-PD photomixers now deliver 1–5 mW at 300 GHz in discrete modules; plasmonic nano-antenna PCAs have raised emitter power by orders of magnitude (300 µW pulsed); PIC-based links with UTC-PDs reach µW-level emitted power at 0.4 THz; and foundry-fabricated laser PICs with integrated semiconductor optical amplifiers have pumped external photodiode emitters up to 1 THz. No room-temperature, fully integrated PIC THz source (laser + on-chip optical amplifier + co-integrated high-efficiency photomixer) has yet delivered application-useful absolute THz power, especially above the 300 GHz band, where every demonstrated photonic source falls to microwatts or below.

Background

The terahertz band (roughly 0.1–10 THz) sits in the long-recognized 'THz gap', where neither electronic oscillators nor semiconductor lasers conveniently deliver power, and envisioned applications — high-capacity wireless links, spectroscopy, imaging, sensing — need sources that are compact, low-cost and room-temperature. One established route is photomixing: two near-infrared laser lines whose frequency difference equals the desired THz frequency illuminate a fast optoelectronic converter, which turns the optical beat into a THz current radiated by an attached antenna. In a photoconductive antenna (PCA), the converter is a photoconductive switch on a short-carrier-lifetime semiconductor; its continuous-wave optical-to-THz conversion efficiency at room temperature is only of order 10−510^{-5}. A uni-travelling-carrier photodiode (UTC-PD), in which only electrons cross the high-field collector, saturates at far higher currents and is the standard higher-efficiency alternative mixer. Photonic integrated circuits (PICs) integrate the optical side on one chip: distributed-feedback (DFB) lasers (a grating inside the cavity selects the wavelength), dual-wavelength DFB lasers emitting two tones, and mode-locked laser diodes whose pulse repetition rate is itself a THz frequency. Hou et al. (IET Optoelectronics 2020) reported four such InP PICs emitting at 1.5 µm: a four-DFB optoelectronic synthesiser continuously tunable 0.254–2.723 THz, a dual-grating DFB laser with electroabsorption modulator producing an 820 GHz beat, a dual-wavelength phase-shifted sampled-grating DFB laser driving a PCA at 560 GHz, and a sampled-grating distributed-Bragg-reflector mode-locked laser driving a PCA at 640 GHz. In every case the PIC output had to be amplified in an external erbium-doped fibre amplifier (EDFA — a bench-top optical amplifier) before pumping the PCA. Detected THz powers were only 20–30 nW: with the PCA efficiency near 10−510^{-5} and only about 10% of the emitted THz beam collected (30° beam divergence, 140 mm free-space path, 11 mm detector aperture), the total emitted power was estimated at 0.2–1.4 µW. The authors' stated path forward is to adopt UTC-PDs as mixers (a UTC-PD module had already delivered more than 1 mW at 300 GHz; Song et al. 2012) and to note that a semiconductor optical amplifier (SOA — the on-chip counterpart of an EDFA) can reach the saturation powers of such photodiodes, eliminating the external amplifier. The unresolved objective behind these observations is to turn PIC pump generators into sources of useful absolute THz power.

Problem Statement

Can the absolute THz output power of room-temperature photonic-integrated-circuit photomixing sources be raised to levels useful for practical wireless links and sensing, with the optical pump amplification on chip rather than in an external bench-top amplifier? Building on the PIC pump sources of Hou et al. (tunable 0.254-2.723 THz; 560-640 GHz photomixing demonstrations whose emitted power is 0.2-1.4 µW), the source itself identifies the following route toward it: (i) replace the photoconductive antenna, whose room-temperature optical-to-THz conversion efficiency is about 10−510^{-5}, with a higher-efficiency mixer such as a uni-travelling-carrier photodiode, which the source notes operates with better efficiency at high frequencies than other photomixers; and (ii) replace the external EDFA with an on-chip semiconductor optical amplifier, which can readily reach the saturation powers of photodiode and PCA detectors, eliminating the need for off-chip pump amplification. The objective is a room-temperature, fully integrated PIC-based THz photomixing source, with no off-chip optical amplification and the photomixer co-integrated with the laser PIC, whose calibrated absolute THz output power reaches application-useful levels: at least 1 mW at 300 GHz and at least 100 µW at 1 THz. These target levels are set against the current records — roughly 1-5 mW at 300 GHz from discrete UTC-PD modules and microwatt-level emitted power at 0.4 THz from the best PIC-plus-UTC-PD link — but must be reached by a source that itself satisfies the on-chip-amplification and co-integration constraints. Pushing useful power beyond 1 THz, toward the upper end of the tuning range where no integrated photonic source currently reaches it, would be additionally desirable but is not required. Related challenges beyond this objective — antenna and collection-optics design delivering usable on-axis power density rather than a 30°-divergence beam, and manufacturable, packageable, long-term-stable operation — remain open.

References

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