Hybrid Optical/THz Communication Links

• Hybrid optical/THz links are systems that convert and transmit signals between optical and terahertz domains using mechanisms like electro-optic modulation, photomixing, and optical rectification.
• They encompass diverse architectures—including radio-over-fiber, integrated photonic chips, and hybrid fibers—enabling applications from broadband wireless transmission to secure network backhauls.
• Efficient design requires precise modal confinement, phase matching, and low-loss conversion to achieve high data rates, robust performance, and practical implementation.

Searching arXiv for recent and foundational work on hybrid optical/THz links. Search query: hybrid optical THz link photonics integrated terahertz optical conversion Hybrid optical/THz links are communication, transduction, and source architectures that directly couple optical carriers, photonic circuits, or free-space optical channels to terahertz electromagnetic waves. In the literature represented here, the term covers several distinct but related classes of systems: radio-over-fiber transceivers that map sub-THz signals onto optical carriers with plasmonic modulators (Burla et al., 2018, Ummethala et al., 2018), monolithic thin-film lithium niobate platforms that support bidirectional THz–optical interaction and continuous THz generation on chip (Zhang et al., 2024), photonics-integrated terahertz transmission lines that co-confine optical and THz modes for broadband optical rectification (Lampert et al., 2024), molecular-modulation sources in hydrogen-filled hybrid anti-resonant fibers that convert near-infrared pump pulses into narrowband THz output (Loranger et al., 2023), and network-level hybridization schemes in which THz links are paired with visible light communication or free-space optics for resilience, secrecy, or backhaul reliability (Prabhakar et al., 22 May 2026, Illi et al., 2024, Singya et al., 2023, Singya et al., 2022). Across these variants, the central technical problem is the same: achieving efficient, phase-matched, low-loss, and application-specific transfer between optical and THz domains.

1. Conceptual scope and architectural classes

Hybrid optical/THz links span both device-level and network-level realizations. At device level, the optical and THz domains are linked through electro-optic modulation, photomixing, optical rectification, stimulated Raman scattering, or guided-wave $\chi^{(2)}$ interactions. At system level, the same term also encompasses composite links in which THz transmission is integrated with optical fiber, free-space optical, or visible-light segments (Ummethala et al., 2018, Illi et al., 2024, Prabhakar et al., 22 May 2026).

One major class is the radio-over-fiber architecture. In a representative sub-THz implementation, a remote antenna unit receives a 220–325 GHz wireless signal, a plasmonic-organic-hybrid Mach–Zehnder modulator encodes that electrical tone onto a continuous-wave optical carrier, and the modulated light is transported over single-mode fiber to a central office, where a uni-travelling-carrier photodiode recovers the 220–325 GHz signal (Burla et al., 2018). A closely related fiber-to-wireless and wireless-to-fiber realization demonstrated direct terahertz-to-optical conversion of a 50 Gbit/s data stream transmitted on a 0.2885 THz carrier over a 16 m-long wireless link, with optical-to-terahertz conversion at the transmitter provided by photomixing in a uni-travelling-carrier photodiode (Ummethala et al., 2018).

A second class is monolithic integrated photonics. A thin-film lithium niobate on quartz platform was designed to support efficient bidirectional interaction between THz and optical waves, combining THz-optic modulation and continuous THz-wave generation at up to 500 GHz on a single chip (Zhang et al., 2024). A related thin-film lithium niobate platform integrates phase-matched terahertz transmission lines with photonic circuits and demonstrates broadband terahertz emission spanning four octaves from 200 GHz to 3.5 THz through broadband down-conversion of optical signals at telecommunication wavelengths (Lampert et al., 2024).

A third class comprises hybrid waveguides and fibers that co-confine optical and THz modes. In hydrogen-filled hybrid hollow-core anti-resonant fibers, a near-infrared pump and its first Stokes band are confined in the optical core, while a concentric THz waveguide confines the TE$_{11}$ THz mode; stimulated Raman scattering and molecular modulation then generate narrowband THz pulses as the second Stokes band (Loranger et al., 2023).

A fourth class is hybridized networking. In indoor integrated sensing and communication, THz communication may be paired with visible-light communication so that THz provides ultra-high-rate links under line-of-sight conditions while VLC maintains coverage under blockage (Prabhakar et al., 22 May 2026). In aerial and backhaul settings, THz feeder links are paired with FSO to improve secrecy or reliability under atmospheric impairments, pointing error, and multi-hop constraints (Illi et al., 2024, Singya et al., 2023, Singya et al., 2022).

This suggests that “hybrid optical/THz link” is best understood ոչ as a single topology but as a family of cross-domain interfaces whose design objectives differ: analog transport, direct data conversion, source generation, broadband emission, integrated transduction, or robustness through multi-technology diversity.

2. Physical mechanisms for optical–THz coupling

The coupling mechanisms in hybrid optical/THz links are diverse, but most reduce to a small set of physical processes: electro-optic modulation, photomixing, second-order nonlinear generation, optical rectification, and Raman/molecular-modulation conversion.

In photomixing-based transmitters, two optical fields with frequencies $\omega_1$ and $\omega_2$ are incident on a uni-travelling-carrier photodiode or photomixer. The photocurrent contains a beat term at $\omega_{\mathrm{THz}}=\omega_1-\omega_2$, which radiates the THz carrier (Ummethala et al., 2018, Nallappan et al., 2017). In the 138 GHz demonstration assembled from commercially available components, two tunable DFB lasers operating near 1534 nm were combined and injected into a UTC-photomixer to generate the THz carrier, while one optical beam carried a 5.5 Gbps NRZ-OOK data stream imposed by a LiNbO$_3$ Mach–Zehnder modulator (Nallappan et al., 2017).

In electro-optic THz-to-optical conversion, an incoming THz signal drives an optical modulator. For a push-pull Mach–Zehnder modulator biased at quadrature, the normalized intensity modulation index is written as

$m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$

in the small-signal regime (Ummethala et al., 2018). The plasmonic-organic-hybrid modulator used for terahertz-to-optical conversion had a measured electro-optic 3 dB bandwidth in excess of 0.36 THz (Ummethala et al., 2018). A related balanced plasmonic Mach–Zehnder interferometer on silicon photonics exhibited a flat frequency response with ripple $<\pm 3$ dB and no roll-off observed up to $500$ GHz (Burla et al., 2018).

In thin-film lithium niobate, the dominant mechanism is the second-order nonlinearity. For THz generation by difference-frequency interaction or guided-wave mixing, the nonlinear polarization is

$$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$

with the dominant tensor element in X-cut lithium niobate and TE polarization along $_{11}$0 given as $_{11}$1 pm/V in the monolithic chip platform (Zhang et al., 2024). In the photonics-integrated transmission-line platform, optical rectification is written as

$_{11}$2

with $_{11}$3 pm/V at optical and $_{11}$4 pm/V in the formula (Lampert et al., 2024).

In hydrogen-filled hybrid anti-resonant fibers, the mechanism is molecular modulation mediated by vibrational Raman coherence. The ground-state-to-vibrational-level Raman shift is $_{11}$5 THz. The process proceeds in two steps: the pump at $_{11}$6 generates a first Stokes band at $_{11}$7 and coherence $_{11}$8, and then the first Stokes field generates a second Stokes field at $_{11}$9 in the THz range (Loranger et al., 2023). The coherence dynamics are described by a Bloch equation,

$\omega_1$0

with coupled Maxwell–Bloch equations for the field envelopes and coherence waves given in Eqs. (1–2) of the paper (Loranger et al., 2023).

A more speculative active-device direction is the graphene–superconductor hybrid “optical transistor,” where amplification arises from Coulomb-coupled surface plasmons and graphene quantum capacitance. The normalized absorption or gain is

$\omega_1$1

and amplification requires $\omega_1$2 (Villegas et al., 2018). The reported negative-absorption region spans approximately $\omega_1$3 to $\omega_1$4 THz, with doping shifting the gain band upward to $\omega_1$5 THz (Villegas et al., 2018).

3. Modal confinement, phase matching, and overlap engineering

The main performance determinants in hybrid optical/THz links are simultaneous modal confinement, phase matching, and overlap. Because optical and THz wavelengths differ by orders of magnitude, obtaining large nonlinear interaction without prohibitive THz attenuation is a recurring design constraint.

In hydrogen-filled hybrid anti-resonant fibers, the waveguide comprises a hollow-core anti-resonant fiber for the near-infrared pump and first Stokes field, surrounded by a concentric THz waveguide formed by Bragg or photonic-bandgap rings and/or a metal capillary (Loranger et al., 2023). The optical core diameter is $\omega_1$6m; the ideal THz core diameter is $\omega_1$7m and the realistic value is $\omega_1$8m (Loranger et al., 2023). The optical LP$\omega_1$9-like mode at $\omega_2$0m and the first Stokes mode at $\omega_2$1m have confinement loss below $\omega_2$2 dB/m, while the THz TE$\omega_2$3 mode near cutoff at $\omega_2$4 THz has attenuation tunable from $\omega_2$5 dB/m in the ideal case up to approximately $\omega_2$6 dB/m, with a realistic value around $\omega_2$7 dB/m (Loranger et al., 2023).

The phase-matching condition for three-wave mixing in that fiber is

$\omega_2$8

in the ideal case (Loranger et al., 2023). In practice, a small residual $\omega_2$9–$\omega_{\mathrm{THz}}=\omega_1-\omega_2$0 is imposed to avoid coherent gain suppression (Loranger et al., 2023). The spatial overlap integrals are

$\omega_{\mathrm{THz}}=\omega_1-\omega_2$1

with ideal values $\omega_{\mathrm{THz}}=\omega_1-\omega_2$2, $\omega_{\mathrm{THz}}=\omega_1-\omega_2$3, and realistic values $\omega_{\mathrm{THz}}=\omega_1-\omega_2$4, $\omega_{\mathrm{THz}}=\omega_1-\omega_2$5 (Loranger et al., 2023).

In the monolithic lithium-niobate photonic chip, collinear guided-wave phase matching requires

$\omega_{\mathrm{THz}}=\omega_1-\omega_2$6

with a slow-wave coplanar GSG transmission line engineered to yield $\omega_{\mathrm{THz}}=\omega_1-\omega_2$7 over 220–500 GHz (Zhang et al., 2024). The optical rib waveguide is a 250 nm etched rib on 500 nm LN with top width $\omega_{\mathrm{THz}}=\omega_1-\omega_2$8m, while the THz electrode is a coplanar GSG slow-wave transmission line with signal-line width $\omega_{\mathrm{THz}}=\omega_1-\omega_2$9m, gap $_3$0m, and periodic T-shaped capacitive loads (Zhang et al., 2024). The nonlinear overlap factor is

$_3$1

with finite-element simulations yielding $_3$2 (Zhang et al., 2024).

In the photonics-integrated terahertz transmission-line platform, the optical rib waveguide has top width $_3$3–$_3$4m, mode area $_3$5, and group index $_3$6, while the THz stripline consists of two parallel gold strips of width $_3$7m separated by a center-to-center gap $_3$8m (Lampert et al., 2024). The effective THz mode area is $_3$9, and the effective THz index $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$0 is tuned to approximately $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$1 over $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$2–$m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$3 THz (Lampert et al., 2024). Phase matching is expressed as

$m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$4

with coherence length

$m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$5

The reported values are $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$6 mm at 1 THz and $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$7 mm at 4 THz (Lampert et al., 2024).

These examples show that the hybrid-link problem is fundamentally a multi-scale mode-engineering problem. The optical mode must remain low loss and compact, the THz mode must be confined without excessive ohmic, dielectric, or radiative loss, and the two must satisfy a velocity- or momentum-matching condition over the desired bandwidth.

4. Representative platforms and quantitative performance

The current literature contains several experimentally realized or numerically validated platform families with markedly different operating regimes.

Platform	Primary function	Representative performance
Hydrogen-filled hybrid anti-resonant fiber	Narrowband THz generation by stimulated Raman scattering and molecular modulation	Peak quantum efficiency up to $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$8 under ideal conditions; $m(\omega)=\pi V_{\mathrm{THz}}(\omega)/V_\pi$9 under current material constraints; tunable from 1–10 THz (Loranger et al., 2023)
Monolithic thin-film LN chip on quartz	THz-optic modulation and continuous THz-wave generation	$<\pm 3$0 V; $<\pm 3$1 V; 3 dB bandwidth 145 GHz; 6 dB bandwidth 310 GHz; generation efficiency $<\pm 3$2 at 500 GHz (Zhang et al., 2024)
Photonics-integrated THz transmission lines on TFLN	Broadband THz emission via optical rectification	10 dB bandwidth $<\pm 3$3 THz, flat from 200 GHz to 3.5 THz; dynamic range $<\pm 3$4 dB; measured field efficiency $<\pm 3$5 (Lampert et al., 2024)
Plasmonic POH-MZM radio-over-fiber link	THz-to-optical analog transport	Flat response from 75 MHz to 500 GHz; ripple $<\pm 3$6 dB; IIP$<\pm 3$7 dBm; RoF ripple $<\pm 3$8 dB over 220–325 GHz (Burla et al., 2018)
Wireless THz-to-optical conversion with POH MZM	Fiber-integrated wireless link	50 Gbit/s over 16 m at 0.2885 THz; modulator 3 dB bandwidth $<\pm 3$9 THz (Ummethala et al., 2018)
Commercial-component photonics-based THz link	Short-range OOK transmission and video streaming	5.5 Gbps NRZ-OOK at 138 GHz; BER $500$0 at 1 m after threshold optimization; uncompressed HD and 4K streaming demonstrated (Nallappan et al., 2017)

In the hybrid anti-resonant fiber source, numerical modelling used a 3 m fiber, a 3 ns, 0.5 mJ pump at 1189 nm, and yielded a peak quantum efficiency up to approximately $500$1 when coherent gain suppression was avoided by choosing $500$2 (Loranger et al., 2023). When current material constraints were included, especially THz attenuation and reduced overlap, the attainable efficiency relaxed to $500$3 (Loranger et al., 2023). The power efficiency is further reduced by the quantum defect according to

$500$4

(Loranger et al., 2023).

In the monolithic thin-film lithium-niobate chip, the THz-optic modulator had $500$5, corresponding to $500$6 V for $500$7 mm (Zhang et al., 2024). Measured THz values were $500$8 V and $500$9 V (Zhang et al., 2024). The 3 dB bandwidth was 145 GHz and the 6 dB bandwidth was 310 GHz (Zhang et al., 2024). Continuous-wave THz generation efficiencies were $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$0, $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$1, and $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$2, with continuous tuning from 220 to 500 GHz (Zhang et al., 2024). The measured linewidth was limited to about 10–20 MHz by unlocked pump lasers and could be reduced to below 100 kHz by electro-optic or Kerr comb locking (Zhang et al., 2024).

In photonics-integrated terahertz transmission lines, measured spectra were flat from 200 GHz up to 3.5 THz, corresponding to four octaves and a 10 dB bandwidth of approximately 2.5 THz (Lampert et al., 2024). The dynamic range was approximately 50 dB in intensity, the peak-to-peak field at the detector was approximately 57 V/m, and the actual on-chip field was estimated as $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$3–$$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$4 V/m once geometry and Fresnel losses were inverted (Lampert et al., 2024). Field build-up scaled proportionally to $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$5 in the phase-matched case (Lampert et al., 2024).

For analog radio-over-fiber using the 500 GHz plasmonic Mach–Zehnder modulator, $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$6 dB from 75 MHz to 500 GHz (Burla et al., 2018). The insertion loss was approximately 22.2 dB, the optical 3 dB bandwidth exceeded 100 nm in the C-band, and the modulator exhibited IIP$$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$7 dBm (Burla et al., 2018). In the sub-THz radio-over-fiber demonstration, the final link had ripple $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$8 dB over 220–325 GHz (Burla et al., 2018).

For direct wireless THz-to-optical conversion, the hybrid link at 0.2885 THz supported 15–25 GBd QPSK, corresponding to 36–50 Gbit/s over 16 m (Ummethala et al., 2018). At 15 GBd, the BER was below $$P^{(2)}(\omega_{\mathrm{THz}})=\epsilon_0\sum_{i,j,k}\chi^{(2)}_{ijk}(\omega_{\mathrm{THz}};\omega_1,-\omega_2)E_j(\omega_1)E_k^*(\omega_2),$$9; at 25 GBd, the BER was approximately $_{11}$00 (Ummethala et al., 2018). The net THz power at the plasmonic modulator input reached up to $_{11}$01 dBm, sufficient to achieve BER below the 7% FEC threshold (Ummethala et al., 2018).

In the 138 GHz system assembled from commercial components, the THz power at the photomixer output was approximately 0.1 $_{11}$02W, the free-space loss over 1 m was approximately 75.2 dB, and the link reached BER below $_{11}$03 at 1 m after threshold optimization (Nallappan et al., 2017). It also supported uncompressed HD 1080p60 and 4K2160p30 video streaming, with 4K error-free at distances up to 0.3 m (Nallappan et al., 2017).

5. End-to-end link architectures and network-level hybridization

At the system level, hybrid optical/THz links are often organized as concatenated optical-fiber, photonic-chip, wireless-THz, and receiver-processing segments. The details vary between analog fronthaul, digital wireless-fiber convergence, and backhaul networks.

A canonical radio-over-fiber chain uses THz electrical reception at a remote antenna unit, direct optical modulation, fiber transport, and photonic or optoelectronic recovery at a central office (Burla et al., 2018). The reported block diagram is TLS$_{11}$04–TLS$_{11}$05 $_{11}$06 PMC $_{11}$07 UTC-PD$_{11}$08 $_{11}$09 GSG probe $_{11}$10 POH-MZM $_{11}$11 SMF $_{11}$12 EDFA $_{11}$13 UTC-PD$_{11}$14 $_{11}$15 mm-wave ESA (Burla et al., 2018). In a simple link-budget example with $_{11}$16 dBm, insertion loss 22 dB, $_{11}$17 V, responsivity 0.3 A/W, and EDFA gain 15 dB, the RF output was estimated as approximately $_{11}$18 dBm; the resulting SNR per 1 Hz was 61 dB·Hz, and the SFDR was approximately 69 dB·Hz$_{11}$19 (Burla et al., 2018).

A direct wireless-to-fiber scheme instead performs terahertz-to-optical conversion at the wireless receiver. In the demonstrated architecture, the incoming THz signal is amplified and applied to a plasmonic-organic-hybrid Mach–Zehnder modulator, which maps the THz QPSK signal onto an optical carrier; a narrowband optical filter then selects one sideband, yielding a standard optical QPSK signal for intradyne detection (Ummethala et al., 2018). A plausible implication is that such architectures relocate high-speed digital signal processing away from the antenna site and into the optical network, reducing the electronic burden at distributed wireless endpoints.

The all-commercial 138 GHz link shows an alternative low-complexity architecture in which a LiNbO$_{11}$20 Mach–Zehnder modulator intensity-modulates one optical tone, a UTC photomixer generates the THz carrier, a zero-bias Schottky detector recovers the baseband, and conventional RF amplification and reconversion handle display or recording interfaces (Nallappan et al., 2017). This architecture favors simplicity over spectral efficiency.

Monolithic integrated photonic chips provide a further degree of integration. In the thin-film lithium-niobate platform for efficient THz-optic modulation and THz generation, a notional transmitter chain is explicitly described as optical pump sources $_{11}$21 on-chip modulator $_{11}$22 on-chip THz generator $_{11}$23 THz antenna $_{11}$24 free-space link (Zhang et al., 2024). A link-budget example gives $_{11}$25 mW, $_{11}$26 dBm, $_{11}$27 dBi, and free-space loss at 300 GHz over 1 m of about 60 dB, yielding a received power of about 1 nW, stated as sufficient for a low-noise THz receiver (Zhang et al., 2024).

At the network level, hybridization is often used to mitigate propagation fragility. In indoor THz/VLC communication with sensing, a THz$_{11}$28-AP senses users and computes detection metrics such as $_{11}$29, $_{11}$30, and $_{11}$31; users satisfying a detection threshold are assigned to a THz communication AP, while the others are assigned to the VLC communication AP with the highest SNR (Prabhakar et al., 22 May 2026). Under THz sensing, most users are connected to the THz communication AP in the absence of blockages, whereas in the presence of blockages the majority are served by VLC communication APs (Prabhakar et al., 22 May 2026).

In HAP-aided and terrestrial backhaul networks, FSO/THz hybridization is performed at the feeder-link level. In the secrecy-enhancement scheme, the ground station computes secrecy capacities for both FSO and THz to each HAP and uses FSO by default, switching to THz when FSO cannot meet the secrecy-rate target (Illi et al., 2024). In multi-hop and mesh backhaul, the hybrid THz/FSO link may be used with switch-and-select or combining, and the end-to-end DF outage occurs if any hop is in outage (Singya et al., 2023). In a related dual-hop model, the AP selects between FSO and THz branches using hard or soft switching before forwarding over an mmWave access link (Singya et al., 2022).

6. Performance limits, impairments, and trade-offs

The dominant impairments differ across implementations, but they fall into a recurring set: THz propagation loss, molecular absorption, dielectric and conductor loss in guided THz structures, mode mismatch, coherent gain suppression, optical insertion loss, pointing or alignment error, and blockage.

In guided and on-chip systems, THz loss is often the principal practical constraint. In the hydrogen-filled hybrid anti-resonant fiber, the main loss mechanisms are THz absorption in the polymer cladding and scattering, represented through $_{11}$32 in the propagation equations (Loranger et al., 2023). This is the main reason the predicted ideal quantum efficiency of approximately 60% relaxes to approximately 0.2% under current material constraints (Loranger et al., 2023). In photonics-integrated THz transmission lines, LN bulk absorption at 1 THz is approximately 10 cm$_{11}$33, corresponding to about 0.5 dB/mm; gold ohmic loss contributes approximately 1 dB/mm at 1 THz; radiative and substrate leakage add approximately 0.5–2 dB/mm (Lampert et al., 2024). Total loss rises rapidly above approximately 2 THz (Lampert et al., 2024).

In electro-optic modulators, insertion loss and RF loss constrain link efficiency and noise. The plasmonic Mach–Zehnder modulator has total insertion loss of approximately 22.2 dB, dominated by grating couplers, photonic–plasmonic tapers, and plasmonic-slot loss (Burla et al., 2018). In the terahertz-to-optical conversion experiment, the total optical insertion loss from the receiver to fiber was approximately 29 dB (Ummethala et al., 2018). In the monolithic lithium-niobate chip, RF loss was approximately 14 dB/cm at 300 GHz and approximately 2 dB/mm at 500 GHz (Zhang et al., 2024).

For free-space wireless THz segments, path loss and alignment dominate. At 0.2885 THz over 16 m, the free-space path loss is approximately 105 dB, partially offset by antenna-plus-lens directivities of approximately 40 dBi each and about 40 dB of THz amplifier gain (Ummethala et al., 2018). At 138 GHz over 1 m, the free-space loss is approximately 75.2 dB (Nallappan et al., 2017). In FSO/THz backhaul models, FSO suffers from atmospheric turbulence and pointing error, while THz suffers from high path loss and misalignment error; both also incur weather-dependent attenuation (Singya et al., 2022, Singya et al., 2023).

In indoor communication and sensing, blockage is a defining issue. The THz/VLC study models human blockage using a Matérn hard-core process and shows that as blocker density increases, both detection probability and sensing coverage probability degrade, with more users offloaded to VLC (Prabhakar et al., 22 May 2026). With blockages at density $_{11}$34, average spectral efficiency drops from approximately 8.3 bps/Hz to approximately 5.2 bps/Hz, and average energy efficiency drops from approximately 4.7 bps/J/Hz to approximately 3.8 bps/J/Hz (Prabhakar et al., 22 May 2026).

A common misconception is that hybridization automatically implies simultaneous use of optical and THz channels for throughput maximization. In several cited systems, hybridization instead denotes fallback or selection. The HAP feeder link uses FSO by default and switches to THz only when the secrecy-rate target cannot be met with FSO (Illi et al., 2024). The AP in the hybrid FSO/THz backhaul selects the branch through hard or soft switching, with soft switching intended primarily to minimize back-and-forth toggling rather than to increase spectral efficiency by coherent combination (Singya et al., 2022). Conversely, other systems do employ simultaneous or co-designed confinement, as in the anti-resonant fiber and the integrated transmission-line platforms (Loranger et al., 2023, Lampert et al., 2024).

7. Research directions and comparative outlook

Current research directions separate into three broad trajectories: higher integration density, better loss management and phase matching, and broader network-level adaptation.

In integrated photonics, thin-film lithium niobate is emerging as a central platform because it supports both efficient electro-optic modulation and optical THz generation. The monolithic chip study identifies several future improvements: integrated high-$_{11}$35 optical resonators that could raise $_{11}$36 toward $_{11}$37, monolithic on-chip Mach–Zehnder modulation for full photonic integration, laser frequency locking for THz linewidth below 100 kHz, and inverse-taper fiber couplers for insertion loss below 1 dB (Zhang et al., 2024). The transmission-line platform similarly highlights compatibility with integrating modulators, EO comb sources, and femtosecond pulse generators on the same TFLN chip (Lampert et al., 2024).

In waveguide and fiber sources, the central challenge remains THz attenuation. The hybrid anti-resonant-fiber scheme is in principle power and energy scalable, tunable from 1 to 10 THz without spectral gaps, and complementary to mature technologies such as quantum cascade lasers (Loranger et al., 2023). Yet its practical efficiency is currently limited by polymer and metal cladding loss (Loranger et al., 2023). This suggests that material advances in low-loss THz dielectrics and improved concentric waveguide geometries would have immediate leverage on source viability.

In wireless-optical convergence, photonic receivers and transceivers are moving toward tighter phase coherence and lower electronic overhead. The all-photonic W-band receiver based on THz-to-optical carrier conversion with soliton microcomb dual carriers reported a 106-GHz, 2.97-Gb/s OOK link with error-free transmission, $_{11}$38, and BER $_{11}$39, substantially outperforming a single-wavelength configuration (Matsumura et al., 26 Oct 2025). Comparative modeling indicated scalability of the transmission distance beyond 100 m (Matsumura et al., 26 Oct 2025). Although this is a W-band rather than high-THz demonstration, it fits the same architectural trend: replacing electronic local oscillators and mixers with photonic downconversion referenced to integrated combs.

In networked backhaul and feeder links, hybrid FSO/THz strategies continue to be motivated by complementary propagation physics. In HAP-assisted secrecy, the hybrid FSO/THz scheme reduces secrecy outage probability by four orders of magnitude using four HAPs relative to a single-HAP benchmark and manifests a 5 dB secrecy gain relative to a THz-feeder benchmark (Illi et al., 2024). In multi-hop and mesh THz/FSO backhaul, hybrid implementation improves network reliability significantly under different switching and combining methods (Singya et al., 2023). In terrestrial hybrid FSO/THz backhaul with mmWave access, the hybrid backhaul gains 4–10 dB over the best single link at outage probability $_{11}$40, while soft switching provides near-optimal performance with limited switching overhead (Singya et al., 2022).

Taken together, these results indicate that hybrid optical/THz links are not converging toward a single dominant implementation. Instead, the field is differentiating into at least four stable niches: ultra-broadband photonic transduction on integrated lithium-niobate platforms, analog and coherent radio-over-fiber front ends using plasmonic or electro-optic modulators, specialty THz sources based on nonlinear waveguides and fibers, and network-level hybrid infrastructures that combine THz with FSO, VLC, or fiber to manage blockage, weather, secrecy, and coverage. The common enabling themes remain phase matching, confinement, overlap, and loss engineering; the dominant open problems remain practical THz attenuation, packaging, coupling, and system-level robustness.