Letting light take extra laps: mode circulation unlocks a broadband, energy-efficient waveguide electro-optic comb

A team of researchers has demonstrated a broadband, energy-efficient electro-optic (E-O) frequency comb on a thin-film lithium niobate chip, using a clever "mode circulation" scheme that sends light through the same modulator eight times. The work is published in PhotoniX.

Optical frequency combs—light sources that emit thousands of evenly spaced, mutually coherent spectral lines—act as a ruler that bridges optical and microwave frequencies. They underpin high-speed optical communications, precision spectroscopy, microwave photonics, and timekeeping. On the thin-film lithium niobate (LNOI) platform, E-O combs built from traveling-wave waveguide modulators offer major practical advantages over microresonator-based combs: freely tunable pump wavelength and repetition rate, near-100% optical conversion efficiency, high per-line power, and flat spectral envelopes.

Their Achilles' heel, however, is bandwidth. A broad comb requires a large modulation index, which scales with the length of the modulation region—but simply lengthening the electrode increases microwave loss and shrinks the E-O bandwidth. A "multi-pass" strategy, in which light traverses the same modulation region several times, sidesteps this trade-off, yet scaling to many passes has been blocked by mode hybridization in anisotropic lithium niobate, by the difficulty of matching phase and group velocity across multiple waveguide modes, and by accumulated optical loss.

The researchers resolved these challenges to build an eight-pass comb. The key idea is to let the light circulate through the modulation region as four different waveguide modes—TE0, TE1, TE2, and TE3—so that the effective modulation length is multiplied without physically extending the electrode. Four four-channel mode multiplexers are arranged to form seven optical loops and two multimode phase modulators, routing the light through the modulation region a total of eight times.

Three innovations make this possible. First, the team designed a high-performance mode multiplexer oriented along the crystal's Z-propagation axis, which strongly suppresses the mode hybridization that plagues anisotropic LNOI waveguides—allowing the scheme to be scaled to four mode channels. Twelve cascaded multiplexers showed less than 2 dB of total insertion loss and crosstalk below −16 dB. Second, by co-optimizing the traveling-wave GSG electrodes, multimode waveguides, and optical delay lines, the design maintains high, phase- and velocity-matched modulation efficiency for all four modes simultaneously. Third, the round-trip time of each of the seven optical loops was precisely matched to an integer multiple of the RF drive period, so that the phase modulation accumulates coherently on every pass.

Driven by a 25 GHz RF signal at about 37 dBm, the fabricated comb generated 128 comb lines spanning 25.6 nm—corresponding to a modulation-index enhancement factor of roughly 7.5 (approaching the theoretical limit of 8), an effective half-wave voltage of just 1.24 V, and a total on-chip loss of only 7 dB, despite a total optical path exceeding 18.5 cm. The comb also proved highly reconfigurable, operating across pump wavelengths from 1510 to 1630 nm and at multiple RF frequencies.

The researchers further showcased the comb in a demanding application: high-precision optical spectrum measurement. Exploiting a multi-heterodyne approach with two slightly detuned combs, they measured wavelengths across 1530–1570 nm with an average deviation below 0.15 nm and resolved signal spacings as small as 2 MHz (about 0.016 pm)—roughly three orders of magnitude finer than typical on-chip dispersive spectrometers. Finally, using a time-to-frequency mapping method, they generated a broadband flat-top comb with 41 lines within a 4 dB flatness.

Combining broad bandwidth, low drive voltage, high efficiency, and reconfigurability, the mode-circulating E-O comb offers a compelling new route for integrated comb sources. The authors note that further reducing insertion loss and raising the repetition rate could make the approach valuable for optical communications, LiDAR, and precision spectroscopy.