Agile THz-range spectral multiplication of frequency combs using a multi-wavelength laser

Optical frequency combs (OFCs) are foundational for applications such as spectroscopy, lidar, and optical communications. Multiple photonic platforms can generate combs from the visible to the THz spectral range and, coupled with photonic integration, provide compact, energy-efficient sources. However, a core trade-off persists: broadband comb sources like mode-locked lasers or microresonators offer wide bandwidths but limited tunability—particularly a fixed free-spectral range (FSR), limited control of the spectral envelope and center wavelength—while electro-optic (EO) combs are highly controllable yet restricted in frequency range, struggling to reach mm-wave and THz bands without complex cascading or nonlinear processing. For versatile systems, tunability of four parameters is particularly desirable: (1) FSR (comb line spacing), (2) center wavelength, (3) spectral envelope (to direct power into bands of interest), and (4) comb coherence. Existing approaches such as filtering broad combs waste optical power and offer limited bandwidth and center-frequency control; nonlinear conversion can be power-hungry; and EO approaches typically require complex cascades to expand bandwidth. Optical injection schemes in semiconductor lasers have shown promise for comb processing (e.g., broadening, polarization changes). In this work, the authors demonstrate frequency-agile spectral multiplication of a narrowband EO comb using a semiconductor multi-wavelength laser (MWL). By injecting a narrow comb into a suppressed mode of the MWL and exploiting intrinsic carrier coupling between MWL modes, they multiply the comb around other MWL wavelengths, achieving offsets from tens of GHz up to 1.3 THz while preserving the RF coherence within each sub-comb. Using on-chip, phase-controlled optical feedback enables nanosecond switching among multiplied combs. They further introduce techniques to phase-lock neighboring multiplied sub-combs and corroborate observations with multi-mode rate-equation simulations, highlighting the role of inter-mode coupling. This addresses the controllability–bandwidth trade-off and enables programmable, scalable THz-range comb processing.

Photonic integrated circuit and feedback design: The multi-wavelength laser (MWL) with monolithically integrated feedback cavity was fabricated on the SMART Photonics InP generic foundry platform using only standard building blocks. The laser comprises a two-port multimode interference broadband reflector (BR, 40% reflectivity), a 500 µm semiconductor optical amplifier (SOA), and two sequential distributed Bragg reflectors (DBRs) with lengths L1 = 500 µm and L2 = 250 µm. DBR pitches are 0.236 µm (DBR1) and 0.234 µm (DBR2), yielding ~10 nm (~1.3 THz) spectral separation between the two cavity modes. Total cavity lengths are 1396.26 µm (DBR1-defined cavity) and 1082.43 µm (DBR2-defined cavity). The MWL couples to a short external feedback cavity via an 85/15 MMI splitter (15% of light to feedback). The external cavity includes a 300 µm SOA, an electro-optic phase modulator (EOPM), and a one-port BR (80% reflectivity); total external cavity length is ~2.42 mm. At SOA transparency and ideal components, ~1.8% of light is fed back to the MWL cavity. The EOPM is designed for ~π phase shift at ~12 V (2π phase for a full round-trip). The chip (MWL and optical feedback) is packaged and temperature-stabilized at 22 °C.

Experimental setup and operating conditions: A narrowband EO comb is generated from a tunable ECL (Keysight N7776C) modulated by a LiNbO3 MZM (iXBlue MXAN-LN-40, 40 GHz BW, Vπ = 6 V) driven by an AWG (Keysight M8194A). The polarization is set with a PC and power boosted with an EDFA. Light is coupled to the PIC via a lensed fiber and fiber optic circulator to suppress back-reflections. Outputs are analyzed with a high-resolution OSA (APEX AP2083, down to 5 MHz or 40 fm RBW) and a 42 GHz PD (Thorlabs RXM42AF) connected to an ESA (Keysight MXA-N9021B). Typical MWL bias: threshold ~21 mA; the shared SOA (MWL) operated at I_SOA1 = 30 mA. In a free-running configuration (I_DBR1 = 0 mA, I_DBR2 = 1 mA), the dominant emission is at λ4 = 1547.58 nm (DBR2 cavity), with residual modes at λ1 = 1536.70 nm, λ2 = 1536.92 nm, and λ3 = 1537.13 nm (DBR1 cavity), each suppressed by >30 dB. The injected EO comb typically has 5 tones with 1 GHz FSR, injected around λ3 (a strongly suppressed mode) to achieve partial locking without extinguishing other modes. For representative conditions, detuning Δ = 2.3 GHz between the comb center and λ3, and injection strength K_inj = 9.5 dBm (measured at the lensed-fiber input). Spectral multiplication occurs via carrier population modulation and inter-mode carrier coupling in the MWL, producing multiplied combs around other MWL wavelengths (e.g., λ4) with frequency separations up to 1.3 THz. Phase-controlled weak optical feedback provides agile selection and switching among multiplied combs using the EOPM voltage V_EOPM (with SOA2 current I_SOA2 = 21 mA). Switching voltages produce sequential activation of multiplied combs around λ1, λ4, and λ2, with extinction ratios >30 dB and stable center wavelengths (±100 MHz). Peak multiplied comb power at the fiber output is about −12 to −13 dBm; re-generated comb power around the injected wavelength is ~11 dBm. Comb bandwidths up to ~10 GHz are observed.

Coherence measurements and cascaded phase locking: Coherence within each re-generated or multiplied comb is assessed by optically filtering a sub-comb and detecting on a PD/ESA, revealing sub-Hz (RBW-limited at 1 Hz) RF linewidths at 1 GHz, confirming preservation of injected comb coherence. Inter-comb coherence (e.g., λ2 and λ3) initially shows a broad ~25 GHz beat note with ~38 MHz 3 dB linewidth, indicating lack of phase correlation. Two strategies demonstrate phase locking between sub-combs: (1) adding an extra RF tone to the injected signal via the AWG to inject a strong tone near a neighboring MWL mode (e.g., ftone ~26.1 GHz near λ2) locks the multiplied comb to the re-generated comb, yielding sub-Hz RF linewidths; (2) precisely tuning ftone in the 26.14–26.26 GHz range triggers a second multiplied comb at λ1 that becomes phase-locked with the others (“cascaded” phase locking), verified by sub-Hz linewidths in the RF beating between λ1 and λ2. Outside this range, broadband noise dominates. Cascaded locking to the more widely separated λ4 (1.3 THz offset) was not observed under the present configuration.

Numerical model and simulations: A phenomenological multi-mode rate-equation model (adapted for semiconductor lasers) includes cross-saturation coupling between modes, delayed optical feedback, and modulated optical injection. Using normalized variables (time/frequency scaled to photon lifetime τ), key parameters include linewidth enhancement factor α = 3, carrier lifetime τ = 1000 (normalized), and pump P = 0.5 (~2× threshold). Two-mode cases explore different inter-mode coupling (cross-saturation) strengths: Case A β = 0.95 with g1 = 0.995, g2 = 0.954; Case B β = 0.88 with g1 = 0.995, g2 = 0.827. These choices give ~40 dB suppression of one mode without injection/feedback. The injected comb has five tones; for comparison to experiments, an assumed photon lifetime of 3 ps maps a normalized FSR of 0.019 to 1 GHz. Simulations reproduce comb re-generation and spectral multiplication, showing strong dependence on detuning and injection strength. Spectral multiplication emerges for positive detuning (outside CW-locking range), with bandwidth and power balance tunable by injection parameters; dynamics and broad bandwidths can arise near negative detuning. Decreasing coupling (lower β) shrinks and shifts the locking region toward stronger injection, while preserving the qualitative geography of multiplication and dynamical regions. Results agree with experimental trends and suggest weaker coupling may favor spectral multiplication and wider bandwidth under suitable detuning.

• Demonstrated agile spectral multiplication of a narrowband EO comb using an on-chip InP multi-wavelength laser over frequency offsets from ~26 GHz up to 1.3 THz (≈10 nm around 1550 nm).
• Achieved on-demand switching among multiple multiplied combs (e.g., around λ1, λ2, λ4) solely via phase-controlled weak optical feedback, with extinction ratios >30 dB and nanosecond-scale switching times (<4 ns without RF optimization).
• Preserved the RF coherence of the injected comb within each re-generated and multiplied sub-comb: sub-Hz RF linewidths at 1 GHz (limited by 1 Hz ESA RBW).
• Initially, neighboring sub-combs are not phase correlated; their RF beating (e.g., ~25 GHz) shows ~38 MHz 3 dB linewidth.
• Introduced and validated two phase-locking strategies: (1) injecting an extra tone (e.g., ftone ~26.1 GHz) to phase-lock a neighboring multiplied comb to the re-generated comb; (2) fine-tuning ftone (26.14–26.26 GHz) to trigger cascaded phase locking of an additional multiplied comb. Resulting RF beat notes display <1 Hz linewidth (ESA-limited), confirming strong inter-comb coherence.
• Multiplied comb peak powers measured at the lensed fiber are ~−12 to −13 dBm; re-generated comb around the injected wavelength ~11 dBm; bandwidths up to ~10 GHz; center wavelengths stable within ±100 MHz when switching.
• Numerical multi-mode rate-equation simulations (two-mode and three-mode in SI) qualitatively reproduce re-generation, spectral multiplication, and feedback-induced switching. The cross-saturation coupling parameter β strongly influences the locking range and dynamics: weaker coupling shrinks the CW-locking region and can suppress certain dynamics, while maintaining regions of spectral multiplication on the positive-detuning side.

The work addresses a core limitation in frequency comb technology: the controllability–bandwidth trade-off. By injecting a narrow, highly controllable EO comb into a multi-wavelength semiconductor laser and harnessing inter-mode carrier coupling, the authors multiply the comb spectrum to widely separated on-chip wavelengths, effectively transferring the comb’s controllable properties across GHz–THz offsets. Phase-controlled weak optical feedback enables agile, programmable selection of which wavelengths host multiplied combs and allows nanosecond switching without disrupting the comb characteristics. The approach preserves intra-comb coherence and, with the addition and tuning of a single extra RF tone, can phase-lock neighboring multiplied combs via a cascaded mechanism, creating coherent multi-comb outputs. Simulations corroborate the mechanisms and clarify the role of inter-mode coupling in determining locking regions and dynamical regimes. The results indicate a scalable, integration-friendly route to flexible, power-efficient THz-range comb processing and generation, compatible with generic InP foundry platforms. This is particularly relevant for THz photomixing, agile spectroscopy, flexible-grid communications, and applications requiring programmable FSR, center wavelength, and spectral envelope.

This study demonstrates on-chip, frequency-agile spectral multiplication of narrowband EO combs using a multi-wavelength semiconductor laser with phase-controlled optical feedback. The approach achieves comb multiplication from tens of GHz to 1.3 THz, preserves intra-comb coherence, enables nanosecond switching among target wavelengths with >30 dB extinction, and provides methods for cascaded phase locking between neighboring multiplied combs. Numerical modeling supports the experimental observations and highlights the influence of inter-mode coupling on locking and dynamical behavior. The technique is scalable with the gain bandwidth (potentially several THz on generic InP platforms) and amenable to full photonic integration. Future work should focus on quantifying and engineering inter-mode coupling, extending cascaded phase locking to larger spectral separations (including THz-scale), optimizing RF and packaging for faster, lower-power switching, and co-integrating photomixers to realize compact, programmable THz comb sources for wireless communications and spectroscopy.

Phase correlation between different multiplied sub-combs is not inherent; without additional measures, neighboring sub-combs exhibit broad RF beat notes (~tens of MHz). Cascaded phase locking was demonstrated only for neighboring modes within ~tens of GHz; phase locking to the 1.3 THz-separated mode (λ4) was not observed with the present configuration. The effectiveness of spectral multiplication depends sensitively on injection detuning/strength and MWL gain balance. The inter-mode coupling (cross-saturation) parameter is not experimentally quantified; its precise impact requires further investigation. Reported center-wavelength stability and bandwidth assessments are constrained by instrument resolution and absolute accuracy.