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

Optical frequency combs (OFCs) are crucial for various applications, including spectroscopy, lidar, and optical communications. On-chip OFC sources offer advantages in compactness, energy efficiency, and cost-effectiveness. However, existing sources face a fundamental limitation: a trade-off between bandwidth and controllability. Broadband combs generated in microresonators lack control over free-spectral range (FSR) and spectral envelope, while combs generated using electro-optic modulators offer precise control but limited bandwidth. This research aims to address this limitation by developing a method for agile spectral multiplication of narrowband combs. The tunability of four key OFC parameters is highly desirable: FSR (for flexible grid transmission networks), center wavelength (for spectroscopy), frequency envelope (for THz generation), and coherence (for lidar and ranging). Current methods, such as filtering broadband combs or using electro-optic modulators, have limitations in bandwidth, tunability, and energy efficiency. Optical injection schemes in semiconductor lasers offer a promising alternative for comb processing, enabling comb broadening and polarization control. This work explores a novel approach using optical injection to achieve agile spectral multiplication.

The paper reviews existing OFC generation techniques, highlighting the trade-off between bandwidth and controllability. It discusses the limitations of broadband combs generated using microresonators and narrowband combs generated using electro-optic modulators. The authors also mention previous research on optical injection schemes in semiconductor lasers, demonstrating their potential for comb processing. The review emphasizes the need for a versatile OFC source with tunable FSR, center wavelength, envelope, and coherence, capable of meeting the requirements of diverse applications.

The proposed method uses a multi-wavelength laser (MWL) and optical injection of a narrowband frequency comb. The MWL, a dual-cavity laser with two detuned distributed Bragg reflectors (DBRs), a shared semiconductor optical amplifier (SOA), and a broadband reflector (BR), provides multiple modes. A narrowband comb is injected around one of the suppressed modes of the MWL. Due to mode coupling within the MWL, the injected comb is spectrally multiplied around the different wavelengths emitted by the MWL, modifying the output comb's envelope. A control mechanism, using weak optical feedback from a monolithically integrated feedback cavity (with an SOA and an electro-optic phase modulator (EOPM)), allows for nanosecond-scale switching between different wavelengths and thus different multiplied combs. The authors use a multi-mode rate equation model to simulate the system's dynamics, focusing on the impact of mode coupling on comb generation. Experimental results are obtained using an external cavity laser (ECL), a Mach-Zehnder modulator (MZM), an arbitrary waveform generator (AWG), polarization controller (PC), an erbium-doped fiber amplifier (EDFA), an optical spectrum analyzer (OSA), and an electrical spectrum analyzer (ESA). The injection strength and detuning are carefully controlled to achieve spectral multiplication. Phase-locking between multiplied combs is achieved either by adding an extra tone to the injected signal or by carefully adjusting the tone spacing of the injected comb.

The research demonstrates THz-scalable spectral multiplication of a frequency comb using an on-chip MWL. Comb multiplication is achieved from tens of GHz up to 1.3 THz, preserving the RF coherence of the injected comb. The system enables nanosecond switching between different multiplied combs with over 30 dB side-mode suppression ratio using phase-controlled optical feedback. The authors show that phase locking between multiplied combs can be achieved by adding an extra tone to the injected signal or adjusting the tone spacing of the injected comb, resulting in cascaded phase locking of neighboring combs. Numerical simulations using a multi-mode rate equation model confirm the experimental results and highlight the crucial role of mode coupling in enabling spectral multiplication. The experimental setup achieves sequential emission of three multiplied combs in the mm-wave and THz range (around 26 GHz, 54 GHz, and 1.3 THz), with an extinction ratio above 30 dB. Switching times below 4 ns are achieved. The RF linewidth of the original EO-comb is preserved in the multiplied combs, although phase correlation between different sub-combs is limited unless measures like adding an extra tone are implemented. The simulations explore the parameter space for a two-mode model, showing that the locking region shrinks when coupling is decreased, but spectral multiplication is still possible. The study shows that weaker coupling between wavelengths might be desirable for spectral multiplication.

The findings demonstrate a novel approach for agile and efficient spectral multiplication of OFCs, overcoming the limitations of existing methods. The ability to achieve THz-range spectral multiplication with nanosecond switching speed opens up exciting possibilities for various applications. The scalability of the approach, limited only by the gain medium bandwidth, suggests potential for even broader THz coverage. The use of weak optical feedback for control minimizes power requirements compared to other methods. Although phase-locking between sub-combs isn't inherent, techniques like adding an extra tone demonstrate effective cascaded phase locking. The agreement between experimental and simulation results validates the underlying mechanism and provides insights into the crucial role of mode coupling.

This work presents a significant advance in on-chip OFC generation by demonstrating agile THz-range spectral multiplication using a multi-wavelength laser and optical injection. The method offers high flexibility, scalability, and efficiency, enabling compact and low-cost THz comb sources. Future research could focus on extending cascaded phase locking to larger frequency differences and integrating the system onto a single photonic chip for improved performance and compactness. The technology holds immense potential for applications such as high-speed wireless communication and biomolecular spectroscopy.

The current study primarily focuses on a dual-cavity MWL configuration. Although the model extends to multiple modes, experimental validation for larger numbers of modes and wider THz ranges needs further investigation. While cascaded phase locking is demonstrated, optimization for even larger frequency differences remains a challenge. The experimental setup uses a relatively simple feedback mechanism; more sophisticated control strategies might enhance performance. The impact of mode coupling needs more detailed experimental quantification.