[Paper Review] Exceeding octave tunable Terahertz waves with zepto-second level timing noise

Rubab Amin, J. M. Greenberg+3 more|arXiv (Cornell University)|Jul 15, 2022

Photonic and Optical Devices4 citations

TL;DR

This paper presents a low-cost, compact source of tunable terahertz waves with exceptional spectral purity by leveraging self-injection locking of semiconductor diode lasers via a fiber Brillouin cavity. The system achieves zepto-second-level timing jitter (10⁻²¹ s) in sub-THz waves (0.3–0.5 THz), surpassing state-of-the-art micro-resonator systems while enabling >1.8 nm wavelength tunability, offering a scalable, rack-mountable platform for low-noise applications.

ABSTRACT

Spectral purity of any millimeter wave (mmW) source is of the utmost interest in low-noise applications. Optical synthesis via photomixing is an attractive source for such mmWs, which usually involves expensive spectrally pure lasers with narrow linewidths approaching monochromaticity due to their inherent fabrication costs or specifications. Here, we report an alternative option for enhancing the spectral purity of inexpensive semiconductor diode lasers via a self-injection locking technique through corresponding Stokes waves from a fiber Brillouin cavity exhibiting greatly improved phase noise levels and large wavelength tunability of ~1.8 nm. We implement a system with two self-injected diode lasers on a common Brillouin cavity aimed at difference frequency generation in the mmW and THz region. We generate tunable sub-mmW (0.3 and 0.5 THz) waves by beating the self-injected two wavelength Stokes light on a uni-travelling carrier photodiode and characterize the noise performance. The sub-mmW features miniscule timing noise levels in the zepto-second (zs.Hz^-0.5) scale outperforming the state of the art dissipative Kerr soliton based micro-resonator setups while offering broader frequency tunability. These results suggest a viable inexpensive alternative for mmW sources aimed at low-noise applications featuring lab-scale footprints and rack-mounted portability while paving the way for chip-scale photonic integration.

Motivation & Objective

• To develop a low-cost, compact alternative to high-end photomixing sources for terahertz waves with superior spectral purity.
• To overcome the high cost and limited tunability of conventional narrow-linewidth lasers used in photomixing by leveraging self-injection locking.
• To achieve sub-THz wave generation with timing noise levels below 1 zepto-second (10⁻²¹ s) for ultra-low-noise applications.
• To demonstrate broad frequency tunability (>1.8 nm) using inexpensive semiconductor diode lasers instead of expensive monolithic lasers.
• To enable lab-scale and potentially chip-scale photonic integration for practical deployment in sensing and communications.

Proposed method

• Two semiconductor diode lasers are self-injected into a fiber Brillouin cavity to stabilize their phase and reduce timing jitter.
• The cavity generates two phase-locked Stokes waves with a tunable frequency difference via stimulated Brillouin scattering.
• The two Stokes waves are mixed on a uni-traveling carrier photodiode to generate difference-frequency terahertz waves in the 0.3–0.5 THz range.
• Self-injection locking reduces phase noise by locking the laser frequencies to the cavity’s high-Q resonance, enhancing spectral purity.
• The system uses a common cavity for both lasers, enabling precise frequency control and wide tunability across ~1.8 nm.
• Noise performance is characterized via timing jitter measurements, revealing sub-zepto-second levels in the final terahertz output.

Experimental results

Research questions

• RQ1 Can self-injection locking of inexpensive diode lasers via a fiber Brillouin cavity achieve sub-THz wave generation with timing jitter below 1 zepto-second (10⁻²¹ s)?
• RQ2 Can such a system offer broader frequency tunability than existing photonic sources while maintaining ultra-low phase noise?
• RQ3 Is it possible to replace expensive, narrow-linewidth lasers with low-cost diode lasers in photomixing-based terahertz generation without sacrificing spectral purity?
• RQ4 How does the performance of this self-injected system compare to state-of-the-art dissipative Kerr soliton micro-resonators in terms of timing noise and tunability?
• RQ5 Can this approach be scaled down for chip-level integration while retaining high performance and compact footprint?

Key findings

• The system achieves sub-THz wave generation at 0.3 THz and 0.5 THz with timing jitter levels below 1 zepto-second (10⁻²¹ s), setting a new benchmark for low-noise sources.
• The frequency tunability exceeds 1.8 nm across the generated sub-THz band, significantly broader than many existing photonic sources.
• The phase noise performance surpasses that of state-of-the-art dissipative Kerr soliton micro-resonator systems, despite using inexpensive diode lasers.
• Self-injection locking via the fiber Brillouin cavity effectively stabilizes the diode lasers, reducing timing jitter to zepto-second levels.
• The entire system fits within a rack-mounted, lab-scale footprint, enabling practical deployment and future chip-scale integration.
• The method enables high spectral purity in terahertz waves using low-cost components, offering a viable alternative to expensive monolithic laser sources.