[Paper Review] Photonic chip-based continuous-travelling-wave parametric amplifier

Johann Riemensberger, Junqiu Liu+4 more|arXiv (Cornell University)|Jul 20, 2021

Advanced Fiber Laser Technologies6 references4 citations

TL;DR

This paper demonstrates the first chip-based continuous-travelling-wave parametric amplifier using a 2-meter-long, dispersion-engineered silicon nitride waveguide that achieves a net optical gain of 12 dB in the C-band, overcoming propagation and coupling losses. The device leverages ultra-low propagation loss (0.15 dB/m) and high Kerr nonlinearity in Si3N4 to enable quantum-limited, broadband, and unidirectional amplification with potential for integration with semiconductor lasers.

ABSTRACT

The ability to amplify optical signals is of pivotal importance across science and technology. The development of optical amplifiers has revolutionized optical communications, which are today pervasively used in virtually all sensing and communication applications of coherent laser sources. In the telecommunication bands, optical amplifiers typically utilize gain media based on III-V semiconductors or rare-earth-doped fibers. Another way to amplify optical signals is to utilize the Kerr nonlinearity of optical fibers or waveguides via parametric processes. Such parametric amplifiers of travelling continuous wave have been originally developed in the microwave domain, and enable quantum-limited signal amplification with high peak gain, broadband gain spectrum tailored via dispersion control, and ability to enable phase sensitive amplification. Despite these advantages, optical amplifiers based on parametric gain have proven impractical in silica fibers due to the low Kerr nonlinearity. Recent advances in photonic integrated circuits have revived interest in parametric amplifiers due to the significantly increased nonlinearity in various integrated platforms. Yet, despite major progress, continuous-wave-pumped parametric amplifiers built on photonic chips have to date remained out of reach. Here we demonstrate a chip-based travelling-wave optical parametric amplifier with net signal gain in the continuous-wave regime. Using ultralow-loss, dispersion-engineered, meter-long, silicon nitride photonic integrated circuits that are tightly coiled on a photonic chip, we achieve a continuous parametric gain of 12 dB that exceeds both the on-chip optical propagation loss and fiber-chip-fiber coupling losses in the optical C-band.

Motivation & Objective

• To develop a photonic chip-based optical parametric amplifier that operates in the continuous-wave regime, enabling net signal gain without pulsed pumping.
• To overcome the limitations of fiber-based parametric amplifiers, which suffer from low Kerr nonlinearity and high loss, by using integrated Si3N4 waveguides with engineered dispersion.
• To achieve net gain that exceeds both on-chip propagation loss and fiber-chip-fiber coupling losses in the telecommunications C-band.
• To demonstrate a scalable, compact, and design-tailorable platform for optical amplification across visible to mid-infrared wavelengths, outside conventional rare-earth amplifier bands.
• To enable future integration with semiconductor lasers and pave the way for quantum-limited, high-gain, broadband amplifiers in optical communication and quantum information systems.

Proposed method

• The amplifier is realized using a 2-meter-long, dispersion-engineered, ultra-low-loss Si3N4 photonic integrated circuit fabricated via deep-ultraviolet (DUV) stepper lithography and a photonic Damascene process.
• Waveguide sidewall roughness is reduced to sub-nanometer levels (0.3 nm RMS) through preform reflow and chemical-mechanical planarization (CMP) to minimize scattering loss.
• The device uses a continuous-wave pump at 1550 nm to drive the parametric process via the χ(3) Kerr nonlinearity in the Si3N4 waveguide, enabling four-wave mixing (FWM) for signal amplification.
• Dispersion engineering eliminates second- and fourth-order dispersion terms, enabling broad gain bandwidth and phase-matching for efficient FWM.
• The amplifier is operated in a tightly coiled 5×5 mm² footprint, with pump and signal co-propagating in the same direction, enabling traveling-wave operation.
• Numerical simulations based on the generalized nonlinear Schrödinger equation (GNLSE) are used to model gain, loss, and nonlinear interaction, validating experimental results.

Experimental results

Research questions

• RQ1 Can a chip-based parametric amplifier achieve net continuous-wave gain in the absence of pulsed pumping, overcoming losses in integrated photonic circuits?
• RQ2 What level of optical loss and nonlinearity is required in a Si3N4 waveguide to enable net parametric gain in the C-band?
• RQ3 To what extent can dispersion engineering in Si3N4 waveguides enable broadband, phase-matched four-wave mixing for efficient amplification?
• RQ4 Can the performance of integrated parametric amplifiers surpass that of fiber-based counterparts in terms of footprint, design flexibility, and scalability?
• RQ5 What is the potential for integrating such amplifiers with on-chip semiconductor lasers for future optical communication and quantum systems?

Key findings

• The device achieves a net continuous-travelling-wave parametric gain of 12 dB in the C-band, exceeding both on-chip propagation loss and fiber-chip-fiber coupling losses.
• The ultra-low propagation loss of 0.15 dB/m in stoichiometric Si3N4 waveguides enables theoretical parametric gain exceeding 70 dB with only 500 mW of pump power.
• The amplifier operates with a quantum-limited noise figure, enabling phase-sensitive and noiseless amplification, crucial for quantum applications.
• The device demonstrates broadband gain through dispersion engineering that suppresses second- and fourth-order dispersion, enabling efficient four-wave mixing.
• The integration of high nonlinearity and low loss in Si3N4 enables a path toward compact, singly-resonant, and pump-enhanced TWOPAs with reduced signal-signal FWM.
• The results demonstrate the feasibility of scaling chip-based parametric amplifiers to high-gain, time- and spectrum-continuous operation, suitable for future optical communication and quantum information systems.