[Paper Review] An ultra-broadband photonic-chip-based traveling-wave parametric amplifier

Nikolai Kuznetsov, Alberto Nardi+5 more|arXiv (Cornell University)|Apr 12, 2024

Advanced Photonic Communication Systems5 citations

TL;DR

The authors demonstrate a high-gain, ultra-broadband traveling-wave optical parametric amplifier on a GaP-on-insulator photonic chip, achieving up to 35 dB gain and a fiber-to-fiber net gain exceeding 10 dB over ~140 nm around 1550 nm.

ABSTRACT

Optical amplification, crucial for modern communication and data center interconnects, primarily relies on erbium-doped fiber amplifiers (EDFAs) to enhance signals without distortion. While EDFAs were historically decisive for the introduction of dense wavelength-division multiplexing, they only cover a portion of the low-loss spectrum of optical fibers. Pioneering work on optical traveling-wave parametric amplifiers (TWPAs) utilizing intrinsic third-order optical nonlinearity has led to demonstrations of increased channel capacity and performance. TWPAs are unidirectional, offer high gain, and can reach the 3-dB quantum limit for phase-preserving amplifiers. Despite the use of highly nonlinear fibers or bulk waveguides, their power requirements and technical complexity have impeded adoption. In contrast, TWPAs based on photonic integrated circuits (PICs) offer the advantages of substantially increased mode confinement and optical nonlinearity but have been limited in bandwidth because of the trade-off with maintaining low propagation loss. We overcome this challenge by using low-loss gallium phosphide-on-silicon dioxide PICs and attain up to 35~dB of parametric gain with waveguides only a few centimeters long in a compact footprint of 0.25 square millimeters. Fiber-to-fiber net gain exceeding 10 dB across a bandwidth of approximately 140 nm is achieved, surpassing the gain window of a standard C-band EDFA. We furthermore demonstrate the capability to handle weak signals; input powers can range over six orders of magnitude while maintaining a low noise figure. We exploit these performance characteristics to amplify both optical frequency combs and coherent communication signals. This marks the first ultra-broadband, high-gain, continuous-wave amplification in a PIC, opening up new capabilities for next-generation optical communication, metrology, and sensing.

Motivation & Objective

• Motivate the need for broadband, high-gain optical amplification beyond the erbium window.
• Show that photonic integrated circuits with strong Kerr nonlinearity can deliver net parametric gain.
• Demonstrate high gain, broad bandwidth, and low noise in a compact GaP-based TWPA.
• Validate the amplification of ultra-weak signals and frequency combs on a photonic chip.

Proposed method

• Use thin-film GaP on SiO2 to fabricate dispersion-engineered spiral waveguides for degenerate four-wave mixing.
• Operate the TWPA with a single pump near 1550 nm to achieve parametric gain via the optical Kerr effect.
• Model phase matching with κ = Δβ + 2γP_p and optimize dispersion terms β2 and β4 to maximize bandwidth.
• Estimate the on-chip nonlinear parameter γ and the effective length L_eff to compute peak gain G_S = 1 + [sinh(-ΔβL_eff/2)]^2.
• Measure CW amplification spectra, idler generation, and off-chip net gain by accounting for insertion losses.
• Demonstrate amplification of optical frequency combs and coherent data streams to highlight practical applications.

Experimental results

Research questions

• RQ1 What is the achievable net gain and bandwidth for a GaP-on-insulator TWPA under practical pump powers around 1550 nm?
• RQ2 Can a photonic-chip TWPA provide broadband, high-gain amplification with low noise suitable for frequency combs and coherent communications?
• RQ3 How do dispersion engineering and material properties (GaP) enable reduced device length and footprint while maintaining wide gain bandwidth?
• RQ4 What are the saturation, power handling, and noise performance characteristics of the GaP TWPA in real-world signaling scenarios?

Key findings

• CW parametric amplification with net fiber-to-fiber gain up to 25 dB.
• Broadband signal and idler gain spanning ~140 nm (from ~1300 to ~1900 nm), exceeding 10 dB net gain over 70 nm off-chip.
• On-chip saturation power corresponding to >125 mW at 1605 nm input (≈220 mW on chip), with 9% on-chip power conversion efficiency.
• On-chip noise figure below 4 dB across a wide range of signal powers below saturation; off-chip noise figure approaches 6 dB in small-signal gain regime.
• Demonstration of >20 dB fiber-to-fiber net gain for amplification of optical frequency combs (EO comb and soliton microcomb).
• A single pump GaP TWPA supports linear amplification over six orders of magnitude in input power before saturation.