Mid-infrared optical parametric amplifier using silicon nanophotonic waveguides (1001.1533v1)

Abstract: All-optical signal processing is envisioned as an approach to dramatically decrease power consumption and speed up performance of next-generation optical telecommunications networks. Nonlinear optical effects, such as four-wave mixing (FWM) and parametric gain, have long been explored to realize all-optical functions in glass fibers. An alternative approach is to employ nanoscale engineering of silicon waveguides to enhance the optical nonlinearities by up to five orders of magnitude, enabling integrated chip-scale all-optical signal processing. Previously, strong two-photon absorption (TPA) of the telecom-band pump has been a fundamental and unavoidable obstacle, limiting parametric gain to values on the order of a few dB. Here we demonstrate a silicon nanophotonic optical parametric amplifier exhibiting gain as large as 25.4 dB, by operating the pump in the mid-IR near one-half the band-gap energy (E~0.55eV, lambda~2200nm), at which parasitic TPA-related absorption vanishes. This gain is high enough to compensate all insertion losses, resulting in 13 dB net off-chip amplification. Furthermore, dispersion engineering dramatically increases the gain bandwidth to more than 220 nm, all realized using an ultra-compact 4 mm silicon chip. Beyond its significant relevance to all-optical signal processing, the broadband parametric gain also facilitates the simultaneous generation of multiple on-chip mid-IR sources through cascaded FWM, covering a 500 nm spectral range. Together, these results provide a foundation for the construction of silicon-based room-temperature mid-IR light sources including tunable chip-scale parametric oscillators, optical frequency combs, and supercontinuum generators.

• The paper presents a mid-infrared optical parametric amplifier that mitigates two-photon absorption by using a 2200 nm pump, resulting in a 25.4 dB on-chip gain.
• It employs silicon nanophotonic waveguides to boost nonlinear optical effects by up to five orders of magnitude over conventional glass fibers.
• Engineered dispersion in the waveguides yields a broadband gain bandwidth exceeding 220 nm, enabling versatile applications in biochemical detection and free-space communications.

Mid-Infrared Optical Parametric Amplifier Using Silicon Nanophotonic Waveguides

The paper presents an investigation into all-optical signal processing via silicon nanophotonic waveguides, specifically focusing on a novel mid-infrared optical parametric amplifier (OPA). This research addresses the challenges associated with two-photon absorption (TPA) at telecom wavelengths (~1550 nm), critically hindering parametric gain. The authors circumvent this limitation by utilizing a pump source at mid-infrared wavelengths (~2200 nm), thus significantly reducing TPA effects and achieving unprecedented parametric gain values.

Key Findings and Methodology

1. Enhanced Nonlinear Optical Effects:
   • The use of silicon waveguides allows an enhancement in optical nonlinearities by up to five orders of magnitude compared to conventional glass fibers. This enhancement is critical for the realization of integrated chip-scale all-optical signal processing functions.
2. Elimination of Two-Photon Absorption:
   • By operating at a pump wavelength of ~2200 nm, which is near one-half the band-gap energy of silicon, the TPA-related absorption is practically eliminated. This leads to a significant on-chip parametric gain of 25.4 dB, enabling 13 dB net off-chip amplification.
3. Broadband Gain Bandwidth:
   • Carefully engineered dispersion in the waveguides results in a gain bandwidth exceeding 220 nm. This broadband capability is vital for applications requiring the simultaneous generation of multiple wavelengths on chip through cascaded four-wave mixing (FWM).
4. Design and Fabrication:
   • The silicon waveguides, with core dimensions of 700 nm x 425 nm, are fabricated using advanced CMOS-compatible processes. This precise engineering allows control over parameters such as effective nonlinearity and dispersion, which are foundational for phase matching in FWM processes.
5. Experimental Verification:
   • Experimental data confirms a peak parametric gain of over 25 dB, effectively overcoming fiber-chip coupling losses and demonstrating net off-chip amplification. The waveguide configuration provides effective nonlinearity values that surpass those found in other materials by orders of magnitude.

Theoretical and Practical Implications

Theoretical implications of this work extend to the potential of silicon nanophotonics to support mid-IR applications, fundamentally broadening the scope of silicon photonics beyond conventional telecom applications. The reduction of TPA opens avenues for enhanced nonlinear optical processes, making silicon a viable material for diverse photonic applications previously restricted to other more expensive materials.

Practically, the work lays the groundwork for new, compact mid-infrared light sources. Such sources include, but are not limited to, tunable parametric oscillators, optical frequency combs, and supercontinuum generators. These devices have considerable applications in biochemical detection, environmental monitoring, and free-space communication, owing to their extended operational wavelength range.

Future Prospects

The development and implementation of a silicon-based optical parametric oscillator (OPO) leveraging the high-gain OPA demonstrated here would represent a significant advancement. Such an OPO would provide a low-threshold, portable solution for generating coherent mid-IR light, expanding the practical applications of integrated photonic devices in spectroscopic and communication technologies.

In conclusion, the research detailed in the paper showcases the potential of silicon nanophotonic waveguides as efficient components for mid-infrared signal processing. With the promising results achieved in terms of gain and bandwidth, further exploration into device optimization and application-specific designs is warranted to capitalize on the significant technological possibilities this work presents.