[Paper Review] Mid-infrared dispersive wave generation in silicon nitride nano-photonic waveguides

Clemens Herkommer, Adrien Billat+5 more|arXiv (Cornell University)|Apr 8, 2017

Advanced Fiber Laser Technologies3 citations

TL;DR

This paper demonstrates efficient, coherent mid-infrared (2.5–4 µm) dispersive wave generation in silicon nitride waveguides using a 1550 nm femtosecond fiber laser with only 0.6 nJ pulse energy. By engineering anomalous group velocity dispersion via a thick (2.3 µm), dispersion-tuned waveguide fabricated with a modified photonic Damascene process, the method enables broadband mid-IR emission with high spectral coherence, offering a compact route to mid-IR frequency combs and spectroscopy applications.

ABSTRACT

The generation of supercontinua using photonic chip-based waveguides enables efficient broadening at low pulse energy owing to the large effective nonlinearity of nanoscale waveguides made of materials with large Kerr nonlinearity. While photonic waveguides for supercontinuum generation have been extensively studied in the visible and the near-infrared spectral range, there is a growing interest in the mid-infrared (mid-IR) range, which is particularly suitable for molecular spectroscopy. Yet to date, accessing the mid-IR using photonic chip-based supercontinua has been challenging, and typically has only been accessed with this method by operating the pulsed source directly within the mid-IR. Here we demonstrate a simple, yet effective method for the direct generation of high coherence mid infrared radiation in the 2.5--4 micron region from a femtosecond laser in the telecommunication band (i.e. 1550 nm). The wavelength conversion is based on dispersive wave generation in dispersion engineered silicon nitride waveguides, pumped by a femtosecond fiber laser with pulse energies of 0.6 nJ. Efficient wave guiding in the mid-IR and dispersion engineering is achieved using a modified photonic Damascene fabrication process, allowing waveguides with an unprecedented height exceeding 2.3 micron and resulting in broadband anomalous group velocity dispersion extending into the mid-IR. Lithographic tuning of the mid-IR dispersive wave position is demonstrated and numerical simulations indicate a high level of spectral coherence in the mid-IR, thereby providing a route for synthesis of mid-IR frequency combs based on this method. Such a simple and versatile mid-IR source is suitable for spectroscopic applications in the first mid-IR atmospheric window, and may also serve to synthesize mid-IR frequency combs.

Motivation & Objective

• Address the challenge of generating broadband, coherent mid-infrared radiation on a photonic chip for spectroscopic applications.
• Overcome the limitations of existing on-chip supercontinuum sources, which are largely confined to the visible and near-infrared ranges.
• Enable direct mid-infrared generation by leveraging dispersive wave emission in dispersion-engineered silicon nitride waveguides.
• Develop a scalable, chip-integrated platform for mid-infrared frequency comb synthesis using low pulse energy and standard telecommunication wavelengths.
• Achieve broadband anomalous group velocity dispersion extending into the mid-infrared through waveguide geometry engineering.

Proposed method

• Employ a modified photonic Damascene fabrication process to create silicon nitride waveguides with unprecedented thickness exceeding 2.3 µm.
• Engineer the waveguide dispersion to achieve anomalous group velocity dispersion extending into the mid-infrared (2.5–4 µm) region.
• Pump the waveguide with a 1550 nm femtosecond fiber laser at 0.6 nJ pulse energy to excite nonlinear processes including dispersive wave generation.
• Use lithographic tuning to control the central wavelength of the generated mid-infrared dispersive wave across the 2.5–4 µm range.
• Apply numerical simulations to predict and confirm high spectral coherence of the mid-infrared output.
• Leverage the high effective nonlinearity of nanoscale waveguides to achieve efficient spectral broadening at low pump energy.

Experimental results

Research questions

• RQ1 Can dispersive wave generation in dispersion-engineered silicon nitride waveguides produce coherent mid-infrared radiation directly from a telecom-band femtosecond laser?
• RQ2 To what extent can waveguide thickness and geometry be optimized to extend anomalous group velocity dispersion into the mid-infrared?
• RQ3 How does lithographic tuning affect the central wavelength of the generated mid-infrared dispersive wave?
• RQ4 What level of spectral coherence can be achieved in the mid-infrared output from such a chip-based system?
• RQ5 Can this platform serve as a viable route for on-chip mid-infrared frequency comb generation?

Key findings

• The authors achieved efficient mid-infrared dispersive wave generation in the 2.5–4 µm range using only 0.6 nJ pulse energy from a 1550 nm femtosecond laser.
• Waveguides with a thickness exceeding 2.3 µm were fabricated using a modified photonic Damascene process, enabling broadband anomalous group velocity dispersion into the mid-infrared.
• Lithographic tuning allowed precise control over the central wavelength of the generated dispersive wave across the 2.5–4 µm spectral window.
• Numerical simulations confirmed a high level of spectral coherence in the mid-infrared output, indicating suitability for frequency comb applications.
• The platform enables direct, low-energy mid-infrared generation on a chip, bypassing the need for mid-IR pump sources.
• The method provides a scalable and versatile route for on-chip mid-infrared frequency comb synthesis and spectroscopic sensing in the first atmospheric window.