Revisiting Raman Gain and Amplification in a Silicon Photonic Platform: The Promise of Lossless Light

Stars emit light that travels through empty space without appreciable attenuation. A visual signal is essentially lossless until it is detected. After many years and billions of kilometers, photons of starlight may finally encounter Earth’s atmosphere and be decoded as a speck in the night sky by the retina and brain of a lucky person.

The light from the pump laser might want to take the same lossless flight, but alas. For optical circuits essential to computing and communications, light is transmitted through waveguides. In a silicon waveguide, the light will weaken after a few centimeters. With the right glass fibers (amorphous quartz silica, silicon oxide), photons can travel hundreds of kilometers before attenuation becomes critical to decoding the information.

So how does the journey through 9,000km of fiber-optic connections across the oceans work? Answer: Every ten kilometers, a section of the fiber is loaded with erbium, a rare earth element that enhances the travel signal. (Oh, and lots of layers of protection for the fibers because sharks are notorious for them Biting communication / internet cables. The exact reasons are not clear.)

You might think we would have an answer to signal amplification on a photonic integrated circuit (PIC). The chips are relatively small and the chance of a shark attack is minimal. Unfortunately, erbium-doped waveguide amplifiers had to be abandoned because their gain and output power could not be matched by other amplifier technologies. Even worse, their fabrication is not compatible with contemporary photonic integration fabrication techniques.

Therefore, the path to a lossless PIC technology has remained a topic of academic research for the past 20 years, even as “good enough” techniques for amplifying/enhancing signals are being used in today’s chips and we are powering their needs for a lot of electricity.

For silicon-based PICs, nonlinear photophysical properties have shown much promise. In essence, nonlinear effects enable light to be manipulated with light. Being intrinsic properties, the implementation on nonlinear behavior requires no special material handling. It can be achieved directly inside a standard waveguide with the right stimulation.

Enter the collaborative work between Canadian professors Shi and La Rochelle V IEEE Journal of Selected Topics in Quantum Electronics: “Non-reciprocal quasi-micron waveguide Raman amplifiers, towards lossless silicon photonics.” The team is taking advantage of new manufacturing offerings from a foundry that makes pics. The capabilities of this foundry allowed the team to revisit and amplify Raman gain in a silicon photonic platform.

Typically, Raman scattering transfers the photon energy from the pump laser to the vibrational modes. It is known to spectroscopy experts who are looking for a special signature of materials. This energy transfer can enhance the signal intensity at resonance energy with the grating vibration mode. Stimulated Raman emission in silicon becomes a strong nonlinear effect, especially when the waveguide dimensions are shrunk.

Co-propagation of a 20-100 mW laser pump allows the signal to maintain its intensity, or even be amplified. Using the same waveguide dimensions configured in an optical resonator, the same team demonstrated an efficient laser covering an extended wavelength range of about 1,550 nanometers.

The battle to mitigate losses in the light circuit finds many angles. Like glass fiber engineering, materials with lower losses such as silicon nitride have been developed and are essential for applications where amplification is not an option, in quantum photonics for example. Combinations of hybrid materials are emerging as possible ways to modulate and amplify light. Proposed solutions should be guided to some extent by their integration with established manufacturing methods as well as smart modifications such that this team has been able to exploit.