This article was originally featured in the edition: PIC Magazine Issue 9 - July 2018

Photonic Integrated Comb Sources Enable High Volume Applications

Pilot Photonics is making laser comb sources suitable for coherent optical transceivers for the first time. As CEO Frank Smyth explains, the techniques and products his company have developed have the potential to transform photonic integration.

An optical comb source is a laser that produces a spectrum consisting of multiple equidistant frequencies. The huge potential of optical comb lasers was recognised in 2005, when they were the subject of the Nobel Prize for Physics, an accolade reserved for highly disruptive discoveries, efforts and inventions. The prize was awarded to John L. Hall and Theodor W. Hänsch "for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique".

Optical combs have been demonstrated as useful tools to enhance a diverse range of applications including precision spectroscopy, microwave synthesis, frequency metrology, optical clocks, and communications.

At its simplest, an optical comb can be used to replace an array of independent lasers, like those of a wavelength division multiplexed (WDM) communications system. However, it is their more subtle properties; the coherence of the comb lines, and the precision of the frequency spacing between them, that create new possibilities and offer unique advantages over existing laser sources.

While the promise of comb lasers has been recognised, most commercial optical comb sources have been the preserve of scientists to date; they are expensive pieces of laboratory equipment which has limited their use in high volume applications.

Recent advances in photonic integration have enabled several demonstrations of on-chip optical combs that have the potential to open up volume markets for the first time. Opportunities for combs are found in optical communications, low cost optical sensing for the internet of things, and chip-scale atomic clocks that can benefit from photonic integrated optical combs.

Comb generation techniques

Optical combs can be generated using many different techniques, and the suitability of each depends on the application.

They can be generated via mode-locking, in either solid-state (including fibre), or diode laser cavities. The solid-state type is the most widely deployed commercial comb source and offers excellent optical properties and a wide comb bandwidth. They are an excellent choice for laboratory and scientific work, but are less suited to low-cost, high-volume applications.

Mode-locked laser diodes on the other hand, do offer the potential for photonic integration. They have been explored deeply in the research community, for example through the EU FP7 project Big Pipes [1], and they are being actively commercialised for short reach communications.

A draw-back of mode-locked techniques in general is that they are based on a fixed physical cavity, and therefore the comb spacing is generally static. This limits swept-mode operation which can prove useful in applications such as spectroscopy and frequency metrology.

Optical combs can also be generated by over-driving electro-optic modulators. In order to get a high quality comb, numerous modulators (each requiring their own RF driver,) are typically used in series. This drives up the cost of the comb source, and also increases the optical loss, often necessitating the use of an optical amplifier which also limits commercial applicability.

Comb generators based on the Kerr effect in ring resonators are receiving much attention and many impressive results have been published recently on optical combs based on micro ring resonators. The efficiency of such devices can prove challenging, with external high-power pump lasers or amplifiers often required. However, more efficient devices, requiring less optical pump power are being actively developed, which provide excellent potential for low-cost commercial devices over time [2,3,4].

The reader is referred to the following references [5,6,7,8] for several far more detailed surveys and reviews of the various types and applications of optical combs. The remainder of this article focuses on gain switched comb generation from a monolithically integrated photonic chip a technique that offers a balance of simplicity, flexibility and performance.