Photonics and optics: interchangeable yet different

Why is it fibre-optics, and why is it not the semiconductor photonic amplifier? It is all to do with a systems perspective versus a device perspective. An industry debate about optics and photonics.

Many terms in telecom are used interchangeably. Terms gain credibility with use but over time things evolve. For example, people understand what is meant by the term carrier [of traffic] or operator [of a network] and even the term incumbent [operator] even though markets are now competitive and 'telephony' is no longer state-run.

"For me, optics is the equivalent of electrical, and photonics is the equivalent of electronics - LSI, VLSI chips and the like" - Mehdi Asghari

Operators - ex-incumbents or otherwise - also do more that oversee the network and now provide complex services. But of course they differ from service providers such as the over-the-top players [third-party providers delivering services over an operator's infrastructure, rather than any theatrical behaviour] or internet content providers. 

Google is an internet content provider but with its gigabit broadband service it is rolling out in the US, it is also an operator/ carrier/ communications service provider. And Google may soon become a mobile virtual network operator.  

So having multiple terms can be helpful, adding variety especially when writing, but the trouble is it is also confusing.

Recent discussions including interviewing silicon photonics pioneer, Richard Soref, raised the question whether the terms photonics and optics are the same. I decided to ask several industry experts, starting with The Optical Society (OSA).

Tom Hausken, the OSA's senior engineering & applications advisor, says that after many years of thought he concludes the following:

• People have different definitions for them [optics and photonics] that range all over the map.

• I find it confusing and unhelpful to distinguish them.

• The National Academies's report is on record saying there is no difference as far as that study is concerned.

• That works for me.

Michael Duncan, the OSA's senior science advisor, puts the difference down to one of cultural usage. "Photonics leans more towards the fibre optics, integrated optics, waveguide optics, and the systems they are used in - mostly for communication - while optics is everything else, especially the propagation and modification of coherent and incoherent light," says Duncan. "But I could easily go with Tom's third bullet point."

"Photonics does include the quantum nature, and sort of by convention, the term optics is seen to mean classical" - Richard Soref

Duncan also cites Wikipedia, with its discussion of classical optics that embraces the wave nature of light, and modern optics that also includes light's particle nature. And this distinction is at the core of the difference, without leading to an industry consensus.

"Photonics does include the quantum nature, and sort of by convention, the term optics is seen to mean classical," says Richard Soref. He points out that the website Arxiv.org categorises optics as the subset of physics, while the OSA Newsletter is called Optics & Photonics News, covering all bases.

"Photonics is the larger category, and I might have been a bit off base when throwing around the optics term," says Soref. If only everyone was as off base as Professor Soref.

"We need to remember that there is no canonical definition of these terms, and there is no recognised authority that would write or maintain such a definition," says Geoff Bennett, director, solutions and technology at Infinera. For Bennett, this is a common issue, not confined to the terms optics and photonics: "We see this all the time in the telecoms industry, and in every other industry that combines rapid innovation with aggressive marketing."

That said, he also says that optics refers to classical optics, in which light is treated as a wave, whereas photonics is where light meets active semiconductors and so the quantum nature of light tends to dominate. Examples of the latter would be photonic integrated circuits (PICs). "These contain active lasers components, semiconductor optical amplifiers and photo-detectors " says Bennett. "All of these rely on quantum effects to do their job."

"We need to remember that there is no canonical definition of these terms, and there is no recognised authority that would write or maintain such a definition" - Geoff Bennett

Bennett says that the person who invented the term semiconductor optical amplifier (SOA) was not aware of the definition because the optical amplifier works on quantum principles, the same way a laser does. "So really it should be a semiconductor photonic amplifier," he says.

"At Infinera, we seem for the most part to have abided to the definitions in terminology that we use, but I can’t say that this was a conscious decision," says Bennett. "I am sure that if our marketing department thought that photonic sounded better than optical in a given situation they would have used it."

Mehdi Asghari, vice president, silicon photonics research & development at Mellanox, says optics refers to the classical use and application of light, with light as a ray. He describes optics as having a system-level approach to it.

"We create a system of lenses to make a microscope or telescope to make an optical instrument using classical optics models or we use optical components to create an optical communication system," he says. This classical or system-level perspective makes it  optics or optical, a term he prefers. "We are not concerned with the nature - particle versus wave - of light, rather its classical behaviour, be it in an instrument or a system," he says.

But once things are viewed closer, at the device level, especially devices comparable in size of photons, then a system-level approach no longer works and is replaced with a quantum approach. "Here we look at photons and the quantum behaviour they exhibit," says Asghari.

In a waveguide, be it silicon photonics (integrated devices based on silicon), a planar lightwave circuit (glass-based integrated devices), or a PIC based on III-V or active devices, the size of the structure or device used is often comparable or even smaller than the size of the photons it is manipulating, he says: "This is where we very much feel the quantum nature of light, and this is where light becomes photons  - photonics - and not optics."

ADVA Optical Networking's senior principal engineer, Klaus Grobe, held a discussion with the company's physicists, and both, independently, had the same opinion.

"Both [photonics and optics] are not strictly defined," he says. "Optics clearly also includes classic school-book ray optics and the like. Photonics already deals with photons, the wave-particle dualism, and hence, at least indirectly, with quantum mechanics, and possibly also quantum electro-dynamics (QED)."

Since in fibre-optics for transport, ray-propagation models no longer can be used, and also since they rely on the quantum-mechanical behaviour, for example of diode receivers, fibre-optics are better filed under photonics, says Grobe: "But they are not called fibre-photonics".

So, the industry view seems to be that the two terms are interchangeable but optics implies the classical nature of light while photonics suggests light as particles.  Which term includes both seems to be down to opinion. Some believe optics covers both, others believe photonics is the more encompassing term.

Mellanox's Asghari once famously compared photons and electrons to cats and dogs. Electrons are like dogs: they behave, stick by you and are loyal; they do exactly as you tell them, he said, whereas cats are their own animals and do what they like. Just like photons. So what is his take?

He believes optics is more general than photonics. He uses the analogy of electrical versus electronics to make his point. An electronics system or chip is still an electrical device but it often refers to the integrated chip, while an electrical system is often seen as global and larger, made up of classical devices.

"For me, optics is the equivalent of electrical, and photonics is the equivalent of electronics - LSI, VLSI chips and the like," says Asghari. "One is a subset or specialised version of the other due to the need to get specific on the quantum nature of light and the challenges associated with integration."

"Optics refers to all types of cats, be it the tiger or the lion or the domestic pet. Photonics refers to the so called domestic cat that has domesticated and slaved us to look after it" - Mehdi Asghari

To back up his point, Ashgari says take a look at older books and publications that use the term optics. The term photonics started to be used once integration and size reduction became important, just as how electrical devices got replaced with electronic devices.

Indeed, this rings true in the semiconductor industry: microelectronics has now become nano-electronics as CMOS feature sizes have moved from microns to nanometer dimensions.

And this is why optical fibre or the semiconductor optical amplifier are used because these terms were invented and used when the industry was primarily engaged with the use of light at a system level and away from the quantum limits and challenges of integration.

"In short, photonics is used when we acknowledge that light is made of photons with all the fun and challenges that photons bring to us and optics is when we deal with light at a system level or a classical approach is sufficient," says Asghari.

Happily, cats and dogs feature here too.

"Optics refers to all types of cats, be it the tiger or the lion or the domestic pet," says Asghari. "Photonics refers to the so called domestic cat that has domesticated and slaved us to look after it."

Last word to Infinera's Bennett: "I suppose the moral is: be aware of the different meanings, but don’t let it bug you when people misuse them."