Mysterious Abbreviations in the Nonlinear Fiber Optics

Photonics solutions are present in our daily lives today. Whenever we browse the Internet, we enjoy the vast amount of information transmitted through optical fibers. Nonlinear optics also manifests itself in the use of lasers, even low-power ones like green laser pointers that help us present information at conferences or corporate meetings. Why don’t scientists use all this goodness to create a solution to make life easier for more important things? That’s what they’re already trying to do. Nonlinear optics is finding applications in example in medical research. Here the main role is played by a phenomenon called soliton self-frequency shift.

But what is the fuss all about? Optical solitons are intense light pulses that propagate in a nonlinear medium over very long distances. They retain their shape due to the balance between dispersion (broadening of the pulse in time) and self-modulation (broadening of the pulse in frequency). In 1986, Mitschke and Mollenauer discovered that the ultrashort soliton undergoes a self-modulation frequency shift caused by the transfer of energy from the lower to higher wavelengths due to Raman scattering. This is called soliton self-frequency shift, or SSFS in short. This phenomenon allows continuous tuning of the soliton’s spectral position and makes it easier to reach wavelengths inaccessible to typical laser sources, but very important from a medical point of view, as it helps to scan human tissues.

Scientists have a specific vocabulary and many “familiar” keywords and abbreviations. It is much easier to read papers knowing the meaning of these mysterious words, so let’s clarify them. Below is a list of the most common technical phrases.

Basic concepts

Let’s start with the basic concepts. In this group we have…

NLO — NonLinear Optics — is the branch of optics that describes the behaviour of light in nonlinear media. High-powered light interacts with this type of matter in such a way that the structure of the molecules that build it changes.

SMF, FMF and MMF — these terms refer to Single–Mode Fiber, Few–Mode Fiber and Multi–Mode Fiber — fibers that support one, several, or multiple guided modes in a given spatial electromagnetic distribution of a light wave. The light is electromagnetic radiation, which can be a collection of several frequencies (for example, white light is a mixture of many colors of the rainbow) or have a specific wavelength, that is, one specific color. Inside a waveguide, light can propagate in a well-defined form, which we call a mode. In a nutshell, we can say that the larger the core (the part of the waveguide in which the light propagates), the more modes it will accommodate.

Modes can be TM, TE, HEM — Transvers Magnetic, Transverse Electric or Hybrid ElectroMagnetic — propagating in a propagation medium in such a manner that the magnetic (electric) field vector is directed entirely transverse or be a hybrid of transverse electric and transverse magnetic mode solutions.

On the other hand, MOF — Microstructured Optical Fiber — is an optical fiber where guiding is obtained through manipulation of waveguide structure. This manipulation is associated with the introduction of multiple air-filled channels in the fiber’s cladding, which prevents light from escaping the core. This principle of guiding light is called modified total internal reflection.

Those were the basics, now let’s try with something more difficult…

FWM — Four-Wave Mixing — is a nonlinear effect arising from a third-order optical nonlinearity, in which two beams of interact, initiating the formation of two more beams; the sum of the frequencies of the interacting beams is equal to the sum of the frequencies of the excitation beams.

And this can be even more compex for IM-FWM — Inter-Modal Four-Wave Mixing — which is the four wave mixing process supported by two or more spatial modes. And …

IM-V-FWM — Inter-Modal-Vectorial Four-Wave Mixing — as the four wave mixing process supported by two or more spatial and polarized modes. Even a single mode optical fiber actually supports more than one mode — it has two orthogonal polarization modes. This means that they have the same spatial distribution, but the vibrations of the electromagnetic field are orthogonal to each other. In that sense the polarization modes differ.

Now let’s move on to algorithms…

GNLSE — Generalized NonLinear Schrödinger Equation — a single equation that describes the propagation of the wave through a single-mode fiber. It takes into account terms that are responsible for temporal broadening (dispersive terms) and frequency shifts or changes in general (nonlinear terms). Of course we can have multimode version as MM-GNLSE — MultiMode Generalized NonLinear Schrödinger Equation.

Want to hear more? Colaborate!

This, of course, is not a complete list of mysterious shortcuts that you may come across while browsing the specialist literature. It’s just a fraction of our everyday language that we use in technical meetings. A few months ago, I started to collect acronyms and abbreviations related to the field of nonlinear optics with their brief explanations ✍️ Now my list is long enough that it is good time to share it.

The abbreviations are collected in the form of a table with English and Polish translations and a brief definitions. I am sure that this is the tip of the iceberg. Feel free to add a Pull Request or an Issue to contribiute 😊