Theoretical knowledge

Why phase modulating the lasers used in ICF

Within the past few decades, several major Inertial Confinement Fusion (ICF) facilities using high power lasers have been built around the world. The two largest ICF lasers currently in operation, the Laser Mégajoule (in Bordeaux, France), and the National Ignition Facility (in Livermore, CA, USA), both generate enough energy (1015 Watts/cm2) to initiate the fusion reaction of two hydrogen isotopes, or to replicate the conditions of a nuclear explosion. Figure 1 : Inertial confinement fusion process Courtesy of National Ignition…


How to choose a laser for Raman Spectroscopy

In specific spectroscopy techniques like the Raman or LIBS (Laser-Induced Breakdown Spectroscopy) ones, the choice of the color and of the optical specifications of the excitation laser can be very important. The Raman spectrum of a material or a gas sample represents the energy shift of its vibration modes in relation to the laser excitation and is usually expressed in wavenumbers (cm-1 unit). In Raman spectroscopy, the effective radiated power varies as the excitation frequency to the fourth (due to the…

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What makes my laser mode hop?

Mode hops are often provoked by external influences as laser case temperature, length drifts of the laser resonator, injection current and optical feedback. And as a matter of fact, mode hops often result from attempts to tune the wavelength of a laser. Let’s see here the two main causes behind laser mode hopping: temperature and injection current. Temperature The laser cavity can support many different wavelengths or longitudinal modes. In laser diodes, these modes are separated by typically 10 to…

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Why does my laser mode hop?

Mode hopping is characterized by a stochastic exchange of power between two longitudinal modes of a laser, inducing a high-level intensity noise in the laser’s output. You can read more about why is laser mode hopping so bad for many applications in a previous article. We will review here why laser mode hopping occurs in semiconductor lasers. The mode wavelengths and the gain peak wavelength depend on the laser’s temperature: the mode wavelengths shift with temperature at about typically 0.06 nm/°C, while…

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Why is laser mode hopping so bad?

Semiconductor lasers have found widespread use in fiberoptic communications, entertainment (videodisc and compact disc players), merchandising (bar-code scanners), and in the scientific field as well (spectroscopy) thanks to their variety of wavelengths, compact size, low price and ease of control. Unfortunately some applications require a minimum degree of stability of wavelength that is not always met by semiconductor lasers, especially when mode hopping occurs. Before reviewing why laser mode hopping is so bad, let’s remember what mode hopping refers to….


Units conversion for spectrum representation

Wavelength, Wavenumber, Frequency and Photon Energy Conversion                                    The x-axis of a spectrum should be scaled in frequencies (in Hertz). The frequency representation is dedicated to applications such as measurements of atomic and molecular transitions, heterodyne spectroscopy or TeraHertz generation. The optical frequencies are so large (from 400 to 750 TeraHertz in the visible range) that it is common to use wavelengths in micrometers…

Monitoring a laser wavelength for strain and temperature

How a laser enables you to monitor temperature and strain variations?

Lasers are also used in set-ups aimed for monitoring strain and temperature. In those set-ups strain and temperature variations are directly proportional to the shift of a reflected laser wavelength. Optical Fiber Bragg Gratings (FBG) are known for many years and allow using an optical fiber as a Temperature + Strain + Pressure sensor. An optical fiber Bragg grating acts as a mirror at a specific wavelength depending on the distance between its lines, and transmits all others, or acts…