Theoretical knowledge


How is spectral purity of a laser assessed?

Spectral purity expresses the degree of monochromaticity of a radiation, and has been early identified as one of the main remarkable characteristics of lasers even before the experimental demonstration of the first laser source. However, since no generic standard definition is available, complementary aspects have to be considered in order to assess spectral purity of a laser, mostly depending on the approach and on the application aimed with a laser set-up. Linewidth is probably the most usual way of quantifying…


Frequency Comb characterization with High-Resolution Fourier Transform Spectroscopy (FTS)

– Explanation and applications of Frequency Combs – Frequency combs are very precise light sources which can be used for the measurement of absolute optical frequencies, so they can serve as an optical clockwork for precision spectroscopy, optical sensing, distance measurements, laser noise characterization, telecommunications, and in fundamental physics. The spectrum of an optical frequency comb consists of a train of equidistant lines, i.e. it has equidistant optical frequency components, while the intensity of the comb lines can vary substantially…

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Wavelength Meter Resolution vs Optical Spectral Resolution

When discussing the characteristics of the laser characterization tool, a really common confusion occurs between the terms “Optical Spectral Resolution” and “Wavelength Meter Resolution”. Indeed, both of these specifications contain the word “resolution” and are amongst the most common specifications of these characterization tools, but are referring to two widely different characteristics. The aim of this article is thus to well define both of the specifications and illustrate them by an example to avoid any mix up between the 2….


Efficient multi-mode fiber to single-mode fiber coupling: High speed VCSEL characterization

This article aims at illustrating the MMF to SMF coupling using a multimode scrambler, as introduced in a previous article, to couple a Vertical Cavity Surface Emitting Diode Laser (VCSEL) laser into a ZOOM Spectra high resolution Laser Spectrum Analyzer. Indeed, VCSEL laser are really compact semiconductors laser diodes with large Numerical Aperture emission beam perpendicular to their top surface, as illustrated figure 1. VCSEL are generally fabricated by batch of millions units on a wafer using standard semiconductor technologies….


Efficient multi-mode fiber to single-mode fiber coupling: comparison between the existing solutions

As described in a previous article, achieving an efficient light coupling from multimode fiber (MMF) to single mode fiber (SMF) is a difficult task, especially when trying to avoid any spectral losses while keeping an acceptable power conversion. The fiber to fiber coupling option, as well as the tapered fiber solution and the lenses system being discarded, other alternatives coupling systems have to be investigated. In the following article, the most common coupling setup, the integrating sphere as well as…


Efficient multi-mode to single-mode fiber coupling: introduction

The coupling of the optical signal from a multi-mode to a single-mode fiber is a critical issue in application fields that involve numerical aperture optical systems and high accuracy optical characterization tools, such as astronomy and laser testing. Indeed, such multi-mode to single-mode coupling is virtually impossible without loss of spectral information while keeping an acceptable power output. These articles will be aimed at screening the possible setups for such coupling, and finding the ones that offer the best trade-off…


High-Depth Optical Coherent Tomography (OCT)

In the last 25 years, Optical Coherence Tomography (OCT) has advanced to be one of the most innovative and most successful optical imaging techniques, reaching very high performance required for challenging applications like ophthalmic medicine, dermatology, angiography, and material analysis. OCT principle and domains The OCT technique is classically based on the principle of a Michelson interferometer, with a broadband light source as an input. The interferometric signature of this low coherence signal obtained with a Fourier Transform is a…

What is a Laser Spectrum Analyzer?

A Laser Spectrum Analyzer (LSA) can be defined as a high resolution spectrometer aimed at the characterization of narrow linewidth light sources. In other words it measures the spectral density of a laser as a function of the wavelength. These instruments are typically used to check the spectral purity of laser sources, or to measure their Full Width Half Maximum. It exists two main methods to perform spectral analysis: Grating based Laser Spectrum Analyzers: Using one or several diffraction grating…