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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….

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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…

Strainmeter to detect earth tide and seismic waves from earthquakes

Strainmeter based on laser wavelength monitoring

We have seen in a previous article that a set-up based on a laser and Optical Fiber Bragg grating (FBG) sensors can be used for strain and temperature monitoring. This technique is based on the monitoring of reflected laser wavelength. FBG sensors find one application in seismology. Strain monitoring is well known in the field of seismology for earth science observation purposes. Strainmeters measure deformation over periods of days, months, and years. This allows them to measure signals at lower…

Dual frequency laser Terahertz

A new dual-frequency laser for THz generation

THz waves are today used for Imaging (security) and Spectroscopy. The IPR in France recently demonstrated a new dual-frequency 780 nm Ti:Sa laser optimized for high spectral purity tunable CW-THz generation. Optimization of the dual –frequency laser has been based on real time and high-resolution instrument. For dual-frequency laser, more than one laser peak need to be controlled. If a high resolution laser spectrum analyzer is used, one can control the spacing between the two peaks and their absolute wavelength….

Multimode VCSEL unstability

VCSEL are now used as singlemode and multimode laser sources in many application fields. Many users are not laser expert and may discover VCSEL unstability late in a development …VCSEL Unstability can be related to two mains parameters : drive current and temperature. The following videos can help feeling how sensitive VCSEL unstability can be for a specific application. Here is a video of the spectrum of VCSEL when changing its current from 90 mA to 140 mA. From a…

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…