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. Laser diodes (DFB, DBR, VCSEL, VeCSEL, ECDL, QCL, ICL, DH, quantum well lasers), which are semiconductor lasers, are available in many different shapes and sizes with laser powers ranging from a few mW to hundreds of watts. The emitted wavelength depends mainly on the semiconductor material of the laser diode cavity and its length, providing laser wavelength from the visible to the infrared range. Under some external influence, a laser diode can operate on a single resonator mode for some time and then switch to some other modes. This rapid switch in wavelengths can happen discontinuously in a back and forward manner. This is so called mode hopping.
Is mode hopping only a rapid discontinuous wavelength shift? Yes and no. Mode hopping introduces a wavelength shift that causes an intensity noise, directly correlated to the occurrence of mode hopping. When mode hopping occurs, the other mode suddenly takes over all the optical power and at some point there may even be power in both modes.
Figure 1. Mode hopping of a DFB laser after temperature/wavelength drift
So you see why mode hopping causes intensity noise and also make it difficult to obtain continuous wavelength tuning. And this is undesirable in many applications. Let's review some of the applications that suffer from laser mode hopping.
For videodisc systems, mode hopping causes wavelength dispersion which causes variations in beam direction which leads to variations in the location of data written to the optical surface. And this finally results in a degraded signal-to-noise ratio (SNR), causing degradation in the quality of the data (image/video) derived from the disk.
For video transmission via fiber optics, mode hopping also causes intensity noise.
For telecommunications, mode hopping affects the maximum data transmission rate as shift in wavelength causes variations in velocities in those single-mode fibers with high dispersion.
For spectroscopy, mode hopping causes small or large laser wavelength shifts. And one knows that spectroscopy usually requires great wavelength stability.
Fortunately, there are combinations of laser case temperature and injection current that lead to mode hopping and other combinations for which the laser is stable. We will see in a next article when and why laser mode hopping occurs.
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