Deeper Dive into TEC Controllers
Directly mounting a TEC to the diode package can have detrimental effects for applications requiring extremely low noise and long coherence length.
As most people already understand, a laser diodes performance is highly dependent on its operating temperature, which is why most mid- to high-end laser packages utilize embedded thermoelectric coolers (TECs). The strong temperature dependence arises from two fundamental properties of the diode itself. First, as the temperature of the semiconductor varies, it’s gain band shifts up or down, this is known as redshifting and blueshifting. Secondly, as the temperature increases or decreases so does both the materials index of refraction and its physical volume, causing variations in the optical path length and resulting in changes in the longitudinal mode structure.
For most laser diode applications mounting a TEC at the base of the TO-can, or directly under the laser platform in a 14-pin butterfly package, provides enough temperature stability to produce a stable output. But, for high-end applications requiring extremely low noise and extremely long coherence lengths, the TEC itself can end up inducing higher frequency noise.
While on the surface this may sound counter-intuitive, when you take a look at how a standard DC switching power supply works, you will see that they inherently generate high-frequency noise due to the large current swings and the internal inductance and capacitance. In fact, this is the same fundamental principle that most pulse forming networks (PFNs) utilize on a much larger scale to drive the flashlamps in high energy pulsed laser systems. The diagram below shows a simplified example of a typical switching mode DC power supply, which would be used to drive a TEC. Read more on this topic in this article.
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