A very important feature of the fiber-coupled laser diode modules without active thermal stabilization is the stability of the coupling efficiency with temperature variation. Typically, a laser diode includes an internal photodiode that monitors optical power and it is possible to stabilize the output optical power by stabilizing the signal of the monitor photodiode. However, for the fiber-coupled modules the coupling efficiency can vary with temperature due to thermal expansion of comprising materials and, consequently, optical fiber misalignment.
In order to measure the degree of optical misalignment, the so-called Tracking Error (or front-to-rear tracking error) is used:
where P(T) is the optical power depending on temperature T, T0 = 25 C is the reference temperature, Tmin and Tmax are the minimal and maximal temperature in the test. The typical value of the tracking error given by the manufacturers is 0.8-1.0 dB in the industrial temperature range -40 - +85 C. Our laser diode modules LDI operating in the telecom range 1270 – 1650 nm with the single-mode fibers exhibit the tracking error 0.4-0.6 dB.
For the modules of LDS series we use a new specially developed coupling optics that allows for the very small tracking error value 0.15-0.25 dB in the whole industrial temperature range – 40 - +85 C at a cost of 25% smaller coupling efficiency.
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