Beam characterization for single-mode fiber coupling of tapered lasers
Tapered lasers show a high market potential for many applications. They deliver a much higher output power than ridge-waveguide (RW) lasers and a significantly better beam quality than broad area (BA) lasers at the same power level. However, for many applications it is necessary to transport the beam using single-mode fibers. Tapered lasers feature a more complex beam profile than RW and BA lasers. This complexity is of such a high level that the commonly used beam propagation factor (M²) is insufficient to predict the coupling efficiency into a fiber.
At the FBH, it could be shown that a more in-depth description of the beam profile using the Wigner distribution function (WDF) allows to precisely predict the coupling efficiency. As an example, a tapered laser with two contacts, one for the ridge-waveguide section and one for the tapered section, was operated using two different RW currents.
While the power is constant and the M² increases only slightly from 4.2 to 5.0, the coupling efficiency deteriorates from 68% to 37% for RW currents of 350 mA and 500 mA, respectively. This difference in coupling efficiency of almost a factor of 2 is not adequately reflected in the M² value. However, when measuring the Wigner distribution function of the beam at the two operation points, coupling efficiencies of 65% and 34% were calculated. The error in the prediction of our WDF based coupling model was 3% only.
This demonstrates the power of the new WDF method, making it a suitable tool to design coupling optics and paving the way for efficient fiber-coupled laser sources powered by tapered lasers.
M. Uebernickel, B. Eppich, K. Paschke, G. Erbert, G. Tränkle, "Prediction of Single-Mode Fiber Coupling Efficiencies of a Tapered Diode Laser From Measured Wigner Distribution Functions", Photonics Technology Letters, IEEE Volume 24 , Issue 14 (2012).