Beam-shaping design for multi-wavelength diode laser stack system coupled into optical fiber for biomedical applications

Abstract

Multispectral diode laser sources are extensively used for a variety of applications involving the identification of small objects based on their spectral signature. Although the power scaling of single emitters is severely limited, they are easily stackable as diode laser bars and stacks, allowing the combination of wavelengths and power levels required for each application. However, a critical drawback given by this topology is the asymmetry between the fast and the slow axes in the beam profile, leading to poor beam quality and possibly poor fiber coupling efficiency. In this regard, a suitable beam shaping is required to maximize the power coupling in the smallest possible fiber core. In this work, we propose an innovative beam shaping method for the homogenization of the beam quality of six 8-bar diode laser stacks at wavelengths from 790 nm to 980 nm. We performed realistic simulations to examine the shaping method with, when possible, commercially available components. Fast-axis collimating (FAC) lenses and beam twisters are designed in Zemax to remodel the far-field beam emitted by each bar. The beam of each diode laser stack is halved in the vertical axis using polarization beam combiners, and then three quartz-plate stacks combine and rearrange the beams coming from each diode laser stack pair in the horizontal axis to eliminate the lightless regions. A single multispectral beam is then obtained by using reflective and dichroic mirrors and effectively coupled into an optical fiber with a core diameter of 1 mm and a numerical aperture (N.A.) of 0.5 using a doublet of cylindrical lenses. A maximum power density of ~ 0.73 MW/cm2 is calculated at the output of the fiber with a fiber coupling of 89 %. A number of applications can benefit from the proposed topology, in particular biomedical applications using fiber probes are identified as potential candidates for the implementation of the proposed system.