Fiber-coupling optical system for high-power and multi-wavelength diode laser bars oriented to integrated biomedical imaging systems

Abstract

The use of pulsed high-power diode lasers (HPDLs) in the near infrared (NIR) range as multispectral sources has attracted much interest in biomedical applications due to their relatively low cost and small size compared to nanosecond Nd:YAG optical parametric oscillator (OPO) lasers. One of the main limitations of these sources is the availability to combine different wavelengths with high power density in the same beam. Various works have shown that the use of linear arrays of emitters as diode laser bars (DLBs) or stacked arrays of emitters as diode laser stacks (DLSs) allows the combination of multiple wavelengths while maintaining high power densities. Nevertheless, the highly asymmetric beam profile emitted by such laser sources between fast and slow axes implies the need for suitable beam shaping for efficient fiber coupling. In this work, we investigate a novel beam shaping technique to homogenize the beam quality of six DLBs in the wavelength range between 790 nm and 980 nm.We consider fast-axis collimating lenses (FAC) and beam twisters to reduce the beam asymmetry of the individual bars. The beams from the DLBs are then combined into a single multispectral beam using reflective mirrors, dichroic mirrors, and a polarizing beam splitter cube (PBC), and effectively coupled into a 400 µm core-diameter/N.A. = 0.22 optical fiber using a pair of cylindrical lenses. Simulation shows high coupled power densities with ~ 1.8 MW/cm2 at the output of the fiber. The coupling efficiency reaches 89.4%. The use of submillimeter fiber optic probes is particularly promising for photoacoustic endoscopy (PAE) applications requiring minimally invasive examination of internal organs.