Achieving high power continuous wave and short pulses of semiconductor laser for use in photodynamic therapy: theoretical work

The semiconductor laser (SCL) is a promising light source in photodynamic therapy (PDT). Overcoming the limitation of the low-penetration depth of PDT and enhancing its therapeutic effect on cancer treatment at a deep level requires enhancing the laser power beyond 100 mW in the CW and/or pulse mode. In this paper, we presented a theoretical guide for designing and optimizing SCL parameters to achieve high CW power and high-power short-time pulses to promote laser performance in PDT. High-power CW output was achieved by optimizing the parameters that control the light–current (L–I) characteristics. Picosecond high-power pulses were predicted based on the simulation of the laser signal under sinusoidal current modulation using the optimized CW parameters along with an appropriate selection of the modulation frequency and index signal class. The characteristics of the laser signal under intensive simulations over wide ranges of modulation frequency and index were used to classify the dynamic types of the laser signal. The operating domain of each of these types was mapped over a (modulation frequency versus index), and bifurcation diagrams were constructed to illustrate the flow among these domains. We spotted the variation of both peak power and pulse width of the periodic pulses with modulation parameters. CW output with power reaching ~ 360 mW was predicted using facet reflectivities of 0.01 and 0.99, cavity length as short as 120 µm, internal loss as small as 100 m⁻¹, and confinement factor greater than 0.2. Periodic picosecond pulses with peak power reaching ~ 440 mW were predicted when the modulation index exceeds unity and the modulation frequency is higher than one-half of the relaxation oscillation. The pulse gets narrower with the increase in the modulation index and/or frequency. The obtained results would help design SCL with high power for efficient use in PDT.