Laser diode systems for photodynamic therapy and medical diagnostics

Abstract: This work concerns techniques for improvement of the coherence properties of diode lasers so that they may be used in two specific medical applications: (Interstitial) photodynamic therapy (PDT) and laser-induced fluorescence diagnostics. In the first application, spatial coherence is crucial, since the treatment light should be delivered through relatively thin optical fibers to optimize treatment and to facilitate minimally invasive treatment. In the second application, a pulsed, blue source at 405nm is desired. This has been approached by frequency doubling the output from an external-cavity laser diode at 810nm. Thus, both spatial and temporal coherence are important to ensure that the second harmonic generation is efficient. Two red (635nm) laser diode systems for PDT, a first and a second generation, have been developed and tested in preliminary clinical trials. The first generation system is based on asymmetrical optical feedback from a barium titanate crystal. This system is coupled into a thin (50µm core diameter) optical fiber and tested in preliminary experimental trials involving interstitial PDT of solid tumors in rat. The second generation system couples two similar systems by means of polarization coupling, whereby the output power is doubled. The individual systems are based on asymmetrical feedback from ordinary mirrors. This system has been tested in PDT treatment of skin cancer in human and has been compared with a conventional treatment. For laser-induced fluorescence diagnostics, an 810nm external-cavity laser diode system with improved spatial and temporal coherence has been constructed. The system is based on a new configuration that employs double feedback from the first diffracted and the zeroth reflected order of a diffraction grating. The output from this system is frequency doubled to 405nm using a single pass configuration with a periodically poled KTP crystal. It has been shown that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared with the freely running laser.