Abstract: The present invention relates to a system and a method for emitting plane laser light to biological tissue, wherein in high power lasers are modulated in order to provide an output beam consisting of high temporal and/or spatial coherence and at the same time is capable of delivering an output beam having a high power when being delivered through fibres having a core diameter of less than 400 &mgr;m. The method and system may in particular be used for photodynamic treatment (PDT) Optical fibers and/or thermal treatment. In a preferred embodiment the system and method also includes a monitoring system for the effect actually delivered to the biological tissue. Furthermore, the invention relates to a method of treatment using the system, as well as a method for diagnosing using the system.