Compact Integrating Sphere for Total Flux of Radial Emitting Fibres used for Photodynamic Therapy
Introduction of Photodynamic therapy (PDT)
Photodynamic therapy (PDT) can be used to treat certain types of cancer as well as some skin and eye conditions. In PDT abnormal cells are destroyed by the use of light-sensitive medication, known as photosensitizers, in conjunction with a suitable light source. Typically, the activation spectrum of the photosensitizer falls within the 630nm to 850nm wavelength range. Individually, the photosensitizer medication and light source are harmless, but when the medication is exposed to the light in the presence of tissue oxygen, it activates and causes a reaction that damages the nearby cells.
Delivering sufficient optical power to the target tissue area has been a topic of much research and development over the years. Any normal tissue covering the target tissue will be highly scattering. Consequently, only limited penetration of the visible/near-IR light occurs. Therefore, PDT techniques rely heavily on fibre optic technology to provide suitable light delivery solutions. Laser diodes offering power levels in the 1W to 10W range are now typically coupled to fibre-optic light guides for PDT. Radial emitting fibres are used for the endoscopic delivery of PDT light within hollow organs.
Special Geometry of PDT Fibres
Such side emitting fibres produce a radial light pattern at the end of the fibre, ideally with a high uniformity. The length of the light emitting portion of the fibre is determined by the treatment area required and is typically less than 80 mm in length.
Measurement Equipment for the Characterization of PDT Fibres
Integrating sphere based laser power meters are most commonly used for the measurement of total flux from radially emitting sources. To precisely measure the radiation output from the side emitting section of the fibre, a conventional integrating sphere would need to be around 200mm (8 inches) in diameter and would require a very specific design of internal baffling. Whilst such a sphere may be acceptable within a laboratory, it is too large to integrate into the laser source equipment used for PDT. For this purpose Gigahertz-Optik GmbH has designed elongated integrated spheres that have a small diameter of only 30mm. Model UPK-30S60-L accommodates fibres with light emitting length of up to 60mm and the model UPK-30S105-L is suitable for up to 80mm length fibres. The UPK-30S60 is also supplied with a removable protective quartz insert to prevent the fibre from touching the sphere’s coating. A stretched integrating sphere does not offer the same uniformity of light distribution than spherical integrating spheres. Therefore, particular attention has to be given to ensuring appropriate calibration procedures are used. This is best achieved using radial emitting fibres as the reference source.