RF-hyperthermia is a promising cancer therapy which is plagued by the difficulties involved in focusing RF energy on tumors while avoiding inadvertent heating of healthy parts of the body. Minimally-perturbing RF probes which can measure both amplitude and phase of all three components of the RF E-field, can be used in vivo in common catheters, are sensitive enough to measure field of 100V/m, and which are available at a reasonable cost are vitally needed by the RF hyperthermia R&D hyperthermia R&D and therapy community. Such probes would be used both singly to monitor RF energy near tumors in vivo, and in probe arrays to help focus applicator-array energy in real time. MRC proposes to use photonic-RF probe techniques MRC has investigated and demonstrated, together with state-of-the-art optical-modulator technology demonstrated by members of the MRC team to develop low-cost probes meeting all of these requirements. Similar probes will also be useful in a variety of other biomedical and non-biomedical applications. PROPOSED COMMERCIAL APPLICATION The probe system will facilitate RF hyperthermia R&D and therapy; R&D in MRI, electro-convulsive shock therapy, and defibrillation; and RF-and pulsed-field biological effects investigations. Non-biomedical uses include antenna characterization; RF and pulsed-field electromagnetic compatibility testing, RF safety monitoring; and analysis of many kinds of distributed, high-frequency circuits.