Abstract: To protect a laser diode (LD) against both excessive input signal levels and electrostatic charges, a high-frequency limiting circuit (BE) and a high-frequency damping circuit (AT) are interconnected, and additionally coupled to a source of bias voltage (U.sub.DC). The high-frequency limiting circuit has two Schottky diodes (D1, D2), biassed for non-conduction, and the damping circuit a PIN diode (D3) likewise biassed for high-resistance value. Upon exceeding a threshold as set by the bias voltage, diodes (D1, D2) become conductive and symmetrically limit the signal, and diode (D3) additionally dampens the signal; an electrostatic charge to the input control the diodes into conductive state and limits the value of energy transferred to the laser diode (LD) to a safe value.

Abstract: Conventional transformers for coupling a transmission line to a laser diode have an upper frequency limit of about 600 MHz. In order to extend bandwidth up to 860 MHz, an improved transformer features a first winding W1 and a second winding W2 twisted together and wrapped through apertures L1 and L2 of a double-aperture ferrite core DK. The twisting together of the windings suppresses stray inductances which would otherwise limit bandwidth. Preferably, a first grounding C1 is connected to the input terminal E and a second grounding capacitor C2 is connected to the output terminal O. Further, at the output terminal O, a laser diode LD is connected. The improved structure will perform anywhere within a bandwidth from 47 MHz up through 860 MHz.