Laser diodes present low impedance, of the order of a few ohms, whereas the external electrical feed lines used for conveying an electrical signal to a laser diode are generally of higher impedance, typically 50 ohms.
In order to limit reflection losses on transmission of the signal from the external electrical feed line to the laser diode due to said different impedance values, it is necessary to match the impedance of the laser diode and of the external electrical feed line. Conventionally, such impedance matching has been performed by means of a connection device comprising a resistor deposited as a thin film on a substrate that also has the laser diode deposited thereon, said resistor being electrically connected in series between one of the conductors of the external electrical feed line and one of the electrodes of the laser diode. A bias T is also usually provided comprising a decoupling capacitor and an inductor for feeding the laser diode with bias current. The substrate, the resistor, the laser diode, and the bias T are usually mounted in a metal box that provides screening, that is provided with an external connector for connection to the external electrical feed line, and that is provided with a passage for receiving an optical fiber that is optically coupled to the laser diode. The box provided with the above-specified elements is called a "laser head".
Nevertheless, a laser diode of that type does not provide full satisfaction since the resistor deposited as a thin film presents parasitic capacitance and inductance that distort the signal conveyed by the external electrical feed line, particularly when said signal is a binary signal at a very high data rate, typically greater than 10 Gbits/s. In addition, since the resistor is deposited on the substrate, it prevents distributed feedback (DFB) or distributed Bragg reflector (DBR) type laser diodes being mounted on the substrate since they are particularly sensitive to temperature, so local heating of the substrate due to electricity being dissipated in the thin film resistor would cause the operation of the laser to be unstable.
Proposals have been made to mitigate those drawbacks by progressively changing the shape of the external electrical feed line so as to achieve continuous variation of impedance therealong until it matches the impedance of the laser diode. However, although such a solution eliminates problems of substrate heating, it gives rise to considerable radiation losses and it presents a narrow passband that is ill-suited to the frequency ranges covered by binary signals at high data rates.