Voltage Drop Across High Current Interconnect Cables
For a large percentage of laser diodes, the drive current and compliance voltage required for operation is on the order of tens to a few hundreds of milliamps at a compliance of typically 2 - 4 Volts. In these situations, cabling is not necessarily a critical component of the system. Shielded cables and twisted pairs within the cables may still be required or desired for improved noise immunity but voltage drop and power loss within the cable may not be a concern. Most laser drivers have sufficient overhead to overcome cable voltage drops and power these typical diodes. This is not necessarily true with laser diodes that require several amperes or when several diodes are connected together in series. In the first case, the large current creates an increased voltage drop due to Ohm’s Law and the small, but no longer negligible, cable resistance. In the second case, even though the current has remained constant, the increased number of diodes in series requires a larger voltage across the group in order to operate them. In certain circumstances, there may be just enough voltage drop within the cable to keep the operational voltage required for each device from being reached. Cable resistance can be minimized by using cabling with a larger number of conductors and conductors of a larger size and by reducing the overall cable length. Wire and cable spec sheets typically call out a resistance per unit length and choice should be based on the lowest resistance possible while maintaining cable flexibility.