  We present a current-modulation technique for diode-laser systems that is
specifically designed for high-bandwidth laser-frequency stabilization and
wideband frequency modulation with a flat transfer function. It consists of a
dedicated current source and an impedance-matching circuit both placed close to
the laser diode. The transfer behaviour of the system is analysed under
realistic conditions employing an external-cavity diode laser (ECDL) system. We
achieve a phase lag less than 90$\deg$ up to 25 MHz and a gain flatness of
$\pm$ 3 dB in the frequency range of DC to 100 MHz while the passive stability
of the laser system is not impaired. The potential of the presented current
modulation scheme is demonstrated in an optical phase-locked-loop between two
ECDL systems resulting in an improved phase noise of 42 mrad$_{rms}$. The
design files are available as an open-source project.
