  Higher repetition-rate optical pulse trains have been desired for various
applications such as high-bit-rate optical communication, photonic
analogue-to-digital conversion, and multi- photon imaging. Generation of multi
GHz and higher repetition-rate optical pulse trains directly from mode-locked
oscillators is often challenging. As an alternative, harmonic injection locking
can be applied for extra-cavity repetition-rate multiplication (RRM). Here we
have investigated the operation conditions and achievable performances of
all-fibre, highly tunable harmonic injection locking-based pulse RRM. We show
that, with slight tuning of slave laser length, highly tunable RRM is possible
from a multiplication factor of 2 to >100. The resulting maximum SMSR is 41 dB
when multiplied by a factor of two. We further characterize the noise
properties of the multiplied signal in terms of phase noise and relative
intensity noise. The resulting absolute rms timing jitter of the multiplied
signal is in the range of 20 fs to 60 fs (10 kHz - 1 MHz) for different
multiplication factors. With its high tunability, simple and robust all-fibre
implementation, and low excess noise, the demonstrated RRM system may find
diverse applications in microwave photonics, optical communications, photonic
analogue-to-digital conversion, and clock distribution networks.
