  A high power pulsed laser system has been installed on the high magnetic
field muon spectrometer (HiFi) at the ISIS pulsed neutron and muon source,
situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade
enables one to perform light-pump muon-probe experiments under a high magnetic
field, which opens new applications of muon spin spectroscopy. In this report
we give an overview of the principle of the HiFi Laser system, and describe the
newly developed techniques and devices that enable precisely controlled
photoexcitation of samples in the muon instrument. A demonstration experiment
illustrates the potential of this unique combination of the photoexcited system
and avoided level crossing technique.
