Frequency-conversion crystal for femtosecond-laser pulses

摘要

Optical apparatus for converting a laser pulse of a first wavelength (PF) to a second wavelength (P2H), the apparatus comprising: an optically non-linear crystal having first and second parallel facets, 14 and 16, and reflective coatings 20 and 22, the coatings partially covering the facets; wherein the laser pulse of the first wavelength enters the crystal through a portion of the first facet 14A not covered by the first coating, makes at least a forward pass (path A) and at least a reverse pass (path B), the two passes being at an angle; wherein the second wavelength is generated from the first wavelength during the forward pass; wherein there is a changing temporal separation between the two pulses while passing through the crystal and wherein reflection of at least one of the reflective coating reduces the temporal separation. In this way, conversion efficiency may be improved. At least one of the reflective coatings may be a multi-layer coating and reduction of the temporal separation between the two pulses is due to different penetration depths of the pulses into the multi-layer coating. The second wavelength pulse may exit the crystal from the first facet 14 or the second facet 16. The crystal may be lithium borate (LBO) in a type-I critical frequency-doubling arrangement converting 1040 nm pulses to 520 nm pulses. A further invention concerns minimizing the temporal separation by introducing an opposite group delay using a combination of passes through the crystal and reflective coating properties. A further application of this arrangement may concern sum-frequency, type 0 or type II conversion operations.