Total Power Control (TPC)
Electro-Optic modulation and Acousto-Optic modulation are two leading approaches to modulate the laser output power of femtosecond lasers used in two-photon microscopy.
An Electro-Optic Modulator (EOM) modulates laser power by applying a phase retardation to the beam by use of the Pockels effect.
Figure- Simplified representation of transverse Pockels cell operation. Transmission through the analyzer is modulated by adjustment of the applied electric field.
An Acousto-Optic Modulator (AOM) comprises a transparent crystal or glass onto which a piezoelectric transducer is attached. A radiofrequency (RF) wave applied to the transducer induces an acoustic wave that strains the crystal resulting in a refractive index grating. Light travelling through the cell then undergoes Bragg diffraction.
Most “home-builders” have so far chosen EOMs because of the relative simplicity of deploying this high-voltage powered device in the optical path. Various microscope vendors offer either EOMs or AOMs partially integrated in their laser delivery train, with their software architecture controlling both microscope and laser. Using manufacturing experience with high power fiber lasers designed for 24/7 manufacturing environments,
Coherent recognized that the advantages of the AOM approach in term of size, cost, speed and overall performance would also fulfil two-photon imaging applications. By integrating the TPC (Total Power Control) sophisticated control of the AOM within the laser software and hardware architecture of Discovery NX, Axon and Monaco systems, two-photon users – both home builders and scope companies – benefit from a greatly simplified and easier to control optical set-up, for applications ranging from advanced neuroscience to medical diagnostics.