Features

• automatic acquisition/tracking operation
• allows ultra-narrow linewidth cavity locking
• designed for RF-optical heterodyne lock
• 3-path controller : high-speed, piezo and TEC actuators
• nested TEC compensator
• fully compatible with SMB30 and SMB20 compensator modules
• 8-bit microcontroller state machine
• predefined operating modes: sweep, acquisition…
• EMI noise immunity
• ideal for rapid prototyping
• schematic diagrams included
• C source files available on request

Overview

In RF-optical heterodyne lock (also known as Pound-Drever-Hall lock), the non-linear frequency discriminator range is given by the frequency of the phase modulating signal. Thus, if the detuning frequency between the laser and the resonator is greater than this modulation frequency, which typically lies in the megahertz range, the error signal is null and the closed-loop cannot operate. Furthermore, for a frequency control loop using some integrators, an out-of-range error signal leads to the saturation of the actuators. In such operation, the laser remains unlocked until a proper action from the user has been carried out. This behaviour becomes problematic with high-finesse cavities because the resulting sensor range is reduced to the resonator bandwidth.

The SMA91 module is able to automatically acquire lock for frequency control systems featuring ultra narrow linewidth cavities. Neither computer nor user actions are required since the module includes its own algorithm. The autolock controller is designed to operate with the high-speed (SMB20) and piezo (SMB30) compensators. It features a thermo-electric cooler (TEC) compensator to provide a third nested loop in order to prevent saturation of the piezo actuator.

Several operating modes are provided to help the user during the tuning phase: for example, it is possible to choose which actuators are engaged when the loop is closed. This feature can be useful to find the right polarity of the error signal.

Like all SM-Series modules, the SMA91 is shipped with the schematic diagrams of its electronic circuitry providing all required information for advanced users. In addition, all C source files of the microcontroller firmware are available for our customers on simple request.