Optical Engineering

One of the first applications in which lock-in amplifiers were widely used was for detecting signals from optical experiments. A typical investigation might use a photo detector, such as photomultiplier tube or photodiode, with the output signal being amplified and measured using a DC voltmeter. However, using DC coupled methods makes the system prone to thermal drift, particularly when using high gains to measure weak signals. It is also necessary to provide complete optical screening to prevent errors due to detection of unwanted stray light.

The lock-in amplifier overcomes these difficulties by allowing the measurement frequency to move from DC to a selected AC value. The optical signal to be measured is modulated at this frequency, by either applying an AC drive signal to the source, such as an LED or semiconductor laser, or by means of a rotating blade optical chopper.

The detected signal plus an electrical reference signal, for example from the optical chopper, is then applied to the lock-in amplifier. The reference is used to drive a phase locked loop, the output of which is a signal that is phase locked to the applied detected signal, since they are derived from the same frequency source. The lock-in then uses this information to measure the in-phase and quadrature components of the detected signal that are phase-coherent with the reference, and from the values can compute and display the corresponding vector magnitude and signal phase. However, and most importantly, detected signals at other frequencies, such as would be caused by stray light pick-up, interfering signals, and drift, are eliminated.

Another way of visualizing the operation is to consider that the modulation of the source at the selected frequency "marks" that as being the signal of interest, and the lock-in uses the reference signal, which is also at that frequency, to extract and measure the detected signal.

Signal Recovery supplies one of the widest commercially available ranges of lock-in amplifiers, as well as several optical choppers, that are suitable for optics applications. We also offer current to voltage preamplifiers that can be used to convert the current signals from typical optical detectors to voltage signals suitable for subsequent measurement using a lock-in amplifier. Typically, these satisfy most optical application demands: