1550nm 80MHz acousto-optic frequency shifter.Acousto-optic frequency shifters are devices for shifting the optical frequency of a laser beam by a fixed or variable amount.
Acousto-optic Frequency Shifters
When light is diffracted at the traveling refractive index grating in an acousto-optic modulator, the diffracted light experiences a shift of optical frequency which is plus or minus the acoustic (or drive) frequency. That effect (which can be interpreted as a Doppler shift) is exploited in acousto-optic frequency shifters.
Drive frequencies are typically between some tens and hundreds of megahertz, rarely more than 1 GHz. The resulting change of optical wavelength is quite small. For larger frequency shifts, or for realizing very small frequency shifts (e.g. only a few MHz), one may cascade two or more devices. It is also possible to use a double pass through a single device in order to obtain twice the frequency shift.
Frequency shifters may either be operated with a fixed drive frequency, generating a fixed optical frequency offset, or with a variable drive frequency. In the latter case, one needs to consider the fact that the beam direction will change with the drive frequency; if that is detrimental, one may use methods to minimize such effects. It is also possible to operate a frequency shifter with several drive frequencies at the same time.
The optical input beam is typically a laser beam from a single-frequency laser. However, a frequency shifter would also work with a multimode beam, if its bandwidth is not too large.
Most acousto-optic frequency shifters are bulk devices, but there are also compact fiber-coupled versions (fiber-pigtailed AOFS). Light from the input fiber is first collimated, then sent through the modulator crystal and finally focused into the output fiber. There are also all-fiber frequency shifters (perhaps not commercially available) where the frequency shift is created within an optical fiber.
