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.
RF Drivers for Frequency Shifters
In contrast to acousto-optic modulators, frequency shifters are usually operated with a constant drive power. The drive frequency is often also fixed, but there are drivers for a variable frequency.
A variable frequency driver may contain a voltage-controlled oscillator (VCO), the frequency of which can be adjusted with an analog input drive signal. For higher frequency precision and stability, direct digital drivers are available. In other cases, the input signal is an RF signal with the wanted frequency, and the driver acts only as an RF power amplifier.
Acousto-Optic Frequency Shifters from amstechnologies
Wavelength 415-1550 nm; Type Visible, IR; Operating Frequency 40-200 MHz; Active Aperture 1.5-6 mm; Optical Material Tellurium Dioxide
Transmission through an acousto-optic (AO) device causes the input light to experience a frequency shift equal to the RF drive frequency. The acousto-optic frequency shifters (AOFS) from Gooch & Housego (G&H) are optimized for the needs of applications like interferometry, with the ability to achieve high extinction ratio between modes.
Frequency shifters are highly efficient acousto-optic devices, requiring very low drive power to achieve the desired frequency shift. G&H’s AOFS designs typically use the slow shear mode, resulting in slow rise time, but requiring minimal power consumption (typically < 100 mW power draw). This allows to often package the driver with the AOFS for a compact and power-efficient solution.