FREQUENCY SHIFTERS

An acousto-optic frequency shifter (AOFS) modifies the frequency of the optical beam as a result of momentum transfer between the acoustic waves (phonons) and photons interacting in an acousto-optic device. As the light scatters off the diffraction grating created by the acoustic wave, it experiences a Doppler shift. If the light is propagating through the crystal in the same direction as the acoustic wave, the diffracted beam is increased in frequency by the RF drive frequency. If the light and acoustic wave are traveling in opposite directions, the diffracted beam is decreased in frequency by the RF drive frequency.   The acoustic wave frequency is very small relative to the light frequency (tens or hundreds of MHz vs ~100 THz), and thus the frequency shift is almost impossible to detect in most applications. It is very useful, however, for interference-based optical techniques like optical heterodyne detection,laser Doppler velocimetry, and laser Doppler vibrometry (LDV).   At G&H, we fabricate our frequency shifters using high quality tellurium dioxide (TeO2), grown and polished in-house, for lowest insertion loss and excellent power handling. We offer standard AOFS products for wavelengths in the visible and NIR for use with fundamental and doubled Argon Ion, Nd:YAG, He:Ne, diode, dye, and Ti:Sapphire lasers.   With standard frequency shifts of over 300 MHz and custom frequency shifts up to 600 MHz, as well as models for double-pass configurations, we have a frequency shifter to match most research and industrial needs. For unusual requirements, we can draw on our lines of standard modulators and beam deflectors to often find off-the-shelf solutions for unusual requirements.  For frequency shifting with an AOFS product, the anisotropic interaction of the slow shear mode in TeO2 generates the frequency shift. Therefore, the unshifted input beam is polarized orthogonally to the diffracted, frequency shifted beam. A polarizer can then be used external to the AOFS to achieve high extinction ratio between the diffracted and undiffracted beams. This is useful to eliminate light leakage and avoid beat patterns between the two beams. It is important to note that the achievable extinction ratio (ER) is a function of the polarizer used, and not the AOFS.  Frequency shifters are highly efficient acousto-optic devices, requiring very low drive power to achieve the desired frequency shift. Our 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 us to often package the driver with the AOFS for a compact and power-efficient solution. In applications where power consumption is an issue, we can advise on how to best balance performance and RF drive power.  As straightforward as frequency shifting may seem, our experience in acousto-optic device design and applications can help to identify the best off-the-shelf or custom solution and complementary RF driver.