NLS-Glossary

Achromatic doublet lenses "achromat" are designed to eliminate chromatic and spherical aberrations inherent in singlet lenses.                                                                               

An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths typically red and blue into focus on the same plane.

Acoustic sensors are a class of microelectromechanical systems which rely on the modulation of surface acoustic waves to sense a physical phenomenon. The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena. The device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon. Acoustic sensors detect defined interference and switching noises on machines. and an adjustable switching threshold the sensor is also qualified for industrial outdoor applications.

An acousto-optic deflector spatially controls the optical beam. In the operation of an acousto-optic deflector the power driving the acoustic transducer is kept on, at a constant level, while the acoustic frequency is varied to deflect the beam to different angular positions. The acoustic wave of an acousto-optic deflector is frequency modulated. Unlike an acousto-optic modulator, which is an amplitude modulator, an acousto-optic deflector is a frequency modulator, which allows its acoustic frequency to be varied electronically.
Acousto-optic deflectors have many applications. A frequency shifter, which has the sole purpose of generating a diffracted optical beam at an optical frequency shifted by the amount of the acoustic frequency from the input optical frequency, can be considered as the simplest form of an acousto-optic deflector. Acousto-optic deflectors are also used in such diverse applications as optical scanners, spatial light modulators, RF pulse compressors, and programmable optical interconnecters.

The acousto-optic Frequency Shifter (AOFS) with RF driver is used to modify the frequency of the optical beam. Due to the Doppler shift, the frequency shift of 1st order diffracted light (variation quantity of wavelength) equals to the frequency of acoustic (wavelength) after the AO frequency shifters.

makes use of the photoelastic effect to modulate a light beam. Suppose that we generate traveling acoustic or ultrasonic waves on the surface of a piezoelectric crystal (such as LiNbO3) by attaching an interdigital electrodes onto its surface and applying a modulating voltage at radio frequencies (RF). The piezoelectric effect is the phenomenon of generation of strain in a crystal by the application of an external electric field. The modulating voltage V(t) at electrodes will therefore generate a surface acoustic wave (SAW) via the piezoelectric effect. These acoustic waves propagate by rarefactions and compressions of the crystal surface region which lead to a periodic variation in the density and hence a periodic variation in the refractive index in synchronization with the acoustic wave amplitude. Put differently, the periodic variation in the strain S leads to a periodic variation in n owing to the photoelastic effect. We can simplistically view the crystal surface region as alternations in the refractive index. An incident light beam will be diffracted by this periodic variation in the refractive index. If the acoustic wavelength is Λ , then the condition that gives the angle θ for a diffracted beam to exist is given by the Bragg diffraction condition, 2Λsinθ=λ/n where n is refractive index of the medium. Suppose that ω is the angular frequency of the incident optical wave. The optical wave reflections occur from a moving diffraction pattern which moves with a velocity Vacoustic As a result of the Doppler effect, the diffracted beam has either a slightly higher or slightly lower frequency depending on the direction of the traveling acoustic wave. If Ω is the frequency of the acoustic wave then the diffracted beam has a Doppler shifted frequency given by ω'=ω±Ω When the acoustic wave is traveling towards the incoming optical beam, then the diffracted optical beam frequency is up-shifted, e.a.ω'=ω+Ω If the acoustic wave is traveling away from the incident optical beam then the diffracted frequency is down-shifted ω'=ω-Ω It is apparent that we can modulate the frequency (wavelength) of the diffracted light beam by modulating the frequency of the of the acoustic waves. (The diffraction angle is then also changed.) Traveling acoustic waves create a harmonic variation in the refractive index and thereby create a diffraction grating that diffracts the incident beam through an angle 2θ. Consider two coherent optical waves A and B being "reflected" (strictly, scattered)from two adjacent acoustic wavefronts to become A' and B'. These reflected waves can only constitute the diffracted beam if they are in phase. The angle θ is exaggerated (typically this is a few degrees).