Acousto-optic Device

Acousto-optics is a branch of physics that studies the interactions between sound waves and light waves,
especially the diffraction of laser lightby ultrasound (or sound in general) through an ultrasonic grating.

Three categories of acousto-optic devices will be discussed. They include the acousto-optic modulator, 
filter and deflector.

1.Acousto-optic modulator
  By varying the parameters of the acoustic wave, including the amplitude, phase, 
  frequency and polarization, properties of the optical wave may be modulated. 
  The acousto-optic interaction also makes it possible to modulate the optical beam 
  by both temporal and spatial modulation.

An acoustic wave can induce non-reciprocal light modulation in a silicon waveguide. Now, the acoustic wave has been induced optically in a neighbouring silicon waveguide, opening the way for a silicon-based optical isolator.

Laser power fluctuations can significantly reduce the device performances in various applications. High frequency fluctuations impact the signal-to-noise ratio, while slow variations can reduce the device repeatability or accuracy. Here we report experimental investigations on the power stabilization of a diode laser with an acousto-optic modulator. In the frequency domain, the relative power noise is reduced at the level of 2.2 × 10−8 Hz−1/2 in the range 1-100 kHz. The slow variations are studied in the time domain. The relative Allan standard deviation is measured at the level of 6 × 10−7 at 100 s averaging time. Above 100 s, the instability increases and reaches 2 × 10−6 at 10 000 s.

Double pass Acousto-optic modulator system Double pass AOM DPAOM

A practical problem that arises when using acousto-optic modulators to scan the laser frequency is the dependence of the beam diffraction angle on the modulation frequency. Alignment problems with AOM-modulated laser beams can be effectively eliminated by using the AOM in the double-pass configuration, which compensates for beam deflections. On a second pass through the AOM, the beam with its polarization rotated by 90° is deflected back such that it counterpropagates the incident laser beam and it can be separated from the input beam with a polarizing beam splitter.
Here we present our design for a compact, stable, double-pass AOM with 75% double-pass diffraction efficiency and a tuning bandwidth of 68 MHz full width at half maximum for light transmitted through a single-mode fiber. The overall efficiency of the system sdefined as the optical power out of the single-mode fiber divided by the optical power into the apparatusd is 60%.

2.Acousto-optic filter
  The principle behind the operation of acousto-optic filters is based on the wavelength of 
  the diffracted light being dependent on the acoustic frequency. 
  By tuning the frequency of the acoustic wave, the desired wavelength of the optical wave can 
  be diffracted acousto-optically.

3.Acousto-optic deflectors
  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. 

Due to its strong piezoelectric effect and photo-elastic property, lithium niobate is widely used for acousto-optical applications. However, conventional bulk lithium niobate waveguide devices exhibit a large footprint and limited light–sound interaction resulting from the weak guiding of light. Here, we report the first acousto-optical modulators with surface acoustic wave generation, phononic cavity, and low-loss photonic waveguide devices monolithically integrated on a 500 nm thick film of lithium niobate on an insulator. Modulation efficiency was optimized by properly arranging the propagation directions of surface acoustic waves and optical guided modes. The effective photo-elastic coefficient extracted by comparing the first and third harmonic modulation signals from an on-chip Mach–Zehnder interferometer indicates the excellent acousto-optical properties of lithium niobate are preserved in the thin film implementation. Such material property finding is of crucial importance in designing various types of acousto-optical devices. Much stronger amplitude modulation was achieved in a high Q (>300,000) optical resonator due to the higher optical sensitivity. Our results pave the path for developing novel acousto-optical devices using thin film lithium niobate.