v. 2013/2014

Acousto-optical modulator with traveling and standing acoustic wave

Introduction

Optical scheme with AOM.

Fig. 9.1: The schematic of a measurement setup. A distance between the AOM and the laser is about 20 cm and a distance between the AOM and the observation plane is about 1 m. He-Ne laser is working at its standart wavelength 632.8 nm.

Instrumentation is based on:

  • AOQS with the driver 45 MHz, max. output power 20 W
  • AOML with the driver 60–80 MHz, max. 5 V / 50 Ω

Goal

To measure the most important parameters of an acousto-optic modulator based on traveling acoustic wave (used as AOQS – Acousto-Optic Q-Switch) and of an acousto-optic modulator based on standing acoustic wave (used as AOML – Acousto-Optic Mode-Locker).

Instructions

Theoretical home preparation
  1. Think of an acousto-optic modulator made of fused silica with interaction length of l = 40 mm and index of refraction of n = 1.46. The speed of ultra-sound in fused silica is vA = 6 km/s. What is the wavelength of the sound wave Λ at frequency 47 MHz and 75 MHz?
  2. Calculate the product l·λ for both frequencies. Compare it with square of the acoustic wavelength Λ2. Decide whether the Raman-Nath condition or Bragg condition is fulfilled.
  3. Determine the frequency interval between the two closest resonant frequencies for the AOML of the thickness of L = 1 cm. Use the following formula Δf = vA/2L (where Δf is frequency interval and vA is speed of sound in the modulator).
  4. Calculate the Bragg angle for both frequencies inside the modulator of refraction index n. What is the angular deviation of the first diffraction maximum outside the modulator?
Laboratory measurements
  1. Adjust the AOQS modulator to observe a diffraction pattern at the observation plane with the highest intensity. Record its shape by camera, cell phone or sketch it.
  2. Measure the angle between zero and first diffraction maximum.
  3. Measure the depth of modulation (percentage portion of power diffracted from zero maximum) for minimal and maximal driving power of hf generator 45 MHz (with AOML maximal only).
  4. Repeat tasks 5-7 with the AOML modulator and different hf driver at 75 MHz.
  5. Change the driving frequency of the hf source in range 74.630 – 74.990 MHz and measure the frequency interval Δf between the two closest frequency peaks of diffraction. Read the exact value of frequency by counter.
  6. Measure the shape in detail of one frequency peak and determine its width ΔF. From measured data compute finesse of AOML acoustic resonator F = Δf/ΔF. Decide what should be the frequency stability of a hf driver.

Requested results

  1. All results of the theoretical home preparation
  2. Schematic of the measurement setup.
  3. Images of observed diffraction patterns for both AOM.
  4. Table with measured and calculated angular distances of adjacent diffraction maximums for both AOM.
  5. Depth of modulation [%] and diffraction efficiency [%/W] of both AOM. For AOQS a pair (min and max), for AOML single values. The finesse of AOML.
  6. Plot the measured dependence of depth of modulation of the AOML on driving frequency in range of 100 kHz around maximum.

References

W. Koechner: Solid State Laser Engineering
online version of 5th edition on Google books, see chapter 8.4, ie. pages 501–504
B.E.A. Saleh, M.C. Teich: Fundamentals of Photonics
Josef Blažej - contact - blazej   fjfi.cvut.cz - phone: +420 224 358 659
Czech Technical University in Prague - Faculty of Nuclear Sciences and Physical Engineering
Brehova 7, 115 19 Prague 1, Czech Republic