Pair Of Acousto-optic Modulators Research Articles

High speed, high power 2D beam steering for mitigation of optomechanical parametric instability in gravitational wave detectors.

In this paper, we propose what we believe to be a novel strategy to control optomechanical parametric instability (PI) in gravitational wave (GW) detectors based on radiation pressure. The fast deflection of a high-power beam is the key element of our approach. We built a 2D deflection system based on a pair of acousto-optic modulators (AOMs) that combines high rapidity and a large scan range. As a fast frequency switching configurable AOM driver, we used a Universal Software Radio Peripheral (USRP) combined with a high-performance personal computer (PC). In this way, we demonstrate a 2D beam steering system with flat efficiency over the whole scan range and with a transition time of 50 ns between two arbitrary consecutive deflection positions for a beam power of 3.6 W.

Doppler-shifted laser self-mixing interferometry for enhanced detection sensitivity

ABSTRACTLaser self-mixing interferometry (SMI) is a well-known measurement technique having been applied to the fields of geometrical quantities detection, medical treatment and industry manufacturing. Its detection sensitivity can be improved by frequency shifting towards the laser's relaxation oscillation frequency (ROF), which has always been implemented by a pair of acousto-optic modulators (AOM). This manuscript presents a novel method based on Doppler effect for frequency shifting, which is based on a rotating disk and has the advantages of convenient adjustment and low cost. The theoretical analysis in the form of Doppler shift and time-delayed rate equations is followed, and simulative and experimental results are included to prove its validity. To our knowledge, this technique has not been used in practice till now, and the proposed structure, with these advantages, can find significant applications in designing high performance SMI sensors, and can extend SMI measurement to more kinds of non-cooperative surfaces.

The Approach of Compensation of Air Refractive Index in Thermal Expansion Coefficients Measurement Based on Laser Feedback Interferometry**Supported by the National Natural Science Foundation of China under Grant No F050306.

We present the thermal expansion coefficient (TEC) measurement technology of compensating for the effect of variations in the refractive index based on a Nd:YAG laser feedback system, the beam frequency is shifted by a pair of acousto-optic modulators and then the heterodyne phase measurement technique is used. The sample measured is placed in a muffle furnace with two coaxial holes opened on the opposite furnace walls. The measurement beams hit perpendicularly and coaxially on each surface of the sample. The reference beams hit on the reference mirror and the high-reflectivity mirror, respectively. By the heterodyne configuration and computing, the influences of the vibration, distortion of the sample supporter and the effect of variations in the refractive index are measured and largely minimized. For validation, the TECs of aluminum samples are determined in the temperature range of 298–748 K, confirming not only the precision within 5 × 10−7 K−1 and the accuracy within 0.4% from 298 K to 448 K but also the high sensitivity non-contact measurement of the lower reflectivity surface induced by the sample oxidization from 448 K to 748 K.

Study of non-contact measurement of the thermal expansion coefficients of materials based on laser feedback interferometry.

The noncooperative and ultrahigh sensitive length measurement approach is of great significance to the study of a high-precision thermal expansion coefficient (TEC) determination of materials at a wide temperature range. The novel approach is presented in this paper based on the Nd:YAG microchip laser feedback interferometry with 1064 nm wavelength, the beam frequency of which is shifted by a pair of acousto-optic modulators and then the heterodyne phase measurement technique is used. The sample is placed in a muffle furnace with two coaxial holes opened on the opposite furnace walls. The measurement beams are perpendicular and coaxial on each surface of the sample, the configuration which can not only achieve the length measurement of sample but also eliminate the influence of the distortion of the sample supporter. The reference beams inject on the reference mirrors which are put as possible as near the holes, respectively, to eliminate the air disturbances and the influence of thermal lens effect out of the furnace chamber. For validation, the thermal expansion coefficients of aluminum and steel 45 samples are measured from room temperature to 748 K, which proved measurement repeatability of TECs is better than 0.6 × 10(-6)(K(-1)) at the range of 298 K-598 K and the high-sensitive non-contact measurement of the low reflectivity surface induced by the oxidization of the samples at the range of 598 K-748 K.

Phase difference in modulated signals of two orthogonally polarized outputs of a Nd:YAG microchip laser with anisotropic optical feedback

We present an experimental observation of the output responses of a Nd:YAG microchip laser with an anisotropic external cavity under weak optical feedback. The feedback mirror is stationary during the experiments. A pair of acousto-optic modulators is used to produce a frequency shift in the feedback light with respect to the initial light. The laser output is a beat signal with 40 kHz modulation frequency and is separated into two orthogonal directions by a Wollaston prism. Phase differences between the two intensity curves are observed as the laser works in two orthogonal modes, and vary with the external birefringence element and the pump power. Theoretical analyses are given, and the simulated results are consistent with the experimental phenomena.

Near-field amplitude and phase measurements using heterodyne optical feedback on solid-state lasers

Heterodyne optical feedback on a solid-state laser is experimentally investigated as an efficient tool to characterize coherently near-field evanescent waves. A well-known topography of evanescent field is obtained via a total internal reflection of the light beam emitted by a class B Yb:Er glass laser. A subwavelength size optical fiber tip is scanned to locally probe the resulting evanescent wave in the near field. After a frequency shifting using a pair of acousto-optic modulators, the collected light is optically reinjected to excite the relaxation oscillations of the laser. The resulting dynamical response simultaneously allows very sensitive measurements of the amplitude and the phase of the evanescent wave. Extension of these preliminary results to near-field optical microscopy is suggested and discussed.

Heterodyne wavelength meter for continuous-wave lasers

A wavelength meter based on a heterodyne interferometer is presented. A single-wavelength test laser beam is modulated to two orthogonal linearly polarized components with different frequencies by a pair of acousto-optic modulators. Then the modulated laser beam and a two-wavelength laser beam are sent to a heterodyne interferometer in a common path. The ratio of two laser interference phase shifts in the heterodyne interferometer is equal to the ratio of their wavelengths. The heterodyne technique measures the heterodyne interference phase but not the interference intensity, which means that it could measure a light source whose intensity is not stable. The heterodyne interference signal is an alternating signal that can easily magnify and process the circuit that makes up the heterodyne wavelength meter and could be used to measure the low-intensity light source even when there are environmental disturbances. A tunable diode laser wavelength range of 630-637 nm has been measured to an accuracy of 5 parts in 10(7).

Optical phase shifting with acousto-optic devices

A novel optical phase-shifting method based on a well-known acousto-optic interaction is proposed. By using a pair of acousto-optic modulators (AOMs) and properly aligning them, we construct an optical phase shifter that can directly control the phase of a collimated beam. The proposed phase shifter is insensitive to the polarization of the incident beam when polarization-insensitive AOMs are used, and no calibration is necessary. The proposed approach is confirmed with experimental results.

Optimization of optical spectral throughput of acousto-optic modulators for high-speed optical coherence tomography

We describe a generic method to optimize the optical spectral throughput of a pair of acousto-optic modulators (AOM) which can be used for introducing a frequency shift in ultrahigh-resolution heterodyne optical coherence tomography. Systematic and quantitative analysis of a pair of AOMs indicates that a configuration with a spectral bandwidth of more than 200 nm at a center wavelength of 825 nm (tunable) can be achieved. Using a pair of AOMs in conjunction with a broadband low coherence light source, real-time imaging of biological tissues with an axial resolution ~3 microm (in air) has been experimentally demonstrated with a high-speed OCT system.

Multi-frequency and multiple phase-shift sinusoidal fringe projection for 3D profilometry

In this paper, we report on a laser fringe projection set-up, which can generate fringe patterns with multiple frequencies and phase shifts. Stationary fringe patterns with sinusoidal intensity distributions are produced by the interference of two laser beams, which are frequency modulated by a pair of acousto-optic modulators (AOMs). The AOMs are driven by two RF signals with the same frequency but a phase delay between them. By changing the RF frequency and the phase delay, the fringe spatial frequency and phase shift can be electronically controlled, which allows high-speed switching from one frequency or phase to another thus makes a dynamic 3D profiling possible.

High-speed random access laser tuning

We have developed a technique for laser tuning at rates of 100 kHz or more using a pair of acousto-optic modulators. In addition to all-electronic wavelength control, the same modulators also can provide electronically variable Q-switching, cavity length and power stabilization, chirp and linewidth control, and variable output coupling, all at rates far beyond what is possible with conventional mechanically tuned components. Tuning rates of 70 kHz have been demonstrated on a radio-frequency-pumped CO2 laser, with random access to over 50 laser lines spanning a 17% range in wavelength and with wavelength discrimination better than 1 part in 1000. A compact tuner and Q-switch has been deployed in a 5-10-kHz pulsed lidar system. The modulators each operate at a fixed Bragg angle, with the acoustic frequency determining the selected wavelength. This arrangement doubles the wavelength resolution without introducing an undesirable frequency shift.

10 μm frequency-referenced saturation spectroscopy using a single carbon dioxide laser

We report a new and simple method to obtain saturation spectra at extreme resolution. A single carbon dioxide laser and a pair of acousto-optic modulators are used to provide two μm beams from a single primary source. One of these is used to lock to a molecular transition as a reference while the other is used to interrogate the same transition at a spectral linewidth of a few kHz. Examples of hyperfine structure in PH 3 are presented. The experimental complexity usually associated with such high resolution is greatly simplified, the tuning range of the laser is increased, and the technique obviously has very wide applicability.

Dynamics of the line-tunable 12-μm continuous-wave NH_3 laser as measured with a tunable-diode laser

A cw CO2 laser operating on the R(30) 9-μm transition is downshifted by 180 MHz in a pair of acousto-optic modulators to give an exact coincidence with the sR (5, 0) transition in NH3. The downshifted radiation optically pumps mixtures of NH3 in N2 and creates vibrational inversion in the ν2 mode of NH3. This inversion is monitored with a tunable-diode laser operating in the 11–12-μm region. Gain coefficients as large as 3.4%/cm are measured in the P branch of the ν2 band, and the measured gain coefficients are found to be in good agreement with a rate-equation model based on rotational thermalization. Detailed measurements of gain coefficients are presented as a function of NH3 transition, pump power, and gas mixture and pressure. These results will aid in the optimization of cw NH3 lasers operating in the 11–13-μm region.

Line-tunable oscillation of a cw NH3 laser from 10.7 to 13.3 μm

A cw CO2 laser operating on the R(30) 9-μm transition is down shifted by 180 MHz in a pair of acousto-optic modulators, and used to optically pump sR(5,0) transition of NH3 at line center. Vibrational inversion is created in the ν2 mode, and cw oscillation is observed on 20 different NH3 lines, spanning the region between 10.7 and 13.3 μm. Single line output powers as high as 760 mW were measured in a waveguide cavity. The operation of this laser is explained using a simple thermalization model, and several potential applications are discussed.