Acousto-optic Modulator Research Articles

Interference-Fading-Suppressed Pulse-Coding Φ-OTDR Using Spectrum Extraction and Rotated-Vector-Sum Method

With phase-sensitive optical time-domain reflectometry (Φ-OTDR), an optic fiber can be used as a distributed vibration sensor to perceive the external disturbance along the fiber through the phase variation of the Rayleigh backscatter light. However, due to the inevitable interference fading phenomenon, the signal-to-noise ratio (SNR) of Rayleigh backscatter light at certain points is insufficient, thus affecting the fidelity of vibration sensing. In this paper, we propose the spectrum extraction and the rotated-vector-sum (SERVS) method to suppress the interference fading in the pulse-intensity-coding Φ-OTDR, which applies Golay complementary sequence through acousto-optic modulator (AOM). Benefited from the easier implementation, the experimental results demonstrate that the percentage of fading points can be reduced from 2.98% to 0.33% significantly and the standard deviation of differential phase is also decreased from 0.6556 rad to 0.2901 rad. Also the minimum level of averaged power spectral density (PSD) of the differential phase at whole locations along the fiber is reduced about 6 dB/Hz effectively. For the demodulated disturbance signal, no influence on the SNR is observed thus it maintains a SNR improvement of 8.6 dB compared to the traditional single pulse Φ-OTDR system, achieving a noise floor of 2.38 pϵ/√Hz.

455-nm tunable narrow linewidth Q-switched frequency-doubled Cr:LiSAF laser

We present a narrow linewidth frequency-doubled Cr:LiSAF laser with a 450- to 460-nm tunability and maximum repetition frequency (RF) of 63 kHz. Under a pump power of ∼900 mW, the fundamental wavelength could be tuned at the range of 883 to 1020 nm, with a maximum output power of 180 mW at 910 nm. The pulsed operation was achieved by using an acousto-optical modulator. An LBO crystal was adopted for intra-cavity frequency doubling and a maximum output power of 44.8 mW was obtained at 455 nm, indicating a slope efficiency of 11.2%. The spectral linewidth was <0.1 nm in the whole tuning range.

Attosecond-precision balanced linear-optics timing detector.

A new timing detection method based on acousto-optic modulation is demonstrated. The timing detector is immune to dispersion effects and the environmental and laser amplitude noise can be well suppressed by a balanced configuration. With 1 mW power per pulse train, the measured timing noise floor is about 1×10-10 fs2/Hz, which is close to the shot noise limit. The integrated timing jitter is 26 as at [1 Hz, 1 MHz]. With 170 fs pulse width and typical detector parameters, the calculated detector's timing noise floor is more than 5 and 12 orders of magnitude lower than that of a BOC, at 1 mW and 1 µW input power, respectively. This timing detector has a variety of potential applications in ultra-long fiber link stabilization, quantum metrology, weak signal timing control, etc.

Temperature Effects in Acousto-Optic Modulators of Terahertz Radiation Based on Liquid Elegas

Temperature Effects in Acousto-Optic Modulators of Terahertz Radiation Based on Liquid Elegas

Dependence of the energy efficiency of the acousto-optic modulator of terahertz radiation on the dimensions of the ultrasound transducer

The paper presents results of experimental investigation of an acousto-optic modulator of terahertz radiation based on liquefied sulfur hexafluoride. It was found that the intensity of diffracted radiation at a fixed ultrasound power depends on the dimensions of the ultrasound transducer. The optimal size of the ultrasound transducer in the direction orthogonal to the plane of the acousto-optic interaction was determined, at which the maximum energy efficiency of the acousto-optic modulator was achieved.

Dual-frequency pulse laser based on acousto-optic modulation.

Non-collinear stimulated Brillouin scattering (SBS) amplification can obtain high peak power Stokes output while ensuring the stability, but the frequency mismatch reduces the energy conversion efficiency of the system. In this paper, a dual-frequency pulse laser based on acousto-optic crystal modulation is designed. The output pulse pair can be used as pump and Stokes light, respectively, which realizes the active frequency matching of the gain medium Brillouin frequency shift during the SBS amplification process and helps to maintain ideal energy conversion efficiency. The dual-frequency laser finally produced a laser pulse pair with a pulse width adjustment range of 100 ps-50 ns, a frequency shift range of 0 GHz-2 GHz, and the polarization extinction ratio (PER) reaches 20.82dB.

Artifact-free holographic light shaping through moving acousto-optic holograms

Holographic light modulation is the most efficient method to shape laser light into well-defined patterns and is therefore the means of choice for many intensity demanding applications. During the last two decades, spatial light modulators based on liquid crystals prevailed among several technologies and became the standard tool to shape light holographically. But in the near future, this status might be challenged by acousto-optic deflectors. These devices are well known for their excelling modulation rates and high optical power resilience. But only few scattered precedents exist that demonstrate their holographic capabilities, despite the many interesting properties that they provide. We implemented a holographic acousto-optic light modulation (HALM) system, that is based on displaying holograms on acousto-optic deflectors. We found that this system can eliminate the ubiquitous coherent artifacts that arise in holography through the inherent motion of acousto-optic holograms. That distinguishes our approach from any other holographic modulation technique and allows to reconstruct intensity patterns of the highest fidelity. A mathematical description of this effect is presented and experimentally confirmed by reconstructing images holographically with unprecedented quality. Our results suggest that HALM promotes acousto-optic deflectors from highly specialized devices to full-fledged spatial light modulators, that can compete in a multitude of applications with LC-SLMs. Especially applications that require large optical output powers, high modulation speeds or accurate gray-scale intensity patterns will profit from this technology. We foresee that HALM may play a major role in future laser projectors and displays, structured illumination microscopy, laser material processing and optical trapping.

Double-Frequency-Shift Acousto-Optic Modulator with Controllable Pulse Pair Frequency Difference

A scheme for controlling the frequency difference of output pulse pair with double frequency shift loops is proposed. The frequency shift system includes two loop elements of 20 and 200 MHz. The first one carries out a single selective positive frequency shift of 1–20 MHz, and the second one can satisfy a single fixed positive frequency shift of 200 MHz. The reverse cascade technology of two acousto-optic crystals is introduced to solve the limitation of the small frequency shift of crystal size. A multichannel synchronization signal completes the time domain control of each acousto-optic modulator. Finally, the frequency shift difference of the output pulse pair ranges of 0–2 GHz, and the frequency shift accuracy is 5 MHz.

Demonstration of on-chip gigahertz acousto-optic modulation at near-visible wavelengths

Abstract Lithium niobate integrated photonics has recently received significant attention because it exploits the attractive properties of lithium niobate on an integrated platform which provides strong optical confinement as well as high photonic integration density. Although many optical functionalities of lithium niobate have been demonstrated on a chip in the telecom band, the visible and near-visible regimes are less explored. This is mainly because devices with a relatively smaller feature size are required which increases fabrication difficulty. Here, we explored the acousto-optic effect of lithium niobate on a chip at near-visible wavelengths (765–781 nm) and demonstrated acousto-optic modulation with the modulation frequency up to 2.44 GHz. We adopted an etchless process for the device fabrication and applied the principle of bound states in the continuum to optimize the device performance. By demonstrating functionality at near-visible wavelengths, our devices will enable many on-chip applications ranging from frequency metrology to quantum information processing.

Temperature Effects in an Acousto-Optic Modulator of Terahertz Radiation Based on Liquefied SF6 Gas.

The acousto-optic (AO) diffraction of terahertz (THz) radiation in liquefied sulfur hexafluoride (SF) was investigated in various temperature regimes. It was found that with the increase in the temperature from +10 to +23 C, the efficiency of the AO diffraction became one order higher at the same amplitude of the driving electrical signal. At the same time, the efficiency of the AO diffraction per 1 W of the sound power as well as the angular bandwidth of the efficient AO interaction were temperature independent within the measurement error. Increase of the resonant sound frequency with decreasing temperature and strong narrowing of the sound frequency bandwidth of the efficient AO interaction were detected.

Mid IR Holmium, Praseodymium - doped Fluoride Fiber Laser Pulse Operation

Abstract: Pulsed operation of a novel single-clad mid IR Ho3+, Pr3+ co-doped ZBLAN fiber laser ‎excited by a Nd:YAG laser at 1064 nm has been demonstrated using pulse-pumping. An acousto-optic modulator (AOM) has been used to modulate the CW input pump ‎laser and used in either zero-order or first-order operation. A regular output train of pulses was generated for modulation frequencies over 1 Hz - 100 kHz ‎with zero-order operation of the AOM. A maximum peak power of ~ 50 W and pulse ‎duration of ~200 ns have been obtained for a modulation frequency of 45 kHz and 9.3 ‎W pump power. Single-end and double-end pumping of the ‎first-order AOM operation for the fiber have also been demonstrated to avoid the overheating of the modulator which ‎reduces the modulation efficiency. The same results as the zero AOM operation were obtained when the fiber laser was pumped with a pumping power of about 5 watts. Keywords: Fiber lasers, Pump pulsed operation, Acousto-optic modulator (AOM), Single-end and double-end pumping. PACs: Fiber lasers, 42.55.Wd, Acousto-optical devices, 42.79.Jq.

One-thousandth-level laser power stabilization based on optical feedback from a well-designed high-split-ratio and nonpolarized beam splitter.

Laser power stabilization plays a significant role in atomic and molecular physics, quantum precision measurement, and optical sensing and measurement. In the classical method of using a feedback control loop to stabilize the laser power, the beam splitter is the conjunction element to connect the feedback beam inside the loop and the output beam outside the loop. The stability of its split ratio will directly affect the result of power stabilization, especially in demand of high split ratios for high-efficiency output. For the compatibility of a high split ratio and high stability in a power-stabilized system, we designed and manufactured a high-split-ratio nonpolarized plate beam splitter, whose split ratio was insensitive to variations of beam intensity, polarization, and ambient temperature. Based on the optical feedback of the designed beam splitter, the light intensity was closed-loop controlled by an acousto-optic modulator; finally, the power outside the loop was stabilized as well. The output power was stabilized at 537 mW and a 6 h long-term test was performed. The relative stability of laser power outside the loop in terms of root mean square and peak to peak was 2.72×10-4 and 1.60×10-3, respectively. The relative Allan standard deviation reached 2.78×10-5 at an average time of 200 s. These results will greatly benefit many practical fields that require laser power stabilization with high split ratios and one-thousandth-level stability.

Generation of nanosecond pulsed azimuthally and radially polarized beams with an actively Q-switched rotating disk laser

We have directly generated two kinds of vector polarized beam pulses from an end pumped rotating Nd:YAG disk laser design acousto-optic Q-switch pulse modulation. About 20 ns pulse width with peak power 29 kW and stable pulse vector polarized beams are achieved at the continuous absorbed pump power. The laser crystal rotary effectively alleviates thermal effect that affects the power scaling with high beam quality. The output state is controlled by an intra-cavity polarization selector birefringent crystal. The polarization degrees of the azimuthally and radially polarized beams are 98.6% and 98.2%, respectively. The study suggests that this new method would constitute a laser light source for producing power and single pulse energy scaling, short pulse width, and high-purity structured vector beams and serve this function.

Focusing light into scattering media with ultrasound-induced field perturbation

Focusing light into scattering media, although challenging, is highly desirable in many realms. With the invention of time-reversed ultrasonically encoded (TRUE) optical focusing, acousto-optic modulation was demonstrated as a promising guidestar mechanism for achieving noninvasive and addressable optical focusing into scattering media. Here, we report a new ultrasound-assisted technique, ultrasound-induced field perturbation optical focusing, abbreviated as UFP. Unlike in conventional TRUE optical focusing, where only the weak frequency-shifted first-order diffracted photons due to acousto-optic modulation are useful, here UFP leverages the brighter zeroth-order photons diffracted by an ultrasonic guidestar as information carriers to guide optical focusing. We find that the zeroth-order diffracted photons, although not frequency-shifted, do have a field perturbation caused by the existence of the ultrasonic guidestar. By detecting and time-reversing the differential field of the frequency-unshifted photons when the ultrasound is alternately ON and OFF, we can focus light to the position where the field perturbation occurs inside the scattering medium. We demonstrate here that UFP optical focusing has superior performance to conventional TRUE optical focusing, which benefits from the more intense zeroth-order photons. We further show that UFP optical focusing can be easily and flexibly developed into double-shot realization or even single-shot realization, which is desirable for high-speed wavefront shaping. This new method upsets conventional thinking on the utility of an ultrasonic guidestar and broadens the horizon of light control in scattering media. We hope that it provides a more efficient and flexible mechanism for implementing ultrasound-guided wavefront shaping.

A High-Precision Offset Frequency Locking Technique With Delay Line Reference and AOM-Based Compensation

We demonstrate a high-precision, high robustness frequency offset locking method,which made the frequency offset between mode-locked laser and continuous-wave laser below less than 3 Hz. The coarse frequency lock control is realized by the feedback control of PZT with electrical delay line as the reference. The fine frequency compensation is realized by feed-forward control of an acousto-optic modulator. The fractional frequency instability was 7.4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> for an averaging time of 1 s, 3.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> for an averaging time of 10 000 s when the narrow linewidth laser is free-running. In this experiment, the fractional frequency instability can be achieved at 1.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> for an averaging time of 1 s, at 3.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-18</sup> for an averaging time of 10 000 s when the system is fine frequency locked. Compared with the unlocked laser, the fractional frequency instability can be improved about 5-6 orders of magnitude. This work lays the foundation for simple structure, high robustness and high precision laser frequency control situation, such as quantum precision measurement and optical lattice clocks.

Development of an Optical set-up for 3D PIV with a Large Volume

Measurements of 3D volumetric velocity fields are of great theoretical interest with numerous practical applications. These measurements are essential for studying volumetric flows that do not exhibit inherent flow symmetry, such as turbulence or vortex breakdown. In the past decade, several technological innovations facilitated the emergence of 3D-PTV techniques for measuring velocity fields at kHz rate with volumes of interest up to 104 cm3 that contain 300 µm helium-filled soap bubbles. However, when a commercial laser beam with millijoule pulse-energy is expanded and shaped to fill volumes above 102 cm3 for 3D-PTV experiments with 15 µm air filled soap bubbles, one finds that the power density of the laser source is insufficient to generate a signal image. This is because the power density of the laser beam falls inversely with respect to its cross-section area and due to the quadratic dependence of Mie-scattering on the particle diameter. Here, we report of the analysis and development of two optical techniques for extending the volume of measurement in volumetric PTV. In particular, when a volume about 103 cm3 is seeded with 15 µm air-filled soap bubbles and a laser with a pulse energy of few single mJ illuminates it. The first technique uses multi reflections between two opposing parallel mirrors. The second technique is a development of laser scanning PIV for volumetric scanning: The potential to increase the scanned volume is examined by experimenting with an acousto-optic modulator for fast scanning. Furthermore, by employing an off-axis parabolic mirror, we obtain parallel beam scanning, which increases the efficiency and quality of the scanning.

High-performance acousto-optic modulator based on environmentally favorable Ge20Sb15Se65 chalcogenide glass

Acousto-optic (AO) properties of an environmentally friendly Ge20Sb15Se65 chalcogenide glass were investigated, and the results show that Ge20Sb15Se65 glass was a potential material for manufacturing AO modulator (AOM) because its composition possesses excellent performance of high optical quality and transmission in the infrared region spectral, non-toxic, high glass transition temperature, strong mechanical strength, easy to fabricate in large bulk, and isotropic homogeneity. Particularly, its outstanding AO figure of merit (M2 = 407 × 10−15 s3/kg) was approximately 270 times that of fused quartz glass, making it attractive for AO device designs. Herein, the construction of the AOM based on Ge20Sb15Se65 glass as AO material was performed, and we obtained a high diffraction efficiency of 73.4% at a high acoustic frequency of 160 MHz and an extremely low radio-frequency (RF) power of 0.25 W when the laser beam at 1550 nm was controlled. To our best knowledge, the results of initial tests on the AOM prepared from Ge–Sb–Se chalcogenide glass were first reported. Such an AOM design enriches the family of AO devices and exploits the route for the practical application of chalcogenide glass.

Axial Error of Spindle Measurements Using a High-Frequency-Modulated Interferometer

In this paper, a novel, compact, and high-precision axial error measurement using a frequency-modulated interferometer is developed. Normally, heterodyne interferometers are a powerful system for small displacement measurements due to their property of being less sensitive to temperature and pressure variations. However, the maximum measurement speed of the heterodyne interferometer is around 5 m/s because it is usually limited by the difference in frequency between the two components of the laser beam, which is no larger than 3 MHz or 20 MHz corresponding laser source based on the Zeeman effect and acousto-optic modulator, respectively. The proposed measuring system is realized by modulating the frequency of the laser diode source at a high modulation frequency and using lock-in amplifiers to extract the harmonics of the interference signal. The measurement speed is proportional to the modulation frequency. Thus, the higher the modulation frequency, the higher the measuring speed attains. The frequency-modulated interferometer is then applied to measure the axial error of an ultra-precision spindle. The proposed system can be a capable solution for noncontact and high-precision spindle error measurements in the machining process.

Acousto-optic Modulators with a Broadened Frequency Band for Fiber-Optic Lines

Acousto-optic Modulators with a Broadened Frequency Band for Fiber-Optic Lines

Enhanced sound focusing with single-slit lens

Sound transmission efficiency and beaming capability are the most important considerations in the design of single-slit lens toward high-performance acoustic imaging and detection. Here, we demonstrated numerically and experimentally enhanced sound focusing through a single subwavelength slit surrounded by grooves on both surfaces. A microscopic coupled-wave model was employed to optimize the slit-groove structure for both transmission enhancement and beam focusing. The simulated and measured results were in good agreement and readily verified the effectiveness of the proposed method in enhancing sound focusing with a single-slit lens. This work paves an alternative avenue for the design of single-slit lens, which may find potential applications in the broad field of acoustics, such as tumor boundary treatment, edge detection, and subwavelength acousto-optic modulation.