Acousto-optic Phase Modulator Research Articles

High-sensitivity fluorescence-detected multidimensional electronic spectroscopy through continuous pump-probe delay scan.

Fluorescence-detected multidimensional electronic spectroscopy (fMES) promises high sensitivity compared to conventional approaches and is an emerging spectroscopic approach toward combining the advantages of MES with the spatial resolution of a microscope. Here, we present a visible white light continuum-based fMES spectrometer and systematically explore the sensitivity enhancement expected from fluorescence detection. As a demonstration of sensitivity, we report room temperature two-dimensional coherence maps of vibrational quantum coherences in a laser dye at optical densities of ∼2-3 orders of magnitude lower than conventional approaches. This high sensitivity is enabled by a combination of biased sampling along the optical coherence time axes and a rapid scan of the pump-probe waiting time T at each sample. A combination of this approach with acousto-optic phase modulation and phase-sensitive lock-in detection enables measurements of room temperature vibrational wavepackets even at the lowest ODs. Alternative faster data collection schemes, which are enabled by the flexibility of choosing a non-uniform undersampled grid in the continuous T scanning approach, are also demonstrated.

Phase-modulated rapid-scanning fluorescence-detected two-dimensional electronic spectroscopy.

We present a rapid-scanning approach to fluorescence-detected two-dimensional electronic spectroscopy that combines acousto-optic phase-modulation with digital lock-in detection. This approach shifts the signal detection window to suppress 1/f laser noise and enables interferometric tracking of the time delays to allow for correction of spectral phase distortions and accurate phasing of the data. This use of digital lock-in detection enables acquisition of linear and nonlinear signals of interest in a single measurement. We demonstrate the method on a laser dye, measuring the linear fluorescence excitation spectrum as well as rephasing, non-rephasing, and absorptive fluorescence-detected two-dimensional electronic spectra.

Superconducting acousto-optic phase modulator.

We report the development of a superconducting acousto-optic phase modulator fabricated on a lithium niobate substrate. A titanium-diffused optical waveguide is placed in a surface acoustic wave resonator, where the electrodes for mirrors and an interdigitated transducer are made of a superconducting niobium titanium nitride thin film. The device performance is evaluated as a substitute for the current electro-optic modulators, with the same fiber coupling scheme and comparable device size. Operating the device at a cryogenic temperature (T = 8 K), we observe the length-half-wave-voltage (length-Vπ) product of 1.78 V·cm. Numerical simulation is conducted to reproduce and extrapolate the performance of the device. An optical cavity with mirror coating on the input/output facets of the optical waveguide is tested for further enhancement of the modulation efficiency. A simple extension of the current device is estimated to achieve an efficient modulation with Vπ = 0.27 V.

Tandem-pulsed acousto-optics: an analytical framework of modulated high-contrast speckle patterns

Recently we presented acousto-optic (AO) probing of scattering media using addition or subtraction of speckle patterns due to tandem nanosecond pulses. Here we present a theoretical framework for ideal (polarized, noise-free) speckle patterns with unity contrast that links ultrasound-induced optical phase modulation, the fraction of light that is tagged by ultrasound, speckle contrast, mean square difference of speckle patterns and the contrast of the summation of speckle patterns acquired at different ultrasound phases. We derive the important relations from basic assumptions and definitions, and then validate them with simulations. For ultrasound-generated phase modulation angles below 0.7 rad (assuming uniform modulation), we are now able to relate speckle pattern statistics to the acousto-optic phase modulation. Hence our theory allows quantifying speckle observations in terms of ultrasonically tagged fractions of light for near-unity-contrast speckle patterns.

Laser frequency stabilisation by the Pound — Drever — Hall method using an acousto-optic phase modulator operating in the pure Raman — Nath diffraction regime

Frequency stabilisation of diode laser radiation has been implemented by the Pound — Drever — Hall method using a new acousto-optic phase modulator, operating in the pure Raman — Nath diffraction regime. It is experimentally shown that, as in the case of saturated-absorption spectroscopy in atomic vapour, the spatial divergence of the frequency-modulated output spectrum of this modulator does not interfere with obtaining error signals by means of heterodyne frequency-modulationspectroscopy with a frequency discriminator based on a high-Q Fabry — Perot cavity with finesse of several tens of thousands.

Fluorescence-detected two-dimensional electronic coherence spectroscopy by acousto-optic phase modulation

Two-dimensional electronic coherence spectroscopy (ECS) is an important method to study the coupling between distinct optical modes of a material system. Such studies often involve excitation using a sequence of phased ultrashort laser pulses. In conventional approaches, the delays between pulse temporal envelopes must be precisely monitored or maintained. Here, we introduce a new experimental scheme for phase-selective nonlinear ECS, which combines acousto-optic phase modulation with ultrashort laser excitation to produce intensity modulated nonlinear fluorescence signals. We isolate specific nonlinear signal contributions by synchronous detection, with respect to appropriately constructed references. Our method effectively decouples the relative temporal phases from the pulse envelopes of a collinear train of four sequential pulses. We thus achieve a robust and high signal-to-noise scheme for phase-selective ECS to investigate the resonant nonlinear optical response of photoluminescent systems. We demonstrate the validity of our method using a model quantum three-level system-atomic Rb vapor. Moreover, we show how our measurements determine the resonant complex-valued third-order susceptibility.

Wave packet interferometry and quantum state reconstruction by acousto-optic phase modulation

Studies of wave packet dynamics often involve phase-selective measurements of coherent optical signals generated from sequences of ultrashort laser pulses. In wave packet interferometry (WPI), the separation between the temporal envelopes of the pulses must be precisely monitored or maintained. Here we introduce a new (and easy to implement) experimental scheme for phase-selective measurements that combines acousto-optic phase modulation with ultrashort laser excitation to produce an intensity-modulated fluorescence signal. Synchronous detection, with respect to an appropriately constructed reference, allows the signal to be simultaneously measured at two phases differing by 90 degrees. Our method effectively decouples the relative temporal phase from the pulse envelopes of a collinear train of optical pulse pairs. We thus achieve a robust and high signal-to-noise scheme for WPI applications, such as quantum state reconstruction and electronic spectroscopy. The validity of the method is demonstrated, and state reconstruction is performed, on a model quantum system--atomic Rb vapor. Moreover, we show that our measurements recover the correct separation between the absorptive and dispersive contributions to the system susceptibility.

Actively mode-locked visible upconversion fiber laser

Active mode locking of a Pr(3+)/Yb (3+) -doped upconversion fluoride fiber laser with an all-fiber ZnO acousto-optic phase modulator is demonstrated for the first time to the authors' knowledge. Optical pulses of ~550-ps duration with a repetition rate of 239 MHz at a wavelength of 635 nm have been generated.

Broadband frequency response of an all-fiber acousto-optic phase modulator using aluminum nitride (AIN) coating

For the first time, we present results on an all-fiber acousto-optic phase modulator using aluminum nitride (AlN) piezoelectric coating. Finite-element analysis has been employed to determine the response of the modulator from 10 kHz to 600 MHz. At frequencies higher than 28 MHz, the response is dominated by radial resonances of the fiber–coating composite. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 25: 124–125, 2000.

Optical response of an all-fibre acoustooptic phase modulator using aluminium nitride coating

For the first time we present results on an all-fibre acoustooptic phase modulator using aluminium nitride (AlN) thin film as the piezoelectric coating. Two-dimensional axisymmetric finite element analysis was employed to determine the response of the modulator over a wide frequency range from 10 kHz to 700 MHz. Phase shifts of 0.823 rad/V/m in the low frequency region and 0.374 rad/V/m in the high frequency region were observed. At frequencies higher than 28 MHz the response is dominated by radial resonances of the fibre-coating composite. A theoretical investigation of the acoustic impedance showed that there is good impedance matching between the AlN piezoelectric coating and the glass fibre resulting in a high acoustic energy transfer of ∼82% at the coating–fibre interface.

Broadband frequency response of an all-fiber acousto-optic phase modulator using aluminum nitride (AIN) coating

For the first time, we present results on an all-fiber acousto-optic phase modulator using aluminum nitride (AlN) piezoelectric coating. Finite-element analysis has been employed to determine the response of the modulator from 10 kHz to 600 MHz. At frequencies higher than 28 MHz, the response is dominated by radial resonances of the fiber–coating composite. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 25: 124–125, 2000.

Design of high-frequency ZnO-coated optical fiber acoustooptic phase modulators

In this paper, we present a theory of an optical fiber acoustooptic phase modulator using piezoelectric zinc oxide (ZnO) coating. The effects of changing device parameters in the acoustooptic resonator are investigated for transducer design. These results are entirely different from those of the well-known; planar device structure since every part plays an important role in the resonator structure. Optical phase modulation in the frequency region up to 1 GHz is also demonstrated. Multiple sharp resonance peaks with a maximum phase shift of 0.7 radians are observed.

Phase Modulation of the Light Wave Propagating in an Optical Fiber by Acoustic Waves

The optical fiber acoustooptic phase modulator offers the advantages of small size, light weight and low diffraction and optical coupling loss. In this paper, we present the fabrication process and results of our proposed device. A ZnO thin-film bulk acoustic wave transducer was fabricated on a glass substrate which was then bonded on the optical fiber. We experimentally obtained an amplitude-modulated lightwave signal with an input of 9 V at 94 MHz with our measurement system.

All-fiber acoustooptic phase modulators using chemical vapor deposition zinc oxide films

For the first time we report an all-fiber acoustooptic phase modulator using a zinc oxide (ZnO) film deposited by modified single source chemical vapor deposition. This technique allows deposition over the full 360/spl deg/ fiber surface without the need for sample rotation, greatly simplifying the manufacturing process. The maximum phase shift measured for our 6 mm long devices was 3.5 rads for a drive power of 580 mW. Unlike devices fabricated using sputtered ZnO films the maximum attainable phase shift is not significantly limited by thermal and mechanical loss effects at higher driving powers which we attribute to the excellent chemical composition of our chemical vapor deposition (CVD) grown films. A maximum efficiency of 0.28 rad//spl radic/(mW)/cm of device length was measured which may be attributed to the relatively thin films used in this experiment (0.4-0.9 /spl mu/m).

Asymmetric double-quantum-well phase modulator using surface acoustic waves

An AlGaAs-GaAs asymmetric double-quantum-well (DQW) optical phase modulator using surface acoustic waves is investigated theoretically. The optimization steps of the DQW structure, which so far have not been reported in detail, are discussed here. The optimized phase modulator structure is found to contain a five-period QDW active region. A surface acoustic wave induces a potential field which provides the phase modulation. Analysis of the modulation characteristics show that by using the asymmetric DQW, the large change of the induced potential at the surface and thus large modification of the quantum-well (QW) structure can be utilized. The modification of each QW structure is consistent, although this consistency is not always preserved in typical surface acoustic wave devices. Consequently, the change of refractive index in each of the five DQW's is almost identical. Besides, the change of effective refractive index is ten times larger here in comparison to a modulator with a five-period single QW as the active region and thus produces a larger phase modulation. In addition, a long wavelength and a low surface acoustic wave power required here simplify the fabrication of surface acoustic wave transducer and the acoustooptic phase modulator.

ZnO-coated fiber acoustooptic phase modulator

We have developed an approximate theory of all-fiber acoustooptic phase modulator using ZnO coating. The induced axial and radial strains by acoustic waves dominantly affect low- and high-frequency modulation, respectively. The prediction of the optical phase modulation agrees well with the experimental published data.

Acousto-optic phase modulation of a laser Doppler vibrometer signal resulting from the radiated pressure produced by a submerged cylindrical shell: Numerical predictions and experimental results.

It has been proposed that large stand-off laser Doppler vibrometer (LDV) measurements have significant errors resulting from index of refraction modulation due to the acoustic pressure distribution along the laser light paths between the structure and the probe [Y. H. Berthelot et al., J. Acoust. Soc. Am. 97, 3347(A) (1995)]. A high fidelity numerical model has been used to predict the pressure distribution and shell motions for a submerged finite ribbed cylinder under point excitation. Generally speaking, these predictions show low levels of contamination even for large stand-off probe distances. In addition, an experimental study has been conducted at NRL using a newly integrated three-axis vibrometer and near-field pressure probe where shell motions and near-field pressure distributions were measured for the case of a submerged ribbed cylinder under point excitation. The experiments and the associated numerical modeling both indicate low levels of acousto-optic contamination of the shell velocity signals. These findings contrast previously reported predictions of the level of acoustically induced error for large stand-off in-water LDV designs. [Work supported by ONR.]

Modeling of a high frequency acousto-optic fiber phase modulator

The theory of a cylindrical acousto-optic phase modulator in the form of a thin ZnO film coated at the surface of a single mode optical fiber has been presented for high efficiency modulator design. The phase modulation at the frequency around 600 MHz with 15 dBm drive power was achieved in a simple acousto-optic device. The validity of acousto-optic modulator modeling is confirmed by experimental results. The device performance for various parameters has been theoretically investigated.

Acoustooptic in-fiber modulator using acoustic focusing

A new efficient in-fiber acoustooptic phase modulator is demonstrated in which the acoustic wave is focused onto the fiber by reflection from a cylindrical surface. The device exhibits a phase modulation of 2.63 rad at a light wavelength of 1550 nm, using an RF frequency of 86 MHz and power of 1 W. >

All-fiber acoustooptic phase modulators using zinc oxide films on glass fiber

Experiments with an acoustooptic phase modulator for single-mode optical fibers are described. The device, which operates in the 500-MHz frequency range, uses a zinc oxide transducer sputter-deposited on top of a gold film evaporated on the fiber. The device is heat cooled and partially acoustically terminated by gallium, which is also used as the top electrode of the transducer. The maximum modulation observed on a 2-mm length of fiber is 2 rad with an input power of 2 W. >