O1.4 - Adaptive interferometer with optoelectronic feedback

Abstract

Detection of low-amplitude acoustic vibrations of real objects, such as ultrasonic transducers, composite materials, biological objects, metal production, MEMS is an important scientific and technical problem. Homodyne laser vibrometers are suitable for practical applications in this regard for their high sensitivity which is limited in principle only by shot-noise of the laser used [1]. These vibrometers allow detection at a distance, with high spatial localization of the measurement region and wide frequency range. However, their utilization is restrained by several problems such as slow phase drifts in the interferometer arms due to environmental reasons, necessity of fine optical adjustment and suppression of laser amplitude noise. Wide-gap photorefractive materials offer an elegant way to solve the problem of keeping operation interferometer point constant [2-4]. The crystal replaces the conventional beamsplitter and can be controlled not only electrically but also optically, i.e., based on the principles of nonlinear optics. In addition, it can be multilayered. Such a multilayered adaptive beamsplitter is nothing else but a volume dynamic hologram recorded by the reference and signal waves. Adaptive detectors based on the effect of the non-steady-state photo-electromotive force (photo-EMF) [5] are promising for sensor applications. The nonstationary holographic photocurrent (or non-steady-state photo-EMF) is a holography related effect and it appears in a semiconductor material illuminated by an oscillating light pattern. Such illumination is usually created by two coherent light beams one of which is phase modulated with frequency . Among the number of the nonlinear phenomena observed in photorefractive crystals there are effects which appearance or specific behavior is determined by additional feedback loop. Stabilization of a holographic setup was one of the earliest and the most important application of the optoelectronic feedback. It is implemented using the detection scheme and the piezoelectric mirror placed in one of the arms of the setup. The mirror is driven by the amplified voltage of the detector providing the necessary feedback loop. Such feedback keeps the difference of the optical path lengths to be constant and correspondingly stabilizes position of the interference fringes. As a result the holograms are recorded with high modulation index. Generally speaking, the implementation of the feedback in optical schemes reveals features typical to those in common dynamical systems (mechanical, electronic, biological) considered in the theory of oscillation and control theory [6, 7]. For example, the negative feedback in Michelson interferometer provides the ability to linearize the sensor (photodiode) output allowing detection of large displacements [8]. The conventional electronic operational amplifier demonstrates similar behavior with the feedback loop. Various optical schemes with positive feedback such as laser, molecular cell inside Fabry-Perot resonator, hybrid schemes with birefringent materials [9] reveal the oscillatory or bistable behavior which also has analogs in electronics and mechanics (harmonic oscillators, triggers). Since the presence of the properly chosen feedback in dynamical system not only improves its performance but can also provide observation of new nonlinear phenomena, we have applied feedback operation concept to the sensor based on the effect of the non-steady-state photo-EMF. The non-steady-state photoelectromotive force (photo-EMF) is a holography related effect and it appears in a semiconductor material illuminated by an oscillating light pattern. Such illumination is usually created by two coherent light beams one of which is phase modulated with frequency . The alternating current is resulted from the periodic relative shifts of the photoconductivity and space charge gratings which arise in the crystal's volume under illumination. Like the holographic recording in photorefractive crystals this effect demonstrates adaptive properties that promote its application in such areas as vibration monitoring, velocimetry, etc. [10]. In contrast to the holographic methods this technique allows the direct transformation of phase modulated optical signals into the electrical current and can be applied for characterization of centrosymmetrical and even amorphous materials. Since the photocurrent is originated from the interaction of both the photoconductivity and space charge gratings a lot of photoelectric parameters can be measured [11]. 6.5 1 4