Laser Phase Noise Rejection by Two-wave Mixing Photorefractive Interferometry

INTERFEROMETRY M. Dubois 1 and T. E. Drake General Electric Global Research, Schenectady, NY, USA; Lockheed Martin Aeronautics Co., Fort Worth, TX, USA Abstract: Laser-ultrasound is a technique currently used for the ultrasonic inspection of composites during manufacturing of advanced jet fighters. This technique is based on a shortpulse laser to generate ultrasonic waves and a long-pulse laser coupled to an interferometer to detect the resulting displacements. In theory, the signal-to-noise ratio of the signal is proportional to the square root of the collected detection light power. In practice however, noise from the laser limits the signal-to-noise ratio above a certain collected power level. When using conventional Fabry-Perot interferometers to demodulate ultrasonic information, one type of noise, amplitude relaxation noise, can be rejected using a differential configuration based on two cavities stabilized on opposite sides of the resonance peak. Another type of laser noise, phase noise, cannot be rejected using Fabry-Perot interferometers and currently limits signal-to-noise ratio. A new type of interferometer based on a photorefractive crystal can be made almost completely insensitive by matching the optical path lengths of the pump and signal beams while still rejecting amplitude noise. In this paper, the phase noise rejection capability of a commercial two-wave mixing photorefractive interferometer is experimentally demonstrated. Results will be presented that demonstrate that speckle effect is not an issue for composite inspection using this interferometer and that it can operate in the photon statistics noise regime for higher collected light powers than conventional differential Fabry-Perot systems. Introduction: A new technique called laser-ultrasound has been developed since the beginning of the 1980’s for the ultrasonic inspection of composite parts in the aircraft industry. This technique is more than an order of magnitude faster than conventional techniques based on piezoelectric transducers and water jets for the non-destructive inspection of complex-shape composite parts during manufacturing. Laser-ultrasound is based on a short pulse laser that generates ultrasonic waves by thermal expansion and on a second long-pulse laser coupled to an interferometer that detects the corresponding mechanical displacements. The laser-ultrasound technology developed at Lockheed Martin Aeronautics Co. and named LaserUTTM is the inspection method for more than 90% of the composite parts of the F/A-22 and will also be used for the F-35 production, to begin in the next few years, resulting in hundreds of millions of dollars in savings. One disadvantage of the laser-ultrasound technology compared to conventional ultrasonic techniques is the lower sensitivity. For most current applications, sensitivity is satisfactory but for some challenging cases, higher sensitivity is required. For those special cases, sensitivity needs to be improved. Sensitivity is characterized through the signal-to-noise ratio (SNR). The SNR of a Fabry-Perot interferometer used for laser-ultrasound composite inspection can be expressed in the following form: