Single-frequency Pulsed Laser Oscillator and System for Laser-ultrasonics

We present a new pulsed laser oscillator and system for the optical detection of ultrasound in materials. A single-frequency laser oscillator based on a pulse pumped Nd:YAG rod inside a ring cavity is proposed. The laser delivers single-frequency pulses of 35 W power. Power of about 1 kW can be obtained when the second rod of a dual-rod pumping chamber is used as an amplifier. Performance of the system is then investigated with a GaAs photorefractive crystal-based two-wave mixing phase demodulator. In particular, the intensity noise of the laser can be made low enough to allow the detection limit to be set by the shot-noise of the laser. The coherence length of the laser is about 20 m, which makes it a versatile laser-ultrasonic inspection system operated with a two-wave mixing based phase demodulator. A complete compact and affordable system is obtained when the second rod of the pumping chamber is used as a generation laser of ultrasound. Tests of this laser-ultrasonic system on metallic samples are presented. In that case the second rod of the pumping chamber is used as an ultrasound generation laser. Introduction: Laser generation and detection of ultrasound presents numerous advantages for material inspection and characterization over classical piezoelectric-based techniques [1]. In particular, lasers allow probing parts with complex shape in severe conditions, such as high temperature or parts moving at high speed on a production line [2]. A laser-ultrasonic system is typically made of two units, one for generation and another one for detection of ultrasound. The generation unit is essentially a short pulse laser with sufficient power to generate adequately ultrasound in the material. The detection unit typically includes a detection laser, cw or long pulse, and an optical interferometer. The interferometer, working as a phase demodulator, is used to measure the small ultrasonic displacements (typically in the Angstrom or nanometer range) produced at the surface of the tested part by the ultrasonic waves. Since insensitivity to laser speckle caused by surface roughness is preferable for practical use in industry, speckle insensitive phase demodulator schemes based on a confocal Fabry-Perot have been developed more than a decade ago [3]. More recently, adaptive phase demodulator schemes based on two-wave mixing in a photorefractive crystal were developed [4, 5, 6]. In a photorefractive two-wave mixing-based phase demodulator (TWMPD), a pump beam interferes inside the crystal with a signal beam that has acquired a phase modulation after reflection by the inspected part. An interference grating or more generally a hologram, is created and diffracts the pump beam in the direction of the signal beam, thus creating a local oscillator beam which is speckle-adapted to the signal beam. Photorefractive-based adaptive phase demodulators are a good alternative to confocal Fabry-Perot interferometers for ultrasonic detection [5]. In addition to the phase demodulator, the laser-ultrasonic detection unit requires a detection laser. This detection laser should meet stringent conditions regarding its power and noise figure. The detection laser should be sufficiently powerful to get an adequate sensitivity for remote measurement of the small ultrasonic surface displacements on highly scattering and absorbing industrial surfaces. In practice, such a power is obtained with a pulsed system; a pulse duration of about 50 μs is usually sufficient to capture several ultrasonic echoes bouncing back and forth in the material. The laser intensity and phase fluctuations should also be sufficiently small so that the dominant noise source is the shot-noise. The spectrum of these fluctuations is usually dominated by the frequency peaks at the relaxation oscillation of the laser and its harmonics. With a proper laser design, the relaxation oscillation frequency is kept outside the ultrasonic frequency range, allowing to reduce this noise source by electronic filtering. Finally, the coherence length of the laser should be also sufficiently large for adequate operation of the demodulator, although this coherence requirement is substantially diminished with two-wave mixing demodulators since the signal and beam paths could be made nearly equal in this case. To fulfill these requirements, current practice is to start from a very stable single-frequency, cw diode-pumped Nd:YAG laser oscillator and to amplify it through a series of flashlamp-pumped or diode-pumped Nd:YAG amplifiers, this approach is commonly named Master-Oscillator Power Amplifier or MOPA. These laser sources can deliver pulses up to a few kW peak power and of very low intensity and phase noises. The coherence length of these laser sources is several km, which is actually much more than needed. These laser sources include many optical elements (several amplifier stages or multi-passes schemes and Faraday isolators to prevent spurious lasing) and consequently tend to have a high cost, which is a significant impediment for broader use of laser-ultrasonics. We present here the results of an effort to make this technology more affordable, particularly by a simplification of the detection laser. Instead of starting from an extremely stable cw diode-pumped oscillator but relatively low power, we have developed a pulsed oscillator with much higher power and adequate characteristics. Depending upon the application, this pulsed oscillator can be used directly without further amplification. In one version, this pulsed oscillator is followed by an amplifier to provide output power and characteristics which, for laser-ultrasonic measurements, are essentially equivalent to the presently existing kW units composed of a cw oscillator followed by amplifiers. In another version, the amplifier stage is turned into a Q-switch generation laser, thus providing an even more cost effective system. This pulsed oscillator, which is affected by some intensity and phase noise and has a limited coherence length, is preferably associated with a TWMPD rather than a confocal Fabry-Perot. TWMPDs have the advantage to provide excellent rejection against intensity noise from the laser source by the associated balanced detection scheme. Reduction of the intensity fluctuations of the laser by 30 dB is readily feasible. Furthermore, by having the signal and pump beam paths substantially equal makes the system insensitive to phase fluctuations and relaxes the requirements on phase noise and the coherence length of the laser source. Single-frequency detection laser: The detection laser is preferably operated on a single-frequency mode (one longitudinal and one transverse mode). Operating the laser in the multimode regime results in a smaller coherence length, which can be overcome with the TWMPD by making the pump and signal beampaths smaller than the coherence length. However, in practice, having to keep the beampath difference under 1 meter can be very difficult while inspecting contoured parts. More importantly, multimode lasers are noisier than single-frequency lasers due to the mode beatings that produce both intensity and phase fluctuations. The single-frequency laser cavity design is based on two main features. First, the cavity should be as short as possible to make the longitudinal modes selection easier or at least possible. Second, the power inside the laser cavity should be as large as possible to reduce the laser spiking damping time constant and then get a quiet zone in the laser pulse substantially free of intensity fluctuations, and also to get as more power output as possible. In theory, a homogeneous gain medium placed inside a linear cavity oscillates in one single longitudinal mode. In effect, even solid-state lasers are not perfectly homogeneous, and additional modes can occur due to spatial hole burning. Several methods are used to produce single-frequency operation. The most useful are based on the insertion of interferometric filters inside the cavity so that only one frequency is above laser threshold [7]. The most common method of obtaining single-frequency output is to insert an etalon inside the cavity [8]. However, the etalon has to be tilted away from normal incidence such that the laser oscillation due to feedback between the cavity mirrors and the etalon reflections is suppressed. This tilt degrades beam spatial profile and power inside the cavity as well as the etalon selectivity because the multiple reflections of the beam do not recover completely [9]. Three-mirror configurations like Fox-Smith interferometers are efficient because of the high tunability of the free spectral range, but suffer from difficulties to get the proper alignment [10]. Furthermore high finesse interferometers necessitate highly reflecting mirrors. A loss of 1 % for each interferometer mirror involves a roundtrip overall loss of more than 10 %. This involves, along with less laser power, less stability.