An effective configuration for interferometric measurement of pulsed laser-induced plasma densities

Pulsed laser-induced plasmas evolve in nanoseconds, so instruments to observe that evolution must be very high speed. We have used a Wollaston prism to create interference fringes, and set up a electronic system to obtain synchronization between the main laser and probing laser. By adjusting the delay unit to make the pulse output of probing laser lag behind the pulse output of main laser for different times, the situation of the plasma at different stages can be recorded. The individual interferometric image is captured by a camera frame grabber that is triggered by the incidence of a probing laser signal on a CCD camera; so the induced phase shift pattern is easy to be photographed and analyzed in terms of plasma density. A demonstration of pulsed laser induce plasma from a graphite target is provided.     

High dynamic range imaging method for interferometry

We demonstrate a method to easily and quickly extend the dynamic range imaging capabilities of the camera in a typical interferometric approach. The camera dynamic range is usually low and limited to 256 gray levels. Also, it is well known that one may have over or under-exposed regions in the interferogram (due to non-uniform illumination) which makes these image regions not reliable. In our proposed method it is not necessary to obtain or use the non-linear camera response curve in order to extend the camera dynamic range. We obtain a sequence of differently exposed interferograms, typically five or six; after that, we compute the corresponding normalized fringe patterns and modulation maps using a typical normalization method. These normalized patterns are combined through a temporal weighted average using as weights the corresponding modulation maps. We show a set of experimental results that prove the effectiveness of the proposed method.     

Multiple beam Fizeau interferometer with filtered frequency comb illumination

Real wedge interferometers of the Fizeau-type do not allow for fringes in case of a spectral broadband source - or in short: for white light fringes. Here, the use of a suitable frequency comb source will help to overcome this limitation on the one hand and on the other will offer the capability for enhanced phase sensitivity in high precision measurements of surface deviations. Frequency combs can be produced either by using a pulse train from a fs-laser or by passive filtering of the light emitted by a broadband source as a superlum-diode or a fs-laser. The frequency comb produced by a common fs-laser is extremely fine, i.e., the frequency difference of consecutive peaks is very small or the distance of consecutive pulses of the pulse train might be of the order of 1 m. Therefore, the pulse train produced by passive filtering of a broadband source is better adapted to the needs of surface testing interferometers. White light fringes are either applied for the profiling of discontinuous surfaces and/or can serve as an indication for the correct choice of multiplication factors in superposition interferometry. During the last decennium it became more and more clear that spatially incoherent sources provide better measuring accuracy in surface measurements due to the reduced influence of dust diffraction patterns. The advantage of laser illumination can nevertheless be maintained if the laser light is made spatially incoherent through moving scatterers in the light path. Here, we will discuss the application of spatially incoherent broadband light frequency filtered through a Fabry-Perot filter. The main applications are in the following fields: (1) surface profiling applications using two-beam Fizeau interferometers, (2) selection of single cavities out of a series of interlaced cavities, and (3) sensitivity enhancement for multi-beam interferometers for planeness or sphericity measurements. Some of the discussed possibilities will be experimentally demonstrated.     

Profilometry with compact single-shot low-coherence time-domain interferometry

We describe the performance of a compact single-shot low-coherence interferometric scheme that can be capable of measuring three-dimensional surface profiles and shape. This technique utilizes a polarizing Michelson interferometer and a four-channel polarization phase-stepper optics, which is based on a paired wedge prism, a combined wave plate and a Wollaston prism. The coherence gated surface image can be calculated by the simultaneous acquisition of two interferograms and a DC image on a single CCD camera. The image calculation is based on a novel algorithm to calibrate the imbalanced intensity as well as the deviated arbitrary relative phase of each of the imaging channels. The system can display the transverse cross-sectional images in real-time. To demonstrate the feasibility of this system, a Japanese coin is presented as a 3-D shape measurement example with an image size of 4 mm (horizontal) × 4 mm (vertical) × 160 μm (depth).