We report on a laser communications experiment over a kilometre optical range where we have used a retro-reflective
transponder incorporating an optical modulator based on silicon micro-electro-mechanical systems (MEMS) device. This
employs interference to provide modulation and relies on performing as a coherent array to modulate incident light in the
near-IR band (1550nm) over a wide angular range (120 degrees). Modulation is achieved by tuning a large array of
Fabry-Perot cavities via the application of an electrostatic force to adjust the gap between a moveable mirror and the
underlying silicon substrate.
The micro-mirrors have a strong mechanical resonance, and modulate light by adjusting the spacing between the micromirrors
and the substrate. We use a 'release and catch' technique to exploit the mechanical resonance, and we time the
motion of the micro-mirrors to be synchronised with the arrival of an interrogator pulse to ensure that the etalon spacing
provides the required modulation, whatever the angle of incidence.
We describe experiments over a one kilometre path where simple strings were sent at 200kbit per second. We also
discuss approaches to adapting the link to a given angle of incidence.
Measurement of the laser beam propagation factor M<sup>2</sup> is essential in many laser applications including materials
processing, laser therapy, and lithography. In this paper we describe the characterisation of a prototype device using a
cross-distorted diffraction grating known as an Image Multiplex (IMP<sup>(R)</sup>) grating, to measure the M<sup>2</sup> value of laser beams.
The advantage of the IMP<sup>(R)</sup> grating instrument lies in its ability to simultaneously image nine positions along the beam
path. This enables beam propagation parameters to be calculated both for pulsed lasers and lasers with rapidly changing
propagation characteristics. This is in contrast to the scanned technique recommended by the ISO, which is relatively
slow and in practice can only be easily used with cw sources. The characterisation was accomplished by comparison of
results from the IMP<sup>(R)</sup> grating device with those obtained using the accepted methodology described in the ISO 11146
series of standards through measurements conducted by the National Physical Laboratory. The scope of the work also
included provision of a traceability route to international standards, and an uncertainty budget, to allow the intended user
community to have confidence in measurements obtained when using the device, and to enable them to use it as part of
their quality framework.
We describe a camera capable of recording 3D images of objects. It does this by projecting thousands of spots onto an object and then measuring the range to each spot by determining the parallax from a single frame. A second frame can be captured to record a conventional image, which can then be projected onto the surface mesh to form a rendered skin.
The camera is able of locating the images of the spots to a precision of better than one tenth of a pixel, and from this it can determine range to an accuracy of less than 1 mm at 1 meter. The data can be recorded as a set of two images, and is reconstructed by forming a 'wire mesh' of range points and morphing the 2 D image over this structure. The camera can be used to record the images of faces and reconstruct the shape of the face, which allows viewing of the face from various angles. This allows images to be more critically inspected for the purpose of identifying individuals. Multiple images can be stitched together to create full panoramic images of head sized objects that can be viewed from any direction. The system is being tested with a graph matching system capable of fast and accurate shape comparisons for facial recognition. It can also be used with "models" of heads and faces to provide a means of obtaining biometric data.
As part of a government funded multidisciplinary project, a sensor has been developed to allow the measurement of the ultrasonic amplitude at the tip of the bonding too. A laser interferometer has been designed with a fiber coupled miniature measurement head which can be mounted on the rotating bond head. A modulated visible diode laser is employed to generate a direction-sensitive Doppler signal according to the pseudo-heterodyne principle.
This paper describes the application of the well-known scanning laser interferometric technique to the vibration measurement of small objects. The technique has been optimized to measure small biological specimens, in which sizes can range from 100 micrometers to 10 mm, but proves to be also suitable for vibrational studies on micromechanical systems. In the field of sensory physiology, it is here applied to the investigation of the biomechanical properties of small tympanal systems. Some results are briefly presented.
Laser interferometric vibrometers are now well known and accepted as sensitive, accurate, high bandwidth and linear measurement system. For many applications the internal complexity and resultant size of the interferometric sensor head limits the widespread use. This paper describes the performance and principle of operation of a new miniaturized interferometric sensor head which retains the important characteristics of the previously mentioned systems, but embodied in a robust compact housing no larger thana typical torchlight. Velocity resolution in the acoustic range has been found to be up to 50 nanometers/sec in a 10 Hz RBW. The size of this new sensor head allows it to be mounted on balanced microscope assemblies or within machinery, and the waterproof design allows disinfectant cleaning in clinical applications or operation in industrial environments.
With the new approach described here, the two tangential velocity components needed are acquired separately using the principle of optical heterodyne interferometry, and combined to provide rotational information after having been converted into electrical signals. Due to the heterodyning with the unshifted reference beam, this provides an optical amplification of each measurement beam, and thus significantly increases the optical sensitivity of the system.
Many reports concerning Laser Doppler Vibration Sensing describe the detection systems as 'High-Sensitivity', and imply this is a correct manner of specifying the performance of the optical system. In this paper, we describe how the performance of a vibrometer is determined by two parameters, which we have (arbitrarily) defined sensitivity and resolution. An analysis of these performance parameters for two different Vibrometer configurations shows that a fiber-optic based system which provides marginally lower sensitivity than its bulk optic counterpart, can actually provide a better resolution. An analysis of various applications and their system requirements, shows that Vibrometer users must understand which of these parameters is the most important. For example, NDT testing using an Ultrasonic Detecting LDV is shown to be mostly critical to resolution (small signal detection) and not so critical in terms of sensitivity (performance from poor quality surfaces). In this paper we present a method of defining sensitivity and resolution and outline the results obtained with fiber and non-fiber systems.
Previously described interferometric techniques for the measurement of rotational motion include the use of a differential mode interferometer directly acquiring the difference of two parallel velocity vectors at separate points of the rotating object. The difference value is immediately obtained in the optical domain and thus the resulting beat frequency at the detector output is directly proportional to the absolute value of the angular velocity. Despite the ingeniously simple design, this technique has some disadvantages: (1) Due to the direct heterodyning, the reflected beams must have a sufficient intensity to generate a useful signal at the interferometer's output. This results in the necessity of a retroreflective coating at the surface under investigation. (2) Acquisition of rotational vibrations is only possible if the rotational speed exceeds a certain value, because no useful carrier frequency will be generated at lower velocities. (3) This technique does not provide directional information. With the new approach described here, the two tangential velocity components needed are acquired separately using the principle of optical heterodyne interferometry and combined to provide rotational information after having been converted into electrical signals. Due to the heterodyning with the reference beam, this provides an optical amplification of each measurement beam and thus significantly increases the optical sensitivity of the system. As a result, measurements from most uncooperative surfaces are possible without any retroreflective coating. Furthermore, this technique allows us to define rotational direction and to acquire rotational vibrations at zero RPM as well.