In modern scientific and industrial laser applications, inter-alignment of multiple optical devices is frequently a basic requirement to meet a certain specification and performance.
However, the designed optical system combining mechanical elements, lasers and optical sights in various wavelengths frequently deviates from specified goals due to real life imperfections and effects. These may include mechanical tolerances, optical distortion, heating, laser cavity misalignment, overall instabilities, and non-linear effects.
In order to deliver accurately and produce intricate optical systems, a carefully designed method for inter-alignment is required completing and updating the already existing methods. Thus, we designed and upgraded the performance of an electronic autocollimator and combined it with innovative mechanical manipulation of optical invariants such as a Lateral Transfer Hollow Periscope to greatly improve and expand inter-alignment procedures.
Depending on the combination of optical sights, laser types, and mechanical requirements, an appropriate method will be analyzed. For example, several layouts will be analyzed such as high power CO2 laser cavity alignment, laser delivery system, and mechanical rollers alignment. By completing the presented gear in this article other instruments such as Align Meter, Lateral Hollow Periscope (LTHP™), Lateral Hollow Retro-reflector (LTHR™) are available for applications such as alignment of articulated beam delivery systems.
This article will cover the aspects of laser beam and optical devices inter-alignment especially where parallax between elements poses a great challenge.
1. FUNDAMETALS OF INTER-ALIGNMENT GEAR
To avoid over-specifying mechanical and optical tolerances, it is helpful to use final alignment and assembly processes that can achieve high levels of accuracy and superior overall performance. The system’s assembly level should provide the engineer with the most appropriate assembly technique and instruments to perform the alignment task that will improve performance without the added cost of manufacturing of very accurate parts.
Advances in sensor technology have opened the door to a new measuring instrument, based on autocollimation principles combining laser beam collimation and direction measurement, accurate telescopic measurement, focusing techniques with sophisticated software and computing techniques.
1.1 Laser analyzing autocollimator
Of special interest is the laser analyzing autocollimator since it combines laser-analyzing technology with angular reflection technology.
The technology presented allows combining data of the alignment autocollimator with incoming laser beam information to increase the overall performance and possible applications. Laser beam deviation is compared with autocollimator center and allows intricate alignment of lasers and optical elements to mechanical datum planes.
Due to the above features, the proposed device is potentially usable in aligning a laser beam to its mirrors as well as the alignment of a laser cavity and laser rods.
Moreover, the demand for greater precision in high-performance applications, especially in high power CO2 laser systems for automotive industries, has increased the use of these lasers. As higher accuracies for mechanical alignment combined with higher power levels can be achieved, optical alignment is a necessity.
The autocollimator device comprises of two instruments fused into a single unit; one being an electronic autocollimator and the second is a laser beam profiling system. This combination with some additional optics is very useful in the alignment and angular analysis of lasers and optical devices.
1.2 Description of Laser Autocollimator
Known autocollimators are high-precision instruments for measurement of angular reflections. Main applications are calibration and measurement tasks in optical and mechanical engineering.
An autocollimator projects a reticle pattern as a collimated beam to infinity by a telescopic lens. A mirror positioned in its optical path reflects the beam into the autocollimator’s aperture.
The reflected beam generates the autocollimation image that is observed by the user via an eyepiece. In general, the autocollimation effect and name are derived from the fact that the same telescopic lens is used to both project the image and collect its back reflection from a mirror element.
The shift of the autocollimation image from the center of the reticle in both an x and y direction is proportional to the mirror’s tilt angle. An electronic autocollimator presents the angular deviation using CCD cameras with resolutions down to 0.01 arc sec. For increased capabilities, a new breed of autocollimators was designed with built-in laser beam profiling for analyzing incoming collimated beams for measuring divergence and incoming angle with respect to autocollimator’s center. Moreover, by a builtin focusing knob, accurate center position of mechanical systems is possible.
Figure 1: Principals of Electronic Autocollimator
1.3 Lateral Offset Device (LOD)
The uniqueness of the Lateral Offset Device is that it can cancel the parallax between several lines of sights over an almost continuous distance. The LOD (Fig. 2) illustrates two Rotary Mounts orthogonally mounted with respect to each other and a Hollow Periscope fixed to the top Rotary Mount. The orthogonally mounted Rotary Mounts' purpose is to steer the periscope mounted on the top Rotary Mount. By doing so, the distance between an incoming light beam into the periscope and its outgoing direction is controlled.
Due to the above features, the proposed device is potentially usable for alignment or Boresighting between two parallel lines of sights or laser beams as well as the alignment of mechanical datum planes relative to other optical devices.
The Monolithic Hollow Periscope is based on two parallel mirrors with a given distance in between. The Rotary Mount is preferably mounted on a mechanical device serving as an optical bench.
For further understanding of the complexity of inter-alignment, the characteristics of several applications are to be described, namely: High power CO2 laser cavity alignment, testing accuracy of a beam delivery system and testing parallelism of rollers for mechanical processing.
Combining the laser analyzer autocollimator with an LOD opens the door to numerous intricate alignments that would otherwise be quite difficult and time consuming to produce.