Accuracy in laser beam collimation is very important in systems used for precision measurements. The present work reports a technique for collimation testing of laser beams using two proximately placed holographic optical elements (HOEs). The required HOEs are designed and fabricated such that upon illumination with the test beam, they release two laterally sheared wavefronts, at desired angles from the directly transmitted beam, that superimpose each other to generate straight interference fringes. Deviation from the collimation of the test beam results in orientation of these otherwise horizontal fringes. The novelty of this setup comes from the fact that HOEs are lightweight, as well as easy to fabricate as compared to conventional wedge plates used for collimation testing, and generate high contrast fringes compared to other interferometry, holography, Talbot and Moiré based techniques in a compact manner. The proposed technique is experimentally validated by measuring the orientation of fringes by an angle of 16.4° when a collimating lens of focal length 200 mm is defocused by 600 μm. The accuracy in the setting of this collimation position is obtained to be 10 μm.
High sensitivity of collimation testing equipment is desirable where collimated beams are used for precise and accurate measurements. Precision in the setting of collimation depends on the sensitivity of the testing equipment. In the present work, sensitivity to beam collimation of the recently reported holographic shearing interferometer (HSI) [J. Opt. 20, 055603 (2018)JOOPDB0150-536X10.1088/2040-8986/aab6dc] is measured and compared with sensitivities of other collimation testing techniques based on the wedge plate shearing interferometer and the Talbot shearing interferometer. For a test beam of diameter 25 mm from an He–Ne laser and displacement of the collimating lens by 1 mm from the collimation position, the Talbot shearing interferometer shows a rotation of interference fringes from the horizontal direction by 2°, the wedge plate shearing interferometer shows 20°, and the HSI shows 25°. Sensitivity is also presented in terms of measured slopes of phase maps of the recorded interferograms for a 1 mm displacement of the collimating lens and is obtained as 0.98 mrad, 15 mrad, and 19 mrad corresponding to the Talbot shearing interferometer, the wedge plate shearing interferometer, and HSI, respectively. The effect of decollimation of the laser beam on the interference fringes of diffraction of the Lloyd mirror interferometer is also demonstrated. Theoretical concepts and experimental results are presented and discussed for the above-mentioned beam collimation testing techniques.
Due to improvements in the technology of laser diodes, nowadays they have become an integral part of most physics and engineering undergraduate courses. Laser diodes, which generally have low coherence, are extensively used in various educational experiments due to their compactness, ease of operation and handling, as well as their cost-effectiveness. Thus, knowledge about the coherence length of beams from these laser diodes becomes important when designing experiments using these as light sources. Generally, students find it difficult to measure coherence length with most of the existing techniques. In the present work an experimental arrangement using holographic optics is designed and demonstrated to measure the temporal coherence of a laser diode beam in a compact and easy-to-align manner. The proposed scheme is particularly suitable to measure short coherence lengths. The temporal coherence of a laser diode is measured to be 2.1 ± 0.02 mm, by varying separation between two holographic optical components. The results obtained are experimentally compared with results from a Michelson interferometer.
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