Sample interview questions: How would you approach characterizing the temporal and spatial coherence of a laser beam?
Sample answer:
- Temporal Coherence Measurement:
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Intensity Autocorrelation: Deploy an intensity autocorrelator to capture the temporal coherence of the laser beam. In this technique, a Michelson interferometer arrangement is used, where a fraction of the beam is delayed and then recombined with the main beam, resulting in an interference pattern. The temporal coherence length can be determined from the width of the resulting autocorrelation function.
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Heterodyne Beat Measurement: Employ a heterodyne beat technique, where two lasers with slightly different frequencies are interfered to produce a beat signal. The beat frequency provides information about the temporal coherence of the lasers. A stable and narrow beat frequency indicates high temporal coherence, while fluctuations or broadening of the beat frequency suggest reduced coherence.
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Spatial Coherence Measurement:
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Young’s Double Slit Experiment: Utilize Young’s double-slit experiment to assess the spatial coherence of the laser beam. By observing the interference pattern produced by the two slits, the spatial coherence length can be estimated. A sharp and distinct interference pattern indicates high spatial coherence, while a washed-out or smeared pattern suggests poor coherence.
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Spatial Filtering: Employ spatial filtering techniques to isolate and analyze different spatial modes of the laser beam. A pinhole or a slit can be used as a spatial filter to select a specific portion of the beam, allowin… Read full answer