Sample interview questions: Describe the principles behind mode selection and control in laser resonators.
Sample answer:
Mode Selection and Control Principles in Laser Resonators:
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Transverse Mode Selection:
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Unstable Resonators:
- Employ curved mirrors to introduce transverse mode discrimination.
- Larger modes experience higher losses due to reduced overlap with the gain region.
- Favors the selection of lower-order (fundamental) modes.
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Stable Resonators:
- Utilize flat or slightly curved mirrors to achieve stable operation.
- Employ intracavity apertures to block higher-order modes.
- Smaller apertures promote the selection of lower-order modes.
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Longitudinal Mode Selection:
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Fabry-Perot Resonators:
- Consist of two parallel mirrors separated by a distance.
- Act as a wavelength-selective filter, allowing only specific longitudinal modes to resonate.
- Determined by the condition of constructive interference: mλ = 2Ln, where m is the mode number, λ is the wavelength, L is the cavity length, and n is the refractive index.
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Grating-Based Resonators:
- Utilize diffraction gratings to achieve longitudinal mode selection.
- Different wavelengths diffract at different angles, allowing for the selection of specific modes.
- Can provide narrow spectral linewidths and mode stability.
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Hybrid Mode Selection:
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Coupled-Cavity Resonators:
- Combine elements of unstable and stable resonators to achieve both transverse and longitudinal mode control.
- Typically consist of a stable resonator section for mode discrimination and an unstable section for spatial filtering.
- Enable the selection of specific transverse and longitudinal modes.
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Active Mode Control Techniques:
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