Frequency-Doubling Crystal with Reflective Facets for Femtosecond Pulses
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Solution Overview
Problem
Frequency conversion of femtosecond laser pulses in optically nonlinear crystals is limited by temporal walk-off, which causes efficiency reduction and pulse shape distortion due to different group velocities for the original and converted wavelengths, and existing solutions require additional crystals or precise translation stages to compensate for this issue.
Innovation Solution
The use of an optically nonlinear crystal with highly reflective coatings on its facets, allowing multiple passes through the crystal at angled paths, where the coatings introduce controlled group delays to minimize temporal separation between the original and converted pulses, thereby extending the interaction length and reducing peak intensity requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the crystal length is increased to improve frequency conversion efficiency, then the conversion efficiency improves, but temporal walk-off increases causing pulse separation and distortion
Solution Approach 1:
The patent implements multiple passes of the laser beam through the crystal by introducing reflective surfaces at specific angles. This allows the frequency conversion process to occur repeatedly over an extended effective interaction length, maintaining temporal overlap between fundamental and harmonic pulses throughout each pass and across multiple passes, thereby achieving high conversion efficiency without excessive temporal walk-off per pass
2Productivity
If the crystal length is increased to extend interaction length, then frequency conversion efficiency improves, but peak intensity requirements increase reducing crystal lifetime
Solution Approach 1:
The patent divides the frequency conversion process into multiple sequential passes through the crystal rather than requiring a single long crystal. Each pass operates at moderate peak intensity levels, accumulating conversion efficiency over time. This segmentation allows the use of a shorter physical crystal that experiences lower cumulative stress, thereby extending crystal lifetime while maintaining high overall conversion efficiency
3Loss of time
If additional crystals are used to compensate temporal walk-off, then temporal overlap improves, but device complexity increases
Solution Approach 1:
Instead of adding more crystals in sequence to compensate for temporal walk-off, the patent introduces a spatial dimension solution by tilting reflective surfaces to create multiple passes through the same crystal. This dimensional approach to pulse recirculation achieves temporal overlap compensation within a single crystal, avoiding the complexity of multiple crystal assemblies while maintaining effective temporal synchronization
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively compensates for temporal walk-off without additional crystals, increasing the interaction length and reducing the intensity of the laser beam, thus extending crystal lifetime and potentially eliminating the need for precise translation stages, while maintaining efficient frequency conversion.
Implementation Method 1
the coatings introduce controlled group delays to minimize temporal separation between the original and converted pulses
Implementation Method 2
Reflection from at least one of the reflective coatings changes the temporal separation. The crystal and the reflective coatings are also configured to cooperatively minimize the temporal separation
Implementation Method 3
Frequency conversion of femtosecond laser pulses in optically nonlinear crystals
Implementation Method 4
One common frequency-conversion operation is generating radiation having a wavelength in the visible region of the electromagnetic spectrum from radiation having a having a wavelength in the near infrared (NIR) region of the electromagnetic spectrum. By way of example, radiation from a laser having a fundamental wavelength of 1040 nanometers (nm) can be converted to radiation having a wavelength of 520 nm by second-harmonic generation (frequency-doubling) in an optically nonlinear crystal.
Implementation Method 5
Optically nonlinear crystals are birefringent. The radiation being frequency-converted and the frequency-converted radiation are orthogonally polarized with respect to each other.
Implementation Method 6
First and second reflective coatings are provided, each thereof being highly reflective at the first and second wavelengths
Data Source
AI summary
An optically nonlinear crystal is arranged for frequency-doubling an input pulse. The crystal has parallel facets each coated with a reflective coating. The crystal is arranged with respect to the input pulse such that the input pulse makes a plurality of forward and reverse passes between the coatings. A frequency-doubled pulse is generated on the forward passes. The input pulse and the frequency-doubled pulse propagate with different group velocities in the crystal such that temporal separation the pulses occurs. The crystal and reflective coatings are configured such that the temporal separation does not exceed a predetermined value.