Intra-cavity SHG Laser with Interferometric End Reflectors
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Solution Overview
Problem
Existing intracavity Second Harmonic Generation (SHG) laser devices face challenges in achieving efficient frequency conversion due to chaotic mode switching and noise in the laser output, particularly in continuous wave lasers with moderate power, where the cavity length is too short to accommodate nonlinear crystals and additional components for effective frequency doubling.
Innovation Solution
The laser device incorporates a Michelson or Fabry-Perot interferometric layout for the end reflectors, utilizing multilayer dielectric mirrors and volume Bragg gratings to enhance spectral selectivity, allowing single longitudinal mode operation with longer cavity lengths, thereby accommodating nonlinear crystals and optimizing SHG efficiency without introducing additional losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a spectral selector is placed inside the laser cavity to eliminate green-noise, then single longitudinal mode operation is achieved, but the radiation power circulating through the nonlinear crystal decreases
Solution Approach 1:
A volume Bragg grating (VBG) is introduced as an intermediary spectral selector with high reflectivity (>95%) at the fundamental frequency. The VBG acts as a mediator that provides strong spectral selection while minimizing power loss compared to traditional spectral selectors, thereby maintaining high circulating power in the nonlinear crystal while achieving single longitudinal mode operation
Solution Approach 2:
The reflectivity parameter of the spectral selector is optimized by using a VBG with reflectivity >95% at the fundamental frequency. This parameter change allows the spectral selector to be more effective at mode selection while being more transparent to the circulating power, resolving the contradiction between mode stability and power circulation
2Productivity
If the cavity length is extended to accommodate nonlinear crystals and additional components, then SHG efficiency is improved, but chaotic mode switching and noise increase
Solution Approach 1:
A volume Bragg grating (VBG) is placed at one of the cavity end mirrors to pre-select the longitudinal mode before the light enters the cavity. This preliminary spectral selection ensures that only a single longitudinal mode oscillates throughout the extended cavity, preventing chaotic mode switching even when the cavity is lengthened to accommodate nonlinear crystals and other components for improved SHG efficiency
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 configuration enables stable single longitudinal mode operation over extended cavity lengths, enhancing SHG efficiency and reducing noise, allowing for optimal placement and geometry of nonlinear crystals, thus improving the quality and efficiency of frequency conversion.
Implementation Method 1
a volume Bragg grating (VBG) recorded in a photo-thermo-refractive glass, as one of the laser cavity end reflectors
Implementation Method 2
In a SHG process, the efficiency of the conversion of laser power at a fundamental frequency into power at the second harmonic frequency is strongly dependent on the intensity of radiation applied to the nonlinear optical material
Implementation Method 3
a Michelson or Fabry-Perot interferometric layout for the end reflectors
Data Source
Figure 1
Figure 1A
Figure 1B
AI summary
This invention provides a laser device for intra-cavity frequency conversion, in particular, for the second harmonic generation (SHG), the device comprising a laser cavity, formed by the first (10) and the second (100) laser cavity end reflectors wherein the said laser cavity end reflectors are highly reflective for the radiation about the laser fundamental frequency and as such provide for a predetermined level of circulating inside the cavity fundamental frequency power; an active medium (20) provided within the said laser cavity; at least one non-linear crystal (40), in particular, for the second harmonic generation provided within the same laser cavity; tuning means (202) adapted for tuning at least one of laser cavity end reflectors; wherein at least one of the laser cavity end reflectors (100) comprises an interferometric layout providing spectrally selective reflection for the radiation about the laser fundamental frequency with the value of reflectivity nearly to 100% within a spectral range close to the free spectrum range (FSR) of the laser cavity and with a sufficiently lower reflectivity outside the above spectral range such that the laser cavity restricts the spectrum of the laser radiation to a single longitudinal mode.