PPLN for Second Harmonic Generation of Fiber Lasers
Tom Legg, G&H Systems Technology Group
Second harmonic generation (SHG) fiber laser systems provide a robust and stable solution to access spectral regions currently relatively inaccessible but with potential. Gooch & Housego’s Systems Technology Group (STG) are developing fiber coupled periodically poled Lithium Niobate (PPLN) modules for second harmonic generation (SHG). Systems using these modules will allow for the exploitation of these spectral regions with optical powers, pulse energies and durations which are currently unavailable with other laser sources.
The project has benefited greatly from cross-site collaboration and pulls on several core capabilities from across G&H, including:
- Modelling nonlinear optical performance in PPLN
- PPLN design optimization and fabrication
- Polarization maintaining (PM) fiber alignment and precision collimator assembly
- Proprietary fiber coupling technology
The challenge of developing a module to double the frequency output of a fiber laser lies in the necessity to optimize the design for each individual system. To maximize the SHG interaction the PPLN crystal length, poling period and focusing parameters all have to be optimized based on the parameters of the pump laser (wavelength, linewidth, pulse duration, etc.).
To date three modules, working with different fiber laser systems and at different wavelengths, have been designed and tested. The first module to be developed was used to frequency double a narrow linewidth CW Er:Yb fiber laser. The module had a conversion efficiency approaching 15% at the highest output power, the maximum doubled power was 1.8W.
The second module was designed to frequency double a CW laser to visible wavelength and achieved a conversion efficiency of 20% at an output power of 2W. The laser is used in a variety of bio-medical applications. The first two modules were configured as fiber-in fiber-out devices, the return loss of the modules was >45dB and the extinction ratio between the fundamental and second harmonic wavelength was >40dB. The PER of the fiber output was >20dB.
The third module was designed to frequency double a mode-locked fiber amplifier operating at 1120nm with a 200ps pulse width; a 40MHz repetition rate and an average power of 1.8W. This module achieved a conversion efficiency of almost 30%, outputting 450-500mW at 560nm. The output of this module was a 1mm diameter collimated free-space beam. This wavelength, pulse duration and pulse energy is ideal for use as a depletion source in stimulated emission depletion (STED) microscopy. For further specifications please view the product page from where the datasheet can be downloaded.
The extension of the PPLN module concept to new wavelengths for both CW and pulsed applications and to other types of nonlinear interaction such as sum and difference frequency generation is of great interest for future development.