Abstract
A specially-designed chirped periodically poled lithium niobate nonlinear crystal was fabricated with a phase-matching bandwidth as large as 50 nm for sum frequency generation to operate at room and higher temperatures. This device also benefits from insensitivity to laser frequency drift and fine alignment. The loosely-focused beam position of a high-power CW laser at around 1550 nm is optimized within the grating for maximum up-conversion efficiency, to realize a super-tunable source in the range of 770–778 nm by tuning a narrowband control signal over 30 nm in the communication band. This device is demonstrated to be fully phased-matched simultaneously for both second-order nonlinear up-conversion processes, namely second harmonic generation and sum frequency generation. The measurement of the generated sum-frequency power versus wavelength agrees well with the theory. The device allows for the creation of tunable broadband CW sources at shorter wavelengths with potentially high power.
Highlights
There is increasing demand for tunable and broadband sources at shorter wavelengths due to many applications in biomedicine and spectroscopy[1,2,3]
We demonstrate super-tunable broadband frequency up-conversion of an available high-power continuous wave (CW) fiber laser for the first time to the best of our knowledge, in a specially-designed nonlinear crystal namely SC-periodically poled lithium niobate (PPLN) device, demonstrating wideband sum frequency generation (SFG) by tuning of a tunable monochromatic laser as a signal source
The results are compared with the computed spectra for SFG and second harmonic generation (SHG) simulated by the real pump spectrum and a monochromatic wavelength at 1545 nm as the inputs to the step-chirped PPLN (SC-PPLN)
Summary
There is increasing demand for tunable and broadband sources at shorter wavelengths due to many applications in biomedicine and spectroscopy[1,2,3]. High-power fiber lasers have attracted huge attention[7], these lasers in the form of continuous wave (CW) generally possess a bandwidth (BW) in the order of hundreds of GHz proportional to the laser output power due to the use of fiber Bragg gratings[8] or four-wave mixing (FWM) between the different longitudinal modes[7, 9] These high-power fiber lasers are only available in limited wavelength ranges[10]. We demonstrate super-tunable broadband frequency up-conversion of an available high-power CW fiber laser for the first time to the best of our knowledge, in a specially-designed nonlinear crystal namely SC-PPLN device, demonstrating wideband SFG by tuning of a tunable monochromatic laser as a signal source. A tunable source at shorter wavelengths is realized in which the central frequency is controlled by tuning the control signal laser without concern of the frequency drift of the high-power CW laser pump
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