Super-tunable, broadband up-conversion of a high-power CW laser in an engineered nonlinear crystal

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.There is increasing demand for tunable and broadband sources at shorter wavelengths due to many applications in biomedicine and spectroscopy [1][2][3] . Owing to the lack of suitable sources, a feasible approach is the frequency conversion of available lasers in nonlinear materials [4][5][6] . Recently, high-power fiber lasers have attracted huge attention 7 , however, 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 . Further, these high-power fiber lasers are only available in limited wavelength ranges 10 . In addition, they experience a drift in their central wavelength when the output power varies 11,12 . Nevertheless, in order to realize tunable CW sources at desired higher frequencies, up-conversion based on second-order nonlinearities using engineered quasi-phase-matching (QPM) in nonlinear crystals can be exploited with several advantages [13][14][15][16][17] . There are many works on second harmonic generation (SHG) and sum frequency generation (SFG) using QPM in uniform gratings in the form of periodically poled crystals 15, 18-23 . However, uniform gratings limit the BW of up-conversion process (and consequently the tunability) as it is inversely proportional to the length of a grating 24 . Since the up-conversion efficiency (assuming an undepleted pump) is increased by the length squared, using a long grating leads to a reduction in the up-converted BW of a few-nm-wide pump and consequently results in a waste of power whilst a short grating up-converts the whole BW with a very low efficiency. Also, at higher pump powers, the use of a short grating may lead to the crystal damage in order to achieve efficiencies comparable to those of longer gratings. Moreover, as the effective period of uniform grating changes by angular rotation and temperature, fine alignment and a controlled temperature are required t...

Fano Resonance on Nanostructured Lithium Niobate for Highly Efficient and Tunable Second Harmonic Generation

“…Consequently, one can estimate the EO effect by only taking into account the largest EO coefficient r 33 . Under this assumption, the EO effect can be quantified as follows [18,36]:Δne=12ne3r33fopt2E…”

“…As for the choice of material, lithium niobate (LN) is a biocompatible material which has a large second-order nonlinearity (the largest one is d 33 = 27 pm/V at 1550 nm) [16,17,18]. Even though it shows slightly weaker nonlinear properties than some semiconductor materials, it has a wide transparency spectral range spanning visible to infrared light (0.42–5 μm).…”

Highly efficient broadband second harmonic generation mediated by mode hybridization and nonlinearity patterning in compact fiber-integrated lithium niobate nano-waveguides

“…The conventional method of temperature or external DC electric field control is not suitable for high-density on-chip integration due to the lack of sufficient resolution. Adiabatic variation of period 17 and other types of chirping of QPM gratings 18 , 19 can be implemented to expand the bandwidth, however, it adds to the technical complexity, and ultimately production costs, of the setup and requires much longer waveguides. Apart from the practical issues due to incongruity with compact design and dense on-chip integration, longer waveguides suffer from stronger walk-offs between interacting harmonics due to group velocity mismatch (GVM).…”

Broadband and Triple-Wavelength Continuous Wave Orange Laser by Single-Pass Sum-Frequency Generation in Step-Chirped MgO:PPLN

“…Until now, the different ways of dealing with executing QPM range from single-period structures to Fibonacci optical superlattice [17], non-uniform [18], linearly chirped [19], apodized and un-apodized [20,21], and multi-sectioned [22]. A widely-tunable source within the 770 nm wavelength region has additionally been implemented based on chirped periodically poled lithium niobate with a phase matching bandwidth as extensive as 50 nm [23]. This advancement has prompted the use of a single crystal for the generation of several wavelengths in the visible range that are very reasonable for entertainment applications.…”