Periodically poled lithium niobate (PPLN) is an excellent nonlinear crystal for laser wavelength conversion. Conventional nonlinear crystals typically require high peak pulse power input. However, because of its periodic non-critical phase matching characteristics, PPLN has high conversion efficiency. It is extremely suitable for continuous wave (CW) laser wavelength conversion and widely used in CW laser systems. In addition, PPLN can realize full-color laser output by flexibly designing its quasi-phase matching (QPM) period, which has strong practical value. There has recently been a gradual shift from bulk PPLN to PPLN thin-film optical waveguides to improve the nonlinear frequency conversion efficiency of CW lasers. In recent years, many domestic research institutions, such as Nanjing University, East China Normal University, Shandong University, and the Chinese Academy of Sciences, have conducted in-depth detailed research on the preparation and application of PPLN thin film optical waveguide devices. Nonlinear frequency conversion devices based on PPLN waveguides have been used in various applications, such as optical communication, quantum optics, microwave optics, and spectroscopy. As applications continue to grow, new requirements are set for the volume and portability of waveguides. This study briefly introduces the basic structure and principle of a silicon-based PPLN thin film ridge waveguide, and a commercially available compact fiber-in-fiber-out PPLN waveguide package module is designed and fabricated.

The fabrication process of the silicon-based PPLN thin-film ridge waveguide is as follows. First, a Z-cut lithium niobate wafer (0.5 mm thick) doped with MgO is poled at high voltage. According to the FDTD software analysis results, the poled period is chosen to be 18.7 μm to obtain phase matching of the pump wavelength near 1560 nm. After poling, a silicon dioxide buffer layer with a thickness of approximately 600 nm is deposited on one side of the PPLN wafer, subsequently, a gold layer of approximately 300 nm thickness is sputtered. Then, another 0.5-mm thick precision polished silicon wafer is coated with a layer of gold of approximately 300 nm thickness and bonded to the PPLN wafer. This process is realized at room temperature, avoiding mechanical stress caused by the different thermal expansion coefficients of both wafers. Next,thinning and polishing are conducted to form the PPLN film. Finally, the PPLN ridge waveguide with the desired size is prepared based on a precision cutting mechanism. The waveguide direction is X direction (Fig. 4). The PPLN ridge waveguide prepared herein has a cross section of 10 μm×10 μm and a length of 20 mm. A single-mode polarization-holding fiber with a core diameter of 8.5 μm, numerical aperture (NA) of 0.125, and mode field diameter of 10.1 μm is used for end-face direct coupling, and the packaged device is shown in Fig. 5.

A tunable laser source is used to tune the wavelength to 1560 nm. Subsequently it is incident into the PPLN ridge waveguide through a narrow-band erbium-doped fiber amplifier (EDFA). The light at the output of the waveguide passes through a 1560-nm high reflection and 780-nm high transmission filter and enters the optical power meter (Fig. 6). Because the refractive index of PPLN is a function of temperature, it is necessary to control the crystal temperature. Here, a temperature controller (the accuracy is 0.01 ℃, temperature control range is from room temperature to 200 ℃) is used to control the temperature of the PPLN waveguide package module. As shown in Fig. 7(a), when the temperature is 24.8 ℃, the output wavelength of the module is 780 nm (the deviation of the spectrometer used in the experiment is 0.2 nm). When the pump power Ppin (shown in Fig. 6) at the output of EDFA reaches 1.6 W, the input pump power Pp0 is calculated to be 1.2 W after deducting coupling loss between the fiber and waveguide at the input, while the coupling pump power PpL [without second harmonic generation (SHG)] at the output of the waveguide is 0.9 W. The power of SHG is 653 mW [Fig. 7(b)], the optical-optical conversion efficiency is 54.4% (Pp0 to SHG power). The normalized conversion efficiency is 20.2%?W^-1?cm^-2 (PpL to SHG power). According to the input pump power Pp0, after deducting the coupling loss between the input fiber and the waveguide, the optical-optical conversion efficiency of the waveguide is 72.5%.

This study simulate and analyze the relationship between the QPM period of the PPLN ridge waveguide with a ridge height or width of 10 μm at 25 ℃ and the corresponding ridge width or height. The QPM period of the PPLN waveguide increases with the increase of ridge height or width at the same pump wavelength and ridge height or width and finally tends to a constant, that is, the period of bulk PPLN crystal. The relationship between the QPM period and temperature of the PPLN ridge waveguide with constant ridge height and width at the same pump wavelength is analyzed. The QPM period decreases gradually with the increase in temperature, and the QPM period decreases by approximately 3 nm when the temperature rises by 1 ℃. Here, the fabrication process of the PPLN thin-film ridge waveguide is improved. For example, the thickness of the silicon dioxide buffer layer is 600 nm. The waveguide package module with compact fiber in and out is fabricated, and its performance tested. When the temperature is 24.8 ℃ and the input power of 1560 nm pump light is 1.2 W, the maximum power of SHG is 653 mW, the optical-optical conversion efficiency is 54.4%, and the normalized conversion efficiency is 20.2%?W^-1?cm^-2.