The experiment was performed with a dual wavelength Er-doped fiber laser, a high power EDFA, and two parallel PPLN channels in a temperature-controlled oven. shows the experimental setup. The seed source was configured with a dual wavelength ring laser system using two FBGs with center wavelengths of 1,547.20 nm and 1,554.48 nm, respectively. As shown in , the lasing wavelengths of the seed source are exactly determined from the spectrum of two FBGs. These two lasing wavelengths can be controlled accurately within a few nm range by the external pulling stretcher on each FBG filter. The center peak wavelength of FBG is linearly shifted to a longer wavelength region when we apply a pulling strain on both end positions of the FBG component. For the high intensity input to the energy efficient CW PPLN conversion, a 33 dBm EDFA is additionally prepared at the output of a dual wavelength laser source.
Spectrum of Seed EDFL output. Inset shows the spectrum of two tunable fiber Bragg gratings.
The free space optical beam of fundamental output was optimally shaped to a line beam using a cylindrical lens of 8 cm focal length because it was necessary to simultaneously transmit through two parallel channels of the PPLN. In this experiment, a circular beam with a diameter of 1.6 mm from a beam collimator (HPUCO-23A-1300/1550-S-11AS) was shaped to the oval line beam with a height of 0.14 mm and width of 1.6 mm at the focal position of 8 cm from the cylindrical lens (LJ1105L1, Thorlabs). Since each PPLN channel has a height of 0.5 mm, width of 0.2 mm and length of 20 mm, the simultaneous SHG can be easily obtained as positioning multiple parallel channels of the PPLN waveguide at the central focal position of a cylindrical lens. The multi-period PPLN device used in this experiment (97-02355-01, Crystal Technology) includes 10 parallel PPLN channels with sequential poling periods of 18.6, 18.8, 19.0, 19.2, 19.4, 19.6, 19.8, 20.0, 20.2 and 20.4 μms. Since there is a separation space of 1.06 mm between each channel, the whole width of the PPLN waveguide device is 11.5 mm.
The quasi-phase-matching condition can be simply described with the following equation:
. Here, Λ is the poling period in a PPLN channel, λω
is the input light wavelength, and λ2ω
is the converted SHG light wavelength. The refractive indexes, n2ω
, at both wavelengths depend on the temperature, T
, of the PPLN crystal.
From the above relation, it is clear that there exists only one pair of input light wavelengths, λω
, and PPLN periods, Λ, under a certain temperature condition, T
. Thus, in order to generate two SHG wavelengths simultaneously using two adjacent PPLN periods, it is necessary to find an optimal condition such that two input wavelengths in a single light beam satisfy this quasi-phase-matching condition under the same temperature simultaneously. Compared with a conventional dual SHG configuration based on two different input wavelengths from each independent light source, this method has a relatively higher efficiency to align the input light beam into the adjacent parallel PPLN channels simultaneously. We performed a simulation in MATLAB to find this optimal condition using an iterative Sellmeier equation code. As a result, the optimal poling periods of 18.6 μm and 18.8 μm were obtained for the input wavelengths of 1,547.20 nm and 1,554.48 nm, respectively, under the phase-matching temperature of 105 °C [5
]. shows the simulation result of this optimization process.
Simulation between the temperature (°C) and PPLN period (Λ) for each input wavelength of (a) 1,547.20 nm and (b) 1,554.48 nm.
For the output beam through PPLN, a dichroic filter (850FG07-25, Andover Corp.) was used to separate the 775 nm region SHG beam from the 1,550 nm region fundamental beam. The spectrum of SHG was measured using an optical spectrum analyzer (OSA) by the focus collimation from the free space beam to the optical fiber.