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• Out Line PPLN
  

1. Nonlinear and high efficiency wavelength conversion crystal.
2. Used for frequency doubling , difference frequency generation , sum frequency generation , optical parametric oscillation and optical parametric amplification etc.
3. Robust , transparent crystal that must be temperature controlled when in use.

• Simple Principles of Nonlinear Properties

This processes is different from other ,not determined by input wavelength. Output wavelength is selected according to which wavelengths are phase matched.

• Getting the phase matching wavelength

　　　　　　Conventional way

1. Most materials have different refractive induces at the fundamental and second harmonic wavelengths.
2. Light at FHW(fundamental) and SHW is becoming out of phase through the crystal.
3. In fact, the intensity of SHWto zero as the light propagates along the crystal.
4. The covnventional way to get phase matching wavelengths is touse the birefringence crystal which gives the same refractive index for FHW and SHW.
5. This is why most conventional crystals have to be carefully angle tuned to achieve optimal conversion efficiency.

　　　 　　New technique by using PPLN crystal

1. This technique relies on the fact that as if the light travels through the crystal structure is inverted when the generated second harmonics is at a maximum.
2. We note that period with which the crystal needs to be inverted depends on the wavelengths and temperature (100〜200℃).

• Fabrication of PPLN

	Step1 Apply electrodes to Lithium Niobate
	Step2 Apply high voltage to periodical pole lithium niobate
	Step3 Remove electrodes to produce finished PPLN

• Lithium Niobate is a ferroelectric crystal
• The electric field is very large(〜22kV/mm), is applied a few milliseconds.
• The structure is permanently imprinted.

• Example uses of PPLN

• How to use PPLN

1. Perpendicular to the end face of the crystal.
2. A good rule of thumb is that for a CW, Gaussian beam , the spot size is chosen such that the Rayleigh range is twice the length of the crystal.
3. The spot size can be reduced in small increments untill the maximum efficiency is obtained.
4. PPLN material has a high index of refraction, resulting in a 14% Fresnel loss per surface.

• Polarization

1. PPLN will only use the fraction of input light that is linearly polarized in the direction of the thickness of the crystal.
2. It will not affect the operation of the crystal if light of the opposite polarization is present as well as light of the correct polarization.
3. The light with the incorrect polarization will just pass straight through the crystal, without any effect.

• Temperature and Period

1. The period of the crystal is determined by the wavelengths being used.
2. This wavelength can be tuned slightly by varying the temperature.
3. PPLN have multiple PPLN sections in crystal allowing different wavelengths and temperature to be used.
4. This is easily done using the PPLN oven.
5. Temperature in this range are used to reduce the photorefractive effect, which causes the output beam tobe distorted if the optical power is too high.