PPLN 晶体,非线性晶体

      基于准相位匹配（QPM），使新的波生成和xx型谱是困难的或不可能的工程实现由传统的非线性材料。xx型芯片和全谱（LN、LT：氧化镁：镁和适当的非线性频率转换计划）（DFG SFG，倍频、OPO，收购，联合，一个CAN，等），实现期望的输出波长（紫外/可见到/从太赫兹光谱反演和特殊功能（），两个频谱转换。频谱工程等）有效。

      HCP提供以下全光谱configurations xx型散装芯片来满足你的应用要求和规格。我们可以帮助你设计结构合适的外加电压和外加电压为选定的时间获得所需的材料/ PPLT极化相匹配指定的操作温度和光谱.思考与输入和输出功率/能量/脉冲以及它们的光谱特性。请为您的特殊要求，也具有挑战。

Ref-2:  Chip Structure

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1. 什么是 PPXX 技术

    PPXX is an advanced technology for high efficiency and arbitrary wavelength conversion based on making engineered microstructure on the ferroelectric materials.

2. 非线性波长转换

Nonlinear wavelength conversion means the phenomenon that input light generates new wavelength via passing through the nonlinear material. For the common 2^nd order nonlinearity, photon energy of the related wavelength is defined to be conserved as the relation below:

 

Here are the common nonlinear process appellations and its expression of frequency relation respectively.

3. 相位匹配条件

To have high conversion efficiency, the photon momentum should be conserved as well. Otherwise, incorrect phase will lead to destructive interference causing very low efficiency.  The relation of photon momentum conservation is shown as below, which is called the phase matching condition.

   

4.双折射相位匹配(BPM)

Typically, due to the material feature of dispersion, phase matching condition could only be achieved in the birefringence material, which has different refractive index of its o-polarization (perpendicular to the optical axis) and e-polarization (parallel to the optical axis).

The phase matching condition could be achieved by changing temperature or the incident angle on the birefringence material with correct polarization.  Two types of phase matching condition are shown as below:  

Type I:  The polarization of the two low frequency photon is the same,

i.e.

Type II:  The polarization of the two low frequency photon is different,

i.e.

 

5. 准相位匹配（QPM）和周期性极化（PP）

Quasi phase matching is a technique that the photon momentum conservation is achieved through additional artificial structure.  The structure could provide an extra vector K_g which matches the momentum conservation as below.

 

Periodical poling is a special technique to form periodical microstructure on the ferroelectric material with a designed K_g. Not only the spatial walk-off issue in BPM material becomes eliminated through QPM technique but the phase matching temperature could also be designed.  Moreover, the  type of phase matching condition (Type 0), which is never existing in BPM (Birefringence phase matching) could be demonstrated through QPM. Type 0 is bringing about several tens of times for conversion efficiency enhancement.

 

6. 什么样的色散方程，我们使用的折射率计算？

We simulate the refractive index according to the reference below, which is the most appropriate function through our experience.

 

Gayer, O., et al, "Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO3." Appl. Phys. B 91, 343-348 (2008)

 

7. 如何计算 QPM 期?

 

First you will need the information of the refractive index to calculate the wave vector mismatch of the involved wavelength.  For example (up-conversion):

 

Then, the QPM period equals to

 

Which the final wave vector mismatch is canceled

 

 

8. 如何实现最优转换效率?

 

To achieve the optimal conversion efficiency, the incident light should be focused on the center of the chip with the focusing condition L/b~2.84, where L is the chip length, b is the Gaussian beam focusing parameter.  The above condition is for SHG/SFG only and with the assumption of Gaussian beam M²=1.  For DFG, the optimal condition is more complicated and will change according to the wavelength, for further study, one can read the reference below.

 

T. -B. Chu and M. Broyer, "Intracavity cw difference frequency generation by mixing three photons and using Gaussian laser beams." J. Phys. (Paris) 46, 523 (1985)

9. How to know the acceptance of the chip?

The phase matching spectrum is the square of a sinc function

 

We know that once we design a QPM period for a target wavelength conversion, , and then we could calculate the wave vector mismatch near the designed wavelength to see when the  becomes large enough that the square of sinc function becomes smaller than 0.5. 

 

10.  如何实现波长调谐?

 

There are two ways for wavelength tuning.  One is changing the period, the other one is changing the temperature (because the refractive index is a function of  temperature).

 

Typically, the temperature tuning could not have wide tuning range, so we have special structures - multiple and fan-out, to achieve broadband wavelength tuning.

Multiple structure is a chip with multiple channel, each channel has different period, through the change of the incident channel and modifying the temperature, one can achieve large tuning range then a single period chip.

 

Fan-out structure is a chip with continuous period change in the width.  One can tune the phase matching wavelength through moving the chip without changing the temperature.