Efficient wavelength conversion is an attractive approach for obtaining coherent radiation in regions of the spectrum where conventional lasers are unavailable or impractical. It is well known that the attenuation loss coefficient of a material and walk-off effect have enormous influence on the wavelength conversion performance. However, we have previously observed that the effective length of a nonlinear optical material is limited by the THz absorption and has restricted the growth of the THz power spastically in the backward THz difference frequency generation (DFG) process.
In order to address the effect of the QPM crystal length on the forward and backward THz-DFG, we used a periodically poled LiNbO3 (PPLN) crystal with different lengths varying from 1 mm to 25 mm in 2 mm increment and same grating period of 65 μm. We have developed a theoretical model taking into account the spatial distribution of pump, signal and output waves. In our model we ignored the pump depletion in the nonlinear crystal, because the conversion efficiency is usually low. Experimental results are in good agreement with the theoretical model. The results show that in the case of forward THz DFG in the PPLN crystal, the THz power continuously grows with the increase of the crystal length, even under the condition where the THz absorption loss exceeds the parametric gain.
The experimental results also show that in the backward DFG case the THz radiation power becomes saturated in a few absorption lengths in the PPLN crystal, as long as the pump intensity is below the oscillation threshold. Moreover, for the long crystal length, the THz wave diffracts rapidly when propagates along the crystal. In the present work, we used a powerful pump source to provide a high parametric gain to overcome the strong absorption within the PPLN crystal. Specifically, we used a passively Q-switch Nd:YAG laser followed by a flashlamp pumped Nd:YAG amplifier as a pump source to perform two stages of optical parametric amplifier (OPA) with a PPLN crystal as the gain medium and generated a long pulsewidth of~450 ps. By using an Nd:YAG laser amplifier and the dual-OPA system, we were able to generate the signal and idler powers with suitable peak power, pulse width, and wavelength tunability to perform the forward and backward THz-wave DFG in this thesis.

兆赫波段就是頻率為0.1到10兆赫茲，波長為30毫米到3000毫米的電磁波段。兆赫波段的雷射光源在環境偵測、國防安全、美容科技等應用上都有著十分重要的用途。舉例來說，兆赫波段(中紅外波段)的雷射對於分辨可燃物質或是在戰爭中分析有毒物質都有相當大的幫助；而遠紅外波段的雷射則是在監視系統以及藥物應用上有相當有用。但是由於缺乏商品化的雷射增益晶體的緣故，中、遠紅外光雷射光源的發展進度遠遠落後目前常見的一微米左右的近紅外光雷射。但是幸運的是透過非線性光學的方式，我們可以把目前已經發展成熟的近紅外光雷射源轉換成中、遠外光雷射源。現階段，產生兆赫波段的方式大致可分為光整流效應，自由電子雷射，還有非線性差頻效應。

在本次實驗中使用非線性差頻效應來產生兆赫波。然而，在普遍大家認之中，利用差頻效應產生兆赫波，兆赫波的功率會受到有效晶體長度的限制，有效長度等於單位吸收係數的倒數，以至於當增加晶體長度後，無法獲得更高功率的兆赫波。在正向兆赫波段的產生過程中，儘管面對強吸收效應，正向兆赫波仍然能夠在增益效果不如吸收效果時緩緩成長。本實驗目的是為了證實此的想法，於是我們設計了長度從1mm到25mm極化反轉的鈮酸鋰(LiNbO3)晶體，利用非線性差頻效應來產生兆赫波。實驗中，我們觀察到，在幫浦光與兆赫波在時間上分開前，兆赫波功率會隨著長度變長而增加。這結果直接證明了我們可以利用增加晶體長度來增加兆赫波功率。

再者，由於兆赫波在鈮酸鋰晶體內產生後，會有很大的發散，使得幫浦光與兆赫波交互作用的區域變小了，加上鈮酸鋰晶體在兆赫波段有強烈的吸收，導致產生的兆赫波功率降低。爲了提高兆赫波功率，我們使用釹參釔鋁石榴石雷射放大器產生更強的幫浦雷射光源並提供更高的增益效果給兆赫波。從結果可以看出背向兆赫波克服了強吸收效應，兆赫波的功率隨著長度的增加而增加。然而，在背向兆赫波的情況下，當幫浦雷射光源的增益係數尚未達到背向兆赫波的震盪臨界值時，背向兆赫波的能量會被晶體吸收效應限制而其功率成長達到飽和。

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