• The population inversion is usually achieved by electric current injection in a p-n junction diode; a forward bias voltage causes carrier pairs to be injected into the junction region, where they recombine by means of stimulated emission.
• The process, recombination, occurs when an electron decays from the conduction band to fill a hole in the valence band. The energy released by the electron may take the form of and emitted photon.
• The recombination is the induced emission an identical photon.

● What kind of material can emit photon ?

• Direct- and Indirect-Gap Semiconductors
  • Semiconductors for which the valance-band maximum and the conduction-band minimum correspond to the same momentum (same k) are called direct-gap material.
  • Semiconductors for which this is not the case are known as indirect-gap material.

Where E is energy; K is Wavevector

The vector form K=[kx , ky, kz];

The magnitude k = (kx + ky+ kz) 1/2

回雷射光束檢測內容大綱

• Photon Emission Is unlikely in an Indirect-Gap Semiconductor
  • Radiative electron-hole recombination is unlikely in an indirect-gap semiconductor. This is because transitions from near the bottom of the conduction band to near the top of the valence band (where electrons and holes, respectively, are most likely to reside) requires an exchange of momentum that cannot be accommodated by the emitted photon.Momentum may be conserved, however, by the participation of phonons in the interaction. Phonos can carry relatively large momenta but typically have small energies, so their transitions appear horizontal on the E-k diagram. The net result is that momentum is conserved, but the k-selection rule is violated.

    ps. The momenta of electron and the hole involved in interaction with the photon are therefore roughly equal. This condition, k2 ~ K1, is called the K-selection rule.

This figure shows that if photon emission in an indirect-gap semiconductor, it must be obey the pathwhich is showen in the fig. But this type of multiparticle interaction is unlikely.

2. An introduction to the trait of semiconductor laser beam

● The divergent angle

The figure is showen following: 請按

● Gaussian beam

Most of laser beams are gaussian beams.

請按

回雷射光束檢測內容大綱

● FWHM：Full-width at half-maximum

請按

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3.利用影像處理做雷射光束檢測

●程式撰寫流程圖

    程式流程圖如下：

請按

●低通濾波器

• 目的：濾掉雜訊，使影像更加清楚。
• 遮罩：[1/9 1/9 1/9; 1/9 1/9 1/9; 1/9 1/9 1/9]

    處理後的圖形如下：

• 使用方法：衝量保持法(Moment-Preserving Thresholding)

    所使用的演算法運算式如下：

• 利用雷射光有對稱的關係，故求分離背景後圖形的重心當作雷射 光束的中心。

●求 FWHM

• 1.先求雷射光的中心點(理論上為整個光束最亮之處)，並將該 處的灰階值計錄下來。
• 2.往左右開始掃描，直到灰階值為中心值一半時便停止，並計 錄此二位置的座標，將這兩座標值相減即為FWHM。

    程式處理後的圖形如下：

• 1.由中心點往上掃描，直至灰階值為0時則停止，計算此段距離為L1， 並由(L1/480)*(CCD晶片之垂直距離)求得真實距離laxis。
• 2.由中心點往右掃描，直至灰階值為0時則停止，計算此段距離為L2， 並由(L2/640)*(CCD晶片之水平距離)求得真實距離saxis。
• 3.擴束角1 =2*argtan(laxis/d)，其中d為雷射光到CCD的距離。
• 4.擴束角2 =2*argtan(saxis/d)

    程式處理後的圖形如下：

• 1.從螢幕上取得座標及亮度資料。
• 2.座標轉換
• 3.將轉換後的座標和最高曲線做比較，如較高則畫出並修改最高曲線，如較低則不 畫出。