本實驗是研究氦原子從2S的亞穩態能階(metastable state)躍遷至3D的雙光子躍遷光譜，首先利用放大功率後的射頻電波(radio frequency)將電子從1S基態能階激發到2S的亞穩態能階，在其中檢驗氣體放電系統所放出的螢光光譜，來確認氣室內的氣體是否為氦氣，確定之後再把外腔雷射(ECDL)分成2道互相對打且相同頻率的雷射光將氦原子激發到3D，接著偵測3D掉到2P的587 nm的螢光訊號，但本實驗只偵測到雷射散射光(Laser scattering),無法得到氦原子2S-3D雙光子躍遷頻譜，深究其原因可能為ECDL外腔雷射線寬太寬、電子處在2S能階數量以及氦原子氣室造型。

為了改進實驗，提出幾個解決方案有幾項，首先，發現ECDL之外腔並無效用，因此重新調整光柵必須將一級光回射回雷射二極體內，產生Injection的效果，ECDL的Ith (the threshold current)必須比原先單顆二極體雷射的Ith來得低，才能保證外腔真正有效用，接著微調光柵使雷射頻率到實驗所需頻率後，測試其穩定性。再來是改變TTL脈衝的時序，增加氣體放電的時間，以增加電子處在亞穩態的氦原子密度，但必須注意增加氣體放電時間，放大功率後的射頻( radio frequency ) 訊號會影響其他實驗儀器。最後改用圓形的氣室，減少氦原子跟管壁的碰撞，雷射入射的窗口改用與雷射入射方向呈90度之窗口，以避免雷射路徑的偏移。

本實驗是利用一台自製的外腔雷射( ECDL )系統來量測2S-3D的氦原子雙光子躍遷，其優點在於自製的外腔雷射( ECDL )之費用非常便宜，也有不錯的輸出功率，同時在實驗中我利用市售的電子零件製作了一台脈衝產生器，可控制輸出的頻率、脈衝寬度以及相對的相位，且可同時控制四套需要TTL脈衝控制的系統或儀器，優點除了便宜之外，也非常穩定；為了隔絕磁場，利用了磁場隔絕金屬箔，我也在實驗室中建立了一組氣體放電系統，其系統可以用脈衝產生器來控制開關時序，最後，我建造了一個氦原子能階的量測系統，而且費用低廉，可與本實驗室的另外一門研究光梳雷射系統做結合。

The thesis is for measuring the 2S-3D two photon transition spectrum of Helium. In the experiment, I needed to use helium discharge to pump electrons from 1S ground state to 2S metastable state , then examined the fluorescence spectrum from the gas discharge system . After made sure that the gas in air cell is 4He, the external cavity diode laser (ECDL) was split into two laser beams have the same frequency and overlapped to excite electrons of helium to 3D state, then measured the He 2S-3D two-photon transition by detecting the cascaded 587 nm fluorescence decayed from 3D-2P. However, I only detected the laser scattering and I can’t see any signal of the He 2S-3D two-photon transition. The reasons I can’t see any signal of the He 2S-3D two-photon transition is the laser line width of ECDL is too broad, the population of electrons in 2S metastable state and the shape of helium cell.

To solve the experiment problems I propose several solutions: first discovered that the external cavity of ECDL does not work , therefore I need to readjust the grating to reflect the first order beam back to the laser diode to produce the injection effect. The threshold current of ECDL must be lower than the threshold current of a single laser diode to guarantee the external cavity of ECDL can work effectively and then fine tuning the grating to find the laser frequency . Second , changing the time sequence of the TTL pulse to increase the time of helium discharge to get more populations of electron in 2S metastable state. But it will be need to pay attention to the amplifying power radio frequency will affect other experiment instruments when increasing the time of helium discharge. Finally using the circular cell to reduce the collision helium atoms hit the wall of the cell and using the window the laser incident assumes 90 degrees with the direction the laser incidents to avoid the displacement of the laser beam.

I used the homemade ECDL to measure the 2S-3D two photon transition spectrum of Helium , the advantages are that the expense of homemade ECDL is cheap and have good output power. At the same time, I used the electronic part can be got in the market to manufacture a pulse generator in the experiment, the pulse generator can control the output frequency, the pulse width, the relative phase and also can simultaneously control four sets of systems or instruments need the TTL pulse control. The advantage is that the cost of the pulse generator is cheap and the pulse generator is stable. To isolate the magnetic field I used the magnetic foil to helium cell. I also built a gas discharge system, the switch sequence of this system can be controlled by the pulse generator. Finally I built a detecting system can measure the helium energy state. It is cheap and can be combined with the comb laser system in my laboratory.

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