Simple Wavemeter with sub-GHz accuracy
具有GHz (1×10-7)以上精確度的極簡單波長儀

在運用可調雷射(tunable laser)時，wavemeter是個極為重要的工具。我們必須藉由它才能將雷射調整到正確的波長，達到與原子或分子躍遷的都卜勒線寬(Doppler linewidth)之內，即GHz的精確度。Wavemeter的基本結構是一個 Michealson 干涉儀，利用比較與已知波長雷射(632.8nm HeNe Laser)的干涉條紋數，得到待測雷射的波長。我們利用電磁鐵驅動一個由鋼片鋸構成的單擺 ，取代了一般wavemeter中的線性滑軌。並以鎖相迴路(PLL)倍增干涉條紋數。使得單擺的實際擺幅雖然只有1-2cm，仍然可以達到10-7的精確度。同時在干涉條紋的計算與處理上，是利用National Instrument的多功能DAQ卡(PCI-6035E)內建的兩個高速計數器，進行記錄。再將數據以Labview程式進行季算處理。由於DAQ卡並不昂貴，甚至在許多實驗室中，高速計數器的功能並沒有被加以利用。我們的方法完全省去了發展繁複的全硬體的電子計數器(尤其是當頻率高於MHZ，到達TTL或CMOS原件的限制時，對於非電子專業的物理學家而言更是困難重重)，且可運用電腦網路存取的功能將波長訊息即時傳遞到遠方。

The reference He-Ne laser is 632.8nm, single longitude mode with a short cavity. The laser frequency has not been calibrated directly, but we calibrated the measured result of a diode laser with a wavelength on the well known rubidium D2 line hyperfine transitions. To avoid frequency instability due to the thermal fluctuation of cavity, the laser was protected using a plastic box. No further frequency stabilization is needed, as the required accuracy is only GHz.

The central part of the wavemeter is a swing corner cube prisms pair mounted on an aluminum block. 4 steel blade saws support entire corner cube block and provide smooth swing, as a pendulum. The oscillation frequency can be controlled by adjusting the weight of the block. It is important to maintain the maximum velocity of the block not to exceed 2.5cm/sec, which is corresponds to roughly 200kHz of the interference fringes. A frequency multiplier will multiply this frequency by 10 using a phase lock loop with HEF4046 and programmable divider HEF4059. The swing range should be longer then 1cm to have enough counts of fringes and accuracy.

This pendulum is pushed using a pair of coils on the base to interact with a magnet mounted on the block. The force is given while the pendulum returns in the end of each swing, driven a driver with TIP31. That is, it is leaving the opto-switch in the ends.

The labview program (screen shot) receives multiplied TTL fringes signal from both reference laser and unknown laser. The opto-switch also send the "start" and "end" to the program to define the conting period.

Note:The work is inspired by the paper of P.J. Fox("A reliable, compact, and low-cost michelson wavemeter for laser wavelength measurement”, Am. J. Phys. 67, 624–630 (1999). ) and a design from PEG Baird's group of Atomic and laser physics in Oxford University. The cost of this SWING wavemeter is about NTD40000.

Poster in 2004 annual meeting of physics, Taiwan.