Electro-Optic Modulator And Its Effect In Laser Frequency Stabilization

In high precision measurements,frequency stabilized laser is an essential tool.However,laser’s frequency has a drift when environment changed,in order to avoid this drift,laser’s frequency is often locked to atomic transition frequency.If we want to get the error signal of frequency stabilization,we should modulate the laser firstly.The Electro-Optic Phase Modulator（EOM）is such a kind of important modulator.In modulation transfer frequency stabilization,the modulation frequency is also an important parameter.Different modulation frequency means the error signal has different amplitude and slope,a large signal with big slope always means we can lock the laser in a better point.So,based on the EOM we made,we researched EOM’s influence on frequency stabilization.Before we researched EOM’s effect in frequency stabilization,it’s important to have a EOM whose characteristic is known.So we made an EOM by ourselves at first.We choose lithium niobate crystal as our electro-optic crystal;and then,we use an Electron-beam Evaporator to gild the crystal to form an electrode;we also use LCR resonant circuit to amplify the input signal,and at last we made a resonant EOM.Through beating with a light,we measured our EOM’s resonant frequency and the modulation depth,the resonant frequency and modulation depth is about 2-4 MHz and0.1-0.7 rad/V,which is larger than some commercial product.In latter experiment,we tried thinner crystal and made a Broadband EOM（DC-160MHz,0.03rad/V）,which can satisfied our experiment requirement.At last,in order to make our EOM more easily to connect to a fiber system,we use a pair of collimator to couple the EOM’s light into a fiber,the coupling efficiency is about 80%.Lithium niobate crystal in EOM has both Electro-optic effect and piezoelectric effect.In order to investigate piezoelectric effect on EOM’s modulation,we derive the crystal’s piezoelectric equation theoretically,the result shows if a crystal’s thickness is 3mm and1mm,the crystal will resonant with the electric field at modulation frequency 1.24×9)9)MHz and 3.71×9)9)MHz（n=1,3,5…）,and the modulation depth will increase at the same time.This theory analysis meets out experiment result.With this effect,we can improve the EOM’s modulation depth.At the same time,this phenomenon also remind us in the experiment of measuring the modulation transfer spectrum in different modulation frequency,we should skip those resonant frequency.We applied our EOM in a modulation transfer frequency stabilization experiment,and achieved laser’s frequency stabilization.In order to increase the precision of the frequency stabilization,we change the modulation frequency of the EOM,and then get the MTS error signal of ⁸⁷Rb atoms in different modulation frequency.When the modulation frequency is about 6.1-9.2 MHz,error signal has big amplitude and slope,this result also meets the theory.In order to increase the stability of frequency stabilization,we also researched the temperature and the magnetic field’s influence to frequency stabilization.We found the laser’s polarization should parallel to the crystal’s axis,or it will cause residual amplitude modulation and make the locked frequency changed.The angle between the polarization and crystal’s axis should smaller than 5degree if we want a small frequency fluctuation（<300 kHz）.As for magnetic field,no matter whether the polarization is parallel or not,the frequency shift is smaller than 100kHz when we modulate magnetic field in 0-8 G.At last,we made a prospect about our EOM.First,we can use RF type inductor to increase the EOM’s resonant frequency;second,in fiber coupled EOM,we can use better glue,to increase the stability of connection.