Laser Frequency Stabilization By Using Modulation Transfer Spectroscopy

In recent years, laser cooling and trapping gets rapid improvement. Semiconductor lasers are widely used in atom optics and optical communications, spectroscopy and other fields, with advantages such as small size, simple configuration, long life, good output mode, stable power and so on,. Many experiments require a laser with a well-defined frequency. But over time, the central frequency of a diode laser with grating feedback will drift, it may reach to GHz. This drift is caused by fluctuations in temperature and injection current and mechanical vibrations. Stabilizing the laser by locking it to an external reference to reduce this drift is very important to our experiment because we expect narrow bandwidth. The basic principle of stabilizing the laser is as follows. The diode laser is locked to the stable reference frequency such as high stable transition spectroscopy of atom or molecule, locked diode laser and so on. We can get the error signal which can control current or PZT of diode laser through PID (proportion-integration-differentiation) feedback controls.In this paper, we focus on modulation transfer spectroscopy (MTS) which puts modulation on the devices of frequency shift and phase shift with electro-optic modulator (EOM) instead of on the laser frequency to reduce the bandwidth. The linewidth of the laser locked by the MTS is about 400 kHz which is better than the past way like the saturation absorption dither locking and the dichroic atomic vapor laser lock(DAVLL). A simple and cheap method is applied into the system which can get the frequency shifting up to 150MHz from the modulation transfer spectrum. The system can achieve frequency locking and tuning (including red and blue detuning) simutaniously within a large frequency range with low power loss. In chapter 1, the technology of laser cooling and trapping and presentation of the diode laser are introduced. Also, the significance of laser frequency stabilization, the purpose and contents of this study are included.In chapter 2, the basic principle and the configuration of the diode laser is briefly summarized. Then the comparison of two diode lasers with different configurations is introduced.In chapter 3, we focus on the normal way of stabling the laser included saturation absorption dither locking, the DAVLL and Frequency-to-Votage Conversion Wavelength Stabilization. Comparison of the three ways is introduced.In chapter 4, the modulation theory, electro-optic effect and EOM are summarized.In chapter 5, we briefly present the MTS. Compared with the DAVLL, MTS have its advantage, for instance, with high stabilization. Then, we get frequency shift with Zeeman Effect. Finally, analysis of the noise is obtained, from which we can improve the system.In chapter 6, the whole work is summarized and the advice of the improvement of the system is given.