http://optics.ph.unimelb.edu.au/atomopt/publications/littrow_rsi_vol72_p4477_2001.pdf The picture of the ECDL I explain can be found from the above web site. The purpose of piezo is to change the cavity length of the diode laser, so that the wavelength can be varied. From the picture, there are two PZT. One is PZT stack, and the other one is PZT disk. PZT disk: change the cavity length. I have one question. I wonder why when I turn on the laser, my photodiode can only detect the signal of the laser only if I turn on the PZT stack? Thank you.