I designed 3 different microstrip lines given below and measured S21 parameters via Network. The S21 measurement of the microstrip line in the on.jpg file is close to 0dB. The S21 measurement of the microstrip line in the off.jpg file is close to -30dB. The S21 measurement of the microstrip line in the photodiode file is around -10dB under the light, and the S21 measurement is around -10dB in the dark. I would expected it to tend to approach 0 dB in the light and -30 dB in the dark. Why not? I used the laser driver circuit in the Laser_driver.jpg file. I used QSB34GR as a photodiode.
Properties of the laser: 785nm wavelength, Pmax=90mW, Imax=160mA