# Understanding the principle of photo diode signal generation

I have two lasers, both illuminate on a photo diode, i'm not very sure what's physically going on in the photo diode. My understanding is the following: The electric fields of both lasers add up and get destroyed in the photo detectors, the electrons excited by photons in the process generates a photo current, and these charge carriers now oscillate like an antenna to emit microwave frequency and travelled via the microwave cable to and Oscilloscope. And the microwave again induce a oscillating voltage to the charge carriers inside the Oscilloscope.

I don't understand how a photo current turns into microwave and back into photo current.

• I don't think there is a microwave. Maybe a waveguide in the form of an optical fiber that transfers the light to the diode? All diodes are photovoltaic, in principle. – Pieter Jan 19 '18 at 16:09
• @Pieter, notice he has two lasers illuminating the photodiode. – The Photon Jan 19 '18 at 16:26

the electrons excited by photons in the process generates a photo current,

I'm with you so far.

Each photon absorbed in the detector generates an electron-hole pair, which is quickly swept out of the depletion region, forming a photocurrent.

and these charge carriers now oscillate like an antenna to emit microwave frequency

I think you have cause and effect reversed here.

You illuminated the photodiode with two lasers. These two lasers will create an interference pattern on the surface of the photodiode, although this pattern will be changing much too fast for the eye to see.

If the laser emission frequencies are separated by some difference $\Delta f$, then the electric field at the photodiode surface will be fluctuating at this same rate. Seen at the quantum level, the rate of photon absorption will be fluctuating at this frequency also.

If $\Delta f$ is in the microwave region (300 MHz - 300 GHz or so) then the rate of photon absorption will be varying at a microwave frequency (It typically takes special care to tune two lasers to be as near in frequency to each other as a few GHz). If the diode capacitance isn't high enough to damp out this variation, you'll also see this frequency in the photocurrent.

If you set up an oscilloscope to measure the photocurrent, and the oscilloscope is fast enough to measure microwave frequencies, then you should see a (quite noisy) microwave signal on your oscilloscope screen. Given the random fluctuations of frequency you're likely to see in this experiment, it might make more sense to measure the signal with a spectrum analyzer rather than an oscilloscope.

Main point: the current doesn't start oscillating after the absorption occurs. The rate of absorption depends on the interference between the two laser beams, and the photocurrent is just proportional to the photon absorption rate.

• very good answer, in my case i have a feedback control to keep two lasers at a fixed frequency offset, so I won't be seeing random fluctuations but rather predictable interference pattern varying at delta f on the screen. Am I correct to say that the Photodiode detects only the intensity variation since you said it is proportional to photon absorption rate so I think you meant Power proportional to Current. – el psy Congroo Jan 21 '18 at 13:39
• Side note: the reason why I said the interference varies at $\Delta f$ is because I proportional to square of |E1+E2|, where I is the intensity and $E(t)$ and $E(t+\tau)$ are the electric field of the two lasers. $I_{M}(\tau)=\int_{-\infty}^{\infty}| E(t)+E (t+\tau)|^{2}dt$ and the oscillation term is proportional to the produce $E(t)E(t+\tau)$ hence varies at $\Delta f$ – el psy Congroo Jan 21 '18 at 13:47
• One thing i still don't quite understand is how this photo current variation of absorption generates microwave that propagates down the microwave cable? To my understanding, current is a flow of charged particles whereas microwave is electromagnetic wave, the thing that propagates in the cable is the microwave not the current. – el psy Congroo Jan 21 '18 at 14:06
• @elpsyCongroo, 1. What is the linewidth of your lasers? That's the random fluctuations I referred to. It can easily be GHz wide, depending on the laser type. – The Photon Jan 21 '18 at 15:55
• 2. We don't usually talk about "microwave" as a noun unless we're re-heating breakfast. In EE, "microwave" is an adjective defining a range of frequencies. In a coaxial transmission line we can either describe the behavior in terms of EM waves propagating in the dielectric, or in terms of currents on the conductors and voltages between them. See the Wiki article Telegrapher's equations. The voltage/current model is a simplification, as it is in any circuit. – The Photon Jan 21 '18 at 15:56