Modulator in lasercom: from theory to practice

Question

Hello everyone!

We are working on a realisation of an optical modulator and have a doubt. Mr Noel Miller posted already a question, but it was suggested this community. I will repost his question because he have not an account here.

PS I have copied.

Question version 1. In lasercom we use mostly OOK or PPM modulation.

Theoretically I understand what the purpose of a modulator is and how it works (mathematically). How does a modulator look like in practice?

Input of the modulator is optical or electrical signal, depends on if modulator is first or optical source is.

In practice, modulator is a device with 100s part…

Could you as engineer explain what modulator is? How was it implemented? What is optical modulator?

For me as someone who works with mathematical model it is difficult to imagine.

EDIT 1

I have found we can modulate the optical or electrical signal. Modulation could be after or before a laser source. What is the difference? Are there advantages or disadvantages? Does it better modulate optical signal or electrical?

EDIT 2

I have the following realisation of the free space communication. The transmitter here are signal generator, driver, bias tree, laser diode, isolator and collimator.

How do the collimator and the isolator connect with laser diode? It seems they are connected via smth...

Answer 1

In comments you said,

theory of OOK modulation I understand. I need to understand its practical realisation

For moderate data rates (say, below 10 Gbps), and when high spectral purity is not required at the output, OOK can be as simple as turning on and off the power supply to the laser. This is called direct modulation.

For higher rates, or when the laser linewidth can't be compromised, the laser is usually allowed to run continuously and an external modulator device is used. This could be an electro-absorption modulator, in which the absorption of a material placed in the beam is controlled by an electrical signal. It could be a Mach-Zehnder modulator in which the beam is split, and one arm is phase-modulated so that when they are re-combined you have either constructive or destructive interference depending on the state of the electric signal controlling the phase modulator.

In practice, modulator is a device with 100s part…

For any of these modulation techniques, the modulator itself is typically considered to be a single part. The driver that controls it is generally just an ordinary RF amplifier, which may involve 5 or 10 transistors. Certainly not 100's. The main requirements for this driver amplifier compared to other RF amplifiers is that it must be relatively broad-band (possibly requiring bandwidth down to DC), and linearity might not be particularly important.

Answer 2

How do the collimator and the isolator connect with laser diode? It seems they are connected via smth...

Those are fiber optic components, so they're connected by an optical fiber. The isolator prevents back reflections from the collimator from damaging the laser. The collimator is a lens and an air gap that takes the diverging laser from the fiber and creates a collimated beam. Together those components take a fiber-based laser and make it act like a free space laser.