Sorry for the late reply, I've just picked up on this thread.
Q1: Suppose two laser beams being used for the purpose of communication cross each other vertically. Would there be any interference? Would the information contained in those laser beam be affected?
No. This one is pretty simple. Photons don't interact with each other. You can't bounce photons off other photons.
The saying is; You can't "see" light.
Q2: We know that radio communication is affected by electromagnetic interference. I think this interference comes into play at the receiving end such as antenna where both information radio signal and interference radio signal is picked up by an antenna and this combined reception of both signals produces a new noisy signal at the receiving circuit. Do I have it right?
Radio interference can come from many sources. They can be directly on the same frequency like another signal jamming. Or it can be on another frequency and for a number of reasons signals can mix (intermodulate) to produce a new signal which interferes. And then there is sources of noise. Both internal to the equipment and external from the rest of the universe.
Q3: What kind of laser is mostly used for optic fiber communication? I believe it's a laser diode which provides continuous laser beam and not a pulsed one, right?
Rumpfy is pretty close to the money with regards to fibre. Laser diodes are what is used for fibre communications. They are monochromatic lasers, there's plenty of information on places like wiki with regards to how they work. Specific wavelengths are used because of the attenuation windows in the fibre.
The laser diode is not actually switched off. The reason for this is that it takes longer for the laser to turn on rather than transition between power levels. So technically it's AM (Amplitude Modulated).
Though it's not uncommon for it to be described as 'pulsed'. Most of the laser diodes I see are 0.5mW power. Though these days SFP's come in a few different ratings.
But to correct rumpfy, the first fibre comms in Australia did use 144MBps. I installed a lot of them in the 80's. The 565MBps systems didn't start being installed until about 1990.
These were PDH systems. SDH started to be installed in the late 90's. The first systems were 2.5GBps rings. Then 10GBps. I stopped working specifically with fibre in 2000.
Now I see 100MBps line systems and DWDM. Where several wavelengths are used on the same fibre to increase the capacity. I haven't even stopped to look at the long haul submarine cables.
I heard somebody say 1TBps a couple of years ago.
Now there are developments with Frequency modulating the lasers and even phase shifting them to modulate QAM. I makes my head spin these days.
Q4: Which laser is most popular for optic fiber communication in terms of wavelength? Is it red one? Can they also use infrared laser in optic fiber communication? Ultraviolet lasers also exist so why not use them because they can provide higher data rate than visible light lasers?
The reason as rumpfy mentioned is the fibre windows. Glass is a very good attenuator of UV light. It's pretty hard to get sunburn through a closed window
In Wave Division Multiplexing the lasers are said to be different colours, but they are actually all so close together in frequency that they would be much the same colour if they visible to the eye.
So you might say the first laser is red, the second, redder and the third reddist.
I can shed a bit of light on the difference between single mode and multimode fibres. It's not correct to think of them in terms of laser beams shinning down the fibres.
It's best that you forget that lasers are even used. Think of it in terms of ordinary light. Though just a single colour for simplicity.
Single mode fibres are almost exclusively used these days. Their construction is now cheap and their losses acceptable.
Multimode fibres were cheaper to manufacture and have (had) lower losses. There problem is that the wave front smears causing optical dispersion.
This puts a limit on how fast you can modulate the light.
Single mode fibre doesn't suffer from this problem, well at least not at the same rate that multimode fibre does, and not at lower data rates.
As data rates and distances are increased they become more of an issue with single mode fibres.
A really good description is to think of fibres as pipes and light waves like ping pong balls.
If the pipe is large enough (multimode) the balls fall down the pipe with almost no friction. Some bounce off the walls on the way down and the balls arrive at slightly different times at the bottom.
A single mode fibre the balls can only travel single file. They arrive exactly as they left.
If the fibre mode is too small, then obviously the balls of a lower frequency are too big to fit down the pipe. If they are a high frequency (small balls) then the pipe will multimode.
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When it comes to atmospheric propagation, it's a whole new story.
Generally we look at the properties of the atmosphere. It conducts orange light best and blue light worst.
The sky is blue from reflection and orange sunsets give you a hint of it's path losses.
So the people I know playing with atmospheric communications choose high power red LED's for their propagation. They also choose PIN diodes for their receivers because they are most
sensitive to red light. I've been slowly working away at the other end of the spectrum with blue lasers and PMT tubes which are much more sensitive, but more sensitive to blue light.
By comparison I have a lot more ground to cover than my solid state friends, and they're not at all threatened for distance records yet. I have much more trouble keeping noise out than
getting light into the tube.