Dynamic beam focusing

[Lynx516]

I am looking at Ladar(lidar). And I am wondering how can you scan a laser beam precisly and quickly. Can you use a mutating lence if so how do these work. I have exhauseted google so time to hit teh profesionalls.

 

[sgtroyer]

Not sure of a way to do this electronically. Many radar systems just use a mechanically rotated dish, why couldn't you do that with your laser? Or keep the laser fixed but rotate a mirror. It's low tech, but it gets the job done.

 

[RossGr]

Originally posted by: sgtroyer
Not sure of a way to do this electronically. Many radar systems just use a mechanically rotated dish, why couldn't you do that with your laser? Or keep the laser fixed but rotate a mirror. It's low tech, but it gets the job done.

That is exactly how it is done. It is referred to as a Galvo system, as the mirrors are driven by a Galvanometer. That is a spring mechanism which changes deflection with a change in applied voltage. They can be precisely driven by electronics.

 

[CTho9305]

Originally posted by: sgtroyer
Not sure of a way to do this electronically. Many radar systems just use a mechanically rotated dish, why couldn't you do that with your laser? Or keep the laser fixed but rotate a mirror. It's low tech, but it gets the job done.

A group here bought a REALLY expensive one... it just has a very-quickly spinning mirror.

 

[Lynx516]

Thank you very much. There are other ways of doing this for Radar. You can get electronically directing radar systems but I cannot remember what they are called.

 

[sgtroyer]

With an array of antennas, you can steer a beam electronically. There are radar systems that use this method, so there is no mechanical movement. In theory, since light is just another form of elecromagnetic radiation, you could do the same for light, but the frequency of visible light is in the hundreds of terahertz, so coming up with an antenna and a signal source is a bit of a problem.

 

[RossGr]

Dynamic beam focusing

Just a note.

The Galvo Mirrors do not focus the beam, they direct it. For high energy densitys a lens can focus a laser beam to a point at the focal length of the lens. I believe for most purposes the beam remains parallel and is not focused. If a laser beam is focused it will lose one of the key features of a laser beam, upon emerging from the lens it will no longer be a parallel beam, but a convergent one. Beyond the focal distance of the lens it will become divergent and energy density will drop rapidly.

 

[Geniere]

An easy way to direct the beam is to use speakers. Glue small mirrors to the center. The speaker diaphagm only moves in and out, so two are needed. Music will generate changing patterns. With more circuitry, you can computerize and spell names or draw pictures or whatever.

 

[CTho9305]

Originally posted by: sgtroyer
With an array of antennas, you can steer a beam electronically. There are radar systems that use this method, so there is no mechanical movement. In theory, since light is just another form of elecromagnetic radiation, you could do the same for light, but the frequency of visible light is in the hundreds of terahertz, so coming up with an antenna and a signal source is a bit of a problem.

Signal source? LED 😉. Wouldn't the antennas have to be spaced distances on the orders of nanometers?

 

[Lynx516]

Thats the problem

 

[sgtroyer]

Yeah, exactly, the antennas would be sized on the order of a few hundred nanometers, and spaced similarly. I guess you could do it with MEMS, nanotech stuff, but that still doesn't get you the 500THz signal source and the ability to control the phase relationship between each of the antennas.

Anyone want to try this for your senior project? You might even get a trip to Sweden out of it.

 

[f95toli]

Antennas of that size can (and have been) been built (gold antennas on silicon or sapphire). It is actually quite simple if you have the right tools, a few hundred nanometers is quite big considering what can be done using modern nanolithography.
I have worked with planar antennas (dipole and double.slot) with center frequencies of around 1 THz (it was a part of my master thesis project). You should be able to find more information about this on the web.

 

[RossGr]

Originally posted by: sgtroyer
With an array of antennas, you can steer a beam electronically. There are radar systems that use this method, so there is no mechanical movement. In theory, since light is just another form of elecromagnetic radiation, you could do the same for light, but the frequency of visible light is in the hundreds of terahertz, so coming up with an antenna and a signal source is a bit of a problem.

The trouble with this is that light enteracts with materials quantumechanically. So all events are governed by propability, photons are emitted in a random direction. The reason electronic beam steering work is due to large scale effects, you would not get these with a beam of light.

Consider a single atom as the antenna for a photon of light.

 

[sgtroyer]

I'm sort of out of my depth here, so it's all just speculation. Ross, I think you're right that interactions with atoms complicates the problem. It's not because the wavelength of light is so much smaller than the wavelength of other radio frequencies. 600 nanometers is still huge compared to the size of an atom, it's still large enough to be treated classically, and neglect most quantum phenomena. The reason, I assume, that light interacts with atoms is because the energy of a photon is on the order of the energy levels for electrons in atoms. So photons are absorbed and emitted by atoms (electrons jumping up and down.) Presumably this is also the property that makes visible light visible. If it didn't interact with the atoms in our retina, we wouldn't be able to see it.

I would contend that this doesn't preclude producing light from an antenna, but it might complicate issues. I would further contend that if you could produce a working antenna array for 600 nanometers, that you could steer the beam with phase relationships. That should work just fine. But, like I said, I'm woefully out of my depth, so I could be wrong.

 

[f95toli]

You can use microwave-antennas at least up to a few THz. You can for example use antennas to detect laser-light from IR-lasers.