11 replies to this topic

[antennaboy]

Hello Forum,

I am new to this topic (laser) but I would like to learn. I went to this website,

http://www.dragonlas...-p-1-c-355.html

and I saw that two different laser, similar wavelength, were so different in price. One is 1W in power, the other, only 200 mW, is more expensive though....

why?

I am trying to set up a lab experiment to show amplitude modulation, free space, with a laser and a detector....
What equipment do I need?

a laser: what type?
a detector: what type?
a modulator: what type?

thanks,
antennaboy

[Cyparagon]

antennaboy, on 10 February 2012 - 08:48 PM, said:

and I saw that two different laser, similar wavelength, were so different in price. One is 1W in power, the other, only 200 mW, is more expensive though....

why?

Short answer: different techniques are used to create different colors, and some techniques cost more to implement than others.

antennaboy, on 10 February 2012 - 08:48 PM, said:

I am trying to set up a lab experiment to show amplitude modulation, free space, with a laser and a detector.

Please be more specific.

[-Elite-]

Hi, welcome to the Laser Community!

Please try not to post links to other laser companies.

Simply put, 447nm is cheaper to manufacture while 473nm is more expensive

[antennaboy]

Thanks to everyone,
sorry for the reference to the laser vendor...

447 nm and 473 nm don't seem so much different in wavelength....but their price is....I guess the construction is quite different...

As far as the free space laser link goes, I am quite open to suggestions (since I am not sure how to go about it). Imaging a computer with some data, a song, that we want to transmit to another point of the room, via laser.
I guess we need a laser modulator to connect to the computer. The modulator then connects to the laser which transmits to a laser detector (photodetector), attached to another computer or speaker directly...

I would like to understand how to send information through a laser....

While radio can be AM, FM (and phase) modulated, laser are usually only amplitude modulated to create pulses OOK.....

What type of equipment do I need to buy? What are the minimum specs?
thanks
antennaboy

[Cyparagon]

Well, the wavelength doesn't seem to matter at all for your application, so go 650/660nm (cheapest). The modulation speed on a typical chinese lab unit will only be 10-30kHz according to most of the specs I've seen. If you can send your data with that kind of bandwidth, great. If not, you need to build your own or pay buku mullah. They're not "amplitude modulated to create pulses," they are either analogue modulated (the output power is proportional to the input signal, generally 0-5V) or they are TTL - digitally modulated. The simplest way is to simply send the digital data to the modulation input of whatever you buy. Just be sure it has modulation capability.

DPSS cannot be modulated past a certain speed. I want to say ~100KHz, but I'm not sure about that. This is because of the fluorescence time of the Nd. Direct injection lasers (laser diodes) themselves can be modulated at many MHz, but the drivers are generally the bottleneck. This is because a common usage is laser shows, where 10-30KHz is sufficient for the display. I've built a driver that can follow a 50kHz square wave, and there is a driver available for ~$40 that can follow a 200kHz square.

[antennaboy]

Ok great, so I need a laser diode that allows a driver to connect to it, since we are talking about direct intensity modulation.
The driver is what will turn the laser on and off, it is the modulator itself...
In my case, I just need to put together a proof of concept situation....

The driver then attaches directly to the computer ethernet card????
I bet i am missing some parts here....

(there are also external modulators, much faster than direct modulators).

Does the laser frequency affect the speed of modulation? The higher the frequency the higher the data rate? I don't think so as long as the carrier frequency is >> than the bandwidth of the signal...

thanks

[jhawins]

antennaboy, on 11 February 2012 - 04:35 AM, said:

Ok great, so I need a laser diode that allows a driver to connect to it, since we are talking about direct intensity modulation.
The driver is what will turn the laser on and off, it is the modulator itself...
In my case, I just need to put together a proof of concept situation....

The driver then attaches directly to the computer ethernet card????
I bet i am missing some parts here....

(there are also external modulators, much faster than direct modulators).

Does the laser frequency affect the speed of modulation? The higher the frequency the higher the data rate? I don't think so as long as the carrier frequency is >> than the bandwidth of the signal...

thanks

facepalm... Google is your friend.

[antennaboy]

I am facepalming a lot these days

Google is everyone's friend, it drops lots of knowledge on everyone....

thanks

[Cyparagon]

antennaboy, on 11 February 2012 - 04:35 AM, said:

The driver then attaches directly to the computer ethernet card????

I can help you with the laser. I can't help you understand your own signal.
If you don't know what your signal is, I sure as hell don't know what your signal is.

antennaboy, on 11 February 2012 - 04:35 AM, said:

there are also external modulators, much faster than direct modulators

Yes, Acousto-optic modulators (AOM for short) are very fast, but also expensive.

antennaboy, on 11 February 2012 - 04:35 AM, said:

Does the laser frequency affect the speed of modulation?

What frequency? Are you talking about the frequency of the light itself? It's in the THz range, and doesn't have anything to do with the modulation speed. A problem you seem to have is your preconcieved ideas about AM radio. They don't translate well to the visible spectrum. A laser data-link will almost always be digital.

[antennaboy]

Thanks Cyparagon,

as far as the signal, it does not really matter: it could be the simplest signal, like a song, or a word document. I am trying to set something up for educational purposes.
I have my pc with the ethernet card...what do I attach to it? The driver? And the driver to the laser?
that's it?

Trivial question: why do lasers carry so much data rate? Is it because they can be turned on and off faster than a Rf source or is it simply a matter of the ratio f_c/W, where f_c is the carrier frequency and W the signal banwidth?

thanks!

[Cyparagon]

antennaboy, on 11 February 2012 - 04:54 PM, said:

it could be the simplest signal, like a song

A song is not a signal. A song may be translated into a signal, but it can be done in many, many ways. You need to figure that part out before you start engineering the link.

antennaboy, on 11 February 2012 - 04:54 PM, said:

...where f_c is the carrier frequency and W the signal banwidth?

You missed my comment about radio. Read it please. There is no carrier frequency in a digital signal. The closest approximation might be baud rate.

[antennaboy]

Well, a song, as an mp3 file, or an image ,as a jpeg file, are a bunch of bits that needs to be converted to an electrical signal which will eventually modulate the laser.....

As far as carrier frequencies: we start with bits. Consider ASK modulation and OOK in its simplest form. When we talk about digital modulation, be it for RF signals or optical signals (LED or laser) we are always using an analog signal that gets digitally modulated. A pulse (1s) contains several cycles of the carrier frequency, which for a laser is THz.
Often in the optical regime they don't talk about frequency but wavelength. If your look at WDM, it is equivalent to FDM in radio: multiple channels (multiple carrier frequencies) correspond to different frequencies (wavelengths). Each frequency is modulated the way we prefer...