Optics, lenses, and light reflection

[Ray_Xu]

Hi, I'm new to this place, and I'm not sure if my problem fits with this forum, but I'll give it a try.

Right now, I'm building a laser range finder, but my biggest concern will be focusing the reflected laser beam onto a small silicon photodiode. And to add to that, I have barely any knowledge in optics except for the types of lenses, and some terms used in optics. It would be best if there is some manufacturer/distributor that sells lens assemblies (custom or standard). Here are my requirements, so
far:

1. diameter: between/equal 5 inches and 2 inches 2. type: convex, or a lens assembly to focus light 3. no filtering (for now).
4. can be focused by using a rotating motion (such as those on a nikon D70 camera)

Also, I'm having trouble on calculating the amount of reflected light if I were to fire a laser beam at a white surface. Can anyone help me on this?

Ray Xu

 

[Andy Resnick]

You are untaking a project that is going to be extremely difficult, given your level of preparation. At least look around and see what information about range finders is out there (e.g. principles of operation). For example, what is your requirement for distance accuracy? What wavelength? Field of view? Speed of acquisition?

Focusing the (reflected) light onto a photodiode is the least of your problems.

 

[Ray_Xu]

You are untaking a project that is going to be extremely difficult, given your level of preparation. At least look around and see what information about range finders is out there (e.g. principles of operation). For example, what is your requirement for distance accuracy? What wavelength? Field of view? Speed of acquisition?

Focusing the (reflected) light onto a photodiode is the least of your problems.

I already have all the electronics and everything planned out and to be simulated (electronics is basically my living). The only part where I really lack is optics; which I need to patch it up before I can move on.

 

[Andy Resnick]

If you make the laser source confocal with the photodiode, then your problem is solved. Use whatever lens system you use to launch and steer the source to collect and focus the return signal.

 

[Bob S]

Interesting and good Science Fair project. How long a distance do you want to range? What time resolution do you want? What are properties of the photodiode - speed (risetime), sensitive area, noise, etc? What are the properties of the laser- pulse length, joules, wavelength, divergence?

 

[Ray_Xu]

Interesting and good Science Fair project. How long a distance do you want to range? What time resolution do you want? What are properties of the photodiode - speed (risetime), sensitive area, noise, etc? What are the properties of the laser- pulse length, joules, wavelength, divergence?

Actually, this isn't a science fair project (although I did enter part of my project into the science fair). This is my hobby and I plan to sell it to some major company to mass produce it. To answer your questions, the range I'm aiming for is from one or two inches to 100 feet. Accuracy will depend on the range setting (nature of the phase detector's design). I'm using a avalanche photodiode in my receiver circuit from Hamamatsu; costs $80 (pretty cheap but yet its perfect for my application). Right now, I'm using a prototype laser module that is 659nm in wavelength, 3.5mW maximum output power, and modulation capable; bought it from Meredith Instruments.

Edit: and I have a blog here if you are interested in my progress on it; http://rayxublogs.blogspot.com/ (yes I have bad English)