# Need help with gaussian beam

1. Mar 28, 2009

### Jeff123

I have a transmitting fiber and a receiving fiber. Now the transmitting fiber core diameter is 50 micrometer and I want to know how many receiving fibers should be used so all light from transmitted fiber is received properly.

As suggested by some friends beam follows the Gaussian distribution. So I tried searching and got these link http://www.mellesgriot.com/products/optics/gb_2_2.htm

where I assumed that w(z) should be radius of my receiving fiber as I was thinking using 2 of receiving fiber of 200 micrometer so I thought maybe I should use 200 micrometer as radius or w(z) value. The wavelength I used was from 400nm to 900nm and got z that is separation distance as 20 mm to 38 mm. I dont know if it is right. I assumed starting beam radius which is w0 as 25 mm as core diameter of transmitting tube is 50 mm

Now I have a power output of 1w so I need to know what should be intensity which my recieveing fibers will recieve. I have got the formula for intensity here http://www.mellesgriot.com/products/optics/gb_2_1.htm
but it has a parameter r which if i have understand correectly is same as w. so I am confused.

2. Mar 28, 2009

### Redbelly98

Staff Emeritus
Welcome to PF.

If you're just coupling the beam from one fiber to another, why not use a lens to focus the beam onto the receiving fiber? Also, is there a reason for having the beam exit the fiber at this point? What is the purpose of the receiving fiber?

The Gaussian beam calculation would require more information. For example, the beam is probably already diverging when it exits the transmitting fiber, and you cannot assume the waist w0 is located at the end of the fiber.

If you can measure the divergence angle of the beam, that would be useful.

3. Mar 28, 2009

### Jeff123

Actually the transmitting beam will have some displacement and these will be recieved by recieving fiber. now if I introduced lens in between them then lens will be fixed and how it will work I am not sure.

The purpose of recieving fiber is to recieve the light from the transmitting fiber and give it to phototransistor. we are trying to see hwo much displacemnt is occuring by transmitting fiber which is linked to some biological object

4. Mar 30, 2009

### Redbelly98

Staff Emeritus

Since you're trying to measure displacement, I am thinking the best approach is to first calibrate the setup by measuring the signal for several (known) displacements of the receiving fiber. I would not depend on calculations and assumptions about the beam intensity for something like this.

I don't know what magnitude of displacements you are expecting, but using a micrometer screw to move the receiving fiber for the calibration might be helpful. You could get one of these:

http://www.thorlabs.com/thorProduct.cfm?partNumber=MS1

A full turn of the screw provides a 250 um (1/4 mm) displacement.

Regards,

Mark

5. Mar 30, 2009

### Jeff123

I am little confused let me tell you what I need to achieve.

I have transmitter fiber on one end of which will be light source. Transmitting fiber will take that light and emit on other end. The biological tissue is attached to transmitting fiber. Now the biological tissue is continuously creating some force which will make the transmitter move a lil bit as it will create displacement of transmitting fiber. Now the force is in micronewton range.

The recieveing fiber will receive the light but due to small change in displacement the intensity of light incident on receiving fiber will change which will lead to change in voltage generated by phototransister. What I am going to do is apply some known forces and measure the voltage. These voltages will be sued as references when actual force from biological tissue is applied.

I have got almost everything just I am confused how to provide calibrate the system as well as what should be distance between reciever and transmitter so maximum intensity can be used

Thanks man. These will be counted as favor from you as well as these community

6. Mar 31, 2009

### Redbelly98

Staff Emeritus
Okay, thanks for clarifying the procedure. I would still just proceed experimentally with the calibration. Try calibrating at different distances between the two fibers, and see where you get a reasonable sensitivity for the range of forces you are expecting.

A couple of things worth mentioning, in case you are not aware of them ... sorry if these are obvious to you:
• You'll want to locate the receiving fiber away from the central maximum in intensity. Move it laterally so that the signal is roughly 1/2 of the central maximum, then do the calibration. This will give much better sensitivity than if you are near the maximum.
• Does your light source fluctuate in power? That may be something to check out and be aware of.

EDIT:
Without knowing how the transmitting fiber's displacement and/or angle changes with applied force, detailed knowledge about the beam's intensity pattern is not really useful.

7. Mar 31, 2009

### turin

I totally agree with Red, here. However, now I'm confused. From the OP I thought that you would use an array of receiving fibers. But now it sounds like you will use only one receiving fiber. If you are only using one fiber, then Red's suggestion to maximize sensitivity is important. However, if you use an array of fibers, then you don't need to worry much about this.