How Do You Calculate the Output Power of a Laser After Multiple Adjustments?

[stunner5000pt]

Homework Statement

A laser produces 15.6 dBm of power. The light is propogated through a 10dB neutral density filter that attenuates the signal so that it can be detected by a photodiode without saturation. The optical signal is steppeddown by 15dB when captured by teh photodiode and converted to an electric signal. This signal is later amplified by 20.2 dB by an inverting amplifier. What is the output signal power from teh amplifier in mW?

Homework Equations

$$G [dB] = 10 \log_{10} \left(\frac{P_{out}}{P_{in}}\right)$$
where P is the power

$$G [dBm] = 10 \log_{10} (P[mW])$$

The Attempt at a Solution

15.6 is in dBm while the others are in dB. How do we convert from dBm to dB??

Since G represents the gain or loss
then the total gain by the signal is -10 - 15 + 20.2 = -4.8

Thus we would use hte first equation
with G = -4.8 , Pin = 15.6 dBm and calculate Pout

would that be correct??

 

[andrevdh]

The power of a laser is usually stated in units of watts, or more likely milliwatts. Even 15.6 mW will be a reasonably powerful laser.

See bottom of this webpage:

http://www.lumalaser.com/laser-power_recommendations.htm"

 

[m2003]

Yes, your approach is correct. To convert from dBm to dB, you can use the formula dB = dBm - 30, since 1 mW = 0 dBm. So in this case, the input power in dB would be 15.6 - 30 = -14.4 dB. Then using the first equation with G = -4.8 dB, you can calculate the output power in dB and convert it back to mW using the second equation. This will give you the output signal power from the amplifier in mW.