Link budget: receive power > 0

Question

I want to compute link budget in free space laser communication.

My input parameters: wave length 700 nm, transmitter aperture: 0,88/0,15/0,2 m, receiver aperture: 0,3/0,5/1

My results:

I have a question regarding these numbers

You see, I got a positive receive power. Could they be wrong results?

EDIT 1

My equations:

$${P_{RX}}_{dBW} = {P_{TX}}_{dBW} + {G_{TX}}_{dB} + {G_{RX}}_{dB} + {L}_{dB}$$

$L$ - free space path loss, $${L}_{dB} = 20 log_{10}(\frac{4 \pi D}{\lambda})$$

${G_{TX}}_{dB}$ - gewinn $${G_{TX}}_{dB}= 10 log_{10}(\frac{4 \pi A}{\lambda^2})$$

Answer 1

Without seeing what equations you used, its hard to say how correct your results are. Especially without knowing what the transmit power / transmit gain is.

But, those numbers are almost certainly in dB space since it is widely used when discussing power levels of RF/optical systems. The equation to convert a given power, P, to a dB scale is \$10 \log_{10}(P/P_0)\$ where \$P_0\$ is a reference power. Power levels are often given in dBW or dBm where \$P_0\$ is 1 W and 1 mW respectively.

So a positive power likely just means that the receiver is seeing a power level greater than whatever the reference in your scale is.

< 0 dBm -> Power levels less than 1 mW
= 0 dBm -> Power level is 1 mW
> 0 dBm -> Power is greater than 1 mW

And as a sidenote, I'm not sure why you would be surprised at a receiver seeing power > 0 W. Receiving power less than 0 W doesn't make much sense.