Juan,
I am not sure I clearly understand what your question was, or how you were measuring the power levels. Can you describe it further? The TX power from a ZX GBIC should be between 0dBm (minimum) and 5dBm (maximum). It may not be steady, and it may not be the same from one GBIC to another, but it can be expected to fall within that 0 to 5dBm range. And it must be attenuated to at least -3dBm in order to be useful on the RX port of a ZX GBIC.
Also, I tried to follow the above link in the above post, it got me an "Error #404: File Not Found"
Here's another link documenting the optical power details:
Cisco Gigabit Interface Converter Data Sheet
http://www.cisco.com/en/US/products/hw/modules/ps872/products_data_sheet09186a008014cb5e.html
Basically, the data sheet shows a ZX GBIC normally transmitting at a power level somewhere between 0dBm and 5dBm. And, the ZX GBIC can normally receive at a power level somewhere between -3dBm and -23dBm.
So, using these values, you can say that a pair of ZX GBICs will operate 100% reliability over a fiber span that has a measured loss at 1550nm of between 8dBm and 23dBm. If you're interested in understanding how these last two numbers are calculated and what they mean, read further; otherwise, save yourself some time.
I realize that what follows may sound like a "Microsoft tech support answer" -- interesting, and absolutely 100% accurate, but not answering the original question -- but it may help somebody out there. So here it is.
Optical Power Budget
The optical power budget of 23 dB is derived from the worst-case scenario, the lowest power level on transmit from one GBIC (0dBm) being attenuated over fiber to the lowest power level that can be received on another GBIC (-23dBm). When you measure your cable plant with an optical loss test set to verify the optical loss of the cable plant (including connectors and splices), if the loss is less than or equal to 23dBm there's a 100% chance your connection will work.
If the loss is greater than 23dBm, there's still a chance your link may work, but the odds diminish as the value of the loss increases, approaching 28dBm. At 28dBm and higher, there is a 0% chance your connection will work. 28dBm is the best-case scenario, derived from the maximum transmit power level (5dBm) and the minimum receive level (-23dBm).
Personally, I would not want to set use a link that was less than 100% reliable: it depends on you finding a matched pair of GBICs that are transmitting at maximum power. I just mention it here to give you a way to interpret the power level information from the data sheet.
Minimum Attenuation
Minimum attenuation required on the link is 8dBm. This is derived from the worst-case scenario: maximum transmit of one ZX GBIC (5dBm), feeding into the maximum receive of another ZX GBIC (-3dBm). If you have at least 8dBm attenuation measured inline, your link will be 100% reliable.
If your measured attenuation is less than 8dBm but more than 3dBm, there's a chance your link may work, but the odds diminish as you approach 3dBm. 3dBm is the best-case scenario: minimum transmit power (0dBm) into maximum receive power (-3dBm). At attenuation levels 3dBm and less, there is 0% chance the link will work, you are definitely overdriving the receive optics.
So, what's this all mean? If you use a 2-meter SMF patch cord to connect two ZX GBICs, make sure you have at least 8dBm inline attenuators installed on the RX ports of your ZX GBICs. 10dBm inline attenuators would be even better, they leave you a margin for safety.
