How do fiber Ethernet transmitters set their power output?

I've noticed that fiber-optic SFP modules from Cisco and the like often specify a range of power outputs, such as -3 to -9 dBm. What determines the actual power output we should expect to see from such modules? Is it determined dynamically, or configurable statically, or is it just a manufacturing tolerance and we can't expect to know exactly how much power a given module will emit before we install it?

An example specification is here: http://www.cisco.com/en/US/prod/collateral/modules/ps5455/ps6577/product_data_sheet0900aecd8033f885.html - see "Table 2. Main Optical Parameters".

Solution 1:

This is a manufacturing tolerance. It's not configurable or adjustable.

Solution 2:

Given the actual watts involved - by definition, negative dBm is < 1mW - the implementation details are probably not important to any problem you're trying to solve in a real-world scenario (unless you're installing tens of thousands of these in a hypersensitive environment).

Edit:

According to this Cisco forums thread this is not configurable via software; I'd suggest contacting your vendor if you have specific issues to see if adjustment is possible on the module itself, or whether different hardware is required.