Hello blackmetal,
To answer your question, I think you have a AFI system (FRU-to-port airflow) and I would say the cooling system and airflow mechanism is designed in such a way that it doesn't heat up the optics/fiber and since the system has fans and sensors installed, the speed of fans spinning is varied based on the heat producted or the inputs from sensors within.
Spoiler
The chassis includes a fan speed-control system. Under normal operating conditions, fans operate at reduced speed to reduce noise and power consumption and the temperature sensors in the chassis monitor the temperature within the chassis. The system raises an alarm if a fan fails or if the temperature inside the chassis rises above permitted levels. If the temperature inside the chassis rises above the threshold, the device shuts down automatically.
You can see the status of fans and the temperature remotely through the CLI by issuing the operational mode command show chassis environment.
Also, you could mount it on a cabinet and prevent any overheating by ensuring sufficient ventilation.
Refer Page 37 and 60 of this document for details: QFX 3500 Hardware Guide
Hope this helps.
Please mark "Accept as solution" if this answers your query.
Kudos are appreciated too!
Regards,
Sharat Ainapur