Disable XFP laser in JUNOS

Unfortunately JUNOS doesn’t provide a built-in command to disable laser on XFP ports.  If you have to shutdown the laser anyway, you can do this in the “pfe shell” with the command test xfp <index> laser off:
adm@MX80> start shell pfe network tfeb0

TFEB platform (1000Mhz MPC 8544 processor, 1024MB memory, 512KB flash)

TAZ-TBB-0(MX80 vty)# test xfp
[...]

Using two HWIC-4ESW in a Cisco 1841

Recently, there was a need for additional FastEthernet ports on a Cisco 1841. The router had already one  HWIC-4ESW attached, but all ports were occupied.
Fortunately, there was a second HWIC-4ESW at stock which was inserted immediately. Two additional ports was configured quickly to a vlan already in use on the other HWIC.
But then the problem [...]