Out of the box, Juniper EX4650 devices cannot use 1G interfaces. Hardware limitations mean that in order to use 1G interfaces, blocks of 4 ports have to have the port speed set to 1G in the chassis configuration.
The Juniper forums has a thread about the issue for further information.
As an example, port 0 has a 1GB SFP plugged in. The SFP will show in the chassis hardware as usual:
{master:0}
me@switch> show chassis hardware
...
Xcvr 0 REV 01 740-011782 F2010956657 SFP-SX
...
But the interface does not seem to exist at all in any show commands:
{master:0}
me@switch> show interfaces ge-0/0/0
error: device ge-0/0/0 not found
{master:0}
me@switch> show interfaces xe-0/0/0
error: device xe-0/0/0 not found
Performing a soft reset on the interface shows this:
Soft removing pic/port: 0/0!!!
FPC0(switch vty)# [Wed Mar 2 11:43:15.833 LOG: Info] sfp-0/0/0 SFP unplugged
[Wed Mar 2 11:43:15.833 LOG: Info] optic_sfp_check_sfp_destory: Detaching sfp-0/0/0
[Wed Mar 2 11:43:16.038 LOG: Warning] CMQFX: FPC0:PIC0:PORT0 Default (10g) config on port but XCVR doesn't support 10g speed. IFD creation skipped
Soft inserting pic/port: 0/46!!!
FPC0(switch vty)# [Wed Mar 2 11:43:37.821 LOG: Info] sfp-0/0/0 SFP plugged in
[Wed Mar 2 11:43:39.826 LOG: Info] PORTDEV: 1G cable_type for 0/0/0 port_type 7
[Wed Mar 2 11:43:39.826 LOG: Warning] CMQFX: FPC0:PIC0:PORT0 Default (10g) config on port but XCVR doesn't support 10g speed. IFD creation skipped
[Wed Mar 2 11:44:21.374 LOG: Notice] sfp-0/0/0 link 46 SFP output power low alarm set
[Wed Mar 2 11:44:21.374 LOG: Notice] sfp-0/0/0 link 46 SFP receive power low alarm set
[Wed Mar 2 11:44:21.374 LOG: Notice] sfp-0/0/0 link 46 SFP output power low warning set
[Wed Mar 2 11:44:21.374 LOG: Notice] sfp-0/0/0 link 46 SFP receive power low warning set
To enable 1G mode on the port, set the interface speed for port 0 to 1G in the chassis configuration:
set chassis fpc 0 pic 0 port 0 speed 1G
This will cause ports 0, 1, 2 and 3 to only be usable at 1G. You will get a warning notifying you that this is the case:
'speed 1G'
warning: 1g config will be applied to ports 0 to 3
The speed configuration can only be set on the first of the block of 4 interfaces, if you try and define it on the other 3 interfaces in the block you will get an error:
{master:0}[edit]
me@switch# set chassis fpc 0 pic 0 port 3 speed 1G
^
syntax error, expecting <data>.
Once the configuration has been added, the port can then be addressed as ge-0/0/0 (NOT xe-0/0/0 as it is no longer a 10/25G port).