How to find a commercial LED with 1625nm wavelength

What you're looking for is also called a "light pipe" or "light guide." If you're designing a product for high volume production, the mechanical design team can create custom ones. They're just little pieces of clear plastic that go between an LED on the main circuit board and the edge of the product case. Most off-the-shelf ones are designed for generic high-volume uses where custom mechanical design doesn't add much to the product value, like an array of lights on the front of rackmount equipment. It seems unlikely you'd find exactly what you're looking for since it sounds like an unusual scheme. If you're a hobbyist you might be able to cut a piece of Plexiglas, or custom mold something that works reasonably well out of epoxy. The optical properties of the material won't be the same but it might work well enough.

Maximising brightness:

Brightness is a function of amount of light and area illuminated.

• More light = more brightness.

• Less area = more brightness.

So

For more light obtain as many lumens as you can afford.

For less illuminated area use LEDs with low "radiation angle" or "cone angle".

Lenses:

If that is not bright enough you can use a lens or reflector.
Many companies provide lenses for LEDs.
Deal Extreme have a range here

Effect of colour: Note also that brightness is related to eye response to colour - yukky yellow green is about the best colour for optimum eye perceived brightness - but if you want a specific colour this is not much help in optimising brightness.

Related: Mind plasticity add on for the ever young:

Lime Green is close - Lime Green Fire Engine from here - BUT more recent studies suggest that familiarity is more important than visibility for emergency vehicles.