Handbook for Sound Engineers

464 Chapter 15

ditions, to place a microscopic glass bead that acts as a
lens on top of the diode’s microchip structure. This

microlensed device has the advantage of direct compati-

bility with a very wide range of possible fibers. There

are also double-lensed versions which allow light to be

concentrated into the output fiber pigtail.

Dominant Wavelengths. Most LEDs will have a maxi-

mum power at a dominant wavelength lying somewhere

within the range of 800 to 850 nm (first window). Some

LEDs are available for other wavelengths: either around

Figure 15-19. Packaging techniques attempt to couple
maximum light into a fiber. Courtesy AMP Incorporated.

Q

Epoxy

Resin

Reflective

Parabolic

Surface

Chip

NA = sin

Glass Lens Window

Chip

Glass

Window

Glass

Window

Be0

Ceramic

TO-46

Header

LED Chip

Lens

380 Mm

Dia.

Junction

63.5 Mm Dia. (TYP)

A. Reflecting parabolic surface and glass lens window.

C. Pigtail.

B. Microlens.

D. Burrus diode.

Fiber

Epoxy

Microlens

Figure 15-20. Surface and edge emitting LEDs. Courtesy

AMP Incorporated.

Metal

Area of carrier

confinement

Light emitting area

Substrate

Metal

A. Surface.

B. Edge.

Emission

pattern

Contact

GaAs

AIGaAs

AIGaAs

AIGaAs

GaAs

GaAs

Contact

Heterojunction

Heterojunction

Metal

Oxide insulation (S 1 O 2 )

AIGaAs

AIGaAs

AIGaAs

AIGaAs

GaAs

Metal