90 60 30 0 - 30
1. 4 948
0 20 40 60 80 100
Optical power (m W)
VERTICAL-CAVITY SURFACE-EMITTING LASERScontinued
very narrow and stable emission when com-
pared to the wide spectrum of LEDs and
the fivefold-larger thermal shift of LEDs
and edge-emitting laser diodes. A VCSEL-
based system needs significantly less power
in the same application, as the background
can be reduced more effectively.
Some technical particulars
Many applications are best served by a
wavelength of 940 nm rather than 850
nm, as the background from the sun is
lower, the red glow becomes less visible,
and silicon CMOS sensors are still sufficiently sensitive at the longer wavelength.
The measurement shown in Fig. 2 demonstrates the good stability of wavelength
and power output over a wide temperature range of - 30° to 110°C.
Time-of-flight measurements require
short pulses of 1 to 10 ns or pulse trains with
modulation up to and above 100 MHz.
VCSEL arrays are well suited for fast switch-
ing—we have demonstrated rise and fall
times < 1 ns, even at array areas on the order of a square millimeter and currents of
many amps. While the VCSEL array itself
allows pulses <0.1 ns, electrical connections to the driver and impedance in general are the limiting factors for pulse length.
Short pulse operation at low duty cy-
cle pushes out the limitations posed by
self-heating in continuous-wave (CW) op-
eration. When pulsed, relatively small chips
can operate at much higher currents and
peak power than in CW mode. Figure 3
shows a measurement of increasing power
per single mesa with shorter pulse duration.
An array of 500 to 1000 mesas can emit sev-
eral tens of watts from a 1 to 2 mm2 area.
VCSEL-array chips can either be as-
sembled directly into the sensing system
FIGURE 3. Power curves of a single 8 µm
VCSEL are measured for short pulses of
FIGURE 2. Measured power and wavelength
is shown as a function of temperature at a
current of 2. 5 A in CW operation for the VCSEL
array shown in Fig. 4.