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 Notes
An electroluminescent IR LED is a product which requires
care in use. IR LEDs are fabricated from narrow band heterostructures with
energy gap from 0.25 to 0.4 eV. That's why the bias used to initiate current
flow is low compared to the well known visible or NIR LEDs. Typical forward
bias is V~0.1- 1 V only for mid-IR LEDs!
Be sure not to exceed I*max which is given in each LED
specification and do not use test instrument that contain sources/batteries
with voltage greater that Vcw max given in specification. For LED
current restriction and further LED current measurement we recommend to use
resistor (1-5 Ohms) connected in serial to LED. This is important to note that
un-grounded devices (e.g. computers) can give V=1-5 V that is enough to destroy
the LED!
It is highly desirable that the user has I-V meter for
small currents (10-100 x10-6 A). We guarantee the existence of the
LED output as long as V-I characteristic shows saturation in the reverse bias
(10-100 x10-6 A).
We recommend activating pulse generator prior connecting
LED to generator. On switching off the procedure is reversed: disconnect LED,
switch off pulse generator. Long wires connecting LED with pulse generator may
be the reason for LED failure because of unexpected voltage surges when
switching on and off the LED supply.
Please test all elements and circuits before applying
voltage to LED. Remember that ground (T0-18 or another holder) should be biased
positively (if not specially designed). Usually the negative electrode is made
shorter than the positive one.
The expected signal is not very big and it is important
to test and eliminate noise in the detector circuits.
In some cases it is possible to increase pulse duration.
Imax in such cases can be estimated using the following equation: Imax=I* max
/20*SQRT(f*t), where f-is the frequency (Hz), t-is the pulse duration (s), I*
max-is the maximum current (A) for t=5 us and f=500 Hz. The equation gives an
order of magnitude and may be used for t< 0.1 ms only. Pulses with t >
0.15 ms should be considered as adequate to CW operation and Imax and Vmax
should be taken close to CW operation parameters. Please, note that long pulses
can increase heat dissipation and the chip temperature. This effect decreases
LED emission power and can be traced due to the LED resistance decrease during
each pulse. CW power often decreases with time due to heatsink temperature
increase.
Microimmersion LEDs are made with chalcogenide glass
that have low melting temperature (50-70oC). That’s why, please, avoid any
heater source close to the LED. Even sunlight concentrated onto the lens can
melt glass the lens. That’s why we recommend vertical position for the
LEDs at the initial stage of the research work. We are working now to increase
the glass melting temperature or/and to strengthen its position and shape.
Be patient in adjusting the optical system. It is only
experience that allows fast work. |
