User's Manual

Fiber Optics Powerwave
4 - 4 Rev. P1A9-Draft 2004-11 VM100 56/EN – User’s Manual
5. Optical transmitter
The optical transmitter converts the electrical RF modulated signal to a 1310 or 1550nm
optical RF modulated signal. The transmitter ends with an optical female connector.
The transmitter has a laser diode for transmitting the optical signal, and a back-facet
monitor photodiode that provides a real-time monitoring of the optical output.
The back-facet monitor photodiode is used to control the laser treshold current that is
temperature dependent. See the treshold current bends of the optical power output
curves for some different temperatures in the left figure. The values shown in the
diagram are typical values that can vary for diffierent devices.
By using the back-facet monitor photodiode, the optical transmitter is compensated for
different operating temperatures and a temperature non-dependent electrical-to-optical
curve can be used, see Figure 4-3.
Figure 4-3. Electircal to optical signal conversion
The RF modulated optical output signal P
O-RF
has the same shape as the RF modulated
electrical input signal I
RF
, see Figure 4-3. The I
BIAS
current is set to keep the dynamic
'I
RF
current range within the straight part of the curve, provided the input power is kept
on about 0dBm (or lower). If the input power is much higher, then the P
O-RF
will be
distored.
The output signal can be the default power range or be set to a low power range via an
O&M software. Default power range means 3.5 – 5dBm, low power range 0.5 – 2dBm.
The noise figure for the optical transmitter is 30 – 35dB.
The IP3 level is 30 – 35dBm.
TX
0
2
4
6
8
10
0 20406080100
P
o
(
m
W
)
I
F
(mA)
0°C
25°C
50°C
P
O
I
I
RF
P
O-RF
I
BIAS