EL6204
8
FN7219.3
October 28, 2015
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Applications Information
Product Description
The EL6204 is a solid state, low-power, high-speed laser
modulation oscillator with external resistor-adjustable operating
frequency and output amplitude. It is designed to interface easily
with laser diodes to break up optical feedback resonant modes
and thereby reduce laser noise. The output of the EL6204 is
composed of a push-pull current source, switched alternately at
the oscillator frequency. The output and oscillator are
automatically disabled for power saving when the average laser
voltage drops to less than 1.1V. The EL6204 has the operating
frequency from 60MHz to 600MHz and the output current from
10mA
P-P
to 100mA
P-P
. The supply current is only 18.5mA for the
output current of 50mA
P-P
at the operating frequency of
350MHz.
Theory of Operation
A typical semiconductor laser will emit a small amount of
incoherent light at low values of forward laser current. However,
after the threshold current is reached, the laser will emit
coherent light. Further increases in the forward current will cause
rapid increases in laser output power. A typical threshold current
is 35mA and a typical slope efficiency is 0.7mW/mA.
When the laser is lasing, it will often change its mode of
operation slightly, due to changes in current, temperature or
optical feedback into the laser. In a DVD-ROM, the optical
feedback from the moving disk forms a significant noise factor
due to feedback-induced mode hopping. In addition to the mode
hopping noise, a diode laser will roughly have a constant noise
level regardless of the power level when a threshold current is
exceeded.
The oscillator is designed to produce a low noise oscillating
current that is added to the external DC current. The effective AC
current is to cause the laser power to change at the oscillator
frequency. This change causes the laser to go through rapid
mode hopping. The low frequency component of laser power
noise due to mode hopping is translated up to sidebands around
the oscillator frequency by this action. Since the oscillator
frequency can be filtered out of the low frequency read and serve
channels, the net result is that the laser noise seems to be
reduced. The second source of laser noise reduction is caused by
the increase in the laser power above the average laser power
during the pushing-current time. The signal-to-noise ratio (SNR)
of the output power is better at higher laser powers because of
the almost constant noise power when a threshold current is
exceeded. In addition, when the laser is off during the pulling
current time, the noise is also very low.
R
AMP
and R
FREQ
Value Setting
The laser should always have a forward current during operation.
This will prevent the laser voltage from collapsing and ensure
that the high frequency components reach the junction without
having to charge the junction capacitance.
Generally it is desirable to make the oscillator currents as large
as possible to obtain the greatest reduction in laser noise. But it
is not a trivial matter to determine this critical value. The
amplitude depends on the wave shape of the oscillator current
reaching the laser junction.
If the output current is sinusoidal and the components in the
output circuit are fixed and linear, then the shape of the current
will be sinusoidal. Thus the amount of current reaching the laser
junction is a function of the circuit parasitics. These parasitics
can result in a resonant increase in output depending on the
frequency due to the junction capacitance and layout. Also, the
amount of junction current causing laser emission is variable
with frequency due to the junction capacitance. It can be
concluded that the sizes of the R
AMP
and R
FREQ
resistors must
be determined experimentally. A good starting point is to take a
value of R
AMP
for a peak-to-peak current amplitude less than the
minimum laser threshold current and a value of R
FREQ
for an
output current close to a sinusoidal wave form (refer to the
“Typical Performance Curves” beginning on page 5
).
FIGURE 19. OUTPUT CURRENT AT 600MHz
FIGURE 20. OUTPUT SPECTRUM-WIDEBAND
Typical Performance Curves V
DD
= 5V, T
A
= 25°C, R
L
= 10Ω, R
FREQ
= 5.21kΩ, R
AMP
= 2.54kΩ, V
OUT
= 2.2V unless
otherwise specified. (Continued)
R
FREQ
= 3.03kΩ
R
AMP
= 2.54kΩ
40mA 0.4ns
RELATIVE AMPLITUDE (dB)
-90
10
FREQUENCY (MHz)
340 360
-30
-70
-10
-50
348 352 356344
EL6204
