EL6205
oscillator takes on the role of supplying the total laser
current.
Applications Information
Product Description
The I current is the previous read current (reduced in
IN
amplitude). Thus it does not need to be set, since it is within
the control loop. The current capability of the external source
for I should be made large enough to power the worst,
IN
The EL6205 is a solid state, low-power, high-speed laser
modulation oscillator with external resistor-adjustable
operating frequency. It is designed to interface easily to laser
diodes to break up optical feedback resonant modes and
thereby reduce laser noise. The output of the EL6205 is
composed of a push current source switched at the oscillator
frequency. The output and oscillator are automatically
disabled for power saving when the average input current
drops to less than 100µA. The EL6205 has the operating
frequency from 60-600MHz and the output current from
hottest old laser.
R
Pin Interfacing
FREQ
Figure 1 shows an equivalent circuit of pins associated with
the R resistor. V is roughly 1.27V. The resistor
FREQ
REF
R
should be connected to the non-load side of the
FREQ
power ground to avoid noise. This resistor should also return
to the EL6202’s ground very directly to prevent noise pickup.
They also should have minimal capacitance to ground.
Trimmer resistors can be used to adjust initial operating
points.
10mA
to 100mA . The supply current is only 30mA
P-P
P-P
(includes laser current) for the output current of 50mA
the operating frequency of 350MHz.
at
P-P
Theory of Operation
A typical semiconductor laser will emit a small amount of
incoherent light at low values of forward laser current. But
after the threshold current is reached, the laser will emit
+
V
REF
-
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.
PIN
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.
FIGURE 1. R
PIN INTERFACE
FREQ
External voltage sources can be coupled to the R
to effect frequency modulation or adjustment. It is
pin
FREQ
recommended that a coupling resistor of 1k be installed in
series with the control voltage and mounted directly next to
the pin. This will keep the inevitable high-frequency noise of
the EL6205's local environment from propagating to the
modulation source, and it will keep parasitic capacitance at
the pin minimized.
The oscillator is designed to produce a low noise oscillating
current that is provided to the laser diode. The 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 no output current time, the noise is also very low.
Supply Bypassing and Grounding
The resistance of bypass-capacitors and the inductance of
bonding wires prevent perfect bypass action, and 150mV
noise on the power lines is common. There needs to be a
lossy series bead inductance and secondary bypass on the
supply side to control signals from propagating down the
wires. Figure 2 shows the typical connection.
P-P
L Series: 70Ω reactance at
300MHz (see text)
V
+5V
S
EL6205
GND
0.1µF
Chip
0.1µF
Chip
Setting the I Current
IN
By looking the typical application circuit, it can be seen that
the push only oscillator is more efficient at the laser than the
conventional push-pull oscillator. The significant current from
FIGURE 2. RECOMMENDED SUPPLY BYPASSING
the main board is reduced to be I (≤2mA), while the
IN
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