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EL6204 Datasheet(PDF) 8 Page - Intersil Corporation

Part # EL6204
Description  Laser Driver Oscillator
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Manufacturer  INTERSIL [Intersil Corporation]
Direct Link  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

EL6204 Datasheet(HTML) 8 Page - Intersil Corporation

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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 to 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 10mAP-P to 100mAP-P.
The supply current is only 18.5mA for the output current of
50mAP-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. But
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
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.
RAMP and RFREQ 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. But 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. In conclusion, the sizes of the RAMP and
RFREQ resistors must be determined experimentally. A good
starting point is to take a value of RAMP for a peak-to-peak
current amplitude less than the minimum laser threshold
current and a value of RFREQ for an output current close to a
sinusoidal wave form (refer to the proceeding performance
RAMP and RFREQ Pin Interfacing
Figure 1 shows an equivalent circuit of pins associated with
the RAMP and RFREQ resistors. VREF is roughly 1.27V for
both RAMP and RFREQ. The RAMP and RFREQ resistors
should be connected to the non-load side of the power
ground to avoid noise pick-up. These resistors should also
return to the EL6204'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.
External voltage sources can be coupled to the RAMP and
RFREQ pins to effect frequency or amplitude modulation or
adjustment. It is 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 EL6204's local environment from
propagating to the modulation source, and it will keep
parasitic capacitance at the pin minimized.

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