8

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

The supply current is only 18.5mA for the output current of

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

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.

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

current amplitude less than the minimum laser threshold

sinusoidal wave form (refer to the proceeding performance

curves).

Figure 1 shows an equivalent circuit of pins associated with

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.

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.

-

+

PIN

EL6204