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Applications Information

Product Description

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

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

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 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.

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

oscillator takes on the role of supplying the total laser

current.

amplitude). Thus it does not need to be set, since it is within

the control loop. The current capability of the external source

hottest old laser.

Figure 1 shows an equivalent circuit of pins associated with

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.

to effect frequency 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 EL6205's local environment from propagating to the

modulation source, and it will keep parasitic capacitance at

the pin minimized.

Supply Bypassing and Grounding

The resistance of bypass-capacitors and the inductance of

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.

-

+

PIN

FIGURE 2. RECOMMENDED SUPPLY BYPASSING

+5V

L Series: 70

Ω reactance at

300MHz (see text)

0.1µF

Chip

EL6205

GND

0.1µF

Chip

EL6205