EL6204 View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

EL6204

Laser Driver Oscillator

Renesas Electronics 

EL6204

Typical Performance Curves

otherwise specified. (Continued)

VDD = 5V, TA = 25°C, RL = 10Ω, RFREQ = 5.21kΩ, RAMP = 2.54kΩ, VOUT = 2.2V unless

10

40mA

0.4ns

-10

-30

-50

RFREQ = 3.03kΩ

RAMP = 2.54kΩ

FIGURE 19. OUTPUT CURRENT AT 600MHz

-70

-90

340

344

348

352

356

360

FREQUENCY (MHz)

FIGURE 20. OUTPUT SPECTRUM-WIDEBAND

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

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. 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.

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. 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 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

“Typical Performance Curves” beginning on page 5).

FN7219 Rev 3.00

October 28, 2015

Page 8 of 12

Share Link: