TZA3041 Просмотр технического описания (PDF) - Philips Electronics

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TZA3041

Gigabit Ethernet/Fibre Channel laser drivers

Philips Electronics 

Philips Semiconductors

Gigabit Ethernet/Fibre Channel laser

drivers

Product specification

TZA3041AHL; TZA3041BHL;

TZA3041U

The reference current and the resistor for the optical 1

modulation current control loop is calculated using the

following formulae:

Iref(ONE) = 1--1--6-- × IMPD(ONE) [A]

(1)

RONE = I--1O----.N-5--E-- = I---M----P---2D---(4-O----N---E---) [Ω]

(2)

The reference current and resistor for the optical 0 bias

current control loop is calculated using the following

formulae:

Iref(ZERO) = 14-- × IMPD(ZERO)

[A]

(3)

RZERO = I--Z--1-E--.-R-5--O-- = I--M----P---D---(-6-Z---E---R---O---) [Ω]

(4)

In these formulae, IMPD(ONE) and IMPD(ZERO) represent the

MPD current during an optical 1 and an optical 0 period,

respectively.

EXAMPLE

A laser operates at optical output power levels of 0.3 mW

for laser HIGH and 0.03 mW for laser LOW (extinction ratio

of 10 dB). Suppose the corresponding MPD currents for

this particular laser are 260 and 30 µA, respectively.

In this example, the reference current flowing into

pin ONE is:

Iref(ONE) = 1--1--6-- × 260 × 10–6 = 16.25 µA

This current can be set using a current source or simply by

a resistor of the appropriate value connected between

pin ONE and VCC(R).

In this example, the resistor is:

RONE = 1----6---.--2---5-1---.-×-5----1---0----–--6- = 92.3 kΩ

In this example, the reference current at pin ZERO is:

Iref(ZERO) = 14-- × 30 × 10–6 = 7.5 µA

and can be set using a resistor:

RZERO = -7---.--5---1-×--.--5-1----0---–---6 = 200 kΩ

It should be noted that the MPD current is stabilized rather

than the actual laser optical output power. Any deviations

between optical output power and MPD current, known as

‘tracking errors’, cannot be corrected.

Designing the modulation and bias current control

loop

The optical 1 and 0 current control loop time constants are

determined by on-chip capacitances. If the resulting time

constants are found to be too small in a specific

application, they can be increased by connecting a

capacitor between pins TZERO and TONE.

The optical 1 modulation current control loop time

constant (τ) and bandwidth (B) can be estimated using the

following formulae:

τONE = (40 × 10–12 + CTONE) × -8-η--0--L---×A---S--1-E--0-R--3- [s]

(5)

BONE = -2---π-----×---1--τ---O---N----E- [Hz]

(6)

BONE

=

----------------------------------------η----L---A---S---E---R-----------------------------------------

2π × (40 × 10–12 + CTONE) × 80 × 103

[Hz]

The optical 0 bias current control loop time constant and

bandwidth can be estimated using the following formulae:

τZERO = (40 × 10–12 + CTZERO) × 5--η--0--L---×A---S--1-E--0-R--3- [s]

(7)

BZERO = 2----π-----×----1-τ---Z---E---R---O-- [Hz]

(8)

BZERO

=

------------------------------------------η---L---A---S---E---R-------------------------------------------

2π × (40 × 10–12 + CTZERO) × 50 × 103

[Hz]

The term ηLASER (dimensionless) in the above formulae is

the product of the following two terms:

• ηEO is the electro-optical efficiency which accounts for

the steepness of the laser slope characteristic. It defines

the rate at which the optical output power increases with

modulation current, and is measured in W/A.

• R is the MPD responsivity. It determines the amount of

MPD current for a given value of optical output power,

and is measured in A/W.

EXAMPLE

A laser with an MPD has the following specifications:

PO = 1 mW, Ith = 25 mA, ηEO = 30 mW/A, R = 500 mA/W.

The term Ith is the required threshold current to switch on

the laser. If the laser operates just above the threshold

level, it may be assumed that ηEO near the optical 0 level

is 50% of ηEO near the optical 1 level, due to the slope

decreasing near the threshold level.

2002 Aug 13

8

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