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Applications:

Optical Receiver Stress Test

The fundamental test for these network elements is the bit error ratio,

demonstrating reliable operation in digital data transmission systems and

networks. The basic principle is simple: the known transmitted bits are

compared with the received bits over a transmission link including the

device under test. The bit errors are counted and compared with the total

number of bits to give the bit error ratio (BER). The applied test data signal

can be degraded with defined stress parameters, like transmission line

loss, horizontal and vertical distortion to emulate worst-case operation

scenarios at which the device under test has to successfully demonstrate

error free data transmission. Obviously, this test is of fundamental impor-

tance for receiving network elements, due to the manifold impairments

occurring on optical transmission lines. Therefore, many all optical trans-

mission standards define such stressed receiver sensitivity on the basis of

a BER measurement. The basic test methods and setups are usually very

similar. However, the test conditions, the stress parameters or methods

of stress generation vary from standard to standard, depending on the

application area, transmission medium, data rate or data protocol.

The basic setup is sketched in the block diagram and consists

of the following elemental building blocks:

• The frequency synthesizer: creates sinusoidally jittered clock, Periodic

Jitter (PJ)

• The clock output from the clock source will be modulated with the

sinusoidal jitter

• The electrical pattern generator creates the defined test pattern at the

required rate

• The electrical stress conditioning setup adds various kinds of signal

distortion onto the test pattern

• The E/O conditioning setup modifies the electrical stress signal

depending on the standard:

• The electrical-to-optical- converter converts the electrical

stressed test signal into the corresponding optical stressed signal

(10 GbE, 10 GFC)

• The tunable E/O source, optical multiplexer and modulated

test sources are used to emulate other lanes for higher speed

standards (40 GbE, 100 GbE)

• The optical attenuator emulates the transmission line loss and

sets the optical modulation amplitude to the required level

• The optical stressed signal is fed to the optical receiver under test

• The receiver’s data output signal is lead to the error detector, which

compares the input and output data test patterns, detects errors and

calculates the bit error ratio

What is optical stress?

Figure 3 illustrates an optical stressed signal which has to be applied to an

optical receiver. While such a signal is applied to the optical input, the bit

error ratio at the receiver’s output has to be below a certain level (typically

1e-12) to be compliant.

Figure 3. Definition of the optical parameters

OMA:

Optical Modulation Amplitude, measured in [µW] (“average signal

amplitude“)

ER:

Extinction Ratio, high-level to low-level, measured in [dB] or [%]

UI:

Unit Interval (one bit period)

LR, SR, ER:

Flavors of 10 Gb Ethernet standard for Long Reach (10 km), Short Reach

(300 m), Extended Reach (40 km)

A0:

Vertical eye opening (“innermost eye opening at center of eye“) [dBm or

µW]

VECP:

Vertical Eye Closure Penalty

Jitter histogram

(at waveform average,

may not be at waist)

Vertical

eye-closure histograms

(at time-center of eye)

Approximate OMA

(difference of

means of histograms)

Jitter

OMA

VECP = 10 log (OMA/AO)

1 2 3 4 5 6 7 8 9 10 ... 39 40

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