What are fiber optic systems power penalties? (video)

There are six physical phenomena that degrade the signal-to-noise ratio at the far-end receiver on high-bit-rate fiber optic systems. These sources of degradation are:

• Modal noise
• Dispersion broadening
• Mode-partition noise
• Frequency chirp
• Reflection feedback and noise
• Extinction ratio

1. Modal Noise

Modal noise is an impairment typical on multimode fiber systems. The reason is the interference among the various propagation modes in multimode fiber. The effect shows as the degradation of signal-to-noise ratio at the receiver because of the fluctuating amplitude of the received signal.

This is why most multimode systems use LEDs rather than laser diodes, since LEDs have very wide spectral bandwidth that can minimize the modal noise problem.

2. Dispersion Broadening

Dispersion broadening can limit the bit-rate—distance (BL) product. This pulse broadening affects receiver performance in two ways.

The first effect is that the broadening can spill pulse energy into the subsequent bit slot which will cause intersymbol interference (ISI).

The second effect is that the pulse peak energy is reduced because of the broadening. So we have to increase incoming signal level at the receiver to compensate for it. To quantify the power penalty, we define δd as:

δd = 10 log fd

where fd is the pulse broadening factor.

The power penalty δd can be calculated by using the formula:

δd = -5 log 10[1-(4BLDσλ)2]

where B is the bit rate, L is the length of the link in kilometer, D is the dispersion factor in ps/nm-km, and σλ is the rms width of the source spectrum.

Here are a few values for δd as follows:

3. Mode-Partition Noise

Mode-partition noise MPN occurs in multimode fiber systems and is brought about by the semiconductor laser source. It is a phenomenon that occurs due to anticorrelation among pairs of longitudinal modes.

There may be many such mode present, and individual modes shows notable power fluctuation even though the total power remains relatively constant.

These various modes, as they travel down the fiber, become desynchronized because they are traveling at different velocities. This causes fluctuations in the receiver signal current degrading the signal-to-noise ratio and a power penalty must be paid for this.

A calculation of this MPN power penalty is complex, involving the mode-partition coefficient k whose value ranges from 0 to 1.

δmpn is the power penalty in dB for MPN. The power penalty can be reduced to a negligible level (<0.5 dB) by designing the optical system such that BLDσλ < 0.1.

4. Frequency Chirp

With directly modulated transmitters there is an optimal setting of bias current which will achieve minimum chirp. This is a adjustment to obtain a certain rex value.

rex is the extinction ratio or ratio of the “on” power to the “off” power. We have rex = P0 / P1 or the laser output power in the binary 1 condition to its output power in the binary 0 condition.

The total power penalty caused by frequency chirp can be reduced to below 2 dB by operating the system with an extinction ratio of about 0.1.

5. Reflection Feedback and Noise

Reflection feedback is the light reflected back into the laser source. This reflected light, even at low levels, can be a source of system upset or cause major degradation in performance.

In fact it can degrade system performance to the point that the system cannot achieve the required Bit Error Rate (BER) despite an infinite increase in receive power.

Most reflection in a fiber link occurs at the fiber-air interfaces. This reflected feedback problem can be easily removed by using an optical isolator in the transmitter assembly fiberguide well prior to using the pigtail connector.

6. Extinction Ratio

An insufficient extinction ratio can incur a power penalty. A fiber optic light source has an on state and an off state. We assign a binary 1 to the on state and a binary 0 to the off state.

The trouble lies in the fact that in the off state the light transmitter is not completely off.

The reason for doing this is to greatly reduce the initial rise time of the light transmitter, allowing the transmitter to have a much greater bit rate than it would have if it was completely off in the binary 0 state.

If we allow P0 to be the off-state output power and allow P1 to be the on-state output power of the light transmitter, then the extinction ratio is defined as:

rex = P0 / P1

Stated another way using decibels, the extinction ratio is defined as:

EX = 10 log (A/B)

A = average optical power at logical 1

B = average optical power at logical 0

Based on the values of rex , the following are equivalent extinction ratio penalties based on using a PIN photodetector.