The Fiber Optic Link

The simple schematic diagram above consists of an optical transmitter and receiver connected by a length of optical cable in a point-to-point link.

The optical transmitter converts electronic signal voltage into optical power which is launched into a fiber by a light emitting diode (LED), laser diode (LD) or laser.

At the photodetector point, either a positive-intrinsic-negative (PIN) or avalanche photodiode (APD) capture the lightwave pulses for conversion back into electrical current.

It is the systems designer's job to determine the most cost and signal efficient means to convey this optical power, knowing the tradeoffs and limits of various components. He must also design the physical layout of the system.

The first of these concerns, signal quality, involves such factors as signal-to-noise ratio (SNR) in analog systems. When designing a system "from scratch" the designer must determine the required SNR or acceptable BER necessary to transfer the data. The next step is to determine the minimum optical power necessary at the receiver end. This can be obtained from component manufacturer's published data.

Link design consists basically of two functions: (1) calculating optical power losses occurring between the light source and the photodetector. (2) determining bandwidth limitations on data carrying abilities imposed by the transmitter, fiber and receiver.

Reductions in optical power loss, or attenuation, as the light pulse travels through the fiber are expressed in dB/km (decibels per kilometer).

The decibel is a logarithmic expression of the ratio for the power entering a component and the power leaving it.

A 3 dB loss means that half the power is lost. For example, starting with 500µW, you would now have 250µW. A 10 dB loss means that 1/10 of the power arrives at the receiver, a 90% loss.

If the source emits sufficient power and the receiver is sensitive enough, the system can operate with high losses. How much loss can be tolerated will be determined by the stated minimum requirements of the receiver selected.

The prime causes of optical attenuation in fiber systems are:

The sum of the losses of each individual component between transmitter and receiver comprise the Optical Link Power Budget shown below.

* The optical power ratio is related to the signal voltage ratio by a factor of two because -

dB = 10 log P1 / P2 = 10 log 11 2 R / 12 2 R.

Since V = IR then dB = 20 log V1 / V2.

The designer must consider these losses and select a transmitter and receiver combination that will deliver enough power to faithfully reproduce the signal.

However, these losses are not exact, and manufacturers typically state ranges, or "best" and "worst" case situations in order to account for product variations. Also some allowance may be required for such things as temperature variations.

Some safety margins should also be made to future repairs or splices to the system, and age degradation of the source emitter. For example, a 3 to 6 dB margin for repairs and aging of the emitter is commonly employed.

The amount of optical power coupled into the fiber is dependent on the physical nature of the fiber used, and the source emitter.

Obviously, the larger the core diameter of the fiber, the more potential for accepting light. However, larger core fibers suffer bandwidth limitations that may outweigh coupling efficiency.

A change in core diameter from 50mm to 100µm (microns) represents an increase of four times in the amount of light coupled to the fiber.

Besides core size, the other measure of the fibers ability to collect optical power is called numerical aperture (NA). This is a mathematical measure of the fiber core's ability to accept lightwaves from various angles and transmit them down to the core.

A large difference between the refractive indices of the core and cladding means a larger NA. For equal core sizes, a fiber with a larger NA will accept more lightwaves. A power increase by about a factor of two is achieved by going from an NA of 0.20 to one of 0.29.

We've combined the effects of core size and NA into an Optical Collection Factor which can be considered a measure of the fiber's efficiency for optical radiation.