Fiber-Optic Communications System
As depicted in Figure 3-16, information (voice, data, and video) from the source is encoded into electrical signals that can drive the transmitter. The fiber acts as an optical waveguide for the photons as they travel down the optical path toward the receiver. At the detector, the signals undergo an optical-to-electrical (OE) conversion, are decoded, and are sent to their destination.
Figure 3-16 Fiber-Optic Communication System
The transmitter component of Figure 3-16 serves two functions. First, it must be a source of the light launched into the fiber-optic cable. Second, it must modulate this light to represent the binary data that it receives from the source. A transmitter's physical dimensions must be compatible with the size of the fiber-optic cable being used. This means that the transmitter must emit light in a cone with a cross-sectional diameter of 8 to 100 microns; otherwise, it cannot be coupled into the fiber-optic cable. The optical source must be able to generate enough optical power so that the desired BER can be met over the optical path. There should be high efficiency in coupling the light generated by the optical source into the fiber-optic cable, and the optical source should have sufficient linearity to prevent the generation of harmonics and intermodulation distortion. If such interference is generated, it is extremely difficult to remove. This would cancel the interference resistance benefits of the fiber-optic cable. The optical source must be easily modulated with an electrical signal and must be capable of high-speed modulation; otherwise, the bandwidth benefits of the fiber-optic cable are lost. Finally, there are the usual requirements of small size, low weight, low cost, and high reliability. The transmitter is typically pulsed at the incoming frequency and performs a transducer electrical-to-optical (EO) conversion. Light-emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) are used to drive MMF systems, whereas laser diodes are used to drive SMF systems. Two types of light-emitting junction diodes can be used as the optical source of the transmitter. These are the LED and the laser diode (LD). LEDs are simpler and generate incoherent, lower-power light. LEDs are used to drive MMF. LDs generate coherent, higher-power light and are used to drive SMF.
Figure 3-17 shows the optical power output, P, from each of these devices as a function of the electrical current input, I, from the modulation circuitry. As the figure indicates, the LED has a relatively linear P-I characteristic, whereas the LD has a strong nonlinearity or threshold effect. The LD can also be prone to kinks when the power actually decreases with increasing input current. LDs have advantages over LEDs in the sense that they can be modulated at very high speeds, produce greater optical power, and produce an output beam with much less spatial width than an LED. This gives LDs higher coupling efficiency to the fiber-optic cable. LED advantages include a higher reliability, better linearity, and lower cost.
Figure 3-17 LED and LD P-I Characteristics
A key difference between the optical output of an LED and a LD is the wavelength spread over which the optical power is distributed. The spectral width, σ, is the 3-dB optical power width (measured in nanometers or microns). The spectral width impacts the effective transmitted signal bandwidth. A larger spectral width takes up a larger portion of the fiber-optic cable link bandwidth. Figure 3-18 shows the spectral width of the two devices. The optical power generated by each device is the area under the curve. The spectral width is the half-power spread. An LD always has a smaller spectral width than an LED. The specific value of the spectral width depends on the details of the diode structure and the semiconductor material. However, typical values for an LED are around 40 nm for operation at 850 nm and 80 nm at 1310 nm. Typical values for an LD are 1 nm for operation at 850 nm and 3 nm at 1310 nm.
Figure 3-18 LED and LD Spectral Widths
Other transmitter parameters include packaging, environmental sensitivity of device characteristics, heat sinking, and reliability. With either an LED or LD, the transmitter package must have a transparent window to transmit light into the fiber-optic cable. It can be packaged with either a fiber-optic cable pigtail or with a transparent plastic or glass window. Some vendors supply the transmitter with a package having a small hemispherical lens to help focus the light into the fiber-optic cable. Packaging must also address the thermal coupling for the LED or LD. A complete transmitter module can consume more than 1 watt, which could result in significant heat generation. Plastic packages can be used for lower-speed and lower-reliability applications. However, high-speed and high-reliability transmitters need metal packaging with built-in fins for heat sinking.
There are several different schemes for carrying out the modulation function. These include intensity modulation (IM), frequency shift keying (FSK), phase shift keying (PSK), and polarization modulation (PM). Within the context of a premise fiber-optic data link, the only one really used is IM. IM is used universally for premise fiber-optic data links because it is well matched to the operation of both LEDs and LDs. The carrier that each of these sources produces is easy to modulate with this technique. Passing current through them operates both of these devices. The amount of power that they radiate (sometimes referred to as the radiance) is proportional to this current. In this way, the optical power takes the shape of the input current. If the input current is the waveform m(t) representing the binary information stream, the resulting optical signal looks like bursts of optical signal when m(t) represents a 1 and the absence of optical signal when m(t) represents a 0. This is also known as direct modulation of the LED or LD.
Figure 3-19 shows a schematic of an optical receiver. The receiver serves two functions: It must sense or detect the light coupled out of the fiber-optic cable and convert the light into an electrical signal, and it must demodulate this light to determine the identity of the binary data that it represents. The receiver performs the OE transducer function.
Figure 3-19 Schematic of an Optical Receiver
A receiver is generally designed with a transmitter. Both are modules within the same package. The light detection is carried out by a photodiode, which senses light and converts it into an electrical current. However, the optical signal from the fiber-optic cable and the resulting electrical current will have a small amplitude. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There might even be filters and equalizers to shape and improve the information-bearing electrical signal.
The receiver schematic in Figure 3-19 shows a photodiode, bias resistor circuit, and a low-noise pre-amp. The output of the pre-amp is an electrical waveform version of the original information from the source. To the right of this pre-amp is an additional amplification, filters, and equalizers. All of these components can be on a single integrated circuit, a hybrid, or discretely mounted on a printed circuit board.
The receiver can incorporate a number of other functions, such as clock recovery for synchronous signaling, decoding circuitry, and error detection and recovery. The receiver must have high sensitivity so that it can detect low-level optical signals coming out of the fiber-optic cable. The higher the sensitivity, the more attenuated signals it can detect. It must have high bandwidth or a fast rise time so that it can respond fast enough and demodulate high-speed digital data. It must have low noise so that it does not significantly impact the BER of the link and counter the interference resistance of the fiber-optic cable transmission medium.
There are two types of photodiode structures: positive intrinsic negative (PIN) and the avalanche photodiode (APD). In most premise applications, the PIN is the preferred element in the receiver. This is mainly due to fact that it can be operated from a standard power supply, typically between 5 and 15V. APD devices have much better sensitivity. In fact, APD devices have 5 to 10 dB more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply, which increases their cost. APD devices are usually found in long-haul communication links and can increasingly be found in metro-regional networks (because APDs have decreased in cost).
The demodulation performance of the receiver is characterized by the BER that it delivers to the user. The sensitivity curve indicates the minimum optical power that the receiver can detect compared to the data rate, to achieve a particular BER. The sensitivity curve varies from receiver to receiver. The sensitivity curve considers within it the SNR parameter that generally drives all communications-link performance. The sensitivity depends on the type of photodiode used and the wavelength of operation. Figure 3-20 shows sensitivity curve examples.
Figure 3-20 Receiver Sensitivity Curves
The quantum limit curve serves as a baseline reference. In a sense it represents optimum performance on the part of the photodiode in the receiverthat is, performance in which there is 100 percent efficiency in converting light from the fiber-optic cable into an electric current for demodulation. All other sensitivity curves are compared to the quantum limit.