The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 3.1 shows a graphical representation of these two basic schemes [1].

The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). In a fiber-optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light-emitting region to couple as much light as possible into the fiber. In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus “every last drop” of light onto the fiber and, in other cases, a fiber is “pigtailed” directly onto the actual surface of the emitter [1].

LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input, and stable in terms of light output versus ambient operating temperature. In contrast, LDs have very small light-emitting surfaces and can couple many times more power to the fiber than LEDs. LDs are also linear in terms of light output versus electrical current input; but, unlike LEDs, they are not stable over wide operating temperature ranges and require more elaborate circuitry to achieve acceptable stability. Also, their higher cost makes them primarily useful for applications that require the transmission of signals over long distances [1].

LEDs and LDs operate in the infrared portion of the electromagnetic spectrum and so their light output is usually invisible to the human eye. Their operating wavelengths are chosen to be compatible with the lowest transmission loss wavelengths of glass fibers and highest sensitivity ranges of photodiodes. The most common wavelengths in use today are 850, 1310, and 1550 nm. Both LEDs and LDs are available in all three wavelengths [1].

Figure 3.1 Basic optical modification methods.

Figure 3.2 Methods of modulating LEDs or LDs.

LEDs and LDs, as previously stated, are modulated in one of two ways: on and off, or linearly. Figure 3.2 shows simplified circuitry to achieve either method with an LED or LD [1]. As can be seen from Figure 3.2a, a transistor is used to switch the LED or LD on and off in step with an input digital signal [1]. This signal can be converted from almost any digital format, by the appropriate circuitry, into the correct base drive for the transistor.

Overall speed is determined by the circuitry and the inherent speed of the LED or LD. Used in this manner, speeds of several hundred megahertz are readily achieved for LEDs and thousands of megahertz for LDs. Temperature stabilization circuitry for the LD has been omitted from this example for simplicity. LEDs do not normally require any temperature stabilization [1].

Linear modulation of an LED or LD is accomplished by the operational amplifier circuit of Figure 3.2b [1]. The inverting input is used to supply the modulating drive to the LED or LD while the noninverting input supplies a DC bias reference. Once again, temperature stabilization circuitry for the LD has been omitted from this example for simplicity.

Digital on/off modulation of an LED or LD can take a number of forms. The simplest is light-on for a logic “1” and light-off for a logic “”0.” Two other common forms are pulse-width modulation and pulse-rate modulation. In the former, a constant stream of pulses is produced with one width signifying a logic “1” and another width, a logic “0.” In the latter, the pulses are all of the same width but the pulse rate changes to differentiate between logic “1” and logic “0” [1].

Analog modulation can also take a number of forms. The simplest is intensity modulation where the brightness of an LED is varied in direct step with the variations of the transmitted signal [1].

In other methods, a radio frequency (RF) carrier is first frequency-modulated with another signal, or, in some cases, several RF carriers are separately modulated with separate signals, then all are combined and transmitted as one complex waveform. Figure 3.3 shows all the preceding modulation methods as a function of light output [1].

The equivalent operating frequency of light, which is, after all, electromagnetic radiation, is extremely high—on the order of 1,000,000 GHz. The output bandwidth of the light produced by LEDs and laser diodes is quite wide [1].

Unfortunately, today’s technology does not allow this bandwidth to be selectively used in the way that conventional RF transmissions are utilized. Rather, the entire optical bandwidth is turned on and off in the same way that early “spark transmitters” (in the infancy of radio) turned wide portions of the RF spectrum on and off. However, with time, researchers will overcome this obstacle and “coherent transmission” will become the direction of progress of fiber optics [1].

Next, let us look at the story of long-wavelength vertical cavity surface-emitting lasers (VCSELs). VCSELs should remind one of an age-old proverb with a small modification: where there is a will (and money), there is a way. Although the realization of long-wavelength VCSELs was once considered nearly impossible, the progress of the field during the past 6 to 7 years has been tremendous, in part due to the abundance in funding. Although at present it is difficult to forecast the market, industry analysts believe that the technical ground for potential applications of long-wavelength VCSELs is sound. This section provides an overview of recent exciting progress and discusses application requirements for these emerging optoelectronic and wavelength division multiplexing (WDM) transmitter sources [2].

Figure 3.3 Various methods to optically transmit analog information.