A typical digital fiber optic transmission application is illustrated in the block diagram of Figure 1.1. Starting at the upper left, we assume that the information to be transmitted originates in analog form, as, for example, in 4-kHz telephone channels. Several such analog signals can be converted simultaneously to individual sequences of binary digits, each at a fixed data rate (bits/s), using a piece of standard electronic terminal equipment known as a channel bank. Such conversion from analog to digital form is done routinely in transmission systems using twisted pairs, coaxial cables, radio, etc. These individually encoded voice signals are then interleaved to form a single (combined) time-multiplexed digital signal which can be used to modulate an optical source.(1) There are two ways to perform the modulation.(2) One approach is to generate a continuous optical signal and then modulate that signal in amplitude or phase using an optical modulator following the source. Another approach is to modulate the electrical power driving the source in order to modulate the source output power directly. The former approach is generally more expensive and difficult. External modulation typically would be used only when it is impractical to directly modulate the source. One limit on direct modulation is speed. Some sources respond very slowly to the variations in their electrical drive power.
KeywordsDepletion Region Fiber Axis Delay Difference Index Fiber Light Output Power
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- 1.A step index fiber has a core index of refraction of 1.5 and an index step A of 0.02. Calculate the maximum angle which a guided ray may have relative to the axis inside the fiber; outside the fiber. Calculate the maximum delay difference per unit length between the axial and a guided nonaxial ray. For the above use both “exact” and approximate relationships, and compare the results [see equation (1.2.1)].Google Scholar
- 2.Consider a step index fiber with a core index of refraction of 1.5, and an index step A of 0.01. How small must the core be so that only a single mode is guided (assume). = 0.9 pm and the angular separation between modes is approximately 2/D)?Google Scholar
- 3.Calculate the center frequency, f, of a light source emitting at = 0.9-pm wavelength. Calculate kT/hf at T = 300 K. Calculate the bandwidth of an LED which is approximately (kT/hf)fGoogle Scholar
- 4.Solve equation (1.4.2) for W, as a function of V, p,,, and p,,. For V = 25 V and Wp = 1 mm, calculate the required immobile charge density p,, (assume p„» pp). Convert this to a doping level in donor atoms per cm’. (Assume a silicon device.)Google Scholar
- 5.Suppose a digital pulse transmitter emits 1 mW of average light power, and a corresponding digital receiver requires an average optical signal input energy of 10-16 J per received pulse (for satisfactory reception). Suppose a fiber has a loss of 10 dB km-1. Calculate and plot the maximum fiber length between the transmitter and the receiver vs. the pulse rate, for rates between 103 and 109 pulses per second. (Ignore coupling losses into and out of the fiber.)Google Scholar
- 6.In the above example, assume that, in addition, pulses traveling through the fiber spread in time due to delay distortion by 5 ns km-1. If the maximum allowable spreading must not exceed 0.5/pulse rate, calculate and plot the maximum allowable fiber length vs. the pulse rate.Google Scholar