FIBER-OPTIC COMMUNICATIONS

Assignment #1 forFiber-Optic Communicationsdue at April 27, 2005(1). A 15 km optical fiber link uses fiber with a loss of 1.5 dB/km. The fiber is jointed every kilometer with connectors which given an attenuation of 0.8 dB each. Determine the minimum mean optical power which must be launched into the fiber in order to maintain a mean optical power of 0.3 W at the detector.

(2). A graded index fiber with a core axis refractive index of 1.5 has a characteristic index profile () of 1.90, a relative refractive index difference of 1.3% and a core diameter of 40 m.

(a). Estimate the number of guide modes propagating in the fiber when the transmitted light has a wavelength of 1.55 m, and

(b). Determine the cutoff value of the normalized frequency for a single mode transmission in the fiber.

(3). Consider a fiber with a 100 m core diameter and a 140 m cladding diameter. If n1 = 1.48 and = 1%,

(a). Calculate the V-parameter if the operating wavelength is 850 nm.

(b). Find the value of V at a wavelength of 1300 nm if the diameter of core is 50 m;

(c). Calculate the number of modes if the operating wavelength is 850 nm and the core diameter is 50 m;

(d). Calculate the percentage of the optical power that is carried in the cladding for the above cases of (b) and (c).

(4). A double-heterojunction InGaAsP LED emitting at a peak wavelength of 1310 nm has radiative and nonradiative recombination times of 25 and 90ns, respectively. The drive current is 35 mA.

(a). Find the internal quantum efficiency and the internal power level.

(b). If the refractive index of the light source material is n = 3.5, find the power emitted from the device.

(5). Consider a silicon avalanche photodiode with parameters as given below, operating in a link with no intersymbol interference present.

Parameter

|Value

F(M)

|M0.35 Responsivity (at M = 1)|0.25 A/W

Surface dark current

|1 A

Temperature

|300 oK

Load resistor RL

|1.2 K

Bulk dark current

|1.0 nA

Bandwidth of receiver

|10 MHz

(a). Calculate the dc optical power that must be incident on the detector to make the optimum gain of this APD have a value of 80.

(b). For a gain value of 80, calculate the ratio (in dB) of the mean-square noise current due to the shot noise caused by the bulk dark current to the mean-square noise current due to the thermal noise.

(6). In optical PIN receiver dominated by thermal noise, the decision circuit compares the sampled current I with a threshold ID and calls it bit 1 (bit 0) if I > ID (I < ID). An error occurs if I < ID (I > ID) for bit 1 (bit 0) because of receiver noise. Let I1 (I0) and 12 (02) denote the average detected current and the corresponding variance for bit 1 (bit 0). If bit 1 and bit 0 are equally likely and the receiver noise is Gaussian distributed,

(a). With optimum threshold setting of ID = (I1+I0)/2 = 1Q, derive out the bit-error-rate (BER) as a function of Q-parameter;

(b). With detected current I1=2RPrec and I0=0, R being detector responsivity, find out the receiver sensitivity Prec as a function of Q-parameter.