Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: DEC 2015

i. Power budgeting for a digital optical fiber communication system is performed in a similar way to power budgeting within any communication system.

ii. When the transmitter characteristics, fiber cable losses and receiver sensitivity are known, the relatively simple process of power budgeting allows the repeater spacing or the maximum transmission distance for the system to be evaluated.

iii. However, it is necessary to incorporate a system margin into the optical power budget so that small variations in the system operating parameters do not lead to an unacceptable decrease in system performance.

iv. The operating margin is often included in a safety margin M_(a ) which also takes into account possible source and modal noise, together with receiver impairments such as equalization error, noise degradations and eye-opening impairments.

v. The optical power budget for a system is given by the following expression: $P_i= P_0 + C_L + M_a dB$

Where $P_i$ is the mean input optional power launched into the fiber, $P_0$ is the mean incident optical power required at the receiver and $C_L$ (or $C_{LD}$ when there is a dispersion–equalization penalty) is the total channel loss.

vi. Therefore the expression may be written as:

$P_i = P_0 + (α_{fc} + α_j) L + α_{cr} + M_a dB ---- Eq. (1)$

vii. Alternatively, when a dispersion–equalization penalty is included Eq. becomes:

$P_i = P_0 + (α_{fc} + α_j) L + α_{cr} + D_L +M_a dB ---- Eq. (2)$

Equations (1) and (2) allow the maximum link length without repeaters to be determined

The basic system design verification can be done through:

1. Power budget

2. Rise time budget

Power budget:

i. Each component in the optical link has a specific loss in dB. If $P_i$ and $P_o$ are the power in and out to the component respectively, the loss $L_i$ of the component is given by

$L_i= 10 \log (P_o/P_i)$

ii. Apart from the component losses, a certain amount of power margin $P_{sm}$ called as system margin, is required for unexpected losses.

iii. Thus, the power budget equation can be written as

$P = P_s- P_R = L_s+L_d+NL_j+ P_0 + αL + P_{sm}$

P = Power margin, $L_s$ = Source Coupling Loss, α=Fiber attenuation, $P_s$ = Source Power, $L_d$= Detector Coupling Loss, $P_{sm}$ = system margin, $P_R$ = Received Power, $L_j$ = Joint Loss, L = Total Fiber Length, N = No. of Joints

Rise time budget:

i. Rise time is defined as the time it takes for the response to rise from the 10% to 90% of maximum amplitude.

ii. Fall time is the time the response needs to fall from 90% to 10% of the maximum.

iii. Pulse width is the time between the 50% marks on the rising and falling edges.

iv. The power budget involves the power level calculations from the transmitter to the receiver.

v. Determines the bandwidth carrying capability. Bandwidth is limited by the component with the slowest rise time.

vi. Rise time of transmitter is based on the response time of the LED or laser diode.

vii. Rise time of the receiver is primarily based on the semiconductor device used as the detector.

1. Attenuation 2.Coupled power 3.Other losses 4.Equalization penalty $(D_L)$ 5.SNR requirements 6.Minimum power at detector 7.BER 8.Safety margin

$(M_a)$

Rise time budget includes the following:

1. Rise time of the source, $T_S$

2. Rise time of the fiber (dispersion), $T_F$

3. Rise time of the amplifier, $T_A$

4. Rise time of the detector, $T_D$

The rise time budget is assembled as:

$T_{syst} = 1.1 (T_S^2+ T_F^2 + T_D^2+ T_A^2)^{1/2}$

For non-return-to-zero (NRZ) data

$T_{syst} = 0.7/B_T $

For return-to zero (RZ) data

$T_{syst} = \dfrac {0.35}{B_T }$