In this presentation I have explained various types of transmission system used for optical transmission and also described about the budget method that has to be followed while selecting an source for optical fibers and also about the factors that should be consider while selecting an source.

1. Laxmi Institute of Technology, Sarigam
Department of Electronics & Communication Engineering
SUB: Optical Communication
Presentation
On
Transmission Systems
Submitted by:-
Name: EnrollnmentNo.
 JayBaria 150860111003
Approved by AICTE, New Delhi; Affiliated to Gujarat Technological University,
Ahmedabad

2. POINT-TO-POINT LINKS :-
 The simplest transmission link is a point-to point line having a
transmitter at one end and a receiver on the other, as shown
below:
 The design of an optical link involves many interrelated
variables such as the fiber, source, and photodetector operating
characteristics, so that the link design and analysis may require
several iterations before they are working satisfactorily.

3. POINT-TO-POINT LINKS :-
 The key system requirements needed in analyzing a link are:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The bit error rate (BER)
 The major requirement to transfer the data from source to detector
is to select the following components:
1. Light Source
2. Fiber Optic Cable
3. Detector

4. 1) Light Source :-
1. Selection of LED or laser diode optical source depends on the
following factors:-
(a) Emission wavelength
(b) Output power
(c) Number of emitting modes
2. Generally the LED or LD is used as light source.
3.The cost of LED is less as compared to laser diode.
4. LDs have advantages over LED's :
 Can be modulated at very high speeds.
 They produce greater optical power.
 They have higher coupling efficiency to the fiber

5. 2) Fiber Optic Cable :-
1. Selection of the Multimode or single-mode optical fiber depends on the
following factors:-
(a) Core size
(b) Core refractive-index profile
(c) Bandwidth or dispersion
(d) Attenuation
(e) Numerical aperture or mode-field diameter
2. Fiber Optic Cable can be one of two types Multi-mode or Single-mode.
• These provide different performance with respect to both attenuation and time
dispersion.
3. Glass fiber optic cable has the lowest attenuation and comes at the highest cost,
4. Plastic fiber optic cable has the highest attenuation, but comes at the lowest cost.

6. 3) Detector :-
1. Selection of the detector or Pin or avalanche photodiode depends on the
following factors:-
(a) Responsivity
(b) Operating wavelength
(c) Speed
(d) Sensitivity
2. There are two types of photodiode structures;
 Positive Intrinsic Negative (PIN) and
 Avalanche Photo Diode (APD).
3. In most premises 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 15 V.
4. APD devices have much better sensitivity. In fact it has 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. This makes cost higher.

7. System Consideration:-
While Designing the communication system, using optical cable, the
following points must be considered:-
 Determine the correct optical transmitter and receiver combination based upon the
signal to be transmitted (Analog, Digital, Audio, Video, RS-232, etc.)
 Determine the operating power available (AC, DC etc.)
 Determine the special modifications (if any) necessary (Impedances, Bandwidths,
Special Connectors , Special Fiber Size, etc.)
 Calculate the total optical loss (in dB) in the system by adding the cable loss, splice
loss, and connector loss. These parameters should be available from the
manufacturer of the electronics and fiber.

8. System Consideration:-
1 ) Wavelength of operation:
 For Short Distance : 800 to 900 nm
 For longer Distance : 1300 to 1550 nm
2) Selection of particular Fiber Type :
 Numerical Aperture
 Amount of attenuation
 Losses in connectors & Splices
 Environmental effects like temp. variations, radiation effects etc.

9. System Consideration:-
3) Photo Detector :
 Compared to APD, pins are less expensive and more stable with
temperature. However pins have lower sensitivity.
4) Optical Source:
 LEDs: 150 (Mb/s)·km @ 800-900 nm and larger than 1.5 (Gb/s)·km
@1330 nm
 In GaAsP lasers: 25 (Gb/s).km @ 1330 nm and ideally around
500(Gb/s).km @ 1550 nm. 10-15 dB more power. However more costly
and more complex circuitry.

10. Budget methods for design of optical link :-
(I) Link power budget analysis :
 In the link power budget analysis one first determines the power margin
between the optical transmitter output and the minimum receiver sensitivity
needed to establish a specified BER.
(II) Rise-time budget analysis :
 Once the link power budget has been established, the designer can perform a
system rise-time analysis to ensure that the desired overall system
performance has been met.

11. 1) Link power budget analysis :
 The optical power received by the receiver depends on the power
transmitted and on the various losses occurring over the fiber.
 The power received at the output must be sufficiently higher than the
receiver sensitivity (after accounting for safety margin).

12. 1) Link power budget analysis :
 The optical power received by the receiver depends on the power
transmitted and on the various losses occurring over the fiber. The power
received at the output must be sufficiently higher than the receiver
sensitivity (after accounting for safety margin).
Ptx = Prx + CL + Ms
where Ptx is the transmitter power, Prx is the sensitivity of the receiver, CL is
the total link loss or channel loss (including fiber splice and connector
loss), and Ms is the system’s safety margin.
 Channel Loss may be expressed as :
CL = f L + con + splice
where f is the fiber loss (dB/km), L is the link length, con is the sum of the
losses at all the connectors in the link, and splice is the sum of losses at all
splices in the link.

13. Example :- Design as optical fiber link for transmitting 15
Mb/sec of data for a distance of 4 km with BER of 10-9.
Soln:
 Bandwidth x Length = 15 Mb/sec x 4 km = (60 Mb/sec) km
 Selecting optical source: LED at 820 nm is suitable for short distances.
 The LED generates – 10 dBm optical power.
 Selecting optical detector: PIN-FER optical detector is reliable and has –
50 dBm sensitivity.
 Selection optical fiber: Step-index multimode fiber is selected.
 The fiber has bandwidth length product of 100 (Mb/s) km.

14. Links power budget:
Assuming:
Splicing loss ls = 0.5 dB/slice
Connector loss lc = 1.5 dB
System link powr margin Pm – 8 dB
Fiber attenuation αf = 6 dB/km
Actual total loss = (2 x lc) + αfL + Pm
PT = (2 x 1.5) + (6 x 4) + 8
PT = 35 dB
Maximum allowable system loss:
Pmax = Optical source output power- optical receiver sensitivity
Pmax = -10 dBm – (-50 dBm)
Pmax = 40 dBm
Since actual losses in the system are less than the allowable loss, hence the
system is functional.

15. 2 ) Rise Time Budget :
 Rise time gives important information for initial system design. Rise-
time budget analysis determines the dispersion limitation of an
optical fiber link.
 Total rise time of a fiber link is the root-sum-square of rise time of
each contributor to the pulse rise time degradation.
 The link components must be switched fast enough and the fiber
dispersion must be low enough to meet the bandwidth requirements
of the application adequate bandwidth for a system can be assured by
developing a rise time budget.
 As the light sources and detectors has a finite response time to inputs.
The device does not turn-on or turn-off instantaneously. Rise time
and fall time determines the overall response time and hence the
resulting bandwidth.

16. 2 ) Rise Time Budget :
 A rise-time budget analysis determines the dispersion limitation of an
optical fiber link.
 The total rise time tsys is the root sum square of the rise times from each
contributor ti to the pulse rise-time degradation:
 The transmitter rise time ttx
 The group-velocity dispersion (GVD) rise time tGVD of the fiber
 The modal dispersion rise time tmod of the fiber
 The receiver rise time trx

17. Example: For a multimode fiber following parameters are recorded.
i) LED with drive circuit has rise time of 15 ns.
ii) LED spectral width = 40 nm
iii) Material dispersion related rise time degradation =21 ns over 6 km link.
iv) Receiver bandwidth = 235 MHz
v) Modal dispersion rise time = 3.9 nsec
Calculate system rise time.
SOLn: ttx = 15 nsec
tTmat = 21 nsec
tmod = 3.9 nsec
NOW,
Since

18. Bit Error Rate(BER):
 A bit error rate is defined as the rate at which errors occur in a
transmission system.
 The bit error rate (BER) is the number of bit errors per unit time.
The bit error ratio (also BER) is the number of bit errors divided by
the total number of transferred bits during a studied time interval.
 BER is a unitless performance measure, often expressed as a
percentage.
 The bit error probability pe is the expectation value of the bit error
ratio. The bit error ratio can be considered as an approximate
estimate of the bit error probability. This estimate is accurate for a
long time interval and a high number of bit errors.

19. Bit Error Rate(BER):
 BER estimation is one of the valuable ways of viewing parametric
performance of digital communication systems at high speeds.

20. Thank you…!!