Optical communication system General Optical Fiber System An

Optical communication system

General Optical Fiber System An optical fiber communication system is similar in basic concept to any type of communication system i. e. to convey the information over the transmission medium to the destination. Light is used as the carrier.

BASIC MODEL FIBER OPTIC COMMUNICATION SYSTEM

BASIC MODEL : The bandwidth of the fiber optic communication system, which determines the maximum data rate, depends on the major components of the system. Fig. shows the block diagram of fiber optic communication system. The information signal to be transmitted may be voice, video or computer data.

Countd. The first step is to convert the information into a form compatible with the communications medium. This is usually done by converting continuous analog signals such as voice and video (TV) signals into a series of digital pulses. An Analog – to – Digital (A/D) converter is used for this purpose. Computer data is already in the digital form.

Countd. These digital pulses are then used to flash a powerful light source (i. e. ) off and on very rapidly. In a simple low – cost system that transmits over short distances, the light source is usually a light emitting diode (LED). This is a semiconductor device that puts out a low – intensity red light beam. Other colours are also used.

Countd. Infrared beams like those used in TV remote controls are also used in transmission. Another commonly used light source is the solid state laser. This is also a semiconductor device that generates an extremely intense single frequency light beam.

The light beam pulses are then fed into a fiber – optic cable where they are transmitted over long distances. At the receiving end, a light sensitive device known as a photocell or light detector is used to detect the light pulses. This photocell or photo detector converts the light pulses into an electrical signal. The electrical pulses are amplified and reshaped back into digital form. 8

Countd. Both the light sources at the sending end and the light detectors on the receiving end must be capable of operating at the same data rate. The circuitry that drives the light source and the circuitry that amplifies and processes the detected light must both have suitable high -frequency response. The fiber itself must not distort the high-speed light pulses used in the data transmission. They are fed to a decoder, such as a Digital – to – Analog converter (D/A), where the original voice or video is recovered. 9

Countd. In very long transmission systems, repeater units must be used along the way. Since the light is greatly attenuated when it travels over long distances, at some point it may be too weak to be received reliably. To overcome this problem, special relay stations are used to pick up light beam, convert it back into electrical pulses that are amplified and then retransmit the pulses on another beam. Several stages of repeaters may be needed over very long distances. But despite the attenuation problem, the loss is less than the loss that occurs with the electric cables. 10

Optical communication system This is an example of an optical comm. system that uses external modulation (because it uses an external modulator at the transmitter) and multiplexing of the signal in the electrical domain.

optical modulator An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide (optical fibre). Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase modulators, polarization modulators etc. Often the easiest way to obtain modulation of intensity of a light beam, is to modulate the current driving the light source, e. g. a laser diode. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason light modulators are, e. g. in fiber optic communications, called external light modulators. With laser diodes where narrow linewidth is required, direct modulation is avoided due to a high bandwidth "chirping" effect when applying and removing the current to the laser

Optical fiber comm. system (In general) • The optical source is generally a semiconductor laser or light-emitting diode (LED) that provides the electrical–optical conversion. • The electrical signal can be converted into optical signal either by means of optical modulation which can be in the form of: • Direct modulation - the laser drive circuit directly modulates the light intensity (power/area) of the laser / LED with the signal, or • External modulation –In this case, the laser is biased at a constant current to provide the continuous wave (CW), and an optical modulator which is placed next to the laser converts the CW light into a data-coded pulse train with the right modulation format. A CW wave has a continuous output (as opposed to a pulsed output beam). • Laser emits constant optical power. This then passes through an optical modulator (external modulator) – this is a voltage driven device. As we adjust the voltage, the amount of optical power absorbed will vary. In this way, we achieve modulation of the optical power coming out of the modulator

Advantages: This optical modulation is simple. It is cheaper as no complex circuitry is involved during modulation process. Disadvantages: This method is slower compare to indirect or external modulation type. It can be used below 3 GHz.

Advantages: It is much faster in processing. It can be used with high power laser devices. It can be employed in high speed applications e. g. long haul telecom or cable TV head ends. Disadvantages: It is more expensive. .

General description cont … • The optical carrier is modulated using either analog or digital electrical signal. • Analog modulation - involves the variation of the light emitted from the optical source in a continuous manner. • Digital modulation - involves discrete changes in the light intensity (i. e. on–off pulses). • Although analog modulation is often simpler to implement, this is less efficient in an optical fiber communication system because it requires a far higher signal- to-noise ratio at the receiver than digital modulation. • Also, the linearity needed for analog modulation is not always provided by semiconductor optical sources, especially at high modulation frequencies. For these reasons, analog optical fiber communication links are generally limited to shorter distances and lower bandwidth operation than digital links.

General description cont … • The optical signal is launched into the optical fiber cable. The optical fiber is the transmission medium in an optical fiber comm. system. • During transmission, the optical signal can be attenuated and this can cause bit-error at the receiver. To ensure reliable operation, optical amplifiers are used to amplify the optical signal. Examples of optical amplifiers are erbium doped fiber amplifier, EDFA. • At the receiver, the optical signal is detected by the photodetector at the other end of the fiber and is converted into photocurrent (photocurrent means that the electrical signal is caused by photons/light and photocurrent is an electrical signal). The photocurrent is converted into voltage by an amplifier that is connected to the output of the photodetector. • An equalizer at the receiver compensates for distortion of the signal due to the combined transmitter, medium and receiver characteristics. The equalizer is often a frequency-shaping filter which has a frequency response that is the inverse of the overall system frequency response. • Finally, the signal is decoded to give the original information.

EMT 462/3 Optoelectronic System Optical Fiber System: Wavelength division multiplexing (WDM)

Multiplexing • Multiplexing is a technique to combine multiple signals from different channels into a single channel. • The purpose of multiplexing in optical communication is to maximize (shared medium) the information transfer over the transmission line. • In optical communication, channel multiplexing techniques used in the electrical domain (prior to the intensity modulation of the optical source or electrical multiplexing) are such as a) Time domain multiplexing (TDM) b) Frequency division multiplexing (FDM) c) Wavelength division Multiplexing (WDM)

The Concept of Multiplexing 22

Multiplexing in networks Sharing the medium Main purpose is ?

Multiplexer example

Wavelength Division Multiplexing

Wavelength Division Multiplexing

Wavelength Division Multiplexing Each color can be used as a channel Today's WDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation.

Wavelength Division Multiplexing

a&b Wavelength Division Multiplexing of an optoelectronic system c d e g f

Wavelength Division Multiplexing

Wavelength Division Multiplexing

ITU wavelength grid – CWDM and DWDM • A grid wavelength contains all the central wavelengths (and corresponding frequencies) of the channels recommended in the WDM system. • An example of the grid wavelength with a channel spacing of 20 nm for a Coarse Wavelength Division Multiplexing (CWDM) system is shown in the figure below. 2 categories of WDM: a) Coarse wavelength division multiplexing (CWDM) Channel spacing : 20 nm. Use dry fiber in the E band to minimize optical loss. b) Dense wavelength division multiplexing (DWDM) Channel spacing : the typical values for the channel spacing are 0. 8 nm (100 GHz) and 0. 4 nm (50 GHz) that uses wavelengths at the 1. 55 μm region.

WDM cont … • Each signal is transmitted over the channel with a recommended channel spacing to guarantee a minimum interchannel crosstalk. Crosstalk can give rise to bit error. • The multiplexed signal is launched into the optical fiber for transmission to the other end of the fiber and the demultiplexer will send each channel to its own receiver. • This technique allows many channels to be transmitted through a single optical fiber. • When N channels with the bit rates of B 1, B 2, . . . , and BN are transmitted simultaneously over an optical fiber of length L, the total bit rate–distance product, BL, becomes BL = (B 1+B 2+· · ·+BN)L. • For example, with 10 channels and each operating at 40 Gb/s over a distance of 100 km, capacity of the WDM system is 10 40 = 400 Gb/s and the BL is = 10 40 100 Gb/s-km = 40 Tb/s-km.

Relation between range of wavelengths and frequencies • The frequency and wavelength of the light is given by the relation c = f 0 where c = 3 108 ms-1 is the speed of light, f is the frequency and 0 is the wavelength. • The relation between the range of wavelengths, 0, and the range of frequencies, f, occupied by the optical signal can be obtained by differentiating c = f 0 w. r. t. f. This yields the relation c f 2 0 0 • or 0 c f 2 f where the deviation in frequency f around f corresponds to the wavelength deviation around .

Example 1 1. Typical modern DWDM systems have channels working in the 1. 55 μm wavelength region are spaced 0. 8 nm apart. The corresponding band of frequencies occupied by the optical signal in a channel can be obtained from the following steps: Solution: a) Use the relation that | f| = [c/ 02]| 0|. In this example, 0 = 0. 8 nm, c = 3 108 ms-1, and 0 = 1. 55 μm and by substituting these values into the relation yields f = 100 GHz. Therefore, the band of frequencies occupied by the optical signal in the 0. 8 nm channel is 100 GHz.

Question 1 • The C and L spectral bands cover a wavelength range from 1. 53 to 1. 61 μm. How many channels can be transmitted when the channel spacing is 0. 8 nm? What is the bit rate – distance product when a WDM signal covering the two bands using 10 Gb/s channels is transmitted over 2000 km? • Solution: • Number of channels = (1. 61 -1. 53) μm/0. 8 nm = 100 channels. • Bit rate – distance product = (2 10 Gb/s) 2000 km = 40 (Tb/s)-km

Limiting factors in fiber optics communication system • So far, we have covered the topics on optical dispersion and optical attenuation. These 2 factors are the main limiting factors in fiber optics communication system. a) Signal attenuation due to optical loss can limit the distance in which the signal can travel (longest distance without “boost”). Signal can be attenuated in the fiber cable, splice and other connectors. When the received optical signal is too low, the transmitted optical data cannot be correctly received. This causes bit-error. b) Signal distortion due to optical dispersion can limit the bit rate and the transmission distance.

Loss-limited and dispersion-limited optical comm. system • When the dispersion-limited transmission distance, Ld, is shorter than the loss-limited distance, L , i. e. (Ld < L ) the system is said to be dispersion- limited. • To increase the transmission distance, • use a light source with a narrower linewidth • use an optical fiber with lower dispersion such as a dispersion compensated fiber (DCF) • When the loss-limited transmission distance, L , is shorter than the loss limited distance, Ld, i. e. (L < Ld) the system is said to be losslimited / attenuation-limited. • Use repeaters / amplifier such as erbium doped fiber amplifier (EDFA) • Increase the power of the LED / laser at the transmitter • Improve the sensitivity of the receiver e. g. by using an photodetector e. g. avalanche photodiode (APD) with internal gain. This means that the receiver can differentiate 1 and 0 although the signal is low. (to be covered in the sensing system section if time permits)

Optical power budget (link budget) • The purpose of power budget is to ensure that enough power from the transmitter will reach the receiver to maintain reliable performance during the entire system lifetime. • This is given as Pout = Pin + CL (channel loss) + MS (system margin). • The MS (system margin) is to allocate a certain amount of power to additional sources of power penalty that may develop during the system lifetime because of component degradation of other unforeseen events. • CL (channel loss) takes into account of all possible sources of power loss e. g. due to fiber, splices and connectors.

Optical power budget (link budget)

Example

Example • Consider a single-mode fibre link operating at 1. 5 μm with a fibre loss of α = 0. 3 d. B/km. The fibre used has a dispersion coefficient of 3 ps/(km. nm). A Fabry-Perot laser with a spectral linewidth of 2 nm is used as the transmitter. The transmitter will have an average power of 2. 5 m. W. The receiver requires an average power of 300 n. W. A power budget margin of 8 d. B is required within the system. The system is designed for a data rate of 2. 5 Gbit/s. A maximum pulse broadening of 50% of the bit slot is permitted for the dispersion. • (i) Calculate the maximum optical loss allowed for the system. • (ii) Calculate the maximum transmission distance with regard to loss in the system. • (iii) Calculate the maximum transmission distance with regard to dispersion in the system.

Answer I. The optical loss due to the optical fiber is P transmitter Loss 10 log l og 10 Preceiver • The maximum optical loss allowed (Plossmax) = the loss due to the optical fiber (Pfiber)– noise power margin required (PMs). P loss max II. 2. 5 10 3 39. 21 d. B 9 300 10 P fiber P Ms 39. 21 d. B 8 d. B 31. 21 d. B The maximum transmission distance in terms of optical loss: L 31. 21 / 0. 3 km 104 km

Answer cont … for part (iii) 0. 5 T 0. 5 / B (optical pulse broadening; T: bit slot; B: bit rate) In single mode fiber, there is only chromatic dispersion. Therefore, the pulse spread is given by L × × D ch 0. 5 / B The length of the fiber is given by L 0. 5 B× × D ch The maximum length of the fiber in terms of optical dispersion is given by 0. 5 Be careful of the units L max 33. 33 km 2. 5 109 × 2× 3 10 12 Since this is a dispersion limited system, to increase the transmission distance of higher than 33. 33 km, either use a source with a smaller linewidth or use another fiber with a smaller Dch.

Summary • Loss and dispersion are the key factors in determining the bit rate and data transmission distance. • Loss causes bit error because the receiver cannot between distinguish 1 and 0. This gives rise to a “power budget” to ensure that enough power will reach the receiver to ensure a reliable operation. • Dispersion causes the broadening of the bit within it’s bit slot and possibly lead to intersymbol interference and bit-error.

FTTH Fiber To The Home

Fiber to the home (FTTH), also called "fiber to the premises" (FTTP), is the installation and use of optical fiber from a central point directly to individual buildings such as residences, apartment buildings and businesses to provide unprecedented high-speed Internet access. FTTH dramatically increases the connection speeds available to computer users compared with technologies now used in most places.

Assignment 1 Question: As engineer, what type of the fiber will you use to implement Fiber-tofiber (FTTH). Discuss your answer.