Historical Development of Optical Communications 1790 – Claude Chappe invented ‘optical telegraph’. 1880 – Graham Bell invented ‘photophone’. 1930 – Heinrich Lamm presented unclad-fibers, but it showed poor performance. 1954 – van Heel and Kapany reported about the 1 st clad- fibers by covering a bare fiber with a transparent of lower refractive index.
Historical Development of Optical Communications 1960 – Maimen demonstrated the 1 st laser for communications. 1966 – Kuo and Hockham introduced fiber communications with low attenuation (< 20 dB/km). 1970 – Maurer, Keck, and Schultz made a single-mode fused silica fiber (very pure with high melting point and a low refractive index) for 633 nm wavelength of HeNe laser. 1977 – Fibers used at 850 nm from GaAlAs laser.
Historical Development of Optical Communications 1980’s – A 2 nd generation of optical communication at 1300 nm with 0.5 dB/km for fiber attenuation. 1990’s – A 3 rd generation operates at 1550 nm with fiber loss of 0.2 dB/km with EDFA serving as an optical amplifier. Signals also could be sent via WDM.
Preview on Fiber Optic Communication Basic schematic diagram
Preview on Fiber Optic Communication The advantages of optical fiber communication over electrical based system are Low attenuation High bandwidth Immune to electro-magnetic interference Short circuiting, Earthing, and Fire Free Low in weight and volume Data security
Preview on Fiber Optic Communication The transmission passbands for installed fibers today are 0.85, 1.3, and 1.55 μ m (near-infrared). Wavelength of 1.6+ μ m can be seen in some applications. There are more than 25,000 GHz of capacity in each of the three wavelength bands.
Preview on Fiber Optic Communication Digital transmission – The sampling theorem says that an analog signal can be accurately transmitted if sampling rate is twice the highest frequency contained in that signal. Let R be the required transmission rate. R can be expressed by where m = number of bits/sample f s = sampling frequency = 2( f)
Preview on Fiber Optic Communication Message TypeUsed bandwidth(B) Voice (telephone)4 kHz Music -- AM10 kHz Music -- FM200 kHz TV (Video + Audio)6 MHz
Preview on Fiber Optic Communication Number of Voice channels Transmission Designation Signaling DesignationData Rate 1--64 kb/s 24T1DS-11.544 Mb/s 48 (2-T1 systems)T1CDS-1C3.152 Mb/s 96 (4-T1 systems)T2DS-26.312 Mb/s 672 (7-T2 systems)T3DS-344.735 Mb/s 1344 (2-T3 systems)T3CDS-3C91.053 Mb/s 4032 (6-T3 systems)T4DS-4274.175 Mb/s
Example 1 A telephone system has m = 8 bits/sample. Find R. Sol n
Preview on Fiber Optic Communication A transmission standard developed for optical communication is called SONET (Synchronous Optical NETwork). Transmission (electrical) Designation (optical) SDH systemData Rate(Mb/s) STS-1OC-1-51.84 STS-3cOC-3STM-1155.52 STS-12OC-12STM-4622.08 STS-24OC-24STM-81,244.16 STS-48OC-48STM-162,488.32 STS-96OC-96STM-324,976.64 STS-192OC-192STM-649,953.28 STS-768OC - 768STM-12839,813.12
Preview on Fiber Optic Networks Fiber-To-The-Home (FTTH) 2.5 Gbps Mid 90’s 10 Gbps y2k 40 Gbps and beyond state of art
Preview on Fiber Optic Networks Now a number of channels per fiber is more than 128. This was increased from 32 channels/fiber in 2004. The link attenuation is less than 0.2 dB/km at 1.55 μ m wavelength. BER can be achieved at 10 -15 with a help of Er-doped fiber amplifier (EDFA).
Optical communication systems Multiplexing refers to transmission of multiple channels over one fiber. Channels can be data, voice, video, and so on. We may classify the communication systems into 3 classes as: Point-to-point link Multipoint link Network
Example 2 A cable consists of 100 fibers. Each fiber can carry signals of 5 Gbps. If audio message encoded with 8 bits/sample is being sent, how many conversations can be sent via one cable? Sol n
Example 3 By using the same cable as previous example, how many TV channels could be sent via a cable. Sol n
Generations of Fiber Usage Bandwidth and error rate improved (fatter links), but propagation delay not changed (same length). Source: Fiber Optic Network Paul E. Green, Prentice Hall.
Generations of Fiber Usage First generation: no fiber (copper link) 2 nd generation: Fiber used for point-to-point link only. Multiplexing & switching carried out electronically. 3 rd generation: Fiber used for multiplexing and switching as well as point-to- point transmission.
Generations of Fiber Usage Copper links Copper links are more vulnerable to outside influence since moving electrons influence each other. It is also affected by electromagnetic wave (EM wave). Fiber links Moving photons of light in a fiber do not interact with other moving photons. EM wave has no effect on a fiber as well.
Fiber Bandwidth We all know that where λ = free-space wavelength ν = optical frequency c = speed of light at free-space
Fiber Bandwidth At = 1.5 µm, the attenuation is about 0.2 dB/km, and there is a window about = 200 nm wide between wavelengths having double that number of dB per kilometer. The useful bandwidth is about 25,000 GHz.
Fiber Bandwidth This can applied to = 1.3 µm and 0.85 µm as well. For 0.85 µm, this band is not defined by an attenuation standpoint, but by the range which GaAs components can be easily made.
Multiplexing Space Division Multiplexing Frequency Division Multiplexing Time Division Multiplexing Wavelength Division Multiplexing
Wavelength-Division Multiplexing For example, 16 channel WDM using 1,300 nm or 1,550 nm with 100 GHz channel spacing. Therefore, bandwidth = 16 x 100 = 1,600 GHz. LAN = Local Area Network (< 2 km) MAN = Metropolitan Area Network ( < 100 km) WAN = Wide Area Network (unlimited)