2015.7.15 Optical Transmission Technologies presented by K. Inoue
Optical communication is widely spreading 1
Number of subscribers for broadband service optical line 2
All trunk transmission lines are optical this talk What is optical communication? Why it is beneficial? 3
Signal transmission is made via a physical quantity Voice transmission via the air density of the air Voice transmission via a string vibration of a string 4
Copper wired communication electrical vibration within copper Wireless communication electromagnetic wave in the air A physical quantity conveying signal is called carrier 5
Carrier vibrating at a high frequency can convey a large amount of information 1 1 0 time 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 time 6
Light is an electromagnetic wave with a quit high frequency 10 4 10 6 10 8 10 10 10 12 10 14 10 16 10 18 frequency(hz) term: ultraviolet longwavwave short- mediumwave ultrashortwave microwave milliwave infrared visible light X- ray Lightwave is potentially preferable for signal carrier 7
by the way Frequent misunderstanding Light is good for communication because its traveling speed is fast But, the traveling speed has nothing to do with the date speed. fast slow The arriving time is different, but the number of pulses is the same. Data speed = Frequency of pulses representing data 8
How to convey signal using light? A primitive way is to ON/OFF shutter a signal lamp. But, this method has crucial defects for high speed communication. - Shuttering speed is quite slow. - Transmission is not made when there is an obstacle. - Transmission is not made on rainy or cloudy days. - Light spreads and becomes week while traveling long. - How to convert optical signal to electrical signal? Issues are signal light generation/transmission/detection 9
Laser was invented in 1960 It generates light with quite unique properties. good directionality high power density single wavelength (color) single color directionality sun prism laser prism laser possibility for multi-channels 10
Optical communication research started in 1960 s Space transmission experiment Lens waveguide Impractical those days, unfortunately 11
breakthrough 1 Semiconductor laser firstly operated at room temperature in 1970 Semiconductor laser is suitable for communication systems - Compact - Light emission is ON/OFF by injection current laser diode average lifetime (hour) year 12
breakthrough 2 Low-loss glass fiber was fabricated in 1970 loss (db/km) corning co. Bell lab. loss L transmittance T 0 db 1.0 10 db 0.1 20 db 0.01 30 db 0.001 NTT NTT T = 10 L /10 year Research activity was triggered by these two innovations 13
Optical fiber High-refractive index glass (core) is surrounded by low-index glass (clad) Light propagates along fiber, being totally reflected. cladd (n 2 ) core (n 1 ) cladd (n 2 ) refraction/reflection n 2 n 1 n 1 > n 2 n 2 n 1 n 1 > n 2 14
The propagation loss in fiber is quite low 0.2dB-loss @ 1km (transmittance = 95.5%) 2.0dB-loss @ 10km (transmittance = 63%) on the other hand Copper wire is lossy for high frequencies. (ex. 10dB/km for 10MHz) 15
Why low-loss is preferable? Optical transmitter receiver Electrical repeater repeater repeater transmitter rec. trans. rec. trans. rec. trans. receiver many repeaters high cost Fiber transmission is superior for long-distance & high-speed communication 16
Optical communication has been developed, pursuing to fully utilize the low-loss property. (fiber loss property) 2nd generation 3rd generation Transmission medium is the most important matter in general 1st generation (0.8µm): first semiconductor laser 2nd generation (1.3µm): zero-dispersion 3rd generation (1.5µm): minimum loss 17
Crucial issue is dispersion Dispersion: The property that light velocity is not unique. Mode-dispersion The propagation velocity is different for different propagation angles. Chromatic-dispersion The propagation velocity is different for different wavelength (color). 18
When the velocity is different... Pulse width broadens. In case of a pulse train, Data are not correctly received 19
By the way Rainbow is caused by dispersion Light is refracted at the boundary between materials with different refractive indices. The refraction angle is determined by the ratio of the refractive indices. n 2 θ 2 sinθ 2 sinθ 1 = n n 1 2 n 1 θ 1 Thus, the refraction angle is different for different color due to the dispersion. Then, sun water drop 20
research effort Combat with mode-dispersion A fiber with a small core-area allows just one propagation angle. multi-mode fiber single-mode fiber 50 µm 10 µm 1 10 µm = mm 100 Several techniques have been studied and developed. Fabrication process How to input laser light into a fiber How to connect fibers 21
Combat with chromatic-dispersion (1) Strategy 1: Use of signal light with a narrow wavelength width Development of single-mode lasers Fabry-Perot type DFB type Develop of optical modulators laser diode laser diode modulator 22
Combat with chromatic-dispersion (2) Strategy 2: Use of a wavelength at which fiber dispersion is zero. dispersion value (ps/km/nm) normal fiber 分散フラットファイバ dispersion-shifted fiber wavelength (µm) Development of laser diodes emitting 1.3-µm wavelength light Unfortunately, however, 1.3 µm is not loss-minimum wavelength. Development of fibers with zero-dispersion at the loss-minimum wavelength normal fiber dispersion-shifted fiber ref. index ref. index n 3 n 2 n 1 n 2 n 3 n 2 n 1 n 2 radius axis radius axis 23
Even though fiber is low-loss, researchers wanted to expand the transmission length. Combat with fiber loss Optical amplifiers have been developed 24
WDM technologies have been developed to fully utilize the low-loss property of fiber 光分波low-loss region Wavelength Division Multiplex (WDM) System transmitter (λ 2 ) transmitter (λ 3 ) transmitter (λ 4 ) simultaneous amplification transmitter (λ1) receiver (λ1) receiver (λ 2 ) 器receiver (λ 3 ) receiver (λ 4 ) The transmitted data amount is increased by using a number of wavelengths (colors) 25
With the above technologies, the transmission capacity has increased. 光ファイバ容量 [bit/s] 10 P 1 P 100 T 10 T 1T 100 G 10 G 1 G 100 M 単一コアファイバの容量限界 時分割多重 波長多重 : 研究 ( マルチコアファイバ ) : 研究 ( 従来ファイバ ) : 実用システム 1980 1990 2000 2010 2020 年 空間多重?! (100G x 80 波 : 8T) 2030 26
Then Optical technologies are basic infrastructure supporting the present communication networks 27