2. Digital Optical Systems based on Coherent and Direct Detection

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1 1/ 2. Digital Optical Systems based on Coherent and Direct Detection Optical Communication Systems and Networks

2 2/ 12 BIBLIOGRAPHY Fiber-Optic Communications Systems Govind P. Agrawal, Chapter 10, pp , John Wiley & Sons, 2002, Third Edition. Optical Fiber Communications. Principles and Practice John M. Senior. Chapter 12, pp , Ed. Prentice Hall, 1992, Second Edition,

3/ 12 Modulation formats Optical carrier: E(t) = A 0 cos( 0 t 0 ) ê Amplitude modulation A 0 : ASK,

(Bit sequence) ASK Frequency modulation 0 : FSK, Frequency-shift keying Polarization modulation ê:

FSK Most commercial systems are based on ASK (These systems are also known as on off keying, OOK)

3 3/ 12 Modulation formats Optical carrier: E(t) = A 0 cos( 0 t 0 ) ê Amplitude modulation A 0 : ASK, Amplitude-shift keying Phase modulation 0 : PSK, Phase-shift keying Electric signal (Bit sequence) ASK Frequency modulation 0 : FSK, Frequency-shift keying Polarization modulation ê: PoSK, information coded by polarization state (not allowed in optical systems based on fiber) PSK FSK Most commercial systems are based on ASK (These systems are also known as on off keying, OOK) IM/DD (intensity modulation and Direct Detection) First Differential PSK (DPSK) are being deployed recently

4 Direct Detection Lecture 2: Digital Optical Systems 4/ 12 ELECTRICAL RECEPTION E(t)=f(t)cos(t) Antenna Electrical receiver weak electric signal Amplifiers (& filter/s) i(t)=ce(t) f(t) E(t)=f(t)cos(t) OPTICAL RECEPTION Optical receiver weak electric signal Amplifier (& filter/s) Lens (Focusing /collimation) i(t)=c E(t) 2 =CP(t) f 2 (t)

5 5/ 12 Thermal Variation of radiation makes a change of temperature T (bolometers). They are slow for communication applications Photodetectors Direct Detection: Detectors Vacuum devices As the photomultiplier tubes, which have high sensitivity but require high voltages, are expensive, slow and large. They also have difficulty operating at > 1 m Photoconductors Semiconductors Incident radiation causes variation of carriers, which in turn causes the modification of the conductivity of the material. They have low sensitivity fotodiodes Best option in Optical Communications

6 6/ 12 Coherent Systems ADVANTAGES: Coherent detection can provide a potential improvement up to 20 db in the receiver sensivity unlike direct-detection-based systems For a given power budget, this would allow to increase the total length of an optical link (or spacing between repeaters/aplifiers) Higher transmisssion rates over existing optical links without reducing repeater spacing is achieved Efficent use of the available bandwidth 6 Allows to transmit simultaneously several carriers (frequency multiplexing) Channel spacing can be reduced to 1-10 GHz. In IM/DD systems, 100 GHz channel spacing has been proposed. Latest recomendations (G.694.1) include 50, 25 and 12.5 GHz versions DISADVANTAGES: Receivers become more complex Sensitivity to the optical carrier s phase and frequency degradation in reception

7 7/ 12 Diagram of a Coherent Detection System Beam combiner Received optical signal (modulated) Detector Electronic driver CW Local oscillator Electrical bit secuence

8 Coherent Systems Lecture 2: Digital Optical Systems 8/ 12 The optical carrier carries modulated/coded information (phase and/or frequency) At receiver: coherent mixing between the incoming signal and optical wave generated by a stable and reduced spectral width local oscillator. Incoming signal: Local oscillator: E E s OL As exp j( 0t s ) A LO exp j( t LO LO ) Assuming perfect optical mixing, and recalling than optical power is proportional to the square of the electrical field strength, we have : P( t) 2 cos( IFt s LO), P s 2 2 KAs KALO IF 0 LO if IF = 0, coherent system with homodyne detection if IF 0, coherent system with heterodyne detection

9 Coherent Systems. Homodyne detection 9/ 12 When the local oscillator frequency equals to optical carrier frequency: w FI = w s - w OL = 0 The photocurrent generated by the optical detector is proportional to the optical power (or optical intensity) : I( t) ( ) 2 cos( s LO) I ( t ) 2 P P s LO cos( s LO) (P LO >> P s P LO + P s P LO, where DC can be eliminated) Assuming: s = LO, there is an improvement of SNR: This information can not be detected in direct detection systems I I HomodyneDetect Direct Detect 2 2 P sp LO 2 4P P LO s 1 Main disadvantage: Very sensitive to phase variations Accurate control of LO and s could be a solution unless both do not fluctuate Solution: Phase control so that the difference between OL y s remains constant (by using phase locked-loops)

10 10/ 12 Coherent Systems. Heterodyne detection Typically, the local oscillator frequency is chosen so that intermediate frequency values range from 0,1 to 5 GHz. I( t) ( ) 2 cos( wift s LO) (following the same considerations we made in homodyne detection) I( t) 2 cos( wift s LO) A SNR improvement is obtained with regard to IM/DD systems. However, this improvement (3dB) is lower compared to the one obtained in homodyne detection Adventage: Simpler optical receivers Suitable for optical communications systems Unable to demodulate directly optical signal to baseband (it is required a previous demodulation from intermediate frequency to baseband in the electrical domain)

11/ 12 Coherent Systems. Heterodyne detection Typically, the local oscillator frequency is chosen so that intermediate frequency values range from 0,1 to 5 GHz.

11 11/ 12 Coherent Systems. Heterodyne detection Typically, the local oscillator frequency is chosen so that intermediate frequency values range from 0,1 to 5 GHz. I( t) ( ) 2 cos( wift s LO) (following the same considerations we made in homodyne detection) I( t) 2 cos( wift s LO) A SNR improvement is obtained with regard to IM/DD systems. However, this improvement (3dB) is lower compared to the one obtained in homodyne detection If the power level P LO dominates and can be controlled: I Photocurrent which depends on the detection process (homodyne or heterodyne) The homodyne case ( OL = s ) produces an increase of 3dB with regard to the heterodyne case

12 Coherent Systems. Heterodyne detection 12/ 12 Incoming optical signal (modulated) Beam combiner Photodetector Bandpass filter Lowpass filter CW Local Oscillator subcarrier recovery (IF) Data out Diagram of a coherent system based on heterodyne detection

Lecture 7 Fiber Optical Communication Lecture 7, Slide 1

Lecture 7 Fiber Optical Communication Lecture 7, Slide 1 Dispersion management Lecture 7 Dispersion compensating fibers (DCF) Fiber Bragg gratings (FBG) Dispersion-equalizing filters Optical phase conjugation (OPC) Electronic dispersion compensation (EDC) Fiber