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Prof. Bogdan Galwas Warsaw University of Technology RoFS-Pforzheim 2007 1 R-o-F – Basic Structure of System Data transmission between Central Station and Base Station via fiber link Data transmission between Base Station and Terminal via radio link Data Output Data Output Base Station Central Station Optical Transceiver Data Input Fiber link Fiber Optical Transceiver Data Input Mobile wave Terminal Mobile wave Terminal Radio System RoFS-Pforzheim 2007 2 R-o-F – Basic Structure of System Central Station transmits optical carriers (fO) modulated at RF (fC & data) over fiber links toward remote base stations Photodiode PD converts the optical signal into an electrical RF signal (fC,2fC... nfC & data) RF signal is amplified and transmitted by an antenna (fC,2fC... nfC & data) RoFS-Pforzheim 2007 3 Outline of lecture: 1. Introduction 2. Optical Transmission of μwave Signals 3. Optical Generation of μwaves 4. Optical- μwave Mixing 5. Examples 6. Conclusions RoFS-Pforzheim 2007 4 1. Introduction Modulation frequency of of laser diodes LD is limited to the 40 GHz by the internal resonance between the electrons and photons. The push-pull principle solves partially these problems. Two types of the external optical modulators are widely used: The electro-optic (EOM) ridge-type travelling wave LiNbO3 Mach-Zender modulators, Electro-absorption (EAM) optical modulators. The new types of travelling-wave PIN photodectors have moved the bandwidth above 100 GHz . Special constructions of Metal-Semiconductor-Metal photodetectors have banwidth above 300 GHz. LD 3 10 30 P-I-N 100 EAM EOM RoFS-Pforzheim 2007 MSM 300 f [GHz] 5 2. Optical... Analog optical link (1) Problem: microwave signal (fRF,PIN,POUT ) is transmitted by an analog optical link. Fiber fRF,PIN fRF,POUT fOPT,PT WN Laser fOPT,PR WO L,=+j Photodetector The simplest technique for the distribution of the RF signal modulated with date is an intensity modulation scheme via direct modulation of laser. We will discuss the overall gain G of the system. POUT f RF POUT f RF PR f OPT PT f OPT G ; PIN f RF PR f OPT PT f OPT PIN f RF Modulation RoFS-Pforzheim 2007 Transmission Detection 6 2. Optical... Analog optical link (2) PR PT e 2 f Attenuation by fiber is simply expressed: POPT [mW] SL [W/A] IL [mA] t POPT(t) ID [A] RD[A/W] OPT L ID [mA] POPT[W] t ; t t Principle of operation of optical analog link with direct intensity modulation of laser optical power. Gain of analog link: 2 2 G SL R D ; RoFS-Pforzheim 2007 7 2. Optical... Analog optical link (3) An intensity modulation of laser optical power may be realised by external electrooptical modulator. fRF,PIN Fiber WN Laser fOPT,PT L,=+j PO T 1 fOPT,PR V WO Photodetector ID [A] POPT SMZ[V-1] fRF,POUT RD[A/W] POPT t I(t) t V(t) t RoFS-Pforzheim 2007 t 8 2. Optical... Analog optical link (4) The transmission of M-Z modulator can be described as: In the point of inflexion of the T(V) characteristic there is a long straight line section at V0 = V/2 and with a slope SMZ : Gain of analog link is proportional to the level of optical power P0 : RoFS-Pforzheim 2007 TV SMZ TMAX 2 V 1 cos V ; T TV MAX ; V VV 2V 2 P G S2MZ R 2D 02 R 2D ; V 9 2. Optical... Analog optical link (5) Analog link with external electro-optical modulator offers high gain 30 Gain G[dB] 20 External modulation 10 0 High SL laser -10 Laser modulation -20 -30 0,01 Typical link 0,1 1 10 100 Photodetector current [mA] RoFS-Pforzheim 2007 10 2. Optical Transmission of μwaves (1) a). A conventional FO link in which the data signal is up-converted by the MMW carrier reference before laser bias current modulation b) The date signal and carrier are transmitted separately over different FO links. Separation of signals can significantly increase dynamic range DATA SIGNAL FIBER PHOTO-DIODE OUTPUT CARRIER REFERENCE DATA SIGNAL CARRIER REFERENCE RoFS-Pforzheim 2007 LASER DIODE LASER DIODE LASER DIODE OUTPUT FIBER PHOTO-DIODE FIBER PHOTO-DIODE GAIN 11 2. Optical Transmission of μwaves (2) c) The photodiode is used as W mixer. This solution reduces numbers of elements and local oscillator power d) The photo-detector output signal is filtered and carrier reference signal is separated, next amplified and directed to the W mixer RoFS-Pforzheim 2007 DATA SIGNAL CARRIER REFERENCE LASER DIODE PHOTO-DIODE FIBER OUTPUT GAIN LASER DIODE FIBER PHOTO-DIODE DATA SIGNAL OUTPUT + LASER DIODE CARRIER REFERENCE PHOTOFIBER -DIODE FILTER GAIN 12 2. Optical Transmission of μwaves (3) e) It is also a conventional FO link in which the data signal is up-converted by the W carrier reference and external modulator is used f) The structure of the circuit was discussed earlier, external modulator is also used. LASER DIODE EO MODUL. FIBER DATA SIGNAL LASER DIODE PHOTO-DIODE CARRIER REFERENCE EO MODUL. FIBER PHOTO-DIODE DATA SIGNAL LASER DIODE OUTPUT EO MODUL. FIBER OUTPUT GAIN PHOTO-DIODE CARRIER REFERENCE RoFS-Pforzheim 2007 13 2. Optical Transmission of μwaves (4) 40-58 GHz 0-18 GHz LD - 1=1,3 m PD - 1=1,3 m 0-18 GHz 40-58 GHz Amp Mux x8 Demux DM x8 Amp 5 GHz LD - 1=1,5 m 5 GHz PD - 1=1,5 m 5 GHz Very interesting and professional system for transmission signal from 40-58 GHz millimeter-wave region by optical link. RoFS-Pforzheim 2007 14 2. Optical ... Chromatic-Dispersion Effect Laser optical power is modulated to generate an optical field with the carrier and two sidebands ET A0 J 0 m cos0 t 0 J1 m cos0 RF t 1 J1 m cos0 RF t 2 ; If the signal is transmitted over fiber, chromatic dispersion causes each spectral component to experience different phase shifts depending on the fiber-link distance L, modulation frequency fRF, and dispersion parameter D[ps/nm.km] 1 2 2 0 0 ... L; E R E T exp L j0 2 2 0 At the PIN output the amplitude of the mm-wave power is given by: POUT RoFS-Pforzheim 2007 2 f RF 2 1 2 ; cos cos LcD 2 f 0 15 2. Optical ... Chromatic-Dispersion Effect fTO f0 N / 2LcD ; PRF = 0 at frequency fTO, where N = 1, 3, 5... Problem: The standard amplitude modulation of optical carriers generates double-sideband signals. Due to the chromatic dispersion effects the sidebands arrived at the BS are phase shifted. In consequence periodical fading of PFR is observed. The techniques of Optical Single-Sidebands OSSB generation have been developed. RoFS-Pforzheim 2007 16 2. Optical... Subcarrier Multiplexing Base Station Central Station Optical Transceiver Fiber Selective Terminal Optical Transceiver Selective Terminal Data 2 – f2 M U X f1 f2 fN Data 1 Data 2 Data N RoFS-Pforzheim 2007 Data 1 – f1 Subcarrier multiplexing may be used for multichannel transmission 17 3. Optical Generation...Optical mixing Photodetector is responsive to the photon flux, is insensitive to the optical phase. Two optical signals (EM fields): the first signal: E S Re A Se j2 f S t Re A S e j2 f S t S ; The second signal, local oscillator, ELO, |ALO|, fLO i LO. The signal directed to the photodetector: Local Oscillator fLO Photodetector Coupler 3dB, 1800 Signal fS E ES E LO ; Intermediate Frequency fIF Photocurrent I is proportional to the incident power P and detector’s sensitivity R : I RP RPS PLO 2 PS PLO cos2f IF t S LO ; - PS and PLO are the powers, is intermediate frequency. The name of the process: optical mixing, optical heterodyning, photomixing, coherent optical detection. RoFS-Pforzheim 2007 18 3. Optical Generation...Two Optical Carriers Process of optical mixing may be used for generation of microwave frequency signal. The simplest way is to use 2 lasers with frequency f1 and f2, to transmit the optical signals by fiber to a photodiode and to extract the intermediate frequency fIF. Coupler Laser Carier & Data Amp f2 fIF Data Laser f1 fopt 0 0 f1 - f2 f1, f2, One optical signal may be modulated by data. The spectrum of optical signals must be “pure”, it is not easy to satisfy this condition . RoFS-Pforzheim 2007 19 3. Optical Generation...Two Optical Carriers It is possible to construct a specially modified distributed feedback semiconductor laser (DFB) in which oscillation occurs simultaneously on two frequencies, for two modes. Microwave Signal Double-mode Laser f1 & f2 Amp fIF fopt 0 f1, f2, 0 f1- f2 Dual-Mode DFB semiconductor laser for generation of microwave signal The mode separation is adjusted to the desired value by proper choosing the grating strength coefficient. RoFS-Pforzheim 2007 20 3. Optical Generation...Two Optical Carriers The spectral purity of the microwave signal may be really improved by synchronising the laser action. The master laser is tuned by stable microwave source of frequency f. f fOPT - 10f PD Slave Laser 1 Tunable Master Laser fOPT nf Slave Laser 2 20f Fiber fOPT + 10f The slave laser 1 and laser 2 are synchronized for different sidebands: upper sideband fOPT + 10f, and lower sideband fOPT - 10f, The frequency of output signal is equal to 20 f.. RoFS-Pforzheim 2007 21 3. Optical Generation of μwaves Laser on Nd:LiNbO3 electrooptical material placed inside microwave cavity changes its frequency of optical oscillation. M-Z Modulator Date (fSi Bi) fm Microwave Generator Laser Nd:LiNbO3 Laser inside microwave cavity fOPT f0 fm fOPT f0 (fPi Bi) Optical transmitter with Nd:LiNbO3 laser with frequency modulated by Microwave Generator and with external Mach-Zehnder modulator RoFS-Pforzheim 2007 22 3. Optical Generation of μwaves It is possible to transmit reference frequency fREF and to control a frequency of VCO by Phase Detector and PLL system. With using frequency multiplication process we can obtain every frequency from millimetre-wave region. Amp fREF, PIN Phase Detector m Frequency Divider PD VCO fOUT= n m fREF POUT >> PIN xn Frequency Multiplier Amp Complex and universal circuit for optical controlling of frequency from millimetre-wave region. RoFS-Pforzheim 2007 23 4. Optical- μwave Mixing (1) Transmission of an optical power by Mach-Zehnder interferometer may be written as: TN V0 , VRF POUT 1 V 1 cos0 V0 RF ; PIN TMAX 2 V Above formula will be the the starting point for a theoretical analysis of nonlinear mixing processes. a) POUT b) T(V) TMAX Planar Optical Waveguide C RF PIN RoFS-Pforzheim 2007 RF A B Coplanar Line 0 V0 V V 24 4. Optical- μwave Mixing (2) System to perform optical-microwave mixing process with the use of M-Z modulator M-Z Modulator POUT [W] Laser P0 Fiber V0 Photodiode f1, f2, 2f1, 2f2, 2f1-f2, 2f2+f1, 2f2-f1 Combiner V1,f1 Filter V2,f2 A combiner and bias circuit allow inputting the bias voltage and two alternating sine-form voltages into the modulator. VRF V1 sin 1t V2 sin 2 t; The amplitude of the first of them, called also the signal, is small. The second signal at the amplitude V2 plays role of a heterodyne and usually V2 >> V1. RoFS-Pforzheim 2007 25 5. Examples (1) Amp Laser M-Z Modulator Remote Antenna ..... Data f f1, f2,... fN Fiber Amp Data Receiver at Base Station Optical link for transmitting the received signal to the base station. RoFS-Pforzheim 2007 26 5. Examples (2) ..... f Data Antenna f1, f2,... fN Laser M-Z Modulator Transmitter at Base Station Fiber Amp Data Receiver at Remote Antenna Optical link for transmitting microwave signal to remote antenna. RoFS-Pforzheim 2007 27 5. Examples (3) Millimetre-wave radio signals Central Station Picocell Fiber 100...200 m Optical coupler Radio-over-fiber system delivers the broad-band services to the customers by a radio RoFS-Pforzheim 2007 28 5. Examples (4) By using wavelength division multiplexing WDM techniques into the fiber access network each BS can be addressed by a different wavelength. fIF1 fIF2 fIFN LD1 LD2 Transponder 1 Transponder 2 1 2 N LDN M-Z Modul MUX f0 Base Station Optical Coupler & Filter 2 1 PD1 f0 xN fIF1 PD2 f0 fIF2 xN Block diagram of the system which uses dense WDM RoFS-Pforzheim 2007 29 5. Examples (5) Base-station fD,D Amp Amp Photodiode 1 xN fC Amp Customer Unit WDM 2 Amp fD Amp Laser DFB Block diagram of base-station circuit with multiplication of carrier frequency for full-duplex, mm-wave fiber-radio network RoFS-Pforzheim 2007 30 5. Example (60 GHz P-MP) Base Station Central Station T/R module 156 Mb/s/60GHz Transceiver E/O system T/R module T/R module BS Point-to-multipoint radio-over-fiber full duplex system transmits data between computer systems RoFS-Pforzheim 2007 T/R module 31 5. Examples (60 GHz P-MP) λ2 Central Station LD λ1 156 Mb/s DPSK Modem 60 GHz Transceiver Base Station LD EAM PD EDFA EDFA DWDM Mux LD – Laser diode, EAM – Electro-absorption modulator, EDFA – Fiber amplifier, DWDM Mux – Multiplexer, PD Photodiode RoFS-Pforzheim 2007 32 5. Examples (60 GHz P-MP) Base Station λ1 λ2 PD 60 GHz Transceiver 156 Mb/s DPSK Modem 60 GHz Transceiver 156 Mb/s DPSK Modem EAM EAM – Electro-absorption modulator, PD - Photodiode RoFS-Pforzheim 2007 33 5. Examples (125 GHz/10 Gb/s) PD EDFA 125 GHz Receiver DATA DATA Terminal EOM EDFA EOM LD 2fM fOPT fM RoFS-Pforzheim 2007 fM=62,5 GHz f0 fOPT The last experimental system 34 6. Conclusions Photonic technology opens new possibilities to generate and to transmit the microwave signals, especially in millimeter-wave region New wideband communication systems are developed on the basis of mm-wave and optical technologies The gap between what is theoretically possible and what we experimentally demonstrated has narrowed considerably in the last decade RoFS-Pforzheim 2007 35