Warsaw University of Technology Radio-over-Fiber Systems Prof. Bogdan Galwas Warsaw University of Technology RoFS-Pforzheim 2007

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 Mobile wave Terminal Base Station Data Input Fiber Central Output Optical Transceiver Data Output Data Input Radio System Fiber link RoFS-Pforzheim 2007

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

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

1. Introduction LD MSM P-I-N EAM EOM 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 bandwidth above 300 GHz. LD P-I-N MSM 3 10 30 100 300 f [GHz] EAM EOM RoFS-Pforzheim 2007

2. Optical... Analog optical link (1) Problem: microwave signal (fRF,PIN,POUT ) is transmitted by an analog optical link. Fiber fRF,PIN WN Photodetector WO fRF,POUT fOPT,PT L,=+j fOPT,PR Laser 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. Modulation Transmission Detection RoFS-Pforzheim 2007

2. Optical... Analog optical link (2) Attenuation by fiber is simply expressed: IL [mA] POPT [mW] SL [W/A] POPT(t) t ID [A] POPT[W] RD[A/W] ID [mA] Principle of operation of optical analog link with direct intensity modulation of laser optical power. Gain of analog link: RoFS-Pforzheim 2007

2. Optical... Analog optical link (3) An intensity modulation of laser optical power may be realised by external electrooptical modulator. Laser PO Fiber fOPT,PT L,=+j fOPT,PR Photodetector WO fRF,POUT fRF,PIN WN V(t) 1 T t POPT SMZ[V-1] V ID [A] RD[A/W] POPT t I(t) RoFS-Pforzheim 2007

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

2. Optical... Analog optical link (5) Analog link with external electro-optical modulator offers high gain Photodetector current [mA] Gain G[dB] -10 -20 -30 0,01 0,1 1 10 100 20 30 High SL laser Typical link External modulation Laser modulation RoFS-Pforzheim 2007

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 PHOTO- -DIODE FIBER LASER DIODE OUTPUT CARRIER REFERENCE DATA SIGNAL FIBER LASER DIODE OUTPUT CARRIER REFERENCE GAIN PHOTO- -DIODE RoFS-Pforzheim 2007

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 FIBER OUTPUT GAIN PHOTO- -DIODE DATA SIGNAL LASER DIODE CARRIER REFERENCE FIBER LASER DIODE OUTPUT CARRIER REFERENCE GAIN + FILTER DATA SIGNAL PHOTO- -DIODE RoFS-Pforzheim 2007

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. DATA SIGNAL FIBER OUTPUT CARRIER REFERENCE LASER DIODE PHOTO- -DIODE EO MODUL. DATA SIGNAL OUTPUT CARRIER REFERENCE GAIN FIBER LASER DIODE PHOTO- -DIODE EO MODUL. EO MODUL. RoFS-Pforzheim 2007

2. Optical Transmission of μwaves (4) LD - 1=1,3 m 5 GHz DM x 8 Mux Demux 40-58 GHz 0-18 GHz PD - 1=1,3 m LD - 1=1,5 m PD - 1=1,5 m Amp Very interesting and professional system for transmission signal from 40-58 GHz millimeter-wave region by optical link. RoFS-Pforzheim 2007

2. Optical ... Chromatic-Dispersion Effect Laser optical power is modulated to generate an optical field with the carrier and two sidebands 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] At the PIN output the amplitude of the mm-wave power is given by: RoFS-Pforzheim 2007

2. Optical ... Chromatic-Dispersion Effect 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

2. Optical... Subcarrier Multiplexing Selective Terminal Base Station Fiber Central Optical Transceiver Data 1 – f1 M U X fN Data N Data 2 Data 1 f2 f1 Data 2 – f2 Subcarrier multiplexing may be used for multichannel transmission RoFS-Pforzheim 2007

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: The second signal, local oscillator, ELO, |ALO|, fLO i LO. The signal directed to the photodetector: Photocurrent I is proportional to the incident power P and detector’s sensitivity R : - PS and PLO are the powers, is intermediate frequency. The name of the process: optical mixing, optical heterodyning, photomixing, coherent optical detection. Signal fS Intermediate Frequency fIF Coupler 3dB, 1800 Local Oscillator fLO Photodetector RoFS-Pforzheim 2007

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. Data Laser f2 f1 Coupler f1, f2, fopt Carier & Data Amp f1- f2 fIF 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

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. f1, f2, Double-mode Laser f1 & f2 fopt Microwave Signal Amp f1- f2 fIF 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

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. Tunable Master Laser fOPT  nf Fiber PD Slave Laser 1 f Laser 2 fOPT + 10f fOPT - 10f 20f 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

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) fOPT f0  fm fm Laser Nd:LiNbO3 Laser inside microwave cavity Microwave Generator f0  (fPi  Bi) Optical transmitter with Nd:LiNbO3 laser with frequency modulated by Microwave Generator and with external Mach-Zehnder modulator RoFS-Pforzheim 2007

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. PD Amp fREF, PIN fOUT= n m fREF POUT >> PIN VCO x n Frequency Multiplier m Divider Phase Detector Complex and universal circuit for optical controlling of frequency from millimetre-wave region. RoFS-Pforzheim 2007

4. Optical- μwave Mixing (1) Transmission of an optical power by Mach-Zehnder interferometer may be written as: Above formula will be the the starting point for a theoretical analysis of nonlinear mixing processes. RF PIN Coplanar Line Planar Optical Waveguide POUT a) V C A B V b) TMAX T(V) V0 RoFS-Pforzheim 2007

4. Optical- μwave Mixing (2) System to perform optical-microwave mixing process with the use of M-Z modulator A combiner and bias circuit allow inputting the bias voltage and two alternating sine-form voltages into the modulator. 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. Fiber POUT [W] Photodiode Laser P0 Combiner Filter V2,f2 V0 M-Z Modulator f1, f2, 2f1, 2f2, 2f1-f2, 2f2+f1, 2f2-f1 V1,f1 RoFS-Pforzheim 2007

5. Examples (1) Data M-Z Modulator Laser Amp ..... f1, f2,... fN f Fiber Remote Antenna Receiver at Base Station Optical link for transmitting the received signal to the base station. RoFS-Pforzheim 2007

5. Examples (2) Data M-Z Modulator Laser Antenna Amp ..... f1, f2,... fN f Fiber Transmitter at Base Station Receiver at Remote Antenna Optical link for transmitting microwave signal to remote antenna. RoFS-Pforzheim 2007

5. Examples (3) Fiber 100...200 m Picocell Millimetre-wave radio signals Optical coupler Central Station Radio-over-fiber system delivers the broad-band services to the customers by a radio RoFS-Pforzheim 2007

Block diagram of the system which uses dense WDM 5. Examples (4) By using wavelength division multiplexing WDM techniques into the fiber access network each BS can be addressed by a different wavelength. f0 MUX M-Z Modul Optical Coupler & Filter  fIF1 LD1 1 fIF2 LD2 2 fIFN LDN N Base Station PD1 xN PD2    Transponder 1 Transponder 2 Block diagram of the system which uses dense WDM RoFS-Pforzheim 2007

5. Examples (5) Amp Base-station fC fD,D x N Laser DFB fD WDM 2 1 Photodiode Customer Unit Block diagram of base-station circuit with multiplication of carrier frequency for full-duplex, mm-wave fiber-radio network RoFS-Pforzheim 2007

T/R module 156 Mb/s/60GHz Transceiver 5. Example (60 GHz P-MP) T/R module Base Station Central Station T/R module 156 Mb/s/60GHz Transceiver E/O system T/R module BS T/R module Point-to-multipoint radio-over-fiber full duplex system transmits data between computer systems RoFS-Pforzheim 2007

5. Examples (60 GHz P-MP) Central Station λ2 λ1 EAM EDFA PD LD λ1 λ2 DWDM Mux Central Station Base Station 156 Mb/s DPSK Modem 60 GHz Trans-ceiver LD – Laser diode, EAM – Electro-absorption modulator, EDFA – Fiber amplifier, DWDM Mux – Multiplexer, PD Photodiode RoFS-Pforzheim 2007

EAM – Electro-absorption modulator, PD - Photodiode 5. Examples (60 GHz P-MP) Base Station λ1 156 Mb/s DPSK Modem 60 GHz Trans-ceiver PD λ2 EAM 156 Mb/s DPSK Modem 60 GHz Trans-ceiver EAM – Electro-absorption modulator, PD - Photodiode RoFS-Pforzheim 2007

The last experimental system 5. Examples (125 GHz/10 Gb/s) PD EDFA EOM LD fM=62,5 GHz fOPT f0 fM 2fM DATA 125 GHz Receiver DATA Terminal The last experimental system RoFS-Pforzheim 2007

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