RoFS-Pforzheim Prof. Bogdan Galwas Warsaw University of Technology

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

RoFS-Pforzheim R-o-F – Basic Structure of System  Central Station transmits optical carriers (f O ) modulated at RF (f C & data) over fiber links toward remote base stations  Photodiode PD converts the optical signal into an electrical RF signal (f C,2f C... nf C & data)  RF signal is amplified and transmitted by an antenna (f C,2f C... nf C & data)

RoFS-Pforzheim Optical Transmission of μwave Signals Outline of lecture: Introduction 1. Introduction 4. Optical- μ wave Mixing 5. Examples 6. Conclusions 3. Optical Generation of μ waves

RoFS-Pforzheim 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 LiNbO 3 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 f [GHz] LD EAMEOM MSM P-I-N

RoFS-Pforzheim Optical... Analog optical link (1)  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.   Problem: microwave signal (f RF,P IN,P OUT ) is transmitted by an analog optical link. Fiber Laser f RF,P IN WNWN Photodetector WOWO f RF,P OUT f OPT,P T L,  =  +j  f OPT,P R Modulation Transmission Detection

RoFS-Pforzheim Optical... Analog optical link (2)  Principle of operation of optical analog link with direct intensity modulation of laser optical power.  Gain of analog link: I L [mA] P OPT [mW] S L [W/A] P OPT (t) t t I D [A] P OPT [W] R D [A/W] t t I D [mA]   Attenuation by fiber is simply expressed:

RoFS-Pforzheim Optical... Analog optical link (3)   An intensity modulation of laser optical power may be realised by external electrooptical modulator. Laser POPO Fiber f OPT,P T L,  =  +j  f OPT,P R Photodetector WOWO f RF,P OUT f RF,P IN WNWN I D [A] R D [A/W] P OPT t I(t) t V(t) 1 T t t P OPT S MZ [V -1 ] VV

RoFS-Pforzheim Optical... Analog optical link (4)   The transmission of M-Z modulator can be described as:   Gain of analog link is proportional to the level of optical power P 0 :   In the point of inflexion of the T(V) characteristic there is a long straight line section at V 0 = V  /2 and with a slope S MZ :

RoFS-Pforzheim Optical... Analog optical link (5) Photodetector current [mA] Gain G[dB] ,01 0, High S L laser Typical link External modulation Laser modulation  Analog link with external electro-optical modulator offers high gain

RoFS-Pforzheim Optical Transmission of μwaves (1) DATA SIGNAL FIBER LASER DIODE OUTPUT CARRIER REFERENCE FIBER GAIN LASER DIODE PHOTO- -DIODE PHOTO- -DIODE 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

RoFS-Pforzheim Optical Transmission of μwaves (2) FIBER LASER DIODE OUTPUT CARRIER REFERENCE GAIN + FILTER DATA SIGNAL PHOTO- -DIODE FIBER OUTPUT GAIN FIBER PHOTO- -DIODE DATA SIGNAL LASER DIODE CARRIER REFERENCE LASER DIODE PHOTO- -DIODE 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 Optical Transmission of μwaves (3) DATA SIGNAL OUTPUT CARRIER REFERENCE GAIN FIBER LASER DIODE LASER DIODE PHOTO- -DIODE PHOTO- -DIODE EO MODUL. EO MODUL. 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.

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

RoFS-Pforzheim 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 f RF, and dispersion parameter D[ps/nm.km]   At the PIN output the amplitude of the mm-wave power is given by:

RoFS-Pforzheim Optical... Chromatic-Dispersion Effect  P RF = 0 at frequency f TO, 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 P FR is observed.  The techniques of Optical Single-Sidebands OSSB generation have been developed.

RoFS-Pforzheim Optical... Subcarrier Multiplexing Selective Terminal Base Station Fiber Central Station Optical Transceiver Data 1 – f 1 M U X fNfN Data N Data 2 Data 1 f2f2 f1f1 Selective Terminal Data 2 – f 2 Subcarrier multiplexing may be used for multichannel transmission

RoFS-Pforzheim Optical Generation... Optical mixing Signal f S Intermediate Frequency f IF Coupler 3dB, Local Oscillator f LO Photodetector   The second signal, local oscillator, E LO, |A LO |, f LO i  LO. The signal directed to the photodetector:   Photocurrent I is proportional to the incident power P and detector’s sensitivity R : - P S and P LO are the powers, is intermediate frequency.   The name of the process: optical mixing, optical heterodyning, photomixing, coherent optical detection.  Photodetector is responsive to the photon flux, is insensitive to the optical phase.  Two optical signals (EM fields): the first signal:

RoFS-Pforzheim Optical Generation... Two Optical Carriers Data Laser f 2 Laser f 1 Coupler 0 f 1, f 2, f opt Carier & Data Amp 0 f 1 - f 2 f IF   One optical signal may be modulated by data.   The spectrum of optical signals must be “pure”, it is not easy to satisfy this condition.  Process of optical mixing may be used for generation of microwave frequency signal.  The simplest way is to use 2 lasers with frequency f 1 and f 2, to transmit the optical signals by fiber to a photodiode and to extract the intermediate frequency f IF.

RoFS-Pforzheim Optical Generation... Two Optical Carriers f 1, f 2, Double-mode Laser f 1 & f 2 0 f opt Microwave Signal Amp 0 f 1 - f 2 f IF   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.  It is possible to construct a specially modified distributed feedback semiconductor laser (DFB) in which oscillation occurs simultaneously on two frequencies, for two modes.

RoFS-Pforzheim Optical Generation... Two Optical Carriers Tunable Master Laser f OPT  nf  Fiber PD Slave Laser 1 ff Slave Laser 2 f OPT + 10f  f OPT - 10f  20f    The slave laser 1 and laser 2 are synchronized for different sidebands: upper sideband f OPT + 10f , and lower sideband f OPT - 10f ,   The frequency of output signal is equal to 20 f ..  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 .

RoFS-Pforzheim Optical Generation of μwaves M-Z Modulator Date  (f Si  B i ) f OPT f 0  f m fmfm Laser Nd:LiNbO 3 Laser inside microwave cavity Microwave Generator f OPT f 0   (f Pi  B i )  Laser on Nd:LiNbO 3 electrooptical material placed inside microwave cavity changes its frequency of optical oscillation. Optical transmitter with Nd:LiNbO3 laser with frequency modulated by Microwave Generator and with external Mach-Zehnder modulator

RoFS-Pforzheim Generation 3. Optical Generation of μwaves PD Amp f REF, P IN f OUT = n m f REF P OUT >> P IN VCO Amp x n Frequency Multiplier m Frequency Divider Phase Detector Complex and universal circuit for optical controlling of frequency from millimetre-wave region.  It is possible to transmit reference frequency f REF 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.

RoFS-Pforzheim Optical- μwave Mixing (1) RF P IN Coplanar Line Planar Optical Waveguide P OUT a) 0 VV C A B V b) T MAX T(V) V0V0  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.

RoFS-Pforzheim Optical- μwave Mixing (2) V 1,f 1 Fiber P OUT [W] Photodiode Laser P0P0 Combiner Filter V 2,f 2 V0V0 M-Z Modulator f 1, f 2, 2f1, 2f2, 2f 1 -f 2, 2f 2 +f 1, 2f 2 -f 1 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 V 2 plays role of a heterodyne and usually V 2 >> V 1.

RoFS-Pforzheim Examples (1) Optical link for transmitting the received signal to the base station. Data M-Z Modulator Laser Amp Data..... f 1, f 2,... f N f Fiber Remote Antenna Receiver at Base Station Amp

RoFS-Pforzheim Examples (2) Optical link for transmitting microwave signal to remote antenna. Data M-Z Modulator Laser Antenna Amp Data..... f 1, f 2,... f N f Fiber Transmitter at Base Station Receiver at Remote Antenna

RoFS-Pforzheim Examples (3) Fiber 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 Examples (4) Block diagram of the system which uses dense WDM f0f0 MUX M-Z Modul Optical Coupler & Filter  f IF1 LD1 1 f IF2 LD2 2 f IFN LDN N Base Station PD1 1 f0f0 f IF1 xN PD2 2 f0f0 f IF2    Transponder 1Transponder 2 xN   By using wavelength division multiplexing WDM techniques into the fiber access network each BS can be addressed by a different wavelength.

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

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

RoFS-Pforzheim Examples (60 GHz P-MP) 156 Mb/s DPSK Modem 60 GHz Trans- ceiver Base Station Central Station LD – Laser diode, EAM – Electro-absorption modulator, EDFA – Fiber amplifier, DWDM Mux – Multiplexer, PD Photodiode EAM EDFA PD LD LD λ1λ1λ1λ1 λ2λ2λ2λ2 DWDM Mux

RoFS-Pforzheim Examples (60 GHz P-MP) Base Station PD EAM 156 Mb/s DPSK Modem 60 GHz Trans- ceiver 156 Mb/s DPSK Modem 60 GHz Trans- ceiver λ2λ2λ2λ2 λ1λ1λ1λ1 EAM – Electro-absorption modulator, PD - Photodiode

RoFS-Pforzheim Examples ( 125 GHz/10 Gb/s ) PD EDFA EOM LD f M =62,5 GHz f OPT f0f0 fMfM 2f M EOM EDFA DATA 125 GHz Receiver DATA Terminal The last experimental system

RoFS-Pforzheim 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