1 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Optoelectronic Oscillators for Communication Systems Bruno Romeira * and José M. L. Figueiredo Centro de Electrónica, Optoelectrónica e Telecomunicações, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 8005-139 Faro, Portugal *E-mail: [email protected] Collaborations: University of Glasgow, UK (Prof. Charles Ironside) University of Seville, Spain (Prof. José Quintana) Support: DoCEIS’10 Caparica – Lisboa 23/02/2010 2 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Contribution to technology innovation: Photonic RF Systems. Resonant Tunneling Diode Optoelectronic Oscillator (RTD-OEO). RTD-OEO optical-electro conversion. RTD-OEO electro-optical conversion. RF Photonics Interfaces for Telecommunications. RTD-OEO Self-Injection Locking. Conclusion and Future Work. Outline 3 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Optoelectronic oscillators for microwave photonics systems: novel technologies for pico/femtocellular transmission systems. Key elements of microwave photonics systems: optical sources and optical detectors or optically controlled microwave devices. Photo diode Laser Uplink RF signal Downlink RF signal Uplink optical signal Passive pico/femtocell Downlink optical signal Communication link using radio-over-fiber transmission Contribution to Technology Innovation 4 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Schematic of a Photonic System Oscillator Photonic RF systems involve RF signals in both optical and electrical domains and control of oscillator by both electrical and optical signals. X. Yao and L. Maleki, IEEE JQE 32 (7), 1141. OEO Functional Diagram Optical out P OUT Optical fiber Optical Coupler BIAS P IN Photodetector RF amplifier Electrical in Electrical out Fiber stretcher RF driving port RF coupler Filter Optical in “Classical” Optoelectronic Oscillators 5 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The optoelectronic oscillator (OEO) consists of a resonant tunneling diode (RTD) embedded in a integrated-optical waveguide (OW) and a laser diode (LD) and has both optical and electrical input and output ports. The RTD can operate as a voltage controlled oscillator (VCO) Novel Optoelectronic Oscillators (OEO) Schematic of RTD-OEO configuration 6 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Resonant Tunneling Diodes (RTDs) are nonlinear devices that use quantum effects to produce negative differential resistance (NDR). n InGaAlAs n InGaAlAs Emitter n+ InGaAs Substrate InP AlAs AlAs InGaAs Double-Barrier { n+ InP Collector DBQW-RTD Structure ~10 nm Conduction band profile 2 nm 2 nm 6 nm Collector 0.5 mm 0.4 mm RTD-OW die top view Resonant Tunneling Diode Electron transmission probability silica Ridge Waveguide gold Slope<0 (NDR) V I NDR P diss <0 (GAIN) RTD I-V characteristic 7 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The effect of bias on the conduction band profile. NDR R=1/G<0 Typical I-V characteristic Zero Bias (i) Off Resonance (iii) Resonance (ii) E 0 V=V v V=V p E F E F E F E C E C E C E F E C E F E C E C E F I V Conduction band profile under applied voltage Conduction band profile I p I v V p V v How does an RTD works? 8 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Operation as photo-detector (RTD-OWPD) Current voltage (I-V) characteristic (valley point) Energy-band diagram Light out Light in RF out RTD – Optical Waveguide (RTD-OW) Embedding an RTD within an optical waveguide core we obtain a monolithic integrated RTD photodetector. 9 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira R=1/G <0 slope<0 V I Nonlinear V Z + - I=GV I G: Conductance Transistors linear P diss <0 = GAIN Amp-ops 831 GHz Conventional electronic oscillators S. Susuki et al., Appl. Phys. Express 2 (2009) Novel RTD based solid-state electronic oscillators Voltage Controlled Oscillators 10 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira Combination of a RTD-OW and a laser diode (LD) in a optoelectronic circuit providing electrical-to-optical (E/O), optical-to-electrical (O/E), and optical-to-optical (O/O) conversion in the same circuit layout. LD Light out RF in Au wire RF out (to the PCB strip line) 0.5 mm DC current-voltage (I-V) characteristics Light in RTD-OW-LD top view RTD-OW Hybrid RTD-OEO Prototype 11 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The RTD relaxation oscillator undergoes repetitive switching between the lower and upper positive-differential-resistance (PDR) regions. Self-sustained relaxation oscillations 1/2 f LC π = Characteristic frequency: ~ 1.2 GHz RTD Optoelectronic VCO (1) L~5 nH C~3 pF RTD oscillator 12 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The optoelectronic voltage controlled oscillator (VCO) shows self- sustained GHz oscillations in the optical and electrical domains controlled by DC bias voltage. Electrical RF output power up to 1 mW at GHz frequencies. VCO Electrical and optical Spectra RTD Optoelectronic VCO (2) 13 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The optical control signal (λ=1530 nm - 1570 nm) was intensity modulated up to 12 dB extinction ratio using a 10 Gb/s external modulator. RTD-OEO Optical-Electro Conversion 14 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The I-V characteristic, RF injection locking capture and responsivity of an optical signal modulated at 1.2 GHz (~9.5 dB extinction ratio). RTD-OEO Response to Optical Signals Optical power level ~1 mW Optical power level ~1 mW and λ=1550 nm 14 dB 15 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The optical power was fixed at 2 mW (λ=1550 nm) and modulated at 1.2 GHz with an extinction ratio of 0.5 dB and 10.5 dB. Single Side Band (SSB) phase noise (SSB phase noise of RF reference source was -118 dBc/Hz) Spectra (RF electrical output) V b =2.537 V V b =2.530 V RTD-OEO Optical Injection Locking Locking @ 1.2 GHz - O/E Conversion 16 DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira The optical injection locking range was investigated for an optical signal λ=1550 nm varying the incident optical (in fiber) between 1 mW and 10 mW and fixing the extinction ratio at 5 dB and 10 dB. 16 0 0 2 = Δ P P Q f f inj Adler’s equation Δf – locking range f 0 – oscillator frequency P inj – power of injected signal P 0 – output power of oscillator Q – oscillator quality factor RTD-OEO Optical Injection Locking Locking range as a function of optical power level cold cavity bandwidth