Optical Fibres and Telecommunications Lecture 9 External Modulators and Detectors Introduction Where are we? A look at some real laser diodes. External modulators Mach-Zender Electro-absorption modulators Detectors Photodiodes PiN Structures Dark current and noise 1
Last time Fabry-perot laser structures Longditudinal modes Single mode laser operation. DBR lasers DFB lasers VCSEL s Modulating laser diodes Relaxation oscillations Chirp Real laser diodes 2
External modulators. Relaxation frequency limits bandwidth of directly modulated (DM) diodes. DM also chirps the output pulse chromatic aberration. Can be difficult to modulate the high currents needed for long distance links. Answer is to separate the modulation from the source. Adds more components (increased circuit complexity.) Allows laser diode to run CW with feedback for ultrastable operation. No power restriction on diode. Diode properties don t affect bandwidth. External Modulator Block Diagram Feedback Control Laser Diode CW Light External Modulator Modulated Output Current Source Interface Electronics Information 3
LiNbO 3 Mach-Zehnder Interferometer Modulator Contact Contact Mach-Zehender interferometer splits incoming pulse into two components. LiNbO 3 has a large electro-optic effect. If no voltage is applied to the contact, the two pulses interfere constructively at the output. To switch the pulse off, a voltage is applied at the contact. A change in refractive index gives a phase shift in the bottom pulse. Destructive interference occurs at the output arm. LiNbO 3 Modulators Operating speed: 10 s Gbit/s Typical loss 4dB may require amplification. Extinction ratio (difference between on and off) >20dB Operating voltage ~6V Maximum optical power ~100 s mw 4
Electro-Absorption Modulators Want to be able to include modulator on the same chip as the laser. LiNbO 3 based technology is not suitable for integration. Make use of the Franz-Keldysh effect in semiconductor waveguide. Shift of bandgap with applied voltage. When voltage is applied, bandgap shifts so diode wavelength is absorbed. Can be fabricated on the same substrate as the DFB laser. Capable of operating at bit rate ~100Gbit/s May provide the technology for many future communications applications. Real Modulators 5
Detectors After transmission, light must be detected Semiconductor based devices are the norm Photodiodes (PD) p-i-n Diodes Avalanche Photodiodes (APD) Noise in detectors Bit error rate measurements Receiver design Simple Photodiodes (PD s) p-type n-type Generated Current Very similar to LED s Light striking p-n junction creates electrons and holes This gives rise to a current Device operating in photovoltaic mode Add reverse bias to improve performance Photoconductive mode 6
PD Characteristics I p / ma Linear Regime Saturation effects R (A/W) P/mW In linear regime: I p =RP R Responsivity (A/W) Long λ E p <E g λ (nm) Short λ E p >> E g Charge carriers promoted to bands far above CB No recombination. PD Wavelength Response II I p = N e /t (Current = number of electrons flowing per unit time) P incident =(N p E p ) / t R= I p /P incident = (N e /N p )(λ/hc)=η(λ/hc) = (η/1248) x λ(nm) η = quantum efficiency P abs =P in (1-exp(-α abs w)) w = width of active region, α abs absorption coefficient. η = P abs /P in =1-exp(-α abs w) α abs is a function of wavelength giving the behaviour shown in the graph. 7
Variation of absorption coefficient with wavelength. PD Dark Current I p / ma I p / µa Dark Current Background P/mW P min P/µW Dark current is the current produced with no light Arises from thermal and other excitations Provides the minimum sensitivity of the PD 8
Biasing PD s Speeds up the process of getting the liberated charge carriers from the depletion zone. Photocarriers created away from the depletion region are slowed down reducing noise. Reduces recombination of created carriers. Controls the dark current of the PD. Need to keep the voltage low for practical devices P-i-N Devices Front Illuminated p-type i-layer n-type Back Illuminated p-type i-layer n-type n + -type In practice most detectors rely on P-i- N structure. A thick layer of undoped (intrinsic) material sandwiched between p and n type layers. Thick absorption layer means efficient device (most of the incident photons absorbed.) No free charge carriers in i-layer large electric field. Thermally generated carriers swept away. Reduced dark current. Lower reverse bias needed. Wide i-layer decreases bandwidth. 9
Summary Real laser diodes External modulation why? Detectors Photodiodes PiN Structures Dark current Biasing photodiodes 10