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Active photonics devices - detectors Devices on optical tract that require a source of energy for their operation and have an output that is a function of present and past input signals. Examples include controlled power supplies, transistors, photodiodes, receiver modules. Copying and processing permitted for noncommercial purposes, on condition that proper reference to the source is given. © Sergiusz Patela, 2000-5 List of active photonic devices 1. Lasers and light emitting diodes 2. Detectors 3. Amplifiers 4. Modulators 5. Switches 6. Tunable filters + Passive devices Σ. Integrated optoelectronic circuits © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 2 Detector - definition Person or thing that detects. UV detectors, IR detectors, … telecommunications detectors, … Classification ! © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 3 Optical-to-electrical converters O/E converters: A device used to convert optical signals to electrical signals. Also known as OEC Optical Input © Sergiusz Patela 2000-5 O/E Conversion Photonics devices - Active photonics devices Electrical Output 4 Detectors – main classification Quantum detector A photodetector in which an electrical charge is produced when incident photons change electrons within the detecting material from nonconducting to conducting states. E.g. photoconductive detector; photovoltaic cell. Thermal detector The functional process includes absorption of infrared radiation, which causes a temperature change, consequently altering the physical properties of the detector's elements © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 5 Detectors sensitivity Thermal detector Ideal quantum detector 1 Sensitivity [a.u.] Actual quantum detector 0 0 200 400 600 800 1000 wavelength [nm] 1,24 E [eV ] = λ [µm] © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 6 Quantum efficiency Photocurrent is proportional to the power of incident light: I p = RPin where R - sensitivity [A/W] Quantum efficiency: Ip q number of generated electrons hν = = R η= number of incident photons Pin hν q sensitivity: R= ηq ηλ ≈ , hν 1,24 (ν = c / λ ) λ = [µm], q = 1,602*10-19 C, c = 3*108 m/s, h = 6,62*10-34 J.s © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 7 Absorption coefficient of semiconductor materials 10-1 105 GaAs GaAs 102 Si 101 In.53Ga.47As 103 101 0,4 © Sergiusz Patela 2000-5 0,6 0,8 1,0 1,2 1,4 wavelength [µm] Photonics devices - Active photonics devices 1,6 102 Penetration depth [µm] 100 104 In.7Ga.3As.64P.36 Absorption coefficient [cm-1] Ge 103 1,8 9 PIN photodiode Anti-reflection coating Fiber cladding Fiber core Incident light Metal contact (-) P+ region Intrinsic region Electron Hole Electron - Hole N+ region Metal contact (+) PIN photodiode is constructed from materials layered by their positive, intrinsic, and negative electron regions © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 10 APD photodiode Fiber cladding Fiber core Anti-reflection coating Incident light Metal contact (+) N+ region Electric field P region P+ region Metal contact (-) Avalanche Photodiode (APD): A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 11 Other types of semiconductor detectors Ring contact p+ p+ i i n+ π n+ contact n PIN APD (Avalanche Photo Diode) Semitransparent contact Au contact (n) Interdigital electrodes + - + - + - n n+ n+ SI (semi insulating) Schotky diode © Sergiusz Patela 2000-5 SI MSM Photonics devices - Active photonics devices 12 Photodiode parameters 1. Sensitivity 2. Spectral response 3. Capacitance 4. Dark current © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 13 Dark current Dark Current: The induced current that exists in a reversed-biased photodiode in the absence of incident optical power. Usually caused by the shunt resistance of the photodiode. A bias voltage across the diode (and the shunt resistance) causes current to flow in the absence of light. © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 14 Receiver Sensitivity Receiver Sensitivity: The minimum acceptable value of received power needed to achieve an acceptable BER or performance. It takes into account power penalties caused by use of a • transmitter with worst-case values of extinction ratio, jitter, pulse rise times and fall times, • optical return loss, • receiver connector degradations, and • measurement tolerances. © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 15 BER BER (Bit Error Rate): The fraction of bits transmitted that are received incorrectly. The bit error rate of a system can be estimated as follows: I2 MN BER = Q 4 N 0 B Where N0 = Noise power spectral density (A2/Hz). IMIN = Minimum effective signal amplitude (Amps). B = Bandwidth (Hz). Q(x) = Cumulative distribution function (Gaussian distribution). © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 16 Dispersion penalty Dispersion Penalty: The result of dispersion in which pulses and edges smear making it difficult for the receiver to distinguish between ones and zeros. This results in a loss of receiver sensitivity compared to a short fiber and measured in dB. The equations for calculating dispersion penalty are as follows: Where: ω = Laser spectral width (nm) Dλ = Fiber dispersion (ps/nm/km) τ = System dispersion (ps/km) f = Bandwidth-distance product of the fiber (Hz • km) L = Fiber length (km) FF = Fiber bandwidth (Hz) C = A constant equal to 0.5 FR = Receiver data rate (b/s) dBL = Dispersion penalty (dB) © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 17 Spectral characteristics of semiconductor photodiodes UV VIS NIR 0.7 Sensitivity [A/W] 0.6 0.5 0.4 Blue+ 0.3 0.2 Standard silicon photodiode UV+ 0.1 0 200 300 400 500 600 700 800 900 1000 1100 Wavelength [nm] © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 18 Detector sensitivity specified in dBm Diody PIN Power1E-7 Photomultiplier 500 nm, Quantum eff. 25% [W] 1E-8 APD 1E-9 SNR = 5 BER = 10-9 1E-10 1E-11 1E-12 1E-13 10k 100k 1M 10M 100M 1G Frequency [Hz] © Sergiusz Patela 2000-5 Photonics devices - Active photonics devices 19 Receiver module Receiver: A terminal device that includes a detector and signal processing electronics. It functions as an optical-to-electrical converter. RECEIVER TRANSMITTER Electrical interface Electrical interface • Video • Video • Audio • Audio • Data • Data Data Encoder/ Modulator Data Encoder/ Modulator • AM Optical Fiber • AM • FM • FM • Digital • Digital Light Emitter Light Detector • LED: 780nm, 850nm, 1300nm • PIN Diode • Laser: 1310nm, 1550nm, 1625nm • Materials: Si, Ge, In, GaAs Electrical to optical conversion © Sergiusz Patela 2000-5 User output(s) • APD Optical to electrical conversion Photonics devices - Active photonics devices 20