Topic Report Photodetector and CCD Tuan-Shu Ho Add a lambda^2 in the interpolation process Laser diode = 2:15

Introduction Photodetector CCD (Charge-coupled device) CMOS Photodiode: photon > electron Thermocouples photon > heat > electron CCD (Charge-coupled device) CMOS CCD camera Photodetector

Basic Principle of Photodiode Reverse biased PN or PIN diode Material (i.e. Si) absorbs photon, and generating electron (current) by photoelectric effect Avalanche photodiode (APD) with a built-in first stage of gain through avalanche multiplication photodiode Avalanche photodiodes

Basic Principle of CCD Composed with pixels Pixel: photoactive element Exposure: charge generated by photoelectric effect Photocharge is stored in the pixel before readout Readout Charge is moved by shift register Output to Charge amplifier EECCD

Feature of Photodetector Responsibility & Noise Bandwidth Responsibility and noise determine the signal to noise ratio (SNR). Bandwidth time limits the temporal resolution Tradeoff: SNR and temporal resolution

Responsibility Responsibility of a photodetector: electrical output per optical input A/W: ampere per watt Quantum efficiency: # of e- / # of photon Material dependent Silicon (<1100nm), Ge, InGaAs … etc. A/W of Thorlabs PDA36A QE of e2v EM4 CCD

Noise of Photodiode Thermal noise Electronic noise Thermal-generated electrons > dark current Electronic noise The noise generated in the circuit (i.e. amplifier) Higher responsibility reduces the effect of electronic noise to the signal Noise-equivalent power (NEP): It is defined as the optical signal power that gives a SNR of 1 in a 1Hz output bandwidth. State-of-the-art: ~1fW/Hz0.5 (10-15W/Hz0.5, APD) Avalanche Photodiode

Bandwidth of Photodetector Limit by RC time constant of circuit and charge collection time State-of-the-art: hundreds of GHz Gain-bandwidth product Bandwidth ↓ as gain ↑ NEP ↓ as gain ↑

Noise of CCD Operation of CCD Exposure Readout by shift register charge amplifier Saturation: the photo-charge is accumulated in each pixel between readout Well depth: How many e- can be stored in a pixel Readout noise: the amplifier noise Dynamic range: max. / min. optical power (noise) Typical: 30~40dB State-of-the-art: ~60dB (Andor EMCCD)

Speed of CCD The “bandwidth“ of CCD is limited by the readout time The time required to read a whole line (with N pixel, N= 256, 512, 1024, 2046, 4096 … etc.) The readout time ↑ as number of pixels ↑ The readout time ↑ as pixel size ↑ The larger the pixel size, the larger the well depth Typical pixel size: >10micron Typical: tens of ns/pixel > hundred of MHz Compare to hundred of GHz of photodiode

EMCCD Andor iXon3 897 EMCCD Pixel number: 512 x 512 Pixel size: 16 x 16 micron Frame rate: 4~40fps Digitization: 16bit Well depth: 800,000 e- Noise level: 4fps: 21 e- 20fps: 49 e- With Electron Multiplication ON: <1 e-

e2v EM4 Pixel number: 4096 x 1 Pixel size: 10 x 10 micron Line rate: <30kHz Digitization: 12bit Well depth: 117,500 e- Noise level: 43 e-

Summery The performance of photodiode is better than CCD. Larger the bandwidth Higher the saturation power, larger the dynamic range Noise CCD offers special features like: Detector array Application in imaging and spectroscopy