1 Topic Report Photodetector and CCD Tuan-Shu Ho

2 Introduction  Photodetector Photodiode:photon > electron Thermocouplesphoton > heat > electron  CCD (Charge-coupled device)  CMOS CCD cameraPhotodetector

3 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

4 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. QE of e2v EM4 CCD A/W of Thorlabs PDA36A

5 Noise of Photodiode  Thermal 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 ( W, APD)

6 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 ↑

7 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)

8 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

9 EMCCD  Andor iXon 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-

10 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-

11 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