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Introduction to Optoelectronics Optical communication (3) Optical components

Introduction to Optoelectronics Optical communication (3) Optical components. Prof. Katsuaki Sato. Optical components. Previous lectures (1) Optical fibers Transmission of light by total reflection (2) Laser diodes The pn-junction is forward biased Above threshold current lasing occurs

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Introduction to Optoelectronics Optical communication (3) Optical components

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  1. Introduction to OptoelectronicsOptical communication (3)Optical components Prof. Katsuaki Sato

  2. Optical components • Previous lectures • (1) Optical fibers • Transmission of light by total reflection • (2) Laser diodes • The pn-junction is forward biased • Above threshold current lasing occurs • High density of carriers and photons are confined in thin active layer (DH structure)

  3. Component (3)Optical detectors • Using photodiode • Very fast response required • pin photodiode or Schottky junction photodiode are used • As material for photodiode InGaAs semiconductors are used

  4. Optical signal Photocurrent Incident photon Reverse bias Electric signal Load resistance Bias voltage Depletion layer p n Structure and band diagram of photo-diode Photodiode 佐藤勝昭編著「応用物性」p.152

  5. Photodetection • Pin-PD • Schottky PD • Response is determined by capacitance of depletion layer where photocarrier flows • Thinning of depletion layer and reduction of junction area is necessary pin photodiode Schottky diode Andrew Davidson, Focused Research Inc. and Kathy Li Dessau, New Focus Inc.

  6. - - - - + + + + ｐ-type ｎ-type Depletion layer Fundamentals of photodiode • Illuminate the pn junction • Electrons and holes are generated by an excitation across the gap • Generated electrons and holes are separated and drift to electrodes by diffusion potential

  7. EDF Optical isolator Light input Composer Band pass filter Pumping laser Component ４Fiber amplifier • Light signal traveling in optical fiber for 100 km suffers 20 dB（1/100）attenuation. Therefore the light intensity should be recovered. Optical fiber amplifier is used for this purpose. • Optical amplifier consists of an erbium doped fiber (EDF) and a pumping laser. By introducing the strong pumping light to EDF the signal light can be amplified by stimulated emission from Er ion. Asahi Glass Company HPhttp://www.agc.co.jp/news/2000/0620.htmlより

  8. Amplification by Er ion • EDF absorbs light with wavelength of 980nm or1480nm and emits infrared light with wavelength of1530nm. Optical amplification is possible utilizing stimulated emission of the 1530-nm luminescence. • Inputting pumping laser light into EDF, Er ion become excited by absorbing the laser light and the signal light stimulate to make a transition to the ground level emitting the light with wavelength around 1530 nm, which is close to the signal light wavelength. Thus the incident light is amplified utilizing the emitted light. • Luminescent intensity and spectrum width differ from sample to sample according to the concentration of doped Er-ion. The broader the bandwidth of the emission band the broader the bandwidth of communication. From the Web-site of Asahi Glass Inc. HPhttp://www.agc.co.jp/news/2000/0620.html

  9. Component ５Optical isolator • Optical isolator is an optical component that makes the light direction oneway. • Operation of laser diodes (LD) and optical amplifiers (EDFA) become unstable and generate noise when returned light enters. • Optical isolator utilize Faraday effect to cut off the returned beam and stabilize the operation of lasers and amplifiers. Shinkosha http://www.shinkosha.com/products/optical/

  10. Polarization-dependent isolator analyzer Magnetic field Returned beam polarizer Faraday rotator Forward direction Incident light Reversed direction

  11. Faraday rotator F ½ waveplate C Birefringent plate B1 Birefringent plate B2 Fiber 1 Fiber 2 Forward direction B2 B1 C F Fiber 1 Fiber 2 Reverse direction Polarization-independent isolator

  12. Component ６WDM=wavelength division multiplexing • WDM technique can increase communication capacity by transmitting many different light signal of different wavelength simultaneously. • Fiber cables can utilize wavelength region from 1450 to1650nm since the transmission loss is very low (less than 0.3dB/km) in this region.

  13. Grating optical fiber Optical circulator Optical circulator Optical add-drop • Optical add-drop can separately drop desired wavelength from multiplexed-signal network or can add a particular wavelength to the network

  14. Optoelectronic integrated circuits (OEIC) • Integration of optical and electronic semiconductor devices • Two types of OEIC exist • One is integration of light emitting devices (example: LD) and driving FET circuits • The other is integration of optical detection device like PD and electronic circuits for amplification and signal processing • Compound semiconductors such as GaAs-based and InP-based alloy semiconductors are used. http://www2.nsknet.or.jp/~azuma/o/o0028.htm

  15. Faraday rotator Prism polarizer A Reflection prism Half wave plate Port 2 Port 1 Port 4 Port 3 Prism polarizer B Magneto-optical circulator

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