Cellulases: Biosynthesis and applications

Publisher Summary This chapter discusses the sources of noise in receivers employing either p–i–n or avalanche photodiodes (APDs) and compares the sensitivities that can be obtained using these two device structures. The effects that the primary dark current and the ionization-rate ratio have on the sensitivity of an APD receiver are considered in the chapter. The chapter compares the performance of Germanium (Ge) and In0.53Ga0.47 As/InP devices and discusses several transmission experiments using APD and p–i–n receivers. The chapter concludes by considering several mechanisms that limit the transmission–system sensitivity at the highest bit rates presently envisioned. The most important mechanisms for degrading receiver performance at high bit rates are the APD gain-bandwidth response-time limit, transmitter bandwidth, laser noise, and fiber dispersion.

Chapter 4 Sensitivity of Avalanche Photodetector Receivers for High-Bit-Rate Long-Wavelength Optical Communication Systems

The key components of long wavelength monomode systems to operate in the 1300 and 1550 nm windows have been under study for some years to establish the best combination of operating parameters for intercity land and undersea use. Recently, two prototype systems have been assembled with 37 and 49 km repeater spacings, operating at 140 Mbits/s and at 1300 and 1500 nm, respectively. Higher bit rate systems are also under study.

Studies of monomode long wavelength fiber systems at the British Telecom Research Laboratories

For two commercial shallow-junction n+-p Ge APDs, it is shown empirically that the photocurrent multiplication and shot noise can be described over a wide range of wavelengths using expressions derived for single-carrier-type injection. Multiplied bulk leakage current and its associated shot noise may be similarly described, the closest agreement following from the use of photocurrent parameters applying at the longest wavelengths, ∼ 1.8μm. These empirical relationships simplify the estimation of optical receiver sensitivity using Ge APDs. For bit rates up to several hundred megabaud, the performance of Ge APD receivers is expected to be inferior to that demonstrated by the high-impedance InGaAs pin—FET hybrid, particularly where operation in the range 40–60° C is specified. The two alternatives become competitive at ∼ 1 Gbaud, but the bias voltage for the Ge APD must be stable within several hundred millivolts and must be compensated for variations in temperature. Where unbounded disparity coding systems are employed, a hybrid receiver comprising a pin photodiode with a transimpedance preamplifier should still outperform the Ge APD for bit rates up to several hundred megabaud.

Characterization and estimated performance of commercial n+-p Ge APDs for long-wavelength optical receivers

Dark current noise causes degradation in the signal-to-noise ratio (SNR) in optical receiver performance. The shot noise power induced by multiplied dark current was measured for germanium avalanche photodiodes (Ge-APD's). The noise power has a different excess noise factor F d from that for photocurrent multiplication noise; F d was M_{p}^{1.0} for a p+-n type Ge-APD and M_{p}^{0.9} for an n+-p type APD where M p is the photocurrent multiplication factor. The dark current noise has a strong frequency dependence at frequencies lower than 300 MHz. The temperature dependence of the noise was also measured.

Dark current noise characteristics and their temperature dependence in germanium avalanche photodiodes

Excess noise in four types of commercially obtained avalanche photodiodes (a.p.d.s) has been measured absolutely, by comparing avalanche noise from the a.p.d. with shot noise from an illuminated p-i-n diode. The method used yields directly the noise-current spectral density, simplifies the deduction of the quantum efficiency keff and hence the true value of the multiplication factor, and ultimately yields a measured value of the noise parameter x.

Absolute noise characterisation of avalanche photodiodes

Mixed carrier injection in a commercial n+-p germaniun a.p.d. causes the excess multiplication noise to be wavelength dependent, with averaged k-values of around unity. The quantum efficiency is 70% between 1.1 and 1.5 μm, and the unmultiplied bulk leakage current density is estimated to be 3.5×10-4 A cm-2

M.C. Brain

Papers

Absolute noise characterisation of avalanche photodiodes

Responsivity and noise characterisation of ge avalanche photodiode throughout wavelength range 1.1–1.7 μm