2 3 4 5 6 7 8 9 10
Power Penalty, dB
ER, dB
Figure 1-17: Power Penalty As A Function of Extinction Ratio. (40296)
1.4.3 Modulation Approaches
The light in simple on-off keying modulation schemes is generally modulated in one of two ways. The first is direct modulation in which the laser current is modulated by the OOK signal to generate a modulated light stream. VCSELs and some DFB lasers are examples of directly-modulated lasers (DMLs). The second approach is to use a separate modulator to modulate the continuous wave (CW) laser. In general any laser can be used with an external modulator and thus become an indirectly modulated laser. One example is the electro-absorption modulated laser (EML) which is a DFB laser integrated with an electro-absorption modulator (EAM).
Both types of modulation have their advantages and disadvantages. Directly-modulated lasers are less complicated because a separate modulator is not required, but they suffer from dynamic effects on the emitted spectrum, such as changes in peak wavelength, spectral
bandwidth and the amplitude of the individual cavity modes. Also for higher speed operation, the lasers are not turned all the way off for the ‘0’ bit level resulting in lower ER and OMA. The indirectly-modulated lasers are more complicated due to the modulator but they operate at higher data rates (modulation above 100 Gbps is possible) and one can employ a CW laser with low relative intensity noise (RIN), such as a DFB laser, so that there is less noise in the light output.
Different types of modulators are discussed in more detail in Section 3.3.
1.5 Light Detection
1.5.1 Role of Photodetectors
The role of the photodetector is to convert an optical signal to an electrical signal. The three basic processes are 1) carrier generation by incident light, 2) carrier transport to electrical
A fourth process that is present in some devices is carrier multiplication by whatever current-gain mechanism may be present.
1.5.2 Photon Absorption
Carrier generation by incident light is essentially the reverse process of light emission in semiconductors. An incident photon with energy greater than the semiconductor bandgap energy is absorbed by moving an electron from the valence band to the conduction band and leaving behind a hole in the valence band as shown in Figure 1-18. Light with a wavelength shorter than the cutoff wavelength (= hc/ Eg = 1.24/ Eg) will be absorbed while longer wavelength light will pass through the semiconductor. Despite silicon’s indirect bandgap which prevents light
emission, it can be used as a photodetector. To conserve momentum, the electron being excited from the valence band to the conduction band interacts with the silicon atoms to gain the
momentum needed to reach the conduction band minimum. The probability of this process occurring is high enough to make a useful device but at reduced efficiency compared to direct bandgap semiconductors. Figure 1-19 shows the absorption coefficients for several materials commonly used to make photodetectors. The absorption coefficient (α) is used to calculate the penetration depth of light in the material using the Lambert-Beer law:
I=I0exp(-αx),
where I is the intensity in the material, I0 is the incident intensity and x is the depth. A higher absorption coefficient corresponds to higher absorption near the surface of the material. As can be seen in the figure, the absorption coefficient of GaAs is approximately 16 times that of Si at a wavelength of 850 nm.
Valence Band Conduction Band
Electron Energy Light
λ (µm) ≤ Eg hc
Eg(eV)
= 1.24 λ (µm) ≤
Eg hc
Eg(eV)
= 1.24
Figure 1-18: Illustration Showing Light Absorption In A Semiconductor. (40310)
600 800 1000 1200 1400 1600 1800
Absorption Coefficient, cm-1
Wavelength, nm
Ge GaAs InP Si
In0.53Ga0.47As 106
105
104
103
102
101
Figure 1-19: Absorption Coefficients of Several Photodetector Materials. (40335)
1.5.3 Photodiodes
The photo-generated carriers discussed in the previous subsection are transported to the contacts using the electric field in the depletion region of a reverse biased diode. Photodiodes are generally used because the electric field generated at the P-N junction in a reverse biased diode effectively moves the electrons and holes to the electrical contacts. Several photodiode characteristics are:
Quantum efficiency (η) = number of electro-hole pairs generated per photon,
Responsivity (r) [A/W] = ratio of the output current to the incident optical power (formerly referred to as sensitivity).
Electrons in the valence band can also gain enough thermal energy to move into the conduction band, resulting in an electron-hole pair without the presence of light. These charge carriers are then swept to the contacts by the electric field resulting in what is known as dark current. The dark current has a strong dependence on the temperature and also has an inverse dependence on the bandgap of the semiconductor. It is a performance limiting characteristic for the photodetector since small photon fluxes will be masked by the dark current. Variations on the photodiode, which will be discussed in the following paragraphs, are the PIN diode and the avalanche photodiode (APD).
1.5.4 PIN Diodes
The P-N junction depletion region of a diode has a relatively small volume for absorbing photons so an undoped or lightly doped layer, referred to as an intrinsic layer, is added between the P-type and N-type layers. Hence the name P-I-N diode. The equilibrium and reverse biased
increased. The P-type and N-type regions are made relatively small because photo-generated electrons and holes do not experience an electric field and are more likely to recombine with each other giving their energy to the semiconductor atoms as heat.
Electron Energy
Distance Conduction Band
Valence Band Electrons
Holes
Fermi Level
P-Type Intrinsic (Undoped) N-Type
Electron Energy
Distance
Electrons
Holes
Reverse Biased Zero Bias
Figure 1-20: PIN Diode Band Diagram In Equilibrium and Reverse Biased. (40311)
1.5.5 Avalanche Photodiodes (APDs)
The APD is similar to the PIN diode except it is operated at large reverse biases (30-300 V) so that it is near the reverse breakdown voltage. The large bias results in a high electric field in the junction region. Relatively thin n+ and p layers form the junction and the intrinsic layer is lightly doped p-type and is referred as the π region as shown in Figure 1-21. Electron/hole pairs are photo-generated in the π-region and the electric field causes the electrons to drift toward the junction. Here they are accelerated in the high electric field and acquire enough energy to dislodge additional electron-hole pairs in collisions with the semiconductor atoms in a process known as impact ionization. These new electrons and holes are then accelerated and cause more impact ionization and eventually result in an avalanche multiplication effect (photoelectric current gain) where large numbers of electrons and holes are generated. This amplification can be as high as 10 – 100 times and gives APDs a higher responsivity than PIN diodes. The multiplication factor has a strong dependence on temperature and voltage so these need to be stabilized. Dark current is also multiplied in these devices resulting in higher noise levels than PIN photodiodes.
Distance
Electron Energy
Conduction Band
Valence Band Avalanche Multiplication
p+ π (Lightly Doped) p n+
Figure 1-21: APD Band Diagram Under Reverse Bias. (40312)
1.5.6 Metal-Semiconductor-Metal (MSM) Photodiodes
Another type of photodiode is the Schottky-Barrier or Metal-Semiconductor-Metal (MSM) photodiode. In these photodiodes, one side of the P-N junction is replaced with a Schottky metal. The resulting band diagram is shown in Figure 1-22. Electron/hole pairs are photogenerated in the n-type semiconductor when photons are absorbed. The electrons reach the Schottky metal by thermionic emission over the barrier or tunneling through the barrier. These photodetectors have the potential to be faster than P-N junction based photodiodes but they have not found widespread use yet.
Distance
From: Low-Dimensional Semiconductors By M. J. Kelly From: Understanding Optical Communications
By H. J. R. Dutton
Figure 1-22: MSM Structure and Band Diagram. (40313)
1.5.7 Transimpedance Amplifiers (TIAs)
The output of the photodetector is basically a current source that needs to be converted to a voltage source before being amplified. This is accomplished using a transimpedance amplifier.
Figure 1-23 shows the block diagram incorporating an op amp and a feedback resistor. The figure also shows the schematic diagram for a differential implementation using bipolar transistors. For systems operating at high speeds (>20 Gbps), TIAs are typically made using GaAs, InP or SiGe technologies.
Iin
Figure 1-23: Block Diagram and Schematic Diagram of A TIA. (40314)