8.2 Concepts of Link Power Penalties
331
rapidly. However, this results in a lower extinction ratio (the ratio of on-state power
to off-state power), which leads to an extinction-ratio power penalty at the receiver
because of a reduced signal-to-background noise ratio.
However, current systems operating above about 2.5 GB/s now use the external
modulation techniques described in Sect. 4.3.9. These external modulators are available in standard miniaturized electronic packages in which they are integrated
together with the laser diode.
8.2.7 Link Instabilities from Reflection Noise
When light travels through a fiber link, some optical power gets reflected at refractiveindex discontinuities such as in splices, couplers, and filters, or at air–glass interfaces
in connectors. The reflected signals can degrade transmitter and receiver performance. In high-speed systems, this reflected power causes optical feedback, which
can induce laser instabilities. These instabilities can show up as intensity noise (output
power fluctuations), jitter (pulse distortion), or phase noise in the laser, and they can
change its wavelength, linewidth, and threshold current. Because they reduce the
signal-to-noise ratio, these effects cause two types of power penalties in receiver
sensitivities. First, as shown in Fig. 8.12a, multiple reflection points set up an interferometric cavity that feeds power back into the laser cavity, thereby converting phase
noise into intensity noise. A second effect created by multiple optical paths is the
appearance of spurious signals arriving at the receiver with variable delays, thereby
causing intersymbol interference. Figure 8.12b illustrates this.
Unfortunately, these effects are signal-dependent, so that increasing the transmitted or received optical power does not improve the bit-error rate performance.
Thus one has to find ways to eliminate reflections. The first step is to look at their
magnitudes. As shown by Eq. (5.10), a cleaved silica-fiber end face in air typically
will reflect about
R =
1.47 − 1.00
1.47 + 1.00
2
= 3.6%
This corresponds to an optical return loss of 14.4 dB down from the incident
signal. Polishing the fiber ends can create a thin surface layer with an increased
refractive index of about 1.6. This increases the reflectance to 5.3% (a 12.7-dB
optical return loss). A further increase in the optical feedback level occurs when
the distance between multiple reflection points equals an integral number of halfwavelengths of the transmitted wavelength. In this case, all roundtrip distances equal
an integral number of in-phase wavelengths, so that constructive interference arises.
This quadruples the reflection to 14% or 8.5 dB for unpolished end faces and to over
22% (a 6.6-dB optical return loss) for polished end faces.
331
rapidly. However, this results in a lower extinction ratio (the ratio of on-state power
to off-state power), which leads to an extinction-ratio power penalty at the receiver
because of a reduced signal-to-background noise ratio.
However, current systems operating above about 2.5 GB/s now use the external
modulation techniques described in Sect. 4.3.9. These external modulators are available in standard miniaturized electronic packages in which they are integrated
together with the laser diode.
8.2.7 Link Instabilities from Reflection Noise
When light travels through a fiber link, some optical power gets reflected at refractiveindex discontinuities such as in splices, couplers, and filters, or at air–glass interfaces
in connectors. The reflected signals can degrade transmitter and receiver performance. In high-speed systems, this reflected power causes optical feedback, which
can induce laser instabilities. These instabilities can show up as intensity noise (output
power fluctuations), jitter (pulse distortion), or phase noise in the laser, and they can
change its wavelength, linewidth, and threshold current. Because they reduce the
signal-to-noise ratio, these effects cause two types of power penalties in receiver
sensitivities. First, as shown in Fig. 8.12a, multiple reflection points set up an interferometric cavity that feeds power back into the laser cavity, thereby converting phase
noise into intensity noise. A second effect created by multiple optical paths is the
appearance of spurious signals arriving at the receiver with variable delays, thereby
causing intersymbol interference. Figure 8.12b illustrates this.
Unfortunately, these effects are signal-dependent, so that increasing the transmitted or received optical power does not improve the bit-error rate performance.
Thus one has to find ways to eliminate reflections. The first step is to look at their
magnitudes. As shown by Eq. (5.10), a cleaved silica-fiber end face in air typically
will reflect about
R =
1.47 − 1.00
1.47 + 1.00
2
= 3.6%
This corresponds to an optical return loss of 14.4 dB down from the incident
signal. Polishing the fiber ends can create a thin surface layer with an increased
refractive index of about 1.6. This increases the reflectance to 5.3% (a 12.7-dB
optical return loss). A further increase in the optical feedback level occurs when
the distance between multiple reflection points equals an integral number of halfwavelengths of the transmitted wavelength. In this case, all roundtrip distances equal
an integral number of in-phase wavelengths, so that constructive interference arises.
This quadruples the reflection to 14% or 8.5 dB for unpolished end faces and to over
22% (a 6.6-dB optical return loss) for polished end faces.