Overview of Analog Links
- Elements of analog links are,
- Optical amplifier.
- Optical detector.
- The incoming information signal, speech, music video etc. is used to control the power output from the LED or the laser. The light output is as near as possible, a true copy of the electrical variations at the input. At the far end of the fiber, the receiver converts the light back to electrical pulses which is the true replica of input
- Any non-linearity either in transmitter or receiver will affect the accuracy of the transmission or reception of signal.
- The other problem is noise. Since the receiver received an analog signal, it must be sensitive to any changes in amplitude. Any random fluctuations in light level caused by light source, the fiber at the receiver will cause unwanted noise in the output
- Electrical noise due to lightening will give rise to electrical noise in the non-fiber parts of the
- As the signal travels along the fiber, it is To restore signal amplitude, amplifiers (repeaters) are added at regular intervals. The repeater has a limited ability to reduce noise and distortion present.
Carrier – to – Noise Ratio (CNR)
- Carrier – to – Noise Ratio (CNR) is defined as the ratio of r.m.s. carrier power to r.m.s. noise power at the
- CNR requirement can be relaxed by changing the modulation format from AM to FM. The BW of FM carrier is considerably larger (30 MHz in place of 4 MHz). The required CNR for FM receiver is much lower (16 dB compared to 50 dB in AM) because of FM advantage. As a result, the optical power needed at the receiver can be small as 10 µW. But the receiver noise of FM system is generally dominated by the thermal noise.
- The important signal impairments includes –
- Laser intensity noise fluctuations.
- Laser clipping
- Optical Amplifier Noise (ASE noise).
- To calculate carrier power signal generated by optical source is The optical source is a square law device and current flowing through optical source is sum of fixed bias current and a time varying current (analog signal).
- If the time-varying analog drive signal is s(t), then the instantaneous optical output power is given by,
Pt is optical output power at bias level,
M is modulation index
- The received carrier power C is given by,
is responsivity of photodetector. M is gain of photodetector.
P is average received optical power.
Photodetector and Preamplifier Noises
- Photodetector noise is given by,
Ip is primary photocurrent
ID is detector dark current.
M is gain of photodetector.
F(M) is noise figure.
B is bandwidth.
- Preamplifier noise is given by,
Req is equivalent resistance.
Ft is noise factor of preamplifier.
Relative Intensity Noise (RIN)
- The output of a semiconductor laser exhibits fluctuations in its intensity, phase and frequency even when the laser is biased at a constant current with negligible current The two fundamental noise mechanisms are
- Spontaneous emission and
- Electron-hole recombination (shot noise).
- Noise in semiconductor lasers is dominated by spontaneous Each spontaneously emitted photon adds to the coherent field a small field component whose phase is random, and thus deviate both amplitude and phase in random manner. The noise resulting from the random intensity fluctuations is called Relative Intensity Noise (RIN). The resulting mean-square noise current is given by,
- RIN is measured in dB/Hz. Its typical value DFB Lasers is ranging from -152 to -158 dB/Hz.
Reflection Effects on RIN
- The optical reflections generated within the systems are to be minimized. The reflected signals increases the RIN by 10 – 20 Fig. 6.1.2 shows the effect on RIN due to change in feedback power ratio.
- The feedback power ratio is the amount of optical power reflected back to the light output from source. The feedback power ratio must be less than -60 dB to maintain RIN value less than -140 dB/Hz.
- When optical power level at receiver is low, the preamplifier noise dominates the system
- The quantum noise of photodetector also dominates the system
- The reflection noise also dominates the system noise.
- The carrier-to-noise ratio for all three limiting conditions are shown in
- 6.1.3 shows carriers-to-noise ratio as a function of optical power level at the receiver with limiting factors. For low light levels, thermal noise is limiting factor causes 2 dB roll of in C/N for each 1 dB drop in received power. At intermediate levels, quantum noise is limiting, factor causing 1 dB drop in C/N for every 1 dB decrease in received optical power. At high received power source noise is dominator factor gives a constant C/N.
Multichannel Transmission Techniques
- Multiplexing technique is used to transmit multiple analog signals over the same higher capacity fiber cable.
- Number of baseband signals are superimposed on a set of N sub-carrier of frequencies f1, f2, f3 …fN.
- Channel or signal multiplexing can be done in the time or frequency domain through Time-Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM). The methods of multiplexing includes Vestigial Sideband Amplitude Modulation (VSB-AM), frequency Modulation (FM) and Sub-Carrier Multiplexing (SCM). All the schemes have different advantages and
Multichannel Amplitude Modulation
- In some applications the bit rate of each channel is relatively low but the number of channels are quite large. Typical example of such application is cable television (CATV). 6.1.4 shows the technique for combining N independent channels. Different channel information are amplitude modulated on different carrier frequencies.
- Power combiner sums all amplitude modulated carriers producing a composite FDM. The composite FDM signal is used to modulate the intensity of semiconductor laser directly by adding it to the bias At optical receiver, a bank of bandpass filters separates the individual carriers.
- Optical modulation index m is given by
N is no. of channels
mi is per channel modulation index
- Since the laser diode is a non-linear device and when multiple carrier frequencies pass through such device, the analog signal is distorted during its transmission, the distortion is referred to as intermodulation distortion (IMD). The IMD causes undersirable signals to produce called intermodulation product (IMP). The new frequencies (IMPs) are further classified as
- Two-tone IMPs and
- Triple-beat IMPs.
The classification is depending on whether two frequencies coinside or all three frequencies are distinct.
- The triple-bear IMPs tend to be a major source of distortion because of their large An N-channel system generates N (N – 1) (N – 2)/2 triple-beat terms compared with N (N – 1) two-tone terms. Depending on channel carrier spacing some of Imps fall within the bandwidth of a specific channel and affect the signal recovery. This is called as beat-stacking.
- The beat stacking result in two types of distortions, which adds power for all IMPs that fall within the passband of a specific channel, these distortions are:
- Composite Second Order (CSO) and
- Composite Triple Bear (CTB)
- CSO and CTB are used to describe the performance of multichannel An links. CSO and CTD are expressed in dBc units, where ‘c’ in dBc denotes normalization with respect to the carrier power. Typically, CSO and CTB distortion values shoud be below – 60 dBc for negligible impact on the system performance. Both CSO and CTB increases rapidly with increase in modulation index.
Multichannel Frequency Modulation
- The CNR requirement can be relaxed by changing the modulation format from AM to The BW of FM carrier is considerably larger (30 MHz in place of 4 MHz). This results in S/N ratio improvement over C/N ratio.
- S/N ratio at the output of FM detector is :
B is required bandwidth.
Δfpp is peak to peak frequency deviation of modulator. fv is highest video frequency.
W is weighing factor for white noise.
- The total S/N improvement is ranging between 36-44
Sub-Carrier Multiplexing (SCM)
- Sub-Carrier Multiplexing (SCM) is employed in microwave engineering in which multiple microwave carriers for transmission of multiple channels are If the microwave signal is transmitted optically by using optical fibers, the signal bandwidth can be exceeded up to 10 GHz for a single optical carrier. Such a scheme is referred to as SCM. Since multiplexing is done by using microwave sub-carrier rather than the optical carrier.
- The input can be analog or digital baseband signal. The input signals are modulated sub- carriers are then combined to give FDM signal. The FDM signals are then combined in microwave The combine signal is then modulates the intensity of semiconductor laser by adding it to bias current. Fig. 6.1.5 shows this arrangement.
- The received optical signal is then passed through low noise pin photodetector to convert it to original signal.
Advantages of SCM
- Flexibility and upgradability in design of broadband networks.
- Analog or digital modulation or combination of two for transmitting multiple voice, data and video signals to large number of users.
- Both AM and FM techniques can be used for
- A combination of SCM and WDM can realize DW upto 1
- SCM technique is also being explored for network management and performance