BIBLIOGRAFIA CONSULTADA
14 5 ACOPIO, PROCESAMIENTO Y COMERCIALIZACIÓN
Wireless optical communication systems have attracted many researchers since the
early 1960s, when the optical communications ‘explosion’ effectively began. Recently,
with the rapid development of optical systems such as radio over fibre, that operate at
millimetre-wave frequencies, passive optical networks and SCM systems also seem to be
potential technologies for large scale implementation. The interest in this technology has
increased again, and become very important as we move towards being “always on” and
“always connected”. Optical communication systems use different optical detection
techniques; these techniques are used to convert the received optical signal into a signal in
the electronic domain, since this is the most appropriate domain for further signal
processing.
Optical detection can be classified according to two main techniques known as
Direct Detection Intensity Modulation (DD/IM) and Coherent Detection (CD). In DD/IM,
the term DD indicates that the receiver measures the optical power of the input signal,
where ‘intensity’ refers to optical power and ‘modulation’ refers to the electro-optical
conversion process. DD is easily obtained because the photodetector generates a current
proportional to the received optical power, as shown in figure 2.1a. Simply put, it is a
photon counting process where each detected photon may be converted into an electron-
hole pair. The DD receiver responds only to fluctuations in the power in the received field,
where the phase, frequency and polarization information are ignored; unless other optical
signal processing elements are employed, ahead of the photodetector device, such as an
interferometer or polarizers that makes the DD receiver more sensitive to the optical phase,
The Coherent Detection (CD) technique, which may potentially improve receiver
sensitivity, together with wavelength selectivity, compared with the DD technique,
generally limited by noise generated in the detector and pre-amplifier except at very high
SNR. Optical Coherent Detection (OCD) is based on the mixing of two optical waves prior
to the detecting with the implication to use a phase synchronous local oscillator, as shown
in figure 2.b; the weak incoming optical signal field is mixed with the strong local laser
signal (i.e. Local Oscillator LO), a third signal is generated at their frequency difference,
called intermediate frequency (IF), The photodetector responds as a square-law detector for
the electrical field, and generates a photocurrent; the resulting photocurrent is a replica of
the original signal, which is translated up or down in frequency from the optical domain to
the electrical domain for further signal processing and demodulation; this technique
showed an improvement in receiver sensitivity with more than a 20dB over DD[46]. This
technique was shown to work well in both free space optical communication and optical
fibre communication, and provided an increase in repeater spacing, improved
sensitivity/selectivity, increased the power budget and provided high transmission rates
over the existing route. The theory and coherence properties of signal detection by optical
mixing has been studied in detail in [47-49]; the EDFA is an example of this technique.
The Optical-Electrical Incoherent Heterodyne Detection (OEIHD) technique is
based on mixing the weak optical received signal (incident field) with a strong local
electrical signal (i.e. electrical local oscillator LO). This technique shares both the
properties of the DD and OCD techniques; the OEIHD receiver responds only to
fluctuations in the power in the received field as DD, which at the same time shares the
advantages of OCD with respect to frequency mixing and power flow transfer, as shown in
figure 2.1c. Photoparametric amplification is an example of this system. Although optical
optical wireless mobile systems is limited because of the required matching of the
wavefronts of the signal and local laser [50]. Conversely, OEIHD seems to be more
applicable to optical wireless communication, particularly for mobile terminal devices, as
its does not suffer from the matching issue.
Figure 2.1Optical detection techniques of (a) Direct Detection (DD); (b) Optical Coherent Beamspliter- (reflection mirror) Received-Field Ei(t) i- photo(t) Incident-Field Av=1 Vout(t) (b) Local Oscillator Laser Es(t) Elo(t) Photodetector fIF =nfLO±mfRF Ei(t) i- photo(t) Incident-Field Av=G Vout(t) Local Oscillator Electrical Elo(t) (c) fIF=nfLO±mfRF i- photo(t) Ei(t) Incident-Field Photodetector Vout(t) Av=1 (a) fRF
As mentioned in the previous chapter, the photo-detectors used in optical
communications that perform the best arePINandAPD. Although an electrically-pumped
APD with down-conversion optoelectronic mixing was practically demonstrated in [51,
52], the main drawback in the use of an APD is the very high reverse bias needed, as it is
shown to operate under 100 and 160 volts respectively ( i.e. not suitable for mobile
devices). Therefore, a photoparametric amplification technique (i.e. OEIHD) based on the
pn/pinstructure was chosen for further investigation.
A basic requirement in the design of a baseband optical wireless receiver is the
achievement of high sensitivity/selectivity, as well as a wide-bandwidth; a parametric
amplification (PA) technique was shown to work well in handling a low level received
signal with minimum degradation of SNR with a substantial conversion gain and frequency
conversion at the same time. A very low noise optical detection may be implemented at the
same time as frequency selectivity in a single junction PD, creating a new definition
known as the PPA. The concept of the PPA was inherited from the conventional electronic
PA, as mentioned before; both the amplifiers can be considered to be mixers (modulators),
in that the input signal causes variation in the energy flowing from the amplifier’s energy
source. The mixer must deliver more power to the output load than the input signal delivers
to the mixer if it is to provide useful gain. Both techniques are based onparametric effect
devices (i.e , capacitance or inductor) which use a nonlinear reactance impedance (i.e.
modulator) for amplification, frequency conversion, oscillations or harmonic generation at
MW frequencies [53-59]. It is necessary to review the microwave PA technique with a
brief discussion of the established Manley and Rowe theory. This can help to facilitate
comparisons of the PPA mechanism with the conventional PA used in microwave systems.
Considerable work has been undertaken to investigate the theory and practice of
by Howson and Smith [32] (i.e. considered as PA “Bible”). Parametric amplification has
been incorporated in many amplifier configurations, such as up-converters, down-
converters, or travelling-wave PAs. These amplifiers are based on nonlinear capacitance
(pure reactance), and not on nonlinear resistance, therefore avoids Johnson noise, resulting
in low noise amplifiers [60]. This low noise amplification technique has been widely used
as a front-end preamplifier for microwave ground receivers in the satellite link, and in
many radar applications, particularly with the new improvement in varactor diode
fabrication that leads to better receiver performance.
In later years, with the development of more powerful satellite transmitter and
beam forming techniques, the need for ultra-low noise-cooled parametric amplifiers has
diminished substantially [37]. Except in scientific and some radar applications, the demise
of PA in microwave receiver systems was assisted by an improvement in low noise GaAs
FET technology, such as MESFETs and HEMETs, that are adequate for these applications.
The complexity of the PA configuration contrasted sharply with the simplicity of the GaAs
FET approach (i.e. less complicated and required less maintenance).
In parallel with the idea of the GaAs FET technique, the application of three-
terminal transistor-based preamplifiers has been adapted to low-noise optical fibre receiver
amplifiers; many techniques have been developed with respect to bandwidth, gain and
sensitivity as mentioned in the previous chapter. However, when large bandwidths in such
applications as multiple channel television and broad-band ISDN, are considered, and also,
due to increased interest in optical and broadband wireless services, BISDIN and video in
demand (VOD), there is a real need for a suitable front-end receiver technique that
amplification occurrs at a second stage (pre-amplifier), as well as the signal selectivity,
based on the resistive/transistor based mixer (i.e. interface issues). Reverse biasedpin PDs
are mostly widely used as photo-detectors in broad-band receivers, as they reach few PF
under high reverse bias, and are capable of better optical efficiency and a better frequency
response; hence, it is interesting to investigate the performance of photoparametric
amplification based on pin structures. The two main sources of noise in the OW receiver:
are background noise (i.e. ambient light as in optical domain), and preamplifier noise (i.e.
thermal noise as in electrical domain); hence, photoparametric amplification seems to be
potentially attractive to simplify the optical front-end receiver using the parametric
amplification. The next section will review the conventional parametric amplifier
technique prior to the photoparametric technique.