of the bandpass can be programmed providing full amplitude
and wavelength tunability of the resonances that compose the
shaped spectrum and, therefore the time characteristics of the
burst signal. Some interesting figures of the chip are the following:
SOI technology with a footprint of 0.1 mm * 1.2 mm,
total optical loss of the PIC: 25 dB with a main contribution (10
dB) from each fiber to chip coupling interface. Tuning speed
(millisecond to microsecond range). The most salient feature of
this configuration is its versatility in terms of the burst shape
which can be reconfigured and the output central frequency
which can be easily tuned up to 60 GHz.
Another application field of interest is the photonic generation
of chirped microwave pulses featuring a broad frequency
operation range and high values of time-bandwidth product.
Several applications benefit from these features, such as spread
spectrum communications, pulsed compression radars or tomography
for medical imaging [94]–[96]. We can find an
approach to implementing microwave pulse compression using
a photonic microwave filter with a fiber Bragg grating with a
specific nonlinear phase response [94]. The key advantage of
this approach is that the system can be implemented using pure
fiber-optic components, which has the potential for integration.
Also, chirped microwave pulses can be generated using
nonuniformly spaced multiwavelength source and a length of
dispersive fiber [95] and employing broadband optical sources
with dispersive elements which second order dispersion is not
negligible [96].
MWP signal processors have been proposed by researchers
from Purdue University for the simultaneous waveform compression
and dispersion post-compensation of ultrawideband
antenna links [97] where, even, if the antenna gains are frequency
independent, the propagation medium is dispersive.
This has been achieved by using a RF photonic phase filter
shown in Fig. 18. Initially, the impulse response of the antenna
link was obtained using a 30 ps pulse [see Fig. 18(a)]. The
dispersed RF signal detected by the antenna was then amplified
by a broadband RF amplifier and then up-converted to optical
frequencies using a MZ-modulator. The output of the MZ-modulator
was the injected to an SLM [97]. Only the phase of the
optical signals was controlled, yielding a programmable MWP
phase filter that allowed the realization of any arbitrary phase
response. Fig. 18(b) shows the compressed voltage pulse with
duration of 65 ps full-width at half-maximum (FWHM) (blue
curve). The RF peak power gain was 5.52 dB. Assuming an
ideal photonic phase filter with no limitations the pulse could
be compressed up to 64 ps FWHM yielding a 8.67 dB of power
gain (red curve) [97].
To experimentally validate the phase filter in radar applications,
two propagation paths, line-of-sight and reflection from
a target, were implemented [see Fig. 18(a)]. The antennas were
placed separated by about 1 m and they were tilted upwards in
order to obtain two paths with equal transmission amplitude.
The target was implemented using a metal plate placed 19 cm
away from the line-of-sight path. The response from the two
paths is displayed in Fig. 18(c). As it can be seen it was not
precise enough to specify the presence of a target and its location.
Fig. 18(d) shows the link response after applying the MWP
phase filter. Two distinct pulses are clearly resolved, with separation
of about 223 ps, which was in reasonable agreement with
the calculated value [97].
A microwave bandpass differentiator is another key application
recently proposed. A version based on an FIR photonicmicrowave delay-line filter with nonuniformly spaced taps
has been reported and experimentally demonstrated [98].
Fig. 19(left) shows the ideal response of a differentiator, while
Fig. 19(a) displays the FIR of the required bandpass differentiator
using uniformly spaced taps and Fig. 19(b) shows the
FIR using nonuniformly spaced taps.
The six taps were generated using six wavelengths traveling
through a dispersive fiber. The spectrum of the six-wavelength
laser array is shown in Fig. 20(a). The measured magnitude and
phase responses of the differentiator are shown in Fig. 20(b) and
(c), respectively. The time delay has been chosen to obtain the
center frequency of the passband filter at 9.95 GHz. The simulated
magnitude and phase response of the six-tap FIR filter have
been calculated using uniformly spaced taps generated with the
coefficients shown in Fig. 19 (Lower right). To show recon-
figurability of the differentiator a similar implementation has
been done at the center frequency of 8.53 GHz. Fig. 20(d)–(f)
shows the taps and the magnitude and phase responses of the
differentiator.
A broadband incoherent light, previously filtered, is temporally
modulated by the microwave signal to be transformed. The
output is injected to a linear optical dispersive medium. In order
to achieve the desired impulse response, the output from the
dispersive medium is filtered with a dispersion-unbalanced optical
interferometer as shown in the upper part of Fig. 21. A
balanced detection scheme was used to cancel out the common
background light. To validate the concept a real-time Fourier
transformation (RTFT) of GHz-bandwidth microwave signals
was implemented [100], including a square-like waveform, a
sinusoidal pulse and a double pulse waveform.
Another example of application is connected to microwave
radars where the pulses are usually phase-coded and a matched
filter (correlator) is used to detect the signal. A nonuniformly
spaced MWP delay-line filter has been proposed in for this application
in [99].
An interesting field for the MWP application is the development
devices performing a real time operations over microwave
signals [100]–[103]. A first example is the Fourier transformation
(RTFT) in the microwave region [100].
The lower part of Fig. 21 shows the experimental results for
the RTFT of a sinusoid electrical pulse: The measured inputintensity waveform after light modulation, and the measured
intensity waveform at the output of the created microwave
dispersive filter. The measured waveform in full time scale
is also shown in the inset together with the numerically calculated
Fourier transform amplitude of the measured input
time waveform. A second example is a method for ultrafast
photonic time-intensity integration of an arbitrary microwave
temporal waveform which has been demonstrated [101]. Themethod is based on the superposition of mutually incoherent,
continuously time-delayed replicas of the optical intensity
waveform to be processed.
Finally, in connection to telecommunication applications, it is
worth noticing a transceiver design based on the adaptation of
a selective MWP filter by the incorporation of a tunable interferometric
structure between the light source and a phase modulator.
This transceiver offers the possibility to transmit and select
SCM electrical signals in a frequency range established by
the phase-to-intensity conversion response [104].
VII. SUMMARY, CONCLUSIONS, AND FUTURE DIRECTIONS
We have presented and reviewed the most significant advances
in the field of MWP signal processing developed during
the last years. Special attention has been paid to describe the
novel techniques developed in order to overcome the major
limitations of these subsystems under incoherent regime. The
new and emergent field of integrated MWP filters has been
presented and though still in its infancy, it is clearly an exciting
future direction of research. The considerable development of
this field during the period under review has crystalized as well
in a number of novel application fields, both in the time and
the frequency domain, some of which have been presented and
reviewed as well. Next years will for sure witness novel efforts
directed towards the implementation of a fully integrated
MWP signal processor on a chipset. Novel materials and the
exploitation of nonlinear effects in integrated waveguides will
most probably enable future developments. This will, no doubt,
open the way to new and cost-effective subsystems capable of
supporting current application needs as well as future ones yet
to be unveiled.