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 x 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 photonic microwave 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 magnitudeandphase responseof thesix-tap FIRfilterhave been calculated using uniformly spaced taps generated with the coefficients shown in Fig. 19 (Lower right). To show reconfigurability 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 input intensity 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]. The
method 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.