In addition, a multitap MWP filter approach comprising six
complex-valued coefficients has been reported in [12]. It is com-
posed of a multiwavelength source and its frequency response
has been shown to be continuously tunable within an FSR/5
range, limited by the available Stokes and pump power. On
the other hand, the use of slow light-induced tunable true time
delay for the realization of microwave filters has been limited
since the product between the maximum achievable group delay
and the signal bandwidth is essentially constant. Thus, only
modest group delays can be achieved for broadband baseband
signals. A theoretical solution was proposed to overcome this
limitation known as the separate carrier tuning (SCT) technique
[13]. The SCT technique predicts that significant improvement
for the generation of TTDs can be achieved by separately per-
forming an MWP phase shifter for the carrier and the RF side-
band. The above principle was experimentally demonstrated for
the first time by means of SBS processing [14], [15]. In [14],
a two-tap MWP filter was implemented in which the phase of
a single-sideband signal was modified using SBS and that of
the optical carrier was adjusted with a fiber Bragg grating. The
work was later extended to implement a 100 MHz wide SBS
slow light delay across a sideband that was centered at 6 GHz
[15]. In this contribution, the carrier phase was adjusted through
a second SBS process. The SBS-based SCT technique allowed
the implementation of a reconfigurable MWP notch filter. The
tunable TTD induced allowed a 20% tunability of the FSR of the
filter, and the optical carrier phase shifting enabled to control the
origin of the FSR variation, matching the reference frequency.
The above demonstrations require additional devices to gen-
erate the optical waves involved and rather high optical powers
to pump the nonlinear interaction. On the other hand, no sig-
nificant impact is produced in the noise figure as the Brillouin
amplification bandwidth is very narrow. In [18], an alternative
technique was introduced to obtain multitap complex-coeffi-
cient filters, which avoids the inherent complexities of using
SBS processing. The technique is based on optically processing
OSSB modulated signals and linear optical filtering instead of
a nonlinear optical effect. The cascade of phase-shifted fiber
Bragg gratings is employed for the implementation of a four-tap
MWP filter. By detuning the laser wavelengths, a continuously
tuning range of the filter of approximately FSR/4.36 can be
obtained.
An overview of the design and implementation of complex-
valued MWP FIR filters with nonuniform sampling is presented
in [19]. An arbitrary bandpass frequency response was demon-
strated with all-positive tap coefficients. The results showed
that the FIR filters having all-positive coefficients with nonuni-
formly spaced taps can achieve equivalently the functionalities
that require negative or complex tap coefficients. Using this con-
cept, a 50-tap bandpass filter with a flat top and a quadratic phase
response was designed and analyzed. In addition, a seven-tap
nonuniformly spaced photonic microwave filter with a flat top
and chirp-free bandpass response was experimentally demon-
strated. The key significance of this technique is that a nonuni-
formly spaced FIR filter can be used to perform advanced mi-
crowave signal processing.
Among others, the semiconductor waveguide has emerged
as one of the preferred technologies for the implementation
of MWP phase shifters due to its performance, maturity, the
possibility of both electrical and optical control and ease of
integration [20]. The demonstration of MWP true-time delay
lines [21] and phase shifters [22]–[24] has been performed
using SOA-based schemes. In particular, the cascade of func-
tional blocks, which are composed of an SOA followed by an
optical notch filter (ONF) [22], [23], has been presented by Xue
et al. as a suitable solution for implementing fully and contin-
uously tunable MWP phase shifters operating at bandwidths
comprising from few to several over 40 GHz [24], as illustrated
in Fig. 6. Tunability ranges exceeding radians for phase
shifters have been demonstrated, leading to the implementation
of filtering schemes involving fully adjustable complex-valued
coefficients. This fact enables the achievement of tuning ranges
over the filter FSR greater than 100%. The main drawback,
however, stems from the system complexity since five SOAs
and three ONFs are required [24]. Furthermore, a typical noise
figure degradation of 10 dB is observed despite the fact that
optical filtering is performed after each amplification stage. To
overcome these limitations, a customized approach involving
a single SOA followed by a tailored ONF, which accurately
exploits the combination of both slow and fast light effects,
has been recently demonstrated in order to implement a fully
and continuously tunable MWP phase shifter [25]. This con-
figuration has been employed to implement a complex-valued
coefficient based MWP filter [26]. In this case, phase shifts
have been accomplished in combination with the phase inver-
sion phenomenon that can occur in the EOMs [26]. These two
novel approaches pave the way toward the implementation of
SOA-based complex-valued coefficients with low complexity
and reduced power consumption.
A parallel-connected tandem composed of two EOMs, which
In addition, a multitap MWP filter approach comprising sixcomplex-valued coefficients has been reported in [12]. It is com-posed of a multiwavelength source and its frequency responsehas been shown to be continuously tunable within an FSR/5range, limited by the available Stokes and pump power. Onthe other hand, the use of slow light-induced tunable true timedelay for the realization of microwave filters has been limitedsince the product between the maximum achievable group delayand the signal bandwidth is essentially constant. Thus, onlymodest group delays can be achieved for broadband basebandsignals. A theoretical solution was proposed to overcome thislimitation known as the separate carrier tuning (SCT) technique[13]. The SCT technique predicts that significant improvementfor the generation of TTDs can be achieved by separately per-forming an MWP phase shifter for the carrier and the RF side-band. The above principle was experimentally demonstrated forthe first time by means of SBS processing [14], [15]. In [14],a two-tap MWP filter was implemented in which the phase ofa single-sideband signal was modified using SBS and that ofthe optical carrier was adjusted with a fiber Bragg grating. Thework was later extended to implement a 100 MHz wide SBSslow light delay across a sideband that was centered at 6 GHz[15]. In this contribution, the carrier phase was adjusted througha second SBS process. The SBS-based SCT technique allowedthe implementation of a reconfigurable MWP notch filter. Thetunable TTD induced allowed a 20% tunability of the FSR of thefilter, and the optical carrier phase shifting enabled to control theorigin of the FSR variation, matching the reference frequency.The above demonstrations require additional devices to gen-erate the optical waves involved and rather high optical powersto pump the nonlinear interaction. On the other hand, no sig-nificant impact is produced in the noise figure as the Brillouinamplification bandwidth is very narrow. In [18], an alternativetechnique was introduced to obtain multitap complex-coeffi-cient filters, which avoids the inherent complexities of usingSBS processing. The technique is based on optically processingOSSB modulated signals and linear optical filtering instead ofa nonlinear optical effect. The cascade of phase-shifted fiberBragg gratings is employed for the implementation of a four-tapMWP filter. By detuning the laser wavelengths, a continuouslytuning range of the filter of approximately FSR/4.36 can beobtained.An overview of the design and implementation of complex-valued MWP FIR filters with nonuniform sampling is presentedin [19]. An arbitrary bandpass frequency response was demon-strated with all-positive tap coefficients. The results showedthat the FIR filters having all-positive coefficients with nonuni-formly spaced taps can achieve equivalently the functionalitiesthat require negative or complex tap coefficients. Using this con-cept, a 50-tap bandpass filter with a flat top and a quadratic phaseresponse was designed and analyzed. In addition, a seven-tapnonuniformly spaced photonic microwave filter with a flat topand chirp-free bandpass response was experimentally demon-strated. The key significance of this technique is that a nonuni-formly spaced FIR filter can be used to perform advanced mi-crowave signal processing.Among others, the semiconductor waveguide has emergedas one of the preferred technologies for the implementationof MWP phase shifters due to its performance, maturity, thepossibility of both electrical and optical control and ease ofintegration [20]. The demonstration of MWP true-time delaylines [21] and phase shifters [22]–[24] has been performedusing SOA-based schemes. In particular, the cascade of func-tional blocks, which are composed of an SOA followed by anoptical notch filter (ONF) [22], [23], has been presented by Xueet al. as a suitable solution for implementing fully and contin-uously tunable MWP phase shifters operating at bandwidthscomprising from few to several over 40 GHz [24], as illustratedin Fig. 6. Tunability ranges exceeding radians for phaseshifters have been demonstrated, leading to the implementationof filtering schemes involving fully adjustable complex-valuedcoefficients. This fact enables the achievement of tuning rangesover the filter FSR greater than 100%. The main drawback,
however, stems from the system complexity since five SOAs
and three ONFs are required [24]. Furthermore, a typical noise
figure degradation of 10 dB is observed despite the fact that
optical filtering is performed after each amplification stage. To
overcome these limitations, a customized approach involving
a single SOA followed by a tailored ONF, which accurately
exploits the combination of both slow and fast light effects,
has been recently demonstrated in order to implement a fully
and continuously tunable MWP phase shifter [25]. This con-
figuration has been employed to implement a complex-valued
coefficient based MWP filter [26]. In this case, phase shifts
have been accomplished in combination with the phase inver-
sion phenomenon that can occur in the EOMs [26]. These two
novel approaches pave the way toward the implementation of
SOA-based complex-valued coefficients with low complexity
and reduced power consumption.
A parallel-connected tandem composed of two EOMs, which
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