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318 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 3, FEBRUARY 1, 2009
Fig. 7. A dual-wavelength single-longitudinal-mode fiber ring laser for mi-
crowave generation. (a) Schematic of the laser, (b) The spectral response of the
two cascaded FBGs.
are low. Three dual-wavelength ultranarrow transmission-band
FBGs with wavelength spacing of 0.148, 0.33, and 0.053 nm
were incorporated into the laser cavity. Microwave signals at
18.68, 40.95, and 6.95 GHz were generated. The spectral width
of the generated microwave signals as small as 80 kHz with
frequency stability better than 1 MHz in the free-running mode
at room temperature was obtained.
III. A
LL-OPTICAL MICROWAVE SIGNAL PROCESSING
It was proposed by Wilner and Van den Heuvel [20] that op-
tical fibers could be utilized as delay lines for signal processing,
since optical fibers have low loss and large bandwidth.
In the last few years, extensive efforts have been directed
to the design and implementation of photonic microwave fil-
ters with different architectures to fulfill different functionali-
ties. Fig. 8 shows a diagram of a generic photonic microwave
delay-line filter with a finite impulse response (FIR). It consists
of a light source, a modulator, a delay-line module, and a pho-
todetector. The key device in a photonic microwave filter is the
optical delay-line module, which can be implemented using op-
tical couplers [21]–[23], FBGs [24]–[32], arrayed waveguide
(AWG) [33], [34], Mach–Zehnder lattices [35], or a length of
dispersive fiber [36], [37].
To avoid optical interference, most of the proposed filters
are operating in the incoherent regime. It is known a photonic
Fig. 8. A diagram showing a generic photonic microwave delay-line filter with
a finite impulse response.
Fig. 9. A photonic microwave delay-line filter with a negative coefficient using
differential detection. MZM: Mach–Zehnder modulator.
microwave delay-line filter operating in the incoherent regime
would have all-positive tap coefficients. Based on signal pro-
cessing theory, an all-positive-coefficient microwave delay-line
filter would only operate as a low-pass filter. To overcome this
limitation, considerable efforts have been taken to design and
implement a photonic microwave delay-line filter with negative
or complex tap coefficients, to achieve bandpass filtering func-
tionality in the incoherent regime. A comprehensive overview
of photonic microwave delay-line filters has been published re-
cently [38], [39]. In this Section, our discussion will be fo-
cused only on techniques for the implementation of photonic
microwave delay-line filters with negative and complex coeffi-
cients.
A. Photonic Microwave Delay-Line Filters With Negative
Coefficients
A straightforward way to generate a negative coefficient is
to use differential detection [22], [40]. As shown in Fig. 9, the
lightwave from an optical source is modulated by a microwave
signal, which is then time delayed by optical fiber delay lines
with a time delay difference of
. The output signals from
the fiber delay lines are fed to a differential photo-detection
module, which consists of two matched photodetectors with
the detected microwave signals combined and subtracted elec-
trically, leading to the generation of a positive and a negative
coefficient. In this approach, the negative coefficient was not
generated directly in the optical domain, the filter is not all-op-
tical, but hybrid. The two-tap photonic microwave delay-line
filter shown in Fig. 9 can be extended to have multiple taps if
the single-wavelength source is replaced by a multiwavelength
source and the 3-dB coupler is replaced by a WDM demulti-
plexer.
A few techniques have been proposed to implement an all-op-
tical photonic microwave delay-line filter with negative coef-
ficients. One approach to generating negative coefficients pro-
posed in 1997 is to use wavelength conversion based on cross-
gain modulation in an SOA [41]. As shown in Fig. 10, a tunable
laser source operating at
is modulated by an input microwave
signal, and then split into two parts. One part goes through a
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