A novel photonic microwave notch filter with capability of frequency tuning is proposed and experimentally demonstrated. The scheme is based on a fiber Bragg grating (FBG)-based, single longitudinal mode, wavelength-spacing tunable dual-wavelength fiber laser and a dispersive fiber delay line. By using a symmetrical S-bending technique along the FBGs, the wavelength spacing of the laser can be tuned, which enables the microwave notch frequency tuning. Experimental results show that the notch rejection of more than 30 dB and the flexible tunability of notch frequency can be readily achieved in the range of 1.2 ~6.7 GHz.
© 2009 OSA
In the past decades, microwave photonics filters have attracted considerable attention due to their potential applications in radio-over-fiber systems, broadband wireless access, and sensor networks. The general concept behind microwave photonic filters is to replace the ordinary approach towards radio frequency (RF) signal processing with new techniques inherent to photonics such as low loss, high bandwidth, light weight, and immunity to electromagnetic interference [1–3]. Besides above advantages, these filters can be made tunable and reconfigurable, features not possible with traditional microwave technologies. Many approaches to implement microwave photonic filters have been reported recently [4–14].
In the practical implementation of photonic microwave filters, it is important how to get a set of optical carriers. The most common way is to use an array of lasers . The use of individual optical sources as filter taps allows for the flexible control of weighting of each filter tap and therefore the reconfiguration of the filter transfer function. However, the requirement of multiple lasers makes the practical applications difficult because of high cost. An alternative method to overcome the cost issue of arrayed tunable lasers is use of spectrum-sliced broadband light source such as light-emitting diode (LED) or erbium-doped fiber amplified spontaneous emission source . However, since the amplitude of sliced optical sources is fixed, it is very difficult to get a reconfiguration capability of transfer function .
In this work, we propose and experimentally demonstrate a novel photonic microwave notch filter that can provide flexible frequency tuning and reconfigurability. Unlike the previous demonstrations, we implemented a wavelength-spacing tunable dual wavelength ring laser by incorporating two FBGs as wavelength selection filters. Moreover, this dual wavelength laser adopts two sub-ring cavities coupled to a main ring cavity to ensure a single longitudinal mode at each lasing wavelength, resulting into improving the relative intensity noise (RIN) characteristics. Based on a symmetrical S-bending technique along the FBGs, we can continuously tune the wavelength-spacing, resulting into the change of a relative time delay at dispersive fiber, finally the notch filter frequency is changed. Owing to the above low cost, SOA-based gain and FBG filtering-based dual wavelength laser and simple frequency tuning mechanism, our proposed photonic microwave filter scheme is distinguished from a vast of previously demonstrated schemes.
2. Operation principle and experimental configuration
Figure 1 shows the configuration of the proposed photonic microwave notch filter. The optical source for the filter is a dual-wavelength fiber ring laser incorporating a semiconductor optical amplifier (SOA). The fiber ring laser is composed of three ring cavities. The main ring cavity is composed of a SOA, a circulator, 90/10 fiber coupler, two polarization controllers (PCs), and two FBGs with different Bragg wavelengths. A commercial 1 mm long SOA with antireflection coated (~10−4) on both facets (OPA-20-N-C-FA from Kamelian) serves as a gain medium. It has a small signal fiber to fiber gain of ~22 dB at 1550 nm with 0.6 dB polarization dependence gain and a maximum saturation power of 12.4 dBm at a bias current of 200 mA. Two FBGs are fabricated by placing a photosensitive fiber directly behind the phase mask in the field of the frequency-double Ar + laser beam (wavelength ~244 nm). Both FBGs have a 3-dB bandwidth of 0.16 nm, a reflectance of ~95% at Bragg wavelengths of 1549.73 nm and 1552.52 nm, respectively, and a length of 10 mm. The FBGs are mounted onto a thin metal plate whose length and thickness are 20 cm and 0.3 mm, respectively. Both ends of the metal plate are clamped on two metal supporters with four pivots . By rotating the movable pivots with the rotation stage, the thin metal plate is bent into an S-shape as shown in Fig. 1. Accordingly, tensile or compressive strain is induced on each FBG, respectively and is expressed as followsFigure 2 shows the calculated strain experienced by the two gratings as a function of rotation angle. For positive rotation angles, FBG1 and FBG2 experiences tensile and compressive strain, respectively, and vice versa. Also the wavelength spacing of two FBGs is given by
Sub-ring-1 and subring-2 are composed of 50:50 coupler with a different cavity length. The free spectral ranges of three ring cavities are 18, 310, and 635 MHz, respectively. Due to the Vernier effect in the coupled cavities, the single longitudinal mode operation at each lasing wavelength can be achieved . Two optical carriers from dual-wavelength fiber laser are externally modulated using a LiNbO3 electro-optic modulator (EOM) driven by a frequency swept microwave signal, which was generated from a network analyzer. The modulated optical signal was amplified by an EDFA and then launched into a 5 km single mode fiber which serves as a dispersive medium (D ~17 ps/nm/km). Due to the time delay between two optical carriers in the dispersive medium, the normalized output power P(f) at the photo detector can be written as Eq. (3), the free spectral range of the filter is
Figure 3 shows the theoretical spectrum of the notch filter, when the wavelength spacing of two optical carriers are 2 and 3 nm, respectively with 5 km delay line (Note that it corresponds to delay time are 170 and 250 ps, respectively). The theoretical results show that the rejection level of the filter could reach infinity. However, in the experiment, there is a rejection level degradation due to the unequal power of two carriers and noise of the fiber laser system.
3. Experiments and discussion
Figure 4(a) shows the measured optical spectra of the dual-wavelength fiber laser for three wavelength-spacing values when the driven current of the SOA is 124 mA. In the spectrum, we can observe two side modes, which come from four-wave mixing in the SOA . The side mode suppression ratio increases from 30 to 37 dB when the wavelength spacing increases from 1.3 to 4.0 nm. We carefully adjusted PCs to make the lasing power equally excited. However, there existed still about 1~2 dB power difference between two lasing carriers. To tune the wavelength spacing, we rotated the movable pivots with the rotation stage. In the experiment, the wavelength spacing was observed to increase from 0.9 to 4.9 nm as the rotation angle varied from −5° to 5° as shown in Fig. 4(b). These experimental results fairly agree with the values calculated from Eqs. (1) and (2).
Figure 5 shows the measured frequency responses of the microwave notch filter with a positive tap when employing a SMF with length of 5 km as a dispersive delay line for various rotation angles of the S-bending stage. From the experimental results, one can clearly see that the notch filter with frequency tuning capability is readily achieved by adjusting the rotation angle. For most cases, a rejection higher than 30 dB at the first notch is observed. In the experiment, the rejection level degradation may be caused by the power unbalance of two modulated optical carriers, polarization state distortions, and undesirable noise induced by long length of fiber .
When the rotation angle of the moving pivot was adjusted from −5° to 5°, the RF frequency response was also measured. The RF notch frequency was observed to be inversely proportional to the rotation angle as shown in Fig. 6 . It agrees well with the calculated results in the previous section. It is clearly seen that the notch frequency can be readily tuned by adjusting the rotation angle of the moving pivot in S-bending stage.
We have experimentally demonstrated a novel photonic microwave notch filter with a positive tap coefficient. It was based on wavelength spacing tunable, single longitudinal mode, dual wavelength fiber laser. The wavelength spacing tunable dual-wavelength laser was implemented by using two FBGs embedded onto a metal plate, where strain is added by the symmetrical S-bending technique. The proposed configuration provided a notch filter with more than 30 dB of rejection and tunability over the range of 1.2 to 6.7 GHz. Comparison between calculated and experimentally measured responses showed very good agreement. In addition, tunable multitap filters based on the same principle are possible if wavelength-spacing tunable multiwavelength laser is employed as the optical carrier . It has a potential application in some radio over fiber systems that require continuous notch frequency tuning.
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