1. Introduction

Microwave photonic filters have been the subject of active research and interest in the last years since they bring the unique advantages of photonic devices and waveguides to the processing of radiofrequency, microwave and millimetre wave signals [1]–[7]. Besides their lightweight, electromagnetic immunity and potential broadband characteristics [2], these filters offer the possibility of fast electronically controllable transfer function tunability and reconfigurability, features not possible with current standard microwave technologies [1].

Potential full reconfigurability and tunability of these filters has been proposed by means of using an array of tunable lasers [3], however this configuration is of high cost since tunable sources are quite expensive. A low cost alternative to filter reconfigurability consists in combining fixed lasers or spectrum-sliced broadband sources [4–5] with variable optical attenuators. This solution however does not provide easy resonance tunability since the wavelengths of the optical taps are fixed. A dispersive delay line with variable dispersion is required, but experiments [6] have shown the difficulty of obtaining a ripple-free controllable group delay spectral characteristic and therefore a considerable increase in the filter noise floor and a limitation in the transfer function contrast ratio is unavoidable. Moreover, the above solutions have only been demonstrated for filters with positive coefficients.

In this paper we propose two configurations of microwave photonic transversal filters that employ low-cost sources (sliced or fixed laser diodes) and provide simultaneously the possibility of filter transfer function reconfigurability, tunability and the possibility of implementing negative coefficients. The structures are based on photonic components already available in the market.

2. Filter layout and description

The proposed filter layouts with the required photonic components and the experimental setup assembled for their measurement are shown in Fig. 1.

 

Fig. 1. Transversal filter configurations and experimental layouts

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The figure shows a first layout (configuration 1) based on the spectral slicing of a broadband source. The output from the source is fed to an AWG-based wavelength demultiplexer that performs the spectral slicing. The source spectral slices are then independently apodised by means of a variable optical attenuator array (VOAA). Each apodised slice is the passed though a 2×2 optical switch whose cross or bar state can be electronically reconfigured by means of an external computer. Depending on the sate of the switch the slice will be sent to the upper or the lower AWG-based wavelength multiplexer. In the first case it will arrive to the upper port of a 2×1 Mach Zehnder modulator (MZM) and will be modulated in-phase by the RF input signal, implementing a positive coefficient, whereas in the second it will be fed to the lower port of the 2×1 MZM and will be modulated by the RF input with a π phase shift, thus implementing a negative coefficient. The operation of the 2×1 MZM device and its ability to implement the π phase shift depending on the input port has been described in [8]. This structure allows maximum flexibility, since any filter sample can be independently apodised from the others and can take a positive or negative value also independently. The above guarantees filter reconfigurability and arbitrary positive and negative coefficients.

Filter tunability requires the possibility of implementing a variable delay T between adjacent wavelength samples. In this way the filter basic delay T and thus the filter resonances can be changed accordingly. Tunability can be easily achieved if tunable sources are used in combination with a dispersive fiber link as it has been shown in [3]. However, it is harder to attain when the wavelengths are fixed, since this requires the use of dispersive delay lines with variable and controllable dispersion [6]. An intermediate solution is to use switched delay lines as proposed in [7]. Although MEMs switches have been used in the past for the implementation of delay lines [10] here we propose the use of this component in the context of a microwave photonic filter which by suitable operation can implement tunability in discrete steps. This solution is efficient, since long delays can be switched to change the resonance position from one spectral band to another, whereas short delays can be switched (in addition to long delays) to change the resonance position within the allowed frequencies inside a given spectral band. The operation of the configuration based on fixed laser sources (configuration 2) is similar but a demultiplexer is saved since there is no need to slice the input optical spectrum before the RF modulation. Also, since the output power from the fixed laser sources is generally in the mW range there is no need to employ optical amplifiers inside the filter structure to compensate for device insertion losses. In the case of using a sliced source, the power of each spectral slice is much lower and therefore the use of a compensating optical amplifier is necessary.

3. Experimental results and discussion

Both layouts shown in Fig. 1 were assembled in the laboratory to implement a 9 tap transversal filter in the sliced configuration and a 7 tap filter in the fixed lasers configuration. The MUX/DEMUX devices were 1×40 Array Waveguide Grating devices from AVANEX featuring insertion losses of 3.5 dB. Up to 9 optical 2×2 spatial switches from JDS-Uniphase and the 8×8 MEMS OMM Inc. switch were electronically controlled via an RS-232 interface with a computer. Each 2×2 switch could be independently addressed featuring a maximum insertion loss of 1 dB and the internal connections of the 8×8 MEMS cross-connect were established via a specific software application. Three output ports of the device were connected to three different fiber coils with lengths given by 24, 5.8 and 24 km and feedback to three different inputs. Thus by proper internal switching we could establish dispersive delay lines of 24, 29.8 and 48 km. The insertion losses per pass were around 3.5 dB. In both configurations as well we employed a 2×1 balanced electrooptic Mach-Zehnder modulator, the characteristics of which were reported in [8]. The sample apodization was obtained by means of an electronically controlled variable attenuator array featuring 8 independent channels. An additional variable attenuator was added to have the possibility of changing the power of the 9 filter taps in the configuration based on the sliced source. The attenuation coefficients of each attenuator were electronically controlled via an RS-232 interface and could range from 0 to 20 dB. In both configurations the input RF signal (frequency swept sinusoid) was produced by an optical component analyser and the output RF signal was recovered by the same instrument after detection of the output optical signal. Both RF signals are clearly marked with red traces in the configurations of Fig. 1. The comparison of the input and output RF signals provided the desired filter transfer function. These were compared in each case with the theoretical results expected from the analytical formulas. The optical sources employed were the following: In the sliced source configuration a linearly polarized superluminiscent LED centred at 1550 nm and featuring a total output power of 7 mW and a linewidth in excess of 60 nm was used. In the fixed laser configuration, 7 semiconductor lasers providing 1 mW output power each were used. The lasers wavelengths were λ(1)=1543.180 nm, λ(2)=1544.060 nm, λ(3)=1544.820 nm, λ(4)=1545.560 nm, λ(5)=1546.400 nm, λ(6)=1547.200 nm and λ(7)=1548.000 nm which were compatible with the centre wavelengths of seven different outputs of the 1×40 AWG devices. Thus in both configurations the wavelength separation between consecutive filter taps was approximately 0.8 nm. For each filter configuration we measured the filter response for four different alternatives corresponding to mixing two different coefficient structures, uniform and apodised, and two different coefficient sign structures, all positive and alternating positive and negative. These measurements were carried out varying the length of the dispersive fiber coil. The following lengths were selected by selected by cross-connecting the MEMS switch: 24 km, 29.8 km and 48 km. The change between the first and the second illustrates a filter tuning to attain an in-band change of the resonance, while the change between the second and the third illustrates a filter tuning to attain a change of RF band. Figures 2, 3 and 4 show the results obtained for the case of using fixed lasers (7 filter taps) and different dispersive line lengths. In each figure the results for the four aforementioned configurations are depicted. The coefficient values were (in dB) [0, 0, 0, 0, 0, 0, 0] and [-1.5, -1, -0.5, 0, -0.5, -1,-1.5]. In all the cases the black trace corresponds to the measured result and the red traces correspond to the expected result from the theory.

 

Fig. 2. Transfer functions of a 7 tap transversal filter with a switched dispersive delay line of 24 km using fixed laser diodes

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Figure 2 shows the results when employing a dispersive fibre delay line of 24 km which corresponds to an incremental delay between wavelength samples of T=17psec/(km.mn)×24 kmX0.8 mn=32.6 nsec and therefore to a RF spectral period of 3.06 GHz. This can be appreciated in the transfer functions corresponding to the structures featuring positive only coefficients. The first filter resonance is at baseband and the second is placed at exactly the value of the spectral period. Note that for structures with negative coefficients the resonance at baseband disappears. Switching from a (uniform/apodised) positive configuration to a (uniform/apodised) negative configuration was as simple as changing the status of alternate 2×2 switches electronically. Changing from an uniform to an apodised configuration was as simple as electronically changing the values of the variable optical attenuators in the array. Note that in all the case there is an excellent agreement between the results predicted by the theory and the experiments. In Fig. 3 the 8×8 MEMS crossconnect is switched to obtain a delay line of 29.8 km and thus an incremental delay between wavelength samples of T=17psec/(km.mn)×29.8 kmX0.8 mn=40.5 nsec (RF spectral period 2.47 GHz). This illustrates for example the shift of the filter resonance within a given RF band. Switching higher delays results in considerable resonance displacement, which can correspond to tuning from one RF band to another. Fig. 4 shows the results when the 8×8 MEMS crossconnect is switched to obtain a delay line of 48 km and thus an incremental delay between wavelength samples of T=17psec/(km.mn)×48 kmX0.8 mn=65.2 nsec (RF spectral period 1.53 GHz).

 

Fig. 3. Transfer functions of a 7 tap transversal filter with a switched dispersive delay line of 29.8 km using fixed laser diodes

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Similar results have been obtained for the configuration based on the sliced broadband source. Here the number of spectral slices were 9 instead of 7 but the values of the uniform and apodised samples followed the same values of the former case. For instance, Fig. 5 shows similar results for the configuration employing a delay line of 24 km. Similar results to those of Figs. 3 and 4 were obtained for the configurations with switched delay lines of 29.8 and 48 km showing the possibility of tuning the filter resonance.

4. Summary and conclusions

We have proposed and experimentally demonstrate two configurations of photonic filters for the processing of microwave signals featuring tunability, reconfigurability and negative coefficients based on the use of low cost optical sources. The first option is a low power configuration based on spectral slicing of a broadband source. The second is a high power configuration based on fixed lasers. Tunability, reconfigurability and negative coefficients were achieved by means of a MEMS cross-connect, a variable optical attenuator array and simple 2×2 switches respectively. These filters are based on photonic components already available in the market. Only a low number of samples or taps (7 and 9) were demonstrated in the experiments and thus the transfer functions show limited Q factor. However higher filter selectivity and contrast ratios can be obtained by cascading these structures with other incoherent filter configurations. First experiments have shown promising results [9] and the topic is currently under investigation in our laboratory.

 

Fig. 4. Transfer functions of a 7 tap transversal filter with a switched dispersive delay line of 48 km using fixed laser diodes

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Fig. 5. Transfer functions of a 9 tap transversal filter with a switched dispersive delay line of 24 km using a sliced broadband source

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