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A novel high-order FIR photonic signal processor
Optics Communications 273 (2007) 339–343 www.elsevier.com/locate/optcom
A novel high-order FIR photonic signal processor Ningsi You *, Robert A. Minasian School of Electrical and Information Engineering, University of Sydney, NSW 2006, Australia Received 10 October 2006; received in revised form 9 January 2007; accepted 15 January 2007
Abstract A new topology for realizing a high-order FIR RF photonic filter is presented. It is based on a novel structure that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal to be obtained through a single photonic subfilter. The sub-filter can be of low order, however the routing of the output of the sub-filter back to itself several times together with some additional coefficients, realizes a high-order filter. Experimental results are presented which demonstrate a high-order FIR photonic filter realization comprising 39 taps. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Photonic signal processing is attractive due to its high time-bandwidth capability and immunity to electromagnetic interference, and its potential to solve the limitations of electronic approaches. Moreover it offers direct signal processing in the optical domain. Finite impulse response (FIR) microwave photonic filters are of particular interest because of their flexibility [1–6]. The advantages of FIR type filters include that they are inherently stable, they can realize a wide range of filter specifications, they can be designed with linear phase, and they can generate flat-top filter characteristics. However, their main disadvantage is that in order to achieve high resolution, they require a large number of taps. This is an inherent issue, which means that a large amount of hardware is required, and which makes their implementation very costly. The object of this paper is to report a new topology for realizing a high-order FIR RF photonic filter. The concept is based on using identical sub-filters as a fundamental
* Corresponding author. Address: Bitline System, Pty. Ltd., Bexley, NSW 2207, Australia. Tel.: +61 2 9597 2689; fax: +61 2 9597 1959. E-mail addresses: [email protected] (N. You), [email protected]. edu.au (R.A. Minasian).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.01.045
building block, together with a structure that enables a multiple-pass transmission of the signal through the same sub-filter. The multiple-pass transmission is achieved through a new wavelength conversion and routing technique. Experimental results are presented which demonstrate a high-order FIR filter realization comprising 39-taps. This paper is organized as follows. Section 2 presents the principles of the new high-order photonic signal processor structure and its analysis. Section 3 presents the design. Experimental results which demonstrate the ability of this new structure to generate a large number of taps, is described in Section 4. Finally, conclusions are presented in Section 4. 2. High-order photonic signal processor principles and structure We commence with the technique of interconnecting a number of identical sub-filters with the aid of a few additional adders and multipliers, which is known in electronic FIR filter design [7]. Fig. 1 shows a general structure for implementing an FIR filter as a tapped cascaded interconnection of N identical sub-filters. The sub-filter has a transfer function FM(z), and forms the basic building block for the entire FIR filter.
340
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
IN (z) FMF(z) M
a[0]
a[1]
FM(z) a[2]
a[N-1] .
a[N]
OUT Z-M
Z-M
+
+
+
+
Z-M
Fig. 1. General structure for implementing an FIR filter as a tapped cascaded interconnection of N identical sub-filters.
The transfer function of the entire structure is [7] H ðxÞ ¼
N X
a½n½F M ðxÞ
n
ð1Þ
n¼0
where FM(z) is the sub-filter transfer function that is given by F M ðzÞ ¼
2M X
f ½nzn
ð2Þ
n¼0
H(x) can be obtained from the polynomial P ðxÞ ¼
N X
a½nxn
ð3Þ
n¼0
using the substitution in (1) x ¼ F M ðxÞ
ð4Þ
The additional tap coefficients a[n] can be determined so as to meet the required filter characteristics. It can be noted that the passband and stopband regions of the sub-filter FM(z) and H(x) are the same, and all that happens is that the multiple use of the same sub-filter reduces the large passband and stopband variations in FM(z) to small variations in H(x) [7]. Unfortunately, in the optical domain, such a cascade interconnection of sub-filters as shown in Fig. 1 for the electronic version, is not possible because it would cause coherent optical interference problems. Hence, we propose a new topology, shown in Fig. 2, for a photonic processor, which can overcome this problem. The key idea in this novel structure is to use wavelength conversion and a rout-
Subfilter
Coupler
λ1
Wavelength Converters
Amplifiers Attenuators λ2 Z-M
The design procedure for FIR filters using identical subfilters as building blocks is described in [7], and the reader is referred to the reference. As an example, a filter was designed having a flat-top response, a good shape factor, and a high stopband attenuation, with a passband frequency of 400 MHz. The sub-filter was chosen to be of low order for ease of implementation, and had a 13-tap response. Fig. 3 shows the
0.16
WDM
FM(z)
OUT
3. Design
Subfilter coefficient
INPUT WDM
ing technique to enable a multiple-pass transmission of the signal to be obtained through a single sub-filter. In Fig. 2, the sub-filter FM is implemented using a number of optical delay lines. The modulated input at wavelength k1 is passed through the WDM MUX and enters the sub-filter where it is processed and then exits through a WDM DEMUX. Here it passes through a wavelength converter that changes its wavelength to k2 and is looped back through the WDM MUX so that it passes through the sub-filter again. This process is repeated N times using the WDM MUX and DEMUX to select and feed optical signals with different wavelengths into the sub-filter. Together with the wavelength converters that produce N wavelengths in total, the sub-filter is thus reused N times. Note that after each pass the output corresponding to each wavelength is further weighted with an amplifier or attenuator, delayed (zM) with optical delay lines, and combined to form the final output. By means of this approach, it is possible to choose a low order for the sub-filter, so that it is practical to implement. Moreover, since a sub-filter implemented using optical components and fibre delay lines has a wide transmitting window, a large number of wavelengths can be multiplexed using WDM into the same sub-filter simultaneously. Therefore, we only need to construct a single sub-filter, while using wavelength conversion and WDM techniques to route the output of the sub-filter back to itself for a number of times to realize a high-order filter.
Coupler
λN
0.12 0.1 0.08 0.06 0.04 0.02
Z-M Fiber Delay-Lines
0.14 Impulse response (linear)
FM(z)
Z-M
Fig. 2. New photonic processor topology that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal through a single sub-filter.
0 1
2
3
4
5
6 7 8 9 coefficient index
10
11
12
Fig. 3. The sub-filter impulse response in the time domain.
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N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
341
Fig. 4. The sub-filter response in the frequency domain.
Fig. 6. The overall frequency response of the proposed processor.
designed sub-filter response in the time domain, and Fig. 4 shows the simulated sub-filter response in the frequency domain. Note that the taps in Fig. 3 are all positive as is desirable for practical optical implementation, hence the sub-filter itself cannot produce a flat-top filter response, as is evident in Fig. 4. Also, because the sub-filter has a low order of only 13 taps, it cannot produce is a narrow passband, which again is evident in Fig. 4. The polynomial P[x] was designed using the procedure in [7] to be a high pass filter with a flat passband at x = 1 and a deep notch at x = 0.
Hence, only two coefficients, a[2] and a[3] needed to be implemented in the experimental setup. Finally, Fig. 6 shows the simulated overall frequency response of the complete structure of Fig. 2 using only three passes of the sub-filter, i.e. with the original input wavelength plus two wavelength conversions, together with the two coefficients. This corresponds to a 39-tap filter response. Comparison of Fig. 6 for the overall filter with Fig. 4 for the sub-filter, shows that the overall filter can realize a flattop response, with a much improved shape factor and also with a greatly increased out-of-band suppression.
P ðxÞ ¼
N X
a½nxn 4. Experimental results
n¼0
¼ 6:40 10
6
2
þ 0:0176x þ 3:110x 2:128x
3
ð5Þ
Fig. 5 shows designed transfer function of this intermediate function P[x]. In order to simplify the system further, for subsequent experimental implementation, the coefficients a[0] and a[1] in (5) were taken to be zero, which is a very good approximation since their values are very small.
In order to verify the proof of principle, the new processor structure was experimentally implemented. Fig. 7 shows the experimental setup. The input optical signal at k1, which is modulated with RF signals to be processed, is coupled into the processor through a WDM multiplexer. The input then passes
WDM
P[x]
WDM
10
Response (dB)
-0.2
λ2
FM(z)
0 0.3
0.8
1.3
-10
λ3
1.8 λ1
Input λ1
-20
λ2 Coupler
-30
SOA
-40
SOA
Laser Source
Laser Source
λ3
-50 Z-M
-60 x (linear)
Fig. 5. The transfer function of the intermediate function P[x].
Output
Fig. 7. Experimental setup of the new processor structure.
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N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
through the sub-filter. At the output of the sub-filter, another WDM demultiplexer is used to separate the wavelengths into different loops. The signal at k1 is first coupled into a semiconductor optical amplifier (SOA) that is configured to give wavelength conversion through cross-gain modulation (XGM). The RF signal on k1 is converted to k2 and is coupled back to pass through the sub-filter again. Then the signal at k2 is converted to k3 using another SOA, and k3 is also fed back into the sub-filter again to produce the third pass. The final output of the processor is obtained by combining the outputs of k2 and k3. By correctly setting the power levels of these wavelengths and the driving current of the SOAs, different RF gain values can be achieved through the wavelength conversion, and this sets the required coefficient weights. Therefore, no additional optical attenuators or amplifiers are required for the coefficient weighting. Also, since XGM inverts the signs of the RF signal, the opposite signs of a[2] and a[3] are naturally implemented. The wavelengths used in the experiment were k1 ¼ 1556:9 nm, k2 ¼ 1559:3 nm, and k3 ¼ 1548:2 nm. These wavelengths were chosen so that they corresponded to the channel wavelength and range of the WDM MUX and DEMUX. The sub-filter was constructed using the direct form. Because of the symmetric nature of the impulse response of the sub-filter, only half the number of optical weights is required. In the experimental setup, variable optical attenuators (VOA) were used to set the tap coefficient weights. A two-tap delay line was used after each VOA to generate and locate the two taps at the symmetric time locations. Star couplers were used to split and combine the optical power at both ends of the sub-filter. Fig. 8 shows the experimental setup of the sub-filter. The measured frequency response of the sub-filter is shown in Fig. 9. This is similar to simulated response shown in Fig. 4, however there are some differences including a somewhat asymmetric response, which arose to errors in setting the tap weights and delays. Finally, the measured overall frequency response of the filter is shown in Fig. 10. This is broadly similar to the simulated response shown in Fig. 6. However there are some differences, primarily in the lower stopband suppression level obtained in the measurements. This is believed to be caused by the reduction in signal response level because Star Coupler
Delay Lines
Star Coupler
Input
Output VOA
Fig. 8. The experimental setup of the sub-filter.
Fig. 9. Measured frequency response of the sub-filter.
Fig. 10. Measured overall frequency response of the processor.
the sub-filter loss was high and also the wavelength conversion efficiency was not optimized, coupled with the high experimental noise floor due to some phase induced intensity noise in the sub-filter and also ASE noise. Also the asymmetric response of the sub-filter directly affected the response of the following stages and reduced the improvement of the rejection. Nevertheless, the measured results demonstrate that in comparison to the sub-filter, a flattop filter response has been achieved, the filter shape has been significantly improved, the transition band of the filter is considerably reduced, and the rejection level in the stopband has been increased by around 15 dB. The experimental implementation passed the signal three times through the sub-filter, which had 13 taps. Hence, effectively the overall filter has 39 taps. It can be pointed out that the real power of this new structure becomes evident in its ability to realize an even greater number of taps if more passes through the sub-filter are used, though ultimately this will be limited by noise build up. Nevertheless, increasing the number of passes through the sub-filter can significantly increase the order of the overall filter, leading to continuous improvement in the frequency response, reduction of the transition band, increase
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
of the stopband rejection, and improvement of the filter shape factor. 5. Conclusion A new topology for realizing a high-order FIR RF photonic filter has been presented. It is based on a novel structure that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal to be obtained through a single photonic sub-filter. Hence a single low order sub-filter can be used in conjunction with wavelength conversion and WDM techniques to route the output of the sub-filter back to itself several times to realize a high-order filter. Experimental results for a three-pass implementation have demonstrated a high-order FIR filter realization comprising 39-taps. Moreover, this new structure has the ability to realize an even greater number of taps if more passes through the sub-filter are used. This offers the prospect of high resolution microwave photonic signal processors.
343
Acknowledgement This work was supported by the Australian Research Council. References [1] J. Capmany, D. Pastor, B. Ortega, IEEE Trans. Microw. Theory 47 (July) (1999) 1321. [2] J.L. Chen, R.A. Minasian, IEEE Photon. Technol. Lett. 17 (4) (2005) 896. [3] A.P. Foord, P.A. Davies, P.A. Greenhalgh, Electron. Lett. 32 (February) (1996) 390. [4] J. Capmany, D. Pastor, B. Ortega, Electron. Lett. 35 (March) (1999) 494. [5] B.A.L. Gwandu, W. Zhang, J.A.R. Williams, Electron. Lett. 38 (October) (2002) 1328. [6] D.B. Hunter, R.A. Minasian, IEEE Microw. Guide. Wave Lett. 6 (February) (1996) 103. [7] T. Saramaki, in: S.K. Mitra, J.F. Kaiser (Eds.), Handbook for Digital Signal Processing, Wiley, New York, 1993.
A novel high-order FIR photonic signal processor Ningsi You *, Robert A. Minasian School of Electrical and Information Engineering, University of Sydney, NSW 2006, Australia Received 10 October 2006; received in revised form 9 January 2007; accepted 15 January 2007
Abstract A new topology for realizing a high-order FIR RF photonic filter is presented. It is based on a novel structure that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal to be obtained through a single photonic subfilter. The sub-filter can be of low order, however the routing of the output of the sub-filter back to itself several times together with some additional coefficients, realizes a high-order filter. Experimental results are presented which demonstrate a high-order FIR photonic filter realization comprising 39 taps. Ó 2007 Elsevier B.V. All rights reserved.
1. Introduction Photonic signal processing is attractive due to its high time-bandwidth capability and immunity to electromagnetic interference, and its potential to solve the limitations of electronic approaches. Moreover it offers direct signal processing in the optical domain. Finite impulse response (FIR) microwave photonic filters are of particular interest because of their flexibility [1–6]. The advantages of FIR type filters include that they are inherently stable, they can realize a wide range of filter specifications, they can be designed with linear phase, and they can generate flat-top filter characteristics. However, their main disadvantage is that in order to achieve high resolution, they require a large number of taps. This is an inherent issue, which means that a large amount of hardware is required, and which makes their implementation very costly. The object of this paper is to report a new topology for realizing a high-order FIR RF photonic filter. The concept is based on using identical sub-filters as a fundamental
* Corresponding author. Address: Bitline System, Pty. Ltd., Bexley, NSW 2207, Australia. Tel.: +61 2 9597 2689; fax: +61 2 9597 1959. E-mail addresses: [email protected] (N. You), [email protected]. edu.au (R.A. Minasian).
0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.01.045
building block, together with a structure that enables a multiple-pass transmission of the signal through the same sub-filter. The multiple-pass transmission is achieved through a new wavelength conversion and routing technique. Experimental results are presented which demonstrate a high-order FIR filter realization comprising 39-taps. This paper is organized as follows. Section 2 presents the principles of the new high-order photonic signal processor structure and its analysis. Section 3 presents the design. Experimental results which demonstrate the ability of this new structure to generate a large number of taps, is described in Section 4. Finally, conclusions are presented in Section 4. 2. High-order photonic signal processor principles and structure We commence with the technique of interconnecting a number of identical sub-filters with the aid of a few additional adders and multipliers, which is known in electronic FIR filter design [7]. Fig. 1 shows a general structure for implementing an FIR filter as a tapped cascaded interconnection of N identical sub-filters. The sub-filter has a transfer function FM(z), and forms the basic building block for the entire FIR filter.
340
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
IN (z) FMF(z) M
a[0]
a[1]
FM(z) a[2]
a[N-1] .
a[N]
OUT Z-M
Z-M
+
+
+
+
Z-M
Fig. 1. General structure for implementing an FIR filter as a tapped cascaded interconnection of N identical sub-filters.
The transfer function of the entire structure is [7] H ðxÞ ¼
N X
a½n½F M ðxÞ
n
ð1Þ
n¼0
where FM(z) is the sub-filter transfer function that is given by F M ðzÞ ¼
2M X
f ½nzn
ð2Þ
n¼0
H(x) can be obtained from the polynomial P ðxÞ ¼
N X
a½nxn
ð3Þ
n¼0
using the substitution in (1) x ¼ F M ðxÞ
ð4Þ
The additional tap coefficients a[n] can be determined so as to meet the required filter characteristics. It can be noted that the passband and stopband regions of the sub-filter FM(z) and H(x) are the same, and all that happens is that the multiple use of the same sub-filter reduces the large passband and stopband variations in FM(z) to small variations in H(x) [7]. Unfortunately, in the optical domain, such a cascade interconnection of sub-filters as shown in Fig. 1 for the electronic version, is not possible because it would cause coherent optical interference problems. Hence, we propose a new topology, shown in Fig. 2, for a photonic processor, which can overcome this problem. The key idea in this novel structure is to use wavelength conversion and a rout-
Subfilter
Coupler
λ1
Wavelength Converters
Amplifiers Attenuators λ2 Z-M
The design procedure for FIR filters using identical subfilters as building blocks is described in [7], and the reader is referred to the reference. As an example, a filter was designed having a flat-top response, a good shape factor, and a high stopband attenuation, with a passband frequency of 400 MHz. The sub-filter was chosen to be of low order for ease of implementation, and had a 13-tap response. Fig. 3 shows the
0.16
WDM
FM(z)
OUT
3. Design
Subfilter coefficient
INPUT WDM
ing technique to enable a multiple-pass transmission of the signal to be obtained through a single sub-filter. In Fig. 2, the sub-filter FM is implemented using a number of optical delay lines. The modulated input at wavelength k1 is passed through the WDM MUX and enters the sub-filter where it is processed and then exits through a WDM DEMUX. Here it passes through a wavelength converter that changes its wavelength to k2 and is looped back through the WDM MUX so that it passes through the sub-filter again. This process is repeated N times using the WDM MUX and DEMUX to select and feed optical signals with different wavelengths into the sub-filter. Together with the wavelength converters that produce N wavelengths in total, the sub-filter is thus reused N times. Note that after each pass the output corresponding to each wavelength is further weighted with an amplifier or attenuator, delayed (zM) with optical delay lines, and combined to form the final output. By means of this approach, it is possible to choose a low order for the sub-filter, so that it is practical to implement. Moreover, since a sub-filter implemented using optical components and fibre delay lines has a wide transmitting window, a large number of wavelengths can be multiplexed using WDM into the same sub-filter simultaneously. Therefore, we only need to construct a single sub-filter, while using wavelength conversion and WDM techniques to route the output of the sub-filter back to itself for a number of times to realize a high-order filter.
Coupler
λN
0.12 0.1 0.08 0.06 0.04 0.02
Z-M Fiber Delay-Lines
0.14 Impulse response (linear)
FM(z)
Z-M
Fig. 2. New photonic processor topology that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal through a single sub-filter.
0 1
2
3
4
5
6 7 8 9 coefficient index
10
11
12
Fig. 3. The sub-filter impulse response in the time domain.
13
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
341
Fig. 4. The sub-filter response in the frequency domain.
Fig. 6. The overall frequency response of the proposed processor.
designed sub-filter response in the time domain, and Fig. 4 shows the simulated sub-filter response in the frequency domain. Note that the taps in Fig. 3 are all positive as is desirable for practical optical implementation, hence the sub-filter itself cannot produce a flat-top filter response, as is evident in Fig. 4. Also, because the sub-filter has a low order of only 13 taps, it cannot produce is a narrow passband, which again is evident in Fig. 4. The polynomial P[x] was designed using the procedure in [7] to be a high pass filter with a flat passband at x = 1 and a deep notch at x = 0.
Hence, only two coefficients, a[2] and a[3] needed to be implemented in the experimental setup. Finally, Fig. 6 shows the simulated overall frequency response of the complete structure of Fig. 2 using only three passes of the sub-filter, i.e. with the original input wavelength plus two wavelength conversions, together with the two coefficients. This corresponds to a 39-tap filter response. Comparison of Fig. 6 for the overall filter with Fig. 4 for the sub-filter, shows that the overall filter can realize a flattop response, with a much improved shape factor and also with a greatly increased out-of-band suppression.
P ðxÞ ¼
N X
a½nxn 4. Experimental results
n¼0
¼ 6:40 10
6
2
þ 0:0176x þ 3:110x 2:128x
3
ð5Þ
Fig. 5 shows designed transfer function of this intermediate function P[x]. In order to simplify the system further, for subsequent experimental implementation, the coefficients a[0] and a[1] in (5) were taken to be zero, which is a very good approximation since their values are very small.
In order to verify the proof of principle, the new processor structure was experimentally implemented. Fig. 7 shows the experimental setup. The input optical signal at k1, which is modulated with RF signals to be processed, is coupled into the processor through a WDM multiplexer. The input then passes
WDM
P[x]
WDM
10
Response (dB)
-0.2
λ2
FM(z)
0 0.3
0.8
1.3
-10
λ3
1.8 λ1
Input λ1
-20
λ2 Coupler
-30
SOA
-40
SOA
Laser Source
Laser Source
λ3
-50 Z-M
-60 x (linear)
Fig. 5. The transfer function of the intermediate function P[x].
Output
Fig. 7. Experimental setup of the new processor structure.
342
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
through the sub-filter. At the output of the sub-filter, another WDM demultiplexer is used to separate the wavelengths into different loops. The signal at k1 is first coupled into a semiconductor optical amplifier (SOA) that is configured to give wavelength conversion through cross-gain modulation (XGM). The RF signal on k1 is converted to k2 and is coupled back to pass through the sub-filter again. Then the signal at k2 is converted to k3 using another SOA, and k3 is also fed back into the sub-filter again to produce the third pass. The final output of the processor is obtained by combining the outputs of k2 and k3. By correctly setting the power levels of these wavelengths and the driving current of the SOAs, different RF gain values can be achieved through the wavelength conversion, and this sets the required coefficient weights. Therefore, no additional optical attenuators or amplifiers are required for the coefficient weighting. Also, since XGM inverts the signs of the RF signal, the opposite signs of a[2] and a[3] are naturally implemented. The wavelengths used in the experiment were k1 ¼ 1556:9 nm, k2 ¼ 1559:3 nm, and k3 ¼ 1548:2 nm. These wavelengths were chosen so that they corresponded to the channel wavelength and range of the WDM MUX and DEMUX. The sub-filter was constructed using the direct form. Because of the symmetric nature of the impulse response of the sub-filter, only half the number of optical weights is required. In the experimental setup, variable optical attenuators (VOA) were used to set the tap coefficient weights. A two-tap delay line was used after each VOA to generate and locate the two taps at the symmetric time locations. Star couplers were used to split and combine the optical power at both ends of the sub-filter. Fig. 8 shows the experimental setup of the sub-filter. The measured frequency response of the sub-filter is shown in Fig. 9. This is similar to simulated response shown in Fig. 4, however there are some differences including a somewhat asymmetric response, which arose to errors in setting the tap weights and delays. Finally, the measured overall frequency response of the filter is shown in Fig. 10. This is broadly similar to the simulated response shown in Fig. 6. However there are some differences, primarily in the lower stopband suppression level obtained in the measurements. This is believed to be caused by the reduction in signal response level because Star Coupler
Delay Lines
Star Coupler
Input
Output VOA
Fig. 8. The experimental setup of the sub-filter.
Fig. 9. Measured frequency response of the sub-filter.
Fig. 10. Measured overall frequency response of the processor.
the sub-filter loss was high and also the wavelength conversion efficiency was not optimized, coupled with the high experimental noise floor due to some phase induced intensity noise in the sub-filter and also ASE noise. Also the asymmetric response of the sub-filter directly affected the response of the following stages and reduced the improvement of the rejection. Nevertheless, the measured results demonstrate that in comparison to the sub-filter, a flattop filter response has been achieved, the filter shape has been significantly improved, the transition band of the filter is considerably reduced, and the rejection level in the stopband has been increased by around 15 dB. The experimental implementation passed the signal three times through the sub-filter, which had 13 taps. Hence, effectively the overall filter has 39 taps. It can be pointed out that the real power of this new structure becomes evident in its ability to realize an even greater number of taps if more passes through the sub-filter are used, though ultimately this will be limited by noise build up. Nevertheless, increasing the number of passes through the sub-filter can significantly increase the order of the overall filter, leading to continuous improvement in the frequency response, reduction of the transition band, increase
N. You, R.A. Minasian / Optics Communications 273 (2007) 339–343
of the stopband rejection, and improvement of the filter shape factor. 5. Conclusion A new topology for realizing a high-order FIR RF photonic filter has been presented. It is based on a novel structure that uses wavelength conversion and a routing technique to enable a multiple-pass transmission of the signal to be obtained through a single photonic sub-filter. Hence a single low order sub-filter can be used in conjunction with wavelength conversion and WDM techniques to route the output of the sub-filter back to itself several times to realize a high-order filter. Experimental results for a three-pass implementation have demonstrated a high-order FIR filter realization comprising 39-taps. Moreover, this new structure has the ability to realize an even greater number of taps if more passes through the sub-filter are used. This offers the prospect of high resolution microwave photonic signal processors.
343
Acknowledgement This work was supported by the Australian Research Council. References [1] J. Capmany, D. Pastor, B. Ortega, IEEE Trans. Microw. Theory 47 (July) (1999) 1321. [2] J.L. Chen, R.A. Minasian, IEEE Photon. Technol. Lett. 17 (4) (2005) 896. [3] A.P. Foord, P.A. Davies, P.A. Greenhalgh, Electron. Lett. 32 (February) (1996) 390. [4] J. Capmany, D. Pastor, B. Ortega, Electron. Lett. 35 (March) (1999) 494. [5] B.A.L. Gwandu, W. Zhang, J.A.R. Williams, Electron. Lett. 38 (October) (2002) 1328. [6] D.B. Hunter, R.A. Minasian, IEEE Microw. Guide. Wave Lett. 6 (February) (1996) 103. [7] T. Saramaki, in: S.K. Mitra, J.F. Kaiser (Eds.), Handbook for Digital Signal Processing, Wiley, New York, 1993.