wdm optical interfaces for future fiber radio systems phần 10 pps

6.9 shows the combined spectrum of the signals after multiplexing, which confirms the functionality of the proposed H-MUX, enabling wavelength interleaving for the modulated multiband si

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The modulated signals were then applied to the AWG, as shown in Fig 6 7 The allocation of the input ports and the selection of the loop-back paths are maintained

in such a way that the resultant output of the AWG is the desired interleaved signals Fig 6.9 shows the combined spectrum of the signals after multiplexing, which confirms the functionality of the proposed H-MUX, enabling wavelength interleaving for the modulated multiband signals in an integrated DWDM access network The spectrum also indicates that the multiplexing of the signals using such

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H-MUX reduces the CSR of optical RF signal to 5 dB, attaining an effective reduction by 8 dB

The composite signal was then amplified by an erbium-doped-fibre-amplifier (EDFA) and followed by a 4-nm optical band pass filter (BPF) prior to transmission over 10 km of singlemode fibre(SMF) to a BS, where each of the multiplexed signals was recovered using a suitable optical add-drop-multiplexing (OADM) interface The OADM interface, which is comprised of a double-notch tunable fibre Bragg grating (FBG) and a 3-port optical circulator, recovers each of the signals separately

(b)

0

-40 -30 -20 -10

1555.8 1556.2 1556.6

Wavelength (nm)

S RF C RF BB

(c)

0

-40 -30 -20

(b)

0

-40 -30 -20 -10

1555.8 1556.2 1556.6

Wavelength (nm)

S RF C RF BB

(c)

0

-40 -30 -20 -10

1555.8 1556.2 1556.6

Wavelength (nm)

S RF C RF BB

(c)

0

-40 -30 -20

(a)

0

-40 -30 -20

(a)

Fig 6.11: Measured optical spectra for the signals passing through while recovering: (a): RF, (b):

BB and (c): IF signals using the OADM interface.

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from the interleaved signals by shifting the centre frequencies of the FBG The spectra of recovered signals can be seen from Fig 6.10 (a) - (c)

The spectra for the signals passing through the OADM interface are also shown in Fig 6.11 (a)-(c) The optical spectra shown in Fig 6.10 and 6.11 indicate that the recovered signals are contaminated by unwanted -24 dB to -30 dB optical crosstalk, which however, can be further minimised by proper selection of the FBG comprising the OADM interface

-6 -7 -8 -9

-17.4 -17 -16.6 -16.2 -15.8 -15.4

RF Signal with Data Rate 155Mb/s

10 KM SMF 0.0 KM SMF

Received Optical Power (dBm)

Baseband Signal with

Data rate 1Gb/s

(b)

-6 -7 -8 -9

-29.2 -28.8 -28.4 -28 -27.6

IF signal with Data Rate 155Mb/s

(c)

-6 -7 -8 -9

-17.4 -17 -16.6 -16.2 -15.8 -15.4

Data rate 1Gb/s

(b)

-6 -7 -8 -9

-29.2 -28.8 -28.4 -28 -27.6

-6 -7 -8 -9

-29.2 -28.8 -28.4 -28 -27.6

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In order to quantify the signal degradation in bit error ratio (BER), each of the recovered signals was detected and data was recovered using suitable photodetector (PD) and data recovery circuits The recovery of data from the BB and IF signals have used 1 GHz and 2.5 GHz optical receivers respectively The PD and data recovery circuit used in recovering data from the optical RF signal is shown in the dotted line inset of Fig 6.7 The RF signal was first detected with a 45 GHz PD, then amplified, down-converted to an IF of 2.5 GHz, and filtered with an electrical BPF with a bandwidth 400 MHz, from which the data was recovered using a 2.5 GHz phase locked loop (PLL)

Fig 6.12 (a) – (c) shows the measured BER curves for the recovered signals both for the back-to-back case (having the H-MUX, but no fibre) and after transmission over 10 km of SMF The results exhibit negligible power penalties of 0.2 to 0.3 dB

at a BER of 10-9 that can be attributed to experimental errors

Therefore, a simple H-MUX is proposed and demonstrated with the capacity to interleave optically modulated BB, IF and RF signals with a DWDM channel separation of 12.5 GHz, which has the potential to combine multiband optical access technologies together, leading to an integrated DWDM network in the access and metro domain The proposed H-MUX also reduces the CSR of the interleaved RF signals that improves the overall RF transmission performance significantly

6.5 Demultiplexing of Multiband Signals

Section 6.3 has introduced a wavelength interleaved hybrid multiplexing scheme for integrated optical access network, which has been demonstrated experimentally

in Section 6.4 In this demonstration an OADM interface comprised of a tunable double-notch FBG and a 3-port optical circulator was used as the means of recovering the desired signals at the BS OADM interfaces of this kind however, exhibit poor performances while used as cascaded units in star-tree and ring/bus network, as described in detail in Chapter 4 As a way to overcome, AWG-based demultiplexing schemes, suitable to recover multiple multiband signals together both

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in the CO and the RANs, can be used This section thus focuses in introducing such hybrid multiband demultiplexers (H-DEMUXs), by which the recovery of the desired multiband signals from an integrated optical access network can be easily realised

6.5.1 H-DEMUX with WDM Channels Larger than the RF Carrier Frequency

The schematic depicting the multiplexing scheme of multiband signals with a WDM channel spacing larger than the mm-wave RF carrier frequency is shown in

Fig 6.1 As f RF is much smaller than ∆f, similar to the multiplexing scheme, the

demultiplexing of the signals also can be realised by using standard demultiplexing technologies using a suitable AWG demultiplexer, where both the optical carrier and the modulation sideband of an optical RF signal will be considered together as a single channel, same as the BB and IF signals

Fig 6.13 shows the schematic of a H-DEMUX that effectively demultiplexes the

3N-2 3N-1 3N

1 × 3N AWG

5 6

3N-2 3N-1 3N

1 × 3N AWG

5 6

Fig 6.13: Proposed hybrid demultiplexer (H-DEMUX) enabling demultiplexing of multiband

WDM signals in an integrated access network with a WDM channel spacing larger than the

mm-wave RF carrier frequency.

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multiband signals from the spectral scheme shown in Fig 6.1 Similar to the MUX shown in Fig 6.2, it is also comprised of a 1 × 3N AWG with a channel

H-bandwidth, ≤∆f and a channel spacing, ∆f, equal to the WDM channel spacing of the

multiplexed multiband signals The output ports of the AWG are numbered from 1

to 3N The multiplexed BB, IF and RF signals, shown in inset of Fig 6.13, enters the AWG via the input port and is demultiplexed to the output ports, the similar way it is demultiplexed in a convention WDM network

6.5.2 H-DEMUX with DWDM Channels Smaller than the RF Carrier Frequency

The schematics depicting the multiplexing schemes of multiband signals with

OUTPUT INPUT

BB 1 ,BB 2 ,…BB N ,IF 1 ,IF 2 ,…IF N

AWG Channel Separation = ∆f

MM-Wave RF = 3∆f DWDM Separation = ∆f

BB 1 ,BB 2 ,…BB N ,IF 1 ,IF 2 ,…IF N

AWG Channel Separation = ∆f

MM-Wave RF = 3∆f DWDM Separation = ∆f

Fig 6.14: Proposed H-DEMUX enabling demultiplexing of multiband wavelength-interleaved

signals in an integrated DWDM access network, which also reduces the CSR of the demultiplexed

RF signals through optical loop-backs.

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DWDM channel spacings equal to or smaller than the mm-wave RF carrier frequency are shown in Figs 6.3 and 6.5 Although both of these schemes are different in spectral configuration, the desired signals from such schemes can be recovered using similar demultiplexers Therefore, to avoid repetition, description of

a separate demultiplexer for the multiplexing scheme shown in Fig 6.3 is ignored Fig 6.14 shows the schematic of the multiband H-DEMUX that realises demultiplexing of the wavelength interleaved signals from the spectral scheme shown in Fig 6.5 It comprises a 4N × 4N AWG with a channel bandwidth, ≤∆f and

a channel spacing, ∆f, equal to the DWDM channel spacing of the interleaved

multiband signals The input (A) and output (B) ports of the AWG are numbered from 1 to 4N The characteristic matrix of the AWG that governs the allocation and distribution of different channels at different ports is illustrated in Table 6.2

Table 6.2: Input/output characteristic matrix of 4N x 4N arrayed waveguide grating

The wavelength interleaved BB, IF and RF signals, shown in the inset of Fig 6.14, enters the AWG via the input port, A1 The AWG then distributes the optical

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carriers and the respective modulation sidebands of the RF signals as well as the BB and IF signals to the output ports, B1 –B4N as per their respective positions in the interleaved spectrum To realise demultiplexing for the RF signals, the distributed optical carriers C1, C2,….,CN are looped back to the AWG via the input ports A4,

A8,….,A4N, respectively and the resultant outputs at ports B1, B5,….,B4N-3 are the optical carriers and the modulation sidebands of the RF signals demultiplexed together Thus the proposed H-DEMUX successfully demultiplexes the multiband signals in an integrated DWDM access network, suitable to be used both in the CO and the RANs

Also due to the loop-backs, the optical carriers of the demultiplexed RF signals are suppressed by as much as equal to the IL of the AWG (typical IL = 4 - 5 dB) compared to the respective modulation sidebands Therefore, the proposed H-DEMUX also enhances the performance of the optical RF signals by reducing the CSRs by 4 to 5 dB, in addition to demultiplexing them from an integrated DWDM

4N-2 4N-1 4N

1 × 4N AWG

5 6 7 8

4N-2 4N-1 4N

1 × 4N AWG

5 6 7 8

4N-3

OUTPUT

Fig 6.15: Proposed H-DEMUX enabling demultiplexing of multiband wavelength interleaved

signals in an integrated DWDM access network, where 3 –dB couplers are used to combine the optical carrier and the respective modulation sideband of an optical RF signal at the output ports of

the AWG.

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network in the access and metro domain

The proposed H-DEMUX shown in Fig 6.14 can also be realised by using a 1 x 4N AWG, where additional 3-dB optical couplers are needed to be inserted for each

of the RF signals that combine the optical carrier and the respective modulation sideband of an RF signal together at the output ports of the AWG The schematic of such a scheme can be seen from Fig 6.15 This scheme, however, causes additional

3 –dB attenuation for the optical RF signals before demultiplexing, in addition to ignoring the performance enhancement of the RF signals through optical loop-backs The following section demonstrates the proposed wavelength interleaved H-DEMUX (shown in Fig 5.14) experimentally and presents the experimental results from which the performance of the proposed multiplexer can be quantified

6.6 Demonstration of Wavelength-Interleaved Hybrid Demultiplexer

Fig 6.16 shows the setup used to demonstrate the proposed scheme experimentally Similar to the demonstration of hybrid multiplexer, three narrow linewidth tunable light sources LS1, LS2, and LS3 at the corresponding wavelengths 1556.2, 1556.3 and 1556.4 nm followed by separate polarization controllers were used as the input to two low-speed (0 - 5 GHz) MZMs and one high-speed (0 - 40 GHz) DE-MZM to generate optical BB, IF and RF signals, respectively The optical BB signal was generated by using 1 Gb/s data, whereas the optical IF and RF signals were generated

by using 2.5 GHz microwave and 37.5 GHz mm-wave signals respectively The 2.5 GHz and 37.5 GHz signals were generated respectively by mixing 155 Mb/s PRBS data with 2.5 and 37.5 GHz LO signals in BPSK format The mixer outputs were then amplified prior to applying to the respective modulators, as shown in Fig 6.16 The RF inputs and biasing of the DE-MZM was controlled in such a way that the resultant output of the DE-MZM was an optical RF signal in OSSB+C modulation format The generated optical BB, IF and RF signals were then interleaved by using two 3-dB optical couplers, the composite spectrum of which can be seen from Fig

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6.17 Like before, the optical RF signal clearly shows a CSR of 13 dB with a suppression of the unwanted modulation sidebands by almost 30 dB The spectrum also indicates the 12.5 GHz DWDM channel spacing (irrespective of carrier or sideband) in addition to the RF carrier frequency of 37.5 GHz

10 KM SMF EDFA

LO Data

155 Mb/s GHz 2.5

PC: polarization controller LO: local oscillator AWG: arrayed waveguide grating

BPF: band pass filter SMF: singlemode fiber PD: photo detector

IF BB RF(S,C)

PD and Data Recovery

A1 A2 A3 A4 A5 A6 A7

B1 B2 B3 B4 B5 B6 B7 B8 A8

C 1

<

8 x8 AWG

10 KM SMF EDFA

LO Data

155 Mb/s GHz 2.5

PC: polarization controller LO: local oscillator AWG: arrayed waveguide grating

BPF: band pass filter SMF: singlemode fiber PD: photo detector

IF BB RF(S,C)

PD and Data Recovery

A1 A2 A3 A4 A5 A6 A7

B1 B2 B3 B4 B5 B6 B7 B8 A8

C 1

<

8 x8 AWG

Fig 6.16: Experimental setup for the demonstration of a wavelength interleaved H-DEMUX that

enables recovery of desired multiband signals from an integrated DWDM network in the access

and metro domain.

The interleaved multiband signals were amplified by an EDFA and then filtered using a 4 nm optical BPF to minimise out-of-band asynchronous spontaneous emission (ASE) noise The filtered signal was transported over 10 km of SMF to the proposed wavelength interleaved H-DEMUX, comprised of an 8 × 8 AWG with a channel separation of 12.5 GHz and a channel bandwidth of ≈10 GHz, the characteristics of which has already been described in Chapter 5 The allocation of

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the input port and the selection of the loop-back path for the optical carrier of the RF signal are as shown in Fig 6.16, which result in the desired demultiplexed RF, BB, and IF signals at the output ports B1, B2 and B3 respectively

of the undesired optical carriers to the desired optical carriers at the demultiplexed signals Crosstalk levels of -13.6 to -14.7 dB were observed The impacts of these crosstalk on the transmission performance of the demultiplexed signals are quantified later of the section The spectrum in Fig 6.18(a) also indicates that the demultiplexing of the signals using such H-DEMUX reduces the CSR of demultiplexed RF signal from13 dB to 8.4 dB

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In order to quantify the signal degradation in bit error ratio, each of the demultiplexed signals was detected and data was recovered using the same PD and data recovery circuits, used in measuring the BER performances in Section 6.4 Fig 6.19 (a) –(c) shows the measured BER curves for the recovered signals both for the back-to-back case (having the H-DEMUX, but no fibre) and after transmission over

10 km of SMF The results exhibit negligible power penalties of 0.15 to 0.4 dB at a BER of 10-9 that can be attributed to experimental errors

IF 0

BB 0

-20

-40

1555.8 1556.1 1556.4 1556.7

Wavelength (nm) (c)

(a)

C RF 0

IF 0

BB 0

-20

-40

1555.8 1556.1 1556.4 1556.7

BB 0

-20

-40

1555.8 1556.1 1556.4 1556.7

(a)

C RF 0

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Also, the effects of the presence of optical crosstalk in the demultiplexed signals, caused by the neighbouring DWDM multiband signals, are quantified by measuring another set of BER curves, transmitting each of the RF, IF and BB signals separately and comparing them with the BER curves measured for the demultiplexed signals

-6 -7 -8 -9

10 km SMF 0.0 km SMF

-18.6

Optical Baseband with 1 Gb/s data

(c)

-6 -7 -8 -9

(a)

-6 -7 -8 -9

-18.6

(c)

-6 -7 -8 -9

-18.6

(c)

Fig 6.19: Measured BER curves as a function of received optical power for: (a): RF, (b): IF, and (c): BB signals demultiplexed from the three wavelength-interleaved multiband signals after

transmission over 10 km SMF with the back to back curves as the reference.

The new set of BER curves is shown in Fig 6.20 It indicates that the integrated DWDM access network incorporating the proposed H-DEMUX causes crosstalk

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induced penalties of 0.5, 0.6, and 0.5 dB for the respective optical RF, IF, and BB signals, which however can be minimised by proper selection of the AWG, the main building block of the proposed demultiplexing scheme

BB +IF +RF Only RF -6

-7 -8 -9 -14.6 -14.2 -13.8 -13.4 -13

-7 -8 -9

BB +IF +RF Only RF -6

-7 -8 -9 -14.6 -14.2 -13.8 -13.4 -13

multiband signals in an integrated access network incorporating the proposed H-DEMUX.

Therefore, the recovered optical spectra and the BER curves clearly demonstrate the functionality of the proposed demultiplexing scheme that offers a practical solution for an integrated DWDM network in the access and metro domain The

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scheme also reduces the CSR of the demultiplexed RF signals that improves the overall RF transmission performance significantly

6.7 Conclusion

This chapter presented hybrid multiplexing and demultiplexing schemes for simultaneous transmission of multiband signals together towards the realisation of an integrated network in the access and metro domain To reduce the WDM channel spacing smaller than the mm-wave RF carrier frequency, hybrid wavelength interleaving technique was proposed, by which integrated access network with DWDM channel spacing can be realised The proposed schemes are based on AWG technology and are suitable to be installed in the standard optical access or metro infrastructure, irrespective of the network technologies and architectures Moreover, the schemes incorporating hybrid wavelength interleaving reduce the CSRs of the optical RF signals while multiplexing and demultiplexing that improve the overall

RF transmission performance significantly

The functionality of the wavelength interleaved hybrid multiplexer and demultiplexer were verified experimentally with three DWDM multiband signals (BB, IF and RF), comprising a baseband signal with 1 Gb/s baseband, a 2.5 GHz IF signal with 155Mb/s BPSK data and a 37.5 GHz RF signal with 155Mb/s data, spaced at 12.5 GHz transported over10 km of fibre link The error free (at a BER of

10-9) data recovery confirm the functionality of the proposed schemes without any noticeable power penalty observed while transporting over 10 km of SMF

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6.8 References

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[3] F Effenberger, H Ichibangase, and H.Yamashita, “Advances in broadband passive optical

networking technologies,” IEEE Communications Magazine, vol 39, no 12, pp 118, 2001

[4] G Wilson, T wood, A Stiles, R Feldman, J Delavaux, T Dausherty, and P Magill,

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[5] P Mahonen, T Saarinen, Z Shelby, and L Munoz, “Wireless Internet over LMDS:

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[9] Y Maeda and R Feigel, “A standardization plan for broadband access network transport,”

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[10] A Martinez, V Polo, and J Marti, “Simultaneous baseband and RF optical modulation

scheme for feeding wireless and wireline heterogeneous access networks,” IEEE Trans

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[11] G H Smith, D Novak, and C Lim, “A millimeter-wave full-duplex WDM/SCM fiber-radio

access network,” Proc Conference on Optical Fiber Communication (OFC'98), Washington

DC, USA, vol 2, pp 18-19, 1998

[12] R A Griffin, P M Lane, and J J O’Reilly, “Radio-over-fiber distribution using an optical

millimeter-wave/DWDM overlay,” Proc Conference on Optical Fiber Communication and

the International Conference on Integrated Optics and Optical Fiber Communications (OFC/IOOC'99),San Diego, CA, USA, vol 2, pp 70-72, 1999

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[13] K Kojucharow, M Sauer, H Kaluzni, D Sommer, and C G Schaffer, “Experimental investigation of WDM channel spacing in simultaneous upconversion millimeter-wave fiber

transmission system at 60 GHz-band,” IEEE MTT-S Int Microwave Symposium Digest,

vol.2, pp 1011-1012, 2000

[14] M A Al-mumin and G Li, “WDM/SCM optical fiber backbone for 60 GHz wireless

systems,” Proc IEEE Top Meet on Microwave Photonics (MWP2001), Long Beach, CA,

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[15] C Lim, A Nirmalathas, D Novak, R S Tucker, and R Waterhouse, “Technique for increasing optical spectrum efficiency in millimeter wave WDM fiber-radio,” Electron Lett

, vol 37, pp 1043 –1045, 2001

[16] H Toda, T Yamashita, K Kitayama, T Kuri, “A DWDM MM-Wave Fiber Radio system

by optical frequency interleaving for high spectra efficiency,” IEEE Top Meet On

Microwave Photonics (MWP '01), pp 85-88, 2001

[17] M Bakaul, A Nirmalathas, and C Lim, “Multifunctional WDM optical interface for millimeter-wave fiber-radio antenna base station,” Journal of Lightwave Technol., vol 23,

no 3, pp 1210-1218, 2005

[18] C Lim, A Nirmalathas, M Attygalle, D Novak, and R Waterhouse, “On the merging of

millimeter-wave fiber-radio backbone with 25-GHz WDM ring networks,” Journal of

Lightwave Technol., Vol 21, no 10, pp 2203-2210

[19] V Polo, A Martinez, J Marti, F Ramos, A Griol, and R Llorente, “Simultaneous baseband

and RF modulation scheme in Gbit/s millimeter-wave wireless-fiber networks,” in Proc

MWP'00 Oxford, U.K., pp 168-171, 2000

[20] T Kamisaka, T Kuri, and K Kitayama, “Simultaneous modulation and fiber-optic transmission of 10 Gb/s baseband and 60-GHz-band radio signals on a single wavelength,”

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[21] K Ikeda, T Kuri, and K Kitayama, “Simultaneous three-band modulation and fiber-optic transmission of 2.5 Gb/s baseband, microwave-, and 60-GHz-band signals on a single

wavelength,” Journal of Lightwave Technol., Vol 21, no 12, pp 3194-3202, 2003

[22] D J Blumenthal, J Laskar, R Gaudino, S Han, M D Shell, and M D Vaughn, optic link supporting baseband data and subcarrier-multiplexed control channels and the

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[23] R D Esman and K J Williams, “Wideband efficiency improvement of fiber optic systems

by carrier subtraction,” IEEE Photon Technol Lett., vol 7, no 2, pp 218-220, Feb 1995

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