OFC/NFOEC 2008 OThP3.pdf signal, heterodyning in the photodiode at the receiver generates the modulated microwave signal at fRF. This od suppresses the phase noise of the laser diode, and hence creates a clean microwav A third method deploys the generation of many harmonics by the so-called Optical Frequency Multiplying (OFM)technique. Its principle is shown in Fig. I. The optical frequency of a wavelength-tunable laser diode in the headend station is periodically swept over an optical frequency range B-Om with a harmonic sweep signal having a frequency Om. After passing the fixed MZl, by FM-to-IM conversion many harmonics of the sweep frequency are generated. The strengths of the harmonics can be set by adjusting the frequency modulation index B. At the antenna site, when neglecting the fibers attenuation and dispersion, the output current signal iour(t) of the photodiode contains all these harmonics n @m with n-th order Bessel function weight Jn(2B- sin(@m [/2)), according to[2] in which lo is the dC current proportional to the received optical power, t is the delay difference in the Mzi arms =do(t/dt equine Fig 2 38. 4 GHz microwave carrier The data signal is present on all the harmonics. After photodetection, a bandpass filter extracts the desired harmonic or sever al harmonics), which is amplified and emitted as a microwave carrier bearing the data signal. The laser phase noise has negligible impact if or <<r/2, i.e. when the laser linewidth is much smaller than a quarter of the Free Spectral Range AoFSR=2T/T of the MZI. This condition is easily met, as the MzIs FSR is typically around 0 GHz, and thus the OFM process effectively suppresses the laser phase noise. Hence very pure microwave signals can be generated. Linewidths below the measurement resolution(< 100 Hz) have been obtained when generating a 8. 4 GHz carrier as the 6 harmonic of a 6. 4 GHz sweep signal, whereas the linewidth of the laser diode was more than I MHz, see Fig. 2. Moreover, by using a high order harmonic, OFM permits to use only a relatively low- frequency sweep generator in the headend, which can be of high quality while still at low cost. 3. Radio over single-mode fiber in access networks Direct RF intensity modulation of a laser diode yields a double-sideband (IM-DSB) spectrum. Fiber dispersion causes phase shifts between the two sidebands, which leads to severe fading of the microwave at certain fiber lengths, as shown in Fig. 3 3]. On the other hand, the OFM technique is very robust against chromatic dispersion in single-mode fiber, as shown also in Fig 3 over 72km of SMF. This dispersion- robustness is very useful operating in link-switched networks, where the fiber connecting the headend to an antenna may vary in length when TH-BPEE r = Fig 3 Measured impact of sMF dispersion on relative strength of 30 GHz carrier, using IM-DSB or 画心 OFM (excluding fiber losses) Fig 4 Flexible routing of radio capacity in Fixed wireless Access Fig 4 shows such a network where at the headend station different microwave signals are generated by OFM in different wavelength channels. Using routing by means of tunable wavelength add-drop modules, one or more of these wavelength channels can be dropped to an antenna site, and thus the radio capacity can be flexibly assigned mong these antennas depending on the local traffic demands. Moreover, the frequency chirping of the sourcedata signal, heterodyning in the photodiode at the receiver generates the modulated microwave signal at fRF . This method suppresses the phase noise of the laser diode, and hence creates a clean microwave. A third method deploys the generation of many harmonics by the so-called Optical Frequency Multiplying (OFM) technique. Its principle is shown in Fig. 1. The optical frequency of a wavelength-tunable laser diode in the headend station is periodically swept over an optical frequency range β⋅ωm with a harmonic sweep signal having a frequency ωm . After passing the fixed MZI, by FM-to-IM conversion many harmonics of the sweep frequency are generated. The strengths of the harmonics can be set by adjusting the frequency modulation index β. At the antenna site, when neglecting the fiber’s attenuation and dispersion, the output current signal iout(t) of the photodiode contains all these harmonics n·ωm with n-th order Bessel function weight Jn(2β ·sin(ωmτ /2) ) , according to [2] = ⋅{ } + [ β ⋅ ( ω τ )⋅ (ω − τ )+ω τ +ϕ&τ ] 2 0 1 2 1 i (t) I0 1 cos 2 sin cos (t ) out sw sw in which I0 is the DC current proportional to the received optical power, τ is the delay difference in the MZI arms, ω0 is the central frequency of the laser diode, and ϕ& = dϕ(t)/dt is its phase noise. fibre LD BPF PD n·ωm λ sweep signal ω m ampl. i out (t) τ MZI +ϕ -ϕ + data - data RF n·ω m fibre LD BPF PD n·ωm λ sweep signal ω m ampl. i out (t) τ MZI +ϕ -ϕ + data - data RF n·ω m Fig. 1 Optical Frequency Multiplying Freq. offset from 38.4 GHz carrier [Hz] 38.4GHz < 100Hz RF power [dBm] -30 -60 -90 -500 0 +500 Freq. offset from 38.4 GHz carrier [Hz] 38.4GHz < 100Hz RF power [dBm] -30 -60 -90 -500 0 +500 Fig. 2 38.4 GHz microwave carrier generated by OFM The data signal is present on all the harmonics. After photodetection, a bandpass filter extracts the desired harmonic (or several harmonics), which is amplified and emitted as a microwave carrier bearing the data signal. The laser phase noise has negligible impact if ϕ&τ <<π / 2 , i.e. when the laser linewidth is much smaller than a quarter of the Free Spectral Range ∆ωFSR = 2π / τ of the MZI. This condition is easily met, as the MZI’s FSR is typically around 10 GHz, and thus the OFM process effectively suppresses the laser phase noise. Hence very pure microwave signals can be generated. Linewidths below the measurement resolution (< 100 Hz) have been obtained when generating a 38.4 GHz carrier as the 6th harmonic of a 6.4 GHz sweep signal, whereas the linewidth of the laser diode was more than 1 MHz; see Fig. 2. Moreover, by using a high order harmonic, OFM permits to use only a relatively low￾frequency sweep generator in the headend, which can be of high quality while still at low cost. 3. Radio over single-mode fiber in access networks Direct RF intensity modulation of a laser diode yields a double-sideband (IM-DSB) spectrum. Fiber dispersion causes phase shifts between the two sidebands, which leads to severe fading of the microwave at certain fiber lengths, as shown in Fig. 3 [3]. On the other hand, the OFM technique is very robust against chromatic dispersion in single-mode fiber, as shown also in Fig. 3 over 72km of SMF. This dispersion-robustness is very useful when operating in link-switched networks, where the fiber connecting the headend to an antenna may vary in length. 0 20 40 60 80 -80 -60 -40 -20 0 20 Fibre Length, [km] Signal Strength, [dB] IM-DSB OFM Fig. 3 Measured impact of SMF dispersion on relative strength of 30 GHz carrier, using IM-DSB or OFM (excluding fiber losses) τ MZI LD λ1 sweep freq. f1 data 1 MZI mod. LD λ2 sweep freq. f2 data 2 MZI mod. LD λN sweep freq. fN data N MZI mod. WDM mux λ-multicasting tun. OADM fmm,x BPF PD λ-multicasting tun. OADM λx fmm,y , fmm,z BPF PD λy, λz BPF λx PD λ-multicasting tun. OADM fmm,y λy OADM control τ MZI LD λ1 sweep freq. f1 data 1 MZI mod. LD λ2 sweep freq. f2 data 2 MZI mod. LD λN sweep freq. fN data N MZI mod. WDM mux λ-multicasting tun. OADM fmm,x BPF PD λ-multicasting tun. OADM λx fmm,y , fmm,z BPF PD λy, λz BPF λx PD λ-multicasting tun. OADM fmm,y λy OADM control Fig. 4 Flexible routing of radio capacity in Fixed Wireless Access Fig. 4 shows such a network where at the headend station different microwave signals are generated by OFM in different wavelength channels. Using routing by means of tunable wavelength add-drop modules, one or more of these wavelength channels can be dropped to an antenna site, and thus the radio capacity can be flexibly assigned among these antennas depending on the local traffic demands. Moreover, the frequency chirping of the source a828_1.pdf OThP3.pdf OFC/NFOEC 2008