Multi-Services Hybrid Access Network System Using A DP-MZM

In last section, a multi-service hybrid access network system using a dual-electrode MZM was proposed. However, a narrow band optical filter is still required in the HEU to control the OPR between the PSK sideband and the optical carrier. In this section, another multi-service hybrid access network system using a DP-MZM will be proposed. All of the optical sidebands in this system are generated from dif-ferent driving signals. Therefore, the OPR between the optical sidebands can be freely adjusted by controlling the driving signals and no narrow band optical filter is required to suppress the optical carrier power.

6.2.1 Experimental setup

Figure 6.9 shows the experimental setup of the multi-services hybrid access net-work system using a DP-MZM. A DFB laser is used as the optical source. Both 8PSK and QPSK modulation formats with 312.5-MSymbol/s symbol rates are demonstrated for the RF signals. The M-ary PSK signals are generated form an AWG with 2.5-GHz carrier frequency and up-converted to 10-2.5-GHz using an electrical mixer with a 7.5-GHz LO signal. The 1.25-Gb/s OOK PRBS signal with word length of2³¹− 1 is generated using a pattern generator and up-converted to 5-GHz. A 10-GHz sinusoidal signal is also employed for the generation of new optical subcarrier. The signals are separated using 90^◦ hybrid couplers. The 90^◦ phase delays are added on the upper path of OOK and PSK to generate USB signals, and the90^◦ phase delay is added on

MZ-a

FIG. 6.9. Experimental setup of the multi-services hybrid access network system using a DP-MZM.

the lower path of the 10-GHz sinusoidal signal to generate LSB signal. The half-wave voltages (Vπ) of MZ-a and MZ-b in theDP-MZM are 2.9 volts. In addition, the mod-ulation indices (Vm/Vπ,Vm is the amplitude of the driving signal) for all the driving signals are about 0.1.

At the output of the modulator, an EDFA is utilized to boost the optical power and adjust to 0 dBm before sent into fiber. After transmission over 25-km SSMF, only an optical coupler is employed to separate the optical power of the hybrid signal for different applications. For FTTx BB application, the BB OOK signal is directly detected using a commercial 2.5-Gb/s photo receiver and sent into a bit error rate tester (BERT) for performance analysis. For radio-over-fiber applications, the 20-GHz M-ary PSK signal is detected using a high speed photo receiver, down-converted with a 17.5-GHz sinusoidal signal, and then sent into a real time scope to capture the time domain waveform for off-line analysis. In this work, 1024 symbols of PSK

signals are captured. The BER of the Gray-coded M-ary PSK are calculated using BER = (1/log2M) × erfc[

E/N₀sin(π/M)] [76, 77]. Where M is the order of the M-ary PSK signal, E is the energy of the modulation symbol and N₀ is the noise power spectrum density.

6.2.2 Results and discussion

In this system, all of the optical sidebands are generated from different driving signals. Therefore, the OPR between the optical sidebands can be freely adjusted. To optimize the RF PSK signal, the RF OOK signal is turned off first. Only PSK and the 10-GHz sinusoidal signals are sent into the dual-parallel MZM. FIG. 6.10 illustrate the -log(BER) of both the 8PSK and QPSK signals with different PSK optical sideband to optical carrier OPR. The BER values of RF 8PSK and QPSK signals are obtained with -18.5 and -17-dBm optical power, respectively. The best receiver sensitivity of both the 8PSK and QPSK signals are obtained when the OPRs are 0 dB.

-10 -8 -6 -4 -2 0 2 4 6 8 10

10 9 8 7 6 5 4 3 2

8PSK (-18.5 dBm) QPSK (-17 dBm)

-log(BER)

OPR (dB)

FIG. 6.10. -log(BER) versus Optical Power Ratio between PSK optical sideband and new carrier.

-1 0 1 2 3 4 -18

-17 -16 -15 -14

OOK w/ 8PSK 8PSK

OOK w/ QPSK QPSK

Sensitivity at BER of 10-9 (dBm)

OPR (dB)

FIG. 6.11. Optical Power Ratio between 8PSK and new carrier.

After optimization of the OPR between PSK and new optical carrier sidebands, the OPR between the RF OOK and optical carrier sidebands are also needed to be optimized. Compared with wired transmission media, wireless signal is more sensi-tive to the transmission environment. Therefore, system operators need to be able to freely adjust the power ratio between wired and wireless services according to geo-graphic variation. FIG. 6.11 illustrates the receiver sensitivity of both RF PSK and BB OOK signals at BER of10⁻⁹with different new carrier to OOK signals OPR. The receiver sensitivity trade-off between the RF PSK and BB OOK signals are observed.

In this work, RF and BB signals with equal receiver sensitivities are set as the optimal condition, where the 8PSK and QPSK to OOK OPR are about 0 dB and 2 dB.

Figure 6.12 shows the -log(BER) curves of both the 8PSK-OOK and QPSK-OOK systems at BTB and following 25-km transmission of SSMF. No significant receiver power penalties of both the RF and BB signals in 8PSK-OOK and QPSK-OOK sys-tems are observed after the transmission. The constellations of the RF 8PSK and QPSK signals, the eye diagrams of the BB OOK signals in 8PSK-OOK and

QPSK--23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -10 10

9 8 7 6 5 4 3

OOK w/ 8PSK BTB OOK w/ 8PSK 25km 8PSK BTB

8PSK 25km OOK w/ QPSK BTB OOK w/ QPSK 25km QPSK BTB

QPSK 25km

-Log(BER)

Sensitivity(dBm)

FIG. 6.12. BER curves of OOK (w/ 8PSK), 8PSK, OOK (w/ QPSK) and QPSK.

OOK systems are also shown in FIG. 6.13. After the transmission, no significant distortions are observed in both the RF and BB signals.

6.2.3 Summary

Simultaneous generation and transmission of wireless RF M-ary PSK vector sig-nal and FTTx BB OOK sigsig-nals for hybrid access networks are experimentally demon-strated. No additional narrow-band optical filter is needed in the proposed system to separate the RF and BB signals. Only a typical low-speed photo receiver is required to recover the BB OOK single. Therefore, the proposed system is compatible with the existing WDM PON system.

In addition, frequency doubling is also achieved in the proposed system. Gen-erations of 20-GHz RF signals are experimentally demonstrated using 10-GHz RF components in this work. Moreover, high spectral efficiency QPSK and 8PSK mod-ulation formats can be used in the proposed system which is compatible with the existing wireless communication system. After the transmission over 25-km SSMF,

(a) (b) (c) (d)

(e) (f) (g) (h)

FIG. 6.13. Constellations and eye diagrams. (a) 8PSK BTB; (b) 8PSK 25km; (c) OOK w/ 8PSK BTB; (d) OOK w/ 8PSK 25km; (e) QPSK BTB; (f) QPSK 25km; (g) OOK w/ QPSK BTB; (h) OOK w/ QPSK 25km. (Eye diagrams 100 mV/div; 200 ps/div)

no significant receiver penalties are observed in both RF and BB signals.

6.3 Conclusion

Two architectures for mulit-service hybrid access network were proposed in this chapter. Simultaneous generation and transmission BB OOK signal for FTTx and RF PSK signal for RoF applications were achieved. No narrow band optical filter which severely hinders the implementation in WDM systems and is not compatible with the existing passive optical network (PON) system is require at remote node to separate the BB and RF signals. In summary, the proposed systems provide attractive and cost-effective solutions for the next generation hybrid access networks.

CONCLUSIONS

The demand and concept of optical millimeter-wave signal generation has been discussed in the thesis. Conventional optical millimeter-wave signal generations based on DSB, SSB and DSB-CS modulation schemes are discussed. DSB modulation is the simplest architecture, but chromatic dispersion induced RF fading issue restricts the fiber transmission distance. SSB modulation scheme has no chromatic dispersion induced RF fading issue. However, the week MI and high optical carrier power limit the OMI of SSB signal. Because the high OMI and no chromatic dispersion induced RF fading issue, DSB-CS modulation scheme becomes a popular choice for optical millimeter-wave signal generation. Moreover, frequency doubling can be achieved in DSB-CS modulation scheme.

An optical millimeter-wave signal generation system with frequency quadrupling was proposed in this thesis. The impact of the imbalance of the DP-MZM was inves-tigated. 40-GHz optical millimeter-wave signal with 38-dB carrier and undesired op-tical sideband suppression was experimentally demonstrated. Since no opop-tical filter is needed, the proposed approaches can be adopted for optical up-conversion in WDM radio-over-fiber systems and continuously tunable millimeter-wave signal systems.

Four channels WDM up-conversion of 1.25-Gbps OOK signals were experimentally demonstrated. Additionally, receiver sensitivity degradation due to MZM bias drifts

125

is also investigated for 20-GHz WDM signals. The receiver power penalty can be less than 1 dB when bias deviation ratios are less than 20% of the half-wave voltage, which can be achieved using a bias feedback control system. After transmission over a 50-km SSMF, the receiver power penalties of both the BB and 20-GHz RF OOK signals are less than 1 dB. The 40- and 60-GHz WDM up-conversion using 10- and 15-GHz RF driving signals are also demonstrated. The proposed system is compati-ble with existing WDM PON system. Since only low-frequency RF components and equipment are required, the proposed system is a potential solution for future WDM up-conversion system.

Optical millimeter-wave generation with frequency octupling was theoretically analyzed and experimentally demonstrated. Two commercially available DP-MZMs are keys to the proposed system. V-band 60-GHz and W-band 80-GHz optical MMW signal are experimentally generated from 7.5- and 10-GHz driving signals with 30-dB undesired sideband suppression ratios. Time domain waveform with a 50% duty cy-cle is observed. After transmission of 25-km SSMF, no significant signal distortion is observed. Optical MMW signal with frequency up to 320-GHz can be generated using the state-of-the-art 40-GHz DP-MZMs. The proposed frequency octupling opti-cal MMW generation is a potential solution for the future ultra-high frequency MMW applications. W-band wireless communication based on the frequency octupling sys-tem was experimentally demonstrated. Single-carrier 1.25-GSymbol/sec QPSK and 8QAM wireless data transmission at 102.5-GHz under direct bias modulation and 9-mA photocurrent can be achieved with 170-cm wireless transmission distance.

Optical millimeter-wave signal generation with frequency 12-tupling was pro-posed. The key to the proposed system is the optical high-purity millimeter-wave signal generation with frequency quadrupling using a commercially available dual-parallel MZM. 40- and 70-GHz high purity two-tone optical signals were experimen-tally demonstrated from 10- and 17.5-GHz RF driving signals. Modulation depth

trim-ming between MZ-a and MZ-b are employed to compensate the amplitude imbalance between the MZM arms and improve the optical carrier suppression ratio. Following optical frequency quadrupling, optical four-wave mixing was promoted utilizing an SOA. Since the excellent optical carrier and harmonic distortion suppression ratio of the FWM pump signals, only the second and sixth order sidebands were obtained at the output of the SOA. After filtering out the undesired sidebands using optical in-terleavers, high-purity optical two-tone signals separated by 120 and 210 GHz were obtained. The optical carrier and harmonic distortion ratios of the generated 210- and 120-GHz optical two-tone signals were 20 and 30 dB, respectively. The frequency of the generated optical millimeter-wave signal is 12 times that of the RF driving signal.

Millimeter-wave signals with frequency beyond 100 GHz can be easily achieved using low frequency RF equipments and components. Since the frequency response of the state-of-the-art MZM is up to 40 GHz, the proposed system provides a reliable and cost-effective solution for optical millimeter-wave generation with frequencies of up to 480 GHz.

To demonstrate the feasibility of 60-GHz RoF system, several 60-GHz RoF ar-chitecture were proposed. Frequency doubling, high spectral efficiency vector signal, and full-duplex systems were achieved in the TSSB system. The TSSB system does not suffer from dispersion induced performance fading issue. An-symmetrical full-duplex with 13.75-Gbps down-link and 1.25-Gbps up-link transmissions was achieved in the TSSB RoF system. No significant receiver power penalties was observed after transmission of 25-km SSMF.

To reduce the bandwidth of the transmitter, an 60-GHz RoF system with fre-quency sextupling was proposed based on a modified SSB modulation scheme and optical up-conversion system with frequency quadrupling. The generation and trans-mission of 13.75-Gb/s QPSK-OFDM and 20.625-Gb/s 8QAM-OFDM signals occu-pying the full 7-GHz license-free band at 60 GHz was experimentally demonstrated.

After transmission over 25-km of standard single-mode fiber, no significant received power penalty was observed with both OFDM signal formats. Based on the frequency sextupling, the bandwidth requirement of the transmitter is only 10-GHz.

To demonstrate the symmetrical full-duplex system, 60-GHz RoF system based on very simple IMDD architecture was proposed. A symmetrical2 × 21 Gbps full-duplex bidirectional RoF system using simple double-sideband IMDD links was suc-cessfully demonstrated at 60 GHz. High speed single-electrode Mach Zehnder and Reflective Electro-Absorption modulators were employed in the downlink and uplink transmission, respectively. OFDM-8QAM signal modulation format was utilized. At least 500m fiber transmission and 2.5m wireless transmission distances were achieved, without any dispersion compensation or optical filtering. The proposed full-duplex RoF system is very simple and fully transparent, making it ideal for in-building high data-rate wireless applications, which are characterized by short fiber spans.

To achieve the multi-service hybrid access network, simultaneous generation and transmission of wireless RF M-ary PSK vector signal and FTTx BB OOK signals for hybrid access networks were experimentally demonstrated. No additional narrow-band optical filter is needed in the proposed system to separate the RF and BB signals.

Therefore, the proposed system is compatible with the existing WDM PON system.

After the transmission of 25-km SSMF, no significant receiver penalties are observed in both RF and BB signals in the proposed systems. In summary, the proposed system provides an attractive and cost-effective solution for the next generation hybrid access networks.

IMBALANCE OF CONVENTIONAL MZM

Due to the manufacturing imperfection, the power splitting ratios of the Y-splitters are usually imbalanced and provide limited extinction ratio in a real MZM.

FIG. A.1 shows a conceptual diagram of a conventional MZM with imbalanced Y-splitters. γ₁ and γ₂ denote the couple factors of the input and output Y-splitters, re-spectively. Assume that the optical field at the input of the integrated MZM is defined as E_in(t) = E_ocos(ω_ot). The upper and lower arm optical field after the input Y-splitter can be expressed as

E₁ =

1 − γ1· E_o· exp[jω_ot] E₂ =√

γ₁· Eo· exp[jωot]

(A.1)

When driving voltages are applied to the MZM, optical carrier phase shifts of φ₁ and

(LQ ʒ

(

(

(ć

(ć

ɴʥ

ɴʥ

ʒ

FIG. A.1. Conceptual diagram of an imbalanced MZM.

129

φ₂ are introduced to the upper and lower arms signals. The phase shifts are

,where V_πis the half-wave voltage of the MZM, V_biasn are the bias voltages, and V_RFn are the RF driving voltages. In a single-electrode MZM,Δφ1 = −Δφ2 = Δφ. After modulation of the electrodes, the optical fields in the upper and lower arms can be expressed as

These two optical fields are combined at the output Y-splitter. Then, the output optical field becomes Besides, the extinction ratio (ER) of this MZM is defined as

ER =

With an ideal case, the couple factors are γ₁ = γ2 = 0.5. Therefore, the optical field after the modulation of the MZM can be presented as

Re{Eout} = Re{1

2 · Ein[j · exp(jΔφ) + j · exp(−jΔφ)]}

= Re{j · E_o· exp(jω_ot) · cos(Δφ)}

= −Eo· sin(ωot) · cos(π 2

1

V_π(Vbias+ VRF))

(A.6)

CHROMATIC DISPERSION INDUCED RF FADING IN DSB MODULATION SCHEME

Due to the chromatic dispersion in a dispersive SSMF, the group velocity of an optical signal transmitting in an optical fiber is wavelength dependent. The wavelength dependent group velocity causes a phenomenon referred to as group-velocity disper-sion (GVD). When an optical millimeter-wave signal is transmitted over a SSMF, different phase shifts are introduced to each optical sidebands due to the chromatic dispersion. When a DSB optical signal is transmitted over the SSMF, the generated millimeter-wave signal fades out with the increasing of the fiber distance due to the chromatic dispersion.

ʨ



ʨ



ʨ

5)

ʨ



ʨ

5)

> @

0

cos

0

A ˜ Z t

 

1

cos

0 _RF

A ˜ ª ¬ Z Z  ˜ t º ¼

 

1

cos

0 _RF

A ˜ ª ¬ Z Z  ˜ t º ¼

FIG. B.1. Conceptual diagram of a DSB optical spectrum.

133

When an optical signal with wavelength of ω transmits over a SSMF, the propa-gation constant of the signal can be expressed as

β(ω) = n(ω)ω c

= β₀+ β₁(ω − ω_o) + 1

2β₂(ω − ω_o)²+ · · ·

(B.1)

where β_m = (d^mβ/dω^m)|ω=ωo is the derivative of the propagation constant evaluated at a angular frequency of ω= ωo, and c is the speed of light. To simplify the analysis, the high order terms can be ignored at 1550 nm. For a DSB signal as shown in FIG.

B.1, the propagation constants for the sidebands with frequencies of ω = ωo± ωRF

are

β(ωo± ωRF) ∼= β0 ∓ β1(ωo)ωRF +1

2β₂(ωo)ω_RF² (B.2) and

β₂(ωo) = − c

2πf_o² · D(ωo) (B.3)

, where D is the chromatic dispersion parameter, f_o is the frequency of the optical carrier, ω_RF is the angular frequency of the millimeter-wave signal. For the SSMF, D is 17ps/(nm · km).

After the transmission over a SSMF of distance z, the electrical fields of the DSB tones can be written as

OpticalCarrier : A0· cos [ωot− β0] · t LowerSideband : A1· cos



(ωo− ωRF) · t − β0 + β1· ωRF · z − 1

2· β2· ω_RF² · z



UpperSideband : A1· cos



(ωo+ ωRF) · t − β0− β1· ωRF · z − 1

2· β2· ω_RF² · z



(B.4) After the PD square-law detection, the millimeter-wave with the frequency of ω_RF are the combination of the beating term of the optical carrier and lower optical sideband,

and the beating term of the optical carrier and the upper sideband. The photo-current with the frequency of ω_RF becomes

I_ωRF = R · A₀· A1 with frequency of ω_RF can be simplified as

I_ωRF = R · A₀· A₁

Fading of a 60.5- GHz DSB signal VPI

Matlab

FIG. B.2. Simulation results of dispersion induced fading of a 60.5-GHz DSB optical millimeter-wave signal.

Due to the chromatic dispersion, the generated photo-current with the frequency of ω_RF is related to cos [(1/2) · β2· ω_RF² · z]. Therefore, the generated millimeter-wave signal fades out with a certain distance of fiber transmission distance.

Figure B.2 shows the analytical (Matlab) and numerical (VPI) simulation fading results of a 60.5-GHz DSB optical millimeter-wave signal. The analytical and nu-merical results are matched to each other. After transmission over 1-km SSMF, the 60.5-GHz millimeter-wave signal fades out. In addition, a fading period of 2 km is observed from the simulation results.

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