Results and Discussion

Although the MZM has been used for several decades, bias drift, which affects MZM performance, remains an important issue when the MZM is biased at the full or null point. In the proposed optical up-conversion system, both MZ-a and MZ-b are biased at the full point, and MZ-c is biased at the null point. Therefore, the single channel harmonic distortion suppression ratio and performance degrading due to bias drift are investigated.

Since MZ-a is biased at the full point, bias drift of MZ-a decreases the sup-pression of odd-order optical sidebands. FIG. 3.13 illustrates the 20-GHz optical millimeter-wave signal receiver sensitivities at BER of10⁻⁹ and harmonic distortion suppression ratio versus different MZ-a bias voltage deviation ratios. Optical spec-trum with optimized, -40%, and 40% bias deviation ratios are also shown in Fig. 3.13.

The voltage deviation ratio is defined as (ΔV/Vπ × 100%) , where ΔV is voltage deviation and V_π is the half-wave voltage of the sub-MZ (i.e. 4.2 volts in this case).

The harmonic distortion suppression ratio declines from 39 dB to 7.5 dB when bias

deviation ratio becomes 40%, and the receiver sensitivity penalty of the 20-GHz RF OOK signal is about 2 dB. However, no significant reductions in receiver sensitivities exist for the 1.25-Gb/s BB OOK signals. The behavior of MZ-b voltage deviation is similar to the results of that in MZ-a. FIG. 3.14 shows the 20-GHz optical millimeter-wave signal receiver sensitivities at BER of10⁻⁹and harmonic distortion suppression ratio versus different MZ-c bias voltage deviation ratios. Optical spectra with -50%

and 50% bias deviation ratios are also shown in Fig. 3.14. Differing from the voltage drift effects of MZ-a and MZ-b, the harmonic distortion suppression ratio decreases form 39 dB to 17 dB when bias drift is almost 50%, which corresponds to a voltage deviation of roughly 2.1 volts and can be considered an extreme case. However, no significant sensitivity penalties of the 20-GHz RF OOK and BB OOK signals are ob-served. This work also investigates the declines of harmonic distortion suppression ratio and receiver sensitivities when all biases of the dual-parallel MZM drift from the optimal point. FIG. 3.15 shows the 20-GHz optical millimeter-wave signal receiver sensitivities at BER of10⁻⁹and harmonic distortion suppression ratio versus different bias voltage deviation ratios in all sub-MZM. Optical spectrum with -30% and 30%

bias deviation ratios are also shown in FIG. 3.15. When the biases of all sub-MZM drift 30% from the optimal point, the harmonic distortion suppression ratio degrades from 39 dB to 5 dB, and the 20-GHz RF OOK signal has a penalty of about 3 dB. And there are still no significant power penalties of the BB OOK signal.

After the square-law detection, the RF OOK signal comes from the beating terms of the two second-order optical sidebands. However, the receiver sensitivity is defined as the total received optical power. If the harmonic distortion suppression ratio de-grades due to bias drifts of the dual-parallel MZM, a portion of the total optical power is taken by the undesired optical sidebands; thus, the receiver sensitivity of the desired millimeter-wave signal is diminished. In the case of bias drifting in a and MZ-b, odd order optical sidebands steal a significant portion of the total optical power,

1549.75 1550.00 1550.25 1550.50

FIG. 3.13. Sensitivities and harmonic distortion suppression ratio versus MZ-a bias drifts, and optical spectrum with different bias drifts.

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

FIG. 3.14. Sensitivities and harmonic distortion suppression ratio versus MZ-c bias drifts, and optical spectrum with different bias drift.

-30 -20 -10 0 10 20 30

FIG. 3.15. Sensitivities and harmonic distortion suppression ratio versus all sub-MZ bias drifts, and optical spectrum with different bias drift.

resulting in receiver sensitivity penalties. Nevertheless, in the case of bias drifting in MZ-c, only the optical carrier and 4^th order optical sidebands emerge with bias drifting. In the proposed system, the original optical carrier can be totally suppressed due to the advantage of the modulation depth trimming of the proposed frequency technique. The modulation depth trimming is done by adjusting the amplitudes of electrical signals for driving MZ-a and MZ-b. Therefore, only the 4th order optical sidebands contribute to the decline of RF receiver sensitivity when MZ-c bias drifts.

Base on the properties of Bessel function, the optical power of 4th order optical side-bands are quite small compared with the desired 2nd optical sideside-bands. The harmonic distortion suppression ratio still exceed 20 dB when the MZ-c bias drifts about 50%.

With such high harmonic distortion suppression ratio, no significant power penalty of RF signals exists when the MZ-c bias drifts. Receiver sensitivities of the proposed system can be less than 1 dB when bias drifts are controlled within 20% of the half-wave voltage using a bias feed-back control system. On the other hand, the BB OOK signal is related to the square terms of all optical sidebands; therefore, the BB OOK signal is insensitive to the biased drifts.

Figure 3.16 shows the optical spectrum of the4 × 1.25-Gb/s BB WDM signals and the up-converted4×1.25-Gb/s 20-GHz WDM signals. All WDM channels are

up-1550 1551 1552 1553

FIG. 3.16. Optical spectra of the (a) BB WDM signals; (b) 20-GHz WDM signals.

-46 -44 -42 -40 -38 -36 -34 -32 -30 -28 (i) BTB (100mV/div; 200 ps/div)

(ii) 50km (100mV/div; 200 ps/div) -44 -42 -40 -38 -36 -34 -32 -30 -28 (i) BTB (68uW/div; 200 ps/div)

(ii) 50km (68uW/div; 200 ps/div)

FIG. 3.17. BER curves of the 20-GHz WDM signals at (a) 20-GHz; (b) Baseband.

converted simultaneously using only one dual-parallel MZM. FIG. 3.17 (a) shows the BER curves and eye diagrams of the BTB and transmitted 20-GHz WDM RF OOK signals. After transmission over 50-km SSMF, the receiver power penalty of each channel is less than 1 dB. FIG. 3.17 (b) shows the BER curves and eye diagrams of the BTB and transmitted WDM BB OOK signals. Receiver power penalties are neg-ligible after transmission over the 50-km SSMF. 40-GHz and 60-GHz WDM optical up-conversion systems are also demonstrated. Nevertheless, the receiver sensitivities and BER analysis are not shown here due to the unavailability of 40- and 60-GHz receiver systems in the laboratory. FIG. 3.18 (a) shows the optical spectra of the 40-GHz up-converted WDM signals. The harmonic distortion suppression ratio of the up-converted WDM signals are more than 35 dB. Figure 3.18 (b) shows the spectra of up-converted 60-GHz WDM signals. The harmonic distortion suppression ratio of the WDM channels are also more than 35 dB.

3.5 Conclusion

A frequency quadrupled optical millimeter-wave generation system was pro-posed in this chapter. Two tone optical millimeter-wave signal with frequency separa-tion four times of the modulasepara-tion frequency can be generated using only one external

1550 1551 1552 1553

FIG. 3.18. Spectra of Up-converted WDM signals at (a) 40-GHz; (b) 60-GHz.

modulator without optical filtering. Theoretical derivations of ideal and imbalanced DP-MZM were demonstrated. 40- and 72-GHz optical millimeter-wave signals were experimentally demonstrated using the proposed system. The harmonic distortion suppression of the generated optical millimeter-wave signals are higher than 36 dB.

Since no narrow-band optical filter is required, this system can be utilized in WDM up-conversion. 20-GHz dual-service hybrid access network which can simultaneously provides RF and BB transmissions were experimentally demonstrated. The harmonic distortion suppression of the generated 20-GHz signals are higher than 39 dB. Addi-tionally, receiver sensitivity degraded due to MZM bias drifts is also investigated in this work 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 compatible with ex-isting WDM PON system. Since only low-frequency RF components and equipment are required, the proposed system is a potential solution for high frequency optical millimeter-wave signal generation and WDM up-conversion system.

OPTICAL MILLIMETER-WAVE GENERATION SYSTEM WITH HIGHER ORDER

MULTIPLICATION

The tremendous increase in the desired bandwidth of wireless data-transmission has attracted considerable attention on various methods to utilize the millimeter-wave bands with ultra high carrier frequency beyond 100 GHz [21, 57]. Not only the millimeter-wave band wireless communication, but many other applications, such as Large Scale Phase Array Antenna [36], millimeter-wave imaging [37], and Tera-hertz applications, require the ultra-high frequency up to several hundred GHz.

Although some CMOS technology can generate MMW signal beyond 60 GHz, the attenuation of millimeter-wave signal in copper wires is extremely high, which restricts the signal transmission distance. To transmit MMW signals over a long dis-tance, optical millimeter-wave signal based on low transmission loss optical fiber net-work is a cost-effective and viable solution. Conventional optical millimeter-wave signalg generation based on DSB, SSB and DSB-CS modulation schemes have been discussed in Chapter 2. Nevertheless, the generated optical millimeter-wave frequen-cies are still restricted by the bandwidth ofLiNbO3modulators, which is typically less than 40 GHz. Moreover, radio-frequency (RF) components with frequency response

41

over 26 GHz are considerably more expensive than those below 26 GHz. In order to achieve optical MMW generation with frequency beyond 60 GHz cost-effectively, optical millimeter-wave signal generations with frequency multiplication are highly desirable. In Chapter 3, an frequency-quadrupled optical millimeter-wave genera-tion system without any narrow-band optical filter was proposed. Optical millimeter-wave signals with frequency up to 72 GHz was experimentally demonstrated. How-ever, to support higher frequency applications, optical millimeter-wave generation systems with higher order frequency multiplication are required. In this chapter, op-tical millimeter-wave generation system with frequency octupling and 12-tupling will be proposed. Theoretical derivation and experimental demonstration of the optical millimeter-wave generation systems will be performed. Optical millimeter-wave sig-nals with frequency up to 210 GHz are generated. Utilizing the generated 100-GHz optical millimeter-wave signals, W-band wireless communication system is experi-mentally demonstrated with 3.75-Gb/s single carrier 8 Quadrature Amplitude Modu-lation (8-QAM) signals.

4.1 Optical Millimeter-Wave Generation with Frequency Octupling

4.1.1 Concept and Theoretical Model

Figure 4.1 shows the experimental setup of the optical millimeter-wave genera-tion with frequency octupling. The frequency-octupled optical millimeter-wave gener-ation system consists of two cascaded frequency-quadrupled optical millimeter-wave generation systems. Assume that the optical field at the input of the first DP-MZM is defined as E_in(t) = Eocos(ωot), where Eo is the amplitude of the optical field and ω_o is the angular frequency of the optical carrier. In the first stage, both of the sub-MZMs (MZ1-a and MZ1-b) are biased at the full point while the main MZM of the first DP-MZM is biased at the null point and introduces a180^◦ phase difference

be-tween the output of the two sub-MZMs. A90^◦phase difference is introduced between the RF driving signals of MZ1-a and MZ1-b. Therefore, the electrical RF driving sig-nal sent into MZ1-a and MZ1-b can be expressed as V_a1(t) = Vm1· cos(ωRFt) and V_b1(t) = Vm1 · cos(ωRFt+ π/2), respectively, where Vm1 denotes the amplitude of the RF driving signal of the first dual-parallel MZM and ω_RF denotes the angular fre-quency of the RF driving signal. From equation (3.4) , the optical field at the output of the first DP- MZM can be expressed as:

E_out−1 = 1

,where the phase modulation index m₁ is πV_m1/2V_π and J_4n−2 is the Bessel function of the first kind with order 4n-2. Only optical sidebands with the order of 4n-2 will be obtained. Due to the properties of Bessel function, without causing significant errors, it is reasonable to ignore the sidebands with orders higher than the second one.

Therefore, the optical field at the output of the first DP-MZM can be further

MZ2-a

FIG. 4.1. Conceptual diagram and experimental setup of the frequency-octupled optical millimeter-wave signal generation system.

simplified as

E_out−1(t) = −Eo· {J2(m1) cos[(ωo+ 2ωRF)t] + J2(m1) cos[(ωo− 2ωRF)t]} (4.2)

After the first stage, a frequency-quadrupled optical millimeter-wave signal is ob-tained. The generated optical millimeter-wave signal from the first DP-MZM is then send into the second DP-MZM. Both of the sub-MZMs (MZ2-a and MZ2-b) are bi-ased at the full point while the main MZM is bibi-ased at the null point. Note that a 45^◦phase delay is introduced between the driving signals of the first and second DP-MZMs. Therefore, the electrical RF driving signal sent into MZ2-a and MZ2-b can be expressed asV_a2(t) = Vm2· cos(ωRFt+ π/4) and Vb2(t) = Vm2· cos(ωRFt+ 3π/4), respectively. Then, the optical field at the output of the second DP-MZM can be ex-pressed as ,where the phase modulation index m₂ is πV_m2/2Vπ. After the frequency-octupled optical millimeter-wave signal generation system, only the optical sidebands with the order of4n will be obtained. Based on the characteristic of Bessel function, the higher

MZ2-a

FIG. 4.2. Principle of the frequency-octupled optical millimeter-wave signal generation system.

order terms can be ignored without causing significant effect. The generated optical millimeter-wave signals can be simplified as

E_out−2 =Eo· [J2(m1)J2(m2) − J2(m1)J6(m2) − J6(m1)J2(m2)]

· {− sin [(ωo+ 4ωRF) t] + sin [(ωo− 4ωRF) t]}

(4.4)

Notably, no optical filter is required to remove undesired optical sidebands. After square-law detection using a photo diode, the electrical signal with frequency eight times that of the RF driving signal is obtained.

Figure 4.2 schematically depicts the principle of the proposed millimeter-wave generation system. Since MZ1-a and MZ1-b are biased at the full point, optical spec-tra with two second-order sidebands are performed after MZ1-a and MZ1-b as shown in insets (ii) and (iii) of FIG. 4.2. The90^◦ phase delay between the driving signal of MZ1-a and MZ1-b causes the180^◦phase difference of the two second order sidebands at the output of MZ1-a and MZ1-b. Since the main MZM is biased at the null point, an additional180^◦phase delay is introduced between the output signals of MZ1-a and MZ1-b. After the combination at the output of the first DP-MZM, the original optical

carrier is inherently suppressed. Optical millimeter-wave signal with frequency qua-drupling is obtained at the output of the first DP-MZM as shown in the inset (vi) of FIG. 4.2. The generated second order sidebands from the first DP-MZM are treated as two new optical carriers and send into the second DP-MZM. Due to the45^◦ phase delay of the second DP-MZM driving signals, optical spectra as shown in insets (vii) and (viii) of FIG. 4.2 are generated from MZ2-a and MZ2-b, respectively. Optical sidebands with the same frequency as the original optical carrier are suppressed. The second main MZM, which is biased at null-point, introduces a180^◦ phase difference between the output signals of MZ2-a and MZ2-b as shown in insets (ix) and (x) of FIG.

4.2. After combination at the output of the second DP-MZM, an optical millimeter-wave signal with frequency eight times that of the driving signal is obtained as shown in inset (xi) of FIG. 4.2.

4.1.2 Experimental Setup and Results

Following the setup shown in FIG. 4.1, a CW laser with 4-dBm optical power is used as the optical source. To generate 60-GHz optical millimeter-wave signal, a 7.5-GHz sinusoid driving signal is utilized. The modulation indices of both DP-MZMs are about1.6 × (Vπ/2). As shown in FIG. 4.3 (a), a 30-GHz optical millimeter-wave signal with 29-dB harmonic distortion suppression ratio and -6.8-dBm optical power is obtained after the first DP-MZM. The generated optical millimeter-wave signal is sent into the second DP-MZM. At the output of the second DP-MZM, a 60-GHz optical millimeter-wave signal with 30-dB undesired sideband suppression ratio and -19-dBm optical power is obtained. The optical spectrum is shown in Fig. 4.3 (b). After the frequency octupling system, an EDFA is utilized to boost the optical power. Because of the slight different V_πof the PD-MZMs, tunable attenuators are employed after the first electrical splitter to control the driving power of two DP-MZMs.

The45^◦ phase difference between the driving signal of the first and second

DP-1550.6 1550.8 1551.0 1551.2 1551.4

1550.6 1550.8 1551.0 1551.2 1551.4 -90

FIG. 4.3. Optical spectra of (a) the 30-GHz millimeter-wave signal generated from the first stage of the frequency octupling system; (b) the generated 60-GHz millimeter-wave signal from the frequency octupling system.

MZM is a key factor which affects the suppression of the original optical carrier. The optical carrier suppression ratio of the original carrier will degrade if the phase dif-ference is not equal to 45^◦. Fig. 4.4 (a) shows the degradation of the optical carrier suppression ratio due to imperfect phase delay between two DP-MZM driving signals with varying phase delay from 30^◦ to 60^◦. The best carrier suppression ratio is ob-tained with45^◦ phase delay.The optical spectra with 30^◦ and 60^◦ are also shown in insets (b) and (c) of Fig. 4.4. Fig. 4.5 (a) and (b) shows the waveform of the generated 60-GHz millimeter-wave signal using a V-band photo-diode at back-to-back (BTB) and after transmission of 25-km standard single mode fiber (SSMF), respectively. Be-cause of the high undesired sideband suppression ratio, a 60-GHz millimeter-wave signal with a 50% duty cycle is observed. Moreover, no significant signal distortion is observed after fiber transmission. The electrical spectrum of the generated 60-GHz millimeter-wave signal is shown in FIG. 4.6.

To demonstrate W-band wave signal generation, 80-GHz millimeter-wave signal is generated using 10-GHz driving signal. FIG. 4.7 shows the optical spectrum of the generated 80-GHz optical millimeter-wave signal. The undesired sideband suppression ratio of the 80-GHz optical millimeter-wave signal is about 30

1550.6 1550.8 1551.0 1551.2 1551.4

Phase Delay ( Deg ) ^{-70 1550.6 1550.8 1551.0 1551.2 1551.4}

-60

FIG. 4.4. (a) Optical carrier suppression ratio versus phase delay of two DP-MZMs and optical spectra of (b) 30 deg (c) 60 deg phase delay.

dB.

To evaluate the performance of the proposed system, undesired sideband sup-pression ratio degradation with DP-MZM bias drifts are also investigated. FIG. 4.8 depicts the undesired sideband suppression ratio degradations with bias drifts of the first-stage DP-MZM. The undesired sideband suppression ratios degrade from 30 dB to 3 dB with 25% bias voltage deviation ratio. The bias voltage deviation ratio is

(a) (b)

FIG. 4.5. Time domain waveform of the generated 60-GHz millimeter-wave signal (a) BTB (b) 25-km SMF Transmission. (100 mV/div; 5 ps/div)

58 59 60 61 62 -140

-120 -100 -80 -60

Level (dBm)

Frequency (GHz)

FIG. 4.6. Electrical spectrum of the generated 60-GHz millimeter-wave signal.

1549.5 1550.0 1550.5

-80 -70 -60 -50 -40 -30 -20

Level (dBm)

Wavelength (nm) 30.2 dB

FIG. 4.7. Optical spectrum of the generated 80-GHz optical millimeter-wave signal.

-25 -20 -15 -10 -5 0 5 10 15 20 25

Undesired Sideband Suppression Ratio (dB)

Bias Drift (%)

FIG. 4.8. Undesired sideband suppression ratio versus first DP-MZM bias deviation ratio, and optical spectra.

Undesired Sideband Suppression Ratio (dB)

Bias Drift (%)

FIG. 4.9. Undesired sideband suppression ratio versus second DP-MZM bias deviation ratio, and optical spectra.

(i) (ii)

Undesired Sideband Suppression Ratio (dB)

Bias Drift (%)

FIG. 4.10. Undesired sideband suppression ratio versus all DP-MZM bias deviation ratio, and optical spectra.

defined as(ΔV/Vπ) × 100%, where ΔV is bias voltage deviation and Vπ is the half-wave voltage of the sub-MZs. Optical spectra with the optimal and 25% bias drift conditions are shown in insets of FIG. 4.8. The undesired sideband suppression ra-tio degradara-tions and optical spectra with second-stage DP-MZM bias drifts are also shown in FIG. 4.9. 30-dB degradation is observed with 25% bias voltage deviation ratio. FIG. 4.10 shows the undesired sideband suppression ratio degradation with all the DP-MZM bias drafts. About 30-dB undesired sideband suppression ratio degra-dation is observed with 25% bias voltage deviation ratio. The undesired sideband suppression ratio will be higher than 15 dB, which is sufficient for most

defined as(ΔV/Vπ) × 100%, where ΔV is bias voltage deviation and Vπ is the half-wave voltage of the sub-MZs. Optical spectra with the optimal and 25% bias drift conditions are shown in insets of FIG. 4.8. The undesired sideband suppression ra-tio degradara-tions and optical spectra with second-stage DP-MZM bias drifts are also shown in FIG. 4.9. 30-dB degradation is observed with 25% bias voltage deviation ratio. FIG. 4.10 shows the undesired sideband suppression ratio degradation with all the DP-MZM bias drafts. About 30-dB undesired sideband suppression ratio degra-dation is observed with 25% bias voltage deviation ratio. The undesired sideband suppression ratio will be higher than 15 dB, which is sufficient for most