Chapter 1 INTRODUCTION
1.5 Objective and Outlines of the Thesis
The application and the challenge of the generation of the optical wave signal have been discussed in this chapter. The desire of the optical millimeter-wave generations using reliable external modulators with frequency multiplication are also discussed. Moreover, the advantages of wireless communication at millimeter-wave bands are stated. Therefore, the optical millimeter-millimeter-wave signal generation along
with millimeter-wave RoF communication systems are of great interests. In this the-sis, optical millimeter-wave signal generation with high order multiplication will be proposed. Theoretical and experimental demonstration will be performed. 60-GHz RoF technologies based on different optical millimeter-wave generation system and multiplication system will be proposed. To demonstrate ultra-high data-rate transmis-sion, high spectral efficiency orthogonal-frequency-division-multiplication (OFDM) modulation formats will be utilized.
In Chapter 2, the basic idea of optical millimeter-wave signal generation based on conventional dobule-sideband (DSB), single-sideband (SSB), and double-sideband with carrier suppression (DSB-CS) modulation schemes will be discussed. The ad-vantage and disadad-vantages among these modulation schemes will be discussed. The basic ideal of the optical millimeter-wave signal detection will also been discussed in Chapter 2.
A novel frequency-quadrupled optical millimeter-wave signal generation system without narrow-band optical filtering will be proposed in Chapter 3. Theoretical anal-ysis and experimental demonstration of this system will be performed. The impact of the MZM imbalance will also be investigated. Since no narrow band optical fil-ter is needed in this system, Wavelength-Division-Multiplexed (WDM) optical up-conversion system with frequency quadrupling can be realized based on the proposed system.
To meet ultra-high frequency wave applications, two optical millimeter-wave signal generation system with frequency octupling and 12-tupling will be pro-posed in Chapter 4. 80-GHz optical millimeter-wave signal generation using the frequency-octupled optical millimeter-wave generation system will be experimentally demonstrated. W-band wireless communication system using the generated 100-GHz optical millimeter-wave signal along with a Near-Ballistic Uni-Traveling Carrier Pho-todiode (NBUTC-PD) will also been demonstrated. 210-GHz optical millimeter-wave
signal generation using a frequency-12-tupled system will also be proposed in this chapter.
In Chapter 5, 60-GHz RoF technologies will be discussed. To achieve high data-rate transmission and overcome the uneven frequency response, OFDM signal gener-ation and modulgener-ation techniques will be developed. 60-GHz RoF systems based on Tandem-Signle-Sideband(TSSB) with frequency doubling, optical up-conversion with frequency sextupling and Intensity Modulation Direct Detection (IMDD) systems will be proposed. Experimental demonstration of bidirectional full-duplex system at 60-GHz based on IMDD system will experimentally demonstrated.
In Chapter 6, two multi-service hybrid access network systems without narrow-band optical filter at remote nodes will be proposed. Since no narrow narrow-band optical filter is required at the remote nodes, the proposed system can be implemented in WDM systems.
This thesis ends with Chapter 7, where conclusions will be drawn.
OPTICAL MILLIMETER-WAVE GENERATION USING EXTERNAL MODULATOR AND
SQUARE-LAW DETECTION
Millimeter-wave signals have been extensively utilized in various applications, such as broadband wireless communication [35], Atacama large millimeter arrays (ALMA) [36], radars, millimeter-wave imaging [37], radio-over-fiber systems [38], and tera-hertz applications. With the increasing of the millimeter-wave signal fre-quency, the transmission loss in copper wire becomes extremely high and restricts the signal coverage. In addition, all-electronic millimeter-wave signal generation beyond 100 GHz remains a serious challenge because of restrictions on frequency responses of electronic devices and equipments. Accordingly, optical millimeter-wave signal generation techniques have become extremely attractive, due to the ability to generate optical millimeter-wave signals with frequencies of higher than 100 GHz. Moreover, the coverage of the optical millimeter-wave signal can be readily extended thanks to the low transmission loss of optical fibers. Many optical millimeter-wave generation approaches have recently been developed, including optical heterodyning with two laser sources [36], mode-locked lasers [39–41], and external modulation using electro-absorption modulators (EAM), phase modulators (PM), or Mach-Zehnder modulators
11
(MZM) with a single wavelength laser source [13, 38, 42–48].
Using external optical modulators represent the most reliable solutions for opti-cal millimeter-wave signal generation because these modulators have been extensively used in telecommunication system with proven stability and reliability. There are three main conventional modulation schemes which generate optical millimeter-wave signals, including Sideband (DSB), Single-Sideband (SSB), and Double-Sideband with Carrier Suppression (DSB-CS) [42, 43]. In this chapter, optical millimeter-wave signal generations using external electro-optical modulators base on these three modulation schemes will be discussed and compared. When the optical millimeter-wave signals are obtained, high speed Photo-Diodes (PD) are usually uti-lized for Optical/Electrical conversion. The basic concept of PD square-law detection will also be introduced in this chapter.
2.1 Double-Sideband Modulation Systems
DSB modulation scheme is the most direct method to generate an optical millimeter-wave signal. Figure 2.1 shows an example of 40 GHz DSB optical sig-nal generation using a single-electrode MZM. The MZM is biased at quadrature point (0.5 Vπ), and a 40 GHz sinusoidal signal is utilized to drive the MZM. After the MZM, a DSB signal as shown in FIG. 2.1 is obtained.
Although the DSB signal can be easily generated, there are several disadvantage in DSB modulation schemes, such as chromatic dispersion induced performance fad-ing issue, higher modulator bandwidth requirement, and limited Optical Modulation Index (OMI). Since there are two identical optical sidebands with different wave-length in the DSB signal, the generated Radio-Frequency (RF) power might fade with different fiber transmission distance due to chromatic dispersion. The detail of the chromatic dispersion induced RF fading issue will be discussed in Appendix B.
Be-DFB
40 GHZ
MZM
0.5 V
40 GHZ
FIG. 2.1. Double sideband (DSB) modulation scheme
cause no frequency multiplication is achieved in DSB modulation scheme, the trans-mitter bandwidth requirement is higher compared with the modulation schemes with frequency multiplication. In addition, the optical carrier in a DSB signal is usually higher than the optical sidebands which leads to a limited OMI and provides limited signal conversion efficiency after the PD square-law detection.
2.2 Single-Sideband Modulation Systems
To overcome the chromatic dispersion induced RF fading issue, optical millimeter-wave generation based on SSB modulation scheme is proposed. FIG 2.2 shows an example of 40 GHz SSB optical signal generation. To generate the SSB optical signal, a dual-electrode MZM is utilized. The MZM is biased at quadrature point (0.5 Vπ).
The 40 GHz driving signal is divided into two paths to drive to upper and lower arm of the dual-electrode MZM. An additional π/2 phase difference is added on the lower arm driving signal. After the MZM, a SSB signal as shown in Fig. 2.2 is obtained.
Since only one optical sideband is obtained, there is no chromatic dispersion in-duced RF fading issue in the SSB signal. However, the Modulation Index (MI) in SSB modulation scheme is limited to prevent the nonlinear effects from MZM. Therefore,
DFB
40 GHZ
MZM 2 0.5 V
/2
40 GHZ
FIG. 2.2. Single sideband (SSB) modulation scheme
the optical carrier is higher than the optical sideband and the OMI is also limited. To increase the OMI, a fiber Bragg grating is usually utilized to suppress the optical car-rier power. In addition, no frequency multiplication is achieved in SSB modultiaon scheme. Moreover, the modulation system is much more complex compared to DSB modulation scheme.
2.3 Double-Sideband with Carrier Suppression Systems
To generate optical millimeter-wave signal using external modulators with fre-quency doubling and without chromatic dispersion induced RF fading issue, DSB-CS modulation scheme was proposed. FIG. 2.3 shows the setup of a 40 GHz DSB-CS signal generation using a single-electrode MZM. To achieve the DSB-CS modulation scheme, the MZM is biased at null point (Vπ), and a 20 GHz sinusoidal signal is uti-lized to drive the MZM. After the MZM, a DSB-CS signal as shown in Fig. 2.3 is obtained.
Since the RF signal is generated from the two second order optical sidebands after the PD square-law detection, there is no chromatic dispersion induced RF fading issue in DSB-CS modulation scheme. The optical power between these two optical
side-DFB
V
40 GHZ 20 GHZ
MZM
FIG. 2.3. Double sideband with carrier suppression (DSB-CS) modulation scheme
bands are equal to each other, the DSB-CS signal provides an OMI of one. Moreover, optical frequency doubling is achieved in DSB-CS modulation scheme. Therefore, the DSB-CS modulation scheme is one of the most popular choice for high frequency optical millimeter-wave signal generation.
2.4 Square-Law Detection using Photo Diode
When the optical millimeter-wave signals are generated, a PD is usually utilized to receive the optical signal and perform the O/E conversion. In this section the basic concept of the O/E conversion using a PD will be discussed.
Photon Current Electrical Spectrum DC 1– 2
Optical Signal
Optical Spectrum
1
2
FIG. 2.4. Double sideband with carrier suppression (DSB-CS) modulation scheme
Figure 2.4 shows the conceptal diagram of the O/E conversion. A two tone opti-cal millimeter-wave signal with angular frequencies of ω1 and ω2 is received using a PD. The electrical fields of the two tone signal can be expressed as E1 = A1cos(ω1t) and E2 = A2cos(ω2t), where A1 and A2 represent the amplitude of the electrical fields. After the square-law detection of the PD, the generated photo current can be expressed as
iP D = R · (E1 + E2)2
= R · A1· A2· cos[(ω1− ω2)t] + R · A1 · A2cos[(ω1+ ω2)t] + DC T erms (2.1) Ignore the DC terms, the generated millimeter-wave signal is related to the first term of equation (2.1). Therefore, the electrical field of the generated millimeter-wave signal can is proportional to the first term of equation (2.1) and can be expressed as
VRF ∝ R · A1· A2· cos[(ω1− ω2)t] (2.2)
2.5 Conclusion
Optical millimeter-wave signal generation using external modulator is one of the most reliable technique. Optical millimeter-wave signal generation based on DSB, SSB and DSB-CS modulation schemes were discussed in this chapter. DSB modu-lation 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 weak MI and high optical car-rier 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 is achieved in DSB-CS modulation scheme.
OPTICAL MILLIMETER-WAVE GENERATION SYSTEM WITH FREQUENCY QUARDRUPLING
Optical millimeter-wave signal generation at frequencies beyond 40 GHz remains a major challenge because of the frequency response ofLiNbO3Mach-Zehnder mod-ulator (MZM) or phase modmod-ulator are usually less than 40 GHz. Moreover, the elec-trical components and equipment at frequencies beyond 40 GHz, such as amplifiers, mixers, and synthesizers, are very expensive. Therefore, a cost-effective means of generating high frequency optical millimeter-wave signal is of great interest.
Many optical millimeter-wave signal generation schemes based on MZM or phase modulator to achieve frequency multiplication have recently been demonstrated [13, 48–56]. However, these proposed systems with frequency multiplication of more than two times either depend on more than one optical filter to remove undesired op-tical sidebands [13, 48, 50–56] or need two cascaded external modulators [13], which significantly increase the complexity and cost of the system. Besides, the required optical filtering severely hinders the implementation of optical up-conversion in a wavelength-division-multiplexer (WDM) radio-over-fiber (RoF) system as shown in FIG. 3.1. Although WDM up-conversion using only one external modulator with op-tical filters is demonstrated in [11], only frequency doubling is achieved.
In this chapter, a carrier-suppressed optical millimeter-wave signal generation 17
1
Optical Up-Conversion
2
3
n
1 2 3 n
1
2
3
n
1 2 3 n
Laser PD
Optical Fiber
FIG. 3.1. Optical colorless up-conversion using a frequency multiplication technique for WDM RoF systems.
with frequency quadrupling is proposed. No narrow band optical filter is required to suppress the undesired optical sidebands, and only one dual-parallel (DP-MZM) is utilized in the architecture. High undesired optical sideband suppression ratio can be achieved, and the high-purity two-tone optical millimeter-wave signal does not suffer from impairment due to fiber dispersion. In addition, WDM optical up-conversion with frequency quadrupling will also be demonstrated in this chapter.
3.1 Concept and Theoretical Model
Figure 3.2 shows a conceptual diagram of the optical millimeter-wave generation with frequency quadrupling. A DP-MZM that comprises of three sub-MZMs is key to generating optical millimeter-wave signals. One sub-MZM (MZ-a or MZ-b) is embedded in each arm of the main modulator (MZ-c). The optical field at the input of the integrated MZM is defined as
Ein(t) = Eocos(ωot) (3.1)
MZ-a
FIG. 3.2. Setup of the optical frequency-quadrupled millimeter-wave signal generation system.
,where Eo is the amplitude and ωois the angular frequency of the optical field.
In the DP-MZM, both MZ-a and MZ-b are biased at the full point. The driving signal is generated from a RF signal generator, and separated into two ways. An additional π/2 phase delay is added on the lower path. Therefore, the electrical driving modula-tion signals sent into MZ-a and MZ-b can be expressed as Va(t) = Vmcos(ωRFt) and Vb(t) = Vmcos(ωRFt+ π/2), respectively. After the modulation of MZ-a and MZ-b, the output signal from MZ-a and MZ-b can be expressed as
Eout−a= √1
FIG. 3.3. (a) Illustrated optical spectrum and (b) Illustrated electrical spectrum of of frequency-quadrupled optical millimeter-wave signal
,where Vπ is the half-wave voltage of the MZM and m is the modulation index. Ex-panding Eo · cos[m · cos(ωRFt)] and Eo · cos[m · sin(ωRFt)] using Bessel function enables the output optical field to be rewritten as
Eout = Eo
2 · cos(ωot) · {2 ·
∞ n=1
J2n(m) · (−1)ncos(2n · ωRFt)
− 2 ·
∞ n=1
J2n(m) · cos(2n · ωRFt)}
= −Eo·∞
n=1
J4n−2(m) · {cos[(ωo+ (4n − 2)ωRF)t]
+ cos[(ωo− (4n − 2)ωRFt)]}
(3.4)
,where Jn is the Bessel function of the first kind with order n. From equation (3.4), only optical sidebands with the order of4n − 2 will be obtained after the DP-MZM
as shown in FIG. 3.3 (a). Due to the properties of Bessel function, without causing significant errors, it is reasonable to ignore sidebands with orders higher than the second one. Therefore, the optical field at the output of the first DP-MZM can be further simplified as
Eout = −Eo· {J2(m) cos[(ωo+ 2ωRF)t] + J2(m) cos[(ωo− 2ωRF)t]
+ J6(m) cos[(ωo+ 6ωRF)t] + J6(m) cos[(ωo− 6ωRF)t]} (3.5)
Then, the generated optical millimeter-wave signal is detected using a PD with a re-sponsivity of R. The generated photocurrent can be expressed as
i4ωRF = R · Eo2· [J2(m) · J2(m) + 2 · J2(m) · J6(m)] · cos(4ωRFt) i8ωRF = R · Eo2· [2 · J2(m) · J6(m)] · cos(8ωRFt)
i12ωRF = R · Eo2· [J6(m) · J6(m)] · cos(12ωRFt)
(3.6)
FIG. 3.3 (b) displays the electrical spectrum of the generated millimeter- wave sig-nal after the square-law PD detection. Only the desired millimeter-wave sigsig-nal with frequency of4ωRF and the harmonic distortion signals with frequency of4nωRF are observed in the electrical spectrum, where n is an integer that exceeds two.
Figure3.4 schematically depicts the principle of optical millimeter-wave genera-tion with frequency quadrupling. Since MZ-a and MZ-b are biased at full point, the optical carrier and even-order sidebands are observed, as shown in insets (ii) and (iii) of FIG. 3.4. The optical sidebands with orders of more than two are neglected for simplicity. The 90◦ phase difference between the sinusoidal signals that drive MZ-a and MZ-b causes the polarities of the two second-order sidebands at the output of MZ-a to oppose those at the output of MZ-b. As the MZ-c is biased at the null point, an extra180◦ phase difference is added to all optical sidebands of the lower arms of MZ-c, as shown in insets (iv) and (v) of FIG. 3.4. Notably, the two optical carriers are
ŎśĮŢ
ŎśĮţ ŎśĮŤ ĩŪĪ
ĩŪŪĪ
ĩŪŪŪĪ
ĩŪŷĪ
ĩŷĪ
ĩŷŪĪ 4RF
2RF
FIG. 3.4. Schematic principle of the optical millimeter-wave generation with frequency quadrupling
out of phase, but the second-order sidebands are in phase. Therefore, after MZ-c, the optical carrier is eliminated and only two second-order sidebands remain, which can be converted into an electrical quadruple-frequency millimeter-wave signal following square-law PD detection.
3.2 Impact of The PD-MZM Imbalance
Theoretical derivation of the frequency-quadrupled optical millimeter-wave gen-eration using an ideal DP-MZM has been discussed in the last section. In the ideal case, only the optical sidebands with the order of ±(4n − 2) can be observed after the DP-MZM. However, the other optical sidebands can also be observed due to the limited extinction ratios (ERs) of the DP-MZM in a real case. The limited ERs come from the manufacturing defect of the Y-splitters in the DP-MZM which introduce im-balance of split ratio to the Y-splitters. Theoretical derivation of the impact of the MZM imbalances will be performed in this section.
(LQ
FIG. 3.5. DP-MZM with Y-spliter imbalance.
Figure 3.5 illustrates a conceptual diagram of a DP-MZM with Y-splitter imbal-ances, where γ denote the coupling factors at different Y-splitters of the sub-MZMs (ie. MZ-a and MZ-b)and main MZM (ie. MZ-c), andΔφ denotes the phase shifts in-troduced by the applied driving voltages. Assume that the electrical field of the laser source is Ein(t) = Eocos(ωot). After the input Y-splitter of MZ-c, the electrical fields become
E1 =
1 − γc1· Eo· Exp(jωot) E2 =√
γc1· Eo· Exp(jωot) (3.7)
Then, E1 and E2 are sent into MZ-a and MZ-b, respectively. MZ-a and MZ-b can be treated as two imbalanced MZM as shown in Appendex A. Therefore, the optical signal after the modulation of MZ-a and MZ-b can be expressed as
E1 =Eo· Exp[jωot] ·
In the main MZM, a phase difference is introduced between the upper and lower arm signal. After the combination at the output Y-splitter, the generated optical signal can
be expressed as
Then, the real part of the optical output signal can be simplified as
Re(Eout) =W · cos(ωot+ Δφa+ Δφc) + X · cos(ωot− Δφa+ Δφc) +Y · cos(ωot+ Δφb− Δφc) + Z · cos(ωot− Δφb − Δφc)
(3.11)
Equation (3.11) is the general form of an optical signal generated by an imbalanced DP-MZM. In the proposed optical millimeter-wave generation with frequency qua-drupling system, MZ-a and MZ-b are biased at the full point and MZ-c is biased at the null point. Therefore,
,where ma= (π · Vma)/(2Vπ) and mb = (π · Vmb)/(2Vπ) are the modulation index of MZ-a and MZ-b, respectively. Then,
Re(Eout) = − W · sin(ωot) · cos[ma· cos(ωRFt)] − W · cos(ωot) · sin[ma· cos(ωRFt)]
− X · sin(ωot) · cos[ma· cos(ωRFt)] + X · cos(ωot) · sin[ma· cos(ωRFt)]
+ Y · sin(ωot) · cos[mb· sin(ωRFt)] − Y · cos(ωot) · sin[mb· sin(ωRFt)]
+ Z · sin(ωot) · cos[mb· sin(ωRFt)] + Z · cos(ωot) · sin[mb· sin(ωRFt)]
(3.13) Expanding equation (3.13) using Bessel function, the output signal becomes
Re(Eout) = − W · sin(ωot) ·
Due to the imbalanced splitting ratios of the Y-splitters, not only the optical side-bands with the order of 4n-2, but also the other optical sideside-bands are obtained at the output of the DP-MZM. The electrical fields of the optical sidebands with the order
(a) (b)
FIG. 3.6. Simulated optical spectra of 40-GHz optical millimeter-wave signals using the quadrupling system (a) with 25-dB extinction ratios; (b) with 25-dB extinction ratios and driving signal trimming.
up to4this shown as the following:
• Optical carrier (zero order sideband) :
sin(ωot) · [−J0(ma) · (W + X) + J0(mb) · (Y + Z)].
• 1stsideband :
2 · cos(ωot) · [J1(ma) · (−W + X) · cos(ωRFt) + J1(mb) · (−Y + Z) · sin(ωRFt)]
• 2nd sideband :
2 · sin(ωot) · cos(2ωRFt) · [J2(ma) · (W + X) + J2(mb) · (Y + Z)]
• 3rdsideband :
2 · cos(ωot) · [J3(ma) · (W − X) · cos(3ωRFt) + J3(mb) · (−Y + Z) · sin(3ωRFt)]
• 4thsideband :
2 · sin(ωot) · cos(4ωRFt) · [−J4(ma) · (W + X) + J4(mb) · (Y + Z)]
In an ideal case, γa1 = γa2 = γb1 = γb2 = γc1 = γc2 = 0.5 when the DP-MZM has infinity extinction ratios. Therefore, W = X = Y = Z and optical sidebands ex-cept for the2 + 4n ones can be eliminated. However, the extinction ratios are always limited due to the imblalnced Y-spliter splitting ratio. Figure 3.6 (a) shows an simu-lated optical spectrum of a 40-GHz optical millimeter-wave signal which is generated
using the proposed quadrupling system with 25-dB MZM extinction ratios. The op-tical carrier can not be perfectly suppressed because of the limited MZM extinction ratios. In the proposed quadrupling system, the limited extinction ratio of the main MZM (MZ-c) can be compensated by trimming the driving powers of the sub-MZMs (MZ-a and MZ-b). From the equations in the last page, the optical carrier can be to-tally suppressed when J0(ma) · (W + X) = J0(mb) · (Y + Z). Figure 3.6 (b) shows a simulated optical spectrum. The optical carrier and the4th optical sideband are to-tally suppressed by trimming the driving powers. However, the1st and the3rdoptical sidebands are alos related to the limited extinction ratios of MZ-a and MZ-b which can not be compensated by the driving power trimming.
3.3 Experimental Results
To verify the proposed methods, optical millimeter-wave generation using the
To verify the proposed methods, optical millimeter-wave generation using the