INTRODUCTION
For the future broadboard access networks, generation and distribution of high
frequency millimeterwave become a key techniques. High power loss of high frequency
millimeterwave has make base station more costly. As consequence, Radio over
Fiber (RoF) technique has been considered a costeffective and promising candiation
for the distribution of the future access networks. Now, reasearch about generation
and distribution millimeterwave in RoF system is intensively conducted. All
optical photonic upconversion has become a promising resolution for optical
millimeterwave generation such as using fourwave mixing in high nonlinear
dispersion fiber (Ma et al., 2006; Dong
et al., 2009; Jia et al., 2005; Yu
et al., 2005a) or semiconductor optical amplifer and crossgain modulation
or crossphase modulation in EAM (Yu et al., 2005b;
Seo et al., 2006) and optical heterodyne techniques
(Yu et al., 2006), using external modulation
to realize frequency doubling (O’Reilly and Lane,
1994), frequency quadrupling and sextupling using Optical Frequency Multiplication
(OFM) (He et al., 2009; Chi
and Yao, 2008; Lin et al., 2008; Wang
et al., 2006; Zhang et al., 2007;
Shih et al., 2009; Chang
et al., 2008; Mohamed et al., 2008).
The principle of OFM is to modulate the power of sidebands through settting
the value of peaktopeak amplitude of RF signal and bias voltage of MZM and
then through O/E conversion, desired millimeterwave is generate. It can use
only low frequency oscillator to generate high frequency. OFM techniques is
based on the inherent nonlinearality of the response of modulator and this can
be well studied in microwave photonic fields which can reduce the bandwidth
requirement of modulator. Futhermore, signal processing in optical domain can
eliminate the effect of electronic bottleneck and shift more complex component
to Central Station (CS) which can reduce the cost of Base Station (BS). O’Reilly
and Lane (1994) proposed another method to generate a frequencyquadrupled
electrical signal. Recently, an approach using an optical phase modulator to
generate a frequencyquadrupled electrical signal was proposed (Shen
et al., 2003). In this scheme, a FP filter was proposed to select
two second order sidebands. Qi et al. (2005)
is using a intensity modulator and a notch filter to generate a wideband continuous
frequency tunable millimeterwave. When the frequency multiplication factor
is higher than four, the method so far typically depended on fourwave mixing
using semiconductor combined with optical filter or using two cascaded single
arm intensity modulators which could result in complicate structure and high
cost.
In this study, we have comprehensively demonstrate two schemes for the generation
of millimeterwave using frequency sixupler with a dualelectronic modulator
(MZM) theoretically and simulatedly. At first, the generation of millimeterwave
and its dispersion performance when transimitting SMF, is analyzed theoretically
and then performance of the generation millimeterwave for two schemes is evaluated
by simulation. With comparing system, performance at different modulation index
and in the case of whether system has optical filter or no optical filter and
the impact of extinction ratio of modulator is inverstiagted. At last, it has
been concluded that the performance of the generated millimeterwave for scheme
two is better than that for scheme one.
COMPARISON BETWEEN TWO SCHEMES
Theoretical analysis: It is assumed that wavelength launched from DFB
laser is continuous wave in nature, it can expressed as E_{in} (t) =
E_{0} cos(ω_{0}t) and the electrical RF signal can be expressed
as V_{RF} (t) = V_{m} cos(ω_{RF}t) and then the
electrical signal is splitted into two parts by an electrical splitter and the
two outputs directly drives the two electrodes of the MZM. For scheme one and
scheme two, odd optical sidebands are generated. The output electrical field
after dualelectrode MZM can be expressed as:
where, splitting ratio:
ε is the MZM extinction ratio, g (t) is digital signal. If the value of
ε is bulky, splitting ratio γ = 1/2. When optical power is boosted
by amplifer and undesirable sidebands is removed by optical filter, the electrical
field of optical signal can be expressed as:
where, H_{F} (nω_{RF}) is the transfer function of optical
filter.
After fiber transmission, the optical field can be expressed as:
Due to limited bandwidth of electrical spectrum analyser, higher order sidebands
is not visable. For scheme one, the electrical field of photocurrent at 6ω_{RF}
after O/E conversion can be expressed as:
Equation 4 clearly shows that the generated mmwave at frequency
60 GHz mainly be composed of harmonics that is result from beating of optical
components at ±3ω_{RF} which is almost independent of intersidebands
beating interference caused by fiber chromatic dispersion but walk off which
will induce broadening of optical pulse width and result in InterSymbol Interference
(ISI) and harmonics that is generated from beating of optical components at
±ω_{RF} and ±5ω_{RF} which has a dispersion
term which
can lead to periodic destructive and constructive interaction caused by fiber
chromatic dispersion. When optical filtering, the output of the photocurrent
at the frequency of 6ω_{RF} is given by:
For scheme two, when optical filtering, the ouput of the photocurrent at the
frequency of 6ω_{RF} is given by:
Where:
N is order of filter, B is bandwidth of filter, f_{c} is the center
frequency of optical bandpass filter.
Simulation analysis: Figure 1a and b
show that the principle of generation mmwave using dualelectrode modulator
for two modulation schemes.

Fig. 1(ab): 
Principle of three optical modulation
techniques for mmwave generation using a dual electrode MZM (a) Scheme
one and (b) Scheme two 
For the simulation of scheme one and scheme two, a Continuous Wave (CW) laser
is assumed to have a wavelength of λ_{0} = 1552.52 nm, a linewidth
of 10 MHz, the frequency of RF signal is 10 GHz and the phase of electrical
signal is shifted by 180 degrees and then the RF signal is split into two parts
by an electrical splitter and the two outputs directly drives the two electrodes
of the MZM. The DC voltage of two arms of MZM is set at different values for
scheme one and scheme two.

Fig. 2(ac): 
Electrical spectra of the generated
photocurrent though O/E conversion after fiber transmission over (a) 3.95
km, (b) 6.19 km and (c) 11.85 km 
For scheme one, the bias voltage of MZM is set for V_{DC} = 6V, the
power ratio between thirdorder sidebands and firstorder is 8.5 dBm and for
scheme two, the bias voltage of MZM is set for V_{DC} = 9.76V, the power
ratio between thirdorder sidebands and firstorder is 52 dBm. The extinction
ratio of MZM is 35 dB. In simulation, the filter has a 3rd order Gaussian transfer
function with a bandwidth of 68 GHz and a central wavelength of 1552.52 nm.
After optical filtering, through optical signal transmission SMF, the photocurrent
is generated with O/E conversion by photodiode and BER analysis is made. Optical
receiver is composed of photodiode, electrical bandpass filter, lowpass electrical
filter and mixer. The dark current is I_{d} = 10 nA, responsibility
is
= 1A/W and the bandwidth of electrical Gaussian bandpass filter is 1.5 bitrate,
centered at 60 GHz the bandwidth of electrical lowpass Gaussian bandpass filter
is 0.75 bitrate. In practice, this mmwave is launched into air by antenna.
The frequency component of photocurrent after O/E conversion is 20, 40 and 60
GHz.
The mmwave signal at 60 GHz is selected by electrical bandpass filter and
is mixed with 60 GHz LO signal and then baseband signal is selected by lowpass
filter. At last, baseband signal is BER analyzed.
Simulation results analysis: Figure 2 shows the spectrum
that the photocurrent at different components through transmission different
lengths of SMF. With theoretical analysis, the power of frequency component
at 40 and 60 GHz has increased due to L = 6.19, that is fading node when optical
signal transmission is 6.19 km in SMF due to term of
from this equation, it can be calucated that L = 6.19xk but when L = 11.85 km,
the power of frequency at 40 GHz is increasing continuously and the power of
frequency at 60 GHz is minimized due to term of
it can be calculated that transmission length L = 3.95x(251). These constructive
and destructive interactions between the two contributions are periodic due
to the term
Now we investigate the impact of optical filter on the performance of mmwave
for two schemes. Figure 3 shows that the impact of fiber length
transmission on Qfactor of mmwave due to fiber chromantic dispersion for scheme
one. This figure shows that the fluctuation of Qfactor is different between
using optical filter and no optical filter. It is resulted from that, after
optical filtering, beating interference among undesirable sidebands is removed
which can lead to reduce fluctuation of Qfactor of mmwave. When no optical
filter is used, the fluctuation of Qfactor is large and the curve of Qfactor
of mmwave is in irregular status. From above theoretical analysis, when transmission
length of 3.95 km in SMF, the Qfactor is at the lowest point of curve due to
transfer function of mmwave power at fading node, so Qfactor is decreasing
when transimission length is range from 03.95 km. When transmission length
of SMF is 6.19 km, the curve of Qfactor is at the second peak due to transfer
function of mmwave power at fading loop, so Qfactor is increasing when transmission
length is range from 3.956.19 km. But when transmission lenth of SMF is L =
11.85 km and L = 12.38 km, the curve of Qfactor is decreasing which is contrast
to above theoretical analysis. It shows that, when no optical filter is used
in simulation, the performance of mmwave is easily affected by fiber chromatic
dispersion and intersidebands beating interference. But when optical filter
is used in system, the curve of Qfactor is increasing slightly. It is implied
that the performance of mmwave is not affected by intersidebands beating interference
and fiber chromatic dispersion for scheme two.
Figure 4 shows the comparsion of simualted Qfactor of mmwave
versus fiber length for scheme two between the two cases of using optical filter
and no optical filter. It shows that the fluctuation of curve of Qfactor is
nearly the same when no optical filter and no optical filter is used in the
simulation.

Fig. 3: 
Two curves of Qfactor for scheme one
vs. fiber length about two cases of using optical filter and no optical
filter 

Fig. 4: 
Two curves of Qfactor for scheme two
vs. fiber length about two cases of using optical filter and no optical
filter 
It shows that the power of firstorder sidebands is small which can lead to
suppression of frequency component beating between firstorder sidebands and
other undesirable sidebands, so this scheme is hardly suffered from data inferference
from other components falling within the same mmwave frequency.
Harmonic Convertion Ratio (HCR) is a parameter that is used to evaluate the
performance of generated mmwave. It is defined as “the average power ratio
between power of generated frequency component that intersidebands beating
and power of generated frequency component that selfbeating”. In simulation,
for the power of 60 GHz mmwave, the power of generated spectral component beating
between +3 and 3 order sidebands is power of generated frequency component
that is through selfbeating. But +1rd and 5rd sidebands and frequency electrical
component generated from beating between +1rd sidebands and +7rd sidebands is
the power of frequency component of selfbeating. So, the performance of generated
mmwave is dependent on the power ratio between the two values. The more value
of HCR, the more serious the beating interference which can lead to deterioration
of the performance of generated mmwave and when the value of HCR is decreasing,
the performance of mmwave is better.
For scheme one, it is assumed that the modulation index β = 1.5π,
extinction ratio is 35 dB, when no optical bandpass filter is established, it
is calculated that HCR value of generated mmwave is:
when, 3rd optical filter is used, whose bandwidth is 68 GHz, its HCR is:
It is showed that, for scheme one, the difference between two cases that optical
filter is used and no optical filter is used is large. It is implied that the
performance of mmwave is easily affected by intersidebands beating, so the
quality of mmwave is reduced.
For scheme two, it is assumed that modulation depth is β = 2.44π,
extinction ratio is 35 dB, when no optical bandpass filter is established, it
is calculated that HCR value of generated mmwave is:
when 3rd optical filter is used, whose bandwidth is 68 GHz, its HCR is:

Fig. 5: 
Simulated Qfactor vs. MZM extinction ratio using scheme one
and scheme two 
The theoretical analysis shows that for scheme two, the fluctuation of HCR
is slight between the case of optical filter is used and no optical filter is
used. The HCR for scheme two is smaller than that of scheme one, it shows that
the performance of generated mmwave for scheme two is better than that for
scheme one and the generated mmwave for scheme two is hardly sensitive to fiber
chromantic dispersion.
In the above analysis, a constant extinction ratio of MZM is used. It is known
that sidebands suppression ratio is dependent on extinction ratio of MZM and
thus the performance of mmwave may also be affected. To investigate the performance
of the two modulation schemes impacted by extinction ratio of the MZMs, we have
measured the Qfactor versus MZM extinction ratio ranged from 550 dB by simulation
and the results is show in Fig. 5. It shows that for two schemes,
Qfactor is immune to extinction ratio if more than 25 dB. Moreover, scheme
two leads to better performance than scheme one and thus the curve of Qfactor
for scheme two is smooth.
CONCLUSION
In this study, we have demonstrated two schemes using for the generation of
millimeterwave using frequency sixupler with a dualelectronic modulator (MZM).
At first, the generation mmwave and the performance of mmwave being impacted
by fiber chromantic dispersion is analyzed theoretically and then compare the
two schemes by simulation and the performance of two schemes is investigated
by measuring Qfactor and comparing two cases betweem optical filter is used
and no optical filter is used simulatedly. According to theoretical analysis,
the fluctuation of the power of generated mmwave is periodic due to intersidebands
beating if no optical filter is established in the system and thus the performance
of mmwave is instable. When optical filter is required, the performance of
mmwave is improved. Then, the HCR for scheme two is calculated and make comparsion.
The result shows that the performance of generated mmwave for scheme two is
better than that of scheme one. Then make simulation and the simulation results
show that for scheme one, the power of generated mmwave is easily sensitive
to fiber chromatic dispersion due to intersidebandes beating and for scheme
two, the performance of generated mmwave is immune to chromantic dispersion
due to reduction of the power of firstorder sidebands which can lead to destruction
of interference resulted from beating among sidebands. Moreover, we have considered
the performance of the generated mmwave impacted by extinction ratio of MZM.
At last, we found that the performance of generated mmwave for scheme two is
better than that of scheme one.
ACKNOWLEDGMENTS
This study was financially supported by the Hunan Provincial Natural Science
Foundation of China (12JJ2040), the Construct Program of the Key Discipline
in Hunan Province, China, the Aid Program for Science and Technology Innovative
Research Team in Higher Educational Institute of Hunan Provinve, China.