A Phase Modulation Scheme for Millimeter Wave Generation Based on Frequency Octupling using LiNbO3 Mach- Zehnder Modulator.

A 80 GHz millimeter wave (mmw) has been generated with a single Lithium niobate Mach Zehnder Modulator (LiNbO3 MZM) using phase modulation (PM) scheme based on frequency octupling with a modulation voltage of 9V. The main advantage of this experiment is that the carrier is suppressed by changing the phase of the signal in the modulator and the need for an optical filter for suppressing the carrier signal is eliminated. Moreover the experiment shows a better data rate, less hardware complexity and less cost for its realization. The generated optical signal is modulated by the data signal in the optical domain which is transmitted through a single mode fiber for a distance of 70 km. The received signal is detected using a photodiode to reconstruct the mmw and further its optical spectrum, RF spectrum and eye diagram are obtained by means of simulation to ensure the robustness of the proposed method. The method achieves error free transmission which is evident from the eye diagram and the efficiency is ensured by the quality factor plotted by the BER analyzer. KeywordLithium Niobate Mach-Zehnder modulator; Radio over Fiber; Phase Modulation; Optical Spectrum; Millimeter wave generation; Frequency Octupling.


II. EXPERIMENTAL SETUP AND ANALYSIS
The figure 1 shows the arrangement used to generate mmw based on frequency octupling in experiment basis. In this setup a continuous wave (CW) laser with 193.1 THz frequency is given as input to the LiNbO 3 MZM with a 10 GHz frequency from a source generator and to the phase modulator. In the modulator the bias and modulation voltage is set as 9 V and the phase deviation for the phase modulator is set as 90°. The data rate of 100 Gbps is incorporated with the phase modulator and thus the output coming from the whole setup in the central station (CS) is a carrier suppressed mmw based on frequency octupling. The carrier signal and the even order optical sidebands are suppressed by the phase modulator so that the resultant signal has a frequency which is eight times that of the electrical signal.
The modulator electric field applied is expressed as cos t) The voltage of the millimeter wave signal is given as = cos ( + ) Where is the electric field amplitude while and are amplitude and phase of electrical signal respectively. The bias voltage of phase modulator is varied in order to suppress all the even optical sidebands, the electric field output of the modulator can be expressed by cos ( π 2 cos

≈ ( ) Cos cos
Where . , is the phase modulation index of the MZM.
is the first order Bessel function of the first kind, is the phase modulation index. and are the critical phase of the signal applied to the first and second modulator respectively.
The suppressed optical mmw signal is transmitted through a 70 km single mode fiber to reach the base station (BS). In the BS an amplifier is fixed as the first component so that the mmw coming from the optical fiber will fall directly on the amplifier in order to prevent the signal from fading. The optical signal from the amplifier is given as the input to the photo-diode to convert it into an electrical signal so that the signal can be given to the antenna for further transmission. In between an amplifier and the photo-diode an optical spectrum analyzer is connected to check whether the generated signal is preserved through the transmission. The final output from the photodiode is expressed mathematically as ( cos 8 4 4 It is evident from the above expression that the proposed phase modulation scheme achieves frequency octupling.

III. EXPERIMENTAL RESULTS
The laser frequency of 10 GHz is used as the input with a 10 GHz signal generator to the MZM modulator, in the modulator a 9V is applied and a 90° phase shift is given for the source signal in order to generate the mmw signal. The output of the modulator converts the 10 GHz carrier signal into 80 GHz with a data rate of 100 Gbps and performs a error free transmission through a 70 km long single mode fiber. The conversion of frequency is clearly observed in the optical and RF spectrum analyzer. The quality as well as the efficiency of the signal is analyzed by using the visualizers and in the BER analyzer the value of the Q-factor is high which indicates that the energy loss is low during transmission.
The figure 2 shows the observational results of the generated optical signal in terms of optical spectrum, electrical spectrum, eye diagram and Q-factor. The optical spectrum and the electrical spectrum of the generated mmw signal for the frequency 80 GHz at a distance of 70 km is shown in figure 2 (a,b) and the eye diagram shown in fig. 2 (c) has a wide opened structure like an eye which clearly indicates the data transmitted using optical signal through an 60 km optical fiber is preserved and can be reconstructed effectively. The high value of Q-factor shown in figure 2 (d) obtained by using BER analyzer indicates the less energy loss occurred during the transmission of mmw from an optical fiber.

IV. FIBER LENGTH VERSUS QUALITY FACTOR
The quality factor plays an important role in the transmission of signal through an optical fiber which graphically represents the energy loss during the transmission of mmw data signal. In table 1 the iteration method is used between the length of the fiber and the quality factor. By using this table it is clear that the energy loss occurs due to fading effect during the transmission of 100 Gbps mmw signal for different fiber length. From table 1 analysis up to the length of 60 km the quality factor is maintained above the value range of 50 km but for 70 km a drastic change in the quality factor is observed. From table 1, it is understood that the drastic change in the value of quality factor from 60 km to 70 km is due to fading effect. To avoid this fading effect an amplifier is used in the base station and thus the range of quality factor is successfully maintained for a length of 70 km distance and it is shown in table 2. The use of an amplifier in this setup will help to reduce the fading effect and the fiber length can be increased for better data transmission.  The table 3 shows bit error rate (BER) versus extinction ratio done as an experiment basis to identify the changes occur in the data signal transmission to the base station. This experiment helps to describe the need of an extinction ratio to achieve error free transmission. The values for the extinction ratio is given in the modulator used to generate mmw and the BER is analyzed by using BER analyzer. According to the figure 3 analysis when extinction ratio increases the BER get reduced and this shows the prevention of message signal from losing throughout the distance of an optical fiber.

VI. CONCLUSION
A novel method for mmw generation using phase modulation with a single LiNbO 3 intensity modulator based on frequency octupling is proposed. A data rate of 100 Gbps is successfully transmitted through a single mode fiber for a length of 70 km and this cost effective model is analyzed to ensure the robustness of the generated mmw signal. The proposed work has a simple setup which is efficient and provides error free transmission of mmw through an optical fiber. The quality of the generated mmw signal for a distance of 70 km is verified by using the spectrum analyzers and the simulation result ensures the quality of the generated mmw signal. The results on Fiber length versus Quality factor and BER versus Extinction ratio gives more detailed information about the robustness of the proposed method and thus achieves a good quality mmw signal in the receiver side.