Received November 16, 2017, accepted January 16, 2018, date of publication January 23, 2018, date of current version March 12, 2018. Digital Object Identifier 10.1109/ACCESS.2018.2797087 Microwave Photonic Downconversion With Improved Conversion Efficiency and SFDR FADIL PALOI1, SHYQYRI HAXHA 1,2, (Senior Member, IEEE), TAIMUR N MIRZA 1,2, AND MOHAMMED SHAH ALOM1,2 1Department of Computer Science and Technologies, University of Bedfordshire, Luton LU1 3JU, U.K. 2Department of Electronic Engineering Egham, Royal Holloway, University of London, Egham TW20 0EX, U.K. Corresponding author: Shyqyri Haxha ([email protected]) ABSTRACT In this paper, we report a novel approach of microwave frequency downconversion with improved conversion efficiency and high dynamic range, using two different configuration schemes. The first proposed scheme is designed by using a dual-parallel dual-drive Mach–Zehnder modulator and the second one using dual-parallel dual-phase modulator. The radio frequency (RF) message signal and the local oscillator (LO) signal are feeding these two parallel connected modulators. By using a tight control of the system parameters, we have reported an effective optical carrier suppression, resulting in high conversion efficiency. We show that when the link is amplified, the relation between mLO and mRF plays a vital role and gives a high value of conversion efficiency, where key parameters lead to the LO and RF modulators modulation index. The conversion efficiency is improved by 5.72 dBm, compared with previously published work using DP-MZM, and 28.4 dBm, compared with the cascaded connected modulator. An experimental demonstration of a proof of concept is also carried out where the intermediate frequency to noise ratio of 69.5 dB is reported. INDEX TERMS Microwave photonic downconversion, dual parallel phase modulation, optical carrier suppression. I. INTRODUCTION suffers from high optical power [3], [4]. Nevertheless, the cas- The photonic microwave is a powerful paradigm for signal cade electro-optic modulator connection allows modulators processing due to low transmission losses, high isolation, to be located at the different position but on the other hand, wide bandwidth and immune to the electromagnetic interfer- they suffer from high nonlinear properties, resulting in very ence. For many applications, the crucial task when designing low conversion efficiency [5]–[7]. Furthermore, the paral- a RoF link is downconverting the high-frequency microwave lel connection of two or more modulators with RF and signal to a lower intermediate frequency (IF), with the least LO signal connected at the base station, have provided higher possible losses, with a reduction of the bandwidth require- downconversion efficiency, but still, there is space to improve ment, and an enhancement of the DR [1]. the system's efficiency and DR [8]. The previously reported To recover the high-frequency signal, and digitise with techniques are suffering from very low conversion efficiency high resolution, it is necessary to downconvert signal to by increasing the system's nonlinearities and reducing the a lower intermediate frequency (IF), which can be easily system's bandwidth [9]. matched. A variety of microwave photonic downconver- In this paper, by applying a stringent control on the sys- sion structures have been reported, employing two or more tem parameters, we present a configuration obtaining high modulators connected at the various configuration [2]. Each conversion efficiency, wide bandwidth, and stable output configuration has advantages and disadvantages. The most signal. We are proposing schemes with improved conver- common is downconversion with conventional electrical sion efficiency and spurious free dynamic range photonic mixing. Following with the optical downconversion using microwave downconversion, using DP-DDMZM, first driven heterodyne mixing of two coherent optical carriers at the pho- by a stronger LO and second driven by an RF message signal. todetector (PD), where higher frequency signals are down- We have simulated our proposed model and benchmarked converted to the low-frequency signals as a result, the system with experimental work published by [5] and [10] (as shown 2169-3536 2018 IEEE. Translations and content mining are permitted for academic research only. VOLUME 6, 2018 Personal use is also permitted, but republication/redistribution requires IEEE permission. 8089 See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. F. Paloi et al.: Microwave Photonic Downconversion With Improved Conversion Efficiency and SFDR TABLE 1. Integrated DP-DDMZM based photonic mixer performance compared to the [10], DP-MZM. in Table 1). High conversion efficiency has been achieved by suppressing the optical carrier at the DP-DDMZM, con- figured as phase modulators, or DP-DPhM, enabling high amplitude of the LO and RF sidebands into the receiver, achieving high conversion efficiency, high Spurious Free Dynamic Range (SFDR) and the capability to work over a wide frequency range. In the first proposed configura- tion, with DP-DDMZM working as a full phase modulator, by using a tight control on the Gallium Arsenide (GaAs) MZMs parameters, and controlling the electrical signal at both electrodes [11]–[13], the residual carrier has been effec- tively suppressed. The second method, configured with DP- DPhM [14] is considered with the adequate phase difference between each sets of dual phase modulator, achieving high IF amplitude with the low spur and high SFDR. Furthermore, the simulation design approach alongside the theoretical anal- ysis of the proposed structure, should allow the optimisation of the microwave photonic downconversion performance, for the benefits of photonic signal transmission. FIGURE 1. IF and Dynamic Range in function of (a) LO input power for the DP-DDMZM and (b) RF input power for the DP-DDMZM. II. METHODOLOGY In our proposed configuration, we have used two different configuration schemes. The first scheme is designed by using suppression in the DP-DDMZM photonic mixer can be DP-DDMZM configured as a phase modulator. The second achieved by controlling the modulator's bias voltage and ade- scheme is designed by replacing both dual drive parallel quate time delay [16]. In our proposed structure, by applying modulators of the first scheme with Dual Parallel Phase Mod- the technique of carrier suppression we keep the output spec- ulators (DPDPM) [14], [15]. At both configuration schemes, trum simple, not putting high optical power into the receiver the first modulator is driven by a stronger LO and the which adds to the noise. second by an RF message signal, generating downconverted Another efficient technique utilised in dual parallel con- IF signal. figurations is biasing the system at the minimum transmis- A thought-provoking approach within this system config- sion point (mTP), in the push-pull operating mode [4]. The uration is the way shifting and biasing is utilised to down- DP-DDMZM forms push-pull configuration mode when the convert the signal frequency [4], [16]. We have deployed VPI LO signal (VLOcos !LOt), at the first modulator or an RF mes- simulation software packages to implement the analytical sage signal (VRF cos !RF t) at the second modulator into two model and to analyse the proposed system performance [17]. equal branches (V11(t) and V12(t)), is defined as cosine-wave Initially, we studied a few action points, investigating the and inverse cosine-wave signal. behaviour of the harmonics in function of RF and LO input The DP-DDMZM operates at the minimum Transmission power, utilised in the conventional DP-MZM configuration, Point (mTP) when LO or RF message signal together with as illustrated in the Fig. 1 (a, b). The LO and RF input DC switching voltages of V π D VDC D Vb11 − Vb12 D power significantly affects the system's performance, par- π D 5:6[V], for both branches (Vb11 D 2:8[V] and Vb12 D ticularly impacting the harmonic distortion and resulting in −2.8[V]) is applied, resulting in low gain, low spur product, adequate SFDR. and high SFDR, as shown in table 2, [16]. Other efficient At the same structure, it is impossible to obtain constant techniques, achieving carrier suppression in the DP-DDMZM conversion efficiency given that input RF power is dependent configuration is biasing the modulator at the quadrature on the RF modulation index. transmission point, in the push-pull operation mode. The The optimisation of the proposed RoF system is performed DP-DDMZM can operate at the Quadrature Transmission by applying techniques of carrier suppression. Optical carrier Point (QTP) when the LO or RF message signal together 8090 VOLUME 6, 2018 F. Paloi et al.: Microwave Photonic Downconversion With Improved Conversion Efficiency and SFDR TABLE 2. Integrated DP-DDMZM based photonic mixer performance simulated both our proposed models and benchmarked them compared to the various DP-MZM based photonic mixer. with experimental work published by [5] and [10], as shown in table 2. Our simulation results show that the carrier is the dominant output optical component with the highest consumed power. For the parallel modulator connection, the carrier is not required at the frequency downconversion. The average power travelling to the receiver can be reduced by decreasing input power, such as the carrier power. By optical carrier suppression in the modulated link, the modulated depth will increase and will result in an increase of SFDR. The high conversion efficiency can also be obtained by car- rier suppression, subsequently, we should find the means to suppress the carrier to the maximum extent. The parallel connection of phase modulators can be used to convert the phase modulated signal into intensity, and suppress or cancel the carrier [4], [14], [19]. The residual carrier can be cancelled by a combination of differential signal detection of both modulators. This can be achieved by the phase shifting, and at some configuration by proper bias- ing. One means of carrier suppression is the inverse biasing of both DP-DDMZM.