Xiupu Zhang

Linearization of radio-over-fiber systems by using two lasers with different wavelengths

A linearization technique using two lasers with different wavelengths is further studied to improve output signal and suppress second and third order nonlinearities in radio-over-fiber (RoF) systems simultaneously, if an optical device or a subsystem generates nonlinear distortion and also has wavelength dependent transmission characteristic. In this paper, for an RoF system using an EAM that has wavelength dependent transmission, it is found that the optimum suppression of second order nonlinearity can be achieved by adjusting the power ratio of the two lasers. Compared to using single laser at 1552.6 nm, 1.6 dB improvement of signal power, 23 dB suppression of second order nonlinearity, and 2.1 dB suppression of third order nonlinearity are obtained experimentally. More than 11.5 dB improvement of spurious free dynamic range, 3 dB improvement of input power at 1 dB compression (P1dB), and 4 dB improvement of output P1dB are achieved.

Broadband Predistortion Circuit Design for Electro-Absorption Modulator in Radio over Fiber System

A low-cost broadband predistortion circuit using zero bias diodes is designed to remove 3rd order intermodulation distortion of electro-absorption modulator in radio-over-fiber system. Experiments show that input power at 1-dB compression point is improved by 0.8-3.8 dB from 8 to 17 GHz; and spurious free dynamic range is improved by ~9 dB from 7 to 14 GHz and ~4 dB from 15 to 18 GHz, limited by 5th order intermodulation distortion. The power consumption is reduced and no matching network is required because of characteristics of zero bias diode.

Novel broadband analog predistortion circuit for radio-over-fiber systems

A novel broadband analog predistortion circuit (PDC) is designed to linearize radio-over-fiber (RoF) systems. Dual Schottky diode is used as predistorter. The PDC with high bandwidth and small dimension is achieved by removing quarter-wavelength transformers and power dividers. Measured S-parameters show that the PDC can work from 10 MHz to up to 40 GHz. The PDC is verified using two-tone and WiFi signals in an RoF system. The spur-free dynamic range with respect to third order intermodulation distortion is improved by more than 10 dB from 1 to 5 GHz and more than 5 dB from 1 to 30 GHz, and it is found that the RoF system is limited by fifth order nonlinearity. Error vector magnitude is improved by 1 dB for WiFi signals at 2.4 and 5 GHz by using the PDC.

Design of substrate integrated gap waveguide

To miniaturize the conventional air gap waveguide (GW), a substrate integrated gap waveguide (SIGW) is proposed. Moreover, SIGW makes up the defects in the microstrip-ridge air GW, such as inconstant gap height, holes in strip, as well as strip electroless nickel immersion gold (ENIG) coating. To verify the proposed SIGW, a prototype was designed and manufactured using the multi-layer printed circuit board (PCB) technology. It contains a very simple transition between SIGW and microstrip lines for measurements. Further, the experimental results show that the measured performance agrees well with the simulated.

Linearization of radio-over-fiber systems using directly modulated and electro-absorption modulator integrated lasers

A low-cost linearization technique is proposed to improve RF signal power and suppress third order intermodulation distortion (IMD3) in radio-over-fiber (RoF) transmission systems. A directly modulated laser (DML) and an electro-absorption modulator integrated laser (EML) in C-band are both used for optical subcarrier modulation. The IMD3s induced by the two optical subcarrier modulations are suppressed by each other through adjusting the bias voltage of the electro-absorption modulator in the EML. In our initial experiments, the spurious free dynamic range (SFDR) is improved by more than 3.5 dB and output power at 1 dB compression point is improved by 4.3 dB.

An UWB antenna using modified Sierpinski-carpet Fractal Antenna

A novel UWB antenna is presented, which is formed by the modified Sierpinski-carpet fractal with the 50Ω micro-strip. Four resonant frequencies and three operation frequency bands can be observed via measurement between 1GHz-20GHz when return loss is smaller than -10 dB. The bandwidth of lower frequency band is from 1.1GHz to 10.8 GHz.

Design of packaged microstrip line

In this paper, a new-version microstrip line is proposed, named packaged microstrip line. It is comprised of three dielectric substrates. This novel microstrip line is no longer suffering from the issues of radiation losses, surface waves, or cavity resonances that exist in the standard microstrip line without or with a metallic shielding box. Instead of the packaging way based on the standard air gap waveguide technology, this novel packaged microstrip line has the following merits: greatly reduced weight and size, easy manufacturing, constant gap height and more flexibility in design. We present how to adopt a proper dielectric substrate for each layer of the packaged microstrip line considering both stopband and line losses, and how to design the line width for a given characteristic impedance. One prototype of this proposed novel packaged microstrip lines is fabricated and tested, showing a good agreement with the simulated results.

A small UWB antenna with triple band-notched characteristics

A small CPW fed triple band-notched (Ultra Wide Band) UWB antenna is presented. The proposed antenna consists of a shovel-shaped radiating patch with three C-shaped slots, a feed line and a modified ground plane. By etching three C-shaped slots with variable dimensions on the radiating patch, frequency band-stop performance is generated and we can control its band-notched frequency and bandwidth through modifying the dimensions of C-shaped slot. The proposed antenna has a very compact size of 14×22×1.6mm3. Simulated VSWR shows that the proposed antenna could operate from 2.88 to 10.67 GHz with VSWR less than 2, except 3.20 to 3.97GHz, 5.26 to 5.75GHz and 7.57 to 8.37 GHz.

Review of linearization techniques for fiber-wireless systems

Optical and electrical linearization techniques are reviewed for broadband radio over fiber transmission. The optical linearization includes optical mixed polarization and two-wavelength linearization, and the electrical linearization includes analog pre-distortion circuit. The optical linearization leads to more than 10-dB improvement of spur-free dynamic range (SFDR). The analog pre-distortion circuit that is simple and of low-cost results in about 10 dB improvement of SFDR.

Reduction of intermodulation distortion in directly modulated lasers by RF predistortion

In this paper, a new method of suppressing the third order intermodulation distortion in directly modulated lasers by RF predistortion has been presented. In our scheme, the predistortion circuit is designed in an RF design tool — Advanced Design System (ADS), then we use the co-simulation between ADS and Optisystem for validation the effectiveness of the design. Using this scheme, we achieved the expected reduction in third order intermodulation distortion (IMD3) of laser.