Frequency combs are optical spectra composed of a set of discrete equally spaced lines. Such spectra can be generated by diverse sources such as mode-locked lasers, resonators, or electro-optic modulators. This last possibility has shown a growing interest in the recent years for its advantageous features in providing high repetition rates, intrinsic mutual coherence, or high power per comb lines. Moreover, applications of electro-optic modulator-based combs have flourished in fundamental physics, spectroscopy, or instrumental calibrations. In this paper, we present the most recent progresses made on frequency combs generated by electro-optic modulators, along with the applications where these combs have shown a particular interest.

Electro-optic frequency combs

Radio-over-fiber (RoF) systems comprise light modulation and transmission of millimeter-wave signals over fiber links. The aim of this study is to investigate the performance of external and direct intensity modulation in RoF links and to analyze the drawbacks induced by different components of the optical system. In external modulation, the Mach–Zehnder modulator (MZM) is used, whereas the vertical-cavity surface-emitting laser (VCSEL) is utilized in direct modulation. Both modulation schemes are tested for a vector modulation format, i.e., the quadrature amplitude modulation (QAM), where an orthogonal frequency-division multiplexing (OFDM) scheme is used to generate signal subcarriers. The simulations are carried out with the same values of common global parameters for both schemes of intensity modulation. Although VCSEL is a promising device for future RoF systems, the external modulation shows a more robust performance compared with that of VCSEL when implemented with the OFDM modulation technique.

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Direct and External Intensity Modulation in OFDM RoF Links

We propose and experimentally demonstrate a power efficient dual-stage optical frequency comb using laser gain switching followed by a dual-drive Mach-Zehnder modulator (DD-MZM). The laser is initially gain switched at ∼ 9.5 GHz and the resultant comb is then expanded using a dual-drive Mach-Zehnder modulator driven at ∼ 19 GHz with signal amplitudes below 1.5 V. The setup generates an optical frequency comb, with 12 lines within 3 dB flatness, in a power efficient manner. Theoretical analysis is presented and verified through simulation and experimental results.

Power efficient optical frequency comb generation using laser gain switching and dual-drive Mach-Zehnder modulator.

A novel design of planar balance-shaped slot antenna suitable for triple-band WLAN/WiMAX applications is proposed in this article. It consists of a slotted ground plane, an FR4 substrate, and an L-shaped microstrip feedline. The radiating element of the proposed antenna is composed of a horizontal H-shaped slot and an M-shaped slot, like a balance. The horizontal H-shaped slot mainly contributes to the 2.4- and 3.4-GHz resonant modes while the resonant modes at 5.4 and 5.8 GHz are generated by the M-shaped slot. The overall size of the proposed antenna is only 30 × 28 mm2, very compact, making it easy to be integrated with other RF front-end circuits. The measured results agree well with the simulated ones and demonstrate that the antenna has a triple-band characteristic entirely covering the 2.4/5.2/5.8-GHz WLAN and 2.5/3.5/5.5-GHz WiMAX operating bands. Furthermore, monopole-like radiation patterns with acceptable gains make it a good candidate as an internal antenna for WLAN/WiMAX applications. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1078–1082, 2016

Planar triple‐band balance‐shaped slot antenna

In this paper, filterless photonic generation of millimeter-wave (mm-wave) signal through frequency 12-tupling via two cascaded dual-parallel polarization modulators (DP-PolMs) is proposed. The principle of operation of the proposed system is theoretically analysed and verified by computer simulation. From the theoretical analysis, the frequency 12-tupling mm-wave signal is shown to be generated by properly adjusting the modulation indices of PolMs, polarization directions of the modulated optical signals, and the phases of the RF drive signals applied to the DP-PolM. The simulation results show that the generated frequency 12-tupling optical mm-wave has high purity with an optical sideband suppression ratio (OSSR) of 25.1 dB and radio frequency spurious suppression ratio (RFSSR) of 19.1 dB. Theoretically obtained results are in good agreement with the simulation results. Furthermore, taking into consideration the practical implementation, the influence of non-ideal parameters on the OSSR and RFSSR are analyzed.

Photonic generation of millimeter-wave signal through frequency 12-tupling using two cascaded dual-parallel polarization modulators

We report the highest transmission rate of 92 Gbps/λ PAM4 signal over 1 km SMF using a low cost, free-running multi-wavelength FP-laser and show the performance within the standard FEC limit.

Demonstration of 92 Gbps/λ PAM4 with Free running Fabry-Pérot Laser for DCI Applications

A phase precoded QPSK modulated frequency tripled microwave signal at 3 GHz is generated optically using an external Mach Zehnder intensity modulator. The modulator can be biased differently to produce double and quadrupled microwave output in a software defined way. Optically generated microwave signal is transmitted wirelessly about 3 m and the received signal is decoded successfully in a software defined receiver. The BER measurement of the received 3 GHz signal demonstrates 2.5 dB power penalty for 30 Mbps data rate at 10−6.

Software Defined Optically Generated Pre-coded QPSK Modulated Microwave Signal

A simple scheme for Millimeter wave generation employing optical heterodyning with Fabry Perot laser based optical frequency comb is proposed. We experimentally demonstrate the generation and distribution of 1.2 Gbps 64 QAM-OFDM on 65 GHz carrier over 25 km fiber, achieving BER within HD-FEC limits.

Optical Heterodyning with FP Laser based Comb Source for 65 GHz MMW Generation

Agilent advanced design systems (ADS) software® is employed to model Mach Zehnder intensity modulator (MZM) based radio frequency generation at 6 GHz. Linearity analysis of the system is implemented in ADS and verified experimentally. Third order intercept point (IP3) is found within acceptable limit of 51dBm which matches closely with the experimental obtained value.

Rangana Banerjee Chaudhuri

Papers

Demonstration of 92 Gbps/λ PAM4 with Free running Fabry-Pérot Laser for DCI Applications

Software Defined Optically Generated Pre-coded QPSK Modulated Microwave Signal

Optical Heterodyning with FP Laser based Comb Source for 65 GHz MMW Generation

Linearity analysis of a frequency tripled optically generated radio frequency signal

Design and analysis of photonic radio frequency multiband generation using transfer function approach in advanced design system software