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A broadband two-thirds frequency divider based on two cascaded carrier-suppressed double sideband (CS-DSB) modulators is proposed and experimentally demonstrated. The first CS-DSB modulator is aimed to introduce an RF signal under frequency division to the system, and the second one is employed in an optoelectronic oscillator (OEO) loop to avoid the OEO from free running. By mixing the injection frequency and the oscillation frequency in the OEO loop, oscillation of a signal with its frequency equal to two-thirds of the injection frequency is established. An experiment is carried out, RF signals with frequencies from 12.3 to 17.7 GHz are successfully divided into 8.2 to 11.8 GHz. The maximum power ripple is 2.79 dB and the harmonic components suppression ratio is larger than 36.13 dB. Introduction: Frequency dividers play a significant role in optical communication systems and microwave photonic areas for agile frequency manipulation and signal processing [1-3]. Traditionally, frequency dividers are realised in the electrical domain with purely electronic active components [1]. Nevertheless, the active electrical components would introduce additional noise and deteriorate the signal-to-noise ratio of the systems. Also, electrical frequency dividers would also suffer from limited operational bandwidth and poor harmonic components suppression ratio. To deal with these problems, frequency dividers realised using photonic techniques are highly desired, thanks to the intrinsic characteristics brought by photonic techniques such as large bandwidth, high frequency, and electromagnetic interference immunity. Optical frequency division could be implemented based on the non-linear dynamics of a semiconductor laser [4], a non-linear fibre loop mirror [5], or an injection-locked optoelectronic oscillator (OEO) [6]. Such approaches, however, are lacking flexibility for the division of different frequencies due to the mechanism of narrowband filtering. Recently, we proposed a wideband frequency divider with a division factor of two by an OEO-based divider [7]; however, the harmonic components suppression is poor. In this Letter, a two-thirds frequency divider with large frequency tuning range and good harmonic suppression is proposed and experimentally demonstrated based on cascaded carrier-suppressed double sideband (CS-DSB) modulations, realised by two Mach-Zehnder modulators (MZMs) biased at the minimum transmission point (MITP). When an RF signal with a frequency of f 1 is applied to the first MZM (MZM1), two sidebands that are separated by 2f 1 would be generated, which is then sent to an OEO containing the second MZM (MZM2). Assume the frequency of the oscillation signal is f 2 , at the output of MZM2 four sidebands with offset frequencies to the optical carrier of −f 1 −f 2 , −f 1 + f 2 , f 1 −f 2 , f 1 + f 2 would be generated. To maintain the oscillation of the OEO, the beating frequencies of − f 1 + f 2 and f 1 − f 2 should equal to f 2 , leading to f 2 = 2/3f 1. As a result, two-thirds frequency division can be real...

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The coherent digital radio-over-fiber (DRoF) system is a promising candidate for future mobile fronthaul networks (MFNs) due to its high receiver sensitivity and excellent robustness against nonlinearities. However, conventional coherent receivers with complicated structure and heavy algorithms are too expensive and power-hungry for cost-sensitive MFN applications. In addition, currently deployed digital MFNs based on common public radio interface (CPRI) suffer from low spectral efficiency and high data rate. Towards these issues we propose a novel DRoF downlink scheme employing a simple self-coherent receiver. In baseband unit (BBU), the radio signal is converted to a digital bit stream by a band-pass delta-sigma modulator (BP-DSM), which can be simply recovered with the utilization of a band-pass filter at the receiver. In remote radio unit (RRU), an electro-absorption modulated laser (EML) acts as a low-cost coherent homodyne receiver in virtue of injection locking technique. In the experiment, the injection-locked operation of the DSM signal is successfully achieved, and two modified schemes are proposed for the DSM signal to increase the locking range with a tolerable sensitivity penalty. The experimental results demonstrate the superiority of our approach in two aspects: 1) the EML-based coherent receiver outperforms a PIN photodiode in terms of receiver sensitivity; 2) compared to the analog RoF system, a 5-dB improvement in loss budget is obtained when DSM is employed with the aid of a simple equalizer.

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