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May 19, 2015 01:29 AM

A 1 Gbps 105.4 GHz Link with a Directly Modulated Photonic Integrated Dual Laser Source

The increasing data rates required for wireless data communications, along with the need for short distance point-topoint wireless connections, are stimulating research in wireless links with carrier frequencies in the sub-terahertz range [1]. Such links will require compact and low-cost sub-THz transmitters operating at gigabits per second data rates. Furthermore, the carrier frequencies of the links should be widely tuneable if multi-channel communications is to be provided. These requirements, however, are challenging for all-electronic approaches [2] and terahertzquantum-cascade-lasers [3]. Resonant tunnelling diodes have been reported with a data rate of 2 Gbps at 542 GHz within a back-to-back connection, though the carrier frequency of this particular sub-THz source however has a limited tuning range [2]..
As a result, in recent years, research interest has focused on using photonic techniques. For example, studies have been reported into an integrated tunable sub-THz transmitter [4,5] comprising two distributed feedback (DFB) lasers and a passive combiner. A sub-THz signal is generated by detecting the combined optical signals from the lasers using a high bandwidth photodiode connected to a horn antenna. The signal frequency is set by the separation of the laser wavelengths, which enables wide frequency tuneability of such devices of up to 120 GHz being reported [4]. A link based on a dual laser source has been demonstrated by applying 1 Gbps on-off-keying (OOK) data onto a Mach–Zehnder modulator [5]. In that case, however, the photonic link components were not fully integrated.
In this paper, we demonstrate a transmission link consisting of a compact dual-laser optical source with an onchip modulator. The optical source generates a beat frequency of 105.4 GHz which is then transmitted into free space. 1 Gbps OOK data with a run length of 27 -1, similar to the block code run length of datacoms standards such as GbE, is directly applied to the on-chip modulator. A chirp-enhanced direct modulation scheme [6] is employed to enhance the received signal extinction ratio. The resultant signal captured is evaluated to have a bit error rate (BER) of 10-3 , the forward error correction (FEC) error rate limit, at a transmission length of 40 cm. The achieveable link length can be predicted to be 50 m at 1 Gbps with optimised link antenna and RF amplifier gains and minimised waveguide losses.

This article has been originally posted on the University of Cambridge's Open Source Research Repository on 2014-12-16. The full length article can be viewed here.

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