Quantum Key Distribution (QKD) is attracting much interest for the distribution of cryptographic keys using single photon signals. Currently QKD is often used to provide secure distribution of cryptographic keys for the encryption of data transmitted using conventional classical communication systems.
This paper reports major field trials carried out over several months on the Cambridge UK Quantum Network showing the operation of QKD systems alongside high-speed classical transmission systems encrypted QKD derived with AES keys. Quantum Key transmission at record secure key rates of 3.3Mbps, 3.2Mbps and 2.5Mbps has been achieved over 5km, 9.5km and 10.5km long links respectively with corresponding average Quantum bit error rates (QBER) of 2.9%, 2.4% and 3%.
Using a 33km link attached to the network with a loss of 7.5 dB, a secure key rate of 1.4 Mbps is achieved with an average QBER of 3.4%. Under loop back conditions this link provides a 66 km transmission path with a 16dB fibre loss, enabling a field trial using the QKD signals multiplexed with two wavelengths each transmitting 100Gb/s classical data to be carried out. This achieves an average secure key rate of 80.2 kbps and a mean QBER of 6.6%, in line with theoretical predictions. During the trial duration, the statistics of the QBER were found to be Gaussian distributed with a standard deviation of 0.5. The results of the field trial suggest that the system works stably and has considerable potential for applications in metropolitan networks. Further measurements will be reported at the conference.
In Europe a number of technology platforms for generic integration are being created for photonic integrated circuits (PICs); in Silicon, in passive dielectrics, and in Indium Phosphide. Such platforms are on the brink of commercialization, they offer a range of calibrated building blocks from which application specific PICs can be built and allow simplified, reduced cost access to a standardised technology, but presently only InP based platforms allow the integration of optical gain blocks; the essential feature of a semiconductor laser. The wavelength is constrained by the platform, usually C-band, but in the near future we expect other wavelengths in the 1.3μm-2.0μm range will be addressed. A frozen platform technology may not seem an ideal starting point for novel laser research but for what may be appear to be lost in epitaxial and process flexibility, much more is gained through a new-found ability to build up complex circuits quickly to deliver new and interesting laser based functionality. Building blocks such as reflectors (a distributed Bragg reflector (DBR) or a multimode interference reflector (MIR)), an amplifier section, and passive waveguides, can be built up by designers into integrated semiconductor lasers of a wide variety of types. This ready integration of novel sources with other circuit functionality can address a wide range of applications in telecoms, datacoms, and fibre based sensing systems. In this paper we describe a number of recent developments on generic InP-based platforms ranging from the fabrication of simple Fabry-Perot lasers, through tuneable DBR lasers, multi-wavelength comb lasers, picosecond pulse lasers and ring lasers.
Optical interconnects are being considered for short link data networks as a solution enabling higher aggregate bit rates and lower power consumption. For short link length interconnects, as used in chip to chip interconnects, internal system backplanes and inter-system interconnects such as blade server backplanes, storage area networks and processing clusters, requirements are quite different to those for long distance telecommunications systems. Low power consumption, latency, and size become important criteria in addition to ultra high bandwidth. In order to achieve the projected ultra high capacity and low latency needs, we are considering optical switching fabrics. The optical switch, however, brings significant changes to the interconnect architecture in terms of how routing decisions are made and how contention resolution is managed. We discuss these issues and present our results for a multiwavelength optically switched interconnect.
There is currently interest in using novel modulation formats for high bit-rate datacoms systems. 4-level modulation is an attractive method of halving the line-rate required for 40Gb/s systems. This 20GBaud line rate enables reduced bandwidth direct modulation of semiconductor lasers, thus reducing laser chirp, increasing transmission distances and also enabling simplified drive electronics to be used. In this experiment the 4-level signal is generated by electrically combining 2 de-correlated 20Gb/s data streams of differing amplitude from a pattern generator and then used to modulate a DFB laser. The directly modulated source is a DFB laser, emitting at 1310nm with a 3dB frequency response of 20GHz. This laser also has a very linear modulation response, with a spurious free dynamic range of over 100dBHz2/3 at 25°C and over 90 dBHz2/3 at 85°C. This highly linear behaviour is necessary to allow direct 4-level modulation source even at high temperature. The 40Gb/s 4-level signal is then transmitted along standard fibre and detected with an electrical receiver. In order to overcome the attenuation limited transmission distance of 20km a semiconductor optical amplifier, with a saturation power of 11dBm and fibre to fibre gain of 20dB, is used. The addition of an SOA enables transmission distances of 40km to be achieved with transmission penalties of as low as 2.6dB, even with the laser operating at 70°C. The robustness of the 4-level modulation is compared to NRZ and the impairments to both signals upon optical amplification are examined.
Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids.
We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz.
GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2.
Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers.
Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained.
Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 µm, probably up to 1.55 μm.
We report the analysis and application of uncooled, directly-modulated high-speed DFB lasers with emphasis on their analogue transmission performance. Fibre-optic links employing such lasers are shown to meet the most stringent requirements of analogue systems at both high carrier frequencies and high temperatures. Spurious-free dynamic ranges (SFDR) exceeding 100dB×Hz2/3 and 90dB×Hz2/3 and input third-order intercept points (IIP3) above 20dBm and 18dBm are reported for carrier frequencies up to 20GHz at 25°C and up to 10GHz at 85°C, respectively. The error-vector magnitude (EVM) for a 256-QAM modulated signal transmitted over 15km of SMF remains below 1.9% for carrier frequencies of both 2GHz and 5GHz for all measured temperatures. The link performance is assessed by using 3GPP W-CDMA, IEEE 802.11a and IEEE 802.11b signals. In all cases the EVM remains within the standard specification, for fibre-optic link lengths of up to 10km and laser operating temperatures of up to 70°C. Finally, an IEEE 802.11b WLAN demonstrator is presented, allowing antenna remoting over up to 1000m of 62.5/125μm MMF.
Threshold reduction and enhanced mode selectivity are demonstrated in pulsed GaN-based lasers upon the introduction of 5(lambda) /4 air/nitride Bragg gratings defined by focused ion beam (FIB) etching. A 13% reduction in threshold current is obtained from a laser with a 5 micrometers wide ridge by introducing a deep-etch air/nitride mirror. The presence of a reduced-depth Bragg grating, etched across 4 micrometers wide ridge structure using a lower FIB dose, results in single-peak spectral characteristics for currents up to 1.14(DOT)ITh. The introduction of the Bragg mirrors always results in a broadening of the near field parallel to the epitaxial planes.