Optical sources for the forthcoming terabit/s era of optical communications and networking will require multiple frequency-locked carriers, each with low phase noise, in order to minimize the spectral occupancy of the overall channel bandwidth. One method to construct a highly reconfigurable version of such a source is to use an optical frequency comb from a gain-switched laser to simultaneously injection-lock many different single mode lasers. The outputs from the single-mode lasers are all mutually frequency locked and possess the same low-phase noise properties of the gainswitched comb. In this submission, we present numerical simulation results from the entire system of simultaneously injection-locked single mode lasers by firstly simulating an optical frequency comb from the gain-switched laser and then using that frequency comb to injection lock the single-mode lasers. The simulation approach is to use lumped rate equations with the appropriate stochastic Langevin terms for spontaneous carrier recombination and for spontaneous emission. The inclusion of the stochastic terms are vital when identifying the locked states of the entire system. Using the simulator we are able to identify important criteria to maximize the frequency locking range that suppresses the cross talk from adjacent comb lines to greater than 30 dB, and avoiding the carrier-photon resonance of the single mode lasers is vital to achieve this. The relative simplicity of the simulator has the advantage of being exploited within optical communication simulators to predict the communication system performance when using these sources, which would be of advantage to designers of such systems.
We report on discrete mode laser diodes designed for narrow linewidth emission and demonstrate a linewidth as low as
96 kHz. A discrete mode laser diode with a minimum linewidth of 189 kHz was also characterised in a coherent
transmission setup using quadrature phase shift keying modulation. Similar performance to an external cavity laser is
demonstrated at baud rates as low as 2.5 Gbaud. The effect of increased linewidth on transmission performance is also
investigated using lasers with linewidths up to 1.5 MHz.
Optical Packet Switched (OPS) networks employing Optical Label Switching (OLS) techniques have the potential to enable an all-optical internet. In these networks, data remains in optical format throughout the entire network and routing is performed using a separate optical label. The label information is used to control fast tunable lasers that will transfer data packets to different wavelengths for routing and contention resolution. In this paper we investigate interference between subcarrier multiplexed (SCM) labels in such a network, due to switching events in the tunable laser transmitter. This interference may place a limitation on the channel spacing and subcarrier frequency used.
Two 50GHz spaced optical carriers were modulated with 2.5Gbit/s SCM labels at 20GHz. Bit error rate measurements were taken with two lasers fixed 50 GHz apart, and also with one of the lasers (an SG-DBR) switching between this channel and another one 800GHz away. When the SG-DBR laser is not switching, a power penalty of approximately 0.25 dB is introduced due to interference through the optical filter. However, when the SG-DBR laser is switching between wavelengths an error floor of 1x10-5 is introduced due to the time it takes the tunable laser to settle to its target channel. In a systems application, this would result in packets being incorrectly routed.
With the increasing demand for broadband services, it is expected that hybrid fiber/radio systems may be employed to provide high capacity access networks for both mobile and fixed users. Third generation (3G) mobile systems for example, will operate around the 2.4Ghz band, while fourth generation (4G) systems may operate in the 5.8GHz band or beyond. To make these future generation systems commercially viable it is important to keep costs as low as possible. One method of keeping costs to a minimum is to have a central station (CS) where the radio frequency (RF) data signals are modulated onto an optical carrier and sent to a number of base stations (BS) over optical fiber, before being transmitted over air to the users. This allows the BS complexity to be kept to a minimum. A possible solution for generating the optical RF data signals for distribution over fiber is to directly modulate the RF signal onto an optical carrier using a laser diode. The major problem with this technique is that broadband microwave systems are likely to use frequency division multiplexing for transmitting very high data rates. This will thus involve modulating the laser with electrical data signals at multiple frequencies, which will result in serious interference due to dynamic nonlinearities in standard laser diodes around the electrical transmission frequencies. This distortion, known as intermodulation distortion (IMD), can significantly degrade the performance of optically fed microwave systems for high-speed access networks. This paper examines how this laser nonlinearity degrades the performance of hybrid fiber/radio systems operating in various RF transmission bands, and investigates possible techniques to overcome these problems.