L. Raddatz

Influence of restricted mode excitation on bandwidth of multimode fiber links

It is shown that multimode optical fiber links that use laser-based transmitters may be limited in bandwidth to values less than half those of the over-filled launch bandwidth. A solution to this problem, based on an offset launch, is therefore proposed, which increases the bandwidth to more than the overfilled launch.

Overcoming the modal bandwidth limitation of multimode fiber by using passband modulation

It is demonstrated that the bandwidth limitation of multimode fibre links arising from modal dispersion can be overcome by using passband modulation in the frequency range beyond the 3dB bandwidth. Transmission of 1.25GBit/s over 500m of near-worst case 62.5μm multimode fibre is achieved using two subcarrier multiplexed channels at the 850nm wavelength. In contrast the conventional baseband transmission distance is limited to 300m at this data rate.

Mode control in vertical-cavity surface-emitting lasers by post-processing using focused ion-beam etching

Single-mode emission is achieved in previously multimode gain-guided vertical-cavity surface-emitting lasers (VCSELs) by localized modification of the mirror reflectivity using focused ion-beam etching. Reflectivity engineering is also demonstrated to suppress transverse mode emission in an oxide-confined device, reducing the spectral width from 1.2 nm to less than 0.5 nm.

Fiber-optic m-ary modulation scheme using multiple light sources

In conclusion, we have demonstrated a multilevel digital modulation scheme in a fiber-optic communications link, using multiple light sources integrated on a single chip and enabling enhanced transmission performance. This scheme allows correction for changes in the output power characteristics of the light sources and is easy to implement, particularly with arrays of laser diodes.

High bandwidth multimode fiber links using subcarrier muitiplexing in vertical-cavity surface-emitting lasers

Summary form only given. There is currently much interest in low-cost high-speed and short-haul links for local area networks and computer interconnect applications, with standards bodies such as those setting the Gigabit Ethernet standard now considering transmission rates in excess of 1 Gbit/s. Because of cost, particular emphasis has been placed on the use of multimode fiber (MMF) links using vertical-cavity surface-emitting lasers (VCSELs). With increasing data rates however the achievable link lengths become limited due to the modal bandwidth of the MMF. Efforts to overcome this limitation have included the use of wavelength-division multiplexing, multilevel modulation and restricted mode launches. In this paper, however we believe we describe for the first time how subcarrier multiplexing (SCM) techniques can be used to allow link transmission significantly in excess of the specified fiber bandwidth. The technique is robust and has been assessed using a representative range of fibres. A series of 200-Mbit/s channels with carrier frequencies of up to more than twenty times the 3-dB fiber bandwidth have been successfully used, the maximum being limited by the electronics currently available.

Overcoming the model bandwidth limitation of multimode fiber by using passband modulation

It is demonstrated that the bandwidth limitation of multimode fiber links arising from modal dispersion can be overcome by using passband modulation in the frequency range beyond the 3 dB bandwidth. Transmission of 1.25 Gb/s over 500 m of near-worst case 62.5 /spl mu/m multimode fiber is achieved using two subcarrier multiplexed channels at the 850-nm wavelength.

Bandwidth enhancement of multimode fiber Gbits/s networks using conditioned launch

There is currently much interest in high-speed LANs, with standards for gigabit Ethernet and fiber channel under development. The 62.5-/spl mu/m core multimode fiber (MMF), which is widely used for LANs, has minimum bandwidth specifications of 160 and 500 MHz/spl middot/km at wavelengths of 850 and 1300 nm, respectively. These values translate into link-length limitations of /spl sim/300 and 800 m, respectively, for the gigabit Ethernet rate of 1.25 Gbits/s. The over-filled launch (OFL) bandwidth, for which these values are specified is, however, not the worst-case bandwidth for all launch conditions. In particular, underfilled launches as obtained with current laser transceivers can reduce the link bandwidth to less than half the OFL value. This effect has led recently to specifications of the worst-case link lengths for gigabit Ethernet of 260 and 440 m for 62.5 /spl mu/m MMF operating at 850 and 1300 nm, respectively. This paper demonstrates that the link bandwidth can be restored using an offset launch scheme, so that link lengths of 800 m with gigabit Ethernet are possible. In this work, the effect of the use of laser-based transmitters in MMF links was studied using 13 different representative fiber samples, all of 62.5-/spl mu/m core diameter. Measurements have shown that in eight cases, the link bandwidth with a laser transmitter module is far below the bandwidth for the OFL. To study this effect, detailed impulse response measurements have been carried out with a launch from a single-mode fiber patch cord with variable radial position into the MMF.

Transmission Of Subcarrier Multiplexed Signals Over Multimode Fibre Links Using Vertical Cavity Surface Emitting Lasers

We show that low-cost multimode VCSElLs show sufficiently linear operation for operation in multimode fibre SCM systems. Dual source SCM systems are demonstrated with carrier frequencies up to 3GHz, indicating that multi-channel systems can readily be achieved for use in self-routing networks. Introduction Recently there has been much interest in the transmission of subcarrier multiplexed (SCM) signals over optical fibre, for example ini applications including cable television distribution and remote antenna addressing in mobile phone systems. SCM systems could also be employed in future computer interconnects and access netwoirks, where the ease of implementing carrier frequency changes readily allows for broadcasting and network reconfiguration. A schematic of such an interconnect, based on a bus-configuration, is shown infigure 1. Fig. 1: Schematic of SCM fibre optic network The dominant type of optical fibre currently installed in buildings is multimode fibre (MMF) [ 13 and hence there is interest in the potential of high speed SCM systems using this installed fibre base. As the insertion loss associate:d with MMF couplers and splitters can cause modal noise [2], low coherence light sources h,ave to employed in such systems. Self-pulsing laser diodes (SPLDs), which were developed for Compact Disk applications, are one option to meet this requirement and a MMF SCM link has been demonstrated using SPLDs [3]. However the

Post-processing of vertical-cavity surface-emitting lasers for transverse mode and polarization control

The operation on how high quality single-mode operation can be readily attained on etching circles in multimode devices is discussed. Arrays of such spots can also be envisaged. Control of the polarization state is also achieved by use of deep line etches. The output filaments and beam shapes of the conventional multimode vertical cavity surface emitting lasers (VCSEL) is shown to be engineered in terms of their positions, widths, and polarizations by use of focused ion beam etching (FIBE). Several GaAs quantum well top-emitting devices with cavity diameters of 10 μm and 18 μm were investigated.

Beam control and manipulation in vertical cavity surface emitting lasers

This paper reviews novel techniques which are used to modify the properties of the optical beam generated by vertical cavity surface emitting laser diodes. The main emphasis within the paper concerns the use of etches applied to the device surface either adjacent to or across its optical aperture. Such etches can be used to modify facet reflectivities sufficiently to pin the optical mode precisely and reduce the onset of higher order modes. In addition, single polarization operation is achieved by using deeper etches placed outside the light emitting region. Using this technique, previously multimode components may be transformed to single mode components with precisely fixed polarization states. The effect of these etches on other lasing properties such as spectral performance and power efficiency is also discussed.