L.D. Westbrook

20-nm optical wavelength conversion using nondegenerate four-wave mixing

Wavelength conversion of optical signals over 20 nm is demonstrated using highly nondegenerate four-wave mixing in a semiconductor traveling-wave optical amplifier. This technique has the potential for extremely-high-speed operation and allows continuous tuning of both input and output wavelengths over the amplifier gain bandwidth. It is demonstrated that, even for such a large wavelength conversion range, it is possible to obtain conversion efficiencies in excess of -10 dB and high extinction ratios. The feasibility of the technique is demonstrated by system measurements at 622 Mb/s, showing a 1.1-dB power penalty at 10/sup -9/ bit error rate (BER).<<ETX>>

Resonance frequency, damping, and differential gain in 1.5 mu m multiple quantum-well lasers

A systematic investigation is presented into the intrinsic frequency response of quantum-well lasers, using parasitic-free relative intensity noise (RIN) measurements. There is shown to be a strong dependence of the resonance frequency on the number of quantum wells in the active region, originating from variations both in internal losses and in differential gain. The differential gain is found to have values higher than in corresponding bulk lasers, but only in devices with a large number of wells. The damping is also found to vary in a manner consonant with the changes in differential gain; however, comparison with bulk lasers indicates substantially stronger gain suppression in the quantum-well lasers studied. >

The "gain-lever" effect in InGaAsP/InP multiple quantum well lasers

The gain-lever-effect has been assessed in both multiple quantum well (MQW) and bulk active region lasers having a range of lengths and split ratios in the top contact. Compared with a conventional single-contact laser, a 500-/spl mu/m MQW FP device with a top-contact split ratio of 8:1 exhibited >15-dB improvement in AM efficiency and a signal-to-noise ratio improvement of 7.5 dB. With proper impedance matching, these figures may be improved. A reduced noise figure is obtained at the expense of dynamic range with an additional nonlinearity seen experimentally due to thermal and carrier leakage when compared with simulations carried out using a harmonic balance model. Used carefully, these gain-lever lasers are useful components for future analogue fiber-optic systems. >

Spectral properties of strongly coupled 1.5 &#181;m DFB laser diodes

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficient k in these devices was determined to be 80 cm-1for lasers with \lambda = 1.12 \mu m cladding layers and 160 cm-1for devices with \lambda = 1.3 \mu m cladding layers. These values for k are believed to be the largest reported for 1.5 μm DFB lasers. CW spectral linewidths as low as 10 MHz at 15 mW output power were obtained, and the linewidth was observed to vary approximately as the inverse of the device length cubed. Spectral measurements performed under 2 Gbit/s direct modulation exhibited a side mode suppression ratio of >38 dB. The effects of transient wavelength chirping were also investigated in detail and the maximum wavelength deviation was found to be ≃1.5 A.

Overcoming fibre chromatic dispersion in high bit rate transmission

We have demonstrated that mid-span spectral inversion by four-wave mixing in a semiconductor optical amplifier can be used to effectively eliminate dispersion in high bit rate transmission. The technique has been used to achieve transmission of 1OGb/s signals from a directly modulated DFB over 200km dispersive fibre, and of 25ps gain-switched pulses over 100km dispersive fibre. The results show a complete elimination of degradation resulting from fibre chromatic dispersion. The former result represents more than a 40-fold increase in the bit rate distance product for the directly modulated signals by means of MSSI.

Linearity and noise of InGaAs multi-quantum-well lasers

Measurements are presented which demonstrate that multiple-quantum-well (MQW) lasers can offer significantly lower noise and distortion levels than equivalent double-heterostructure lasers. Improvements of approximately=10 dB in the laser intensity noise and approximately=8 dB in the second-harmonic distortion have been achieved. These figures are close to the predicted improvement of approximately=12 dB expected in MQW lasers as a result of doubling the laser resonant frequency.<<ETX>>

A novel current injected strained quantum well laser grown by MOVPE

Conventional long wavelength (1.3 and 1.55 μm emitting) GalnAsP alloy lasers suffer from two disadvantages. Firstly, carriers in the highest lying valence band have a heavy effective mass relative to carriers in the conduction band. This asymmetry leads to an increase in the carrier density required for lasing action to occur. Secondly, non-radia-tive recombination processes, such as Auger Recombination (AR) and Inter Valence Band Absorption (IVBA), which involve occupancy of the heavy-hole (HH) states, are thought to be significant in these materials. These again lead to higher thresholds and lower values ofT0than might otherwise be the case. Recently, there has been considerable interest in the prospect of “engineering” the band structure of a 1.5 μm emitting device so as to overcome these problems. It has been reported that for a quantum well under biaxial compression, the light-hole/heavy-hole (LH/HH) degeneracy at the gamma point will be lifted such that the highest lying valence band will be LH-like in the in-plane direction. This should reduce both the effective mass asymmetry and the thermal occupancy of the HH states, lowering the threshold carrier density and reducing the AR and IVBA rates. This paper describes MOVPE growth and characterisation of the first 1.55 μm emitting current injected strained layer laser structure. The active region contains 3.5 nm thick strained quantum wells of Gao.3Ino.7As situated in the central region of a quaternary waveguide and grown on InP. TEM micrographs and x-ray data demonstrate that the lattice mismatch (approximately 1%) has been accommodated elastically, without the formation of misfit dislocations. Broad area lasers have been fabricated with lengths of 200–1200 μm and threshold current densities as low as 930 Acm-2 have been measured from the longer devices. Similar 1.55 μm emitting structures containing unstrained 7.5 nm thick Gao.47Ino.53As wells have also been grown and characterised for comparison. As yet, no significant improvement in either threshold current orT0has been observed for strained lasers over unstrained devices.