Calculation and measurement of resonant-mode blueshifts inoxide-apertured VCSELs

As the aperture size of oxide-apertured vertical-cavity surface-emitting lasers (VCSELs) shrinks, the lasing wavelength blueshifts. We have calculated this effect using both a scalar effective index model and a full-vector weighted index model. Results were compared against experimental data for two different VCSEL designs emitting near 780 and 850 nm. We find that the full-vector weighted index calculation matches the data remarkably well, while the scalar effective index calculation underestimates the blueshift     

Photonic crystal heterostructure cavity lasers using kagome lattices

The lasing characteristics of a photonic crystal membrane heterostructure cavity that utilises a kagome type lattice is demonstrated. A heterostucture cavity is formed by interfacing two photonic crystals such that the dispersion maximum of the inner lattice falls within the photonic bandgap of the surrounding lattice. Feedback to slow Bloch modes allows for localisation of band edge modes and a reduction of in-plane losses. Singlemode lasing is observed in relatively large area lasers at 1550 nm.     

Temperature-dependent characteristics and single-mode performanceof AlGaInP-based 670-690-nm vertical-cavity surface-emitting lasers

We report on temperature dependent characteristics and single mode performance of one-wave cavity, planar implanted, AlGaInP-based vertical-cavity surface emitting lasers. By optimizing the overlap between the gain peak and the cavity mode of the structure, we demonstrate record device performance, including 8.2 mW maximum output power and 11% power conversion efficiency for multimode operation and 1.9 mW and 9.6% power conversion efficiency for single mode operation at 687 nm. Improved performance at elevated temperatures is also achieved, with 1.5 mW output power demonstrated at 50°C from a 15-μm-diameter device     

Direct Modulation Characteristics of Composite Resonator Vertical-Cavity Lasers

We report the small-signal modulation characteristics of a monolithic dual resonator vertical cavity surface emitting laser. The modulation response is described by a system of rate equations with two independent carrier populations and a single longitudinal optical mode. The independent optical overlaps and differential gains of the two active regions can each be adjusted to maximize the output response. We show that under certain conditions, the composite resonator may achieve a higher bandwidth than a single cavity laser with the same photon density. We find the relaxation oscillation frequency to depend mainly on the total photon density and not the individual currents in the two cavities. With appropriate current injection, the composite resonator laser achieves a maximum -3-dB bandwidth of 12.5 GHz and a maximum modulation current efficiency factor of approximately 5GHz/ma^1/2      

Minimisation of carrier frequency acquisition time in QAM systems

Two recent carrier recovery loops for high-order quadrature amplitude modulation (QAM) and optimisation of their parameters for minimum frequency acquisition times are analysed. All variations on the parameters were analysed and optimised to obtain optimal loop bandwidths and damping factors, for given signal-to-noise ratios. Simulation results show that after optimisation, the average acquisition time for 64-QAM is kept below 1000 symbols over the entire range of initial frequency offsets from 0 to 1/8 the symbol rate. This number increases to 3200 symbols for 256-QAM.     

OC-48 capable InGaAsN vertical cavity lasers

A selectively oxidised InGaAsN/GaAs three quantum well vertical cavity laser (VCSEL) demonstrated continuous wave (CW) lasing with a single-mode output power of 0.749 mW at 1266 nm. This is the first reported InGaAsN VCSEL capable of meeting the power and wavelength requirements for both OC-48 SR and OC-48 IR-1 compliant links. The VCSEL uses two low absorption n-type GaAs/AlGaAs distributed Brag reflectors and a tunnel junction to achieve current injection into the active region. A multimode version of the VCSEL had a output power of 1.43 mW at 1.26 μm. CW lasing continued up to temperatures as high as 107°C. The VCSEL material was grown by solid source molecular beam epitaxy with an RF nitrogen plasma source     

GaAs vertical-cavity surface emitting lasers fabricated by reactive ion etching

GaAs quantum well vertical-cavity surface emitting lasers fabricated using low damage reactive ion etching are discussed. Lasers which are partially and completely etched through their structure are compared. The surface recombination velocity of exposed GaAs is not exacerbated in deep etched lasers; other loss mechanisms in shallow etched lasers have comparable impact on laser performance. Etched lasers exhibit low voltage and small differential series resistance at threshold, while devices fabricated by a combination of etching and ion implantation possess lower threshold current. It is found that reactive ion etching has little additional effect on laser operation, whereas the different device structures considered do influence laser performance.<>     

Dual-channel wavelength-division multiplexing using a composite resonator vertical-cavity laser

We demonstrate the use of a dual-resonator vertical-cavity laser diode as a coarse wavelength-division-multiplexing optical source. Binary data is encoded onto each of the two longitudinal modes, which can be independently modulated through current injection into each cavity. Using a single laser, we demonstrate data transmission on two independent channels.     

Automated spectral smoothing with spatially adaptive penalized least squares

A variety of data smoothing techniques exist to address the issue of noise in spectroscopic data. The vast majority, however, require parameter specification by a knowledgeable user, which is typically accomplished by trial and error. In most situations, optimized parameters represent a compromise between noise reduction and signal preservation. In this work, we demonstrate a nonparametric regression approach to spectral smoothing using a spatially adaptive penalized least squares (SAPLS) approach. An iterative optimization procedure is employed that permits gradual flexibility in the smooth fit when statistically significant trends based on multiscale statistics assuming white Gaussian noise are detected. With an estimate of the noise level in the spectrum the procedure is fully automatic with a specified confidence level for the statistics. Potential application to the heteroscedastic noise case is also demonstrated. Performance was assessed in simulations conducted on several synthetic spectra using traditional error measures as well as comparisons of local extrema in the resulting smoothed signals to those in the true spectra. For the simulated spectra, a best case comparison with the Savitzky-Golay smoothing via an exhaustive parameter search was performed while the SAPLS method was assessed for automated application. The application to several dissimilar experimentally obtained Raman spectra is also presented.     

One- and Two-Dimensional Coherently Coupled Implant-Defined Vertical-Cavity Laser Arrays

Loss between elements of coherently coupled vertical-cavity surface-emitting laser (VCSEL) arrays typically causes out-of-phase operation with on-axis intensity nulls in the far-field. We show that in-phase evanescently coupled VCSEL arrays may be defined by proton implantation. An advantage for implanted in-phase coherently coupled VCSEL arrays is that this approach employs a simple and reliable fabrication process where the absence of loss between elements leads to in-phase coupling. We present data for 2, 3, and 4 element in-phase implant-defined coherently coupled VCSEL arrays.