Peter Modh

Direct high-frequency modulation of VCSELs and applications in fibre optic RF and microwave links

With the rapid development of wireless communication networks there is an increasing demand for efficient and cost-effective transmission and distribution of RF signals. Fibre optic RF links, employing directly modulated semiconductor lasers, provide many of the desired characteristics for such distribution systems and in the search for cost-effective solutions, the vertical cavity surface emitting laser (VCSEL) is of interest. It has therefore been the purpose of this work to investigate whether 850 nm VCSELs fulfil basic performance requirements for fibre optic RF links operating in the low-GHz range. The performance of single- and multimode oxide confined VCSELs has been compared, in order to pin-point limitations and to find the optimum design. Fibre optic RF links using VCSELs and multimode fibres have been assembled and evaluated with respect to performance characteristics of importance for wireless communication systems. We have found that optimized single-mode VCSELs provide the highest performance and that links using such VCSELs and high-bandwidth multimode fibres satisfy the requirements in a number of applications, including cellular systems for mobile communication and wireless local area networks.

Dynamic behavior of fundamental-mode stabilized VCSELs using a shallow surface relief

An extensive theoretical study was performed on the dynamic behavior of 850-nm-wavelength oxide-confined fundamental-mode stabilized vertical-cavity surface-emitting lasers (VCSELs), using a shallow surface relief. The surface relief is used to provide lower mirror loss for the fundamental mode, thus acting as a mode discriminator. In this way, single-mode operation at high power levels can be obtained. We utilized a comprehensive model that includes the detailed epitaxial layer structure and device geometry when calculating the optical fields and that accurately accounts for the dynamic effects of carrier density and temperature on the modal distributions. Modulation response, eye diagrams, bit error rate (BER), and relative intensity noise (RIN) were simulated and compared to the performance of VCSELs without a mode discriminator, i.e., conventional multimode VCSELs. The fundamental-mode stabilized VCSELs are associated with a higher out-coupling, which lowers the relaxation oscillation frequency and damping, and strong spatial hole burning, which induces a low-frequency roll-off in the modulation response and contributes to the damping of the relaxation oscillation at low bias. However, their dynamics is fully competitive with conventional multimode VCSELs at both 2.5 and 10 Gb/s although they exhibit a slightly higher eye closure. We only found a 0.5-dB power penalty in the BER. The RIN is enhanced, with a peak that is about 10-15 dB higher, caused by the lower damping of the relaxation oscillation. It should be noted that in the comparison we assume that all modes are equally captured from the multimode VCSEL. A mode-selective loss can severely degrade its performance.

Suppression of Higher Order Transverse and Oxide Modes in 1.3-

It is shown, by a systematic variation of design parameters, that the use of an inverted surface relief is very effective for suppressing higher order transverse modes in oxide confined 1.3-mum InGaAs vertical-cavity surface-emitting lasers (VCSELs). Single-mode emission is achieved for a large variety of oxide aperture and surface relief diameters, with optimum designs, having a surface relief with a diameter half of that of the oxide aperture, producing 1.1-1.3 mW of single-mode power. It is also shown that the anti-phase layer employed to enable the use of an inverted surface relief is effective for suppressing oxide modes that otherwise appear in oxide confined VCSELs with a large detuning between the gain peak and the cavity resonance

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We have studied the modulation bandwidth of high-speed GaInNAs double-quantum-well lasers emitting at 1.28–1.30μm. A 400μm long ridge waveguide laser exhibits a small signal modulation bandwidth of 14 GHz. The intrinsic damping limited modulation bandwidth is as high as 25 GHz (K=0.35ns), and the actual modulation bandwidth is limited by thermal effects under continuous operation. The saturation of the resonance frequency at 10 GHz was found to be the result of a thermal reduction of the differential gain and a rapid increase of the threshold current when the temperature exceeds 80 °C.

m InGaAs VCSELs by an Inverted Surface Relief

Grating coupled surface emitting lasers (GCSELs) are in- plane lasers monolithically integrated with grating outcouplers for beam shaping and image generation. Highly directional and efficient outcouplers can be formed using various grating and waveguide geometries. Beam shaping features are incorporated using a computer generated waveguide hologram that allows the wavefront of the emitted light to be tailored for the required beam shape. Requirements on the integrated in-plane laser include a wide and spatially coherent guided wave with a minimum of wavefront distortion and a stable emission wavelength. Promising lasers for this purpose are master-oscillator power-amplifier configurations as well as various kinds of unstable resonator designs. Here we present results from modeling and experiments on the key elements of GCSELs as well as fully integrated GCSELs of linear and circular geometries.

High-frequency modulation and bandwidth limitations of GaInNAs double-quantum-well lasers

We report on the fabrication and operation characteristics of a grating-based, surface-emitting, tapered unstable resonator laser with a trumpet shaped taper and a focusing outcoupler. The laser has a low threshold current density of 225 A/cm/sup 2/. It operates with a spatially coherent field and the near-field intensity indicates stable, filamentation free behavior up to at least 3/spl times/ the threshold current under continuous operation. At low drive currents the laser produces a nearly diffraction limited focused spot. With increasing current, the focused spot size increases and multiple side-lobes develop at high currents. We attribute this to a change in the phase of the emitted wavefront associated with thermal effects in the gain region.

Grating coupled surface emitters: integrated lasers, amplifiers, and beam shaping outcouplers

A grating-coupled surface-emitting semiconductor laser has been integrated with a focusing diffractive beamforming element. A hyperbolic unstable resonator is used to introduce mode discrimination and suppress filamentation in a broad geometry, resulting in a 200-/spl mu/m-wide coherent output. The light is focused 500 /spl mu/m above the laser surface to a spot size of 5/spl times/7 /spl mu/m. A remarkable beam stability is observed with very small spot size variation and beam-steering up to three times the threshold current under continuous operation.

Surface-emitting tapered unstable resonator laser with integrated focusing grating coupler

The threshold and gain characteristics of GaInNAs single quantum well (QW) lasers with GaNAs and GaAs barriers, both emitting at 1300 nm, have been compared. The threshold current density for the laser with GaAs barriers is twice as high, presumably because of a higher monomolecular recombination rate caused by the higher N concentration in the QW. A significant difference in the spectral gain characteristics was also observed. Calculations show that this is due to a modification of the confinement potential for the conduction band electrons when incorporating N in the barriers and reducing the N concentration in the QW. An additional inhomogeneous broadening also had to be included in the calculations to obtain quantitative agreement between measured and calculated gain spectra.

Grating-coupled surface-emitting laser with a hyperbolic unstable resonator producing a stable focused output beam

We demonstrate that a carefully designed fundamental-mode stabilized vertical-cavity surface-emitting laser (VCSEL) using a shallow surface relief can maintain a large sidemode suppression ratio (SMSR) under strong radio frequency modulation. A number of VCSELs emitting at 850 nm, with different surface relief and oxide aperture diameters, have been experimentally investigated. For certain combinations of these diameters, the SMSR exceeds 30 dB at 2- and 5-GHz operation, even for as much as 15 dBm of input modulation power.

Direct Comparison of Threshold and Gain Characteristics of 1300 nm GaInNAs lasers with GaNAs and GaAs Barriers

A monolithically integrated master oscillator power amplifier with a grating outcoupler for emitting a collimated beam is demonstrated. In order to improve the device performance, theoretical simulation is performed based on the beam propagation method, and both compensation for wavefront distortion produced in the amplifier and optimization of the radiation decay factor are incorporated in a new design of the grating outcoupler. The device was fabricated by a simple process without regrowth. Stable single-mode lasing up to 183 mW output power at a wavelength of 985 nm was obtained under CW operation. As a result of optimization to minimize the divergence angle of the output beam, a well-collimated beam, with a nearly circular profile and divergence angles smaller than 0.28°×0.38°, was maintained in high-power operation.