A. Ramakrishnan

Development of InGaAsN-based 1.3 μm VCSELs

We review the status of InGaAsN-based vertical-cavity surface-emitting lasers (VCSELs) emitting in the wavelength range 1.2–1.3 μm and compare them with similar devices that have been realized using other approaches. To prove the potential of InGaAsN-based VCSELs, we present our results for monolithically MBE- and MOVPE-grown and electrically pumped VCSELs on GaAs substrates. Our MBE-grown devices emit at a wavelength of up to 1305 nm with cw output power at room temperature exceeding 1 mW and a threshold current of 2.2 mA. With an oxide-confined current aperture of about 5 μm diameter, they emit up to 700 μW in single-mode operation at room temperature. Bit-error rates of less than 10−11 are achieved for transmission over 20.5 km of standard single-mode fibre at 2.5 Gbit s−1. Our MOVPE-grown VCSELs with a similar device structure emit single mode at a wavelength of 1293 nm with a cw output power of 1.4 mW and a threshold current of 1.25 mA at room temperature. In back-to-back transmission, we reach a data rate of 10 Gbit s−1, proving the feasibility of high-speed data transmission using InGaAsN VCSELs.

Quantitative spectroscopy of substitutional nitrogen in GaAs1−xNx epitaxial layers by local vibrational mode absorption

Fourier transform infrared absorption measurements have been performed on thin films of GaAs1−xNx grown by metalorganic chemical vapour deposition or molecular beam epitaxy on semi-insulating GaAs substrates. The local vibrational mode absorption due to NAs is used to assess the substitutional nitrogen fraction. Based on a comparison with secondary ion mass spectroscopy and x-ray diffraction analysis, the calibration factor for the integrated absorption is derived. Quantitative determination of substitutional nitrogen is possible up to x ≈ 0.05, in epitaxial layers of thicknesses down to 10–100 nm.

Hybrid 1285 nm GaInNAs VCSELs with 1.2 mW singlemode output power at 85 °C

We present a new hybrid design for GaInNAs-based vertical-cavity surface emitting lasers (VCSELs) at 1285 nm emission wavelength. The VCSELs show record singlemode optical output powers of 1.2 mW from room temperature to 85 °C with series resistance of 120 . The remarkable overall performance is confirmed by a modulation capability at 6 Gbps demonstrated up to 85 °C.

Carrier recombination processes in MOVPE and MBE grown 1.3 μm GaInNAs edge emitting lasers

By measuring the spontaneous emission (SE) from metal organic vapour phase epitaxy (MOVPE) grown ∼1.3 μm GaInNAs/GaAs-based lasers during normal operation, we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 to 370 K and compared these with results previously obtained for molecular beam epitaxy (MBE) grown GaInNAs lasers. From the SE measurements we determine how the current, I, close to threshold, varies as a function of carrier density, n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination respectively. We find that at room temperature (RT), defect-related recombination contributes ∼360 A/cm2 (MBE) and ∼565 A/cm2 (MOVPE) to the total current density at threshold. Radiative recombination accounts for ∼110 A/cm2 (MBE) and 195 A/cm2 (MOVPE) of Jth with the remaining ∼180 A/cm2 (MBE) and 760 A/cm2 (MOVPE) are due to non-radiative Auger recombination. Our results suggest that a larger threshold carrier density in the MOVPE grown device in comparison to the MBE lasers, can reasonably explain the larger current densities of the different recombination processes at RT. We tentatively associate this with higher optical loss processes in the MOVPE grown material.

Development of GaInNAs-based 1.3-μm VCSEL

In this paper the realization, development and production of 1.3μm vertical cavity surface emitting lasers (VCSEL) with datacom suitable performance are presented. These low cost laser diodes are well suited for optical interconnect applications for LAN and MAN with transmission distances up to 15 km. The possibilities as well as the advantages and limits of shifting the wavelength from commercially available VCSEL emitting at 850nm to 1300nm are discussed. 1300nm VCSELs in a low cost SMD plastic package assembled into an intelligent SFP-module developed by Infineon Technologies are demonstrated.

GaInNAs quantum well VCSELs

Summary form only given. We have succeeded to produce high performance VCSELs emitting at 1300nm with bitrates up to 10 Gbit/s and output-powers above 1 mW with both approaches. Our long-wavelength devices comprise intracavity contacts in order to reduce absorption losses due to doped layers. We present different devices and discuss the impact of design and growth methods.

Characterization of 1.3-um wavelength GaInNAs/GaAs edge-emitting and vertical-cavity surface-emitting lasers using low temperature and high pressure

By measuring the spontaneous emission from normally operating ~1.3um GaInNAs/GaAs-based lasers grown by MBE and by MOVPE we have quantitatively determined the variation of monomolecular (defect-related ~An), radiative (~Bn2) and Auger recombination (~Cn3) as a function of temperature from 130K to 370K. We find that A, B and C are remarkably independent of the growth method. Theoretical calculations of the threshold carrier density as a function of temperature were also performed using a 10 band k·p Hamiltonian from which we could determine the temperature variation of A, B and C. At 300K, A=11x10-8 sec-1, B=8x10-11 cm3 sec-1 and C= 6x10-29 cm6 sec-1. These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating defect-related currents and reducing optical losses, the threshold current density of these GaInNAs/GaAs-based edge-emitting devices would be more than halved at room temperature. The results from studies of temperature and pressure variation of ~1.3um VCSELs produced by similar MBE growth could also be explained using the same recombination coefficients. They showed a broad gain spectrum and were able to operate over a wide temperature range.

VCSEL technologies and applications

VCSEL devices for 850nm and 1300nm emission wavelength are presented, suitable for operation in single-channel interconnects as well as parallel optical links. Necessary properties for applications such as 10 Gigabit Ethernet and actual limits for the use of VCSELs are discussed in some detail. Recent progress is demonstrated in developing devices with production-friendly diameters larger than 5µm for 10Gbit/s operation. Also devices with a temperature insensitive monolithically integrated monitordiode are presented and discussed. In order to reach the emission wavelength of 1300nm with a GaAs-based monolithic VCSEL-structure, we use GaInNxAs1-x quantum-wells with a small nitrogen concentration x between one and two percent. We have two different growth approaches, such as solid source MBE with a rf-plasma source to produce reactive nitrogen from nitrogen gas N2 and MOCVD with unsymmetrical di-methylhydrazine as a precursor for nitrogen. The long-wavelength devices comprise intracavity contacts in order to reduce absorption losses due to doped layers. Bitrates up to 10Gbit/s per channel can be achieved within both wavelength regimes.