Red Vertical-cavity Surface-emitting Lasers Research Articles

Spatial coherence properties of pulsed red VCSELs

In this letter, we discuss the near-field, far-field, and spatial coherence properties of pulsed broad-area red vertical-cavity surface-emitting lasers (VCSELs) emitting at 680 nm. We will show experimentally that the spatial coherence of the tested devices is strongly reduced when using a pulsed pump current. Under these conditions, the VCSEL emits as a partially spatially coherent, quasi-homogeneous source with a Gaussian-shaped spatial coherence function and with the coherence area being substantially smaller than the laser’s aperture area.

Red VCSEL for high-temperature applications

Measured power–current curves of 660 nm AlGaInP-based oxide-confined vertical-cavity surface-emitting lasers (VCSEL) are compared with calculated data by a cylindrical heat dissipation model to improve heat removal out of the device. Pulsed lasing operation of a 670 nm VCSEL at high temperatures is demonstrated. At +120 °C heat sink temperature we exceeded 0.5 mW and at +160 °C still 25 μ W output power were achieved.

Vertical Cavity Light Emitters for Plastic Optical Fibers

Plastic optical fibres, which have a local attenuation minimum at 650 nm, have attracted much interest for low-cost short-haul communication systems. Red vertical-cavity surface-emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) provide a potential solution as light sources for these systems. The operation principle of vertical cavity emitters is based on a Fabry-Perot microcavity, which is formed by placing an optically active region inside of two parallel mirrors. The RCLEDs with emission windows of 84 μm in diameter, suitable for standard POFs, exhibit bandwidths up to 200 MHz and light power of 3 mW (cw). The maximum external quantum efficiency is 9.5%. Larger devices, ø500 μm in size, launch light power up to 15 mW. Record high transmission rates, 622 Mbit/s, with bit-error-rate <1 × 10-11 have been demonstrated for a 1-m-long step-index POF. Neither sudden unexpected failure, nor gradual power degradation has been observed after operation under accelerated ageing conditions for about 100 000 device-hours. The alignment tolerance of the coupling efficiency is found to be very large, ±0.5 mm in all directions x-y-z, suggesting that the fibre pigtail packaging is inexpensive. First MBE-grown visible AlGaInP vertical-cavity surface-emitting lasers have been demonstrated. A laser with a 10-μm emitting window has external quantum efficiency of 6.65% under continuous wave operation and it is still lasing at 45°C. Furthermore, a threshold current less than 1.0 mA is obtained for a device, which has an 8-μm emitting window.

Self-heating effects in red (665 nm) VCSELs

The threshold current (Ith) of GaInP/AlGaInP based vertical cavity surface emitting lasers (VCSELs) is very sensitive to temperature. The extent to which self-heating in a compact VCSEL structure couples with temperature sensitive loss mechanisms is investigated. In particular, the authors consider the strong increase in threshold current (Ith) due to carrier leakage into the X-minima and gain-cavity detuning effects. They find, using two experimental methods and from a thermal model, that the current-induced increase of the active region temperature (ΔT) increases linearly with Ith at a rate of ∼ 14.5 K mA−1 over a limited substrate temperature range. They estimate that the additional losses due to self-heating rise from 5% of the total Ith at room temperature (20°C) to 30% at 47°C. To uncouple the effects of leakage and gain-cavity alignment, they have calculated the gain spectra to find the radiative current as a function of temperature. They thus verify that leakage coupled to self-heating is the major cause of the increase in Ith in these devices at room temperature and above. Gain-cavity detuning has minimal effect on Ith due to the broad short wavelength tail of the quantum well gain spectrum.

Red vertical-cavity surface-emitting lasers grown by solid-source molecular beam epitaxy

Plastic optical fibres, which have a local attenuation minimum at 650 nm, have attracted much interest for low-cost short-haul communication systems. Red vertical-cavity surface-emitting lasers (VCSELs) provide a potential solution as light sources for these systems. The operation of vertical cavity emitters is based on a Fabry–Perot microcavity, which is formed by placing an optically active region inside of two parallel mirrors. These mirrors are usually formed epitaxially. So far, metal organic chemical vapour deposition (MOCVD) has been the major technology used for growing visible VCSELs. Recently, an alternative growth method—solid-source molecular beam epitaxy (SSMBE)—has been introduced to be a viable solution to the fabrication of these structures. The authors present the first MBE-grown visible AlGaInP vertical-cavity surface-emitting lasers. A laser with a 10 μm emitting window has an external quantum efficiency of 6.65% under continuous wave operation and it is still lasing at 45°C. Furthermore, a threshold current less than 1.0 mA is obtained for a device, which has an 8 μm emitting window.

Low threshold current densities in red VCSELs

In this work, we have investigated the design and metalorganic vapor-phase epitaxial growth of red vertical cavity surface-emitting lasers (VCSEL, emission wavelength around 650–680 nm) with respect to low threshold current densities and a reduced temperature dependence. A VCSEL in the red regime implies a combination of AlGaInP (cavity) and AlGaAs/AlAs (distributed Bragg reflectors). In order to check the influence of the epitaxially critical P/As-interface on the laser performance, we have grown edge-emitting lasers where we varied the distance between this interface and the active quantum wells. We observed no increase of the threshold currents even for small distances as necessary in VCSEL structures when applying an optimized switching sequence at these interfaces. Consequently, red VCSELs could be grown which showed continuous wave operation at room temperature (670 nm, 0.15 mW at 20°C). In order to improve the temperature sensitivity of these VCSELs, we have studied a large number of devices with well-defined misalignment between gain and Bragg mirror reflectivity. For lasers where this misalignment resulted in somewhat worse room temperature properties (e.g. threshold current densities around 800 A/cm 2), we found a clearly enhanced temperature stability.

Room-temperature CW operation of red vertical-cavity surface-emitting lasers grown by solid-source molecular beam epitaxy

The authors present the first report of MBE-grown AlGaInP vertical-cavity surface-emitting lasers. The lasers exhibit continuous wave operation at 690 nm with a maximum output power of 0.56 mW. With a 10 µm optical aperture the threshold current is only 1.3 mA, and CW lasing is achieved up to 45°C.

Improved AlGaInP-based red (670–690 nm) surface-emitting lasers with novel C-doped short-cavity epitaxial design

A modified epitaxial design leads to straightforward implementation of short (1λ) optical cavities and the use of C as the sole p-type dopant in AlGaInP/AlGaAs red vertical-cavity surface-emitting lasers (VCSELs). Red VCSELs fabricated into simple etched air posts operate continuous wave at room temperature at wavelengths between 670 and 690 nm, with a peak output power as high as 2.4 mW at 690 nm, threshold voltage of 2.2 V, and peak wallplug efficiency of 9%. These values are all significant improvements over previous results achieved in the same geometry with an extended optical cavity epitaxial design. The improved performance is due primarily to reduced optical losses and improved current constriction and dopant stability.