Novel Waveguide Increases Superluminescent Output
Blue superluminescent diodes deliver 200 mW of power
A partnershipbetween Polish and Japanese researchers claims to have set a new benchmark for the combination of output power and smoothness of spectral output for GaN superluminescent diodes.
This team’s emitter features a ‘J-shaped’ waveguide and could be used in fibre-optic gyroscopes and optical coherence tomography. Both applications require a light source that combines a high degree of spatial coherence with low time coherence. These conditions are met with the superluminescent diodes. They were developed in the arsenide material system in the 1970s, but the shorter wavelengths emitted by GaN-based materials are beneficial.
Superluminescent diodes emit a spectra with a peak at about 407 nm and an output power of 200 mW
“In the case of fibre-optic gyroscopes, the advantage of nitride superluminescent diodes is the possibility of using plastic fibres, which may increase the robustness of the entire system," explains Anna Kafar from the Institute of High Pressure Physics in Warsaw, Poland. For optical coherence tomography, a technique that is often used to generate three-dimensional images of biological samples, the emission from the GaN-based source increases spatial resolution. “However, due to absorption, this imaging method will be better for imaging non-biological, more transparent samples," says Kafar.
The team’s J-shaped device combines a straight waveguide that contains the rear facet with a curved waveguide that has the output facet.“This geometry gives the benefits of a double-pass device − such as a long amplification path − while the chip length remains short, which is important from a packaging point of view," remarks Kafar.
She says that the collaboration, which includes researchers from TopGaN and Kyoto University, is not the first to make a GaN-based J-shaped superluminescent diode − but they have taken its performance to a new level by optimising the architecture of these chips. This propelled the output power to 200 mW, twice that of the previous record held by researchers at Osram Opto Semiconductors, and it also enabled a smooth emission profile from this diode.
According to Kafar, the fabrication of the epitaxial structure is analogous to that of a laser. One of the biggest challenges is to optimise the bend angle, which varies with wavelength and governs the quality of the emitted spectrum.
Superluminescent diodes produce a spectral output that narrows with increasing drive current
Construction of J-shaped superluminescent diodes begins with the MOCVD growth of an epistructure on bulk GaN. This epitaxial stack comprises: an 800 nm-thick, silicon-doped Al0.08Ga0.92N bottom cladding layer; a 140 nm-thick GaN waveguide; an active region with three In0.1Ga0.9N quantum wells separated by In0.02Ga0.98N barriers; an AlGaN electron-blocking layer that is 28 nm-thick; a 150 nm-thick waveguide; and a 430 nm-thick, magnesium-doped Al0.05Ga0.95N top cladding.
Superluminescent diodes were formed with chip lengths of 700 µm, 1000 µm and 1500 µm. Angles between the waveguide axis and the axis perpendicular to the chip facet varied from 5.5° to 7.5°. Mounting these chips on a two-side copper heatsink ensured effective heat spreading of the diodes.
A 1 mm-long device with a 7.5° bend angle produced the highest optical power. Output increased exponentially with drive current up to about 300 mA, and from then on increased in a linear fashion.
Measurements of emission spectra revealed that the device is not lasing, but its emission spectra is not entirely smooth. There are modulations in the emission spectra that have a 0.025 nm period, and their depth increases as the current is cranked up. This increase in modulation depth is attributed to a rise in oscillating light in the waveguide. Another consequence of increasing the drive current is a reduction in full-width half maximum from 7 nm to 2.5 nm.
The Poland-Japan collaboration found that operating temperature strongly influences the quality and stability of the light emitted by the diode. Increasing the current in a device mounted in a standard TO56 package produced a red-shift in emission, due to an increase in the chip’s temperature. However, if the diode is mounted in a two-side copper heatsink, cranking up the current leads to a blue-shift in emission, due to compensation of the built-in electric fields.
“We plan to optimise the thermal properties of our diodes, so that we can report 200 mW or more of optical power from a TO-56 can," says Kafar.
A. Kafar et. al.
Appl. Phys. Express 6092102 (2013)