26 Oct 2007
Superluminescent diodes combine the advantages of LEDs and laser diodes in a single compact package. Christian Vélez and Chris Armistead detail the factors to consider when purchasing SLEDs and the applications that are already putting the source to good use.
Superluminescent light-emitting diodes (SLEDs) are semiconductor-based light sources that combine the broadband optical spectra characteristics of LEDs with the diffraction-limited and spatially coherent emission of an edge-emitting laser diode. This combination is extremely useful for applications that require a good beam quality without unwanted interference effects such as speckle, which are related to the high temporal coherence of narrowband sources.
An SLED is based on a p-n junction that is embedded into an optical waveguide. When electrically forward-biased, the device shows optical gain and generates amplified spontaneous emission (ASE). This ASE or superluminescence is light that has been optically amplified by the process of stimulated emission in a gain medium.
SLEDs are designed with high single-pass amplification for the spontaneous emission generated along the waveguide but, unlike laser diodes, insufficient feedback to achieve lasing action. The cavity modes are suppressed by tilting the waveguide with respect to the end facets and also by applying an antireflection coating to each facet (see figure 1). Suppressing the cavity modes results in a smooth emission spectrum with low ripple. For SLED manufacturers, the main technological challenge is to achieve high optical output powers combined with a smooth spectrum.
Because an SLED emits light from a waveguide, the spatial coherence of the light is similar to that seen from an edge-emitting laser. This spatial coherence means that typically more than 50% of the chip facet power can be coupled into a singlemode fibre – a similar efficiency to an edge-emitting laser diode.
SLEDs are particularly advantageous for applications requiring high spectral power density with very good beam quality. The large optical bandwidth (equivalent to low temporal coherence) also benefits applications where interference causes problems such as speckle or ghost signals.
Today's state-of-the-art SLEDs have singlemode output powers of several tens of milliwatts and 3 dB optical bandwidths of typically 60 nm. The material composition of the light-generating active region determines the SLED's emission wavelength. Devices emitting in the range of 650–1000 nm are based on gallium arsenide, while devices emitting between 1000 and 1700 nm are based on indium phosphide.
Choosing an SLED
Most of the applications that rely on SLEDs typically use devices emitting around the centre wavelengths of 800, 1300 and 1550 nm. It is important to note that many other wavelengths are available that give users substantial flexibility (see figure 3). SLEDs operating in the 800 nm range are used in fibre-optic gyroscopes and medical applications, while at 1300 and 1550 nm the main markets are datacoms, telecoms and sensors.
As well as the centre wavelength, the other important parameters to keep in mind when specifying an SLED are its power, 3 dB bandwidth, the ripple and package type. In terms of output power, it is important to clarify if it is ex-facet or from a singlemode or multimode fibre.
It is also worth remembering that there is a significant trade-off between high output power and large bandwidth. This can be overcome by producing an SLED with a flat-top-shaped spectrum instead of a Gaussian. However, this can be detrimental for applications requiring very short coherence lengths.
When specifying ripple (the suppression of the cavity modes) you should define not only the maximum acceptable value but also the spectral resolution that is used for the measurement.
It should be noted that SLEDs are very temperature sensitive and this is crucial when choosing the package type. Because an SLED essentially behaves like a laser diode below threshold, it must be actively temperature controlled. Pigtailed devices are normally delivered with a built-in temperature sensor and thermoelectric cooler. If not cooled properly, the SLED's output power will drop and its wavelength will increase with temperature. If your design includes uncooled SLED modules, such as TO-CAN, then this must be accounted for in the optical system design. Some typical packaged devices are shown in figure 2.
Optical coherence tomography (OCT) is an emerging technology for producing high-resolution cross-sectional medical images of tissue on the micron scale in real time. OCT is the direct optical analogue of ultrasound but as light is unable to penetrate beyond 2 mm in most non-transparent tissues, OCT is limited to optically transparent tissues and ophthalmic or endoscopic examinations.
An SLED's large optical bandwidth together with its good suppression of the cavity modes allows short coherence lengths from some tens of micrometres down to just a few micrometres. Its high output powers also permit the fast scan times required in ophthalmologic, cardiovascular and gastrointestinal imaging applications.
EXALOS is leading the development of new SLEDs with increased spectral bandwidth, Gaussian shape and high optical output power. Such characteristics will lead to higher image resolutions and allow tissue characterization on a scale never before possible within the human body. This technology has the potential to dramatically change the way that scientists see and understand the human body in order to diagnose and treat diseases more effectively.
Another key SLED application is in navigation systems, primarily those in avionics and aerospace that use fibre-optic gyroscopes (FOGs) to make precise rotation measurements. FOGs measure the Sagnac phase shift of optical radiation propaga-ting along a fibre-optic coil when it rotates around the winding axis. When a FOG is mounted within a navigation system, it tracks changes in orientation.
The basic components of a FOG (shown in figure 4) are a light source, a singlemode polarization-maintaining fibre coil, a coupler and a detector. Light from the source is injected into the fibre in counter-propagating directions using the optical coupler. When the fibre coil is at rest, the two light waves interfere constructively at the detector and a maximum signal is produced. When the coil rotates, the two light waves take different optical paths that depend on the rotation rate. The phase difference between the two waves varies the intensity at the detector and provides information on the rotation rate.
SLED-based gyroscopes rely on the large bandwidth of the source to reduce both the scattering along the fibre and reflections at the facets of the internal optical components, which could decrease the sensitivity at very low rotation rates.
A FOG and its associated embedded optical source must be low cost, stable, reliable, compact and have low power consumption. In the past, wavelength stability over time was a major drawback of semiconductor-based broadband light sources like SLEDs compared with fibre-based light sources such as erbium-doped fibre-amplifiers.
At EXALOS, we have successfully overcome this drawback by developing SLEDs that are less sensitive to wavelength shifts induced by temperature or aging. Novel materials and material structures have made it possible to improve the wavelength stability of our devices by a factor of four compared with conventional devices. Further work is in progress with the target of achieving wavelength stabilities down to the 10 parts per million range. SLEDs with this performance will help to reduce the size and cost of FOGs substantially.
Broadband optical sources are also used to test fibre-optic components. One example is characterizing optical components used in coarse wavelength division multiplexing (CWDM), which requires multiple wavelength sources. Our broad range of SLEDs means that several sources with wavelengths spanning the whole CWDM band can be integrated in a single piece of test equipment. In optical networks, SLEDs are used to measure the polarization mode dispersion of optical fibres.
Many features inherent to SLEDs make them the preferred sources for these applications. First, SLEDs are fully compatible with the wavelengths used in optical communication systems. Second, the SLED's optical waveguide allows light to be coupled efficiently into singlemode optical fibres. Finally, we believe that our SLEDs are effective solutions to the pressures of today's tele-communications carrier market, which requires products that are cost effective, reliable, enable better productivity and reduce the number of testers in the field.
The final main SLED application is fibre-optic sensors for strain and temperature measurements in civil engineering, structural analysis and composite material manufacturing. Fibre-optic sensors have a number of advantages over conventional sensors. For example, they are immune to electromagnetic fields, they have the ability to measure at many points along a single fibre and they can be embedded within, or bonded to, structures making them a highly flexible solution. Over the past 25 years there has been continual improvement in the quality and performance of fibre-optic sensors. EXALOS SLEDs are used for these applications because of their large optical bandwidth and robustness.
• This article originally appeared in the October 2007 issue of Optics & Laser Europe magazine.