superluminescent diodes (original) (raw)

Acronym: SLD or SLED

Definition: broadband semiconductor light sources based on superluminescence

Alternative terms: superluminescence diodes, superluminescent LEDs

Categories: photonic devices, optoelectronics, non-laser light sources

• light sources
  • superluminescent sources
    * superluminescent diodes

Related: superluminescent sourcessuperluminescencewhite light sourceswhite light interferometersoptical coherence tomography

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📦 For purchasing superluminescent diodes, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What are Superluminescent Diodes?

Superluminescent diodes (also sometimes called superluminescence diodes or superluminescent light-emitting diodes = superluminescent LEDs) are optoelectronic semiconductor devices which emit broadband optical radiation based on superluminescence.

In terms of their construction, they are similar to laser diodes, containing an electrically driven p–n junction and an optical waveguide. Importantly, however, SLDs are made to avoid any optical feedback by reflections, so that no laser action can occur. Parasitic optical feedback from the facets, which could lead to the formation of resonator modes and thus to pronounced structures in the optical spectrum and/or to spectral narrowing, possibly also to parasitic lasing, is suppressed by means of tilting the facets relative to the waveguide, and can be suppressed further with anti-reflection coatings. Essentially, an SLD is a semiconductor optical amplifier with no input signal, where weak spontaneous emission into the waveguide mode is followed by strong laser amplification (→ amplified spontaneous emission).

A frequently used acronym for the superluminescent diode is SLD. The alternative acronym SLED is also used for surface-emitting LED, i.e., with a totally different meaning, so that confusion can arise.

Wavelength, Power, and Optical Bandwidth

Most superluminescent diodes emit in one of the wavelength regions around 800 nm, 1300 nm, and 1550 nm. However, devices for other wavelengths are available, also in the visible domain.

Typical output powers are in the range from a few milliwatts to some tens of milliwatts, and spatially the emission is close to diffraction-limited, i.e., the spatial coherence and beam quality are very high. Therefore, the broadband output can be easily launched into a single-mode fiber. Fiber-coupled SLDs are in fact most common.

The optical bandwidth of an SLD is usually some tens of nanometers, sometimes even above 100 nm. This corresponds to a coherence length of a few tens of microns, sometimes even only a few microns. Due to gain narrowing, there is a trade-off between high output power and broad bandwidth, which can however be improved with various methods. That trade-off, in addition to the lack of optical feedback, is the main reason why SLDs deliver lower optical powers than laser diodes.

Another factor, which is important for some applications, is wavelength stability, particularly under conditions of variable temperatures and aging. Typically, the center wavelength drifts by some fraction of a nanometer per Kelvin temperature change, following the drift of the semiconductor's gain spectrum.

Various Technical Issues

SLDs should be carefully protected against external optical feedback. Even small levels of feedback can reduce the overall emission bandwidth, or possibly lead to parasitic lasing, causing narrow spikes in the emission spectrum. Some devices may even be damaged by optical feedback. Note that the Fresnel reflection from a perpendicularly cleaved fiber end is already well above the level of feedback which can be tolerated.

To a similar extent as laser diodes, SLDs are sensitive to electrostatic discharges and current spikes e.g. from ill-designed driver electronics. However, when treated carefully and operated well within the specifications, SLDs can easily last for tens of thousands of hours of operation.

Applications

SLDs are applied in situations where a smooth and broadband optical spectrum (i.e. low temporal coherence), combined with high spatial coherence and relatively high intensity, is required. Some examples are:

• Optical coherence tomography (OCT), e.g. for cornea and retina diagnostics, for cardiovascular imaging, for other biomedical purposes or for biology research, requires a broad bandwidth for a high spatial resolution of the images, and a sufficiently high optical power for fast image acquisition with good signal-to-noise ratio.
• The chromatic dispersion of fibers and other optical components is often measured with techniques which require some broadband light source (see also: white light sources). An example is white light interferometry.
• Broadband light can be used for testing of optoelectronic components, e.g. in terms of transmission or reflection spectra, amplification factors, dispersion, and the like. This is used e.g. for diagnosing problems in fiber-optic links by measuring chromatic dispersion or polarization mode dispersion. For such purposes, devices emitting around 1.3 μm or 1.5 μm are usually applied.
• Some types of fiber-optic sensors, e.g. for measuring temperature, strain or pressure in buildings, oil pipelines or oil fields, are interrogated with broadband light sources. High output powers can be beneficial, as they allow the interrogation of multiple sensors over long distances of optical fiber.
• Fiber-optic gyroscopes, as used e.g. for navigation of large airplanes, benefit from a broadband light source. The Sagnac phase shift associated with a rotation can in principle also be measured with a ring laser. However, a broadband source allows one to implement a principle of operation which does not suffer from a phase locking phenomenon at low rotation rates, despite some scattering in the single-mode fiber. Further, the setup is fairly simple and robust and does not contain particularly expensive parts. However, wavelength stability of the source is an issue which has to be carefully observed.

Possible Alternatives

For higher output powers, an SLD may be replaced by an unseeded fiber amplifier. However, fiber-based sources are substantially more expensive.

In cases where very low optical powers are sufficient, a simple incandescent lamp may be used. However, the brightness of a bulb is orders of magnitude smaller than that of an SLD, so the difference in, e.g., signal-to-noise ratio or speed of some measurement can be huge.

In principle, an SLD is a semiconductor optical amplifier (SOA) with no input signal, but it is optimized for a good combination of output power and bandwidth, and therefore better suited for broadband light generation than an all-purpose SOA.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 28 suppliers for superluminescent diodes. Among them:

⚙ hardware

Innolume’s superluminescent diodes (SLDs) deliver high optical power of >250 mW. They offer excellent coupling efficiency into single-mode fibers thanks to their high spatial coherence.

Designed for versatility and seamless system integration, these SLDs are available in a wide range of packaging configurations, including 14-pin butterfly modules with PM or HI fiber, loose tube, and integrated photodiode, as well as chip-level options such as submount, C-mount, and TO-can.

We support customization of peak wavelength (830–1330 nm) and spectral width (up to 110 nm), enabling tailored performance for specialized applications across medical imaging, optical sensing, and interferometry.

⚙ hardware

Serving North America, RPMC offers a selection of broadband Superluminescent Diodes (SLEDs). With broad-spectrum options (≈770-1700 nm), bandwidths up to 460 nm, and output power up to 130 mW (free-space) and up to 50 mW (fiber-coupled), these devices provide high brightness, low coherence & low noise, and speckle-free performance. (types/superluminescent-diodes/).

These diverse, configurable, and fully customizable SLED broadband sources are available in OEM or turnkey versions, with a single SLED or multiple SLEDs in the same device providing ultimate broadband and high-power output. Turnkey versions are provided with advanced features and options like an intuitive GUI, dual-stage isolator, TEC, monitor diode & more.

Our broad-spectrum Superluminescent Diodes are perfect for medical/industrial OCT, optical component testing, telecom test equipment, industrial/biomedical imaging systems, optical sensing, test & measurement, and R&D.

Let RPMC help you find the right laser today!

⚙ hardware

SHIPS TODAY: SLED diodes at 1310 or 1550 nm are offered as stock items or associated with a CW laser diode driver or pulsed laser diode driver. They are compatible with our high speed nanosecond pulsed drivers . The single-mode laser SLD diodes can reach high powers in the nanosecond pulse regime. Most turn-key diode & driver solutions are optimized for single-shot to CW performances with pulse width lengths down to 1 ns. The laser diode precision pulses are generated internally by an on-board pulse generator, or on demand from an external TTL signal.

Bibliography

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