A superluminescent diode (SLED or SLD) is an edge-emitting
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
light source based on
superluminescence Amplified spontaneous emission (ASE) or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers.
Origins
...
. It combines the high power and brightness of
laser diode
file:Laser diode chip.jpg, The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a di ...
s with the low
coherence of conventional
light-emitting diode
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
s. Its emission optical bandwidth, also described as full-width at half maximum, can range from 5 up to 750 nm.
History
The superluminescent diode was reported for the first time by Kurbatov et al. (1971)
and Lee, Burrus, and Miller (1973).
By 1986 Dr.
Gerard A. Alphonse Gerard A. Alphonse is a Haitian electrical engineer, physicist and research scientist, and was the 2005 president of the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or Americ ...
at RCA Laboratories (now
SRI International
SRI International (SRI) is an American nonprofit scientific research institute and organization headquartered in Menlo Park, California. The trustees of Stanford University established SRI in 1946 as a center of innovation to support economic d ...
), invented a novel design enabling high power superluminescent diodes.
This light source was developed as a key component in the next generations of
fibre optic gyroscope
A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed throug ...
s,
low coherence tomography for
medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
, and external cavity tunable
lasers
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
with applications to
fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...
s. In 1989 the technology was transferred to GE-RCA in
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, which became a division of
EG&G
EG&G, formally known as Edgerton, Germeshausen, and Grier, Inc., was a United States national defense contractor and provider of management and technical services. The company was involved in contracting services to the United States government ...
.
Superluminescent light emitting diodes are also called sometimes superluminescent diodes, superluminescence diodes or superluminescent
LEDs
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
.
Principles of operation
A superluminescent light emitting diode is, similar to a laser diode, based on an electrically driven
p-n junction that, when biased in forward direction, becomes optically active and generates
amplified spontaneous emission Amplified spontaneous emission (ASE) or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers.
Origins ...
over a wide range of
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s. The peak wavelength and the intensity of the SLED depend on the active material composition and on the injection current level. SLEDs are designed to have high single pass amplification for the spontaneous emission generated along the
waveguide
A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...
but, unlike laser diodes, insufficient feedback to achieve lasing action. This is obtained very successfully through the joint action of a tilted waveguide and anti-reflection coated (ARC) facets.
When an electrical forward voltage is applied, an injection current across the active region of the SLED is generated. Like most semiconductor devices, a SLED consists of a positive (
p-doped) section and a negative (
n-doped) section.
Electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
will flow from the p-section to the n-section and across the active region that is sandwiched in between the p- and n-section. During this process, light is generated through spontaneous and random recombination of positive
(holes) and negative (
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s) electrical carriers and then amplified when travelling along the waveguide of a SLED.
The pn-junction of the
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
material of a SLED is designed in such a way that electrons and holes feature a multitude of possible states (
energy bands
In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or ' ...
) with different energies. Therefore, the recombination of electron and holes generates light with a broad range of optical
frequencies
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, i.e. broadband light.
The output power performance of an ideal SLED can be described with a simple model, not taking spectral effects into account and considering both a uniform distribution of carrier densities and zero reflections from the facets.
Where h is the
Planck constant
The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivale ...
, ν the optical frequency, Π the size of the
optical mode, R
sp the
spontaneous emission rate into the guided mode, g the modal
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
, α the non-resonant optical losses, L the length of the active channel and c the
velocity of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special relativity, special theory of relativity, is ...
.
So the output power depends linearly on the spontaneous emission rate and exponentially on the optical gain. Obviously a high modal gain is required to obtain high optical output power.
Main characteristics
Dependence of power on current
The total
optical power
In optics, optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens (optics), lens, mirror, or other optical system converges or diverges light. It is equal to the Mu ...
emitted by an SLED depends on the drive current. Unlike laser diodes, the output intensity does not exhibit a sharp threshold but it gradually increases with current. A soft knee in the power vs. current curve defines a transition between a regime dominated by spontaneous emission (typical for surface emitting LEDs) and one that is dominated by amplified spontaneous emission (i.e. superluminescence). Even if the output power is based on spontaneous emission it has to be noted that the amplification mechanism affects the
polarization state of the emitted
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
in a way which is related to the SLED structure and on the operating conditions.
The maximum value of the current that allows a safe operation of the device depends on the model and ranges between 70 mA (for low power SLED) and 500 mA for the most powerful devices.
Centre wavelength and optical bandwidth
The optical power emitted by SLEDs is distributed over a wide spectral range. Two useful parameters that are related to the power density distribution at different wavelengths are the optical
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
(BW) and the peak wavelength,
peak. The first is defined as the
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve mea ...
(FWHM) of the power density vs. wavelength curve at the nominal operating conditions while the latter corresponds to the wavelength having the highest intensity. The
centre wavelength,
centre is defined as the central point between the two FWHM points of the spectral curve; it can be different from the peak wavelength since it is related to the spectrum asymmetry.
Typical values for SLED modules are for the BW between 5 nm and 100 nm with central wavelengths covering the range between 400 nm and 1700 nm. A trade off between maximum output power and bandwidth exists, however, the latter being larger for devices with lower output power.
Spectral ripple
The spectral ripple is the measure of the variation of the spectral power-density that can be observed for small change of the wavelength. It can be detected using high-resolution optical
spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
s and can be ascribed to the residual reflectivity of the chip facets and of the coupling fibre. Spectral ripple is more evident in high-power devices and mainly around the peak wavelength where the device gain is higher. It is always present to some extent but undesirable since it has strong effects on the coherence properties of SLED (see section
coherence length
In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ ...
).
Some SLEDs from certain manufacturers exhibit an extremely low value of the ripple even at the highest power levels. An excessive level of optical back-reflection can cause unexpected irregularities of the spectral distribution of SLEDs that have not to be confused with the ripple. During operation it is therefore important to carefully limit the feedback from any additional equipment.
Polarization
As described above, superluminescent light emitting diodes are based on the generation and on the amplification of spontaneous emission in a semiconductor waveguide. The structure and the material composition used for the SLED chip affect the gain that the radiation experience during the propagation and lead to different amplification factors for different orientations of the
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
(
polarization dependent gain). SLEDs operating in the wavelength range of 1300 and 1400 nm are mostly based on a bulk material and a chip structure both characterized by a low polarization dependence of the gain. On the contrary, devices operating in the 1550 and 1620 nm range make mostly use of a
quantum well
A quantum well is a potential well with only discrete energy values.
The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...
(QW) active region that has a strong polarization-dependent gain. The optical field emitted by the SLED chips, being a combination of unpolarized spontaneous emission and amplified radiation, has therefore a certain degree of polarization (DOP).
A useful quantity that describes the polarization characteristics of the SLED emission is the polarization extinction ratio (PER). This is the ratio between the maximum and the minimum intensities measured after a rotating linear polarizer.
The polarization extinction ratio of bulk chips is around 8–9 dB while it can be as high as 15–20 dB for QW chips. When SLED chips are coupled to pigtail fibres the pigtail bending and coiling will in general modify the polarization state at the fibre output. Modules provided with polarization maintaining (PM) fibre pigtails exhibit high values (>15 dB) of the polarization extinction ratio that are independent on the fibre bending. The polarization extinction ratio of the emission depends also on the drive current, having its highest value at the maximum driving current. On the contrary, the polarization state at the output of standard SM fibre pigtail is arbitrary but can be simply modified with a polarization controller and extinction ratios of about 10 dB can be easily achieved.
Relative intensity noise (RIN)
The optical power emitted by semiconductor active devices is always affected by fluctuations (intensity noise) that are induced by the spontaneous emission. When the emitted power is detected with a wide-bandwidth
square-law detector
In electronic signal processing, a square law detector is a device that produces an output proportional to the square of some input.''IEEE Std. 100 Authoritative Dictionary of Standards Terms Seventh Edition'', IEEE, 2000, ,''Square law detection' ...
the intensity noise will be converted into current fluctuations and the measured photocurrent will include a constant term, I
0, proportional to the mean optical intensity and a time dependent term, I
n, related to the intensity fluctuations.
The spectral distribution of the noise term in the photocurrent can be measured by means of an electrical spectrum analyzer over a radio frequency (RF) range that is limited by the electrical bandwidth of the detector used. The resultant noise spectrum is directly related to the optical intensity noise and in general depends on the RF frequency,
.
From this measurement a useful parameter that provides quantitative information on the noise of the optical source can be evaluated: it is the
relative intensity noise Relative intensity noise (RIN), describes the instability in the power (physics), power level of a laser. The noise term is important to describe lasers used in fiber-optic communication and LIDAR remote sensing.
Relative intensity noise can be gen ...
(RIN), that is the ratio between the power spectral density of the noise current, I
n, measured over a given bandwidth, and the square value of the average photocurrent, I
0
The RIN therefore represents the ratio between the noise power and the average power after detection; the measurement unit used is the dB/Hz. Typical values measured for SLEDs in a frequency range extending from DC up to 500 MHz are reported in the table.
They depend on the injection current (more correctly on the output power) and on the RF frequency range. The highest measured values never exceed −119 dB/Hz for frequencies higher than 5 GHz, while the lowest value (around 127 dB/Hz) is attained by the most powerful SLEDs in the 1310 nm window and in the frequency range limited to values less than 500 MHz. The frequency dependence of RIN is thought to be related to spatial correlation effects induced by the gain saturation.
It has to be noted that, while the use of narrow band optical filters in front of a detector will usually result in the reduction of the detected noise, the relative intensity noise of SLEDs can exhibit an increase. This behaviour, present mainly in high power SLEDs, is similar to what is observed with multimode Fabry-Perot laser diodes where filtering makes evident the presence of mode partition noise (mostly at low RF frequencies) due to competition among several lasing modes.
Modulation characteristics
Intensity modulation In optical communications, intensity modulation (IM) is a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal. The envelope of the modulated optical signal is an ...
of SLEDs can be easily achieved through direct modulation of the bias current. SLED modules do not include terminating
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s inside because, operating at relatively high currents, excessive cooling would be required to compensate for the heat dissipation of the resistor. In order to achieve the best performance some external network that reduces the impedance mismatch between the driver amplifier, that usually requires 50 Ohm loads, and the low impedance of the chip (a few Ohm) would be preferable. As shown in Fig. , response times of about 1 ns, extinction ratios of 27 dB and 3 dB bandwidths exceeding 200 MHz can be easily achieved.
Similar results can be obtained also for direct modulation of butterfly packaged SLEDs as shown in Fig. . Optically induced modulation allows to exploit the high speed modulation capabilities of the chip when they are not affected by package parasitics; as shown in Fig. , a 3 dB bandwidth exceeding 10 GHz also for packaged SLEDs can be achieved in this case.
Coherence length
SLEDs are optical sources with a rather wide optical bandwidth. In that they differ from both lasers, that have a very narrow spectrum, and white light sources, that exhibit a much larger spectral width. This characteristic mainly reflects itself in a low
temporal coherence of the source (which is the limited capability of the emitted light wave to maintain the phase over time). SLEDs may however exhibit a high degree of spatial coherence, meaning that they can be efficiently coupled into
single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholtz ...
s. Some applications take advantage of the low temporal coherence of SLEDs sources to achieve high
spatial resolution
In physics and geosciences, the term spatial resolution refers to distance between independent measurements, or the physical dimension that represents a pixel of the image. While in some instruments, like cameras and telescopes, spatial resolutio ...
in imaging techniques. The coherence length, L
c, is a quantity frequently used to characterize the temporal coherence of the light source. It is related to the path difference between the two arms of an optical
interferometer
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
over which the light wave is still capable to generate an interference pattern. For sources having a
Gaussian spectral distribution, the value of L
c is inversely proportional to the spectral width, BW, so that the full width at half maximum (FWHM) of the power spectral density can be related to L
c through the equation
,
where
is the central wavelength of the emitted radiation. As an example, an SLED operating around 1300 nm and with an optical bandwidth of 100 nm is expected to have a coherence length of about 17 μm.
From a practical point of view a definition independent on the spectral distribution (non-Gaussian spectrum) of the source is more suitable. If an optical interferometer is used for the coherence length evaluation (see Fig. 11 a and b) a useful quantity is the FWHM value of the visibility, that is the relative amplitude
peak - Ivalley) / (Ipeak + Ivalley)">Ipeak - Ivalley) / (Ipeak + Ivalley) of the intensity variations evaluated as a function of the interferometer imbalance.
SLEDs exhibit a large spectral width even at the highest power levels so that corresponding FWHM values of the visibility less than 20 μm are easily achieved.
The presence of an excessive spectral ripple (see section spectral ripple) in the power spectral density results in the presence of side lobes ) in the visibility curve that can limit both the spatial resolution and the sensitivity of SLED based measurement systems. SLEDs of certain manufacturers have very low side lobes and allow measurements with high dynamic ranges.
Technical challenges
On the one hand SLEDs are semiconductor devices that are optimized to generate a large amount of
amplified spontaneous emission Amplified spontaneous emission (ASE) or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers.
Origins ...
(ASE). In order to do that, they incorporate high-power gain sections in which seeding spontaneous emission is amplified with high gain factors of 30 dB or more.
On the other hand, SLEDs lack optical feedback, so that no laser action can occur. Optical feedback resulting from back-reflections of light from optical components such as e.g. connectors into the cavity is suppressed by means of tilting the facets relative to the waveguide, and can be suppressed further with anti-reflection coatings. The formation of resonator modes and thus pronounced structures in the optical spectrum and/or to spectral narrowing are avoided.
It is therefore natural that even small amounts of back-reflections are amplified inside the SLED chip in a similar manner, producing optical power levels of several tens of milliwatts at the back facet, which may destroy the SLED device. SLEDs should be carefully protected against external optical feedback. Even small levels of feedback can reduce the overall emission bandwidth and the output power, or sometimes even 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. If back reflections cannot be avoided, an optical isolator must be installed directly behind the SLED module. The isolator provides a low insertion loss from the SLED to the fiber and a high insertion loss in the back direction. However, SLEDs from certain component manufacturers are on the market featuring intrinsically safe designs with high robustness against optical back reflections.
To a similar extent as laser diodes, superluminescent light emitting diodes are sensitive to
electrostatic discharge
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an short circuit, electrical short or dielectric breakdown. A buildup of static electricity can be caused ...
s and
current spikes e.g. from ill-designed driver electronics. When selecting the current source to operate the SLED, special attention should be paid to low-noise specifications. Again certain suppliers are offering driver electronics especially designed to handle on the one hand the high-power, low-noise requirements and on the other hand protect the light sources against discharge and spikes. When treated carefully and operated well within the specifications, SLEDs can easily last for tens of thousands of hours of operation.
Availability of SLEDs
By means of the above-mentioned optimized
optical cavity An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors or other optical elements that forms a cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and provi ...
design the SLEDs exhibit high output power, large bandwidth and low residual spectral ripple, making them an ideal light source for a number of applications. Based on the application's requirements and specifications, SLED devices are available in various packages or form factors covering a broad range of wavelengths and power levels. The packages include cooled 14-pin dual-in-line (DIL) and butterfly (BTF) modules or low-cost uncooled TOSA and TO-56 devices. The SLED modules includes
indium phosphide
Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic ("zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors.
Manufacturing
Indium phosphide ca ...
(InP) based superluminescent light-emitting diodes operating in the high wavelength range (1100 nm to 1700 nm) as well as
gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
(GaAs) based devices operating from 630 to 1100 nm. Usage of
gallium nitride
Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it ...
(GaN) based designs is breaking ground for SLEDs in the ultraviolet and blue spectral range.
SLEDs are commercially available from a number of suppliers, e.g. Denselight (Singapore), EXALOS (Switzerland), InPhenix (US), Superlum (Ireland), or Thorlabs Quantum Electronics (US). The product portfolio offered varies greatly from supplier to supplier by wavelength, power, and bandwidth. Other examples include Zeiss Plex Elite 9000 SLD at 750 nm, and LD-PD inc SLDs at 1480 nm and 1530 nm.
Applications of SLEDs
SLEDs find application in situations demanding high intensity and
spatial coherence but where a need for a broad, smooth optical output spectrum makes
laser diodes
The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
unsuitable. Some examples include
optical coherence tomography
Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is used for medical ...
,
white light interferometry
As described here, white light interferometry is a non-contact optical method for surface height measurement on 3D structures with surface profiles varying between tens of nanometers and a few centimeters. It is often used as an alternative name f ...
,
optical sensing and
fibre optic gyroscope
A fibre-optic gyroscope (FOG) senses changes in orientation using the Sagnac effect, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed throug ...
s.
External links
Encyclopedia of Laser Physics and Technologyentry
Short overviewof device operation principles and performance parameters (PDF).
References
Optical diodes