Superluminescent diode
Encyclopedia
A superluminescent diode (SLED or SLD) is an edge-emitting semiconductor
light source based on superluminescence. It combines the high power and brightness of laser diode
s with the low coherence
of conventional light-emitting diode
s. Its emission band is 5–100 nm wide.
at RCA Laboratories (now Sarnoff Corporation
), invented the superluminescent diode. This light source was developed as a key component in the next generations of fibre optic gyroscope
s, low coherence tomography
for medical imaging
, and external cavity tunable lasers with applications to fiber-optic communication
s. In 1989 the technology was transferred to GE-RCA in Canada
, which became a division of EG&G
.
Superluminescent light emitting diodes are also called sometimes superluminescent diodes, superluminescence diodes or superluminescent LEDs
.
over a wide range of wavelength
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
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. Electrical current
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
s) electrical carriers and then amplified when travelling along the waveguide of a SLED.
The pn-junction of the semiconductor
material of a SLED is designed in such a way that electrons and holes feature a multitude of possible states (energy bands
) with different energies. Therefore, the recombination of electron and holes generates light with a broad range of optical frequencies
, 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
, ν the optical frequency, Π the size of the optical mode
, Rsp the spontaneous emission rate
into the guided mode, g the modal gain
, α the non-resonant optical losses, L the length of the active channel and c the velocity of light
.
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.
emitted by an SLED depends on the injected current (bias). 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 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.
(BW) and the peak wavelength,
peak. The first is defined as the full width at half maximum
(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.
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
).
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 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
(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 bias (i.e. injected current level), 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.
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
(RIN), that is the ratio between the power spectral density of the noise current, In, measured over a given bandwidth, and the square value of the average photocurrent, I0
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.
of SLEDs can be easily achieved through direct modulation of the bias current. SLED modules do not include terminating resistor
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.
of the source (which is the limited capability of the emitted light wave to maintain the phase over time). Some applications take advantage of the low coherence of SLEDs sources to achieve high spatial resolution in imaging techniques. The coherence length, Lc, is a quantity frequently used to characterize the coherence of the light source. It is related to the path difference between the two arms of an optical interferometer over which the light wave is still capable to generate an interference pattern. For sources having a Gaussian spectral distribution, the value of Lc 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 Lc 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 [(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.
It is worth noting that the presence of an excessive spectral ripple (see section spectral ripple) in the power spectral density results in the presence of side lobes (see Fig. ) 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.
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
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.
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 (InP) based superluminescent light-emitting diodes operating in the high wavelength range (1100 nm to 1700 nm) as well as gallium arsenide (GaAs) based devices operating from 630 to 1100 nm. Usage of gallium nitride (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), or Superlum (Russia). The product portfolio offered varies greatly from supplier to supplier by wavelength, power, and bandwidth.
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
light source based on superluminescence. It combines the high power and brightness of laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
s with the low coherence
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
of conventional light-emitting diode
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s. Its emission band is 5–100 nm wide.
History
In 1986 Dr. Gerard A. AlphonseGerard A. Alphonse
Gerald A. Alphonse is an electrical engineer, physicist and research scientist, and was the 2005 president of the United States division of the Institute of Electrical and Electronic Engineers . He has served on several of IEEE's committees and boards....
at RCA Laboratories (now Sarnoff Corporation
Sarnoff Corporation
Sarnoff Corporation, with headquarters in West Windsor Township, New Jersey, was a research and development company specializing in vision, video and semiconductor technology....
), invented the superluminescent diode. This light source was developed as a key component in the next generations of fibre optic gyroscope
Fibre optic gyroscope
A fibre optic gyroscope senses changes in orientation, thus performing the function of a mechanical gyroscope. However its principle of operation is instead based on the interference of light which has passed through a coil of optical fibre which can be as long as 5 km.Two beams from a laser...
s, low coherence tomography
Optical coherence tomography
Optical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media . Optical coherence tomography is an interferometric technique, typically employing near-infrared light...
for medical imaging
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
, and external cavity tunable lasers with applications to fiber-optic communication
Fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information...
s. In 1989 the technology was transferred to GE-RCA in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, which became a division of EG&G
EG&G
EG&G, formally known as Edgerton, Germeshausen, and Grier, Inc., is 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 during World War II, and conducted weapons research and...
.
Superluminescent light emitting diodes are also called sometimes superluminescent diodes, superluminescence diodes or superluminescent LEDs
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
.
Principles of operation
A superluminescent light emitting diode is, similar to a laser diode, based on an electrically driven pn-junction that, when biased in forward direction, becomes optically active and generates amplified spontaneous emissionAmplified spontaneous emission
Amplified spontaneous emission 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....
over a wide range of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
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
P-type semiconductor
A P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....
) section and a negative (n-doped
N-type semiconductor
N-type semiconductors are a type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material . This creates an excess of negative electron charge carriers....
) section. Electrical current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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)
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
and negative (electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) electrical carriers and then amplified when travelling along the waveguide of a SLED.
The pn-junction of the semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
material of a SLED is designed in such a way that electrons and holes feature a multitude of possible states (energy bands
Electronic band structure
In solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...
) with different energies. Therefore, the recombination of electron and holes generates light with a broad range of optical frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, 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
Planck constant
The Planck constant , also called Planck's constant, is a physical constant reflecting the sizes of energy quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory, who discovered it in 1899...
, ν the optical frequency, Π the size of the optical mode
Transverse mode
A transverse mode of a beam of electromagnetic radiation is a particular electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam...
, Rsp the spontaneous emission rate
Spontaneous emission
Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...
into the guided mode, g the modal gain
Gain (lasers)
Gain in laser physics is a process, where the medium transfers part of its energy to the emitted electromagnetic radiation, resulting in an increase in laser power...
, α the non-resonant optical losses, L the length of the active channel and c the velocity of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
.
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.
Power dependence of current
The total optical powerOptical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...
emitted by an SLED depends on the injected current (bias). 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
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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 bandwidthSpectral linewidth
The spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of an acoustic or electronic system...
(BW) and the peak wavelength,
peak. The first is defined as the full width at half maximumFull width at half maximum
Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....
(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 analyzerSpectrum 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...
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
Coherence length
In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. The significance is that interference will be strong within a coherence length of the source, but not beyond it...
).
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 fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
(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
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
(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 bias (i.e. injected current level), 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-low detector the intensity noise will be converted into current fluctuations and the measured photocurrent will include a constant term, I0, proportional to the mean optical intensity and a time dependent term, In, 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
Relative intensity noise , describes the instability in the power level of a laser. The noise term is important to describe lasers used in fiber-optic communication and LIDAR remote sensing....
(RIN), that is the ratio between the power spectral density of the noise current, In, measured over a given bandwidth, and the square value of the average photocurrent, I0
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.
| SLED center wavelength | 100 mA | 150 mA | 200 mA | 300 mA | 400 mA | 500 mA |
|---|---|---|---|---|---|---|
| 1550 nm | −121.5 | −123.5 | ||||
| 1550 nm | −124.5 | −127.5 | −128.0 | −129.5 | −130.0 | |
| 1300 nm | −123.5 | −125.0 | −126.5 | −127.0 | −127.5 | |
| 1300 nm | −124.0 | −124.5 | ||||
| 1600 nm | −123.0 | −123.0 |
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 modulationIntensity modulation
In optical communications, intensity modulation 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....
of SLEDs can be easily achieved through direct modulation of the bias current. SLED modules do not include terminating resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
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 coherenceCoherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
of the source (which is the limited capability of the emitted light wave to maintain the phase over time). Some applications take advantage of the low coherence of SLEDs sources to achieve high spatial resolution in imaging techniques. The coherence length, Lc, is a quantity frequently used to characterize the coherence of the light source. It is related to the path difference between the two arms of an optical interferometer over which the light wave is still capable to generate an interference pattern. For sources having a Gaussian spectral distribution, the value of Lc 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 Lc 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 [(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.
It is worth noting that the presence of an excessive spectral ripple (see section spectral ripple) in the power spectral density results in the presence of side lobes (see Fig. ) 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 (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
Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...
s and current spikes
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
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 cavityOptical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
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 (InP) based superluminescent light-emitting diodes operating in the high wavelength range (1100 nm to 1700 nm) as well as gallium arsenide (GaAs) based devices operating from 630 to 1100 nm. Usage of gallium nitride (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), or Superlum (Russia). The product portfolio offered varies greatly from supplier to supplier by wavelength, power, and bandwidth.
External links
- Encyclopedia of Laser Physics and Technology entry
- Short overview of device operation principles and performance parameters (PDF).