FAQ

General Questions

How can I select the right EXALOS SLED product for my application?

The most common criteria for selecting the right SLED for a given application are output power, central wavelength and spectral bandwidth. Our standard products are listed on our EXALOS's website http://www.exalos.com, where typical performance data and product datasheets can be downloaded.
EXALOS offers a wide variety of standard SLEDs with wavelengths ranging from 400 to 1750 nm with different powers and bandwidths. The SLEDs are available in standard 14-pin dual-in-line (DIL) or Butterfly packages as well as low-cost TOSAs (pigtailed) and TO-56 cans (window devices) packages.
In addition to our standard products, EXALOS can also provide devices with custom performance and custom housing/packaging solutions. Please contact our Sales team, sales@exalos.com , to discuss your specific requirement. Our Sales and Applications team will work with you to determine the product best suited for your particular application.

Where can I find the general specifications and typical performance curves for EXALOS’ standard products?

The general specifications and typical performance curves can be accessed from the summary table under the sub-menu of "PRODUCTS" Menu or from the links listed below.
SLED Modules
SLED Driver Boards
Broadband Light Sources
Transceivers
Swept Sources
OCT Engines
Balanced Receivers
Please contact the EXALOS Sales team, sales@exalos.com , with any questions.

At what wavelengths does EXALOS offer SLEDs?

EXALOS' portfolio of SLEDs covers wavelengths ranging from 400 to 1750 nm.

Are SLEDs available in TO-can case?

Yes, we offer several SLED types as pigtailed TOSA devices or mounted in TO‑56 cans as standard. Please refer to our website http://www.exalos.com for a complete list. Exalos can also perform custom packaging in different TO cans.

Are devices available with Polarization Maintaining Fiber (PMF)?

Yes. Please contact EXALOS Sales, sales@exalos.com , for more information.

Are EXALOS products supplied with an optical connector? Which type?

All standard SLEDs are supplied with a fiber length of approximately 1 meter and are connectorized with an angle-polished FC/APC connector. Other connector types are available on request.

Are EXALOS SLEDs supplied with a monitor photodiode?

Yes, all EXALOS SLEDs feature a silicon or InGaAs photodiode inside the optical module to monitor the output power, unless otherwise specified.

Which fiber is used as standard fiber?

The default optical fiber is a single-mode fiber, for example SMF-28 for the long-wavelength range (1250-1750 nm), HI-780 for the short-wavelength range (700-950 nm), 1060-XP for wavelength range (980-1230 nm) , S405-XP for the visible range (400-680 nm), or other single-mode fibers for other wavelengths. Please contact Sales team (sales@exalos.com) for more information.

Where can I buy EXALOS products?

Please contact the EXALOS Sales team, sales@exalos.com, for more information on our products or a quotation. A list of international sales offices can be found on our website  http://www.exalos.com.

What are the key advantages of an SLED?

SLEDs provide a broadband optical power spectrum (low temporal coherence or small coherence length) at high power levels coupled into single-mode fiber. The small coherence length is particularly useful in all applications where unwanted interference effects may occur. An example of this type is the reduction of speckle effects in imaging applications when replacing the laser with an SLED.
In addition, SLEDs have a small form factor, are rugged and can be manufactured cost efficiently, while still offering superb performance.  These unique properties enable SLEDs to be used in a number of different applications. Due to their improved spectral performance, higher power levels and extended range of wavelengths, SLEDs are seeing a growth in the number of applications.  These new applications range from scientific research to commercial applications. The applications for SLEDs will continue to grow as designers become more aware of the performance and cost improvements in today's SLEDs. 
What are the advantages of SLEDs in FOG
The low temporal coherence is advantageous in applications where unwanted interference effects within the system can be a problem. An example is the Fiber Optic Gyroscope (FOG). Here, undesired interference effects from backreflections at the interface between internal fiber optic components and scattering effects along the fiber loop can be significantly reduced when replacing the laser source with an SLED.
What are the advantages of SLEDs in OCT
SLEDs also provide advantages in interferometric applications, where the short coherence length of the SLED enables the localization of a reflective point with high accuracy. The SLEDs short coherence length property enables high-resolution imaging systems such as Optical Coherence Tomography (OCT) equipment to be built.  Several companies are now offering OCT instrumentsquipment for both medial and industrial applications.
What are the advantages of SLEDs in fiber based sensors
Further, SLEDs are being employed in fiber optic sensing systems based on Fiber Bragg Gratings (FBG). In the simplest case, an SLED is illuminating an array of FBGs that have been written along a single optical fiber. Each FBG is characterized by its own Bragg wavelength and can be identified in the system by spectrally resolving the return signal using an Optical Spectrum Analyzer. Strain and/or temperature changes at the location of individual FBGs are measured as shifts in the Bragg wavelength of the corresponding FBG. The wide bandwidth of SLEDs allows a large number of FBGs to be addressed simultaneously.  In addition, optical component manufacturers are increasingly using SLEDs for the characterization of WDM components as they offer the best choice between measurement speed, accuracy and cost.

What are the main differences between a laser diode and an SLED?

Superluminescent light emitting diodes (SLEDs) are unique semiconductor light emitters in the sense that they combine the spatial coherence properties of laser diodes with the temporal incoherence of an LED. They could be considered as a broadband and incoherent laser diode or as LED with high optical intensities.
The high spatial coherence translates into a small divergence angle of the light beam emitted by the SLED chip. This allows for high coupling efficiencies into single-mode or polarization-maintaining fibers. Typically, 50% of the optical power emitted from the SLED chip is coupled into a single-mode fiber.
The temporal incoherence (short coherence time) is equivalent to a short coherence length. This means that when light from an SLED is split into two beams and one of the beams is delayed with respect to the other by more than the coherence length, the two beams will not interfere with each other any longer when recombined. In the spectral domain, this property manifests itself as a broadband spectrum, i.e. light is emitted over a wide range of wavelengths.
In contrast, the optical spectrum of a semiconductor laser diode consists of a few longitudinal modes within a narrow wavelength span, resulting in long coherence lengths and, for example, unwanted  coherent interference (speckle) noise.
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Operating EXALOS SLEDs and Controllers

Does EXALOS provide a laser diode controller or a TEC driver?

EXALOS is offering a variety of compact but high-performance driver boards that feature a low-noise, zero-drift and stable SLED current driver and a high-efficiency TEC driver, including for example theu00a0 EBD5000 for R&D testing that can be operated with various adapter boards for the individual types of optical modules, or the credit card-sized OEM driver board EBD5200 for 14-pin butterfly modules. More information can be found on our websiteu00a0http://www.exalos.com.

Are EXALOS products thermally stablized?

All SLED modules in a 14-pin butterfly or DIL form factor feature an integrated thermo-electric cooler (TEC) and a standard 10-kOhm NTC temperature sensor such that the SLED chip can be stabilized to a set temperature withu00a0 an accuracy of 0.01u00b0C. Lower-cost SLED modules offered as TOSAs (fiber-pigtailed) or TO cans (free space) typically do not have an integrated TEC and require external heat sinking and temperature stabilization. Heat sinking is also required for butterfly and DIL modules.
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What is the maximum drive current ?

The maximum forward current depends on the exact type of SLED and can be found in the corresponding datasheet. Any values in excess of the stated maximum ratings should be avoided.
Some devices, especially uncooled SLEDs, may also have a maximum specified output power as the maximum drive current at lower chip temperatures may result in excessive optical output power values that can damage the output facet of the device. The maximum drive current for short-wavelength GaAs devices is typically in the range of 120-250 mA while many long-wavelength InP devices can be operated up to 500-600 mA.

How can I use the Monitor Photo Diode (MPD)?

The MPD current is typically proportional to the optical output power of the SLED. Therefore, the MPD current signal can be used to realize an automatic power control (APC) where the output power of the SLED is kept constant over a long operating time or over a wide temperature range. Typically, the MPD current is in the range of 0.1-1.5 mA. EXALOS recommends to use a low-impedance termination (TIA gain) of max. 100 Ohm for the MPD current. If larger electrical control signals are required, a low-noise voltage amplifier shall be used.

How should I control the SLED temperature?

All standard EXALOS products packaged in DIL or Butterfly package contain a Thermo Electric Cooler (TEC) inside the package and a negative temperature coefficient (NTC) thermistor is placed in close proximity to the SLED chip. Employing a suitable external heat sink and control electronics allows for the stabilization of the chip temperature against environmental variations. Unless otherwise specified, the default chip temperature for short-wavelength SLEDs (400-1100 nm)u00a0 is 25 u00b0C while it is 20 u00b0C for long-wavelength SLEDs (1250-1750 nm).u00a0
Please download our Applications Notes for more details.

What is the operating temperature range?

Theu00a0operating temperatureu00a0range over which anu00a0SLED moduleu00a0can be operated is given in the specification for that SLED. It is typically between -20u00b0C and 65u00b0C.
Theu00a0chip temperatureu00a0should be set to 25u00b0C for the short-wavelength SLEDs (650 to 1100nm) and to 20u00b0C for the long-wavelength SLEDs (1250 to 1700nm). The SLEDs can be operated at slightly modified temperatures, but care should be taken not to exceed the maximum current ratings and temperature given in the specification and for the uncooled devices theu00a0 maximum output power ratings, especially at lower chip temperatures. This is especially true when operating the devices at lower temperatures, where higher power levels can be achieved.

Where is the TEC located and how is the heat dissipated?

The SLED chip is mounted on a submount that is fixed on top of Thethermo-Electric Cooler (TEC). Depending on the type of package, the generated heat is transferred either directly to the bottom of the package (Butterfly) or to the back face of the package (DIL).

Why is the Butterfly package better for high power devices than DIL?

High-power devices are generally operated at higher drive currents and therefore, generate more heat. In a Butterfly package the generated heat is transferred directly to the bottom of the package where it can be dissipated efficiently. In contrast, in a DIL package the heat path is less efficient as the heat is transferred to the back face.

 

Performance of Superluminescent Light Emitting Diodes (SLEDs)

Are SLEDs sensitive to optical feedback?

Yes, in general SLEDs are sensitive to optical back-reflections. Due to EXALOS innovative design our SLEDs can still be operated under optical feedback and do not get damaged. However, optical feedback of lower than -30dB is generally recommended in order to fully guarantee the specifications. Higher values of optical feedback can induce modifications to the spectral density distribution and reduce output power. The use of an optical isolator with isolation > 30dB over the operational wavelength band is recommended. Without an optical isolator, the use of angled fiber connectors is necessary for optimal device performance. It is worth noting that the degree of sensitivity to back-reflection also depends on the state of polarisation of the returned light. Please download our Applications Notes for more details.

Are SLEDs temperature sensitive?

Yes, both optical output power and spectral properties depend on the SLED chip temperature. All standard EXALOS products packaged in a DIL or a Butterfly package contain a Thermo-Electric Cooler (TEC) inside the package to thermally stabilize the SLED. Standard low-cost EXALOS SLEDs packaged in a TO-CAN or TOSA devices do not have a TEC included. As a rough guide, the center wavelength changes with temperature at a rate of approximately 0.3nm/°C (0.7nm/°C) for short (long) wavelength devices.

Can EXALOS SLEDs be modulated? What is the maximum modulation frequency?

Yes, EXALOS' SLEDs can be directly modulated via the bias current. The devices can be modulated by using the available modulation input of commercially available laser drivers. The maximum bandwidth for packaged devices is 200-500 MHz, depending on the package, and is limited by parasitics of the optical package. Higher modulation frequencies will require pre-emphasis to compensate for the limited RF bandwidth of the optical package. Please note that the maximum operating conditions specified by EXALOS are for cw operation and may not apply for pulsed or modulation operation Please download our Applications Notes for more details.
When the SLED is used on the EXALOS driver boards, the modulation frequency is limited to 50 Hz as these driver boards have been optimized for lowest-noise performance. Higher modulation frequencies may be realized upon request.

Does the Monitor Photo Diode (MPD) current depend on the reverse bias voltage?

Yes, the relation between the MPD current and the emitted optical power depends on the reverse bias voltage applied to the MPD. As a general rule, the MPD current increases with increasing reverse bias. Unless specified otherwise in the datasheet of the particular SLED, EXALOS recommends a reverse voltage of -1 V to -5 V  to ensure that the photodetector is not driven into saturation and is providing a rather linear relationship between optical output power and MPD current.

What is spectral ripple?

Spectral ripple refers to a small spectral modulation of the ASE spectrum of an SLED. The primary cause for the spectral ripple is residual reflections at the SLED chip interfaces or at the fiber tip that is facing the SLED chip. Spectral ripple can be measured by using a high-resolution optical spectrum analyzer (OSA) with a resolution of 0.1 nm or better. Spectral ripple is more pronounced in high-power SLED devices and typically has the highest amplitude at the peak of the ASE spectrum. Spectral ripple are generating secondary coherence peaks in the autocorrelation or coherence function that can have unwanted effects for the application that the SLEDs are used at. For example in spectral-domain OCT, spectral ripple and related secondary coherence peaks can generate horizontal lines in the OCT image.
EXALOS SLEDs exhibit a low amount of spectral ripple due to an advanced chip design. Even at high power levels, the spectral ripple is typically less than 0.1 dB, corresponding to a secondary peak suppression of around 35 dB.

What is the coherence length of an SLED?

Coherence lengths of light sources are typically measured with an interferometer featuring an adjustable length for at least one of the interferometer arms, thereby introducing an optical path length difference (OPD) of the two interfering light waves at the output. The coherence length of any light source, including SLEDs, is defined as the OPD value at which the detected fringe amplitude of the interference pattern drops from its maximum (at zero OPD) to 50%. This is equivalent to the half-width at half maximum (HWHM) of the coherence function or autocorrelation, which is the Fourier transform (FFT) of the optical ASE spectrum (Wiener-Khinchin theorem). The coherence function is often plotted on a 10-log vertical scale, which means that the coherence length is found at the 3-dB drop from the maximum.
For OCT applications it is common to describe the imaging performance of the system by plotting the point spread function (PSF) of the electrical power signal as a function of OPD (or imaging depth). Here, the coherence length is equivalent to the 6-dB drop of the PSF amplitude (on a 20-log vertical scale) from the maximum.
The coherence length is proportional to the square of the center wavelength of the SLED and inversely proportional to the width of the optical ASE spectrum. The coherence length also depends on the shape of the ASE spectrum.  For example, a 840-nm SLED having a flat-top ASE spectrum with a FWHM of 50 nm has a coherence length of 7.4 microns (in air), while a 650-nm SLED with a Gaussian ASE spectrum and a FWHM of 10 nm has a coherence length of 18.6 microns. At twice the center wavelength (1300 nm), a 4-times broader spectrum (40 nm FWHM) is required to achieve the same coherence length.

What is the degree of polarization of an SLED?

The degree of polarization (DOP) of a light source describes the percentage of polarized light relative to the total emitted light. For example, black body radiation (e.g., sun light) or light from an LED is unpolarized  and therefore has a DOP of zero. On the other hand, light from a laser is typically highly polarized and therefore has a DOP close to one.
The DOP value can be converted into a maximum polarization extinction ratio (PER), which can be measured when the polarized portion of the light is aligned linearly along one polarization axis. In this case, the PER is maximum and is only defined by the DOP. For example, linearly polarized light with a PER of 20.0 dB corresponds to a DOP of 98%. Or, a DOP of 90% corresponds to a maximum PER of 12.8 dB. But, a light source with a DOP of 90% can also emit light that is circularly polarized and hence results in a lower PER measurement.
SLEDs are typically polarized in TE and have a PER between 10 and 20 dB. Higher-power SLEDs show higher PER or DOP values, lower-power SLEDs are usually less polarized

What is the lifetime of EXALOS products?

EXALOS SLEDs are extremely reliable and have a typical Mean Time To Failure (MTTF) of 100k hours at 25C, based on an end-of-life criteria of 50% power drop.
All our SLEDs offer excellent reliability and fully comply with the Telcordia GR-468 or other MIL standards.
Typical tests that are carried out include verification of mechanical integrity and bond strength, temperature cycling, robustness against electro-static discharge (ESD) and hermiticity of sealed modules.

Why is the ripple high in the 1400nm range?

When measuring broadband SLEDs in the wavelength range of 1340 nm to 1490 nm, the ASE spectrum may show high amount of spectral ripple or distinct narrow and deep dips in the optical power spectral density. However, these dips are typically not residual reflections from the SLED module but are absorption lines of OH molecules that are present in the optical beam path, for example the water vapor in the air when working with free-space beam optics. Even fiber-coupled SLED modules may show such absorption lines when the spectrum is measured with an optical spectrum analyzer (OSA) that is operated in standard air. Reducing the humidity level of the ambient air or operating the OSA in dry air or nitrogen will remove those absorption lines.


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