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FAQs: Performance of Superluminescent Light Emitting Diodes (SLEDs)
PERFORMANCES OF EXALOS superluminescent light emitting diodes (SLEDs)
Are superluminescent light emitting diodes (SLEDs) sensitive to optical feedback?
What is spectral ripple?
Why is the ripple high in the 1400nm range?
Can EXALOS superluminescent light emitting diodes (SLEDs) be modulated? What is the maximum modulation frequency?
Is an SLED temperature sensitive?
What is the coherence length of an SLED?
What is the degree of polarization of an SLED?
Does the Monitor Photo Diode (MPD) current depend on the reverse bias voltage?
Is the monitor photo diode (MPD) current sensitive to temperature?
Is the Monitor Photo Diode (MPD) current proportional to the SLED output power?
What is the lifetime of EXALOS products?
PERFORMANCES OF EXALOS superluminescent light emitting diodes (SLEDs)
Are superluminescent light emitting diodes (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 -35dB 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 ask for our Application Notes for more details.
What is spectral ripple?
Spectral ripple refers to the modulation that can be seen on top of the spectral density curve. The primary causes of the spectral ripple are the residual reflections from the SLED chip facets and from the fiber facet facing the SLED chip. Typically, spectral ripple is measured by using a high-resolution Optical Spectrum Analyzer (OSA) with 0.1nm resolution. Spectral ripple is more pronounced in high-power SLED devices and is highest around the peak wavelength.
Thanks to their advanced design, EXALOS's SLEDs exhibit an extremely low level of ripple. Even at high power levels, the spectral ripple is typically 0.1dB.
Low values of spectral ripple are important in order to obtain a clean coherence function, with minimum secondary coherence peaks.
Why is the ripple high in the 1400nm range?
When measured with a standard OSA, the optical spectrum of superluminescent light emitting diodes (SLEDs) in the region of 1400nm is characterized by many deep and narrow dips. However, these dips are not due to residual reflections, they are caused by absorption lines from OH molecules that are located in the optical path. The main contribution to these dips originates from absorptions suffered by the optical beam when propagating from the exit fiber to the detector array within an optical spectrum analyzer.
Can EXALOS superluminescent light emitting diodes (SLEDs) be modulated? What is the maximum modulation frequency?
Yes, EXALOS's 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 about 200MHz and is limited by package parasitics, rather than by the SLED itself. Please ask for our Application Notes for more details.
When the SLED is used on the EXALOS driver board, EBD2000-0000, the modulation bandwidth is limited to 10kHz due to the employed electrical circuit that is designed to minimize noise.
Are superluminescent light emitting diodes (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 temperature.
Standard low-cost EXALOS SLEDs packaged in a TO-CAN or TOSA devices do not have a TEC included to stabilize SLED temperature. As a rough guide, the center wavelength changes with SLED at a temperature rate of approximately 0.3nm/°C (0.7nm/°C) for short (long) wavelength devices.
What is the coherence length of an SLED?
The coherence length is a quantitative measure of the path difference between two arms of an interferometer over which the light wave is still able to generate some interference pattern.
The coherence length of a light source is inversely proportional to the width of its emission spectrum. Due to its large bandwidth, an SLED has a much shorter coherence length than a laser. Typically, values for the coherence length of SLED may vary between 5mm and 40mm.
Please contact EXALOS and ask for the Application Notes for more detail on the coherence length and properties of our SLEDs.
What is the degree of polarization of an SLED?
The Degree Of Polarization (DOP) of a light source describes what portion of the total emitted light power Ptotal = Ppol + Punpol is polarized and is defined as
Natural sunlight and light from a LED is completely unpolarized (DOP=0%) whereas light from a DFB laser is completely polarized light (DOP=100%). The light emitted by an SLED is a combination of unpolarized spontaneous emission and amplified spontaneous emission, generally resulting in a DOP considerably less than 100%.
Does the Monitor Photo Diode (MPD) current depend on the reverse bias voltage?
Yes, the relation between the monitor photo diode (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.
Is the monitor photo diode (MPD) current sensitive to temperature?
Yes, the relation between the MPD current and the emitted optical power depends on temperature. If a cooled SLED is used, the MPD's temperature dependence should not be an issue as the MPD is kept at a constant temperature. However, for an uncooled device, the MPD current will depend on the operating temperature.
Is the Monitor Photo Diode (MPD) current proportional to the SLED output power?
The monitor photo diode (MPD) current increases with increasing SLED power. The exact relationship is not always linear and one should refer to the corresponding datasheet where the measured dependence is presented. As noted above, the MPD current may have a temperature dependence for uncooled SLEDs.
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 very high reliability, fully comply with the Telcordia GR-468 standard and meet the MIL-STD-883 requirements. Test carried out included checks for mechanical integrity, endurance, ESD and hermeticity.
