FAQ about SLD

Which type of optical fiber is used in SLDs, single-mode fiber (SMF), multi-mode fiber (MMF), or polarization maintaining fiber (PMF)? What are the characteristics of these fibers?

SMF is used as standard, but PMF is also available. As an example, the short-wave SLD modules use a fiber with a cut-off wavelength of 730 nm and a mode field diameter of 5.0 µm at 850 nm.

Can a Peltier element (TEC) be built into a cylindrical module?

It is difficult to build in due to the TEC size. Consider using a butterfly module if temperature control is required.

Does the module incorporate an optical isolator?

All long-wave SLD modules have built-in optical isolators, however 0.8-µm band short-wave SLDs cannot be incorporated due to the size and insertion loss of the applicable optical isolators.

Is the pattern of the SLD light emitted into space elliptical similar like an LD?
Is there any astigmatism?

Short-wave SLDs emit in an elliptical shape, while long-wave SLDs emit in a nearly circular elliptical shape.

Astigmatism, which is a gap between the horizontal and vertical emission points, is more noticeable in devices with a wide active layer device. Our products are designed on the premise of optical fiber coupling, and we believe that astigmatism is either non-existent or almost negligible.

Can the unit products be applied to all wavelength bands?

Unit product lines are currently for short-wave SLDs. If you have any requests for long-wave SLDs, contact us.

Is the output light of the SLD emitted from the front side only?

The output light actually propagates in both directions and is emitted from the rear facet as well.

What is the luminescence difference between LDs, SLDs, and LEDs?

LDs, SLDs, and LEDs have light outputs of hundreds mW, tens mW, and several mW, respectively, each with a difference of around 10 times. Since the light-emitting areas of LDs and SLDs are generally the same, the luminance difference is about the same as the output difference.The luminance of LEDs is much lower due to their larger emitting area.

What precautions should be taken when combining multiple SLDs?

Careful consideration is required to balance the level of each SLD output spectrum, control the length of the fiber including the combining coupler, and suppress reflections from each optical component. We have experience of combining two 0.8-µm SLDs into one unit with a driver; contact us for more information.

Why does the spectral shape of the 0.8-µm SLD look different for products with a half-width of 14 nm and 50 nm?

SLDs with a spectral half-width of 14 nm are used for general sensing applications, while 50-nm products are designed for OCT (Optical Coherence Tomography) with a wider bandwidth by using two different emission layers with different center wavelengths to create a bimodal spectral shape, which is expected to enable high-resolution measurements.

Is the lack of a 1.4-µm product lineup due to no applications or a technical problem?

The 1.4-µm band is not widely used in optical communications due to the high transmission loss of optical fibers, and there are no applications for it. Manufacturing is possible without technical problems.

What is the difference between spatial coherence and temporal coherence, and which coherence is higher in SLDs?

High spatial coherence means that the light does not spread over a long distance and can be focused to a small area, while high temporal coherence means that light generated at different times can interfere with each other. Compared to LEDs, SLDs have higher spatial coherence and can be coupled to optical fibers with higher efficiency. On the other hand, the temporal coherence is low, allowing for high resolution at interferometer measurement.

If the LED emitting area is reduced, will spatial coherence be improved and fiber coupling will be possible?

Only light emitted from a small area of the light source can be coupled to an optical fiber, and the coupling efficiency of LEDs with a large emitting area and low directivity is not good. Although the coupling efficiency improves when the emitting area is reduced, the optical fiber output does not change much. If the luminance, which is the intensity per area, is increased, the optical fiber output will be improved.

What is the correlation between estimated lifetime and operating temperature?

The relation between operating temperature and lifetime can be calculated based on the Arrhenius formula.

where, ML: median life, k:Boltzmann's constant, A: constant, T: absolute temperature

The activation energy Ea is 0.4 eV in compliance with Telordia GR468-CORE.

Contact us for details.

How does it behave as it degrades? What precautions should I take to prevent degradation?

The degradation modes of SLDs include COD degradation in which the emitting facet is suddenly destroyed, crystal defects in which the crystal in the active region is degraded, and damage to the rear facet caused by external reflected light. To prevent such degradation, be careful not to apply surge current, suppress the return light, and do not exceed the absolute maximum ratings. In particular, 0.8-µm products are delicate and must be handled with extreme care.

Are there any examples of applications for data communications?

SLDs have a lower modulation frequency than LDs and are affected by wavelength dispersion during transmission due to their wide wavelength range. Therefore, the advantages of using SLDs in communications are not high. However, the installation of SLDs on a ceiling has been examined as a secure and eye-safe spatial light source for short-range communications. SLDs are free of interference noise, and stable communications quality has been confirmed for this purpose.

When configuring an OCT system, which is more appropriate to use as a light source, SLD or LED?
What are the advantages and disadvantages of each.

LEDs have a lower fiber output power than SLDs, resulting in lower signal-to-noise ratio in fiber interferometers, as well as lower spatial coherence, resulting in lower resolution in spatial interferometers. Although LEDs have the advantage of low-cost configuration, SLDs clearly have superior performance.

What is the difference between a resolution of a few µm for OCT (Optical Coherence Tomography) and a few nm for AFM (Atomic Force Microscopy)?

The measurement principle is completely different between the two. For AFM, the position of a probe with a tip radius of several nm is detected by an SLD and a sensor. Meanwhile, OCT is a type of interferometry in which the resolution is determined by the wavelength of the light source and the spectral linewidth; the accuracy for SLD light sources is a few micrometers.

What wavelengths are mainly used for optical fiber sensors? What wavelengths can be supported?

The 0.8-µm band is used for fiber optic sensor applications requiring high resolution, while the 1.3 and 1.55-µm communication bands are often used for cost-sensitive applications using low-cost available optical components. Technically, we can support 0.8 to 2 µm. If you have a specific application, contact us.