Superluminescent Diodes – Preferred Choice for Imaging Applications

If one is associated with the light source or laser industry, they are likely familiar with superluminescent diode light sources and their diverse applications. These diodes are often the first choice for imaging applications due to their unique characteristics. However, even those new to the industry can quickly grasp the significance of superluminescent diodes.

This article explores what superluminescent diodes are, their salient characteristics, and why they are the preferred choice for imaging applications. Before delving into their relationship with imaging applications, it’s essential to understand what a superluminescent diode is and how it differs from or relates to laser diodes (LDs) and light-emitting diodes (LEDs).

What is a Superluminescent Diode?

A Superluminescent Diode (SLD) or Superluminescent Light Emitting Diode (SLED) is an edge-emitting semiconductor light source that operates on the principle of superluminescence. In simple terms, SLDs are broadbandsemiconductor light sources based on superluminescence. They combine the properties of a laser diode and an LED, offering a broad spectrum of light emission with high brightness, making them suitable for applications requiring both high power and low coherence.

How are SLDs Different from LDs and LEDs?

Although superluminescent diodes, LDs, and LEDs belong to the same ‘family,’ there are considerable differences in their workings and characteristics:

• Laser Diodes (LDs): In LDs, a laser cavity is formed with an optical waveguide and reflective facets, essential for directing the light. The narrowband output results from amplified stimulated emission, where light waves reinforce each other through multiple passes within the cavity, creating a highly coherent and focused beam.
• Light Emitting Diodes (LEDs): LEDs have zero amplified gain, and light emission occurs through spontaneous emission only, where electrons recombine with holes and release energy in the form of photons. This process results in a broad spectrum of light but lacks the coherence and directionality seen in laser diodes.
• Superluminescent Diodes (SLDs): In SLDs, the waveguide is used to capitalize on the process of amplified spontaneous emission in a single pass, unlike the multiple round trips required in laser diodes. This design allows SLDs to generate a broad spectrum of light with high output power while maintaining low coherence, making them suitable for applications like Optical Coherence Tomography (OCT), where broad bandwidth and low speckle are essential.

What Makes Superluminescent Diodes a Preferred Choice for Imaging Applications?

Here are the key characteristics that make superluminescent diodes (SLDs) a preferred choice for imaging applications:

1. Short Temporal Coherence: SLDs have a short coherence length, which reduces speckle noise in imaging, allowing for clearer images in applications like Optical Coherence Tomography (OCT).
2. Efficient Coupling into Single-Mode Fibers: The design of SLDs allows for efficient coupling of light into single-mode fibers, ensuring minimal loss of intensity and maintaining the integrity of the signal over long distances.
3. Divergence-Limited Light Emittance: SLDs emit light that is well-collimated, meaning it diverges less compared to other sources. This characteristic enhances image quality by focusing light more effectively on the target area.
4. Broadband Optical Spectrum: SLDs generate light across a wide range of wavelengths, crucial for imaging applications that require high resolution and depth information, allowing for detailed structural analysis of biological tissues.
5. Presence of the Optical Waveguide: The incorporation of an optical waveguide enables efficient light propagation within the diode, enhancing overall performance by maximizing light output while minimizing losses.
6. Linear Polarization State: SLDs often produce linearly polarized light, which can be beneficial in certain imaging techniques that rely on polarization for improved contrast and clarity in the captured images.
7. Amplified Spontaneous Emission: This mechanism allows SLDs to achieve high output power without the coherence of laser light, providing the advantages of high brightness and a wide spectral range, essential for accurate and detailed imaging in various applications.

The short temporal coherence helps reduce speckle and interference noise. One such application is optical coherence tomography (OCT), which demands the least speckle and interference noise to generate real-time, cross-sectional images of high resolution. This is critical in medical imaging, particularly for visualizing retinal structures, where clarity and precision are paramount for accurate diagnosis and treatment planning. By minimizing noise, OCT can provide clearer images, allowing clinicians to better assess conditions such as glaucoma or macular degeneration.

SLDs are also very useful in making fiber optic gyroscopes. Gyroscopes are often used in Navigation Systems, Satellite Systems, and Aerospace to measure the degree of rotation. These devices rely on the principles of angular momentum and the Coriolis Effect to determine changes in orientation. A gyroscope is also a kind of imaging technique, as it can provide real-time data on the movement and positioning of objects, essential in applications like inertial navigation and stabilization in various technologies, including drones and autonomous vehicles.

The short temporal coherence of the SLDs also makes them an ideal choice for measuring load and vibration—such as in bridges. This characteristic allows for precise detection of minute changes in structural integrity over time. It is also used in measuring the temperature of various structures, including suspension bridges and skyscrapers. By monitoring temperature variations, engineers can assess thermal expansion and contraction, crucial for maintaining the safety and longevity of these structures.

Conclusion

The amplified spontaneous emission and short temporal coherence are the two critical properties that make superluminescent diodes a preferred choice for imaging applications. The amplified spontaneous emission provides a broad optical spectrum, enhancing image quality and resolution, while the short temporal coherence minimizes speckle and interference noise. Together, these attributes enable SLDs to deliver high-performance results in various fields, including medical imaging and structural monitoring.

Inphenix is a prominent laser and light source manufacturer headquartered in California, USA. Along with superluminescent diodes, the company offers a wide range of lasers and light sources, including semiconductor optical amplifiers, distributed feedback lasers (DFBs), swept-source lasers, broadband light sources, Fabry-Perot (FP) lasers, and vertical-cavity surface-emitting lasers (VCSELs), among others. These products are utilized in various applications, such as telecommunications, medical imaging, and sensing technologies. Visit the company’s official website to learn more about its services and products.