Lasers vs. Light Emitting Diodes (LEDs) vs. Superluminescent Diodes (SLEDs)

Examining the differences between these three powerful light sources

Lasers, Light Emitting Diodes (LEDs) and Superluminescent Diodes (SLDs or SLEDs) are all highly useful sources of light, and they all have distinct characteristics and applications.

Lasers and LEDs are opposites in terms of the type of light they produce, while SLEDs sit somewhere in the middle of the two and can be applied to a wide range of applications.

Here we highlight the unique features of each light source and provide insights into when it's best to use one over another.

How Lasers, LEDs and SLEDs work

A laser emits coherent light that generates the light of a single wavelength while an LED’s non-coherent source of light generates a diverse beam of different wavelengths. SLEDs bridge the gap between lasers and LEDs, providing a necessary solution when a mix of excellent beam quality and low coherence is required.

Optical Power Density refers to optical power spectral densities, defined as the optical power per optical frequency (or wavelength) interval (e.g., specified in mW/THz or mW/nm). In simple terms, optical power density measures a light source’s luminescence, or how much light something emits. It’s important to note that a lasers’ optical power density is high, an LEDs’ is low, and a SLEDs’ optical power density is at a medium.

Lasers vs. LEDs vs. SLEDs

1. Laser: A laser (Light Amplification by Stimulated Emission of Radiation) works based on the principle of stimulated emission. It produces coherent, monochromatic light through the stimulated emission of photons.
   Stimulated emission is a phenomenon that occurs when a photon, carrying a specific frequency, interacts with an excited electron within an atom or a similarly excited state in a molecule. As a result of this interaction, the electron transitions to a lower energy level. The excess energy possessed by the electron is released and transferred to the surrounding electromagnetic field. Consequently, a new photon is generated, possessing identical characteristics to the original incident wave, including its frequency, polarization, and direction of propagation. This process leads to the amplification and coherent emission of light in the laser cavity.
   Stimulated emission is the mechanism that allows the laser to produce a highly focused, intense, and monochromatic beam of light.
2. LED: A light-emitting diode generates light through the process of electroluminescence and spontaneous emission. It produces incoherent light and emits a broad spectrum of colors depending on its design.
   Spontaneous emission refers to the mechanism through which a molecule, atom, or subatomic particle moves from an excited state to a lower energy state, like its ground state. During this transition, energy is released via photons. Spontaneous emission plays a vital role in generating most of the light we perceive in our environment. This process is so commonplace that it has acquired different names despite being essentially the same phenomenon. When atoms or molecules are excited by methods other than heating, this type of emission is known as luminescence.
3. SLED: A superluminescent diode is like a laser but designed to produce broad-spectrum light. It combines the characteristics of both a laser and an LED, emitting amplified spontaneous emission (ASE), which is useful in many applications.
   ASE produces light sources that encompass a broad spectrum of wavelengths. If the area where the light is generated lacks reflective properties, such as in a SLED with anti-reflection coated (ARC) facets, the process of lasing cannot occur. This leads to a broader range of emitted light due to the properties of the material producing the light.
   Because of this broader range, the light has lower consistency over time, which means less interference noise when compared to a laser. However, the light can still be tightly focused, making it useful for technologies like fiber optic systems and optical coherence tomography.

Speckle

Speckle, also known as speckle pattern or speckle noise, is a kind of grainy texture that can make the quality of images worse in certain imaging systems. It happens because the waves of light interfere with each other, causing this unwanted texture. Speckle can be found in systems like biomedical imaging, projection, and optical coherence tomography. It's important to note that speckle is not an external type of noise; instead, it occurs naturally because of the way light bounces off surfaces. This is because the surfaces have different properties, and even tiny changes in the waves can have a big effect on the interference pattern.

The idea of speckles dates to the time of Isaac Newton, but it gained more prominence with the invention of the laser. On account of their high optical power density, lasers produce a lot of speckles, or optical noise, making SLEDs the preferred light source in various applications. SLEDs’ lower optical power density, combined with their medium coherence, means they can reduce speckle in imaging applications while remaining effective.

Applications

1. Laser: Lasers are coherent, monochromatic light sources with narrow spectral bandwidth and high power.
   Overall, the combination of laser-like properties (such as partial coherence and higher output power) with the broader spectral emission of LEDs makes SLDs a versatile light source that can be tailored to specific applications that require both bandwidth and moderate coherence.
   

   Applications that require a laser include:

   1. Laser printers
   2. Barcode scanners
   3. DNA sequencing instruments
      
   4. Fiber-optics
      
   5. Laser surgery and skin treatments
      
   6. Military devices
2. Light Emitting Diodes: LEDs are known for their efficiency since they convert a significant portion of the supplied electrical energy into light.
   LEDs have a longer lifespan, faster switching times, and are more rugged compared to traditional incandescent or fluorescent light sources. The versatility, efficiency, durability, compact size, and controllability have made LEDs the lighting technology of choice in various industries and everyday settings.
   
   Applications where LEDs are most widely used include:
   1. Lighting and displays
   2. Automotive lighting
   3. Low-power optical communications
   4. Flashlights and portable lighting
   5. Back lighting

A Light Source for Every Application

While lasers are suitable for precise work requiring a strong, coherent light, LEDs produce incoherent light with a broad spectrum and lower power, commonly used for lighting and low-power applications.

SLEDs are widely used for applications where moderate-to-high power and wider bandwidth are needed.

No light source ranks superior over any other, but rather different scenarios require a different type of light, and the key is knowing what you need.

Want to learn more about your options when it comes to lasers and SLEDs? Contact DAYY Photonics anytime to talk about the specialized needs in your industry.