Lasers vs. Light Emitting Diodes

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:Laser printers
   Barcode scanners
   DNA sequencing instruments
   Fiber-optics
   Laser surgery and skin treatments
   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:

Lighting and displays
Automotive lighting
Low-power optical communications
Flashlights and portable lighting
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.