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Fiber Optic Light Source Tutorial

  • Fiber Optic Light Source Tutorial Fiber-Mart.com
  • Post on Wednesday 04 February, 2015
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Fiber Optic Testers & Tools

 

Fiber Optic Light Source Tutorial

 

Introduction

 

As we know, Fiber Optic Light Source is a important device which can provide a continuous wave (CW) and stable source of energy attenuation measurements. Fiber optic light source is a fiber optic test equipment to measure the fiber optic loss for both single mode fiber cable and multimode fiber cables, usually the fiber optic light source is used with the fiber optic power meters. Fiber Optic Light Source includes a light emitting diode (LED) or laser that it is stable to use an automatic gain control mechanism.

Fiber Optic Light Source

 

Different Types

 

Optical sources are one of the most researched areas in fiber optic communication. The following types of fiber optic light sources are usually used in telecommunications and data communications.

 

1. Fabry-Perot lasers (FP lasers)

 

Fabry-Perot lasers (FP lasers) is the most widely used light source for lightwave telecommunication systems. It is named for the French scientists Charles Fabry and Alfred Perot.

In a Fabry-Perot (FP) laser, light is reflected and re-reflected between two "mirrors" at either end of a semiconductor material that has been biased electrically. The material and two mirrors form a resonant cavity that roughly determines the wavelength of the light produced. One of the mirror is only partly reflective, allowing some portion of the light to "leak" out into an external fiber, whereas most is internally reflected. This is directly analogous to a "high-Q" resonant L-C tuned circuit where the circulating energy is much higher than that coupled into a load.

The following figure illustrates a section view of an FP laser parallel to the direction of light emission. The laser has two parts: a semiconductor optical amplifier to provide gain and mirrors to form a resonator around the amplifier.

FP lasers

 

2. Distributed Feedback Lasers (DFBs)

 

A Distributed Feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fiber laser where the active region of the device is periodically structured as a diffraction grating. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback for the laser.

The DFB laser is designed to overcome the spectral shortcomings of the FP laser. The following figure shows the structure of a DFB laser. It is very similar to an FP laser with the addition of a Bragg reflector structure located near the light-emitting active region. The Bragg reflector grating provides a periodic change in the index of refraction in the waveguide.

DFBs

 

3. Vertical Cavity Surface-Emitting Laser (VCSEL)

 

The vertical cavity surface-emitting laser (VCSEL) was originally developed as a low cost alternative to FP and DFB lasers. The first commercial applications of these lasers is in the area of high-speed data communication links replacing LEDs. Vertical cavity lasers emit perpendicular to the top plane of a semiconductor wafer. It uses a multilayer dielectric mirror that is grown directly on the semiconductor surface as shown below.

This mirror consists of alternations of high and low index of refraction layers to form a Bragg reflector. The distinguishing feature of this structure is its extremely short optical amplifier length (on the order of 100nm). This length is compared to the 300um length typical of an FP and DFB laser. This short amplifier length limits the available gain from the amplifier to a very small value.

VCSEL

 

4. Surface-Emitting LEDs

 

The SLED has low-bandgap semiconductor materials sandwiched between high-bandgap materials as is found in semiconductor lasers. The major difference in an LED is that there are no mirrors to provide feedback. Current is passed through the active region to create hole and electron pairs in the low-bandgap active region. The electrons from the conduction band lose energy spontaneously and emit photons in all directions. A fraction of the generated light is coupled into the multimode fiber. The most common multimode source is the surface-emitting LED (SLED) shown below.

surface-emitting LED

 

5. Edge-Emitting LEDs

 

Edge-Emitting LED (EELED) is very similar to an FP laser without mirrors. Its structure shown as the following diagram. This EELED configuration shows two segments. One segment is forward biased to produce gain in a semiconductor optical amplifier. The other segment is reverse biased to produce an optical absorber.

The absorber prevents the optical amplifier from becoming a FP laser. The output of the semiconductor optical amplifier is also antireflection-coated to further prevent a mirror from forming. The EELED optical amplifier produces ASE. Spontaneously emitted light at the input of the amplifier produces ASE at the amplifier output.

Edge-Emitting LEDs

 

Warm Tips: Fiber-Mart's fiber optic light source can provide wavelength output according to the specific requirements including the 650nm red source, 1310nm/1550nm wavelength for the single mode fiber and 850nm/1300nm wavelength for the multimode fiber. Used together with our fiber optic power meter, they act as an economic and efficient solution for the fiber optic network works.

 


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