Difference between Laser Light Source and LED Light Source

As the wide application of fiber optic system, optical light source plays a more and more important part in it. We known a basic optical fiber system consists of a transmitter, an optical fiber and a receiver. The fiber optic light source, as an important component of the transmitter is modulated by a suitable drive circuit in accordance with the signals to be transmitted. Optical light source are also needed for performing fiber optic network testing to measure the fiber optic loss in the cable plant. Light source are offered in a variety of types including LED, halogen and laser. Among which, LED and Laser light source are two types of semiconductor light sources. The following article will discuss about some differences between laser and Led light source.
Basically, both kind of light source must be able to turn on and off millions to billons of times per second while projecting a near microscopic beam of light into an optical fiber. During the working process of optical signals, they are both supposed to be switched on and off rapidly and accurately enough to properly transmit the signals.
General difference between them as that LEDS is the standard light source which is short for light-emitting diodes. Laser light source like gas lasers may be mainly used in some special cases. Lasers are more powerful and operate at faster speeds than LEDs, and they can also transmit light farther with fewer errors. Laser are also much more expensive than LEDs.
LED fiber optic light source are made of materials that influence the wavelengths of light that are emitted. A basic LED light source is a semiconductor diode with a p region and an n region. When the LED is forward biased, current flows through the LED. As current flows through the LED, the junction where the p and n regions meet emits random photons. LEDs emitting in the window of 820 to 870 nm are usually gallium aluminum arsenide (GaAIAs). Laser is also a semiconductor diode with a p and an n region like LED, but it provide stimulated emission rather than the simplex spontaneous emission of LEDs. The main difference between a LED and a laser is that the laser has an optical cavity required for lasting. The cavity is formed by cleaving the opposite end of the chip to form highly parallel, reflective, mirror like finishes.
VCSEL is a popular laser source for high speed networking, which consist of two oppositely oppositely-doped Distributed Bragg Reflectors (DBR) with a cavity layer. It combines high bandwidth with low cost and is an ideal choice for the gigabit networking options.
Different wavelengths travel through a fiber at different velocities as a result of material dispersion. What should always keep in mind is that both Laser and LED will not emit a single wavelength, but a range of wavelength that is known as the spectral width of the source. Fiber optic light source is always works with the fiber optic power meter. During the working process, it collimated beams of light and aim right down the center of the narrow single mode core and propagates in essentially a single mode transmission. By more questions about fiber optic test equipment, such as visual fault locators, optical power meter, OTDR testers, and more. please go for FiberStore webstore.

Working Principle and Characteristics of OTDR

OTDR, the full name of which is Optical Time Domain Reflectometer, is a precise optoelectronic integrated fiber optic test equipment that produced by use of the backscatter during the Rayleigh scattering and Fresnel reflecting in the optical transmission. OTDR tester are widely used for optical cable maintenance and construction, and it can be used for the evaluating the fiber cable length, measuring optical transmission and connection attenuation, detecting the fault location of the fiber links, etc.
During the process of OTDR testing, the instrument inject a higher power laser or fiber optic light source pulse into a fiber from one end of the fiber cable, at the OTDR port to receive the return information. When the optical pulse is transmitted through the fiber, due to the nature of the fiber itself, the connector, the engagement points, bending or other similar event, there will be a scattered reflection. Part of the scattering and reflection will return to the OTDR. Useful information returned will be measured by the OTDR detector, and act as the time or curve segments of fibers at different positions. By recording the time used of the signals from transmission to returning, the transmission speed of the light in the glass fibers, the distance can be calculated.
OTDR testing has some limitation when it come to the applications for measuring the outside able plant loss. The OTDR tester will not be always sufficiently for testing. The OTDR will not work well with short cables in a building or LAN environment. The source and power meter should be used for these tasks as a result of the OTDR is not equipped to show actual cable plant loss.
OTDR use Rayleigh scatting and Fresnel reflection to characterize fibers’ characteristics. Rayleigh scattering refers to the irregular scattering generated when the optical signals transmitting in the fiber. OTDR only measure the scattered light back on the OTDR port. The backscatter signal show the attenuation degree (loss/distance) of the optical fiber, and will be tracked as a downward curve, illustrating the power of backscatter is decreasing, this is because that both transmission signal and backscatter loss are attenuated.
Given the optical parameters, Rayleigh scattering power can be marked, if the wavelength is know, it is proportional with the pulse width of the signal: the longer the pulse width, the stronger backscatter power. Rayleigh scattering power is also related to the wavelength of transmitted signal: the shorter the wavelength, the power is stronger. That is to say, the backscatter loose generated by the trajectory of 1310nm will higher than that of 1550nm signals.
In the higher wavelength region (more than 1500nm), the Rayleigh scattering will continue to decrease, and the other one phenomenon which called infrared attenuation (or absorption) will appear to increase and cause an increase the overall attenuation values. Therefore, 1550nm wavelength is the lowest attenuation, this also explains why it is a long distance communication wavelength. Naturally, these phenomena will return to affect the OTDR. OTDR of 1550nm wavelength is also have low attenuation, so it can be used for long distance testing. While as the high attenuation wavelength 1310nm or 1625nm, OTDR testing distance is bound to be limited, because the test equipment need to test a sharp front in the OTDR trace, and the end of the spikes will quickly fall into the noise area.
Fresnel reflection is discrete reflection, which is caused by the individual point of the whole fibers. These points are caused by a change in reverse coefficient elements such as glass and the air gap. At these points, there will be a strong backscattering light reflected back. Therefore, OTDR is using the information of Fresnel reflection to locate the connection point, fiber optic terminal or breakpoints.
An OTDR tester is essentially an optical radar: it sends out a flash of bright light, and measures the intensity of echo or reflections. This weak signal is averaged to reduce detection noise, and computation is used to display a trace and make a number of mathematical deductions.

Better Knewing Optical Power Measurement

As fiber optic system becomes more and more common and has increasingly sophisticated, optical power measuring, the most basic actions for the fiber optic testing process, will also grow more and more complex. To make reliable measurements, people must consider the characteristics and interactions of optical power meter, fiber optic light source, detector types, attenuation, back reflection, interference, and beam divergence.

Optical Power Meter Measurement

An optical power meter is used to measure the absolute optical power or the relative length of optical fiber optical power loss. Through the measurement of the absolute power of the transmitting end optical network, a power meter to be able to evaluate the performance of the light end equipment. The basic assumption for optical power measurement is the at the meter output reading is directly proportional to the optical input power. This proportionality property is defined as linearity, and the departure from this direct proportionality is defined as nonlinearity.

Optical Light Source
Optical measurements can be made with a wide variety of light sources. Fiber optic light sources are designed to cover a variety of wavelength ranges to suit all optical testing needs, Light sources are offered in a variety of types including LED, halogen and laser. With the combination of a power meter and stabilized light source, it is possible to measure the connection loss, test continuity and help evaluate the transmission quality of fiber link.Optical Multimeter Fiber optic multi meter integrates both an optical power meter module and an optical light source module and can perform closed-loop tests by using both modules, and can also work individually.

Optical metering system
Typical optical power measurement system consists of a detector and a display unit that calculates the optical power or energy represented by the electrical signal. The measurements are displayed or stored in convenient formats, such as analog or digital output, or entries in a data-collection file.
The optical detector, which converts an optical signal into an electrical signal, The most common types of optical-signal detectors are photodiodes, thermopiles, and pyroelectric detectors.
Photodiodes use the photon’s energy to create an electron-hole pair, Thermopile detectors are used for high-power laser sources, up to tens of kilowatts of optical power. Pyroelectric sensors are popular for pulsed laser sources. Among these detector types, photodiode sensors are the most widely used.

Light source back inflection
Accurately measuring a modulated or a pulsed light is very difficult as well as the CW (Light source back inflection) light source. When making peak-to-peak measurements, since waveforms can be greatly distorted. Rather tan reading directly from the detector. It is better to record the raw data, and than process it with digital filtering or statistical averaging with the fact of the limitations of the detector’s response time, and the speed of the meter’s circuitry.

To sum up, optical power measurement involved in the application of optical power meter, a stable fiber optic light source as well as the good understanding of the optical setup, the choice of detector types, detector saturation and noise, attenuation.

Optical Attenuator and Optical Power Meter Calibration

In the fiber optic network systems, that is the true that the power level of an optical signal, either in free space or in an optical fiber are not always the same. For example, for fiber optic receivers, too much light can overload it and degrade the bit error ratio. In order to achieve the best bit error ratio, the light power must be reduced, this time an optical attenuator is need to balance the power of the light by reducing the higher power and fit for the fluent signal transmissions between different devices.
The working principle of the fiber optic attenuator can be compared to a sunglass, which absorbs the extra light energy and protect you eyes from being dazzled. Attenuators typically have a working wavelength range (usually from 1310 to 1550nm) in which they absorb the light energy equally. An important characteristic of a good fiber attenuator is that they should not reflect the light instead, they should absorb the extra light without being damaged. Since the light power used in fiber optic communications are fairly low, they usually can be absorbed without noticeable damage to the attenuator itself.
Fiber optic attenuators are used for reducing the power of the light in the fiber optic network. two types fiber optic attenuators, fixed value fiber optic attenuator refer to the attenuator that can reduce the power of fiber light at a fixed value loss, for example, 2dB. Their applications include telecommunication networks, optical fiber test facility, LAN and CATV systems. Fixed value fiber optic attenuators are composed of four groups: Fixed-Plug in type, fixed adapter Type attenuator, fixed In-line type attenuator, and Fixed Hybrid Adapter type. While adjustable fiber optic attenuator refer to the attenuator that can generate an adjustable Loss to the fiber optic link, they are general used for testing and measurement, but they also have a wide usage in EDFA for equalizing the light power among different channels. Variable Fiber optic attenuators are composed of three groups: Variable plug-in type, Variable adapter type, Variable In-line type and Manual VOA type.
The attenuators can be female to female which is called bulkhead fiber optic attenuator or male to female which is also called a plug fiber optic attenuator. Bulkhead and plug types are designed without cables; another type inline fiber optic attenuator is designed with a piece of fiber optic cable.Wide range variable & inline fiber optic attenuators .the inline fiber optic attenuators are with more accurate attenuation compared with traditional connector type fiber optic attenuators. Variable in-line Optical attenuators are often used with an optical power meter in the optic amplifier systems to balance the gain across the different operating wavelengths.
A wide range of fiber optic attenuators are supplied at ingellen.com which includes LC, SC, and ST, FC, MU, E2000, Fixed Adaptor Type Attenuators, Fixed In Line Type Attenuators, Variable Attenuator Instrument and Variable attenuators. Our fiber optic attenuators are manufactured according to international standard and are compartible with the fiber optic products currently available in the international market.
The optical attenuator is an essential component where optical signal levels need to be adjusted over a wide range of powers. To make the reliable measurements, one must consider the characteristics interactions of fiber optic light source.  Optical light sources are used with the fiber optic power meter to test the fiber system loss, which can offered in a variety of types including LED, halogen and laser. With the optical attenuator and optical power meter calibration system, the attenuation can be easily modified to any level required for telecom and CATV network.