Optical fiber laser

OPTICAL FIBER LASER

What is a LASER ?• LIGHT AMPLIFICATION BY STIMULATED EMISSION OF

RADIATION.• A device that generates an intense beam of coherent

monochromatic electromagnetic radiation by stimulated emission of photons from excited atoms or molecules. 

• 3 Main Processes: • 1)Stimulated absorption/ absorption.• 2) Spontaneous Emission• 3) Stimulated Emission

History of LASER ?1917: A Einstein postulated stimulated emission and laid the foundation for theinvention of the laser by re-deriving Planck’s law

1954: J P Gordon, H J Zeiger and C H Townes and demonstrate first MASER operating as a very high resolution microwave spectrometer, a microwave amplifier or a very stable oscillator.

1958: A Schawlow and C H Townes, extend the concept of MASER to the infrared and optical region introducing the concept of the laser.

1960: T H Maiman realizes the first working laser: Ruby laser

1961: Elias Snitzer wrapped a flashlamp around a glass fiber (having a 300-μm core doped with Nd3+ ions clad in a lower index glass) and when suitable feedback was applied, the first fiber laser was born.

Component of a LASER ?  1) Active medium -

collection of atoms, molecules, or ions (in solid, liquid, or gaseous form), which acts as an amplifier for light waves

2) Pumping source-For amplification, the medium has to be kept in a state of population inversion. The pumping provides population inversion .

3) Optical resonator-for population inversion ,the medium is to act as an oscillator, so a part of the output energy must be fed back into the system.

What is an Optical Fiber?• Thin glass fiber . Fiber Size -

referred to by the outer diameter of its core, cladding and coating. Example: 50/125/250 indicates a fiber with a core of 50 microns, cladding of 125 microns, and a coating of 250 microns.

• Works on total internal reflection. The angle of incidence is greater than critical angle. The refractive index of core , n1 should be greater than that of cladding,n2. (n1>n2). The varying the size of the core diameter we can increase the acceptance angle(numerical aperture).

What is an Optical Fiber?• Depending on the core size, refractive index

and wavelength. Core is single mode (core size- 8-10 micron) and multi mode(core size- 50-100 micron) optical fiber. The multimode has two types – Step index and graded index fiber.

What Fiber Bragg Reflector?• A fiber Bragg grating (FBG) is a type of

distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others.

What Optical Fiber Laser Device? • For commercial

products, it is common to use fiber Bragg gratings, made either directly in the doped fiber, or spliced to the active fiber.

• Pumped by Laser diode 980nm

Optical Fiber Laser : Types and

Characteristics?• High beam quality• High gain due to long interaction

in fibers• High thermal management and

stable• Due to small core dimension, the

light is well confined and only fundamental mode can propagate

• operating at high power levels (greater than 100 W) with high efficiency.

• Pumped with diode laser• Silica fibers doped with • Ytterbium (Yb; 1 µm

operating wavelength), • Erbium (Er; 1.5 µm)• Thulium (Tm; 2 µm)• Holmium (Ho greater

than 2.1 µm)

• EDFA/Laser is a 3 Level Laser system

• Population inversion cannot be achieved by 2-level system.

• In 3 level system, E2 is the metastable state and it is an energy level of guest atom.

Fabrication of optical fiber? 2 Stage process

• Produces a preform• Drawing preform into a

optical fiber of the size desired.

Preform • is a cylinder of silica

composition • 10 to 20 cm in diameter • about 50 to 100 cm long• consists of a core

surrounded by a cladding with• desired refractive

index profile• a given attenuation

in short, preform a desired optical fiber but on a much larger scale.

Vapor-phase oxidation technology

Fabrication Vapor-phase oxidation technology for optical fiber

1. Outside vapor deposition (OVD)- Corning USA,19722. Modified chemical vapor deposition (MCVD)- AT&T

Bell Labs,19743. Plasma chemical vapor deposition (PCVD)-

Philips,19754. Vapor phase axial deposition (VAD or AVD)-

Japanese,19775. Plasma outside deposition(POD)

Modified chemical vapor deposition (MCVD) technique is capable of creating very low loss,single mode, rare earth doped preforms for the fabrication of fiber lasers

In CVD (used in Si industry), we need to need the substrate also and temperature is lower . But in MCVD, no substrate is heated. Here simultaneously Silicon dioxide is melted and with dopants and rare earth dopants.

MCVD Setup:

• Two parts chemical delivery section and the glass working lathe.• Chemical delivery – halides are stored in bubblers within a sealed

glove box.• The control of the rate of Oxygen low is done by computer and

mass flow controller. • The working glass lathe has rotating clamp that hold the glass and

a burner which can traverse along the length of the glass being held.

• The rotary seal has at one end to allow the passage of gas into the tubes and extract at the other end to remove the hazardous gases.

Fabrication OF MCVD Preform• Use a flow SF6 which etches 100 micron of

Silica from the inside of the tube. This removes the layer where the diffusion of OH- will have occurred and helps to ensure that the inside of the tube is perfectly smooth.

• The hot zone is moved back and forth along the tube allowing the particles to be deposited layer by layer basis giving a sintered transparent silica film on the walls of the tube.

• Soot particles formed during the reaction travel with the gas flow and are deposited on the walls of the Silica tube. When a soot particle placed in a field with a temperature gradient, tends to move towards the cooled area, as a result of the impact with particles at higher temperature called thermophoretic effect.

• 3rd Step - the collapse of the tube at a very high temperature into the final preform. The tube collapses down due to the surface tension of the glass, and the decreased viscosity of the glass. (1700-1900deg C)

Solution Doping in MCVD :• incorporation of rare-earth ions into fiber preforms Er3+ ions

• Added procedure with MCVD as the inorganic rare-earth compounds has low vapour pressure.

• 3 Steps - • Soot formation by MCVD• Solution Doping• Consolidation

• 1st step – regular MCVD.• The silica layer is generated at a lower temperature, so that

whilst the soot is formed and deposited, the temperature of the tube is not high enough to consolidate these particles. The result is a layer of porous silica into which the rare earths can be added in solution.

• The temperature of deposition of the soot layer is critical,• if the temperature is too low the soot tends to disassociate

from the tube during sintering, • if the temperature is too high the absorption of the rare-earth

is impaired as the soot becomes sintered as it is deposited

Solution Doping in MCVD :• The tube is removed from lathe

• 2nd Stage – Soaking in the solution (almost an 1 hour)

• Solution of methanol and rare earth (chloride) compounds is poured slowly into the tube.

• After complete diffusion of the rare-earth has occurred the tube is then drained slowly and left to dry completely.

• Final Stage is Consolidation or sintering of the soot into a transparent glass layer

• Last stage of collapse of the tube.

PREFORM PROCESSING:

FIBER DRAWING:

Challenges in MCVD:• The collapse (solid rod) must be carefully controlled to ensure no

deformation of the tube and no alteration of the chemical composition. • The collapse rate should not be too high, either by using low tube

pressure or high temperature, since the tube is not perfectly circular and this can cause elliptical cross sections.

• The porous silica layer is only loosely attached to the interior of the tube and care must be taken whilst transferring the preform from lathe to solution doping clamp not to dislodge any particles.

• The solution is pumped in slowly for the same purpose, a fast flow could dislodge particles and cause refractive index changes or bubbles to form later in the preform fabrication process.

• Fiber drawing - speed of the rotating parts of the machinery need to be controlled with high degree of accuracy

• Preform problems- neckdown,lumps,discontinuity,bubbles

Applications of fiber laser:• to make high-performance surface-acoustic

wave (SAW) devices. These lasers raise throughput and lower cost of ownership in comparison to older solid-state laser technology.

• material processing (marking, engraving, cutting)

• telecommunications, • spectroscopy, • medicine, • directed energy weapons

References:

http://www.orc.southampton.ac.uk/61.htmlhttps://www.rp-photonics.com/fiber_lasers.html S. Nagel et al., “An overview of the modified

chemical vapor deposition (MCVD) process and performance”, IEEE J. Quantum Electron. 18 (4), 459(1982)

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