Optical fiber lasers operate based on stimulated emission of photons from excited atoms or molecules within an active medium, such as rare earth doped silica fibers. They were first developed in the 1960s and have several advantages over solid-state lasers including high beam quality, efficiency, and thermal management. Fiber lasers are fabricated by first making a preform via modified chemical vapor deposition to dope the silica with rare earth ions. The preform is then drawn into an optical fiber, which can be structured using fiber Bragg gratings to form the laser cavity. Applications include materials processing, telecommunications, medicine, and directed energy weapons.

1. OPTICAL FIBER LASER

2. 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

3. History of LASER ?
1917: A Einstein postulated stimulated emission and laid the foundation for the
invention 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.

4. 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.

5. 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).

6. 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.

7. 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.

8. 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

9. 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.

10. 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

11. Fabrication
 Vapor-phase oxidation technology for optical fiber
1. Outside vapor deposition (OVD)- Corning USA,1972
2. Modified chemical vapor deposition (MCVD)- AT&T Bell
Labs,1974
3. Plasma chemical vapor deposition (PCVD)-Philips,1975
4. Vapor phase axial deposition (VAD or AVD)-Japanese,1977
5. 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.

12. 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.

13. 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)

14. 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

15. 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.

16. PREFORM PROCESSING:

17. FIBER DRAWING:

18. 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

19. 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

20. References:
 http://www.orc.southampton.ac.uk/61.html
 https://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)
 Wikipedia
 Youtube
 Corning Incorporation