Optical fibers utilize total internal reflection (TIR) to confine and guide light. They are typically used in materials processing as a means of delivering light from a laser source to a processing head. Other applications include telecommunications, spectroscopy, illumination and sensors. A particularly common form of optical fiber is a step-index fiber, shown in Figure 1. Step-index fibers have an inner core made from a material with a refractive index (ncore) that is higher than the surrounding cladding layer (nclad). Within the fiber, a critical angle of incidence (θcrit) exists such that light will reflect off the core/cladding interface by TIR, as opposed to refracting into the fiber cladding. To fulfil the conditions for TIR to occur, the angle of incidence of light launched into the fiber must be less than a certain angle, which is defined as the acceptance angle (θacc). The critical angle for TIR in a given fiber can be calculated on the basis of the refractive indices of the fiber cladding and core materials according to Snell’s law. The acceptance angle is related to the numerical aperture (NA) of the fiber, a dimensionless quantity used by fiber manufacturers to specify the acceptance angle of an optical fiber. The acceptance angle in a given medium of refractive index n can be calculated. Laboratory exercise For this assessment, you will collect experimental data as part of a group, which will form the basis of an individual submission. Laser source • Type: Diode (U22000, 3B Scientific) • Wavelength: 650 nm • Beam profile: See additional information provided • Initial beam diameter: See additional information provided Launch configuration • Single plano-convex focussing optic with effective focal length of 50 mm Fiber optic • Type: SMA connectorized patch cable • Core diameter: 200 µm • Refractive index of core: 1.47 • Refractive index of cladding: 1.4543 • Maximum power density at air-glass interface: 1 GW/cm2 (based on peak power, determined at a wavelength of 1064 nm and a pulse duration of 10 ns) Questions 1) Considering all information provided regarding the laser source, launch configuration and fiber optic, determine whether or not the entirety of the incident beam is launched effectively into the fibre optic. You may use any additional measurements from the laboratory session to support your answer to this question. 2) Using an LBA camera, profile, describe and comment on the intensity distribution of the beam exiting the fiber relative to the beam from the laser source. 3) Taking measurements at various distances from the fiber exit, determine the quality of the exit beam. 4) This fiber optic is to be used for delivery of a short-pulsed laser beam as part of a laser ignition experiment, utilising a TEM00 operating at 532 nm wavelength, 4.5 ns pulse duration, <180 mJ pulse energy and 10 Hz repetition rate. Appraise the suitability of this fiber optic for this purpose. lab results Beam diameter (mm) z (cm) Beam profile 1(mm) Beam profile 2(mm) 15 1.47 2.81 20 1.39 2.72 25 1.34 2.59 30 1.27 2.49 35 1.23 2.41
Optical fibers utilize total internal reflection (TIR) to confine and guide light. They are typically
used in materials processing as a means of delivering light from a laser source to a
processing head. Other applications include telecommunications, spectroscopy, illumination
and sensors.
A particularly common form of optical fiber is a step-index fiber, shown in Figure 1. Step-index
fibers have an inner core made from a material with a refractive index (ncore) that is higher than
the surrounding cladding layer (nclad). Within the fiber, a critical angle of incidence (θcrit) exists
such that light will reflect off the core/cladding interface by TIR, as opposed to refracting into
the fiber cladding. To fulfil the conditions for TIR to occur, the angle of incidence of light
launched into the fiber must be less than a certain angle, which is defined as the acceptance
angle (θacc).
The critical angle for TIR in a given fiber can be calculated on the basis of the refractive
indices of the fiber cladding and core materials according to Snell’s law.
The acceptance angle is related to the numerical aperture (NA) of the fiber, a dimensionless
quantity used by fiber manufacturers to specify the acceptance angle of an optical fiber. The
acceptance angle in a given medium of refractive index n can be calculated.
Laboratory exercise
For this assessment, you will collect experimental data as part of a group, which will form the
basis of an individual submission.
Laser source
• Type: Diode (U22000, 3B Scientific)
• Wavelength: 650 nm
• Beam profile: See additional information provided
• Initial beam diameter: See additional information provided
Launch configuration
• Single plano-convex focussing optic with effective focal length of 50 mm
Fiber optic
• Type: SMA connectorized patch cable
• Core diameter: 200 µm
• Refractive index of core: 1.47
• Refractive index of cladding: 1.4543
• Maximum power density at air-glass interface: 1 GW/cm2 (based on peak power,
determined at a wavelength of 1064 nm and a pulse duration of 10 ns)
Questions
1) Considering all information provided regarding the laser source, launch configuration and
fiber optic, determine whether or not the entirety of the incident beam is launched effectively
into the fibre optic. You may use any additional measurements from the laboratory session
to support your answer to this question.
2) Using an LBA camera, profile, describe and comment on the intensity distribution of the
beam exiting the fiber relative to the beam from the laser source.
3) Taking measurements at various distances from the fiber exit, determine the quality of the
exit beam.
4) This fiber optic is to be used for delivery of a short-pulsed laser beam as part of a laser
ignition experiment, utilising a TEM00 operating at 532 nm wavelength, 4.5 ns pulse duration,
<180 mJ pulse energy and 10 Hz repetition rate. Appraise the suitability of this fiber optic
for this purpose.
lab results
|
|
Beam diameter (mm) |
|
|
|
z (cm) |
Beam profile 1(mm) |
Beam profile 2(mm) |
|
|
15 |
1.47 |
2.81 |
|
|
20 |
1.39 |
2.72 |
|
|
25 |
1.34 |
2.59 |
|
|
30 |
1.27 |
2.49 |
|
|
35 |
1.23 |
2.41 |
|
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