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1
Designing High Power
Single Frequency Fiber Lasers
Dmitriy Churin
Course OPTI-521
2
Overview
βWhat is a single frequency laser
βApplications of single frequency lasers
β Identifying the limitation of the fiber single frequency lasers
β Stimulated Brillouin Scattering
β Gradient of temperature of the fiber
βApplying strain to the fiber and choice of the epoxy
3
Single frequency laser
πΈ=πΈ0 exp (πππ§β πππ‘ )+π .π .
But we really have:
Lc β coherence length
πΏπ=π ππππππ hπππ π‘
hπππππ€πππ‘ ππ hπ‘ ππππ ππ
4
Displacement measurement
Record accuracyβ 2 nm
znII 2
2cos10
5
Velocity measurementDoppler effect:
V
Dimensions, mm 300 x 120 x 110Wavelength 690 nm Laser power max. 25 mWWorking distance [mm] 200 1,500Min. velocity [m/min] 0.3 2.11Max. velocity [m/min] 875 6,211
POLYTEC Velocimeter:
0f
V
f2
0
6
Laser Atom Cooling
2 mm
Six laser beams converge from three orthogonal directions to slow the atoms that happen to pass through the volume where the beams intersect. To hold and trap the atoms in this region, a magnetic induction field is created by two coils positioned on either side of the overlap volume.
Rubidium atoms:
eVPSE 59.1)55( 2121
2121
nm786
k
fmTkTEk 3
)(
2
3 22
oKMHzfT 21.0)10( oKMHzfT 21)1.0(
oKrecord 1
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Acoustic Vibration Measurements
~1 mW DFB fiber laser
kHzf 10
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Other Applications
Coherent Beam Combining
Spectroscopy with high resolution
Fiber Optic Communication
GHz1012 RyEFine structure:
10 Gbit/s per channel. Up to terabit/s with wavelength division multiplexing (100 channel per one fiber) β Stable single source is required
MHz11.04 RyEHyperfine structure:
kHz 100~ W,150
LaserFiber CW :IPG
f
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What is Stimulated Brillouin Scattering (SBS)?
1. Field of a single frequency laser
Ξ» Ξ»L
2. Medium βseesβ the intensity of the light
3. It creates variation of the density of the material (electrostriction) that travels with the speed of sound -> variation of refractive index. Effectively we have an induced moving Bragg mirror.
Pump Brillouin scattering
4. Incident light reflects back. We can get up to 99% of reflection. Shift due to the Doppler effect.
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Brillouin Spectrum Profile
Ξ©B is a frequency shift from the laser signal and it is defined by the medium properties.
ΞB is full width at half maximum (FWHM) level of the Brillouin spectrum.
gp is Brillouin gain value at the maximum. It has a value of ~5Β·10-11 m/W. It cannot be modified significantly.
Ξ©π΅=2ππ£π
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Threshold for SBS
What can we do to get Pcr as large as possible?
1. Decrease the effective length (interaction length between medium and light).Even with the highest concentration of dopants in active fiber the minimal length is about 30cm.
2. Increase the modal area of the fiber.The limit of the mode diameter for the single mode fiber is ~30 microns. At larger diameters the fiber stops guiding the light.
Pcr for such fiber amplifier would be at the level of ~1kW. If we need to build a higher power laser or pulsed laser with peak power >1kW and pulse duration >10ns we have to develop other methods to suppress the SBS.
ππ΅πππ πΏππππ΄πππ
β21
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Temperature dependence
Temperature gradient along the fiber
Enhancement by factor of 5
CT=1.05MHz/K
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Applying strain to the fiber
ππ΅πππ πΏππππ΄πππ
β21Split into 6 individual fibers with 1/6 of the total length
CS=0.464GHz/%
14
Choice of the epoxy
For 2% strain:
π=πΉπ΄ βπΈ
πΉ=π β π΄ βπΈ=17.7π
πΉ
π΄=π π2
2=Ο 125ππ2
4 (Fused Silica)
Long term strength is (2216 epoxy)
π
Fiber
Epoxy
πΏ
How much of the epoxy we need?
π΄π=π βπ βπΏ
πΉπ΄π=
3.5πππ4
=β«πΏ=5ππ
Safety Factor
Need to use another epoxy
Something else to consider:1. Shrinkage of the epoxy2. Using epoxy at high temperatures
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Conclusion
Higher power/peak power single frequency fiber lasers need to have high suppression of Stimulated Brillouin Scattering.
Common methods to reduce SBS are:1. Use of high gain active fiber to reduce the effective length of the fiber2. Use large core fiber.
To further suppress the SBS we need to βmodifyβ the fiber:3. Apply strain to fiber in steps (enhancement factor of 20).4. Apply temperature gradient (enhancement factor of 5).
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Thank you
17
Applying strain to the fiber
ππ΅πππ πΏππππ΄πππ
β21
β«π (π§ ) ππ§=ππππ π‘
Enhancement by factor of 20!