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Designing High Power Single Frequency Fiber Lasers . Dmitriy Churin Course OPTI-521. Overview. What is a single frequency laser Applications of single frequency lasers Identifying the limitation of the fiber single frequency lasers Stimulated Brillouin Scattering

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slide1

Designing High Power

Single Frequency Fiber Lasers

DmitriyChurin

Course OPTI-521

slide2

Overview

  • What is a single frequency laser
  • Applications of single frequency lasers
  • Identifying the limitation of the fiber single frequency lasers
  • Stimulated Brillouin Scattering
  • Gradient of temperature of the fiber
  • Applying strain to the fiber and choice of the epoxy
slide3

Single frequency laser

But we really have:

Lc – coherence length

slide4

Displacement measurement

Record accuracy– 2 nm

slide5

Velocity measurement

Doppler effect:

POLYTEC Velocimeter:

Dimensions, mm 300 x 120 x 110

Wavelength 690 nm

Laser power max. 25 mW

Working distance [mm] 200 1,500

Min. velocity [m/min] 0.3 2.11

Max. velocity [m/min] 875 6,211

slide6

Laser Atom Cooling

Rubidium atoms:

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.

slide7

Acoustic Vibration Measurements

~1 mW DFB fiber laser

slide8

Other Applications

Coherent Beam Combining

Spectroscopy with high resolution

Fine structure:

Hyperfine structure:

Fiber Optic Communication

10 Gbit/s per channel.

Up to terabit/s with wavelength division multiplexing

(100 channel per one fiber) – Stable single source is required

slide9

What is Stimulated Brillouin Scattering (SBS)?

λ

λL

1. Field of a single frequency laser

2. Medium “sees” 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.

4. Incident light reflects back. We can get up to 99% of reflection. Shift due to the Doppler effect.

Brillouin scattering

Pump

slide10

Brillouin Spectrum Profile

ΩB is a frequency shift from the laser signal and it is defined by the medium properties.

ΓBis full width at half maximum (FWHM) level of the Brillouinspectrum.

gpis Brillouin gain value at the maximum. It has a value of ~5·10-11 m/W. It cannot be modified significantly.

slide11

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.

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.

slide12

Temperature dependence

Temperature gradient

along the fiber

CT=1.05MHz/K

Enhancement

by factor of 5

slide13

Applying strain to the fiber

Split into 6 individual fibers with 1/6 of the total length

CS=0.464GHz/%

slide14

Choice of the epoxy

Epoxy

For 2% strain:

Fiber

(Fused Silica)

How much of the epoxy we need?

Something else to consider:

Shrinkage of the epoxy

Using epoxy at high temperatures

Long term strength is (2216 epoxy)

Need to use another epoxy

Safety Factor

slide15

Conclusion

Higher power/peak power single frequency fiber lasers need to have high suppression of Stimulated Brillouin Scattering.

Common methods to reduce SBS are:

Use of high gain active fiber to reduce the effective length of the fiber

Use large core fiber.

To further suppress the SBS we need to “modify” the fiber:

Apply strain to fiber in steps (enhancement factor of 20).

Apply temperature gradient (enhancement factor of 5).

slide17

Applying strain to the fiber

Enhancement by factor of 20!