1

Designing High Power

Single Frequency Fiber Lasers

Dmitriy Churin

Course OPTI-521

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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

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Single frequency laser

𝐸=𝐸0 exp (𝑖𝑘𝑧− 𝑖𝜔𝑡 )+𝑐 .𝑐 .

But we really have:

Lc – coherence length

𝐿𝑐=𝑠𝑝𝑒𝑒𝑑𝑜𝑓 h𝑙𝑖𝑔 𝑡

h𝑙𝑖𝑛𝑒𝑤𝑖𝑑𝑡 𝑜𝑓 h𝑡 𝑒𝑙𝑎𝑠𝑒𝑟

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Displacement measurement

Record accuracy– 2 nm

znII 22cos10

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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

Vf 2

0

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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

kfmTkTEk 3

)(23 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) – 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

λ λL

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.

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

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

ΓB 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 ~5·10-11 m/W. It cannot be modified significantly.

Ω𝐵=2𝑛𝑣𝜆

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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.

𝑔𝐵𝑃𝑐𝑟 𝐿𝑒𝑓𝑓𝐴𝑒𝑓𝑓

≈21

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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

𝑔𝐵𝑃𝑐𝑟 𝐿𝑒𝑓𝑓𝐴𝑒𝑓𝑓

≈21Split into 6 individual fibers with 1/6 of the total length

CS=0.464GHz/%

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Choice of the epoxy

For 2% strain:

𝜀= 𝐹𝐴 ∙𝐸

𝐹=𝜀 ∙ 𝐴 ∙𝐸=17.7𝑁

𝐹

𝐴=𝜋 𝑑2

2=π 125𝜇𝑚2

4 (Fused Silica)

Long term strength is (2216 epoxy)

𝑑

Fiber

Epoxy

𝐿

How much of the epoxy we need?

𝐴𝑒=𝜋 ∙𝑑 ∙𝐿𝐹𝐴𝑒=

3.5𝑀𝑃𝑎4 =≫𝐿=5𝑐𝑚

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 “modify” 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

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Applying strain to the fiber

𝑔𝐵𝑃𝑐𝑟 𝐿𝑒𝑓𝑓𝐴𝑒𝑓𝑓

≈21

∫𝑃 (𝑧 ) 𝑑𝑧=𝑐𝑜𝑛𝑠𝑡Enhancement by factor of 20!