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
Single Frequency Fiber Lasers
But we really have:
Lc – coherence length
Record accuracy– 2 nm
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
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.
~1 mW DFB fiber laser
Coherent Beam Combining
Spectroscopy with high resolution
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
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.
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.
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.
along the fiber
by factor of 5
Split into 6 individual fibers with 1/6 of the total length
For 2% strain:
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
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).
Enhancement by factor of 20!