Designing High Power Single Frequency Fiber Lasers Dmitriy Churin Course OPTI-521 1

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 2

Single frequency laser 3 But we really have: L c – coherence length

Displacement measurement 4 Record accuracy– 2 nm

Velocity measurement Doppler effect: Dimensions, mm300 x 120 x 110 Wavelength 690 nm Laser powermax. 25 mW Working distance [mm] 200 1,500 Min. velocity [m/min] Max. velocity [m/min] 8756,211 POLYTEC Velocimeter: 5

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: 6

Acoustic Vibration Measurements ~1 mW DFB fiber laser 7

Other Applications Coherent Beam Combining Spectroscopy with high resolution Fiber Optic Communication Fine structure: 10 Gbit/s per channel. Up to terabit/s with wavelength division multiplexing (100 channel per one fiber) – Stable single source is required Hyperfine structure: 8

What is Stimulated Brillouin Scattering (SBS)? 1. Field of a single frequency laser λ λLλL 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. 9

Brillouin Spectrum Profile Ω B is a frequency shift from the laser signal and it is defined by the medium properties. Γ B is full width at half maximum (FWHM) level of the Brillouin spectrum. g p is Brillouin gain value at the maximum. It has a value of ~5· m/W. It cannot be modified significantly. 10

Threshold for SBS What can we do to get P cr 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. P cr 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. 11

Temperature dependence Temperature gradient along the fiber Enhancement by factor of 5 C T =1.05MHz/K 12

Applying strain to the fiber Split into 6 individual fibers with 1/6 of the total length C S =0.464GHz/% 13

Choice of the epoxy For 2% strain: Fiber Epoxy How much of the epoxy we need? Safety Factor Need to use another epoxy Something else to consider: 1.Shrinkage of the epoxy 2.Using epoxy at high temperatures 14

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 fiber 2.Use large core fiber. To further suppress the SBS we need to “modify” the fiber: 1.Apply strain to fiber in steps (enhancement factor of 20). 2.Apply temperature gradient (enhancement factor of 5). 15

Thank you 16

Applying strain to the fiber Enhancement by factor of 20! 17