### Document

```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
= 0 exp  −  + . .
But we really have:
Lc – coherence length
=
ℎ
ℎ  ℎ
3
Displacement measurement

 2
I  I 0  1  cos 
 


2 n  z   

Record accuracy– 2 nm
4
Velocity measurement
Doppler effect:
f0 
2V

f0
V
POLYTEC Velocimeter:
Dimensions, mm
Wavelength
Laser power
Working distance [mm]
Min. velocity [m/min]
Max. velocity [m/min]
300 x 120 x 110
690 nm
max. 25 mW
200
1,500
0.3
2.11
875
6,211
5
Laser Atom Cooling
Rubidium atoms:
E ( 5 S 1 2  5 P1 2 )  1 . 59 eV
1 2
1 2
2 mm
  786 nm
Ek 
3
m (f ) 
2
kT  T 
2
2
3k
T (  f  10 MHz )  0 . 21 K
o
T (  f  0 . 1 MHz )  21  K
o
record  1  K
o
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.
6
Acoustic Vibration Measurements
~1 mW DFB fiber laser
 f  10 kHz
7
Other Applications
Coherent Beam Combining
IPG : CW Fiber Laser
150 W,  f ~ 100 kHz
Spectroscopy with high resolution
Fine structure:
 E   Ry     1  10 GHz
Hyperfine structure:
 E   Ry     0 . 1  1 MHz
2
4
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
8
What is Stimulated Brillouin Scattering (SBS)?
λ λ
L
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.
Pump
Brillouin scattering
9
Brillouin Spectrum Profile
2
Ω =

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.
10
Threshold for SBS

≈ 21

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
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.
11
Temperature dependence
CT=1.05MHz/K
along the fiber
Enhancement
by factor of 5
12
Applying strain to the fiber

≈ 21

Split into 6 individual fibers
with 1/6 of the total length
CS=0.464GHz/%
13
Choice of the epoxy
=

∙

For 2% strain:
2
1252
=
=π
2
4
= 82 (Fused Silica)

Epoxy

Fiber
=  ∙  ∙  = 17.7
How much of the epoxy we need?
Long term strength is 3.5 (2216 epoxy)
=  ∙  ∙
Something else to consider:
1. Shrinkage of the epoxy
2. Using epoxy at high temperatures

3.5
Need to use
=
=≫  = 5
another epoxy

4
Safety
Factor
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!

≈ 21

=
17
```