# An Optical Fiber Mode-Locked Figure Eight Laser

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"An Optical Fiber Mode-Locked Figure Eight Laser"

```					An Optical Fiber Mode-Locked Figure Eight Laser
K-State Physics REU
Daniel Nickel
Mentor: Brian Washburn
Overview

•   Mode locked lasers
– Produce soliton-like pulses
•   The soliton
– The optical soliton
Dispersion
Self-phase modulation
•   The figure eight laser
– Description
– Principles of operation
– Results
Pulse Train from a Mode-Locked Laser

Gain
S D
A C

DC = Dispersion Compensation
SA = Saturable Absorber
The Soliton

•   First Observation
– John Scott Russell, 1834
– Deemed the “Wave of Translation”

Photos courtesy of www.amath.washington.edu/~bernard/kp/waterwaves
The Optical Soliton

•   F8L produces soliton-like pulses

•   Balance between Self-Phase Modulation (SPM)
and Dispersion
1

0.8        sech2
0.6

0.4

0.2

0

-4       -2   0   2     4
Material Dispersion

Characterized by the mode propagation constant:

                 1 d m
n( )   ( )  0           (  0 ) m
m m! d
m
c

• Positive (normal)
– High frequencies: slow
– Low frequencies: fast
•   Negative (anomalous)
– High frequencies: fast
– Low frequencies: slow

1.0                                                                                                             1.0

1.0                                                                                                             1.0
Temporal Intensity

Temporal Intensity
0.8                                                                                                             0.8
0.5                                                                                                             0.5
(a)                                                                                                             (b)
0.6                                                                                                             0.6
0.0                                                                                                             0.0

0.4                                                                                                             0.4
-0.5                                                                                                            -0.5

0.2                         Intensity                     -1.0                                                  0.2
Intensity                     -1.0
Phase                                                                                                           Phase
0.0                                                                                                             0.0
-400 -300 -200 -100   0   100     200   300   400                                                               -400 -300 -200 -100   0   100     200   300   400
Time (fs)                                                                                                       Time (fs)

d                                                                                                           d
0                                                                                                            finite
dt                                                                                                           dt
Self-Phase Modulation

Arises from the
Optical Kerr Effect

n( I )  n0  n2 I
d
to lower frequencies    dt
Trailing edge shifts
to higher frequencies

Net negative dispersion needed in F8L
cavity to form solitons
The Figure Eight Laser

980 nm pump

PC             Er fiber
isolator
isolator

50/50     NALM
50/50
PC
PZT

Output
NALM: nonlinear amplifying loop mirror
The Optical Coupler (50/50)

Beamsplitter                 Directional Coupler
b2                     a1                     b1
a1                     b1

a2               a2                      b2
Both 4 port devices
Coupler needs a π phase difference between pulses to completely
switch out of one port
Nonlinear Loop Mirror: Linear Operation

A                PC
PC

50/50
Gain
B
Linear Operation:
No phase shift between interferometer arms
Nonlinear Loop Mirror: Nonlinear Operation

A             PC
PC

50/50
Gain
B
Nonlinear Operation:
Phase Difference:   n2 (G  1) I (t ) L
Building the Figure Eight Laser

•   Fiber length requirements
– Net negative (anomalous) dispersion
– NALM π phase shift
•   Fiber splicing using arc fusion splicer
– Spliced fibers and components together
– Spliced fiber that the polarization
controllers broke
•   Mode-locking
– Many days of manipulating the PC’s until
sech2 shaped spectrum appeared
Results

• Mode-locked Operation
– Inherently Stable                               1.0
Spectral FWHM : 12.9 nm

– Tunable

Normalized Intensity
0.8

0.6
Center Wavelength = 1567 nm
0.4
Bandwidth (FWHM)= 12.9 nm
0.2

Repetition Rate= 57.753 MHz                           0.0

Power = 10 mW                                          1540 1550 1560 1570 1580 1590 1600
Wavelength (nm)

Kelly Sidebands


Results

• Nearly transform-limited
pulses                                              1.0                      AC FWHM : 322 fs
2
Sech FWHM : 209 fs
– Shortest possible

Normalized Intensity
pulse with the given                             0.8
bandwidth
– From the uncertainty                             0.6
principle
0.4
Transform limited pulse:                             0.2
t  0.315                                      0.0
for   sech2   pulses
-600   -400   -200   0     200   400     600
Our pulses:                                                           AC Delay (fs)

t  0.329
What’s Next?

•   Amplification and compression to < 70 fs pulses

Amplifier

LD                          LD
from
EDFL

SMF            EDF           SMF
section 1                    section 2
Thanks for your time.

References:
1.   I. Duling III, Opt. Lett. 16, 539 (1991)
2.   M.E. Fermann, F. Haberl, M. Hofer, and H. Hochreiter, Opt. Lett. 15,
752 (1990)

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