ModeLocked Fiber Lasers and their Applications

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Mode-Locked Fiber Lasers and their Applications
Ph.D. Thesis Proposal
Nick Usechak
1/17/2003
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Overview

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

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A little motivation
Continuous main pulse has higher intensity and
lower bandwidth (2-5 Å?)
foot composed of picket pulses with Dl 11Å
Picket intensityPower on target
main
foot
t
Dtmain 1-4 ns
Dtfoot 2-5 ns
Dtfoot 3-7 ns
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Progress

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

5
Background

• The first fiber laser was built in 19611

• The field remained relatively unexplored until
  the late 1980s when the

availability of EDFAs made the construction of
fiber lasers not only feasible but also
potentially useful due to the application of
mode-locking techniques to such lasers

• Some fiber lasers developed in the late 1990s
  possessed the ability to

have their operation extended to different
wavelength regimes by using prism, or grating
pairs or CFBG for dispersion compensation

• Commercially IMRA is producing fiber lasers in
  the 1053 nm regime by

using a fiber laser mode-locked at a different
wavelength in conjunction with nonlinear effects
and a Ytterbium based CPA system.
1. E. Snitzer, Phy. Rev. Lett. 7, 444 (1961).
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Progress

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

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Mode-Locking Techniques
Mode-Locking may be classified into the following
categories
Passive Mode-Lockers

• Saturable Absorbers (SA)

• Saturable Absorbers (SA)

• Nonlinear Polarization Rotation (NLPR)

• Nonlinear Polarization Rotation (NLPR)

• Additive Pulse Mode-Locking

• Colliding Pulse Mode-Locking

• Kerr Lens Mode-Locking

Active Mode-Lockers

• AM Modulation

• AM Modulation

• FM Modulation

• FM Modulation

• Synchronous Pumping

• Optically modulated saturable absorption

• Optically modulated saturable absorption

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Saturable Bragg Reflector
Of practical importance to us is a Saturable
Bragg Reflector (SBR), a device which consists of
a semiconductor containing quantum wells (or
dots) and a Bragg grating which acts as a mirror
A schematic of such a device is depicted here
The effect of a SBR on a laser cavity is crudely
depicted below
Mirror
Gain Medium
SBR
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NLPR
This process relies on an intensity dependent
polarization rotation
Where
Is the phase difference between the fields Ax and
Ay.
And ix, jy or vice versa
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NLPR continued
Hence the effect of NLPR on a pulse is depicted
below
This effect may be exploited to mode-lock a laser
by introducing a polarization dependent loss into
the cavity such that the center of the pulse
experiences minimal loss whereas the wings are
attenuated. This results in shortening the pulse.
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Optically Modulated Saturable Absorption2
This technique uses a semiconductor laser source
to modulate the reflection of a SBR through a
periodic optical pumping of the SBR
In practice the semiconductor laser pump
introduces a time dependent loss into the cavity
which can be used to synchronize the mode-locked
pulses as well as increase the cavity repetition
rate
Gain Medium
Semiconductor Laser
Mirror
SBR
This method has the advantage over AM modulation
that it does not introduce chirp or excess loss
into the cavity. It will self start as opposed
to NLPR and produce solitons in contrast to AM or
FM modulation.
2. N. H. Bonadeo, Opt. Lett. 25, 1421 (2000).
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Progress

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

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Experimental Plan
L. Lefort et. al. Opt. Lett. 27, 291 (2002).
The laser required for the LLE application
alluded to during the third slide must possess

• Pulse width ? 1-4 ps

• Repetition rate of 8-10 GHz

• Center wavelength 1053 nm

• Fixed output polarization

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Experimental Plan continued-
M. Fermann et. al. Opt. Lett. 20, 1625 (1995).
L. Nelson et. al. Appl. Phys. B 65, 277 (1997).
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Experimental Plan
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Progress

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

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Modeling
The following two coupled nonlinear equations can
be adapted to model any of the proceeding lasers
as well as vector pulses propagating in
birefringent optical fiber.
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AM Mode-Locking a scalar cavity from noise
Modeling
FWHM 10 ps
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Progress

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation

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Some More Motivation
Dispersion compensation by FM modulation?
Last summer we questioned the feasibility of
using a FM modulator to produce transform limited
pulses in a cavity operating in the normally
dispersive regime.
Although some related work was done at 1550 nm,
showing that a FM modulator can cancel chirp in a
cavity,3 our hope is to be able to introduce a FM
modulator as a device that we can think of as an
electronically tunable dispersion compensator in
addition to a mode-locker in fiber laser cavities
where no dispersion shifted fiber exists.
The advantages of such a device are obvious when
its use is compared to that of other dispersion
compensating techniques. Since FM modulators can
be purchased with fiber pig tales this technique
is more environmentally stable and introduces
less loss than either a prism or grating pair
configuration. Although the same can be said of
CFBG we note that unlike CFBG we have control
over the amount of chirp introduced by such a
device once it is constructed.
3. K. Tamura, Eiji Yoshida and M. Nakazawa, IEICE
Trans. On Elec. 195 (1998)
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Dispersion compensation by FM modulation?
To try and probe the potential use of this
technique we have modeled the following cavity
FWHM 7 ps
phase
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Dispersion compensation by FM modulation?
To try and probe the potential use of this
technique we have modeled the following cavity
FWHM 18 ps
phase
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Some More Motivation
Intrapulse Raman Scattering (IRS)
Does IRS exist in the normal dispersion regime?
IRS manifests itself as a shift in the spectrum
of a pulse due to SRS pumping the lower frequency
components of a pulse at the expense of the
higher frequency components (as depicted above).
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Ultrashort pulse propagation
output
input
output
input
Although IRS has only been observed in anomalous
fiber (where it is commonly referred to as
soliton self frequency shift) theory predicts its
existence, although greatly reduced, in normal
dispersion fiber as well4
4. Y. S. Kivshar and B. A. Malomed, Opt. Lett. 18
485 (1993)
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Some More Motivation
Ultrashort pulse propagation in PCF
With an ultrashort 1?m fiber laser we posses a
new tool which can be exploited to probe for
previously undiscovered physics. Here we propose
an experiment with photonic Crystal Fiber (PCF).
By changing the polarization of the pulses
incident on a PCF it has been observed that the
structure of the resulting supercontinuum
changes.5 By performing such an experiment in
conjunction with a comprehensive program we feel
that additional insight could be gained as to the
dynamic interplay between nonlinear processes.
Input polarization will effect phase matching
conditions and thus the nonlinear processes that
rely on such conditions such as FWM, etc..
5. A. Ortigosa-Blanch et. al., Quantum Elec. And
Laser Sci. conference paper 120 (2001)
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Conclusion

• A little motivation

• Background

• Mode-locking techniques

• Experimental plan

• Modeling of a fiber laser

• Some more motivation