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Ultrafast Chromium-Forsterite Laser and its Application to Frequency Metrology Ahmer Naweed Group: M. Faheem, K. Knabe, R. Thapa, A. Pung, B. R. Washburn, and K. L. Corwin Thanks: M. Wells, R. Reynolds, and JRM Staff (KSU) S. Diddams and N. Newbury (NIST) J. Nicholson (OFS) Funding: NSF AFOSR. - PowerPoint PPT Presentation
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Ultrafast Chromium-Forsterite Laserand its Application to Frequency Metrology
Ahmer Naweed
Group: M. Faheem, K. Knabe, R. Thapa, A. Pung,
B. R. Washburn, and K. L. Corwin
Thanks: M. Wells, R. Reynolds, and JRM Staff (KSU)
S. Diddams and N. Newbury (NIST)J. Nicholson (OFS)
Funding: NSF
AFOSR
Ti:sapphire Laser Verdi
5 - 10 W 530 nm
= 800 nm
Cr:forsterite LaserFiber Laser
10 W1075 nm
= 1270 nm
Cr:forsterite LaserFiber Laser
10 W1075 nm
= 1270 nm
Frequency standards for the telecom wavelengthsFrequency standards for the telecom wavelengths
Cr:forsterite LaserFiber Laser
10 W1075 nm
= 1270 nm
Frequency standards for the telecom wavelengthsFrequency standards for the telecom wavelengths
Cr doped forsterite
Cr:forsterite LaserFiber Laser
10 W1075 nm
= 1270 nm
Frequency standards for the telecom wavelengthsFrequency standards for the telecom wavelengths
Cr doped forsterite
Poor thermal conductivity
Cr:forsterite LaserFiber Laser
10 W1075 nm
= 1270 nm
Frequency standards for the telecom wavelengthsFrequency standards for the telecom wavelengths
Cr doped forsterite
Poor thermal conductivity
Sensitive to environmental perturbations
OutlineOutline
• Fundamentals of ultrafast lasers– Mode locking– Dispersion management
• Frequency combs and their realization
• Chromium-forsterite lasers: – Benefits and Challenges
• Optimizing Chromium-forsterite laser – Operation at KSU
• Supercontinuum generation
• Laser performance• Future work
Ultrafast Lasers: BasicsUltrafast Lasers: Basics
t
f
S. Diddams et al., Science 306, 1318 (2004)
Tr
Constant depends upon the pulse shape
For a Gaussian pulse,
Time Bandwidth ProductTime Bandwidth Product
constantpulset
0.441pulset f
t
Propagation of Ultrafast Laser PulsesPropagation of Ultrafast Laser Pulses
20 0exp( )exp( )inE E i t t
20 0 0
1( ) ( ) .....
2k k k k
0
g
dk v
dk
0 0
2
2
1
g
d dk
dk d v
xx
exp( )i k x
2
0 2 2
22
2 2
α exp exp1 4
2exp
1 4
out
p g
g
x xE i t t
v k x v
k x xi t
k x v
Propagation of Ultrafast Laser PulsesPropagation of Ultrafast Laser Pulses
20 0exp( )exp( )inE E i t t
xx
Propagation of an ultrafast laser through a transparent material can lead to:
• Pulse broadening• Pulse delay• Chirp
Propagation of Ultrafast Laser PulsesPropagation of Ultrafast Laser Pulses
• Material dispersion is positive.• A prism (or a grating) pair can have both positive or negative dispersion• By using a pair of prisms (or gratings) one can control net cavity dispersion.
Frequency CombsFrequency Combs
tr.t = 1/fr
t
E(t)Time domain
Frequency domain
0fn = nfr + fo
I(f)
f
fo fr
Supercontinuum generation in microstructure fiber preserves frequency comb.
T. Udem, J. Reichert, R. Holzwarth, and T.W. Hänsch, OL 24, 881, (1999).D. J. Jones, et al. Science 288, 635 (2000).
Carrier-envelopephase slip from pulseto pulse because:
vg vp
It is critical to have an octave spanning spectrum.
1( ) ( ) ( )g p
c dnv v
n dn d n d
( )p
cv
n
www.nobel.se
Existing portable wavelength references for the telecom industry
laser
or LEDPressure-broadened
Line centers:±130 MHz or ±13 MHzUsed to calibrate Optical Spectrum
Analyzers (OSA’s)Line widths ~5 GHz (OSA resolution)pressure → broadening & shift
C2H2
W.C. Swann and S.L. Gilbert, JOSA B 17, 1263 (2000)
Saturation spectroscopy in hollow optical fiber
zPump Probe
Saturation spectroscopy in hollow optical fiber
-1000 -500 0 500 1000
0.0
0.2
0.4
0.6
0.8
1.0 112 mW (+ 0.4) 83 mW (+ 0.3) 40 mW (+ 0.2) 20 mW (+ 0.1) 10 mW
Fra
cti
on
al A
bso
rpti
on
Frequency (MHz)
-400 -200 0 200 400 600 800-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
112 mW (- 0.2) 83 mW (- 0.1) 40 mW (- 0.1) 20 mW (- 0.05) 10 mW
Fra
ctio
nal
Ab
sorp
tio
n
Frequency (MHz)
Significant signal strength at 10 and 20 mW pump powers!
10 m core
R. Thapa, K. Knabe, M. Faheem, K. L. Corwin
I(f)
f
fo
0
fr
Self-Referenced Optical Frequency Comb
• fo is generated from a heterodyne beat between the second harmonic of the nth mode and the 2nth mode.
• Once fr and fo are referenced to a known oscillator, all the frequency modes of the fs comb are fixed.
D. J. Jones, et al. Science 288, 635 (2000)
fnf2n
fn = n fr + fo f2n = 2nfr + fo
fo
2nfr + 2fox2
lasing medium Ti:sapphire Cr:forsterite
pump laser 10 W Green (>$ 60,000) 10 W fiber laser (<$ 15,000)
optical fiber microstructured highly-nonlinear Dispersion-shifted
frequency range 500 – 1100 nm 1100 – 2200 nm
Crystal temp room temp -5 oC
Ti:sapphire vs. Cr:forsteriteTi:sapphire vs. Cr:forsterite
S. Diddams et al., Science 293 (2001) I. Thomann et al., OL 28, 1368 (2003)
Zhang et al, 90 nm FWHM; 20 fs; 60 mW
IEEE J Q. Electronics 1997
V. Yanovsky et al, 90 nm FWHM; 80 nm FWHM; 25 fs, 400 mW
OL 1993
Haus et al., 90 nm FWHM; 250 nm FWHM; 14 fs, 80 mW, OL
Chromium-forsterite Lasers: A Brief HistoryChromium-forsterite Lasers: A Brief History
Net cavity dispersion = Cr:f dispersion + prism (SF6 ) dispersion
+ angular dispersion
net cavity dispersion* = - 260 fs2
Cr:f dispersion = 277 fs2 Prism dispersion = - 588 fs2
angular dispersion = -1155.13 fs2
optimal prism separation = 32.5 cm
third order dispersion = 240.77 fs2
Optimizing Cr:fr Laser: DispersionOptimizing Cr:fr Laser: Dispersion
Pump laser
Cr:forsterite Laser
*I. Thomann et al., OL 28, 1368 (2003)
Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.
Optimizing Cr:fr Laser: StabilityOptimizing Cr:fr Laser: Stability
1 /
0 1
d n
h
refractive index n
d
1 0
1/ 1f
Lens of focal length f
Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.
Optimizing Cr:fr Laser: StabilityOptimizing Cr:fr Laser: Stability
1 1 2 2
1 1 2 2
........ n n
n n
A BA B A B A B
C DC D C D C D
Ray matrix (ABCD) analysis performed to yield optimal cavity parameters that is essential for stable laser operation.
Optimizing Cr:fr Laser: StabilityOptimizing Cr:fr Laser: Stability
Self consistentsolution:
2
2
0 12
B
n A Dq
Pump laser
Cr:forsterite Laser
Optimizing Cr:fr Laser: AstigmatismOptimizing Cr:fr Laser: Astigmatism
Because of a lack of axial symmetry, the beam waist along the sagittal and tangential planes may not necessarily be equal and spatially overlap (astigmatism). Therefore, the effects of astigmatism must be taken into account in cavity stability analysis.
2 1/ 21 /( sin )
0 1ct n
2 2 2 3/ 21 (1 sin ) /( sin )
0 1c ct n n
Optimizing Cr:fr Laser: AstigmatismOptimizing Cr:fr Laser: Astigmatism
4.5 5.5 6 6.5dcm
-0.2
-0.1
0.1
0.2
beam waist mmBeam diameter (mm)
d2 (cm)
Mode Locking Cr:fr LaserMode Locking Cr:fr Laser
Unlike Ti-sapphire laser, no well established method for mode-locking the Cr:fr laser is known.
Observation of strong and periodic fluctuation in output laser power. This is an indication that the laser is close to ML regime.
I. Thomann et al., OL 28, 1368 (2003)
76.43 nm FWHM Bandwidth 59 nm FWHM Bandwidth
1100 1200 1300 1400 1500 1600 1700
-80
-70
-60
-50
-40
-30
-20
-10
0
Inte
nsi
ty (
dB
m/n
m)
Wavelength (nm)
103.452 nm FWHM Bandwidth
Rep. Rate Measurements: 115 MHzRep. Rate Measurements: 115 MHz
Hyperbolic Secant Pulse: Hyperbolic Secant Pulse: 38 fs.38 fs.
Transform limited pulse for Transform limited pulse for 105 nm bandwidth: 16.5 fs.105 nm bandwidth: 16.5 fs.
Stability of Mode Locked LaserStability of Mode Locked Laser
0 2 4 6 830
45
60
75
90
105
120
Mod
eloc
ked
Spe
ctra
l Ban
dwid
th (
nm)
hours
Spectral width: 90-105 nm Pulse Duration: 38 fsRep. Rate: 115 MHzOutput Power: 220 mWCenter Wavelength: 1275 nm
Laser ParametersLaser Parameters
Supercontinuum GenerationSupercontinuum Generation
Nonlinear Effects cause creation of new optical frequencies
Honeycomb Microstructure Optical Fiber
J. Ranka, R. Windeler, A. Stentz, Opt. Lett. 25, 25 (2000).
courtesy of Jinendra Ranka
Aeff =13.9 mm2
Dispersion slope = 0.024 ps/(nm2 km)
Nonlinear coefficient = 8.5 ( W km)-1
J. W. Nicholson et. al, Opt. Lett 28, 643, 2003
• Broadest continuum is generated by the fiber when the ultrafast laser pulse is in the anomalous dispersion region.
• The pulse intensity begins to self Raman shift to longer wavelengths.
• Due to break up of these higher order solitons, four-wave mixing generates frequencies at wavelengths shorter than zero dispersion wavelength.
Highly Nonlinear FiberHighly Nonlinear Fiber
1000 1100 1200 1300 1400 1500 1600 1700 1800
-80
-70
-60
-50
-40
-30
-20
-10
0
Inte
nsi
ty (
dB
m/n
m)
Wavelength (nm)
1000 1200 1400 1600 1800
-80
-70
-60
-50
-40
-30
-20
-10
0
Inte
nsi
ty (
dB
m/n
m)
Wavelength (nm)
Laser output
88.892 nm FWHM Bandwidth
Supercontinuum
Supercontinuum Generation from Cr:fr LaserSupercontinuum Generation from Cr:fr Laser
Current Research StatusCurrent Research Status
1800 1900 2000 2100 2200 2300 24000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Po
we
r (a
rb. u
nits
)
Wavelength (nm)
Fiber in
Fiber Laser10 W
1075 nm
Cr:forsterite Laser
Current Research StatusCurrent Research Status
Fiber in
Fiber Laser10 W
1075 nm
Cr:forsterite Laser
Fiber out
HNLF
SC BS
DMnonlinearcrystal
stabilized opticalfrequency comb
Synthesizer
frep LoopFilter
PhaseDetector
Synthesizer
f0 LoopFilter
Current Research StatusCurrent Research Status
Current Research StatusCurrent Research Status
Saturation SpectroscopySaturation Spectroscopy
0
1 ( / )s
s
dP
Pdz P P
0 0( )( )zP P A B z
zPump Probe
B =
A =
2P0P0PsP02Ps
arctanhP0PsPs Ps P0 Ps
P0 PsP0 Pz
2PzPsPzPsPz2
arctanhPsPzPs Ps Ps Pz
P0 PzPs Pz
Saturation SpectroscopySaturation Spectroscopy
0.25 0.5 0.75 1 1.25 1.5 1.75Distance m5
10
15
20
25
30
35
40mW Probe
sat no sat
no saturation
saturation
Pump Power (mW)
Distance (m)
Saturation SpectroscopySaturation Spectroscopy
ConclusionsConclusions
Future WorkFuture Work
Robust and efficient Cr:fr femto second laser.
FWHM bandwidth of up to 105 nm and output energy of about 220 mW.
Realized supercontinuum generation by coupling Cr:fr pulses to a HNLF.
Octave spanning spectrum.
Laser Stabilization.
Installation of piezo mounted mirror in laser cavity.
1( ) ( ) ( )g p
c dnv v
n dn d n d
( )p
cv
n
0 2CE
rf f
ULTRAFAT LASER BASICSULTRAFAT LASER BASICS2
0 0exp( )exp( )inE E i t t
2
0 2 2
22
2 2
α exp exp1 4
2exp
1 4
out
x xE i t t
v k x v
k x xi t
k x v
20 0 0
1( ) ( ) .....
2k k k k
0
g
dk v
dk
0 0
2
2
1
g
d dk
dk d v
Chromium-forsterite Lasers: A Brief HistoryChromium-forsterite Lasers: A Brief History
4.5 5.5 6 6.5dcm
-0.2
-0.1
0.1
0.2
beam waist mm
Optimizing Cr:fr Laser: AstigmatismOptimizing Cr:fr Laser: Astigmatism
3 2
2 22
d nk
c d
Frequency Combs for frequency metrology
• Transfer stability and accuracy between optical and microwave regimes.
• Ti:sapph comb commercially available.• Fiber lasers at 1.5 m increasingly interesting.
– near IR (telecom)– cheaper– more portable– will require portable references
• near-IR comb being developed at Kansas State for characterization of new standards.
Microwave OpticalFrequency Comb5 x 104 (500 THz)(9.2 GHz)