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
Ti:sapphire Laser
Verdi
5 - 10 W
530 nm
l = 800 nm
Cr:forsterite Laser
Fiber Laser
10 W
1075 nm
l = 1270 nm
Frequency standards for the telecom wavelengths
Cr:forsterite Laser
Fiber Laser
10 W
1075 nm
l = 1270 nm
Frequency standards for the telecom wavelengths
Cr:forsterite Laser
Fiber Laser
10 W
1075 nm
l = 1270 nm
Cr doped forsterite
Frequency standards for the telecom wavelengths
Cr:forsterite Laser
Fiber Laser
10 W
1075 nm
l = 1270 nm
Cr doped forsterite
Poor thermal conductivity
Frequency standards for the telecom wavelengths
Cr:forsterite Laser
Fiber Laser
10 W
1075 nm
l = 1270 nm
Cr doped forsterite
Poor thermal conductivity
Sensitive to environmental
perturbations
Outline
• 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: Basics
f
Tr
t
S. Diddams et al., Science 306, 1318 (2004)
Time Bandwidth Product
t
   t pulse  constant
Constant depends upon the
pulse shape
For a Gaussian pulse,
f
   t pulse  0.441
Propagation of Ultrafast Laser Pulses
x
2
E in  E 0 ex p ( i  0 t ) ex p (   t ) exp( i k x )
k  k 0  k  (   0 ) 
k 
2
k  (   0 )  .....
2
d 
2
d
dk
1
 vg
0
k  
dk
2
0
d  1


d  vg



0
Propagation of Ultrafast Laser Pulses
x
E in  E 0 ex p ( i  0 t ) ex p (   t )
2





x 

x 

α ex p  i  0  t 
t

 ex p  



2
2 


 


v

1

4

k
x
v
p
g



 


2
E out
2

2  k  x
ex p  i
2
2
 1  4  k  x



x 
 t 
 
vg  


2
Propagation of Ultrafast Laser Pulses
Propagation of an ultrafast laser through a
transparent material can lead to:
• Pulse broadening
• Pulse delay
• Chirp
• 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 Combs
Time domain
f
E(t)
Carrier-envelope
phase slip from pulse
to pulse because:
2f
vg  vp
t
vp 
tr.t = 1/fr
Frequency domain
I(f)
fo
c
n ( )

 dn 
 v p 1 

n ( )   ( dn d  )
n ( ) d  

c
vg 
fr
It is critical to have an
octave spanning spectrum.
0
fn = nfr + fo
f
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).
www.nobel.se
Existing portable wavelength references for the telecom industry
laser
C2H2
or LED
Pressure-broadened
Line centers:±130 MHz or ±13 MHz
Used to calibrate Optical Spectrum
Analyzers (OSA’s)
Line widths ~5 GHz (OSA resolution)
pressure → broadening & shift
W.C. Swann and S.L. Gilbert, JOSA B 17, 1263 (2000)
Saturation spectroscopy in hollow optical fiber
Pump
z
Probe
1.0
112 mW (+ 0.4)
83 mW (+ 0.3)
40 mW (+ 0.2)
20 mW (+ 0.1)
10 mW
0.8
0.6
0.4
Fractional Absorption
Fractional Absorption
Saturation spectroscopy in hollow optical fiber
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
112 mW (- 0.2)
83 mW (- 0.1)
40 mW (- 0.1)
20 mW (- 0.05)
10 mW
-0.6
-400 -200 0 200 400 600 800
Frequency (MHz)
0.2
10 mm core
0.0
-1000
-500
0
500
Frequency (MHz)
1000
Significant signal strength
at 10 and 20 mW pump
powers!
R. Thapa, K. Knabe, M. Faheem, K. L. Corwin
Self-Referenced Optical Frequency Comb
fr
I(f)
fo
f2n
fn
f
0
fn = n fr + fo
x2
2nfr + 2fo
f2n = 2nfr + fo
fo
• 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)
Ti:sapphire vs. Cr:forsterite
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
S. Diddams et al., Science 293 (2001)
I. Thomann et al., OL 28, 1368 (2003)
Chromium-forsterite Lasers: A Brief History
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
Optimizing Cr:fr Laser: Dispersion
Net cavity dispersion = Cr:f dispersion
+ prism (SF6 ) dispersion
+ angular dispersion
Pump laser
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
Cr:forsterite Laser
*I. Thomann et al., OL 28, 1368 (2003)
Optimizing Cr:fr Laser: Stability
Ray transfer matrix (ABCD) analysis is
performed to yield optimal cavity parameters
that is essential for stable laser operation.
refractive index n
h
f
Lens of focal length f
d
1

0
d /n

1 
 1

 1 / f
0

1
Optimizing Cr:fr Laser: Stability
Ray transfer matrix (ABCD) analysis is
performed to yield optimal cavity parameters
that is essential for stable laser operation.
 A1

 C1
B1   A 2

D1   C 2
B2 
 An
 ........ 
D2 
 Cn
Bn   A

Dn   C
B

D
Optimizing Cr:fr Laser: Stability
Ray matrix (ABCD) analysis performed to yield
optimal cavity parameters that is essential for
stable laser operation.
Pump laser
Self consistent
solution:
 
2
l
B
n
q0
 AD 
1 

2


2
Cr:forsterite Laser
Optimizing 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.
1

0
t /( n c  sin  )
2
1
1/ 2



1

0
t n c (1  sin  ) /( n c  sin  )
2
2
2
1
3/2



Optimizing Cr:fr Laser: Astigmatism
mm
beam waist(mm)
Beam diameter
0.2
0.1
5.5
4.5
-0.1
-0.2
6
6.5
dd 2 cm(cm)
Mode 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.
76.43 nm FWHM Bandwidth
59 nm FWHM Bandwidth
I. Thomann et al., OL 28, 1368 (2003)
0
-10
103.452 nm FWHM Bandwidth
Intensity (dBm/nm)
-20
-30
-40
-50
-60
-70
-80
1100
1200
1300
1400
1500
Wavelength (nm)
1600
1700
Rep. Rate Measurements: 115 MHz
Hyperbolic Secant Pulse:
38 fs.
Transform limited pulse for
105 nm bandwidth: 16.5 fs.
M o d e lo c k e d S p e c tra l B a n d w id th (n m )
Stability of Mode Locked Laser
120
105
90
75
60
45
30
0
2
4
h o u rs
6
8
Laser Parameters
Spectral width:
Pulse Duration:
Rep. Rate:
Output Power:
Center Wavelength:
90-105 nm
38 fs
115 MHz
220 mW
1275 nm
Supercontinuum 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
Highly Nonlinear Fiber
• 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.
Aeff =13.9 mm2
Dispersion slope = 0.024 ps/(nm2 km)
Nonlinear coefficient g = 8.5 ( W km)-1
• Due to break up of these
higher order solitons, four-wave
mixing generates frequencies
at wavelengths shorter than
zero dispersion wavelength.
J. W. Nicholson et. al, Opt. Lett 28, 643, 2003
Supercontinuum Generation from Cr:fr Laser
Laser output
Supercontinuum
0
0
-10
-10
-20
-20
Intensity (dBm/nm)
Intensity (dBm/nm)
88.892 nm FWHM Bandwidth
-30
-40
-50
-30
-40
-50
-60
-60
-70
-70
-80
-80
1000
1100
1200
1300
1400
1500
Wavelength (nm)
1600
1700
1800
1000
1200
1400
Wavelength (nm)
1600
1800
Current Research Status
1.4
Power (arb. units)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1800
1900
2000
2100
Wavelength (nm)
2200
2300
2400
Current Research Status
Fiber in
Fiber Laser
10 W
1075 nm
Cr:forsterite Laser
Current Research Status
Fiber in
Fiber Laser
10 W
1075 nm
Fiber out
Cr:forsterite Laser
SC
BS
HNLF
stabilized optical
frequency comb
Synthesizer
frep Loop
Filter
nonlinear
crystal
Synthesizer
f0 Loop
Filter
Phase
Detector
DM
Current Research Status
Saturation Spectroscopy
Pump
Probe
z
s  
dP
P dz

0
1  ( P / Ps )
( Pz  P0 )( A  B )   0 z
Saturation Spectroscopy
2 Pz
Ps Pz
Ps P2
z
Ps Pz
arctanh
Ps
A=
P0
2 P0
B=
P0 Ps
P2
0 Ps
Ps
Pz
arctanh
Ps
Ps
P0 Ps
P0
Pz
Pz
Ps
P0
Ps
Pz
Ps
P0
Ps
Saturation Spectroscopy
sat
Pump
mWPower
Probe (mW)
40
saturation
35
30
25
20
15
10
5
no saturation
0.25
0.5
0.75
1
1.25
1.5
1.75
Distance
Distance m(m)
no sat
Conclusions
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.
Future Work
Octave spanning spectrum.
Laser Stabilization.
Installation of piezo mounted mirror in laser cavity.

 dn 
vg 
 v p 1 

n ( )   ( dn d  )
n
(

)
d



c
vp 
f0 
c
n ( )
 fCE
2
fr
ULTRAFAT LASER BASICS
E in  E 0 ex p ( i  0 t ) ex p (   t )
2
k  k 0  k  (   0 ) 
k 
d
dk
E out
1
2
k  (   0 )  .....
2
d 
2
 vg
0
k  
dk
2
0
d  1


d  vg



x 


α exp  i  0  t 
  exp 
2
2





v
1

4

k
x
f 



2

2  k  x
exp  i
2
2
 1  4  k  x



x 
 t 
 
vf  


2



0


x 
 t 
 
vf  


2
Chromium-forsterite Lasers: A Brief History
Optimizing Cr:fr Laser: Astigmatism
k  
l
3
2
d n
2 c d l
2
2
Frequency Combs for frequency metrology
• Transfer stability and accuracy between optical and
microwave regimes.
Microwave
(9.2 GHz)
•
•
Optical
(500 THz)
Ti:sapph comb commercially available.
Fiber lasers at 1.5 mm increasingly interesting.
–
–
–
–
•
Frequency
Comb
5 x 104
near IR (telecom)
cheaper
more portable
will require portable references
near-IR comb being developed at Kansas State for characterization of
new standards.