Low-power Portable Laser Spectroscopic Sensor for Atmospheric CO2 Monitoring
Current Tuning Self Osc Vcsel
1. NanoStructure Laboratory CLEO/QELS
PRINCETON UNIVERSITY May 03, 2011
Drive-Current Tuning of Self-Oscillation
Frequency of External Cavity VCSEL
Clinton J. Smith, Wen-Di Li,
Gerard Wysocki, and Stephen Y. Chou
Department of Electrical Engineering,
Princeton University, Princeton, NJ 08544
pulse.princeton.edu http://www.princeton.edu/~chouweb/
2. Motivations: Optical clock source for
atomic clocks
GPS Current state of the art
Handheld & satellite
Telecommunications
High-speed all optical clock signal
Underwater & underground drilling
Military applications 115 mW operating power
35 g
Functionality of the current state
of the art Goal: Create a more power-efficient,
compact atomic clock by replacing
the microwave synthesizer, local
oscillator, and physics package with a
VCSEL self-oscillator.
www.symmetricom.com
http://www.eetimes.com/electronics-news/4212176/Chip-scale-atomic-clock-approaches-performance-of-modules-2
NanoStructure Laboratory PRINCETON UNIVERSITY 2
3. Outline
• Motivation
• Create an all-optical atomic clock to further reduce device size and power
consumption
• Use the polarization self-switching property of VCSELs to create a self-
modulating optical clock.
• Schematic of optical clock design
• Changing VCSEL drive current as a method of self-modulation frequency
tuning
• Discussion of “current tuning”
• Representation of self-modulation as a heterodyne beatnote between
orthogonal polarization modes
• Conclusion and future directions
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4. VCSEL polarization self-switching can be used
as oscillator source
VCSELs have an isometric cavity & circular aperture
• Lase with modes in both horizontal and vertical polarizations
• Corresponds to [011] & [01/1] crystal directions
Isometry can lead to semi-random polarization self-switching
• Like polarization “mode-quenching”
• Usually occurs at ~100% above threshold current
6 μm
SEM image of Avalon Photonics single-mode Typical Optical Power vs. Drive Current curve of
850nm VCSEL a VCSEL that polarization self-switches.
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5. Background: Create an atomic clock using
VCSEL external cavity self-oscillations
Feedback Loop
L
850nm VCSEL QWP PR Cs Vapor Cell QWP POL
||
R
Clock
R
R
||
||
||
f=c/4L
L
L
L
L
||
R
||
Bai & Chou, 2005
4.6 GHz modulations create sidebands
separated by Cs hyperfine frequency
Use frequency (f=c/4L) and Cs
absorption in feedback loop to maximize
resonance
Can fine tune oscillation frequency to D.K. Serkland, G.M. Peake, K.M.
Geib, R. Lutwak, R.M. Garvey, M.
match resonance by changing cavity Varghese, & M. Mescher, “VCSELs
for atomic clocks,” Proceedings of the
length SPIE, vol. 6132, pp. 66-76, 2006
Goal of 30 mW power consumption
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6. New: Changing VCSEL drive current can alter
self-oscillation frequency
Selection of frequency tuning
measurements
What causes this effect?
Only the VCSEL drive current is changed.
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7. L-I measurements
QWP PR POL
VCSEL Detector
The polarizer position
relative to the VCSEL
polarization axes is 45 or
135
VCSEL
VCSEL orthogonal
polarization axes
NanoStructure Laboratory PRINCETON UNIVERSITY 7
8. Self-oscillations as the heterodyne beatnote
VCSEL
VCSEL orthogonal
polarization axes
QWP neutral axes 45°
offset from VCSEL
polarization axes
QWP
•Orthogonally polarized standing waves are oriented along the QWP neutral axes
•Jones Matrices can be used to describe the steady-state roundtrip frequency
beatnote selected by the polarizer
2 kL
1 0
MV V rVCSEL rPR e J rot , f JVCSEL J QWP J QWP JVCSEL J rot ,i J QWP i
0 e 2
MV V Eigenvalues MV V Eigenfrequencies
c
2k L
rVCSEL rPR e 2L
rVCSEL rPR e i
e i 2k L c c
2L 4L
Steady-state beatnote
c Does not account for
frequency:
4L changing cavity phase
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9. Rotating the QWP tunes self-oscillation
frequency
Changing External Cavity Length Rotating Quarter Wave Plate
L
Rotate QWP orientation
f=c/4L about optical axis
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10. Calculating the degree of self-oscillation
frequency tuning
θ
*
1 r1r2
2 arctan
r1,2 are the VCSEL mirror/facet reflectivity's
2 cos 2
• Based on the vectorial Airy function of the field inside the VCSEL, the
change in cavity phase can be calculated as a function of VCSEL
birefringence (δ) and QWP offset angle (θ)*
• Calculation matches well with experiment measurements of changing
QWP offset
*F. Ginovart, F. Robert, and P. Besnard, "Surface-emitting lasers coupled to external cavities with a phase plate: dependence
on the orientation of the plate axes," Journal of Optics B: Quantum and Semiclassical Optics 1, 646-649 (1999).
NanoStructure Laboratory PRINCETON UNIVERSITY
11. Changing VCSEL birefringence as cause of self-
oscillation tuning
θ=36 QWP offset
5x10-4 radians
Θ=38 QWP offset
θ=40 QWP offset
Θ=42 QWP offset
Θ=44 QWP offset
δ
*
1 r1r2
2 arctan r1,2 are the VCSEL mirror reflectivity's
2 cos 2
• Birefringence change of 25 GHz (5x10-4 radians) is high but not
unreasonable**
• Changing VCSEL birefringence (δ) and QWP offset angle (θ) produce self-
oscillation frequency changes consistent with experiment
*F. Ginovart, F. Robert, and P. Besnard, "Surface-emitting lasers coupled to external cavities with a phase plate: dependence on the orientation of the plate axes," Journal of Optics B: Quantum and Semiclassical Optics
1, 646-649 (1999).
**T. Ackemann and M. Sondermann, "Characteristics of polarization switching from the low to the high frequency mode in vertical-cavity surface-emitting lasers," APPLIED PHYSICS LETTERS 78, 3574-3576 (2001).
**B. R. Bennett, R. A. Soref, and J. A. Del Alamo, "Carrier-induced change in refractive index of InP, GaAs and InGaAsP," Quantum Electronics, IEEE Journal of 26, 113-122 (1990).
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12. Conclusion and Future Work
•An optical, external cavity VCSEL based self-oscillator was presented
•The device was demonstrated to consistently change its self-oscillation
frequency with changing VCSEL drive current
•Useful as a variable for a frequency tuning feedback loop
•The drive-current tuning observations are found to be dependent on the QWP
rotational position and current-dependent changing VCSEL birefringence
Future Improvements
•Investigate the link between VCSEL drive current and birefringence
•Investigate ultra-stable current sources to eliminate jitter to narrow the
linewidth of the self-oscillation signal
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13. Acknowledgements
This work was sponsored in part by:
DARPA
The National Science Foundation’s MIRTHE Engineering Research Center
under Grant No. EEC0540832
National Science Foundation Grant No. 0903661 “Nanotechnology for Clean
Energy IGERT”
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14. Questions?
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15. Equivalent Cavity Length Tuning
µm-sized VCSEL vs.
cm-sized external cavity
Theoretical
Experimental Equivalent
Cavity Center Oscillation
Center Cavity Length
Length Frequency Tuning (Δf)
Frequency (fc) Change
f=c/4L
14.8cm 507MHz 500 MHz 64.2 MHz 19.2 mm
7cm 1.07GHz 1 GHz 74.3 MHz 5 mm
3.5 cm 2.14GHz 1.4 GHz 190 MHz 7.6 mm
3.75 cm 2.00GHz 2 GHz 74 MHz 1.39 mm
2.5cm 3.00GHz 2.5 GHz 56 MHz 0.64 mm
2.2cm 3.40GHz 3 GHz 134 MHz 1.08 mm
2cm 3.75GHz 3.5 GHz 157 MHz 0.91 mm
1.55cm 5.21GHz 4 GHz 188 MHz 0.83 mm
1.2cm 6.25GHz 4.6 GHz 285 MHz 0.98 mm
• Thermal expansion of the VCSEL cannot account for range of oscillation tuning
NanoStructure Laboratory PRINCETON UNIVERSITY 15
Editor's Notes
Now we show that changing the VCSEL drive current can also alter this self-oscillation frequency. We have observed this behavior for a every cavity length we have been able to build. A selection of such frequencies can be seen above. But what causes this effect?
We have observed that the self-oscillation frequency tuning correlates with L-I measurements. A polarizer rotate along the QWP neutral axes shows drastically different results for different cavity lengths. For longer cavities, the major and minor polarizations co-exist for very small current ranges; whereas, for smaller cavity they tend to co-exist over larger current ranges. This co-existence correlates pretty well with the self-oscillation tuning behavior. This data suggests that we cannot look at this cavity system as polarized light switching on and off but as a cavity of two standing waves. But it does not explain the tuning behavior…
Can the external cavity system be described as two-standing waves? Yes. Using Jones Matrix algebra we can show that this external cavity system consists of two orthogonally polarized standing waves each of which is aligned along the QWP neutral axes. Their separation frequency is f=c/4L. Still this does not account for changing cavity phase. It turns out that this is a 0-order description.
Changing the length of the external cavity clearly changes the self-oscillation frequency. This frequency has been shown to be defined by f=c/4L. Additionally, it has also been shown that changing the QWP rotation with respect to the VCSEL polarization axes can change the self-modulation frequency. I bring this up because it will figure into our discussion later.
We can remember the plot from before where we tuned the QWP and changed the VCSEL self-oscillation frequency. I’ve shown it here again. Ginovart et al. originally documented this behavior and they were able to come up with both an analytical and numerical model to explain it. Using their analytical model (which is based on using the the vectorial Airy function of the field inside the VCSEL) we can describe how in our experiment, the QWP’s rotation changes the self-oscillation frequency. This change in cavity phase heavily depends on the VCSEL facet reflectivity, the VCSEL birefringence, and the rotational angle of the QWP. You can see that (using reasonable values of r1,r2, and delta) we are able to reasonably match our analytical result with experiment. Can we use this to predict how the current affects the self-oscillation frequency?
Yes! It has already been shown in the literature that the VCSEL birefringence changes with drive current. Here we have plotted how changing birefringence for diff. QWP rotational positions can change the self-oscillation frequency of this system. You can see that to match our experimental results in this plot requires a birefringence change of at much as 25 GHz. I realize this is pretty high, but it is not unreasonable and can serve as a good starting point for explaining this self-oscillation tuning behavior. In the end,these results beg the question of “How does the birefringence in this device change with drive current?” Investigating this will be the purpose of future work.
Thermal expansion of the VCSEL cannot because the cavity length change required to see these changes in self-oscillation frequencies is larger than that of the laser cavity (taking into account index of refraction).
