Transcript Frequency Bistability in a Diode Laser Using Diffraction Gratings Forrest Smith1 , Weliton Soares2, Samuel Alves2, Itamar Vidal2, Marcos Oria2 1State University of.

Frequency Bistability in a Diode Laser
Using Diffraction Gratings
Forrest Smith1 , Weliton Soares2, Samuel Alves2, Itamar Vidal2, Marcos Oria2
1State University of New York at Geneseo, 2Federal University of Paraiba
Methods and Theory
Objectives
To achieve and demonstrate frequency
bistability in a diode laser system using
entirely mechanical means, hopefully
improving on the quality of previous
successes.
Introduction
• Bistability is a useful and important phenomenon
exploited in experimental physics using diode
lasers – Amplitude, Polarization, and Mode
bistability are well established
• In 2005 Farias et al. demonstrated the first instance
of frequency bistability at UFPB using orthogonal
feedback modulated in frequency by atomic cells.
[1]
• The cost and complexity of
• To ensure noticeably large feedback
atomic cells make a mechanical
modulation, two diffraction gratings were [2]
2
𝑘𝜆
Δ𝜈
1
cos 𝛾+𝛽1 1
alternative attractive.
used.
Δ 𝛽2 =
[ +
]
𝑐 cos 𝛽2 𝑑2
cos 𝛽1 𝑑1
• This project uses diffraction
• The final change in output direction for
gratings, which disperse light at
some given feedback was theoretically
angles dependent on frequency:
modeled to discover dependencies and
a feature which is analogous to
inform the placement of gratings in the
Final equation for angle
atomic absorptive behavior.
experimental set up.
change emerging from two
diffraction gratings.
Example of Atomic Absorption Intensity
[1]
𝜷𝟏 is the first angle of
reflection , 𝜷𝟐 is the second
angle of reflection, 𝜸 is the
angle between the diffraction
gratings , 𝚫𝝂 is the frequency
in the laser caused by
feedback , k is diffraction
order (same for both) , 𝝀 is
laser wavelength , c is the
speed of light , and 𝒅𝟐 is the
grating slit width for the
second width.
Example of Diffraction Grating Transmission Intensity
-1
0
1
Diffraction Order
Results
Frequency Shift (GHz)
• The feedback of diode lasers are sensitive to both
temperature, current, and feedback
• Great care was taken to control ambient
temperature and cool the diode.
• Current and feedback are both dynamic quantities
during this experiment. As such, the relationship
between frequency and each value was plotted.
Frequency shift (GHz)
• First, theory was confirmed using a simplified system
without feedback. Only two diffraction gratings, a 20
µm pinhole, and an intensity detector were used.
• The second diffraction grating face was
approximately 35 cm away from the pinhole, which
itself was close (~2 cm) from the detector to avoid
diffraction.
• By modulating the diode's current in this simple
system, feedback was emulated, and the laser
underwent spatial oscillations along the face of the
pinhole, which had a width of 20 µm.
4
Conclusions & Prospectives
• Bistability appears to have been achieved, with this
experiment achieving an oscillatory amplitude of
approximately 18.2 MHz in frequency shift. This
corresponds, in Eq. [2], to a Δ 𝛽2 value of 1.83x10-9
radians
• However, to confirm this result, a full mathematical
model must be created, and predictions therein must
be compared to this and replications of this
experiment
• Further work can also be done to examine the time
evolution of the frequencies in order to characterize
the nature of the frequency oscillations.
Oscilloscope image without feedback. Yellow line represents scanning
cavity length, which causes green spikes where frequency is in
resonance.
Frequency Shift v. Current
3
2
y = 1.4882x - 135.08
R² = 0.9971
1
0
90.5
25
91
91.5
92
92.5
Current (mA)
93
93.5
94
• With feedback, clearly defined and separated pairs
of peaks were produced implying bistability, evolving
over time as pictured below.
Frequency Shift v. Feedback Power
20
15
y = 1.845x + 2.7517
R² = 0.9799
10
Oscilloscope image with feedback. Dual peaks where there were
singular peaks implies successful oscillation between two frequencies.
5
0
0
• The wave with humps shows the intensity of laser
signal passing through the pinhole over time.
• The distinct bumps, separated by long flat periods
confirms that the laser is oscillating well past and
back over the pinhole, confirming theory.
• Pictured below, the green peaks represent the
frequency of the laser beam without feedback. The
sharp profile implied a very narrow distribution, and
thus essentially a singular frequency
2
4
6
8
Feedback Power (mW)
10
12
14
• The feedback power relationship is crucial for
analyzing the dynamics which diffractive feedback
produces as the feedback power is a measurable
which informs the maximum and minimum dynamics
References
[1] B. Farias, T. Passerat de Silans, M.
Chevrollier, and M. Oria, Physical
Review Letters 94, 173902 (2005):
Frequency Bistability of a …
[2] C. Masoller, T. Sorrentino, M.
Chevrollier, and M. Oria, IEEE Journal
of Quantum Electronics, Vol. 43, No. 3,
March 2007:
Bistability in Semiconductor …
Acknowledgements