Phys 726: JQI Rotation 1
Michael Jarret & Jiehang Zhang
Table of Contents
- Experimental Setup
- The Basic Setup
- Notes, Details, and
- Discussion of Results
- Moving Forward
First discovered by Penning in 1928, the
Ogtogalvanic Effect (OGE) is a useful laboratory technique for
obtaining spectroscopic data without the use of absorption or emission
spectroscopy. Despite an early discovery, the technique did not become
popular until 1976, when the tunable dye laser made the technique
practical. Since then, optogalvanic spectroscopy (OGS) has seen many
applications, including laser calibration and stabilizaton.[1,2]
In the long term, this project will create a laser
locking apparatus for a 399nm laser. Because OGS allows us to access
the spectrum of our discharge directly through an electronic signal, we
can use it as a feedback mechanism to stabilize our laser. If
implemented properly, this technique can lock the laser to a known
frequency to within a high degree of accuracy.
The first task in the development of this
apparatus is to effectively measure the optogalvanic spectrum of our
neutral Ytterbium hollow-cathode lamp. We constructed an apparatus to
observe the OGE.
The OGE is a cumulative effect of one or more
different ionizing reactions.  (For a list of the
ionizing mechanisms, see .) When a laser at or near
transition wavelength strikes the discharge, the ionization balance of
the discharge changes. Because electrons are excited to higher energy
states, there is a net effect on the electrical properties of the
discharge. Although it can be observed in multiple ways, we observe it
as a change in impedence across the discharge.  The
electrical effect is sufficiently small that we can consider our
observed effect as "directly proportional to the numer of photons
In the case of Ytterbium, we can stimulate a
transition between (6s 6p)1P1
and (6s2)1S0 by using 399nm light.  A diagram of the energy levels of neutral Ytterbium
(taken from ) is shown below:
The Basic Setup
We adopt the standard experimental setup for an OGE
measurement. Light from a 399nm laser is
collimated and sent through a chopper wheel set to around 1.1kHz. This
light is then focused on the Ytterbium lamp. A reference frequency is
provided to a Lock-in Amplifier  by a Sharp
A high voltage supply (HVS) set to ~300V is
connected across a
Hollow Cathode Ytterbium Lamp in series with a 30kΩ Ballast Resistor.
The lock-in measures the potential across the resistor, with a 100nF
capacitor acting as a high-pass filter. (See Figures 1 and 2)
|Figure 1. A
power supply is
connected to a hollow cathode Ytterbium lamp in series with a Ballast
Resistor. The OGE is measured using a Lock-in Amplifier.
Actual realization of
Notes, Details, and
The HVS could be limited by either current or potential.
We found that when current limited, the signal was highly unstable, so
that we typically voltage limited the supply. Also, we observed a more
discernable signal for lower currents, and thus we chose to operate at
around 300V with an approximate current of 6mA. (Note: the lamp
requires about 210V to fire and about 130V to maintain operation.)
The chopper wheel has a slight wobble, which causes
some complications when attempting to lock the signal. Using the
photointerrupter improved the stability of the reference frequency and,
when operated at around 1kHz, allowed us to lock to the chopper
frequency. (The layout of the pins for the photointerrupter are
diagrammed elsewhere. See References.)
Additionally, there might be a warm-up time
associated with the lamp (discussed in detail below). After attempting
to observe the OGE for a brief period, we observed the signal make a
180 Degree phase change. Only after this phase change did the effect
Last, we discovered that to observe the OGE the
laser light must land directly in the center of the cathode. We are
still unsure of the reason, but when the light lands elsewhere an
effect (not believed to be the optogalvanic effect) is produced. This
effect may disappear if the orientation of the lamp is altered. (See
Our first observation of the OGE was incomplete. (Figure 3) The
spectrum produced by the impedance change is approximately what one
would expect for the doppler-broadened spectrum of Neutral Ytterbium.
We were unable to complete this data set because of an observed shift
in impedance while at constant frequency. (Figure 4)
|Figure 3. The first observation
of OGS. The data is incomplete between 751526 and 751527 GHz.
Impedance shift as a function of time at constant frequency. The
observed discontinuity around t=300s is because of an intentional
retuning of the laser frequency.
After seeing this time dependent nature of the
impedence, we altered our experimental procedure. In between
consecutive data points, we returned the laser to a reference
frequency. Then, our data were plotted with respect to the observed
impedence at this reference. (Figures 5 and 6)
|Figure 5. Second run, fitted
with a Lorentzian.
Second run, fitted with a gaussian
Discussion of Results
We believe that we have observed the OGE. Our
measurements were, however, not precise enough to form a practically
useful spectrum. From Figure 5 we can see that the Lorentzian
profile does seem to fit the data, with an observed width of around
1.5GHz. Figure 6 shows that a
Gaussian seems to more accurately describe the data of the right-hand
This time-dependency is quite strong and, as one can
readily observe from Figure 4, fairly consistent. (Although, this
signal was observed to both increase and decrease at different
occassions.) Because we observed an anomalous 180 Degree phase shift
and a continual change in impedance, we are led to speculate that there
may be some continual, potentially predictable, dephasing between the
lock-in and the OG signal. Alternatively, there may be some thermal
effects or signal contributions not specifically observed.
Before proceeding to use this technique as a
laser-locking method, we need to work out either precisely what causes
the observed time-dependent effects or a means of counteracting them. A
first step would be to attempt a longer term observation of the
impedance change as a function of time with the laser held at constant
frequency. Additionally, we might change the orientation of the lamp.
Because we are unsure of the precise construction of
the lamp, we are not convinced that the light is landing on the
Ytterbium alone. It is possible that, in addition to the OGE we are
also seeing the photoelectric effect, or some other interaction between
the laser and the electronics. Changing the direction that the laser
enters the lamp may enable the beam to land primarily upon the
Ytterbium while avoiding other electronic components.
Once the time-dependency issue is worked out, one
might proceed by attempting to sharpen the spectrum peak. One means of
accomplishing this is to use saturation spectroscopy. 
(As a final note, it might be worthwhile to attempt
to find a means of mounting the chopper wheel in a more stable fashion.)
 Goldsmith, J. E. M., and Lawler, J. E., 1981,
Contemp. Phys., 22, 235.
 Barbieri, B., and Beverini, N., 1990, Rev. Mod.
Phys., 62, 603.
 Andrés David Cimmarusti's Rotation Page
 Ohmukai, R. et al., 1994, Jpn. J. Appl. Phys., 33,
 Dunlap, R. A., 1988, Experimental Physics, 102-106
(photointerrupter pin layout)
Thanks to Prof. Chris Monroe for providing us with a project and
workspace for this rotation.
Thanks to Dr. Wes Campbell for his imeasurable amount of help and
Thanks to Dr. Qudsia Quraishi for her help and advice.
Thanks to Dr. Kihwan Kim for his help in using the 399nm laser.
Thanks to Prof. Luis Orozco for organizing the course.