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A frequency-stabilised diode laser and its application to atomic spectroscopy of caesium : a thesis presented in partial fulfilment of the requirements for degree of PhD in Physics at Massey University
This thesis examines the hypothesis that a commercially available single-chip AlGaAs
diode laser can be rendered suitable for atomic physics research through the addition of
external circuitry alone. This circuitry is discussed and the frequency-stabilised laser is
applied to some interferometric and spectroscopic studies. These studies also include
the development of theoretical models.
The design and construction of an ultra-stable constant current source is discussed,
along with an evaluation of its effectiveness. The current source is capable of
supplying up to 100-mA de with the addition of a ramp current of up to 20 rnA to allow
frequency scanning of the laser, and exhibits short-term fluctuations of ±8 JlA with a
drift of less than 2 J1A in ten hours. A temperature controller, capable of both heating
and cooling the laser, is described. This can maintain temperature stability to ±1 mdeg.
A data acquisition and control unit is designed and constructed to enable the interfacing
of both the laser and other measurement apparatus to a micro-computer.
A system for automatically measuring wavelength maps as a function of injection
current and laser temperature is developed, utilising the data acquisition and control unit
in conjunction with a grating spectrometer. This is applied to four diode lasers.
Linearity measurements are made of the frequency scan with injection current. The
linewidth of one laser is measured as a function of its output power by heterodyning its
output with that of a second laser operated at fixed injection current and temperature,
and observing the beat note on a rf spectrum analyser. The linewidth can be reduced to
below 10 MHz. The absolute frequency stability of the laser is measured by
monitoring the change in absorbed power over time when the laser is tuned to the D2
transition of caesium. The drift is measured to be 10 kHz/s over five hours, and is
attributed largely to temperature drift.
An analysis is presented which allows the spectral linewidth of a light source with a
Lorentzian or Gaussian profile to be calculated from the measured fringe visibility of
the transmission fringes of a high-finesse scanning Fabry-Perot interferometer when
these fringes are not fully resolved. The method is also applied to truncated spectral
profiles, which may provide a more appropriate model for the spectral profile of some
lasers, and the differences are characterised. The analysis is then extended to
interferometers with low finesse and also to include the Voigt profile. An experimental
verification of the analysis is made using a diode laser whose spectral profile is
demonstrated to be Lorentzian in shape.
The frequency-stabilised diode laser is applied to the study of saturated absorption
spectroscopy of the caesium D2 line at 852.1 nm. Both single-beam and two-beam
experiments are performed using an absorption cell containing caesium vapour with no
buffer gas. A theoretical model is developed, based on rate equations for the
population densities of the 48 sublevels involved in the transition, including a rate for
ground state relaxation due to transit of atoms through the laser beam. The model
facilitates the analysis of the effects of optical pumping and extent to which the ground
state relaxation rate limits the optical pumping. For the medium-resolution single-beam
experiments the laser is shown to be a suitable source for resolving the Doppler-broadened
transitions, and good agreement is obtained with the model for sufficiently
low laser powers. However, for the sub-Doppler two-beam experiments the limited
frequency stability is revealed. An analysis of the lock-in detection process for the two-beam
experiments is made, emphasising the relationships between the detection
frequency, the ground state relaxation rate, and the shape of the detected signal. It is
discovered that if the detection frequency is of the order of the ground state relaxation
rate then a simple subtraction of the Doppler-broadened background from the probe
absorption signal does not adequately model the lock-in detection process. A technique
is described to experimentally determine the ground state relaxation rate from the
detected signal as a function of the lock-in phase.