Assessment of a Rubidium ESFADOF Edge-Filter as ... - tuprints

70 4 Experimental Investigations of the Rubidium ESFADOF
monitors the pump laser wavelengths. In the following only the injected pump
powers will be quoted. The corresponding pump intensities can be computed by
relating these powers to the measured cross section of the pump laser.
In contrast to the pump ECDL of the reference cell, this one is connected
through a custom feed forward electronic device to an additional frequency generator.
The feed forward electronics allow for a mode–hop free frequency scan of
the ECDL by establishing a linear dependency between the injection current and
the piezo voltage, while simultaneously inducing a phase shift of π between both
entities [94, 162]. In order to systematically investgate the influence of all accessible
experimental parameters on the transmission spectrum of the ESFADOF
device, two operational schemes of its pump ECDL have been employed:
1. Fixed pump wavelength scheme: Among all other experimental parameters
the frequency detuning of the pump ECDL with respect to the 5S 1/2 → 5P 3/2
transition center is a very important parameter. Hence it has to be held fixed
when investigating the influence of all other parameters. The ECDL’s temperature
and injection current have been appropriately tuned in order to operate
on the 5S 1/2 → 5P 3/2 transition. In addition, a fine tuning of the operating
wavelength within the ECDL’s mode–hop free tuning range is achieved by adjusting
the flat-top output voltage of the frequency generator. This defines via
the feed forward device the ECDL’s injection current and piezo voltage simultaneously.
Again, this scheme benefits from the robust setup of the employed
ECDL design, so that no additional locking mechanism is required.
2. Scanning the pump wavelength scheme: Complementary to scheme 1, all
other experimental parameters are held fixed when scanning the wavelength of
the ESFADOF pump laser. A mode–hop free tuning of the laser wavelength is
achieved by changing the output voltage of the frequency generator to a sawtooth
shape, which drives the ECDL injection current and the piezo voltage simultaneously
through the same feed forward device [94]. The frequency scan
of the pump ECDL is monitored permanently by feeding the output voltage of
the frequency generator, U FG , to the A/D-card of the measurement electronics,
while simultaneously recording the output of the wavelength meter. A correlation
between both entities is always possible as long as the turning points
have been recorded. This task, together with the linearization of the frequency
scale is carried out by the post-processing software (cf. Sec. 4.4).
4.4 Data Acquisition and Evaluation
The described setup allows for precise measurements of the transmission spectra
of the ESFADOF device. A spectrum is achieved by simultaneously tuning the
injection current and the piezo voltages of the tripod ECDL seeding the fiber amplifier
by two identical computer controlled 16 bit D/A- and A/D-converters [167].
Each D/A- and A/D-converter offers two output channels and 64 input channels.
Programmed by a custom developed LabVIEW interface they have been configured
to work synchronously in order to control the experimental setup and record