Redstone™ Optical Spectrum Analyzer Chapter 4: Description
Page 7 STN053070-D02
Figure 2833. Pulsed Source Measurement Dialog
Wavelength Calibration.
The instrument is factory-aligned so that the reference and unknown input beams experience the same optical
path length change as the interferometer is scanned. The effect of any residual alignment error on wavelength
measurements is less than 0.5 ppm; the input beam pointing accuracy is ensured by a high-precision ceramic
receptacle and a robust interferometer cavity design. No optical fibers are used within the scanning
interferometer. Similarly as described in Section 4.5, each measured wavelength in the spectrum is corrected
for the difference in refractive indices for the reference and measured wavelength. The refractive indices are
calculated using Ciddor’s formula as given by NIST with the temperature, pressure, and relative humidity
measured by sensors internal to the instrument.
4.7. Resolution and Sensitivity
The resolution of this type of instrument depends on the optical path difference (OPD) between the two paths
in the interferometer. In this case it is easier to work with wavenumbers (inverse centimeters) than wavelength
(nanometers) or frequency (terahertz).
Assume we have two narrowband sources, such as two lasers with a 1 cm
-1
energy difference: 6500 cm
-1
and
6501 cm
-1
. To distinguish between these signals in the interferogram, we would need to move 1 cm away from
the point of zero path difference (ZPD). The Redstone OSA305 can move 16 cm in OPD and thus spectral
features that are 0.0625 cm
-1
apart can be resolved. The resolution of the instrument can be calculated as
follows:
Here, is the resolution in pm, is the OPD in cm
-1
and is the wavelength in µm.
The Redstone OSA305 instrument can be set to one of four resolution modes: High, Medium High, Medium
Low, or Low, which correspond to OPDs of 16 cm, 4 cm, 1 cm, and 2.5 mm, respectively. See Figure 2 for a
visual comparison of the different resolutions for the OSA20xC and Redstone OSA305 instruments.
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