Del Mar Photonics - Ge etalons
Del Mar Photonics featured customer presentation:
A New Quantum-Cascade Laser Based Spectrometer for High-Precision Airborne CO2 Measurements
Rodrigo Jiménez et al. Harvard University – Department of Earth and Planetary Sciences
We present a new,
compact, fast response mid-IR laser spectrometer for high-precision airborne
measurements of CO2. The instrument is based on a
thermoelectric-cooled, pulsed-operated DFB quantum-cascade (QC) laser. Unlike conventional cryogenically-cooled Pb-salt diode lasers, QC lasers
display high mode purity and wavelength stability, and can be operated at near room temperature. This last attribute allows for a compact design and
The CO2 concentration is derived from direct absorption dual-cell spectra obtained by electrical modulation at ~5-10 kHz of the laser wavelength across
a selected ν3-band transition at around 4.3 μm (typically 2311 or 2314 cm-1). The measurements are thus fully specific of the CO2 molecule and free
from interference of H2O or other mid-IR light absorbers. Sample gas humidity is nevertheless reduced to less than ~100 ppmv in order to restrain
density variation effects.
Absorption spectra of the sample and a flowing standard (reference) along a 10-cm (or 5-cm) path are simultaneously detected by LN2-cooled InSb
detectors. The CO2 concentration difference is retrieved from the differential spectrum (sample/reference). The advantages of this “null” mode
operation are discussed in detail.
The spectrometer includes a mechanism that allows directing the sample beam either to the cell or to a 25-mm Ge etalon for accurate wavelength tuning
rate determination. Additional technical details are discussed, including the impact of laser linewidth on the linearity of the measurements.
The spectrometer is enclosed in a temperature controlled, hermetically sealed vessel. The enclosure is flushed with CO2-free dry air previous operation
in order to avoid light absorption in the external path. Details on the gas temperature, pressure, and flow rate controls are also presented.
The demonstrated short-term precision of the instrument is better than ~75 ppbv Hz-1/2 (1-sigma in 1-s integration time) for CO2 concentrations within
±100 ppmv of the reference concentration. An accuracy of ±0.2 ppmv or better is insured through periodic calibration with high, low and “archival”
Quantum Cascade Lasers Calibration
My application is calibrate the relative wavelength of tunable quantum cascade lasers in the mid-infrared (~10 microns). The laser itself is used for remote sensing of atmospheric gases.
FSR (Free Spectral
Range) is determined by etalon length L and wavelength of operation
FSR= (λ2) / (2*n*L)
The most popular
standard etalon is 1 inch long
For longer wavelength (> 10 micron) it's often necessary to use etalons 2-3 inches long to get required accuracy of calibration.
Del Mar Photonics offer variety of standard as well as custom germanium etalons.
Here are few examples of Ge etalon specifications:
etalon part number #GEFPE1/0.5
request a quote - sold in
Greyhawk Optics nline store as part number
Length 1.00 +/- 0.01 inch
Diameter 0.50 +/- 0.05 inch
1/4 lambda flat each end, HeNe
parallel to 3 ARC SEC or better
length measure to 0.0005"
surface quality F-f(80/50) per MIL-F-48616
Ge etalon part number #GEFPE3/1.25 - request a quote
Length 3.00 +/- 0.01 inch
Diameter 1.25+/- 0.05 inch
Other specification as in #GEFPE1/05
Ge etalon part number #GEFPE20mm/1.5
Ge etalon part number #GEFPE20mm/1.5 - request a quote
dia 38.1 (-0.1) mm
thickness: 20 (+/-0.1) mm
Flatness: 1 fringe ( Lambda/2 at 633 nm) , delta N = 0.2 fringes
Surface quality: 60-40 s/d
Parallelism: < 2 sec
Perpendicularity: 5 min
|Model||Product Name+||Buy Now|
|CPPE-Ge-38.1-4||Germanium circular plane parallel etalon, ø 38.1 mm x 4 mm|
|E-Ge-12.7-25.4||Germanium etalon, ø 12.7 mm x 25.4 mm (ø 0.5" x 1")|
Using Ge etalon in gas sensing project involving quantum cascade lasers
Jagadeeshwari (Esha) Manne, PhD student, University of Alberta
Project: Infrared absorption
spectroscopy with Quantum cascade laser.
I am a graduate student pursuing my doctoral studies at the University of Alberta. The Ge- etalon will be used for non-profit research here at the university. I am presently working on a gas sensing project involving Quantum cascade laser.
The mid-IR region is ideally suited for absorption spectroscopy since the fundamental vibrational transitions of a number of molecules lie here. Quantum cascade (QC) lasers fabricated by band structure engineering offer an attractive new option for sensitive IR absorption spectroscopy. Distributed feed-back pulsed QC lasers are of particular interest for they allow the realization of a room temperature, compact IR source with relatively high output power. In particular, their combination with the pulsed-CRDS technique has the potential for high sensitivity trace gas analysis. The goal of our present work is to develop a technique for quantitative online analysis of trace constituents in exhaled breath. We are using pulsed-CRDS technique in combination with a mid IR QC laser operating near 970 cm-1 to measure ammonia levels in exhaled breath. An unlocked ring-down cavity was designed for these studies which can be operated at atmospheric or reduced pressure. We have already attained a detection limit of 25 ppb for ammonia. We are in a process of improving the system to get down to sub ppb levels with this technique.
The etalon will be used for characterization of the laser which would be an important step in choosing the right parameters for the laser for gas-sensing applications.
Del Mar Photonics>