Village Project Pages:Technology Details

Village Project Technology Details

Orientation Patterned GaAs

The picture shows a cross-section of an orientation-patterned gallium arsenide (OP-GaAs) crystal which has been grown using Hydride Vapour Phase Epitaxy (HVPE). The crystal growth direction is from the top to the bottom. OP-GaAs consists of GaAs where the crystal orientation is alternating periodically along the length of the crystal. In the picture the domains are apparent as vertical stripes in two different shades of grey. The brighter horizontal lines are results of the growth stops during the crystal growth process.

GaAs has a wide transparency range in the infrared region and a large non-linear coefficient which makes the material well suited for use with non-linear wavelength conversion techniques. Unlike traditionally used crystals like periodically poled lithium niobate (PPLN), GaAs cannot be poled electrically, so in order to achieve the periodic reversal of the effective non-linear coefficient required for quasi-phase matching (QPM) the GaAs crystal needs to be grown with the crystal orientation alternating in a periodic pattern along the length of the crystal, hence the name orientation-patterned gallium arsenide.

Fibre Lasers

Fibre laser.

The traditional approach for generating laser output in the ~2µm wavelength regime, using Tm-doped (Tm=Thulium) crystals or Tm, Ho co-doped (Ho=Holmium) crystals as the laser gain medium, suffers from the disadvantage that the transition line widths are typically rather narrow. As a consequence, the wavelength tuning range is very limited.

An alternative and more promising approach is to use a fibre-based source. Fibre sources benefit from a geometry that allows very simple thermal management and hence immunity from the effects of heat generation. Moreover, the relatively high gain in combination with the broad transition line width in a glass host allows very broad wavelength tunability.

Preliminary studies also indicate that Tm-doped fibre lasers have the potential for scaling to very high output powers and can be operated in a distributed feedback (DFB) cavity configuration to produce a single-frequency output, in a compact, robust and cost effective manner.

OP GaAs growth using HVPE

OP-GaAs is prepared from a template that imposes the alternating crystal orientation with the desired period along the length of the crystal during the following growth. On a single wafer several OP-GaAs crystals may be grown. The crystal growth takes place in a hydride vapour phase epitaxy (HVPE) chamber. After growth the wafer with the crystals is cleaved and polished to produce the final OP-GaAs crystals.

OPO growth steps OP GaAs Fabrication using Hydride Vapour Phase Epitaxy.

HVPE Machine HVPE chamber in clean room for OP GaAs fabrication.

OP GaAs Crystal

What is DFG?

A setup for Difference Frequency Generation (DFG) consists typically of two input lasers (called pump and signal) and a nonlinear optical crystal. The two laser beams are superimposed in the conversion crystal. If the conditions for momentum and energy compensation are fulfilled (so called phase matching) a third wave with the difference frequency of pump and signal waves will be generated. This third wave is also called idler wave. A typical setup is shown in the picture.

DFG set-up picture DFG set-up in the lab. The light grey box with the red wires contains the OP GaAs crystal.

What is an OPO?

An optical parametric oscillator (OPO) consists typically of an optical pump source, an optical resonator, and a nonlinear optical crystal. The latter is able to convert the frequency of the pump laser into two lower frequencies called signal and idler frequency, respectively. Signal and idler frequency depend on the temperature and grating of the nonlinear optical crystal (so called phase matching). The interaction of the three waves in the nonlinear optical crystals inside the resonator (which is resonant for at least one of signal and idler waves) leads to an amplification of the signal and/or idler wave and an attenuation of the pump wave. When the pump power reaches a specific threshold level, oscillation inside the resonator occurs and the output power of the OPO increases very rapidly with the pump power. Thus very powerful radiation with a very good beam quality can be generated.

See also at Wikipedia: OPO and Nonlinear optics, phase matching

What is single line spectroscopy?

If done properly single line spectroscopy will ensure that the measurement of gas is free from interference from other gases i.e., you will not get a higher reading of methane (CH4) if the humidity increases if your target gas is methane.

Using single line spectroscopy you will measure the gas concentration using one single absorption line that is free from interference from other gases i.e., other absorption lines for other gases must be located a certain distance away from your selected line. Also for this to work you must have a sufficiently good spectral resolution so that you can discriminate between the different lines. Tunable diode lasers (TDL) and lasers in general are well suited for this kind of applications since all of the energy is emitted in a very narrow wave length range giving high spectral resolution.

The figure below shows a spectrum of water vapour (H2O) and hydrogen fluoride (HF) illustrating single line spectroscopy. Plotted Spectrum This figure shows water vapour (thin line) and hydrogen fluoride, HF, (thick line). As you can see the HF is well separated from the other absorption lines.

Let us assume that you work at a petrochemical plant and that you have installed gas detectors that should detect leaks of methane or other hydro carbons. Let us also assume that these sensors have problems with interference from water vapour which actually is the case for many commercial gas detectors. What then will happen is that all is well during the winter, but when the summer sets in the relative humidity rises and the temperature rises which leads to a significant increase in the absolute humidity and therefore the response from water vapour lines is much greater than usual and then the detector detects "something" and triggers an alarm. Then the complete petrochemical plant has to evacuate due to a gas alarm which of course is a false alarm. So if you get a gas alarm 3 times a day in the warmest season, buy gas detectors based on single line spectroscopy next time.

Why go for the Mid Infra Red (MIR) wavelength range?

Today many lasers exists around 760 nm, 1.3, 1.5, 1.7, 2.0 and 2.3 µm making it possible to measure many gases using Tuneable Diode Laser (TDL) spectroscopy. Why then try to develop new lasers for the Mid Infra Red region?

The gas absorption lines in the 1.3 to 2.3 µm region are typically overtones of lines at longer wavelengths and these lines at longer wavelengths have much stronger absorption making it possible to detect lower gas concentrations. This is of special importance since allowed and recommended emission levels are getting lower and lower. Often new research tends to conclude that emissions have greater impact on peoples health and the environment than previously thought. To comply with new regulations the industry needs new instrumentation with lower detection limits to measure gas emissions.

In addition some gases do not have absorption lines in the 1.3 to 2.3 µm region, but they have absorption lines in the MIR region. One example of this is SO2, which is an unhealthy gas in addition to causing acid rain.

One important additional aspect with the MID IR lasers developed in this project is that they will have a wide tunability range i.e., you can scan the laser wavelength over a larger wavelength interval. This makes it possible to scan across absorption lines for several absorption lines making it possible to measure several gases using one laser. This is possible for some gas combination even today, but it typically limited to 2 or 3 gases, -often a combination of water vapour and one other gas. New lasers developed in the VILLAGE project make it possible to make one gas monitor capable of measuring multiple gases with superior accuracy and extremely low detection limits. This will improve instrument performance compared to current technology and at the same time reducing cost for installations requiring monitoring of many gases.

As a summary the development of MIR lasers in the 4 to 9 µm region for the VILLAGE project will give us the following advantages:

significantly better (lower) detection limits
capability to detect new gases not possible with current TDL technology
capability to detect many gases with one laser in one instrument giving cost reductions

About the VILLAGE name

VILLAGE is an acronym for Versatile Infrared Laser source for Low-cost Analysis of Gas Emissions.

VILLAGE (Versatile Infrared Laser source for Low-cost Analysis of Gas Emissions) is a research project supported by the European Commission under the Information Society Technologies priority of the Sixth Framework Program (FP6).