Sources for MIR-NDS

As stated above the most common light sources for MIR-NDS are thermal sources, LEDs and lasers. Each of these has very different characteristics such as spectral profile, emitted power, level of power consumption, etc. Therefore let us review some of the basic characteristics of these sources.

1.4.1.1.1 Blackbody Sources

These probably are the most commonly utilized sources for MIR-NDS since there is a very wide range of them on the market. These sources are relatively simple, for instance a coiled resistance wire filament or a tungsten filament, they are fairly inexpensive and they radiate a considerable amount of power. The blackbody spectral radiance in terms

of power is given by [49]

(1.5)

where is the Planck’s constant , is the light speed

in vacuum , is the Boltzmann’s constant

, is the frequency in wavenumbers, and is the absolute source temperature. It is important to recall that . Consequently a hotter source radiates more power (Figure 1.7). Some of the main advantages of these sources are: a) emitted power over a wide spectral region, b) usually they do not require complex electric supply for operation, c) their life time is reasonably

large (around 3 years) and d) they do not require frequent calibration. The main limitation of these sources is their relatively high power consumption which can limit their use in portable instruments. In Table 1.5 we list some thermal emitters that are commercially available including some of their characteristics.

Figure 1.7- Spectral radiance of a blackbody at different temperatures.

1.4.1.1.2 Light Emitting Diodes LED

LEDs are another source option for MIR-NDS, these are small with low power consumption and their time response it is of the order of 10-9 s. These sources emit over a narrow spectral region and their power emission is relatively small in the order of µWatt. LEDs usually consume less power than thermal sources which can make them more suitable for portable instruments; however their application suitability can be limited by their lower power emission. Moreover LEDs spectral profiles are usually affected by temperature and current fluctuations. These fluctuations produce a spectral shift of the emission profile (Figure 1.8). These shifts should be taken into consideration when designing a MIR-NDS since it is necessary to analyse how much these possible shifts affect the overall sensor response. In Table 1.5 some examples of MIR-LEDs with some of their characteristics that are commercially available are listed.

1.4.1.1.3 Lasers

Lasers represent another source option for MIR-NDS, especially the semiconductors lasers. Lasers can have multiple longitudinal multimode or single mode emissions. Spectrally these emission modes can be very narrow (narrow line width) in the order of a very small fraction of a wavenumber (Figure 1.9). Using some laser characteristics it is possible to tune the emission modes, this means shift spectrally the emissions modes. For instance, in a semiconductor laser the emission mode can be shifted by changing the temperature of the laser ‘chip’ [50]. Usually for gas sensing the most used are the single mode lasers, since it is possible to tune the mode exactly to the frequency where one transition of the target gas occurs, in a later section we review this technique with more detail. Solid state lasers have power outputs in the order of mW (quantum cascade lasers). These lasers are sensitive to temperature fluctuations and as a consequence their spectral emission profile can shift (Figure 1.9b). The main advantages of the solid state lasers are their small size, high power emission and very narrow bandwidth. The main disadvantage is that they need more sophisticated temperature and current drivers to keep them operating at the correct wavelength.

Figure 1.8- Emission spectral profile for a LED with peak emission at µm. Graphic taken from Ioffe LED LTd.

Figure 1.9- Spectral Emission Laser a) Multimode (bottom) and single mode (top) laser. b) Spectral emission profile of a quantum cascade laser DFB electrically tuned along the wavelength. Graphs taken

Table 1.5- Examples of sources that can be used in NDS gas sensors.

Type of

Source Model and Supplier

Temperature / radiated power

Driver requirements Source Area

Modulated

Wavelength region

Thermal IR-12 by Scitec Instruments. K DC 4.5 V at 1.8 A (8 watts) _{3.5 3.5 mm}2

Steady All MIR

Thermal CS-IR-20 by Boston Electronics _K DC 5V at 0.8 A (4 watts) 1.5 3.5 mm2 Steady All MIR Thermal IR-55 by HawkEye Technologies K DC or AC 6.4V at 0.135A

(0.9watts)

1.5 1.5 mm2

Pulsed 2 - 20µm

LED LED42SC by Ioffe LED LTd 70 µWatt / 25 µWatt 0.4V @ 1A / 0.3 @ 0.2A mm Pulsed / CW 3.8 - 4.4 µm (FWMH)

LED LED47SC by Ioffe LED LTd 25 µWatt / 5 µWatt 0.9V @ 1A / 0.25 @ 0.2A mm Pulsed / CW 4.4 – 5.0 µm (FWMH)

LED LED30SC by Ioffe LED LTd 250 µWatt / 50 µWatt 0.4V @ 1A / 0.26 @ 0.2A mm Pulsed / CW 2.8 – 3.1 µm (FWMH)

Laser (QC-DBF) sb640 UP by Alpes Lasers SA _{0.5 mW @ 15ºC} 12.5V @ 1.26A Pulsed 2322 cm

-1

(4.304 µm) with FWMH 1 cm-1 Laser (QC-DBF) Sbcw1428 UP by Alpes Lasers SA 2.7 mW @ -20ºC 12.7V @ 0.45A CW 2305 cm

-1

(4.338 µm) with FWMH <1 cm-1

Laser (QC-DFB) nanoplus 9.5µm 1 mW @ <260K 6.2A 8 20 µm

2

Pulsed 9.55µm

Note: FWMH is for Full Width at Medium High