TELS employs a quartz tuning fork (mostly hidden by the metal top) to sense a targeted gas made to vibrate by a pulsating laser. The two wires visible in the opening between the two screws are electrical leads, not the tuning fork.

In a process called "resonant photoacoustic spectroscopy," the Tunable Environmental Laser Spectrometer (TELS) detects and identifies specific, targeted gases by making them produce sound waves that it can "hear." It can be used to continuously monitor the air inside the International Space Station, the Orion crew exploration vehicle scheduled to carry astronauts to the Moon, or any other habitat.

How it works

Pulses of laser light are beamed into an air sample within TELS. The light is tuned to a wavelength that is characteristically absorbed by one particular chemical of concern (for example, formaldehyde).

If the air sample contains the target chemical, those molecules absorb the light energy and heat up when the laser pulses on, causing the gas to expand. When the laser pulses off, the gas cools and contracts. These cycles of heating and cooling, expansion and contraction, produce sound waves that ripple through the gas. The frequency of the sound waves is that of the laser's pulsation rate (the number of on-off cycles per second). The amplitude of the wave (the "loudness" of the sound) varies directly with the concentration of the gas in the air sample.

The tuning fork.

Within the air sample, an electronic tuning fork -- the same kind that is used in quartz wristwatches -- vibrates at its own characteristic frequency (about 32 kHz). If the air sample surrounding it carries sound waves caused by laser excitation of the target chemical, those waves interact with the tuning fork and damp its oscillations. The tuning fork is extremely sensitive to such changes in its frequency. Measuring the change in the tuning fork's frequency thus reveals the amplitude of the sound waves and the concentration of the target gas in the air sample.

If the gas poses a threat or warns of an imminent fire, the device can alert the astronauts and mission control, and possibly activate an automatic system to correct the problem.

A bank of lasers, each tuned to a different wavelength, could test sequentially for a variety of substances -- such as oxygen, carbon dioxide and fire-precursor gases -- with a single sensor.

The lasers

The mid-infrared interband-cascade laser module, including a thermoelectric cooler, is about the size of a quarter. The laser chip itself is less than 2 mm long and about 500 microns wide. The whole integrated module consumes only about 5 watts of electricity.

TELS employs tunable diode lasers (TDLs) which are able to step through a range of wavelengths surrounding that at which each target chemical absorbs best, since it is difficult or impossible for a laser to hit the single optimal wavelength consistently.

To excite most of the targeted gases, TELS employs a variety of TDLs that are used commercially for telecommunications. For formaldehyde, the instrument uses a TDL originally developed at JPL for the Tunable Laser Spectrometer that will be part of the Mars Science Laboratory rover, which is scheduled to begin exploring Mars in 2010.

In contrast to the telecommunication lasers which operate at near-infrared wavelengths of 1.5 to 1.6 microns, this latter TDL employs a novel approach called "interband cascade" (IC) to generate laser light with wavelengths of about 3 to 5 microns, which is in the mid-infrared range. Lasers of these wavelengths are much more difficult to make than those that produce visible or near-infrared light.

The heart of the IC TDL is a series of very thin layers of semiconductor materials, constructed in such a way that when an electric current is applied, electrons move from one layer to the next, then to the next, and so on. In each succeeding stage (comprising a number of layers), the electron occupies a lower and lower quantum energy state. It can be thought of as an energy staircase, with each electron acting like a marble that drops from step to step to step. With each drop, the electron gives off energy in the form of a photon of infrared light.

The "energy staircase" of the interband-cascade laser. An electron drops to successively lower energy states, and emits a photon (hv) with each step.

The photon travels along a waveguide to a crystal mirror that, given the wavelike aspect of light's nature, allows some light to pass through and reflects some light back into the waveguide. At the opposite end of the waveguide, the light is reflected by another mirror, which sets it traveling back toward the first mirror.

The reflected light interacts with the electron as it makes its next jump to a lower energy state, and imparts its wavelength and other characteristics to the new photon that the electron releases. The new photon joins the rest of the light headed for the first mirror. Constructive interference of the light waves increases the intensity of the light.

When the light reaches the mirror, the process repeats, with some light passing through and some reflected back. The reflected light again interacts with an electron, producing another "identical" photon. Thus each electron, as it drops from step to step to step, produces multiple "identical" photons.

While all the photons are theoretically identical, in reality they are all within a narrow range of wavelengths -- close enough for many uses, but not for highly sensitive spectroscopy. So the light is filtered by a grating that permits only light within a still narrower range. This is known as single-mode (i.e., single frequency) emission.

Image produced by a scanning electron microscope, showing an interband-cascade laser. The DFB grating filters the laser light down to a very narrow range of wavelengths.

The interband-cascade laser has high quantum efficiency (producing multiple photons per electron) and high output power. Less than 2 mm long and about 500 microns wide, its tiny mass and volume make it ideal for space travel.

Unlike visible and near-infrared diode lasers, the mid-infrared IC laser needs to be operated at temperatures of about 240 to 260 K (about -33 to
-13 °C or about -28 to 8 °F). Ours is integrated with a thermoelectric cooler and can be conveniently operated at room temperature with a total power consumption not higher than 5 watts. This is a significant advance over the much less convenient liquid-nitrogen cooling system normally used for mid-IR lasers.

Advantages

TELS has high sensitivity, able to detect chemicals in concentrations as low as a few parts per billion in less than 5 minutes. It is rugged, with no high-precision optical alignment issues, and small -- about 0.8 liters for the sensor head, detector, laser head, and gas system.