BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the invention can be obtained from the following description of preferred embodiments in connection with the claims and the drawings. The single features can be realised alone or in combination in embodiments of the invention. The figures show:
FIG. 1 a principle depiction of an infrared photo acoustic gas sensor;
FIG. 2 a principle depiction of a near-infrared photo acoustic gas sensor
FIG. 3 the laser line scanned during each modulation cycle across the absorption features of the gas;
FIG. 4 the comparison of the laser wavelength width to the absorption features of the gas;
FIG. 5 a block diagram of a first embodiment of a gas sensor;
FIG. 6 a block diagram of another embodiment of a gas sensor, and
FIG. 7 a block diagram of a further embodiment of a gas sensor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 and FIG. 2 show the well-known principles of the different photo acoustic sensors as discussed above.
FIG. 3 and FIG. 4 show the gas absorption strength GAS and the laser intensity LI in dependence of the wavelength WL. The figures show the variations of the gas absorption. The laser intensity peak locked to a defined wavelength as shown in FIG. 3 only detects the gas absorption at this defined wavelength. In view of the differences of the absorption strength of the broadband spectra, a 0.1 nm shift to one side of the laser wavelength may lead to differences in the absorption strength of about 30-50%. Thus the results also can vary in a wide range. By sweeping the laser peak across the wavelength as disclosed in FIG. 4, the absorption features of the target gas are levelled out to a mean value.
Although in the following the invention is described in connection with a laser diode as laser source, the invention is not limited to this device and other appropriate laser sources having similar features might be used for obtaining similar results. Further, as known in the art, more than one laser source 1 with appropriate detection means may be used.
FIG. 5 shows the principle of the gas sensor with processing means 15 for the processing of the signals. A laser source 1, preferably a diode laser, which is in connection with temperature unit 7 acting as heater or cooler, emits light through a chamber 5 providing an absorption volume 4 for a target gas to be detected. A microphone 3 is arranged near the absorption volume 4. A photo diode 6 serves as photo detector and receives the light from the laser source 2. The laser source 1 is set by its temperature, via the temperature unit 7, to a wavelength, which corresponds to the absorption features of interest. A modulation frequency generator 9 comprises a square modulation means 10 providing a square modulation signal SSM and a saw tooth modulation means 11 providing a saw tooth modulation signal SSTM. The drive current of the laser source 2 is then modulated with a modulation signal SM, which is the multiplication of the signals SSM and SSTM. The square modulation means 10 provides an on-off modulation of the laser source 1 with a duty cycle of 50% which is ramped, due to the saw tooth modulation 11, by its drive current from its wavelength at threshold current to its maximum wavelength corresponding to the maximum drive current.
The microphone 3 provides a signal SA, which is proportional to the absorption of the gas in the absorption volume 4 and the photo diode 6 provides a signal SI, which is proportional to the light intensity of the laser source 1. The signal SA provided by the microphone 3 is fed to a lock-in-amplifier 12 for multiplying this signal with a reference signal SRef received from the modulation frequency generator 9 and finally integrating of the resulting signal. The signal from the photodiode 6 is fed to an amplifier 13. The signals from the lock-in-amplifier 12 and from the amplifier 13 are fed to a control unit 14. The control unit 14 the absorption signal SA after having processed by the lock-in-amplifier 12 is normalised by dividing through the intensity signal SI from the photodiode 6 after having amplified by amplifier 14. The resulting signal SGC is the required signal for the concentration of the gas in the chamber 5. The control unit 14 further provides respective signals ST to the temperature control 8 to keep the laser source 1 on its temperature to a wavelength, which corresponds to the absorption features of interest.
FIG. 6 shows another embodiment in which the modulation frequency generator 9 only comprises a sine wave modulation 16 providing the laser source drive current modulation signal SM. The sine wave modulation is a rectified sine wave modulation, which is obtained either by reversed negative parts of the sine wave or by deleting of the negative parts, e.g. by a laser diode.
A further embodiment is depicted in FIG. 7. There the modulation frequency generator 9 comprises the square modulation means 10 and a temperature modulation means 17. The square modulation means 10 provides a first modulation signal SM1 for modulating the laser source drive current and the temperature modulation means 17 provides a second modulation signal SM2 for modulating the temperature of the laser source 1 via the temperature unit 7.
Patent Application
20080073536
