Pulse laser spectrometer

Abstract

A method of sensing a gas using a semiconductor diode laser, the method comprising applying a current pulse to the semiconductor diode laser, thereby causing the laser to generate a laser output pulse, where applying the electrical pulse to the semiconductor diode laser results in an increase in temperature, the generated laser output pulse comprises continuous wavelength chirp in a wavelength range; providing the generated laser output pulse to the gas sample region, wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan; the wavelength chirp is at least partially caused by an increase in laser temperature caused by the applied electrical pulse, the current pulse comprises an electrical pulse length or duration of at least 5 microseconds, and / or the length of the current pulse causes the increase in temperature to slow over the length of the current pulse, the continuous wavelength chirp is continuously slowed over the length of the pulse from an initial first rate of change to a second, slower rate of change, and wherein the detection and / or further sampling of the light output takes place at the second, slower rate of change.

Description

Field of the Invention

[0001] The present invention relates to a semiconductor diode laser spectrometer arrangement, particularly an infrared semiconductor diode laser spectrometer. Background Art

[0002] WO 03 / 087787 describes applying short-duration current pulses to a quantum cascade laser operating near room temperature to provide narrow-wavelength pulses. WO 03 / 087787 describes applying a sequence of electrical pulses that are substantially step functions to a semiconductor laser to output one or more laser pulses having a continuous wavelength chirp. WO 03 / 087787 describes applying pulses in the range of 150 to 300 ns (preferably 200 to 300 ns) to provide a tuning range of approximately 60 GHz. In WO03 / 087787, the wavelength variation provided by the wavelength chirp itself is used to provide wavelength scanning, so there is no need to tune the effective emission linewidth (up-chirp) across a spectral region by, for example, superimposing a slow DC current ramp on the pulse sequence.

[0003] WO 03 / 087787 describes that: due to controllable and predictable characteristics, an almost linear wavenumber chirp (as a function of time) allows the construction of a high-speed, sub-microsecond semiconductor diode laser absorption spectrometer. In this document, a pulse generator provides a plurality of pulses (rectangular) at the input of the laser. The generator provides a sequence of rectangular current drive pulses of fixed amplitude and sub-microsecond duration to cause a rapid laser heating effect, thereby causing the emitted semiconductor diode laser radiation to have a continuous wavelength chirp at a certain rate over time. The laser heating caused by the current pulse results in, for each pulse emitted from the laser, a continuous and almost linear spectral change of the chirp from short wavelength to long wavelength (defined as a continuous spectrum or wavelength scan). Summary of the Invention

[0004] According to a first aspect, a method for sensing a gas using a semiconductor diode laser is provided, the method comprising: applying an electrical pulse to the semiconductor diode laser, thereby causing the laser to generate a laser output pulse, wherein applying the electrical pulse to the semiconductor diode laser causes a temperature change such that the generated laser output pulse includes a continuous wavelength chirp within a wavelength range; providing the generated laser output pulse to a gas sample region, wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan; and detecting the light output from the gas sample region, wherein the current pulse includes an electrical pulse length of at least 1 microsecond. The wavelength chirp may be at least partially caused by a temperature increase of the laser due to the applied electrical pulse. The current pulse includes an electrical pulse length or duration of at least 5 microseconds. The length of the current pulse may cause the temperature increase to slow down over the length of the current pulse, thereby causing the rate of change of the continuous wavelength chirp to continuously slow down from an initial first rate of change to a slower second rate of change over the length of the pulse, and wherein the detection and / or further sampling of the light output is performed at the slower second rate of change.

[0005] The temperature change may include a temperature increase. Applying a current pulse to the semiconductor can have a heating effect on the semiconductor laser. The method may include sensing a gas, and / or sensing one or more compounds in a sample gas, and / or sensing one or more materials in a sample gas. The method may include identifying one or more compounds in a sample gas.

[0006] The method may include applying one or more current pulses to the semiconductor diode laser, thereby causing a temperature change of the semiconductor laser. The laser may output a continuous wavelength chirp, wherein the continuous wavelength chirp is at least partially generated due to a temperature change of the semiconductor laser.

[0007] The temperature increase may slow down over the length of the current pulse, thereby causing the rate of change of the continuous wavelength chirp to continuously slow down from an initial first rate of change to a slower second rate of change over the length of the pulse, and wherein the detection and / or further sampling of the light output is performed at the slower second rate of change. The temperature increase may slow down over the length of the current pulse, thereby causing the rate of change of the continuous wavelength chirp to monotonically decrease over the length of the pulse.

[0008] Detecting the light output may include: using one or more photodetectors to generate an electrical detection signal in response to the laser incident on the detector, and performing digital processing on the electrical detection signal.

[0009] The length of the electrical pulse can be long enough such that the rate of change of the generated wavelength chirp is slowed down on the generated wavelength chirp. The length of the electrical pulse can have a duration such that the rate of change of the generated wavelength chirp is slowed down on the generated wavelength chirp. The length of the electrical pulse can have a duration such that the generated wavelength chirp has a first rate of change at a first time and a lower second rate of change at a second time. The rate of change of the generated wavelength chirp can continuously slow down from the first rate of change to the second rate of change over the length of the generated consecutive wavelength chirps. The rate of change can slow down from the first rate of change to the second rate of change. The rate of change can be monotonically decreasing.

[0010] The length of the electrical pulse can be long enough such that the rate of change of the generated wavelength chirp is slowed down on the generated wavelength chirp by at least 25%, optionally 50%, and further optionally 75%.

[0011] The temperature can vary over the pulse length. The rate of change of the temperature can decrease as the pulse length gets longer. The rate of change of the generated wavelength chirp can be slowed down over the duration of the generated wavelength chirp. The temperature can vary over the pulse length due to the self-heating effect of the laser during pulse generation. The rate of change of the temperature can decrease as the pulse length gets longer. The rate of change of the generated wavelength chirp can be slowed down over the duration of the generated wavelength chirp.

[0012] The change in temperature can include an increase in temperature. The magnitude of the increase can depend on the laser. The change in temperature can include a change from a first temperature to a second temperature. The increase in temperature can be an increase in the range of 50 o °C to 80 o °C. The increase in temperature can include an increase in the range of 1 degree Celsius to 100 degrees Celsius. The increase in temperature of the laser can be slowed down over the electrical pulse length.

[0013] The current pulse can be long enough such that the increase in temperature of the laser is slowed down over the electrical pulse length, resulting in the chirp being non-linear, and / or the rate of change of the generated wavelength chirp is slowed down on the generated wavelength chirp.

[0014] The chirp can be represented by a polynomial and / or exponential function and / or other non-linear functions, and / or the length or duration of the current pulse is such that the consecutive wavelength chirps are non-linear in time.

[0015] The wavelength scan can include a portion of the wavelength chirp. The wavelength scan can include or correspond to a detection time window. The wavelength scan and / or the detection window can include up to 75%, up to 50%, up to 25%, up to 10% of the wavelength chirp.

[0016] The method can include: detecting and / or sampling the light in a portion of the wavelength chirp that has a slower rate of change.

[0017] The method may include: detecting and / or sampling light in a portion of a wavelength chirp having a rate of change in the range of 0.1 to 2 cm -1 per microsecond.

[0018] One or more characteristics of the wavelength chirp may depend at least on the temperature of the laser, and / or wherein the rate of change of temperature over the length of the electrical pulse decreases when an electrical pulse is applied, and / or wherein the temperature change may include a rise to a maximum temperature, where the maximum temperature depends on one or more of the ambient temperature and / or the cooling effect.

[0019] The pulse may be such that the frequency of the corresponding etalon trace decreases over time.

[0020] The wavelength chirp may include a decreasing chirp rate on the wavelength chirp or be characterized by such a chirp rate. The wavelength chirp may include a portion that occurs within or defines a detection time window. The portion may be characterized by a chirp rate within a desired range. The method may include detecting an output from a sensing region and / or processing a detection signal during the detection time window. The chirp may be non - linear.

[0021] The detection window may start at a time after 1 microsecond after the start of the pulse. The detection window may start at a time after 2 microseconds after the start of the pulse, optionally after 5 microseconds after the start of the pulse, optionally after 10 microseconds after the start of the pulse.

[0022] The wavelength chirp may include a decreasing chirp rate on the wavelength chirp, where the wavelength chirp includes a portion characterized by a chirp rate within a desired range, where the portion occurs at least within the detection time window, where the method includes detecting an output from a sensing region and / or processing a detection signal at least during the detection time window. The portion may include an available portion. The duration of the portion may be at least the size of the detection time window. The portion may define the detection time window. The duration of the portion may be the size of the detection time window. The desired chirp rate may be in the range of 0.1 to 2 cm -1 per microsecond.

[0023] The start of the detection time window may be fixed and / or may be synchronized with the laser output.

[0024] The method may include detecting and / or sampling the emitted light within the chirp length and discarding data collected outside the detection time window and / or only processing data collected during the detection time window.

[0025] The electrical pulse length can be between 1 microsecond and 200 microseconds, optionally between 100 microseconds and 10 milliseconds, and further optionally between 100 microseconds and 1 millisecond.

[0026] The current pulse can include an electrical pulse length of, optionally, at least 5 microseconds, optionally at least 10 microseconds, further optionally between 5 microseconds and 1 millisecond, and further optionally between 10 microseconds and 1 millisecond.

[0027] The first rate of change can be higher than 2 cm -1 per microsecond, and the second rate of change can be lower than 2 cm -1 per microsecond. Optionally, wherein the first rate of change is higher than 2 cm -1 per microsecond, and the second rate of change is in the range of 0.1 to 2 cm -1 per microsecond. Further optionally, wherein the second rate of change is lower than 0.1 cm -1 per microsecond.

[0028] The first rate of change can be higher than a predetermined value, and the second rate of change can be lower than a predetermined value. The predetermined value can depend on one or more parameters, such as the laser system.

[0029] The electrical pulse can include a current pulse.

[0030] The chirp rate can be slowed down by up to 25%, optionally up to 50%, and optionally up to more than 100%.

[0031] The detection and digitization circuit can include a photodetector with a bandwidth in the range of 1 to 10 MHz, and a digitization circuit with a bandwidth in the range of 1 to 10 MHz. The detection and digitization circuit can include a photodetector and a digitization circuit with a bandwidth in the range of 1 to 20 MHz.

[0032] The detection of the optical input can include detection and / or digitization by the photodetector and the digitization circuit, wherein the detection and / or digitization is performed at a wavelength chirp rate of 0.1 to 2 cm -1 per microsecond.

[0033] A reflective optical component can be used to direct the laser to the gas analysis region. The laser can propagate through free space between the laser and the gas analysis region. The laser can propagate through free space between the gas analysis region and the detector.

[0034] The pulse can be substantially constant in electrical pulse length, and / or wherein the pulse substantially includes a step function. The electrical pulse can include at least one of the following: a rectangular pulse shape and / or a flat top and / or a triangular top.

[0035] Electrical pulses can minimize the attenuation of the optical output power. The method can also include selecting one or more parameters of the electrical pulses.

[0036] The method can include changing the rate of change of the wavelength per unit time, for example, by changing the amplitude of the current / voltage drive pulse.

[0037] The method can include adjusting the wavelength scan length, for example, by changing the duration of the current / voltage drive pulse.

[0038] The method can include changing the temperature of the semiconductor diode laser.

[0039] The method can also include adjusting one or more of the pulse width, duty cycle, temperature, and voltage / current of the pulse.

[0040] The method can include controlling the starting temperature of the laser using a thermoelectric controller.

[0041] The starting temperature can range between -20°C and 50°C. The starting temperature can be controlled to control the initial wavelength.

[0042] The magnitude of the current pulse can depend on the laser. The current range of the current pulse can be between 0.4 A and 1.5 A. The current pulse can be below 2.5 A.

[0043] The magnitude of the wavelength scan can be between 0.1 and 10 cm -1 between.

[0044] The wavelength scan can include light having a wavelength characterized by a wavenumber range. The magnitude of the wavenumber range can be within a region of 0.1 to 10 cm -1 within.

[0045] The method can also include changing the temperature of the laser.

[0046] The wavelength of the output radiation can be in the range of 0.5 µm to 14 µm.

[0047] The semiconductor diode laser can be configured to emit laser light in the near-infrared to mid-infrared range, optionally emit laser light in the visible to near-infrared range, optionally emit laser light in the near-infrared to mid-infrared range.

[0048] The magnitude of the generated wavelength chirp can be in the range of 0.1 to 10 cm -1 The rate of change of the generated wavelength chirp can be in the range of 0.1 cm -1 to 10 cm -1 per microsecond. The detector and / or digitizing circuit can be configured such that the rate of change of the wavelength chirp is between 0.1 and 2 cm -1in the range of per microsecond and / or when the magnitude of the wavelength chirp is in the range of 0.1 to 5 cm -1 to sample the emitted light.

[0049] The optical unit may include a Herriott cell. The laser may include a semiconductor laser. The laser may include a quantum cascade laser, an interband cascade laser, or a tunable diode laser.

[0050] The method may include selecting and / or adjusting one or more operating parameters of the laser to adjust the duty cycle of the laser and / or substantially maintain the duty cycle of the laser, for example, maintaining the duty cycle below a target value. The target value may be less than 10%, optionally between 2.5% and 7.5%, further optionally less than 5%, and further optionally between 1% and 5%.

[0051] The method may include controlling the laser to reduce the pulse repetition frequency of the laser, optionally where the pulse repetition frequency is in the range of 0.01 to 5 kHz.

[0052] The laser output may be provided to the sample area without further modulation by another laser.

[0053] The wavelength chirp may be at least partially (optionally, substantially) caused by a temperature change of the laser induced by an applied pulse. The change in wavelength is caused by the temperature change and not by the modulation of the laser (e.g., a second external laser). The generated laser may include an unmodulated laser. The method may include wavelength and / or wavenumber tuning due to pulse chirping caused by temperature.

[0054] According to a second aspect, there is provided a semiconductor diode laser spectrometer for measuring the radiation absorption of a sample, comprising: a semiconductor diode laser; an electrical pulse generator configured to apply an electrical pulse to the laser, thereby causing the laser to generate a laser output pulse for a gas sample area, wherein applying the electrical pulse to the semiconductor diode laser causes a temperature change such that the generated laser output pulse includes a continuous wavelength chirp within a wavelength range, wherein the generated laser output pulse is provided to the gas sample area, and wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan; and a detector for detecting the light output of the gas sample area, wherein the electrical pulse length of the current pulse is at least 1 microsecond. The electrical pulse length of the current pulse may be at least 5 microseconds.

[0055] The wavelength chirp can be caused, at least in part, by a temperature increase of the laser induced by an applied electrical pulse. The temperature increase can be slowed over the length of the current pulse, resulting in the rate of change of the continuous wavelength chirp slowing continuously from an initial first rate of change to a slower second rate of change over the pulse length, and wherein the optical output is detected and / or further sampled at the slower second rate of change. The temperature increase can be due to an increase in the temperature of the gain or active medium of the laser.

[0056] The spectrometer can also include an optical unit, and at least a portion of the sample area is located within the optical unit. The optical unit can include a non-resonant optical unit.

[0057] The spectrometer can be part of an open path sensing system. The spectrometer can be part of a cross-stack system.

[0058] The detector can include a photodetector and / or digitizing circuitry. The bandwidth of the photodetector can be in the range of 1 to 10 MHz, and this bandwidth can be higher or lower than this range depending on the application. The bandwidth of the digitizing circuitry can be in the range of 1 to 10 MHz. This bandwidth can be higher or lower than this range depending on the application. The detector can include detection and digitizing circuitry with a bandwidth in the range of 1 to 20 MHz.

[0059] The detection and digitizing circuitry can include: a photodetector with a bandwidth in the range of 1 to 10 MHz; and digitizing circuitry with a bandwidth in the range of 1 to 10 MHz. The detection and digitizing circuitry can include: a photodetector and digitizing circuitry with a bandwidth in the range of 1 to 20 MHz. The detection of the optical input can include detection and / or digitization by the photodetector and digitizing circuitry, wherein the detection and / or digitization is performed while the wavelength chirp is in the range of 0.1 to 2 cm -1 per microsecond.

[0060] The bandwidth of the digitizing circuitry can be greater than the bandwidth of the digitizing circuitry. The sampling rate of the digitizing circuitry can be at least ten times, optionally at least four times, the bandwidth of the photodetector.

[0061] The laser can include an intra-pulse laser spectroscopy system, optionally wherein the intra-pulse spectroscopy is configured to detect compounds using direct absorption.

[0062] According to a third aspect, there is provided a method of sensing a gas using a semiconductor diode laser, the method comprising: applying a current pulse to the semiconductor diode laser so as to cause the laser to generate a laser output pulse, wherein applying the electrical pulse to the semiconductor diode laser causes a temperature rise such that the generated laser output pulse includes a continuous wavelength chirp within a wavelength range; providing the generated laser output pulse to a gas sample region, wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan; and detecting the light output from the gas sample region, wherein the wavelength chirp is at least partially caused by the temperature rise of the laser due to the applied electrical pulse, wherein the current pulse includes an electrical pulse length or duration of at least 5 microseconds, and / or wherein the length of the current pulse is such that the temperature rise slows down over the length of the current pulse, such that the rate of change of the continuous wavelength chirp continuously slows down from an initial first rate of change to a slower second rate of change over the length of the pulse, and wherein the detection of the light output and / or further sampling is performed at the slower second rate of change.

[0063] According to a fourth aspect, there is provided a semiconductor diode laser spectrometer for measuring the radiation absorption of a sample, comprising: a semiconductor diode laser; an electrical pulse generator configured to apply an electrical pulse to the laser so as to cause the laser to generate a laser output pulse for a gas sample region, wherein applying the electrical pulse to the semiconductor diode laser causes a temperature change such that the generated laser output pulse includes a continuous wavelength chirp within a wavelength range, wherein the generated laser output pulse is provided to the gas sample region, and wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan, wherein the wavelength chirp is at least partially caused by the temperature rise of the laser due to the applied electrical pulse; and a detector for detecting the light output from the gas sample region, wherein the electrical pulse length of the current pulse is at least 5 microseconds, and / or wherein the temperature rise slows down over the length of the current pulse, such that the rate of change of the continuous wavelength chirp continuously slows down from an initial first rate of change to a slower second rate of change over the length of the pulse, and wherein the detection of the light output and / or further sampling is performed at the slower second rate of change.

[0064] The features of one aspect may be provided as features of any other aspect. For example, the features of the method may be provided as features of the spectrometer and vice versa. Additionally, the features of the first aspect may be provided as features of the third aspect and vice versa. Additionally, the features of the second aspect may be provided as features of the fourth aspect and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Aspects of the present invention will be described by way of example only and with reference to the accompanying drawings, in which:

[0066] Figure 1Shows a spectrometer arrangement 10 for measuring the radiation absorbed by a species (e.g., a gas sample);

[0067] Figure 2 where (a) is a plot of a laser pulse having a pulse length exceeding 100 microseconds, and Figure 2 where (b) is a plot of a laser pulse having a pulse length exceeding 10 microseconds;

[0068] Figure 3 Shows three pulses ramping with different rise time constants;

[0069] Figure 4(a) is a plot showing a calibrated pulse, and Figure 4(b) shows the relationship between time and wavenumber for a portion of the calibrated pulse of Figure 4(a);

[0070] Figure 5 is a graph of voltage, current, and optical power;

[0071] Figure 6(a) is a plot of pulse versus time, and Figure 6(b) is a plot of pulse versus wavenumber; and

[0072] Figure 7(a) is a plot of a current pulse having a flat top, and Figure 7(b) is a plot of a current pulse having a shaped top. Detailed Description

[0073] WO 03 / 087787 describes using a pulse generator to provide a plurality of pulses (rectangular) at the input of a semiconductor diode laser. The generator provides a sequence of rectangular current drive pulses of fixed amplitude and sub-microsecond duration to cause a rapid laser heating effect, such that the emitted semiconductor diode laser radiation chirps in time at a certain rate over a continuous wavelength range. The laser heating caused by the current pulses results in, for each pulse emitted from the laser, a chirp that is a continuous and nearly linear spectral change from short wavelength to long wavelength (defined as a continuous spectrum or wavelength scan). WO03 / 087787 describes applying pulses in the range of 150 to 300 ns (preferably 200 to 300 ns) to provide a tuning range of approximately 60 GHz.

[0074] In the context of gas analyzers, it was previously thought that pulsed lasers such as semiconductor diode lasers having a pulse length exceeding about 1000 ns offered little benefit. This view was due in part to the design of the drive circuits used. Another factor was the behavior of the lasers available at that time. For example, the data in WO 03 / 087787 indicate that the characteristics of the lasers at that time were significantly different from those of the lasers currently in use. Additionally, in WO 03 / 087787, the chirp was shown to be linear.

[0075] In addition, in the context of quantum cascade lasers, it has been understood that if such long pulses or high duty cycle pulsed lasers are used, the temperature of the laser will become too high, which is related to the structure and design of the laser at that time. It should be understood that the efficiency of the laser decreases as the temperature increases, thereby reducing the light emitted by the laser. However, as described below, it has been found that if the laser pulse is long enough, an available spectral window can be generated. Although this requires a larger time window for sampling, since the tuning rate within the window of interest is much slower, the sampling rate can be significantly reduced. This enables the use of electronic circuits with lower bandwidth and sampling rate, thereby reducing the component cost. Specifically, this alleviates the burden on the electronics required in the system, especially in terms of detection.

[0076] The available spectral window can correspond to a wavenumber range that covers the typical absorption lines of the desired target gas. Such examples are provided in Table 1 (but are not limited thereto). This range is typically about 0.05 to 0.1 cm -1 (but not as a limitation), so a minimum total chirp size of 0.1 to 0.2 cm -1 is required. For more complex spectroscopy examples, a larger wavenumber range may be required. The available spectral window will thus be understood as a window that allows for the accurate identification and measurement of one or more gases of interest.

[0077] It should be understood that the generated wavelength chirp can have a duration measured in microseconds. The generated wavelength chirp can also be characterized by the magnitude of the wavenumber. In some embodiments, the magnitude of the generated wavelength chirp is in the range of 0.1 to 10 cm -1 . In some embodiments, the rate of change of the wavelength chirp is in the range of 0.1 to 10 cm -1 per microsecond.

[0078] As described above, only a portion of the chirp provides available data. In some embodiments, this portion can correspond to a certain proportion of the wavelength chirp, for example, one-quarter, one-half, or three-quarters of the wavelength chirp. In some embodiments, the available portion corresponds to a portion of the wavelength chirp whose rate of change is below a threshold or within a suitable or desired range. In some embodiments, the available portion corresponds to the rate of change within the wavelength chirp being in the range of 0.1 to 2 cm -1 per microsecond. In some embodiments, the magnitude of the available portion is in the range of 0.1 to 5 cm -1 . The available portion of the chirp can correspond to the available spectral window. The duration of the available portion can define or correspond to the detection window.

[0079] Figure 1Figure 10 shows a spectrometer arrangement 10 for measuring radiation absorbed by a substance (e.g., a gas sample). The spectrometer 10 has: a semiconductor diode laser 12; and a current-driven pulse generator 14 connected to the input of the laser 12. The pulse generator 14 is operable to provide a substantially rectangular pulse to the laser 12. In this embodiment, the laser 12 is a single-mode semiconductor diode quantum cascade laser (QC laser). The laser 12 is mounted on a Peltier temperature device (not shown in Figure 1 ), and is enclosed within a suitably heat-sinking laser package (or housing). The Peltier element is controlled by a thermoelectric controller 16 (TEC). In this embodiment, the wavelength range of the output radiation is from 0.5 µm to 14 µm. The Peltier element is controlled by the thermoelectric controller 16, which allows the starting temperature of the laser to be configured. This allows the starting wavelength of the laser to be selected.

[0080] Figure 1 The spectrometer has an enclosed non-resonant optical unit 20 for collecting the gas sample to be analyzed. According to an embodiment, the gas to be analyzed can be contained within a Herriot cell, an astigmatic Herriot cell, other types of multi-channel cells, or an open path arrangement can be used.

[0081] In Figure 1 the arrangement, the radiation emitted by the semiconductor laser passes through the optical path of the optical unit 20. The light output of the optical unit 20 is directed (if necessary, through guiding optics) to a photodetector 22, also referred to as a detector. The detector 22 detects the post-absorption light pulse output of the optical unit 20. Connected to the detector 22 is a digitizer 24, which is also connected to a control and data processing system 26. The detector and the digitizer form a data acquisition system 25. The control and data processing system 26 provides overall control of the spectrometer. In addition to the digitizer, the control and data processing system 26 is also connected to the current-driven pulse generator 14.

[0082] In some embodiments, more than one laser is provided. In these embodiments, each laser can have its own pulse generator and TEC. In some embodiments, more than one detector and digitizer are provided.

[0083] As part of its function, the control system 26 is operable to set the amplitude and duration of the pulse applied to the laser and to monitor the resulting output detected from the optical unit 20.

[0084] In this embodiment, the control system 26 is operable to determine the ratio I a / I o . This can be achieved, for example, by the Beer-Lambert law, which can be written as Ia / I o = exp(-aL). Of course, those skilled in the art will understand that other techniques can be used. Under low-intensity limits, the spectrometer determines the absorption coefficient of a substance by measuring the ratio of the light intensity I o incident on the sample gas cell to the light intensity I a that propagates through the sample gas cell containing the absorbing substance. Under low-intensity limits, the change in light intensity passing through the gas is described by the Beer-Lambert relationship, I a = I o exp(-aL), where the absorption coefficient is a and L is the optical path length. It should be noted that a is a function of the wave number and is independent of intensity at low intensities of the incident radiation.

[0085] It should be understood that the controller can be configured to sense a gas or one or more compounds in a gas sample, or detect one or more materials in a gas sample. Non-limiting examples are provided in reference to Table 1. Additionally, the controller can be configured to identify one or more compounds in a gas sample. The photodetector 22 and the digitizer 24 can together form or be referred to as a detection system or module.

[0086] In this embodiment, the photodetector 22 is configured to generate an electrical signal in response to the laser incident on the detector 22. The digitizer 24 is connected to the detector 22 and is configured to perform a sampling or digitization process on the electrical signal of the detector 22. The sampling and digitization system generates data for processing by a processing resource. The processing resource can be provided as part of the controller or can be provided as an additional processing resource.

[0087] Turning to the pulse generator 14, the pulse generator 14 drives the laser 12 to generate a laser. The up-chirp (also known as the "effective emission linewidth") is caused by the duration of the drive current pulse applied to the laser. The term "effective emission linewidth" here refers to the observable / measurable spectral width (FWHM) of a semiconductor diode laser caused by the current / voltage pulse applied to its electrical contacts. For example, a 10 µs pulse will have a chirp of 6 wave numbers, which corresponds to a frequency change of approximately 60 GHz. As another example, a 1 µs pulse typically has a chirp of 2 wave numbers, corresponding to a frequency change of approximately 20 GHz.

[0088] In this embodiment, the pulse generator 14 is configured to generate a plurality of substantially rectangular pulses, which are applied to the input of the laser 12. More specifically, the generator 14 provides a sequence of rectangular current drive pulses of a fixed amplitude. The electrical pulse length of each pulse is at least 1 microsecond (1000 ns). This results in a laser heating effect, which causes a continuous wavelength chirp in the emitted semiconductor diode laser radiation. In an embodiment, the duration of the electrical pulse length is at least 5 microseconds (5000 ns). The duration of the pulse length is long enough to slow down the generated chirp rate to a slower rate.

[0089] The pulse remains substantially constant over the electrical pulse length. Each pulse can also be mathematically represented as a step function or a suitable approximation. The pulse can be considered to be of a rectangular pulse shape.

[0090] The laser heating caused by the rectangular current pulse results in a continuous spectral change of the chirp from a short wavelength to a long wavelength for each pulse emitted from the laser. This can be considered to define a continuous spectrum or wavelength scan. Chirp is understood as the change of wavelength over time. Chirp can be characterized by a chirp rate. The chirp rate is the change of chirp over time. In WO03 / 087787, the chirp is linear, so the chirp rate is constant and does not change over time. Instead, as described below, the chirp is non-linear and the chirp rate slows down during the pulse.

[0091] It should be understood that one or more characteristics of the wavelength chirp depend on the temperature of the laser. The temperature of the laser changes during laser generation. In particular, when an electrical pulse is applied, the temperature of the laser increases until it reaches a maximum temperature. The possible maximum temperature will depend on the characteristics of the laser and environmental factors, such as the ambient temperature and / or any cooling effects, such as the cooling effect from a TEC. An electrical current pulse is applied to the semiconductor, thereby providing a heating effect to the semiconductor laser.

[0092] It has been found that a pulse length of more than 1 microsecond is sufficient to slow down the rate of increase in the temperature of the laser during the application of the pulse. In an embodiment, this effect can be enhanced by using a chirp duration of more than 1 microsecond, such as greater than 5 microseconds. In other embodiments, the chirp duration can reach or exceed 1 millisecond. Therefore, the rate of change of the wavelength of the generated chirp slows down over time. This effect is shown, for example, in the following Figure 2 、 Figure 3This is shown in FIGS. 4(a) and 4(b). Accordingly, the chirp rate is reduced on the wavelength chirp. This brings advantages. For example, the portion of the sampled photodetector signal being analyzed has a wavelength chirp rate that allows for optimization of the detector and acquisition system. As another example, the laser can be configured to scan the wavelength of interest at a rate where the signal has a lower electrical bandwidth, thus allowing the use of a less costly photodetector and acquisition system. As another example, it can also allow for a higher spectral resolution because the chirp rate is slower.

[0093] In operation, the photodetector detects the response to the entire pulse signal to generate a photodetector signal, and the entire photodetector signal is sampled and / or digitized. However, due to the bandwidth of the photodetector and the digitizing circuitry, only a portion of the sampled signal is useful.

[0094] The system can target a desired spectral window by adjusting the laser parameters. The system can be configured such that the useful portion of the spectral window corresponds to one or more spectral features of interest, such as one or more absorption lines. The system can be configured such that the spectral features of interest occur during the slower portion of the pulse.

[0095] In some embodiments, the wavelength chirp includes a portion that corresponds to a chirp rate within a desired range. This portion occurs during a time period referred to as the detection time window. The controller can be configured to perform sampling and detection at least during the desired detection time window in order to allow for the acquisition of detection data from the desired window. As described in further detail, for example, with reference to Figure 3 , the TEC can control the temperature to define the start of the chirp. In some embodiments, a synchronization process is performed to ensure that detection is performed at least during the desired detection time window.

[0096] It should be understood that data is captured, but only some are selected for processing. As described above, all detection data is captured through the detection and sampling process to provide reference values because relative amplitudes are being measured. The detector can have a non-zero offset.

[0097] The slower chirp rate reduces the frequency content of the electrical signal from the photodetector, thus reducing the bandwidth required for both the photodetector and the electronics used to digitize the signal. The lower bandwidth can bring many advantages. As described above, the chirp in a long laser pulse is fast at the start of the pulse but slows down later in the pulse. By calibrating the laser to sweep the wavelength of interest when the chirp is slower, the electrical frequency content of the resulting pulse signal is reduced.

[0098] In this embodiment, one or more operating parameters of the pulse generator or the laser are selected to adjust and / or substantially maintain the duty cycle of the laser. In particular, the laser operates at a low duty cycle to maintain a low average thermal load. In this embodiment, the pulse repetition frequency is controlled within a desired range. In this embodiment, the pulse repetition frequency is maintained within the desired range of 0.01 to 5 kHz. This allows the duty cycle to remain below the desired level or a predetermined value. In this embodiment, the desired level is a target value of 5%. In additional embodiments, the target value can be a value below 10% or, for example, a value between 2.5% and 7.5%. Further, the temperature is controlled by the TEC to define the start of the chirp and / or the position of the desired spectrum or detection time window. As described above, the TEC defines the start of the chirp and the laser reaches equilibrium under specific operating conditions.

[0099] In addition to the advantage of reducing the burden on the electronic device, the wavelength chirp itself is relatively long, exceeding the chirp generated using shorter pulses. This allows a larger wavelength range or a larger wavenumber range to be measured from a single laser.

[0100] In operation, an electrical pulse is applied to the semiconductor diode laser, causing the laser to generate a laser output pulse. The electrical pulse has a pulse length of at least one microsecond. The applied electrical pulse causes a temperature change in the laser, such that the generated laser output pulse has a continuous wavelength chirp over a wavelength range. The generated output pulse is then provided to the gas sample region (in this embodiment, the gas sample region is located in the optical unit 20) and interacts with the gas sample.

[0101] After interacting with the gas, the light passing through the optical unit 20 is incident on the photodetector 22. The detector 22 is configured to generate a detection signal in response to light being incident on the detector 22. In this embodiment, the control system 26 is configured to synchronize the operations of the pulse generator 14 and the digitizer 24 such that the digitizer performs a sampling process on the detection signal. In this embodiment, the sampling process is performed on the wavelength chirp, thereby allowing detection data to be captured during a period when the wavelength chirp changes at a slower rate. The detection data from this portion is subsequently processed to determine, for example, whether one or more compounds of interest are present in the sample gas.

[0102] It should be understood that multiple parameters of the electrical pulse can be selected. For example, the pulse length can vary. As described above, the electrical pulse length exceeds 1 microsecond. In some embodiments, the electrical pulse length is between 1 microsecond and 200 microseconds. In other embodiments, the electrical pulse length is between 100 microseconds and 10 milliseconds. In other embodiments, the electrical pulse length is between 100 microseconds and 1 millisecond. The variation of the electrical pulse length allows adjustment of the wavelength scan length. Other parameters of the pulse can also be adjusted by the control system. For example, one or more of the pulse width, duty cycle, temperature, and current of the pulse can be controlled.

[0103] In an embodiment, the electrical pulse length is between 5 microseconds and 1 millisecond. In an embodiment, the electrical pulse length is between 10 microseconds and 1 millisecond.

[0104] Furthermore, the parameters of the pulse or other parameters of the system can be varied to change the rate of change of wavelength over time. For example, the amplitude of the current pulse can be varied to provide a change in the rate of change of wavelength per unit time.

[0105] Compared with known systems where the laser pulse frequency is in the range of 1 to 50 kHz, the laser pulse frequency can be in the range of 10 Hz to 5 kHz. Compared with known systems where the bandwidth of the photodetector is typically in the range of 80 to 100 MHz, it has been found that the bandwidth of the photodetector can be an order of magnitude lower, for example, which can be in the range of 1 to 10 MHz.

[0106] In some embodiments, the laser pulse frequency is between 100 Hz and, for example, 40 kHz. As a non-limiting example, the pulse width can be 10 microseconds, which would be a 5% duty cycle at 5 kHz.

[0107] It should be understood that due to the reduced bandwidth requirements, a photodetector and / or digitizer with a lower bandwidth can be used. The required bandwidth is reduced because the chirp rate of the spectral portion of interest is slower because the pulse is longer.

[0108] In some embodiments, the electrical pulse minimizes the attenuation of the optical output power. It should be understood that the laser driving system is optimized to drive pulses with a pulse length between 5 - 100 µs. More specifically, any drop in the laser driving current exhibited by some laser drivings (e.g., laser drivings for sub-microsecond pulses) is minimized to avoid excessive attenuation of the optical output of the pulse.

[0109] In the embodiments described above, a rectangular current pulse shape was described. In additional embodiments, the shape of the current pulse can be adjusted to improve the effectiveness of the optical signal generated by the laser. This includes, but is not limited to, controlling the rise time of the current pulse to optimize the optical power output of the laser and the wavelength tuning during the first portion of the pulse, where the chirp rate is faster than the rate at which the photodetector and data acquisition system are configured to correctly capture. In some embodiments, the current pulse can have a flat top (i.e., a period of constant current), or can be shaped (such that the current changes during the middle portion of the pulse). The shape of the pulse can be selected to adjust the wavelength chirp exhibited by the laser to achieve optimal behavior for a particular laser or measurement. FIGS. 7(a) and 7(b) depict current pulses having different shapes.

[0110] Figure 2 (a) shows the detector signal generated by a long laser pulse. Figure 2 The pulse length in (a) is 100 microseconds. The upper trace 102 is the detector signal of the quantum cascade laser. The upper trace can be regarded as a time pulse or a time response. The lower trace 104 is the etalon trace, depicting the fringes observed after the pulse passes through the etalon. In this embodiment, the etalon is a half-inch-long germanium etalon. The frequency of the etalon trace decreases with time, demonstrating the non-linear spectral change from short wavelength to long wavelength on the pulse.

[0111] As shown in FIG. 2(a), the signal amplitude increases rapidly in the first portion 102a of the pulse. The first portion can also be referred to as the fast turn-on portion. The pulse has a second portion 102b and a last portion 102c. The corresponding portions of the etalon trace 104 show the non-linear spectral change from short wavelength to long wavelength during the pulse duration. In particular, the chirp rate slows down during the pulse.

[0112] Figure 2 (b) shows the detector signal of a long pulse with a pulse length of 10 microseconds. The upper trace 202 is the detector signal, and the lower trace 204 is the etalon trace showing the etalon fringe pattern. The detector signal has a first portion 202a, a second portion 202b, and a third portion 202c. There is a local minimum corresponding to absorption between the second portion 202b and the third portion 202c. The corresponding portions of the etalon trace 104 show the non-linear spectral change from short wavelength to long wavelength during the pulse duration. In particular, the chirp rate slows down during the pulse.

[0113] It should be understood that Figure 2 (a) andFigure 2 The curve in (b) shows the averaging of the signal response over 100 to 1000 pulses per second. These were subsequently recorded using an oscilloscope, averaging 32 consecutive pulse trains.

[0114] Figure 3 Illustrates the effect of applying different rate ramps to the pulsed current. Compared to known systems where the current and pulse are suddenly turned on and controlled by a square voltage pulse, Figure 3 Shows the ramp applied to the pulsed current.

[0115] Figure 3 Shows three pulses ramping up with three different rise time constants. In Figure 3 (a) of, Figure 3 (b) of, Figure 3 (c) of, each upper trace (labeled 302a, 302b, and 302c) in the figure is a plot of the pulse versus time, while the lower trace is the etalon trace. The rise time constant is applied to the current by configuring the pulse driver.

[0116] Figure 3 (a) of shows the result of the first pulse 302a ramping up at the first rate (rise time of 3.2 µs). Figure 3 (b) of shows the result of the second pulse 302b ramping up at the second rate (rise time of 1.7 µs). Figure 3 (c) of shows the result of the third pulse 302c ramping up at the third rate (rise time of 0.7 µs). Each figure has a first rising portion (303a, 303b, 303c), followed by a second falling portion (304a, 304b, 304d).

[0117] For these three pulses, different TEC temperatures were used to control the timing of the reference line (in this example, the reference line is the first reference line from the left). In this example, the reference line is held at 7.5 µs after the end of the pulse. The reference lines are labeled 306a, 306b, and 306c in each figure respectively.

[0118] Figure 3 Demonstrates a faster rise time (e.g., Figure 3(a)) results in a smaller available spectral window from the reference line to the end of the pulse. However, for the same temperature, the faster the slope, the more overall available tuning will result. Thus, to achieve a compromise between achieving long tuning and maximizing the second part of the pulse (the slower tuning part), an intermediate rise time was selected. It should be understood that the value of the intermediate rise time can depend on the laser. Faster and slower rise times can each have advantages, and thus the value that is optimal for the overall system needs to be optimized. 1.7 µs is an arbitrary number that has been shown to be effective. As a non-limiting example, the value 1.7 µs was used. However, it should be understood that different values can be used, for example, values in the range between 1 and 2 µs can be used.

[0119] Figure 4(a) depicts an etalon pulse trace 402 showing tuning. The y-axis is the photodiode signal and the x-axis is time. A portion 404 of the pulse trace 402 is marked out. Portion 404 can be referred to as the available portion and can correspond to the available spectral window. The available spectral window refers to the portion of the pulse trace that is generally more suitable for spectral analysis. In some embodiments, data collected outside of the available portion 404 is discarded and not used for data processing.

[0120] Figure 4(b) depicts the relative time-to-wavenumber conversion for the entire pulse. The y-axis is the relative wavenumber in cm -1 units. The x-axis is time in µs. The chirp rate corresponds to the change in wavenumber over time and continuously slows during the pulse duration. During the pulse duration, the chirp is non-linear. Thus, the chirp rate significantly decreases over the pulse length. The chirp rate continuously decreases over the pulse length. The chirp rate can be considered to be monotonically decreasing.

[0121] The non-linearity of the chirp can be caused by multiple factors. Without being limited by theory, these factors can be related to the heating effect of the laser, external cooling effects, and changes in the refractive index. Thus, accurately modeling the non-linearity of the chirp can be complex. To obtain an available output from the detector, an approximation can be used. This can be a polynomial function or an exponential function. In some embodiments, a polynomial of at least third order may be required to represent the non-linearity due to the longer pulse length. A multi-exponential function can be used to represent the chirp.

[0122] Figure 5Depicts the I-V-L curve. The first graph 502 shows the variation of voltage (in volts) with laser current (in amperes). The second graph 504 shows the variation of optical power with laser current. The limiting value of the optical power is defined at the lower end by the current / voltage amplitude required to achieve the available output power, and at the upper end by the current / voltage amplitude that causes a reduction in the output power. The upper limit is based on the manufacturer's maximum specification, which can be the point where the output power decreases, or where other characteristics of the laser output can no longer be maintained. The lower limit is defined by the optical threshold of the laser, i.e., the minimum current required to emit laser light.

[0123] Figure 6(a) depicts a graph of the photodiode signal versus time. The curve or signal 602 corresponds to the detector signal obtained with the measurement cell filled with the target gas (NO in this example). The curve 602 has multiple signal features (602a, 602b, 602c, 602d), corresponding to the absorption of the laser by the target gas. The signal features correspond to the absorption lines of the target gas. The signal features can also be referred to as characteristic signals.

[0124] Figure 6(b) depicts the pulsed photodiode signal versus relative wavenumber. Both curves were obtained using 10 µs pulses of a 5 µm laser. The curve 604 corresponds to the signal obtained when the measurement cell is filled with N2. The curve 606 corresponds to the signal obtained when the measurement cell is filled with the target gas (NO) and N 2 is used as the balance gas. Thus, the curve 606 exhibits some signal features (606a, 606b, 606c, 606d), corresponding to the absorption lines of the target gas.

[0125] Comparing Figure 6(a) and Figure 6(b), the signal features of the signal obtained over time (represented by 602a, 602b, 602c, 602d) correspond to the signal features of the signal obtained for the wavenumbers (606a, 606b, 606c, 606d). This correspondence can be understood as due to the continuous wavelength chirp generated by the laser being provided to the measurement cell. The generated laser output provided to the measurement cell has a varying wavenumber (over time), thus providing a wavelength or wavenumber scan.

[0126] Figure 7(a) depicts a current pulse with a flat top. The current pulse 702 with a flat top has a first part 702a, a second part 702b, and a third part 702c. The amplitude of the first part 702a increases with time. The flat top part has a portion where the current value remains constant for a period of time (corresponding to the second part 702b).

[0127] Figure 7(b) depicts a current pulse 704 with a shaped top. The current pulse has a first part 704a, a second part 704b, and a third part 704c. Compared to the current pulse of Figure 7(a), the second part 704b is not flat and not constant over time, but increases over time. The first part 704a increases at a first rate, and the second part 704b increases at a greater second rate.

[0128] Table 1 shows example values of the wavelength ranges of compound groups and their corresponding wavenumber ranges. It should be understood that these examples are provided only as non-limiting, and alternative compounds and / or compound groups can be detected using the methods and systems described above.

[0129]

[0130] Table 1

[0131] Those skilled in the art will understand that variations of the disclosed arrangements are possible without departing from the scope of the invention. For example, in the embodiments described above, the laser is a quantum cascade laser. However, it should be understood that other semiconductor lasers configured to produce wavelength chirps can be used, such as, for example, interband cascade lasers, tunable diode lasers. Additionally, in the above embodiments, a single-mode laser is described. However, in alternative embodiments, a multi-mode laser is provided. In such an embodiment, a spectral filter is optionally provided on the optical path between the diode laser and the optical unit, and the filter is controlled by the control and acquisition system. The spectral filter can be, for example, a small grating monochromator and can be used to provide a single-mode laser output in the case of using a multi-longitudinal mode laser. In some embodiments, additional optical elements can be provided in the arrangement, for example, between the laser and the unit.

[0132] According to an embodiment, the wavelength range of the laser can be between near-infrared and mid-infrared. The near-infrared can include the range from 800 to 2500 nm. The mid-infrared can include the range from 2.5 - 3 microns to about 25 microns. The far-infrared can correspond to the range exceeding 25 microns up to millimeters. The application of the present system and method in molecular spectroscopy can fall within the near-infrared to mid-infrared range. In a non-limiting example, the output wavelength range of the quantum cascade laser used is between 3 - 10.1 µm. Additionally, in some examples, tunable diode lasers and ICLSs in the range from 0.76 to 4 µm can be used.

[0133] Furthermore, in the embodiments described above, a system using an optical unit is described. However, it should be understood that the system can be extended to alternative gas sensing systems, such as, for example, open path sensing systems, such as, for example, cross-stack systems.

[0134] As a further non-limiting example, in the embodiments described above, a rectangular pulse shape was described. However, it should be understood that other pulse shapes may be used, such as, for example, flat top and / or triangular top.

[0135] By a slow turn-on, the light intensity within the pulse unavailable region is reduced, thus allowing optimization of the signal level. The turn-on shape of the pulse can also be controlled in order to optimize the chirp rate characteristics of the lasers that can be used.

[0136] In addition, in the embodiments described above, the gas sample region was described as being located within a multi-channel optical unit. However, it can be understood that other configurations may also be provided.

[0137] The temperature rise caused by the pulse application depends on the laser used. In some embodiments, for a particular laser, under particular operating conditions, the change in temperature can correspond to the chirp magnitude. Approximately and without limitation, a temperature rise of 1 degree Celsius can correspond to a wavelength chirp magnitude of 0.1 cm -1 portion.

[0138] In an example embodiment, the temperature rise can be between 50 and 80 degrees. This temperature rise can correspond to a wavelength chirp between 5 cm -1 and 8 cm -1 .

[0139] Although depending on the laser, such as depending on laser characteristics including the laser wavelength, in some embodiments, for every 10 degree Celsius increase in temperature, the wavelength chirp can span one wave number. In an example embodiment, the chirp can span 3 to 4 wave numbers (3 to 4 cm -1 ), corresponding to a temperature increase of approximately 30 to 40 degrees Celsius.

[0140] According to the above embodiments, the available window will be a portion of the wave numbers spanned by the chirp. The available window can be 0.5 to 1 wave number (0.5 to 1 cm -1 ), but is not limited thereto. The available window can be up to 75% of the magnitude of the total wavelength chirp, optionally up to 50%, optionally up to 25%, optionally up to 10%.

[0141] As a comparative example, pulse lengths below 1 µs generally result in chirp rates between 2 cm -1 / µs and 7 cm -1 / µs. Such a system requires a faster detection and / or digitization system with a bandwidth of approximately 100 MHz.

[0142] By providing a method that allows the use of a lower bandwidth detector, a more cost-effective and technically efficient method and system can be provided.

[0143] According to another aspect, a method of using QCLs or ICLs in a gas analyzer is provided, where the pulses are longer than may have been possible when using pulsed QCLs previously. Utilizing the wavelength shift caused by the temperature rise induced by the pulses, which shows a significant slowdown at longer pulses, allows for the use of a detection system with a lower speed and / or lower bandwidth.

[0144] In the embodiments described above, different types of semiconductor diode lasers were discussed, which are configured to output laser light at different wavelengths. In an embodiment, a quantum cascade laser can output mid-infrared range laser light. In an embodiment, a quantum cascade laser can output light with a wavelength between 3 and 11 micrometers. In additional embodiments, the system can use tunable diode lasers and interband cascade lasers. In additional embodiments, the system can use tunable diode lasers and interband cascade lasers. TDLs and ICLs that output wavelengths between 0.7 and 4 micrometers can be used.

[0145] In some embodiments, the laser is configured to output laser light having a visible to near-infrared wavelength. The laser is configured to output laser light having a near-infrared to mid-infrared wavelength.

[0146] The detection system can be an in-pulse laser spectroscopy system. In this context, in-pulse can be understood as performing gas detection during the pulse duration. The system can also be described as a direct absorption system, where the absorption of light having a wavelength indicates the presence of a compound of interest. In an in-pulse system or modulation scheme, the chirp can be resolved by the detector to scan the absorption lines / features. Thus, the description of the above specific embodiments is for illustrative purposes only and not for purposes of limitation. It will be apparent to those skilled in the art that minor modifications can be made without materially changing the operations described.

Claims

1. A method for sensing a gas using a semiconductor diode laser, the method comprising: applying a current pulse to a semiconductor diode laser so that the laser generates a laser output pulse, wherein the application of the electric pulse to the semiconductor diode laser causes a temperature increase such that the generated laser output pulse includes a continuous wavelength chirp within a wavelength range; providing the generated laser output pulses to a gas sample region, wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan; as well as detecting a light output from the gas sample region, wherein the wavelength chirp is caused at least in part by an increase in the temperature of the laser caused by the applied electrical pulses, wherein the current pulse comprises an electrical pulse length or duration of at least 5 microseconds, and / or The length of the current pulse causes the temperature rise to slow down over the length of the current pulse, thereby causing the rate of change of the continuous wavelength chirp to continuously slow down from an initial first rate of change to a slower second rate of change over the length of the pulse, and the light output is detected and / or further sampled at the slower second rate of change.

2. The method according to claim 1, wherein the length of the current pulse is long enough to slow down the increase in temperature of the laser over the length of the electric pulse, causing the chirp to exhibit nonlinearity, and / or the rate of change of the generated wavelength chirp to slow down over the generated wavelength chirp.

3. A method according to any preceding claim, wherein: a) the chirp can be represented by a polynomial or exponential function or other non-linear function; and / or b) The length or duration of the current pulse is such that the continuous wavelength chirp is non-linear in time.

4. A method according to any preceding claim, wherein the temperature change comprises an increase to a maximum temperature, wherein the maximum temperature is dependent on one or more of ambient temperature and a cooling effect.

5. A method according to any preceding claim, wherein the wavelength chirp comprises a portion characterised by a chirp rate within a desired range, wherein the portion occurs at least during a detection time window, wherein the method comprises: An output from the sensing region is detected and / or a detection signal is processed at least during the detection time window.

6. The method according to claim 5, wherein: a) the detection window starts 1 microsecond after the start of the pulse; and / or b) The start of the detection portion is fixed and / or synchronized with the laser output.

7. The method according to any preceding claim, wherein the electric pulse length of the current pulse satisfies one of the following: at least 5 microseconds, at least 10 microseconds, between 5 microseconds and 1 millisecond, and between 10 microseconds and 1 millisecond.

8. A method according to any preceding claim, wherein the first rate of change is higher than 2 cm -1 per microsecond, and the second rate of change is less than 2cm -1 per microsecond, optionally, wherein the first rate of change is higher than 2 cm -1 per microsecond, and the second rate of change is 0.1 to 2 cm -1 per microsecond, further optionally, wherein the second rate of change is less than 0.1 cm -1 Every microsecond.

9. A method according to any preceding claim, wherein the chirp rate can be slowed down by up to one of: 25%, 50%, more than 100%.

10. A method according to any preceding claim, wherein the pulse remains substantially constant over the electrical pulse length, and / or wherein the pulse substantially comprises a step function.

11. The method according to any preceding claim, wherein the electrical pulse comprises at least one of: a rectangular pulse shape and / or a flat top and / or a triangular top.

12. The method according to any preceding claim, further comprising at least one of the following a), b), c), d): a) Changing the wavelength change rate per unit time, such as by changing the amplitude of the current / voltage driving pulse to change the wavelength change rate per unit time; b) adjusting the wavelength scanning length, for example, by changing the duration of the current / voltage driving pulse; c) changing the temperature of the semiconductor diode laser; d) adjusting one or more of pulse width, duty cycle, temperature and voltage / current of the pulse; e) controlling the starting temperature of the laser using a thermoelectric controller.

13. A method according to any preceding claim, wherein the wavelength scan has a size range of 0.1 to 10 cm -1 .

14. A method according to any preceding claim, wherein: a) The wavelength of the output radiation is in the range of 0.5µm to 14µm; and / or b) The semiconductor diode laser is configured to emit laser light in the near infrared to mid infrared range, optionally in the visible to near infrared range, optionally in the near infrared to mid infrared range.

15. A method according to any preceding claim, wherein: a) The wavelength chirp produced is between 0.1 and 10 cm -1 to the extent that; and / or b) The rate of change of the generated wavelength chirp is 0.1cm -1 Up to 10cm -1 per microsecond range; and / or c) the detector and / or digitizing circuit is configured to detect the rate of change of the wavelength chirp between 0.1 and 2 cm -1 per microsecond range and / or the size of the chirp in the wavelength range of 0.1 to 5 cm -1 The emitted light is sampled when it is within the range of 16. A method according to any preceding claim, wherein the gas sample region comprises an optical cell, optionally a Herriott cell.

17. A method according to any preceding claim, wherein the laser comprises a semiconductor laser, optionally wherein the laser comprises a quantum cascade laser, an interband cascade laser, a tunable diode laser.

18. The method according to any preceding claim, further comprising: One or more operating parameters of the laser are selected and / or adjusted to regulate and / or substantially maintain a duty cycle of the laser.

19. The method of claim 18, wherein the duty cycle is maintained below a target value, optionally wherein the target value is one of: 5%, between 1% and 5%, between 2.5% and 7.5%, below 10%.

20. The method of any preceding claim, further comprising: The laser is controlled to reduce a pulse repetition frequency of the laser, optionally wherein the pulse repetition frequency is in the range of 0.01 to 5 KHz.

21. A method according to any preceding claim, wherein laser output is provided to the sample region without further modulation by other lasers.

22. A semiconductor diode laser spectrometer for measuring radiation absorption of a sample, comprising: Semiconductor diode lasers; an electrical pulse generator configured to apply electrical pulses to the laser so that the laser generates laser output pulses for a gas sample region, wherein applying the electrical pulses to the semiconductor diode laser causes a temperature change such that the generated laser output pulses include a continuous wavelength chirp over a wavelength range, wherein the generated laser output pulses are provided to the gas sample region, wherein at least a portion of the wavelength range of the wavelength chirp is used as a wavelength scan, wherein the wavelength chirp is caused at least in part by an increase in the temperature of the laser caused by the applied electrical pulses, and a detector for detecting light output from the gas sample region; wherein the current pulse comprises an electrical pulse length of at least 5 microseconds, and / or wherein the temperature increase is slowed over the length of the current pulse, thereby causing the rate of change of the continuous wavelength chirp to continuously slow down from an initial first rate of change to a slower second rate of change over the length of the pulse, and wherein detection and / or further sampling of the light output is performed at the slower second rate of change.

23. The spectrometer of claim 22, wherein: a) the spectrometer further comprises an optical unit, such as a non-resonant optical unit, and wherein at least a portion of the sample area is located in the optical unit; and / or b) the laser comprises an intra-pulse laser spectroscopy system, optionally wherein the intra-pulse spectroscopy is configured to detect compounds using direct absorption; and / or c) The spectrometer is part of an open path sensing system, such as a cross-stack system.

24. A spectrometer according to claim 22 or 23, wherein the detector comprises a photodetector and / or a digitising circuit, wherein: a) the bandwidth of the photodetector is in the range of 1 to 10 MHz, optionally above or below this range depending on the application; and / or b) the bandwidth of the digitising circuit is in the range of 1 to 10 MHz, optionally above or below this range depending on the application; and / or c) The detector comprises detection and digitisation circuitry with a bandwidth in the range of 1 to 20 MHz.

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