An apparatus for providing a modulated light beam, an arrangement for analyzing a sample and a method for providing a modulated light beam

Abstract

An apparatus for providing a modulated light beam, the apparatus comprising at least a first light source (102) configured to provide a first light beam and a modulating system (104) configured to provide a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation, and provide a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation, wherein the modulating system provides the first modulation by providing a first control signal and provides the second modulation by providing a second control signal, wherein the first control signal and the second control signal comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation, wherein the modulating system comprises at least a reference clock (106) for providing a clock signal and a driving unit (108) configured to obtain the clock signal, wherein the driving unit is configured to provide the first control signal and the second control signal based on at least the obtained clock signal.

Description

[0001] AN APPARATUS FOR PROVIDING A MODULATED LIGHT BEAM, AN ARRANGEMENT FOR ANALYZING A SAMPLE AND A METHOD FOR PROVIDING A MODULATED LIGHT BEAM

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The invention relates to optics in general. More specifically, the invention relates to providing a modulated first light beam by applying a first modulation and second modulation to a first light beam, where the first modulation is provided through a first control signal and the second modulation is provided through a second control signal, wherein the first control signal and second control signal are based on at least an obtained clock signal.

[0004] BACKGROUND OF THE INVENTION

[0005] Frequency modulation (FM) of light, specifically laser light, is useful related to many applications such as spectroscopy and telecommunication. In many applications, the frequency modulation is desirable to carry out without affecting intensity or amplitude of the frequency-modulated light. FM can be implemented as direct modulation or external modulation.

[0006] Solutions relying on external modulation may be more complex and more expensive than direct modulation, as expensive additional components are needed and such solutions can e.g. require difficult alignment optimization (for instance especially in connection with mid-infrared (MIR) lasers). Additionally, external modulation is not completely free of amplitude modulation (AM) and may also cause interference signals.

[0007] Implementations for direct frequency modulation of laser light also suffer from undesired amplitude modulation of the produced light, especially those relying on modulation of current being supplied to the light source. Some solutions that attempt to cancel out AM are known, but amplitude modulation of the produced light is still present in these solutions to some degree.

[0008] Quantum cascade lasers (QCL) are advantageous in many applications and may be utilized to provide compact instruments working in the MIR range for various applications. Due to intrinsic short relaxation times, QCLs are applicable for fast radiofrequency (RF) modulation, also beyond the GHz range. Yet, the bandwidth for modulating the laser current is often limited by parasitic inductances and capacitance of the wires connecting the QCL chip.

[0009] SUMMARY OF THE INVENTION

[0010] An object of the invention is to alleviate at least some of the problems relating to the known prior art. In one aspect of the invention an apparatus for providing a modulated light beam is provided, the apparatus comprising at least a first light source configured to provide a first light beam and a modulating system configured to

[0011] - provide a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation, and

[0012] - provide a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation, wherein the modulating system is further configured to provide the first modulation by providing a first control signal and provide the second modulation by providing a second control signal, wherein the first control signal and the second control signal comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation, wherein the modulating system comprises at least a reference clock for providing a clock signal and a driving unit configured to obtain the clock signal, wherein the driving unit is configured to provide the first control signal and the second control signal based on at least the obtained clock signal.

[0013] With the present invention, the target modulation to a first light beam may be achieved in a simple, economical, and controlled manner. The first control signal and second control signal, namely the selected amplitude ratio and selected phase difference, may be accurately set independently and digitally based on a common clock signal. This may provide benefits over approaches where e.g. analog phase shifters are utilized, as better phase stability may be obtained and simple digital control is possible, allowing for better control and integration of the system. The invention may be utilized in laser spectroscopy application relating for instance to ultra-sensitive detection of molecular species, radio-carbon biofraction measurements, or radioactive waste monitoring (e.g. tritium & radiocarbon).

[0014] The first control signal and the second control signal may be mutually configured such that the target amplitude modulation is a minimized amplitude modulation, preferably essentially corresponding to no amplitude modulation.

[0015] With the invention, cancellation of AM may be kept stable and efficient, while a phase between the first amplitude modulation second amplitude modulation may be kept stable and easy to adjust. Analog phase shifters can have electronic drifts that cause instability of the AM cancellation and require electronic control of the phase requiring stable analog control electronics.

[0016] The driving unit may comprise at least a first direct digital synthesizer (DDS) and a second direct digital synthesizer, wherein the first direct digital synthesizer and second direct digital synthesizer are coupled to the reference clock. A common clock signal provided by the reference clock may be split up and fed to two synchronized DDS integrated circuits. The first DDS and second DDS may be configured to generate the same frequency by setting a phase increment to the same value. Two phase- locked modulation signals may be provided for providing the first modulation and second modulation. As an alternative to DDSs, phase-locked loops may be employed in the driving unit.

[0017] The first light source may be a quantum cascade laser (QCL). The invention enables RF modulation of QCLs, which may be provided for use e.g. in laser spectroscopy applications. In many known solutions that provide RF modulation for QCLs suffer from the problem that AM is significant for current modulation and cancellation schemes are difficult to adjust or insufficient. The present invention may allow more efficient AM cancellation and better control of the provided modulation, due to the digital control provided by the reference clock. AM may be problematic for sensitive laserbased instruments, as it causes a constant background signal at the modulation frequency that carries the laser intensity noise. In the present invention, a first modulation and second modulation may be utilized to cancel out AM while keeping the FM of a QCL at RF frequencies up to 1 GHz.

[0018] The modulating system may be configured to provide the first modulation by manipulating the first light source or by interacting with the first light beam. The modulating system may be configured to provide the second modulation by manipulating the first light source or by interacting with the first light beam.

[0019] The first modulation and second modulation may be provided by utilizing a first actuator and a second actuator. An intrinsic response of the first and second actuators to the control signals or effect of the first and second actuators on the first light source should not be identical. The first and second actuators may have different response to the control signals, such that there may be a phase shift between AM and FM provided and / or a difference in modulation indices for AM or FM for a specific control signal may be provided.

[0020] The first actuator may comprise a second light source and the second actuator may comprise a third light source that provides a different modulation response to a given control signal than that provided by the second light source, or the second actuator may comprise means for modulating a current being provided to the first light source, as will be discussed further below.

[0021] Advantageously, the first modulation and second modulation are provided by manipulating the first light source, i.e. the modulations are provided as direct modulation. The modulating system may comprise at least a second light source, optionally a first near-infrared laser light source, configured to receive the first control signal and provide a second light beam with the intensity modulated by the first control signal. The modulating system may then additionally be configured to provide the second modulation using either a third light source or by providing current modulation. The modulating system may comprise at least a third light source, optionally a second nearinfrared laser light source, configured to receive the second control signal and provide a third light beam with the intensity modulated by the second control signal, or the modulating system may be configured to provide the second modulation by modulating a current being delivered to the first light source based on the second control signal. The modulating system may comprise at least one first dichroic mirror for directing the at least second light beam to a waveguide of the first light source.

[0022] The apparatus may additionally comprise at least one detector and at least one control unit, wherein the detector is configured to detect the first modulated light beam and the at least one control unit is configured to facilitate adjusting the selected amplitude ratio and / or the selected phase difference between the first control signal and the second control signal if the detected first modulated light beam does not exhibit the target frequency modulation and target amplitude modulation until the detected first modulated light beam essentially exhibits the target frequency modulation and target amplitude modulation.

[0023] A control unit may be configured to provide the first modulation and second modulation simultaneously and facilitate adjusting the selected amplitude ratio and / or the selected phase difference such that target amplitude modulation is a minimized amplitude modulation or wherein the control unit is configured to provide the first modulation and the second modulation one at a time and detect, for each of the first modulation and second modulation, respectively, the resulting modulated first light beam, wherein adjusting of the first modulation and second modulation are facilitated during said detecting such that the first amplitude modulation and / or second amplitude modulation is adjusted such that the amplitude of the first amplitude modulation is essentially equivalent to the amplitude of the second amplitude modulation and a phase of the first amplitude modulation and / or a phase of the second amplitude modulation is adjusted such that a phase difference between the first amplitude modulation and the second amplitude modulation is essentially 180 degrees.

[0024] Using a photodetector, the AM at the modulation frequency can be acquired and minimized by adjusting the phase-offset of e.g. one of the DDSs.

[0025] A control unit may be configured to demodulate the first modulated light beam and determine an in-phase component and a quadrature component for the first modulated light beam. The same clock signal obtained from the reference clock that is used to provide the first signal and second signal may be used for digital demodulation. This may allow for a digital low-phase- noise signal acquisition. Also, digital demodulation that avoids offsets and 1 / f noise present in analog electronics may be avoided.

[0026] The modulating system may comprise a feedback control unit configured to determine at least one feedback signal based on the in-phase component and the quadrature component, wherein the at least one feedback signal is used in controlling the first control signal and second control signal to obtain in-phase component and quadrature component that maintain or approach a selected target value, optionally zero.

[0027] A feedback control unit may comprise at least a first PID controlling unit configured to receive an indication of the in-phase component and a second PID controlling unit configured to receive an indication of the quadrature component, wherein the first PID controlling unit is configured to provide a first feedback signal for controlling the amplitude ratio between the first control signal and the second control signal and the second PID controlling unit is configured to provide a second feedback signal for controlling a phase difference between the first control signal and the second control signal.

[0028] In a further aspect of the invention, an arrangement for analyzing a sample may be provided, the arrangement comprising an apparatus and at least one chamber for receiving a sample, wherein the apparatus is configured to direct the first modulated light beam through the chamber, the arrangement comprising at least one detector for detecting the first modulated light beam that has traversed through the chamber.

[0029] In one more further aspect of the invention, a method for providing a modulated light beam is provided, the method comprising at least:

[0030] - providing a first light source for providing a first light beam,

[0031] - providing a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation by providing a first control signal,

[0032] - providing a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation by providing a second control signal, and

[0033] - obtaining a clock signal and providing the first control signal and the second control signal based on at least the obtained clock signal, wherein the first control signal and the second control signal are provided to comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to provide a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation.

[0034] A method for analyzing a sample is also provided, the method comprising at least:

[0035] - providing a first light source for providing a first light beam,

[0036] - providing a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation by providing a first control signal,

[0037] - providing a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation by providing a second control signal, and

[0038] - obtaining a clock signal and providing the first control signal and the second control signal based on at least the obtained clock signal, wherein the first control signal and the second control signal are provided to comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to provide a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation, the method further comprising providing a chamber comprising a sample, providing the first modulated light beam through the chamber, and detecting the first modulated light beam after it has passed through the chamber.

[0039] The exemplary embodiments presented in this text are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

[0040] The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific example embodiments when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

[0042] Figure 1 schematically illustrates one example of an apparatus,

[0043] Figure 2 shows a further example of an apparatus,

[0044] Figure 3 shows a further example of an apparatus,

[0045] Figure 4 shows a further example of an apparatus,

[0046] Figure 5 shows a further example of an apparatus,

[0047] Figure 6 shows examples of arrangements for analyzing samples, and

[0048] Figure 7 depicts a flow chart of a method.

[0049] DETAILED DESCRIPTION

[0050] Figure 1 shows a schematic illustration of an apparatus for providing a modulated light beam. The apparatus comprises at least a first light source 102 and a modulating system 104. The first light source 102 is configured to provide a first light beam. The first light source 102 may advantageously be a QCL. The first light beam refers to a light beam that the first light source 102 would provide without modulation.

[0051] The modulating system 104 is configured to provide a first modulation to the first light beam, the first modulation comprising a first amplitude modulation and a first frequency modulation. The modulating system 104 is further configured to provide a second modulation to the first light beam, the second modulation comprising a second amplitude modulation and a second frequency modulation. The modulating system 104 may comprise a plurality of components of which a portion may be embodied in the same entity, while others may be provided as separate entities. The dashed rectangle 104 of Fig. 1 is schematically used to illustrate the modulating system. The modulating system 104 may comprise means for providing the first modulation and the second modulation. The means for providing the modulating may comprise e.g. at least one light source and optionally means for modulating a current being provided to the first light source.

[0052] The modulating system 104 is configured to provide the first modulation by providing a first control signal and provide the second modulation by providing a second control signal. The first control signal and the second control signal comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation.

[0053] The modulating system 104 comprises at least a reference clock 106 and a driving unit 108. The reference clock 106 is configured to provide a clock signal that is transmitted to the driving unit 108. The driving unit is configured to provide the first control signal and the second control signal based on at least the obtained clock signal. The first control signal and second control signal may be provided as phase locked signals for providing the first modulation and second modulation.

[0054] The driving unit 108 may comprise at least a first direct digital synthesizer and a second direct digital synthesizer, wherein the first direct digital synthesizer and second direct digital synthesizer are coupled to the reference clock 106. The driving unit 108 may alternatively comprise phase- locked loops.

[0055] Figure 2 shows an apparatus where the modulating system 104 further comprises at least one second light source 110. The second light source 110 may be configured to receive the first control signal from the driving unit 108 and provide a second light beam with the intensity modulated by the first control signal. The modulating system 104 may then comprise at least one first dichroic mirror 112 for directing the at least second light beam to a waveguide of the first light source 102.

[0056] The second light source 110 may be a first near-infrared (NIR) laser light source. The first light beam can thus be modulated by non-resonant injection of intensity modulated NIR light into a front or back facet of the QCL. By utilizing at least a second light source 110 in the modulating system, the bandwidth for modulating the first light beam with suppressed AM may be higher than a bandwidth that may be obtained by using only current modulation. A wavelength of the second light source 110 may be selected to provide an optimized frequency and amplitude modulation response that differs from the first frequency modulation and amplitude modulation response. The difference in response can be a difference in the phase between AM and FM and / or a difference in the ratio of the modulation indices of AM and FM.

[0057] In the embodiment of Fig. 2, the second modulation is provided by modulating the current being provided to the first light source 102 based on the second control signal.

[0058] The driving unit 108 may be a dual output frequency generator circuit that is capable of providing the first and second control signals based on the obtained clock signal.

[0059] Fig. 3 shows an alternative embodiment of an apparatus where the second modulation is provided using a third light source 114 for providing light with a wavelength differing from the wavelength of light provided by the second light source. The modulating system 104 may thus comprise at least a third light source 114 configured to receive the second control signal and provide a third light beam with the intensity modulated by the second control signal. The third light source may be a second near-infrared laser light source. The modulating system 104 may further comprise a second dichroic mirror 116 for directing the third light beam to a waveguide of the first light source 102. Alternatively, two fiber coupled NIR lasers may be combined using a fiber coupler and then emitted to free-space using a single fiber-collimator, eliminating the need for a second dichroic mirror.

[0060] The selection of which actuators are utilized for providing the first and second modulations may depend on characteristics of the first light source, such as wavelength or manufacturer of the first light source 102. For some laser devices the modulation response of two NIR lasers with different wavelength might not induce a response different enough. The response for current modulation as compared to NIR modulation is for some first light sources very different and therefore may allow for a simpler cancellation of the AM.

[0061] Figure 4 shows a further apparatus that otherwise corresponds to the apparatus of Fig. 2 but shows further possible components and specifically, the modulating system 104 also comprises at least one detector 116 and at least one control unit 118. The at least one detector 116 and at least one control unit 118 may also be used in combination of other embodiments of the apparatus, such as that shown in Fig. 3. The first light source 102 may include a current driver and RF coupling electronics.

[0062] The control unit 118 may comprise at least one processing unit and may comprise e.g. at least one microcontroller or field-programmable gate array (FPGA) unit.

[0063] The apparatus or modulating system may additionally comprise at least one chamber 120 through which the first modulated light beam is directed. The chamber may be configured to host a reference gas, such as N2O and may be used for detecting FM of the first modulated light beam. The chamber 120 may be a part of the apparatus that is used upon need for hosting a reference gas during adjustment of the first and second modulation.

[0064] The chamber 120 may also be used for hosting a sample gas, e.g. if the apparatus is used for analyzing a sample after an adjustment phase where the first and second modulation are adjusted. An arrangement for analysing a sample may be provided by providing an apparatus, such as that one depicted in Fig. 4, but where the chamber 120 is is configured to host a sample gas. In such an arrangement, the first modulation and second modulation may have been pre-set to exhibit selected characteristics as described elsewhere herein.

[0065] An arrangement for analyzing a sample may also be provided where the arrangement comprises an apparatus as shown in Fig. 4, where a (first) chamber 120 is used for hosting a reference gas and a (first) detector 116 is used to detect the detect the first modulated light beam that has passed through the reference gas, the arrangement additionally comprising a second chamber for hosting a sample and a second detector for detecting the first modulated light beam that has passed through the sample chamber.

[0066] As in the example of Fig. 2, the apparatus of Fig. 4 comprises a second light source 110 configured to receive the first control signal from the driving unit 108 and provide a second light beam with the intensity modulated by the first control signal. A first dichroic mirror 112 for directing the at least second light beam to a waveguide of the first light source 102 is also provided. The second modulation is provided by modulating the current being provided to the first light source 102.

[0067] The driving unit 108 (schematically illustrated with dashed rectangle) is being shown as comprising at least a first DDS 120 and second DDS 122. The first DDS 120 is configured to provide the first control signal and the second DDS 122 is configured to provide the second control signal.

[0068] The driving unit 108 may comprise a first amplitude control component 124 and second amplitude control component 126. The first amplitude control component 124 and second amplitude control component 126 may be separate components or they may be integrated in the first and second DDSs.

[0069] The modulating system 104 may also comprise a phase locked loop (PLL) component 128. The PLL component 128 may be a separate component or it may be integrated in the first and second DDSs 120, 122. If the reference clock 106 is compatible with the first DDS 120 and second DDS 122, the PLL component 128 is optional.

[0070] The first modulated light beam may be transmitted through the reference chamber 120 and detected by the first detector 116. The first detector 116 may be a photodetector, from which a signal is transmitted to the at least one control unit 118. The control unit 118 may comprise or be in connection with e.g. oscilloscope or other electronics required for signal processing.

[0071] The control unit 118 may be configured to demodulate the detected first modulated light beam and determine an in-phase (I) component and a quadrature (Q) component for the first modulated light beam. The same clock signal obtained from the reference clock 106 that is used to provide the first signal and second signal may be used for digital demodulation. The signal may be demodulated at the modulation frequency or a multiple of it (using a digital or analog or mixed demodulation scheme).

[0072] The AM at the modulation frequency can be acquired and minimized by adjusting the phase-offset of one of the first DDS 120 or second DDS 122. The AM can be further minimized by adjusting the amplitude of the first and / or second modulation. The amplitude adjustment of the first modulation and second modulation can be done using the amplitude setting of the DDS or an external variable attenuator or amplifier. Here, the first detector 116 may thus be configured to detect the first modulated light beam and the at least one control unit 118 may be configured to facilitate adjusting the selected amplitude ratio and / or the selected phase difference between the first control signal and the second control signal if the detected first modulated light beam does not exhibit the target frequency modulation and target amplitude modulation until the detected first modulated light beam essentially exhibits the target frequency modulation and target amplitude modulation.

[0073] The control unit 118 may be configured to facilitate providing the first modulation and second modulation simultaneously and enable adjusting the selected amplitude ratio and / or the selected phase difference such that the target amplitude modulation is a minimized amplitude modulation.

[0074] Alternatively, the control unit 118 may be configured to provide the first modulation and the second modulation one at a time and detect, for each of the first modulation and second modulation, respectively, the resulting modulated first light beam, and adjusting of the first modulation and second modulation may be facilitated during said detecting such that the first amplitude modulation and / or second amplitude modulation is adjusted so that the amplitude of the first amplitude modulation is essentially equivalent to the amplitude of the second amplitude modulation and a phase of the first amplitude modulation and / or a phase of the second amplitude modulation is adjusted such that a phase difference between the first amplitude modulation and the second amplitude modulation is essentially 180 degrees.

[0075] If the amplitude of the frequency modulation of the target modulation should be minimized, the demodulation phase may be adjusted such that one of the I or Q component signals detects the amplitude modulation. The other signal only includes the frequency modulation response and may therefore be minimized to minimize the frequency modulation.

[0076] Solid lines between the different components of the apparatus shown in the figures may refer to direct physical coupling between the components or the lines may refer to data or signals that are transmitted between the different components. The data or signals may be transmitted in wired or wireless manner, depending on the embodiment, the components in question, and the transmitted information. It may also be noted that for example the terms “modulating system” and “driving unit” are used to refer to one or more components that may be different depending on the embodiment. Of the components, some may be integrated with each other, and some may be separate, whereby e.g. a specific component, such as a detector 116 could be considered as being part of the modulating system 104 or it could be considered as being part of the apparatus or an arrangement in general.

[0077] Figure 5 shows an example of an apparatus where the modulating system is otherwise similar to the modulating system of Fig. 4 but a chamber for reference gas is not utilized, while the modulating system further comprises at least a feedback control unit 130. The apparatus of Fig. 4 may be used in a set-up phase to adjust the first modulation and second modulation to achieve the target modulation for a specific use case, while the apparatus of Fig. 5 may be used in a set-up phase or during specific use for continuous adjusting of the first modulation and second modulation. A beam splitter 132 may be provided for sampling the modulated light for further use.

[0078] The modulating system of Fig. 5 additionally comprises a feedback control unit 130. The feedback control unit 130 is configured to determine at least one feedback signal based on the in-phase component and the quadrature component determined by the control unit 118. The at least one feedback signal is used in controlling the first control signal and second control signal to obtain in-phase component and quadrature component that maintain or approach a selected target value, optionally zero.

[0079] The feedback control unit 130 may comprise at least a first PID controlling unit configured to receive an indication of the in-phase component and a second PID controlling unit configured to receive an indication of the quadrature component. The first PID controlling unit may then be configured to provide a first feedback signal for controlling the amplitude ratio between the first control signal and the second control signal and the second PID controlling unit may be configured to provide a second feedback signal for controlling a phase difference between the first control signal and the second control signal. The PID controlling units may facilitate adjustment of the phases and amplitudes of the control signals to essentially maintain both I and Q components at zero.

[0080] An arrangement for analyzing a sample may comprise an apparatus with a feedback control unit 130 as shown in Fig. 5 and a chamber for receiving a sample to be analyzed through which the first modulated light beam is directed. The chamber may be provided after the first detector 116. The arrangement may then comprise a second detector for detecting the first modulated light beam that has passed through the chamber comprising the sample.

[0081] Figures 6A and 6B show alternative examples of arrangements for analyzing samples. The arrangement of Fig. 6A comprises an apparatus with at least first light source 102, reference clock 106, and driving unit 108. In Fig. 6A, the first modulation is provided by second light source 110 and the second modulation is provided through current modulation. A chamber 120 is provided for receiving a sample to be analyzed, while the first modulated light is configured to be directed through the chamber and to a detector 116.

[0082] Fig. 6B shows an arrangement comprising an apparatus with at least first light source 102, reference clock 106, driving unit 108, first detector 116, control unit 118, and feedback control unit 130. The arrangement additionally has beam splitter 132 for providing a portion of the first modulated light towards a chamber 120 for receiving a sample to be analyzed. The arrangement further comprises a second detector 134 for detecting the first modulated light that is directed through the chamber 120.

[0083] A flow chart for a method of providing a modulated light beam is shown in Fig. 7. The method comprises providing 202 a first light source 102 for providing a first light beam. A clock signal is also obtained at 204. The method then comprises providing 206 a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation by providing a first control signal and providing 208 a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation by providing a second control signal.

[0084] The method comprises providing the first control signal and the second control signal based on at least the obtained clock signal. In the method, the first control signal and the second control signal are further provided to comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to provide a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation.

[0085] A method of analyzing a sample comprises the method steps associated with Fig. 7, the method further comprising providing a sample chamber comprising a sample, providing the first modulated light beam through the sample chamber, and detecting the first modulated light beam after it has passed through the sample chamber.

[0086] The invention has been explained above with reference to the aforementioned embodiments, and several advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims.

[0087] The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

Claims

CLAIMS1. An apparatus for providing a modulated light beam, the apparatus comprising at least a first light source (102) configured to provide a first light beam and a modulating system (104) configured to- provide a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation, and- provide a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation, wherein the modulating system is further configured to provide the first modulation by providing a first control signal and provide the second modulation by providing a second control signal, wherein the first control signal and the second control signal comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation, wherein the modulating system comprises at least a reference clock (106) for providing a clock signal and a driving unit (108) configured to obtain the clock signal, wherein the driving unit is configured to provide the first control signal and the second control signal based on at least the obtained clock signal.

2. The apparatus of claim 1 , wherein the first control signal and the second control signal are mutually configured such that the target amplitude modulation is a minimized amplitude modulation, preferably essentially corresponding to no amplitude modulation.

3. The apparatus of any previous claim, wherein the driving unit comprises at least a first direct digital synthesizer (120) and a second direct digital synthesizer (122), wherein the first direct digital synthesizer and second direct digital synthesizer are coupled to the reference clock.

4. The apparatus of any previous claim, wherein the first light source is a quantum cascade laser.

5. The apparatus of any previous claim, wherein the modulating system is configured to provide the first modulation by manipulating the first lightsource or by interacting with the first light beam and wherein the apparatus is configured to provide the second modulation by manipulating the first light source or by interacting with the first light beam.

6. The apparatus of claim 5, wherein modulating system is configured to provide the first modulation by manipulating the first light source, wherein the apparatus comprises at least a second light source (110), optionally a first near-infrared laser light source, configured to receive the first control signal and provide a second light beam with the intensity modulated by the first control signal, and wherein the apparatus is configured to provide the second modulation by manipulating the first light source, wherein the modulating system comprises at least a third light source (114), optionally a second near-infrared laser light source, configured to receive the second control signal and provide a third light beam with the intensity modulated by the second control signal, or wherein the modulating system is configured to provide the second modulation by modulating a current being delivered to the first light source based on the second control signal.

7. The apparatus of claim 6, wherein the modulating system comprises at least one first dichroic mirror (112) for directing the at least second light beam to a waveguide of the first light source.

8. The apparatus of any previous claim, wherein the apparatus additionally comprises at least one detector (116) and at least one control unit (118), wherein the detector is configured to detect the first modulated light beam and the at least one control unit is configured to facilitate adjusting the selected amplitude ratio and / or the selected phase difference between the first control signal and the second control signal if the detected first modulated light beam does not exhibit the target frequency modulation and target amplitude modulation until the detected first modulated light beam essentially exhibits the target frequency modulation and target amplitude modulation.

9. The apparatus of claim 8, wherein the control unit is configured to provide the first modulation and second modulation simultaneously and facilitate adjusting the selected amplitude ratio and / or the selected phase difference such that target amplitude modulation is a minimized amplitude modulation or wherein the control unit is configured to provide the first modulation and the second modulation one at a time and detect, for each ofthe first modulation and second modulation, respectively, the resulting modulated first light beam, wherein adjusting of the first modulation and second modulation are facilitated during said detecting such that the first amplitude modulation and / or second amplitude modulation is adjusted such that the amplitude of the first amplitude modulation is essentially equivalent to the amplitude of the second amplitude modulation and a phase of the first amplitude modulation and / or a phase of the second amplitude modulation is adjusted such that a phase difference between the first amplitude modulation and the second amplitude modulation is essentially 180 degrees.

10. The apparatus of claim 8, wherein the control unit is configured to demodulate the first modulated light beam and determine an in-phase component and a quadrature component for the first modulated light beam.11 . The apparatus of claim 10, wherein the modulating system comprises a feedback control unit configured to determine at least one feedback signal based on the in-phase component and the quadrature component, wherein the at least one feedback signal is used in controlling the first control signal and second control signal to obtain in-phase component and quadrature component that maintain or approach a selected target value, optionally zero.

12. The apparatus of claim 11 , wherein the feedback control unit comprises at least a first PID controlling unit configured to receive an indication of the in-phase component and a second PID controlling unit configured to receive an indication of the quadrature component, wherein the first PID controlling unit is configured to provide a first feedback signal for controlling the amplitude ratio between the first control signal and the second control signal and the second PID controlling unit is configured to provide a second feedback signal for controlling a phase difference between the first control signal and the second control signal.

13. An arrangement for analyzing a sample, the arrangement comprising an apparatus according to any previous claim, the arrangement comprising at least one chamber (120) for receiving a sample, wherein the apparatus is configured to direct the first modulated light beam through the chamber, the arrangement comprising at least one detector (116, 134) for detecting the first modulated light beam that has traversed through the chamber.

14. A method for providing a modulated light beam, the method comprising at least:- providing (202) a first light source for providing a first light beam,- providing (204) a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation by providing a first control signal,- providing (206) a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation by providing a second control signal, and- obtaining (208) a clock signal and providing the first control signal and the second control signal based on at least the obtained clock signal, wherein the first control signal and the second control signal are provided to comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to provide a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation.

15. A method for analyzing a sample, the method comprising at least:- providing a first light source for providing a first light beam,- providing a first modulation to the first light beam comprising a first amplitude modulation and a first frequency modulation by providing a first control signal,- providing a second modulation to the first light beam comprising a second amplitude modulation and a second frequency modulation by providing a second control signal, and- obtaining a clock signal and providing the first control signal and the second control signal based on at least the obtained clock signal, wherein the first control signal and the second control signal are provided to comprise a selected amplitude ratio and selected phase difference, such that the first light beam is modulated by a target modulation to provide a first modulated light beam, wherein the target modulation comprises a target frequency modulation and target amplitude modulation, the method further comprising providing a chamber comprising a sample, providing the first modulated light beam through the chamber, and detecting the first modulated light beam after it has passed through the chamber.

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