A device and method for controlling a frequency of light being provided by a laser apparatus

Abstract

A device for controlling a frequency of light being provided by a laser apparatus, the device comprising at least one control unit, wherein the control unit is configured to provide a periodic control signal for controlling a frequency of light being provided by a laser apparatus, to scan a frequency of light being provided by the laser apparatus to obtain controlled laser light, and to obtain an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity. Upon determining that the input signal exceeds a predetermined first trigger value, the control unit is further configured to determine an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase value or triggered signal value of the periodic control signal essentially corresponds to a phase or signal value of the periodic control signal at a time when the input signal exceeds the first trigger value, and utilize the error parameter to facilitate performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity.

Description

[0001] A DEVICE AND METHOD FOR CONTROLLING A FREQUENCY OF LIGHT BEING PROVIDED BY A LASER APPARATUS

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The invention relates to spectroscopy in general. More specifically, the invention relates to a device and method for controlling a frequency of light being provided by a laser apparatus via a periodic control signal, said laser apparatus being configured to illuminate a resonant optical cavity, such as a cavity ring-down spectroscopy (ORDS) optical cavity. The invention also relates to an arrangement for performing laser spectroscopy.

[0004] BACKGROUND OF THE INVENTION

[0005] Cavity ring-down spectroscopy (CRDS) is a method that can be used in highly sensitive measurements of e.g. trace gases. To efficiently implement the CRDS method, frequency of light provided by a laser used for irradiating the optical resonant cavity should be stabilized close to the resonance frequency of the cavity. The most sensitive CRDS instruments are in general laboratory-based systems where space requirements and transportability do not restrict the choice of components used in the instruments. Many known frequency stabilization methods employ additional optical components and involve complex electronics.

[0006] Quantum cascade lasers (QCLs) may be beneficial to use in CRDS measurements due to the possibility of providing light in the mid-infrared (MIR) wavelength region. QCLs are also readily usable in field deployable CRDS instruments. However, frequency stabilization of QCLs is particularly challenging due to noisiness of QCLs and requirements relating to optical components that are utilized in known solutions.

[0007] It would be advantageous to be able to perform CRDS measurements using a compact, field deployable solution but which is still capable of providing high sensitivity.

[0008] SUMMARY OF THE INVENTION

[0009] 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 a device for controlling a frequency of light being provided by a laser apparatus, said laser apparatus being configured to illuminate a resonant optical cavity, such as a cavity ring-down spectroscopy (CRDS) optical cavity, the device comprising at least one control unit, wherein the control unit is configured to:

[0010] - provide a periodic control signal for controlling a frequency of light being provided by a laser apparatus, to scan a frequency of light being provided by the laser apparatus to obtain controlled laser light,

[0011] - obtain an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity, and

[0012] - upon determining that the input signal exceeds a predetermined first trigger value, said first trigger value preferably essentially corresponding to an intensity of light being detected indicating that a frequency of light being provided by the laser apparatus essentially corresponds to a resonance frequency of the resonant optical cavity o determine an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase value or triggered signal value of the periodic control signal essentially corresponds to a phase or signal value of the periodic control signal at a time when the input signal exceeds the first trigger value, and o utilize the error parameter to facilitate performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity, preferably resulting in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity.

[0013] With the present invention, frequency stabilization of a laser that is used to irradiate a resonant optical cavity may be carried out in a simple and efficient manner. The solution may be implemented without the use of further optical components and / or complicated electronics that are utilized in some prior art solutions. The invention may thus enable provision of an arrangement for carrying out e.g. CRDS measurements that is more compact than currently available sensitive instruments, while still providing higher sensitivity than currently available field deployable instruments. All control and data acquisition is advantageously purely electronic. The present invention may, for example, enable detection of THO for nuclear waste monitoring or14CC>2 for biofraction measurements.

[0014] The invention is based on utilizing inherent CRDS signals as an input signal in performing of at least one adjustment action resulting in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity, which provides a simple way of tuning the frequency of the controlled laser light or tuning a resonance frequency of the resonant optical cavity.

[0015] As a frequency of light being provided through the controlled laser light may be characterized through either a phase value of a signal value of the periodic control signal, either predetermined phase values or predetermined signal values may be utilized, while the error parameter may be determined using phase values or signal values, through using triggered phase values or triggered signal values.

[0016] The adjustment action may lead to the frequency of the controlled laser light and the resonance frequency of the resonant optical cavity matching each other periodically during essentially each period of the periodic control signal. The invention may reduce the amount of dead time when performing e.g. CRDS spectroscopy, as a decay time measurement may be carried out essentially at every period of the periodic control signal.

[0017] Frequency stabilization of the frequency of the controlled laser light, where changes, such as e.g. noisiness of the laser apparatus, which affect matching of the laser frequency and resonance frequency of the resonant optical cavity, may be compensated for efficiently.

[0018] The adjustment action may comprise utilizing the error parameter in providing a feedback signal for adjustment of the resonance frequency of the resonant optical cavity.

[0019] The adjustment action may comprise utilizing the error parameter in providing a feedback signal for a laser frequency control entity, said laser frequency control entity being configured to control the frequency of the controlled laser light.

[0020] The adjustment action may comprise utilizing the error parameter in providing a feedback signal for providing the periodic control signal, said feedback signal being utilized for adjustment of a signal offset of the periodic control signal.

[0021] The predetermined phase value may essentially correspond to or be within a selected distance from a phase value where the periodic control signal has a mean value or the predetermined signal value may essentially correspond to or be within a selected distance from the mean value of the periodic control signal.

[0022] The adjustment action may lead to an adjustment of a mean frequency of the controlled laser light or of the resonance frequency of the resonant optical cavity being such that the laser frequency and the resonance frequency of the resonant optical cavity periodically match each other close to a predetermined phase value or signal (amplitude) value of the periodic control signal.

[0023] An adjustment action may lead to a mean frequency of the controlled laser light and the resonance frequency of the resonant optical cavity being stabilized close to each other.

[0024] The matching of frequencies may be carried out essentially at every period of the periodic control signal regardless of changing or drifting of laser frequency and / or of a resonance frequency of the resonant optical cavity over time. Changes such as frequency noise, drift, tuning, or jittering may be compensated for and matching of frequencies may be ensured.

[0025] After performing said adjustment action and upon subsequently determining that the input signal exceeds the predetermined first trigger value, a triggered signal value of the periodic control signal may be closer to the predetermined signal value or a signal value corresponding to the predetermined phase value.

[0026] Upon determining that the input signal exceeds a predetermined second trigger value, the control unit may be configured to provide an interruption signal, wherein the interruption signal is utilized to deactivate the laser apparatus, to deflect the laser light from the resonant optical cavity, or to obtain controlled laser light comprising a frequency which deviates from the resonance frequency of the resonant optical cavity by a selected amount, and the interruption signal is applied for a selected period of time. The predetermined second trigger value may be equivalent to the predetermined first trigger value or the predetermined second trigger value may differ from the predetermined first trigger value.

[0027] Upon determining that the input signal exceeds the predetermined second trigger value, the control unit may additionally be configured to facilitate evaluation of a decay time constant of the resonant optical cavity.

[0028] The device may therefore be utilized to decouple the laser light from the resonance frequency of the resonant optical cavity in order to perform a CRDS measurement, i.e. measurement of the decay of light intensity in the optical resonant cavity. The same device, essentially the same control unit, may thus enable locking / stabilization of the laser frequency close to the frequency of the resonant optical cavity and may enable recording of a CRDS decay signal.

[0029] The device may be configured to obtain the input signal essentially continuously and utilize the input signal to determine an error parameter and facilitate performing of at least one adjustment action upon need essentially continuously for a plurality of consecutive periods of the periodic control signal, preferably wherein each adjustment action takes into account a plurality of, optionally all, previously determined error parameters.

[0030] Regarding a plurality of periods of the periodic control signal, for a first period in the plurality, an associated predetermined first trigger value of the input signal may be higher than an associated predetermined first trigger value of the input signal for at least a second period. Here, initiation of the locking may be carried out such that the locking of the laser frequency to higher-order modes may be avoided, while not missing matching of the laser frequency under locked condition.

[0031] The control unit may be configured to utilize the error parameter in providing a feedback signal for facilitating performing of the adjustment action, wherein the control unit is configured to deliver the error parameter to a servo filter, optionally an integrator filter or Proportional-integral-derivative filter, the output of which is used in the feedback signal.

[0032] If upon determining that the input signal exceeds a predetermined first trigger value and further determining that the triggered phase value or signal value of the periodic control signal does not correspond to a predetermined phase range, the error parameter may not be determined in some embodiments and regarding periods of the periodic control signal where this additional condition is not fulfilled.

[0033] The device may be configured to control a frequency of light being provided by a laser apparatus being a tunable laser apparatus, such as a QCL device. The invention may enable the use of tunable lasers, QCL lasers in particular, in e.g. CRDS arrangements, allowing for compact arrangements with high sensitivity.

[0034] An arrangement for performing laser spectroscopy may also be provided, the arrangement comprising at least a resonant optical cavity, a laser apparatus, a detector, and a device as described herein, wherein the device is configured to control a frequency of light being provided by the laser apparatus, the laser apparatus is configured to illuminate the resonant optical cavity, the detector is configured to detect intensity of light in the resonant optical cavity, the detector additionally being configured to provide the input signal to the device.

[0035] A method for controlling a frequency of light being provided by a laser apparatus may further be provided, said laser apparatus being configured to illuminate a resonant optical cavity, such as a cavity ring-down spectroscopy (CRDS) optical cavity, the method comprising:

[0036] - providing a periodic control signal for controlling a frequency of light being provided by a laser apparatus to scan a frequency of light being provided by the laser apparatus to obtain controlled laser light, wherein the method comprises,

[0037] - obtaining an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity, and

[0038] - determining if the input signal exceeds a predetermined first trigger value, said first trigger value preferably essentially corresponding to an intensity of light being detected indicating that a frequency of light being provided by the laser apparatus that corresponds to a resonance frequency of the resonant optical cavity, and if yes o determining an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase value or triggered signal value of the periodic control signal essentially corresponds to a phase value or signal value of the periodic control signal at a time when the input signal exceeds the first trigger value, and o utilizing the error parameter to facilitate performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity, preferably resulting in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity.

[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.

[0041] The previously presented considerations concerning the various embodiments of the device may be flexibly applied to the embodiments of the method mutatis mutandis, and vice versa, as being appreciated by a skilled person.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which:

[0044] Figure 1 illustrates a device and arrangement, Figure 2 illustrates how laser frequency locking can affect the relationship between laser frequency and resonance frequency of a resonant optical cavity,

[0045] Figure 3 shows schematically how a feedback signal may be used in facilitating an option of an adjustment action,

[0046] Figure 4 shows schematically how a feedback signal may be used in facilitating an option of an adjustment action,

[0047] Figure 5 shows schematically how a feedback signal may be used in facilitating an option of an adjustment action,

[0048] Figure 6 shows one example of how an error parameter may be determined and used,

[0049] Figure 7 shows one further example of how an error parameter may be determined and used,

[0050] Figure 8 shows the effect of performing adjustment actions compared to a case where adjustment actions are not carried out, and

[0051] Figure 9 illustrates a flow chart of a method.

[0052] DETAILED DESCRIPTION

[0053] Figure 1 shows a device 100 comprising at least one control unit 102. The device is configured to control a frequency of light being provided by a laser apparatus 104, e.g. by controlling a current being delivered to the laser apparatus 104, where the laser apparatus 104 is configured to illuminate a resonant optical cavity 106, such as a cavity ring-down spectroscopy (CRDS) optical cavity. A device 100 may essentially consist of only the control unit 102, while an arrangement 200 may be considered as comprising further components, such as at least the laser apparatus 104. Alternatively, a device 100 may be considered as comprising the control unit 102 and any further components that may be utilized in connection with an arrangement 200 as described herein. In a practical implementation, a device 100 or arrangement 200 may comprise further components required for operation which may not be explicitly discussed herein, which will be clear to the skilled person. The control unit 102 may advantageously be a Field Programmable Gate Array (FPGA). The control unit 102 may alternatively comprise other digital electronics or analog electronics. With digital implementations, however, the invention enables “digital locking” of the laser frequency. An FPGA allows simple and compact implementation of the device 100, but any processing unit, for example, may be used.

[0054] The device 100, specifically the control unit 102, is configured to provide a periodic control signal for controlling a frequency of light being provided by the laser apparatus 104 to scan a frequency of light being provided by the laser apparatus 104 to obtain controlled laser light. The periodic control signal comprises a mean signal value, an amplitude, and a frequency. The periodic control signal may be in the form of e.g. a sawtooth waveform or a triangular waveform. The periodic control signal may be coupled to a laser controller 108 which modulates the frequency of light provided by the laser apparatus 104.

[0055] The periodic control signal advantageously may result in the controlled laser light comprising frequencies which allow light to build up inside the resonant optical cavity 106 once the mean frequency of the controlled laser light is near a resonance frequency of the resonant optical cavity 106.

[0056] An amplitude of the periodic control signal may be selected such that after an adjustment action is carried out, matching of the frequency of controlled laser light and a resonance frequency of the resonant optical cavity may be ensured.

[0057] The frequency of the periodic control signal may be selected so that it is high enough to obtain sufficient control-bandwidth. For example, for a 10 kHz periodic ramp signal one can perform an adjustment at maximum once every 100 ps, i.e. this has a bandwidth of up to 5 kHz.

[0058] The frequency and amplitude of the periodic control signal may be chosen such that the laser apparatus 104 is tuned sufficiently slowly across the resonance frequency of the resonant optical cavity 106 to allow for efficient in-coupling of light into the resonant optical cavity 106. If the tuning rate across the resonance frequency of the resonant optical cavity 106 is too fast, the laser apparatus 104 will not have enough time to couple a significant amount of light into the resonant optical cavity 106. This decreases the average pulse peak intensity detected, which is an unwanted effect especially for CRDS measurements, since it decreases the signal-to- noise ratio for evaluation of a ring-down time.

[0059] Advantageously, the control unit 102 may obtain an input signal, determine an error parameter, and facilitate performing of at least one adjustment action as further described herein essentially continuously or at least regarding a plurality of periods of the periodic control signal, such that the device 100 enables repeated measurement of decay of light in the resonant optical cavity 106. In general, the error parameter may be processed by a feedback control system comprising an integrator filter, such as servo filter. The servo filter may comprise an integrator that sums up all consecutive error parameters. Thereby, a determined error parameter will affect all future periods.

[0060] The control unit 102 is further configured to obtain an input signal being indicative of light intensity in the resonant optical cavity 106 being effectuated by provision of the controlled laser light into the resonant optical cavity 106. The input signal may be provided based on a signal from a detector 110, which measures the intensity of light inside the resonant optical cavity 106.

[0061] Upon determining that the input signal exceeds a predetermined first trigger value, the control unit 102 is further configured to determine an error parameter. The error parameter is based on a difference between a predetermined phase value or signal value and a determined triggered phase value or signal value of the periodic control signal. The triggered phase value or signal value of the periodic control signal essentially corresponds to a phase value or signal value at a time when the input signal exceeds the first trigger value. The manner in which the error parameter is generated may prevent so called “integrator windup” due to a potential offset in the error parameter. The error parameter is only generated once the frequencies of the laser apparatus and the resonant optical cavity are close to each other.

[0062] The first trigger value may be selected such that it is well above the detector noise and above signals detected because of sudden laser frequency changes induced either by the control mechanism or noise sources. The first trigger value may be selected and optimized during an initial setup process. The first trigger value may also be automatically or manually adjusted to compensate for laser apparatus or resonant optical cavity transmission intensity changes.

[0063] The control unit 102 is then configured to utilize the error parameter to facilitate performing of at least one adjustment action. The adjustment action shall result in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity.

[0064] The adjustment action may result in the frequency of light being provided by the laser apparatus 104 matching a resonance frequency of the resonant optical cavity 106 at the time corresponding to the predetermined phase value or signal value or is at least closer to the time corresponding to the predetermined phase value or signal value than without the adjustment action.

[0065] Several different options for at least one adjustment action that is to be performed are available.

[0066] In general, the control unit 102 may be configured to provide a feedback signal based on the error parameter. The feedback signal may be used to enable carrying out at least one adjustment action. The feedback signal may be generated using a PID control filter or any other suitable feedback filter. In one implementation, the error parameter may be multiplied with a constant factor, after which it may be summed to the mean value of the control signal, i.e. essentially a digital integrator filter may be provided.

[0067] The feedback signal may be used to alter the periodic control signal or the feedback signal may be provided to a component separate from the control unit 102.

[0068] A first option for the adjustment action is that the adjustment action comprises adjustment of the resonance frequency of the resonant optical cavity 106. The adjustment action may comprise using the error parameter in providing a feedback signal to a cavity control member for adjustment of the resonance frequency of the resonant optical cavity 106, such as a cavity control member for piezo controlling the length or temperature of the resonant optical cavity 106.

[0069] A second option for the adjustment action is that the adjustment action comprises utilizing the error parameter in providing a feedback signal to a laser frequency control entity, said laser frequency control entity being configured to control the frequency of the controlled laser light.

[0070] In the first and second options, the control unit 102 may provide an auxiliary output for providing a feedback signal.

[0071] A third option for the adjustment action is that the adjustment action comprises utilizing the error parameter in providing a feedback signal for providing the periodic control signal, said feedback signal being utilized for adjustment of a signal offset of the periodic control signal of the periodic control signal. In the third option, the feedback signal is utilized by the control unit 102 itself.

[0072] In advantageous embodiments, the predetermined phase value may essentially correspond to or be within a selected distance from a phase value where the periodic control signal has a mean value or the predetermined signal value may essentially correspond to or be within a selected distance from the mean value of the periodic control signal.

[0073] Changes in e.g. components of a device or apparatus leading to changes in matching of the laser frequency and the resonance frequency of the resonant optical cavity 106 may lead to shifts associated with either direction regarding a period of the periodic control signal and thus, a predetermined phase value essentially corresponding to a phase value where the periodic control signal has a mean value or choosing the predetermined signal value essentially corresponding to a mean value of the periodic control signal may be advantageous.

[0074] After adjustment of a signal offset of the periodic control signal and subsequently determining that the input signal exceeds the predetermined first trigger value, a triggered phase value or triggered signal value of the periodic control signal is advantageously closer to the predetermined phase value or predetermined signal value than the triggered phase value or triggered signal value regarding a previous time when the input signal has exceeded the first trigger value.

[0075] An arrangement 200 may further comprise e.g. a digital-to-analog converter 112 for converting an output signal from the control unit 102 to an analog signal and an analog-to-digital converter 114 for converting the input signal from an analog signal into a digital signal.

[0076] In some embodiments, feed-forward signals to be used for the laser apparatus 104 and / or to be used for resonance frequency of the resonant optical cavity may be used to reduce the load on the feedback control system.

[0077] The arrangement may constitute a closed loop controller system. The control unit 102 may monitor the input signal to determine a triggered phase value or triggered signal value as a process variable. The predetermined phase value or predetermined signal value may be a setpoint for the closed loop controller system. The setpoint may correspond to a target difference between the mean laser frequency and the resonance frequency of the resonant optical cavity. The target difference may advantageously be zero. A feedback signal may be used to facilitate an adjustment action to adjust the difference. With the present invention, low frequency noise may be well compensated for, while noise at frequencies higher than the frequency of the periodic control signal can still induce jittering of the laser frequency.

[0078] Figure 2 illustrates how laser frequency locking / stabilization can affect the relationship between laser frequency and resonance frequency of a resonant optical cavity 106. Solid vertical lines along the frequency axis represent resonance frequencies of the resonant optical cavity. The resonance frequency that is the second on the left represents a target cavity resonance, which corresponds to a frequency that is aimed to be provided by the laser apparatus 104 in order to perform an e.g. CRDS measurement.

[0079] Fig. 2 illustrates one example of frequencies of controlled laser light that may be scanned e.g. during one period of a periodic control signal. The dashed vertical line corresponds to a frequency of light being provided that corresponds to a predetermined phase value (or predetermined signal value). The predetermined phase value or predetermined signal value has been selected to correspond to a laser frequency that is about at the center of the frequencies being scanned, i.e. corresponds to about a mean frequency being provided by the controlled laser light. At Fig. 2A, locking of the laser frequency is not active, and the laser frequency is scanned along frequencies that do not correspond to the targeted resonance frequency of the resonant optical cavity 106. Here, an e.g. CRDS measurement cannot be obtained. Fig. 2B shows a situation where locking may be initiated. Here, the frequencies of light being scanned by the controlled laser light comprise a frequency that corresponds to the targeted resonance frequency of the resonant optical cavity 106. A frequency corresponding to the predetermined phase value or predetermined signal value is not equivalent or even near to a frequency corresponding to the targeted resonance frequency of the resonant optical cavity 106. A CRDS measurement may still be carried out, but without locking of the laser frequency, a shift in frequencies being provided by the controlled laser light may lead to the laser frequency not scanning a frequency that corresponds to the targeted resonance frequency of the resonant optical cavity 106 e.g. during the next period of the periodic control signal.

[0080] Fig. 2C shows how after initiating locking after the situation of Fig. 2B and subsequently providing e.g. a next period of the periodic control signal, through determining an error parameter and performing an adjustment action, a frequency of light being provided by the controlled laser light where the frequency corresponds to the targeted resonance frequency of the resonant optical cavity 106, the frequency also essentially corresponds to or is near (at least nearer than for a previous period of the periodic control signal) to the predetermined phase value or predetermined signal value.

[0081] The adjustment action leading to a selected frequency (corresponding to the target resonance frequency of the resonant optical cavity) being provided through controlled laser light being nearer to a frequency corresponding to the predetermined phase value or predetermined signal value may not necessarily occur at a next consecutive e.g. period or a subsequent time that the adjustment action is performed, but may occur for example after several periods of the periodic control signal.

[0082] To initiate locking, the mean frequency of controlled laser light provided by the laser apparatus 104 and the resonance frequency of a selected cavity resonance of the resonant optical cavity may be tuned close to each other. The tuning may be carried out by tuning the mean frequency of controlled laser light or by tuning the frequency of the selected cavity resonance. The tuning of the mean frequency of controlled laser light may be realized by tuning the mean voltage of the periodic control signal or by any other means (such as temperature piezo actuators etc.). The tuning of the frequency of the selected cavity resonance can be done by controlling the optical path length of the optical resonator by tuning the length using a cavity control entity, such as piezo actuator or any other means. Once close enough the periodic control signal may repeatedly tune the frequency of controlled laser light across the selected cavity resonance frequency.

[0083] Initiation of the locking may be carried out in one embodiment by, regarding a plurality of periods of the periodic control signal, for a first period in the plurality, having an associated predetermined first trigger value of the input signal being higher than an associated predetermined first trigger value of the input signal for at least a second period.

[0084] The present invention may enable locking of the laser apparatus frequency that compensates for inherent changes that are known / expected to occur in the device or apparatus components, such as drifting of laser frequency. If locking or frequency stabilization of the laser frequency is however lost or broken, reinitiating of the locking may be carried out automatically, for example in case a rate of determining that the input signal exceeds the predetermined first trigger value drops to zero or is below a selected rate. Losing of the locking may occur due to larger, unexpected events.

[0085] Using the light intensity in the resonant optical cavity 106 in an input signal and detecting that a first trigger condition is fulfilled is efficient for resonant optical cavities since the build-up time of the light intensity is significantly shorter as compared to the decay time. This makes the bandwidth of the locking of the laser frequency according to the present invention up beyond tens of kHz possible. At these frequencies dither locks known in the prior art suffer from a relatively weak signal.

[0086] Upon determining that the input signal exceeds a predetermined second trigger value, the control unit 102 may further be configured to provide an interruption signal. The interruption signal may be utilized to obtain controlled laser light comprising a frequency which deviates from the resonance frequency of the resonant optical cavity by a selected amount. The interruption signal may be applied for a selected period of time, which may essentially correspond to a time at which the intensity of light in the cavity decays to under a threshold value, such as under a detector noise level, after fulfilment of the first trigger condition has been determined, to allow measurement of the decay time and enable e.g. a CRDS measurement. The predetermined second trigger value may be equivalent to the predetermined first trigger value or the predetermined second trigger value may differ from the predetermined first trigger value.

[0087] Upon determining that the input signal exceeds the predetermined second trigger value, the control unit 102 may thus additionally be configured to facilitate evaluation of a decay time constant of the resonant optical cavity.

[0088] In some embodiments, if the interruption of the laser light was not triggered earlier than a selected phase value of the periodic control signal it will be triggered anyway. This may ensure that the interruption is triggered for every period of the periodic signal. This may be beneficial in a case where the mean frequency of the controlled laser light is affected by an interruption signal. Ensuring a fixed periodic triggering of the interruption signal may prevent oscillation of the mean laser frequency due to a feedback coupling into the error and control signals.

[0089] Figure 3 shows a schematic illustration of how a feedback signal may be generated by the control unit 102 when the adjustment action comprises adjustment of the resonance frequency of the resonant optical cavity 106. The feedback signal may be provided to a cavity control entity (not depicted).

[0090] Figure 4 shows a schematic illustration of how a feedback signal may be generated by the control unit 102 when the adjustment action comprises utilizing the error parameter in providing a feedback signal to a laser frequency control entity, said laser frequency control entity being configured to control the frequency of the controlled laser light. The feedback signal may be provided to a laser frequency control entity (not depicted).

[0091] Figure 5 shows a schematic illustration of how a feedback signal may be generated by the control unit 102 when the adjustment action comprises utilizing the error parameter in providing a feedback signal for providing the periodic control signal, said feedback signal being utilized for adjustment of a signal offset of the periodic control signal. Figure 5 shows the feedback signal being used to control a mean signal amplitude, which is one option for carrying out the laser locking / stabilization.

[0092] Figure 6 shows one example of how an error parameter may be determined and used. Here, the periodic control signal comprises a sawtooth / ramp signal waveform. When the input signal exceeds the first trigger value, a triggered phase value or triggered signal value of the periodic control signal may be determined. An error parameter (indicative of a phase error or signal value error) may be determined as a deviation between a predetermined phase value or signal value and the triggered phase value or triggered signal value. The predetermined phase value may be a phase value that essentially corresponds to a center of the ramp, i.e. where the periodic control signal has a mean value, or at least is within a selected distance from a phase value that corresponds to the center of the ramp. The predetermined signal value may be a mean signal value of the periodic control signal, or at least be within a selected distance from a mean signal value of the periodic control signal.

[0093] The error parameter may be used to facilitate performing of an adjustment action. In Fig. 6, the effect of an adjustment action (that is carried out in response to the input signal exceeding the first trigger value regarding the first period of the periodic control signal that is depicted) may be seen during the second period of the periodic control signal. Regarding the second period, after the input signal exceeds the first trigger value, i.e. the laser frequency matches the resonance frequency of the resonant optical cavity, a triggered signal value may be closer to the predetermined signal value and / or the triggered phase value may be closer to the predetermined phase value than regarding the first period.

[0094] Figure 7 shows a one further example of how an error parameter may be determined and used. In the example of Fig. 7, the periodic control signal comprises a triangular waveform. In cases where the periodic control signal waveform is such that the signal has a plurality of phase values that correspond to the same signal value, an error parameter (and subsequent adjustment action) may be determined a plurality of time during one period of the periodic control signal. In the case of Fig. 7, an error parameter may be determined twice per period (if the first trigger value of the input signal is exceeded). Here, one predetermined signal value may be utilized, while there may be two predetermined phase values (corresponding to one predetermined signal value). In using a predetermined phase value to determine an error parameter, for a negative slope of the periodic control signal, the error parameter may be inverted to obtain a correct feedback signal. Otherwise, the example of Fig. 7 may essentially correspond to the example of Fig. 6. Figure 8 illustrates the effect of performing adjustment actions. Figs. 8A, 8B, and 8C show, as a function of time, a determined error parameter which in this case is an error parameter determined using signal values (predetermined signal value and triggered signal value). In Fig. 8A, a servo feedback control system comprising e.g. servo filter is active and is used to facilitate the performing of adjustment actions in response to determining the error parameter. In Figs. 8B and 8C a servo feedback control system is not active, and thus adjustment actions are not facilitated and performed. In Fig. 8B, the effect of random noise is shown, while in Fig. 8C, a strong laser drift is induced. In Fig. 8B, the laser is not tuned into resonance with a resonant optical cavity for the time between 22 and 27s and therefore no error values are determined for this time, while for Fig. 8C, the laser is not tuned into resonance with a resonant optical cavity for the time between 71 and 82 seconds. Fig. 8 clearly illustrates how the error parameter stays significantly smaller in the case of Fig. 8A where the present invention is utilized and adjustment actions are facilitated.

[0095] Figure 9 depicts a flow chart of method according to an embodiment of the invention. The method comprises providing 002 a periodic control signal for controlling a frequency of light being provided by a laser apparatus to obtain controlled laser light. The method also comprises obtaining 004 an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity.

[0096] At 006, the method comprises determining if the input signal exceeds a predetermined first trigger value, said first trigger value preferably essentially corresponding to an intensity of light being detected indicating that a frequency of light being provided by the laser apparatus that corresponds to a resonance frequency of the resonant optical cavity. As long as the input signal does not exceed the first trigger value, the method may not continue.

[0097] Yet, upon at step 006 determining that the input signal does exceed the first trigger value, the method further comprises determining 008 an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase or triggered signal value of the periodic control signal essentially corresponds to a phase value or signal value of the first period of the periodic control signal at a time when the input signal exceeds the first trigger value.

[0098] The method further comprises utilizing the error parameter to facilitate 010 performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity. The adjustment action may result in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity, by resulting in an error parameter preferably being smaller than a previous error parameter that has been determined at a time when the input signal subsequently exceeds the first trigger value.

[0099] 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.

[0100] The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.

Claims

CLAIMS1. A device for controlling a frequency of light being provided by a laser apparatus, said laser apparatus being configured to illuminate a resonant optical cavity, such as a cavity ring-down spectroscopy (CRDS) optical cavity, the device comprising at least one control unit, wherein the control unit is configured to:- provide a periodic control signal for controlling a frequency of light being provided by a laser apparatus, to scan a frequency of light being provided by the laser apparatus to obtain controlled laser light,- obtain an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity, and- upon determining that the input signal exceeds a predetermined first trigger value, said first trigger value preferably essentially corresponding to an intensity of light being detected indicating that a frequency of light being provided by the laser apparatus essentially corresponds to a resonance frequency of the resonant optical cavity o determine an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase value or triggered signal value of the periodic control signal essentially corresponds to a phase or signal value of the periodic control signal at a time when the input signal exceeds the first trigger value, and o utilize the error parameter to facilitate performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity, preferably resulting in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity.

2. The device of claim 1 , wherein the adjustment action comprises utilizing the error parameter in providing a feedback signal for adjustment of the resonance frequency of the resonant optical cavity.

3. The device of any previous claim, wherein the adjustment actioncomprises utilizing the error parameter in providing a feedback signal for a laser frequency control entity, said laser frequency control entity being configured to control the frequency of the controlled laser light.

4. The device of any previous claim, wherein the adjustment action comprises utilizing the error parameter in providing a feedback signal for providing the periodic control signal, said feedback signal being utilized for adjustment of a signal offset of the periodic control signal for at least the second period of the periodic control signal.

5. The device of any previous claim, wherein after performing said adjustment action and upon subsequently determining that the input signal exceeds the predetermined first trigger value, a triggered signal value of the periodic control signal is closer to the predetermined signal value or a signal value corresponding to the predetermined phase value.

6. The device of any previous claim, wherein the predetermined phase value essentially corresponds to or is within a selected distance from a phase value where the periodic control signal has a mean value or the predetermined signal value essentially corresponds to or is within a selected distance from the mean value of the periodic control signal.

7. The device of any previous claim, wherein upon determining that the input signal exceeds a predetermined second trigger value, the control unit is configured to provide an interruption signal, wherein the interruption signal is utilized to deactivate the laser apparatus, to deflect the laser light from the resonant optical cavity, or to obtain controlled laser light comprising a frequency which deviates from the resonance frequency of the resonant optical cavity by a selected amount, wherein the interruption signal is applied for a selected period of time.

8. The device of claim 7, wherein upon determining that the input signal exceeds the predetermined second trigger value, the control unit is additionally configured to facilitate evaluation of a decay time constant of the resonant optical cavity.

9. The device of any previous claim, wherein the control unit is configured to obtain the input signal essentially continuously and utilize the input signal to determine an error parameter and facilitate performing of at least one adjustment action upon need essentially continuously for a plurality ofconsecutive periods of the periodic control signal, preferably wherein each adjustment action takes into account a plurality of, optionally all, previously determined error parameters.

10. The device of any previous claim, wherein regarding a plurality of periods of the periodic control signal, for a first period in the plurality, an associated predetermined first trigger value of the input signal is higher than an associated predetermined first trigger value of the input signal for at least a second period.11 . The device of any previous claim, wherein the control unit is configured to utilize the error parameter in providing a feedback signal for facilitating performing of the adjustment action, wherein the control unit is configured to deliver the error parameter to a servo filter, optionally an integrator filter or Proportional-integral-derivative filter, the output of which is used in the feedback signal.

12. The device of any previous claim, further wherein if upon determining that the input signal exceeds a predetermined first trigger value and further determining that the triggered phase value or signal value of the periodic control signal does not correspond to a predetermined phase range, the error parameter is not determined.

13. The device of any previous claim, wherein the device is configured to control a frequency of light being provided by a laser apparatus being a tunable laser apparatus, such as a quantum cascade laser (QCL) device.

14. An arrangement for performing laser spectroscopy, the arrangement comprising at least a resonant optical cavity, a laser apparatus, a detector, and a device of any previous claims, wherein the device is configured to control a frequency of light being provided by the laser apparatus, the laser apparatus is configured to illuminate the resonant optical cavity, the detector is configured to detect intensity of light in the resonant optical cavity, the detector additionally being configured to provide the input signal to the device.

15. A method for controlling a frequency of light being provided by a laser apparatus, said laser apparatus being configured to illuminate a resonant optical cavity, such as a cavity ring-down spectroscopy (CRDS) optical cavity, the method comprising:- providing a periodic control signal for controlling a frequency of light being provided by a laser apparatus to scan a frequency of light being provided by the laser apparatus to obtain controlled laser light, wherein the method comprises,- obtaining an input signal being indicative of light intensity in a resonant optical cavity being effectuated by provision of the controlled laser light into the resonant optical cavity, and- determining if the input signal exceeds a predetermined first trigger value, said first trigger value preferably essentially corresponding to an intensity of light being detected indicating that a frequency of light being provided by the laser apparatus that corresponds to a resonance frequency of the resonant optical cavity, and if yes o determining an error parameter being indicative of a difference between a predetermined phase value or predetermined signal value and a determined triggered phase value or triggered signal value of the periodic control signal, wherein the triggered phase or triggered signal value of the periodic control signal essentially corresponds to a phase value or signal value of the periodic control signal at a time when the input signal exceeds the first trigger value, and o utilizing the error parameter to facilitate performing of at least one adjustment action, said adjustment action relating to adjustment of the controlled laser light or adjustment of a resonance frequency of the resonant optical cavity, preferably resulting in the frequency of the controlled laser light periodically matching a resonance frequency of the resonant optical cavity.

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