Patent Application
20140264031
The present invention relates to trace gas detection system based on frequency combs.
In recent years interest in high resolution optical spectroscopy has increased. The following exemplary patents, published patent applications, and publications relate to light sources for precision optical frequency measurement and applications of the same in high resolution spectroscopy:
Holzwarth et al., U.S. Pat. No. 6,724,788 entitled ‘Method and device for generating radiation with stabilized frequency’;
Holzwarth et al., U.S. Pat. No. 6,785,303, entitled ‘Generation of stabilized, ultra-short light pulses and the use thereof for synthesizing optical frequencies’;
Haensch et al., U.S. Pat. No. 6,897,959, entitled “Frequency comb analysis”;
Fermann et al., U.S. Pat. No. 7,190,705, entitled ‘Pulsed laser sources’;
Fermann et al., U.S. Pat. No. 7,649,915, entitled ‘Pulsed laser sources’;
Hartl et al., U.S. Pat. No. 7,809,222, entitled ‘Laser based frequency standards and their applications’;
Gohle et al., U.S. Pat. No. 8,120,773, entitled ‘Method and device for cavity enhanced optical vernier spectroscopy’;
Fermann et al., U.S. Pat. No. 8,120,778: entitled ‘Optical scanning and imaging systems based on dual pulsed laser systems’;
Giaccari et al. U.S. Patent Application Pub. No. 2011/0043815, entitled ‘Referencing of the Beating Spectra of Frequency Combs’;
Vodopyanov et al., U.S. Patent Application Pub. No. 2011/0058248, entitled ‘Infrared frequency comb methods, arrangements and applications’;
T. Sizer, ‘Increase in laser repetition rate by spectral selection’, IEEE J. Quantum Electronics, vol. 25, pp. 97-103 (1989);
S. Diddams et al., ‘Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb’, Nature, vol. 445, pp. 627 (2007);
R. Gebs et al., ‘1 GHz repetition rate femtosecond OPO with stabilized offset between signal and idler frequency combs’, Opt. Expr., vol. 16, pp. 5397-5405 (2008)
F. Adler et al., Phase-stabilized, 1.5 W frequency comb at 2.8 μm-4.8 μm, Opt. Lett., vol. 34, pp. 1330-1332 (2009),
A. Foltynowicz et al., ‘Optical frequency comb spectroscopy’, Faraday Discussions, vol. 150, pp. 23-31, 2011
Kohlhaas et al., ‘Robust laser frequency stabilization by serrodyne modulation’, Opt. Lett., vol. 37, pp. 1005 (2012); and
N. Leindecker et al., Opt. Expr., ‘Octave-spanning ultrafast OPO with 2.6-6.1 μm instantaneous bandwidth pumped by femtosecond Tm-fiber laser’, Opt. Expr., 20, 7046 (2012).
Advances in frequency measurement methods and systems have occurred over the past several years with the use of optical frequency combs. However, high resolution, broadband measurement in the mid-IR spectral region and beyond remains challenging.
Cavity enhanced spectroscopy systems are difficult to implement in a reliable fashion due to the complexity of the required electronics and components required to facilitate stable coupling of a source to the cavity. To reduce the complexity of the electronics required to couple a frequency comb laser into a cavity, and to reduce amplitude noise from cavity length fluctuations, a dither lock to the cavity can be implemented. Dither locking of enhancement cavities to modelocked lasers is well known in the state of the art and was for example described in T. Gherman and D. Romanini, ‘Modelocked Cavity—Enhanced Absorption Spectroscopy’, Opt. Express, vol. 10, 1033 (2002). In some configurations, when implementing dither locking, the comb spacing of the frequency ruler and the enhancement cavity are adjusted to be integer multiples of each other. The relative location of the cavity or source comb modes is then scanned in frequency space by about one free spectral range (FSR) of the cavity, though smaller and larger scan ranges can also be implemented. As the cavity length is swept, the resonant frequencies of the cavity change as well, such that every comb line will be coupled into the cavity for some small period of time. In the absence of dispersion, all comb lines would be coupled in at the same point in time. The presence of the sample gas in the cavity and other intracavity components may introduce dispersion. With dispersion, a slight mismatch between the cavity mode spacing and the frequency ruler means that different comb lines will couple to the cavity at slightly different times. With this kind of coupling, the average cavity transmission is reduced significantly compared to a system configuration where the cavity is locked to the frequency ruler. For a dither scan range of 10 MHz and a cavity line width of 10 kHz, the average cavity transmission can be reduced by up to a factor on the order of 1000. Therefore, dither-locked cavity enhanced spectroscopy requires relatively high laser powers and has not been demonstrated due to the lack of appropriate laser sources in the mid-IR.
Frequency combs comprise a highly developed technology platform that has been used in many advanced optical technologies. Here we present a system configuration based on frequency combs that can be used for cavity enhanced and cavity enhanced direct comb spectroscopy.
At least one embodiment of a trace gas detection system includes an optical source which produces as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range. An enhancement cavity, containing a sample gas for spectroscopic measurement, is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity may be characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range. The first 1st spectral range and 2nd spectral range overlap. The system further includes a dither mechanism configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A feedback mechanism is coupled to the dither mechanism to stabilize the location of the 1st comb lines with respect to the resonances of the 2nd comb on a time scale much greater than a dither period, Td=1/fd. A Fourier transform spectrometer is configured to receive the secondary output, and to measure the spectrum of a time-averaged signal transmitted by the cavity over a time scale much longer than Td.
At least one embodiment of a trace gas detection system includes an optical source producing as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range. The first spectral range includes wavelengths>1600 nm. An enhancement cavity, containing a sample gas for spectroscopic measurement, is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity may be characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range. The 1st spectral range and 2nd spectral range overlap. The system further includes a dither mechanism configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A feedback mechanism is coupled to the dither mechanism to stabilize the location of the 1st comb lines with respect to the resonances of the 2nd comb on a time scale much greater than a dither period, Td=1/fd. A spectroscopic measurement tool, which includes an optical detection system, is arranged for frequency resolved detection of a time-averaged signal transmitted through the enhancement cavity.
At least one embodiment of a trace gas detection system includes an optical source producing as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range. An enhancement cavity, containing a sample gas for spectroscopic measurement, is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity may be characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range. The 1st spectral range and 2nd spectral ranges overlap. The system further includes a dither mechanism configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A spectroscopic measurement tool is configured to receive the secondary output, and to measure the spectrum of a time-averaged signal transmitted by the cavity over a time scale much longer than Td=1/fd. The spectroscopic tool is arranged to provide a signal for synchronization of dithering with spectroscopic data acquisition.
Any form of frequency comb can be implemented. For example, frequency combs based on quantum cascade lasers, micro-resonators, or mode locked lasers can be used. Mode locked lasers based on fiber, semiconductor or solid-state technology can be implemented. Appropriate amplification stages can further be used for signal amplification.
To shift the spectral output of the modelocked lasers into a spectral region of interest, a frequency shifting device such as supercontinuum generator, difference frequency generator, optical parametric oscillator (OPO), or optical parametric amplifier (OPA) can be used.
To couple the light from a frequency comb system into an enhancement cavity a dither lock is implemented, where the comb modes of either the cavity or the frequency comb system are rapidly dithered around an average value.
The comb modes can be dithered using a modulation of the carrier envelope offset frequency of the frequency comb, its comb mode spacing or the cavity length of the enhancement cavity. Additional optical frequency shifter(s) can also be incorporated between the comb source and the enhancement cavity.
To facilitate spectroscopic measurements the enhancement cavity is filled with a gas and the spectrum transmitted through the cavity is detected using dispersive optical systems such as diffraction gratings or VIPAs and one or two dimensional detector arrays.
Alternatively, spectral detection can be performed with conventional Fourier transform spectrometers.
In order to minimize amplitude fluctuations when using a Fourier transform spectrometer, it is beneficial to synchronize the zero-crossings in the Fourier transform detection system with the dither function of the enhancement cavity.
The spectroscopy system as discussed here can be used for trace gas detection such as that used in medical breath analysis. Of particular interest is the detection of molecules and volatile organic compounds (VOC) with absorption bands in the 3-5 μm and the 5-12 μm spectral ranges, with endogenous compounds being of particular interest.
a schematically illustrates an output spectrum of an OPO.
Optical spectroscopy has experienced a great resurgence in interest since the introduction of optical frequency combs as, for example, exemplified in U.S. Pat. No. 6,785,303: ‘Generation of stabilized, ultra-short light pulses and the use thereof for synthesizing optical frequencies’ and U.S. Pat. No. 6,724,788: ‘Method and device for generating radiation with stabilized frequency’. Frequency combs are disclosed in '303 as having an output spectrum which can be written as fn=nfrep+f0 (e.g.: column 2, lines 4-11), where n is an integer and fn denotes the frequencies of individual comb modes. The frequency spectrum 100 of such a conventional frequency comb laser is further illustrated in
Some frequency comb systems utilize mode-locked fiber oscillators to produce an output spectrum as illustrated in
The pump light for the oscillator 801 can be directed via a polarization-maintaining wavelength division multiplexing coupler 128 from a coupler arm 129 attached to a preferably single-mode pump diode 130. The pump current to the pump diode 830 can be modulated to stabilize the beat signal frequency and the carrier envelope offset frequency using feedback based on the signal at one selected frequency.
Monitoring and control of f0 and frep provide for full characterization of the comb. The oscillator output, which may be amplified with an optional fiber amplifier, may be supplied to an f-2f interferometer (not shown) in which the well-known self-referencing technique can be used to extract f0 via detection of a beat signal. The repetition rate frep may be monitored or stabilized in an arrangement having an electronic phase locked loop comprising high speed photodetector(s), RF-amplifier(s), RF bandpass filter(s), phase detector(s) and loop filters, as discussed in '222.
Electronic feedback loops may be used stabilize the comb. In particular,
As shown in
Many possibilities exist. Further information regarding the above arrangements may be found in '915 and '222. In accordance with the present invention such arrangements may be utilized in various embodiments or modified in various ways for use in high resolution spectroscopy systems, as will be further discussed below. For example, as will become apparent in the present disclosure, control circuits for rapidly modulating pump current supplied to the oscillator, grating pressure, and/or grating temperature may be advantageously used to vary f0, and such rapid modulation may be over a small modulation depth compared to the operating range of the device.
For any instrumentation applications, frequency combs based on mode-locked fiber lasers have several advantages over both mode-locked bulk solid state lasers and mode-locked diode lasers. Mode-locked fiber lasers offer typically superior noise properties compared to mode-locked diode lasers and can be packaged in smaller spaces than mode-locked bulk solid state lasers. Mode-locked fiber lasers can be produced with excellent thermal and mechanical stability. In particular, passively mode-locked fiber lasers can be constructed with few and inexpensive optical components, suitable for mass production, as disclosed in U.S. Pat. No. 7,190,705 ('705) and U.S. Pat. No. 7,809,222 ('222). The dispersion compensated fiber lasers as disclosed in '705 provide for the construction of low noise frequency comb sources. Also disclosed were designs of fiber lasers operating at repetition rates in excess of 1 GHz. As a compact alternative to mode locked fiber lasers, frequency combs based on micro-resonators or quantum cascade lasers can also be used.
Low-noise operation of fiber lasers limits timing jitter, allowing optimized control of the timing of the pulses. The '705 patent disclosed the first low noise fiber-based frequency comb source. Low noise operation was obtained by controlling the fiber cavity dispersion in a certain well-defined range. Low noise operation of fiber frequency comb sources reduces the noise of the carrier envelope offset frequency f0 of the laser to a negligible level, and also facilitates measurement and control of f0.
Exemplary applications of optical frequency combs have been demonstrated in Fourier transform spectroscopy based on two frequency comb lasers operating at slightly different repetition rates as discussed in U.S. Patent Application Pub. No. 2011/0043815, entitled ‘Referencing of the Beating Spectra of Frequency Combs’, Other spectroscopy applications include measuring the response function of samples with frequency combs as discussed in ‘Frequency comb analysis’, U.S. Pat. No. 6,897,959. Many other examples can be found in the literature.
In addition to the construction of frequency combs as discussed in '303, other implementations of frequency combs have been demonstrated. One such implementation of a frequency comb is shown in
Other nonlinear optical devices have been demonstrated where the relation fn=nfrep+f0 also holds. Examples of such optical devices are highly nonlinear optical fibers that generate a supercontinuum output, for example as described in U.S. Pat. No. 7,809,222. Another example can be a degenerate synchronously pumped optical parametric oscillator (DOPO), for example as described in ‘Infrared frequency comb methods, arrangements and applications’, U.S. Patent Application Pub. No. 2011/0058248, and in N. Leindecker et al., Opt. Expr., ‘Octave-spanning ultrafast OPO with 2.6-6.1 μm instantaneous bandwidth pumped by femtosecond Tm-fiber laser’, Opt. Expr., 20, 7046 (2012). Because the DOPO is synchronously pumped, its repetition rate is the same as the repetition rate of the pump laser.
In some other devices, the relation fn=nfrep+f0 does not hold for the output frequency range of the device. An example of such a device is a non-degenerate OPO (NOPO), where generally the idler and signal frequency have different unstable carrier envelope offset frequencies f0i and f0s respectively, even when the pump f0p is stabilized. As described in F. Adler et al., ‘Phase-stabilized, 1.5 W frequency comb at 2.8 μm-4.8 μm’, Opt. Lett., vol. 34, pp. 1330-1332 (2009), additional electronic feedback loops need to be implemented inside the NOPO that stabilize the carrier envelope offset frequencies of either the signal or idler frequency, f0s or f0i respectively. If f0p is stabilized, and either f0s or f0i is also stabilized, the carrier envelope offset frequencies at both signal and idler frequencies can be determined because, due to energy conservation, f0p=f0s+f0i.
In other devices, such as weakly non-degenerate OPOs (WOPOs), the difference of f0s and f0i can also be stabilized by taking advantage of overlapping signal and idler spectra, as described in R. Gebs et al., ‘1 GHz repetition rate femtosecond OPO with stabilized offset between signal and idler frequency combs’, Opt. Expr., vol. 16, pp. 5397-5405 (2008).
In yet other devices, degenerate doubly resonant synchronously pumped OPOs (DOPOs) were suggested as versatile mid IR sources for operation with stable carrier phase when pumped with a fiber laser comb source, see N. Leindecker et al., Opt. Expr., ‘Octave-spanning ultrafast OPO with 2.6-6.1 μm instantaneous bandwidth pumped by femtosecond Tm-fiber laser’, Opt. Expr., 20, 7046 (2012).
In yet other devices, synchronously pumped non-degenerate optical parametric oscillator (DNOPOs) were suggested as versatile mid IR sources for operation with stable carrier phase in U.S. Patent Application No. 61/764,355, ('355), entitled “Optical frequency ruler”, filed Feb. 13, 2013, which is hereby incorporated by reference in its entirety. An example of a typical optical arrangement for a DNOPO is shown in
A generic frequency ruler generated by such a DNOPO is shown in
Examples of optical sources for spectroscopy applications, and more particularly for embodiments directed to cavity enhanced spectroscopy, include: frequency combs, mode locked lasers, DFG, OPOs, OPAs and frequency shifted mode locked lasers based on, for example, supercontinuum generation.
Various embodiments of frequency comb lasers can be constructed at comb spacings of >300 MHz, or preferably >500 MHz and most preferably at comb spacings>1 GHz for applications in direct comb spectroscopy. Methods for direct comb spectroscopy were for example disclosed in U.S. patent application Ser. No. 12/955,759, ('759), entitled: ‘Frequency comb source with large comb spacing’, filed Nov. 29, 2010. In brief, when using a frequency ruler with a comb spacing>500 MHz, bulk optic components can be readily used to resolve individual comb lines and the individual comb lines can then be detected with a detector array. One such implementation was discussed in '759.
A scheme with a solid-state laser based multi-GHz repetition rate comb system and a two dimensional angular dispersion element as well as a two dimensional detector array was previously described in S. Diddams et al., ‘Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb’, Nature, vol. 445, pp. 627 (2007). However, a system with a fiber laser pumped GHz-level repetition rate OPO was not considered. With advancements as described herein, low noise OPO frequency rulers at repetition rates of 1 GHz and higher can be constructed which make such schemes very attractive.
A frequency resolution equivalent to the ruler line width can be obtained by slowly scanning the comb spacing or carrier envelope offset frequency of the frequency ruler, and detecting with a resolution approximately twice higher than the repetition rate of the frequency ruler, sufficient to separate individual comb lines. Integrating the signal over adjacent frequencies that are identified as belonging to a comb line gives the signal at that comb line. This results in a frequency resolution that is several orders of magnitude better than the frequency resolution of the detection system with a standard light source.
For example, a typical Fourier transform spectrometer (FTS) can have a resolution of 500 MHz, which is sufficient to resolve comb lines for comb spacings of 1 GHz. Using a Fourier transform spectrometer provides a significant cost benefit from using a single channel detector rather than a two dimensional detector array, and Fourier transform spectrometers can have very broad bandwidths, up to the entire bandwidth of the detector. For example, HgCdTe detector provides for detection over an optical bandwidth from about 2 to 13 μm.
For comb spacings larger than around 10 GHz, individual comb lines can, for example, be resolved using two or more conventional diffraction gratings in series or multiple passes or reflections from a single grating. A grating system has the cost advantage of a single-dimensional detector array rather than a two-dimensional array. Compared to a Fourier transform spectrometer, it can have a faster acquisition rate, and does not have moving parts, but it has the disadvantage of a much lower detection bandwidth.
Large comb spacings further allow the implementation of broadband differential absorption spectroscopy. In such a system, the position of the comb lines can be slowly scanned and at the same time modulated at high frequencies in frequency space in order to generate a time dependent modulation of the signal impinging on the comb resolved detection system. Such schemes are well known from single laser spectroscopy. Many other spectroscopic techniques can be adapted to broadband detection where the principle requirement is the optical resolution of individual comb lines.
A particularly attractive scheme for broadband trace gas detection is based on cavity enhanced trace gas detection as disclosed in U.S. Patent Application No. 61/617,482, ('482), entitled ‘Methods for precision optical frequency synthesis and molecular detection’, filed Mar. 29, 2012, to Fermann et al. The '482 application is hereby incorporated by reference in its entirety. When combined with a frequency ruler or frequency comb with large frequency spacing, broadband detection of multiple gas species can be performed simultaneously.
As described in patent and non-patent literature a frequency comb source has sometimes been associated with arrangements in which one or both of the repeitition rate, frep, or carrier envelope offest, fo, are phased locked to reference signals, for example, with phase locked loop(s). It is to be understood that such phase locking is not necessary to the practice of each and every embodiment of the present invention. For example, unless other specified, carrier envelope offest, fo, may slip or be free-floating with allowable variation (which may be pre-determined). A frequency comb source (or frequency ruler) may operate in the absence of phase locking. In some embodiments one or both of the repetition rate, frep, or carrier envelope offest, fo, are phased locked to reference signals, and may be preferred for certain high-resolution spectroscopy applications.
The output of the frequency ruler (e.g.: a frequency comb) is coupled to an enhancement cavity 420. The enhancement cavity may contain a gas sample for spectroscopic measurement. The enhancement cavity may be characterized by having a comb of approximately equidistant spectral reonances 460-a within a second spectral range (as determined by the coatings of the cavity mirrors). The second spectral range is to overlap the first spectral range associated with the frequency comb output 450.
In certain embodiments a frequency comb source 410 may produce a comb spacing in the range from about 50 MHz to greater than 1 GHz. Enhancement cavity 420 may be configured such that a linewidth of a resonance is in the range from about 1 kHz to 100 kHz. In some embodiments the enhancement cavity may have a comb spacing (e.g.: second comb spacing) which is an integer multiple of the first comb spacing, or an integer fraction of the first comb spacing.
The arrangement 400 employs various mechanisms for monitoring and stabilizing at least the frequency ruler 410 and cavity 420. A frequency dither mechanism is included to lock the ruler or comb frequencies (1st comb) to the resonances (2nd comb) of the enhancement cavity. Thus, with comb 450 as input to enhancement cavity 420, the output of the enhancement cavity will include a second frequency comb corresponding to a secondary output 460 with the comb lines 460-b spaced at the approximately equidistant spectral resonances 460-a and centered, on the average, at the peaks of the enhancement cavity resonances 460-a. The spectrum of the time-averaged signal transmitted by enhancement cavity 420 is an output available for downstream spectroscopic measurement as will be further discussed below.
A control unit 440 as schematically represented in
In particular, a frequency dither mechanism is included which modulates the relative position between the first comb produced by the frequency comb source 450 and the second comb of spectral resonances 460-a. The modulation occurs at a dither frequency, fd, and a corresponding dither period, Td. A dither frequency may be in the range from about 100 Hz to about 100 kHz, and in some embodiments may be at or near 10 kHz. A feedback mechanism, which may include one or more servo loops for monitoring/controlling the comb source and/or enhancement cavity, is arranged to center, on the average, comb lines of comb 450 within cavity resonances 460-a of the enhancement cavity 420.
The resulting output 460 of the enhancement cavity, as illustrated in
The system further includes a tool for spectroscopic measurement, for example Fourier transform spectrometer (FTS) 430 which provides as an output the spectrum 470 of a sample. In a conventional FTS arrangement a time delay is introduced between two arms of an interferometer. The time delay between the arms can be varied by translating a reflector. The recombined light intensity is then detected and recorded as a function of path delay, which is measured, for example, by a HeNe reference laser which simultaneously propagates through the interferometer. In accordance with the present invention the spectrometer is advantageously arranged for operation with the frequency comb and corresponding optical pulses by synchronizing FTS data acquisition to the dithering via control unit 440 and feedback mechanisms as associated therewith, as discussed above. Other aspects, features, and advantages of various embodiments and arrangements will become more apparent from the following examples, discussion, and the accompanying drawings.
A system configuration implementing a frequency ruler dither locked to an enhancement cavity for cavity enhanced spectroscopy in the mid IR spectral region is shown in
The round trip time of the cavity is further locked to the frequency comb spacing of the cavity with a dither lock and the servo loop which may be included in or interfaced to control unit 440. An appropriate electronic locking scheme for implementing a dither lock was for example described with respect to FIG. 8 of M. J. Thorpe et al., ‘Cavity-enhanced direct frequency comb spectroscopy’, Appl. Phys. B., vol. B91, pp. 397-414 (2008). In brief, the cavity is dithered across the resonance using a triangle waveform applied to the piezo-electric transducer (PZT) that controls the cavity length. Since the cavity drifts slowly, an additional DC offset is applied to the PZT. This DC offset is regulated such that the cavity dither is always approximately centered around the resonant length of the input light.
An electronic control scheme generally referred to as a flip flop servo loop may be utilized and is well known in the state of the art. The feedback circuit is implemented as follows: From the triangle scan waveform applied to the PZT, an auxiliary square wave SQW1 is generated which flips at each change of the scan direction. A photodetector-comparator combination generates a second square wave SQW2 with rising edges aligned to the points where the cavity transmission reaches a pre-set threshold from below. The threshold may be set at about 3-10 times the peak-peak noise level so that a stable square wave is obtained. This signal is used as the clock on a D-flip-flop, sampling SQW1 applied to the D-input of the flip-flop at the rising edges of SQW2. The mark-to space ratio of the D-flip-flop's output wave SQW3 is now a measure of the alignment of the cavity resonance to the triangle dither scan. That is, if the transmission threshold is reached exactly at the center of the scan, the mark-to-space ratio would be 1:1. An integrator converts the mark-to-space ratio of SQW3 to a proportional DC voltage which is used for slow feedback control of the PZT offset voltage. The above description is to serve only as an example and many alternative implementation of flip flop servo loops or similar arrangements may be used in the servo loop of
A dither scan range of one free spectral range ensures that the frequency comb will be coupled into the cavity at some point during the sweep. This scan range also means that most of the time, light will not be coupled into the cavity. To increase cavity transmission, the dither scan range can be reduced to a fraction of the free spectral range of the cavity. The flip flop circuit described above can be implemented to keep the dither centered on resonance, however, other electronic control loops may also be used for the same purpose. In the case of a flip-flop circuit, the light reflected from or transmitted through the cavity can be sampled with detector D1 to keep the cavity dither on resonance. The dither frequency and the dither magnitude are easily controlled by the frequency and magnitude applied to the drive signal of the intra-cavity PZT. The useful dither frequency is limited by the finesse of the cavity. The cavity must be resonant for enough time for the intracavity field to become large enough to enable strong coupling into the cavity.
Moreover, the cavity spacing can be adjusted depending on which spectral region is being detected; this accounts for a mismatch in mode spacing between the frequency ruler and the enhancement cavity due to dispersion.
An alternative to dithering the cavity length is to dither the laser frequencies as shown in
In an exemplary implementation, a DNOPO can be implemented as a frequency ruler. The comb spacing of the DNOPO can be dithered by dithering the cavity length of the DNOPO pump laser (e.g.: the frequency ruler or comb source). Alternatively, or in combination, the carrier envelope offset frequency of a DNOPO can be changed by changing the carrier envelope offset frequency or repetition rate of the DNOPO pump laser or the DNOPO cavity length. Controlling the laser comb source 410 rather than the cavity 420 has the advantage that the laser frequency combs can be controlled at a much faster rate than the speed of moving an enhancement cavity mirror. For example, the carrier envelope offset frequency of a mode locked laser is often controlled by adjusting the power of its pump laser, which can be done quickly by combining the main pump with a faster supplementary pump.
For example, referring back to
There are also many methods of controlling the repetition rate, for example, using piezoelectric transducers to move a laser cavity mirror, or to stretch a spool of optical fiber, thereby controlling the cavity length as, for example, illustrated and discussed with respect to
The frequency comb lines can further be modulated using external modulators. For example an acousto-optic frequency shifter (AOFS in
Using an AOFS is particularly beneficial because it separates the dithering function from the control of the frequency comb laser, The faster response time enabled by such comb dithering can be used to reduce the dither range, increasing transmission through the enhancement cavity. The faster response time of comb dithering can also be used to lock the comb laser to the cavity, yielding less amplitude noise than when locking the cavity to the comb laser. For this method to provide single comb-line resolution the cavity length must fluctuate enough so that all frequencies are occasionally transmitted through the cavity.
For some applications, the frequency dependent beam pointing from the AOFS may be a limitation. However, as discussed in the '482 application, this can be eliminated by double passing the AOFS as described, for example, in E. A. Donley et al., ‘Double-pass acousto-optic modulator system’, Rev. of Scientific Instruments, vol. 76, pp. 063112 (2005). A double-pass through an AOFS effectively doubles the modulation frequency, therefore the AOFS drive frequency needs to be divided by two to produce the right frequency correction to the cw laser.
Frequency dithering and cavity length scanning can be combined to yield a high-throughput, high-resolution system, with relatively simple locking requirements, as shown in
A low cost spectroscopic detection system for cavity-enhanced spectroscopy is a Fourier transform spectrometer (FTS), which can provide high resolution, and broad bandwidth. A standard FTS includes an interferometer where the time delay between the two arms can be scanned by a moving carriage (e.g.: translation stage) with a reflector. The recombined light intensity is detected and recorded as a function of path delay, which is measured by, for example, a HeNe reference laser which simultaneously propagates through the interferometer. However, a conventional FTS operates with continuous, or effectively continuous light, rather than the intermittent, time-separated pulses that result from dithered transmission through a cavity.
In at least one embodiment of the present invention the system 400 is arranged for use with dither-controlled cavity enhanced detection schemes by synchronizing the FTS data acquisition to the dithering based on a control signal. The FTS 430 may be configured to sample the signal transmitted through the enhancement cavity 420 in synchronism with zero crossings of an interference signal generated with the FTS internal reference laser (not shown). A dither period, Td, may also be derived from the zero crossings and used to control a dither mechanism coupled to optical source 410 and/or cavity 420 via the feedback mechanism. In one implementation, the FTS detector is coupled with a long time constant, for example about 1 msec. As such, the pulses arriving at the dither rate appear effectively continuous for the detection. The carriage speed is then synchronized to the dither rate to have the same number of bursts of light for each acquired point. For example, if one data point is acquired for every FTS reference-laser wavelength of path delay, and a group of pulses arrive at a group rate of 1 kHz, the carriage speed will be an integer fraction of (reference-laser wavelength*1 kHz). By synchronizing the carriage speed to the dither rate, shot noise problems from acquiring irregular numbers of pulses per data point are avoided.
In a related implementation, as shown in
An example of timing signals for synchronizing the FTS, dither, and acquisition are illustrated in
In at least one embodiment, a more flexible implementation is provided. Synchronization of the reference laser zero crossings, dither and signal acquisition, is replaced by relatively fast data acquisition, at a rate higher than the dither rate, such that the burst of light for each dither period is well-resolved. The multiple peaks within a burst due to the dispersion do not need to be resolved. For example, all signals are acquired simultaneously at 1 MHz, while the dither rates are on the scale of 10 kHz. Two signals are acquired for the two quadratures of the reference laser, and the two signals at both interferometer outputs are measured. The reference laser provides the path delay for each measurement. Acquiring both quadratures provides the absolute path delay for each measurement, as is common in FTS. Acquiring both interferometer outputs has the advantage of reducing noise by taking the difference of the two intensities, as is common for FTS. The sum of both outputs also provides a measurement of the cavity transmission, providing the function of detection system D1. In at least one embodiment, at least 1 MHz detection may be implemented using commercially available data converters and associated digital processing hardware. Higher data acquisition rates are feasible, for example operation in the range from about 1 MHz to 50 MHz in embodiments for very high speed operation.
In this implementation, most data points have low light levels, and correspond to non-resonant cavity lengths. These can be ignored, thereby reducing noise. Data points identified as corresponding to resonant cavity transmission, for example by thresholding the sum of the two interferometer outputs, are kept and used in calculating the spectrum. If the acquisition rate is fast enough that a transmission burst lasts for more than one data point, the intensities can be summed and treated as a single data point, with the requirement that the carriage speed is slow enough that the burst is complete before the path delay has changed by the desired minimum path delay interval, for example, a reference laser wavelength. In the case where absolute position is measured, acquired data points can be averaged over multiple scans. Beyond the usual benefit of scan averaging on noise, there is the additional benefit that the data points at different positions do not need to be taken in succession, but can come from combining many scans. The localization of a burst to within a path delay interval is still required, and this can be achieved by faster dithering of, for example, the laser frequency.
An exemplary design of a gas delivery system appropriate for use with enhancement cavities is further shown in
In some configurations, the system can be configured to measure the concentration of volatile organic compounds (VOCs), with endogenous compounds being or particular interest. Some VOC's of particular interest include acetaldehyde, acetone, benzene, toluene, ethylbenzene, formaldehyde, decane, dodecane, undecane, 1,2,4-trimethylbenzene, hexanal and isopropanol. Spectroscopy has an inherent advantage over mass spectrometry in that it can discriminate between molecules with the same nominal mass to charge ratio (m/z), for example ethane and formaldehyde (m/z=30), methanol/methylamine (32), nitrogen dioxide/dimethylamine (47), acetone/isobutane/butane (58), carbonyl sulfide/isopropanol (60), dimethylsulfide, trimethyl amine/ethanethiol (62), carbondisulfide/propanethiol/isopropanethiol (76), hexene/methyl cyclopentane (84), and propylbenzene/1,2,4-trimethyl benzene (120). These molecules with the same nominal mass can often be differentiated with high-resolving-power mass spectrometers, which come with increased time, cost, and complexity, such as dual MS followed by collisions as in MS/MS or by selected ion flow-tube mass spectrometry (SIFT). Dual MS, MS/MS as well as SIFT are well known in the state of the art and not further explained here. Such techniques require complicated analysis of the fragmentation patterns of a single selected ion, which further increases the analysis time, and wastes sample. Sensitivity is expected in the ppmv or ppbv range, where ppmv or ppmb stands for parts per million or billion volume fraction in air.
To further simplify a broadband trace gas detection system, the enhancement cavity can further be substituted with a multi-pass gas cell such as a Herriott or White cell as well known in the state of the art.
For mass applications of trace gas detection systems, the frequency rulers as described here can further be substituted with other mid IR light sources, such as quantum cascade laser based frequency combs, micro-resonators, fiber or waveguide based supercontinuum sources or sources based on difference frequency generation (DFG). For example mid IR continuum light sources have been disclosed in U.S. patent application Ser. No. 13/458,058, ('058), entitled “Broadband generation of coherent continua with optical fibers”, filed Apr. 27, 2012. Mid IR sources based on DFG have been disclosed in the following U.S. patents and applications: U.S. patent application Ser. No. 13/232,470, ('470), entitled Optical parametric amplification, optical parametric generation, and optical pumping in optical fibers systems”, filed Sep. 14, 2011; U.S. patent application Ser. No. 13/682,309, ('309), entitled “A compact, coherent, high brightness light source for the mid-IR and Far-IR”, filed Nov. 20, 2012; and U.S. Pat. No. 8,237,122, entitled “Optical scanning and imaging systems based on dual pulsed laser systems”.
Thus, the invention has been described in several embodiments. It is to be understood that the embodiments are not mutually exclusive, and elements described in connection with one embodiment may be combined with, or eliminated from, other embodiments in suitable ways to accomplish desired design objectives.
In at least one embodiment the present invention features a trace gas detection system. The trace gas detection system includes an optical source producing as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range. An enhancement cavity contains a sample gas for spectroscopic measurement. The enhancement cavity is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity is characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range, wherein the 1st spectral range and 2nd spectral range overlap. The system includes a dither mechanism configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A feedback mechanism is included, and coupled to the dither mechanism to stabilize the location of the 1st comb lines with respect to the resonances of the 2nd comb on a time scale much greater than a dither period, Td=1/fd. The system further includes a Fourier transform spectrometer configured to receive the secondary output, and to measure the spectrum of a time-averaged signal transmitted by the cavity over a time scale much longer than Td.
In any or all embodiments a first comb may be characterized by having a carrier envelope offset frequency, fo, and allowable variations thereof in the absence of phase locking of a carrier envelope offset frequency.
In any or all embodiments an enhancement cavity may have a comb spacing which is an integer fraction or integer multiple of a 1st comb spacing.
In any or all embodiments a trace gas detection system includes optical source which may include a mode-locked laser, an OPO, OPA, DFG system, quantum cascade laser or micro-resonator.
In any or all embodiments a gas delivery system may be included to insert and optionally extract a gas sample into or from a cavity.
In any or all embodiments a trace gas detection system may be configured for detection of optical spectra at wavelengths>1600 nm.
In any or all embodiments a trace gas detection system may be configured for detection of optical spectra at wavelengths in the wavelength range from 3 to 6 μm.
In any or all embodiments a trace gas detection system may be configured for detection of optical spectra at wavelengths in the wavelength range from 5 to 15 μm.
In any or all embodiments a dither period, Td, may be derived from the zero-crossings of an interference pattern generated by a reference laser within a Fourier transform spectrometer.
In any or all embodiments a Fourier transform spectrometer may be configured to sample a signal transmitted through a cavity in synchronism with zero-crossings of an interference pattern.
In any or all embodiments a feedback mechanism may be configured for detecting transmission peaks from an enhancement cavity and to provide electronic feedback to a cavity mirror so as to produce an approximately uniform time spacing of transmission peaks.
In any or all embodiments a dither mechanism may be configured to modulate the position of cavity spectral resonances of an enhancement cavity via movement of one of the cavity mirrors.
In any or all embodiments a dither mechanism may be configured to modulate the comb spacing of a 1st comb.
In any or all embodiments dither mechanism may be configured to modulate a carrier envelope offset frequency of a 1st comb.
In any or all embodiments an optical source may be diode pumped, and a carrier envelope offset frequency modulated by dithering diode power with a supplementary pump signal.
In any or all embodiments a carrier envelope offset frequency may be modulated with a graphene modulator.
In any or all embodiments an acousto-optic frequency shifter may be provided to modulate a carrier envelope offset frequency of a 1st comb.
In any or all embodiments dither period, Td, may be greater than about 100 μs, corresponding to a dither frequency less than about 10 kHz.
In any or all embodiments a dither period may be in the range from about 1 μsec to about 100 μs, corresponding to a dither frequency in the range from about 10 kHz to 1 MHz.
In any or all embodiments a dither mechanism may be configured to modulate the position of a 1st or 2nd frequency comb by about one free spectral range of an enhancement cavity.
In any or all embodiments a dither mechanism may be configured to modulate the position of the 1st or 2nd frequency comb by a fraction of a free spectral range of an enhancement cavity.
In any or all embodiments a dither mechanism may be configured to modulate a position of a 1st or 2nd frequency comb by more than a free spectral range of an enhancement cavity.
In any or all embodiments a Fourier transform spectrometer may be configured to sample more than two cavity transmission peaks between two zero-crossings.
In any or all embodiments a Fourier transform spectrometer may be configured to sample a uniform number of cavity transmission peaks between two zero-crossings.
In any or all embodiments a Fourier transform spectrometer may be configured to sample a signal transmitted through a cavity at time intervals much smaller than the time intervals between two adjacent zero crossings.
In any or all embodiments an optical source may be configured as a frequency comb source with repetition rate, frep, and carrier envelope offset frequency, fo, phase locked to reference signals via phase locked loop(s).
In any or all embodiments feedback loops may be arranged in a feedback mechanism.
In any or all embodiments a trace gas detection system may be configured for breath analysis.
In any or all embodiments a trace gas detection system may be configured for detection of volatile organic compounds.
In any or all embodiments a trace gas detection system may be configured for detection of endogeneous compounds.
In any or all embodiments a trace gas detection system may be configured for cancer detection via breath analysis of volatile organic and/or endogenous compounds.
In at least one embodiment the present invention features a trace gas detection system. The trace gas system includes an optical source producing as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range, the 1st spectral range including wavelengths>1600 nm. An enhancement cavity contains a sample gas for spectroscopic measurement. The enhancement cavity is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity is characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range. The 1st spectral range and 2nd spectral range overlap. A dither mechanism is included and is configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A feedback mechanism is coupled to the dither mechanism to stabilize the location of the 1st comb lines with respect to the resonances of the 2nd comb on a time scale much greater than a dither period, Td=1/fd. The trace gas detection system includes a spectroscopic measurement tool including an optical detection system. The tool is configured for frequency resolved detection of a time-averaged signal transmitted through the enhancement cavity.
In any or all embodiments an optical detection system may include a one dimensional detector array or a two dimensional detector array.
In at least one embodiment the present invention features a trace gas system. The system includes an optical source producing as a primary output a frequency spectrum having a 1st comb with a 1st comb spacing within a 1st spectral range. An enhancement cavity contains a sample gas for spectroscopic measurement. The enhancement cavity is configured to receive the primary output of the optical source and to produce a secondary output. The enhancement cavity is characterized by having a 2nd comb of approximately equidistant spectral resonances and a 2nd comb spacing within a 2nd spectral range. The 1st spectral range and 2nd spectral range overlap. The system includes a dither mechanism configured to modulate the relative position between the 1st comb and the 2nd comb at a dither frequency, fd, and to impart variations of the relative position in optical frequency space larger than an optical linewidth of the cavity resonances. A spectroscopic measurement tool is included and configured to receive the secondary output, and to measure the spectrum of a time-averaged signal transmitted by the cavity over the time scale much longer than Td=1/fd. The spectroscopic tool is configured to provide a signal for synchronization of dithering with spectroscopic data acquisition.
In any or all embodiments a spectroscopic tool may include a Fourier transform spectrometer (FTS) having a reference laser from which an interference signal is generated, and the FTS may be configured to sample a signal transmitted through an enhancement cavity in synchronism with zero crossings of an interference signal.
In any or all embodiments the system may include a feedback mechanism coupled to a dither mechanism, wherein a dither period, Td, is derived from zero crossings of an interference signal and used to control a dither mechanism via a feedback mechanism.
For purposes of summarizing the present invention, certain aspects, advantages and novel features of the present invention are described herein. It is to be understood, however, that not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, the present invention may be embodied or carried out in a manner that achieves one or more advantages without necessarily achieving other advantages as may be taught or suggested herein.
The term “or” is used in this application its inclusive sense (and not in its exclusive sense), unless otherwise specified. In addition, the articles “a” and “an” as used in this application and the appended claims are to be construed to mean “one or more” or “at least one” unless specified otherwise
Thus, while only certain embodiments have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the invention. Further, acronyms are used merely to enhance the readability of the specification and claims. It should be noted that these acronyms are not intended to lessen the generality of the terms used and they should not be construed to restrict the scope of the claims to the embodiments described therein.
This application is based on and claims priority under 35 USC 119(e) from U.S. Provisional Application No. 61/771,346 filed Mar. 1, 2013, the contents of which are incorporated herein by reference in their entirety
| Number | Date | Country | |
|---|---|---|---|
| 61793913 | Mar 2013 | US |
