Technical Field

[0001] The present invention relates generally to a spectroscopic detection system and method and, in particular, to a spectroscopic detection system and method incorporating a polarization mode impedance matching methodology.

Background

[0002] Optical absorption spectroscopy is an active branch of research which finds important applications in atmospheric, biochemical and geophysical sensing. The fundamental challenge is to attain ever lower detection thresholds. Towards this end, optical resonators are excellent transducers for amplifying the effects of small optical absorption and loss.

[0003] There are several established ways to measure optical absorption. For example, the simplest is by comparing the intensity of an optical beam before and after the beam is passed through a substance, the optical density or absorption of that substance can be determined. However, more sophisticated ways have been developed in recent years, using optical resonators, to enhance the absorption signal.

[0004] The most widely investigated cavity enhanced technique suitable for broadband loss is time-domain ring-down spectroscopy. However, this technique typically yields sub-optimum sensitivity due to limited measurement duty cycle and large effective noise bandwidth.

[0005] Other techniques include monitoring small changes in transmission power through the optical cavity. However, these types of techniques are again limited in their sensitivity and so are limited when detecting small changes in optical absorption and loss.

[0006] In order to increase sensitivity, techniques have been developed using resonant optical cavities and the generation of error signals at, or around, resonance. One such technique developed by the applicant of this application involved radio-frequency modulation of the laser source to generate non-resonant sidebands for sideband optical interference. This generates an impedance matching error signal, and hence absorption readout. However, this technique required the use of high frequency electro-optic modulators as well as high speed

photodetectors. [0007] It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages, or at least to provide a useful alternative.

Summary

[0008] Disclosed are arrangements which seek to address the above problems by generating interference between a first polarization mode and an orthogonal second polarization mode in a resonant birefringent optical cavity to generate an impedance matching error signal or read out that is associated with an optical absorption or loss within the cavity.

[0009] According to a first aspect of the present disclosure, there is provided a spectroscopic detection method for detecting optical absorption and/or optical loss within an optical cavity, the method comprising the steps of: generating an optical beam from an optical source; injecting the optical beam into a birefringent optical cavity, wherein the optical beam comprises a resonating first polarization mode that is orthogonal to a non-resonating second polarization mode; adjusting the polarization of the optical beam to generate both the first polarization mode and the second polarization mode of the optical cavity, and generating an impedance matching error signal that is associated with optical absorption and/or optical loss within the optical cavity based on reflected fields of the first polarization mode and the second polarization mode that are reflected from the optical cavity.

[0010] According to a second aspect of the present disclosure, there is provided a spectroscopic detection system for detecting optical absorption within an optical cavity, the system comprising: an optical beam source for generating an optical beam; a birefringent optical cavity for receiving the generated optical beam, wherein the optical beam comprises a resonating first polarization mode that is orthogonal to a non-resonating second polarization mode; a polarization controller arranged to adjust the polarization of the optical beam to generate both the first polarization mode and the second polarization mode, and an analyser arranged to generate an impedance matching error signal that is associated with optical absorption and/or optical loss within the cavity based on reflected fields of the first polarization mode and the second polarization mode that are reflected from the optical cavity.

[0011 ] Other aspects of this disclosure invention are also disclosed.

Brief Description of the Drawings [0012] At least one embodiment of the present invention will now be described with reference to the drawings and appendices, in which:

(0013] Fig. 1 shows expected spectra of the reflected power from a cavity as the laser frequency is tuned across resonance;

[0014] Fig. 2 shows a schematic diagram of a polarization impedance matching spectroscopic system according to the herein disclosure;

[0015] Fig. 3 shows a schematic diagram of a linear polarization analyser according to the herein disclosure;

[0016] Fig. 4 shows the frequency response of various signals according to the herein disclosure;

[0017] Fig. 5 shows an error signal according to the herein disclosure;

Detailed Description including Best Mode

[0018] Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears.

[0019] Described herein is a technique for real time, dynamic high resolution cavity-enhanced laser absorption spectroscopy. The technique uses the two polarization modes of a cavity to detect small changes in dynamic intra-cavity loss such as, but not limited to, those caused by absorption of a sample gas species in real time. This therefore enables real-time, continuous wave, high duty cycle, and high precision spectroscopic measurements to take place.

[0020] The method and system described can be used for ultra-sensitive detection of atomic and molecular absorption, as well as material losses and scattering, in gaseous, liquid and solid phase samples. For example, the method and system may be used to quantify extremely weak absorption transitions in isotopic ratio measurements of gaseous molecules. This measurement can then be used for industrial instruments for, but not limited to. ● biomedical diagnostics: such as breath testing for diseases and testing for illicit drugs and substances; ● oil and gas exploration; ● greenhouse gas monitoring: including carbon dioxide and methane;

● hydrology: including ice core sampling and monitoring of heavy water; and ● nanoparticle detection.

[0021] Fig. 1 shows the changes in the reflected power of the cavity with different intra-cavity absorption. Fig. 1 (a) shows the response of the cavity with unequal mirror reflections, i.e. the cavity is over-coupled where are the reflection coefficients of the three

cavity mirrors, a is the optical absorption or loss coefficient, and L is the optical path length of the cavity. As the absorption within the cavity is increased the amount of power reflected from the cavity input coupler changes. When the absorption is increased so tha the

reflected field on resonance goes to zero, Fig. 1 (b). This case is commonly referred to as impedance matched, or critically coupled. As the absorption further increases light is

once again reflected from the cavity at resonance, i.e. the cavity is under-coupled as shown in Fig. 1 (c). it can be clearly seen that the light reflected on resonance changes with varying absorption. However, this power change is approximately parabolic with varying absorption. In the case where the cavity is close to impedance matched, one cannot tell whether there was an increase or decrease in the absorption within the cavity based only on a measurement of the reflected power.

(0022] Polarization Impedance Matching Spectroscopy (PIMS) obtains spectroscopic measurements by monitoring a cavity around the impedance matching condition (also referred to as the cavity coupling conditions). A cavity's impedance matching condition is a measure of how well light is coupled into the cavity on resonance and affects the reflection response of a cavity. The complex reflection response, R _s and R _p , of a three-mirror ring cavity for S polarization and P polarization respectively is given by,

[0023] where r ₁ , r ₂ and r ₃ are the polarization dependent amplitude reflection coefficients of the three mirrors respectively, ω is the angular frequency of the light incident on the cavity, a is the absorption within the cavity, and is the polari2ation dependent round trip optical path length of tiie cavity.

[0024] Although the optical cavity in the herein described example is a three mirror ring cavity, it will be understood that any other suitable optica! cavity or resonator may also be used to perform the methods described herein.

[0025] PIMS uses the non-degenerate polarization mode of an optical cavity, a ring cavity in this example, to detect changes in the cavity impedance conditions. This can be understood using Jones vector representation of polarized light. Fig. 2 shows a schematic diagram of a polarization impedance matching spectroscopic system. An optical beam, i.e. light, in the form of a laser beam 201 is generated by any suitable optical source 202. For example, the optical source may be a tunable laser of one of the following types: DFB laser, diode laser, QCL laser, fibre laser, OPO, ECDL.

[0026] The laser beam 201 is injected into the birefringent optical cavity 203 via an input coupler ri and the frequency is adjusted until the cavity reaches resonance. This can be done by simply scanning the frequency of the laser across resonance. Alternatively the system can include an active feedback system that is used to maintain resonance within the optical cavity 203. For example, the active feedback system could include the Pound-Drever-Hall (PDH) frequency locking system.

[0027] The laser beam 201 can be decomposed into two orthogonal polarization modes: S and P modes. Further, the S and P modes are non-degenerate within the cavity 203 and so resonate at well separated frequencies.

[0028] The polarization of the input beam 201 can be slightly rotated using an input polarization controller 209 to generate both the S and P modes 211 of the optical cavity. It will be understood that one of the modes is excited when resonant and so only one polarization mode can be excited at a time. The reflected fields of the S and P modes are then reflected from the optical cavity 203 and passed through a linear polarization analyser 213 to extract the PIMS error signal 215, as explained in more detail below. [0029] An explanation of the process is now provided using Jones Matrix formalism. To start, the input light to the cavity is first considered as

[0030] where and E _P are the real electric field amplitudes of the light for the S and P polarization modes respectively, and φ represents the phase difference between the two polarization modes just before the optical cavity 203. By using a polarization controller, the input optical power can be distributed between E _s and E _pi and the phase difference between the two polarization modes can also be changed arbitrarily. This input light is injected into the ring cavity via the input couple The optical cavity's complex reflection response is represented

by a Jones matrix with

[0031] where R _p and R _s correspond to the complex reflection response of the two orthogonal polarizations as explained in Eqn. 1.

[0032] Introducing intra-cavity absorption by injecting gas results in changes in both R _p and Applying the reflected plane wave from the cavity can be obtained as

[0033] The analyser 213 is then used to extract the PIMS spectroscopic signal, as illustrated in Fig. 3 and explained below.

[0034] Fig. 3 shows a schematic diagram of a linear polarization analyser and the signal flow of the linear polarization analyser to obtain a PIMS error signal. Fig. 3(a) shows the light reflected from the cavity when impedance is not matched. In the example illustrated in Fig. 3, the reflected light is linearly polarized and composed of reflected fields of the S (301 A) and P (301 B) polarization modes. At Fig. 3 (b), a half-wave plate 305 is used to rotate the reflected fields, such that S becomes S' (308A) rotated field and P becomes P' (308B) rotated field. For example, the reflected field of the S polarization mode is rotated by 45°, while the reflected field of the P polarization mode is rotated by 135°. At Fig. 3 (c), the rotated fields S' and P' are put through a polarizing beam splitter (PBS) 307 to produce first and second polarized optical beams, where the first polarized optical beam is vertically polarized and the second polarized optical beam is horizontally polarized. At Fig. 3 (d), it can be seen that a first output of the PBS is vertically polarized and Interferes the vertical components of S ^' and P' which are detected by the photodetector PDa 309. At Fig. 3 (e), it can be seen that a second output of the PBS is horizontally polarized and interferes the horizontal components of S' and P ^' , which are detected by the photodetector PDb 311. At Fig. 3 (f), the PIMS error signal 215 is obtained through the subtraction of the photodetector signals obtained from photodetector PDa 309 and

photodetector PDb 311 using a subtractor module 313.

[0035] The above is explained in further detail with reference to Fig. 3, where the reflected field E _r (301) is linearly polarized, and is a superposition of S and P polarization modes (303). The S and P polarization modes are then rotated by 45° and 135° respectively by a half-wave plate 305. The rotated field E' _r is

[0036] and is passed through a polarizing beam splitter (PBS) 307. One output of the PBS 307 only reflects the vertical components of the rotated field.

[0037] This is illustrated in Fig. 3(d), and can be represented as

[0038] Similarly, another output of the PBS is the horizontal components of the rotated field E' _r , which is illustrated in Fig. 3(e). given by

[0039] The light beams reflected and transmitted by the PBS 307 are then received by photodetectors PD _a and PD _b respectively. The signals measured on the photodetectors are the respective optical powers multiplied by a constant k that accounts for photodiode response and transimpedance gain. Therefore [0040]

[0041] The PIMS error signal is obtained by taking the difference between output of PD _a and PD _b> which contains the interfering term as

where <p' is a phase difference caused by the birefringence of the cavity. Because of the ability to arbitrarily change φ using the input polarization controller 209, φ' can be easily compensated so that (φ+φ'=0), resulting in

[0042] Equation 11 is a general description of the PIMS error signal. It can be used to extract the cavity impedance matching condition given that one polarization is resonant with the cavity mode. According to this example, the S polarization field is resonant, and hence its reflection response becomes

[0043] with mirror reflectivity specific to S-polarized light. Because of the non-degeneracy of the S and P polarization modes of the cavity, the P polarization field is ideally non-resonant and completely reflected (R _? *1). Substituting these conditions into Eqn. 11 gives the error signal on S resonance, [0044] which characterizes the impedance matching condition of the cavity in the S polarization field, and is zero crossing at the cavity impedance matched (R _sres = 0) condition. This enables a zero-background, continuous readout to be provided for detecting optical absorption in the cavity.

[0045] It will be understood that, as an alternative, the S and P polarizations may be interchanged. P polarization field may be resonant instead of S polarization, and the PIMS error signal is then read out according to Eqn. 11.

[0046] In one example implementation, prior to the gas being injected into the optical cavity, the vertical mode (S polarization) is resonant in the impedance matched cavity and no S polarization is reflected from the cavity. The non-resonant horizontal mode (P polarization) is reflected towards the polarization analyser (213). The resultant error signal is zero.

[0047] However, when impedance mismatch occurs due to the optical absorption of the gas in the cavity, a portion of the vertical polarization {S mode) is reflected towards the polarization analyser (213) together with the non-resonant horizontal polarization (P mode). Analysis (using 213) of the combined S and P polarization fields produces a non-zero error signal which is associated with or related to the optical absorption.

[0048] Fig. 4 shows a frequency response of the signals of (a) the reflected power of the cavity for both polarizations, i.e. the sum of the power of both polarizations after being reflected off the cavity, and (b) the PIMS error signal, as the laser frequency is swept over the cavity resonances. To highlight the absorption sensitivity of the two signals, the signals are plotted with intra-cavity absorptions, aL, of 0 and 0.01. We can see that the power reflected from the cavity shows little variation with change in the absorption, whereas a significant difference is seen in the PIMS error signal when the laser is resonant with the cavity. It is important to note that the PIMS error signal is optimally sensitive to absorption variation whilst the laser is resonant with the cavity. To ensure this is the case, laser locking techniques can be used.

[0049] Fig. 5 shows the on-resonance PIMS error signal as the intra-cavity absorption is varied. A portion of the plot is magnified to show approximate linearity of the signal for small absorption levels. At these absorption levels, below ~1x10 ³ , the PIMS error signal approximates a linear response with respect to optical absorption and/or loss and can be used to directly measure the absorption within a cavity. [0050] We note that the initial operating condition exemplified in above and Fig. 5 is at the impedance matched point. However, it will be understood that the technique will also function when the cavity initial conditions are over-coupled or under-coupled.

[0051] Therefore, according to the herein disclosure, a resonant cavity is utilised with non- degenerate polarization modes to provide a cavity impedance matching readout. One of the two orthogonal polarization modes for the cavity is non-resonant. By causing the resonant and non-resonant polarization modes to interfere through a polarization adjustment, an error signal is generated that is proportional to the infra-cavity loss or absorption with a zero background, where the zero crossing corresponds to the impedance matched condition.

[0052] It will be understood that the methods described herein may be applied using any suitable birefringent optical cavity. For example, the optical cavity may be an optical ring cavity, a linear cavity with birefringence, a monolithic cavity, a micro cavity or macro cavity. It will also be understood that any suitable non-birefringent optical cavity may be used where that non birefringent optical cavity is configured to operate as a birefringent cavity.

Industrial Applicability

[0053] The arrangements described are applicable to the spectroscopic detection industries.

[0054] The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

[0055] in the context of this specification, the word "comprising" means "including principally but not necessarily solely ^* or "having ^* or "including", and not "consisting only of. Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.