WO2009127250A1

WO2009127250A1 - Method and device for vibrational circular dichroism spectroscopy

Info

Publication number: WO2009127250A1
Application number: PCT/EP2008/054585
Authority: WO
Inventors: Hubert Brändle; Klaus Bohnert
Original assignee: ABB Research Ltd Switzerland
Current assignee: ABB Research Ltd Switzerland
Priority date: 2008-04-16
Filing date: 2008-04-16
Publication date: 2009-10-22
Anticipated expiration: 2010-10-16

Abstract

A circular polarisation spectrometer is disclosed comprising a source (1) of essentially monochromatic electromagnetic radiation preferably in the infrared range, an element (21, 37-45) for the generation of altematingly left and right circularly polarised radiation therefrom, and at least one detector (7) for detecting radiation transmitted by a sample to be analysed spectroscopically in terms of differential absorption of left and right circularly polarised radiation. The spectrometer is versatile and allows compensation of almost any otherwise detrimental offset effects if the element (21, 37- 45) for the generation of altematingly left and right circularly polarised radiation is an electro-optical device in which by means of the application of voltage and a correspondingly generated electric field the birefringence can be adapted as a function of time.

Description

SPECIFICATION

TITLE Method and device for vibrational circular dichroism spectroscopy

TECHNICAL FIELD

The present invention relates to the field of spectroscopy and spectrophotometers. Specifically, the invention relates to the field of near-infrared vibrational circular dichroism spectroscopy. So more specifically, the invention relates to the field of circular polarised light spectroscopy in the near-infrared wavelength range, The term near infrared shall include wavelengths in the range from about 800 nm to about 2500 nm. More specifically, it relates to a circular polarisation spectrometer comprising a source of linearly polarised monochromatic electromagnetic radiation in the infrared range, an element for the generation of alternatingly left and right circularly polarised radiation therefrom and at least one detector for detecting radiation transmitted by a sample to be spectroscopically analysed in terms of differential absorption of left and right circularly polarised radiation. It furthermore relates to uses of electro-optical devices for vibrational circular dichroism spectroscopy and it relates to methods of operation of a spectrometer of the above type.

BACKGROUND OF THE INVENTION

Vibrational circular dichroism (VCD) is the differential absoiption by a chiral molecule of left and right circularly polarised infrared radiation during vibrational excitation. VCD is the extension of electronic circular dichroism (CD or ECD) from the ultraviolet and visible regions to vibrational transitions in the near-infrared regions. Compared to infrared (IR) spectroscopy, VCD is much more sensitive to the stereochemical details of the molecular structure. The principal area of application of VCD is structure elucidation of biologically significant molecules including peptides, proteins, nucleic acids, carbohydrates, natural products and pharmaceutical molecules.

For a general introduction into the field of circular polarisation spectroscopy of molecules reference is made to the introductory portion of US 6,480,277, This US 6,480,277 specifically deals with a particular set up of a circular dichroism spectrometer which eliminates linear birefringent interference by having a first polarisation modulator before the sample and a second polarisation modulator after the sample. The two polarisation modulators vibrate at different frequencies such that the signals can be distinguished and manipulated. The addition of the second polarisation modulator, an additional lock in amplifier, and software to manipulate the two signals corresponding to the two vibrational frequencies allow real-time circular dichroism spectra to be determined free from interference.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an improved spectrometer for circular polarisation spectroscopy as well as methods of operation of such spectrometers.

The present invention specifically relates to an improvement of a circular polarisation spectrometer comprising (among other elements): a source of linearly polarised monochromatic electromagnetic radiation, preferably in the infrared range, which can e.g. be a wavelength-tuneable source of linearly polarised monochromatic electromagnetic radiation; an element for the generation of alternatingly left and right circularly polarised radiation therefrom (either one sense of rotation of one specific frequency alternating with the other sense of rotation of the same specific frequency, or ramping through a certain range of frequencies keeping left sense of rotation and subsequent ramping through the same or a following range of frequencies with right sense of rotation); and at least one detector for detecting radiation transmitted by a sample to be spectroscopically analysed in terms of differential absorption of left and right circularly polarised radiation.

According to the invention, a spectrometer of the above type is in particular characterised in that the element for the generation of alternatingly left and right circularly polarised radiation is an electro-optical device in which by means of the application of voltage and a correspondingly generated electric field the birefringence can be adapted as a function of time. In contrast to the state-of-the-art, in which for the generation of alternatingly left and right circularly polarised radiation for the measurement of vibrational circular dichroism a photoelastic modulator is used, in which the modes of operation are highly limited by the resonance frequency due to the mechanical nature of the device, the newly proposed spectrometer makes use of hitherto for these purposes never used electro -optical devices. The use of electro-optical devices in this context has many beneficial advantages: the electro-optical devices are much more easily controllable, they are not limited to mechanical resonance frequencies, and the phase shift or birefringence can be adapted much more easily to the specific wavelength to be used for the spectroscopy, to temperature effects, to phase shifts induced by other components, and the like. Furthermore, the transition from one sense of polarisation to the opposite sense is almost instantaneous rather than gradual in contrast to the devices according to the state of the art, so that the available optical power is used with significantly higher efficiency. Surprisingly, therefore, the use of an electro-optical device in this context not only simplifies the setup of the corresponding spectrometer but it also makes the spectrometer much more easily controllable, adjustable and reliable.

Alternatively to the electro-optical device it is possible to use a light path as follows: a polarisation maintaining fiber carrying the linearly polarised light from e.g. a tuneable laser is spliced with a 45° splice to a second polarisation maintaining fiber, i.e. with a rotation angle about the fiber axis of 45° between the birefringent axes of the two fibers, respectively. As a result both orthogonal polarisation modes are excited in the second fiber with equal amplitudes. The second fiber being part of a piezoelectric modulator is wrapped on a cylinder-shaped tube made of a piezoelectric ceramic (or other suitable piezoelectric material). By applying a control voltage to the piezoelectric element the phase shift between the two orthogonal polarisation modes can be adapted and as a result time- dependent alternating circular polarisation can be generated. A polarisation control adjusts the voltage to the piezoelectric modulator such that the exiting light is left or right circularly polarised with sufficient precision.

According to a first preferred embodiment of the invention the electromagnetic radiation is in the infrared, preferably in the near infrared range, typically from about 800 nni to about 2500 run, for example in a range of 1200-1700 nm. However, the method can also be applied to spectroscopy in the visible and ultraviolet spectral range. In principle, the source of linearly polarised monochromatic electromagnetic radiation can be a conventional source of electromagnetic radiation followed by a frequency selection element like a grating or an FTIR and a linear polariser. It is, however, preferred and in accordance with a further preferred embodiment of the invention, that the source of linearly polarised monochromatic electromagnetic radiation is a tuneable laser. Behind this laser a polariser may be located for generating linearly polarised radiation. If this laser generates linearly polarised radiation itself, the polariser is not necessary but may also be used for optimization. One advantage of using a tuneable laser is the fact that fiber optics can be used very conveniently. Preferably, the light from the source of linearly polarised monochromatic electromagnetic radiation is coupled into the electro-optical device by means of a polarisation maintaining fiber.

One further preferred embodiment of the present invention is characterised in that the electro-optical device is a birefringence modulator. Such a device is preferably based on a lithium niobate (LiNbO₃) waveguide structure. According to specific embodiment, the electro-optical device is a lithium niobate modulator with the waveguide produced by titanium in-diffusion into a lithium niobate substrate, as e.g. described in detail in M. Lawrence, "Lithium niobate integrated optics", Reports on Progress in Physics, 363- 429, 1993, the disclosure of which is herewith included into the specification.

One still further preferred embodiment of the present invention is characterised in that the electro-optical device includes an electro -optical phase modulator, preferably with a

Y-type waveguide, the output of which is collected by at least two individual polarisation maintaining fibers which are subsequently combined in a polarisation maintaining fiber coupler, wherein in only one of the at least two polarisation maintaining fibers a 90°-splice is inserted. The phase modulator in this context can also be based on a lithium niobate waveguide structure, which is preferably generated by proton exchange.

For optimum control of the spectrometer it is preferred if downstream of the electro- optical device a polarisation analyser is provided, the output of which is coupled to a polarisation controller (polarisation analyser and polarisation controller need however not be individual components, but can also be integrated in one component), which polarisation controller is connected to the electro-optical device for controlling the applied voltage. A further input of the polarisation controller can be the frequency, for example the tuning frequency of the laser for a pre-calibration. Preferably, a beam splitter is located in the output light path of the electro -optical device, and a fraction (typically small, e.g. less than 30%, preferably less than 10%) split off by this beam splitter is coupled into the polarisation analyser. Even more preferred, the polarisation analyser comprises a second beam splitter and two linear polarisers, the polarisation directions of which are shifted by 90° with respect to one another for the two light beams generated by the second beam splitter, as well as two individual photodetectors, the output of which photodetectors is used in a differential amplifier for the generation of a signal proportional to the phase shift between the two orthogonal polarisation components. The two polarisers in the polarisation analyser are preferably oriented under + 45° and -45° with respect to the polarisation directions of the transverse electric and/or transverse magnetic modes of the electro-optical device.

According to a still further embodiment of the invention, the spectrometer further comprises an element for compensating the group delay between orthogonal polarisation modes in the element for the generation of alternatingly left and right circularly polarised radiation. Without such a compensation the wavelength tuning itself would result in significant differential phase shifts. The compensation of such phase changes might exceed the possibilities of inducing a differential phase behaviour in the waveguide structure and necessitate complicated control using the periodicity of the signals.

The element for compensating the group delay can for example be a polarisation maintaining fiber section in the optical path, in which the slow and fast axis of propagation is inverted (90° shift) with respect to the slow and fast axis of propagation in the element for the generation of alternatingly left and right circularly polarised radiation. The length of such a polarisation maintaining fiber section and/or its (differential) birefringence is adapted to essentially compensate the group delay between orthogonal polarisation modes in the element. Preferably, the polarisation maintaining fiber section is located between the source of essentially monochromatic electromagnetic radiation which is coupled into an polarisation maintaining fiber, in which the linearly polarised light propagates along one of the main axes, and the polarisation maintaining fiber section is coupled to this polarisation maintaining fiber via a 45° splice and is coupled to the element under a 90° offset with respect to the birefringence axes. The polarisation maintaining fiber section can also be located behind the waveguide structure, or compensation fibers can be located before and behind the waveguide structure for in total compensating for the differential behaviour in the waveguide structure. The present invention furthermore relates to the use of an electro-optical device in which by means of the application of voltage and a correspondingly generated electric field the birefringence can be adapted as a function of time, for a circular polarisation spectrometer, preferably for vibrational circular dichroism spectroscopy in the near infrared wavelength region. Preferably, in this case the electro- optical device is a birefringence modulator based on a lithium niobate waveguide structure. It can also be a lithium niobate modulator with the waveguide produced by titanium in-diffusion into a lithium niobate substrate (as described in the reference given above). Alternatively or additionally, the electro-optical device includes an electro-optical phase modulator typically with a Y-type waveguide, the output of which is collected by at least two individual polarisation maintaining fibers which are subsequently combined in a polarisation maintaining fiber coupler, wherein in only one of the at least two polarisation maintaining fibers a 90°-splice is inserted, wherein preferably the phase modulator is based on a lithium niobate substrate, wherein further preferably it is generated by proton exchange. Last but not least the present invention relates to a method for operating a spectrometer as given above. In this method preferably the voltage applied to the electro-optical device is alternatingly ramped such as to induce an alternating positive and negative phase shift, wherein preferably the frequency of alternation is in the range of 1 Hz - 1 MHz, most preferably in the range of 1 kHz - 100 kHz. The present invention furthermore relates to a method for operating a spectrometer as given above, wherein the source of linearly polarised monochromatic electromagnetic radiation is a tuneable laser, and wherein the tuneable laser is periodically ramped over a frequency range (which can be the full scanning range of the spectrometer or only a partial range) in the near infrared window, and wherein preferably the ramping frequency of the tuneable laser is identical or an integer fraction or an integer multiple of the alternation frequency of the voltage applied to the electro-optical device.

Further embodiments of the present invention are outlined in the dependent claims. SHORT DESCRIPTION OF THE FIGURES

In the accompanying drawings preferred embodiments of the invention are shown in which:

Figure 1 is a schematic representation of a conventional VCD spectroscopy setup; Figure 2 is a schematic representation of a first embodiment of the invention wherein in a) generation of circularly polarised light with an integrated optics phase modulator is shown; in b) for the case of switching the phase between -90° and + 90°: the wavelength of the tuneable source (b.l), the voltage applied to the electro-optic birefringence modulator for the situation where the phase is switched between individual sweeps of the tuneable source of light (b.2) and for the situation where the phase is switched several times within an individual sweep of the tuneable source of light (b.3); in c) for the case of switching the phase without zero-crossing so for example between + 90° and + 270°: the wavelength of the tuneable source (c.l), the voltage applied to the electro-optic birefringence modulator for the situation where the phase is switched between individual sweeps of the tuneable source of light (c.2) and for the situation where the phase is switched several times within an individual sweep of the tuneable source of light (c.3), all the functions in b) and c) given as a function of the time for the modes of operation; and in d) a schematic representation of the polarisation analyser is given;

Figure 3 is a schematic representation of a second embodiment of the invention; and Figure 4 is a partial schematic representation of a third embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same, figure 1 illustrates the basic aspects of the conventional measurement of VCD. The measurement of VCD involves determining the differential response of e.g. a chiral molecule to left and right circularly polarised radiation. Therefore, an optical setup is needed to provide alternately right and left circularly polarised radiation in the entire spectral region of interest. Due to the usually weak signal intensity it is preferred to switch quickly between the two polarisation states in order to recover the small difference in the absorbance of a chiral sample for left versus right circularly polarised infrared radiation. In this way it is possible to avoid common mode effects while increasing the sensitivity to differential effects. For VCD the source of electromagnetic radiation 1 is thermal, such as a SiC glower, or an electric arc as in a xenon lamp. In VCD measurement, the spectrometer, or generally element for frequency selection 2, either a dispersive grating monochromator or a Fourier transform infrared (FT-IR) spectrometer, precedes the polarisation-modulation stage of the instrument. Behind this element for frequency selection first follows an infrared optical filter 3 (optional). Subsequently, the monochromatic unpolarised radiation is converted into linearly polarised radiation 9 by means of a linear polariser 4. The creation or selective detection of left (LCP) and right circularly polarised (RCP) radiation is carried out with the combination of a polariser 4 and a modulating quarter- wave plate 5. The modulating waveplate 5 is a photo-elastic modulator (PEM) operating between 35 kHz and 60 kHz, wherein this resonance frequency is essentially predetermined by the specific device chosen. In this photoelastic modulator 5 a birefiϊngent body is mechanically vibrated in its resonance frequency and the mechanical distortion due to this vibration leads to a time-dependent modulation of the birefringence and correspondingly of the polarisation state of the radiation exiting this mechanical device 5. Specifically, the state of polarisation of the output light 10 as a function of time with the resonance frequency of the device 5 oscillates for the transitions between lambda/4 and - lambda/4 between left circularly polarised light - linearly polarised light - right circularly polarised light - linearly polarised light - and so forth.

After the infrared radiation passes through the sample 6, the transmitted and thus not absorbed electromagnetic radiation 11 is focused on a detector 7, normally a liquid- nitrogen-cooled InSb or HgCdTe (MCT) detector which is fast enough to follow the high frequency of the polarisation modulation. The processing electronics employ lock- in amplifiers, and in the case of FT-VCD also necessarily involve digitization, phase correction and Fourier transformation, all prior to spectral presentation. The present invention proposes methods to measure vibrational circular dichroism in the near-infrared region based on electro-optical generation of alternatingly left and right circularly polarised light preferably in combination with tuneable laser spectroscopy. A preferred implementation is shown in Fig. 2a. A tuneable laser 16, eventually followed by an isolator 17, sends narrow-band linearly polarised light 20 through a polarisation maintaining (pm) fiber 18 to an integrated- optics phase modulator with a pm fiber pigtail 57, For optimum preservation of the linear polarisation generated in the tuneable laser the birefringent axes of the fiber 18 are preferably arranged with respect to the linear polarisation direction as given in the figure, i. e. the polarisation direction is parallel to one of the fiber axes (for example major or minor core axis in case of an elliptical-core pm fiber). The tuning range of the tuneable laser may range e. g. from 1200 nm to 1700 run, it may however also be broader, as for example generally within the window of the near infrared range, e,g, from about 800 nm to 2500 nm. Several sources may be combined to extend the tuning range.

The fiber link 18 may include a polariser, e. g. a piece of polarizing fiber, in order to improve the degree of linear polarisation. The cut-off wavelength of higher order modes of the different fiber segments is preferably below the lowest wavelength of operation. As mentioned above, the proposed setup comprises a pm fiber pigtail 57 just in front of the phase modulator 21. The transition between the fiber 18 which guides the linearly polarised light generated in the tuneable laser 16 and this pigtail section 57 is a 45° splice 55. The polarisation direction of the linearly polarised light guided in the fiber 18 (which is parallel to one of the main axes of the fiber as indicated by the arrow in the schematic representation 19, where it is parallel to the small axis) is thereby split into two components, one travelling along the slow axis of this polarisation maintaining fiber 57 (indicated with the arrow s in the schematic representation 58) and one travelling along the fast axis of this polarisation maintaining fiber 57 (indicated with the arrow f in the schematic representation 58). At the entry of the phase modulator 21 this polarisation maintaining fiber 57 is coupled into the waveguide structure of the phase modulator 21 under a 90° offset 56, so the fast and the slow axes are interchanged at this transition as indicated in the schematic representation 22.

The idea behind this pigtail section 57 is to compensate for the different propagation within the waveguide structure 21 along the fast and the slow axes already. This compensation is achieved by interchanging the fast and the slow axes between the pigtail 57 and the modulator 21. The length and/or differential birefringence properties of the pigtail section 57 is adjusted such as to make sure that the differential propagation delay of the two components of the light in the pigtail section 57 (section a in figure 2 a)) essentially completely compensates for the differential propagation delay of the two components in the modulator 21 (section b in figure 2 a)).

It should be noted that this compensation need not necessarily take place before entry into the modulator 21. The compensation can actually be located at any position in the light pathway between the source and the sample, Correspondingly therefore, the same effect can be achieved if the light modulator 21 is connected to fiber 18 under a 45° splice with its entry and on the output side of the light modulator 21 is attached to a pigtail output fiber with a 90° offset. It should be noted that this compensation may even be integrated into the modulator 21.

The pm fiber pigtail thus serves to compensate the group delay between the orthogonal polarisation modes. The fast birefringent axis f of the pigtail 57 is aligned parallel to the slow axis s of the modulator 21. The pigtail length (and/or its birefringence properties) is chosen such that the group delay of the orthogonal modes in the pigtail 57 is the same as in the modulator 21. Under these conditions the total group delay of the orthogonal modes is zero or close to zero. Group delay compensation is advisable in the context of the proposed setup, because otherwise the wavelength tuning can result in excessive differential phase changes which cannot be balanced by the applied voltage as it would exceed the accessible range. Note: It is also possible in principle to operate the system without such a compensation of the modulator group delay. In this case one needs to reset the control voltage by amounts of V2π (V2π being the voltage needed for a phase reset of 2π), each time the phase variation introduced by the wavelength tuning exceeds the control range of the modulator. Here, it is assumed that the coherence length of the light source is much longer than the optical path length difference of the two waveguide modes.

Preferably, the birefringence modulator or more generally phase modulator 21 is a lithium niobate modulator with preferably the waveguide produced by titanium in- diffusion into the lithium niobate substrate. As concerns the production as well as the functioning of this birefringence modulator 21, reference is made to M. Lawrence, Lithium niobate integrated optics, Reports on Progress in Physics, 363-429, 1993, the disclosure of which is herewith included into the specification. The waveguide 21 supports the orthogonal polarisation modes of the fundamental spatial mode (TE and TM modes). The cut-off wavelength of higher order waveguide modes is again below the lowest wavelength of operation. Preferably, the substrate is x- cut or z-cut (x-axis or z-axis perpendicular to the plane of the substrate). The light propagates in y-direction. The fiber link 18 is attached to the pigtail 57 under a 45° splice 55 such that the polarisation direction of the incoming light is at 45° with respect to the birefringent pigtail axes, so that both fiber modes and subsequently both waveguide modes are excited with the same amplitude (see schematic representation of the relative orientation in the two fibers 18 and 57 as well as in the waveguide in the cross-sections in 19, 58 and 22). As a result of remaining uncompensated birefringence of the modulator 21 and pigtail 57 the two modes can have an arbitrary phase difference after the propagation through the waveguide, i.e. the light 24 emerging from the modulator 21 has some arbitrary elliptical state of polarisation.

The phase difference of the two orthogonal polarisations is controlled by a voltage applied to the modulator 21 via electrodes 23. The voltage introduces extra birefringence and thus an extra phase shift as a result of the electro-optic effect. In order to generate left of right circular light the voltage is chosen such that the phase shift Δφ of the two orthogonal modes corresponds to 90° + N . 180° or -90° + N . 180° wherein N is an integer. A control circuit 13-15 including a polarisation analyzer 15 controls the voltage of the birefringence modulator 21.

It should be noted that in contrast to a photoelastic device this electro-optical device 21 not only allows the modulation frequency to be adapted independently from resonance frequencies of the corresponding device (thus allowing for example a free adaptation to the ramping frequency of the tuneable laser), but it also allows adaptation of the minimum and maximum value of the phase shift induced in one cycle. Furthermore, the voltage and thus the phase shift may be switched in a stepwise manner. The sample is thus continuously probed with either perfectly left or perfectly right circularly polarised light. In contrast, in systems according to the state of the art (Fig. 1), the phase shift varies sinusoidally. As a result the transition from left to right circular light and vice versa is gradual which reduces the efficiency of the method. Figures 2 and 3 show examples for the wavelength tuning between two extreme wavelengths X₁ and λ₂. In the figures 2 b) b.l and 2 c) c.l the sweeping of the laser frequency as a function of time is indicated.

The voltage applied to the device 21 as a function of time is schematically given in figure 2 b) b.2 and figure 2 c) c.2, respectively, for the case that the time periods for left and right circular light are equal to the duration of the wavelength ramps. Fig. 2 b) generally shows the control voltage if the difference phase is switched between -90° and +90°, that is with zero crossing. Fig. 2 c) shows the voltage if the differential phase is switched for example between 90° and 270°, i. e. without zero-crossing. Here, V_π(λi) is the voltage change that is required to switch from left to right circular light at wavelength λi. V_π(λ₂) is the voltage change that is required to switch from left to right circular light at wavelength X₂. It should be noted that the voltage ramps do not necessarily have to be linear in the positive and/or negative range, but can be non-linear and adapted for corrections of non-linearities in the system and other effects. Figures 2 b) b.3 and 2 c) c.3, respectively, show the corresponding voltage for the case that the light is switched several times between left and right circular polarisation within the duration of the wavelength ramp. In the case of Figure 2 c) there is an asymmetric situation where the absolute value of the slope of the positive and the negative ramp are different, the voltage change that is required to switch from left to right circular light at a specific wavelength is a function of the specific wavelength and so is the voltage change that is required to switch from left to right circular light. The closed-loop control of the voltage applied to the device automatically account for wavelength-dependent birefringence and phase retardations in the various components in the optical path thereby allowing exact establishment of left and right circularly polarised light at the location of sample 6 with only little or virtually none elliptical contributions.

In order to control the state of polarisation and its variation as a function of time in the output light beam 24 (which can also be coupled into a fiber) a small fraction of the light 24 is sent via a first beam splitter 27 to the polarisation analyzer 15, as schematically illustrated in more detail in figure 2 d). Preferably the analyzer 15 consists of a second beam splitter 31 and two polarisers 32 and 33 aligned at +45° and -45°, respectively, with regard to the polarisation directions of the TE and TM modes. Two photodiodes 34 and 35 detect the light transmitted by the polarisers. The photodiode signals S₁ and S₂ are given by

5₁ = (S_o/2) (1+ cos Δφ)

5₂ = (S_o/2) (l- cos Δφ) wherein S₀ is proportional to the light power and Δφ is the induced phase shift.

A differential amplifier 36 sends the difference of the two signals ΔS = Si - S₂ = S₀ cos Δφ to a controller 14. The controller 14 adjust the voltage applied to the modulator 21 in such a way that ΔS is zero. The phase shift Δφ is then +/-90° modulo 180°, i.e. the light is circularly polarised. To switch from left to right circularly polarised light the controller switches the working point from the positive to the negative slope of the cosine function (or vice versa depending to signs of the polariser angles).

The closed loop control also automatically compensates for variations in Δφ as a result of the temperature dependence of the modulator birefringence and the electro-optic effect or due to ageing, etc. Furthermore, it also compensates for variations in Δφ due to the wavelength tuning. The information on the instantaneous wavelength may be used by the controller as a further input for the polarisation control, but is not needed as a matter of principle.

Some extra phase shifts may be introduced at the beam splitters, These phase shifts, if any, are stable, however, and therefore can be corrected for by the polarisation controller 15.

In figure 2a) the light 24 leaving the phase modulator 21 is collimated by a lens 54, preferably a graded index lens, and then propagates through free space to the polarisation analyzer 15 and the sample 6 to be investigated. Alternatively, the light may be coupled into another polarisation-maintaining fiber and guided by this fiber to the polarisation analyzer 15 and the sample 6. The birefringent fiber axes are then aligned parallel to the axes of the modulator. Preferably, the fast axis of the fiber is aligned parallel to the slow axes of the modulator and its length is selected such that the input pm fiber pigtail, if there is any, and the exit pm fiber together again fully or partially balance the group birefringence of the modulator. So in this case it is on one hand possible that this output polarisation-maintaining fiber may completely or partially take over the function of the above- described pigtail 57 and compensate for the group delay between the orthogonal polarisation modes in the modulator 21. It is also possible that one fraction of the compensation is taken over by a pigtail 57 and the remaining fraction is taken over by such an additional fiber behind the modulator 21. The fiber and modulator end faces are prepared with a properly chosen oblique angle in order avoid back reflections.

Another implementation of the invention is shown in Figure 3. To avoid duplication of the specification, functionally identical elements are indicated with the same reference numerals as before and according to figure 2a) and only the differences shall be pointed out. Here, the modulator 37 is a phase modulator which has a y-type waveguide as known from modulators for fiber gyroscopes. Preferably, the waveguide 37 is generated by proton exchange. This type of waveguide 37 only guides the extraordinary polarisation mode (TE in case of an x-cut substrate). The input fiber is aligned such that the polarisation of the incoming light is parallel to the polarisation direction of the guided mode. The polarised light waves, which are coupled out by means of two individual polarisation maintaining fibers 39 and 40, respectively, both with cross- sections aligned with respect to the direction of polarisation as indicated in the schematic representations 41 and 42, respectively, are combined in a polarisation maintaining fiber coupler 45, such that the light waves have orthogonal polarisations after the coupler 45. For this purpose there is a 90°-splice 43 in one arm of the coupler where fast and slow fiber axes are exchanged. In respect of this specific arrangements and method of operation, reference is made to K. Bohnert, et al, "Highly accurate fiberoptic dc current sensor for the electro-winning industry", IEEE/IAS Transactions on Industry Applications 43(1), 180-187, 2007, the disclosure of which is herewith expressly included into the present specification. The polarisation controller 15 now controls the relative phase between the two branches 39 and 40, respectively of the modulator 37 and thus again the resulting polarisation state at the sample, with essentially the same advantages and possibilities as described above in the context of the birefringence modulator 21. One can thus say that the elements 37-47 are essentially functionally similar to the birefringence modulator 37.

A still further possible embodiment is illustrated in figure 4, wherein only the light path between the tuneable laser and the sample (possibly with beam splitter 27 being present) is indicated. In this case polarisation maintaining fiber 18 carrying the linearly polarised light 20 from the tuneable laser 16 is spliced in the 45° splice 48 to a second polarisation maintaining fiber 49 with an angle of 45° between the birefringent axes of the two fibers 18, 49, respectively. As a result both orthogonal polarisation modes of the second fiber 49 are excited with equal amplitudes. The second fiber 49 in the piezoelectric modulator 50 is wrapped on a cylinder shaped (preferably hollow) tube made of a piezoelectric ceramic (or other suitable piezoelectric material). By applying a control voltage (schematically illustrated by the arrow) to the piezoelectric element the phase shift between the two orthogonal polarisation modes can be adapted and, as a result, time-dependent alternating circular polarisation can be generated. Preferably, there is a third polarisation maintaining fiber 53 spliced to the second fiber 49 such that the slow and fast axis are exchanged for compensation of the group birefringence of fiber 49 for the reasons given above. A change in the voltage applied to the modulator slightly changes the length and birefringence of the second pm fiber and thus introduces a change in the differential phase. The polarisation control adjusts the voltage such that the exiting light is left or right circularly polarised.

Instead of a modulator based on an electro-optic waveguide structure a bulk electro- optic crystal (e.g. lithium niobate, bismuth germanium oxide, bismuth silicon oxide) could be used.

LIST OF REFERENCE NUMERALS

1 source of electromagnetic radiation

2 element for frequency selection (e.g. monochromator or FTIR)

3 infrared optical filter 4 polariser

5 photoelastic modulator

6 sample

7 detector

8 electromagnetic radiation of selected frequency in infrared region, unpolarised 9 electromagnetic radiation of selected frequency in infrared region, linearly polarised

10 electromagnetic radiation of selected frequency in infrared region, alternatingly left/right circularly polarised

11 transmitted portion of electromagnetic radiation

12 creation and modulation of circularly polarised light (CPL)

13 system controller

14 polarisation controller

15 polarisation analyser

16 tuneable laser

17 isolator

18 polarisation maintaining optical fiber behind laser

19 schematic representation of the fiber-core cross-section of 18 and of polarisation direction

20 monochromatic linearly polarised radiation, wavelength determined by wavelength of 16

21 electro-optical birefringence modulator

22 schematic representation of the core cross-section of the waveguide of 21 and of the fast (f) and slow (s) axes of the waveguide input side

23 schematic representation of electrodes of 21

24 output beam of 21

25 left circularly polarised light of 24

26 right circularly polarised light of 24

27 beam splitter

28 control beam

31 second beam splitter 32 polariser, first component

33 polariser, second component

34 photo detector for first component

35 photo detector for second component 36 differential amplifier

37 electro-optical phase modulator

38 schematic representation of the electrodes of 37

39 first output fiber of 37

40 second output fiber of 37 41 schematic representation of the core cross-section of 39 and of polarisation direction

42 schematic representation of the core cross-section of 40 and of polarisation direction

43 90° splice 44 schematic representation of the core cross-section of 40 downstream of 43 and of polarisation direction

45 polarisation maintaining fiber coupler

46 schematic representation of the core cross-section of the collecting fiber downstream of 45 and of polarisation directions 47 collecting fiber

48 45° splice

49 second polarisation maintaining fiber section

50 piezoelectric modulator

51 tube made of piezoelectric ceramic 52 90° splice

53 third polarisation maintaining fiber section

54 lens

55 45° splice

56 90° offset 57 pm fiber section, pigtail

58 schematic representation of the fiber-core cross-section of 57 and of the fast (f) and slow (s) axes of the fiber

V voltage applied in 21 t time

Claims

1. A circular polarisation spectrometer comprising a source (1) of essentially linearly polarised essentially monochromatic electromagnetic radiation; an element (21 , 37-45) for the generation of alternatingly left and right circularly polarised radiation therefrom; at least one detector (7) for detecting radiation transmitted by a sample to be spectiOscopically analysed in terms of differential absorption of left and right circularly polarised radiation; wherein the element (21, 37-45) for the generation of alternatingly left and right circularly polarised radiation is an electro-optical device in which by means of the application of voltage and a correspondingly generated electric field the birefringence can be adapted as a function of time,

2. A spectrometer according to claim 1, wherein the electromagnetic radiation of the source (1) is in the infrared range, preferably in the near infrared range from about 800 nm to 2500 nm.

3. A spectrometer according to any of the preceding claims, wherein the source (1) of linearly polarised monochromatic electromagnetic radiation is a tuneable laser.

4. A spectrometer according to any of the preceding claims, wherein the light from the source (1) of essentially linearly polarised essentially monochromatic electromagnetic radiation is coupled into the electro-optical device (21, 37-45) by means of a polarisation maintaining fiber (18, 57).

5. A spectrometer according to any of the preceding claims, wherein the electro- optical device (21) is a birefringence modulator based on a lithium niobate waveguide structure.

6. A spectrometer according to any of the preceding claims, wherein the electro- optical device is a lithium niobate modulator (21) with the waveguide produced by titanium in-diffusion into a lithium niobate substrate.

7. A spectrometer according to any of the preceding claims, wherein the electro- optical device (37-45) includes an electro-optical phase modulator (37) with a Y- type waveguide, the output of which is collected by at least two individual polarisation maintaining fibers (39, 40) which are subsequently combined in a polarisation maintaining fiber coupler (45), wherein in only one of the at least two polarisation maintaining fibers (39, 40) a 90°-splice is inserted.

8. A spectrometer according to claim 7, wherein the phase modulator (37) is based on a lithium niobate waveguide structure, wherein preferably it is generated by proton exchange.

9. A spectrometer according to any of the preceding claims, wherein downstream of the electro-optical device (21, 37-45) a polarisation analyser (15) is provided, the output of which is coupled to a polarisation controller (14), which polarisation controller (14) is connected to the electro-optical device (21, 37-45) for controlling the applied voltage.

10. A spectrometer according to claim 9, wherein a beam splitter (27) is located in the output light path of the electro-optical device, and wherein a small fraction split off by this beam splitter (27) is coupled into the polarisation analyser (15), wherein preferably the polarisation analyser (15) comprises a second beam splitter (31) and two linear polarisers (32, 33) the polarisation directions of which are shifted 90° with respect to each other for the two light beams generated by the second beam splitter (31), as well as two individual photo detectors (34, 35), the output of which is used in a differential amplifier (36) for the generation of a control signal proportional to the deviation of the phase shift between the two orthogonal polarisation components from +π/2, eventually modulo 2π, or -π/2, eventually modulo 2π, wherein more preferably the two polarisers (32, 33) in the polarisation analyser (15) are oriented under + 45° and -45° with respect to the polarisation directions of the transverse electric and transverse magnetic modes of the electro-optical device (21, 37-45).

11. A spectrometer according to any of the preceding claims, wherein it further comprises an element (57) for compensating the group delay between orthogonal polarisation modes in the element (21, 37-45) for the generation of alternatingly left and right circularly polarised radiation.

12. A spectrometer according to claim 11, wherein the element for compensating the group delay is a polarisation maintaining fiber section (57) in the optical path, in which the slow and fast axis of propagation is inverted with respect to the slow and fast axis of propagation in the element (21, 37-45) for the generation of alternatingly left and right circularly polarised radiation, and the length of the polarisation maintaining fiber section (57) and/or the birefringence of it is adapted to essentially compensate the group delay between orthogonal polarisation modes in the element (21, 37-45), wherein preferably the polarisation maintaining fiber section (57) is located between the source of essentially monochromatic electromagnetic radiation which is coupled into an polarisation maintaining fiber (18) in which the linearly polarised light propagates along one of the main axes, and wherein the polarisation maintaining fiber section (57) is coupled to this polarisation maintaining fiber (18) via a 45° splice (55) and is coupled to the element (21 , 37-45) under a 90° offset as concerns the birefringence axes.

13. Use of an electro-optical device in which by means of the application of voltage and a correspondingly generated electric field the birefringence can be adapted as a function of time for a circular polarisation spectrometer, preferably for vibrational circular dichroism spectroscopy in the near infrared wavelength region, particularly preferred in a spectrometer according to any of the preceding claims.

14, Use according to claim 13, wherein the electro -optical device (21) is a birefringence modulator based on a lithium niobate waveguide structure, most preferably a lithium niobate modulator (21) with the waveguide produced by titanium in-diffusion into a lithium niobate substrate, or wherein the electro- optical device (37-45) includes an electro-optical phase modulator (37) with a Y- type waveguide, the output of which is collected by at least two individual polarisation maintaining fibers (39, 40) which are subsequently combined in a polarisation maintaining fiber coupler (45), wherein in only one of the at least two polarisation maintaining fibers (39, 40) a 90°-splice is inserted, wherein preferably the phase modulator (37) is based on a lithium niobate waveguide structure, wherein preferably it is generated by proton exchange.

15. Method for operating a spectrometer according to any of claims 1-12, wherein the voltage applied to the electro-optical device is alternatingly switched such as to induce alternatingly left and right circular light, wherein preferably the frequency of alternation is in the range of 1 Hz -I MHz, most preferably in the range of I kHz - IOO kHz.

16. Method for operating a spectrometer according to any of claims 1-12, preferably in a method according to claim 13, wherein the source (1) of essentially linearly polarised essentially monochromatic electromagnetic radiation is a tuneable laser (16), and wherein the tuneable laser (16) is periodically swept over a wavelength range in the near infrared window, and wherein preferably the sweeping frequency of the tuneable laser (16) is essentially identical or an integer fraction or an integer multiple of the switching frequency of the voltage applied to the electro-optical device (21, 37-45).