WO2004019017A1

WO2004019017A1 - Spectroscopic measurement of dispersion

Info

Publication number: WO2004019017A1
Application number: PCT/AU2003/001061
Authority: WO
Inventors: Malcolm Bruce Gray; Nicholas Peter Robins; John David Close
Original assignee: Australian National University
Priority date: 2002-08-23
Filing date: 2003-08-20
Publication date: 2004-03-04
Also published as: AU2002950968A0

Abstract

A device for spectroscopically measuring the refractive index of a gas or Doppler broadened medium, the device comprising an optical input unit (26) for a light beam (12), from a laser (10), a splitting unit (30) for splitting a light beam into two beams, and an interferometer (18) arranged, in use, for interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the refractive index of the gas or Doppler broadened medium. The Sagnac interferometer (18) includes a gas cell (20) and is arranged such that one of the beams utilised as a pump beam and the other as a probe beam in a Doppler-free saturated absorption spectroscopy configuration. The device can also be used to produce an interferometric error signal for a wavelength stabilisation of the laser (10).

Description

Spectroscopic measurement of dispersion

Field of the invention

The present invention relates broadly to a device and method for spectroscopically measuring the refractive index of a gas or Doppler broadened medium, to a device for providing an error signal for wavelength stabilization of a light source, to a method of determining an error signal for wavelength stabilization of a light source, and to a wavelength stabilized light source.

Background of the invention

The spectroscopic measurement of refractive index (dispersion) of materials finds application in the monitoring of gases or Doppler broadened material emitted from factories, engines or machinery of any type. In such applications, it is desirable to facilitate monitoring with sub-Doppler precision, e.g. to distinguish between elements of the gas or Doppler broadened material for which the Doppler broadened spectroscopic resonances overlap.

On the other hand, the locking of lasers to narrow atomic or cavity resonances is a prerequisite for many experiments in physics and precision industrial applications. Several methods for obtaining an error signal from an atomic transition are in use. These generally rely on modulation of either the laser frequency or atomic reference source and subsequent electronic demodulation to produce the error signal required for locking.

More recently, modulation-free differencing techniques have come into use, whereby an error signal is produced by subtracting two frequency or phase shifted signals generated from the same atomic reference source. Modulation free schemes have the advantage of simplicity over more traditional methods as they potentially do away with the need for lock-in electronics and various modulation apparatus. The benefit is two-fold, a simplified experimental setup, and a reduction in broadening effects on the frequency reference and/or laser.

In some modulation free setups, expensive apparatus such as acousto-optic modulators (AOMs) are used to produce a frequency difference between two probe beams used to generate the error signal, in other methods, strong magnetic fields are used to split a Doppler broadened absoφtion feature to provide an error signal. In other techniques, the setup is very sensitive to the alignment of probe and pump beams used to probe different velocity classes in a Doppler broadened sample, and some rely on the non-linear interaction of two laser beams in an absorbing gas.

In at least preferred embodiments, the present invention seeks to provide a novel, geometrically stable, technique that makes a true measurement of the phase difference between a pump and a probe beam to provide a sub-Doppler precision measurement of the refractive index of the material being probed. The technique can also be used in the wavelength stabilization of a light source.

Summary of the invention

In accordance with a first aspect of the present invention there is provided a device for spectroscopically measuring the refractive index of a gas or Doppler broadened medium, the device comprising an optical input unit for a light beam, a splitting unit for splitting the light beam into two beams, and an interferometer arranged, in use, for interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the refractive index ofthe gas or Doppler broadened medium.

In one embodiment, the interferometer is arranged, in use, such that one of the beams is utilised as a pump beam and one of the beams as a probe beam in a Doppler-Free saturated absoφtion spectroscopy configuration, and such that a phase shift of the probe beam when compared with the pump beam is interferometrically detected.

Preferably, the interferometer comprises a Sagnac loop terminated at the splitting unit.

The splitting unit may comprise a beam splitter. In such an embodiment, the interferometer is preferably misaligned to an extent sufficient, in use, to facilitate detection of the phase shift through tilt-locking. The device may comprise a detector unit arranged, in use, such that vertically displaced TEM_01 lobes in the interferometer output are independently detected for determining the phase shift through subtraction ofthe respective lobe intensities.

The splitting unit may comprise a diffraction grating. In such an embodiment, the interferometer is preferably arranged, in use, such that the output signals form the Sagnac loop are diffracted at the diffraction grating and interfere in quadrature at two detection elements of the device for detecting the phase shift through subtraction of electrical outputs ofthe respective detection elements. The input unit of the device may be in the form of a single mode PM fibre for coupling the light beam into the interferometer.

The input unit ofthe device may be in the form of an optical assembly comprising a first lens for Fourier transforming the input field of the light beam, a first aperture for clipping the focal plane field of the transformed beam, a second lens for collimating and retransforming the clipped beam and a second aperture for clipping the retransformed beam.

In one embodiment, the input unit of the device is in the form of an optical assembly comprising a first lens and a second lens disposed in a beam enlarging telescope configuration, and an aperture for clipping the enlarged telescope output beam.

The device may be arranged for use in wavelength stabilisation of a laser source emitting the light beam, wherein the device comprises the gas or Doppler broadened medium as a reference medium, whereby, in use, the refractive index measured is a measure of a wavelength detuning ofthe laser source. The reference medium may comprise Cs contained in a gas cell.

In accordance with a second aspect of the present invention there is provided a method of spectroscopically measuring the refractive index of a gas or Doppler broadened medium, the method comprising the steps of (a) splitting an input light beam into two beams, and (b) interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the refractive index of the gas or Doppler broadened medium.

In one embodiment, step (b) comprises utilising one of the beams as a pump beam and one of the beams as a probe beam in a Doppler-Free saturated absoφtion spectroscopy configuration comprising a test medium, and interferometrically detecting a phase shift of the probe beam when compared with the pump beam.

Preferably, step (b) is conducted utilising a Sagnac interferometer configuration.

In one embodiment, in step (b) the phase shift is determined utilising a tilt-locking technique. Step (b) may comprise independently detecting vertically displaced TEM_01 lobes in the interferometer output for determining the phase shift through subtraction of the respective lobe intensities.

In another embodiment, step (b) is conducted in a manner such that output signals form the Sagnac loop are diffracted at a diffraction grating and interfere in quadrature at two detection elements for detecting the phase shift through subtraction of electrical outputs of the respective detection elements.

The method may be used for wavelength stabilisation of a laser source emitting the light beam, wherein the gas or Doppler broadened medium acts as a reference medium, whereby, in use, the refractive index measured is a measure of a wavelength detuning ofthe laser source.

In accordance with a third aspect of the present invention there is provided a device for providing an error signal for wavelength stabilisation of a light source, the device comprising an optical input unit for a tapped of portion of a light beam generated by the light source, a splitting unit for splitting the tapped off portion into two beams, and an interferometer arranged, in use, for interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the wavelength detuning ofthe laser source.

In accordance with a fourth aspect ofthe present invention there is provided a method of determining an error signal for wavelength stabilisation of a light source, the method of comprising the steps of (a) tapping off a portion of a light beam generated by the light source, (b) splitting the tapped off portion into two beams, and (c) interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the wavelength detuning ofthe laser source.

In accordance with a fifth aspect ofthe present invention, there is provided a wavelength stabilized light source incoφorating a device for providing an error signal for wavelength stabilisation ofthe light source as defined in the third aspect.

Brief description of the drawings

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.

Figure 1 is a schematic diagram illustrating a device embodying the present invention.

Figure 2 shows (a) saturated absoφtion signals and (b) error signals generated from the embodiment shown in Figure 1.

Figure 3 shows a theoretical prediction of (a) saturation absoφtion, and (b) error signals generated by the embodiment shown in Figure 1. Figure 4 shows a time trace of the interferometer error signal illustrating the time- stability ofthe embodiment of Figure 1.

Figure 5 is a schematic drawing of another error signal detection circuit embodying the present invention.

Figure 6 is a schematic drawing of another error signal detection circuit embodying the present invention.

Detailed description of the embodiments

An experimental setup embodying the present invention is shown in Fig. 1. The laser 10 is a lOOmW SDL-5700-TO3 DBR diode operating at 852nm. A small portion ofthe laser beam 12 is split off and passed through a two mirror (14, 16) Sagnac interferometer 18. A cesium vapor cell 20 and a neutral density filter 22 are placed inside the interferometer 18. In this configuration, the counter clock-wise beam passing through the filter 22 before the cell 20 acts as a probe, and the clock-wise beam as the pump, creating the conditions for Doppler-free saturated-absoφtion spectroscopy.

The embodiment shown in Figure 1 combines the precision of spectroscopy with the sensitivity of interferometry to produce a robust, zero crossing signal that can either be used as a zero crossing error signal for the frequency stabilisation of the laser 10 to the frequency of a transition in the material in the cell 20, or, in another embodiment, to detect, with sub-Doppler precision, the presence of a refractive material. In such an embodiment, the cell 20 takes on the function of a sample cell as opposed to a reference cell. It will be appreciated by a person skilled in the art, that in such embodiments, the sample cell may e.g. be configured to be in fluid communication with an ambient around the device, with the remaining components of the device incoφorated into a casing structure for gas detection applications.

It will be appreciated by a person skilled in the art that in Doppler-free saturated- absoφtion spectroscopy the pump and probe beams only interact with the same atoms in the cell 20 when the frequency of the pump and probe beams matches an absoφtion frequency of the atoms within the cell 20. More particularly, the pump and probe beams under those conditions react with those atoms in the cell 20 that do not move along the pump-probe axis, because for these atoms there is no Doppler shift of the beams in the atom's frame. If the frequency of the pump and probe beams are detuned from the absoφtion resonance, the pump and probe beams interact with different groups of atoms within the cell 20, because a Doppler shift of a particular sign is required for absoφtion, i.e. an atom moving along the pump and probe axis that absorbs light from one beam, cannot absorb light from the other beam. The exceptions to this simple description of the origin of the Doppler free signal are the well-known Doppler free crossover peaks that are also observed with the example embodiment.

Between 0.1 and 2mW of power incident on the input port 24 of a single mode PM fibre coupler 26 was used, with a beam diameter of 1mm and a filter 22 with transmissivity 0.1. A simple CCD camera (not shown) was initially used to check the output of the interferometer 18 for coarse alignment of the dark port and tilt mode. The tilt output of the interferometer 18 is achieved by misaligning the beamsplitter 30 (and/or one or both of the mirrors 14, 16) on the horizontal axis until the dark port changes to a clearly visible TEM_01 mode. The detector 28 is a commercial quadrant photodiode (EG\&G C30843E) with the quadrants appropriately added to produce a vertically split two lobed tilt detector. The output of the lobes is then summed or subtracted allowing both the saturated-absoφtion and error signals to be monitored simultaneously. Finally, the error signal is amplified and passed to a servo-locking circuit 29 which feeds back to the laser diode current.

The embodiment shown in Figure 1 combines the precision of spectroscopy with the sensitivity of interferometry to produce a robust, zero crossing error signal from an atomic transition. The technique is modulation and Doppler free, polarization independent, requires no lock-in amplifier and consists of only a photodetector, gas cell and inexpensive optical components. A Sagnac configuration is used in the example embodiment because it is interferometrically insensitive to environmental drift due to the optical beams travelling the same path in opposite directions.

In operation, the pump beam saturates the vapor cell 20 and is then attenuated, while the probe beam is first attenuated and then probes the saturated transitions of atoms in the vapor cell 20. Upon recombination at the Sagnac beam splitter 30, both beams have experienced the linear absoφtion of the vapor cell 20 and neutral density filter 22. However, under Doppler- free resonance condition the probe beam experiences/probes the population in the atoms which have been saturated at the Doppler-free resonance by the pump beam. Associated with an atomic absoφtion feature there is a shaφ change in n(x), the refractive index, which in turn effects the phase of the interacting light, i.e. the probe beam. The phase change associated with the absoφtion feature can be expressed as

2πv φ = j[«(*x) - ljctf

where v is the frequency ofthe light and x is the coordinate along the beam propagation path. The integral is taken over the length ofthe interaction region. The refractive index, [n(x)- 1], has a typical dispersive form with the zero crossing coinciding with the peak in absoφtion. By placing the vapor cell 20 inside the interferometer 18, the phase change associated with a resonance and its resultant effect on the output of the interferometer 18 is used to frequency stabilize the laser 10.

This Doppler-free phase shift is used to produce an interferometric error signal at the Sagnac output, achieved through the use of Tilt locking in the example embodiment. This scheme is described in the applicant's co-pending patent application entitled "Interferometer control and laser locking", filed in Australia on 30 June 2000, application no. PCT/AU00/00739, International publication no. WO01/03258. The contents of that application are hereby incoφorated by cross reference. The concept of Tilt locking can be extended to two beam interferometers. The output of the Sagnac 18, or any two beam interferometer, can be described in the basis of TEM modes, as can a cavity. In the case of a Sagnac, a TEM_00 mode incident on the input of the interferometer will result in a dark output. If the interferometer 18 is misaligned to produce a tilt error on recombination at the beamsplitter 30, the output will be a TEM_01 mode ^π 12 out of phase with the original beams. Any phase shift introduced within the interferometer 18 will lead to a power mismatch between the TEM_01 lobes due to constructive interference between the TEM_00 mode and TEM 01 mode in one half of the beam while producing destructive TEM_00/ TEM_01 interference in the other half.

This effect is shown schematically in insets (a)-(c) in Figure 1. As the laser frequency is scanned across the spectroscopic resonance, the phase shift induced by the Doppler-free dispersion inside the interferometer 18 adds (a) or subtracts (c) to the phase of the TEM_01 mode resulting in an intensity mismatch in the lobes 32, 34. The magnitude ofthe phase shift is directly proportional, in sign and magnitude, to the detuning of the frequency of the pump and probe beams, i.e. the laser frequency with respect to the resonance. If the laser frequency matches that ofthe atomic resonance, both lobes 32, 34 have equal intensity (b).

A typical signal from the interferometer 18 is shown in Fig. 2. The transition is the Cs 6² S_ι;2 ,F = 4 - 6²P_3/2 used for laser cooling. An extraordinary feature of the example embodiment is the large signal-to-noise ratio of the true zero crossing error signal (curve (b)), generated without the use of any modulation ofthe laser light or atoms. The inset 100 in Figure 2 shows the laser scan across the Cs 6² S_{1 / 2} , E = 4 → 6² P_3/2 , F = 5 transition, illustrating the true zero crossing nature of the error signal. Once the system is operating, the error signal was found to be insensitive to intensity fluctuations of the laser beam, and is strikingly immune to acoustic perturbations.

Figure 3 shows the results of a simple analytic model of both the saturated absoφtion (a) and the resulting tilt locked error signal (b). In this model, we explicitly derive the real and imaginary coefficients for the complex refractive index for each transition within the Cs 6² S_{1/ 2} , E = 4 — 6² P_{3/ 2} hyperfine manifold and then cascade these transitions before calculating the total complex refractive index as a function of optical frequency. Propagation around the Sagnac and through the Cs cell in one direction (pump) yields the Doppler broadened absoφtion spectrum while propagation in the other direction (probe) yields the saturated absoφtion spectrum. These two fields are then subtracted at the Sagnac dark fringe ( π out of phase) output. Then, the tilt fields are vectorially added (at π and -π phase respectively) on each half of the output plane to give the resulting tilt error signal. The effects of polarization on population pumping have been included by matching the transition strengths to the experimental spectrum. This model gives a zero crossing error signal for each hyperfine transition (transitions 3 and 4 are almost merged).

Fig. 4 shows the error signal of a single laser stabilized in accordance with an embodiment of the present invention as a function of time, the system being locked at t=20 seconds. A heterodyne beat measurement of two independently tilt locked lasers was also made. One laser was locked to the / = 4 - F = 5 crossover 125MHz to the red of the second laser which was locked directly to the / = 4 - F = 5 transition. The first laser was down shifted with an AOM by 64MHz. The beat signal at 189MHz, shown in the inset 200 to Fig. 4, was detected on an RF photodiode, fed to a spectrum analyzer (HP 8568B) and then to a computer data logger. It was found that the centre frequency is stable to the sub-MHz level indefinitely. The laser linewidth of the diodes was measured to be 5MHz. This linewidth is consistent with measurements made using a prior art locking techniques, and is not due to the tilt locking mechanism. The bandwidth of the servo-locking circuit is 100 Hz, providing good low frequency control of the central laser frequency, but has no effect on fast dynamics which determine the laser linewidth.

In the embodiment shown in Figure 1 , a single mode PM fibre 26 is used to couple the input light into the Sagnac interferometer 18 in a flexible fashion. This also spatially filters out any beam jitter and removes this noise source from affecting the tilt locking detector.

In an alternative embodiment shown in Figure 5, Fourier transform spatial filtering is utilised. In that embodiment, a focusing lens 300 is used to Fourier transform the input field 302 (including beam jitter) and an aperture 304 to clip the focal plane field. This not only removes the periphery of the field but also attenuates beam tilt as well; the focus lens 300 converts tilt to offset at the focal plane while the clipping aperture 304 then attenuates the effects of offset on the transmitted field. Another lens 306 is then used to collimate the clipped beam. This retransforms the field so that offset beam jitter is now transformed back to offset at the collimated output. A second aperture 308 then clips this collimated beam, again removing the periphery of the field and attenuating the offset of the field transmitted through the second aperture. The total lens-aperture assembly in the example embodiment is located immediately prior to the Sagnac interferometer beam splitter 310.

It will be appreciated by a person skilled in the art that the Fourier transform spatial filtering technique depicted in Figure 5 can be modified to utlise lens 300 and 306 to form a beam enlarging telescope. Beam enlargement telescopes in this configuration, reduce beam tilt by the reciprocal of the enlargement factor, eliminating the need for aperture 304 in Figure 5. Aperture 308 still clips the enlarged telescope output beam and thereby reduces beam offset errors entering the beam splitter 310.

Unlike the single mode fibre coupler, these approaches necessarily couples the power loss to the filtering quality of the system: the greater the power loss due to clipping, the greater the attenuation of both beam jitter and beam tilt on the transmitted beam. The main advantage of the embodiment shown in Figure 5 is that it avoids the need to use a single mode fibre and hence avoids the associated phase noise that this creates. For high sensitivity applications this fibre phase noise may well be unacceptable and hence the use of spatial filtering may be an attractive solution.

Figure 6 shows a schematic of an alternative embodiment. A diffractive phase mask 400 is used to split the incident light 402 into the two Sagnac beams, instead of the standard beamsplitter used in the previous embodiments. When optimised, this can create a bulk equivalent of a 6-port coupler. Further, it is then possible to derive a phase shift for this "beam splitter" so that the CW and CCW beams interfere approximately in quadrature on the two signal detectors 406, 408. Hence a subtraction of the electrical output of these two detectors 406, 408 yields an error signal that is proportional to the phase difference experienced by the two beams within the interferometer 410.

A potential advantage of the embodiment shown in Figure 6 is that it is free of spatially sensitive detectors and hence may be robust to input beam jitter.

It will be appreciated by the person skilled in the art that numerous modifications and/or variations may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

For example, it will be appreciated that other interferometer configurations could be used in different embodiments, other than a Sagnac interferometer. In one such alternative embodiment, the tapped-off portion of the laser beam from the light source to be stabilized is split into three beams. One of the beams is used as a pump beam in conjunction with another one of the beams as a probe beam for creating the Doppler-free saturated absoφtion spectroscopy condition, whereas the probe beam and the third beam could be configured in a Mach-Zehnder interferometer configuration to determine the phase shift in the probe beam.

It will be appreciated that modulation free locking of the frequency difference between two lasers can be achieved using the interferometric measurement of dispersion around a dark state transition in a tilt lock setup embodying the present invention. In an alternative embodiment, the pump light could comprise two frequencies whose difference is equal to the difference in the energy of two metastable levels, for example the 9.2 GHz hyperfine splitting in the ground state of Cs. The probe in such an embodiment can comprise a single frequency and will be strongly phase shifted when it satisfies the Raman condition with either ofthe two pump frequencies. Furthermore, while the embodiments described herein present modulation-free techniques, it will be appreciated that known modulation techniques may be utilised in different embodiments to facilitate the interferometrical detecting ofthe modification in one ofthe beams with respect to the other.

In the claims that follow and in the summary of the invention, except where the context requires otherwise due to express language or necessary implication the word "comprising" is used in the sense of "including", i.e. the features specified may be associated with further features in various embodiments ofthe invention.

Claims

1. A device for spectroscopically measuring the refractive index of a gas or Doppler broadened medium, the device comprising:

- an optical input unit for a light beam,

- a splitting unit for splitting the light beam into two beams, and

- an interferometer arranged, in use, for interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the refractive index ofthe gas or Doppler broadened medium.

2. A device as claimed in claim 1, wherein the interferometer is arranged, in use, such that one of the beams is utilised as a pump beam and one of the beams as a probe beam in a Doppler-Free saturated absoφtion spectroscopy configuration, and such that a phase shift of the probe beam when compared with the pump beam is interferometrically detected.

3. A device as claimed in any one of the preceding claims, wherein the interferometer comprises a Sagnac loop terminated at the splitting unit.

4. A device as claimed in any one ofthe preceding claims, wherein the splitting unit comprises a beam splitter.

5. A device as claimed in claim 4, wherein the interferometer is misaligned to an extent sufficient, in use, to facilitate detection ofthe phase shift through tilt-locking.

6. A device as claimed in claim 5, wherein the device comprises a detector unit arranged, in use, such that vertically displaced TEM 01 lobes in the interferometer output are independently detected for determining the phase shift through subtraction of the respective lobe intensities.

7. A device as claimed in any one of claims 1 to 3, wherein the splitting unit comprises a diffraction grating.

8. A device as claimed in claim 7, wherein the interferometer is arranged, in use, such that the output signals form the interferometer are diffracted at the diffraction grating and interfere in quadrature at two detection elements of the device for detecting the phase shift through subtraction of electrical outputs ofthe respective detection elements.

9. A device as claimed in any one of the preceding claims, wherein the input unit of the device is in the form of a single mode PM fibre for coupling the light beam into the interferometer.

10. A device as claimed in any one of claims 1 to 8, wherein the input unit of the device is in the form of an optical assembly comprising a first lens for Fourier transforming the input field ofthe light beam, a first aperture for clipping the focal plane field ofthe transformed beam, a second lens for collimating and retransforming the clipped beam and a second aperture for clipping the retransformed beam.

11. A device as claimed in any one of claims 1 to 8, wherein the input unit of the device is in the form of an optical assembly comprising a first lens and a second lens disposed in a beam enlarging telescope configuration, and an aperture for clipping the enlarged telescope output beam.

12. A device as claimed in any one of the preceding claims, wherein the device is arranged for use in wavelength stabilisation of a laser source omitting the light beam, wherein the device comprises the gas or Doppler broadened medium as a reference medium, whereby, in use, the refractive index measured is a measure of a wavelength detuning ofthe laser source.

13. A device as claimed in claim 12, wherein the reference medium comprises Cs contained in a gas cell.

14. A method of spectroscopically measuring the refractive index of a gas or Doppler broadened medium, the method comprising the steps of:

(a) splitting a light beam into two beams, and

(b) interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the refractive index of the gas or Doppler broadened medium.

15. A method as claimed in claim 14, wherein step (b) comprises utilising one of the beams as a pump beam and one of the beams as a probe beam in a Doppler-Free saturated absoφtion spectroscopy configuration comprising a test medium, and interferometrically detecting a phase shift ofthe probe beam when compared with the pump beam.

16. A method as claimed in claims 14 or 15, wherein step (b) is conducted utilising a Sagnac interferometer configuration.

17. A method as claimed in any one of claims 14 to 16, wherein in step (b) the phase shift is determined utilising a tilt-locking technique.

18. A method as claimed in claim 17, wherein step (b) comprises independently detecting vertically displaced TEM_01 lobes in the interferometer output for determining the phase shift through subtraction ofthe respective lobe intensities.

19. A method as claimed in claims 14 to 16, wherein step (b) is conducted in a manner such that output signals form the interferometer are diffracted at a diffraction grating and interfere in quadrature at two detection elements for detecting the phase shift through subtraction of electrical outputs ofthe respective detection elements.

20. A method as claimed in any one of claims 14 to 19, wherein the method is used for wavelength stabilisation of a laser source omitting the light beam, wherein the gas or Doppler broadened medium acts as a reference medium, whereby, in use, the refractive index measured is a measure of a wavelength detuning ofthe laser source.

21. A device for providing an error signal for wavelength stabilisation of a light source, the device comprising:

- an optical input unit for a tapped off portion of a light beam generated by the light source,

- a splitting unit for splitting the tapped off portion into two beams, and

- an interferometer arranged, in use, for interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the wavelength detuning ofthe laser source.

22. A method of determining an error signal for wavelength stabilisation of a light source, the method comprising the steps of:

(a) tapping off a portion of a light beam generated by the light source,

(b) splitting the tapped off portion into two beams, and

(c) interferometrically detecting a modification in one of the beams with respect to the other, wherein the modification is a measure for the wavelength detuning ofthe laser source.

23. A device for spectroscopically measuring the refractive index of a gas or Doppler broadened medium, substantially as herein described with reference to the accompanying drawings.

24. A method of spectroscopically measuring the refractive index of a gas or Doppler broadened medium, substantially as herein described with reference to the accompanying drawings.

25. A wavelength stabilized light source incoφorating a device as claimed in any one of claims 1 to 13 for providing an error signal for wavelength stabilisation of the light source.

26. A device for providing an error signal for wavelength stabilisation of the light source substantially as herein described with reference to the accompanying drawings.

27. A method of providing an error signal for wavelength stabilisation of the light source substantially as herein described with reference to the accompanying drawings.

28. A wavelength stabilized light source substantially as herein described with reference to the accompanying drawings.