GB2347756A

GB2347756A - A radiation source with frequency conversion member and imaging system

Info

Publication number: GB2347756A
Application number: GB9903307A
Authority: GB
Inventors: Donald Dominic Arnone; Craig Michael Ciesla
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 1998-11-03
Filing date: 1999-02-12
Publication date: 2000-09-13
Anticipated expiration: 2019-02-12
Also published as: GB2347756B; GB2343964A; GB9824058D0; GB9903307D0; GB2343964B; JP2000162656A

Abstract

A beam of radiation, preferrably in the THz waveband, is emitted from a frequency conversion member C2 in response to irradiation from one or more input beams 7. The frequency conversion member is contained within an optical cavity defined by M3, M4, M5, M6 and 23, from within which the input beams are produced. Phase matching means having spatially varying refractive indices may be included, and the invention also extends to an imaging system using such a radiation source.

Description

2347756 1 A Radiation Source and Imaging System

The present invention is concerned with the field of radiation sources and imaging systems using radiation sources. More specifically, the present invention is concerned with radiation sources which operate in the Terahertz (THz) frequency range.

Difficulties arise in producing electro-magnetic radiation in several commercially important wavelength bands, one of these is the Terahertz frequency range i.e. 100 GHz to 20 THz.

A possible method of producing a signal in the above frequency range is to use nonlinear optical effects. The polarisation P in a material radiated by incident radiation can be expressed in terms of E the electric field exciting the material as the power series:

P = X (')E + X ME 2 + X (3)E 3......

Generally, the relationship PccE is used as the higher order terms are assumed to be negligibly small. This approximation does not hold for large E. Non linear optics is concerned with these higher order terms.

If a material is irradiated with two different frequencies, the second order term allows the material to emit frequencies which are the sum of the input frequencies (known as Sum Frequency Generation), the difference between the input frequencies (Difference Frequency Generation). The second order susceptibility can also result in the generation of different optical frequencies when the material is irradiated by a single input frequency. For instance, second harmonic generation results from self sum generation. For optical parametric conversion, two frequencies are generated from the input frequency.

2 Also, the third order term y,(3)E3 can also be "cited to produce third harmonic frequencies and other third order terms.

The present invention is particularly concerned with the field of difference frequency generation, e.g. where two beams at visible frequencies covis, and covis2 are converted to THz radiation at coTH, via non-linear difference frequency generation (Oniz CO,j,j- Ovisl-

Generating THz radiation using difference frequency generation suffers from the problem that in order to produce THz radiation with a commercially useful power level, either a large power density of input radiation is required or the THZ signal must be amplified.

The conversion efficiency p from cDvjj and (Ovi,2 to COTHz is given by:

Y3 P(CO ") 2 2 2 2 sin 2 (Ak P = 2 9 Y2)P(O CO 0) 2 CO Visd I 3 (Ak 71 21 A Where A= area of the beam, d = second order non-linear optical coefficient, 11 is the refractive index at a)vj, I is the length of the non-linear crystal, and where (Ovis O)visl ' (OviQ Ak = k((ovi.,2) k(covisl) k(o)THz) (expresses momentum conservation).

It is difficult and expensive to provide an input beam at o)vjr, with a very high power density outside of a conventional laser cavity. Using amplifiers to amplify either the input radiation or the emitted THz signal increases the bulkiness and the cost of the source. It is precisely this increased size and cost which currently limits the proliferation of THz imaging systems in potential commercial applications.

3 The present invention addresses the above problems and in a first aspect provides a radiation source comprising: a frequency conversion member for emitting a beam of radiation in response to irradiation by one or more input beams, the emitted beam having a frequency different to that of the one or more input beams, the one or more input beams all being produced within a lasing cavity and said frequency conversion member being located within said lasing cavity.

In the present invention, the frequency conversion member is actually placed within the laser cavity of the input beam or beans, and thus it is termed an intra-cavity structure. Thus, high powers of the input beam are accessed inside the laser cavity, resulting in larger THz powers, without having to resort to bulky or expensive amplifiers placed external to the cavity to realise high input beam powers.

In the following description, the input beam or beams will be referred to as having a frequency of o)vjj, COvi,2 etc., or more generally covj as, in general, covj.'j;t: (o,j.'2. The emitted beam will be referred to as having a frequency of (oni, However, it will be appreciated by a person skilled in the art that although itis preferred that a TRz signal is emitted, radiation of other wavelengths could be produced using the radiation source of the present invention. In particular, radiation could include mid-infrared (20THz90THz), near-infrared (90TIh-300THz), and millimetre wave/microwave frequencies (IOOGHz-10GHz). A wide bandwidth signal with powerful components at frequencies ranging from the millimetre wave to near-infrared is achievable with this invention, and this would have wide ranging applications in imaging and spectroscopy.

The intensity of the input beam at covj inside a laser cavity exceeds its value outside the lasing cavity by (I-R)-', where R is the lowest reflectivity of the mirrors (at (ovi') within the cavity for the input beam itself. If R = 1, the intensity enhancement inside the cavity is very large and hence any THz power at coTju generated by placing a frequency conversion member inside the cavity is also greatly enhanced.

4 The apparatus can be configured such that one can extract the total available power of the laser at coTH, instead of at o),i, and obtain 100% conversion efficiency However, the frequency conversion member located within the lasing cavity inevitably results in additional losses within the cavity at co,i, This results in a reduction of the power density (P(co,iJ) in the cavity.

One of the mirrors in the input beam (laser) resonator is referred to as an output coupler and reflects a majority of the input beam back into the laser cavity, allowing (with other components) for lasing action in the cavity at (o,i, The reflectivity of the output coupler is changed (increased) to cancel any losses of power at coq, inside the resonantor which arise when the frequency conversion member is inserted. If no input beam power at co,i, is required, the reflectivity R of the output coupler is ideally set to 100% so that all the power at co,i, remains in the cavity and contributes to the THz generation from the frequency conversion member. However, many practical imaging and spectroscopy systems will require output from the system at both as well as o)m, and in this case R is preferably set between 90-100% at (O,i,.

The output coupling means may be provided on the opposing side of the frequency conversion member to the side which the input beam or beams first enter the frequency conversion member.

The drop in power occurs at the laser output of covi., external to the cavity. This is not of direct concern in making a more powerful THz source.

The output coupler is preferably provided by a member which has substantially zero reflectivity to the emitted radiation and highly reflective to the radiation of the input beam or beams. Preferably, this high reflectivity is between 90% and 100%. Thus, the output coupler allows the THz radiation to exit from the cavity, but confines the input I beam radiation to within the optical cavity to generate further THz radiation from the frequency conversion member.

The output coupler can be configured to allow a source according to the present invention to emit both THz radiation and input beam radiation. (The advantages of this will be described with reference to imaging systems later in this description.)

Preferably, the output coupler is arranged so that the THz radiation is emitted from the cavity before it is reflected back onto the lasing element which generates the input beam or beams.

To produce THz radiation, the frequency conversion member will preferably be a non linear crystal which is preferably- configured to emit a frequency which is substantially equal to the difference of two frequencies provided by the input beam or beams.

The incident radiation generates a time-dependent polarisation via the second order non linearity of the-material. A simplified view of the mechanism is to picture the electrons in the material as being on springs. The incident radiation causes the electrons to vibrate with frequencies corresponding to the incident frequencies, their sum and their difference. Vibration occurs at sum and difference frequencies due to the non linear nature of the spring vibration.

Which frequency is emitted is dependent on the non-linear coefficients of the material at the fundamental frequency, and phase matching between the non-linear polarisation and generated/converted radiation at the difference frequencies.

The efficiency of the THz generation from two visible photons cDvil and covi,2 is governed by two key material properties, which are summarised below:

I. The second order susceptibility, X (2) 6 The magnitude of X (2) determines the conversion strength of the visible electric field to THz electric field, and is related to the degree of asymmetry of the electric potential at the microscopic level. This is evident from the fact that the THz power generated is proportional to the polarisation of the material p(coTH,) oscillating at coTiiz given by:

P (CO Tff -) Oc X (2).Evisl EViS2 Crystals which have a large x (2) which are suitable for the frequency conversion member are: Li103, NH4H2PO4, ADP, KH2PO4, KH2As04, Quartz, AIP04, ZnO, CdS, GaP, GaAs, BaTi03, LiTa03, LiNb03+, Te, Se, ZnTe, ZnSe, Ba2NaNb5Ol5, AgAsS3, proustitie, CdSe, CdGeAs2, AgGaSe2, AgSbS3, ZnS, BBO, KTP and organic crystals such as DAST (4-N-methylstilbazolium).

2. The refractive indices for the visible and THz fields and 71ni,).

These govern the degree of phase matching between the optically induced non-linear polarisation and the generated THz, which need to interfere constructively throughout the crystal. Materials with a large difference between ij,j, and ilm suffer from poor conversion efficiencies. Phase matching ensures that the momentum k is conserved between the visible photons with momentum at or near k(covij, and the THz photons with momentum k(com) which are generated by the frequency conversion member. Phase matching for generating THz radiation from the difference frequencies of the incident radiation is expressed as:

Ak = k(co vi.,+co m) - k(cD vi.) - k(o) m) - 0.

The coherence length, 1,, which is a measure of the distance over which the opticallyinduced non-linear polarisation and the generated THz electric fields remain in phase, is defined:

Ic= 7tC/ (O)TH.ITIWs - T1MI) 7 where'q,j, is the refractive index of the material at visible frequencies and TITH, is the refractive index of the material at THz frequencies.

Thus, for the long coherence length necessary for efficient THz generation,,q,i,, - 71 must be small. However, inorganic non-linear optical materials have a large index mismatch and so 1, may be no larger than a few microns.

Therefore, it is preferable if the frequency conversion means of the present invention further comprise phase matching means for enhancing the phase matching between at least two different frequency signals propagating in the frequency conversion member in response to illumination by the one or more input beams, the phase matching means having a spatial variation in its refractive index along a component of the input radiation beams.

Some materials have a natural degree of phase matching due to birefringence properties. In this case, phase matching can be achieved over at least a certain length of the material. However, many materials with large optical non-linearities nevertheless suffer from having no birefringence or other properties which allow some degree of phase matching and thus the full realisation of the material as frequency converter.

Preferably, the phase matching means are provided in the frequency conversion member to reduce this variation in the refractive indices at the different frequencies.

Alternatively, or in addition to the spatial variation in the refractive index, the phase matching means may be provided for by a periodic modulation in the frequency converting non-linear coefficient along the axis of the input beam or beams.

Preferably, there will be a single input beam which provides two frequency components. This can be achieved by a pulsed laser. Preferably, the pulsed laser will have a pulse width ranging from 10 fs to 10 ps.

8 Losses due to the frequency conversion member can be reduced if the frequency conversion member is cut at the Brewster's angle for the polarisation And frequency of the input radiation. Alternatively, the frequency conversion member could be antireflection coated to reduce losses at (o,j, Preferably, the frequency conversion member is also a material which has minimum absorption at (oTH, due to mechanisms such as phonon absorption etc.

Preferably the two different frequency components are provided by a pulsed laser source. A pulse laser source is desirable because mode matching and beam overlap between the input beams are required to obtain optimum conversion to THz. This problem is circumvented if a single beam provides both frequency components (0),isl and (Ovis2). Also, the higher fields obtainable by pulsed lasers allow the (progressively smaller) non linearities in the polarisation term to be accessed and the radiation pulse produced by a pulsed laser inherently contains a number of different frequencies making it ideal for difference frequency generation over a broad range.

The radiation source of the first aspect for the present invention is particularly useful in a THz imaging system. Previous THz imaging systems have been bulky because amplifiers external to the laser cavity are required to produce sufficiently large THz powers for applications. Such external amplifiers are bulky, very expensive, and very difficult to operate. Moreover, the addition of such an amplifier results in a reduction of the pulse repetition rate, which lowers the signal to noise ratio associated with the TIU image and its quality. These aspects have made the widespread use of THz imaging prohibitive in terms of size, cost, ease of use, quality of images, and image acquisition times. Therefore, an imaging system which uses the source of the first aspect of the present invention provides considerable advantages in that the system is a) more compact, b) less expensive, c) more user-friendly and d) may provide better signal to noise ratios due to the higher pulse repetition rate and higher THz power levels, and e) may provide faster data acquisition times, allowing for collection of THZ images at video frame rates.

I 9 Therefore, in a second aspect, the present invention provides an imaging system comprising a radiation source and a detector, the radiation source comprises a frequency conversion member for emitting a beam of radiation in response to a radiation by one or more input beams, the input beam having a frequency different to that of the one or more input beams, the one or more input beam or beams being produced within a lasing cavity and said frequency conversion member being located within said lasing cavity.

It will be appreciated that the radiation source can be configured as described with reference to the first aspect of the present invention. The imaging system basically comprises three main sections, a generation section for generating the imaging radiation (including the THz beam and visible detection beam), an imaging section for imaging a sample and a detection section for detecting the radiation once it has passed through the imaging section. The generation section will be provided by a source in accordance with a first aspect of the present invention.

Preferably, wherever possible mirrors will be used instead-of transmission optics to minimise losses associated with transmission optics, i.e. (i) frequency dependent refraction losses and amplitude pattern distortion at dielectric (e.g. air-lens) interfaces (ii) frequency dependent absorption losses (iii) diffraction effects and distortions of the field distributions due to power falling on the lens surface at an angle.

Preferably, the THz radiation is directed onto the sample by means of a off axis parabolic (OAP) mirror. In such a mirror, there is a constant phase difference between the incident and reflected beam across the surface of the mirror.

More preferably, an even number of OAP mirrors are used and each adjacent pair are separated by the sum of their focal lengths. In this configuration, the size of the beam waist (minimum beam diameter normal to the beam axis), after reflection from the second mirror in the sequence, is frequency independent. This is also true for the last optical element in a chain providing that there are an even number of optical elements in the chain.

This configuration is particularly advantageous for THz imaging because a THz pulse is made up from a large number of frequency components and it is required to keep the size of the imaging beam constant for all THz frequencies in the pulse. Similar considerations apply for directing the THz radiation, collected from the object which is the subject of the imaging, towards the detection section.

Alternatively or in addition to OAP mirrors, condenser cones may also be used, which may be made of brass or copper, highly polished on the inside and with electro-plating and/or gold/silver evaporated coating. These are preferably located next to-the sample which is to be imaged. More preferably within a few wavelength of the sample i.e. 50 im to 100 tm. The cones preferably have an entrance aperture of diameter about 2mm and an exit aperture of between 50 tm to 100 gin.

Lenses may also be added, preferably these are chosen from non dispersive materials such as high density polyethylene (HDPE), polytetrafluorethylene (PTFE) and high resistivity Silicon. The materials are preferably non dipersive to avoid temporal broadening of the THz pulse.

Preferably, the imaging system comprises a motorised stage for supporting a sample which is to be imaged. The stage is preferably moveable in two directions orthogonal to the beam axis.

Preferably, the detector is a non linear crystal and is preferably configured to detect THz radiation using the AC Pockels effect. Here, the detector is configured to rotate the polarisation vector of a first input beam in response to illumination with a THz beam and emit a beam with the polarisation vector rotated.

I I I The first input beam can be thought of as a reference beam, the second beam is the detected beam. Before entering the detector, the polarisation of the reference beam and the detected beam are rotated such that they are parallel to one another and have a component along both the ordinary and extraordinary axes of the modulation region.

The reference beam and the detected beam are preferably linearly polarised before entering the detector. However, one or both of them may also be circularly polarised.

The detector is configured so that the detected beam (if present) rotates the polarisation of the reference beam by an angle. If the detected beam is not present, the reference beam may become slightly elliptically polarised during its passage through the detector crystal. To compensate for this effect, the beam emitted from the detector crystal is preferably passed through an optical correction circuit.

Preferably, the optical correction circuit converts the linearly (or elliptically) polarised beam into a circularly polarised beam.

The emitted linearly (or elliptically) polarised beam is preferably converted into a circularly polarised beam by a variable retardation waveplate e.g. a quarter waveplate. Preferably, this circularly polarised beam is then passed into a polarisation splitting device (such as a Wollaston prism) which separates the circularly polarised radiation into two linear components. If the detected beam is not present, these linear components are equal: If the detected beam is present, these linear components are not equal to one another.

Preferably, the two output beams from the prism are incident on a balanced photodiode assembly, which produces a non-zero output signal if there is a differenc e in magnitude between the two beams.

Alternatively, the reference beam can be circularly polarised before entering the detector. This can be achieved by putting the variable retardation waveplate in the path 12 of the reference beam before it reaches the detector, and replacing the variable retardation waveplate after the detector with for example, a half waveplate if the variable retardation waveplate is a quater waveplate. The detected beam (if present) changes the circularly polarised reference beam into an elliptically polarised beam. The elliptically polarised beam is then separated into its two components by a Wollaston prism or the like as described previously.

A variation on the prism and balanced photodiode configuration uses two crossed polarizers situated on either side of the detector. The reference beam is passed through the first polarizer, and transmitted through the detection crystal along with the THz beam. If the THz beam is present, the polarisation of the reference beam will be rotated such that the beam has a component in a transmission direction of the second polarizer. If the THz beam is not present, the polarisation of the reference beam is not rotated and hence is blocked by the second polarizer.

The reference beam is preferably produced by the generation section. The radiation carrying the image information is then detected by the detector using the reference,beam. Preferably, a control system is provided which provides a time variation between the input beam and the reference beam. The control system may be inserted into either the generation or the reference beam paths.

The control system may comprise a motorised mirror which can be oscillated backwards and forwards along the beam axis in order to increase or decrease the optical path length of the reference beam.

The system may also comprise optics to enlarge the cross sectional area of the reference beam before it enters the detection system. Preferably, the cross sectional area of the reference beam will be larger than that of the imaging radiation, to ensure that the whole of the imaging beam cross-section is detected. The detection section may also comprise a CCD camera with a detection area which is larger or the same size than the I 13 area of the reference beam. Thus, the CCD camera reads a 2D image and there is no need to move the sample during imaging.

Alternatively, the control system may comprise a grating pair configured to extend the pulse width of the reference beam. An optical fibre cable can also be configured to extend the pulse width of the reference beam. These cause the different wavelength components of the pulsed reference beam to travel through the detector crystal at very different times.

The detection section may comprises a grating spectrometer, to disperse the wavelengths. The detection section may also comprise a CCD camera to record the spatial distribution from the spectrometer.

Possible materials which posses good non-linear characteristics for either difference frequency generation or detection are GaAs or Si based semiconductors. More preferably, a crystalline structure is used. The following are further examples of possible materials: - NH4H2P04, ADP, KH2PO4, KH2AS04, Quartz, AIP04, ZnO, US, GaP, BaTi03, LiTa03, LiNb03, Te, Se, ZnTe, ZnSe, lBa2NaNb5Oj5, AgAsS3, proustite, CdSe, CdGeAS2, AgGaSe2, AgSbS2, ZnS, organic crystals such as DAST (4-Nmethylstilbazolium).

In general, non-centrosymmetric crystals are used for second order effects. Third order effects are found in a variety of different crystals with varying strengths.

The present invention will now be described in more detail with reference to the following non-limiting embodiments, in which:

Figure I is a schematic of a mode locked pulse laser; Figure 2a to 2c are radiation sources in accordance with a first aspect of the present invention-, 14 Figure 3 is a schematic diagram of a terahertz imaging system; Figure 4 is an imaging system in accordance with a second aspect of the present invention; Figure 5 shows a schematic of a detection section which can be used with the imaging system of figure 4- Figure 6 shows the imaging system of figure 4 with the detection section of figure 5; Figure 7 shows a variation on the imaging section of Figure 6; Figure 8 shows a variation on the imaging section of Figure 6; Figure 9 shows a variation on the imaging section of Figure 6; Figure 10 shows a further embodiment of an imaging system in accordance with the second aspect of the present invention; and Figure I I shows another example of an imaging system in accordance with the second aspect of the present invention.

Figure I shows a conventional pulsed laser e.g. here the schematic of a pulsed laser based on a Ti:Sapphire crystal CI as the lasing medium. (Examples of alternative crystals to Ti:Sapphire include Cr:LiSaF and Cr:LiSGaF: Cr:YAG and Cr:Fosterite. Alternatively, a cavity based upon optical fibres is also possible). The laser crystal C I is housed in laser cavity I which is defined by dichoric mirror M4 at one end and output coupler 9 at the other end. A pump beam 3 is directed on to said laser crystal C I. The pump beam 3 excites transitions within the laser crystal C I which then emits a coherent light beam, the input beam, 7. The input beam 7 is reflected back onto the crystal CI by dichoric mirrors M3, M4 and the output coupler 9, to excite further transitions within the crystal C I and the crystal C I emits further coherent radiation at the input beam 7 frequency (o,j..

The pump beam 3 is transmitted through Brewster window 5 and steered via steering mirrors MI, M2 into laser cavity I defined by mirror M4 and output coupler 9. The pump beam 3 can pass through mirrors M3 and M4 and is not reflected by them. The input beam 7, produced by the crystal C I is reflected back onto the crystal via mirrors I M3, M4 and the output coupler 9. Other mirrors may also appear in the cavity I to steer and focus the beam through the various optical components in the path of the input beam 7. In this example, the pump beam 3 is green and the input beam is red. Mirror M3 is positioned such that it steers the input beam 7 between the crystal C I and the rest of the lasing cavity 11, terminating on the far side by the output coupler 9 and on the near side by the mirror M4.

The output coupler 9 has a reflectivity between 90 and 100%, the exact value is dictated by the particular application of the invention. For imaging systems which can cope with moderately enhanced levels of THz radiation at o)nu and also require an output at (o,js for the reference beam, R is best kept near 90%. For applications where having the maximum amount of THz power is required, R,&100% is used. In this specific example, R is about 90%. This means that 90% of the light which reaches the output coupler 9, is reflected back into the cavity and 10% of the input beam exits the cavity. This maintains the power within the cavity 1.

Such an output coupler 9 can achieve average output levels of 500 mW and peak powers of 75 kW for the input beam at covi, The intra-cavity power (the one way power in the cavity) is of the order of 750 kW (5 watt average power). In electro-optic detection systems used for THz imaging, average power levels of 10 to 100 mW, or less, are typically utilised. Thus, R may be considerably increased, resulting in a larger value intracavity power P (co,jj. Hence, enhanced P (o)TH') may be achieved whilst still maintaining a reasonable degree of output power (pext(.vi,)) necessary for electrooptic detection of THz radiation for use in a THz imaging system.

The laser also has mode locking means 11. For example, the means I I may be an acoustic optic modulator which produces a periodic modulation of the loss in the laser cavity. The frequency of this modulation is half the round trip frequency, 0),, of the laser cavity defined by c/2L, 16 where cis the speed of light and L is the cavity length. This produces a stable train of pulses from the laser at (o,,. Those skilled in the art will be aware of alternative mode locking configurations that could be used.

Positive group velocity dispersion effects in the laser cavity results in the temporal broadening or chirping, of the pulses. Pulse width adjustment and wavelength tuning means 10 are provided for the control of the pulse duration by introducing a negative group velocity dispersion in the cavity. One example uses 4 prisms arranged to provide negative group velocity dispersion to the laser cavity pulses, the prisms arranged such that wavelength selection is also possible. An alternative design uses the chirped dielectric mirrors in the cavity, where the mirror coating provides to necessary negative group-velocity dispersion; the mirror coating also determines the lasing wavelength. Following dispersion control, pulse widths ranging from several ps to a few fs are achieved, and typical visible/near-infrared wavelengths over the range 690 to 1, 000nm are possible, typically centred on 800nm.

Figure 2a shows the cavity of Figure I which has been modified to accommodate a frequency conversion member C2. The generation of the coherent input beam 7 from C I, is identical to that described in Figure 1. Therefore, these details will not be repeated here.

Instead of output coupler 9, a gecond output coupler 23 is provided. This output coupler has a reflectivity of between 95 and 100% to the input beam 7 and has zero reflectivity but 100% transmittivity to the required THz output. The input beam 7 is reflected at off normal incidence onto the output coupler 23 via steering mirror 25.

The output coupler reflects input beam 7 onto crystal C2 which is a non linear crystal. The input beam 7 is focused on the crystal C2 via lens L L The input beam 7 emitted from crystal C I is reflected in a closed loop defined by mirrors M3, M4, M5, M6 and output coupler 23. Therefore, the cavity keeps reflecting the input beam 7 onto both I 17 crystals C I and C2 and maintains the power within the cavity defined by mirrors M3, M4, M5, M6 and output coupler 23.

The THz radiation emitted from crystal C2 is transmitted via output coupler 23. Output coupler 23 should have zero reflectivity to THz radiation as it is not advisable for THz radiation to be reflected back within the cavity onto crystal C! and also,it is desirable to extract as much THz signal as possible.

The output coupler 23 may be a dichoric mirror which ideally passes radiation at (om unattenuated, while being highly reflective at co,j, to ensure high P (a),jj in the cavity.

Possible materials for the output coupler include highly polished silicon or dielectric coated silicon, z-cut quartz with a Ge/Zn/S coating or the like. The angle of incidence may not be parallel to the surface normal for the visible and/or THz radiation. As the THz beam is likely to be much larger in diameter than the visible beam, it may be possible to use a small diameter (< 2mm) silver (or otherwise metallic) layer in the centre of the output coupler with anti-reflection coating, mounted on a z- cut quartz substrate; the silver mirror reflects all the power at the fundamental (R (o),jj equal 100%) whilst being of a sufficiently small diameter to allow most of the larger THz beam to pass around it (R (com) 0%).

C2 is a non linear crystal possessing a large second order non linear coefficient and is configured to emit a frequency which is the difference of two frequency components in the pulsed input beam 7. Here, the crystal is made from ZnTe which has been shown to have a good conversion efficiency for visible wavelengths light to THz frequencies when pumped external to a laser cavity. Thus, average power levels of several 4W in the THz are produced for input power levels of -3 00 mW in the visible range at (D Vi., for frequency conversion crystals placed external to the cavity. Even higher power levels will be achieved with a crystal inside the cavity.

18 in order to minimise losses, C2 is cut at the Brewster's angle for the polarisation and frequency of the visible radiation @,i,1 and (O,i,2. Alternatively, or in addition to cutting at the Brewster's angle, C2 could be anti-reflection coated to minimise these losses.

ZnTe is a good material for crystal C2 as it has minimum absorption at (om due to mechanisms such as phonon absorption and transparent at 800nm, the centre wavelength of TiSapphire based ultrafast lasers. Alternatively, GaAs maybe used in conjunction with a pulsed laser source operating at a wavelength where GaAs is transparent for example Cr:LiSaF, or Cr:YAG could be used for crystal C L The length of C2 should also be optimised for the system. Phase matching frequency conversion optimally occurs over the whole length of the crystal. In practice, the useful length is given by the coherence length Ic which is a measure of the distance over which the optical ly-in duced non-linear polarisation and the generated THz electric fields remain in phase. It is defined by:

lC=7EC/ ((J)TH, lllvi - 710- Optics Ll and M6 are arranged such that the input beam 7 is focused down on the crystal C2. The alignment of these and other components is such that there is no spatial walk-off of the input and THz beams in the crystal C2. The latter point ensures that high spatial mode quality is ensured and also favours a long interaction length i.e. full distance 1. is utilised. L I is anti-reflection coated for (ovi., and as a non- absorbing/nonreflecting as possible as (om. Materials such as anti- reflection coated TPX or Z-cut quartz are possible candidates for dichoric, transmissive optics such as L1.

Figure 2b shows a variation of the source of figure 2a. Mirror M6 is used as the output coupler, removing the requirement for output coupler 23 of figure 2a. This causes the outputted THz pulse to be generated by the visible pulse on its first pass through C2.

I 19 In Figure 2a, two THz pulses are generated by the visible pulse during the two passes it makes through C2. These pulses may, in some cases, destructively interfere thus reducing the efficiency of the THz generator. The design of figure 2b addresses this problem. Further, the THz pulse only passes through mirror M6 (i.e. a single element) before leaving the cavity thus reducing dispersion and absorption effects at conu.

Figure 2c shows a further modification of the design in figures 2a and b. This design uses chirped dielectric mirrors MCI and MC2 in the cavity I to control the pulse dispersion/length and the wavelength, thus removing the need for the pulse length adjustment means 10 of Figure 2a. In Figure 2c, mirrors MCI and MC2 have chirped dielectric coatings to produce the negative group velocity dispersion required to,shorten the pulses. The mode locking means I I is still required.

In Figures 2a to 2c, two frequencies are provided by beam 7. This irradicates the problems caused by using two separate beams. For example in V Petrov and F. Seifert Optic letters, 19, 40-42 (1994). Here, two pulsed lasers are used and problems are encountered due to non-optimised beam overlap due to the fact that the two beams at covi,l and COA,2 are of different origin, i.e. not part of the same beam. This design also suffers because one of the beams (e.g. co vi 1) is not of intra-cavity origin and hence the power available at covi,,, is greatly reduced, causing a corresponding drop in the power of the difference frequency radiation.

This THz source can be used in a THz imaging system. Figure 3 shows the basics of a THz imaging system. The system can be simplified into three main sections, a generator 3 1, an imaging section 33 and a detection section 35. THz radiation is generated in the generating section 31 by using a THz emitter which is supplied by a visible pulsed laser 37.

A THz beam 39 is emitted from generation section 31 and is directed onto sample 41 of the imaging section 33. The THz beam 39 is then directed via further optics 45 into the detection section 35. The system of Figure 3 is an example of a near field imaging system, where the sample to be imaged is placed immediately behind the THZ source.

The detection section reads the information carried in the detected THz signal via a visible light signal and AC Pockels effect. The visible light is ideally obtained from laser 37 via beam splitter 47. A time delay is added to the THz pulse via time delay line 34. The system (e.g. the control of the sample 41 movement, the time delay 34 and the detected signal processing) is controlled by computer 36.

Details of the AC pockets effect will be described with reference to Figure 5.

An imaging system in accordance with the present invention will be described with reference to Figure 4. Here, for simplicity, details of the detection part of the system will be omitted. These will be described with reference to Figure 5.

The THz generation section is indicated by the components within box 5 1. The individual components of the generation system 51 have been described with- reference to Figure 2. Therefore, the details of these will be omitted from the description of Figure 4. The imaging system requires both a visible light pulse and a THz- -pulse to be emitted from the generation section 5 1. Therefore, the output coupler 23 should not 100% reflective to visible radiation to allow some visible radiation to be emitted from the THz generation section 5 1.

The emitted THz beam 53 and visible 55 from the generation system are incident on beam splitter 57. This beam splitter 57 allows transmission of the THz beam 53 but reflects the visible light beam 55 onto mirror 59 which reflects the beam 55 into optical delay line 61. The delayed beam 55 is then inputted into the THz detection unit 63.

The THz beam 53 is directed into the imaging section 52 and onto sample 65 via THz imaging optics 67. The sample 65 is located on a motorised X-Y translation stage (not shown) so that the whole sample 65 can be imaged. (The x-y plane is othogonal to the I 21 beam axis). The THz radiation 69 carrying the imaging information from the sample is reflected into the THz detection system 63 via THz imaging optics 71.

The presence of visible radiation 55 as well as THz radiation 69 allows for imaging and electro-optic detection to be performed inside a single nitrogen-purged unit.

The sample 65 is mounted on a X-Y motorised translation stage (not shown) which is controlled by a PC computer (not shown). Each section (pixel) of the object may then be imaged. To improve the spatial resolution of the technique, off-axis parabolic mirrors, condenser cones, and lenses may be used to focus the beam to a diffraction limitspot. By mounting the sample in the near field of a condenser cone, the diffraction limit may be overcome and spatial resolution of about 50 Lm may be achieved. The imaging system can function with or without such objects depending on the nature of the object to be imaged and the nature of the detection circuit. These variations on the imaging section will be discussed with reference to figures 6 to 9.

Figure 5 shows the detection system in detail. The THz beam 69 carrying the imaging information and a visible light beam 55 are inputted into the THz detection system. The visible light beam 55 acts as a reference beam which is incident on the detection crystal 73. The reference beam 55 is linearly polarised and the polarisation is orientated such that it has components along both the ordinary and extraordinary axis of the detection crystal 73. Each of the axes has distinct refractive indices n(, and n, along the ordinary and extraordinary axis of the crystal 73 respectively. In the absence of a second (THz) radiation beam 69, the linearly polarised reference beam 55 passes through the detection crystal 73 with negligible change in its polarisation.

The applicant wishes to clarify that the angle 19 through which the polarisation is rotated by is negligible. However, the linearly polarised beam can become slightly elliptical. This effect is compensated for by a variable regardation waveplate, e.g. a quarter waveplate 8 1.

22 The emitted beam 77 is converted into a circularly polarised beam 83 using quarter wave plate 81. This is then split into two linearly polarised beams by a Wollaston Prism 79 (or equivalent device for separating orthogonal polarisation components) which directs the two orthogonal components of the polarised beam onto a balanced photodiode 85. The balanced photodiode signal is adjusted using wave plate 81 such that the difference in outputs between the two diodes is zero.

However, if the detector 73 also detects a secondary beam 69 (in this case a beam with a frequency in the THz range) as well as a reference beam, the angle 0 through which the polarisation is rotated by is not negligible. This is because the THz electric field modifies the refractive index of the visible (fundamental) radiation along one of the axes n., n.. This results in the visible field after the detector 73 being elliptical and hence the polarisation components separated by the prism 79 are not equal. The difference in the voltage between the output diodes gives a detection voltage.

The reference beam 55 and the THz beam 69 should stay in phase as they pass through the crystal 73. Otherwise the polarisation rotation 0 is obscured. Therefore, the detection crystal 73 has phase matching means to produce a clear signal.

Figure 6 shows a complete THz imaging system. The individual components of the detection section 63 and the THz generation section 51 have been described with reference to Figures 2 and 5, described and discussion of them will not be repeated here.

All of the items shown in Figure 6 sit on an optical bread board of dimensions 36 inches by 36 inches. The only external units required are a power supply for the diode laser and a cooling unit for the generation section 51 The imaging section 91, has a motorised stage which is movable in the x-y plane, i.e. along two orthogonal axis which are perpendicular to the incident beam of THz radiation.

I 23 The imaging section 91 has two mirrors MI I and M12. Mirror M12 directs the THz beam 53 onto the sample 65. Mirror MI I is positioned to reflect the THz radiation transmitted through sample 65 onto the detection crystal 73. Mirrors MI I and M12 are off axis parabolic (OAP) mirrors. Such mirrors are configured so that the phase difference between the incident and reflected beams is the same at all points on the mirror. The parameters resulting in an off axis parabolic surface are characterised by the focal length of the mirror.

An optical delay section 93 is also shown. The visible light beam emitted from the generating section is reflected by bearnsplitter 57 into delay section 93. The delay section 93 has a comer cube mirror M9 which is moveable along the beam axis. The beam is directed onto comer cube mirror M9 via mirror M8. The beam is reflected off comer cube mirror M9 onto mirror MIO. Comer cube mirror M9 is oscillated back and forth along the beam direction with an oscillation frequency of several 10s of Hz. This increases or decreases the path length of the visible beam 55 as required. A Clark ODL- 150 system may be used to drive the mirror, this is capable of delays of 150ps. The emitted beam is then combined with the emitted TE[z beam at mirror M1 1. Alternatively the THz and visible beams may be combined colinearly using a beam splitter, for example, a pellicle beam splitter. Such a device would be placed before or after MI I and would eliminate the requirement for a hole in MI 1.

Figure 7 shows a variation on the imaging section 91 of Figure 6. The extended path length over which the TE[z beam 53 travels is purged with nitrogen to remove water vapour and hence improve the quality of the image..

Due to diffraction effects associated with the large wavelengths in the THz range, the cross-sectional size of the THz beam 53 and imaging applications is not sufficiently large that it may be treated as plain parallel. If diffraction effects re such that the L__ radiation is paraxial so that it can be represented by a scalar field distribution, Gaussian beam mode optics and optical techniques can be used. The simplest case for system

24 design is to assume that the fundamental mode dominates the beam profile. The use of Gaussian mode optics and design applied to conventional THz radiation and systems (generated in the Fourier transform machines, far-infrared lasers or Gunn diodes) is applicable and important to THz imaging systems.

A number of design rules or guidelines should be followed when constructing a THz imaging system to obtain a good quality image. For transmission optics such as lenses, geometric losses are kept to a minimum by ensuring that the ratio of the lens thickness to focal length and diameter to focal length is less than 0.2. If this is satisfied, then losses in the lenses will be primarily due to absorption and reflection. In this case, choice of materials is important.

A requirement which arises in pulsed systems is the need for the material to be nondispersive so that pulse broadening does not occur. Given these requirements, high density polyethylene (DHPE), polytetrafluorethylene (PTFE), high resistivity silicon (Si), and TPX are some of the best materials and can also be machined in a lathe; any material combining low absorption and low dispersion at THz frequencies is a good candidate for fabrication of transmission optics, provided its shape can be suitably fabricated for a lens. Reflection losses in lenses tend to be highly frequency dependent at THz frequencies. Therefore care must be taken in lens design to ensure that all frequencies -across the pulse bandwidth undergo the same reflection (and absorption) losses.

Ideally, reflective optics (mirrors) are used wherever possible instead of transmission optics (lenses) in order to minimise a number of losses associated with transmission optics, which include (i) frequencydependent reflection losses and amplitude pattern distortion at dielectric (e.g. air-lens) interfaces, (ii) frequency-dependent absorption losses, (iii) diffraction effects and distortions to field distribution due to power falling on the lens's surface at an angle.

I An additional property of importance in imaging (and not particular to Gaussian mode beam optics) is that if two mirrors are separated by the sum of their focal lengths, then the size of the beam waist (minimum beam diameter in plane normal to optical axis) on the optical axis after the reflection from the second mirror will be frequencyindependent. This is true of the last mirror (focusing element) in a chain provided there are an even number of focusing elements in the chain. This provides a major advantage for THz imaging as the pulse is comprised of a wide range of frequency components, and it is desired to keep the object at a fixed position on the optical axis whilst images are being recorded at various (x, y) points and at all THz frequencies in the pulse. This is particularly important THz imaging as the spectral coverage (bandwidth) of TE[z pulses increases into the mid-infrared and even higher frequencies.

The system in Figure 7 will produce beams with I/e diameters (for the fundamental Gaussian mode in the beam) of 1-2 mm at the sample in the THz frequency range (e.g. at 300GHz, diameter = 2mm). In the system of Figure 7, six mirrors and two lenses are used as opposed to the two mirrors of Figure 6. The direction of the beam. in Figure 7 is reversed to that in Figure 6. In the imaging section, the beam is first reflected off first OAP mirror 101 onto second OAP mirror 103 and then onto third OAP mirror 105. Second 103 and third OAP mirrors each have a focal length of 250 rim. They are separated by 500 nm.

The beam is reflected from the third OAP mirror 105 onto plano-convex lens 107 which has a focal length of 10 mm and a diameter of 10 mm. Third OAP mirror 105 is separated from piano convex lens 107 by 260nm (i.e. the sum of their focal lengths). The lens 107 is made from polyethylene or high resistivity Si. The lens 107 is placed 10 mm from the motorised stage (not shown) on which sample 109 is mounted. The beam has traversed through an even number of focussing optics and mirrors (10 1, 103, 105 and 107) which are all spaced apart by the sum of their focal lengths. Hence, the waist of the beam at the sample is independent of the frequency. Here, the beam diameter is chosen to be 2 mm, independent of frequency in the frequency range of about 300 GHz (0.30THz).

26 Once the beam has passed through sample 65, the transmitted THz radiation falls onto second plano convex lens I 11. Plano convex lenses 107 and I I I are identical in optical characteristics. Lens I I I focuses the THz radiation onto the fourth OAP mirror 113. Fourth OAP mirror 113 has a focal length of 250 mm and reflects the THz beam onto fifth OAP mirror 115. Fifth OAP mirror 115 also has a focal length of 250 mm and lies 500 nm away from the fourth OAP mirror 113 (i.e. the sum of the focal length of the fourth and fifth OAP mirrors).

The beam is reflected from the fifth OAP mirror 115 to the sixth OAP mirror 117. Sixth OAP mirror has a focal length of 30 mm and is located 280 mm away from the f ifth OAP mirror (i. e. the sum of the focal length of the, f ifth and sixth OAP mirrors).

The sixth OAP mirror 117 is provided with a hole 119. The visible light beam 55 is passed through this hole to combine it with the THz beam 69 for detection.

Further improvements in spatial resolution may be achieved by inserting condenser cones (made of brass or'copper, highly polished on insides, with electro-plating and/or gold/silver evaporated coating) adjacent to the sample to be imaged as shown in Figure 8. In Figure 8, condenser cones 121 and 123 are located on either side of the sample 125 between the sample 125 and plano convex lenses 127 and 129 respectively. The plano convex lenses are the same as those described with reference to Figure 7. They have a focal length of 10 mm and are placed 10 mm away from condenser cones 121 and 123. The cones have a typical entrance aperture of 2 mm and an exit aperture of between 50 to 100 pm.

The sample 125, is typically placed within a few wavelengths of the exit aperture of condenser cone 121 e.g. about 100 gm, such that near field imaging techniques may be used to realise THz spot sizes at the sample which are less than the diffraction-limited spot size.

I 27 Another advantage of this design is that the beam waist size is frequency independent at the aperture entrance, so that all frequencies in the pulse should fit into the condenser cone.

The plano-convex lenses 127, 129 condenser cones 123, 121 and sample 125 are placed between OAP mirrors 131, 133. The mirrors have a focal length of 250 mm. THz beam 53 is reflected from OAP mirror 131 onto plano convex lens 127 which focuses beam 153 onto condenser cone 121. The beam 53 enters through the widest aperture of the condenser cone and exits through the narrowest aperture onto sample 125. Once beam 53 has passed through sample 125 it enters condenser cone 123 and exits the condenser cone 123 through its narrowest aperture onto plano convex lens 129. The beam is then reflected off OAP mirror 133 onto the detection crystal 73. The OAP mirror 133 has a hole 135. Visible light from the generator is combined with the THz: beam 69 at mirror 133. It should be noted also that the optical configuration in Figure 8 can also be used with a multiplicity of other mirrors, such as the arrangement in Figure 7. It should be noted, however, that a variety of different focal lengths are possible for mirrors 133 and 131.

Also, the arrangement of condenser cones used here can easily be inserted into the system of Figure 7 using similar guidelines to beam size and mirror placement as those already elucidated.

It should be noted that simpler coupling systems such as that in Figure 9 are possible which utilise only two off-axis parabolic mirrors. These reduce the path length of the beam and therefore minimise losses due to any water vapour or other absorbing gases in the beam path. However, transmission optics are necessary in order to create frequency independent beam waists at the same and/or to realise higher spatial resolution.

In Figure 9, the THz beam 53 is reflected from OAP mirror 141 onto sample 143. The focal length of OAP mirror 141 is 30 mrn and sample 143 is placed 30 mm away from 28 OAP mirror 141. Optionally, further optical components such as lenses and condenser cones as described in Figure 8 may be added between mirror 141 and sample 143.

Once beam 53 has passed through sample 143 it is encoded with imaging information and is referred to as beam 69. Beam 69 is reflected from OAP mirror 145 onto the detection crystal. The OAP mirror is provided with a hole 147 which allows the visible beam 55 to be mixed with the TE[z beam 69 for detection.

Figure 10 shows a further example of an imaging system. The generation and imaging sections have been described with reference to figures 2 to 4 and 6 to 9. The details of these components will not be repeated here. In Figure 10, the delay section of Figure 6 is replaced with a grating pair or an optical fibre 151 which chirps the visible pulse, extending its temporal width from 50 fs to about 20 p. The different wavelength components in the visible pulse travel through the detection crystal 73 at different times.

Thus, when a grating spectrometer 153 is used to spatially disperse the wavelengths and a CCD camera 155 is used to record the spatial diversion, each pixel in the (for example) X-direction corresponds to a different wavelength and hence a different time. The result is that a given row of pixels in the x-direction on the CCD 155 effectively map out the temporal form of the THz beam which co-propagates through the detector crystal 73 and rotates the polarisation of the visible beam at different times during the pulse by varying amounts. Thus, transmission through the object being imaged is plotted as a function of time along one direction in the CCD array. Hence, the rotation of the polarisation of the reference beam 55, is measured by crossed polarises 161, 163 which are arranged on either side of the detection crystal 73.

The imaged object may then be stepped in the y direction on the translation stage in the usual way to develop a 2D THz image. Alternatively, if the probe beam is focused down by a cylindrical lens to a line (say 400 pm in x by 10mm in y) on the sample, the THz transmission along the y axis of the sample can be measured by the pixels along y I 29 direction of the CCD, i.e. the y-pixels of the CCD may then be used to image the object in the y-direction without resorting to the translation stage moving in y. A full image is then completed by stepping the translation stage in x. Both of these abilities (to measure time delay along the x-axis of the CCD and y image information without mechanical movement) resulting in much quicker acquisition times if sufficient THz power is available as in this intra-cavity design to affect higher signal to noise ratios. Quicker data acquisition and potentially cheaper cost for more compact systems are the result.

The primary advantage of this system is the fast data acquisition owing to the lack of moving parts such as translation stages; using this system, both a) imaging along the ydirection of the object and b) the sampling of the time domain is very fast, limited the creation of a time delay are very fast, limited only by the speed of the CCD camera and the need to average many frames from the camera to get adequate signal to noise rations (SNR) on the images. The latter is the chief mechanism which limits the application of this technique, and hence the realisation of real-time imaging. Poor SNR results in part from the fact that the balanced photodiode detection scheme outlined in Figure 5 can no longer be used because the quarter wave plate would introduce background light onto the CCD as strong as one-half of the total probe power. Small signaldetection in this scenario will be overwhelmed by photon shot noise if a CCD camera is used. To reduce the "ambienf' light level on the camera, crossed polarizers are used in which the signal on the CCD falls to zero in the absence of a THz electric field. Such as detection system is optimal for a CCD, but still does not provide as high signal-to-noise as the system in Figure 5, especially if lock-in detection is used in the latter case.

To overcome this SNR problem, regenerative amplifiers are used (not shown) to boost the optical peak power which nonlinearly generates the THz pulse, resulting in a larger THz field. Such a system suffers, however, from numerous disadvantages. Regenerative amplifiers are extremely expensive (-IIOOX) and tend to be large and bulky. Also, a second pump laser to drive the amplifier is required. Lastly, such systems operate at low repetition rates (50E[z-250kHz), resulting in a relative decrease in average power. The bright intracavity sources designed here would overcome all of these disadvantages. The intracavity design could therefore be a major step forward in the realisation of a THz imaging system with sufficiently quick data acquisition at sufficiently high signal to noise ratios to realise THZ images at video frame rates (-38 frames/sec), so-called "THz movies".

Figure I I shows a further example of the imaging system of Figure 10. In Figure 11, the motorised stage (of Figure 10) is redundant, Instead, the imaging area of the CCD camera 155 is matched to the imaging area of the detection crystal 73. This area is 2 typically several MM. The reference beam 55 is expanded by optics 170 (e.g. telescopic or analogous optics), such that the reference beam has a larger cross sectional area than the THz beam and ideally fills all the pixels in the CCD camera 155. The distribution of the rotated polarisation of the reference beam in the x-y plane (proportional to the THz power transmitted through the sample 65 in the x-y plane) is transferred to the pixels of the CCD camera, resulting in a THz image of the object appearing on a CCD or a computer screen (not shown) attached to the output of CCD camera 155. The time delay in this system is created by an optical delay line (as described with reference to Figure 6). This is the only mechanical moving part of the system.

I 31

Claims

CLAIMS:

1. A radiation source comprising: a frequency conversion member for emitting a beam of radiation in response to irradiation by one or more input beams, the emitted beam having a frequency different to that of the one or more input beams, the one or more input beams all being produced within a lasing cavity and said frequency conversion member being located within said lasing cavity.

2. A source according to claim 1, wherein the frequency conversion member is an optically non-linear member.

3. A radiation source according to claim 2, wherein the frequency conversion member is configured to emit radiation with a frequency substantially equal to the difference between two frequency components of the one or more input beams.

4. A radiation source according to claim 3, wherein the frequency conversion member is configured to emit radiation with a frequency of between 100 GHz to 20THz.

5. A radiation source according to any preceding claim, wherein a single input beam with two frequency components is provided.

6. A radiation source according to claim 5, wherein the single input beam is a pulsed beam.

7, A radiation source according to claim 6, wherein the pulsed beam has a pulse width of between 10 ps to 10 fs.

8. A radiation source according to any preceding claim wherein the frequency conversion member comprises at least one of the following UI03, NH4H2PO4, ADP, 32 KH2, A504, Quartz, AIP04, ZnO, CdS, GaP, GaAs, BaTi03, LiTa03, LiNb03, Te, Se, ZnTe, ZnSe, Ba2NaNb5Ol5, AgAsS3, proustite, Cd, Se, CdSe, CdGeAs2, AgGaSe2, AgSbS3, ZnS, BBO, KTP, DAST (4-N-methylstilbazolium), L4NbO3.

9. A radiation source according to any preceding claim, wherein the frequency conversion member comprises phase matching means, for enhancing the phase matching between at least two different frequency signals propagating in the frequency conversion member in response to illumination by the one or more input beams, the phase matching means having a spatial variation in the refractive index along a component of the one or more input beams.

10. A radiation source according to any preceding claim, wherein the frequency conversion member is crystalline and is cleaved at its Brewster angle for one of the frequency components of the input beam or beams.

11. A radiation source according to any preceding claim, wherein the frequency conversion member has an anti-reflection coating, provided to reduce reflections at a frequency component of the one or more input beams.

12. A source according to any preceding claim, wherein an element is provided to stop the emitted beam irradiating the lasing element.

13. A source according to any preceding claim, wherein an output coupler is provided which is capable of transmitting the emitted beam and reflecting the input beam or beams.

14. A source according to claim 13, wherein the output coupler has substantially 0% reflectivity to the emitted beam and between 90 and 100% reflectivity to the input beams or beams.

I 33

15. A source according to either of claims 13 or 14, wherein the output coupler is positioned on an opposing side of the frequency conversion member to the side which the input beam or beams first enter the frequency conversion member.

16. A source according to any preceding claim wherein the source is configured to emit both the beam emitted from the frequency conversion member and at least one or more input beams.

17. An imaging system comprising a source according to any of claims I to 16 for supplying a beam of imaging radiation onto an object and a detector.

18. An imaging system according to claim 17, comprising a motorised stage for supporting an object which is to be imaged. -

19. An imaging system according to either of claims 17 or 18, wherein the imaging radiation is directed onto the sample by at least one off axis parabolic mirror.

20. An imaging system according to claim 19, wherein the imaging radiation is directed onto the sample by an even number of off axis parabolic mirrors arranged with each adjacent pair of mirrors separated at the sum of the focal lengths of the two mirrors.

21: An imaging system according to any of claims 17 to 20, wherein the system further comprises a condenser cone.

22, An imaging system according to claim 2 1, wherein the condenser cone is placed between 50 and 150 Lm from the sample,

23. An imaging system according to any of claims 17 to 22, wherein the system further comprises lenses made from one or more of the following: high density polyethylene (HDPE), polytetrafluorethylene (PTFE) or high resistivity Silicon.

34

24. An imaging system according to any of claims 17 to 23, wherein the detector comprises a non-linear crystal.

25, An imaging system according to claim 24, wherein the imaging radiation is detected by the non-linear crystal using a reference beam with a different wavelength than that emitted by the frequency conversion member.

26. An imaging system according to claim 25, wherein the reference beam corresponds to one or more of the input beams.

27. An imaging system according to either of claims 25 or 26, wherein the nonlinear crystal is configured to rotate the plane of polarisation of the reference beam in the presence of the imaging radiation.

28. An imaging system according to claim 27, wherein the rotation of the polarisation of the reference beam is detected by a waveplate, prism and balanced photodiode assembly.

29. An imaging system according to claim 27, wherein the rotation of the polarisation is detected by two crossed polarizers located on either side of the non-linear crystal.

30. An imaging system according to any of claims 25 to 29, further comprising delay means for the reference beam.

31. An imaging system according to claim 30, wherein the delay means comprises a mirror which can be oscillated to increase or decrease the optical path of the reference beam.

I

32. An imaging system according to any of claims 25 to 29, wherein the system further comprises means to increase a pulse width of the reference beam.

33. An imaging system according to claim 32, wherein the means to increase the pulse width comprise a grating pair.

34. An imaging system according to claim 32, wherein the means to increase the pulse width comprise an optical fibre.

35. An imaging system according to any of claims 32 to 34, wherein the imaging system further comprises a grating spectrometer.

36. An imaging system according to claim 35, further comprising a CCD camera.

37. An imaging system according to any of claims 25 to 34, wherein optical elements are provided to increase the reference beam cross sectional area to be larger than the cros s sectional area of the imaging radiation at the detector crystal.

38. An imaging system according to claim 37, the system further comprising a CCD camera with a detection area similar to that of the cross sectional area of the reference beam.

39. A radiation source as substantially herein before described with reference to Figures 2a, 2b and 2c.

40. An imaging system as substantially hereiribefore, described with reference to any of figures 4 to 11.