GB2336938A

GB2336938A - A device for the generation of coherent radiation

Info

Publication number: GB2336938A
Application number: GB9909928A
Authority: GB
Inventors: Achim Nebel; Jochen Schmitt; Richard Wallenstein
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-04-30
Filing date: 1999-04-29
Publication date: 1999-11-03
Anticipated expiration: 2019-04-29
Also published as: AT500694A3; GB9909928D0; FR2778278A1; DE19819473C2; GB2336938B; ITMI990904A1; DE19819473A1; AT500694A2; IT1312257B1; AT500694B1; FR2778278B1

Abstract

A device for the generation of coherent radiation comprises a resonator which is tuned to a fundamental frequency w and in which are arranged a laser or optical parametric oscillator (7), which is suitable for the generation of this fundamental wave by excitation by means of a pumping wave, and a frequency multiplying material (10) for the generation of a wave m*#, which is multiplied in frequency with respect to the fundamental wave, the resonator having an output coupling mirror (1) for the fundamental with the frequency #. The frequency multiplying material (10) is selected and dimensioned for a phase mismatch of the fundamental and the frequency multiplied waves. The material (10) may be a non-linear frequency doubling crystal such as LBO, located in a temperature controlled oven (12). Such a device can be used in the display of rastered television pictures.

Description

1 i 2336938 A Device for the Generation of Coherent Radiation The present

invention relates to a device for the generation of coherent radiation with a resonator, which is tuned to a fundamental wave with a f requency (o and in which a material, is arranged which is suitable for the generation of this fundamental wave by excitation with a pumping wave, and a frequency multiplying material for the generation, with io respect to the fundamental wave, of a frequency multiplied wave, with the frequency mo), whereby the resonator has further an output coupling mirror for the fundamental wave with the frequency o, in which the frequency multiplying material is selected and dimensioned for a phase mismatch between the fundamental wave and the frequency multiplied wave.

Such devices are for instance applicable as lasers, whereby the fundamental wave is then the radiation to be generated. The present invention is, however, not restricted to this. Other devices for the generation of coherent radiation can for instance be optical parametric oscillators (OPO), in which an incident wave generates two new wavelengths, due to non-linearities, the so-called signal and idler waves. one of these two waves is here understood as the 2 fundamental wave in the sense of the invention, whilst the incident wave then represents the pumping wave.

In the commercial area there is today a great need for lasers, for example in simple applications in CD players, laser printers and light indicators. For such applications relatively low power lasers, such as helium-neon lasers and laser diodes are sufficient.

Higher technology.

powers are especially If indeed for simple vector lasers of this type are successful, then rastered television pictures by pixels is higher powers, since in this technology time of an entire picture is limited by pixels displayed per pixel in the medium. have been proposed for this. In DE 195 04 required in display graphics, low power the presentation of possible only with the power per unit the number of the Various suggestions 047 C, WO 96/08116, and DE 197 13 433 A, the application of optical parametric oscillators (OPO) is recommended, with which high power output radiation of all three colours is generated by the combination of a fundamental wave of high power with the signal and idler waves of the OPOs.

A further problem exists in display techniques of this 25 construction due to interference of the laser beam illuminating the pixels, which is visible as so-called speckle 3 and has a disturbing effect on the picture. To avoid the problem of speckle it is proposed in DE 196 45 978 A1 to keep the coherence length small to suppress interference effects of this type. This can be achieved for instance with a short pulse duration, however other methods are also known. For instance in DE 195 35 526 A1 a double core ribbon laser in proposed, which by the selection of suitable doping materials, emits a large wavelength spectrum, whereby the coherence length becomes smaller.

The prior art mentioned above relates to lasers with mode coupling to achieve high laser power in a pulse, which can possibly be applied to such commercial applications.

A multiplicity of different methods for mode coupling is proposed in the literature. Above all the patents US 4,914,658, DE 37 36 881 Al, DE 38 26 716 Al, EP 0 314 171 A2, EP 0 314 171 A3, JP-SHO 63- 274348, US 4,993,944, US 5,119,383, EP 0 235 950 Al, US 5,046,184 and US 5,054,027 should be quoted as the state of the technology.

In mode coupling, active and passive methods can be distinguished. Active mode coupling is realised with the aid of an acoustic-optic modulator, which effects an amplitude modulation. Further, for these methods electro-optic modulators can be applied, with which frequency modulation is 4 produced. In passive mode coupling a laser pulse circulating in the resonator modulates its own amplitude. Passive mode coupling is achieved with "Kerr lens mode locking- (KLM), "additive pulse mode-locking- (APM), a saturable absorber, or a so-called non-linear mirror. The various mode coupling methods are differentiated with respect to their universal applicability, the power which can be achieved, the pulse duration which can be achieved and the technical expenditure required.

The KLM method uses the intensity dependent refractive index of a component within the resonator. The non-linear conversion effect is of the third order. At high intensities self-focusing of the radiation arises in the resonator because of the non-linear conversion effect, and thereby leads to a lessening of the mode diameter. Against this, the selection of the mode, for instance by a filter, leads to high losses in continuous operation and to low losses in mode coupled operation.

The essential disadvantage of this method lies in that the non-linearity utilised here delivers only small amounts because of the third order applied in, it and therefore a high intensity must be present to effect a mode coupling in the resonator. Further, the KLM method for increasing the power intensity requires a very small mode radius. The resonator must, on this account, be adjusted such that it works on the edge of optical stability. Self-starting of the laser is, furthermore, impossible. Such characteristics make this type of mode coupling unsuitable for a commercial laser with a high output power.

Furthermore the KLM method is not usable on all known lasers, for instance not when the pump radiation does not possess good focusability. It is not possible, therefore, to use a fibre coupled high power diode laser for this area of application. The normally achievable non-linearity is also relatively weak, so that typically greater than 1 effective mode coupling.

for pulses with pulse lengths of ps it is difficult to obtain an Additive mode coupling (APM), on the other hand, uses an interferometric method. Onto a resonator including the active laser medium a second resonator is coupled, which includes an element with an intensity- dependent index of refraction.

Typically for this element a single mode glass fibre is utilised, since this has a sufficiently long conversion length. The intensity dependent refractive index leads hereby to self-phase-modulation of the light. Due to coherent overlaying of the phase modulated radiation thus arising with other radiation, which is emitted by a main resonator, interference is generated, based on which a shortened pulse 6 arises. In any case the coherent overlaying of the out and return-running pulse requires stabilisation of the resonator length of the coupled-on resonator with an accuracy of a fraction of the wavelength. This method is therefore expensive and liable to disturbance and requires an especially stable form of construction. A further disadvantage, which limits this method to systems using only a small output power, is the necessity for coupling the laser light into a single mode fibre, which could be damaged by high laser power and thus only permits a short lifetime. For these reasons additive mode coupling is not suitable for the applications mentioned above.

Mode coupling with a so-called -non-linear mirror,, uses second order nonlinearity for the mode coupling. This method and the non-linear mirror is also identified in the literature as the "Stankov Mirror". The -non-linear mirror" used here is a part of an optical resonator and comprises a non-linear parametric crystal and a dichroic mirror, which reflects the wave generated in the non-linear crystal completely and the laser wave only in part. On re-entry of both waves into the crystal during the return passage, the conversion effect is dependent on the relative phase between the fundamental wave and the harmonics. If the non- linear crystal is a frequencydoubling crystal, then the frequency-doubled wave is converted back to the fundamental on reentry to the non-linear crystal, when the relative phase between the fundamental wave and the 7 frequency-doubled wave is phase-shifted by 900. The returning fundamental wave is thereby amplified. The combination of frequency doubler and mirror works, with phase-correcting positive feedback, as an intensity dependent reflector, whose amplitude modulation effects the mode coupling. The relative phase is adjusted by the dispersion of a glass plate, which is between the doubler crystal and the mirror or by dispersion in the air. TO this end a modification in the distance of the mirror to the crystal is required.

For this type of mode coupling the highest possible conversion in the nonlinear crystal and the smallest possible reflection of the mirror at the wavelength of the laser wave are required, so that a sufficiently large separation between the "non-linear,, reflective capability of the crystal/mirror combination and the -normal- reflection capability of the mirror results. In order to minimise the pulse duration of the corresponding pulses, a suitable optical element, such as a doublerefracting crystal, can additionally be inserted between the crystal and the mirror, so as to compensate for the different group velocities of the fundamental wave and the second harmonic.

This mode coupling method uses two second order non linear processes connected one behind the other. In this the strength of the non-linear amplitude modulation is 8 proportional to the square of the non-linear coefficient for the conversion of the fundamental wave into the second harmonic. It is thus typically larger by two orders of magnitude than the non-linear conversion effect in such mode coupling methods, as have already been described for KLM methods.

It is a disadvantage in the mode coupling method using a -non-linear mirror" that a special dichroic mirror is required, which must be matched to the individual parameters of the laser in this amplifying medium. Apart from this, the choice of non-linear crystals is limited due to the necessity of a high conversion efficiency. Furthermore it can, especially with high power lasers with several watts of output power, give rise to stability problems due to the high power of the higher harmonics. Small absorption in the crystal at the typical wavelengths of the harmonics lead to changes in the refractive index and thus to an unwanted phase mismatch, reducing the conversion.

In a continuous mode coupled light source for the generation of a fundamental wave of frequency co various requirements have to be stipulated for the above mentioned commercial applications areas:

a high average power of a few watts 9 a short pulse length of the laser pulse of typically less than 20 ps; a high repetition rate of the laser pulse of greater than 40 MHz; a good ray quality, with the highest possibl bending limitation; the most simple and compact construction; the application of a mode coupling method, which can be applied for different laser materials and emission wavelength ranges; is robust against outside interference and adjustments of the parameters of the laser and the resonator; goes into the operational condition of emission of ultra short pulses by itself (selfstarting).

e To realise these aims a mode coupling method is desired, which does not exhibit the limitations of previously proposed methods. With this new method ultra short pulses should be created, so that at the same time both a high average power is available and a low coherence length is produced, by which speckle in the picture presentation arises to a lesser degree.

It is an aim of the present invention to produce a device, in which a mode coupling method comes into application, with which a laser for ultra short pulses and at the same time high average power is possible.

Accordingly the present invention is directed to a device as described in the opening paragraph of the present specification, in which the frequency multiplying material is selected and dimensioned for a phase mismatch between the fundamental wave and the frequency multiplied wave.

The frequency multiplying material can be a suitable crystal. In the sense of the present invention it can also be of a completely different type, such as, for instance, several pieces of crystal with added dispersive pieces of other materials pushed between them. In the present invention only the characteristic of the frequency conversion and the dispersion of the phases of the fundamental wave and the frequency-multiplied wave are important.

It is firstly unexpected that for the desired laser to avoid speckle a move is made in the direction of very short pulses. It would have been possible instead of going back to mode coupled lasers also to have further developed the quoted double core fibre lasers. AS is plain from the introductory discussion of mode coupling, the mode coupling method appeared to have little promise as the desired laser for picture presentation. Only from the recognition that phase mismatching 11 in the frequency multiplying material, which previously had been regarded as unsuitable, permitted a mode coupling, led to the possibility of producing the desired devices, especially lasers, for commercial purposes.

Due to the phase mismatch according to the present invention there remains after the passage through the frequency multiplying material almost the whole energy in the fundamental wave, which first makes the desired high power possible. Dichroic mirrors and sensitive tuning can be dispensed with. so that devices of this construction can be produced for commercial application with the usual variations of the environment. The lack of the dichroic mirror according to the state of the technology and the noncritical tuning of the resonator lead to a great cost advantage, so that it is first possible with the help of the invention, to apply devices of this type commercially, especially in video equipment with rastered light groups.

Because of the phase mismatch the fundamental wave does not have to pass twice through the non-linear frequency multiplying material, to become reconverted into the fundamental wave. Thereby the desired mode coupling is substantially more effective.

12 Advantageously, the frequency multiplying material is a non-linear crystal, which is designed for frequency doubling m = 2 and which is dimensioned with respect to its length L for a minimum for the generation of the frequency doubled wave and for the arrangement of its crystal orientation for the direction of light propagation for phase mismatch.

Thereby a non-linear crystal can be selected as stated, from known crystals and adjusted in a known manner with respect to its length and crystal orientation. Due to the restriction of the frequency multiplication to frequency doubling, only the first non- linear coefficient goes into the conversion. The mode coupling process is thereby extraordinarily effective. Frequency doubling is indeed, as stated, already previously proposed in the state of the art. By the phase matching effected there, with a single pass of the light, the non-linear coefficient is however entered squared, so that the resulting advantage of the effective mode coupling achieved here could not previously be attained.

with Preferably the length L is determined by a whole number n in the equation Ak L/2 = n n 25% n:# 0 13 Ak = k(2o) - k(co) whereby k(2o)) is the wave number of the frequency doubled wave and k(o) the wave number of the fundamental wave taking into consideration the propagation speed given at the individual frequency in the non-linear crystal.

Based on the equation given above the phase matching is at a minimum. The tolerance given of 25% is in practice also easy to realise. Furthermore the device works at the minimum in its very flat range of the conversion curve, so that this tolerance departs only little from the optimum phase mismatch conditions. This would be entirely different with the 900 shift between the frequency multiplied wave and the fundamental wave in accordance with the prior art, since one is then in a conversion maximum. Due to this selection the adjustment of the phase condition is essentially non-critical. It should be stated further that the selection of a noncritical range of this type is only possible on the basis of the selected condition of phase mismatch according to the invention.

As can be seen clearly from the above equation, various whole numbers n for the adjustment of phase mismatch can be selected. In a preferred embodiment the length L is chosen 14 according to a number n, at which the pulse length of the output-coupled fundamental wave is a minimum.

Because of the selection of the minimum pulse length the highest possible power is available within the pulse, so that the conversion to a frequency multiplied wave for mode coupling is also extraordinarily effective. Thereby especially the quoted aim of a high average power with a short pulse length is promoted to an exceptional extent.

Advantageously, a control device is provided, which regulates at least one of those physical parameters which determine the phase mismatching. Due to this control the device is stabilised, so that it can be applied even under major variations of the environmental conditions. This is above all required in commercial applications.

Physical parameters which have an influence on the sought-after phase mismatch can for instance be the temperature of the frequency multiplying material or to a lesser extent also the crystal propagation direction, which could alter, if, for instance, a carrier for the crystal or the mirrors in the device distorts due to variations in the environmental conditions.

Preferably, one of the parameters is the length of the frequency multiplying material, which is traversed by the fundamental wave and the frequency multiplied wave. Especially, the phase mismatch, as already stated above, depends on the length of the frequency multiplying material, so that a control is here most effectively performed.

It is conceivable that the frequency multiplying material can be divided into various pieces and a dispersive material provided between them, so that any movement of the individual pieces against each other would cause a change of the effective length for the mismatching. In a preferred embodiment the parameter determining the phase mismatch is the temperature of the frequency multiplying material and a temperature controlled oven, heating the frequency multiplying material, is provided for the control of the phase mismatch. Even the temperature changes the length very little. The regulation of the temperature has, however, the further advantage that the wave number difference Ak is temperature dependent and is controlled with it. The regulation could also be effected by cooling, for instance by the arrangement of a Peltier element on the frequency multiplying material, possibly with additional heating. The application of a temperature controlled oven is, against this, extremely simple and effective.

16 The regulation could be performed in various ways. For instance it would be possible to branch off from the device a partial ray of the fundamental wave, determine its power as a reference value and compare it with an actual value, so as to maintain the phase mismatch conditions to the optimum. Here it must especially be distinguished in the control algorithm, in which direction, towards higher or lower temperature, the phase mismatch deviates from the minimum of the matching. This makes the regulation expensive.

Advantageously, the oven controls the temperature of the frequency multiplying material directly by comparison of a temper ature-proport ional reference value with an established actual value. Experience with experimental models has unexpectedly shown that such regulation is sufficient, such that recourse to power regulation is not required. This may possibly be traced back to the phase mismatch being essentially less critical than, for instance, the known 900 shift between the fundamental wave and the frequency multiplied wave.

Preferably, an output coupling mirror is provided for coupling the output, which is on a substrate for the wavelength of the fundamental wave and is anti-reflection coated on the side away from the resonator, whereby the substrate has a wedge angle. The mode coupling characteristic 17 is thereby lessened, with regard to the shortness of the pulses and pulse quality.

The frequency multiplying material can be added to the resonator at various positions. Furthermore the most diverse optical forms of construction can be applied for the generation or input coupling of the fundamental wave. Advantageously, the device further comprises a second mirror at the opposite end of the resonator and two mirrors reflecting the fundamental wave arranged within the resonator on both sides of the material generating the fundamental wave, whereby the output coupling mirror, the second mirror and the mirrors bordering the material generating the fundamental wave are transparent to the frequency multiplied wave.

Preferably, the frequency multiplying material is provided between a mirror lying opposite the output coupling mirror, and borders the resonator and the mirror matched to the fundamental wave.

In a preferred embodiment the input coupling of a pumping wave, for instance with a laser, is achieved via a folded ray path, with at least one mirror, is generated for the resonator, which is tuned to the fundamental wave, whereby the mirror is transparent to the pumping wave, and the pumping wave is coupled in via its rear side.

18 Advantageously, diode lasers are provided to generate the pumping wave. These permit an especially laser high power and stability in the device.

Preferably, the fundamental wave lies in the infrared region and especially has a longer wavelength than 80Onm. This is contrary to the possible expectation that for the application of a device constructed as a laser for laser video projection the fundamental wave would lie in the visible frequency range, that is red, green and blue.

Such a longer wavelength can advantageously be excited by diode lasers. Furthermore there are available sufficiently 15 many materials for the frequency multiplying material. on this account it is far more favourable to construct this device as an infra-red laser and with the aid of the infra-red light thus generated to generate at least three colour groups, for instance red, blue and green, for instance using the 20 previously quoted OPO technique, for laser projection. This is especially possible since the pulses are ultra short and are applied with high non-linear frequency conversion processes, as they in the known technology, can be exploited efficiency.

Examples of devices made in accordance with the present invention will now be described with reference to the accompanying drawings, in which Figure 1 shows an example of construction of a laser; Figure 2 shows a graph of the power of the second harmonic in a frequency doubling crystal of length L as a function of the position z in the crystal for phase matching and anti phase matching with different ordinal numbers n; Figure 3 shows a graph of the pulse duration of the fundamental wave and the power of the second harmonic as a function of the crystal temperature; Figure 4 shows a graph of pulse duration as a function of crystal temperature for different degrees of output coupling; and Figure 5 shows a further example of construction of the invention for an optical parametric oscillator (OPO).

In Figure 1 the construction of a laser is shown, which works by means of the previously described mode coupling process. In a resonator comprising the mirrors 1, 2, 3, 4, 5, and 6 there is a Nd:W04 crystal as lasing material 7. The crystal is pumped by diode laser pumping light, which is coupled in via the mirrors 3 and 4 in the direction shown by arrows 8 and 9.

The mirror 1 serves as the output coupling mirror in this arrangement. mirrors 3 and 4 do not only f old the ray path within the resonator but at the same time permit an especially favourable input coupling of the diode laser light. The mirrors 2 to 6 are highly reflecting for the emission wavelength of the Nd:W04 laser of 1064 nm constructed with the resonator in this way.

Between the mirrors 4 to 6, especially between the mirrors 5 and 6 a lithium borate (LBO) crystal 10 is arranged, with which a frequency multiplied wave is generated. The arrangement shown in Figure 1 is designed for a frequency doubled wave, that is a second harmonic at 532 nm. All the mirrors 1 to 6 have a high transmittivity of T > 85% at this wavelength. The mirrors 3 and 4 are also designed for high transmission at the wavelength of the pumping laser diodes of 808 nm and are especially anti-reflection coated on the rear side for wavelengths of 808 nm.

The mirror 1, that is the output coupling mirror, has a transmittivity of T 18% at a wavelength of 1064 nm in this embodiment. It is specially anti-reflection coated on its rear side for the wavelength of 1064 nm. The substrate of the mirror 1 further has a wedge angle of 0. 50, so as to suppress back coupling of the fundamental wave into the resonator. The 21 Nd:W04 crystal forming the material 7 is anti-reflection coated for the wavelengths 1064 nm and 808 nm.

The frequency multiplying crystal 10 is oriented such that the propagation direction of the laser light lies along the x-axis of the LBO crystal. The tuning of the phase mismatch is effected by a temperature changing of the crystal. For this the latter is positioned in a casing, which can be heated by an oven 12. Using this the LBO crystal is temperature controlled to an accuracy of better than 0.10C. Based on conversion effects with the fundamental wave, a wave with the wavelength of 532 nm arises in the frequency multiplying crystal 10, which however is fully suppressed due to the phase mismatch in the ideal case. Due to the transparency of the mirrors 1 to 6 for this wavelength, any possibly remaining portion of the frequency doubled wave will also leave the resonator and not be available for a further excitation. Therefore only the frequency Multiplying LBO crystal 10 serves for the mode coupling.

The invention is however not limited to LBO crystals, but can be replaced by any crystal or any medium, which exhibits a non-linear behaviour at the field strengths arising from the fundamental wave. The phase matching or phase mismatch is obtained by double refracting non-linear crystals by a suitable choice of the direction of propagation of the laser 22 radiation in the crystal and/or a suitable crystal temperature and a corresponding change of further physical parameters.

The requirements on the components are less than those according to the prior art for the mode coupling process in the embodiment. No special dichroic mirror is required as the end mirror of the laser resonator, since the fundamental wave always runs out of the crystal applied in the frequency multiplication without any essential accompaniment of second io harmonic, whilst with the non-linear mirrors according to the prior art the fundamental wave must pass twice through the crystal, so as then again almost completely to reconvert to the fundamental wave.

The requirements on the non-linear crystals are less in this new process than for other processes. Since the intensities of the harmonics outside the crystal are comparatively small, the crystal surfaces are not exposed to high powers of the harmonics. This applies also for the coatings applied to the surfaces, since the phase mismatch in this process means that always a minimum of harmonics and thereby no or only very little power is present on the surface. Apart from this the phase mismatch provides stable operation, depending very little on the adjustment of the laser and the resonator parameters, as becomes clear below.

23 TO improve the mode coupling the mirrors 5 and 6 are made focusing, so that a focussed ray radius of about 160gm results in the LBO crystal 10 and the radius of the ray in the Nd:W04 crystal corresponds somewhat to the radius of the ray of the pumping radiation. This focusing in the LBO crystal leads to a higher power concentration, which increases theefficiency of the generation of the second harmonic. In particular the mirrors have radii of curvature of 150 mm for mirror 6, 350 mm for mirror 5 and 600 mm for mirror 2.

In order better to understand the arrangement of radii of curvature and the focusing, attention is called directly to Figure 1. From this Figure it should be especially recognised that all other resonator mirrors 1, 3, 4 have plane surfaces.

In Figure 1 the individual mirror inclinations for optimal coupling are however not shown in the simplification of the schematic drawing. These are essentially dependent on the construction and known to a man skilled in the art.

The pulsed operation for the non-linear coupling and therewith the desired mode coupling is effected in the example of construction in Figure 1 by an acousto-optic modulator 14, which is situated inside the resonator near to the output coupling mirror 1. The modulator was obtained from the NEOS Company, Type N12054-TE with the associated driver N11054 1ACL. The modulation frequency of this modulator was 54 MHz.

24 With this a repetition rate for the laser pulse of the Nd:YV04 laser of the doubled modulation frequency, that is 108 MHz, is obtained. The length of the resonator constructed by the mirrors 1 to 6 was matched to the given conditions in each case in the following trials on an X- Translation table, on which mirror 1 was positioned:

without a doubling crystal in the resonator the Nd:YV04 laser was actively mode coupled in the construction of Figure 1. The pulse duration then amounted to 35 ps with an output power of 10.3 W. The necessary pumping radiation from laser diodes with a pump power of 2 x 12 W = 24 W was here, and in the following trials also, led via an optical light fibre and transfer optics to the Nd:YV04 crystal. The pulses had a gaussian-similar intensity curve, such as is known for active mode coupled lasers. Such a laser is relatively sensitive to changes in resonator length. For resonator length changes greater than 20 to 30 pm for instance an increasing pulse duration was observed, whereby the output radiation showed strong intensity fluctuations. Especially for variations of more than 30 gm an operating condition was reached, which no longer corresponded to a continuous mode coupled laser.

After insertion of the doubling crystal into the resonator and an adjustment of the temperature for a minimum phase matching the pulse duration shortened from 40 ps to some 10 ps. The tolerance of the mode coupling with respect to resonator length changes about the working point with stable short pulses was strongly increased. Based on the trials results it amounts to about 200 to 250 gm. The average power achieved was then about 9 W. The decrease of power as against the previous condition is essentially to be traced back to the additional losses on the not ideally anti-reflection coated surfaces of the LBO crystal, which could still be 10 substantially improved.

The graphical representation in Figure 2 shows inter alia the method of operation of the frequency doubled crystal. In this Figure the power of the second harmonic P21 is plotted 15 against position along the length of the crystal z.

The curve shown in Figure condition of phase matching. It power of the second harmonic length of the frequency doubled 2 with Ak = 0 describes the can clearly be seen that the increases strongly with the crystal. with the lengthening of the crystal a great part of the laser energy under phase matching is transferred into the second harmonic and thus the power of the fundamental, even when it is partly excited by a second pass through, is no longer available. The case is different with phase mismatch, on the other hand, as is shown 26 by the curves represented by the ordinal numbers n = 1, n = 2 and n = 3.

These ordinal numbers n describe, as can also be seen in Figure 2, how often a second harmonic is maximally excited. This number n for phase mismatch exists in relationship to the length L and the difference of the wave numbers Ak for the frequency doubled wave and the fundamental wave according to the equation AkL/2 = nn Thus the difference number Ak is calculated as k(2o))-k(o)), where k(2o)) is the wave number of the frequency doubled wave and k(co) is the wave number of the fundamental wave, whereby the individual wave numbers at the propagation velocity given for the individual frequency, dependent on the refractive index of the material of the non-linear crystal 10, have to be taken into account.

AS can be further seen from Figure 2, the amplitudes for the power of the second harmonic in the propagation within the frequency doubled crystal decrease sharply with the ordinal number n. There is thus less power available in the second 25 harmonic, so that the losses due to the conversion against the 27 case of phase matching are greatly reduced. It is also especially to be recognised that the power of the second harmonic at the ends of the crystal, i.e. at the points z = 0 and z = L completely disappears independently of the value of 5 n.

For the selection of the optimum number n attention is drawn to Figure 3. In Figure 3 the pulse duration c of the fundamental wave and the power of the second harmonic P2, are shown as a function of the temperature of the LBO crystal 10. The LBO crystal 10 was 20 mm long for this trial and the degree of coupling of the resonator amounted to 18%. For the trial the temperature was varied, as entered on the abscissa, in a range from 1400 to 1690C. The range of the phase matched temperature lay under the selected conditions at about T = 1570C. This range was cut out from Figure 3 since the pulses in this region of phase matching exhibit a substantially greater pulse duration than 20 ps and could not be shown in the drawn diagram. As already set out using Figure 2, this is because the conversion efficiency is large in phase matching. In the trial the power of the second harmonic amounted to substantially more than 1 W.

Figure 3 shows the wave-shaped dependency of the power of the second harmonic on the crystal temperature. It can be 28 further seen from Figure 3 that in the regions designated by the ordinal numbers -4 to +4 of the phase matching the pulse duration is also a minimum, whereby it similarly shows an oscillating curve. Especially the pulse length over the whole temperature range was shorter than 20 ps, which was less than half of the pulse duration in operation with an unheated crystal. The shortest pulse duration was reached in the test set up at T= 149.80C, exactly at a minimum of the phase matching function at n = 3. In general one would select the ordinal number n optimally such that the pulse duration corresponded to the minimum.

In order to study the influence of the length L on the pulse duration, different LBO crystals of varying lengths of 14.5 mm, 20 mm, and 25 mm were included. For each of these crystals the temperature, as shown in Figure 3, was varied and that working point determined at which a stable mode coupling operation with a minimum pulse duration resulted. The results are summarised in Table 1, whereby for the determination of pulse duration a sech 2_ shaped curve was assumed and the given ordinal number corresponds to the minimum pulse duration.

In the trials the shortest pulses thus resulted from a crystal length of 25 mm with a higher ordinal number of n = -4, which indicated that higher ordinal numbers Inj 2 for the laser operation with mismatch are optimal. The power 29 of the laser amounted in this to 8.6 W. It was shown especially in the trials that a continuous mode coupled laser operation with ultra short pulses is only possible when the temperature of the crystal does not correspond to the phase matching temperature, on the contrary a phase mismatch is set up via the temperature.

In Table 1 the temperatures for phase matching are entered in the column headed Ak = 0. The deviation in the temperature for phase matching from the temperature for which the laser delivers the given pulse durations, is dependent on the crystal length. The reason is the change of the temperature acceptance surface, which decreases with 1/L.

is So as to investigate the dependency of the pulse duration and the output power at the fundamental wave at 1064 nm, the power internally in the resonator was changed by decreasing the degree of output coupling. In Table II and Figure 4 the results are presented for pulses with minimal pulse duration T for a crystal length of 20 mm. The degree of output coupling was then varied from 9% to 22%. At small output coupling the power available externally sinks and the pulse duration T decreases. Furthermore, if small degrees of output coupling are selected, a different minimum must be chosen for the phase matching. The ordinal number rises accordingly from n = -3 for T = 22% up to n = -9 for T = 9%. Correspondingly the temperature of the LBO crystal can be reduced from 149.80C to 134.80C.

With reducing output coupling, the power of a pulse in the resonator rises further from 33.9 kw to 87.8 kW. It is here especially noteworthy, that with the same ordinal number and almost the same external power a reduction in the pulse duration from 11.4 ps to 10.4 ps, was possible, solely by changing the transmission.

This laser emits pulses both of higher average power and also of shorter duration. The pulse duration is reduced by the non-linear crystal in the resonator from 40 ps to less than 10 ps, without a reduction in the average power. With the mode coupling process applied here the output values of the laser system can be optimised independently of each other. AS can be seen from the data in Table II and Figure 3 and Figure 4, the output power is very stable against large changes of the resonator parameters, whereby the pulse duration can be minimised solely by the choice of the ordinal number n. A laser of this type is therefore extraordinarily suited for commercial applications with little expense.

This method offers also the possibility of varying the pulse duration over a large range without power 1OSS. TO this 31 end the phase mismatch of the non-linear crystal can be adjusted by an angle or temperature change. A further possibility exists in the application of a turning of the direction of polarisation of the laser light to change the phase mismatch. By changing the direction of the polarisation the power component of the light is changed, which has the necessary direction of polarisation for phase matching in the crystal. Such a turning of the direction of polarisation can be carried out using suitable optical components, such as for instance a half-wave (X/2) plate. The possibility also exists of turning the non-linear crystal about an axis, which is parallel to the resonator axis of the laser.

In Figure 5 an optical parametric oscillator, which relies on the mode coupling process represented, is shown schematically as a further example of construction. In principle, the same elements are used, as are shown in Figure 1. A substantial difference exists in that instead of the Nd:YV04 crystal, a suitable OPO crystal 71 is employed and no pumping light is coupled in for the excitation of a laser process. Instead of this, as shown by the reference number 81, a wave suited to the functioning of the OPO is introduced the OPO crystal. Because of the non linearity of the OPO crystal a suitable wave 81 can be split into two parts, the so-called signal and idler waves. The frequencies of the signal and idler waves are determined by the physical 32 conditions, so that the energy of a single photon in the exciting wave 81 is the same as the sum of the energies in the photons of the signal and idler waves.

For the greatest possible excitation of the signal and idler waves a resonator is employed, which is here again constructed from the mirrors 1 to 6 and operates in a similar manner to the resonator in Figure 1, however here it is tuned to the signal and idler waves. An acousto-optic modulator 14 is similarly not needed with the OPO. However an optical system 16 is provided for input coupling. Further the mirror curvatures are dimensioned here for optimal OPO operation.

The two examples above in Figure 1 and Figure 5 show, as well as the advantage for the mode coupling in the process represented, also that the invention can be applied in many ways. Not only lasers and OPO applications permit mode coupling with a non-linear crystal 10, but every process with a suitable fundamental wave, to which a resonator is tuned, wherein a higher frequency is generated in a non-linear crystal and further excited by phase mismatching.

33 Table I

Length Output Power Autocorrelation Pulse duration Spectral Temperature Temperature Ordinal Width (sech 2) width for Alk = 0 for n number n 14.5mm 9.1 W 17.5 ps 11.4 ps 69 GHz 156.20C 150.40C -2 mm 9.1 W 14.7ps 9.6 ps 85 GHz 156.60C 149.80C -3 mm 8.6 W 13.4 ps 8.7 ps 85 GHz 157.80C 148.20C -4 Table II

Transmission Output min. Pulse Intracavity Spectral Temperature Ordinal From mirror 1 Power duration pulse width number n Gaussian) power T = 9% 7.6 W 8.9 PS 87.8 kW 111 GHz 134.81C -9 T = 12.5% 7.85 W 8.4 ps 69.2 kW 117 GHz 142.411C -6 T = 18% 9.1 W 10.4 ps, 45.1 kW 85 GHz 149.811C -3 T = 22% 9.2 W 11.4 ps, 33.9 kW 88 GHz 149.811C -3 34

Claims

Claims

1 Adevice f or the generation of coherent radiation with a resonator, which is tuned to a fundamental wave with a frequency c) and in which a material, is arranged which is suitable for the generation of this fundamental wave by excitation with a pumping wave, and a frequency multiplying material for the generation, with respect to the fundamental wave, of a frequency multiplied wave, with the frequency mo), whereby the resonator has further an output coupling mirror for the fundamental wave with r the frequency co, in which the frequency multiplying material is selected and dimensioned for a phase mismatch between the fundamental wave and the frequency multiplied wave.

2. A device according to Claim 1, in which the frequency multiplying material is a non-linear crystal, which is designed for frequency doubling m = 2 and whichis dimensioned with respect to its length L for a minimum for the generation of the frequency doubled wave and for the arrangement of its crystal orientation for the direction of light propagation for phase mismatch.

A device according to Claim 2, in which the length L is determined by a whole number n in the equation Ak L/2 = n n 25% with n # 0 Ak = k(2o) k(co) whereby k(2co) is the wave number of the frequency doubled wave and k(o)) the wave number of the fundamental wave taking into consideration the propagation speed given at the individual frequency in the non-linear crystal.

4. A device according to Claim 3, in which the length L is chosen according to a number n, at which the pulse length of the output-coupled fundamental wave is a minimum.

A device according to any one of Claims 1 to 4, in which a control device is provided, which regulates at least one of those physical parameters, which determine the phase mismatching.

A device according to Claim 5, in which one of the parameters is the length of the frequency multiplying material, which is traversed by the fundamental wave and the frequency multiplied wave.

36 7.

A device according to Claim 5 or Claim 6, in which the parameter determining the phase mismatch is the temperature of the frequency multiplying material and that a temperature controlled oven, heating the frequency multiplying material, is provided for the control of the phase mismatch.

8. A device according to any one of Claims 1 to 7, in which an output coupling mirror is provided for coupling the output, which is on a substrate for the wavelength of the fundamental wave and is antireflection coated on the side away from the resonator, whereby the substrate has a wedge angle.

9. A device according to any one of Claims 1 to 8, and further comprising a second mirror at the opposite end of the resonator and two mirrors reflecting the fundamental wave arranged within the resonator on both sides of the material generating the fundamental wave, whereby the output coupling mirror, the second mirror and the mirrors bordering the material generating the fundamental wave are transparent to the frequency multiplied wave.

10. A device according to Claim 9, in which the frequency multiplying material is provided between a mirror lying 37 opposite the output coupling mirror, and borders the resonator and the mirror matched to the fundamental wave.

11. A device according to any one of Claims 1 to 10, in which a folded ray path, with at least one mirror, is generated for the resonator, which is tuned to the fundamental wave, whereby the mirror is transparent to the pumping wave, and the pumping wave is coupled in via its rear side.

12. A device according to Claim 11, in which diode lasers are provided to generate the pumping wave.

13. A device according to any one of Claims 1 to 12, in which is the fundamental wave lies in the infrared region and especially has a longer wavelength than 80Onm.

14. A device substantially as herein described with reference to and as illustrated in the accompanying drawings.