GB2338569A

GB2338569A - Fixing plate for optic fibres and lens array

Info

Publication number: GB2338569A
Application number: GB9914093A
Authority: GB
Inventors: Ehrhard Dammann; Juergen Bauer
Original assignee: Jenoptik Jena GmbH; Jenoptik AG
Current assignee: Jenoptik AG
Priority date: 1998-06-18
Filing date: 1999-06-16
Publication date: 1999-12-22
Anticipated expiration: 2019-06-16
Also published as: GB2338569B; FR2780167B1; DE19827125A1; CH694707A5; FR2780167A1; GB9914093D0

Abstract

A fibre-fixing arrangement (4, Fig 2) consists of a plane parallel plate (42) having at least two recesses (43) in pairs symmetrically with respect to an optical axis predetermined by a transmitting optical device (71) in order to receive the transmitting fibres (31) fixed orthogonally in the plate (42), the end surfaces of the transmitting fibres (31) coinciding with the plane of connection (45, Fig 2) which is adjoined by a rigidly coupled lens array (5, Fig 2), and the lens array consists of a plane parallel supporting plate (52) with collimating lenses (51) attached by cementing, wherein the number and position of collimating lenses (51) corresponds to the number and position of the transmitting fibres (31) and the parameters of the subsequent transmitting optical device (71), and the optical path length as far as the collimating lens (51) is adapted by adjustment of the thickness of the supporting plate (52) to the aperture of the transmitting fibres (31) and the focal length of the transmitting optical device (71) so that the laser narrowings of the transmitting beam (6) focused through the transmitting optical device (71) coincide upon superimposition in a volume for measurement (8) in a laser Doppler measuring device eg. an anemometer.

Description

1 - 2338569 Apparatus for generating parallel collimated beams from

fibre-coupled laser diodes and method of producing this apparatus The invention relates to apparatus for generating parallel collimated laser beams which are produced from fibre-coupled laser diodes. Laser beams which are precisely adjusted in this way so that they are parallel and collimated are preferably used in laser Doppler devices as transmitting beams for the illumination of moving scattering particles. The quality of the collimation is a prerequisite for high accuracy of measurement.

Since the introduction of the laser, laser Doppler measuring devices, such as laser Doppler anemometers (LDA),, velocitneters, (LDV), vibrometers and interferometers, have been determinedly developed as means for measuring speed and length- In this case moving objects of measurement are illuminated by a laser beam and the frequency shift of the scattered light achieved by the Doppler effect is evaluated. In particular the so-called cross beam principle has been successful as a laser Doppler measuring assembly because of its relative insusceptibility to faulty adjustments.

However, the accuracy of measurement of such arrangements depends not only on the constancy of the wavelength but also essentially upon the curvature of the wavefronts of the two transmitting beams in the volume for measurement. This situation is described by F. DURST et al. in Appl. Opt. 18 (1979) 4, pages 516524, "Influence of Gaussian beam properties on laser Doppler signals". The ideat therefore - because of the evenness of the wavefronts in the laser narrowing - is an exact imaging of the laser narrowings of the focused 2 transmitting beams into the volume for measurement. If a gas laser is used as the light source, its natural slight divergence can be exploited in order for the two transmitting beams generated via one beam splitter to be superimposed with a convergent lens (transmitting lens) m the volume for measurement.

1 With the development of the laser diodes and their increasingly long service life and high performance, these are used ever more frequently in laser Doppler devices. However, the great asymmetry of the laser diode radiation and the large aperture angle (numerical aperture) lead to great difficulties in the generation of a Gaussian beam. Such a beam transformation is normally only possible with costly multi-lens systems. For this reason there is an increasing changeover to the transmission of two transmitting beams with polarisation-retaining monomode fibres, since fibre-coupled lasers are available which transform the transmitting beams into a Gaussian beam. This guiding of the beam simultaneously facilitates a compact arrangement and a galvanic separation of electronics and optical probe.

In his paper "Anwendungen von Halbleiterbauelementen in der LaserDopplerAnemometrie, Teil IF' (Laser Magazin 6191, pages 8-15) C. TROPEA shows an optical arrangement in which the receiving lens has bores through which the transmitting beam is passed unhindered. The larger receiving lens diameters which can be used in this case improve the scattered light signal. In this case each transmitting fibre is cemented in a ferrule, and this ferrule/fibre arrangement must be provided with an oblique grinding in order to suppress any reflection back into the fibres. If only because of the oblique exit of light such an arrangement requires a costly adjustment of the collimating optical system to j 3 the fibre outlets in order to collimate the two transmitting beams, to image them exactly into the volume for measurement and to fix this state. Also the chosen use of two separate collimating lenses cannot provide sufficient stability and reproducibility of the adjusted state because of the high adjustment requirements (in twice 3 degrees of freedom in Wanslation and 3 in rotation), so that to some extent considerable restrictions in the accuracy of measurement must be taken into account.

The object of the invention is to find a novel possibility for generating parallel collimated beams from fibre-coupled laser diodes which meets the high requirements for the state of coflimation of transmitting beams in laser Doppler arrangements and is simple to adjust. The invention is further directed to the provision of a method for reproducible manufacture of such apparatus for generating collimated laser beams.

According to the invention, in apparatus for generating parallel collimated beams from a fibre-coupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, the object is achieved in that a fibre-fixing arrangement and a lens array are in surface contact with one another in a plane of connection, that the fibre-fixing arrangement is a plane parallel plate and has at least two recesses which are disposed in pairs symmetrically with respect to an optical axis predetermined by the transmitting optical device in order to receive the transmitting fibres provided by branching from the laser diode, wherein the transmitting fibres are fixed rigidly in the plate in such a way that the end surfaces of the fibres coincide exactly with the plane 4 - of connection, and that the lens array consists of a plane parallel supporting plate with rigidly fixed collimating lenses, wherein the number of collimating lenses and the position thereof corresponds to the number and position of the supplied transmitting fibres m the fibrefixing arrangement and the parameters of the subsequent transmitting optical device and the optical path length through the supporting plate as far as the collimating lens is adapted by adjustment of the thickness of the supporting plate to the aperture of the transmitting fibres and the focal length of the transmitting optical device so that the laser narrowings of the beams focused through the transmitting optical device coincide upon superimposition in the volume for measurement.

In the fibre-fixing arrangement a further optical fibre is preferably provided as a receiving fibre centrally between the transmitting fibres and along the optical axis predetermined by the transmitting optical device in order to transmit light to a receiver, whereby light scattered on objects of measurement in the volume for measurement is imaged via the transmitting optical device and an equivalent receiving optical device onto the end of the receiving fibre. In this case the receiving optical device advantageously has openings for the transmitting beam to pass through.

In an advantageous construction the fibre-fixing arrangement and the lens array are produced from a uniform plane parallel plate, the plane of connection being defined by highly precise recesses on the one hand from the front face of the plate for the fixing of the fibres and on the other hand from the rear face of the plate for the lens mounting. The recesses for the lens mounting extend with reduced diameter as far as the plane of connection, the laser beams emerging from the transmitting fibres into the air space of the particular recess and being collimated by the collimating lens. In order to avoid a complicated adjustment of the collimating lenses, spherical lenses are preferably used.

In a further advantageous variant the fibre-fixing arrangement and the lens array are rigidly connected to one another by means of a layer of cement which is adapted to the refractive index. In this construction the collimating lenses of the lens array are preferably glued in the form of plano-convex lenses with their plane surface on the surface of the supporting plate which is at the rear in the path of the light. For this purpose both the fibre-fixing arrangement and the lens array are advantageously ground and polished in the plane of connection for them to be cemented, so that in the fibre-fixing arrangement all fibre ends are exactly disposed in a plane and in the lens array the optical path length of the assembly consisting of the layer of cement supporting plate and lens cement layer as far as the collimating lens is adjustable.

It has also proved advantageous if the fibre-fixing arrangement is a plane parallel plate which is essentially divided in half along a straight line, at least one of the part-plates has notches along the dividing surface which are disposed orthogonally with respect to the plate surfaces and are dimensioned so that the when the part-plates are subsequently joined together and glued the fibres are secured without play by the part-plates. Advantageous shapes of the notches are rectangular, wedge-shaped or trapezoidal.

When the collimating apparatus according to the invention is used in a laser Doppler device (e.g. an anemometer) the transmitting fibres preferably exit from i - 6 an integrated optical chip, the input of which is supplied by the fibre- coupled laser diode and on which are provided a junction and a phase modulator for frequency shifting between the two light waves of the transmitting fibres.

The object of the invention is achieved by a method of producing apparatus for generating parallel collimated beams from a fibre-coupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, in that the collimating apparatus is produced in two essential part-processes, the production of a fibre- fixing arrangement and of a lens array, that for production of the lens array tubular recesses are made in a plane parallel plate for mounting of collimating lenses, the positions and the depth of the recesses being adapted to the parameters of the transmitting fibres and transmitting optical device which are used, so that the narrowings of the laser beams entering the parallel tubular recesses are brought by the transmitting optical device precisely into superimposition in the volume for measurement and due to the uniform depth of the tubular recesses a plane of connection is produced which is parallel to the surface of the plane parallel plate and constitutes the contact surface with the fibre-fixing arrangemen that in order to produce the fibre-fixing arrangement recesses are made in a plane parallel plate for precise fixing of transmitting fibres coupled to the laser diode and these recesses guarantee a reproducible locking of the end surfaces of the fibres in the plane of connection, the positions of the recesses being aligned with respect to the axes of the tubular recess. es m such a way that the axes of the transmitting fibres coincide with the tube axes and the transmitting fibres open into the tubular recesses in the plane of connection in each case.

7 - In a first advantageous embodiment of the method the fibre-:FLxing arrangement and the lens array are produced from a uniform plane parallel plate, the recesses of the two components being made from different faces of the plane parallel plate and touching one another in a virtual plane of connection. The recesses for receiving the fibres are in each case preferably constructed as double cylinders, wherein a smaller cylinder diameter which touches the plane of connection only receives the fibre core and with respect to a larger cylinder diameter functions as a stop when the fibres are introduced. For production of the recesses a socalled LIGA process can advantageously be used, in which threedimensional microstructures are produced by a succession of lithography with synchrotron radiation (X-ray depth lithography), electroforming and forming techniques with plastics materials.

In a second advantageous variant of the method the fibre-ffixing arrangement and the lens array are produced from two different plane parallel plates, the two different plates being produced so that they are co-ordinated with one another as regards the fibre axes and the axes of the tubular recesses, are aligned with one another in the plane of connection and are glued to one another.

According to the invention, in a method of producing apparatus for generating parallel collimated beams from a fibre-coupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, the object is further achieved in that the collimating apparatus is produced in two essential part-processes, the production of a fibre-fixing arrangement and of a - 8 lens array, that for production of the lens array plano-convex collimating lenses are glued onto a plane parallel supporting plate, the position and focal length of the said lenses being adapted to the parameters of the incident beam and of the subsequent transmitting optical device so that the narrowings of the laser beams collimated by the collimating lenses are brought to superimposition in the volume for measurement by way of the transmitting optical device, and the surface of the supporting plate lying opposite the glued-on collimating lenses is used as a plane of connection for connecting the lens array to the fibre- fLxffig arrangement, wherein for adjustment of the optical thickness of the assembled supporting plate, collimating lens and interposed layer of cement which is suitable for the defined superimposition of the laser beam in the volume for measurement the said surface of the supporting plate is ground flat until the necessary thickness is achieved, that for production of the fibre-fixing arrangement through recesses are made in a fin-ther plane parallel plate for precisefixing of transmitting fibres which are coupled to the laser diode, and these recesses guarantee an orthogonal locking of the transmitting fibres relative to the surface of the plate in the recesses, the positions of the recesses being aligned with respect to the axes of the collimating lenses Mi such a way that the axes of the transmitting fibres coincide with those of the collimating lenses, the transmitting fibres are in each case pushed into the recesses and then glued, wherein the transmitting fibres penetrate through the plane of connection predetermined by the rear face of the plate, and the protruding ends of the transmitting fibres are abraded as far as the rear surface of the plate as plane of connection, that the fibre-fLxing arrangement and the lens array are joined together and glued with their plate surfaces defined as plane of connection, the - 9 axes of the fibres being brought into coincidence with the axes of the collimating lenses.

When light backscattered on objects is utilised, it has proved advantageous for a further recess to accommodate a receiving fibre to be provided centrally between the recesses for the transmitting fibres.

For all of the recesses in the fibre-:FLxing arrangement there are various possibilities available for making these in the plane parallel plate.

On the one hand the recesses can be made by laser drilling, in which case they are conveniently of cylindrical or conical construction. With a suitable choice of cylinder diameter (as a compromise between guiding the fibre with an accurate fit and reliable introduction of the fibre with glue) the glueing of the fibre cladding simultaneously achieves a traction relief For certain conditions of strain on the fitbres it proves advantageous to glue the fibre core separately in the plane parallel plate, since the recess is constructed as a double cylinder which is adapted on the one hand to the core diameter and on the other hand to the cladding diameter of the fibre. The fibre core is exposed at the end of the fibre for glueing in.

In the case of a conical construction the recesses are preferably produced with their large diameter greater than the external diameter of the transmitting fibres and with their small diameter in the plane of connection adapted to the external diameter of the transmitting fibres, so that as in the case of the cylindrical recess traction relief is provided and assembly is simplified. If extra locking is also required here for the fibre core, the small cone diameter should be co- ordinated with the core diameter and the end of the fibre should be correspondingly stripped. Furthermore this provision achieves automatic centring of the fibres.

A particularly reliable and precise adjustment of the mitting fibres in the fibre-fixing arrangement takes place by dividing the plane parallel plate employed perpendicular to the plate surfaces, introducing notches to receive the fibres in at least one of the dividing surfaces and joining together the dividing surfaces after the fibres have been inserted in the notches. In this case the notches are preferably made in a wedge shape or trapezoidal and are dimensioned so that when the part-phates are joined together the fibres laid in them are enclosed substantially without play and are additionally glued.

i The invention is based on the thought that the adjustment and fixing of fibres from fibre-coupled laser diodes relative to the necessary collimating lenses and a subsequent focusing optical device can only be simplified and reproducibly carried out if the number of necessary degrees of freedom of adjustment (for two transmitting fibres: twice three degrees of freedom in translation and three in rotation) is markedly reduced. For this purpose according to the invention - coordinated with the focusing optical system (transmitting optical device) and the parameters of the fibres used - an assembled system is proposed which consists of a fibre-fixing arrangement and a lens array within close manufacturing tolerances and with which degrees of freedom in translation and rotation of the fibres and of the collimating lenses amongst themselves as well as the fibre ends and the collimating lenses with respect to one another are omitted, so that only one degree of freedom in rotation and two degrees of freedom Mi translation have to be ad usted by mechanical/optical means for the specific use (e.g. in a laser Doppler anemometer). By simple production steps, in which manufacturing tolerances can be kept small or can be subsequently corrected (e.g. by surface grinding), a very high degree of reproducibility can be achieved in the production and adjustment of the collimating apparatus according to the invention.

The apparatus according to the invention facilitates the generation of parallel collimated beams from fibre-coupled laser diodes which meet the high standards required of the state of collimation of transmitting beams in laser Doppler arrangements, and the apparatus is simple to adjust. The invention further ensures reproducible adjustrnent in the production of simple assembly components for the coupling of optical fibres and collimating lenses.

The invention will be explained in greater detail below with reference to examples of embodiments. In the drawings:

Figure 1 shows an optical scheme of a cross beam LDA according to the prior art,

Figure 2 shows an optical scheme according to the invention for generating collimated transmitting beams with an assembly consisting of two plane parallel plates for fibre fixing and for arrangement of the collimating lenses, Figure 3 shows an optical scheme according to the invention for generating collimated transmitting beams with a uniform plane parallel plate, Figure 4 shows a complete optical scheme for application of the apparatus according to the invention in a LDA, 12 - Figure 5 shows a variant of the fibre-fixipg arrangement with conical recesses for receiving four transmittingfibres and one receiving fibre, Figure 6 shows a variant of the fibre-fixing arrangement with cylindrical recesses for receiving two transmitting fibres and one receiving fibre, Figure 7 shows a variant for construction of the fibre-fixing arrangement from two part-plates with notches to receive the fibres.

For greater clarity of the invention a diagram corresponding to the prior art is shown in Figure 1 for a fibre-optic laser Doppler anemometer GLDA) according to the cross beam principle utilising the light backscattered from moving objects. The optical fibre 3 emerging from a fibre-coupled laser diode 1 is guided into a light divider and modulator component 2 which divides the light to two transmitting fibres 31 and produces a frequency difference between both transmitting fibres 31. The transmitting fibres 31 are polarisation-retaining monomode fibres of any length, so that the subsequent elements can be combined in a handy probe head. The ends of the transmitting fibres 31 are placed for highly precise adjustment in so-called ferrules 41. In order to avoid reflections back on the end surface of the fibre, each ferrule is provided with an oblique grinding at the end, resulting in the beam path shown (exaggerated) in the enlarged circled detail. The divergent beam which emerges is refracted on the oblique end of the fibre and thus leaves the transmitting beam 31 at the designated angle to the fibre axis. Consequently for the generation of a collimated transmitting beam 61 the axis of the collimating optical device 51 is also slightly inclined and makes the adjustment of the entire free beam optical system considerably more difficult. This is improved only slightly by the compact optical unit consisting of the transmitting optical device 71 and the

13 receiving optical device 72 with the same parameters. The receiving optical device 72, which in order to receive as much scattered light as possible should be larger than the distance between the transmitting beams 3 1, has openings for the transmitting beams 31 to pass through unaffected. This results in very strict requirements for the state of collimation of the transmitting beams 31 as well as the adjustment of the transmitting beam in the volume for measurement 8, the crossing point of the beams in the cross beam principle to which this gives its name. These requirements make the adjustment very costly, susceptible to disruption and hardly reproducible because of the large nurnber of necessary individual adjustments (ferrules 41 with tilting of the axis to collimating lenses 51 and to the transmitting and receiving optical devices 71 and 72) and the number of degrees of freedom of each individual element (3 degrees of freedom in translation and 3 in rotation).

In the backscatter principle which is generally used and is illustrated in Figure 1 an intensity modulation is produced in the volume for measurement 8 by scattering on moving object particles. This development of intensity over time is irnaged by way of the transmitting optical device 71 and the receiving optical device 72 as focused scattered light 62 onto the receiving fibre 32 which is likewise retained by a ferrule 41. By means of the receiving fibre 32, a multimode fibre, the receiver 9 which frequently contains an avalanche photodiode (APD) is supplied with the intensity-modulated scattered light and causes an evaluation unit to evaluate Doppler frequency shifts in order to determine the Doppler frequency and derived values, such as speed or length of the movement.

By contrast with this the invention - with the same construction in principle (cross beam principle) of any laser Doppler measuring device is especially directed to simplifying the critical adjustment between the ends of the transmitting fibres 31 and the collimating lenses 51 and reducing the number of degrees of freedom of adjustment.

1 1 1 Apparatus according to the invention for generating parallel collimated beams consists in its basic construction - as shown in Figure 2 - of a fibre-fixing arrangement 4 and a lens array 5 which are rigidly connected to one another in a plane of connection 45. The special feature of the fibre-fixing arrangement 4 lies in the fact that for all transmitting fibres 31 a plane parallel plate 42 is provided in which orthogonally penetrating recesses 43 are made accurately to size in conformity with the geometricalloptical imaging conditions of the transmitting optical device 71 and the dimensions of the transmitting fibres 3 1. The transmitting fibres 31 are fixed orthogonally in the plane parallel plate 42 so that the end surfaces of the transmitting fibres 31 terminate exactly flush with the rear surface of the plate which at the same time forms the plane of connection 45 to the fibre array 5. This state of adjustment is achieved exactly using production techniques first of all by insertion and gluemig of the transmitting fibres 31 with the fibre ends protruding and then by abrading the transmitting fibres 31 as far as the surface of the plate 42.

The lens array 5 contains collimating lenses 51 which are co-ordinated with the transmitting fibres 31 in the fibre array 4 as regards number and spacing and in this variant it consists of a plane parallel supporting plate 52 made from optical glass (preferably quartz glass) onto which the collimating lenses 51 are glued.

- is The collimating lenses 51 are plano-convex lenses, preferably hemispherical lenses, which form an optical assembly with the supporting plate 52 by means of a layer of cement 53. This assembly is adapted in the refractive indices of its supporting plate 52, lens cement layer 53 and collimating lenses 51 to the refractive index of the transmitting fibres 3 1. A layer of cement 54 by which the lens array 5 is rigidly connected to the fibre-fixing arrangement 4 likewise has a refractive index co-ordinated with the refractive index of the transmitting fibres 3 1. so that reflections back on the end surfaces of the fibres are largely avoided thereby.

The optical thickness of the supporting plate 52 together with the adjoining cement layers, the lens cement layer 53 and the cement layer 54, has a significant ftinction in the lens array 5, because the distance ratios of the collimating lenses 51 to the ends of the transmitting fibres 31 can be adjusted thereby. Deviations with respect to the calculated thickness of the supporting plate 52 and lens cement layer 53 which occur due to manufacturing tolerances of the lens cement layer 53 are eliminated in the production of the lens array 5 by subsequent surface grinding of the surface of the supporting plate 52 lying opposite the collimating lenses 5 1. During cementing of the fibre-fixing arrangement 4 and the lens array 5 only one degree of freedom in rotation and two degrees of freedom in tranation have to be adjusted in the plane of connection 45, this adjustment operation is carried out once and the achieved state then remains stable. In this case the adjustment takes place by feeding light into the transmitting fibres 31 and monitoring the state of collimation of the parallel transmitting beam 61 or - as set out more especially below in relation to Figures 5 and 6 - monitoring the state of the laser narrowings focused by way of the - 16 transmitting optical device 71 in the volume for measurement 8 including a receiving fibre 32, transmitting optical device 71 and receiving optical device 72 as complete adjustment of the transmitting and receiving channel within one operation.

1 1 Figure 3 shows the same basic construction of the collimating arrangement according to the invention consisting of fibre-fixing arrangement 4 and lens array 5, wherein in this case the fibre-fixing arrangement 4 has in addition to the recesses 43 for the transmitting fibres 31 a further re. cess 43 positioned centrally between the transmitting fibres 31 in the same plane (drawing plane) to accommodate a receiving fibre 32. In the lens array 5 there are also recesses which are provided as tubular recesses 55 for mounting the collimating lenses 5 1. This variant can be produced appropriately, i.e. with suflicient accuracy, for example by the so-called LIGA method. The LIGA method is a special method of producing microstructures by the processes of lithography with synchrotron radiation (X-ray depth lithography), electroforming and forming of plastics materials. In this case the fibre-fixing arrangement 4 and lens array 5 can be produced as separate plane parallel plates in this way within particularly narrow tolerance limits. However, it is also advantageous, as shown in Figure 3, to produce a uniform plane parallel plate by forming of the recesses 43 and 55 by means of two differentforming tools which are opposite one another and touch one another in the plane of connection 45. As can also be seen from Figure 3, the recesses 43 for the transmitting fibres 31 and the receiving fibres 32 are produced in each case in two different diameters, so that after defined exposure of the fibre cores 33 for the transmitting and receiving fibres 31 and 32 a stop is provided which positions the ends of the fibres exactly in the plane of 17 - connection. The collimating lenses 5 1, preferably in the form of spherical lenses, are likewise reliably positioned by correspondingly placed stops within the tubular recesses 55 and - Eke the transmitting and receiving fibres 31 and 32 also - secured by glueing. In order for the light backscattered by the moving object particles to be imaged onto the receiving fibre 32 with as little attenuation as possible, a suitable recess is advantageously provided for the scattered light 62 focused by the transmitting and receiving optical devices 71 and 72 (not shown in Figure 3).

In Figure 4 a collimating arrangement according to the invention is used in the optical scheme of a fibre-optic laser Doppler anemometer (LDA). The light from a fibre-coupled laser diode 1 is led onto an integrated optical chip (10Q 21 for dividing into two transmitting beams 31 and producing a frequency shift. The IOC 21 contains a Y junction 22 as well as an electro-optical phase modulator 23 which is represented by its electrodes. The transmitting fibres 31 leaving the IOC 21 are polarisation-retaining monomode fibres and are glued with their ends into the recesses 43 of the plane parallel plate 42 of the fibreExing arrangement 4. By way of the rigidly coupled supporting plate 52 with the collimating lenses 51 cemented thereon as lens array 5, parallel collimated transmitting beams 61 are produced which pass unaffected through a receiving lens 72 having openings and are focused with their laser narrowings into the volume for measurement 8 by means of a transmitting optical device 7 1. In the volume for measurement 8 an interference fringe pattern is produced due to the frequency difference between the two transmitting beams 31 which is produced m the phase modulator 23. During traversing of the volume for measurement 8, moving object particles scatter the laser light failing onto them with the intensity modulated because of the flinge pattern. By way of the system consisting of the transmitting optical device 71 and the receiving optical device 72, which are optically equivalent, a part of the scattered light (in this case backscattered light) is imaged as focused scattered light 62 onto the receiving fibre 32 which is glued in centrally between the transmitting fibres 31 in the plate 42. The receiving fibre 32 passes the modulated light to the receiver 9, which is preferably an avalanche photodiode (APD), which is followed by the usual evaluation of the Doppler frequency and calculation of the derived values, such as speeds and path lengths, of the moving objects.

Figure 5 shows in the upper part a plan view of the fibre-fixing arrangement 4 which in this example is a circular plane parallel plate 42 with one receiving fibre 3 2 and four transmitting fibres 3 1. The symmetrical arrangement of two pairs of transmitting fibres 31 which is chosen here on the one hand results in a higher heat stability and on the other hand facilitates the measurement of particle movements with components of motion in two co-ordinate directions in the volume for measurement 8. Various materials such as glass, glass ceramic and certain types of steel can be used as materials for the plate 42, with the prerequisite that their linear expansion coefficients are adapted to those of the lens array 5 and the transmitting and receiving fibres 3 1 and 3 2.

In the lower part of Figure 5 a section is shown along the line A-A of the plan view. In this case the recesses 43 are conical in shape. In order to achieve a simpler and precise centring of all fibres 3, the small diameter of the conical recess 43 on the surface of the plate 42 which forms the plane of connection 45 with the lens array 5 is co- ordinated with the diameters of the fibre cores 33 of - 19 transmitting fibres 31 and receiving fibres 32 and the large diameter is greater than the external diameter of the transmitting and receiving fibres 31 and 32, which must then be stripped at the ends of the fibres. The ends of all fibre cores 33 are flush with the plane of connection 45. The production procedure can be of uncomplicated design if the external diameter of the fibres 3 in the conical recesses 43 lies approximately in the centre of the plate 42. In this case the transmitting and receiving fibres 31 and 32 centre themselves if they are introduced already provided with glue into the conical recesses 43. In this case the dimensioning of the exposed fibre core 33 has no significance, since the fibre core 33 can penetrate any distance through the surface of the plate 42 which forms the plane of connection 45 to the lens array 5, since this protrusion can be abraded after the glueing of the fibres 3.

In Figure 6 the fibre-fixing arrangement 4 is shown in the same way (in plan view and in section) as in Figure 5. The fibre-fixing arrangement 4 is again a circular plane parallel plate 42 which has two transmitting fibres 31 and one receiving fibre 32 positioned centrally therebetween along a diameter line (sectional plane B-B). As the sectional representation B-B shows, the recesses 43 here are cylindrical for all fibres 3 and are made using production techniques with sufficient accuracy preferably by laser drilling into the plate 42. The fibres, 3 are then pushed through the recesses 43 and glued so that they penetrate the surface of the plate 42 which serves as the plane of connection 45. After hardening of the glue the ends of the transmitting and receiving fibres 31 and 32 are subjected to surface grinding and brought to coincidence exactly flush with the plane of connection 45.

1 1 i For the configurations of the fibre-fixing arrangement 4 according to Figures 5 and 6 with the receiving fibre 32 introduced between the transmitting fibres 3 1, apart from the actual positioning advantages of the fibres 3 a uniform adjustment of the transmitting beam 61 for the irnaging of the focus scattered light 62 is also possible. In this case the laser narrowings of the collimated transmitting beams 61 are focused into the volume for measurement 8 and simultaneously the receiving fibre 32 is used as fight inlet during the adjustment and cementing of the flibre-fixing arrangement 4 and lens array 5 and the fibre end of the receiving fibre 32 is imaged onto the same point in the volume for measurement 8. The state of adjustment is observed on a monitor, optimised and held until it is finally fixed with the hardening of the cement layer 54.

Figure 7 shows a further plan view of the plane parallel plate 42 of the fibrefixing arrangement 4 which again retains two transmitting fibres 31 and one receiving fibre 32 in a defined manner. The special feature Mi this example lies in the special way of producing the plate 42, which in this case consists of two part-plates 47. Although both part-plates 47 can in principle be made from different materials, since they have no optical function, a uniform plate 42 is preferably divided along a plane orthogonally with respect to the plate surfaces. Notches 48 are made in one of the resulting dividing surfaces according to the optical requirements described above, and the transmitting fibres 31 are placed into them at the same distance on either side of the receiving fibre 32. The notches 48 are wedge-shaped in this example. They could also have a trapezoidal or rectangular cross-section. In any case they should be introduced into the dividing surface to such a depth that the fibres 3 even in the event of differing thicknesses of the transmitting and receiving fibres 31 and 32 - project Dui of the dividing surface by the same amount. As a result during glueing to the other part-plate 47 all fibres 3 are secured orthogonally and without play. The glueing 49 of the plate serves only for fixing the secured state. If the diameters of the transmitting and receiving fibres 31 and 32 respectively are nevertheless too different so that the deviation from the uniform plane of the transmitting and receiving fibres 31 and 32 can no longer be tolerated because of the losses to be expected when receiving the focused scattered light 62 (see Figure 4), notches 48 can advantageously be made in both part-plates 47.

After the glueing of the part-plates 47 the entire plate 42 is surfaceground, particularly for levelling the fibre ends of the transmitting and receiving fibres 31 and 32,, and thus again acquires the necessary coincidence of the fibre ends with the plane of connection 45 for coupling to the lens array 5.

With the manner according to the invention of producing defined conditions for the fibre ends of the transmitting fibres 31 and the receiving fibres 32 a further advantage is produced for the optical adjustment procedure which is in any case already simplified (only two degrees of freedom in lation and one degree of freedom in rotation). In this case the assembly consisting of the fibre-fixing arrangement 4 and the lens array 5 is produced most simply during glueing by the layer of cement 54 along the plane of connection 45 (see figures 2 and 4), with light being fed into the receiving fibre 32. By positioning of a sufficiently well reflecting or scattering (stationary) object in the volume for measurement 8 the state of focusing of the returning beam 6 focused by the collimating lenses 51 onto the ends of the transmitting fibres 31 is then optimised during the cementing procedure. Thus a sufficiently precise, reproducible and particularly simple method of producing a collimating arrangement is provided.

-23 List of reference numerals used laser diode (fibre-coupled) 2 light divider and modulator component 21 IOC (integrated optical chip) 22 Y junction 23 phase modulator 3 fibres 31 transmitting fibres 32 receiving fibres 33 fibre core 4 41 42 43 44 45 46 47 48 49 fibre-fixing arrangement ferrule (plane parallel) plate recess laser drilled bore plane of connection fibre glue g part-plate notches plate glueing 51 1 52 53 54 55 6 61 62 71 72 lens array coIllimating lenses supporting plate lens cement layer cement layer tubular recess beam transmitting beam focused scattered light transmitting optical device receiving optical device 8 volume for measurement 9 receiver

Claims

1. Apparatus for generating parallel collimating beams from a fibrecoupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, characterised in that a fibre-fixing arrangement (4) and a lens array (5) are in surface contact with one another in a plane of connection (45), the fibre-fixing arrangement (4) is a plane parallel plate (42) and has at least two recesses (43) which are disposed in pairs symmetrically with respect to an optical axis predetermined by the transmitting optical device (7 1) in order to receive the transmitting fibres (3 1) provided by branching firom the laser diode (1), wherein the transmitting fibres (31) are fixed orthogonally and rigidly in the plate (42) in such a way that the end surfaces of the fibres coincide exactly with the plane of connection (45), and the lens array (5) consists of a plane parallel supporting plate (52) with collimating lenses (5 1) rigidly attached by cementing, wherein the number of collimating lenses (5 1) and the position thereof corresponds to the number and position of the supplied transmitting fibres (31) in the fibrefixing arrangement (4) and the parameters of the subsequent transmitting optical device (7 1) and the optical path length through the supporting plate (52) as far as the collimating lens (51) is adapted by adjustment of the thickness of the supporting plate (52) to the aperture of the transmitting fibres (31) and the focal length of the transmitting optical device (71) so that the laser narrowings of the beam (6) focused through the transmitting optical device (71) coincide upon superimposition in the volume for measurement (8).

2. Apparatus as claimed in Claim 17 characterised in that in the fibrefixing arrangement (4) a fiHffier optical fibre (3) is provided as a receiving fibre (32) centrally between the transmitting fibres (31) and along the optical axis predetermined by the transmitting optical device (7 1) in order to transmit light to a receiver (9), whereby light scattered on objects of measurement in the volume for measurement (8) is imaged via the transmitting optical device (71) and an equivalent receiving optical device (72) onto the end of the receiving fibre (32).

3. Apparatus as claimed in Claim 2, characterised in that the receiving optical device (72) has openings for the transmitting beam (6) to pass through.

4. Apparatus as claimed in Claim 1, characterised in that the fibrefixing arrangement (4) and the lens array (5) are produced from a uniform plane parallel plate, the plane of connection (45) being defined by highly precise recesses (43; 55) on the one hand from the front face of the plate for the fixing of the fibres and on the other hand from the rear face of the plate for the lens mounting.

5. Apparatus as claimed in Claim 4, characterised in that the recesses for the lens mounting extend with reduced diameter as far as the plane of connection (45), the laser beams (6) diverging as they emerge from the transmitting fibres (31) into the particular recess (55) and being collimated by the collimating lens (51).

6. Apparatus as -daimed in Claim 5, characterised in that the collimating lenses (5 1) are spherical lenses.

7. Apparatus as claimed in Claim 1, characterised in that the fibrefixing arrangement (4) is rigidly connected to the lens array (5) by means of a layer of 1 cement (54) which is adapted to the refractive index.

8. Apparatus as claimed in Claim 7, characterised in that the collimating lenses (5 1) are glued in the form of plano-convex lenses with their plane surface on the surface of the supporting plate (52) which is at the rear in the path of the light.

9. Apparatus as claimed in Claim 7, characterised in that both the fibrefixing arrangement (4) and the lens array (5) are ground and polished in the plane of connection (45) for them to be cemented, so that in the fibre-fixing arrangement (4) all fibre ends are exactly disposed in a plane and in the lens ariay (5) the optical path length of the assembly consisting of the layer of cement (54), supporting plate (52) and lens cement layer (53) as far as the collimating lens (5 1) is adjustable.

10. Apparatus as claimed in Claim 7, characterised in that the fibrefixing arrangement (4) is a plane parallel plate (42) which is essentially divided in half along a straight line, at least one of the part-plates (47) has notches (48) in the dividing surface which are disposed orthogonally with respect to the plate surfaces and are dimensioned so that the when the part- plates (47) are subsequently joined together and glued the fibres (3L. 32) are secured without play by the part-plates (47).

11. Apparatus as claimed in Claim 10, characterised in that the notches (48) in the dividing surface are rectangular notches (48).

12. Apparatus as claimed in Claim 10, characterised in that the notches (48) in the dividing surface are wedge-shaped notches (48).

13. Apparatus as claimed in Claim 10, characterised in that the notches (48) in the dividing surface are trapezoidal notches (48).

14. Apparatus as claimed in Claim 3 and 8, characterised in that the transmitting fibres (31) exit from an integrated optical chip (21), the input of which is supplied by the fibre-coupled laser diode (1) and on which are provided a junction (22) and a phase modulator (23) for frequency shifting between the two light waves of the transmitting fibres (3 1).

15. Method of producing apparatus for generating parallel collimated beams from a fibre-coupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, characterised in that 1 1 the collimating apparatus is produced in two essential part-processes, the production of a fibre-fLxing arrangement (4) and of a lens array (5), for production of the lens array (5) tubular recesses (55) are made in a plane parallel supporting plate (52) for mounting of collimating lenses (5 1), the positions and the depth of the tubular recesses (55) being adapted to the parameters of the transmitting fibres (3 1) and transmitting optical device (71) which are used, so that the narrowings of the laser beams (6) entering the parallel tubular recesses (55) are brought by the transmitting optical device (7 1) precisely into superimposition in the volume for measurement (8) and due to the uniform depth of the tubular recesses (55) a plane of connection (45) is produced which is parallel to the surface of the plane parallel supporting plate (52) and constitutes the contact surface with the fibre-fixing arrangement (4), in order to produce the fibre-fixing arrangement (4) recesses (43) are made in a plane parallel plate (42) for precise fixing of transmit ting fibres (3 1) coupled to the laser diode (1) and these recesses guarantee a reproducible locking of the end surfaces of the fibres in the plane of connection (45), the positions of the recesses (43) being aligned with respect to the axes of the tubular recesses (55) in such a way that the axes of the transmitting fibres (3 1) coincide with the axes of the tubular recesses (55) and the transmitting fibres (3 1) open into the tubular recesses (55) in the plane of connection (45) in each case.

16. Method as claimed in Claim 15,, characterised in that the fibrefixing arrangement (4) and the lens array (5) are produced from a uniform plane parallel plate, the recesses (43; 55) of the two components being made from different faces of the plane parallel plate and touching one another in a virtual plane of connection (45).

17. Method as claimed in Claim 16, characterised in that the recesses (43) for receiving the fibres (3 1; 32) are in each case preferably constructed as double cylinders, wherein a smaller cylinder diameter which touches the plane of connection (45) only receives the fibre core (33) and with respect to a larger cylinder diameter functions as a stop when the fibres (3 1; 32) are introduced.

18. Method as claimed in Claim 16, characterised in that for production of the recesses (43; 55) a so-called LIGA process is used.

19. Method as claimed in claim 15, characterised in that the fibre-fixing arrangement (4) and the lens array (5) are produced from two different plane parallel plates (42; 52), wherein the two different plates (42; 52) are aligned with one another as regards the axes of the fibres (3 1) and of the tubular recesses (55) in the plane of connection (45) and are glued to one another.

20. Method of producing apparatus for generating parallel collimated beams from a fibre-coupled laser diode which when superimposed in a volume for measurement by means of a transmitting optical device are suitable for illumination of the object in laser Doppler devices, characterised in that the collimating apparatus is produced in two essential part-processes, the production of a fibre-fixing arrangement (4) and of a lens array (5), for production of the lens array (5) 0 0 plano-convex collimating lenses (5 1) are glued onto a plane parallel supporting plate (52), the position and focal length of the said lenses being adapted to the parameters of the incident beam (6) and of the subsequent transmitting optical device (71) so that the narrowings of the laser beams (61) collimated by the collimating lenses (51) are brought to superimposition in the volume for measurement (8) by way of the transmitting optical device (7 1), the surface of the supporting plate (52) lying opposite the glued-on collimating lenses (51) is used as a plane of connection (45) for connecting the lens array (5) to the fibre-fixing arrangement (4), wherein for adjustment of the optical thickness of the assembled supporting plate (52), collimating lens (5 1) and interposed layer of cement (53) which is suitable for the defined superimposition of the laser beam (6) in the volume for measurement (8) the said surface of the plate is ground flat until the necessary thickness of the supporting plate (52) is achieved, for production of the fibre-fixing arrangement (4) through recesses (43) are made in a flather plane parallel plate (42) for highly precise fixing of transmitting fibres (3 1) which are coupled to the laser diode (1), and these recesses guarantee an orthogonal locking of the transmitting fibres (31) relative to the surface of the plate in the recesses (43), the positions of the recesses (43) being aligned with respect to the axes of the collimating lenses (5 1) in such a way that the axes of the transmitting fibres (3 1) coincide with those of the collimating lenses (5 1), a the transmitting fibres (3 1) are in each case pushed into the recesses (43) and then glued, wherein the transmitting fibres (31) penetrate through the plane of connection (45) predetermined by the rear face of the plate and are then glued, and the ends of the transmitting fibres (3 1) which penetrate through the rear face of the plate are abraded to the level of the surface of the plate as plane of connection (45), that the fibre-fLxing arrangement (4) and the lens array (5) are brought into contact with the plate surfaces defined as the plane of connection (45) and are glued, the axes of the fibres (3) being brought into coincidence with those of the collimating lenses (5 1).

21. Method as claimed in Claim 20, characterised in that a further recess (43) to accommodate a receiving fibre (32) is provided centrally between the recesses (43) for the transmitting fibres (3 1).

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22. Method as claimed in Claim 20 or 21, characterised in that the recesses (43) are produced in the fibre-fixing arrangement (4) by making a laser-drilled bore (44).

23. Method as claimed in Claim 22, characterised in that the recesses (43) in the fibre-fixing arrangement (4) are of cylindrical construction.

24. Method as claimed in Claim 22, characterised in that the recesses (43) in the fibre-fixing arrangement (4) are of conical construction.

25. Method as claimed in Claim 24, characterised in that the conical recesses (43) of the fibre-fixing arrangement (4) are constructed with their large diameter greater than the external diameter of the fibres (3 1; 32) and with their small diameter in the plane of connection (45) adapted to the diameter of the fibre cores (3 3), wherein after subsequent glueing the cladding of the fibres (3 1; 3 2) serves as an aid to centring and for traction relief.

26. Method as claimed in Claim 20 or 21, characterised in that the recesses (43) are produced in the fibre-fixing arrangement (4) by dividing the plane parallel plate (42) which is employed perpendicular to the plate surfaces to form two part-plates (47), introducing notches (48) to receive the fibres (3) in at least one of the dividing surfaces of the part-plates (47) and 34 joining together the part-plates (47) after the fibres (3) have been inserted into the notches (48).

27. Method as claimed in Claim 26, characterised in that the notches (48) are made in a wedge shape or trapezoidal and are dimensioned so that when the part-plates (47) are joined together the fibres (3) laid in them are enclosed substantially without play and are additionally glued.

28. Apparatus for generating parallel collimating beams from a fibrecoupled laser diode comprising:

a fibre fixing plate having a connection face in a plane of connection; first And second transmitting fibres fixed in the plate and having ends flush with the connection face and axes orthogonal to the connection face; and a lens array plate having a connection face secured in contact with the fibre fixing plate connection face in the plane of connection and comprising first and second collimating lenses operatively aligned with the respective first and second transmitting fibres.

29. A method of producing apparatus for generating parallel collimating beams- from a fibre coupled laser diode comprising the steps of.. forming first and second recesses in a fibre fixing plate having a connection face in a plane of connection; fixing first and second transmitting fibres in the respective first and second fibre fixing plate recesses; mounting first and -second collimating lenses on a lens array plate on a face opposed to a connection face; and securing the respective connection faces of the fibre fixing plate and the lens array plate in the plane of connection with the respective first and second transmitting fibres in operative alignment with the first and second collimating lenses.

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30. A method as claimed in Claim 29 flirther comprising the step of forming first and second recesses in the lens array plate between the connection face and the opposing face, mounting the collimating lenses over the recess, and securing the fibre fixing plate and the lens array plate in contact with the fibres, recesses and lenses in operative alignment.

31. A method as claimed in Claim 29 or 30 comprising the step of fixing the transmitting fibres in the fibre fixing plate recesses with ends projecting from the connection face and abrading or reducing the projecting ends until they are flush with the connection face.