OPTICAL DEVICE WITH PHASE-LOCKED DIODELASER ARRAY

Abstract: An optical device comprising a phase-locked diodelaser array (10) and a collimatorlens (17) is described. By arranging behind the collimatorlens a prism system (30, 34) of at least one prism to broaden the far field radiation pattern in the lateral plane (XY) and a spatial filter (19) to select a favoured mode, the laser radiation can be concentrated into a single, round and diffraction limited spot (V).

Description: The present invention relates to an optical device comprising a phase-locked diodelaser array emitting radiation the far field distribution of which comprises, in the lateral plane through the axis of the emitted radiation and parallel to the p-n junction layer of the diodelaser array, a central lobe and a number of side lobes, a collimator lens and means for converting the radiation from the array into a single radiation beam with circularly symmetrical cross-section and intensity distribution. The invention also relates to an optical apparatus for scanning an information medium, such as an optical disc recorder and/or player or a laser printer, comprising such optical device. The European patent EP-A-0.100.242 discloses such optical device with a phase-locked diodelaser array, which is a special kind of multi-emitter semi-conductor laser. There is an increasing interest in phase-locked diodelaser arrays because they can deliver a considerably higher output power than a single-emitter diodelaser. For many applications it is necessary to focus the output power of all of the emitters into a single circularly and diffraction limited spot with as high efficiency as possible. Such focussing cannot be carried out by a high-aperture objective lens solely, because of the specific radiation pattern of the phase-locked laser array. Such an array with N emitters radiates in N so-called supermodes, and the laser array itself selects that, or those supermode(s) for which its threshold current is minimum. The threshold current is the value of the current perpendicular to the p-n-junction layer at which the laser action starts. A diodelaser array with a specific internal construction will radiate in one supermode. The most interesting modes for practical use are the fundamental, or zero order, single-lobe supermode which has a uniform near field wavefront and the highest order double-lobe supermode which shows a great stability. In the theory of diodelaser arrays the terms "near field distribution" and "far field distribution" are used. The near field phase-or intensity distribution is the distribution at the position of the emitting facet of the diodelaser array, or in a plane conjugate to this facet. The far field phase-or intensity distribution is the result of the interference of the individual beams emanating from the individual slit-shaped emitters of the array. The far field distribution is formed at some distance from the emitting facet. The near- and far field in this application are reserved for those fields in the lateral plane, i.e. the plane through the axis of the radiation pattern and parallel to the p-n-junction layer. The plane perpendicular to said layer and through said axis is the transversal plane. In order to obtain a single circular spot from a phase-locked diodelaser array, it is proposed in the European patent EP-A-0.100.242 to use one or more cylindrical lenses and a spatial filter, in the form of a limiting aperture, arranged in the image plane, or its conjugate, of the laser emitting facet. The spatial filter, which should select one of the lobes is critical as to its dimension and position, because it is arranged in the lateral beam waist plane, where the beam is very small. The use of cylindrical lenses provides some disadvantages. A cylindrical lens is quite critical as to its orientation with respect to the optical axis of the device and to shifting of its central line with respect to said axis. A small tilting or shifting of a cylindrical lens with respect to said axis may influence the optical behaviour of this lens. Moreover, when introducing cylindrical lenses in an optical system to enlarge a beam, the optical path length of this system increases. In the case of two cylindrical lenses the magnification is proportional to the quotient of their focal lengths. When the optical length would be decreased, spherical aberrations would occur. Cylindrical lenses which are corrected for such aberrations are not available for reasonable prices. It is the purpose of the present invention to provide an optical device according to the preamble of this specification which is compact and moreover enables a diffraction limited radiation spot, which is free of aberrations, to be obtained. The optical device is characterized in that said means comprise a prism system for enlarging a selected lobe in said lateral plane and a spatial filter arranged in the far field of the diodelaser array for transmitting only the enlarged selected lobe, said prism system comprising at least one prism which prism has in said lateral plane an entrance face being inclined with respect to the axis of the incident radiation and an exit face being perpendicular to the axis of the emergent radiation. The prism system broadens the beam in the lateral direction so that its lateral width becomes equal to its transversal width. The spatial filter blocks the unwanted lobes and transmits only the enlarged selected lobe. Because of its position in the far field this filter is less critical as to its dimension and position. Besides of its anamorphotic function the prism system also has a de-astigmatism effect. The spherical collimator lens is so arranged and dimensioned that the radiation leaving this lens still has some wavefront-sphericity. As is known (see for instance "Laser Focus/Electro-Optics" Vol. 20, No. 3, March 1984, Newton, Massachusetts, USA: "Specifying Laser Diode Optics" by D. Kuntz), when a divergent, or convergent, beam passes through a prism, this prism will impart astigmatism to the beam. The astimgatic effect of the prism is used to correct for the astigmatism of the diodelaser array. Preferably the optical device may be further characterized in that the the angle opposite said entrance face of said at least one prism is 90 degrees. Such a prism is easier to manufacture than a prism wherein said angle is different from 90 degrees. Moreover the 90 degrees angle simplifies positioning of the prism during assembling the optical device. The spatial filter may be formed by a diaphragm arranged behind the last prism of the prism system. A preferred embodiment of the optical device is however characterized in that the spatial filter is constituted by a black coating, leaving a central circular opening, on the exit face of the last prism of the prism system. Then no separate diaphragm is needed so that the optical unit becomes simpler and easier to manufacture. The same advantages are obtained in an optical device which is characterized in that the spatial filter is constituted by the boundaries of the last prism of the prism system. If the optical device is part of an optical apparatus wherein the diodelaser array radiation should be focussed in a diffraction limited spot, the spatial filter may be constituted by the entrance pupil of the spot forming lens of the apparatus. As stated hereinbefore the prism system may consist of only one prism. However such a prism causes the axis of the emergent beam to have a direction different from that of the axis of the incident radiation. Especially for an optical device to be used in an apparatus for recording and/or reading an optical record carrier this is less appropriate, because in that case it is preferred to accommodate the optical device into a tubular holder with a small cross-section. Preference is therefore given to an optical device which is characterized in that the prism system comprises two prisms whereby the exit face of the last prism is perpendicular to the axis of the radiation incident on the first prism. Then the axis of the beam emanating from the last prism is parallel to that of the incident radiation and the beam enlargment can be greater than in a one prism system, which is of importance when the diodelaser array comprises an increased number of radiating slits and consequently an increased transversal to lateral far field beam dimension quotient. On the other hand using for the same lateral beam enlargment two prisms instead of one means that the enlargment power of each prism can be halved so that less stringent requirements are to be set on alignment and tolerances of the prisms. An embodiment which is even further improved, is characterized in that it comprises four prisms, whereby in the lateral plane the exit faces of the second and fourth prism are perpendicular to the axis of the radiation incident on the first prism. In this embodiment the axis of the emergent beam extends in the direction of the axis of the radiation incident on the first prism. The invention may advantageously be used in an apparatus for optically recording an optical record carrier and comprising an optical head for generating a radiation spot modulated in intensity according to the information to be recorded, because of the relative high recording intensity involved. This apparatus is characterized in that the optical head comprises an optical device as described hereinbefore and an objective lens for focussing the beam from said device into a diffraction limited round spot on the record carrier. The invention may also be used in an apparatus for reading a record carrier and comprising an optical head wherein a radiation spot for scanning the record carrier is generated. This apparatus is characterized in that the optical head comprises an optical device as described hereinbefore and an objective lens for focussing the beam from said device into a diffraction limited round spot on the record carrier. The invention may also be used advantageously in an apparatus for laser printing information on a recording medium comprising a recording medium carrier movable in a first direction and a beam deflector for deflecting a radiation beam along a second direction perpendicular to the first direction. This apparatus is characterized in that it comprises an optical device as described hereinbefore for generating said radiation beam and an objective lens for focussing the beam from said device into a diffraction limited radiation spot on the recording medium. The three mentioned apparatus may be further characterized in that the spatial filter of the optical device is constituted by the entrance pupil of the objective lens. The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows, in perspective view, a semiconductor laser unit, Figure 2a illustrates the grating-like structure of the laser array, Figure 2b represents the far field radiation distribution of the laser array, Figures 3a and 3b show a first embodiment of the optical device according to the invention in a lateral and transversal section respectively, Figures 4a and 4b show a second embodiment of this device in the same sections, Figures 5a and 5b show a third embodiment again in a lateral and transversal section, Figure 6 represents an apparatus for recording an optical disc comprising the device of the invention, and Figure 7 shows a laser printer comprising this device. Figure 1 shows schematically and in perspective view a multi-emitter semiconductor laser unit, a so-called phase-locked diodelaser array. This laser unit comprises, amongst others, a first layer 11 of the p-type material, a second layer 12 of te n-type material and an intermediate layer 13, or p-n-junction layer, wherein the laser action takes place when an electric current above a certain threshold value is sent through the laser unit in the Z-direction. The laser radiation emanates through the first facet, or cleaved surface, 14 of the layer 13 and propagates in the Y-direction. This radiation shows a first, small, divergence in the XY-plane, called the lateral plane, and a second, greater, divergence in the YZ-plane, called the transversal plane. The p-n-junction layer comprises a number of radiation guiding stripes 13₁, 13₂ etc. which are separated from each other by intermediate stripes. For clearness sake in Figure 1 only four guiding stripes are shown but their number may amount to ten or more. As is known, for example from the paper: "Phased Array Diode Lasers" in "Laser Focus / Electro-Optics", June 1984, W. Streifer et al., such phase-locked laser array provides the advantage of a considerably increased power output relative to a single stripe diodelaser. This laser unit is therefore very well suited to record information on an optical medium like an optical disc or the recording medium of a laser printer. The radiation of all of the individual stripes should then be concentrated into a single, round and diffraction limited radiation spot. Besides a combination of a first lens which collimates the radiation and a second lens which focusses the radiation, additional optical elements are needed, because of the specific radiation pattern of a diodelaser array. As is known, for example from the mentioned article in "Laser Focus / Electro-Optics", June 1984 issue, an N-emitter laser array may radiate in N supermodes and a laser array with a specific internal structure selects itself one of the supermodes to minimize its threshold current. The present invention is of special interest for a diodelaser array which radiates in the fundamental, or zero order, supermode. This mode for which the radiations of all of the lasing stripes are in phase is of great interest in practice because its near field phase distribution, or wavefront, is uniform. The near field is the field at the emitting facet 14 of the laser array, which may be regarded as a grating g on which a plane wavefront WF is incident, as shown in Figure 2a. The individual sub-beams of the radiating stripes interfere with each other so as to form an interference pattern, or intensity distribution, in the far field at the entrance pupil of the collimator lens. This pattern is shown in Figure 2b, wherein I is the intensity and ϑ is the angular position in the far field with respect to the optical axis of the system. The far field pattern is given by the product of λ/D multiplied by the radiation pattern of a single emitting stripe. λ is the wavelength of the radiation and D is the period of the grating. The intensity pattern in the lateral plane is composed of a central, main, lobe and a number of side lobes on both sides of the main lobe. The FWHM, i.e. the width of the lobe at the intensity which is 1/e² of the maximum intensity is given by wherein N is the number of radiating stripes. In the transversal plane the far field distribution has only one broad lobe. The main far field lateral lobe has its beam waist, or its single virtual point of origin, situated at a small distance behind the laser facet. A single spot can be obtained when the radiation pattern is expanded such that the side lobes shift outside the optical system and only the main lobe is passed through this system. This can be called spatial filtering in the far field. Masking of the sides lobes can also be regarded as a reduction of the numerical aperture of the spot forming lens, such that the resolving power of this lens is too small to discriminate the intermediate stripes between the radiating stripes of the laser array. By the spatial filtering interference between the main lobe and the other lobes is avoided. Because in the lateral plane the point of origin of the said main lobe is situated behind the laser facet, whereas in the transversal plane this point lies at the laser facet, the lobe, and the laser array radiation as a whole, shows astigmatism in an analog way as a single emitter diodelaser of the gain guided type. In order to obtain a single diffraction limited, aberration free, spot this astigmatism should be eliminated. The laser array is a linear shaped emitter; an array of ten emitters for example measures for example 40 µm x 2 µm. The aperture of the composed beam emitted by the diodelaser array is relative small in the lateral plane and relative large in the transversal plane. Downstreams the emitting facet the composed radiation beam has an elliptical cross-section and for obtaining a round spot the beam should be resphaped so that its cross-section becomes circular. According to the present invention the radiation of a phase-locked laser array is converted into a beam suited for forming a single, round and diffraction limited radiation spot by enlarging the radiation pattern in the lateral plane and by filtering the favoured, for example the fundamental, supermode, whereby the functions of enlarging, beam shaping and astigmatism elimination are performed by one and the same prism system. In a device with a phase-locked diodelaser array radiating in the highest order supermode with two lobes and wherein the adjacent stripes have a phase difference of 180 degrees, in the radiation path a phase plate can be arranged having alternating first and second areas introducing phase shifts of 180 degrees and 0 degrees respectively. Then a radiation pattern with one central lobe instead of two central lobes is obtained. From this radiation pattern a single, round, and diffraction limited radiation spot can be obtained by using the present invention. This invention is not limited to cases wherein a central lobe is selected; it can also be used for diodelaser arrays wherein other radiation modes are selected. Moreover the diodelaser array may be of the index guided- or gain guided type. The Figures 3a and 3b show a first embodiment of the optical device in the lateral and transversal cross-section respectively. In these Figures 10 is the diodelaser array with guiding stripes 13. The composed radiation beam b emitted by the array has an opening angle γ₁ in the lateral plane and a point of origine 0₁ in this plane, whereas in the transversal plane this angle is γ₂ and the point of origin is 0₂. The distance Δy between 0₁ and 0₂ is the astigmatic distance. In the radiation path of the composed beam is arranged a lens 17 such that it nearly collimates the beam in the transversal plane. In the lateral plane however the wavefront of the beam after passage through the lens 17 is more curved, due to the Δy shift of the point of origin 0₁ in this plane. Behind the collimatorlens 17 a prism 20 is arranged. In the transversal plane this prism only slightly affects the beam because the collimated beam is nearly perpendicular to the entrance face 21 and the exit face 22 of the prism. In the lateral plane however the entrance face 23 is arranged at an angle α with the axis h of the beam. At said plane 23 refraction occurs such that the beam is widened, in dependency of the angle α. This means that the far field radiation pattern of the laser array radiation is spread, as is illustrated in the upper part of Figure 3a wherein the far field pattern at different positions along the optical path are shown. Behind the prism 20 a diaphragm 19 is arranged. The opening of this diaphragm and the orientation and parameters of the prism 20 are so adapted to each other that the diaphragm passes for example only the central lobe l₀ of the radiation pattern and blocks all of the sides lobes l₁, l ' / 1 etc. Thus a beam is obtained which has such distribution that it can be concentrated by an objective lens 18 in a single spot V. By adapting the numerical aperture of this lens to the beam cross-section, so that the lens pupil is adequately filled, a diffraction limited spot having minimal dimensions is obtained. Care is taken that the enlargment of the beam in the lateral plane is such that the beam dimension in this plane is equal to that in the transversal plane, so that the beam leaving the prism has a circular cross-section and consequently the spot V is round. Because especially in the lateral plane the beam passing through the skew prism is not collimated, the prism imparts astigmatism to the beam. It can be arranged such that this astigmatism compensates the astigmatism of the diodelaser array, so that the wavefront of the beam from the prism has the same curvature in the lateral and transversal planes. The angle β between the faces 24 and 25 of the prism 20 may have an arbitrary value. In practice it is preferred that this angle is 90 degrees because of easier manufacturing and assembling. These remarks hold also for the other embodiments of the prism system to be described yet. The first of these embodiments is shown in Figures 4a and 4b, in lateral and transversal cross-section respectively, and comprises two prisms 30 and 34. By using a second prism it becomes possible to bend the axis h′ back so that it is parallel to the axis h of the incident radiation. Then the transversal dimension of the device is decreased, which is of importance especially when the device is to be used in an optical disc recorder/player. Moreover in the embodiment of Figures 4a and 4b for each prism less beam-widening power of the prisms is needed. Figures 5a and 5b show an embodiment of the optical device with four prisms 38, 39, 40 and 41. Besides that the beam-widening power needed for each prism is further reduced, the embodiment has the advantage that axis h′ of the emergent radiation is coincident with axis h of the incident radiation, so that the device can be housed in a tubular holder with small cross-section. In the embodiments with two or four prisms the beam-widening power is preferably equally divided over the prisms. The prisms preferably have the same shape and dimensions, which considerably decreases the manufacturing costs of the device. In the embodiments of Figures 3, 4 and 5 the spatial filter may be constituted by a separate element (19) in the form of a diaphragm on a separate holder. It is also possible that such diaphragm is fixed to the exit face, 24 respectively 36, of the last prism in the embodiments of Figures 3 and 4 respectively and of Figure 5. As indicated in Figure 4a it is also possible to apply an opaque coating 42, leaving a central opening, on said faces. Another possibility is that the boundaries of said last prism form a natural stop for the radiation towards the spot-forming lens 18 as indicated in Figure 4a by the interrupted lines 43, 44. Finally in an apparatus wherein the spot-forming lens 18 is arranged close to the last prism 41 of the prism system, the spatial filter may be formed by the pupil of the spot forming lens, as indicated by 45 in Figures 5a and 5b. Figure 6 shows schematically an apparatus for recording information in a surface 51 of an optical record carrier 50. The surface 51 may be provided with pre-recorded servo tracks 52. The phase-locked diodelaser array designated 10 emits a beam b which should be focussed by an objective lens 18 to a high-intensity, round and diffraction limited radiation spot V on the information surface 51. According to the present invention between the laser array 10 and the objective lens 18 a unit 60, comprising a collimator lens a prism system and a spatial filter as shown in Figures 3a and 3b, or Figures 4a and 4b, or Figures 5a and 5b is arranged to convert the radiation form the laser array into a collimated, rotationally symmetrical beam. Recording of an information track is performed by modulating the beam intensity in accordance with the information to be recorded and by rotating the record carrier around its axis 53. By gradually moving the whole optical head with elements 10, 60 and 18, or part of it, in the radial direction of the record carrier, all of the tracks can be successively recorded. Modulation of the recording beam can be performed for example by modulating the electric current through the laser unit. By arranging a beam separator, for example a beam dividing mirror 55, before or behind the block 60, which separator directs part of the beam reflected from the record carrier towards a photosensitive detector 56, the apparatus of Figure 6 can be used for reading the information recorded on the record carrier. During reading the beam has a constant intensity of such level that it cannot provoke any change in the recording surface. Figure 7 shows schematically a laser printer with a recording medium 71, for example wrapped around a drum 70 which, for recording successive lines, is rotated around its axis 72. Line scanning is performed for example by a polygon 73 carrying a number of mirror facets 74 and rotating around its axis 75. 18 is an objective lens which should focus the radiation from a phase-locked diodelaser array 10 and reflected by a facet 74 to a round spot V of small diameter on the recording medium. The laser beam is modulated in intensity according to the information to be recorded, either by the electric current through the laser unit or by a separate modulator in the path of the beam. To convert the composed beam from the laser array, which beam is divergent and has an elliptical wavefront, into a collimated beam with a rotationally symmetrical wavefront, a block 60 comprising a collimatorlens a prism system and a spatial filter as shown in Figures 3a and 3b, or Figures 4a and 4b, or Figures 5a and 5b, is arranged between the laser unit 10 and the objective lens 18. That the invention has been described on the hand of information recording apparatus does not mean that it is restricted to these apparatus. The invention can be used in all apparatus wherein a high-intensity small and round spot should be formed out of the radiation of a phase-locked diodelaser array. As an example can be mentioned a projection TV apparatus with light valves or Liquid Crystal Displays for modulating the laser radiation with (colour) TV information. Also in long-haul fiber optical communication systems or in chirurgical instruments the present invention can be used.

Claims: Optical device comprising a phase-locked diodelaser array (10) emitting radiation the far field distribution of which comprises, in the lateral plane (xy) through the axis of the emitted radiation (h) and parallel to the p-n junction layer (3) of the diodelaser array, a central lobe and a number of side lobes, a collimator lens (17) and means for converting the radiation from the array into a single radiation beam with circularly symmetrical cross-section and intensity distribution, characterized in that said means comprise a prism system (20; 30, 34; 38, 39, 40, 41) for enlarging a selected lobe in said lateral plane and a spatial (19, 45) filter arranged in the far field of the diodelaser array for transmitting only the enlarged selected lobe, said prism system comprising at least one prism which prism has in said lateral plane an entrance face (23) being inclined with respect to the axis of the incident radiation and an exit face (24) being perpendicular to the axis of the emergent radiation. Optical device as claimed in claim 1, characterized in that the angle (β) opposite said entrance face of said one prism is 90 degrees. Optical device as claimed in Claim 1 or 2, characterized in that the spatial filter is constituted by a black coating, leaving a central circular opening, on the exit face of the last prism of the prism system. Optical device as claimed in Claim 1 or 2, characterized in that the spatial filter is constituted by the boundaries of the last prism of the prism system. Optical device as claimed in Claim 1, 2, 3 or 4, characterized in that the prism system comprises two prisms (30, 34) whereby the exit face of the last prism is perpendicular to the axis of the radiation incident on the first prism. Optical device as claimed in Claim 1, 2, 3 or 4, characterized in that it comprises four prisms (38, 39, 40, 41), whereby in said lateral plane the exit faces of the second and fourth prism are perpendicular to the axis of the radiation incident on the first prism. Apparatus for optically recording an optical record carrier (50), comprising an optical head (10, 60) for generating a radiation spot modulated in intensity according to the information to be recorded, characterized in that the optical head comprises an optical device as claimed in any one of the preceding Claims and an objective lens (18) for focussing the beam from said device into a diffraction limited round spot (V) on the record carrier. Apparatus for reading a record carrier (50) comprising an optical head (10, 60) wherein a radiation spot is generated for scanning the record carrier, characterized in that the optical head comprises an optical device as claimed in any one of Claims 1 to 6 and an objective lens for focussing the beam from said device into a diffraction limited spot (V) on the record carrier. Apparatus for laser printing of information on a recording medium (71), comprising a recording medium carrier (70) movable in a first direction and a beam deflector (73) for deflecting a radiation beam along a second direction perpendicular to the first direction, characterized in that it comprises an optical device (10, 60) as claimed in any one of Claims 1 to 6 for generating said radiation beam, and an objective lens (18) for focussing the beam from said device into a diffraction limited radiation spot (V) on the recording medium. Apparatus as claimed in Claim 7, 8 or 9, characterized in that the spatial filter (45) of the optical device is constituted by the entrance pupil of said objective lens (18).

Inventors: ACKET, GERARD ADRIAAN C/O INT. OCTROOIBUREAU B.V., TATSUNO, KIMIO C/O INT. OCTROOIBUREAU B.V., VAN DER POEL, CAROLUS JOHANNES
Applicants: HITACHI, LTD., N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: G02B27
IPC codes: G11B7, G02B27