PRISM OPTICS AND OPTICAL INFORMATION PROCESSING APPARATUS

Abstract: An optical system for converting shape of a light beam by making use of refraction of prisms comprises a prism optic which includes a complex prism (10; 11) constituted by com­ bination of at least two types of prisms (10A, 10B; 11A, 11B) made of materials differing from each other in respect to dispersion of refractivity. An optical information processing apparatus includes the optical system employing the prisms. Geometric optical properties of the optical system is insus­ ceptible to being changed irrespective of changes in wavelength of the light beam brought about by dispersion in oscillation wavelength of a light source, change in the course of time lapse, change in ambient temperature and/or change in emitted light energy. With the optical system, information processing is realized which is insusceptible to the influence of the change in wavelength of the light beam.

Description: The present invention relates to an optical information recording apparatus which is destined to record information on a prescribed recording surface by changing a laser beam corresponding to information to be recorded and which employs a special prism system which is suited for modifying or changing the shape of a light beam having a two-dimensional distribution. Hitherto, two types of optical systems have been used for modifying or varying or converting the cross-sectional shape of a light beam. The first type includes a pair of cylindrical lenses which are disposed confocally. The magnification of an optical system of that type is adjusted in dependence on the mutual ratio of the focal lengths of the cylindrical lenses. An example for an optical system of that type is disclosed in US A 4 203 652. The other type of optical systems employs a triangle prism and uses the difference between the angle of incidence and the output angle of the light beam due to the refraction of the prism for converting the shape of the light beam. An example for an optical system of that second type is disclosed in US A 4 333 173. The present invention is contemplated to improve an optical system of the said second type, and it is for better understanding the special problems to be overcome in that connection that reference is now made to the accompanying drawings. In the said drawings, Fig. 1 illustrates the refraction of a light beam by a triangle prism, in which a light beam is incident on one face of a triangle prism along a direction inclined relative to the one face. Referring to the figure, when the incident angle (i.e. angle of incidence) and the output angle at the boundary between air and prism media are represented by ϑ₁ and ϑ₂, respectively, the following relation applies valid (in accordance with the Snell's law). sin ϑ₁ = n·sin ϑ₂ (1) where n represents the refractive index of the medium or material constituting the triangle prism. Further, magnification ratio M of the beam diameter brought about by the refraction is given by M = D₂/D₁ = cos ϑ₂/cos ϑ₁ (2) where D₁ and D₂ represent the diameters of the incident beam and the output beam, respectively. By making use of the relation mentioned above, modification or conversion of the beam shape can be realized. On other prism face opposite to the inclined one, the light beam is incident or outputted in a direction perpendicular to said other prism face, as will be seen in Fig. 1, so that the ratio of beam diameter generated at the inclined face undergoes no change at said other face. In case the beam leaves the prism in the direction perpendicular to the output face thereof, as described above, there apply valid among the incident angle ϑ₁, apex angle , refractive index n and the magnification ratio of beam diameter M the following relations: provided that M ≧ 1. In this connection, it is noted that when the magnification ratio M of beam diameter is excessively large (e.g. about 3 or more), the reflection factor of the prism at the incident face will become too large to be useful in practical applications. Under the circumstance, with the aim of realizing a greater magnification ratio of beam diameter while evading the above problem, an attempt has been proposed in which a plurality of prisms made of a material exhibiting one and same refractive index are used to constitute a prism system. By way of example, Fig. 2 of the accompanying drawings shows an optical system in which a pair of prisms 1 and 2 are employed. The relations mentioned above are invariable so far as the wavelength of the light beam remains constant. However, when the wavelength is varied, the refractive index of the medium constituting the triangle prism undergoes variation or change, bringing about a corresponding change in the output angle ϑ₂ of the light beam, which in turn exerts significant influence to the devices and instruments used in combination with the optical system, to a great disadvantage. The variation or change in the wavelength of the light beam is ascribable to various causes such as dispersion of wavelength of the light emitted by a light source such as a semiconductor laser, change of the emitted wavelength in the course of time lapse, change in in the ambient temperature, change in the light output energy and the like. US A 2 855 819 discloses a composite achromatic light-deviating prism which is specifically designed to obtain a wavelength independent magnification of white light. For the said purpose the said prism comprises two outer faces opposite each other and a cemented interface disposed therebetween. One of the said three faces has a sawtooth-like profile, and the composite prism has a base defined as that side of the prism toward which light rays entering the prism through one face and leaving through the opposite face are deviated and has an apex defined as the side of the prism opposite the base. The basic idea is to reduce the influence of the chromatic aberration by constituting the prism through bonding a crown glass body and a flint glass body to each other that incident white light is refracted to mutual opposite directions remaining white light. There is no connection, however, to any of the specific problems that arise when a laser beam is used for optical information recording through changing the shape of the said beam, as described above. Considering the prior art described above, it is an object of the present invention to provide an optical information recording apparatus in which there are no changes in the geometric optical properties of the incident light beam independent of changes in the wavelength thereof. The said object according to the present invention is achieved by means of a design of the optical information recording apparatus as defined in the independent claims 1 and 8. Further developments of the invention are defined in corresponding dependent claims. The gist and the advantages of the present invention will be further apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Figs. 1 and 2 are views for illustrating functions of hitherto known optical systems employing prisms; Fig.3 is a view showing an optical prism system designed for being used in an exemplary embodiment of the present invention; Figs. 4 and 5 are views showing optical prism systems to be used in further embodiments of the present invention; Fig. 6 shows a block diagram of an optical disk apparatus employing the prism system of Fig. 3; Fig. 7 is a block diagram showing an arrangement of a laser printer employing the prism system of Fig. 3; Fig. 8 is a block diagram showing an arrangement of an optical disk apparatus employing the prism system of Fig. 5 and Fig. 9 is a block diagram showing an arrangement of a laser printer employing the prism system of Fig. 5. Before in detail entering into a description of the preferred embodiments of the invention, examination will be made on the change in the output angle of a light beam leaving an optical prism system which is brought about in dependence on change or variation in the wavelength of the incident light beam, by taking as an example the optical prism system shown in Fig. 2. Referring to the figure, it is assumed that a pair of prisms having a same refractive index n and respective apex angles  equal to each other (i.e. having the same configuration) are disposed such that the incident angle α₁ of a light beam at the prism 1 is equal to the incident angle δ₁ at the other prism 2 while the output angle is perpendicular to the output face in both of the prisms 1 and 2. From the Snell's law, the following relations apply valid: sin α₁ = n sin α₂ (5) n sin β₁ = sin β₂ (6) sin δ₁ = n sin δ₂ (7) n sin ε₁ = sin ε₂ (8) Further,  = α₂ + β₁ (9)  = δ₂ + ε₁ (10) ϑ = δ₁ - β₂ (11) α₁ = δ₁ (12) When changes of angles α i , β i , δ i and ε i (where i = 1, 2) brought about by a change Δλ in the wavelength are represented by Δα i , Δβ i , Δδ i and Δε i , respectively, differentiation of the equations (5) and (7) results in the following expressions: Through differentiation of the equations (6) and (8) and application of the condition for the perpendicular output of the light beam (i.e. β₁ = β₂ = 0 and ε₁ = ε₂ = 0), following expressions are derived: nΔβ₁ = Δβ₂ (15) nΔε₁ = Δε₂ (16) Further, differentiation of the equations (9), (10) and (11) results in 0 = Δα₂ + Δβ₁ (17) 0 = Δδ₂ + Δε₁ (18) 0 = Δδ₁ - Δβ₂ (19) By substituting the term Δα₂ of the expression (17) by the expression (13) to determine Δβ₁ with the result of determination being placed in the expression (15), Δβ₂ can be determined. On the basis of the determined Δβ₂, the change Δδ₁ is determined in accordance with the expression (14) to thereby determine Δδ₂. Further, on the basis of the determined Δδ₂, the change Δε₁ is determined in accordance with the expression (18), the result being placed in the expression (16) to determine Δε₂ which is then given by the following expression: Now considering the second prism 2, the condition of the perpendicular output is satisfied by  = δ₂. Accordingly, the above expression (20) can be rewritten by taking account of the expression (2) as follows: Δε₂ = Δntan (1 - 1 M ) (21) On the assumption that a glass material commercially available under trade name "SF-11" is used as the medium constituting the prism and that the magnification ratio M equal to 2 is to be realized, Δε₂ will be determined through calculatoin. The refractive index of the prism medium "SF-11" undergoes changes in dependence on changes in the wavelength of the light beam in the manner listed in the following table 1. Wavelength (nm) Refractive Index n 800 1.764710 830 1.763066 860 1.761582 As will be seen, in case the change of wavelength Δλ is equal to 4 nm, the corresponding change Δn of the refractive index amounts to 0.000209. On the other hand, for the magnification ratio M equal to 2 with the refractive index corresponding to the wavelength λ = 830 nm, it is derived from the expressions (3) and (4) that α₁ = δ₁ = 64.56° and that = 30.79°. On the basis of determined Δn,  and M, it is calculated that Δε₂ = 0.00356°(Δλ = 4 nm) from the expression (21). When the optical prism system under consideration is employed, for example, in an optical disk apparatus, change Δε₂ in the output angle brought about by change in the wavelength will make appearance as a positional change or deviation of a light spot focussed on a disk surface. In case a focussing lens having a focal length of 4.5 mm, for example, is employed, the positional change or deviation Δd of the focussed spot due to the change in the output angle is calculated as follows: Δd = f·tan ε₂ = 0.28 µm Such positional change of the focussed spot is undesirable for the detection of focussing deviation and/or tracking deviation because of its influence exerted to the offset. In light of the foregoing, the present invention teaches that a complex prism composed of at least two types of prisms made of materials differing each other in respect to the dispersion of refractivity is used as the prism constituting the concerned optical system to thereby make the output angle of the light beam leaving the optical system be substantially constant or invariable independent of the changes in the wavelength of the light beam. Now, description will first be made on an exemplary embodiment of the complex prism realized by bonding two types of prisms made of materials which differ from each other in the dispersion of refractivity (i.e. change of the refractive index in dependence on change in the wavelength of the incident light beam). Fig. 3 is a view showing an exemplary embodiment of the optical prism system according to the invention, in which two types of prisms are bonded together to constitute a complex prism. Referring to Fig. 3, a reference numeral 10 denotes the complex prism which is constituted by a first prism 10A made of a first material of which refractive index is represented by n₁ and a second prism 10B made of a second material having a refractive index n₂, both prisms 10A and 10B being bonded together along a plane M. In the following description, it is assumed that the light beam travelling in the air impinges on the complex prism 10 at the angle of incidence ϑ₁ and leaves the complex prism in the direction perpendicular to the output or exit face thereof. In order that the output light beams leaves the complex prism in the direction perpendicular to the output or exit face thereof, arrangement must be made such that the angle indicated by α₂ in Fig. 3 undergoes no changes regardless of changes in the wavelength of the light beam. From the Snell's law, the following expressions apply valid: sin ϑ₁ = n₁ sin ϑ₂ (22) n₁ sin α₁ = n₂ sin α₂ (23) and ϑ₂ + α₁ = c (constant) (24) When the changes in the refractive indexes n₁, n₂, angles α₁, α₂, ϑ₂ and others brought about by a change in the wavelength of the incident light beam are represented by Δn₁, Δn₂, Δα₁, Δα₂, Δϑ₂ and so forth, respectively, (with the assumption that the angle ϑ₁ is constant), the change Δn₁ is determined from the expression (22) as follows: Further, from the expression (23), the following expression can be derived. Δn₁sinα₁ + n₁cosα₁Δα₁ - Δn₂sinα₂ = n₂cosα₂Δα₂ (26) Since it is necessary that Δα₂ = 0 for any change in the wavelength, the expression (26) must satisfy the following condition: Δn₁sinα₁ + n₁cosα₁ - Δn₂sinα₂ = 0 (27) Further, differentiation of both sides of the equation (24) results in: Δϑ₂ = -Δα₁ (28) Accordingly, from the expressions (23) and (27), is derived. The relations given by the expressions (29) and (28) can be combined and rewritten as follows: By taking into account the relation represented by the expression (25), the above expression can be rewritten as follows: This expression can again be rewritten as follows: The above expression defines the condition for which the output beam angle undergoes no change irrespective of changes in the wavelength of the light beam. The medium or material which has the refractive index and the dispersion of refractivity which satisfies the above condition is available for use for the reason mentioned below. Assuming, for example, that ϑ₂ = α₁ and taking it into consideration that n₁ n₂, the following relation applies valid. On the basis of data of commercially available glass of optical quality provided by Ohara Optical Glass Manufacturing Co., Ltd., value of Δn₁/Δn₂ can be selected from the range defined below. Thus, it is apparent that the optical system which satisfies the condition given by the expression (32) can be implemented for practical purpose. The structure of the complex prism 10 which is constituted by a pair of prisms 10A and 10B, as described hereinbefore, involves increasing in the number of the refracting faces, which in turn means that loss due to reflection is increased. In order to minimize the loss, the materials constituting the two prisms should be so selected that the condition n₁ n₂ is satisfied at a predetermined wavelength of the light beam. By way of example, in case the first prism 10A of the complex prism 10 is made of optical glass commercially available from Ohara Optical Glass Manufacturing Co., Ltd. in Japan under the trade name "LaK10" and exhibiting the refractive index (n₁) of 1.70889 at the wavelength (λ) of 830 nm while the second prism 10B is made of optical glass material commercially available from the same company under the trade name "SF11" of which refactive index (n₂) is 1.763066 at the wavelength (λ) of 830 nm, wherein angle γ (see Fig. 3) is selected equal to 33.40° or alternatively ϑ₁ = 64.56°, it is determined that ϑ₂ = 31.90°, α₁ = 40.70° and that α₂ = 32.90°. In this case, the changes Δn₁ and Δn₂ in the refractive indexes (i.e. dispersion of refractivity) brought about by the change of the wavelength λ from 800 nm to 860 nm are -0.001759 and -0.003128, respectively, which apparently satisfies the aforementioned condition given by the expression (32). In reality, it has been established that the change of the output angle is only 0.000708° when the wavelength changes from 830 nm to 800 nm, while the change of the output angle is as small as 0.000717° when the wavelength changes from 830 nm to 860 nm. It is apparent that the change of the output angle is reduced by one order of magnitude as compared with that of the hitherto known prism. At that time, the beam diameter is magnified by a factor of 2.19 in the direction in which the output angle is changed. Accordingly, when the light beam of an elliptical cross section having a diametrical ratio of 1:2 is employed, the light beam is modified in the cross-sectional shape to the diametrical ratio of 2.19:2, which approximates to a true circle. Next, description will be made of an optical system in which the complex prism constituted by bonding a pair of prisms made of materials having mutually different dispersions of refractivity is combined with a unitary or simplex prism. Fig. 4 shows an optical system according to another embodiment of the present invention. In the figure, a reference numeral 11 denotes a complex prism constituted by bonding together a first prism 11A having an apex angle A and made of a material having a refractive index n₁ and dispersion of refractivity Δn₁ and a second prism 11B having an apex angle B and made of a material exhibiting a refractive index n₂ and dispersion of refractivity Δn₂. A numeral 12 denotes a third unitary or simplex prism having an apex angle C and made of a material having a refractive index n₃. In the illustrated optical system, the first prism 11A is bonded to the second prism 11B with the second face of the former being in opposition to and in physical contact with the first face of the latter, wherein the light beam is so introduced as to impinge on the first face of the first prism 11A at an incident angle α₁ and transmit through the bonded faces of both prisms to leave the second prism 11B at the second face thereof which is oriented at an angle D relative to the first face of the third simplex prism 3 so that the light beam is incident on the first face of the third simplex prism 12 and exits from the second face thereof at the output angle ε₂. In this connection, it should be mentioned that the apex angles A, B and C of the first, second and third prisms are angles formed between the first and second faces thereof, respectively. In the arrangement mentioned above, it follows from the Snell's law that sin α₁ = n₁ sin α₂ (33) n₁ sin β₁ = n₂ sin β₂ (34) n₂ sin γ₁ = sin γ₂ (35) sin δ₁ = n₃ sin δ₂ (36) n₃ sin ε₁ = sin ε₂ (37) and that A = α₂ + β₁ (38) B = β₂ + γ₁ (39) C = δ₂ + ε₁ (40) D = γ₂ + δ₁ (41) ϑ = α₁ - ε₂ - A + B + C - D (42) When changes of refractive indexes n₁, n₂ and n₃ and the angles α₁, α₂, β₁, β₂, γ₁, γ₂, δ₁, δ₂, ε₁, ε₂ and so forth brought about in accompaniement with a change Δλ in the wavelength of the incident light beam are represented by Δn₁, Δn₂, Δn₃, and Δα₁, Δα₂, Δβ₁, Δβ₂, Δγ₁, Δγ₂, Δδ₁, Δδ₂, Δε₁, Δε₂ and so forth, respectively, with the assumption that the angles A, B, C and D as well as the incident angle α₁ are invariable, the expression (42) can be rewritten as follows in light of the expression (33). That is, 0 = Δn₁sinα₂ + n₁cosα₂·Δα₂ (43) Δn₁sinβ₁ + n₁cosβ₁·Δβ₁ = Δn₂sinβ₂ + n₂cosβ₂·Δβ₂ (44) Δn₂sinγ₁ + n₂cosγ₁ = cosγ₂·Δγ₂ (45) cosδ₁·Δδ₁ = Δn₃sinδ₂ + n₃cosδ₂·Δδ₂ (46) Δn₃sinε₁ + n₃cosε₁·Δε₁ = cosε₂·Δε₂ (47) 0 = Δα₂ + Δβ₁ (48) 0 = Δβ₂ + Δγ₁ (49) 0 = Δδ₁ + Δε₁ (50) 0 = Δγ₂ + Δδ₁ (51) Δϑ = Δα₁ + Δε₂ (52) From the expression (43), it follows By determining Δβ₁ in accordance with the expressions (53) and (48) and rewriting the expression (44) by taking into account the determined Δβ₁, Next, Δγ₁ is determined on the basis of the expression (54) and (49). Then, taking into account the determined Δγ₁, the expression (45) can be rewritten as follows: Further, Δδ₁ is determined from the expressions (55) and (51) and placed in the expression (46), which then can be rewritten as follows: Since it is necessary that Δε₂ = 0 irrespective of the change in the wavelength λ, it follows from the expression (47) that Δn₃sinε₁ + n₃cosε₁·Δε₁ = 0 (57) Accordingly, from the expressions (56) and (57), Thus, it is established that in the arrangement shown in Fig. 4, the output beam angle ε₂ remains unchanged irrespective of the change in the wavelength λ of the light beam so far as the individual angles satisfy the conditions given by the expression (58). It should here be added that the prism system can further be simplified when the three conditions mentioned below are satisfied. Condition 1: Output light beam is parallel with the incident light beam (ϑ = 0). Condition 2: Output light beam leaves the exit face of the third simplex prism in the direction perpendicular thereto (γ₂ = ε₂ = 0). Condition 3: n₁ = n₃ By taking the above conditions into consideration, the expressions (33) to (42) can be rewritten as follows: sinα₁ = n₁ sinα₂ (59) n₁ sinβ₁ = n₂ sinB (60) sinD = n₁ sinC (61) A = α₂ + β₁ (62) B = β₂ (63) C = δ₂ (64) D = δ₁ (65) 0 = α₁ - A + B + C - D (66) The expression (58) also can be rewritten as follows: By combining the expressions (66) and (67) to eliminate A, When A and β₁ are eliminated through combination of the expressions (62), (66) and (67), it follows: In summary, the prism system which can satisfy the aforementioned conditions 1, 2 and 3 and the condition that the change in the output beam angle Δε₂ be zero has to satisfy the relations mentioned below. sin α₁ = n₁ sin α₂ (70a) sinD = n₁ sinC (70b) Since the expressions (70a), (70b) and (70c) are three simultaneous equations including four variables, it is possible to determine values of three variables for a given value of one variable. The prism materials exhibiting the refractive indexes and dispersions of refractivity which meet the conditions metnioned above are commercially available. For example, the first prism 11A of the complex prism 11 may be made of a material commercially available from Ohara Optical Glass Manufacturing Co., Ltd. in Japan under the trade name "LaSF016", while the second prism 11B may be made of a material "SF11" also available from the same company, and the third simplex prism 12 may be made of the material "LaSF016", refractive indexes of these media are listed in the following table 2. LaSF016 SF-11 λ = 830 nm n₁ = 1.760298 n₂ = 1.763066 λ = 800 nm n₁₁ = 1.761312 n₂₁ = 1.764710 λ = 860 nm n₁₂ = 1.759361 n₂₂ = 1.761582 Δn i = n i2 - n i1 Δn₁ = -0.001951 Δn₂ = -0.003128 It is assumed that the magnification ratio M of beam diameter is to be about 2. On the assumption, when n = n₁ (LaSF016), it follows from the expression (3) that  = C = 30.871°. Thus, the apex angle C may be selected equal to 31.00°. Then, from the expression (70b), it is determined that D = 65.042°. By solving the expressions (70a) and (70c) simultaneously for the known values of the angles C and D, it is established that α₁ = 64.913°. Accordingly, from the expression (68), B = -45.933°, while A = -15.063° from the expression (66). A prism system designed in accordance with the values thus obtained is illustrated in Fig. 5. In the prism system shown in Fig 4, the magnification ratio M of beam diameter is given by In the case of the numerical example mentioned above, the ratio M equals 4.1. For solving the expressions (70), one of the variables can be set at an appropriate value. Accordingly, by determing  (= C) for the desired value of M in accordance with the expression (3) and solving the expressions (70) for the value of  now known, it is possible to implement the optical prism system whose output beam angle can remain invariable irrespective of change in the wavelength of the light beam for a given value of the magnification (or diminution) ratio. In the case of the numerical example mentioned above, the change in the output beam angle Δε₂ is only 0.0000640° for the change in the wavelength Δλ = 4 nm. This change of the output beam angle is about 1/40 of that of the prior art optical system, which tells how advantageous the present invention is. In the foregoing description, the complex prism is constituted by bonding together the first and second elementary prism. It is however important to note that so far as the second face of the first prism (11A) is maintained in parallel with the first face of the second prism (11B), utterly same effect can be accomplished even when a single or multiple flat layers of other material are interposed between the second face of the first prism and the first face of the second prism. In that case, the material of the interposed layer or layers should preferably exhibit high transmittivity. Of couse, a layer of air may be interposed between the first and second prisms. Further, it goes without saying that the invention is not restricted to the optical system composed of the complex prism and the simplex prism but may equally be embodied as the optical system constituted by a combination of complex prisms. Further, in the optical system using prism according to this invention, the same operation or function as described above can be obtained even when light is made incident from the direction which is reverse to the above described direction. In the following, description will be made of an optical information processing apparatus to which the optical system according to the invention can be effectively applied. Fig. 6 is a block diagram showing schematically a general arrangement of a write-once type optical disk apparatus according to an embodiment of the present invention. In the figure, a reference numeral 21 denotes a semiconductor laser, 22 denotes a lens system, and 23 denotes a complex prism for shaping the light beam, which prism may be realized in the structure shown in Fig. 3. A numeral 24 denotes a beam splitter, 25 and 27 denote lenses, respectively, 26 denotes an optical disk, and 28 denotes a photo-detector. The optical disk apparatus of the arrangement mentioned above operates in the manner which will be described below. A laser light beam of an elliptical shape in cross section emitted from the semiconductor laser 21 is collimated by the lens 22 to a parallel light beam and introduced to the prism (complex prism) 23 which is characterized in that the output beam from the prism undergoes no change in the output beam angle irrespective of changes in the wavelength of laser light. The light beam leaving the prism 23 is guided to impinge on the optical disk 26 for irradiation thereof through the beam splitter 24 and the lens 25. In the recording mode, information is recorded on the optical disk in the form of pits formed in a recording film deposited on the disk under irradiation of the laser beam. On the other hand, in the case of the reproducing (playback) mode, a reflected light modulated in correspondence to the information as recorded is picked up by the beam splitter 24 and received by the photo-detector 28 to be converted into a corresponding electrical signal. Since the operation described above is essentially same as that of the conventional optical disk apparatus, any further description will be unnecessary. The arrangement of the embodiment mentioned above is particularly effective for such applications where change in the wavelength is brought about due to the change in the power of the semiconductor laser between the reading or reproducing operation and the writing or recording operation as in the case of the optical disk apparatus of write-once and erasable type. Fig. 7 is a block diagram showing schematically an arrangement of a laser printer according to another embodiment of the present invention. In the figure, a reference numeral 31 denotes a semiconductor laser, 32 denotes a lens, and 33 denotes a complex prism for shaping the light beam, which prism may be implemented in such a structure as shown in Fig. 4. A numeral 34 denotes a polygon mirror, 35 denotes a Fϑ-lens, and 36 denotes a photoconductive drum. In the laser printer of the arrangement as illustrated, the laser beam of an elliptical shape in cross section emitted by the semiconductor laser 31 is collimated by the lens 32 to a parallel beam which is then shaped through the complex prism 33 to be used for scanning the photoconductive drum 36 by way of the polygon mirror 34 and the Fϑ-lens 35. In the case of the hitherto known laser printer, there may arise such situation in which change in the wavelength of laser light brings about a change in the output beam angle, involving thus distortions in the patterns and characters imprinted on the photo-conductive drum 36 to a disadvantage. This problem can be solved quite satisfactorily by using the complex prism 33 according to the teaching of the invention. Fig. 8 is a block diagram showing schematically an erasable type optical disk apparatus according to still another embodiment of the invention, in which the prism system of the structure shown in Fig. 5 is employed. In the figure, a numeral 100 denotes a semiconductor laser, 101 denotes a lens, 102 and 104 denote, respectively, first and second optical systems each of the structure illustrated in Fig. 5, 103 denotes an acousto-optic modulator (AOM), 105 denotes a lens, and 106 denotes an optical disk. The optical disk apparatus of the above arrangement operates in the manner described below. Laser light emitted from the semiconductor laser 100 is collimated to a parallel beam by the lens 101 and converted by the first prism system 102 to a light beam of a width or diameter smaller than the effective aperture of the AOM 103 to be subsequently applied to the latter. The prism system 102 characteristically features the insusceptibility to change in the output beam angle irrespective of any change in the wavelength of laser light, as described hereinbefore. The light beam exiting from the AOM 103 is shaped into the beam of a circular shape in cross section by the second prism system 104 and focussed onto the disk 106 by means of the focussing lens 105. In the reproducing mode, the reflected light from the optical disk 106 is picked up by a beam splitter (not shown) interposed between the shaping prism system 104 and the focussing lens 105 to be received by a photo-detector (not shown) for reading out the recorded information. In the case of the conventional optical disk system of this sort, change in the output angle of a beam exiting from a prism due to change in the wavelength of laser light makes appearance in the form of positional change or deviation of the focussed light spot on the optical disk, as described hereinbefore. The prism system according to the invention is very effective for preventing or suppressing such undesirable positional change of the light spot. Additionally, because of the parallelism existing between the incident light beam and the output light beam, structure as well as the optical head can be simplified and facilitated, to another advantage. Fig. 9 is a block diagram showing schematically an arrangement of a laser printer according to a further embodiment of the present invention, in which the prism system of the structure shown in Fig. 5 is employed. In the figure, 31, 32, 34, 35 and 36 denote, respectively, like elements shown in Fig. 7 attached with the same reference numerals, and 200 denotes the beam shaping prism optic. The laser printer operates in substantially same manner as the one shown in Fig. 7. Accordingly, any further description will be unnecessary. As will be now be appreciated from the foregoing description, there can be realized according to the invention the prism optic system which includes a complex prism constituted by combination of at least two prisms made of materials differing in respect to the dispersion of refractivity and which undergoes no appreciable change in the geometric optical properties thereof irrespective of change in the wavelength of the light beam, to great advantage. When the prism optic or system is employed in the optical information processing apparatus, the latter can be made insusceptible to the influence of changes in the wavelength of the light source as employed.

Claims: An optical information processing apparatus for recording information on a prescribed recording surface by changing a laser beam corresponding to information to be recorded, the apparatus comprising a semiconductor laser member (21, 31, 100); a first optical member (22, 32, 101) for rendering parallel the laser beam output from the semiconductor laser member; a second optical member (23, 33) for modifying the cross-sectional shape of the laser beam from the first optical member; a condensing optical member (25, 35, 105) for condensing the laser beam from the second optical member to the prescribed recording surface; wherein the second opical member (23, 33) comprises a complex prism unit (10, 11) constituted by two types of prisms made of materials having mutually different dispersions of refractivity, characterised in that the complex prism unit (10, 11) comprises a first and a second prism (10A, 11A, 10B, 11B) the different dispersions (Δn₁, Δn₂) of refractivity of the first and second prism (10A, 11A; 10B, 11B) satisfy the condition given by where n₁, n₂ are refractive indices at a predetermined wavelength, ϑ₂ is the angle of refraction at the first surface of the first prism when the beam is first incident on the first prism, and the angle of incidence at the first surface of the first prism when the beam is first incident on the second prism and α₁ is the angle of incidence at the second surface of the first prism when the beam is first incident on the first prism, and the angle of refraction at the second surface of the first prism when the beam is first incident on the the second prism. An optical information processing apparatus according to claim 1, characterised in that the complex prism unit (10, 11) is constituted by bonding a second face of the first prism (10A, 11A) and a first face of the second prism (10B, 11B) to each other, the laser beam is incident on a first face of the first prism, transmitted through the second face of the first prism and the first face of the second prism and outputted from a second face of the second prism in a direction perpendicular to the second face of the second prism. An optical information processing apparatus according to claim 1, characterised in that the complex prism unit (10, 11) is constituted by bonding a second face of the first prism (10A, 10B) and a first face of the second prism (10B, 11B9 to each other, the laser beam is incident on a second face of the second prism along a direction perpendicular to the second face of the second prism, transmitted through the first face of the second prism and the second face of the first prism and outputted from a first face of the first prism. An optical information processing apparatus according to one of the claims 1 to 3, characterised in that the first and second prisms (10A, 11A, 10B, 11B) are made of materials exhibiting substantially equal refractive indices (n₁ and n₂) at the predetermined wavelength. An optical information processing apparatus according to one of the preceding claims, characterised in that the prescribed recording surface includes a surface of an optical disk (26, 106) having a recording layer for optically recording information. An optical information recording apparatus according to claim 5, characterised in that an acousto-optic modulator (103) for modulating the laser beam from the second optical member (23, 33) and another optical member (104) for converting a cross-sectional shape of the laser beam from the acousto-optic modulator are provided between the second optical member and the condensing optical member (105). An optical information recording apparatus according to one of the claims 1 to 4, characterised in that the prescribed recording surface includes a surface of a photosensitive drum (36). An optical information processing apparatus for recording information on a prescribed recording surface by changing a laser beam corresponding to information to be recorded, the apparatus comprises: a semiconductor laser member (21, 31, 100); a first optical member (22, 32, 101) for rendering parallel the laser output beam from the semiconductor laser member; a second optical member (23, 33) for converting a cross-sectional shape of the laser beam from the first optical member; a condensing optical member (25, 35, 105) for condensing the laser beam from the scond optical member to the prescribed recording surface; wherein the second optical member (23, 33) comprises a complex prism unit (10, 11) constituted by two types of prisms made of materials having mutually different dispersions of refractivity, characterised in that the complex prism unit (10, 11) is constituted by bonding a second face of the first prism (10A, 11A) and a first face of the second prism (10B, 11B) to each other, and the laser beam incident on the first face of the first prism (10A, 11A) is transmitted through the bonded faces, exits from the second face of the second prism (10B, 11B), impinges on the first face of a third prism (12), and wherein the arrangement satisfies the following condition: where α₁ represents the angle of incidence of the laser beam at the first face of the first prism (10A, 11A), α₂ and β₁ represent, respectively, the angles of the first and second faces of the first prism, β₂ and γ₁ represent, respectively, the angles of the first and second faces of the second prism (10B, 11B), γ₂ represents an angle at which the laser beam exits from the second face of the second prism, δ₁ represents the angle of incidence of the laser beam at the first face of the third prism (12), δ₂ and ε₁ represent, respectively, the angles of the first and second faces of the third prism, ε₂ represents an angle at which the laser beam exits from the third prism (12), A represents an angle formed between the first and second faces of the first prism, Δn i (i = 1, 2, 3) represents dispersions of refractivity of materials constituting the first, second and third prism, respectively, and n i (i = 1, 2, 3) represents refractive indices of the materials, respectively, at the predetermined wavelegth of the laser beam. An optical information processing apparatus according to claim 8, characterised in that the arrangement is such that the condition α₁ - ε₂ - A + B + C - D = 0 is satisfied, where B represents an angle formed between the first and second faces of the second prism, C represents an angle formed between the first and second faces of the first and second faces of the third prism, and D represents an angle formed between the second face of the second prism and the first face of the third prism.

Inventors: ARIMOTO, AKIRA, NAKAMURA, SHIGERU
Applicants: HITACHI KOKI CO., LTD., HITACHI, LTD.
Application year: 1985
Main classification: G02B17
IPC codes: G02B27, G02B17, G11B7