Optical system for semiconductor laser beam

Abstract: An optical system for a semiconductor laser beam comprises a semiconductor laser which generates both laser light and spontaneous emission in accordance with the amount of electric current applied thereto, a convergent lens disposed in the light path of the beam emitted by the semiconductor laser, and an optical element disposed in the light path of the beam from the convergent lens which cuts more spontaneous emission than laser light.

Description: This invention relates to a method of operating an arrangement comprising a semiconductor laser more particularly, the present invention relates to a method of operating an arrangement comprising a semiconductor laser which enables the laser beam to be converged to a spot of small diameter over a broad output range of the semiconductor laser. US-A-3,663,087 discloses a device for improving the homogeneity of a laser light beam. A Faraday rotator and two polarizers are placed in the laser light beam. This patent does not disclose the use of polarizers in a semiconductor laser beam. JP-A-59 205 785 discloses a device in which the beam of a semiconductor laser is transmitted in a reflecting body. The reflected light is converged to the end surface of a resonator in the semiconductor laser element. An interference film filter is used to determine the rate of reflected rays to transmitted rays. The interference filter is not selective of the wavelength of the laser light. EP-A-0019 778 discloses a laser beam optical system wherein the beam has a guassian cross sectional profile. A focusing lens and a stop having an aperture are arranged in the laser beam whereby the outer portion of the stop stops the outer portion of the laser beam. By this arrangement, the focal range of the laser beam is influenced. This device does not use a semiconductor laser which generates laser light and spontaneous emission. From the Article "Irradiance uniformity and power loss with a spatially filtered laser beam," B. J. Pernick, Rev. Sci. Instrum., Vol. 45, No. 11, November 1974, page 1344, a device is disclosed in which a gaussian laser beam is expanded and then the outer edge region is discarded. In this device, no semiconductor laser generating laser light and spontaneous emission is disclosed. Semiconductor lasers fabricated using semiconductor chips are used for generation of scanning beams in different kinds of equipment such as optical scanners for reading and writing of information. The semiconductor laser has numerous advantages over other types of lasers, including the gas laser. Aside from being more compact and less expensive, it also consumes less power and its output can be modulated by control of the driving current supplied thereto, in what is referred to as direct analogue modulation. The semiconductor laser is especially convenient for use in an optical scanning device for writing information since it allows the aforesaid direct modulation to be conducted using a signal generated in accordance with the image information to be recorded. In this connection, however, it is known that the light produced by a semiconductor laser includes both laser light spontaneous emission. The relation between the amounts of these two kinds of light and the amount of current applied to the semiconductor laser will be explained with reference to Figure 4. In the graph, the line a represents the relation between the driving current and the spontaneous emission output, while the line b represents that between the driving current and the laser light output. As will be noted, no laser light is produced until the driving current reaches a threshold current value Io, so that up to this current value, only spontaneous emission is generated. While the amount of spontaneous emission output increases gradually with increasing driving current, once the driving current has gone beyond the threshold value Io and the amount of laser light output becomes large, the spontaneous emission comes to account for such a small percentage of the total light output that for all intents and purposes the output can be considered to consist solely of laser light. In the graph, the line c shows the relationship between the amount of driving current and the total light output (i.e. the combined spontaneous emission and laser light output) of the semiconductor laser. On the other hand, as disclosed for example in United States Patent Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, the applicant previously proposed a radiation image recording and reproduction system in which recording, read-out and reproduction of radiation images are conducted using a stimulable phosphor sheet as the recording medium. In this system, since the images to be recorded and reproduced have a wide density range, it is necessary in recording the images on the recording medium to modulate the recording beam over a broad dynamic range of 1 : 100 - 1000. Thus when the light source used for producing the recording beam in the radiation image recording and reproduction system is a semiconductor laser that is directly analogue modulated, it becomes necessary to use the semiconductor laser even in the low output region where the influence from the spontaneous emission produced is large. Differently from the laser light, however, the spontaneous emission includes a mixture of various angular components. Moreover, as in the case of a vertical multimode semiconductor laser, for example, the spontaneous emission consistes of spectral components ranging over about 40 nm, in contrast to the spectral range of approximately 2 nm for the laser light. As a result, it is impossible using a convergent lens to converge the spontaneous emission to a spot of such small diameter as is possible in the case of the laser light. Therefore, when the semiconductor laser is used both in the high output region where the laser light dominates and in the low output region where the spontaneous emission dominates, there arises a problem of degraded spatial resolution in the scanning operation. Further, the semiconductor laser beam source equipped with a semiconductor laser is generally provided with a collimator lens, convergent lens or the like. Thus when, for example, there is used a multimode semiconductor laser which produces spontaneous emission having a spectral range extending over 40 nm, as against a 2 nm range for the laser light component, the spontaneous emission is apt to be affected by the chromatic aberration of these lenses, making it difficult to converge the light to a spot of small diameter without employing expensive lenses that have been precision compensated for chromatic aberration. It is the object of the invention to provide a method of operating an arrangement comprising a semiconductor laser which reduces the influence of the spontaneous emission produced by the semiconductor laser, expands the output range in which the influence of the laser light produced thereby is large and the influence of chromatic aberration by lenses of the optical system is avoided, thereby enabling light produced in the low output region to be converged to a spot of small diameter. A first method according to the present invention allows the operation of an arrangement comprising a semiconductor laser including a semiconductor laser chip, a convergent lens disposed in the light path of the beam emitted by the semiconductor laser, and a polarizing filter which passes only light polarized in the direction parallel to the junction plane of the semiconductor laser chip, wherein the driving current of the semiconductor laser is controlled over a broad dynamic range from a low output region where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. It is known that the laser light which a semiconductor laser emits is linearly polarized in the direction parallel to the junction plane of the semiconductor laser. In contrast, the light within the spontaneous emission region is randomly polarized. Therefore, when a polarizing filter of the aforesaid characteristics is provided, almost all of the laser light is passed by the polarizing filter while only about one-half of the spontaneous emission generated by the semiconductor laser is passed thereby. Thus, when both laser light and spontaneous emission are incident on the polarizing filter, about one-half of the spontaneous emission will be cut upon passing through the polarizing filter and, as a result, the ratio of the amount of laser light to the total amount of light will increase. Therefore, the dominance of the laser light will extend into a lower region of the light output than heretofore, making it possible to obtain a spot of adequately small diameter even when light in the low output region is used. This is particularly advantageous in a recording optical scanner in which it is necessary to conduct high-precision scanning using a beam that is modulated over a wide dynamic range. A second method according to the present invention allows the operation of an arrangement comprising a semiconductor laser, a convergent lens disposed in the light path of the beam emitted by the semiconductor laser, and an interference filter which selectively passes only light of wavelengths within a wavelength region that includes the wavelength region of the laser light and wavelengths in the vicinity thereof, wherein the driving current of the semiconductor laser is controlled within the range from a low output region where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. With this arrangement, the interference filter disposed in the path of the light emitted by the semiconductor laser passes nearly all of the laser light but cuts a major portion of the spontaneous emission. Thus, when both laser light and spontaneous emission are incident on the interference filter, most of the spontaneous emission will be cut upon passing through the interference filter and, as a result, the ratio of the amount of laser light to the total amount of light will increase. Therefore, the dominance of the laser light will extend into a lower region of the light output than heretofore, making it possible to obtain a spot of adequately small diameter even when light in the low output region is used. Moreover, since the spontaneous emission which passes through the interference filter is almost all of wavelengths within or in the vicinity of the laser light wavelength region, it is almost totally unaffected by the chromatic aberration of the convergence lens, making it possible to obtain a spot of adequately small diameter at the point of convergence using a relatively low-cost lens which has not been specially compensated for chromatic aberration. Further research carried out by the applicant toward achieving the aforesaid objects led to the discovery that it is possible to converqe even the spontaneous emission to a spot of a diameter remarkably smaller than has been possible in the past by providing in the path of the light emitted by the semiconductor laser a light control plate having an aperture located so as to pass only the central portion of the light (the laser light and the spontaneous emission). It was also found that the smaller the aperture of the light control plate is made and consequently the less the amount of light passed, the smaller becomes the diameter of the spot to which the spontaneous emission can be converged. In this connection, however, although the diameter of the spot formed by the converged spontaneous emission can be reduced by reducing the size of the aperture, as such reduction in aperture size also results in a reduction in the amount of light passing through the light control plate, it is necessary to determine the size of the aperture in consideration of the maximum permissible diameter of the spot of converged light and the minimum permissible amount of light. The relationship between the aperture and the diameter of the spot holds only within a predetermined range. A third method according to the present invention allows the operation of an arrangement comprising a semiconductor laser, a convergent lens disposed in the light path of the beam emitted by the semiconductor laser and a light control plate which has an aperture which passes only the central portion of the light emitted by the semiconductor laser, each of the longitudinal and transverse magnitudes of the aperture of the light control plate being independently set at a value of 0.2 to 1.5 of the full width of half maximum (FWHM) of the laser light intensity distribution, wherein the driving current of the semiconductor laser is controlled within the range from a low output region where the spontaneous emission region to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. It is known that the laser light component of the light emitted by a semiconductor laser spreads out over a certain angular range to form a beam of elliptical cross section. Therefore, using the aforesaid value of the amount of laser light passed as a reference, it is possible to determine the proper size of the aperture of the light control plate. As will be explained in greater detail later, where the size of the aperture is set at 0.2, the transmission rate of the light control plate becomes 20 %, and where it is set at 1.5, the diameter of the spot of the converged spontaneous emission relative to the diameter of the spot of the converged laser light taken as 1 becomes 1.6 and the transmission rate becomes nearly 100 %. Therefore, in accordance with this embodiment of the invention, it becomes possible to maintain the diameter of the spot of converged spontaneous emission at a relative value of not more than 1.6 with respect to the diameter of the laser beam spot while maintaining the amount of the laser light emitted by the semiconductor that is utilized at not less than 20 %. Provision of the light control plate further has the effect of reducing the beam diameter and thus increasing the depth of focus, and also the effect of eliminating the influence of any slight variance that may be present in the propagation of the laser light, whereby the a stable, constant spot can be obtained. The use of the light control plate is also advantageous from the point that it is inexpensive to fabricate and the point that, because of the relatively wide range of tolerance regarding its positioning, it is easy to install. As a result, the practical value of the embodiment of the invention provided with the light control plate is very high. A fourth method according to the present invention allows the operation of an arrangement comprising a semiconductor laser, a collimator lens disposed in the path of the light emitted by the semiconductor laser, a dispersing optical element provided in the path of the light which has passed through the collimator for angularly changing the path of the light issuing therefrom in accordance with the wavelength of the light incident thereon, a convergent optical element disposed in the path of the light issuing from the dispersing light element, a spatial filter disposed in the focal plane of the convergent optical element which spatial filter selectively transmits or rejects only a predetermined wavelength component of the light converged by the convergent optical element a collimator optical element disposed in the path of the light of the predetermined wavelength from the spatial filter for converting the light incident thereon to a parallel light beam, and a reverse-dispersing optical element disposed in the path of the light which has passed through the collimator element (34) for subjecting the light incident thereon to dispersion opposite to that by the dispersing optical element (33,43), wherein the driving current of the semicoductor laser (1) is controlled over a broad dynamic range from a low output region, where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. As was mentioned earlier, even in the case where the semiconductor laser is a multi-vertical mode laser, the spectral width of the emitted laser light is no more than 2 nm while that of the generated spontaneous emission has a spectrum extending over 40 nm. Thus, in the aforesaid arrangement, by making the path of the light having the same wavelength band as the laser light (namely all of the laser light and a part of the spontaneous emission) different from that of the paths of the other light and by using the spatial filter to separate out the light having the same wavelength band as the laser light, it is possible to eliminate all components of the spontaneous emission differing in wavelength from the laser light so that, particularly at the beginning of laser light emission, it is possible to shift the ratio between the amount of laser light and the amount of spontaneous emission such that the proportion accounted for by the laser light is increased. As a result, the laser light will dominate down into a lower output region than in the past and, moreover, since only the light components having wavelengths in the wavelength region of the laser light remain, it become possible to obtain a spot of adequately small diameter even when using light in the low output region. Also, the light extracted by the dispersing optical element and the spatial filter and converged by the convergent optical element is further finely dispersed within its wavelength band by the spatial filter. Therefore, after the extracted light has been passed through the collimator element and converted into a beam of parallel light, it is further reverse-dispersed by the reverse-dispersing optical element to have the dispersion compensated for, whereby there is obtained a single parallel beam. In the following, the invention is described in more detail by the drawings. Figure 1 is a side view of a first optical system for a semiconductor laser beam operated by the first method according to the present invention. Figure 2 is a graph showing how the relation between driving current and semiconductor laser output in the case where a polarizing filter is provided compares with the same relation in the case where one is not provided. Figure 3 is a graph showing how the relation between the amount of light and the diameter of the spot of converged light in the case where a polarizing filter is provided compares with the same relation in the case where one is not provided. Figure 4 is a graph showing the relation between the driving current of a semiconductor laser and the spontaneous emission and laser light outputs thereof. Figure 5 is a side view of a second optical system for a semiconductor laser beam operated by the second method according to the present invention. Figure 6 is a graph showing the spectra of laser light and spontaneous emission as well as the spectral transmission rate characteristics of an interference filter. Figure 7 is a graph showing how the relation between the semiconductor laser driving current and the amount of light after passage through the interference filter compares with the relation between the amount of driving current and the amount of light before passage through the interference filter. Figure 8 is a graph showing how the relation between the amount of light and the diameter of the spot of converged light in the case where an interference filter is provided compares with the same relation in the case where one is not provided. Figure 9 is a side view of a third optical system for a semiconductor laser beam operated by the third method according to the present invention. Figure 10 is a graph showing the angular distribution of laser light intensity. Figure 11A and 11B are schematic views showing example configurations of the aperture of a light control plate. Figure 12 is a graph showing the relation between the light control plate aperture width and each of the relative spot width and the light control plate transmission rate. Figure 13 is a graph showing how the relation between the light output and the light spot in the case where a light control plate is provided compares with the same relation when one is not provided. Figure 14 is schematic perspective view of a fourth optical system for a semiconductor laser beam operated by the fourth method according to the invention. Figure 15 is a plan view of an important part of the optical system of Figure 14. Figure 16 is a graph showing how the relation between the driving current and the output of the selectively extracted light in the optical system of Figure 14 compares with the same relation in the conventional optical system. Figure 17 is a schematic view showing a convergent diffractive grating used in another optical system according to the invention. Figure 1 shows a side view of a first optical system for a semiconductor laser beam operated by a first method according to the present invention. The optical system comprises a semiconductor laser 1 consisting of a semiconductor laser chip 3 sealed within an airtight case 2. When an electric current is applied to the semiconductor laser 1 it produces light in an amount depending on the amount of applied current. The front face of the case 2 is formed as an emission window 2a made of glass or the like and the light emitted through the emission window 2a is converted into a parallel beam by a collimator lens 4 disposed in the light path of the emitted light. This parallel beam advances through a convergent lens 5 to be converged at convergence point A. Between the collimator lens 4 and the convergent lens 5 there is further provided a polarizing filter 6 which passes only light which is polarized in the direction parallel to the junction plane 3a of the semiconductor laser chip 3 (i.e. in the direction perpendicular to the drawing sheet in Figure 1). As mentioned above, the semiconductor laser 1 generates laser light and spontaneous emission dependent on the electric current applied thereto and as the light is linearly polarized in the direction parallel to the junction plane 3a nearly 100 % thereof is passed by the polarizing filter 6. On the other hand, since the spontaneous emission produced is randomly polarized only about one-half of this light impinging on the polarizing filter 6 is passed thereby. Therefore, the relation between the amount of light Po impinging on the polarizing filter 6 and the amount of light P passed by the polarizing filter 6 can be expressed as Po > P. The relation between the amount of current applied to the semiconductor laser 1 and each of the light amounts Po and P is shown by the graph in Figure 2. In the graph, the solid line curve shows how the light amount Po varies with the amount of current applied, while the single dot chain line shows how the light amount P varies therewith. Since as mentioned above the polarizing filter operates to selectively cut a part of the spontaneous emission, in the region where the driving current is below the threshold current Io at which lasing begins, the value of P is about one-half that of Po. Further, during the interval from the time that the current value reaches Io and generation of laser light begins and the time that the amount of laser light reaches a level at which the influence of the spontaneous emission becomes negligible (the interval within the circle in Figure 2), the ratio of the amount of laser light to the amount of spontaneous emission becomes larger. Therefore when the light which has passed through the polarizing filter is used, the laser light comes to dominate into a lower light output region than in the case where the light is not passed through the polarizing filter. As a result, when the light which has passed through the polarizing filter is used, it becomes possible to adequately converge the light generated in the low light output regions at the convergence point A, and to do so even with respect to the light in low light output regions which could not heretofore be adequately converged to a beam spot at the convergence point A. The effect of this inventive method will now be explained with respect to a specific example. As the semiconductor 1 there was used a Mitsubishi ML4402A semiconductor laser, as the collimator lens 4 there was used one with a wavefront aberration of not more than λ/10 at wavelengths of 760 - 810 nm, and as the convergent lens 5 there was used a laser scanning fϑ with a wavefront aberration of not more than λ/8 at wavelengths of 760 - 810 nm. As the polarizing filter 6 was used a polarizing beam spitter with a polarization ratio of 1000 : 1. Using the optical system constituted with the foregoing components, the relation between the diameter of the spot of converged light obtained at the scanning position A and the amount of light after passage through the polarizing filter was investigated. The results obtained are represented by the solid line curve in Figure 3. The broken line in the same figure represents the results obtained when for sake of comparison the polarizing filter was removed from the aforesaid optical system. The lasing threshold of the semiconductor laser used was about 0.2 mW. In the graph of Figure 3, the focused spot diameter is represented as a relative diameter with respect to the spot diameter at an output of 3 mW taken as 1, it being understood that at an output of 3 mW the laser output is adequately large and the spot diameter adequately small regardless of whether the polarizing filter is used or not. It was found that when the polarizing filter was provided, it was possible to reduce the influence of the spontaneous emission and to keep the diameter of the focused spot small in the low output region, as compared with the case where one was not used. While in the aforesaid embodiment the polarizing filter is disposed between the collimator lens and the convergent lens, it can of course alternatively be disposed between the semiconductor laser and the collimator lens, or behind the convergent lens. Figure 5 shows a second optical system for a semiconductor laser beam operated by a second method according to the present invention. When an electric current is applied to a semiconductor laser 11, it produces light in an amount proportional to the amount of applied current. The light emitted by the semiconductor laser 11 is converted into a parallel beam by a collimator lens 12 disposed in the light path of the emitted light. This parallel beam then enters an interference filter 13 which selectively passes only light of specific wavelengths. In response to the application of electrical current, the semiconductor laser 11 generates both laser light and spontaneous emission, the spectra of which are shown in Figure 6. As will be noted from the line d , the spectrum of the laser light extends over a 2 nm range having its center at 785 nm, while as shown by the curve c , the spontaneous emission has a spectrum extending over about 40 nm. The interference filter 13 which receives the laser and spontaneous emission with the wavelength characteristics just described has spectral transmission rate characteristics as shown by the single dot chain line in Figure 6. Specifically, the interference filter selectively passes light of wavelengths within a wavelength region that includes the wavelength region of the laser light and wavelengths in the vicinity thereof. It thus passes nearly 100 % of the laser light. On the other hand, of the spontaneous emission, only that part in the wavelength band near the wavelength of the laser light is passed by the interference filter 13 while all other parts are cut thereby. Therefore, the relation between the amount of light Po impinging on the interference filter 13 and the amount of light P passed by the interference filter 13 can be expressed as Po > P. The relation between the amount of current applied to the semiconductor laser 11 and each of the light amounts Po and P is shown by the graph in Figure 7. In the graph, the solid line curve shows how the light amount Po varies with the amount of current applied, while the single dot chain line shows how the light amount P varies therewith. Since as mentioned above, the interference filter operates to selectively cut a part of the spontaneous emission, in the region where the driving current is below the threshold current Io at which lasing begins, the value of P is much smaller than that of Po. Further, during the interval from the time that the current value reaches Io and generation of laser light begins and the time that the amount of laser light reaches a level at which the influence of the spontaneous emission becomes negligible (the interval within the circle in Figure 7), the ratio of the amount of laser light to the amount of spontaneous emission becomes larger. Therefore when the light which has passed through the interference filter is used, the laser light comes to dominate into a lower light output region than in the case where the light is not passed through the interference filter. The light which has passed through the interference filter 13 passes through a convergent lens 14 as shown in Figure 5 to be converged at a convergence point A. In this embodiment, since the influence of the spontaneous emission is reduced by the interference filter 13, it becomes possible to adequately converge the light generated in low light output regions at the convergence spot A, and to do so even with respect to the light in low light output regions which could not heretofore be adequately converged to a beam spot at the convergence point A. Lenses such as the collimator lens and the convergent lens used in the aforesaid optical system generally have chromatic aberration. Thus there is the problem that if light having a broad wavelength band is passed through these lenses, the beam spot formed at the convergence point tends to become large. In the optical system according to the present invention, however, the interference filter 13 serves to extract only light of predetermined wavelengths so that it is possible to avoid any problems arising from the chromatic aberration of the lenses. Therefore, in accordance with this embodiment it is possible to realize a spot of small diameter at the convergence point without need to employ expensive lenses which have been specially compensated for chromatic aberration. As will be understood from the foregoing, in second method of the invention a part of the spontaneous emission is cut so as to expand the region in which the influence of the laser light is large and also so as to limit the wavelength band of the utilized light. Using the optical system with these features, the relation between the diameter of the spot of converged light obtained at the scanning position A and the amount of light after passage through the interference filter was investigated. The results obtained are shown by the solid curve in Figure 8. The single dot chain line in the same figure represents the results obtained when for sake of comparison the interference filter was removed from the aforesaid optical system. In the graph of Figure 8, the focused spot diameter is represented as a relative diameter with respect to the spot diameter (defined as a diameter of 1.0) within the region wherein the laser output is adequately large and the spot diameter adequately small regardless of whether the interference filter is used or not. It was found that when the interference filter was provided, it was possible to reduce the influence of the spontaneous emission and to keep the diameter of the focused spot small in the low output region, as compared with the case where one was not used. While the results shown in Figure 8 were obtained using an interference filter exhibiting a half-width of about 5 nm, the diameter of the spot in the low light output region can be made smaller in proportion as the half-width centered on the wavelength of the laser light is made smaller. Therefore, the half-width of the interference filter should be appropriately selected based on the cost of producing the interference filter, the wavelength band of the light to be used, the range of permissible light spot diameters and the like. While in the aforesaid method the interference filter is disposed between the collimator lens and the convergent lens, it may of course alternatively be disposed between the semiconductor laser and the collimator lens, or behind the convergent lens. Figure 9 is a side view of a third optical system for a semiconductor laser beam operated by a third method according to the invention. When an electric current is applied to a semiconductor laser 21, it produces an amount of light 21A proportional to the amount of applied current. The light 21A emitted by the semiconductor laser 11 is converted into a parallel beam by a collimator lens 22 disposed in the light path of the emitted light. This parallel beam then impinges on a light control plate 23 having a centrally located aperture 23a which passes only the central portion 21A′ of the light 21A emitted by the semiconductor 21. In response to the application of electrical current, the semiconductor laser generates both laser light and spontaneous emission. As shown in Figure 10, the laser light exhibits a specific intensity distribution within a prescribed range of emission angles, the intensity being maximum at the center of the angular emission range. The emitted laser light spreads out to different angles in the longitudinal and transverse directions and thus forms a beam having an elliptical cross section. As a result, the curve 1 in Figure 10 has different slopes in the longitudinal and transverse directions. On the other hand, differently from the laser light, the spontaneous emission includes a variety of different angular components and is emitted in directions in which the laser light is not. In each of the longitudinal and transverse directions, the magnitude of the aperture 23a of the light control plate 23 which results in passage through the aperture of the laser light which, in the intensity distribution shown in Figure 10, ranges between the maximum intensity of L and an intensity L/2 equal to one-half the maximum intensity is defined as 1 and the dimensions of the aperture are selected to fall within the range of 0.2 to 1.5 of FWHM. The specific configuration of the aperture 23a can be freely selected, the only condition being that it must permit passage of an amount of the light at the center of the beam falling within the range defined above. The aperture may, for example, have a rectangular shape as shown in Figure 11A or an elliptical shape as shown in Figure 11B. The light which passes through the aforesaid light control plate 23 enters a convergent lens 24 and is converged at a prescribed convergence point A. When the light control plate 23 is not disposed in the path of the light emitted by the semiconductor laser 21, the relation between the output of the semiconductor laser 21 and the diameter of the focused spot becomes as represented by the solid line curve in Figure 13. In the graph of Figure 13, the vertical axis represents the spot diameter relative to the spot diameter at an output of 3 mW taken as 1, the light produced at an output of 3 mW being substantially constituted of laser light. As can be seen from Figure 13, when the light control plate is not provided, the diameter of the focused spot becomes very large in the low output region where the output consists solely of, or is dominated by, spontaneous emission. In the method according to the present invention, however, the effect of the light control plate 23 ensures that the diameter of the spot is maintained small in the low output region. In the case where the light control plate 23 is used, if the focused spot diameter at a semiconductor laser output of 3 mW is defined as d o and the spot diameter at a semiconductor laser output of 0.02 mW (a value at which the spontaneous emission dominates) is defined as d, then the relationship between the size of the aperture 23a of the light control plate 23 and the relative value d/d o becomes as shown by the solid line curve in Figure 12. It should be noted that the aperture width shown in Figure 12 is represented relative to the aforesaid reference magnitude taken as 1. As can be seen from Figure 12, the smaller the aperture width, the smaller becomes the relative diameter of the focused spot. Where the aperture width is not greater than 1.5, the relative diameter of the focused spot is 1.6 or less, which represents an improvement over the prior art. Moreover, the percentage of the light passing through the aperture decreases as the aperture becomes smaller. The relation between the size of the aperture and the ratio of the proportion of the total amount of light impinging on the light control plate to the amount thereof passing through the light control plate is represented by the single dot chain line curve in Figure 12. As is clear from this figure, insofar as the aperture width is 0.2 or larger, the transmission rate of the light control plate is 20 % or higher, a range which is permissible depending on the application. It should be noted that the aperture width can be different in the longitudinal and transverse directions and that it can be freely selected within the range of 0.2 to 1.5 in accordance with the low output range to be utilized and the required degree of precision in beam focusing. By way of example, the longitudinal and transverse dimensions of the aperture 23a of the light control plate 23 were both set at 0.5 and the relation between the semiconductor laser output and the relative diameter of the focused spot as based on the spot diameter at an output of 3 mW was investigated. As can be seen from the results shown by the broken line in Figure 13, the diameter of the focused spot in the low output region was remarkably smaller than that of the prior art. While in the method described in the foregoing, the light control plate is disposed between the collimator lens 22 and the convergence lens 24, it can be disposed at any position in the light path between the semiconductor laser 21 and the convergence point A. It is further possible to provide the light control plate 23 internally of the collimator lens 22 or the convergence lens 24. Also, while it is necessary that both the longitudinal and transverse dimensions of the aperture be within the aforesaid range of 0.2 to 1.5, it is obvious that the range of absolute dimensional values of the aperture which satisfy this condition will vary depending on the position at which the light control plate is disposed. Figure 14 is a schematic perspective view of a fourth optical system for a semiconductor laser beam operated by a fourth method according to the invention and Figure 15 is a plan view of an important part of the same. When an electric current is applied to a semiconductor laser 31, it produces light in an amount proportional to the amount of applied current. The light emitted by the semiconductor laser 31 is converted into a parallel beam by a collimator lens 32 disposed in the light path of the emitted light. This parallel beam then enters a prism 33 serving as a dispersing optical element which changes the path of the light issuing therefrom in accordance with the wavelength of the light incident thereon. In response to the application of electrical current, the semiconductor laser generates both laser light and spontaneous emission. The spectrum of the laser liaht extends over 2 nm, while the spontaneous emission has a spectrum extending over about 40 nm. As a result, the laser light passing through the prism 33 substantially follows a single light path while the spontaneous emission passing therethrough follows a number of different light paths in accordance with the wavelength components thereof. More specifically, when the parallel rays of light of various wavelengths enter the prism 33 as shown in Figure 15, the rays of laser light, which have a wavelength of λ₁, follow the light path indicated by the solid lines in the figure, while the rays of a light of a wavelength λ₂ taken as one example of light of a wavelength different from that of the laser light follows the light path indicated by broken lines. The light rays issuing from prism 33 in various directions are converged by a convergent lens 34 disposed in the light path to serve as a convergent optical element. As a result, the rays are converged at different points according to their wavelengths. A slit mirror 35 serving as a spatial filter and capable of selectively extracting only light of wavelengths in the vicinity of λ₁ is provided at the position where the rays converge. As shown in the figures, the slit mirror 35 has a reflection mirror 35a provided only at the portion corresponding to that at which light with wavelengths in the vicinity of λ₁ converge, and other portions thereof are arranged either to pass or absorb the incident light. Therefore, owing to the effect of the slit mirror 35 only the components of the light from the semiconductor laser 31 having wavelengths in vicinity of λ₁, i.e. only the laser light and one portion of the spontaneous emission, are extracted by reflection. As the light reflected by the slit mirror is finely dispersed within the range of its wavelengths, it is passed back through the convergence lens 34 and the prism 33 to be reverse-dispersed. More specifically, the light reflected from the slit mirror 35 is first returned to the convergent lens 34, which in this case acts as a collimator optical element for converting the light passing therethrough into a parallel beam. Further, the parallel beam exiting from the convergent lens 34 is sent to the prism 33 to be reverse-dispersed by passing through the prism 33 in the opposite direction from the direction it passed through the prism 33 before impinging on the slit mirror 35. As a result, all of the rays entering the prism 33 exit therefrom as a parallel beam following one and the same light path. In other words, in this case the prism 33 acts as an reverse-dispersing optical element which subjects the light to reverse-dispersion. When light of a specific wavelength is reflected by the slit mirror 35, a spatial separation arises between the paths of the impinging and reflected rays. Therefore, the slit mirror 35 is disposed as slightly tilted so that the paths of the impinging and reflected light will be vertically offset from each other (in the direction of the arrow a ). The reverse-dispersed light issuing from the prism 33 is, if necessary, deflected by a mirror 36 or the like and then converged as by a convergent lens for use in scanning or the like. In the method according to this invention, only light having the same wavelength as that of the laser light is selectively extracted and as a result nearly 100 % of the laser light is extracted. On the other hand, as the spontaneous emission has a broad wavelength band extending over 40 nm, only one part thereof is extracted. Therefore, the relation between the total amount of light Po emitted by the semiconductor laser 31 and the amount of extracted light P can be expressed as Po > P. The relation between the amount of current applied to the semiconductor laser 11 and each of the light amounts Po and P is shown by the graph in Figure 16. In the graph, the solid line curve shows how the light amount Po varies with the amount of current applied, while the single dot chain line shows how the light amount P varies therewith. Since in the optical system of this embodiment the spontaneous emission is selectively cut, in the region where the driving current is below the threshold current Io at which lasing begins, the value of P is much smaller than that of Po. Further, during the interval from the time that the current value reaches Io and generation of laser light begins and the time that the amount of laser light reaches a level at which the influence of the spontaneous emission becomes negligible (the interval within the circle in Figure 16), the ratio of the amount of laser light to the amount of spontaneous emission becomes larger. Therefore when the optical system according to this embodiment is used, the laser light comes to dominate into a lower light output region and only light having the same wavelength components as the laser light are extracted. As a result, by converging the extracted light, it becomes possible to adequately converge the light generated in low light output regions into a focused beam spot and to do so even with respect to the light in low light output regions which could not heretofore be adequately converged. While in the aforesaid method, a slit mirror which selectively reflects light of a prescribed wavelength is used as the spatial filter, the spatial filter may alternatively be one that selectively passes light of a prescribed wavelength. While in this case it is not possible to return the selectively extracted light to the convergence lens and the prism, the same effect can be obtained by passing the light transmitted through the spatial filter through a lens or other type collimator element provided in the path of the light exiting from the spatial filter so as to convert the light into a parallel beam and then to pass the parallel beam through a prism or other type reverse-dispersing optical element for subjecting the parallel beam to dispersion reverse from that carried out by the first-mentioned prism. Moreover, instead of the aforesaid prism, it is also possible to use a diffraction grating as the dispersing optical element. If a convergent diffraction grating such as that denoted by the reference numeral 43 in Figure 17 is used, the light of wavelengths λ₁′ λ₂ .... extracted as primary rays are simultaneously converged. That is, the diffraction grating can provide both the function of a spatial filter and the function of a convergent optical element such as the aforesaid convergent lens.

Claims: A method of operating an arrangement comprising a semiconductor laser (1) including a semiconductor laser chip (3), a convergent lens (4) disposed in the light path of the beam emitted by the semiconductor laser (1), and a polarizing filter (6) which passes only light polarized in the direction parallel to the junction plane of the semiconductor laser chip (3), wherein the driving current of the semiconductor laser (1) is controlled over a broad dynamic range from a low output region where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. A method of operating an arrangement comprising a semiconductor laser (11), a convergent lens (12) disposed in the light path of the beam emitted by the semiconductor laser (11), and an interference filter (13) which selectively passes only light of wavelengths within a wavelength region that includes the wavelength region of the laser light and wavelengths in the vicinity thereof, wherein the driving current of the semiconductor laser (11) is controlled within the range from a low output region where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. A method of operating an arrangement comprising a semiconductor laser (21), a convergent lens (22) disposed in the light path of the beam emitted by the semiconductor laser (21), and a light control plate (23) which has an aperture (23a) which passes only the central portion of the light emitted by the semiconductor laser (21), each of the longitudinal and transverse magnitudes of the aperture (23a) of the light control plate (23) being independently set at a value of 0.2 to 1.5 of the full width of half maximum (FWHM) of the laser light intensity distribution, wherein the driving current of the semiconductor laser (21) is controlled within the range from a low output region where the spontaneous emission region to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. A method of operating an arrangement comprising a semiconductor laser (31), a collimator lens (32) disposed in the path of the light emitted by the semiconductor laser (31), a dispersing optical element (33,43) provided in the path of the light which has passed through the collimator (32) for angularly changing the path of the light issuing therefrom in accordance with the wavelength of the light incident thereon, a convergent optical element (34) disposed in the path of the light issuing from the dispersing light element (33,43), a spatial filter (35) disposed in the focal plane of the convergent optical element (34), which spatial filter selectively transmits or reflects only a predetermined wavelength component of the light converged by the convergent optical element (34), a collimator optical element (34) disposed in the path of the light of the predetermined wavelength from the spatial filter (35) for converting the light incident thereon to a parallel light beam, and a reverse-dispersing optical element (33,43) disposed in the path of the light which has passed through the collimator element (34) for subjecting the light incident thereon to dispersion opposite to that by the dispersing optical element (33,43), wherein the driving current of the semicoductor laser (1) is controlled over a broad dynamic range from a low output region, where the spontaneous emission dominates, to a laser light emission region according to an image signal to effect an analogue intensity modulation, wherein the modulated laser beam is used to record an image. A method of operating the arrangement of claim 4, wherein the dispersing optical element is a prism (33) and the spatial filter is a slit mirror (35) which selelctively reflects light of the predetermined wavelength, the light reflected by the slit mirror (35) being passed again through the convergent optical element (34) and the prism (33), the convergent optical element (34) being then used as the collimator element (34) and the prism (33) being then used as the reverse-dispersing element. A method of operating the arrangement of claim 4, wherein the dispersing optical element and convergent optical element are constituted by a convergent diffraction grating (43) capable of carrying out the functions of both said elements.

Inventors: MIYAGAWA ICHIROU FUJI PHOTO FI, NOGUCHI MASARU FUJI PHOTO FILM, MIYAGAWA, ICHIROU FUJI PHOTO FILM CO., LTD., NOGUCHI, MASARU FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1987
Main classification: G02B27
IPC codes: G02B27, G02B6