Abstract:
A color mixing system is proposed for use in an optical-fiber laser-diode assembly comprising at least two semiconductor laser diodes, optical fiber light input and output couples, a system of spatial superposition of laser beams of different wavelength with at least one semi-transparent mirror, and a system for electronic control of light power in monochromatic light components to be mixed. The electronic control system makes it possible to produce a plurality of different colors. The basic colors, i.e., blue, green, and red, are produced by respective laser diode assemblies provided with means for adjusting output light power on each individual assembly. The electronic system contains a microprocessor connected to a pulse width modulation unit capable of modulating the duration and shape of the light pulse emitted from the laser diode. This allows for selecting a required ratio of energetic brightnesses of light beams produced by individual laser diode assemblies. The electronic system contains a microprocessor connected to a pulse width modulation unit capable of modulating the duration of the light pulse emitted from the laser diode for controlling chromaticity of the color and to a current source for controlling the luminescence, i.e., the light power. The aforementioned control of chromaticity and light power is carried out simultaneously in real time with reproduction of perfect colors based on the use of single-mode pure stabilized and frequency double wavelengths with narrow linewidths of the light spectra.

Description:
FIELD OF THE INVENTION 
     The present invention relates to color mixing, in particular to color-image formation in an optical fiber laser system for use in display units, virtual displays, laser printing, laser TV, laser lithography, scanning, etc. 
     BACKGROUND OF THE INVENTION 
     For better understanding the principle of the invention, it would be advantageous to briefly describe some fundamentals of color mixing in an image-formation process. It is well known that perception of colors by a human eye is a complicated physiological process, which involves participation of an optical system of an eye and our brain. In other words, color is the external factor, a sensation existing only in our minds. 
     The remarkable repertoire of color effects seen by the eye, and interpreted by the brain, is our response to certain wavelengths of electromagnetic energy that makes up the visible spectrum of light. While we are able to measure quite accurately the wavelengths of light, our response to those wavelengths is affected by a whole multitude of factors that are beyond the scope of our invention. What is important to note is that, when we observe a non-self-luminescent object, i.e., an object illuminated by an external light source, the color perception of an eye from the light reflected from this object will depend essentially only on the spectral composition of the light that illuminates this object. Thus, a color is a property of spectral characteristics of light radiation common for the entire radiation spectrum, irrespective of visible or invisible. Generally speaking, light can be expressed as a combination of three interconnected parameters, i.e., by colors of three-color components, which are known as base colors. Each of these base colors can be the one that cannot be reproduced by mixing two others. There are a great variety of three basic colors that can reproduce the entire range of the color spectrum. Among those, three basic colors, i.e., red (R), green (G), and blue (B) form the most popular system known as an RGB system. By mixing the RGB colors we can produce practically any color shade of a visible color. The principle of RGB color mixing system find wide practical application in color TV, flat panel displays, color printing, etc. 
     There are known various color definition and classification systems. These systems define various colors and standardize them. In 1931, the International Commission on Illumination has standardized the following wavelengths for the RGB color mixing system: 700 nm for R, 546.1 for G, and 435.8 for B. Therefore development of new effective color mixing systems still remains an actual problem of the technology. 
     It is important to notice that, in the aforementioned standards, unit quantities of basic colors are selected so that a ratio of their energetic brightnesses is equal to a predetermined value, i.e., 72.1:1.4:1.0. This ratio, however, will be changed, if we chose, e.g., 653 nm for red, 530 nm for green, and 458 nm for blue. 
     Heretofore color-mixing systems with electronic control have been known. One of the latest developments in this field is a color image formation system described in U.S. Pat. No. 5,832,155 issued in 1998 to Rasch, et al. This invention concerns an integrated-optical junction splitter, in particular for applications in the wavelength range of visible light, which ensures a spatial and wideband combination of light in a wavelength spectrum Δλ greater than 75 nm (value given applies to short-wave visible light). The system uses a single white light source or several light sources with a wide wavelength distance. In the case of a usable wavelength range comprising the entire spectrum of visible light, the junction splitter is a white light junction splitter. The aforementioned junction splitter consists of at least three channel waveguides, at least one of which must be a single-mode integrated-optical wideband channel waveguide (SOWCW). Two channel waveguides, with each having a respective input, are combined into a common SOWCW at their outputs in a coupling point, which common SOWCW features a common light output at its end. This wideband junction splitter is used as a wavelength-selective or wavelength-independent switch or modulator, in interferometric and photometric devices, sensors, and microsystem-technical solutions. 
     The aforementioned U.S. Pat. No. 5,832,155 describes only passive optical waveguide splitting and mixing of various wavelengths in a solid-state waveguide embedded in a solid-state material, such as LiNbO 3 . No electronic control is mentioned. 
     The system described in U.S. Pat. No. 5,832,155 has the following disadvantages. The white light junction splitters used in the system are optically and electrically inefficient because only few microwatts of light energy (less than 1% of the entire light energy emitted from the source) can be coupled from the white source. This inefficiency generates heat-removal problems, since the unused energy is converted into heat inside the device. For the same reason, electrical consumption of the system of the aforementioned patent is not suitable for portable devices since a high-capacity power source is needed. 
     Furthermore, embedding of titanium waveguides into the matrix of TiNbO 3 , as well as processing of electrodes used in the aforementioned patent, are expensive and complex processes that include photolithography and other complicated operations. 
     The only light source for all spectral (wavelength) components of the system is a white light source that must be always ON for operation of the system. This means that when modulation signals, i.e., high-voltage pulse signals U 1 , U 2 , U 3 , are sent to the electrodes for adjusting output light powers of individual waveguides, the process consists in “closing” unnecessary waveband portions while passing to the output the selected portions. This means that a major part of light energy is lost, and the system operates with low efficiency. 
     U.S. Pat. No. 5,802,222 issued in 1998 to the same applicants as U.S. Pat. No. 5,832,155 discloses a color image generation system for the reproduction of real or virtual, two-dimensional or three-dimensional, color, or monochrome image, in particular for television, video, or printing applications. Integrated-optical structures are used on a mount, in particular a substrate. An image is generated by deflecting color light beams generated into a viewing space, which deflection is effected in synchronism with color setting and intensity- or amplitude modulation of the light. The patent does not describe but only shows the electronic control assembly and describes in more detail the color image generation system that may be integrated on a mount either monolithically or in hybrid fashion. The color generation system may be implemented as an encased module. 
     Similar to U.S. Pat. No. 5,832,155, the system of U.S. Pat. No. 5,802,222 is based on adjusting the light intensity signals of at least two wavelengths which are to be mixed so that the final light intensity signal ratio on the output of the color mixer is obtained in accordance with the value required for a specific color. To achieve this purpose, the inventors use various control means such as amplitude attenuators, amplitude modulators and modulators of other types, pulse duration adjusters, etc. Although the description of the electronic control unit is essential for understanding the operation of the system, no disclosure of electronic control is contained in the description. Nevertheless, it can be assumed that for obtaining a great variety of color shades, such an electronic system, would be very complicated and expensive, since it is based on amplitude modulation of component light power signals prior to their mixing. 
     Known are various systems for electronic control of intensity of light emitted from a laser diode. Thus, U.S. Pat. No. 5,715,021 issued to Gibeau at al. describes a method and apparatus for a multi-application, laser-array-based image system, which utilizes three linear laser arrays. Each linear array generates multiple (N&gt;1) parallel output beamlets at one of the three primary colors (red, green, blue). The corresponding 1 to N output beamlets of the three linear arrays, each individually modulated in luminance according to a specific encoding scheme representing the video image to be produced on the viewing screen, are combined spatially to form a single white light linear array source. Through a projection/scanner optical system, the N output beamlets of the white light source are simultaneously directed to, and swept horizontally across a distant viewing screen, resulting in a swath of N lines of a graphic video image. By producing M contiguous swaths vertically down the viewing screen, a full image of M×N lines is produced. The red, green, and blue linear laser arrays may consist of arrays of semiconductor laser diodes made of suitable semiconductor materials so as to directly emit radiation at wavelengths corresponding to red, green, and blue colors, respectively. Alternatively, the red, green and blue color arrays may be formed by arrays of semiconductor laser diodes emitting radiation at twice the desired red, green and blue wavelengths, whose radiation is coupled to arrays of second harmonic generation crystal elements. 
     The device described in the aforementioned patent is complicated in construction since it relates to three color laser arrays, i.e., to arrays of red, green, and blue colors. Furthermore, some components of the optoelectronic circuits used in the system of U.S. Pat. No. 5,715,021, such as the array of blue color, which corresponds to the basis blue of the standard RGB system, are not yet commercially produced. The optical system for collimation and delivery of light beamlets is also extremely complicated and requires the use of special couplings and optical elements, which also are not yet industrially produced. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide an apparatus for color mixing in a laser diode system which has miniature dimensions and therefore is suitable for use in portable devices, possesses high light power efficiency, is free of heat-generation problems, has low electrical consumption, has simple construction and is inexpensive to manufacture, ensures modulation of wavelength signals directly on a laser-diode light source, and ensures mixing of a great number of different colors and color shades with adjustment of output color tones. Another object is to provide a new method of color mixing in a laser diode optical system for obtaining a great variety of colors and color. Another object of the invention is to provide aforementioned apparatus and method, which, in view of their versatility, are suitable for application in different fields from mere color mixing to high-definition TV sets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of the apparatus of the invention, which shows layout of the components of the optical system. 
     FIG. 2 is a longitudinal sectional view of a semiconductor laser diode assembly for generating red light in the apparatus of the invention. 
     FIG. 3 is a longitudinal sectional view of a semiconductor laser diode assembly for generating green and blue lights in the apparatus of the invention. 
     FIG. 4 is a block diagram of an electronic circuit for setting up and controlling intensity of light in one of monochromatic optical units shown in FIGS. 1,  2 , and  3 . 
    
    
     SUMMARY OF THE INVENTION 
     A color mixing system is proposed for use in an optical-fiber laser-diode assembly comprising at least two semiconductor laser diodes, optical fiber light input and output couples, a system of spatial superposition of laser beams of different wavelength with at least one semi-transparent mirror, and a system for electronic control of light power in monochromatic light components to be mixed. The electronic control system makes it possible to produce a plurality of different colors. The basic colors, i.e., blue, green, and red, are produced by respective laser diode assemblies provided with means for adjusting output light power on each individual assembly. The electronic system contains a microprocessor connected to a pulse width modulation unit capable of modulating the duration of the light pulse emitted from the laser diode for controlling chromaticity of the color and to a current source for controlling the luminescence, i.e., the light power. The aforementioned control of chromaticity and light power is carried out simultaneously in real time with reproduction of perfect colors based on the use of single-mode pure stabilized and frequency-doubled wavelengths with narrow linewidths of the light spectra. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS.  1 - 3 —Structure and Operation of the Optical System 
     An optical color-mixing system of the present invention is shown schematically in FIG. 1, which shows three optical fiber assemblies that generate component lights of three basic RGB colors and a system of beam superposition on a common output coupler. 
     As shown in FIG. 1, the system has three semiconductor laser diode assemblies  10 ,  12 , and  14 , which correspond to R, G, and B of the RGB system, respectively. Each assembly contains a semiconductor laser ( 10   a ,  12   a , and  14   a ) that generates a light signal of an appropriate wavelength corresponding to R, G, and B, appropriate power supply unit ( 10   b ,  12   b , and  14   b ), respectively, as well as temperature control means, which is described below. 
     An example of a semiconductor laser diode assembly  10  suitable for generating red light in the system of the invention is shown in FIG.  2 . Such laser diode assembly is described in U.S. patent application Ser. No. 09/658,619 filed by the same applicant on Sep. 8, 2000. 
     As shown in FIG. 2, a laser-diode assembly  10  for generating a frequency-stabilized narrow-bandwidth light comprises a light source in the form of a semiconductor laser diode  10   a  coupled via a first optical coupling device  18  to one end of a first optical fiber  20 . The other end of this fiber is coupled to a second or an output fiber  22  via a second optical coupling device  24 . The assembly is characterized by the fact that a long inner laser cavity L 1  is formed by a section of the optical system between two oppositely directed mirror coatings M 1  and M 2 . The first mirror coating M 1  is applied onto the back side of the semiconductor laser diode  10   a , and the second mirror coating M 2  is applied, e.g., onto a flat front side of an optical lens element  26  or onto the back side of another optical lens element  28  included into the laser-diode assembly  10 . These optical lens elements  26  and  28  are parts of an optical coupling between the first and the second fibers  20  and  22 , respectively. The first mirror coating M 1  completely reflects the entire light incident onto this mirror coating, whereas the second mirror coating M 2  reflects a major part of the light, e.g., about 90% and passes only a small part, e.g., 10% of the light incident onto this mirror. The Bragg grating  30 , written in the fiber  20 , is designed so that, in combination with the laser cavity L 1 , it suppresses the side modes of the wavelength bands and transforms them into the central mode of the narrow wavelength band, which can be passed through this grating. The light processed by the Bragg grating is passed through the second mirror coating M 2  to the output fiber  22 , while the reflected light performs multiple cycles of reflection between both mirrors M 1  and M 2  which thus form a laser resonator which amplifies the laser light output at the selected narrow waveband. 
     The laser diode assembly  10  has a housing H with a central longitudinal groove that is used for mounting and aligning optical components of the assembly, i.e., the laser diode  10   a , the couplers  18 ,  24 , etc. The housing H is used for stabilization of temperature of the optical components and is connected to a temperature control means that consists of an electrically controlled cooling device  19  (FIG.  2 ), such as a Peltier-type device, and an electronic temperature control unit  10   c  (FIG.  1 ). In FIG. 1, the electronic temperature control unit  10   c  is shown conventionally as a separate block. Reference numeral  25  in FIG. 2 designates a heat-removing radiator. 
     Although the laser diode assembly  10  is used in the system of FIG. 1 specificaly for generating the basic red-light component of 700 nm that corresponds to the standard of the International Commission on Illumination, it is understood that the above-described construction can be realized for a basic color of another three basic component color system. 
     Laser diode assemblies  12  and  14 , which are intended for generating G and B components of the RGB system used in the present invention, have identical constructions and differ only by the types of laser diodes that generate light of 1092.4 nm for G, and 871.6 for B. The non-linear crystals will double the frequencies and thus produce on the output fibers wavelengths of 546.1 nm and 435.8 nm. For example, a laser diode assembly produced in a laboratory could generate 458 nm for blue by doubling 916 nm, 530 nm for green by doubling 1060 nm, and by using 635 nm as it is for red. 
     The construction of laser diode assemblies  12  and  14  for generation of green and blue lights is shown in FIG.  3  and is described in more detail in U.S. patent application Ser. No. 09/698,619 by the same applicant on Sep. 8, 2000. FIG. 3 illustrates only one of the assemblies  12  and  14  which are structurally identical for both green and blue light generation assemblies and differ from each other only by characteristics of some components such as laser diodes, non-linear crystals, Bragg gratings, etc. Therefore, the description of the green/blue light-generation assemblies will be common for both assemblies, wherein the components of the green-light assembly will be shown without parentheses, while the components of the blue-light assembly will be shown in parentheses. 
     It can be seen from FIG. 3 that the assembly consists of a light source  32  ( 132 ) in the form of a semiconductor injection laser, which is coupled via an anamorphotic objective  38  ( 138 ) and via a butt-connection unit  40  ( 140 ) directly to an optical fiber  42  ( 142 ). The other end of this fiber is coupled to the output optical fiber  44  ( 144 ) via an air gap  46  ( 146 ), a first circular aspherical lens element  48  ( 148 ), a first dichroic coating  50  ( 150 ) on the back side of the lens element  48  ( 148 ), a layer of a UV-curable glue  52  ( 152 ), a non-linear crystal  54  ( 154 ), a layer of a UV-curable glue  56  ( 156 ), a second dichroic coating  58  ( 158 ), a second microlens element  60  ( 160 ), and an air gap  62  ( 162 ). The aforementioned dichroic coatings  50  ( 150 ) and  58  ( 158 ) located on both sides of the non-linear crystal  54  ( 154 ) are oppositely directed dichroic coatings of high reflectivity for lights of different wavelengths. The back facet of the semiconductor laser  32  ( 132 ) has a high-reflectivity coating  64  ( 164 ) for reflecting the fundamental wavelength. The section of the fiber  42  ( 142 ) with other appropriate components between the aforementioned high-reflectivity coating  64  ( 164 ) and the last dichroic coating  58  ( 158 ) in the direction of light propagation forms the so-called laser cavity L 2  (L 3 ), which contains a Bragg grating  66  ( 166 ). This grating selects a desired mode of the fundamental wavelength from the possible modes of the spectral range. By using different orientations of the non-linear crystal, it is possible to double, triple, or quadruple the wavelengths, and by replacing the crystals, it is possible to obtain lights of different colors. Such a construction makes it possible to form a very compact and efficient frequency-multiplied laser diode. 
     In the system of the present invention shown in FIG. 3 the assemblies  12  and  14  produce on the output optical fibers  44  and  144  green (G) and blue (B) lights of RGB system, respectively. More specifically, the semiconductor laser diodes  32  and  132  of assemblies  12  and  14 , respectively, are selected so that the light produced on the output of optical fiber  44  has a wavelength of 530 nm, and the light produced on the output of optical fiber  144  has a wavelength of 458 nm. 
     The laser diode assembly  12  ( 14 ) has a housing which consists of two parts H 1   a  (H 1   b ) and H 2   a  (H 2   b ) with central longitudinal grooves  70  ( 170 ) and  72  ( 172 ) that are used for mounting and aligning optical components of the assembly, i.e., the laser diode  32  ( 132 ), the objectives  38  ( 138 ), etc. The housing parts H 1   a  (H 1   b ) and H 2   a  (H 2   b ) are used for stabilization of temperature of the optical components and are connected to electrically controlled cooling devices  41   a  ( 141   a ) and  41   b  ( 141   b ), such as Peltier-type devices, and an electronic temperature control units  12   c  ( 14   c ). In FIG. 1, the electronic temperature control units  12   c  and  14   c  are shown conventionally as separate blocks. In FIG. 3 reference numeral  74   a  ( 174   a ) and  74   b  ( 174   b ) designate heat-removing radiators. 
     As shown in FIG. 1, the aforementioned laser diode assemblies  10 ,  12 , and  14  are mounted on a common support or plate  200 , which also supports a color combining assembly  202 . This assembly includes a mounting plate  204  made of glass, quartz, ceramic, or another material with a low coefficient of thermal expansion. The mounting plate  204  has a longitudinal groove  206  of a rectangular cross section and two transverse grooves  208  and  210 , which are perpendicular to the groove  206  and are parallel to each other. The transverse grooves  208  and  210  also have a rectangular cross section. All three grooves  206 ,  208 , and  210  serve for precisely positioning respective beam collimating units  212 ,  214 , and  216  for the formation of diffractionally-limited collimated red, green, and blue beams BR, BG, and BB. Respective units  212 ,  214 , and  216  consist of ferrules  212   a ,  214   a , and  216   a  aligned and centered in the aforementioned grooves  206 ,  208 , and  210 , ends of the output fibers  22 ,  44 , and  144  (FIGS. 1,  2 , and  3 ) inserted into the central openings  212   b ,  214   b , and  216   b  of respective ferrules  212   a ,  214   a , and  216   a , and microlens elements  212   c ,  214   c , and  226   c  with microlenses  212   d ,  214   d , and  226 d inserted into the openings  212   b ,  214   b , and  216   b  from the opposite sides of the ferrules  212   a ,  214   a , and  216   a . The end faces of output fibers  22 ,  44 , and  144  have to be located at strict distances from the respective microlenses  212   d ,  214   d , and  226   d  so as to form the aforementioned diffractionally-limited collimated laser beams BR, BG, and BB. 
     Self-alignment of respective beam collimating units  212 ,  214 , and  216  is achieved due to precise cross-sectional dimensions of the grooves, accurate dimensions of the ferrules, and accurate mutual (parallel or perpendicular) positions of the grooves. More specifically, the diameters of the ferrules  212   a ,  214   a , and  216   a  ensure precise sliding fit of the ferrules into the grooves, and the heights of the grooves are equal to the outer radii of the ferrules. 
     In FIG. 1, symbol Z designates an optical axis of the beam collimating unit  212  which coincides with the longitudinal axis of the groove  206 , i.e., with the direction of propagation of light from the output optical fiber  22  of the laser diode  10 , and symbols X 1  and X 2  designate optical axes of the beam collimating units  214  and  216  which coincide with the longitudinal axes of the grooves  208  and  210 , i.e., with the directions of propagation of light from the output optical fibers of the laser diodes  12  and  14 , respectively. It is understood that the optical axes X, Y, and Z lie in the plane of the upper surface of the plate  202 . 
     Grooves  228  and  230 , which are arranged at an angle of 45° to the axis Z, are cut in the plate  202  at points of intersection of the axes X 1  and X 2  with the axis Z. Inserted into these grooves  228  and  230  are semitransparent mirror plates  232  and  234 , respectively. The mirror plate  232  has a coating  232   a  on the side that faces the beam-collimating unit  212  and a coating  232   b  on the opposite side of the plate. At the same time, these coatings reflect a small amount, e.g., 3% of the light power of the beam BR, towards a feedback circuit  256   a , which is connected via a photodiode  258   a  and an analog/digital converter  260   a  to a microprocessor  248 , described later in connection with the description of the electronic control system. As will be described later, this feedback is used for controlling the luminescent characteristics, i.e., the light power of the beam emitted from the laser diode assembly  10 . At the same time, the coatings  232   a  and  232   b  ensure maximum possible reflection of the green light of 546.1 nm emitted from the laser diode assembly  12  with minimal losses. 
     An additional partially-transparent mirror  233  is installed in the optical path of the green beam BG from the output optical fiber  44  toward the mirror  232 . The mirror  233  passes the beam BG, but reflects a small amount, e.g., 3% of the light power, towards a feedback circuit  256   b , which is connected via a photodiode  258   b  and an analog/digital (A/D) converter  260   b  to the aforementioned microprocessor  248 . As will be described later, this feedback is used for controlling the luminescent characteristics, i.e., the light power of the beam emitted from the laser diode assembly  12 . 
     The mirror plate  234  has a coating  234   a  on the side that faces the beam-collimating unit  212  and a coating  234   b  on the opposite side of the plate  234 . In combination with coatings  234   a  and  234   b , the coatings  232   a  and  232   b  ensure passage of red light of 700 nm from the laser beam assembly  10  in the direction of the axis Z with minimal losses. 
     An additional partially transparent mirror  235  is installed in the optical path of the blue beam BB from the output optical fiber  144  toward the mirror  234 . The mirror  235  passes the beam BB, but reflects a small amount, e.g., 3% of the light power, towards a feedback circuit  256   c , which is connected via a photodiode  258   c  and an analog/digital (A/D) converter  260   c  to the aforementioned microprocessor  248 . As will be described later, this feedback is used for controlling the luminescent characteristics, i.e., the light power of the beam emitted from the laser diode assembly  14 . 
     In combination, the coatings  234   a  and  234   b  ensure passage of the red light beam that has passed through the semitransparent mirror plate  232  and the green light beam reflected from the coating  232   b  of the plate  232 . At the same time, the coating  234   b  ensures maximum possible reflection of the blue light of 435.8 nm emitted from the laser diode assembly  14  with minimal losses. 
     Thus, downstream of the semitransparent mirror plate  234 , a three-component beam BR—BG—BB consisting of beams BR, BG, and BB propagates in the direction of axis Z. Installed on the way of the three-component beam BR—BG—BB is a common output coupler  236  which consists of a self-aligned ferrule  238  positioned in the groove  206  and having a through opening  238   a , a microlens element  240  with a microlens  242  inserted into one end of the opening  238   a , and a common output fiber  244  of the entire system which is inserted into the opening  238   a  from the other side of the ferrule  238 . 
     In order to provide the maximum intensity of the green and blue beams BG and BB for matching with the intensity of the red beam BR, the distances from respective beam collimating units  212 ,  214 , and  216  to the flat back side  240   a  of the microlens element  240  should be substantially equal. The fine adjustment of the light beam intensities is performed only once during assembling of the system by micropositioning the respective beam collimating units with reference to the maximum output light signal separately for each wavelength and then fixing the units in the adjusted positions, e.g., by glue. 
     As has been mentioned above, for controlling luminescent properties, i.e., the light power, in each laser diode assembly  10 ,  12 , and  14 , a portion of light energy, e.g., about 3%, is extracted from the main beam and is sent via respective feedback circuits  256   a ,  256   b , and  256   c  (FIG. 1) to a microprocessor  248 . 
     It should be noted that the units for coupling the beams reflected from the respective mirrors  232 ,  233 , and  235  to the feedback fibers  257   a ,  257   b , and  257   c  between the respective mirrors and the photodiodes  258   a ,  258   b , and  258   c  are performed through the use of couplings  259   a ,  259   b , and  259   c . These couplings are identical to beam collimating units  212 ,  214 , and  216  shown in FIG.  1 . Furthermore, matching of the photodiodes  258   a ,  258   b , and  258   c , which received the feedback signals from the reflecting mirrors  232 ,  233 , and  235 , with the feedback fibers  257   a ,  257   b , and  257   c  is carried out with the use of the same ferrules and lenses as those shown in connection with the beam collimating units  212 ,  214 , and  216 . Thus, the same standard fiber/fiber or fiber/optical component unit is used for assembling the optical system. This simplifies assembling and reduces the cost of the system. 
     FIG.  4 —Structure and Operation of the Electronic Control System 
     FIG. 4 is a block diagram of an electronic circuit for setting up and controlling intensity and chromaticity of light in one of monochromatic optical units shown in FIGS. 1,  2 , and  3 . It is understood that three such units are incorporated into the general system of FIG.  1 . Since, in principle, the set up and control of the light intensity and chromaticity in any of the aforementioned units is identical, the diagram of FIG. 4 will be considered as a generalized circuit for all three component colors. 
     In the diagram of FIG. 4, reference numerals  10 ,  12 , 14  designate laser diode assemblies shown in FIGS. 1,  2 , and  3 . The input current which controls operation of the diodes  10   a  ( 12   a ,  14   a ) of the assemblies  10 ,  12 , 14  is supplied to each of them from their respective power supply unit  10   b  or  12   b  or  14   b  (FIGS.  1  and  4 ), which, in turn, is controlled from a microprocessor  248  via a digital/analog (D/A) converter  250 . The microprocessor  248  is common for all three monochromatic systems. The subsequent description will be in singular, as it is common for each laser diode assembly. Examples of the microprocessor  248  suitable for the purposes of the present invention are TI-430-32, TI-430-33, and TI-430-34 units produced by Texas Instrument Co., USA. 
     The microprocessor  248  is connected to a programming device, such as a computer  249  (also common for all three assemblies), and is connected in serial to respective microprocessors with an auxiliary 8-bit output which interfaces with a high speed 8-bit pulse-width modulating logic circuits (PWMLC)  254   a ,  54   b ,  254   c  which determine chromaticity of the colors based on duration of light pulses generated by laser diodes  10   a ,  12   a ,  14   a . For this purpose, the PWMLC  254   a ,  254   b ,  254   c  are connected to the laser diodes for controlling the pulse duration by turning transistor circuits T 1   a , T 1   b , T 1   c  of the laser diodes ON on a rising edge of the pulse and OFF on the falling edge of the pulse. The function of these transistor circuits can be fulfilled by commercially produced laser diode drivers  10   b ,  12   b ,  14   b  suitable for use in conjunction with laser diodes for causing them to operate in a pulse mode. Thus, the microprocessors  252   a ,  252   b ,  252   c  in combination with the 8-bit PWMLC  254   a ,  254   b ,  254   c  makes it possible to obtain 2 8  (i.e., 256) combinations of light intensity gradations in light emitted from each laser assembly via their respective output optical fibers  22 ,  44 , and  144  (FIG.  1 ). 
     Levels of currents in the laser diodes  10   a ,  12   a , and  14   a  are controlled by means of appropriate power supply units, such as current sources  10   b ,  12   b , and  14   b  (FIG. 4) for controlling the output optical signals of beams BR, BG, and BB (FIG. 1) so that they correspond, e.g., to the aforementioned 72.1:1.4:1.0 ratio. The outputs of these current sources are connected to respective laser diodes  10   a ,  12   a , and  14   a , while their inputs are connected to the microprocessor  248  via respective digital/analog (D/A) converters  250   a ,  250   b ,  250   c . In each respective laser-diode assembly, the current source ( 10   b ,  12   b , and  14   b ) determines the luminescent characteristics or light power. 
     As has been shown above with reference to FIG. 1, each laser diode assembly  10 ,  12 , and  14  is connected to the microprocessor  248  via the feedback circuit  256   a ,  256   b ,  256   c  which contain a photodiodes  258   a ,  258   b , and  258   c  and the 12-bit A/D converters  260   a ,  260   b ,  260   c  connected in series in the optical fibers  257   a ,  257   b ,  257   c.    
     The microprocessor  248  is also connected to the aforementioned electronic temperature control unit  10   c  or  12   c  or  14   c  (FIG. 1) of its respective laser assembly. It should be noted that separate electronic temperature control units should be used for each housing part H 1   a  (H 2   a ) and H 1   b  (H 2   b ) in the laser assembles of the blue and green lights shown in FIG. 3. A single electronic temperature control unit will be used for the red laser diode assembly. 
     Reference numeral  271  (FIG. 4) designates a decoder unit which is common for all the laser diode assemblies and which makes it possible to adjust luminescent properties and chromaticity from voice commands, digital commands, etc., e.g., via a cellular phone or the like. 
     The electronic control system of the apparatus of the invention operates as follows: 
     After the manufacturing of the apparatus of the invention shown in FIGS. 1-4 is completed, it is subjected to initial set-up at the manufacturer&#39;s facility. When the apparatus is turned on for this set up, the microprocessor  248  begins to operate in a self-diagnostic mode in which it checks all the parameters of the system, performs functional tests, and compares the measured parameters with those of the last set-up data. The aforementioned set up parameters include a temperature set points for the laser diode ( 10   a ,  12   a , and  14   a ) and the non-linear crystal ( 54 ,  154 ). This self-diagnostic procedure is fulfilled under control of the computer  249 . 
     Next step is to set up a standard color in each laser diode assembly so that a ratio of energetic brightnesses of the component beams corresponds to 72.1:1.4:1.0. This is achieved by selecting levels of currents in appropriate power supply units  10   b ,  12   b , and  14   b  (FIG. 4) for controlling the output optical signals of beams BR, BG, and BB (FIG. 1) so that they correspond to the aforementioned 72.1:1.4:1.0 ratio. At this stage the initial set up of the system is completed. It is understood that the 72.1:1.4:1.0 ratio is given only as an example which is in compliance with recommendations of the International Commission on Illumination and that any other ratio can be chosen. For example, in a laboratory model the laser assemblies generated beams of 635 nm, 530 nm, and 458 nm, which are different from those mentioned above. 
     Once the system is calibrated to the selected color ratio standard, the optical power level is permanently set in the microprocessor  248  of each color system. With the aging of the system, the aforementioned initial setup is periodically adjusted by the microprocessor  248  in order to provide colors per selected standards. 
     The performance of the optical energy is periodically compared to the pre-set values in the microprocessor  248 , and the power levels in  10   c ,  12   c , and  14   c  is automatically adjusted. 
     The operation of all three-laser diode assemblies  10 ,  12 , and  14  (FIG. 1) will mix the beams BR, BG, and BB of the component standard basic colors and will produce a white beam WB (FIG. 1) in the general output optical fiber  244 . 
     It is understood that for obtaining any selected color shades, it is necessary to adjust levels of output optical signals in the output optical fibers  22 ,  44 , and  144  (FIG. 1) in predetermined proportions. The levels of the output optical signals in the output optical fibers  22 ,  44 , and  144  are proportional to the level of current amplitude as controlled through optical feedbacks  258   a ,  258   b ,  258   c , the A/D converters  260   a ,  260   b ,  260   c , the microprocessor  248 , the D/A converters  250   a ,  250   b ,  250   c , and the current sources ( 10   c ,  12   c ,  14   c ), The level of the mean current can be regulated via the PWMLC  254   a ,  354   b ,  254   c . For this purpose, a command is sent from the computer  249  to the microprocessor  248 , and the latter, in turn, sends a command via the auxiliary microprocessors  252   a ,  252   b ,  252   c  with an 8-bit output to the PWMLC  254   a ,  254   b ,  254   c . The PWMLC  254   a ,  254   b ,  254   c  control the width of the current pulses in the laser diodes  10   a ,  12   a , and  14   a , thus defining the laser light power at the output of the laser diodes 
     Once the levels of currents in the individual laser diodes  10   a ,  12   a , and  14  are adjusted, maintaining of these levels is ensured through the feedback circuits  256   a ,  256   b ,  256   c . For this purpose, a feedback signal with an intensity of about 1% of the total optical power is sent from the output optical fibers  22 ,  44 ,  144  via the photodiodes  258   a ,  258   b ,  258   c  and the 12-bit analog/digital (A/D) converters  260   a ,  260   b ,  260   c  to the microprocessor  248 , where the feedback signal is compared with the initial setting. In the case of disagreement, the microprocessor  248  will adjust the level of the current in the appropriate laser diode via the microprocessors  252   a ,  252   b ,  252   c  and the 8 PWMLC  254   a ,  254   b ,  254   c  in a manner described above. 
     If necessary, the chromaticity and luminescent properties of the system can be adjusted via the decoder unit  271  by means of a voice command, digital command, etc., e.g., via a cellular phone or the like. 
     It can be seen that the correction of the current level via the feedback circuit  256  occurs in the position prior to the mixing portion of the circuit in direction of propagation of the light. Therefore the current correction procedure can be performed at any time during operation of the system without interruption of the operation. If necessary, this procedure can be automated. 
     Since the main components of the apparatus of the invention are based on microoptical elements interconnected through optical fibers, the overall dimensions of the entire apparatus of the invention can be reduced to miniature dimensions. 
     For the example, the entire optical and electrical system may have overall dimensions, which allow to built this system into any portable device. 
     Thus, it has been shown that the invention provides an apparatus for combining monochromatic color beams in a laser diode system which has miniature dimensions and therefore is suitable for use in portable devices, possesses high light power efficiency, is free of heat-generation problems, has low electrical consumption, has simple construction and is inexpensive to manufacture, ensures modulation of wavelength signals directly on a laser-diode light source, ensures mixing of a great number of different colors and color shades with adjustment of output color tones. The invention also provides a new method of color mixing in a laser diode optical system for obtaining a great variety of colors and color shades adjustable in a stepless manner. 
     Although the invention has been shown and described with reference to specific embodiments, it is understood that these embodiments were given only for illustrative purposes and that any changes and modifications with regard to shapes, designs, materials, and combinations thereof are possible, provided these changes and modifications do not depart from the scope of the patent claims. For example, the set-up procedure described in the specification is not necessarily performed for obtaining of only basic colors but can also be used for changing the intensities of the basic color beams and thus for obtaining different shades of the output colors. The microprocessor can be common or all three-color systems. The microprocessor is not necessarily an 8-bit processor and can be a 64-bit or any other processor. The color ratio may be selected in accordance with any other standard. Laser diode current modulation can be carried out in accordance with any waveform such as square, sinusoidal, etc. Laser diode current modulation and pulse width modulation can be performed simultaneously for optimum color mixing and for electrical power consumption from the battery of a portable system.