Abstract:
The invention concerns a method of operating a laser diode ( 2 ) producing a beam ( 9 ) whose intensity is controlled by a control signal and which is directed along a first beam path ( 10 ) at the target surface ( 5 ), stray reflections from the target surface ( 50  which are unstable with respect to the diode ( 2 ) in time and/or space acting on the diode ( 2 ). An element ( 19, 28, 31, 37, 38 ) which is stable in time and with respect to the diode ( 2 ) in space is used to feed part of the light emitted by the diode back along a second beam path ( 11 ) to the active zone ( 42 ) of the diode ( 2 ) so that the diode ( 2 ) is kept in a state of coherence collapse.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related in subject matter to the U.S. patent application Ser. No. 08/805,992, filed Feb. 24, 1997 now U.S. Pat. No. 5,946,333 and entitled “METHOD AND CIRCUIT FOR OPERATING A LASER DIODE”. The present application and this prior application are commonly assigned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a device for operating a laser diode, whereby the intensity of the laser beam produced by the diode is adjusted with an adjustment signal and directed along a path to stock, such as printer stock or photographic stock, that is to be illuminated. Under certain conditions unstable reflections from the stock, acting as a reflector, act on the diode with respect to time and/or space, thus interfering with the laser intensity. 
     Laser diodes are employed in many fields of communications. They are particularly practical because the intensity of the light they emit can be modulated. Since they are so small and easy to control electronically, laser diodes can be employed to transmit data very rapidly. This feature is of significant advantage not only for transmitting but also for reproducing information. 
     Laser diodes, however, do have one drawback in that they tend to shift spontaneously from one mode to another at specific ranges of intensity. This tendency results in the low-frequency intensity fluctuations called mode hopping. Mode hopping is particularly annoying when laser diodes are employed to switch statically or dynamically, especially for signal modulation. 
     Several ways to avoid mode hopping have already been suggested. The U.S. Pat. No. 4,817,098 for example proposes a system that involves various means of maintaining laser outputs constant. The temperature of the diode can be measured for instance and regulated to minimize temperature fluctuations and accordingly maintain its output as constant as possible. 
     The U.S. Pat. No. 5,283,793 suggests superposing a high-frequency signal over an imaging signal being applied to printer stock. 
     US Pat. Nos. 4,799,069 and 5,386,124, finally, suggest preventing mode hopping by briefly turning the laser diode off between every pair of pixel signals. 
     These approaches to the prevention of mode hopping are either technically rather complicated or lead to relatively unsatisfactory results. One object of the present invention is accordingly a method and device for preventing the sudden low-frequency disruptions in laser intensity that mode hopping causes. A more specific object is to stabilize the light emitted by a laser diode scanning a surface with respect to mode hopping when reflections from the surface interfere with the diode. 
     These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by producing intentional and controlled feedback of enough of the light emitted by the laser diode back to the diode&#39;s effective surface, via another path, to maintain the laser diode in a state of coherence collapse. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the awareness that mode hopping can be prevented by establishing as many different modes as possible inside the laser diode. This approach generates high-frequency noise and suppresses the low-frequency noise generated by mode hopping. 
     The light emitted from the laser diode is fed back to its effective surface by way of an optical component that is stable with respect to time, and, in terms of the diode, with respect to space, maintaining the diode in a state of coherence collapse. A little of the emitted light is accordingly reflected by an external mirror and hence to the laser resonator by appropriate optical components. The light is fed back over a different path that can be at least partly separate from the utility path. The diode&#39;s light-emitting area constitutes, in conjunction with the light-feedback component, an external resonator that is considerably longer than the diode&#39;s resonator alone. The resonance frequencies (equivalent to modes) of the external resonator are accordingly much closer together than those of the laser resonator itself. The mode intervals can for example be 500 MHz for an external resonator 30 cm long and 170 GHz for a GaAlAs laser diode 0.2 mm long. 
     The light fed back to the laser diode over the utility path will always exhibit slight fluctuations in amplitude and phase provoked not only by optical noise (spontaneous emission) from the diode itself but also by time-variant optical reflections from the various moving components (pivoting mirrors and film). 
     Once the output of the light being intentionally fed back over the second path exceeds a prescribed minimum, the fluctuations in amplitude and phase at the effective surface will be amplified to the point of static inherent modulation in the light output (AM) and in the laser frequency (FM). The period T of this inherent modulation will be dictated by the time 2·L extern /c the lightwave takes to travel in the additional external resonator, whereby L extern  is the length of the external resonator and c the speed of light (3·10 10  cm/sec). The reciprocal of period T, however, is precisely the frequency interval 
     
       
           df= 1/ T=c/ (2· L   extern ) 
       
     
     of the external resonator&#39;s modes. 
     The combined AM and FM inherent modulation generates a large number of sidebands, that coincide precisely with the external resonator&#39;s modes. A large number of external-resonator modes can accordingly be simultaneously established in a laser system comprising a laser diode and an external resonator, and the emitted laser energy can be uniformly distributed over a large number of modes and maintained constant in the time-based means. 
     One effect that can be intentionally achieved and maintained time-constant in accordance with the present invention is that the large number of modes will considerably decrease the coherence length of the resulting radiation. This state of the laser diode is called coherence collapse because the radiation will convert abruptly from a state of higher coherence (monomodal operation with static mode hopping) to one of lower coherence (multimodal operation without mode hopping) once the beam has exceeded a critical threshold in the fed-back light output. 
     In systems intended to stabilize the frequency of the laser beam, the optical coupling is intended to ensure that the optical path is at least as short as the light&#39;s coherence. Systems of this type are described in German Patents Nos. 3,442,188 A1 and 3,410,729 A1 l for example. Feeding the light back in the system in accordance with the present invention, which exploits coherence collapse, is intended on the other hand to ensure that the path of the fed-back light is actually longer than the coherence, and the feedback in such a system is accordingly called incoherent. 
     How much optical-feedback output is need to achieve coherence collapse, depends on the type of laser diode and on how much light the external resonator can reflect. It is on the order of a small percentage of the emitted output. R. W. Tkach, Regimes of Feedback Effects in 1.5 □μm Distributed Feedback Lasers,  Journal of Lightware Technology,  LT-4, 11 (November 1986), 1655-61 and K. Petermann, “Laserdiode Modulation and Noise”, Kluwer Academic Publishers, Dordrecht (NL), 1988, 251-90 describe studies of laser diodes operating subject to coherence collapse and discuss appropriate levels of reflection. K. Petermann, External Optical Feedback Phenomena in Semiconductor Lasers,  IEEE Journal of Selected Topics in Quantum Electronics,  1, 2 (June 1996) 480-89, describes further studies of laser diodes operating in the same state. 
     The present invention can be employed to particular advantage in what are called monomodal lasers, the internal resonators of which normally generate light of a single wavelength. Monomodal lasers usually exhibit abrupt mode hopping at specific points when the current strength changes. The laser&#39;s activity, in other words, jumps from one mode to an adjacent mode. It was discovered in accordance with the present invention that intentionally inducing a coherence collapse can keep the light output in the low-frequency range much more constant than is possible in monomodal operation. The emitted light will also be much less sensitive to undesired reflection from other components of the optical equipment. 
     Mode hopping can be classified as internal or external. The internal mode hopping in monomodal lasers is affected by the laser&#39;s internal design. In distributed Bragg reflectors (DBR&#39;s) for example, internal mode hopping is caused by the structure of the internal resonator. The index of refraction of the laser-active semiconductor material varies not only with the temperature of the diode but with current strength. When these parameters exceed a specified threshold, the resonance frequency will change powerfully enough to generate mode hopping. These discrepancies occur at a very high frequency range, typically of some hundred GHz. The amplification curve of the internal resonator in such a laser diode, its amplification as a function of wavelength, that is, will shift with the diode&#39;s temperature. This effect contributes significantly to mode hopping in monomodal lasers. 
     External mode hopping is dictated by effects outside the diode&#39;s internal resonator. These effects can occur for example when a reflecting surface (from a drawing for example) briefly moves into the path of the light inside a laser printer and reflects light back into the diode. In other words, an external resonator is briefly created that affects the laser&#39;s overall output and results in mode hopping. 
     Both aforesaid varieties of mode hopping can be suppressed by the intentional initiation of coherence collapse. 
     The light can be fed back to the laser diode in accordance with the present invention by either fiber optics or reflection. It is of advantage in attaining incoherent feedback for the optical path of the fed-back light, the external resonator or the feedback path that is, to be essentially longer than the coherence. It is of particular advantage for the optical path to be at least 100 mm long. It is also of advantage to feed at least 0.01% of the diode&#39;s light output back into the diode. Feeding 0.1 to 1% of the emitted light back into the diode will definitely produce coherence collapse. A third parameter that governs the effectiveness of the feedback is how much of the fed-back light will cover the diode&#39;s effective surface. It has been demonstrated that coherence collapse will be particularly extensive when the cross-section of the reflected beam covers at least 50% of the laser&#39;s surface. Such coverage will ensure that the internal feedback factor, the percentage of light intensity fed back to the effective surface, that is, will be between 10 −5  and 1%, resulting in the desired collapse. 
     For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the principle of the present invention as employed in a laser printer. 
     FIG. 2 illustrates various versions of laser-feedback devices. 
     FIG. 3 shows details of laser-feedback devices. 
     FIG. 4 illustrates a double-sided laser. 
     FIG. 5 illustrates a feedback system employing light waveguides, 
     FIG. 6 shows details of a laser diode. 
     FIG. 7 comprises graphs of various mode spectra. 
     FIG. 8 comprises graphs of two noise spectra. 
     FIG. 9 illustrates a circuit for noise modulating a laser diode. 
     FIG. 10 is a graph comparing the results of coherence collapse with those of noise modulation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates how the present invention operates in a laser printer  1  by way of example. A laser diode  2  is activated over a graphic-signal line  3  that provides gray-scale signals. Diode  2  generates a laser beam  9  intensity-modulated with respect to the signal and having a time-modulated output P A (t). The graphics signals can derive from a CCD scanner or from such a medical-diagnostics device as a CT, NMR, or ultrasound generator. A beam splitter  4  separates original laser beam  9  into a utility beam  10  with an output P nutz  and a feedback beam  11  with an output P ruck . Utility beam  10  is periodically deflected in an in-itself known procedure by an unillustrated deflector, a polygonal or pivoting mirror for example, to printing stock  5 . Stock  5  can for example be a excitable-phosphor coated image-memory film or a photographic film being transported in direction A by rollers  6 . Such application require laser diodes of relatively high output, approximately 100 mW or more. 
     In the embodiment illustrated in FIG. 1, feedback beam  11  is forwarded to an optical input component  8  through an optical feedback component  7 . Optical input component  8  directs the beam to the effective surface of laser diode  2 . Optical feedback component  7  is for this purpose mounted time-stable and, with respect to diode  2 , spatially stable on a support. 
     FIG. 2 illustrates various versions of the feedback system. The feedback beam  11  in the version illustrated in FIGS. 2 a  and  2   b  travels at least to some extent over the same optical path and through the same optical components as original laser beam  9 . The feedback beam  11  in the version illustrated in FIGS. 2 c  and  2   d  travels to the effective surface of laser diode  2  over a separate optical path and through different optical components. 
     The beam splitters  4  in the versions illustrated in FIGS. 2 a ,  2   b , and  2   d  all include a reflecting component. 
     The reflecting component illustrated in FIG. 2 a  is an optical transmitter and reflector  12 . It reflects a prescribed portion of the light shining on it from original laser beam  9  such that feedback beam  11  is coaxial with or at least parallel and next to the optical axis of the original beam. 
     Part of the feedback beam  11  in the version illustrated in FIG. 2 b  is deflected by a semitransparent mirror  13  positioned at an angle to the optical axis to a fully reflecting mirror  33 . Fully reflecting mirror  33  reflects beam  11  back coaxial with or at least parallel and next to the optical axis. The optical path of feedback beam  11 , and hence the laser&#39;s overall external resonator, is accordingly considerably longer than that of the embodiment illustrated in FIG.  2 . 
     In the version illustrated in FIG. 2 c , light is subtracted from the internal resonator in laser diode  2  both upstream and downstream, by a mirror at each end, that is. The entire beam  10  leaving the internal resonator downstream is employed for the intended application (e.g. to print an image). The beam  15  leaving the internal resonator upstream on the other hand is entirely reflected in the form of a feedback beam  11  by a totally reflecting mirror  14  to the effective surface of diode  2 . Both resonator mirror in this version must have a reflection coefficient of less than 1. 
     The external resonator in this version is as long as the optical path between the upstream-reflecting mirror in laser diode  2  and totally reflecting mirror  14 . 
     FIG. 2 d  illustrates still another appropriate feedback system. In this version as well, only one laser beam  9  leaves laser diode  2 . Laser beam  9  is separated into a utility beam  10  and a feedback beam  11  by the semitransparent mirror  13  in beam splitter  4 . Feedback beam  11  is directed into an optical feedback component  7 . Optical feedback component  7  can include both deflectors and reflectors or fiber-optical devices. Optical feedback component  7  transmits feedback beam  11  to optical input component  8 , a series of lenses for example. The lenses focus feedback beam  11  onto the upstream mirror of the resonator in diode  2  and returns the beam the diode&#39;s effective surface. 
     FIG. 3 provides details of a beam feedback system appropriate for generating coherence collapse inside laser diode  2 . Diode  2  is mounted on a support  17  (heat sink) in a module  16 . The laser beam  9  generated in diode  2  leaves it divergent through a window in module  16 . Laser beam  9  is then shaped into a parallel beam by a collimating lens  18 . The focal point of lens  18  is located precisely at the deflection mirror of the laser diode&#39;s internal resonator. The lens&#39;s focal length is f 1 . Parallel laser beam  9  shines on a semitransparent plane mirror  19 . The front  20  of plane mirror  19  is dielectrically coated to reflect between 1 and 20% and preferably 5% of the beam straight back. The reflected component constitutes a feedback beam  11 . The back  21  of mirror  19  transmits all the wavelengths characteristic of diode  2 , all the modal wavelengths it can emit, that is. The parallel beam  10  leaving plane mirror  19  is refocused by another lens  22 . At the focal point of lens  22  is the stock (e.g. Film) that the image is to be printed on. The stock is located in a plane  24  tilted at an angle α to a plane perpendicular to optical axis B, preventing the stock from reflecting too much light back to the effective surface of diode  2  along the optical axis. Such random reflections have turned out to be especially detrimental to the quality of reproduction. The present invention on the other hand ensures intentional and controlled reflection of specific wavelengths onto the diode&#39;s effective surface. 
     FIG. 3 b  illustrates a focusing system that can be incorporated into the version illustrated in FIG. 3 a  instead of the afocal system in the vicinity of plane mirror  19 . This system features a focusing lens  25  upstream and another focusing lens  26  downstream of mirror  19 . The positions and focal lengths of the lenses ensure that they will focus directly on the semitransparent front  20  of mirror  19 . 
     The semitransparent mirror  19  in the version illustrated in FIG. 3 a  must be precisely perpendicular to optical axis B to ensure satisfactory feedback of beam  11  to the effective surface of laser diode  2 . The relation of the semitransparent mirror to that axis in the version illustrated in FIG. 3 b  on the other hand is not as critical. The focal relations between lenses  25  and  26  and the front  20  of mirror  19  ensure that the feedback beam  11  will extend precisely along optical axis B even when mirror  19  is not precisely perpendicular to that axis. 
     FIG. 3 c  illustrates another improved version of laser-diode feedback system. A cubical beam splitter  27  diverts approximately 10% of the beam  9  leaving diode  2  into a diverted beam  29  and allows the other 90% through to the stock in the form of a utility beam  10 . Diverted beam  29  shines onto a plane surface  28 , which can constitute either an opaque mirror or the semitransparent incident surface of a photodiode  31 . If surface  28  is an opaque mirror, diverted beam  29  will be entirely fed back to the effective surface of diode  2 . If surface  28  is semitransparent on the other hand, the light that enters photodiode  31  can be exploited to regulate any optical fluctuations in diode  2  by way of an appropriate signal-feedback line  31   a  with associated controls. Another advantage of this version is that the signal from photodiode  31  can be exploited to detect low-frequency noise suppression or coherence collapse. The same signal can then be exploited to advantage to adjust the overall system for example even during the illumination of an operation surface to control the diode itself. 
     The separated unfocused beam  29  is focused onto surface  28  by a condensing lens  30 . It has been discovered important in this version for surface  28  to be precisely at the focal point of condensing lens  30  at focal length f 2 . 
     The diverted beam  29  can be fed back very precisely to diode  2  in the form of a feedback beam  11  in this version as well. 
     The laser diode  2  in this version can be very precisely positioned at the focal point of collimating lens  18  because module  16  is mounted on a base  23  that can be displaced both along optical axis B and above and below it. 
     Photodiode  31  is also mounted on a base  32  that can be displaced in the x, y, and z directions in relation to lens  30 . One version that is simpler than the version illustrated in FIG. 3 c  entirely lacks a condensing lens  30 , and diverted beam  29  is reflected directly back by a reflecting surface  28  perpendicular to its path. 
     Displaceable bases  23  and  32  can be provided with stepped motors or piezo-electric actuators. Their position can be varied by known controls such that surface  28  and that of diode  2  will be directly at the focal point of their particular adjacent lens. 
     FIG. 4 is a detail illustrating a laser diode  2  mounted on a base  35  such that the diode can emit light from each end of its optical resonator. In this chip-on-base system, utility beam  10  is diverted along optical axis B at one end of the diode&#39;s resonator and feedback beam  11  at the other end. Collimating lens  18  unfocuses feedback beam  11 , and condensing lens  30  focuses it on surface  28 , precisely at the end of focal length f 2 . Surface  28  subsequently directs feedback beam  11  to the effective surface of diode  2 , generating coherence collapse. In this system as well it is necessary for the effective surface of or the deflecting mirror in diode  2  to be directly at the end of the focal length f 1  of collimating lens  18 . Another lens  36  collimates utility beam  10 , making it available for its intended purpose, printing an image on a film for example. 
     FIG. 5 illustrates another chip-on-base system wherein feedback beam  11  is separated from the beam  9  leaving laser diode  2  by a cubical beam splitter  27 . Feedback beam  11  is introduced into a fiber-optical light guide  38  by a gradient-index rod (GRINROD)  37 . Light guide  38  forwards the light through another GRINROD  39  to the upstream surface of diode  2  and to its effective surface. 
     FIG. 6 illustrates the essential components of laser diode  2 . The diode comprises a doped-on positive semiconductor layer  40  and a doped-on negative n semiconductor layer  41 . The laser&#39;s activity occurs in an effective surface  42  at the interface between layers  40  and  41 . At least one of the terminal surfaces of the crystal has a reflectivity of less than 100%, allowing light to be diverted outside from effective surface  42 . The result is a deflection-dictated light-emitting surface  43  at the laser mirror. The light emitted from effective surface  42  is fed back to it in accordance with the present invention. Feedback is preferably achieved through one of the two laser mirror. To ensure total coherence collapse, the cross-section ( 44 ) of the fed-back light beam must overlap as much of the cross-section ( 43 ) of the emitted-light beam as possible. An overlap of 50 to 100% has been proven effective. 
     FIG. 7 illustrates the result of laboratory tests of the present invention. FIGS. 7 a ,  7   b , and  7   c  show spectra  45 ,  46 , and  47  measured by optical analysis (with a Fabry-Perot interferometer) of a free spectral range (FSR) of 4.95 GHz. These spectra derive from a distributed Bragg-reflection (DBR) monomodal laser diode. The feedback is of the nature illustrated in FIG. 3 c . Spectra  45  and  46  were produced by positioning the surface  28  illustrated in FIG. 3 20 and 10 □μm away form the precise focal point of condensing lens  30 . 
     The spectrometer was positioned where a film would normally be positioned. As will be evident from spectrum  45 , the laser diode  2  in this system has generated just one mode. The interferometer was jiggled in this test over approximately five free spectral ranges. The monomodal spectral line representing the DBT laser is accordingly indicated five time at time intervals ΔT of approximately 8 msec. As surface  28  approaches the precise focal point of condensing lens  30 , spectrum  45  merges into spectrum  46 , and it will be obvious that the laser has begun activity in three different resonance modes. 
     Once surface  28  arrived precisely at the end of the focal length f 2  of condensing lens  30 , the laser activity changed to multimodal, with a difference Δf in frequency of 525 MHz between the various modes. In this spectrum  47  accordingly, the laser&#39;s output has been distributed over several different modes. 
     FIG. 8 illustrates the results of various modal spectra. FIG. 8 a  schematically shows two noise spectra over a frequency f. They were obtained with the system illustrated in FIG. 3 c  with a reflecting surface  28  in the form of a photodiode. Spectrum  48  represents the signal-to-noise ratio (relative-intensity noise, RIN) that accompanies mode hopping. Spectrum  49  represents the RIN at coherence collapse. It will be evident that the signal-to-noise ratio is very low and constant at low frequencies, of less than 0.5 GHz, that is. This ensures very constant image quality with a low pixel frequency of less than 0.5 Ghz. 
     FIG. 8 b  shows the empirical low-frequency noise level  50  in the 0-10 MHz range as a function of time. Coherence collapse occurs in approximately 2 minutes, with the noise level dropping rapidly by more than 25 dB. This result confirms the stabilizing effect of coherence collapse. 
     Coherence collapse can also be employed to stabilize diode outputs in a system wherein a laser diode is operated extensively statically but can be switched back and forth between two different outputs. Such a system is employed for example in medicine to excite stimulable phosphor-coated films that carry a latent x-ray image, generating a laser beam point by point that allows the image to be read. 
     Various examples of how a laser diode can be operated in accordance with the present invention in coherence collapse have been specified. It will accordingly be evident that various variations can occur to one of skill in the art. Complete fiber-optical separation can be provided inside a laser-diode module instead of the external fiber-optical system specified herein. In such systems a focusing lens would be unnecessary, and the feedback could be achieved by simple reflection at the cable faces. Combining the individual features of the various embodiments specified herein is also absolutely possible. It is also absolutely possible to combine the various methods and devices specified herein with state-of-the-art methods and devices to stabilize laser diodes against mode hopping. 
     It has been demonstrated that the system specified herein can be employed in conjunction with the method specified in German Patent Application 19 607 880.6, which patents have also been applied for worldwide (e.g in Japan and the U.S.A.). The contents of these applications are accordingly hereby incorporated by reference into the present specification. Operating the laser diode with both a noise signal and an adjustment-or-modulating signal is in particular specified therein. Since the noise signal is not a determinate signal but a random broad-band signal, a number of modes is generated inside the diode. The result is that the overall laser energy is uniformly distributed over several different modes, leading to an essentially lower tendency on the part of the diode&#39;s output to fluctuate suddenly and irregularly due to mode hopping. It is in this application of advantage for the noise signal to have the widest possible band in the range of 1 to approximately 100 MHz. 
     The laser diode in the system specified in the German Application No. 19 607 880.6 is in particular actuated not only by an intensity-adjustment signal but by a wide-band bandpass noise signal. The spectrum for this non-periodic incidental signal is continuous, with all frequencies occurring at the same amplitude from a lower threshold f u  to an upper threshold f o . The spectrum of the noise-modulated laser beam accordingly exhibits continuous sidebands wherein all frequencies are uniformly represented. The continuous spectral distribution of the also occasions sideband frequencies that math the resonance frequencies of a parasitic external resonator. The active laser diode will accordingly amplify the energy available at the external-resonator frequencies until unattenuated laser fluctuations occur. Such noise modulation also makes it possible to distribute the output of the diode to a large number of external-resonator modes. Mode hopping and its associated abrupt changes in output will accordingly be extensively suppressed. 
     Frequency-selective superimposition circuitry is employed to superimpose the noise signal over the adjustment or modulating signal. It acts like a low-pass filter with respect to the utility signal and as a high-pass filter in terms of the noise signal. This circuitry constructs, preferably additively, the combined signal that engages the laser diode. 
     FIG. 9 illustrates one way to additively superimpose a noise signal over an intensity-adjustment (modulation-imaging or static-adjustment) signal in the electronic controls  61  employed with the laser module  16  illustrated in FIG. 2 or  3 . Imaging or adjustment signals arriving at an imaging-signal amplifier  63  are forwarded to a frequency-selective superimposition circuit  64 . Circuit  64  superimposes a noise signal generated by a noise generator  62  over the imaging signal. Superimposition circuit  64  accordingly prevents mutual interference by the signal generators. The combined signal is then forwarded to the laser diode  2  in a module  66 , which produces a laser beam  65 . 
     With the diode in the static state, the difference between the optical amplification created by the lasering material and the optical losses experienced by the laser&#39;s resonator, the gain difference, that is, is zero. During high-frequency disturbance, whereby the frequency spectrum is on the same order of magnitude as that of the laser&#39;s inherent resonance, gain difference will vary over time. The effective level of this time-related fluctuation, the dynamic gain difference, is a measure of the actuation of further internal and external diode modes. 
     FIG. 10 illustrates dynamic gain difference Δg as a function of the current flowing through a laser diode. Dynamic gain difference is influenced by the strength of the optical feedback and by the effective level of any external noise signal. If mode hopping is to be effectively suppressed, 
     
       
           Δg&gt; 0.01 cm −1   
       
     
     must hold for external, and 
     
       
           Δg&gt; 1.0 cm −1   
       
     
     for specifically mode hopping. 
     These curves can be calculated theoretically in a model and experimentally confirmed. Curve  70  results from wide-band noise modulation. Curves  71 ,  72 ,  73 , and  74  derive from optical feedback in the laser diode, whereby curve  71  is based on an internal-feedback factor of 2·10 −7 , curve  72  on a factor of 2·10 −5  curve  73  on one of 2·10 −4 , and curve  74  on one of 2·10 −3 . 
     It will be evident that noise modulation will suppress more effectively at less powerful currents, at lower light intensities, that is, than a powerful optical feedback (coherence collapse) will. At more powerful currents, however, at higher light intensities, a sufficiently powerful optical feedback is more effective than noise modulation. 
     The method of noise modulation specified in German 91 607 880.6 does not need special harmonizing measures to ensure optimal mode-hopping suppression. This is also true in the presence of several external resonators with different types of mode hopping as long as the particular resonance frequencies are within the band width of the optical spectrum. Furthermore, superimposing the noise signal over the adjustment signal increases the available light output beyond what can be achieved at the state of the art, which employs scanning. 
     With semiconductor lasers, the band of the optical spectrum is essentially wider than twice the width of the noise signal because the direct modulation of such component results not only in amplitude modulation but in frequency modulation as well. The band width B of the optical spectrum it establish by the frequency-modulation slope S FM  and by the effective level I eff  of the noise current: 
     
       
           B=S   FM   ·I   eff .  (B3) 
       
     
     This relationship makes it possible to cite a simple requisite for effective suppression of mode hopping by noise modulation. If L ext  is the distance between the laser diode and the first optically active surface (point of reflection), a noise current with an effective level of 
     
       
           I   eff   &gt;c/ ( S   FM   ·L   ext )  (B4) 
       
     
     will be of particular advantage. At this distance, all mode hopping provoked by farther-downstream points of reflection will also be suppressed. The FM slope S FM  of a laser diode usually ranges from 100 to 500 MHz/mA. At a typical slope of 200 MHz/mA for example, a noise current with an effective level of at least 5 mA will be especially satisfactory for a resonator 30 cm long. Since the effective level of a noise factor is proportional to the root of the product of band width and spectral-output density, both wide-band noise at low densities and narrow-ban noise at higher densities can be employed for noise modulation. This can be experimentally confirmed, with mode hopping suppressed at an effective noise current of approximately 5 mA independently of noise band width (5 and 50 MHz). 
     Combining the initially described method using coherence collapse with the finally described method using noise modulation results in a decisive advantage for applications in the fields of image scanning, image transmission, and image printing. The overall result is uniformly satisfactory suppression of mode hopping over the total usable range of laser-diode outputs. 
     There has thus been shown and described a novel method of and device for operating a laser diode which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims which follow.