Abstract:
Both a system and method are provided for modulating the intensity of an output beam generated by semiconductor laser. The exemplary system includes a source of pulsating current connected to the laser that generates a pulsating beam of laser light, an external modulator having an input that receives the pulsating beam, and an output controlled by pulsating control signal, wherein the output beam transmitted by the external modulator output is modulated by changing a relative phase angle between the pulsating current powering the laser, and the control signal of the external modulator over time. The external modulator may be an intensity-type modulator whose output is controlled by a gate signal having a constant phase, and the source of pulsating current powering the laser may be variable phase in order to modulate the output beam with an external modulator having a simple structure. Both the system and method are advantageously compatible with DFB lasers, and avoid wavelength drift and the consequent thermally induced patterning effect by powering the laser with a pulsating current having a constant duty cycle.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Ser. No. 11/002,978, filed Dec. 3, 2004 by HU et al. and assigned to Corning, Inc. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to the modulation of a semiconductor laser and more specifically to a system and method for modulating a distributed feedback (DFB) or a distributed Bragg reflector (DBR) semiconductor laser without undesirable, thermally induced wavelength drift.  
         [0004]     2. Background  
         [0005]     Lasers find particular utility in display technologies such as computer screens and televisions and the like. In such displays, lasers generate the three primary colors (red, blue and green) which are mixed in various combinations to provide a color image. The output beam of each laser can be raster-scanned across the screen or can be stationary and employed to illuminate pixels forming an image (e.g., a motion picture film or spatial light modulator containing an image). The ability of a laser to provide a beam having excellent brightness characteristics leads to projectors that are more efficient and better performing compared to projectors utilizing incandescent bulbs.  
         [0006]     Semiconductor lasers such as DBR and DFB lasers are particularly useful for laser-based displays as their output beams can be efficiently converted to useful primary color wavelengths. For example, a 1060 nm DBR or DFB semiconductor laser tuned to a spectral center of a second-harmonic-generation (SHG) device such as a non-linear crystal may be used to generate a 530 nm beam. This provides a low-cost, compact and efficient non-linear source of green light.  
         [0007]     Generally, for technologies involving video displays, the optical power used to generate the intensity of a primary color needs to be modulated at a fundamental frequency of approximately 50 MHz and with an extinction ratio (i.e., the ratio of highest to lowest optical power) of approximately 40 dB. The practical and economical achievement of such a combination of high modulation speed and large extinction ratio has proven difficult in the prior art.  
         [0008]     One prior art technique for achieving a fast modulation and a large extinction-ratio in the combination of a semiconductor laser and second harmonic generator (SHG) is to rapidly modulate the wavelength of the output beam of the semiconductor laser. Such a modulation technique exploits the fact that non-linear SHG devices are typically capable of converting only a very narrow range of incoming laser wavelengths into longer wavelength light. In operation, the wavelength of the semiconductor laser beam rapidly scans across the narrow spectral width of a non-linear SHG device to produce the necessary intensity modulation. For example, if maximum green power is needed, the wavelength is tuned to the center wavelength of the non-linear crystal while, if zero green power is needed 10 ns later, the wavelength is tuned to one side or the other of the center wavelength that is outside of the spectral width of the SHG device.  
         [0009]      FIG. 1A  schematically illustrates a conventional DBR semiconductor laser  100  and a second harmonic generation (SHG) device  150 . The DBR semiconductor laser  100  includes a DBR portion  110 , a phase portion  120  and a gain portion  130 . The gain portion  130 , when injected with a continuous wave (CW) current, generates continuous optical power for the laser. The current injected into the DBR portion  110  makes large changes to wavelengths output from the laser and the current into the phase portion  120  makes small changes to the wavelength of the beam output of the laser. The SHG device  150  receives the beam produced by the semiconductor laser  100 , whose output intensity of the converted wavelength (green, for example) depends upon alignment of the DBR laser wavelength and the SHG device&#39;s spectral center. The beam output from the SHG device  150  is then directed to an output such as a display screen.  
         [0010]      FIG. 1B  schematically illustrates a conventional DFB semiconductor laser  160  and an SHG device  170 . The current injected into the DFB semiconductor laser  160  controls the output intensity from the laser, and the SHG device  170  receives the beam produced by the semiconductor laser  160 . The output intensity of the converted wavelength (green, for example) depends upon the current input into the DFB semiconductor laser  160 . The beam output from the SHG device  170  is then directed to an output such as a display screen.  
         [0011]     The simplest way to rapidly tune the DBR semiconductor laser&#39;s output wavelength is by injecting modulated current into the DBR portion and phase portion of the DBR semiconductor laser  100  while keeping the gain-portion current continuous and constant. As illustrated in the chart provided in  FIG. 2A , a video signal can require green light with an intensity of up to 100% within each bit period of the signal. The bit period width is the inverse of the system frequency, for example, the resident time of each pixel of a raster scan on a display screen. For the example shown in  FIG. 2A , an intensity of 100% is the brightest possible signal while 0% is dark. Thus, as illustrated in  FIG. 2A , the video intensity required for the first bit period is 100%, the intensity reduces to 0% for the second bit period and is increased to 40% for the third bit period.  
         [0012]     With conventional systems, the current injected into the DBR portion  110  is pulse width modulated based on the required intensity in each bit period. That is, the duration within one bit period in which the current is “on” is proportional to the intensity of the video signal in that bit period (shown in the first waveform from the top of  FIG. 2A ). Ideally, the wavelength of the output of a DBR semiconductor laser is shifted based on the carrier-induced effect and output to the SHG device  150  (shown in the second waveform from the top of  FIG. 2A ). The SHG device  150 , based upon the received beam, outputs a converted beam having an ideal intensity signal for display, as illustrated in  FIG. 2A . However, the simple scheme described above ignores the possible adverse thermal effect that the injection of current into the laser causes.  
         [0013]     Similarly, for a DFB laser, the current injected into the DFB laser  160  is pulse width modulated based on the required intensity in each bit period, as shown in  FIG. 2B . Ideally, the wavelength of a DFB semiconductor laser is constant. However, the wavelength changes according to temperature, as shown in  FIG. 2B .  
         [0014]     Generally, current injection into the DBR portion of a DBR semiconductor laser generates two effects within the DBR semiconductor laser. First, a carrier effect is generated that provides more carriers in the portion increasing carrier density and reducing the refractive index within the laser. As a result, a shorter wavelength beam is generated. Current injection also causes a heating effect which causes the temperature of the DBR semiconductor laser device to rise. More specifically, currents higher than zero rise the temperature in the DBR semiconductor laser, thereby increasing the refractive indices, which tend to generate a longer wavelength beam. The collective wavelength shift is produced by the combined effect of the carrier effect and thermal effect. For large current values that are needed to achieve large wavelength shift, the temperature rise is severe enough to reduce and sometimes completely reverse the carrier-induced wavelength shift. For the case of a DFB laser, the current injection causes DFB laser temperature to change and therefore red shifts the wavelength (i.e. shifts to a longer wavelength).  
         [0015]     Another feature of current induced thermal effect is that it provides a slow wavelength modulation process. The thermal effect, which causes the temperature of the laser to increase, has μs-to ms response time compared to the carrier effect that has ns response time. The degree of thermal effect also depends upon the current amplitude and the heat sinking conditions associated with the laser. The slow response of the thermal effect means that the wavelength does not change for pulse widths much smaller than 1 μs, for example, 20 ns. Slow thermal effect results in an undesirable patterning effect because the average heating depends upon the width of pulses and therefore on the pattern of the video signal. In other words, the DBR or DFB semiconductor laser wavelength at a particular bit of the video signal depends on the history of the previous bits of data.  
         [0016]      FIGS. 2A and 2B  illustrate the effect that the injection of current, and the resulting increase in temperature, can have on the operation of the laser. The adverse effects resulting from a temperature rise in the laser are also shown in the charts of  FIGS. 2A and 2B . Specifically, in  FIG. 2A , when injection current is applied to the DBR portion  110  of the DBR semiconductor laser  100 , and the current is constantly on, the DBR temperature rises as shown by the DBR temperature waveform in  FIG. 2A . As a result, the actual DBR wavelength waveform provided from the laser to the SHG device  150  will be distorted, and the resulting output from the SHG device  150  will also be distorted, and the required intensity of the original video signal is not achieved at the output of the SHG device  150 . These same adverse effects accompany the operation of DFB semiconductor lasers, shown in  FIG. 2B . The actual DFB wavelength provided from the laser  160  to the SHG device  170  will be distorted, and the resulting output from the SHG device  170  will also be distorted.  
         [0017]     Another example of modulating a semiconductor laser output is by modulating the output intensity instead of the wavelength that was discussed in previous paragraph. A modulating current is applied to the entire length of a DFB laser or the gain section of a DBR laser. High-level amplitude of the modulating current results in high output intensity and lower amplitude results in reduced output intensity. The laser wavelength is ideally maintained constant. For the case of an optical system consisting of a DBR or a DFB laser and a SHG, the laser output wavelength is desired to be constant and aligned to the spectral center of a SHG.  
         [0018]     Accordingly, what is needed is a way to modulate the output of a semiconductor laser, or the combination of such a laser and an SHG, without the creation of thermal drift that in turn creates an undesirable thermal patterning effect. Ideally, such a technique should be compatible with short pulse widths on the order of 10 ns and an extinction ratio of 40 db so that a high bit rate of information can be transmitted. Finally such a modulation technique should be easy and inexpensive to implement, and compatible not only with DBR lasers, but lower-cost DFB lasers as well.  
       SUMMARY OF THE INVENTION  
       [0019]     The invention is both a system and a method for the modulating of the intensity of an output beam generated by a semiconductor laser that overcomes the aforementioned shortcomings associated with the prior art. To this end, the system generally comprises a source of pulsating current connected to the laser that generates a pulsating beam, an external modulator having an input that receives the pulsating beam, and an output controlled by a pulsating control signal that transmits the output beam, wherein the output beam is modulated by changing a relative phase angle between the pulsating current and the control signal over time. Preferably, the pulsating current is variable phase while the control signal of the external modulator is constant phase so that a relatively simple and inexpensive external modulator may be used. While the external modulator may be either an intensity modulator or a wavelength modulator, an intensity-type modulator is again preferred for simplicity and cost reasons.  
         [0020]     According to some of the embodiments of the present invention, both the pulsating current and the control signal may operate at a constant 50% duty cycle. Additionally, while the pulse width of the pulsating current and the control signal may be equal, the pulse width of the pulsating current may be slightly smaller than that of the control signal in order to improve the extinction ratio of the modulated output beam.  
         [0021]     When an intensity-type external modulator is used, the control signal may be a gate signal, and the external modulator may advantageously be a video component such as a reflective wheel or prism that rotates at a constant rpm which has faces which are either light reflective or light absorptive.  
         [0022]     The invention further encompasses a method for modulating the intensity of a pulsating beam generated by a semiconductor laser by means of an external modulator that comprises the steps of (1) providing a pulsating current to the laser; (2) receiving the pulsating beam into an input of an external modulator having an output that is controlled by a pulsating control signal, and (3) varying a relative phase angle between the pulsating current and the pulsating control signal to modulate laser light transmitted by the output of the external modulator.  
         [0023]     Both the system and method advantageous eliminate wavelength drift and the previously described, thermally-induced patterning effect, and are compatible with relatively low cost DFB semiconductor lasers.  
         [0024]     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.  
         [0025]     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The above and other aspects, features and advantages of this invention will be described in relation to the following figures in which like reference characters refer to the same parts throughout the different views:  
         [0027]      FIGS. 1A and 1B  are schematic diagrams of a conventional 3-portion DBR semiconductor laser and a conventional DFB semiconductor laser with an SHG device;  
         [0028]      FIGS. 2A and 2B  are charts illustrating the thermally induced patterning effect of the DBR semiconductor laser and SHG and the thermally induced patterning effect of the DBR semiconductor FB semiconductor laser and SHG;  
         [0029]      FIGS. 3A and 3B  illustrate exemplary video display systems employing the external optical intensity modulator of the present invention;  
         [0030]      FIGS. 4A, 4B  and  4 C illustrate characteristics associated with the differential phase external wavelength modulation scheme of the present invention; they show various embodiments of the present invention;  
         [0031]      FIG. 5  illustrates the optical intensity of an optical system comprising of a DFB semiconductor laser and an external intensity modulator as a function of phase between current injection into DFB laser and gate signal into external intensity modulator in accordance with the present invention;  
         [0032]      FIG. 6  illustrates a DFB semiconductor laser and AOM used to model the present invention;  
         [0033]      FIG. 7  illustrates a flow diagram providing a method of differential phase modulation in accordance with the present invention;  
         [0034]      FIGS. 8A, 8B  and  8 C illustrate characteristics associated with a differential phase modulation scheme of the present invention with a constant duty cycle of 50% for the current pulse to the DFB laser and to the external modulator; and  
         [0035]      FIGS. 9A, 9B  and  9 C illustrate characteristics associated with a differential phase modulation scheme of the invention with a constant duty cycle of ≦50% for the current pulse to the DFB laser and 50% to the external modulator. 
     
    
     DETAILED DESCRIPTION  
       [0036]     An exemplary embodiment of the present invention relates to a method and associated system to enable efficient operation of a DBR or DFB semiconductor laser and to reduce the thermal effects associated with semiconductor lasers. Although specific embodiments will be illustrated and described herein with regard to controlling the relative phase angle of current injected into a DBR semiconductor laser or into a DFB semiconductor laser and the control signal (also referred to as a pulsating gate signal herein) to an external modulation, it should be appreciated by those of ordinary skill in the art that such a system and method would also be advantageous, for example, in applications for any semiconductor laser device in which current injection can cause adverse thermal effects.  
         [0037]     Additionally, while the DBR and DFB semiconductor lasers of the present invention are employed within an application directed to video signal processing and display, this disclosure is intended to cover any adaptations or variations of the present invention that generally relate to semiconductor lasers. For example, this invention can also be employed in such areas as optical data storage, image reproduction, optical communications as well as sensing instruments and the like.  
         [0038]     In the following detailed description of the exemplary embodiments, reference is made to the accompanying drawings that form part hereof, and in which is shown by way of illustration, specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and/or electrical changes may be made without departing from the spirit and scope of the present invention. The following detail description is therefore not to be taken in a limiting sense.  
         [0039]     In accordance with an exemplary embodiment of the present invention, phase modulated current injection into a DBR or DFB semiconductor laser can be efficiently employed within a video display system, as shown generally in  FIGS. 3A and 3B .  
         [0040]     In  FIG. 3A , a video signal  310  enters a video processor  320  that sends a signal to a controller  330 . The controller  330  interfaces with analog driver circuits  340 , which send current I G  to the semiconductor DBR or DFB laser  350 , and control (gate) signal I M  (for example, modulation voltage, or current) to the external modulator  360  (for example, an acoustic optical modulator). The output of the semiconductor laser  350  is likewise sent through the external modulator  360 , to a SHG device  370  and then is displayed  380 . Similarly, in  FIG. 3B , a video signal  310  enters a video processor  320  that sends a signal to a controller  330 . The controller  330  interfaces with analog driver circuits  340 , which send current I M  to the external modulator  360  and current I G  to the semiconductor DBR or DFB laser  350 . The output of the semiconductor laser  350  is sent first a SHG device  370 , then through the external modulator  360 , and then is displayed  380 .  
         [0041]     Additionally while an SHG device is discussed in accordance with exemplary embodiments, other types of wavelength-selective devices could be employed to provide an output. For example, passive optical filters could also be employed. Passive optical filters do not convert the wavelength of the output from a laser.  
         [0042]      FIG. 4A  illustrates the characteristics of current to a DFB laser (or the gain section of the DBR laser) and control signal (i.e., pulsating gate signal) to the external modulator. As shown in the figures, the current pulses to the DFB laser (or the gain section of the DBR laser) have a fixed duty cycle. The duty cycle is the ratio of the time when the current is “on” to the total time of bit period (time “on” divided by a sum of “on” plus “off”). The thermal load to the DFB laser is constant for each bit period, and as a result, the wavelength of the DFB laser output is constant. The net output intensity of the system in each bit period is determined by the differential phase angle, or the overlap integral as shown in  FIG. 5  below, between the current pulse to the DFB laser and the pulsating gate signal (i.e. pulsating control signal) to the external modulator. Because the external intensity modulator does not change the wavelength of the DFB output, the wavelength of the system is constant and aligned to the central wavelength of an SHG device without a patterning effect. The maximum conversion efficiency with the SHG is always realized.  
         [0043]      FIG. 5  shows a graph of the differential phase, also represented as an overlap integral, between the current pulse to the DFB semiconductor laser and the gate pulse to the external modulator.  FIG. 5  shows that the wavelength of the semiconductor laser (e.g, DBF semiconductor laser) is kept constant at 1060 nm and that the differential phase values change, resulting in changes (modulation) of the optical beam intensity.  
         [0044]      FIGS. 4A, 4B ,  4 C show various embodiments of present invention, using either a DBR or a DFB laser and an external modulator to create a differential phase, resulting in optical intensity modulation of the final output. In each of the embodiments, the control signal (i.e., pulsating gate signal) to the external modulator has a constant duty cycle, modulating either intensity or wavelength modulation. The intensity or wavelength of a laser, either DBR or DFB, are modulated by current with a variable time delay in a bit period that allows adjustment of the differential phase, because the changing of the phase angle for a laser modulation is more easily performed or less expensive than the changing of the phase angle of the control signal to the external modulator.  
         [0045]     For applications that require high-speed operation, such as laser projection display, the current pulses to the DBR or DFB laser carry the fast video signal with phase adjusted in each bit period. A low-cost and simple external modulator is used to simply turn on and off at constant speed. For the video display application, the second intensity modulator can possibly be a part of an imaging system so that only a small additional cost is added to the overall system cost. For example, each fact of a polygon scanning mirror popular in many laser projection systems can be made with a “good” (or “on”) surface and a “bad” (or “off) surface. When it is incident on the “good” surface, the light beam is directed onto the target display screen. When it is incident on the “bad” surface, the light beam is lost through scattering, absorption, diffraction, or reflection. In another example, a bi-position mirror can be integrated into the scanning mirror system to reflect the laser beam on and off at a 50% or less duty cycle. At each bit period, the phase angle of the current pulse to a DBR or DFB laser is adjusted to achieve the desired optical intensity.  
         [0046]      FIG. 4A  illustrates characteristics associated with the differential modulation scheme according to one embodiment of the present invention. In this embodiment both the semiconductor laser and the SHG device are intensity modulated (i.e., the optical beams from these devices are intensity modulated). More specifically, in this embodiment either the DFB semiconductor laser or the gain section of a DBR laser is modulated with current pulses. The current pulses to the DFB laser or gain section of a DBR laser have a fixed duty cycle but a variable phase angle. The fixed duty cycle results in a constant thermal load to the DFB laser or a DBR laser and a constant wavelength output. The external modulator (e.g. acoustic modulator) is modulated using a pulsating control (gate) signal with a fixed duty cycle and a constant phase angle that results in intensity modulation. The differential phase angle between the intensity modulation of the DFB laser or the gain section of a DBR laser and the intensity modulation of the external modulator determines the intensity of the final beam output (i.e, the system comprising the laser, the external modulator, SHG device; or the system comprising the laser, SHG device and the external modulator). When the differential phase angle is 0°, the pulse width of the SHG output is 50% of the bit period corresponding to 100% of the maximum intensity. Similarly, when the differential phase angle is 90°,—the pulse width of the SHG output is 25% of the bit period corresponding to 50% of the maximum intensity, and when the differential phase angle is 180°, the pulse width of the SHG output is 0% of the bit period corresponding to 0% of the maximum intensity.  
         [0047]      FIG. 4B  illustrates characteristics associated with the differential modulation scheme according to another embodiment of the present invention. In this embodiment the DBR semiconductor laser is wavelength and the external modulator is intensity modulated (i.e., the optical beams provided by the laser has a changing wavelength and the external modulator provides an output beam that is intensity modulated). More specifically, in this embodiment, the semiconductor laser used is a DBR semiconductor laser. In a DBR semiconductor laser, unlike a DFB laser which can only be intensity modulated, the wavelength and the intensity of the DBR laser can both be modulated, by changing the current to the DBR section of the laser and by changing the current to the gain section of the laser, respectively. In this embodiment, the wavelength of the DBR laser is modulated using current pulses with a fixed duty cycle and a variable phase angle. This may produce two different wavelengths, one corresponding to the “on” current and the other corresponding to the “off” current to the DBR section of a DBR laser. The intensity of the external modulator is modulated with a fixed duty cycle and a fixed phase angle, as in the previous embodiments. This embodiment takes the advantage that the SHG device is a wavelength-discriminating device and it is set to convert the wavelength corresponding to the laser&#39;s “on current” to a different (desired wavelength, for example green light), while the laser beam corresponding to other wavelength (i.e., to the “off” current) beam is passed through the SHG device unchanged. The result is a SHG device output with an optical intensity similar to the results of the previous embodiments. The differential phase angle between the wavelength modulation of the DBR laser and the intensity modulation of the external modulator determines the intensity of the output after the SHG device. When the differential phase angle is 0°, the pulse width of the SHG output is 50% of the bit period corresponding to 100% of the maximum intensity. Similarly, when the differential phase angle is 90°, the pulse width of the SHG output is 25% of the bit period corresponding to 50% of the maximum intensity, and when the differential phase angle is 180°, the pulse width of the SHG output is 0% of the bit period corresponding to 0% of the maximum intensity.  
         [0048]      FIG. 4C  illustrates characteristics associated with the differential modulation scheme according to another embodiment of the present invention. In this embodiment the DBR semiconductor laser is intensity modulated and the external modulator is wavelength modulated (i.e., the optical beams provided by the laser has a changing intensity and the external modulator that can be a SHG device provides an output beam that is changing wavelength). More specifically, in this embodiment, the semiconductor laser used is a DFB laser or a DBR laser. The DBF semiconductor laser or gain section of a DBR laser that is intensity modulated, exactly as described in reference to  FIG. 4A . The current pulses to the DFB laser or gain section of a DBR laser have a fixed duty cycle but a variable phase angle. The fixed duty cycle results in a constant thermal load for the DFB laser or a DBR laser and a constant wavelength output. However, the external modulator that can be the SHG itself modulates the wavelength of the laser output with fixed duty cycle and fixed phase angle rather than the intensity. The resulting laser intensity after the SHG device is the same as the intensity of the previous embodiments. This embodiment takes the advantage that the SHG device is a wavelength-discriminating device and it is set to convert the wavelength corresponding to the external modulator&#39;s “on signal” to a different (desired wavelength, for example green light), while the laser beam corresponding to other wavelength (i.e., to the “off” signal) beam is passed through the SHG device unchanged. The differential phase angle between the intensity modulation of the DFB laser or the gain section of the DBR laser and the wavelength modulation of the external modulator determines the intensity of the output the pulse width of the SHG output. When the differential phase angle is 0°, the pulse width of the SHG output is 50% of the bit period corresponding to 100% of the maximum intensity. Similarly, when the differential phase angle is 90°, the pulse width of the SHG output is 25% of the bit period corresponding to 50% of the maximum intensity. When the differential phase angle is 180°, the pulse width of the SHG output is 0% of the bit period corresponding to 0% of the maximum intensity.  
         [0049]     In testing, an acoustic optical modulator (AOM)  710  was used as the external intensity modulator, as shown in  FIG. 6 . The DFB laser  700  receives a modulation current with pulses of constant duty cycle and varying phase. The output, which has been intensity modulated, is received by the AOM  710  where the laser output is intensity modulated with a constant duty cycle and constant phase, to produce the modulated output  720 . The procedure used is shown in  FIG. 7 . Initially, a video signal is received and processed in step  810 . Then, the optical power, or required video intensity, associated with the video signal is obtained in step  820 . Then, based upon the obtained intensity, the phase differential is determined in step  830 . Next, current pulses are injected into the DFB laser in step  840 , and the pulsating control signal (gate signal) is injected into the external intensity modulator in step  850 . Finally, the output beam is passed through an SHG device and displayed in step  860 .  
         [0050]     The results of several tests are shown in  FIGS. 8A-8C  and  9 A- 9 C. In  FIGS. 8A, 8B  and  8 C, the DFB duty cycle is 50% and the AOM duty cycle is 50%, similar to those duty cycles shown in  FIG. 3A  above. In  FIG. 8A , the differential phase angle is 0°; in FIG.  8 B, the differential phase angle is 90°; and in  FIG. 8C , the differential phase angle is 180°. In  FIG. 8C , the output optical intensity is not zero due to the non-ideal rise and fall times of the AOM. In  FIGS. 9A, 9B  and  9 C, the DFB laser&#39;s current pulse duty cycle is &lt;50% in order to improve the extinction ratio, and the AOM duty cycle is 50%. In  FIG. 9A , the differential phase angle is 0°; in  FIG. 9B , the differential phase angle is 90°; and in  FIG. 9C , the differential phase angle is 180°.  
         [0051]     The present invention provides several distinct advantages over conventional systems. By employing the present invention, under high-frequency current pulse injection, the temperature of the DBR or DFB laser is constant. By changing the phase differential angle between the current pulse to the laser and the pulsating control signal to the external modulator, the output intensity of the output beam provided by either the DBR or DFB laser in conjunction with the external modulator, such as or an acoustic modulator and/ for SHG example, can be changed without a thermally-induced patterning effect.  
         [0052]     It is, therefore, apparent that there has been provided, in accordance with the present invention, a method and system for intensity modulation. While this invention has been described in conjunction with a number of illustrative embodiments, it is evident that many alternatives, modifications, and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, the disclosure is intended to embrace all such alternatives, modifications, equivalents and variations that are within in the spirit and scope of this invention.