Abstract:
A system and method of stabilizing laser output levels includes a laser ( 10 ), an injection circuit for injecting a radio frequency waveform, and a control circuit for energizing and stabilizing the laser. The radio frequency waveform injected by the injection circuit has a high duty cycle to maintain high output power while providing a stable multimode spectrum. A back facet photodiode sensor ( 102 ) detects radiation emitted from a back facet semiconductor laser ( 101 ) and provides a feedback signal to the control circuit ( 41 ) for maintaining the laser output power. The response of the photodiode is not fast enough to track intensity variations due to the RF waveform, and thus provides feedback to the control circuit ( 41 ) only when there is a substantial need to adjust laser power.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a Continuation-in-Part of U.S. patent application Ser. No. 09/788,747, filed Feb. 20, 2001 now abandoned, entitled SYSTEM AND METHOD FOR IMPROVING LASER POWER AND STABILIZATION USING HIGH DUTY CYCLE RADIO FREQUENCY INJECTION, by Roddy et al. 

   FIELD OF THE INVENTION 
   This invention relates in general to stabilized semiconductor lasers for imaging applications and in particular, to a high duty cycle radio frequency waveform injected into a semiconductor laser with a back facet closed loop control circuit. 
   BACKGROUND OF THE INVENTION 
   In many imaging applications, it is often desirable to have an inexpensive semiconductor laser device that provides constant wavelength and power output, as well as low noise. In one type of laser raster printing system, a photosensitive media is placed on a drum and is written to by a semiconductor laser. A light beam from the laser is typically deflected from a polygon or galvanometer, and focused through an imaging lens. The image is written pixel by pixel using a raster scan technique onto a photosensitive media. 
   Controlling the amount of laser energy delivered is important in achieving quality images. Unwanted variations in laser energy delivered to a photosensitive media can introduce objectionable artifacts, such as dark and light streaks or spots in the image printed on the media. In many image writing applications, laser optical power must be controlled to better than 0.5% accuracy in order to obtain a reasonable image quality. 
   Optical power is affected by many parameters, such as semiconductor laser driving current and operating temperature. In order to ensure that a laser operates at a stable condition, an operating temperature is chosen in which the laser operates at a wavelength which is relatively constant. For example, assume a particular laser has a relatively stable operating wavelength of 685 nm only over a narrow temperature range of 3° C. Outside this range, there would be variations in intensity of the laser output power as the laser hops from one mode to another. A thermoelectric cooler must be used to keep the laser in its stable range of operation. 
   Another problem which may cause variations in laser power output is caused by optical feedback, which is unwanted light reflected back into a laser by optical elements external to the laser. Optical feedback can disturb laser operation and cause intensity fluctuations which may amount to as much as 10% or 20%. As more components are added, such as in a collimator lens and beam forming optics, the stable temperature range in which the laser can operate may be decreased significantly from the 3° C. noted above, to only a few tenths of a degree. 
   Other factors may affect the stability of laser operating systems. For example, characteristics of some components change with age, and small contaminants may accumulate on the surfaces of the optical elements. This change can cause variations in reflectivity which results in optical feedback to the laser. 
   Past attempts to stabilize laser performance have met with mixed results. For example, thermoelectric coolers have been used to prevent drift with ambient temperature. However, over the operating life of the equipment, lasers still may change modes because laser characteristic changes or external optical elements shift, causing optical feedback. Furthermore, thermoelectric coolers add additional cost and complexity. 
   Another method of stabilizing laser is using back facet photosensors which detect a portion of the light leaving a back facet of the laser to control laser output. This has not been entirely successful because the layers of dielectric mirror coating on the back facet of the laser are wavelength specific. Therefore, small changes in the wavelength of the light leaving the back facet can result in large changes in power to the back facet sensor, while the actual laser output is essentially unchanged. The power control loop on the laser ends up making a light level correction where none should be made. 
   Another attempt at stabilization of lasers has used radio frequencies (RF) to stabilize low power level lasers, for example, laser printing in the range of 1 to 2 mW. These low power RF stabilization schemes, however, are not suitable for high power laser stabilization because of intensity control problems. This type of RF stabilization in a high power laser has a possibility of back-biasing the laser diode and destroying it. See U.S. Pat. Nos. 5,197,059; 5,386,409; and 5,495,464. Other undesirable features in RF control are decreased lifetime and overdriving of the laser. See U.S. Pat. Nos. 5,495,464 and 5,175,722. 
   A further attempt at stabilization of low power lasers has used radio frequencies with low duty cycle waveforms. U.S. Pat. No. 5,386,409 discloses the use of low duty cycle radio frequency waveform to stabilize a semiconductor laser for reading and writing to an optical disk. 
   In addition, attempts have been made to stabilize high power semiconductor lasers at approximately 20 to 100 mW using RF injection. U.S. Pat. No. 6,049,073 discloses a system and method for high power semiconductor laser stabilization using RF injection, where the RF waveform is a sine wave. This method of stabilization, however, only allows half the laser&#39;s rated output power to be available as stabilized power because 50% duty cycle sine wave is used as the RF drive. Driving the laser at higher current levels to increase power results in overdriving the laser and decreasing lifetime. Increasing the RF drive to the laser can result in back biasing the laser and destroying it. 
   It is, therefore, desirable to stabilize a high power semiconductor laser at or near its rated maximum power against changes in temperature, current variations, effects of aging, and optical feedback. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a high power output radio frequency injected stabilized semiconductor laser. It is another object of the invention to provide a laser with a stable spectrum output that allows for accurate back facet photodiode control. It is a further object of this invention to eliminate the need for thermoelectric cooling to control the output of the laser. It is an additional object of the invention to confine any inherent laser noise within each pixel of an image when the stabilized semiconductor laser is used as part of a raster scan image writing system, thus rendering the resultant spatial noise invisible to the eye. 
   The present invention is directed to overcoming one or more of the problems set forth above. According to one aspect of the present invention, a system for stabilizing laser output levels comprises a laser responsive to a control signal for generating a radiation beam. A control circuit connects to the laser providing the control signal to the laser. An injection circuit connects to the output of the control circuit injecting a high duty cycle radio frequency waveform into the laser. A back facet photodiode sensor detects radiation from a back facet of the laser and provides a feedback signal to the control circuit to maintain a power level of the laser constant. A radio frequency waveform causes the laser to oscillate above and below a DC bias point between the levels of a lasing threshold level and an asymmetrical level above a DC bias point. An injection circuit injects the radio frequency waveform with a duty cycle greater than 50%. 
   According to one embodiment of the present invention a radio frequency signal is injected into a semiconductor laser, wherein the waveform has a duty cycle greater than 50%. A control circuit connected to the laser provides the control signal and an injection circuit injects a radio frequency signal into the laser. A back facet photodiode sensor detects radiation emitted from a back facet of the laser diode and provides a feedback signal to the control circuit for adjusting laser output power. 
   The advantage of injecting a radio frequency waveform with a high duty cycle into a semiconductor laser is that the laser will have both high output power and stability without exceeding the maximum rated parameters of current or power. For example, a 50 mW laser with an RF waveform with a 90% duty cycle will allow 45 mW of stabilized power without driving the current above I op , the maximum rated current. In order to achieve high power, the laser is operated predominantly above laser threshold, and will only operate near the lasing threshold for short intervals of the duty cycle. 
   To achieve stability, the injection of the radio frequency waveform will force the laser to mode hop at high frequency, essentially forcing the laser to have a stable multimode spectrum. This result is accomplished by driving the laser down to or slightly below threshold, forcing it out of lasing and then allowing it to re-establish lasing at a rate of millions of times per second. Because the spectral output is stable over time, the current from the photodiode is truly representative of the laser output power. A shift in current represents a drift in laser output power, not a hop in laser wavelength. The rate of intensity fluctuation will be greater than that which a back facet photodiode detects because of the photodiode&#39;s response characteristics. From the low frequency perspective of the photodiode and feedback circuit, the laser intensity is stable. Since the spectrum detected by the photodiode is stable, historical problems associated with using a back facet photodiode and control circuit as a means of stabilizing a laser will be solved. Only significant slow drifts in the laser output power, not wavelength, will be detected, and the control circuit will make appropriate adjustments to the current supplied to the semiconductor laser. 
   The added complexity and cost associated with thermoelectric cooling can be eliminated. Because radio frequency injection creates laser stability, it eliminates the need to have a thermoelectric cooler control the temperature of the laser. Any changes in the output wavelength of the laser will be very minor, and it is unnecessary to introduce the expense and complexity of a thermoelectric cooler to control the laser. 
   Laser noise associated with mode hop that may normally appear in an image can be shifted to higher frequencies where it is not noticeable by the human eye. The present invention uses a circuit to inject a high duty cycle radio frequency waveform to force the laser to a stable multimode spectral output. Any mode hopping that occur will be at the injected radio frequency. Increasing the mode hopping frequency of a laser shifts the noise spectrum of the laser such that the intensity noise is averaged within each pixel, thus making the noise less visible in images that are written with lasers. 
   The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cut away perspective view of a semiconductor laser; 
       FIG. 2  is a schematic view of a radio frequency (RF) stabilized laser according to the present invention; 
       FIG. 3  is a graph of power versus input current for a stabilized semiconductor laser with a RF injected sine wave; 
       FIG. 4  is a graph of power versus input current for a stabilized semiconductor laser with a RF injected waveform having a 90% duty cycle; 
       FIG. 5  is a graph of input current versus time for a stabilized semiconductor laser with a high duty cycle RF injected waveform; 
       FIG. 6   a  is a schematic of a control circuit and RF injection circuit; 
       FIG. 6   b  is a more detailed schematic of the control circuit shown in  FIG. 6   a;    
       FIG. 7   a  is a schematic of a distorted sine wave oscillator circuit used to generate a high duty cycle RF waveform to be injected into a semiconductor laser; 
       FIG. 7   b  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th ; 
       FIG. 8   a  is a schematic of a shunt modulator circuit used to generate a high duty cycle RF waveform used to generate a high duty cycle RF waveform to be injected into a semiconductor laser; 
       FIG. 8   b  is a graph of a pulsed input signal to the shunt modulator circuit; 
       FIG. 8   c  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th ; 
       FIG. 9   a  is a schematic of a fast pulse network modulator circuit used to generate a high duty cycle RF waveform to be injected into a semiconductor laser; 
       FIG. 9   b  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be directed in particular to elements forming part of, or in cooperation more directly with, the apparatus in accordance with the present invention. It is understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     FIG. 1  shows a semiconductor laser  12 . Laser  12  is in a container defined by a cap  104  having an aperture  103  in a stem  106  and terminal  107 . A semiconductor laser element  101  is mounted on a heatsink  105  with a light-emitting face on the side of aperture  103 . A back facet photodiode  102  is fixed to stem  106  with a light receiving surface facing the semiconductor laser element  101 . A laser beam  110  and a light power output (P o ) is emitted from the semiconductor laser element  110  through aperture  103 . At the same time, a monitor beam  120  with a light power output (P m ), at usually about 3% of P o , is emitted from the semiconductor laser element  101  toward the photodiode  102 . The laser beam  110  is directed through writer optics, not shown. 
     FIG. 2  shows a RF stabilized laser configuration  10 . A laser diode  12  and laser drive assembly  40  are attached to an aluminum block  16  which is screwed to a heatsink plate  18 . The heatsink plate  18  is attached to a collimator mount  24 , which in turn is attached to mounting bracket  20 . Collimator mount  24  also holds a collimator lens  22 . The stabilized laser  10  is aligned to writer optics, not shown. In an alternate embodiment, the stabilized laser  10  is coupled to an optical fiber allowing the stabilized laser  10  to be mounted at a remote location. 
     FIG. 3  shows a graph of power versus input current for a 50 mW Mitsubishi 1413 R01 semiconductor laser, with a threshold just above 30 mA and a maximum current of 90 mA. When the DC level is set to 60 mA, the laser provides 25 mW of output, which is half of the rated value. The AC signal, provided by a Colpitts RF oscillator, is added to the applied DC level to swing the laser current from 30 mA (I th , laser threshold) to 90 mA (I op , the maximum rated optical power out). Laser power I op  is 50 mW for this laser and at I th  it is approximately 0.1 mW optical power. The semiconductor laser is turned on to maximum power and essentially turned off during each cycle of the RF. The RF frequency is typically about 200 MHz for a writer system with a pixel clock of approximately 20 MHz, thereby turning the laser on and off about 10 times during each pixel. The sine wave generated by the Colpitts oscillator has a 50% duty cycle, because it is easy to generate but has little or no higher harmonic content. Only the fundamental 200 MHz signal is generated, making it easier to deliver the signal to the laser diode. 
     FIG. 4  shows a graph of power versus input current with a 90% duty cycle waveform. When the duty cycle of the injection waveform is increased, the average power level of the stabilized power will be increased. For example, if the waveform has a duty cycle of 90%, then 45 mW out of a possible 50 mW would be stabilized output power. 
     FIG. 5  shows a waveform where the drive signal is predominantly at a high level, and only occasionally goes low in a very short duration spike. The spike must be low enough to take the laser below threshold and just long enough to disrupt lasing. 
     FIG. 6   a  shows a laser drive system  30 , a power level adjust  42 , and a control circuit  41  to provide constant laser power output by utilizing the feedback signal  50  from the photodiode  102 . A high duty cycle RF source  44 , commonly called an injection circuit, is injected into the semiconductor laser  101  to induce a stable multimode spectrum. 
     FIG. 6   b  shows the laser drive system  30  in more detail. It consists of a control system  41  with a power level adjust  42  and a high duty cycle RF source  44 . The power level adjust  42  is used to set a nominal reference level to drive amplifier  51  to provide a nominal DC drive current  52  to the diode laser  101 . The photodiode  102  senses the laser power output and generates a feedback signal  50  which is conditioned and amplified by amplifier  53  and sent to a summing junction  55 . Amplifier  51  varies the DC drive current  52  to the laser  101  such that the feedback signal  50  from photodiode  102  will match the signal from the power level adjust  42  at the summing junction  55 , a well known feature of this commonly used analog servo circuit. As the laser output varies with heat and aging, the variation in laser power is sensed by photodiode  102  and the drive current level  52  is automatically adjusted to keep the laser power constant. Because the laser wavelength can vary slowly or rapidly as a result of changes in temperature, drive current, laser aging effects and unwanted optical feedback, an erroneous signal can be generated by photodiode  102  causing undesirable fluctuations in laser output power. 
   To stabilize the wavelength spectra from the laser and thus avoid erroneous feedback signals from photodiode  102 , an RF drive current  56  from the high duty cycle RF source  44  is combined with the DC drive current  52  and the combined current is sent to drive the diode laser  101 . The DC current level, I bias , is set to approximately the middle of the lasing range, halfway between I th  and I op . As shown previously in  FIGS. 3 and 4 , I th  is 30 mA, I op  is 90 mA, and I bias  is approximately 60 mA. The RF level from RF source  44  is then adjusted such that the combined laser drive current now swings about the nominal DC level of 60 mA, down to or slightly below threshold at 30 mA and up to the maximum current I op  at 90 mA. The output power of the laser is now flashing at the RF frequency rate, typically around several hundred megahertz, with the output power varying from 0.1 mW to 50 mW at the RF rate. The photodiode  102  is too slow to respond to such a high frequency and controls the power based on the DC level. Forcing the laser out of the lasing and back into lasing tends to bring the laser up sequentially into the few favored modes. The mode structure for each lasing event may not be the same, but combinations of the same five or six favored modes are selected. If, in writing one pixel, ten of these mode combinations occur, then the wavelength and power output will tend to average. Since the averaging is occurring during a pixel in the raster scan writing device, the power variation caused by mode hopping in the laser will not be seen.  FIG. 3  represents prior art and shows an RF sinewave drive. A sinewave, by its very nature, has a 50% duty cycle. As shown in  FIG. 3 , the average stabilized power of the 50 mW laser is 25 mW, half of the rated power.  FIG. 4  shows that if a 90% duty cycle squarewave is used, the stabilized output power can be raised to 45 mW. Increasing the laser output power simply by increasing the DC current or the RF current level will respectively result in shortening the life of the laser or destroying it by reverse biasing. Increasing the duty cycle will cause neither of these two deleterious effects, and it will increase the power output. 
   The inductor  61  allows the DC current to pass to the laser while blocking RF from getting to amplifier  51 . The capacitor  62  allows the RF current to pass to the laser while blocking any DC current from getting to the RF source. The combination of the two is commonly known as a bias tee. 
     FIG. 7   a  shows a schematic of a distorted sine wave oscillator circuit used to generate a high duty cycle RF waveform to be injected into a semiconductor laser. A sine wave oscillator with excess feedback and altered bias is used to create an asymmetrical sine wave. When injected into the semiconductor laser, the asymmetrical radio frequency sine waveform is capable of stabilizing the output spectrum of the laser and increasing the laser&#39;s output power.  FIG. 7   b  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th . For example, if a 200 MHz RF distorted sine waveform is injected, the semiconductor laser is driven down to or slightly below threshold and forced to come back up into lasing at 200 million times a second. Based on the DC level, the RF is adjusted to drive the laser to operate at threshold or slightly below threshold. However, the drive current should stay above 0 mA. If the drive current is below zero, the laser could become back biased and be destroyed. Likewise, driving the laser above its rated I op  can cause damage or reduce the lifetime of the laser. Moreover, the multimode operation of the semiconductor laser will transfer the intrinsic noise of said laser to higher frequencies, thus substantially reducing their visibility when such a laser is integrated into a laser raster system capable of writing images. Furthermore, the back facet photodiode, which is used in combination with the control circuit to monitor and control the output of the laser, is not responsive to the fast switching at the radio frequency. The back facet photodiode cannot detect the rapid changes in the output of the laser, and therefore continues to supply the same amount of current. Changes in laser output are therefore only detected within the response characteristics of the photodiode. Because the laser spectral output is stable over time, the current from the photodiode is truly representative of the laser output power. A shift in current now represents a drift in laser output power, not a hop in laser wavelength. Thus, the historical unreliability of back-facet photodiodes to control laser output power is remedied. 
     FIG. 8   a  is a schematic of a shunt modulator circuit used to generate a high duty cycle RF waveform to be injected into a semiconductor laser. The shunt modulator circuit is comprised of a DC current source and an active device. The active device in  FIG. 8   a  is a single NPN bipolar transistor. However, other active components could be combined to produce the same effect in the shunt modulator circuit. The DC current is momentarily shunted by an active device connected in parallel with the ground or a suitable alternate load for a brief period of time.  FIG. 8   b  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th . When the pulsed input of the active device briefly shunts the current from the semiconductor laser, the laser operates at or below lasing threshold. While the current is not being shunted, the semiconductor laser operates above lasing threshold. Frequent switching between operation near lasing threshold and above lasing threshold will induce multimode operation in the laser. Adjusting the pulsed input signal to the active element of the circuit will affect the duration that the laser is lasing above threshold, and a stable laser with a high power will result. In addition, the multimode operation of the semiconductor laser will transfer the intrinsic noise of said laser to higher frequencies, thus substantially reducing their visibility when such a laser is integrated into a laser raster system capable of writing image. Furthermore, the back facet photodiode, which is used in combination with the control circuit to monitor and control the output of the laser, is not responsive enough to the fast switching. The back facet photodiode cannot detect the changes in the output of the laser caused by the high frequency RF injection, and therefore continues to supply the same amount of current. Changes in laser output are therefore only detected within the response characteristics of the photodiode. Because the laser spectral output is stable over time, the current from the photodiode is truly representative of the laser output power. A shift in current now represents a drift in laser output power, not a hop in laser wavelength. Thus, the historical unreliability of back-facet photodiodes to control laser output power is remedied. 
     FIG. 9   a  is a schematic of a fast pulse network modulator circuit used to generate a high duty cycle RF waveform to be injected into a semiconductor laser. The circuit consists of a DC current source, a transformer, and a diode wherein said diode is “fast clamping” and sensitive to large pulses that occur rapidly over time. A fast pulse generator, such as a blocking oscillator, is used to create narrow pulses that are superimposed onto the DC drive current to the semiconductor laser. Additional control circuitry is required to control the pulses, as well as to prevent reverse polarity on the semiconductor laser.  FIG. 9   b  is a graph of the semiconductor laser drive current showing laser operating current I op  and lasing threshold current I th . The graph shows that the laser current drive signal will allow the laser to operate above threshold, and operates near threshold for short periods. Frequent switching between operation near lasing threshold and above lasing threshold will induce multimode operation in the laser. Adjusting the pulsed input signal to the active element of the circuit will affect the duration that the laser is lasing above threshold, and a stable laser with a high power output will result. In addition, the multimode operation of the semiconductor laser will transfer the intrinsic noise of the laser to higher frequencies, thus substantially reducing their visibility when such a laser is integrated into a laser raster system capable of writing images. Furthermore, the back facet photodiode, which is used in combination with the control circuit to monitor and control the output of the laser, is not responsive to the fast switching. The back facet photodiode cannot detect the changes in the output of the laser, and therefore continues to supply the same amount of current. Changes in laser output are therefore only detected within the response characteristics of the photodiode. Thus, the historical unreliability of back-facet photodiodes to control laser output power is remedied. 
   Single longitudinal mode operation is the quietest method of laser operation. However, it is difficult to keep the laser from mode hopping for long periods of time. Driving the laser to multiple longitudinal mode operation with RF injection is the next quietest method of operation. In noise level tests on a semiconductor laser, it is believed that the laser is not necessarily operating multimode at any instant it is turned on. Rather, operating the laser at or slightly below lasing threshold allows it to resume lasing in any of the approximately 4 or 5 likely longitudinal modes. Cycling between near threshold and lasing many times during the writing of one pixel allows an averaging effect to take place. The RF frequency should be several times the pixel clock frequency or pixel rate, with 10 times being a reasonable value. Thus, if all mode possibilities are not of the same intensity, the exposure from the average of ten samples should not vary significantly. The noise is not completely eliminated, but it is effectively confined to each written pixel, and does not show up as light and dark spots in an image. In addition, driving the laser to essentially multimode operation yields a stable output, which eliminates the cost and complexity of laser output control by thermoelectric cooling. 
   Thus, it is seen that a stabilized laser according to the present invention using radio frequency signal injection is able to produce high power output, produce a stable output spectrum that eliminates the need for thermoelectric cooling, and confine the inherent laser noise within each pixel of an image. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
   PARTS LIST 
   
       
         10 . Laser configuration 
         12 . Semiconductor laser 
         16 . Aluminum block 
         18 . Heatsink plate 
         20 . Mounting bracket 
         22 . Collimator lens 
         24 . Collimator mount 
         30 . Laser drive system 
         40 . Laser drive assembly 
         41 . Control circuit 
         42 . Power level adjust 
         44 . High duty cycle RF source 
         50 . Feedback signal 
         51 . Amplifier 
         52 . DC drive current 
         53 . Amplifier 
         55 . Summing junction 
         56  RF drive current (high duty cycle RF waveform) 
         61 . Inductor 
         62 . Capacitor 
         101 . Semiconductor laser element 
         102 . Photodiode 
         103 . Aperture 
         104 . Cap 
         105 . Heatsink 
         106 . Stem 
         107 . Terminal 
         110 . Laser element 
         120 . Monitor beam