Abstract:
A diode-pumped solid-state laser (DPSSL) has self-maintained multi-dimensional optimization. The output property of the DPSSL, including optical power, noise level, and the operation conditions of its individual components, including the drive current and temperature of the laser diode, the temperature of the laser crystals and laser cavity, the drive current of the thermoelectric coolers, is monitored and systematically optimized in real time through automatic electronic control using a microprocessor. Such monitoring and optimization enable the DPSSL to maintain its optimum performance in output power, beam quality, noise level, and stability, throughout its lifetime regardless of component aging and change of environmental conditions. A highly accurate temperature monitoring and control method is also developed.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims the benefit of U.S. Provisional Application No. 60/609,244, filed Sep. 14, 2004, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention generally relates to a laser apparatus, and more specifically, to a diode-pumped solid-state laser with self-maintained multi-dimensional optimization.  
       DESCRIPTION OF RELATED ART  
       [0003]     A diode-pumped solid-state laser (DPSSL) is a complex system composed of multiple components including: a diode laser or laser array as the pump source; a laser cavity, which further includes a gain medium, such as a laser crystal or fiber, and a nonlinear medium such as a nonlinear optical crystal or other nonlinear optical media for non-linear optical frequency conversion (including but not limited to frequency doubling, tripling or parametric oscillation); one or more thermal-electric coolers (TECs) for temperature control; and electronic circuits to drive the laser diode and the TECs. Correspondingly, the performance of the DPSSL in its output power, beam quality, noise level, and stability is a complex function of the conditions of these components. With environmental variation and the aging of its constituent components, a monitoring and control system is required to keep the DPSSL in its optimum performance.  
         [0004]     The prior arts for DPSSL monitoring and control are limited to monitoring and control of only one or two parameters for some specific components of the DPSSL. Thus the overall performance of the system cannot be guaranteed. Those prior arts include U.S. Pat. No. 5,754,574 to Lofthouse-Zeis, et al., in which the temperature of the diode laser is controlled in accordance with its current to match its wavelength with the external cavity resonance. In U.S. Pat. No. 6,292,498 to Pfaff, et al., the wavelength of the diode laser is monitored and controlled by employing a temperature modulation/detection technique. In U.S. Pat. No. 5,177,755 to Keith Johnson, the noise level of the DPSSL is monitored and used to control the drive current of the diode laser to achieve the lowest relative intensity noise (RIN). In U.S. Pat. No. 5,265,115 to Amano, the drive current of the diode laser is controlled to maintain a constant output power for the DPSSL. In U.S. Pat. No. 5,267,252 to Amano and U.S. Pat. No. 5,181,214 to Berger, et al., the temperature of the laser components is maintained at a constant level by mounting all the components on a common thermal conductive support. In summary, none of these references provides a systematic solution to monitor and control the overall performance of the DPSSL.  
       SUMMARY OF THE INVENTION  
       [0005]     The primary object of the present invention is to systematically and automatically monitor and control the overall performance of the DPSSL, which is a complex multi-dimensional function determined by the operation condition of its individual components, such as the current and temperature of the laser diode, the temperature of the laser crystals, the temperature and length of the laser cavity, and the drive current of the thermal electric coolers (TECs). The method comprises a first step of utilizing multiple sensor elements to monitor the output of the DPSSL and the operation condition of its components and a second step of utilizing a microprocessor to collect the sensor information and adjust the parameters of the individual components to find and maintain an optimum overall performance of the DPSSL. The sensor elements include, but are not limited to, photo detectors to monitor the output power and noise level of the DPSSL, thermistors to monitor the temperature of the laser diode(s), the laser crystal(s), and the laser cavity, and current sensors to monitor the drive current of the laser diode and the TECs.  
         [0006]     It is another goal of the current invention to provide a highly accurate temperature monitoring and control method to solve the problem that the thermistors may not reflect the real temperature of the target components as they are generally placed a small distance way from the targets. The method involves utilizing the drive current of the TECs that are used to control the temperature of components as another temperature indicator, since the TEC drive current will vary with environmental temperature to maintain the temperature of the thermistors.  
         [0007]     The DPSSL disclosed in this invention can operate at its fundamental lasing frequency (wavelength) or at other frequencies (wavelengths) through non-linear optical conversion such as harmonic generation and parametric oscillation.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Preferred embodiments of the present invention will be set forth in detail with reference to the drawings, in which:  
         [0009]      FIG. 1  illustrates a schematic view of the multi-dimensional optimized DPSSL;  
         [0010]      FIG. 2  illustrates a schematic view of the multi-dimensional optimization mechanism;  
         [0011]      FIG. 3  shows one example of the optimization process, where the noise levels of the DPSSL are measured under different laser diode and nonlinear crystal temperatures;  
         [0012]      FIG. 4  shows another aspect of the optimization process, where the drive currents of the laser diode are measured under different laser diode and nonlinear crystal temperatures;  
         [0013]     FIGS.  5  ( a ) and  5  ( b ) show the mode patterns of the DPSSL under different laser diode and nonlinear crystal temperatures; and  
         [0014]      FIG. 6  illustrates the mechanism of using TEC current (voltage) as another temperature indicator for accurate component temperature control.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Preferred embodiments of the invention will now be set forth in detail with reference to the drawings.  
         [0016]     In one preferred embodiment of the current invention as shown in  FIG. 1 , the DPSSL comprises a laser head  10  and a laser driver  11 . The laser head  10  further comprises a laser diode  12  as the pump source; a laser cavity  13  formed by laser gain crystal  14 , nonlinear optical crystal  15  and two mirrors  16 ,  17 ; thermal control units  18 ,  19  for monitoring and controlling the temperature of the laser diode  12  and the laser cavity  13 , respectively (the thermal control unit  19  further comprising thermal control units  20 ,  21  for monitoring and controlling the temperature of the laser gain crystal  14  and the nonlinear optical crystal  15 , respectively); a servo-control unit  22  for fine tuning of the laser cavity length; an optical filter  23  to select the desired DPSSL output wavelength; and a beam splitter  24  to tap a portion of the output power and send it to a photo detector  25  to monitor the power and noise level of the DPSSL. Each thermal control unit comprises a heat sink  29 , a thermistor  30  attached to the heat sink  29  as temperature sensor, and a TEC  31  for temperature control.  
         [0017]     In this specific embodiment, the laser diode  12  emits at a center wavelength of 809 nm. The laser gain crystal  14  is a Nd: YVO 4  crystal, which emits at 1064 nm under the diode pump. The nonlinear optical crystal  15  is a KTP crystal, which doubles the laser frequency and results in a laser output at 532 nm. The two mirrors  16 ,  17  are formed by depositing thin film reflectors on the corresponding surfaces of the laser gain crystal  14  and the nonlinear crystal  15 , respectively. The servo-control unit  22  is composed of piezo-electric micro-positioners to fine tune the relative position of the laser gain crystal  14  and the nonlinear optical crystal  15 . The laser driver  11  further comprises a driver circuit board  26  to drive the laser diode, the TECs, and the micro-positioners, and a microprocessor  27  for intelligent monitoring and control. The information collected by the sensor elements, including the laser power and noise level that is obtained by the photo detector  23 , the temperature of the laser diode and the laser cavity that is obtained by the thermal control units  18 ,  19 , the drive current of the laser diode and the TECs, the drive voltage of the micro-positioners, are all fed into the microprocessor  27  via A/D conversion to monitor the overall performance of the DPSSL. Based on the collected information, the microprocessor controls and adjusts the operating conditions of individual components in the laser head  10 , including the drive current and temperature of the laser diode, the temperature of the gain crystal and nonlinear crystal, the drive current of the TECs, and the drive voltage of the micro-positioners, by controlling the driver circuit board  26  via D/A conversion to achieve and maintain the optimum overall performance of the DPSSL. The microprocessor  27  also has the capability of sending alarm signals to an external system  28 , which in most cases corresponds to the instrument the laser is embedded inside, through its communication port when the DPSSL performance does not reach the specifications. The microprocessor  27  can also receive and react up on any particular command from the external system  28 .  
         [0018]      FIG. 2  illustrates the multi-dimensional optimization mechanism for the DPSSL. The operating conditions for individual components of the DPSSL, such as the drive current applied on the laser diode (or diode array) and on the TECs (I 1 , I 2 , . . . , I 1 ), the drive voltage applied on the piezo-electric micro-positioners (V 1 , V 2 , . . . , V k ), the temperature of the laser diode and the laser crystals (T 1 , T 2 , . . . , T n ), the length of the laser cavity, the relative position between the laser crystals (S 1 , S 2 , . . . , S m ), form a multi-dimensional system. There are some operating conditions (the shadowed regions in  FIG. 2 ) under which the DPSSL has the best overall performance. In the current embodiment, the microprocessor can automatically locate these optimum conditions by scanning the setting of each component in certain ranges. This is preferred for manufacturing automation, repair and replacement, and field maintenance, as different laser heads and different electronic parts become interchangeable with the coordination of the microprocessor, which acts as an automatic ‘match maker’ for the laser head and laser driver. The microprocessor can also correct the preset operating conditions (P 0  in  FIG. 2 ) by monitoring the output status of the DPSSL and adjusting the settings for each component in real time until an optimum operation condition is found (P 1  in  FIG. 2 ). In case of environmental variation (such as temperature change) and component aging, the microprocessor can automatically change the component settings to maintain the optimum performance for the DPSSL over its whole lifetime. To increase the searching speed, the microprocessor may scan in the vicinity of certain optimum conditions that are preloaded in its memory during the manufacturing process.  
         [0019]     One example of the multi-dimensional optimization process is shown in  FIG. 3  and  FIG. 4 , in which the DPSSL operates in a constant output power mode. A feedback control circuit on the driver circuit board  26  is used to automatically adjust the laser diode drive current according to the power level of the DPSSL that is measured by the photo detector  25 . In this example, the temperature of the laser diode and the temperature of the nonlinear crystal (KTP in the current embodiment) are the two target variables that need to be optimized. The drive current of the laser diode, and the noise level of the DPSSL, which is measured from the DPSSL power fluctuation over a time period, are used as the two criteria for the optimization process. The goal of the optimization process is to achieve a low noise level and a low drive current simultaneously at an optimized laser diode and nonlinear crystal temperature setting. The microprocessor  27  performs a two-dimensional scan on the temperature setting of the laser diode and the nonlinear crystal by adjusting the corresponding TEC drive current and records the variation of the noise level of the DPSSL, which is measured by the photo detector, and the drive current of the laser diode, which is automatically adjusted by the feedback control circuit to maintain the DPSSL output power.  
         [0020]      FIG. 3  shows the measured noise level variation of the DPSSL at different laser diode and nonlinear crystal temperatures. The temperature values are measured from the resistance values of the thermistors in the corresponding thermal control unit. The resistance range of the thermisters corresponds to a temperature range from 37° C. to 16° C. In  FIG. 3 , the noise level is represented by the gray scale of the data points, where a dark shade corresponds to a high noise level while a light shade corresponds to a low noise level.  
         [0021]      FIG. 4  shows the measured drive current variation of the laser diode during the scan process. The current value is represented by the gray scale of the data points in a similar way as in  FIG. 3 . It can be seen that both the noise level and the drive current are complicated functions of the two temperature settings. At certain temperature settings, the DPSSL achieves low noise level and low drive current simultaneously. These points are treated as ‘good’ temperature settings. The function of the microprocessor is to locate an optimum temperature setting (such as point P 0  in  FIG. 3  and  FIG. 4 ), where both low noise level and low drive current are achieved and maintained over a wide temperature range around the optimum setting. The optimum position can be located through many well-known mathematical methods.  
         [0022]     In the above embodiment, the output power, noise level, and the stability of the DPSSL can be directly monitored by the microprocessor. The beam quality, the mode pattern, the polarization and other parameters of the laser output are treated as implicit functions of the first three parameters as they are closely related. An example of this relationship is shown in FIGS.  5  ( a ) and ( b ), where the transverse mode pattern and intensity profile of the DPSSL are measured by a laser beam profiler at a non-optimized temperature setting (left) and an optimized temperature setting (right).  FIG. 5  ( a ) shows the transverse mode pattern.  FIG. 5  ( b ) shows the transverse intensity profile, i.e., intensity variation of the laser beam over its two main axes. The elliptically shaped mode pattern for the non-optimized temperature setting indicates the existence of higher order transverse modes. In other embodiments of the present invention, the single element photo detector can be replaced with a photo detector array to monitor the beam quality and the mode pattern directly. More components, including but not limited to cavity length stabilizers, polarization controllers, and transverse and longitudinal mode regulators, can be added in the laser head with their status monitored and controlled by the microprocessor. The laser cavity and crystals can also be replaced with optical fibers to form a diode-pumped fiber laser, which can be viewed as a special case of the DPSSL.  
         [0023]     To make the DPSSL work in consistent performance under environmental temperature changes, the temperature sensors in the laser head, such as the thermistors in the present embodiment, must be able to measure the temperature of the components very accurately. However, owing to the physical separation between the thermistor and the component, their temperature may differ due to thermal resistivity. The current invention provides a new method to compensate this temperature gap. In the laser head, the current (voltage) applied on the TECs is adjusted in proportion to environmental temperature to stabilize the temperature of the thermistors that are placed close to the components. Thus the TEC current (voltage) can be used as another temperature indicator for certain thermistor settings (T 1 , T 2 , T 3  . . . ), as illustrated in  FIG. 6 . In a preferred embodiment, the microprocessor adjusts the temperature setting of the thermistors according to the voltage on the TECs when the environmental temperature changes to keep the temperature of the components stable. In another embodiment, the temperature of the laser case (or temperatures at other parts of the laser) is monitored, which reflects the environmental temperature changes. The microprocessor will use the temperature in combination with a preloaded look-up-table to generate a new thermistor setting temperature so that the temperature of the components can be accurately maintained at changing environmental temperatures  
         [0024]     While various exemplary embodiments of the current invention have been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the present invention. For example, numerical values and recitations of particular substances are illustrative rather than limiting. Therefore, the present invention should be construed as limited only by the appended claims. we claim: