Patent Publication Number: US-2012039349-A1

Title: Circuit for controlling a gain medium

Description:
RELATED INVENTIONS 
     This application claims priority on U.S. Provisional Application Ser. No. 61/374,228, filed on Aug. 16, 2010, and entitled “DRIVE CIRCUIT FOR CONTROLLING A QUANTUM CASCADE LASER MODULE”. As far as permitted, the contents of U.S. Provisional Application Ser. No. 61/374,228 are incorporated herein by reference. 
    
    
     BACKGROUND 
     Mid Infrared (“MIR”) laser sources that produce a fixed wavelength output beam can be used in many fields such as, thermal pointing, medical diagnostics, pollution monitoring, leak detection, analytical instruments, homeland security and industrial process control. 
     Often, these MIR laser sources include a circuit having a switch which causes the laser to operate in a pulsed fashion. A common, pulsed MIR laser source includes a gain medium, a regulated voltage source, and a switch that selectively directs power from the voltage source to the gain medium. Unfortunately, existing switch designs are not entirely satisfactory because with certain types of gain media, e.g. quantum cascade gain media, can be easily damaged by current spikes. 
     Moreover, as provided in “Transport and gain in a quantum cascade laser: model and equivalent circuit” written by Khurgin and Dikmelik, Optical Engineering 49(11), 111110 (November 2010), “cascade” type gain media (quantum cascade and interband cascade) present new challenges for achieving functional control because they present a reactive load that is complex compared to traditional gain media such as laser diodes. Thus, in certain conditions, exiting circuit designs do not adequately control a cascade type gain medium. 
     SUMMARY 
     An assembly that generates a laser beam includes a voltage source, a quantum cascade (“QC”) gain medium, a closed loop current regulator, and a controller. The QC gain medium generates a laser beam when medium current flows through the QC gain medium. The current regulator regulates the medium current that flows through the QC gain medium from the voltage source independent of a voltage of the voltage source. The controller directs a command input to the current regulator that is used to control the current regulator. 
     As an overview, the assembly is uniquely designed so that the current regulator regulates a medium current that flows through the QC gain medium in a closed loop fashion, and this regulation is independent of variations in voltage from the voltage source. Further, in certain embodiments, the current regulator regulates the magnitude of the medium current to be proportional to an amplitude of the command input. Thus, the medium current that is flowing through the laser can be adjusted by adjusting the command input. With this design, the current regulator allows for the individual control of the QC gain medium to account for variations in the QC gain medium and specific adjustment of the laser beam. 
     In one embodiment, the current regulator includes a transistor that is positioned in series with the QC gain medium, and an amplifier that receives the command input and that controls the transducer. Further, the amplifier can include an amplifier output that is electrically connected to a gate of the transistor, a positive amplifier input that receives the command input, and a negative amplifier input that receives feedback that relates to the medium current. 
     Additionally, the assembly can include a feedback system that provides feedback that relates to the medium current to the amplifier. In one embodiment, the feedback system includes a first feedback and a second feedback that provide a differential measurement of a feedback voltage across a sense resistor. 
     In one embodiment, the command input is a pulsed signal having an amplitude that varies over time to selectively pulse the QC gain medium. With this design, the controller can selectively adjust an amplitude, a pulse width and a repetition rate of the command input to control a magnitude, a pulse width and a repetition rate of the laser beam. 
     The present invention is also directed to a method for generating a laser beam. In this embodiment, the method can include the steps of (i) providing a voltage source; (ii) electrically connecting a QC gain medium to the voltage source, the QC gain medium generating a laser beam when medium current flows through the QC gain medium; (iii) regulating the medium current that flows through QC gain medium with a closed loop current regulator; and (iv) directing a command input to the current regulator that is used to control the current regulator and the medium current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a simplified circuit illustration of an assembly having features of the present invention; 
         FIG. 2  is a simplified graph that illustrates a command input, a medium current, and laser output versus time; 
         FIG. 3  is a simplified circuit illustration of another embodiment of an assembly having features of the present invention; 
         FIG. 4  is a simplified circuit illustration of still another embodiment of an assembly having features of the present invention; 
         FIG. 5  is a simplified circuit illustration of another embodiment of an assembly having features of the present invention; and 
         FIG. 6  is a simplified circuit illustration of yet another embodiment of an assembly having features of the present invention. 
     
    
    
     DESCRIPTION 
       FIG. 1  is a simplified circuit illustration of an assembly  10  that generates a laser beam  12  (illustrated as a dashed arrow). In one embodiment, the assembly  10  includes a voltage source  14 , a laser  16 , a controller  18  that generates a command input  20 , and a current regulator  22 . The design of these components can be varied pursuant to the teachings provided herein. 
     As an overview, the assembly  10  is uniquely designed so that the current regulator  22  is able to regulate a medium current that flows through the laser  16  in a closed loop fashion, and this regulation is independent of variations in voltage from the voltage source  14 . This leads to better current regulation, a more accurate output for the laser beam  12 , and protection of the laser  16  from damage from current spikes that can result from variations in the voltage source  14 . 
     Further, in certain embodiments, the current regulator  22  is uniquely designed so that the current regulator  22  regulates the medium current to be proportional to an amplitude of the command input  20 . Thus, the medium current that is flowing through the laser  16  can be adjusted by adjusting the command input  20 . With this design, the current regulator  22  allows for the individual control of the laser  16  to account for variations in the laser  16  and specific adjustment of the laser beam  12 . 
     Moreover, the current regulator  22  provided herein allows for relatively fast on/off switching for precise operation in a pulsed mode, while maintaining the desired current. In certain embodiments, the current regulator  22  provided herein has a relatively high bandwidth to provide the fast on/off switching. For example, in alternative non-exclusive embodiments, the current regulator  22  can have a bandwidth of at least approximately 20, 25, 30, or 35 megahertz for a QC gain medium. However, the desired bandwidth can be varied to achieve the design requirements of the system, including rise times. For example, a rise time of ten nanoseconds can be achieved with 25 megahertz bandwidth. 
     It should be noted that the circuits provided herein are also relatively insensitive to the transient response of the voltage source  14 . Moreover, in certain embodiments, the circuit can be adjusted to compensate for any other transient events that occur within the QC gain medium or in the system. 
     Additionally, the circuits are designed so that current is regulated during turn-on and turn-off so that current spikes do not occur in the laser  16  due to parasitic inductance or capacitance in the circuit. 
     Further, with the circuits provided herein, the device can be operated just below threshold and then pulsed above threshold to achieve very fast turn-on of the optical pulse. 
     There are a number of possible usages for the assembly  10  disclosed herein. In one embodiment, the assembly  10  can be used as part of a thermal pointer (not shown) that generates the laser beam  12  that in is the infrared range, e.g. the mid-infrared range. In this example, the thermal pointer can be used on a weapon (e.g. a gun) in conjunction with a thermal imager to locate, designate, and/or aim at one or more targets. 
     Alternatively, for example, the assembly  10  can be used for a free space communication system in which the assembly  10  is operated in conjunction with an IR detector located far away, to establish a wireless, directed, invisible data link. Still alternatively, the assembly  10  can be used for any application requiring transmittance of directed infrared radiation through the atmosphere at the distance of thousands of meters, to simulate a thermal source to test IR imaging equipment, as an active illuminator to assist imaging equipment, or any other application. Still alternatively, the assembly  10  can generate an infrared beam  12  that is used in medical diagnostics, pollution monitoring, leak detection, analytical instruments, homeland security and industrial process control. 
     The voltage source  14  provides a voltage to the laser  16 . For example, the voltage source  14  can include one or more batteries (not shown), a generator, or another type of power source. In one embodiment, the voltage source  14  provides DC power. The voltage source  14  can be regulated or unregulated. As provided herein, an adjustable output voltage is not required because the current regulator  22  is used to control the flow through the laser  16 . One non-exclusive example of a voltage source  14  provides a voltage of between approximately two and thirty volts. Alternatively, other voltages can be utilized. 
     In  FIG. 1 , the voltage source  14  includes a positive terminal  14 A and a ground terminal  14 B. 
     The laser  16  is electrically connected to the voltage source  14 . In  FIG. 1 , the laser  16  includes a gain medium  24 A having (i) a first connector  24 B that is electrically connected to the positive terminal  14 A of the voltage source  14 , and (ii) a second connector  24 C. The gain medium  24 A generates the laser beam  12  when the medium current is flowing through the gain medium  24 A. 
     For example, the gain medium  24 A can be a Quantum Cascade gain medium that generates a laser beam  12  that is in the mid-infrared range. With this design, electrons transmitted through the QC gain medium  24 A emit one photon at each of the energy steps. In the case of a QC gain medium  24 A, the “diode” has been replaced by a conduction band quantum well. Electrons are injected into the upper quantum well state and collected from the lower state using a superlattice structure. The upper and lower states are both within the conduction band. Replacing the diode with a single-carrier quantum well system means that the generated photon energy is no longer tied to the material bandgap. This removes the requirement for exotic new materials for each wavelength, and also removes Auger recombination as a problem issue in the active region. The superlattice and quantum well can be designed to provide lasing at almost any photon energy that is sufficiently below the conduction band quantum well barrier. In one, non-exclusive embodiment, the semiconductor QCL laser chip is mounted epitaxial growth side down. A suitable QC gain medium  24 A can be purchased from Alpes Lasers, located in Switzerland. 
     In contrast with typical semiconductor diodes, QC gain media typically exhibit higher capacitance and a higher dynamic resistance. These characteristics can lead to a problem with spikes in current during switching on and off that can damage the QC gain medium. Further, with the quantum cascade gain medium, the active medium relies on intersubband transitions in the quantum wells instead of some naturally occurring atomic or molecular transition. Thus, the reactions in the QC gain medium are much more complicated than in a typical semiconductor laser, and as a result thereof, the QC gain medium is much more difficult to safely control. The circuits provided herein are uniquely designed to accurately control the current to the QC gain medium, while protecting the quantum cascade gain medium from current spikes. 
     Further, a quantum cascade gain medium is a high current device. Further, the circuits provided herein prevent droop that can occur when a switch initially switches on the high current quantum cascade gain medium. 
     Moreover, in certain embodiments, the circuits provided herein can provide a faster transition to “ON” because the current can be held just below a threshold which, typically, is at nearly half the operational current (usually chosen near the peak of efficiency). This is possible in part because the QC device has remarkably low Amplified Spontaneous Emission (ASE) compared to laser diodes. As one non-exclusive example, for a QC gain medium, it may be desired to direct one amp of current to the QC gain medium during the ON part of the cycle. With the present design, the circuit can direct less than a threshold current that causes the QC gain medium to generate significant light (e.g. at less than approximately one half amp of current, the QC gain medium does not generate significant light) to the QC gain medium during the OFF part of cycle. This will allow for fast switching between OFF and ON. 
     Alternatively, in certain embodiments, the gain medium  24 A can be an Interband Cascade (“IC”) Lasers. IC gain medium use a conduction-band to valence-band transition as in the traditional diode laser. 
     As used herein, “cascade type gain medium” shall include both QC gain medium and IC gain medium. 
     As used herein, the term mid-infrared range has a wavelength in the range of approximately 3-14 microns. 
     In certain embodiments, the laser  16  can be tuned to adjust the primary wavelength of the laser beam  12 . For example, the laser  16  can include a wavelength selective element (not shown) that allows the wavelength of the laser beam  12  to be individually tuned. The design of the wavelength selective element can vary. Non-exclusive examples of suitable wavelength selective elements include a diffraction grating, a MEMS grating, prism pairs, a thin film filter stack with a reflector, an acoustic optic modulator, or an electro-optic modulator. Further, a wavelength selective element can be incorporated into the gain medium  24 A. A more complete discussion of these types of wavelength selective elements can be found in the Tunable Laser Handbook, Academic Press, Inc., Copyright 1995, chapter 8, Pages 349-435, Paul Zorabedian, the contents of which are incorporated herein by reference. 
     Additionally, in certain designs, the laser  16  can be tuned slightly by adjusting the medium current with the controller  16 . 
     As provided herein, the controller  18  is electrically connected to and provides the command input  20  to the current regulator  22  to control the flow of the medium current through the gain medium  24 A. Further, the controller  18  can include a processor that can be used to selectively adjust the characteristics of the command input  20  to selectively adjust the medium current and the resulting laser beam  12 . For example, the controller  18  can adjust an amplitude, a pulse width and a repetition rate of the command input  20  to control a magnitude, a pulse width and a repetition rate of the laser beam  12 . With this design, analog modulation can be achieved by varying the command input  20 . 
     In one embodiment, the controller  18  causes the medium current to be directed to the laser  16  in a pulsed fashion. As a result thereof, the intensity of the laser beam  12  is also pulsed. In one, non-exclusive embodiment, the duty cycle is approximately 12.5 percent. In this embodiment, for example in one cycle, the controller  18  can direct the command input  20  to the current regulator  22  so that medium current flows through the gain medium  24 A for approximately 25 milliseconds, and medium current does not flow through the gain medium  24 A for approximately 175 milliseconds. 
     With this design, the QC gain medium  24 A lases with little to no heating of the core of the QC gain medium  24 A, the average power directed to the QC gain medium  24 A is relatively low, and the desired average optical power of the output beam  12  can be efficiently achieved. It should be noted that as the temperature of the QC gain medium  24 A increases, the efficiency of the QC gain medium  24 A decreases. With this embodiment, the pulsing of the QC gain medium  24 A keeps the QC gain medium  24 A operating efficiently and the overall system utilizes relatively low power. 
     Alternatively, the duty cycle can be greater than or less than 12.5 percent. With this design, the controller  18  selectively adjusts a pulse width and a repetition rate of the laser beam  12 . Further, the controller  18  can control the magnitude of the medium current (and the laser beam  12 ) by adjusting the magnitude of a control current of the command input  20 . 
     The current regulator  22  (under the control of the controller  18 ) regulates the medium current that flows through the gain medium  24 A. In  FIG. 1 , the current regulator  22  provides a fast switching time of the gain medium  24 A while maintaining a constant current regulation. Further, because the current regulator  22  regulates the medium current, the current regulator  22  protects the gain medium  24 A by inhibiting spikes in the medium current. In this embodiment, the current regulator  22  includes a transistor  26 A, an amplifier  28 A, and a feedback system  30 . 
     In one embodiment, the transistor  26 A includes (i) a source terminal  26 B that is electrically connected to the second connector  24 C of the gain medium  24 A, (ii) a gate  26 C that is electrically connected to the amplifier  28 A, and (iii) a drain  26 C that is electrically connected to the feedback system  30 . For example, the transistor  26 A can be a field effective transistor. In this embodiment, the transistor  26 A is connected in series with the voltage source  14 , the gain medium  24 A, and the feedback system  30 , and the transistor  26 A is electrically connected between the gain medium  24 A and the feedback system  30 . 
     Alternatively, the transistor  26 A can be a bi-polar junction transistor. 
     The amplifier  28 A receives the command input  20  from the controller  18  and controls the gate  26 C of the transistor  26 A to selectively control the medium current to the gain medium  24 A. In  FIG. 1 , the amplifier  28 A includes (i) a positive amplifier input  28 B that is electrically connected to the controller  18  and receives the command input  20  from the controller  18 , (ii) a negative amplifier input  28 C that is electrically connected to and receives feedback from the feedback system  30 , and (iii) an amplifier output  28 D that is electrically connected to the gate  26 C. In one embodiment, the amplifier is an operational amplifier. 
     As provided herein, the command input  20  is applied to the positive amplifier input  28 B. In order to turn off the laser  16 , the amplitude of the command input  20  is set to zero volts. When the command input  20  is zero, the amplifier output  28 D will turn off the transistor  26 A, preventing current from flowing through the gain medium  24 A. Alternatively, to turn on the laser  16 , the amplitude of command input  20  is increased. This will cause the amplifier output  28 D to increase, and the amplifier  28 A will drive the gate  26 C of the transistor  26 A, so that medium current begins to flow through the gain medium  24 A and through the feedback system  30 . 
     In one embodiment, the amplifier  28 A is designed to control the gate  26 C so that a feedback voltage across the feedback system  30  is equal to a command voltage of the command input  20 . Stated in another fashion, the operation amplifier  28 A is designed to control the gate  26 C so that the voltage at the positive amplifier input  28 B (the command voltage) is equal to the voltage at the negative amplifier input  28 C (the feedback voltage). 
     The feedback system  30  provides feedback to the amplifier  28 A so that the current regulator  22  can precisely control the medium current that flows through the gain medium  24 A. In one embodiment, the feedback system  30  includes a sense resistor  30 A. In this embodiment, the medium current that flows through the sense resistor  30 A creates the feedback voltage that is fed back to the negative amplifier input  28 C. From the feedback voltage across the sense resistor  30 A, the medium current can be determined. 
     With the present design, the feedback voltage across the sense resistor  30 A is proportional to the medium current, and this feedback voltage is connected back to the negative amplifier input  28 C of the amplifier  28 A. The amplifier  28 A will act to increase the medium current flows through the sense resistor  30 A until this feedback voltage is equal to the command voltage of the command input  20 . Thus, the magnitude of the medium current flowing through the gain medium  24 A will be proportional to the magnitude of the command voltage of the command input  20 . Thus, the command input  20  can be adjusted to adjust the medium current. 
     In  FIG. 1 , the sense resistor  30 A includes a first connector  30 B that is electrically connected to the transistor  26 A and a second connector  30 C that is electrically connected to the ground terminal  14 B of the voltage source  14 . Further, in this embodiment, the negative amplifier input  28 C is electrically connect to the circuit near the first connector  30 B of the sense resistor  30 A. With this design, the amplifier  28 A receives the feedback voltage from near the top of the resistor  30 A. 
     In one embodiment, the current regulator  22  is designed to be very small. Further, the current regulator  22  is placed in close proximity to the laser  16 . As a result thereof, any parasitic capacitance and inductance can be minimized allowing for the best performance characteristics for this current regulator  22 . The result is improved pulse performance while maintaining strict current regulation. This will, in turn, provide better protection for the laser  16 . Further, the current regulator  22  is able to provide shorter pulses with less chance of damaging voltage spikes. 
       FIG. 2  is a graph that illustrates the command input  232 , the medium current  234 , and the laser beam output  236  versus time. In this embodiment, the command input  232  is pulsed. As a result thereof, the medium current  234  and the laser beam output  236  are also pulsed. Further, it should be noted that with certain embodiments of the present invention, the circuit is designed so that the medium current  232  is proportional to the command input  232 . 
     It should be noted that the amplitude, the pulse width and the repetition rate of the command input  232  can be selectively controlled to selectively control a magnitude, a pulse width and a repetition rate of the medium current  234  and the output of the laser beam  236 . 
       FIG. 3  is a simplified circuit illustration of another embodiment of an assembly  310  that provides fast switching time of the laser  316  device while maintaining a constant current regulation. In this embodiment, the circuit includes the voltage source  314 , the laser  316 , the current regulator  322 , and the controller  318  that are similar to the components described above and illustrated in  FIG. 1 . In this embodiment, the positive terminal  314 A of the voltage source  314  is connected to the first connector  324 B of the gain medium  324 A, and the second connector  324 C of the gain medium  324 A is connected in series to the transistor  326 A and the sense resistor  330 A of the feedback system  330 . 
     Further, in this embodiment, the command input signal  320  from the controller  318  is applied to the positive amplifier input  328 B of the amplifier  328 A. With this design, in order to turn off the laser  316 , the command input  320  is set to zero volts. The operational amplifier  328 A output will turn off the transistor  326 A, preventing current from flowing. To turn on the laser  316 , the command voltage of the command input  320  is increased. The operational amplifier  328 A will drive the gate  326 C of the transistor  326 A, so that current begins to flow through the laser  316  and through the sense resistor  330 A. The feedback voltage across sense resistor  330 A is proportional to the current flowing and this feedback voltage is connected back to the negative amplifier input  328 C of the operational amplifier  328 A. The amplifier  328 A will act to increase the medium current flow through the sense resistor  330 A until this voltage is equal to the command voltage. Thus, the magnitude of the medium current flowing through the laser  316  will be proportional to the amplitude of the command voltage of the command input  320 . 
     However, it should be noted that the circuit illustrated in  FIG. 3  differs from the circuit illustrate in  FIG. 1  in that the circuit in  FIG. 3  includes a different feedback system  330 . More specifically, in  FIG. 3 , the feedback system  330  provides feedback from each side of the sense resistor  330 A. This differential measurement of the feedback voltage across the sense resistor  330 A reduces and/or cancels out any effects due to parasitic inductance in the power supply connections to the circuit. 
     In the embodiment illustrated in  FIG. 3 , the feedback system  330  includes (i) a first feedback  338 A that provides the feedback voltage (at the top of the sense resistor  330 A near the first connector  330 B) across the sense resistor  330 A to the negative amplifier input  328 C, and (ii) a second feedback  338 B that provides the feedback voltage (at the bottom of the sense resistor  330 A near the second connector  330 C) across the sense resistor  330 A to the positive amplifier input  328 B. 
     With the design illustrated in  FIG. 3 , the feedback system  330  includes a resistor network having (i) a first resistor  340 A electrically positioned between the controller  418  and a junction with the second feedback  338 B; (ii) a second resistor  340 B electrically positioned between the junction of the positive amplifier input  428 B and the second connector  330 C of the shunt resistor  330 A; (iii) a third resistor  340 C electrically positioned between the negative amplifier input  428 C and the ground terminal  314 B of the voltage source  314 ; and (iv) a fourth resistor  340 D electrically positioned between the negative amplifier input  328 C and the first connector  330 B of the shunt resistor  330 A. With this design, the resistor network is used for scaling. 
     By designing this circuit to be very small, and placing it in close proximity to the QC gain medium  324 A, parasitic capacitance and inductance can be minimized allowing for the best performance characteristics for this current regulator  322 . The result is improved pulse performance while maintaining strict current regulation. This will, in turn, provide better protection for the QC gain medium  324 A. 
       FIG. 4  is a simplified circuit illustration of another embodiment of an assembly  410  that provides fast switching time of the laser  416  device while maintaining a constant current regulation. In this embodiment, the circuit includes the voltage source  414 , the laser  416 , the current regulator  422 , and the controller  418  that are somewhat similar to the components described above and illustrated in  FIG. 3 . However, in this embodiment, the position of the current regulator  422  and the laser  416  are switched. 
     More specifically, in this embodiment, (i) the positive terminal  414 A of the voltage source  414  is electrically connected to the first connector  430 B of the sense resistor  430 A, (ii) the second connector  430 C of the sense resistor  430 A is electrically connected to the source terminal  426 B of the transistor  426 A, (iii) the drain  426 D of the transistor  426 A is electrically connected to the first connector  424 B of the gain medium  424 A, and (iv) the second connector  424 C of the gain medium  424 A to the ground terminal  414 B of the voltage source  414 . With this design, the gain medium  424 A is connected between the ground terminal  414 B and the transistor  426 A, and the sense resistor  430 A is connected between the voltage source  414  and the transistor  426 A 
     In this embodiment, the transistor  426 A is a P-type MOSFET. Further, in this embodiment, the assembly  410  includes a programmable current source  450  that is connected in parallel with the laser  416 . In this embodiment, the controller  418  provides a negative command input into the programmable current source  450  which pulls current from the voltage source  414  through a resistor  452  that is in series with the programmable current source  450  and positioned electrically between the voltage source  414  and the programmable current source  450 . The positive amplifier input  428 B is electrically connected at a junction  454  electrically positioned between the resistor  452  and the current source  450 . This causes a reduction of the voltage on the non-inverting positive amplifier input  428 B. The amplifier  428 A responds by reducing the voltage from the amplifier output  428 D applied to the gate  426 C of the transistor  426 A, turning it on and allowing current to flow through the sense resistor  430 A, and the gain medium  424 A. 
     Moreover, in this embodiment, the negative amplifier input  428 C receives feedback from the sense resistor  430 A so that the system is a closed loop current regulator  422 . 
     A benefit of this type of circuit is that the second connector  424 C of the gain medium  424  is connected to ground potential, e.g. the ground terminal  414 B. Typically, this would be the connection which is connected to the heat sink of the device. Because the heat sink is, thus, connected to ground potential, device operation is safer and less prone to damage of the gain medium  424  by accidentally short circuiting the heat sink to ground. 
       FIG. 5  is a simplified circuit illustration of yet another embodiment of an assembly  510  that provides fast switching time of the laser  516  device while maintaining a constant current regulation. In this embodiment, the circuit includes the voltage source  514 , the laser  516 , the current regulator  522 , and the controller  518  that are somewhat similar to the components described above and illustrated in  FIG. 4 . However, in this embodiment, the circuit includes two additional resistors  570 ,  572  that lower the common mode voltage on the amplifier inputs  528 B,  528 C. More specifically, the resistor  570  is in series with the negative amplifier input  528 C, and the resistor  572  is in series with the positive amplifier input  528 B. With this design, the common mode voltage on the inputs  528 B,  528 C are shifted downward so the amplifier  528 A is compatible with the voltage source  514 . Stated in another fashion, with this design, the common mode voltage is lower to be within the operating range of amplifier  528 A. In one embodiment, the resistors  570 ,  572  have approximately the same resistance. 
       FIG. 6  is a simplified circuit illustration of yet another embodiment of an assembly  610  having features of the present invention. In this embodiment, the assembly  610  includes the voltage source  614 , multiple lasers  616 A,  616 B,  616 C, multiple current regulators  622 A,  622 B,  622 C, and the controller  618 . The number of lasers  616 A,  616 B,  616 C in the assembly  610  can be varied. In  FIG. 6 , the assembly  610  includes three lasers  616 A,  616 B,  616 C. Alternatively, the assembly  610  can include more than three or fewer than three lasers  616 A,  616 B,  616 C. 
     More specifically, in  FIG. 6 , the assembly  610  includes (i) a first laser  616 A that generates a first beam  612 A, (ii) a first current regulator  622 A that is in series with the first laser  616 A and that regulates the current in the first laser  616 A, (iii) a second laser  616 B that generates a second beam  612 B, the second laser  616 B being in parallel with the first laser  616 A, (iv) a second current regulator  622 B that is in series with the second laser  616 B and that regulates the current in the second laser  616 B, (v) a third laser  616 C that generates a third beam  612 C, the third laser  616 C being in parallel with the first laser  616 A and the second laser  616 B, and (vi) a third current regulator  622 C that is in series with the third laser  616 C and that regulates the current in the third laser  616 C. 
     In this embodiment, the beams  612 A,  612 B,  612 C can be combined to generate a combined output beam. For example, the beams  612 A,  612 B,  612 C can be redirected to be parallel to each other (e.g. travel along parallel axes), and/or fully overlapping, partly overlapping, or are adjacent to each other. 
     Moreover, in one embodiment, the controller  618  independently directs (i) a first command input  620 A to the first current regulator  622 A to selectively control the current through the first laser  616 A, (ii) a second command input  620 B to the second current regulator  622 B to selectively control the current through the second laser  616 B, and (iii) a third command input  620 C to the third current regulator  622 C to selectively control the current through the third laser  616 C. It should be noted that the controller  618  can be used to control the command inputs  620 A,  620 B,  620 C so that all of the command inputs  620 A,  620 B,  620 C are the same or different. With this design, a common voltage source  614  can be used to save space, while still allowing for the individual control of the lasers  616 A,  616 B,  616 C via individual control of the command inputs  620 A,  620 B,  620 C to account for variations in the lasers  616 A,  616 B,  616 C, and specific adjustment of the individual laser beams  612 A,  612 B,  612 C. As a result thereof, the current through each laser  616 A,  616 B,  616 C can be controlled to be the same or different. 
     Further, with this design, the controller  618  can simultaneous direct pulses of power to each of the lasers  616 A,  616 B,  616 C so that each of the lasers  612 A,  612 B,  612 C generates the respective beam at the same time. Alternatively, the controller  618  can direct pulses of power to one or more of the lasers  616 A,  616 B,  616 C at different times so that the laser  616 A,  616 B,  616 C generate the respective beam at different times. 
     It should be noted that the design of the current regulators  622 A,  622 B,  622 C and the relative position of the components of the assembly  610  can be similar to that illustrated in  FIG. 1 ,  3 ,  4 , or  5  and described above. 
     Finally, the designs provided herein are merely non-exclusive examples of possible designs. While the particular assembly as shown and disclosed herein is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.