Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser
  • H01S3/0975—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser using inductive or capacitive excitation

Definitions

• the present invention relates to lasers, more particularly to providing a power supply for driving a radio frequency excited gas laser.
• Radio frequency (RF) excited gas lasers can be driven by a RF signal generated by a laser power supply.
• Conventional laser power supplies for RF excited gas lasers typically use a complex control circuit which may include both digital and analog circuit elements. Such conventional laser power supplies can be time consuming and expensive to design and produce. Conventional laser power supplies can also constrain the operating characteristics (maximum power output, duty cycle control, operating mode) due to limitations in the power supply design.
• the present invention has been made to address drawbacks afflicting conventional laser power supplies.
• a drive system for a radio frequency excited gas laser can comprise a microprocessor and/or microcontroller and a RF power amplifier.
• the microprocessor can be configured to generate an amplifier drive signal and to gate the amplifier drive signal in accordance with a received command input.
• the RF power amplifier can be configured to receive the amplifier drive signal and to provide a laser drive signal proportional to the amplifier drive signal.
• the drive system is operable in a plurality of operation modes. These modes can include a first mode in which the microprocessor and/or microcontroller is operable to gate the amplifier drive signal. A second mode can also be provided, in which the microprocessor and/or microcontroller is operable to generate an ungated amplifier drive signal. A third mode can be provided in which the microprocessor and/or microcontroller is operable to generate a constant amplifier drive signal.
• the microprocessor and/or microcontroller is operable to gate the amplifier drive signal between a plurality of signal levels, the-signal levels representing different ones along a scale between fully off and fully on. This arrangement allows an output power of a laser to be closely controlled without direct control over a DC power source for the amplifier.
• the microprocessor and/or microcontroller is operable to interpret the received command input to take account of latencies within the radio frequency excited gas laser. His arrangement provides for more accurate control over a laser output for high tolerance applications.
• drive system also comprises a network interface.
• This arrangement provides for control of the drive system and a laser driven thereby from a remote terminal.
• the laser can be situated in an inaccessible location or a location harmful to a human operator and full control over the system can be achieved.
• a radio frequency excited gas laser can comprise a drive system operable to provide a laser drive signal and a beam generating system.
• the drive system can include a microprocessor configured to generate an amplifier drive signal and to gate the amplifier drive signal in accordance with a received command input.
• the drive system can also include a RF power amplifier configured to receive the amplifier drive signal and to provide a laser drive signal proportional to the amplifier drive signal.
• the beam generating system can be configured to output a laser beam in response to receiving said laser drive signal.
• a method of generating a coherent light beam can comprise generating an amplifier drive signal in accordance with a received command input; gating the amplifier drive signal in accordance with the received command input; generating an RF laser drive signal in response to the gated amplifier drive signal; and generating a coherent light beam by exciting a gas using said RF laser drive signal.
• Figure 1 is a schematic representation of an RF excited gas laser apparatus
• Figure 2 is a schematic representation of a laser power supply for the RF excited gas laser of Figure 1 ;
• Figure 3 is a schematic representation of a user interface part of the laser power supply of Figure 2;
• Figure 4 is an example of a command input signal for a first operation mode
• Figure 5 is an example of a laser drive signal in the first operation mode
• Figure 6 is an example of a command input signal for a second operation mode
• Figure 7 is an example of a laser drive signal in the second operation mode
• Figure 8 is another example of a laser drive signal in the second operation mode
• Figure 9 is an example of a command input signal for a third operation mode
• Figure 10 is an example of a modupuise power level control input signal for the third operation mode
• Figure 11 is an example of an RF drive signal in the third operation mode
• Figure 12 is an example of a laser drive signal in the third operation mode
• Figure 13 is an example of a laser output in the third operation mode
• Figure 14 is another example of a command input signal for the third operation mode
• Figure 15 is another example of a modupuise power level control input signal for the third operation mode
• Figure 16 is another example of an RF drive signal in the third operation mode
• Figure 17 is another example of a laser drive signal in the third operation mode.
• Figure 18 is another example of a laser output in the third operation mode.
• the RF excited gas laser 1 includes a housing 11 in which the beam generation components of the laser can be disposed.
• the housing 11 can have ends 10a and 10b, one of which can have a reflective surface directed into the housing 11 and the other of which can have a partially reflective surface directed such that this end acts as an output coupler.
• an RF feed through 12 can be provided. This protrudes into the housing 11 (as shown by the dotted line in Figure 1) and can be encircled by an insulating ceramic casing 13.
• the ceramic casing 13 can consist of one or more a number of insulating and/or dielectric materials, such as BeO, AIN or AI 2 O 3 .
• a separate housing may not be required where the laser is housed within a sealed discharge structure containing reflective elements, where the sidewalls or electrodes additionally form the housing.
• a RF power source 30 can be coupled to the RF feed through 12 to deliver an RF excitation signal to the beam generating components within the housing.11.
• the RF power source 30 can be controlled by a microprocessor 32 to deliver an RF excitation signal in accordance with one of a number of operation modes for the RF excited gas laser 1.
• Example lasers to be driven by the power supply or supplies discussed herein are disclosed in U.S. Patent Application Publication No. 2004/0218650, the disclosure of which is hereby incorporated herein by reference.
• the RF power source 30 of the present example includes a laser control board 31 on which the microcontroller 32 is mounted.
• the microcontroller is a PIC18F series microcontroller produced by Microchip Technologies Inc. In other examples, alternative microcontrollers or microprocessors could be used.
• the laser control board 31 of the present example also has a dedicated processor 33 mounted thereon.
• the dedicated processor is a pre-programmed field programmable grid array (FPGA).
• FPGA field programmable grid array
• a custom designed hardware DSP, or a microprocessor controlled in real-time by locally stored software could be used in place of the FPGA.
• a single microprocessor or microcontroller could be provided to carry out the functions of the microcontroller 32 and FPGA 33 in a single device.
• the laser control board 31 and all of the components thereon are supplied with power by a DC power supply 34.
• the laser control board 31 can receive a command input signal via a command input 35 of the RF power source 30.
• the laser control board 31 can also receive a mode select signal via a mode select input 36 of the RF power source 30.
• the laser control board 31 can receive a modupulse power level control signal via a modupulse power level control input 37 of the RF power source 30.
• the laser control board 31 can also output a drive signal for a user interface 38.
• the user interface can include optical and audio indicators of the RF power source 30 (such as LEDs and audio signal generators), or can be a remote user interface device such as a display screen (not shown) for displaying operational information for the RF excited gas laser 1.
• the microcontroller 32 In accordance with the command input signal, mode select signal and, ⁇ if appropriate, the modupulse power level control signal, the microcontroller 32 generates an RF drive signal which is output to a RF power amplifier 39.
• the RF power amplifier 39 is powered by the DC power supply 34 and outputs a laser drive signal proportional to the RF drive signal to the beam generating components within the housing 11 via the RF feed through 12.
• the RF power amplifier 39 of the present example can output a RF drive signal having a peak- power above the maximum output capacity of the RF power amplifier 39 provided that the average output power is maintained less than the maximum output capacity by limiting the duty cycle of the output.
• the RF drive signal generated by the RF power source 30 can have a frequency in the range 13MHz to 175MHz. This range is set in the present example to avoid an excessively large discharge sheath being required for the laser, which can occur at very low frequencies, and to avoid difficulties in maintaining a uniform discharge which can occur at very high frequencies. In other examples, a wider or narrower range may be applicable, depending on what size of discharge sheath and how much effort to control uniformity of discharge are required and/or acceptable to the particular implementation.
• the command input signal received via the command input 35 is a bi- level signal for controlling when the laser is turned on and when it is turned off.
• the signal can be switched between an "off" state represented by OV and an "on” state represented by +5V.
• the laser can be turned on and off.
• the delay between switching the laser on or off via the command input signal and the laser being switched on or off is generally less than 500 nanoseconds, although the skilled addressee will recognize that altering the behavior of different components can increase or reduce this delay. Thereby complete control over the activation of the laser can be provided.
• the mode select signal can be used to select an operation mode of the RF excited gas laser 1.
• the signal can be a muttklevel signal with different voltage levels representing different modes of operation.
• the signal can be a parallel digital signal with different binary codes representing different operation modes.
• the signal can be a serial digital signal with different binary codes representing different operation modes.
• the signal can be applied once to enter the desired mode with no further mode select signal being required until a mode change is desired.
• the RF excited gas laser of the present example is operable in three modes.
• the first mode is a true CW (continuous wave) mode
• the second mode is a superpulse mode
• the third mode is a modupulse mode. Details of these modes will be presented below.
• the mode descriptions which follow are illustrative in the context of the present example, and are. not intended to limit the scope of the present disclosure or invention to a specific number of modes for other applications and uses.
• the modupulse power level control signal received via the modupulse level control input 37 is an analog input which can be used to control the laser output in modupulse mode. Mode details of this mode of operation will be given below.
• the microcontroller 32 can drive a user interface 38. Further details of the user interface components of the present example are shown in Figure 3.
• the user interface 38 comprises an array of LEDs 41.
• these LEDs 41 are driven by a dedicated LED driver integrated circuit 42 under control of the microcontroller 32.
• a device suitable for performing this function is the TLC5921 produced by Texas Instruments, Inc. In other examples, the LEDs can be driven directly by the microcontroller 32.
• the LEDs of the user interface can be individually driven to indicate various operational states of the laser.
• individual LEDs can represent a laser on/off condition (41 a), a fault condition (41b), a warning signal (41 c), an over temperature condition (41 d), a selected operation mode 41 (e), and a power output level (41f-o).
• the user interface 38 can also include a network connection for control of the RF excited gas laser by a remote computer.
• the network connection can include an Ethernet MAC/PHY 44 under the control of the microcontroller 32.
• the Ethernet MAC/PHY 44 can be connected to an RJ45 socket 45 to allow a network cable such as a CAT5E Ethernet cable (not shown) to be attached to the laser 1 to enable remote network-based control of the laser 1.
• the Ethernet MAC/PHY 44 can provide a 10/100 base T Ethernet connection. In other examples a 10 base 2, GbE (Gigabit Ethernet) or 10GbE (10 Gigabit Ethernet) connection can be provided.
• GbE Gigabit Ethernet
• 10GbE 10 Gigabit Ethernet
• the microcontroller 32 can be operable to provide an http based "web-interface" for network management and control of the laser 1 via the network connection.
• a text-based terminal interface can be provided. Where such an interface is provided, the functionality of the Command Input, Mode Select Input and Modupulse Power Level Control Input can be replicated or replaced by the network interface.
• complete control over the laser can be achieved from a remote computer workstation without any specialist or dedicated control equipment or cabling.
• a power source for a radio frequency (RF) excited gas laser such as a RF excited CO 2 laser.
• This power source can be controlled to provide three operational modes for the laser, thereby providing enhanced flexibility and control of the laser.
• This first operation mode is a true continuous wave (CW) mode.
• the DC power supply 34 is controlled to output a constant voltage sufficient to cause the RF power amplifier 39 to run at peak power at 100% duty cycle.
• the laser control board 31 outputs a control signal to the RF power amplifier 39 to provide the output at 100% duty cycle.
• the RF power amplifier 39 outputs a constant drive signal of peak power equal to the maximum continuous output capacity of the RF power amplifier.
• the RF drive signal has a frequency in the range of 13-175 MHz (as discussed above), the laser is considered to be operating constantly at this frequency.
• the Command Input 35 the laser can be turned on and off using the bi-level control signal described above. As shown in Figures 4 and 5, when the command input control signal is "ON" (+5V in the present example) the laser will operate and when the command input control signal is "OFF" (OV in the present example) the laser will not operate.
• a 32V DC power supply which causes the RF power amplifier to produce a drive signal of 150W peak power.
• the RF power amplifier of this example has a maximum output capacity of 150W, a 100% duty cycle will causing the average RF power output to be 150W.
• the command input signal ( Figure 4) is at the on position, the RF power supply outputs 150W ( Figure 5).
• this 150W RF drive signal will cause the laser to output a constant laser beam in the region of 1 1 -14W power and with a maximum of 17W.
• the gas laser in general, and the CO2 laser in particular is considered to be a very efficient laser, it is still only about 30% efficient at best, due to the molecular energy levels involved.
• a maximum output of 17W from a 150W drive signal represents a high efficiency for a laser of this type. All of the waste energy is given off as heat. Removal of this waste heat by some form of active or passive cooling of the laser apparatus can aid in maintaining an optimum efficiency of operation.
• This second mode is a superpulse mode in which higher peak power from the laser can be achieved.
• the peak power from the RF power amplifier 39 is increased beyond the maximum continuous output capacity of that amplifier.
• the output power from the RF power amplifier 39 is dependent solely upon the DC drive voltage which it receives from the DC power supply 34.
• the RF power amplifier 39 of the present example outputs 150W RF power when driven at 32V DC. When 48V DC is applied to the RF power amplifier 39, 300W peak RF power can be produced but the duty cycle must be limited to 50% in order not to exceed the 150W average power rating of the amplifier.
• the RF power amplifier 39 outputs peak RF power between 150W and 300W, increasing as the DC voltage is increased.
• the duty cycle must therefore be adjusted to between 100% and 50% respectively in order not to exceed the 150W average power rating of the amplifier.
• the output RF power is approximately proportional to the square of the voltage.
• a user is able to choose a desired peak power by selecting a voltage supply of between 32V and 48V.
• the laser control board 31 monitors the voltage and operates to protect the RF power amplifier 39 from overload by limiting the duty cycle and maximum pulse width of the RF drive signal supplied to the RF power amplifier 39.
• the duty cycle can be controlled by the FPGA 33 on the laser control board 31.
• the FPGA can respond to the voltage supplied to the RF power amplifier 39 and limit the duty cycle and maximum allowed pulse width in the RF drive signal supplied to the RF power amplifier 39 to maintain the operation of the power transistors within operation tolerances. In the present example, this is performed in real time at pulse frequencies of up to approximately 100KHz.
• the microcontroller 32 monitors the voltage supply to the RF power amplifier 39 and communicates the necessary limits to the FPGA 33 based on that voltage. If the duty cycle or maximum pulse width limits are exceeded by the RF drive signal to be provided to the RF power amplifier 39, that RF drive signal is limited to ensure that the average power rating is not exceeded. A warning that the drive signal has been altered to prevent an overload situation can also be provided to an operator to indicate that an override control has been performed.
• the FPGA 33 of the present example can also be used to protect the RF power amplifier 39 from excessive reflected power that could cause damage to the amplifier.
• the RF power amplifier 39 generates signals proportional to the forward and reflected power. These signals are compared to predetermined values so as to produce a reflected power warning signal at any time that the Voltage Standing Wave Ration (VSWR) of the RF power amplifier 39 exceeds 1.6 (representing approximately 10% reflected power).
• VSWR Voltage Standing Wave Ration
• the VSWR is an unavoidable feature of a RF system. In general terms, it is caused by mismatches in component impedances in the signal path.
• the circumstances when such a value of the VSWR might be expected to reach such a level in the present example include a failure in the laser or matching network (the components within the laser arranged to balance the input impedance for the RF laser drive signal with the output impedance of the RF power amplifier).
• the reflected power warning signal is monitored by the FRGA 33.
• the FPGA 33 is operable to interrupt the RF drive signal from the laser control board 31 to the RF power amplifier 39 in the case that the reflected power is too great for too long.
• the RF power amplifier 39 has a tolerance for high reflected power such that it cannot operate with a VSWR greater than 1.6 for more than 200 microseconds or a duty cycle above 10%.
• the FPGA of the present example is operable to control the RF drive signal to prevent these conditions being exceeded in RF drive signals having pulses at a frequency of up to
• the command input provided to the microcontroller 32 includes a number of pulses, each of a given duration.
• the pulse frequency can be up to 100kHz as the FPGA 33 is operable to protect against excessive reflected power in the RF power amplifier 32 at pulse frequencies up to that limit.
• the maximum frequency which can be monitored to protect against excessive reflected power in the RF power amplifier may be higher or lower than this figure, depending upon the intended capabilities of the laser. If the RF power amplifier 39 is driven so as to produce an output power equal to the maximum output capacity, then a laser drive signal as shown in Figure 7 will be produced, with pulses corresponding to the pulses of the command signal.
• the RF power amplifier 39 has a maximum output capacity of 15OW and produces a 15OW output when driven by 32V DC, as illustrated by Figure 7.
• a peak power output of 300W is produced.
• a duty cycle of 50% is introduced, as illustrated in Figure 8.
• a peak laser output beam of 3OW power can be achieved in this mode of operation.
• the pulse frequency can be any frequency up to the reflected power monitoring limit of the FPGA 33. It is not necessary that a discreet frequency be used for the pulses, the command input can include randomly generated pulses. The limits for calculating duty cycle can be based upon the time since the previous pulse. FOF- example, if a supply of 48V DC is supplied to the RF power amplifier 39, then the maximum duty cycle is 50%. In order to meet this requirement without knowing a pulse frequency, the microcontroller 32 can count the delay since a pulse last ended and allow the next pulse to be of up to that duration (thereby achieving a 50% duty cycle).
• Similar logic tailored to the correct duty cycle limit can be applied to supply voltages to the RF power amplifier 39 of between 32V and 48V DC, where the duty cycle limit is between 100% and 50%.
• the maximum pulse width which ensures that the average power output from the RF power amplifier is maintained below the average power limit is 1 ms at a supply voltage of 48V. At lower supply voltages, a correspondingly wider pulse width can be used until the supply voltage reaches 32V where true continuous wave operation is possible as outlined above. In other examples, where RF power amplifiers having different operation characteristics are used, this maximum pulse width may be greater or smaller than 1ms.
• This third mode is a modupulse mode in which the laser control board can generate a pulsed drive signal, which can be gated on and off using the command input.
• the duty cycle of this pulsed drive signal can be controlled using the modupulse power level control.
• the laser output thus created is a continuous wave laser beam with a ripple at the frequency of the pulses of the drive signal.
• This combination of control features allows great control to the user of the laser.
• the user can drive the laser using a command input of any duty cycle or pulse width, even when driving the RF power amplifier 39 with a DC voltage sufficiently large to cause the average power rating to be exceeded in a constant output situation, as all issues concerning power output management are handled by the laser control board 31.
• a low (less than maximum) power continuous wave signal can be produced.
• continuous wave laser output can be achieved, albeit with the ripple at the drive signal pulse frequency.
• complex pulse shapes can be produced.
• pulsing the Command Input, and varying the Modupulse Power Level Control extremely precise control of a laser pulse can be achieved. This can be advantageous in situations where the laser is being targeted at, for example, a curved surface or a moving uneven surface to ensure than the power delivery at the target exactly matches the desired level. In the present example, there is no limit on the relative frequencies of the pulsed signals.
• Figures 9 to 13 illustrate an example of the low power continuous wave operation made possible by the Modupuise operation mode.
• the Command Input signal is switched to "ON" for the duration for which it is desired that the laser be switched on.
• the Modupulse Power Level Control Input is controlled to various levels, each corresponding to a desired laser power output, as shown in Figure 10.
• the laser control board 31 In response to the Command Input and the Modupulse Power Level Control Input, the laser control board 31 generates an RF drive signal having a duty cycle proportional to the Modupulse Power Level Control for a time corresponding to the duration of the Command Input being "ON", as shown in Figure 11.
• the RF power amplifier 39 causes the RF power amplifier 39 to output a laser drive signal having an average power proportional to the Modupulse Level Control Signal as shown in Figure 12.
• the laser output beam has a ripple at the frequency of the pulses in the RF drive signal.
• the ripple has been emphasized in Figure 13 for clarity, the actual ripple variation in power level on the laser drive signal is likely to fall within the range of 0 to 30%, although this will be dependent upon the actual pulse spacing of the drive signal.
• the varied duty cycle of the pulsed RF drive signal directly translates into the different laser drive signal strengths to produce the desired continuous wave laser output.
• the laser has a rise and fall time of approximately 50 microseconds. At high frequencies, the laser output cannot faithfully follow the RF drive signal, which creates the pseudo-continuous wave effect in the output. The depth of the ripple depends upon the frequency and duty cycle of the drive signal.
• the microcontroller 32 interprets the applied Command Input and Modupuise Power Level Control to output a RF drive signal that will result in the laser output which must have been the desired result given the signals received at the control inputs, rather than the laser . output which would result were those control inputs applied directly.
• the microcontroller 32 is operable to cause the laser output to behave in the manner expected by a user by compensating and adjusting to overcome the imperfections in the laser drive system.
• the microprocessor adjusts the RF drive signal pulse width, pulse position and pulse spacing to take account of the latencies inherent in the system.
• the microcontroller 32 has access to a look-up index or matrix table of data describing the behavior of the laser output in response to different drive signal characteristics. The microcontroller 32 can then, when producing the RF drive signal, use the data held in the look up table to provide a drive signal which will cause the laser to output the correct beam strength and duration.
• the laser has a 50 microsecond rise and fall time.
• a longer initial pulse may be provided in order to cause the laser output to rise to the desired level quickly.
• the, for example, 100kHz pulsed drive signal can be sued to achieve the pseudo-continuous wave output. If such a scheme is not deployed, the laser output will ramp up slowly to the desired level, over the course of as many as five hundred pulses. Such a slow-ramping output may be desirable, but for circumstances where an "instant start-up" to a desired operating level is required, providing a longer initial pulse can avoid the slow ramp-up period.
• the necessary pulse width can be determined by performing a look-up in a pulse width data table accessible by the microcontroller 32.
• the look-up table can contain pulse width values for different combinations of desired laser output properties.
• the values of the present example can be determined for the combination of RF power amplifier and laser generating components present.
• a calculation may be performed by the microcontroller 32 or FPGA 33 to determine a suitable initial pulse width.
• the microcontroller 32 can be programmed to calculate the latencies which will occur in response to different drive signal characteristics. The microcontroller can then calculate in real time the drive signal characteristics which will be necessary to produce the desired beam output.
• a laser power supply operable to control the output of an RF excited gas laser to produce a variety of beam strengths, pulse durations, and pulse shapes.
• Such a laser power supply can allow a user great control over the delivered power from the laser to allow more wide and varied use of the laser, particularly in applications where low power consumption is desired (e.g. battery powered systems) or where power delivery tolerances are critical (e.g. medical and marking applications).

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