WO2008003997A1

WO2008003997A1 - Laser control systems

Info

Publication number: WO2008003997A1
Application number: PCT/GB2007/050356
Authority: WO
Inventors: Stephen Matthews; Chris Parsons; David Judd; Stephen Keen
Original assignee: Gsi Group Ltd
Priority date: 2006-07-03
Filing date: 2007-06-25
Publication date: 2008-01-10
Also published as: GB2439758A; GB0613143D0

Abstract

A laser control system comprising an energy source (10) adapted to feed a gain medium of a laser, wherein the energy source is arranged to supply continuous power at a level below laser threshold, further comprising means (13, 15) for boosting the level of the continuous power to a higher level for a predetermined period before the laser is fired and closed loop feedback circuit comprising means (17, 22) for monitoring the output laser energy of the laser and monitoring the magnitude of the power supplied by the power supply unit (11) in a sense to maintain the output from the laser substantially uniform.

Description

Laser Control Systems

This invention relates to laser control systems. In particular, it relates to systems for controlling the laser pump power supply of a laser for maintaining or improving overall operational stability and for producing a constant train of output pulses.

Material processing applications for lasers increasingly require laser sources with exceptionally good pulse energy stability. It is also important that the stability of pulses (that is consistency, reliability and repeatability) is achieved immediately or as soon as possible after start up so that preferably every pulse can be utilised in a process without any need to discard a number of initial pulses.

Start up performance of a solid state pulse laser is very unpredictable and generally results in pulses with inconsistent pulse energy, as is shown in Figure 1. This shows a plot of pulse energy against the number of pulses and shows that in a typical solid state pulse laser the initial few pulses are very unstable and consistency is only achieved after a number of start up pulses.

Thermal lensing effects in a laser resonator can be responsible for laser cavity instability when lasing from start up. Whilst the laser rod can be preloaded with required energy to provide a consistent thermal load prior to lasing, and this can reduce transient thermal effect within the resonator and eliminate inconsistency due to thermal lensing, this technique is not always practical for high energy pulses. The power supply has to be capable of providing a suitable output whilst ensuring that the resonant chamber of the laser stays below a laser threshold value and also the complex nature of thermal effects also means that the exact thermal profile across a laser rod is not possible to match in most situations.

It is an object of the present invention to provide an improved control system which produces a more consistent train of output pulses.

According to the present invention there is provided a laser control system comprising an energy source adapted to feed a gain medium of a laser, wherein the energy source is arranged to supply continuous power at a level below laser threshold, further comprising means for boosting the level of the continuous power to a higher level for a predetermined period before the laser is fired and closed loop feedback circuit comprising means for monitoring the output laser energy of the laser and monitoring the magnitude of the power supplied by the power supply unit in a sense to maintain the output from the laser substantially uniform.

In a further aspect, the invention provides a method of controlling a source of energy feeding a gain medium of a laser, comprising the step of supplying continuous power at a level below laser threshold, boosting the level of the continuous power to a higher level for a predetermined period before the laser is fired and using a closed loop feedback circuit including a monitor for monitoring the output energy of the laser to modify the magnitude of the supplied energy pulses to maintain the outputs from the laser substantially uniform. According to the present invention in a further aspect, there is provided a method of controlling a laser, comprising using an active simmer technique in conjunction with a closed loop feedback system.

Preferably, the method also comprises applying predetermined values to alter the simmer value, by predetermined values according to, for example, a look up table or other means.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows samples of laser pulse energies against number of pulses for a previously proposed laser control system;

Figure 2 shows samples for a system having active simmer; Figure 3 shows pulse levels of an active simmer system having a boosted simmer level;

Figure 4 shows pulse energies against number of pulses for a system having an active simmer and a closed loop feedback system;

Figure 5 shows a system having active simmer control, closed loop feedback and optimised predetermined feedback values with an increased simmer level; Figure 6 shows energy levels against pulse numbers in a system in accordance

Figure 5; and

Figure 7 shows a control system. - A -

Control systems as described in the present specification may be used for any type of laser system, in particular solid state lasers such as Nd: YAG lasers or other lasers.

Figure 1 shows a plot of pulse energy against number of pulses from start up for a prior art system as described.

Figure 2 shows a similar plot but for a system in which active simmer is used. "Active simmer" is a method of preloading a laser rod (or other laser gain medium) by boosting a simmer current prior to the first pulse. This is shown in Figure 3. The figure shows a nominal simmer level Sl which is boosted by active simmer to a boosted level S2. The boosted level is somewhere between the nominal simmer level S 1 and the pulse level S3 which generates laser output pulses and this (S3) is otherwise known as the laser threshold energy. As is shown in Figure 2, the energy level and repeatability of the initial pulses is better (ie more consistent) than that where no active simmer is used but there is still a noticeable period Pl where the pulse energy is still low and thus these pulses would normally have to be discarded or would produce unsatisfactory results. The number of pulses that would therefore have to be discarded is, however, significantly less than with the prior art (shown as P_a in Figure 1).

Active simmer does not have the output current capability to match the input power for all output pulse energies and powers. It is possible to adjust the simmer to optimise initial thermal lensing to produce a first pulse of good stability, although its energy may be lower than the nominal free running value and this may not always be appropriate. It is generally desirable to optimise the period of the boosted simmer to be as short as possible due to process speed and pump source reliability. The length of time will normally be dependent on the thermal characteristics of the resonator in question. On seeing the first pulse, an active simmer control reduces the simmer level to the nominal running level.

Active simmer therefore improves the predictability of initial laser pulses from start up. To improve further, a closed loop from an energy monitor within the laser may be incorporated. The closed loop measures the output energy of each laser pulse and is used to adjust the power supply output level accordingly.

In preferred embodiments, the closed loop uses an algorithm, typically a PID (proportional integral differential) based algorithm correcting the delivered power supply output energy on a pulse by pulse basis. The closed loop is modified during the first few pulses by including lensing compensation parameters input into a look up table or otherwise to boost the energy delivered by the power supply. This may be done by using a look up table. The method is typically as below.

First Pulse: Output = h x comp(l) Integral(2) = (Demand - Energy( I))

Second Pulse: Output = h x comp(2) + (Prop x (Demand - Energy(l) ) + Int x Integral(2) + (Diff x (Energy(l)) Integral(n) = Integral + (Demand - Energy(n-l)) nthPulse: Output = h x comp(n) + (Demand - Energy(n-l)) + Int x Integral(n) + Diff x (Energy(n-l) - Energy (n-2))

where n = number of pulse comp(n) - compensation term for thermal lens variation for pulse n Prop = proportional gain

Int = integral gain Diff = Differential Gain

Energy(n) = Energy feedback from energy monitor for pulse n Integral(n) = Integral term to be applied to pulse n h = nominal demand

After the first few pulses, once thermal lensing has reached a stable point, the compensation term comp(n) is fixed for the remaining calculations.

In the example, the first term of the look up table for the first pulse may be one which increases the simmer level to, say, 110%. The second term of the look up table may or may not be the same value but may typically be lower, say 107%. By the nth pulse (eg 5^th pulse), the compensation value may alter the boosted simmer level to, say, 104% of the nominal simmer level and this level is kept throughout the operation of the laser to process a material.

It should be noted in passing that the material processing may be any sort of material processing such as welding, cutting or otherwise. The closed loop compensates for irregularities in lensing stabilisation and can also compensate for other medium to long term effects which cause variation in laser output such as water temperature, pump source ageing and so on. The effect of introducing a compensated closed loop in addition to active simmer are shown in Figure 4. It is seen that pulse stability is achieved much better than before.

As an additional improvement, an increased running simmer level may be used. For low output powers, only a small amount of thermal lensing is established whilst the laser is running. This can lead to increased instabilities both at start up and during free-running operations. Therefore, optimising the control values and compensation terms and increasing a running simmer level can produce very good stability at low power levels. Figure 5 shows this. A nominal simmer level Sl and an initial boosted simmer level S2 are generated, as in Figure 4. Once the first pulse has been established, then the system generates an intermediate running simmer level S4, which will be between the boosted simmer level and the nominal simmer level. Figure 6 shows energy against pulse number for a system operated in accordance with this and it is immediately seen that excepted stability is achieved and that all pulses are useable.

Thus, three different simmer levels can be produced by a simmer circuit and adjusted to different values depending upon the use to which the laser is put. In variation, more than three simmer levels can be produced.

Figure 7 shows an example of power supply and control apparatus. A pumping lamp 10 is used to provide pumping energy to a laser gain medium in normal manner. This is provided with power through a power supply 11 which has a control unit or CPU 12 having both an active simmer control 13 and output control 14. The active simmer control 13 generates simmer current 15 in accordance with the current shown in the previous figures and the output control 14 is used to generate output pulses 16 also in accordance with these. An energy monitor 17 is used to monitor the energy output by the laser. The laser itself is not shown in the diagram. This may typically be a photodiode or other means which can monitor the energy output of a laser. The output from the energy monitor is applied to a control system 18 which has a controller 19 connected to an output pulse control 20 and from then to output pulse generation unit 21. The control unit 19 is also connected to the energy monitor 22, which is a processed version of output from energy monitor 17.

The output pulse control unit 22 includes the closed loop described above and the look up table. Values from this are applied to the output pulse generation unit 21 and the output pulse generation unit 21 provides an output to the output pulse generation unit 16 of the power supply to thereby modify the output pulse in accordance with the predetermined values from the look up table. The closed loop within unit 20 is linked to the energy monitor so that output levels of the laser can be used to modify subsequent pulse energies.

Claims

1. A laser control system comprising an energy source adapted to feed a gain medium of a laser, wherein the energy source is arranged to supply continuous power at a level below laser threshold, further comprising means for boosting the level of the continuous power to a higher level for a predetermined period before the laser is fired and closed loop feedback circuit comprising means for monitoring the output laser energy of the laser and monitoring the magnitude of the power supplied by the power supply unit in a sense to maintain the output from the laser substantially uniform.

2. A laser control system as claimed in Claim 1 , further including means for altering the level of first and subsequent pulses by varying predetermined predicted values.

3. A laser control system as claimed in Claim 2, including a look-up table provided with a series of values for adjusting the level of pulses to thereby adjust the energy supplied to the gain medium.

4. A system as claimed in Claim 2 or Claim 3, wherein the output energy is adjusted by increasing or decreasing the power supply output energy by a known amount.

5. A system as claimed in any preceding claim, including an algorithm correcting the delivered power supply output energy on a pulse by pulse basis.

6. A system as claimed in Claim 5, wherein the algorithm is a PID (proportional integral differential) based algorithm.

7. A laser control system as claimed in Claim 6, wherein the PID based algorithm is as follows:

First Pulse: Output = h x comp(l)

Integral(2) = (Demand - Energy(l)) Second Pulse: Output = h x comp(2) + (Prop x (Demand - Energy(l) ) + Int x Integral(2) + (Diff x (Energy(l)) Integral(n) = Integral + (Demand - Energy(n-l)) nth Pulse: Output = h x comp(n) + (Demand - Energy(n-l)) + Int x Integral(n) + Diff x (Energy(n-l) - Energy (n-2))

where n = number of pulse comp(n) - compensation term for thermal lens variation for pulse n

Prop = proportional gain

Int = integral gain

Diff = Differential Gain

Energy(n) = Energy feedback from energy monitor for pulse n

Integral(n) = Integral term to be applied to pulse n h = nominal demand

8. A system according to any preceding claim, wherein the continuous power is supplied at least at three different levels.

9. A system according to any preceding claim, wherein said monitoring means comprises a photodiode positioned to monitor the outputs of said laser, an integrator for integrating the output of the photodiode and a comparator for comparing the output of the integrator with a reference value, the output of the comparator being used by a control means to adjust the magnitude of the output of each output pulse of said succession of pulses after the first accordingly.

10. A method of controlling a source of energy feeding a gain medium of a laser, comprising the step of supplying continuous power at a level below laser threshold, boosting the level of the continuous power to a higher level below a laser threshold level for a predetermined period before the laser is fired and using a closed loop feedback circuit including a monitor for monitoring the output energy of the laser to modify the magnitude of supplied energy pulses to maintain the outputs from the laser substantially uniform.

11. A method of claimed in Claim 10, including the step of boosting first and subsequent pulses by varying predetermined predicted values.

12. A method as claimed in Claim 10 or Claim 11, further include the use of an algorithm to adjust the energy supply to maintain a consistent energy at the output of the laser.

13. A method as claimed in Claim 12, wherein the algorithm is overlaid on top of the predetermined values.

14. A method as claimed in any of Claims 10 to 13, wherein the algorithm used is a follows:

First Pulse: Output = h x comp(l)

Integral(2) = (Demand - Energy(l)) Second Pulse: Output = h x comp(2) + (Prop x (Demand - Energy(l) ) + Int x Integral(2) + (Diff x (Energy(l))

Integral(n) = Integral + (Demand - Energy(n-l)) nth Pulse: Output = h x comp(n) + (Demand - Energy(n-l)) + Int x Integral(n) + Diff x (Energy(n-l) - Energy (n-2))

Prop = proportional gain

Int = integral gain

Diff = Differential Gain

Energy(n) = Energy feedback from energy monitor for pulse n

Integral(n) = Integral term to be applied to pulse n h = nominal demand

15. A method as claimed in any one of Claims 10 to 14, including means generating an internal boosted simmer level and, after generating of a first laser pulse, a subsequent simmer level of a level higher than a nominal simmer level but lower than the initial boosted level.

16. A method of controlling a laser, comprising using an active simmer technique in conjunction with a closed loop feedback system.

17. A method as claimed in any preceding claim, applying predetermined values to alter the level of pulses supplied to a pumping means of the laser by predetermined amounts.

18. An apparatus or a method substantially as hereinbefore described with reference to, or as illustrated by, the accompanying drawings.