GB1601355A

GB1601355A - Lasers

Info

Publication number: GB1601355A
Application number: GB18577/77A
Authority: GB
Original assignee: BOC Ltd
Current assignee: BOC Ltd
Priority date: 1977-05-03
Filing date: 1977-05-03
Publication date: 1981-10-28
Also published as: IT1095702B; DE2819206A1; ES469358A1; DE2819249A1; JPS5416200A; NL7804791A; IT7822901A0; FR2390033A1

Description

(54) LASERS (71) We, Boc LIMITED, of Hammersmith House, London W6 9DX, England, an English company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to lasers and in particular to lasers of the continuouswave type provided with a Q-switch for generating a series of relatively-high energy pulses at a high frequency.

By a 'Q-switch' in this specification is meant a device for suddenly increasing the Qfactor of the oscillator cavity of a laser having a low Q-factor, so as to generate a very short output pulse of high intensity.

When using a rod of yttrium-aluminiumgarnet (YAG) in the cavity of the laser, with a Q-switching frequency greater than one kHz the period between successive pulses is less than the population inversion lifetime. This means that the energy stored in the rod between successive pulses is less than when the frequency is lower. Thus the effective energy per pulse can be controlled by varying the frequency of Q-switching.

With some applications of the laser, it is necessary to switch off the laser from time to time. One known method of switching off a Q-switched laser (in the sense of reducing its output to zero) is to activate the Q-switch continuously instead of repetitively. When this happens the energy stored in the YAG rod reaches a maximum. When the Q-switch has its mode of operation changed from continuous to repetitive, the first pulse of the consequent train of pulses releases all the energy in the rod, producing a very high energy pulse, which can be disadvantageous in some applications of the laser.

The present invention aims at suppressing this high initial pulse so that, irrespective of the time for which a laser is switched off by the Q-switch, when switched on it functions as a source of pulses of substantiallyuniform energy.

Accordingly to the present invention a laser apparatus comprises a continuouswave optically-pumped laser provided with a Q-switch for generating a series of high energy pulses at a predetermined frequency and means for controlling the operation of the Q-switch so that the Q-switch is capable of switching the laser on and off at intervals greater than the intervals between successive pulses, and for reducing the efficiency of the laser oscillation cavity for a selected period when the laser is switched on, thereafter returning the laser cavity to its normal efficiency.

An embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a diagrammatic side elevation of a cylindrical film resistor which has been trimmed by a known laser apparatus; Figure 2 is a cross-section of the resistor shown in Figure 1, but to a slightly-larger scale, and Figure 3 is a block diagram of the circuitry for controlling a laser apparatus of the present invention.

Figure 4 is a sketch illustrating the output from a laser apparatus which is not provided with means for controlling the operation of the Q-switch forming part of the laser apparatus; Figure 5 is a sketch illustrating the output from a laser apparatus provided with means for controlling the operation of the Q-switch forming part of the laser apparatus; and Figure 6 is a circuit diagram of a Q-switch drive system with means for controlling the operation of the Q-switch shown within dotted lines.

The laser apparatus of the present invention is particularly useful for the 'phototrimming' of cylindrical film resistors. Such resistors consist of a cylindrical body 2 on the curved surfaces of which is deposited a uniform film 4 of electricity resistive material.

In practice, the resistor has electric terminals applied to opposite ends of the body 2, so that the electric resistance between the terminals is dictated by the electrical properties of the film. Normally the resistance presented by the film is significantly lower than desired. Accordingly the film has to have portions of it removed in order to increase the length of the electrical path between the terminals, and hence the resistance.

One convenient method of 'trimming' the resistor to bring it to the desired value is to remove a helical strip of film so as to form a helical conductive path between the terminals. In a finished resistor, such as is shown in Fig. 1, the film 4 has been removed from a helical strip 6 having a start point 8 and an end point 10.

The strip is conveniently produced by mounting the body 2 between rotatable supports which are capable also of being moved in unison along a linear path. The supports (not shown) and the body 2 are positioned in the path of a beam 12 from a continuous wave YAG laser, with the beam being brought to a focus at a point on the film 4.

Before the trimming operation temporary electrodes or terminals are secured to the ends of the body 2, and the electrical resistance monitored. Thereafter the body is both rotated and translated, and the laser beam allowed to fall on the film, producing the start point 8. From this point the impact point of beam 12 starts to trace out a helix on the outer surface of body 2. This process continues until the associated measuring instruments indicate that the electrical resistance between the terminals has risen to the desired value, at which point the laser is switched off, the body 2 removed from between the supports, and a fresh body placed in position for the trimming process to be repeated.

As has already been mentioned, when the beam 12 comes from a continuous-wave Q-switched laser, without the present invention the first output pulse from the laser has a very high energy. This results in vaporisation of the film 4 over a relatively-large area as indicated by the start point 8 in Fig. 1. Because the succeeding pulses are of siguificantly-lower energy, they each vaporise a considerably smaller amount of film, so that the width of the path 6 is significantly less than the diameter of the start point 8.

This enlarged start point causes the helical strip of film to be of non-uniform width in the region 14 between the start point and the adjacent part of path 6. This leads to an undesirably-high concentration of the electrical resistance in region 14, which could result in the production of 'hot spots'. In any case, the non-uniformity of width of the strip of film functioning as the resistor is undesirable.

It will be appreciated that the same problem is not produced when the laser is switched off, so that the end point 10 is merely a rounded termination to path 6. It is highly desirable to make the start point 8 similar to end point 10 in configuration.

To enable this to happen, it is essential to suppress the initial high-energy pulse.

A further disadvantage of the unsuppressed initial pulses is illustrated diagrammatically in Fig. 2.

The material of body 2 may be of a dielectric material which is transparent to the laser radiation. When the incoming beam 12 is of the normal pulse amplitude, substantially the whole of the pulse is used to vaporise a small crescent shaped section of the film, with there being very little unabsorbed energy left to travel on into the body 2.

However, in the case of the unsuppressed initial pulse, while a lot of energy is used to produce the unnecessarily large start point 8, a significant amount of energy is able to pass through the body 2 and vaporise some of the film at the rear of the body.

Although this secondary vaporisation may not be so extensive as to burn a hole in this part of the film, the random vaporisation is able to affect the electrical characteristics of that area of the film, so that it degrades the resistor eventually produced. This is another reason for suppressing the initial pulse.

In the block diagram shown in Figure 3, suppression of the initial pulse is effected by controlling the manner in which the Qswitch is controlled.

In a continuous-wave YAG laser using a Q-switch, the latter can be regarded as an adjustable diffraction grating positioned between the effective mirrors at opposite ends of the resonant cavity of the laser.

When the Q-switch is activated it, as it were, prevents the photons in the cavity from 'seeing' both mirrors. In other words, it prevents the energy stored in the rod from stimulating the emission of photons causing an increase in the energy stored in the YAG rod.

When the Q-switch is deactivated, resonance and stimulation of emission can restart, leading to the stored energy being discharged. Normally the Q-switch is activated vibrating it at a radio-frequency by means of piezo-electric transducers and a radio-frequency generator. Pulsing the drive to the generator results in the laser emitting pulses of coherent radiation.

As already mentioned, in a conventional photo-trimming process using a Q-switched YAG laser, in the 'on' mode, the Q-switch is operated at a frequency higher than one kHz. In the 'off' mode, the Q-switch is energised continuously.

In accordance with the present invention, when the laser is switched from its 'off' mode to its 'on' mode by operation of the Q-switch, the operation of the Q-switch, and hence the efficiency of the laser cavity, is modified so that the laser produces a train of pulses of nearly-equal pulse energy, irrespective of the time for which the laser has been switched off by the Q-switch.

In accordance with the present invention, operation of the Q-switch is modified by not completely deactivating it, and by reducing the power to it slowly. Both modifications result in the Q-switch acting as an inefficient diffraction grating, so that some of the energy in the laser rod will be lost while still allowing the gain in the oscillator to be greater than unity. It may be found necessary to apply both forms of control in order to bring about the small charges in the gain in the oscillator cavity necessary to control the smplitude of the initial pulse.

In the block diagram shown in Fig. 3, block 16 represents a variable frequency pulse generator, 18 represents a pulse shaper 20 represents a driver, 22 represents a radio frequency oscillator, 24 represents a Qswitch and 26 represents a laser on/off signal interface. All of these components are known in conventional photo-trimming laser apparatus. The modifications of the present invention are the insertion of a rise time suppressor 28 between signal interface 26 and pulse generator 18, and the insertion of a pulse amplitude controller 30 between the signal generator 26 and the driver 20.

Both the rise time suppressor 28 and the pulse amplitude controller 30 contain components such as RC networks, giving a specified time constant. The effect of the rise time suppressor 28 is to reduce the rate at which the radio frequency generator is switched off and hence the rate at which the Q-switch turns on the laser. The effect of the pulse amplitude controller 30 is to prevent the driver to the radio frequency generator from being turned off completely and hence preventing the laser from turning completely on.

In this invention, it is of no consequence if the energy stored in the YAG rod at the end of an "off" period is sufficient only to give an initial output pulse of lower than the usual amplitude: what is important is that the amplitude of this pulse must not be significantly greater than usual.

Referring to Fig. 6, Ql normally briefly (6u sec) short circuits the transducer drive circuit, making the Q-switch block transparent and enabling the output of a laser pulse. Resistor R1 reduced the efficiency of this short circuit, and so permits partial activation of the Q-switch during the brief interval it would normally be transparent.

In this way, the efficiency of the laser cavity is reduced, and hence the amplitude of the output pulse. This inefficiency is progressively reduced by a gradual switching on of Q2 (after the beginning of the 'Laser On' control signal) by means of the timing components R2 and C1. R3 enables the threshold level to be adjusted.

The rise time suppressor network degrades the sharpness with which the piezoelectric driver is short circuited, by slugging the output of the pulse generator with C2. The timing components R4 and C3 progressively turn Q8 off and so disable the slugging action of C2.

The combined effect of these modifications result in the modulation envelope shown in Fig 5. This can be compared with the modulation envelope shown in Figure 4 which results when the means for controlling the Q-switch is absent from the laser apparatus.

WHAT WE CLAIM IS: 1. A laser apparatus comprising a continuous-wave optically pumped laser provided with a Q-switch for generating a series of high energy pulses at a predetermined frequency and means for controlling the operation of the Q-switch so that the Qswitch is capable of switching the laser on and off at intervals greater than the intervals between successive pulses and for reducing the efficiency of the laser oscillation cavity for a predetermined period when the laser is switched on thereafter returning the laser cavity to its normal efficiency.

2. A laser apparatus as claimed in claim 1, in which the means includes an amplitude network for reducing the amplitude of pulses driving the Q-switch when the laser is switched off and a rise time network for controlling the speed at which the driving pulses are reduced.

3. A laser apparatus constructed and arranged substantially as herein before described with reference to Figures 3 and 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. mode to its 'on' mode by operation of the Q-switch, the operation of the Q-switch, and hence the efficiency of the laser cavity, is modified so that the laser produces a train of pulses of nearly-equal pulse energy, irrespective of the time for which the laser has been switched off by the Q-switch. In accordance with the present invention, operation of the Q-switch is modified by not completely deactivating it, and by reducing the power to it slowly. Both modifications result in the Q-switch acting as an inefficient diffraction grating, so that some of the energy in the laser rod will be lost while still allowing the gain in the oscillator to be greater than unity. It may be found necessary to apply both forms of control in order to bring about the small charges in the gain in the oscillator cavity necessary to control the smplitude of the initial pulse. In the block diagram shown in Fig. 3, block 16 represents a variable frequency pulse generator, 18 represents a pulse shaper 20 represents a driver, 22 represents a radio frequency oscillator, 24 represents a Qswitch and 26 represents a laser on/off signal interface. All of these components are known in conventional photo-trimming laser apparatus. The modifications of the present invention are the insertion of a rise time suppressor 28 between signal interface 26 and pulse generator 18, and the insertion of a pulse amplitude controller 30 between the signal generator 26 and the driver 20. Both the rise time suppressor 28 and the pulse amplitude controller 30 contain components such as RC networks, giving a specified time constant. The effect of the rise time suppressor 28 is to reduce the rate at which the radio frequency generator is switched off and hence the rate at which the Q-switch turns on the laser. The effect of the pulse amplitude controller 30 is to prevent the driver to the radio frequency generator from being turned off completely and hence preventing the laser from turning completely on. In this invention, it is of no consequence if the energy stored in the YAG rod at the end of an "off" period is sufficient only to give an initial output pulse of lower than the usual amplitude: what is important is that the amplitude of this pulse must not be significantly greater than usual. Referring to Fig. 6, Ql normally briefly (6u sec) short circuits the transducer drive circuit, making the Q-switch block transparent and enabling the output of a laser pulse. Resistor R1 reduced the efficiency of this short circuit, and so permits partial activation of the Q-switch during the brief interval it would normally be transparent. In this way, the efficiency of the laser cavity is reduced, and hence the amplitude of the output pulse. This inefficiency is progressively reduced by a gradual switching on of Q2 (after the beginning of the 'Laser On' control signal) by means of the timing components R2 and C1. R3 enables the threshold level to be adjusted. The rise time suppressor network degrades the sharpness with which the piezoelectric driver is short circuited, by slugging the output of the pulse generator with C2. The timing components R4 and C3 progressively turn Q8 off and so disable the slugging action of C2. The combined effect of these modifications result in the modulation envelope shown in Fig 5. This can be compared with the modulation envelope shown in Figure 4 which results when the means for controlling the Q-switch is absent from the laser apparatus. WHAT WE CLAIM IS:

1. A laser apparatus comprising a continuous-wave optically pumped laser provided with a Q-switch for generating a series of high energy pulses at a predetermined frequency and means for controlling the operation of the Q-switch so that the Qswitch is capable of switching the laser on and off at intervals greater than the intervals between successive pulses and for reducing the efficiency of the laser oscillation cavity for a predetermined period when the laser is switched on thereafter returning the laser cavity to its normal efficiency.