IMPROVEMENTS IN LASERS

Abstract: A resonant power circuit which supplies pulsed power to a load, such as a pulsed laser, from a d.c. supply by cyclically charging a capacitor. The capacitor is discharged through the load on closure of a switch which should then open to allow the capacitor to re-charge. Occasionally the switch fails to open and the circuit of the invention then develops and applies a reverse voltage to the switch to force it to become open circuit.

Description: IMPROVEMEUTS II LASERS DESCRIPTION This invention relates tα an electric circuit which provides protection to a power source supplying power to a load through a resonant circuit. In such circuits a cyclical operation takes place in which a direct current power supply charges a capacitor and the power stored in the capacitor is then discharged through the load. The discharge supplies operating power to the load and the present invention applies in particular tα a load which needs repetitive discharges over an extended period or for continuous operation. In such circuits the discharge of the capacitor through the load can be initiated by a switch device which connects * £he fully-charged capacitor across the load. At the appropriate time it is necessary for the switch device tα become open circuit so that the capacitor can recharge and the regular operation of the circuit requires that this should occur. If the switch device fails tα become open circuit it effectively presents a short circuit to the power supply and unless the switch becomes open circuit within a predetermined time, the protective circuits of the power supply will cause the power supply tα switch off to prevent it being damaged. The present invention provides a protective circuit to prevent just such an occurrence. Accαrding to the invention there is provided an electrical circuit comprising first capacitive means arranged to be charged from a power supply, switch means arranged to close for discharging said first capacitive means through a load and then tα open to enable said first capacitive means to re-charge, characterised in that second capacitive means (C3) are connected across said switch means (T) and current limiting means (L2) are arranged in the circuit between the power supply and said second capacitive means <C3), whereby upon continued closure of said switch means (T. said second capacitive means (C3) first discharges through said switch means (T) and then applies a reverse voltage tα said switch means (T) causing opening of said switch means (T). Preferably, said power supply is prevented from charging said second capacitive means in the event αf continued closure of said switch means by said current limiting means (L2) . Preferably also, means are provided for discharge of the energy in said current limiting means (L2). Preferably also, a detector is arranged tα indicate if operation αf said circuit occurs in excess of a predetermined rate. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 is a circuit diagram of a laser discharge resonant circuit constructed in accordance with the invention, and Figures 2 to 4 are voltage and current wave form diagrams. Referring first to the circuit outside (to the left and right of) the two broken lines of Figure 1, there is shown a capacitor Cl arranged to be charged from a direct current voltage supply VO. When voltage VO is applied current flows through inductor LI and diode Dl tα charge capacitor Cl. The voltage VI (Figure 2) across capacitor Cl rises towards VO; the current II (Figure 3) reaches a maximum and starts tα decay. Inductor LI, which initially resists the current II flowing when voltage VO is first applied, now resists decay of this current with the result that the voltage across capacitor Cl continues to rise and eventually reaches twice VO. At time tl thyratron T closes and connects capacitor Cl directly across the load, The load in this case is a pulsed metal vapour laser using a material such as capper as the vapour lasing medium. The charge which has been developed across capacitor Cl causes a discharge to travel down the discharge tube of the laser initiating lasing. Normally thyratron T then becomes open circuit after a short period and capacitor Cl is then recharged from the d.c. power supply. This mode αf operation continues at a pulsed repetition rate αf about 20 Hz, Capacitor C2 is connected across the power supply for smoothing purposes and when thyratron T closes, capacitor C2 does not discharge through it tα any great extent because inductor LI limits the speed at which the current from capacitor C2 can build up through the short circuit of thyratron T. From time to time quite randomly, thyratron T malfunctions and remains short circuited when the system requires it to go open circuit. In these circumstances the discharge from capacitor C2 continues and the short circuit which is continued to be applied across the power supply causes protective devices (not shown) αf the power supply tα operate and isolate the circuit thus causing the laser to cease operation. Metal vapαur lasers which are caused tα stop operating then have tα be fully re-initiated and this can take up to an hour to complete before the laser can be made tα operate again. Unwanted closedown for this reason can be a major disadvantage with this kind αf laser. The components within the two broken lines in Figure 1 constitute a circuit which provides protection to the system in the case αf thyratron malfunction αf this kind. When the cycle starts capacitors Cl, C2 and C3 are all charged from the voltage VO. Vhen thyratron T closes capacitor Cl is discharged through the load but capacitors C2 and C3 are prevented from discharging tα any great extent by the inductances LI and L2. If thyratron T malfunctions and remains closed after the system requires it tα go open circuit, capacitor C3 discharges through thyratron T while capacitor C2 is still prevented from discharging to any great extent due to the higher inductance of L2 as compared tα LI. Ψhen the voltage V3 (Figure 4, note the different timescale) across capacitor C3 reaches zero (at approximately time 10 * tl) and if thyratron * T is still closed, the current 13 flowing starts to decay but this decay is resisted by the inductor LI: as a result, the voltage V3 reverses and the current 13 decays tα zero and tries tα reverse. The reverse voltage V3 is applied across thyratron T forcing it to become open circuit. Because current flow from capacitor C2 is prevented from building up by inductor L2, capacitor C2 is discharged only slowly and the current from it is insufficient to affect the over-swing αf voltage across capacitor C3 described above. Operation αf the circuit as described above does, however, build up energy in inductor LI and after thyratron T has again become open circuit, this energy is discharged into resistors Rl and R2 through diode D2. Also, light emitting diode D3 flashes tα give a visible indication αf the discharge αf L2 and detector D4 is arranged tα count the rate αf malfunction αf the circuit. If this exceeds a predetermined rate, αf the order of eight times per day, detector D4 gives an indication that the laser ought to be closed down for maintenance. Recharging of capacitors Cl and C3 will take place in the normal manner after operation αf the circuit tα force open circuit αf the thyratron T. One additional aspect is that capacitor C3 would be capable αf recharging to more than voltage VO (up tα twice voltage VO) by current flow- from capacitor C2 through inductor L2; this would interfere with the-desired functioning of the thyratron T. However, for low/average current circuits resistor Rl provides sufficient damping tα limit this current and limit the voltage to VO. In high power circuits diode D2 and resistor R2 remove any excess voltage from capacitor C3 and, in this case, resistor Rl can.be reduced to zero or at least to the resistance provided by the windings of inductor L2. Resistor R2 is then chosen in value sufficient tα damp out quickly the current which would circulate through inductor L2, diode D2 and resistor R2. Typical circuit values of these components are:- Ll , 100 mH L2 1.5 H Cl 8 nanα Farads C2 10 micro Farads C3 2 micro Farads Rl 0 - 10 K Ohms R2 0 - 10 K Ohms

Claims: CLAIMS An electrical circuit comprising first capacitive means arranged tα be charged from a power supply, switch means arranged to close for discharging said first capacitive means through a load and then to open to enable said first capacitive means tα re-charge, characterised in that second capacitive means (C3) are connected across said switch means (T) and current limiting means (L2) are arranged in the circuit between the power supply and said second capacitive means (C3), whereby upon continued closure αf said switch means (T) said second capacitive means (C3) first discharges through said switch means (T) and then applies a reverse voltage tα said switch means (T) causing opening of said switch means (T). An electrical circuit as claimed in claim 1, characterised in that said power supply is prevented from charging said second capacitive means (C3) in the event αf continued closure of said switch means (T) by current limiting means (L2). An electrical circuit as claimed in claim 2, characterised in that means (Rl, R2) are provided for discharge αf the energy in said current limiting means (L2). An electrical circuit as claimed in claim 1, characterised in that a detector (D3) is arranged to give a visible indication when said switch means (T) is caused to ga open circuit. An electrical circuit as claimed in claim 1, characterised in that a detector (D4) is arranged tα indicate if continued closure of said switch means (T) occurs in excess of a predetermined rate.

Inventors: KEARSLEY ANDREW JOHN, NAYLOR GRAHAM ALEXANDER, KEARSLEY, ANDREW, JOHN, NAYLOR, GRAHAM, ALEXANDER
Applicants: OXFORD LASERS LTD, OXFORD LASERS LIMTED
Application year: 1986
Main classification: H03K3
IPC codes: H02M9, H03K3