Europaisches Patentamt number: 0191856 J) European Patent Office Publication B1 Office europeen des brevets EUROPEAN PATENT SPECIFICATION Date of publication of patent specification: 30.08.89 Intel.4: H 01 S 3/125 Application number: 85904897.7 Date of filing: 24.07.85 International application number: PCT/US85/01409 International publication number: WO 86/01347 27.02.86 Gazette 86/05 A CO2 TEA LASER UTILIZING AN INTRA-CAVITY PRISM Q-SWITCH. Priority: 02.08.84 US 637097 Proprietor: Hughes Aircraft Company 7200 Hughes Terrace P.O. Box 45066 Los Angeles, California 90045-0066 (US) Date of publication of application: 27.08.86 Bulletin 86/35 Inventor: DEWHIRST, Donald, R. 4822 White Court Publication of the grant of the patent: Torrance,CA 90503 (US) 30.08.89 Bulletin 89/35 Inventor: DUVALL, Robert, L, III 2649 West 233 Street Torrance, CA 90505 (US) Designated Contracting States: BECHDEFRGBITLINLSE Representative: Kuhnen, Wacker & Partner Schneggstrasse3-5 Postfach 1553 References cited: D-8050 Freising (DE) FR-A-1 537 891 FR-A-2331801 US-A-3434073 References cited: US-A-3548253 Japanese Journal of Applied Physics, volume CO US-A-3 609 588 7, no. 12, December 1968, Tokyo, (JP). Y. US-A-3 725 817 Ohtsuka et al.: "ACO2 Q-switched laser and its CO US-A-3 995 230 nonlinear amplification characteristics", pages in US-A-4355394 1510-1517 00 Applied Physics Letters, volume 8, no. 3, 1 February 1966, New York (US). G.W. Flynn et 5> al.: "Vibrational and rotational studies using Q-switching of molecular gas lasers", pages 63-65 o Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall Q. be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been Ill paid. (Art. 99(1 ) European patent convention). Courier Press, Leamington Spa, England. EP 0 191 856 B1 Description output. The difficulty of creating the discharge in the higher pressure gas is offset by the reduced Background of the invention path length of the transverse discharge. The high 1. Field of the invention peak power of the CO2 TEA laser is not accom- The present invention relates to a rotating 5 plished by a Q-switch, but results from the fast prism Q-switch for use with CO2TEA (carbon discharge which causes the gain to build up faster dioxide transversely excited atmospheric) lasers. than the laser pulse. This method is called "gain The rotating prism Q-switch angularly sweeps switching". through alignment with the resonator mirrors The fast discharge method is undesirable for once per revolution. An opto-electronic timing io many laser applications because sufficient nitro- device with imaging optics rotating with the gen excitation remains after the initial laser pulse prism triggers the gas discharge at the proper to sustain laser oscillation at a power level 1/10 to time prior to resonator alignment. 1/4 of the peak. The output energy after the main pulse is referred to as the "tail" and typically 2. Description of related art 15 contains more than half the energy and lasts up to The CO2 laser has long been available and can several microseconds. In laser range finder be configured to produce a continuous or pulsed applications, the tail is backscattered into the laser beam. It is capable of high average power receiver, thus "blinding" the receiver for the few output while at the same time maintaining the microseconds that the tail exists, which blinding high degree of spectral purity and spatial coher- 20 is unacceptable. ence, characteristics of the lower power atomic The tail can be eliminated by the addition of a gas lasers. An electric discharge is the most Q-switch wherein the Q-switch is on for the main common means of excitation. Operating effi- pulse, and then turned off to prevent the tail. The ciency and output power are greatly increased by Q-switch also can increase the peak output power adding nitrogen and helium to the fill gas. Helium 25 by delaying the switch opening so that the laser aids depopulation of the terminal laser level and pulse occurs near peak gain. For the gain nitrogen excites the carbon dioxide molecules by switched laser, the pulse can occur well before collisional energy transfer. To facilitate the dis- peak gain, thus increasing the tail energy. charge, the CW (continuous-wave) excited CO2 From US— A— 3 434 073 a mechanically Q- laser is operated at low pressure, on the order of 30 switched laser is known in which the timing pulse 13.3 kPa (100Torr). for triggering excitation of the laser medium is Because of the long lifetime of the vibration generated by directing a light beam to the rotat- levels, it is possible to store energy in the dis- ing prism Q-switch and the light beam reflected charge medium by blocking the path of the laser from the prism Q-switch hits a photodetectoronly beam within the resonator, thereby preventing 35 if the prism Q-switch is in a certain position. the laser oscillation. If the block is suddenly From US — A — 3 548 253 an apparatus for con- removed, then the output from the laser occurs in trolling the resonator gain of a pulse laser device the form of a sharp pulse with peak power two to is known, which is the basis for the preamble of three orders of magnitude larger than the average claim 1. Said known apparatus comprises means continuous-wave power obtainable from this 40 for rotating a prism Q-switch whereby resonator laser. This mode of operation is called Q-switch- mirrors of the laser are angularly aligned periodi- ing. In a typical prior art device in which the gas is cally by the rotating prism and an optical imaging excited by CW discharge, Q-switching is accom- device including a photodetector, a light source, plished by replacing one of the laser cavity and an optical guideway coupled to said rotating mirrors with a rotating mirror. A laser pulse at 45 means for optically coupling light from the light 10.6 microns is produced every time the rotating source to the photodetector. The optical pathway mirror lines up with the opposite stationary is a hole in a drum which rotates together with the mirror. prism Q-switch. The photodetector and the light A more efficient method of producing high peak source are arranged such that once per revolution power pulses from the CO2 TEA laser is the use of so of the rotating means the light source and the a pulsed high voltage discharge in a gas medium photodetector are optically coupled. at much higher pressure. As is known, a C02 TEA Since the CO2 TEA laser has an excited state (transversely excited atmospheric) laser is a type lifetime of only a few microseconds, a timing of C02 laser in which excitation of the active accuracy of a few hundred nanoseconds is medium is transverse to the laser beam axis and, 55 required for the time delay between the gas because of a shorter breakdown length, can oper- discharge and Q-switch opening. Heretofore, only ate in a gas pressure range higher than that for the electro-optic Q-switch was capable of provid- longitudinally excited gas lasers, thus achieving a ing this degree of timing accuracy. However, the higher power output per unit volume because of electro-optic Q-switch has serious disadvantages the greater density of lasing molecules. In this 60 when used with C02 TEA lasers due to its cost, laser the gas pressure is near one atmosphere complexity, fragility and susceptibility to laser and the discharge is very fast and transverse to damage. the beam axis. By operating at higher pressure, What is desired is an arrangement wherein a Q- the density of excited CO2 molecules is increased, switch can be utilized with the CO2TEA laser thereby proportionally increasing the peak power 65 without the aforementioned disadvantages of the EP 0 191 856 B1 schematic of the electro-optic Q-switch, the timing accuracy Fig. 4 is a simplified optical the required for laser operation still being provided. components which comprise opto-electronic timing device of the present invention; the Summary of the invention Fig. 5 is a timing diagram illustrating trig- of the These problems are overcome with an gering pulse timing to initiate lasing shown in 3 and the resultant apparatus controlling the resonator gain of a CO2 TEA laser Fig. and pulse laser device in accordance with claim 1. laser pulse output; An embodiment of the present invention pro- Figs. 6a and 6b are reproductions of photo- shape vides a CO2 TEA type laser which is adopted to graphs of CO2TEA laser temporal pulse 6a) and Q- with a simple rotating prism Q-switch. 10 respectively gain switched (Fig. operate 200 The Q-switch is interposed switched (Fig. 6b), both of whose scales are rotating prism KW between the laser resonator mirrors and is nanoseconds per division horizontally and 60 arranged such that rotation of the Q-switch per division vertically. reference numerals identify identical sweeps the resonator mirrors through alignment The same with one another once each revolution of the Q- 15 components in each of the figures. switch. The speed of rotation is selected so that the time interval of good resonator alignment is Detailed description of the invention sufficient to allow for the main pulse buildup, but For purposes of understanding the background discussion of of not long enough to support the tail.