Electric heating – Metal heating – By arc
Reexamination Certificate
1999-10-20
2002-02-12
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121700
Reexamination Certificate
active
06346686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an apparatus and method for processing material using lasers and more particularly to removal of material using lasers and more particularly to removal of material using a laser with optimally configured pulses.
2. Description of the Related Art
Lasers and laser systems for drilling materials are commercially available. Many of these lasers typically cause excessive heating of the material on which they are incident and this may not be desirable for certain types of material processing. Other commercially available lasers typically produce Q-switched or mode-locked pulses and it is difficult to control the duration, shape and spacing of the pulses generated by such lasers.
In principle, it is possible to control the pulse shape by using an active modulator in the laser resonator, an approach used, for example in the DP-11 laser available from TRW Inc. The DP-11 laser can produce a series of short (~100 ns), spaced (>20 &mgr;sec) pulses under the ~400 &mgr;s long diode pump pulse envelope.
However, the use of a modulator in such an actively controlled system results in certain drawbacks such as increase in the cost and complexity of the system and power and efficiency limitations. Specifically, the DP-11 laser generates 350-500 W average power when unmodulated (400 Hz repetition rate quasi-CW operation with 400-500 &mgr;s long pulses), whereas the output power is limited to ~80 W with the modulated pulse format. Further, such a modulator is not usable at high power since the active modulator device probably cannot be scaled. The DP-23 laser, also available from TRW Inc., is more powerful (nominally 2-3 kW average with 400 &mgr;s long pulses), but does not use such modulation.
Electro-Discharge Machining (EDM) is also presently used for drilling small holes. However, duration times on the order of tens of seconds or longer are required for each hole and diameters smaller than ~0.006″ and depth to diameter ratios greater than ~50 are very difficult to achieve.
Phase conjugate master oscillator power amplifier laser architectures (PC-MOPA) capable of achieving high average output power with near diffraction-limited beam quality are known. See, for example, the article “A Review of Phase-Conjugate Solid-State Lasers” by David A. Rockwell, IEEE Journal of Quantum Electronics, vol. 24, no. 6, June 1988, pp. 1124-1140, the content of which is hereby incorporated herein by reference.
Loop phase conjugate mirrors (Loop PCMs) are also known. See, for example U.S. Pat. No. 5,729,380 entitled “Loop Phase-Conjugate Mirror For Depolarized Beams”, inventors: Alexander A. Betin and Metin S. Mangir, issued Mar. 17, 1998 to Hughes Electronics Corporation, the assignee of the present invention. Also see U.S. Pat. No. 5,726,795, entitled “Compact Phase-Conjugate Mirror Utilizing Four-Wave Mixing In a Loop Configuration”, inventors: Alexander A. Betin, Metin S. Mangir and David A. Rockwell, issued Mar. 10, 1998, and assigned to Hughes Electronics Corporation, the assignee of the present invention. The subject matter of U.S. Pat. Nos. 5,726,795 and 5,729,380 are incorporated herein by this reference.
Additional information regarding loop PCM can be found in A. A. Betin and O. V. Mitropol'sky, “Generation of radiation by four-wave interaction in a feedback system in the &lgr;=10 &mgr;m range,” Sov. J. Quant.Electron. 17, 636 (1987) and the article by A. S. Dement'ev and E. Ya. Murauskas, “Emission from a YAG:Nd laser with a four-wave thermal mirror in a ring resonator,” Sov. J. Quant. Electron. 18, 631 (1988), each of which is hereby incorporated by reference herein.)
FIG. 1
is a diagram of a system employing Loop PCM. The input beam E
1
100
first passes through a nonlinear medium
102
, which can be a simple absorption cell
344
. The input beam
100
is then directed through an amplifier
104
having a gain G by two or more mirrors
106
,
108
to form a loop or ring. The amplified wave
110
, labeled E
3
, is directed to intersect E
1
100
at a small angle in the cell
102
. These propagating waves, having sufficient coherence length, form an interference pattern in the nonlinear medium
102
that produces an associated index grating of modulation dn ~E
1
E
3
*. The grating is characterized by a reflectivity R which closes the loop and allows ring laser oscillation under the condition RG>1. Not shown, but used in many cases, is a non-reciprocal optical diode that prevents saturation of the loop amplifier
104
by the incoming input beam
100
and preferentially selects the ring oscillation to be in the opposite direction from the input beam
100
. Being the laser oscillation mode, beam E
2
112
starts from spontaneous noise, diffracts from the grating to become beam E
4
114
and is amplified as it passes around the loop and becomes E
2
112
again. The grating and loop resonator select wave E
2
112
to be phase conjugated to the input beam
100
. The portion of E
2
112
that is transmitted by the grating is, finally, the output wave E
out
116
, which is phase conjugate to E
1
100
and may be larger in amplitude.
Any kind of nonlinear mechanism for recording a grating hologram can be used, but most of the work reported in the literature has been done using the thermal nonlinearity in liquids and the gain saturation effect in the active medium of the amplifier itself. References which discuss the use of thermal nonlinearity in liquids include: A. A. Betin and O. V. Mitropol'sky, “Generation of radiation by four-wave interaction in a feedback system in the &lgr;=10 &mgr;m range,” Sov. J. Quant.Electron. 17, 636 (1987); A. S. Dement'ev and E. Ya. Murauskas, “Emission from a YAG:Nd laser with a four-wave thermal mirror in a ring resonator,” Sov. J. Quant. Electron. 18, 631 (1988); A. A. Betin and A. V. Kirsanov, “Spatial structure of radiation from a neodymium glass four-wave feedback oscillator,” Sov.J.Quant.Electron. 22, 715 (1992); A. A. Betin and A. V. Kirsanov, “Selection of a phase-conjugate wave in an oscillator based on a four-wave interaction with feedback in an extended nonlinear medium,” Quantum Electronics 24, 219 (1994); K. V. Ergakov and V. V. Yarovoy, “Energy optimization of an Nd:YAG-based four-wave-mixing oscillator with feedback and investigation of its adaptive properties in the pulse-periodic regime,” Quantum Electronics 26, 389 (1996), each of which is hereby incorporated by reference herein. References which use the gain saturation effect in the active medium of the amplifier itself include: A. A. Betin, “Phase conjugation based on thermal nonlinearity,” Nonlinear Optics, Maui, Hawaii, July 1996, Techn. Digest v. 11, p.336-339; R.P.M. Green, G. J. Crofts and M. J. Damzen, “Holographic Laser resonators in Nd:YAG, “Optics Letters 19, 393 (1994); A. V. Berdyshev, A. K. Kurnosov, and A. P. Napartovich “Formation of amplitude grating in the medium of a CO laser subject to the field of its own multifrequency radiation,” Quantum Electronics 24 87 (1994); A. A. Ageichik, et al. “Self-phase conjugation of middle-infrared radiation by four-wave mixing in active medium of CO2 laser with feedback loop,” in Laser Optics '95: Phase Conjugation and Adaptive Optics, Vladimir E. Sherstobitov, Editor, Proc. SPIE 2771, 119-125 (1996), each of which is hereby incorporated by reference herein.
There is a continuing need for laser systems and methods that improve the efficiency of laser materials processing, particularly with respect to material removal such as laser drilling.
SUMMARY OF THE INVENTION
To address the requirements described above, the present invention provides an apparatus for generating radiation comprising a master oscillator and a phase conjugator for controlling transient relaxation oscillations to form sustained pump pulses. In a preferred embodiment, the phase conjugator is a loop phase conjugate mirror (Loop PCM). The present invention uses the oscillatory output of the Loop-PCM for material processing applications by contro
Betin Alexander A.
Bruesselbach Hans W.
Mangir Metin S.
Duraiswamy V. D.
Heinrich Samuel M.
Hughes Electronics Corporation
Sales M. W.
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