Laser based material processing methods and scalable...

Details Laser based material processing methods and scalable... Laser based material processing methods and scalable...

2002-11-13

2004-05-18

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular beam control device

Mode discrimination

C372S006000, C372S064000, C372S034000

Reexamination Certificate

active

06738396

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to pumped solid-state lasers, amplifiers, and laser processing devices and methods for using same and, particularly, when such pumping is provided by one or more laser diodes. The invention also relates to amplification of pulsed laser beams such as those produced by q-switched and/or mode locked lasers. Several embodiments include planar waveguide devices and the use of the same for high and low power laser material processing and micro-machining applications.
2. Background Art
Most solid state laser applications benefit from the use of laser sources which have high beam quality, high efficiency, and high reliability, and which are low in cost. When compared to lamp-pumped, solid state lasers, LPL's, diode-pumped, solid state lasers, DPL's, offer significant advantages in terms of beam quality, efficiency, and reliability, but their cost effectiveness is hampered by the high cost of laser diodes.
An improved solid state laser architecture will provide for scalability to high power while maintaining high beam quality and stability over a wide range. Certain material processing applications, for instance micro-machining, have stringent requirements so as to machine miniature features with tight tolerance. Fiber lasers offer good power scalability but are limited at high power as a result of non-linear interactions in the gain medium, which is often several meters in length. It is generally difficult to achieve the requirements with rods, slabs, and thin disk lasers.
Various solid-state laser architectures are possible candidates for high power and low power diode pumped laser systems. One of these architectures is a planar waveguide laser architecture. Planar waveguide technology is scalable to high power while maintaining beam quality over a wide range of power levels. U.S. Pat. No. 6,160,824 entitled “Laser Pumped Compound Waveguide Lasers and Amplifiers” and co-pending U.S. application Ser. No. 09/912,214 entitled “Waveguide Laser with Mode Control and Pump Light Confinement”, filed Jul. 24, 2001, describe various aspects of planar waveguide technology. The co-pending Ser. No. '214 application is assigned to the assignee of the present invention with a common inventor. Both the '824 patent and the disclosure of Ser. No. '214 are both incorporated by reference in their entirety. Data regarding diode pumped planar waveguide lasers and amplifiers has been published by ORC Southampton, and Maxios Laser Corporation among others: Shepherd et. al., “A Diode Pumped, High Gain Planar Waveguide Nd: Y
3
Al
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O
12
Amplifier”, University of Southhampton, ORC Research Center Review, Mar. 26, 2001, and R. J. Beach et. al., “CW and Passively Q-Switched Cladding Pumped Planar Waveguide Lasers” (Maxios Corporation).
High beam quality is a desirable feature of many laser processing systems, particularly for precision micromachining. Some methods for controlling beam quality in solid state lasers are disclosed in patent publications WO 0152367, WO 0027000, and U.S. Pat. Nos. 5,818,630 and 6,163,558.
In high power solid state lasers long term stability of output power and/or beam quality is affected by the temperature profile of the gain medium. Planar waveguides are typically face cooled by attaching a heatsink to the outer face of the substrate and/or cladding. Face cooling causes the heat to flow perpendicular to the plane of the core resulting in an essentially one-dimensional thermal gradient in the core. Thermal effects during laser operation are minimized because the lasing region is about 2 orders of magnitude thinner than that used in rod or slab lasers. Temperature differences between the center of the guide and the edge are on the order of 0.1° C., and can be neglected. The minimal temperature gradient in the guided direction, combined with the guiding effect of the waveguide structure eliminate any thermally induced optical effects like the thermal lensing seen in rod geometry lasers. The thermal gradient within the core in the transverse direction is dependent on the pumping and cooling arrangement. The transverse gradients, though relatively small, can be large enough to produce undesirable lens effects, particularly in a side pumped arrangement. Exemplary U.S. patents related to compensation of thermal effects in high power solid state lasers include U.S. Pat. Nos. 4,617,669; 6,418,156; and 6,002,695.
High peak power pulses are often produced by solid state lasers using well known q-switching techniques. Two main classes of q-switches exist, active and passive. Active q-switching is often implemented with an acousto-optic or electro-optic modulator. The following exemplary patents relate to q-switching: U.S. Pat. Nos. 4,057,770; 4,742,523; 4,860,296; 5,408,480; 5,495,494; and 6,160,824.
A plane polarized laser output may be advantageous in certain laser material processing applications. Without some polarization loss control mechanism the output of a planar waveguide laser will be randomly polarized. Patent publication WO 0027000 relates to a technique for producing a polarized output from planar waveguides and other laser structures.
Fast rise time modulators with low delay can be used in a variety of laser based systems, including laser material processing, measurement, and telecommunication systems. In typical laser processing systems electro-optic (Pockels cells) or Acousto-optic modulators are used. The Pockels cells require Kilovolt level signals for on-off switching and acouto-optic modulators are often limited by acoustic transit time. Waveguide technology offers potential for high speed switching with simplified electronics.
In high gain lasers and amplifiers, for instance power amplifiers in Master Oscillator-Power Amplifier or q-switched configurations, the maximum possible gain is limited by Amplified Spontaneous Emission (ASE) and/or paristic oscillations. These effects deplete the stored energy and effectively clamp the gain. The following patents relate to techniques for suppressing ASE and/or parisitic oscillations in lasers: U.S. Pat. Nos. 3,946,128; 4,849,036; 4,918,703; 5,084,888; 5,317,585; 5,335,237; 5,569,399; 5,636,053; 5,852,622; 6,141,475; and 6,418,156.
Improved limits of performance of present material processing systems are expected with use of at least one embodiment of the present invention set forth in the following sections. For instance, various embodiments may be used in (a) high power diode pumped lasers for applications such as laser welding and soldering, (b) lower power diode pumped lasers for applications such as marking, cutting, drilling, machining, and communications (c) in emerging micro-machining applications, for instance in applications wherein metal or dielectric materials are micro-machined in a non-thermal manner with high energy, short (sub nanosecond) or ultrashort (femtosecond-picosecond) pulses, (d) in high gain amplifiers for amplifying laser beams and (e) for beam manipulation and control.
SUMMARY OF THE INVENTION
An object of the invention to provide an improved method and system for laser based material processing systems.
One aspect of the invention features a method laser processing. The method includes: generating a pulsed laser beam having a pre-determined wavelength, pulse width, peak energy, and repetition rate. In at least one embodiment a combination of two or more of the pre-determined characteristics provides for an improved figure of merit for a material processing application.
Another aspect of the invention features a method of producing, using a solid state waveguide section, at least one of a polarized, modulated, q-switched, and mode controlled output beam at an output of the section.
Another aspect of the invention features a method of producing a laser output beam having a pre-determined level of beam quality. The method includes: swapping the guided and unguided axes of a guided laser beam between passes through a waveguide structure to produce a pre-determined level of output beam quali

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