Coherent light generators – Particular beam control device – Mode discrimination
Reexamination Certificate
2000-01-12
2001-11-27
Font, Frank G. (Department: 2877)
Coherent light generators
Particular beam control device
Mode discrimination
C372S020000, C372S029011, C372S030000, C372S031000
Reexamination Certificate
active
06324191
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to oscillators which are useful for generating pulsed laser beams. More particularly, the present invention pertains to devices and methods which are useful for isolating a transverse electromagnetic (TEM) zero—zero mode pulsed laser beam from a multi-mode pulsed laser beam. The present invention is particularly, but not exclusively, useful for generating a zero—zero mode pulsed laser beam which is useful for ophthalmic surgery.
BACKGROUND OF THE INVENTION
It is known that oscillators can be incorporated into laser systems to generate laser beams which comprise a train of laser pulses. More specifically, it is known that pulsed laser beams are useful for ophthalmic laser surgery. For example, U.S. Pat. No. 4,764,930 which issued to Bille et al. for an invention entitled “Multiwavelength Laser Source” and which is assigned to the same assignee as the present invention, discloses a system for use in ophthalmic laser surgery which generates a beam of laser pulses wherein the pulses are of extremely short duration and are generated at an extremely high frequency.
The object in creating a pulsed laser beam which can be effectively used for ophthalmic laser surgery is to quickly and efficiently create a beam of highly concentrated laser light pulses. Importantly, the individual laser light pulses in such a beam should be uniform in intensity (pulse to pulse). Stated differently, it is desirable to have a so-called zero—zero mode pulsed laser beam which does not have the detracting characteristics caused by multiple transverse modes wherein the amplitudes of the pulses vary over time.
In longitudinal diodepumped lasers it is very difficult to focus into a small lasermode because of the relative bad beam quality of the laserdiode.
In high power single strip laserdiodes it is particularly the axis parallel to the strip which has a bad beam quality and therefore makes modematching with a small lasermode in that axis very difficult. Bad modematching (in this case a too big pumpbeam compared to the lasermode in the gain medium in one axis) will result in the creation of higher modes in addition to the transverse electromagnetic (TEM) zero—zero mode (TEM
00
-Mode) in that axis. Thus, heretofore it has been necessary to choose between a quick and easy self-starting generation of a pulsed laser beam with a small gainvolume, and accepting the consequences of the resultant multi-mode condition, or getting a single TEM
00
Mode by increasing the gainvolume but accepting the slower and less reliable self starting condition of the laser pulses.
Multi-mode pulsed laser beams have certain specific characteristics which are relatively easily identifiable. For one, there is a central pulse mode (the zero—zero mode). Additionally, there are linked transverse modes which can exist in addition to the central pulse mode. More specifically, the higher modes are diametrically symmetric to the central pulse and, when there is more than one, all have the tendency to align linearly with the bad pumpdiode axis.
Another characteristic of all multi-mode pulsed laser beams is that due to small frequency differences in the laser light between the transverse modes, the interferometrical addition of these modes result in a beating signal which has a frequency that is about five or six times slower than the frequency of pulses in the laser beam. Further, due to the symmetry of the transverse modes in the multi-mode beam, if one side of the mode is removed from the beam on one side of the central pulse, the other side is also removed from the other side of the central pulse.
In light of the above it is an object of the present invention to provide an oscillator for isolating a transverse electromagnetic (TEM) zero—zero mode pulsed laser beam which can benefit from the relatively quick response time for initial generation of a multi-mode pulsed laser beam. It is also an object of the present invention to provide an oscillator for isolating a transverse electro-magnetic (TEM) zero—zero mode pulsed laser beam which can quickly convert an induced multi-mode laser beam into a zero—zero mode laser beam. Still another object of the present invention is to provide an oscillator for isolating a transverse electromagnetic (TEM) zero—zero mode pulsed laser beam which uses closed loop control to maintain a zero—zero mode pulsed laser beam. It is yet another object of the present invention to provide an oscillator for isolating a transverse electromagnetic (TEM) zero—zero mode pulsed laser beam which is simple to operate, relatively easy to manufacture and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
An oscillator for isolating a transverse electromagnetic (TEM) zero—zero mode pulsed laser beam in accordance with the present invention includes a lasing medium which is mounted on a base, and a laser pumping diode which is also mounted on the base. Preferably the lasing medium is Nd:glass (1%-3%) and the laser pumping diode is a ≦200 &mgr;m stripe diode which is capable of generating light having a wavelength of approximately eight hundred and eight nanometers (808 nm). With this combination, the result is an induced beam of laser light which has a wavelength of approximately one thousand and sixty nanometers (1060 nm). Within the oscillator, the induced beam of laser light is reflected back and forth on a path that is established by a plurality of coated mirrors. As the laser light reflects back and forth in the oscillator-cavity, pulses of multi-mode laser light are created in a manner well known in the art.
An optical detector is provided to sense the pulses of multi-mode laser light that are induced in the oscillator. Additionally, the optical detector is used to generate a signal that is characteristic of these pulses. Specifically, the detector works in combination with electronics to isolate a beating signal that is indicative of pulse amplitude variations in the multi-mode laser light beam. As mentioned above, it is typical that the frequency of this beating signal will be approximately five to six times slower than the frequency of laser pulses in the beam. Thus, the beating signal can be relatively easily identified, when present (i.e. when there is a multi-mode laser beam), and the variations in pulse amplitude and can, therefore, be converted into an error signal.
The oscillator of the present invention also includes an interrupter that is mounted on the base. Specifically, this interrupter is formed with a straightedge, and it is mounted on the base for advancement toward the path of the multi-mode laser beam in response to the error signal that is generated by the electronics. As intended for the present invention, as the straightedge is advanced toward the path of the multi-mode laser beam, it will shave or remove the transverse modes from the laser beam. This action is then continued until all of the transverse modes have been removed. When this is accomplished the error signal will be substantially a null and the zero—zero mode of the laser beam will have been isolated.
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pat
Font Frank G.
Fulbright & Jaworski L.L.P.
IntraLase Corp.
Rodriguez Armando
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