Systems for generating short-pulse laser light

Details Systems for generating short-pulse laser light Systems for generating short-pulse laser light

2001-07-13

2004-06-15

Ip, Paul (Department: 2828)

Coherent light generators

Raman laser

C372S021000, C372S022000, C372S025000

Reexamination Certificate

active

06751240

ABSTRACT:

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 213587/2000, filed Jul. 14, 2000, the entire contents of this application are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to the fabrication of the lasers described below, as well as their use in various applications including micromachining, analyses by high-intensity short-pulse x-rays and medical diagnoses and treatments.
(1) Fabrication of Industrially Applicable Short-pulse High-peak Power Lasers
Short-pulse high-peak power lasers have a history of more than 10 years on the commercial market and yet they have not been used at industrial sites.
FIG. 10A
shows a conventional laser system constructed by incorporating a Ti:sapphire laser into the CPA (chirped pulse amplification) technology; as shown, the system consists of a mode-locked laser (and a pump laser), a diffraction grating paired pulse stretcher, a regenerative amplifier (and a pump laser), a master amplifier (and a pump laser), and a diffraction grating paired pulse compressor. The system can produce a peak power as high as several tens of terawatts but the repetition rate is only about 10 Hz. Other disadvantages of the system include bulkiness, high cost, difficult adjustments and low reliability for consistent use over a prolonged period.
FIG. 10B
shows a Yb-fiber laser which consists of a fiber mode-locked laser, a fiber pulse stretcher, a fiber amplifier and a fiber pulse compressor. Although this system is compact and has repetition rates on the order of MHz, the limitation of fiber resistance to intense light makes it difficult for the system to output power in excess of mJ per pulse which is a minimum requirement for industrial use.
In order to solve these problems, the present inventors took two approaches. First, in order to shorten the pulse duration, the diffraction grating which was bulky, difficult to adjust and costly was replaced by SBS (stimulated Brillouin scattering) cells (or crystals) and nonlinear Raman crystals; second, the Ti-sapphire crystal was replaced by a Raman cell amplifier to reduce thermal load and enhance reliability.
The new systems are compared with the conventional systems in the following table, in which the problems with the conventional systems are labelled with dots and the features of the new systems with open circles.
TABLE 1
Advantages (∘)
Function
Design considerations
and disadvantages (&Circlesolid;)
Amplification
Laser crystal
thermal source lased on quantum
efficiency
Nonlinear crystal
∘ small thermal load due to
negligible heat absorption
Production of
Diffraction grating
&Circlesolid; suitable for producing ultra-
shorter pulses
pair
short pulses but the system is
bulky and involves difficulty in
precise adjustments
SBS
compression
Single
&Circlesolid; SRS occurs and is amplified
to increase system instability
Tandem
∘ use of materials having
different Raman shifts prevents
SRS amplification
SRS
compression
Single
&Circlesolid; high threshold for SRS
increases vulnerability to optical
damage
Tandem
∘ generation of seed light
combined with amplifier lowers
threshold
Reflection
method
At full waist
∘ ultra-short pulses are difficult
to generate but the system is
compact
∘ high Raman threshold required
At half waist
∘ ultra-short pulses are difficult
to generate but the system is
compact
∘ low Raman threshold
(2) Various Uses of the Invention
a) non-thermal fine-machining: as a light source for precision machining of semiconductors and marking of electronic-grade glass
b) microscopes of exotic function: as a light source for multi-photon microscope
c) lithography: as a light source for USLI fabrication
d) x-ray fluorescence spectroscopy: enables analysis of elements in ultra-low levels
e) high intensity x-ray nondestructive analyzers: as a light source enabling the measurement of low radiation doses
f) short-pulse x-ray diffractometer: as a light source for measuring ultra-fast structural changes
g) dental x-ray imaging apparatus: as a light source enabling imaging at low radiation dose
h) dental scale removers: as a light source for selective removal of scale without adverse effects on the enamel
i) high-precision x-ray imaging apparatus: as a light source for projecting high-resolution x-rays
j) sterilizing apparatus: as a light source for noninvasive local sterilization
k) apparatus for removing or transplanting hair: as a light source for painless surgery
l) removal of fouling films, oxide films, plates and paints: no damage to surfaces
To produce high-peak power short-pulse laser beams, the following CPA-based methods have been used but they have several problems from a practical viewpoint.
(1) Fiber Chirping Method
In this method, laser oscillation is performed with ordinary lasers and the generated pulses are compressed by eliminating chirping over fibers; the method is implemented by a Nd:YAG laser, Nd:glass laser, Nd:YLF laser, etc. Alternatively, a fiber laser is directly oscillated and the generated pulses are compressed over fibers; this method may be implemented by a Yb fiber laser which employs Yb glass. Whichever method is used, high-intensity light is passed through an extremely thin fiber, so in order to avoid optical damage, the laser power is limited and the pulse energy is no more than about 1 mJ.
(2) Grating Chirping Method
a) Ti:sapphire Laser (see
FIG. 10A
)
This is a typical CPA-based laser and the technique which uses a pair of diffraction gratings is quite common. However, the diffraction gratings require not only a wide installation space but also precise adjustments, so the technique is too costly to be useful in general industries. In addition, the mode-locked laser as an oscillator and the regenerative amplifier also require precise adjustments. As a further problem, when a high average power is produced, the lasing crystal gives off heat in an amount corresponding to quantum efficiency and the thermal distortion from the heat generated in the crystal must be taken into account.
b) Optical Parametric Short-pulse Laser (OPCPA, or Optical Parametric CPA)
This laser performs amplification based on nonlinear effect, so the thermal load imposed on it is much smaller than what is experienced by the Ti:sapphire laser. However, it still needs a mode-locked laser as an oscillator, a pulse stretcher using a diffraction grating pair, and a pulse compressor. Furthermore, the conditions for optical parametric amplification are so rigorous that efficient light emission is not easy to realize.
To solve these problems, the following techniques are used in the present invention.
(1) In place of the bulky diffraction grating pair, a very small nonlinear crystal is used to compress pulses. In extending and compressing pulses, precise adjustments are necessary but this requires considerable space and adds to the cost. Therefore, to minimize the required space, pulses should be compressed within a very small crystal.
(2) High power far in excess of the limit on fiber output is produced. Use of fibers in place of the diffraction grating pair has been known for years but high power cannot be produced since the passage of light through a thin fiber can cause optical damage. To cope with this problem, the present inventors developed an optical system capable of generating short pulses in high power by making use of the stimulated Raman scattering in crystal.
The Raman compression technology is known but the Raman light to be amplified usually has power with an extremely low noise level, so it has been necessary to use extremely intense pump light but then the threshold for the generation of Raman light is high enough to increase the chance of optical damage. To avoid this problem, particularly in the case of using crystals having high thresholds for the generation of Raman light, the present inventors adopted the half-waist reflection method and the tandem crystal method. In the half-waist reflection method, the crystal is so cut that the emerging laser beam ha

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