Optical pulse waveform shaper

An optical pulse waveform shaper capable of providing an optical pulse waveform of a desired intensity distribution with little loss. The optical pulse beam split by a beam splitter is directed into the beam splitter with a phase difference to combine with the transmitted optical pulse beam. The phase difference is controllable. The optical pulse waveform shaper is applicable between a laser oscillator with different exciting conditions and an amplifier.

TECHNICAL FIELD 
The present invention relates to an optical pulse waveform shaper provided 
between a laser oscillator with different exciting conditions and an 
amplifier or between a pair of amplifiers to provide an optical pulse 
waveform of a desired intensity distribution with little loss. 
BACKGROUND ART 
A conventional optical pulse waveform shaper such as shown in the Laser 
Handbook, page 365, published by the Laser Gakkai (Laser Society) in 1982 
is illustrated in FIG. 1. It includes three beam splitters 1, 2, 3 for 
splitting an input beam L, and a pair of reflectors 4, 5 and three beam 
splitters 6, 7, 8 for combining the split beams L.sub.1, L.sub.3, L.sub.5, 
L.sub.6 to provide an output beam L.sub.9. 
The operation will now be described below. 
An input optical pulse beam L is split into a reflected beam L.sub.1 and a 
transmitted beam L.sub.2 by the beam splitter 1. The transmitted beam 
L.sub.2 is split into a reflected beam L.sub.3 and a transmitted beam 
L.sub.4 by the beam splitter 2. The transmitted beam L.sub.4 is split into 
a reflected beam L.sub.5 and a transmitted beam L.sub.6 by the beam 
splitter 3. Thus, the respective beams L.sub.1, L.sub.3, L.sub.5, L.sub.6 
are different from each other in phase and attenuation. The beam L.sub.1 
is reflected by the reflector 5 and combined with the beam L.sub.3 into a 
beam L.sub.7 by the beam splitter 6. The beams L.sub.7 and L.sub.5 are 
combined into a beam L.sub.8 by the beam splitter 7. The beam L.sub.8 is 
combined with the beam L.sub.6 reflected by the reflector 4 into an output 
beam L.sub.9 by the beam splitter 8. Thus, the output beam L.sub.9 has 
been shaped to have a desired intensity distribution. 
In such conventional optical pulse waveform shaper, part of the beam 
L.sub.3 reflected by the beam splitter 2 is transmitted by the beam 
splitter 6 resulting in an optical pulse energy loss. Part of the beam L1 
reflected by the reflector 5 is reflected by the beam splitter 6 resulting 
in an optical pulse energy loss. In addition, two sets of beam splitters 
1-3 and 6-8 for splitting and combining, respectively, must be designed 
and manufactured. Moreover, it is not easy to align all these optical 
axes. 
Although the optical pulse waveform L.sub.9 of a desired intensity 
distribution may be shaped when the input optical pulse waveform L has an 
even intensity distribution, it has been impossible to provide an optical 
pulse waveform of a desired intensity distribution for an input optical 
pulse waveform of an intensity distribution in which the central portion 
leads the peripheral portion or vice versa. 
Accordingly, it is an object of the invention to provide an optical pulse 
waveform shaper capable of combining the split optical pulse beams with 
little loss. 
It is another object of the invention to provide an optical pulse waveform 
shaper capable of providing an optical pulse waveform of a desired 
intensity distribution from that of a given intensity distribution with 
little loss. 
DISCLOSURE OF THE INVENTION 
According to one aspect of the invention there is provided an optical pulse 
waveform shaper which includes a beam splitter for splitting an input 
optical beam into a transmitted optical beam and a reflected optical beam; 
and a plurality of reflectors for reflecting the reflected optical beam to 
provide a phase difference with respect to the transmitted optical beam 
before the reflected optical beam is combined with the transmitted optical 
beam by the beam splitter. 
With the optical pulse waveform shaper, the optical pulse beam reflected by 
the beam splitter is combined with the optical pulse beam transmitted by 
the beam splitter by a plurality of reflectors thereby minimizing the 
optical pulse energy loss. 
According to another aspect of the invention there is provided an optical 
pulse waveform shaper which includes a beam splitter for splitting an 
input optical pulse beam into first and second optical pulse beams, the 
transmittance of a portion of the beam splitter being different from that 
of another portion, and at least three reflectors for providing a phase 
difference to the first optical pulse beam and combining the first and 
second optical pulse beams into an output optical pulse beam through the 
beam splitter. 
In this embodiment, an input optical pulse beam is split into two optical 
pulse beams by the beam splitter. One of the split beams is provided with 
a phase difference by the reflectors and reflected by the beam splitter to 
be combined with the second beam into an output optical pulse beam.

BEST MODE FOR PRACTICING THE INVENTION 
In FIG. 2, a beam splitter 10 for splitting an input optical pulse L is 
made by coating a surface of a plane parallel plate with a reflective 
material and the other surface with a non-reflective material. Four total 
reflectors 11-14 (hereinafter simply "reflectors") cooperate to provide 
the split optical pulse beam with a phase difference and combine it with 
the transmitted optical pulse beam. The reflectors 11-14 are plane 
mirrors. The axis of the optical pulse reflected by the reflector 14 
agrees with the axis of the optical pulse reflected by the beam splitter 
10. The axis of the optical pulse reflected again by the beam splitter 10 
agrees with the axis of the optical pulse transmitted by the beam splitter 
10. 
In operation, an optical pulse is split by the beam splitter 10. The 
reflected optical pulse is provided with a desired phase difference and 
then combined by the reflectors 11-14. The desired phase difference is 
determined by controlling the length of an optical path; namely, the 
distance between the beam splitter 10 and the reflectors 11, 14, the 
distance between the respective reflectors, and the number of reflectors. 
Part of the optical pulse reflected by the reflector 14 is transmitted by 
the beam splitter 10 but reflected the reflectors 11-14 for a few cycles 
and output after all. 
FIG. 3 shows the operation of the above optical pulse waveform shaper. a is 
an input optical pulse waveform. 15 is the waveform of the optical pulse 
transmitted by the beam splitter 10. 16 is the waveform of the optical 
pulse reflected by the beam splitter 10 and the reflectors 11-14 and then 
the beam splitter 10 after one round trip. 17 is the waveform of the 
optical pulse reflected by the beam splitter 10 and then by the beam 
splitter 10 after two round trips through the reflectors 11-14. 18 is the 
phase difference controlled by the aforementioned techniques. 19 is the 
shaped pulse waveform. 
b is the shaped optical pulse waveform 19 provided with a large 
transmittance of the beam splitter 10. The rise time of the optical pulse 
b is equal to that of the input pulse a, but the waveform b has a long 
tailing portion. 
c is the shaped optical pulse waveform 19 provided with a small 
transmittance of the beam splitter 10. It became wider and shorter than 
the input waveform a. 
As has been described above, according to the invention, the reflected and 
transmitted light from a beam splitter are combined so that the optical 
pulse energy loss is minimized. 
Alternatively, the number of reflectors may be three or more as shown in 
FIG. 4. 20, 21 are reflectors. Two or more optical pulse waveform shapers 
may be cascaded or connected in series. With a partially coated beam 
splitter, a spatially shaped optical pulse may be provided. With a 
polarizing beam splitter, an optical pulse with polarizing spectrum in 
terms of time may be provided. With a spectrum beam splitter, an optical 
pulse with spectrum in terms of time may be provided. The beam splitter 
may have one or more apertures. These alternatives may be combined. 
The optical pulse energy loss may be further reduced by providing the beam 
splitter 10 with non-reflective coating on the surfaces other than the 
reflective surface. Part or all of the reflectors 11-14 may be flat or 
curved reflectors. The substrate of the beam splitter may be a parallel or 
wedge plate. It may also be a plane or curved plate. With a curved plate, 
it is possible to provide a given diameter of the optical pulse beam by 
controlling the curvature of the beam splitter. 
As shown in FIG. 5, the optical pulse L.sub.12 transmitted by the beam 
splitter 10 is reflected by the reflectors 22-25 to provide a phase 
difference. Then, it is combined with the optical pulse L.sub.11 reflected 
by the beam splitter 10 to provide the same results as those of the 
aforementioned embodiment. 
In FIG. 6, a beam splitter 26 not only splits an input optical pulse beam L 
into two beams a reflected pulse beam L.sub.11 and a transmitted pulse 
beam L.sub.12, but also combines these two beams L.sub.11, L.sub.12 into 
an output optical pulse beam L.sub.13. The reflective coating A is 
provided on a plane parallel plate such that the transmittance changes 
continuously. A non-reflective coating B is provided on the area other 
than that of the reflective coating A. Four total reflectors 27, 28, 29, 
30 (hereinafter simply "reflectors") are provided to give the reflected 
beam L.sub.11 a phase difference and combine it with the transmitted beam 
L.sub.12. These reflectors may be flat mirrors. 
The reflectors are arranged so that the optical axis of the beam L.sub.11 
reflected by the reflector 30 agrees with the optical axis of the beam 
L.sub.11 reflected by the beam splitter 26 and that the optical axis of 
the beam L.sub.11 reflected again by the beam splitter 26 agrees with the 
optical axis of the beam L.sub.12 transmitted by the beam splitter 26. 
In operation, an input optical pulse beam L is split into two beams 
L.sub.11, L.sub.12 by the beam splitter 26. The reflected beam L.sub.11 is 
provided with a proper phase difference by the reflectors 27-30 and 
combined with the transmitted beam L.sub.12 by the beam splitter 26 to 
provide an output optical pulse beam L.sub.13. A desired value of the 
phase difference is obtained by controlling the length of an optical path; 
namely the distance between the beam splitter 26 and the reflectors 27, 30 
and between the respective reflectors 12-15, and the number of reflectors 
used. 
FIGS. 7(a) through 7(d) show schematically the intensity distributions of a 
few optical pulse waveforms. FIG. 7(a) shows an optical pulse waveform 
L.sub.21 at a certain time of an intensity distribution whose central 
portion leads the peripheral portion. FIG. 7(b) shows an optical pulse 
waveform L.sub.22 at a certain time obtained by shaping the optical pulse 
waveform L.sub.21 with a beam splitter 26 whose transmittance is greater 
in the peripheral portion than in the central portion. FIG. 7(c) shows an 
optical pulse waveform L.sub.31 at a certain time of an intensity 
distribution whose peripheral portion leads the central portion. FIG. 7(d) 
shows an optical pulse waveform L.sub.32 at a certain time obtained by 
shaping the optical pulse waveform L.sub.31 with a beam splitter 26 whose 
transmittance is greater in the central portion than in the peripheral 
portion. In each case, the transmittance of the beam splitter 26 is 
designed to adjust the phase difference P.sub.2 or P.sub.3 as described 
above providing the optical pulse waveform L.sub.22 or L.sub.32 of an even 
intensity distribution. 
According to the invention, only one beam splitter 26 is necessary for 
splitting an optical pulse beam L and combining the split beams L.sub.11, 
L.sub.12. In addition, the reflected beam L.sub.11 is totally reflected by 
the reflectors 27-30 so that an optical pulse of a desired intensity 
distribution is obtained from that of a given intensity distribution with 
little loss. 
Alternatively, the reflectors 28, 30 may be arranged as shown in phantom in 
FIG. 6 to reflect the reflected beam L.sub.11 to the reflector 30 as shown 
in phantom, so that the three reflectors 27, 28, 30 provide the same 
results as the four reflectors 27-30. Thus, the number of reflectors may 
be three or more. For example, six reflectors 12-17 may be used to make an 
optical pulse waveform shaper. 
The transmittance of the beam splitter 26 may be made to change 
continuously to shape an optical pulse waveform of a continuously varying 
distribution into an optical pulse waveform of an even intensity 
distribution. The same results may be obtained by providing a hole in the 
beam splitter depending on the intensity distribution of an optical pulse 
waveform. 
The substrate of the beam splitter 26 may be a wedge, plane, or curved 
plate to provide the same results. With the wedge plate, the combined 
optical pulse waveform L.sub.13 has no or little ghost. The curvature of a 
curved plate may be constant or variable. The reflectors 27-30 may be a 
combination of curved and plane mirrors to provide the same results. When 
the beam splitter and the reflectors employ a curved plate and curved 
mirrors, respectively, a desired diameter of an optical pulse beam may be 
obtained by controlling the curvatures. The curvature of curved mirrors 
may be constant or variable. 
The transmitted beam L.sub.12 may be provided with a phase difference and 
combined with the reflected beam L.sub.11 to provide the same results. The 
optical pulse waveform shaper according to the invention may be used to 
shape an optical pulse waveform of an even intensity distribution into an 
optical pulse waveform of an uneven intensity distribution. 
The coating materials for providing varying transmittance include 
(Al+MgF.sub.2), (Al+SiO), Al, (Cr+Au), (Cr+Pt), Cr, and dielectric 
multilayer films. The non-reflective coating materials include MgF.sub.2. 
This MgF.sub.2 is coated in a single or multiple layers. The optical pulse 
waveform shapers may be connected in series. 
INDUSTRIAL APPLICABILITY 
The optical pulse waveform shaper according to the invention is able to 
provide an optical pulse waveform of a desired intensity distribution from 
an optical pulse waveform of a given intensity distribution with little 
loss. Thus, it is applicable between a laser oscillator and an amplifier 
or between amplifiers.