Rapid scanning autocorrelation detector

Autocorrelation traces of laser pulses of short duration are produced by a method and apparatus that uses a rotating glass block to vary the path length on a time scale rapid enough to display the traces on a synchronized oscilloscope. Both paths of a split input beam of a laser pulse pass through the glass block. Rotation of the glass block changes the relative time of travel for light pulses along the two paths. The paths of the two beam arms pass through the block from different directions at a relative angle chosen so that the relative travel time difference is nearly linearly related to the angular position of the block during its rotation. Autocorrelation with rapid scanning through rotation of the block enables measurement of repetitive laser light pulses of very short duration, down to the order of one picosecond. An oscilloscope display of the autocorrelation traces allows the user to make laser adjustments while continuously monitoring the pulse correlation function so that the laser can be tuned in real time to produce ultra-short pulses.

BACKGROUND OF THE INVENTION 
The invention relates to an autocorrelator for connection to a pulsed laser 
for generating a display signal which enables the laser to be tuned in 
real time (with reference to a display of the display signal) down to the 
order of one picosecond pulse width. The invention also encompasses a 
method of producing autocorrelation traces. 
The prior art techniques for the tuning of a pulsed laser for very short 
pulse widths was difficult because of the lack of efficient 
instrumentation and methods for tuning the laser in real time, i.e., with 
simultaneous display of the traces for reference in making tuning 
adjustments. 
A trial-and-error procedure was generally used for tuning a pulsed laser 
for operation with pulse widths in the range of one to 100 picosecond. The 
laser was connected to a strip chart recorder, and a three to five minute 
long delay in readout on the recorder was often required. Tuning 
adjustments to the laser were made after the strip recorder printout was 
analyzed. The procedure was then repeated until the strip recorder finally 
indicated the desired result. 
U.S. Pat. No. 4,190,366 to Doyle discloses an interferometer having a 
moving refractive element in one arm for scanning. The moving refractive 
element is a glass wedge which presents a greater or lesser thickness of 
glass for a light beam to pass through. The Doyle patent interferometer 
has the refractive element in one arm only, and motion of the element is 
reciprocal linear. Both of those features differ from the present 
invention. 
The autocorrelator system of the present invention does not use an 
interference effect, as does the Doyle interferometer, and the refractive 
element of the present invention is used as a variable pulse time delay 
rather than as a variable phase shift. 
Other prior art interferometers have used the well known linear motion of a 
mirror in one of the two split beam paths. 
An article entitled "Real-Time Intensity Autocorrelation Interferometers", 
by R. L. Fork and F. A. Beisser, published in Applied Optics, Vol. 17, No. 
22, pp. 3534-35, Nov. 15, 1979, describes an autocorrelator for obtaining 
real time performance by varying the path length of one interferometer 
arm. The path length is varied at audio frequencies by an oscillating 
glass corner cube mounted on the armature of a shaker device. A display of 
the temporal shape of pulses of one picosecond and less is obtained by a 
phase-matched sum frequency generation in a KDP crystal. The apparatus 
described in the article provides for calibration of the real time display 
by using a stepping motor to adjust the path length by a known distance. 
SUMMARY OF THE PRESENT INVENTION 
It is an object of this invention to improve on prior interferometer and 
autocorrelator apparatus through methods and apparatus which enable 
contemporaneous monitoring of an autocorrelation trace during fine tuning 
of a pulsed laser for very short pulse widths, utilizing a simple and 
improved variable beam delay apparatus. 
The autocorrelator apparatus and method of the present invention enable 
tuning of a pulsed laser in real time, down to a pulse width of one 
picosecond or less. The system is capable of displaying the trace of an 
autocorrelation function on any high impedance oscilloscope for continous 
monitoring of pulse characteristics while fine-tuning laser performance. 
By scanning the relative delay between the two paths of a split beam in a 
Michelson arrangement the autocorrelator uses the light to measure itself. 
The autocorrelator utilizes a polarized input beam and a means for 
splitting the beam into first and second separate beams. A rotating, 
light-transmitting, refractive block is positioned in the paths of both 
beams which strike the block at a fixed angle relative to each other but 
at varying angles to the block faces. Light travels more slowly through 
the block than through air; and the greater the thickness of block to 
penetrate, the greater the delay. The relative travel time of the two 
beams is thus varied as the block rotates, and the approach angles of the 
two beams are chosen so that the relative travel time of the beams is 
nearly linearly related to the angle of rotation of the block over the 
scan range. 
The time-delayed beams are then reflected into a device for detecting the 
difference in travel time between the beams. This may be accomplished with 
a non-linear crystal which receives the two beams and transmits an output 
whose intensity varies in response to the amount of overlap of the pulses 
from the first and second beams. Output intensity is measured for use in 
displaying the function on an oscilloscope. 
The input beam into the autocorrelator is vertically polarized so that 
light transmission factors at the surfaces of the refractive block (e.g., 
glass) do not adversely affect the autocorrelation function. The 
tranmission factors change with the angle the beam makes with the glass 
surfaces. Since it is desired to obtain a relatively flat response of beam 
intensity versus rotational position of the block, it is important that 
the light be properly polarized to eliminate any significant 
reflection/transmission factors at the surfaces of the block. It is also 
important that both beams pass through the block so that both pass through 
the same surfaces. 
Because both beams in the present invention go through the rotating block 
(rather than just one beam as in prior art linear-driven devices) a 
substantially linear relative delay of the two beam arms is produced 
during rotation of the block. If only one beam passed through the block, 
the response would be nonlinear; and the proper calibration of the 
autocorrelation trace on an oscilloscope (which has a uniform rate of 
sweep in the horizontal direction) would not be possible. 
Each beam as it passes through the rotating block encounters a delay which 
plots as a curve against time. However, the net effect of the two beam 
delays (i.e., the relative subtractive delay of the two beams) is nearly 
linear--assuming the beams are properly oriented. 
The two beams are ultimately directed into a nonlinear, frequency doubling 
crystal which transmits an output whose intensity varies in response to 
the degree of overlap of the pulses from the two separate beams. If there 
is no overlap at all, i.e., if the waves of the two beams cancel out, 
there is no output from the nonlinear crystal. Partial overlap produces 
some output, and maximum output is realized when there is no delay, with 
the two waves reinforcing each other. Under that condition peak intensity 
is produced and the nonlinear crystal produces a doubled-frequency, 
halved-wavelength ultraviolet output from the two beam inputs. 
The output of the nonlinear crystal is sensed by a photomultiplier tube and 
is connected to an oscilloscope for display. The oscilloscope displays 
intensity versus time. 
The time base on the oscilloscope must be converted to time base in 
autocorrelation space. In a preferred embodiment, the conversion factor is 
15 picoseconds in autocorrelation space per millisecond on the 
oscilloscope. This depends upon the speed of rotation of the block and the 
thickness of the block. 
The thickness of the block is selected in accordance with the size range of 
the pulse durations to be monitored. Thicker blocks are used for pulses of 
larger width or duration, while thinner blocks are used for shorter 
duration pulses. 
The rate of rotation of the refractive block is fixed, and it may rotate at 
30 cycles per second. This corresponds to 60 hertz repetition of the 
relative delay scan, since the block has two identical sides. The ends of 
the block are frosted or opaqued to prevent transmission except during the 
relative delay scan. 
To check the calibration of the oscilloscope, a fixed, known delay is put 
into one beam or arm of the autocorrelator; and the resulting shift in the 
scope trace is observed. When the fixed delay is a known quantity, the 
scope can be calibrated. 
It is an important object of the present invention to pass the two arms of 
a split beam through a single rotating block simultaneously to produce a 
substantially linear relative delay response and to rotate the block at a 
speed which permits the traces to be displayed on a synchronized 
oscilloscope in real time. This facilitates fine tuning of the pulsed 
input beam for very short pulse widths. 
Rotating block autocorrelator apparatus and methods which incorporate the 
structure and techniques described above and which are effective to 
function as described above constitute further, specific objects of this 
invention. 
Other and further objects of the present invention will be apparent from 
the following description and claims and are illustrated in the 
accompanying drawings which, by way of illustration, show preferred 
embodiments of the present invention and the principles thereof and what 
are now considered to be the best modes contemplated for applying these 
principles. Other embodiments of the invention embodying the same or 
equivalent principles may be used and structural changes may be made as 
desired by those skilled in the art without departing from the present 
invention and the purview of the appended claims.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
A rapid scanning autocorrelator system constructed in accordance with one 
embodiment of the present invention is indicated generally by the 
reference numeral 11 in FIG. 1 and also in the schematic view of FIG. 2. 
The system 11 includes an alignment window 13 for receiving an input beam 
15 in proper orientation, a beam splitter 17, and a rotating refractive 
block 19 preferably of fused silica (glass) for affecting simultaneously 
the path lengths of both the first and second beams 21 and 23. The beams 
are ultimately directed to autocorrelator detection apparatus 25 which 
produces an intensity signal which, when connected to an oscilloscope (not 
shown), is capable of displaying the autocorrelation function in real 
time. 
The input beam striking the beam splitter 17, of conventional construction, 
is partially transmitted as the first beam 21 and partially reflected as 
the second beam 23. These two beams are reflected by mirrors 27 toward the 
rotating block 19, as indicated in FIGS. 1 and 2. The block is 
rotationally driven by an AC-synchronous motor 29, producing a reliably 
constant rate of rotation about an axis 31. 
The block 19 has two opposed flat faces 33 which pass the light beams and a 
pair of frosted or otherwise opaqued ends 35 which interrupt the travel of 
the beams. The two beams are oriented at an optimum angle with respect to 
each other as they approach the block 19, preferably about 70.degree. and 
more specifically, about 72.degree.. This produces a substantially linear 
response of relative delay as a function of the angular position of the 
block (or as a function of time), as depicted in FIG. 7 and as described 
in more detail below. 
Path length (or delay) variation is effected by variation in thickness of 
block traversed by each beam due to the angle of the block, and by the 
corresponding change in the length that the beam travels through air. 
Light travels more slowly through glass; beam delay is least when a beam 
enters and leaves the glass block at 90.degree., and greatest when it 
travels through the glass most obliquely. 
The beams 21 and 23 emerging from the block 19 strike reflecting device 37 
and reflecting device 38, which may be simple mirrors but preferably 
comprise retro-reflecting prisms, as explained below. From there the beams 
21' and 23' are reflected back through the block 19 and off the mirrors 27 
back to the beamsplitter 17, generally along the same path as before but 
somewhat offset due to the use of the retro-reflectors 37 and 38. 
As in a conventional interferometer, the returning beams 21' and 23' run 
parallel as they leave the beamsplitter, but separated somewhat due to 
preferred positioning of the retro-reflectors 37 and 38, the beam 21' 
being reflected (partially) and the beam 23' being transmitted 
(partially). Spaced apart slightly as indicated in FIG. 1, they pass 
through a convex lens 39 which converges them toward a frequency-doubling 
crystal 41; and this may be via a mirror 43 for compactness of the system 
11. 
The frequency doubling crystal is a non-linear crystal which produces 
frequency-doubled, ultraviolet components from the beams 21' and 23', a 
function which is well known and was used, for example, in the 
autocorrelator disclosed in the Applied Optics article referenced above. 
If there is no overlap between the two beams, i.e., their wave patterns 
cancel, there will be no output from the nonlinear crystal. The intensity 
of the output depends on the degree of overlap, and zero delay between the 
beams produces peak intensity. An ultraviolet-pass filter 45 filters out 
all but the ultraviolet output, which is then sensed by a photomultiplier 
tube 47. 
The resulting signal from the photomultiplier tube 47, output via a 
terminal 49, may be input to an oscilloscope (not shown) and displayed, 
with intensity on the vertical axis against time on the horizontal axis. 
There is a time base conversion from autocorrelation space to the 
oscilloscope, and this may be, for example, 15 picoseconds per millisecond 
on the oscilloscope. The conversion factor varies if the input current 
frequency varies, and the 15 picosecond/millisecond factor is given as an 
example for 60 Hz current as is used in the United States. 
The calibration of the scope may be checked by putting a fixed, known delay 
into one of the beam paths, and checking the trace on the scope to see how 
much shift occurs. Since the fixed delay is known, the scope can be 
calibrated. A fixed-delay calibration device 51 is shown in FIG. 1. It 
preferably consists of a glass element or etalon which may be interjected 
into the path of the beam 23 or retracted from it by a driving unit 53. 
The autocorrelator apparatus 11 is mounted on a frame or housing 55 as 
indicated in FIG. 1. It may be in vertical orientation, with FIG. 1 being 
a side elevation view, and leveling legs 57 may be included at the bottom 
for leveling the system 11 relative to an input beam 15. 
For proper function of the system 11, the input beam 15 is polarized. 
Polarization is in the plane parallel to the plane in which the split 
beams 21 and 23 lie, i.e., vertical polarization if the unit is oriented 
vertically as described above. This eliminates adverse effects of surface 
transmission factors at the glass block, as discussed previously. 
FIGS. 3 and 4 relate to the use of retro-reflective prisms 37 and 38 in the 
system 11 to produce an offset between the beam paths travelling in 
opposite directions. Both these views are rough schematics to show the 
beam offsets and the manner in which inteference between the beams is 
avoided. 
In FIG. 3 the mirror 43 is shown reflecting the two returning beams 21' and 
23' from offset positions toward convergence (by the lens 39, FIGS. 2 and 
4) in the nonlinear crystal 41. 
In FIG. 4, both beams 21, 23 are represented as a simple line leaving the 
beamsplitter 17 and passing through the rotating block 19. The beam 23 
strikes the retro-reflector 38, while the beam 21 strikes the 
retro-reflector 37. Both retro-reflectors reflect the beams back at an 
offset from the approaching beam, as the return beams 21' and 23', which 
pass back through the block 19 offset from the approaching beams 21 and 
23. The beam 21' is reflected from the beamsplitter 17, while the beam 23' 
is transmitted through it, both beams being depicted as straight lines at 
that point in the schematic diagram of FIG. 4. The offset here avoids 
feedback of either return beam into the beam generator. 
The lens 39 converges the beams 21' and 23', which are then reflected off 
the mirror 43 to focus in the nonlinear crystal 41. 
As indicated in FIG. 4 by the lines of convergence of the two beams 21' and 
23', the mirror 43 can be concave to add to the convergence, if desired, 
although the lens 39 can simply be made more convex, with the mirror 43 
planar, to accomplish this same purpose. 
The offset of the return beams 21' and 23' provides a means of separating 
the two beams so that the focusing lens 39 directs the beams into the 
nonlinear crystal at different angles. This then separates the normal 
frequency doubled output obtained from a nonlinear crystal as a response 
to a single beam, from the response needed to display the frequency 
doubling resulting from overlap of two beams. The background free sum 
frequency generation proportional to the product of the two beam 
intensities is detected at an angle bisecting the angle between the two 
beams. 
FIGS. 5, 6 and 7 show the effect of the rotating block on the two beams. 
Preferably, these beams 21 and 23 approach the block 19 at a fixed angle A 
(FIGS. 5, 6 and 2) of about 72.degree. relative to each other, in this 
specific embodiment. This helps produce a linear response of relative beam 
delay versus time (or angular rotation), as explained below with reference 
to FIG. 7. 
FIG. 5 shows the beam 21 passing straight through the block 19, 
perpendicular to its faces 33, so that delay is at a minimum in this beam. 
This preferably is where the scan of relative delay commences--at the 
point where one beam is perpendicular--and it ends at the point where the 
other beam is perpendicular, at its minimum delay. 
At the same time the beam 21 is at its minimum delay as shown in FIG. 5, 
the beam 23 is at the maximum delay in the scan, i.e., at the most oblique 
angle. At this block position, the block can be considered to be at the 
origin of the particular scan underway, at 0.degree. in a scan of 
0.degree. to 72.degree. (preferably) of block rotation. In this position 
the relative delay between the beams is at maximum, as depicted in the 
lower curve (nearly straight line plot) of the FIG. 7 graph. 
In FIG. 6 the block is midway in the scan, with its faces at the same angle 
to both beams. At this point the relative delay between the beams is zero, 
assuming, as is preferred, the total remaining components of the two path 
lengths are equal to each other. This is where the lower curve in FIG. 7 
crosses the horizontal axis or zero point. 
As the block progresses to the end of the scan shown in FIGS. 5 and 6, it 
moves toward a position opposite that shown in FIG. 5, i.e., with the beam 
23 perpendicular to the block and the beam 21 at the most oblique angle. 
The relative delay is again at maximum, but in the other direction, as 
shown at the upper right end of the lower curve of FIG. 7. 
FIG. 7 is a plot of path lengths of the two beams, first individually and 
then subtracted, giving the relative variation in pathlength. The relative 
variation function is nearly a straight line in the preferred embodiment, 
due to both beams passing through the block and their being at the 
preferred angle relative to each other. When the curve of the beam 23 
delay is subtracted from the curve of the beam 21 pathlength, the nearly 
linear function, relative delay, results. The linear response, as noted 
above, permits display on an oscilloscope having a uniform rate of 
horizontal sweep. 
A polarization rotator (not shown in the drawings) can be used between the 
beam splitter 17 and the lens 39 when a particular crystal 41 is used 
which requires a different input polarization. 
While we have illustrated and described the preferred embodiments of my 
invention, it is to be understood that these are capable of variation and 
modification, and we therefore do not wish to be limited to the precise 
details set forth, but desire to avail myself of such changes and 
alterations as fall within the purview of the following claims.