Total optical loss measurement device

The total optical loss caused by laser induced damage to an optical component is measured by monitoring the phase shift during mirror reflectance or transmission. The phase shift is directly proportional to the amount of loss. A secondary laser illuminates the area under test with a coherent light beam well below the component's damage threshold. This reference beam is modulated. The reflected or transmitted reference beam is monitored by a photomultiplier tube whose output is fed to a lock-in circuit. The lock-in circuit compares the phase of the received light to the induced modulation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to a device that measures the optical loss at a 
specific location on a mirror surface or at a specific location on a lens 
surface. In particular, it pertains to a device for measuring the 
degradation of optical loss in real-time by the amount of phase shift in 
coherent light. This loss is the fractional reduction in optical power 
which occurs upon reflection of light from the mirror surface. 
2. Description of the Prior Art 
Previously, each optical characteristic of a sample mirror has been 
measured separately on a different system. Reflectance, absorption, 
transmission and scatter have each required separate evaluation 
techniques. These quantities could be combined for the total optical loss 
of a sample mirror. In particular, total optical loss was measured as a 
composite of these characteristics and if a total optical loss system was 
evaluated, there was no way to measure changes in total optical loss 
caused by strong or damaging illumination of the surface. Thus, whether 
the individual characteristics were being measured or an overall figure of 
merit was being determined, the individual characteristics of a sample 
mirror were not determined as a function of induced damage. Measurements 
simply identified before and after characteristics. 
SUMMARY OF THE INVENTION 
A limited area of a sample mirror or lens within a resonant cavity is 
evaluated for total optical loss. Evaluation of total optical loss in each 
section of a large mirror or lens permits an overall figure of merit to be 
determined for large optical components. The evaluation of the total 
optical loss also permits measurements of the change in optical loss as 
damage is induced at the point of measurement. A test laser provides a low 
level illumination of the area to be evaluated. This illumination is well 
below the damage threshold and is modulated in a predetermined manner. A 
damaging laser with appropriate focusing optics is then directed to the 
same area and damage is commenced by illuminating with the damaging laser 
past the damage threshold levels. A photomultiplier tube is used to 
observe the output from the resonant cavity of the test laser system and 
its modulated light. The modulation of the test laser system is controlled 
by a signal generator. The signal generator and the output of the 
photomultiplier tube are input to a lock-in amplifier which compares the 
phase of the modulated illumination to the output illumination. Any other 
state-of-the-art monitoring means such as an oscilloscope or other 
monitoring device may also be used to compare the output intensity and 
phase shift with respect to the input intensity and phase shift. For an 
ideal situation with no loss, there would be an infinite phase shift.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a block diagram of the present invention for testing of mirrors. 
For testing of lenses, the transmittance requires the exit mirror to be 
behind the sample. A test laser system 10 emits a coherent light beam. The 
phrase laser system means that all elements of a necessary coherent light 
emission system are present. For a typical laser this includes resonant 
cavity, active media, flash tube, entrance and exit mirrors. Test laser 
system 10 has an output intensity well below the damage threshold of a 
sample which is to be evaluated. A modulator 12 is used to vary the output 
intensity from test laser system 10. Modulator 12 is driven by a modulator 
driver 14. Modulator 12 may be two Pockels cells 6 and 8 with a polarizer 
7 between them. Pockels cells 6 and 8 are rotated such that by reversing 
the voltage across them they produce opposite phase shifts of equal 
magnitude. First Pockels cell 6 and polarizer 7 produce amplitude and 
phase modulation. Second Pockels cell 8 demodulates the phase leaving only 
amplitude modulation. Both modulator 12's components and modulator driver 
14 are standard optical devices commercially available. The modulated beam 
passes through an entrance mirror 16 which is one end of a resonance 
cavity whose second end is defined by an exit mirror 18. A sample mirror 
20, which has a surface to be tested, is placed in the resonance cavity to 
form a fold. If a transmissive optic, such as a lens or antireflection 
coating, is to be tested exit mirror 18 is behind sample 20 so that the 
path extends oppositely from the sample and no fold occurs. A fold in the 
resonance cavity means that light traveling along a path reflects off the 
mirror and either back along itself or back in a different direction. 
Sample mirror 20 is mounted in a two-axis mount 21 which permits sample 
mirror 20 to be translated within a plane. This permits all sections of 
its surface to be used at the same place in the resonant cavity. In this 
case, light traveling between entrance mirror 16 and exit mirror 18 via 
reflection from sample mirror 20 will follow a resonant path. The total 
loss for a round trip in the resonant cavity is equal to the loss for the 
entrance mirror 16 and the exit mirror 18 plus twice the loss of the 
sample mirror 20 because light reflects from the sample mirror twice in 
one round trip. The output from this resonant cavity is monitored by a 
photomultiplier tube 22. Photomultiplier tube 22 is placed in line with 
light passing through exit mirror 18. The overall length of a resonant 
cavity is defined as L. Photomultiplier tube 22 has a preamp 24 connected 
to its output to serve as a signal amplifier or signal conditioner. Preamp 
24's output is fed to a lock-in circuit 26 which is driven by a signal 
generator 28. Signal generator 28 is also the signal source for modulator 
driver 14. Thus, signal generator 28 serves to synchronize lock-in circuit 
26. The synchronization permits lock-in circuit 26 to compare the initial 
amplitude and phase of light from modulator 12 with the arrival amplitude 
and phase of light at photomultiplier tube 22. For purposes of example, 
signal generator 28 provides a sinusoidal modulation shown as input on 
FIG. 2. 
In FIG. 2, input signal 25 is compared to output signal 27 which is 
adjusted to allow for the brief time delay in light traveling from 
modulator 12 to photomultiplier tube 22. The maximum amplitude H.sub.o of 
the output beam is reduced with respect to the maximum input amplitude 
H.sub.i. The phase shift of peaks by an amount .delta. is due to the total 
optical loss during reflection off the sample mirror. As the sample mirror 
undergoes damage, both the reduction in amplitude and phase shift will 
change. 
A monitoring means such as oscilloscope 30 or other reference devices 
connected to signal generator 28 and preamp 24 to observe the real-time 
signal. Lock-in circuit 26 measures amplitude and phase of the reference 
signal to the output after total optical loss. The total optical loss 
appears as a reduced amplitude and as a phase shift. Lock-in 26 records 
the input and output intensity versus time similar to that shown in FIG. 
2. 
Phase is measured by nulling out the signal. This forces the reference 
frequency to be in quadrature with the output signal so the difference in 
phase is measured. If there is no loss, there is no phase shift. The loss 
is defined by the equation l=2L/c.delta.t where 
l=round trip fractional loss for whole resonator; 
2L=round trip, which is the resonator cavity length doubled; 
c=speed of light; 
.delta.=phase difference in radians between input and output modulated 
light power; and 
t=period of modulation. 
Test laser system 10 can be any laser which is subject to reasonable 
modulation, approximately 10 microseconds for 100 KHz. If the sample being 
tested does not have high loss, modulation at higher frequencies than 100 
KHz will gain more sensitivity. 
Sample mirror 20 is subject to destructive testing by a damaging laser 32. 
Damaging laser 32 is a laser system capable of output above the damage 
threshold of sample mirror 20. Damaging laser 32 is on a mount 36 which 
provides a path for shifting the angle of incidence for light from 
damaging laser 32. This permits damage due to different angles of 
incidence to be determined. Varying the angle of incidence may have 
significant changes in varying the destructive illumination. The angle of 
incidence may also be changed by pivoting the resonant cavity. Focusing 
optics 34 are placed in the path of light from damaging laser 32 to 
control the focus and illumination on sample mirror 20. The area 
illuminated by test laser system 10 will coincide with the focus spot for 
focusing optics 34. A periscope 38 of known total loss permits input to be 
measured by photomultiplier tube 22 without resonance between entrance 
mirror 16 and exit mirror 18. Periscope 38 is emplaced as shown in FIG. 1 
to calibrate photomultiplier tube 22 and adjust lock-in circuit 26. 
Periscope 38 is moveably mounted about said exit mirror to direct light to 
tube 22 without resonating in the cavity and is removed to permit actual 
operation. 
This permits a very small portion of a mirror to be evaluated for total 
loss due to reflectance, absorption, transmission, and scatter. The change 
in the total loss of the test sample during damage is measured in real 
time. This during lifetime measurement permits monitoring of the rate of 
change of total loss for a given type of mirror. By shifting the mirror in 
the reflection plane the entire surface can be evaluated for good and bad 
portions and to produce an overall figure of merit. 
It is obvious to those skilled in the art that numerous modifications to 
the above may be made.