Device for producing an electric potential having a difference frequency of a self-mixed signal in a laser resonator

A device and method for measuring a difference frequency having a simple structure and utilizing optogalvanic effect for measuring a difference frequency when carbon dioxide laser light of a single wavelength is varied in frequency by a measuring object. When a part of frequency varied laser light is mixed with an original frequency of laser light by self-mixing in a laser resonator, a strength of the laser light in the resonator is modulated to a difference frequency between the original frequency and the variation frequency. Since a current change in response to strength of light is generated by the optogalvanic effect in the laser resonator, when a frequency of optogalvanic current change is measured, the difference frequency can be measured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device and method for measuring a 
difference frequency generated when a light having a frequency different 
from the original laser frequency is self-mixed to a laser resonator, and 
more particularly, to a device and method for measuring a difference 
frequency between two lights by measuring a current change, since a 
current flowing in the interior of a resonator is modulated to a 
difference frequency by an optogalvanic effect at a time when a mixing 
with original light frequency occurs by self-mixing a frequency varied 
light by a laser resonator in a measuring device utilizing carbon dioxide. 
2. Description of the Related Art 
Since a conventional frequency variation measuring device of a carbon 
dioxide laser is detected by using generally an MCT (Mercury cadmium 
Telluride) detector cooled by liquid nitrogen, the liquid nitrogen should 
be replenished at predetermined time. This fact has presented a hindrance 
in manufacturing a movable device due to a large volume of the liquid 
nitrogen storing container. 
SUMMARY OF THE INVENTION 
Therefore, the present invention solves; such a problem as above, and it is 
an object of the present invention to provide a device and method capable 
of measuring a difference of frequency without using a detecting device 
cooled by liquid nitrogen. 
Another object of the present invention is to provide a device and method 
for measuring a difference frequency capable of extending the life of the 
measuring device by using a laser generating device itself as a detector, 
which is simple in structure and capable of decreasing the cost of 
manufacturing. 
In order to attain the above object, the device and method for measuring a 
frequency variation of the present invention comprises laser discharging 
tube, a high voltage direct current supply device, a resistor or a 
transformer which is connected to a cathode of the laser discharging tube 
and which is capable of measuring a discharging current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described in detail with 
reference to the accompanying drawings. 
FIG. 1 and FIG. 2 are schematic drawings showing a device and method for 
measuring a difference frequency of the present invention, and FIG. 3 
shows spectrum of a difference frequency measured by a device and method 
for measuring frequency variation quantity of the present invention. 
In FIG. 1, a resistor for measuring a difference frequency and a carbon 
dioxide laser are shown for explaining a device and method for measuring a 
difference frequency. In FIG. 2, a transformer for measuring a difference 
frequency and a carbon dioxide laser are shown for explaining a device and 
method for measuring a difference frequency. 
The carbon dioxide laser used for the above object is a water cooled gas 
circulation type. As a total reflecting mirror 1, a gold coated concave 
mirror was used, and as an output mirror 8, a flat surface mirror of ZnSe 
material having a 70% reflection rate was used. CO2:N2:He being a medium 
gas were continuously supplied at inlet 3 so as to maintain 31 Torr 
pressure in a ratio of 1:5.5:24.5 to a discharging tube 6. A high voltage 
was applied to both side electrodes 4, 7 of the discharging tube by using 
a direct current high voltage power supply means 10, whereby a discharging 
excitation was executed. As a discharging electrode, a tungsten bar was 
used for a positive electrode 4 for preventing a spattering, while a 
negative electrode 7 is attached by making a nickel plate in cylindrical 
form to tungsten bar. A Brewster's window 13 of ZnSe material is attached 
to both sides of the discharging tube in order to obtain a lineally 
polarized light from output light and to separate the reflecting mirror 
from the discharging tube. A ballast resistor 5 of 600 k.OMEGA. was 
connected in series with the discharging tube, and then at a time of 
supplying a current of 10 mA, approximately 4.5 W was obtained as a 
maximum output, and at a basic lateral mode and 10.59 .mu.m P 20, it was 
operated by a single longitudinal mode. 
A frequency F1 of light 9 generated from a carbon dioxide laser was mixed 
with a light F2 14 being frequency varied by an external source. A laser 
light strength within the resonator was modulated to a difference 
frequencies of F1 and F2 by self mixing whereby optogalvanic current 
varying in response to the laser light strength was generated. Since the 
optogalvanic current appeared as an electric potential difference at both 
ends of resistor 11, when it was passed through a capacitor, a difference 
of frequencies F1 and F2 excluding the direct current component could be 
measured. When a transformer 15 was connected to a negative electrode of 
the laser by applying the same principle as in FIG. 2, a difference of 
frequencies F1 and F2 excluding the direct current component could be 
measured at a secondary side of the transformer. 
In accordance with the device and method for measuring a difference 
frequency generated upon self-mixing a laser light of the present 
invention, a difference frequency was measured by utilizing a method for 
measuring an optogalvanic current generated at a time when a part of 
frequency varied laser light was re-applied to the carbon dioxide laser 
and mixed oneself with laser inherent frequency. 
Since the conventional difference frequency measuring device utilizing a 
carbon dioxide laser has measured by using generally MCT (Mercury Cadmium 
Telluride) detector cooled by liquid nitrogen, the liquid nitrogen should 
be replenished at predetermined time intervals, thereby presenting a 
hinderance in manufacturing a movable device due to a large volume of the 
liquid nitrogen storage container. Therefore, since the present invention 
can measure a difference frequency without using a detecting device cooled 
by liquid nitrogen and uses a laser generator itself as a detector, it has 
a very excellent effect that a reliability of the measuring device can be 
increased and the lifetime of the device can be, and the structure is 
extending while simplifying its structure and reducing a manufacturing 
cost.