Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-11-17
2003-06-03
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S032000
Reexamination Certificate
active
06574252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radio frequency excited slab waveguide laser comprising a detector and method of stabilizing the laser power and frequency using the optogalvanic effect, and more particularly to a radio frequency excited slab waveguide laser comprising a detector and method of stabilizing the laser power and frequency using the detector, which use a variation in incident or reflecting radio frequency signal depending on an optogalvanic effect generated from the laser itself as a reference for the stabilization. Especially, the present invention comprises the steps of splitting radio frequency energy with high power, inputted for powering a laser medium gas and a weak signal reflected from each other by an optogalvanic effect; detecting them in order to stabilize the laser power and frequency by using the detected signals.
2. Description of the Related Art
A laser generates electromagnetic waves shorter than that of which is created by an optical maser, though they have the same principle. The laser is generated by such a process in which a laser medium is put between two reflection mirrors of a cavity and energy such as radio frequency is induced thereto to generate a laser beam, and this beam is repeatedly reflected between the reflection mirrors, formed in opposite directions to each other to be amplified, thereby being stimulated-emitted. The laser is classified into the gas laser, solid-state laser, liquid laser, ion laser, ruby laser, He—Ne layer, nitrogen laser, semiconductor layer, etc. according to kinds of laser medium.
The laser is generally called as a beam because its frequency is higher than microwaves, which does not disperse and travels straight without interference, so that it is widely used for optical communication, multiplex communication, space communication, etc. Further, a focused laser beam is used for processing refractory materials since it can concentrate a large amount of energy on a very small range. Frequency of the laser depends on an interval between the two reflection mirrors, which constitute a cavity of the laser. In accordance with above principle, the stabilization of the laser power and frequency is achieved when the above interval, namely, cavity length, is kept constant. However, the cavity length may be minutely changed due to the thermal expansion or by vibrations that would cause troubles on stabilization of the laser power and frequency.
In order to keep the cavity length be constant during its correspondence with the variation in the cavity length caused by the thermal expansion or vibrations, said two conventional methods are used as representative examples for a radio frequency-excited slab-wave guide laser in which includes a radio frequency generator
1
, connected to an impedance matching circuit
2
, which is for powering a laser, an unstable resonator
3
which is configured of a slab waveguide
6
having a medium, such that is excited by the radio frequency provided by the radio frequency generator
1
, a slab laser reflection mirror
5
located at one side of the slab waveguide
6
and a slab laser output mirror
41
placed on the other side of the slab waveguide
6
, being opposite to the slab laser reflecting mirror
5
, a piezo electric transducer
7
located on back of the slab laser output mirror
4
, and a lock-in stabilizer
8
for applying a signal for a vibration of the piezoelectric transducer
7
.
The first stabilization method is that the piezoelectric transducer
7
is attached to the slab laser reflection mirror
5
, constituting one side of the unstable resonator, and it is vibrated by 520 Hz approximately. Then, the cavity length varies within the range of said vibration numbers, such that output of the laser is oscillated due to the variation of the cavity lengths. And the output of the laser gets split by means of an optical splitter, and it is partially transmitted to an optical detector. The transmitted oscillating signal is detected with the optical detector, such that it is subsequently used as a reference signal for stabilization of the laser power and frequency. The theoretical basis of this method using the piezoelectric transducer
7
is as follows.
The oscillating frequency and power of a gas laser using a Fabry-Perot cavity depend on an optical length of the cavity. Here, the optical length means an interval between the slab laser reflection mirror
5
and the output mirror where the repeated reflection of laser beam substantially occurs. Accordingly, in order to stabilize the oscillating frequency and power of the laser, it is important to prevent the optical length of the cavity from being affected by the environment where the laser is used. Otherwise, a variation in the optical length of the cavity should be compensated for. In other words, the optical length of unstable resonator
3
, which may vary depending on the environment involving a variation in ambient temperature around a discharge tube used, mechanical vibrations, and a variation in the pressure of laser gas used, should be kept constant.
A variation in laser radiation flux occurring in the unstable resonator
3
results in variations in laser beam intensity, spectrum distribution, gas pressure, discharge current, and discharge impedance. In this regard, when vibrations of a certain frequency (for example, 522 Hz) are applied to the piezo electric transducer
7
attached to the back of the slab laser reflection mirror
5
located on the optical axis of the unstable resonator
3
, the optical length of the unstable resonator
3
vibrates at the above frequency. The intensity of laser beam is modulated at the same frequency as that of the cavity length, along a laser gain characteristic. When the average mode frequency of the laser beam intensity being modulated is lower than the central frequency of the same laser beam intensity, the laser beam intensity has a phase opposite to that of the impedance variation. On the other hand, where the average mode frequency is higher than the central frequency, the impedance variation increases correspondingly to a difference of the mode frequency from the central frequency. In such a way, a considerable variation in the radio frequency discharge impedance occurs even when a slight variation in laser beam intensity occurs in the laser cavity. For instance, an intensity variation of only 1% results in a considerable variation in the radio frequency discharge impedance corresponding to about 0.1%. Based on such an impedance variation in the unstable resonator
3
, a variation in incident or reflecting radio frequency energy is detected. Based on the detected signal, the lock-in stabilizer
8
generates an error signal that is, in turn, fed back to the piezo electric transducer. In such a way, stabilization of the laser power and frequency is carried out.
In the second stabilization method, a high-voltage direct current (DC) discharge tube is arranged in the unstable resonator in place of the optical detector used in the first stabilization method. In this case, an oscillating signal generated from the discharge tube is used as a reference signal for the laser power and frequency stabilization. Further, there was a method of which attenuates all laser outputs with a level higher than a minimum power using an optical attenuator arranged outside the laser. The above conventional stabilization methods are disclosed in U.S. Pat. Nos. 4,694,458, 4,856,010, 4,972,425, 6,084,893.
However, the stabilization method, in which a reference signal for stabilization is generated based on a signal split from an output from the laser by the optical splitter, involves a reduction in laser power because the laser output is partially used. Furthermore, since the optical splitter is arranged on the optical path of the laser, it is difficult to obtain an accurate optical axis alignment for the optical splitter. In addition, a variation in the transverse mode of the laser may occur.
The stabilization method, in which the high-voltage DC discharge tu
Choi Jong Woon
Chung Young Boong
Shin Dong Ju
Al-Nazer Leith
Ip Paul
Korea Research Institute of Standards and Science
McCormick Paulding & Huber LLP
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