Bi-directional short pulse ring laser

Details Bi-directional short pulse ring laser Bi-directional short pulse ring laser

2000-04-27

2003-11-18

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular resonant cavity

Folded cavity

C372S092000, C372S025000

Reexamination Certificate

active

06650682

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to the field of short pulse ring lasers and applications therefor.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The gyroscope (gyro) has been in use for navigational purposes since the 1920's, its accuracy improving as technology advanced. Today's state-of-the-art gyroscopes use two counter-propagating beams of continuous wave (CW) light in a ring configuration and take the beat frequency &Dgr;&ugr; between the two beams to be a measure of the rotation rate &OHgr;.
The principle behind the laser gyro is known as the Sagnac effect. This effect is the phase shift induced by a light beam as it completes a loop, when the plane of the loop is given a rotation. Consider the case where the loop is part of a laser cavity. One condition for lasing is that the cavity length which, to be precise, should be the optical path length of the laser cavity, and the cavity length must be an integral multiple of the wavelength of the light. Hence, the lasing frequency will adjust itself to match the cavity length. When the ring experiences a rotation (&OHgr;) or other non-reciprocal effects, the light travelling in one direction senses a different optical path length than the one travelling in the opposite direction. The light which travels against the rotation sees a slightly shorter path, and hence its frequency is upshifted while the reverse is true for the light travelling with the rotation. By recombining these two beams outside of the ring cavity, one observes a beat frequency &Dgr;&ugr;, which is the difference of the two light frequencies. The following relation is easily derived from consideration of path length changes:
υ
0
=
4
⁢
A
P
⁢
 
⁢
λ
⁢
 
⁢
Ω
=
R
⁢
 
⁢
Ω
where A is the area of the ring which is perpendicular to the rotation axis, &lgr; is the wavelength of the light within the ring and P is the ring perimeter. From this equation, note that a) the beat frequency due to rotation (Sagnac effect) is directly proportional to &OHgr; with the proportionality constant R being determined by the cavity geometry and that b) the plot of &OHgr; versus &Dgr;&ugr; should pass through the point (0,0). Deviations from these two conclusions are addressed by the present invention.
The first problem is that of linearity. It was found from the beginning that the counter-propagating beams of light become locked in frequency when the rotation rate was small, leading to the existence of a “dead band” where the gyro has zero response. The solution currently common in the art is to dither the ring thereby shaking the two beams out of the lock-in regime at small rotation rates. Of course, one consequence is that the gyro becomes more cumbersome due to the mechanical dithering devices. In addition, the gyro response near the dithering frequency does not reflect the “actual” rotation rate &OHgr;. Hence, the linear response is still not assured.
If the scattering that causes the lock-in is phase conjugated, the dead band can be reduced. U.S. Pat. No. 4,525,843 to Diels, entitled “Ring Laser with Wavefront Conjugating Beams”, discloses a phase conjugating coupling element inside the laser cavity to reduce lock-in. This patent has not been implemented in standard CW laser gyros, but provides background related to laser gyros based on short pulses.
Since the lock-in results from the scattering from either circulating beam into the one circulating in the opposite direction, one solution to eliminate the coupling is to use short pulses, and to ensure that they do not encounter a scattering medium where they cross. One implementation presently put in practice with dye lasers and Ti:sapphire lasers is to use phase conjugated coupling, such as disclosed in the '843 patent, at one crossing point, and ensure that the other crossing point is in air as disclosed by U.S. Pat. No. 5,363,192 to Diels et al., entitled “Mode-Locked Active Gyro Solid State Lasers”.
Some possible implementations of the '192 patent use saturable absorbers at a crossing point. The scattering associated with these media can cause lock-in, which can be eliminated by moving the saturable absorber transversally to the laser beam as disclosed in U.S. Pat. No. 5,367,528 also to Diels et al., entitled “Motion Induced Elimination of Dead Band in a Short Pulse Laser Gyro”. The ring laser that is mode-locked with a nonlinear substance of the present invention improves upon these prior art patents and provides applications for the unique bi-directional short pulse ring laser of the present invention.
In the case of a short pulse laser gyro as in the '192 patent, an artificial rotation can be obtained by inserting an electro-optic modulator (phase modulator) in the cavity, and pulsing it at the cavity round-trip time, in such a manner as to give a different cavity (optical) length for the pulses circulating clockwise and counterclockwise as disclosed in U.S. Pat. No. 5,251,230 to Lai et al., entitled “Resonant Cavity Dither with Index of Refraction Modulator.” As a result of this difference in cavity length, the two trains of pulses will exhibit a different frequency, resulting in a beat note undistinguishable from that corresponding to a rotation. A sufficient artificial rotation will force the laser gyro out of its “dead band”. This type of electro-optic dither can be implemented with a short pulse laser gyro of the type mentioned in the '192 patent.
Commercial laser gyros are all He—Ne gas lasers, operating in continuous mode. These gyros are plagued by a dead band, even using very expensive optics that have much less scattering than normal mirrors. Because of this dead band, one has to physically move the laser in order to have the laser operate outside of that range. It defeats the purpose of making a laser gyro, if moving parts are to be used. Further, vacuum tubes such as He—Ne lasers are an obsolete technology. The electrodes have a short lifetime and the laser itself has very low efficiency.
The embodiments of the present invention make it possible to use much more efficient solid state lasers, in particular diode pumped lasers. The dead band is suppressed without introducing a motion of the whole laser. In one of the embodiments, a small motion of one very small component (a solid state saturable absorber, or nonlinear crystal, or a flowing absorber, of a flowing nonlinear liquid) of the laser is used to eliminate a possible dead band. In another embodiment (electronic dithering), the dead band is completely eliminated electronically. In yet another embodiment, the laser is actively mode-locked by one or two modulators. Several applications are provided as well including detection of magnetic susceptibility; measurement of small displacements, measurement of the time derivative of n(t); measurement of high voltage; measurement of the derivative of voltage; and measurement of magnetic and electric fields.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION
The present invention is a method of making a bi-directional pulse ring laser and comprises the steps of providing a substance with an index of refraction that is light intensity-dependent; locating the substance in proximity to a beam waist of the laser cavity; and interacting bi-directional light pulses such that they are phase conjugated. The method further comprises the steps of altering the beam diameter within the cavity with the self-lensing effect of the substance and producing bi-directional short light pulses by inserting an aperture in the cavity where the beam diameter decreases with i

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