Rotation and acceleration sensor based on superluminal ring lasers

Abstract

Ring laser gyroscopes, in which rotation is detected by the Sagnac effect between counterpropagating lasers, are in common use in navigation applications. The invention disclosed here uses an induced strong anomalous dispersion inside the ring laser cavities to create a group velocity of as much as 10.sup.6 times greater than the vacuum speed of light, with a corresponding increase in gyroscope sensitivity; the resulting device is referred to as a Superluminal Ring Laser Gyroscope (SRLG). The invention disclosed here also incorporates an acceleration-sensing element that modifies the path length of the ring lasers in the gyroscope, the effects of which on the output of the gyroscope can be separated from those of rotation. The resulting composite device is a Superluminal Ring Laser Gyroscope/Accelerometer (SRLGA).

Claims

1. A device comprising: two counter propagating ring lasers sharing a common cavity, each incorporating an element that produces a strong anomalous dispersion sufficient to cause the laser fields to propagate with group velocities in excess of the vacuum speed of light, wherein the anomalous dispersion is generated using a resonant Raman process in an alkali vapor, wherein the cavity contains a mirror mounted on an acceleration-sensitive element to generate acceleration detection; and an optical pump module, wherein the optical pump module includes a first 780 nm Distributed Bragg Reflector (DBR) laser and a second 780 nm DBR laser, wherein the output of the first 780 nm DBR laser is locked to an .sup.85Rb absorption transition, wherein the output of the second 780 nm DBR laser is locked to an .sup.87Rb absorption transition, wherein the output of the lasers is passed through a heated rubidium vapor cell.

2. The device according to claim 1, wherein the outputs of the two ring lasers are combined and the resulting beat frequency is measured by a readout module, wherein the readout module includes a clockwise demodulator for receiving the clockwise resonant Raman laser signal and a counter clockwise demodulator for receiving a counter clockwise resonant Raman laser signal, wherein the readout module determines the difference in frequency between the clockwise demodulator and counter clockwise demodulator.

3. The device according to claim 1, wherein the device is used to measure rotational rate by a readout module, wherein the readout module includes a clockwise demodulator for receiving the a clockwise resonant Raman laser signal and a counter clockwise demodulator for receiving a counter clockwise resonant Raman laser signal, wherein the readout module determines the difference in frequency between the clockwise resonant Raman laser and the counter clockwise resonant Raman laser, wherein the difference is proportional to the rotational rate of the ring laser, wherein the readout module generates a gyroscope signal based on the difference.

4. The device according to claim 1, wherein the two lasers are operated at different frequencies.

5. The device according to claim 1, wherein the laser gain medium is the vapor of an alkali metal element.

6. The device according to claim 1, wherein the laser gain is generated using a resonant Raman process in .sup.85Rb atoms.

7. The device according to claim 1, wherein the anomalous dispersion is generated using a resonant Raman process in .sup.87Rb atoms.

8. The device, according to claim 1, wherein the laser gain and anomalous dispersion generating processes occur in the same vapor cell using different coexisting alkali metal isotopes.

9. The device according to claim 1, wherein the mirror mounted on an acceleration-sensitive element to generate acceleration detection is a mirror attached to a MEMS-fabricated spring, wherein motion of the mirror is used to measure acceleration.

10. The device according to claim 1, wherein the device is used to measure rotation and acceleration simultaneously.

11. A combined superluminal ring laser gyroscope and accelerometer device comprising: a clockwise resonant Raman laser pump module; a counter clockwise resonant Raman laser pump module; an optical pump module; a ring laser module; and a readout module, wherein each of the clockwise resonant Raman laser pump module and the counter clockwise resonant Raman laser pump module incorporate an element that produces a strong anomalous dispersion sufficient to cause the laser fields to propagate with group velocities in excess of the vacuum speed of light, wherein the anomalous dispersion is generated using a resonant Raman process in an alkali vapor, wherein the output from the clockwise resonant Raman laser pump module and the counter clockwise resonant Raman laser pump module combine with one of the two optical pump outputs on a dichroic beam combiner to form a combined set of beams, wherein the ring laser module receives the combined set of beams in a single cavity, wherein the ring laser module generates a pair of counter-propagating resonant Raman lasers in a single cavity, wherein the cavity contains a mirror mounted on an acceleration-sensitive element to generate acceleration detection, wherein the readout module receives the output of the ring laser module to produce a desired rotation and acceleration signal wherein the optical pump module includes a first 780 nm Distributed Bragg Reflector (DBR) laser and a second 780 nm DBR laser, wherein the output of the first 780 nm DBR laser is locked to an .sup.85Rb absorption transition, wherein the output of the second 780 nm DBR laser is locked to an .sup.87Rb absorption transition, wherein the output of the lasers is passed through a heated rubidium vapor cell.

12. The device of claim 11, wherein the clockwise resonant Raman laser pump module and the counter clockwise resonant Raman laser pump module produce an output of different frequencies.

13. The device of claim 11, wherein the optical pump module provides an output of 780 nm.

14. The device of claim 11, wherein the ring laser module includes three mirrors defining a triangular cavity, wherein the mirror mounted on an acceleration-sensitive element is a gold mirror on MEMS-fabricated springs, wherein a second mirror is a high reflector attached to a PZT cylinder, wherein a third mirror is a concave output coupler.

15. The device of claim 11, wherein the ring laser module includes two rubidium vapor cells, wherein the output of the clockwise resonant Raman laser pump module passes through a first rubidium vapor cell, and the output of the counter clockwise resonant Raman laser pump module pass through a second rubidium vapor cell.

16. The device of claim 11, wherein the readout module includes a counter clockwise length demodulator and a clockwise length demodulator, wherein the readout module determines the difference in signal between the clockwise and counter clockwise demodulators, wherein the readout module provides a measure of the cavity length change due to movement of the MEMS-mounted mirror based on the difference.

17. The device of claim 11, wherein the readout module generates an accelerometer signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the invention will be readily obtained by reference to the following Description of the Preferred Embodiments.

(2) FIG. 1 illustrates the concepts behind superluminal laser enhancement.

(3) FIG. 2 shows the transitions in rubidium that are used to generate lasing and superluminal group velocity in the invention.

(4) FIG. 3 shows the combined laser gain and index of refraction profiles of the sort used to generate superluminal lasing in the invention.

(5) FIG. 4A shows the layout of the various modules and the optics and laser beam lines connecting the modules. FIG. 4B shows a schematic of a Raman pump module. FIG. 4C shows the schematic of the optical pump module. FIG. 4D shows the schematic of the ring laser module. FIG. 4E shows the schematic of the readout module.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) The new system for generating the requisite gain profile for an SRL is illustrated in FIG. 2, using a single cell containing two naturally occurring isotopes of Rb (.sup.85Rb and .sup.87Rb). Note first that the energy difference between the two ground states |1 and |2 is different for the two isotopes (3.035732439 GHz=F.sub.85 for .sup.85Rb and 6.834682611 GHz=F.sub.87 for .sup.87Rb). Furthermore, the absolute transition frequency for the 1-4 transition in .sup.85Rb is lower by about 1.5 GHz than that for .sup.87Rb. Similarly, the absolute transition frequency for the 1-3 transition in .sup.85Rb is lower by about 1.5 GHz than that for .sup.87Rb. These non-degeneracies make it possible to use two distinct optical pumps (shown by the purple lines) for the two isotopes. Use of a neon buffer gas can reduce the population relaxation rate between the ground states to as low as 10 kHz. We apply two different Raman pumps (.sub.1A at frequency F.sub.A and .sub.1B at frequency F.sub.B) for the two isotopes and a common Raman probe .sub.2. When the optical pump .sub.PA resonant with the transition from level |1> to |4> in .sup.85Rb is applied, the depletion of atoms in level |1> induces population inversion between |1> and |2>, thereby producing a Raman gain for the signal. We also apply the other optical pump .sub.PB resonant with the transition from |2> to |4> in .sup.87Rb, which produces depletion of atoms in level |2>, thereby generating Raman absorption for the same signal. The width of the Raman gain in .sup.85Rb and Raman absorption in .sup.87Rb can be independently controlled, with a lower limit of 10 kHz for each, set by the decay rate between levels 1 and 2 (when the residual Doppler width is taken into account, this is increased by about 5 kHz for .sup.85Rb and 11 kHz for .sup.87Rb, as discussed later in this subsection). With suitable choices of parameters, it is possible to achieve a narrow dip on top of a broad gain in the transmission profile. We show a typical susceptibility as a function of the signal detuning for such a case in FIG. 3. The rather narrow (max about 5 MHz) width of the envelope gain profile would guarantee single mode oscillation.

(7) It is necessary to ensure that the two Raman pumps are offset-phase-locked with respect to each other. The frequency difference, F.sub.OPL, between these two pumps should be such that F.sub.OPL=F.sub.AF.sub.B=F.sub.87F.sub.85=3.798950172 GHz. Of course, the degree of precision in this number would be determined by the stability of the VCO used in the offset-phase-locking system.

(8) Furthermore, a stable AOM (or a sideband from an EOM), would be used to ensure that these pumps are offset stabilized to an absolute frequency. Specifically, the Raman pump for .sup.85Rb would be locked at a frequency that is below the 2-3 transition by a frequency offset of F.sub.OS. The value of F.sub.OS should be chosen to ensure that both Raman pumps are away (e.g., by more than the Doppler half-width of 300 MHz) from absorption in both .sup.85Rb and .sup.87Rb.

(9) Given this system, we now describe how to realize overlapping, counter-propagating, non-degenerate lasers for producing a robust SRLG. It is well know that a Raman laser is unidirectional; it only lases in the direction of the pump (this aspect is discussed in greater detail later on in this subsection). Thus, we can use two different cells, each of which will have the components described above. However, the direction of the Raman pumps will be reversed. The pumps will be dumped with polarizing beam splitters (PBSs) so that pumps for one cell cannot enter the other. Furthermore, in one, the Raman pump for .sup.85Rb will be tuned above resonance, while in the other it will be tuned below resonance. The difference between these two will be tuned to be one free spectral range (FSR) of the cavity. This will ensure that the laser in one direction will not affect the dynamics of the other laser. The non-degeneracy between the counter-propagating lasers will provide a natural offset frequency. Such an offset enables one to determine the sign of rotation, and eliminates the lock-in problem. For concreteness, let us assume that the Raman laser in the clock-wise direction has a longitudinal (integer) mode number m (i.e., m=L/, where L is the cavity length and is the wavelength of the Raman laser), and the Raman laser in the counter-clock-wise direction has a longitudinal mode number of (m1). The value of m can be determined easily, given the known value of the 1-3 transition frequency in .sup.85Rb, the value of F.sub.OS, and the FSR of the cavity.

(10) To detect the SRLG signal, the outputs of the two Raman lasers are mixed on a photodetector. The output of the detector is then mixed with a VCO at a frequency that is the sum of the values of F.sub.OS for the two directions, which is equal to the FSR of the cavity, and sent through a low-pass filter to eliminate the sum frequency. A voltage proportional to the output frequency would be generated by using the well-known PLL-FM demodulation technique (PLL: Phase-locked-loop; FM: Frequency Modulation). The sign of the rotation can be determined, for example, by changing the frequency of the VCO in one direction and monitoring the corresponding increase (for one sign) or decrease (for the other sign) in the final output signal.

(11) For long observation periods, or for applications demanding extreme precision, fluctuations in the cavity length cannot be ignored. A change in the cavity length will produce a shift in the resonance frequency for each mode, and the corresponding change in the FSR. In order to account for and rectify errors resulting from such a change, we will employ the following common-mode rejection servo mechanism, made possible by the fact that the SRLG excitation beams include lasers locked to absolute frequency references. First, for each direction, we will detect the beat signal between the .sup.87Rb Raman pump and a small part of the Raman laser output (while the rest of the outputs of the Raman lasers will continue to be used for detecting and processing the beat signal between them, as described in the preceding paragraph). This signal will then be mixed with a stable VCO at f.sub.87 (=6.834682611 GHz), followed by a low pass filter, so that we get only the difference frequency. This beat frequency is converted to a voltage using a PLL-FM demodulator. We denote as f.sub.m (f.sub.m-1) the resulting voltage signal for the clock-wise (counter-clock-wise) laser. Furthermore, we define the signals f.sub.m+f.sub.m-1 and f.sub.mf.sub.m-1, which can be easily generated. Consider now a situation where the sum of the two Raman laser frequencies have changed by an amount that corresponds to a voltage signal .sub.CM due to a change in the cavity length (a common-mode effect), and the difference between the two Raman laser frequencies have changed by an amount that corresponds to a voltage signal .sub.SE due to the Sagnac effect (i.e., rotation). A straight-forward analysis shows that .sub.SE=Z[(2m1)] and .sub.CM=Z[(2m1)], where Z=(2m1)/[(2m1).sup.21]. For m>>1, this simplifies to .sub.SE(+/2m), and .sub.CM(+E/2m), each of which can be generated as a voltage signal, given the determined value of m. The value of .sub.CM will be used as a feedback signal to correct for the fluctuations in the cavity length, producing the condition that .sub.CM=0. Under this condition, we then also get .sub.SE(1-4/m.sup.2). However, in order to ensure that the quantum noise in the final SRLG signal is from the Raman lasers only, we will use the output of the detection system described in the preceding paragraph, since it involves mixing of the Raman lasers only.

(12) FIG. 4A shows the overview of the entire system and the lasers and optics that couple them together. There are five modules in the system: The clockwise (CW) Raman laser pump module, the counterclockwise (CCW) Raman laser pump module, the optical pump module, the ring laser module, and the readout module. Each Raman laser pump module produces an output that is the combined pair of 795 nm lasers necessary to pump the .sup.85Rb Raman gain process responsible for lasing and the .sup.87Rb Raman loss process responsible for providing the dispersion necessary for superluminal lasing. They also provide a pickoff from the laser pumping the .sup.87Rb Raman loss process for locking and stabilization. The optical pump module provides two identical outputs, each of which is the combined output of the 780 nm lasers necessary for the optical pumping of both .sup.85Rb and .sup.87Rb. Each Raman pump output is combined with one of the two optical pump outputs on a dichroic beam combiner and the resulting combined set of beams is directed into one input to the ring laser module. The outputs from the ring laser module, along with the pickoffs of the .sup.87Rb Raman pump lasers, are directed into the readout module. The readout module then produces the desired rotation and acceleration signals, as well as providing a lock signal to the PZT in the ring laser module. Each module of this system will be described in detail individually.

(13) The two Raman pump modules are functionally identical, differing only in the frequency offsets provided to the lasers. The schematic of one Raman pump module is shown in FIG. 4B. It contains two 795 nm DBR lasers. The output of one of these lasers (795DBR1) will be the pump for the Raman lasing process. It is collimated, passed through an isolator and a half-wave plate, and split (BS1) into two equal-powered beams. One of these beams is focused into the fiber input of an electro-optic intensity modulator (EOMI). A voltage-controlled oscillator connected (VCO1) is connected, through appropriate amplifiers, to the EOM, which splits the laser into a carrier and a series of odd-ordered sidebands. One of the first-order sidebands will be selected for locking to an .sup.85Rb resonance. The output of the modulator is then sent back into free space and into a saturated absorption system. An uncoated, thick flat window (W1) reflects two low-power pickoffs of the beam through a heated rubidium vapor cell (RC1). The remainder of the beam is reflected into a 50/50 beamsplitter (BS2), which directs part of it back along the same path as one of the two low-power pickoffs. One photodetector (PD1) measures the unsaturated absorption signal, while a second (PD2) measures the saturated absorption signal. A locking servo (PLS1) dithers the laser frequency across a small portion of the .sup.85Rb absorption spectrum and uses the difference between the saturated and unsaturated outputs to lock the EOM-shifted laser frequency to one of the hyperfine resonances of .sup.85Rb. This means that the frequency of the DBR output itself is thus locked at a constant offset frequency (determined by the VCO and the choice of sidebands) from the .sup.85Rb resonance. A second laser (795DBR2) provides the Raman pump for the .sup.87Rb Raman loss process. It is collimated and passed through an isolator and a half-wave plate, then a pickoff from it is extracted (BS4) and sent to the readout module. The remainder of the beam is combined on a 50/50 beamsplitter (BS3) with the output from the first DBR. One output from this beamsplitter is directed into a fiber-coupled high speed photodetector (HSPD1), which provides the beatnote between the lasers to an offset phase locking servo. This servo controls the frequency of the second DBR so that the beatnote between the two DBRs is held constant at the frequency determined by a second voltage-controlled oscillator (VCO2). The other output from the beamsplitter is directed (after combination with one of the outputs of the optical pump module) into the ring laser module.

(14) The optical pumps for the clockwise and the counterclockwise laser are provided by the two outputs of the optical pump module, shown in FIG. 4C. There are two 780 nm DBR lasers in the optical pump module. Both follow identical paths, but one is locked to a .sup.85Rb absorption transition and the other is locked to a .sup.87Rb absorption transition. Each laser is collimated and passed through an isolator and half-wave plate, then a pickoff from it is passed through a heated rubidium vapor cell (RC2/RC3) and into a photodetector. A servo (PLS2/PLS3) uses the signal from the photodetector to lock the frequency of the DBR to the appropriate transition. Each beam is then directed into a fiber-coupled tapered amplifier (TA1/TA2) to increase its power to appropriate levels. The outputs of the two tapered amplifiers are combined on a 50/50 beamsplitter (BS5), and the two outputs of the beamsplitter are directed individually (after combination with one of the Raman pump module outputs) into the ring laser module.

(15) The heart of the SLRG/A is the ring laser module, a schematic of which is shown in FIG. 4D. It produces a pair of counterpropagating superluminal Raman lasers that share a common path. Each combined set of Raman and optical pump lasers is directed in the vertical polarization into a polarizing beamsplitter (PBS1/PBS2) and is dumped out of the cavity before completing a round trip by a common polarizing beamsplitter (PBS3). The superluminal lasers propagate in the horizontal polarization and pass through the polarizing beamsplitters unaffected. Gain is provided by two rubidium vapor cells (BRC1 and BRC2) that contain a helium buffer gas to allow for narrower transitions. The triangular cavity is defined by three mirrors. One mirror (CM1) is a high reflector attached to a PZT cylinder to allow for active control of the cavity length by a servo in the readout module. The second mirror (MM1) is a gold mirror on MEMS-fabricated springs; the motion of this mirror allows for acceleration detection. The third mirror (OC1) is a concave output coupler whose curvature and reflectivity are chosen to provide optimal laser performance. The superluminal laser outputs are directed into the readout module for measurement. The entire module is contained within a monolithic machined aluminum body that is contained within a mu metal shield to protect against interference from external magnetic fields. This body is heated with resistance wire to the temperature necessary to provide the rubidium vapor pressure needed for optimal laser performance.

(16) The readout module measures the frequency differences induced in the two superluminal lasers in order to extract the rotation rate and linear acceleration experienced by the ring laser module; a schematic of this module is shown in FIG. 4E. There are four inputs to this module: The two superluminal laser outputs and the pickoffs from the two .sup.87Rb Raman pump lasers. A pickoff from each optical pump (BS6/BS7) is combined on a 50/50 beamsplitter (BS9/BS10) with its corresponding .sup.87Rb Raman pump and the resulting beatnote is detected with a high-speed photodetector (HSPD3/HSPD4). The output of each detector is mixed with the output of a voltage-controlled oscillator (VCO3) set at the 6.834682611 GHz frequency corresponding to the spacing between the two .sup.87Rb ground-state hyperfine levels. This signal is then demodulated using a phase-locked loop to provide a DC signal proportional to the departure of each superluminal laser frequency from the value expected of from an unmoving system. The difference between the demodulated signals for the clockwise and counterclockwise lasers is then taken. This difference signal provides a measure of the cavity length changes due to movement of the MEMS-mounted mirror, from which the acceleration on the system can then be determined. This signal is then fed back through a servo (PZTS1) to the PZT in the ring laser module to correct the cavity length. The two superluminal lasers are then combined with each other on a 50/50 beamsplitter (BS8) and its output is directed into another high-speed photodetector (HSPD2). The beatnote measured by this detector is then mixed with the output of a voltage controlled oscillator (VCO2) set to match the frequency difference between the two superluminal lasers expected from an unmoving system and is demodulated using a phase-locked loop demodulator. The signal from this demodulator is proportional to the rotation rate of the ring laser module, and thus provides the gyroscope signal.

CITED WORKS

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