Patent Publication Number: US-11022440-B2

Title: Cladding-pumped waveguide optical gyroscope

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/408,074, filed Jan. 17, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/398,255, filed Sep. 22, 2016, all of which are incorporated by reference in their entirety. 
    
    
     FIELD 
     This application is generally related to inertial navigation devices including optical gyroscopes. 
     BACKGROUND 
     Optical gyroscopes are important for inertial navigation. Optical gyroscopes are expected to play more important roles for applications where GPS is denied or compromised. 
     The Sagnac effect is directly related to optical gyroscopes. According to the Sagnac effect, the phase or frequency of light circulating in a loop geometry is modified by the rotation of the loop. The effect is manifested by a change in interference between the two counter propagating lights in the loop. 
     Optical gyroscopes can be broadly categorized into the interferometric type where the phase shift is measured, and the resonant type where a frequency shift due to rotation is measured. In both cases, the sensitivity of the gyroscope is linearly proportional to the length of the loop. 
     Chip-scale optical gyroscopes have been advanced to reduce the size of the device to adopt optical gyroscope technologies to other application areas. However, the optical propagation loss of typical waveguides is greater than 1 dB/m. This is several orders of magnitude inferior to optical fiber loss of about 0.2 dB/km. As a result, the loop length, and in turn the sensitivity, of the waveguide optical gyroscope is limited. 
     SUMMARY 
     The foregoing needs are met, to a great extent, by a waveguide optical gyroscope (WOG) utilizing a rare-earth doped waveguide core and cladding pumping. In so doing, a loss-free and symmetric gyroscope loop is obtained. 
     There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to facilitate a fuller understanding of the invention, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the invention and intended only to be illustrative. 
         FIG. 1  illustrates a cross-sectional view of a waveguide layer of a WOG according to an aspect of the application. 
         FIG. 2  illustrates a configuration of a cladding-pumped WOG according to an aspect of the application. 
         FIG. 3  illustrates a WOG according to an aspect of the application. 
         FIG. 4  illustrates a multi-layer Sagnac loop with cladding pumping according to an aspect of the application. 
         FIG. 5  illustrates a cross-sectional view of the cladding pumped waveguide loop along a dashed line of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments or embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     Reference in this application to “one embodiment,” “an embodiment,” “one or more embodiments,” or the like means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of, for example, the phrases “an embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by the other. Similarly, various requirements are described which may be requirements for some embodiments but not by other embodiments. 
     In one aspect of the application, interferometric waveguide optical gyroscope (I-WOG) sensors are described. The I-WOG sensors may be useful for munitions navigation in the absence of GPS. One feature of the I-WOG sensors is a three dimensional (3-D) integrated Sagnac interferometer implemented with a rare-earth (RE)-doped silica waveguide on silicon. The RE-doped waveguide is illuminated by an athermal pump laser. The pump laser light is guided in and through the cladding layer to provide compensation for the loss of the signal guide by the waveguide core. The light enables an optical path longer than 120 m necessary for achieving navigation-grade inertial guidance. A similar RE-doped waveguide structure, with pump laser light directly injected to the waveguide core provides a signal input with wavelength stability better than 1 ppm. This helps obtain the scale factor stability under a severe operating environment. 
     A WOG has no moving parts and therefore possesses distinct advantages over MEMS-based inertial sensors in providing capability for operating through the harsh shock and vibration stresses expected during launch conditions. Silica PLCs have excellent mechanical properties comparable to silicon-based electronics historically shown to be survivable up to 50,000 g. Accordingly it is envisaged that a waveguide implementation of an interferometric FOG can produce a navigation-grade sensor. The techniques compensate for the low loss of 2-3 dB/m by weakly pumping active RE-doped waveguides to achieve completely lossless propagation. This can also provide overall signal gain. The doped waveguide also provides means for implementing an on-chip light source with much reduced environmental sensitivity. 
     According to another aspect of the application, hybrid integration using a silica planar lightwave circuit (PLC) on silicon is provided for integrating the necessary optical components on chip. This includes a micro-optic isolator and phase modulators with sufficient mechanical strength and low insertion losses. 
     WOG 
       FIG. 1  illustrates a waveguide optical gyroscope (WOG)  100  in accordance with aspects of the present disclosure. The WOG  100  may comprise a waveguide  102  (e.g., rare-earth (RE) doped waveguide) and dual clad layers  104  and  106 . The inner clad layer  106  may be pumped using cladding pumping, as described herein. As an example, the WOG  100  may be configured as a loss-free loop, such as a loss compensated loop. As shown, the waveguide  102  may be embedded in a high-index (n 1 ) cladding  106  that is sandwiched between lower-index (n 2 &lt;n 1 ) cladding layers  104 . The WOG  100  may further comprise a silicon substrate  108 . As such, a pump light  200  (e.g., laser) may irradiate in the plane of the high-index cladding layer  106  and the irradiated energy may pump the waveguide  102 , as shown in  FIG. 2 . 
     As shown in  FIG. 2 , the waveguide  102  may be configured in a loop configuration and coupled to one or more of a signal source  202  (e.g., ASE source) and a detector  204  (e.g., photodetector). It is to be understood that the pump light  200  may be propagated in a manner such that the reciprocity of the clockwise (CW) and counter-clockwise (CCW) rotation of a signal beam received from the signal source  202  is not violated. The sensitivity of the WOG  100  may be dependent on the symmetry between the CW and CCW propagation in the loop configuration of the waveguide  100 . Near uniform illumination of the waveguide  100  by the cladding pumping facilitates nominal to no longitudinal gain/loss variation along the waveguide  100  and maintains the symmetry between the CW and CCW propagation. 
       FIG. 3  illustrates an interferometric waveguide optical gyroscope (I-WOG)  300  in accordance with aspects of the present disclosure. The I-WOG  300  may include an amplified spontaneous emission (ASE) source  302 , one or more phase modulators  304  (e.g., for biasing), a Sagnac interferometer  306  or sensor, and a photodetector  308 . Other components may be included and configured in various manners such as isolators and polarizers, for example. As an example, the I-WOG  300  may be configured as a loss-compensated Sagnac sensor using cladding-pumped RE-doped waveguide is described. As a further example, one or more components of the I-WOG  300  may be disposed on a planar lightwave circuit (PLC) such as hybrid-integrated on a silica PLC  318   
     The ASE source  302  may include a waveguide  301  and a pump source  303  configured to energize the waveguide  301 . The pump source  303  of the ASE source  302  may be or comprise an athermal pump laser. The waveguide  301  may comprise a rare-earth (RE) doped silica. 
     The Sagnac interferometer  306  (e.g., sensor) may include a waveguide loop  321  disposed in a first cladding material interposed between layers of at least a second cladding material having an index of refraction lower than an index of refraction of the first cladding material. As an example, the waveguide loop  321  comprises a rare-earth (RE) doped silica. The Sagnac interferometer  306  may be configured to receive an output signal of the ASE source  302 . As an example, the output of the ASE source  302  may pass through one or more of a micro-optic isolator  314 , a thin-film optical filter (not shown) a micro-optic polarizer  316 , and the phase modulators  304  before guided into the Sagnac interferometer  306 . A pump source  320  may be configured to pump at least the first cladding material of the Sagnac interferometer  306  with an in-plane pump signal. As such, the propagation loss in the Sagnac interferometer  306  may be compensated by the pump source  320 . The pump source  320  may be or comprise an athermal pump laser. In certain aspects, the Sagnac interferometer  306  may include a trench  322  formed along a transverse axis of the multilayers of cladding material and spaced from the pump source  320  such that the waveguide loop is disposed between the pump source and the trench  322 . A reflective coating, such as metal, may be disposed in the trench  322  to reflect the pump light back into the cladding layer, effectively confining the pump light. Pump rays may be fully absorbed by the RE-doped waveguide core, while they traverse the waveguide loop that is optically confined by the reflective side walls surrounding  321 . 
     As an illustrative example, the ASE source  302  may include an athermal pump laser  310  and a ˜2-m long RE-doped waveguide  312 . A micro-optic isolator  314  and thin-film optical filter may be configured to lock the ASE wavelength. A high polarization extinction (&gt;40 dB) micro-optic polarizer  316  may be configured to control the polarization before transmitting the light into the Sagnac interferometer  306 . For phase modulation, an approximate 1.5 cm X-cut lithium niobate phase modulators  304  may be hybrid-integrated to the silica PLC  318 . The Sagnac interferometer  306  may include active loops of multi-layer waveguides pumped by two pumps  320  (e.g., 980-nm pump signals), facing in opposite directions, located at/near the center of the active loop. The outer and inner perimeters of the loops may be surrounded by trenches  322  configured with metal coating to operate as reflectors and thereby serving to confine the pump beam to improve the pump efficiency. A COTS InGaAs photodetector  308  (˜10 MHz bandwidth) is hybrid-integrated for detecting the gyro signal. 
     As provided in the exemplary embodiment as illustrated in  FIG. 4 , a three dimensional (3-D) integrated waveguide  402  may be configured a fiber loop in an interferometric fiber-optic gyroscope (I-FOG)  400 . The I-FOG  400  of  FIG. 4  may be configured as the Sagnac interferometer  306  of  FIG. 3 . The I-FOG  400  may be configured with extremely low propagation loss (e.g., 0.2 dB/km) of silica fiber at or near 1550 nm. It is envisaged in this application that achieving chip-scale long optical delays may leverage a low-loss (2 dB/m) silica waveguide (e.g., waveguide  402 ) disposed on silicon (e.g., with index contrast ˜1.5-2%). A tight bend radius (e.g., about 1-2 mm) of the waveguide loop  403  and small confined mode size (˜5 μm) facilitates high density packing of waveguides with negligible (−60 to −70 dB) overlap between the modes of neighboring waveguides (15-20 μm spacing). As an example, up to a 30-min delay is implemented in a single waveguide layer. Waveguide loss may be overcome by doping the waveguide  402  with Er or Er/Yb and pumping the waveguide  402  with ˜980-nm pump light. 
     In reverence the  FIG. 4 , a pump beam  404  may be side-coupled and uniformly distributed in a cladding  406  surrounding at least a portion of the waveguide  402  interposed between cladding layers  408  configured to maintain the symmetry of the waveguide  402 . Minimizing the background loss of the waveguide  402  may facilitate reduction of the pump power required to compensate for the waveguide loss. As an example, RE-doped waveguides may exhibit background loss lower than 3 dB/m. It is calculated that about 60 mW of 980 nm pump light (e.g., pump beam  404 ) may be absorbed in 120 m of an Er-doped (Er concentration ˜10 25 /m 3 ) waveguide to generate sufficient optical gain for compensating the background waveguide loss. The pump beam  404  may be sourced from 980-nm pump lasers operated athermally over the entire relevant temperature range by hybrid integrating the laser and an athermal micro-optic volume-grating wavelength locker (Δλ=0.01 nm/° C.) onto a silica PLC. 
     3D Photonic Integration 
     In another embodiment, the waveguide  402  may comprise a multi-stack design to implement Sagnac sensors having multiple waveguide layers. Such a configuration multiplies the length of the delay to longer than 120 m in order to achieve a shot-noise limited bias stability of less than 0.01 degree/hr. The multi-layer design may have no waveguide crossings which could contribute to detrimental back reflection and excess loss, nor in-plane or inter-plane modal overlap creating cross talk, except at the inter-layer transition region. The mode size may be expanded from ˜5 μm to larger than 10 μm only locally in the transition area using a mode-size expander ( FIG. 5 ), for example. This controlled transition between the layers without waveguide/mode crossings will be utilized to optimize the loop-winding geometry to minimize the impact of non-reciprocal perturbations. 
     The waveguide  402  in the Sagnac loop may include a RE-doped silica waveguide core (Δn˜2.5%) and may be surrounded by cladding (Δn˜0.5%) layers  406 ,  408  for guiding the pump light  404 . The refractive indices are measured relative to that of the cladding layer  408  (e.g., buffer glass layer (n 0 )) between the cladding  406  and a silicon substrate  401 . The index of refraction of the waveguide  402  may be controlled from about 0.5% up to 4% by adjusting the concentration of glass ingredients, such as P and Ge, as well as the active ions such as Er and Yb. The waveguide  402  spacing (˜15 μm) is sufficiently larger than the optical mode size such that unwanted power cross talks between the neighboring waveguides will be negligible 
     Athermal RE-Doped Waveguide Light Source 
     One of the challenges in I-FOGs is scale-factor stability. This may be compounded in harsh environments due to extreme temperature cycling, shock and vibration. In view of this, the RE-doped waveguide technology and hybrid-integrated athermal 980-nm pump laser described in this disclosure generates amplified spontaneous emission (ASE) from the doped waveguide. For efficient ASE generation, pump light is injected into the core of the doped waveguide. The mode expander on the doped waveguide side and active alignment in flip-chip bonding of the athermal pump laser will minimize the insertion loss. A thin-film optical filter whose passband (1540-1560 nm) overlaps with the spectrally flat part of ASE from Er ions will be integrated in the micro-optic isolator package, which will be flip-chip bonded to the silica PLC. Optimizing the thin-film optical filter helps achieve the required wavelength band and temperature stability. 
     According to another embodiment, a hybrid integration process similar to that used for the cladding pumps may be used to integrate an athermal pump laser to core-pump a ˜2 m doped, tightly coiled, doped waveguide. A micro-optic isolator package (˜2 mm size) includes a double-stage isolator (40-dB isolation) and collimating lens coated with optical bandpass filter (1540-1560 nm) to stabilize the central wavelength of the ASE irrespective of the temperature. Approximately 4-mW ASE generation per polarization within the band is expected using about 250 mW electrical power. Insertion loss of the isolator is minimized by using waveguide spot-size converter on the silica PLC. 
     Cladding-Pumping with Athermal Pump Lasers 
     According to another aspect of the application, a cladding pumping scheme may be used to achieve uniform population inversion along the doped waveguide cores. The Sagnac interferometer  306  (e.g., sensor) is illustrated in  FIG. 5 , which may be a representation of a cross section taken across line  5 - 5  in  FIG. 3 . As shown in  FIG. 5 , Sagnac interferometer  306  the may include the waveguide loop  321  disposed in a first cladding material  323  interposed between layers of at least a second cladding material  325  having an index of refraction lower than an index of refraction of the first cladding material  323 . As an example, the waveguide loop  321  comprises a rare-earth (RE) doped silica, which may maintain the reciprocity of the loop. Efficient pump geometry is may be used to achieve power consumption management. First, the light from the athermal pump source  320  may be coupled to the cladding layers,  323 . Vertical alignment may be achieved by flip-chip bonding of the athermal pump source  320  on solder stacks whose thickness is precisely controlled by deposition. 
     The numerical aperture (NA) of the pump light of the pump source  320  may be controlled such that the pump light is vertically guided in the cladding layer  323 . Second, the pump source  320  may operate athermally with the output wavelength varying less than 1 nm around the Er/Yb absorption peak over 100° C. temperature swings. The perimeter of the doped Sagnac loop area may form a cavity or trench  322  to trap the pump light. The trench  322  may comprise a metallic mirror coating to form a vertical reflecting surface around the waveguide loop  321 . The shape of the trench  322  may be configured to promote chaotic pump propagation and even distribution of the pump light throughout the Sagnac loop. To uniformly excite all RE-doped waveguide cores, the trajectory of the pump rays in  306 , as viewed from above in  FIG. 3 , is advantageous to be chaotic so that it ergodically covers the loop area. One problem of tracing rays confined in a reflective two dimensional cavity, formed by the reflective trench walls, may be similar to the problem of dynamical billiards in Newtonian mechanics. It is understood in the art that certain geometry, such as a racetrack, promotes chaotic trajectory. Two pump sources  320  may be used for symmetric illumination of the area. With ideal coupling (˜1 dB coupling loss) and negligible potential pump leakage via scattering at the core waveguide walls, or losses at the mirror surfaces, about 80 mW pump power is needed to fully compensate for 2 dB/m passive propagation loss. Assuming about 40% wall plug efficiency of COTS pump lasers, 200 mW electrical power is needed. Any reduction in passive waveguide loss results in commensurate reduction in the pump power requirement. 
     Hybrid Integration 
     According to another aspect of the application, the maturity of silica PLC provides versatile tools for engineering ruggedized athermal I-WOG devices. For example, the symmetry of the power splitting of a Y-branch or directional coupler can be much more accurately controlled in silica PLC than in other platforms such as lithium niobate or III-V materials. In addition, mode size engineering can be reliably achieved with negligible excess loss, which in turn allows facile integration of various micro-optic components, such as isolators and polarizers, whose performance far exceed what can be achieved with waveguide equivalents. Various bonding methods, including solder bonds and hardened epoxy, can be used to achieve ruggedized integration of optical components having performance optimized for the task. In addition, local substitution in the over-clad materials, using polymer or other glasses, can achieve athermal performance in optical delay or power splitting ratio in the couplers. 
     In an embodiment, similar mode expanders also facilitate hybrid integration of the remaining components including the micro-optic polarizer and an array of X-cut lithium niobate (LN) modulators. Again, these components will be solidly bonded to the silica PLC using solder stacks similar to those used for integrating the pump lasers. It is envisaged that while the power split ratio of a silica Y-branch is quite symmetric and nearly independent of the temperature, the hybrid integration of lithium niobate modulators may introduce asymmetry. This can be addressed by various trimming techniques such as controlled scoring of the silica waveguide cladding. 
     LN modulators are subject to phase drift/noise from pyro- and piezo-electric effects. Good modulator design such as the X-cut geometry with buffer and shielding layers can significantly reduce the pyroelectric effect to &lt;1 mrad/° C. The pyroelectric phase variation can be calculated using a temperature monitor or compensated by fast bias adjustments using signal processing, such as the four-phase modulation technique. It is expected the effect of the pyroelectric drift of the LN modulator will be manageable and should not seriously degrade the performance of the gyro. Piezoelectric phase variations are more difficult to cope with owing to potentially high-frequency vibration. experience in modeling the effects of vibration on electronic components in high-shock environments, which will be used for estimating and mitigating the piezoelectric effects. 
     High-Bandwidth Gyroscope Operation 
     Inertial guidance of projectile or UAV that may be subject to rapid rotation may require additional high-bandwidth control; maintaining 1-ppm scale factor stability/linearity over the wide dynamic range up to ±100,000°/sec in the case of guided munition is extremely challenging. The scale factor stability is directly related to the stability of the key system elements, specifically the loop area, the source center wavelength, and the modulator&#39;s V π . The loop area will vary with temperature, but it can be calibrated out by directly measuring the loop temperature. Excessive stress on the coil due to centripetal forces may cause small deformations in the shape if not adequately managed in the mechanical design. The source wavelength stability will be derived from a thin film optical filter properly packaged so as to ensure negligible rotation relative to the waveguide. In addition, the ASE source must be very stable to minimize any spectral power variation over the passband of the filter. Achieving this stability requires a stable pump laser with stable coupling into the waveguide. 
     Achieving both the required sensitivity and supporting ±100,000°/sec axial rotation also means that the gyro must operate over multiple fringes. One option is to directly count the fringes either in open- or closed-loop mode. Alternately, a lower resolution gyro, or even the accelerometers, could be used to evaluate which fringe the high resolution gyro is operating on. Closed loop operation will be used for tip and tilt monitoring during and after the launch. However, careful analysis is required to determine an optimal mode of operation for the axial rotation during the launch, given the system constraints. 
     In an embodiment, a SWAP per axis is: (i) Volume/Weight: ˜0.5 cm 3 /˜1 g; (ii) Power consumption: &lt;470 mW (cladding pump laser: 200 mW, ASE source: 250 mW, Phase modulator driver: &lt;20 mW). 
     While the system and method have been described in terms of what are presently considered to be specific embodiments, the disclosure need not be limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.