ROMY: A Multi-Component Ring Laser for Geodesy and Geophysics

1 Single-component ring lasers have provided high-resolution observations of Earth’s rotation rate as well as 2 local earthquakeor otherwise-induced rotational ground motions. Here we present the design, construction, 3 and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical 4 Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 5 m side length provide rotational motions that can be combined to construct the complete vector of Earth’s 6 rotation from a point measurement with very high resolution. Combined with a classic broadband seismometer 7 we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local 8 and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific 9 design and construction details are discussed as are the resulting consequences for permanent observations. We 10 present seismic observations of local, regional, and global earthquakes as well as seasonal variations of ocean11 generated rotation noise. The current resolution of polar motion is discussed and strategies how to further 12 improve long-term stability of the multi-component ring-laser system are presented. 13

each corner exiting through a viewport at the back of the vacuum enclosure. To establish a closed optical path 112 an external alignment laser is injected into the cavity. All corner boxes can be rotated and tilted gently to obtain 113 lasing. Note the bottom installation of the three corners, fixed to a rigid base plate attached to the concrete 114 basement connected to the bedrock. This gives a rigid geometrical reference for the ring laser orientation. 115 The (temporal) stability and noise level of each ring laser component depends strongly on keeping the 116 triangular geometry as rigid as possible. The G-ring (Schreiber et al., 2009a) is very stable because the entire 117 body of the interferometer is built as a monolith from Zerodur, thermally almost a zero-expansion material. 118 To apply the same design to ROMY would be prohibitively expensive, therefore we applied a heterolithic 119 approach, where a solid concrete foundation provides the geometrical reference. One of the corner boxes in 120 each ring laser component is adjustable by a piezo actuator in order to compensate thermal expansion. Utilizing 121 an active control of the optical frequency in the cavity will eventually make ROMY a virtual monolithic 122 structure. 123 In principle, three ring lasers would be sufficient to reconstruct the Earth's rotation vector and observe the 124 complete rotational ground motion. While three sub-horizontal triangular cavities are enough to reconstruct 125 the full Earth rotation vector, the final design included an additional interferometer in the horizontal plane, thus 126 providing the vertical component of rotation additionally. This allows us to directly compare observations with point in the easterly and westerly direction respectively. This achieves nearly equal projections on the Earth rotation axis (see Fig. 1a). 136 3 ROMY: Construction 137 ROMY is a highly sensitive rotation sensor, which is operated in a strap-down configuration. This means that 138 the ring laser structure has to be rigidly attached to the Earth's crust in order to guarantee that the recorded 139 rotations in fact represent the ground motion. A design goal is the reliable detection of rotation rates of less 140 than 1 prad/s in all three spatial directions. With an arm length of 12 m for each of the sides of the tetrahedron, 141 this sets high requirements for the mechanical monument structure. At the same time it requires a careful 142 procedure for the construction process itself, in order to ensure as little ground settling motions as possible. 143 Furthermore, excavations had to be reduced to a bare minimum in order to maintain the overall terrain stability. 144 In the first phase, the seamless integration of the concrete monument into the local terrain took place (Fig. 2a). 145 This provided a rigid mounting platform for the beam lines of the laser interferometers. Since the scale factor 146 of the gyros depend on the size of the enclosed area, the size had to be as stable and as large as possible. 147 Therefore it was important to make the concrete support massive. embankment with a shotcrete reinforcement, anchored to the surrounding terrain by long bolts, 3) construction 155 of a massive concrete structure from bottom to top rigidly supporting the inclined and horizontal beam lines of 156 the laser cavities, and 4) adding a large circular access shaft at the center and smaller vaults at each top corner 157 and halfway between them. This provides the necessary service access points for the alignment of the laser 158 cavities and the gain sections of the laser excitation.

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Due to the fact that the mirror supports are on two different floor levels, approximately 10 m apart in 160 height and that the entire structure has to be stable to within a few wavelengths (≈ 3 µm), it was required that 161 the entire soil structure around the ROMY monument had to be left intact as far as possible. Removing and 162 subsequently refilling large quantities of soil for a larger building structure would destabilize the terrain and 163 gives rise to a subtle and continuous creep motion over many years until the soil has compacted again. For the 164 installation of a laser interferometer this is clearly not adequate. Since the surrounding terrain was supported 165 by a strong retaining wall during the excavation process, the creep of the terrain could be minimized. When 166 the terrain was refilled after the integration of the monument, care was taken to properly compact the refill 167 material. The construction phase took approximately six months. The final installation is illustrated in Fig. 2a 168 and also shown in Hand (2017), supplemented by a video on youtube (https://youtu.be/MXYV6wNdZm8).

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This video also contains a compressed account of the entire construction phase. 14 m deep, where three corners are joined together (Fig. 2d). Angled granite support structures carry the 174 mirror holder boxes (Fig. 2c), while the corner boxes for the horizontal ring are directly bolted to the concrete 175 monument. The location of the corner boxes define the physical size of the structure and the corners are 176 joined by stainless steel pipes to form the beam enclosure. Short bellows near the corner boxes (Fig. 2c) reduce deformations from strain and make sure that the mechanical rigidity of the corner construction is not 178 compromised. In the middle of the uppermost side of each triangle, there is a 5 mm wide and 20 cm long 179 capillary for laser excitation (Fig. 2b). The width of the capillary also acts as a spatial mode filter and has been 180 designed to minimize the loss for the desired transverse TEM 0,0 laser mode. Higher-order transversal modes 181 with a larger mode volume, however, are discouraged by increased loss. There are no additional Brewster 182 windows or other loss increasing components anywhere inside the cavity. In fact, there are only the three 183 curved super mirrors (radius-of-curvature = 12 m) as interacting intra-cavity components with a specified total 184 loss of approximately 12 ppm per mirror (scatter, transmission and absorption) in the system.

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Since the entire beam path is enclosed by an UHV (ultra-high vacuum) compatible enclosure (pipe and 186 mirror box housing), the resonator can be evacuated and then filled with a mixture from (0.2 hPa) neon and 187 (6.3 hPa) helium. Lasing is achieved by radio frequency excitation. Figure 3 depicts the basic sensor concept. In order to obtain a stable beat note, the laser beam power in the cavity has to be stabilized. A small portion 202 of the light leaking through one of the mirrors is detected and amplified by a photo-multiplier. The resulting 203 voltage is then fed back to drive the power of the radio frequency transmitter such that the laser radiation in the The principle of ring lasers and the history have been well documented in recent review papers (e.g., Schreiber 213 and Wells, 2013). We focus here on the essential aspects. to an inertial frame of reference the gyro is frequency degenerate and the beat note between the two counter-219 propagating waves disappears. However, when the gyro is rotated, the effective co-rotating cavity becomes 220 slightly longer, while the anti-rotating cavity gets shorter by the same amount. The laser oscillation responds 221 by adjusting the optical frequency of each sense of propagation to fit an integer number of wavelengths within 222 the cavity, a necessary condition to satisfy laser coherent amplification. This means that the rotation rate ex-223 perienced (Ω) around the normal vector n on the laser plane is strictly proportional to the frequency splitting 224 (δf ) of the gyro: where A is the area circumscribed by the beams, λ the wavelength and P the perimeter of the gyro contour.

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The inner product accounts for the projection of the axis of rotation on the normal vector on the laser plane.

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Since ROMY has the shape of an inverted tetrahedron, each ring has a different projection angle to the Earth's 228 rotation axis. Each ring laser is operated at low beam powers of approximately 20 nW in order to obtain a stable interfer-231 ogram. After mixing the two counter-propagating laser beams in a beam combiner, the beat note is detected by 232 a photo-multiplier through the application of a trans-impedance amplifier, then digitized. The resultant wave-233 form of all four rings is digitized at a rate of 5 kHz by a 24-bit digitizer unit (Kinemetrics Obsidian System).

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The analog-to-digital processing flow is described in the section 5.1. knowledge that strategies exist to stabilize these effects in a second construction phase (see Discussion). normal vector is increasingly aligned with the Earth's rotation vector, thus giving better resolution.

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Given the tetrahedral ROMY setup we have to deal with carrier frequencies between 300-554 Hz (Table 1) 277 with the highest frequency signal originating from the horizontal ring (vertical normal). In order to allow a 278 precise and broad-band rotation rate signal reconstruction, these carrier frequencies have to be sampled with    the Allan deviation σ(τ ) can be calculated as follows: withȳ k (τ ) as the k-th average value of the time series y of length τ and denoting the average over all k along

Earth's Rotation
ROMY has twice the scale factor (the proportional factor in Eq. 1) of the G ring laser. So it is sensitive 342 enough to measure variations in the rate of Earth rotation, provided that the stability of the entire installation 343 can be improved to take the knee of the Allan deviation of Fig. 8 down to 1 part in 10 9 of Earth's rotation.

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Expressed differently, a laser gyro for Earth rotation monitoring has to resolve a rotation rate of less than

Regional Seismic Event
An example of a regional seismic event at a distance of around 1500 km that occurred in Turkey September 26, 405 2019 with a magnitude M w 5.7 is shown in Fig. 11. The processing and graphical representation is identical to 406 the teleseismic event. The data have been bandpassed in the interval [0.01−0.2Hz]. The waveform fit between 407 appropriately rotated acceleration and rotation rate signals is less pronounced than in the teleseismic case. Due 408 to the higher frequencies involved we expect stronger effects due to non-planar wavefronts and scattering in 409 general. There is also consequently more scattering of the back-azimuth that has highest correlation and there 410 seems to be a systematic shift away from the true backazimuth in both SH and P-SV type setups (except for 411 some time windows with very high correlations).

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While the Rayleigh wave phase velocity estimates in the time windows with high correlations (e.g., t = 610 s) 413 are comparable with those for the teleseismic event, the Love wave phase velocity estimates (Fig. 11g), are 414 estimated at the lower end of the correlation scale and are questionable.