detección de problemas

design.pdf

fine adjustment

1’’ lens tube course adjustment 1’’ lens tube

1’’ - 2’’ lens

tube adapter 2’’ lens tube

Interchangable lens _{Interchangable lens}

Telesc ope

rifle sc ope!

Figure 6.4: Design of the Kepler telescope and integration onto the optical breadboard.

The following estimate of optical losses was calculated for a 24km roundtrip with and without the telescope. We were fairly optimistic with these calculations, assuming a refractive index structure parameter of C_n2 = 10−14 was which corresponds to relatively weak turbulence, loss from absorption and scattering was estimated as 0.43dB/km corresponding to a clear day, and the fibre coupling efficiency was calculated as the fraction of the beams power over the initial beams waist which was coupled into the fibre. Fraunhofer diffraction from a circular aperture was used to model the return divergence loss from the corner cube.

24km Round-Trip Estimates of Optical Losses Loss Mechanism without telescope

(dB)

without telescope (dB)

absorption and scattering 10.4 10.4 divergence loss to corner cube 24.4 9.6 divergence loss from corner cube 29.0 12.8

fibre coupling loss 5.4 5.4

loss from splitters 12.0 12.0

total 81.2dB 50.2dB

Rifle Scope

A Target & Varmint rifle scope (10-40x50mm, model: TG104050DS) was also connected to the optical breadboard (as shown in figure 6.4). This was used to initially align the beam within the range of the FSM (±26mrad).

6.3

Results & Discussion

During the field trial a general approach was taken of beginning with short links, and incrementally increasing the links distance after each successful demonstration. Theoretical predictions for the optical loss along channels were reasonably consistent with those measured. There were a number of challenges encountered along the way; One very

time consuming issues was that one of the collimators translation mounts seemed to be also changing the collimators angle when small translations were made. This made the alignment calibration a tedious task and also meant the rifle scope had to be recalibrated a number of times. There were also issues with spurious reflections (primarily from fibre ends) which sometimes compromised the modules ability to stabilise the phase noise, the challenge of isolating these was compounded by the temperament of the atmosphere which fundamentally changed the performance of the phase stabilisation module on a day-by-day basis over identical links. Additionally it was suspected that there was some damage on one of the spliced-in variable optical attenuators used for controlling the signal strength of the 240MHz beatnote (used to generate an error signal). This was suspected due to its unusually high sensitivity to even just touching the attenuator knob, and was potentially incurred from the flights between Perth and Canberra. Since multiple beatnotes were detected on this detector (from various round trips, reflections etc) the performance of the phase stabilisation system was critically influenced by the attenuation applied here. Again, this made the task of optimisation long and tedious.

Despite these we were able to successfully stabilise free space channels all the way up to 600m, with received fractional frequency stabilities on the order of 10−17at 1s, and also able to dither lock the beams pointing for free space channels over 1km. However little testing was done for links over 1km due to placing priority on getting results that demonstrated the system stabilising phase noise induced in the channel. Figure 6.5 shows the stabilisation of the received frequency over a 600m channel by UWAs system. Interesting it was found that results for the frequency stability were significantly better when not using the telescope (despite having higher received powers). This is likely due to coherence radius (ρo) of channel. Atmospheric channels are significantly different to

fibre links in the sense that the phase perturbation incurred in a single mode fibre will be relatively uniform over the wavefront, however in a turbulent channel (especially if the beam size is larger than the coherence diameter) the phase perturbations may be wildly varied over the phase front. The phase stabilisation system can only actively stabilise some mean value of this noise across the wavefront, and therefore when using the telescope it was likely that the rms phase noise across the wavefront was too large and temperamental, and therefore could not be compensated. Doing a back of the envelope calculation for this is quite revealing; under the Rytov plane wave approximation, the spatial coherence can be calculated as a function of propagation length L:

ρo = 0.423k2 Z L C_n2(z)dz −3/5 (6.4)

Assuming reasonable turbulence C_n2 = 10−13 thats constant along a 600m channel provides an estimate of ρo = 6.7mm, i.e. approximately the waist size of the collimator

(7mm) without the telescope. Hence in this case phase perturbations along the channel would be applied relatively uniformly over the part of the beams phase front which is received by the collimator, hence why it could be compensated by the system. Using the telescope, and/or increasing the length beyond this 600m channel effectively decreased the spatial coherence radius below the receiving apertures waist size, and therefore the system had trouble stabilising the relative fluctuations.

§6.3 Results & Discussion 51

Phase stabalised Un-stabilised

Dither locked over 600m channel

Figure 6.5: The fractional frequency for the transmitted 70MHz signal over a 600m channel (without the telescope) when the phase was stabilised and un-stabilised by UWAs module. Dither locking was used for locking the beams pointing over the channel.

mators can be used that are below the coherence radius for the given link length. There will be, however, a practical limit on how small you can go, and also when other effects such as random atmoshperic focusing of beam through the channel begin to play a dominate role. Nevertheless, based on the obtained results, it is reasonable to conclude that UWA’s method of phase stabilisation could be used for free space transfers of stabilised frequency references for relatively short (∼km’s) channels.

On the final day of the field a serious attempt at acquiring the corner cube on Mount McDonald was conducted. During this attempt the atmosphere was quite hazy although the weather was calm and wind speeds were relatively low (measured as 19km/hr at Canberra airport). However in these conditions we were not able to acquire a link to Mount McDonald even when using the telescope. It is hard to assess the reason for this given the short period that it was trialled, it may have been issues with calibration of rifle scope such that the breadboard was not pointed at the corner cube within the range of the FSM, or fundamentally more attenuation along the channel than predicted.

6.4

Field Trial Summary

Despite not being able to acquire Mount McDonald, overall the field trial was considered a success. The dither locking scheme was demonstrated through turbulent free space channels over 1km, and a stabilised frequency transfer was demonstrated using dither locked beam pointing over a 600m channel, acheiving fractional frequency stabilities on the order of 10−17 _{at 1s. For potential future field trials demonstrating coherent phase}

stabilisation techniques, it would be beneficial to have atmospheric data for the location of the trial so enough power can be provided to allow the receiving/transmitting apertures to be designed below the coherence radius. The results from this field trial indicate that with enough power such that suitable receivers/transmitters can be used, this method of stabilised frequency dissemination in free space holds potential for link lengths on the order of a few kilometres.

Chapter 7

Conclusion & Future Work

This thesis investigated a general dither locking scheme for acquisition, tracking and point- ing for applications in free space optics. A transceiver (self beacon) set-up was imple- mented with this dither locking scheme and tested both in the laboratory and during a field trial at Mount Stromlo Observatory. The developed system was capable of locking the beam pointing to murad precision, and successfully tracked vibrations in a corner cube over short link lengths, showing up-to a 40dB suppression of the induced jitter at low (¡20Hz) frequencies. Strong turbulence was also introduced into the channel and our analysis showed that the system had a good capability of tracking the turbulence induced beam wander, even in the presence scintillation. For the field trial at Mount Stromlo Ob- servatory the system was integrated with a phase stabilisation module developed at the University of Western Australia (UWA). The combined system was tested over horizontal free space channels. Due to potentially stronger than predicted optical loss through the atmosphere, and/or poor calibration of a rifle scope which was used for initial (coarse) alignment, we were not able acquire a planned 6km link to Mount McDonald. However, the dither locking system successfully acquired and locked the alignment for free space links over 1km, and with the links alignment locked, the phase stabilisation system de- veloped at UWA was able to demonstrate a stabilised free space frequency transfer with a averaged fractional frequency on the order of 10−17 at 1s. Throughout this work there were a number of potential avenues discussed for future work and applications of the ATP dither locking scheme. Some of these are enumerated below:

1. miscilaneous improvements to the current implementation including additional fil- ters, as well as more comprehensive testing of the scheme under coherent detection.

2. an additional control loop to account for beam ellipticity.

3. integrate with fibre bundles or otherwise to use spatial diversity techniques.

4. incorporate with optical phased arrays or other faster beam steering mechanisms to provide faster dithering and therefore bandwidth.

5. implement on a bi-directional link.

Of these, implementing the dither locking scheme on a bi-directional link would be the most useful in-terms of demonstrating the utility and general applicability of this method.

A recommended design would be two self-beaconing nodes where the dither signal is provided by corner cubes mounted on a rotating donut. In this set-up the received signal at each station would be through the centre of this donut, and thus the size of

the dithers would not influence the received power at each station as was the case in this demonstration. Furthermore, by not dithering the beam, the issues found with the beams ellipticity causing fluctuations at the second harmonic of the dither frequency would be completely mitigated. For below a 10cm radius, dither speeds of over 1kHz would be quite achievable for the donuts which would allow high bandwidth tracking. However a potential trade off with this would be the vibrations induced by the spinning donut. This could be somewhat mitigated through passive vibration isolation. In-regards to absolute pointing precision, a limiting factor may be how concentric the receiving aperture and rotating donut could be made. Therefore precise machining and assembly of the components would be desirable. Due to the additional weight of the rotating platform and associated mechanics, this set-up would be best suited for terrestrial links. Since each node in this set-up receives two signals; a returned beacon and the received signal from the opposite node, it would be recommended that each node transmit at different frequencies. For large frequency differences a dichroic beam splitter could be used to separate the signals, alternatively for nearby frequencies heterodyne detection could be used, where for instance, frequency offset optical local oscillators could be utilised to generate separate beat notes. Signals could then be differentiated post processing. As mentioned in refDither Locking for Acquisition, Tracking & Pointing the returned beacon signal in this case will have both amplitude and phase modula- tion determined by the nodes alignment. Therefore both amplitude and phase signals could be used for actuating on the links alignment. Figure 7.1 shows the proposed set-up.

ωd1 Node 1 D D ωd2 Node 2 Δφ Rotating corner cube

ωb1

ωb2

ωL1

ωL2

Figure 7.1: proposed ATP dither locked scheme for a bi-directional link. Each station is self- beaconing with the dither signals being provided by corner cubes on rotating donuts. The received signal, and the return beacon signal is passed through the centre of the donut to the receiving equipment.

Having nodes transmit signals with a slight frequency offset could also be used to demonstrate a bi-directional, frequency stabilised transfer through free space. The idea

55

would be to have one node carry the master laser, and the other the slave laser such that the slave laser is phase locked to the master. This is essentially the proposed set-up for the GRACE follow on mission [8] where the two lasers forming the satellites interferometer are phase locked relative to each-other. To the authors knowledge no such set-up has been demonstrated through free-space turbulent channels, and based on the results from the field trial this would be an interesting avenue for future research in this field.

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