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| Fig. 1. Simple Diagram of an Acousto-Optic Modulator [3] | Fig. 2. Acousto-Optic Modulator Example |
By Andrés David Cimmarusti candres@umd.edu
Four neutral density attenuators were tested, using a 780 nm diode laser, for polarization dependence and interference effects due to changes in ordering. This was done by directing polarization-tunable laser light through each attenuator (and combinations of 2 to probe for interference effects) to be then collected by a power meter detector. The results indicate the reflective type attenuators tend to have the most pronounced polarization dependence as well as power loss due to interference effects.
Measurements of pulse rise, fall time and signal delay were carried out on an amplitude modulated Te02 acousto-optic modulator. The laser passed through the AOM and was then focused on a detector. Using a fast oscilloscope, comparison between signals from the detector and the pulse generator allowed the measurement of the desired quantities. The rise and fall times where very close as those reported by the AOM manufacturer. However, the delay was cut in half from the initial measurement by carefully directing the beam as close as possible to the AOM transducer.
As part of the improvements being done on the cavity QED experiment, four neutral density attenuators were tested for polarization and interference problems. An ideal neutral density attenuator is simply a filter that absorbs light equally on all wavelengths. They are characterized by their optical density ND and the type of attenuating material in use (reflective or absorptive). In this case, three attenuators (Labeled A, C, D) were of the reflective type, leaving only one absorptive (Labeled B). The fractional transmittance can be calculated as follows: 10-ND [1].
Secondly, an acousto-optic modulator (AOM) was studied. This device roughly consists of a crystal (in this case TeO2) placed between an absorber and transducer material. The transducer is nothing more than a piezoelectric material which can be controlled to generate a sound wave in the crystal. This wave causes areas of compression and rarefaction in the crystal [2], thus inducing a roughly sinusoidal variation of the index of refraction (see fig. 1). This variation causes incoming light to difract. There are two regimes of operation, one is called the Raman-Nath difraction and the other bragg difraction. Most devices operate in the bragg regime, and this case is no exception.
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| Fig. 1. Simple Diagram of an Acousto-Optic Modulator [3] | Fig. 2. Acousto-Optic Modulator Example |
The first order difracted beam is not only bent but its frequency changes according to the sound wave frequency. To see this, consider the laser as a stream of photons, and the sound waves in the crystal as phonons, we can apply conservation of momentum and energy to obtain the following expressions for the first order :
where the +/- sign corresponds to the following geometries:
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| Fig. 3. Addition of wave vectors: frequency up-shift (left) and down-shift (right) |
A 780 nm diode laser was used. It had a temperature controller that was set at 12 degrees Celsius and a current controller set at 100 mA. This produced a beam of roughly 63 mW of power. The experimental setup for the attenuator test consisted of a couple of mirrors, used to steer and properly align the laser beam. The partially polarized beam then passed through a PBS (Polarizing Beam Splitter) and later a half wave plate before arriving at a power meter (See fig. 4).
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| Fig. 4. Setup for attenuator testing |
Firstly, each attenuator (see table 1 for specs) was tested individually. That is, the power of the transmitted laser beam was measured while rotating the half wave plate in intervals of 20 degrees. Naturally, before testing the attenuators, the power meter was tested for polarization dependence. This was done by switching off its internal attenuator (which was also tested and found to have a polarization dependence) and placing one of the attenuators (B) to be tested before the PBS.
Secondly, the attenuators were placed in all possible pairs before the power meter, and discrepancies in power readings due to ordering were recorded.
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| Table 1. Specifications of the Neutral density attenuators |
The second experimental setup was put in place for the study of the AOM. The same laser was used as well as the steering mirrors. The beam was focused by a lens (f = 15 cm) into the AOM crystal. Then, another lens (f = 17.5 cm) was used to further focus the light to be able to effectively filter the first order diffraction using an iris. Finally, a short-focal-length lens (f = 6.5 cm) was used to converge as much light as possible into the detector (see fig. 5 and/or 6). The output of the detector was connected to Channel 1 of an oscilloscope. The AOM's piezoelectric transducer was controlled by an RF driver that needed a constant 24 V. The driver permitted sound wave frequency modulation via a tunable DC voltage (which we set to 0 V), and amplitude modulation induced with the help of a pulse generator (the duration of the pulse used was roughly 1 µs and an amplitude of 3 V). This junction was connected to channel 4 of the oscilloscope for comparison purposes.
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| Fig. 5. AOM experiment setup |
The following table contains several important specifications about the AOM and its RF driver. These devices were manufactured by Crystal Technology, Inc. AOMO 3080-122 (part #: 97-01280-01) and AODR 1200AF-AEF0-1.0 (part #: 97-02207-17)
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| Table 2. Specifications of the AOM and its RF driver |
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| Fig. 6. AOM table setup |
The raw data collected for each attenuator including the power meter was averaged. A percent difference in power was calculated from the average, then plotted vs the half wave plate angle. See below for the results:
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| Fig. 7. % Difference in power transmitted versus half wave plate angle (power meter check) |
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| Fig. 8. % Difference in power transmitted versus half wave plate angle (attenuator A) |
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Fig. 9. % Difference in power transmitted versus half wave plate angle (attenuator B) |
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| Fig. 10. % Difference in power transmitted versus half wave plate angle (attenuator C) |
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| Fig. 11. % Difference in power transmitted versus half wave plate angle (attenuator D) |
All the above results pertain the polarization effects. On the other hand, the interference effects were quantified, by taking averages of power readings for differing orders in the attenuator placing. These averages were then substracted, and a percent difference found. The table below summarizes these results.
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| Table 3. Inteference effects in attenuators | |||||||||||||||||||||||||||||||||||||||
Turning to the AOM experimental setup, with the use of the oscilloscope we were able to trigger on channel 4 (original signal) and thus measure the rise and fall time of the signal from the detector and its delay with respect to the original. Below are the snapshots of the oscilloscope screen:
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| Fig. 12. Rise time (39.85 ns) |
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| Fig. 13. Fall time (41.60 ns) |
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Fig. 14. Rise delay (1.143 µs) |
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| Fig. 15. Fall delay (1.144 µs) |
Identifying that the delay is caused by the time it takes for the signal to propagate through the crystal from the transducer to the location of the beam, we were able to cut in half this delay, by directing the beam much closer to the transducer. The snapshots of the oscilloscope can be observed below:
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Fig. 16. Improved rise delay (562.1 ns) |
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| Fig. 17. Improved fall delay (563.4 ns) |
By careful inspection of the fig. 7 - 11, it is apparent that attenuator B and the power meter have no polarization preference. The fluctuations in power for these two cases can be explained by laser fluctuations (which were very common). D showed very little dependence (0.20 %). On the other hand, the almost sinusoidal dependence on the angle exhibited by A and C and their high fluctuations from mean (5 % and 1.5 %, respectively) are clear indications of their dependence on polarization.
Once again, attenuator B performed best when probing for interference effects. Table 3 shows that the percent difference was smallest when B was present. It is also worth noting that when C was included, the differences were very high. All these results coincide with the fact that B is the only attenuator that is absorptive.
The results for the first experimental setup indicate the absorptive type attenuator is best for avoiding unwanted polarization and interference losses in power (at 780 nm). Nevertheless, it seems a good idea to probe with a more stable and powerful laser, for one, to be able to probe combinations of at least 3 attenuators and secondly, to avoid power fluctuations as much as possible.
The rise and fall time achieved were satisfactory (the goal was to obtain less than 50 ns). This quantity depends on the beam waist, and thus it is difficult to lower it without sacrificing difraction efficiency. The detailed spec sheet for the AOM, states that for a 830 nm laser, and a beam waist of 250 µm, a rise time of 41 ns can be achieved at 80 % efficiency. This closely resembles our results.
The delay was successfully reduced to half its value, by reducing the distance between the transducer and the laser beam. Initially, the beam was at about 5 mm from it. Using the speed of sound in the crystal (see table 2) together with this distance, one can estimate the delay to be of about 1.2 µs, which was our initial delay. Once again, our results coincide with what we expected.
There was a slight discrepancy between rise and fall time. Perhaps this was due to the not completely homogeneous nature of the beam, or to some dust present in the optics. Also, a shorter rise and fall time, might have been achieved by a more collimated and focused beam which could have been obtained by the use of an optical system designed for this purpose.
The notorious transient obtained for the signal coming from the pulse generator (see fig. 14 -17) was, most likely, due to a signal reflection at the junction (see fig. 5). The junction, consisted of a simple T-connector, when it should have also had 50 Ohms resistors on each branch, to equalize the impedance and thus prevent a signal reflection.
I would like to thank David Norris for all his help and guidance in the lab, Prof. Luis Orozco for his help, for suggesting the project and for the opportunity to take this course and Elohim Becerra also for his assistance.