Name: in situ measurement of vacuum window birefringence using 25mg+ fluorescence
Uploaded: 2020-06-13T14:00:00
Description: Watch this Scientific Journal Video about In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence at JoVE.com

This work was partially supported by the National Key R&#38;D Program of China (Grant No. 2017YFA0304401) and the National Natural Science Foundation of China (Grant Nos. 11774108, 91336213, and 61875065).

1. Set up the reference directions for polarizers A and B
<ol>
	<li>Put polarizer A and polarizer B into the laser beam (280 nm fourth harmonic laser) path.</li>
	<li>Ensure that the laser beam is perpendicular to the surfaces of the polarizers by carefully adjusting the polarizer holders to keep the back-reflection light coincident with the incident light. 
	NOTE: All the following alignment procedures for the optics components must follow the same rule. The placement of polarizer A and B in the laser path is not...

<ol>
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This manuscript describes a method to perform in situ measurement of the birefringence of the vacuum window and the polarization states of the laser light inside the vacuum chamber. By adjusting the azimuthal angles of the HWP and the QWP (&#945; and &#946;), the effect of the birefringence of the vacuum window (&#948; and &#952;) can be compensated so that the laser inside the vacuum chamber is a pure circularly polarized light. At this point, there exists a definite relationship between the birefringence of the vacuum ...

In many research fields such as cold atom experiments1, measurement of the electric dipole moment2, test of parity-nonconservation3, measurement of vacuum birefringence4, optical clocks5, quantum optics experiments6, and liquid crystal study7, it is important to precisely measure and accurately control the polarization states of laser light.
In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of vacuum windows will affect the polarization states of laser light. It is not feasible to put a polarization analyzer inside the vacuum chamber to directly measure the polarization states of the laser light. One solution is to use atoms or ions directly as an in situ polarization analyzer to analyze the birefringence of vacuum windows. The vector light shifts of Cs atoms8 are sensitive to the degrees of linear polarization of the incidence laser light9. But this method is time consuming and can only be applied to the linearly polarized laser light detection.
Presented is a new, quick, precise, in situ method to determine the polarization states of laser light inside the vacuum chamber based on maximizing single 25Mg+ fluorescence in an ion trap. The method is based on the relationship of the ion fluorescence to the polarization states of the laser light, which is affected by the birefringence of the vacuum window. The proposed method is used for detecting the birefringence of vacuum windows and degrees of circular polarization of laser light inside a vacuum chamber10.
The method is applicable to any atoms or ions whose fluorescence rate is sensitive to the polarization states of laser light. In addition, while the demonstration is used to prepare a pure circularly polarized light, with the knowledge of the birefringence of the vacuum window, arbitrary polarization states of laser light can be prepared inside the vacuum chamber. Therefore, the method is quite useful for a wide range of experiments.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>280 nm Doppler cooling laser</td>
			<td>Toptica</td>
			<td>SYST DL-FHG Pro 280</td>
			<td>Doppler cooling laser</td>
		</tr>
		<tr>
			<td>285 nm ionization laser</td>
			<td>Toptica</td>
			<td>SYST DL-FHG Pro 285</td>
			<td>ionization laser</td>
		</tr>
		<tr>
			<td>Ablation laser</td>
			<td>Changchun New Industries Optoelectronics Technology</td>
			<td>EL-532-1.5W</td>
			<td>Q-switched Nd:YAG laser</td>
		</tr>
		<tr>
			<td>AOM</td>
			<td>Gooch &#38; Housego</td>
			<td>AOMO 3200-1220</td>
			<td>wavelengh down to 257 nm</td>
		</tr>
		<tr>
			<td>EMCCD camera</td>
			<td>Andor</td>
			<td>iXon3 897</td>
			<td>imaging of 25Mg+ in ion trap</td>
		</tr>
		<tr>
			<td>Glan-Taylor polarizer</td>
			<td>Union Optic</td>
			<td>Custom</td>
			<td>distinction ratio 1e-6</td>
		</tr>
		<tr>
			<td>Half waveplate</td>
			<td>Union Optic</td>
			<td>Custom</td>
			<td>made of quartz</td>
		</tr>
		<tr>
			<td>Photon multiplier tube</td>
			<td>Hamamatsu</td>
			<td>H8259-09</td>
			<td>fluorescent counting</td>
		</tr>
		<tr>
			<td>Power meter</td>
			<td>Thorlabs</td>
			<td>PM100D</td>
			<td>laser power monitor</td>
		</tr>
		<tr>
			<td>Quarter waveplate</td>
			<td>Union Optic</td>
			<td>Custom</td>
			<td>made of quartz</td>
		</tr>
		<tr>
			<td>Mirror</td>
			<td>Union Optic</td>
			<td>Custom</td>
			<td>dielectric coated for 280 nm</td>
		</tr>
		<tr>
			<td>Stepper motor roation stage</td>
			<td>Thorlabs</td>
			<td>K10CR1/M</td>
			<td>rotating wave plates</td>
		</tr>
		<tr>
			<td>Vacuum chamber</td>
			<td>Kimball Physics</td>
			<td>MCF800-SphSq-G2E4C4</td>
			<td>made of Titanium</td>
		</tr>
		<tr>
			<td>Vacuum window</td>
			<td>Union Optic</td>
			<td>Custom</td>
			<td>made of fused silica</td>
		</tr>
	</tbody>
</table>

in situ measurement of vacuum window birefringence using 25mg+ fluorescence

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of the vacuum windows will affect the polarization states of laser light inside the vacuum system, and it is very difficult to measure and optimize the polarization states of the laser light in situ. The purpose of this protocol is to demonstrate how to optimize the polarization states of the laser light based on the fluorescence of ions in the vacuum system, and how to calculate the birefringence of vacuum windows based on azimuthal angles of external wave plates with Mueller matrix. The fluorescence of 25Mg+ ions induced by laser light that is resonant with the transition of |32P3/2,F = 4, mF&#160;= 4<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;&#8594;&#160;|32S1/2,F = 3, mF&#160;= 3<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;is sensitive to the polarization state of the laser light, and maximum fluorescence will be observed with pure circularly polarized light. A combination of half-wave plate (HWP) and quarter-wave plate (QWP) can achieve arbitrary phase retardation and is used for compensating the birefringence of the vacuum window. In this experiment, the polarization state of the laser light is optimized based on the fluorescence of 25Mg+ ion with a pair of HWP and QWP outside the vacuum chamber. By adjusting the azimuthal angles of the HWP and QWP to obtain maximum ion fluorescence, one can obtain a pure circularly polarized light inside the vacuum chamber. With the information on the azimuthal angles of the external HWP and QWP, the birefringence of the vacuum window can be determined.

Figure 3 shows the beam path of the experiment. Polarizer B in Figure 3a is removed after angle initialization (Figure 3b). The laser passed through a polarizer, an HWP, a QWP, and the vacuum window, sequentially. The Stokes vector of laser is <img align="center" alt="Equation" src="/files/ftp_upload/61175/61175eq6.jpg" />, where <img align="center" alt="Equation" sr...

Watch this Scientific Journal Video about In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence at JoVE.com

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Presented here is a method to measure the birefringence of vacuum windows by maximizing the fluorescence counts emitted by Doppler cooled 25Mg+ ions in an ion trap. The birefringence of vacuum windows will change the polarization states of the laser, which can be compensated by changing the azimuthal angles of external wave plates.

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum ...

The method presented here, can be used to compensate in a birefringence of vacuum windows, to generate a purely polarized light, inside a vacuum chamber. It can have many applications in proceeding experiments involving cold atoms or ions. The main advantage of this technique is, not atoms or ions are used not only as objects to be started but also, as in-situ detectors of polarization states.So, we can avoid putting a polarization analyzer into the vacuum system. Demonstrating the procedure will be Wen Hao Yuan, a engineer from our laboratory, Zhi Yu Ma and Peng Hao, two PhD students from our laboratory. To set up the reference directions for the A and B polarizers, place the polarizers into the path of a 280 nanometer fourth harmonic laser beam, and carefully adjust the polarizer holders to keep the back reflection light coincident with the incident light, to keep the laser beam perpendicular to the polarizer surfaces.Place a power meter behind polarizer A, and rotate the polarizer to maximize the output power. Record the angle of polarizer A by Stepper Motor Rotation Stage and the power behind polarizer A, by power meter. Next, place the power meter behind polarizer B and rotate polarizer B to maximize the output power.Record the angle of polarizer B, by Stepper Motor Rotation Stage and the power behind polarizer B, by power meter. To set up the reference directions for the azimuthal angles of the wave plates, place a half wave plate into the beam path between the polarizers and rotate the wave plate to maximize the output power. Use the power meter behind polarizer B, to record the rotation angles and the output laser powers.Place a quarter-wave plate into the beam path between the half wave plate and polarizer B, and rotate the quarter-wave plate to maximize the output power. Use the power meter behind polarizer B, to record the rotation angles and the output laser powers. Then remove polarizer B and the power meter from the beam path.Use two mirrors to direct the laser beam into the vacuum chamber that houses an ion trap, to interact with magnesium-25 ions. For Doppler cooling of single magnesium-25 ions, first turned a 532 nanometer ablation laser and a 285 nanometer ionization laser. To make sure only one ion is trapped in the ion trap, look at the image of an Electron Multiplied Charged Coupled Device.Magnesium atoms have three isotopes, so we'll be sure to analyze magnesium-25 isotopes, using proper ionization frequency. Then adjust the HEM holds coil current, to set the magnetic field to 6.5 Gals. To lock the Doppler cooling laser frequency to a wavelength meter, use a frequency counter to skin the frequency of the 280 nanometer laser and the photon multiplier tube, while using a wavelength meter to record the frequency of the laser.When the resonant frequency at which the fluorescence rate reaches a maximum, use a digital servo control program, to lock the laser frequency to the wavelength meter. To set the intensity of the laser to equal the saturation intensity, adjust the driving power of an acousto-optic modulator, to set the power of the laser. Record the power and the fluorescence counts and use the appropriate equations to fit the curve of the power and the fluorescence counts, to obtain the saturation power.Then adjust the acousto-optic modulator driving power, to set the laser power to the determined saturation power. Alternatively, adjust the azimuthal angles of the half and quarter-wave plates close, to maximize the fluorescence counts by hand and record the, azimuthal angles of the plates, at their maximum counts. Then use the Stepper Motor Rotation Stages to rotate the plates and record the rotation angles and the corresponding fluorescence counts.Because magnesium-25 ion has 48 Zeeman levels, analytical solutions cannot be derived from the rate equations. These data can however, be simulated by a numerical program. Here, the relationships between the polarization states and the fluorescence counts under different light intensities are shown, indicating that the polarization state of the light inside the vacuum chamber for this analysis, was greater than 0.999, when the fluorescence counts were maximized.At this position, the fluctuation of the fluorescence count was less than 2%Here, the relationship of the laser power and the fluorescence counts under different D tuning frequencies, is shown. Plotting the data into curves allows determination of the saturated power value at each frequency. By fixing the azimuthal angle of one wave plate, rotating the other, and recording the angles and the fluorescence counts, differences between the theoretical and experimental results, can be assessed.In this analysis, the theoretical and experimental data were closely matched, demonstrating the reliability of the method. When putting the polarizers and the wave plates into laser paths, remember that the laser beam must be perpendicular to the surfaces on each polarization element. As birefringence or window is affected by temperature, we can use this simple and the quick method, to compensate some of effects in real time, for your feedback two wave plates.This method provides measure for obtaining measurements, of birefringence of windows in vacuum based fields, such as optical clocks and experiments.

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