Technology for Polarization Measurement

Detector Questions

Modulator Questions

Repair Questions

Detector Questions

What is the difference between photoconductive and photovoltaic detectors?

The difference between these two classifications is that photoconductive detectors use the increase in electrical conductivity resulting from increases in the number of free carriers generated when photons are absorbed (generation of current), whereas photovoltaic current is generated as a result of the absorption of photons of a voltage difference across a p-n junction (generation of voltage).

In general, photoconductive detectors have a higher frequency response, however they also have a higher signal to noise ratio. We recommend using photoconductive detectors if you are using a high-powered laser.

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Modulator Questions

How does the PEM work?

The PEM is a resonant device whose precise oscillating frequency is determined by the physical properties of the optical element/transducer assembly. The electronic head, optical head and the cables that connect them make up a circuit that operates like a crystal-controlled oscillator circuit.

The PEM controller controls the amplitude of the PEM oscillations and generates a reference signal. A feedback signal from the head assembly is used by the controller to monitor the PEM oscillation amplitude and to provide timing for the generation of the reference.

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How Does My Optical Head Work?

The basic principle of operation of the PEM is the photoelastic effect.  A mechanically stressed sample exhibits birefringence proportional to the strain caused by the induced stress. As such, the PEM can be described as a birefringence modulator. Light moves along one axis faster than another because, as a result of resonating the optic, birefringence is induced along the axis that is either compressed or stretched. See Principles of Operation.  Fused silica exhibits the phenomenon that light moves faster along the horizontal component when compressed and slower along the horizontal axis (the axis parallel to modulation) when stretched.

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What are the advantages of PEMs over other forms of polarization modulation?

PEMs have several unique features such as wide spectral range, large aperture, wide acceptance angle, and high precision of phase modulation. The PEM is typically used for fast, high sensitivity measurements. Fused silica PEM optical elements are isotropic when no stress is applied (as contrasted with Pockels cells and electro-optic modulators). This gives the PEM a large acceptance angle.  Click here for more information.

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Are PEMs thermally stable?

PEMs have a feedback circuit to help control thermal stability. In addition, we recommend a 15 minute warm up time for the PEM to reach resonance equilibrium and stabilize before measurements are taken. However, if there is a rapid or large change in ambient temperature, it is possible there will be a slight drift associated with the thermal change. For this reason Hinds Instruments has recently introduced the PEM-ATC, a PEM with Advanced Thermal Control. This optional accessory provides two heaters controlled by a PID controller to make the PEM stable to within 0.5 C which holds the PEM retardation value stable to 0.02%. Please contact Hinds Instruments for more information on this new product.

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How long is the stationary acoustic wave in the optical element?

Photoelastic modulators are resonant devices which produce oscillating birefringence. The optical bar shaped element vibrates along its long dimension at a frequency determined by the length of the bar and the speed of a longitudinal sound wave in the optical element material. In other words, a free standing acoustic vibration mode is used, of acoustic wavelength twice the linear dimension of the modulator element.

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What spectral range do Hinds’ PEMs cover?

Hinds manufactures over a dozen standard PEM models.  Collectively, they cover the range from 130 nm – 57µm.

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I ONLY want right and left circular light. Can you help me?

All PEMs are capable of generating left and right circularly polarized light. In the case of the Hinds I/FS50 PEM, you would be able modulate light in a continuously varying fashion between left-circular and right-circular at a frequency of 50 kHz.

The PEM modulates polarization as the function:
f(t) = A*cos(wt), where w = 2π *50kHz.

The polarization output of the PEM is not a step function but varies sinusoidally in time. The operating frequency of the PEM is nonadjustable, being equal to the resonant frequency of the optical element.

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What are my options for minimizing modulated interference effects (using my PEM with a laser)?

When PEMs are used with lasers as light sources, modulated interference effects may occur. These manifest themselves in the form of spurious signals at the fundamental and harmonic frequencies of the PEM. Modulated interference is not a problem with any light source other than lasers. This effect is caused by multiple reflections of the light beam at surfaces of the PEM optical element. This interference then, in turn, becomes modulated because the two parallel surfaces of the PEM optical element are in relative motion.  Several straight-forward approaches have been developed with the phenomenon of modulated interference.  Please click here for a discussion of solutions.

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What is the maximum laser power I can use with my PEM?

The maximum laser power is determined by the PEM optical material required for your application. If the laser is powerful enough to damage the material from which the optical assembly is made, then it cannot be used with a PEM. This is seldom a problem, however if you have a question about your particular laser, please Contact Us.

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Can I use my optical head with a different controller than it came with?

Yes, but it will require recalibration. The PEM is a resonant device, with the resonant frequency being determined by the size of the optical assembly. In spite of very tight manufacturing tolerances, each optical assembly is slightly different in size and this results in a slightly different frequency. The controller, electronic head, and optical head are all calibrated to work together as one resonant circuit. Changing the controller would change this precise calibration and PEM performance can no longer be guaranteed. If you have further questions, please Contact Us.

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Can a PEM be used to provide a steady retardation level (can they be used as static waveplates)?

As the name would suggest, a PEM provides polarization modulation, that is, a changing retardation that follows a sinusoidal pattern. As such, applications that need a steady retardation level are not well suited for PEMs.

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How close can my electronic head be to a magnetic field?

Based on our testing, we recommend that the magnetic field strength at the electronic head and controller be no more than 100 gauss field. We provide an MFC option for PEMs that includes an 8 ft (244 cm) head to head cable which allows removing the electronic head and controller from a magnetic field.

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Can I use my electronic head with a different optical head?

No. The PEM is a resonant device, with the resonant frequency being determined by the size of the optical assembly. In spite of very tight manufacturing tolerances, each optical head is slightly different in size and this results in a slightly different frequency. The controller, electronic head, and optical head are all calibrated to work together as one resonant circuit. Changing the pairing of the electronic and optical heads would change this precise calibration and PEM performance can no longer be guaranteed. If you have further questions, please Contact Us.

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Can I use my electronic head in a vacuum?

The electronics are not meant to be operated in a vacuum. The optical head itself contains no electronics and is safe to use in a vacuum. We can provide you with longer head to head cables so that the electronic head is not in the vacuum. Please click here to look at Hinds PEM vacuum options.

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Are my dual coax cables interchangeable?

Yes. If your PEM has 2 dual coax cables instead of one blue triax head to head cable, the 2 cables are interchangeable.

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Can I use standard cables in place of the cables that you provide?

The cables that we provide are standard cables. However, the cables are part of the entire PEM resonant circuit. The length of any replacement cables must be precisely matched to the length of Hinds cables in order for the resonant circuit to be maintained. Hinds can provide you with replacement cables that have been designed to work with the PEM.

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Can my optical head be close to an electric field?

Yes. The optical head itself contains no electronic components and is safe to use in an electronic field. However, depending on the strength of the field, it may be advisable for you to have longer head to head cables so that the electronic head can be sufficiently removed from the electrical field so that performance is not affected.

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Repair Questions

My PEM is not providing stable retardation and/or there is noise if I look at my signal on an oscilloscope. What could be wrong?

There are several possibilities. A list of questions to begin troubleshooting is:

  • What is your optical and electronic setup?
  • What is the light source you are using and the wavelength or wavelengths (in particular, are you using a laser)?
  • What detector are you using?
  • When the PEM is operating, is the limit light off, on steady or flashing? If it is flashing, what is the approximate time interval between flashes?
  • What is the operating frequency of the PEM? You may take this from the controller, but if you have a separate frequency meter, this is preferable.

Please Contact Us with the answers to these questions.

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I unplugged my blue cable while the modulator was turned on.

If the blue head to head cable (or either one of the black triax cables for systems that do not have a blue triax cable) was removed during operation of the PEM, there is a good chance that your PEM will need to be returned for repair. Follow the test procedure that is on page I-v of your PEM User’s Manual to see if the PEM is functioning properly. If it is not, email Hinds Instruments for an RMA number to return your PEM system. Return shipping instructions are included in the User Manual. If you are unable to locate this, please Contact Us and we’ll provide the information.

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