Noise Eater Operation
Thorlabs’ Liquid Crystal Noise Eater is a precision instrument for stabilizing and attenuating laser power. The noise eater consists of a variable attenuator (liquid crystal wave plate and polarizer), a calibrated beamsplitter, and a servo controller to control the modulator, as depicted in the block diagram to the right.
Linearly polarized light is input into the liquid crystal retarder, which, together with the output polarizer, acts as a variable retarder. A beamsplitter then sends a small part of the beam to a feedback loop consisting of a photodiode and control servo. The servo compares the optical signal to a preset signal level and applies the appropriate adjustment voltage until the optical signal reaches the desired level.
The noise eater can also be used as a variable attenuator, even without the presence of noise. By adjusting the resistance of the potentiometer, the user can set the desired output power level.
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The Noise Eater can be post mounted in two different orientations to match the input light's direction of polarization. A CRM1
Cage Rotation Mount and four ER2
Cage Rods can mount a half waveplate
for fine tuning the polarization alignment.
Mounting and Alignment
The noise eater is designed to work with linearly polarized input light aligned with the direction of the arrow engraved on the noise eater near the input aperture. Linearly polarized light and proper alignment of the direction of polarization are important for achieving the best results from the noise eater.
In order to minimize optical losses, the noise eater does not have an input polarizer. If the incident light is not linearly polarized, a linear polarizer (such as our LPVIS or LPNIR polarizers) before the noise eater will polarize the incident light.
If the incident light is linearly polarized but is not aligned exactly vertically or horizontally, a half-wave plate can be used before the noise eater to rotate the polarization axis. As shown in the photo to the right, the noise eater’s cage mount can be used along with a CRM1 cage rotation mount to rotate the half-wave plate, thus aligning the polarization axis with the noise eater.
For post mounting, the noise eater is equipped with two 8-32 (M4) threaded holes. These holes are offset by 90° so that light with a vertical or horizontal polarization axis can be aligned with the noise eater. The four 4-40 holes on the front of the noise eater can also be used to mount the noise eater in either a horizontal or vertical orientation using the Thorlabs 30 mm Cage System.
For best performance of the noise eater, it is recommended that the beam is well centered in the input aperture. Due to the optical path inside the noise eater, the output beam will be shifted down by 1.0 mm if the noise eater is mounted vertically.
Selecting the Power Range
The selection switch at the top of the noise eater is used to select the input power range. The power selector should be set to the lowest value that is still higher than the actual power of your laser. For example, if you are using the LCC3111 and your beam power is 8 mW at 635 nm, set the selector to 10 mW.
The LCC3111 and LCC3112 noise eaters use a Silicon detector as part of the feedback loop, while the LCC3113 uses a Germanium detector. The responsivity of the detectors is different for different wavelengths, and so the power settings on the selector only correspond to the design wavelength of the detector (635 nm for LCC3111, 780 nm for LCC3112, 1550 nm for the LCC3113). The power range at a given wavelength is inversely proportional to the responsivity (a higher responsivity value will result in a lower power range value). The graph to the right shows the relative responsivity of both detectors at a range of wavelengths. The tables below show a rough estimate of the power settings at various wavelengths for each model.
The noise eater operates by varying how much of the signal is attenuated in order to reach the target output power and attenuate the noise. Since the noise eater can attenuate the signal but not amplify it, the clean output beam can only have a power as high as the minimum power level of the noisy signal. In practice, to remove all noise without unnecessarily attenuating the signal power, the output power level should be set to slightly lower than the minimum power of the noisy signal. See the operating manual on the Documents & Drawings tab for more information.
|LCC3111 Power Ranges at Various Wavelengths|
|Power Level at 450 nm*||Power Level at 550 nm**||Power Level at 635 nm|
(Marked on Noise Eater)
|2 mW||1.5 mW||1 mW|
|6 mW||4.5 mW||3 mW|
|20 mW||15 mW||10 mW|
|60 mW||45 mW||30 mW|
*For 450 nm light, the values will be ~100% higher than marked.
**For 550 nm light, the values will be ~50% higher than marked.
|LCC3112 Power Ranges at Various Wavelengths|
at 650 nm
at 700 nm
|Power Level at 780 nm|
(Marked on Noise Eater)
at 900 nm
at 1000 nm
at 1100 nm
|1.2 mW||1.1 mW||1 mW||0.9 mW||0.9 mW||3.3 mW|
|3.5 mW||3.3 mW||3 mW||2.6 mW||2.7 mW||10.0 mW|
|11.8 mW||11.1 mW||10 mW||8.8 mW||8.9 mW||33.3 mW|
|35.5 mW||33.3 mW||30 mW||26.3 mW||26.8 mW||100.0 mW|
| LCC3113 Power Ranges at Various Wavelengths|
at 1050 nm
at 1150 nm
at 1250 nm
at 1350 nm
at 1450 nm
|Power Level at 1550 nm|
(Marked on Noise Eater)
at 1620 nm
|1.9 mW||1.6 mW||1.4 mW||1.2 mW||1.0 mW||1 mW||1.6 mW|
|5.8 mW||4.8 mW||4.1 mW||3.5 mW||3.1 mW||3 mW||4.8 mW|
|19.2 mW||16.0 mW||13.7 mW||11.7 mW||10.4 mW||10 mW||16.0 mW|
|57.6 mW||48.0 mW||41.1 mW||35.1 mW||31.3 mW||30 mW||48.0 mW|