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| In order for Absolute Length Measurement to be effective, the differential frequency (beat frequency) between prestabilized laser (PSL) and Auxiliary Laser (AUX) must remain constant. | In order for the Arm Length Stabilization (ALS) system to be effective in its purpose, the beat frequency between transmitted AUX and PSL must be within the working range of ALS. That is to say, up to 150 MHz. Thus, our purpose in Frequency Offset Locking (FOL) is to design a feedback-control circuit that will keep this beat frequency well within the working range of ALS, so that manual tuning of auxiliary (AUX) laser frequencies may be avoided. |
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| However, over time, the AUX frequency tends to wander due to things like temperature change. Thus, we employ our frequency offset locking (FOL) system. Frequency offset locking works by sampling light from each laser source, PSL and AUX, and combining them, in-line, to produce a beat note. The value of which corresponds to the difference between the frequencies of either laser. |
AUX frequency tends to wander due to things like temperature change. Frequency offset locking works by sampling light from each laser source, PSL and AUX, and combining them in-line, to produce a beat note. The value of which corresponds to the difference between the frequencies of either laser. |
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| The error is then converted back to an analog signal, which actuates upon the frequency of the AUX laser, keeping it within the desired range. | The error is then converted back to an analog signal, which actuates upon the frequency of the AUX laser, keeping it within the desired range. The actuator is a temperature control, which controls the dimensions of the crystal resonator within the NPRO via thermal expansion. |
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| The actuator is a temperature control, which controls the dimensions of the crystal resonator within the NPRO via thermal expansion. | Through this scheme, we hope to effectively achieve a controlled frequency offset between the AUX and PSL. |
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| Through this scheme, we hope to effectively achieve a constant frequency offset between the AUX and PSL. |
=== Optics -- Project Plan === |
FREQUENCY OFFSET LOCKING
Overview
In order for the Arm Length Stabilization (ALS) system to be effective in its purpose, the beat frequency between transmitted AUX and PSL must be within the working range of ALS. That is to say, up to 150 MHz. Thus, our purpose in Frequency Offset Locking (FOL) is to design a feedback-control circuit that will keep this beat frequency well within the working range of ALS, so that manual tuning of auxiliary (AUX) laser frequencies may be avoided.
AUX frequency tends to wander due to things like temperature change. Frequency offset locking works by sampling light from each laser source, PSL and AUX, and combining them in-line, to produce a beat note. The value of which corresponds to the difference between the frequencies of either laser.
This light is then fed into a photodiode, the signal from which is digitized, and sent into a digital PID control loop. The PID Controller uses this signal as the current state of the system, which is further analyzed to produce an error signal.
The error is then converted back to an analog signal, which actuates upon the frequency of the AUX laser, keeping it within the desired range. The actuator is a temperature control, which controls the dimensions of the crystal resonator within the NPRO via thermal expansion.
Through this scheme, we hope to effectively achieve a controlled frequency offset between the AUX and PSL.
Optics -- Project Plan
