Ultrastable metrology laser at 633 nm using an optical frequency comb

Abstract

We propose a wavelength standard for highly precise dimensional measurements. An internal-mirror helium-neon laser is offset-locked to a frequency comb line in order to carry out a secondary standard with reduced phase noise and high optical power. Additional lasers can be traced back to this secondary standard, which will enable us to disseminate the accuracy and stability to the metrology lasers of our nanopositioning and -measuring machine, the so-called NPMM-200. First measurements revealed that the stability of the secondary standard is restricted by the time standard of the optical frequency comb to a value of 2.4·10 -12 (τ = 1 s), which is a significant improvement in comparison to the stability of the existing metrology lasers. In further measurements a metrology laser was locked onto the secondary standard with a relative instability of 0.6·10 -15 (τ = 1000 s).

A GPS-Referenced Wavelength Standard for High-Precision Displacement Interferometry at λ = 633 nm

“…The beat signal is detected with an avalanche photo diode (Hamamatsu C5658). Due to the high SNR of the beat signal, a conventional edge-detection method can be used for frequency determination [ 74 , 75 ]. For this purpose, the analog signal from the photodiode is transformed into a TTL-signal via a Schmitt–Trigger circuit and is subsequently fed into an FPGA-based control loop (card: PXI-7853R and chassis PXI-1042Q).…”

“…Therein, the number of counted pulses within a well-defined time frame is compared to a user defined frequency determining the frequency offset between the two lasers. The resulting frequency difference is fed into a digital PID-controller and the digital output signal is converted into an analog voltage [ 74 , 75 ]. The analog voltage is finally transformed into a current by a transconductance amplifier (U/I converter (UIC)) and drives the heater of the second laser tube to adapt its cavity length to follow the frequency of the first laser [ 74 , 75 ].…”

“…The resulting frequency difference is fed into a digital PID-controller and the digital output signal is converted into an analog voltage [ 74 , 75 ]. The analog voltage is finally transformed into a current by a transconductance amplifier (U/I converter (UIC)) and drives the heater of the second laser tube to adapt its cavity length to follow the frequency of the first laser [ 74 , 75 ]. If the first laser is kept in its two-mode frequency stabilization regime, the maximum frequency deviation over one day limits the frequency stability of the heterodyne source to 2 × 10 −9 (see Section 3.2 ).…”

“…The implemented servo loop allows to correctly determine the beat frequency even at a low SNR ≤ 10 dB. The detection algorithm is currently designed with a ± 1-MHz bandpass filter around a fixed center frequency of 62.5 MHz [ 75 ]. The phase data of the input signal is used to generate a frequency variable by subtracting subsequent values over a time slot of 64 ns.…”

Calibration of Displacement Laser Interferometer Systems for Industrial Metrology

“…An even more accurate reference frequency can be obtained using a frequency comb that gives a direct connection to the realization of the second and takes down the uncertainty from the 10 −11 to the 10 −12 level [14]. Although an iodine-stabilized laser is less suitable for using in a displacement laser interferometer directly, it is possible to establish a direct link to a primary standard when operating a laser interferometer [15]. In that case, a calibration as mentioned in this section becomes obsolete.…”