Digital frequency offset-locked He–Ne laser system with high beat frequency stability, narrow optical linewidth and optical fibre output

Abstract

We report on the enhancement of a previously-presented heterodyne laser source on the basis of two phase-locked loop (PLL) frequency coupled internal-mirror He-Ne lasers. Our new system consists of two digitally controlled He-Ne lasers with slightly different wavelengths, and offers high-frequency stability and very narrow optical linewidth. The digitally controlled system has been realized by using a FPGA controller and transconductance amplifiers. The light of both lasers was coupled into separate fibres for heterodyne interferometer applications. To enhance the laser performance we observed the sensitivity of both laser tubes to electromagnetic noise from various laser power supplies and frequency control systems. Furthermore, we describe how the linewidth of a frequency-controlled He-Ne laser can be reduced during precise frequency stabilisation. The digitally controlled laser source reaches a standard beat frequency deviation of less than 20 Hz (with 1 s gate time) and a spectral full width at half maximum (FWHM) of the beat signal less than 3 kHz. The laser source has enough optical output power to serve a fibre-coupled multi axis heterodyne interferometer. The system can be adjusted to output beat frequencies in the range of 0.1 MHz-20 MHz.

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

“…Instead, we decided to implement a laser-array configuration as originally described in [ 72 ]. The laser module of the NPMM-200 was thus extended with a fiber-coupled digitally frequency offset-locked He-Ne heterodyne source consisting of two commercially available He-Ne lasers (model: SL02/1, SIOS Meßtechnik GmbH [ 73 ]) with the same mechanical design and similar transfer behavior [ 74 ]. The basic control concept of this heterodyne source is indicated in Figure 6 and was described in detail in [ 74 ].…”

Optical Sensors for Multi-Axis Angle and Displacement Measurement Using Grating Reflectors

“…By aligning the laser beam to comply with the Abbe principle, the influence of the Abbe error can be minimized [18,20,107,108]. Meanwhile, attention should be paid to stabilize the light wavelength of the laser beam since it is employed as the ruler for displacement measurement in the laser interferometers [109].…”

Ultrastable metrology laser at 633 nm using an optical frequency comb

“…In contrast to off-the-shelf laser systems, the switching power supplies of both laser tubes were upgraded each with a state-of-the-art model (Model: 121T-1700-4.9, Martek Power) and the power cablings were optimized to suppress side peaks and high frequent distortions (fPS > 700 kHz), which provided an instrumentation noise floor level of less than -90 db [34]. Referring to [34], the linewidth of each He-Ne laser light source have also been subject of investigations and was determined to a value of about 1.5 kHz at this setup. In other words, the gas laser represents a wavelength standard with low phase noise and high optical power [1].…”

“…This analog signal is transferred into a discrete signal with TTL level by using a Schmitt-Trigger circuit and represents the input for another FPGA-based control system (referred to as "PXI-system") to lock the metrology laser onto the secondary standard. At this point, the authors refer to previous publications [34,38], in which the developed control system has already been described in detail. The basic method makes use of only a single digital input and one digital-to-analog converter to offsetlock two lasers with a beat frequency fBeat/ML between 0.1 MHz and 20 MHz at our setup.…”