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NANOMETROLOGY COORDINATE MEASUREMENT MACHINES UNCERTAINTIES CAUSED

BY FREQUENCY FLUCTUATIONS OF THE LASER

Jan HRABINA*, Josef LAZAR, Ondřej ČÍP

Institute of Scientific Instruments, v.v.i., Academy of Sciences of the Czech Republic, Královopolská 147,

61264 Brno, Czech Republic, *hrabina@isibrno.cz

Abstract

One of considerable sources of displacement measurement uncertainty in nanometrology systems such as

multidimensional interferometric positioning for local probe microscopy is the influence of amplitude and

especially frequency noise of a laser source which powers the interferometers. We investigated the noise

properties of several laser sources suitable for interferometry for micro- and nano-CMMs (coordinate

measurement machines) and compared the results with the aim to find the best option. The influences of

amplitude and frequency fluctuations were compared together with the noise and uncertainty contributions of

other components of the whole measuring system. Frequency noise of investigated laser sources was

measured by two approaches – at first with the help of frequency discriminator (Fabry-Perot resonator)

converting the frequency (phase) noise into amplitude one and then directly through the measurement of

displacement noise at the output of the interferometer fringe detection and position evaluation. Both

frequency noise measurements and amplitude noise measurements were done simultaneously through fast

and high dynamic range synchronous sampling to have the possibility to separate the frequency noise and to

compare the recorded results.

Keywords:

Interferometry, Nanometrology, Laser noise, Frequency discriminator, Atomic Force Microscopy

1. INTRODUCTION

The main goal of this work is to measure and compare the short-term noise properties of different laser

sources used for powering nanometrology interferometric measurement tools like metrological AFM (atomic

force microscope). Metrological AFM tool consists of the AFM microscope itself and a sample holder (table)

and also a system for precise measurement of coordinates and dimensions of the sample in all six axes of

freedom [1-7]. In case of a design of a nanometrology tool combining local probe (or other high-resolution)

microscope and positioning with position monitoring the problem of traceability is quite important one.

System designed to operate in a laboratory of fundamental metrology should fulfill the demand for

traceability even if it does not represent a significant improvement in resolution of position measurement.

The only answer here is (multiaxis) interferometric system being the technique based on light and thus

reflecting the present definition of the SI-unit metre for measuring the coordinates of the table [8]. Suitable

laser source for interferometric system should have minimal amplitude and also frequency noise which can

influence accuracy, resolution and finally the uncertainty of the whole measurement and also should be able

to supply sufficient power level for powering all interferometers contained in the system.

2. FREQUENCY STABILITY

One of the most crucial conditions for correct operation of nanometrology CMM machines is to have

sufficient frequency stability of the laser source which powers the interferometers. Traditional interferometry

usually uses He-Ne laser which offers visible laser light, low price and relative good long term frequency

stability [9,10,21]. Main advantages of frequency doubled Nd:YAG lasers in comparison to He-Ne are: more

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laser power needed for powering all interferometers, shorter wavelength which means better resolution of

the measured distances and also coincidence with stronger and narrower iodine absorption lines in the 532

nm region of spectra which allow using of linear absorption spectroscopy technique to stabilize the frequency

of the laser [11,12]. The relative frequency stability of DPSSL (diode pumped solid state laser) monolithic

Nd:YAG lasers stabilized by this technique was measured in range of 4x10-7

for an integration time of 100 s

and measurements performed on air conditions (frequency modulation of the laser was done through slow

thermal tuning only) [13-16]. This frequency stability is fully sufficient for measurements which are done on

air conditions – the influnce of the refractive index of air fluctuations is in order of 10-6

. Next, the relative

frequency stability of frequency doubled Nd:YAGs stabilized by some more powerful technique like saturated

subdoppler spectroscopy in iodine vapor can be in range of 10-14

for 100 s integration times [17-25].

We set up an experimental arrangement for simultaneous measuring both amplitude and short-term

frequency noise of several He-Ne and also frequency doubled Nd:YAG lasers. The optical power amplitude

fluctuations were measured directly by a photodetector, frequency noise of laser light was measured with the

help of a passive Fabry-Perot cavity used as an optical frequency discriminator. This cavity contained a

mirror holder equipped with piezoelectric element which allowed tuning of the cavity length. The length of

tuneable Fabry-Perot cavity was in the next step stabilized by slow servo-loop ( ~ 3 s) to the investigated

laser optical frequency so the frequency of the laser matched the middle point of the slope of the resonant

transmission curve of the cavity. In this regime the cavity operated as a frequency discriminator suitable for

fast laser noise fluctuations measurement. The free spectral range of the cavity was 2 GHz and measured

linewidth of the cavity was 25 MHz. Because the output signal of this frequency discriminator contained both

amplitude and frequency noises, the amplitude noise was measured directly by photodetector and frequency

fluctuations investigated through the cavity were recorded simultaneously. This gave us an opportunity to

subtract the influence of amplitude noise from the frequency discriminator output. The cavity was inserted

into an evacuated and thermal-shielded chamber to suppress the influences of refractive index of air

fluctuations.

The opportunity to compare the results of these noise measurements through the Fabry-Perot cavity was

established by extension of the measurement setup with a next part - measurement of the performance of a

length measuring interferometer under the presence of laser noise by investigation of the output signal of an

interferometric unit with homodyne quadrature detection. The interferometer reacts to variations of the input

laser frequency by variations of the phase of the signal on quadrature unit so it can be seen as another kind

of a frequency discriminator. The key advantage of this approach is only negligible influence of amplitude

fluctuations of the laser power to the output signal because the detection system response to these

fluctuations is a few times smaller in comparison to frequency noise. We used Michelson interferometer in

four-pass configuration. The length difference between reference and measurement arm of the

interferometer was 2 meters.

Both measurements (with the help of the Fabry-Perot cavity and also with the help of interferometer) were

done in the same time to have possibility to compare the results. Measured signals were simultaneously

recorded with the digital acquisition card with the 16 bit resolution and 600 kSa/s per channel sampling.

Whole experimental setup schematic is in Fig. 1.

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(Fig. 1) The schematic of experimental setup. PBS-polarizing beam splitter, M-mirrors, Det-photodetectors.

3. EXPERIMENTAL RESULTS

The amplitude and frequency noise was measured for different laser sources (Tab. 1). The amplitude noise

recordings are referenced to optical power of 1mW (0 dB = 1mW), the frequency noise recordings are

referenced to 1 MHz frequency drift (0 dB = 1 MHz). The Innolight Prometheus laser is ultra-stable ring

Nd:YAG intended for metrology applications. It offers “noise-eater” option which means switchable internal

filter of amplitude noise. Measurement with activated and also deactivated filter was done (measurement 1

and 2). ILP/TimeBase laser is ultra-stable diode-pumped ring laser with an intracavity frequency doubling.

Both Innolight and ILP laser are primarily intended for saturated subdoppler spectroscopy in iodine vapor at

532 nm wavelength and are normally used as laser standards [17,22,23]. The Oxxius laser is simple DPSSL

laser with alignment-free monolithic resonator equipped with slow thermal frequency tuning option which

allows linear absorption spectroscopy frequency stabilization technique. Some results of the long-term

frequency stability of this laser stabilized by linear absorption technique are in [13,14,16]. SIOS and Thorlabs

lasers are commonly used single-mode He-Ne lasers. Metra is approximately 25 years old laser, where

worse properties due to degradation of laser tube during long time period can be expected. ISI He-Ne-I2

laser is iodine stabilized standard for 633 nm wavelength. The interferometric measurement of the frequency

noise was not done for laser no. 7 and 8 due to insufficient optical power needed for powering the

interferometer. Correct operation of both frequency discriminators was evaluated by measurement of

frequency modulated laser. The Innolight laser includes tuning option through the mirror holder equipped

with piezoceramic element (PZT). This PZT was modulated by sinusoidal signals of different frequencies (1 –

30 kHz) and amplitudes and results from cavity and also interferometer measurements were compared. The

results for 1st harmonics of modulation show very good correlation (differencies on level of 2 dB (/1MHz)).

(Tab. 1) List of tested laser sources

Meas. no. Description Wavelength [nm] Optical power [mW]

1 Innolight Prometheus 20 532, Nd:YAG with “noiseeater” 20

2 Innolight Prometheus 20 532, Nd:YAG without “noiseeater” 20

3 ILP 1064/532-30/80-3S 532, Nd:YAG 30

4 Oxxius 532S-50-COL-PP 532, Nd:YAG 50

5 Sios SL series He-Ne 633, He-Ne 5

6 Thorlabs HRP008-1 633, He-Ne 0.8

7 Metra TK6 633, He-Ne 0.2

8 He-Ne-I2 ISI 633, He-Ne 0.2

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Amplitude noise measurements (range 0-100 kHz) were done for free-running lasers directly by measurement of optical power with the help of low noise photodetector after few hours of stable operation of all of the laser heads. The best results were obtained with Innolight and also ILP lasers with noise floor level below -70 dBm, respectively -62 dBm. Oxxius laser show LF fluctuations in region of 0-10 kHz, caused probably by acoustic noise due to mechanical vibrations of fan-cooled laser head. In case of He-Ne lasers the best result was obtained with He-Ne-I2 laser head from ISI, with noise floor at -65 dBm and few harmonics (about -55 dBm) especially in region of 35 kHz. The rest of He-Ne lasers show amplitude fluctuations in range of -45 to -50 dBm. We suggest that these fluctuations were mainly caused by using of switched power-supplies and in case of Metra’s laser also by ageing of old laser tube.

The frequency noise measurements (Fig. 2-15) were done through the Fabry-Perot cavity and interferometer

simultaneously to have the possibility of comparison of the results.

Fig. 2-15 Measured frequency noise of investigated laser sources.

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4. CONCLUSION

The amplitude and frequency noise of different laser sources intended for interferometry were measured.

The experimental setup with two frequency discriminators was assembled. First discriminator was

represented by Fabry-Perot cavity with length stabilized through the slow servo-loop to the frequency of the

investigated laser’s optical frequency, the second frequency discriminator was the Michelson interferometer.

Measurements of amplitude and frequency noises were done simultaneously with the help of high-speed

digitalization card. The detection chain contained interfering unwanted signal with 20 kHz frequency which

was not possible to suppress completely, so the frequency components of this outside signal are present in

measurement spectrums. The best results of the amplitude noise performs the Innolight laser Prometheus

and ILP 532 nm standard, in case of He-Ne and 633 nm wavelength the best laser is ISI He-Ne-I2 standard.

All of these lasers are intended to work as laser standards for realization of fundamental etalons of length at

532 and 633 nm respectively in laboratory environment. Their design is more complicated, these lasers are

more expensive in comparison to other tested laser sources. The correct operation of frequency

discriminators was tested with the help of additional modulation of the laser frequency. The results of

frequency measurements show very good correlation between Fabry-Perot measurements and

interferometer measurements. However it can be seen that there are differences especially at low

frequencies of spectrum recordings which are caused by insufficient vibration isolation and acoustic shielding

of the interferometer. The Oxxius laser contains internal servo loops for driving the laser and also a fan for

cooling the laser which caused huge frequency variations in the low frequency range. The worst results of

frequency stability show the laser Metra, mainly due to degradation due to the long history of this laser, old

construction of the laser itself and its power supply too. Next aspect is the resolution limit of the

interferometer and its detection chain, which is higher than the noise of the investigated lasers (especially

less noisy Nd:YAGs). We expect that most of the maximums in the recordings of frequency noise are caused

by switching power supplies of the laser heads. The results show improved performance in frequency

stability for measuring systems with Nd:YAG lasers in comparison to traditionally used He-Ne. This

corresponds to measurements of long-term frequency stabilities which were done before [13,14,16]. Higher

power available with Nd:YAG is not only high enough for feeding all of the interferometers but high power

level incident on the photodetectors improves the noise performance of the detection chain which can be

designed with low gain. Two of the last lasers (Metra and ISI) have insufficient power for powering the

interferometer. The best choice of laser source for multiaxis interferometric laser systems are DPSSL lasers

which are good compromise between frequency stability, purchase cost, and simple construction.

ACKNOWLEDGEMENTS

The authors wish to express thanks for support to the grant projects from the Grant Agency of CR,

projects: GPP102/11/P820 and GA102/09/1276, Academy of Sciences of CR, project: KAN311610701,

Ministry of Education, Youth and Sports of CR, projects: CZ.1.05/2.1.00/01.0017 and LC06007,

European Social Fund and National Budget of the Czech Republic, project: CZ.1.07/2.4.00/31.0016

and Technology Agency of CR, project: TA02010711.

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