NMI TR 3
Characterisation and Calibration of Wavelength Measuring
Instruments Based on Grating Spectrometers
Dr Philip B. Lukins
First edition — June 2005
Bradfield Road, Lindfield, NSW 2070
PO Box 264, Lindfield, NSW 2070
Telephone: (61 2) 8467 3600
Facsimile: (61 2) 8467 3610
Web page: http://www.measurement.gov.au
© Commonwealth of Australia 2005
CONTENTS
Preface........................................................................................................................... iv
1
Wavemeter ............................................................................................................. 1
1.1 Reference Laser Sources .............................................................................. 1
1.2 Wide-range Calibration ................................................................................ 2
1.3 Medium-range Calibration ........................................................................... 4
1.4 Narrow-range Calibration ............................................................................ 4
1.5 Short-term and Long-term Drift................................................................... 6
1.6 Conclusions .................................................................................................. 6
2
Optical Spectrum Analyser .................................................................................... 6
2.1 Calibration of the OSA ................................................................................ 6
2.2 Calibration of a Multimode Diode Laser – An Example ............................. 7
2.3 Conclusions .................................................................................................. 7
Acknowledgements ........................................................................................................ 7
References ...................................................................................................................... 7
iii
PREFACE
A new laser measurement service is currently being established at NMI. This service
will provide measurement of various laser parameters including power, energy,
wavelength, linewidth, coherence length, pulse parameters, beam divergence and
spatial mode. Customers using this service may require wavelength calibration of
their lasers or calibration of their power meters at precisely defined wavelengths.
Therefore, appropriate wavelength measuring devices, such as wavemeters and optical
spectrum analysers must be used.
This report describes the characterisation and calibration of both a wavemeter and an
optical spectrum analyser, and caters for the need for a traceable calibration of
wavelength for this calibration service. Both the wavemeter and the optical spectrum
analyser are based on grating spectrometers. As we will see in clause 1, some
interesting and important issues arise in relation to the characterisation and calibration
of, in particular, grating-type wavemeters.
iv
1
WAVEMETER
Laser wavemeters are used to measure the wavelength of continuous or pulsed visible or
infrared lasers, and are used widely in laser-based metrology and in high-resolution
applications in industry and research. Accurate calibration at the 0.001 – 0.01 nm or <10 ppm
level is necessary for most applications.
Traditionally, laser wavemeters have been of the interferometric type (eg. Michelson, Fizeau,
Mach-Zender). For this type, calibration at a single wavelength is straightforward and
generally sufficient. However, wavemeters based on high-order grating spectrometers are
now becoming commonplace because of their simplicity, robustness and lower cost.
Both types of wavemeter can suffer systematic errors at the ppm level, and while these are
well-known and well-characterised for interferometric-type wavemeters, the same is not true
for grating-type wavemeters. Unfortunately, wavelength-dependent non-idealities in the
spectrometer and the use of proprietry firmware and optoelectronic designs that are not
transparent in terms of their operation means that accurate evaluation and calibration of this
type of wavemeter is usually not available and would, in general, require calibration at many
wavelengths across the range over which the wavemeter would be used.
As a demonstration of these issues, a commercial grating-type wavemeter (Coherent Inc
‘Wavemaster’ laser wavemeter, serial number WO223 [1]) was calibrated at ~20 laser
wavelengths across the range 399 – 935 nm. The wavemeter is based on a grating
spectrometer operating in a high-order diffraction mode. A laser beam is coupled to the
spectrometer via a 50 m core diameter multimode fibre cable terminated at the instrument
end by an ST connector. The input to the fibre is achieved by a post-mounted probe head
which has a choice of two angular aperture settings and a rotatable fitting housing a 45 flat
silica optic which acts as a beam sampler with a reflectance ~5%. This rotatable beam
sampler allows the laser to be coupled into the fibre at either 0 or 90 relative to the
direction of the laser beam. The spectrometer incorporates a linear diode array as the lightsensing element, a microcontroller-based acquisition and display system, and an internal He–
Ne laser for autocalibration. Firmware is included to calculate the wavelength from the diode
array data, calculate related quantities such as frequency, and to apply corrections to
compensate for the refractive index of air and spectrometer nonidealities. The units displayed
are air wavelength (nm), vacuum wavelength (nm), wavenumber (cm–1) and frequency
(GHz). The manufacturer’s specifications [1] are:

wavelength range
380 – 1095 nm

wavelength resolution
0.001 nm

wavelength accuracy
0.005 nm

laser linewidth
<5 nm

optical input power
20 W – 100 mW
This wavemeter is currently used for laser wavelength calibration, wavelength stabilisation of
laser/sphere sources and wavelength measurement of diode, ion, dye and Ti:sapphire lasers.
1.1
Reference Laser Sources
A Spectra-Physics 165 argon/krypton mixed gas laser was used to obtain the 487.990,
496.512, 501.716, 514.536 and 520.832 nm argon-ion lines and the 476.243, 482.518,
530.866, 568.189, 647.089 and 676.442 nm krypton-ion lines. The blue He–Cd line at
441.565 nm was provided by a Kimmon IK5651R-G laser. Three He–Ne lasers with lines at
632.817, 543.516 333 and 611.970 770 nm were used: the 633 nm line being from a standard
2 mW Spectra-Physics He–Ne laser while the 543 nm and 612 nm lines being from custommade I2-stabilised He–Ne lasers in NMI’s Length Group. Reference wavelength values for
NMI TR 3
1
these lines were obtained from compilations [2–5] of standard laser wavelengths. The
uncertainties in the reference wavelengths were <0.005 pm for the I2-stabilised He–Ne lasers
and <0.5 pm for the other laser lines.
Four further reference wavelengths were obtained by tuning Ti:sapphire and diode laser
systems to resonances of atomic Yb and Yb+ ions in a magnetically-shielded trap in NMI’s
Time and Frequency Group. The three transitions used were [2, 6–8]:

Yb
4f146s2 1S0

4f146s6p 1P1
(398.911 42 nm)
+
14 2

Yb
4f 6s S1/2

4f146s2 P1/2
(369.524 3 nm)
+
14 2
14
3
3

Yb
4f 5d D3/2 
4f 5d6s ( D) [3/2]1/2 (935.186 nm)
The uncertainties in the wavelengths of these transitions are <0.01 pm to <0.3 pm [2, 9–17].
A frequency-doubled CW Ti:sapphire ring laser system (Coherent 899–21) pumped by an
argon-ion laser (Coherent I400–20) was used to produce radiation at four wavelengths :
739.048 2 and 797.822 84 nm (fundamental), and 369.524 3 and 398.911 42 nm (second
harmonic). The 369.524 1 nm radiation was not measured directly because this wavelength is
outside the usable range of the wavemeter, but the relevant transition was still excited by this
wavelength so that the fundamental at 739.048 nm could be used as a wavemeter calibration
point. An extended-cavity diode laser system was temperature and grating tuned to the
935.186 nm resonance.
Preliminary measurements indicate that the wavemeter firmware uses a dispersion relation
for nair() to calculate vac from air. That is, the wavemeter measures air, then calculates nair
for this wavelength then evaluates vac from air = vac / nair().
1.2
Wide-range Calibration
The wavemeter was calibrated at 19 wavelengths across the range 399 – 935 nm which is
almost the whole of its operating range of 380 – 1095 nm. In most cases, air reference
wavelength values were used and so the wavemeter was set to read air. In the case of six
high-precision reference wavelengths which are quoted for vacuum conditions, the
wavemeter was set to read vac. For the measurements using Yb and Yb+ transitions, the trap
is, of course, at vacuum so the transitions are detected in vacuo even though the laser is
measured at ambient conditions. This approach means that a separate refractive index
dispersion correction is not required: this correction is done by the wavemeter firmware and
so is integral to the overall wavemeter accuracy. Reference wavelengths (ref), wavemeter
readings (meas) and differences (ref – meas) are shown in Table 1 and Figure 1. The
wavelengths are also defined as vac or air in Table 1.
Reference wavelengths were chosen that are well away from atmospheric molecular
absorption lines. This eliminates any possibility that absorption and dispersion effects may
change the spectral shape of the transmitted laser beam causing a small shift in the effective
centroid and hence the wavemeter reading.
The scatter in the data points in Figure 1 is apparently random and uncorrelated. This means
that interpolation approaches will not yield an effective improvement in accuracy, nor any
useful estimate of the accuracy over ranges of ~1 nm or more. There is no correlation
between the accuracy of the reference source and the deviation of the wavemeter reading
from the reference wavelength.
The mean difference between reference and measured wavelengths is <ref – meas> = –0.2
pm (standard deviation  = 2.8 pm). These wide-range calibrations showed that the
wavemeter readings deviated from the known reference wavelengths by <5 pm.
NMI TR 3
2
Table 1. Wide-range wavelength calibration of the wavemeter
Reference wavelength, ref (nm)
Measured wavelength, meas (nm)
398.911 42*
441.565
476.243
482.518
487.990
496.512
501.716
514.536
520.832
530.866
543.516 333*
568.189
611.970 770*
632.817
647.089
676.442
739.048 2*
797.822 84*
935.186*
*
398.911
441.569
476.243
482.519
487.986
496.508
501.716
514.530
520.836
530.868
543.515
568.190
611.970
632.817
647.092
676.444
739.050
797.822
935.190
ref – meas (nm)
0.000 42
–0.004
0
–0.001
0.004
0.004
0
0.006
–0.004
–0.002
0.001 5
–0.001
0.000 5
0
–0.003
–0.002
–0.001 8
0.000 84
–0.004
vac values; other wavelengths are air values.
0.010
reference - measured (nm)
0.005
0.000
-0.005
-0.010
400
500
600
700
800
900
1000
Wavelength (nm)
Figure 1. Difference between the reference and wavemeter-measured wavelength
as a function of the reference wavelength
NMI TR 3
3
1.3
Medium-range Calibration
To evaluate the wavemeter over smaller wavelength ranges of ~1 nm, the frequency-doubled
CW Ti:sapphire ring laser system was again used and tuned to the 399 nm Yb transition as in
clause 1.2. In this calibration, the laser was then tuned away from the transition by a single
longitudinal mode (thin etalon mode spacing of 30 GHz) at a time and the fundamental laser
wavelength near 798 nm measured using the wavemeter. The results are shown in Table 2
and Figure 2.
Since the laser mode spacing, and hence the laser wavelength shift, is fixed, the incremental
wavelength difference as given by the wavemeter is a measure of the ability of the wavemeter
to measure small sequential wavelength differences accurately. The mean incremental
difference is 21.0 pm (standard deviation  = 0.8 pm). There is a single outlying data point at
24 pm due to residual wavemeter systematic uncertainties. These medium-range calibrations
indicated wavemeter deviations of typically <2 pm over a range of ~1 nm.
1.4
Narrow-range Calibration
The 399 nm transition in Yb neutrals has a width of ~3 GHz. This transition has six hyperfine
lines due to 171Yb and 173Yb with a splitting of <1 GHz [6]. The laser was tuned to each of
these hyperfine transitions using the fluorescence visible in the trap as an indicator. The six
hyperfine transitions occur at 398.910 – 398.911 nm as measured by the wavemeter
corresponding to a fundamental laser beam at 797.820 – 797.822 nm. These narrow-range
measurements indicated wavemeter deviations of <1 pm over wavelength ranges of a few pm.
798.1
Wavemeter reading (nm)
798.0
797.9
797.8
797.7
797.6
797.5
797.4
797.3
0
5
10
15
20
25
30
35
mode number
Figure 2. Wavemeter readings as a function of the actual wavelength expressed as
consecutive longitudinal laser modes with spacings of 30 GHz
NMI TR 3
4
Table 2. Medium-range calibration of the wavemeter
Laser mode number n
(nm)*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Measured wavelength
(nm)
797.339
797.359
797.380
797.401
797.425
797.446
797.467
797.488
797.509
797.530
797.550
797.571
797.593
797.614
797.635
797.655
797.676
797.697
797.718
797.740
797.760
797.782
797.802
797.823
797.844
797.865
797.887
797.907
797.928
797.949
797.970
797.991
798.012
Incremental difference
(nm)#
–
0.020
0.021
0.021
0.024
0.021
0.021
0.021
0.021
0.021
0.020
0.021
0.022
0.021
0.021
0.020
0.021
0.021
0.021
0.022
0.020
0.022
0.020
0.021
0.021
0.021
0.022
0.020
0.021
0.021
0.021
0.021
0.021
*
Laser mode number (thin etalon) relative to the shortest wavelength used; Yb
resonance occurs at n = 24.
#
(n+1 – n) where is the wavelength measured by the wavemeter and n is the laser
mode number.
NMI TR 3
5
1.5
Short-term and Long-term Drift
The short-term (10 min) drift and reproducibility of the wavemeter was 1 pm for all of the
wavelength measurements.
Possible variations in the calibration of the wavemeter were checked by periodically
recalibrating over a period of 18 months at 476.243, 487.990, 514.536, 530.866, 632.817,
647.089 and 676.442 nm. Not surprisingly, the calibration at 632.817 nm was both stable and
accurate to 1 pm partly because the wavemeter uses an internal He–Ne laser for selfcalibration. For the other wavelengths at which periodic recalibration was performed, the
wavemeter readings also changed by no more than 1 pm over 18 months. These long-term
variations were random, uncorrelated and near the resolution limit. No systematic trend was
seen.
1.6
Conclusions
This wavemeter is accurate to 5 pm over its full operating range of 380 – 1095 nm with no
apparent correlation between uncertainties for wavelengths separated by more than a few nm.
This result confirms the manufacturer’s specification for accuracy [1]. Interestingly, however,
its reproducibility for wavelengths within a band of ~1 nm is typically within 1 pm.
Furthermore, if the wavemeter is calibrated against a reference laser line then the apparent
uncertainty in subsequent wavelength measurements within ~1 nm of this reference
calibration is <2 pm. This demonstrates that enhanced accuracy can be obtained by
calibrating the wavemeter against reference laser lines within ~1 nm of the wavelength with
which the wavemeter would subsequently be used. The origin of this enhanced narrow-range
and medium-range performance probably lies in the wavemeter’s optoelectronic design or
firmware effectively integrating fluctuations over this ~1 nm range.
The long-term drift results suggest that the wavemeter is reproducible to ~2 pm over several
years. For a given wavelength, we therefore recommend a recalibration interval of three to
five years. However, grating-type wavemeters should be calibrated for the specific
wavelengths for which they are to be used or should be calibrated at several wavelengths
across their usable range if they are to be used for broadband measurements.
2
OPTICAL SPECTRUM ANALYSER
While many lasers are single-line sources, there are an increasing number of broadband (eg.
dye, crystalline and diode lasers, and ASE sources) and multiline (eg. diode lasers and some
crystalline lasers such as Nd:YVO4) sources. These lasers cannot be measured directly with a
wavemeter because of their multiline character which would lead to a potentially erroneous
result. This means that an optical spectrum analyser (OSA) is required to measure such
sources. OSAs display the emission spectrum of the source rather than a numerical value of
the wavelength of the laser line as in the case of a wavemeter. Typically, wavemeters have
accuracies ~1 pm while OSAs have accuracies ~10 – 100 pm. Despite their lesser accuracy,
OSAs display the actual laser emission spectrum. OSAs can be calibrated against either
wavemeters or reference laser lines. Here we perform a calibration of an Anritsu MS96A
OSA (serial no. M22691); which has a resolution of 0.1 nm and a sensitivity of ~100 pW
over the range 0.6 – 1.6 m.
2.1
Calibration of the OSA
The OSA was calibrated against known laser lines from a 5 mW 1523.0 nm Melles–Griot 05LIP-171 He–Ne laser and a 100 mW 1064 nm SUWTech DPIR-3100 Nd:YVO4 laser. The
1523.0 nm He–Ne line gave a reading of 1522.95 nm while the 1064 nm laser doublet in
NMI TR 3
6
Nd:YVO4 gave readings of 1064.205 nm and 1064.36 nm. The deviations of the OSA results
from the known laser wavelengths were in the range 0.05 to 0.1 nm.
2.2
Calibration of a Multimode Diode Laser — An Example
We evaluated a 2 mW Fujitsu fibre-coupled single-emitter multiple longitudinal mode diode
laser operating near 1.3 m. It was necessary to measure the laser spectrum to establish an
effective wavelength for InGaAs detector calibrations [18]. The laser spectrum consisted of
13 detectable modes and the wavelengths and peak powers for each of these modes were
measured. An initial estimate of the effective wavelength was obtained by calculating the
power-weighted average wavelength which was 1294.8 nm. The peak powers were then
plotted against wavelength and fitted. The mode distribution was symmetric and gaussian.
The fitted parameters at 21.5C are:

centre wavelength
1294.91  0.12 nm

envelope width
1.95  0.11 nm

mode spacing
0.772  0.022 nm

mode line width
0.04  0.01 nm
2.3
Conclusions
There is agreement between the known laser lines and the OSA to within 0.1 nm which
agrees with the OSA manufacturer’s specification. At this uncertainty level, grating
spectrometer based systems are generally linear so that this uncertainty of 0.1 nm is very
likely to apply across the wavelength range of the OSA of 600 – 1600 nm.
ACKNOWLEDGEMENTS
The author is indebted to Dr Bruce Warrington for valuable discussions and the use of the
Ti:sapphire and external cavity diode lasers, and to Dr Nick Brown for the use of the I2stabilised He–Ne lasers.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
TM
Coherent Wavemaster Users Manual, Coherent Inc. Auburn Division (2000)
W.C. Martin, R. Zalubas and L. Hagan, Atomic Energy Levels — The Rare Earth
Elements, NSRDS-NBS 60, US Government Printing Office (1978)
CRC Handbook, Vol. 2
E. Jaatinen and N. Brown, Metrologia 32, 95 (1995)
T.J. Quinn, Metrologia 36, 211 (1999)
A. Banerjee et al., Europhys. Lett. 63, 340 (2003)
A.M. Martensson-Pendrill et al. Phys. Rev. A 49, 3351 (1994)
M.J. Sellars et al., unpublished data
V. Kaufman and J. Sugar, J. Opt. Soc. Am. 63, 1168 (1973)
J.-F. Wyart and P. Camus, Physica Scripta 20, 43 (1979)
P. Taylor et al., Phys. Rev. A 56, 2699 (1997)
P. Taylor et al., Phys. Rev. A 60, 2829 (1999)
M. Roberts et al., Phys. Rev. A 60, 2867 (1999)
M. Roberts et al., Phys. Rev. A 62, 020501 (2000)
J. Stenger et al., Opt. Lett. 26, 1589 (2001)
S.A. Webster et al., Phys. Rev. A 65, 052501 (2002)
P.J. Blythe et al., Phys. Rev. A 67, 020501 (2003)
P.B. Lukins, CSIRO Technical Report TIPP-1699 (2003)
NMI TR 3
7