design of the laser assembly of the prima metrology...
TRANSCRIPT
INSTITUT DE MICROTECHNIQUE UNIVERSITE DE NEUCHATEL Rue A.-L. Breguet 2 Phone: +41 32 718 3211 CH - 2000 Neuchâtel, Switzerland Fax: +41 32 718 3201 http://www-optics.unine.ch
Institute of Microtechnology, Neuchâtel ARP, 8.10.03
Design of the Laser Assembly of the PRIMA Metrology System
Doc No. VLT-TRE-IMT-15731-3154.
Technical representative : Samuel Lévêque
Written by: A. R. Pourzand, Y. Salvadé, O. Scherler
Supervised by: R. Dändliker
Address: Institute of Microtechnology University of Neuchâtel Rue A.-L. Breguet 2 2000 Neuchâtel Switzerland Phone: +41 32 718 3200 Fax: +41 32 718 3201
INSTITUT DE MICROTECHNIQUE UNIVERSITE DE NEUCHATEL Rue A.-L. Breguet 2 Phone: +41 32 718 3211 CH - 2000 Neuchâtel, Switzerland Fax: +41 32 718 3201 http://www-optics.unine.ch
Institute of Microtechnology, Neuchâtel ARP, 8.10.03
Design of the Laser Assembly of the
PRIMA Metrology System A. R. Pourzand, Y. Salvadé, O. Scherler, and R. Dändliker
1 Applicable documents....................................................................................................................... 3
2 Reference documents ........................................................................................................................ 3
3 Acronyms .......................................................................................................................................... 4
4 Introduction ...................................................................................................................................... 5
5 SHG crystal....................................................................................................................................... 8
6 Electro-optic modulator ................................................................................................................... 8
7 Iodine cell ........................................................................................................................................ 10
8 Optical detection............................................................................................................................. 13
9 Laser assembly design proposal ..................................................................................................... 14
10 Lock-in amplifier ............................................................................................................................ 17
11 Regulation scheme .......................................................................................................................... 18
12 Test plan.......................................................................................................................................... 21 12.1 Absolute frequency calibration and stability measurement....................................................... 21 12.2 Fast frequency fluctuations ...................................................................................................... 23
13 Electronics and metrology rack ..................................................................................................... 25
14 Annexes ........................................................................................................................................... 28 14.1 New Focus 4001M EOM ......................................................................................................... 28 14.2 New Focus 3363-B resonant EOM driver................................................................................. 29 14.3 Preview of the optical setup..................................................................................................... 30 14.4 Optical detector and High voltage bias supply ......................................................................... 31 14.5 Power supplies......................................................................................................................... 32 14.6 Metrology rack ........................................................................................................................ 33 14.7 Warning sticker and enclosure ................................................................................................. 34
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1 Applicable documents
[AD1] “Technical specifications snd ststement of the work for the laser assembly of the
PRIMA metrology system”, VLT-SPE-ESO-15731-2852
[AD2] “Feasibility study for the frequency stabilization of the PRIMA metrology laser”,
VLT-TRE-IMT-15731-2868
2 Reference documents
[RD1] “Characterization of the performance of PPKTP for the second harmonic generation
of a 1319nm Nd-Yag laser”, VLT-TRE-ESO-15731-3065 Issue 1.0, 1/8/03
[RD2] “Characterization of iodine transitions around 659nm”, VLT-TRE-ESO-15731-
3064, issue 1, 26/5/03
[RD3] “Analysis of second harmonic generation in KTP”, VLT-TRE-IMT-15731-3006,
11/3/03
[RD4] A. Arie et al., “Iodine spectroscopy and absolute frequency stabilization with the
second-harmonic of the 1319-nm Nd:YAG laser”, Opt. Lett. 18, 1757 (1993).
[RD5] New Focus, “Practical Uses and Applications of Electro-Optic Modulators”,
Application note
[RD6] S. Lévêque, Y. Salvadé, R. Dändliker, O. Scherler, “High-accuracy laser metrology
enhances the VLTI”, Laser Focus World, April 2002.
[RD7] “PRIMA-Metrology, Phase-meter prototype, Progress report V”
[RD8] Nicolas Schuhler, “Second harmonic generation and iodine spectroscopy for the
frequency stabilization of an Nd:YAG Laser emitting at 1.319µm”, DEA report 2003
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3 Acronyms EOM Electro-Optical Modulator KD*P Potassium Dideuterium phosphate KTP Potassium titanyl phosphate CW Continuum Wave PPLiNbO3 Periodically Poled Lithium Niobate AM Amplitude Modulation PM Phase Modulation FM Frequency Modulation SHG Second-harmonic generation MFD Mode Field Diameter NA Numerical Aperture SM Single Mode VSWR Voltage Standing Wave Ratio PRIMA Phase Referenced Imaging and Micro-arcsec Astronomy TBD To Be Defined PID Proportional-Integrator-Differentiator PSD Power Spectral Density ADC Analog to Digital Conversion DAC Digital to Analog Conversion
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4 Introduction
The document reports on the design of the laser assembly based on the technical
specifications and the selection of the most convenient components, according to the work
package description WP3211 [AD1]. The following table summarizes the requirements for
the laser source [AD2]. Comments have been added, to indicate the potential critical aspects.
Aspects Requirements Comments
Wavelength Between 1.1 –1.5 µm to
avoid straylight on existing
stellar photodetectors
No major problems.
Achieved with NPRO
Nd:YAG laser emitting at
1.319 µm
Coherence length > 260 m (maximal optical
path difference)
No major problems. Easily
achieved by commercial
NPRO Nd:YAG laser.
Optical power Given by the power losses
along the VLTI paths, the
losses of the fiber couplers
and AOMs, beam injection
and extraction, as well as the
power required by the laser
stabilization part.
The highest power available
for NPRO Nd:YAG lasers is
200 mW at 1319 nm.
According to the recent tests
performed at Paranal, this
optical power is amply
sufficient.
Frequency stability
(over at least the measuring
time of 30 min)
better than 1.10-8 to achieve
1 nm accuracy over 100 mm.
Corresponds to a frequency
instability ∆ν < 2 MHz
Critical aspect
Wavelength calibration
accuracy
better than 1.10-8
A frequent calibration is
required only if a long-term
stability is not ensured.
Critical aspect
Short term frequency
fluctuations
The corresponding phase
fluctuations must be lower
than 2π/132 (5 nm) for the
highest specified bandwidth,
i.e. 8 kHz
Critical aspect
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The solution consists of using the concepts described by Arie [RD4]. The second-harmonic of
the emitted 1319 nm beam from the laser is stabilized on one of the absorption lines of an
iodine absorption cell around 659.5 nm [RD2]. Saturation spectroscopy is not necessary,
since the accuracy of 2 MHz can be achieved with Doppler-broadened lines. The second-
harmonic is generated by a periodically poled non-linear crystal [RD1][RD3] . Only a few
100 nW of second-harmonic light is sufficient by using a low-noise photodetector and a
synchronous detection such as the FM sideband technique[AD2]. The frequency modulation
is performed by means of an electro-optic modulator. The absorption cell should be slightly
heated (about 60°C) to get an internal pressure of 4 Torr. The frequency repeatability of the
Nd:YAG laser (typically < 1 GHz) should allow to stabilize always on the same absorption
line (for instance, the line at 659.588 nm is separated by 3 GHz from the closest weak
absorption lines).
The laser frequency stabilization on the center of an absorption line is widely used. The FM
sideband technique is probably the most commonly used technique for that purpose. The
principle consists of using the first derivative of the frequency reference transmission as
frequency discriminator. Figure 1 shows the transmission of an absorption line, as well as its
first derivative. The value of the first derivative goes to zero when ν is equal to the center
frequency ν0, and changes sign whenever (ν – ν0) changes sign. Therefore, this is a
convenient error signal for the feedback loop of frequency stabilization. The laser frequency
is thus modulated to obtain an error signal proportional to the first derivative. The laser
frequency νl becomes then
)ft2sin(FMl πν+ν=ν , (1)
where νFM is the frequency excursion and f is the modulation frequency. Assuming a
monochromatic wave, the transmitted light is given by
)(TI)(I linlout ν=ν (2)
where Iin is the intensity of the incident beam. By expanding T(νl) in a Taylor serie around the
average frequency ν, the transmitted intensity becomes to a first-order approximation
[ ])ft2sin()(T)(TI)(I FMin1out πνν′+ν=ν , (3)
where ′ T (ν) is the first derivative of the transmission curve with respect to the laser
frequency. Amplitude and sign of the sinusoidal function at the frequency f is thus
proportional to the first derivative of the transmission function.
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0.95
0.90
0.85
0.80
0.75
-2 -1 0 1 2
Frequency detuning [GHz]
Fig. 1. Transmission of an absorption cell (upper part) and its first derivative (lower part)
The stabilization principle is shown in Fig. 2 in the case of an iodine stabilization technique.
The intensity at the output of the frequency reference is synchronously detected at the
frequency f in order to obtain an error signal proportional to T’(ν). Here, an external
frequency (or phase) modulator is employed to perform the frequency modulation at the
electrical frequency f. From Eq. (3), we can show that the slope of the frequency discriminant
is
SFM = ′ ′ T (ν0 )νFM = −
8C2
∆ν2 νFM , (4)
assuming frequency excursion smaller than the linewidth.
We note that this technique is insensitive to the laser power fluctuations and to the change of
the absorption coefficient of the cell. In addition, the synchronous detection allows to work at
relatively high frequencies (f > 10 kHz), where the 1/f noise of electronic components is not
any more dominant. The expected signal-to-noise ratio is therefore much higher than for the
side-of-fringe locking technique.
Laser I2
PI
PZTT
Mod
Lock-in
Freq. f
Fig. 2. Principle of the FM sideband technique for an iodine stabilization.
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5 SHG crystal
The second harmonic is generated by sending the infrared laser beam in a non linear crystal.
The SHG crystal is defined by ESO. Currently a bulk PP-KTP (from HC-Photonics) has been
tested [RD1]. It generates sufficient second harmonic of the 1319 nm fundamental beam to
provide at least 300 nW required on the stabilization detector as identified in the feasibility
study [AD2]. A PP-LiNbO3 (from HC-Photonics) will be purchased and tested. Based on the
experimental results the optimum crystal will be implemented in the final system.
Meanwhile, ESO has provided the PP-KTP crystal to IMT.
For an optimum efficiency, the crystal must be heated to a temperature of 80 °C. The crystal
will be placed within the oven (model OV03145305). The connection to the oven is made
with a SubD9 male connector. Only 4 pins out of 9 are used, 2 (pins 1 and 2) for the
pt100/RTD sensor (R=110 Ω±5%) and 2 (pins 4 and 5) for the heating wire resistance
(200 Ω<R<250Ω). The temperature controller chosen for the regulation of the oven is a
Newport Omega CN77352-C4. The controller can read the temperature from a pt100/RTD
like the one used in the oven. The regulation can be done with an auto tunable PID used with
an analogic output (0-10V). Unfortunately this output does not furnish enough power to reach
the required temperature. Indeed in order to heat the crystal to a temperature of 100 °C in a
few minutes the voltage should be ~40V (7 W) [RD8]. Hence the analog output of the
controller will be used to command a programmable power supply CUI60.1 from
KNIEL.This power supply can provide up to 60 V voltage and 1 A current, proportional to 0
– 10 V of the temperature controller. The oven will be connected to the front panel of a 3U
19” sub-rack containing the controller and the power supply (see section 13). With the PID,
the temperature stability should be ±0.1°C which is largely enough compared to the
temperature dependence of the conversion efficiency.
The oven will be mounted on a multi-axis stage described. The output of the beam from the
crystal will be monitored using a regular PIN photodetctor (with a dichroic filter cutting the
remaining 1319 nm radiation) in order to find out the optimum conversion efficiency by
alignment of the crystal with respect to the incident beam [RD3].
6 Electro-optic modulator
An electro-optic modulator acts as a phase modulator. Therefore, the phase of the laser light
will be given by
φ(t) = φ0 sin(2πft) (5)
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where φ0 is the phase modulation amplitude. The induced frequency modulation is then
ν(t) = ν + (1/2π) dφ(t)/dt = ν + f φ0 cos(2πft). (6)
The frequency excursion depends therefore on the phase modulation amplitude φ0 and the
modulation frequency f. Assuming a phase amplitude of π (typical value), we must use a
modulation frequency f of 25 MHz to get the 80 MHz frequency excursion. The New Focus
resonant type phase modulating EOM (4001), with a transmission range of 500 – 900 nm,
seems to be the most suitable for our application due to the following assets:
• Compact size
• Original phase modulator
• Low drive voltage for π phase modulation amplitude
• Dedicated driver supplied by the manufacturer
The characteristics of the EOM are shown in the following table:
Manufacturer Model Type Crystal Size (mm) Vπ (V)
New Focus 4001-M Resonant PM MgO:LiNbO3 55 x 38 x 32 10 – 31
Resonant Phase Modulators operate at a single user-specified frequency anywhere in the
range 0.01 to 250 MHz. This device can only be operated at their resonant frequency but
require much lower drive voltages, on the order of 16 volts, to achieve a π phase shift. The
crystal is combined with an inductor to form a resonant tank circuit. On resonance, the circuit
looks like a resistor whose value depends on the inductor’s losses. A transformer is used to
match this resistance to the 50-Ω driving impedance. Putting the crystal in this resonant
circuit results in a voltage across the crystal electrodes that can be more than ten times the
input voltage across the connector. This leads to reduced half-wave voltages and larger
modulation depths compared with broadband modulators.
New Focus 3363-B EOM driver, consisting of a frequency source and RF power amplifier in
a compact package, has been selected to be used with the 4001M resonant modulator. The
driver can provide frequencies from 50 kHz to 40 MHz. An RS232 connection allows to set
and read out the parameters of the driver. The driver signal will be split using a power
divider, one connected to the EOM and the other one used as the reference signal for the FM
sideband detection. According to the manufacturer, there is no limitation on the length of the
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cables between the driver and EOM or the lock-in amplifier as long as the cables are well
shielded, in order to prevent the effects of environmental RF noise.
A limitation when using a phase modulator is residual amplitude modulation (RAM) [RD5].
An ideal phase modulator should not modulate the intensity of an optical beam. Amplitude
modulation will be induced by sources of back-reflection placed after the phase modulator.
Back-reflections result in weak étalons which will alter the harmonic content of the
modulated optical beam by introducing a measurable amplitude modulation component onto
the beam. Unwanted amplitude modulation can be minimized by properly aligning the input
polarization state to the principal axis of the modulator, which is vertical in the case of New
Focus modulators. The RAM can be reduced further by using a collimated optical beam
positioned down the center of the modulator. In practice, the RAM will be monitored by the
712A-2 detector and minimized by aligning the input polarization and the position of the
crystal with respect to the of the incoming beam. The characteristics of the 4001M EOM (M
for metric version) and 3363-B driver are shown in sections 14.1 and 14.2, respectively.
7 Iodine cell
Since no suitable transition line is available near 1319 nm, the use of an Iodine absorption
cell at the second harmonic wavelength as the frequency reference has been selected [AD2].
Arie [RD4] has already stabilized a 1319 nm Nd:YAG laser by locking its second harmonic
at iodine transition near 659.5 nm. As mentioned earlier, the second harmonic is generated by
sending the infrared laser beam in a non linear crystal (PP-KTP or PP-LiNbO3).The
requirement for the Iodine cell are detailed in the following table
Length 150 mm Clear aperture 50 mm Vapor pressure 4 Torr Transmission @ 659.5 nm T > 90% AR coating No Windows Brewster angle Optical flatness λ / 10 Working temperature 60°
A fused silica cell with Brewster windows made at HELLMA (Fig. 3) and filled at
Physikalisch-Technische Bundesanstalt, Braunschweig, Germany (PTB), has been
recommended by Dr. Thalmann from METAS. The cell can be delivered with a calibration
certificate from PTB at 633 nm.
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1 5 0 m m
Ø 8 m m2 5 m m
8 0 m m
~ 1 1 5 m m1 5 0 m m
~ 1 8 5 m m
Fig. 3. Fused cilica cell as described by HELLMA
The cell must be heated to a temperature of 60 °C in order to obtain an internal pressure of 4
Torr. Accordingly, heating bands and sensors manufactured by MINCO have been
recommended by the members of ESO-HARPS, already involved in the heating of a similar
Iodine cell.
Figure 4 shows the housing and the heating mechanism of the PRIMA Iodine cell. The cell
will be enveloped by a thermo-conductor gel foil from FISCHER ELEKTRONIK (0.5 mm
thick). The cell will then be placed within two Aluminum hemi-cylinders which forms a tube
(3 mm thick). The tube will be slightly longer than the cell. Four heating bands will cover the
external surface of the Aluminum tube and an insulating mounting ring will grip the hemi-
cylinders together. An insulating foil will cover the whole tube in order to minimize the heat
dissipation by the Aluminum. The housing will be made and assembled at IMT.
Iodine cel l
Thermoconduc tor ge l
A lumin ium hous ing
Heat ing e lement
Insulator
Iso lat ing mount ing r ing
(a)
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H K 5 4 2 6 R 1 1 . 2 L 1 2 A H K 5 3 9 0 R 4 . 7 L 1 2 AH K 5 4 2 6 R 1 1 . 2 L 1 2 AH K 5 3 9 0 R 4 . 7 L 1 2 A
Senso rS 6 5 1 P D Y 2 3 A (b)
Fig. 4. Cross-section (a), and side view (b) of the Iodine cell mount
Due to the cold-finger arm of the cell, there will be a gap in the middle of the cell where no
heating element is operating. The sensor will be placed in this gap on the aluminum cylinder.
This will result in a gradient of the temperature during the heating process such that, the
temperature near the Brewster windows is slightly higher that in the middle of the cell. This
is very convenient for us, since it will prevent the condensation of the Iodine on these
windows.
The characteristics of selected heating bands are shown in the following table:
Model X (mm) Y (mm) Resistances in Ohm Effective area (cm2)
5426 50.8 177.8 120 60.4 36.5 17.5 11.2 8.3 76.3
5390 38.1 177.8 50.1 25.1 15.2 7.2 4.7 3.5 53.8
These elements are chosen according to the external diameter and the length of the
Aluminum tube of the housing. In order to determine the resistance values of the elements,
we use the basic formula to calculate required power to bring an object to a given
temperature in a given time (warmup power)
( )t
TTmCP ifp −
= (7)
Where m is the weight of the object, Cp the specific heat, t the warmup time, and Tf and Ti
the final and initial temperatures, respectively. Considering a warmup time of 10 min, the
required warmup power has been estimated to be 30 W, at most. With an available voltage of
30 V, the global resistance of the heating elements will be 30 Ω. This resistance can be
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obtained by connecting in series two 5426 model with a resistance of 11.2 Ω, and two 5390
model with a resistance of 4.7 Ω. A voltage controlled power supply (CUI60.1) from KNIEL
which supplies 60V (1A) has been selected as the programmable power source. Newport-
Omega CN-77352-C4 controller will be used in order to stabilize the temperature of the cell
to the desired temperature. The set point and PID parameters of the controller can be set
through an RS232 connection to a PC. The sensor is connected to the controller which
supplies a 0 – 10 V regulation signal at the output. This signal commands the 60 V output of
the CUI60.1 power supply, connected to the heating elements of the Iodine cell. The
controller and the Power supply will be placed in a 3U 19” sub-rack described in section 12.
8 Optical detection
A commercially available photodetector with a voltage sensitivity SVP of 0.7 V/µW and a
noise-equivalent power (NEP) of 2.1 pW/Hz0.5 has been considered as the optical detection
device (Analog Modules, model no 712A-2) [AD2].
Model no. Photodiode Active area diameter Peak optimum Reverse
bias Bandwidth K] WR
Nominal gain Typical noise
712A-2 Si PIN 1mm 900nm +45V (1) 60MHz 0.7V/µW 2.1pW/¥+]
According to [RD1], the available input intensity Iin after the second-harmonic generation
with the tested PP-KTP crystal at ESO is 1.7 µW. Assuming a linewidth of 800 MHz (∆ν)
and an absorption coefficient of 25% (Amax) for the Iodine cell, the voltage sensitivity at the
output of the photodetector will then be
V/GHz 215.0SA8
ISSIS VPFM2max
INVPFMINVF =νν∆
== . (8)
Where νFM is the frequency excursion corresponding to 10% of ∆ν. The detection bandwidth
does not need to be very high, since the cut-off frequency of the regulator does not need to be
higher than 1 Hz. Therefore a cut-off frequency of 100 Hz for the lock-in amplifier is high
enough. For a bandwidth B of 100 Hz, the voltage noise is
mV 015.0BSNEP VPV ==σ (9)
The signal-to-noise ratio of the detected signal for a frequency drift δν is
( ) ( ) ( )
2V
2VF
2V
22VF
ac
S21ft2cosS
SNRσ
δν=σ
πδν= (10)
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The minimal detectable frequency drift δνmin is the value for which SNRac= 1. We find a value
of δνmin = 100 kHz, which is well below the required 2 MHz frequency stability. Note that
even with the critical value of 300 nW discussed in the feasibility report, δνmin still remains
low enough (400 kHz).
An internal bias of +12V is provided within the detector. However the bandwidth decreases
as the reverse bias decreases. For best bandwidth, the use of Model 521 high voltage bias
supply is recommended by the manufacturer in order to apply optimum reverse bias. Internal
bias is protected by diode when external supply is used. The model used for our application is
521-1 with an input voltage of 12 – 15 V and an output of +10 to +300 V. The output voltage
is linearly proportional to the 0 to +5V control input. For positive output units, +5V gives
maximum output and 0V gives minimum output. A +5V internal reference is provided which
will be used through a resistive voltage divider to set the required +45 V bias voltage at the
output (Fig. 5).
Vref
Control
+V
G N DR2
R1
Model 521-1
Fig. 5. Cabling scheme of high voltage bias supply.
The required power supply for the detector and the high voltage bias supply is +15 V. This
voltage will be supplied by a 3U board (KNIEL CK15.06) plugged in a 3U 19” sub-rack
described in section 12. The drawings of the detector and the bias supply are shown in section
14.4.
9 Laser assembly design proposal
The initial design proposed in [RD1] has been slightly modified, such that the traveling beam
through the EOM is collimated. The reason is to avoid windowing effect on a divergent beam
at the input or at the output of the EOM aperture and minimizing the residual amplitude
modulation of the EOM, as recommended by the manufacturer [RD5].
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E O M I2 cel lS H G2 5 %
7 5 %L asph L ach1
Det
L ach2dichroic
filter
powersplitter
laser
do di
O c
λ/2
Fig. 6. Schematic of the optical system for the laser assembly (Lasph: aspheric lens, Lach1 and Lach2: achromate lenses)
The system has then been designed (Fig. 6) regarding following criteria:
• The MFD and NA of the polarization maintaining fibers used in the power splitter from
CIRL, i.e. 9.5 µm and 0.11, respectively (SM PM Fujikura Panda fiber @ 1300 nm).
• The size of the spot inside the SHG crystal providing the best conversion ratio. In the
case of the PP–KTP crystal tested at ESO, this value has been calculated to be 36 µm.
• The aperture of the EOM housing (Ø = 2 mm).
The aspheric lens Lasph (f = 15 mm) will image the core of the fiber at the center of the SHG
crystal. The outgoing beam will then be collimated by Lach1 (f = 40 mm) to match the 2 mm
aperture of the EOM.
A paraxial raytrace estimation shows that the spot size of 63 µm required at the center of the
crystal results in an available space of ~94 mm between the output of the fiber and the center
of the crystal according to the following table.
do (mm) di (mm) spot (mm) Oc (mm) do + di (mm)
19.42 74.40 0.036 2.34 93.82 19.44 74.10 0.036 2.35 93.54 19.46 73.81 0.036 2.36 93.27 19.48 73.53 0.036 2.37 93.01 19.50 73.24 0.036 2.38 92.74
Collimating the output beam from the crystal using a 40 mm achromat lens results in a beam
diameter slightly larger than the aperture of the EOM. This can either be matched by tuning
the position of the achromat, or by choosing a smaller focal length, if the enough space is
available. As mentioned in section 6, the polarization of the beam must be vertical before the
EOM, in order to reduce the residual amplitude noise to a minimum. This will be granted by
a half-wave plate placed before the EOM. The outgoing beam from the Iodine cell will be
focused on the detector surface through Lach2 (f = 20 mm). In order to avoid the saturation of
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the detector by the remaining 1319 nm radiation, a dichroic filter will be placed in the optical
path. The filter will be tilted in order to avoid back reflections. The reflected 1319 nm
radiation from the filter will be blocked by a screen in order to avoid hazardous exposure.
The system will be build on a 800 x 800 mm2 optical breadboard from Thorlabs.
Commercially available Optics and optomechanics are mostly used in the setup. Some
custom mounts will be made at the mechanical workshop at IMT.
Element Model / Characteristics Manufacturer
Single Mode fiber Aligner 9091 New Focus
Aspheric Lens F = 15.4 mm 5726-H-C New Focus
Aspheric hoder Custom IMT
Multi-axis stage 9971 New Focus
Multi-axis stage holder Custom IMT
Base plate 9021 New Focus
Fixed pedestrals 995x New Focus
Pedestral shim set 9950 New Focus
Holding forks 9909 New Focus
Iodeine cell Holder Custom IMT
Achromat lens with holder F = 40 mm AR coated Spindler and Hoyer
Achromat lens with holder F = 20 mm AR coated Spindler and Hoyer
Dichroïc filter λ = 1319 nm Spindler and Hoyer
Breadboard 800 x 800 mm2 Thorlabs
Black enclosure Modified at IMT Thorlabs
A first preview of the final system is shown at section 14.3. The 25% output of the fibre power
splitter will be mounted on a 9091 fibre aligner (X,Y,Z = 3mm, θx, θy = 5°). The SHG crystal
within the oven and the electro-optic crystal of the EOM are mounted on 9071 kinematic
stages and can be aligned to the beam (X,Y = 3mm, θx, θy = 8°). In order to improve the
mechanical stability of the stages, custom massive mounts will be used instead of fixed
pedestrials. A custom designed mount will also be used for the Iodine cell. The overall length
of the system (fiber aligner – detector) is about 650 mm and matches the dimension of the
breadboard. The system will be covered with a black painted enclosure with a sticker which
warns about the danger of the invisible laser radiation (see section 14.7). Holes will be made at
the lower part of the enclosure in order to ventilate the setup and also for cable passthroughs.
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10 Lock-in amplifier
According to the feasibility report [AD2], the modulation frequency of the modulator, hence
the signal on the detector will be 25 MHz. SR844 Dual Phase RF Lock-In Amplifier from
Stanford Research Systems, fulfills this requirement with a frequency range of 25 kHz to 200
MHz. This instrument can be interfaced to a PC through a RS232 bus, which is convenient for
the control system developed at ESO (VME – ISER12). Time constant setting, output channel
configurations and readouts, and automatic phase matching can be performed through RS232.
The instrument can select a Reference Phase that matches the phase of the input signal. This
results in a measured phase of the input signal that is close to zero. The Reference Phase will
not track changes in the phase of the input signal. However the R function always provides
the magnitude of the input signal, even as the phase moves, as long as the phase moves
slowly compared to the measurement time constant. According to the manufacturer, at high
frequencies the difference in path length between the signal and the external reference
contributes large amounts of phase shift. For example, even 1” of difference contributes 6° of
phase shift. For instance, the phase imbalance of a BNC T shape power splitter may be as
high as 3°. The two signal paths must be as identical as possible. 10° phase matching can be
achieved without difficulty.
The SR844 has two analog outputs, CH1 and CH2, on the front panel. These outputs can be
configured to output ±10 V full scale voltage proportional to R, θ, X, and Y which can be
read out by the ESO MPV955 analog data acquisition borad.
R is the amplitude of the input signal. θ, X, and Y are the phase, the In-Phase component, and
the quadrature component of the input signal, respectively, and are given by
IR θθθ −= (11)
( )IRRX θθ −⋅= cos (12)
( )IRRY θθ −⋅= sin (13)
X will be used as the error signal for the regulation. In the case of slow phase changes, only
the gain will be slightly affected, but the integration will maintain the regulation. The
instrument provides time constant from 100 µs to 30 ks determining the detection bandwidth.
The a automatic phase matching order will be sent to the lock-in at the start of each
measurement sequence. A periodic monitoring of X and Y values through the RS232 interface
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
will be used to determine if a new automatic phase matching order should be sent to the
lock-in.
11 Regulation scheme
To achieve the desired accuracy of 5 nm the maximal phase variations must be less than
2π/132. Therefore, the standard deviation must be less than 2π/400. The power spectral
density of the remaining frequency fluctuations must therefore be less than 5x109 Hz2/Hz (or
7.1x104 Hz/Hz0.5) for frequencies f < 8 kHz. The power spectral density shown in Fig. 7
allows therefore to fulfill this requirement. As it can be seen, the cut-off frequency of the
regulator does not need to be higher than 1 Hz.
100
102
104
106
108
10-4 10
-2 100 10
2 104
Frequency [Hz]
Free-running laser Stabilized laser Noise limit
Fig. 7 Required frequency noise spectrum for the stabilized Nd:YAG laser[AD2].
An appropriate feedback loop must be used to get the required psd. As long as the regulator
does not introduce additional noise, the psd of the frequency noise with electronic feedback is
given by
S∂ν(f)with feedback
=1
1+ H(f) 2 S∂ν(f)free − running
, (14)
where H(f) is the transfer function of the loop. As in most regulated systems, PID
(Proportional-Integrator-Differentiator) servo loops is used for the stabilization. The transfer
function of the feedback loop is given by the product of the transfer function of the regulator
with the transfer function of the error signal detector.
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10-3
10-1
101
103
Tra
nsfe
r fu
nctio
n
10-4 10
-2 100 10
2 104
Frequency [Hz]
Fig. 8. Example of transfer function for the feedback loop [AD2]
Figure 8 shows the example of the transfer function of a system composed of an integrator
dominant for frequencies lower than 1 Hz, a proportional stage with a gain of 5 and a
detection bandwidth of 10 Hz. Figure 9 shows the expected frequency noise psd, calculated
from Eq. (14). We see that the noise level for frequencies f < 8 kHz is lower than the required
level of 7.1 104 Hz/Hz0.5. A PI type regulator seems therefore to be appropriate for this
application.
101
103
105
107
109
Freq
uenc
y no
ise
PSD
[H
z/H
z0.5 ]
10-4 10
-2 100 10
2 104
Frequency [Hz]
Expected Free-running
Fig. 9. Expected frequency noise psd [AD2].
The stabilization principle is shown in Fig. 10 in the case of an iodine stabilization technique.
The intensity at the output of the frequency reference (I2 cell) is synchronously detected at the
frequency f in order to obtain an error signal proportional to the first derivative of the
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
frequency reference transmission. Here, an external phase modulator is employed to perform
the frequency modulation at the electrical frequency f.
T Laser EOM I2 cell Det
Lock-in
Freq. ref
ADCPI
PZT
+–
+–
CPU
DAC
DAC
Fig. 10. Principle of the FM sideband technique for an iodine stabilization.
The error signal X described in the previous section will be read out by an analog data
acquisition card and processed though the soft-implemented PI loop. The output of the loop
will be filtered trough a soft implemented low pass (-3dB at 10 Hz) filter in order to drive the
temperature input of the laser driver. The non filtered output will be used to drive the PZT
cavity input of the laser driver. The obtained values will be converted to a ±10 V analog
signal and connected to the appropriate inputs on the laser driver.
Hardware and software used to implement the regulation loop during the test phase at IMT
and for the final system at ESO are listed in the following table
Device IMT ESO
Control unit PIII PC under windows 98 MVME – 2606 64 Mb
Communication interface COM port RS232 VME – ISER2
ADC board NI AT MIO16 E – 2
16 channels, 12 bits, ±10 V
VMIVME – 3123
16 channels, 16 bits, ±10 V
DAC board NI AT MIO16 E – 2
2 channels, 12 bits, ±10 V
MPV – 955
8 channels, 16 bits, ±10 V
Development software Labview + PID toolbox TBD
Since the loop time is below 1ms (10 – 100 ms), PID toolbox will be sufficient to implement
the PID on a PIII computer. No real time interface is needed.
Considering the fast and slow tuning coefficients of the laser and the resolutions of the DAC
boards, the following conversion characteristics are expected from the test and final systems:
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
Tuning resolution
Board Slow (3.8 GHz/V) Fast (2 MHz/V)
IMT : AT MIO16 E – 2 18.6 MHz 9.8 kHz
ESO : MPV – 955 1.1 MHz 600 Hz
The parameters of the loop will be supplied to ESO under the standard form, such that those
can be easily implemented in the software.
12 Test plan
The test plan will consist of three different tests:
1) Calibration and stability measurement of the wavelength (accuracy 10-8)
2) Measurements of the slow frequency fluctuations, i.e. verify that the power spectral
density of the frequency noise is below the required level.
3) Measurement of the remaining 1/fα part of the frequency noise spectrum between 10
Hz and 10 kHz.
12.1 Absolute frequency calibration and stability measurement
The wavelength calibration can be made by comparison with a commercially available
Agilent laser interferometer (model no 5527B). Since the absolute frequency stability of the
Agilent laser is better than 10-8, a calibration accuracy with the same accuracy can be
achieved. The proposed set-up is described in Fig. 1.1 and is the same set-up that was used
for the tests of the PRIMA metrology prototype in a polarizing configuration [RD7]. A
heterodyne interferometer can be made by means of acousto-optic modulators generating a
frequency difference of 450 kHz, as foreseen for the PRIMA metrology [AD1]. This
interferometer can share with the Agilent interferometer a common optical path difference, as
sketched in Fig. 1.1. By comparing the results obtained with the Agilent interferometer and
the results obtained with the Nd:YAG interferometer, we can calibrate the frequency of the
Nd:YAG laser with respect to the Agilent laser wavelength. We propose to use the PRIMA
phasemeter prototype for the detection. The phase measurement accuracy was measured to
be 2π/800 [RD6], which is better than the phase resolution of the 5527B Agilent
interferometer (2π/32). The remaining 650 kHz signals, required by the phasemeter, can be
generated either optically (see Fig. 11) or electrically (by means of a function generator). In
both cases, the same 650 kHz signal must feed the reference and probe inputs of the PRIMA
phasemeter.
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
The polarizing configuration has the advantage to ensure a common optical path. However,
the polarization crosstalks may limit the accuracy. Despite this problem, we could
demonstrate an accuracy of ±13 nm [RD7]. Therefore, a common optical path of 1.5 m is
sufficient to demonstrate the 10-8 accuracy.
The manufacturer specifies a wavelength stability better than ±0.02 ppm (2x10-8) over the
lifetime of the laser, and better than ±0.002 ppm (2x10-9) over 1 hour. The wavelength
stability will therefore remain 2x10-9 during the calibration time. Therefore, the above-
mentioned resolution of 13 nm will be reached. In addition, the Agilent laser head will be re-
calibrated at METAS before the test to ensure the required 10-8 wavelength accuracy.
In order to minimize the so-called “cosine error”, the Agilent laser beam must be superposed
to the Nd:YAG laser beam with a relatively high precision. The angle between the two beams
must be less than 0.1 mrad to achieve the accuracy of 10-8. The alignment between the two
beams can be controlled by visualizing the two spots at relatively large distance (e.g. at 10 m,
the two spots will be shifted by 1 mm for an angle of 0.1 mrad).
This calibration can be repeated several times in different environmental and experimental
conditions, to test the reproducibility of the absolute stabilization.
The set-up described in Fig 1.1 can also be used to check the slow frequency fluctuations, by
recording the phase fluctuations of the Nd:YAG interferometer with respect to the Agilent
interferometer results. Assuming a sampling period of 50 ms for the phase fluctuations, the
power spectral density between 0.1 mHz and 10 Hz can be deduced with a FFT algorithm.
Indeed, the power spectral density of the phase fluctuations Sδφ(f) are related with the power
spectral density of the frequency noise Sδν(f) by the equation
)f(Sc
L4)f(S 2
22
δνδφπ= , (15)
where L is the optical path difference. Note that this technique may be limited by the error
caused by vibrations or by mechanical instabilities that may happen during the delay between
the Agilent interferometer measurement and the phasemeter output. The interferometric
stability should be better than 13 nm during this delay.
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
HP
laser
PBSC C
C C
HP det.
PBS
PBS
ν−40 MHz
ν−39.55 MHz
ν+38.65 MHz
ν+38 MHz
Probe 450 kHz
Ref 450 kHz
Probe 650 kHz
Ref 650 kHz
FC
Fig 11. Absolute stability measurement using an Agilent laser interferometer as a reference.
12.2 Fast frequency fluctuations
The laser frequency noise psd at higher frequency (typ. 10 Hz – 10 kHz) can be estimated
from the measurements of the interferometric phase fluctuations at large optical path
difference (e.g. > 1 km), as explained in [RD7]. Figure 12 shows a possible measuring set-up
for that purpose. The set-up is again based on heterodyne detection. The long optical path
difference is provided by a 1 km fiber delay. The interferometric phase is directly measured
by measuring the phase difference between the reference and probe signals.
Figure 13 shows the power spectral density (PSD) of the phase fluctuations that was observed
for a free-running laser, as explained in [RD7]. Although the measurement is affected by
other contributions such as vibrations or electronic noise, the remaining 1/fα component of
the frequency noise spectrum can be easily seen in this PSD.
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
ν−39.55 MHz
(from AOM2)
ν−40 MHz
(from AOM1) Reference signal
450 kHz
Probesignal
450 kHz
FC
1 km fiber delay
PC
P
P
Figure 12 : Possible set-up for measuring the phase fluctuations at large optical path difference.
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
104
105
106
Phas
emet
er P
SD [
digi
t^2/
Hz]
101
102
103
104
Frequency [Hz]
Individual phase Phase difference
Fig. 13. Power spectral density of the phase fluctuations.
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
13 Electronics and metrology rack
Electronic and computing units will be placed within a 19” cabinet from Knuerr (47U x
600mm x 800 mm). A generic scheme of the elements within the cabinet is shown in section
14.6.
IMT will deliver a 3U 19” format sub-rack (Fig. 14) containing:
− 1 CK15.0,6: the power supply for the detector 712A-2 and high voltage bias supply 521
− 2 CN-77352-C4: the temperature controllers for the crystal oven and the Iodine cell
− 2 CUI60.1: programmable power supplies for the crystal oven and the Iodine cell
− 2 Lemo 00302 connectors for the detector and the bias supply
− 2 Lemo 0B304 connectors for the crystal oven and Iodine cell
Det.
Bias
CN-77352-C4 CUI60.1 CK15.0,6
Oven
CUI60.1
I2 Cell
Fig. 14. 3U 19” sub-rack for power supplies and temperature control
The drawing of the CK15.0,6 and CUI60.1 are shown in section 14.5. Two Lemo 0B302
connectors on the front panel of the sub-rack will supply the +15 V for the detector and the
bias supply. Two Lemo 0B304 connectors will be used to read out the thermistor values and
supply the drive voltage to the heating elements of the crystal oven and the Iodine cell.
The free cables from the temperature sensor and the heating elements of the Iodine cell, and
also from the voltage entries of the 712A-2 detector and the 512 Bias supply will be welded
to corresponding Lemo connectors on a panel on the breadboard (Fig. 15).
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
Det. BiasI2 Cell
B readboard Breadboard
Side v iew Front v iew
Fig. 15. Breadboard side connector panel for the detector, bias supply and Iodine cell
The elements of the optical system (breadboard) are connected to the electronic cabinet as
following:
Breadboard connectors Electronic cabinet L (m)
Laser head SubD9 – SubD9 Laser driver 5
Oven SubD9 – 0B304 IMT 19” sub-rack 5
I2 cell 0B304 – 0B304 IMT 19” sub-rack 5
EOM 4001 SMA – BNC EOM driver 3306 – B 5
Detector 712A-2 (signal) BNC – BNC Lock-in amplifier SR844 5
Detector 712A-2 (power) 00302– 00302 IMT 19” sub-rack 5
Bias supply 521 (power) 00302 – 00302 IMT 19” sub-rack 5
The units within the cabinet are also connected to each other as following
Electronic cabinet connectors Electronic cabinet L (m)
Controller CN-77352-C4 RS232 – RS232 ISER12 board 2
Controller CN-77352-C4 RS232 – RS232 ISER12 board 2
Laser driver RS232 – RS232 ISER12 board 2
EOM driver 3306 – B RS232 – RS232 ISER12 board 2
Lock-in amplifier SR844 RS232 – RS232 ISER12 board 2
Lock-in amplifier SR844 BNC – Distribution box VME-WMI-3123 2
EOM driver 3306 – B BNC – BNC Lock-in amplifier SR844 2
MPV-955 Distribution box – BNC Laser Driver (T ) 2
MPV-955 Distribution box – BNC Laser Driver (PZT ) 2
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
The following table shows the known power dissipation amount of the components on the
breadboard and within the electronic cabinet
Component Model Location Power dissipation (W)
Oven OV03 Breadboard 7
I2 cell I2 cell Breadboard 20 – 30
EOM 4001M Breadboard 1
Laser head Laser head Breadboard 20 – 30
EOM driver 3306 – B Electronic cabinet 50
Laser driver Laser driver Electronic cabinet 100
Power supply CK.15.0,6 Electronic cabinet 10
Power supply CUI60.1 Electronic cabinet 60
Temperature controller TC 038 Electronic cabinet 4
INSTITUT DE MICROTECHNIQUE UNIVERSITE DE NEUCHATEL Rue A.-L. Breguet 2 Phone: +41 32 718 3211 CH - 2000 Neuchâtel, Switzerland Fax: +41 32 718 3201 http://www-optics.unine.ch
Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14 Annexes
14.1 New Focus 4001M EOM
50 mm
Wavelength 0.5-0.9 µm Type Resonant PM Operating Frequency 0.01 to 250 MHz Modulation Depth (at 1 µm) 0.1-0.3 rad/V Maximum Vp (at 1 µm) 10-31 V Material MgO:LiNbO3 Maximum Optical Intensity (in a 1-mm beam)
4 W/mm2 (647 nm)
Aperture 2 mm RF Bandwidth 2-4% freq. Connector SMA Impedance 50 Ω Maximum RF Power 1 W VSWR <1.5
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14.2 New Focus 3363-B resonant EOM driver
Maximum RF Output Power 30 dBm (typical)
Output Power Range 0-30 dBm
Max. Modulation with Resonant EOM 3 rad (typical)
Ideal for EOM Models 4001, 4003, 4103
Compatible with EOM Models 4002, 4004, 4104
Frequency Range 0.05 - 40 MHz in 0.001 Hz Steps
Frequency Stability ± 1.5 ppm
Frequency Adjust Range Fixed at Factory*
Frequency Lock Indicator Yes
Max. Output VSWR 2 : 1
Output RF Connection SMA
Reflected RF Monitor 16 V/Vrms (typical)
Reflected RF Connector BNC
External Oscillator Input Level 10 dBm (maximum)
External Oscillator Input Connector SMA
Disable Input Standard TTL Logic,High to Disable
Disable RF Attenuation 45 dB (typical)
Disable Connector BNC
AM Modulation Input 0-5 V (5 V produces max. RF output)
AM Modulation Range 0-30 dBm
AM Modulation Connector BNC
Option Connector RS-232, DB-9 Female
Power Requirements 100-250 VAC, 50 W (max.)
*User-adjustable via option port in rear (inquire about accessible frequency range)
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14.3 Preview of the optical setup
9071
M
9071
M
9071
M
9071
M
9952M
9952M
98
34
M9
83
4M
99
52
M9
95
2M
99
52
M9
95
2M
9952M9952M
9952M
9952M
1
fiber
alig
ner
Asp
heric
f=11
mm
Ove
n+
SH
G c
ryst
al+
kine
mat
ic s
tage
Ach
rom
atf =
40
mm
EO
M+
kine
mat
ic s
tage
I2 c
ell
+cus
tom
hol
der
Lens
dete
ctor
50 m
m
98
34
M 99
52
M
Z
Y
Z
YX
X
Sid
e vi
ew
To
p v
iew
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14.4 Optical detector and High voltage bias supply
Analog Module 712A-2 optical detector:
Analog Module 521-1high voltage bias supply:
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14.5 Power supplies
2
5
8
11
14
17
20
23
26
29
32
30.48 mm
128.
4 m
m
CK15.0,6
Front panel Rear panel
2
5
8
11
14
17
20
23
26
29
32
101.6 mm
128.
4 m
m
CUI60.1
Front panel Rear panel
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Institute of Microtechnology, Neuchâtel ARP, 8.10.03
14.6 Metrology rack
IR PSD
0
1
2
3
5
6
7
8
9
1 0
1 1
1 2
1 3
1 5
1 6
1 7
1 8
1 9
2 0
2 1
2 2
2 3
2 5
2 6
2 7
2 8
2 9
3 0
3 1
3 2
3 3
3 5
3 6
3 7
3 8
3 9
4 0
4 1
4 2
4 3
4 5
4 6
4 7
2 4
4 4
3 4
1 4
4
AO Driver
AO Driver
Phasemeter
LCU lpmm
LCU lpma
SR844
EOM Dr iver Laser Driver
Fans
Fans
Fans
Temp. controllers / Ampli / Power supplies
Laser E O M I2 cell Det Bias
1000 mm
800 mm
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14.7 Warning sticker and enclosure