design of the laser assembly of the prima metrology...

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



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

3


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

5


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

6


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.

7


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.

8


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)

9


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

10


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)

12


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

13


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)

14


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].

15


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

16


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.

17


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

18


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.

19


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

20


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


DAC board NI AT MIO16 E – 2


MPV – 955


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:

21


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.

22


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.

23


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.

24


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.

25


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).

26


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

27


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



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

29


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)

30


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

31


14.4 Optical detector and High voltage bias supply

Analog Module 712A-2 optical detector:

Analog Module 521-1high voltage bias supply:

32


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

33


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

34


14.7 Warning sticker and enclosure

design of the laser assembly of the prima metrology...

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