ISOMET RFA2120 Series User manual

Feb 2020
Acousto-Optic Modulator Driver
Including: Basic Modulator Alignment
for M1340 series
Instruction Manual
RFA2120 Series
Models -
RFA2120-x, (RFA2121-1-25) : 120MHz, > 25W output
RFA2120-L-x (RFA2121-1-12) : 120MHz, > 12W output
Options -x:
- V : 0-5V analog modulation range
- A : analog modulation only. No RF gate
- D : digital modulation only. No RF gate
- R : coolant fittings on rear face
ISOMET CORP, 10342 Battleview Parkway, Manassas, VA 20109, USA.
Tel: [phone], Fax: [phone], e-mail: [email protected]
www.ISOMET.com
ISOMET (UK) Ltd, 18 Llantarnam Park, Cwmbran, Torfaen, NP44 3AX, UK.
Tel: [phone], Fax: [phone], e-mail: [e-mail]
ISOMET

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1. GENERAL
The RFA2120 combined Analog Driver and Power Amplifier is a fixed frequency RF power source
specifically designed to operate with Isomet acousto-optic devices such as the M1340 series. The
driver accepts two independent digital and analogue modulating signals and provides a double-
sideband amplitude modulated RF output to the acousto-optic modulator. A summary of the driver
specification is shown in the following table:
Model Use With Center Frequency Output Power
RFA2120 M1340-aQ120-5 120MHz 12.0 Watts at 355nm
RFA2120 M1340-aQ120-7 120MHz 17.0 Watts at 355nm
Figure 2 is a functional block diagram of the driver. The center frequency of the driver is determined
by the free-running quartz-crystal oscillator at 120MHz. This frequency is accurate to within ±
25ppm and its stability is better than ±25ppm; the oscillator is not temperature stabilized.
A high-frequency, diode ring modulator is used to amplitude-modulate the RF carrier. The single turn
potentiometer provides gain control for adjusting the maximum r-f power at the Driver output.
A solid state switch provides the Digital Modulation or RF Gating function. A TTL high level will gate
the RF ON. The MMIC r-f pre-amplifier stage isolates the low level modulation and control circuitry
from the power amplifier stage.
The rise and fall times for the amplifier from either modulation input is identical (approx 200nsec).
The video analog input level must not exceed 15 volts
The digital input level must not exceed 7 volts
The RF modulation functions are shown in Figure 3

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This amplifier is designed to operate at full rated power into a 50Ωload with 100% duty cycle.
Water cooling is mandatory. The heatsink temperature must not exceed 70°C.
SERIOUS DAMAGE TO THE AMPLIFIER MAY RESULT IF THE TEMPERATURE EXCEEDS 70°C.
SERIOUS DAMAGE TO THE AMPLIFIER MAY ALSO RESULT IF THE RF OUTPUT CONNECTOR
IS OPERATED OPEN-CIRCUITED OR SHORT-CIRCUITED.
A low impedance d-c power supply is required. The operating voltage is +24V or +28Vdc at a current
drain of approximately 4 A. The external power source should be regulated to ±2% and the power
supply ripple voltage should be less than 200mV for best results.
Higher RF output power is achieved at 28Vdc.
The output power level is set by the power adjust potentiometer (PWR ADJ)
NOTE : Maximum power = fully clockwise

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2, CONTROL
Two inputs directly control the RF output; Gate and Modulation.
The Gate response time (tgm, Fig1) is approximately 200nsec
The RF Modulation response time is < 50nsec
The relationship between the driver control inputs, the RF waveform and AO response is shown below
for analog modulation type.
Time
Analog
Modulation
Input
(0 - 10V)
AM Modulated
RF Output
Zero Order
Control Range
Max Laser O/P
0 W
Ton
Vmod
Minimum
(not 0)
Maximum
Maximum
(= Laser O/P)
First Order
Control Range
0 W
Minimum
FIRST ORDER
ZERO ORDER
RF
tgm
GATE
DRIVER INPUTS
POT Set Level
Figure 1: Typical Modulation Waveforms

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Gate (active high enables the RF amplifier)
TTL compatible input
Default condition is RF Off.
A high level (1.7V < V < 5V) will gate the RF ON.
A low level (0V < V < 0.8V), or not connected will gate the RF OFF.
Analog Modulation (0 - 10V)
Provides high speed proportional amplitude control of the RF output.
Lower limit: an input voltage of less than 0.4V will drive the RF Off.
Upper limit: an input of 10V will result in the maximum RF output,
(For RFA2120D type, modulation control is On:Off and requires a 5V CMOS / TTL compatible input).
The amplitude level is defined by the selected RF power adjustment POT.
RF Power Adjustment (POT set level)
The maximum RF power limit is set using a manual adjust multi-turn potentiometer ‘PWR ADJ’.
Maximum RF power = fully clockwise

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3 LED INDICATORS
The front panel LEDs serve to indicate a number of possible operating states.
The LEDs [C] and [D] illuminate when the DC power is applied and the Interlocks are valid.
•[A] Shows GREEN when the RF output is live PROVIDED:
a) the modulation duty cycle is more than 20% (approx).
b) the RF CW power is > 20% (approx) of the driver maximum power
•[B] Not applicable – for future options
•[C] shows RED when the DC power is applied
•[D] shows GREEN when the (thermal) Interlocks are enabled
Caution, the RF output may be live even if these LED’s are not illuminated.
LEDs Off
The LEDs [C] and [D] will not illuminate if :
a) the internal driver thermal interlock switch is open (Over temperature fault)
b) the AOM thermal interlocks switch is open (Over temperature fault)
c) the AOM thermal interlock is not connected to the driver interlock input
d) the DC supply is off.
The thermal interlocks will reset once the AO device and / or RF driver are cooled below the switching
temperature.
- The driver thermal switch over temperature is 50deg C
- The AOM thermal switch over temperature is 35deg C
The hysterisis of the thermal switches is 7-10deg C.
Once in a fault state the coolant temperature may need to be reduced to reset the thermal switches.
Power
Adj Pot
B C
A D
LED's

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4. INSTALLATION AND ADJUSTMENT
4.1 Connect cooling water to the RFA2120
Connect cooling water to the AO device.
Due to the high RF power dissipated in the AO modulator, it is paramount that the
device is operated only when water cooling is circulating.
For optimum AO performance ensure the flow rate is not less than 0.5 litres/minute
at < 25 degC
4.2 With no d-c power applied, connect the + 24V (or +28V) DC in to the center terminal of the
feed-thru terminal. DO NOT APPLY POWER.
4.3 Connect the RF output BNC jack to the acousto-optic modulator (or a 50ΩRF load, if it is
desired to measure the modulator RF output power).
4.4 Connect the Interlock of the acousto-optic modulator (SMA, SMC or mini 3pin connector) to
the enable inputs on the 9-pin D-type connector of the RFA. Connect pin 4 of ‘D’ to the centre
pin of the SMA/SMC and pin 5 of ‘D’ to the outer ground of the SMA.
The interlock connection becomes open circuit, disabling the RF output if the temperature of
the modulator exceeds 35ºC or the internal driver temperature exceeds 70ºC. LED indicator
illuminates when the Interlocks are closed and the RF is enabled (see Section 3). In addition,
a CMOS ‘interlock valid’ signal output is provided on pin 1 of the D-type connector for remote
monitoring purposes.
4.5 Adjustment of the RF output power is best done with amplifier connected to the acousto-optic
modulator. The Amplifier maximum output power is factory preset to approx 9W.
The optimum RF power level required for the modulator to produce maximum first
order intensity will be different at various laser wavelengths. Applying RF power in
excess of this optimum level will cause a decrease in first order intensity (a false
indication of insufficient RF power ) and makes accurate Bragg alignment difficult. It
is therefore recommended that initial alignment be performed at a low RF power level.
4.6 Locate the PWR ADJ access on the driver end plate.

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4.7 With an insulated alignment tool or screwdriver rotate the PWR ADJ potentiometer fully anti-
clockwise (CCW) , then clockwise (CW) approx 1/4 turn.
4.8 Apply DC to the amplifier.
4.9 Apply a 10.0V constant signal to the Analog modulation input on the D-type connector.
Connect pin 8 of ‘D’ to the signal and pin 9 of ‘D’ to the signal return.
4.10 Apply a constant TTL high level to the digital Gate input on the D-type connector.
Connect pin 6 of the ‘D’ to the signal and pin 7 of the ‘D’ to the signal return.
Input the laser beam toward the centre of either aperture of the AOM. Ensure the polarization is
vertical with respect to the base and the beam height does not exceed the active aperture height of
the AOM.
Start with the laser beam normal to the input optical face of the AOM and very slowly rotate the AOM
(either direction). See Figure 4 below for one possible configuration.
4.11 Observe the diffracted first-order output from the acousto-optic modulator and the undeflected
zeroth order beam. Adjust the Bragg angle (rotate the modulator) to maximise first order
beam intensity.
4.12 After Bragg angle has been optimized, slowly increase the RF power (rotate PWR ADJ CW)
until maximum first order intensity is obtained.
4.13 The driver is now ready for use for modulation using both the digital and the analog inputs.

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5. MAINTENANCE
5.1 Cleaning
It is of utmost importance that the optical apertures of the deflector optical head be kept clean and
free of contamination. When the device is not in use, the apertures may be protected by a covering of
masking tape. When in use, frequently clean the apertures with a pressurized jet of filtered, dry air.
It will probably be necessary in time to wipe the coated window surfaces of atmospherically deposited
films. Although the coatings are hard and durable, care must be taken to avoid gouging of the surface
and leaving residues. It is suggested that the coatings be wiped with a soft ball of brushed (short
fibres removed) cotton, slightly moistened with clean alcohol. Before the alcohol has had time to dry
on the surface, wipe again with dry cotton in a smooth, continuous stroke. Examine the surface for
residue and, if necessary, repeat the cleaning.
5.2 Troubleshooting
No troubleshooting procedures are proposed other than a check of alignment and operating
procedure. If difficulties arise, take note of the symptoms and contact the manufacturer.
5.3 Repairs
In the event of deflector malfunction, discontinue operation and immediately contact the manufacturer
or his representative. Due to the high sensitive of tuning procedures and the possible damage which
may result, no user repairs are allowed. Evidence that an attempt has been made to open the optical
head will void the manufacturer's warranty.

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RFA 2120 Standard Version
Connection Summary
1.0 ‘D’ Type Control Connection
Signal Type Pin out connection
Digital Modulation / GATE Input Signal pin 6
TTL high (>1.7V) = ON Return pin 7
TTL low (<0.8V)
or no connection = Off
Analogue Modulation Input Signal pin 8
0.0V(off) to 10.0V(on) Return pin 9
Interlock Input Signal pin 4
(connect to AO Return pin 5
modulator ‘INT’)
‘Interlock Valid’ monitor Output Signal pin 1
(CMOS compatible Return pin 2
~15V = OK)
Minimum Connections shown below:
(Note: AOM thermal interlock connector type is model dependent)
PL1
3-way Mini Plug
1
23
P1
RFA 9-way D-type
5
9
4
8
3
7
2
6
1J1
1
2
J2
1
2
J1 = GATE input
TTL High = ON
J2 = Analog MOD'n input
10V = Max ON
AOM Thermal Interlock Plug
(OK = connected contacts 1-2)
Notes:
Both Digital GATE and Analog Modulation signals need to be applied.
The interlock signal must be connected. Contacts closed for normal operation.
2.0 Mounting Holes
4 x M5

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PWR A DJ
+24V 0V
RF
Analog Mod'n nput
BNC
9-way male
D-type
240
230
2080 71
120
Mounting Holes (4 places)
M5 Thread
Water cooled Heatsink must not exceed 50deg C
Driver Amplifier Outline
Dimensions : mm
1" = 25.4mm
220
Underside View
CTRL
ISOMET
RF DRIVER
MODEL :
S/N :
DC Supply Input
Screw terminals
+Vdc GND
G 1/8" Thread Water Fittings
Front placement : RFAXXX
Rear placements : RFAXXX-R
RF Output
BNC
Figure 1: Driver Installation
+Vdc
+Vdc
Gnd
nc
En
Out
no
Vcc
o
+
BNC
RF
Output
Modulation Input
Oscillator
Pre-Am p
PA Transistor
Mixe r
Digital
(Gate) Input
PWR
Adj
Figure 2: Driver Block Diagram

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-1
0
1
RF Carrier
-1
0
1
Modulation Signal Input
-1
0
1
Amplitude Modulated RF Output
Typical analog modulation RF waveforms are shown above.
For the RFA2120 and similar drivers, the modulation input is a combination of analog and digital
control as illustrated below.
Peak Level
set by driver
PWR ADJ
potentiometer
RFOutput
Power
(BNC)
Analog Input
(Pin 8)
Range = 0 - 10V
10V
0V
0W
Digital Gate Input
(Pin 6)
High to enable RF output
Set W
Figure 3: Typical Analog Modulation RF Waveforms

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1st
0th
Input
Orientation options
Input
Input
Input
1st
1st
1st
0th
0th
0th
Modulation Input
(RF Gate Input)
RF
INT
First Order (-1)
Diffracted Beam
Zero Order
Input Laser Beam
M1340 series
RFA2110 / RFA2120
Separation
Angle
Coolant circuit not shown for clarity.
Flow rate > 0.5 liter / min at less than 25deg C
DC supply : 24Vdc / 4A
1
23
AOM Interlock Connector
Normally Closed Contacts : Pins 1 , 2
Not connected : Pin 3
Bragg
Angle
(-2)
(+1)
Figure 4: Typical Connection Configuration

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Basic AO Modulator Parameters
Driver
RF
Intensity
Modulation
Input Laser Beam
1st Order Deflected Beam
0th Order
θ
θ
BRAGG
SEP
The input Bragg angle, relative to a normal to the optical surface and in the plane of deflection is:
θ BRAGG = λ.fc
2.v
The separation angle between the Zeroth order and the First order is:
θ SEP = λ.fc
v
Optical rise time for a Gaussian input beam is approximately:
tr = 0.65.d
v
where: λ = wavelength
fc = centre frequency = 110MHz / 120MHz / 140MHz
v = acoustic velocity of interaction material = 5.7mm/usec (Quartz)
= 5.96mm/usec (Fused Silica)
d = 1/e2beam diameter
Figure 5. Modulation System