Vescent Photonics D2-105 User manual

Laser Controller
Vescent Photonics, Inc.
www.vescentphotonics.com
4865 E. 41st Ave
Denver, CO 80216
Phone [phone]
Fax [phone]

General Warnings and Cautions
The following general warnings and cautions are applicable to this instrument.
WARNING
This instrument is intended for use by qualified personnel who
recognize shock hazards or laser hazards and are familiar with
safety precautions required to avoid possible injury. Read the
instruction manual thoroughly before using to become familiar
with the instrument’s operations and capabilities.
CAUTION
There are no serviceable parts inside the instrument. Work
performed by persons not authorized by escent Photonics may
void the warranty.
CAUTION
Although ESD protection is designed into the instrument,
operation in a static-fee work area is recommended.
WARNING
To avoid electrical shock hazard, connect the instrument to
properly earth-grounded, 3-prong receptacles only. Failure to
observe this precaution can result in severe injury or death.
WARNING
Do not clean outside surfaces of any escent Photonics products
with solvents such as acetone. Front panels on electronics
modules may be cleaned with a mild soap and water solution.
Do not clean optics modules.

Limited Warranty
escent Photonics warrants this product to be free from defects in materials and workmanship for a period of one year
from the date of shipment. If this product proves defective during the applicable warranty period, escent Photonics, at
its option, either will repair the defective product without charge or will provide a replacement in exchange for the
defective product. The customer must notify escent of the defective product within the warranty period and prior to
product return. The customer will be responsible for packaging and shipping the defective product back to escent
Photonics, with shipping charges prepaid.
escent Photonics shall not be obligated to furnish service under this warranty from damage caused by service or repair
attempts made without authorization by escent Photonics; from damage caused by operation of equipment outside of
its specified range as stated in either the product specification or operators manual; from damage due to improper
connection to other equipment or power supplies.
This warranty is in lieu of all other warranties including any implied warranty concerning the suitability or fitness of
the product for a particular use. escent Photonics shall only be liable for cost of repairs or replacement of the
defective product within the warranty period. escent Photonics shall not be liable for any damages to persons or
property resulting from the use of the product or caused by the defect or failure of this product. escent Photonics'
liability is expressly limited to the warranty set out above. By accepting delivery of this product, the purchaser
expressly agrees to the terms of this limited warranty.
escent Photonics
Printed Feb 22, 2012

Absolute Maximum Ratings
Note: All modules designed to be operated in laboratory environment
Parameter Rating
Environmental Temperature >15°C and <30°C
Environmental Humidity <60%
Environmental Dew Point <15°C

Vescent Photonics D2 Series Product Manual
1. Laser Controller
Model No. D2-105 / D2-105-500
Document Revision: 4
1.1. Description
The laser controller has two temperature controllers capable of sub-mK stability1 and a 200 mA or 500
mA precision current source based on the Libbrecht-Hall2 circuit. The laser controller is designed for
very fast current modulation via the servo input enabling high-speed servo control of the laser’s
frequency. The current servo input can accommodate input frequencies over 10 MHz and is limited by
the 1 kΩ input impedance. Additionally, an RF port is available for higher frequency modulation.
1 Sub-mK stability requires a proper thermal design and proper tuning of the temperature controller to the thermal
plant. If you did not purchase the Laser Controller with a Laser Diode, please read the section on tuning the
temperature controller.
2 Libbrecht and Hall, A Low-Noise, High-Speed Current Controller, Rev. Sci. Inst. 64, pp. 2133-2135 (1993).
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Vescent Photonics D2 Series Product Manual
1.2. Specifications3
D2-1 5 D2-1 5-5 Units
Current Source
Current range 0-200 0-500 mA
Current noise density <100 <200
pA/

Hz
RMS Noise (10Hz - 100kHz) <50 <100 nA
RMS Noise (10Hz - 1MHz) <100 <150 nA
RMS Noise (10Hz - 10MHz) <300 <500 nA
Monitor Resolution (Display) 0.1 1 mA
Absolute accuracy 2 2 %
Temperature coefficient <1 <5 µA/°C
Current Servo Input
Input impedance 1000 1000 Ω
Bandwidth >10 >10 MHz
Modulation coefficient 1 1 mA/
RF Input Bandwidth 0.5 - TBD 0.5 - TBD MHz
Temp Servo Input
Input impedance 100 100 kΩ
Temp modulation coefficient
Gain = Low -20 -20 mK/
Gain = High -600 -600 mK/
Temperature Control
Temperature setpoint range 1-50 1-50 °C
Temperature isolation (T2)4TBD TBD mK/°C
Long term stability (T2) ~1 ~1 mK/day
Temperature coefficient 5
(controller, T2)
TBD TBD mK/°C
Max TEC current (voltage) 1 (4) 1 (4) A ( )
3All current noise measurements made with Laser Controller powered by D2-005 placed 5ft away from Laser Controller. Placing
D2-005 closer to Laser Controller may increase 60 Hz noise and their harmonics. Specifications not guaranteed when Laser
Controller is powered by device other than D2-005, or when placed near power supply or other noise source..
4Laser module temperature changed while Laser Controller module held fixed.
5Laser module temperature fixed while Laser Controller temperature change.
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Vescent Photonics D2 Series Product Manual
1.3. Inputs, Outputs, and Controls
Front Back
1.3.1. Monitor Section
The monitor section contains a 3 ½ digit display and an output BNC for monitoring by an oscilloscope or
voltmeter. A selector knob controls which of six signals are relayed to the display and BNC. An LED
indicator is below the display to show which signal is being displayed and its units.
9
Vescent Photonics, Inc.
Denver, Colorado USA
MADE IN U.S.A.
Power I/O Power I/O
Remote Interlock
Setpo nt
Input
Setpo nt
Enable
Short
Open
= On
= Off
Model: D2-105
Laser
Enable
T1
set
I I
l m
∆ T1 T2
set
∆ T2
mA mA C mK C mK
T1 = Hous ng Temp
T2 = Laser Temp
∆ = Temp Error
Mon tor
Laser Controller
DIM
Temp Control
T2 Set
T2
T1
Power
Servo
Standby
Temp
Status
Laser
On
on
off/reset
Current Control
RF
Input
Current
Servo
Input
Coarse
Current
Laser
Laser
Current
Output
Current
L m t
Set
Temp
Lock
Temp
Servo
Input
Laser
Temp
Output
F ne
Current
Current
L m t
Mon tor
Output
199.9
Vescent Photonics, Inc.
Denver, Colorado USA
MADE IN U.S.A.
Power I/O Power I/O
Remote Interlock
Setpo nt
Input
Setpo nt
Enable
Short
Open
= On
= Off
Model: D2-105
Laser
Enable
T1
set
I I
l m
∆ T1 T2
set
∆ T2
mA mA C mK C mK
T1 = Hous ng Temp
T2 = Laser Temp
∆ = Temp Error
Mon tor
Laser Controller
DIM
Temp Control
T2 Set
T2
T1
Power
Servo
Standby
Temp
Status
Laser
On
on
off/reset
Current Control
RF
Input
Current
Servo
Input
Coarse
Current
Laser
Laser
Current
Output
Current
L m t
Set
Temp
Lock
Temp
Servo
Input
Laser
Temp
Output
F ne
Current
Current
L m t
Mon tor
Output
Vescent Photonics, Inc.
Denver, Colorado USA
MADE IN U.S.A.
Power I/O Power I/O
Remote Interlock
Setpo nt
Input
Setpo nt
Enable
Short
Open
= On
= Off
Model: D2-105
Laser
Enable
T1
set
I I
l m
∆ T1 T2
set
∆ T2
mA mA C mK C mK
T1 = Hous ng Temp
T2 = Laser Temp
∆ = Temp Error
Mon tor
Laser Controller
DIM
Temp Control
T2 Set
T2
T1
Power
Servo
Standby
Temp
Status
Laser
On
on
off/reset
Current Control
RF
Input
Current
Servo
Input
Coarse
Current
Laser
Laser
Current
Output
Current
L m t
Set
Temp
Lock
Temp
Servo
Input
Laser
Temp
Output
F ne
Current
Current
L m t
Mon tor
Output
199.9

Vescent Photonics D2 Series Product Manual
Name Symbol Units Resolution
Current I mA 0.1 mA / 1mA6
Current limit Ilim mA 0.1 mA / 1mA6
Housing Temperature Set T1set °C 0.1 °C
Housing Temperature Error ∆T1 mK 0.1 mK
Laser Temperature Set T2set °C 0.1 °C
Laser Temperature Error ∆T2 mK 0.1 mK
NOTE When the Laser Switch is in the “Off / Reset” position, the laser diode is shorted to ground and
no current is flowing through the laser diode. However, the Current Monitor will read up to 30 mA of
current flowing through the short. This is normal. When the switch is in the “ON” position, the current
monitor accurately measures the current flowing through the laser diode.
1.3.2. Current Control
Current Lim (LED)
The current limit indicator lights when the current limit circuit is activated. If the user attempts to
set the current over the current limit setpoint the circuit shunts the excess current through a
transistor to ground
Fine Current (knob)
The fine current adjusts the diode injection current by ½-1 % of the course control setting. Use
this control for fine positioning of the laser frequency prior to locking.
Course Current (Scale Dial)
The course current control sets the laser diode injection current between 0 and 200 mA (0 and
500mA for 500mA version). To set the current, switch the selector knob in the monitor section
so that the current LED (I) is lit. Then adjust the course current to the desired setting.
Current Limit Set (Trimpot)
The current limit set is a front panel trimpot adjustment. Set the selector knob to the Ilim position
and adjust the trimpot to the desired value. The Current Limit should be set below the maximum
current for the laser diode.
RF Input (BNC)
The RF input is ac coupled to the laser diode through a 10 nF capacitor. Over ~3 MHz the
impedance of this input will approach the ac impedance of the laser diode of ~5Ω. This input is
normally connected to the RF output from the Laser Servo module, which applies FM sidebands
at 4 MHz to the laser output.
Note a large voltage transient to this input could possibly cause damage to the laser diode. If
you are connecting other equipment to this input do not exceed 0.25 Vrms from a 50 Ω source or
1 mW of power.
Current Servo Input (BNC)
The current servo input adds or subtracts current though 1 k Ω connected to the laser diode giving
a modulation coefficient of 1 mA/ . A bias circuit sets the voltage of the current servo input to
6 1mA resolution with D2-105-500
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Vescent Photonics D2 Series Product Manual
zero volts. (Normally, a connection to the laser diode would place this voltage at ~ 2 or equal
to the forward diode drop.) Therefore, leaving the input open or grounded does not alter the
current to the laser diode.
The bandwidth of the current servo input is >10 MHz.
Laser On (LED indicator)
Turns ON 5 seconds before laser light turns on. If light is on, laser is on (or will be in <5secs).
Laser ON-Off/Reset (switch)
When the switch is in the Off/Reset position, the laser diode is turned off and the laser is shorted
to ground. When flipped into the On position, the Laser ON (LED indicator) will turn on and 5s
later, the laser will turn on. If the laser diode is turned off from the laser enable or remote
interlock, this switch must to placed into the Off/Reset position and then into the ON position to
turn the laser back on.
Laser Out (SMA)
The laser current is output through an SMA connector and returns through the cable shielding to
ground.
1.3.3. Temperature Control
Power (LED indicator)
All electronic modules have a blue LED power indicator on the top right side of the front panel
control section. The LED requires +15 and 5 in order to light.
Temp Status (dual LED indicators)
The temperature status LED indicators turn red whenever the temperature servo loop has been
disengaged. The temperature servo loop will disengage when the temperature is below -1°C,
above 50°C, or the thermistor is shorted or open. Additionally, when the TEMP LOCK is in
standby mode, the servo loop is off and the LED's will be red.
The temperature status LED indicators will turn green when the temperature is within a narrow
temperature window of the setpoint. The window is typically set to a 100mK for stage 2 and
500mK for stage 1 and can be monitored via T4 on the side-panel.
T2 Set (Trimpot)
The temperature of the laser diode (T2) is set with a front panel trimpot. When a D2-100 laser is
purchased with the controller, T2 is factory set to put the laser on transition with the Rb D2
hyperfine lines and should not need trimming. However, as the diode ages the user might need to
adjust this value. Set the display selector knob to read T2set and set to the desired value.
T1 is also set at the factory and should not require further adjustment. However, the T1 trimpot
can be accessed by removing the right panel from the enclosure.
Temp Lock (Dual position switch)
The temperature servo can be placed into standby mode if desired. In this mode no current is
supplied to the TEC elements.
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Vescent Photonics D2 Series Product Manual
Temp Servo Input (BNC)
The temperature servo input is summed to the T2 temperature setpoint signal and can be used to
make electronic perturbations to the laser diode temperature. The Temp Servo Input, has two
settings: “Low” and “High” gain. The default settings in “Low” but can be changed by a switch
accessible on the right side panel.
When the gain is set to “LOW”, the slope for changing the setpoint is ~-20 mK/ .
When the gain is set to “HIGH”, turn the setpoint to the lowest desired temperature. Apply a
voltage between 0 and 10 to TEMP SER O INPUT to raise the setpoint temperature. With
this configuration, you can sweep the setpoint all the way from the low temperature limit (-1°C)
to the high temperature limit (58°C). NOTE that the Vescent Photonics Lasers should not be
operated above 50°C. The slope for changing the setpoint is ~-600 mK/ .
The “Low” mode is designed for slow temperature feedback for long-term (days) stability of the
locked laser. Normally the Temp Servo Input is used to drive the dc value from the Current
Servo Ouput on the Laser Servo to zero over long time scales. In other words, temperature tuning
is used to remove large, slow variations in the laser frequency. To accomplish this connect the
Temp Servo Output from the Laser Servo module to the Temp Servo Input of the Laser
Controller (with TEMP SER O INPUT Gain is set to “Low”). This connection is only important
if the user is trying to maintain a laser lock continuously over many days or even weeks. Without
feedback to Temp Servo In the Laser Servo can eventually run out of range. (The Current Servo
output of the Laser Servo is clamped at 1.2 . See discussion in the Laser Servo manual.)
Laser Temp Output (8-pin connector)
An 8-pin Hirose connector (HR25-7TR-8SA) carries the signals for the temperature control of the
Laser module. The wiring diagrams are shown in the table below, where 1 (2) refer to stage 1 (2)
temperature control, which stabilizes the Laser Housing (Laser Diode). Rth and Rth_Rtn are the
two ends of a 10k thermistor.
Pin Signal
1 TEC1+
2 TEC1-
3 Rth1
4 Rth1-RTN
5 TEC2+
6 TEC2-
7 Rth2
8 Rth2-RTN
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Vescent Photonics D2 Series Product Manual
1.3.4. Back panel I/O
Power I/O (9-pin D-sub)
The power to each electronics module is through a 9-pin D-sub connecter through a power bridge
unit. The unit can also be powered through any serial cable with 9-pin D-sub connectors, which
is convenient when the unit must be taken out of line for access to the side panels. The pin outs
are shown in the following figure:
Laser Enable
This key-switch is required to be in the enable position for the laser to turn on. Normally, the key
is left in the enable position.
Remote Interlock
The remote interlock can be used to disable the laser diode output via an interlock control. When
this input is shorted the laser diode output is ON. When the input is open the diode output is
OFF. If not used, leave a shorting cap over this BNC. Once the interlock has been tripped, the
laser will stay off until it is manually reset with the front panel switch.
Setpoint In
SETOINT IN is an analog input. When the SETPOINT ENABLE is LOW (0 ) the SETPOINT
IN voltage value sets the injection current instead of the front panel dial. Zero volts sets zero
current and 6 sets the maximum value of 200 mA (500mA for 500mA version).
This input is rolled off at 1 kHz, which is a much higher frequency than the front panel dial,
which rolls off at 8 Hz, with a second pole at 4 Hz. Therefore, noise can enter the circuit at this
point and this input should be used with caution. It is primarily intended for sweeping the current
in order to measure PI curves and threshold values of laser diodes. We do not recommend using
this input for computer control of the injection current.
Setpoint Enable
SETPOINT ENABLE is a TTL input. 5 puts the front panel dial in control of the injection
current and 0 gives control to SETPOINT IN.
When disconnected, the SETPOINT ENABLE is at 5 .
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Vescent Photonics D2 Series Product Manual
1.4. Turning on the Laser Diode
In compliance with FDA requirements for a Class 3B laser, the Laser Controller has two safety interlocks.
If either interlock is tripped, the laser will turn off and stay off until the interlocks are reset AND the laser
switch is switched from the “off / reset” position to the “on” position. Additionally, if the Laser Controller
loses power, the laser diode will remain off when power is restored.
To turn on the laser diode, follow these instructions:
1) Flip the Laser switch into “off/reset” position
2) Insert the key into the keyhole and rotate the key 90 degrees (keyhole located on back panel)
3) Place grounding BNC terminator on “Remote Interlock” BNC (located on back panel)
4) Flip the Laser switch into the “on” position. The green light above the switch should turn on and
after a 5 second delay, the laser will turn on.
1.5. Tuning the Temperature Controller
If you purchased the Laser Controller with a Laser Module, then the Laser Controller is properly tuned
for the Laser Module's thermal load and you can skip this section. This section will describe the basic
theory about tuning the temperature servo response and provide step-by-step instructions for tuning the
servo response to your thermal load.
To get good temperature stability, the temperature servo response needs to be tuned to match the thermal
load. Access to tuning the temperature response is provided on the right side panel of the Laser Controller
and requires removing that side panel to access the controls. The Laser Controller provides two
independent temperature controllers that are nominally identical. However, stage 2 has front panel
adjustment of the temperature set-point, while the stage 1 temperature set-point is a side-panel
adjustment. Additionally, the front panel TEMP SER O INPUT adjusts the stage 2 set-point while stage
1 does not have an equivalent function. Stage 2 is accessed in the middle of the side-panel, while stage 1
is near the back of the side panel. Typically, stage 2 is used to control the laser temperature and stage 1 is
used to control the temperature surrounding stage 2. In this way temperature gradients between the laser
diode and the thermistor measuring the laser temperature are stabilized and temperature changes caused
by room temperature drift is greatly reduced.
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Vescent Photonics D2 Series Product Manual
1.5.1. Transfer Function and Poles
Each stage of temperature control has a transfer function shown below:
The three poles (ω1, ω2, ω3) and the overall gain can be adjusted using trimpots and click-switches on the
side panel. The first pole (ω1) is the Proportional-Integrator (PI) pole, or the frequency where the gain
switches from being an integrator to proportional. The second pole (ω2) is the Differential (D) pole, or the
frequency where the gain becomes differential. The final pole (ω3) is where the differential gain ends and
the gain becomes proportional again.
15
Transfer Function
ω
1
ω
2
ω
3
Frequency
Gain
Transfer Function
ω
1
ω
2
ω
3
Frequency
Gain

Vescent Photonics D2 Series Product Manual
1.5.2. User Control of the Poles and Gain
If you remove the right side panel on the Laser Controller, for each stage of temperature control, you will
see the panel shown in Figure 1. The set of click switches labeled “Integral” control the PI (ω1) pole.
Clicking the first switch, labeled “proportional,” into the on position removes the integral gain. If the
“proportional” switch is in the off position, then the sum of the times for all switches in the on position
gives the RC time-constant for the PI pole. For example, if the 2nd (0.47s) switch and the 4th (2.2s) switch
are in the on position (and the rest off), then the time constant is 2.7s and ω1=
1/2.7s
= 0.37 Hz.
Similarly, the switches labeled “Differential” control the D (ω1) pole. If the first switch, labeled “Diff On”
is in the off position, then there is no differential pole. If the “Diff On” switch is on, then the D pole has
an RC time-constant given by the sum of the times of all the switches in the on position, same as with the
Integral bank of switches.
The “PROPGAIN” trimpot tunes the overall gain of the system and is adjustable by a factor of 200.
Additionally, the TSET1 trimpot is used to adjust the set-point temperature for stage 1 (stage 2 is
controlled on the front panel).
The “STBY / SER O” switch can disable temperature controller for either stage by placing the switch
into STBY (standby) mode. In this mode, a red light will be shown on the front-panel to show that the
stage has been disabled.
1.5.3. Tuning the Thermal Loop
Although there are numerous methods for tuning the loop parameters, these instructions will use the
Ziegler-Nichols tuning method.
For nested stages (one stage is inside a housing whose temperature is controller by another stage), we
recommend tuning the outside stage first with the inside stage turned off. Then tune the inside stage while
the outside stage is turned on. For the stage directly controlling the laser temperature, we recommend
16
Figure 1: Side Panel Adjustments for Temperature Control.

Vescent Photonics D2 Series Product Manual
running the tuning procedure while the laser is on. For each stage, follow the steps below to tune the
plants according to the Ziegler-Nichols tuning method:
1) Connect thermal load to Laser Controller.
2) Place Loop in proportional-only mode: Switch labeled “Proportional” is on, switch labeled “Diff
On” is off.
3) Turn the gain all the way down (trimpot labeled “PROPGAIN” all the way CCW).
4) Turn on temperature loop.
5) Adjust set-point to approximately desired temperature.
6) Turn up the gain. Keep increasing the gain until the temperature error (front panel BNC) just start
to oscillate or ring with very little damping. If oscillation too large, reduce gain. Measure the
period of oscillation.
7) Turn off the Laser Controller. Measure resistance between “GAIN” testpoint and “GND”
testpoint. Turn down the “PROPGRAIN” until this resistance reads 1.7 time less than its original
value (i.e. from 500Ω to 295Ω).
8) Take the measured oscillation period in step 6 and divide by two. Set the Integrator time constant
to this value. If you measured a period of oscillation of 14 seconds, turn on the 4th (2.2s) and 5th
(4.7s) switches in the integrator bank, to get a time constant of 6.9s.
9) Turn off the “proportional” switch.
10) Turn the “DiffGain” trimpot all the way CW.
11) Set the “Differential” switches to the same position as the “integral” one. This works out to
setting a D time-constant roughly equal to 1/8 of the period of oscillation. For the previous
example, set the 4th (0.47s) and 5th (1.0s) switches on to get a time constant of 1.5s.
12) Turn on the “Diff On” switch.
13) Your thermal loop is now tuned. Power and Laser Controller wait for temperature to stabilize.
Change the setpoint and observe the temperature error and verify that the oscillations of damped
and the temperature stabilizes. You may be able to get better performance by tweaking the poles
and gain.
NOTE Depending on the thermal design, nested temperature loops can fight each other, causing
oscillations and instability. If you observe this, you will need to reduce the gain and/or increase the time-
constants on the slower stage.
17