THORLABS LD2000 User manual

LD2000, LD2000R
OEM Laser Diode Driver
with Analog Modulation
Operating Manual

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 2 www.thorlabs.com
Table of Contents
Part 1. Description ........................................................................................... 3
Compatible Laser Diode Styles............................................................................................ 3
Part 2. LD2000 Overview................................................................................ 4
2.1. Slow Start Circuit............................................................................................ 4
2.2. Limit Current Circuit...................................................................................... 4
2.3. Constant Power Feedback Loop..................................................................... 4
2.4. Theory of Operation........................................................................................ 5
Part 3. Operating the LD2000......................................................................... 7
3.1. Setup ................................................................................................................. 7
Setting the Feedback Resistor .............................................................................................. 9
3.2. Operating Modes ........................................................................................... 10
CW Operation .................................................................................................................... 10
Analog Modulation ............................................................................................................ 10
External Modulation Operation ......................................................................................... 11
Part 4. Specifications...................................................................................... 13
Part 5. Regulatory.......................................................................................... 14
Part 6. Thorlabs Worldwide Contacts.......................................................... 15

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 3 www.thorlabs.com
Part 1. Description
The LD2000 is a low-noise, stable laser diode current source that can be operated with
laser diodes having a common laser anode and monitor photodiode cathode. The driver
operates in an automatic power control (APC) mode using the built-in monitor
photodiode integrated in the laser diode for feedback or a constant current (CC) mode.
On board trimpots are provided for controlling the laser power and current limit. Both
functions can also be controlled via an external voltage source. The LD2000 supports a
wide range of laser diodes with drive currents up to 100 mA and photodiode currents
from 20 µA to 2 mA. The LD2000 also has an external modulation input to support
applications that require modulating the laser output.
The LD2000R is a RoHS compliant version of the LD2000 and features the same
operation and specifications as the LD2000
Compatible Laser Diode Styles
NOTE
Read the section on operating modes (page 10) before attempting to
operate LD2000
WARNING
The LD2000 uses CMOS circuitry to minimize power drain. Use
anti-static precautions while handling the LD2000 to prevent
permanently damaging the device.
Style F
Style A Style D

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 4 www.thorlabs.com
Part 2. LD2000 Overview
The LD2000 is composed of three independent circuits: slow start circuit, limit current
circuit, and output control circuit. Each is described below.
2.1. Slow Start Circuit
The slow start circuit is used to monitor the supply voltage and keep the laser output off
until the power supply stabilizes. The slow start circuit uses a voltage reference and a
comparator to monitor the supply voltage. An internal 2.5 V reference is compared to the
voltage at the ON/OFF pin (pin 17). When this voltage exceeds 2.5 V, the laser is
enabled. The comparator input (pin 17, ON/OFF) has an input impedance of 20 kΩ. This
resistance is used with an external resistor to form a voltage divider that sets the LD2000
dropout voltage. For most applications a 15 kΩresistor tied from the 12 V power supply
to the ON/OFF pin which disables the laser when the power supply drops below 4.5 V is
adequate.
Note, the ON/OFF pin can also be used to disable the laser by pulling this pin low to 0 V.
The slow start circuit uses an internal time constant formed by a 1 MΩand a 1 µF
capacitor to yield a 50 ms turn on delay. This can be extended by adding an external
capacitor to the SLOW_START pin.
2.2. Limit Current Circuit
The limit current circuit is a constant current source which can be set by the on-board
trim pot or an external control voltage. This determines the maximum drive current that
can be supplied to the laser. The transfer function for this control is 40 mA/V. The
current limit also determines the laser current when operating in the constant current
mode.
2.3. Constant Power Feedback Loop
The constant power feedback loop circuit uses the laser monitor photodiode current
(which is proportional to the laser output power) to regulate the laser output power. An
internal transimpedance amplifier converts the photodiode current to a voltage used by
the feedback circuit. The feedback loop varies the drive current to the laser such that the
voltage derived from the photodiode monitor current matches an adjustable setpoint
voltage (described below). The laser output can be adjusted by varying the setpoint
voltage.
When the current limit is set higher than the laser current needed by the feedback loop the
laser is operating in a constant power mode. If the current needed by the feedback loop is
higher than the current limit, the laser drive current will be clipped to the current limit
and the laser will then be operating in the constant current mode.
The photodiode transimpedance amplifier has an internal gain of 20 kΩwhich yields a
50 µA/V output. Since the maximum voltage of the feedback loop is 2.5 V, this limits the

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 5 www.thorlabs.com
maximum photodiode current to 125 µA. This upper limit can be easily increased by
adding an external resistor (see page 9).
The setpoint voltage used by the feedback loop is the difference between the PD
CURRENT SETPOINT voltage and the Analog Modulation Voltage as follows:
 
  
The control loop integrator has a time constant of approximately 60 µs set by an 0.012 µF
integrating capacitor. The loop time constant can be extended by adding an external
capacitor across CX1 and CX2 .
Note, all control signals are based on the photodiode current. The user must refer to the
manufactures spec sheets of the particular diode that will be used to correlate this to the
laser output power.
2.4. Theory of Operation
The LD2000 uses the internal monitor photodiode provided on most low power diode
lasers for feedback when operating in the Constant Power Mode. The following figure is
a block diagram of the LD2000 laser driver.
Figure 1: LD2000 Simplified Block Diagram
R
F INT 20 K
Ω
Constant Current
Source
LD
PD
V
CC
/2
2.5 V
R
F EXT
Lvl
Shift
∑

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 6 www.thorlabs.com
The laser power is regulated through an integrating feedback loop. The setpoint of the
feedback is determined by the PWR LIMIT control trimpot and the OUTPUT ADJUST
knob (in external modulation mode, the external voltage is used in place of the OUTPUT
ADJUST). An internal transimpedance amplifier converts the laser feedback current to a
voltage that is used as the error signal for the feedback loop.
Since all analog signal levels are based on a 2.5 V internal reference, we will use this to
derive the feedback gain setting resistor value:
The LD2000 has an internal transimpedance gain of 20 kΩ. Without a user installed
feedback resistor, the transimpedance gain is:
RF= 20 kΩ
and VError is equal to:
 󰇛󰇜
where IMon is the feedback monitor photocurrent. The total transimpedance gain should be
set so that the photocurrent at the maximum laser power equals 2.5 V. Since VError has a
maximum value of 2.5 V, we can derive the value of an external feedback resistor needed
to set the transimpedance for any laser:
2.5V

where RFis the transimpedance gain needed, and IMon is the monitor photocurrent for
your laser.
RF= RF EXT || 20 kΩ
 20kΩ
󰇛 20kΩ󰇜
solving for RF EXT,
 20,000
󰇛8,000󰇛󰇜1󰇜

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 7 www.thorlabs.com
Part 3. Operating the LD2000
3.1. Setup
The LD2000 is packaged as a component which, with minimal external components, can
be integrated into a system to make a complete laser diode driver system. We recommend
using printed circuit board construction to achieve optimum results. The pinouts for the
LD2000 are provided in Figure 1 and described below. The LD2000 can be used with A,
D, or F style diodes. The D and F style diodes will need to be configured like an A style
diode (see page 3 for diagrams). Please note that the EK2000 is not compatible with the
F Pin Style
LD2000 Pin Out
1 CX1
2 CX2
3 Analog Modulation
4 Slow Start
5 PD Current Trimpot
6 PD Current Setpoint
7 REF Out (2.5 V)
8 PD AMP Out
9 V+
10 GND
11 LD A / PD K
12 PD A
13 LD K
14 Limit Setpoint
15 Limit Trimpot
16 NO Connection
17 ON / OFF
18 Limit Out
N.C.
N.C.
PD Setpoint
Adjustment
Current Limit
Adjustment

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 8 www.thorlabs.com
Pin Name Description
1 CX1 These pins are provided for connecting an external capacitor to the control loop
integrator to extend the integrator time constant. This may be necessary to get
maximum bandwidth when using TTL modulation. Connect the positive terminal of the
cap to CX2.
2 CX2
3 Analog
Modulation
This pin is used with an external voltage signal source to provide analog modulation.
The transfer function (referenced to the photodiode current) is -50 µA/V with 0 V
being the laser completely on. The laser output decreases as this voltage increases with
the laser being completely off at 2.5 V. Connect this pin to ground when not using the
analog modulation.
4 Slow Start This output pin is high during the startup period and goes low when the laser is
enabled. It can be used as a LASER EMISSION indicator. An external capacitor can be
connected from this pin to ground to extend the slow start delay time.
Note: this output will not drive an LED directly and must be buffered. Contact Tech
Support for more details.
5 PD Current
Trimpot
This pin is connected to the wiper of the on-board PD Current Trimpot. Connect this
pin to the PD Current Setpoint to control the PD current with the on-board trimpot.
6 PD Current
Setpoint
This pin controls the PD Current according to a transfer function of 50 µA/V with 0 V
being the laser is completely off (0 PD current). The laser output increases as this
voltage increases.
7 REF Out This is a buffered 2.5V voltage reference.
8 PD AMP
Out
This is an analog voltage proportional to the photodiode current and referenced to one
half the supply voltage as follows:
 
󰇛󰇜,
where RF= PD Amp Transimpedance gain (internal 20 kΩin parallel with RF EXT)
9 V+Positive supply voltage (8 to 12 VDC).
10 GND Power supply common.
11 LD A/PD K Common laser diode anode, photodiode cathode.
12 PD A Photodiode anode.
13 LD K Laser diode cathode.
14 Limit
Setpoint
This voltage determines the maximum laser drive current according to the transfer
function 40 mA/V.
15 Limit
Trimpot
This is connected to the wiper of the Limit Current trimpot. Connect this to the Limit
Setpoint pin to use the on-board trimpot to set the current limit.
16 Not used This must be tied to ground to operate the laser.
17 ON/OFF This pin is used to externally turn the laser on and off through the slow start circuit and
to set the low voltage dropout point. It has an internal 20 kΩresistor to ground.
Connect a 15 kΩresistor to the power supply voltage to set the dropout voltage to
4.5 V.
18 Limit Out This is an output voltage proportional to the limit current with a transfer function of 40
mA/V. Use this pin to assist in setting the laser current limit.

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 9 www.thorlabs.com
Setting the Feedback Resistor
The LD2000 is configured at the factory for a maximum feedback gain. This gain setting
is appropriate for lasers that have low monitor currents in the range of 20 to 120 μA.
For most lasers, the photodiode current is greater than 120 μA and the feedback gain will
have to be reduced to drive the laser at full drive current. This can be done by following
the procedure below.
1. Determine the appropriate feedback gain using the following calculation:
 20,000
󰇛8,000 1󰇜
where RF EXT is the external gain setting resistor to be added in ohms and IMON
is the monitor current for a particular laser in Amps.
2. Pick the nearest standard value resistor (0.25 W, 5% or better).
3. Connect RF EXT across pins 8 and 12 of the LD2000.
WARNING
The LD2000 uses CMOS circuitry to minimize power drain. Use
anti-static precautions while handling the LD2000 to prevent
permanently damaging the device.
0.1 1 10
0.1
1
10
100
1 103RFext vs. PD Monitor Current
PD Monitor Current (mA)
RFext (kohms)
! !

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 10 www.thorlabs.com
3.2. Operating Modes
NOTE
For all modes of operation, we recommend using a linear DC
power supply or battery. Although the LD2000 provides over
-60 dB of power supply suppression, switch-mode power supplies
should be avoided due to the inherent transients in their output.
CW Operation
To operate the LD2000 in a CW mode, follow the steps below.
1. Connect your laser diode to the appropriate pins (11, 12, 13)
2. Attach a suitable DC voltage supply to pins 9 and 10. The power supply should
be bypassed near the LD2000 with a 10 µF tantalum capacitor and a 0.1 µF
ceramic capacitor.
3. Short pins 5 and 6 to use the on-board PD Current Trimpot.
4. Short pins 14 and 15 to use the on-board Current Limit Trimpot.
5. Short pin 3 to ground to set analog modulation to FULL ON.
6. Short pin 16 to ground.
7. Turn both the PD Current Trimpot and the Current Limit Trimpot
counterclockwise 20 turns each to set these at their minimum operating points.
8. Turn the DC power supply on and use a voltmeter to monitor the LIMIT OUT
on pin 18.
9. Adjust the Current Limit Trimpot clockwise slowly while observing the LIMIT
OUT to set the maximum operating current for your laser (refer to laser
manufacturer's data sheets). Note, this output is 40 mA/V.
10. Using a calibrated power meter to monitor the diode laser output, slowly adjust
the PD Current Setpoint trimpot clockwise to obtain the desired operating
power level. The laser will begin to emit upon reaching the drive current
threshold.
Analog Modulation
To operate the LD2000 analog modulation feature, follow the setup procedures for CW
Operation to establish the laser operating conditions. Once the LD2000 has been setup for
your laser, remove the short from pin 3 to ground and apply a positive voltage to pin 3 to
modulate the laser.

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 11 www.thorlabs.com
1. The analog modulation voltage has a negative transfer function characteristic.
That is, at 0 V, the laser is fully on, at 2.5 V the laser should be completely off.
2. The linear operating range of the analog modulation is determined by the
transimpedance gain of the PD Amplifier, RF. The appropriate transimpedance
gain for your laser can be calculated as follows:
2.5V

Where IMON is the PD current specified by the laser manufacturer for the
maximum operating output power.
Note, the LD2000 includes an internal 20 kΩresistor. RFis the net resistance of
the internal 20 kΩresistor with any external resistance added in parallel on pins
8 and 12. To calculate the external resistance (RF EXT) needed to operate at a
particular monitor current (IMON), use the following equation:
 50,000
󰇛20,000 2.5󰇜
External Modulation Operation
The laser output power can be controlled via an external modulation voltage while
operating in the Constant Power Mode. The laser output is inversely proportional to the
modulating voltage with 0 V being the laser fully on and 2.5 V turning the laser fully off.
1. To use the external modulation, perform the following steps:
2. Set up the LD2000 for Constant Power.
3. Attach a external modulation source (e.g. function generator, D/A converter,
etc.) to the Analog Modulation Input (pin 3).
4. Apply power to the LD2000 and adjust the modulation input amplitude and
frequency for the desired output.
5. Set the toggle switch on the rear panel for external modulation (up position).
6. The laser output will now be controlled by the external modulation voltage.
The graph above describes the characteristic of the modulation voltage. If the LD2000 is
set up to match a particular laser, the solid curve would represent the output power of the
laser as a function of the modulation voltage. A couple of notes of interest:

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 12 www.thorlabs.com
1. If the PWR Limit control is set below the maximum output power, the laser
output will plateau (clip) at the PWR Limit level for modulations below the
PWR limit.
2. If the total feedback gain, RF, is not optimized for the operating laser, the laser
turn off point of the output will be different than 2.5V (usually somewhere
below 2.5V since the default feedback gain is usually too high for most lasers).
3. If the feedback gain is too low for a laser (i.e. the maximum laser power can be
reached at a point somewhere below the maximum setting of PWR Limit, than
use care to set the PWR Limit control to the maximum desired operating power
before applying the modulating voltage.
Modulaon Voltage
Output Power
P
max
2.5 V
Clipping due to Current
Limit being set too low.
The turn-off voltage is dependent on
matching Rfext to the laser. If the
feedback gain is too high, the laser
will turn off at a lower voltage
(Voff = RF(IMon),
where RF=RF EXT in parallel with
20 kΩ).

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 13 www.thorlabs.com
Part 4. Specifications
Specification Value
Current Output
Limit Current Control Trimpot or external analog voltage
Limit Current Range 0 to 100 mA
Limit Accuracy ±1%
Compliance Voltage 󰇧
2󰇨5
Power Output
Photodiode Current Control Trimpot or external analog voltage
Photodiode Current Range 20 to 125 μA (Factory configured - see note 1).
Long Term Drift (24 hrs) < 0.1%
Temperature Coefficient <100 ppm/°C
Analog Bandwidth
3db Bandwidth (Nominal) 10 kHz (Actual Bandwidth is Laser Dependent)
Power Supply
Supply Voltage (V+) 8 to 12 VDC
Supply Current 30mA Plus Laser Current
General
Dimensions 2" x 1.3" x 0.5"
Operating Temperature -20 to 60 °C
Storage Temperature -65 to 150 °C
Packaging PCB DIP, Plastic Encapsulated
Note 1: Higher Photodiodes currents are easily supported by adding a single external resistor.
Mechanical Drawing
18 Pins
0.025" (0.635 mm) Typical
On 0.10" (2.54 mm) Centers
0.18"
(4.6 mm)
0.18"
(4.6 mm)
0.15"
(3.8 mm)
0.15"
(3.8 mm)
0.42"
(10.7 mm)
0.43"
(10.9 mm) 0.77"
(19.6 mm)
1.15"
(29.2 mm)
0.72"
(18.3 mm)
1.30"
(33.0 mm)
1.63"
(41.4 mm)
1.90"
(48.3 mm)
2.05"
(52.1 mm)
1.70"
(43.2 mm)
Photodiode Current
Laser Current

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 14 www.thorlabs.com
Part 5. Regulatory
As required by the WEEE (Waste Electrical and Electronic Equipment Directive) of the
European Community and the corresponding national laws, Thorlabs offers all end users
in the EC the possibility to return “end of life” units without incurring disposal charges.
•This offer is valid for Thorlabs electrical and electronic equipment:
•Sold after August 13, 2005
•Marked correspondingly with the crossed out
“wheelie bin” logo (see right)
•Sold to a company or institute within the EC
•Currently owned by a company or institute within
the EC
•Still complete, not disassembled and not
contaminated
As the WEEE directive applies to self contained operational
electrical and electronic products, this end of life take back
service does not refer to other Thorlabs products, such as:
•Pure OEM products, that means assemblies to be built into a unit by the user
(e.g. OEM laser driver cards)
•Components
•Mechanics and optics
•Left over parts of units disassembled by the user (PCB’s, housings etc.).
If you wish to return a Thorlabs unit for waste recovery, please contact Thorlabs or your
nearest dealer for further information.
5.1. Waste Treatment is Your Own Responsibility
If you do not return an “end of life” unit to Thorlabs, you must hand it to a company
specialized in waste recovery. Do not dispose of the unit in a litter bin or at a public
waste disposal site.
5.2. Ecological Background
It is well known that WEEE pollutes the environment by releasing toxic products during
decomposition. The aim of the European RoHS directive is to reduce the content of toxic
substances in electronic products in the future.
The intent of the WEEE directive is to enforce the recycling of WEEE. A controlled
recycling of end of life products will thereby avoid negative impacts on the environment.
Wheelie Bin Logo

LD2000(R)
0187-D01 Rev F, 3/22/2011 Page 15 www.thorlabs.com
Part 6. Thorlabs Worldwide Contacts
USA, Canada, and South America
Thorlabs, Inc.
435 Route 206
Newton, NJ 07860
USA
Tel: [phone]
Fax: [phone]
www.thorlabs.com
email: [e-mail]
Europe
Thorlabs GmbH
Hans-Böckler-Str. 6
85221 Dachau
Germany
Tel: [phone]
Fax: [phone]
www.thorlabs.de
email: [email protected]
UK and Ireland
Thorlabs LTD.
1 Saint Thomas Place, Ely
Cambridgeshire CB7 4EX
Great Britain
Tel: [phone]
Fax: [phone]
www.thorlabs.de
email: [email protected]
France
Thorlabs SAS
109, rue des Côtes
78600 Maisons-Laffitte
France
Tel: [phone]
Fax: [phone]
www.thorlabs.de
email: [e-mail]
Scandinavia
Thorlabs Sweden AB
Box 141 94
400 20 Göteborg
Sweden
Tel: [phone]
Fax: [phone]
www.thorlabs.de
email: [email protected]
Japan
Thorlabs Japan Inc.
5-17-1, Ohtsuka
Bunkyo-ku, Tokyo 112-0012
Japan
Tel: [phone]
Fax: [phone]
www.thorlabs.jp
email: [email protected]
China
Thorlabs China
Oasis Middlering Centre
3 Building 712 Room
915 Zhen Bei Road
Shanghai
China
Tel: [phone]
Fax: [phone]
www.thorlabs.com
email: [e-mail]

Thorlabs, Inc.
435 Route 206N
Newton, NJ 07860 USA
Phone: [phone] ♦Fax: [phone]
www.thorlabs.com