THORLABS ULN15 Series User manual

ULN15 Series
Ultra-Low-Noise Lasers
User Guide

Table of Contents
Chapter 1 Warning Symbol Definitions.............................................................................................2
Chapter 2 Safety...................................................................................................................................3
Chapter 3 Description .........................................................................................................................4
3.1. General Characteristics................................................................................................ 4
3.2. Operating Characteristics............................................................................................ 5
3.2.1. “C” Configuration – Optimized for Mode-Hop-Free Current Tuning .....................................7
3.2.2. “T” Configuration – Optimized for Longer-Range Temperature Tuning................................8
3.3. Frequency Noise Characteristics .............................................................................. 11
3.4. Intensity Noise Characteristics.................................................................................. 12
Chapter 4 Setup .................................................................................................................................13
4.1. Control Electronics..................................................................................................... 13
Chapter 5 Operation..........................................................................................................................15
Chapter 6 Trouble Shooting.............................................................................................................16
Chapter 7 Specifications...................................................................................................................17
Chapter 8 Mechanical Drawing........................................................................................................18
Chapter 9 Regulatory ........................................................................................................................19
Chapter 10 Thorlabs Worldwide Contacts........................................................................................20

Ultra-Low Noise Laser Chapter 1: Warning Symbol Definitions
Page 2 QTN039956-D02
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your device.
Symbol
Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or Chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution: Risk of Electric Shock
Caution: Hot Surface
Caution: Risk of Danger
Warning: Laser Radiation
Caution: Spinning Blades May Cause Harm

Ultra-Low Noise Laser Chapter 2: Safety
Rev B, October 23, 2020 Page 3
Chapter 2 Safety
Adequate instructions for the safe installation of this laser component are the responsibility of the party
assembling a complete laser system from it. This item is designated for use solely as a component of such a
system, and therefore does not comply with the requirements of the U.S. Code of Federal Regulations, title 21,
sections 1040.10 and 1040.11.
A laser system based on the ULN component is capable of producing sufficient radiation to be classified as a
Class IIIb laser. Avoid eye or skin exposure.
WARNING
Device emits invisible laser radiation. Avoid eye or skin exposure.
CAUTION
The ULN component should be properly mounted to a thermally conductive surface to prevent heat
buildup. See Chapter 4 for additional details on the proper setup of for the ULN component.

Ultra-Low Noise Laser Chapter 3: Description
Page 4 QTN039956-D02
Chapter 3 Description
3.1. General Characteristics
Thorlabs’ Ultra-Low Noise (ULN) laser (US Patents 10193306, 10483718, 10476233, and 10454248) is an
external cavity diode laser consisting of a single angled facet gain chip coupled to a fiber Bragg grating (FBG).
The laser’s output is provided through a fiber pigtail. The ULN laser features ultra-low frequency and intensity
noise, along with high optical power. This product is provided in a hermetically-sealed, 14-pin, extended butterfly-
style package. The ULN laser includes two integrated thermoelectric cooler / thermistor pairs (one each for the
gain chip and the FBG), an auxiliary thermistor for monitoring the case temperature, and a photodiode for
monitoring the back facet power.
The ULN15PC and ULN15PT lasers operate in the ITU C band (1530 - 1565 nm). The center wavelength of each
device is tested and reported on a serialized data sheet. Most devices will operate at or near 1550 nm. Customers
that require a specific wavelength may contact technical support to place a special order (see Chapter 10).
ULN lasers are available with polarization-maintaining fiber; please contact technical support for a single mode
pigtailed fiber. The lasers are available in either a current tuning or a temperature tuning configuration, and details
on the configurations are discussed in the following sections.
Further details of the design of the ULN laser are available in the following publication:
Paul A. Morton and Michael J. Morton, “High-Power, Ultra-Low Noise Hybrid Lasers for Microwave Photonics
and Optical Sensing”, Journal of Lightwave Technology, vol. 36, no. 21, pp. 5048—5057, 2018.

Ultra-Low Noise Laser Chapter 3: Description
Rev B, October 23, 2020 Page 5
3.2. Operating Characteristics
For any given choice of temperature setpoints, the ULN operates as a single mode laser over a number of ranges
of drive current. As the current is adjusted upwards from an initial operating point, the temperature of the gain chip
increases, which in turn changes the round-trip phase of the cavity mode. This causes the center wavelength of
the mode to increase. As the current is further increased, the lasing mode will shift far enough away from the
center of the FBG’s reflection peak that single mode operation is no longer possible. At this point, the laser enters
multimode operation. As the current is further increased, a new cavity mode will move into resonance with the
FBG reflection peak, and the laser will once again move into single mode operation. This power vs. current (LI)
behavior is shown in Figure 1. The single mode regions are consistent and stable as long as the device’s
temperature is adequately controlled. The figure also plots the ULN’s LI behavior as the current is decreased.
With decreasing current, the device is able to stay in single mode operation within each particular single mode
range down to a lower current. This hysteresis is characteristic of the ULN.
Figure 1 Typical power vs. current (LI) plot. Only single mode power is plotted. This is a current-
tuning C-configuration device operated at the optimal ΔT, which is the difference between TFBG
and TCHIP.
The ULN laser’s spectrum is much narrower than can be measured with a typical optical spectrum analyzer (20
pm resolution). However, side-mode suppression ratio (SMSR), center wavelength, and single mode behavior can
all be measured with such a tool. A typical single mode spectrum is shown in Figure 2. The SMSR is well over 70
dB.
0100 200 300 400 500 600 700 800
0
20
40
60
80
100
120
140
160
180 Increasing Current
Decreasing Current
Single Mode Operating Regions
Power (mW)
Current (mA)

Ultra-Low Noise Laser Chapter 3: Description
Page 6 QTN039956-D02
Figure 2 Typical ULN spectrum at single mode operation with
an operating point above 100 mW. TCHIP and TFBG are 25 °C.
Current adjustment provides only a limited tuning range before the ULN laser hops to a new mode, as shown in
Figure 3. The actual tuning is smoother than shown; these data are impacted by the 4 pm sampling resolution of
this measurement. Longer-range tuning can only be achieved by adjustment of the FBG temperature. This
changes the center wavelength of the FBG’s resonant frequency.
Figure 3 LI plot overlaid with current-tuning characteristics. Note that the optical spectrum analyzer
used in this measurement was set to a sampling resolution of 4 pm; the actual tuning is smoother
than shown here. This is a C-configuration device operated at the optimal ΔT, which is the
difference between TFBG and TCHIP.
1548 1549 1550 1551 1552
-65
-55
-45
-35
-25
-15
-5
5
15 Typical Optical Spectrum
Amplitude (dB)
Wavelength (nm)
0100 200 300 400 500 600 700 800 900
0
20
40
60
80
100
120
140
160
180 Decreasing Current
Center Wavelength
Laser Output Characteristics vs. Current
Power (mW)
Current (mA)
1549.91
1549.92
1549.93
1549.94
Center Wavelength (nm)

Ultra-Low Noise Laser Chapter 3: Description
Rev B, October 23, 2020 Page 7
The ULN is available in two tuning configurations, each of which are optimized for a particular tuning method:
Current (C) and Temperature (T). The configuration is set at the time of manufacture and can not be altered. It is
indicated in the item number: “ULN15PC” is a C configuration device, while “ULN15PT” is a T configuration
device.
3.2.1. “C” Configuration – Optimized for Mode-Hop-Free Current Tuning
The C configuration is optimized to provide the user with the widest possible range of currents at which the laser
operates in a single mode. The mode-hop-free current tuning range is therefore maximized. In this configuration,
it is not possible to independently adjust the temperature set points of a C configuration ULN’s gain chip and FBG
without reducing the mode-hop free range. We report the optimal ΔT, which is the difference between the FBG
temperature (TFBG) and chip temperature (TCHIP), for each device on its individual datasheet. Note that operation
with TCHIP > 25 °C will reduce the device’s output power. Users wishing to thermally tune the device’s wavelength
without impacting the output power should therefore select the T configuration.
The plots in Figure 1 and Figure 3 show a C-configuration device operated at the optimal ΔT. The multimode
operation ranges are quite narrow, especially in the decreasing-current direction. Figure 4 shows the change in
behavior of a C configuration ULN as the device is moved away from optimal ΔT. We see that the multimode gap
between single mode regions increases away from the optimum. We also see that the start and end currents of
the single mode regions move as TFBG is adjusted. This behavior is normal.
Figure 4 Change in single mode ranges as TFBG is changed for a C-configuration device. TCHIP is
25 °C for all measurements. -1 °C is the optimum ΔT for the specific device shown in this figure,
corresponding to 24 °C for TFBG.
500 550 600 650 700
100
120
140
160
180 TFBG = 22 °C
TFBG = 23 °C
TFBG = 24 °C
Effect of Changing TFBG
Single Mode Power (mW)
Current (mA)

Ultra-Low Noise Laser Chapter 3: Description
Page 8 QTN039956-D02
3.2.2. “T” Configuration – Optimized for Longer-Range Temperature Tuning
The T configuration allows for independent adjustment of TFBG without impacting the laser’s output power.
However, the current span of each mode-hop free range is considerably reduced in the T configuration. Users
requiring the maximum possible mode-hop free tuning and/or current range should select the C configuration.
Figure 5 shows a typical LI curve for a T-configuration device. The reduction in the size of the single mode ranges
can be seen when compared to the C-configuration device behavior shown in Figure 1. OSA measurements of
the spectrum are shown at various TFBG in Figure 7. The tuning of the center wavelength is shown in Figure 8. It
should be noted that the single mode regions do move as TFBG is adjusted, as shown in Figure 6; a ULN kept at a
single current will therefore not operate single mode at all temperatures. To ensure continued single mode
operation, we recommend monitoring the laser’s output with an OSA.
Figure 5 LI Measurement of a T-Configuration ULN at Constant TFBG
0100 200 300 400 500 600 700 800
0
20
40
60
80
100
120
140
160
180 Increasing Current
Decreasing Current
Single Mode Operating Regions
Power (mW)
Current (mA)