Sacher Lasertechnik TEC-500 Guide

Technical Documentation
Page 1
Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
Alignment Procedure for Littman/Metcalf (TEC-500)
Laser Cavities
1 Security Precautions................................................................................................................ 2
1.1 Radiation Protection............................................................................................................ 2
1.2 ESD Protection.................................................................................................................... 2
1.3 Protection of Optical Elements............................................................................................ 2
1.4 Identification of Each Element within the Laser Head........................................................3
2 Collimation of the Laser Diode ............................................................................................... 5
2.1 Laser Diode Beam Propagation........................................................................................... 5
2.2 Determining the Status of the Laser Collimation................................................................ 6
2.3 Adjustment of the Laser Collimation .................................................................................. 7
3 Cavity Alignment....................................................................................................................... 8
3.1 Pre-Adjustment of the Reflection Prism.............................................................................. 8
3.2 Minimizing the Laser Threshold....................................................................................... 10
4 Mode-Hop Free Tuning.......................................................................................................... 12
4.1 Principles of Mode-Hop Free Tuning................................................................................ 12
4.2 Monitoring the Tuning Performance via Fabry Perot Interferometer ............................... 12
4.3 Monitoring the Tuning Performance via Power Monitoring............................................. 12
4.4 Monitoring the Tuning Performance via Wavemeter........................................................ 14
4.5 Optimizing the Tuning Performance................................................................................. 14
5 Summary.................................................................................................................................. 16
Thank you for ordering a laser system with Sacher Lasertechnik. The provided alignment
procedure will enable you to achieve the best performance with the laser system.

Technical Documentation
Page 2
Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
1 Security Precautions
Before starting to work on the alignment procedure of the Littman/Metcalf laser system,
there will be two types of security precautions required, protection against laser radiation
and protection of the laser diode against electro-static discharge (ESD).
1.1 Radiation Protection
Make sure to provide adequate protection against laser radiation. E.g. protective eyewear
is absolutely required. Further precautions are required for higher power laser systems.
Consult your laser security officer for detailed instructions before removing the protective
cover of the laser head and before the starting the alignment procedure.
1.2 ESD Protection
Laser diodes are electro-static discharge sensitive (ESD) devices. Most premature laser
diode damages are caused by ESD damages. Make sure to take adequate precautions
against ESD. This can be done e.g. via a wristband which connects you to the laser
ground. Consult your security officer for detailed instructions before starting the alignment
procedure.
1.3 Protection of Optical Elements
The optical elements within the laser head are sensitive components. A simple finger print
will cause a irreversible damage. Do not touch an optical surface of any of the optical
components.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
1.4 Identification of Each Element within the Laser Head
Identify each element inside of the laser head and compare its position and function with
the description within the operation manual of the laser system.
Fig. 1: CAD drawing of the Littman/Metcalf laser head. The red arrow indicates
laser radiation (A). Laser beam (A) is the zero order diffraction of the diffraction
grating. Green arrows indicate fine adjustments screw. Black arrows indicate
optical elements.
(2) Cavity
Alignment Screw
(1) Wavelength
Alignment Screw
(A) output beam
(3) Collimation
Adjustment Screw
Reflection PrismDiffraction
Grating
Optical Isolator
(optional)
Fiber Coupler
(optional)

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
Fig. 2: CAD drawing of the Littman/Metcalf laser head. The green arrow indicates
the fine adjustment screw for the wavelength adjustment.
Fig. 1 and Fig. 2 show a CAD drawing of our Littman/Metcalf type of laser head. The red
arrow indicates laser radiation. Laser beam (A) is the laser output beam. It is the zeroth
order diffraction of the diffraction grating. Green arrows indicate fine adjustment screws.
Fig. 3: Photo of the Littman/Metcalf laser head. The green arrows indicates the fine
adjustment screws for the collimation and the cavity alignment. Black arrows
indicate optical elements.
Fig. 3 shows a photo our Littman/Metcalf type of laser head. The green arrows indicate
fine adjustment screws. Contact us in case you feel uncertain on any of the described
elements.
(1) Wavelength
Alignment Screw
(2) Cavity
Alignment Screw
(3) Collimation
Adjustment Screw
Optical Isolator
(optional)
Fiber Coupler
(optional)

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
2 Collimation of the Laser Diode
Proper beam collimation of the laser is required before the cavity alignment can be
performed. The collimation procedure consists of several steps. Before discussing details
of the collimation procedure, the physical basics of laser diode emission will be briefly
reviewed.
2.1 Laser Diode Beam Propagation
Laser diodes are epitactic grown multi-layer semiconductor crystals. The epitactic layers
define multiple electric p-n-junctions. Only a small part of the laser diode crystal generates
the laser radiation. This active area of the laser diode crystal has a typical size of 1µm x
3µm x 1500µm (height x width x length). The width is measured parallel to the p-n-junction.
The height is measured perpendicular to the p-n-junction.
Due to the large difference between height and width of most laser diodes, the emission
point for the laser radiation measured parallel and perpendicular and to the p-n-junction is
different. This effect is commonly referred to as astigmatism of laser diodes.
The emission point of the laser radiation measured in the direction parallel to the p-n-
junction is located almost at the facet of the laser diode. The emission point measured
perpendicular to the p-n-junction is located behind the laser facet, inside of the laser diode.
A typical distance between these two emission points is between 5µm and 10µm for single
mode laser diodes.
Emitter
Width
Fast
Axis
Slow
Axis
Fig. 4: Schematic view of a laser diode together with the laser emission. The blue
indicated area shows the p-n-junction of the laser diode. The yellow indicated area
shows the active region of the laser diode. The laser radiation is indicated on the
right hand side of the laser diode. The schematic indicates the beam profile of the
astigmatic beam propagation.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
Due to the astigmatism of laser diodes, the divergence of the laser beam is different for the
direction parallel and perpendicular to the n-p-junction. The collimated laser beam
measured in the direction parallel to the p-n-junction shows a larger divergence than
measured perpendicular to the p-n-junction. Fig. 4 shows a schematic of a laser diode with
the astigmatic beam propagation.
Commonly, the direction of the laser beam measured parallel to the p-n-junction is referred
to as slow axis of the laser beam. The direction of the laser beam measured perpendicular
to the p-n-junction is referred to as fast axis of the laser beam.
2.2 Determining the Status of the Laser Collimation
When collimating laser diodes with a spherical lens, it is not possible to perform a
simultaneous collimation for fast and for slow axis. The laser diode is either collimated for
the fast axis with accepting a beam divergence of the slow axis, or the laser diode is
collimated for the slow axis with accepting a beam divergence of the fast axis.
Fast
Axis
Slow
Axis
Emitter
Width Collimation
Lens
Fig. 5: Schematic view of the beam propagation of a collimated laser diode. Due to
the astigmatism of the laser diode, the slow axis shows a focus behind the
collimation lens. After the focus, the slow axis diverges and becomes larger than
the fast axis.
Commonly, a proper collimation of the fast axis is chosen with accepting a slight beam
convergence for the slow axis.
For analyzing the beam propagation, a laser camera is required. In case a laser camera is
not available, a screen together with a beam viewer may give some indication of the beam
propagation.
Make sure to wear eye protective glasses and a ESD protective wrist band before starting
any work at the collimator.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
2.3 Adjustment of the Laser Collimation
The collimator offers a fine adjustment screw indicated as (3) in Fig. 1 for adjusting the
beam collimation. Clockwise rotation of the fine adjustment screw will increase the
distance between the emission facet of the laser diode and the collimation lens.
Fig. 6: Photo of the laser collimator together with the inside-hex adjustment tool.
Rotation of the adjustment tool will result into an adjustment of the lens relative to
the laser diode. Do not rotate the adjustment screw more then +/- ¼ of a rotation.
Verify the beam propagation according to the schematic given in Fig. 5. The laser diode is
well prepared for using it within the external cavity if the beam propagation shows the point
of lowest diameter at a distance of approximately 1.5m distance from the collimator. The
beam propagation be adjusted via rotation of the adjustment tool shown in Fig. 6. Do not
rotate the adjustment screw more than +/- ¼ of a rotation.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
3 Cavity Alignment
This section describes the preparation of the prism arm for the alignment procedure as
well as the alignment procedure itself. Optional optical components as optical isolator or
fiber coupler may need to be removed.
3.1 Pre-Adjustment of the Reflection Prism
The following steps need to be taken for performing the cavity alignment. As tools, a beam
viewer and/or a NIR detection card are required.
A) Rotate the wavelength adjustment screw until angle between the prism arm and the
grating is close to the expected value for the operating the laser system at the desired
wavelength.
B) Preset the laser current with a value between the expected threshold current with
optical feedback and the threshold current without optical feedback.
C) Make sure to wear eye protective glasses and a ESD protective wrist band.
D) Enable the laser injection current.
(1)
(2)
(3)
(4)
Fig. 7: Schematic view of the laser spot visible on a screen placed into the zeroth order
beam of the laser head. The steps 1 to 4 describe the steps of the alignment
procedure.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
E) As soon as the injection current is enabled, the zeroth order diffraction of the
diffraction grating will be visible. A good distance to observe the zero order diffraction
is approximately 0.5m from the grating. In case you cannot see the spot, you need to
darken the laboratory.
F) When rotating the prism arm, the zeroth order diffraction beam will not move due to
the Littman/Metcalf design.
G) Rotate the prism arm until the first order diffraction directs into the collimation lens of
laser collimator after passing the grating a second time.
H) As soon as the first order diffraction of the grating is directed into the lens of the
collimator, a second weak spot will be visible close to the zero order diffraction at
approximately 0.5m distance from the diffraction grating. Fig. 7 (1) shows an
approximate view of the two spots.
I) Rotate the prism arm in such a way that the two spots are close to each other, and
both spots visible as indicated in Fig. 7 (2).
J) Adjust the cavity alignment screw in such a way that the two spots are on the same
horizontal line as shown in Fig. 7 (3). As soon as the strong moving spot and the weak
spot are close enough to each other, the weak spot will ‘jump’ into the strong spot and
the laser will start to operate under external cavity conditions.
K) Keep the position of the weak spot in mind and rotate the prism arm in such a way that
the strong spot is in an identical position as the weak spot. The spot should become
much brighter as soon as the laser is above threshold as indicated in Fig. 7 (4).
Laser Power
Cavity Alignment
Fig. 8: Schematic of the dependence of the laser power as a function of the cavity
alignment.

Technical Documentation
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Sacher Lasertechnik GmbH Tel.: [phone] Sacher Lasertechnik, LLC Tel.: [phone]
Rudolf Breitscheid Str. 1-5 Fax: [phone] 5765 Equador Way Fax: [phone]
D-35037 Marburg, Germany Email: [email protected] Buena Park, CA 90620, USA Email: [email protected]
L) The optimization of the cavity will be performed via the cavity alignment screw. Once
laser action is found, no more than +/- 1 rotation of the cavity alignment screw is
required.
M) A typical dependence of the laser power on the cavity alignment is shown in Fig. 8.
Usually, the best adjustment is achieved as soon as the laser power reaches its
maximum value. Make sure that the laser is not operating at a side maximum by
verifying the cavity alignment according to Fig. 8. Otherwise you will not be able to
achieve the specified values.
N) After optimization of the cavity alignment screw, you may try to optimize the laser
collimation for improving the laser power. No more than 1/8 of a turn of the collimator
adjustment screw will be required.
O) Repeat step L) and M) until the maximum power is achieved.
Please note that it may be difficult to determine which cavity alignment maximum will
provide the provides the highest power and the best laser performance.
Furthermore, there are types of laser diodes where the alignment for the highest power
does not coincide with the alignment for the best laser performance.
Therefore after optimizing the laser for the highest power, it is recommended to check the
minimization of the threshold current due to the feedback of the external cavity.
3.2 Minimizing the Laser Threshold
When optimizing the cavity adjustment via the cavity alignment screw, you may find
several maximum within the laser power as a function of the cavity alignment screw as
indicated in Fig. 8. Only one of these power maximum will provide you the best laser
performance.
Please note that the maximum with the highest power does not necessarily provide the
best over all performance of the laser system. A procedure for choosing the best
alignment is to check which power maximum offers the lowest threshold current.
A simple method for monitoring the threshold current as follows. Required items are an
oscilloscope, a power meter with a monitor exit and a signal generator.
A) Connect the laser current modulation input of the laser controller to signal generator
which provides a triangular voltage signal.
B) Alternatively, you may use the internal Ramp Generator function of the laser controller
together with the Current Coupling function of our laser controller for generating the
laser current modulation.