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Technical note: Extended Cavity Diode Laser in
Litrow Configuration at the TRIP Facility
S. De, L. Willmann
August 14, 2008
1 Laser diodes
We are using several Diode Lasers (DL) at wavelengths 1500.4 nm, 1130.6 nm,
667.7 nm and 659.7 nm (see Table 1) for performing atomic spectroscopy as well
as for laser cooling and trapping of barium. Diode lasers are advantageous for the
atomic physics research because of their size, relatively simple to use and lower in
cost [3, 4]. Now a days laser diodes are available in a wide range of wavelengths
and with different optical power. Such a plot for available diode lasers at differ-
ent wavelengths can be found out elsewhere [2]. These diodes have unique name
according to their stabilization technique or fabrication procedure in the semicon-
ductor industry. A bare laser diode (LD) without any other stabilization mechanism
is known as solitary diode. In that case one needs external cavity for stabilizingits frequency. Such a system is named as Extended Cavity Diode Laser (ECDL).
Some laser diodes are frequency stabilized with a grating within a semiconduc-
tor chip and commercially available in a rather compact form, they are named as
Distributed Feed Back (DFB) laser.
A commercially available QFBLD-1550-20 distributed feedback diode laser
(from QPhotonics, VA, USA) produces light at wavelength 1500.4 nm with a max-
imum output power of 17 mW. The frequency of this laser is stabilized with a
grating within the semiconductor chip. The light is coupled into a single mode
fiber attached to the diode chip. The frequency can be changed by altering the
lasers temperature or its operating current. The laser is tunable over a wide fre-
quency range without any mode hops. A commercially available mount LM14S2(from Thorlabs. Inc., NJ, USA) interfaces the laser in a 14 pin butterfly package to
the temperature and the current controller.
Visible laser light is generated with the laser diodes QLD-660-80S (from QPho-
tonics, VA, USA) and DL3149-057 (from Thorlabs. Inc., NJ, USA) at the wave-
lengths 659.7 nm and 667.7 nm. For infra-red light at wavelength 1130.6 nm a laser
diode LD-1120-0300-1 (from TOPTICA Photonics AG, Grafelfing, Germany) is
used. The output powers are 8 mW, 5 mW and 250 mW respectively. The laser
diodes are in commercially standardized packages of 5.6 mm and 9 mm diame-ter. They are stabilized in extended cavity diode laser configuration in home made
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Diode lasers
Part No. QLD-660 DL3149- LD-1120 QFBLD-80S 057 -0300-1 -1550-20
Supplier QPhotonics Thorlabs TOPTICA QPhotonics
USA USA Germany USA
Wavelength 659.7 nm 667.7 nm 1130.6 nm 1500.4 nm
Power 8 5 300 17
(mW)
Typical frequency tuning coefficients
Current 1200 1000 175 106
(MHz/A)
Tuning 560 MHz/V 500 MHz/V 70 MHz/V 21 MHz/
Actuator (PZT) (PZT) (PZT) (thermistor)
Table 1: Characteristics of the diode lasers used in the experiments.
mounting systems. The compact diode laser systems are user friendly for spec-
troscopy experiments because of their simplicity, size and cost [3, 4]. Detailed
descriptions of grating stabilized diode lasers can be found elsewhere [?, ?].
The commercially available solitary diodes of diameter 5.6 mm or 9 mm (see
Table 1) need a biasing through cathode and anode relative to a common terminal
for working. Output frequency of a LD is very sensitive to both operating current
and temperature. The front and back side of the diode have high reflectivity due
to the large dielectric constant of the semiconductor material. The reflectivity is
greater than 10%. The resonator cavity of a LD is built up by two opposite facet
perpendicular to the semiconductor junction. The typical cavity length of a DL is
on the order of 250 micon. This results typical mode spacing of the spectrum of
about 120 GHz or 0.25 nm at 800 nm lasing wavelength. Changing the temperature
changes band gap of the semiconductor material, which is directly related to the
change in lasing frequency typically 0.10.3 nm/K, depending on diodes. Changein diode current changes carrier density in the junction and that changes refractive
index of the cavity and hence changes frequency. Typically change in frequency
of a LD with current is 0.01 nm/mA. We set the diode up in Extended CavityDiode Laser (ECDL) configuration for frequency stabilizing them using an external
cavity.
We mount the LD and all other necessary optics in ECDL configuration on a
home made mounting system (Fig. 1). Since the output of a DL is diverging, we use
an aspheric lens of focal length f = 511 mm, very close to the diode to collimatethe beam. The divergence of a DL is not symmetric in all the directions because of
the shape of the laser diode facet, which is typically 10 50m. Thus the spatial
extension of the collimated beam is more in one axis than the other. After the lens,
we use a reflection grating to diffract the laser in higher orders. The diffraction
angle (m) is proportional to wavelength of the incident light and the m-th order
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Groove
normal
Gratingnormal
0-order
diffracted
Incidentllight
-1 orderdiffracted
blazeangle
i r=i
Diffraction from a grating in Littrow configuration
d = 1/groove density
p/2
+1 orderdiffracted
Figure 2: Diffraction phenomena from a grating in Littrow configuration, whichhas been used for external frequency stabilization of a diode laser.
diffraction equation from a grating is
dsin(i) + dsin(m) = m, (1)
where d is the distance between two consecutive grooves, i.e, d= 1groove density
.
We stabilize the output frequency of the diode laser by feeding back the light
of the -1 order into the diode. Hence we force the laser to resonate at that particular
frequency that we feed back from the grating. There are many ways to build up an
external cavities for DL stabilizations. We have chosen the Littrow configuration
(Fig. 2). In this configuration the diffraction angle of the -1 order relative to the
grating normal (1) is same as incident angle (i). Thus the above diffraction
equation reduces to
2dsin() = , (2)
considering i = 1 = . Tuning the grating angle changes the effective lengthof the external cavity , which selects different wavelength in the diffracted beam.
Thus frequency of the output laser changes accordingly. We use a piezo connected
to the grating mount for homogenous tuning of the grating angle, hence it scans the
frequency of the DL. There we use a low voltage piezo with an extension coefficient
ofd(length)
d(voltage) 30 nm/V. (3)
If the lever arm of the mount on which the grating rotates freely in the plane of
the laser diode base has a length l, a voltage dv applied to the piezo will produce a
rotation angle
d() =1
l
d(length)
d(voltage)d(V). (4)
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In that case, the change of wavelength with changing the piezo voltage can be
estimated from the grating equation,
d = 2dcos() d() (5)
= 2dcos() 1
l
d(length)
d(voltage)d(V). (6)
Taking an example, that a LD at wavelength 1130 nm seated up in ECDL using
a grating of 1200 lines/mm at an angle = 8
, we get tuning of wavelength by
the piezo is 3.2(1) pm/V or 0.75(2) GHz/V at wavelength 1130 nm. The loose
end of this configuration is, changing angle of the grating changes the pointing of
the diffracted zero order beam. Over one meter distance from the DL the pointing
changes typically few ten micron for scanning over a range of 30 MH z. Since
for spectroscopic purpose we scan the laser by some 20 30 MH z, change ofthe pointing of the laser does not affect the experiment that much. That is why we
have chosen Littrow configuration other that Littman-Metcalf configuration, where
pointing does not change while scanning the laser by angle tuning of the grating.
At the cost of the available output power from the diode laser for doing exper-
iment, the choice of the grating with respect to the angle between grating normal
and the groove normal (blaze angle,B) is important. Commercial grating are char-
acterized by groove density, i.e., number of grooves/mm (n) and by blaze angle.
The blaze angle affects the diffraction efficiency of the grating at any wavelength.
For a particular order, the diffraction angle only depends on and on d, which
is inversely proportional to n. The relative position of the LD and the grating is
such that one likes to have the first order diffraction angle close to 45 o, so thatthe light coming out from the LD os not blocked by the diode laser holder (Fig.
6). That means, the zero order reflected light from the grating on the order of 90 o
relative to the incident light. In this configuration we only have 0-th order and -1
order after diffraction, thus we do not loose power into the other diffraction modes.
The diffraction efficiency from a grating depends on total number of illuminated
grooves and on the polarization of the incident light. Thus commonly people use
the minor axis of the laser beam cross-section aligned parallel to the grooves of
the grating. The output of the diode laser is polarized in the direction parallel to
the semiconductor junction of the diode, hence along the narrow dimension of the
spatial mode. From the definition of the polarization the light incident on grating
is s-polarized. In the Fig. 3, we show polarization dependent diffraction efficiencyfor the light of wavelength at 1100 nm on a 1200 lines/mm grating with different
blaze angle.
In a grating stabilized diode laser the feedback from the grating has to be larger
than the light reflected back from the facet of the diode itself. In practice 20
30% feedback from the grating is sufficient for infrared lasers and some higher
percentage, up to 50%, at the visible wavelength. Here the advantage of the Anti
Reflection (AR) coated laser diode facets are clear, because then less feedback from
the grating would be needed. In Fig. 3, we see that s-polarization have always more
diffraction efficiency than the p-polarization. One has to choose a grating so that
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Figure 3: Polarization dependent absolute diffraction efficiency of ruled gratings
with fixed number rulings per cm (1200 linem/mm) but blaze for different wave-
lengths.
there is sufficient power available into the -1 order beam for the stabilization. On
the other hand one can use a half wave plate to rotate the polarization to make the
standard Littrow configuration simple, one can choose a right grating depending
on wavelength to make the system reasonably efficient.
An example of the choice of the grating for a laser at 1100 nm wavelength is
1200 lines/mm. This corresponds to the first diffraction order at an angle of42o.
A grating which is blazed for 400 nm would be appropriate.
Now we are moving to some more detail on the electrical connections for run-
ning the DL. We connect the current controller and the temperature controller to
the diode laser through the male sub D-15 connecter mounted in the diode laser
mounting table. We use cables of different colors for different types of diodes to
connect the three pins of a commercially brought solitary diode to the sub D-15
connector fixed on the diode laser table. There are two different types of diodes
available, anode ground (AG) and cathode ground (CG) types. Sometimes there
is a photodiode (PD) connected with the laser diode, which measures the emitted
light power from the diode. Since they are also diodes, they can be two different
types AG and CG. So, in total there are six different combinations possible between
different types of LD and PD. We use unique color codes of the wires connecting
the diodes and the sub D-15 connectors for identifying them easily. That allows to
select the mode (AG or CG) of the current controller without touching the diodes
every time.
The female sub D-15 mainly provides connection of the current controller to
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Figure 4: Color code of the wires we use in our lab for connecting different types
of commercially available TO-3 type laser diode to the female sub D-15 connector
mounted on the diode laser mounting table. (a), (b), (c) laser diodes are for cathode
ground (CG) type and (d), (e), (f) are for anode ground (AG) type.
the LD and the TEC cooler connection to the peltier element. There is s red LED in
the female sub D-15 connector which glows only when the LD is switched on and
also there is a green LED which glow up when the LD is broken. The protection
diode connected between pin-4 and pin-10 protects the LD from any large current
flow through the diode due to any accidental short circuit. The connections are
shown in Fig. 6(a).
A special cable has been made, which connects the temperature controller
(KVI, custom design or TED 200C, Thorlabs) through a male sub D-9 and also
connects the current supply (LDC 200, LDC 500, Thorlabs) through the other fe-
male sub D-9 connector. The sub D-9 connectors has been chosen to be compatible
with commercial available temperature and current controllers, for example tem-
perature and current controllers from Thorlabs Ins. Other side of the cable has a
male sub D-15 connector for making the connection to the female sub D-15 at-
tached to the DL mounting table. The pin connection of the custom cable is given
in Table 3. A 4 wire LEMO cable is needed for connecting the Male sub D-9 con-
nector of the special cable with the temperature controller, which is custom made
at KVI. One end of this cable will have female sub D-9 connector and the other
end will have a male 4 pin LEMO connector. The pin configurations of the cable
is given in Tab. ??.
Other important thing to know is the mounting of the laser diode into the home
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1 2 3 4 5 6 7 8
9 10 11 12 13 14 15
Blue (AD590 -Ve)
Red (TEC +Ve)
White (AD590 +Ve)
Black (TEC -Ve)
Red LED
GreenLED
ProtectionDiode
(See color code in thetable) photo diode
(See color code in thetable) laser diode
(Green) ground
(a) Female Sub D-15 attached to the laser diode base (TOP view)
1 2 3 4 5 6 7
9 10 11 12 13 14
(b) Special dual connector cable for temperature and current controller (TOP view)
1 2 3 4 5
6 7 8 9
15
1 2 3 4 5
6 7 8 9
Male Sub D-15 connecter for thefemale sub D-15 attached to the diode laser base
Female sub D-9connector for the current
controller LDC500
Male sub D-9connector for the diode laaser
temperature controller (TC200)
Figure 5: Schematic of the electrical connections (a) Sub D-15 connector mounted
to the diode laser mounting table (b) Connections of the custom made cable for
providing connections to the temperature controller and to the diode laser driving
current.
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7 wire sub D-9 sub D-15 sub D-9 7 wire
LEMO cable Male Male Female LEMO cable
wire color pin number pin number pin number wire color
- - 1 1 pink
- - 2 2 white
- - 3 3 gray
- - 4 4 yellow
- - 5 5 green
pink 7 6 - -
- - 7 - -green 4 8 - -
- - 9 7 brown
- - 10 8 blue
- - 11 - -
- - 12 - -
gray 9 13 - -
white 1 14 - -
blue 5 15 - -
Table 2: Pin connection of the custom made cable connecting the current and the
temperature controller.
4 pin male Female sub D-9
LEMO connector connector
pin number wire color pin number
1 brown 42 white 5
3 yellow 9
4 green 7
Table 3: Pin connection of the cable connecting the KVI temperature controller to
the female sub D-9 connector of the custom made special cable.
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7
S3
S1
S2
S41. Diode laser holder2. Lens holder3. Grating4. Grating holder5. Piezo6. Adjustment screw7. Strain relief of diode laser8. Diode laser out putS1. M3X12 hex screwS2. M2X8 hex screwS3. M3X16 hex screwS4. M2X15 circular screw
8
Figure 6: Drawing of the diode laser base specially made for ECDL Litrow config-
uration
made mounting systems for ECDL configuration. Solitary diode in a TO-3 pack-
age are mounted in a diode laser holder. The holder has to be of right diameter
according to the 5.6 mm or 9 mm package type of the diode, so that the laser diode
fits into it. A press ring on the back of the diode which fixes the position inside
the holder. Before pressing the ring we have to check the orientation of the spatial
mode of the laser beam relative to the grating plane. The smaller spatial exten-
sion has to be along the grooves of the grating and the wider spatial extension of
the beam has to the perpendicular of the grooves to get maximum diffraction effi-
ciency. The diffraction efficiency depends on total number of illuminated grooves.Once we place the diode at a right angle we fix it by pressing the ring. In front of
the diode we use an aspheric lens for collimating the diverging beam. The screws
of the lens mount, which fixes the lens holder to the diode laser base, allows to
change the distance from the DL to the lens within few mm. In this way we make
the laser beam as good collimated as possible. A grating is mounted on the grating
mount and is screwed up with the diode laser base. First we fix the grating angle
and then fix the grating mount with the diode laser base by the screw. The part
of the diode laser base which holds the grating has an fine adjustment screw, that
allows the fine adjustment of the grating in the vertical plane. The screw connected
with the piezo mount also has a fine adjustment for fine tuning of the grating angle
in the horizontal plane. That changes the grating angle relative to the propagationdirection of the laser.The hardware for the ECDL has been shown below in Fig. 7
and Fig. 8. Different parts and screws are also indicated there in the figure.
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1. Female Sub D -15: electrical connection for controlling diode laser2. 2-pin female LEMO ENG .OB: high voltage for piezo connected with grating3. White box: base for setting the diode laser in Littrow configuration4. Dotted box: TEC cooler sitting under the neath of the DL base5. S1: stainles steel screw M3X16, hex head6. S2: stainles steel screw M3X12, hex head7. S3: stainles steel screw M2X8, hex head8. S4: stainles steel screw M2X15, circular head9. P1: set screws for keeping diode laser holder fixed at a position10. P2: set screw for veretical alignment of the grating
Sub D - 15Female
2-pinLEMO
S1 S1 S1
S1 S1 S1
P1
P1
S2
S2
S3
S3 P2
S4 S4
S4
Diode laser base specially designed for ECDlittrow configuration
Figure 7: Schematic of the diode laser mounting table, which contains diode laser
base and other necessary electrical connections for running the laser.
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2 Wide band frequency tuning of a laser diode in ECDL
configuration
To know about the frequency tuning of a diode laser, first we have to understand
little more about the lasing mechanism. The emission of light for a particular
diode laser is limited within a frequency range. The amplitude of such an emission
spectrum has a maximum at a particular frequency and that falls on either side
of that frequency. That is called gain profile. There are several resonator modes
with different center frequencies and with a certain spectral width are oscillating
through the gain medium. Each of these resonator modes experience different gain
thus the gain profile has a spectral width. That has been described else where in
more detail [?].
In case of a ECDL we build up an external cavity using a grating and feed backa fraction of output light into the laser cavity. The detail of the ECDL have been
described in section-1 of this chapter. So, in this case we feed back a particular
resonator mode coming out of the diode into the laser diode cavity and force the
diode to lase at that mode. In this way we make the laser diode operational in
a single mode and in a narrow spectral range, of the order of 1 MH z. In this
configuration changing the length of the external cavity matches with a different
mode, i.e., a different frequency, in the diode cavity. On the other hand changing
of the temperature and injection current also changes the lasing frequency. That
has also been described in section-1.
Taking example of our 1130.6 nm laser diode (LD112003001), it tunes
frequency (f) with the injection current (i) as,
d f
di= 172(5) MH z/mA. (7)
The temperature (T) tuning of the frequency is
d f
dT= 20(10) MH z/mK. (8)
Since we use this laser diode in an ECDL set up, that also allows us to frequency
tune of that laser by tuning the grating angle. The grating angle changes with
the tuning of piezo voltage, which is connected with the grating. This provides
frequency tuning rate of
d f
dV= 70(20) MH z/V. (9)
The tuning rate is very much dependent on the lasing mode, thats why the error
bar on the tuning rate is rather large. From these tuning rates we see that the
laser frequency is very sensitive to the current as well as temperature. That means
for stabilizing a diode laser within the natural line width of a transition (on the
order of 20 MH z), it requires a very stable current supply and a stable temperature
controlling unit. Changing any of these three parameters, current, temperature
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Figure 8: Tuning of the DL frequency by changing injection current to the laser
diode. Each valley in the plot is corresponds to one mode-hop-free tuning range.
or piezo voltage, scans the frequency of the laser. Wider tuning range of these
parameters causes jump of the laser from one mode to another mode, that is called
as mode-hop. Mode-hop-free tuning range are very different among different lasers
and also this range is different in different laser modes. The theory behind this
phenomena are rather complex and we are not interested to know that in detail.
For wider scanning one has to change more than one parameters at the same time.Thus, when laser is appearing to jump from one to another mode due to changing
of one parameter the other parameter changes in a way to keep the mode fixed. So,
this process requires a synchronized tuning of these parameters. There is not any
particular theory behind the way of changing them. That varies diode to diode,
even if they are categorically identical. For a particular DL it is rather trial and
error method in the beginning and once it is working we have fixed this process.
For our 1130.6 nm diode laser the average mode-hop-free tuning range is on the
order of 800 1000 MH z. Since changing of temperature requires some time for
the temperature controller to stabilize the case temperature of the laser diode, we
have chosen changing of current and grating angle in a synchronized way for the
above mentioned technique for widening the mode-hop-free scan range. In Fig. 8and Fig. 9 we see the typical characteristics of the 1130.6 nm DL. The wavelength
of the diode jumps between two different range of wavelengths, 1129 .800.15 nmand 1130.500.15 nm, in steps of 6001000 MH z while changing either the piezovoltage or the current.
The schematic of the electrical connection for the synchronized tuning of the
current and piezo voltage has been shown in the Fig. 10. We generally use a trian-
gular wave front, ramp signal, out of a frequency synthesizer and pass it through
two individual amplifiers. The amplifiers are made out of operational amplifiers
(OPAMP) and gain of the amplifiers are defined by ratio between feedback resis-
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Figure 9: Tuning of the DL frequency by tuning piezo voltage, which connected
with the grating. Each valley corresponds to one mode-hop-free tuning range.
4
4
7
72
2
3
3
R4
R3
R2
R1
R0
R0
6
6
Ground
Ground
+
_
+
_
Frequencysynthesizer
Diode lasercurrent controller
Modulation in
High voltageamplifier
Piezo connectedto grating
Injection currentfor laser diode
+15
+15
-15
-15
OPAMP
OPAMP
Figure 10: Block diagram of the electrical connections for the synchronized tuning
of diode current and piezo voltage.
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Figure 11: Distribution of output power along the spatial axes of a diode laser
output.
tors (R2 and R4) to the bias resistors (R1 or R3).
gain o f the OPAMP = f eedback resistancebias resistance
. (10)
We use output of one OPAMP directly to the modulation IN port of the current
supply of the laser diode for external modulation of the injection current. Gain of
these two amplifiers has been chosen such that change of one volt in the ramping
changes frequency of the DL by the same amount in both ways by changing of
current and piezo voltage. The fine tuning on the gains can be done by changing
bias resistance on the potentiometer. The output from other OPAMP we amplify
using a high voltage amplifier (HV amplifier) and then apply that to the piezo for
tuning the grating angle. In this way one can choose different mode of the laser by
changing offset voltage of the ramp signal. That shifts the tuning range for both
current and piezo at the same time. The HV amplifier also has a separate offset,
which changes the scan range of the piezo voltage only. For fine adjustment of the
mode-hop-free tuning, we use the offset of the HV amplifier. In this way we have
achieved to scan the 1130.6 nm DL by 1 GHz in a mode-hop-free tuning.
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Bibliography
[1] W. Demtroder, Laser Spectroscopy, Springer, Third Edition, chapter 5 (2002).
[2] http://www.qphotonics.com/.
[3] K. MacAdam, A. Steinbach, and C. E. Wieman, Am. J. Phys. 60, 1098 (1987).
[4] L. Ricci, M. Weidemuller, T. Esslinger, A. Hemmerich, C. Zimmermann, V.
Vuletic, W. Konig, and T. W. Hansch, Opt. Comm. 117, 541 (1995).
[5] C. E. Wieman, and L. Hollberg, Rev. Sci. Instrum. 62, 1 (1991).
6 Diodelaser6L. Hollberg, R. Fox, S. Waltman, and H. Robinson, NIST Technical
Note 1504, May (1998).
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