Diode Laser Setup

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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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Diode Laser Setup