Blue Diode Pumped Pr:YLF Laser - ALKAAD - Homealkaad.com/assets/um-le10-manual-prylf-laser.pdfBlue Diode Pumped Pr:YLF Laser Compiled by Akanksha Goyal and Mukul Goyal New Delhi -

ALKAAD Photonics

Teaching and Operation Manual

Blue Diode Pumped Pr:YLF Laser

Compiled byAkanksha Goyal

andMukul Goyal

New Delhi - December 26, 2014www. alkaad.com

2 CONTENTS

Contents

1 Introduction 3

2 Praseodymium YLF Laser 42.1 Energy level system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Experimental Set-up 63.1 Description of the components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1.1 Optical rail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1.2 The diode laser module LD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1.3 Collimator (CO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.1.4 Focusing lens (FL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.5 De-focusing lens (L2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.6 Laser mirror adjustment holders “left” and “right” . . . . . . . . . . . . . . . . . . . . . . . 83.1.7 Set of laser mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.1.8 Pr:YLF crystal in five axes adjustment holder (LC) . . . . . . . . . . . . . . . . . . . . . . 93.1.9 The GG495 filter module (FI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.10 Crossed hair target (CH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.11 Si PIN photodetector module (PD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.12 Photodetector Signal Box ZB1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.13 Birefringent tuner (BFT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.14 Littrow prism tuner (LPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.15 Active q-switch with Pockels Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.1.16 Digital Diode Laser Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.17 Diode laser controller operating software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.18 Laser Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.1.19 Main screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.20 Information screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.21 Injection current settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.22 Temperature settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.23 Modulation settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.24 Duty Cycle settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.1.25 Overheating warning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Experimental Set-up and Measurements 154.1 Characterisation of the blue diode laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Collimating the blue diode laser beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3 Preparing the pump laser focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.4 Inserting the Pr:YLF crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.5 Measurement of absorbed pump laser power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.6 Absorption spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.7 Recording the excitation spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.8 Measuring the lifetime of the excited states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.9 Completion of the set-up for laser operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.10 Stability criteria and laser power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.11 Measuring threshold and slope efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.12 Measuring dynamic laser behaviour, spiking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.13 Wavelength selection and tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.13.1 Wavelength selection by wavelength selective laser mirror . . . . . . . . . . . . . . . . . . . 234.13.2 Wavelength selection with birefringent tuner . . . . . . . . . . . . . . . . . . . . . . . . . . 254.13.3 Wavelength selection with Littrow prism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.13.4 Generation of short pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3

1 Introduction

The first ever operated laser was an optically pumped solid state laser. This laser has been discovered by TheodoreMaiman in 1960 [1]. The active material was element 24, chromium, which was embedded into a transparenthost crystal. The host crystal is a transparent corundum crystal also known as aluminium oxide (Al2O3). Thechromium dopant replaces some aluminium atoms thus changing the optical properties of the crystal. The sodoped crystal shows a red colour and is also known as ruby.

UH pHevLi mBe EB yC kN 8O 9F UDNe

UUNa UpMg UvAl UmSi UEP UyS UkCl U8ArU9K pDCa pUSc ppTi pvV pmCr pEMn pyFe pkCo p8 Ni p9Cu vDZn vUGa vpGe vvAs vmSe vEBr vyKr

vkRb v8Sr v9Y mDZr mUNb mpMo mvTc mmRu mERh myPd mkAg m8Cd m9In EDSn EUSb EpTe EvI EmXeEECs EyBa 57..71 kpHf kvTa kmW kERe kyOs kkIr k8Pt k9Au 8DHg 8UTl 8pPb 8vBi 8mPo 8EAt 8yRn87Fr 88Ra 89..103 UDmRf UDEDb UDySg UDkBh UD8Hs UD9Mt UUDDs UUURg UUpCn UUvUut UUmFl UUEUup UUyLv UUkUus UU9Uuo

↓EkLa E8Ce E9Pr yDNd yUPm ypSm yvEu ymGd yETb yyDy ykHo y8Er y9Tm 70Yb kULu89Ac 9DTh 9UPa 9pU 9vNp 9mPu 9EAm 9yCm 9kBk 98Cf 99Es UDDFm UDUMd UDpNo UDvLr

Figure 1: Periodic table of the elements

E1

E3

E2

fast radiationlesstransfer

optic

al p

umpi

ng

694 nm

Figure 2: Simplified three level system of the ruby laser

The ruby laser boosted a tremendous research effort and initiated a hunt for other more promising lasermaterials. One of the major drawbacks of the ruby laser was the fact that it could only operate in pulsed mode.This is due to its three laser level system as shown in figure 1. By excitation with suitable light chromium ionsof the ground state (E1) are excited and consequently populate the excited state (E3).

From here the only way back to the ground state is via the E2 energy level. In a first step the excited Cr ionsare transferring a fraction of their energy to the lattice of the host crystal and are assembling in the energy levelE2. The transfer from E3 → E2 is very fast and takes place in a few picoseconds.

Note: Population inversion is hard to achieve in a three energy level system like the ruby laser.

However the energy level E2 is a so called metastable state. This means that the Cr ions are trapped in thisstate, since an optical transition to the ground state is forbidden due to the selection rules for electromagnetictransitions involving an emission of a photon according to quantum mechanics. Nature is not strictly mercilessand a forbidden transition still has a certain probability and can be considered as a weak optical transition.Nevertheless the Cr ions will remain approximately five micro seconds in the E2 state (which is fairly long foroptical transitions) before they reach the ground state again.

N2 −N1 =2πn2ν2

c2(1)

We learned that a laser process can only start, if the so called Schawlow-Townes [2] oscillation condition (1)is fulfilled. The equation shows a simplified version of it. In this equation n stands for the index of refraction,ν for the laser frequency and c for the speed of light, N2 is the population density of energy level E2 and N1

accordingly. Only if N2 − N1 is greater zero the equation yields useful results. In other words the populationdensity of state E2 must be greater than that of state E1. This situation is also termed as population inversion,see [3].

4 2 PRASEODYMIUM YLF LASER

Such an inversion is hard to reach since N1 is the population of the ground state, which is always populated.Only under “hard pumping” most Cr ions will be transferred to E2. We have just five microseconds time toalmost empty the ground state before the delayed transfer from E2 starts to populate the ground state.

This is one of the reasons that a ruby laser in general emits pulsed laser radiation. On the search for a moresuitable laser material element 60 (neodymium) turned out to be a good candidate. Laser operation of neodymiumwas first demonstrated by J. E. Geusic et al. at Bell Laboratories in 1964 [2].

In the same way as for chromium atoms of the ruby laser the neodymium atoms are embedded in a host crystalwhich in this case is a composition of yttrium, aluminium and oxygen (Y3Al5O12) forming a clear crystal of thestructure of a garnet. Neodymium atoms are replacing a small fraction of the yttrium atoms. Inside the garnetlattice they are triply ionized (Nd3+).

The outstanding property of such a Nd:YAG laser lies in the fact that the laser process takes place inside afour level energy system (fig. 2). This and the possibility of creating more than 10.000 W output power madethis laser an indispensable tool for a great variety of applications.

Furthermore this laser system is an integral part of the lectures in photonics since it exhibits the importantfour level laser system [1].

From the figure 2 we can conclude that the laser oscillation condition (1) is already fulfilled once the opticalpumping takes place. In this system the population inversion is created between the energy levels E3 and E2 andsince E2 is far above the ground state its population is zero. So even a single excited neodymium ion provides anpopulation inversion. The Nd:YAG lasers began their triumphant success as workhorse in medicine and industry.

Based on the energy levels of neodymium only invisible laser radiation could be created. However the technol-ogy of optically second harmonic generation (SHG) also termed as frequency doubling could bring visible laserradiation. The most important one has been the green 532 nm radiation created by SHG of the strong 1064nm radiation of the Nd:YAG laser. Even by third and fourth harmonic generation deep UV radiation could becreated based on non-linear optical effects.

Because multimedia applications demand powerful RGB (red green blue) light sources, industrial researchbecame interested in these. Along this road the praseodymium laser has been reinvented again since this materialhas the potential to directly emit visible laser radiation on many interesting wavelengths.

Whereas in the past this material has been more of scientific interest [3], it is nowadays considered as anoteworthy candidate for RGB applications. The recent new developments of compact Pr:YLF lasers have beenenabled due to the presence of powerful blue emitting laser diodes. Such blue laser diodes actually have beendeveloped for powerful RGB data projectors.

The aim of the experimental laser diode pumped praseodymium doped YLF laser is to demonstrate its greatpotentials as well as the exciting effect to study a four level laser system with visible radiation.

2 Praseodymium YLF Laser

2.1 Energy level system

Whereas neodymium as colour centre is usually hosted in an yttrium aluminium garnet (YAG), praseodymium isdoped into an yttrium lithium fluoride (YLF) crystal because of different requirements for the phonon spectrum.

The first optically pumped praseodymium laser has been reported ?? in 1977. It used a pulsed dye laser with anemission wavelength of 444 nm as pump source. This way of optically pumping could only be done in a scientificlaboratory equipped with high power laser systems. The development of Pr:YLF laser - the only solid state laseremitting in the visible part of the spectrum - was boosted by the availability of powerful GaN (gallium nitride)laser diodes [5] in 2007 with up to 0.5 W and nowadays (2014) up to 2 W. .

Figure 3 shows an overview of the excitation spectrum of Pr:YLF when pumped with 444 nm. The pumpprocess starts from the ground state 3H4 and populates the 3P2 state. From here the very fast radiationlesstransfer populates the initial laser levels. Depending on the wavelength (energy) the transition terminates in avariety of final states. From here the transition back to the ground state 3H4 takes place as fast radiationlesstransfer. The given laser transitions are just a few among the most important strongest ones. A variety of muchmore lines is possible due to the strong Stark splitting of the involved energy levels. Some more spectroscopicassignments of laser lines can be found in [6]. Now it is time to go to the practical realisation of a Pr:YLF lasersystem.

2.2 Principle of operation 5

3P23P1

1I6

1G4

3F43F33F23H6

3H5

444

nm

fast radiationlesstransfer (FRT)

FRT3H4

3P0

523

nm

607

nm

640

nm

689

nm

721

nmFigure 3: Four level system of the Pr:YLF laser

2.2 Principle of operation

The radiation of the blue emitting laser diode (LD) is collimated by the collimator (CO) which is a high precisionaspheric lens with a short focal length and a high numerical aperture. The resulting beam is parallel in one axisshowing a more or less rectangular to elliptical intensity cross section. The focusing lens (FL) is used to focus theblue pump laser radiation into the praseodymium doped YLF crystal (LC). The Pr:YLF crystal is coated onlywith a broadband anti reflection coating on both sides, so called ARB coating. The wavelength range for lowestreflection covers the entire emission range of the Pr:YLF material including the pump radiation at 445 nm. Theoptical resonator cavity is formed by a flat mirror on the left (M1) and a curved mirror on the right side (M2) asshown in fig. 4.

LD COFL M1

M2LC

Figure 4: Principle of operation

In principle the laser mirror M1 could also be directly coated onto the left side of the laser crystal (LC). Butthis will reduce the flexibility for the operation at different wavelengths because for each particular wavelengthan extra laser crystal would be required.

LC

pump radiation

laser radiation M2

Figure 5: Mode and pump volume

We consider as pump volume the space the pump beam occupies within the laser crystal (LC). In the sameway we consider as mode volume the space a stable resonator mode occupies within the laser crystal. The modevolume is defined by the structure of the cavity. In our example we are using a flat (M1) and a curved mirror

6 3 EXPERIMENTAL SET-UP

(M2) resulting in a hemispherical cavity. In such a cavity the beam waist (where the beam diameter is smallest)lies always on the surface of the flat mirror.

The laser radiation is fed by an population inversion inside the crystal which exists only where the crystal iscovered by the pump radiation.

So laser radiation is only or efficiently created when the pump volume is slightly larger than the mode volume.Within a practical setup one has to choose a proper focusing lens and a suitable curvature of the spherical cavitymirror. As usual the things are a bit more complicated than just mentioned, however we will keep in mind thatthe pump and mode volume overlap can be optimised by moving the focusing lens, laser crystal as well as themirror M2.

3 Experimental Set-up

LD CO FL M2 FI PD ORM1 LC

Diode Laser Controller

Photdetector Signal Box

Figure 6: Diode laser pumped Pr:YLF experimental laser setup

Fig. 6 shows the set-up of the praseodymium YLF experimental laser. All optical components sit on carriermounts with “optics click” mechanisms and are placed onto an optical rail (OR). This allows a convenient butvery precise positioning of carriers. Optic parts are mounted in “click holders” held by spring loaded steel balls.

Laser diode injection current, temperature and modulation are controlled by the diode laser controller MK1.The XY adjustable collimator (CO) is used to collimate the divergent blue radiation of the pump laser diode(LD). The focusing lens (FL) focusses the collimated beam on a point behind the flat mirror (FL) where thepraseodymium doped YLF laser crystal (LC) is positioned. The second mirror of the laser cavity (M2) is followedby a filter (FI), which suppresses the pump radiation and transmits wavelength greater than 495 nm.

The optical signals like pump radiation, fluorescence as well as created laser radiation can be detected by thephotodiode (PD) which is connected to the photodetector signal box. From here the signal is transferred via aBNC cable to an optional oscilloscope to display time dependent signals. In the same way the modulator referenceof the diode laser controller is connected to the oscilloscope.

3.1 Description of the components

3.1.1 Optical rail

The ALKAAD rail and carrier system (figure 12) provides a high degree of integral structural stiffness andaccuracy. The idea for this structure is based on the form of the reference meter of Paris and was furtheroptimised for daily laboratory use. The optical height of the optical axis is chosen to be 75 mm above the tablesurface. The optical height of 32.5 mm above the carrier surface is compatible with many other systems like thosefrom MEOS, LUHS, MICOS. OWIS and LD Didactic. Consequently a high degree of system compatibility isachieved.

3.1 Description of the components 7

Figure 7: Platinum-Iridium meter bar [7]

32.5 mm

75 mm

Figure 8: Rail system

3.1.2 The diode laser module LD

For the efficient optical excitation of the praseodymium doped YLF crystal a pump wavelength of 444 nm at highpower is required. The pump laser diode is mounted onto a Peltier element to control the operating temperaturein a range of 10...50 C . The output power is 1 Watt at a wavelength of 444 nm. The emitted wavelengthdepends for blue diode lasers particularly strong on injection current with 3.3 nm/A. It depends on temperaturewith 0.05 nm/C.

LD

CR

CN

Figure 9: Diode laser module (DL)

DANGER

INVISIBLE LASER RADIATIONAVOID DIRCET EXPOSURE

TO BEAM

DIODELASER

PEAK POWER 0.5 W

WAVELENGTH 808 nm

CLASS IV LASER PRODUCT

Invisible Laser Radiationpower max. 0.5 W 808 nm

Avoid eye or skin exposure todirect or scattered radiation

class 4 laser product

Figure 10: Laser warning labels

This device can emit highly concentrated visible light which can be hazardous to the human eye. When themodule is connected to the controller, the operators of the diode laser module have to follow the safety precautionsfound in IEC 60825-1 “Safety of laser products Part 1: Equipment classification, requirements and user’s guide”[8].

The diode laser is connected by a 15 pin SubD HD connector (CN) to the controller MK1. Inside the connectoran EPROM contains the data of the laser diode and when connected to the controller, these data are read anddisplayed by the controller.

3.1.3 Collimator (CO)

XY

A,B

Figure 11: Collimator module withXY adjuster

MP

FL

Figure 12: Focusing module with60 mm lens (FL)

L2

Figure 13: Optional lens L2

A high precision aspheric glass lens is mounted into a click holder (A,B) which is inserted into the XY adjuster(XY). By means of two fine pitch screws the collimator can be adjusted accordingly.

focal length 4.6 mm clear opening 4.9 mmnumerical aperture 0.53 anti reflex (AR) coating 300...700 nm, ≤ 0.5 %


3.1.4 Focusing lens (FL)

To obtain a very high intensity of the pump light the collimated blue laser beam is focused by using a biconvexlens with a focal length of 60 mm. The lens is mounted into a so called click mount (FL) with a mounting diameterof 25 mm. The mount is clicked into the mounting plate (MP) where three spring loaded steel balls keep the lensprecisely in position.

3.1.5 De-focusing lens (L2)

The optional mounted lens L2 is used in combination with the Littrow prism module as intra cavity element. Ithas a focal length of 50 mm and a high quality anti-reflexion coating R ≤ 0.3 % in a range of 425...700 nm. Thelens is mounted into a 25 mm click mount which can be inserted in any mounting plate for 25 mm click mounts.

3.1.6 Laser mirror adjustment holders “left” and “right”

LM

AH

Figure 14: Laser mirror adjustment holder “left”with mirror M1

LMAH

Figure 15: Laser mirror adjustment holder “right”with mirror M2

The adjustment holders (AH) comprise two high precision fine pitch screws. The upper screw is used to tiltthe moveable plate vertically and the lower one to tilt it horizontally.

The mounting plate provides a M16 mount into which the laser mirror holders (LM) are screwed. The mirroris pressed against a mechanical reference plane inside the M16 mount in such a way that the mirror is alwaysaligned perfectly when removed and screwed in again.

The adjustment holder is either mounted to the carrier in the “left” operating mode to hold the left cavitymirror M1 or in the “right” mode to hold the right cavity mirror M2.

Due to the symmetry of the adjustment holder (AH) it can also be changed from the “right” to the “left” modeand vice versa if required.

3.1.7 Set of laser mirrors

PC LM

MH

Figure 16: Mounted laser mirror

The set-up comprises two sets of mirrors each mounted separately as shown in figure 16. Each mirror has thestandard diameter of 12.7 mm (1/2 inch) and a thickness of 6.35 mm (1/4 inch).

The laser mirror (LM) is mounted into the holder (MH) and kept in position by two spring loaded flaps. Asoft O-ring provides a soft seat of the mirror inside the holder (MH) especially when screwed into the adjustmentholder.

The mirrors are of supreme quality, coated by ion beam sputtering (IBS) yielding the highest degree of reflec-tivity and lowest scatter losses achievable until date. A cap (PC) protects the sensitive mirrors when not in use.Each mirror is labelled and the meaning of the markings is given in the following table:


Marking and labelling of the mirrors:

Mark Coating Label GeometryRED HT 445 / HR 580...720 nm RED FLAT flat mirrorGREEN HT 445 / HR 500...570 nm RED 100 ROC 100 mmHR R > 99.98 % RED 150 ROC 150 mmHT T > 80 % GREEN FLATflat mirrorROC Radius of curvature GREEN 100 ROC 100 mm

3.1.8 Pr:YLF crystal in five axes adjustment holder (LC)

RR

θ

φ

CM

CR

x

y

Figure 17: Five axes adjustment holder

A praseodymium doped yttrium lithium fluoride crystal (CR) with a diameter of 5 mm and a length of 6 mmis mounted into a disk with 3 mm thickness and gently clamped. The disk holding the crystal is set into themount (CM) where it is fixed by using the ring (RR).

The crystal mount (CM) is inserted into the five axes adjustment holder. It is kept in position by a springloaded steel ball in the same way as the lens click mounts. Four precise fine pitch screws for translative (x, y)and azimuthal (θ, φ) adjustment allow repeatable accuracy positioning.

The crystal mount (CM) can be rotated free of play around its axis. This is important to rotate the crystalwith respect to the polarisation of the pump laser radiation. The Pr dopant level is 0.7 % and the crystal is cutalong its c axis termed as c-cut orientation. The end faces of the crystal are polished better I/10 and are coatedwith a high bandwidth anti reflection coating for 440...740 nm with a residual reflectivity R of ≤ 0.1 %.

3.1.9 The GG495 filter module (FI)

FH

FP

Figure 18: Filter module (FI) withplate holder

1.00

400 500 600 700 800 900 1000 nmWavelength →

Tra

nsm

issi

on →

0.95

0.90

0.50

0.10

10-510-410-3

10-2

Figure 19: Transmission curve of the 3 mm thick GG495 filter


The coloured glass filter (FP) GG495 has a thickness of 3 mm and is used to suppress the pump radiationwhich is not absorbed by the Pr:YLF crystal. It is for instance important for the measurement of the lifetime ofthe excited state.

3.1.10 Crossed hair target (CH)

MP

CH

Figure 20: Crossed hair target

A crossed hair target screen is part of a 25 mm click holder (CH) which can be inserted into the mountingplate (MP). By means of three spring loaded steel balls the screen is kept in position. It is used to align a lightbeam by eye sight with respect to the optical axis of the ALKAAD rail and carrier system.

3.1.11 Si PIN photodetector module (PD)

PDMP

Figure 21: Photodetector module

Wavelength λ

0

relS

400445

20

40

60

80

%

100

500 600 700 800 900 nm 1100

Figure 22: Sensitivity curve of the BPX61 photodiode

A Si PIN photodiode is integrated into a 25 mm housing with two click grooves (PD). A BNC cable andconnector is attached to connect the module to the photodetector signal box ZB1. The photodetector module isplaced into the mounting plate (MP) where it is kept in position by three spring loaded steel balls.

Parameter Symbol Value UnitCurrent rise, fall time atRL = 50 Ω;VR = 5 V ;λ = 850 nm and Ip = 800 µA tr, tf 20 nsForward voltage IF = 100mA,E = 0 VF 1.3 VCapacitance at VR = 0, f = 1 MHz C0 72 pFWavelength of max. sensitivity λSmax 850 nmSpectral sensitivity S 10% of Smax λ 1100 nmDimensions of radiant sensitive area L x W 7 mm2

Dark current, VR = 10V IR ≤ 30 nASpectral sensitivity, λ = 850 nm S(λ) 0.62 A/W

Table 1: Basic parameters of Si PIN photodiode BPX61


3.1.12 Photodetector Signal Box ZB1

PO PI 9V

Figure 23: Signal Box ZB1

BNCIN

PD

UP

IP

+

BNCOUT

1 32 4 5 6 7 8 9 10 11 12

Figure 24: Signal box schematic

The signal box contains a resistor network and a replaceable 9 V battery. It can work with all kind ofphotodiodes provided they are connected to the BNC input (PD-IN) as shown in the schematic of figure 24. Atthe output (PD-OUT) of the signal box a signal is present which is given by the following equation:

IP =Up

RL

IP is the photo current created by illuminating the photodiode with light. Up is the voltage drop across theselected load resistor RL. To convert the measured voltage into a respective optical power we have to make useof the spectral sensitivity S(λ) [A/W] which depends on the wavelength of the incident light according to figure19. The detected optical power Popt in W can be given as:

Popt =IpS(λ)

Assuming a wavelength of 700 nm we take the value of Srel from figure 22 as 0.8 and the spectral sensitivityfrom the last row of table 1 as 0.62 A/W, so the value of S(λ = 700 nm) is 0.62 A/W · 0.8 = 0.496 A/W . If wemeasure a voltage Um of 5 V with a selected load resistance RL of 1 kΩ, the optical power will be

Popt =IpS(λ)

=Up

RL · S (λ)=

5 V

1000 VA · 0.496 A

W

= 10mW

It must be noted that the measured power is correct only if the entire light beam hits the detector.

Based on the selected load resistor the sensitivity will be high for higher resistances, but the rise and fall timewill be longer, because the diode capacitance with the load resistor forms a RC low pass filter. For fast signals alow resistance should be used, however the sensitivity will be lower.

3.1.13 Birefringent tuner (BFT)

A detailed description of the property and function of a double refractive tuning element can be found in [9] or[10].

The double refractive or birefringent plate (P) is mounted in a dual rotational stage. For the intra-cavityoperation the birefringent plate (P) needs to be aligned in such a way that the laser beam hits the plate underthe Brewster angle to minimize the reflection losses. This can be accomplished by turning the rotary plate (B).

In addition the birefringent plate can be rotated around its optical axis by tilting the lever (L).

By rotating the plate (P), its optical retardation δ, the path difference between ordinary and extraordinaryray, is changed. Only if the retardation after two passes through the plate is a mulitiple integer of the wavelength,the plane of polarisation remains unchanged and there are no additional losses at the Brewster windows.


3.1.14 Littrow prism tuner (LPT)

Another way to select different lines of a laser is to use a Littrow prism. A detailed description of tuning a HeNeLaser with a Littrow prism is given by [10].

In this experiment we are using such a module to tune the helium neon laser. The Littrow prism is made fromfused silica which is the required substrate for ion beam sputtering (IBS) coating.

The spectral range of the IBS coating covers 580...720 nm with a reflectivity ¿99.98 %. The prism is mountedinto a precise adjustment holder where it can be smoothly tilted in vertical or horizontal direction.

L

B

P

Figure 25: Optional birefringent tuner

V

H

LP

Figure 26: Optional Littrow prism tuner

3.1.15 Active q-switch with Pockels Cell

Delay

ON

OFFPower Trigger

Input H.V. [kV]

PC

PCD

Figure 27: Active q-swicth with Pockels cell

The active q-switch consists of a lithium niobate crystal. Applying a high voltage to it, a phase retardation resultsthe value of which depends on the applied voltage. Further details and a nice description of the fundamentals aregiven by Luhs [11]. The properties of the crystal (PC) are as follows:

Material: LiNbO3 Z-0cut, 6 x 6 x 30 mmHalf wave voltage: 800 V at 633 nmContrast ratio: 200:1Clear aperture: 6 x 6 mmCapacitance: 11.2 pF

The crystal is operated with the controller (PCD) having the following properties: Output voltage: 320..1940 V

Switching time: 11 ns @ 1.500 V and 30 pF cap. loadDelay: 54 . . . 1090 µsRepetition rate: max. 20 kHzTrigger input: TTL


3.1.16 Digital Diode Laser Controller

12V LDUSB

MOD

DISP

Figure 28: Digital Diode Laser Controller MK1

The laser diode module is connected via the 15 pin HD SubD jacket (LD). The controller reads the EEPROMof the laser diode and sets the required parameters accordingly. The MK1 is powered by an external 12 V/1.5A wall plug supply. A USB bus allows the connection to a computer for remote control. Furthermore firmwareupdates can be applied simply by using the same USB bus.

The MK1 provides an internal modulator which allows the periodic switch on and off of the diode laser.A buffered synchronisation signal is available at the BNC jacket (MOD). Furthermore the duty cycle of themodulation signal can be varied in a range of 1 . . . 100 % to enable the measurement of thermal sensitivity of theoptically pumped laser crystal.

The controller is equipped with industrial highly integrated circuits for the bipolar Peltier cooler (Maxim,MAX 1978) as well as for the injection current and modulation control (iC Haus, iC-HG) of the attached laserdiode. Further detailed specifications are given in the following section of the operation software.

3.1.17 Diode laser controller operating software

When the external 12 V are applied, the controller starts displaying thescreen as shown in fig. 3.23. This will take approximately 3.5 seconds.

Figure 29: Start screen

3.1.18 Laser Safety

The first interactive screen requires the log in to the device because due tolaser safety regulations unauthorized operation must be prevented. In gen-eral this is accomplished by using a mechanical key switch. However, thismicroprocessor operated device provides a better protection by requestingthe entry of a PIN.After entering the proper key the next screen is displayed and the systemis ready for operation.

Figure 30: Combination lock


3.1.19 Main screen

For immediate “Laser OFF” just press the yellow button. To set the

injection current press the button and the injection current settingsscreen shows up. The same is true also for the “Set Temperature” as wellas the “Modulation” section. When in operation and connected to thelaser diode the actual laser diode temperature is shown in the “ActualTemperature [C]” section. Furthermore the actual current through thePeltier element is displayed to show if the element is cooling or heating.

Figure 31: Maincontrol screen

3.1.20 Information screen

When pressing the Info button of the main screen this screen comes up.It again reads and displays the information stored in the EEPROM of theattached diode laser. Each successful tap of the screen is confirmed by abeep the volume of can be set and tested by the slider in the “Settings”section. Pressing the “BACK” button brings back the main screen.

Figure 32: Information screen

3.1.21 Injection current settings

By pressing the numerical pads the injection current is selected. For in-stance entering 023 sets the injection current to 0.23 A which is displayedand applied instantly to the diode laser. Pressing the “CLEAR” buttonsets the value of the injection current to zero and switches off the diodelaser. Tapping on the “ENTER” pad the main screen is invoked again. Ifan entry exceeds the maximum current as retrieved from the EEPROMof the attached diode laser the entry is decreased to the maximum value.

Figure 33: Injection current setting

3.1.22 Temperature settings

By pressing the numerical pads the temperature is selected. For instanceenteringing 235 sets the temperature to 23.5 C which is displayed andapplied instantly to the Peltier element controller. Pressing the “CLEAR”button sets the value of the temperature to the default value of 25.0C.Pressing the “BACK” pad the main screen is invoked again. If an entryexceeds the maximum or minimum value retrieved from the EEPROM ofthe attached diode laser the entry is set to the respective minimum ormaximum value.

Figure 34: Temperature setting

3.1.23 Modulation settings

The diode laser can be switched periodically on and off. This is of interestfor a couple of experiments. By pressing the numerical pads the modula-tion frequency is selected. For instance entering 235 sets the frequency to23.5 kHz which is displayed and applied instantly to the injection currentcontroller. Tapping on the “CLEAR” button stops modulation. Pressingthe “BACK” pad leads back to the main screen. A buffered TTL referencesignal is present at the BNC jack on the rear of the controller.

Figure 35: Modulation setting

15

3.1.24 Duty Cycle settings

For some experiments it is important to keep the thermal load on theoptically pumped laser crystal as low as possible. For this reason the dutycycle of the injection current modulation can be changed in a range of 1. . . 100 %. A duty cycle of 50 % means that the OFF and ON period hasthe same length. For instance entering 235 sets the duty cycle to 23.5 %which is displayed and applied instantly to the injection current controller.

Figure 36: Duty cycle setting

3.1.25 Overheating warning

This screen you should never see. It appears only when the chip of theinjection current controller overheats.Tap on the “ACKNOWLEDGE” button to return to the main screen.Wait a couple of minutes and try again. If the error persists please contactALKAAD for service.

Figure 37: Overheating warning

4 Experimental Set-up and Measurements

In the following we will explain step by step the set-up of the different experiments and measurements. Pleasenote that we will not publish measured results. However we will give wherever possible qualitative informationon the values or curves to be expected.

4.1 Characterisation of the blue diode laser

LD

OR

PD

Figure 38: Characterization of the blue diode laser

In this experiment we want to measure the optical power versus the injection current for a set of fixed temper-atures.

To measure the output power in relative units the photodetector module (PD) is placed onto the rail as shownin fig. 38. The detector is connected to the signal box (see also section 3.1.12 on page 11). The output of the boxis connected either to an oscilloscope or to a digital multimeter set to voltage measurement. For a set of differenttemperatures such as 10, 30 or 40 C the voltage Um on the signal box output is recorded.

16 4 EXPERIMENTAL SET-UP AND MEASUREMENTS

Wavelength 445 nm

Temperature 30 C

S(λ)rel 0.23 see fig. 22

S(λ) S(λ)rel · 0.62 A/W see table 1

Injection current [ mA ] Voltage Um [ V ] RL [Ω] Popt [ mW ]

0

...

1000

Table 2: Example for a diode laser output measurement data table

→ Injection current0.2

0.5

0.0

1.0

W

0.4 0.6 0.8 1.0A

→ D

iode

lase

r po

wer T=10°C→

T=30°C → ←T=40°C

Figure 39: Laser power versus injection current and temperature as parameter

Note: Set the distance of the photodetector to the diode laser in such a way that the detector is not saturated.The measured power is just a fraction of the actual power because only a part of the laser light reaches thedetector.

4.2 Collimating the blue diode laser beam

Y

CO

X

Z

CH

LD

OR

Figure 40: Collimating and centring the diode laser beam

Place the collimator module (CO) in front of the diode laser with a free space of 10 mm between both. Switchon the diode laser and select not more than 200 mA injection current or such a value that the diode laser juststarts emitting laser radiation in addition to the blue LED radiation.

Move the collimator towards the diode laser and observe the image on the crossed hair target (CH). Align ifnecessary the X and Y fine pitch screws of the collimator module (CO) in such a way that the image is centredto the crossed hair target (CH).

4.3 Preparing the pump laser focus 17

Go closer with the collimator to the diode laser and you will notice that the beam cross section on the crossedhair target becomes smaller and smaller. If you continue to move the collimator against the diode laser the imageof the beam on the crossed hair target start to grow again. If you reached this point, stop the movement andcheck with a piece of paper if the beam is almost parallel along its way to the target.

If not fine tune the position of the collimator and fix it by fastening the clamping screw. If required also realignthe spot of the blue laser beam to the centre of the target screen.

4.3 Preparing the pump laser focus

f=60 mm

FL

LD

OR

CH

Figure 41: Inserting the focusing module (FL) and creating the pump laser focus

As next step we will create a focus of the diode laser beam as shown in fig. 41. The position of the focusing lensmodule (FL) is not critical, since the initial beam has been parallelised. In a distance of 60 mm which correspondsto the focal length of the applied lens a focus is created and can be viewed on a piece of paper. The position ofthe focus is noted down by reading the scale on the ruler and this is where in the next step the praseodymiumdoped YLF crystal rod will be placed. Before doing this next step, switch off the blue laser.

4.4 Inserting the Pr:YLF crystal

LD

CO FL

LC

OR

PD

Figure 42: Inserting the Pr:YLF crystal

The praseodymium YLF crystal (LC) is placed onto the optical rail (OR) in such a way that the focus of theblue diode laser radiation lies inside the laser crystal. The crossed hair target (CH) is exchanged against thephotodetector (PD).

A series of experiments will be performed to characterise the laser crystal.

4.5 Measurement of absorbed pump laser power

To measure the output power in relative units the photodetector module (PD) is placed onto the rail as shown infig. 42. The detector is connected to the signal box (see figure 23 on page 11). The output of the box is connectedeither to an oscilloscope or to a digital multimeter which is set to voltage measurement. For a set of temperatures


such as 10 . . . 50 C in steps of 2 C the voltage Um of the signal box is recorded. The measurements will be takenwith laser crystal out and in to properly determine the absorbed power.

Note: The laser crystal should be placed always at the same position!

→ Injectionpcurrent0.20.0

0.5

0.0

1.0

W

0.4 0.6 0.8 1.0A

→ab

sorb

edpla

serp

pow

erT=10°C →

T=30°C →

T=40°C →

Figure 43: Absobed laser power in dependence on injection current

Wavelength 445 nm

Temperature 30 C

S(λ)rel 0.23 see fig. 22

S(λ) S(λ)rel · 0.62 A/W see table 1

Temperature [ C] Voltage Um [V] RL [Ω] Popt [mW] Pabsorbed [mW]

Laser crystal OUT IN OUT IN

0

. . . 12 8 4

1000

Table 3: Example for an absorption measurement data table

4.6 Absorption spectrum

A very elegant way to measure a global absorption spectrum is to make use of an optical spectrum analyser whichis available with a USB bus. Such a spectrum analyser displays a spectrum from 400...1000 nm in almost realtime.

To create an absorption spectrum, first the dark spectrum is measured and stored. Then a white lamp is usedto provide a continuous white spectrum. The lamp is fixed with respect to the spectrometer in such a way thatthe spectrometer is illuminated, but not saturated. The white light spectrum is stored as reference spectrum.After that we carefully place the Pr:YLF crystal mounted in its disk on top of the spectrometer opening. Thecrystal should completely cover the spectrometer entrance. Again the spectrum is recorded and stored.

The provided software allows the processing of all three spectra yielding the pure absorption spectrum.

4.7 Recording the excitation spectrum 19

white light lampwhite light lamp

light blocker

absorption spectrumreference spectrumdark spectrum

Pr:YLF crystal

Figure 44: Measuring the absorption spectrum of the Pr:YLFcrystal by using a white light lamp and a spectrum analyser

400 450 500 550 600 650 700

443

467

478

584

594

nm

Wavelength →

rel.

Inte

nsity

→

Figure 45: Measured absorption spectrum ofa Pr:YLF crystal.

4.7 Recording the excitation spectrum

LD

SA

COFL

LC

F

Figure 46: Set-up to measure and record the excitation spectrum for pumping with 445 nm

720

800700 nm600Wavelength →

rel.

Inte

nsity

→

500400

697

639

606

587546

521479

excitation

Figure 47: Excitation spectrum

We are using again a spectrum analyser now equipped with an optical fibre (F). The excitation fluorescence isstrong enough that the fibre only needs to be held in direction of the pumped laser crystal to obtain an almostnoise free signal. Such a spectrum is shown in fig. 45. The resolution of the spectrum analyser is just 1 nm and


a better one will resolve more lines. However, with this simple spectrum analyser the fluorescence lines can beassigned to the transitions of the energy level diagram as shown in fig. 3.

By observing the fluorescence spectrum almost perpendicular to the excitation beam its anyway strong intensityis reduced and the weaker fluorescence lines can be observed.

4.8 Measuring the lifetime of the excited states

LD

CO FLLC

FIPD

OR

Figure 48: Set-up to measure the lifetime of excited states

In this experiment we are interested in the time course of the fluorescence light. To suppress unwanted pumpradiation we are inserting the filter module (FI) in front of the photodetector (PD). The photodetector is connectedto the signal box (fig. 23) and the output of it to the oscilloscope to channel 1. Channel 2 is connected via theprovided BNC cable with the modulation reference of the diode laser controller MK1 (fig. 28).

The oscilloscope is set to trigger on channel 2 (modulation reference) at falling edge. The life time τS of theexcited state is defined as the time, when the fluorescence intensity IF drops to I0/e. This time can be takenfrom the oscilloscope display as shown in fig. 49. In [12] the value of the lifetime is mentioned to be 50 µs.

I0→

If→

τS

Figure 49: Fluorescence decay curve

4.9 Completion of the set-up for laser operation

After completing the spectroscopy related measurement we are going to prepare the set-up for laser operation. Asfirst step we need to align the mirror M1 perpendicular to the blue pump radiation. For this purpose we removethe focusing lens (FL). We note down its position so that we can place it back when the laser mirror M1 willplaced into its final position. After the removal of the lens we place the laser mirror module (M1) onto the opticalrail (OR) as shown in fig. 50.

By turning the fine pitch screws for vertical (V) as well horizontal (H) tilt the back reflected beam until it iscentred to the incident beam. When this has been done, the focusing lens is placed back into its position. The

4.9 Completion of the set-up for laser operation 21

note position

V

Hback reflexLD

CO FLM1

OR

Figure 50: Basic alignment of laser mirror M1

laser mirror mount (M1) is moved towards the focusing lens in such a way that the focus lies just behind the lasermirror.

The laser mirror module M2 is placed onto the rail. The distance d should be chosen such that it is less thanthe radius of curvature (RM2) of the mirror M2. If d exceeds the value of RM2 the cavity is optically unstableand no laser radiation will be obtained.

The back reflex of M2 is now centred to the spot on M1 by adjusting the fine pitch screws for horizontal (H)and vertical (V) movement.

d < RM2

back reflex

LD

CO FLM1

M2

V

H

Figure 51: Inserting mirror M2

After aligning M1 and M2 we place the mounted Pr:YLF crystal (LC) into the cavity as close as possible tothe mirror M1. Turn the Pr:YLF in its holder about the optical axis (see also fig. 17) in such a way that thetransmitted blue light becomes minimum - the diode laser light is polarised and the YLF crystal birefringent, sothe crystal has to be aligned accordingly.

After powering up the pump laser, we will notice again a very bright white fluorescence. Behind the mirrorM2 we will notice a mixture of unabsorbed blue diode laser light as well greenish fluorescence which however wewill notice only if we are placing the GG495 filter onto the rail. By means of a small sheet of white paper we willnotice a greenish spot in the centre. The greenish colour results from the coating of the laser mirror M2 whichreflects the red radiation of the strong white fluorescence light but transmits the green fraction.

If we are turning the adjustment screws (H, V) of the mirror M2, we will notice another green spot movingaccordingly. If we now adjust this spot to the centre of the fixed one, red laser oscillation occurs suddenly.

Once the red laser light occurs, the entire set-up will be aligned for best performance. The distance δ and theposition of the focusing lens is optimised. Furthermore the Pr:YLF crystal is aligned perpendicular to the laseraxis by turning the adjustment screws CV for vertical as well as CH for horizontal tilt - observe the multiplereflections from the crystal faces on the cavity mirrors or around the red beam and tune the m away completely.The better the alignments are, the less the laser threshold will be. Good values are well below 300 mA for theinjection current of the diode laser.


δ

LD

CO FLM1 LC

CV

CH

M2

FI

Figure 52: Adding the Pr:YLF laser crystal into the cavity

4.10 Stability criteria and laser power

Once the laser threshold has been optimised, the stability criterion of the optical cavity is validated. As theresonator cavity is of hemispherical type, the mirror distance d must be less or equal than the radius of curvatureRm of the second mirror. A nice description and derivation of the stability criterion of an optical cavity is givenin [11], [10] or [13].

The output power is measured versus the position of the mirror M2. The experiment may come with twodifferent radius of curvature (ROC) 100 mm and 150 mm. For each ROC the measurement is recorded like Figure53.

4.11 Measuring threshold and slope efficiency

For an optimised and well adjusted set-up the laser output power is measured for a set of different temperaturesof the laser diode. From the resulting graph (like figure 54) the threshold and the slope efficiency is obtained fromthe linear regression of the respective curve.

→ PositionpofpM27050

0.5

0.0

1.0

90 100 120 140mm

→P

r:Y

LGF

prel

.plas

erpp

ower

ROC=100pmm→

← ROC=150pmm

Figure 53: Pr:YLF laser power versus cavity mirrorM2 position

→ Injectionlcurrent0.20.0

0.5

0.0

1.0

0.4 0.6 0.8 1.0A

→P

r:Y

LGF

lrel

.llas

erlp

ower

T=10°C→

T=30°C →←T=40°C

Figure 54: Measurement of laser parameter

4.12 Measuring dynamic laser behaviour, spiking

To measure the time course of the laser power of the Pr:YLF laser we are going to modulate the injection current ofthe diode laser by periodically switching it periodically on and off, so also the pump radiation will be modulated.The photodetector (PD) is placed behind the GG495 Filter (FI) to pick up the generated Pr:YLF laser lightand connected to the signal box (ZK1, PD-IN). The output (PD-OUT) is connected to the first channel of an

4.13 Wavelength selection and tuning 23

Figure 55: Demonstration of spiking

oscilloscope to display the light intensity over time. The oscilloscope is triggered by the second channel which isconnected to the buffered modulation reference signal from the diode laser controller.

The scope is set to trigger on the rising edge of the modulation signal. When powering on the diode laser wewill observe the so called spiking which results from the long lifetime of the excited state.

4.13 Wavelength selection and tuning

4.13.1 Wavelength selection by wavelength selective laser mirror

720

800700 nm600Wavelengthd→

rel.d

Inte

nsity

d→

mirr

ordr

efle

ctiv

ityd→

500400

697

639

606604

“greendmirror” “reddmirror”

587546

521479

100%

excitation

Figure 56: Selective mirror coating

The experiment comes with mirrors with two types of coating. One type is the “green mirror” and the otherone the “red mirror”. In fig. 56 both types are shown with respect to the fluorescence lines of the Pr:YLF crystal.

A strong fluorescence line does not necessarily result also in a strong laser transition. Besides other parametersthe gain of a transition increases linear with its so called effective cross section. Example values for these are givenin the above table. This confirms what we already noticed with our laser experiments using the “red mirrors”:The 639 nm line is the strongest one. Also here is valid that “the winner takes it all”. To force the laser tooscillate on another line we need to introduce losses for the unwanted line.


Cross section of the main lines and their transitions taken from reference [14]

Wavelength [nm] Transition Effective cross section [10−19 cm2]479 3P0 → 3H4 1.9523 3P1 → 3H5 0.3607 3P0 → 3H6 1.4639 3P0 → 3F2 2.2698 3P0 → 3F3 0.5721 3P0 → 3F4 0.9

In the next chapter we will demonstrate how this can be accomplished. A simple but expensive method is touse cavity mirrors which have a high reflectivity only for the wanted line. As an example for this method weuse special coated mirrors to operate the “green line” at 523 nm. The effective cross section for this transition isonly 0.3 ·10−19 cm2 and is the weakest among all other lines. Low cross section means high threshold and carefulcavity alignment.

LD

CO FLM1 LC

CV

CH

99 mm

M2

FI

Figure 57: Preparation to operate the green line

Before we can start to operate the weak green line, we first optimise the set-up with the red mirrors. Thecavity is set close to its optical stability limit at 100 mm mirror distance. By means of a good ruler or vernier thedistance from face to face of the mirror adjustment holder is set to 99 mm. The Pr:YLF crystal is placed about2 mm apart from the Mirror M1. Moving the focusing lens slightly, the position for best laser performance canbe found.

Switch off the pump laser and without changing the positions of the components replace mirror M1 and M2are against the “green mirrors”. Switch on the blue diode laser. Set the modulation to 60 Hz and the duty cycleto 10 %. You might not see any green emission for the moment. Try to scan with the screws of M2 and if youare lucky, the green line comes up. If not, go for the detailed instruction as follows:

A few things should be taken care of:

1. Collimation of the blue light: Watch the spot size in a distance of 2-3 m on a wall and create an almostsame spot size as just behind the collimator and centred to the crossed hair target.

2. Insert the focusing lens

3. Reduce the blue power as much as possible, but with laser emission

4. Place the crystal (carefully cleaned) onto the rail. Observe the beam inside the crystal, move the crystal sothat the beam focus lies at 1-2 mm behind the entrance surface

5. Turn the crystal and maximise absorption of the blue light

6. Place the left mirror adjustment holder with the flat red mirror (cleaned) directly in front of the crystal.The crystal is close (almost touching the mirror)

7. Place the right adjustment holder with the “RED 100” mirror (cleaned) onto the rail in a distance less than100 mm to the left one.

8. Operate the red line

9. Optimise the crystal position, optimise the focus lens position and finally the flat mirror position


10. Align the crystal perpendicular by means of the two horizontal screws of its holder, you will see the resulton a piece of paper, the multiple reflections in red will be reduced to the centre spot.

11. Fix all components if not done before.

12. Without changing anything exchange the red flat against the green flat and the red 100 against the green100 mirror

13. Set the modulation to 60 Hz, duty cycle 10 %

14. Set the power to 600 mA or more.

15. Monitor the wavelength of the blue laser: It should be 444 nm, it depends on injection current as well astemperature.

16. Increase the pump power to achieve 444 nm. Higher current → longer wavelength, lower temperature →lower wavelength

17. An optimum can be seen when rotating the crystal, there should be a significant drop in intensity when youturn the crystal

18. Darken the room as much as possible

19. Take the yellow GG filter and look through it into the cavity to see the right laser mirror.

20. Turn the knob for the horizontal adjustment until you will see a small green spot on the rim of the lasermirror mount

21. Align this spot while still visible on the rim vertically to the centre of the mirror

22. Move back the green spot horizontal to the centre of the mirror

23. You should see now two spots on the mirror (weak) while aligning the moving accordingly. Bring bothtogether and the green line should appear.

Figure 58: Aligning the green spot viewing throughthe GG495

Figure 59: Finally it lases!

4.13.2 Wavelength selection with birefringent tuner

As already mentioned in chapter 2.0 the Praseodymium laser has the potential to oscillate on different visiblewavelengths. The goal of this experiment is to tune to as many as possible of these wavelength. In principle alaser oscillates on a wavelength for which the gain is the highest and losses are the lowest. The gain is determinedbasically by the laser material and the losses mainly by the laser cavity. So for each wavelength a set of mirrorswith appropriate coating could be created that is high reflective for the intended wavelength and transparentotherwise, but this is a quite cumbersome way to select a specific wavelength. It would be much better just toturn a knob to tune to a different wavelength.

Such devices exist, one of it is the so called Littrow Prism and another one is the birefringent tuner. In theset-up of fig. 60 we use a birefringent tuner which is simply placed into the cavity. To avoid any insertion lossesthe birefringent or double refractive plate is turned to the Brewster’s angle. Instead of observing the fluorescencespot at the output of M2 we are using now the reflexions caused by the BFT and align the mirror M2 in such away that the spots of the fluorescence light fully overlap. When rotating the birefringent plate by tilting the lever(L) laser emission should occur.

Gently tune to the maximum of performance and optimise the alignment of the mirror M2. By tilting thelever some other wavelength should show up. It is important that the birefringent plate is cleaned and no dustparticles are visible. It is recommended to modulate the diode laser to obtain also weaker lines.


LD

BFT

SC

CO FL

LCM1

M2L

Figure 60: Setup with birefringent tuner

fC

fC

fL

fL

Littrow prismPr:YLF crystalM1L1 L2

Figure 61: Cavity design for Littrow prism tuning

4.13.3 Wavelength selection with Littrow prism

Using a Littrow prism for laser line tuning requires a modification of the laser cavity as the reflecting surface ofthe prism is flat resulting in a cavity with two flat mirror. Such a cavity is just at the brim of optical stability andis extremely hard to align and thus to maintain laser oscillation is difficult. In order to make the arrangementhemispherical, we place a lens (L2) in front of the littrow prism creating a parallel beam. With the parallel beamthe position of the Littrow prism is not critical.

LDCO

FLLC

M2

M1LP

V

H

Figure 62: Alignment of the littrow prism by using the running Pr:YLF laser as alignment reference.

When the Pr:YLF laser is operating, place the Littrow prism tuner at the end of the optical rail and align itby using its fine pitch screws (H, V) in such a way that the Pr:YLF laser beam is reflected back to itself.

Remove the laser mirror M2. The lens L2 is set into the mounting plate for the photodetector (PD) whichis not in use in this arrangement. Place the lens inside the cavity at the proper distance dF. The Pr:YLF lasershould flash up, optimise the position of the lens and the alignment of the Littrow prism (LP).


LDCO

FL L2LCM1 LP tune

Figure 63: Arrangement with Littrow prism tuner

Once the entire system is optimised, one can start to tune to other lines by turning the fine pitch screw for thehorizontal tilting. Also here it might be a good idea to modulate the diode laser.

4.13.4 Generation of short pulses

LDCO

FL

LC

M1

M2FL PD

PC

Figure 64: Set-up with Pockels cell as active q-switch

The Pockels cell is inserted into the cavity and its HV BNC plug is connected to the HV power supply withthe provided cable. The Pockels cell driver is switched on and the voltage controller knob is set to the lowestvalue. The Pr:YLF laser should work properly. Now increase the voltage to such a value that the Pr:YLF laserstops oscillating.

Figure 65: q-switch pulse

Connect the trigger input of the Pockels cell driver to a frequency generator with a buffered TTL output. Set

28 REFERENCES

the frequency to 250 Hz for the beginning. The photodetector (PD) is connected to the signal box. The outputBNC-OUT is connected to the first channel of an oscilloscope. The second channel is connected to the bufferedmodulation reference signal of the frequency generator. The scope is set to trigger on the falling edge of thetrigger signal.

On the scope we will observe a single peak of the Pr:YLF laser. Depending on the setting of the delay of thePockels cell driver a certain time difference between the switching off of the trigger pulse and the occurrence ofthe laser peak will be noticed. Since the rise time of the laser pulse is only a couple of nanoseconds, the loadresistor RL should be switched to low values until the shape of the oscilloscope track of the laser pulse does notchange any more.

References

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[2] A. L. Schawlow and C. H. Townes. Infrared and optical masers. Phys. Rev., 112(8):1940, 1958.

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