Diatomic Cesium in a Diode-Pumped Alkali Laser System

Jamey ChristyEric MartinezTanner OakesJake Smith

Kendrick Walter

Submitted in Partial Fulfillmentof the Requirements for the

Bachelor of Science in Electrical Engineering

New Mexico Institute of Mining and TechnologyDepartment of Electrical Engineering

May, 2008

Abstract

Diode Pumped Alkali Laser (DPAL) systems combine the positive characteristics of chemical

and diode lasers. These systems create a laser that is compact and efficient, while working

well at high temperatures and high powers. In conjunction with the Air Force Research

Laboratory (AFRL), an attempt was made to improve DPAL technology by using a diatomic

alkali metal as a gain medium. By switching to the diatomic species, it was theorized that

the system would be able take advantage of the diatomic’s wider absorption spectrum. This

would better match the line width of the diode pump and improve the power scalability.

Unfortunately, diatomic cesium will not lase due to very high input power requirements,

partially due to quenching, and pre-disassociation affects. However, during testing it was

discovered that diatomic cesium works as a relaxing agent for atomic rubidium.

In typical DPAL systems, ethane is used for a relaxing agent. This is creates problems

in current DPAL systems because ethane does not work well at high temperatures or high

powers because it burns and creates a tar-like coating inside the cell. The discovery that

diatomic cesium can be used as a relaxing agent, without having the negative effects of

ethane at high temperatures, allows the ethane to be replaced in rubidium based DPAL

systems. This addition greatly increases power scaling and efficiency.

Fully investigating this discovery is beyond the scope of this project, but the following

actions are recommended. First is testing different ratios of diatomic cesium to atomic

rubidium in the chemical cell to find the optimum ratio. Additional research is also needed

into the electro-physics occurring inside the diatomic cesium/atomic rubidium cell, which

may lead to additional discoveries with different combinations of alkali metals and diatomic

molecules for future DPAL systems.

Contents

1 Introduction 5

1.1 Current Laser Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 DPAL Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.3.1 Deliverables and Specifications . . . . . . . . . . . . . . . . . . . . . . 10

2 Literature Review 12

2.1 Laser Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.1.1 Electron Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.1.2 Laser Inversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 Alkali Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3 Diatomic Cesium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3.1 Energy Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.2 Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3 Experiment Procedure 21

3.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Pumping a Diatomic Cesium Cell . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3 Lase with Atomic Rubidium . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.4 Attempts to Lase Diatomic Cesium . . . . . . . . . . . . . . . . . . . . . . . 29

3.4.1 Pump with Ti:Sapph . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1

3.4.2 Maximize Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.4.3 Add Diode Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.5 Cell Burn Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Results and Discussion 32

4.1 Unable to Lase Diatomic Cesium . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2 Lase Atomic Rubidium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2.1 Diatomic Cesium as a Relaxant . . . . . . . . . . . . . . . . . . . . . 36

5 Conclusion 38

6 Future Work 39

2

List of Tables

1.1 Deliverables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.2 Final System Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3

List of Figures

1.1 Diode Pumped Alkali Laser System . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 DPAL Setup at AFRL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 Periodic Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Alkali Laser Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Diatomic Cesium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4 Diatomic Alkali Laser Transitions . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5 Diatomic Cesium Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.6 Expected Emission Spectrum of Diatomic Cesium . . . . . . . . . . . . . . . 20

3.1 Expected Emission Spectrum of Diatomic Cesium . . . . . . . . . . . . . . . 21

3.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3 Ti:Sapph Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Picture of Optical Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.5 Diatomic Cesium Laser Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.6 Diatomic Cesium Laser Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.7 Refined Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.1 Curvature of a Laser’s Input/Output Power . . . . . . . . . . . . . . . . . . 33

4.2 Power In vs Power Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.3 Rubidium Lasing Inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.4 Cell Temperature and its Effects on Cell Activity . . . . . . . . . . . . . . . 36

4

Chapter 1

Introduction

This paper presents the results and discoveries of a project involving the use of diatomic

alkali metals to improve Diode-Pumped Alkali Laser (DPAL) systems. It will focus first on

the original goal of using a diatomic alkali as a lasing gain medium in order to better utilize

the wide line width of currently available diode lasers. This concept was proved impractical

because lasing could not be achieved with test lasers that were significantly higher power

than the diode lasers that would be used in a real system. However, during the testing

process, it was discovered that diatomic cesium can be used as a relaxing agent for the

atomic rubidium laser system. While further testing of this discovery is outside the scope

of the project, the impact of the discovery and how it could be tested and utilized will be

discussed.

1.1 Current Laser Technology

Since their invention, lasers have had a great impact in creating many new commercial

devices. Optical storage, such as CDs and DVDs, allows large amounts of data to be stored

in compact form, while at the same time being easily and cheaply produced. In medicine,

lasers have been used to perform eye surgery that would be impossible with traditional

means. Lasers can also be used in other forms of surgery, making them less invasive and

5

allowing faster recovery. Heavy industry has adopted lasers, with lasers making extremely

precise cuts through some of the toughest materials. Despite this variety of applications,

there are areas in which a laser system is wanted, but there is no laser able to meet the

demands of the application. By looking at two of the most common types of commercial

lasers, diode lasers and chemical lasers, it is easy to see where these gaps are, and what a

new laser would have to do to fill those gaps.

In diode lasers, an electrical current is used to excite a semi-conducting material to release

a stream of photons. Diode lasers are relatively inexpensive, small in size, and require small

amounts of power to operate. This makes them ideal for consumer devices such as optical

storage. Their downfall is that the beam produced is of poor quality, and they have low

maximum power output.

A second major class of lasers is the chemical laser. In a chemical laser, a chemical gain

medium is excited to produce the laser beam. The gain medium can be a gas such as helium

or neon, a liquid such as dyes, or a crystalline solid such as ruby. These gain mediums are

capable of handling large amounts of power, and they produce a high quality beam with a

uniform wavefront. This makes them ideal as cutting lasers. Their downfall is that they are

expensive, large, and often require elaborate cooling systems that add to the cost, size and

complexity of the overall system. This limits them to semi-stationary locations for use with

specialized systems in order to justify the high cost.

In between these two classes of lasers exists a range of applications that needs a laser

that is small and inexpensive enough to be mass-produced for commercial devices but with a

medium power output in the hundreds of watts. One of the new technologies that attempts

to bridge this gap is Diode Pumped Alkali Laser (DPAL) technology.

6

1.2 DPAL Technology

The DPAL laser system creates a hybrid laser by combining a diode laser with a chemical

cell. This process balances advantages and disadvantages of each parent system, creating

a final system with properties between those of diode and chemical lasers. In the DPAL

system, a diode laser is used to pump a chemical cell containing vaporized alkali metal as

shown in Figure 1.1. The chemical cell acts as a gain medium, increasing the output power

of the system. DPAL systems have been successfully made with all of the alkali metals.

However, problems have been encountered in the implementation of the system that makes

them commercially inviable The goal of this project is to try to eliminate or reduce some of

these problems in an attempt to make DPAL systems commercially viable.

Figure 1.1: Diode Pumped Alkali Laser System

While there are several problems with current DPAL systems, there are two that will be

the focus of this project. The first is a mismatch between the line width of currently available

diode pumps and the atomic alkali’s absorption spectrum. The second is temperature and

power limits due to the use of ethane as a relaxing agent. Overcoming either one of these

problems would be a major step towards making DPAL systems commercially viable.

The first issue with current DPAL implementations is a mismatch between the diode

pump’s line width and the absorption line width of the alkali metals. The most advanced

diode lasers have a line width of 2 nm. However, the absorption line for a monatomic alkali

is on the order of two magnitudes narrower. The line width on the diode lasers can be

7

reduced by a factor of ten using an external grating, but this is still higher than desired, and

the grating increases the cost and complexity of the system [1]. Thus, even with the best

available technology, DPAL systems are highly inefficient. Figure 1.2 shows a typical DPAL

laser setup.

Figure 1.2: DPAL Setup at AFRL

The original project attempted to resolve this problem by switching from a monatomic

alkali to its diatomic form. This was done because the diatomic form adds additional energy

levels from the atoms ability to rotate and vibrate on the covalent bond between the two

atoms. These additional energy levels have the effect of broadening the absorption spectrum.

This creates a better match to the line width of the diode laser, making the system more

efficient.

The second problem with current DPAL implementations is the need for a relaxing agent

8

in order to make the alkali metals lase. The most common and currently most effective agent

is ethane gas. The problem with ethane is that it will break down if it is subjected to too

much heat or power. If the ethane does break down, it forms a tar like coating on the inside

of the cell, making the cell opaque, and essentially ruining the cell. This has two detrimental

effects on the system. First, it limits the amount of power that can be pumped into the

cell, and thereby the amount of power that can be output from the cell. Second, it limits

the temperature inside of the cell, reducing the energy of the alkali atoms, and ultimately

reducing the efficiency of the system. This immediately affects the power output, and it also

introduces a reliability problem. If the system operates below the threshold temperatures

and powers, the ethane could still break down if a ”hot spot” were to develop, which requires

the possibility of regular replacement of the cell.

While using the diatomic alkali as a gain medium was mainly being looked at for solving

the line width problem, it also has the secondary benefit of not requiring that ethane be

added as relaxing agent. This allowed much higher temperatures and input powers to be

used. While the original plan of using the diatomic alkali as a lasing gain medium was proved

not to work. It was discovered that diatomic cesium does act as a relaxant agent for atomic

rubidium. This will allow the replacement of the ethane in atomic rubidium DPAL systems,

allowing them to run at higher powers and temperatures.

1.3 Purpose

The purpose of this project was to improve DPAL technology. The original project looked

at enhancing three characteristics of a DPAL laser system. The first characteristic that was

examined was improving line width matching between the diode laser and the absorption

spectrum of the gain medium. This was accomplished by switching from atomic rubidium

to diatomic cesium. The second improvement was that diatomic cesium works without the

addition of an additional relaxing agent, increasing the power and temperature range of

9

the system. These are important because it would allow a laser system based off of this

design to use commercially available diode pumps instead of expensive test pumps. While

this design was proved to be inviable, a discovery was made that provides a new avenue for

future research. This discovery that diatomic cesium could be used as a relaxant for atomic

rubidium allows the system to be used at high temperatures than with the system using

ethane as the relaxing agent.

1.3.1 Deliverables and Specifications

Originally, the project was to build a system based around using diatomic cesium as a lasing

gain medium. The deliverables for such a system are shown in Table 1.1.

Deliverable Additional InformationFeasibility Study A feasibility study of an diatomic alkali laser using diode-

pumped alkali laser technology will be produced. Thestudy will entail assessing possible output wavelengths,determining pumping wavelengths, and determining theavailability of suitable diode pumps.

Diode Pump Laser System The laser will then incorporate a diode pump at the op-timal optical pumping wavelength as determined by thefeasibility study. The final laser system will be fine tunedand optimized. A demonstration of the system will thenbe given.

Table 1.1: Deliverables

Since the project was primarily a proof of concept, the final specifications for the system

were to show lasing with the diatomic cesium, with commercial considerations such as power

scaling to be left to later research. To be judged as successful, the final system was to meet

the specifications shown below in Table 1.2.

10

Specification Additional InformationWavelength between 0.8 - 3.0 microns A wavelength will be picked to provide the

greatest power efficiency.Power efficiency of 35% This will provide enough power to make a

diode pumped alkali laser useful.Power output of 5 mW Only 5 mW are needed to prove the system

as chemical lasers can be effectively powerscaled.

Diode widths between .1 nm and 10 nm The laser will be able to use commerciallyavailable diode pumps.

Table 1.2: Final System Specifications

Since diatomic cesium did not lase, the project specifications and deliverables were

changed to focus on the discovery of diatomic cesium as a relaxing agent. The sponsor

approved these changes. The final deliverables included the results of the initial feasibility

study and recommendation for future work. The results of the feasibility study are that

the diatomic cesium could not be made to lase with the current system. Furthermore the

energy requirements to make the diatomic cesium lase are too high to be considered for a

practical system. The recommendations for future work are to determine the ideal ratio of

atomic rubidium to diatomic cesium to make an efficient cell, and to determine the reaction

mechanism that makes diatomic cesium work as a relaxing agent.

11

Chapter 2

Literature Review

There is a large amount of research dealing with the chemistry of lasers. The fundamental

laser principles are well understood, and current research focuses on new materials and better

pumping methods. Much of the theoretical background research behind diode-pumped alkali

lasers has been completed, and there is ongoing research being done on diatomic alkali gain

mediums.

2.1 Laser Chemistry

A laser is composed of an energy source, a series of optics, and a gain medium. Energy is

pumped from the energy source into the gain medium exciting electrons to a higher state. A

laser is formed when a population inversion is created and the excited electrons fall back to

a lower state releasing energy in the form of electromagnetic waves. A series of optics is used

to collimate the released energy into a beam. The energy source can be an electrical current,

bright light, a heat source, or even another laser. The power density of the energy pump is

more important than the overall output power. Power density is the amount of power over

an area. In the case of a laser pump source the power density can be increased by either

reducing the beam size by using a telescope, or by increasing the output power of the laser.

12

2.1.1 Electron Transitions

Atoms are composed of a nucleus and a number of electrons occupying discrete energy levels.

Each energy level an electron can occupy in an atom is defined by a set of quantum numbers.

To describe an energy level within an atom term symbols are used. Term symbols take the

form 2S+1LJ , L represents the total orbital angular momentum quantum number, 2S + 1

is the multiplicity of the term with S being the total spin quantum number, and J is the

total angular momentum quantum number of the energy level [2]. The electron structure of

diatomic molecules is more complex. The energy levels are still defined by term symbols but

the symbols take the form 2S+1∆+/−Ω,(g/u), with the +/- and g/u terms showing the symmetry

of the energy level [3]. Electrons can change energy levels by the absorption or emission of

a photon. The absorption of a photon causes an electron to move to a higher energy level,

and the emission of an electron causes an electron to a lower level. Because the energy levels

in an atom are discrete only photons with wavelengths corresponding to the difference of

energy between two states can be absorbed or emitted. The relationship between wavelength

and energy is shown in equation 2.1.

λ = hc/E (2.1)

where λ is the wavelength, h is Planck’s constant, c is the speed of light, and E is energy.

Often wavenumbers are used instead of energy and wavelength. Wavenumbers are equal to

1 over the wavelength.

Lasers involve at least two electron transitions called the pumping transition and the las-

ing transition. In the pumping transition photons with certain wavelength are used to excite

electrons to a higher energy state. The electrons then relax to a lower energy state emitting

a photon with a wavelength corresponding to the change of energy. These emitted photons

are amplified in an optical cavity and focused into a beam of collimated light. Electrons

will make some intermediate transitions between the pumping and lasing transitions. Lasers

13

with one intermediate transition are three level lasers, two intermediate transitions are four

level lasers [4]. The amount of energy lost during the intermediate transitions is one of the

factors in determining the efficiency of the laser, and is referred to as the quantum defect of

the laser.

2.1.2 Laser Inversions

In order to form a laser there needs to be more electrons in the excited states than in

the target lasing state. This situation is called a laser inversion. This requires that the

intermediate transitions happen faster than the pumping transition, leaving the highest

energy level open for another electron to be pumped into it. For some gain mediums, such

as atomic alkali gain mediums, a buffering gas must be added to increase the intermediate

transition rates. The electron loses energy due to collisions with the buffering gas and drops

to lower energy levels quicker than it would otherwise. It is also desired that the lasing

transition happen at a slower rate to ensure that there is time for multiple electrons to be

in excited states. If electrons fall to the target lasing level too quickly an inversion cannot

be formed, this is known as quenching.

2.2 Alkali Lasers

Alkali metals are the elements in the first column of the Periodic Table (Figure 2.1). The

alkali metals all have the same electronic structure of a single valence electron.

14

Figure 2.1: Periodic Table

This leads to a 2S1/2 ground energy level and to a 2P1/2 and 2P3/2 first and second excited

energy levels. The alkali is pumped to the 2P3/2 energy level called the D2 transition and

then relaxes to the 2P1/2 energy level, see Figure 2.2. From there the alkali then lases back

to the 2S1/2 ground level, called the D1 transition. The quantum defect of these materials is

quite small, 2% for Rb and 5% for Cs [4], therefore alkalis have the potential to make very

efficient lasing mediums.

Figure 2.2: Alkali Laser Transitions

Three issues must be overcome in order to lase an alkali metal [4]. First, the pumping

energy needs to be absorbed in the narrow D2 transition. Currently this is accomplished

by diffraction grating the diode pump laser to narrow its bandwidth, and using a relatively

15

long optical path through the cell to allow as much absorption volume as possible. Second,

the transition between the 2P3/2 and 2P1/2 levels needs to happen in an efficient and rapid

manner. This transition can be sped up by adding a small molecule gas such as ethane to

increase the collisional mixing rate by an order of magnitude. Third, the energy from the

D1 lasing transition needs to be efficiently extracted without burning a spectral hole in the

gain medium. This is caused by standing waves canceling each other out in the gain medium

and effectively reducing the gain of the medium at the lasing wavelength. To counter this

the vapor can be buffered by a noble gas to broaden out the D1 and D2 transitions which

disrupts the interference caused by the standing waves.

2.3 Diatomic Cesium

Diatomic cesium, shown in Figure 2.3, consists of two cesium atoms bonded together with

a covalent bond. The properties of the bond give rise to a more complex energy structure

than the energy structure of a single cesium atom. Due to the many energy levels available,

diatomic cesium has wide bands in certain regions of its spectrum.

Figure 2.3: Diatomic Cesium

The high number of energy levels allows many different wavelengths to be emitted. Each

wavelength is associated with a corresponding electron transition. Due to the high number

of electron transitions there are many energy levels that electrons can fall back to as they

relax, a process known as quenching, and time between transitions can be short. Electrons

16

pumped to the higher B energy levels can also escape and cause the diatomic molecule to

break apart. This is known as pre-dissociation, and it happens when the time need to escape

the bond is shorter than the relaxing time to a lower energy state. The high number of

states in diatomic cesium is both an advantage and a disadvantage.

2.3.1 Energy Structure

Diatomic cesium is formed by a covalent bond between two cesium atoms. The energy

levels of each atom overlap and create energy wells occupied by electrons. In addition to

the electronic energy levels there are additional energy levels caused by the vibration and

rotation of the atoms around the covalent bond. Diatomic cesium has a ground level of

X1Σ+g , a first excited energy level of A1Σ+

u and a second excited energy level of B1Πu [5].

Each of these levels is split into numerous sub energy levels by the vibrational and rotational

energy levels. The energy of each energy level can be approximated using equation 2.2 [6].

Constants for diatomic cesium can be found in [7].

T = Te + we (v + .5)− wexe (v + .5)2 + [Be − αe (v + .5)] J (J + 1) (2.2)

where T is the calculated wavenumber, Te is the wavenumber of the electronic transition, we

and wexe are vibrational kinematic constants, v is the vibrational energy level, Be and αe

are rotational kinematic constants, and J is the rotational energy level.

The wavelength associated between two transitions is determined by equation 2.3 [8]. The

bigger the energy gap between two transitions the shorter the wavelength of light emitted.

λ =1

T2 − T1

(2.3)

where λ is the wavelength of the resulting transition, and T2 and T1 are the wavenumbers of

each energy level.

Lasing with a diatomic cesium gain medium is similar to lasing with an atomic alkali

17

gain medium. The pumping transition excites an electron from the X ground state to the B

excited state. The lasing takes place when the electron relaxes to the A excited or to the X

ground state, see Figure 2.4. The three lasing issues that atomic alkali gain mediums have

are not present with a diatomic molecule gain medium due to the multiple energy levels in

the diatomic molecule.

Figure 2.4: Diatomic Alkali Laser Transitions

2.3.2 Spectrum

The spectrum of diatomic cesium has been measured in various experiments. The absorption

spectrum of cesium is shown in Figure 2.5 [9]. The 750 nm to 800 nm peak is caused by the

transitions between the X and B excited states. The peaks around 850 nm are the atomic

cesium transitions that have been broadened out by inter-atomic interactions.

18

Figure 2.5: Diatomic Cesium Spectrum

When pumped at 775 nm 780 nm and 785 nm, diatomic cesium emits the spectrum shown

in Figure 2.6 [10]. The largest peak in each spectrum is at the wavelength of the pumping

laser, and the rest of the peaks are emissions from diatomic cesium as the electrons relax to

lower energy levels. The intensity units are arbitrary and only give a relative measurement

of the peak’s intensity.

19

Figure 2.6: Expected Emission Spectrum of Diatomic Cesium

Peaks to the left of the pumping wavelength are toward the blue side, to the right are on

the red side. It is impossible to lase with peaks on the blue side. This indicates more energy

is coming out of the laser than is being put into the laser. Peaks toward the red side are the

viable lasing peaks.

20

Chapter 3

Experiment Procedure

Figure 3.1 shows the critical path that was followed in this project. The project was divided

into three main phases: Preparatory Works Phase, Test Pump Phase, and Max Power Phase.

Figure 3.1: Expected Emission Spectrum of Diatomic Cesium

The preparatory work phase consisted of determining the spectrum of diatomic cesium.

This was done by pumping the cell with a Ti:Sapph laser. Based on the results of this test

21

, three possible pumping wavelengths were selected for further examination in later phases.

These wavelengths are 757 nm, 765 nm, and 768 nm. Once this phase was completed, the

team moved onto the Test Pump Phase.

The Test Pump Phase consisted of building the optical system and optimizing it. During

this phase, several tests were done to try and lase with the diatomic cesium cell. All of

these tests proved unsuccessful, and it was determined that the next step was to increase

the amount of power being pumped into the cell. This led the project into the Max Power

Phase.

In the Max Power Phase, an additional diode pump was added to the optical setup. The

diode pump added an additional watt of input power into the cell. This additional power

was still not enough to make the diatomic cesium lase. However, during this test, atomic

rubidium was present as a contaminant and it was lasing without the ethane usually needed.

After further testing, diatomic cesium was acting as a relaxing agent for the atomic rubidium.

With these results, the project moved away from attempting to lase with the diatomic cesium

and towards recommending what to do with the discovery of diatomic cesium as a relaxing

agent.

The critical path also includes a timeline that shows when each specific phase of the

project was completed. The Prep Work Phase was completed on February 20, 2008, the Test

Pump Phase was completed on March 20, 2008, and the Max Power Phase was completed

on April 18, 2008.

Figure 3.1 also includes as the expenditure rate of the budget. The test equipment was

located at AFRL in Albuquerque, NM, so it was necessary to make trip to the lab in order

to work on the project. The 400 dollars of internal funds were used to cover some of the

travel cost from going from Socorro to Albuquerque.

The tasking is also described in Figure 3.1. The group collaborated and worked together

on the entire project. However, specific individuals led each key part of the project. The

legend on the bottom of the diagram indicates which individual led and co-led each portion

22

of the project. For example, the spectrum part was led by Tanner and the co-leader was

Eric.

3.1 Experimental Setup

The optical setup went through many revisions during the experimentation process. Fig-

ure 3.2 presents the basic design. The Ti:sapph system is fed with up to 15 Watts from the

diode-pumped solid-state Nd:YVO4 laser. This is then frequency doubled with a KTiOPO4,

or KTP crystal. The resultant output is the green 532 nm pump source for the Ti:Sapph

laser (Figure 3.3).

Figure 3.2: Experimental Setup

23

The Ti:Sapph was used because of its wavelength output adjustability using a birefringent

crystal system. Through these conversions, the output power of the Ti:Sapph was reduced

to a maximum of 3 Watts. It is important to note that the lasers progressively move toward

the red because electrons need to be pumped at least to the next shell in order to create an

inversion. The 1/2 Waveplate, to the right of the pump source, is a polarizer. This gives

the beam the correct orientation needed to travel correctly through the beam splitter. The

polarizer used was adjustable and when combined with the polarization beam splitter, it

could be used to adjust the power level. This method is more accurate and faster to use

than trying to adjust the Ti:sapph to have the desired output. The focusing lens seen below

was usually a set of optics setup as a telescope. They were used to bring the beam width

down to the theoretical minimum inside the cell. The PBS then acts as a one-way valve,

allowing the beam to go through the splitter on the way in, and then when it bounce off the

high reflector and goes back, acts as a mirror and deflects all the light towards the output

coupler. The oven was used to vaporize the cesium dimer inside of the cell and was typically

heated to 265 degrees Celsius, but some of the experiments were run at up to 300 degrees

Celsius. The output coupler and the high reflector comprised the sides of the lasing cavity,

and were changed based on the length of the cell so that the beam stayed focused through

the entire length.

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Figure 3.3: Ti:Sapph Laser

Other optical components were part of the setup including irises, isolators, and periscopes.

Irises were used at numerous points to help in aligning the beam and to reduce the impacts

of reflections from other parts of the system. The isolator on the output of the Ti:Sapph

was used to protect the laser from back reflections. Two periscopes were implemented to

correctly align the beam one for the vertical detection the other for the lateral. When a

diode pump was added to this setup, an additional set of periscopes and PBS were added

to get both beams to follow the same path. In addition beam blocks were used to contain

the beam while adjustments were being made to the system, and a low power Helium-Neon

laser was used to check alignment of components

Figure 3.4 shows all of the components of the experimental setup. The periscope and

mirrors are towards the bottom right. They sent the laser from the Ti:Sapph through an

25

Figure 3.4: Picture of Optical Setup

iris for alignment purposes. Then, the laser went through a telescope, which has the lenses

placed far apart. The polarization beam splitter, high reflector, output coupler, and alkali

cell are all close to each other to minimize alignment errors. A CCD camera detects the

activity inside the cell, and the final laser after output coupler was sent to a power meter.

3.2 Pumping a Diatomic Cesium Cell

The first step in the testing process was to build the test cell and oven and to determine

the cells excitation in the region of interest. The initial diatomic cesium cell was four inches

long, and about an inch in diameter. The oven was constructed by placing the cell into a

metal shell, which was then wrapped with a heating tape. A thermocouple was placed next

26

to the cell to control its temperature.. A picture of the cell is shown below in Figure 3.5.

Figure 3.5: Diatomic Cesium Laser Cell

First, the cell was heated to 250C, and then it was pumped with the Ti:Sapph laser. The

Ti:Sapphs output was swept over the region of interest 750 nm to 800 nm. A CCD camera

looked through a hole in the side of the cell and measured the different wavelengths of light

inside the cell to display it on a computer monitor. The results with the most transition

levels for diatomic cesium are shown in Figure 3.6.

The top panel of Figure 3.6 shows the transition levels for when the cell was pumped

with a 757 nm wavelength, and heated to 266C. The large peak at 757 nm is due to the

Ti:Sapph pumping. This graph shows a large emission at 759 nm in comparison to the other

two graphs and observations. Since an electron emission is necessary to lase, the first graph

showed a possible wavelength and temperature for lasing. The middle panel shows higher

27

Figure 3.6: Diatomic Cesium Laser Cell

peaks farther away from the pumping wavelength. The bottom panel shows several emission

lines with relatively large magnitudes. Only the spectral lines to the right of the pumping

wavelength may be used for lasing because lasing in the gain medium occurs at a higher

wavelength than the pumping due to the transition energy levels.

3.3 Lase with Atomic Rubidium

To test that the experimental setup was working and to gain familiarity with the lasing

process, the system was first built and tested using atomic rubidium. A rubidium cell was

heated to around 200C, and it was pumped from the Ti:Sapph laser at 780 nm wavelength.

The system then lased at the expected 795 nm wavelength. With the correctly operating

28

system, the optical setup was optimized to improve the power efficiency. Then the cell was

replaced with a diatomic cesium cell.

3.4 Attempts to Lase Diatomic Cesium

In attempting to get the cesium dimer to lase, a procedure was used that was based off of

the previously successful attempt to build the atomic rubidium DPAL system. This process

benefited from use of the CCD camera, which allowed the conditions to be observed and

monitored inside of the cell. The test facility did not have the CDD camera when the atomic

rubidium DPAL was built, and the ability to observing the transitions greatly improved the

testing process.

3.4.1 Pump with Ti:Sapph

The particular model of Ti:Sapph that was used was the MBR-110. This is a broadband laser

that can be tuned from 650 nm to 1100 nm, but operate most efficiently at about 800 nm.

In these experiments the laser had a crystal installed that allowed for an adjustable output

wavelength from 750-820 nm. The first step was to determine an ideal cell temperature

and wavelength at which to lase at. A number of measurements were made at different

temperatures to determine the emission spectrum of diatomic cesium.

3.4.2 Maximize Power

Initial tests showed that the cesium dimer cells were being excited by the test system but

without enough power to actually produce the lasing threshold or inversion. First, the main

laser was increased to its maximum power determined by the Ti:Sapph laser. The next

step was to narrow the width of the beam to concentrate as much power as possible in the

smallest area. This was accomplished using a series of telescopes. The first telescope system

used first lens with a focal length of 250 mm, with the second lens placed so that the focal

29

points of the two lenses coincided. This process produces a collimated beam with the new

beam width proportional to the ratio of focal lengths of the two lenses. A series of telescopes

were used to decrease the beam width from one-half to one-forth and finally to one-eighth

its original size. This focusing still did not create a high enough power density to create the

laser. As a final step, one of the employees in the lab disassembled and cleaned the Ti:sapph

to increase its power back up to its original maximum, which became slightly degraded due

to dust and other imperfections.

Figure 3.7: Refined Telescope

In Figure 3.7, the lenses for the new telescope are shown. The laser from the Ti:Sapph

laser goes from right to left. First, it goes through an iris and then through the telescoping

lenses. After these focusing lenses, it goes to the polarizing beam splitter, alkali cell, high

reflector, and output coupler.

30

3.4.3 Add Diode Pump

In an attempt to further increase the power in the system, a diode pump was added to the

setup. The diode pump was from the Satur tiger series and has a 1 Watt output, 3 MHz line

width, and an ideal output wavelength of 780 nm. While this increase in the input power

of the cell did not cause diatomic cesium to lase, another discovery was found during this

process. This discovery was that there was atomic rubidium in the cell, and it was lasing

without the ethane usually needed as a relaxing agent.

3.5 Cell Burn Out

In order to improve the output of the cell, the oven temperature was increased multiple

times. The output power did increase as the temperature increased. However, when the

oven reached too high of a temperature, the cell burned out. Cesium is an extremely reactive

element; thus, it will react strongly with other elements. The cell walls were made out of

quartz glass which cesium will not readily react with. However, once the temperature of the

cell passed a certain threshold, the cesium was able to react with the crystal quartz in the

cell walls and left a brown residue on the inside of the cell. This ruined the cell, and further

testing was impossible.

31

Chapter 4

Results and Discussion

The project found that diatomic cesium does not work as a gain medium using the experi-

mental setup. One reason is that the power threshold for the alkali cell was never reached.

It was never saturated. This was true for multiple pumping wavelengths and beam sizes.

However, diatomic cesium works well as a relaxing agent for atomic rubidium. It allowed

the alkali rubidium lase even though there was only trace amounts of rubidium in the alkali

cell.

4.1 Unable to Lase Diatomic Cesium

Measurements of the power that was input into the oven versus the power that was present

on the output of the oven were taken. During this process, the Ti:Sapph laser was used.

The purpose of this procedure was to determine if the diatomic cesium cell was saturating.

Ideally, the laser’s input/output power should resemble the curve in Figure 4.1.

32

Figure 4.1: Curvature of a Laser’s Input/Output Power

At first, the output power rises linearly with the input power. Once the alkali cell is

saturated, the output power remains fairly flat with changing peak output power for different

cell temperatures. If there is too much power going into the cell, it will burn out, and the

curve will drop at that temperature.

The Beam size of the Ti:Sapph is considered. When the beam goes through a telescope,

the beam gets smaller in size. A beam size of 0.5 indicates that the beam is half the size

of the original beam, a beam size of 0.25 indicates that the beam size is a quarter of the

original beam. By decreasing the size of the beam, the intensity of the beam is increased and

the energy is more concentrated. Figure 4.2 show that a saturation level was never reached.

Therefore, there is not enough input power present to lase diatomic cesium.

33

Figure 4.2: Power In vs Power Out

Figure 4.2 shows pumping wavelengths of 765 nm and 757 nm with two beam sizes. The

plots compare the input power to the output power. As can be seen the line continues to

increase in a fairly linear fashion. If the plot had a drop off point then the diatomic cesium

could potentially lase.

Other research on lasing diatomic cesium has also concluded that diatomic cesium is not

a viable lasing medium [11]. Energy that is pumped into diatomic cesium is lost in two ways.

First, enough energy is being absorbed by the electrons to allow them to escape fragmenting

the molecule. As the vapor is in equilibrium the molecule can eventually reform, but the

absorbed energy is lost. Second, due to quenching the electrons fall back to random lower

energy states. This is seen as the multiple peaks in the emission spectrum as diatomic cesium

is pumped. Because much of the energy was being used in these two processes, very little

energy was available to create a laser.

34

4.2 Lase Atomic Rubidium

During the process of adding the diode pump laser to the setup it was found that atomic

rubidium was present in the cell. Figure 4.3 shows an inversion. This means that the Rb (795

nm) had a stronger magnitude that the pumping lasers (780 nm). Both the taller Ti:Sapph

and the shorter diode laser lines can be seen at 780 nm, just slightly at different wavelengths.

Figure 4.3: Rubidium Lasing Inversion

To verify that diatomic cesium was causing the atomic rubidium to lase the temperature

of the oven was slowly lowered. This is shown in Figure 4.4.

35

Figure 4.4: Cell Temperature and its Effects on Cell Activity

As the temperature decreased the emissions seen from both the diatomic cesium and

atomic rubidium also decreased. Atomic rubidium in an ethane environment will lase at

temperatures above 120C. When the temperature of the cell decreased to 180C, all emis-

sions ceased. This showed that without stimulation from the diatomic cesium the rubidium

in the cell can not lase.

4.2.1 Diatomic Cesium as a Relaxant

Diatomic Cesium creates an environment for atomic rubidium in which all three conditions

for rubidium to lase are fulfilled. Collisions between diatomic cesium molecules and rubidium

atoms causes the rubidium atoms to lose some of their energy. As a result, this speeds up

the intermediate energy transition. Diatomic cesium also broadens out the lasing transition

36

of rubidium thereby inhibiting a spectral hole from forming. In addition, diatomic cesium

absorbs the pumping energy that rubidium cannot absorb and some of that extra energy is

released at 780 nm providing more energy to the rubidium. The exact mechanisms involved

are not yet fully understood, and further research is necessary.

37

Chapter 5

Conclusion

There are several important conclusions to be drawn from the work done in this project. The

first is that despite best efforts, it is impossible to lase with diatomic cesium with the setup

used. Most of the energy pumped into diatomic cesium is lost in other transitions before a

lasing inversion can form. With a higher power system or the addition of a quenching gas

it may be possible to get the diatomic cesium to lase. However, such a system would be

impractical and provide little real benefit.

The second discovery was that diatomic cesium acts as a relaxing agent for atomic rubid-

ium based DPAL system. This possibility has never been explored before, and this will allow

the ethane currently being used to be replaced. This replacement will improve the quality

of the current systems in several key ways such as the ability to run at higher temperatures,

and higher input powers.

There is still work to done to fully utilize this discovery in current systems. There were

plans to investigate this property more extensively. Unfortunately, while testing the reaction

at higher temperatures, the cell was destroyed when the cesium metal reacted with the quartz

glass cell wall. Due to time and budgetary concerns, it was not possible to acquire another

test cell. Recommendations for future work will be included as to how to determine optimal

cell properties and investigate the interactions behind the discovery.

38

Chapter 6

Future Work

This project tapped the surface of the potential for using diatomic cesium for Diode-Pumped

Alkali Laser technology. Since this project showed diatomic cesium works well as a relaxant,

this new technology should be investigated more. The next step is to combine different

proportions of diatomic cesium and atomic rubidium into the same cell, and then test the

effectiveness.

The cell used for this project was supposed to contain 100% diatomic cesium according

the cell manufacturer. Tests show that it contained atomic rubidium, and this slight amount

in the cell was able to lase since diatomic cesium is a good relaxing agent. Therefore, only a

small percentage of atomic rubidium is needed. The exact amounts or relative percentages

are unknown because the amount inside the cell for the project remains unknown. One way

to figure out the required amount is to research the interaction between the diatomic cesium

molecules and rubidium atoms inside the cell.

Details about the chemistry inside an atomic rubidium and diatomic cesium cell are

unknown. Also, the reason that diatomic cesium is a good relaxant is unclear. These

topics should be investigated, and this research may lead to discoveries about using different

diatomic alkalis as a relaxant for atomic rubidium or cesium. Additionally, a possibility of

exploring similar results in other columns of the periodic table might be performed.

39

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