Wavelength Tunable Passively Q-Switched Alexandrite Laser with

Direct Diode-Pumping at 635 nm

Ufuk Parali

1,2*

, Gabrielle M. Thomas

, Ara Minassian

, Xin Sheng

and Michael J. Damzen

Photonics Group, The Blackett Laboratory, Imperial College London, London, SW7 2AZ, U.K.

Department of Computer Engineering, Adnan Menderes University, Aydin, 09010, Turkey

Unilase Ltd, 1 Filament Walk, Unit G02, London, SW18 4GQ, U.K.

Keywords: Alexandrite, Passive Q-Switch Lasers, Tunable Lasers, Diode-Pumped Lasers.

Abstract: We report on a wavelength tunable passively Q-switched Alexandrite laser directly red-diode-pumped at

635 nm. Passive Q-switching was achieved with a semiconductor saturable absorber mirror (SESAM) and

wavelength tuning with a birefringent tuner. The pulse repetiton rate was variable on the pump power and

wavelength and a maximum 27 kHz rate was achieved in fundamental TEM

mode. The maximum average

output power obtained was 41 mW. The Q-switched wavelength tuning band was studied between 740 nm

and 755 nm. To the best of our knowledge, this is the first time that tunable TEM

passive Q-switched

operation of a diode-pumped Alexandrite laser has been achieved. The results obtained in this study can be

significantly further optimised for performance. A new cavity configuration for this optimisation is

described. Future work is expected to lead to the development of higher power, more efficient tunable

passive Q-switched (and potentially passive mode-locked) diode-pumped Alexandrite laser sources in the

near-infrared band and also ultraviolet region through frequency conversion.

1 INTRODUCTION

Laser sources possessing broad emission bandwidths

can provide wavelength tunable in a wide range with

capability of different pulse durations (ns/ps/fs) and

capability to potentially offer significantly benefit to

many technological and scientific studies. Nonlinear

microscopy, optical coherence tomography,

frequency conversion, generation of high power

ultrashort optical pulses and remote sensing

applications can be given as some examples

(Damzen, 2014; Teppitaksak, 2014; Koechner,

2003; Ghanbari, 2016;). In air-borne (and also in

space-borne) remote sensing with laser-based lidar

and altimetry techniques, such as resonant

backscatter lidar, ground vegetation bio-mass/bio-

health detection etc., such laser sources could

provide a powerful tool for 3-D mapping of

atmospheric species and physical attributes, spectral

indicators of Earth features and high precision

ground topography (Damzen, 2014; Eitel, 2011; Lu,

2016; Pelon, 1986; Milton, 1997; Chen, 2014). This

would provide a valuable source for understanding

the atmospheric science and health of the Earth

vegetation and ecological system (Eitel, 2011).

However, these remote sensing techniques requires

the utilization of cutting-edge laser technologies

having space-borne qualification. The exacting

requirements for spaceborne operation severely

restricts the class of lasers compatible in the space

environment. The primary laser system with long

space heritage is the diode-pumped Nd:YAG laser

(Damzen, 2014; Eitel, 2011). However, this laser

system has narrow linewidth and does not allow for

wavelength tunability providing only a discrete

single frequency at its primary fundamental laser

line 1064 nm and its higher harmonics at 532 nm

and 355 nm. Thus, accessing other wavelength

regions at these systems is only possible by optical

parametric conversion methods leading to higher

complexity and cost and lower reliability and

efficiency (Teppitaksak, 2014; Koechner, 2003;

Ghanbari, 2016; Teppitaksak, 2015). An alternative

approach for obtaining wavelength tunability and

pulse generation capabilities by directly diode laser

pumping is the utilization of vibronic solid-state

laser materials. Today, the most commonly used

vibronic laser systems are Ti:Sapphire solid-state

lasers, which have the broadest gain bandwidth

permitting direct generation of a few cycle optical

Parali U., Thomas G., Minassian A., Sheng X. and Damzen M.

Wavelength Tunable Passively Q-Switched Alexandrite Laser with Direct Diode-Pumping at 635 nm.

DOI: 10.5220/0006155500820089

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 82-89

ISBN: 978-989-758-223-3

pulses (Teppitaksak, 2014; Koechner, 2003;

Ghanbari, 2016; Beyatli, 2013; Demirbas, 2009).

However, these lasers usually requires complex

pump sources causing disadvantages such as

complex system structure, bulky and large physical

size, and low efficiency of electrical-optical

conversion (Teppitaksak, 2014; Demirbas ,2009). As

an alternative class of vibronic laser crystals, there

are the well-known Cr-doped colquiriites

(Cr:LiSAF, Cr:LiCAF and Cr:LiSGaF) and Cr-

doped chrysoberyl (Cr:BeAl

) more commonly

known as Alexandrite (Teppitaksak, 2014; Ghanbari,

2016; Beyatli, 2013; Sennaroglu, 1998; Demirbas,

2009; Sennaroglu, 2007). Among these crystals,

however there are some performance-limiting

disadvantages such as upper-state lifetime quenching

at elevated temperatures, poor thermal conductivity

and excited state absorption, Cr-doped colquiriite

lasers are not attractive alternatives (Koechner,

2003; Ghanbari, 2016). Conversely, Alexandrite has

a number of superior optical and thermo-mechanical

properties making it an attractive vibronic solid-state

laser gain medium. Alexandrite can be direct diode-

pumped by red, green and blue laser diodes due to

its broad absorption bands in the visible range

(Koechner, 2003; Ghanbari, 2016). However for

highest efficiency and lowest heating factor the red

diodes provide the most favourable pump sources. It

has a fracture resistance five-times bigger than that

of Nd:YAG. And its thermal conductivity (23Wm

-1

) (Koechner, 2003) is almost twice that of

Nd:YAG and five-times that of the Cr doped

colquiriites. The laser emission of the Alexandrite is

highly linearly polarized due to the birefringence of

the crystall eliminating the depolarization problems

(Teppitaksak, 2014; Koechner, 2003). Its relatively

longer upper-state lifetime (~260μs) at room-

temperature provides a good energy storage

potential for Q-switched operation (Teppitaksak,

2014). Moreover, Alexandrite’s laser performance is

increased at elevated temperatures due to its unique

spectroscopic properties (Loiko, 2016). It has a

relatively low stimulated emission cross section (0.7

× 10

-20

) requiring intense diode pumping

(Koechner, 2003). Historically, it is the first

wavelength tunable solid state laser operated at room

temperature. Alexandrite has a broad emission

wavelength range from ~ 700 nm to 850 nm

(Walling, 1985; Sam, 1980). This spectral band is

especially important. It is in the biological window

for tissue transmission and for remote sensing of

vegetation sits across the so-called red-edge band

(~700-750nm) of chlorophyll (Eitel, 2011). This

band is the steep rising transition band between high

red and visible absorption and high near-IR

reflection. Changes in this spectral band are well-

known as indicators of plant health (Eitel, 2011). It

is very interesting for a vegetation lidar to have a

laser with emission wavelength range covering this

red-edge band range.

Recently, in our prior work at Imperial College,

continuous-wave (cw) red diode-pumped

Alexandrite laser with 26W of average output power

was obtained and the first active Q-swithcing with

an electro-optic Pockels cell (Teppitaksak, 2014).

Interestingly, the first femtosecond Kerr-lens mode-

locked Alexandrite laser was recently reported in

(Ghanbari,2016) producing pulses as short as 170 fs.

In this study, we report the first successfully

demonstration of a wavelength tunable passive Q-

switched Alexandrite laser with semiconductor

saturable absorber mirror (SESAM) and using a red

diode (AlGaInP) pump laser at 635 nm. For tuning

the wavelength, a birefringent filter (BiFi) was used

in the cavity. The highest repetiton rate achieved

was 27 kHz in fundamental TEM

mode. The

maximum average output power obtained was 41

mW. The shortest pulse generated was 550 ns

(FWHM). The wavelength tuning band spanned was

between 740 nm and 755 nm. To the best of our

knowledge, this is the first wavelength tunable

TEM

passive Q-switched operation of a red diode-

pumped Alexandrite laser. Our results open the way

for further development, optimization and power

scaling of this new generation passive Q-switched

(and also potentially for passive mode-locked)

Alexandrite lasers. One interesting application for

developing a low-cost, compact and wavelength

tunable pulsed laser source is space-borne remote

sensing applications, especially for the next

generation vegetation lidar systems.

2 EXPERIMENTAL SETUP

A simple linear cavity was designed to study the

wavelength tunable passive Q-switching operation

mode of the direct diode-pumped Alexandrite laser.

Two experimental systems are described. The first is

a wavelength tunable cw setup shown in Figure 1 as

a precursor study. The second setup was a

wavelength tunable passive Q-switched using a

semiconductor saturable absorber mirror (SESAM)

as shown in Figure 2.

The wavelength tunable cw laser had a plane-plane

mirror cavity. The Alexandrite gain medium was a

10 mm-long and 4 mm-diameter Alexandrite rod

crystal doped with 0.22% of Cr

and c-axis cut. The

Wavelength Tunable Passively Q-Switched Alexandrite Laser with Direct Diode-Pumping at 635 nm

Figure 1: Schematic layout of the plane-plane mirror cavity wavelength tunable continuous-wave direct red diode-pumped

Alexandrite laser operating in fundamental TEM

mode.

Figure 2: Schematic layout of the wavelength tunable passive Q-switched direct red diode-pumped Alexandrite laser. Here,

OC in Figure 1 is replaced with SESAM. The output coupling is provided from the reflection of the BiFi in the cavity.

end faces of the rod were plane-parallel and anti-

reflection coated at the Alexandrite wavelength

(~755 nm). The rod was mounted in water-cooled

copper heat-sink and an indium foil interface used

for enhanced thermal contacting to the copper.

The cavity length ~142 mm including an intracavity

plano-convex lens (PCX, f=70 mm). The intracavity

lens design was chosen to form a stable cavity

configuration and to optimise for TEM

operation.

A dichroic back mirror (BM) was highly-reflecting

(R>99.9%) at laser wavelength (~755nm) and

highly-transmitting (R<0.2%) for pump diode laser

(~635 nm). Both the temperature of the crystal and

pump laser diode were cooled to 16

C using a

single water chiller in the experiments.

The small-signal absorption coefficient (α) of the

crystal measured ~6cm

-1

with a He-Ne laser at 633

nm for light polarized parallel to the b-axis of

Alexandrite crystal. The crystal was pumped by a

red diode module, operating nominally at central

wavelength 635nm with bandwidth (FWHM) of ~

1.5nm and capable of providing max ~5W (5180

mW full power) in cw mode. The diode module is

fibre coupled in a multi-mode fiber with core

diameter of 105 µm and numerical aperture of 0.22.

An aspheric fiber collimator with 35mm of focal

length was used for collimating the pump beam

output from the fiber. The circularized pump beam

was focused into a ~120 μm spot size diameter

inside the crystal by an aspheric pump lens (ASP) of

50 mm focal length.

The output of the fibre was not a pure polarisation

but > 60% was in the highly absorbing b-axis

direction of the Alexandrite crystal. About 83% of

the pump power was absorbed in the crystal. The

confocal parameter of the b-axis component of the

pump (~4 mm) was sufficiently longer than the

absorption depth of the crystal to allow good laser

mode overlap with the pump. The overlap of the

laser mode to the other polarisation however would

be poor.

The wavelength tuning of the laser is provided by

using a quartz plate with a thickness of 0.5 mm

acting a birefringent filter (BiFi) tuner. The BiFi sits

at Brewster angle (to minimize insertion losses) and

wavelength tuned by rotation of the BiFi in the plane

of the plate using a goniometer to alter the

birefringence of the plate. In this way it was simple

to tune the wavelength of the laser output of both

cavities (Figures 1 and 2) working in cw and passive

Q-switched mode.

For optimizing the laser cavity and as a reference,

we first obtained the cw mode of operation by using

an output coupler (OC) with 99.5% reflectivity

(Figure 1). The output of the wavelength tunable

cavity working in cw mode is obtained through the

65 mm

75 mm

BiFi

Fibre

collimator

50 mm

fibre coupled

red diode

λ/2

waveplate

ASP=

50mm

Crystal

0.22%

PCX=70m

SESAM

2.5 mm

Output

Bea

Dichroic

70 mm

BiFi

Fibre

collimator

50 mm

fibre coupled

red diode

λ/2

waveplate

ASP=

50mm

Dichroic

PCX=70mm

99.5%

2 mm

Crystal

0.22%

Output

Bea

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

OC. Once the cavity was optimized in cw mode, we

replaced the OC with a SESAM to obtain passive Q-

switching mode (Figure 2). The output beam was

obtained from the BiFi since the SESAM was used

as the end mirror in the cavity and was non-

transmitting as it was attached on an aluminium disc

using thermal adhesive. This helped to increase the

heat transfer between SESAM and the aluminum.

The beam spot size diameter on the SESAM was

calculated about to be ~250 µm. The SESAM had a

saturable absorption with modulation depth ~ 0.5%

and had also some nonsaturable loss (~1%). Both the

nonsaturable loss and modulation depth depends on

wavelength. The SESAM structure had a ~60 nm

reflectivity bandwidth centered around 740 nm

(R=~99%) and a ~10 nm (FWHM)

photoluminescence band centered around ~736 nm.

The dynamic response (absorption recovery) of the

SESAM was biexponential with <0.5 ps for fast

component and ~300 ps for slow component. The

saturation intensity was around ~30 µJ/cm

3 RESULTS AND DISCUSSION

In the initial experiments, we investigated the

variation of the cw output power as a function of

emission wavelength. The wavelength of the

Alexandrite laser with the 99.5% reflectivity output

coupler was tuned in the range extended from 730

nm to 790 nm and the output power shown in

Figure 3. Maximum output power was obtained at

740 nm lasing wavelength at full pump power ~5W

(5180 mW) at 635 nm. It can be seen from Fig.3 that

a much wider tuning range was possible, however

the main focus of this study was operation around

the wavelength band of the SESAM device.

Figure 3: Tuning curve for the cw Alexandrite laser with

0.5 mm BiFi tuner plate. Alexandrite crystal temperature

was 16

C, output coupler was 0.5% transmitting, and

incident pump power was 5180 mW.

By fixing the angle of the BiFi so that the laser

output wavelength is constant at 740 nm, the

measured cw output power versus the pump power

curve of the cavity is depicted in Figure 4. The slope

efficiency was ~14% and shows no sign of

decreasing at the highest output power. This

suggests that thermal lensing effects did not limit the

output power. As can be further seen in Figure 4, the

slope efficiency and output power is increasing with

the available pump power, suggesting that the power

performance can be further improved with more

powerful pump lasers. The optical-to-optical

efficiency with respect to incident pump power can

also be expected to be considerably increased by

optimising output coupling, reducing intracavity loss

and attention to spectral profile of coated cavity

optics.

Figure 4: Output power vs Pump power curve of cw

Alexandrite laser cavity at constant wavelength of λ=740

nm. Laser crystal is at 16

C and 0.5% transmitting output

coupler.

After the characterisation of the wavelength tunable

cw laser cavity, the OC mirror was replaced with the

SESAM for passively Q-switching the Alexandrite

laser. The output of the laser was provided by a

reflection loss from the BiFi plate. With the

dimensions of the cavity as shown in Figure 2, the

system started working in a self-Q-switched mode.

In particular, after the alignment of the cavity was

optimized, the output power showed a strong

dependence on the position of the intracavity lens

that might be to do the spot size on the SESAM.

For constant wavelength λ=747 nm (by fixing the

BiFi at appropriate angle), the evolution of the

output power, pulse width and the repetiton rate with

respect to the pump power are given in Figure 5. It

is noted that the cavity has a higher threshold due to

the larger losses incurred by insertion of the

SESAM. As the available pump power increases

730 740 750 760 770 780 790

120

140

160

180

200

220

240

Wavelength (nm)

Output Power (mW)

Output characteristics of wavelength tunable

cw Alexandrite laser direct diode-pumped at 635 nm

Output Pow er (mW) at 5180 mW

full pump power

2000 2500 3000 3500 4000 4500 5000 5500

100

150

200

250

Pump Power (mW)

Output Power (mW)

Output characteristics of wavelength tunable

cw Alexandrite laser direct diode-pumped at 635 nm

cw output power (mW) at 740 nm

Wavelength Tunable Passively Q-Switched Alexandrite Laser with Direct Diode-Pumping at 635 nm

above threshold, shorter pulses with higher

repetition rate were obtained.

Figure 5: For constant wavelength λ=747 nm, the

evolution of the output power, pulse width and the

repetiton rate of the passive Q-switched diode-pumped

Alexandrite laser with respect to the 635 nm pump power.

The slope efficiency was limited by the intracavity

losses and can be improved by careful intracavity

loss management. Moreover, the output is obtained

from the BiFi in the cavity meaning that the actual

power generated by the cavity is higher than the

measured value. Although this is not the best

configuration for obtaining the output pulse from a

laser cavity, we preferred to use this cavity

configuration as shown in Figure 2. Our objective in

choosing this setup was to make the system as

simpler as possible in order to just show

experimentally for the first time that the diode-

pumped Alexandrite crystal can be passively Q-

switched. In our future studies, we are planning to

use an X-Cavity (as described in Section 4) and/or

L-cavity configurations with higher crystal

temperature to obtain higher efficiency and higher

pump power to provide higher output power in

passive Q-switching of diode-pumped Alexandrite

laser.

In the tunable passive Q-switching experiments, the

BiFi was again used to tune the wavelength but was

also restricted by the limited reflectivity band of the

SESAM and range over which Q-switching could be

accomplished. The tuning bandwidth achieved was

between 740 nm and 755 nm (Figure 6). Compared

with the cw tuning range shown in Figure 3, Q-

switching tuning bandwidth is narrower. Figure 6

shows the evolution of the output power, pulse

width, repetition rate and spectral width of the pulses

obtained from the passive Q-switched diode-pumped

Alexandrite laser in the wavelength tunability range

for full pump power.

Figure 6: The output power, pulse width, repetition rate

and spectral width of the pulses obtained in the

wavelength tunability range from the passive Q-switched

Alexandrite laser.

The highest pulse repetition rate was 27 kHz at 755

nm and the shortest pulse width was 550 ns at 740

nm. The observed spectral width was <1 nm for the

tuning range of the passive Q-switch mode. Figure 7

compares the pulse widths and spatial profiles

obtained at the wavelengths 743 nm and 750 nm for

full pump power. The laser output beams for both

wavelengths had TEM

beam profile with M

values

<1.7 and <1.9 for 743 nm and 750 nm, respectively.

The M

beam quality was determined using the ISO

11146-1 method, based on the second moment beam

size. When the mode of the beam profile was

adjusted to higher spatial modes, higher output

power was achieved due to the increased mode-gain

overlap in the cavity.

Figure 7: Pulse profiles and spatial profiles obtained at

743 nm and 750 nm from the tunable passive Q-switched

Alexandrite laser.

Figure 8 shows the pulse train and the repetition

rates obtained at the wavelengths 743 nm and 750

nm for full pump power. At 743 nm a pulse

repetition rate of 14 kHz was obtained, whilst at 750

nm the repetition rate was higher at 23 kHz.

A photograph of the developed laser is shown in

Figure 9 below (see Figure 2 for dimensions and

600

800

1000

1200

1400

1600

1800

2000

Pulse Width - FWHM (ns)

Repetition Rate (kHz)

3000 3500 4000 4500 5000 5500

Pump Power (mW)

Output Power (mW)

Pulse Width - FWHM (ns)

Repetiton Rate (kHz)

550

600

650

700

750

800

Pulse Width - FWHM (ns)

Repetition Rate (kHz)

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

ectral Width

(

)

740 745 750 755

Wavelength (nm)

Output Power (mW)

Output

Power (mW)

Pulse Width

FWHM (ns)

Repetiton

Rate (kHz)

Spectral

Width (nm)

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

schematic layout). We noted that in order to

demonstrate this wavelength tunable passive Q-

switched direct diode-pumped Alexandrite laser

experimentally for the first time in the literature, we

choice to utilize this very simple cavity

configuration. However this linear cavity

configuration we used has some disadvantages

especially in the means of achieving relatively

higher output power and higher efficiency. In

particular, the use the beam coming from the BiFi

was not ideal. This caused lower output power

values to be detected in the measurements. In order

to overcome this issue, we developed an X-cavity

setup for our future studies as explained in the next

section.

Figure 8: Pulse train observed from passively Q-switched

Alexandrite at 743 nm and 750 nm with repetition rates 14

kHz and 23 kHz, respectively.

4 FUTURE STUDY

We established an astigmatically compensated 4-

mirror X-cavity direct diode-pumped Alexandrite

laser for cw experiments shown in Figure 10. The

intention of this configuration was for obtaining

higher output power from the wavelength tunable

passive Q-switched Alexandrite laser in our future

studies. It is important to have an optimized cw

cavity to obtain passive Q-switching or passive

mode-locking. As we have done previously in

Figure 1, as a preliminary to Q-switching we first

developed a stable X-cavity cw Alexandrite laser

operating in fundamental TEM

mode. The cavity

consisted of Brewster angle cut Alexandrite crystal,

two curved mirrors (each with a radius of curvature

of 75 mm), a highly reflecting flat back mirror (BM)

and an output coupler mirror with reflection 99.5%.

We used the same diode pump source as in Figure 1.

Moreover, as we have done previously, we kept the

crystal and pump diode temperature equal and

constant at 16

C using the same single water cooling

system.

Two different cavity configurations were

investigated. In the 1

setup, the internal cavity

dimensions are arranged as: L

=37.5 mm, L

=35

mm, L

=62 mm and L

=55 mm. The beam quality of

the laser output is measured (with ISO 11146-1

method) as 





9.6

and 





 10.4. Since these

beam quality factors are not satisfactory, we tried to

re-optimize the X-cavity with new lengths. In this

configuration: L

=53 mm, L

=44 mm, L

=135

mm and L

=96 mm. For this configuration, the laser

output beam had a TEM

beam quality and its beam

propagation factor was measured as 





1.34

and







1.17

which are far better than the results

found for the 1

setup. The free running laser

wavelength was around ~755 nm in both cavities.

Figure 9: Annotated photograph of the wavelength-tunable passive Q-switched direct diode-pumped Alexandrite laser.

-1 0 1

x 10

-4

0.2

0.4

0.6

0.8

time (sec)

Normalized Intensity (a.u.)

743 nm pulse train, Repetition Rate=14 kHz

750 nm pulse train, Repetition Rate=23 kHz

Output

Beam

Crystal

holder

Back mirror

Intracavity

lens

λ/2 Wave plate

Pump lens

Cavity

axis

Pump Beam

SESAM

BiFi

Brewster

angle

Surface

normal

of BiFi

Wavelength Tunable Passively Q-Switched Alexandrite Laser with Direct Diode-Pumping at 635 nm

Figure 10: X-cavity cw Alexandrite laser setup with Brewster angle cut crystal: 5W fibre coupled red diode laser module

(1), Fibre collimator (2), λ/2 wave plate (3), Plano convex pump lens (PCX) with f=75 mm (4), curved dichroic mirror HT

at 635 nm and HR at 755 nm with radius of curvature (ROC) 75 mm (5 and 7), Brewster angle cut Alexandrite rod with

crystal

= 8 mm length doped with 0.22% of Cr

(6), back mirror – BM (8), Output coupler OC with reflection 99.5% (9).

Figure 11 given below shows the cw output power

of the X-cavity for the two different setups.

Figure 11: Output power vs Pump power curve of cw X-

cavity Alexandrite laser at free running laser wavelength

around ~755 nm in the two cavity setups. The crystal

temperature was 16

C in the experiments and output

coupling was 0.5%.

As can be seen in Figure 11, the oscillation threshold

of the 2

setup is much higher than the 1

setup. On

the other hand, as can be further seen its slope

efficiency (~30%) is higher than the slope efficiency

of the 1

setup and output power is increasing with

the available pump power. This suggests that the

power performance and slope efficiency of the 2

setup can be further improved with more

optimization, adjustment and by utilizing more

powerful pump lasers with higher crystal

temperatures. After having completed this step, we

will plan to replace the back mirror with the SESAM

we used in Figure 2, but can now utilise and

optimise the output coupler mirror to obtain more

efficient passive Q-switched (or potentially passive

mode-locked) diode-pumped Alexandrite laser in

our future studies.

5 CONCLUSIONS

This work demonstrated experimentally for the first

time that a wavelength tunable passive Q-switched

TEM

operation of Alexandrite laser can be

achieved with direct red diode-pumping. The

shortest pulse generated was 550 ns (FWHM) and

the highest repetiton rate achieved was 27 kHz in

fundamental TEM

mode. The maximum average

output power obtained was 41 mW and 0.56 nm was

measured as the shortest spectral width. The

wavelength tuning band spanned was between 740

nm and 755 nm. The results obtained in this study

pave the way for further development, optimization

and power scaling of this new generation tunable

diode-pumped passive Q-switched (and also

potentially for passive mode-locked) Alexandrite

laser. In our future studies; higher crystal

temperature, higher pump power, higher doping

concentration, longer gain medium, optimized

(reduced) output coupler reflectivity, enhanced

cavity design (X-cavity) and careful intracavity loss

management will provide us to obtain better

performance in tunable Q-switched operation of

diode-pumped Alexandrite laser. Due to the superior

optical and thermo-mechanical properties comparing

with its counterparts, this new generation direct

diode-pumped wavelength tunable, compact, low-

cost and efficient passive Q-switched (and also

passive mode-locked) Alexandrite laser has the

potential to become an attractive source for

addressing the requirements of many near-future

crucial scientific, medical and game-changing

industrial applications such as next generation

space-borne vegetation lidar systems.

crystal

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

ACKNOWLEDGEMENTS

We would like to acknowledge support from the

British Council and Newton Fund under Project No.

215277462. U.P. would like to acknowledge support

from TÜBİTAK (The Scientific and Technological

Research Council of Turkey) for his visit to Imperial

College London. The authors thank Dr Arkady

Major from the University of Manitoba for

providing the SESAM that was used in this study.

REFERENCES

Damzen, M.J., Thomas, G.M., Teppitaksak, A.,

Minassian, A., 2014. Progress in diode-pumped

Alexandrite lasers as a new resource for future space

lidar missions. In ICSO 2014, International

Conference on Space Optics.

Teppitaksak, A., Minassian, A., Thomas, G.M., Damzen,

M.J., 2014. High efficiency >26 W diode end-pumped

Alexandrite laser. In Vol. 22, No.13, Optics Express.

OSA.

Koechner, W., Bass, M., 2003. Solid-State Lasers:

Graduate Text, Springer-Verlag. New York.

Ghanbari, S., Akbari, R., Major, A., 2016. Femtosecond

Kerr-lens mode-locked Alexandrite laser. In Vol. 24,

No. 13, Optics Express. OSA.

Eitel, J.U.H., Vierling, L.A., Litvak, M.E., Long. D.S.,

2011. Broadband, red-edge information from satellites

improves early stress detection in a New Mexico

conifer woodland. In 115, 3640-3636, Remote Sensing

of the Environment.

Lu, T., Li, H., 2016. Atmospheric turbulence induced

synthetic aperture lidar phase error compensartion. In

381, 214-221, Optics Communications. ELSEVIER.

Pelon, J., Megie, G., Loth, C., Flamant, P., 1986.Narrow

bandwidth Q-switch alexandrite laser for atmospheric

applications. In Vol. 59, Issue 3, 213-218, Optics

Communications. ELSEVIER.

Milton, M.J.T., Gardiner, T.D., Molero, F., Galech, J.,

1997. Injection seeded optical parametric oscillator for

range-resolved DIAL measurements of atmospheric

methane. In 142, 153-160, Optics Communications.

ELSEVIER.

Teppitaksak, Thomas, G.M., Damzen, M.J., 2015.

Investigation of a versatile pulsed laser source based

on a diode seed and ultra-high gain bounce geometry

amplifiers. In Vol. 23, No. 9, Optics Express. OSA.

Ghanbari, S., Major, A., 2016. High power continuous-

wave Alexandrite laser with green pump. In 26,

075001, Laser Physics. IOP PUBLISHING.

Beyatli, E., Baali, I., Sumpf, B., Erbert, G., Leitenstorfer,

A., Sennaroglu, A., Demirbas, U., 2013. Tapered

diode-pumped continuous-wave alexandrite laser. In

Vol. 30, No. 12, J. Opt. Soc. Am. B. OSA.

Demirbas, U., Li, D., Birge, J.R., Sennaroglu, A., Petrich,

G.S., Kolodziejski, A., Kartner, F.X., Fujimoto, J.G.,

2009. Low-cost, single-mode diode-pumped

Cr:Colquiriite lasers. In Vol.17, No. 16, Optics

Express. OSA.

Sennaroglu, A., Kaertner, F.X., Fujimoto, J.G., 2007.

Low-threshold, room-temperature femtosecond

:forsterite laser. In Vol. 15, No. 20, Optics

Express. OSA.

Loiko, P., Major, A., 2016. Dispersive properties of

alexandrite and beryllium hexaaluminate crystals. In

Vol. 6, No. 7, Optics Express. OSA.

Walling, J.C., Heller, D.F., Samelson, H.,Harter, D.J.,

Pete, J.A., Morris, R.C., 1985. Tunable Alexandrite

lasers: Development and Performance. In Vol. QE21,

No. 10, J. Quantum Electronics. IEEE.

Sam, C.L., Walling, J.C., Jenssen, H.P., Morris, R.C.,

O’Dell, E.W., 1980. Characteristics of alexandrite

lasers in Q-switched and tuned operations. In Vol.247,

130-136, Advances in Laser Eng and Appl. SPIE.

Wavelength Tunable Passively Q-Switched Alexandrite Laser with Direct Diode-Pumping at 635 nm