DEVELOPMENT OF HIGH-POWER PICOSECOND FIBER-BASED …upcommons.upc.edu/.../handle/2099.1/23052/JosepCanals_Master_… · Abstract. We report a high-power, picosecond ultraviolet (UV)

MSc in Photonics PHOTONICSBCN

Universitat Politècnica de Catalunya (UPC) Universitat Autònoma de Barcelona (UAB) Universitat de Barcelona (UB) Institut de Ciències Fotòniques (ICFO)

http://www.photonicsbcn.eu

Master in Photonics

MASTER THESIS WORK

DEVELOPMENT OF HIGH-POWER PICOSECOND FIBER-BASED ULTRAVIOLET SOURCE

Josep Canals Casals

Supervised by Dr. S. Chaitanya Kumar (ICFO) and Prof. M. Ebrahim-Zadeh (ICFO)

Presented on date 8th September 2014

Registered at

http://www.photonicsbcn.eu/

Development of high-power picosecondfiber-based ultraviolet source

Josep Canals CasalsOptical parametric oscillators Group, ICFO - Institut de Ciències Fotòniques,Av. Carl Friedrich Gauss, 3, 08860 Castelldefels, Barcelona, Spain.

E-mail: [email protected]

Abstract. We report a high-power, picosecond ultraviolet (UV) source at 266 nm in acompact design based on single-pass fourth harmonic generation (FHG) and mode-lockedYb-fiber laser. The configuration has two stages: second harmonic generation (SHG) of1064 nm to 532 nm based on BIBO crystal followed by SHG of 532 nm to 266 nm basedon BBO crystal. The obtained power is ∼ 55 mW at 80 MHz for a fundamental averagepower of 15 W at a green to UV conversion efficiency ∼ 1.5%. The long term UV powerstability is recorded to be 4.65% rms in TEM00 spatial mode profile.

Keywords: non-linear optics, frequency conversion, harmonic generation, non-linearmaterials, fiber lasers, visible lasers, UV lasers

1. IntroductionSince the demonstration of the first working laser by Maiman in 1960 [1], major

research in non-linear optics was initiated and several non-linear processes could beobserved such as the second harmonic generation (SHG) which was demonstrated in1961 by Franken et al [2]. The coherent ultra-fast optical sources in the ultraviolet(UV) are of great interest for a variety of applications including quantum optics, opticaldata storage, atmospheric sensing, combustion diagnostics and bio-imaging [3]-[4]. Theaccess to this spectral region has been achieved using bulky, complex and power-hungrygas lasers such as excimer lasers. Semiconductor lasers as diode laser can also achievecoherent light in this spectral region but with low quality beam and with low power.Hence, the development of high-repetition-rate ultra-fast sources in the UV at practicalaverage power and efficiency, in simple, compact, cost-effective and practical architectureremains challenging.

Non-linear frequency conversion techniques represent a potentially viable andeffective approach to the direct generation of ultra-fast radiation in UV applying frequencydoubling tripling or quadrupling of a pulsed laser with a suitable non-lineal crystal.

Earlier reports demonstrate a 355 nm source generation at low repetition rate 25

Hz using a third harmonic generation (THG) of mode-locked and amplified picosecondNd:YAG laser and a BIBO crystal [5]. Efficient generation of tunable pulses from 375

Development of high-power picosecond fiber-based UV source 2

to 435 nm are also shown using a mode locked Ti:sapphire laser at 76 MHz and a BIBOcrystal in femptosecond regime and in picosecond regime [6]-[7].

Here we report a picosecond UV source at 266 nm, with ∼ 55 mW output powerin a compact design based in two stages. Firstly, SHG of Yb-fiber laser using a 10 mmlength BIBO crystal is obtained as in [8]. Secondly, fourth harmonic generation (FHG)of the fundamental fiber laser beam is obtained with a SHG process from the generatedgreen at 532 nm to ultraviolet at 266 nm using a 5 mm length BBO crystal. Single-passconfiguration is used recording long term stability of 4.65% rms over 70 minutes and aTEM00 spatial profile.

2. Basics of non-linear opticsNon-linear optics is the study of the phenomena derived from the modification of the

optical properties of a material system by the presence of light. When a electromagneticradiation E(t) is applied to the material, the electrons which are inside are displaced withrespect to the nuclei and the system is thus polarized. This polarization P (t) is a functionof the field P = P (E), and can be expanded in a power series in terms of it, following(1) from [9].

P (t) = ε0

[χ(1)E(t) + χ(2)E2(t) + χ(3)E3(t) + ...

]≡ P (t)(1) + P (t)(2) + P (t)(3) + ...

(1)The constant of proportionality χ(n) is known as the susceptibility and ε0 is the

permittivity of free space. In general χ(n) is a tensor and it is independent on frequency ωof the optical radiation. In the linear optics case, the induced polarization depends linearlyon the electric field strength and only the first term P (t)(1) in (1) is considered. In non-linear optics case, the electrical field strength is comparable to intra-atomic electric fieldand the optical response of the matter needs to consider all terms in (1).

In this work we use SHG, which only involves second-order non-linearsusceptibilities χ(2) and two input beams with ω = ω1 = ω2 in (2).

E(t) = E(t)e−iω1t + E(t)e−iω2t (2)

With this input beam, the second order polarization can be expressed as (3) wherea term at frequency 2ω called second harmonic or SH beam and other independent of ωcalled optical rectification can be observed.

P (t)(2) = ε0χ(2)E(t)2 = ε0χ

(2)E21e

−2iωt + 2ε0χ(2)E1E

∗1 (3)

2.1 Second harmonic generation (SHG) and fourth harmonic generation (FHG)SHG is a process in which a beam at frequency ω interacts with matter to generate

an output beam at 2ω. If this beam at frequency 2ω interacts with other non-linear crystalcreating other SHG process, the output beam has a frequency at 4ω. Thus, in figure 1 theblock diagram of FHG process using two stages of SHG interactions is represented.

The efficiency of this process depends on several parameters such as the non-linearoptical coefficient of the crystal, the phase-matching angle in a certain optical plane, the


SHG SHG

FHG

⍵ 2⍵ 4⍵χ(2)χ(2)

Figure 1: Block diagram of FHG using two SHG processes.

length of the non-linear crystal, the spectral and angular acceptance bandwidth or thespatial walk-off.

2.1.1 Phase-matching angle and effective non-linear coefficientsThe phase-matching direction for a yz plane and its corresponding effective non-

linear coefficient deff to have SHG and FHG are calculated using the Sellmeier equationsin [10]-[11] for BIBO and BBO respectively and the refractive index in each axis arerepresented as a function of the wavelength in figure 2(a) for BIBO and figure 2(b) forBBO.

20 40 60 80

0.2

0.4

0.6

0.8

1

(degrees)

FH

Wav

elen

gth

(

m)

20 40 60 800

0.5

1

1.5

2

SH W

avel

engt

h (

m)

120 140 160 1800

1

2

3

(degrees)

Fu

nd

amen

tal W

avel

engt

h (

m)

120 140 160 1800

0.5

1

1.5

SHG

Wav

elen

gth

(

m)

0 1 2 3 41.7

1.8

1.9

2

2.1

Wavelegth (m)

Ref

ract

ive

Ind

ex

nx

ny

nz

Transparency range0.29 μm -2.5 μm

BIBO(a)

0 1 2 32.2

2.4

2.6

2.8

3

Wavelength (m)

Ref

ract

ive

Ind

ex

no

ne

Transparency range0.196 μm -2.2 μm

BBO(b)

BIBOΦ=90º, θType I (eeo)

1.064

168.9

(c)

0.532

47.4

BBOΦ=90º, θType I (ooe)

(d)

1064 nm → 532 nm 532 nm → 266 nm

Figure 2: Refractive index as a function of the wavelength for (a) BIBO and (b) BBO andphase-matching angle as a function of the input and output wavelength in (c) BIBO and(d) BBO. The shadowed areas show the transparency range of (a) BIBO and (b) BBO.


The θ angle represents the polar angle relative to the optical z-axis and φ representsthe azimuthal angle measured from x-axis. The polarization direction normal to theplane is called ordinary o and the other allowed direction parallel to the plane is calledextraordinary e. BIBO is a positive biaxial crystal with type I (eeo) interaction and BBOis a negative uniaxial crystal with type I (ooe) interaction. In yz plane, φ = 90◦ and angleθ is that satisfy the phase-matching condition in (4),

kω + kω = k2ω →2πnω(θ)

λω+

2πnω(θ)

λω=

2πn2ω

λ2ω→ nω(θ) = n2ω (4)

where nω and n2ω are the refractive index at ω and 2ω, λω and λ2ω are thefundamental and second harmonic wavelength and kω and k2ω are the wave vectors atω and 2ω respectively. The refractive index of the e polarized wave depends on the angleθ whereas the refractive index of the o does not. The refractive index expression of the epolarized wave in yz plane in type I (eeo) interaction as function of θ is (5).

1

n2ω(θ)

=cos2(θ)

n2ω,y

+sin2(θ)

n2ω,z

(5)

The phase angle θpm as a function of the wavelength is represented for BIBO andBBO crystals in figure 2(c) and figure 2(d). In figure 3(a) and 3(b) is graphically solvedthe phase-matching condition in (4) for BIBO and BBO crystals.

0.8 0.9 1 1.1 1.20

1

2

3

4

Wavelength (m)

def

f (p

m/V

)

My

Str3

0.4 0.6 0.8 1

0.8

1

1.2

1.4

1.6

Wavelength (m)

def

f (p

m/V

)

0 50 100 150 2001.75

1.8

1.85

1.9

1.95

n(

)

(degrees)

BIBOType I (eeo)

BIBOΦ=90º, θType I (eeo)λ=1064 nm

𝑛𝑒 = 𝑛(θ, 𝑛⍵,𝑦, 𝑛⍵,𝑧)

𝑛𝑜 = 𝑛2⍵,𝑥

10.9º 169º

(a)

0 50 100 150 2001.6

1.65

1.7

1.75

1.8

(degrees)

n(

)

47º 132º

BBOΦ=90º, θType I (ooe)λ=532 nm

𝑛𝑒 = 𝑛(θ, 𝑛2⍵,𝑥 , 𝑛2⍵,𝑧)

𝑛𝑜 = 𝑛⍵,𝑥

(b)

3.36

2.12

1.65

1.14

0.532


(d)

Φ=90º, θ

Φ=90º, 180º-θ

Φ=0º, θ

(c)

1.064

1064 nm → 532 nm 532 nm → 266 nm

Figure 3: Phase-matching condition solution for (a) BIBO and (b) BBO and effectivenon-linear coefficient in (c) BIBO and (d) BBO.


Comparing the results, can be observed that both satisfy the same phase-matchingangle for the given wavelength. Since there are two possible solutions, we take the phase-matching angle in which its effective non-linear coefficient is larger for the same crystallength which is θ = 168.9◦ for BIBO and θ = 47◦ for BBO. The effective non-linearcoefficient for these phase-matching angles in the working wavelength are 3.36 pm/Vin BIBO and 1.14 pm/V for BBO using the relevant formulae in [11]-[12]. They arerepresented as a function of the wavelength in figure 3(c) and figure 3(d) for BIBO andBBO respectively.

2.1.2 Angular and spectral acceptance bandwidthsAngular and spectral acceptance bandwidth are defined as the tolerance of phase-

matching to the spatial and spectral spread of the input beam. They have a huge effect inthe conversion efficiency and output power of the crystal. They are calculated representingthe normalized efficiency curve as function of the angle θ and the input wavelengthrespectively and calculating the full width at half maximum (FWHM) of the resulting sincfunction. Hence, the angular acceptance bandwidth for SHG is 1.03 mrad and 0.17 mradfor FHG both represented in figure 4(a) and figure 4(b) respectively. On the other hand,the angular acceptance bandwidth for SHG is 0.97 nm and 0.074 nm for FHG and theyare represented in figure 4(c) and figure 4(d). All these calculations are done consideringa BIBO and BBO crystal length of 10 mm.

2.1.3 Spatial walk-offThe spatial walk-off causes angular separation between the orthogonally polarised

o and e waves within the non-linear crystal reducing the efficiency of the process. Tocharacterize this effect the angular walk-off is represented as a function of the inputwavelength for SHG and FHG in figure 4(e) and figure 4(f) respectively.

3. Experimental set-upThe schematic of the high power picosecond fiber-based UV source is shown in

figure 5. The primary pump source is an Yb-fiber laser providing up to 20 W of averagepower at 1064 nm in pulses of ∼ 20 ps duration at 80 MHz repetition rate. Since theperformance of the Yb-fiber laser with regard to pulse duration and spectral stability isoptimum at the highest power, we operate the laser at the maximum output power anda combination of a half-wave plate and a polarising beam-splitter as an attenuator areused. The other half-wave plate controls the polarization of the beam to achieve theoptimus phase matching in the crystal. The BIBO crystal is 4 mm × 4 mm × 10 mm-long, cut at θ = 168.9◦ and φ = 90◦ for type-I (eeo) phase-matching in the yz opticalplane. The fundamental beam is focused with L1 at the center of the beam waist radiusof ω0 ∼ 40 µm, corresponding to a focusing parameter of ξ ∼ 0.47. The dichroic mirrorsM1 (R > 99% at 532 nm; T > 99% at 1064 nm) separate and deliver the generated greenfrom the fundamental IR. Two cylindrical lenses L2 and L3 with focal length f = 75 mmand f = 150 mm are used to circularize the green beam obtaining a ellipticity of 0.91.The second part of the experiment is composed for a 4 mm × 4 mm × 5 mm-long BBOcrystal, cut at θ = 47◦ and φ = 90◦ for type I (ooe) phase-matching in the yz optical plane.


47.35 47.4 47.45 47.50

0.2

0.4

0.6

0.8

1

(degrees)

SH

1.062 1.064 1.0660

0.2

0.4

0.6

0.8

1

Wavelength (m)

SH

168.6 168.7 168.8 168.90

0.2

0.4

0.6

0.8

1

(degrees)

SH

BIBOΦ=90º, θType I (eeo)L=10 mm

0.97 nm

0.5316 0.5318 0.532 0.53220

0.2

0.4

0.6

0.8

1

Wavelength (m)

SH

BBOΦ=90º, θType I (ooe)L=10 mm

0.074 nm

(d)

0.5 0.6 0.7 0.850

60

70

80

90

100

Wavelength (m)

Wa

lk-o

ff a

ng

le (

mra

d) 92.07

0.532


0.8 0.9 1 1.1 1.20

10

20

30

40

50

60

Wavelength (m)

Wa

lk-o

ff a

ng

le (

mra

d)

1.064

25.64

BIBOΦ=90º, θType I (eeo)

BBOΦ=90º, θType I (ooe)L=10 mm

0.17 mrad

(c)

BIBOΦ=90º, θType I (eeo)L=10 mm

1.03 mrad

(a) (b)

(f)(e)

1064 nm → 532 nm 532 nm → 266 nm

Figure 4: Theoretical calculations of angular acceptance bandwidth in (a) BIBO and (b)BBO, spectral acceptance bandwidth in (c) BIBO and (d) BBO and walk-off angle in (e)BIBO and (f) BBO.

The output beam is focused with L4 at the center of the crystal with a beam waist radius ofof ω0 ∼ 30 µm, corresponding to a focusing parameter of ξ ∼ 0.5. The dichroic mirrorsM2 (R > 99% at 266 nm; T > 99% at 532 nm) separate and deliver the generated UVfrom the green. A UV filter (FGUV11-�25 mm) with high transmission in wavelengthfrom 275 nm to 375 nm is finally used to separate FH beam from any residual SH beam.


Figure 5: Experimental design of high-power picosecond fiber-based UV source. FI:Faraday isolator, λ/2: Half-wave plate, PBS: Polarizing beam-splitter, L: Lens, M:Mirrors, F:UV filter

4. Results

4.1 SHG characterization

4.1.1 Power scalingThe SHG efficiency and the power scaling measurements for the SHG as a function

of the fundamental power are represented in figure 6(a). As the input power is increased,the output power increases quadratically reaching a maximum SH power of 5.77 W for afundamental power of 15 W. Hence, a single-pass SHG efficiency conversion of ∼ 38.2%

is obtained. Figure 6(b) shows the variation of the SH power as function of the square offundamental power which is expected to be linear.

4.1.2 Spectrum, stability and beam qualityThe SHG spectral measurements carried out in fundamental input power of 15 W is

represented in figure 6(c). The long-term stability of the SH beam at an average power of5.77 W at 532 nm is represented in figure 6(d). The SH generated has excellent passivestability of 0.71% rms over 80 minutes.

The SH beam profile is measured at a distance ∼ 55 cm from the BIBO crystal withan output power of 5.4 W together with the intensity profile and represented in figure 6(e).The green beam presents an ellipticity of ∼ 0.4 due to the spatial walk-off between thetwo beams with different wavelengths interacting in the BIBO crystal seen previously infigure 4(e). Using two cylindrical lens with f = 75 mm and f = 150 mm of focal length(L2 and L3 in figure 5) the green beam is circularized up to ellipticity of ∼ 0.91 as figure6(f) to improve the spatial behaviour of the generated UV beam.

4.2 FHG characterization

4.2.1 Power scalingThe FH efficiency and the power scaling measurements for the FHG as a function of

the SH power are represented in figure 7(a). As the input power is increased, the outputpower increases quadratically reaching a maximum FH power of ∼ 55 mW for a SHpower of 15 W. Hence, a single-pass from green to UV efficiency conversion of ∼ 1.5%


0 5 10 15 200

5

Fundamental Power (W)

SH P

ow

er (

W)

0 5 10 15 200

20

40

SHG

Eff

icie

ncy

(%

)

(a)

BIBO

Φ=90º, θType I (eeo)

0 100 200 3000

2

4

6

8

[Fundamental Power (W)]2

SH P

ow

er (

W)

(b)

BIBO


528 530 532 534 5360

0.2

0.4

0.6

0.8

1

Inte

nsi

ty (

a.u

.)

Wavelength (nm)

Φ=90º, θType I (eeo)BIBO

(c)

Ellipticity=0.912Ellipticity=0.4Power=5.4 W2

(e) (f)

0 20 40 60 800

2

4

6

Time (min)

SH P

ow

er (

W)


BIBO

(d)

SH power stability: 0.71% rms

Figure 6: Experimental measurements of SHG: (a) power scaling and efficiency curve,(b) SH power variation as function of the square of the fundamental, (c) spectrum ofthe generated green beam, (d) long term stability over 80 minutes, spatial profile of thegenerated TEM00 beam, (e) without and (f) with circularization with cylindrical lenses.

is reported. Figure 7(b) shows the variation of the FH power as function of the square ofSH power which is expected to be linear.

4.2.2 Spectrum, stability and beam qualityThe FHG spectral measurements carried out in fundamental input power of 15 W is

represented in figure 7(c).The long-term stability of the FHG beam at average power of55 mW at 266 nm is represented in figure 7(d) and is recorded to have a stability of 4.65%rms over 70 minutes.

The FHG beam profile is measured at a distance∼ 60 cm from the BBO crystal with


0 2 4 60

0.5

1

1.5

SH Power (W)

FH

G E

ffic

ien

cy (

%)

0 2 4 60

20

40

60

FH

Po

wer

(m

W)

Φ=90º, θType I (ooe)BBO

Ellipticity=0.166Power=17 mW2

(a)

0 10 20 300

20

40

60

80

[SH Power (W)]2

FH

Po

wer

(m

W)


(b)

264 265 266 267 268 269

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Inte

nsi

ty (

a.u

)


(c)

(e)

0 20 40 600

20

40

60

Time (min)

FH

Po

we

r (W

)Φ=90º, θType I (ooe)BBO

(d)

FH power stability: 4.65% rms

(f)

Figure 7: Experimental measurements of FHG: (a) power scaling and efficiency curve,(b) FH power variation as function of the square of the SH power, (c) spectrum ofthe generated beam, (d) long term stability over 70 minutes, (e) spatial profile of thegenerated TEM00 beam and (f) picture of experimental set-up in the laboratory where inthe bottom-right corner of the picture infra-red beam crossing L1 and going inside theBIBO. Then the green beam can be observed in the middle of the figure crossing the BBOgenerating the UV beam in the top-left corner of the image.

an output power of 17 mW together with the intensity profile and represented in figure7(e). The UV also presents an ellipticity of ∼ 0.166 due to the circularity of the greenbeam is not 1 and there is also spatial walk-off in between the two beams with differentwavelengths interacting in the BBO crystals presented in figure 4(f).

Finally, figure 7(f) shows a picture of the complete set-up in the laboratory showingthe infra-red, green and UV beams interacting with BIBO and BBO crystals.


5. ConclusionIn conclusion, we have demonstrated a high-power, picosecond UV source based

on single-pass FHG of Yb-fiber laser. The set-up consists in a first stage based on 10

mm length BIBO crystal from 1064 nm to 532 nm followed with a second stage basedon 5 mm length BBO crystal from 532 nm to 266 nm. The generated average power is∼ 55 mW at 266 nm at 80 MHz for a fundamental power of 15 W. The green to UVconversion efficiency is ∼ 1.5% and the long term UV power stability is recorded tobe 4.65% rms over 70 minutes. This results could be improved by optimizing spatialmode-matching in the non-linear crystals, using a UV filter with maximum transmissionin central wavelength of the spectrum and optimizing the transparency of the mirrors ingreen and UV wavelength. This technique represents a single, compact and practicalapproach to the development of ultra-fast source in the UV.

[1] T. M. Maiman, "Simulated optical radiation in ruby," Nature 187, 493-494, (1960).[2] P. A. Franken et al., "Generation of optical harmonics," Physical Review Letters 7 (4), 118-119, (1961).[3] S. Khripunov et al., "Variable-wavelength second harmonic generation of CW Yb-fiber laser in

partially coupled enhancement cavity," Opt. Express 22 (6), 7046-7051, (2014).[4] A. K. Jayasinghe et al., "Holographic UV laser microsurgery," Biomedical Opt. Express 2 (9), 2590-

2599, (2011).[5] M. Ghotbi et al., "Efficient third harmonic generation of microjoule picosecond pulses at 355 nm in

BiB3O6," Applied Physics Letters 89 (17), 173124-173124, (2006).[6] M. Ghotbi et al., "High-average-power femptosecond pulse generation in the blue using BiB3O6,"

Opt. Lett. 29 (21), 2530-2532, (2004).[7] M. Ghotbi and M. Ebrahim-Zadeh, "990 mW average power, 50% efficient, high-repetition-rate

picosecond-pulse generation in the blue with BiB3O6," Opt. Lett. 30 (24), 3395-3397, (2005).[8] S. Chaitanya Kumar and M. Ebrahim-Zadeh, "High-power, fiber-pumped, picosecond green source

based on BiB3O6," Laser Physics 24 (2), 025401-025401, (2014).[9] S. Chaitanya Kumar and M. Ebrahim-Zadeh, "High-power, fiber-laser-pumped, picosecond optical

parametric oscillator based on MgO: sPPLT," Opt. Express 19 (27), 26660-26665, (2011).[10] K. Miyata et al., "Phase-matched pure χ(3) third-harmonic generation in noncentrosymmetric

BiB3O6," Opt. Lett. 34 (4), 500-502, (2009).[11] D. N. Nikigosyan, "Beta barium borate (BBO)," Applied Physics 52 (6), 359-368, (1991).[12] M. Ghotbi and M. Ebrahim-Zadeh, "Optical second harmonic generation properties of BiB3O6," Opt.

Express 12 (24), 6002-6019, (2004).

Documents

DEVELOPMENT OF HIGH-POWER PICOSECOND FIBER-BASED …upcommons.upc.edu/.../handle/2099.1/23052/JosepCanals_Master_… · Abstract. We report a high-power, picosecond ultraviolet (UV)