Dispersion-scan Measurements of the Multiplate Continuum Process

Miguel Canhota

, Rosa Weigand

and Helder Crespo

IFIMUP-IN e Departamento de F

ısica e Astronomia, Faculdade de Ci

encias, Universidade do Porto,

Rua do Campo Alegre 687, 4169-007 Porto, Portugal

Departamento de

Optica, Facultad de Ciencias F

ısicas, Avda. Complutense s/n,

Universidad Complutense de Madrid, 28040 Madrid, Spain

Keywords:

Ultrafast Optics, Dispersion-scan, Pulse Compression, Pulse Measurement, Multiplate Continuum, Supercon-

tinuum Generation.

Abstract:

Multiplate continuum (MPC) is a recent supercontinuum generation technique for spectral broadening of ul-

trashort laser pulses. In this work, we report the ﬁrst direct temporal characterization of ultrashort laser pulses

generated by the MPC process, without any further pulse manipulation apart from dispersion compensation,

using the dispersion scan technique.

1 INTRODUCTION

The ﬁrst reports on large spectral broadening of pi-

cosecond laser pulses - supercontinuum generation -

were done in the 1970s (Alfano, 2013). This nonlin-

ear optical phenomenon occurs in solids (bulk media,

optical ﬁbers, photonics crystal ﬁbers), liquids and

gases. It has major applications in ultrashort pulse

compression, ultrafast laser spectroscopy (Lindfors

et al., 2004), optical coherence tomography (Hum-

bert et al., 2006), telecommunications (Takara et al.,

2005), and application in carrier-envelope phase sta-

bilization of mode-locked lasers (Dudley J.M., 2010).

Supercontinuum generation in bulk media has

several practical advantages, but suffers from a major

drawback, i.e., it cannot be used with high peak power

pulses; the lower damage threshold in solids render

them unattractive for high power applications. While

the generation of supercontinuum in solids is readily

available for peak powers of MW (Silva et al., 2012),

no other method for the generation of high power su-

percontinuum in solids was available until the intro-

duction of the multiplate continuum (MPC), which

uses a set of thin slides of glass (e.g. fused silica).

This enables spectral broadening in each plate, while

avoiding damage to the medium due to self-focusing

(Lu et al., 2014).

The dispersion-scan (d-scan) technique (Miranda

et al., 2012) is a recent but well-established method

for the measurement of ultrashort laser pulses. It re-

lies on the manipulation of the total dispersion in-

curred by the pulse while traveling through a standard

pulse compressor setup comprised of dispersion com-

pensation mirrors (DCMs) and a pair of glass wedges.

The amount of glass traversed by the pulse is an in-

dependent variable that can be controlled with the

simple insertion of one of the wedges. While the

DCMs impart negative dispersion, the variable pos-

itive dispersion introduced by the wedges will vary

the total dispersion of the pulse to be measured. In

the case of the second-harmonic variant of the d-

scan (SHG d-scan), the measurement of the second-

harmonic signal after the compressor results in a two-

dimensional trace of the SHG spectrum versus glass

insertion. With the measured trace, together with

the linear spectrum of the light source, it is possible

to retrieve the spectral phase of the pulse under test

through a mathematical optimization algorithm, and

thus, reconstruct its full temporal proﬁle.

2 EXPERIMENTAL SETUP

Our laser system is composed of a Ti:Sapphire

chirped pulse ampliﬁer (Femtolasers FemtoPower

Compact Pro CEP) seeded by pulses from a few-

cycle, prismless Ti:Sa oscillator (Femtolasers Rain-

bow), with 0.8 mJ of pulse energy in sub-30-fs pulses

at an repetition rate of 1 kHz. Only a fraction of

this output power will be used for the present exper-

iment. The experimental setup (Figure 1) comprises

two main parts: the spectral broadening process, i.e.,

the MPC, and the ultrafast pulse measurement part,

the d-scan. The MPC part begins with a f=1 m lens

Canhota M., Weigand R. and Crespo H.

Dispersion-scan Measurements of the Multiplate Continuum Process.

DOI: 10.5220/0006167802690272

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 269-272

ISBN: 978-989-758-223-3

269

that focuses our input beam onto a stack of six un-

evenly spaced, thin (100 µm) slides of fused silica

placed at Brewster’s angle. The uneven spacing is

motivated by simulations (Cheng et al., 2016), as well

as by the criterion of obtaining the broadest spectrum

as we place each individual slide in the setup while

minimizing spatial wavefront distortions due to self-

focusing. Both factors show that the spacing between

successive slides gets shorter. The long focal length of

the lens gives us a wide Rayleigh range that fully en-

velopes the slide stack, ensuring that the electric ﬁeld

strength is high enough to induce spectral broaden-

ing. The emerging beam, with its successively broad-

ened spectrum is then collimated by a concave mirror

and directed to a pair of dispersion compensation mir-

rors (DCMs) which impart negative dispersion to the

pulse. We record the second-harmonic signal gener-

ated by a 5 µm thick nonlinear BBO crystal cut for

type-I SHG as a function of wedge insertion, hence

obtaining the d-scan trace of the pulses shown in Fig-

ure 2. The simplicity and performance of the d-scan

setup was one of the deciding factors on the choice of

this measurement method, compared with other con-

ventional techniques like SPIDER (Anderson et al.,

2008) and FROG (Trebino et al., 1997). Another con-

venient factor is the fact that after making the sweep

and obtaining the d-scan trace we can immediately

set the necessary amount of glass insertion, so that

the pulse at the exit of the wedges is as compressed as

possible for the given setup.

MPC

D-scan

Spectrometer

DCM

BBO

Wedges

Plates

~25 fs

~12,5 fs

Parabolic

mirror

Figure 1: Experimental setup of the MPC and the d-scan.

3 RESULTS

As mentioned previously, the d-scan trace is a two-

dimensional plot of the second harmonic spectrum as

a function of glass insertion. As the total spectral

phase of the pulse under test changes from negative

to positive, it passes through a point where the pulse

is well compressed, i.e, the second-harmonic signal

achieves a maximum. This position is relabeled as the

point of zero insertion, as shown in Figure 2, which

contains the measured and numerically retrieved d-

scan traces.

Figure 2: Measured and retrieved d-scan traces.

Even though the measured trace has a complex

structure, we were able to get a retrieved trace that

closely resembles the measured one, i.e., it is able to

reproduce the main features of the measured trace.

The spectrum and retrieved spectral phase of the

MPC pulse is shown in Figure 3.

600 650 700 750 800 850 900 950

Wavelength (nm)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Arb. units

-10

-8

-6

-4

-2

Spec.

Phase

Figure 3: Spectrum and spectral phase of the measured

pulse.

Although the broadest high power supercontin-

uum is currently generated in gas-ﬁlled hollow ﬁber

compressors, e.g. (Silva et al., 2014; B

ohle et al.,

2014), the supercontinuum generated by MPC is still

signiﬁcant.

The measured linear spectrum and the retrieved

spectral phase gives us full information to completely

reconstruct the temporal proﬁle of the pulse under

test, as shown in Figure 4.

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

270

-60 -40 -20 0 20 40 60

Time (fs)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Norm. intensity

TL: 9.8 fs

Measured: 12.5 fs

Figure 4: Retrieved pulse and transform-limited pulse.

We see that the retrieved pulse has twice the dura-

tion compared with the transform-limited (TL) pulse

(ie., a pulse with same spectrum but with a ﬂat spec-

tral phase).

The pulses generated by MPC are longer than the

pulses generated in a hollow-core ﬁber (Silva et al.,

2014) by a factor of 3, but nonetheless this kind

of setup has its own advantages, such as compact-

ness (it occupies a small space), simplicity (can be

implemented with off-the-shelf optical components),

robustness (less dependent on alignment, whereas

hollow-core ﬁbers setups normally require feedback

loops to keep a good alignment for a long period of

time), high efﬁciency, and does not involve the han-

dling of gases within a vaccuum system.

The output power of the MPC process is roughly

50 % of the 140 mW of input average power. The

output beam proﬁle is shown in Figure 5; it is homo-

geneous and most of the power is concentrated in the

central part. Hence, the central part was spatially ﬁl-

tered by an iris (not shown in Figure 1) before being

directed to the measurement setup (d-scan).

Figure 5: Spatial mode.

4 CONCLUSIONS

We successfully compressed and measured ultrashort

laser pulses using the multiplate and dispersion-scan

techniques. We were able to generate a broad spec-

trum capable of supporting transform-limited pulses

down to 9.8 fs from a high peak power laser, which

suggests that the output pulse can be further shortened

if additional phase control or pulse shaping is used.

The spatial mode is homogeneous and stable, and the

MPC process has a good efﬁciency compared to tra-

ditional hollow-ﬁber compression setups. The com-

pactness and simplicity of the setup makes the MPC

a good candidate/alternative for application in high-

harmonic generation (HHG), as was demonstrated re-

cently (Huang et al., 2016).

ACKNOWLEDGEMENTS

We acknowledge the funding through the Portuguese

funding agency, Fundac¸

ao para a Ci

encia e a Tecnolo-

gia (FCT) through project NORTE-07-0124-FEDER-

000070 (Task 3). The authors thank Francisco Silva

for the d-scan acquisition and retrieval software.

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