Phasematched Secondharmonic Generation in Periodically Poled Optical Fibers

8/11/2019 Phasematched Secondharmonic Generation in Periodically Poled Optical Fibers

1/4

Phasematched secondharmonic generation in periodically poled optical fibers

Raman Kashyap

Citation: Applied Physics Letters 58, 1233 (1991); doi: 10.1063/1.104372

View online: http://dx.doi.org/10.1063/1.104372

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/58/12?ver=pdfcov

Published by the AIP Publishing

Articles you may be interested inPhase-matched second-harmonic generation in cross-linking polyurethane films by thermal-assistedoptical polingAppl. Phys. Lett. 91, 091105 (2007); 10.1063/1.2776876

Phase-matched second-harmonic generation in bulk azodye-doped polymers by all-optical polingJ. Appl. Phys. 95, 3837 (2004); 10.1063/1.1667271

Phasematched secondharmonic generation by periodic poling of fused silicaAppl. Phys. Lett. 64, 1332 (1994); 10.1063/1.111925

Phasematched secondharmonic generation from freestanding periodically stacked polymer filmsAppl. Phys. Lett. 63, 1462 (1993); 10.1063/1.109656

Phasematched secondharmonic generation in novel corona poled glass waveguidesAppl. Phys. Lett. 60, 2853 (1992); 10.1063/1.106845

s article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP

134.117.39.204 On: Thu, 05 Jun 2014 22:48:42
http://scitation.aip.org/search?value1=Raman+Kashyap&option1=authorhttp://scitation.aip.org/content/aip/journal/apl?ver=pdfcovhttp://dx.doi.org/10.1063/1.104372http://scitation.aip.org/content/aip/journal/apl/58/12?ver=pdfcovhttp://scitation.aip.org/content/aip?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/91/9/10.1063/1.2776876?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/91/9/10.1063/1.2776876?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/jap/95/7/10.1063/1.1667271?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/64/11/10.1063/1.111925?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/63/11/10.1063/1.109656?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/60/23/10.1063/1.106845?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/60/23/10.1063/1.106845?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/63/11/10.1063/1.109656?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/64/11/10.1063/1.111925?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/jap/95/7/10.1063/1.1667271?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/91/9/10.1063/1.2776876?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/91/9/10.1063/1.2776876?ver=pdfcovhttp://scitation.aip.org/content/aip?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/58/12?ver=pdfcovhttp://dx.doi.org/10.1063/1.104372http://scitation.aip.org/content/aip/journal/apl?ver=pdfcovhttp://scitation.aip.org/search?value1=Raman+Kashyap&option1=authorhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/454711315/x01/AIP-PT/COMSOL_APLArticleDL_060414/COMSOL_banner_US_IEEE-Supplement-2014_1640x440.png/5532386d4f314a53757a6b4144615953?xhttp://scitation.aip.org/content/aip/journal/apl?ver=pdfcov


2/4

second-harmonic generation in periodically poled optical

Raman Kashyap

British Telecom ResearchLaboratories, Martlesham Heath, Ipswich IP5 7RE, United Kingdom

(Received 1 October 1990; accepted for publication 18 December 1990)

Phase-matched second-harmonic generation is reported in periodically poled optical fibers for

the first time. A periodic x was induced in optical fibers during phase-matched

periodic-electric field-induced second-harmonic generation at a fundamental wavelength of

1064 nm. Further, it is shown that periodic poling can be achieved by photo excitation

with radiation of 514.5 nm wavelength while applying a spatially periodic static field. In both

cases, the mode interaction HEyi

-*HE:? is quasi-phase matched at 1064 nm. Seeding is

observed in fibers beyond the poled region. A photoinduced index change has been measured

using electric field induced second-harmonic generation.

Optical fibers have been poled by the application of

electric fields in the presence of high-intensity

phase-matched and non-phase-matched fibers.2

fibers by quasi-phase matching3 using spatially pe-

dc electric fields in the presence of (a) 1064 nm

or (b) argon ion laser 514.5 nm radiation. It is

d that after poling of the fiber for the interac-

-+ HE:;, it subsequently

the same mode interaction when

1064 nm radiation alone. The fiber beyond the

region seeds4 in the presence of 1064 nm radiation

competition in second-harmonic generation (SHG)

and poled regions of the fiber is thought to

The fiber device used for poling is similar to the one

in Ref. 5. Two half-coupler blocks (HCB) as used in

tric field induced second-harmonic gen-

in optical fibers were made using two

(core radius 1.19 pm) and B (core radius 1.52

Both fibers had a core-cladding index difference of

o that the fiber cladding was partially removed in

lo-mm-long region in the middle of the HCB. The edge

ore was estimated5 to be approximately 6 pm below

urface for fiber A and 2 pm for fiber B. A chromium

electrode structure, manufactured on a glass

placed on the surface of the HCB with a

spacer to separate the periodic electrode f rom the

etter, seeding refers to the situation

the presence of the fundamental and second-

wavelengths alone in the fiber induces prepara-

al poling as reported by Stolen and Tom,4

self-seeding refers to preparation in the absence of

external second-harmonic seed.

Before beginning the poling experiments, the-electrode

rotated while 1064 nm radiation was present in the

to achieve phase-matched periodic-EFISH generation

HEY, *HE:; interaction. The angle 8 of the elec-

= A/(2 cos 6), where A is the period of the in-

electrodes. For both wavelengths, the electrode

was left in position at the phase-matching angle during

poling so that the poled fiber would be phase matched for

the same mode interaction.

In the poling experiments with 1064 nm radiation, ini-

tially, 130 mW of 150 ns Q-switched 1064 nm radiation at

a repetition rate of 1 kHz was launched into fiber A to

ensure that no self-preparation was taking place. There was

little change in the background signal over a period of an

hour. A pulsed dc field of 10 ps pulse width was then

applied. Poling was performed on a device using fibre A

with a 4 ,um Mylar spacer. Phase-matched SHG was ob-

served at 8 = 10.8. With A = 32 ,um the coherence length

Z, was calculated to be 16.29 pm. The maximum average

field in the core was calculated5 to be 1.95 x lo6 V m - t

(120 V across electrodes). After poling, and in the pres-

ence of the electric field, there are three factors that con-

tribute to the SHG signal-EFISH generation, SHG from

the poled region and SHG from any seeded region. The

poled region should have a grating rr/2 out of phase with

the grating created by the EFISH generated seeding signal

(proportional to x

3&,) as is the case in normally seeded

fibers. After poling for 10 min, the SHG signal with 150

mW of infrared power was monitored, as the spatially pe-

riodic static potential is varied from 120 to 0 V and back up

to 120 V (shown in Fig. 1). The x axis shows arbitrary

time taken to ramp the voltage manually . The top curve

shows that at first the SHG signal decreases with decreas-

ing field (bottom curve). The SHG signal does not reach

zero showing that there is an increase in the background

SHG level which is believed to be as a result of EFISH

generation acting as the seeding radiation. As the dc-field

induced nonlinearity x

(3)Edc becomes smaller by further

reducing Ed0 the second-harmonic signal increases indicat-

ing that the poled nonlinearity is of the opposite sign to the

EFISH nonlinearity (i.e., out of phase by Z- radians). At

zero field, the poled signal is slightly less than the EFISH

signal. Increasing the static field returns the SHG to the

EFISH level by retracing the curve. By cutting back the

fiber it was ascertained that only the length of fiber beyond

the poled region had seeded; it might be concluded that the

fiber had not self seeded but that had undergone EFISH

generation-assisted seeding.

Appl. Phys. Lett. 58 (12), 25 March 1991

0003-6951/91 /I 21233-03 02.00

@ 1991 American Institute of Physics

1233


134.117.39.204 On: Thu, 05 Jun 2014 22:48:42


3/4

Time (Minutes)

FIG. 1. Top curve shows SHG signal from a poled fiber. Bottom curve is

the corresponding voltage applied to the periodic electrodes. The hori-

zontal axis shows the time over which the measurement was performed as

the voltage is varied manu ally between 120 and 0 V.

A second device made with fiber B was also electrically

poled using the same infrared power. The voltage applied

across the electrodes was between 50 and 100 V (average

dc field in the core of between 1.84~ lo5 and 3.68~ lo5

V m - ). Phase-matched periodic EFISH generation ob-

served at an angle of 18.8 corresponds to a coherence

length t, of 16.90 pm. The conversion efficiency for EFISH

generation was measured to

be

6.08 X 10 - 6%. The poling

field was applied for 200 min and the growth of the poling-

induced second-harmonic signal (shown in Fig. 2) mea-

sured by occasionally switching off the dc-electric field.

The final poled SHG signal was approximately 190 x the

initial background SHG signal, giving

a

conversion effi-

ciency of 3.85 X 10 - 6%. A linear growth in the SHG sig-

nal with time of application of the field was seen and no

saturation was observed. Once the poling field was

switched off, the SHG signal was monitored continuously

over a period of 600 min at the same input infrared power.

The data are shown in Fig. 3. Immediately after poling the

4

El

t

cl

g

2

El

a

?

II

lo

fn 1

El

El

l3

a

I 1

I

0

100

200

Field application time (minutes)

FIG. 2. Growth in the SHG signal from a poled fiber as a function of

poling time.

+self

seeding

- IOOIM

z

- 1000

competitionwith

170 mW QS 1064nm

% 100

a

I., ,

. , .

,

0 100 200 300 400 500

time (minutes)

600

FIG. 3. SHG si gnal after poling as a function of time. Initially the signal

decreases slowly. Note that at 1 20 min, the i nfrared power was raised

from 130 to 170 mW.

SHG signal slowly decayed, During this measurement, the

infrared power was raised to 170 mW after 120 min caus-

ing the SHG signal to increase slightly before continuing to

decay. After 220 min, the SHG signal began to increase

and this was attributed to the beginning of seeding of the

fiber beyond the poled region. The decrease in SHG is not

believed to be due to self erasure, since the decrease after

poling is followed by a steady growth in self-seeded har-

monic radiation. Our preliminary experiments indicate

that competition between the SHG from the poled and

seeded regions is the cause of the SHG signal decaying

with time immediately after poling.

The observation of poling in optical fibers with 1064

nm radiation on application of the periodic static field is of

practical interest but also intriguing. Bergot et af. have

reported poling in the presence of infrared light bu t they

gave no reasons for their observation. We believe that pol-

ing is assisted by Four-photon absorption from 1064 nm. In

previous experiments,6 we observed erasure of self-seeded

gratings with infrared radiation alone and recently, the

erasure has been shown to be proportional to the fourth

power of the infrared intensity. The intensity of the infra-

red in the core was approximately 14 GW cm - f 140

W pm - 1, similar to that used in Ref. 7. A possible ex-

planation is that: carriers are generated through four-

photon absorption which are then organized by external

electric fields. With the external field off, the internal

charge distribution remains, creating an internal poling

field.

Electric field poling was also tried with 514.5 nm ra-

diation. At first irradiation of Fiber B with 250 mW of

514.5 nm radiation for 30 min completely erased the pre-

viously poled fiber so that subsequentl y only the back-

ground SHG signal was observed when probed with 1064

nm radiation. Not wi thstanding the use of different fibers,

this observation is contrary to the work of Bergot et al.

who noted that the poled nonlinearity did not erase with

blue-green light irradiation. After e rasure, the EFISH gen-

eration phase-matching angle 8 at 1064 nm had to be al-

tered to 18, indicating a photoinduced change in the mode

indices had occurred. The magnitude of the change in the

index mismatch of the modes A(n - nb) before and af-

ter erasure was calculated to be 7.3~ 10 - * (equivalent to

a change in the core cladding index difference at both fre-

1234 Appl. Phys. Lett., Vol. 58, No. 12, 25 March 1991

Raman Kashyap

1234


134.117.39.204 On: Thu, 05 Jun 2014 22:48:42


4/4

d 2w, of approximately 8 X 10 - 5). Detail ed

be published elsewhere.

Poling of the fiber was then performed using 140 ps dc

and acousto-optically generated 100 ps pulses at

nm wavelength radiation at a repetition rate of 1

The poling field (average of 8.1 x lo5 V m - in the

was applied for a time longer than the optical pulses

avoid optical bleaching during the off period. 200 mW of

was used for a poling time of 205 min. After poling,

final SHG signal was 70.3~ the background when

130 mW of Q switched 1064 nm radiation.

conversion efficiency was approximately

x 10 - 6%, being limited by a photoinduced change in

The intensity of the 514.5 nm radiation while poling

approximately 10 MW cm - (0.1 W pm - 2), well be-

the 1 Wprnw2

threshold required for poling by Bergot

al. in their experiment. By our own experiments of

formation in optical fibers and those re-

recently by LaRochelle et ~f.,~ we believe that pho-

y by two-photon absorption (TPA), is

place at these intensities and the carriers so gener-

if two-photon excitation at 514.5 nm is assisting

process, then the question remains as to whether

532 nm light through EFISH generation is causing

With intensities five orders of

smaller at 532 nm than used in the 514.5 nm

experiment, it is expected that the poling time

be ten orders of magnitude greater, contrary to ob-

EFISH-generated signal on its own does

the poling process.

It has been shown that optical fibers can be poled to

create a spatially periodic nonlinearity by photoexcitation

at a wavelength of 1064 nm or 514.5 nm and simultaneous

application of a periodic static field. The poled nonlinearity

has the opposite sign to that of EFISH generation. A pos-

sible explanation is that electric field poling is assisted by

four-photon excitation from 1064 nm and two-photon ex-

citation from 5 14.5 nm. A photoinduced change in the core

cladding refractive index difference of the fiber after irra-

diation of the fiber with 514.5 nm light has been observed

and estimated to be approximately 8X lo- 5. With better

optimized devices, it should be possible to seed fibers effi-

ciently by using periodic EFISH generation. Periodic pol-

ing of fibers in connection with short wavelength radiation

should allow phase-matching at arbitrary wavelengths.

Frequency mixing from wavelengths at which the fiber is

not photosensitive should then be possible.

I would li ke to thank S. T. Davey, D. L. Williams, and

C. A. Millar for stimulating discussions and comments on

the manuscript.

M. V. Bergot, M. C. Farries, M. E. Fermann, L. Li, L. J. Poyntz-

Wright, P. St. J. Russell, and A. Smith son, Opt. Lett. 13, 592 (1988).

M. E. Fermann, L. Li, M. C. Farries, and D. N. Payne, IEE Conf. Proc.

292, 135 (1988).

3N. Bl oembergen and N. J. Sievers, Appl. Phys. Lett. 17, 483 ( 1970).

4R. H. Stolen and H. W. K. Tom, Opt. Lett. 12, 585 (1987).

5R. Kashyap, J. Opt. Sot. A m. B 6, 313 ( 1989).

J. Lucek, R. Kashyap, S. T. Davey, and D. L. Williams, J. Mod. Opt.

37, 533 (1990).

Y. Hibino, V. Mizrahi, and G. I. Stegeman, Appl. Phys. Lett. 57, 656

(1990).

R. Kashyap, in Proceedings of Topical Meeting on Nonlinear Guided

Wave Phenomenon: Physics and Applications, 1989 Technical Digest Se-

ries, Vol. 2 (Optical Society of American, Washington DC, 1989), pp.

255-258.

9S. LaRochelle, V. Mizrahi, G. I. Stegeman, Appl. Phys. Lett. 57, 747

(1990).

Appl. Phys. Lett., Vol. 58, No. 12, 25 March 1991

Raman Kashyap

1235


134.117.39.204 On: Thu, 05 Jun 2014 22:48:42

Documents

Phasematched Secondharmonic Generation in Periodically Poled Optical Fibers