Optics Communications 241 (2004) 503–506

www.elsevier.com/locate/optcom

Noncritical phase matching of Nb:KTP crystalfor blue light generation

Ge Zhang *, Deyin Zhang, Hongyuan Shen, Wen Liu, Chenghui Huang,Lingxiong Huang, Yong Wei

Fujian Institute of Research on the Structure of Matter, Crystal Centre, Chinese Academy of Sciences, Yangqiao West Road, Fuzhou,

Fujian 350002, PR China

Received 8 May 2004; accepted 21 July 2004

Abstract

The relationships between the temperatures and the principal refractive indices of 7.5 mol% Nb:KTP crystal is

achieved from the measured refractive indices. The cutoff wavelengths of noncritical phase matching at different tem-

peratures are calculated. Compared with the KNbO3 crystal, the Nb:KTP crystal shows the better performance for gen-

erating the blue light.

� 2004 Published by Elsevier B.V.

PACS: 42.70.Mp; 42.79.Nv; 42.65.Ky

Keywords: Nb:KTP; Blue light; Noncritical phase matching; Temperature tuning

1. Introduction

KTiOPO4 (KTP) is regarded as an excellent

nonlinear optical crystal for its broad tempera-

ture tolerance and large angular bandwidth.

Unfortunately, its cutoff wavelength for type II

0030-4018/$ - see front matter � 2004 Published by Elsevier B.V.

doi:10.1016/j.optcom.2004.07.049

* Corresponding author. Tel.: +86 591 371 3114; fax: +86

591 371 4648.

E-mail addresses: zhg@fjirsm.ac.cn (G. Zhang),

chh@fjirsm.ac.cn (C. Huang).

phase matching is 994 nm because of the smallbirefringence [1], which limits its applications in

the blue spectral region. This limitation can be

bypassed by doping the KTP crystal with Nb

[2]. In earlier papers, we reported the refractive

indices, thermal refractive-index coefficients and

properties of second harmonic generation of 7.5

mol% Nb:KTP crystal [3], and the cutoff wave-

length of this crystal has been shortened to 960nm at room temperature [4]. In order to realize

shorter cutoff wavelength, more Nb must be

Table 1

Refractive indices of 7.5 mol% Nb:KTP at different tempera-

tures and wavelengths

Temperature (�C) Nx Ny Nz

539.75 nm

46.1 1.7794 1.7924 1.9033

75.5 1.7799 1.7932 1.9047

111.2 1.7804 1.7942 1.9063

143.0 1.7808 1.7949 1.9080

632.8 nm

46.1 1.7646 1.7760 1.8799

75.5 1.7652 1.7769 1.8811

111.2 1.7655 1.7774 1.8825

143.0 1.7659 1.7781 1.8838

1079.5 nm

46.1 1.7391 1.7481 1.8416

504 G. Zhang et al. / Optics Communications 241 (2004) 503–506

doped into KTP crystal, but it will cause more

difficulties during the crystal growing. For

Nb:KTP crystal, the cutoff wavelength is the

phase matching wavelength at y axis direction.

Raising the temperature of noncritical phasematching at y direction is also an efficient way

to shorten the cutoff wavelength. In this paper,

the wavelengths of noncritical phase matching

of 7.5 mol% at y direction under different temper-

atures are calculated. The cutoff wavelength is

shortened to 939 nm at 400 �C. The properties

of noncritical phase matching are compared with

KNbO3. It can be concluded that the 7.5 mol%Nb:KTP crystal can be used as the frequency

doubler at wavelength range of 960–939 nm and

is easier to be used than KNbO3.

75.5 1.7394 1.7486 1.8425

111.2 1.7398 1.7492 1.8436

143.0 1.7401 1.7498 1.8446

1341.4 nm

46.1 1.7331 1.7417 1.8328

75.5 1.7335 1.7422 1.8338

111.2 1.7338 1.7428 1.8348

143.0 1.7341 1.7434 1.8361

Table 2

The relationships between refractive index and temperature of

7.5 mol% Nb:KTP crystal

Wavelengths (nm) Fitted results

539.75 Nx = 1.7788 + 1.4380 · 10�5 t

Ny = 1.7912 + 2.6046 · 10�5 t

Nz = 1.9011 + 4.8084 · 10�5 t

632.8 Nx = 1.7640 + 1.3310 · 10�5 t

Ny = 1.7752 + 2.0728 · 10�5 t

Nz = 1.8781 + 4.0125 · 10�5 t

1079.5 Nx = 1.7386 + 1.0423 · 10�5 t

Ny = 1.7473 + 1.7462 · 10�5 t

Nz = 1.8401 + 3.0945 · 10�5 t

1341.4 Nx = 1.7327 + 1.0077 · 10�5 t

Ny = 1.7409 + 1.7462 · 10�5 t

Nz = 1.8312 + 3.3358 · 10�5 t

2. Calculation and results

The 7.5 mol% Nb:KTP crystal studied in thispaper was grown by the top-seed flux method,

the principal refractive indices of it were measured

by enhanced autocollimation method at wave-

lengths of 539.75, 632.8, 1079.5 and 1341.4 nm

[3], which is listed in Table 1.

The relationship between the principal refrac-

tive index and the temperature of the above data

obeys the linear equation

n ¼ n0 þdndt

t; ð1Þ

where n0 is the refractive index at 0 �C and dn/dt is

the thermal refractive-index coefficient. The fittedresults of the data list in Table 1 are given in

Table 2.

The dispersive relationship of the principal

indices can be expressed as:

niðk; tÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAiðtÞ þ BiðtÞ

k2 � CiðtÞ�DiðtÞk2

s; i ¼ x; y; z:

ð2Þ

Because the cutoff wavelength of Nb:KTP crys-

tal is the type II phase matching wavelength at yaxis direction, the phase matching condition must

satisfy the equation

0 100 200 300 400935

940

945

950

955

960

Cut

off

wav

elen

gth

of S

HG

(nm

)

Temperature (˚C)

940 945 950 955 960 965

6.926.946.966.987.007.027.047.067.087.107.12

∆T·l

(˚C

·cm

)

λ (nm)

(a)

(b)

Fig. 1. The cutoff wavelength and temperature acceptance of

Nb:KTP. (a) The curve of cutoff wavelengths versus tempera-

ture. (b) The curve of temperature acceptances versus

wavelength.

G. Zhang et al. / Optics Communications 241 (2004) 503–506 505

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAxðtÞ þ BxðtÞ

k2 � CxðtÞ� DxðtÞk2

s

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAzðtÞ þ BzðtÞ

k2 � CzðtÞ� DzðtÞk2

s

¼ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAxðtÞ þ BxðtÞ

k2

� �2 � CxðtÞ� DxðtÞ k

2

� �2

vuut : ð3Þ

Generally, it is helpful for calculating the tem-perature tuning of SHG to get the uniform

expressions for A(t), B(t), C(t), D(t). But to

our acknowledge, the uniform expressions for

A(t), B(t), C(t), D(t) by fitting the A, B, C, D

versus temperature will bring more errors to

refractive indices especially when the thermal

refractive-index coefficients are not big. So we

adopt a step by step calculation. First, usingthe relationships listed in Table 2, we can calcu-

late the principal refractive indices at any tem-

perature at the wavelengths of 539.75, 632.8,

1079.5 and 1341.4 nm, and then the parameters

of A, B, C, D at this temperature. At last, the

phase matching wavelength at this temperature

can be achieved by solving Eq. (3). By this

method, we cannot write an uniform expressionof cutoff wavelength suitable to all the tempera-

tures, but it is easy to calculate the cutoff wave-

lengths step by step with the aid of computer.

Fig. 1(a) shows the noncritical phase-matching

wavelength of Nb:KTP crystal at y direction at

the temperatures from 0 to 400 �C. The shortest

cutoff wavelength is 939 nm at temperature of

400 �C.Temperature acceptance bandwidth is an

important parameter to temperature phase match-

ing. According to Kato [5], the temperature

acceptance bandwidth can be expressed as

DT � l ¼ 2kx2:25

� dne1xdT

þ dne2xdT

� 2 � dne22x

dT

� ��1

; ð4Þ

where kx is the fundamental wavelength, dnx/dT

and dn2x/dT are the temperature derivatives of

the refractive indices at fundamental and SHG fre-

quencies, e1 and e2 refer to the polarization direc-tions. It is easy to calculate the temperature

acceptance from Table 2 and Eq. (4).

Fig. 1(b) shows the temperature acceptances of6.92–7.12 �C cm at wavelength range of 960–939

nm. The longer the wavelength is, the larger the

temperature acceptance is.

The cutoff wavelength at room temperature (20

�C) achieved from Fig. 1(a) was compared with the

result calculated from the refractive indices meas-

ured at room temperature and the measured result

[4] in Table 3. It can be seen that the cutoff wave-length at room temperature calculated in this pa-

per is more accurate than that in [4]. The reason

must be that the temperature disturbance at higher

temperature can be more accurately controlled

than that at room temperature.

By the similar method, we can calculate the

noncritical phase matching wavelength for

blue light generation of KNbO3 crystal and thetemperature acceptance range under different

0 100

840

850

860

870

880

890

900

The

non

criti

cal p

hase

mat

chin

gw

avel

engt

h at

a-ax

is d

irec

tion

(nm

)

Temperature (˚C)

850 860 870 880 890 9000.18

0.20

0.22

0.24

0.26

0.28

∆T·l

(˚C

·cm

)

λ (nm)

(a)

(b)

Fig. 2. The noncritical phase-matching wavelengths and tem-

perature acceptances of KN for the blue light generation. (a)

The curve of noncritical wavelength versus temperature. (b) The

curve of temperature acceptance versus wavelength.

Table 3

The cutoff wavelength of Nb:KTP crystal at room temperature

(20 �C)

Calculated

result

Calculated

result

Measured

result

Cutoff wavelength

(nm)

959.3a 965b 960

a The calculated result in this paper.b The calculated result in [4].

506 G. Zhang et al. / Optics Communications 241 (2004) 503–506

temperatures with the dispersion and temperaturedependence of refractive indices of KNbO3 from

Beat Zysset et al. [6]. The results are shown in

Fig. 2. Because there exist phase changing points

at �50 and 220 �C for KN crystal, the actual tun-

able range is limited from �35 to 115 �C. The

phase-matching wavelengths of KN are from 840

to 900 nm at the temperature range from �35 to

115 �C. The corresponding temperature accept-

ances are from 0.19 to 0.27 �C cm.

Due to the much larger thermal refractive-indexcoefficients of KNbO3 [6], the wavelength range of

the noncritical phase matching of KNbO3 is much

larger than that of Nb:KTP. For the same reason,

the temperature acceptance bandwidth of KNbO3

is much smaller than that of Nb:KTP. The large

temperature acceptance bandwidth causes the eas-

iness in the experiment. In addition, it is well

known that the wavelength acceptance of KTPfamily is larger than that of KN crystal. So the fre-

quency doubling of Nb:KTP is easier to be real-

ized than KN crystal for blue light generation.

3. Conclusion

In this paper, the characteristics of temperaturetuning type II noncritical phase matching of the

7.5 mol% Nb:KTP crystal for blue light generation

are calculated. The tuning range of 7.5 mol%

Nb:KTP crystal is from 960 to 939 nm at the tem-

peratures from 0 to 400 �C. The cutoff wavelength

of 7.5 mol% Nb:KTP crystal at room temperature

calculated in this paper is more accurate than the

previous work. Compared to the KNbO3 crystal,the Nb:KTP crystal shows the better performance

of SHG for blue light generation and is a good

candidate of frequency doubler for blue laser.

References

[1] W.P. Risk, R.N. Payne, W. Lenth, C. Harder, H. Meier,

Appl. Phys. Lett. 55 (1989) 1179.

[2] C.T. Cheng, L.K. Cheng, R.L. Harlow, J.D. Bierlein, Appl.

Phys. Lett. 64 (1994) 155.

[3] H.Y. Shen, D.Y. Zhang, W. Liu, G.F. Zhang, W.Z. Chen,

G. Zhang, Appl. Opt. 38 (6) (1999) 987.

[4] W. Liu, H.Y. Shen, G.F. Zhang, D.Y. Zhang, G. Zhang,

W.X. Lin, R.R. Zeng, C.H. Huang, Opt. Commun. 185

(2000) 191.

[5] K. Kato, IEEE J. Quantum Electron. 28 (1992) 1974.

[6] Beat Zysset, Ivan Biaggio, Peter Gunter, J. Opt. Soc. Am. B

9 (3) (1992) 380.