- Email: [email protected]
Simultaneous wavelength conversion and pulse compression exploiting cascaded second-order nonlinear processes in LiNbO3 waveguides
Optics Communications 259 (2006) 321–327 www.elsevier.com/locate/optcom
Simultaneous wavelength conversion and pulse compression exploiting cascaded second-order nonlinear processes in LiNbO3 waveguides Junqiang Sun *, Dexiu Huang, Deming Liu Department of Optoelectronic Engineering, Huazhong University of Science and Technology, Luo Yu Road No. 1037, Wuhan 430074, PR China Received 20 February 2005; received in revised form 4 August 2005; accepted 24 August 2005
Abstract Simultaneous wavelength conversion and pulse compression are proposed and demonstrated exploiting cascaded second-order nonlinear processes in periodically domain-inverted LiNbO3 waveguides. The influences of initial pulse widths and waveguide length on the conversion efficiency and converted pulse compression are theoretically analyzed. Tunable wavelength conversion is performed for the signal pulse with the temporal width of 7.5 ps and repetition rate of 40 GHz. Conversion efficiency of more than 24 dB is obtained for 35-nm conversion span under average signal power of 10 dBm when a CW control wave is adopted. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Frequency conversion; Optical communications; Waveguides
1. Introduction All-optical wavelength conversion is required in dense wavelength-division-multiplexed (DWDM) networks to fulfill information processing and routing in the optics domain [1]. Among numerous demonstrated wavelength conversion techniques, difference frequency generation (DFG) has attracted considerable interest and manifested distinct advantages: in addition to strict transparency, independence of bit rate and data format, and no intrinsic frequency chirp, it can achieve a broad conversion bandwidth with negligible spontaneous emission noise [2,3]. Simultaneous multiwavelength conversion [3,4], spectral inversion and parametric amplification [5] are also attractive properties of the DFG-based wavelength converters. Various continuous waves (CW) DFG operations of quasi-phase-matching (QPM) devices have been demonstrated utilizing periodically domain-inverted LiNbO3 (PPLN) [3]
*
Corresponding author. Tel.: +862787543355x5; fax: +862787556188. E-mail address: [email protected] (J. Sun).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.08.044
and AlGaAs waveguides [6], showing impressive performances. A great number of research results indicate that adopting cascaded second-order nonlinear processes, such as cascaded DFG and second-harmonic generation (SHG) [7], and cascaded DFG and sum-frequency generation (SFG) [8], DFG-based wavelength conversion is conveniently implemented by overcoming the difficulty in coupling the pump and the signal waves into the fundamental mode of the waveguide. In the conventional DFG-based wavelength converter employing the cascaded SHG and DFG processes, a strong continuous wave (CW) pump wave at kp is used to yield a frequency-doubled wave at kh = kp/2. Meanwhile, the frequency-doubled wave interacts with a signal wave at ks through the DFG process to generate an idler wave at ki with 1/ki = 2/kp 1/ks. The information carried by the signal wavelength is completely copied onto the wavelength of the idler wave and the wavelength conversion is thus realized. So, wavelength conversion takes place between the signal and the idler waves, and the wavelengths of the signal and idler waves satisfy the object–image relationship as if a mirror were positioned at the pump wavelength. For a given pump wavelength,
322
J. Sun et al. / Optics Communications 259 (2006) 321–327
wavelength conversion occurs only in a certain wavelength separation. If the pump wavelength can be varied, the converted wavelength corresponding to the idler wave will be tuned accordingly. Unfortunately, the bandwidth for the pump wavelength is much narrower due to the intrinsic property of PPLN waveguide [9]. Therefore, the flexibility of the DFG-based wavelength converter is restricted to some extent. Exploiting cascaded sum- and differencefrequency generation(cSFG/DFG), a 10-GHz, 5-ps pulse stream has been wavelength-converted with the wavelength shifting range of 15 nm. But in that experimental configuration several laser sources and high pump power are required [10]. Wavelength conversion for a 40-Gbit/s, 5-ps pulse data stream has also been achieved recently by employing cascaded SHG and DFG effects. But the wavelength conversion occurred only between fixed wavelengths [11]. In this paper, a pulsed pumping is adopted and thus the pump pulse can be used to carry the information. Wavelength conversion from the pump to the idler wave is proposed and demonstrated in PPLN waveguide, and is different from those reported in [3,4,10]. In [3], simultaneous multiwavelength conversion is realized from the signal to idler wave and only CW performance is presented. In [4], multichannel wavelength conversion is implemented from the signal to idler wave using intra-cavity generated pump. In the above schemes, wavelength conversion occurred only between fixed wavelengths and the tunable performance cannot be achieved. In [10], although the tunable wavelength conversion is fulfilled from the signal to idler wave based on the cascaded SFG and DFG process, two pump sources are required and experimental setup is complicated. Wavelength conversion in our scheme takes place from the pump signal to the idler and the wide tunable region can be obtained by the change of CW control wave. With this scheme, the injected pump pulse power can be lowered and tunable wavelength conversion with pulse compression can be potentially achieved. The wavelength conversion performance is experimentally demonstrated and theoretically analyzed. 2. Theoretical model and results Since the information can be carried by the pump pulse, in the following discussion the pump pulse is called by the signal wave and the signal in the conventional wavelength conversion scheme is named by the control wave. The wavelength conversion process exploiting the cascaded SHG and DFG interactions can be briefly described as follows: Injected signal wave (wavelength ks) of frequency xs, which serves as pump pulse in the cascaded secondorder nonlinear interactions, together with the continuous wave (CW) or pulsed control wave (wavelength kc) of frequency xc are coupled into the PPLN waveguide. Propagating along the waveguide, the signal pulse is frequency doubled and up-converted to second-harmonic (SH) pulse with the frequency of xh = 2xs. Simultaneously, following the growth of the SH pulse, the control wave is mixed with
the generated SH pulse in the same waveguide to realize the DFG process. As a result, the idler pulse (wavelength ki) corresponding to the frequency of xi = 2xs xc is generated. Consequently, the information carried by ks is completely copied onto the wavelength ki and the wavelength ki satisfies the object image relationship relative to the wavelength kc as if a mirror were positioned at ks (between ki and kc). Thus the idler pulse wavelength can be tuned following the variation of the control wave wavelength if the control signal bandwidth is broad enough. The slowly varying amplitudes As, Ac, Ah, and Ai of the signal, control, SH, and idler waves in the PPLN waveguide are governed by [12] oAs oAs b00 o2 As þ b0s j s ¼ jjh Ah As expðjDk h zÞ; ð1Þ oz ot 2 ot2 oAh oAh b00 o2 Ah þ b0h j h oz ot 2 ot2 ki 2 ¼ jjh fAs g expðjDk h zÞ j jd Ai Ac expðjDk d zÞ; kh ð2Þ oAc oAc b00 o2 Ac ki þ b0c j c ¼ j jd Ah Ai expðjDk d zÞ; 2 oz ot 2 ot kc 00 2 oAi oA b o A i i þ b0i j i ¼ jjd Ah Ac expðjDk d zÞ; oz ot 2 ot2 ob ðxÞ b0m ¼ m m ¼ s; c; h; i; ox x¼xm o2 bm ðxÞ b00m ¼ m ¼ s; c; h; i; ox2 x¼xm 2p Dk d ¼ bh bc bi ; K 2p Dk h ¼ bh 2bs ; K
ð3Þ ð4Þ ð5Þ ð6Þ ð7Þ ð8Þ
where bs, bc, bh, bi are the propagation constants corresponding to the signal, control, SH, and idler fields, respectively. kh and kd are the coupling coefficients of SHG and DFG processes, and we assume kd kh for the case of ks kc. b0m and b00m stand for group velocities and group-velocity dispersions for the signal frequency xm. Dkh and Dkd denote the phase mismatch for the SHG and DFG processes, respectively. K is the period of the periodically domain-inverted structure in PPLN waveguide. By designing an appropriate period K, exactly phasematched SHG (Dkh = 0) and nearly phasematched DFG (Dkd 0) can be realized. Thus, the wavelength of the SH wave is determined by the period K through the expression Eq. (8). The above coupling Eqs. (1)–(4) can be numerically solved using the finite difference beam propagation method (FD-BPM). Once the field amplitudes Am (m = s, c, h), are acquired, conversion efficiency g for converting the input signal pulse into the idler pulse can be calculated with the following expression: R þ1 2 jAi ðL; tÞj dt g ¼ R1 . ð9Þ þ1 2 jAs ð0; tÞj dt 1
J. Sun et al. / Optics Communications 259 (2006) 321–327
oAðz; tÞ oAðz; tÞ b00 o2 Aðz; tÞ þ b0 þ ; ð10Þ oz ot ot2 2 and the recursion formula below is utilized to calculate the field amplitudes Am with the FD-BMP method
F ðz; tÞ ¼
b0 Dz n ðA Anm1 Þ 2Dt mþ1 b00 Dz Anmþ1 þ Anm1 2Anm þ DzF nm ; þ 2 Dt2 F ðmDt; nDzÞ ¼ F nm .
¼ Anm Anþ1 m
ð11Þ ð12Þ
The difference term in Eq. (11) is calculated by using the gradient function in Matlab 6.5. To assure the convergence and stability of the calculated results, we utilize a tentative approach to acquire the appropriate time and distance step lengths for each calculation parameters. Here, the step lengths for time and distance are taken less than 1 ps and 10 lm, respectively. During the calculation, only the group-velocity mismatching is considered and group-velocity dispersion is ignored. This approximation does not impact the accuracy of the model and calculated results. In fact, the group-velocity dispersion length for the PPLN waveguide is several times larger than the waveguide length used in our calculation [13]. Thus, the neglect of influences of group-velocity dispersion on the device performances is reasonable. An experimental expression about the variations of refractive index with the signal wavelengths at room temperature for LiNbO3 is utilized [14]. A typical PPLN waveguide with the length of 50 mm and the waveguide effective area of 50 lm2 is used in our simulations. The nonlinear coefficient d33 is about 27 pm/V and the coupling coefficients are assumed kd kh = 0.086 W1/2 mm1. The uniform QPM grating period K is assumed 14.7 lm to satisfy exactly phasematched SHG at the launched signal wavelength of 1.545 lm. The initial signal and control pulses are assumed with sech2 shapes and their amplitudes As(0, t) and Ac(0, t) are expressed as pffiffiffiffiffiffi 1:76 t ; ð13Þ As ð0; tÞ ¼ P s0 sech ss0 pffiffiffiffiffiffiffi 1:76 t ; ð14Þ Ac ð0; tÞ ¼ P c0 sech sc0 where ss0 and sc0 are the full-width at half-maximum (FWHM) corresponding to the signal and control pulses. A walk-off length is defined by ss0 . ð15Þ Lwalk-off ¼ 0 bh b0s Lwalk-off denotes the propagation length for which the temporal walk-off between the signal and SH pulses caused by the group-velocity mismatching amounts to ss0. Lwalk-off is approximately 18.3 mm for ss0 = 5 ps and ks = 1.545 lm.
Fig. 1 illustrates the dependence of wavelength conversion efficiency on the initial pulse width of the signal wave under different pulse widths of the launched control waves. The wavelength of the control wave is 1.56 lm, and the input optical powers are 100 and 500 mW corresponding to the signal and the control waves, respectively. The waveguide length is 50 mm. It is noticed that the conversion efficiency increases with the increasing of ss0 and sc0. Based on the expression (15) about the walk-off length, the short signal gives rise to the shortening of the walk-off length, leading to the weakening of the nonlinear interaction between the SH and control waves for the waveguide length greater than the walk-off length, and hence the decreasing of the conversion efficiency. The results imply that for high-speed operation the conversion efficiency is restricted at a certain waveguide length. Fig. 2 shows the changes of the converted pulse width with the waveguide length for different pulse widths of the launched control waves. The initial signal pulse width is 5 ps and other parameters are the same in the Fig. 1. It is found that the converted pulse is inevitably compressed at the beginning section of the waveguide and then broadened with the increase of the waveguide length. Additionally, the narrower the control pulse is, the more the converted pulse can be compressed. This indicates that the device should be designed shorter for high speed operation and therefore there is a trade-off between speed and efficiency. The behavior can be explained in qualitative manner as follows: Picosecond control and signal pulses own wide frequency spectra with abundant frequency components. In the starting section of the waveguide, the influences of the walk-off effect on the pulse propagation can be ignored and all the frequency components relative to the traveling pulses involve in the secondorder nonlinear interactions. Consequently, the converted
-10
CW signal τc0=10ps
-11
Conversion efficiency (dB)
Thus, the wavelength conversion characteristics including conversion efficiency and pulse width variations can be analyzed and discussed. Coupling equations (1)–(4) can be considered as a general partial differential equation with the form as follows
323
τc0=5ps -12
-13
-14
-15
-16
-17 2
4
6
8
10
Launched signal pulse width τs0 (ps) Fig. 1. Variations of conversion efficiency with the initial signal pulse width for the different control pulse widths. The waveguide length is 50 mm, and peak powers of the signal and the control waves are 100 and 500 mW, respectively. The wavelengths of the signal and control waves are chosen at 1.545 and 1.56 lm, respectively.
324
J. Sun et al. / Optics Communications 259 (2006) 321–327
under different pulse widths of the launched control waves. The converted pulse width is decreased with the increase of the signal pulse width, and pulse compression is achieved for the larger signal pulse width. This is because the walk-off length is larger for the longer signal pulse than the shorter one according to Eq. (15). It is also found that the pulse compression ratio (sd/ss0) is increased with the decrease of the launched control pulse width. This behavior is attributed to the tolerance of the QPM condition for the narrower control pulse owing to its broader frequency bandwidth.
Converted pulse width τd/τs0
1.1
1.0
CW signal τc0=10ps τc0=5ps
0.9
0.8
0.7
0.6
3. Experiments and results 0.5
1.0
1.5
2.0
2.5
Waveguide length L/LW0 Fig. 2. Changes of converted pulse width against waveguide length for the different control pulse widths. The initial signal pulse width is 5 ps, and the other parameters are the same as in Fig. 1.
pulse within the starting part of the waveguide owns broad frequency band and is accordingly compressed in its time domain. As the propagation length increases beyond the walk-off length, the propagating pulses will not overlap completely and the nonlinear interactions are weakened. On the contrary, the converted pulse is broadened. Fig. 3 gives normalized conversion efficiency as a function of the control wave wavelength. The 3 dB bandwidth is approximate 100 nm, which implies that the tunable wavelength conversion can be performed within the range of 100 nm. As seen from Fig. 3, 3 dB bandwidth for the pulsed control wave is slight narrower than that for the CW control wave. This restriction to the conversion bandwidth is mainly caused by the QPM mismatching for the DFG process. Fig. 4 displays the variations of the converted pulse width against the initial signal pulse width
Fig. 5 shows an experimental arrangement for wavelength conversion based on cascaded SHG and DFG interactions. The CW control wave is generated by a tunable fiber ring laser, which is formed by a tunable filter, an isolator, a variable attenuator, a PPLN waveguide, polarization controllers, and erbium-doped fiber amplifiers (EDFAs). The polarization-independent optical isolator is employed
1.8
CW Signal 1.6
Converted pulse width τd/τs0
0.0
τ c0=10ps τ c0=5ps
1.4
1.2
1.0
0.8
0.6
0.4 2
Normalized conversion efficiency (a.u.)
1.0
CW Signal τc0=10ps
0.8
4
6
10
8
Initial signal pulse width (ps) Fig. 4. Converted pulse width as a function of the initial signal pulse width for the different pulse widths of the control waves. The other parameters are the same as in Fig. 1.
τc0=5ps
0.6
0.4
Tunable pulsed signal source
EDFA2
1/1 FC
PC
PPLN waveguide 1/9 FC
Optical spectrum analyzer
0.2
PC
0.0 1.45
1.50
1.55
1.60
1.65
Control wave wavelength (µm) Fig. 3. Dependence of the normalized conversion efficiency on the wavelength of the control waves for the different pulse widths of the control waves. The initial signal pulse width is 5 ps, and the other parameters are the same as in Fig. 1.
Tunable filter
Isolator EDFA1 Variable attenuator
Fig. 5. Experimental setup for wavelength conversion based on cascaded SHG and DFG interactions. PC, polarization controller; FC, fiber coupler.
J. Sun et al. / Optics Communications 259 (2006) 321–327
Fig. 6. Measured output spectrum from the wavelength converter with the wavelength of the control wave at 1.5637 lm.
1.5637 lm. The central signal wavelength is tuned at 1545 nm to meet the QPM conditions for SHG and DFG processes. Wavelength conversion efficiency is assessed about 21 dB when the average signal power of 10 dBm is launched into the PPLN waveguide. The wavelength of the converted signal can be varied from 1519 to 1562.6 nm as the CW lasing wavelength is tuned from 1534.5 to 1572.1 nm. Fig. 7 depicts the dependence of conversion efficiency on the converted wavelength by keeping the signal wavelength at 1545 nm. The conversion efficiency is measured approximate 24 dB covering 35-nm conversion span under the injected signal power of 10 dBm. As the converted wavelength shifts toward the shorter or the longer wavelength side, the conversion efficiency decreases sharply owing to the breakdown of the QPM condition. Compared with the theoretical results, the reduction of the conversion efficiency is mainly caused by the lower intracavity signal power, residual SHG phase mismatching, and chirping of the launched pulse. Fig. 8 displays the waveforms of the output converted optical signal and its
-22
-24
Conversion Efficiency (dB)
to prevent back reflection and thus to ensure unidirectional laser oscillation in the fiber ring. The tuning bandwidth of the tunable filter is 50 nm at 1.55-lm band and its linewidth is 1 nm. The variable attenuator is utilized here to control the CW lasing power by the change of the cavity loss. A tunable mode-locked fiber laser serves as a pulsed signal source operating at 1.5 lm band with a pulse repetition rate of 40 GHz. The measured signal pulse width ss0 is 7.5 ps. The PPLN waveguide with the length of 50 mm is fabricated by the electrical poling method and thereafter annealing proton-exchanged process. Annealing proton exchanged (APE) channel waveguide in PPLN was made by proton exchanged in pure benzoic acid for 2 h at 190 °C, followed by annealing for 7 h at 300 °C. The PPLN waveguide has QPM period of 14.7 lm, waveguide width of 12 lm, and an initial proton exchange depth of 0.8 lm. The device parameters allow phase matching at room temperature between the fundamental mode of the signal at 1545 nm and the fundamental mode of the secondharmonic wave at 772.5 nm. The fiber-to-fiber coupling loss is estimated about 4.7 dB caused by the reflection losses at the uncoated endfaces, mode mismatching between the fibers and the PPLN waveguide, and intrinsic waveguide losses. Two inline polarization controllers are inserted into the ring cavity to enhance the nonlinear interactions in the PPLN waveguide. The cascaded SHG and DFG processes for the experimental arrangement can be briefly explained as follows. When no external signal pulse is launched into the wavelength converter, the fiber ring laser operates in CW status, and the tunable filter determines its lasing wavelength. Here, both EDFA1 and EDFA2 are used as the gain medium in the fiber laser. When the signal pulse satisfying the QPM condition is injected in the fiber ring laser, it will be amplified by the EDFA2, and thus the intra-cavity power of the CW lasing is reduced accordingly. As the signal pulse propagates through the PPLN waveguide, a frequencydoubled pulse is generated through the second-harmonic process. Meanwhile, the frequency-doubled pulse interacts with the CW lasing wave to produce an idler pulse thanks to the difference frequency generation process, and the magnitude of the idler pulse will change with the injected signal pulse. As a result, the information carried by the signal wavelength is completely copied onto the wavelength of the idler pulse and the wavelength conversion is achieved. Tunable DFG-based wavelength conversion can be fulfilled by tuning the lasing wavelength. The output spectra are monitored by an optical spectrum analyzer (Anritsu MS9710C) with the highest spectral resolution of 0.05 nm, and the optical pulses are observed through a communication signal analyzer. In the following experiments, optical powers are measured at the port of the 1/9 FC by the optical spectrum analyzer. Thus, the tunable pulsed laser output, EDFA output, and PPLN output can be estimated by taking into account the FC coupling ratio, the PPLN total insertion loss, and EDFA gain. Fig. 6 illustrates obtained spectrum of the output waves when the wavelength of the control wave is tuned at
325
-26
-28
-30
-32
Signal pulse area
-34
-36 1520
1530
1540
1550
1560
Converted wavelength (nm) Fig. 7. Dependence of the conversion efficiency on the converted wavelength under signal wavelength of 1545 nm and signal power of 10 dBm. The hatched region corresponds to the launched signal pulse area.
J. Sun et al. / Optics Communications 259 (2006) 321–327
corresponding original injected signal by keeping the signal wavelength at 1545 nm and tuning the lasing wavelength at 1569.6 nm. The waveforms are not completely overlapped and there is a phase offset between the converted signal and launched signal pulses. This phenomenon can be attributed to the group-velocity mismatching between the signal and SH pulses, leading to the walk-off effect in the PPLN waveguide. Fig. 9 plots the variations of pulse duration with the converted wavelength under signal wavelength of 1545 nm and the launched signal power of 10 dBm. Assuming that the initial signal pulse is a sech2-shape pulse, the pulse duration of the converted pulse is measured about 7.3 ps by using an autocorrelator. This value is little less than that measured by the communication analyzer due to the limited bandwidth of the optical detector installed in the communication analyzer. The time jitter value is measured less than 1.7 ps for the converted signal waveforms. The jitter behavior can be attributed to the residual reflection from the PPLN waveguide endfaces, the instability of the lasing wavelength and the time jittering induced by the initial signal pulse. The experimental results are good compliance with the theoretical prediction
8.0
Converted pulse duration (ps)
326
7.5
7.0
Signal pulse area 6.5
6.0 1520
1530
1540
1550
1560
Converted wavelength (nm) Fig. 9. Variations of pulse duration with the converted wavelength under signal wavelength of 1545 nm and signal power of 10 dBm. The hatched region corresponds to the launched signal pulse area.
for the CW control waves. Although the suggested configuration operates only for a single WDM channel, tunable wavelength conversion can be easily implemented by the change of the control signal wavelength, meaning that dynamic wavelength conversion from a fixed wavelength to any other wavelengths can be obtained. This is very important and increases the flexibility in the management of the multi-channel WDM network. If the pulsed control signal is employed instead of the CW lasing wave, the converted signal pulse is expected to be compressed according to our theoretical simulations. As a result, simultaneous wavelength conversion and pulse compression can be potentially implemented by means of cascaded secondorder nonlinear interactions between pulses employing our experimental scheme. 4. Conclusion
Fig. 8. Waveforms for different optical signals: (a) output converted signal, (b) launched signal pulse. The signal wavelength is kept at 1545 nm and the CW control wavelength is tuned at 1569.6 nm.
A novel tunable wavelength conversion scheme for picosecond pulses is proposed and demonstrated based on the cascaded SHG and DFG nonlinear interactions in PPLN waveguides. The influences of launched pulse widths and walk-off length on the performances of the wavelength converters in terms of conversion efficiency and converted pulse width are theoretically analyzed and discussed. The results show that there is a trade-off between the conversion efficiency and speed. With the pulsed control wave, compressed pulses can be obtained during the wavelength conversion. Wavelength conversion together with pulse reshaping can be implemented with the suggested scheme. Tunable wavelength conversion is experimentally realized for the 7.5 ps signal pulse with the repetition rate of 40 GHz when a changeable CW control wave is adopted. The wavelength of the converted signal can be tuned from 1519 to 1562.6 nm as the CW control wavelength is varied from 1534.5 to 1572.1 nm. Conversion efficiency of more
J. Sun et al. / Optics Communications 259 (2006) 321–327
than 24 dB is measured for 35-nm conversion span under average signal power of 10 dBm. The results provide the possibility of simultaneously realizing wavelength conversion and pulse reshaping for optical communication applications. Acknowledgment This work was supported by the Chinese Natural Science Foundation under Grant No. 60177015. References [1] S.J.B. Yoo, J. Lightwave Technol. 14 (6) (1996) 955. [2] C.Q. Xu, H. Okayama, K. Shinozaki, K. Watanabe, M. Kawahara, Appl. Phys. Lett. 63 (9) (1993) 1170. [3] M.H. Chou, I. Brener, M.M. Fejer, E.E. Chaban, S.B. Christman, IEEE Photon. Technol. Lett. 11 (6) (1999) 653.
327
[4] Junqiang Sun, Wei Liu, Jing Tian, J.R. Kurz, M.M. Fejer, IEEE Photon. Technol. Lett. 15 (12) (2003) 1743. [5] L.E. Myers, W.R. Bosenberg, IEEE J. Quantum Electron. 33 (10) (1997) 1663. [6] S.J.B. Yoo, C. Caneau, R. Bhat, M.A. Koza, A. Rajhel, N. Antoniades, Appl. Phys. Lett. 68 (19) (1996) 2609. [7] K. Gallo, G. Assanto, G. Stegeman, Appl. Phys. Lett. 71 (8) (1997) 1020. [8] B. Chen, C.Q. Xu, IEEE J. Quantum Electron. 40 (3) (2004) 256. [9] Wei Liu, Junqiang Sun, J. Kurz, Opt. Commun. 216 (1–3) (2003) 239. [10] Y. Min, J. Lee, Y. Lee, W. Grundkoetter, V. Quiring, W. Sohler, in: Tech. Digest of OFC2003, FP4, vol. 2, 2003, p. 767. [11] W. Sohler, W. Grundkoetter, J.H. Lee, Y.H. Min, V. Quiring, H. Suche, R. Schiek, T. Pertsch, F. Lederer, R. Iwanow, G.I. Stegeman, in: Tech. Digest of ECOC2004, Tu.3.4.1, 2004. [12] H. Ishizuki, T. Suhara, M. Fujimura, H. Nishihara, Opt. Quantum Electron. 33 (7–10) (2001) 953. [13] Zheng Zheng, A.M. Weiner, K.R. Parameswaran, Ming-Hsien Chou, M.M. Fejer, J. Opt. Soc. Am. B 19 (4) (2002) 839. [14] M.V. Hobden, J. Warner, Phys. Lett. 22 (3) (1966) 243.
Simultaneous wavelength conversion and pulse compression exploiting cascaded second-order nonlinear processes in LiNbO3 waveguides Junqiang Sun *, Dexiu Huang, Deming Liu Department of Optoelectronic Engineering, Huazhong University of Science and Technology, Luo Yu Road No. 1037, Wuhan 430074, PR China Received 20 February 2005; received in revised form 4 August 2005; accepted 24 August 2005
Abstract Simultaneous wavelength conversion and pulse compression are proposed and demonstrated exploiting cascaded second-order nonlinear processes in periodically domain-inverted LiNbO3 waveguides. The influences of initial pulse widths and waveguide length on the conversion efficiency and converted pulse compression are theoretically analyzed. Tunable wavelength conversion is performed for the signal pulse with the temporal width of 7.5 ps and repetition rate of 40 GHz. Conversion efficiency of more than 24 dB is obtained for 35-nm conversion span under average signal power of 10 dBm when a CW control wave is adopted. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Frequency conversion; Optical communications; Waveguides
1. Introduction All-optical wavelength conversion is required in dense wavelength-division-multiplexed (DWDM) networks to fulfill information processing and routing in the optics domain [1]. Among numerous demonstrated wavelength conversion techniques, difference frequency generation (DFG) has attracted considerable interest and manifested distinct advantages: in addition to strict transparency, independence of bit rate and data format, and no intrinsic frequency chirp, it can achieve a broad conversion bandwidth with negligible spontaneous emission noise [2,3]. Simultaneous multiwavelength conversion [3,4], spectral inversion and parametric amplification [5] are also attractive properties of the DFG-based wavelength converters. Various continuous waves (CW) DFG operations of quasi-phase-matching (QPM) devices have been demonstrated utilizing periodically domain-inverted LiNbO3 (PPLN) [3]
*
Corresponding author. Tel.: +862787543355x5; fax: +862787556188. E-mail address: [email protected] (J. Sun).
0030-4018/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2005.08.044
and AlGaAs waveguides [6], showing impressive performances. A great number of research results indicate that adopting cascaded second-order nonlinear processes, such as cascaded DFG and second-harmonic generation (SHG) [7], and cascaded DFG and sum-frequency generation (SFG) [8], DFG-based wavelength conversion is conveniently implemented by overcoming the difficulty in coupling the pump and the signal waves into the fundamental mode of the waveguide. In the conventional DFG-based wavelength converter employing the cascaded SHG and DFG processes, a strong continuous wave (CW) pump wave at kp is used to yield a frequency-doubled wave at kh = kp/2. Meanwhile, the frequency-doubled wave interacts with a signal wave at ks through the DFG process to generate an idler wave at ki with 1/ki = 2/kp 1/ks. The information carried by the signal wavelength is completely copied onto the wavelength of the idler wave and the wavelength conversion is thus realized. So, wavelength conversion takes place between the signal and the idler waves, and the wavelengths of the signal and idler waves satisfy the object–image relationship as if a mirror were positioned at the pump wavelength. For a given pump wavelength,
322
J. Sun et al. / Optics Communications 259 (2006) 321–327
wavelength conversion occurs only in a certain wavelength separation. If the pump wavelength can be varied, the converted wavelength corresponding to the idler wave will be tuned accordingly. Unfortunately, the bandwidth for the pump wavelength is much narrower due to the intrinsic property of PPLN waveguide [9]. Therefore, the flexibility of the DFG-based wavelength converter is restricted to some extent. Exploiting cascaded sum- and differencefrequency generation(cSFG/DFG), a 10-GHz, 5-ps pulse stream has been wavelength-converted with the wavelength shifting range of 15 nm. But in that experimental configuration several laser sources and high pump power are required [10]. Wavelength conversion for a 40-Gbit/s, 5-ps pulse data stream has also been achieved recently by employing cascaded SHG and DFG effects. But the wavelength conversion occurred only between fixed wavelengths [11]. In this paper, a pulsed pumping is adopted and thus the pump pulse can be used to carry the information. Wavelength conversion from the pump to the idler wave is proposed and demonstrated in PPLN waveguide, and is different from those reported in [3,4,10]. In [3], simultaneous multiwavelength conversion is realized from the signal to idler wave and only CW performance is presented. In [4], multichannel wavelength conversion is implemented from the signal to idler wave using intra-cavity generated pump. In the above schemes, wavelength conversion occurred only between fixed wavelengths and the tunable performance cannot be achieved. In [10], although the tunable wavelength conversion is fulfilled from the signal to idler wave based on the cascaded SFG and DFG process, two pump sources are required and experimental setup is complicated. Wavelength conversion in our scheme takes place from the pump signal to the idler and the wide tunable region can be obtained by the change of CW control wave. With this scheme, the injected pump pulse power can be lowered and tunable wavelength conversion with pulse compression can be potentially achieved. The wavelength conversion performance is experimentally demonstrated and theoretically analyzed. 2. Theoretical model and results Since the information can be carried by the pump pulse, in the following discussion the pump pulse is called by the signal wave and the signal in the conventional wavelength conversion scheme is named by the control wave. The wavelength conversion process exploiting the cascaded SHG and DFG interactions can be briefly described as follows: Injected signal wave (wavelength ks) of frequency xs, which serves as pump pulse in the cascaded secondorder nonlinear interactions, together with the continuous wave (CW) or pulsed control wave (wavelength kc) of frequency xc are coupled into the PPLN waveguide. Propagating along the waveguide, the signal pulse is frequency doubled and up-converted to second-harmonic (SH) pulse with the frequency of xh = 2xs. Simultaneously, following the growth of the SH pulse, the control wave is mixed with
the generated SH pulse in the same waveguide to realize the DFG process. As a result, the idler pulse (wavelength ki) corresponding to the frequency of xi = 2xs xc is generated. Consequently, the information carried by ks is completely copied onto the wavelength ki and the wavelength ki satisfies the object image relationship relative to the wavelength kc as if a mirror were positioned at ks (between ki and kc). Thus the idler pulse wavelength can be tuned following the variation of the control wave wavelength if the control signal bandwidth is broad enough. The slowly varying amplitudes As, Ac, Ah, and Ai of the signal, control, SH, and idler waves in the PPLN waveguide are governed by [12] oAs oAs b00 o2 As þ b0s j s ¼ jjh Ah As expðjDk h zÞ; ð1Þ oz ot 2 ot2 oAh oAh b00 o2 Ah þ b0h j h oz ot 2 ot2 ki 2 ¼ jjh fAs g expðjDk h zÞ j jd Ai Ac expðjDk d zÞ; kh ð2Þ oAc oAc b00 o2 Ac ki þ b0c j c ¼ j jd Ah Ai expðjDk d zÞ; 2 oz ot 2 ot kc 00 2 oAi oA b o A i i þ b0i j i ¼ jjd Ah Ac expðjDk d zÞ; oz ot 2 ot2 ob ðxÞ b0m ¼ m m ¼ s; c; h; i; ox x¼xm o2 bm ðxÞ b00m ¼ m ¼ s; c; h; i; ox2 x¼xm 2p Dk d ¼ bh bc bi ; K 2p Dk h ¼ bh 2bs ; K
ð3Þ ð4Þ ð5Þ ð6Þ ð7Þ ð8Þ
where bs, bc, bh, bi are the propagation constants corresponding to the signal, control, SH, and idler fields, respectively. kh and kd are the coupling coefficients of SHG and DFG processes, and we assume kd kh for the case of ks kc. b0m and b00m stand for group velocities and group-velocity dispersions for the signal frequency xm. Dkh and Dkd denote the phase mismatch for the SHG and DFG processes, respectively. K is the period of the periodically domain-inverted structure in PPLN waveguide. By designing an appropriate period K, exactly phasematched SHG (Dkh = 0) and nearly phasematched DFG (Dkd 0) can be realized. Thus, the wavelength of the SH wave is determined by the period K through the expression Eq. (8). The above coupling Eqs. (1)–(4) can be numerically solved using the finite difference beam propagation method (FD-BPM). Once the field amplitudes Am (m = s, c, h), are acquired, conversion efficiency g for converting the input signal pulse into the idler pulse can be calculated with the following expression: R þ1 2 jAi ðL; tÞj dt g ¼ R1 . ð9Þ þ1 2 jAs ð0; tÞj dt 1
J. Sun et al. / Optics Communications 259 (2006) 321–327
oAðz; tÞ oAðz; tÞ b00 o2 Aðz; tÞ þ b0 þ ; ð10Þ oz ot ot2 2 and the recursion formula below is utilized to calculate the field amplitudes Am with the FD-BMP method
F ðz; tÞ ¼
b0 Dz n ðA Anm1 Þ 2Dt mþ1 b00 Dz Anmþ1 þ Anm1 2Anm þ DzF nm ; þ 2 Dt2 F ðmDt; nDzÞ ¼ F nm .
¼ Anm Anþ1 m
ð11Þ ð12Þ
The difference term in Eq. (11) is calculated by using the gradient function in Matlab 6.5. To assure the convergence and stability of the calculated results, we utilize a tentative approach to acquire the appropriate time and distance step lengths for each calculation parameters. Here, the step lengths for time and distance are taken less than 1 ps and 10 lm, respectively. During the calculation, only the group-velocity mismatching is considered and group-velocity dispersion is ignored. This approximation does not impact the accuracy of the model and calculated results. In fact, the group-velocity dispersion length for the PPLN waveguide is several times larger than the waveguide length used in our calculation [13]. Thus, the neglect of influences of group-velocity dispersion on the device performances is reasonable. An experimental expression about the variations of refractive index with the signal wavelengths at room temperature for LiNbO3 is utilized [14]. A typical PPLN waveguide with the length of 50 mm and the waveguide effective area of 50 lm2 is used in our simulations. The nonlinear coefficient d33 is about 27 pm/V and the coupling coefficients are assumed kd kh = 0.086 W1/2 mm1. The uniform QPM grating period K is assumed 14.7 lm to satisfy exactly phasematched SHG at the launched signal wavelength of 1.545 lm. The initial signal and control pulses are assumed with sech2 shapes and their amplitudes As(0, t) and Ac(0, t) are expressed as pffiffiffiffiffiffi 1:76 t ; ð13Þ As ð0; tÞ ¼ P s0 sech ss0 pffiffiffiffiffiffiffi 1:76 t ; ð14Þ Ac ð0; tÞ ¼ P c0 sech sc0 where ss0 and sc0 are the full-width at half-maximum (FWHM) corresponding to the signal and control pulses. A walk-off length is defined by ss0 . ð15Þ Lwalk-off ¼ 0 bh b0s Lwalk-off denotes the propagation length for which the temporal walk-off between the signal and SH pulses caused by the group-velocity mismatching amounts to ss0. Lwalk-off is approximately 18.3 mm for ss0 = 5 ps and ks = 1.545 lm.
Fig. 1 illustrates the dependence of wavelength conversion efficiency on the initial pulse width of the signal wave under different pulse widths of the launched control waves. The wavelength of the control wave is 1.56 lm, and the input optical powers are 100 and 500 mW corresponding to the signal and the control waves, respectively. The waveguide length is 50 mm. It is noticed that the conversion efficiency increases with the increasing of ss0 and sc0. Based on the expression (15) about the walk-off length, the short signal gives rise to the shortening of the walk-off length, leading to the weakening of the nonlinear interaction between the SH and control waves for the waveguide length greater than the walk-off length, and hence the decreasing of the conversion efficiency. The results imply that for high-speed operation the conversion efficiency is restricted at a certain waveguide length. Fig. 2 shows the changes of the converted pulse width with the waveguide length for different pulse widths of the launched control waves. The initial signal pulse width is 5 ps and other parameters are the same in the Fig. 1. It is found that the converted pulse is inevitably compressed at the beginning section of the waveguide and then broadened with the increase of the waveguide length. Additionally, the narrower the control pulse is, the more the converted pulse can be compressed. This indicates that the device should be designed shorter for high speed operation and therefore there is a trade-off between speed and efficiency. The behavior can be explained in qualitative manner as follows: Picosecond control and signal pulses own wide frequency spectra with abundant frequency components. In the starting section of the waveguide, the influences of the walk-off effect on the pulse propagation can be ignored and all the frequency components relative to the traveling pulses involve in the secondorder nonlinear interactions. Consequently, the converted
-10
CW signal τc0=10ps
-11
Conversion efficiency (dB)
Thus, the wavelength conversion characteristics including conversion efficiency and pulse width variations can be analyzed and discussed. Coupling equations (1)–(4) can be considered as a general partial differential equation with the form as follows
323
τc0=5ps -12
-13
-14
-15
-16
-17 2
4
6
8
10
Launched signal pulse width τs0 (ps) Fig. 1. Variations of conversion efficiency with the initial signal pulse width for the different control pulse widths. The waveguide length is 50 mm, and peak powers of the signal and the control waves are 100 and 500 mW, respectively. The wavelengths of the signal and control waves are chosen at 1.545 and 1.56 lm, respectively.
324
J. Sun et al. / Optics Communications 259 (2006) 321–327
under different pulse widths of the launched control waves. The converted pulse width is decreased with the increase of the signal pulse width, and pulse compression is achieved for the larger signal pulse width. This is because the walk-off length is larger for the longer signal pulse than the shorter one according to Eq. (15). It is also found that the pulse compression ratio (sd/ss0) is increased with the decrease of the launched control pulse width. This behavior is attributed to the tolerance of the QPM condition for the narrower control pulse owing to its broader frequency bandwidth.
Converted pulse width τd/τs0
1.1
1.0
CW signal τc0=10ps τc0=5ps
0.9
0.8
0.7
0.6
3. Experiments and results 0.5
1.0
1.5
2.0
2.5
Waveguide length L/LW0 Fig. 2. Changes of converted pulse width against waveguide length for the different control pulse widths. The initial signal pulse width is 5 ps, and the other parameters are the same as in Fig. 1.
pulse within the starting part of the waveguide owns broad frequency band and is accordingly compressed in its time domain. As the propagation length increases beyond the walk-off length, the propagating pulses will not overlap completely and the nonlinear interactions are weakened. On the contrary, the converted pulse is broadened. Fig. 3 gives normalized conversion efficiency as a function of the control wave wavelength. The 3 dB bandwidth is approximate 100 nm, which implies that the tunable wavelength conversion can be performed within the range of 100 nm. As seen from Fig. 3, 3 dB bandwidth for the pulsed control wave is slight narrower than that for the CW control wave. This restriction to the conversion bandwidth is mainly caused by the QPM mismatching for the DFG process. Fig. 4 displays the variations of the converted pulse width against the initial signal pulse width
Fig. 5 shows an experimental arrangement for wavelength conversion based on cascaded SHG and DFG interactions. The CW control wave is generated by a tunable fiber ring laser, which is formed by a tunable filter, an isolator, a variable attenuator, a PPLN waveguide, polarization controllers, and erbium-doped fiber amplifiers (EDFAs). The polarization-independent optical isolator is employed
1.8
CW Signal 1.6
Converted pulse width τd/τs0
0.0
τ c0=10ps τ c0=5ps
1.4
1.2
1.0
0.8
0.6
0.4 2
Normalized conversion efficiency (a.u.)
1.0
CW Signal τc0=10ps
0.8
4
6
10
8
Initial signal pulse width (ps) Fig. 4. Converted pulse width as a function of the initial signal pulse width for the different pulse widths of the control waves. The other parameters are the same as in Fig. 1.
τc0=5ps
0.6
0.4
Tunable pulsed signal source
EDFA2
1/1 FC
PC
PPLN waveguide 1/9 FC
Optical spectrum analyzer
0.2
PC
0.0 1.45
1.50
1.55
1.60
1.65
Control wave wavelength (µm) Fig. 3. Dependence of the normalized conversion efficiency on the wavelength of the control waves for the different pulse widths of the control waves. The initial signal pulse width is 5 ps, and the other parameters are the same as in Fig. 1.
Tunable filter
Isolator EDFA1 Variable attenuator
Fig. 5. Experimental setup for wavelength conversion based on cascaded SHG and DFG interactions. PC, polarization controller; FC, fiber coupler.
J. Sun et al. / Optics Communications 259 (2006) 321–327
Fig. 6. Measured output spectrum from the wavelength converter with the wavelength of the control wave at 1.5637 lm.
1.5637 lm. The central signal wavelength is tuned at 1545 nm to meet the QPM conditions for SHG and DFG processes. Wavelength conversion efficiency is assessed about 21 dB when the average signal power of 10 dBm is launched into the PPLN waveguide. The wavelength of the converted signal can be varied from 1519 to 1562.6 nm as the CW lasing wavelength is tuned from 1534.5 to 1572.1 nm. Fig. 7 depicts the dependence of conversion efficiency on the converted wavelength by keeping the signal wavelength at 1545 nm. The conversion efficiency is measured approximate 24 dB covering 35-nm conversion span under the injected signal power of 10 dBm. As the converted wavelength shifts toward the shorter or the longer wavelength side, the conversion efficiency decreases sharply owing to the breakdown of the QPM condition. Compared with the theoretical results, the reduction of the conversion efficiency is mainly caused by the lower intracavity signal power, residual SHG phase mismatching, and chirping of the launched pulse. Fig. 8 displays the waveforms of the output converted optical signal and its
-22
-24
Conversion Efficiency (dB)
to prevent back reflection and thus to ensure unidirectional laser oscillation in the fiber ring. The tuning bandwidth of the tunable filter is 50 nm at 1.55-lm band and its linewidth is 1 nm. The variable attenuator is utilized here to control the CW lasing power by the change of the cavity loss. A tunable mode-locked fiber laser serves as a pulsed signal source operating at 1.5 lm band with a pulse repetition rate of 40 GHz. The measured signal pulse width ss0 is 7.5 ps. The PPLN waveguide with the length of 50 mm is fabricated by the electrical poling method and thereafter annealing proton-exchanged process. Annealing proton exchanged (APE) channel waveguide in PPLN was made by proton exchanged in pure benzoic acid for 2 h at 190 °C, followed by annealing for 7 h at 300 °C. The PPLN waveguide has QPM period of 14.7 lm, waveguide width of 12 lm, and an initial proton exchange depth of 0.8 lm. The device parameters allow phase matching at room temperature between the fundamental mode of the signal at 1545 nm and the fundamental mode of the secondharmonic wave at 772.5 nm. The fiber-to-fiber coupling loss is estimated about 4.7 dB caused by the reflection losses at the uncoated endfaces, mode mismatching between the fibers and the PPLN waveguide, and intrinsic waveguide losses. Two inline polarization controllers are inserted into the ring cavity to enhance the nonlinear interactions in the PPLN waveguide. The cascaded SHG and DFG processes for the experimental arrangement can be briefly explained as follows. When no external signal pulse is launched into the wavelength converter, the fiber ring laser operates in CW status, and the tunable filter determines its lasing wavelength. Here, both EDFA1 and EDFA2 are used as the gain medium in the fiber laser. When the signal pulse satisfying the QPM condition is injected in the fiber ring laser, it will be amplified by the EDFA2, and thus the intra-cavity power of the CW lasing is reduced accordingly. As the signal pulse propagates through the PPLN waveguide, a frequencydoubled pulse is generated through the second-harmonic process. Meanwhile, the frequency-doubled pulse interacts with the CW lasing wave to produce an idler pulse thanks to the difference frequency generation process, and the magnitude of the idler pulse will change with the injected signal pulse. As a result, the information carried by the signal wavelength is completely copied onto the wavelength of the idler pulse and the wavelength conversion is achieved. Tunable DFG-based wavelength conversion can be fulfilled by tuning the lasing wavelength. The output spectra are monitored by an optical spectrum analyzer (Anritsu MS9710C) with the highest spectral resolution of 0.05 nm, and the optical pulses are observed through a communication signal analyzer. In the following experiments, optical powers are measured at the port of the 1/9 FC by the optical spectrum analyzer. Thus, the tunable pulsed laser output, EDFA output, and PPLN output can be estimated by taking into account the FC coupling ratio, the PPLN total insertion loss, and EDFA gain. Fig. 6 illustrates obtained spectrum of the output waves when the wavelength of the control wave is tuned at
325
-26
-28
-30
-32
Signal pulse area
-34
-36 1520
1530
1540
1550
1560
Converted wavelength (nm) Fig. 7. Dependence of the conversion efficiency on the converted wavelength under signal wavelength of 1545 nm and signal power of 10 dBm. The hatched region corresponds to the launched signal pulse area.
J. Sun et al. / Optics Communications 259 (2006) 321–327
corresponding original injected signal by keeping the signal wavelength at 1545 nm and tuning the lasing wavelength at 1569.6 nm. The waveforms are not completely overlapped and there is a phase offset between the converted signal and launched signal pulses. This phenomenon can be attributed to the group-velocity mismatching between the signal and SH pulses, leading to the walk-off effect in the PPLN waveguide. Fig. 9 plots the variations of pulse duration with the converted wavelength under signal wavelength of 1545 nm and the launched signal power of 10 dBm. Assuming that the initial signal pulse is a sech2-shape pulse, the pulse duration of the converted pulse is measured about 7.3 ps by using an autocorrelator. This value is little less than that measured by the communication analyzer due to the limited bandwidth of the optical detector installed in the communication analyzer. The time jitter value is measured less than 1.7 ps for the converted signal waveforms. The jitter behavior can be attributed to the residual reflection from the PPLN waveguide endfaces, the instability of the lasing wavelength and the time jittering induced by the initial signal pulse. The experimental results are good compliance with the theoretical prediction
8.0
Converted pulse duration (ps)
326
7.5
7.0
Signal pulse area 6.5
6.0 1520
1530
1540
1550
1560
Converted wavelength (nm) Fig. 9. Variations of pulse duration with the converted wavelength under signal wavelength of 1545 nm and signal power of 10 dBm. The hatched region corresponds to the launched signal pulse area.
for the CW control waves. Although the suggested configuration operates only for a single WDM channel, tunable wavelength conversion can be easily implemented by the change of the control signal wavelength, meaning that dynamic wavelength conversion from a fixed wavelength to any other wavelengths can be obtained. This is very important and increases the flexibility in the management of the multi-channel WDM network. If the pulsed control signal is employed instead of the CW lasing wave, the converted signal pulse is expected to be compressed according to our theoretical simulations. As a result, simultaneous wavelength conversion and pulse compression can be potentially implemented by means of cascaded secondorder nonlinear interactions between pulses employing our experimental scheme. 4. Conclusion
Fig. 8. Waveforms for different optical signals: (a) output converted signal, (b) launched signal pulse. The signal wavelength is kept at 1545 nm and the CW control wavelength is tuned at 1569.6 nm.
A novel tunable wavelength conversion scheme for picosecond pulses is proposed and demonstrated based on the cascaded SHG and DFG nonlinear interactions in PPLN waveguides. The influences of launched pulse widths and walk-off length on the performances of the wavelength converters in terms of conversion efficiency and converted pulse width are theoretically analyzed and discussed. The results show that there is a trade-off between the conversion efficiency and speed. With the pulsed control wave, compressed pulses can be obtained during the wavelength conversion. Wavelength conversion together with pulse reshaping can be implemented with the suggested scheme. Tunable wavelength conversion is experimentally realized for the 7.5 ps signal pulse with the repetition rate of 40 GHz when a changeable CW control wave is adopted. The wavelength of the converted signal can be tuned from 1519 to 1562.6 nm as the CW control wavelength is varied from 1534.5 to 1572.1 nm. Conversion efficiency of more
J. Sun et al. / Optics Communications 259 (2006) 321–327
than 24 dB is measured for 35-nm conversion span under average signal power of 10 dBm. The results provide the possibility of simultaneously realizing wavelength conversion and pulse reshaping for optical communication applications. Acknowledgment This work was supported by the Chinese Natural Science Foundation under Grant No. 60177015. References [1] S.J.B. Yoo, J. Lightwave Technol. 14 (6) (1996) 955. [2] C.Q. Xu, H. Okayama, K. Shinozaki, K. Watanabe, M. Kawahara, Appl. Phys. Lett. 63 (9) (1993) 1170. [3] M.H. Chou, I. Brener, M.M. Fejer, E.E. Chaban, S.B. Christman, IEEE Photon. Technol. Lett. 11 (6) (1999) 653.
327
[4] Junqiang Sun, Wei Liu, Jing Tian, J.R. Kurz, M.M. Fejer, IEEE Photon. Technol. Lett. 15 (12) (2003) 1743. [5] L.E. Myers, W.R. Bosenberg, IEEE J. Quantum Electron. 33 (10) (1997) 1663. [6] S.J.B. Yoo, C. Caneau, R. Bhat, M.A. Koza, A. Rajhel, N. Antoniades, Appl. Phys. Lett. 68 (19) (1996) 2609. [7] K. Gallo, G. Assanto, G. Stegeman, Appl. Phys. Lett. 71 (8) (1997) 1020. [8] B. Chen, C.Q. Xu, IEEE J. Quantum Electron. 40 (3) (2004) 256. [9] Wei Liu, Junqiang Sun, J. Kurz, Opt. Commun. 216 (1–3) (2003) 239. [10] Y. Min, J. Lee, Y. Lee, W. Grundkoetter, V. Quiring, W. Sohler, in: Tech. Digest of OFC2003, FP4, vol. 2, 2003, p. 767. [11] W. Sohler, W. Grundkoetter, J.H. Lee, Y.H. Min, V. Quiring, H. Suche, R. Schiek, T. Pertsch, F. Lederer, R. Iwanow, G.I. Stegeman, in: Tech. Digest of ECOC2004, Tu.3.4.1, 2004. [12] H. Ishizuki, T. Suhara, M. Fujimura, H. Nishihara, Opt. Quantum Electron. 33 (7–10) (2001) 953. [13] Zheng Zheng, A.M. Weiner, K.R. Parameswaran, Ming-Hsien Chou, M.M. Fejer, J. Opt. Soc. Am. B 19 (4) (2002) 839. [14] M.V. Hobden, J. Warner, Phys. Lett. 22 (3) (1966) 243.