2009 advanced optical limiting function

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ICTON 2009 Th.A2.1

978-1-4244-4826-5/09/$25.00 ©2009 IEEE 1

Advanced Optical Limiting Function Based on

Effective Understanding of Physical Phenomena

Tsuyoshi Konishi†‡, Member, IEEE, Hiroomi Goto†, Takuya Kato†, and Kentaro Kawanishi†

†Osaka University, 2-1 Yamadaoka, Suita Osaka 565-0871, Japan

‡ Laboratoire Charles Fabry de l’Institut d’Optique, Campus Polytechnique R.D. 128, 91127 Palaiseau CEDEX

ABSTRACT

Attention will be paid to cooperative use of linear and nonlinear effects in optical signal processing. This talkreports opportunities and significances of understanding of physical phenomena in order to make furtheradvances in cooperative use of linear and nonlinear effects. In this paper, we focus on optical limiting functionusing self-phase modulation phenomenon and examine carefully its behaviour assisted by time-resolvedspectroscopic technology. Based on the results examined, we successfully achieved high-stable optical limitingfunction. This optical limiting function achieved wavelength conversion based on self-frequency shift

phenomena with preventing influences of input intensity fluctuation.Keywords: optical limiting, nonlinear analysis, self phase modulation, self-frequency shift, optical filtering,

time-resolved spectroscopy.

1.

INTRODUCTION Nonlinear optical signal processing has attracted considerable attention because of its transparency, diversity,and so on. With the popularity of nonlinear optical signal processing, demand for intensity stability of optical

pulses is increasing. Up to now, a lot of techniques for optical intensity stabilization including optical limiters,optical reshaping techniques, and optical regenerators have been reported [1]-[7]. We have also proposed anddemonstrated an ultra-fast all-optical intensity limiting function based on intensity-dependent spectral patternchange by self-phase modulation (SPM) and optical pattern recognition technique [8], [9]. Among thesetechniques, the SPM based ones [1], [2], [5], [8], [9] are promising approaches because of its simplecomposition, stability, and fast operation over 100 Gb/s. Such techniques have already well-studied and

achieved excellent limiting function. Nevertheless, they have a common limitation in the dynamic range (∼3 dBfor output variance of 0.5 dB) and the accuracy of the stability would not be sufficient enough to provide thestabilized optical pulses for the following nonlinear optical signal processing. To relay such stabilized optical

pulses to the following nonlinear optical signal processing, much more accurate optical limiting function is

required.Most of methods based on SPM achieve limiting function by extracting the center frequency part of optical

pulses after SPM. After the input peak intensity reaches a certain threshold, the energy around the centerfrequency is spread out to its both frequency sides. As a result, the intensity around the center frequency canideally keep balance between input and spread out energies. However, the actual behaviour of SPM is not idealand the amount of spread out energy becomes larger or smaller than that of input one after a certain threshold

point and intensity of optical pulses extracted starts varying. While SPM is indeed a well-known phenomenon,the behaviour of SPM is still very complicated and is not easy to freely manage the behaviour of SPM. Torealize advanced optical limiting function based on SPM, it is necessary to effectively examine the mechanism ofSPM in line with the purpose. In this paper, we focus on optical limiting function using SPM phenomena andexamine carefully its behaviour assisted by time-resolved spectroscopic technology.

Figure 1. Schematic diagram of relay to the following nonlinear optical signal processing.

2. SPM-BASED OPTICAL LIMITER

To realize optical limiting function for high repetitive optical pulses, discrimination of intensities of optical pulses without latency is necessary and ultrafast light-driven effects are very attractive like nonlinear effectsduring propagation in a fibre because of its ultrafast response. Among them, especially, SPM is a very attractivenonlinear effect because it can be induced by relatively low peak power of an optical pulse without any optional

equipment. Figure 2 shows a schematic diagram of our proposed all optical limiter. [10], [11]



ICTON 2009 Th.A2.1

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Figure 2. Schematic diagram of our proposed all optical limiter.

Our proposed optical limiter is composed of three parts. They are a pulse-shaping part, intensity-dependentspectral pattern change part, and a spectral filtering part. In the spectral pattern change part, the spectral patternof optical signal is changed depending on its intensity by SPM in a fiber. In the pulse shaping part, an input

pulse waveform is shaped so as to induce appropriate spectral change in a HNLF for the following spectralfiltering part. In the spectral filtering part, various spectral patterns, which are generated from various signalintensities, are filtered with a designed filter and approximately equal-intensity signals are output.

Thus, optical limiting function can be realized by using the intensity-dependent spectral change of SPM. Thisspectral change seems to be too complicated to understand it because it depends on the input waveform of anoptical pulse as wells as an amount of temporal intensity change, the nonlinear and effective propagation lengths.

SPM gives rise to a phase shift due to an induced nonlinear refractive index change. The phase shift amount isdescribed as [12]

( ) ( )2

( , ) 0,eff NL

z T U T z Lφ = , (1)

where z is the propagation length, T is the relative time at the so-called retarded frame, U (0,T ) is the fieldamplitude at z = 0, and L NL is the nonlinear length, z eff is the effective propagation length, respectively. Since the

phase shift amount φ ( z ,T ) is proportional to a temporal intensity of optical pulse |U (0,T )|2, the phase shiftamounts are temporally varying in an optical pulse. As the results, new frequency components are generatedcontinuously and the spectrum of an optical pulse is modulated depending on its intensity. Figure 3 representsa carrier frequency changing behaviour due to a nonlinear phase shift induced by SPM.

Figure 3. Carrier frequency in a pulse (a) before modulation and (b) after modulation by SPM. The arrowsrepresent a travelling direction of the pulses.

Because the phase shift amount is largest at the highest intensity position in an optical pulse, the wavelength atthe leading edge shifts to longer wavelength and the wavelength at the trailing edge shifts to shorter wavelength.

The carrier frequency change δω caused by SPM is described as

( )2(0, )

( )eff

NL

U T z T

T T L

φ δω

∂∂= − = −

∂ ∂. (2)

From equation (2), δω (T) is essentially corresponding to time-frequency distribution and that implies we caneffectively understand it as time-varying spectral behaviour of SPM in order to freely manage it in line with the

purpose of optical limiting. Therefore, we carefully examine its behaviour assisted by time-resolvedspectroscopic technique.

3. EFFECTIVE UNDERSTANDING OF SPM FOR OPTICAL LIMITING

In order to avoid the unwanted spectral change around the center frequency by SPM , we examined the behaviour by observing a series of spectrogram after SPM of output pulses with different input intensities. Toobserve a series of spectrogram, we adopted optical spectrogram scope (OSS) as a measurement tool.[13], [14], [15] The spectrogram is a kind of time-frequency distribution and it produces a visual representationof a time dependence of spectrum in an optical pulse. In this experiment, we used a light source whose pulse-

width is 0.7 ps and an input pulse is generated through 1 nm band-pass filter at the 1559 nm center wavelength.The fiber parameters used in the experiment are following; length L = 92 m, dispersion D = -0.0 ps/km/nm,

(a) (b)



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Figure 4. A series of spectrograms observed by using optical spectrogram scope (OSS) after propagation of HNLF.

Figure 5. Experimental setup of optical limiter designed based on effective understanding.

dispersion slope S = 0.0026 ps/km/nm2, and nonlinearity γ = 16 /W/km. Figure 4 shows a series of spectrogramsand corresponding spectral profiles observed by changing the intensity of an input pulse. We can clearly tracethe behaviour of time-varying spectral change after SPM.

From Figs. 4 (a)-(c), the energy around the center frequency is spread out to its both frequency sides after the

input peak intensity reaches a certain threshold. The amount of spread out energy becomes larger than that ofinput one after a certain threshold point and intensity of optical pulses extracted starts decreasing. This would bea main reason why the dynamic range and the accuracy of the stability are limited in conventional opticallimiters. Here, our purpose is to figure out a solution which can avoid the unwanted energy decreasing aroundthe center frequency by SPM.

In this intensity region, the spectral distribution is finally split into three parts after SPM. Intriguingly, fromFigs. 4 (d)-(e), we can see the center wavelength component is re-increased. During the process, the centerwavelength part interacts with both the tail part of the longer wavelength one the head of the shorter wavelengthone. This phenomenon would derive from the up-chirp profile of an optical pulse at Fig. 4(c). It suggests that wecould avoid the unwanted energy decreasing and design an ideal optical limiter function by giving an adequate

pre-chirping profile to an input pulse.

4. CONTROL OF SPM BASED ON ITS EFFECTIVE UNDERSTANDING FOR OPTICAL LIMITING

To confirm the effect designed by the proposed pre-chirping method, we experimentally verified it and relayedthe pulse obtained to the following wavelength conversion processing based on Soliton self frequency shift(SSFS) [16] . Figure 5 shows the experimental setup.

In this experiment, we used a light source whose pulse-width and center wavelength are 0.7 ps and 1559 nm,respectively. To introduce a pre-chirping profile to an input pulse, we used a dispersion compensation fiber(DCF) which has the following fiber parameters; L = 4.22 m, dispersion D = -106 ps/nm/km. The fiber

parameters of a nonlinear fiber for SPM are following; length L = 92 m, dispersion D = 0.0 ps/nm/km, dispersionslope S = 0.0026 ps/nm2/km, nonlinearity γ = 16 /W/km, and loss α = 1.3 dB/km. The fiber parameters ofa nonlinear fiber for SSFS are following; length L = 1000 m, dispersion D = 7.0 ps/nm/km, dispersion slopeS = 0.02 ps/nm2/km, nonlinearity γ = 13 /W/km, and loss α = 1.5 dB/km. By combination of 2 bandpass filters,we extracted spectral component around 1559 nm whose bandwidth is 1.4 nm. Figure 6 shows the experimentalresults. From Figs. 6, the amount of wavelength conversion based on SSFS keeps the almost same value withthe proposed optical limiter. It confirm that the accuracy of the stability could be sufficient enough to providethe stabilized optical pulses for the following nonlinear optical signal processing.



ICTON 2009 Th.A2.1

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(a) (b)

Figure 6. Experimental results of SSFS for input pulses with various intensities (5.7 – 7.5 µ W):

(a) without, and (b) with the proposed optical limiter.

5. CONCLUSIONS

To demonstrate effectiveness of carefully understanding for advanced applications of optical nonlinear phenomena, we focused on optical limiting function using self-phase modulation phenomenon and examinedcarefully its behaviour assisted by time-resolved spectroscopic technique. Based on the results examined, we

have successfully relayed the pulse obtained to the following wavelength conversion based on self-frequencyshift phenomena with preventing influences of input intensity fluctuation.

REFERENCES

[1] P. V. Mamyshev: All-optical data regeneration based on self-phase modulation effect, in Proc. ECOC1998, Madrid, Spain, Sep. 1998, pp. 475-476.

[2] M. Asobe et al.: Noise reduction of 20 Gbit/s pulse train using spectrally filtered optical solitons, Electron.

Lett., vol. 34, no. 11, pp. 1135-1136, 1993.[3] E. Ciaramella and S. Trillo: All-optical signal reshaping via four-wave mixing in optical fibers, IEEE

Photon. Technol. Lett., vol. 12, no. 7, pp. 849-851, 2000.[4] Y. Ueno et al.: Penalty-free error-free all-optical data pulse regeneration at 84 Gb/s by using a symmetric-

Mach-Zehnder-type semiconductor regenerator, IEEE Photon. Technol. Lett., vol. 13, no. 5, pp. 469-471,2001.

[5]

M. Matsumoto and O. Leclerc: Analysis of 2R optical regenerator utilising self-phase modulation in highlynonlinear fibre, Electron. Lett., vol. 38, no. 12, pp. 576-577, 2002.

[6] K. Nishimura et al.: All-optical regeneration by electro-absorption modulator, IEICE Trans. Electron., vol.E88-C, no. 3, pp. 319-326, 2005.

[7] G. Contestabile et al.: A simple and low-power optical limiter for multi-GHz pulse trains, Opt. Express, vol. 15, no. 15, pp. 9849-9858, 2007.

[8] H. Goto et al.: Amplitude-phase modulation technique using phase-only filter for optical intensity equalizer,in Proc. ODF2008, Taipei, Jun 2008, Paper 10S3-06.

[9] H. Goto, T. Konishi, and K. Itoh: Ultrafast all-optical intensity stabilizer based on self phase modulation-induced spectral pattern change and optical pattern recognition, Jpn. J. Appl. Phys. 47 8834-8837 (2008).

[10] H. Goto, T. Konishi, R. Itoh, T. Enoki, T. Nishitani, and K. Itoh: Expansion of dynamic range of all-opticalintensity equalizer based on effective self-phase modulation by using pre-chirped signals, in Proc. PS 2008,D-03-4, Sapporo, Japan, 2008.

[11]

H. Goto, T. Konishi, and K. Itoh: An all-optical limiter with high-accuracy thresholding based on self- phase modulation assisted by preparatory waveform conversion, J. Opt. A: Pure and Appl. Opt . 10,095306, (2008).

[12] G.P. Agrawal: Nonlinear Fiber Optics 4th Ed., Academic Press, San Diego: Springer-Verlag, 1980.[13] T. Konishi and Y. Ishioka: Optical spectrogram scope using time-to-two-dimensional space conversion and

interferometric time-of-flight cross correlation, Opt. Rev. 6 507-512 (1999).[14] K. Tanimura, T. Konishi, K. Itoh, Y. Ichioka: Amplitude and phase retrieval of ultrashort optical pulse

using optical spectrogram scope, Opt. Rev., 10, 2, pp. 77-81 (2003).[15] T. Konishi, and H. Goto: Response characteristics test of ultra-fast optical devices with optical spectrogram

scope (OSS), in Proc. ICTON 2008, Tu.A 1.2, Athens (2008).[16] J. P. Gordon: Theory of the soliton self-frequency shift, Opt. Lett ., 11, 10, pp. 662-664 (1986).

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2009 advanced optical limiting function

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