Experimental investigation of the Raman induced self-frequency shift of ultrashort Stokes pulses in optical fibres

1 April 1994

OPTICS COMMUNICATIONS Optics Communications 107 (1994) 170-178

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Full length article

Experimental investigation of the Raman induced self-frequency shift of ultrashort Stokes pulses in optical fibres J. Schlitz,W. Hodel, H.P. Weber Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

Received 24 March 1993; revised manuscript received 28 August 1993

Abstract

The properties of Raman generated Stokes bands after propagation through approximately 1 km of standard communication fibre have been investigated experimentally in terms of the input pulse parameters for pico- and subpico-second pulses near 1300 nm wavelength. It has been found that the frequency shift of these Stokes bands is primarily determined by the average input power and shows only a weak dependence on the temporal width (on a picosecond time scale) and on the centre wavelength of the input pulses. On the other hand, the power transferred to the Stokes band depends on both, the average power and the duration of the input pulses. 1. Introduction

The frequency shift of ultra-short pulses induced by Raman self-pumping during propagation in an optical fibre is detrimental for communication applications [ 1-3 ] but bears on the other hand considerable potential for generating wavelength-tunable (sub-)picosecond pulses [4,5]. Gordon [6] developed a theory describing the frequency shift of fundamental solitons known as the "soliton self-frequency shift" (SSFS). However, it is obvious that the initial condition of an isolated soliton, required for Gordon's theory to be valid, is difficult to meet in the experiment. In most experiments the soliton first has to form from initial conditions that do not correspond to those of a fundamental soliton. For examplc, when ultra.~hort pulses with a peak power considerably higher than that required for a fundamental soliton are launched, R a m a n self-pumping will lead to the generation of several Stokes pulses which experience different frequency shifts. During propagation along the fibre these Stokes pulses will eventu-

ally evolve into fundamental solitons with different centre wavelengths [4,7,8]. The formation of these solitons is a very complex process which involves interactions of the individual solitons with each other and with dispersive radiation. This makes it impossible to use Gordon's theory to determine the resulting frequency shift directly from the input pulse parameters. To the best of our knowledge no attempt has been made up to now to analyse systematically the properties of Raman-generated Stokes pulses in terms of the experimental input pulse and fibre parameters. This is the aim of the presented work where we have investigated in particular the dependence of the frequency shift of the generated Stokes pulses on the average power, the pulse width and the centre wavelength of the input pulses for a given fibre length. The results show that for long (approximately 1 kin) communication fibres the frequency shift increases monotonically with the average input power but shows practicallyno dependence on the input pulse width which was varied in the range from 0.4 to 4 ps. Further, itwas found that near the zero-group-vcloc-

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z Schiitz et al. / Optics Communications 107 (1994) 170-178

ity dispersion (zero-GVD) wavelength of the fibre (near 1300 nm) the frequency shift depends only weakly on the centre wavelength of the input pulses (and consequently on the fibre dispersion). We have also studied how the transfer of power to the Stokes pulses depends on the input pulse parameters. Our results indicate that for any given power level the power transfer is most efficient for one specific input pulse duration (and vice versa).

2. Experimental setup

The experimental setup is shown in Fig. 1. A synchronously pumped dye laser, which has been described in more detail in Ref. [ 9 ], was used as the ultrashort pulse source. The centre wavelength of the dye laser pulses can be tuned between 1250 and 1350 nm. Using two different Lyot filters and a pellicle of 7 ~tm thickness, pulses with autocorrelation widths between 0.4 and 4 ps fwhm #t at a repetition rate J'r~p of 82 MHz can be generated. Since the determination of the temporal width of a pulse from the measured autocorrelation crucially depends on the knowledge of the pulse shape, the subsequent values of the pulse duration always refer to the fwhm of the m e a s u r e d autocorrelation traces. T h e input radiation was coupled into the 0.8 m long fibre pigtail by using a selfoc lens. In order to achieve optimized coupling, an additional weak lens was inserted between the laser source and the selfoc lens. In this way approximately 80% of the input power was typically coupled into the pigtail. The coupled radiation was transferred from the pigtail to the fibre under test by optical connec#n Fullwidth at half maximum.

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tors (of PC-standard type) which made it possible to perform measurements on different fibres without major changes in the setup (the additional loss of a PC-PC connection was approximately 10%). The main specifications of the standard communication fibres used in the experiments are listed in Table 1. At the output of the fibre the radiation was collected by an antireflection coated microscope objective. The orientation of the signal polarization was adjusted by a halfwave plate for maximum signal in the analytical devices. The optical spectrum was measured using a 0.5 m Czerny-Turner grating monochromator. In order to reduce the influence of strong absorptions between 1340 and 1420 nm from the humidity in the air the optical path between the fibre end and the detector after the monochromator was flooded with gaseous nitrogen. The background-free autocorrelation function was obtained from the second harmonic signal of a LilO3 crystal. The conversion efficiency of this crystal drops to zero for wavelengths separated by more than + 20 nm from the optimum phasematching wavelength which was generally set to the centre wavelength of the input pulse.

3. Experimental results

In the following we will describe in detail how the properties of the generated Stokes pulses depend on the average input power Pays, the input pulse width (characterized by the fwhm of the autocorrelation) and the input pulse centre wavelength 20 (or the dispersion parameter D at 20). Fig. 2 shows the dependence of the output spectrum on the average input power Pavgfor input pulses of 0.5 ps (left) and 3.9 ps (fight) duration after propagation through fibre # 1.

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J. Schiitz et al. / Optics Communications 107 (1994) 170-178

Table 1 Experimentally determined fibre parameters.

tion of a considerably broadened spectrum with a modulation, which is characteristic for this process. However, with increasing input power the overall spectral width increases and R a m a n self-pumping becomes more effective. In fact, the spectra generated for input power levels above approximately 6 m W show a qualitatively similar dependence on the input power for both pulse durations. In both cases a Stokes band isgenerated which is distinctlyred shifted and separated from the rest of the spectrum. An important point to note is that the frequency shiftof the Stokes band is practically the same although the respective input pulse durations differ by almost one order of magnitude. This is further illustratedby the results for 500 fs and 2.6 ps input pulses at somewhat higher power levels shown in Fig. 3 and by Fig. 4 which shows spectra generated for different input pulse durations at comparable (average) power lev-

Fibre #1 Fibre #2 Length L [m] 1183 Zero-GVD wavelength [nm ] 1303 Attenuation (around 1320 nm) [dB/krn] 0.38 Spot size (1300 nm) [ttm] = 9.5

1000 1317 0.47 10.2

=) Diameter measured at 1/e 2 of the intensity. F r o m this figure it is e v i d e n t that different mechanisms are responsible for the generated spectra at low power levels. The initial spectral w i d t h o f the 500 fs pulses is so large that practically the entire spectrum is shifted to the Stokes side m a i n l y by R a m a n selfpumping. O n the other hand, the spectral evolution o f the 3.9 ps pulses is d o m i n a t e d at low p o w e r levels b y self phase m o d u l a t i o n which leads to the formaI

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Fig. 3. Dependence of the spectrum on the average input power Paw after propagation of 0.5 ps (left) and 2.65 ps (right) input pulses through fibre #1 (for higher average power levels than in Fig. 2). The centre wavelength of the input pulses is 1307 and 1310 nm, respectively. The zero-GVD wavelength of the fibre is 1303 nm.

els. Note also, that all the spectra in Fig. 4 look remarkably similar. In particular, the generated Stokes band is clearly separated from the rest of the spectrum in each case and is located in the vicinity of 1380 nm. The exact values of the frequency shift of the Stokes bands indeed agree within 10% (A~,= 11.5 _+1.0 THz at 16 mW average pulse power). We have determined the values of the Stokes frequency shift (with respect to the input wavelength 20 of the pulses) graphically from the measurements shown in Figs. 2-4 (but only for those power levels for which the Stokes band is clearly separated). The centre wavelengths of Stokes pulses were hereby taken at the maximum values of their spectral envelope. The resulting frequency shifts are summarized in Fig. 5 as a function of the average input power. The figure clearly confirms that the frequency shift in long fibres is primarily determined by the average input power and shows no significant dependence on the input pulse duration. We have also studied the dependence of the frequency shift on the input wavelength 28 (or equiva-

lently on the dispersion parameter D at 20). Fig. 6 shows the measured output spectra of 3.5 ps pulses with different input wavelengths (marked in the spectra by triangles) but comparable average input powers P=vgfor the two different fibres (see Table 1 ). Fig. 6 demonstrates that in the anomalous group-velocity dispersion (GVD) regime and for D smaller than 2 ps/nm km the resulting frequency shifts are practically identical weand thus depend only weakly on the input wavelength 2o (or D at 2o). It is especially interesting to note that Stokes bands are also generated for input pulses with centre wavelengths in the range of normal GVD. Furthermore, the resulting shifts are comparable to those obtained for input wavelengths in the range of anomalous GVD. Only in the bottom most spectrum on the left hand side of Fig. 6 (for D= - 1.86 ps/nm kin) no Stokes band is generated. This suggeststhat for any given power level there exists a lower limit for the input wavelength in the normal GVD regime (or a lower limit for D, re#2 The exact values differ by less than 20%.

J. Schiitz et al. / Optics Communications 107 (1994) 170-178

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Fig. 4. Dependenceof the spectrum on the duration of the input pulses after propagationthroughfibre #1 (right). The respective input spectra (autocorrelations) are shown in the centre (on the left hand side) of the figure. The values for the averageinput powerP.,,~, the fwhm of the input spectrum (A2 in nm) and the fwhm of the autocorrelation (At in ps) are indicated in each case. spectively) below which the formation of a Stokes band in the wavelength range of anomalous GVD is inhibited. Finally, we have investigated how the power transfer into the Stokes band depends on the average input power and on the input pulse duration. The experimental results for 0.5, 2.65 and 3.9 ps pulses are summarized in Fig. 7. In Fig. 7a the ratio of the power in the Stokes band to the total input power is displayed (on a log scale) as a function of the average input power. This ratio has been evaluated from the measured spectra by numerical integration. Note that the figure contains only those results for which the Stokes band is clearly separated from the remaining part of the spectrum. For this reason no points are shown for small input power levels. For Pavg> 2 mW the fraction of power contained in the generated

Stokes bands is well approximated by an exponentially decreasing function of the average input power. As a consequence, the absolute values for the power transferred to the Stokes band - which is shown for each case in Fig. 7b - first increases with increasing input power, reaches a maximum and tends to decrease for even higher power levels. Therefore, for each pulse duration there exists an optimum value for the average input power which maximises the power transfer to the Stokes band. On the other hand, Fig. 7b also shows that for any input power level, the conversion efficiency is always largest for the intermediate pulse duration of 2.65 ps. This behaviour suggests that for a given input power there also exists an optimum value for the pulse duration which maximizes the power transfer to the Stokes band. This point becomes more obvious by inspection of Fig. 8

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which shows the power transferred to the Stokes band as a function of the input pulse duration for an input power level of 16 mW. Clearly, for this specific average input power the conversion efficiency reaches a maximum for an input pulse duration of approximately 2 ps.

4. Discussion

As described above, our experimental results lead to the main conclusion that the frequency shift of the Stokes bands, which are generated (in the anomalous GVD regime) when input pulses of about 1 ps duration and with centre wavelengths in the vicinity of the zero-GVD wavelength are launched into optical fibres, is primarily determined by the average input power and not by the peak power of the input pulses. This finding is somewhat counter-intuitive for the following reasons. If the input pulse and fibre parameters are known, any input pulse can be characterized by its soliton number N which depends on the peak power and on the duration of the input pulse. Under ideal conditions such a pulse would evolve into a soliton of order No (where No is the nearest integer to N) and a dispersive rest-wave [ 1 ]. However, pertur-

175

bations such as third order dispersion and the Raman effect lead to a decay of the input pulse into a number of individual ultra-short pulses which will eventually develop into fundamental solitons [1,4,7,8 ]. The temporal widths of these individual pulses are determined by the value of N and the duration of the input pulse [ 7 ]. Since the differential frequency shift of an ultra-short pulse strongly depends on the pulse duration (for a fundamental soliton it is inversely proportional to the fourth power of the soliton pulse duration [ 6 ] ), one would expect that the frequency shift, which these individual pulses will experience during their further propagation along the fibre, should also strongly depend on the peak power and the duration of the input pulse. Clearly, this is in contrast to our experimental results. This discrepancy may be due to the fact that in the above arguments the Raman effect is implicitly treated as a perturbation which - for picosecond input pulses with high N - may not be justified #3. In fact, the pulse evolution in the early stage (i.e. in the first few meters of fibre) is rather complicated. The generated Stokes pulses can be as short as 50 fs after initial compression [4 ] and are further characterized by soliton numbers N varying typically between 0.6 and 1.3. Moreover, their formation involves interactions between individual pulses and dispersive waves which are at least partially overlapping in time but may have different centre frequencies (like the generated Stokes and anti-Stokes bands in Figs. 2-4, see also Ref. [4] ). If such interactions occur, the subsequent evolution of the generated Stokes pulses will differ from freely propagating solitons. We think that the most reliable way to find an explanation for the experimental findings is to perform numerical simulations. We have made calculations based on an extended version of the nonlinear SchrSdinger equation which takes into account second and third order dispersion and the Raman effect in the response function approach. The normalized Raman response function has been approximated by a single-sided exponential with a decay time of 76 fs (see Ref. [ 10 ] for further details on the model used). First results show that the calcu#3 In our experiments the input pulse duration is in the order of 1 ps and the soliton number N of the input pulse is in the range of N= 5-20. Note, that these high values of N are also due to the very low dispersion at the input wavelength ( D < 3 p s / n m km).

J. Schiitz et al. / Optics Communications 107 (1994) 170-178

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lated and the measured spectra are in good qualitative and quantitative agreement for low input power levels (a few milliwatts) and/or short fibre lengths (tens of meters). For higher input power levels (tens ofmiUiwatts) and long fibres (in the order of 1 km), however, there are significant differences between the numerical and the experimental results. This suggests that additional theoretical efforts are necessary in order to give a physical explanation for our experimental results. Nevertheless, we have chosen to present the experimental results because we believe

that they are interesting in themselves and may stimulate further activities in this field.

5. Conclusion Raman self-pumping is a useful process for generating wavelength tunable ultra-.short pulses. However, it is difficult to predict theoretically the dependence of the resulting frequency shift and the energy content of the generated Stokes pulse on the input pulse and fibre parameters. Therefore we chose to in-

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vestigate this problem in detail experimentally. The most important result is that for long fibres (approximately 1 kin) the frequency shift is primarily determined by the average input power but depends only weakly on the width (on a picosecond time scale) and the centre wavelength of the input pulse. These results have been obtained for input pulse widths between 0.4 and 4 ps and for centre wavelengths of the input pulses ranging from 1283 to 1340 nm (which corresponds to a variation of the dispersion paramet e r D in the range from - 1.86 to +2.55 ps/nm kin). The power transferred to the Stokes band, on the other hand, depends on the input pulse duration and on the average power of the input pulses: For any average power level there exists a specific pulse duration for which the power transfer to the Stokes band is maximized (and vice versa). For an average power of 16 mW the optimum input pulse duration is about 2 ps.

Acknowledgements This work was financially supported by the Swiss PTT. Especially we would like to thank G. Bodmer and B. Bolz for the preparation of the fibres. We also

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3. Schiitz et al. / Optics Communications 107 (1994) 170-178

t h a n k Dr. H.H. Gilgen, Dr. G. O n i s h c h u k o v a n d H. A m m a n n for v a l u a b l e r e m a r k s a n d s t i m u l a t i n g discussions.

References [ 1 ] G.P. Agrawal, Nonlinear Fiber Optics (Academic Press, New York, 1989). [2] D. Wood, J. Lightwave Technol. 8 (1990) 1097. [3] D. Cotter, Opt. Quantum Electron. 19 (1987) 1.

[4] P. Beaud, W. Hodel, B. Zyssct and H.P. Weber, IEEE J. Quantum Electron. QE-23 (1987) 1938. [ 5 ] A.S. Gouveia-Neto, A.S.L. Gomes and J.R. Taylor, IEEE J. Quantum Electron. QE-24 (1988) 332. [6] J.P. Gordon, Optics Lett. 11 (1986) 662. [ 7 ] J.K. Lucek and ILL Blow, Phys. Rev. A 45 (1992) 6666. [8] W. Hodel and H.P. Weber, Optics Lett. 12 (1987) 924. [9] P. Beaud, B. Zysset, A.P. Schwarzenbach and H.P. Weber, Optics Lett. 11 (1986) 24; P. Beaud, B. Zysset and H.P. Weber, Proc. ECOOSA '86, SPIE 701 (1986) 466. [10] J. Schiitz, W. Hodel and H.P. Weber, Optics Comm. 95 (1993) 357.