High-speed tunable photonic crystal fiber-based femtosecond soliton source without dispersión pre-compensation

 

Martín Caldarola,¹* Víctor A. Bettachini,² Andrés A. Rieznik,² Pablo G. Kónig,² Martín E. Masip,¹ Diego F. Grosz,²'³ Andrea V. Bragas¹'⁴

*E-mail: caldarola@df.uba.ar

Laboratorio de Electrónica Cuántica, Departamento de Física, Universidad de Buenos Aires, Pabellón I, Ciudad Universitaria, (C1428EHA) Buenos Aires, Argentina.
Instituto Tecnológico de Buenos Aires, Eduardo Madero 399, (C1106ACD) Buenos Aires, Argentina.
Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.
IFIBA, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.

We present a high-speed wavelength tunable photonic crystal fiber-based source capable of generating tunable femtosecond solitons in the infrared región. Through measurements and numerical simulation, we show that both the pulsewidth and the spectral width of the output pulses remain nearly constant over the entire tuning range from 860 to 1160 nm. This remarkable behavior is observed even when pump pulses are heavily chirped (7400 fs ), which allows to avoid bulky compensation optics, or the use of another fiber, for dispersión compensation usually required by the tuning device.

 

I. Introduction

Light sources based on the propagation of solitons in optical fibers have emerged as a compact solu-tion to the need of a benchtop source of ultrashort tunable pulses [1-3]. The soliton formation from femtosecond pulses launched into an optical fiber is explained in terms of the interplay between self-phase modulation (SPM) and group-velocity dispersion (GVD) in the anomalous dispersion regime [4]. The wavelength tunability is a consequence of the Raman-induced frequency shift (RIFS) produced on the pulse when traveling through the fiber [5]. The term soliton self-frequency shift (SSFS) [6] was coined to name this effect widely used to produce tunable femtosecond pulses in different wave-length ranges, e.g., from 850 to 1050 nm [7], from 1050 to 1690 nm [8], and from 1566 to 1775 nm [1]. In most cases, photonic crystal fibers (PCF) are used for building these sources since their GVD can be easily tailored to produce solitons in a de-sired tuning range [9, 10]. For a given choice of the PCF, full experimental characterization of the pump and output pulses, complemented with theo-retical predictions, is necessary to understand how nonlinear effects modify the output soliton.

The wavelength tunability in a PCF-based light source is provided by the modulation of the pump power injected into the fiber [11-14]. It is worth noting that the wavelength choice of the output pulse is done without moving any mechanical part, which is clearly attractive for all the proposed and imaginable applications of these soliton sources. Moreover, the wavelength of the output pulse can be chosen as fast as one can modulate the power of the pump pulse, as introduced in Ref. [15, 16]. By introducing an acousto-optic modulator (AOM) in the path of the pump pulse, the output wavelength can be changed at a speed which is ulti-mately limited only by the laser repetition rate. This kind of experimental setup has been presented in some previous reports [14,17], with stunning ap-plications as the one presented in Ref. [18], where a pseudo-CW wideband source for optical coher-ent tomography is introduced. However, the need to pre-compress the pump pulse to avoid the chirp produced by the AOM contrives against the com-pact and mechanically robust design of the light source. In this paper, we demonstrate that the PCF-based source presented here is robust against chirped pump pulses. We present a complete set of measurements showing that the temporal and spectral characteristics of the generated solitons in the PCF remain unaltered even when pump pulses are heavily chirped up to 7400 fs². Results are pre-sented for the whole range of tunability (860 nm to 1160 nm). We also present numerical simulations which remarkably fit the experimental data and help to understand the soliton behavior.

This paper is organized as follows: In section II, we describe the experimental setup. The numerical simulations are described in section III. In section IV, we present experimental and numerical results and in section V we further analyze the results with numerical simulations. Finally, in section VI, we present our conclusions.

II. Experimental Setup

A scheme of the experimental setup is shown in Fig. 1. A Ti:Sa laser (KMLabs) generates ultra-short transform-limited (TL) pulses of ∆t = 31 fs (FWHM-sech²), λ_pump = 830 nm, with a spectral width ∆λ = 23 nm, and a repetition rate of 94 MHz.

The AOM not only allows high speed (up to MHz) and accurate control of the soliton wave-length, as previously discussed, but also prevents feedback into the Ti:Sa, replacing the optical isola-tor required in similar setups [19]. As the AOM introduces 56 mm of SF8 glass path, pump pulses gain a positive chirp of about 7400 fs², which leads to a time spread by a factor of 3 in them. This can be pre-compensated, for example, by introducing an optical fiber in the anomalous dispersion regime [8, 20] or a prism compressor in the well-known configuration presented in [21]. In this work, the chirp was compensated by a pair of SF18 prisms with an apex separation of 78 cm. Addition-ally, the prism compressor allowed us to up-chirp pump pulses in a controlled fashion from TL to 1400 fs² by introducing an extra glass path at the second prism of the arrangement [22]. This full or partial compensation of the phase distortion introduced by the AOM allowed us to study the role of different chirp figures in the temporal and spectral characteristics of the solitons generated in the PCF.

Figure 1: Experimental setup. (a) Titanium-Sapphire (Ti:Sa) laser, (b) Prism compressor, (c) Acousto-optic modulator (AOM), (d) Coupling lens, (e) Photonic crystal fiber (PCF), (f) Collima-tor objective, (g) Spatial filter, (h) Flipper mirror, (i) Fast-scan interferometric autocorrelator, (j) Op-tical spectrum analyzer (OSA).

 

Pump pulses were coupled into a non-polarization-maintaining microstructured fiber commercially used for supercontinuum generation (Thorlabs, NL-2.3-790-02). Its main parameters are listed in Table 1 and the dispersion curve and SEM image are shown in Fig. 2.¹ Upon propagation down the fiber, the spectrum is highly broadened so a spatial band-pass filter made of a prism and razor blades, similar to the one presented in [23], allowed to filter the spectral region of the solitonic branch (see Fig. 3) without adding any extra chirp to the solitons.

Once the spectral selection was achieved, a flip-per mirror directed the filtered beam for analysis either by the optical spectrum analyzer (OSA) or by the interferometric autocorrelator. A fast-scan system [24] allows to perform fast interferometric autocorrelations. Briefly, a platform with a hollow retroreflector is moved sinusoidally back and forth, with a stepper motor at 11 Hz, to produce and optical delay in one of the arms of a Michelson interferometer. The autocorrelation signal is recorded by a PMT and averaged with an oscilloscope.

 

 

 

III. Numerical Simulations

In order to further validate experimental results, we simulated the propagation of femtosecond pulses in the PCF by numerically solving the generalized nonlinear Schr¨odinger equation (GNLSE) including dispersive, Kerr, instantaneous and delayed Raman response, and self-steepening effects [25], with a conservation quantity error (CQE) adaptive stepsize algorithm [26].

 

where A(z,t) is the complex envelope of the electric field, /3_n are the expansión terms for the propaga-tion constant around the carrier frequency ujq and 7 is the nonlinear coefficient. /ñ(í) represents the fractional contribution of the delayed Raman effect fiR. Note that Eq. (1) adopts a more accurate de-scription of this effect than the one usually used [4]. In our simulation, we adopted t\ = 12.2 fs, t"2 = 32 fs, Tb = 96 fs, f_a = 0.75, /& = 0.21, f_c = 0.04, and fu = 0.24 [27]. The dependence of the fiber non-linear parameter 7 with the frequency was modeled as a linear function (see Table 1).

IV. RESULTS

i. Transform-limited pump pulses

First, we present the full characterization of the soliton source seeded by TL pump pulses, in an extended wavelength range if compared with the results presented in our previous paper [14].

In order to investígate the dependence of the out-put spectrum with the coupled power, managed by the AOM, we skipped spectral filtering at first. Fig. 3 shows the measured spectrum at the PCF output as a function of the coupled power. The infrared solitonic branch appears at ~ 10 mW and under-goes red-shift with increasing power. The máximum wavelength attained is 1130 nm at 55 mW. Spectra in Fig. 3 also shows show that some of the input energy is converted to visible non-solitonic radiation.

The pulsewidth of the filtered soliton as a function of its wavelength, A_s, is shown in Fig. 4. The pulsewidth remains constant at 45 fs, for the entire tunability range. Numerical simulations are also plotted in the same figure, showing an excellent agreement with experimental measurements.

 

Figure 3: Experimental spectra vs coupled power to the PCF with transforrn lirnited (TL) purnp pulses. The color rnap shows spectral intensity. The rnaxirnurn achieved soliton shift, A_s ~ 1130 nrn, was reached at 55 W.

 

ii. Chirped pump pulses

The effect over the soliton produced by the chirp of pump pulses was studied systematically by in-troducing a known amount of extra glass path on the second prism of the compressor. This scheme allowed to change the GVD of pump pulses from 0 to 1400 fs². Further chirping was achieved by the complete removal of the prism compressor, leading to a total amount of positive chirp 7400 fs².

Figure 5 (a) shows the pulsewidth of solitons with wavelength A_s = 1075 nm upon variation of pump pulses chirp. Even for a ~ 7400 fs² chirp, the soliton output pulsewidth remained around 45 fs. Numerical simulations show very good agreement with these observations, as they predict nearly constant pulsewidth regardless of the input chirp (full line in Fig. 5). Measurements and numerical simulations in the spectral domain also indicate that the band-width of the output solitons is almost unaffected by the pump pulses chirp [see Fig. 5 (b)]. The product ÁtÁis was found to be near 0.315, as it is expected for transform-limited sech pulses.

The effect of this heavy chirping was evident in the auto-correlation traces of pump pulses, as can be seen by comparing Fig. 6 (a) and (c). How-ever, there is not a clear difference between traces of the output solitons for the TL (b) and the highly chirped (~ 7400 fs²) case (d).

A color map of the spectra as function of the coupled power, for a highly chirped pump pulse (~ 7400 fs²), is shown in Fig. 7. As in the TL case, we observe that a solitonic branch is red shifted by increasing the coupled power. However, in this case, 80 mW of coupled power is required to produce a 1160 nm soliton which represents an incre-ment of about ~ 45% in comparison to the TL case.

 

V. DISCUSSION

i. Fiber soliton self-frequency shift effective length

In order to further analyze soliton formation, we studied the pulse evolution along the fiber by per-forming numerical simulations. The spectrum evolution along the fiber, for a given coupled power, in the TL and the chirped cases are shown in Fig. 10 (a) and (b), respectively. These simula-tions show that in the case of chirped pump pulses (7400 fs²), the spectrum broadening and the soli-ton formation take place farther down into the fiber (see Fig. 10), as compared to the TL case.

The delay in the formation of the soliton can be explained by an interplay of opposite chirping effects: the positive chirp acquired by traversing the AOM is compensated as the pulse advances into the PCF, in anomalous propagation, leading to pulse compression. The PCF itself provides pulse com-pression in the first stretch of the fiber previously to the branching of a soliton. Therefore, the SSFS effective length, i.e., the fiber path where nonlinearity broadens the spectrum, is longer in the TL case. If the chirp is overcompensated and a nega-tively chirped pulse is fed into the fiber, these pulses would also be compressed within the first stretch of the fiber due to SPM [28] leading to the same be-havior than in the positively chirped case, resulting in a narrower tunability range.

Once the soliton is formed and the peak power is high enough, intrapulse Raman scattering red-shifts the soliton as it propagates through the re-maining of the fiber. This spectral shift increases with both fiber length and soliton peak power [4]. So the fact that the soliton is formed at different lengths explains the red shifts observed for the same coupled power.

However, as a larger wavelength shift can be achieved with a higher input power, this shorten-ing in the effective length in the chirped case could be compensated by coupling more power into the PCF [1]. Another possibility for compensating this effect on the SSFS is using a longer PCF.

ii. Fiber power conversión efflciency

Figure 11 shows a simulation where the same shift wavelength obtained for TL pump pulses is achieved by increasing the coupled power in the shorter effective length fiber (7400 fs² chirp). Fis-sion of more than one soliton branch is visible in this case, as compared to the case of TL pump pulses, for which only a single soliton branch appears (Fig. 10). Each soliton branch carries a fundamental soliton (N = 1) with a peak power Po given by [4] of the solitons is also the same.

 

The arising of new soliton branches partially ac-counts for the increased pump power required in the chirped case (Fig. 11) to attain the same shift. Indeed, the soliton-pump power ratio is 0.2 in the chirped case and 0.44 in the TL case. This result reveals that the use of the PCF as a compressor decreases its power conversion efficiency.

On the other hand, it is possible to achieve the same soliton shift as in the TL case by increasing the fiber length, and keeping the same pump power. In this case, the power conversion efficiency is even lower, 0.17, as predicted by simulations.

VI. Conclusions

We have presented a high-speed tunable soliton infrared source capable of generating 45 fs transform-limited pulses in the range from 860 to 1160 nm. Both the pulsewidth and the spectral width were shown to remain constant over the en-tire tuning range, even when pump pulses were heavily chirped up to 7400 fs². Insensitivity to the chirp of pump pulses points out to the feasi-bility of avoiding bulky compensation optics prior to the PCF, opening up the possibility to build reli-able and compact high-speed tunable femtosecond sources in the near infrared region. A minor draw-back of this source is that either more power needs to be coupled or a longer PCF needs to be used in order to achieve the same tuning range obtained with transform-limited pump pulses.

 

Acknowledgements - This work was supported by ANPCyT PICT 2006-1594, ANPCyT PICT 2006-497 and UBA Programación Científica 2008-2010, Proyecto N X022.

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