ОПТИКА И СПЕКТРОСКОПИЯ, 2011, том 111, № 4, с. 628-633
КОГЕРЕНТНЫЕ ЭФФЕКТЫ В ОПТИЧЕСКИХ И АТОМНО-МОЛЕКУЛЯРНЫХ ПРОЦЕССАХ
УДК 535.14
FEMTOSECOND SUPERCONTINUUM CHARACTERISTICS CONTROL
© 2011 N. M. Kachalova*, V. S. Voitsekhovich*, A. M. Borodin*, V. V. Khomenko*,
and S. Yu. Pentegov**
*Institute of Physics, Nat. Acad. Sci. of Ukraine, Kyiv, 03028 Ukraine **LTD Lasertrack, Moscow, 142784Russia kachalova.nataliya@gmail.com; val555@mail.ru; mihajlovi4@googlemail.com; khomenko.vadim@igmail:l.com;
serpl@smtp.ru Received April 18, 2011
Abstract—Supercontinuum generation in the spectral range 530—1100 nm in series of photonic-crystal fibers pumped by femtosecond Ti:Sapphire laser Mira Oprima 900-F is achieved. The evolution of spectral characteristics of femtosecond supercontinuum is proofed to be dependent on pump pulse wavelength and power radiation. Polarization characteristics of supercontinuum spectral components are analyzed. We demonstrated experimentally the possibility of control of femtosecond supercontinuum generation.
1. INTRODUCTION
Supercontinuum (SC) generation is a spectrum broadening after propagation of high energy pulses in a nonlinear media which first was observed by Alfano and Shapiro in 1970 [1, 2]. The term "supercontinuum" is definitely not associated with some effect, because it is a combination of various nonlinear phenomena, which produces continuous spectrum broadening of input pulse after mutual interaction of phenomena, with the properties of laser beam. In other words SC is a coherent white light.
SC generation can be observed in drop of water after pumping with the appropriate input pulse power, but nonlinear effects participating in the formation of spectral broadening strongly depend on medium dispersion. In such a case development of materials with high nonlinear properties can reduce the requirements for input power. The broadest spectrum is observed in case of pumping pulse wavelength is close to zero dispersion wavelength of the nonlinear media.
Input of femtosecond laser radiation into nonlinear photonic-crystal fiber (PCF) with wavelength close to
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Fig. 1. Photos of the cross sections PCFs (a) Thorlabs NL-2, 4-800 and (b) H 0710151_25 obtained by a provided by polarizing microscope Olimpus BX-51 (zoom factor is 400).
zero dispersion wavelength of the fiber leads to the generation of femtosecond SC [3]. The first report of SC generation in PCFs was made by Ranka and others [4] in 1999. The fundamental physical processes, which underlie of SC generation in PCF, consist of stimulated Raman scattering [5], four wave mixing [6] and self-phase modulation [7], solitons formation [8] and others.
The need for broadband coherent radiation — SC — occurs when solving various problems: nanotechnolo-gy, metrology, biomedicine, spectroscopy, nearfield microscopy, etc. However, other applied problems pose different requirements for characteristics of SC. For example, for optical frequency metrology required SC with wide range of not less than one octave, which is characterized by high stability of amplitude and phase. Narrow and stable frequency range of intensive SC radiation is enough for realization of a femtosecond CARS spectroscopy. One approach to create universal SC generator is to develop methods of dynamic change of parameters.
Within studies we have achieved SC generation in microstructure fibers with input radiation from femtosecond Ti:Sapphire laser Mira Optima 900-F and investigated its properties.
2. PHOTONIC-CRYSTAL FIBER
To achieve SC generation, we used PCFs. Such a fiber belongs to a class of microstructured optical fibers with a solid quartz core surround by a periodic pattern of cylindrical quartz tubes fulfilled with air.
If in conventional optical fibers total internal reflection provided at a time when the refractive index of fiber cladding is less than the refractive index of core, then in microstructured optical fibers waveguide modes are formed by reflection and scattering on the
Fig. 2. Scheme of a supercontinuum generation: femtosecond TilSapphire laser 900 Mira Optima-F, FI — Faraday optical isolator, M — mirror, PCF — photonic-crystal fiber, F — filter, Co — collimating lenses, PC — personal computer, Spectormeter — Ocean Optics Usb 4000.
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Fig. 3. Multiplexing frequency transformation in microstructure fiber mark H 0710151_25.
microinhomogeneities of refractive index. For a wide class of microstructured optical fibers condition of existence ofwaveguide modes in fiber core, which can be regarded as a defect in microstructure, may be written in the form, similar to existence of the complete internal reflection in conventional fiber: nclad < ncore. Along with the conventional waveguide modes, which are provided with complete internal reflection, in micro-structured optical fibers there are waveguide modes of electromagnetic field, which are derived from high fiber shell reflective ability in the field of two-dimensional-periodic microstructure (two-dimensional photonic crystal).
Microstructured optical fibers have unique properties, especially in the choice of dispersion characteristics. With appropriate choice of fiber parameters (refractive index of fiber material, core size and shell) a zero group velocity dispersion for a given wavelength can be achieved. This leads to an increase of the effi-
ciency of nonlinear interactions and allows observing of the new nonlinear optics phenomena.
In our work was used PCFs Thorlabs NL-2, 4—800 and H 0710151_25, which have zero dispersion wavelength of 800 nm. Fiber Thorlabs NL-2, 4-800 has core diameter of 2.4 microns, and H 0710151_25 fiber — the core diameter of 2.5 microns (see Fig.1).
3. EXPERIMENTAL PART
The laser femtosecond radiation was provided by femtosecond laser complex at the Institute of Physics NAS of Ukraine (Coherent Mira Optima 900-F). Typical scheme of experimental setup is shown on Fig. 2. Basic functional parameters of the complex:
— minimum pulse duration t = 70 fs;
— maximum pulse power W = 2.5 MJ;
— maximum pulse peak power P = 3 x 1011 W;
— maximum radiation intensity I0 =1015 W/cm2;
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Fig. 4. SC spectra generated by FS pulses of Ti:S laser (A) at a wavelength of 820 nm, power 500 mW in microstructured photonic-crystal fibers: (a) Thorlabs NL-2, 4-800 length 14.2 cm and 30 cm (B) and (b) H 071011_25 50 cm in length and 349.3 cm (B). Insertion (C) — experimentally recorded intensity distribution of radiation leaving the fiber.
— pulse repetition rate 76 MHz. Frequency transformation is shown on Fig. 3.
4. SUPERCONTINUUM GENERATION IN PHOTONIC — CRYSTAL FIBER
The main factors for SC generation are: dispersion properties of a fiber with respect to wavelength of a pump laser, pulse duration and peak pulse power. Dispersion, and especially sign of dispersion, determines
the type of nonlinear effects and the nature of the spectrum — the spectral forms and stability.
The fibers Thorlabs NL-2, 4—800 and H 0710151_25 designed with zero dispersion wavelength of 800 nm (see Fig.4). In the normal dispersion region, where the pumping wavelength is less than zero dispersion wavelength, self-phase modulation and Raman scattering are the dominant nonlinear processes, which broaden spectrum into long-wave part. Fast pulses are broadening in the first few centimeters of fi-
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Fig. 5. SC spectra, pumping power of 400 mW: (a) Thorlabs NL-2, 4-800 length 14.2 cm, (b) H 0710151_25 length 50 cm in length for the corresponding wavelengths.
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ber; this limits the peak output power and spectral broadening.
In the anomalous dispersion region, where the pumping wavelength is more than the length corresponding for zero dispersion, the soliton is formed. In this region there is an efficient conversion of incoming energy into SC output radiation. Output spectra for different pumping power and pumping wavelength are presented in Fig. 5.
Apparently, for 780 nm (the normal dispersion region) a narrowed range of SC, most energy is concentrated around the wavelength of pumping. For 800 nm (zero dispersion region) the spectrum is wide and can be used in various tasks. For 820 nm we used pump
pulse with its wavelength in anomalous dispersion region and obtained a very wide spectrum.
5. POLARIZATION CHARACTERISTICS OF THE FEMTOSECOND SUPERCONTINUUM
We investigated polarization characteristics of SC by using Glan-Taylor prism and we showed that a FS laser radiation changes its linear polarization to elliptical after passing through the PCF H 0710151_25 length 50 cm. The ellipticity of polarization is 0.33. Experimental data is shown in Fig. 6.
Spectral broadening for each component of polarization was analyzed. It was shown that the maximal spectral broadening was 385 nm at the minimum pow-
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(a) Output power, mW
Fig. 6. Analyze of radiation polarization on the output fiber. Relative characteristics of radiation changes: (a) output power, (b) broadening, (c) central radiation component transmission regarding to pumping wavelength as functions of angle turn of Glan-Taylor prism.
er after polarizer, and the minimum spectral broadening er angle were analyzed. We get a maximum shift — 94 nm was 310 nm with a maximum power after polarizer. Cen- at the minimum power after polarizer and minimum shift tral components of FS shifting depending on the polariz- of 48 nm at the maximum p
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